Using CH3F/O2/He based chemistries in high density plasmas for silicon nitride spacer etching, loss of silicon in active source/drain regions of CMOS transistors can be observed. Minimizing the so-called silicon recess during nitride spacer etching is extremely difficult to achieve but mandatory since it directly impacts the device performance. In this work, the authors investigated the benefits of CH3F/O2/He/SiCl4 plasma to limit this recess. Using x-ray photoelectron spectroscopy, the mechanism for high Si3N4/Si selectivity is identified as the formation of a preferential thick SiOxFyClz passivation layer, generated at the silicon surface. The silicon damage is reduced by the growth of this thick layer, limiting the transfer of the reactive layer into the silicon film. These results have been confirmed on pattern structures for fully depleted silicon on insulator 14 nm technology showing the benefit of CH3F/O2/He/SiCl4 etch plasma compared to CH3F/O2/He plasma.

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