Optical inspection apparatus and method thereof

An optical inspection apparatus, comprising:

• a light source which radiates light to a sample;
• a light interference device configured to generate transmitted, scattered, or reflected target light from the sample and a reference light to interfere with each other, such that a strength of light after the interference becomes lower than a strength of the target light;
• a photon counter configured to count a number of photons in the light after the interference by the light interference device;
• and
• a defect identifier which calculates a probability distribution of each case in which a defect is present or absent, and identifies the presence or absence of a defect, on the basis of a detected photon number obtained by the photon counter and the calculated probability distributions, wherein
• a controller controls the amplitude and the phase of the reference light based on a required performance parameter threshold, wherein the controller is configured to manipulate a quarter wavelength plate and a half wavelength plate located in the reference arm, so as to provide a decreased average erroneous identification rate in case the defect is present and decreased average erroneous identification rate in case the defect is absent.