This article describes a new negative-tone photoresist, SU-8, for ultrathick layer applications. An aspect ratio of 10:1 has been achieved using near-ultraviolet lithography in a 200-μm-thick layer. The use of this resist for building tall micromechanical structures by deep silicon reactive-ion etching and electroplating is demonstrated. Using SU-8 stencils, etched depths of ≳200 μm in Si and electroplated 130-μm-thick Au structures with near-vertical sidewalls have been achieved.

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Poly(bipheny1 dianhydride-p-phenylenediamine) (BPDA-PDA) polyimide film surfaces are initially modified with KOH aqueous solution to yield a potassium polyamate surface. The reaction of the polyamate surface with acid gives a polyamic acid surface. The starting polyimide surface is reproduced upon curing the modified surface. Modified surfaces are identified with X-ray photoelectron spectroscopy, external reflectance infrared (ER IR) spectroscopy, and contact angle measurement. The average depth of modification, which is measured by a method using an absorbance-thickness relationship established with ellipsometry and ER IR, is controlled by the reaction temperature and time. Surface topography and film thickness can be maintained while a strong polyimide-polyimide adhesion is achieved. A relationship between surface structure and adhesion is established.