X-ray lithography for microelectronics

Abstract

The modern information revolution, which is transforming our societies so rapidly, is based upon silicon integrated circuits. The fabrication of these circuits utilizes the planar-fabrication process, the basic step of which is exposure of a radiation sensitive film, known as the resist, to a pattern of radiation. In the technical fields, the exposure step is referred to as lithography. Electrons, ions, UV photons and x-rays have all been used for the exposure. In addition to providing high resolution by virtue of their short wavelength, x-rays have the unique property of providing absorption without spurious scattering, a significant virtue in high density microlithography. The optimal wavelength range for x-ray lithography is dictated by the tradeoff between resist absorption and mask membrane transmission. The efficacy of x-ray lithography has been demonstrated in the fabrication of high-density deep-submicron ULSI circuits, sub-100nm-channel-length Si MOSFETs, sub-100nm quantum-effect devices, single-electron transistors, optoelectronic devices, and micromechanical structures. The factors that have militated against the utilization of x-ray lithography in ULSI manufacturing are discussed along with prospects for adoption in future generations. Projections are made with regard to the ultimate limits of resolution and accuracy, and the possible role of x-ray projection using zone plate arrays.