William H. Heath

Design of Reversible Cross-Linkers for Step and Flash Imprint Lithography Imprint Resists

Progress in the semiconductor manufacturing industry depends upon continuous improvements in the resolution of lithographic patterning through innovative materials development and frequent retooling with expensive optics and radiation sources. Step and Flash Imprint Lithography is a low-cost, nanoimprint lithography process that generates nanopatterned polymeric films via the photopolymerization of low-viscosity solutions containing cross-linking monomers in a transparent template (mold). The highly cross-linked imprint materials are completely insoluble in all inert solvents, which poses a problem for reworking wafers with faulty imprints and cleaning templates contaminated with cured imprint resist. Degradable cross-linkers provide a means of stripping cross-linked polymer networks. The controlled degradation of polymers containing acetal- and tertiary ester-based cross-linkers is demonstrated herein. The viscosity and dose to cure are presented for several prepolymer formulations, along with imprint resolution and tensile modulus results for the cured polymers. Optimum conditions for de-cross-linking and stripping of the cross-linked polymers are presented, including demonstrations of their utility.

Photo-Reactive Polyimides and Poly(siloxane imide)s as Reversible Polymeric Interfaces

Polyimides and poly(siloxane imide)s containing photo-active cyclobutane diimide units were developed as reversible adhesives for temporary bonding in micro-fabrication processes. Poly(amic acid) formation in dipolar aprotic solvents and subsequent chemical or thermal solution imidization yielded the corresponding organic soluble polyimides. Incorporation of nonplanar (alicyclic or bicyclic) structures and poly(dimethyl siloxane) blocks into the main chain improved the solubility. The synthesized poly(siloxane imide)s afforded tough and ductile films, which exhibited a rubbery plateau in the storage modulus, as observed using dynamic mechanical analysis. This plateau was attributed to the microphase-separation of the poly(dimethyl siloxane) segments and the polyimide segments. Spin-coated thin films on silicon substrates were photo-degraded with ultraviolet-C (UVC) light (254 nm). Nuclear magnetic resonance (NMR) spectroscopy demonstrated a growth in the proton resonance corresponding to the formation of maleimide functionality upon photo-cleavage. This observed photo-cleavage suggests possible applications as temporary structural adhesives.

Photoactive Polyesters Containing o-Nitro Benzyl Ester Functionality for Photodeactivatable Adhesion

The fabrication of modern microelectronic silicon devices mechanically challenges these thin silicon substrates during manufacturing operations. Melt and solution polyesterification enabled the synthesis of polyesters containing photoreactive o-nitro benzyl ester units for use as a potential photocleavable adhesive. Melt transesterification provided a solvent-free method for synthesis of 2-nitro-p-xylylene glycol (NXG)-containing polyesters of controlled molecular weights. 1H NMR spectroscopy confirmed the chemical composition of the photoactive polyesters. Size exclusion chromatography (SEC) determined the number-average molecular weights (Mn) of the polyesters synthesized in the range of 6000 to 12000 g/mol. 1H NMR spectroscopy confirmed increasing levels of photocleavage of the o-nitro benzyl ester functionality with increasing exposure to broad wavelength UV irradiation, and exposure levels ranged from 0–187 J/cm2 UVA. Photocleaveage of approximately 90% of the o-nitro benzyl ester (ONB) units within the backbone of the polymer occurred at maximum dosage. Wedge fracture testing revealed approximately a two-fold decrease in fracture energy upon UV irradiation, suggesting that these structural adhesives offer potential for commercial “flip bonding” applications.

Non-chemically amplified resists for 193 nm lithography

Acid diffusion during the post-exposure bake of chemically amplified resists (CARs) is a major contributing factor to line width roughness (LWR) and resolution limits at the 32 nm node and beyond. To overcome these limitations, non-CAR materials are becoming more attractive because acid diffusion is eliminated. We have therefore focused our effort on the synthesis of copolymers that have both a diacyldiazo side chain unit as well as a hexafluoroalcohol unit. This copolymer shows better contrast than that of copolymers containing lactone units due to their inhibition behavior. Furthermore, polymer blends containing hexafluoroalcohol groups show good 100 nm line and space patterning property for 193 nm lithography. This paper describes the design, synthesis, and characterization of these non-CARs, and thier improvement to photolithography.