LOW-STRESS NITRIDE AS SURFACE ISOLATION IN BIPOLAR-TRANSISTORS

Low stress silicon rich nitride SiNx has been examined as surface isol ation in a 15 GHz washed emitter-base (WEB) bipolar process. The resis tance of SiNx to wet etching in HF simplifies the processing and impro ves the control of device dimensions. The WEB process allows a direct comparison of device characteristics for several SiO2 and SiNx passiva tion layers. Films of SiNx with strain levels in the range 4 x 10(-4)- 3 x 10(-3) were developed using a micromachined test structure. Damage of the silicon surface which induces emitter-collector shorts, is avo ided by using very low stress nitride (strain < 1.8 x 10(-3)). The bas e leakage currents, which are associated with silicon/insulator interf ace traps, are not stress dependent bur ape characteristic of the type of isolation. (C) 1995 The Institute of Materials.