SiO2 thickness measurements Using the NanoSpec Model 200 Automatic Film Thickness tool NanoSpec Model 200 AFT Computer control and thickness reading light source and filter Wafer stage Reference wafers The NanoSpec AFT measures optically transparent thin films on silicon wafers. The intensity of a monochromatic reflected light depends on the thickness of the film. The tool uses a photomultiplier tube to measure the reflected optical spectrum from a bare silicon wafer and the wafer under test. Given the refractive index of the film in question (SiO20 and the two measured spectrums, the computer will determine the thickness. Typically, the tool is set for SiO2 on silicon but addition options are available such as measuring silicon nitride thickness.