An explosion in microsystems technology.
Once largely confined to automotive and printer-cartridge applications, microsystems technology (MST) has expanded to include some functions once believed to be impossible. Several organizations are working on medical applications in which pieces of silicon are used to hold reagents. Other groups are using the technology in switches and relays, and a miniature inductive position sensor has been developed in Europe. One promising new area is the use of MST in photonics for telecommunications applications.
A device or system that incorporates MST consists of either mechanical components or both electrical and mechanical components. The typical size of an MST application is on the order of microns--smaller than the width of a human hair.
In the United States, the acronym MST is often used as a synonym for MEMS--micro electromechanical systems. "MEMS is a regionalism," said Roger Grace, a San Francisco-based marketing consultant who specializes in helping manufacturers commercialize MST. "In Japan, such devices are micromachines, and European countries use the term microsystems."
Strictly speaking, MEMS is a process technology used to create tiny electromechanical devices or systems; as a result, it is a subset of MST. For example, various organizations have demonstrated that they can build a microscopic motor, which can be considered a MEMS device. On the other hand, the head of an ink-jet printer cartridge is made with microplating; while not a MEMS device, it is still an example of MST. "The real difference between MEMS and MST," Grace said, "is that MEMS tends to use semiconductor processes to create a sensor, actuator, or mechanical part. By contrast, if you deposit a resistive material on silicon, that's not a MEMS. Both are applications of MST, however."
Regardless of whether or not an application of MST involves a MEMS device, the uniting factor is the way it is made. MST is an offshoot of the technology developed to fabricate integrated circuits on silicon chips--including techniques such as ion implantation, isotropic and anisotropic etching, anodic bonding, noncontact photo-lithography, material deposition, electroplating, and X-ray radiation. While integrated circuits are designed to exploit the electrical properties of silicon, MST takes advantage of either silicon's mechanical properties or both its electrical and mechanical properties.
Gradually, MST has made its way out of research laboratories and into everyday products. For example, many automobiles now use at least one MST-based sensor--most commonly the accelerometer used to control airbag deployment. Industry experts agree that MST in automobiles will become increasingly common.
Another widespread application is in disposable pressure sensors used for biomedical applications. Estimates for the size of the total MST marketplace by the end of the century range from $8 billion to $15 billion, which would represent explosive growth even taking the most conservative estimate.
The heads of ink-jet printer cartridges are currently the biggest application of MST, beating out even automotive and medical sensors, because one printer can require up to four cartridges per year. A die is created that contains an array of resistors, known as heaters. These resistors can be fired under microprocessor control with electronic pulses of a few microseconds. Ink flows over each resistor, and when a resistor is fired, the ink heats up at a rate of 100 million [degrees] C per second, which causes the ink to boll. An ink bubble is created, which bursts out through a nozzle plate that sits on top of the die, lands on the paper, and solidifies almost instantly. A clear, cohesive image is possible because the microprocessor controls individual heater fires. Modern printers have MST-based devices with more than 300 nozzles, giving a resolution of up to 720 dots per inch. Ink-jet printers have become more popular now that some models--equipped with separate nozzles for different inks--can print in color.
"Which particular parts of the cartridge head constitute MST is open to interpretation," said Douglas Finke, vice president of the Wafer Foundry Business Unit of SMC Corp., a company in Hauppauge, N.Y., that manufactures the heads for cartridge suppliers. According to Finke, the head is a composite of MST and other manufacturing techniques. "MST is used to deposit a thermal barrier, resistive film, conductor, and protective overcoat. A thick film is then put on top by a separate machine, and the nozzle is either molded or drilled with a laser."
MST USES ON THE RISE
As of late, the uses of MST have been rapidly increasing. Even the medical applications of MST have moved beyond the basic pressure sensors used in blood-pressure machines. A new application is in medical devices, in which a small piece of silicon serves as a chemical platform for reagents, with some kind of selective layer on top of the reagents. Given an interface with blood or urine, the layer can transmit material onto a reagent that would indicate the presence or absence or certain chemicals.
EG&G IC Sensors in Milpitas, Calif., has developed a dielectric membrane combined with platinum thin-film elements that can be used both to heat the suspended membrane and to sense its temperature; the membrane, in turn, is affected by a number of gas properties. The device can be optimized by changing the geometry and coating of the suspended mass to sense the presence of catalytic agents by altering the thermal capacity and density involved.
Performing medical tests this way has numerous advantages. By putting a drop of blood onto a card that is inserted into a handheld instrument with the necessary computational power, results can be obtained instantly. In addition, such devices require a much smaller amount of the test sample than other methods, which means that the sample can usually be extracted less invasively. Furthermore, this technology is useful for home-based tests, which will become more common as medical costs continue to rise. Perhaps the biggest advantage is that a cheap, MST-based test device would be disposable, eliminating the need for decontamination of equipment exposed to potentially dangerous substances.
Researchers at Northeastern University in Boston are currently working on another innovative application: fabricating switches and relays with MST. As with the ink-jet printer head, the microrelay has a purely electronic function rather than a mechanical one. Two configurations are now under development: a four-terminal device used for actuation, which is equivalent to an electromechanical relay; and a three-terminal device that is equivalent to a switch. The devices consist of a cantilevered nickel beam suspended over gate and drain electrodes.
The microrelay is smaller and consumes less power than conventional relays. Its biggest advantage, however, is that it can be integrated with other devices on a single die. "These devices do not require any high-temperature steps in their manufacture," said Paul Zavracky, an associate professor at Northeastern. "They can be included as post-process additions to conventional integrated circuits without harming the circuit in any way, which is not possible with many MST applications." Complex switching arrays and devices can be designed to handle high-frequency signals with low loss.
The process for making a microswitch involves depositing gold and chromium on an insulating substrate, such as glass or silicon coated with silicon dioxide. Next, a sacrificial layer of copper is deposited and etched to define the contact tip. The copper layer is then patterned a second time to form the beam base. Photoresist is used to define the plated nickel beam with gold contacts. Making a micro-relay requires all these processes plus an additional photo-masking step and insulating layer.
Prototype devices have been operated for more than 1 billion cycles, and they can conduct as much as 100 milliamperes. Measuring 30 by 65 microns, they can be configured in switching arrays for analog switching applications, or could be applied to digital logic systems for application in extreme environments. The project has been funded by Analog Devices Inc. in Wilmington, Mass.
MST POSITION SENSORS
The Centre Suisse d'Electronique et de Microtechnique SA (CSEM) in Neuchatel, Switzerland, a major research organization, has developed an inductive position sensor that measures, without contact, the lateral position or speed of any metallic target with a structured repetitive pattern, such as teeth or notches. Although magnetic position sensors are commonplace, they have not been made using MST until recently, because miniaturization degrades the performance of coil windings. CSEM makes the miniature sensor using a differential transformer with a thick copper coil for the generator and aluminum coils for detection. This sensor configuration is independent of the resistance and quality factors of the microcoils, and the sensing components can be integrated on a silicon chip.
The device is made by depositing copper coils on silicon. The sensor chip is an arrangement of microcoils in a differential transformer configuration. One microcoil generates a high-frequency magnetic field that is modified by the position of a nearby metallic target. The position of the target marks modulates the amplitude of the induced voltage in coplanar secondary detection microcoils. The combination of a high working frequency and a transformer without any magnetic core enables eddy currents to be used to detect the position of the target. The eddy currents modify the magnetic coupling between the primary coil and the secondary microcoils. The mutual inductances, not relying on ferromagnetism, are relatively unaffected by changes in temperature or by aging. The differential signals and a zero-crossing detection allow the speed to be detected independently of distance between sensor and target with a 50-percent duty-cycle pulse signal.
One possible application of the sensor is in high-speed printing, using coils to print; another would be as a wheel rotation sensor for an antilock-braking system, because the sensor could determine the speed of gear rotation. In addition, the sensor can be applied to speed sensing or tooth-wheel sensing, replacing strain gauges. CSEM is currently working on miniaturizing the system further and realizing a dedicated system interface for accurate signal conditioning.
Another new and growing area of MST is photonics. Since the use of telephone lines for modems has created problems in available bandwidth in the telecommunications network, solutions have been proposed in the area of photonics, such as wavelength-decision multiplexed passive-optical-network systems. These solutions are very expensive, but basing such systems on MST promises to reduce the cost significantly.
In an attempt to reduce network installation and administration costs, Bell Laboratories, the research and development arm of Lucent Technologies in Murray Hill, N.J., is investigating a loopback system that would replace the laser sources at a customer's premises with an optical modulator. In a loopback system, a steady stream of optical power is provided to the subscriber by a source located at the central office. Data applied to this signal by the modulator then loop back through the network to the central office.
The central device for this approach can be relatively expensive, but its cost can be amortized over a number of users. However, the optical-network unit (ONU) at each customer's facility must be inexpensive. One option is to design an ONU based on MST, which Bell Labs has dubbed the mechanical antireflection switch (MARS).
The operating principle of the MARS device is based on the change in an air gap between a suspended silicon nitride film and the underlying substrate. If the air gap equals zero or is an odd multiple of one-quarter the wavelength of the transmitted light, a typical single-layer antireflection coating is achieved. Alternatively, an even multiple of one-quarter of the wavelength suspends the air gap above the film, simulating a high-reflection, dual-layer dielectric mirror.
The MARS device is made with surface micromachining. The film structure consists of an n-doped silicon wafer coated with a 1-micron-thick film of photosilicate glass, a film of sodium nitride, and a metal film. The substrate and the top metal film act as the electrodes to which the electrostatic driving signal is applied. The silicon nitride film is patterned into the desired configuration using reactive ion etching, and the glass under the mechanically active area is removed by means of a sacrificial wet etch in a hydrofluoric acid-based solution.
"There is another benefit besides cost," said Jim Walker, a researcher at Bell Labs. "Since the MARS device has a relatively large active area compared with that of a conventional optoelectronic chip, users can use packaging techniques that allow the optical fiber to be aligned and be kept aligned passively so that misalignment later is not a problem." Passive alignment reduces packing costs by eliminating the skilled labor time spent on active alignment.
MARS modulators have been used experimentally and have demonstrated performance of 3.5 megabytes per second with an error rate of 1 X [10.sup.-8] bits. The number of wavelength channels is about four times that of conventional devices. The device is being evaluated by the system developers for use in telecommunications systems--both long-haul and local access applications.
Although actual development is still far off, MST also has been proposed as a means of electronic data storage. This has the potential to eliminate the size, weight, and power limitations of traditional storage media. Since computers that soldiers would use in the field would have to be very small and light to be practical, the military is taking the lead in researching this technology.
RELATED ARTICLE: Software-Based Design Tools Arrive
The lack of computer design tools thus far has been a significant barrier to the development of microsystems-technology (MST) applications, because most CAD and FEA systems do not have the capabilities to analyze microstructures adequately. As a result, manufacturers have been forced to develop and test prototypes, both of which have been time-consuming and expensive.
Companies such as IntelliSense in Wilmington, Mass., have recently put software design tools for MST on the market. Both Ford Microelectronics in Colorado Springs, Colo., and Dallas-based Texas Instruments have used the MEMCAD package from Microcosm Technologies Inc. in Cary, N.C. The MEMCAD system defines device layout and process, constructs the three-dimensional geometry of the device, assembles a detailed 3-D model, and analyzes device performance as well as device sensitivity to manufacturing and design variations.
Other features are available that, depending on the requirements, can perform additional analyses. For example, the thermomechanical feature applies temperature or heat-flux boundary conditions and solves for the temperature field in the solid model. Application of material thermal-expansion coefficients allows the computation of temperature-induced stresses and deformations. With modal dynamics, the modal frequencies and mode shapes of the solid model can be computed, and the mode's deformed shaped can be used for solid modal analysis. Both conductive and dielectric materials are used throughout dielectric modeling, which enables realistic standard MST materials such as oxides and nitrides to be included.
The residual-stress-gradients package applies an average residual stress and residual-stress gradient to any material. This permits modeling of the shrinkage film and film warpage, both characteristics of MST applications. It also improves modeling of members whose compliance is affected by residual stress, such as straight fixed-fixed tethers or supports.
Ford uses the software to project and perfect the geometries of future automotive devices and enhance processing and manufacturing analysis. Texas Instruments models several micromachined devices, including its spatial lights modulator, optical switches, sensors, and process-monitoring test structures.
Tanner Research Inc. in Pasadena, Calif., also offers a CAD software package, MEMS-Pro. This package was built on top of the company's existing package, which was created to design very-large-scale integrated circuits. An MST application is designed by drawing a schematic, running simulations of electromechanical performance, and creating the physical layout. Designers of microelectromechanical systems (MEMS) can program the cross-section viewer to simulate the grow/deposit, implant/diffuse, and etch steps that simulate the corresponding steps used in the fabrication process.
Future versions of many current software packages should be able to handle the design of mixed-technology devices, which are chips that incorporate MEMS devices right alongside traditional digital and analog circuits. A barrier to placing both the MEMS device and the circuit on the chip is that the manufacturing processes are incompatible. Theoretically, however, design software could be used to surmount many of these obstacles. The Defense Advanced Research Projects Agency in Arlington, Va., is one of the main organizations promoting the development of these tools.
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|Title Annotation:||includes related article on software design tools; advances in microsystems technology|
|Date:||Sep 1, 1997|
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