Showing papers in "Sensors in 1997"

Stabilization of electrostatically actuated mechanical devices

Abstract: We show that the pull-in instability, which significantly limits the operation of electrostatically actuated devices, can be avoided by the simple addition of a series capacitance.

Simple and low cost fabrication of embedded micro-channels by using a new thick-film photoplastic

Abstract: In this paper we describe new ways to realise micro-channels. The used technologies are based on a novel thick-film photoplastic, the SU-8. This photoresist has outstanding properties, allowing the realisation of high aspect ratio microstructures. It is a simple and low-cost fabrication process. The micro channels developed in this work will be used for the realisation of thermal flow-sensors.

Surface-micromachined resonant accelerometer

Abstract: This paper discusses the design and testing of a resonant accelerometer developed for integrated surface-micromachining processes. First- and second-generation designs are presented. The sensors use leverage mechanisms to transfer force from a proof mass to double-ended tuning fork (DETF) resonators used as force transducers. Each fork forms the basis of an integrated oscillator to provide the output waveforms. The DETFs on the first-generation device have a nominal frequency of 175 kHz, and the sensor has a scale factor of 2.4 Hz/g. The oscillators on this device exhibit a root Allan variance floor of 38 mHz (220 ppb). The second-generation, higher-sensitivity sensor uses DETFs with a nominal frequency of 68 kHz and has a measured scale factor of 45 Hz/g.

Power MEMS and microengines

Abstract: MIT is developing a MEMS-based gas turbine generator. Based on high speed rotating machinery, this 1 cm diameter by 3 mm thick SiC heat engine is designed to produce 10-20 W of electric power while consuming 10 grams/hr of H/sub 2/. Later versions may produce up to 100 W using hydrocarbon fuels. The combustor is now operating and an 80 W micro-turbine has been fabricated and is being tested. This engine can be considered the first of a new class of MEMS device, power MEMS, which are heat engines operating at power densities similar to those of the best large scale devices made today.

Embedded interconnect and electrical isolation for high-aspect-ratio, SOI inertial instruments

Abstract: A new technique for providing both electrical isolation and embedded interconnect to SOI-based, single crystal silicon, inertial sensors is described. This technology allows fabrication of high-aspect-ratio, in-plane, capacitive sensors with improved sensitivity suitable for integration with on-chip electronics. Various 45 /spl mu/m-tall MEMS devices with electrical isolation from the silicon substrate and embedded interconnect have been fabricated and tested. The embedded interconnect and electrical isolation enable truly integrated high-aspect-ratio MEMS sensors, and alternatively simplifies packaging in monolithic two-chip approaches. By extending the demonstrated technique to aluminum interconnect, only two additional masks are required to convert a CMOS process into a fully integrated MEMS technology at the incremental cost of an SOI starting material.

Dual axis operation of a micromachined rate gyroscope

Abstract: Since micromachining technology hs raised the prospect of fabricating high performance sensors without the associated high cost and large size, many researchers have investigated micromachined rate gyroscopes. The vast majority of research has focused on single input axis rate gyroscopes, but this paper presents work on a dual input axis micromachined rate gyroscope. The key to successful simultaneous dual axis operation is the quad symmetry of the circular oscillating rotor design. Untuned gyroscopes with mismatched modes yielded random walk as low as 10/spl deg///spl radic/(Hour) with cross axis sensitivity ranging from 3% to 16%. Subsequent mode frequency matching via electrostatic tuning allowed performance of 2/spl deg///spl radic/(Hour), but at the expense of excessive cross axis sensitivity.

A very low-power consumption wireless ECG monitoring system using body as a signal transmission medium

Abstract: A very low-power consumption wireless system for monitoring ECG (electrocardiogram) is proposed. It consists of an ECG detector/transmitter located on the chest and the relay transmitter placed at the wrist. Between the detector part and the relay transmitter, the signal is send as the AC micro current flows through the tissue of the body. Since this method achieves very low-power signal transmission of about 8 /spl mu/W (400 mV/sub RMS//spl times/20 /spl mu/A/sub RMS/), the system can be driven by a very small power source like a thin film battery. A similar wireless transmission method can be applied for other medical signal/data sensing micro systems sharing the same relay transmitter.

Gas-phase Silicon Etching With Bromine Trifluoride

Abstract: We report the first study of gas phase silicon micromachining using pure bromine trifluoride (BrF/sub 3/) gas at room temperature. This work includes both the design of a new apparatus and etching characterization. Consistent etching results and high molecular etching efficiency (80%) have been achieved by performing the etching in a controlled pulse mode. This pure gaseous BrF/sub 3/ etching process is isotropic and has a high etch rate with superb selectivity over silicon dioxide (3000:1), silicon nitride (400-800:1) and photoresist (1000:1). Moreover, gaseous BrF/sub 3/ etching has also been demonstrated in surface micromachining process, where silicon nitride channels and membranes using polysilicon as the sacrificial layer have been successfully fabricated.

Controlled pulse-etching with xenon difluoride

Abstract: A gas-phase, room-temperature, plasmaless isotropic etching system has been used for bulk and thin film silicon etching. A computer controlled multi-chambered etcher is used to provide precisely metered pulses of xenon difluoride (XeF/sub 2/) gas to the etch chamber. Etch rates as high as 15 microns per minute have been observed. The etch appears to have infinite selectivity to many common thin films, including silicon dioxide, silicon nitride, photoresist, and aluminum. The etch rate, profile, and roughness are reported as a function of mask aperture, etch pressure, and duration.

A MEMS electrospray nozzle for mass spectroscopy

Abstract: We present our development of a micron-sized MEMS electrospray nozzle and demonstrate its application for electrospray mass spectroscopy (MS). The fabricated and tested micromachined electrospray nozzles are typically 40 /spl mu/m long and with 1-3 /spl mu/m orifice diameters. They also have built-in particle filters. This micromachined nozzle has been successfully interfaced with a mass spectrometer (Finnigan Mat LCQ Ion Trap) to perform standard characterization using a solution of gramicidin S at a flow rate of 50 nL/min and a voltage potential of 4 kV. This MEMS nozzle has demonstrated valid MS analyses with lower flow rates, and it has many advantages over the traditional complex preparation of a glass-capillary.

A 3-axis force balanced accelerometer using a single proof-mass

Abstract: This paper presents a new method for wideband force balancing a proof-mass in multiple axes simultaneously. Capacitive position sense and force feedback are accomplished using the same air-gap capacitors through time multiplexing. Proof of concept is experimentally demonstrated with a single-mass monolithic surface micromachined 3-axis accelerometer.

Self-assembled microactuated XYZ stages for optical scanning and alignment

Abstract: A novel self-assembled, surface-micromachined micro-XYZ stage with large displacements and fine positioning accuracy has been demonstrated on a Si microoptical bench for optical scanning/alignment applications Continuous lateral scanning up to 120 /spl mu/m and vertical scanning up to 250 /spl mu/m have been achieved with integrated scratch drive actuators (SDA), which have step resolutions of 27 nm The XYZ stage can be fully assembled by applying an electric bias only Focus adjustment of the micro-Fresnel lens integrated on the micro-XYZ stage has also been successfully demonstrated This is useful for two-dimensional scanning, dynamic focusing/tracking, and fine optical alignment

Materials reliability in MEMS devices

Abstract: Reliable operation of MEMS requires development of accelerated testing techniques and protocols. One such technique using resonant fatigue specimens has demonstrated a failure mode that was previously unknown. This work indicates that moisture can decrease the lifetime of cyclicly stressed polysilicon components.

The microfabrication of capacitive ultrasonic transducers

Abstract: The successful fabrication of surface micromachined ultrasonic transducers is reported. In a key step of the microfabrication, amorphous silicon is used as a sacrificial layer to form sealed nitride cavities. The process is fully CMOS compatible and allows for improved geometric control compared to previously reported work. Transmission experiments in both water and air are presented. A dynamic range in excess of 110 dB is observed in air at 2.3 MHz. In water, a single pair of transducers is able to operate from 2 MHz to 20 MHz. When tuned, a 3 MHz tone burst results in a received signal with better than 60 dB signal to noise ratio. The transducer behavior agrees with theoretical understanding of transducer dynamics.

A titanium-nickel shape-memory alloy actuated micropump

Abstract: The shape memory alloy (SMA) titanium-nickel (TiNi) is used in thin film form as the basis of actuation for a reciprocating micropump. A novel configuration comprising two complementary TiNi actuators is used to perform cyclic motion by utilizing the shape memory effect (SME). Check valves are fabricated from polyimide to ensure unidirectional flow through the pump chamber. The pump is driven by an electrical drive signal, which is passed directly through the TiNi thin films, causing a Joule heating-induced phase transformation and initiating the SME. Cyclic motion is achieved by heating the actuators out of phase. The pump was tested with filtered DI water and attained a pump rate of 50 /spl mu/L/min.

Self-assembled fluorocarbon films for enhanced stiction reduction

Abstract: We have developed a fluorinated self-assembled monolayer (SAM) coating process for stiction reduction in polysilicon MEMS that does not use chlorinated solvents. Using this process, cantilever beams up to 2 mm in length emerge from the final water rinse dry and released. Beam arrays fabricated from two types of polysilicon were used to characterize in-use stiction. In the first set of structures (poly A, rms roughness /spl ap/12 nm), all beams out to the maximum length of 1 mm remained unstuck following actuation, giving an adhesion energy per apparent contact area of less than 2.4 /spl mu/J/m/sup 2/. Poly B structures (rms roughness /spl ap/3 nm) gave an apparent adhesion energy of 5.2 /spl mu/J/m/sup 2/ compared to 23 /spl mu/J/m/sup 2/ for the OTS SAM coating. The fluorinated SAMs survive heat treatment in both air and N/sub 2/ at 400/spl deg/C for 5 minutes and are thus compatible with several MEMS packaging processes.

Differential blood cell counts obtained using a microchannel based flow cytometer

Abstract: This paper reports results demonstrating the ability to use single microfabricated silicon flow channels for the differential counting of granulocytes, lymphocytes, monocytes, red blood cells (RBCs), and platelets, in a sample of blood by means of laser light scattering. The microfabrication-based flow cytometer described does not rely on sheath flow in order to align the blood cells.

Silicon angular rate sensor for automotive applications with piezoelectric drive and piezoresistive read-out

Abstract: In this work a silicon angular rate sensor for automotive applications with a new architecture is presented. It is based on the vibrating tuning fork principle with excitation direction of the tines perpendicular to the wafer surface. This arrangement allows the design of tines with significant inertial masses which lead to substantial signal. The oscillation of the tines is excited by a piezoelectric drive using an AlN thin film layer. The angular rate to be measured causes a torsional oscillation of the stem. The torsional amplitude is proportional to the angular rate and is measured by a piezoresistive read-out structure. We use silicon bulk micromachining based on a new twofold SOI-technique.

Fabrication process of high aspect ratio elastic structures for piezoelectric motor applications

Abstract: Briefly reports on advances in the fabrication technology of components for millimeter size ultrasonic piezoelectric motors. A very large height to width aspect ratio with good lateral patterning resolution is required for the active component in acoustic mode conversion. A new fabrication process using a thick epoxy-based material (SU-8) and the electroplating of nickel is demonstrated. The main advantages over past fabrication methods are better flexibility in the design, simplicity of the fabrication process and the combination of metallic materials (Ni) with polymeric materials (SU-8). The mechanical properties of SU-8 have been measured using free cantilever beam structures.

On-chip coils with integrated cores for remote inductive powering of integrated microsystems

Abstract: This paper reports on integrated receiver coils and accompanying circuitry for wireless powering of integrated microsystems, emphasizing implantable biomedical applications. These planar spiral coils consist of an electroplated Cu winding and electroplated NiFe core, and they can be post processed directly on top of integrated circuitry. Ten turn, 20 mm/sup 2/ versions of these coils operating at 4 MHz have a Q of 6.6, and can supply loads of a few k/spl Omega/ with over 15 mW of DC power transmitted a distance of 3 cm. A receiver chip optimized to work with these coils has been fabricated and tested. Together with a coil, this circuit can supply over 3 mA DC at 4 V to an integrated microsystem. It also demodulates 50 kbps of ASK encoded data, and generates a 1 MHz system clock.

Low-order models for fast dynamical simulation of MEMS microstructures

Abstract: In this paper, are describe how a few simulations of fully meshed dynamical problems can be used to construct efficient low-order models for system-level design of microstructures. We report on the use of this method to capture the measured behaviour of a pressure sensor based on the pull-in time of a beam. Results show that the reduced order model decreases simulation time by at least a factor of 37 while achieving good agreement with experimental data.

Electronics for micromachined inertial sensors

Abstract: Capacitive interface circuits for micromachined sensors are described The focus is on the position sense electronics for accelerometers and gyroscopes Special attention is given to the characteristics of the interface between the sensor and electronic circuits, and on practical and fundamental noise mechanisms that limit the achievable resolution

Effect of air damping on the dynamics of nonuniform deformations of microstructures

Abstract: In this paper, we present a methodology for extracting macromodel parameters of compressible isothermal squeezed-film damping (CISQFD) for flexible structures of MEMS under small amplitude oscillation. The theoretical derivation, which is based on structural modal analysis and CISQFD numerical simulation, is presented. The spring and damping components of CISQFD of any oscillation mode can be extracted by this methodology, and thus the generic parameters of CISQFD can be obtained for any flexible structure with given oscillation modes. We successfully formulated an accurate and concise second order dynamics equation for a flexible MEMS devices by using the CISQFD parameters and the modal spring constant and mass. Simulation results of resonance shift and quality factor are consistent with experimental results.

Mechanical properties of LIGA-deposited nickel for MEMS transducers

Abstract: This paper presents a new technique for measuring the stress-strain curve of nickel produced by the LIGA method. The results of nine tests on 200 /spl mu/m thick specimens produced on the 3rd LIGAMUMPS run show a Young's modulus similar to that of pure nickel, but strengths that are considerably higher.

People count system using multi-sensing application

Abstract: A people counting system with human information sensors for use in a multi-sensing application for detecting the number of persons and their directions of movement at a wide door has been developed. The sensor head module of this sensing system consists of a 1-dimensional 8-element detector array fabricated by a sheet forming method of pyroelectric PbTiO/sub 3/ ceramics, a IR-transparent lens and oscillating mechanical chopping parts. The sensor output is binarized by using the background mean method, and the number of passing persons and their directions of movement are recognized by a pattern recognition algorithm. By using this people counting system, the number of passersby at a 200 cm wide door can be detected with more than 95% accuracy.

The effect of release-etch holes on the electromechanical behaviour of MEMS structures

Abstract: When wet etches are used to release a large microstructural element, it is common practice to distribute small holes in the microstructure to promote under-etching and release, and for reduction of squeezed-film damping. In this paper, we demonstrate that while the mechanical properties are strongly affected by the presence of etch holes, because of fringing-field effects at the edges of the etched holes, the electrostatic-force behaviour of structural elements with an array of small etched holes is nearly that of an unetched element. This leads to an "effective medium" approximation for the mechanical material that can then be used with standard electrostatic actuation models to give good predictions of electromechanical behaviour. Comparison with experimental device behaviour shows the validity of this approach.

Contactless angle measurement using four Hall devices on single chip

Abstract: A novel method for contactless angle detection is presented. It is based on four Hall devices on a single industrial CMOS chip in combination with a permanent magnet. The system setup and the Hall devices implemented are described. Measurements show an angular accuracy of /spl plusmn/0.3/spl deg/ over the range of 360/spl deg/. We also measured the magnetic field distribution in the chip plane. Based on this, the robustness of the angular accuracy against mechanical tolerances is investigated. Lateral misalignment below 100 /spl mu/m of the chip or the magnet to the axis of rotation causes an angle error below /spl plusmn/1/spl deg/. A tilt of 1.5/spl deg/ between magnet and chip results in /spl plusmn/0.5/spl deg/. A worst case combination of the three effects still results in an accuracy better than /spl plusmn/3.6.

Pumping gases by a silicon micro pump with dynamic passive valves

Abstract: Pumping gases by a membrane pump requires a strong compression ratio inside the pump chamber. This demand is fulfilled by an optimized micro pump with dynamic passive valves, working at the resonance frequency. The experimental results for the zero-load pump rate and the maximum pump pressure are 7.5 ml/min and 2.8 kPa, respectively. The pump's behaviour is described by a mathematical model. The calculated values coincide with the experimental data quite well.

Microchip Coulter particle counter

Abstract: This paper presents a micro device employing the Coulter principle for counting and sizing of living cells and particles in liquid suspension. The microchip Coulter particle counter (/spl mu/CPC) has been employed in a planar silicon structure covered with glass, which enables detailed observation during operation. By sheathing of a nonconductive liquid on either side of an electrolyte, it is possible to optimize the sensitivity to a specific cell-size. A method for measuring the relative flow-rates between liquid phases with different conductivity is presented. The method utilizes the laminar flow and short contact time of liquids in microchannels. As a result, the width of the liquids can be controlled without knowing the actual flow rates. The /spl mu/CPC has been fabricated by standard microfabrication techniques, including RIE, wet silicon etching, metalization and anodic bonding.

Integrated magnetic microrelays: normally open, normally closed, and multi-pole devices

Abstract: A fully integrated magnetically actuated micromachined relay has been realized. This particular device uses a single layer coil to actuate a movable upper magnetically responsive platform. The minimum current for actuation was 180 mA, resulting in an actuation power of 33 mW. Devices have been tested which can make and break 1.2 A of current through the relay contacts when the relay is electromagnetically switched. Operational lifetimes in excess of 300,000 operations have been observed. A normally closed relay has also been developed through the addition of permanent magnets to the microrelay. These devices have also been able to electromagnetically switch 1.2 A of current. Multi-pole devices, which contain more than one pair of contacts per coil have also been realized, and possess comparable performance characteristics to single-pole devices.