**Thermal Microactuators**

Leslie M. Phinney, Michael S. Baker and Justin R. Serrano *Sandia National Laboratories USA* 

#### **1. Introduction**

414 Microelectromechanical Systems and Devices

Wang, Q, D., Duan, Y, G., Ding, Y. C., Lu, B. H., Xiang, J, W., & Yang, L. F. (2009).

Microelectronics Journal, Vol. 40, Iss. 1, pp. 149-155, ISSN 0026-2692 Wong, W, S., Chabinyc, M. L., Limb, S., Ready, S, E., Lujan, R., Daniel, J., & Street, R, A.

Information Display, Vol. 15, Iss. 7, pp. 463-470, ISSN 1071-0922

Ioffe Physical Technical Institute, http://www.ioffe.rssi.ru

Silicon Light Machine, http://www.siliconlight.com

Acreo, http://www.acreo.se Corning, http://www.corning.com E Ink, http://www.eink.com

Sipix, http://www.sipix.com

Liquavista, http://www.liquavista.com/ Pixtronix, http://www.pixtronix.com Qualcomm, http://www.qualcomm.com

Texas Instruments, http://www.ti.com

*Investigation on LIGA-like Process Based on Multilevel Imprint Lithography*,

(2007). *Digital Lithographic Processing for Large-area Electronics*, Journal of Society for

This chapter discusses the design, fabrication, characterization, modeling, and reliability of thermal microactuators. Microelectromechanical systems (MEMS) devices contain both electrical and mechanical components and are in use and under development for applications in the consumer products, automotive, environmental sensing, defense, and health care industries. Thermal microactuators are standard components in microsystems and can be powered electrically through Joule heating or optically with a laser. Examples of MEMS designs containing thermal microactuators include optical switches (Cochran et al., 2004; Sassen et al., 2008) and nanopositioners (Bergna et al., 2005). Advantages of thermal microactuators include higher force generation, lower operating voltages, and less susceptibility to adhesion failures compared to electrostatic actuators. Thermal microactuators do require more power and their switching speeds are limited by cooling times.

Extensive work has been performed designing, fabricating, testing, and modeling thermal microactuators. Howell et al. (2007) has reviewed the fundamentals of thermal microactuator design. Designs of electrically powered MEMS thermal actuators include actuators fabricated from a single material (Comtois et al., 1998; Park et al., 2001; Que et al., 2001) and bimorphs (Ataka et al., 1993). Thermal actuator designs using a single material are both symmetric, referred to as bent-beam or V-shaped, structures (Baker et al., 2004; Park et al., 2001; Phinney et al., 2009) and asymmetric (Comtois et al., 1998), which have a hot arm and a cold arm. Asymmetric actuators are also referred to as flexure actuators. Some studies investigated both bent-beam and flexure actuators (Hickey et al., 2003; Oliver et al., 2003). In addition to electrical heating, powering thermal microactuators optically using laser irradiation has been demonstrated (Oliver et al., 2003; Phinney & Serrano, 2007; Serrano & Phinney, 2008). Modeling efforts have focused on bent-beam microactuators (Baker et al., 2004; Enikov et al., 2005; Howell et al., 2007; Lott et al., 2002; Wong and Phinney, 2007) and flexure actuators (Mankame and Ananthasuresh, 2001).

This chapter focuses on bent-beam and flexure microactuators. In order for thermal actuators to operate, sufficient heating and thermal expansion of the components must occur. However, device temperatures that are too high result in permanent deformation, damage, and degradation in performance. In addition, packaging processes and conditions affect the performance and reliability of microsystems devices motivating studies on the effects of surrounding gas pressure and mechanical stress on thermal MEMS.

Thermal Microactuators 417

actuators have angled legs that expand when heated, providing force and displacement output as shown in Fig. 1b (Park et al., 2001; Que et al., 2001). The flexure actuator in Fig. 1c contains asymmetric legs, for example of unequal width, that flex to the side due to differential expansion when heated (Comtois et al., 1998). Figure 2 has pictures of

Thermal microactuators are created using various microfabrication techniques including surface micromachining and silicon on insulator (SOI) processing which will be reviewed. Particular designs for surface micromachined thermal microactuators are presented in detail

Surface micromachining involves the sequential growth or deposition of thin films, patterning of features, and etching of the films to create multilayer structures and devices. Surface micromachining results in devices with in-plane dimensions from a few microns to millimeters and thicknesses of microns to 10 microns so they have low aspect ratios, i.e., thickness divided by length or width. Typical surface micromachining processes use polycrystalline silicon (polysilicon) for the structural layers and silicon dioxide for the

The surface micromachined thermal microactuators for which characterization data will be reported were fabricated using the SUMMiT VTM (Sandia Ultra-planar Multilevel MEMS Technology) process (Sniegowski and de Boer, 2000; SUMMiT V, 2008). The SUMMiT V process uses four structural polysilicon layers with a fifth layer as a ground plane. These layers are separated by sacrificial oxide layers that are etched away during the final release step. The two topmost structural layers, Poly3 and Poly4, are nominally 2.25 m in thickness, while the bottom two, Poly1 and Poly2, are nominally 1.0 m and 1.5 m in thickness, respectively. The ground plane, Poly0, is 300 nm in thickness and lies above an 800 nm layer of silicon nitride and a 630 nm layer of silicon dioxide. The sacrificial oxide layers between the structural layers are each around 2.0 μm thick (Sniegowski and de Boer,

Figure 3 pictures schematics of an electrically heated bent-beam thermal microactuator with two legs and the cross-sectional area of an actuator leg with the width and thickness dimensions labeled. The SUMMiT V processing constraints on the sacrificial oxide cut between two polysilicon layers result in an I-beam shape for the thermal actuator legs (SUMMiT V, 2008). In this chapter, mechanical, electrical, and thermal characterization results are presented for bent-beam thermal microactuators with two actuator legs (Phinney et al., 2009). The thermal microactuator designs have the actuator legs fabricated from three laminated structural polysilicon layers: Poly1, Poly2, and Poly3 (Figure 4). This actuator design is referred to as the P123 actuator throughout this chapter. The second thermal actuator design is the same thermal actuator as the first design with a force gauge attached to the actuator shuttle (Figure 5) and is referred to as the P123F actuator. The force gauge consists of a linear bi-fold spring attached to the shuttle of the actuator using the Poly3 layer. Table 1 summarizes the geometries of the thermal microactuators with nominal

electrically and optically powered bent-beam and flexure thermal microactuators.

as characterization data for these designs are reported in Section 4.

**3. Fabrication** 

sacrificial layers.

2000; SUMMiT V, 2008).

**3.1 Surface micromachining** 
