Dose-focus monitor technique using CD-SEM and application to local variation analysis

A dose-focus monitoring technique using critical dimension scanning electron microscope (CD-SEM) is studied for on-product applications. Our technique uses two target structures; one is a dense grating structure with iso-focal pitch for dose determination, and the other is a relatively isolated line grating with no assists for focus determination. The small sizes of these targets enable us to monitor dose and focus variations across the chip on a product wafer. The model which describes how the top and bottom CD depend on dose and focus deviations is the same as that for scatterometry dose-focus metrology, and monitoring precision is estimated to be the order of 1% for dose and 10 similar to 15nm for focus. The method has strong potential to apply to dose and focus monitoring of product wafers. By using a mask with a multitude of these targets, it is possible to study dose and focus variations across the wafer in great detail. The focus variation of pairs of such targets is measured for various separations between the two targets. As the separation distance increases from similar to 100 mu m to similar to 10mm, the focus variation increases from 10nm to 25nm. We think that the true focus variation between targets becomes near zero at the small separation distance, while the focus variation increases as separation distance increases because more variation sources such as wafer thickness variation are included at larger separation distances. Our small CD-SEM targets allow us to explore this kind of local spatial variation analysis.