Grazing and normal incidence interferometer having common reference surface

A method for measuring thickness variations of a specimen comprising a plurality of sides, the method comprising:

• diffracting variable coherence light energy into multiple channels of light energy;
• directing said multiple channels of light energy toward two sides of said specimen and toward multiple reflecting surfaces, the multiple reflecting surfaces comprising a semi-transparent reflecting surface transparent through a first side and less than 90 per cent reflective on a second side;
• sensing light energy received from the two sides of the specimen;
• and
• performing an interferometric normal incidence inspection of the specimen through the semi-transparent reflecting surface.

A specimen thickness measurement method, comprising:

• diffracting variable coherence light energy toward a specimen having a plurality of sides;
• receiving light energy from said diffracting at a plurality of reflecting surfaces and at two sides of said specimen, the plurality of reflecting surfaces comprising a semi-transparent reflecting surface transparent through a first side and less than 90 percent reflective on a second side;
• and
• performing an interferometric normal incidence inspection of the specimen through the semi-transparent surface;
• wherein said diffracting directs energy simultaneously toward one reflecting surface and one side of the specimen.