Microelectromechanical Systems or Structures (MEMS) incorporate a set of attributes which make them uniquely suited for an expanding array of applications.The field of displays is one that has been a particular beneficiary of this class of devices. At least one display product has been introduced, and there are numerous products on the verge of being introduced or which are under development. Many segments of displays are experiencing rapid growth. Reflective displays in particular are growing in importance due to their inherent low-power consumption and ambient light performance. Unfortunately, the technical challenges for all these segments are such that traditional solutions, primarily LCD based, are limited in their ultimate performance. These are challenges for which MEMS are well suited, and there are several efforts underway to exploit this fact. One such effort, which is based on a device known as an Interferometric Modulator (IMod), is described here. A brief review of MEMS based display concepts is also included.
Smart MEMS devices such as pressure, mass flow and yaw rate sensors are presented in detail. One common point of this range of devices is their fabrication technology regarding anisotropic etching. This paper is first meant to give a short review upon the applications processed by using wet chemical etching in KOH-solutions. Furthermore, we will discuss about the impact of material and process related defects in the silicon crystal and on the anisotropic etching behavior.
The small size and possible low cost of micromachined sensors make them attractive for some medical applications. Minimally invasive therapy aims to reduce the damage done to healthy tissue by reaching the affected area through existing pathways through the body. However, information is scarce as direct view or touch is lacking. Small sensors are needed on catheters inside the blood vessels to gather the data such as blood pressure and flow. To this end a combined pressure and flow sensor is fabricated in an epi-poly process that uses a 4 micrometer thick polysilicon membrane grown during epitaxial growth, to form the diaphragm of the pressure sensor and the thermal insulation of the thermal flow sensor. Using RIE etching of holes through the membrane, sacrificial etching and closing of the etch holes by oxide depositions, a closed reference chamber is formed for an absolute pressure sensor. The process is compatible with standard bipolar electronics to enable integration of signal conditioning, multiplexing, etc. Measurements of the two sensors show that fabrication of flow and pressure sensors using epi-micromachining is possible and that the sensors have the required measurement range, but drift necessitates calibration before use.
Pressure sensors serve a variety of automotive applications, some which may experience high levels of acceleration such as tire pressure monitoring. To design pressure sensors for high acceleration environments it is important to understand their sensitivity to acceleration especially if thick encapsulation layers are used to isolate the device from the hostile environment in which they reside. This paper describes a modeling approach to determine their sensitivity to acceleration that is very general and is applicable to different device designs and configurations. It also describes the results of device testing of a capacitive surface micromachined pressure sensor at constant acceleration levels from 500 to 2000 g's.
A system for wireless interrogation of surface acoustic wave (SAW) based humidity sensor is described. The SAW sensor is a Lithium Niobate (LiNbO3) substrate with metallic interdigital transducers (IDTs) and reflectors etched on it. A microstrip antenna is placed in close proximity to the IDT, such that the antenna can excite the IDTs through the air gap by inductive coupling. The sensor antenna receives the FM signal from the transceiver antenna of the wireless system. The electromagnetic signal received by the sensor antenna is converted by the IDT into a surface acoustic wave that propagates in the LiNbO3 substrate. This surface acoustic wave is reflected by the etched metallic reflectors, reconverted into an electromagnetic signal by the IDT and returned to the transceiver system. The SAW velocity on the LiNbO3 substrate varies as a function of the relative humidity and results in a varying time delay in the reflected signal, which is detected by the wireless system. The resolution and accuracy of such a system are investigated and some experimental data is presented. The operating principle and techniques can also be used for other wireless, passive SAW sensors.
The combination of aligned silicon fusion bonding (SFB) with deep reactive ion etching (DRIE) is a flexible technology platform that can be used to fabricate complex multi-layer MEMS services. Silicon wafers can be processed separately and subsequently aligned and bonded and further processed. DRIE technology enables very deep (through-wafer) silicon etching with high-aspect-ratio beams and trenches (1:20), using standard resist masks. DRIE technology can be used in combination with a buried cavity, etched into the bottom substrate before boding, to fabricate a suspended microstructure. Based on this technology platform, a multi- level microfluidics board, thermal actuators, a microvalve, and a high sensitivity accelerometer have been designed, fabricated, and tested.
We present preliminary results for a single-crystal silicon gyroscope with decoupled drive and sense oscillators. The gyroscope is fabricated using a plasma micromachining process on a six-inch MEMS production line at Kionix, Inc. The process yields high-aspect-ratio structures, large structure-to- substrate separation, and low-parasitic electrodes, unlike designs that rely on silicon-on-insulator substrates, polysilicon, or thick epitaxial layers. We describe the fabrication process and emphasize the design, operation, and testing of the sensor. Results to date have yielded resolutions of 0.15 deg/sec over a 100 Hz bandwidth, short term bias stabilities less than 100 deg/hr, and quadrature signals less than 25 deg/sec.
This paper presents the development of accelerometers using the epitaxial layer as the mechanical structure. In this work epi-poly was chosen for the fabrication of the accelerometer structures. Epi-poly is a polycrystalline material deposited in an epitaxial reactor. This means that the mechanical structures can be deposited in the same step as the epitaxial layer used for the electronics. An extension to the epi-poly process has been used where after sacrificial etching to remove the oxide, anodic etching in HF is used to increase the airgap under the mechanical structures. This has the advantages of reducing vertical sticking and reducing parasitic capacitances. The paper describes the basic epi-poly process and the extension to a double sacrificial etching. Accelerometers have bene fabricated using both techniques and measurements have been made for both static and dynamic accelerations.
The integration of MEMS, IDTs (Interdigital Transducers) and required microelectronics and conformal antenna to realize a programmable accelerometers and gyroscopes is presented in this paper. This unique combination of technologies results in novel conformal sensors that can be remotely sensed by an RF system with the advantage of no power requirements at the sensor site. Hybrid Programmable accelerometers and gyroscopes on a single chip are useful for inertial navigation systems. Programmable sensors are achieved with splitfinger interdigital transducers (IDTs) as reflecting structures. If IDTs are short-circuited or capacitively loaded, the wave propagates without any reflection whereas in an open circuit configuration, the IDTs reflect the incoming SAW signal. The programmable accelerometers and gyroscopes can thus be achieved by using an external circuitry on a semiconductor chip using hybrid technology.
We investigate noise and fluctuations in Micro-Electro- Mechanical-Systems (MEMS). MEMS operate on many different subsystem levels and in different energy domains. In the following we analyze two subsystems in more detail: (1) the electronic subsystem represented by the circuitry part of the device, (2) the mechanical subsystem consisting of fragile vibrating mechanical structures. Mode amplitudes of vibrating structures are subject to interaction with their environment, i.e. gaseous, liquid, and electronic systems, each of which shows fluctuations and noisy behavior by itself. Our approach focuses on finding the time-scales and the dominant process in order to determine the noise behavior. By applying the fluctuation-dissipation theorem we are able to extract various response coefficients, such as e.g. the carrier electric and thermal conductivity and the quality factors of the vibrational modes. The impact of response coefficients due to the cross correlations of subsystems may be analyzed. The analysis is performed by numerical simulations of an appropriate model for the different sub-systems in terms of stochastic differential equations of motion for the respective observable quantities. These are the vibrational amplitudes, the electronic densities and the currents. The resulting correlation functions are analyzed.
We use finite-element simulations of a micromachined gas-flow sensor as an example to demonstrate and critically compare various design- and analysis-of-experiments methods for the generation of simple, surrogate functional approximations. Such approximations can be used to greatly reduce computational expense in MEMS design and optimization. The following design methods are included in our study: factorial, alphabetic (A, D, G, I, and S-optimal), maximin, latin hypercube, central composite, and a method using a new program from the University of Michigan named IMSETTM. Analyses include both a parametric method (ordinary least squares or OLS) and a non-parametric method (best linear unbiased predictor or BLUP).
Satellites based on microelectromechanical system (MEMS) technology and tailored to low-cost space missions are investigated to determine their characteristics and feasibility. This work explores an alternative chassis formed from a stack of microfabricated silicon wafers. The outer layers contain optical sensing, micropropulsion and power generation systems whereas internal layers contain computers, RF components and mechanical sensors. This technique has the advantage of saving space and weight while allowing for easy design changes and precise tailoring to mission specifications. This concept is expanded through a design study of the MEMS control moment gyroscope which is used in satellite attitude control. In addition, a feasibility study is performed with special regard to the alternative chassis outlined above. This work (part of a Phase I NASA Institute for Advanced Concepts study) demonstrates that a wide variety of spacecraft components can be fabricated with silicon processing techniques. This approach may lead to batch- fabricated, high-volume, low cost, redundant teams of MEMS spacecraft.
Fixtures are used to locate and hold parts during automated inspection, machining, or assembly. Microelectromechanical systems (MEMS) are tiny devices built in batch processes derived from integrated circuit fabrication. We describe a design for an array of MEMS microfixtures for parallel inspection, transport, and assembly of microfabricated parts. In a microfixture array, parts are brought near the fixture by random motion provided e.g. by vibratory agitation. The fixture clamps actively close when the parts enter the fixture. In large future fixture arrays, electrostatic or optical sensors integrated into the fixture cell can trigger this clamping function. Each cell operates autonomously and no global control is necessary. We fabricated a prototype cell consisting of two upfoldable fixture walls and a bimorph thermal actuator using a standard CMOS process. This approach allows batch fabrication of large numbers of cells on a single silicon wafer, as well as easy integration of sensors and actuators that autonomously close each cell when filled.
Microelectromechanical system (MEMS) RF components for millimeterwave reconfigurable transceivers are investigated. The components include reconfigurable Vee-antennas, sliding planar impedance tuners, microswitches and variable capacitors. These different components are fabricated using the same three-layer-polysilicon surface micromachining techniques. In this work, we focus on the demonstration of component architectures and the integration issues. The reconfigurability of the Vee-antenna is demonstrated with beam-steering and beam-shaping functionality. The antenna characteristics are also studied. The planar feature of the Vee-antennas allows integration of planar impedance tuners. The antenna impedance can be matched dynamically by two shunt sliding planar backshort impedance tuners. The tuning ranges of planar backshort impedance tuners on different transmission lines are studied. The mechanical architectures for microswitches, parallel-plate variable capacitors and circular variable capacitors are also investigated.
Current silicon on-chip inductor have the problems of low quality factors (Q), low self-resonant frequencies, poor electromagnetic isolation and lack of a good radio-frequency (RF) ground plane. To address these issues, we present a new method to fabricate an on-chip copper spiral inductor. The basic structure of the inductor consists of a spiral polysilicon coil suspended over a cavity etched into the silicon substrate. Copper (Cu) is electrolessly deposited onto the polysilicon spiral in order to obtain high conductivity. The formation of the suspended coil is realized by first creating a silicon oxide block embedded in the silicon substrate, then fabricating on the oxide the coil by polysilicon surface micromachining, and in the end removing the embedded oxide by hydrofluoric acid (HF). The benefit of using a suspended spiral structure is two-folded: first, the electrical and magnetic coupling between the inductor and the substrate is reduced dramatically, thus decreasing the substrate loss, and second, by reducing the parasitic capacitance between the inductor and the substrate, the self- resonance of the inductor at an undesirably low frequency can be avoided. The metallized bottom and side-walls of the cavity under the inductor serve both as an electromagnetic shield for isolation and as an RF ground plane. Initial experimental results show that the maximum Q-factor can be as high as 26 for a 2.14 nH inductor. The self-resonant frequency is measured to be 10.3 GHz.
A measurement system for static and dynamic characterization of silicon micromachines was developed. The computer- controlled system, using a HeNe-laser at 633 nm and a two- dimensional position sensor, allows simultaneous measurements of optical beam motion on two axes. To demonstrate the flexibility and functionality of the setup, measurements on various polysilicon surface-micromachined mirrors were performed. These measurements included tilt-angle versus driving-voltage curves, frequency responses, and motion perpendicular to the primary scan axis.
Microsystems, whose main components are made of polymers, have meanwhile found a firm place in microsystems technology, both as important niche products and as bulk commodities. A large number of structuring techniques are available, on the one hand, for primary structuring (electron beam, UV and X-ray lithography, laser patterning, stereo lithography, etc.) and, on the other hand, as replication techniques for low-cost mass production (reactive injection molding, thermoplastic injection molding, hot embossing techniques, etc.). Microstructures made of polymers may be clearly cheaper to manufacture than those made of any other material. Besides polymers structured on a micrometer scale, also membranes made of polymers are becoming increasingly more important in microsystems technology. The development of microstructure technology with the use of polymers began at what is now Forschungszentrum Karlsruhe (Karlsruhe Research Center) in the early eighties. The development of both, deep X-ray lithography and various molding techniques (LIGA) process started back then already. The large variety of possible uses will be shown by examples of developments originating from Forschungszentrum Karlsruhe. These mainly comprise micro- optics (lenses, distance sensors, spectrometers, micro-optical benches, etc.), microfluidics (capillaries for 'labs on chip,' polymer membranes as basis for pumps, valves, pressure sensors, flow sensors, and separating systems), and actuators based on selectively inflatable chambers. Another example for the possible application of polymers in microsystems is their use as functional coating for electrochemical transducers [e.g. surface acoustic wave (SAW) sensors].
This paper reports on a new integration concept for MEMS based on an Additive Electroplating Technology (AET). This technology allows the integration of fixed and movable electroplated microstructure on top of a standard ASIC by a low temperature back-end process. The basic fabrication sequence of the AET including aspects of a first level packaging will be presented. Various examples of novel MEMS for automotive and medical applications will show the capability and the limitations of this integration concept.
A surface-micromachined two-degree-of-freedom system that was driven by parallel-plate actuation at antiresonance was demonstrated. The system consisted of an absorbing mass connected by folded springs to a drive mass. The system demonstrated substantial motion amplification at antiresonance. The absorber mass amplitudes were 0.8 - 0.85 micrometer at atmospheric pressure while the drive mass amplitudes were below 0.1 micrometer. Larger absorber mass amplitudes were not possible because of spring softening in the drive mass springs. Simple theory of the dual-mass oscillator has indicated that the absorber mass may be insensitive to limited variations in strain and damping. This needs experimental verification. Resonant and antiresonant frequencies were measured and compared to the designed values. Resonant frequency measurements were difficult to compare to the design calculations because of time-varying spring softening terms that were caused by the drive configuration. Antiresonant frequency measurements were close to the design value of 5.1 kHz. The antiresonant frequency was not dependent on spring softening. The measured absorber mass displacement at antiresonance was compared to computer simulated results. The measured value was significantly greater, possibly due to neglecting fringe fields in the force expression used in the simulation.
One highly desirable characteristic of electrostatically driven microelectromechanical systems (MEMS) is that they consume very little power. The corresponding drawback is that the force they produce may be inadequate for many applications. It has previously been demonstrated that gear reduction units or microtransmissions can substantially increase the torque generated by microengines. Operating speed, however, is also reduced by the transmission gear ratio. Some applications require both high speed and high force. If this output is only required for a limited period of time, then energy could be stored in a mechanical system and rapidly released upon demand. We have designed, fabricated, and demonstrated a high-density energy storage/rapid release system that accomplishes this task. Built using a 5-level surface micromachining technology, the assembly closely resembles a medieval crossbow. Energy releases on the order of tens of nanojoules have already been demonstrated, and significantly higher energy systems are under development.
A novel approach for the fabrication and assembly of a solid oxide fuel cell system is described which enables effective scaling of the fuel delivery, manifold, and fuel cell stack components for applications in miniature and microscale energy conversion. Scaling towards miniaturization is accomplished by utilizing thin film deposition combined with novel micromachining approaches which allow manifold channels and fuel delivery system to be formed within the substrate which the thin film fuel cell stack is fabricated on, thereby circumventing the need for bulky manifold components which are not directly scalable. Results demonstrating the generation of electrical current in the temperature range of 200 - 400 degrees Celsius for a thin film solid oxide fuel cell stack fabricated on a silicon wafer will be presented.
Micromachining is envisioned as an enabling technology for the miniaturization of chemical analysis systems. It has been pursued for gas phase analysis (GPA) in HP's Chemical Analysis Group and at HP Labs. A thermally actuated, silicon micromachined gas control valve has been jointly developed at HPL and Little Falls Analytical Division (LFAD) for use in a gas chromatograph. This microvalve has 1/10 the weight and 1/5 the package volume of competing solenoid valves. This low cost, high reliability, proportional gas control valve reduces the cost, weight and size of a crucial component for a gas chromatograph system. This paper discusses valve structure and performance, and the resolution of issues over the course of the development.
In the course of a progressive miniaturization of complex measuring systems conventional flame ionization detectors and flame spectrometers are no longer competitive with the new generation of mobile analysis systems. This paper presents a micro flame ionization detector and flame spectrometer structured by methods of the microsystem technology. Main component is a micro burner unit with a reduced oxyhydrogen consumption to realize a stable miniature oxyhydrogen flame. The required oxyhydrogen is generated by electrolysis in a miniaturized electrolysis cell at low energy consumption. Thus the electrolyzer can be battery operated. Due to the reduced amount of explosive oxyhydrogen and the small dimensions of the gas supply micro flame analyzers have an unlimited mobility without safety restrictions and are easy to handle. Furthermore they have a high sensitivity and selectivity similar to conventional systems. Concentrations down to one ppm are detectable up to now with the micro flame ionization detector. The micro flame spectrometer is in its initial stage. First measurements to demonstrate the further possibilities of such a microsystem are presented.
Piezoresistors have been used widely for various microsensors, such as accelerometers and pressure sensors. Conventionally, piezoresistive sensors are fabricated by placing the piezoresistors on the region where largest strain would occur. In order to satisfy the measurement sensitivity as well as the fabrication processes, the substrate underneath the microstructures are usually removed through the back-sided etching. Hence, the fabrication processes are complex and time-consuming. To solve this problem, this article intends to propose an alternative design for piezoresistive sensors. The approach is to form the piezoresistors behind the boundary of the microstructure by diffusion. Therefore, this piezoresistive sensor is compatible with front-side etch process and without depositing an additional polysilicon layer. The proposed design also provides the capability of integrating with various microactuators. In application of the proposed design, a micromachined cantilever was used to provide the strain field to the piezoresistive sensor. The feasibility of the sensor was verified by both simulation and experiment. According to the results predicted by the finite element analysis, the stress at the proposed sensing region is approximate one order of magnitude smaller than that at the end of the microstructure. In other words, a reasonable signal is still available at this region. During the experiment, silicon dioxide microcantilevers were fabricated, and piezoresistors were formed by diffusing n+ on silicon substrate. An experimental setup containing a micropositioner and a piezoelectric shaker was constructed. The output of piezoresistors was then measured when microcantilevers bending or vibrating.
In this work we demonstrate the advantages of using polycrystalline silicon germanium (poly SiGe) as a structural material for surface micromachined devices, and more specifically uncooled Infra-Red (IR) microbolometers. The low stress and the low thermal conductivity of poly SiGe enable the realization of IR microbolometers having an effective detectivity above 2 X 109 cm.Hz1/2/W. Currently, linear arrays of optimized devices included in an on-chip vacuum package are developed. The vapor HF sacrificial etching technique is used to release extremely thin microbolometers with high yield. Combined with the practical advantage of an uncooled system, a low-cost yet sensitive sensor system is the result. Possible applications include space based pushbroom earth sensing, spectral environmental monitoring and process control.
This paper proposed a new direction for the miniaturization of silicon bulk-machined sensors. Herein, the glass substrate bonded with the silicon wafer substitutes the role of the silicon wafer itself. Then the whole silicon wafer with sensing portions, whatever piezo-resistive or capacitive types, could be all machined to the membrane structure without chip-area wasting on the (111)-face slopes. Basically, the chip size by this semi-SOI method would be as small as the surface-machined one in principle, if the silicon-glass bonding process is guaranteed. The other advantages of this strategy include the process compatibility with the different sensing principles and the different techniques of membrane machining. The on-chip circuitry of sensors could be also preserved. Another important issue is that the surface- machining-link membrane now is mono-crystalline, which means the stable mechanical properties and reproducible characteristics. This new strategy practically augmented the mass production of piezoresistive pressure sensors using in tire pressure gauges and other industrial pressure meters. The 1.0 mm X 0.8 mm X 0.5 mm of sensor size with chip density exceeds 5,000 per 4-inch wafer was successfully fabricated. Some engineering applications used this new pressure sensor to solve their corresponding problems.
The recently developed JHU/APL magnetometer, which is based on a free-free (xylophone) resonating bar, is simple, small, light weight, has a low power consumption and utilizes the Lorentz force to measure vector magnetic fields. The device is intrinsically linear and has a wide dynamic range such that it can measure magnetic field strengths from nanoteslas to teslas. Furthermore, its sensitivity is independent of size for resonating bars of the same material and aspect ratio. This makes it ideally suited for miniaturization using MEMS techniques. Various polysilicon xylophone bars have been designed, processed, and characterized. The output response has verified the size-independent scaling law and sensitivities of the order of 100 nanoTesla have been achieved with drive currents as low as 20 microamps. This drive current is limited by the sheet resistance of the polysilicon support electrodes and directly affects the sensitivity. The electrodes also have a dramatic effect on the resonant frequency since they act as torsional stiffening members on the resonating bar. For example, for a 500 X 50 micron xylophone the resonant frequency varies from the designed 69 kHz to over 95 kHz for 10 micron wide support electrodes. The electrodes do not affect the mechanical Q-factors observed and values in excess of 20,000 at reduced pressures have been routinely obtained.
Bi-stable microactuators are necessary to implement optical switch and microrelay with low power and high reliability. In this work, we analyzed the buckling and vibration characteristics of a planar microactuators with shallow arch- shaped leaf springs. To investigate elastic stability of the proposed microactuator, we derived static buckling modes. A concentrated force of 0.35 muN at the center of beam was required for the snap-through motion for the beam length of 1600 micrometer, thickness of 3 micrometer, beam width of 6.5 micrometer and initial rise of 15 micrometer considering only the first buckling mode. We also analyzed vibration characteristics of arch-shaped leaf spring. The resonant frequencies of the first modes across over the second mode and keeps constant resonant frequencies over the cross point. On the contrary, the resonant frequencies of second modes become almost constant regardless of initial rise. The planar microactuator with shallow arch-shaped leaf springs at both sides were fabricated using silicon micromachining technology. The vertical structure of the planar microactuator features simplicity and consists of p-doped polysilicon as a structural layer and LTO (Low Temperature Oxide) as a sacrificial layer. The polysilicon was annealed for the relaxation of residual stress and HF GPE (gas-phase etching) process was finally employed in order to release the microactuators. These bi- stable planar microactuators with shallow arch-shaped leaf springs showed a high stiffness against external disturbance, and would be very useful for the stable operation of micro optical switch and microrelay.
A dynamic model for a vibrating microgyroscope with respect to angular rate is derived. Using the dynamic model, responses of the vibrating microgyroscope with respect to angular rate input is analyzed. A microgyroscope, which vibrates on the substrate plane, is designed and fabricated by simplified fabrication processes using single polysilicon on insulator structure. The validity of the derived dynamic model is tested by comparing simulation results with and experiments. The performance of the fabricated microgyroscope is investigated in a vacuum chamber of 100 mtorr. The obtained sensitivity of the microgyroscope at a typical static angular rate is 5 mV s/degree.
In this work, a linear position sensor with a Hall effect element for automotive applications is presented. The work involves the design of device structure, finite element analysis (FEA) computer modeling of sensor performance, fabrication of sensor prototype, and prototype characterization. The significance of the work is a low cost linear position sensor has been developed with high linearity for use in automobile control systems. The complete sensor system is designed for ease of manufacturing and the harsh automotive environments. The output data from the two- dimensional basic ideal structure model shows high linearity over a range of 11 mm. Models of temperature coefficient effect (TCE), electromagnetic interference (EMI), and several optimized device structures, including reducing the thickness of bottom material, shaping of the corners as well as adding shield material were also analyzed. The sensor prototype was fabricated and characterized. The modeling data were compared with the measurement results from the sensor prototype.