Method Speeds Superconductor Lithography

Laurel M. Sheppard

Superconducting materials have a number of potential applications, including high-performance amplifiers, switches and logic circuits. Tailoring the size, shape and pattern of their microstructures provides control of the critical current, the point at which the superconductivity is lost.

Interferometric lithography produced these 0.2-µm-diameter photoresist posts with 1-µm spacing for superconducting film research. The side-wall ripple is due to standing wave effects; the post heads are enlarged to make the subsequent lift-off process more effective.

Although electron-beam lithography has been the conventional method used to make these precisely controlled structures, researchers at the University of New Mexico's Center for High Technology Materials and Argonne National Laboratory in Argonne, Ill., are developing an alternative method that uses laser interferometric lithography.

Major disadvantages of using electron-beam lithography to make the superconducting lattices include the method's high cost and slow writing speed. The new method can produce patterns over large areas (up to 1-m-square panels have been demonstrated) in minutes rather than hours. The equipment is also at least one order of magnitude less expensive than an electron-beam system. Another advantage of the optics is the very large depth of field, limited only by the laser coherence lengths.

Laser interferometric lithography generates subwavelength patterns by the interference of two or more coherent laser beams (364-nm argon-ion lasers or 355-nm third-harmonic Nd:YAGs for current experiments) to produce nearly perfect periodic structures on a wafer. The two-dimensional structures require two exposures, rotating the wafer 90° after the first exposure. After exposure, the photoresist is developed, and the pattern is transferred as a lattice of holes to a superconducting film. Each hole acts as a strong pinning site for flux lines.

Vitali Metlushko and other researchers at Argonne are investigating laser interferometric lithography for use in manufacturing magnetic transistors and similar applications. The method has a range of potential applications, noted Steven R.J. Brueck, director of the Center for High Technology Materials, including field emitter displays, fiber Bragg gratings, crystal growth on textured substrates, visible/near-IR photonic bandgap materials, and ultralarge-scale integrated circuits. Interserv Co. of Minneapolis built a first-generation system for International Sematech, a semiconductor consortium based in Austin, Texas, which used it to make 300-mm test wafers.

Brueck and his team are also investigating imaging interferometric lithography, which combines the arbitrary pattern capabilities of conventional optical lithography with the high spatial frequency capabilities of interferometric lithography. With it, they have demonstrated a threefold resolution enhancement over conventional optical lithography.