In this study, the evolution and reduction of the surface roughness during the high-speed chemical dry thinning process of Si wafers were investigated. The direct injection of NO gas into the reactor during the supply of F radicals from NF3 remote plasmas was very effective in increasing the Si thinning rate, due to the NO-induced enhancement of the surface reaction, but resulted in the significant roughening of the thinned Si surface. However, the direct addition of Ar gas, together with NO gas, decreased the root mean square surface roughness of the thinned Si wafer significantly. The process regime for the enhancement of the thinning rate and concomitant reduction of the surface roughness was extended at higher Ar gas flow rates. In this way, Si wafer thinning rates as high as 22.8 mu m/min and root-mean-squared surface roughnesses as small as 0.75 nm could be obtained. The results indicate that the high-speed chemical dry thinning process using F radicals and directly injected NO/Ar gases can be applied to ultrathin Si wafer thinning with controlled surface roughness. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3456124]