Design and Fabrication of Reliable Power Efficient Bistable MEMS Switch Using Single Mask Process

Abstract: A latching RF MEMS switch has been fabricated in a multi-user polysilicon surface micromachining process. The switch uses 5 V, 35 mA thermal actuation for < 500 mus to toggle between states and a compliant bistable latching mechanism to hold the state in the absence of applied bias. The switch, including probe pads, measures 1 mm2 and has <0.4 dB insertion loss, >25 dB return loss, and >75 dB isolation at 1 GHz. The switch has potential applications in low duty-cycle, low power RF tuning and switching applications.

Abstract: A limit point behaviour of a metastructure, composed of two double clamped, initially curved beams, coupled via a rigid truss at their respective centres, is studied when subjected to a distributed electrostatic load. The analysis is based on a reduced order (RO) model, resulting from Galerkin’s decomposition, with symmetric buckling modes used as the base functions. To better understand the behaviour of the structure, the electrostatic analysis is preceded by a study of the model under “mechanical”, displacement-independent, load, allowing validation against a finite elements (FE) model, which served as proving ground for the RO model. In addition, results obtained by the RO model are compared with those extracted via finite differences (FD) solutions, facilitating the usage of the latter as the reference in the electrostatic, displacement-dependent, analysis. To accommodate for winding equilibrium paths, all solutions employed the implicit arc-length “Riks” method. The analysis has produced complicated equilibrium paths, which necessitated a corresponding local stability analysis, conducted using the energy method, allowing for limit point characterisation. The analyses have shown that the double beam metastructure is able to possess bistable and tristable properties, usually found in highly complex structures, which can be cumbersome for design and miniaturisation, provided that it meets certain geometrical parameters. Several variations of tristability are disclosed in the study, hitherto unknown, providing insight to the complexity of the structure. The analysis indicates that a model with at least three degrees-of-freedom (DOF) is needed to predict the various critical thresholds with reasonable errors. In so doing, the model can be used for static characterisation and design of various applications, where a simple structure, possessing tristable properties, is required.

Abstract: In this paper, a novel micro-positioning device based on a 3D digital actuator is presented. The proposed system allows realizing planar motions of micro-objects, which could be implemented in several applications where micro-positioning tasks are needed such as micro-component manufacturing/assembly, biomedicine, scanning microscopy, etc. The device has three degrees of freedom, and it is able to achieve planar motions of a mobile plate in the xy-plane at two different levels along the z-axis. It consists of a hexagonal mobile part composed of a permanent magnet that can reach twelve discrete positions distributed between two z-axis levels (six at each level). Two different approaches are presented to perform positioning tasks of the plate using the digital actuator: the stick-slip and the lift-mode approaches. A comparison between these two approaches is provided on the basis of the plate displacement with respect to different current values and conveyed mass. It was observed that for a current of 2 A, the actuator is able to displace a mass of 1.15 g over a distance of 0.08 mm. The optimal positioning range of the planar device was found to be ±5.40 mm and ±7.05 mm along the x- and y-axis, respectively.

Abstract: 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.

Abstract: An analytical model that can accurately predict the performance of a polysilicon thermal flexure actuator has been developed. This model is based on an electrothermal analysis of the actuator, incorporating conduction heat transfer. Heat radiation from the hot arm of the actuator to the cold arm is also estimated. Results indicate that heat radiation becomes significant only at high input power, and conduction heat losses to both the substrate and the anchor are mainly responsible for the operating temperature of the actuator under routine operations. Actuator deflection is computed based on elastic analysis of structures. To verify the validity of the model, polysilicon thermal flexure actuators have been fabricated and tested. Experimental results are in good agreement with theoretical predications except at high input power. An actuator with a 240 µm long, 2 µm thick, 3 µm wide hot arm and a 180 µm long, 12 µm wide cold arm deflected up to 12 µm for the actuator tip at an input voltage of 5 V while it could be expected to deflect up to 22 µm when a 210 µm long cold arm is used.

Abstract: 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.

Abstract: For Part I see L. Que, J.S. Park and Y.B. Gianchandani, ibid., vol.10, pp.247-54 (2001). This paper reports on the use of bent-beam electrothermal actuators for the purpose of generating rotary and long-throw rectilinear displacements. The rotary displacements are achieved by orthogonally arranged pairs of cascaded actuators that are used to rotate a gear. Devices were fabricated using electroplated Ni, p/sup ++/ Si, and polysilicon as structural materials. Displacements of 20-30 /spl mu/m with loading forces >150 /spl mu/N at actuation voltages 200 /spl mu/N at displacements >100 /spl mu/m were measured.

Abstract: 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.