huang_2010_ chamber less plasma polymerization of fluorocarbon thin films

832019 Huang_2010_ Chamber Less Plasma Polymerization of Fluorocarbon Thin Films

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Chamberless plasma polymerization of fluorocarbon thin films

Chun Huang a Chi-Hung Liu b Wen-Tung Hsu b Ta-Hsin Chou b

a Department of Chemical Engineering amp Materials Science Yuan Ze Fuel Cell Center Yuan Ze University 135 Yuan-Tung Road Chung-Li 32003 Taiwan ROC b Mechanical and Systems Research Laboratories Industrial Technology Research Institute Hsinchu 310 Taiwan ROC

a b s t r a c ta r t i c l e i n f o

Article history

Received 12 January 2010

Received in revised form 7 July 2010

Available online 12 August 2010

Keywords

Deposition

Fluorocarbon films

Atmospheric pressure plasma

Plasma polymerization

Hexafluorobenzene

Plasma jet

Fluorocarbon thin films were prepared and investigated using atmospheric pressure plasma jet This

atmospheric pressure plasma jet depositedfluorocarbon films without unfavorable contamination in plasma

source The atmospheric pressure plasma generated with RF frequency power at 1356 MHz was fed with

hexafluorobenzene (C6F6) as the deposition precursor and helium as the carrier gas After deposition the

fluorocarbon films were analyzed for chemical characteristic elemental composition and surface

morphology The chemical structures and surface properties of the films were evaluated by Fourier

transform infrared spectroscopy (FTIR) X-ray photoelectron spectroscopy (XPS) and scanning electron

microscopy (SEM) analyses By reason of the XPS and FTIR data atmospheric pressure plasma deposited

fluorocarbon films are similar to original aromatic ring in chemical structure This investigation

demonstrates that atmospheric pressure plasma jet affords good control over fluorocarbon film properties

copy 2010 Elsevier BV All rights reserved

1 Introduction

Low-temperature plasma polymerization of fluorocarbon thin

films has been of great interest because it can achieve low dielectricconstant excellent chemical stability and hydrophobic properties [1]

Conventionally fluorocarbon films are deposited by low-pressure

plasma polymerization and these films are widely used in ULSI

processing [2ndash4] In spite of the well-developed processes involved

the currently available low-pressure plasma polymerization techni-

ques have several major limitations such as the restricted volume of

the plasma reactor and one or more vacuum and chemical cycles

required Atmospheric-pressure plasma technique is a promising

alternative to conventional low-pressure plasma techniques [56] The

processing can be conducted in the absence of a chamber so that there

is no limitation on the reactor volume or vacuum equipment

However the use of atmospheric-pressure plasma for fluorocarbon

plasma polymerization has not been the mainstream of research and

very few studies have been published

Low pressure plasma polymerized aromatic films attracted much

attention due to their interesting chemical physical electrical and

mechanical properties [7] The simplest aromatic monomer benzene

has been studied in RF plasma polymerization of continuous wave

mode since the 1960s [8] Low-pressure plasma polymerization of

fluorobenzenes has also been extensively investigated and well

documented [910] The low pressure pulsed plasma polymerization

with the control of the variable duty cycle (DC) especially has a great

effect on the aromatic structure of plasma polymers [1112] The goal

of this work is to investigate the use of atmospheric-pressure plasma

jet system to retain aromatic ring functionality in the plasma-

deposited film This is significant for low dielectric constant andintermetal dielectric materials where thermal stability and low

moisture absorption are desired [1] In this paper we report that

fluorocarbon films with aromatic ring structure can be achieved by

chamberless plasma polymerization and its surface characteristics are

discussed on the basis of FTIR XPS and SEM analyses

2 Experimental procedure

21 Atmospheric-pressure plasma deposition system

Fluorocarbon thin films were deposited by an atmospheric-

pressure plasma jet system as shown in Fig 1 This system contains

a plasma jet and a movable table In general atmospheric pressure

plasma system the contaminations are always deposited in a plasma

jet To deal with this problem a double-pipe type quartz tube is used

as the plasma jet system through which various gases flow at

controllable flow rates A quartz capillary (05 mm diameter) is

inserted into the quartz tube with a bigger inner diameter (155 mm

diameter) to form double-pipe quartz tube For this design precursor

and carrier gas do not interfere each other during atmospheric

pressure plasma processing since the quartz capillary acts as a barrier

The high-speed gas flow rate helium (10000 sccm) is introduced

from the upside of the plasma system and passes through the quartz

capillary as the ionization gas Precursor is guided in the plasma

system through the annular space between the quartz tube and the

capillary and serves as a precursor The precursor flow rate is kept by

Journal of Non-Crystalline Solids 356 (2010) 1791ndash1794

Corresponding author

E-mail address chunhuangsaturnyzuedutw(C Huang)

0022-3093$ ndash see front matter copy 2010 Elsevier BV All rights reserved

doi101016jjnoncrysol201007021

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids

j o u r n a l h o m e p a g e w w w e l s e v i e r c o m l o c a t e j n o n c r y s o l



the carrier gas (He)at 300 sccmby a massflow controller Two copper

annular electrodes are placed face to face inside the quartz tube An

electrical field is applied to the two electrodes located inside the

quartz tube to ignite the plasma glow discharge by a 1356 MHz RF

power supply A capacitive coupled RF plasma source power is in

continuous mode (DRESSLER CESAR 1310 Advanced Energy) Hexa-

fluorobenzene (C6F6) precursor was introduced into a mixing system

(evaporator) and the temperature was maintained at 150 degC Static

deposition was carried out with the atmospheric pressure plasma

jet pointing perpendicular to the substrate at a deposition rate of

75 nmmin The surface remains at room temperature The typical

deposition times of 20 min were required to ensure a film thickness

suf ficient for the uniform film analysis For this reason the

atmospheric pressure plasma jet was scanned along the x direction

while the substrate was moved along the y direction to make a large

and uniform film (40times40 mm) at a scan speed of 25 mms

22 Film characterization and analysis

The thickness of the atmospheric-pressure plasma-deposited fluor-

ocarbon thin films was measured using the optical thin-film thickness

detector at a wavelength of 6328 nm The chemical structure of the

atmospheric-pressure plasma deposited fluorocarbon thin films was

characterized using the Fourier transform infrared (FTIR) spectrometer

(Perkin-Elmer LX 20000G)Each spectrum was obtainedfroman average

of64 scans intherangeof 400ndash4000 cmminus1 at a resolution of 4 cmminus1The

chemical composition of atmospheric-pressure plasma-deposited fluor-

ocarbon thin films was investigated using an X-ray photoelectron

spectroscopy (XPS) with the Mg Kα source (12536 eV) The surface

morphology of the atmospheric-pressure plasma-deposited fluorinated

carbon thin films was observed by scanning electron microscopy (SEM)with a JEOL model JSM-5600 apparatus A tungstenfilament was used as

the electron source A 15-KV accelerator voltage was used for scanning

the atmospheric-pressure plasma-deposited fluorocarbon thin film

surfaces

3 Results

Fig 2 shows the FTIR spectrum of an atmospheric-pressure plasma

deposited fluorocarbon thin film It is different from the conventional

FTIR spectra corresponding to the plasma-deposited amorphous a-C F

structure [13] The difference of FTIR spectra between PECVD a-CF

and atmospheric pressure plasma-deposited fluorocarbonfilms is the

thin film with the original molecular shape from the precursor

monomer Under continuous mode PECVD PECVD a-CF film cannot

obtain the original molecular shape of the precursormonomer This

resulted in the precursormonomer structure fully broken by

continuous electrical energy from low pressure PECVD From our

experimental results we succeed to form the fluorocarbon thin films

with the original molecular shape of the precursormonomer from

capacitive coupled RF atmospheric pressure plasma jet in continuous

mode The characteristic absorption peaks can be clearly seen in Fig 2

with the strong peak at 1220 cmminus1 corresponding to the CF CF2 and

CF x groups and the wide peak at approximately 1700 cmminus 1

corresponding to the unsaturated double bonds associated with C O

C C C CF2 and CF CF2 The aromatic C C stretching mode is normally

observed at 1600 cmminus1 in organic compounds [4] Moreover the CndashF

stretching absorption of F2C CF or F2C C bonds appeared around

1220 cmminus1 [14]

The chemical composition of atmospheric-pressure plasma-de-

posited fluorocarbon thin films at core level and circular edge wasinvestigated by X-ray photoelectron spectroscopy (XPS) The core

level and circular edge would be defined as the positions of

atmospheric-pressure plasma polymerization area of thin films

Table 1 represents the atomic percentages of the fluorine carbon

oxygen and nitrogen present in the atmospheric-pressure plasma-

deposited fluorocarbon thin films at the core level and circular edge

As can be seen the films comprise 425 to 256 at fluorine 7046 to

5745 at carbon 2459 to 1592 at oxygen and 07 to 103 at

nitrogen at core level and circular edge Fig 3 and Table 1 show the

XPS survey spectrum and the elemental composition of the deposited

film respectively It was also found that small amounts of N and O

were detected in films deposited by atmospheric pressure plasma

polymerization process implying that the nitrogen and oxygen from

Fig 1 Schematic diagram of atmospheric-pressure plasma jet system Fig 2 FTIR spectrum of atmospheric-pressure plasma depositedfluorocarbon thin film

Plasma condition RF power input of 100 W helium gas flow rate of 10000 sccm and

treatment time of 300 s

Table 1

Chemical composition of atmospheric-pressure plasma deposited fluorocarbon thin

film

O N F C FC ratio

Core

level

246 07 42 705 006

Circular

edge

159 10 256 57 5 0 45

1792 C Huang et al Journal of Non-Crystalline Solids 356 (2010) 1791ndash1794



ambient air was reacted and dissociated with atmospheric pressure

plasma andor C H and F species in the plasma region That is

consistent with the fact that we observed CO and OH peaks of FTIR

spectra (see Fig 2)

C1s XPS-spectra in the energy range of 280ndash300 eV of atmospher-

ic-pressure plasma-deposited fluorocarbon thin films were measured

and evaluated as shown in Fig 4 These spectra revealed the

contribution of CndashCH (2846 eV) CndashCF x (2873 eV) CF (2895 eV)

CF2 (2919 eV) and CF3 (294 eV) groups to the films [24] All

atmospheric-pressure plasma-deposited fluorocarbon thin films at

core level and circular edge show similar XPS spectra In particular

the spectra showed that the content of CF groups is higher at circular

edge This implies that the chemically reactive species in the

atmospheric-pressure plasma polymerization jet are mainly created

by the dissociation of gas molecules which obtained less electrical

energy [6] to cause minor decomposition of gas molecules at circularedge The observation of XPS analysis is well consistent with the

indication of aromatic ring structures which are composed of CF and

CndashCF x Inclusion of CF and CndashCF x may be due to the fragmentation of

hexafluorobenzene (C6F6) precursor into small radicals and their

contribution to film deposition This result shows that only the films

deposited in this plasma polymerization process can preserve the

aromatic ring structure of the C6F6 precursor and the film in circular

edge has the highest retaining degree

Scanning electron microscopy (SEM) offers a directly forward

method for identifying the surface morphology of atmospheric-

pressure plasma-deposited fluorocarbon thin films Fig 5 shows the

scanning electron microscopy (SEM) images of atmospheric-pressure

plasma-deposited fluorocarbon thin films Fluorocarbon thin films

deposited by atmospheric-pressure plasma jet (see Fig 5a) top-view

Fig 3 XPS survey spectrum of atmospheric-pressure plasma deposited fluorocarbon

thin film Plasma condition RF power input of 100 W helium gas flow rate of

10000 sccm and treatment time of 300 s

Fig 4 C1s region of XPSspectraof atmospheric pressureplasma depositedfluorocarbonthin filmsat centerpoint andedgepointPlasma conditionRF powerinputof 100 Whelium

gas flow rate of 10000 sccm and treatment time of 300 s

Fig 5 SEM images of atmospheric-pressure plasma depositedfluorocarbon thin film at

the center point (a) Top-view (b) cross-section Plasma condition RF power input of

100 W helium gas flow rate of 10000 sccm and treatment time of 300 s

1793C Huang et al Journal of Non-Crystalline Solids 356 (2010) 1791ndash1794



of thin film) show continuous and smooth surface morphology

Moreover the fluorocarbonfilms are dense (see Fig 5b cross-section

of thin film) These results indicated that the atmospheric-pressure

plasma jet system did not much ion bombardment damage with

plasma polymerization process It also supports the implication that

the dissociation precedes the ionization in this atmospheric-pressure

plasma polymerization

4 Discussion

As seen in FTIR spectrum the clear carbon ring peaks reveal that

aromatic ring structuresare incorporated in the atmospheric-pressure

plasma-depositedfluorocarbon thin films with the original molecular

shape of the C6F6 precursor This observation indicates that the

ionization in the atmospheric-pressure plasma jet does not achieve

full fragmentation of the monomers This further implies that the

chemically reactive species in this atmospheric-pressure plasma jet

arenot mainlycreated by theionizationwhichshould result in strong

ion bombardments that can fully break monomers but also the

dissociation of gas molecules with low energy electrons The

dissociation plays an important role in atmospheric-pressure plasma

polymerization especially in the decomposition of monomerprecur-

sor Recent investigations on plasma polymerization of DC trimethyl-

silane glow discharge indicate that the glow characteristic of DC

trimethylsilane plasma is entirely different from that of DC argon

plasma [15ndash17] The existence of cathode glow in plasma polymer-

ization systemindicates that themajor chemical reactionsoccurat the

cathode surface butnot in thenegativeglowIt implies that in plasma

polymerization the chemically reactive species are created by the low

energy dissociation near cathode surface rather than by the ionization

of gas molecules which should occur at the fringe of the negative

glow [615] This finding of dissociation glow also infers that the

dissociation could proceed prior to the ionization in plasma

polymerization system

According to XPS analysis it also indicates that the dissociation

seems to play a key role in the deposition of the atmospheric-pressure

plasma polymerization jet system The increasing atomic presence of

fluorine in the film expresses as incomplete decomposition of thehexafluorobenzene (C6F6) precursor because of low-energy dissoci-

ation Table 1 shows the photo-image of fluorocarbon thin films

deposited by the atmospheric-pressure plasma polymerization jet As

can be seen the presence of small vortices inside results in symmetric

deposition profiles For double-pipe atmospheric-pressure plasma jet

design precursor and carrier gas do not interfere with the

atmospheric-pressure plasma polymerization since the quartz capil-

lary acts as a barrier With proper operational parameters the

deposited fluorocarbon thin films are symmetrical and the deposition

process is stable From XPS analysis in Table 1 the increasing atomic

presence of fluorine at circular edge of atmospheric-pressure plasma

polymerization area indicates that the composition of the circular

edge of atmospheric-pressure plasma deposited fluorocarbon thin

film was the same as that of the precursor because the precursor

deposited on the surface with little dissociation or the C6F6 molecule

was dissociated to a small molecule and polymerized

5 Conclusion

In conclusion an atmospheric-pressure plasma jet formed using apower supply of 1356 MHz was employed to deposit fluorocarbon

thin films The FTIR XPS and SEM results shown here reveal that

significant film characteristics occur during atmospheric-pressure

plasma polymerization FTIR spectra of the films have indicated that

the atmospheric-pressure plasma deposited fluorocarbon films

contain aromatic ring structures The low-energy dissociation in the

plasma state is the major cause for deposition of fluorinated carbon

thin filmswith theoriginalmolecular shape of the C6F6 monomer The

use of atmospheric-pressure plasma jet for the fluorocarbon film

deposition has shown to be capable of controlling film composition

This atmospheric-pressure plasma polymerization method opened a

new way for fast and ef ficient deposition of aromatic fluorinated

carbon thin films

Acknowledgments

The authors are thankful for the support of the National Science

Council under grant NSC 99-2221-E-155-077 and NSC 99-2632-E-

155-001-MY3 and the technical support of Mechanical and Systems

Research Laboratories Industrial Technology Research Institute

Taiwan

References

[1] NM Mackie DG Castner ER Fisher Langmuir 14 (1998) 1227ndash1235[2] TC Wei CH Liu Surf Coat Technol 200 (2005) 2214ndash2222[3] T Shirafuji A Tsuchino T Nakamura K Tachibana Jpn J Appl Phys 43 (2004)

2697ndash2703

[4] CH Liu TC Wei J Chin Inst Chem Engr 37 (2006) 169ndash

175[5] Y Duan C Huang QS Yu IEEE Trans Plasma Sci 33 (2005) 328ndash329[6] QS Yu FH Hsieh H Huff Y Duan Appl Phys Lett 88 (2006) 013903ndash013906[7] A Bubenzer B Dischler G Brandt P Koidl Appl Phys 54 (1983) 4590ndash4595[8] JK Stille CE Rix J Org Chem 31 (1966) 1591ndash1594[9] DT Clark D Shuttleworth Polym Sci Polym Chem 18 (1980) 27ndash46

[10] DT Clark MZ Abrahman J Polym Sci Polym Chem Ed 20 (1982) 1729ndash1744[11] CL Rinsch X Chen V Panchalingam RC Eberhart JH Wang RB Timmons

Langmuir 12 (1996) 2995ndash3002[12] P Favia G Cicala A Milella F Palumbo P Rossini R dAgostino Surf Coat

Technol 169 (2003) 609ndash612[13] LM Han RB Timmons WW Lee J Vac Sci Technol B 18 (2000) 799ndash804[14] Yokomichi A Masuda J Appl Phys 86 (1999) 2468ndash2472[15] H Yasuda QS Yu J Vac Sci Technol A 22 (2004) 472ndash476[16] H Yasuda QS Yu Plasma Chem Plasma Proc 24 (2004) 325ndash351[17] H Yasuda QS Yu J Vac Sci Technol A 19 (2001) 773ndash781




the carrier gas (He)at 300 sccmby a massflow controller Two copper

annular electrodes are placed face to face inside the quartz tube An

electrical field is applied to the two electrodes located inside the

quartz tube to ignite the plasma glow discharge by a 1356 MHz RF

power supply A capacitive coupled RF plasma source power is in

continuous mode (DRESSLER CESAR 1310 Advanced Energy) Hexa-

fluorobenzene (C6F6) precursor was introduced into a mixing system

(evaporator) and the temperature was maintained at 150 degC Static

deposition was carried out with the atmospheric pressure plasma

jet pointing perpendicular to the substrate at a deposition rate of

75 nmmin The surface remains at room temperature The typical

deposition times of 20 min were required to ensure a film thickness

suf ficient for the uniform film analysis For this reason the

atmospheric pressure plasma jet was scanned along the x direction

while the substrate was moved along the y direction to make a large

and uniform film (40times40 mm) at a scan speed of 25 mms

22 Film characterization and analysis

The thickness of the atmospheric-pressure plasma-deposited fluor-

ocarbon thin films was measured using the optical thin-film thickness

detector at a wavelength of 6328 nm The chemical structure of the

atmospheric-pressure plasma deposited fluorocarbon thin films was

characterized using the Fourier transform infrared (FTIR) spectrometer

(Perkin-Elmer LX 20000G)Each spectrum was obtainedfroman average

of64 scans intherangeof 400ndash4000 cmminus1 at a resolution of 4 cmminus1The

chemical composition of atmospheric-pressure plasma-deposited fluor-

ocarbon thin films was investigated using an X-ray photoelectron

spectroscopy (XPS) with the Mg Kα source (12536 eV) The surface

morphology of the atmospheric-pressure plasma-deposited fluorinated

carbon thin films was observed by scanning electron microscopy (SEM)with a JEOL model JSM-5600 apparatus A tungstenfilament was used as

the electron source A 15-KV accelerator voltage was used for scanning

the atmospheric-pressure plasma-deposited fluorocarbon thin film

surfaces

3 Results

Fig 2 shows the FTIR spectrum of an atmospheric-pressure plasma

deposited fluorocarbon thin film It is different from the conventional

FTIR spectra corresponding to the plasma-deposited amorphous a-C F

structure [13] The difference of FTIR spectra between PECVD a-CF

and atmospheric pressure plasma-deposited fluorocarbonfilms is the

thin film with the original molecular shape from the precursor

monomer Under continuous mode PECVD PECVD a-CF film cannot

obtain the original molecular shape of the precursormonomer This

resulted in the precursormonomer structure fully broken by

continuous electrical energy from low pressure PECVD From our

experimental results we succeed to form the fluorocarbon thin films

with the original molecular shape of the precursormonomer from

capacitive coupled RF atmospheric pressure plasma jet in continuous

mode The characteristic absorption peaks can be clearly seen in Fig 2

with the strong peak at 1220 cmminus1 corresponding to the CF CF2 and

CF x groups and the wide peak at approximately 1700 cmminus 1

corresponding to the unsaturated double bonds associated with C O

C C C CF2 and CF CF2 The aromatic C C stretching mode is normally

observed at 1600 cmminus1 in organic compounds [4] Moreover the CndashF

stretching absorption of F2C CF or F2C C bonds appeared around

1220 cmminus1 [14]

The chemical composition of atmospheric-pressure plasma-de-

posited fluorocarbon thin films at core level and circular edge wasinvestigated by X-ray photoelectron spectroscopy (XPS) The core

level and circular edge would be defined as the positions of

atmospheric-pressure plasma polymerization area of thin films

Table 1 represents the atomic percentages of the fluorine carbon

oxygen and nitrogen present in the atmospheric-pressure plasma-

deposited fluorocarbon thin films at the core level and circular edge

As can be seen the films comprise 425 to 256 at fluorine 7046 to

5745 at carbon 2459 to 1592 at oxygen and 07 to 103 at

nitrogen at core level and circular edge Fig 3 and Table 1 show the

XPS survey spectrum and the elemental composition of the deposited

film respectively It was also found that small amounts of N and O

were detected in films deposited by atmospheric pressure plasma

polymerization process implying that the nitrogen and oxygen from

Fig 1 Schematic diagram of atmospheric-pressure plasma jet system Fig 2 FTIR spectrum of atmospheric-pressure plasma depositedfluorocarbon thin film

Plasma condition RF power input of 100 W helium gas flow rate of 10000 sccm and

treatment time of 300 s

Table 1

Chemical composition of atmospheric-pressure plasma deposited fluorocarbon thin

film

O N F C FC ratio

Core

level

246 07 42 705 006

Circular

edge

159 10 256 57 5 0 45







spectra (see Fig 2)












































4 Discussion












































5 Conclusion













carbon thin films

Acknowledgments





Taiwan

References


2697ndash2703












spectra (see Fig 2)












































4 Discussion












































5 Conclusion













carbon thin films

Acknowledgments





Taiwan

References


2697ndash2703
















4 Discussion












































5 Conclusion













carbon thin films

Acknowledgments





Taiwan

References


2697ndash2703







huang_2010_ chamber less plasma polymerization of fluorocarbon thin films

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