Plasma damage to gate oxide is studied using the test structures with various length antennas. It is shown that the plasma damage to gate oxide can be monitored quantitatively by measuring charge to breakdown (QBD). From the QBD measurements, it is confirmed that the degradation occurs in the duration of over-etching but not in the duration of main etching. The breakdown spots in gate oxide are detected by a photon emission method. The breakdown are caused by plasma damage at the LOCOS edge. A LOCOS structure plays an important role for the degradation by the plasma damage.