Backbone Degradable Poly(aryl acetal) Photoresist Polymers

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Backbone Degradable Poly(aryl acetal) Photoresist Polymers: Synthesis, Acid Sensitivity, and Extreme Ultraviolet Lithography Performance Matthias S. Ober,*,† Duane R. Romer,† John Etienne,† P. J. Thomas,† Vipul Jain,‡ James F. Cameron,‡ and James W. Thackeray‡ †

Core R&D, The Dow Chemical Company, Midland, Michigan 48674, United States Dow Electronic Materials, 455 Forest St., Marlborough, Massachusetts 01752, United States

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‡

S Supporting Information *

ABSTRACT: A new class of acid labile poly(aryl acetal) polymers has been developed that can be used in photoresist formulations for next-generation microlithography techniques including extreme ultraviolet (EUV) or electron beam lithography. Example polymers have been synthesized by an optimized Suzuki polycondensation protocol. They are soluble in common photoresist solvents but are insoluble in water or aqueous bases that are used to develop positive photoresists. The structural design includes further elements that are aimed at improving photoresist resolution, stability, and etch resistance. Upon acid exposure, the acetal linkages are cleaved, and the polymers degrade into phenolic terphenyl fragments, which are readily soluble in a photoresist developer. Polymer degradation has been studied by NMR and LC-MS. Lithographic formulations have been developed and tested in line-andspace patterning experiments using EUV photolithography. Optimized resist formulations achieved 22 nm resolution with line width roughness values of 5.7 nm.

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INTRODUCTION

quencher base limits proton diffusion and improves line width roughness (LWR).2−4 Among the first commercially successful CAR polymers was poly(4-hydroxystyrene) (PHS) protected with tert-butoxycarbonyl groups (tBOC).5 Since then, many other deprotectable CAR polymer platforms have evolved to meet the needs of process miniaturization and improving processability. Examples include PHS protected with highly acid labile (low Ea) acetal groups,6−11 PHS/protected acrylate copolymers,12 acrylate/acrylic acid copolymers,13,14 and norbonene/maleic anhydride copolymers.15 Besides catalytic deprotection, other concepts of chemical amplification have been successfully demonstrated early on, for example, catalytic cross-linking of functional termini/side chains16 or triggered “unzipping” depolymerization of capped polyacetals.17,18 A mixed approach, utilizing a polymer susceptible to both depolymerization and catalytic deprotection, has been demonstrated at the Bell Laboratories.19 This concept is particularly relevant to the polymer design discussed herein. To date, miniaturization of feature sizes that can be accessed through microlithography has been mainly achieved by

Computer chips and other semiconductors are manufactured by microlithography technology. In this process, silicon wafers coated with photoresists are selectively exposed to radiation to induce chemical transformations that change the solubility of the resist material after exposure. During development, the exposed areas are washed away with a mild aqueous base (positive photoresists) or remain intact (negative photoresists). The patterned photoresist serves in turn as a template during the following etch step in which the underlying silicon wafer is selectively etched only in areas that are not covered by resist material. The process thus allows for transferring a pattern from the photolithographic mask onto the treated silicon wafer. The most common type of high-resolution photoresists used today are chemical amplification resists (CAR) that were developed at IBM in the early 1980s.1 Common positive CAR formulations include a polymer containing acidic side groups that are partially capped with acid-sensitive protecting groups, a photoacid generator (PAG), and a quencher base. Upon exposure to photons, the PAG releases acid, which in turn deprotects the polymer, releases acidic side chains, and thereby imparts polymer solubility in the aqueous base. This acidinduced deprotection reaction is catalytic, with each proton initiating the deprotection of multiple protecting groups. The © XXXX American Chemical Society

Received: May 15, 2018 Revised: August 20, 2018

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DOI: 10.1021/acs.macromol.8b01038 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules reducing the wavelength of the exposing light.4 Technology improvements advanced from Hg-lamp g-line (λ = 436 nm) and i-line (λ = 365 nm) light sources to excimer laser light KrF (λ = 248 nm), ArF (λ = 193 nm), and finally ArF-immersion lithography. For several reasons, the 157 nm F2 excimer laser platform has proven to be economically unfeasible, and the next step in further wavelength reduction will be extreme ultraviolet light (EUV, λ = 13.5 nm) lithography. EUV lithography is a significant departure from classical deep UV exposure. Currently available EUV photon sources have a significantly lower photon output contributing to extended irradiation times and shot noise. Additionally, EUV radiation is ionizing, and therefore exposure has to occur under vacuum. EUV wavelength photons are weakly, but unselectively, absorbed by all photoresist components, resulting in a different solubility-switching photochemistry compared to lithography at higher wavelengths. Despite these challenges EUV lithography has been identified as the patterning method of choice for the next generation of semiconductor devices. Traditional CARs have approached an empirical limit at which any one of the resist characteristicsresolution, line width roughness, and sensitivitycan only be improved at the expense of at least one of the other two (RLS trade-off problem). This problem has proven to be a serious obstacle to EUV lithography. To address RLS trade-off, novel materials with improved pattern definition capability20 must be developed. Current approaches include further reduction in the activation energy of deprotection of the solubility switching groups to improve the efficiency of the catalytic chain reaction, reduction of acid blur by attenuating proton diffusion, improvement of EUV photon absorption of the polymer by introducing elements such as fluorine, or further promotion of acid generation by increasing the PAG loading in the EUV photoresist formulation.21 Another way to improve the RLS trade-off may be through a material that exhibits a significantly higher acid diffusion constant within the exposed area in comparison to the unexposed area.22 In such a system, the acid may travel further within the exposed areas which increases its catalytic activity, whereas excessive diffusion into unexposed areas is discouraged. Such a behavior can be achieved by utilizing a backbone-degradable resist polymer. Regular CARs behave in the opposite fashion because acid diffusion is typically slow in the polymer matrix and is further attenuated if the polarity/ hydrogen bonding of the matrix increases after deprotection.23 Furthermore, as resolution requirements will soon approach the molecular dimensions of classical photoresist polymers, a degradable platform may enable resolution beyond the dimensions of classical resist polymers. The concept of depolymerizable photoresist polymers has been described even before development of CAR. Poly(methyl methacrylate) polymers undergo chain scission upon e-beam exposure,24 although sensitivity is not particularly high. Polymers that unzip upon deprotection as described above or are based on dialdehyde monomers often suffer from either a low glass transisiton temperature (Tg) or poor etch resistance.4 Polymers with specifically inserted breaking points such as carbonates25 and especially sulfones19 may develop gaseous fragments upon vacuum irradiation, which might damage the optics of the EUV tool. Most of the backbone degradable concepts, including polyacetals, have in common that it is not common to alter the type or tune the sensitivity of the cleavable moiety, which is usually created as part of the

polymerization process. As a result, a new polymerization method has to be developed if an adjustment to the cleavable group is required. In this paper, we describe a novel modular EUV photoresist polymer platform, in which the type and sensitivity of the fragmenting groups in the backbone can be freely and independently modified without the need to alter the polymerization method. Key targets for this concept are (1) the polymerization reaction is independent of acid-sensitive group introduced into the backbone, (2) the polymerization reaction is compatible with a variety of functional groups and allows variable polymer design and molecular weight, and (3) the method enables access to relatively rigid polymer structures. All of these requirements are met by Suzuki polycondensation (SPC)26,27 of monomers containing an acid cleavable linkage such as a sensitive acetal, which is described in this paper. Significant molecular weights can be obtained, even with highly acid labile and difficult to synthesize acetals/ketals in the backbonetheir formation is no longer part of the polymerization process. We have sought to include other elements into the polymer design that were believed to be beneficial to EUV lithography (Figure 1). Besides highly acid-sensitive acetal-based side

Figure 1. Key design elements of novel backbone-cleavable photoresist polymer platform include sensitive acetal moieties in side chains (a) and polymer backbone (b), a rigid polyaromatic framework (c), and free phenol groups (d).

chains and backbone moieties, design elements further included a rigid polyaromatic backbone to provide a high Tg and free phenolic groups to provide good adhesion to the silicon wafer as well as modulate hydrophobicity of the polymer. Acetal groups are thought to be advantageous for vacuum exposure methods, such as EUV lithography. Although the reaction is initiated through generation of protons upon exposure, the actual cleavage reaction requires stoichiometric water and therefore likely accelerates significantly after the wafer is removed from the vacuum chamber and exposed to ambient humidity.9 This behavior partially alleviates outgassing that can be detrimental to the tool optics but brings along new challenges that are likely relevant to the system described hereinfor example, the requirement of tight control of postexposure conditions.9,10 The polymer structure was furthermore designed to have favorable Ohnishi28 and ring parameters29 to improve etch resistance, but it should be noted that these empirically derived parameters might not fully predict the properties of a novel polymer type. It is believed that the first step of EUV-facilitated acid generation within a photoresist requires a photon-induced ejection of an electron of matrix material (usually the resist polymer) that is eventually accepted by a PAG. The resulting radical cationic matrix material then releases a proton to B


Article

Macromolecules neutralize the charge.22,30 To facilitate acid generation by this mechanism, phenol or acetal substituents and linking phenylene groups were placed in the ortho- or para-position to one another. The resulting terphenyl moieties are Michael analogues of catechol and hydroquinone. Structures of this nature should be readily oxidizable via a comparatively stable radical intermediate, releasing a phenolic proton that will be catalytically active once stoichiometric water is available for acetal cleavage. Electron ejection also potentially provides a secondary pathway of backbone cleavage through oxidation. Although this hypothetical mechanism has not been systematically studied as part of this work, it is supported by the fact that alkyl and silyl ethers are known to be cleavable in an oxidative environment31 via radical intermediates.32 Upon treatment with aqueous acid, the polymer was designed to degrade into sufficiently base-soluble fragments distinct from the initial monomer set (Scheme 1).33

The cleavable SPC also has solubility parameters in the range of commercial resists and has higher Ohnishi and ring parameters, predicting a good etch resistance (with the caveat that these empirical parameters might not extrapolate to degradable polymers). Additional details are discussed in the Supporting Information. Monomer Synthesis. Synthesis of Acid-Cleavable Bisboronates. An important feature of the proposed photoresist platform is acid cleavability of the backbone, which was introduced using bisboronic ester SPC monomers. The first step was to develop rigid building blocks containing a sufficiently sensitive linker to be readily broken down by photogenerated acid in the exposed formulation but sufficiently stable to provide etch resistance during pattern etch transfer. To this end, formaldehyde diphenylacetal, acetaldehyde diphenylacetal, and benzaldehyde diphenylacetal monomers were synthesized according to Scheme 2. Formaldehyde and benzaldehyde diphenylacetals 4 and 5 were synthesized in excellent yields by implementing mild procedures published for similar compounds.37−39

Scheme 1. Formation and Cleavage of BackboneDegradable Poly(aryl acetal) EUV Photoresist Polymer Concept

Scheme 2. Synthesis of Bisboronates Containing Different Acetal Linkers

The bisboronate esters 9 (pBEFA) and 10 (pBEBA) were accessed in good yields by lithium halogen exchange of the corresponding dibromides 4 and 5 at −78 °C, followed by treatment with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (pinBOiPr) and a rapid aqueous work-up. The synthesis of compound 8 has been previously described,40 but the method was further optimized to improve yield. In the first step, vinyl ether 7 was synthesized by vinyl transfer reaction from vinyl acetate 6 to bromophenol 1 catalyzed by bis(1,5-cyclooctadiene)diiridium(I) dichloride.41 The vinyl ether 7 was then reacted with an additional equivalent of p-bromophenol 1 to give the symmetric acetal. For the final borylation reaction, an aqueous work-up step had to be omitted and yields of the bisboronic ester 11 (pBEAA) were somewhat lower than that of the other two bisboronates. Investigation of Acid Stability. To ensure that the acid sensitivity of the backbone is comparable to the protecting groups of existing photoresists, aqueous degradation kinetics of the backbone acetal monomers (4, 5, and 8) were investigated by using degradation conditions mild enough to only cleave sensitive “low Ea” photoresist protecting groups within the time frame of minutes to hours. A specific amount of aqueous triflic

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RESULTS AND DISCUSSION Modeling. Properties of the degradable SPC polymer were computationally investigated and compared to the properties of a traditional PHS photoresist polymer. As acid switchable side chain protecting groups, cyclohexylvinyl ether (CHVE) acetals were selected, which are highly sensitive and have been successfully used in PHS-based resists. Among the simulated properties were Tg (dependent on molecular weight and degree of protection), Ohnishi and ring parameters (dependent on degree of protection), as well as solubility parameters (Fedors34,35 and van Krevelen36). The Tg of cleavable SPC polymer was predicted to exceed the Tg of a PHS-based resist at comparable molecular weights and degrees of protection. C


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boiling point and the ability to fully solubilize the monomers. Reactions were run in parallel at 60 °C overnight. Results are summarized in Table S3. Polymerizations with (t-Bu)3Pd(crotyl)Cl44 and tripotassium phosphate in 1,4-dioxane gave the highest number-average molecular weight (Mn) and a low polydispersity (PDI) (Table S3f). This catalyst/base combination proved to be general for other monomer combinations and was sufficiently reactive to reliably obtain molecular weights of >7 kDa at room temperature (Table S4). In addition to synthesizing the current polymer targets, the room temperature conditions also proved useful for the development of second-generation materials protected with acid-labile esters (not discussed here) that would otherwise cleave during SPC at higher temperatures. Proof-of-Concept Polymer Synthesis and Degradation. The first material to be scaled up and fully characterized was p-phenylboronic ester benzaldehyde acetal/2,4-dibromophenol copolymer, protected with cyclohexylvinyl ether (pBEBA-2,4-DBP-CHVE p1), which was polymerized by using the optimized method identified above. The molecular weight Mn was limited to ∼5 kDa by adjusting the monomer stoichiometry according to Carothers’ equation.46−48 The polymer was quantitatively end-capped with phenyl groups (details below) to remove pendent boron or bromide functionalities which are undesired in photoresist polymers. To be suitable as a photoresist polymer, the following three key requirements must be met: (1) the polymer must be soluble in propylene glycol methyl ether acetate (PGMEA) or other common solvents typically used in photoresist formulations, (2) the untreated polymer must be insoluble in aqueous tetramethylammonium hydroxide (TMAH) solution (0.26 N TMAH), an approximation of a photoresist developer solution, and (3) after treatment with acid the polymer (or degradation products) must be fully soluble in aqueous TMAH solution. To demonstrate these requirements, 5 mg of pBEBA-2,4DBP-CHVE polymer p1 (4.8 kDa) was dissolved in 200 μL of PGMEA. After a few minutes, a clear solution of pBEBA-2,4DBP-CHVE in PGMEA was formed (Figure 2.1). To assess criteria 2 and 3, 5 mg of pBEBA-2,4-DBP-CHVE p1 was dissolved in 200 μL of THF. The resulting solution was distributed into two vials. The first vial was left untreated (Figure 2.2), and the second vial was treated with ∼50 μL of concentrated trifluoroacetic acid (TFA), upon which the content of the vial turned red (Figure 2.3). Subsequently, 50 μL of the untreated vial (Figure 2.2) was added to 300 μL of 0.9% TMAH in water, upon which a thick precipitate of the insoluble polymer was formed (Figure 2.4). Addition of acidtreated polymer solution (Figure 2.3) to aqueous TMAH resulted in a clear, orange-tinged solution without any visible solid residue (Figure 2.5). To confirm the acid degradation mechanism of pBEBA-2,4DBP-CHVE, we ran a 1 H NMR acid decomposition experiment on a 20 mg sample of polymer p1 in THF-d8. In this study, backbone degradation (yielding benzaldehyde) and side-chain deprotection (yielding acetaldehyde and cyclohexanol) could be easily monitored by time-dependent integration of the acetaldehyde proton peak (visible as a quartet at δ 9.67 by 500 MHz 1H NMR spectroscopy) and the benzaldehyde peak (visible at δ 9.98). The signals of the formed cyclohexanol were not baseline separated. The sidechain CHVE groups are completely removed within ∼30

acid was added to an anhydrous deuterioacetonitrile solution of the investigated substrate, and the decomposition was monitored by 1H NMR. Although this method provides a quick initial assessment of the relative stability of similar acetal functionalities in the presence of acid and moisture, it cannot fully predict cleavage kinetics in a vacuum-exposed thin film because other critical film parameters that will accelerate or attenuate this reaction such as water retention, film hygroscopicity, and diffusion are not captured. Among the tested compounds, the benzaldehyde acetal 5 was the only monomer groups among the three candidates to show promising cleavage kinetics (pseudo-first-order, half-life ∼100 min, Figures S6−S9, Tables S9 and S10). We therefore focused our synthesis and scale-up efforts toward monomer pBEBA 10 and the corresponding Suzuki polycondensates. Synthesis of Protected Dibromophenols. To modulate the density of protected phenol groups in the monomer, two dibromophenol building blocks were synthesized (Scheme 3). Scheme 3. Synthesis of CHVE-Protected Dibromophenols

The synthesis of the single CHVE-protected 2,4-dibromophenol 16 (2,4-DBP-CHVE) was achieved by stirring commercially available 2,4-dibromophenol 14 with (vinyloxy)cyclohexane 15 in the presence of a catalytic amount of acid. The highly acid sensitive product 16 is easily isolated by filtration of the crude reaction mixture through basic alumina, which binds strongly to trace unconverted 2,4-dibromophenol 14, the main impurity in the reaction mixture. The double CHVE-protected 4,6-dibromoresorcinol monomer (2,4-DBR-CHVE) 17 was synthesized in two steps from resorcinol 12 via a clean, regioselective bromination with pyridinium tribromide42 to 13 followed by CHVE protection as described above. Synthesis and Optimization of Suzuki Polymerization. Using s-Phos43 as a catalyst, our first attempts to copolymerize pBEBA 10 and 2,4-DBP-CHVE 16 yielded waxy oligomers with weight-average molecular weights (Mw) of 0.5−2 kDa and Tg in the range 48−85 °C. both of which are insufficient for the desired application. Attempts to condense the resorcinol monomer 17 with pBEBA 10 were unsuccessful. To improve the SPC reaction of 10 with 17, the performance of a series of Suzuki catalysts and bases was investigated, and the molecular weights (Mn and Mw) of the polymers formed in each of the investigated reactions in a fullfactorial library were determined. Catalyst loading was kept constant at 0.1 mol%. To prevent confounding this side-byside comparison by variations in monomer stoichiometry, master solutions of both monomers were prepared at a 1:0.998 ratio, which were used in aliquots in all experiments. 1,4Dioxane was selected as the reaction solvent due to its high D


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visible by NMR. A complete list of compounds that were detected by HPLC-MS and the corresponding HPLC trace is shown in Table S11. Partially Protected Polymers. Initial wafer coating and lithographic experiments with fully CHVE-protected pBEBA2,4-DBR-CHVE p0 (not shown) and pBEBA-2,4-DBP-CHVE p1 polymers were not satisfactory due to the relatively high hydrophobicity of the polymer, resulting in poor substrate adhesion and high water contact angle (WCA). To improve upon these characteristics, the next step was to increase polymer hydrophilicity by leaving some of the phenol groups unprotected. The goal was to find the window at which developer insolubility is retained, but acceptable developer contact angles and substrate adhesion were achieved. A series of pBEBA-2,4-DBP-CHVE polymers were synthesized with different degrees of protection. Table 1 summarizes the properties of the backbone degradable polymers prepared as part of this study. We pursued several synthetic approaches to obtain pBEBA2,4-DBP-CHVE polymers with defined levels of CHVE protection and hydrophilicity. The initial focus was to develop methods to enable orthogonal CHVE deprotection while leaving the polymer intact. With this approach, polymers between a range of 11% and 58% CHVE were successfully synthesized (p2−p5). Further details are given in the Supporting Information. With improvements of the Suzuki polycondensation procedure, de novo synthesis of partially protected polymers became the preferred method. An important discovery was the fact that the catalyst (t-Bu)3Pd(crotyl)Cl in combination with 1,4-dioxane as the solvent and concentrated aqueous tripotassium phosphate solution (50% w/w) as the base permitted incorporation of free phenol monomers into the polymer at high yields. Polymers between 29% and 0% CHVE protection and different molecular weights (p6−p8) were synthesized in this fashion, but the method should be readily extendable beyond this range. This method allowed us to accurately and reproducibly target both a specific molecular weight and CHVE functionalization level by simply adjusting the monomer stoichiometry of pBEBA 10, 2,4-DBP-CHVE 16, and 2,4dibromophenol 14. To develop a recipe to target specific molecular weights and degrees of protection, we first estimated the average reaction yield of the optimized polymerization reaction. We assume that molecular weight of the Suzuki polycondensation reaction follows Carothers’ equation,46−48 wherein Mn is the number-average molecular weight, M0 is the average molecular weight of a repeating unit, Xn is the numberaverage degree of polymerization, and p is the reaction yield of each condensation reaction. With the highest observed Mn of 11.6 kDa (p8) during an exactly stoichiometric polymerization of 2,4-dibromophenol 14 with pBEBA 10, and an average molecular weight of both repeat units of 246.3, and resolving Carothers’ equation for p, a minimum average coupling yield of ≥97.9% during the polymerization reaction can be estimated. Extending for nonstoichiometric bifunctional monomer ratios of the type A−A/B−B, Carothers’ equation can be expanded to include a polymer ratio r that describes the ratio of the concentrations of A−A and B−B.49 The ratio is calculated as r < 1:

Figure 2. Qualitative solubility experiment of untreated and acidtreated pBEBA-2,4-DBP-CHVE to demonstrate solubility switch in aqueous TMAH.

Figure 3. Acid treatment of pBEBA-2,4-DBP-CHVE shows different degradation rates for backbone and side-chain acetal cleavage. Benzaldehyde and acetaldehyde protons were diagnostic in this NMR experiment.

min,45 whereas the backbone degrades at a slower rate (Figure 3). Further analysis of polymer fragments after complete degradation by LC-MS indicated that the main decomposition products in the solutions are the expected [1,1′,3′,1″terphenyl]- 4,4′,4″-triol, benzaldehyde, a few oxidation products of [1,1′:3′,1″-terphenyl]-4,4′,4″-triol, and fragments that can be attributed to polymer end groups. Acetaldehyde and cyclohexanol were not observed by LC-MS but are clearly E


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Macromolecules Table 1. Summary and Key Characteristics of All Poly(aryl acetal) Photoresist Polymers Discussed Herein

name p0 p1c

pBEBA-4,6-DBRCHVE pBEBA-2,4-DBPCHVE

CHVE (%)

R1

R2

100

−OCHVE

OCHVE

100

−OCHVE or −H

−OCHVE if R1 is −H

p2

pBEBA-2,4-DBPCHVE

58

−OCHVE, −OH, or −H

p3

pBEBA-2,4-DBPCHVE

33


p4

pBEBA-2,4-DBPCHVE

11


p5

pBEBA-2,4-DBPCHVE

30


p6

pBEBA-2,4-DBPCHVE

29


p7

pBEBA-2,4-DBP

0

−OH or −H

p8

pBEBA-2,4-DBP

0

−OH or −H

−H if R1 is −OCHVE −OCHVE or −OH if R1 is −H −H if R1 is −OCHVE or −OH −OCHVE or −OH if R1 is −H −H if R1 is −OCHVE or −OH −OCHVE or −OH if R1 is −H −H if R1 is −OCHVE or −OH −OCHVE or −OH if R1 is −H −H if R1 is −OCHVE or −OH −OCHVE or −OH if R1 is −H −H if R1 is −OCHVE or −OH −OH if R2 is −H −H if R2 is −OH −OH if R2 is −H −H if R2 is −OH

Mw (kDa) a

Mn (kDa)

Tg (°C)

method

a

n.d.

8.6

4.7

104

de novo synthesis

6.0

3.8

110

timed deprotection

6.0

4.1

118

timed deprotection

5.7

3.6

110

timed deprotection

8.4

4.5

130

water starved deprotection

13.4

6.4

136

de novo synthesis

14.1

6.1

151

de novo synthesis

11.6

174

de novo synthesis

var

47

var.

b

a Variable results. Tables S3 and S4 summarize the synthesis of polymers p0 by using different reaction conditions. bNot determined. cA separate batch was resynthesized at similar conditions yielding polymer p1′ (Mn = 3.94 kDa, Mw = 6.17 kDa). This batch was used to synthesize p2−p4.

Scheme 4. Optimized Suzuki Polycondensation toward Partially Protected and End-Capped pBEBA-2,4-DBP-CHVE Polymers pn

F


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Macromolecules Xn =

Mn 1+r = M0 1 + r − 2pr

precipitation from methanol. To remove residual palladium, the precipitated polymer is dissolved in ethyl acetate and stirred for 1 h at elevated temperature with a concentrated aqueous solution of N,N-diethyl dithiodicarbamate followed by separation and removal of the aqueous phase. Precipitated palladium black is removed by plug filtration through silica/ Florisil, and the polymer is further precipitated twice more from methanol to obtain high purity. The resulting polymer contains a residual palladium content of 0.87 ± 0.08 ppm of palladium and 14.8 ± 0.5 ppm of bromine as determined by neutron activation analysis. Fragmentation of Unprotected Polymer. We explored the degradation of the unprotected polymer pBEBA-2,4-DBP p7 under acidic conditions by 1H NMR and LC-MS in analogy to the method described above to validate its structure including the polymer termini and confirm acid fragmentation. Acidic degradation of the polymer p7 should mainly yield the compounds (1,1′,3′,1″-terphenyl)-4,4′,4″-triol 22 and benzaldehyde 23 in a 1:1 ratio (along with a small amount of termini fragments, Scheme 5.) Upon acid treatment, the

(1)

Assuming a minimum condensation yield of 97.9%, the prediction for the ideal reaction stoichiometry to give a polymer with a target number-average molecular weight of 5− 6 kDa is therefore 0.95−0.96:1 (e.g., 0.95 equiv of 2,4dibromophenol 14 to 1.00 equiv of pBEBA 10). To target a specific degree of CHVE-protection x, the unprotected dibromophenol 14 and the CHVE-protected dibromophenol 16 are polymerized in a ratio of 1 − x to x, respectively. For example, a 30% CHVE-protected polymer requires 70% of 2,4-dibromophenol 2 and 30% of CHVEprotected 2,4-dibromophenol 2-CHVE as the A−A type monomers. Equation 2 can be solved for r, which is an estimation of the required ratio of A−A type monomers (i.e., dibromides) and B−B type monomers (i.e., bisboronic esters) as a function of the Xn and p: r=

1 − Xn X n − 2pX n − 1

(2)

Scheme 5. Fragmentation Reaction of Unprotected pBEBA2,4-DBP

Therefore, to synthesize a polymer with a desired polymerization degree of Xn and a desired protection degree of x, for each equivalent of bisboronic acid, x × r equivalents of monomer 16 and 1 − x × r equivalents of monomer 14 are required, assuming the relative reaction rates of both monomers are identical. For example, a 30% CHVE-protected polymer with a Mn of 5−6 kDa, 1 equiv of bisboronic ester 10, 0.95 × 0.30 = 0.285 equiv of CHVE-protected 2,4dibromophenol 2-CHVE 16, and 0.95 × (1 − 0.30) = 0.665 equiv of unprotected 2,4-dibromophenol 14 are required. This simple recipe correlated well with the polymerization outcome. Additional Considerations Regarding Copolymerization and Work-Up. To increase the likelihood of random monomer incorporation, the polymerization reaction was conducted in two stages. In the first stage, the minor dibromo monomer (usually the CHVE-protected monomer, e.g., 2-CHVE) is combined with the full equivalent of pBEBA 10. Because of the large excess of 10 at this stage, boronic ester macromonomers 18 are being formed with a very low degree of polymerization (step 1, Scheme 4). During the second stage, the remaining major dibromo monomer (usually the unprotected monomer, e.g., 14) is added to the reaction mixture. As the chemical environment of the bisboronic ester macromonomer 18 and unreacted pBEBA monomers 10 are similar, it is expected that they react at similar reaction rates and random copolymers (pn-Br/B) should form (step 2, Scheme 4). Both bromide- and boron-containing functionalities are undesired in photoresist polymers as they may react with or dope the surface of silicon wafer. Hence, care was taken to cap the bromide or boronic ester termini in two additional steps. First, an excess of phenylboronic acid 19 was added to the polymer, and the Suzuki mixture was allowed to react overnight. This step substitutes any bromide termini with phenyl caps (step 3, Scheme 4). The remaining boronic ester termini were phenyl-capped by adding an even larger excess of bromobenzene 20 to the reaction mixture (step 4, Scheme 4). The bromobenzene reacts both with the boronic ester termini and residual phenylboronic acid from the first capping step. Fully capped polymer pn is produced, with biphenyl 21 as a side product. Biphenyl 21, bromobenzene 20, and phenylboronic acid 19 are soluble in methanol and can be easily removed by polymer

Figure 4. Top: 1H NMR of intact polymer. Bottom: 1H NMR after acid treatment, showing a mixture of the expected fragments. Details are given in the Experimental Section.

initial NMR spectrum of the polymer (Figure 4, top) converts into a spectrum that is consistent with a 1:1 mixture of the expected fragments 22 and 23 (Figure 4, along with a small amount of additional compounds, possibly terminal fragments in the baseline). Time-arrayed NMR further shows that new aldehyde and phenol protons are simultaneously formed in a 1:2 integration ratio consistent with the expected degradation pathway (Figure S11). LC-MS studies (Table S12) confirmed the identity of the expected reaction fragments and allowed a detailed characterization termini fragments. We were also pleased to see that a significant amount of oxidation dimers of the terphenyl fragment 22 is observed as G


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Figure 5. Line-and-space images for (a) photoresist R6 at 24 nm resolution, (b) photoresist R7 at 22 nm resolution, and (c) R8 at 22 nm resolution.

To the best of our knowledge, this is the first example of a Suzuki polycondensate that fully fragments into small, sufficiently base-soluble molecules upon acid treatment. Suzuki polycondensation as a method to generate backbonedegradable polymers has several advantages. First, it decouples the formation reaction of the sensitive acetal from the polymerization reaction, allowing for useful molecular weights and synthetic flexibility. Second, Suzuki reactions tolerate other functional groups that are commonly used in photoresist applications, which allows easy expansion of this concept to other sensitive groups. Most importantly, unprotected hydroxyl and phenol groups are tolerated by this chemistry. Third, Suzuki polycondensation enables the synthesis of rigid polymer structures that contain a high fraction of aryl carbons, giving access to polymers with excellent glass transition temperatures. Finally, all monomers can be prepared from commercial starting materials in one to three steps, making this approach synthetically straightforward and economically viable for the desired application. The most convenient synthetic access to partially CHVEprotected polymers was copolymerization of unprotected and protected dibromophenol with an acetal-linked bisboronic ester at a defined ratio. Among the investigated polymerization conditions, the (t-Bu)3Pd(crotyl)Cl/K3PO4 catalyst/base system gave particularly high condensation yields and was compatible with incorporation of building blocks that contain free phenol functionalities. During initial developer wetting and substrate adhesion tests, it became apparent that protection degrees of pBEBA2,4-DBP-CHVE between 0% and 30% were ideal for lithographic applications. Lithographic formulations of example polymers in this range were developed and performance was tested by EUV photolithography. EUV line and space patterning tests of optimized formulations showed resolutions of 24 nm (30% CHVE) and 22 nm (0% CHVE) with line width roughness values of 8.1 and 5.7 nm, respectively.

main contaminant in the acid-treated polymer. The sensitivity of 22 to oxidation confirmed the predicted hydroquinone/ catechol-like redox activity of 22 discussed in the Introduction, which should be beneficial to EUV performance. Formulation Development and Lithographic Performance. Initial efforts were focused on identifying the optimum level of phenol protection. Using polymers p1−p4, and p7, having CHVE-protection degrees between 100 and 0%, model photoresists R1−R5 were formulated and assessed for film quality, substrate adhesion, and WCA.50 Polymers with 30% CHVE and below exhibited the best performance. Additional details are provided in the Supporting Information. Based on this finding, EUV photoresist formulations were further developed50 using polymers of the p5 or p6 type containing 30% CHVE (resist R6) and fully unprotected polymer of the p7 type (resists R7 and R8). The formulations were spin coated onto silicon wafers and submitted to sensitivity testing, followed by line-and-space patterning testing at Sematech Albany eMET. The 1:1 line space patterns for the submitted resists are shown in Figure 5. Resist R6 (left panel) had a minimum resolution of 24 nm with a LWR of 8.1 nm. Resist R7 (middle panel) exhibits a minimum resolution of 22 nm with a LWR of 8.6 nm, and a more improved resist R8 (right panel) showed minimum resolution of 22 nm with a LWR of 5.7 nm.50 Additional lithographic data are presented in Table S13. The process window of each resist is presented in the form of a lithographic Bossung plot51 shown in Figures S12−S14. The large process window of photoresist R8 is clearly visible in the focus exposure matrix shown in Figure S15. It was demonstrated that the new polymers largely dictate the lithographic performance and successfully enabled a significant advancement in the ultimately attainable EUV resolution and LWR trade-off at the time of this work. In comparison with a then state-of-the-art PMMA based EUV photoresist, this class of photoresist polymers also showed superior behavior with regards to pattern collapse (likely due to a superior Tg) and process window.50 Further improvement of LWR, possibly through formulation improvement, or by understanding the correlation of polymer Mw reduction and resist dissolution, will the focus of future work.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01038. Computational studies, experimental details, including synthesis of monomers and polymers, calculated properties of model polymers, details on selective deprotection of polymers, details on photoresist formulation optimization, details and spectra of monomer and polymer decomposition experiments, spectra and analytical data of monomers (1H NMR, 13C NMR, FTIR, UV−vis, and HRMS spectra), analytical data of polymers (DSC,

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CONCLUSION A novel class of backbone degradable poly(aryl acetal) photoresist polymers was developed using Suzuki polycondensation, and their fragmentation upon acid treatment was demonstrated. The hydrophilicity of the polymers was modulated by varying the degree of CHVE protection of the phenolic side chains to optimize the utility of a polymer matrix in EUV photoresist formulations. H


Article

Macromolecules

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TGA, GPC, 1H NMR, and 13C NMR spectra), lithographic data (sensitivity, process windows, topdown images) (PDF)

imaging with a low activation energy chemically amplified photoresist. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 2004, 22 (6), 3479−3484. (11) Wallraff, G. M.; Medeiros, D. R.; Larson, C. E.; Sanchez, M.; Petrillo, K.; Huang, W.-S.; Rettner, C.; Davis, B. W.; Sundberg, L.; Hinsberg, W. D.; Houle, F. A.; Hoffnagle, J. A.; Goldfarb, D.; Temple, K.; Bucchignano, J. In Studies of Acid Diffusion in Low Ea Chemically Amplified Photoresists, Microlithography 2005, Proc. of SPIE, 2005, Vol. 5753, pp 309−318. (12) Ito, H.; Breyta, G.; Hofer, D.; Sooriyakumaran, R.; Petrillo, K.; Seeger, D. Environmentally Stable Chemical Amplification Positive Resist: Principle, Chemistry, Contamination Resistance, And Lithographic Feasibility. J. Photopolym. Sci. Technol. 1994, 7 (3), 433−447. (13) Allen, R. D.; Wallraff, G. M.; Hinsberg, W. D.; Simpson, L. L. High performance acrylic polymers for chemically amplified photoresist applications. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 1991, 9 (6), 3357−3361. (14) Kunz, R. R.; Allen, R. D.; Hinsberg, W. D.; Wallraff, G. M. In Acid-Catalyzed Single-Layer Resists for ArF Lithography; 1993; p 5. (15) Wallow, T. I.; Houlihan, F. M.; Nalamasu, O.; Chandross, E. A.; Neenan, T. X.; Reichmanis, E. In Evaluation of Cycloolefin-Maleic Anhydride Alternating Copolymers As Single-Layer Photoresists for 193nm Photolithography, Advances in Resist Technology and Processing XIII, International Society for Optics and Photonics: 1996; pp 355− 365. (16) Schlesinger, S. I. Epoxy photopolymers in photoimaging and photofabrication. Polym. Eng. Sci. 1974, 14 (7), 513−515. (17) Fech, Jr., J.; Poliniak, E. S. Electron beam recording media and method of recording. US Patent 3940507A, 1974. (18) Ito, H.; Willson, C. G. Chemical amplification in the design of dry developing resist materials. Polym. Eng. Sci. 1983, 23 (18), 1012− 1018. (19) Tarascon, R. G.; Reichmanis, E.; Houlihan, F. M.; Shugard, A.; Thompson, L. F. Poly(t-BOC-styrene sulfone)-based chemically amplified resists for deep-UV lithography. Polym. Eng. Sci. 1989, 29 (13), 850−855. (20) Kozawa, T.; Tagawa, S. Radiation Chemistry in Chemically Amplified Resists. Jpn. J. Appl. Phys. 2010, 49 (3R), 030001. (21) Thackeray, J. W. Materials challenges for sub-20-nm lithography. J. Micro/Nanolithogr., MEMS, MOEMS 2011, 10 (3), 033009-1−033009-8. (22) Kozawa, T.; Tagawa, S.; Santillan, J. J.; Itani, T. Impact of Nonconstant Diffusion Coefficient on Latent Image Quality in 22 nm Fabrication using Extreme Ultraviolet Lithography. J. Photopolym. Sci. Technol. 2008, 21 (3), 421−427. (23) Wallraff, G. M.; Hinsberg, W. D.; Houle, F. A.; Morrison, M.; Larson, C. E.; Sanchez, M.; Hoffnagle, J.; Brock, P. J.; Breyta, G. In Experimental Method for Quantifying Acid Diffusion in Chemically Amplified Resists, Proc. SPIE, 1999, Vol. 3678, pp 138−148. (24) Haller, I.; Hatzakis, M.; Srinivasan, R. High-resolution Positive Resists for Electron-beam Exposure. IBM J. Res. Dev. 1968, 12 (3), 251−256. (25) Houlihan, F. M.; Bouchard, F.; Frechet, J. M. J.; Willson, C. G. Thermally depolymerizable polycarbonates. 2. Synthesis of novel linear tertiary copolycarbonates by phase-transfer catalysis. Macromolecules 1986, 19 (1), 13−19. (26) Schlüter, A. D. The tenth anniversary of Suzuki polycondensation (SPC). J. Polym. Sci., Part A: Polym. Chem. 2001, 39 (10), 1533− 1556. (27) Sakamoto, J.; Rehahn, M.; Wegner, G.; Schlüter, A. D. Suzuki Polycondensation: Polyarylenes à la Carte. Macromol. Rapid Commun. 2009, 30 (9−10), 653−687. (28) Gokan, H.; Esho, S.; Ohnishi, Y. Dry Etch Resistance of Organic Materials. J. Electrochem. Soc. 1983, 130 (1), 143−146. (29) Kunz, R. R.; Palmateer, S. C.; Forte, A. R.; Allen, R. D.; Wallraff, G. M.; Di Pietro, R. A.; Hofer, D. C. In Limits to Etch Resistance for 193-nm Single-Layer Resists, 1996; pp 365−376. (30) Tagawa, S.; Nagahara, S.; Iwamoto, T.; Wakita, M.; Kozawa, T.; Yamamoto, Y.; Werst, D.; Trifunac, A. D. In Radiation and

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected](until 03/2019), matthias.ober@ corteva.com (thereafter). ORCID

Matthias S. Ober: 0000-0002-4719-8443 Notes

The authors declare no competing financial interest. M.O. is now affiliated with Corteva Agriscience, Agriculture Division of DowDuPont, Dow AgroSciences LLC, 1710 Bldg. 112, Midland, MI 48674.

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ACKNOWLEDGMENTS The authors acknowledge Bruce Bell, Lynn Stiehl, and Mark Rickard for LC-MS and IR analyses and Siaka Yusuf for neutron activation studies. Furthermore, the authors thank Jin Wuk Sung, Suzanne Coley, Matthew Meyer, Matthew Christiansen, Valeriy Ginzburg, John Kramer, and Peter Trefonas for helpful discussions.

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Backbone Degradable Poly(aryl acetal) Photoresist Polymers

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