This paper describes electrothermal microactuators that generate rectilinear displacements and forces by leveraging deformations caused by localized thermal stresses. In one manifestation, an electric current is passed through a V-shaped beam anchored at both ends, and thermal expansion caused by joule heating pushes the apex outward. Analytical and finite element models of device performance are presented along with measured results of devices fabricated using electroplated Ni and p/sup ++/ Si as structural materials. A maskless process extension for incorporating thermal and electrical isolation is described. Nickel devices with 410-/spl mu/m-long, 6-/spl mu/m-wide, and 3-/spl mu/m-thick beams demonstrate 10 /spl mu/m static displacements at 79 mW input power; silicon devices with 800-/spl mu/m-long, 13.9-/spl mu/m-wide, and 3.7-/spl mu/m-thick beams demonstrate 5 /spl mu/m displacement at 180 mW input power. Cascaded silicon devices using three beams of similar dimensions offer comparable displacement with 50-60% savings in power consumption. The peak output forces generated are estimated to be in the range from 1 to 10 mN for the single beam devices and from 0.1 to 1 mN for the cascaded devices. Measured bandwidths are /spl ap/700 Hz for both. The typical drive voltages used are /spl les/12 V, permitting the use of standard electronic interfaces that are generally inadequate for electrostatic actuators.

Bent-beam electrothermal actuators-Part I: Single beam and cascaded devices

A comprehensive thermal model for an electro-thermal-compliant (ETC) microactuator is presented in this paper. The model accounts for all modes of heat dissipation and the temperature dependence of thermophysical and heat transfer properties. The thermal modelling technique underlying the microactuator model is general and can be used for the virtual testing of any ETC device over a wide range of temperatures (300-1500 K). The influence of physical size and thermal boundary conditions at the anchors, where the device is connected to the substrate, on the behaviour of an ETC microactuator is studied by finite element simulations based on the comprehensive thermal model. Simulations show that the performance ratio of the microactuator increased by two orders of magnitude when the characteristic length of the device was increased by one order of magnitude from 0.22 to 2.2 mm. Restricting heat loss to the substrate via the device anchors increased the actuator stroke by 66% and its energy efficiency by 400%, on average, over the temperature range of 300-1500 K. An important observation made is that the size of the device and thermal boundary conditions at the device anchor primarily control the stroke, operating temperature and performance ratio of the microactuator for a given electrical conductivity.

https://iopscience.iop.org/article/10.1088/0960-1317/11/5/303/pdf

Comprehensive thermal modelling and characterization of an electro-thermal-compliant microactuator

Microactuators and their technologies

This paper presents a new type of MEMS (micro-electromechanical systems) actuator consisting of an array of in-plane micro-fabricated thermal buckle-beam actuators. The technology used in MEMS actuators is typically magnetic, electrostatic or thermal. Magnetic actuators may require special materials in the fabrication process while electrostatic actuators typically require high voltages, large chip areas and produce very low forces. Thermal actuators have seen some use in MEMS applications, the most popular being the pseudo-bimorph that relies on differential expansion of a cold and hot arm to cause it to bend in-plane (parallel to the substrate). These thermal actuators typically generate on the order of a few micro-Newtons each but can be combined for larger forces by linking with small tendons. A disadvantage of this type of actuator is that it moves in an are where most desired movements are linear. Also, when combined in an array, the linking tendons consume much of the energy in bending them. Also, arrays of these can still occupy a fairly large chip area. The electro-thermal actuator described here resembles a chevron where an array of buckle-beams are packed close together and link two common anchored arms with a movable third arm. Arrays can be made within a single released micromachined layer and generate many mN of force. Additional actuators can be arrayed with no coupling penalty and occupy much less area that an equivalent pseudo-bimorph actuator. Preliminary tests indicate that a 450/spl times/120 /spl mu/m array consumes 240 mW of power, deflection up to 14 /spl mu/m and can produce many milli-Newtons. A chip of actuator geometry variations and different applications has been fabricated and tested.

A high force low area MEMS thermal actuator

This work addresses a range of issues on modeling electrothermal microactuators, including the physics of temperature dependent material properties and Finite Element Analysis (FEA) modeling techniques. Electrical and thermal conductivity are a nonlinear function of temperature that can be explained with electron and phonon transport models, respectively. Parametric forms of these equations are developed for polysilicon and a technique to extract these parameters from experimental data is given. A modeling technique to capture the convective and conductive cooling effects on a thermal actuator in air is then presented. Using this modeling technique and the established polysilicon material properties, simulation results are compared with measured actuator responses. Both static and transient analyzes have been performed on two styles of actuators and the results compare well with measured data.

Electrothermal properties and modeling of polysilicon microthermal actuators

This paper describes in-plane microactuators fabricated by standard microsensor materials and processes that can generate forces upto about a milli-newton. They operate by leveraging the deformations produce by localized thermal stresses. Analytical and finite element models of device performance are presented along with measured results of fabricated devices using electroplated Ni, LPCVD polysilicon, and p++ Si as structural materials. A maskless process extension for incorporating thermal and electrical isolation is outlined. Test results show that static displacements of =lo pm can be achieved with power dissipation of =lo0 mW, and output forces >300 pN can be achieved with input power <250 mW. It is also shown that cascaded devices offer a 4X improvement in displacement. The displacements are rectilinear, and the output forces generated are lox-1OOX higher than those available from other comparable options. This performance is achieved at much lower drive voltages than necessary for electrostatic actuation, indicating that bentbeam thermal actuators are suitable for integration in a variety of microsystems.

J. S. Park

Papers

Bent- beam electro- thermal a ctua tors for high force applications