Polyimide membrane x-ray lithography masks—Fabrication and distortion measurements

Abstract

A process for fabricating polyimide membrane x-ray lithography masks is described in detail. Thin membranes of polyimide are formed by spinning polyamic acid on Corning 0211, or other glass substrates, and polymerizing in situ. The glass substrate acts as a holder and an efficient heat sink during formation of gold absorber patterns on top of the polyimide, a procedure that usually includes ion beam etching. A support ring is then bonded to the polyimide, and the glass etched in dilute HF. Optically smooth polyimide membranes with thicknesses from 0.5 to 5 μm are readily achieved. Measurements of the distortion of polyimide membrane x-ray lithography masks have been made using a holographic moiré method and a method based on measuring the spread in the angle of diffraction from gold-grating patterns. These indicated a distortion less than two parts in 105 over the central 7.5-mm-diam area of a 19-mm-diam membrane.