Solid state interaction between thin chromium films and silicon—a comparison between amorphous and single-crystal silicon

Abstract

The interaction of a vacuum-deposited thin chromium film with both Si〈100〉 and amorphous silicon (a-Si) during heating was studied by means of electron microscopy, electron diffraction and Rutherford backscattering. Measurements on free polycrystalline chromium films alone revealed that the chromium grain size increases with the thickness of the chromium film, but that very little grain growth occurs when the temperature is increased. A phase change in the chromium crystal structure from simple cubic to b.c.c. was detected around 550°C. During CrSi interaction, in cases where an unlimited supply of silicon is provided, CrSi 2 forms on both single-crystal silicon and a-Si at temperatures around 450°C, i.e. well below 550°C. However, when the silicon supply is limited, a metal-rich phase (Cr 5Si 3) develops from the CrSi 2 and the excess chromium at 550°C. Such a situation may prevail when a thin layer of radioactive (a-) 31Si is used as a tracer in diffusion studies. The accumulation of impurities such as oxygen at the Si〈100〉−a- 31Si interface during the formation of metal-poor silicides where silicon is the diffusing species ( e.g. CrSi 2) may limit the supply of silicon from the substrate. This situation leads to the formation of a more metal-rich silicide, e.g. Cr 5Si 3. As a result the silicide layer usually exhibits bad adhesion and peeling off. To prevent formation of Cr 5Si 3 in such cases it was found that if a thin “glue” layer of chromium (about 200 Å) is first deposited onto the Si〈100〉 substrate, followed by the a- 31Si and the rest of the chromium, complete CrSi 2 formation can be obtained. Rutherford backscattering showed that the thin layer of chromium “glue” first reacts with the Si〈100〉 to form CrSi 2, thus providing a clean interface between the Si〈100〉 and the final CrSi 2 layer, and good adhesion results. A polycrystalline CrSi 2 structure similar to that formed on Si〈100〉 alone thus develops for diffusion studies.