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Amorphous TiAl films for micromirror arrays with stable analog deflection integrated on complementary metal oxide semiconductors

: Schmidt, J.-U.; Friedrichs, M.; Gehner, A.


Journal of micro/nanolithography, MEMS and MOEMS 7 (2008), No.2, Art. 021012
ISSN: 1537-1646
ISSN: 1932-5150
ISSN: 1932-5134
Journal Article
Fraunhofer IPMS ()

Large micromechanical mirror arrays (MMA) with analog pixel deflection integrated onto active CMOS address circuitry require both high-quality planar reflective optical surfaces and a stable deflection versus voltage characteristic. However, for implementing a CMOS-compatible surface-micromachining process, certain obstacles such as a restricted thermal budget and a limited selection of suitable materials must be overcome. Amorphous TiAl is presented as a new actuator material for monolithical MEMS integration onto CMOS circuitry. TiAl films may be sputter deposited at room temperature, have an x-ray amorphous structure, and a low stress gradient. The glassy structure and high melting point make TiAl less vulnerable to stress relaxation, which makes TiAl an ideal spring material. One-level actuators with TiAl or Al-TiAl-Al structural layers and two-level actuators with separate TiAl spring and Al-alloy mirror layers were fabricated and tested with respect to their drift stability. The stability of TiAl-based actuators was found to be superior in comparison to one-level Al-alloy actuators. Two-level actuators with TiAl hinges emerge as the most promising design.