Micromechanical devices with embedded electro-thermal-compliant actuation

Abstract

At the micro-scale, thermal actuation provides larger forces compared to the widely-used electrostatic actuation. In this paper, we highlight another advantage of thermal actuation, viz. the ease with which it can be utilized to achieve a novel embedded electro-thermal-compliant (ETC) actuation for MEMS. The principle of ETC actuation is based on the selective non-uniform Joule heating and the accompanying constrained thermal expansion. It is shown here that appropriate topology and shape of the structures give rise to many types of actuators and devices. Additionally, selective doping of silicon ETC devices is used to enhance the non-uniform heating and thus the deformation. A number of novel ETC building blocks and devices are described, and their analysis and design issues are discussed. The devices were microfabricated using MCNC’s MUMPs foundry process as well as a bulk-micromachining process called PennSOIL (Penn silicon-on-insulator layer). The designs are validated with the simulations and the experimental observations. The experimental measurements are quantitatively compared with the theoretical predictions for a novel ETC microactuator with selective doping.

Introduction

In recent years, thermal actuation in micro-electro-mechanical systems (MEMS) has received considerable attention. When compared to the widely-used electrostatic microactuation, thermal actuation provides larger forces and is also easier to control [1]. In this paper, we demonstrate a third advantage of thermal actuation by presenting a number of micro-devices with embedded actuation. In embedded actuation, the mechanism and the actuator become indistinguishable. That is, the actuation is built into the mechanical structure.

Differential expansion of a laminate made of two materials of unequal thermal expansion coefficients, the bi-metallic or bimorph effect [2], is well known. This has been used effectively in MEMS to generate large forces [3], [4], [5], [6]. External heating or internal Joule heating causes elastic structures to expand. If constrained mechanically, these structures will generate forces making them suitable for actuation. Such actuators were also reported in the literature [1], [7], [8], [9], [10], [11], [12]. Among the latter category of actuators, the ones presented in [13], [14] are most relevant to the type of actuation discussed in this paper. The electro-thermal actuator, also called a heatuator, [8] takes advantage of the shape to create “bi-metallic” effect using a single material as shown in Fig. 1a. When current is passed through the folded-beam structure from one mechanically-anchored electrode to the other, the narrow portion gets hotter than the wide portion because of higher current density and the ensuing larger Joule heating. Therefore, the narrow arm tends to expand more than the wide arm, and they achieve thermo-elastic equilibrium by bending toward the wide arm.

Our modification to the heatuator is shown in Fig. 1b. As shown here, if we pass current through the narrow and wide arms in parallel rather than in series, the structure will then bend towards the narrow arm. This is because the resistance of the wide arm is smaller (note: resistance, R=(resistivity×length)/area=(ρl)/A) and hence draws larger current and gets hotter than the narrow arm. This parallel connection gives rise to new ways of achieving the selective heating of a flexible continuum, and leads to the concept of embedded electro-thermal-compliant (ETC) actuation.

The remainder of the paper is organized as follows. The principle of actuation of ETC devices is described in Section 2. The sequential analysis of current distribution, temperature distribution, and thermo-elastic deformation and the related issues are considered in Section 3. In Section 4, the fabrication of ETC microactuators is discussed. A new bulk-micromachining process that uses silicon-on-insulator (SOI) wafers called PennSOIL (Penn silicon-on-insulator layer) is described. A number of designs for basic ETC building blocks and various ways of combining them into larger systems, including a three degree of freedom parallel manipulator, are described in Section 5. Experimental data is presented in Section 6 for some of the designs. After discussing the results and future work in Section 6, some concluding remarks are made in Section 7.