Acoustic Energy: a New Tool for MEMS Manufacturing
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Acoustic Energy: a New Tool for MEMS Manufacturing D. Dussault1 and V. Dragoi2 1 Product Systems Inc, 1745 Dell Av., Campbell, CA 95008, U.S.A. 2 EV Group, DI E. Thallner Str. 1, 4782 - St. Florian/Inn, Austria ABSTRACT The use of non-standard materials (e.g. specific substrates shapes and dimensions or polymer materials) for MEMS applications imposed a requirement for the development of new techniques for even well-established processes. Acoustic energy in the MHz frequency range has been used in the semiconductor industry for various processes such as photoresist development, substrate cleaning and electro-plating enhancement. The work presented here is focusing on single wafer cleaning and on photoresist development. The cleaning process developed addresses mainly wafer cleaning prior to wafer bonding processes, in which particle contamination is of crucial importance. The photoresist development process was developed mainly for thick resist layers development (few hundreds of µm) in order to improve definition of high aspect ratio features but was used as well as a significant process time reduction factor for development of regular thickness resists (few µm). INTRODUCTION MEMS-specific processes had to adapt standard IC manufacturing technologies in order to accommodate their custom requirements. Wafer cleaning is an important chapter in any semiconductor process flow as contamination typically has a negative impact on process results. The challenges of processes adapted for MEMS manufacturing demand the implementation of advanced cleaning techniques. One process requiring very strict contamination control is wafer bonding. Even extremely low levels of particle contamination prior to wafer bonding result in significant yield losses. For this reason, besides the standard batch cleaning process used to remove various types of contamination, wafer bonding also requires a single wafer cleaning step immediately prior bringing the wafers in contact in order to assure that any airborne particles that may have been added during transfer and handling are removed. Photolithography is a standard patterning technique which has had to adopt the use of thick photoresists (between few tens and up to few hundreds of µm). These thicknesses are required for needed for high aspect ratio features fabrication for and to adapt to the specific high volume manufacturing yield requirements demanded by developing new processing techniques. Thick resists processing require a very long cycle time due to some particular aspects as polymer coating (usually requiring edge bead removal), polymer slow baking for solvent removal after coating and development of high aspect ratio structures. A single wafer spin process station equipped with triangular-shaped megasonic transducer, the MegPie®, was used for both cleaning and photoresist processes. This transducer covers a sector of the wafer and is placed in proximity to substrate surface. The small gap between transducer and substrate is filled with the required process fluid. The shape and posit
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