Temperature measurement using etalons

A system comprising:

• a processing chamber having a support for a sample etalon-object;
• a stage for a reference etalon-object;
• a radiation source;
• and
• a radiation detector, wherein:
• a first optical path is defined from the radiation source to the radiation detector,
• the support disposes the sample etalon-object in the first optical path, and
• the stage disposes the reference etalon-object in the first optical path.

A method comprising:

• exposing a sample silicon wafer to a first incident radiation, resulting in a first transmitted radiation;
• exposing a reference silicon wafer to a second incident radiation, resulting in a second transmitted radiation, wherein the first transmitted radiation becomes the second incident radiation;
• and
• analyzing a resultant radiation for a heterodyned spectrum, wherein the second transmitted radiation becomes the resultant radiation, the analyzing comprising:
• measuring a fringe spacing between two adjacent interference patterns in the heterodyned spectrum;
• and
• utilizing the fringe spacing to estimate a refractive index of the sample silicon wafer.