Experiments for 3-D structuring of thick resists by 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 binary multi-masking technique, laser micro-stereolithography, or direct electron-beam-writing were used, but an inexpensive one-step UV-lithographic method, using the so-called "gray-tone lithography", seems to be the best choice to produce local intensity modulation during exposure process. The paper reports on the study of arbitrary three-dimensional (3-D) shaping of negative and positive thick resists, using this method, and common technologies in standard ICs fabrication. Particular emphasis is placed on the design, manufacturing and use of half-tone 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 reticles, 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 high aspect ratio obtained over 9:1. Experimental results are presented in SEM micrographs, only for positive thick resists: showing different 3-D shapes in positive and negative polarity, and also the results obtained by using the wall-type test structure for aspect ratio evaluation. Finally, by optimization of the lithographic process, interesting applications are shown. (C) 2001 Elsevier Science Ltd. All rights reserved.