Silicon Etching Mechanisms with HBr and Cl2

The effects of exposing a single crystalline silicon surface to a halogen plasma can be studied with spectroscopic ellipsometry (SE). The measurements show that the surface layer is partially amorphized after plasma exposure. In addition, the existence of a halogen containing layer can be shown (slide 1).

Similar information can be obtained by XPS. The thickness of the amorphized layer is derived from the FHWM of the Si-Si peak (slide 2).

Slide 3 compares the thicknesses of the brominated top layer as measured by SE and XPS. Similarity between etch rate and halogen coverage suggests that the etch rate is limited by the ability of halogenated products at the surface to form volatile species. Halogen coverage can be limited by: thermal flux of neutrals, ion flux, steric hindrance. At etch rate saturation (20 As-1), the silicon atom removal rate is 1016 cm-2 s-1, thermal flux of HBr at 2 mTorr is higher than 1017 cm-2 s-1.

From the studies, the following conclusions can be drawn (slide 4):
- XPS and SE both indicate that the effective thickness of amorphized and halogenated layers are in the range 5-30 Å and increases with ion energy.
- The amorphization effect is almost identical for Cl2 and HBr plasmas.
- The halogenated layer is thicker for HBr.
- The etch rate saturates for HBr due to halogen coverage saturation.
- Halogen saturation is most likely attributed to steric hindrance by Br atoms.

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