The unique ability of the tert‐butoxycarbonyl protecting group (t‐BOC) to be removed by a catalytic amount of strong acid has intensified the search towards new chemically amplified resist systems based on this chemistry. A series of new copolymers of t‐BOC‐styrene and sulfur dioxide have been prepared by free radical polymerization. These polysulfones function as chemical amplification positive resists for deep‐UV lithography when mixed with either 2,6‐dinitrobenzyl tosylate or triarylsulfonium salt acid precursors. The lithographic characteristics of 2:1 and 3:1 polysulfones have been evaluated. The new positive deep‐UV photoresists are aqueous base developed and are capable of 0.5 μm resolution. Even though the photoresists containing 2,6‐dinitrobenzyl tosylate are less sensitive than the onium salt formulations, they displayed greater contrast values. For example the poly(t‐BOC‐styrene sulfone) (2:1) 15 wt % tosylate resist formulation exhibits a sensitivity of 26 mJ/cm2 and a contrast of ∼20.
Chlorinated polymethylstyrene has been shown to behave as a negative resist upon electron beam exposure. Polymethylstyrene was prepared by both anionic and free radical techniques and chlorinated to produce a series of polymers with a range from 0 to 2.5 chlorine atoms per polymer repeat unit (0 to 42.5 wt. % chlorine). Sensitivity and contrast were found to be a function of chlorine content, molecular weight, and polydispersity. Sensitivity was maximum for materials with 0.6 to 0.9 chlorine atoms per repeat unit, depending on the chlorination technique chosen. Nearly monodisperse polymers exhibited much higher contrast than broader molecular weight materials and showed improved resolution. Thermogravimetric analysis was employed to study solvent removal from spun films and to determine prebaking and postbaking conditions. A chlorinated polymethylstyrene (0.92 chlorines/monomer unit) with a molecular weight of 70 000 and polydispersity of 1.06 had a sensitivity of 2.0 μC/cm2 and contrast of 2.0. This material demonstrated half-micron resolution for line and space patterns in both single level and trilevel processing.
The thermolysis and acidolysis of three polymers, poly(4-((tert-butoxycarbonyl)oxy)-amethylstyrene) [poly@-BOC-a-methylstyrene)], poly(4( (tert-butoxycarbony1)oxy)styrene) [poly( t-BOC-styrene], and poly(4-( (tert-butoxycarbony1)oxy)styrene sulfone) [poly(t-BOC-styrene sulfone)], were studied by gas chromatography/mass spectroscopy. The acidolysis reaction was investigated with both thermally and photochemically generated acid. The photogenerators of acid considered in this study were triphenylsulfonium hexafluoroarsenate and 2,6-dinitrobenzyl tosylate. The thermal generator of tosic acid was 2-nitrobenzyl hylate. It was found that the presence of acid caused the expected deprotection of the phenol functionality with liberation of COP and isobutene in all three polymer systems. However, in the case of poly(t-BOC-a-methylstyrene), a depolymerization reaction was also observed with liberation of 4-hydroxy-a-methylstyrene. The extent of depolymerization was found to be as high as 20% in some cases, and a secondary product, 5-hydroxy-1,1,3,3-tetramethylindan, was observed.
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