RELAXATION OF SI-SIO2 INTERFACIAL STRESS IN BIPOLAR SCREEN OXIDES DUETO IONIZING-RADIATION

Current gain degradation due to ionizing radiation in complementary si ngle-crystalline emitter bipolar transistors was found to grow progres sively worse upon subjecting the transistors to repeated cycles of rad iation exposure and high-temperature anneal. The increase in radiation sensitivity is independent of the emitter polarity or geometry and is most dramatic between the first and second radiation and anneal cycle s. In parallel with the current gain measurements, samples from a moni tor wafer simulating the screen oxide region above the extrinsic base in the npn transistors were measured for mechanical stress while under going similar cycles of irradiation and anneal. The oxide on the monit or wafer consisted of a 45 nm thermal layer and a 640 nm deposited lay er. The results indicate that ionizing radiation helped relieve compre ssive stress at the Si surface. The magnitude of the stress change due to radiation is smaller than the stress induced by the emitter contac t metallization followed by a post-metallization anneal. Correlation o f radiation sensitivity in the bipolar transistors and mechanical stre ss in the monitor wafer suggests that mechanical stress may be influen tial in determining the radiation hardness of bipolar transistors and lends validation to previously reported observations that Si-SiO2 inte rfaces are increasingly more susceptible to radiation damage with decr easing Si compressive stress. Possible mechanisms for the observed cha nges in stress and their effect on the radiation sensitivity of the bi polar transistors are discussed.