Abstract

An unusual overlay error has been seen in test chips exposed at A ~ 250 nm through pellicles made from polyvinyl butyral (PVB). The error manifests itself as a local excess magnification within each subarray of an integrated circuit (IC) pattern. In the worst case we have seen misregistration increase over time to 600 nm. The phenomenon appears to be the same as recently reported at λ = 365 nm. We have found the cause of the problem to be the steady pellicle thickness loss known to arise under 248 nm irradiation. The lifetime of these pellicles is acceptable by the usual criterion of dose to punch-through, but the image displacement effect is of far greater concern. The pellicle thickness loss is not uniform, since gross features of the mask layout remain visible after several millimeters of defocus. When the resulting phase modulation reaches ~λ/10 during wafer exposure, the sensitive overlay tolerances of photolithography can be exceeded. This can occur at cumulative pellicle doses about 2 orders of magnitude below punch-through. The effect is highly pattern dependent, so different IC levels show placement errors of widely different severity. Within a level, large shifts typically occur near kerfs. We will discuss the theory of pellicle image formation and show interferograms of the phase modulation. We will show limited etch rate measurements from pellicles made of two different materials. The problem can for all practical purposes be eliminated by using pellicles made from fluorocarbon.

© 1992 Optical Society of America