Semiconductor wafer surface measurement
The surface measurement results shown below are from sample GaAs compound semiconductor wafers manufactured by Wafertech Ltd
The requirement from Wafertech was to measure surface distortion, surface finish and other parameters such as TTV (total thickness variation) across both polished and unpolished wafers. For polished wafers the Zygo GPI Fizeau Interferometer provides accurate surface measurement of wafers in a matter of seconds. Measuring unpolished “as cut” GaAs compound semiconductor wafer samples makes use of the Zygo NewView 7300 an optical profiler with a much higher resolution able to not only measure the profile but also the roughness of surfaces. This is achieved by multiple measurement scans of the surface using a high magnification objective lens and stitching the scanned data to create a much larger data set covering the whole of the wafer surface.
Figure 1 is a screenshot from the Zygo GPI showing the distortion across a 4” GaAs compound semiconductor wafer. This surface measurement delivers over 300,000 data points in seconds. A snap line has been placed across the 3D data (top left) and this is displayed underneath as a 2D plot similar to what you would achieve with a much slower stylus surface profiler. This measurement is fast and non-contact and will display a host of parameters (including peak to valley, rms etc) to suite the measurement requirements. The system is able to measure deviations from flatness to better than 16nm across the full 4” aperture of the system.
Figure 2 shows the measurement results and screenshot from the Zygo NewView 7300 non-contact optical surface profiler. With this system an objective lens with a higher magnification is used to measure a smaller area of the surface and multiple measurements are stitched together with the inbuilt software to provide a complete and accurate data set for the whole surface. Each scan takes a few seconds and large surfaces such as this 2” wafer can be covered in 10’s of minutes with a lateral resolution of 11 microns. This approach works well when the surface is unpolished and where surface roughness is required in addition to just a profile. In this example of a GaAs compound semiconductor wafer the peak to valley value across the wafer is 27 microns with a Ra of 1.9 microns