Method of forming self-aligned P contact

Disclosed is a process for forming self-aligned low resistance ohmic contact to a P doped region (e.g., base of an NPN device) in conjunction with forming similar contact to a (highly) N doped region (e.g., emitter of NPN). After forming a P doped region in an N type monocrystalline silicon body and masking it with an insulator (e.g. dual oxide-nitride) layer, the highly doped N region (hereafter, N+ region) is formed in a portion of the P doped region by selectively opening the insulator layer and introducing N dopant therethrough. This opening also serves as contact opening for the N+ region. contact opening for the P region is formed by selectively etching the insulator layer. The structure is subjected to a low temperature steam oxidation to from an oxide layers in the P contact and N+ contact regions, the oxide in the N+ contact being about 3-5 times thicker than that in the P contact region due to the significantly higher oxidation rate of the N+ region relative to the P doped region. The oxide in the P contact is etched off while retaining a substantial portion of the oxide grown in the N+ contact region. P type dopant is then introduced into the P contact opening to achieve solid solubility limit of the P dopant species in silicon. The oxide remaining in the N+ contact region is removed and contact metallurgy is established with all contacts.