By using techniques for near field probes to measure dielectric values of blanket films, the measure of the sidewall damage of the patterned structure is calculated. The interaction between the near field probe and the etched structure is modeled to obtain the model total capacitance. The near field microwave probe is calibrated on a set of blanket films with different thicknesses, and the dielectric constant of the etched trench structure is calculated using the measured frequency shift and calibration parameters. The measured capacitance is further calculated for the etched trench structure using the dielectric constant and the total thickness of the etched trench structure. The effective dielectric constant of the structure under study is extracted where the model capacitance is equal to the measured capacitance. The measure of the sidewall damage is further calculated using the effective dielectric constant.