Stitching errors in diffractive nulltest elements

For a nulltest of aspheric lenses diffractive optical elements (DOEs) can be used as they offer the freedom of design to adapt easily to the desired sample under test. Like every other component in the measurement setup they introduce systematic errors to the measurement. The main error sources concerning the phase of the DOE are the aberrations of the substrate and the structure distortion due to the lithography machine used for fabricating the DOE. Here, a x-y direct writing lithography, where the exposure is done stripewise, was used to fabricate amplitude DOEs of a spherical lens. A local pattern distortion due to the stitching of the stripes and a global distortion due to errors of the global coordinate system could occur. The DOEs were characterized by an absolute test in a Mach-Zehnder Interferometer. A study concerning global and local distortions of the DOE pattern was performed.