Real time wafer deformation detection by burl detection

摘要

A semiconductor wafer is clamped on a wafer table to undergo various process steps. The wafer table has a plurality of burls to support the wafer. Burls are pin-like structures, having a top surface to support the wafer and a bottom surface connected to the wafer table. Ideally, the wafer is perfectly flat. However, due to various reasons, the wafer may be deformed, e.g., not flat or warped. Loading the deformed wafer on the wafer table may cause even more deformation of the wafer. The disadvantage of the deformation is that a process conducted on the wafer, such as an exposure by radiation or a measurement, does not take place on the intended area of the wafer. Further, the wafer may also deform between different process steps applied to the wafer. For example, the process may provide a thermal gradient that causes the wafer to locally extend or contract, which may result in local slip between the wafer and some of the burls.