The speed of thin-film transistors (TFTs) relates directly to their gate length, which must be kept as short as possible to lower electron transport time between electrodes, and improve its high-frequency response characteristics. Since current density is proportional to W/L, where W is the gate width and L is the gate length, reduced gate length improves device current capability. Photolithography, which is the main technology used in defining electrode dimensions for TFTs, is limited in scope to fabrication line widths larger than 1 micrometer.
Advanced lithography techniques applied to modern single-crystal microelectronics rely on extremely well controlled substrate surface flatness and resist uniformity. These conditions cannot be maintained for thin film electronics, and therefore only large gate lengths have been possible in the past.
The present invention is a method for fabricating an ultra-short-gate-length thin-film transistor. Multiple layers are deposited on a substrate including a refractory metal, and a first and second photosensitive material. The second material is sensitive to longer-wavelength optical radiation than the first material, and the first material is not soluble in chemicals used to develop or strip the second material.
A source contact pattern is defined in the second material to mask the first photosensitive material. The first material is processed to produce an undercut of the first material with respect to the second material. A metal layer is deposited at a normal incidence on the second material and an exposed portion of the refractory metal. The second material is removed. Exposed portions of the refractory metal corresponding to the undercut of the first material are removed to form a gap in the refractory metal.