A holographic phase-shifting interferometer technique to measure in-plane distortion

We describe the use of holographic phase-shifting interferometry (HPSI) to measure in-plane distortion of a substrate in which a shallow grating, prod uced by interferometric lithography, has been etched. The diffractive metro logy inherent in HPSI should enable one to study process-induced distortion s down to the level of a few nanometers. We used HPSI to investigate distor tion introduced by the anodic bonding of silicon nitride membranes to Pyrex frames in x-ray lithography masks. This was part of an effort to develop a n inverted x-ray mask configuration. The HPSI technique gave quantitative m easurements of the linear and nonlinear components of distortions in one-di mension. The high levels of distortion found were presumably introduced by the high temperatures used in anodic bonding, combined with the asymmetric manner in which the membranes were brought into contact with the Pyrex fram e. The comprehensive, quantitative measurement of distortion provided by HP SI should enable us to modify the mask making process so as to avoid steps that introduce distortion. (C) 1999 American Vacuum Society. [SO734-211X(99 )17006-9].