Abstract
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.
Highlights
The electrothermal actuation principle is one of the principles, with increasing interest in the MEMS domain
Electrothermal actuators can be fabricated from metal [1,2], silicon [3,4] or polymer materials such as epoxide, polyimide [5] or polyether ether ketone (PEEK) [6]
To determine the influence of the process parameters on cross-linking and to find the parameter set with the highest degree of cross-linking, samples with the six parameters sets are analyzed after postbake and again after hardbake by a Fourier Transform Infrared Spectroscopy (FT-IR) procedure presented by Tan et al [22]
Summary
The electrothermal actuation principle is one of the principles, with increasing interest in the MEMS domain. Electrothermal actuators can be fabricated from metal [1,2], silicon [3,4] or polymer materials such as epoxide, polyimide [5] or polyether ether ketone (PEEK) [6]. Actuators made of metal or semiconductor materials provide high thermal stability up to several hundreds of °C, but low thermal expansion coefficients (Si: 2.3 ppm/K; Ni: 13 ppm/K). This leads to very high operating temperatures and increased fabrication costs. To underline the potential of cost-efficient polymer electrothermal actuators for commercial use, their suitability and stability for hours of operation has to be examined. Samples of actuators are characterized regarding electrical power consumption, maximum operation temperature and long-term operation
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