Hydroxystyrenes, polymerizations

Synthesis of the Polymers Containing Reactive Ether Pendant Groups, We have previously described the synthesis of high purity poly(p-hydroxystyrene) free from deleterious oxidized species by polymerization of 4-t-butyloxycarbonyloxy-styrene followed by the removal of... 

Poly(4-hydroxystyrene) was previously prepared by the authors (2) by the decarboxylation of 4-hydroxycinnamic acid followed by the free radical polymerization using Vazo 67. 

Westover et al. 95 prepared lightly crosslinked nitrated poly(4-hydroxystyrene) microspheres for pH sensors. The microspheres were produced using SPG membranes followed by suspension polymerization and they showed diameters between 1 and 2 micrometers. 

Faced with the shortcomings of the polyphthaldehyde resist (presented helow in chemical amplification resists based on depolymerization), the search for chemically amplified DUV resists resulted in a quick switch to more stable materials based on poly(p-hydroxystyrene), a phenolic polymer that Willson et al. were studying as a potential replacement for novolac. They observed that poly(p-tert-butoxycarbonyloxystyrene) (PBOCST), which is poly(vinyl phenol) protected with tert-butoxycarbonyl groups (t-BOC), is far more stable than the unprotected p-hydroxystyrene and could be purified and polymerized under controlled conditions. The resulting protected polymer could be easily deprotected thermally by heating it to 200°C or to a much lower temperature (100°C) by treatment with acid generated from the exposure of onium salts, just as in the poly(phthaldehyde)... 

Scheme 7.30 shows the synthetic pathways to PHOST, including radical, cationic, and anionic polymerization techniques. The polymerization behavior of the hydroxystyrene monomer (also called vinyl phenol) has been extensively investigated by Sovish, " Overberger, " and Kato. " PHOST can by synthesized via direct radical polymerization of 4-hydroxystyrene, which in turn is obtained from catalytic dehydrogenation of 4-ethylphenol. This was the method used in the preparation of the first commercially available PHOST, which was sold by... 

Kato, Radical polymerization behavior of hydroxystyrenes, 7. Polym. Sci. Part A I 7, 2175... 

PBOCST is readily synthesized from the polymerization of r-hutoxycarhonyl oxystyrene via radical or cationic polymerization in liquid sulfur dioxide or alternatively by reacting poly(hydroxystyrene) with di-tert-h xty dicarbonate in the presence of a base PBOCST polymers with narrow dispersity have been prepared by living anionic polymerization of 5-tert-butyl(dimethyl)silyloxystyr-ene), followed by desilylation with HCl to form PHOST and protection with di-tert-butyl carbonate PBOCST is very transparent around the 250-nm region of the spectrum (absorbance <0.1/ p,m), thus making it an ideal candidate for DUV 248-nm lithography. 

A very prominent example of this type of resist is a copolymer of 4-hydroxy-styrene with tert-butyl ester-protected 4-hydroxystyrene (TBEST) (XXIX) (see scheme 7.33), developed at IBM and sold under the brand name of APEX-E by the Shipley Company. The synthetic route to copolymer (XXIX) can be through direct copolymerization of 4-hydroxystyrene with TBEST or via polymerization of TBEST, followed by partial deprotection to afford a copolymer with repeating units having about 20-30% protecting groups. Partial protection of copolymer... 

A rather unexpected water- and alcohol-insensitive activator, namely boron trifluoride etherate, was developed by Sawamoto for the living polymerization of p-hydroxystyrene in combination with the water adduct of p-methoxystyrene as initiator [78], The polymerizations proceeded even in large excess of water, which is in large contrast with the absolutely dry conditions that are normally required for carbocationic polymerizations. It is proposed that acetonitrile, which is used as polymerization solvent, stabilizes the short-lived carbocationic propagating species. The same polymerization methodology could be applied for the preparation of statistical and block copolymers consisting ofp-hydroxystyrene and b-methoxystyrene [79], as well as for the homopolymerizations of styrene, p-chlorostyrene and p-methylstyrene in the presence of a proton trap [80]. 

DHBCs of the type poly(p-hydroxystyrene-b-methaciylic acid) (PHOS-PMAA) were also synthesized via anionic polymerization followed by acidic hydrolysis [10]. Both blocks of the precursor poly(p-tert-butoxystyrene-b-tert-butylmethacrylate) (PtBOS-PtBMA) copolymers, formed by sequential addition of the protected monomers, could be deprotected in a single step giving the desired pH-responsive block copolymers. Figure 2. Hydrolysis was found to be nearly quantitative and resulted in a series of copolymers with well defined molecular characteristics and of variable composition. 

These short TEMPO capped styrene blocks (7) were then used as "macroinitiators for the further polymerization of 4-acetoxystrene (1) using the same procedure as described above. The length of the acetoxystyrene block is controlled by the ratio of the "macroinitiator (7) to acetoxystyrene monomer. The acetoxystyrene block was then converted to 4-hydroxystyrene by quantitative base hydrolysis of the acetoxy groups as previously described. Table 1. A similar synthetic strategy was used for the preparation of poly(styrene- co-t-butyl acrylate) block copolymers. 

The Choice of a Matrix Resin. As was mentioned earlier, the common matrix resins for today s lithography are phenolic resins such as Novolac and poly(4-hydroxystyrene). Though some of our early work had involved simple water soluble alcohols such as poly(vinyl alcohol), schemes for their reversible in situ insolubilization were sometimes complicated by irreversible processes or side-reactions. As a result we chose to test the water-soluble linear polymer that is obtained by free-radical polymerization of 2-isopropenyl-2-oxazoline, 1. Monomer 1 can be polymerized through a variety of techniques, as shown in Scheme 1 (6,7). Both radical or anionic polymerization conditions lead to a polymer containing pendant oxazoline rings, while a more complex structure is obtained under cationic conditions as both the vinyl and the oxazoline moieties are reactive. 

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