Wafer level forward current reliability analysis of 120GHz production SiGe HBTs under accelerated current stress

A wafer-level reliability (WLR) technique is presented to assess the forward current mode degradation of polysilicon emitter bipolar transistors. Using this technique, we show results for the first time on degradation properties of high-speed self-aligned SiGe HBTs featuring 120 GHz f_T and 100 GHz f_max. Accelerated current stress up to as high as J_C=34 mA/μm² was employed to assess the device degradation. It is shown that current accelerated stress may be used to effectively predict shifts in device characteristics. No catastrophic failures were observed, and thus this technique is used principally to demonstrate parametric shifts rather than end-of-life failures. Since the current stress also involves substantial self-heating of the device, we compare the degradation with temperature-only acceleration at both wafer and module level. It was found that the current acceleration is dominant over the temperature acceleration in the observable mechanisms. The mechanism for the parametric shifts is believed to be related to interfacial properties between the polysilicon and single-crystal portions of the device emitter. Through these studies, it is shown that the device is highly robust to parametric shifts for over 10⁶ hours. The comparison with module level stress results verified their consistency with wafer level stressing, thus providing a viable WLR methodology for parameter shift projection in a relatively short stress time and moderate temperature.