Method and system for measuring patterned substrates

A common path auto-correlation low coherence interferometer method comprising:

• directing a light beam emitted from a light source output toward a first beam splitter;
• directing at least a portion of the light beam output by the light source through the first beam splitter toward a sample having two reflective interfaces including a top surface reflective interface and a feature bottom reflective interface;
• passing each of a reference light beam reflected from the top surface reflective interface and a measurement light beam reflected from the feature bottom reflective interface through the first beam splitter and toward a second beam splitter;
• reflecting the reference light beam through the second beam splitter to a first mirror;
• passing the measurement light beam through the second beam splitter to a second mirror, wherein the first mirror is a first distance from the second beam splitter and the second mirror is a second distance from the second beam splitter, wherein the first mirror is a scanning mirror;
• combining a reflected measurement light beam from the second mirror and a reflected reference light beam from the first mirror in the second beam splitter;
• directing a resulting interference pattern toward a detector;
• and
• scanning the first mirror such that a difference distance between the first distance and the second distance is equal to an optical separation between the feature bottom reflective interface and the top surface reflective interface,
• wherein the light source is a polarizing light source and further comprising a selected polarized light beam reflective and transmissive element is disposed in a beam path between the first beam splitter and the sample and the second beam splitter is a polarizing beam splitter.