A wall film monitoring system includes first and second microwave mirrors in a plasma processing chamber each having a concave surface. The concave surface of the second mirror is oriented opposite the concave surface of the first mirror. A power source is coupled to the first mirror and configured to produce a microwave signal. A detector is coupled to at least one of the first mirror and the second mirror and configured to measure a vacuum resonance voltage of the microwave signal. A control system is connected to the detector that compares a first measured voltage and a second measured voltage and determines whether the second voltage exceeds a threshold value. A method of monitoring wall film in a plasma chamber includes loading a wafer in the chamber, setting a frequency of a microwave signal output to a resonance frequency, and measuring a first vacuum resonance voltage of the microwave signal. The method includes processing the wafer, measuring a second vacuum resonance voltage of the microwave signal, and determining whether the second measured voltage exceeds a threshold value using the first measured voltage as a reference value.