<title>3D resolution gray-tone lithography</title>

Abstract

With the conventional micro machining technologies: isotropic and anisotropic, dry and wet etching, a few shapes can be done. To overcome this limitation, both binary multi- tasking technique or direct EB writing were used, but an inexpensive one-step UV-lithographic method, using a so- called 'gray-tone reticle', seems to be the best choice to produce local intensity modulation during exposure process. Although, by using this method and common technologies in standard IC fabrication it is easy to obtain an arbitrarily 3D shaping of positive thick resists, there are some limitations, too. The maximum number of gray-levels, on projection reticle, achieved by e-beam writing, are only 200. Also, for very thick resists, the limited focus depth of the projection objective gives a poor lateral resolution. These are the reasons why the author prose da new approach to enhance the 3D resolution of gray-tone lithography applied for thick resist. By a high resolution, both for vertical direction, as well as for horizontal direction. Particular emphasis was put on the design, manufacturing and use of halftone transmission masks, required for UV- lithographic step in the fabrication process of mechanical, optical or electronics components. The original design and fabrication method for the gray-tone test reticle were supported by experiments showing the main advantage of this new technology: the 3D structuring of thick resist in a single exposure step and also a very promising aspect ratio obtained of over 9:1. Preliminary experimental results are presented for positive thick resists in SEM micrographs. A future optimization of the lithographic process opens interesting perspectives for application of this high 3D resolution structuring method in the fabrication process of different products, with imposed complex smooth profiles, such as: x-ray LiGA-masks, refractive optics and surface- relief DOEs.