Second-harmonic generation reveals the oxidation steps in semiconductor processing

Monitoring oxidation steps is an important factor during the fabrication of semiconductor devices, because transistor performance can be greatly affected by defects in the passivation layer. As an example, we discuss the formation of a gate stack in metal oxide semiconductor (MOS) devices using Ge as an alternative channel material. Building an MOS gate stack on Ge requires passivation of the interface between the dielectric (typically a high-k material such as Al ₂O ₃ or HfO ₂, grown by means of atomic layer deposition (ALD)) and the Ge channel. Such passivation can be obtained from a very thin Si layer, epitaxially grown on Ge. The Si surface receives an oxidizing clean (O ₃ or wet chemical clean) before the ALD step. In this work, second-harmonic generation (SHG) data are presented for silicon layers with varying thickness, grown with either trisilane (Si ₃H ₈) or silane (SiH ₄) and with various cleaning steps. The trend in second-harmonic response upon azimuthal rotation of the samples was comparable for both silane and trisilane as a Si precursor. Our results show that upon oxidation, the SHG intensity reduces, most likely due to a reduction of the amount of crystalline Si, which is converted to SiO ₂.