Low stress nitride as surface isolation in bipolar transistors

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Abstract
Low stress silicon rich nitride SiNx has been examined as surface isolation in a 15 GHz washed emitter–base (WEB) bipolar process. The resistance of SiNx to wet etching in HF simplifies the processing and improves the control of device dimensions. The WEB process allows a direct comparison of device characteristics for several SiO2 and SiNx passivation layers. Films of SiNx with strain levels in the range 4×10−4–3×10−3 were developed using a micromachined test structure. Damage of the silicon surface, which induces emitter–collector shorts, is avoided by using very low stress nitride (strain −3). The base leakage currents, which are associated with silicon/insulator interface traps, are not stress dependent but are characteristic of the type of isolation. MST/3222