The extendibility of x-ray lithography is limited by the usable proximity gap. The gap is considered to be larger than 10 mum. Practically, a line and space (L/S) pattern of 70 nm linewidth and 140 nm pitch can be formed at a 10 Am gap using a 1:1 exposure mask (1X mask). If a double-pitch mask (2 X mask), whose LIS 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 2X mask, a gap of more than 1000 mum can be used to form a L/S pattern of 200 nm pitch. An exposure test at a gap of 160 mum using the 2X 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. (C) 2003 American Vacuum Society.