Method for compensating for wafer shape measurement variation due to variation of environment temperature

A machine-readable storage medium storing computer-readable instructions that, when executed, cause a computer to perform the functions of:

• determining a first reference cavity tilt value between transmission flats of an interferometric measuring system when a temperature gradient exists between said interferometric measuring system and a reference wafer mounted in the cavity of said interferometric measuring system;
• determining a second reference cavity tilt value between transmission flats of said interferometric measuring system when said reference wafer mounted in the cavity of said interferometric measuring system has thermally stabilized such that said reference wafer is at substantially an equal temperature to said interferometric system;
• calculating a third reference cavity tilt value equal to the difference between the first reference cavity tilt value and the second reference cavity tilt value;
• and
• using said first cavity tilt value and said second cavity tilt value to determine and compensate for thermally induced errors in measurement of a sample by said interferometric measuring system.