Single component deepultraviolet and xray resists: The lithographic behavior of poly[(2methyl4tbutoxycarbonyloxystyrene)sulfone] and poly[(3chloro4tbutoxycarbonyloxystyrene)sulfone]Thomas X. Neenan, Uday Kumar, Janet M. Kometani, and Anthony E. Novembre Citation: Journal of Vacuum Science & Technology B 11, 2779 (1993); doi: 10.1116/1.586601 View online: http://dx.doi.org/10.1116/1.586601 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/11/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Silylation of novolac based resists: Influence of deepultraviolet induced crosslinking J. Vac. Sci. Technol. B 10, 701 (1992); 10.1116/1.586435 Methylated poly(4hydroxystyrene): A new resin for deepultraviolet resist application J. Vac. Sci. Technol. B 8, 1466 (1990); 10.1116/1.585098 Synthesis and lithographic characterization of poly(4tbutoxycarbonyloxystyrenesulfone) J. Vac. Sci. Technol. B 8, 1428 (1990); 10.1116/1.585091 Degradation of poly(methylmethacrylate) by deep ultraviolet, xray, electron beam, and proton beam irradiations J. Vac. Sci. Technol. B 6, 2286 (1988); 10.1116/1.584071 A model for comparing process latitude in ultraviolet, deepultraviolet, and xray lithography J. Vac. Sci. Technol. B 6, 346 (1988); 10.1116/1.583994

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Single component deep-ultraviolet and x .. ray resists: The lithographic behavior of poly[(2-methyl .. 4 .. ta butoxycarbonyloxystyrene )sulfone] and poly[(3 ... chloro",4 .. t .. butoxycarbonyloxystyrene )sulfone]

Thomas X. Neenan, Uday Kumar, Janet M. Kometani, and Anthony E. Novembre AT&T Bell Labs, Murray Hill, New Jer.sey 07974

(Received 16 June 1993; accepted 14 July 1993)

A series of styrene monomers was prepared having the structure R-4-t-butoxy­carbonyloxystyrene, (R=2-CH3' 3-Cl). The monomers were copolymerized with sulfur dioxide to give copolymers (PMTBSS and PCITBSS, respectively) with the ratio of styrene to S02 of ~2:1. The polymers were evaluated as single component deep-ultraviolet COUV) and x-ray resists and compared to PTBSS, the copolymer of 4-t-butoxycarbonyloxystyrene and sulfur dioxide. No enhancement in sensitivity was observed for either new material under standard processing conditions for either OUV or x-ray (1.4 nm) radiation. However, the enhanced thermal stability of PMTBSS allowed higher post exposure bake (PEB) temperatures (150 versus 140°C), and using a PEB of 150 ·C, PMBTSS showed an x-ray sensitivity of 8-10 mJ/cm2 as compared to 18 mJ/cm2 for PTBSS.

t INTRODUCTION

The search for new resist materials capable of being imaged by ever shorter wavelengths and with decreasing feature size remains a potent driving force in lithography. I In particular, interest in resists capable of being imaged by deep ultraviolet (DtJV) (248 nm),2 electron-beam (EB),3 and x-ray radiation4 has grown in recent years. Resist sys­tems based upon chemical ampIification2 have provided materials which exhibit both high sensitivity and good res­olution when used in conjunction with DUV, EB, and x-ray lithography. Chemically amplified systems are gen­erally multicomponent, and require processing conditions different from those required for conventional novalac­diazonaphthoquinone photoresist formulations. Single component chemically amplified resists are less common, but may have the advantage over multicomponent systems of increased homogeneity. We have long been interested in the development of single component resists which are sensitive over a broad range of exposure wavelengths. We have recently shown5 that certain matrix resins, most notably PTESS, the copolymer of 4-tert-butoxy­carbonyloxystyrene (TBS) and sulfur dioxide (S02)' act as highly sensitive, chemically amplified, aqueous base sol­uble positive acting x-ray (A = 1.4 mm) resists, without the addition of specific acid generating components. PTBSS also functions as a moderately sensitive BB resist. A pos­sible mechanism for both of these processes is depicted in Scheme 1. Exposure to or radiation by either x-ray or EE results in homolytic cleavage of the carbon-sulfur bond to produce benzylic and sulfonyl radical species. The sulfonyl radicals may subsequently form sulfonic or sulfonic acid groups (by abstraction of a hydrogen from the matrix) to become the ends of fragmented chains. A second source of acid could arise from the liberation of sulfur dioxide and its subsequent reaction with water in the film to produce sul­furous or sulfuric acid. In the subsequent process postex­posure baking (PEB) step, the photogenerated acids carry out deprotection of the t-butoxycarbonyI (I-BOC) groups.

Main-chain sission and evolution of gaseous products re­sulting from the y-radiolysis of olefin sulfones have been previously reported by O'Donnen, 6,

7 and these results pro­vide justification for the proposed PTBSS radiation­induced degradation mechanism. It is also known that poly(butene-l-sulf6ne) and other poly(olefin-sulfones) be­have as positive acting resists. 8

The 1.4 nm x-ray sensitivity of PTBSS increases with increasing S02 content in the resist and acceptable x-ray sensitivity was achieved for a resist containing no photo­generator of acid (PAG). Although PTBSS shows promise as a one component resist when exposed with x rays cen­tered at 1.4 nm, the same material has reduced sensitivity to harder (A=O.8 urn) x rays, EB, and nuv (248 nm) radiation.

In an effort to understand the underlying chemistry, we found that the addition of certain arylmethyl sulfones to PTBSS increases the sensitivity for both DUV and x-ray exposure. Figure 1 is a plot depicting the x-ray (1.4 nm) sensitivities for PTBSS of varying compositions, and for a two-component resist containing a 3.75:1 TBS:S02 copol­ymer and 19,3 mol % dibenzyl sulfone (1) (The structure of 1 is shown in Fig. 2). Compound 1 was chosen for study because its structure most closely resembles the structure of the PTBSS backbone. Addition of 19.1 mol % 1 yields a resist formulation with a composition equivalent to PTBSS having a 2.2:1 TBS:S02 ratio. The addition of 1 to the 3.75:1 TBS:S02 polymer clearly effects a dramatic im­provement in the sensitivity of this resist. The sensitivity of the two-component formulation is now comparable to that observed for a one component resist having an equivalent S02 content. This result is a clear indication that benzyl sulfones such as 1 act as effective acid generating species and that the radiation induced mechanism depicted in Scheme I is analogous to what is occurring in PTBSS based resists.

The use of arylmethyl sulfones in OUV resist formula­tions was also found to improve the sensitivity of PTBSS to this form of radiation. Specifically, we found that the ad-

2779 J. Vac. Sci. Technol. B 11(6), Nov/Dec 1993 0134-211X/93!11(6)/2119/4!$1.00 (6)1993 American Vacuum Society 2719

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2780 Neenan et sl.: Single component DUV and x-ray resists

Radiation Induced Cbemistry

~02t hv ~ I --- I e'

O-t-Boc O-t-Boc 9

O-t-Boc

Acid Catalysed Deprotection Reaction

--.,;:cH:.....+ _ ~02_ + COZ + Y heat Y

OH

SCHEME I.

dition of 7.5 mol % of 2 to PTBSS increased the DUV sensitivity from 800 to 80 mJ/cm2

, while the addition of 3 increased the sensitivity to 60 mJ/cm2

• In the case of 3, an increase in film absorption at 248 nm and a measured in­creased quantum yield for acid formation combined to pro­duce a more sensitive resist. For 2, only a marginal increase in absorption was observed, and the higher sensitivity was primarily derived from an efficient acid formation process. Themechanism for this increase in sensitivity is believed to involve the formation of sulfinic acids from the arylmethyl sulfone by the same process described above for soft x-ray irradiation of PTBSS (Scheme I). Similar interesting re­sults were observed for two component systems, compris­ing PTBSS and arylmethyl sulfones, when exposed to harder (0.8 nm) x rays. Of particular significance was the observation that a resist containing 2 is almost twice as sensitive as a formulation containing the same molar con­centration of 1 (clearing dose Ds of 175 versus 350 mJ/cm2). Since the molecular formulas, and hence the x-ray absorbance, of 1 and 2 are identical, the observed greater x-ray sensitivity for the resist containing 1 is attrib­uted to the presence of the methyl group at the ortho po­sition of 1. This methyl group provides a readily available

40

35 (3.75:11

30

~ 25 12.2:\1 12.1:11 *

.§ 20

r:r 15 ( ) =TDS:S01

10 • = 3.75:1 1'D8:802 1l.75:U + 19.3 mo(e% DDS

5

0 0.6 0.65 0.7 0.75 0.8

Mole Fraction TBS

FIG. L Effect of PTBSS composition all x-ray (A.= 1.4 11m) sensitivity. The starred point (*) shows the increase ill sensitivity observed when 19.1 mol % of dibenzyl sulfone (1) is added to a 3.73:1 TBS:SOz copolymer.

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FIG. 2. Structures of aryl sulfones acid generators added to PTBSS; dibenzyl sulfone (1), benzyl 2-methylphenyl sulfone (2), and bis(3,4-dichlorobenzyl)sulfone (3).

source of abstractable hydrogens in close proximity to the site of sulfonyl radical generation.

These results suggested that a copolymer consisting of 802 units, and suitably substituted styrenes containing methyl or halogens groups proximal to the S02 units, might show enhanced sensitivity over PTBSS, particularly when exposed to harder x rays. Here we try to combine the advantages of a single component resist which is sensitive over a wide variety of exposure wavelengths, with the enhanced sensitivities of resists containing aryl­methyl sulfones. We describe the synthesis and char­acterization of a series of new t-BOC protected polyhydroxystyrenes having methyl or halogen groups in the aryl ring. Specifically we have prepared 2-methyl-4-t­butoxycarbonyloxystyrene and 3-Cl-4-t-butoxycarbonyl­oxystyrene. We have copolymerized these monomers with sulfur dioxide to form poly[(2-methyl-4-t-butoxycarbonyl­oxystyrene) sulfone J (PMTBSS) and poly[ (3-chloro-4-t­butoxy-carbonyloxystyrene) sulfone] (PCITBSS), respec­tively (Scheme II). In a preliminary study, we have exam­ined PMTBSS and PCITBSS with respect to their behavior as DUV or x-ray resists, and compared them to PTBSS, the copolymer of TBS and sulfur dioxide.

II. EXPERIMENT

A. X-ray and DUV lithography

PTBSS, PMTBSS, and PCITBSS resist solutions (5%-15% solids) were prepared by dissolving the polymers in ethyl 3-ethoxypropionate. The solutions were filtered at least twice through a 1.0, 0.5, and 0.2 pm average pore size Millipore Teflon filters.

Resist films in the range of 0.6-0.75 f.1.m thick were prepared by spin coating onto 5 in. silicon substrates at spinning speeds ranging from 1500 to 3000 rpm. The films were baked after coating at 105°C for 2 min on a vacuum hot plate. Film thickness was measured using a Sloan Dektak Model IIA profilometer. Prior to resist coating the

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2781 Neenan et sl.: Single component DUY and x-ray resists

R=2-CH,

= 3-el

+

SCHEME It

R = 2-CH, PMTBSS

= J-e! PCITllSS

silicon substrates were vapor primed using hexamethyldis­ilazane in a Yield Engineering Systems (YES) oven for 5 min at 90°C, X-ray (A=0.8-2.2 nm centered at 1.4 nrn) exposures in helium (02 content <220 ppm) were per­formed using a Hampshire Instruments series 5000 point source proximity print stepper. The laser pulse rate was set at 1.0 Hz and the flux was measured to be ~ 1,0 mJ/cm2/pulse. An x-ray mask having a structure consist­ing of a patterned 0.4 /l-m gold absorber layer on top of a 1.0/l-m B-doped episilicon membrane was used for imaging purposes. DUV (A=248 nm) exposures were performed using a GCA Laserstep exposure tool (NA=O.35, 5 X reduction optics) equipped with a Cymer excimer laser.

The minimum dose (Ds) necessary to completely de­velop a > 25 /l-m2 exposed area was determined by printing a 3 X 3 exposure on a 5 in. silicon wafer. After PEB (140-150 "C, see text), the wafer was immersion developed in a 0.17-0.26 N tetramethylammonium hydroxide (TMAH) solution for 30-120 s and rinsed in de-ionized water for 60-120 s. Analogous experiments were performed using a patterned x-ray mask. A Hitachi Model 2400 scanning electron (SEM) operating at 25 kV was used to take pho­tomicrographs of the developed images.

m. RESULTS AND DISCUSSION

A. SyntheSis of materials

The materials used in this study were prepared as shown in Scheme II. The synthetic details of the new monomers and polymers will be reported elsewhere.9 The copolymers PTBSS, PMTBSS, and PCITBSS were characterized by a combination of spectroscopic techniques, differential scan­ning calorimetry (DSC), thermal gravimetric analysis (TGA), and elemental microanalysis. All three polymers showed 1 H NMR spectra consistent with the assigned structures, with a ratio of aromatic protons to aliphatic protons of 4:3 for PTBSS, of 1: 1 for PClTBSS, and of 1:2 for PMTBSS, due to the presence of the extra methyl group at 2.65 ppm. Infrared spectroscopy (IRS) showed strong carbonyl stretching vibrations at ~ 1750 cm --I for an three polymers, proving that the t-BOC remained intact under the polymerization conditions. Polymer composi­tions were determined by a combination of elemental anal­yses for Sand Cl atoms and thermal analysis. The molec­ular weights and molecular weight distributions are listed in Table 1.

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TABLE I. Thermal properties of polymers. Tg=gJass transition tempera­ture; T,,=onset of deprotection; T d=onset of decomposition.

Mn Tg Tv Td Composition Polymer (X 103

) MjM" ('C) eel eC) styrene:su!fur dioxide

PTBSS 350 1.55 160 165 235 2.0:! 1 PMTBSS 109 1.51 81 180 206 2.06:1 PC1TBSS 352 1.66 78 162 234 2.22:1

B. Thermal analysis

A critical factor in the design of new resists is an un­derstanding of the thermal stability of the polymeric com­ponents, Thermogravimetric analysis of TBS-based poly­mers generally shows one major weight loss event between 100 and 200·C due to the thermal decomposition of the t-BOC group (which undergoes further decomposition to form carbon dioxide and isobutylene). For the homopoly­mer of TBS, the onset point for the decomposition and loss of the t·BOC group (To) is 188 ·C, while decomposition of the polymer backbone (T d) is of the order of 400 ·C. The incorporation of S02 into TBS polymers significantly low­ers both the onset point of t-BOC removal and the onset temperature for polymer backbone decomposition. Typi­cally, To for PTBSS is of the order of 150-180 "C, while T d

is in the range of 230-260 ·C, depending on composition and molecular weight. The observed To and T d points for the new polymers prepared in this study are listed in Table II. It is interesting to note that while the presence of the chlorine does not exert a significant effect on either the To or the T d of PClTBSS, the introduction of the 2-methyl group into PMTBSS raises the To of the polymer signifi­cantly. The T g values of both PMTBSS and PCITBSS are low, suggesting that the presence of substituents in posi­tions 2 and 3 of the aromatic ring (Scheme II) has a dra­matic effect on the packing of the polymer chains versus PTBSS. The low T g values were of concern because of the necessity to PEB films in order to effect image development.

C, Lithography

1. X~ray lithographic response

The x-ray exposure response, and percent deprotection at a dose equal to Ds for the PTBSS, PMTBSS, and PCIT­BSS resists are summarized in Table II. A prebake of 105 ~C for 2.0 min and PEB of 140·C for 2.5 min was used initially to evaluate each resist; this temperature is the up­per limit to which PTBSS and PClTBSS can be baked without suffering thermolysis of the t-BOC group. The de­protection data listed in Table II are derived from a

TABLE II. X-ray response of PTBSS, PMTBSS, and PCITBSS.

Ds % deprotection Resist (mJ/cm2) (@Ds)

PTBSS 18 70 PMTBSS 30 7S PCITBSS 40 70

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2782 Neenan et al.: Single component DUV and x-ray resists

FIG. 3, Patterning of 0.4 /-lm coded line and space pairs in a 0.7 I'm thick film of PMTBSS, using 0.202 TMAH as developer, expo A = 1.4 nm, expo dose= 15 mJ/cm2

,

Fourier-transform infrared (FTIR) study in which spectra were obtained from the exposed and PEB section of each sample wafer. Monitoring the intensity of the carbonyl stretch at 1760 em - 1 in each sample and taking the ratio of the absorbance of exposed areas to baked only areas leads to the conclusion that -70% deprotection of the t-BOC groups is necessary for development in a 0.17 N TMAH solution. This threshold is constant for the three resists examined, showing that the introduction of the methyl or the chlorine substituents on the aryl ring does not affect solubility in alkali developer. Previous studies have con­firmed the need to achieve high t-BOC conversion for aqueous base development and this behavior has also been used to explain the high contrast values which have been reported for resists based upon PTBSS. 1O

It is clear from the data in Table II that a significant decrease in sensitivity is realized from the introduction of both the methyl group in PMTBSS and the chlorine atom in PCITBSS, a result that runs contrary to the results ob­served earlier when imaging PTBSS in the presence of ar­ylsulfones as photoacid generating species. These unex­pected results may be the consequence of the effect that a ring substituent has on the activation energy for deprotec­tion of the t-BOC group, Since the strength of the acids being generated by x-ray irradiation is low, (pKa of sulfinic acids is - 4), relatively small increases in the ac­tivation energy needed for deprotection may have a pro­found effect on sensitivity. Further experiments in which more powerful photoacid generators are added to each re­sist are in progress.

As described earlier, PMTBSS exhibits improved ther­mal stability over PTBSS, allowing a PEB temperature of 150°C with a concurrent increase in sensitivity. The Ds value for PMTBSS at this higher PEB is of the order of 8-10 mJ/cm2

, and thus offers a significant improvement in performance over PTBSS, albeit for different reasons than those upon which this study was developed, Figure 3 shows some initial patterning of PMTBSS when exposed to (.1.= 1.4 nm) x rays. The dose necessary to image the 0.4

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2782

(.Lm line and space pairs in a 0.70 11m thick resist films was 8 mJ/cm2 when using the more aggressive PEB conditions of 150·C for 2.5 min. For PTBSS, the equivalent dose required to image equivalent sized features was 18 mJ/cm2

•

For both resists, these sensitivities represent a 35% in­crease in dose over what is required to develop a full field exposed area.

2. DUV lithographic response

The sensitivity of PTBSS and PMTBSS to 248 nm ra­diation was determined using the following set of process­ing conditions: prebake at 105°C for 2 min; PEB at 140·C for 2.5 min. followed by development for 60 s in a 0.262 N TMAH solution. Using these conditions, Ds was deter­mined as 550 and 310 mJ/cm2 for PMTBSS and PTBSS, respectively. The PEB condition used in the comparison represents the upper limit at which PTBSS can be baked without undergoing thermolysis of the t-butoxycarbonyl protecting group. PMTBSS however exhibits improved thermal stability over PTBSS, and a PEB temperature of 150·C affords an increase in sensitivity (Ds of 125 mJ/cm2

).

IV. CONCLUSION

Copolymers ofR-t-BOC-styrenes (R=2-CH3 and 3-Cl) and sulfur dioxide were prepared in an effort to develop a one component resist having the sensitivity previously demonstrated for two component systems comprising PT­BSS and aryl sulfones. The new materials prepared PMT­BSS (R=CH3 ) and PCITBSS (R=CI), function as sensi­tive x-ray (A= 1.4 nm) and DUV resists. However, no enhancement in sensitivity of these materials was observed over PTBSS itself. These results suggest that lithographic sensitivity in these resist systems is governed by several factors; more work is needed to fully understand the fac­tors which determine performance in these systems. PMT­BSS does exhibit improved thermal stability over PTBSS, and a PEB temperature of 150·C affords an increase in sensitivity CDs of 125 mJ/cm2 for DUV and a 8 mJ/cm2

for x ray (.1.= 1.4 nm).

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