Chapter 28 D e g r a d a t i o n a n d Passivation o f Poly(alkenylsilane
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sulfone)s i n O x y g e n P l a s m a s A. S. Gozdz, D. Dijkkamp, R. Schubert, X. D. Wu1, C. Klausner, and Murrae J. Bowden Bell Communications Research, Red Bank, NJ 07701-7020
The passivation and degradation of poly(3-butenyltrimethylsilane sulfone) (PBTMSS), a sensitive, positive electron-beam resist has been studied in a parallel plate and barrel type plasma reactor. The resist is passivated in an oxygen plasma only when the oxygen pressure and/or the cathode self-bia exceed certain critical values during the initial exposure of the polymer to the plasma environment. Several oxygen plasma pretreatment procedures have been developed leading to the formation of a high quality silicon dioxide surface passivated layer. The oxide layer was characterized by IR, Auger electron spectroscopy, and Rutherford backscattering spectroscopy. The passivation efficiency is determined by the relative rates of surface oxidation and polymer oxidation which are dependent upon the concentration of active species (ions and neutrals) in the plasma. The reactive environments employed in the plasma-enhanced deposition and etching processes which are widely used at various stages of integrated circuit manufacturing (/), place stringent requirements on the resist materials used in these processes. For example, oxygen reactive-ion etching (RIE) is often used in two-layer microlithography to transfer the image from a thin imaging resist layer into an underlying thick planarizing layer. This requires the top resist to be highly resistant to the oxygen plasma environment with an etch rate ratio between the bottom and top layer greater than 10/1. Such requirements have led to the development of organosilicon polymers - long known for their thermal and chemical resistance - as resists for this application (2-4). These materials are oxidized upon exposure to an oxygen plasma resulting in the formation of a thin surface layer of silicon oxide which passivates the surface (5-7) and greatly retards the rate of resist removal. This surface oxide layer is slowly sputtered during RIE, but the overall resist etch rate is much lower than that of the planarizing layer. Etch rate ratios as high as 100:1 can be obtained. The majority of organosilicon polymers crosslink efficiently upon e-beam exposure (8) and thus behave as negative-tone resists. Poly(alkenylsilane sulfone)s, on the other hand, have been found to degrade easily upon e-beam exposure, which places them in a small class of positive 7
Current address: Department of Physics, Rutgers University, New Brunswick, NJ 08903
0097-6156/87/0346-0334$06.00/0 © 1987 American Chemical Society
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G O Z D Z ET A L .
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sulfone)s in Oxygen
335
Plasmas
organosilicon electron-beam resists (ΙΟΊ3). In view of their high e-beam sensitivity (1 to 2 MC/cm at 20 keV), and superior resolution, (i.e., compared to the negative silicon-containing resists which are generally characterized by inferior resolution and linewidth control because of the extensive swelling and thinning during solvent development), these materials presage potential application in the fabrication of microelectronic devices with submicron geometries via direct e-beam lithography (9). Initial processing experiments showed however, that under typical 0 R I E conditions (power - 0.1 to 0.2 W / c m , self-bias - - 2 5 0 to - 3 5 0 V , pressure - 5 to 20 mTorr 0 ) , these resists are not very resistant, particularly under prolonged etching. Effective pattern transfer may require etch times of 15 to 30 min (11,12). which are sufficient to cause extensive degradation of the resist. Such behavior is reminiscent of poly (olefin sulfone) s such as the well-known P B S e-beam resist, which is etched 5-7 times faster than polystyrene in an oxygen plasma (5). Further studies have shown, however, that under appropriately chosen plasma etching conditions, poly(alkenylsilane sulfone) s can be passivated in an oxygen plasma like other organosilicon polymers (12,13), but the passivation process depends strongly on the plasma processing parameters. In this paper, we report the results of our studies on the degradation and passivation processes of a typical poly(alkenylsilane sulfone), viz., poly(3-butenyltrimethylsilane sulfone) ( P B T M S S ) (13) in oxygen plasmas. 2
2
2
2
Experimental Materials. Poly(3-butenyltrimethylsilane sulfone) ( P B T M S S ) was synthesized by free-radical copolymerization of 3-butenyltrimethylsilane with liquid sulfur dioxide (molar ratio 1:9) initiated with azobisisobutyronitrile ( A I B N ) at 35 °C in a sealed glass ampoule. The detailed preparation procedure and properties of this copolymer have been reported elsewhere. (73) Sample preparation. Thin films of P B T M S S for Rutherford backscattering spectroscopy ( R B S ) and general plasma etching studies were spun on polished silicon wafers from a 3.5% solution in chlorobenzene using a photoresist spinner. The films were baked for 10 to 20 min. at 105-120 C in air. P B T M S S films for Auger electron spectroscopy ( A E S ) studies were spin-coated on silicon wafers previously coated with 2000 A of gold. Films for IR studies were spin-coated onto N a C l plates. e
Oxygen Plasma Etching. P B T M S S films were etched in a Cooke Vacuum Products parallel plate R I E system operating at 13.56 M H z . The stainless steel reactor was 30 cm in diameter and 35 cm high. Both electrodes, 12.5 cm in diameter and spaced 9 cm apart, were covered with a S i wafer and both were water-cooled. The R F power could be coupled either to the lower electrode ( R I E mode) or to the upper electrode (sputter mode, S M E ) . The required oxygen pressure and its flow rate were maintained by the mass flow and exhaust valve controllers. Composition of the gas mixture during R I E was monitored using a quadrupole mass spectrometer (Quadrex 200, Inficon Leybold-Heraeus, Inc.). The pressure was reduced to the 10~ Torr range using a flow-restricting orifice and a turbo-molecular pump. Other parameters of the etching process are reported in the text. Resist films were also etched in a barrel-type plasma reactor (Plasmod, M a r c h Instruments, Inc.) operating at 13.56 M H z at a power density of approx. 0.007 W / c m and with an oxygen pressure of 0.85 Torr. 6
3
Surface Analysis. The resist etching process was studied by measuring changes in the resist thickness versus etching time using a mechanical stylus surface profiler (Alpha-Step 200, Tencor Instruments, Inc.). Infrared spectra were recorded on a Perkin-Elmer Model 983G double-beam spectrophotometer in the transmission mode using 3500 Â thick P B T M S S films spin-coated and processed on polished N a C l plates. Spectral subtraction and absorbance correction to account for the decreased film thickness were used to isolate the silicon oxide absorption band at about
POLYMERS FOR HIGH T E C H N O L O G Y
336
1050 c m . The equivalent thickness of silicon oxide surface layers was calculated by comparing the absorbance at 1050 c m with that of thermally grown oxide films of known thickness. The change in absorbance per unit thickness obtained from the calibration curve was 1.5· 10~ a.u./A. Glancing angle ( a = 8 0 ° ) Rutherford backscattering spectrometry (RBS) with a 2.0 M e V H e beam was used to obtain the depth-resolved elemental composition of the films. In order to obtain sufficient counting statistics, and at the same time minimize beam-induced damage, each spectrum was accumulated from 20 different spots on the sample. The H e ion beam dose was kept below 1 μθ/cm per spot. Spectral simulation was performed using the computer code R U M P (14). Auger electron spectroscopy ( A E S ) data were obtained on a Perkin-Elmer Scanning Auger Multiprobe P H I 600 using a 10 keV, 5 n A electron beam being rastered at 400X, and a 3 keV, rastered 1.1 n A argon ion beam. These electron and ion beams were of sufficiently low power so as not to thermally decompose the polymer when the beams were used in the raster mode of operation. The area sputtered by the argon ion beam was about 2 by 2 mm. The ebeam survey area had a rectangular shape with sides about 0.4 by 0.5 mm; it was located in the center of the ion-sputtered crater. Sputtering rate for the initial P B T M S S film under experimental conditions used was 140 ± 2 0 Â/min. Exposure to the electron beam alone did not cause any detectable change in surface composition. - 1
- 1
4
+
+
2
Results Oxygen Plasma Etching. The change in thickness versus time for a 3600 Α-thick P B T M S S film during 0 R I E is shown in Figure 1 (curve A ) . The R I E parameters were: power: — 0.12 W / c m , bias: —400 V , 0 pressure - 20 mTorr, flow rate - 10 seem. These parameters correspond to what we shall refer to as the "standard" R I E process parameters throughout the remainder of the text. A s seen in Figure 1, the thickness decreased by 30 to 70%. Accompanying this reduction in thickness was the appearance of numerous defects in the form of pinholes and bubbles in the film, particularly after prolonged etching. This behavior was similar to that of the previously reported terpolymer of allyltrimethylsilane with 1-butene and sulfur dioxide (11,12). Reducing the oxygen pressure and/or the bias increased both the rate of resist removal and the number of defects in the residual film. O n the other hand, increasing the self bias resulted in a significant decrease in the etching rate (Figure 1, curves Β & C ) . The number of defects was also significantly reduced. In fact, for a bias greater than —600 V , an entirely defect free surface was obtained provided the etching time was less than 30 sec. W i t h prolonged etching at high bias, numerous defects again appeared in the etched film as shown by dashed curves in Figure 1. If, however, the bias was lowered back to the standard value of —400 V following 10 to 30 sec at high bias (—600 to —900 V ) , etching could be continued without any subsequent defect formation. In fact, films subjected to this short high-bias pretreatment were able to withstand longer than 30 min. standard R I E processing without the formation of surface defects, thus facilitating excellent R I E pattern transfer. In another set of experiments, P B T M S S samples were etched in the reactor which has been configured for sputtering, i.e., with a relatively high R F power (0.8 W/cm ) coupled to the upper target electrode. The wafer potential in this mode is low (80 to 130 V ) . The resist thickness was reduced by about 800 to 1000 Â after 1 min. of such treatment (Figure 1, curve D ) . However, the surface of the polymer was smooth and defect-free. It was also found that a P B T M S S film pretreated by this method would withstand subsequent standard 0 R I E pattern transfer in a manner similar to the sample pretreated by the high-bias process, i.e., without the formation of surface defects. For convenience this pretreatment process will be referred to as "sputter-mode etching" ( S M E ) . We found that there is an R F power density threshold below which this S M E passivation pretreatment does not occur. In our RIE/sputtering system at p=20 mTorr 0 , this threshold was about 0.25 W / c m (30 W ) (Figure 2). A t lower power densities, the resist was again seriously damaged and mostly volatilized. A t higher R F power densities but with longer etch 2
2
2
2
2
2
2
28.
G O Z D Z ET A L .
Poly(alkenylsilane
1
0-1 0 Fig. 1.
1
2
337
sulfone)s in Oxygen Plasmas
1
4 6 ETCHING TIME (min.)
1
1
8
10
P B T M S S film thickness vs. time during R I E and S M E at 20 mTorr 0 . Bias: A : - 3 5 0 V ; B: - 4 5 0 V ; C : - 5 5 0 V ; D : sputter mode etch ( S M E ) at 75 W . 2
6000
/
20 Fig. 2.
40
60 80 100 R. F. POWER (W)
120
140-
160
Thickness of a P B T M S S film remaining after 1 min. of S M E etching (20 mTorr 0 , 10 seem) versus R F power. 2
338
POLYMERS FOR HIGH T E C H N O L O G Y
times (greater than 2-4 min), the resist began to decompose with the formation of numerous bubbles occurs during the prolonged high-bias R I E etching. We found that a passivation time of 15 to 60 s in the S M E mode at a power of 50 to 75 W was sufficient to permit subsequent pattern transfer by "standard" R I E . The S M E passivation pretreatment has two major drawbacks: 1) it introduces additional process complexity in cases where the R I E system cannot be conveniently converted to a sputtering mode, and 2) it causes a relatively large decrease in the resist thickness which may lead to loss of linewidth control. We also studied the etching of P B T M S S in a barrel-type oxygen plasma reactor at a much higher oxygen pressure (850 mTorr) than that used in the R I E process (10 to 20 mTorr). The etching curve of normalized resist thickness remaining as a function of time is shown in Figure 3, curve A . Contrary to the S M E process, the loss of thickness during plasma barrel etching was always less than ca. 300 À even after prolonged exposure to the oxygen plasma. The process again created a surface oxide layer which was durable enough to withstand subsequent "standard" 0 R I E (Figure 3, curve B). N o surface defects were present either after etching in the barrel reactor or after subsequent R I E , provided a barrel reactor pretreatment of — 2 min. was employed. This result suggested that poly(alkenylsilane sulfone) s might undergo passivation under standard R I E conditions of bias and power but at significantly higher oxygen pressures than the standard 20 mTorr used in the earlier studies. The results from a series of experiments designed to determine the range of process parameters (oxygen pressure and self-bias) which cause rapid and defect-free passivation of the polymer are summarized in Figure 4. A l l results refer to etching time of 1 min. A combination of low oxygen pressure and low bias resulted in inefficient passivation of the resist, leading instead to rapid conversion to gaseous products and/or the formation of numerous pinholes. O n the other hand, prolonged etching at high bias caused excessive heating of the surface and bubble formation. However, the combination of processing variables encompassed by the shaded area in Figure 4 favored formation of a defectfree surface, provided etching times were less than 1 minute. For example, a defect-free surface could be obtained under standard conditions of bias (—400 V) and power (0.12 W/cm ) provided the oxygen pressure was greater than 100-140 mTorr. It is important to note, however, that extended etching under these conditions will lead to undercutting of the planarizing layer which is why it is necessary to revert to the "standard" low-pressure conditions to obtain anisotropic etched profiles. A l l samples pretreated under conditions encompassed by the shaded area withstood subsequent "standard" R I E pattern transfer etching with negligible thinning and without the formation of defects in the etched film. A comparative summary of the four passivation pretreatment methods for P B T M S S is given in Table I. 2
2
Mass Spectrometry. The etching and passivation process was studied by measuring the concentration of volatile gaseous products in the plasma by quadrupole mass spectrometry. Sulfur dioxide gives rise to a characteristic peak in the mass spectrum of the degradation products at m/z—64 corresponding to S O j resulting from the oxidation of these poly (olefin sulfone) polymers. Since there are no other interfering mass peaks, the change in the peak intensity at m/z 64 vs. time during etching provides a reliable measure of the progress of resist degradation. Other mass peaks, such as m/z 18 ( H 0 ) , 28 ( C O , but also N j ) , 32 ( O j ) , 44 ( C O J ) , and 48 ( S O ) were more prone to interference from other fragmentation products or were less sensitive than the S O ? peak. 2
+
+
+
Changes in the concentration of S 0 in the plasma during R I E at various cathode bias values as well as during S M E at the same pressure are shown in Figure 5. The change in intensity of the m/z 64 peak with time depended markedly on the bias conditions and electrode configuration and mirrored the changes in the resist thickness shown in Figure 1. S 0 was rapidly eliminated during the initial phase of low-bias R I E (curves A - C ) , although the extent of 2
2
28.
G O Z D Z ET A L .
50 I 0 Fig. 3.
Poly(alkenylsilane
1
1
1
2
4
6
sulfone)s in Oxygen
1
1
1
339
Plasmas
1
8 10 12 14 ETCHING TIME (min.)
1
1
16
18
1
20
Thickness of P B T M S S after etching in a barrel plasma reactor (A) (p * 850 mTorr) versus time. The samples were then etched for 10 min. under 0 R I E conditions (B) (p = 20 mTorr, U - - 4 0 0 V ) . 02
2
0l
1
60G ι
55G [
500 > 450 ΙΛ
< m 400 -
350 3QQ
Fig. 4.
I
ι
ι
0
20
40
ι
ι
ι
ι
1
1
60 80 100 120 140 160 OXYGEN PRESSURE (mtorr)
1
1
180
200
1
220
Effect of pressure and bias on surface quality following 1 min. 0 R I E . Shaded area shows combination of pressure and bias that result in a defect-free surface. 2
340
POLYMERS FOR HIGH T E C H N O L O G Y
Table I. Comparison of P B T M S S Passivation Methods Passivation Method Parameter
Sputter Mode
Barrel Reactor
Hi-Pressure RIE
0 press., mTorr 2
Hi-Bias RIE
20
800-1000
100-200
20
bias, -V
80-130
10-30
400
600-900
power, W
50-100
low
50
50
60
120
30-60
10-30
800-1000
150-200
200
200-300
800-1000
300-400
-300
-300
time, s loss of thickness, A thickness loss after 10 min. R I E
degradation (as judged from the peak intensity) decreased with increasing bias. Above —500 V , very little S 0 was detected in the plasma (curves D and E) indicating a low extent of degradation. This is in agreement with the low thickness loss observed under high-bias conditions. Curves F and G in Figure 5 represent changes in the S 0 concentration during S M E at a R F power of 50 and 100 W , respectively. The intensity of the m/z 64 peak increased sharply at the very beginning of the process during which about 800 to 1000 A of the resist were volatilized. Thereafter, it decreased asymptotically to the background level. The amount of S 0 liberated initially depended on the R F power being somewhat higher at 100 W (curve G) than at 50 W (curve F). 2
2
2
Infrared spectroscopy. The IR transmission spectra of P B T M S S films (original thickness: 3500 A ) in the range 900 to 1200 c m after passivation by various methods are shown in Figure 6. The difference spectra between the initial and etched (for 1 and 10 min.) films corrected for changes in film thickness are also plotted in Figure 6. Curves l a , l b , and l c show the IR spectra after 0 (initial), 1, and 10 min. etching under standard R I E processing conditions (—400 V , 20 mTorr 0 10 seem) respectively. The corresponding difference spectra are represented by curves Id and le. The major difference was the appearance of a peak at 1070 c m which corresponded to the Si—Ο stretching frequency. We attribute this to S i O * formed at the surface of the resist by oxidation in the plasma environment. The intensity of this peak increased with etching time corresponding to increased conversion of S i to S i O . The remaining spectra in Figure 6 show evidence of similar formation of S i O for the different passivation treatments. The extent of oxide formation as judged from the intensity of the peak at 1070 c m depended on the particular passivation treatment as did stability to an additional 10 min. "standard" R I E . For example, S M E resulted in a much greater extent of oxide formation (curve 2d) than high-bias (curves 3d and 4d) or barrel etching (curve 5d). - 1
2
- 1
x
x
- 1
28.
G O Z D Z ET A L .
0
Poly(alkenylsilane
25
sulfone)s in Oxygen Plasmas
50
341
75
Time (s) Fig. 5.
Concentration of sulfur dioxide in an oxygen plasma measured by mass spectrometry vs. time during P B T M S S etching under various conditions. R I E (20 mTorr 0 10 seem): A : -320 V , B: -350 V , C : -380 V , D: -450 V , E : -550 V ; S M E (20 mTorr, 10 seem 0 ) ; F: 50 W , G : 100 W 2
2
100
POLYMERS FOR HIGH T E C H N O L O G Y
342
1100
1000
1200
1000
800
Wavenumber (cm" ) 1
Fig. 6.
IR transmission (a-c) and difference (d,e) spectra of 0 plasma etched P B T M S S films listed in Table II. Samples: 1 - R I E - 4 0 0 V ; 2 - S M E ; 3 - high-pressure R I E ; 4 - high-bias R I E ; 5 - barrel reactor etched. Curves a are for the initial film, b for the film treated as given in Table I, c after additional R I E at 20 mTorr and - 4 0 0 V for 10 min. Curves d—b—a, e c — a . 2
e
28.
G O Z D Z ET A L .
Poly(alkenylsilane
sulfone)s in Oxygen
343
Plasmas
Contrary to the increase in oxidation observed during standard R I E , films subjected to one of the passivation pretreatments (with the exception of the barrel reactor process) showed little further increase in oxide conversion beyond the initial pretreatment conversion. Only the films pretreated by high pressure oxygen plasma in the barrel reactor showed a further increase in oxide conversion on subsequent "standard" R I E (curve 5e). It is recognized that transmission IR only presents an integrated picture of the silicon conversion throughout the whole thickness of the resist. However, the bulk of the oxidation occurs at the surface, and the equivalent oxide thickness obtained from the calibration curve for thermally grown oxide films can be taken to represent the approximate thickness of the plasmatreated films. The thickness of the oxide film formed by the various pretreatments is summarized in Table II. Table II. Thickness of Surface S i O * and P B T M S S After Plasma Passivation process (see Tab.I) RIE -400 V S M E 75 W barrel hi-bias R I E hi-press R I E
After R I E
Passivation absor bance at 1070cm" a.u.
SiO , A
resist
%
absor bance at 1070cm a.u.
72 92 28 48 48
30-60 76 96 92 95
0.0192 0.0156 0.0072 0.0084 0.0084
thickness of 1
x
0.0108 0.0138 0.0042 0.0072 0.0072
-1
thickness of SiO A
x
128 104 48 56 56
resist
%
Ί0 76 88 92 92
Auger Electron Spectroscopy (AES). A E S coupled with argon ion-beam sputtering was used to obtain atomic concentration depth profiles of etched P B T M S S films. Poly (olefin sulfone) s present considerable experimental difficulties for surface analytical techniques involving high energy particle beams because of their facile decomposition when irradiated with such high energy radiation. However, with the precautions outlined in the experimental section, spectra could be obtained which corresponded to the actual composition of the surface. A n Auger electron survey spectrum of an initial P B T M S S film showed the expected composition of the surface layer, namely C , O, S i , and S as shown in Figure 7, curve A . After the exposure to an oxygen plasma the surface composition of the film was vastly different as shown in the A E S survey (Figure 7, curve B). Essentially, only Si and Ο were present in the surface layer. Moreover, the shift in the A E S S i peaks from 92 and 1619 eV to 76 and 1606 eV indicated that the S i was present as S i O as opposed to elemental or organosilicon. Monitoring both S i peaks (labeled as S i (92 eV) and S i (76 eV) enabled us to follow the growth of the oxide layer as a function of time for the different passivation treatments. The atomic composition of the unetched P B T M S S surface shown in Figure 8 closely corresponds to the molecular formula of the polymer. The slight surface enrichment in S i and Ο and corresponding depletion in C may be due to a small degree of oxidation during prebaking. Figure 9 shows the concentration profiles for the different elements as a function of depth for a film pretreated for 1 min. in an oxygen barrel reactor. The results show that the composition of the surface layer is variable to a depth of about 300 A , thereafter conforming to that expected of the virgin polymer. The surface layer is composed mostly of inorganic silicon and oxygen with small amounts of organic silicon and carbon. The sputtering time required for the atomic composition of the surface layer to reach that of the unetched, i.e., virgin P B T M S S depended on the passivation pretreatment process. The time increased in the order S M E > high bias > barrel reactor reflecting the differences in thickness of the oxide layer. These results are also in qualitative agreement with the IR results presented in Figure 6 and Table II. x
e
0
POLYMERS FOR HIGH T E C H N O L O G Y
344
200
400
600
800
1000
1200
1400
1600
1800
2000
Kinetic Energy, eV Fig. 7.
Auger electron survey of P B T M S S surface before (curve A ) and after 0 min, - 6 0 0 V , 20 mTorr 0 ) (curve B)
2
R I E (1
2
0 Fig. 8.
1
2
3
4 5 6 7 SPUTTER TIME (min.)
8
A E S atomic concentration depth profile for an untreated P B T M S S film on A u / S i (1 min. sputtering = 140 A ) .
28.
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Poly(alkenylsilane
sulfone)s in Oxygen
345
Plasmas
Rutherford Backscattering Spectroscopy. The high energy of the H e ions used in R B S gives rise to electronic energy loss (bond breaking) through the entire thickness of the polymer film. Therefore, the precautions described in the Experimental section were taken to minimize loss of P B T M S S film during the measurement. Nevertheless, the spectrum of the untreated film (Figure 10) shows a sloped S i substrate edge below 0.8 M e V which is caused by thinning of the film during the measurement. However, the simulation based upon the elemental composition of P B T M S S ( C H i 0 S S i ) shows excellent agreement above 0.8 M e V . Figure 11 shows an R B S spectrum of a P B T M S S film after 0 R I E (1 min, - 8 0 0 V , 20 mTorr). The substrate signal in this and other spectra is markedly lower and it is similar to the simulated spectrum which indicates that oxygen plasma treatment makes the film more radiation resistant. In addition, the surface layer of plasma treated films does not contain measurable amounts of sulfur. The thickness of this sulfur-free layer, estimated by comparison with simulated spectra, is about 50 A after R I E and S M E and 25 À after the barrel etching. The data also show that the same layer is enriched in oxygen and silicon which was confirmed by simulation. These results compare favorably with those obtained from A E S spectra. +
7
6
2
2
Discussion Passivation and Etching of PBTMSS. Poly (olefin sulfone) s traditionally have shown very poor resistance to plasma environments. This primarily stems from the tendency of these polymers to decompose with extensive depropagation of the polymer chain in a high-energy radiation environment, particularly at elevated temperature, leading to enhanced rates of material loss. The onset of thermal depolymerization of P B T M S S in air determined by thermogravimetric analysis is approx. 170°C (13), but radiation-induced depolymerization occurs at much lower temperatures depending on the ceiling temperature (T ) of the polymer. By analogy with the 1-olefins, T for formation of P B T M S S is probably in the neighborhood of +60 °C which is comparable to the temperature at the wafer surface during R I E . Thus we would expect P B T M S S to be degraded in an R I E environment in a manner similar to the other poly (olefin sulfone)s. Our results indicate that P B T M S S does indeed rapidly degrade (volatilize) in the 0 R I E environment, particularly at low bias and low oxygen pressure. Increases in the mass spectrum peak intensities at m/z equal to 1, 2, 17, 18, 28, 44, 48, and 64 are entirely consistent with the production of S 0 and 3-butenyltrimethylsilane, the latter being oxidized in the oxygen plasma to C O (m/z-28), C 0 (44), H 0 (18) and S i 0 . A uniform passivating S i O layer does not form under these conditions reflecting a much greater rate of polymer degradation compared to surface oxidation (passivation). A t higher oxygen pressures or at higher bias (>—600 V ) however, the situation appears to be reversed, i.e., the rate of surface oxidation is considerably greater than the rate of polymer degradation resulting in efficient surface passivation. A t high bias, for example, very little S 0 is eliminated during the initial stages of etching (curve E , Figure 5), i.e., for etching times
