MEMS Metallization
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MEMS Metallization Christian Lohmann1, Knut Gottfried2, Andreas Bertz1, Danny Reuter1, Karla Hiller1, Michael Kuhn1 and Thomas Gessner1, 2 Chemnitz University of Technology (CUT), Center of Microtechnologies1 Fraunhofer IZM, Department MDE2 Chemnitz, Germany
ABSTRACT Silicon is the dominating material for the fabrication of MEMS devices, especially in high volume production. However, metals with their typical properties are used to enhance or enable the functionality of MEMS. In contrast to microelectronic technologies, not only the electrical but also the mechanical and optical behavior of metals could be helpful. New requirements in MEMS technologies demand optimized processes in metallization for the fabrication of microstructures. This paper presents some metallization applications and related technology development in the field of MEMS.
INTRODUCTION Based on its widespread application in microelectronics technology and the mechanical properties as well, crystalline silicon is the dominant material for MEMS fabrication. Its usage is not limited to act as the base plate (substrate) but as the functional film or component too. The electrical resistivity of crystalline silicon can be varied in a wide range (0.005 up to 1000 Ωcm) offering a large flexibility as MEMS material. However, from an application and technology point of view in certain cases metal films are to be used. For example for rf MEMS a resistivity as low as possible is desired in order to minimize electrical losses. Metals with a resistivity of a few µΩcm meet this requirement. Furthermore light reflection from MEMS components can also be increased by metal coating of surfaces. Several effects for signal transducing offer other application fields: e.g. thermal – electrical, thermal – mechanical (bimorph) and magnetic-electrical transducers. Further fields for metal application are given by technology. The galvanoforming process within the LIGA technology [1] represents a powerful method for shaping materials, even if they are difficult to etch. Electroplating and electroless plating processes can easily be combined with wet etching processes in order to get released structures. For mass fabrication by using bulk technology [2] nowadays metal is often used as an electrode and contact pad material. In this case the requirements are for instance: good adhesion, low thermal mismatch to adjacent films (substrate) and high long-term stability (relaxation, migration). This is quite comparable to IC fabrication. Surface micromachining by sacrificial layer etching underneath metal films as known from rf devices (switches, variable capacitances) [3] and laminated post-CMOS MEMS [4] illustrates much more challenges with respect to the mechanical properties. Thin film stress, yield strength, hardness and Young’s modulus are critical issues. Although structure bending can be minimised by stress compensation, stress hysteresis
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due to relaxation can influence reliability and long-term behaviour deeply. Thus even metal surrounded crystalline sil
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