This kind of configuration is designed for backside illuminated image sensor process control. The Metrology System is equipped with measuring instruments to control the total wafer thickness and individual layers in the stack for all steps of the BSI process. Special IR technology helps to achieve great Gage Repeatability and Reproducibility values even for very thin Si layers.
The SemDex Metrology System is using various types of Spectral Coherence Interferometry instruments with different wavelengths and spectral widths to cover the full range of Silicon thickness expected for typical BSI processes, starting at 1 µm all the way up to full wafer thickness and mm-range stacks. Special IR cameras are used for through silicon pattern alignment to enable on-product measurements and scribe line test box monitoring. An IR microscope in combination with a counterpart IR illumination source can be used for box-in-box wafer bond alignment control.
Typical parameters that customers are checking with this configuration are global wafer thickness variation after grind, polish, CMP and etch. With an extra SW package it is even capable to monitor the edge trim process used in the most advanced BSI processes.