GB1362345A - Semiconductor device manufacture - Google Patents

Abstract

1362345 Manufacture of inverted bipolar transistors MULLARD Ltd 11 May 1973 [20 July 1972] 34035/72 Heading H1K In making the base region of an inverted bipolar transistor by ion implantation through a surface masking layer into a layer of one conductivity type on a higher conductivity region of the same type, the portion of the masking layer immediately adjacent the window is constructed to be semi-permeable to the ions to provide a peripheral region of the base in which the ions are less deeply implanted, while the ions in the unmasked region extend to the higher conductivity region. The described processes start with an epitaxial N layer on an N + silicon substrate which ultimately constitutes the emitter zone. Thick oxide is thermally grown over the layer, apertured by photolithographic and etching steps, a thinner oxide regrown in the aperture and similarly apertured at one side, the etching being such that the apertures have chamfered edges. Bombardment with 500 keV boron ions produces a base zone which extends to the surface under the thin oxide to form a contact region and under the chamfered edge, and an unconverted N-type collector region at the exposed silicon surface. The base contact region may alternatively be formed in a separate diffusion or implantation step. After annealing at 700‹ C. and optionally providing a collector contact region by diffusion or implantation a base contact hole is etched and aluminium deposited and pattern etched to form the base and collector electrodes, and an emitter contact provided on the back of the wafer. In a modification for integrated circuit use the N+ silicon is a diffused inclusion in a P-type substrate and an emitter electrode provided on the upper face contacts the inclusion via a connecting region. In a further variant the ion energy is such that the base zone extends also to the exposed surface, on which a Schottky collector electrode is provided. The mask aperture may alternatively be provided with a stepped edge either by depositing silica on or diffusing phosphorus into the surface of the grown oxide to provide a faster etching layer to increase undercutting of the photoresist etching mask. A similar effect is obtained by depositing metal over the oxide, using it to mask preferential etching of the oxide and then preferentially etching it to undercut the overlying photoresist mask. In another embodiment, Fig. 7, after aperturing the thick oxide, very thin oxide is regrown in the aperture, itself apertured centrally, and a base contact 32 deposited in the aperture. A photoresist mask 33 after development is baked so that it plastically flows to produce graded edges 6 and ion implantation effected as before but with the partial masking effect of the contact 32 providing a self registered contact region. Electrodes are then provided substantially as above. Reference has been directed by the Comptroller to Specification 1,294,515.