The increase in wafer temperature due to plasma heating during etching has been studied. Si and InP were etched using a high ion density discharge generated by an electron cyclotron resonance source. The wafer temperature was measured in-situ using fiberoptic thermometry as microwave power, rf power, chamber pressure, and gas flow were varied. Wafer temperatures increased with both microwave and rf power, and decreased with chamber pressure. For rf power of 50 W, chamber pressure of 1 mTorr, a source distance of 13 cm, and 10 sccm Ar flow, an increase in microwave power from 50 to 500 W caused the temperature to increase from 62 to 186 °C. Additionally, the use of He flowing at the backside of the wafer for temperature control was analyzed. By setting the backside He pressure at 3 Torr, the temperature increased from 20 °C at the beginning of the etch to only 29 °C after 12 min. Time dependent etch characteristics of InP were studied and related to the wafer temperature measurements. At 100 W microwave power, the InP etch rate increased from 100 to 400 nm/min as the wafer temperature rose from 20 to 150 °C. As the temperature increased above 150 °C, the profile became more undercut and the surface morphology improved. By setting the stage temperature to -100 °C and using 3 Torr He pressure at the backside of the wafer, the InP etch rate remained constant during etching and undercutting was suppressed. For 500 W microwave power, a fast InP etch rate of 2 μm/min was obtained when the wafer temperature was <110 °C, and it increased to over 4 μm/min when the temperature was >150 °C.