May 21, 1998

MEMS sensors and mechanical gadgets enter the mainstream

Bill Travis, Senior Technical Editor

Once considered lab curiosities, micromachined-ilicon devices are making the grade in the
commercial market at an increasing rate.   

MEMS (microelectromechanical systems), or micromachined-silicon devices, are rapidly emerging from research labs and finding their way into practical applications. Processing refinements and new mass-production techniques are making the once-esoteric devices economical to produce with tolerable yields. MEMS' small sizes and sterling performance make them the components of choice in many sensing, motion-control, and other designs.

Remember the science-fiction film, Fantastic Voyage? A crew of scientists and a submersible vehicle undergo a microscopic shrink to enter a patient's body to give medical treatment. Except for shrinking the humans, the idea is now not so far-fetched. Research labs everywhere are reporting micromachined devices--cutting tools, drills, and reaming devices, for example--that physicians can control from outside a patient's body. Most of these devices (microminiature motors, ratchets, and gear trains, for instance) are still in the developmental stage, but they're poised for entry to the market in the near future.

Roger Grace, a consultant to the sensor industry, reports that more than 300 companies and research institutes are pursuing MEMS activities (Reference 1). The MEMS include sensors, actuators, and mechanical structures. Accelerometers and pressure sensors were among the first commercially viable MEMS devices; however, MEMS that perform functions other than sensing are rapidly emerging (Table 1).

Before examining available MEMS products, a little insight into the structure and manufacturing of MEMS may be edifying. Reference 2 describes bulk and surface-micromachining processes. In the bulk process, etching produces pits and cantilever structures. Silicon-on-silicon or silicon-on-glass bonding then produces a sensor (pressure, for example) configuration by exploiting the thin diaphragm or cantilever structure. The surface process produces movable microstructures on the silicon wafer. It uses a "sacrificial" material, which, once etched away, leaves a freestanding structure. MEMS accelerometers use surface processing.

A little spray will do it

One of the many nonsensor applications of MEMS technology is in spraying fine jets or droplets of fluid. Ink-jet printers, for example, use micromachined devices for this purpose. SprayChip Systems takes micromachining a step further to produce spray nozzles that offer carefully controlled emitted-fluid parameters (Reference 3): ultrasmall droplets, narrow droplet-size distribution, tight  droplet-density dis-
tribution, uniform velocity distribution, and selectable spray pattern.   

Figure 1 shows two nozzle configurations available in SprayChip's repertoire. In Figure 1a, thousands of small, micromachined nozzles on a chip produce microjets of fluid that break up into uniform droplets. In this configuration, you can obtain 10-µm nozzles with center-to-center spacing as close as 25 µm. The configuration in Figure 1b operates with pressures as low as 10 psi to obtain droplets of 15-µm diameter or smaller. In this arrangement, atomization of the fluid occurs along the edge of a slot between two sandwiched silicon structures.

Another promising nonsensor application for MEMS is in fiber-optic communications. Micromachined structures can provide optical modulation, switching, and other light-control functions. Reference 4 from Bell Labs describes an optical modulator that uses surface micromachining to provide an electrostatically controlled refractive silicon-nitride film (Figure 2). Thefilm has a refractive index equal to the square root of that of the substrate, and a quarter-wavelength thickness. In Figure 2a, the suspended silicon-nitride film provides greater than 70% reflection to the light signal; in Figure 2b, when the film is in contact with the substrate, it forms an antireflective coating.

Bell Labs also has plans for MEMS in fiber-optic systems. Reference 5 describes an optical switch that uses surface micromachining. The two-port reflective switch consists of a movable vertical gold-coated shutter connected to an electrostatic actuator. The actuator inserts the shutter in the light path in a narrow fiber gap. The switch delivers ad-mirable optical performance: 38- to 80-dB transmission isolation and 2.15-dB return loss in the 1550-nm wavelength region, operating from actuation voltages as low as 4V.

Several relay and sensor manufacturers are developing electrostatically actuated MEMS relays. Microrelays from EG&G IC Sensors, for example, consist of a polysilicon relay armature suspended below a glass plate that contains both relay contacts and drive electrodes. Applying a voltage to the drive electrode causes the polysilicon plate to deflect, thereby closing the contacts. Each relay element in an array measures only 0.2×0.3 mm. Actuation time is lower than 20 µsec, and the useful lifetime exceeds 10⁹ cycles. The relays draw no static power, as opposed to reed relays, which draw several milliamps.

One of the most ambitious--and successful--undertakings in MEMS technology is a pixel-mirror system from Texas Instruments (Reference 6). In this system, a static-RAM cell addresses an aluminum micromirror, commanding it (via an electrostatic signal) to rotate through a ±10° angle span. At 10°, light from a projection source impinges on the pupil of a projection lens, and the pixel appears bright on a screen. Figure 3 shows the exceedingly intricate micromachined structure.

The mirror mounts on a cantilevered structure that uses torsion hinges and a yoke with landing tips that determine the endpoints of rotation. Such an elaborate structure requires a correspondingly elaborate manufacturing process. It also gives rise to several possible failure mechanisms, which TI has thoroughly characterized and investigated. The failure mechanisms are hinge fatigue, shock and vibration failure, hinge memory, and stiction (static-friction) failure.

To test hinge fatigue, TI subjected devices to more than 10¹² switching cycles, equivalent to more than 20 years of normal operation. The result: no broken hinges. Shock and vibration results are equally good, because the silicon structure has vibration modes at least two orders of magnitude above the vibration frequencies of normal handling and operation.

Hinge-memory failures can occur when a pixel is switched to the same direction for an extended period. The symptom is the failure of the hinges and yoke to return to the flat position upon removal of the bias and address voltages. A study of this phenomenon led TI to adopt an alloy with a low metal-creepage rate; as a result, the devices  preserve the address margin over their operating lifetime. Stiction failures result from an excessive adhesive force between the yoke landing tip and its landing site. A thin antistick passivation layer eliminates stiction failures in the MEMS.

Theoretically, there's no limit (aside from production yields) on the number of pixels a TI pixel-mirror MEMS can handle. The company has built arrays as large as 2048×1152 pixels, yielding 2.3 million pixels. Production devices offer 848×600 pixels, capable of projecting NTSC, PAL, VGA, and SVGA graphics. Another MEMS chip from TI has a 7056×64 array, which can project a 600-dpi image over a print width of 11.7 in.

MEMS sensors make sense

The movable structures on micromachined-silicon ICs are a natural for configuring sensors. The structures can produce either capacitive or resistive changes, which accompanying circuitry on the chip can easily detect. A wide variety of accelerometers and pressure sensors capitalize on the advantages inherent in MEMS ICs. Analog Devices, the original innovator of MEMS-based accelerometers, continues to add to its line with more sensitive and lower power-consumption devices.

The ADXL202 (Figure 4) from Analog Devices provides x- and y-axis acceleration data while consuming just 0.6-mA supply current. The device operates from a 2.7 to 5.5V supply, over ­40 to +85°C. The surface-machined accelerometer uses differential, 0.1-pF capacitors. The detectable DELTAC range is from 20 zF (10^­21F) to 10 fF full-scale, for a ±2g acceleration. The de-tection method relies on duty cycle--for 0g acceleration, the output pulse train has a 50% duty cycle. When acceleration occurs, the duty cycle changes by a nominal 12.5% per g force. The ADXL202 costs $9.95 (10,000).

For a greater acceleration range, you can use Analog Devices' single-axis ADXL150 or dual-axis ADXL250. These ±50g accelerometers provide a linear voltage output, with a 38-mV/g scale factor. Both the scale factor and the 0g output level are ratiometric with the supply voltage, so you can use them with a ratiometric A/D converter (such as those in most µCs) without a voltage reference. The devices consume less than 2 mA per axis from a 4 to 6V supply. The single- and dual-axis MEMS ICs cost $12.45 (100) and $19.95 (100), respectively.

For several years, the MEMS-based accelerometers from Analog Devices have principally targeted the automotive-air-bag market. According to the company, the new devices will open the door to many new applications. These could include new automotive uses, industrial sensing systems, and consumer applications such as video games. Their low power consumption also makes them amenable to battery-powered systems.

Analog Devices is not alone in the MEMS-based accelerometer sector. EG&G IC Sensors offers a two-chip device that combines a sensor chip and an ASIC in a 16-pin plastic DIP. Model 3265 is available in ±25 and ±50g versions, which have acceleration  sensitivities of 80 and 40 mV/g, respectively. The output is ratiometric over a 5V±0.25V supply range, a convenient feature for use with µCs that contain an A/D converter. The $65 (100) ICs have a self-test feature that provides a voltage that simulates an acceleration.

CSEM offers a MEMS accelerometer having less than 150-µA current draw and available in 2, 5, 10, 20, and 50g full-scale versions. The MS 6100 has sensitivity factors of 0.5, 0.2, 0.1, 0.05, or 0.02V/g, corresponding to the cited full-scale ranges. The device contains a MEMS sensor chip, an ASIC for signal conditioning and calibration, and an EEPROM for storing calibration data. CSEM uses the on-chip EEPROM to program corrections for gain, offset, and linearity during manufacturing. A self-test feature checks the integrity of the sensor and other ICs.

A MEMS-based accelerometer chip from Motorola is available for either z-axis (vertical) or x-axis (lateral) detection. The XMMA1000P (z) and XMMA2000W (x) detect a ±50g acceleration range, with ±40-mV/g sensitivity. The ICs contain an onboard, four-pole, switched-capacitor Bessel filter. A self-test capability verifies system integrity. The output is ratiometric with respect to the supply voltage.

A good example of other-than-automotive applications of MEMS-based accelerometers is a vertical gyro system from Crossbow Technology. The $4000 DMU Turbo measures 3×3.375×3.25 in. and weighs 475g. The gyro provides stabilized roll and pitch (artificial horizon) outputs over ±180° for roll and ±90° for pitch. It uses three accelerometers and three angular-rate sensors, all micromachined-silicon devices. Its angular-rate resolution for roll, pitch, and yaw is 0.05°/second. Look for a MEMS-based gyro system from Microsensors Inc, a subsidiary of Irvine Sensors. The Silicon MicroRing Gyro uses a 2.4-mm² sensor based on Coriolis' tuning-fork gyro principle. It incorporates a capacitive pick-off plate and an oscillating wheel to provide angular-rate information.

MEMS technology also provides a means to design position sensors. CSEM provides an alternative to Hall-effect sensors, in the form of micromachined copper-coil assemblies (Figure 6). The assemblies provide position and speed sensing in automotive and industrial-control applications. The MS1020/21 operates with any structured metallic target (for example, aluminum,copper, or iron) in position-sensing systems.

Bulk-machined pressure sensors

Most micromachined pressure sensors use the bulk process. Etching a pit into a silicon wafer produces a diaphragm at the bottom of the pit. A second wafer fits over the pitted one, using a glass seal. The pit then forms a vacuum reference. The sensing mechanism uses a piezoresistive strain gauge, which senses flexing of the diaphragm when pressure impinges on it. Motorola offers a range of micromachined pressure sensors.

A recent low-pressure model from Motorola targets heating, ventilation, and air-conditioning; medical; food-processing; and appliance applications, to name a few. The MPTXT5006D provides 3% full-scale accuracy for a pressure of 0 to 0.87 psi. The output ranges from 0.2V for zero pressure to 4.5V for 0.87-psi pressure. Motorola designates the two sides of the pressure sensor as the pressure and the vacuum sides. The device operates with positive differential pressure applied. It is available in a top-piston-fit package, having a cap with a pressure access hole, and a top-piston-fit/snap port, having a nozzle extending from the top of the IC.

Another top-piston-fit pressure sensor from Motorola covers 0 to 1.45 psi with full-scale output span of 25 mV. The MPXT2010 works well with ADC-containing µCs, because its output is ratiometric with respect to the supply voltage. Motorola specs ±1% linearity for the sensor, using the "best-fit-straight-line" method. The MEMS sensor costs $10.40 (10,000).

A micromachined pressure sensor for hostile environments measures pressures of 0 to 25,000 psi. The MSP-430 from Measurement Specialties is suitable for harsh applications, such as diesel injection, hydraulic, down-hole drilling, and other high-pressure environments. Fabricated from a piece of stainless steel, the leakproof sensor has no internal O rings, no welds, and no silicon-filled cavities. Accuracy, combining linearity, hysteresis, and repeatability, is ±0.5% of full-scale output. The $70 (1000) MSP-430 measures 2 in. long and 1.25 in. in diameter.

A capacitive MEMS-based pressure sensor from CSEM provides absolute-pressure measurements in versions that cover 0 to 1.2 bar (1 bar is one atmosphere, or 14.7 psi) and 0 to 10 bar. The MS4010 contains no active circuitry but rather delivers a capacitive signal for external sensing. The respective sensitivities for the cited pressure ranges are 2.6 and 0.32 pF/bar. The sensor's equivalent circuit shows a fixed capacitor of 6.4 pF in parallel with a 5.3-pF pressure-sensing capacitance. A CMOS IC from Microsensors Inc provides support for capacitance-readout MEMS, such as the MS4010. The Universal Capacitive Readout device senses capacitance changes as small as 0.05 fF. The IC is an integral part of the Silicon MicroRing gyro, described earlier.

Honeywell Micro Switch also provides a range of MEMS-based pressure sensors. In addition, the company uses micromachined silicon to produce a range of airflow sensors. The sensors use the theory of heat transfer to do their job. Mass airflow directed across the surface of the sensing element produces an output voltage proportional to the air or other gas flow through the inlet and outlet ports of the package. The sensor element consists of a thermally isolated, bridge structure containing heater and temperature-sensing elements. The airflow sensors are available in unamplified and amplified versions.

EDA meets MEMS

Early MEMS were custom-tailored, painstakingly handcrafted devices. With the growing acceptance, complexity, and maturity of MEMS technology, however, designers need more sophisticated design tools. Some recent EDA tools provide modeling and simulation software for the creation of complex MEMS devices. MEMCAD 4.0 from Microcosm Technologies, for example, is an integrated design package comprising 3-D design, modeling, and simulation software tools.

MEMCAD 4.0 has a solver framework that supports such physics disciplines as electrostatics, electromechanics, microfluidics, thermomechanics, piezoresistance, and coupled electrothermomechanics. The basic package includes tools for layout, process, material-property database, and others. Available add-on modules are as follows:

• CoSolve-EM--coupled electromechanics with moving dielectrics
• MemCap--100K panel electrostatics support and dielectrics
• MemMech--upgraded stress/displacement, thermal/stress model
• MemElectroTherm--electro/thermal analysis
• MemPiezo--piezoresistive effects
• NetFlow--fluidic interconnects, diffusions, and flows in mechanical devices, electrokinetics
• Packaging--coupled package-device modeling
• Spring Analysis--automated multidimensional spring analysis
• AutoMM--electromechanical macro-model/Spice-model generation.

MEMSCap, in collaboration with Mentor Graphics, offers MEMS-specific EDA tools that integrate electromechanical modeling with VHDL/Verilog and HDL-A support for mixed-signal modeling (Figure 7). HDL-A is a high-level modeling language specific to MEMS devices. It contains technology-specific layout generators for various mechanical structures. The package supports both bulk and surface micromachining and provides an etching-verification and cross-section viewer.

To sum up, MEMS-based sensors are graduating in increasing numbers from academia and research labs into production. Some of the micromachined parts available now are just this side of astonishing; you can expect many more surprises in the near future.

References

1. Grace, Roger, "MEMS--a perspective on markets, technologies and applications," Proceedings, Sensors Expo 1996, pg 101.
2. Frank, Randy, Understanding Smart Sensors, Artech House, 1996.
3. Skeath, Perry, "Practical application of micromachined spray chips," SprayChip Systems Corp, submitted to Sensors Expo 1998.
4. Walker, JA, JE Ford, and N Basavanhally, "Performance and packaging implications of a MEMS-based optical modulator for WDM fiber-to-the-home System," Bell Laboratories, Lucent Technologies, presented at the 47th Electronic Components and Technology Conference, May 1997.
5. Aksyuk, V, B Barber, CR Giles, R Ruel, L Stultz, and D Bishop, "Low insertion loss packaged and fiber-connectorized Si surface-micromachined reflective optical switch," Bell Laboratories, Lucent Technologies.
6. Hornbeck, Larry, "Digital light processing and MEMS: timely convergence for a bright future," Texas Instruments, presented at Micromachining and Microfabrication '95.  

| EDN Access | Feedback | Table of Contents |