Interferometric thickness variation test method for windows and silicon wafers using a diverging wavefront

摘要

An interferometric apparatus and method are provided for determining a ceal thickness and thickness variations of silicon wafers and other window-like optics. The method includes the steps of (1) emitting a beam of electromagnetic radiation having a diverging wavefront, at an object which is transparent to the radiation, (2) splitting the beam into a reference beam and a test beam, (3) positioning the object in a first position in the test beam to irradiate a central portion of the object, (4) directing the test beam through the central portion, (5) causing the reference beam to interfere with the test beam which has passed through the central portion to form a first interferometric fringe pattern, (6) changing the length of the test beam which has passed through the object until the first interferometric pattern is changed into a desired second interferometric fringe pattern, (7) determining a central thickness of the object in accordance with an amount by which the length of the test beam was changed, (8) moving the object in said test beam to a location in the test beam to irradiate a second portion of the object which is larger than the central portion, (9) causing the reference beam to interfere with the test beam which has passed through the second portion to form a third interferometric fringe pattern, and (10) determining variations relative to the central thickness in accordance with the third interferometric fringe pattern. The method can also be used for measuring thickness variations by taking into account a bow in the object. An interferometer is also provided for carrying out the method.