Optical contrast determination of the thickness of SiO2 film on Si substrate partially covered by two-dimensional crystal flakes

SiO2/Si substrate has been widely used to support two-dimensional (2-D) crystal flakes grown by chemical vapor deposition or prepared by micromechanical cleavage. The visibility of 2-D flakes is very sensitive to the thickness of the SiO2 layer (h(SiO2)) , which can not be determined precisely after the deposit of 2-D flakes. Here, we demonstrated a simple, fast and nondestructive technique to precisely determine h(SiO2) of SiO2 films on Si substrate only by optical contrast measurement with a typical micro-Raman confocal system. Because of its small lateral resolution down to the micrometer scale, this technique can be used to access h(SiO2) on SiO2/Si substrate that has been partially covered by 2-D crystal flakes, and then further determine the layer number of the 2-D crystal flakes. This technique can be extended to other dielectric multilayer substrates and the layer-number determination of 2-D crystal flakes on those substrates.