Synthesis and Polymerizations of N-(tert-Butoxy)maleimide and Application of Its Polymers as a Chemical Amplification Resist

Abstract

N-(tert-Butoxy)maleimide, t-BuOMI was synthesized as a new kind of a protected acid-labile monomer. Its radical copolymerizations were performed and the thermal deprotection behavior of its copolymers were investigated. t-BuOMI was readily copolymerized with styrene derivatives (X-St) to give copolymers, P(t-BuOMI/X-St) in high conversions. The t-BuOMI units of the tert-butyl (t-Bu) protected copolymers were converted into the N-hydroxymaleimide (HOMI) units by heating at about 280 °C releasing 2-methylpropene and the facile deprotection of the side-chain t-Bu groups resulted in a large polarity change in the polymer structure. The deprotected copolymers, P(HOMI/X-St) have very high glass transition temperatures higher than about 250 °C and showed solubilities in aqueous base solutions whereas the protected polymers are soluble only in organic solvents. Acidolytic deprotection of P(t-BuOMI/St) was observed at 130 °C or lower temperatures in the presence of catalytic acids. Resist solutions of P(t-BuOMI/St) containing triphenylsulfonium salts as a photoacid generator were prepared and the films were imagewise exposed to 260 nm light by a contact mode. The films were post-exposure baked at 130 °C and were followed by development with 2.38 wt% TMAH solution to obtain positive tone images.The design and synthesis of protected polymers having acid-labile groups are considered to be of challenging research for obtaining imageable polymers which can serve high sensitivity resist materials for fabricating microelectronic devices (i). Adequately protected polymers bring a remarkable change in polarity, thereof considerable differentiation in solubilities, when the side-chain protecting groups of polymers are deprotected by thermolysis or acidolysis. Those protected polymers are best suited for the application as chemically amplified resists to achieve high sensitivity (2).A typical acid-labile protecting group is terf-butoxycarbonyl (t-BOC) group which is known to be readily deprotected in the presence of catalytic acids. Poly[p-(r-butoxycarbonyloxy)styrene] has been investigated in detail as a prototype t-BOC protected polymer and successfully applied in the manufacture of VLSI devices (3,4). In this case, the acid-labile t-BOC group is utilized to