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Identification of sensitive positive and negative working resist materials for proximity X-ray lithography
MICROELECTRONIC ENGINEERING
ELSEVIER
Microelectronic Engineering 27 (1995) 389-392
Identification of Sensitive Positive and Negative Working Resist Materials for Proximity X-Ray Lithography A.E. Novembrea, J.M. Kometania, C.S. Knurek~, U. Kumar~, T.X. Neenana, D.A. Mixona, O. Nalamasu ~ and N. Miinzelb aAT&T Bell Laboratories, 600 Mountain Ave, Murray Hill, NJ 07974 bOCG Microelectronic Materials AG, Klybeckstrasse 141, CH-4002, Basel, Switzerland Proximity x-ray lithography represents one of the emerging technologies currently under investigation for advanced circuit design fabrication. A pulsed laser point source proximity print stepper is currently being evaluated for the feasibility of fabricating working devices having design rules in the range of 0.12-0.18p.m. Both conventional and chemically amplified (CA) positive and negative working resists have been evaluated on the stepper and exhibit sensitivities in the range of 15-60mJ/cm 2. Multi- and single component positive CA resists represent the leading candidates for use in device fabrication. Negative resists provide high sensitivity but have more limited resolution capability vs. positive working materials. 1. INTRODUCTION Various classes of positive and negative working resist systems have been investigated for use on a pulsed laser point source proximity print stepper [ 1]. The exposure wavelength is centered at 1.4nm and provides an improvement in the resist absorption characteristics over that which is found using synchrotron x-radiation centered at -1.0nm. Exposures at the longer wavelength provide the opportunity to investigate a larger variety of resists capable of meeting the process dose requirement of 25mJ/cm.2 Concurrent with the dose requirement is that the resist must also exhibit resolution in the range of 0.12-0.181am. The resist image aspect ratio must be >_2 and linewidth uniformity must be < + 10% of the minimum mask size feature. For positive resists, two types of chemically amplified materials have been investigated. The first makes use of resists in which the absorbed radiation initiates a chemical transformation and the effects during a subsequent post-exposure bake (PEB) step are amplified [2]. In the second approach a polymeric dissolution inhibitor comprised of an olefm sulfone copolymer is blended with a novolac to produce a high resolution resist [3]. Specific examples of type 1 resists are single component CA resists based on Elsevier Science B.V. SSDI 0167-9317C94)00130-8
protected hydroxystyrene sulfone copolymers, and the multi-component resist ARCH which has been initially identified as a high performance deep UV sensitive material [4]. For type 2 materials the resist referred to as NPR has been initially evaluated as an electron beam sensitive material and its use in x-ray lithography will be highlighted. Negative chemically amplified x-ray resists exhibit high sensitivity as exemplified by the results obtained with OCG LMB-7011 material. The conventional crosslinking resist poly(glycidyl methacrylate-co-3-chlorostyrene) (GMC) was also investigated and the results are compared to those obtained with CA materials. 2. EXPERIMENTAL Resists comprised of sulfur dioxide and a protected hydroxystyrene were prepared by free radical solution polymerization methods [5]. NPR resist was prepared by blending a novolac resin obtained from Borden Chemical with an alternating copolymer of 2-methyl-l-pentene and sulfur dioxide (PMPS). The proportion of PMPS in NPR ranged from 5-15 wt.% based on the novolac weight. The nonchemically amplified negative acting resist GMC was prepared at elevated
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A.E. Novembre et al. / Microelectronic Engineering 27 (1995) 389-392
temperatures (>85 °C) by radical initiated solution polymerization techniques. Resist solutions were prepared either in ethyl-3-ethoxy propionate or propylene glycol monomethylether acetate. The solutions were filtered through a Millipore 0.20 ~tm pore size Teflon filter. For lithographic analysis 0.500.701am thick resist films were spin coated onto HMDS primed 5" silicon substrates. The resist films were hot plate or box oven baked immediately after coating at 80-130 °C for 1-60 min. The coating and hot plate baking were performed on an environmentally enclosed MTI Flexifab wafer track system. X-ray (~,=0.90-1.8nm centered @ 1.4nm) exposures in Helium were performed using a pulsed laser point source proximity print stepper. The laser pulse rate was set at 1.0 Hz and the energy flux at the wafer plane was 0.70~ 1.0mJ/cm2/pulse. X-ray masks consisting of either a 0.40~tm Gold or 0.30~tm Tungsten absorber layer on a polysilicon or epi-silicon membrane were used for patterning purposes. The mask to wafer gapping distance was 30~tm and the exposure field size was 15mm x 15ram. All chemically amplified positive and negative resists were PEB at times after exposure ranging from 1-30 min. In certain cases NPR exposed wafers were vacuum oven baked at 70-90 °C. ARCH and LMB 7011 resists were immersion developed in 0.13-0.17N aqueous tertramethylammonium hydroxide (TMAH) solution and rinsed in deionized water. NPR and GMC resists were spray/spin developed in an APT model 915 resist processor. The NPR developer was 0.20N TMAH and the GMC developer was nbutyl acetate. Mask and patterned wafer metrology was performed on an Hitachi S-6000 SEM. Resist resolution performance was evaluated using an Hitachi 2400 SEM. 3. RESULTS AND DISCUSSION 3.1 Single Component Positive CA Resists Resists containing a protected hydroxystyrene and sulfur dioxide monomers were
shown to act as sensitive positive working CA resists. In-situ acid generation occurs via exposure induced scission of the main chain carbon-sulfur bond. At the scission site a sulfmic acid end group of the fragmented polymer chain is produced and which catalytically removes the t-butoxycarbonyl (t-BOC) protecting group from the copolymer during the subsequent PEB step. This resist exhibited 25mJ/cm2 sensitivity, but showed a 35% film loss in the exposed film regions after the PEB step. Reduction in film loss was achieved by three different approaches: lowering the amount of hydroxystyrene units protected in the polymer, terpolymerization with a radiation insensitive acetoxystyrene monomer and in place of the t-BOC group the use of a tetrahydropyranyl protecting group. Table 1 summarizes the sensitivity and % film loss after PEB at the minimum clearing dose for the various co- and terpolymers investigated. Table 1: X-ray exposure response of single component CA resists Resist
PTBSS PHSTBSS PASTBSS PHSTHPSS
Sensitivity (mJ/cm2) 20 17 27 14
% Film Loss After PEB 35 11 12 <2
All resists listed in Table 1 have an overall aromatic to sulfur dioxide monomer ratio of 2:1. PTBSS, a copolymer of t-BOC styrene and sulfur dioxide represented the initial material version and had the greatest film loss after PEB. Removal of 30% of the t-BOC groups in PTBSS to produce the hydroxystryene containing terpolymer PHSTBSS improved the sensitivity and reduced the film loss to 11%. This resist however exhibited marginal thermal stability and process latitude at the PEB conditions used to produce the results listed in Table 1. Replacement of TBS monomer with acetoxystyrene to yield the terpolymer PASTBSS did not provide a resist having a sensitivity of <25 mJ/cm2. High sensitivity and
A.E. Novembre et al. /Microelectronic Engineering 27 (1995) 389-392
391
minimal film loss was achieved with terpolymers of hydroxystyrene, tetrahydropyranyloxystyrene and SO2 (PHSTHPSS). 0.30pm lines and spaces were imaged at a dose of 16 mJ/cm2 and some evidence of line-edge roughness is apparent in Figure 1. Work is ongoing to improve the overall image quality. Figure 2. SEM micrograph of 0.18pm lines and spaces in ARCH resist. ,'!i
Figure 1. SEM micrograph of 0.30 pm lines and spaces in PHSTHPSS resist 3.2 Multi-component positive CA resist evaluation Initial characterization of the x-ray lithographic response of ARCH resist containing 3% acid generator and at processing conditions of 0.70~tm starting film thickness and 100 °C PEB temperature, resulted in a clearing dose of 60mJ/cm 2. The resist exhibited sub-0.20pm resolution and vertical profiles. Increasing the PEB temperature to 125 °C reduced the minimum exposure dose to 36mJ/cm 2 while maintaining equivalent resolution. In order to meet the required 25mJ/cm 2 exposure dose threshold the ARCH formulation was modified to contain a 6% acid generator loading. Additionally the initial film thickness was reduced to 0.50pm and the PEB temperature was maintained at 125 °C. For this formulation and process, the dose required to print 0.18pm line and space patterns was 22mJ/cm 2. Figure 2 is an SEM micrograph of the sub 0.20pm features.
A time delay study was conducted in which a series of exposed wafers were allowed to stand in the clean room ambient for a period of 1 to 30 min. Prior to this delay time, the exposed wafers were also maintained in the helium environment of the x-ray stepper. The delay time in Helium depended on the total number of fields/wafer which were exposed and for ARCH samples the upper limit was 25 min. The total delay time was therefore in excess of 50 min. Figure 3 is an SEM of 0.30~tm lines and spaces produced with the 50 min delay. A very minimal change in the image profile is observed and represents an acceptable time limit with regards to process latitude considerations.
Figure 3. SEM of 0.30pm lines and spaces in ARCH resist after a 50 min time delay. 3.3 NPR lithographic characteristics Chemical amplification is achieved in this resist as a consequence of the use of the polymeric dissolution inhibitor PMPS. PMPS undergoes spontaneous depolymerization upon exposure to xradiation. The exposed regions are therefore freed of the inhibitor and rendered soluble in an aqueous base developer. No PEB processing is required and there is no sensitivity to the presence of basic airbome contaminants. For the formulation consisting of 10% of a low molecular weight PMPS in a novolac, a minimum clearing dose of
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A.E. Novembre et al. / Microelectronic Engineering 27 (1995) 389-392
40mJ/cm 2 was measured for a starting film thickness of 0.65~tm. An interruptive spray development process was found to improve the contrast and image quality of the resist. Figure 4 depicts 0.25~tm line and space patterns formed in NPR.
m
4. CONCLUSIONS Positive CA resists based on acid catalysis represent the materials closest to meeting the sensitivity and resolution requirements for use in proximity x-ray lithography. Critical dimension control however needs to be further improved upon. The resist NPR offers solutions to problems associated with using a radiation generated acid for imaging, but improvements in sensitivity must first be made. Negative resists provide high sensitivity and improvements in the resolution capability is essential before use of these materials can be realized.
Figure 4. SEM of 0.25 ~tm features in NPR resist. 5. ACKNOWLEDGMENTS Further improvements in NPR sensitivity occurs at the expense of resolution. To produce a more sensitive formulation, higher molecular weight PMPS is required. This however reduces the compatibility between the novolac and PMPS and impairs the pattern resolution. Efforts to improve the phase compatibility between the novolac and PMPS is in progress. 3.4 Negative Resists The multi-component chemically amplified resist OCG LMB 7011 was determined to exhibit sensitivity of <15mJ/cm2 and 0.30~tm resolution in 0.70~tm thick fills. Process latitude with respect to PEB temperature and developer normality has been found be marginal. Lithographic characterization of the conventional crosslinking resist GMC was performed on the version having a weight average molecular weight of 1.3x105 g/mol and comprised of 63 mol% glycidyl methacrylate. Using a 0.50~tm initial f i l l thickness, a process dose of 32mJ/cm2 was required for imaging of 0.35-0.401am size features. Higher resolution is achieved in this resist at the expense of higher operating dose. Future work in the area of negative resists will concentrate on investigating only chemical amplification systems.
The authors would like to thank all the members of the x-ray lithography group at AT&T Bell Labs. This work was partially supported by ARPA DALP through a NAVAIR Contract No. N0019-92-C-0064. 6. REFERENCES 1. J. Frackoviak, et. al., Proc. SPIE,1924 (1993) 258 2. C.G. Willson, J.M.J. Frechet, Y. Tessier and F.M. Houlihan, J. Electrochem. Soc., 133, (1986) 181 3. M.J. Bowden, L.F. Thompson, S.R. Farenholtz and E.M. Doerries ibid, 128, (1981) 1304 4. N. Mtinzel et. al., Proc. SPIE, 2195 (1994) 47 5. R.S. Kanga et. al. Chem. Mater., 4, (1992) 278
ELSEVIER
Microelectronic Engineering 27 (1995) 389-392
Identification of Sensitive Positive and Negative Working Resist Materials for Proximity X-Ray Lithography A.E. Novembrea, J.M. Kometania, C.S. Knurek~, U. Kumar~, T.X. Neenana, D.A. Mixona, O. Nalamasu ~ and N. Miinzelb aAT&T Bell Laboratories, 600 Mountain Ave, Murray Hill, NJ 07974 bOCG Microelectronic Materials AG, Klybeckstrasse 141, CH-4002, Basel, Switzerland Proximity x-ray lithography represents one of the emerging technologies currently under investigation for advanced circuit design fabrication. A pulsed laser point source proximity print stepper is currently being evaluated for the feasibility of fabricating working devices having design rules in the range of 0.12-0.18p.m. Both conventional and chemically amplified (CA) positive and negative working resists have been evaluated on the stepper and exhibit sensitivities in the range of 15-60mJ/cm 2. Multi- and single component positive CA resists represent the leading candidates for use in device fabrication. Negative resists provide high sensitivity but have more limited resolution capability vs. positive working materials. 1. INTRODUCTION Various classes of positive and negative working resist systems have been investigated for use on a pulsed laser point source proximity print stepper [ 1]. The exposure wavelength is centered at 1.4nm and provides an improvement in the resist absorption characteristics over that which is found using synchrotron x-radiation centered at -1.0nm. Exposures at the longer wavelength provide the opportunity to investigate a larger variety of resists capable of meeting the process dose requirement of 25mJ/cm.2 Concurrent with the dose requirement is that the resist must also exhibit resolution in the range of 0.12-0.181am. The resist image aspect ratio must be >_2 and linewidth uniformity must be < + 10% of the minimum mask size feature. For positive resists, two types of chemically amplified materials have been investigated. The first makes use of resists in which the absorbed radiation initiates a chemical transformation and the effects during a subsequent post-exposure bake (PEB) step are amplified [2]. In the second approach a polymeric dissolution inhibitor comprised of an olefm sulfone copolymer is blended with a novolac to produce a high resolution resist [3]. Specific examples of type 1 resists are single component CA resists based on Elsevier Science B.V. SSDI 0167-9317C94)00130-8
protected hydroxystyrene sulfone copolymers, and the multi-component resist ARCH which has been initially identified as a high performance deep UV sensitive material [4]. For type 2 materials the resist referred to as NPR has been initially evaluated as an electron beam sensitive material and its use in x-ray lithography will be highlighted. Negative chemically amplified x-ray resists exhibit high sensitivity as exemplified by the results obtained with OCG LMB-7011 material. The conventional crosslinking resist poly(glycidyl methacrylate-co-3-chlorostyrene) (GMC) was also investigated and the results are compared to those obtained with CA materials. 2. EXPERIMENTAL Resists comprised of sulfur dioxide and a protected hydroxystyrene were prepared by free radical solution polymerization methods [5]. NPR resist was prepared by blending a novolac resin obtained from Borden Chemical with an alternating copolymer of 2-methyl-l-pentene and sulfur dioxide (PMPS). The proportion of PMPS in NPR ranged from 5-15 wt.% based on the novolac weight. The nonchemically amplified negative acting resist GMC was prepared at elevated
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A.E. Novembre et al. / Microelectronic Engineering 27 (1995) 389-392
temperatures (>85 °C) by radical initiated solution polymerization techniques. Resist solutions were prepared either in ethyl-3-ethoxy propionate or propylene glycol monomethylether acetate. The solutions were filtered through a Millipore 0.20 ~tm pore size Teflon filter. For lithographic analysis 0.500.701am thick resist films were spin coated onto HMDS primed 5" silicon substrates. The resist films were hot plate or box oven baked immediately after coating at 80-130 °C for 1-60 min. The coating and hot plate baking were performed on an environmentally enclosed MTI Flexifab wafer track system. X-ray (~,=0.90-1.8nm centered @ 1.4nm) exposures in Helium were performed using a pulsed laser point source proximity print stepper. The laser pulse rate was set at 1.0 Hz and the energy flux at the wafer plane was 0.70~ 1.0mJ/cm2/pulse. X-ray masks consisting of either a 0.40~tm Gold or 0.30~tm Tungsten absorber layer on a polysilicon or epi-silicon membrane were used for patterning purposes. The mask to wafer gapping distance was 30~tm and the exposure field size was 15mm x 15ram. All chemically amplified positive and negative resists were PEB at times after exposure ranging from 1-30 min. In certain cases NPR exposed wafers were vacuum oven baked at 70-90 °C. ARCH and LMB 7011 resists were immersion developed in 0.13-0.17N aqueous tertramethylammonium hydroxide (TMAH) solution and rinsed in deionized water. NPR and GMC resists were spray/spin developed in an APT model 915 resist processor. The NPR developer was 0.20N TMAH and the GMC developer was nbutyl acetate. Mask and patterned wafer metrology was performed on an Hitachi S-6000 SEM. Resist resolution performance was evaluated using an Hitachi 2400 SEM. 3. RESULTS AND DISCUSSION 3.1 Single Component Positive CA Resists Resists containing a protected hydroxystyrene and sulfur dioxide monomers were
shown to act as sensitive positive working CA resists. In-situ acid generation occurs via exposure induced scission of the main chain carbon-sulfur bond. At the scission site a sulfmic acid end group of the fragmented polymer chain is produced and which catalytically removes the t-butoxycarbonyl (t-BOC) protecting group from the copolymer during the subsequent PEB step. This resist exhibited 25mJ/cm2 sensitivity, but showed a 35% film loss in the exposed film regions after the PEB step. Reduction in film loss was achieved by three different approaches: lowering the amount of hydroxystyrene units protected in the polymer, terpolymerization with a radiation insensitive acetoxystyrene monomer and in place of the t-BOC group the use of a tetrahydropyranyl protecting group. Table 1 summarizes the sensitivity and % film loss after PEB at the minimum clearing dose for the various co- and terpolymers investigated. Table 1: X-ray exposure response of single component CA resists Resist
PTBSS PHSTBSS PASTBSS PHSTHPSS
Sensitivity (mJ/cm2) 20 17 27 14
% Film Loss After PEB 35 11 12 <2
All resists listed in Table 1 have an overall aromatic to sulfur dioxide monomer ratio of 2:1. PTBSS, a copolymer of t-BOC styrene and sulfur dioxide represented the initial material version and had the greatest film loss after PEB. Removal of 30% of the t-BOC groups in PTBSS to produce the hydroxystryene containing terpolymer PHSTBSS improved the sensitivity and reduced the film loss to 11%. This resist however exhibited marginal thermal stability and process latitude at the PEB conditions used to produce the results listed in Table 1. Replacement of TBS monomer with acetoxystyrene to yield the terpolymer PASTBSS did not provide a resist having a sensitivity of <25 mJ/cm2. High sensitivity and
A.E. Novembre et al. /Microelectronic Engineering 27 (1995) 389-392
391
minimal film loss was achieved with terpolymers of hydroxystyrene, tetrahydropyranyloxystyrene and SO2 (PHSTHPSS). 0.30pm lines and spaces were imaged at a dose of 16 mJ/cm2 and some evidence of line-edge roughness is apparent in Figure 1. Work is ongoing to improve the overall image quality. Figure 2. SEM micrograph of 0.18pm lines and spaces in ARCH resist. ,'!i
Figure 1. SEM micrograph of 0.30 pm lines and spaces in PHSTHPSS resist 3.2 Multi-component positive CA resist evaluation Initial characterization of the x-ray lithographic response of ARCH resist containing 3% acid generator and at processing conditions of 0.70~tm starting film thickness and 100 °C PEB temperature, resulted in a clearing dose of 60mJ/cm 2. The resist exhibited sub-0.20pm resolution and vertical profiles. Increasing the PEB temperature to 125 °C reduced the minimum exposure dose to 36mJ/cm 2 while maintaining equivalent resolution. In order to meet the required 25mJ/cm 2 exposure dose threshold the ARCH formulation was modified to contain a 6% acid generator loading. Additionally the initial film thickness was reduced to 0.50pm and the PEB temperature was maintained at 125 °C. For this formulation and process, the dose required to print 0.18pm line and space patterns was 22mJ/cm 2. Figure 2 is an SEM micrograph of the sub 0.20pm features.
A time delay study was conducted in which a series of exposed wafers were allowed to stand in the clean room ambient for a period of 1 to 30 min. Prior to this delay time, the exposed wafers were also maintained in the helium environment of the x-ray stepper. The delay time in Helium depended on the total number of fields/wafer which were exposed and for ARCH samples the upper limit was 25 min. The total delay time was therefore in excess of 50 min. Figure 3 is an SEM of 0.30~tm lines and spaces produced with the 50 min delay. A very minimal change in the image profile is observed and represents an acceptable time limit with regards to process latitude considerations.
Figure 3. SEM of 0.30pm lines and spaces in ARCH resist after a 50 min time delay. 3.3 NPR lithographic characteristics Chemical amplification is achieved in this resist as a consequence of the use of the polymeric dissolution inhibitor PMPS. PMPS undergoes spontaneous depolymerization upon exposure to xradiation. The exposed regions are therefore freed of the inhibitor and rendered soluble in an aqueous base developer. No PEB processing is required and there is no sensitivity to the presence of basic airbome contaminants. For the formulation consisting of 10% of a low molecular weight PMPS in a novolac, a minimum clearing dose of
392
A.E. Novembre et al. / Microelectronic Engineering 27 (1995) 389-392
40mJ/cm 2 was measured for a starting film thickness of 0.65~tm. An interruptive spray development process was found to improve the contrast and image quality of the resist. Figure 4 depicts 0.25~tm line and space patterns formed in NPR.
m
4. CONCLUSIONS Positive CA resists based on acid catalysis represent the materials closest to meeting the sensitivity and resolution requirements for use in proximity x-ray lithography. Critical dimension control however needs to be further improved upon. The resist NPR offers solutions to problems associated with using a radiation generated acid for imaging, but improvements in sensitivity must first be made. Negative resists provide high sensitivity and improvements in the resolution capability is essential before use of these materials can be realized.
Figure 4. SEM of 0.25 ~tm features in NPR resist. 5. ACKNOWLEDGMENTS Further improvements in NPR sensitivity occurs at the expense of resolution. To produce a more sensitive formulation, higher molecular weight PMPS is required. This however reduces the compatibility between the novolac and PMPS and impairs the pattern resolution. Efforts to improve the phase compatibility between the novolac and PMPS is in progress. 3.4 Negative Resists The multi-component chemically amplified resist OCG LMB 7011 was determined to exhibit sensitivity of <15mJ/cm2 and 0.30~tm resolution in 0.70~tm thick fills. Process latitude with respect to PEB temperature and developer normality has been found be marginal. Lithographic characterization of the conventional crosslinking resist GMC was performed on the version having a weight average molecular weight of 1.3x105 g/mol and comprised of 63 mol% glycidyl methacrylate. Using a 0.50~tm initial f i l l thickness, a process dose of 32mJ/cm2 was required for imaging of 0.35-0.401am size features. Higher resolution is achieved in this resist at the expense of higher operating dose. Future work in the area of negative resists will concentrate on investigating only chemical amplification systems.
The authors would like to thank all the members of the x-ray lithography group at AT&T Bell Labs. This work was partially supported by ARPA DALP through a NAVAIR Contract No. N0019-92-C-0064. 6. REFERENCES 1. J. Frackoviak, et. al., Proc. SPIE,1924 (1993) 258 2. C.G. Willson, J.M.J. Frechet, Y. Tessier and F.M. Houlihan, J. Electrochem. Soc., 133, (1986) 181 3. M.J. Bowden, L.F. Thompson, S.R. Farenholtz and E.M. Doerries ibid, 128, (1981) 1304 4. N. Mtinzel et. al., Proc. SPIE, 2195 (1994) 47 5. R.S. Kanga et. al. Chem. Mater., 4, (1992) 278