3-D resolution gray-tone lithography

With the conventional micromachining technologies: isotropic and anisotropic, dry and wet etching, a few shapes can be done. To overcome this limitation, both binary multi-masking technique or direct EB writing were used, but an inexpensive one-step W-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's easy to obtain an arbitrarily 3-D 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 proposed a new approach to enhance the 3-D resolution of gray-tone lithography applied for thick resists. By combining binary masking technique with a corresponding focus depth of the projection objective, it was possible to obtain a high resolution, both for vertical direction (by increased number of gray-levels), as well as for horizontal direction (by setting a good focus of the projection objective). 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 3-D structuring of thick resists 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 3-D 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.