Silicon x-ray masks - pattern placement and overlay accuracy

Epitaxial-grown silicon as the membrane and gold as the absorber material fulfils the requirements on an x-ray mask concerning longterm stability and absorber-induced distortions. The pattern placement accuracy of the master masks is primarily determined by the e-beam writing process and yields appr. 0.1 micrometer (3 sigma). The overlay of two different masters meets the target specifications (0.15 micrometer (3 sigma)) even in a non-optimal mask geometry. In case of x-ray copies the deviations from the CAD data amount to 0.2 micrometers and are mainly generated by temperature rise during exposure in a non-optimized exposure system.