Although microelectromechanical system (MEMS) devices started out as sensors—mostly for automotive at first, and later for medical applications—the technology has now mushroomed into commercialization in a number of other arenas. In fact, MEMS technology is proving to be a key enabler for many implementations hitherto not possible or practical with conventional electronic devices. Furthermore, it promises to become even more prevalent in at least three "killer" applications: RF, biomedical, and geophysical/environmental fields.
The key advantage of MEMS technology is its ability to utilize silicon's mechanical and electrical properties. This enables monolithic ICs with both mechanical and electronic functions on the same piece of silicon. In effect, an entire control system can be integrated on one chip. This single chip senses, acquires, and processes data and then feeds it to an actuator or manipulator, which acts upon the data in a closed-loop manner.
A Natural For RF: MEMS will have the greatest impact on applications operating in the RF spectrum, like mobile communications (cell phones), radar, and the life sciences. These all require small, inexpensive, low-power, high-performance devices that MEMS basic components (filters, relays, switches, capacitors, and inductors) can uniquely provide.
For example, the University of Michigan's Center for Wireless Integrated Microsystems has shown that MEMS filters with Qs of 9400 are possible on a silicon chip. The chip will have a width of just a few tens of micrometers and bandwidths in the hundreds of megahertz range, and ultimately in the gigahertz range (Fig. 1).
Cell phones today can only obtain Qs of less than 2000 from conventional SAW filters, which are typically too large to fit on-chip. MEMS filters are a boon for new-generation phones, providing extremely high sensitivities. This would enable high levels of channel selectivity within different bands, a revolutionary capability for wireless communications.
MEMS filters also offer very low power dissipation, promoting long battery life and operation, as well as a smaller form factor. Moreover, MEMS capacitors, resistors, and relays can work with MEMS filters for even greater space savings and improved performance. Variable MEMS capacitors supply performance levels superior to those of present conventional varactor diodes.
Another area where MEMS technology will make a huge impact is in RF switches, where it heralds the construction of less expensive electronically steerable antennas for mobile communications and radar applications. This capability is useful in two-way radios to quickly switch the antenna between the receiver and transmitter at low losses. It's estimated that RF MEMS switches can be fabricated with losses of less than 0.1 dB and typically feature isolation levels of more than 40 to 50 dB in the "off" state. They also are useful for phase shifting in phased-array radars.
Many other applications exist for RF MEMS. In the life sciences, MEMS RF switches can be used to keep track of everything from animals in research settings to espionage agents in the field. RF MEMS gyroscopes for luxury automobiles are already on the drawing board for ride stabilization, rollover sensing, and skid control.
While it may not require RF speeds, automatic test equipment is yet another application for MEMS technology. Here, there's a need to quickly switch the I/O pins of the device under test in as small an area as possible with the least amount of power. MEMS switches do this quite well.
Instant, Comprehensive, And Accurate Diagnostics: No area of MEMS killer applications is more exciting and will have a greater impact on our lives than biomedical/life sciences. One of the earliest biomedical MEMS uses was implantable and disposable blood-pressure sensors, which continues to grow. Such pressure sensors have become far more sophisticated, as demonstrated by the implantable optical pressure sensor from Fiso Technologies, Quebec, Canada (Fig. 2).
On the immediate horizon and awaiting FDA approval are complete MEMS labs-on-a-chip. These devices will let healthcare providers perform point-of-care diagnostics on a patient without the time and cost conventional methods require.
In the Third Biblical Book of Luke in the New Testament, verse 7:22, there's a revelation: "The blind see, the lame walk... the deaf hear." Every one of these prophecies is close to reality. Researchers are hot on the trail of implantable MEMS devices that will make them all possible.
MEMS technology is being harnessed to realize a host of implantable mechanisms that can stimulate paralyzed limbs, improve the treatment of diseases like epilepsy and Parkinson's, diagnose bacterial and viral agents, determine the safety and efficacy of drugs, speed up drug delivery, and deliver drugs more accurately and effectively. Already, an implantable MEMS-based insulin pump has been achieved.
The Cleveland Clinic Foundation in Ohio is investigating MEMS sensors and antennas that will let neurosurgeons accurately study and control the human spine (Fig. 3). Patents are currently pending for MEMS-based orthopedic implants, including a combination spinal pressure sensor/actuator to monitor bone fusion and increase it through stimulation.
Engineers at Detriot's Wayne State University hope to commercialize MEMS-based probes for robotic computer-aided surgery. The probes will allow doctors to determine blood flow in human tissue and differentiate between tumors and normal cells, as well as give them better "touch" feedback during delicate procedures like soft-tissue suturing.
Exploring The Environment: Improved performance of MEMS devices and their miniaturization has steered them into the unchartered waters of geophysics and the environment. Some companies are using highly selective and accurate low-cost MEMS sensors for oil and gas exploration. These sensors have outperformed and replaced oil-based geophones over the last five decades.
Concern about the environment—air and water pollution—is a major driving force for the use of MEMS sensing technology. Environmental monitoring and earthquake detection are only two of the hot areas MEMS researchers are exploring.
Recent fears of biological and chemical warfare have pushed scientists at Sandia National Labs in Albuquerque, N.M., to develop a micromachined sensing system that continually monitors the air and water for harmful compounds in-situ, eliminating the need to collect samples remotely and analyze them elsewhere in a lab. It's based on an array of four miniature sensors, called chemresistors, packed in a DIP and able to detect potentially harmful volatile organic compounds. The DIP is connected to a long weatherproof cable that connects to a data logger to transmit data to a central computer.
Today, we're witnessing just the tip of the iceberg for putting MEMS technology to work. Because of its unique and versatile combined mechanical and electrical properties, it will continue to be an enabling platform for applications limited only by the imagination.