On the Challenges of SiGe HBTs in Advanced BiCMOS Technology Toward Half THz f_MAX

Advanced BiCMOS technology is a cost-effective candidate for wide-range of applications, including Radar (automotive radars, high-speed industrial sensors), high-speed wireless and wireline communication, high-speed instrumentation, and mmWave THz imaging and sensing that require high performance SiGe HBTs with high cut-off frequencies fT and high maximum frequencies fMAX. Development of SiGe HBTs in BiCMOS technology with both high fT and fMAX faces significant challenges. Vertical scaling is employed to increase fT in advanced SiGe HBTs including reduction in base and collector thickness or by increasing the collector doping. All these approaches reduce the carrier transit times, but result in an increase in the base resistance and the collector-base capacitance, which degrade fMAX. To overcome these limits, lateral scaling is necessary to reduce the base resistance and the collector-base capacitance. Reduction in emitter width reduces the intrinsic base resistance and Ccb, self-aligned emitter-base integration schemes reduce the extrinsic base link resistance, and raised extrinsic base helps with lower Rb and Ccb. In short, these scaling rules and approaches would need to reduce both base resistance and collector-base capacitance simultaneously for improved fMAX and fT In this paper, results from two experimental studies are presented in a 90nm SiGe BiCMOS technology. Compared with the baseline, one experiment shows more reduction in collector-base capacitance, while the other shows more reduction in base resistance. Both experiments achieved 300GHz fT and met or exceeded the 360GHz fMAX goal of the technology. In addition to straight vertical and lateral scaling, a series of experiments employing other process techniques and structure innovation have also been studied and are reviewed in this paper, including millisecond anneal techniques, low temperature silicide and low temperature contact processes, and secondary trench isolation, achieving fT around 300GHz and fMAX toward half THz.