Silicon wafer thickness variation measurements using the National Institute of Standards and Technology infrared interferometer

Decreasing depths of focus, coupled with increasing silicon wafer diameters, place greater restrictions on chucked wafer flatness in photolithography processes. A measurement device is described that measures thickness variation of double-sided polished wafers using an IR source and vidicon detector. Various possible instrument configura- tions are described with the focus on a setup that uses a collimated wavefront to produce interference fringes between the front and back surfaces of the plane parallel wafer. Experimental results are presented. These tests include (1) a drift test; (2) comparisons between measure- ments performed using different collimators and, subsequently, wave- fronts; (3) an exploration of the impact of phase change on reflection due to the wafer clamping method; and (4) an intercomparison with thickness measurements recorded by a capacitance gage-based instrument and surface measurements obtained using a separate visible wavelength interferometer. © 2003 Society of Photo-Optical Instrumentation Engineers.