The author investigated the etching characteristics of semi-insulating (SI) and n-doped (n-) 4H-SiC substrates at a high etch rate of about 2 mu m/min using high-density SF6/O-2 inductively coupled plasma. The etch rate of SI-SiC was found to be lower than that of n-SiC, and the etching profile of SI-SiC showed retrograde features with a larger sidewall angle and a rounder etched bottom compared to n-SiC. These characteristics are attributed to the difference in wafer heating and negative charging of the sidewall during plasma etching between both substrates. The temperature of n-SiC increases by radiative heating from the high-density plasma during etching because of the higher free-carrier absorption compared to SI-SiC. Furthermore, the negative charge buildup at the sidewall of SI-SiC becomes stronger because of the lower electrical conductivity compared to n-SiC.