M. Edward Motamedi

Micro-opto-electro-mechanical systems

Micro-optical devices such as diffractive and refractive microlenses fabricated using integrated circuit technology have been highlighted during the past five years. This scientific breakthrough has created a revolution in optical technology. Miniaturizing devices using microoptics promise to revolutionize many electrooptical systems-from video cameras, video phones, and compact disk data storage to robotic vision, optical scanners, and high-definition projection displays. The supporting technologies for these micro-optical systems, such as binary optics and plastic micro-replication, are built on existing or adapted integrated circuit technologies that have been developed over the past five years. In parallel with this optical technology revolution, researchers have also developed another frontier technology called micro-electro-mechanical (MEM). MEM technology has found applications in automotive, machine tools, airbag actuators, and many generic microsensors. Merging microoptics, microelectronics, and micromechanics creates a new and broader class of micro-opto-electro-mechanical (MOEM) devices, which may attract additional industrial demonstrations of commercial devices such as torsional mirrors, laser scanners, optical shutters, and dynamic micromirror displays. Both constituent technologies of MOEM devices have the potential of batch processing and embossed replication, which, again, makes them highly attractive for low-cost commercial applications. Other more complex microsystems that use MOEM devices such as microspectrometers, microinterferometers, and miniature on-machine-inspection subsystems are being investigated. This new technology enables high-performance devices for microsensor systems that are lighter, easier to produce, more efficient, and less expensive than conventional components.

Micro-opto-electro-mechanical devices and on-chip optical processing

Micro-optical components, such as diffractive and refractive microlenses, micromirrors, beamsplitters, and beam combiners, have recently received considerable attention in the optics R&D centers and finally in the manufacturing community. This achievement is due to micro-electro-mechanical (MEM) technology that has demonstrated major improvements in overall performance and cost of optical systems while offering the possibility of relativity rapid transition to products for military, industrial, and consumer markets. Because of these technology advances, an industrial infrastructure is rapidly becoming established to combine micro-optical components and MEM-based microactuators for on-chip optical processing. Optical systems that once were considered to be impractical due to the limitations of bulk optics can now easily be designed and fabricated with all required optical paths, signal conditioning, and electronic controls integrated on a single chip. On-chip optical processing will enhance the performance of devices such as focal-plane optical concentrators, smart actuators, color separators, beam shapers, fiber data distribution interface (FDDI) switches, digital micromirror devices (DMDs), and miniature optical scanners. We review advances in microoptical components developed at the Rockwell Science Center. We also review the potential of on-chip optical processing and the recent achievement of free-space integrated optics and micro-optical bench components developed at UCLA, and DMDs developed at Texas Instruments.

Miniaturized micro-optical scanners

Optical beam scanners are critical components for airborne and space-based laser radar, on-machine-inspection systems, factory automation systems, and optical communication systems. We describe here a laser beam steering system based on dithering two complementary (positive and negative) microlens arrays. When the two microlens arrays are translated relative to one another in the plane parallel to their surfaces, the transmitted light beam is scanned in two directions. We have demonstrated scanning speeds up to 300 Hz with a pair of 6-mm-aperture microlens arrays designed for input from a HeNe laser. The output beam covers a discrete 16 x 16 spot scan pattern with about 3.6 mrad separation and only 400 μrad of beam divergence, in close agreement with design predictions. This demo system is relatively compact; less than 2 in. on a side. We also describe several near-term applications, some critical design trade-offs, and important fabrication and design issues.

Development of micro-electro-mechanical optical scanner

Rockwell is working on the development of a micro-electromechanical optical scanner based on bimorph microactuators. This scanner is lightweight, is small, and has superior scanning performance. The scanner is a low-power (<1 W) device that has large scan angles (?20 deg) and scan rates in the range of 100 to 2000 Hz. It works for all wavelengths and offers the potential for monolithic integration with both electronics and optics for on-chip signal processing and control. The optical scanner consists of two main components—actuator and mirror— which are fabricated on a silicon cantilever beam. The actuator is comprised of a bimorph layer covered with two metal layers, which function as top and bottom electrodes. The mirror can be as large as 12 mm2 in area, is placed at the end of the cantilever beam, and is designed for maximum optical flatness. The optical efficiency of the device is very high and can exceed 90% on proper metallization of the mirror area. The scan angle is a function of beam thickness, power efficiency of the bimorph, and many other design criteria. Through many improvements in these design parameters, a scan angle greater than 20 deg is expected to be achieved with high yield.

Micro-optic integration with focal plane arrays

The large detector size of conventional focal plane arrays (FPAs) often acts as a limiting source of noise currents and requires these devices to run at undesirably low temperatures. To reduce the detector size without reducing the detector’s quantum efficiency (QE), we have developed efficient on-focal-plane collection optics consisting of arrays of thin-film binary-optics microlenses and photoresist-based refractive microlenses on the back surface of hybrid detector array structures. Photodiodes of p/n polarity, of an unusual planar-mesa geometry, were fabricated in epitaxial HgCdTe deposited by molecular beam epitaxy (MBE) on the front side of a CdZnTe substrate. Diffractive (8- to 16-phase-level) Ge microlenses were deposited on 48-µm centers in a registered fashion (using an IR mask aligner and appropriate marks on the front surface of the CdZnTe) on the back side of the substrate using a lifting process. The lifting circumvents some of the process limitations of the more conventional chemical etching methods on diffractivemicrolens processing, allowing the microlenses to approach more closely their theoretical efficiency limit of .95%. Photoresist microlenses were fabricated by reflow of photolithographically defined photoresist islands. Prior to microlens deposition, but after diode fabrication, the test structures were flip-chip bonded or ‘‘hybridized’’ using indium interconnections to metallic striplines that had been photolithographically deposited on sapphire dice (a process equally compatible with a siliconintegrated- circuit readout). After hybridization, the CdZnTe was thinned to equal the focal length of the lenses in the CdZnTe material. Optical characterization has demonstrated that the microlenses combined with the detector mesas concentrate light sufficiently to increase the effective collection area. The optical size of the mesa detectors being larger than the theoretical diffraction limit of the microlenses precludes determining whether the lenses themselves produce the theoretical diffraction-limited gain, but they clearly decrease the required detector area by at least 3 to 6 times. To our knowledge, this is the first successful demonstration of IR detectors and binary optics and of photoresist refractive-microlens integration.

MOEM scan engine for bar code reading and factory automation

Rockwell is in the state of technology transfer to manufacturing of a micro-opto-electro-mechanical scan engine with superior scanning performance for bar code reading and factory automation. The scan engine consists of three main components: actuator, mirrors, and control electronics. The first two components are fabricated on a silicon cantilever beam while the control electronics are presently hybrid. The actuator comprises of a bimorph layer covered with two metal layers. The mirror has a large area (several mm2) and it is micromachined with a surface flatness better than (lambda) /2. Actuator scan-angles greater than 22 degree(s) with high repeatability in performance are achieved. The scan engine was integrated with an existing Rockwell commercial bar code reader/decoder and successfully proven to read a two-character code 39 bar code. The system was capable of decoding the 13-mil label at 360 scans per second with a 100% successful read performance. Environmental testing of the device indicates that the scanner can operate at elevated temperatures up to 70 degree(s)C with minor fluctuations in frequency and scan angle. The scanner has also gone through a lifetime cycle test and it has survived more than 8 billion cycles during a period of 18 months. To increase the yield and the performance level of the device, theoretical study as well as dynamic simulation by finite elements modeling have been investigated and will be reported separately.

Micro-optics development in the past decade

The field of microoptics has grown into an important technology in the past decade as evidenced by the growing range of applications using microoptic compontents. The development of large-volume, batch-processed diffractive and refractive microooptics component manufacturing has created a technology that promises to revolutionize many electro-optical systems. Microoptics is an enabling technology for applications that cannot be addressed using conventional optics and is currently playing a significant role in numerous applications, including lightwave communications, optical interconnects, detector arrays, visible and infrared imagers, beam steering, and display systems. The trend toward miniaturization and integration of conventional optical systems will accelerate the adoption of microoptics technology in commercial, space and military systems. Since microoptic processes are compativle with integrated circuits, a broader range of applications is expected as microoptics become integrated with MEMS devices. Other more complex microsystems that use microoptic devices such as microspectrometers, microinterferometers and miniature on-machine-inspection subsystems are being investigated. This new technology enables high performance optical components that are lighter, easier to produce, more efficient, and less expensive than conventional components. We present a broad survey of developments in microoptics, including design, fabrication, and applications, in the last decade.

Guest Editorial: Special Section on Micro-Opto-Electro-Mechanical Systems

Recent industrial demands in the fields of microsystems and device miniaturization led to development of two major technologies, micro-electro-mechanical (MEM) and micro-optics. A promising team-up of these two technologies combined with microelectronics creates a rich enabling technology of micro-opto-electro-mechanical (MOEM) systems. All constituent technologies in MOEM allow for batch processing and embossing and include a micromachining process that makes them highly fascinating for commercial applications.

Surface micromachined devices for microwave and photonic applications

As an enabling technology, Micro Electro Mechanical Systems (MEMS) have continuously provided new and improved design/implementation paradigms for a variety of scientific and engineering applications. In this paper, we review recent advances made in MEMS and its derivative MOEM devices for both microwave and photonic applications.

Novel highly scalable and very high torque micromotor for MEMS and MOEMS applications using the mechanical rectification of oscillatory motion

A new micromotor that is powered by the mechanical rectification of oscillatory motion is conceived, designed and fabricated. Its operational characteristic is quite rich and exhibits various modes of excitations. The idea explored in the design of this novel device is completely different than other micromotors reported in the literature. It consists of a rotor with elastic fins and linear actuators located on its perimeter and it operates as follows. When the actuators move toward the fins, the friction between the actuator edge and the tip of the fins causes them to deform. This deformation applies a force to the rotor that has both tangential and normal components. The tangential components results in a twisting action on the rotor, setting it into rotation. When actuators move away from the fins, the fins are released and allowed to move back to their original position. Continuous rotation is achieved by matching the oscillations of the actuators with the movement of the adjacent fins. The torque of this device is quite large (mN.m for 0.5 micrometers rotor radius) and can be adjusted by the radius of the actuator, the length and elastic properties of the fins, the tilt angle of the fins with respect to the rotor, the force exerted by the actuators, the vibration frequency and the number of fins, and by adjusting the friction between actuators and the fins. Experimental data using a prototype micrometer will be given and various application of the micromotor in optical MEMS and fluidics will be discussed.