Microfabricated microengine for use as a mechanical drive and power source in the microdomain and fabrication process

A microengine uses two synchronized linear actuators as a power source and converts oscillatory motion from the actuators into rotational motion via direct linkage connection to an output gear or wheel. The microengine provides output in the form of a continuously rotating output gear that is capable of delivering drive torque to a micromechanism. The microengine can be operated at varying speeds and its motion can be reversed. Linear actuators are synchronized in order to provide linear oscillatory motion to the linkage means in the X and Y directions according to a desired position, rotational direction and speed of said mechanical output means. The output gear has gear teeth on its outer perimeter for directly contacting a micromechanism requiring mechanical power. The gear is retained by a retaining means which allows said gear to rotate freely. The microengine is microfabricated of polysilicon on one wafer using surface micromachining batch fabrication

Chemical Vapor Deposition Coating for Micromachines

Two major problems associated with Si-based MEMS devices are stiction and wear. Surface modifications are needed to reduce both adhesion and friction in micromechanical structures to solve these problems. In this paper, the authors will present a process used to selectively coat MEMS devices with tungsten using a CVD (Chemical Vapor Deposition) process. The selective W deposition process results in a very conformal coating and can potentially solve both stiction and wear problems confronting MEMS processing. The selective deposition of tungsten is accomplished through silicon reduction of WF{sub 6}, which results in a self-limiting reaction. The selective deposition of W only on polysilicon surfaces prevents electrical shorts. Further, the self-limiting nature of this selective W deposition process ensures the consistency necessary for process control. Selective tungsten is deposited after the removal of the sacrificial oxides to minimize process integration problems. This tungsten coating adheres well and is hard and conducting, requirements for device performance. Furthermore, since the deposited tungsten infiltrates under adhered silicon parts and the volume of W deposited is less than the amount of Si consumed, it appears to be possible to release stuck parts that are contacted over small areas such as dimples. Results from tungsten deposition on MEMS structures with dimples will be presented. The effect of wet and vapor phase cleanings prior to the deposition will be discussed along with other process details. The W coating improved wear by orders of magnitude compared to uncoated parts. Tungsten CVD is used in the integrated-circuit industry, which makes this approach manufacturable

High Amplitude Secondary Mass Drive

In this paper we describe a high amplitude electrostatic drive for surface micromachined mechanical oscillators that may be suitable for vibratory gyroscopes. It is an advanced design of a previously reported dual mass oscillator (Dyck, et. al., 1999). The structure is a 2 degree-of-freedom, parallel-plate driven motion amplifier, termed the secondary mass drive oscillator (SMD oscillator). During each cycle the device contacts the drive plates, generating large electrostatic forces. Peak-to-peak amplitudes of 54 {micro}m have been obtained by operating the structure in air with an applied voltage of 11 V. We describe the structure, present the analysis and design equations, and show recent results that have been obtained, including frequency response data, power dissipation, and out-of- plane motion

A Low-Voltage Rotary Actuator Fabricated Using a Five-Level Polysilicon Surface Micromachining Technology

The design, fabrication and characterization of a low-voltage rotary stepper motor are presented in this work. Using a five-level polysilicon MEMS technology, steps were taken to increase the capacitance over previous stepper motor designs to generate high torque at low voltages. A low-friction hub was developed to minimize frictional loads due to rubbing surfaces, producing an estimated resistive torque of about 6 pN-m. This design also allowed investigations into the potential benefit of using hard materials such as silicon nitride for lining of both the stationary and rotating hub components. The result is an electrostatic stepper motor capable of operation at less than six volts

Fundamental mechanisms of micromachine reliability

Due to extreme surface to volume ratios, adhesion and friction are critical properties for reliability of Microelectromechanical Systems (MEMS), but are not well understood. In this LDRD the authors established test structures, metrology and numerical modeling to conduct studies on adhesion and friction in MEMS. They then concentrated on measuring the effect of environment on MEMS adhesion. Polycrystalline silicon (polysilicon) is the primary material of interest in MEMS because of its integrated circuit process compatibility, low stress, high strength and conformal deposition nature. A plethora of useful micromachined device concepts have been demonstrated using Sandia National Laboratories' sophisticated in-house capabilities. One drawback to polysilicon is that in air the surface oxidizes, is high energy and is hydrophilic (i.e., it wets easily). This can lead to catastrophic failure because surface forces can cause MEMS parts that are brought into contact to adhere rather than perform their intended function. A fundamental concern is how environmental constituents such as water will affect adhesion energies in MEMS. The authors first demonstrated an accurate method to measure adhesion as reported in Chapter 1. In Chapter 2 through 5, they then studied the effect of water on adhesion depending on the surface condition (hydrophilic or hydrophobic). As described in Chapter 2, they find that adhesion energy of hydrophilic MEMS surfaces is high and increases exponentially with relative humidity (RH). Surface roughness is the controlling mechanism for this relationship. Adhesion can be reduced by several orders of magnitude by silane coupling agents applied via solution processing. They decrease the surface energy and render the surface hydrophobic (i.e. does not wet easily). However, only a molecular monolayer coats the surface. In Chapters 3-5 the authors map out the extent to which the monolayer reduces adhesion versus RH. They find that adhesion is independent of RH up to a threshold value, depending on the coating chemistry. The mechanism for the adhesion increase beyond this threshold value is that the coupling agent reconfigures from a surface to a bulk phase (Chapter 3). To investigate the details of how the adhesion increase occurs, the authors developed the mechanics for adhesion hysteresis measurements. These revealed that near-crack tip compression is the underlying cause of the adhesion increase (Chapter 4). A vacuum deposition chamber for silane coupling agent deposition was constructed. Results indicate that vapor deposited coatings are less susceptible to degradation at high RH (Chapter 5). To address issues relating to surfaces in relative motion, a new test structure to measure friction was developed. In contrast to other surface micromachined friction test structures, uniform apparent pressure is applied in the frictional contact zone (Chapter 6). The test structure will enable friction studies over a large pressure and dynamic range. In this LDRD project, the authors established an infrastructure for MEMS adhesion and friction metrology. They then characterized in detail the performance of hydrophilic and hydrophobic films under humid conditions, and determined mechanisms which limit this performance. These studies contribute to a fundamental understanding for MEMS reliability design rules. They also provide valuable data for MEMS packaging requirements

Chemical vapor deposition coating for micromachines

Two major problems associated with Si-based MEMS devices are stiction and wear. Surface modifications are needed to reduce both adhesion and friction in micromechanical structures to solve these problems. In this paper, the authors will present a process used to selectively coat MEMS devices with tungsten using a CVD (Chemical Vapor Deposition) process. The selective W deposition process results in a very conformal coating and can potentially solve both stiction and wear problems confronting MEMS processing. The selective deposition of tungsten is accomplished through silicon reduction of WF{sub 6}, which results in a self-limiting reaction. The selective deposition of W only on polysilicon surfaces prevents electrical shorts. Further, the self-limiting nature of this selective W deposition process ensures the consistency necessary for process control. Selective tungsten is deposited after the removal of the sacrificial oxides to minimize process integration problems. This tungsten coating adheres well and is hard and conducting, requirements for device performance. Furthermore, since the deposited tungsten infiltrates under adhered silicon parts and the volume of W deposited is less than the amount of Si consumed, it appears to be possible to release stuck parts that are contacted over small areas such as dimples. Results from tungsten deposition on MEMS structures with dimples will be presented. The effect of wet and vapor phase cleanings prior to the deposition will be discussed along with other process details. The W coating improved wear by orders of magnitude compared to uncoated parts. Tungsten CVD is used in the integrated-circuit industry, which makes this approach manufacturable

Integration of Optoelectronics and MEMS by Free-Space Micro-optics

This report represents the completion of a three-year Laboratory-Directed Research and Development (LDRD) program to investigate combining microelectromechanical systems (MEMS) with optoelectronic components as a means of realizing compact optomechanical subsystems. Some examples of possible applications are laser beam scanning, switching and routing and active focusing, spectral filtering or shuttering of optical sources. The two technologies use dissimilar materials with significant compatibility problems for a common process line. This project emphasized a hybrid approach to integrating optoelectronics and MEMS. Significant progress was made in developing processing capabilities for adding optical function to MEMS components, such as metal mirror coatings and through-vias in the substrate. These processes were used to demonstrate two integration examples, a MEMS discriminator driven by laser illuminated photovoltaic cells and a MEMS shutter or chopper. Another major difficulty with direct integration is providing the optical path for the MEMS components to interact with the light . We explored using folded optical paths in a transparent substrate to provide the interconnection route between the components of the system. The components can be surface-mounted by flip-chip bonding to the substrate. Micro-optics can be fabricated into the substrate to reflect and refocus the light so that it can propagate from one device to another and them be directed out of the substrate into free space. The MEMS components do not require the development of transparent optics and can be completely compatible with the current 5-level polysilicon process. We report progress on a MEMS-based laser scanner using these concepts

Effect of W coating on microengine performance

Two major problems associated with Si-based MEMS (MicroElectroMechanical Systems) devices are stiction and wear. Surface modifications are needed to reduce both adhesion and friction in micromechanical structures to solve these problems. In this paper, the authors present a CVD (Chemical Vapor Deposition) process that selectively coats MEMS devices with tungsten and significantly enhances device durability. Tungsten CVD is used in the integrated-circuit industry, which makes this approach manufacturable. This selective deposition process results in a very conformal coating and can potentially address both stiction and wear problems confronting MEMS processing. The selective deposition of tungsten is accomplished through the silicon reduction of WF{sub 6}. The self-limiting nature of the process ensures consistent process control. The tungsten is deposited after the removal of the sacrificial oxides to minimize stress and process integration problems. The tungsten coating adheres well and is hard and conducting, which enhances performance for numerous devices. Furthermore, since the deposited tungsten infiltrates under adhered silicon parts and the volume of W deposited is less than the amount of Si consumed, it appears to be possible to release adhered parts that are contacted over small areas such as dimples. The wear resistance of tungsten coated parts has been shown to be significantly improved by microengine test structures