Impact of isotropic plasma etching on channel Si surface roughness measured by AFM and on NMOS inversion layer mobility

Si wafers were intentionally roughened by isotropic plasma etching to reach RMS values (from 0.15 to 1.4 nm) comparable to those measured by AFM on multichannel devices. Isotropic plasma selective etching degraded mobility by less than 5% for a RMS les 0.7 nm at high transverse electric field. This mobility decrease is enhanced at low field because of fixed charges at the Si/SiO₂ interface (-8x10 q.cm²). Charge pumping measurements revealed that roughening with the developed process conditions does not modify the interface state density. An improved extraction method for surface roughness mobility is also proposed.