Papers

AVS1997 Session VT+MM-ThM: Vacuum MEMS

Thursday, October 23, 1997 8:20 AM in Room C3/4

Thursday Morning

Start	Invited?	Item
8:20 AM		VT+MM-ThM-1 Microfluidic Devices for Semiconductor Processing A.K. Henning (Redwood Microsystems) The advent of MEMS (microelectromechanical systems) enables dramatic changes in semiconductor processing. MEMS-based devices offer opportunities to achieve increased performance, and higher levels of functional integration, at lower cost, with decreased size and increased reliability. Previous research and product development efforts have demonstrated the application of the thermopneumatic actuation of silicon microvalve membranes to problems of industrial gas and liquid control. Wide ranges of pressure, temperature, and flow rate have been demonstrated. Devices such as pressure regulators and mass flow controllers have been achieved, based on both normally-open and normally-closed microvalves. In this work, we describe the achievement of several important devices for use in the semiconductor process equipment industry. They include a low-flow mass flow controller, a pressure regulator, and an integrated gas panel. Compared to current technology, the devices are ultra-small in size, thus minimizing dead volumes and gas contact surface areas. With wettable surfaces comprised of ceramic and silicon (or, silicon coated with Si3N4 or SiC), they become resistant to corrosion, and generate virtually no particles. These devices are created from modular components. The science and technology of these components will be detailed. The modules examined are: normally-open proportional valves; normally-closed, low leak-rate shut-off valves; critical orifices (to extract information of flow rate); flow models (to extract flow rate from pressure and temperature information); silicon-based pressure sensors; and, the precision ceramic-based packages which integrate these modules into microfluidic devices.
9:00 AM		VT+MM-ThM-3 A Study of MEMS-Type Low Pressure Sensors A.P. Miiller, C.R. Tilford, S.A. Tison (National Institute of Standards & Technology) There has been significant progress made during the past decade or so in the development of micromachined silicon-based pressure sensors for applications in selected areas of industrial measurement technology, such as, process control and measurement of flow, pressure, liquid and gas densities, etc. In recent years, these MEMS-type pressure sensors have become available in ever decreasing full scale (FS) ranges and with advertised performance characteristics that make them attractive candidates for use in standards laboratories. Two types of MEMS sensors, namely, resonant silicon diaphragm gages (RSDG’s) and piezoresistive silicon diaphragm gages (PSDG’s) have been included in an ongoing study of measurement performance of low pressure transducers at the National Institute of Standards and Technology. Data obtained on RSDG’s and PSDG’s with FS ranges between 1 and 130 kPa indicate that these sensors have promising long-term repeatabilities of the order of 0.01% per year. In addition to long-term stability (repeatability), data on other performance characteristics of these gages, such as, measurement sensitivity (resolution), tilt sensitivity, zero-pressure stability, and linearity will also be described. Examples of comparative data for more traditional transducers, such as, capacitance diaphragm gages or quartz Bourdon gages, are also included.
9:20 AM		VT+MM-ThM-4 A Micromachined Metal Contaminant Detector for Monitoring Ion Propulsion Plume Re-Impingement T.E. Pfafman, A. Fitzgerald, T.W. Kenny (Stanford University) A sensor was developed to detect the build-up of metal ions, which are present in the exhaust of ion thrusters, from plume reimpingement on a spacecraft surface. The micromachined sensor employs the principles of calorimetry and a closed-loop controller to detect the change in emissivity of the sensing element surface as a result of contaminant metal deposition. The use of a micromachined sensor enables significant reductions in mass, size, and power consumption. The thermal, mechanical, and feedback design are presented, along with calibration data indicating the sensitivity of the instrument. The sensor will be integrated into the NASA Deep Space-1 spacecraft as part of a suite of instruments to analyze the particles and fields in the deep space environment and in the vicinity of a comet and an asteroid.
9:40 AM		VT+MM-ThM-5 Microprobes for a New MEMS Wafer Probe Card Yanwei Zhang, Yongxia Zhang (New Jersey Institute of Technology); D. Worsham (Pacific Western Systems, Inc.); D. Morrow (sp³, Inc.); R.B. Marcus (New Jersey Institute of Technology) A new type of MEMS cantilever wafer probe card consists of an array of microprobe cantilevers individually actuated by bimorph heating to make contact with test pads on a device chip. This probe card is called the CHIPP (Conformable, High-Pin count, Programmable) probe card and can be designed to contact up 800 I/O pads along the perimeter of a 1 cm² chip with a microprobe repeat distance of ~50 microns. Microprobes for a prototype CHIPP probe card have been fabricated with a variety of structures including Al-SiO₂, W-SiO₂ and Al-Si bimorphs, Au, W and conducting diamond tips, and with the resistive heater placed either inside or on the surface of the cantilever. Each microprobe contains four separate layers plus a tip contact metal. Cantilever lengths are 300-500 microns and widths are 30-50 microns. The dynamic performance was tested both in air and in vacuum inside the SEM. The deflection efficiency in air varies from 5.3 to 9.6 microns/mW applied power depending on device parameters, and the corresponding force efficiency varies from 1.4 to 5.5 microNewtons/mW. Maximum reversible deflection is in the range 200-300 microns for Al-SiO₂ bimorphs and 100-150 microns for W-SiO₂ bimorphs. Heat loss for devices operating in air was found to be substantially higher than for vacuum operation with heat loss ratio 2/1 for an internal heater structure and 4.25/1 for a structure with the heater as an outer layer of the cantilever. The resonant frequency of measured devices is 9-10 kHz, and the time constant for thermal response is 2-3 ms. The contact resistance ranges from 0.25 ohms for gold-gold contacts to 0.5 ohms for W-W contacts, and contact force was measured at 45-60 microNewton. Video recording using SEM voltage contrast imaging clearly shows simultaneous mechanical and ohmic contact to a stationery tungsten electrode, deflections over 150 microns in response to 2 V heater signals, the damping of response with increasing frequency, and a strong resonance for one device at 8.16KHz.
10:00 AM		VT+MM-ThM-6 Electron Tunneling Noise in Surface Micromachined Tunneling Tip Devices. J.-C. Wang (Analog Devices, Inc.); M. Zavracky, R.H. Morrison (Northeastern University) This paper reports the investigation of electron tunneling noise in tunneling tip devices. Tunneling tips when used as displacement sensors have already demonstrated resolution as low as a few hundredths of an Angstrom. However, it has been found that tunneling tips suffer from low frequency noise, which may limit their usefulness for high sensitivity low frequency measurements. We have investigated tunneling tip noise using appropriate test structures incorporating different spring constants and masses. To avoid the need for a complicated vibration isolation system, the resonant frequencies are designed to be larger than 10 KHz. The devices are fabricated using surface micromachining technology. The beams and proof masses are formed by eletroplating nickel and the tips are formed by plating gold on a partially etched sacrificial layer. A closed-loop control circuit is implemented for the test structure to obtain a stable operation. The tunneling distance is controlled by an electrostatic force applied between the proof mass and counter electrode. Analysis based on theoretical calculations and measurements shows that the flicker noise level is more than one order of magnitude higher than the total noise level of other well-known noise sources Noise spectra are also obtained in air ambient and low pressure nitrogen ambient. The results show that the tunneling noise level is much lower in vacuum than in air especially for low frequencies. This suggests that the surface condition of the tunneling electrode plays an important role in the noise performance. Noise measurements are implemented for different device structures and different metals (Au and Ru). Surface adsorption of foreign molecules changes the work function of tunneling electrodes and therefore affects the tunneling current. Based on this physical mechanism, a mathematical model is established. The model shows that a random process of surface adsorption-desorption gives rise to a 1/f noise power spectrum of tunneling current. Theoretical results are compared with measured data. Reasonable fits are obtained.
10:20 AM		VT+MM-ThM-7 Ciliary Microactuator Array for Scanning Electron Microscope Positioning Stage R.B. Darling (University of Washington); J.W. Suh, G.T.A. Kovacs (Stanford University) The performance of a ciliary microactuator array as a positioning stage in a scanning electron microscope is detailed. Such arrays are attractive because they can provide micron scale positioning resolution with direct electronic control within the sample chamber. The present array uses thermal actuation for long-travel motion and electrostatic actuation in a low-power hold-down mode. The cilia are fabricated by a novel polyimide bimorph process that has been described elsewhere.¹ Each cell in the array consists of four orthogonally oriented cilia which can provide omnidirectional motion within the plane of the array. By tiling four of the actuator die into a larger array, automatic positioning and centering of objects can be achieved. Adapting this array for use inside a scanning electron microscope requires only a modest feedthrough of 8 conductors plus ground. The cilia deflection versus thermal actuation current and versus electrostatic hold-down voltage will be compared for operation inside the SEM and for that in air. Effects of reduced air cooling and substrate charging by the scanning beam will also be discussed. ¹J. W. Suh, S. F. Glander, R. B. Darling, C. W. Storment, and G. T. A. Kovacs, "Combined Organic Thermal and Electrostatic Omnidirectional Ciliary Microactuator Array for Object Positioning and Inspection," Tech. Digest Solid-State Sensor and Actuator Workshop, pp. 168-173, Hilton Head, SC, June 2-6, 1996.
10:40 AM		VT+MM-ThM-8 A Micromachined Scanning Fabry-Perot Interferometer P.M. Zavracky, K.L. Denis, R.H. Morrison (Northeastern University) In this paper we describe a surface micromachined scanning Fabry-Perot interferometer¹ (FPI). The FPI is assembled using a novel technique of mounting the upper mirror to free standing nickel beams as a final step in the fabrication. This offers the significant benefit that high quality optical mirrors can be incorporated in the design without degradation due other processing steps. The bottom mirror and support beams are fabricated using a simple five level surface micromachining process. The completed device consists of two plane parallel mirrors separated by a small gap (2um). The gap is formed by a copper sacrificial layer which is etched away to free the nickel beams. In present devices, the mirrors are coated with five alternating layers of Si and SiO2. The upper mirror is supported by four nickel beams (100 x 200 microns) which are independently actuated by applying a voltage between the beam and the substrate. Actuating voltages of approximately 100-150 volts are required on present devices. Independent actuation of each beam ensures that the mirrors remain parallel throughout the tuning range. Proposed applications include in -situ measurements of plasma composition, chemical analysis, and thickness measurements of thin films. ¹C. Fabry and A. Perot, "Theorie et applications d'une nouvelle methode de spectroscopie interferentielle", Ann. Chim. Phys., Paris, 7 th series, 1899, 16, 115-144
11:00 AM		VT+MM-ThM-9 Low Voltage, High Torque and Displacement Microactuators J.G. Fleming (Sandia National Laboratories) There is currently a need for low voltage, high torque, high displacement, actuators. Our approach to this problem consists of a series of parallel plate structures. Each plate has two electrically conductive sides separated by an insulator. Sides of the same orientation, are electrically and mechanically connected by two separate sets of leaf spring conductors. When freed, the pairs of plates are pulled together against the spring force by the application of a potential. Since each spring/plate pair is mechanically separate, the total displacement is the sum of the individual displacements. When the voltage is sufficiently high, the plates are displaced by roughly a third of the gap between them, and the structure snaps shut. There are thus two modes of operation. At low voltage, there is a steady displacement with increasing voltage. However, above a critical voltage the pairs suddenly close completely. The actuators were fabricated by a mold micromachining process using only two mask levels. The parts consist of three different components, the conductive springs and plates, the insulators between the plates, and the substrate, and the bond pads. These were formed by filling trench etched silicon with various grown or deposited layers of insulators, thermal oxide and silicon nitride and conductors, titanium nitride and tungsten. The actuators were released by etching the substrate using either a F based plasma or using KOH. The lowest actuation voltage to date is 15V and the maximum displacement was 34 microns. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.
11:20 AM		VT+MM-ThM-10 Mechanical Testing of LIGA and SMM Micro-ElectroMechanical Systems Materials T.E. Buchheit, R.J. Bourcier (Sandia National Laboratories) The design of efficient and reliable MicroElectrical Mechanical Systems (MEMS) based devices requires knowledge of the materials properties used in their fabrication. We are currently developing a suite of experimental testing capabilities to generate material properties data for both electroplated and surface micromachined (SMM) silicon structures. A small scale computer controlled servo-hydraulic test system has been assembled to perform uniaxial and cyclic mechanical characterization of LIGA-fabricated specimens. Specimens with characteristic dimensions as small as 25 µm have been successfully tested. Tensile and fatigue data for LIGA synthesized Nickel, Ni-Fe (Permalloy) and Ni-Co alloys will be presented. To obtain materials data for SMM fabrication, several on chip testing features have been designed and fabricated to examine the fatigue and fracture properties of polysilicon. To capture fatigue properties, cantilever, three point bend and curved beam test specimens have been designed. These geometries use comb drives as their actuator mechanisms and are designed to gain the mechanical advantage necessary for fatigue testing at moderate to high stress levels. Also, using SMM techniques, several cantilever beams were fabricated on a polysilicon shuttle which can be shifted to the edge of the chip thus providing ample space below the beams to conduct fracture tests using an ultra-low load hardness tester. Details of the test specimen designs and some preliminary properties data will be given. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed- Martin Company, for the United States Department of Energy.
11:40 AM		VT+MM-ThM-11 Complete Integrated CMOS Controller for Tunnel Instruments A. Partridge, J.K. Reynolds, B.J. Kane (Stanford University); N. Maluf (Lucas NovaSensor); G.T.A. Kovacs, T.W. Kenny (Stanford University) Micromachined tunnel instruments require electronic circuitry to servo control their tunnel gap width. This paper describes the first integrated and complete tunnel sensor controller. The self-contained CMOS circuit provides all necessary controller functions: It generates a tunnel junction bias, senses the tunnel current by dropping it across a diode-connected well transistor, compares the voltage across this diode to an internally generated setpoint, derives a feedback signal with an externally configurable gain and frequency response, generates a high voltage electrostatic drive, and provides a low voltage output for external circuitry. The chip requires only -4V and -40V power supplies and a compensation RC network on its output. Total die area, exclusive of bond pads, is 0.5 mm². This paper presents circuit performance over supply voltage and temperature, as well as system performance controlling a membrane-type micromachined tunnel sensor. ¹ ¹This work is supported by NSF CAREER (ECS-9502046), and by JPL/NASA.