Temperature Effects in Plasma Etching

The wafer surface temperature depends primarily on the chuck temperature, the ion density and ion energy and the exothermicity of the etching reaction. Surface temperature influences etching processes (slide 1):
- The Reaction probabilities of incident species depends on substrate temperature.
- The vapor pressure of etch products is temperature dependent.
- The re-deposition of reaction products on feature surfaces depends on temperature.

Tight control of the wafer surface temperature is an engineering challenges caused by sudden changes in the plasma condition during the transition between process steps. In addition, the true substrate temperature is difficult to monitor.

Slide 2 summarizes the findings on th selectivities and profile control for silicon etching in a SF6 plasma by S. Tachi et al. (Appl. Phys. Lett. 52 (1988) 617):
-At very low temperature, the silicon etch rate in SF6 based plasmas is not impacted
- SiO2 and photoresist etch rates decrease strongly for decreasing temperatures
- Spontaneous etching reaction between fluorine atomes and silicon are frozen for temperatures below -90°C (no under cut below an SiO2 hard mask).

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