The synergistic combination of micro-electro-mechanical systems (MEMS) technology and opto-electronics has evolved into a class of integrated micro systems expected to create tremendous new application opportunities. The component areas of MEMS are categorized as micro-machines, micro-integrated circuits, micro-optics, and diffractive optics. The latter two are often called MOEMS technologies.
According to our estimates, the world market for MEMS will grow to several billion dollars per year by 2005. Roughly one-third of this market includes micro-optical systems. In addition, the market for equipment and applications using MOEMS leverages the micro-systems markets by a factor of 20 to 100.
MOEMS represent a key technology for many innovations and revitalized applications. The markets in which these changes are taking place are industrial automation, safety and identification, chemical analysis, automotive applications, imaging/display technology, medical and biomedical technology, and tele- and data-communications.
Optical switches for telecommunications are now receiving a lot of attention, but these are not the only applications of MOEMS technology. Others include free-space micro-optical components and systems, displays and spatial light modulators, micro-optical sensors and actuators, optical devices using MEMS technology, adaptive optics and aberration-control systems, nanoscale optics and photonic systems, micro photonics for analytical chemistry, micro-optical biosensors, optical data storage, simulation, packaging, integration, and reliability. applying MOEMS
The technical drivers for sensor development come not only from materials science but from innovations in low-cost, large-scale manufacturing of interconnects, microelectronics, and micromachining. The 1980s saw the development of fiber-optic sensors for chemical, mechanical, and biological applications; in fact, in the middle of that decade, a new fiber-optic sensor application was submitted to patent authorities almost every week.
In the MOEMS sector, low-cost miniature spectrometers are key components in the realization of small-sensor solutions for applications such as color measurement or industrial process control. Optical methods are enabling technologies in this respect. Sensors in development include cameras-on-a-chip, nose-on-a-chip, and devices for mass-spectrometry.
Due to the small scale and extreme precision of MEMS devices, much interest surrounds the development of miniaturized devices to move and process fluids with micropumps. The challenges are great because the effects of viscous drag and friction must be overcome. Obtaining sufficient minimal-clearance space between dynamic members and the walls of the pump cavities, as well as sealing the pump cavities from the exterior, also present challenges.
In recent years, optical-data storage has been gaining on magnetic hard-disk storage, as it offers superior areal densities. In the last few years, magnetic storage has made a comeback with a 60% cumulative annual growth rate of areal density in magnetic-disk storage products. Meanwhile, new approaches combine micromachining technologies with holographic optical storage. For example, the Texas Instruments (Dallas, TX) digital micromirror device (DMD) has been integrated with holographic storage systems.
In one important application, MOEMS devices are used as tracking actuators for magnetic data storage systems. Seagate Technology (Scotts Valley, CA) uses an ultra-compact magneto-optical storage head to implement something it calls an optically assisted Winchester approach, which integrates several closely spaced platters, an advanced light-delivery system, a flying optical head design, a MEMS-based servo system, and ultra-high coercivity media on a preformatted plastic substrate. The company promises to take disk drives beyond the superparamagnetic effect, which is the theoretical areal density limit of traditional magnetic recording technology, believed to be at 20 to 40 Gb/in2.
In terms of cost benefits and performance enhancement, introducing MOEMS into the optical-storage sector could hold great promise.
In a quest to develop a high-resolution display that is small, lightweight, and requires low power consumption, researchers are investigating MOEMS-based technology. One example is grating light valve (GLV) technology from Silicon Light Machines (Sunnyvale, CA). GLV has an external bulk-optical scanning mirror and a 1-D array of micromachined deformable diffraction gratings that modulate pixel intensity. To create a display, this architecture requires a number of diffraction gratings, each composed of several micromechanical beams.
The DMD is considered a showpiece of MOEMS technology at work. Each pixel in the image corresponds to one electrostatically controlled micromirror that can reflect light in one of two directions determined by the state of the underlying memory cell. If the memory cell is in the (1) state, the mirror rotates to +10°. If the memory cell is in the (0) state, the mirror rotates to -10°. When the DMD is combined with a light source and projection optics, the mirror reflects the light either in or out of the pupil of the projection lens. The (1) state makes the mirror appear bright and the (0) state dark. By modulating the incident light, a gray scale is achieved, and the color is provided by color filters and a DMD chip.
Other applications involve MOEMS-based scanners. Conventional optical scannersfor example, the type used in supermarket cash-registersare complex optical systems requiring careful alignment. It behooves the industry to explore ways to miniaturize the dimensions and the weights of these devices for novel applications.
Reducing the phase aberrations introduced when the wavefront travels through atmosphere to a telescope sharpens the resultant image. Adaptive optic systems combine a wavefront phase sensor, focusing optics, and a spatial light modulator to correct phase errors. The systems have become de rigueur for large ground-based telescopes.
Moving adaptive optics technology from its current astronomy niche to imaging and beam-forming systems for applications such as optical communications, biomedical imaging, aviation and aerodynamic control, and laser welding will require a reduction in fabrication cost by a few orders of magnitude. Engineers see the potential for this MOEMS-based technology to transform the field of adaptive optics. Modulation of an optical wavefront can be accomplished with a deformable mirror in combination with a wavefront sensor and a real-time controller. Most deformable mirrors now available with this capacity are macroscopic devices made with flat-glass mirror plates and are supported by actuators. Various types of micromirror arrays are being explored for this purpose.
According to the Defense Advanced Research Project Agency (DARPA), MOEMS have many potential military uses, for example as navigation units on a chip for munitions guidance and personal navigation. Used in sensors, the technology could allow engineers to develop distributed unattended sensors for asset tracking, border control, surveillance, environmental monitoring, and process control. Miniature analytical instruments in fluidics are of interest, as are fluidic systems in hydraulic and pneumatic systems, as well as propellant and combustion control. MOEMS may also appear in weapons systems, in embedded sensors and actuators for condition-based maintenance of machines and vehicles that warn when maintenance is required, and in systems for friend-or-foe identification systems.
Optical switches, variable attenuators, active equalizers, add/drop multiplexers, optical cross-connects, gain-tilt equalizers, and data transmitters are finding widespread application in advanced optical communications systems. To date, MEMS have not been deployed in these systems, but industry representatives say they are poised for great commercial success in this field.
MEMS and MOEMS technology brings the economies of batch processing to a wide range of applications. As R&D brings technologies currently under development to fruition, new applications will doubtlessly emerge in this fast-paced market. oe
Leo O'Connor is director of research with Technical Insights, a division of Frost & Sullivan, San Jose, CA.