Chapter 27 Enhancement of Dry-Etch Resistance of Poly(butene-1 sulfone) William M. Mansfield AT&T Bell Laboratories, Murray Hill, NJ 07974
The etch resistance of poly(butene-1 sulfone) in fluorocarbon-based plasmas can be enhanced by prior treatment of the surface in an ox plasma. This pretreatment inhibits or retards the depolymerization rea that characterizes normal etching influorocarbonplasmas, thereby permitti formation of a surface-modified layer which exhibits a substantially redu etch rate. Pretreating PBS in an oxygen plasma enables it to be us subsequently in selective reactive-ion etch processes involving fluoro plasmas to delineate submicron, anisotropicalîy etched patterns. Poly(butene-l sulfone (PBS) is a highly sensitive, high-resolution electron-beam resist (1-2) which is used primarily as a wet-etch mask in the fabrication of chrome photomasks. PBS has found little use as a dry-etch mask because of its lack of etch resistance in plasma environments (3-8). This primarily stems from the fact that PBS depolymerizes in such an environment which greatly enhances the rate of material loss from thefilm.Moreover, depolymerization is an activated process which causes the etching rate to be extremely temperature dependent. Previous work (3,7) has shown that the etch rate of PBS influorocarbon-basedplasmas varies by orders of magnitude for temperature differentials of less than 30 °C. It has been found, however, that the etch rate of PBS can be reasonably controlled in both oxygen and CF /0 plasmas if the substrate temperature is kept below room temperature (9). This fact has been utilized to reduce the defect density in the manufacture of chrome photomasks by exposing the developed PBS pattern to a low-temperature oxygen plasma (descum) prior to wet-etching the chrome. We have now found that the plasma-etch resistance of PBS in a CF /0 plasma can be markedly enhanced at room temperature simply by exposing the resist to a short oxygen plasma pretreatment prior to exposure to thefluorinatedplasma. This effect can be used in a variety of pattern transfer processes to controllably generate submicron features on wafers and masks. This paper examines the parameters associated with this effect, proposes a mechanism to account for the results and delineates some possible pattern transfer processes. 4
2
4
2
EXPERIMENTAL Sample Preparation All experiments were carried out on three-inch silicon wafers. Details of the various steps involved in the processing of PBS which include spin-coating, exposure, development, and baking are proprietary, and are available under license from AT&T. Etching rates offilmsspun on silicon wafers were determined by measuringfilmthickness as a function of time, using a Nanometrics/Nanospec AFT Micro Area thickness gauge. Selected measurements were verified with a stylus profilometer. Auger and scanning electron microscope analyses (SEM) were made on silicon wafers which had been coated with sputter deposited tantalum/gold/tantalum metallization prior to resist application. 0097-6156/87/0346-0317$06.00/0 © 1987 American Chemical Society
POLYMERS FOR HIGH T E C H N O L O G Y
318
Plasma Etching A H plasma exposures were carried out i n an I P C (International Plasma Corporation) 2005 capacitance-coupled barrel reactor at 13.56MHz. T h e reactor was equipped with an aluminum etch tunnel and a temperature controlled sample stage. Pressure was monitored with an M K S capacitance manometer; R F power was monitored with a Bird R . F . power meter and substrate temperature was measured with a Fluoroptic thermometer utilizing a fiber optic probe which was immune to R . F . noise.
Auger Analysis and Scanning Electron Microscopy The surface composition of the uncoated, patterned wafer was analyzed by Auger spectroscopy. Line quality and surface detail at each key process step were examined by scanning electron microscopy using samples overcoated with 100Â of a platinum/gold alloy.
Gaseous Etchants Two fluorocarbon mixtures were used i n plasma etching studies. The first consisted of 9 6 % C F and 4 % 0 while the second contained 50% helium, 4 9 % C F and 1%0 . 2
4
4
2
RESULTS AND DISCUSSION PBS/Substrate Etching in Fluorocarbon Plasmas The results obtained on etching P B S in fluorine-based plasmas vary quite considerably depending on the temperature of the substrate. Below 1 5 C , both the resist and substrate are attacked (etched), by the etchant species while at an intermediate temperature ( 1 5 - 2 3 C ) , the phenomenon of reversal etching is observed (8) in which the exposed substrate is not etched at all, while the resist (and subsequently the substrate underneath the resist) is readily etched by the plasma thus giving rise to a negative-tone pattern. A t higher temperatures, ( > 2 5 C ) , both the resist and substrate are etched by the plasma as i n the low-temperature case, except that etching completely stops after several seconds. These apparent contradictions can be rationalized in terms of a model which incorporates plasma-induced polymerization along with depolymerization. P B S has long been known to exhibit a marked temperature-dependent etch rate in a variety of plasmas. This is clearly seen in the previously published Arrhenius plots (3,7) for two different plasma conditions (Figure 1). This dependence is characteristic of an etch rate that is dominated by an activated material loss as would occur with polymer depolymerization. The latter also greatly accelerates the rate of material loss from the film. Bowmer et a l . (10-13) have shown in fact that poly(butene-l sulfone) is thermally unstable and degrades by a depolymerization pathway. A similar mechanism had been proposed by Bowden and Thompson (1) to explain dry-development (also called vapor-development) under electron-beam irradiation. The reactive species present i n a plasma possess sufficient energy to break the weak main chain carbon-sulfur bonds in P B S forming propagating radicals which, at the high temperatures encountered in the plasma environment, rapidly initiate depolymerization. The etchant species can also abstract hydrogen atoms from the hydrocarbon moiety (butene) i n the polymer chain facilitating formation of fluorinated species at the surface of the resist. Depolymerization i n this case thus results in sequential elimination of S 0 and fluorinated butene derivatives. Since the gaseous products are continuously removed in the system vacuum, the reaction should proceed in the direction of complete depolymerization resulting in total material loss. These decomposition reactions occur at a sufficiently high enough rate under ordinary conditions to render P B S ineffective as a plasma-etch mask, i.e., the resist is removed long before etching of the substrate is complete. This relatively straightforward process becomes more complicated at elevated temperatures where the rate of depolymerization is correspondingly higher. W e speculate that the evolved e
e
e
2
MANSFIELD
Enhancement
of Dry-Etch
Resistance
(°C) 60
40
30
20
15
10
r
1
1
1
1
Γ
Λ
a
GAS = 9 5 5
\
PRESSURE=.75 TORR POWER = 2 0 0 W IPC 2 0 0 5 W / T U N N E L
" \ \
3.0
\
3.1
CF : 0 4
2
\
3.2
3.3
3.4
10 /T 3
3.5
3.6
3.7
(K" ) 1
F i g u r e 1. PBS e t c h r a t e v e r s u s i n v e r s e t e m p e r a t u r e carbon plasma. Curve a , Ref. 3; curve b, Ref. 7.
in
fluoro
POLYMERS FOR HIGH T E C H N O L O G Y
320
species can participate in a plasma-induced polymerization reaction which competes with the etching reaction. W e further suggest that this competing reaction is initiated only when the concentration of reactive species (which includes species derived from the fluorinated etchant as well as fluorinated degradation products) reaches some threshold level. W e would expect such a threshold to depend on both the system configuration (which determines the residence time of the reactants in the plasma), and the rate at which the fluorinated species are generated by depolymerization. In our experimental setup, the pumping parameters and flow rate were fixed as were the pressure and power. This left substrate temperature as the only variable. Thus we would expect the rate of depolymerization to increase with increasing temperature, eventually reaching a point where the rate of gaseous evolution becomes sufficient to permit plasma polymerization to occur. Accordingly we would expect etching to stop because of the build-up of a deposited fluorinated layer on the surface of the resist and the substrate. In order to check the validity of this model, we analyzed the surface of both the resist and substrate by Auger spectroscopy. Figures 2 A and 3 A show Auger spectra of the resist and substrate, respectively, prior to plasma exposure. Surprisingly, only two of the elements common to the resist composition (C,S) are evident in Figure 2 A . The expected oxygen peak associated with the S 0 structure is absent. In fact, all spectra taken in the course of this work show the. absence or reduction of the expected oxygen peak at the resist surface (see Figures 2B, 6 A , 6 B , 6 C ) . Work is currently ongoing to explain this phenomenon as recent E S C A results indicate that the oxygen associated with the S0 structure is indeed present at the surface. We speculate that the absence of the peak in these spectra results from some complex interaction of the Auger beam with the resist surface. For this reason, extreme care must be taken in using these spectra for quantitative analysis, and it is pointed out that these spectra will only be used to indicate the existence of the carbon and sulfur components at the P B S surface in this work. Inversely, the spectrum of the substrate surface (Figure 3A) reflects the presence of oxygen, presumably due to the formation of the native oxide ( T a 0 ) at the surface. Samples which were ion-milled and analyzed while in the Auger chamber indicate that the oxide film is approximately 100A thick. The presence of the oxygen peak even in the etched samples (Figures 7B and 7C) is attributed to the fact that these samples were exposed to ambient air for more than an adequate amount of time between etching and Auger analysis to allow the oxide to regrow. 2
2
2
5
Samples were subjected to a fluorocarbon plasma for 30 seconds and 90 seconds, respectively, at temperatures spanning the three regions referred to above. Figures 2B and 2C show the changes in the Auger spectrum of the resist surface for plasma exposure (100W, 0.5 Torr) in the intermediate temperature range, i.e., at 1 6 C . The corresponding substrate spectra are shown in Figures 3B and 3C. These spectral changes are characteristic of all three temperature regimes, except that the changes occurred at different rates depending on the particular temperature studied. Thus, whereas it took 90sec for the sulfur peak in the spectrum of the resist to disappear at low and intermediate temperatures (leaving only the fluorinated hydrocarbon (see Figure 2C)), it disappeared after only a few seconds plasma exposure at high temperature ( > 2 5 C ) . A s noted earlier, the resist is etched during exposure at low and intermediate temperatures and is eventually removed entirely. O n the contrary, etching completely stops after a few seconds at high temperatures. W e attribute this to the fact that in the former case, depolymerization does not produce "monomer" at a sufficiently high enough rate to sustain polymerization at a rate greater than the plasma-induced etching rate of the resist. Thus although we see a build-up of fluorinated material at intermediate temperatures and below, its thickness and/or density is presumably not sufficient to prevent etching of the underlying resist. A t high temperatures, the fluorinated surface layer builds up at a considerably greater rate because of the additional contribution from high rate plasma-induced polymerization. The similarity of the spectra after prolonged etching at low temperature with that at high temperature is understandable i f we assume that S 0 does not take part in the plasma polymerization reaction. This would seem to be a reasonable assumption in view of the results of Brown and O'Donnell on the gas-phase copolymerization of butene-1 and S 0 in which they showed that the temperature for formation of P B S from the gas phase is around 0 ° C . Hence it e
e
2
2
MANSFIELD
Enhancement
of Dry-Etch
321
Resistance
ANALYSIS
PBS •^^^SUBSTRATE
0
200
400
600
800
π
1000
1200
1400
1600
1
1
1
1
1800 2000
1
-
b.
A
ANALYSIS ;;CF /He./0 -: 4
_1
0
200
400
600
800
1000
1200 1400
1600
L_
1800 2000
ANALYSIS
°0
200
400
600
800
1000
1200
1400
1600
1800 2000
KINETIC ENERGY F i g u r e 2 . A u g e r s p e c t r a o f PBS s u r f a c e p o s t (a) no p l a s m a e x p o s u r e ( b ) 3 0 s e c o n d a n d ( c ) 90 s e c o n d f l u o r o c a r b o n p l a s m a exposure.
2
POLYMERS FOR HIGH T E C H N O L O G Y
π
1
1
r-
a. ANALYSIS
O
200 400
600
_l I L_ 800 1000 1200 1400 1600 1800 2000
ANALYSIS
: ; · * : · ·ν·.:.· ;CF /He/0£: • ···. ·· · 0
4
PBS ^^^SUBSTRATE^^^
0
200
400
600
800
1000 1200 1400 1600 1800 2000
.V.HV.
ANALYSIS
;CF /He/0 :: 4
2
PBS
200
400
600
800 1000 1200 1400 1600 1800 2000
KINETIC ENERGY F i g u r e 3 . Auger s p e c t r a o f tantalum s u b s t r a t e surface (a) no p l a s m a e x p o s u r e (b) 30 second a n d ( c ) 90 second carbon plasma exposure.
post fluoro
27.
MANSFIELD
Enhancement
of Dry-Etch
323
Resistance
would not be possible for S 0 to participate in a plasma-initiated copolymerization reaction at the temperatures used in our studies. W e therefore suggest that the actual spectrum in Figure 2 C which was observed after 90sec at a temperature of 16 * C , reflects the elemental composition of the resist surface, whereas the similar spectrum observed at higher temperature (25 C ) reflects the composition of the polymerized, deposited material. Figures 4 A and 4 B are micrographs of the resist patterns after 30 and 90 seconds plasma treatment at 16 °C corresponding to the spectra in Figs 2 and 3. It will be recalled that this is the temperature at which the phenomenon of reversal etching is observed. The micrographs clearly show evidence of attack at the resist surface together with surface deposition. Moreover, since we did not observe etching of the exposed substrate, the deposited material must effectively cover and protect it from further attack. Auger analysis of this surface revealed only C , Ο and F (Fig 3B) so it is not surprising that such a material would protect the substrate from further etching. W e speculate that in this complex plasma environment deposition onto the substrate is favored over deposition onto the resist surface and that this difference in deposition rates accounts for the reversal etching mode. Thus, while the substrate does not etch, the resist continues to etch and will eventually result in a tone-inverted etched image. Given the complicated kinetics associated with the process, we were not able to obtain reproducible etch rates under our experimental conditions. Depolymerization introduces a degree of process complexity that precludes useful application of polymers such as P B S in dryetching environments. 2
e
PBS/SUBSTRATE E T C H I N G IN F L U O R O C A R B O N P L A S M A S W I T H PRIOR
OXYGEN
EXPOSURE P B S Etching in Oxygen Plasma Figure 5 shows a plot of etching rate of P B S in an oxygen plasma versus 1 0 0 0 / T ( K ) . The apparent activation energy obtained from a least squares plot of the data was 0.2 eV, which is a factor of two or three lower than that observed for etching in the fluorocarbon plasma discussed in the previous section. Analysis of the exposed substrate shows no evidence of film deposition (Fig. 7 A and 7B). The low activation energy energy for etching in oxygen would suggest that depolymerization is inhibited or retarded in the oxygen plasma. e
Subsequent Fluorocarbon Plasma Treatment Figure 8 shows a plot of thickness of P B S removed versus time in both the C F / 0 and C F / H e / 0 plasmas for samples priorly exposed to an oxygen plasma (100W, 0.5 Torr, 3 minutes 16°C). The etching curves in the fluorocarbon plasma are characterized by two distinct regions. Initially, the etch rate of P B S is quite high being comparable to that of samples not subjected to pretreatment in 0 plasma (cf. Figure 1). The etch rate then quickly diminishes to a low constant value of 12±2À/min (for C F / H e / 0 and 29±5À/min in C F / 0 . When the linear removal rate, obtained from a least-squares plot of the thickness removed versus plasma exposure time, is plotted as an Arrhenius expression at different temperatures (Figure 9), an activation energy of zero is obtained. It would therefore appear that pretreating P B S in an oxygen plasma not only substantially lowers the etching rate in the fluorocarbon-based plasmas, but also eliminates the temperature sensitivity of the process. In order to follow the changes in surface composition, P B S patterns on tantalum/gold/tantalum-coated wafers were subjected to a three-minute oxygen plasma exposure (100W, 0.5 Torr, 16 Ό followed by a 90 sec exposure to the fluorocarbon plasma under conditions identical to those used previously (100W, 0.5 Torr, 1 6 * C ) . The Auger spectra of the resist and substrate surfaces are shown in Figures 6 and 7, respectively. Figure 6 C shows that even after prolonged exposure to the fluorocarbon plasma, a significant sulfur peak still remains which is quite contrary to the case discussed earlier involving samples not pretreated in the 0 plasma, which showed the complete extinction of the sulfur peak at the surface (see 4
4
2
2
4
2
2
4
2
2
324
POLYMERS FOR HIGH T E C H N O L O G Y
F i g u r e 4. PBS f e a t u r e s on t a n t a l u m s u b s t r a t e p o s t and (b) 90 second f l u o r o c a r b o n plasma e x p o s u r e .
(a) 30
second
27.
MANSFIELD
F i g u r e 5. plasma.
Enhancement
PBS e t c h r a t e
of Dry-Etch
vs
Resistance
inverse
temperature
325
i n an
oxygen
POLYMERS FOR HIGH T E C H N O L O G Y
326
-ι
1
1
r a.
ANALYSIS
PBS
0
200
τ
400 600
1
1
800
1
1000 1200 1400 1600 1800 2000
1
1
1
1
r
b.
ANALYSIS Ο
0
^OXYGEN ; PBS SUBSTRATE > y////////////A
0
200
_J
400
I
600
l_
_ι
ι
I—
800 1000 1200 1400 1600 1800 2000
c.
ο • •
ANALYSIS
°* ο °
Λ
.:ΛΥ::>ν::%·. .••CF^/He/Og,:
ο • ο
••OXYGEN; PBS
WW/////WS////>s Ο
200 4 0 0
600
800 1000 1200 1400 1600 1800 2000
KINETIC ENERGY F i g u r e 6 . Auger s p e c t r a o f PBS s u r f a c e p o s t ( a ) n o p l a s m a exposure (b) 3 minute oxygen plasma exposure and (c) 3 minute oxygen plasma and 90 second f l u o r o c a r b o n plasma e x p o s u r e .
MANSFIELD
27.
Enhancement
τ
1
of Dry-Etch
Resistance
327
r
ANALYSIS
PBS ^SUBSTRATE %
0
200 400
τ
_i
600
ι
ι
ι
1
1
r-
— I
800 1000 1200 1400 1600 1800 2000
1—ι—ι
1
ANALYSIS • OXYGEN ; C
° ° +
ο
Ο
+0
PBS
Β.
_J
Ο
200
400 600
π
ν///, SUBSTRATE
POST OXYGEN DESCUM
I
L_
800 1000 1200 1400 1600 1800 2000
1
1
1
1
1
1
-
C
'
* ° OXYGEN; O
0
200
400 600
_Ι
I
ANALYSIS
;:CF /He/o -; 4
1_
800 1000 1200 1400 1600 1800 2000
KINETIC ENERGY F i g u r e 7 . Auger s p e c t r a o f t a n t a l u m s u b s t r a t e s u r f a c e post (a) n o p l a s m a e x p o s u r e (b) 3 m i n u t e oxygen p l a s m a e x p o s u r e a n d (c) 3 m i n u t e oxygen p l a s m a and 90 second f l u o r o c a r b o n p l a s m a exposure.
2
POLYMERS FOR HIGH T E C H N O L O G Y
F i g u r e 8. PBS t h i c k n e s s r e m o v e d ν s t i m e i n f l u o r o c a r b o n plasma post 3 minute oxygen plasma exposure.
27.
MANSFIELD
Enhancement
of Dry-Etch
329
Resistance
CO 60
10
3
10
2
40
GAS=96-4
CF :0 4
10 "T"
20
30
2
28.5A/MIN
Δ
