SU-8 Electrothermal Actuators: Optimization of Fabrication and Excitation for Long-Term Use

• Main Authors: Thomas Winterstein, Matthias Staab, Christian Nakic, Hans-Jürgen Feige, Jürgen Vogel, Helmut F. Schlaak
• Format: Article
• Language: English
• Published: MDPI AG 2014-12-01
• Series: Micromachines
• Subjects: electrothermal actuator; SU-8; long-term stability; pseudo-bimorph; glass transition temperature
• Online Access: http://www.mdpi.com/2072-666X/5/4/1310

In this paper we examine the suitability of SU-8 2000 as a construction material for electrothermal actuators and the actuator stability for long-term operation. The fabrication of SU-8 was optimized for mechanical and thermal stability. Samples with different softbake duration, exposure dose and postbake temperature were evaluated using Fourier-Transform IR-spectroscopy and dynamic-mechanical analysis. The exposure dose and postbake temperature proved to have a strong influence on the cross-linking and the glass transition temperature. A final hardbake levels the effects of the process history. A high degree of crosslinking, a low drop of the dynamic modulus over temperature (30%) up to the glass transition temperature 100–140 °C were achieved for SU-8 with an exposure dose of 1500 mJ/cm², a postbake temperature of 95 °C and hardbake of 240 °C. Electrothermal actuators proved to be stable until the end of the experiment after 2400 duty cycles. Actuator deflections up to 55 μm were measured (actuator length: 4 mm) for input powers up to 160 mW and a maximum operating temperature of 120 °C. Higher temperatures led to permanent deformations and failure. An offset drift of up to 20% occurs during actuation, but converges after a burn-in phase of about two hours.