We have developed a fabrication technique for high-performance high-thermal-stability InP/InGaAs heterojunction bipolar transistors (HBTs) for use in 40-Gb/s ICs. The HBT's T-shaped emitter electrode structure simplifies the fabrication process and enables high controllability of spacing between the emitter and the base electrodes. A highly-C-doped base, grown by gas-source MBE, and a new Pt-based metal system results in a low base resistance. An InP subcollector suppresses thermal runaway of HBTs at high collector current better than a conventional InGaAs subcollector does. Using these techniques, we fabricated a very-high-performance HBT with an extremely high cutoff frequency fT of 235 GHz. The RF measurements show that the collector current at the peak cutoff frequency is inversely proportional to collector thickness. We also fabricated a static 1/2 frequency divider, that can be used for 40-Gb/s optical transmission systems, operating up to 44 GHz. This divider confirmed that the developed HBT is applicable to 40-Gb/s optical transmission ICs.

Synchrotron radiation (SR) irradiation of amorphous SiO2 (a-SiO2) induces continuous removal of the SiO2 film without the use of etching gas. The dependence of the photostimulated evaporation rate on substrate temperature and SR intensity was measured and the reaction mechanism is discussed in detail separately for surface and bulk. Using the high material selectivity of the Sr-stimulated evaporation, a sefl-aligned process to fabricate a 0.6 µm line-and-space pattern is presented. Si surface cleaning is demonstrated as an example of application of this reaction to thin native oxide film grown by wet pretreatment. Si(100)-21 and Si(111)-77 structures were observed by reflection high energy electron diffraction (RHEED) at temperatures as low as 650. The difference between a-SiO2 and native oxide on the evaporation rate is higlighted. Epitaxial Si growth using disilane (Si2H6) gas occurs selectively in the SR-irradiated region on a Si(100) surface. Using SR irradiation in an ultrahigh vacuum, followed by residual oxide reduction by disilane, is proposed as an effective cleaning method.