Method and apparatus for automatic optical inspection

A method of optically measuring the variation in thickness or index of refraction of a thin film structure, comprising the steps of:

• illuminating the structure with light, collecting the light leaving the structure so illuminated using a light sensor, and measuring the grey level signal of the image, characterized by the further steps of:
• disposing a polarizer device in the optical axis between the source and the structure, for linearly polarizing the incident light to have a P-plane component parallel to the plane of incidence of said light, and an S-plane component perpendicular to the P-plane, disposing an analyzer device in the optical axis between the structure and the sensor, and disposing a phase compensator in one of said optical axes for compensating the phase shift between the P-plane component and the S-plane component caused by said thin film structure;
• selecting a reference point on the article surface;
• rotating said polarizer, analyzer and phase compensator devices around their optical axes to obtain a minimum grey level signal in said preselected reference point;
• and
• determining the thickness or index of refraction of said thin film structure responsive to the measurement of the grey level signal.

Apparatus for automatically inspecting the surface of an article particularly a thin film multilayer structure including at least two materials, comprising:

• illumination means including an intense source of polychromatic light of wide spectral range for illuminating the article with incident light;
• a light sensor for collecting light leaving the article and for outputting electrical signals varying responsive to the intensity of the collected light;
• imaging means for optically imaging the illuminated article responsive to the light collected by the light sensor;
• and
• processing means for processing the output of said light sensor in order to indicate defects in said article;
• said illumination means including:
• a polarizer device for linearly polarizing the incident light, so as to include a P-plane component parallel to the plane of incidence of said light, and an S-plane component perpendicular to the P-plane;
• said imaging means including:
• an analyzer device for extinguishing the reflected light;
• said apparatus including:
• a phase compensator device for compensating the phase shift between the P-plane component and the S-plane component caused by one of said materials, disposed in the optical axis of said illumination means between the polarizer and the article or in that of said imaging means between the article and the analyzer;
• and
• means for rotating at least two of said polarizer, analyzer, and phase compensator devices, to obtain the maximum contrast between the two materials in the image of the article.