Publications

A mesoscale dimensional artifact based on silicon bulk micromachining fabrication has been developed with the intention of evaluating the artifact both on a high precision Coordinate Measuring Machine (CMM), and on a video-probe based measuring system. A high accuracy touch-probe based CMM can achieve accuracies that are as good as the 2-D repeatability of video-probe systems. While video-probe based systems are commonly used to inspect mesoscale mechanical components, a video-probe system's certified accuracy is generally much worse than its repeatability. By using a hybrid artifact where the same features can be extracted by both a touch-probe and a video-probe, the accuracy of video-probe systems can be improved. In order to use the micromachined device as a calibration artifact, it is important to understand the uncertainty present in the touch-probe measurements. An uncertainty analysis is presented to show the potential accuracy of the measurement of these artifacts on a high precision CMM.

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This work is the result of a Sandia National Laboratories LDRD funded fellowship at the University of Michigan. Although, guidance and suggestions were offered by Sandia, the work contained here is primarily the work of Brian H. Stark, and his advisor, Professor Khalil Najafi. Junseok Chae, Andrew Kuo, and their coworkers at the University of Michigan helped to record some of the data. The following is an abstract of their work. We have developed a vacuum packaging technology using a thick nickel film to seal MEMS structures at the wafer level. The package is fabricated in a three-mask process by electroplating a 40 micro-meter thick nickel film over an 8 micro-meter sacrificial photoresist that is removed prior to package sealing. Implementation of electrical feedthroughs in this process requires no planarization. The large release channel enables an 800x800 micro-meter package to be released in less than three hours. Several mechanisms, based upon localized melting and lead/tin solder bumping, for sealing the release channel have been investigated. We have also developed Pirani gauges, integrated with this package, which can be used to establish the hermeticity of the different sealing technologies. They have measured a sealing pressure of approximately 1.5 Torr. Our work differs from previous Pirani gauges in that we utilize a novel doubly anchored structure that stiffens the structural membrane while not substantially degrading performance in order to measure fine leak rates.

We describe a post release die separation process for polysilicon surface micromachines using a combination of diamond scribing and breaking. The process resulted in yields above 80% for two types of electrostatic actuators. The paper describes the experimental apparatus and optimization of the process using a four parameter design of experiments. We determined that the two key parameters in the scribe-and-break process are the scribe force and the scribe angle. We also examined the theory of crack creation during the scribing process and determined experimentally that the crack depth in silicon is consistent with the theory developed for the scribing of glass. Copyright © 2004 by ASME.

Many microfabrication techniques are being developed for applications in microelectronics, microsensors, and micro-optics. Since the advent of microcomponents, designers have been forced to modify their designs to include limitations of current technology, such as the inability to make three-dimensional structures and the need for piece-part assembly. Many groups have successfully transferred a wide variety of patterns to both two-dimensional and three-dimensional substrates using microcontact printing. Microcontact printing is a technique in which a self-assembled monolayer (SAM) is patterned onto a substrate by transfer printing. The patterned layer can act as an etch resist or a foundation upon which to build new types of microstructures. We created a gold pattern with features as small as 1.2 {micro}m using microcontact printing and subsequent processing. This approach looks promising for constructing single-level structures such as microelectrode arrays and sensors. It can be a viable technique for creating three-dimensional structures such as microcoils and microsprings if the right equipment is available to achieve proper alignment, and if a means is available to connect the final parts to other components in subsequent assembly operations. Microcontact printing provides a wide variety of new opportunities in the fabrication of microcomponents, and increases the options of designers.

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This paper reports on the design, simulation, and preliminary testing of a three phase variable reluctance stepping motor. This motor is pancake-shaped with an overall outside diameter of 8 mm and a height of 3 mm. The outside diameter of the rotor is 4.7 mm. The rotor and stators occupy 2 mm of the height with the remaining 1 mm reserved for a 6:1 planetary gear reductor. The rotor and stators were constructed of Hyperco 50 using conventional miniature machining. The reductor was assembled using copper and PMMA (polymethylmethacrylate) components that were constructed using the LIGA (Lithographic Galvanoformung Abformung) microfabrication process. The maximum measured stall torque of the motor without the reductor is 0.47mNm at 4W and the maximum speed is 2,400 rpm.

A completely foundry compatible chip-scale package for surface micromachines has been successfully demonstrated. A pyrex (Corning 7740) glass cover is placed over the released surface micromachined die and anodically bonded to a planarized polysilicon bonding ring. Electrical feedthroughs for the surface micromachine pass underneath the polysilicon sealing ring. The package has been found to be hermetic with a leak rate of less than 5 x 10{sup {minus}8} atm cm{sup {minus}3}/s. This technology has applications in the areas of hermetic encapsulation and wafer level release and die separation.

This paper reports on significant advances in electrothermal bent beam actuators. Designs for long throw linear and rotary actuators are described. Silicon p++ devices showed 20--30 {mu}m displacements with 150 {micro}N loads at actuation levels of 6--8 V, and 250--300 mW. An electroplated version provided 15 {mu}m displacements at 0.8 V and 450 mW. Inchworm type devices are reported that had linear displacements of 100 {micro}m with 200 {micro}N loads. Refinements in the modeling to account for non-linear thermal expansion coefficients and buckling are also reported.