Showing papers in "Sensors and Actuators A-physical in 1990"

Electrostatic-comb drive of lateral polysilicon resonators

Abstract: This paper mvestlgates the elcctrostatlc dnve and sense of polynhcon resonators parallel to the substrate, using an mterdlgtated capacitor (electrostatic comb) Three expenmental methods are used nucroscoplc observation mth contmuous or stroboscopic iummatlon, capacltlve sensing using an amphtude-modulation technique and SEM observation The mtnnslc quality factor of the phosphorus-doped low-pressure chemlcal-vapordeposited (LPCVD) polyslhcon resonators is 49 000 + 2000, whereas at atmosphenc pressure, Q < 100 The finger gap IS found to have a more pronounced effect on comb charactenstlcs than finger width or length, as expected from simple theory

Piezoelectric micropump with three valves working peristaltically

Abstract: A silicon micropump, provided with piezoelectric valves, which can be manufactured by established integrated circuit techniques is the subject of this report. The micropump can be used to pump liquids or gases to a higher pressure, which can be relieved through a check valve. The body of the pump and the valves are made in silicon and contain piezoelectric material which allows opening and closing of the valves electrically.

Liquid transport in micron and submicron channels

Abstract: An experimental investigation of fluid flow in extremely small channels is presented. Potential applications for such channels include cooling of electronic circuits, and reactors for modification and separation of biological cells. The immediate goal is to determine at what length scales the continuum assumptions break down and if the Navier-Stokes equations adequately predict fluid behavior. In order to accomplish this, experiments are being conducted in progressively thinner flow channels. We have constructed three channels of rectangular cross-section ranging in area from 7200 to 80 square microns, utilizing the recent advances in microfabrication. In this paper, preliminary results are reported pertaining to friction measurements. It is found that in the relatively large flow channels the experimental observations are in rough agreement with the predictions from the Navier-Stokes equations. However, in the smallest of the channels, there is a significant deviation from the Navier-Stokes predictions.

A thermopneumatic micropump based on micro-engineering techniques

Abstract: The design, working principle and realization of an electro-thermopneumatic liquid pump based on micro-engineering techniques are described. The pump, which is of the reciprocating displacement type, comprises a pump chamber, a thin silicon pump membrane and two silicon check valves to direct the flow. The dynamic pressure of an amount of gas contained in a cavity, controlled by resistive heating, actuates the pump membrane. The cavity, chambers, channels and valves are realized in silicon wafers by wet chemical etching. Experimental results are presented. Maximum yield and built-up pressure equal 34 ?l/min and 0.05 atm, at a supply voltage of 6 V. Results of simulations show good agreement with the actual dynamic behaviour of the pump.

A low-offset spinning-current hall plate

Abstract: An offset-reduction method for Hall plates has been developed which minimizes the influence of the stress and the temperature on the offset. The new method uses only one single symmetrical Hall plate in which the direction of the current is made to spin by contact commutation with steps of π/6 radians or smaller. The consecutive Hall voltages are averaged over time and the offset cancels out. The residual offset is about a factor of 10 less than that specified for commercially available silicon Hall plates and is limited by inhomogeneities in the plate.

Microfabricated electrohydrodynamic pumps

Abstract: Pumping is often cited as a general application which motivates the development of microfabricated motors and other actuators. In that spirit, this paper studies microfabricated electrohydrodynamic fluid pumps. In electrohydrodynamic (EHD) pumping, fluid forces are generated by the interaction of electric fields and charges in the fluid. In contrast to forces generated by mechanical pumping using an impeller or bellows, EHD pumping requires no moving parts and consequently offers the possibility of simplified fabrication and high reliability. This paper discusses electrohydrodynamic pumping and issues concerning its use in micronsize scale systems. The fundamental operating principles of the EHD pump are outlined and examples of configurations which meet the requirement for inducing free electric charge are shown. The possible performance achievable in micron size-scale regimes is indicated. Issues concerning fluid conductivity, instability and surface tension are addressed. A microfabricated structure which demonstrates the EHD pumping of a highly insulating silicone oil is described. The structure consists of an array of 10 μm by 235 μm highly doped, LPCVD polycrystalline silicon electrodes patterned over silicon nitride. The electrode array is excited with a traveling wave of electric potential. Pumping results are qualitatively described. This paper describes a study of electrohydrodynamic pumping, including issues concerning its application to micron size-scale systems. The fundamental operating principles of EHD pumps are outlined and the possible performance achievable in micron size-scale regimes is indicated. Surface and bulk instabilities are addressed. Finally, the preliminary results of an EHD pumping experiment with a microfabricated structure are described. A practical requirement for EHD pumping is the induction of free electric charge in the volume of the fluid to be pumped or on its interface with another material. Charge accumulation on a material interface is readily achievable; however, if one material is a fixed rigid wall, such as the wall of a conduit, no pumping can take place. Consequently, the practical application of microfabricated EHD pumps may require the induction of free charge in the volume of the fluid, possibly by temperature-induced conductivity gradients. A further constraint on the usefulness of EHD pumping is its reduced effectiveness with conducting fluids. Hence its usefulness in many situations, including biological environments, may be limited. Another possible difficulty may be fluid instabilities; however, these instabilities may be useful for mixing and cooling purposes. Yet, it remains to be seen if they can compete with molecular diffusion in micron-size scale systems. Finally, it seems clear that surface tension will be a dominant force in virtually any micron-scale system with a liquid surface. In spite of these difficulties, electrohydrodynamic interactions may prove to be a reasonable way to achieve pumping without moving parts.

Thin-film processing of TiNi shape memory alloy

Abstract: The deposition and processing of thin-film Nitinol, a shape memory alloy (SMA), are discussed and demonstrated. By using sputter-deposition techniques, 0.5 to 8.0-μm-thick films of Nitinol (an alloy of nickel and titanium) were deposited, patterned and etched. Although the processing of the individual constituents of the alloy are well understood, the processing of the shape memory alloy is not a simple extension or combination of the constituent metals processing. Requisite deposition conditions for the film to retain the shape memory effect of the bulk material and avoid stress-induced cracking are discussed. An appropriate wet-etch for patterning the thin-film Nitinol is presented. Results from surface micromachined test structures are presented, which verify the shape recovery property of the thin-film Nitinol using resistive heating as the source of heat energy for the shape recovery. Finally, we discuss an isotropic, dry-etch technique developed for the release of the thin-film test structures that also has potential use in other thin-film applications.

Compensation structures for convex corner micromachining in silicon

Abstract: In order to gain precise information on the convex corner undercutting when using anisotropic etchants for silicon, a test mask has been elaborated, incorporating structures with orientations different from those of the wafer flats. The aim is to determine the behaviour of the etchants in terms of crystal orientation. These masks allow an insight to be gained into crystal-dependent etch rates, focusing in detail on the and directions. A second set of masks has been developed with an integrated edge compensation, using the etch depth as a parameter. This results in a novel compensation technique for preventing convex corner undercutting. Two types of etchant compositions are examined: an aqueous solution of KOH, and the same solution in combination with isopropyl alcohol (IPA); the first etchant has distinct advantages when precise convex corners are required.

Silicon fusion bonding for fabrication of sensors, actuators and microstructures

Abstract: Silicon fusion bonding (SFB) is the joining together of two silicon wafers without the use of intermediate adhesives. The technology has been used to fabricate silicon-on-insulator (SOI) substrates and silicon power devices, and also has wide applications in the fabrication of silicon sensors, actuators and other microstructures. This paper reviews the development and current status of SFB. A history of the technology from the early 1960s to the present is presented. Process techniques necessary to incorporate SFB successfully into silicon micromachining processes are discussed, and examples of successful SFB structures are presented. Comparisons to competing techniques are made, and the potential for future development of SFB structures is discussed. Silicon fusion bonding presents major new possibilities in the design of silicon micromachined structures when combined with other available processing techniques. SFB has already been used in novel accelerometers, high-temperature pressure sensors, ultraminiature pressure sensors and high over-range pressure sensors. SFB does not appear to be the technique of choice for VLSI SOI technology, but it is highly viable for use in silicon microstructures, and it is incumbent on the micromachining community to pursue further development of the technology. With the development of ‘smart’ power devices occurring in parallel with the development of ‘smart’ sensors, it is to be hoped that evolution of SFB for both microstructures and power devices will continue and will provide cross-fertilization between the two fields.

Micropump and sample-injector for integrated chemical analyzing systems

Abstract: A dual pump and a buffer pump have been integrated on a silicon wafer for chemical analyzing systems These pumps realize constant and rippleless liquid flow of a small volume The controllable flow rate of the pumps was up to about 40 μl/min and the maximum pumping pressure was about 1 mH2O A sample injector made up of two three-way valves has also been fabricated with micromachining

In situ friction and wear measurements in integrated polysilicon mechanisms

Abstract: We describe a series of experiments, in situ measurements and theoretical models designed to provide estimates of the coefficients of friction and the nature of wear in integrated polysilicon micromechanisms. A laser-based measurement system was used to monitor the motion of the rotating micromechanisms and steady-state speeds of up to 10 000 rps (600 000 rpm) were recorded. Estimates of the dynamic coefficient of friction were obtained by incorporating sampled motion profiles and derived speed and distance measures into a dynamic model which included both coulomb friction and viscous damping forces. From such an analysis, we estimated dynamic coefficients of friction for polysilicon on silicon ranging in value from 0.25 to 0.35. The results were reproducible on numerous components, across structures of identical and different geometries, produced on different wafers from the same fabrication lot. Life-cycles for some of these structures were determined under various operating conditions, with typical components surviving for almost one million cycles.

The application of fine-grained, tensile polysilicon to mechanicaly resonant transducers

Abstract: Resonant force sensors are devices which convert axially applied forces to changes in resonant frequency. These structures are fundamentally wires or beams or more complicated structures which are in a vacuum envelope. They become interesting and useful if they can be miniaturized, can be fabricated from a single material in a cost effective manner and can be excited and read via simple techniques. The devices which are reported here satisfy most of the above criteria. The construction material involves a silicon substrate, tensile strain polysilicon films and strain-compensated silicon nitride deposits. Clamped-clamped beams of polysilicon, typically 400,μm long, 40,μm wide and 2μm thick are fabricated with an isoplanar process over an oxide filled tub. Low-pressure chemical-vapor-deposited (LPCVD) nitride is used as a second sacrificial layer which also serves to support a second polysilicon layer which is part of the vacuum envelope. Internal surface adhesion problems are avoided by freeze-sublimation procedures which remove surface tension-induced beam deflections. Passivation and sealing is accomplished via LPCVD nitride and reactive sealing. Excitation and sensing is accomplished via ion implanted resistors. Experimental results always produce quality factors, Q , above 35 000. Resonant frequencies to 750 kHz have been achieved. It is estimated that these devices can measure axially applied forces below 0.1 dyne with standard electronic interfaces.

Viscosity and density sensing with ultrasonic plate waves

Abstract: Flexural plate waves propagating on thin silicon-nitride membranes in contact with viscous fluids are studied. The attenuation of the plate wave is shown in an approximate analysis to depend on the amount of viscous dissipation that occurs in the liquid. The effective mass in the viscous boundary layer increases the mass loading of the membrane over the amount due to density variations alone, resulting in a decrease in the frequency of maximum transmission. Measurements of the wave attenuation and frequency response of the plate-wave delay line in water-glycerol solutions demonstrate viscosity-density sensor applications in liquids with viscosities exceeding 1000 cP.

Thermophysical properties of low-residual stress, Silicon-rich, LPCVD silicon nitride films

Abstract: The thermal properties of low-residual stress, low-pressure chemical-vapor-deposited (LPCVD), silicon nitride (Si 1.0 N 1.1 ) have been extracted f

Silicon pressure sensor integrates resonant strain gauge on diaphragm

Abstract: A novel silicon pressure sensor has been developed which will enable high-precision pressure measurement. The sensor, which is based on a new concept, is fabricated from a single silicon crystal and has two resonant strain gauges which are held in vacuum cavities on the surface of the diaphragm to isolate them from the surrounding fluid. The two oscillating frequencies of the resonant strain gauges are differentially modulated by pressure. The sensor's measuring principle, features, its amplitude-controlled self-oscillation circuit, and the results of experiments are given.

Frictional study of IC-processed micromotors

Abstract: A frictional-torque model consisting of two components, one a constant term and the second a position-dependent term, is used to analyze the motion of a variable-capacitance IC-processed micromotor. Values for the position-dependent frictional torque are calculated from a two-dimensional electrostatic simulator using a non-concentric rotor/hub model. The constant torque is calculated from the experimental starting voltage. Predictions of the theory are in agreement with experiment. Frictional coefficients for polysilicon surfaces adjacent to silicon nitride surfaces are calculated to range from 0.21 to 0.38 in the micromotors.

Integrated thermopile sensors

Abstract: This paper is about integrated silicon thermopiles and their applications in silicon sensors. After a short description of the thermoelectric effect and its use in silicon thermopiles, some attention is devoted to the design of micromachined structures for implementing thermal sensors. The various sensing principles based on thermal effects are discussed next. Finally, an impression is given of some of the recently developed silicon-thermopile sensors which implement these sensing principles.

A study of three microfabricated variable-capacitance motors

Abstract: This paper discusses the design, microfabrication, operating principles and experimental testing of three types of rotary variable-capacitance micromotors. The advantages and disadvantages of these motors are discussed. The three motor types are top-drive, side-drive and harmonic side-drive. In this work, the micromotors are surface micromachined using heavily-phosphorus-doped polysilicon for the structural material, deposited oxide for the sacrificial layers and LPCVD nitride for electrical isolation. Frictional forces associated with electric pull-down forces on the rotor are dominant in the side-drive and harmonic side-drive motors fabricated and tested to date. Air drive and electric excitation have been used in studying these effects. Side-drive micromotors have been successfully operated by a three-phase electrical signal with the rotors air-levitated. With air levitation, successful operation is achieved at bipolar excitations greater than 80 V across 4 μm air-gap motors having eight rotor and twelve stator poles, with only half of the stator poles excited. Motor operation is sustained indefinitely.

A capacitive pressure sensor with low impedance output and active suppression of parasitic effects

Abstract: This paper describes the design, operating principles and performance of a capacitive pressure sensor in silicon, combined with a dedicated CMOS interface circuit. The readout circuit is designed to suppress parasitics and to yield an output signal proportional to pressure. The sensor-specific part is fabricated using standard photolithography, silicon micromachining in KOH, and anodic silicon/glass bonding at wafer level. The devices measure 2.2 × 3.5 × 0.8 mm and show a typical zero-pressure capacitance of 10 pF, with pressure-induced changes up to 250%. The interface chip ‘CAPRICE’ (CApacitive Pressure sensor Readout IC) is processed in a 3 μ n-well CMOS process and was designed to anticipate the intrinsic drawbacks of the capacitive transducing principle, i.e. sensitivity to environment noise, nonlinear output response and effects of parasitic capacitances. These drawbacks have always prevented the breakthrough of integrated capacitive mechanical sensors. CAPRICE, however, converts small capacitance variations into a noise-insensitive output voltage and a first-order linearisation is achieved by inversion of the hyperbolic capacitance versus pressure relationship. A second-order linearisation is obtained by the adoption of a novel suppression scheme for parasitic capacitances to the substrate. Parasitic capacitance rejection ratios up to 80 dB can be achieved in this way, enabling the practical feasibility of capacitive pressure sensors with less than 0.5% of full-scale nonlinearity.

Precision accelerometers with μg resolution

Abstract: A highly miniaturized accelerometer for micro- gravity measurements in spacecraft has been developed. It is based on a capacitive micromechanical silicon chip and an associated CMOS measurement integrated circuit. The accelerometer has been modeled in order to select and to optimize the system. The technology for the electromechanical chip, based on a glass-silicon sandwich structure has been developed. The final accelerometer developed, with an analog output signal of 5 V/g and a working range of ± 0.1 g, has a resolution of less than 1 μg at 1 Hz. Transverse sensitivities are below 0.4%. The temperature coefficients of offset and sensitivity are typically 30 μg/°C and 150 ppm/ °C respectively. Bandwidth is d.c. to 100 Hz. The hybrid technique accelerometer package has a size of 33 mm × 15 mm × 5 mm.

An ASIC for high-resolution capacitive microaccelerometers

Abstract: An ASIC (application specific integrated circuit) for high-resolution capacitive accelerometers using switched-capacitor techniques is presented The circuit, realized in a 3 μm CMOS process, is suited for the measurement of the differential capacitance of any capacitor bridge and features a maximum output noise density of −110 dB V/ ✓Hz The cut-off frequency of its first-order low-pass filter characteristic is determined by an external filter capacitor and the sampling rate

Smart temperature sensor in CMOS technology

Abstract: A CMOS monolithic temperature sensor is described, based on compatible lateral bipolar transistors for the sensor part and the reference, and on CMOS circuits for A/D conversion, control and calibration. The accuracy of the analog part is close to the performances obtained in bipolar technology, for example ± 0. 1 °C in a 60 °C temperature range, and it is maintained on conversion. CMOS flexibility allows adaption of the digital output signal to any temperature scale and storage of the calibration bits in an EEPROM on the chip. The circuit works in a 2.5 to 5 V range and draws 50 μA current. A 1.5 V version with reduced accuracy is also feasible.

Capacitive silicon accelerometer with highly symmetrical design

Abstract: The design and performance of a capacitive micromechanical accelerometer, as well as an electronic circuit for the conditioning of the output signal are described. The sensing element consists of a differential capacitance which is formed by a seismic silicon mass and two counter electrodes situated on anodically bonded glass plates. The mass is symmetrically suspended on at least eight cantilever beams located on both sides of the silicon wafer. For a ±5 g device a typical sensitivity of 1 pF/ g with a zero capacitance of 10 pF and a detection limit below 1 m g was achieved. For signal conditioning a switched capacitor CMOS-ASIC was developed, yielding an analogue voltage output signal.

Residual stress and mechanical properties of boron-doped p+-silicon films

Abstract: Annealing was used to relax non-uniform tensile stress in boron-doped p + -Si cantilever beams. The optimum annealing condition was determined. A for

A review of low cost accelerometers for vehicle dynamics

Abstract: In the last few years, vehicle manufacturers have been recognising the advantages, in terms of passenger comfort and driveability, that can be achieved using electronically controlled vehicle suspension systems. A critical component in all of these, other than the manually adjusted ride control systems, is the accelerometer, a number of which are used for measuring vehicle motion in all directions. These accelerometers are required to have a high sensitivity and accuracy and yet be sufficiently robust to operate in the relatively harsh environment of a vehicle. Further to this, the unit cost of the accelerometers must be very low. This paper reviews the main technologies and sensor designs, which have the potential to meet the required suspension systems and other vehicle dynamics performance specifications within the cost constraints.

Investigations of porous silicon for vapor sensing

Abstract: A 440% increase in capacitance in response to a humidity change from 0 to 100% has been measured in a Schottky-barrier r.h. sensor made using an alumin

A sub-micron particle filter in silicon

Abstract: A multi-purpose screen filter structure in silicon has been fabricated and characterized. The fabrication of the filter structure is based on a two-step self-aligning process using lateral boron doping, anisotropic silicon etching and silicon dioxide undercut etching. Filters for filtration of particles down to 50 nm have been fabricated. The flow resistance for gases and liquids has been measured. The fluid passes between two electrically conducting membranes, so other applications such as fluid identification and concentration measurements are possible. A preliminary fluid identification experiment is presented.

NH4Oh-based etchants for silicon micromachining

Abstract: Wet chemical etchants based on ammonium hydroxide-water (AHW) solutions for micromachining monocrystalline silicon are described. The etchants can be applied under clean-room conditions within standard IC fabrication lines. The nature of AHW solutions is discussed with respect to anisotropy and selectivity. At 75 °C and 9 wt.% AHW, a maximum etch rate for (100) silicon of approximately 30μm/h has been determined. (111):(100) etch rate ratios show values around 4%. AHW etchants exhibit an excellent selectivity to SiO 2 , Si 3 N 4 and highly boron-doped silicon. Aluminium will not be attacked by silicon-doped AHW etchants. Therefore, standard IC metallization techniques can be used for device fabrication. The application of AHW solutions to the fabrication of basic micromechanical structures is demonstrated.

The fabrication and use of micromachined corrugated silicon diaphragms

Abstract: Corrugated diaphragms offer much greater linearity and travel than the equivalent flat diaphragms. Corrugations have been, produced in silicon microstructures by etching grooves in the top surface of a wafer and diffusing an etch stop. A back-side etch is then used to form the corrugated diaphragm. Corrugations have been made between 3 and 20 μm deep with diaphragm thicknesses between 0.6 and 8 μm. A capacitive pressure sensor has been fabricated using these techniques with a full-scale sensitivity of 1 mmHg.

Compatibility of zinc oxide with silicon IC processing

Abstract: In this paper we present the passivation of zinc oxide by a thin silicon nitride layer. With this passivation, silicon wafers covered with zinc oxide can be further processed without contamination of the process chambers of the subsequent processes, and without damaging the zinc oxide layer. In addition, we review some process technology concerning zinc oxide: the cleaning and etching of zinc oxide and the etching of aluminium on zinc oxide.