Extreme expansion of proximity gap by double exposures using enlarged pattern masks for line and space pattern formation in x-ray lithography (evolution of exposure method to symmetric illumination)

Abstract

The extendibility of x-ray lithography is limited by the usable proximity gap. The gap is considered to be larger than 10 μm. Practically, a line and space (L/S) pattern of 70 nm linewidth and 140 nm pitch can be formed at a 10 μm gap using a 1:1 exposure mask (1× mask). If a double-pitch mask (2× mask), whose L/S pitch is twice as large as desired what to be printed on the wafer, is used while applying double exposures, the gap can be expanded greatly. A simulation result showed that using a 2× mask, a gap of more than 1000 μm can be used to form a L/S pattern of 200 nm pitch. An exposure test at a gap of 160 μm using the 2× mask proved sufficient to form the L/S pattern of 200 nm pitch. The principle of symmetric illumination explains this result well. The principle is also applicable to two-dimensional pattern formation, for which we propose a concept of mask design, i.e., the symmetric-illumination mask (SIM). Multidot images formed by the SIMs can produce mega- or giga-unit patterns of less than 25 nm linewidth by applying continuous writing.