Precise measurement of the thickness of silicon wafers by double-sided interferometer and bilateral comparison

The Length Standards Group at the National Metrology Institute of Japan has developed a double-sided interferometer (DSI) for non-contact thickness measurement, that is traceable to SI units. The measurement uncertainty of DSI was evaluated. The results of gauge block measurements were compared with those of a conventional gauge block interferometer and their equivalency within the claimed uncertainty was confirmed. The advantage of applying a DSI in measuring the thickness of silicon wafers is that it has absolute measurement traceable to SI units with high precision and independence from the refractive index of the sample material. The expanded uncertainty of silicon wafer thickness measurement was evaluated as 20 nm (the coverage factor k = 2). A bilateral comparison on silicon wafers' thicknesses was conducted with Korea Research Institute of Standards and Science, using a different measurement technique from the spectral-domain interferometer. Results of 100 μm, 300 μm, 600 μm, and 950 μm-thick silicon wafers agreed well with absolute values of En-numbers of 0.15 or less.