Novel gap alignment sensor for high-resolution proximity lithography

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The control of very small distances is essential for many applications and alignment procedures in the field of micro technology, e.g. micro lithography for MEMS or micro optics, where proximity lithography is often used for cost effective mass fabrication. Also in proximity lithography the requirements, especially for resolution, are increasing permanently. Recently new techniques have been developed to get sub-micron resolution even for larger distances between mask and substrate. But then also the proximity distance has to be controlled with sub-micron accuracy. A passive and an active sensor concept have been developed based on triangulation using diffractive structures. The required sensing patterns are implemented directly in the photo mask. In the passive gap alignment the distance can be reconstructed from the resist pattern obtained as a result of a lithographic step in which the diffractive sensor structure is exposed in the Mask-Aligner. In the active configuration the proximity gap can be controlled already during the alignment procedure prior to the lithographic exposure. A collimated laser beam irradiates a diffractive structure in the photo mask, which deflects the beam which will be reflected from the resist coated substrate towards the mask. A second mask structure, which is placed in a defined lateral distance to the first one, acts then as a ruler for the distance between mask and wafer and can be observed through the alignment microscope or a camera module. The design and fabrication of the diffractive structures, the measurement results for the full-wafer proximity distance distribution according to the passive method, as well as the realization of an active sensor module for mask aligners are presented in this paper.