Oxide layers are transparent films. If light is irradiated onto the wafer and reflected, various properties of the light wave are changed which can be detected with metering devices. If there are multiple layers stacked on each other they must differ in optical properties to allow a determination of the materials.
To monitor the film thickness across the wafer, several measuring points are quantified (e.g. 5 points on 150 mm, 9 points on 200 mm, 13 on 300 mm wafers). Thereby not only the absolute thickness is relevant but also the uniformity across the wafer, because the uniformity is important in subsequent processes. If the deposited layer is too thick or too thin material has to be removed (e.g. by etching) or deposited again.