Estimation of Centrifugal-Force Effect on Resist Coat around Wafer Edges

In exposure equipment for production of semiconductor devices or steppers, the accuracy of the alignment between the mask and wafers is one of the most important factors. Since optical information is used in most methods for measurement of the alignment, there remain some intrinsic errors. For example, since a photosensitive resist is spin-coated, its thickness is not constant due to the centrifugal-force effect, and this variation causes an optical distortion. In this letter, we give an analytical estimation of the centrifugal-force effect on the resist surface profile around wafer edges. We show that due to this effect for a concave wafer pattern, the resist surface has an asymmetrical form proportional to βx 3 where β>0 and x is a position, whereas for a convex pattern it is proportional to -βx 3.