Terpolymerization of Styrenic Photoresist Polymers: Effect of RAFT

Article pubs.acs.org/Macromolecules

Terpolymerization of Styrenic Photoresist Polymers: Effect of RAFT Polymerization on the Compositional Heterogeneity Yi Guo,† David J. T. Hill,‡ Andrew K. Whittaker,†,§,∥ Kevin S. Jack,*,⊥ and Hui Peng*,†,∥ †

Australian Institute for Bioengineering and Nanotechnology, ‡School of Chemistry & Molecular Biosciences, §Centre for Advanced Imaging, ∥Australian Research Council Centre for Convergent Bio-Nano Science and Technology, and ⊥Centre for Microscopy and Microanalysis, The University of Queensland St Lucia, Brisbane, Queensland 4072, Australia S Supporting Information *

ABSTRACT: The heterogeneity of the chemical structure of photoresist polymers, both within and between the polymer chains, is believed to contribute to the phenomenon of line edge roughness in high-resolution lithography. The copolymerization and terpolymerization of three important photoresist monomers4-acetoxystyrene (AOST), styrene (Sty), and tert-butyl acrylate (tBA)are examined in detail in this work. In particular, the effect of the use of a reversible addition−fragmentation chain transfer (RAFT) agent in these reactions is reported. The pairwise copolymerization reactivity ratios for the RAFT polymerizations are close to but different from those measured under the same conditions but in the absence of the RAFT agent. The differences are suggested to be related to the effect of the RAFT agent on the environment of the locus of polymerization. The course of the terpolymerization reactions can be well predicted from the copolymer reactivity ratios and the classical Alfrey−Goldfinger expression, indicating that the reactions in this system conform to terminal model of reactivity. Calculations of the expected sequence distributions as a function of composition and conversion clearly demonstrate that the polymers prepared under RAFT conditions are significantly more homogeneous than their counterparts prepared using conventional free radical polymerization. The significance of these results for the performance of the RAFT terpolymers in photoresist formulations is highlighted.

1. INTRODUCTION Chemically amplified resists (CAR), first introduced in the early 1980s,1 are widely used in the semiconductor industry for the production of microelectronic devices.2 The CAR contains a polymer resin that provides most of the properties of the photoresist film, an acid-labile group on the polymer to provide a solubility switch before and after radiation exposure as well as a source of acid, the photoacid generator (PAG).3 The acidlabile group in the unexposed resist polymer completely inhibits the dissolution of the photoresist by protecting the developer-soluble group (e.g., hydroxyl group) with an insoluble group (e.g., tert-butyl group). Under irradiation, the PAG undergoes photolysis to form photoacids. A subsequent postexposure baking process allows the photoacid to diffuse through the polymer matrix and catalytically deprotect the acidlabile groups to change the local solubility in developing solutions. The photochemically induced solubility change (a result of the change in polymer polarity) provides either a positive or negative image of the latent image depending on the polarity of the developer solvent. The versatility and sensitivity of this method are remarkable and currently allow the production of devices with feature sizes below 30 nm.4 Roughness of printed features is an important problem currently confronting the manufacturers of integrated circuits. Such roughness, termed line-edge roughness (LER), or linewidth roughness (LWR) is predicted to significantly degrade © XXXX American Chemical Society

performance of devices with small feature sizes. Low-frequency roughness can potentially increase severely the failure rate of memory devices.5 On the other hand, high-frequency roughness can cause local variations in voltage in transistors and hence lead to variations in leakage or threshold voltages.6,7 The origins of line edge roughness have been the subject of intense investigation and factors such as acid diffusion,8−10 shot noise,11−14 and mask roughness15−21 identified as major contributors.22−25 However, it has been suggested that the inherent heterogeneity of the photoresist may also strongly contribute to the generation of rough features upon processing. This includes heterogeneity in the distribution of photoacid generator (PAG) or other components such as base quencher26 and in the resist polymer itself. Heterogeneity of the resist polymer can be defined as that due to the distribution of molar masses commonly seen in free radical polymerization27 and also the intra- and interchain distribution of placement of different monomer units.28,29 The effect of polymer heterogeneity on photolithographic imaging performance including on LER has been the subject of discussion and investigation within the community for several decades. The first most obvious source of heterogeneity is the Received: April 1, 2015 Revised: May 4, 2015

A

DOI: 10.1021/acs.macromol.5b00683 Macromolecules XXXX, XXX, XXX−XXX

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locus of polymerization, i.e., the terminal radical species. In conventional free radical polymerization (FRP), polymer chains are created continually as long as there is a source of initiating species, and the growing chains have very short lifetimes. Therefore, their composition depends not only on the relative reactivity of the monomers to the chain ends but also on the ever-evolving composition of the mixture of unreacted monomers. Thus, chains rich in the most reactive monomer are formed first, and on progressive depletion of that monomer the composition of the chains tends toward the less reactive monomer. Without control over the monomer feed ratio the structure of the polymers will be varied from chain to chain; that is, the polymer will display interchain heterogeneity. In techniques of controlled radical polymerization such as RAFT, metal-catalyzed, and nitroxide-mediated polymerization,45−47 propagation of each polymer chain continues throughout the polymerization, and thus the composition of the individual polymer chains may evolve as the polymerization proceeds. In this case the polymer chains possess intrachain heterogeneity. This has been exploited by many to produce gradient polymers for a range of potential applications.48−50 The preceding discussion concerning the effects of chemical and structural heterogeneity on the performance of lithographic polymers, and the growing application of controlled radical polymerization to such polymers, necessitates a consideration of how such methods affect the polymer structure and potentially how they can be exploited or controlled. In this study we investigate for the first time the compositional homogeneity in statistical polymers of 4acetoxystyrene (AOST), styrene (Sty), and tert-butyl acrylate (tBA) prepared using both conventional FRP and RAFT polymerization methods. Initially, the copolymerization of the monomer pairs was examined. The reactivity ratios for the binary copolymerizations were determined by nonlinear leastsquares (NLLS) fits to experimental data of the feed ratios and corresponding polymer compositions, measured by 1H NMR spectra. Subsequently, the values of the binary reactivity ratios were used via a probabilistic method51 to calculate the expected compositions in the terpolymerization. The composition of the terpolymers was obtained by quantitative 13C NMR spectroscopy and was found to be significantly more homogeneous, in terms of monomer distribution, for the RAFT terpolymerization to high conversion. The results have important implications for not only polymers for lithography but also any system where chemical heterogeneity is likely to be important.

distribution of resist polymer chain lengths arising in all conventional synthetic processes. In an early study Barclay and colleagues30 examined the effect of molecular weight and architecture (block vs statistical copolymers) of copolymers of styrene and 4-acetoxystyrene prepared by nitroxide-mediated polymerization on the rate of dissolution in aqueous developer. While the results conformed to expected trends on changes in molecular weight, they also observed unusually fast dissolution of a block copolymer which may have been due to microphase separation. This work occurred at roughly the same time as Willson and his colleagues31−33 reported extensively on the effect of molecular weight and molar mass dispersity on the rates of dissolution of phenolic polymers. The authors proposed what they term the “critical-ionization” model, in which a minimum or critical fraction of ionizable sites on a particular polymer chain must be ionized for it to become soluble. Of relevance to this study the model was able to predict the effect of molecular weight on the surface roughness to a remarkably close extent. More recently, Kim et al.34 reported modest improvements in LER in methacrylate resist polymers of narrow molar mass dispersity prepared by RAFT polymerization methods compared with conventional polymers. However, there was no obvious trends in performance with changes in molecular weight under their conditions. Other groups have used the RAFT process to prepare materials for lithographic applications35,36 including Lee et al.,37,38 who prepared methacrylate polymers for contact hole printing. They were able to demonstrate a decrease in the rate of thermal flow for the narrow molar mass dispersed polymers and thus improved depth of focus especially for densely patterned features. Also of note is the work of Sohn and co-workers,39 who have prepared a broad range of resists polymer based on methacrylate chemistry and in particular examined the effect of differences in comonomer reactivity ratios on the polymer composition. In their 2011 paper they report modest improvements in the LER for the polymer of narrow molar mass dispersity; however, they acknowledge that the development conditions and likely the polymer composition had not been fully optimized. Finally, it should be noted that the resist formulation also consists of additives such as the photoacid generator (PAG) and base quencher and that the distribution of these components may also profoundly affect the lithographic processes and impact negatively on the roughness of the features. For example, Fedynyshyn and colleagues40 have investigated the effect of PAG segregation on LER and speculated that changes in the rates of dissolution on the nanometer scale can be significant. These and other observations have led to intensive studies of polymer-bound PAGs which can show improved roughness compared with resists containing low molar mass PAG molecules.41−44 In summary, these studies clearly demonstrate that control over the molecular structure of the polymer, and in particular of the distribution of ionizable groups, can have a significant impact on the performance of lithographic polymers. The objective of this paper is to investigate approaches to minimizing such chemical heterogeneity within resist polymers. However, the findings of this work have implications well beyond the field of lithography. The introduction of methods of so-called controlled radical polymerization is likely to have a profound effect on the structure of polymers intended for use in photoresists. All photoresist polymers are composed of more than one monomer which have different inherent reactivity to the

2. MATERIALS AND CHARACTERIZATION Materials. 4-Acetoxystyrene (AOST; 96%; Sigma-Aldrich), styrene (Sty; 98%; Sigma-Aldrich), and tert-butyl acrylate (tBA; 99%; SigmaAldrich) were purified by passing through an activated basic alumina column to remove the inhibitor. 2,2′-Azobis(2-methylpropionitrile) (AIBN) in toluene solution (0.2 M, Aldrich) was first concentrated under vacuum and then followed by recrystallization from methanol. Chromium(III) acetylacetonate (Cr(III)AcAc, 97%, Aldrich), hydrazine hydrate (50−60%, Sigma-Aldrich), 1,4-dioxane (anhydrous, 99.8%, Sigma-Aldrich), and methanol (99%, RCI Labscan) were used as received. Deionized (DI) water was produced by a Milli-Q reverse osmosis system and had a resistivity of 18.4 mΩ cm−1. Deuterated NMR solvents (CDCl3 and DMSO) were purchased from Cambridge Isotope Laboratories. Characterization. Nuclear magnetic resonance (NMR) was used to determine the structure of the synthesized compounds as well as to determine the polymer compositions. The copolymer compositions B


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Table 1. Copolymer Composition Data for the FRP of AOST, Sty, and tBA Using AIBN as the Initiator in 1,4-Dioxane at 65 °C AOST−Sty conv (%) 5.6 7.2 8.8 4.9 9.8 9.0 7.2 8.2

fAOST

FAOST

0.0801 0.1053 0.1902 0.2307 0.3767 0.4396 0.5201 0.5792 0.6281 0.6827 0.7242 0.7664 0.8102 0.8492 0.9123 0.9322 rAOSTb = 1.254; rSty = 0.773 SFc = 0.0046

AOST−tBA errora

conv (%)

0.0072 0.0083 0.0066 0.0082 0.0111 0.0062 0.0093 0.0067

7.5 6.8 9.2 10.2 5.6 7.7 8,2 7.4

fAOST

FAOST

0.1201 0.2663 0.2376 0.4097 0.4102 0.5545 0.5312 0.6421 0.6300 0.6982 0.7013 0.7615 0.8102 0.8356 0.8901 0.9105 rAOST = 1.082; rtBA = 0.292 SF = 0.0058

Sty−tBA error

conv (%)

0.0127 0.0104 0.0110 0.0093 0.0091 0.0054 0.0098 0.0067

6.6 7.8 9.8 9.1 11.2 8.2 3.5 7.7

fsty

Fsty

0.1087 0.2467 0.2102 0.3692 0.4008 0.5339 0.5187 0.6012 0.6134 0.6682 0.7202 0.7404 0.8062 0.8151 0.9082 0.9045 rSty = 0.872; rtBA = 0.289 SF = 0.0046

error 0.0047 0.0025 0.0029 0.0080 0.0050 0.0022 0.0058 0.0021

a

The standard deviation in the mean copolymer composition (F) determined from three NMR measurements. bReactivity ratios determined using the program REACT. cSF is the standard error in the copolymer mole fraction calculated from the difference between the experimental and calculated values.

Table 2. Copolymer Composition Data for the RAFT Polymerization of AOST, Sty, and tBA Using EMP as the RAFT Agent and AIBN as the Initiator in 1,4-Dioxane at 65 °C AOST−Sty conv (%) 7.6 5.2 9.8 5.9 7.8 7.8 7.2 4.2

fAOST

FAOST

0.1010 0.1464 0.1982 0.2589 0.3021 0.3685 0.3922 0.4815 0.5033 0.5887 0.6108 0.6677 0.7002 0.7751 0.7967 0.8555 rAOSTb = 1.384; rSty = 0.691 SFc = 0.013

AOST−tBA errora

conv (%)

0.0072 0.0090 0.0048 0.0070 0.0036 0.0087 0.0050 0.0091

5.5 4.8 9.2 7.7 11.4 9.7 8.0 8.4

fAOST

FAOST

0.1030 0.1757 0.2063 0.2988 0.3022 0.4035 0.4012 0.4855 0.4981 0.5633 0.7063 0.7323 0.8001 0.8006 0.8920 0.8817 rAOST = 0.896; rtBA = 0.468 SF = 0.0015

Sty−tBA error

conv (%)

0.0019 0.0091 0.0031 0.0020 0.0090 0.0070 0.0087 0.0063

9.2 8.8 7.8 10.1 11.2 7.2 8.5 6.7

f Sty

FSty

0.1078 0.2247 0.2189 0.3631 0.3201 0.4512 0.4087 0.5155 0.4876 0.5674 0.6012 0.6397 0.8081 0.7990 0.9019 0.8971 rSty = 0.799; rtBA = 0.326 SF = 0.018

error 0.0032 0.0040 0.0031 0.0020 0.0090 0.0070 0.0087 0.0063

a

The standard deviation in the mean copolymer composition (F) determined from three NMR measurements. bReactivity ratios determined using the program REACT. cSF is the standard error in the copolymer mole fraction calculated from the difference between the experimental and calculated values. were determined from the 1H NMR spectra in solutions in CDCl3 on a Bruker Avance 300 MHz high-resolution NMR spectrometer (Pulse Program = zg45, p1 = 11.90 μs, NS = 32, d1 = 2 s, AQ = 4.56 s). The compositions of the terpolymer were determined by quantitative 13C NMR spectra on a Bruker Avance 400 MHz high-resolution NMR spectrometer; 20 mg/mL Cr(III)AcAc was used to reduce the longitudinal (T1) relaxation times of the 13C nuclei (Pulse Program = zgig, p1 = 9.10 μs, NS = 1024, d1 = 5 s, AQ = 1.42 s, TD0 = 4). Size exclusion chromatography (SEC) was performed on a Waters SEC (model number 717 with modules 1515, 2414, and 2489) equipped with RI and UV detectors and Styrogel HT16 and HT3 columns run in series. THF was used as an eluent at a temperature of 40 °C and a constant flow rate of 1 mL min−1. The SEC system was calibrated using linear polystyrene standards, ranging from 1350 to 1 300 000 g/mol (Polymer Laboratories).

determine the monomer composition fAOST (Table 1). Anhydrous 1,4-dioxane was used as solvent to keep a constant monomer concentration of 5 mol L−1 and an initiator concentration of 0.05 mol L−1. The polymerization was carried out at 65 °C for 15−30 min and quenched by submerging the reaction vessel into an ice bath. The polymer was collected by precipitation twice using 1,4-dioxane/ methanol at a factor of at least 20 by volume and consequently vacuum-dried at 35 °C overnight. For polymerization with fAOST above 0.5, a mixture of methanol and water (4/1, w/w) was used for precipitation. The conversion was measured by gravimetry and in all cases kept below 10%. The copolymerizations of AOST and tBA were carried out in a similar manner; however, hexane was used as the solvent for precipitation. For the copolymerization of Sty and tBA the polymers were precipitated into hexane, except when f Sty was below 0.5, in which case precipitation was carried out into a mixture of methanol and water (1/1, w/w). Reversible Addition−Fragmentation Chain Transfer (RAFT) Polymerization. Reaction mixtures having feed ratios shown in Table 2 were prepared in a similar manner to above. The monomer feed ratio ( f) was determined by 1H NMR spectra before introducing the RAFT agent and initiator. The ratio of [monomer]:[RAFT agent]:[AIBN] was kept at 800:2:1 in all cases. The polymerizations were carried out in 1,4-dioxane ([monomer] = 1 mol L−1) at 65 °C for 1−2 h, followed by quenching in an ice bath. The conversion was kept below 10%, and polymer product was collected and vacuum-dried as described above. Copolymerization kinetics for the polymerization of AOST with tBA ( fAOST = 0.2) were investigated by collecting the 1H spectra of the

3. EXPERIMENTAL PART Synthesis of the RAFT Agent. 2-Ethylsulfanylthiocarbonylsulfanyl-2-methylpropionic acid (EMP), RAFT agent, was synthesized according to a method described in the literature.52 The purity of the product was confirmed as greater than 98% by 1H NMR spectroscopy. Conventional Free Radical Polymerization (FRP). Copolymers of AOST and Sty were prepared by conventional free radical polymerization with varying initial AOST feed ratio (fAOST) from 10 to 90%. The required amounts of the monomers were weighed into glass vials sealed with rubber septa and equipped with Teflon magnetic stirrer bars. Before introducing the initiator AIBN, a small fraction of monomer mixture was sampled for 1H NMR measurement to C


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Table 3. Summary of Copolymerization Reactivity Ratios Determined in This Work and Values Reported Previously in the Literature this work

literature studies

polymers

FRP

RAFT

method

FRP

method

AOST−Sty AOST−tBA Sty−tBA

rAOST = 1.254; rSty = 0.773 rAOST = 1.082; rtBA = 0.292 rSty = 0.872; rtBA = 0.289


REACT REACT REACT


Mayo−Lewis CONTOUR Mayo−Lewis

crude reaction mixtures sampled every 5 min for FRP and every 15 min for RAFT polymerization. Free Radical and RAFT Terpolymerization. The terpolymerization of AOST, Sty, and tBA was conducted in the same manner as the copolymerization reactions. A series of monomer mixtures were prepared with varying monomer feed ratios (listed in Tables 3 and 4).

lithography. It has been demonstrated for example that molecular weight and molecular weight distribution can influence patterning performance and impact upon line edge roughness (LER).27,31,57 This has led to a growing interest in the use of so-called controlled radical polymerization methods, such as RAFT, in the preparation of resist polymers, and to date some promising results have been obtained. It is well know that techniques such as RAFT afford control over molecular size and molar mass dispersity during free radical polymerization. However, this is not the critical issue to be addressed in this paper, which instead is concerned with the heterogeneity of monomer placement along the polymer chain. The fundamental differences in mode of reaction from conventional free radical polymerization means that more subtle differences in structure may arise during RAFT polymerization. Specifically, because the polymer chains are growing continually in controlled radical polymerization, in a copolymerization reaction changes in the composition of the monomer pool will result in variations in instantaneous polymer composition along the polymer chain. In conventional radical polymerization where the lifetimes of the chains are short, this variation in composition is instead evident from chain to chain. It is the objective of this study to examine the effect of the RAFT process on the polymerization, and specifically monomer placement, of a typical resist polymer, namely a terpolymer of styrene (Sty), 4-acetoxystyrene (AOST), and tert-butyl acrylate (tBA). Initially, the reactivity ratios for the copolymerization of pairs of these monomers were measured for both forms of polymerization, FRP and RAFT. Differences in reactivity ratios were observed and are discussed. The composition of terpolymers of the trio of monomers was examined and compared with the results expected assuming the rate of monomer addition to the growing chain conformed to the reactivity in the copolymerizations. Analysis of the Copolymer Compositions. A series of copolymers spanning the full composition range were prepared by both conventional and RAFT polymerization, as described above. As expected, the addition of the RAFT agent to the polymerization mixture resulted in polymers having wellcontrolled molar masses and narrow molecular weight distributions. Representative molecular weights are listed in Table S2 of the Supporting Information. The composition of these copolymers was determined by quantitative 1H NMR spectroscopy. As described above, the conversion of monomer to polymer was restricted to less than 10% to ensure nearconstant monomer feed composition during the polymerization. Examples of 1H NMR spectra of poly(AOST-stat-Sty), poly(AOST-stat-tBA), and poly(Sty-stat-tBA) copolymers synthesized via conventional free radical polymerization (FRP) are shown in Figures 1, 2, and 3, respectively. The assignments to the spectra are shown in the figures, with the peaks and structures labeled with corresponding letters. In each copolymerization the polymer composition (F) was determined from the integrals of regions 1 (I1) and 2 (I2). In the

Table 4. Summary of Feed Ratios and the Experimental and Theoretical Polymer Compositions for Terpolymers Produced by the Conventional Free Radical Terpolymerization of AOST, Sty, and tBA experimental

a

predicted

conv (%)

fAOST:f Sty:f tBA

FAOST:FSty:FtBA

FAOST:FSty:FtBAa

9.21 7.82 6.43 5.92 7.18

0.17:0.50:0.33 0.32:0.41:0.37 0.34:0.36:0.30 0.55:0.25:0.20 0.63:0.22:0.15

0.18:0.57:0.25 0.26:0.42:0.32 0.33:0.42:0.25 0.58:0.21:0.25 0.60:0.23:0.17

0.19:0.53:0.28 0.34:0.37:0.28 0.39:0.35:0.26 0.60:0.22:0.18 0.65:0.20:0.15

Predicted using the Alfrey−Goldfinger eq 4 based on the feed ratios.

The reported feed ratios are based on 1H NMR measurements, and for the RAFT terpolymerization the [monomer]:[RAFT agent]:[AIBN] was kept at 800:2:1 in all cases. After purification and vacuum drying, the conversion was obtained by gravimetry and was confirmed to be below 10%. The terpolymer composition was determined by quantitative 13C NMR spectroscopy. The average molecular weights and molar mass dispersities for representative copolymer/terpolymer samples synthesized via both FRP and RAFT are summarized in Table S2. As expected, the molar mass dispersities for the copolymers and terpolymers prepared by RAFT were close to 1.1 for all materials prepared. Nonlinear Least-Squares Analysis of Copolymerization Data. The computer program REACT, based on a NLLS approach and developed by Hill and co-workers,53−55 was used to calculate unbiased estimates of values of the reactivity ratios (the so-called best values) for the terminal model. In the calculations the data points were equally weighted across the composition range. Any drift in the feed composition with conversion was modeled explicitly by sequential calculations using a step function with a step size of 0.005% of the conversion interval, described previously in detail by Hill et al.55 Using smaller step sizes did not make significant changes to the calculated reactivity ratios. The uncertainty in the calculated reactivity ratios was plotted as a 95% joint confident interval (JCI) established from Fisher’s criteria and the sum of the weighted squared deviations (SS) calculated for the best values of the reactivity ratios. In addition, for comparison purposes, the program Contour developed by van Herk,56 based on a similar approach, was applied to the experimental data. We note that the REACT and Contour programs produce identical best values for the reactivity ratios for the same data set assuming zero conversion of monomer to polymer.

4. RESULTS AND DISCUSSION It has been postulated by researchers in academia and industry that chemical heterogeneity of resist polymers has the potential to be a significant source of the roughness appearing after patterning and development of resists in commercial D


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Figure 1. 1H NMR spectrum of an AOST−Sty copolymer with FAOST = 0.85 (in CDCl3) synthesized via FRP using AIBN as initiator in 1,4dioxane at 65 °C. The molar ratio of AOST and Sty in the copolymer was calculated based on the integrals I1 (∼6.0−7.2 ppm; peaks d, e, and f) and I2 (∼2.1−2.5 ppm; peak c) of the corresponding peaks using eq 1 as described in the text.

Figure 3. 1H NMR spectrum of a Sty-tBA copolymer with FSty = 0.53 (in CDCl3) synthesized via FRP using AIBN as the initiator in 1,4dioxane at 65 °C. The molar ratio of Sty and tBA in the copolymer was calculated based on the integrals I1 (∼6.0−8.0 ppm; peaks d, e, and f) and I2 (∼0.6−3.0 ppm; peaks a, b, and c) of the corresponding peaks using eq 3 as described in the text.

In the NMR spectra of copolymers of Sty-tBA (Figure 3, f Sty = 0.40), I1 is due to the five protons of the aromatic group in Sty. I2 is the sum of nine protons on tert-butyl group in tBA units and the methylene and methine protons of the polymer backbone for both Sty and tBA repeating units. The mole fraction of Sty in poly(Sty-stat-tBA) (FSty) was determined by using eq 3: 12I1 FSty = 9I1 + 5I2 (3) The copolymer compositions determined from the NMR spectra in this manner for the three pairs of monomers are listed in Table 1. In addition, the copolymer compositions for the materials prepared in the RAFT polymerizations are listed in Table 2. The compositions in Table 2 were calculated after taking into account the small contribution from the signals from the RAFT agent which fall in the region 0.4−2.5 ppm. In Figure 4 the copolymer compositions at low conversion measured by NMR, for both the FRP and RAFT polymerizations, are plotted as a function of the monomer feed ratio. Representative 13C NMR spectra of the copolymers prepared in this work, along with the peak assignments, are shown in Figures S2−S4. In all cases a small but significant difference in the copolymer composition is observed across the whole monomer feed range. The differences are particularly pronounced for the monomer pair AOST with tBA. These effects will be discussed below. Reactivity Ratios of Copolymerization. The terminal model for describing the copolymerization of two monomers assumes that the radical reactivity depends only on the nature of the terminal unit and that it is independent of chain length. It is the simplest model for describing copolymerization, and the relationship between the copolymer composition and the feed composition can be described using just two reactivity ratios.58 In 1965, Tidwell and Mortimer59 first described the advantages of using a nonlinear least-squares (NLLS) regression method to estimate unbiased values for the reactivity ratios of copolymerization for the terminal model. Later, Hill et al.53−55 extended the NLLS method to this and higher-order copolymerization models to account for data of different

Figure 2. 1H NMR spectrum of an AOST-tBA copolymer with FAOST = 0.76 (in CDCl3) synthesized via FRP using AIBN as the initiator in 1,4-dioxane at 65 °C. The molar ratio of AOST and tBA in the copolymer was calculated based on the integrals I1 (∼6.0−7.2 ppm; peaks e and f) and I2 (∼0.6−2.8 ppm; peaks a, b, c, and d) of the corresponding peaks using eq 2 as described in the text.

copolymerization of AOST with Sty (Figure 1, fAOST = 0.81), the integral I1 is due to the four aromatic protons of AOST and the five aromatic protons of Sty. The integral I2 corresponds to the three methyl protons on the acetoyl groups in AOST units. The mole fraction of AOST in the poly(AOST-stat-Sty) (FAOST) is therefore calculated using eq 1:

FAOST =

5I2 3I1 + I2

(1)

In the NMR spectra of copolymers of AOST-tBA (Figure 2, fAOST = 0.70), I1 arises from the four aromatic protons on AOST. I2 is due to contributions from the three methyl protons of AOST, nine methyl protons of the tertiary-butyl group in tBA units, and the methylene and methine protons of the polymer backbone for both AOST and tBA repeating units. The mole fraction of AOST in poly(AOST-stat-tBA) (FAOST) was calculated using eq 2: FAOST =

6I1 3I1 + 2I2

(2) E


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are close to but slightly different from those obtained using REACT (Table S1) because no account has been made for conversion of the monomers in CONTOUR. Again, care was taken to ensure that a true global minimum was found by the NLLS algorithm by selecting numerous initial values of the reactivity ratios (Figure S1). The best values of reactivity ratios determined using the program REACT are listed in Table 1 for conventional free radical polymerization and in Table 2 for RAFT polymerization. The standard errors, SF, in the values of the mole fractions for the copolymerizations obtained from the data fits are given in Tables 1 and 2. The predicted instantaneous copolymer compositions calculated for the full composition range are plotted as solid lines in Figure 4, along with the experimental data points. As demonstrated in Figure 4, the terminal model with the corresponding reactivity ratios represent the experimental data very well for each copolymerization, and in each case the experimental points are randomly distributed about the predicted curve. The error in the experimental copolymer mole fraction (F) arising only from the NMR measurements was found by repeated measurements to be 0.006. The values of the overall standard error in the mole fractions calculated from the reactivity ratios (SF) reported in Tables 1 and 2 are comparable with 0.006, consistent with the NMR measurements being the major source of experimental error, except possibly for the RAFT processes for AOST-Sty and Sty-tBA. In the latter two cases, where the standard errors calculated from the fitted reactivity ratios are greater than 0.006, additional random errors arising from other experimental sources may also contribute to the overall error in the copolymer mole fractions. This is reflected in the larger confidence ellipsoids shown in Figure 5 Figure 4. Experimental copolymer composition (F) vs monomer feed composition (f) for the binary FRP (empty shape) and RAFT (filled shape) copolymerizations of (a) AOST and Sty, (b) AOST and tBA, and (c) Sty and tBA. The full (FRP) and medium dashed (RAFT) lines shows the Mayo−Lewis plots generated from the reactivity ratios determined by fitting to the experimental data. The short dashed lines indicate ideal copolymerization.

reliability (weighting) and to allow for monomer compositional drift during polymerization. The computer program developed by them is called REACT.54,55,60 Subsequently, O’Driscoll and co-workers61 described a NLLS analysis of the terminal model based on an error-in-variables approach (RREVM computer program) which allowed for errors in both the feed and copolymer compositions. More recently, van Herk56,62 has reported an analysis of copolymerization data for the terminal model similar to the methods reported by the earlier workers (CONTOUR computer program); however, their approach does not allow for conversion of monomer to polymer. In the current work the program REACT applied to the terminal model of copolymerization was used to evaluate the best unbiased values of r1, r2, the reactivity ratios as well as the JCIs of those paired values. Care was taken to ensure that a true global minimum was found by the NLLS algorithm by selecting numerous initial values of the reactivity ratios. In addition, we have compared the results obtained with REACT to those from the program CONTOUR developed by van Herk.62 The results of the analysis of the copolymer composition data using CONTOUR are provided in the Supporting Information. As expected, the reactivity ratios determined using CONTOUR

Figure 5. 95% joint confidence intervals associated with the reactivity ratios generated by the REACT program for FRP (full line) and RAFT copolymerization (dashed line) of pairs of AOST, Sty, and tBA using EMP as the RAFT agent and AIBN as the initiator in 1,4-dioxane at 65 °C.

for the corresponding reactivity ratios. However, as stated above, the terminal model with the reactivity ratios given in Tables 1 and 2 adequately describes the experimental data for all of the FRP and RAFT copolymerizations. The reactivity ratios for the copolymerization of AOST-Sty are for FRP, rAOST = 1.254; rSty = 0.773 and for RAFT polymerization, rAOST = 1.384 and rSty = 0.691 (see Table 3). The reactivity ratios for this system reported in the literature63,64 for conventional FRP are rAOST = 1.1−1.4; rSty = 0.89. The small differences can be accounted for by the different reaction conditions. For the polymerization of AOST and tBA the values determined are rAOST = 1.082; rtBA = 0.292 for conventional FRP and rAOST = 0.896; rtBA = 0.468 for RAFT F


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the pre-equilibrium stages of the RAFT reaction, i.e., within the first several percentages of conversion to polymer. It is worthwhile at this point to consider the effect that a change in the macroradical concentrations would have on the measured reactivity ratios. In the analysis presented here we assume that the terminal model of copolymerization applies to the conventional and the RAFT polymerizations. The derivation of the copolymer equation describing the relationship between the concentration of the monomer species and the final copolymer composition is based on consideration of the conditional probabilities of addition of the two monomer species to the respective radical chain ends and importantly makes no assumption of the concentration of the radical species.58 In the Supporting Information we discuss a probabilistic derivation of the copolymer equation considering the involvement of the RAFT agent and conclude that the terminal model copolymer equation is appropriate for RAFT polymerizations. This derivation involves no steady state assumption and relies only on the fact that each monomer sequence in the copolymer has a cross-propagation at its start and finish, so the reactivity ratios should not depend on the ratio of concentrations of the macroradicals. To examine whether the effects observed are indeed restricted to the early stages of polymerization, we have conducted careful polymerizations of AOST with tBA up to approximately 25% conversion and examined the cumulative copolymer composition using 1H NMR (Figure 6). As can be

polymerization. In 2002 Hiroshi Ito and his colleagues reported65 values of rAOST and rtBA as 1.140 and 0.297, respectively, for the conventional FRP in toluene at 60 °C, values very close to those reported here. For the final comonomer pair, Sty and tBA, the values of reactivity were rSty = 0.872 and rtBA = 0.289 for conventional FRP, and rSty = 0.799 and rtBA = 0.326 for RAFT polymerization. RAFT vs Conventional FRP. The RAFT method has previously been applied to the copolymerization of a large range of monomer pairs; however, only a very limited number of detailed reports of copolymerization reactivity ratios have appeared. In a number of reported instances the reactivity ratios determined for polymerization mediated using a RAFT agent are close to those for the corresponding conventional FRP. For example, Luo and Liu66 reported that the evolution of composition during the RAFT copolymerization of styrene and methyl methacrylate conforms to the behavior expected for conventional FRP. Likewise, Favier et al.67 reported that the reactivity ratios for the copolymerization of N-acryloxysuccinimide with N-acryloylmorpholine in the presence of the RAFT agent were close to those previously reported by that group for conventional free radical polymerization. On the other hand, Cuervo-Rodriguez et al.68 studied the copolymerization of MMA with ethyl α-(hydroxymethyl)acrylate (EHMA) and found a lower value of rEHMA in the RAFT copolymerization. The reasons for this discrepancy were not discussed in detail, but it was ascribed to the change in aggregation behavior of the hydrophilic monomer EHMA. Zhang et al.69 reported the copolymerization of nBMA with 2-(perfluorohexyl)ethyl methacrylate and that the reactivity ratios of the copolymerization were slightly but significantly different from the results or conventional copolymerization. Again, the reasons for the discrepancy were not explored in depth. Feldermann and his colleagues70 reported in 2004 that at very low conversions the apparent reactivity ratios in RAFT copolymerization may in some systems differ from the reactivity ratios observed in conventional FRP. The authors used the computer program PREDICI to calculate the evolution of composition in the copolymerizations taking into account the additional reaction pathways of the growing polymer chain. These competing pathways, for example reaction with the RAFT agents, can potentially lead to a change in the macroradical populations; however, this is predicted to be restricted to the pre-equilibrium stage of the polymerization. The 95% JCIs for the three copolymerizations, shown in Figure 5, illustrate that for these systems small but significant differences in apparent reactivity ratios are seen when polymerizations are conducted in the presence of the RAFT agent EMP. It should be noted that for the system Sty−tBA there is partial overlap of the 95% JCIs for the FRP and RAFT polymerizations; however, the 90% confidence intervals (not reproduced here) do not intersect. The most pronounced differences in reactivity ratios are observed for the comonomer pair AOST−tBA with significantly lower rates of incorporation of AOST being observed across the whole composition range in the RAFT polymerization (Figure 4b). Recall again that the results in that figure are for polymerization at low conversion, less than 10%. As discussed above, Feldermann and his colleagues70 suggested that in some RAFT copolymerizations the reactivity ratio of the more reactive species may be enhanced compared with conventional FRP due to the changes in the concentrations of the respective macroradicals. However, they clearly state that these effects are likely to be restricted to

Figure 6. Experimental and predicted (based on Mayo−Lewis equation) copolymer composition (FAOST) as a function of conversion for FRP (full circles: experiment; full line: predicted) and RAFT (empty circles: experiment; dashed line: predicted) copolymerization of AOST and tBA ( fAOST = 0.2) in 1,4-dioxane at 65 °C.

clearly seen, the differences in rate of incorporation of the two monomers for the two methods of polymerization persist up to high conversion. These results are at variance with the predictions of Feldermann et al. and indicate that other explanations for the differences in reactivity ratios should be considered. Potentially the conditions at the growing chain end may be altered by the presence of the RAFT agent. The first possibility to be considered is whether the individual rate parameters may change due to the influence of the polar RAFT agent in the reaction sphere. It is well recognized that the rates of radical reactions can be profoundly affected by the solvent, the so-called kinetic solvent effect.71 As an example, Lalevée et al.72 showed that the transition state barrier in radical addition reactions can be influenced by the polarity of the solvent, by G


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Macromolecules measurement and computer simulation of the addition of an aminoalkyl radical to the double bond methyl acrylate. In a second possibility, the relative concentrations of the monomers in the reaction volume around the radical may be influenced by the presence of the RAFT agent. This is analogous to the “bootstrap effect” introduced by Harwood in 1987.73 Harwood noted in his paper that in the copolymerization of monomers of different polarity the apparent reactivity ratios change markedly depending on the solvent used in the polymerization medium. Critically he observed that polymers produced in different solvents but at the same composition had identical microstructures, i.e., sequence distributions. This suggested that for polymers of the same composition the same set of conditional probabilities applied and that furthermore the reactivity ratios for comonomer addition were the same. It follows from this that the differences in composition in different solvents must arise from differences in the effective concentration of the two monomers at the growing polymer chain end. In other words the monomers are partitioned to different extents at the locus of polymerization. Around the same time Semchikov74 demonstrated in a large number of systems dependence of copolymer composition on initiator concentration and the presence of chain transfer agents. He ascribed these effects to changes in the extent of association of monomers to the growing polymer chain as a function of polymer molecular weight and supported this proposal with measurements of changes in the sorption coefficients of the monomers. Both of these bodies of work demonstrate that changes in the local monomer concentration at the growing polymer chain end can have a large influence on the observed reactivity ratios, and we suggest that the presence of the polar RAFT agent may lead to similar phenomena. Unfortunately, from the experimental data available we are unable to confirm whether such a partitioning effect is acting on these copolymerizations. A more detailed systematic study of the influence of RAFT agent structure and concentration is necessary to understand these phenomena more completely. Terpolymerization of AOST−Sty−tBA. Terpolymers of AOST−Sty−tBA have been important in the manufacture of integrated circuits and remain essential test beds for developments in that field. As discussed in the Introduction, inter- and intrachain variations in composition (compositional heterogeneity) are believed to be in part responsible for the phenomenon of line edge roughness in optical lithography. Accordingly, we have examined the effect of the use of a RAFT agent on the composition of terpolymers of these three monomers. A series of terpolymers were prepared and their compositions determined by quantitative 13C NMR. The 13C NMR spectrum of AOST−Sty−tBA terpolymer ( fAOST:f Sty:f tBA = 0.63:0.22:0.15) synthesized via FRP is reproduced as an example in Figure 7. Assignments to the spectrum are shown on the figure. The mole fractions of AOST, Sty, and tBA repeating units were determined by comparison of the areas of characteristic peaks. The integrated intensities of peaks due to −OOCH3 of the acetoxy group of AOST (21.2 ppm), the methyl carbon (−C−(CH3)3) of tBA (27.8 ppm), and the total integrated intensity of peaks due to aromatic carbons of AOST and Sty (110−130 ppm) were used to calculate the composition of the terpolymers. The results for five different terpolymers are listed in Table 4. Also listed in this table are theoretical polymer compositions calculated using expressions derived by Alfrey and Goldfinger in 1944.75 The Alfrey− Goldfinger equation, which as above assumes the terminal

Figure 7. 13C NMR spectrum (in CDCl3) obtained from the free radical terpolymerization of AOST, Sty, and tBA at 65 °C using AIBN as the initiator at the feed ratio of fAOST:f Sty:f tBA = 0.63:0.22:0.15. The mole ratios of each pair of monomers were calculated from I1 (∼118− 123 ppm, peak h), I2 (∼118−127 ppm, peak o), and I3 = (∼25−31 ppm, peak a) using the relations [AOST]/[Sty] = I1/2I2, [AOST]/ [tBA] = 3I1/2I3, and [Sty]/[tBA] = 3I2/I3.

model of reactivity and uses the reactivity ratios of the three binary pairs, can be expressed as F1: F2: F3 = f1 [f1 /(r31r21) + f2 (r21r32) + f3 (r31r23)][f1 + f2 /r12 + f3 /r13]: f2 [f1 /(r12r31) + f2 (r12r32) + f3 (r32r13)][f2 + f1 /r21 + f3 /r23]: f3 [f1 /(r13r21) + f2 (r23r12) + f3 (r13r23)][f3 + f1 /r31 + f2 /r32] (4)

In this expression F1:F2:F3 are the molar concentrations of the three monomer units in the terpolymer, f1, f 2, and f 3 are monomer feed ratios, and the parameters rij are the corresponding binary copolymerization reactivity ratios. In Table 5 the corresponding experimental and theoretical values Table 5. Summary of Feed Ratios and the Experimental and Theoretical Polymer Compositions for Terpolymers Produced by the RAFT Terpolymerization of AOST, Sty, and tBA experimental

a

predicted

conv (%)

fAOST:f Sty:f tBA

FAOST:FSty:FtBA

FAOST;FSty:FtBAa

7.88 8.23 8.81 5.77

0.15:0.50:0.35 0.29:0.30:0.41 0.55:0.25:0.20 0.59:0.19:0.22

0.18:0.51:0.31 0.28:0.41:0.31 0.55:0.27:0.18 0.61:0.19:0.20

0.19:0.51:0.30 0.27:0.38:0.35 0.55:0.26:0.19 0.60:0.21:0.19

Predicted using the Alfrey−Goldfinger eq 4 based on the feed ratios.

of composition are listed. Strong agreement is observed between experimental and theoretical compositions, validating the assumptions implicit in the Alfrey−Goldfinger equation. Previously one of us has published the derivation of expressions describing the composition and sequence distribution of terpolymers based on the conditional probabilities for monomer addition to the respective radical chain ends.51 This work enables the calculation of the monomer feed composition (Mi), average terpolymer composition (Fi,average), instantaneous terpolymer composition (Fi,instan), and the rate of monomer feed consumption (dF/dc) as a function of conversion of monomer to polymer. These quantities are plotted as a H


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Macromolecules

Figure 8. Predicted (a) instantaneous monomer compositions (M), (b) instantaneous terpolymer compositions (F′i), (c) average terpolymer compositions (Fi), and (d) rate of monomer consumption (dF/dc) as a function of conversion for FRP (solid line) and RAFT terpolymerization (dashed line) of AOST, Sty, and tBA in 1,4-dioxane at 65 °C. In (c) the predicted values are compared with experimentally determined values of Fi as described on the figure, and the molar feed ratios ( fAOST:f Sty:f tBA) are 0.63:0.22:0.15 (full circles) for FRP and 0.59:0.19:0.22 (empty circles) for RAFT.

remains active throughout the entire course of the reaction. The calculations indicate that for the conventional FRP significant interchain variation in monomer composition is to be expected.

function of conversion in Figure 8 for the terpolymerization with a monomer feed ratio (fAOST:f Sty:f tBA) of 0.55:0.20:0.25 for FRP and RAFT copolymerizations. This composition is close to that of the commercial open-source photoresist TER60 from JSR Micro, Inc., which has a composition of FAOST:FSty:FtBA = 0.60:0.20:0.20.76 The calculated behavior shown in Figure 8a shows that for conventional FRP significant drift in the monomer feed ratio is expected as the conversion exceeds approximately 40%. This is naturally reflected in the drift in the terpolymer composition (Figure 8b). In comparison, the compositional drift for the RAFT terpolymerization (dashed line, Figure 8a and 8b) is significantly less up to 80% conversion. These differences are further emphasized in the plots of the calculated rate of monomer consumption shown in Figure 8d. We have also plotted the composition of the terpolymers, measured from the 13 C NMR spectra as a function of conversion for the polymerization at a composition ( f AOST :f Sty :f tBA ) of 0.55:0.20:0.25 (see Figure 8c). Taking into account the scatter in the data points, due to the decreased signal-to-noise ratio in the carbon NMR spectra compared with the proton spectra used to analyze the copolymers, the reduced rate of incorporation of the AOST in the RAFT polymerization is again clearly evident. Overall, the results conform well with the values calculated using the equations of Hill et al.51 In summary, these findings indicate that the RAFT polymerization of this trio of monomers is significantly less susceptible to drift in polymer composition with conversion. This in turn indicates that the polymers synthesized using RAFT in this work are expected to possess a more homogeneous structure on a monomeric level. In the case of RAFT polymerization this refers to intrachain monomer distribution as the growing chain

5. CONCLUSIONS A detailed examination of the conventional FRP and RAFT copolymerization of three pairs of monomers4-acetoxystyrene (AOST), styrene (Sty), and tert-butyl acrylate (tBA)is reported here. The copolymerization reactivity ratios for conventional FRP are in line with values previously reported. However, when the copolymerization reactions were conducted in the presence of the RAFT agent EMP, small but significant differences in the rate of incorporation of the monomers were seen, in particular for the system AOST−tBA. The apparent reactivity ratios were obtained using a NNLS method REACT and show that the 95% JCIs for the FRP and RAFT copolymerizations are separated. Furthermore, the changes in rates of incorporation are not restricted to the early stages of polymerization, as illustrated by a detailed study of the AOST− tBA system. The reasons for this disparity are not clear; however, they may be related to the influence of the RAFT agent on the environment at the radical chain end. The binary copolymerization reactivity ratios were used to predict the composition of terpolymers of AOST−Sty−tBA, including at compositions close to that of a commercial photoresist, TER60. Excellent agreement was observed between the predicted compositions and those measured by quantitative 13C NMR spectroscopy, validating the assumption of the terminal model used in the development of the terpolymerization equations. Most importantly the calculations indicated that the RAFT terpolymer, when polymerized to high conversion, is expected I


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to have appreciably more homogeneous monomer composition and sequence distribution compared with the terpolymer prepared using conventional FRP. The findings have potential important implications for the uniformity of patterned features in high-resolution optical lithography. These aspects are currently being examined and will be reported elsewhere.

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ASSOCIATED CONTENT

S Supporting Information *

Analysis of terminal model for RAFT, derivation of the copolymer equations, the reactivity ratios along with the corresponding JCIs obtained using CONTOUR program, representative 13C NMR spectrum of each copolymers, and GPC results for representative copolymers synthesized via both FRP and RAFT polymerization. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00683.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (H.P.). *E-mail [email protected] (K.S.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge helpful discussions with Professors Michelle Coote (The Australian National University) and Christopher Barner-Kowollik (Karlsruhe Institute of Technology). This work was performed in part at the Queensland Node of the Australian National Fabrication Facility (ANFF), a company established under the National Collaborative Research Infrastructure Strategy to provide nano and microfabrication facilities for Australia’s researchers. Funding for this project was provided by the Australian Research Council (DP110104299, DP130103774, LE0775684, LE110100028, and CE140100036). Dr. Hui Peng thanks the University of Queensland for the awarding of a UQ Postdoctoral Fellowship for Women.

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Terpolymerization of Styrenic Photoresist Polymers: Effect of RAFT

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