Microelectronics Technology - American Chemical Society

Chapter 11

Hydrogen Bonding in Sulfone- and N-Methylmaleimide-Containing Resist Polymers with Hydroxystyrene and Acetoxystyrene Two-Dimensional NMR Studies 1

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Sharon A. Heffner , Mary E. Galvin , Elsa Reichmanis , Linda Gerena , and Peter A. Mirau Downloaded by CORNELL UNIV on August 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0614.ch011

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AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974 Chemistry Department, Mount Holyoke College, Carr Laboratory Park Street, South Hadley, MA 01075

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High resolution solution state NMR has been used to study structure/property relationships in hydroxystyrene and acetoxystyrene resist polymers containing either sulfone or N-methyl maleimide. The properties of resists, including the differential solubility of UV-exposed and nonexposed materials and the glass transition temperature, depend on the local molecular interactions. We have studied hydrogen bonding in the polymer component of selected resists by two dimensional NMR, where hydrogen bond donors and acceptors are identified by the appearance of off-diagonal intensity in the two dimensional nuclear Overhauser effect spectra. The results show that weak intermolecular hydrogen bonds are found in hydroxystyrene/acetoxystyrene/sulfone terpolymers where hydrogen bonding cross peaks are only observed at concentrations above 20 wt%, while hydrogen bonding cross peaks are observed at much lower concentrations in polymers that do not contain sulfone. Stronger intrachain hydrogen bonding is observed in polymers containing N-methyl maleimide. These results show that the materials with the best lithographic performance are those which do not form strong intra- or intermolecular hydrogen bonds. Polymers are a key element in the development of resist technology used in the production of integrated circuits. The sensitivity and contrast in chemically amplified resists are dependent on the difference in solubility between the parent resist matrix, e.g., poly(t-butoxycabonyloxystyrene-co-sulfone) and the deprotected resists containing, for example, hydroxystyrene (i-3). Resists with improved imaging characteristics and lower weight loss have been engineered by introducing a variety of para and meta substituted styrene derivatives (4) into poly(t-butoxycabonyloxystyreneco-sulfone) copolymers, or by replacing the sulfone monomers with maleimide or Nmethyl maleimide (Galvin, R. E., Reichmanis, E. R., and Nalamasu, O., Bell Laboratories, unpublished data). However, little information is available on the 0097-6156/95/0614-0166$12.00/0

© 1995 American Chemical Society

Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Downloaded by CORNELL UNIV on August 4, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0614.ch011

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Sulfone- and N-Methylmaleimide-Containing Resists 167

molecular level properties of the monomers or their functional groups. Intra- and intermolecular hydrogen bonding is potentially an important factor affecting the behavior of these materials because the UV exposed and postexposure baked materials contain high concentrations of hydrogen bond donors and acceptors, and the state of these polar groups (free or hydrogen bonded) can have a large impact on the aqueous base solubility and the diffusion and reactivity of small molecules in the resist matrix. Hydrogen bonding is most frequently studied by infrared spectroscopy, since the frequency and width of the carbonyl absorption is sensitive to the extent and strength of the hydrogen bonding (5). However, molecular level assignment of the donors and acceptors is difficult to achieve by this method when there are many possible donor and acceptor groups. In these studies we have used high resolution solution NMR methods to measure intra- and interchain hydrogen bonding in hydroxystyrene, acetoxystyrene, Nmethyl maleimide and sulfone-containing terpolymers, copolymers and mixtures. The hydroxystyrene materials were chosen for this investigation because this is the unit that is generated upon acid induced t-butoxycarbonyl deprotection and is responsible for allowing aqueous base dissolution of the exposed and post-exposure baked regions of the resists. We have used two dimensional NMR as a function of concentration to identify hydrogen bond donors and acceptors and to measure the effect of polymer microstructure on the hydrogen bonding interactions. The results show that the presence of sulfone, which is required for high contrast and sensitivity, leads to a large decrease in both intra- and intermolecular hydrogen bonding interactions between the hydroxystyrene donors and the acetoxystyrene acceptors. Trie introduction of N-methyl maleimide into the resist polymers leads to a material in which the hydroxyl groups generated by the acid catalyzed deprotection are strongly associated by intramolecular hydrogen bond formation. These results suggest a correlation between the molecular level properties and the imaging characteristics of the resist matrix. Methods and Materials N-methyl maleimide and deuterated solvents were purchased from Aldrich Chemical Co. and used as received, rerf-butoxycarbonyloxystyrene and acetoxystyrene were purchased from Hoechst-Celanese, Inc. and used as received. Polymerizations were conducted in cyclohexanone at concentrations of 2.5 moles of monomer per litre of solvent at 60°C with the free radial initiator azo-bis-isobutylnitrile. Dodecyl thiol was added to the polymerizations to control the molecular weight. The polymers rx)ly(^ara-hydroxystyre^ poly(parahydroxystyrene-co-para-acetoxystyrene-co-sulfone), poly(para-hydroxystyrene-comeia-acetoxystyrene-co-sulfone), poly(pam-hydroxystyrene-co-sulfone), and poly(para-acetoxystyrene) were prepared as previously described (4). In most polymers there was an approximately equal amount of hydroxystyrene and acetoxystyrene, and for the sulfone containing polymers the total ratio of styrene to sulfone was 3:1. Poly(para-hydroxystyrene) (MW=30,000) was obtained from Aldrich. The model polymers for the UV-exposed and deprotected material were obtained by cleaving the ί-butoxycarbonyl (t-BOC) moiety of the poly(t-



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butoxycarbonylstyrene-co-sulfone) parent polymer by heating in a rotary evaporator under vacuum at 170°C. A Perkin Elmer TGA-7 system thermal analyzer was used to insure that all t-BOC groups had been thermally removed. Heating rates of 10 °C/min were used for the thermal analysis. The miscibility of the model resist polymers with the probe polymers poly(phydroxystyrene) and poly(p-acetoxystyrene) was evaluated by the optical clarity of solution cast films. In a typical experiment 2 mL of 3 wt% solutions of the probe polymer and the resist in tetrahydrofuran were mixed for several hours. Films were cast on a microscope slide under a nitrogen atmosphere overnight and annealed at 110°C for 4 h under vacuum to remove all traces of the solvent. The high resolution solution NMR spectra were acquired at 500 MHz on a JEOL GX-500 spectrometer. The 90° pulse widths were 20 |is and the sweep widths were set to 7 kHz. The two dimensional nuclear Overhauser effect (NOESY) spectra were obtained with the (90 -ti-90°-T -90 -t ) pulse sequence in the phase sensitive mode (6). In a typical experiment 256 complex ti points and 512 complex ti points were acquired with a mixing time of 0.5 s and a recycle delay time of 4 s. The data were processed with 5 Hz line broadening in each dimension. o

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Results Hydrogen bonding is a strong interaction that can alter the bulk properties of polymers. Such interactions can affect the processing of resist formulations by altering the local polarity, the glass transition temperature, the mobility of photoacid generators, and the aqueous base solubility. We have studied hydrogen bonding in model resists by comparing the solution NMR properties of several copolymers, terpolymers and blends. These polymers were chosen as model systems to understand the properties of exposed and deprotected resist matrix resins in terms of the local, molecular interactions. It might be expected that hydrogen bonding would affect the behavior of these materials since they contain a high concentration of hydrogen bond donors and acceptors. High resolution NMR in solution is used to measure the strength of the intra- and intermolecular hydrogen bond formation and to identify the donor and acceptor groups. We have studied hydrogen bonding in sulfone (S0 ) and N-methyl maleimide (NMM) resist polymers containing para-hydroxystyrene (pOHSty), paraacetoxystyrene (pAcSty), and meia-acetoxystyrene (mAcSty) (Scheme I) by blend formation and by NMR spectroscopy. Most polymers do not form miscible blends because molecular mixing of the chains is not entropically favored in high molecular weight polymers (7, 8). As a result, films cast from most binary mixtures are phase separated and appear cloudy because the length scale of the phase separation is on the order of the wavelength of visible light. Miscibility in polymer blends can be promoted by introducing low levels (5%) (9) to stabilize the formation of a miscible polymer blend. a


Table I. The miscibility of poly(p-hydroxystrene) and poly(p-acetoxystyrene) with model resist copolymers and terpolymers Polymer pOHSty pAcSty pOHSty/pAcSty pOHSty/pAcSty/S0 pOHSty/pAcSty/NMM pOHSty/Sty/NMM 2

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p-OHSty —

Ο Ο Ο

pAcSty Ο —

Ο ο

••

••

The symbols Ο and · represent miscible and immiscible films.

Table I also shows that poly(p-hydroxystyrene) and poly(p-acetoxystyrene) form miscible blends with the pOHSty//?AcSty/S0 terpolymer while the terpolymers containing N-methyl maleimde monomers show dramatically different behavior. In contrast to the miscibility seen in the above systems, cloudy, phase separated films were obtained for all blends of the probe polymers with the NMM-containing materials. This result was unexpected due to the large number of potential hydrogen bond donor and acceptor groups in the model resist polymer. Immisciblefilmswere also obtained for the NMM-containing polymers cast from dimethylsulfoxide and methyl ethyl ketone. To gain an insight into the hydrogen bonding properties of these materials, we studied the resist polymer solutions by one and two dimensional NMR. We have previously used these methods to identify the interacting groups, to distinguish intrafrom intermolecular association, and to estimate the interaction strength in mixtures of polymers that form miscible blends (12-14). Figure la-c shows the high resolution proton NMR spectra of three examples of the model resist polymers examined in this study, /?OHSty//?AcSty, /?OHSty//?AcSty/S0 and pOHSty/pAcSty/NMM. The peaks of particular interest are the exchangeable hydroxyl protons appearing between 9 and 10 ppm, the aromatic protons at 6-8 ppm, and the N-methyl and acetoxy signals at 2.6 and 2.2 ppm that overlap with the signalsfromthe main chain methylene and methine protons. Differences in the spectra can be attributed to differences in the chemical structure, the chain microstructure, and the chain dynamics. The differences in the chemical shift dispersion of the hydroxyl protons in the sulfone and N-methyl maleimide-containing polymers can be related to the difference in chain architecture between the sulfone and maleimide terpolymers. The environment of the hydroxyl protons in the maleimide polymers is expected to be more uniform as the maleimide and styrene units alternate (15), while there is a statistical distribution of styrene and sulfone units (16). The larger line widths observed in the NMM-containing j

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ppm Figure 2. The 400 MHz 2D NOES Y spectrum of the 20 wt% solution of pOHSty/pAcSty/NMM terpolymer obtained with a 0.5 s mixing time. The hydroxy-N-methyl cross peaks are labeled a/b and the hydroxy-acetoxy cross peaks are labeled a/c.



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polymers may be due either to a difference in chain stiffness or hydrogen bonding (vide infra). The hydroxyl protons of /?ara-hydroxystyrene can be used as a molecular level probe of the hydrogen bonding in deprotected resists. The average distance between the hydroxyl protons of hydroxystyrene and the methyl protons in a nearby acetoxystyrene for random coil copolymers in solution is much larger than the 5 À separations that can be measured by two dimensional nuclear Overhauser effect spectroscopy (10, 11). However, hydrogen bonding brings these groups into close contact (Scheme Π) and cross peaks can be observed in the two dimensional spectra between the hydrogen bond donor protons and protons near the hydrogen bond acceptor. Figure 2 shows the two dimensional NOES Y spectrum for the pOHSty /pAcSty/NMM terpolymer at 20 wt% acquired at 25 °C in DMSO solution with a 0.5 s mixing time. In addition to the off-diagonal peaks expected from the chemical structure of these materials, such as between the OH and aromatic protons, cross peaks between the OH protons and the protons in the vicinity of the hydrogen bond acceptors are also observed. In this terpolymer the cross peaks are observed mainly between the OH protons and the N-methyl protons of N-methyl maleimide (cross peak a/b). Only small cross peaks are observed to the acetoxy protons (cross peak a/c), indicating that the carbonyl group of the N-methyl maleimide is the primary acceptor for the hydrogen bonds. Further evidence for the hydrogen bond formation between the OH protons and the N-methyl maleimide carbonyl are obtained from the observation of similar NOES Y cross peaks in the spectrum of the pOHSty/Sty/NMM terpolymer that lacks the acetoxy group (not shown). Our previous studies have shown that such cross peaks are not expected from random coil polymers in the absence of specific interactions (14). Figure 3 shows a comparable experiment for the pOHSty/pAcSty/S0 terpolymer obtained on a 20 wt% sample with a mixing time of 0.5 s. Under these conditions intermolecular cross peaks are observed, but they are much weaker than those observed for the pOHSty/pAcSty/NMM sample shown in Figure 2. We have previously shown that the concentration dependence of the two dimensional NMR cross peaks can be used to estimate the relative interaction strength in weakly interacting polymer mixtures (12,14). In weakly interacting polymer mixtures, such as polystyrene and poly(methyl vinyl ether) (12) or poly( vinyl chloride) and poly(methyl methacrylate) (14), intermolecular NOES Y cross peaks are observed at concentrations above 25 wt%. In strongly interacting systems, such as the complex of poly(acrylic acid) and poly(ethylene oxide) (13) that is stabilized by hydrogen bond formation at low pH, these cross peaks are observed at concentrations in the range of 1 wt%. We have measured the strength and mode, intra- vs intermolecular, of hydrogen bond formation in the model resist polymers from the concentration dependence of the NOES Y cross peaks. As noted above, larger cross peaks are observed at lower concentrations for strongly interacting polymers. Intra- and intermolecular hydrogen bond formation can be distinguished since intermolecular association is expected to be concentration dependent while intramolecular association is not. 2


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ppm Figure 3. A contour plot of the 2D NOES Y spectrum of the pOHSty/pAcSty/S0 terpolymer acquired with a 0.5 s mixing time. The hydroxy-acetoxy cross peaks are labeled a/b and the area enclosed in the box contains both the hydrogen bonding cross peaks and the intramolecular aromatic-main chain cross peaks that are used as an internal intensity calibration standard.


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The concentration dependence of hydrogen bond formation has been studied by two dimensional NMR for the N-methyl maleimide- and sulfone-containing terpolymers, the pOHSty/pAcSty copolymer, and the solution mixture of poly(phydroxystryene) and poly(p-acetoxystrene). The results of these studies are compiled in Table Π, which lists the minimum concentration that the hydrogen bonding cross peaks were observed and the concentration dependence. Table Π. The minimum polymer concentration ([C]mi )and concentration dependence (A[C]) for the observation of intermolecular hydrogen bonding cross peaks in the two dimensional NOESY spectra of model resist polymers.


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Table Π. The minimum concentration ([C]min) and concentration dependence (A[C]) of the hydrogen bonded cross peaks detected by two dimensional NMR Polymer

[C]min

pOHSty + pAcSty pOHSty/pAcSty pOHSty/pAcSty/S0 pOHSty/mAcSty/S0 pOHSty/pAcSty/NMM pOHSty/Sty/NMM

(wt%) 2

Microelectronics Technology - American Chemical Society

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