1082
Anal. Chem. 1981, 5 3 , 1082-1087
(17) Sugiyama, T.; Suzuki, Y.; Takeuchi, T. J . Chromatogr. 1973, 80, 61. (18) Seiber, J. N.; Woodrow. J. E. Arch. Environm. Contamin. T ~ x i c d . ioai, io, 133.
RECEIVED for review December 29,1980. Accepted March 27, 1981. This paper was presented at the 178th National Meeting
of the American Chemical Society, Paper No. 10 in the Division ofpesticide Chemistry,washington, DC, Sept 10,1979, The authors are grateful for support of this work by NIEHS Training Grant ES 07059, by the Western Regional Pesticide Impact Assessment Program, and by a grant-in-aid from Mobay Chemical Co.
Vacuum Ultraviolet Photochemistry in Thin Resist Films Paul W. Bohn and James W. Taylor" Department of Chemistty, Unlversity of Wisconsin- Madison, Madison, Wisconsin 53706
Henry Guckel Department of Electrical and Computer Engineering. University of Wisconsin- Madison, Madison, Wlsconsin 53706
A method to measure quantitative absorptlon spectra of thin film samples In the vacuum ultravlolet (VUV) is described. Results between 17.5 and 32.0 nm indicate poiy(methy1methacrylate) (PMMA) Is a stronger absorber than either poiy(butene-1 sulfone) (PBS) or Kodak 747 (K747) resists. Fourier transform Infrared (FTIR) difference spectrometry Is used to elucidate the VUV photochemical reaction pathways of these three resists. Mechanisms whereby vapor deveiopment of positlve resists may occur are considered, and the failure to obtaln 100% vapor development for PBS is explained.
Thin filmsof organic polymers have come under increasing scrutiny primarily because of their use as photoresist materials in integrated circuit fabrication. These materials are the medium into which the form of circuitry is written lithographically. Visible radiation has until recently been the source of choice for writing information into the resist. However, the demand for smaller line widths and greater device throughput capability has spurred research into new lithographic methods employing ions, electrons, or X-rays as the exposing radiation (1). X-ray lithography has been developed as a complementary technique to the electron beam approach to avoid some of its inherent difficulties (2). These include the high cost of an electron beam scanning system, the long range of backscattered electrons which can cause exposure to deviate from the desired sample geometry, and the requirement of sequential processing imposed by both vector and raster scanning methods. The use of X-rays to expose resists makes possible high throughput processing because once a mask is available, several wafers may be run at once. In addition, soft X-rays avoid the diffraction limited resolution inherent in visible lithographysand can produce circuit line widths on the same order as electron beam techniques while concurrentlyreducing backscattering problems (3, 4). Thus, X-ray lithography possesses the same potential for application to high density integrated circuit manufacture as the electron beam technology while having greater throughput possibilities. Assembly of an X-ray lithographic system requires an X-ray source, mask, and suitable photoresist. Choices of any of the three components depend on the nature of the other two and, in particular, on an understanding of their optical properties. For the photoresist this entails characterization of the ab0003-2700/81/0353-1082$01.25/0
sorption properties in the soft X-ray region as well as elucidation of the molecular high-energyphotochemical processes occurring upon irradiation. Some data are available for mask and substrate absorber materials in the spectral regions of interest (5, 6), but most polymer molecules have not been investigated below 100 nm (7).Because the first step leading to the ultimate exposure of the resist is absorption of a photon of ionizing radiation (81, the quantitative determination of the linear absorption coefficient in the appropriate spectral region is critical. Qualitative absorption spectra have been studied for three common X-ray resists (91, but quantitative data are needed to compare various resists at comparable wavelength intervals. In addition, although the specific chemistry has been studied in some cases (IO,11),details of the exposure process at short wavelengths are unknown for many photoresists. These details are especially important for positive photoresists such as poly(butene-1 sulfone) (PBS) which have the capability to self-develop (12-14), i.e., not require solvent extraction after exposure. Because the photochemical reactions are wavelength dependent, a continuum source of soft X-ray radiation is needed to explore the molecular consequences of radiation. For these studies an electron storage ring producing synchrotron radiation coupled to a grazing incidence monochromator provided spectrometric capabilities over the wavelength range of 17.5-80 nm. These experiments at relatively low photon energies provide the basis for use of synchrotron radiation to study high-energy photochemical processes and lend themselves naturally to extension to higher energies.
EXPERIMENTAL SECTION Photoresists. Poly(methy1methacrylate) was obtained as a secondary standard (M,= 60000, M,,=I 33 200) from the Aldrich Chemical Co., Milwaukee, WI and prepared for use as 2.56% (w/v) and 10.22% (w/v) solutions in filtered (1km) methyl isobutyl ketone. Poly(butene-1sulfone) was synthesized by the method of Brown and ODonnell (15) and prepared for use as a 4.74% (w/v) solution in filtered (1pm) 1:l methyl ethyl ketone:cyclohexanone. Kodak 747 Micro Negative Resist (60 cst) was a gift from the Eastman Kodak Co., Rochester, NY. It was prepared for use by dilution with ritered (1hm) xylene mixture to a solution of 2:1 (v:v) K747:xylene. All photoresists were stored in the dark and used under safelight illumination only. Thin Film Samples. Samples for absorption measurements were prepared by modifying the previously described process (9). Two 11-milwafers (Siltec Lot F-66354, n type (P), (loo), double polished, p 2-25 cm) of single-crystalsilicon were degreased, cleaned, and oxidized to a depth of 100 nm in an 02/HC1 ambient 0 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
at 950 OC. These wafers were then quartered and processed as follows. Negative photoresist was applied to both sides, and in one side a 3 X 3 array pattern was developed by ultraviolet lithography. In the open areas the SiOz was etched away by dipping in buffered 35% HF. Photoresist was then stripped from both sides. A preferential dicon etch was performed by immersing the wafers in a 1:1mole ratio mixture of hydrazine and water for an average time of 5 h at 100 "C (16, 17). After inspection by optical microscopy to ensure the window areas were clear and unbroken, the experimenital photoresist films were applied. This was accomplished by fixing the windowed substrate to a carrier wafer with Apiezon H grease. The carrier assembly was mounted on a vacuum chuck ancl the resists were spin coated on the windowed substrates with manual acceleration and deceleration of the spinning motor. The fiial result was a series of four samples consisting of nine 100 pm x 100 pm X 100 nm thick SiOzwindows covered with a coat of polymeric resist between 100 and lo00 nm thick (an electron micrograph of the resulting window array structure is shown in ref 9). One sample was not coated with resist in order to measure the portion of the absorption due to the SiOz. Resist film thickness measurements were made with a Dektak pressure-sensitive stylus film thickness monitor. At the levels employed in these films the thicknesses determined are accurate to f10% or better. A wafer with known step size openings in an oxide film was used to check calibration. Instrumentation. Synchrotron radiation from the Tantalus I storage ring located at the Synchrotron Radiation Center (SRC), Stoughton,WI was used for spectrometric measurements and soft X-ray exposures. Characteristics of the Wisconsin storage ring have been described previously (18).Stored beam currents during the course of these experiments ranged from 140 to 40 mA. Radiation was dispersed with the University of Southern California grazing incidencemonochromator which provided a photon flux of approximately 1 O l o photons s-l nm-' mA-' at the exit slit (19). A spectral band-paw of 0.4 nm was employed for spectrometric experiments. A 150 nm thick aluminum window (Luxel Corp., Friday Harbor, WA, Model TF 101 a) which transmits between 17.5 and 82.5 nm was inserted between the exit slit of the monochromator and the experimental chamber to preserve the vacuum integrity of the monochromator and prevent hydrocarbon contamination of the grating. The window also permitted wavelength calibration as the A1 L, edge (20)was visible in first and second orders. The experimental chamber provided a vacuum of lo-' torr for the absorption and exposure experiments. Samples were reproducibly positioned in the beam by means of a linear motion vacuum feedthrough to which three separate samples could be attached. Variationsin transmitted light intensity were monitored by detecting the fluorescence from a sodium salicylate window with an EM1 9524B photomultiplier. Analog data were recorded with a Houston 2000 X-'Y recorder and were processed on the chemistry department's Harris/7 computer. All spectra were corrected for decay of the photon flux and variable resist thickness. The absorption spectra were obtained by dividing the intensity vs. wavelength data for SiOzplus resist by the data for the SiOz window alone. Infrared Measurements. Three 11-mil wafers (Monsanto 1-1.5 Q cm) of 3892, p type (B), (loo), single polished, p single-crystal silicon were degreased, cleaned, and oxidized to a depth of 100 nm in an HzO ambient at 1050 "C. These were then scribed into 1/2-in.squares, and photoresist was applied by spin coating at 2000 rpm followed by an appropriate prebake (14,21, 22). Enough samples were made to allow separate exposure of each polymer for three different time intervals, 1 min, 10 min, and 30 min, under broad band and narrow band irradiation conditions. Narrow band irradiation was achieved by tuning the monochromator to 23 nm and setting the slits to obtain a spectral band-pass of 0.4 nm. Broad band irradiation conditions were obtained by setting the monochromator to transmit zero order. With the A1 filter in place this allowed radiation between 17.5 and 82.5 nm to reach the sample (23). The total exposures of samples run under the same pass-band conditions required correction for the varying beam current in the storage ring. Infrared spectra were obtained on a Digilab FTS-20 Fourier transform infrared spectrometer. Typically a week or more elapsed between irradiation and infrared measurement. A silicon
1083
PMMR
r--20.0
24.0
28.0
LAMBDA [ N M I
Figure 1.
Linear absorption coefficient (cm-') vs. wavelength for
PMMA.
reference spectrum was obtained by signal averaging at double precision the intensity output for loo00 scans of an uncoated wafer taken from the same lot as the polymer-coatedwafers. All subsequent spectra were also run in double precision and divided by the silicon reference spectrum to remove the silicon phonon bands and the channel spectrum in the region from 1500to 500 cm-l (24). All spectra were taken at a resolution of 1cm-' from 3500 to 500 cm-l. Unirradiated resists were run in two forms. Each resist was examined in the form of the Si/Si02 polymer sandwich already described and as a thin film on KBr plates. Irradiated resists were examined only in the form of Si/Si02/ polymer sandwiches. Difference spectra were calculated for each irradiated resist. These were obtained by subtracting the unirradiated polymer spectrum from the spectrum of the irradiated resist. Variations in the sample size and position made it necessary then to normalize the tabulated differences to the Si-0 stretching band near 1100 cm-l.
RESULTS AND DISCUSSION Sample Requirements for Absorption Measurements. The quantitative measurement of the linear absorption coefficient of organic solids in the VUV is severely constrained by the sample fabrication requirements. The basis of the present technique is the use of extremely thin free standing SiOzwindows upon which a thin f i of sample may be coated. Thus, a major requirement is that the thickness of the oxide be the same for the reference wafer and each sample wafer. This is met by fabricating all samples from wafers which were oxidized together in the same tube at the same time. In addition, the removal of silicon from the etch pit must be complete, and all windows must be unbroken. The former requirement is met by careful inspection by optical microscopy during the etch process. The latter is met by growing a more strain-free oxide and using a mask with fewer windows of smaller area (ca. 100 pm X 100 pm) than previously employed (9). In the W V measurements light reflected from the polymer surface is counted as absorbed light since reflectivity measurements were not made. However, reflectivity measurements previously made on PMMA and glassy carbon indicate 1%as an upper bound on the amount of light lost through this channel (25,26). Thus, if these data reflect the situation for carbon-based materials in general, then neglecting the reflectance at the polymer surface is justified. Spectral Results. The absorption spectra are shown in Figures 1-3 for PMMA, PBS, and Kodak 747 films. The spectra are similar in several respects. Each contains a region of singularity near 17.5 nm caused by rapidly changing absorption of the aluminum window a t the A1 LILIIIedge. In
1084
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
PBS
.d
.
4-
20.0
24.0
28.0
LAMBDA I N M I Figure 2.
Linear absorption coefficient (cm-’) vs. wavelength for PBS. K141
20.0
24.0
28.0
LAMBDR I N M I Figure 3.
Linear absorption Coefficient (cm-’)vs. wavelength for K747.
addition, some of the structured region observed previously (9) is obscured for all three resists although structure is still clearly present above 20 nm. In this respect these data more closely approach the expected slowly varying absorption based on studies of small organic molecules in this spectral region (27). The interesting differences concern the magnitude of the linear absorption coefficient, p, at a particular wavelength. Over the region studied, PMMA absorption is stronger by about a factor of 5 than PBS which is somewhat more absorbing than K747. All data are within the range of mass attenuation coefficients for other organic materials near 20 nm. These results have interesting implications for photochemical processes in resists. PMMA has long been known to be a resist with very good resolution and contrast properties but only moderate sensitivity. Lithographic sensitivity refers to the radiation dose, or intensity-time product, which must be administered to the resist to obtain a specified change in normalized film thickness after development. Much effort has been expended in designing sensitivity into a polymer with the other desirable features of PMMA. Thus, the results showing PMMA to be more strongly absorbing in the spectral region studied are surprising. The linear absorption coefficient, which was the quantity determined, measures light loss per unit path length regardless of film properties while the mass attenuation coefficient accounts for film density. A trivial explanation of the observed data would invoke a dif-
ference in film density since the linear absorption coefficient rather than the mass absorption coefficient was measured. It is difficult, however, to imagine a difference large enough to account for the factor of 5 difference in linear absorption. Another possible explanation lies in the possible degradation of the PBS film during measurement of the transmission spectrum. Although the possibility of material loss cannot be discarded, previous experiments have shown that the transmission of a particular thin film sample at several wavelengths does not change substantially as a function of time of irradiation (9). Examination of the exposure process in more minute detail reveals the nature of the discrepancy. When a thin film of poIymeric photoresist is irradiated with high energy, optical radiation characteristic chemical changes occur in the film to alter one or more of its macroscopic properties. The ease with which a certain amount of chemical change is produced is directly related to the lithographic sensitivity of that film, Thus, the efficiency of generating chemical change in the film is really the definition of lithographic sensitivity. However, generation of a scission or cross-linking site in a polymer film may be considered in terms of discrete events: absorption of impinging radiation; generation of secondary electrons; and reaction of the secondary electrons with other portions of the resist (8). The overall sensitivity of a resist is determined by the product of the efficiencies of each of the component steps. On the basis of the s u m of atomic mass absorption coefficient, PBS is expected to absorb more strongly than PMMA due to the greater mass absorption coefficient of sulfur when compared to carbon, oxygen, or hydrogen. However, in the spectral region studied, this was not found. An explanation consistent with the available data involves the efficiency of photoelectron and secondary electron generation as well as the efficiency of producing a differential developability. PBS may be a weaker absorber than PMMA but much more efficient at producing a difference in developing properties due to the greater ease of cleaving a C-S bond (-65 kcal/mol) relative to C-C bonds (-80 kcal/mol), C-0 bonds (-85 kcal/mol), or C-H bonds (-100 kcal/mol) (28). Alternatively, since all sensitivity tests have been done at shorter wavelengths, it could be that PMMA is a more sensitive resist a t the wavelengths employed. Experiments are currently under way to distinguish these two possibilities. Comparison of the two positive resists with K747 indicates that either should be more sensitive based on absorption alone. This is not surprising in light of the partially cyclized poly(cis-isoprene) polymer base of K747 (29). In particular, it contains no epoxidized groups, and several investigators have sought to improve the sensitivity of isoprene and butadiene type polymer molecules to electron beams by both cyclization and epoxidation (30-32). These absorption data point out one reason for the lower sensitivity of the unepoxidized polymer to ionizing radiation. FTIR Spectral Assignments and Interpretation. Assignment of some of the spectral bands for the resists are given in Tables 1-111. A typical difference spectrum is shown in Figure 4. IR difference spectrometry is a powerful tool in these investigations for several reasons. The polymer thickness and area of irradiation ensure that any spectral changes related to alteration of the structure at a molecular level are small changes. The ability to obtain and manipulate data from large numbers of scans for several samples enables the operator to observe small changes in rather large quantities. More importantly, because the difference spectra are obtained by subtracting the transmission of the unirradiated sample from that of the irradiated sample, the direction, or sign, of the deviation of a band from base line is meaningful in terms of the change in the relative number of bonds giving rise to the
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981 PMMR
Table I. PMMA Infrared Spectral Assignmentsa band location, cm-' 2995 2955 1730 1480 1440 1240 1190,1150 750
I
*
assignment
'
[Ipc3
'
'
_
1085
NE
'
1730
OOO 1440
A M
u,(O-CH,) + u,(CH,) u,(C)-CH,) + u,(a-CH,)' u(C.=O) 6,(tu-CH3) 6 (CH,),6 ,(O-CH,1 ua( C-C-0)coupled to u ( C-0)
+++ 1240 1150
+
1
skeletal stretching coupled to internal C-H deformation r(CH,)coupled to skeletal stretch
*
Reference 33. a = antisymmetric; s = symmetric; stretching mode; 6 = bending mode; 7 = rocking mode. a-CH, refers to the methyl group attached to the chiral carbon. a
u=
10' 2
a
1
va (SO, ~S(SO2)
u(C-S)
Table 111. Kodak 74'7 Infrared SpLctral Assignments" band band location, location, cm-' assignmentb cm-' assignmentb
a
u(N=N)
u(C=C)
Reference 34.
0.0
-12.0
..
1
Flgure 4. IR
1460 1370 1280 1115
,See Table I
2000
for abbreviations.
1400
3 15671
id
800
data show the differences starting large and positive indicating bond cleavages, then decreasing to smaller positive values at middle exposure, and increasing upon longer exposure. These data are surprising in terms of a simple model which predicts only increasing bond breakage with increasing exposure. However, a fuller interpretation is possible by considering these results with what is already known about the radiation chemistry of PMMA under electron bombardment (IO). As shown in eq 1-3, the main chain can cleave in three ways:
YH3
I
difference spectrum for PMMA.
absorption. Negative going bands indicate a relative increase in the number of absorbers in the irradiated film and are characteristic of negative working photoresists whereas positive going bands indicate a relative decrease in the number of absorbers in the irradiated sample and are characteristic of positive working photoresists. In each case the relative magnitude of the band indicates the extent of bond formation or scission. These normalized spectral difference data provide one means to monitor the photochemistry of the resist films. Photochemical Implications. Figure 5 shows the normalized spectral difference data for PMMA. The narrow band
r
-CH,C- I
I
bOzCH3
6,(CH,) 6,(CH,) 6(C-H),in plane u(C-CH,)
PMMA Dlfferencr
2600
2
Plot of normalized spectral differences for several bands in PMMA vs. time of exposure for narrow band irradiation.
ua(CHz) V,(CH,), Vs(CH2) 6 (CH,)
u,(CH,) u(-CH-)
3 15678910'
Figure 5.
See Table I for abbreviations.
2945 2860 2110 1600
2
TIME
Table 11. PBS Infrared Spectral Assignments band location, cm-I assignmenta 2940 2865 1450 1310 1125 620
3 4561a910°
-CH, -CH,CCH,
L
t CH$CO,CH,
+
CO,CH,
-CH2CCOzCH3 t CH,
severing the C-C main chain bond; cleaving the ester side chain linkage; and breaking the a-CH3 side chain. Current evidence indicates that the least likely of these primary processes is a-CH3 cleavage. The band at 1480 cm-l due to an antisymmetric bending motion of the pendant methyl group (Table I) shows no change in the difference spectrum under narrow band irradiation conditions. Only at higher exposure levels under broad band conditions is any difference observed. The 1730-cm-' band may be associated with the ester group. Monitoring this frequency should probe the loss of C02CH3or the sequence of CH30 loss followed by GO. Any decrease in the carbonyl group will show as a positive difference at 1730 cm-l. The bands at 1440 and 1240 cm-l are also indicative of the ester group, but since they are weaker, they are less useful. At 1and 10 min of exposure, the largest difference is at 1150 cm-' suggesting main chain scission processes predominate at first with only some ester group cleavage. Any stretch at 1050 cm-' indicative of a GO linkage is completely obscured by the 1085-cm-l SiOzstretch so that band cannot be used to monitor loss of the ester linkage. At 30 min of exposure the situation has changed and the 1730cm-l band shows the largest difference. This implies that at longer exposures the ester group cleavage process is very competitive with main chain scission. These competing processes may be interpreted in terms of kinetic and thermodynamic control of reaction pathways. In any positive resist it is reasonable to expect the first products to be those from the cleavages which are least costly energetically. The present experimentsshow that pathway to be main chain C-C scission. However, if dynamic equilibrium is approached, the final products must be those which allow the system to assume its lowest energy. Because the evolution of GO is highly favorable energetically and the ester group cleavage process is the path
1088
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
PBS A
LIIIpl 1450
OOo 1310
AAA 620
13
0 A
10'' 2
3 4 5678910'
2
3 4 5678910'
2
3 4 5576Jld
TIME
Flgure 6. Plot of normalized spectral differences for several bands
in PBS vs. time of exposure for narrow band irradiation.
which leads to this product, scission of the ester C-C bond is favored. The mechanism by which these two primary processes interact is the various chain recombination reactions which occur in competition with the primary and secondary cleavage processes. Once a reactive site has been generated from a primary cleavage process, several competing paths are available for further reaction. The chain may undergo further degradation or cross-linking and recombination or rearrangement processes may occur to yield a product closer in structure to the unirradiated resist. These latter processes would result in a decrease in the observed difference at the appropriate energy. Thus, at intermediate dose levels chain scission events are dominated by recombination reactions to yield a decrease in the observed spectral differences. Because of the time interval between irradiation and the infrared measurements and because the reactive species are expected to relax very quickly, these data represent only a snapshot picture of the product distribution at any particular photon dose. The overall picture then is one in which scission events are dominant a t low photon dose, recombination processes control at intermediate, and the two compete in an approach to dynamic equilibrium at the higher level. Implicit in this explanation is the establishment of a quasi-equilibriumin the thin resist film. Considering the nonmobile nature of the polymer molecules in the resist film, this result is indeed unexpected. The broad band irradiation results point to increasing scission of every bond with increasing exposure. As expected, the ester group cleavage predominates over the main chain primary pathway. At longer exposures there is even evidence of some a-CH3 side group loss. Results for PBS are shown in Figure 6. The useful bands are at 2940,2865,1450,1310, and 620 cm-l and are assigned as in Table 11. Unfortunately the symmetric stretching mode of SO2at 1125 cm-l is hidden under the Si-0 stretching band. The narrow band irradiation data show the same behavior as that of PMMA with the exception of the 620-cm-' C-S stretching band. Again as shown in eq 4-6, the chemistry may
f
+
-CH,CHC,H,
-CHzCHSOz-
I
'ZH5
-CH,
+
-CH,CHSO,-
(4)
SO-,
CzH5CHSOz-
+
C,H5
(5)
the C-S is expected to be the easiest to disrupt. The 620-cm-' difference band, which is assigned to a C-S stretch, increases monotonically with increasing exposure a t narrow band irradiation conditions. This indicates that increasing exposure simply breaks more C-S bonds. However, as more C-S linkages are disrupted, cross-linking reactions also increase as a competitive route to SO2evolution and subsequent vapor development. Eventually there exists a relative depletion of C-S bonds to break and C-C main chain scission becomes an important competing process both in the remaining uninterrupted polymer seqments and in the newly formed crosslinked segments. Hydrocarbon evolution may occur in the latter case. Secondary pathways explain the increase in the magnitude of the difference bands at 1310 and 1450 cm-l at longer exposures. The band at 1310 cm-' is assigned to the antisymmetric SO2 stretch. The increase in this band may be attributed to the fact that both C-S and C-C main chain cleavage eventually lead to loss of SOz while the competing cross-linking reaction tends to decrease the amount of SO2 evolution. The CH2 bending vibration at 1450 cm-' is affected analogously. Cleaving C-S linkages or C-C bonds leads to a decrease in the number of CH2 moieties available to absorb and to an increase in that band in the difference spectrum, Cross-linking, rearrangement, and recombination processes tend to produce the CHz linkage at longer exposures. Thus, the same arguments made for PMMA irradiated at an intermediate dose are applicable to PBS, as might be expected, since they are both positive working resists. The broad band data show decreasing difference values with increasing exposure. This is probably due to increasing importance of cross-linking and recombination processes at longer exposure times. It also points to the tendency of PBS to become a negative resist at high exposures as has been observed for PMMA (35). Thus, the chosen broad band exposure conditions appear to have produced overexposure. An overall picture of the photochemistry in PBS films can be drawn. The initial phase of irradiation is dominated by C-S bond breaking. Soon after irradiation begins, recombination and cross-linking reactions start to compete. Again, as for PMMA, the quasi-equilibrium situation is established in the film. As the number of C-S bonds available becomes relatively depleted, C-C bonds are broken in primary processes. At extremely high irradiation levels the recombination and cross-linking processes dominate leading to an overexposed resist film. The resulting highly cross-linked polymer has lost its capability to degrade into SO2 and olefin. Thus these latter reactions provide a portion of the explanation for the reason PBS has not been found to vapor develop completely despite the facile loss of SO2. The normalized spectral differences of several bands in the spectrum of narrow band irradiated K747 are shown in Figure 7. The significant IR bands are those at 2945, 2860, 2330, 2110,1460, and 1370 cm-l (Table 111). The bands at 2330 and 2110 cm-l are probably due to the diazide sensitizing moiety added for use as an ultraviolet photoresist (29). Of the other bands only the 1115-cm-' band is not related to a C-H type motion. Unfortunately this band is hidden by the large u(Si-0) of the substrate oxide layer. Each of the other bands becomes increasingly more negative with increasing exposure. This is expected as the result of new bond formation. For negative photoresists there should be a relative increase in the number of moieties absorbing with increasing exposure. The proposed chemistry is shown in eq 7. The polymer
(6)
be interpreted in terms of the major primary reaction pathways: C-S band cleavage; C-C main chain scission; and severing of the ethyl side group. From energetic considerations,
backbone of the resist as it is applied to the substrate is a
ANALYTICAL CHEMISTRY, VOL.
work remains to realize the goal of a self-developing X-ray resist system.
K747
LITERATURE CITED
t33[3 2940
OOO 2330 A A A 2110
Deckert, C. A.; Ross, D. L. J. Electrochem. Soc. 1980, 127, 45C56C. Smith, H. I.; Spears, D. L.; Bernacki, S.E. J. Vac. Sci. Tech. 1973, 70, 913-917. Feder, R.; Spiller, E.; Topalian, J. J. Vac. Scl. Technol. 1975, 12, 1332- 1335. Maydan, D.; Coquln, 0. A.; Maldonado, J. R.; Somekh, S.;Lou, D. Y.; Taylor, G. N. IEEE Trans Electron Devlces 1975, 22, 429-433. Hagemann, H.J.; Gudat, W.; Kunz, C. J. Opt. SOC. Am. 1975, 65, 742-744. 301-306. R.; Kunz, C.; Sasaki, R.; Sonntag, B. Appl. Opt. 1968, 7 , Haensel,
+++ 1460
0 A +
A
[3
? [3
L io1 z
3
rss7aslo' z
53, NO. 7, JUNE 1981 1087
3
A
rso?as]o' z
3 4
s67asld
TIME
Figure 7. Plot of normalized spectral differences for several bands
in K747 vs. time of exposure for narrow band irradiation.
partially cyclized poly(&-isoprene) network. As the macromolecule is irradiated, the C-C double bonds are partially disrupted resulting in a group with less steric rigidity and high reactivity. These thein may react with neighboring groups to yield a cyclized cross-link in the two chains. Because this type of cross-linking would affect all the C-H bands, it would produce the type of dlifference data observed. Further conclusions about the photochemistry of this resist are precluded because of the proprietary nature of the resist formulation. Implications for X-ray Lithography. The results of these studies have important ramifications for practical lithographic systems. In particular they are pertinent to vapor development of resish Previously,it has been suggested that PBS should completely vapor develop (14).However, 100% development has never been observed experimentally. There are two salient points in this regard. Our results show that SO2 evolution is not a primary process. Several successive reaction steps must occur before the SO2 molecule is evolved. At each of these reaction steps the possibility of a competing cross-linking, rearrangement, or recombination reaction exists. In addition, the results indicate that the local environments of reactive groups are sufficient for the establishment of a quasi-equilibrium. This means the polymer backbones do not have to reorient substantially themselves in order for some of the competing reactions to take place. Both of these effects bear directly on the ability of PBS to vapor develop. The results also suggest that thickness of the resist film is an important parameter. In a resist film the ionizing radiation must reach the substrate interface for complete exposure of the resist. The absorption method developed provides a powerful tool in studying this constraint. Any gaseous species evolved at some depth in the film must percolate through the resist in order to escape and be pumped away. During this process the possibility of reaction with highly reactive but relatively immobile plolymer fragments exists. Thus, much
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RECEIVED for review November 10, 1980. Accepted March 2, 1981. P.W.B. was supported by an NSF Fellowship 1977-1980 and by an American Chemical Society Analytical Division Fellowship 1980-1981 courtesy of the Perkin-Elmer Corp. This work was also supported by the Wisconsin Alumni Research Foundation and the National Science Foundation through Grant No. DMR-7924555. The Synchrotron Radiation Center is supported by NSF Grant No. DMR-772188.
