766
Langmuir 1991, 7, 766-773
Substrate Temperatu- Effects on Film Chemistry in Plasma Deposition of rganics. 1. Nonpolymerizable tecursors Gabriel P. L,,ez
and Buddy D. Ratner"
Department of Chemical Engineering and Center for Bioengineering, BF-10, University of Washington, Seattle, Washington 98195 Received J u l y 13, 1990. I n Final Form: October 2, 1990 The effect of reduced substrate temperature on the chemistry of radio frequency (rf) plasma-deposited organic films was examined. The scope of this study was confined to rf plasmas formed by the ionization of organic liquid vapors (acetone and hexafluoro-2-propanol) that are not readily polymerizable by conventional organic chemical reactions. Film chemistries and thicknesses were analyzed primarily by X-ray photoelectron spectroscopy. For both compounds, films deposited at low substrate temperatures show chemical features (atomic composition and functional groups) more closely related to the precursor vapors than films deposited at high temperatures, indicating a reduction in precursor fragmentation at low substrate temperatures. The proposed mechanism for the chemical effects observed involves the increased adsorption or condensation of nonfragmented organic vapors at low substrate temperatures.
Introduction Interest in plasma deposition of thin organic films has increased in recent years. Overlayer films on solid substrates exhibiting desirable characteristics such as uniformity, conformal coverage, excellent adhesion to the substrate, and unique chemistries can be produced by plasma deposition without exposing the solids to solvents or y In addition, the overlayer films do not penetrate significantly into the substrate and, therefore, do not affect its mechanical properties. These films are typically free of leachable components and can be designed to inhibit leaching of low molecular weight components from the substrate. Film thickness is easily controllable and ultrathin films (10-1000 A) are readily achieved. The primary disadvantage of plasma-deposited films is their ill-defined chemistry. Because of the complex composition of the plasma and the many possible reactions that may lead to incorporation of a particular atom or functional group into the growing film, a spectrum of final film chemistries can be obtained. Reactions contributing to film formation have been characterized into plasma, substrate sheath, and surface reactions, and generalized mechanistic schemes have been d e v e l ~ p e d . ~ During %~-~~ plasma deposition, an organic compound (precursor), which may or may not be polymerizable by traditional methods, can be dissociated, rearranged, and deposited on a substrate in a chemistry different from the precursor.
* Author
to whom correspondence should be addressed. (1) d'Agostino, R. Acta Cient. Venez. 1985, 36, 19. (2) Boenig, H. V. Encycl. Polym. Sci. Eng. 1987, 11, 248. (3) Hollahan, J. R.; Bell, A. T. Techniques andilpplications ofPlasma Chemistry; John Wiley and Sons: New York, 1974. (4) Morita, S.; Hattori, S.Pure Appl. Chem. 1985, 57, 1277. (5) Ratner, B. D.; Chilkoti, A.; Lopez, G. P. In Plasma Deposition, Treatment and Etching ofPolymers;d'Agostino, R., Ed.; Academic Press: New York, 1990. (6) Yasuda, H. K. In Plasma Polymerization; ACS Symposium Series 108;Shen, M., Bell, A. T., Eds.; American Chemical Society: Washington, DC 1979; Vol. 108, p 37. ( 7 ) Yasuda, H.; Wang, C. R. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 87. (8) Yasuda, H. Plasma Polymerization; Academic Press, Inc.: New York, 1985. (9) d'Agostino, R.; Capezzuto, P.; Bruno, G.; Cramarossa, F. Pure A p p l . Chem. 1985, 57, 1287. (10) Ferreiro, L. M. Ph.D. Thesis, University of Minnesota, 1988. (11) Jensen, R. J.; Bell, A. T.; Soong, D. S. Plasma Chem. Plasma Process. 1983, 3, 163.
0743-7463/91/ 2407-0766$02.50/0
For example, if an organic amine is introduced into the plasma to incorporate amine functionalities on the surface, a wide range of nitrogen-containing organic functional groups will actually be obtained. Consequently, tailoring the chemistry of plasma-deposited films by this method has often been laborious and empirical in approach. An increased need for chemically specific films and surfaces has intensified the demand for understanding the processes leading to final film chemistry.12 The dependence of film chemistry on operational variables has, thus far, been expressed mainly in terms of the degree of precursor fragmentation that occurs between the time of exposure to the plasma and the incorporation into the film matrix. Thus, if a functionalgroup of interest is to be incorporated into a film without fragmentation or rearrangement, the operational variables that affect fragmentation must be identified and optimized. Almost all controllable deposition variables (e.g., plasma power, pressure, flow rate, precursor, duration of deposition, substrate position) have been found to influence film chemistry to some degree.13-21 The substrate temperature, which is often not controlled in plasma deposition, has also been shown to affect film c h e m i ~ t r y . ~ Investigations ,~~-~~ have dealt almost exclu(12) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (13) Nakajima, K.; Bell, A. T.; Shen, M. J. Appl. Polym. Sci. 1979,23, 2627. (14) Clark, D. T.; Abrahman, M. Z. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 1729. (15) Clark, D. T.; Abrahman, M. Z. J. Polvm. Sci.. Polvm. Chem. Ed. 1982, 20, 691. (16) Clark, D. T. Pure Appl. Chem. 1982, 54, 415. (17) Inagaki, N.; Doyama, M.; Igaki, H. J . Polym. Sci., Polym. Chem. Ed. 1984, 22, 2083. (18) Evans. J. F.;Prohaska. G. W. Thin Solid Films 1984. 118. 171. (19) Yasuda, H.; Hsu, T. J. Polym. Sci., Polym. Chem. Ed: 1977, 15, 81. (20) Pender, M. R.; Shen, M.; Bell, A. T.; Millard, M. In Plasma Polymerization; ACS Symposium Series 108; Shen, M., Bell, A. T., Eds.; American Chemical Society: Washington, DC, 1979; Vol. 108, p 147. (21) Pender, M.; Shen, M.; Bell, A. T.; Millard, M. Polym. Prepr. (Am. Chem. Soc., Diu. Polym. Chem.) 1978, 19, 516. (22) Munro, H. S.; Grunwald, H. J . Polym. Sci., Polym. Chem. Ed. 1985, 23, 479. (23) Munro, H. S.; Till, C. J . Polym. Sci., Polym. Chem. Ed. 1985,23, 1621. (24) Munro, H. S.; Till, C. J . Polym. Sci., Part A: Polym. Chem. 1987, 25, 1065.
0 1991 American Chemical Society
Plasma- Deposited Organic Films
Langmuir, Vol. 7, No. 4, 1991 767
sively with elevated substrate temperatures, however. Other variables (e.g., excitation frequency, substrate bias) may also have effects on film ~hemistry.3~-33However, methods of controlling precursor fragmentation investiI gated thus far have all resulted in decreased film deposition rate, leading to unacceptably long process times. FurCas lnlcl thermore, most methods for reducing precursor fragmentation have been only marginally effective. We have investigated the effect of reducing the substrate temperature on the film chemistry obtained from plasma deposition of organics. Of the few published investigations of the effectof low substrate temperatures on chemi~try,7*~~ Figure 1. A schematic diagram of a tubular reactor modified none has provided a complete, systematic study of chemical for deposition at reduced substrate temperatures. effects a t low temperature, nor have any hypotheses been presented as to the mechanisms a t work a t low substrate (separation = 10 in.) and a 13.56-MHz E.N.I. Power Systems temperatures. Our work has led to the production of thin, Model HF 650 rf generator (Rochester, NY). The generator is organic coatings that exhibit both the good film qualities coupled to the capacitance rings on the reactor tube by a matching associated with plasma-deposited films and the high network assembled in our laboratory. The vacuum system was chemical specificity associated with traditional organic evacuated to -5 mTorr by a Stokes vane pump (Model 009-2, ~ h e m i s t r y . ~ ~ Because --"~ the glow discharge plasma is Penwalt Corp., Philadelphia, PA). Liquid precursors were degassed by repeated freezing and thawing under vacuum. Flow weakly ionized, decreasing the substrate temperature can of precursor gases into the reactor was controlled by mass flow lead to preferential condensation, or adsorption, of lowcontrollers (Vacuum General 80 series, San Diego, CA). Flow of energy gaseous species and, therefore, can increase the hexafluoro-2-propanolvapors wascontrolled by a Teflon stopcock concentration of nonfragmented molecules in the film leak valve. Pressure in the reactor was controllable by a Vacuum growth region. This method can result in the incorporation General throttle valve between the reactor and pump. The of relatively intact precursor molecules into the film matrix. throttle valve was interfaced to a power supply connected to a Therefore, higher probability of incorporation of desired capacitance manometer gauge on the reactor. A basic schematic chemical functionalities from the precursor molecules and diagram of the plasma deposition system has been published more predictable film chemistries are obtained. The films elsewhere.38 in this study were analyzed by X-ray photoelectron Depositionwith Substrate TemperatureControl. Figure 1showsa detailed view of the prototype tubular reactor permitting spectroscopy (XPS) and static secondary ion mass specdeposition at reduced substrate temperatures. Aglass cold stage trometry (SIMS). The static SIMS analysis corroborated for recirculating cooling liquid (e.g., methanol) was fabricated. the results to be reported, but because of the large amount Methanol and other coolant liquids were cooled by an FTS of information in these spectra, they will be presented in Systems FT-100 cold probe refrigeration system (Stone Ridge, a separate publication. NY) capable of maintaining temperatures from room temperature to -80 OC. Experimental Section Substrates to be treated are placed on the cold stage within the vacuum chamber. After the chamber is evacuated to -5 Materials. Films were deposited on Teflon (purchased from mTorr, argon may be introduced to 175mTorr and the substrate Berghof/America Inc., Concord,CA),brass (AlaskanCopper and samples to be coated can be cleaned by etching with an argon Brass Co., Seattle, WA) and glass (V. W. R. Scientific Co., Seattle, plasma (40 W). Coolant can then be pumped from the cold WA) substrates. Teflon substrates were cleaned by sonication reservoir through the cold stage to allow thermal equilibration in 1.0% Ivory soap solution followed by three rinses and soniof the cold stage before the plasma deposition of the organic film. cations in deionized reverse-osmosis-purified water. Glass subAfter the deposition is complete, it may be desirable to allow strates were similarly cleaned in 1.5% Isopanasol (C.R. Callen the freshly coated specimens to remain in the presence of the Co., Seattle, WA) solution. Brass substrates were cleaned by precursor to permit termination of reactive species present in sonication in acetone/methanol solution. Brass substrates were the film. If the samples are still cool, bringing them to ambient sputter-coated with gold bya Denton Vacuum,Inc., Desk 1Model temperature may prevent the adsorption of condensibles upon coater (Cherry Hill, NJ). Acetone(J.T. Baker, Inc., Phillipsburg, exposure to the atmosphere and speed up quench reactions by NeJ)and hexafluoro-Zpropanol (Aldrich Chemical Co., Milwauthe precursor. kee, WI), used as plasma precursors, were both of greater than Analysis of Films. The XPS experiments were done on an 99O;. purity. SSX-100 surface analysis system (Surface Science Instruments, Plasma Deposition. Plasma deposition was conducted in a Mountain View, CA) using a monochromatic A1 Ka X-ray source capacitivelycoupled reactor with symmetrical,external electrodes and a detection system with a 30' solid angle acceptance lens, a hemispherical analyzer, and a position-sensitive detector. (2s) Wrobel, A. M.; Klemberg, J. E.; Wertheimer, M. R.; Schreiber, H. Deposited film samples were typically analyzed at 5 5 O takeoff P. J . Macromol. Sei., Chem. 1981, 15, 197. (26) Sacher, E.; Klemberg-Sapieha, J. E.; Schreiber, H. P.; Wertangle. The takeoff angle is defined as the angle between the heimer, M. R. J . Appl. Polym. Sei. Appl. Polym. Symp. 1984,38,163. surface normal and the axis of the analyzer lens. Survey scans (27) Mukherjee, S. P.; Evans, P. E. Thin Solid Films 1972, 14, 105. (0-1000 eV binding energy) to determine the elemental com(28) Oelhafen, P.; Cutro, J. A.; Haller, I. J . Electron Spectrosc. Relat. position of each sample were run at 150eV analyzer pass energy Phenom. 1984,34,105. and 1000-pm X-ray spot size. High-resolution 0 1s and C 1s (29) Ohno, M.; Ohno, K.; Sohma, J. J. Polym. Sci. 1987,25, 1273. (30)Yasuda, H.; Lamaze, C. E. J. Appl. Polym. Sei. 1971,15, 2277. spectra were obtained at 25eV pass energy. A low energy electron (31) Kay, E.; Dilks, A. J. Vac. Sci. Technol., A 1981, 18, 1. flood gun set at 5 eV was used to minimize sample charging. The (32) Claude, R.; Moisan, M.; Wertheimer, M. R.; Zakrzewski, Z. Am. high-resolution spectra were resolved into individual Gaussian Chem. Soc., Diu. Polym. Mater. Sei. Eng., Proc. 1987.56, 134. peaks using a least-squaresfitting program. All binding energies (3.3) Wrobel, A. M.; Wertheimer, M. R.; Dib, J.; Schreiber, H. P. J . (BE) were referenced by setting the lowest BE component of the Macromol. Sei., Chem. 1980, A14, 321. resolved C 1s peak, corresponding to carbon in a hydrocarbon (34) Czornyj, G. ACS Org. Coat. Appl. Polym. Proc. 1982,47,457. (35)Lopez, G. P.; Ratner, B. D.; Rapoza, R. J.; Horbett, T. A., In environment (CH,), to 285.0 eV. Proceedings of the Ninth International Symposium on Plasma The plasma-deposited acetone C 1s spectra were resolved by Chemistry, Pugnochiuso, Italy; d'Agostino, R., Ed.; 1989; Vol. 2, p 1178. constraining peaks to expected positions estimated from model (36)Lopez, C. P.; Ratner, B. D. Polym. Mater. Sei. Eng. 1990,62,14. (87) Lopez, G. P.; Ratner, B. D. J . Appl. Polym. Sei.: Appl. Polym. Symp. 1990,46,493.
(38) Haque, Y.; Ratner, B. D. J . Appl. Polym. Sci. 1986,32,4369.
L6pez and Ratner
768 Langmuir, Vol. 7, No. 4, 1991 [YO00
Table I. Effect of Substrate Temperature on Elemental ComDosition of Plasma-Deposited Acetonee
'
1
~~
reservoir temp, "C 15
C
70 0
C/O
93.3
6.6
92.0 92.0
90.4
8.0 8.0 9.6
14.1 11.5 11.5 9.4
75.0
25.0
3.0
$0
-15
-50 -75
theoryb
Deposition parameters: pressure = 150 mTorr, power = 30 W, reaction time = 10 min, quench = 10 min. Theory for acetone.
I ' 1 296.7
1 i
Binding Energy (ev)
276.7
Figure 2. XPS C 1s spectrum of an acetone film frozen on a cold stage (-140 "C).
compound studies.39 For example, peaks a t 286.5 eV represent ether or alcohol carbons, those at 288.0 eV represent carbons with two bonds to oxygen, and those a t 288.9 eV represent carboxyl-type carbons. The high-resolution C Is spectrum of acetone was obtained by dosing liquid acetone onto a cooled sample stub (--140 "C) in the XPS preparation chamber. The stub was then transferred into the analysis chamber and cooling was resumed to maintain the stub at -140 "C. Carbonyl functional groups in the plasma-deposited acetone films were derivatized by exposing the films to hydrazine (AldrichChemicalCo., Inc.,Milwaukee,WI)vapor for l h. Elemental composition of the hydrazine-derivatized films was then determined by yuantitation of XPS survey scan spectra. A detailed description of the protocol followed may be found elsewhere.40
1
---/,/
__---' 1
292 8
-
\'L---
\ Binding Energy (eV)
c i d
272 8
Figure 3. XPS C Is spectra of plasma-deposited acetone: (a) low W/P, -75 "C; (b) low W/P, 15 "C; (c) high W/P, -75 "C; (d) high W/P, 15 "C. Electron takeoff angle = 55".
how effectively the molecular integrity of the precursor is retained in the deposited film. The initial film depositions using acetone were for 10 Results and Discussion min on glass substrates at 150 mTorr and 30 W power. Acetone. The goal of these experiments was to create The temperature of the cold stage was decreased increa thin film that appeared, by XPS analysis, to be identical mentally from approximately 15to -75 "C. The elemental with the unreacted, condensed precursor. It is obvious compositions of the films obtained are given in Table I. that the total molecular integrity of precursors cannot be Only carbon and oxygen were detected by XPS (the Si maintained if they are to be covalently bound into a film signal from the substrate was completely obscured), matrix, but it was postulated that precursor fragmentation suggesting that an overlayer film more than 100 A (the could be minimized by deposition at low substrate temtypical maximum sampling depth of XPS) in thickness peratures. A more complete retention of molecular was formed. The carbon to oxygen ratio (C/O) decreased structural integrity is expected when precursors that can slightly as the substrate temperature decreased. In all polymerize by a free radical mechanism are i n ~ e s t i g a t e d . ~ ~ cases, however, the C/O of the films was much higher Acetone was used as a model nonpolymerizable precursor than that for acetone (C/O = 3.0). Moreover, in no case for most of the preliminary experiments for several reasons. did the X P S C 1s core level spectrum of the resultant film Plasma-deposited acetone films have been studied exresemble that of the condensed acetone control film. tensively in our laboratories for use in cell culture High-resolution X P S C 1s spectra (Figure 3c,d) and the a p p l i ~ a t i o n s . ~ 0Correlation ~ ~ 2 ~ ~ ~ of bovine aortic endotheresults presented in Table I suggested that the final lial cell growth with film elemental percent oxygen, and deposited film a t all substrate temperatures was formed film carbonyl concentration in ~ a r t i c u l a rhas , ~ ~provided through loss of oxygen-containing moieties and severe the impetus for finding methods of depositing acetone fragmentation and molecular rearrangement of the acetone with low degrees of fragmentation. Furthermore, conprecursor. By analogy to the systems studied by Yasuda densed acetone has a distinctive XPS C 1s core level et al.,8,45we concluded that the power to pressure ratio spectrum (Figure 2) showing a hydrocarbon peak (CH3) ( W / P ) was too high. By lowering W / P , we anticipated a t 285.0 eV and a ketone (C=O) peak a t 288.0 eV. The higher levels of nonfragmented precursor molecules in the acetone C 1s spectrum can be used as a standard to assess plasma and substrate sheath. Therefore, acetone was deposited at a pressure of 300 (39) Dilks, A. In Electron Spectroscopy: Theory, Techniques, and mTorr and 3 W rf power. Because of the lower deposition Applications;Baker, A. D., Brundle, C. R.,Eds.; Academic Press: London, 1981: Vol.. 4. ~ D 277. ~ . rates expected with these parameters, the deposition time (40) ChilkitLA.; Ratner, B. D. Polym. Mater. Sci. Eng. 1988,59, 258. was increased to 30 min and gold-sputtered brass was used (41) Lopez, G. P.; Ratner, B. D. In preparation. as a substrate. Gold substrates provide distinct peaks (42) Chinn, J. A. M.S. Thesis, University of Washington, 1986. (43) Ertel, S. I.; Ratner, B. D.; Horbett, T. A. J . Biomed. Mater. Res. whose attenuation in the X P S spectrum can be used to 1990, 24, 1637. estimate the thickness of ultrathin overlayer films. Sub(44) Ertel, S. I.; Chilkoti, A.;Horbett, T. A.;Ratner, B. D. In Extended
Abstracts of the 2nd Topical Conference on Emerging Technologies in Materials.; American Institute of Chemical Engineers: San Francisco, CA, 1989; p 61.
(45) Yasuda, H.; Hirotsu, T. J . Polym. Sci., Polym. Chem. Ed. 1978,
16, 743.
Plasma-Deposited Organic Films
Langmuir, Vol. 7, No. 4 , 1991 769
strates were etched with an argon plasma prior to acetone deposition to remove hydrocarbon contamination (see Appendix). Effects due to differences in substrate chemistry (i.e., glass vs gold) on the chemistry of deposited, ultrathin overlayer films were not investigated. Substrate temperatures of 15 and --75 " C were used. The deposited films were thin and the strongest gold substrate lines could be seen in the XPS spectra (5-10 atom 90). Film composition changed significantly as the substrate temperature decreased. The C/O ratio decreased from 3.2 for samples deposited without cooling to 2.0 for samples deposited a t -75 "C. Figure 3 compares the C 1s XPS spectra of acetone-deposited samples at low and high W / P ratios and at low and high substrate temperatures. The spectrum of the acetone deposited a t low substrate temperatures and low W / P ratio shows a significant difference compared to that deposited at high temperature. An increase in the higher binding energy C Is peaks indicates a greater proportion of oxygencontaining carbon functional groups. The reduction in the C / O ratio and the change in the C Is spectrum a t low substrate temperature and low W / P indicate a substantial reduction in precursor fragmentation a t these deposition conditions. The thickness ( d ) of these films may also be estimated, if they are assumed to be uniform, because of the dependence of electron mean free path (A) on kinetic energy (KE). Electron mean free path increases as kinetic energy increases. Hence, electrons from different gold energy levels have different escape depths from the sample surface. By comparison of the intensities (I)of peaks from different energy levels, an estimate of an overlayer thickness may be obtained. The following equation may be solved for d when the intensities for the etched (clean) sample are not available (see Appendix):
-
"
I
0
10
30
20
Power (W)
F i g u r e 4. Effect of rf power on film composition for plasmadeposited acetone (0.3 Torr, 40 sccm). Diamonds indicate normalized percent oxygen ( p~ O*), white boxes indicate normalized percent carbon (BC*), and black boxes indicate % C + c~ 0. Dashed lines indicate rGC and 0 for acetone.
"1 20 0) -
w
0 4 . , 0 200
. , . , . , 400
600
. , . , .
800
I
1000 1200 1 4 0 0
Pressure (millitorr)
F i g u r e 5. Effect of acetone pressure on film composition for plasma-deposited acetone (20 W, 40 sccm). Diamonds indicate normalized percent oxygen (7O*), white boxes indicate normalized percent carbon ( % C*), and black boxes indicate % C + 700. Dashed lines indicate % C and %O for acetone.
I , - I,' exp(d/X, COS (0)) -_ I , I,' exp(d/X, cos(0)) The subscripts refer to any two spectral peaks in the gold spectrum. I" is the intensity of a gold peak from a pure gold sample. 0 is the electron takeoff angle. Use of eq 1 is more convenient for routine estimation of d than the method described in the Appendix because the xenon etching step is not required. The ratio of II" to 12" for any clean gold sample may be used. Assuming a film density of 1.1g/cm3 to estimate X (see Appendix), and comparing intensities of the Au 4d5p peak (KE 1152 eV) to that of the Aulf doublet (KE 1043eV) yields film thicknesses of -70 A for films deposited at both high and low substrate temperature. Subsequent experiments with acetone further explored the effect of rf power, pressure, and flow rate on the XPS spectra of plasma-deposited acetone ultrathin films on gold-sputtered brass substrates. Figures 4,5, and 6 show the effects of these variables on the composition of the deposited films as determined by XPS. Since the oxygen and carbon atoms being sampled are assumed to be in a uniform overlayer, normalized oxygen and carbon percentages (!" O* and $0 C*) are plotted as film compositions. For example, normalized film percent carbon is computed as
-
"bC* =
-
x100
%C + 900 Total carbon plus oxygen (90C + 90 0) is also plotted as an indication of the amount of deposit. Figure 7 is a plot of film thickness calculated by eq 1 (assuming a density of 1.1g/cm3 for all films) versus carbon plus oxygen of the
0
10
30
20
40
50
60
70
Flowrate (sccm)
F i g u r e 6. Effect of acetone flow rate on film composition for plasma-deposited acetone (20 W, 0.3 Torr). Diamonds indicate normalized percent oxygen (%O*), white boxes indicate normalized percent carbon ( % C*), and black boxes indicate % C + 5 0. Dashed lines indicate 7, C and ot 0 for acetone. 100,
90
0
-
$ 00-
+
0
ae
'O-
u
6o 0 50
20
40
60
80
100 120
d (Angstroms)
F i g u r e 7. Correlation of % C + % 0 (XPS atomic percentages) with film thickness as estimated by eq 1.
films whose composition is given in Figures 4, 5, and 6. The simple, logarithmic nature of the graph (R2 = 0.92) supports the assertion that carbon plus oxygen is an indication of the amount of deposited overlayer film. The value of the correlation coefficient for a linear least-squares fit (R2 = 0.78) is much lower for these data. Figures 4 , 5 , and 6 show that low power, high pressure, and high flow rate all favor the deposition of films that are closer in elemental composition to the precursor. Because these conditions correspond to a low level of energy per precursor molecule, it is evident that the reduction in
Lbpez and Ratner
770 Langmuir, Vol. 7, No. 4, 1991 Table 11. Effect of Substrate Temperature on C/O and XPS C 1s Peak Areas of Plasma-Deposited Acetone. reservoir
temp, "C 25 10 -20 -50 -72 theoryb
peak area (7; of total C 1s) C/O 285.0 eV 286.5 eV 288.0 eV 288.9 eV 292.0 eV 6.24 4.97 4.85 4.31 3.72
75.0 72.7 66.7 63.4 39.5
15.7 16.1 20.7 21.0 21.7
5.1 7.6 9.2 12.8 12.6
4.2 3.6 3.4 2.7 5.2
3.0
66.6
-
33.3
-
21.0
a Deposition parameters: pressure = 300 mTorr, power = 20 W, acetone flow = 41.4 sccm, reaction time = 20 min, quench = 10 min. Theory for acetone.
*
292 eV
i I 'ii
25oC h__
278.6
Binding Energy (eV)
298.6
Figure 8. Effect of substrate temperature on the XPS C 1s spectra of plasma-deposited acetone films (20 W, 0.3 Torr, 41.2 sccm, 20 min). Electron takeoff angle = 55".
precursor fragmentation under these conditions is responsible for the observed compositional trends. Figures 4,5, 6, and 7 also reveal, however, that these conditions also lead to diminished deposition rates. Therefore, subsequent experiments with acetone were done by using 20 W rf power, 300 mTorr, and a flow rate of approximately 40 sccm. Depositions were done a t these conditions for 20 min while the substrate coolant reservoir temperature was decreased incrementally from room temperature to -75 "C. In this set of experiments, films were deposited on Teflon because its XPS spectrum is free of oxygen and hydrocarbon. Hence, no argon etch pretreatment is required to remove contaminants. Table I1 shows that the C / O ratio of these films decreases as the substrate temperature is decreased. The C / O approaches that for acetone (3.0) a t low temperatures. Figure 8 shows the progression of C Is XPS spectra as the substrate temperature is reduced. Figure 8 also shows an increase in the C Is BE distribution toward higher binding energies as the substrate temperature is decreased. This shoulder is indicative of the relative amount of oxygen covalently bound to carbon in the film. In agreement with the C / O of the films, the C 1s spectra show that lower substrate temperatures result in more oxygen-containing moieties in the film. The significance of the peak a t 292 eV (which is due to the Teflon substrate) for the sample deposited at -75 "C will be discussed below. T o estimate the relative amounts of different oxygencontaining species, C 1s spectra shown in Figure 8 were resolved by constraining peaks to positions common for the various carbon-oxygen functionalities obtained from conventional polymers. In model polymer systems, hytype carbons yield a peak a t 285.0 eV, drocarbon (CH,)
;5
10
6LL 5 0
h a o
4
s
3
g 4 -
2 1
2 40
-60
-40
-20
0
20
40
Reservoir Temperature (C)
Figure 9. Effect of substrate temperature on the ratio of the 285.0-eV peak area to the 288.0-eV peak area from XPS C Is spectra of plasma-deposited acetone films and the relative degree of fragmentation of carbonyl groups (DF, see eq 2).
while oxygenated carbons (e.g., C-0 -286.5 eV, C=O and 0-C-0 -288.0 eV, O=C-0 -288.9 eV) yield peaks shifted to higher binding energies.39 Peak widths were also constrained to 1.5 eV. The relative areas in percentage of total C 1s area for the resolved peaks are given in Table 11. Figure 9 shows the ratio of the 285.0-eV peak to the 288.0-eV peak as a function of the coolant reservoir temperature during deposition. The graph shows that this ratio approaches that for a pure acetone film (2.0) as the temperature is decreased. It should be emphasized, however, that these films are of highly complex chemistry and resolved peaks correspond only approximately to the assigned functionalities and should only be used as an estimate of the various functionalities. Nevertheless, the XPS spectra suggest that reduction of precursor fragmentation in plasma deposition is possible by decreasing the substrate temperature. Films formed on cold substrates are more similar in functional group concentration to the precursor gas than films prepared by ambient temperature plasma deposition. It is convenient to define a semiquantitative estimate for the relative degree of fragmentation of a precursor functional group that occurs during deposition. This can be done for systems where the precursor has a functional group with a characteristic peak in the XPS spectrum which would be seen in the XPS spectrum of the deposited film if the functional group was deposited without severe fragmentation. For example, for acetone, the ratio of the resolved C 1s 288.0eV peak (indicative of carbonyl groups) to the total C 1s area is equal to 1/3. By comparison of this ratio to that of the plasma-deposited film, an estimate of the relative degree of fragmentation (DF) may be made DF =
(A28k~IA~ls)acetone
(2)
(A288eV/ACla)film
By use of this formalism, the DF for acetone plasma deposited a t different substrate temperatures is given in Figure 9. The DF for the sample deposited a t -72 " C is unexpectedly high because of the 292-eV peak in the C 1s spectrum. Another semiquantitative assay that can be used to verify the relative degree of fragmentation of the acetone precursor is to derivatize the carbonyl groups in the film with hydra~ine.~6 This gas-phase derivatization reaction has been shown to selectively tag carbonyl groups in the presence of a variety of other oxygen containing functional groups. However, it has also been shown that the reaction cannot be considered completely quantitative because it does not proceed to completion for all model polymers.47 Figure 10 shows the percent nitrogen incorporated into (46) Gerenser, L. J.; Elman, J. F.; Mason, M. G.;Pochan, J. M. Polymer 1985,26, 1162. (47) Chilkoti, A.; Ratner, B. D.; Briggs, D. submitted for publication in Chem. Mater.
Plasma-Deposited Organic Films i2
Langmuir, Vol. 7, No. 4, 1991 771
I
-
E
8m ae
-.
I
.
.
-80 -60 -40 -20
. 0
I
20
.
279.2
Figure 11. Variable electron takeoff angle XPS C 1s spectra of acetone deposited using liquid nitrogen cooling. the film after 1 h of exposure to hydrazine. As expected, as the substrate temperature is reduced, more nitrogen is incorporated into the films. If the percent yield of the reactions on the various films is assumed to be approximately equal, these data indicate that more carbonyl groups were present in the films deposited at low temperature. The additional peak at a BE of 292.0 eV in the sample deposited at the lowest temperature is due to the -CF2groups in the Teflon substrate. XPS sampling of photoelectrons from the Teflon substrate could be due to either deposition of a very thin film (e100 A) or deposition of a patchy, nonuniform film. Deposition at these low substrate temperatures and lower substrate temperatures (obtained by liquid nitrogen cooling) resulted in films with similar spectra even when deposition times were extended. It became obvious that for low substrate temperatures (below the condensation point of acetone) a substantial portion of the film was volatilizing when the samples were returned to ambient temperature (after the plasma deposition). To assess whether the film remaining was uniform or patchy, angular-dependent XPS was used to generate compositional depth profiles of the sample surfaces. Figure 11 shows the C Is spectra at various electron takeoff angles. These spectra were resolved into component peaks as before and depth profiles of the likely functional groups were calculated by using the regularization algorithm developed by Tyler e t al.48 The resulting depth profile (Figure 12) indicates that acetone films plasma-deposited a t very low temperatures are both very thin and patchy. Functional groups associated with the deposition (e.g., CH,, C-0, C=O, etc.) as (48) Tyler, B. J.; Castner, D. G.; Ratner, B. D. Surf. Interface Anal. 1989, 14, 443.
20
0
Figure 10. Amount of nitrogen (atom % ) incorporated into acetone films plasma-deposited at different substrate temperatures after hydrazine derivatization.
Binding Energy (eV)
40
c-0 c=o oc=o CF2
n
40
Reservoir Temperature (C)
299.2
60
CHx
25
50 75 100 Depth (Angstroms)
125
Figure 12. Compositionaldepth profile calculated by resolving C 1s spectra of acetone deposited using liquid nitrogen cooling. The smoothing parameter,a,used in the depth profile algorithm was equal to 7.5. well as CF:! are present a t the surface, and patches (islands) are approximately 75 A thick. Film nonuniformity has been observed when using other precursors (e.g., ethylene oxide and tetrahydrofuran) at very low temperatures on a variety of substrates. Methods of obtaining uniform overlayers at very low temperatures are being investigated.41 There are little published data for the effect of reduced substrate temperatures on deposition rate, especially for nonpolymerizable precursors. In general, an increase in deposition rate at lower temperatures has been shown, but experiments with nonpolymerizable precursors have been sparse. Furthermore, methods of measuring deposition rate (in situ, ex situ) differ as to their ability to distinguish deposition rates of nonvolatile film components from film components that might volatilize when the substrate temperature is raised to ambient.7~9~49~50 The data presented here for acetone suggest that, for some nonpolymerizable precursors, films resulting from deposition at low temperatures may be thinner than those deposited a t higher substrate temperatures. This is probably related to low reactivity between condensed molecules at low temperatures. Hexafluoro-2-propanol. Hexafluoro-2-propanol (HFIP) was also used as a precursor because it has a characteristic XPS spectrum and it contains a variety of bonds with different bond strengths. I t is also more easily condensed than acetone. The -CF3 moieties give rise to a large chemical shift in the C 1s spectrum, which can be used as an indicator of the degree of precursor fragmentation during deposition. Fluorine also provides a film signal distinct from the substrate (glass). A computer-synthesized C 1s spectrum for HFIP in Figure 13 shows the relative amounts of the two carbon environments. This spectrum was constructed based on literature values of the BE shifts for the different carbon atoms. These estimates were made by assuming linear addition of BE shifts due to the various atoms bonded to the carbons. The peak corresponding to the -CF3 carbons was placed at 293.7 eV because one fluorine bonded to a carbon is known to shift the carbon BE -2.9 eV from a carbon-carbon bond (285.0 eV).39 Hence, three fluorines bound to carbon would shift the BE -8.7 eV. The BE shift expected of the central alcohol carbon was estimated by adding the BE shift typically seen for an alcohol carbon (1.5 eV) to that typical of carbons in the 0 position to fluorine-containing carbon functionalities (approximately 0.35 eV per f l ~ o r i n e ) . ~Hence, 9 the peak corresponding to the alcohol carbon was placed at 288.6 eV. (49) Deutsch, H.; Kersten, H.; Klagge, S.; Rutscher, A. Contrib.Plasma Phys. 1988,28, 149. (50) Westwood, A. R. Eur. Polym. J . 1971, 7, 363.
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772 Langmuir, Vol. 7, No. 4, 1991
The relative degree of fragmentation of the -CF3 groups in HFIP occurring during deposition for the two substrate temperatures can be estimated by using the area of the CF3 peak and the formalism developed in eq 2. By use of a similar ratio of areas, the relative degree of fragmentation for the films deposited a t low temperature (DF 1.5) is much less than that for the films deposited a t the higher substrate temperature (DF 5.3). XPS analysis also showed that the samples deposited a t low temperature had 2.4 times more fluorine (42.4% vs 17.4% ), indicating an overall composition much closer to HFIP. The C / F / O ratio (HFIP = 3/6/1) for the low temperature coated samples was 4.3/4.0/ 1,while that for the sample coated a t high temperature was 1.0/0.6/1. The high oxygen content of the samples coated a t high temperature is due to XPS sampling of the glass substrate through the
