Selection of ESH solvents for cleaning applicationsin semiconductor manufacturing
E. Kesters1, M. Claes1, Q. T. Le1, K. Barthomeuf 2, M. Lux1, G. Vereecke1*, T. Bearda1, and J.B. Durkee3**
1 IMEC, Kapeldreef 75, 3001 Leuven, Belgium2 INSA, 20 av. A. Einstein, 69621 Villeurbanne, France3 Precisioncleaning, PO Box 847, Hunt TX 78024, USA* [email protected]** [email protected]
E. Kesters et al. imec 2008 2
11th POS 2008
Outline
• Introduction
• Materials & methods
• Results & first discussion
• Analysis of the Hansen “spherical” approach to solvent selection
• Conclusions
E. Kesters et al. imec 2008 4
11th POS 2008
Semiconductor manufacturing
• Introduction of new materials
• Cu wires
• Embedded in a porous dielectric
– SiOC, …
• Encapsulated in a barrier
– TaN/Ta, SiC, WCN, Ru, …
• Transistors, capacitors, etc…
• Novel gate dielectric materials
– HfO, HfSiON, LaO, DyO, …
• Novel gate electrode materials
– TiN, Ta2C, …
• On a substrate (wafer)
– Si, Ge, Ga/As, etc…
E. Kesters et al. imec 2008 5
11th POS 2008
Uses of organic photoresist (PR) layers
• Transistor level
• Patterning of materials by etching
• Local modification of substrate conductivity by ion doping
– Protection of undoped areas
nFET pFET
poly-Si
metal gatehigh-k
resistcrust
As implantation
E. Kesters et al. imec 2008 6
11th POS 2008
Uses of organic photoresist (PR) layers
• Cu wiring
• Patterning of dielectric layers by plasma etching
– Before Cu filling
substrate
dielectricetch
photoresist
E. Kesters et al. imec 2008 7
11th POS 2008
Removal of used PR
• Issues with plasma ashing– Materials compatibility: damaging of exposed materials
� Degradation of properties
– E.g. porous dielectrics
� Material loss in subsequent process steps
�Renewed interest in wet organic stripping– Must meet new ESH criteria
– E.g. n-methyl pyrrolidone (NMP), a good solvent, will be re-classified as reprotoxin (cat. 2)
E. Kesters et al. imec 2008 8
11th POS 2008
This work
• Find ESH replacement solvents– Making use of Hansen approach
• Focus on pristine PR– Model for bulk under crust
Crust
Bulk PR removed
Crust
Bulk PR removed
E. Kesters et al. imec 2008 9
11th POS 2008
Outline
• Introduction
• Materials & methods– PR materials
– Solvents selection
– Experimentals
• Results & first discussion
• Analysis of the Hansen “spherical” approach to solvent selection
• Conclusions
E. Kesters et al. imec 2008 10
11th POS 2008
PR materials: 193 nm DUV
• PR1: polymethacrylate (PMA)
• PR2: made from acrylate and methacrylate monomers
adamantane lactone
adamantane lactone
/H /H
E. Kesters et al. imec 2008 11
11th POS 2008
ESH selection criteria
Criteria set according to EEC classification
• Safety: flash point FP > 55 °C– No F+, F, R10 solvents
• Health: no toxic solvents– T+ (very toxic) and T (toxic) discarded
• No R23-28, R39, R48 solvents
– Carcino/mutagenic & reprotoxic discarded (of all cat.; T, Xn)
• No R40, R45-46, R49, R60-63, R68 solvents
• Environment: no toxic solvents – Aquatic & non-aquatic environment
• No R50-R59 solvents
• No N label
E. Kesters et al. imec 2008 12
11th POS 2008
Selection of solvents
• Method: based on Hansen theory– Hansen solubility parameters (HSP)
– General principle of use: “like dissolves like”
δd
δp δh
2 2 2
p d hδ δ δ δ= + +
Hydrogen bonds
Dispersive interactions (VdWaals)
Polar interactions
E. Kesters et al. imec 2008 13
11th POS 2008
Experimental procedure
• Dissolution of pristine PR– Wafer pieces with PR films
– Beaker tests at RT, afo time
– Visual inspection for complete removal
– Final check by FTIR
0.18
0.2
0.22
0.24
0.26
0.28
0.3
0.32
0.34
5001000150020002500300035004000
Wavenumber (cm-1
)
0.26
0.27
0.28
0.29
0.3
0.31
0.32
5001000150020002500300035004000
Wavenumber (cm-1
)
CHx
lactone+ester
clean
E. Kesters et al. imec 2008 14
11th POS 2008
Outline
• Introduction
• Materials & methods
• Results & first discussion– Selected solvents & mixtures
– Evaluation of organics solvents & mixtures
• Analysis of the Hansen “spherical” approach to solvent selection
• Conclusions
E. Kesters et al. imec 2008 15
11th POS 2008
0
5
10
15
20
0 5 10 15 20 25 30
Hydrogen bonding parameter
Polar parameter
NMP
Ac
PGMEDCM
MIBK
TCE
CHex
EATHF
PC
EG
Tex
BA
Selected solvents & mixtures (1)
• Mixtures of PC with:
–BA, Tex � NMP & Ac
–EG: used in commercial chemistries� good solvents from initial selection
Acronyms: see Annex
• Very few solvents meeting selection criteria & close to good solvents� mixtures
• HSPmixture = volume fraction weighted average of HSPcomponents
E. Kesters et al. imec 2008 16
11th POS 2008
0
5
10
15
20
0 5 10 15 20
Hydrogen bonding parameter
Polar parameter
NMP
Ac
PGMEDCM
MIBK
TCE
CHex
EATHF
THFA
DMSO
Cap
NP
? NEP
? HFE
? HFC
EHA
Selected solvents & mixtures (2)
• Mixtures of THFA with:
–Cap, NEP, NP � NMP
–CHex, EHA, MIBK� DCM
–EA, HFE, HFC � THF & TCE
• Mixtures of DMSO with:
–HFC, HFE � NMP, Ac, CHex, MIBK
� good solvents from initial selection
? Hansen parameters unknown
Acronyms: see Annex
E. Kesters et al. imec 2008 17
11th POS 2008
Dissolution of PR1
Hansen plot
• All solvents & mixtures
–Acronyms: see annex
• Color scale for dissolution time
Discussion
• Hansen approach does not seem to work
–No well defined solubility domain
–Solvents & mixtures with similar parameters do not show same behavior
• See DCM & NMP
0
5
10
15
20
0 5 10 15 20
Hydrogen bonding parameter
Polar parameter NMP
Ac
PGME
DCM
MIBK
TCE
CHex
EATHF
THFA
DMSO
Cap
NP
? NEP
? HFE
? HFC
EHA
PC
BATex
EG
H2O
� ≤ 0.5 min
� 0.5-1 min
� 1 - 2 min
� 2 - 5 min
� > 5 minunlabelled data points are mixtures
? Hansen parameters unknown
E. Kesters et al. imec 2008 18
11th POS 2008
Reasons for apparent failure
• Hansen parameters are thermodynamics
↔ Dissolution tests data gives kinetics– Discrepancy points to kinetic factors
– Effect of size of solvent molecules ? • See later
– Microstructure of PR• PR is a (blend of) copolymer(s)
• PR contains additives
• Hansen parameters are 3-D↔ 2-D analysis– See later
E. Kesters et al. imec 2008 19
11th POS 2008
0
5
10
15
20
0 5 10 15 20
Hydrogen bonding parameter
Polar parameter NMP
Ac
DCM
MIBK
EATHF
THFA
DMSO
Cap
NP
? NEP
? HFE
? HFC
EHA
PC
BATex
EG
H2O
unlabelled data points are mixtures
� ≤ 0.5 min
� 0.5-1 min
� 1 - 2 min
� 2 - 5 min
� > 5 min
Dissolution of PR2
Hansen plot
• All solvents & mixtures
– Acronyms: see annex
• Color scale for dissolution time
Discussion
• Very large solubility domain
• Very different behavior compared to PR1
• Possible reasons
– Different copolymers
– Different additives in PR mixture
? Hansen parameters unknown
E. Kesters et al. imec 2008 20
11th POS 2008
Summary of solubility tests on pristine PR
• Best pure solvents
– Benzyl alcohol
– (1-nitropropane)1
– NEP
– (NMP)2
– DMSO
– Propylene carbonate1 FP = 33 °C
2 Reprotoxin cat.2
• Mixtures with HFC & HFE
– Not miscible with DMSO (down to 10 %)
– Not miscible with THFA (down to 10 %)
• Ranking of the solvent mixtures
1. Propylene carbonate/benzyl alcohol
2. THFA/1-nitropropane
3. THFA/NEP
4. THFA/NMP2
5. THFA/MIBK
6. THFA/Cyclohexanone
7. THFA/Ethyl Acetate
8. Propylene carbonate/ethylene glycol
9. Propylene carbonate/texanol
10.THFA/2-ethylhexyl acrylate
11.THFA/ε-caprolactam
E. Kesters et al. imec 2008 21
11th POS 2008
Outline
• Introduction
• Materials & methods
• Results & first discussion
• Analysis of the Hansen “spherical” approach to solvent selection
• Conclusions
E. Kesters et al. imec 2008 22
11th POS 2008
Outline
• Review of Method– Based on matching impact of intermolecular forces between solvent and soil
– Presentation format is spherical geometry
– An optimization routine & a two-dimensional plot
• Application to PR1 and PR2 Data Sets– Analysis of specific results
• Key Learnings About “Spherical” Approach– Maintain realistic optimization goals
– Recognize / accept “false” results
– Beware of two-dimensional plots
• Conclusions
E. Kesters et al. imec 2008 23
11th POS 2008
Hansen Approach to Solvent Selection
• For polymers, soils, or solvents [solutes]:
– One matches intermolecular forces of solvent & solute
• “Like dissolves like”
• Three intermolecular forces:
– Dispersed throughout the molecule
– Localized between poles of electrostatic charge
– Localized among hydrogen bonds
• Each force characterized by a thermodynamic parameter:
– δ disperse
– δ Polar
– δ H2-Bonding
• Total force characterized by the Hildebrand parameter:
σ =∆ Energy
Volume
δ δ δ δ2 2 2 2Hildebrand Polar Dispersion Hydrogen bonding= + + −
E. Kesters et al. imec 2008 24
11th POS 2008
A Geometric Vision by Hansen & Coworker
• Each solubility parameter is independent– Impacting in one of three independent coordinates
• A polymer / soil / solvent is located in a three-dimensional space at the values of its solubility parameters.
• The limit of action with another chemical is a radial dimension called RO.
• There will be differentiable outcomes– GOOD and BAD solvency are binary outcomes
– Experimental data are of solution time
– “Solubility Grade” = solution time <> value
E. Kesters et al. imec 2008 25
11th POS 2008
Not-Plain Geometry
• The separation between any two chemicals in three-dimensional “Hansen space” is another radial dimension called RA.
• In terms of solubility parameters for solvents and solute:
• Equation derived from data plots
– The “4” has theoretical and empirical significance
– Not RA ≠ ∆ δ Hildebrand
• Mutual solubility is expected when
– RO overlaps RA
– I.e., The data lies within the solubility sphere
{ }[ ] { }[ ] { }[ ]R A disperse disperse polar polar h bonding h bonding= × − + − + −
− −4 2 1 2 1 2 1
2 2
2 2
2
δ δ δ δ δ δ
E. Kesters et al. imec 2008 26
11th POS 2008
The Meaning of Data Analysis
• Data analysis is to inquire:– When RA < RO, is there solubility?
• i.e., “Good” and IN?
– When RA > RO, is there not solubility?
• i.e.., “Bad” and OUT?
• “False Negatives” are:– When RO > RA, & there is solubility!
• i.e., “Good” and OUT?
• “False Positives” are:– When RO < RA, & there is not solubility?
• i.e., “Bad” and IN?
E. Kesters et al. imec 2008 27
11th POS 2008
Data Processing
• Wanted are the parameters which characterize the material being solubilized
– Will allow selection of new solvents/mixtures
• Optimization identifies the solubility parameters which place all the data where it should be placed:
– So all are “Good” and IN; or “Bad” and OUT.
• The parameters are known for each solvent:
– δ disperse
– δ Polar
– δ H2-Bonding
– RA
{ }δ δany i iX= ×∑X Volume Fractioni =
E. Kesters et al. imec 2008 28
11th POS 2008
Optimization Methodology; One Needs:
• To define an error:
– Dfi = 1 for “Good” and IN, or “Bad” and OUT, because then FACTOR = 0. For “False” results, FACTOR = 1
– Without regard to values of RA and RO
• To choose an error function:– The “Desirability Function” (DF)
• To choose an optimization routine– Linear (and non-linear) programming in spreadsheets
• To pay attention– To the distribution of outcomes
( ) { }[ ]df i R R FACTORO A i= − − ×exp
( ) { }[ ]{ } ( ) { }[ ]{ } ( ) { }[ ]{ }DF R R FACTOR R R FACTOR R R FACTOR iO A O A O Ai= − − × × − − × × − − ×exp exp ..... exp1 2
1
E. Kesters et al. imec 2008 29
11th POS 2008
Distribution of Outcomes – PR1
0.730113124780.005.6525.5219.18<5
0.720714144390.0011.1921.9413.52<2
0.83849928340.0011.7619.7610.67<1
1.000008009.589.5222.4923.69>5
0.85405620490.0010.5618.929.35< 0.5
Optimized MPa^(1/2)
DFGood & OUT
Bad & IN
Good & IN
Bad & Out
ð H2 Bonding
ð Polar
ð Dispersion
RASolution Time, Min
HSP Optimization with PR1 Photoresist
• The goal optimum (DF = 1) includes all data, i.e.– No optimization being done
• A significant amount of data is properly considered @ < 0.5 min– 69 of 80 data points
• Targets were methylene chloride and n-methyl pyrrolidone
E. Kesters et al. imec 2008 30
11th POS 2008
Analysis of the “False” Outcomes
• Molar volume is often a significant parameter in solubility performance:– Low volume solvents like methylene chloride, water, and acetone often perform better than expected.
• That’s not the case here.
• But, all but 3 of the “False” outcomes for PR1 are within ± 1 RA
unit of expected performance
E. Kesters et al. imec 2008 31
11th POS 2008
Analysis of Four “False” Outcomes
Better (PR1)98.132.0311.309.2318.80'THFA & Cyclohexanone
@ 50 % THFA (0.17 min)
Poorer (PR1)96.87-2.3410.208.2017.80'Tetrahydrofurfuryl alcohol (THFA), (5 min)
Poorer (PR2)110.7-3.347.4015.0019.70Caprolactam (>100 min, but it’s a solid at room temperature…)
Better (PR1)103.585.0313.706.3018.40Benzyl Alcohol (0.17 min)
MPa^(1/2)
Vs Expectations
Molar Volume, cc/mole
Excess HSP RA
ð H2 Bonding
ð Polar
ð Dispersion
Solvents, or
Mixtures
Solvent Combinations Performing Considerably Outside of Expectations
• Benzyl alcohol is far outside the sphere and performed very well!
• The two Tetrahydrofurfuryl alcohol materials performed differently?
• THE REASONS FOR THIS ARE NOT KNOWN!
E. Kesters et al. imec 2008 32
11th POS 2008
Distribution of Outcomes – PR2
0.9192227368.718.6822.5615.35<5
0.9544616779.466.4521.1912.96<2
0.9470226897.168.1120.4210.39<1
1.0000008109.6612.3621.5523.86>5
0.81035461115.908.6420.209.48< 0.5
Optimized MPa^(1/2)
DFGood & OUT
Bad & IN
Good & IN
Bad & Out
ð H2 Bonding
ð Polar
ð Dispersion
RASolution Time, Min
HSP Optimization with PR2 Photoresist
• All but three other “False” results are within ± 1 RA unit of expected performance
• 80 of 81 results are as expected
• There Are Fewer Anomolies With PR2
E. Kesters et al. imec 2008 34
11th POS 2008
Surprising Outcome
• Hydrogen-bonding HSP is about zero for PR1.– And not for PR2.
• It’s not clear, based on these structures, why this should be so.– Removal of the “False”outcomes doesn’t change this outcome.
adamantane lactone
adamantane lactone
/H /H
E. Kesters et al. imec 2008 35
11th POS 2008
Beware Two-Dimensional HSP Plots
• Commonly used in analysis of solubility problems.
• Based on assumption that δ disperse doesn’t vary among solvents– But it does.
• Plot makes it appear that some “GOOD” & Out are actually in.– They’re not
– Plotted @ δ disperseoptimum = 18.9
E. Kesters et al. imec 2008 36
11th POS 2008
A Two-Dimensional Plot Re-plotted
• Plot shows that some “GOOD” & Out are actually out.– Plotted @ HSP optimum + 4 = 22.9
– Shows just the “end” of the sphere
• Don’t use two-dimensional plots to make final judgments– Use them to select solvents for test
E. Kesters et al. imec 2008 37
11th POS 2008
Outline
• Introduction
• Materials & methods
• Results & first discussion
• Analysis of the Hansen “spherical” approach to solvent selection
• Conclusions
E. Kesters et al. imec 2008 38
11th POS 2008
Conclusions
• Several ESH solvents & mixtures identified that can dissolve the PRs
• Selection is dependent on PR– Kinetics vs. thermodynamics
– PR is a (blend of) different copolymer(s)
– PR is a complex mixture (with different additives)
E. Kesters et al. imec 2008 39
11th POS 2008
Conclusions
• Hansen “spherical” approach shows measured solubility data set “thermodynamically”consistent.– 18 for PR1 (< 0.5 min solution time)
– 62 for PR2 (< 0.5 min solution time)
• The HSP and RA values are known for the PRs– For additional evaluations
• Care must be taken with the “spherical”approach.– It can produce unexplained outliers
– DF =1 is not the optimization target
• Properly-located assignments Vs Solubility Grades are
– Don’t make judgments from 2-D plots
E. Kesters et al. imec 2008 41
11th POS 2008
Solvents acronyms
• Ac: acetone
• BA: benzyl alcohol
• Cap: ε-caprolactam
• CHex: cyclohexanone
• DCM: dichloromethane
• DMSO: dimethyl sulfoxide
• EA: ethylacetate
• EHA: 2-ethyl hexylacrylate
• EG: ethylene glycol
• HFC: Vertrel MCA
• HFE: HFE-7100DL
• MIBK: methyl-isobutyl-ketone
• NEP: n-ethyl pyrrolidone
• NMP: n-methyl pyrrolidone
• NP: 1-nitropropane
• PC: propylene carbonate
• PGME: 1-Methoxy-2-propanol
• TCE: trichloroethylene
• Tex: texanol
• THF: tetrahydrofuran
• THFA: tetrahydrofuranalcohol