GB1079730A - Method of producing smeiconductor contacts - Google Patents

Abstract

<PICT:1079730/C6-C7/1> Ohmic alloyed contacts to two different conductivity type regions of a semi-conductor body are made by a process in which appropriate metals are deposited from the vapour phase on to the heated semi-conductor. The semi-conductor body is first masked by a surface oxide layer which leaves one or more regions of one conductivity type exposed. An alloy suitable to form an ohmic contact to such regions is then evaporated on to the body with the latter at the alloy-semi-conductor eutectic temperature. Subsequently an isolating metal layer is evaporated on to the alloy layer at a lower temperature such that this metal does not alloy with the layer already present. An aperture is then etched through this double metal layer and through the oxide to expose a region of opposite conductivity type. A second metal alloy is then deposited at a temperature such that it will alloy with the semi-conductor but not with the isolating layer. The first conductivity type region now has an alloyed contact surmounted by two non-alloyed layers and the second conductivity type region has a simple alloy contact, though both contacts are interconnected. A further metal layer may be applied before a final etching treatment to separate the electrodes. The process is particularly described in relation to a gallium arsenide NPN planar transistor, though its possible application to integrated switching circuits is also mentioned. The masking oxide layer is of silicon dioxide on which a thin (200<\>rA) chromium layer may be formed. This chromium is partly etched away to leave chromium areas of a required electrode shape which, being adherent both to the oxide and the contact material, ensure that in the completed device the contacts extend over the oxide as strips of the required shape. The P-type base region is then exposed by further etching before the first alloy, 2% Mg 98% Ag, is deposited with the wafer at 620 DEG C. The isolating, non-alloyed layer is of silver deposited at 450 DEG C. and the second alloy, for contacting the N-type emitter and collector regions, is 2% Te 98%Au also deposited at 450 DEG and the final layer is Au deposited at 300 DEG C. Thus in Fig.5-which shows part of the resulting structure after the final etching to separate the contacts-the emitter contact 14 of Te-Au contacts the semi-conductor through layers 11, 9, 6, 5 of Ag, Mg-Ag, Cr and SiO2 respectively and is surmounted by Au 15, whereas the base contact 10 is of Mg-Ag extending through Cr and SiO2 layers 7, 5 and surmounted by Ag, Pe-Au and Au layers 11, 13, 15 respectively. The purpose of the silver layer 11 is to prevent the Te in layer 13 from contaminating the base contact 10.