Image Placement Errors in X-Ray Masks Induced by Changes in Resist Stress During e-Beam Writing

Image placement demands for X-ray masks useable in sub-quarter micron applications have prompted a renewed effort to minimize or eliminate sources of mask distortion. The specification for masks to be used for 0.25 um CD call for a placement accuracy at the mask level of 35 nm (3sigma) For 0.18 um and 0.13 um the placement accuracy is 22 and 15 nm (3sigma), respectively. In order to meet these tight specifications it is necessary to have a full understanding of the various factors that contribute to the placement errors. Through the years efforts have been made to elucidate sources of error during processing and writing. Among the former are the as-deposited stress of the absorber (Chiu), and the effect that annealing during subsequent processing has on the absorber stress (Nash). Writing errors can be due to charging (Puisto 1994), and to instability of the e-beam writer (Groves). In this work we studied a phenomenon that so far seems to have been overlooked among the contributors to poor image placement in X-Ray maks: the stress change that most resists experience upon exposure. Since e-beam writing is a serial process, this stress change induces a dynamic distortion of the mask, because the force applied on the membrane by the resist film varies continuously as writing of the pattern progresses. Particular attention was given to the changes in stress that PMMA experiences upon exposure, because this is the resist presently used at IBM's Advanced Mask Facility for fabrication of X-Ray masks...)