Image placement errors in x-ray masks induced by changes in resist stress during electron-beam writing

Image placement demands for x-ray masks useable in sub-0.25 mu m applications have prompted a renewed effort to further reduce or eliminate sources of mask distortion. In this work we studied a phenomenon that so far had been overlooked among the contributors to poor image placement: the large change in stress of some resists upon exposure. For instance, it was found that the stress of PMMA films changes from similar or equal to 1x10(8) dyn/cm(2) in the unexposed state to similar or equal to 1x10(7) dyn/cm(2) after the exposure dose received during electron-beam (e-beam) writing. Because e-beam writing is a serial process, this stress change induces a dynamic distortion of the mask, since the force applied on the membrane by the resist film varies continuously as writing of the pattern progresses. The stress of PMMA films was determined as a function of dose for exposures using e-beams of 100, 75, and 25 keV energy, broadband UV light (from a Hg lamp), and to x rays from a synchrotron ring with lambda(charac.)=10 Angstrom. In all cases, the stress was found to be a very strong function of the exposure dose at low doses with a slow decay in stress change at larger dose values. The stress changes upon exposure are attributable to changes in molecular weight (MW) distribution, rather than to a reduction in MW only, since the stress of unexposed PMMA films was found to be the same for a wide range (25k to 920k) of initial MW resin. Model experiments demonstrated the correlation between the in-plane distortion (IPD) of membranes and the lowered resist stress. Because of the drastic stress drop at low doses, fogging prior to writing reduces the distortion problem. Other methods thought to reduce resist stress (various annealing cycles, different solvents, plasticizers) proved unsuccessful. The effect of fogging on IPD was measured for several model cases and found to correlate with the lowered resist stress. Initial evaluation of the changes that fogging induces on the lithographic performance of the resist was performed. (C) 1996 American Vacuum Society.