Study On SiGe/Si Epitaxy And SiGe HBT Microwave Monolithic Integrated Circuit

Silicon-Germanium (SiGe) heterojunction bipolar transistors (HBTs) have shown marvelous performance in many areas for high-speed and microwave applications. SiGe HBTs can also be integrated with other devices on a monolithic silicon wafer. As an all-purpose circuit, Darlington-structure SiGe HBT microwave monolithic integrated circuits (SiGe HBT MMICs) has a good many advantages. They have simple structures and they not only can be replaced easily but also can be monolithically integrated in a Si wafer with other active devices and passive components. In this study, SiGe HBT MMIC research was done for the first time in China. A series of efforts was made on SiGe HBT MMIC including UHV/CVD SiGe epitaxial machine, SiGe film characteristics, process improvements, circuit design and fabrication.The structure of the self-made UHV/CVD and the design of the rapid-thermal graphite heater were improved based on the investigation of many SiGe epitaxial machines. The Ge mol fraction and thickness uniformity of SiGe film were obviously improved: the Ge fraction uniformity achieved ±5%, and the thickness uniformity achieved ±8%.High quality SiGe epitaxial layers with uniform or graded Ge profiles were successfully achieved based on the study of the SiGe epitaxial process on the SGE500-type UHV/CVD. The strain relaxation of SiGe films under different thermal processes was thoroughly studied. Compared with annealing time, furnace annealing temperature is much more important to the relaxation of SiGe film with uniform or graded Ge profile. The higher the temperature was, the easier was the strain of the SiGe film to relax. Rapid thermal annealing showed less effect on the strain relaxation of the SiGe film when the temperature was less than 910oC. Innovative study in China was done on the design and fabrication of SiGe HBT MMIC. A new fabrication process for this circuit was developed based on the study of SiGe uniformity, growth process, strain-relaxation and the SiGe HBT fabrication process. The analysis of scattering parameters showed that the SiGe HBT MMIC had good electrical performance: The power gain was 11.6 dB@850 MHz, 8.0 dB@1950 MHz and 4.7 dB@3000 MHz; the input voltage static wave ratio (VSWR) was 3.01:1@3000 MHz and the output VSWR was 2.66:1.Three methods were developed to improve the frequency performance of the SiGe HBT. TiSi2 was introduced to the fabrication process of SiGe HBT. SiGe HBTs with different Ge graded profile were thoroughly studied, and the transition frequency fT of the SiGe HBT was improved by this means. Selective growth of SiGe film on patterned Si wafers was also studied. The anti-radiation performance of SiGe HBT devices was studied for the first time in China. Analysis showed that the anti-radiation performance of SiGe HBTs is remarkably better compared with that of Si BJTs.The transition region thickness was 0.16 μm for the Si epitaxial layer grown at 700oC on the highly arsenic-doped Si substrates, and was only 0.06 μm under 500oC, which was much less than high-temperature Si epitaxy by APCVD. Thus the low-temperature epitaxial Si layer needs less film thickness to achieve the same breakdown voltage.