The use of a disposable double spacer of silicon-nitride/amorphous-silicon to fabricate lightly-doped-drain (LDD) MOSFETs with just two masking steps compared to four in the conventional oxide spacer LDD FET is discussed. The alpha -Si spacer is disposed of after it has been used to pattern the nitride. The nitride acts as a second spacer to block low-energy source/drain (S/D) implants and to shift LDD implants away from the gate edge. Self-aligned metallization can be realized using the nitride as silicidation barrier.
Ronkainen, H., Drozdy, G., & Franssila, S. (1991). The use of disposable double spacer and self-aligned cobalt silicide for LDD MOSFET fabrication. IEEE Electron Device Letters, 12(3), 125 - 127. https://doi.org/10.1109/55.75732