Apparatus and method for determining a depth of a region having a high aspect ratio that protrudes into a surface of a semiconductor wafer

A method of measuring the depth of a region having a high aspect ratio that protrudes into a surface of a semiconductor wafer, comprising steps of:

• providing a semiconductor wafer comprising a first major surface, a second major surface opposing the first major surface and a region protruding into the first major surface, the region having a width and a depth and a high aspect ratio,
• illuminating at least one of the first major surface and the second major surface with light comprising multiple wavelengths;
• measuring light of multiple wavelengths that is reflected by the semiconductor wafer as a function of wavelength, the light reflected by the semiconductor wafer forming an interference pattern, and
• analysing the measured light by applying a Fourier transform to the interference pattern and generating a graphical representation having peaks associated with different interfaces of the semiconductor wafer, and
• determining the depth of the region from at least one of the peaks.