optimization of ammonia-peroxide water...
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Optimization of Ammonia-Peroxide WaterMixture (APM) for High Volume Manufacturing
through Surface Chemical Investigations
Item Type text; Electronic Dissertation
Authors Siddiqui, Shariq
Publisher The University of Arizona.
Rights Copyright © is held by the author. Digital access to this materialis made possible by the University Libraries, University of Arizona.Further transmission, reproduction or presentation (such aspublic display or performance) of protected items is prohibitedexcept with permission of the author.
Download date 13/05/2018 21:13:56
Link to Item http://hdl.handle.net/10150/201511
OPTIMIZATION OF AMMONIA-PEROXIDE WATER MIXTURE (APM) FOR HIGH VOLUME MANUFACTURING THROUGH SURFACE CHEMICAL
INVESTIGATIONS
Shariq Siddiqui
___________________________________________
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING
In Partial Fulfillment of the Requirements For the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
2 0 11
2
THE UNIVERSITY OF ARIZONA
GRADUATE COLLEGE
As members of Dissertation Committee, we certify that we have read the dissertation prepared by Shariq Siddiqui entitled Optimization of Ammonia-Peroxide Water Mixture (APM) for High Volume Manufacturing through Surface Chemical Investigations and recommend that it be accepted as fulfilling the dissertation requirement for the degree of Doctor of Philosophy.
_____________________________________________________Date: 5/13/11
Srini Raghavan
_____________________________________________________Date: 5/13/11
Supapan Seraphin
_____________________________________________________Date: 5/13/11
Jinhong Zhang
_____________________________________________________Date: 5/13/11
Manish Keswani
Final approval and acceptance of this dissertation is contingent upon the candidate’s submission of the final copies of the dissertation to the Graduate College.
I hereby certify that I have read this dissertation prepared under my direction and recommend that it be accepted as fulfilling the dissertation requirement.
_____________________________________________________Date: 5/13/11
Dissertation Director: Srini Raghavan
3
STATEMENT BY AUTHOR
The dissertation has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the library.
Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgement of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.
SIGNED: Shariq Siddiqui
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TABLE OF CONTENTS
LIST OF FIGURES………………………………………..……………………………7
LIST OF TABLES………………………………………..………………………........10
ABSTRACT……………………………………………………………………...……..14
CHAPTER 1: INTRODUCTION………………………………………………………16
CHAPTER 2: LITERATURE REVIEW AND BACKGROUND…..........................24
2.1. Overview of Semiconductor Wafer Cleaning………………………………….24
2.2. Silicon Surface Wettability……………………………………………………….31
2.3 Particle-Wafer Interactions in Wet Cleaning Systems………………………...37
2.3.1 Van der Waals Forces………………………………………………….38
2.3.2. Electrical Double-Layer Interaction Forces………………………….47
2.4 Measurement of Interaction Forces……………………………………………. 53
2.5. Overview of Atomic Force Microscope (AFM)………………………..……….54
2.5.1 Principle of Force Measurements in Atomic Force Microscope…………............................................................................................56
2.6. Literature Review for Interaction Force Measurements using AFM………...58
2.7. Literature Review for the Stability of Ammonia-Peroxide Mixture (APM)…..64
CHAPTER 3: EXPERIMENTAL PROCEDURE AND METHODS………………..70
3.1. Materials …………………………………………………………………………..70
3.2. Silicon Surface and Tip Preparation……………………………………………70
3.3 Contact Angle Measurements.…………………………………………………..71
5
TABLE OF CONTENTS-CONTINUED
3.4. Surface Force Measurements……………..……………………………………72
3.5. Measurements of NH4OH and H2O2 concentrations using the Horiba SC-1 Composition Monitor………………………….……………………………………….75
3.5.1. Monitor Specification…….. …………………………………………...76
3.5.2. Data Acquisition………………………………………………………...77
3.5.3. Experimental Procedure for NH4OH and H2O2 Concentration Measurements.………………………………………...................................79
CHAPTER 4: RESULTS AND DISCUSSION………………………………….…...80
4.1. Interaction Force Measurements between Hydrophobic Si Surface and Si Tip using Atomic Force Microscopy……………………………………………………..80
4.1.1. Interaction Force Measurements between Si Surface and Si Tip in DI-water………………………………………………………………………...82
4.1.2. Interaction Force Measurements between Si Surfaces in NH4OH:H2O (1:100) Solution ….………………………………………….....84
4.1.3. Interaction Force Measurements between Si Surfaces in H2O2:H2O (1:100) Solution……………………………………………………………......86
4.1.3. Interaction Force Measurements between Si Surfaces in NH4OH:H2O2:H2O Solutions……………………………………………….....88
4.2. Analysis of Measured Adhesion Forces between Si Surfaces………………90
4.3. Comparison of Measured Repulsive Forces to Calculated Forces using Electrostatic Double Layer Theory…………………………………........................97
4.4. Comparison of Measured Adhesion Forces to Calculated Forces using JKR Adhesion Force Model……………………………………………………………….99
4.5. Brief Summary of Interaction Force Measurements………………..............104
6
TABLE OF CONTENTS-CONTINUED
4.6. Characterization of the Stability of APM Solutions using the Optical Concentration Monitor……………………………………………………………….105
4.6.1 Effect of Temperature on the Stability of APM Solution………......105
4.6.2. Effect of Dilution on the Stability of APM Solution………………...106
4.6.3. Effect of pH on the hydrogen peroxide decomposition…………...107
4.6.4. Effect of Iron (Fe2+) Ions on H2O2 Decomposition…………….......110
4.7. Kinetic Analysis of H2O2 Decomposition in APM Solutions …………..........111
4.8. Brief Summary of Stability of APM Solutions ….…………………………....119
CHAPTER 5: CONCLUSIONS AND FUTURE WORK ….………………………120
5.1. Interaction Force Measurements using Atomic Force Microscope………..120
5.2. Characterization of the Stability of APM Solutions using the Optical Concentration Monitor …………………………………..……………………….….121
5.3. Suggestions for Future Work..………………………..……………………….122
REFERENCES……………………………………………………………………….123
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LIST OF FIGURES
Figure 1.1: CMOS transistor pitch scaling trend vs. dates of introduction ……...17
Figure 2.1: Contaminated silicon wafer with different types of impurities………26
Figure 2.2: A schematic of typical wafer cleaning process in the front-end-of-line cleaning…………………………………………………………………………………27
Figure 2.3: A schematic of surface forces acting on three phase contact line of a liquid on the wafer surface……………………………………………………………31
Figure 2.4: Representation of water drop on (a) hydrophilic and (b) hydrophobic surfaces.………………………………………………………………………………..32
Figure 2.5: The interaction energy between two surfaces as a function of separation distance………………………………………………………………….. .37
Figure 2.6: Illustration of (a) same materials interacting in a liquid media and (b) two different materials interacting in a liquid media………………………………..41
Figure 2.7: A schematic representation of different potentials associated with a particle in aqueous solutions…………………………………………………………47
Figure 2.8: Zeta potential of particle contaminants as a function of pH…………51
Figure 2.9: Comparison of zeta potential of silicon dioxide (SiO2) surfaces prepared using different treatment methods as a function of pH…………………52
Figure 2.10: (a) A schematic representation of interaction forces between the surface and the tip using AFM (b) An SEM image of a silicon tip………………..54
Figure 2.11: A schematic of AFM controller feedback loop to maintain constant deflection between the tip and the surface………………………………………….54
Figure 2.12: A schematic representation of different stages of force-distance curves…………………………………………………………………………………...57
Figure 2.13: (a) Normalized approach and (b) retract force curves between a silicon nitride tip and a silicon surface as a function of separation distance in DIW and HF solutions……………………………………………………………………….61
Figure 2.14: A schematic representation of iron-catalyzed decomposition of hydrogen peroxide in APM solutions………………………………………………...68
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LIST OF FIGURES-CONTINUED
Figure 3.1: AFM image of silicon surface (2 x 2 µm) after etching in dilute HF solution...………………………………………………………………………………..71 Figure 3.2: Measured interaction forces between silica particle and silicon dioxide surface as a function of separation distance in 5 x 10-4 NaOH solution…………74 Figure 3.3: A schematic representation of Horiba CS-100C monitor coupled with a solution bath interfaced with resistively heated jacked and temperature controller……………………………………..…………………………………………75 Figure 3.4: Measured and calculated (a) H2O2 (b) NH4OH concentrations in 1:1:5 APM solutions at different temperatures…………………………………………….77 Figure 3.5: A graphical representation of ammonium hydroxide, hydrogen peroxide, and water concentrations measured using the Horiba CS-100C concentration monitor…………………………………………………………………78 Figure 4.1: Water contact angle values for silicon surfaces treated with different solutions as a function of time………………………………………........................82
Figure 4.2: Interaction forces as a function of separation distance between Si surface and Si tip in DI-water after 2, 10 and 60 min of immersion time……..….84 Figure 4.3a: Approach force curves as a function of separation distance in aqueous NH4OH:H2O (1:100) solution after 2, 10 and 60 min of immersion time………...........................................................................................................85
Figure 4.3b: Retract force curves as a function of separation distance in aqueous NH4OH:H2O (1:100) solution after 2, 10 and 60 min of immersion time………...........................................................................................................86
Figure 4.4a: Approach force curves as a function of separation distance in aqueous H2O2:H2O (1:100) solution after 2, 10 and 60 min of immersion time………...……………………………………………………………………………87
Figure 4.4b: Retract force curves as a function of separation distance in aqueous H2O2:H2O (1:100) solution after 2, 10 and 60 min of immersion time………...……………………………………………………………………………88
Figure 4.5: (a) Approach and (b) retract force curves as a function of separation distance in dilute NH4OH:H2O2:H2O (1:1:100) solution………………………...….89
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LIST OF FIGURES-CONTINUED
Figure 4.6: (a) Approach and (b) retract force curves as a function of separation distance in dilute NH4OH:H2O2:H2O (1:1:100 – 1:1:500) solutions………………90
Figure 4.7: Representation of an abrupt jump-in distance between the silicon surface and silicon tip marked as “a”. The only data point available after tip jump-in and before making contact with the surface is marked as “b”. The average value of point “a” and “b” is used for the calculating the product of the Hamaker constant and tip radius….....................................................................................93
Figure 4.8: Example of an exponential fit to measured repulsive forces between silicon surface and silicon tip in H2O2:H2O (1:100) solution after 2 min of immersion time…………………………………………………………………………95
Figure 4.9: Measured concentrations of (a) NH4OH and (b) H2O2 for a conventional (1:1:5) APM solution at different temperatures……………………106
Figure 4.10: Measured concentrations of (a) NH4OH and (b) H2O2 in 1:1:50 APM solution at 24°, 40°, 50° and 65 °C………………………………................. ........107
Figure 4.11: Hydrogen peroxide decomposition at 65 °C as a function of time at different pH values…………………………………………………………………...108
Figure 4.12: Measured and calculated [OH-] for 1:1:5 APM solutions…………110
Figure 4.13: Decomposition of hydrogen peroxide at different Fe2+ concentrations in APM solutions maintained at 50 and 65 °C…………………………………….111
Figure 4.14: An example of fitted data of hydrogen peroxide concentration vs. time. Open circles represent the experimental data. A solid line is the fitted second order polynomial…………………………………………………………….112
Figure 4.15: Log-log plots of rate of H2O2 decomposition (mol. L-1 sec-1) vs. H2O2 concentration (mol. L-1) at different solution pH values at 65 C..………………114
Figure 4.16: (a) Log-log plot of rate of hydrogen peroxide decomposition and hydrogen peroxide concentration at 0, 5 and 10 ppb Fe2+ in 1:1:5 APM solutions at 65 °C. (b) First order reaction rate constant ( k’’) as a function of Fe2+ concentration at different APM solution temperatures…………………………...117
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LIST OF TABLES
Table 1.1: Front end processing surface preparation technology requirements..19
Table 2.1: Surface tension and its components (γTOT, γLW, γ+,γ-) of commonly
used probe liquids at 20 °C………………………………………………………….35
Table 2.2: van der Waals interaction energy for common geometries…………..40
Table 2.3: Hamaker constant Aii for two identical materials interacting in vacuum………………………………………………………………………………….42
Table 2.4: Calculated Hamaker constants for two materials 1 and 2 immersed in a liquid medium (3)…………………………………………………………………….43
Table 2.5: Mechanism of decomposition of hydrogen peroxide by Fe3+………...66 Table 3.1: Surface tension and its components of different liquids used for contact angle measurements…………………………………………………………72 Table 3.2: Recommended measurement ranges for the concentration of ammonium hydroxide, hydrogen peroxide and water in Horiba CS-100C APM composition monitor…………………………………………………………………...76 Table 4.1: Measured adhesion force and calculated product of the Hamaker constant and tip radius between silicon surface and silicon tip as a function of immersion time in DI-water………………………………………………………..….91 Table 4.2: Comparison of measured adhesion force and calculated product of the Hamaker constant and tip radius (AH.RT) between silicon surface and silicon tip as a function of immersion time in DI-water……………………………….………..94 Table 4.3: Comparison of the calculated product of the Hamaker constant and tip radius using the measured adhesion force and total interaction force (attractive and repulsive) between silicon surface and silicon tip as a function of immersion time in NH4OH:H2O (1:100) and H2O2:H2O (1:100) solutions…………………….96 Table 4.4: Comparison of the calculated electrostatic forces using the electrical double layer model and experimentally measured repulsive forces between silicon surface and a silicon tip as a function of immersion time in NH4OH:H2O (1:100) and H2O2:H2O (1:100) solution……………………………………………..98
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LIST OF TABLES-CONTINUED Table 4.5: Contact angles (θ) for Si surface treated with DI-water, NH4OH:H2O (1:100) and H2O2:H2O (1:100) solutions measured with water (θw), formamide (θFM) and diiodomethane (θMI) for HF-treated silicon surfaces…………………99 Table 4.6: Calculated surface free energy components (γS
LW, γS+, γS
-) and interfacial tension (γSL) between silicon surface and different solutions as a function of treatment time. The units of calculated values are in N.m-1. ……….101 Table 4.7: Comparison of the calculated adhesion force (FJKR/R) using the JKR model and measured force (Fadhesion/R) between silicon surface and silicon tip in DI-water as a function of immersion time………………………………………….102 Table 4.8: Comparison of the calculated adhesion force (FJKR/R) using the JKR model and measured force (Fadhesion/R) between silicon surface and silicon tip in NH4OH:H2O (1:100) and H2O2:H2O (1:100) as a function of immersion time………………………………………………………………………………..…...103 Table 4.9: Rate constant, [OH-], ratios of rate constants and hydroxyl ions as a function of pH in APM solutions at 65 °C………………………………… ……….115 Table 4.10: H2O2 half-lives in different APM solutions at 65 °C… ………………116
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ACKNOWLEDGMENTS
I would like to begin this acknowledgement by sincerely thanking my advisor and mentor, Professor Srini Raghavan, for his guidance, support and mentorship in the completion of this dissertation. Professor Raghavan has been flexible, patient, helpful and kind throughout my time in his research group. Being part of his research group has truly provided me opportunities that are rare to find.
I would also like to thank Professor Jinhong Zhang for his teaching and training, particularly with atomic force microscope. Without his training, part of my graduate studies, particularly interaction force measurements, would have been a very difficult task. I’d like to acknowledge a few other faculty members, Professors Supapan Seraphin, Jim Farrell, and Anthony J. Muscat, who taught me aspects of science with different views and approaches. In addition, I’d like to thank Avi Fuerst and Barry Brooks from Intel Corporation for their insightful discussions.
I would also like to thank the staff of the Department of Materials Science and Engineering for all of their help and guidance through my education here at the University of Arizona. I would also like to thank the SRC/Sematech Engineering Benign Semiconductor Manufacturing Research Center and staff (Alicia Foley and Karen McClure) for the funding and the opportunities provided to me through conferences and exposure to work conducted in other academic universities and semiconductor industry.
I wish to express my warm and sincere thanks to the following people who have made this dissertation possible and because of whom my duration at U of A has been one of the greatest and most unforgettable experiences of my lifetime: Dr. Manish Keswani, a friend and colleague who has helped and mentored me throughout my graduate studies; the Raghavan research group, particularly Ryan Biggie, and Rajkumar Govindarajan. A few close friends: Rahul Jain, Shweta Agarwal, Greg Cure, Tim Sullivan, Gary Morton, Andrew Abalos Joaquin Cruz, Jeffrey Scogin, James Collins and Arin Leonard.
My greatest thanks goes to my family whose constant love and support has been a key component in finishing graduate studies and this dissertation. I would like to acknowledge my parents, Shahid Siddiqui and Vardah Jamal Siddiqui; my brothers and sister Shahab Siddiqui, Ali Siddiqui, Rohit Tripathi and Mariam Siddiqui; my sister-in-law, Shahla Khan, and my uncle Dr. Junaid Siddiqui. Last but not least, I would like to extend my sincere gratitude and many thanks to Kari Davies-Mason, who has been the biggest supporter and without whose patience, understanding and unconditional love, this dissertation would be impossible to complete.
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DEDICATION
I would like to dedicate this dissertation to my parents Shahid H. Siddiqui and Vardah J. Siddiqui.
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ABSTRACT
Ammonia-peroxide mixture (APM) is a widely used wet chemical system
for particle removal from silicon surfaces. The conventional APM solution in a
volume ratio of 1:1:5 (NH4OH:H2O2:H2O) is employed at elevated temperatures
of 70-80 °C. At these temperatures, APM solution etch es silicon at a rate of ~3
Å/min, which is unacceptable for current technology node. Additionally, APM
solutions are unstable due to the decomposition of hydrogen peroxide and
evaporative loss of ammonium hydroxide resulting in the change in APM solution
composition. This has generated interest in the use of dilute APM solutions.
However, dilution ratios are chosen without any established fundamental
relationship between particle-wafer interactions and APM solutions.
Atomic force microscopy has been used to measure interaction forces
between H-terminated Si surface and Si tip in APM solutions of different
compositions. The approach force curves results show attractive forces in DI-
water, NH4OH:H2O (1:100) and H2O2:H2O (1:100) solutions at separation
distances of less than 10 nm for all immersion times (2, 10 and 60 min)
investigated. In the case of dilute APM solutions, the forces are purely repulsive
within 2 min of immersion time. During retraction, the adhesion force between Si
surface and Si tip was in the range of 0.8 nN to 10.0 nN. In dilute APM solutions,
no adhesion force is measured between Si surfaces and repulsive forces
dominated at all distances. These results show that even in very dilute APM
15
solutions, repulsive forces exist between Si surface and particle re-deposition
can be prevented.
The stability of APM solutions has been investigated as a function of
temperature (24 - 65 °C), dilution ratio (1:1:5 - 1 :2:100), solution pH (8.0 - 9.7)
and Fe2+ concentration (0 - 10 ppb) using an optical concentration monitor. The
results show that the rate of H2O2 decomposition increased with an increase in
temperature, solution pH and Fe2+ concentration. The kinetic analysis showed
that the H2O2 decomposition follows a first order kinetics with respect to both
H2O2 and OH- concentrations. In the presence of Fe2+, hydrogen peroxide
decomposition follows a first order reaction kinetics with respect to H2O2
concentration.
16
CHAPTER 1
INTRODUCTION
The continuous scaling down of silicon complimentary metal-oxide-
semiconductor (CMOS) devices has been the primary means by which the
semiconductor industry has achieved unprecedented gains in productivity and
performance as quantified by Moore’s Law.1 In 1965, Gordon Moore (co-founder
of Intel Corporation) predicted that the minimum feature size could be expected
to reduce by ~0.7 times, while the number of transistors per integrated circuit
chip would double every 18 months, as shown in Figure 1.
Figure 1.1: CMOS transistor pitch scaling trend vs. dates of introduction.
For the past 40 years, the semiconductor industry has met and exceeded
the Moore’s Law requirements, which has been held as the benchmark for
17
integrated circuit (IC) scaling. Silicon CMOS scaling is no longer a simple matter
of shrinking device dimensions. Maintaining the scaling roadmap will require
continual improvement in channel mobility. While advanced materials, such as
germanium (Ge) or III-V semiconductors, may offer potential long-term solutions
a shorter term approach requires novel device structures, new chemistries and
optimized processes in order to meet the requirements for higher transistor
density and performance.2-3
One of the key enablers that has made silicon CMOS device scaling
possible is the advancements in wafer cleaning technology. Wafer cleaning is a
critical step in the fabrication of ultra-large-scale integration (ULSI) circuits due to
its ability to maintain the contamination and defectivity levels within the required
specifications.4 The overall objective of wafer cleaning is the removal of particles,
defects and chemical impurities from the surface without collapsing the patterned
features on the wafer surface.5 It can be achieved either by liquid-phase or gas-
phase cleaning methods depending on the fabrication step. Wet cleaning
systems have been widely used to remove particles and other contaminants
because of their excellent characteristics such as high cleaning performance,
high throughput, and low damage.6 It also offers several other advantages over
their counterpart gas-phase cleaning chemistries that include high solubility of
certain contaminants, drag forces to aid in removal of solid contaminants, and
metal complexation.3, 7
18
Wafer cleaning can have a direct impact on device performance and it has
been reported that over fifty percent of yield losses in device fabrication are due
to micro-contamination.4 With an increase in new materials and processing steps
in device fabrication, the International Technology Roadmap for Semiconductors
(ITRS)8 has set stringent requirements for wafer cleaning to ensure high device
yield. The ITRS requires that the killer defect density, critical particle diameter
and total particle count be 0.043 #/cm2, 17.9 nm and 135.3 #/wafer, respectively,
for the front surface of a 300 mm wafer in logic devices. In addition, at the 45 and
32 nm technology nodes, the material loss target for silicon and silicon oxide is
less than 0.4 Å and 0.3 Å, respectively per cleaning step. This requirement of
minimized material loss while maintaining high particle removal efficiency for
nanosized particles is currently one of the most difficult challenges in wafer
cleaning. The front end of the line surface cleaning requirements set by the ITRS
roadmap is summarized in Table 1.1.
19
Table 1.1: ITRS roadmap for front end processing surface preparation technology requirements.8
Year of Production Near-term
2007 2008 2009 2010 2011 2012 2013
DRAM ½ pitch (nm) 65 57 50 45 40 36 32
MPU/ASIC Metal 1 ½ pitch (nm) 68 59 52 45 40 40 36
MPU Physical gate length (nm) 25 23 20 18 16 14 13
Front Surface Particles
Killer defect density (#/cm2) 0.022 0.027 0.017 0.022 0.027 0.017 0.022
Critical particle diameter, (nm) 31.8 28.5 25.3 22.5 20.1 17.9 15.9
Critical particle count (#/wafer) 75.2 94.8 59.6 75.2 94.8 135.3 170.4
Metallic and Surface Contamination
Critical GOI surface metals 0.5 0.5 0.5 0.5 0.5 0.5 0.5
Critical surface metals 1.0 1.0 1.0 1.0 1.0 1.0 1.0
(1010 atoms/cm2)
Mobile ions (1010 atoms/cm2) 2.0 2.2 2.4 2.5 2.3 2.5 2.4
Surface carbon (1010 atoms/cm2) 1.2 1.0 0.9 0.9 0.9 0.9 0.9
Surface oxygen (1010 atoms/cm2) 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Cleaning effects on materials
Surface roughness, RMS (Å) 0.4 0.4 0.4 0.2 0.2 0.2 0.2
Silicon loss (Å) per cleaning cycle 0.5 0.4 0.4 0.3 0.3 0.3 0.3
Oxide loss (Å) per cleaning cycle 0.5 0.4 0.4 0.4 0.3 0.3 0.2
20
A key challenge in FEOL cleaning is the removal of particles, metallic
impurities and organic contaminants from the wafer surface. Particle removal is
critical since they can cause “killer” and “latent” defects during subsequent
processing steps and results in a severe yield loss at the end of the line.9
Particles can locally block or mask photolithography, implant, or etch steps.
These particles can adhere to surfaces and may become embedded during film
formation, leading to pinholes, micro-voids, micro-cracks, and other structural
defects, depending on their chemical composition.9 Other contaminants, such as
sodium ions and trace metal ions are also detrimental, particularly during high-
temperature processing steps (thermal oxidation, diffusion, epitaxial growth)
because they can diffuse into the wafer and cause electrical defects and device
degradation.10-11,12 Organic contaminants such as photoresist and solvent
residues can alter film properties and also cause device degradation.
One of the most widely used wet cleaning systems in semiconductor
manufacturing is based on the RCA Standard Cleans (SC-1 or APM and SC-2 or
HPM) developed by Kern and Puotinen in 1970s.7, 13 Ammonia-peroxide mixture
is used for the removal of particle and organic contaminants from the silicon
surface. The conventional APM is a mixture NH4OH (29 wt%), H2O2 (30 wt%)
and de-ionized water in a volume ratio of 1:1:5 and generally employed at
elevated temperatures of 70-80°C. The addition of m egasonic energy has been
used to enhance particle removal.4, 14 The cleaning mechanism in APM solution
is based on the oxidation of the hydrophobic silicon surface by hydrogen
21
peroxide. This oxidation step is followed by the etching of SiO2 film by ammonium
hydroxide in the cleaning solution, which facilitates particle removal. As a result
of oxidation and dissolution rates, a thin chemical oxide of 10 Å is formed after
treatment with APM solutions.15 Additionally, high pH (~10) of APM solution
provides a condition under which dislodged particles and surface experience
electrostatic repulsion, which prevents re-deposition of particles onto the
surface.16-17
One of the disadvantages of using a conventional APM (1:1:5) solution for
cleaning is that it etches silicon oxide at a rate of 2.5-3 Å/min at 70-80°C. 15 For
32 nm technology node and lower, such etch rates become unacceptable. As
indicated in Table 1.1, the ITRS roadmap dictates that the loss of silicon and
silicon oxide to be less than 0.4 Å and 0.3 Å for 45 and 32 nm technology,
respectively per cleaning cycle. Therefore, there is a trend in semiconductor
industry to use dilute APM solutions for wafer cleaning to meet the strict
requirement set by the ITRS roadmap. Currently, dilution levels (1:1:50 to
1:1:100) are chosen based on particle removal efficiency data. An alternative
approach to choose an optimal APM ratio is through a systematic study of
interaction forces between particles and surfaces. The surface force apparatus18-
19 and the atomic force microscope (AFM)20-22 have provided direct methods to
measure the interaction force between two surfaces. In particular, AFM has
emerged as a powerful tool for measuring interaction forces between two
surfaces in vacuum, air and liquid media. AFM can also be employed to measure
22
the adhesion force between particles and surfaces. In general, the adhesion
force between a particle and a substrate in wet cleaning scenario is mainly due to
van der Waals force.
Another disadvantage associated with APM solutions at elevated
temperatures is that there is a change in composition due to the evaporative
losses of ammonium hydroxide and the decomposition of hydrogen peroxide.
This change can lead to higher silicon etching, surface roughness and insufficient
particle removal. For example, APM solutions with a hydrogen peroxide
concentration below 1 wt% can lead to undesired silicon etching at a rate of 0.4
um min-1.23 In addition to temperature, the stability of hydrogen peroxide can also
be influenced by pH of the solution and the presence of metallic contaminants
typically at 0-10 ppb levels. In particular, iron can act as a catalyst and has the
most significant effect on H2O2 decomposition followed by Cu with about an order
of magnitude smaller effect.24 Due to the lack of APM composition control, the
solutions are re-spiked with NH4OH and H2O2 or replaced in order to maintain
process stability. This approach is undesirable due to the high cost of chemical
consumption and the lost production time from bath replacement. With an
increased interest in using dilute APM solutions, it is important to monitor and
control composition of APM solutions to ensure uniform processing, device
reliability and yield.24
The objective of this research is to systematically study the interaction
forces between silicon surfaces in dilute ammonium hydroxide-hydrogen
23
peroxide solutions using atomic force microscope (AFM). Additionally, the
stability of ammonium hydroxide and hydrogen peroxide in APM solutions was
continuously and simultaneously measured using an optical concentration
monitor.
24
CHAPTER 2
LITERATURE REVIEW AND BACKGROUND
2.1. Overview of Semiconductor Wafer Cleaning
Wafer cleaning has played a vital role in order to meet the requirements of
scaling set by the ITRS roadmap. In advanced integrated circuit manufacturing,
wafer cleaning processes represent more than 25% of the operations and are
considered to be key to the performance of the final device.16 Wafer cleaning in
ultra-large scale integration (ULSI) technology has been divided into two main
categories, namely Front-End-of-the-Line (FEOL) and Back-End-of-the-Line
(BEOL). Cleaning processes in FEOL include steps that extend from a bare
silicon wafer to the first metal contact. In contrast, BEOL cleaning includes post-
etch residue removal and post-CMP cleaning.
In particular, cleaning prior to gate dielectric formation is an important
step. It is necessary to minimize not only particles but also to minimize silicon
etching and surface roughness. In addition to the removal of particles, other
contaminants such as organic, metallic and photo-resist residual must be
removed in FEOL to increase the device yield. Figure 2.1 shows some of the
contaminants on silicon wafer that are removed during wet cleaning processes.
25
Figure 2.1: Contaminated silicon wafer with different types of impurities. Used with permission of Manish Keswani.
Wet cleaning chemistries based on the use of strong inorganic acids,
bases and oxidizers have been extensively used in cleaning processes.9 These
systems include sulfuric acid-hydrogen peroxide mixtures, hydrofluoric acid (HF)
based solutions, ozone based systems, ammonium hydroxide-hydrogen peroxide
mixtures (APM) and hydrochloric acid-peroxide mixtures (HPM).7, 13 Cleaning
processes should be able to remove particles and contaminations from the
surface without changing wafer physical and chemical properties. In this context,
each of these wet cleaning systems is unique and has played a vital role in the
advancement of wafer cleaning technology. The mechanism of liquid phase
cleaning can be purely through a physical removal process and/or through
chemical reaction dissolution.25-26 Figure 2.2 shows the schematic of a typical
cleaning sequence using wet cleaning systems in FEOL wafer cleaning. Each of
the cleaning steps in this figure could be repeated many times during the process
flow, depending on the fabrication scheme.
26
Figure 2.2: A schematic of typical wafer cleaning process in the front-end-of-line cleaning.
Sulfuric-acid peroxide (SPM) mixture, also known as piranha is used as a
first step to remove organic contaminants from the wafer surface. SPM solutions
consist of sulfuric acid (98%) and H2O2 (30%) with volume ratios ranging from 2:1
to 4:1 H2SO4:H2O2 and are typically used at elevated temperatures of 90-120 °C
for 10-15 min, followed by DI-water rinsing. SPM solutions are also used to
remove photoresist that is un-implanted or only lightly implanted up to about 1 x
1014 ions/cm2.27 Stripping high dose-ion implant (HDI) photoresists is one of the
most challenging processes in the semiconductor manufacturing due to the
difficulty of removing the dehydrogenated, amorphous carbon layer that forms on
27
the surface during the ion implantation.28 A combination of SPM solutions with
other processes such as either low pressure plasma ashing or O3-DIW has been
used to remove HDI photo-resists.
Ozone based aqueous solutions have been used in semiconductor
industry for organic contamination removal and photoresist stripping, largely due
to their low cost and environmental benefits. The high oxidation potential (E0red =
2.08 V) of ozone in liquid solutions makes it an effective cleaning system. There
is a growing interest in the use of ozone-sulfuric acid mixtures due to the higher
solubility of ozone in sulfuric acid compared to that in water.14
Hydrofluoric acid (HF) solutions are used to remove the native oxide
(SiO2) film from the silicon surface since it is a poor quality oxide film. The
etching rate and uniformity of SiO2 film depends on the composition and
temperature of the solution. Typically, oxide layers are removed in a HF: H2O
mixture with a volume ratio ranging from 1:50 to 1:100 at room temperature.29-31
Etching by HF leaves the silicon surface terminated with Si-H or Si-H2 groups.
The etching of silicon dioxide by HF occurs according to the following reaction:
SiO2 + HF SiF62- + 2H+ + 2H2O [2.1]
It was first initially suggested that a silicon surface treated with HF solution
resulted in a chemically stable surface due to fluorine termination.32 This
argument was supported based on the fact that the Si-F bond strength of 138.4
28
kcal/mol is much greater than that of Si-H (80.8 kcal/mol). However, numerous
experimental studies using different analytical techniques, such as Fourier
transform-infrared spectroscopy (FT-IR), high-resolution electron energy-loss
spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS) showed
that the chemical stability of a silicon surface could be attributed to surface
passivation by hydrogen atoms. It was concluded that etching of SiO2 in HF
solutions is a kinetically controlled reaction.29-30
The next step in cleaning sequence involves removal of particles from the
wafer surface using a mixture of ammonium hydroxide (NH4OH), hydrogen
peroxide (H2O2) and de-ionized water (DIW) known as APM or SC-1 solution.
The conventional APM mixture consists of NH4OH (29%), H2O2 (30%) and DI-
water in the ratio of 1:1:5 and is typically employed at ~70-80 °C with or without
megasonic energy.24 The cleaning mechanism in APM solutions is based on
simultaneous oxidation and etching of the silicon dioxide surface. Previously
reported experimental results show that the H-terminated silicon surface
becomes hydrophilic instantly, therefore indicating that rate of oxidation is faster
than silicon dioxide etching in APM solutions.33
One of the drawbacks of using APM solutions is a high surface roughness
(0.3 Å) and silicon loss (0.4 Å) which is not acceptable by the ITRS roadmap for
32 nm and beyond technology nodes. The other disadvantage of 1:1:5 APM
solution is that it has a higher ionic strength, which reduces the electrical double-
layer repulsion of the particles resulting in a less efficient particle removal. In
29
recent years, dilute APM solutions have been used to decrease etching of silicon
and surface roughness while maintaining high particle removal efficiency. In one
study, etching of silicon surface in APM solutions with composition ratios in the
range of 1:1:5 to 0.0001:1:5 (NH4OH:H2O2:H2O) at 80 °C has been reported. 34
The results show that the silicon etch rate decreased from 0.8 nm/min to 0.50
nm/min with a decrease in ammonium hydroxide concentration. In another study,
it was reported that lowering the ammonium hydroxide concentration by half in
1:1:5 APM solution increased the particle removal efficiency by a factor of two
without changing the surface roughness.35 Another drawback of using APM
solution is that a trace metal contaminant such as iron on the wafer surface can
act as a catalyst and increase the decomposition of hydrogen peroxide. This can
lead to a change in APM composition resulting in higher surface roughness and
decreased particle removal efficiency.
Hydrochloric acid-hydrogen peroxide mixture (HPM) or SC-2 cleaning step
is used to effectively remove metallic contaminants from the wafer surface. This
second set in RCA cleaning was designed to remove alkali ions, and cations,
such as Al3+, Fe3+, and Mg2+, that form NH4OH-insoluble hydroxides in basic
solutions. This second step also eliminates metallic contaminants not entirely
removed by APM treatment. Although oxidation of a silicon surface from
hydrogen peroxide is possible in HPM solution, there is no etching of silicon and
silicon dioxide.
30
The last steps in wafer cleaning technology are rinsing and drying. The
quality of a cleaning sequence is dependent on these two steps because clean
wafers can very easily become re-contaminated. The purpose of rinsing is to
remove chemical residues that might be left on the wafer surface. Commonly
used rinsing techniques in semiconductor industry include overflow rinse and
quick dump rinse for batch systems, and spray rinse for single wafer systems.
The last step in the wafer cleaning processes is wafer drying. Recently, wafer
drying has become a critical step in cleaning technology due to an increase in
pattern density and rapid decreases in pattern size. Removing water can be
achieved by various methods including evaporation, wafer spinning at high
velocity and displacement by another liquid with a lower surface tension.
However, drying by evaporation can leave watermarks on the wafer surface. Spin
and hot isopropyl alcohol (IPA) vapor drying have been replaced by Marangoni-
type IPA drying technique, which utilizes the surface pressure gradient at the air-
liquid interface at room temperature. IPA has been used in wafer drying because
of its low surface tension and high solubility in water.
31
2.2. Silicon Surface Wettability.
Surface wettability plays an important role in the attraction of particles to
the surface. For example, H-terminated silicon surface easily attracts particles
due to its hydrophobic nature. Another reason wettability is important in wafer
cleaning is because of an increase in high aspect ratios in current technologies,
which require wet cleaning systems to penetrate through the fine patterns and
trenches.3 The wettability of a surface depends upon the surface tension of both
the solid and the liquid. When the surface tension of the solid is greater than or
equal to the surface tension of the solution, surface wettability increases. The
wettability of a silicon surface can be evaluated by using the contact angle
technique. Contact angle is defined by the equilibrium of the three surface
tension vectors at a solid-liquid-vapor interface as shown in Figure 2.3.
Figure 2.3: Surface forces acting on the three phase contact line of a liquid droplet placed on the wafer surface.
32
Contact angle is related to the surface and interfacial tension free energies
through Young’s equation30, as shown in Eq. 2.2.
SLVSVL γγθγ −=cos [2.2]
In this equation, γLV is the surface tension of the liquid (subscript L), θ is
the contact angle, γSV is the surface free energy of the solid (subscript S) and γSL
is the solid-liquid interfacial energy. Surfaces that exhibit water contact angle of
less than 20° are known as hydrophilic whereas surfaces tha t have water contact
angle close to or greater than 90° are considered hydrophobic. For example, Si
wafer surface treated with a conventional APM solution results in a complete
wetting by DI-water and has contact angle value of less than 5°. Removal of the
oxide film by etching in HF solution results in H-terminated Si surface, which is
hydrophobic in nature with a contact angle value in the range of 70-80°. 36 A
schematic representation of hydrophilic and hydrophobic surfaces is shown in
Figure 2.4.
Figure 2.4: Representation of water drop on (a) hydrophilic and (b) hydrophobic surfaces.
a b
33
There are several different methods to measure a contact angle, including
the static and dynamic sessile drop method, the Wilhelmy plate method and the
powder contact angle method.37 Among these various measurement methods,
the sessile drop technique is one of the most commonly used methods. In this
method, the contact angle is directly measured from the liquid droplet profile on a
flat surface using a goniometer.38
Van Oss, Chaudhury, and Good39 developed an acid-base theory that
describes the interaction between two surfaces immersed in liquid media.
According to this theory, the change in free energy upon immersion of a solid (S)
immersed into liquid (L), ∆GSL can be calculated by adding the non polar or
Lifshitz-van der Waals (LW) component and a polar or acid-base (AB)
component, as shown in equation 2.3.
ABSL
LWSLSL GGG ∆+∆=∆ [2.3]
The LW component of the change in free energy can be described in
terms of the surface tension of both the solid (γS) and liquid (γL), as shown in
equation 2.4.
LWL
LWS
LWSLG γγ2−=∆ [2.4]
The polar component can be described in two separate parameters: polar
or acid parameter (γ+) of surface tension and a non-polar or base parameter (γ-)
of surface tension for both solids and liquids.
34
)(2 +−−+ +−=∆ LSLSAB
SLG γγγγ [2.5]
The combining rule for both the polar and non-polar components can be
obtained by substituting equation 2.4 and 2.5 into equation 2.3.
++−=∆ +−−+
LSLSLW
LLW
SSLG γγγγγγ2 [2.6]
The interfacial tension between a solid and a liquid can also be described
in terms of polar (LW) and non-polar (AB) components, as shown in equation 2.7.
( ) )(22
+−−+−+−+ +−++−= LSLSLLSS
LW
L
LW
SSL γγγγγγγγγγγ [2.7]
The quantity ∆GSL is the work of adhesion of solid to liquid and can also
be described in terms of Dupre equation:
LSLSSLG γγγ −−=∆− [2.8]
Inserting equation 2.8 into Young’s38 equation, the free energy can be
related to the contact angle, as shown in equation 2.9.
)cos1( θγ +=∆− LVSLG [2.9]
Combining equations 2.7 and 2.9, the Young-Dupre equation can be
written in the form, as shown in equation 2.10.
35
)(2)cos1( +−−+ ++=+ LSLS
LW
L
LW
S
TOT
L γγγγγγγθ [2.10]
where θ is the contact angle; γLTOT is the total surface tension of a liquid.
The Lifshitz-van der Waals (γsLW ) and the acidic (γ+) and basic components (γ-) of
a solid surface can be determined experimentally by performing contact angle
measurements using three well-characterized probe liquids with known
parameters (γLTOT
, γLLW, γL+,γL-) and by solving equations 2.10a-c. Table 2.1
summarizes the parameters for some of the commonly used probe liquids.
)(2)cos1( 11111
+−−+ ++=+ LSLS
LW
L
LW
S
TOT
L γγγγγγγθ [2.10a]
)(2)cos1(22222
+−−+ ++=+ LSLS
LW
L
LW
S
TOT
L γγγγγγγθ [2.10b]
)(2)cos1( 33333
+−−+ ++=+ LSLS
LW
L
LW
S
TOT
L γγγγγγγθ [2.10c]
Table 2.1: Surface tension and its components (γTOT, γLW, γ+,γ-,γAB) of commonly used probe liquids at 20 °C. 40
γTOT γLW γ+ γ- γAB=2−+γγ
DI-water 72.8 21.8 25.5 25.5 51.0
Formamide 58.0 39.0 2.28 39.6 19.0
Glycerol 64.0 34.0 3.92 5.74 9.5
Bromonaphthalene 44.4 44.4 0 0 0
Diiodomethane 50.8 50.8 0 0 0
36
Surface free energy of HF etched silicon and its components have been
calculated from contact angle data obtained from water, diiodomethane and
bromonapthelene.41 The total surface free energy of silicon etched in HF
solutions in the range of 0.1 to 5.0 % was in the range of 50-54 mJ/m2. The polar
component of surface free energy significantly decreased from 40 mN/m2 to 5
mN/m2 with an increase in HF concentration from 0.1 to 5.0%. This decrease in
polar component of surface free energy was due to progressive H-passivation of
silicon surface. In contrast, the non-polar component slightly increased from 30
mN/m2 to 50 mN/m2.
In another study, H-terminated silicon surface energy was calculated after
treatment with HNO3, H2SO4-H2O2, HNO3-HF and H2SO4-H2O2-HF cleaning
systems.42 It was reported that the basic component of surface tension (γS-)
increased continuously through 60 min for surfaces treated with oxidizing acid
mixtures (HNO3 and H2SO4-H2O2) indicating an increase in hydrophilicity. This
increase in hydrophilicity was due to the formation of Si-OH groups on silicon
surfaces. In the case of surfaces treated with oxidizing acid mixtures containing
HF (HNO3-HF and H2SO4-H2O2-HF), a decrease in γS- was observed with time.
The change in free energy (∆GSL) for H-terminated silicon surface increased from
96 mN/m2 to 132 and 135 mN/m2 after treatment in HNO3 and H2SO4-H2O2
solutions, respectively. In oxidizing solutions containing HF (HNO3-HF and
H2SO4-H2O2-HF), the free energy decreased from 96 mN/m2 to 90 and 85
mN/m2, respectively in these solutions.
37
2.3. Particle-Wafer Interactions in Wet Cleaning Systems.
In general, particles adhere to a silicon surface mainly due to the van der
Waals and electrostatic interaction energies.16 The interaction energy between a
particle and the wafer surface is a function of the separation between two
surfaces. At short separation distances of less than 10 nm, the van der Waals
energy is dominant while the electrical double-layer energy governs the
interaction at larger separation distances. Figure 2.5 is a schematic illustration of
van der Waals, electrostatic interaction energy and sum of the two components
(total interaction energy, represented as “W” in Figure 2.5) as a function of
separation distance between a particle and a surface. In this figure, the positive
energy represents repulsive energy and the negative energy represents
attractive energy.
Figure 2.5: Interaction energy between two surfaces as a function of separation distance.
38
2.3.1. Van der Waals Forces
In 1873, van der Waals first proposed that intermolecular interactions
between neutral atoms or molecules originate from the instantaneous dipole
interactions due to the position of the electrons surrounding the nuclei. This was
to account for non ideal gas behavior in the continuum between gaseous and
liquid states of matter.43-44 These interactions include permanent dipole-dipole or
Keesom potential, permanent dipole-induced dipole (Debye interactions) and
induce dipole-induce dipole interactions or London dispersion interactions, and
the sum of all three interactions are known as the van der Waals (vdw)
interaction energies. All three interaction energies are inversely proportional to
the sixth power of the closest separation distance.
The permanent dipole-dipole interaction or Keesom interaction (Uk)
originates from the angle averaged dipole-dipole interaction and is shown in
Equation 2.11.
6221
6 )4(3)(
Trk
uu
r
CrU
Bo
Kk επε
−=−= [2.11]
where u1 and u2 are the dipole moments of the molecules, ε the dielectric
constant of the medium, ε0 is the permittivity of the vacuum, kB the Boltzmann’s
constant and T is the temperature. The averaged dipole-induced dipole
interaction (UD) between two atoms or molecules:
39
62
012021
6 )4()(
r
uu
r
CrU
o
DD επε
αα +−=−= [2.12]
In this equation, α01 and α02 are the electronic polarizabilities of the
molecules. The London dispersion (UL) is the most important contribution to van
der Waals interaction energy, because it acts between all types of molecules or
atoms, as shown in Equation 2.13.
−=−=
21
21
62
012021
6 )4(2
3)(
νννν
πεαα h
r
u
r
CrU
o
LL [2.13]
In Equation 2.13, v1 and v2 are the frequencies calculated from the
ionization potentials of the molecules and h is the Planck’s constant. The total
van der Waals interaction (UVDW) is the summation of all three individual
(Keesom, Debye and London) interactions and can be written in the form UvdW =
(UK + UD+ UL)/r6, as shown in Equation 2.14.
21
21
620
020102
2
201
2
1
620 )4(2
3)
3)(
3(
)4(
3)(
vv
vhv
rkBT
u
Tk
u
r
TkrU
B
Bvdw +
−++−=πε
αααα
επε [2.14]
In 1937, Hamaker45 developed a method to calculate interaction energy
between macroscopic bodies (spherical particles) based on a pair-wise
summation of the potential energy between individual atoms in each body. The
interaction is described in terms of the Hamaker constant, which is dependent on
the material. Another approach, developed by Lifshitz,44 uses the interacting
40
bodies and an intervening medium as a continuous phase and determines the
strength of the interaction from bulk dielectric properties of the materials.46 The
interaction energy between two macroscopic bodies depends on their
geometrical features and on the Hamaker constant.21 A list of van der Waals
(vdW) interaction energy between some of the common geometries is shown in
Table 2.2.
Table 2.2: van der Waals interaction energy for common geometries.46 Geometry Energy (J)
Two flat surfaces 212 D
AE H
π−= per unit area
Two spheres )(6 21
21
3 RR
RR
D
AE H
+−=
Sphere Particle-flat surface D
RAE H
6−=
AH is the Hamaker constant, R1 and R2 are the particle radii between two spheres, D is the closest separation distance between two surfaces.
The Hamaker constant of a material, AH, can be calculated based on
molecular properties, as shown in equation 2.15.
11
2
1
2
11βπ qA = [2.15]
In this equation, q1 is the number of atoms per cm3 and β11 is the
London/van der Waals constant (CL) for the interaction between two molecules.
41
The Hamaker constant, A12, between two bodies of different materials (1 and 2)
can also be estimated, as shown in Equation 2.16.
221112AAA = [2.16]
Hamaker constants of materials (1 and/or 2) interacting in a liquid media
represented as 3, can also be determined using the geometric combining
relation, as displayed in Figures 2.6a and b.
(a)
(b)
Figure 2.6: Illustration of (a) same material (1) interaction in liquid media (3) and (b) two different materials (1 and 2) interacting in a liquid media (3).47
In the case where the same materials are interacting in a liquid media, the
effective Hamaker constant can be described by the following equation:
1333111312AAAA −+= [2.17]
42
The interaction between two different materials in a liquid media can be
approximated by:
23133312132
AAAAA −−+= [2.18]
These equations can be simplified to:
( )2
3311131AAA −=
[2.19] and
( )( )
33223311132AAAAA −−=
[2.20]
A list of the individual Hamaker constants, Aii for a number of materials of
relevance to integrated chips (ICs) is given in Table 2.3.
Table 2.3: Hamaker constant Aii for two identical materials interacting in vacuum.48
Materials AII (10-20J)
Si3N4 16.4 SiO2 6.3
Al2O3 14 Polystyrene latex (PSL) 6.5 Teflon 3.8 Si 25.6 Metals (Cu, Ti, W) 25-40 Water 3.7
The introduction of a medium between two surfaces generally decreases
the van der Waals energy. A strong reduction in interaction between two
43
materials occurs when the Hamaker constants are close to that of water. For
example, silicon-silicon interaction in vacuum is approximately 25.6 x 10-20 J
whereas introducing water as a media decreases the Hamaker constant from
25.6 x 10-20 J to 9.9 x 10-20 J. In Table 2.4, the Hamaker constants calculated for
different materials interacting in liquid medium (water) are summarized.
Table 2.4: Calculated Hamaker constants A132 for two materials 1 and 2 immersed in water (3).
Materials A132 (10-20J) SiO2-Si3N4 1.6
SiO2- SiO2 0.34
SiO2-Al2O3 1.07
SiO2-PSL 0.39
SiO2-Teflon 0.015
SiO2-Si 1.8
SiO2-Metals (Cu, Ti, W) 1.8-2.5
Si- SiO2 6.7
Si- Si3N4 1.8
Si-Al2O3 5.7
Si-PSL 1.97
Si-Teflon 0.08
Si-Si 9.9
Si-metals 10-14
Based upon the data in Table 2.4, it is expected that silicon (Si), silicon
nitride (Si3N4) and aluminum oxide (Al2O3) particles (most common contaminants
found in semiconductor processing) would adhere strongly to silicon wafers. For
44
polymeric materials such as PSL and Teflon, the van der Waals attraction is
expected to be less important.
Several other models have also been proposed to predict the adhesion
force between two macroscopic bodies immersed in a liquid medium. If the
macroscopic bodies behave as rigid solids, then the adhesion force can be
computed using the Derjagun-Muller-Toporov (DMT)49 model, as shown in
equation 2.21 :
SLR
Fπγ4−= [2.21]
In this equation, γSL is the interfacial tension between solid and liquid. Most
solids are not completely rigid, and elastic deformation can significantly affect the
adhesion force, in which case the Johnson-Kendall-Roberts (JKR) theory50 can
be used to calculate the adhesion force using the following relationship.
SLR
Fπγ3−= [2.22]
Surface roughness plays an important role in the vdW contribution to
particle-surface interaction. This is because the contact area between two
macroscopic bodies depends on the roughness. At large separation distances,
where the roughness height or asperities is much smaller than the separation
distance, the effect of roughness may be small or neglected. However, the van
45
der Waals interaction can be significantly reduced if the surface roughness is the
same order of magnitude or higher than the separation distance between the
particle and the substrate. A model that accounts for surface roughness was first
introduced by Rumpf.51 In this model, the adhesion between spherical particle
and a surface with a small hemispherical asperity centered below the particle
was used. The interaction force in this model is given by Equation 2.22:
+
++
=22
1
1
6
D
rRr
r
d
RAF H
vdw [2.22]
In this equation, AH is the Hamaker constant, R is the particle radius, r is
the radius of the asperity, and D is the distance of separation between the
particle and the asperity. One of the limitations of Rumpf model is that the radius
of the asperity is not easily measured, while surface roughness can be measured
in terms of a root mean square (RMS) values. Rabinovich et al.52 modified the
Rumpf model based on the RMS values for hemispherical asperities, as shown in
Equation 2.23. The RMS values can be easily measured using an atomic force
microscope (AFM).
46
+
++
=22
132
48.11
1
48.11
1
6
D
RMSRMS
Rd
RAF [2.23]
Cooper et al.53 developed a model for the adhesion of a rough
particle on a smooth surface by assuming homogeneous hemispherical
asperities on a spherical particle, as shown in Equation 2.24.
232
4/
1
2
)(6
21
)(6)2
11arccos(
3601
dr
AR
r
d
rdd
Ar
i
Fxai
i ++
++
−+= ∑
<
=
[2.24]
where d is the contact radius, r is the asperity radius, and d is the distance
between the particle and surface. This model was experimentally validated for
systems relevant to semiconductor processing using an atomic force microscopy
(AFM). Adhesion force was measured between polystyrene latex (PSL) particle
and a silicon dioxide surface in solutions with varying pH (2-11) that caused
different surface roughness. A strong adhesion force of 127 nN was measured in
solutions at pH values lower than 4.0 whereas adhesion force of less than 12 nN
was measured in basic solutions with pH values ranging from 5-11. The
decrease in adhesion force in high pH solutions was attributed to high silicon
surface roughness caused by etching.
47
2.3.2. Electrical Double-Layer Interaction Forces
Understanding phenomena near the interface of a solid surface and an
aqueous solution is important in particle removal from the wafer surface. When a
substrate is immersed in an aqueous solution containing an electrolyte, the solid
surface can acquire a charge through either adsorption of ions (for example OH-),
or by dissociation or ionization of surface groups.54-55 An excess charge at the
solid surface balanced by a diffuse region of equal but opposite charge on the
liquid side. Composed of surface charge and its counter ions, this region is called
the electrical double-layer.
Figure 2.7: Schematic representation of different potentials asscociated with a particle in aqueous solutions. Used with permission of Manish Keswani.
48
The separation of charge at the interface leads to a surface potential. The
potential of the charged surface depends on the activity of the potential
determining ions and can be calculated by the Nernst equation which is given by
the following equation:
=Ψ
PZCa
a
F
RTlog
303.20 [2.25]
where Ψ0 is the potential difference between the charged surface and the
bulk solution, and a and aPZC are the activities of the potential determining ions at
the solution condition and at the point of zero charge (PZC), respectively. In the
first term, R is the gas constant, T is the temperature and F is the Faraday’s
constant. The variation of electrostatic potential Ψ in the double layer
surrounding a surface is governed by the Poisson-Boltzmann equation:
∑=
Ψ−
−=∇N
I
Tk
qz
iib
i
enzq
10
2 4
εεπ
ψ [2.26]
where ε is the dielectric constant of the medium, ε0 is the permittivity of the
vacuum, q is the electron charge, N is the number of types of ions, ni is the
number of ions i in the bulk solution per volume, zi is the valence of ion i, kb is the
Boltzmann’s constant, and T is the absolute temperature. If the surface potential
is small, less than 25 mV, for a 1:1 electrolyte solution, the Poisson-Boltzmann
equation can be reduced to a linear differential equation.
49
ψψ 22 k=∇ [2.27]
In this equation, κ is the inverse Debye length, which is given by the
following equation:
Tk
znq
B
ii
0
22
εεκ
∑= [2.28]
The inverse Debye length or the double layer thickness depends on the
properties of the electrolyte solution. For 1:1 electrolytes at 298 K, the Debye
length in nm is given by c/304.0 , where c is the concentration of an
electrolyte in mol.L-1. The Debye length decreases with an increase in electrolyte
concentration.
Hogg, Healy and Fuerstenau (HHF)56 developed expressions for
interaction forces between a particle (1) and a surfaces (2) for constant potential
and constant charge cases, as shown in Equations 2.29 and 2.30.
−
+−+= −
−
−kd
oo
kd
kd
oo
el ee
keRF 2
2
2
1
0201
2
2
2
2
10
2
1)(2
ψψψψ
ψψπεεψ [2.29]
+
+−+= −
−
−kd
oo
kd
kd
ooel e
e
keRF
2
2
2
1
0201
2
2
2
2
10
2
1)(2
ψψ
ψψψψπεεψ
[2.30]
50
At large separation distances (d), these two expressions are nearly
identical and show an exponential decay in the repulsive or attractive force.
However, at short separation distances of less than 10 nm, these expressions
are not valid. It is therefore important to note that the linear approximation for the
Poisson-Boltzmann equation fails as the separation distance approaches zero.
In an aqueous solution, interaction between a particle and a wafer surface
depends on the potential of both the particle and the surface. If the potentials are
of the same signs, the repulsion occurs, and if they are of opposite signs,
attraction occurs. It is important to control the potentials of both the particle and
the substrate in order to prevent particle deposition in aqueous solutions. It can
be controlled by adjusting the pH of an aqueous solution or by adding additives
such as surfactants. The surface potential is not experimentally measureable. A
more commonly used and experimentally measured quantity is the zeta potential.
Figure 2.8 shows the zeta potential of particle contaminants commonly found in
semiconductor processing as a function of solution pH.
51
Figure 2.8: Zeta potential of particle contaminants commonly found in semiconductor processing as a function of solution pH.48
It can be seen from Figure 2.8 that in acidic solutions with pH ~ 3, Si is
characterized by a zeta potential value of ~ -20 mV, whereas SiO2 and Si3N4
particles have a positive ZP values, which leads to particle deposition due to the
electrostatic attractive forces. In high pH solutions such as APM (pH~10)
solutions, most of the particles and surfaces have a negative ZP and re-
deposition of removed particles is prevented due to the presence of repulsive
forces.
Surface treatment or synthesis procedure can significantly change the
zeta potential of materials.57 Figure 2.9 shows the zeta potential as a function of
pH in 1 x 10-3 M KCl solution for silicon oxide films prepared using different
52
treatment methods. The measured zeta potential of thermally grown silicon oxide
(TOX) film showed an isoelectric point close to pH 4. In contrast to thermal
oxides, chemically grown oxide film generated in APM solution (1:1:5
(NH4OH:H2O2:H2O) at 80 °C for 10 min) did not exhibit an isoelectri c point in the
pH range of 2 to 10. Measurements made on SiO2 particles (0.4 µm) showed that
the zeta potential was more negative compared to that of thermal and chemical
oxides in the pH range of 4 to 10. Based on these results, it is evident that the
value of IEP is dependent on the type of surface preparation.
Figure 2.9: Comparison of zeta potential of silicon dioxide surfaces prepared using different treatment methods as a function of pH.57
53
2.4. Measurements of Interaction Forces
Interactions forces between two surfaces can be measured by various
techniques. One of the early methods used to measure van der Waals force
involved two polished glass bodies, where one glass surface was fixed and the
other was mounted to a spring.36 The distance between the glass surfaces and
the deflection of the spring were measured, and multiplication of the deflection by
the spring constant yielded the interaction force. Using another approach,
Derjagun, Rabinovich, and Churaev39 measured the forces between two metal
wires that correlated well with the theoretically calculated van der Waals force for
metals. However, there were several experimental limitations including the
difficulty of obtaining precise measurements at short separation distances of less
than 20 nm and making measurements in liquid media.
In the last 2 to 3 decades, the development of the surface force apparatus
(SFA) and the atomic force microscope (AFM) has provided the means to
measure interaction forces between two surfaces at short separation distances.
In particular the development of the atomic force microscope was an important
advancement in force measurement because it facilitates direct measurement of
the interaction forces between two surfaces with different shapes and in various
media.58-59
54
2.5. Overview of Atomic Force Microscope (AFM)
The atomic force microscope (AFM) has emerged as a powerful tool for
the measurements of interaction forces between two surfaces with high lateral
(25 nm),60 vertical (0.1 nm) and force (1 pN) resolution61 in vacuum, air or liquid
media. In the AFM method, interaction forces between a tip or particle and a
surface are measured. A schematic representation of the tip-surface interaction
is shown in Figure 2.10a. The cantilever materials used in AFM are constructed
either from silicon or silicon nitride. An SEM of a typical silicon tip is shown in
Figure 2.10b.
Figure 2.10: (a) A schematic representation of interaction forces between the surface and the AFM tip. (b) An SEM image of a silicon tip obtained at the Center for Surface and Interface Imaging (KECK) at the University of Arizona.
AFM can be used in different modes depending upon the nature of the
materials and surfaces that are being characterized. The most commonly used
modes of operation of an AFM are the contact and tapping modes. In the contact
b b a
55
mode, the AFM tip is in physical contact with the surface and is typically used for
hard samples and when a vertical resolution of greater than 50 nanometers is
required. It has been shown that rough samples with extreme change in vertical
topography can be scanned more easily in contact mode. A drawback of using
the contact mode is that it can damage soft surfaces due to high pressure and
high scan speed.
In the tapping mode, the cantilever is driven to oscillate at near its
resonance frequency by a small piezoelectric element mounted in the AFM tip
holder. At the end of the cantilever, where the tip is, the vibration amplitude is
typically 1-10 nm. As the tip approaches the surface with a vibrating cantilever,
the frequency amplitude decreases when the tip comes into contact with the
surface due to van der Waals, electrostatic or the presence of other forces.
Instead of being scanned at constant deflection or constant height, the surface is
scanned at a constant reduction of the vibration amplitude. The force-distance
measurements using the vibration mode are determined by the changes in
frequency and amplitude. The tapping mode is often less destructive than the
contact mode because the contact time is very short and shear is prevented. One
disadvantage of the tapping mode is the slightly lower resolution; therefore force
measurements between the probe and surface are conducted with contact mode
rather than with tapping or vibrating modes. A feedback controller is used to
maintain a constant deflection between the tip and the surface, as shown in
Figure 2.11.
56
Figure 2.11: A schematic of AFM controller feedback loop to maintain constant deflection between the tip and the surface.
2.5.1. Principle of Force Measurement in Atomic Force Microscope (AFM)
The cantilever deflection, Zc, converted from the voltage measured by the
photodiode, can easily be transformed to the interaction force, F, between the tip
and the surface by multiplying with the spring constant of the cantilever Kc and
separation distance, d, according to Hooke’s law:
dKF C= [2.31]
The separation distance between the surface and the tip can be
determined from the piezo position. Separation distance is the summation of
piezo position (Zp) and cantilever deflection (Zc), where Zc is defined as the
57
intercept divided by the slope, as shown in Figure 2.12. The interaction forces
are measured by cantilever deflection as a function of the piezo position normal
to the surface. The interaction forces are measured in two steps; during
approach and during retraction, as displayed in Figure 2.12.
Figure 2.12: A schematic representation of approach and retract force curves measured using atomic force microscope.
In an approach curve, at large separation distances, where the tip and the
substrate do not interact, the cantilever is un-deflected because it experiences no
forces. At this stage, the displacement of the surface is only a physical
separation between two surfaces by that distance. This defines as a zero-force
line and is marked as “1” in Figure 2.12. As the surface approaches the tip, the
cantilever is deflected due to the interaction between the tip and the sample
58
surface. If the interaction is strongly attractive, where the gradient of the force
exceeds the spring constant, the tip suddenly jumps onto the surface
(represented as “2” in Figure 2.12). After the tip jumps onto the surface, the tip
and the surface are in contact, represented by “3”, and their movements are
coupled so that a piezo displacement causes an equal displacement in the
cantilever deflection. This region is also known as the constant compliance, “3”
region. The compliance region where the tip and the surface are in contact is
used to define the zero distance of separation. Upon retraction of the piezo, the
surfaces may continue to move together past the jump-in distance due to the
presence of an adhesion force that exists between two surfaces, (Figure 2.12
and point “4”). At the point, “5”, the tip jumps off the surface, and this hysteresis
is the adhesion force between the tip and the surface.
The most interesting regions of force-displacement curves are the jump-
to-contact and the jump-off-contact. The non-contact region in the approach force
curves provides information about attractive or repulsive forces whereas the
jump-off contact provides the magnitude of adhesion force that exists between
the tip and the surface.
2.6. Literature Review for Interaction Force Measurements using Atomic Force Microscope (AFM).
Numerous studies have been published on the interaction force
measurements between two surfaces in various media using AFM.20-21, 27, 58, 62-66
59
Ducker et al.20 first reported the interaction force between a silica particle (40 µm)
and silicon dioxide surface in 10-3 M NaCl solutions. The results showed that
interaction forces between hydrophilic silica surfaces were purely repulsive at all
separation distances. Evidence for an additional repulsive force at small
separation distances (< 3 nm) was also seen and this was ascribed to hydration
forces. The measurements were also conducted using the SFA between silica
surfaces under the same conditions. The results were comparable giving further
confirmation of the use of AFM for the measurements of interaction forces.
Hartley et al.67 reported the interaction force results between a silica
particle (4-6 µm) and silica surface as a function of separation distance in 1 x 10-4
M sodium nitrate solution with pH ranging from 3.1 to 9.4. Repulsive forces were
measured at long separation distances between silica surfaces in all solutions,
whereas attractive forces dominated and the tip jumped onto the surface at a
short separation distance of 2-6 nm. The jump in distance was found to decrease
with increasing pH and at pH 8.8 and higher, no jump was measured. The
experimentally measured forces between silica surfaces correlated well with the
theoretically calculated forces using the van der Waals and electrostatic repulsive
forces.
Toikka and Hayes 68 reported force measurement results between silica
particles (5 µm) and silicon dioxide surfaces in 1 x 10-3 and 1 x 10-4 M NaCl
solutions. Interaction forces were purely repulsive at all separation distances.
60
The repulsive forces were fitted to electrical double layer model and the
magnitude of forces decreased with an increase in solution concentration due to
the compression of the electrical double layer. Larson et al.69 have reported
similar results between silica surfaces in electrolyte solutions. The experimental
data were fitted to the non-linear Poisson-Boltzman equation that yielded the
silica surface potential value of -45 mV in 10-3 M KNO3 solution at pH 6.3. It must
be noted that van der Waals attractive forces were not incorporated in the fitting
procedure.
Several authors have also published AFM studies relevant to
semiconductor processing.70-72 In one study, interaction forces between a
hydrophilic silica particle (20 µm) and a hydrophilic silica surfaces prepared using
three different cleaning (APM, UV/ozone and H2O-vapor plasma) procedures
were reported.73 The approach force curves between a silica particle and wafer
surfaces show repulsive forces at all separation distances. During retraction, the
force-distance curve follows the same profile as that obtained during approach.
The surface potentials were estimated by fitting the approach force curves with
the DLVO theory, and found to be between -65 mV and -75 mV for silica surfaces
using all three cleaning procedures.
Interaction force measurements between a silicon nitride tip (~500 nm)
and hydrophobic silicon surface in DI-water and 0.5 wt% HF solutions at a pH of
1.88 have been reported.71 Figures 2.13a and b show the normalized interaction
forces by the tip radius as a function of separation distance in DI-water and HF
61
solutions. In DI-water, the results showed repulsive forces starting at a
separation distance of ~ 40 nm due to the electrostatic interaction forces. At short
separation distances of less than 10 nm, the van der Waals interaction becomes
dominant and the tip jumps onto the surface. In HF solution, the interaction
forces were attractive at all separation distances as can be seen in approach
force curve displayed in Figure 2.13b. During retraction, a higher magnitude of
adhesion force (6 mN/m) was measured in HF solution than in DI-water.
Figure 2.13: Normalized approach and retract force curves between a silicon nitride tip and a silicon surface as a function of separation distance in (a) DIW and (b) 0.5 wt% HF solution.71
AFM technique can also be used to measure adhesion forces between
particles and substrates using retract force curves.63, 65, 74 For example, adhesion
forces between a polystyrene sphere (5 µm) and a SiO2 surface in 0.03 M KNO3
solution in the pH range from 2 to 10 have been reported.75 The results showed a
strong dependence of adhesion forces on pH. For solutions at pH values lower
a b
62
than 4.0, a strong adhesion force of 127 nN was measured. The decrease in
adhesion force at alkaline pH solutions was attributed to higher silicon surface
roughness caused by etching. The same authors have also reported the
adhesion forces between alumina particles and silicon dioxide surface in DI-
water, 0.2 wt% NH4OH and 0.2 wt% H2O2 solutions.76 The results showed that
the adhesion forces in NH4OH and H2O2 solutions are ~ 2.5 times higher than
those in DI-water. This effect was attributed to the change in the surface
chemistry of the alumina particle in solutions of different pH.
Adhesion force between silica particles and a copper surface in different
cleaning solutions of relevance to post CMP cleaning has been reported.77 In
citric acid containing tetramethyl ammonium hydroxide (TMAH), an adhesion
force of 9 nN was measured whereas a much lower adhesion force of 0.012 nN
was measured in citric acid containing ammonium hydroxide. It was concluded
that the appropriate selection of the pH and the chemical additives is important in
the control of adhesion force between a particle and surface. In general, the
measured adhesion force between a particle and a substrate in a wet cleaning
scenario is mainly due to van der Waals force, 30-33 which is given by Equation
2.32.
26D
RAF TH
vdW −= [2.32]
In this equation, AH is the effective Hamaker constant between the particle
and surface in a particular medium, RT is the radius of the particle or tip and D is
the closest separation distance between the particle and the substrate. For a
63
system where the precise value of the AH is not known, AFM has been employed
to calculate the AH from adhesion force during the retraction of the particle from
the surface.16, 31 An alternative method uses the experimentally measured jump-
in distance between the tip and the surface during the approach of the particle
towards the surface to calculate AH.16 The tip jump onto the surface occurs when
the force gradient exceeds the spring constant (ks) of the cantilever, as shown in
Equation 2.33.
skdD
dF≥ [2.33]
The total force of interaction between a surface and a tip would typically
consist of attractive and repulsive components. In systems where repulsive
forces are due to electrical double layer (EDL) interaction and the attractive force
is mainly due to van der Waals (vdW) interaction force, then the total interaction
force, F, may be expressed as follows:
221 6)exp(
D
RADkkF TH−−= [2.34]
The first term in this equation represents EDL interaction force, and the
second term is the attractive vdW force. The electrical double layer interaction
force, which has an exponential relation to distance, can be calculated if zeta
potential of interacting surfaces is known. Alternatively, based on the work of
Israelachivili and Adams,18 repulsive forces can be calculated by fitting an
64
exponential curve of the function )exp( 21 Dkk − to force values to distance in the
range of 5 to 60 nm, where k1 and k2 are fitting constants. By differentiating
Equation 3 with respect to separation distance (D) and equating it to the
cantilever spring constant (ks), AH can be calculated as shown in Equation 2.35.
[2.35]
In the case where only attractive forces are present between a surface
and a particle, AH values can be calculated using the van der Waals interaction
force as shown in Equation 2.36.
T
sinumpj
H R
kDA
3)(3−=
[2.36]
The Hamaker constant values for materials of interest in semiconductor
processing have been reported using an atomic force microscope.78 The values
were calculated using both the approach and retract (adhesion force) curves in a
vacuum and nitrogen environment. The measured values were compared to
theoretical DLVO theory for comparison and the Hamaker constants using both
methods were in good agreement.
2.12. Literature Review for the Stability of Ammonia-Peroxide Mixture (APM)
Hydrogen peroxide decomposition over a wide range of experimental
conditions (temperature, pH, H2O2 and metal ion concentrations) using different
T
injumpinjumps
H R
DDkkkkA
−−−+=
3
2213)]exp([
65
techniques have been reported.79-82 Acid base titration method83 has shown that
hydrogen peroxide decomposition in a conventional 1:1:5 APM solution in the
temperature range of 30-80 °C is first order with resp ect to hydrogen peroxide
concentration with an activation energy of 82 kJ/mol. The hydrogen peroxide
half-lives of 888, 84, 16 and 9.3 min at 30°, 50°, 7 0°, and 80 °C, respectively
were calculated in a conventional APM solution. The decomposition was
proposed to occur via the following reaction:
2222 2
1OOHOH +→
[2.37]
In alkaline solutions, hydrogen peroxide decomposition has been described
as a base catalyzed reaction84-85 involving its reaction with perhydroxyl ion (HO2-)
as shown in Equation 2.38.
22222 OOHOHHOOH ++→+ −−
[2.38]
The data obtained at pH 11.6 and 35 °C were found t o fit with a rate equation
of the type, rate = k2 [H2O2] [HO2-]. The values of k2 range from 3.0 x 10-6 L mol-1
s-1 to 7.4 x 10-6 L mol-1 s-1. In another publication, peroxide decomposition in 1-7
M KOH and 1-3 M NaOH solution containing 1 M H2O2 at 20 °C was reported.
The results show that peroxide decomposition rate in these solutions was also of
first order with respect to H2O2. The authors proposed that the hydrogen peroxide
reacted with OH- to produce perhydroxyl ions and water, as shown in Equation
2.39.
66
OHHOOHOH 2222 +↔+ −−
[2.39]
Hydrogen peroxide decomposition can also be significantly enhanced by the
presence of metallic contaminants in APM solutions. In particular, iron acts as a
catalyst and has the most significant effect on H2O2 decomposition, with an order
of magnitude higher effect than other metals such as Cu. Iron catalyzed
decomposition of hydrogen peroxide in acidic conditions has been well-
documented in terms of free-radical mechanisms, known as Fenton reaction,86-87
as shown in Table 2.5.
Table 2.5: Mechanism of Decomposition of H2O2 initiated by Fe3+.81
+•++ ++→+ HHOFeOHFe 22
223
−•++ ++→+ OHOHFeOHFe 3
22
2
−+•+ +→+ OHFeOHFe 32
OHHOOHOH 2222 +→+ ••
223
22 OHFeHHOFe +→++ ++•+
++•+ ++→+ HOFeHOFe 22
23
Under alkaline conditions, iron-induced hydrogen peroxide decomposition is
less understood. It has been reported that hydrolyzed iron species are formed in
alkaline solutions and are highly active in the decomposition of hydrogen
peroxide. At the pH of APM solution (~10.6 at 25 °C) , hydrolyzed iron species
67
begin to form at a critical concentration of 5 x 10-8 mol L-1, which corresponds to
0.5 ppb of Fe ion.88 It is important to point out that most of the previous studies
have focused on understanding the effect of Fe3+ on decomposition of peroxide
in APM solutions. Since cleaning chemical solutions may contain either Fe3+ or
Fe2+. The presence of Fe2+ in APM solutions can have a significant effect on
catalytic decomposition of peroxide.
A gasometric technique has been used to follow the decomposition rate of
H2O2 in Fe3+ containing (0.08 to 1 ppb) APM solutions based on the
measurement of evolved oxygen gas. Increasing the Fe3+ ion concentration from
0.08 to 1 ppb increased the hydrogen peroxide decomposition and decreased the
hydrogen peroxide half-life from 6.8 days to 1.7 h. An activation energy of 103
kJ/mol was calculated for APM solutions containing less than 0.08 ppb Fe3+
ions.60 In another publication, the decomposition of H2O2 in 1:1:5 APM solutions
using different metal ions (Fe3+ and Cu2+) in the concentration range of 0.1 to 10
ppb and various temperatures has been reported.17 The reaction order and rate
constants were dependent on the type of the metallic contaminants present in the
solution. For example, peroxide decomposition induced by the presence of Cu2+
ions follows first-order reaction kinetics, whereas iron-ion catalyzed
decomposition is a mixture of first and second order reaction. The activation
energy of 65 and 71 kJ/mol for first and second order reaction kinetics
respectively was calculated. Based on the similar activation energies for the first
and second order kinetics, it was proposed that the mechanism for peroxide
68
decomposition is similar. Figure 2.14 shows a schematic representation of the
proposed mechanism for iron catalyzed decomposition of hydrogen peroxide in
APM solutions.
Figure 2.14: A schematic representation of iron-catalyzed decomposition of hydrogen peroxide in APM solutions.17
The first order reaction path was explained based on peroxide reaction
with a single Fe ion bonded to four hydroxyl groups in complex 1. In complex 2,
intra-molecular nucleophilic substitution takes place which results in the
formation of complex 3. If the reaction follows first-order reaction kinetics, then
complex 3 decomposes and forms O2 and the reduced metal catalyst (complex
69
4). In strong oxidizing conditions, the reduced metal catalyst will be oxidized by
hydrogen peroxide and form an active catalyst, complex 1 again. Therefore, the
described reaction scheme explains for the first order reaction kinetics. It was
concluded that the addition of complexing agents can reduce the peroxide
decomposition and extend the bath life of the APM solution.
Different analytical techniques such as electrochemical, UV absorption
spectroscopy89 and Raman spectroscopy24 have been used to monitor the
composition of APM solutions. However there are limitations using these
experimental techniques. One of the drawbacks to using electrochemical
techniques is that it requires direct contact of sensing probe with the solution,
which increases the contamination risk. In addition, bubble formation from
hydrogen peroxide decomposition can introduce measurement errors due to the
sensitivity of electrochemical probes. Details of experimental limitations using
above mentioned techniques can be found elsewhere.90
70
CHAPTER 3
MATERIALS, EXPERIMENTAL TECHNIQUES AND PROCEDURES
3.1 Materials
Silicon (100) samples (p-type, 38-50 Ω-cm) were used for all interaction
force experiments using an atomic force microscope. De-ionized water (18 MΩ-
cm) was used to prepare all the experimental solutions for this research. The
different chemicals used in various stages of this work include hydrogen peroxide
(30 wt%), ammonium hydroxide (29 wt%) and hydrofluoric acid (49 wt%), all of
which were purchased from Ashland Chemicals. All the chemicals used were
electronic grade with purity greater than 99%. Anhydrous Fe2SO4 salt (99.999%)
was purchased from Sigma-Aldrich and used for metal-ion catalyzed hydrogen
peroxide decomposition experiments. AFM tips (SNL-10) were purchased from
Veeco Instruments (Santa Clara, California).
3.2. Silicon Surface and Silicon Tip Preparation
Si (100) wafers were diced into 1 x 1 cm2 pieces and cleaned by
sonicating in acetone and methanol for 5 min, respectively. Samples were rinsed
with DI-water and blown dry with nitrogen. The H-terminated silicon surface was
prepared through the removal of the native oxide layer in a mixture of
71
hydrofluoric acid and DI-water at a volume ratio of 1:100. Finally, the samples
were rinsed thoroughly with DI-water and dried with ultra-pure N2 gas. AFM
imaging of the silicon surface after etching in a dilute HF solution showed a
smooth surface with a root mean square (RMS) roughness value of ~ 0.20 ±
0.05, nm as shown in Figure 3.1. A freshly prepared Si sample was used for
each force measurement experiment. Silicon tips were prepared using the same
preparation method as that used for silicon wafers.
Figure 3.1: AFM image of silicon surface (2 x 2 µm) after etching in dilute HF solution
3.3. Contact Angle Measurements
Contact angle measurements were conducted with a goniometer (Ramé-
Hart Instrument CO., Mountain Lakes, NJ) using the sessile drop. Hydrophobic
72
silicon samples were immersed in DI-water, NH4OH:H2O (1:100), H2O2:H2O
(1:100) and dilute NH4OH:H2O2:H2O solutions for 2, 10 and 60 min, followed by a
rinse with DI-water, and blown dry with N2 gas. Different liquids used for contact
angle measurements include DI-water, formamide (CH3NO) and diiodomethane
(CH2I2). Table 3.1 shows the total surface tension and a breakdown of the
components (γLLW, γL
+,γL-) of surface tension for all three probe liquids. Several
drops of liquid (0.25 µL) were placed on the sample and the average values of
contact angle are reported.
Table 3.1: Surface tension and its components of different liquids used for contact angle measurements. The units are in mN.m-1
Probe Liquids γL γLLW
γL+
γL-
γLAB = 2
−+γγ
Water
Formamide
Diiodomethane
72.8
58
50.8
21.8
39
50.8
25.5
2.28
0
25.5
39.6
0
51.0
19.0
0
3.4. Surface Force Measurements
Surface forces were measured using a Digital Instrument Nanoscope IIIa
atomic force microscope (Veeco Instruments, CA). A sealed liquid cell (volume
less than 0.2 mL) was used for the measurements which were carried out at
73
room temperature (24 ±° 1°C). The experimental solut ions were injected by
means of a syringe into the liquid cell. In-situ measurements in hydrogen
peroxide solutions were challenging due to bubble formation caused by peroxide
decomposition. Hence, experiments in dilute hydrogen peroxide and ammonia-
hydrogen peroxide solutions were carried out first by injecting the solution for the
desired immersion time and then replacing with DI-water before capturing force-
distance measurements. Force measurements were conducted within 10 minutes
of Si sample and tip preparation. Each experiment was repeated three times and
reported adhesion values are the average values.
Before force measurements between hydrophobic Si tip and Si surface
were conducted, a well-known hydrophilic silica particle and hydrophilic silica
surface system20, 67 was used to measure interaction forces in order to validate
the experimental technique and procedures. This system was chosen because
interaction forces between silica surfaces in different aqueous media have been
extensively reported in published literature.20, 68, 91 In this experiment, a silica
particle with a radius of 5 µm was attached to the AFM cantilever using electronic
grade epoxy glue. The silicon dioxide (SiO2) surface was cleaned by sonication
for 5 min in acetone and methanol followed by rinsing in DI-water. Samples were
blown dry with ultra-pure nitrogen after each sonication step. Silicon samples
were cleaned in H2SO4:H2O2 solution (4:1 by volume) for 10 min followed by a
thorough rinsing with DI-water and blown dry with nitrogen gas.
74
Figure 3.2 shows the normalized force (F/R) by the particle radius (5 µm)
as a function of separation distance between the particle and the surface in a 5 x
10-4 sodium hydroxide solution. The force measurement results show a purely
repulsive interaction force at all separation distances. Measured interaction
forces were fitted using the well-known DLVO theory. The electrostatic repulsive
forces were fitted using the constant surface potential model as shown in
equation 2.30. The published surface potential value of -65 mV92 and the
Hamaker constant value of A131 = 8 x 10-21 J was used. It can be seen from
Figure 3.2 that the measured forces correlated well with the calculated forces
using the DLVO theory. These results confirm that the experimental technique
and procedures were accurate and reproducible using the atomic force
microscope.
Figure 3.2: Measured interaction forces between silica particle and silicon dioxide surface as a function of separation distance in 5 x 10-4 NaCl solution.
75
3.5. Measurements of Ammonium Hydroxide (NH4OH) and Hydrogen Peroxide (H2O2) Concentrations using the Horiba SC-1 Composition Monitor.
A Horiba CS-100C optical monitor was used to continuously and
simultaneously measure the concentrations of ammonium hydroxide (NH4OH)
and hydrogen peroxide (H2O2) in APM solutions. Figure 3.3 shows a schematic
of the concentration monitor coupled with heating jacket interfaced with a
temperature controller (Model # E5CK, Omron Inc).
Figure 3.3: A schematic representation of an experimental set-up to measure NH4OH and H2O2 concentrations.
76
The concentrations of individual components of APM solutions were
obtained from the intensity of characteristic absorption peaks in the spectra
measured at intervals of minimum 2 seconds.
3.5.1. Monitor Specification
This monitor uses the near infrared (NIR) technique. The CS-100C
monitor can be operated in three different concentration ranges for ammonium
hydroxide and hydrogen peroxide, depending on the APM solution composition,
as shown in Table 3.2. For example, concentration data for a conventional 1:1:5
APM solution were collected using range “1” for both NH4OH and H2O2. The CS-
100C monitor can also be used to measure APM solution concentrations at
different temperatures; however, heated APM solution is cooled between 24-
27°C through an internal cooling fan before entering the optical cell.
Table 3.2: Recommended measurements ranges for the concentration of ammonium hydroxide, hydrogen peroxide and water in Horiba CS-100C APM composition monitor.
NH4OH H2O2 H2O
Range 1 0-5% 0-10% 94-100%
Range 2 0.2-0.8 0.2-0.8% 94-100%
Range 3 0-0.3% 0-0.5% 94-10%
77
Hydrogen peroxide and ammonium hydroxide concentrations were also
calculated to compare with the measured concentrations using the optical
monitor. Figure 3.4 shows the comparison between calculated and measured for
1:1:5 APM solutions at 24°, 40°, 50° and 65 °C. Publ ished literature values of
hydrogen peroxide and ammonium hydroxide density at a given temperature
were used to calculate concentrations. The highest error for H2O2 and NH4OH
concentrations were ~15% and 20%, respectively at 65 °C.
Figure 3.4: Measured vs. calculated hydrogen peroxide and ammonium hydroxide concentrations in 1:1:5 APM solutions at different temperatures.
3.5.2. Data Acquisition
CS-measure software (v 6.0.2), which allows the setting of such
parameters as concentration, range and time were used to acquire data. A time
interval of 2 seconds was set for all measurements in this study. Figure 3.5
78
shows a typical graphical representation of concentrations of ammonium
hydroxide, hydrogen peroxide and water as a function of time in a conventional
1:1:5 APM solution at 65°C. It can be seen from this figure that there was a
change in NH4OH concentration (70%) as well H2O2 concentration (80%),
respectively in four hours.
Figure 3.5: Graphical representation of ammonium hydroxide, hydrogen peroxide, and DI-water concentrations measured from a 1:1:5 APM solution at 65°C using the Horiba CS-100C concentration monitor.
H2O2
NH4OH
DI-water
79
3.5.3. Experimental Procedure for NH4OH and H2O2 Concentration Measurements
All experiments were carried out in 300 mL of DI-water contained in a 500
mL PTFE vessel. A resistively heated jacket interfaced with a temperature
controller, as shown in Figure 3.3, was used to heat the solution. Ammonium
hydroxide and hydrogen peroxide were then injected by means of a medical
grade polypropylene syringe just below the surface of continuously stirred water.
The solution temperature was maintained constant to ± 1°C of the desired
temperature within 5 min of adding ammonium hydroxide and hydrogen peroxide.
Experimental solutions were pumped and re-cycled through the monitor using a
peristaltic pump (Cole Palmer, Model # EW-07523-80) at a flow rate of 20
mL/min. In the case of iron contaminated APM solution measurements, iron
sulfate (FeSO4) solution was added to DI-water before heating such that the
concentration of the Fe2+ ion was 5 or 10 ppb in the APM solution. Each
experiment was repeated at least three times and average values are reported.
Solution pH measurements were performed with a glass electrode using the
OAKTON® pH 5 meter coupled with automatic temperature compensation (ATC)
probe. Experiments for hydrogen peroxide decomposition at a given pH value
were conducted by adding ammonium hydroxide to adjust the solution pH.
80
CHAPTER 4
RESULTS AND DISCUSSION
This chapter of the dissertation is divided into two sections. In the first
section, silicon surface wettability and interaction force measurements between
silicon surface and silicon tip in different aqueous solutions (DI-water, aqueous
NH4OH, H2O2 and APM solutions) are described. In addition, interaction forces
between Si surface and Si tip were theoretically calculated using the electrostatic
double layer theory and the JKR model.
In the second section, stability of ammonium hydroxide and hydrogen
peroxide in APM solutions is discussed. The concentrations of ammonium
hydroxide and hydrogen peroxide were measured as a function of temperature
dilution ratio and solution pH. The effect of metal (Fe2+) ion on hydrogen peroxide
decomposition is also discussed in this section.
4.1 Interaction Force Measurements between Hydrophobic Silicon Surface and Silicon Tip using Atomic Force Microscopy.
As a first step in this work, wettability of HF treated silicon surface after
treatment with DI-water, aqueous ammonium hydroxide, hydrogen peroxide and
ammonium hydroxide-hydrogen peroxide (APM) solutions was characterized
using contact angle measurements. Figures 4.1a and b show contact angle of
different solutions on silicon surfaces as a function of contact time. Water contact
angle values of 76° ± 2° and 64° ± 3° were measured after 2 and 10 min of
81
immersion time, respectively, indicating that the surface was hydrophobic in
nature. As treatment time increased from 2 to 60 min, the contact angle
decreased from 76° ± 2° to 41°± 3°, as a result of nat ive oxide growth. In the
case of NH4OH:H2O (1:5) solution, contact angle value of 65°± 4° was me asured
after 2 min of contact time. Increasing the time to 10 and 60 min resulted in lower
contact angle values of 61° ± 3° and 39° ± 7°, respect ively. Similar trends in
contact angle were observed for surfaces treated with dilute ammonium
hydroxide (1:50 and 1:100) solutions.
Contact angle data collected with aqueous hydrogen peroxide solutions
show lower values when compared to DI-water and ammonium hydroxide
solutions. The results, displayed in Figure 4.1b, show that the contact angle
values of 10° ± 4°, 37° ± 2° and 40° ± 2° were measur ed for 1:5, 1:50 and 1:100
peroxide solutions, respectively after 2 min. As treatment time increased to 10
and 60 min, lower contact angle values, between 25° ± 3° and 20° ± 4° were
measured for both 1:50 and 1:100 solutions, whereas 0° (complete wetting) was
measured in 1:5 peroxide solution. This decrease in contact angle is due to the
initially rapid oxidation (1 Å/sec) of silicon surfaces in aqueous hydrogen
peroxide solution. The wettability of hydrophobic silicon surface by ammonium
hydroxide-peroxide solutions (APM) ranging from 1:1:5 to 1:1:500 was measured
at different contact times (2, 10 and 60 min). Complete wetting was observed in
all APM solutions after 2 min, indicating hydrophobic silicon surface was rapidly
transformed to a hydrophilic state.
82
Figure 4.1: Contact angle values for silicon surfaces treated with (a) aqueous ammonium hydroxide and (b) hydrogen peroxide solutions as a function of treatment time.
4.1.1 Interaction Force Measurements between Si Surfaces in DI-water. In the first series of force measurement experiments, interaction forces
were measured between Si surface and Si tip in DI-water (pH ~ 5.8) as a function
of immersion time. The approach force curves as a function of separation
distance after 2, 10 and 60 min are shown in Figure 4.2a. Clearly, only attractive
forces exist between the surface and the tip after 2 and 10 min. The tip jump onto
Si surface occurs at distances of roughly 4.9 and 2.8 nm after 2 and 10 min
respectively, as indicated by down arrows in Figure 4.2a. When the immersion
time was increased to 60 min, a weak repulsive force was measured starting at a
distance of ~ 15 nm. However, ultimately the force becomes attractive and the tip
a
b
83
suddenly jumps onto the surface at a distance of ~ 2.0 nm. As shown earlier,
measured water contact angle values of hydrophobic silicon surface were 76°
and 64° after 2 and 10 min of immersion time. Theref ore, it is reasonable to
expect that the hydrophobic Si-H sites are not completely replaced by hydrophilic
Si-OH sites after 2 and 10 min of immersion time. Hence, attractive forces exist
between silicon and silicon tip after 2 and 10 min of contact time. After 60 min of
immersion time, a contact angle value of 40° was measur ed indicating silicon
surface is more hydrophilic in nature due to the formation of native oxide film.
This native oxide film would acquire a negative charge by dissociation of silanol
groups (Si-OH) to SiO- and H+.93 Such a changing phenomenon is confirmed by
approach force curves results for 60 min of immersion time in DI water that show
weak repulsive forces between the surface and the tip.
Adhesion forces between silicon surface and silicon tip in DI-water were
measured from the retract curves as shown in Figure 4.2b. It can be seen from
this figure that a strong adhesion force of 10.0 nN was measured after short
immersion time of 2 min. The results also show that as the immersion time
increased from 2 to 10 and 60 min, the adhesion force decreased from ~10.0 nN
to 3.0 nN and ~ 1.0 nN respectively. This decrease in adhesion force is due to
the oxidation of silicon surface.
84
Figure 4.2: Interaction forces as a function of separation distance between Si surface and Si tip in DI-water (a) Approach force curves and (b) Retract force curves after 2, 10 and 60 min of immersion time.
4.1.2. Interaction Force Measurements between Si Surfaces in NH4OH:H2O (1:100) Solution
The next set of experiments was carried out in aqueous ammonium
hydroxide solutions. The approach force curves between the surface and the tip
in NH4OH:H2O (1:100) solution (pH ~ 10.4) after 2, 10 and 60 min of immersion
time are shown in Figure 4.3a. Repulsive forces were measured starting at a
distance of ~ 10 nm after 2 and 10 min. At distances of less than 5 nm, the force
becomes attractive, and the tip suddenly jumps onto the silicon surface, as
indicated by down arrows in Figure 4.3a. This is in contrast to DI-water where
85
only attractive forces were measured at these immersion times. The repulsive
forces in dilute ammonium hydroxide solutions at shorter immersion times can be
explained based on higher surface density of Si-O- groups in alkaline ammonia
solutions as compared to DI-water.94 After 60 min, presence of repulsive forces
between the surface and the tip was measured. At short separation distances of
less than 4 nm, the tip suddenly jumps onto surface for all immersion times as
indicated by down arrows in Figure 4.3a.
Figure 4.3a: Measured interaction force as a function of distance during approach of a Si surface towards a Si tip in dilute NH4OH:H2O (1:100) solution.
Figure 4.3b shows the retract force curves during the retraction of Si tip
from the Si surface in NH4OH:H2O (1:100) solution as a function of immersion
86
time. A lower adhesion force of 2.0 nN was measured after 2 min of contact time
in dilute ammonia solution when compared to DI-water (~ 10.0 nN). It may be
seen from this figure that there was no significant change in measured adhesion
force values (2.3 and 1.9 nN) after 10 and 60 min of immersion time.
Figure 4.3b: Retract force curves measured in aqueous NH4OH:H2O (1:100) solution after 2, 10 and 60 min of immersion time.
4.1.3. Interaction Force Measurements between Si Surfaces in H2O2:H2O (1:100) Solution
Similar set of experiment between Si surface and Si tip in H2O2:H2O
(1:100) solution was also conducted. The approach force results, as displayed in
Figure 4.4a, show that the addition of hydrogen peroxide to DI-water significantly
87
increased the magnitude of repulsive forces when compared with DI-water and
NH4OH:H2O (1:100) solution at all immersion times. Repulsive forces were
measured at long separation distances after all immersion times. Hydrogen
peroxide is a stronger oxidizing agent than aerated ammonium hydroxide and DI
water; hence the nature and surface charging behavior of the chemical oxide
formed in hydrogen peroxide solution is different compared to that oxide formed
in other two aqueous solutions. This is responsible for the higher magnitude of
repulsive forces measured between Si surfaces in H2O2:H2O (1:100) solution.
However, at separation distances of less than 4 nm, the tip suddenly jumps onto
the surface as indicated by down arrows in Figure 4.4a.
Figure 4.4a: Force during approach as a function of separation distance in aqueous H2O2:H2O (1:100) solution after 2, 10 and 60 min of immersion time.
88
The retract force curves in a H2O2:H2O (1:100) solution are shown in
Figure 4.4b. It can be seen that the adhesion force between the surface and the
tip decreased from 7.8 nN to 0.8 nN after 60 min of contact time. This decrease
in adhesion force is consistent with contact angle measurements that show
increased wettability of hydrophobic silicon surface with time in peroxide
solutions.
Figure 4.4b: Retract force curves as a function of separation distance in aqueous H2O2:H2O (1:100) solution after 2, 10 and 60 min of immersion time.
4.1.4. Interaction Force Measurements between Si Surfaces in NH4OH:H2O2:H2O Solutions.
In order to understand the combined effect of both components of APM
mixture, measurements were also conducted in dilute APM solutions. Figure 4.5a
shows the approach force curves as a function of separation distance between Si
89
surface and Si tip in 1:1:100 APM solution after 2, 10 and 60 min of immersion
time. Repulsive forces exist at all separation distances for the three different
immersion times. The retract curves shown in Figure 4.5b show no adhesion
force between the surface and the tip. This behavior is not surprising considering
the surface and the tip are completely hydrophilic due to oxide formation.
Figure 4.5: Interaction forces as a function of separation distance in dilute NH4OH:H2O2:H2O (1:100) solution. (a) Approach force curves and (b) Retract force curves after 2, 10 and 60 min of immersion time.
Figure 4.6 (a) and (b) display approach and retract force curves as a
function of separation distance in APM solutions ranging from 1:1:50 to 1:1:500.
Only repulsive forces were measured between silicon surface and silicon tip at all
separation distances in the presence of APM solutions of different composition.
90
During retraction, there was no adhesion force measured between silicon surface
and silicon tip in APM solutions as dilute as 1:1:500 solution. These results
indicate that repulsive forces responsible for the prevention of particle re-
deposition during cleaning can be obtained even in very dilute APM solutions.
Figure 4.6: (a) Approach force curves and (b) Retract force curves as a function of separation distance between Si surfaces in APM solutions with different composition ratios after 2 min of immersion time.
4.2. Analysis of Measured Interaction Forces between Silicon Surface and Si Tip.
The consistency in force measurements made during approach and
retract curves can be verified by the calculation of effective Hamaker constant
(AH). If it is assumed that the adhesion between the surface and the tip during
retraction is due to van der Waals interaction, then it is given by Equation 4.1.
a b
91
26D
RAF TH
vdW −= [4.1]
where AH is the Hamaker constant, RT is the tip radius, D is the separation
of distance between the tip and the surface at contact. Due to the difficulty in
determining the exact tip radius, the product AH.RT was calculated and used as a
metric for determining the measurement consistency. Considering the case of DI-
water, it was shown earlier that the adhesion forces between the surface and the
tip decreased with time. The product AH.RT was calculated from the measured
values of adhesion force and by assuming a separation distance of 0.5 x 10-9 m
at contact. The adhesion force values and AH.RT are listed in Table 4.1.
Table 4.1: Measured adhesion force and calculated product of the Hamaker constant and tip radius (AH. RT) between silicon surface and silicon tip as a function of immersion time in DI-water.
Time (min) Fadhesion (N) AH.RT (N.m2)
2 10.5 x 10-9 1.6 x 10-26
10 3.0 x 10-9 4.5 x 10-27
60 1.2 x 10-9 1.8 x 10-27
It is also possible to estimate the product AH.RT using experimentally
determined jump-in distance in approach force curves. The tip jump onto the
92
surface occurs when the force gradient exceeds the spring constant (ks) of the
cantilever. The total force of interaction between a surface and a tip is typically
consists of repulsive and attractive components as shown in equation 2.33. By
differentiating the total force with respect to separation distance and equating to
the cantilever spring constant (ks), the Hamaker constant (AH) can be calculated
as shown in equation 4.2.
T
injumpinjumps
H R
DDkkkkA
−−−+=
3
221 3)]exp([ [4.2]
In the case where only attractive forces are present between a surface
and a tip, AH values can be calculated using van der Waals interaction force, as
shown in equation 4.3.
T
sinumpj
H R
kDA
3)(3−= [4.3]
In the case of DI-water, only attractive forces exist, therefore the tip jump
onto the surface occurs when the force gradient exceeds the spring constant (ks)
of the cantilever as shown in Equation 4.2. In this equation, the AH.RT is a
function of jump-in distance and cantilever spring constant. In approach force
curves, the tip jump-in occurs rather abruptly on the surface, as indicated by
down arrow in Figure 4.7. It may also be noted that there is only one data point
available after jump-in occurs and before the tip making contact with the surface.
93
Therefore, average values of data points represented as “a” and “b” in Figure 4.7
are used in the calculations of jump-in distance.
Figure 4.7: Representation of an abrupt jump-in distance between the silicon surface and silicon tip marked as “a”. The only data point available after tip jump-in and before making contact with the surface is marked as “b”. The average value of point “a” and “b” is used for the calculating the product of the Hamaker constant and tip radius.
Using the jump-in distance values of 4.9, 2.8 and 2.1 nm for 2, 10 and 60
min of immersion time in DI-water respectively, and cantilever spring constant
(ks) value of 0.12 N. m-1, values of AH.RT were also calculated. Table 4.2
compares AH.RT values calculated from approach and retract force curves. It can
94
be seen from this table that the values of the product AH.RT calculated using both
methods are roughly the same and they decrease with an increase in DI-water
contact time due to the oxidation of silicon surface.
Table 4.2: Comparison of measured adhesion force and calculated product of the Hamaker constant and tip radius (AH.RT) between silicon surface and silicon tip as a function of immersion time in DI-water.
Time (min) AH.RT (N.m2)-adhesion AH.RT (N.m2)-approach
2 1.6 x 10-26 4.2 x 10-26
10 4.5 x 10-27 7.9 x 10-27
60 1.8 x 10-27 3.3 x 10-27
Similar computations were performed for the data collected during
approach in ammonium hydroxide and hydrogen peroxide solutions. In both
solutions, repulsive forces were measured between silicon surfaces at long
separation distances at all immersion times. Therefore, extraction of AH.RT
values calculated from approach force curves need recognition of the fact that
the measured force has both electrical double layer and van der Waals
components. The electrical double layer interaction force, which has an
exponential relation to distance, can be calculated if zeta potential of silicon is
known. Alternatively, based on the work of Israelachivili and Adams,18 an
exponential line of the function form )exp( 21 Dkk − can be fitted to data points in
95
the range of 5 to 60 nm, where k1 and k2 are constants. The total force will then
by given by the expression 4.4.
221 6)exp(
D
RADkkF TH−−= [4.4]
The first term in this equation represents electrostatic double-layer
interaction force, and the second term is the attractive van der Waals force. The
measured repulsive forces between silicon surface and silicon tip were fitted with
an exponential line with correlation coefficient (R2) values ranging from 0.962 to
0.985. An example of resulting fit to experimental data is shown as solid line in
Figure 4.8.
y = 1.2072e-0.0774x
R2 = 0.9625
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 10 20 30 40 50 60
Distance (nm)
Fo
rce
(nN
)
Figure 4.8: An exponential fit to measured repulsive forces between silicon surface and silicon tip in H2O2:H2O (1:100) solution after 2 min of immersion time. Open circles represents the experimental data. A solid line is the fitted exponential line.
96
Calculated values of the product (AH.RT)approach are tabulated in Table 4.3.
Also given in this table are the values of the product (AH.RT)retract calculated from
the retract force curve (adhesion data). Considering the error involved in
estimating the jump-in distance from the approach force curves, the values of
AH.RT calculated using both methods agree well. This validates the consistency
of experimental data. Additionally, the values of AH.RT decrease with increasing
immersion time in both solutions. This may be expected since the effective
Hamaker constant in both solution systems would decrease due to oxidation.
Table 4.3: Comparison of the calculated product of the Hamaker constant and tip radius using the measured adhesion force and total interaction force (attractive and repulsive) between silicon surface and silicon tip as a function of immersion time in NH4OH:H2O (1:100) and H2O2:H2O (1:100) solutions.
Time (min) AH.RT (N.m2) retract. AH.RT (N.m2)-approach
NH4OH:H2O(1:100)
2 3.3 x 10-27 9.9 x 10-27
10 3.4 x 10-27 4.2 x 10-27
60 2.7 x 10-27 3.8 x 10-27
H2O2:H2O(1:100)
2 1.2 x 10-26 2.2 x 10-26
10 4.5 x 10-27 8.0 x 10-27
60 1.9 x 10-27 3.9 x 10-27
97
Even though the product AH.RT is calculated, the individual values of the
Hamaker constant (AH) and tip radius (RT) are not known. If AH can be estimated,
the tip radius can be calculated. At a short immersion time of 2 min in DI water,
the substrate can be considered to be nearly oxide-free silicon surface. In this
case, the effective Hamaker constant for silicon-water-silicon system can be
calculated to be 9.9 x 10-20 J from the Hamaker constant of silicon (A11: 25.6 x
10-20 J) and water (A22: 3.7 x 10-20 J) using the geometric mean rule.46 This yields
an effective tip radius of ~ 150 x 10-9 m. This value of tip radius was cross-
checked using the data collected in dilute H2O2 solution after 60 min of contact
time, when the surface had developed an oxide layer. In this case, the effective
Hamaker constant for silica-water-silica is calculated to be 8.0 x 10-21 J. The
effective tip radius was then computed to be 150 x 10-9 m; confirming the
consistency of experimental data.
4.3. Comparison of Measured Repulsive Forces to Calculated Forces using Electrostatic Double Layer Theory
The measured repulsive forces between silicon surface and silicon tip in
aqueous ammonium hydroxide and hydrogen peroxide solutions were also
calculated using the electrostatic double-layer (EDL) interaction. A constant
potential model was used to calculate the repulsive forces between silicon
surfaces. The surface potential values of -40 mV and -70 mV were used for
calculations in ammonium hydroxide and hydrogen peroxide solutions,
respectively. Based on chemical concentrations, the double layer thickness was
98
calculated to be 26.5 x 10-9 m for NH4OH:H2O (1:100) solution. In the case of
dilute hydrogen peroxide solution, measurements were carried out by first
injecting H2O2:H2O (1:100) solution, which was then replaced with DI-water
before capturing force-distance curves. This procedure was used because
measurements in dilute hydrogen peroxide solutions were challenging due to the
bubble formation caused by peroxide decomposition. Hence, the double layer
thickness was calculated to be 40 x 10-9 m, which corresponds to an electrolyte
concentration of 6 x 10-5 M in DI-water. A comparison of calculated repulsive
forces between silicon surface and silicon tip using the electrostatic double-layer
force and experimentally measured repulsive forces is shown in Table 4.4.
Table 4.4: Comparison of the calculated electrostatic forces using the electrical double layer model and experimentally measured repulsive forces between silicon surface and silicon tip as a function of immersion time in NH4OH:H2O (1:100) and H2O2:H2O (1:100) solutions
Time (min) FEDL (nN) Fexp.measured (nN)
NH4OH:H2O(1:100)
2 2.6 x 10-1 1.4 x 10-1
10 2.9 x 10-1 2.1 x 10-1
60 3.7 x 10-1 4.1 x 10-1
H2O2:H2O(1:100)
2 2.8 x 10-2 8.5 x 10-1
10 5.2 x 10-1 2.0 x 10-1
60 8.5 x 10-1 3.0 x 10-1
99
4.4. Comparison of Measured Adhesion Forces to Calculated Forces using JKR Adhesion Force Model.
The Johnson-Kendall-Roberts (JKR)50 force model was used to calculate
adhesion force between silicon surface and silicon tip, as shown in Equation 4.5.
SLR
Fπγ3−= [4.5]
In this equation, γSL is the interfacial tension between solid (S) and liquid
(L). In order to calculate the interfacial tension, the contact angles using DI-water,
diiodomethane (MI) and formamide (FM) were measured and Lifshitz-van der
Waals/acid-base approach was used. Table 4.5 summarizes the contact angle
results for HF-treated silicon surfaces treated with DI-water, NH4OH:H2O (1:100),
and H2O2:H2O (1:100) solutions.
Table 4.5: Contact angles (θ) for Si surface treated with DI-water, NH4OH:H2O (1:100) and H2O2:H2O (1:100) solutions measured with water (θw), formamide (θFM) and diiodomethane (θMI) for silicon surfaces. Time (min) DI-water treated NH4OH:H2O(1:100)treated H2O2:H2O (1:100) treated θW θFM θMI θW θFM θMI θW θFM θMI
2 76±1 45±2 19±1 64±4 43 ±3 10±7 42±3 19±3 27±2
10 64±2 38±3 25±2 65±3 31± 5 19±3 26±3 18±2 28±4
60 41±1 29±2 27±2 39±3 26±2 21±4 22±3 16±4 32±1
These measured contact angle data were further used to calculate the
values of surface free energy components, non-polar and polar (both acidic and
basic) by using equations 2.4 to 2.10, and these are tabulated in Table 4.8. It can
100
be seen from this table that there was no significant change in values of non-
polar (γSLW) and acidic component (γS
+) of silicon surface free energy at different
treatment times (2 to 60 min). However, the basic component of surface energy
(γS-) increased from 4 mN/m to 50 mN/m with an increase in treatment time.
Similar γS- values have been reported in literature for HF-treated silicon surfaces
treated with other oxidizing solutions such as HNO3 and H2SO4:H2O2.95 Based on
calculated surface energy components (polar and non-polar), the interfacial
tension between silicon surface and different solutions was calculated using
equation 4.6.
( ) )(22
+−−+−+−+ +−++−= LSLSLLSS
LW
L
LW
SSL γγγγγγγγγγγ [4.6]
The calculated values are shown in Table 4.6. It can be seen from this
table that interfacial tension between silicon surface and aqueous solutions
decreased with time due to the oxidation of silicon surface.
101
Table 4.6: Calculated surface free energy components (γSLW, γS
+, γS-) and
interfacial tension (γSL) between silicon surface and different solutions as a function of treatment time. The units of calculated values are in mN.m-1. DI-water Time (min) γS
LW γS+ γS
- γSL
2 48 2.5 4 32
10 46 4.0 14 12
60 45 3.0 28 2
NH4OH:H2O (1:100)
Time (min) γSLW γS
+ γS- γSL
2 46 4.6 12 19
10 47 8.3 13 17
60 47 2.3 35 0
H2O2:H2O (1:100)
Time (min) γSLW γS
+ γS- γSL
2 45 1.0 28 2.0
10 43 6.0 32 1.5
60 43 4.0 49 0
For silicon surfaces treated with DI-water, the interfacial tension values of
32, 16 and 2.0 mN.m-1 for 2, 10 and 60 min of immersion time, respectively were
used to calculate the adhesion force using the JKR adhesion force model. Table
102
4.7 compares the calculated and measured adhesion force adhesion force
values in DI-water. The measured adhesion force was normalized with respect to
the effective tip radius of 150 x 10-9 m. It can be seen from this table that the
calculated adhesion forces using the JKR model are roughly the same as the
measured adhesion forces.
Table 4.7: Comparison of the calculated adhesion force (FJKR/R) using the JKR model and measured force (Fadhesion/R) between silicon surface and silicon tip in DI-water as a function of immersion time.
Time (min) Fadheshion/R (N.m-1) FJKR/R (N.m-1)
2 1.5 x 10-1 3.0 x 10-1
10 3.0 x 10-2 9.0 x 10-2
60 1.5 x 10-2 1.8 x 10-2
Similar computations were performed to calculate the adhesion force
between silicon surface and silicon tip in ammonium hydroxide and hydrogen
peroxide solutions. A comparison of calculated adhesion force values in these
solutions are shown in Table 4.8. It may be noted that the calculated adhesion
forces are ~ 10 times larger than experimentally measured adhesion forces in
ammonium hydroxide solution. This discrepancy is most likely due to a higher
silicon surface roughness in aqueous ammonium hydroxide solutions. Surface
roughness causes the contact between the tip and surface to decrease resulting
in a decrease in the measured interaction forces between two surfaces.76 In
103
hydrogen peroxide solution, the calculated adhesion forces using the JKR
adhesion model are in good agreement with the measured force after 2 and 10
min of contact time. After 60 min, the JKR model predicted no adhesion force,
whereas a small adhesion force of 0.8 nN was measured between silicon surface
and silicon tip.
Table 4.8: Comparison of the calculated adhesion force (FJKR/R) using the JKR model and measured force (Fadhesion/R) between silicon surface and silicon tip in NH4OH:H2O (1:100) and H2O2:H2O (1:100) as a function of immersion time
Forces Measured in NH4OH:H2O (1:100)
Time (min) Fadhesion/R (N.m-1) FJKR/R (N.m-1)
2 1.5 x 10-2 1.7 x 10-1
10 1.3 x 10-2 1.5 x 10-1
60 1.2 x 10-2 3.6 x 10-1
Forces Measured in H2O2:H2O (1:100)
2 5.0 x 10-2 2.5 x 10-2
10 1.3 x 10-2 1.6 x 10-2
60 5.3 x 10-3 (0.8 x 10-9 N) 0
104
4.6. Brief Summary of Interaction Force Measurements.
Using atomic force microscope (AFM), interaction forces between silicon
surface and silicon tip in the presence of DI-water, dilute NH4OH, H2O2 and
NH4OH:H2O2:H2O (1:1:50 to 1:1:500) solutions were measured after 2, 10 and 60
min of immersion time. The approach force curves results showed attractive
forces at short separation distances of less than 10 nm between Si surfaces in
DI-water, NH4OH and H2O2 solutions. A strong adhesion force of 10.5 nN was
measured in DI-water after 2 min of contact time. Lower adhesion force values
were measured in aqueous ammonium hydroxide and hydrogen peroxide
solutions. The magnitude of adhesion force decreased with an increase in
solution contact time. Theoretically calculated adhesion force using the van der
Waals and the JKR adhesion force models agree reasonably well with the
measured adhesion forces between silicon surfaces in different solutions.
105
4.6. Characterization of the Stability of APM Solutions using the Optical Concentration Monitor.
4.6.1. Effect of Temperature on the Stability of APM Solutions.
In the first series of experiments, the concentration of ammonium
hydroxide (NH4OH) and hydrogen peroxide (H2O2) was measured in 1:1:5 APM
solutions as a function of time in the temperature range of 24 to 65 °C. Figures
4.9a and b show the measured ammonium hydroxide and hydrogen peroxide
concentrations normalized with respect to initial concentrations ([NH4OH]0 and
[H2O2]0) as a function of time. The initial NH4OH and H2O2 concentrations were
measured to be 0.94 ± 0.1 and 1.65 ± 0.05 mol/L, respectively at all
temperatures. These measured concentrations agree well with the calculated
concentrations of NH4OH (0.96 mol/L) and H2O2 (1.55 mol/L) for 1:1:5 APM
solutions. As shown in Figure 4.10a, there was no significant change in NH4OH
concentration at the ambient temperature of 24 °C. Ho wever, increasing the
temperature from 24° to 40 °C resulted in an approx imately 30% decrease in
ammonium hydroxide concentration in four hours. A similar level of decrease in
NH4OH concentration was also seen in 1:6 NH4OH:H2O at 40 °C, indicating that
this change is most likely due to the evaporation loss of ammonium hydroxide.
The decrease in hydrogen peroxide concentration was much higher
than that of ammonium hydroxide at all temperatures. As displayed in Figure
106
4.10b, the highest peroxide decomposition of ~80% was measured at an
elevated temperature of 65 °C in a four hour period .
Figure 4.9: Measured concentrations of (a) ammonium hydroxide and (b) hydrogen peroxide in a conventional 1:1:5 APM solutions at 24°, 40°, 50° and 65 °C as a function of time.
4.6.2. Effect of Dilution on the Stability of APM Solutions.
The next set of experiments was carried out in 1:1:50 APM solutions (pH ~
10.5 at 24 °C) at different temperatures. Figures 4.1 0a and b show the measured
concentrations of NH4OH and H2O2 in a dilute (1:1:50) APM solution as a function
of time at different temperatures. At lower temperatures of 24° and 40 °C, no
significant change in hydrogen peroxide concentration (less than 10%) was
measured in four hours. However, increasing the solution temperature to 50° and
107
65 °C resulted in a higher change in peroxide concent ration (20-35%) as well as
ammonium hydroxide concentration (~70%) in four hours, as displayed in Figure
4.10a. Comparing these results with those obtained for 1:1:5 APM solution, it is
clear that the peroxide decomposition and ammonium hydroxide loss in 1:1:50
APM solution was decreased by 15-20 and 3-6 times respectively at 50 and 65
°C.
Figure 4.10: Measured concentrations of (a) ammonium hydroxide (b) hydrogen peroxide in 1:1:50 APM solutions at 24°, 40°, 50° and 65 °C.
4.6.3. Effect of pH on Hydrogen Peroxide Decomposition.
In order to understand the effect of pH on hydrogen peroxide
decomposition, measurements were conducted at different pH values. The
108
measurements were conducted by using solutions, which were made by mixing 1
part of NH4OH with varying amounts of H2O2 and water in the ratio of 1:5. Figure
4.11 shows the normalized hydrogen peroxide concentration as a function of time
at solution pH values of 8.0, 9.0, 9.25, 9.5 and 9.7 at a temperature of 65 °C.
These pH values correspond to APM solutions in the range of 1:1:5 to 1:20:100.
It can be seen from Figure 4.11 that pH (hydroxyl ion concentration) has a
significant effect on H2O2 decomposition in four hours. Increasing the solution pH
from 8.0 to 9.7 increased the hydrogen peroxide decomposition from 5% to
almost 80% in four hours.
Figure 4.11: Hydrogen peroxide decomposition at 65 °C as a function of time at different pH values.
109
It is pertinent to note that there was a minimal change in pH measured for
all APM solutions at all temperatures in a four hour of period. Additionally, the pH
was also calculated based on relevant chemical equilibria and charge balance,
as shown in equations 4.7 to 4.10.
NH4OH NH4+ + OH-, where Kb = [NH4+] [OH-]/[NH4OH] [4.7]
H2O2 HO2- + H+, where Ka = [HO2
-] [H+]/[H2O2] [4.8]
H2O OH- + H+, where Kw = [OH-][H+]/[H2O] [4.9]
[NH4+] + [H+] = [HO2-] + [ OH-] [4.10]
In these equations, [X] denotes the concentration of chemical species and
Kb, Ka and Kw are the equilibrium constants. Solving Equations 4.7 to 4.10
resulted in an expression for the concentration of OH- at a given time as shown in
Equation 4.11, provided the chemical equilibrium values are known.
]22
[
]4
[][
OHaK
OHNHwKb
KOH =−
[4.11]
The published literature values33 of the chemical equilibrium constants (Kb,
Ka and Kw) and experimentally measured concentrations of NH4OH and H2O2
were used to calculate [OH-]. Figures 4.12a and b shows the measured and
Kb
Ka
Kw
110
calculated OH- at different temperatures. It may be noted from eq. 4.11 that the
hydroxide ion concentration at any given time depends on the ratio of NH4OH
concentration to H2O2 concentration at that time. Because of this simultaneous
decrease in the concentrations of H2O2 and NH4OH, calculated OH-
concentration remained constant in APM solutions at all temperatures
investigated in this study. It can also be seen from this figure that there is
discrepancy between measured and calculated pH values at 65 °C. One possible
reason for this discrepancy is that measuring the pH of APM solution at high
temperature is challenging due to the bubble formation on pH electrode.
Figure 4.12: Measured and calculated [OH-] for 1:1:5 APM solutions in four hour.
4.6.4. Effect of Fe2+ ions on the Stability of APM Solutions.
Hydrogen peroxide concentration in APM solutions containing Fe2+ ions
was also measured as a function of time at different temperatures. The results
obtained at 50 and 65 °C, as displayed in Figures 4.12 a and b, show that the
0.00E+00
1.00E-04
2.00E-04
3.00E-04
4.00E-04
5.00E-04
6.00E-04
0 5000 10000 15000
Time (sec)
calc
ula
ted
[O
H-]
24C
40C
50C
65C
0.00E+00
1.00E-04
2.00E-04
3.00E-04
4.00E-04
5.00E-04
6.00E-04
0 5000 10000 15000
Time (sec)
mea
sure
d [
OH
-]
24C
40C
50C
65C
111
addition of Fe2+ in APM solutions significantly increased the hydrogen peroxide
decomposition. It can also be seen from Figure 4.12b that increasing the Fe2+
concentration and temperature resulted in an 80% change in peroxide
concentration within one hour. Although not shown in these plots, H2O2
decomposition in iron containing APM solutions at lower temperatures of 24° and
40 °C was higher when compared to APM solutions without Fe2+ ions. The
addition of Fe2+ had no effect on the decrease of NH4OH concentration at all
temperature investigated.
Figure 4.13: Decomposition of hydrogen peroxide at different Fe2+ concentrations in APM solutions maintained at 50 and 65 °C.
4.7. Kinetic Analysis of Hydrogen Peroxide Decomposition in APM Solutions.
A kinetic analysis was performed to determine the reaction order and rate
constants for H2O2 decomposition in APM solutions. In this analysis, the rate of
112
hydrogen peroxide decomposition at different times was first calculated using the
differential method. The experimentally measured H2O2 concentration data as a
function of time were first fitted to an expression of the type
[ ] cbtatOH ++= 222 , with correlation coefficient (R2) values ranging from 0.988
to 0.996 for all fitted data, as displayed in Figure 4.14.
Figure 4.14: An example of fitted data of hydrogen peroxide concentration vs. time. Open circles represent the experimental data. The solid line is the fitted second-order polynomial.
The rate of peroxide decomposition was then calculated by differentiating
this equation with respect to time. The next step was to determine the reaction
order and rate constant for hydrogen peroxide decomposition. Results obtained
113
clearly show that peroxide decomposition increased with temperature as well as
pH. If decomposition of hydrogen peroxide in alkaline solution occurs through the
reaction of hydroxyl ions (OH-) with hydrogen peroxide (Equation 2.35), an
empirical rate equation can be written as:
βα ][][][
2222 −=
−OHOHk
dt
OHd [4.11]
In this equation, k is the rate constant and α and ß are the reaction order
with respect to H2O2 and OH-, respectively. The concentration of OH- was
determined by experimentally monitoring the pH of APM solutions. Since the
hydroxyl ion concentration remained constant in a four hour period, the empirical
rate expression for hydrogen peroxide decomposition can be approximated as
follows:
α]['][
2222 OHk
dt
OHd=
−
[4.12]
where k’ is k[OH-]ß. Plots of log of H2O2 decomposition rate vs. log of H2O2
concentration in the time range of 0 to 7200 sec at different pH values, as
displayed in Figure 4.15, resulted in straight lines with a slope in the range of 0.9
to 1.2, indicating that the peroxide decomposition follows a first order reaction
kinetics with respect to hydrogen peroxide concentration.
114
Figure 4.15: Log-log plots of rate of peroxide decomposition (mol. L-1 sec-1) versus peroxide concentration (mol. L-1) at different solution pH values at 65 °C.
Values of k’ were obtained from the intercept of the fitted experimental
data and are shown in Table 1. The order of the reaction with respect to [OH-], β,
was calculated from the k’ values and OH- concentration. It may be noted that the
ratio of k’ at two different pH values is simply the ratio of [OH-] concentration
raised to the power of β. The calculations yielded β values in the range of 0.96 to
1 with respect to [OH-], as tabulated in Table 4.9.
115
Table 4.9: Rate constant (k’), [OH-], ratios of rate constants and hydroxyl ions as a function of pH in APM solutions at 65 °C.
pH k’(sec-1) [OH-] k’ratio [OH-]ratio β
9.0 2.8 x 10-5 1.0 x 10-5 2.1 x 10-1 2.0 x 10-1 1.0
9.25 5.0 x 10-5 1.8 x 10-5 3.5 x 10-1 3.5 x 10-1 1.0
9.50 6.3 x 10-5 2.5 x 10-5 4.8 x 10-1 5.0 x 10-1 0.96
9.70 1.3 x 10-4 5.0 x 10-5 - -
Therefore, the overall decomposition of hydrogen peroxide in APM
solutions can be described by the following rate equation:
0.10.122
22 ][][][ −=
−OHOHk
dt
OHd
[4.13]
The half-live of hydrogen peroxide in different APM solutions was
calculated and is shown in Table 4.10. It can be seen from this table that the half-
life values decreased from 150 min to 80 min with an increase in solution pH
from 9.5 to 9.7. However, at lower pH values of 9.0 and 9.25, the peroxide
decomposition was lower than 50% in four hours.
116
Table 4.10: Hydrogen peroxide half-lives at different pH values and APM solutions at 65°C.
pH APM dilution ratio [H2O2]0(mol/L) half life (min)
9.0 1:6:30 1.68 -
9.25 1:3:15 1.65 -
9.50 1:2:10 1.63 150
9.70 1:1:5 1.64 80
Addition of Fe2+ to 1:1:5 APM solution accelerates the decomposition of
H2O2. If it is assumed that iron acts as a catalyst then the empirical rate law for
the decomposition of hydrogen peroxide can be written as:
γ][''][
2222 OHk
dt
OHd=
−
[4.14]
In this equation, k’’ is a function of Fe2+ concentration and temperature,
and γ is the reaction order with respect to [H2O2]. Figure 4.17a shows the fitted
data for hydrogen peroxide decomposition versus hydrogen peroxide
concentration in the presence and absence of Fe2+ ions at 65 °C. Plots are
straight lines with a slope of 1, indicating that the hydrogen peroxide
decomposition follows a first order reaction kinetics with respect to H2O2
concentration at all Fe2+ concentrations and temperatures studied. Rate
constants for hydrogen peroxide decomposition were also obtained and they
increased with an increase in Fe2+ concentration and temperature, as shown in
117
Figure 4.16b. From the rate constant values of different temperature, the
activation energy for H2O2 decomposition in 1:1:5 APM solution was calculated to
65 kJ/mol in the absence of Fe2+ and 50 kJ/mol in the presence of Fe2+ ions.
Figure 4.16: (a) Log-log plots of rate of hydrogen peroxide decomposition and hydrogen peroxide concentration at 0, 5 and 10 ppb Fe2+ in 1:1:5 APM solutions at 65 °C. (b) First order reaction rate constant ( k’’) as a function of Fe2+ concentration at different APM solution temperatures.
In published literature, data for hydrogen peroxide decomposition in 1:1:5
APM solutions have been fitted with first, second order and a combination of first
and second order reaction kinetics.17, 83 For example, Knotter et al.17 measured
the decomposition of hydrogen peroxide in a conventional 1:1:5 APM solution at
different temperatures (30 to 90 °C) and Fe 3+ ion concentrations (0.1 to 10 ppb).
118
In their study, data were taken one hour after adding hydrogen peroxide and
ammonium hydroxide because of an initial increase in solution temperature
resulting from the exothermic nature of peroxide decomposition reaction. They
regressed the data obtained in the presence of Fe3+ to fit an equation of this type:
[ ] 2.222221
22 ][][ OHkOHkdt
OHd+=− . The values of the rate constants for k1 (s
-1)
and k2 (L mol-1.s-1) were in the range of 10-4 to 10-7 depending on temperature
and Fe3+ ion concentration. In the absence of Fe3+ ion, the decomposition of
peroxide followed a second order kinetics. In this present study, the rate
constants for H2O2 decomposition in 1:1:5 APM at different temperatures were in
the range of 10-5 to 10-6 s-1 and 5 x 10-5 to 5 x 10-4 s-1 in the presence of Fe2+ ion
concentrations. These rate constants are in agreement with the values reported
in published literature.
The activation energy for peroxide decomposition in 1:1:5 APM solution
has been reported in the range of 80 to 170 kJ/mol. In the case of iron catalyzed
decomposition, activation energies of 65 and 70 kJ/mol were calculated for first
and second order reactions, respectively. In this study, the apparent activation
energy of H2O2 decomposition was calculated to be 65 ± 3 kJ/mol. In the
presence of Fe2+ ion, the activation energy was reduced to 50 ± 5 kJ/mol.
119
4.8. Brief Summary for the Stability of APM Solutions.
The stability of APM solutions was investigated by simultaneously
monitoring the concentrations of ammonium hydroxide and hydrogen peroxide as
a function of temperature, dilution ratio, solution pH and Fe2+ concentration. The
results show hydrogen that peroxide decomposition increased with an increase in
temperature, solution pH and Fe2+ concentration. The kinetic analysis showed
that the peroxide decomposition follows first order reaction kinetics with respect
to both H2O2 and OH- concentrations in all APM solutions. In the presence of
iron, peroxide decomposition was first order with respect to peroxide
concentration.
120
CHAPTER 5
CONCLUSIONS AND FUTURE WORK
5.1. Interaction Force Measurements using Atomic Force Microscope (AFM)
Using atomic force microscope (AFM), interaction forces between silicon
surface and silicon tip were measured in the presence of DI-water, dilute
NH4OH:H2O (1:100), H2O2:H2O (1:100) and NH4OH:H2O2:H2O (1:1:50 to 1:1:500)
solutions after 2, 10 and 60 min of immersion time. Measurements made during
approach force showed attractive forces at short separation distances of less
than 10 nm between Si surfaces in DI-water, NH4OH and H2O2 solutions. A
strong adhesion force of ~10.0 nN was measured in DI-water after 2 min of
contact time. Lower adhesion force values were measured in aqueous
ammonium hydroxide and hydrogen peroxide solutions. The magnitude of
adhesion force in DI-water, ammonium hydroxide and hydrogen peroxide
solutions decreased with an increase in contact time. Theoretically calculated
adhesion force using the van der Waals and the JKR adhesion force models
correlate well with the measured adhesion forces between silicon surfaces in
different solutions.
The silicon-silicon interaction in APM solutions showed purely repulsive
forces at all separation distances within 2 min of immersion time. At all
separation distances, the magnitude of repulsive forces was roughly the same in
121
dilute APM solutions in the range of 1:1:50 to 1:1:500. This indicates that
repulsive forces responsible for the prevention of particle re-deposition during
cleaning can be obtained in APM solutions as dilute as 1:1:500 solutions.
5.2. Stability of Ammonium Hydroxide-Hydrogen Peroxide Solutions (APM) using an Optical Spectroscopy.
The stability of ammonia-peroxide mixtures (APM) was investigated by
simultaneously monitoring the concentrations of ammonium hydroxide and
hydrogen peroxide as a function of temperature (24 - 65 °C), dilution ratio (1-1-5 -
1:2:100) and Fe2+ ion concentration (0 - 10 ppb) using an optical concentration
monitor. The results show hydrogen that peroxide decomposition increased with
an increase in temperature, Fe2+ ion. The decomposition rate of hydrogen
peroxide increased with an increase in solution pH in the range of 8.0 to 9.7. The
kinetic analysis showed that the H2O2 decomposition reaction follows a first order
with respect to both H2O2 and OH- concentrations. In the presence of iron,
hydrogen peroxide decomposition follows first order reaction kinetics with respect
to H2O2 concentration. The calculated rate constant increased with an increase in
temperature and Fe2+ ion concentrations. The apparent activation energy of H2O2
decomposition was calculated to be 65 ± 3 kJ/mol. In the presence of Fe2+ ion,
the activation energy was reduced to 50 ± 5 kJ/mol.
122
5.3. SUGGESTIONS FOR FUTURE WORK
On the basis of the research that was conducted for this dissertation, the
following areas for future research are suggested:
1. Investigate the interaction forces between silicon surface and silicon tip
in dilute APM solutions at high temperature ranging from 40° to 65 °C.
2. Study the interaction forces between different materials such as a
silicon nitride tip (commonly used contaminant standard) and silicon
surface in dilute APM solutions at different solution temperatures and
immersion times.
3. An attempt should be made to configure the Horiba CS-100C APM
solution monitor for the measurements of ammonium hydroxide and
hydrogen peroxide in further dilute APM solutions ranging from 1:1:200
to 1:1:500.
4. Investigate the effect of chelating agents on the stability of ammonium
hydroxide and hydrogen peroxide.
123
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