Microengineering Aerospace Systems

Henry Helvajian, editor

 

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Chapter 1: Introduction to MEMS (cont.)

 

M. Mehregany and S. Roy

 

1.3    MEMS Components

The miniaturization, multiplicity, and microelectronics characteristics of MEMS technology make it especially attractive to realize small-size, low-cost, high-performance systems integrated on one chip. Microfabricated pressure sensors have dominated the MEMS application market for the last two decades. With advances in IC technology and corresponding progress in MEMS fabrication processes in the last decade, additional integrated microsensor and microactuator systems are now being commercialized, and even more applications are expected to benefit. In this section, we present examples of some commercially available MEMS components selected on the basis of fabrication technique and system complexity. First, pressure sensors are presented as an example of a MEMS device fabricated using bulk micromachining, followed by integrated accelerometers that are fabricated by surface micromachining. Next, the suitability of MEMS technology in complex, array-type application systems is demonstrated using the example of a digital micromirror device (DMD). Finally, the potential of MEMS components in aerospace applications is discussed, and some promising devices are listed.

1.3.1    Pressure Sensors

MEMS technology has been utilized to realize a wide variety of differential, gauge, and absolute pressure microsensors based on different transduction principles. Typically, the sensing element consists of a flexible diaphragm that deforms due to a pressure differential across it. The extent of the diaphragm deformation is converted to a representative electrical signal, which appears at the sensor output.

Figure 1.10 shows a manifold absolute pressure (MAP) sensor for automotive engine control, designed to sense absolute air pressure within the intake manifold (manufactured by Motorola, Schaumburg, Illinois). This measurement can be used to compute the amount of fuel required for each cylinder in the engine. The microfabricated sensor integrates on-chip, bipolar op-amp circuitry and thin-film resistor networks to provide a high output signal and temperature compensation.

Fig. 1.10. Commercially available absolute pressure sensor. (a) Sensor package; (b) cross-sectional schematic. (Motorola, Inc.)25

The sensor die/chip consists of a thin Si diaphragm fabricated by bulk micromachining. Prior to the micromachining, piezoresistors are patterned across the edges of the diaphragm region using standard IC processing techniques. After etching of the substrate to create the diaphragm, the sensor die is bonded to a glass substrate to realize a sealed vacuum cavity underneath the diaphragm. Finally, the die is mounted on a package such that the top side of the diaphragm is exposed to the environment through a port. A gel coat isolates the sensor die from the environment while allowing the pressure signal to be transmitted to the Si diaphragm. The ambient pressure forces the diaphragm to deform downward, resulting in a change of resistance of the piezoresistors. This resistance change is measured using on-chip electronics; a corresponding voltage signal appears at the output pin of the sensor package.

1.3.2    Accelerometers

Acceleration sensors are relatively newer applications of MEMS technology. Typically, the sensing element consists of an inertial mass suspended by compliant springs. Under acceleration, a force acts on the inertial mass, causing it to deviate from its zero-acceleration position, until the restoring force from the springs balances the acceleration force. The magnitude of the inertial-mass deflection is converted to a representative electrical signal, which appears at the sensor output.

Figure 1.11 shows a monolithic accelerometer (manufactured by Analog Devices, Inc., Norwood, Massachusetts), the ADXL-50, fabricated by surface micromachining and BiCMOS (a combination of bipolar junction transistor [BJT] and complementary metal-oxide semiconductor [CMOS]) processes. The inertial mass consists of a series of 150-µm-long fingerlike beams connected to a central trunk beam, all suspended 2 µm above the substrate by tether beams. The ADXL-50 uses a capacitive measurement method: the deflection of the inertial mass changes the capacitance between the finger beams and the adjacent cantilever beams. The sensor structure is surrounded by supporting electronics, which transduce the capacitance changes due to acceleration into a voltage, with appropriate signal conditioning.

Fig. 1.11. Surface micromachined integrated accelerometer. (a) Chip overview, (b) close-up of sensor structure showing central trunk beam and fingers. (Analog Devices, Inc.)

The analog output voltage is directly proportional to acceleration, and is fully scaled, referenced, and temperature compensated, resulting in high accuracy and linearity over a wide temperature range. Internal circuitry implements a forced-balance control loop that improves linearity and bandwidth. Internal self-test circuitry can electrostatically deflect the sensor beam upon demand, to verify device functionality.

1.3.3    DMDs

The DMD (manufactured by Texas Instruments, Inc., Dallas, Texas) is a microchip consisting of a superstructure array of Al micromirrors functionally located over CMOS memory cells. The DMD digital light switch moves between the "on" and "off" states to create and reflect digital gray-scale images from its surface when light is applied. These digitally created images are transferred through appropriate optics and filters to create projected and/or digitally printed images.

The Al micromirror superstructure is realized by surface micromachining, while the underlying memory cells are fabricated using standard CMOS processes. The mirrors are hermetically sealed beneath nonreflecting glass to prevent contamination-induced failure. Figure 1.12 shows the details of the DMD microchip. Each mirror is 16 µm square with a 1-µm space between mirrors on all sides. The number of mirrors in use on a single chip can range from 307,200 to 1.3+ million (with one mirror per pixel).

Fig. 1.12. DMD microchip. (a) Portion of the micromirror array, (b) exploded view of a single (16-µm-edge length) micromirror element. (Texas Instruments, Inc.)

To achieve digital operation, the DMD micromirrors are designed to be bistable. In the "on" mode, the mirrors deflect +10 deg, while in the "off" position, the mirrors rest at –10 deg. When a given CMOS memory cell is loaded with a digital 1, electrostatic forces switch the corresponding mirror "on" to reflect light into the aperture of an imaging lens. Memory cells loaded with a digital 0 cause the mirror to switch "off," and to direct incident light away from the imaging lens. In conjunction with appropriate optics, a color wheel, and electronic control circuitry, the DMD can be used to display high-quality projection images.

 

1.3.4    Sensors and Actuators in Aerospace Applications

Sensors are required in a variety of aerospace instrumentation, including fuel measurement and monitoring, landing gear, ice protection, and navigation. In small, private aircraft, the instrumentation is simple and may consist only of an altimeter to register height, an indicator to register airspeed, and a compass. The most modern airplanes and manned spacecraft, in contrast, have fully automated "glass cockpits," in which a tremendous array of sensor information is continually presented on the aircraft's height, attitude, heading, speed, cabin pressure and temperature, route, fuel quantity and consumption, and on the condition of the engines and the hydraulic, electrical, and electronic systems. Aerospace vehicles are also provided with inertial guidance systems for automatic navigation from point to point, with continuous updating for changing weather conditions, beneficial winds, or other situations. This array of instrumentation is supplemented by vastly improved meteorological forecasts, which reduce the hazard from weather, including such difficult-to-predict elements as wind shear and microburst.

Attitude and direction of aerospace vehicles are handled by flight controls that actuate elevators, ailerons, and rudders through a system of cables or rods. In sophisticated modern aircraft, there is no direct mechanical linkage between the attitude and direction devices used by the pilot and the actual controls used to achieve the changes in attitude and direction; instead, these controls are actuated by electric motors. The catch phrase for this arrangement is "fly by wire." In addition, in some large and fast aircraft, controls are boosted by hydraulically or electrically actuated systems. In both the fly-by-wire and boosted controls, the feel of the control reaction is fed back to the pilot by simulated means.

The use of MEMS devices in aerospace systems is expected to be highly application specific and would typically aim to reduce size, weight, and power consumption at the component level. Changes in both commercial and military markets for fixed-wing and rotor-wing aircraft demand increased performance with less weight. The cost advantage and electronic integration capabilities of MEMS enables the feasibility of distributed measurement and actuation. These features would be based on flexible location of smart transducers and decreased reliance on pneumatics, which would, in turn, lead to more accurate measurements, reduced vulnerability through redundancy, fewer moisture drain traps, and considerable weight savings.

In addition to conventional aircraft, evolution of MEMS technology should lead to the development of micro, unmanned aerial vehicles (UAVs). These small flight vehicles would perform as aerial robots whose mobility could be used to deploy micropayloads to a remote site or to otherwise hazardous locations.

Fig. 1.13. Photograph of the Micro-Air Data Transducer, a commercially available MEMS-based sensor for use in aircraft. (BF Goodrich Co.)

Figure 1.13 shows a commercially available pressure measurement instrument, the micro, air data transducer (manufactured by BF Goodrich Company, Richfield, Ohio), which measures static and total pressures using micromachined Si-based sensors. This instrument is only 25% of the size and weight of its conventional non-MEMS-based counterparts, and exhibits a 0.02% full-scale pressure accuracy. Applications include primary accuracy air data for flight control, cockpit display, navigation, and fire control.

Fig. 1.14. Microfabricated ice detection sensor.

A number of other MEMS devices have been developed for aerospace applications. Although most of these devices are still at the research stage, their eventual integration into aerospace systems will revolutionize flight safety and performance. One of the devices that has been realized, the miniaturized ice detector, uses bulk micromachining and wafer bonding techniques. The detector is shown in Fig. 1.14.²⁶ The sensing element is 2 mm square and can detect ice films as low as 0.1 mm thick. Table 1.2 presents examples of MEMS devices with potential aerospace applications.

 

Table 1.2. Examples of MEMS with Potential in Aerospace Applications

Device Application	Fabrication Method	Transduction Principle	Organization
Shear Stress Sensor	Surface micromachining	Capacitive detection	Case Western Reserve University, Cleveland, OH
	Bulk micromachining	Optical detection	Massachusetts Institute of Technology (MIT), Cambridge, MA
Accelerometer	Integrated surface micromachining	Capacitive detection	Analog Devices, Norwood, MA
	Surface micromachining	Capacitive detection	Motorola, Phoenix, AZ
	Bulk micromachining	Piezoresistive detection	Endevco, Capistrano, CA
Pressure Sensor	Bulk micromachining	Piezoresistive detection	Lucas Novasensor, Sunnyvale, CA
	Bulk micromachining	Piezoresistive detection	Motorola, Phoenix, AZ
Angular Rate Gyroscope	Surface micromachining, micromolding	Capacitive detection	Draper Labs, Cambridge, MA
	Surface micromachining	Capacitive sensor	University of California, Berkeley
Drag Reduction	Bulk micromachining, micromolding	Magnetic flap actuator	University of California, Los Angeles
Fuel Atomization	Bulk micromachining	Precision nozzle	CWRU
Screech Control	Bulk micromachining	Electrostatic microactuator	University of Michigan, Ann Arbor
Communication Filters and Oscillators	Surface micromachining	Electrostatic resonators	University of Michigan
Microrelays	Surface micromachining, micromolding	Electrostatic actuator	CWRU
	Surface micromachining	Electrostatic actuator	Hughes Research Labs, Malibu, CA
	Surface micromachining, micromolding	Magnetic actuator	Georgia Institute of Technology, Atlanta
Optical Scanners	Surface micromachining	Electrostatic micromotor	CWRU
	Surface micromachining	Electrostatic resonator	UC, Berkeley

 

 

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Next: 1.4    Commercial Applications

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