A plasma processing tool for fabricating a semiconductor device on a semiconductor wafer includes an optical window disposed on a plasma chamber, remotely from a plasma region. The window is thermally connected to an electrical heater element capable of maintaining the window at a temperature of at least 30° C. A heater controller provides electrical power to the heater element. During operation of the plasma processing tool, the heater controller provides power to the heater element so as to maintain the window at a temperature of at least 30° C. during at least a portion of a plasma process step in which by-products are produced in the plasma chamber.