Raytheon Company

Sperry Corporation

Walter Fabian

Frank H Spooner

A method for forming a field effect transistor having a submicron gate length. A gate electrode is formed by angularly depositing metal through an aperture formed in a thick masking layer. A substrate upon which the gate electrode is to be formed is placed in an apparatus for depositing a stream of evaporated metal through the aperture onto portions of the substrate surface exposed by the aperture. The stream is directed at a selective oblique angle .theta. with respect to a normal to the surface of such substrate. Portions of the exposed surface of the substrate are shadowed from the obliquely directed stream of evaporated metal by an edge of the aperture formed in the thick masking layer. Thus, only selected portions of such obliquely directed stream of evaporated metal are deposited onto unshadowed portions of the substrate to thereby provide the gate electrode. The width w' of the portion of the gate electrode in contact with the substrate, and hence the gate length of the field effect transistor, is related to the angle .theta., the aperture width w, and aperture height h by w'=w (h)(tan .theta.). Such parameters are selected to provide a gate length which is less than a micron even though the aperture width is greater than or equal to a micron and hence one which can be formed using conventional optical photolithographic techniques. Edges of the masking layer are formed to overhang portions of the sidewall of the aperture to thereby shield such portions of the sidewall, thus providing separation between the evaporated metal deposited through the aperture onto the substrate and the evaporated metal deposited on the masking layer.

Forming sub-micron electrodes by oblique deposition

A method of fabricating gallium arsenide devices in which contact isolation is provided by a deep mesa step structure. Step coverage of deposited conductive films is facilitated by preferential orientation of the non-centrosymmetric crystal substrate and wet anisotropic etching that provides a sloped step. Problems of fine line definition of the Schottky anode contact in the photolithographic process are addressed by a two-step exposure of a single layer of thick photoresist followed by a chlorobenzene soak prior to development that ensures a retrograde resist profile needed for good lift-off of undesired evaporated metal. Mesas as deep as 7 .mu.m have been obtained, which permit the fabrication of monolithic planar mixer millimeter-wave diodes with low series resistance and reduced parasitic capacitance.

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Burlington, MA, United States of America

Walter Fabian