The use of diffraction techniques for the study of in-plane distortions of x-ray masks

Abstract

We describe the use of gratings, placed on boron–nitride membranes and exposed with electron beam lithography, as a method of obtaining both qualitative and quantitative information about x-ray mask in-plane distortions. The mask is placed at an angle α≂arcsin λ/d0 measured with respect to an incident plane wave front (λ=6328 Å for the He–Ne laser and d0=1.0 μm for our grating pitch). A mirror, placed at right angles to the mask, is positioned so that it also intersects the incident plane wave front. Both the resultant diffracted wave front and the reflected diffracted wave front are then collected by a lens system and video camera. The grating is written on all or part of the active area of the mask. The process of developing and patterning the trilayer resist structure and absorber film is identical to that of processing an x-ray mask. Patterned gratings allow us to observe: 1) stage placement errors to <0.1 μm, 2) in-plane errors induced by the mounting fixture, 3) in-plane distortions due to thermal gradients generated at e-beam write-time, 4) in-plane distortions due to the absorber stress. Results indicate that for a 0.9 μm tungsten film the worst-case strain due to the absorber, ∼20 ppm, is seen at visual endpoint; however, complete etching of the interlayer lowers this strain to ∼2–3 ppm.