AVS1996 Session MM+NS-WeM: Electromechanical Actuation on the Micro/Nano Scale
Wednesday, October 16, 1996 8:20 AM in Room 204B
MM+NS-WeM-1 Precision Microsensors Based on Tunneling Transducers
T. Kenny (Stanford University)
Since the invention of the STM, the use of tunneling as a displacement transducer has been discussed. A number of operational tunneling sensors have been demonstrated, including accleerometers, infrared sensors, magnetometers, and pressure sensors. In this talk, the measured performance characteristics of tunneling displacement transducers will be discussed. An important issue with all tunneling transducers is the drift which is expected due to rearrangements of the atoms in the tip. Recent measurements have shown that transducers operated with very wide-bandwidth controllers (bandwidth>50 kHz) are exceedingly stable over long time periods. The measured drift in these devices is dominated by thermal expansion of the support structures, and is better than 0.1 Angstroms over hour-long measurement periods. THis presentation will describe these measurements, and their implications for a variety of sensor applications.
MM+NS-WeM-3 Microelectromechanical Force Probe Arrays
S. Miller, K. Turner, N. MacDonald (Cornell University)
Arrays of microelectromechanical force probes with integrated actuators and capacitive sensors for scanning force microscopy have been fabricated and characterized. The arrays were fabricated using the SCREAM silicon micromachining process. Each array element consists of a single crystal silicon tip on a torsional cantilever with integrated z actuators. The current device design consists of a 2x2 array of force probes. Each element in the array is about 125\mu\m by 125\mu\m in size. The tip-tip spacing in the array is 150\mu\m. Mechanical testing has been done and the first resonant mode of the cantilevers occurs at approximately 280 kHz for each of the array elements. The effective stiffness of the cantilevers is about 70 N/m. Parallel operation of the force sensors can be performed since each array element is independent of the others. Applications of such arrays include nanolithography and nanomachining. The use of torsion allows for a wide range in cantilever design. The stiffness of the cantilever scales with the length of the cantilever and the width of the support beams. Since the support beams have a high aspect ratio, their width can be scaled down without weakening the out of-plane stiffness of the structure as a whole. The torsional design also allows motion of the tip in the \+-\z directions. Interdigitated electrode capacitors supply the force for actuation in these directions and provide the means for sensing deflections of the cantilever. The device is compact and suitable for array architectures since there is no external deflection sensor.  K. A. Shaw, Z. L. Zhang, and N. C. MacDonald, Sensors and Actuators A 40, 63-70 (1994).
MM+NS-WeM-4 Advanced Micromachined Cantilevers for Thermomechanical Data Storage
B. Chui (Stanford University); H. Mamin, B. Terris (IBM Almaden Research Center); T. Stowe (Stanford University); D. Rugar (IBM Almaden Research Center); T. Kenny (Stanford University)
Silicon cantilevers have been developed which are suitable for use with data storage schemes based on the atomic force microscope (AFM), in particular thermomechanical recording. The principle of thermomechanical recording is based on the storage of digital data as nano-indentations on a rotating polycarbonate disk. Bit densities up to 30 gigabit/sq.inch (50X CD-ROM) have previously been achieved. Low-stiffness cantilevers for data readback have been fabricated combining a sharp tip with an integrated piezo- resistive sensor. A novel process was developed to produce very shallow piezoresistors using a low-energy boron implant through thin oxide followed by rapid thermal annealing. These 0.4-micron-deep piezoresistors are compat- ible with cantilever thicknesses of 1 micron, significantly thinner than possible with previous AFM piezoresistive cantilevers. The measured sensitivity of the cantilevers, at up to 0.7 ppm/angstrom, is in excellent agreement with predictions. A noise floor of 0.016 angstrom/sqrt(Hz) was measured, very close to the Johnson noise floor of 0.013 angstrom/sqrt(Hz). In a bandwidth of 1 Hz-100 kHz, the minimum detectable displacement was found to be under 10 angstroms. Separate devices have been developed for thermo- mechanical writing, in which a heated tip is used to write features into the polycarbonate disk. To form these devices, resistive heaters were integrated onto the end of the cantilevers. The thermal time constant of these heaters was measured to be approximately 30 microseconds. Reading and writing on a rotating poly-carbonate sample has been demonstrated at linear velocities up to 120 mm/s.
MM+NS-WeM-5 Micro Latching Accelerometer with Optical Readout
X. Sun, S. Zhou, W. Carr (New Jersey Institute of Technology)
A mechanically-latching accelerometer based on silicon surface micromachining technology has been designed and fabricated for the measurement of large peak accelerations. The device consists of a seismic-mass-loaded cantilever beam and an array of distributed latches, which are made of a 3.0 micron thick polysilicon layer. The cantilever beam latches at specific threshold accelerations. The cantilever is latched with increasing acceleration at successive tether points to provide a measure of peak acceleration. The threshold acceleration is thus stored and can be read out using optical techniques. These surface micromachined structures of low stress polysilicon permit useful design of sensitivity over the range from 10 to 5000 G. A closed form analytical solution of the acceleration sensitivity has been derived and compared with experimental data. The model is confirmed experimentally within the limit of estimated frictional forces for several acceleration levels.
MM+NS-WeM-6 Electrostatic Actuation of a Microgravity Accelerometer
B. Dolgin, F. Hartley, B. Lurie (Jet Propulsion Laboratory); P. Zavracky (Northeastern University)
The paper presents the design of the bi-directional electrostatic actuation and controls for a Microgravity Accelerometer (MGA). The MGA is design to measure acceleration up to 10 mg with a 10ng accuracy in a frequency domain from 0.0001 Hz to 20 Hz. The MGA verification requires a space-based experiment. A rough device characterization has been performed on a vibration isolation table. The MGA performance characteristics exceed the capabilities of the test setup used in its verification. The operation has been demonstrated for frequencies between 0.01Hz and 20Hz and micro-g level accelerations. The device appears to maintain stability better than 0.003% in the thermal environment of +/-5C over several weeks of operation. Both high and low demonstrated limits of operation appear to be test setup related as opposed to MGA design related. The accuracy of the accelerometer exceeds the capabilities of the test equipment. The MGA performance was demonstrated by measuring the response of the MGA to small accelerations along the vertical direction in a 1g field. The tunneling tip controller was tested in a 1g mode of operation. The device is primarily aimed at zero-g operations. The controller modifications introduced to accommodate the 1g offset field did not change its frequency characteristics. Thus, all the results of the 1g measurements may be translated into 0g operations. The tests demonstrated the feasibility of the MGA.
MM+NS-WeM-7 Electromechanical Actuation of a 1 mm\super 2\ LIGA-based Shuttle for use as a Tangential Tactor
R. Ghodssi, E. Harris, I. Glasgow (University of Wisconsin, Madison); D. Beebe (University of Illinois, Urbana-Champaign); V. White, D. Denton (University of Wisconsin, Madison)
Two different suspension style flexures (folded beam and parallelogram) are used for the development of a MEMS-based tangential tactor using electrostatically driven linear microactuator technology. Both designs have a free moving shuttle of 1mm\super 2\ area. The devices are designed to generate a 1 mN force at an applied voltage of 100 volts for a travel distance of 30-200 microns in the lateral direction. The tactor devices are fabricated using an optimized LIGA/MEMS process. The microstructures are made of 30-50 micron thick electroplated nickel with a minimum linewidth of 5 microns. The electroplated nickel 1 mm\super 2\ shuttle was released with no circular holes on its surface in order to provide a solid surface area for a microspherical plastic object to be assembled on it. The extended sacrificial removal process did not attack the other thin film materials on the sample. Neither suspension style exhibited any stiction to the substrate after the sacrificial layer removal. The shuttle has exhibited a repeatable movement of 30-200 microns with an applied rms voltage of 48 volts at a frequency of 10 Hz. Finite element analysis was performed in order to investigate both the static and dynamic characteristics of the microdevices. Results for both electromechanical actuation and modeling analysis will be presented.
MM+NS-WeM-8 Electrostatically Actuated Micromechnical Switches
N. McGruer, P. Zavracky (Northeastern University)
Micromechanical switches realized by micromachining processes have many desirable properties : they have a high off-impedance to on-impedance ratio compared to semiconductor switches, they have extremely low power consumption, and they are capable of higher switching speeds and are potentially more reliable than conventional mechanical relays. A number of researchers have recently reported micromechanical switches that are either electrostatically actuated [1,2,3], or magnetically actuated [4,5]. We report an electrostatically actuated micromechanical switch fabricated using a very simple 4 level surface micromachining process. Switches have been fabricated with threshold voltages less than 30 V, and cut-off frequencies greater than 100 KHz. The devices have been operated while switching current up to 5 x10 \super 7\ cycles prior to failure in air, and for greater than 10 \super 8\ cycles without failure in a nitrogen ambient. Contact resistance is less than 10 miliohms. Switches have been designed and tested in several configurations, including three-terminal configurations (analogous to the gate, source and drain of a FET), and in four-terminal configurations with the driving signal and output isolated from each other. The performance of devices with different contact metal combinations has been studied. In this paper we discuss the operating principle of the switches on the basis of a simple analytical model, and then present more accurate finite-element and semi-empirical models. The fabrication process is then described. Finally, the performance of the switches is discussed on the basis of measurement results.References 1. J. Drake, et al., "An electrostatically actuated micro-relay", Transducers '95 Eurosensors IX, Stockholm, Sweden(1995). 2. M. Gretillat, et al., "Integrated circuit compatible electrostatic polysilicon microrelays", J. Micromech. Microeng., 5 156-60(1995). 3. J. Yao and M. Chang, "A surface micromachined miniature switch for telecommunications applications with signal frequencies from DC up to 4 GHz", Transducers '95 Eurosensors IX, Stockholm, Sweden(1995). 4. H. Guckel, private communication. 5. H. Hosaka, et al., "Electromagnetic microelays: concepts and fundamental characteristics", Sensors and Actuators A, 40 41-47(1994).