Victor M. Bright

Surface tension-powered self-assembly of microstructures - the state-of-the-art

Because of the low dimensional power of its force scaling law, surface tension is appropriate for carrying out reshaping and assembly in the microstructure size domain. This paper reviews work on surface tension powered self-assembly of microstructures. The existing theoretical approaches for rotational assembly are unified. The demonstrated fabrication processes are compared. Mechanisms for accurately determining the assembled shape are discussed, and the limits on accuracy and structural distortion are considered. Applications in optics, electronics and mechanics are described. More complex operations (including the combination of self-assembly and self-organization) are also reviewed.

Applications for surface-micromachined polysilicon thermal actuators and arrays

Abstract Electro-thermal (E-T) actuators have been developed to complement the capabilities of electrostatic actuators. The thermal actuators presented here can be arrayed to generate high forces. Equally significant is that the single actuators and arrays of actuators operate at voltages and currents that are directly compatible with standard microelectronics. This paper demonstrates how combinations of two or more electro-thermal actuators can be applied to a variety of basic building-block micromechanical devices: array of ten lateral actuators; array of ten vertical actuators; vertically actuated two-axis tilting mirror; corner-cube retroreflector actuated by lateral array; grippers over the die edge with flip-over wiring; rotary stepper motor; flip-up optical grating on rotary stepper motor; linear stepper motor; and a linear stepper motor for assembly of hinged structures.

Fabrication of SiCN MEMS by photopolymerization of pre-ceramic polymer☆

This paper describes the use of photopolymerization of a liquid polysilazane as a novel, versatile and cost-effective means of fabricating SiCN ceramic MEMS. SiCN is a new class of polymer-derived ceramics whose starting material is a liquid-phase polymer. By adding a photo initiator to the precursor, photolithographical patterning of the pre-ceramic polymer can be accomplished by UV exposure. The resulting solid polymer structures are then crosslinked under isostatic pressure, and pyrolyzed to form an amorphous ceramic capable of withstanding over 1500°C. By adding and curing successive layers of liquid polymer on top of one another, multi-layered ceramic MEMS can be easily fabricated. The use of photopolymerization can also be used to make thin, membrane-like ceramic structures. Key issues concerning the fabrication process are discussed. By combining photopolymerization with other in-house developed techniques such as polymer-based bonding and flip-chip bonding, three SiCN MEMS devices for high-temperature applications have been fabricated: an electrostatic actuator, a pressure transducer, and a combustion chamber. These represent a wide range of MEMS, demonstrating the versatility of this technique.

Fabrication of SiCN ceramic MEMS using injectable polymer-precursor technique

In this paper, a novel and cost-effective technology for the fabrication of high-temperature MEMS based on injectable polymer-derived ceramics is described. Micro-molds are fabricated out of SU-8 photoresist using standard UV-photolithographic processes. Liquid-phase polymers are then cast into the molds and converted into monolithic, fully-dense ceramics by thermal decomposition. The resultant ceramic, based on the amorphous alloys of silicon, carbon and nitrogen, possess excellent mechanical and physical properties for high-temperature applications. This capability for micro-casting is demonstrated in the fabrication of simple single-layered, high aspect ratio SiCN microstructures. A polymer-based bonding technique for creating more complex three-dimensional structures is also presented.

Solder self-assembly for three-dimensional microelectromechanical systems

Abstract A solder technology has been developed that utilizes molten solder surface tension forces to self-assemble MEMS 3-D structures. Using solder, a single batch reflow process can be used to accomplish hundreds or thousands of precision assemblies, and the cost per assembly can be reduced considerably. A model, based on surface energy minimization of molten liquids, has been developed for predicting assembly motion. The modeling, combined with experimental studies, have demonstrated ±2° assembly angle control is possible when the MEMS structures are assembled by solder alone. To improve the self-assembly angle precision, a self-locking mechanism can be added, which reduces the assembly angle variation down to ±0.3°.

Thermal microactuators for surface-micromachining processes

Microelectromechanical systems must include large-deflection actuators to create devices which can interact mechanically with their surroundings. Electrostatic actuators with large nonresonant motion require high voltages and large surface areas to produce useful forces. Large-deflection thermal actuators have been fabricated in electroplated metal processes, but the choice of materials is limited, and the high conductivity of metal means these devices need a high current to generate the required heat. Thus, there is a need for a low voltage actuator with a large nonresonant deflection which can be fabricated in integrated circuit compatible surface-micromachining processes. The device described in this paper has a simple, flexible, and reliable design which overcomes the limitations of other actuators. It is a low voltage, medium current thermal actuator which can be designed to move laterally in a controllable, nonresonant motion. Typical actuators are 200 micrometers long, 18 micrometers wide, and deflect 10 micrometers at the tip with a drive current of 5 mA at 5 volts. A wide range of layout geometries allows this type of actuator to be fabricated in any surface-micromachining process that includes a releasable, current-carrying layer. An empirical model is presented to describe the devices fabricate in a commercially available surface-micromachining process. Arrays of actuators were fabricated to test the effect of different device dimensions on maximum achievable deflection as a funciton of applied current. Actuators can be combined to produce more force and have been integrated with other micromechanical structures. Applications of these actuators include linear and rotating motors, compliant micromechanisms, latches, micro-relays, variable capacitators and tweezers.

Automated assembly of flip-up micromirrors

Abstract Batch fabrication of microelectromechanical systems utilizing flip-up microstructures is limited by the manual assembly process required to lift the structures off the substrate. This problem is solved with the use of an automated assembly system integrated on the die with the flipup structures. A microelectromechanical automated assembly system consisting of a vertical thermal actuator, a linear assembly micromotor, and a self-engaging locking mechanism is presented in this paper. The vertical thermal actuator is used to lift one end of the plate off the substrate. This provides the linear assembly micromotor with the leverage needed to push the plate up into a position where the self-engaging locking mechanism secures the plate. A CMOS drive system has been developed to control the automated assembly system. When combined with the automated assembly system, CMOS control allows flip-up microstructures to be assembled without any operator intervention. The automated assembly system is demonstrated with a scanning micromirror and a corner cube reflector.

Design and modeling of RF MEMS tunable capacitors using electro-thermal actuators

A series mounted MEMS tunable capacitor in a CPW line is reported. An electro-thermal actuator has been used for driving the top plate of the parallel plate capacitor. The MEMS structure is bonded on an alumina substrate using flip-chip technology so that the silicon on the backside of the MEMS can be removed to reduce the RF losses. The lumped-element model of the capacitor up to 40 GHz has been developed based on Y-parameters, which are derived from measured S-parameters. The measured Q-factor is 256 at 1 GHz for a 0.102 pF capacitor and C/sub max//C/sub min/ ratio of the capacitor is about 2:1.

Development of microelectromechanical variable blaze gratings

Abstract Two types of microelectromechanical variable blaze gratings (VBGs) have been designed, modeled, fabricated, and tested. The gratings operate by adjusting the blaze angle of each slat so specular reflection of the incident light matches a particular grating diffraction order. The VBG blaze angle is adjustable with either electrostatic or thermal actuators. VBGs direct incident light in discrete directions, and are useful for steering light with beam diameters greater than 1 mm and power levels greater than 1 W. Both electrostatically and thermally actuated VBGs have been constructed from gold and polysilicon using a surface-micromachining process and tested with a 20 mW continuous wave HeNe laser operating at a wavelength of 632.8 nm. The diffraction efficiency and far-field pattern have been modeled and measured. Drive voltages for both types of gratings are measured as a function of blaze angle and selected diffraction order.

An embedded overlay concept for microsystems packaging

An embedded overlay concept for packaging hybrid components containing microelectromechanical systems (MEMS) is described. This packaging process is a derivative of the chip-on-flex (COF) process currently used for microelectronics packaging. COF is a high performance, multichip packaging technology in which die are encased in a molded plastic substrate and interconnects are made via a thin-film structure formed over the components. A laser ablation process has been developed which enables selected areas of the COF overlay to be efficiently ablated with minimal impact to the packaged MEMS devices. Analysis and characterization of the ablation procedures used in the standard COF process was performed to design a new procedure which minimized the potential for heat damage to exposed MEMS devices. The COF/MEMS packaging technology is well-suited for many microsystem packaging applications such as micro-optics and radio frequency (RF) devices.