Dissolution, 4-hydroxystyrene-styrene copolymers

O-methylated PHOST [179]. This terpolymer was originally developed as a chemically amplified laser resist for circuit board application [180] and then as a single layer 193 nm positive resist [181], which will be described in more detail later. Another interesting three-component approach is the use of a N-acetal polymer as a dissolution inhibitor of poly(3-methyl-4-hydroxystyrene) [182]. A deep UV resist consisting of poly(3-methyl-4-hydroxystyrene-co-4-hydroxystyrene), poly(N,0-acetal), bis(arylsulfonyl)diazomethane, and a photobase was reported from Hoechst (currently Clariant). The function of the photobase is described later. A copolymer of 4-hydroxystyrene with styrene was also employed as a matrix resin. 

Copolymerization is the most important synthetic strategy available to tailor the physical, solution and mechanical properties of macromolecules. In this paper the synthesis of well defined random and block copolymers containing monomer units of very different polarity are described. Novel macromolecular architectures incorporating 4-hydroxystyrene, t-butylacrylate and styrene have been prepared to investigate the combined effects of macromolecular architecture and hydrophobicity on thin film aqueous base dissolution. 

The copolymer architecture of poly(4-hydroxystyrene-co-styrene) was found to have insignificant effect on its dissolution rate (Fig. 57.5 Table 57.12) [36]. On the other hand, incorporation of inert styrene unit into poly(4-hydroxystyrene) drastically reduces dissolution rate. This method of incorporating inert unit has been employed to optimize the dissolution of base polymers for advanced DUV photoresists. 

FIGURE 57.5. Effect of copolymer composition on the dissolution rates of poly(4-hydroxystyrene -co-styrene) in 0.26N TMAH at room temperature [36]. 

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