Method for the formation of high density memory cells using ion

Abstract

A semiconductor structure is fabricated using a process involving all ion implantation and using only five masks prior to metallization. A buried contact mask is used to form a buried contact layer (114), an isolation mask is used to form grooves (130a, 130b) in an epitaxial layer of silicon (113), a self-aligned transistor mask is used to form a mask (134a to 134e) to define the areas in which emitters (138a, 140b, 140c) bases (113, 139) and contact regions (140a) are to be formed, a base exclusion mask (135a,b) is provided to exclude certain impurities from being implanted into a region to be formed of one conductivity type, and a second exclusion mask (137a, 137b) is provided to exclude impurities to be implanted in a region of opposite conductivity type from the prohibited regions of the structure. The last ion implantation of the device is a two-level implantation to yield a shallow implant which provides good ohmic contact to certain to-be-formed metal contact areas and a deep implant to provide current gain control for the base of the NPN transistors (Q.sub.1, Q.sub.2). The metal contacts to active regions on the device are formed merely by etching the device to remove a thin oxide (131) over these regions. A thicker oxide (134a through 134e) overlying all other regions of the device is substantially unaffected by this etching.