Modern microelectronic semiconductors and integrated circuits provide remarkable computational and memory capabilities but lack the capability to interface with the physical world directly. MEMS (MicroElectroMechanical Systems) and the even smaller NEMS (Nano ElectroMechanical Systems) integrate sensors and actuators on microelectronic circuitry and enabling these products, awareness of the surroundings in the physical world, and allow them to perform physical tasks and send/receive inputs/outputs. Further,
Micro-Opto-Electro-Mechanical Systems (MOEMS) or optical microsystems are integrated devices or systems that interact with light through actuation or sensing at a micro- or millimeter scale. MOEMS merges MEMS with micro-optics as required for sensing and/or manipulating optical signals or light on a very small scale using integrated mechanical, optical, and electrical systems. Examples include digital light-projection devices and mini spectrometers, with applications such as projection systems, 3D printers, and instrumentation.
MOEMS are not a special class of Micro-Electro-Mechanical Systems (MEMS) but rather the combination of MEMS merged with Micro-optics. This high degree of integration is possible due to the progress in two areas, miniaturization of integrated circuits, which has been going for decades and now reached ultra large scale integration (ULSI) in which the size of the transistor is at the submicron scale and the development of micro-opto-electro-mechanical systems (MOEMS) and microelectromechanical system (MEMS) that can be fabricated in silicon using modified microelectronic processing derived from IC fabrication.
The MOEMS endow systems with the ability to alter or modulate the path of a light beam and in some cases, to temporally or spectrally modify the light beam. The movement of a micro-optical element permits the dynamic manipulation of a light beam. This dynamic manipulation can involve (amplitude or wavelength) modulation, a temporal delay, diffraction, reflection, refraction or simple spatial re-alignment. The most common micro-optical elements are those that reflect diffract or refract light.
The major advantages of MOEMS include faster switching and sensing operations, Wider bandwidth, multiplexed data transmission, low power consumption, reduction of the electrical interferences, and the lowest possible sizes. The reliability of MOEMS is higher than MEMS as reported in many pieces of research.
This has enabled the realization of new and complex structures that can be used as sensing, actuating or manipulating optical signals on a very small size scale using integrated mechanical, optical, and electrical systems. Examples include digital light-projection devices and mini spectrometers, with applications such as projection systems, 3D printers, and instrumentation.
These micro-optical devices and systems are inherently suited for cost-effective wafer-scale manufacturing as the processes are derived from the semiconductor industry. The ability to steer or direct light is one of the key requirements in optical MEMS.
MOEMS includes a wide variety of devices including optical switch, optical cross-connect, tunable VCSEL, microbolometers amongst others. Other commercialized applications include displays e.g. Head up displays, and vehicle displays; shutter arrays for switching; tunable filters for WDM / Broadband systems; and waveguides for Biomedical / diagnostics. These devices are usually fabricated using micro-optics and standard micromachining technologies using materials like silicon, silicon dioxide, silicon nitride and gallium arsenide.
The emerging applications of MOEMS shall be in portable devices, IoT devices, augmented reality systems and ‘Google Glass’ like contact lens.
The rapid, worldwide installation of optical fiber-based telecommunication systems has given rise to phenomenal growth in the number and size of manufacturers of optical components and devices. Initially, such manufacturers relied on costly precision-based engineering to produce optical fibre connectors, splices, and alignment structures. Such manufacturing techniques have, however, evolved to encompass
micromachining as the basis of manufacturing for low-cost, mass-produced components. Currently, micromachining methods, combined with IC-based processing techniques, enable the fabrication of complex opto-electronic integrated circuits and micro-electromechanical alignment devices in production quantities. Some of the components are:
The design of Optical Couplers, the components for coupling optical power from a single fibre (or optical source) onto a number (N) of output fibres is based on integrated optics waveguides (e.g. silica).
WDM Devices, are Wavelength selective components used for filtering out and/or separating specific wavelengths from within a frequency/wavelength band. Such components rely on the use of dichroic filters or gratings. The latter are, usually, micromachined onto the surface of optical substrates (e.g. silica).
Optical Switches / Routers & Shutters: Optical switches enable optical signals to be routed from N inputs to M outputs. The routing/switching mechanisms are, on the whole, based on micromechanical mirrors/shutters and /or positioning structures. In this context, microtechnologies form an essential part of such systems for both functionality and interconnection.
Optical Alignment Systems and Fibre Positioning Devices: Techniques for aligning optical fibre to either other fibre or to components are, increasingly, reliant on the use of micromachining and microtechnologies to satisfy the necessary sub-micron tolerances. Typically, such devices allow components (and fibre) to be micro-manipulated and positioned with high accuracy. Linear, rotational and 3-dimentinal movements are realised using microsystem-based mechanical transducers.
Micro-Lenses: Microtechnologies enable the fabrication of high precision lenses and lens arrays used for focussing and / or redirecting optical beams. Such micro-lenses may be used to maximise optical coupling between (laser) sources and fibre or between the input and output fibres of an optical switch. The lens structures are most likely to be integration with precision alignment microsystems.
Some of the companies developing Optical MOEMS for telecommunication applications are ADC Telecom, Alcatel, Astarte, Axsum, C Speed, Calient, Corning / Intellisence, Cypress, Semiconductor / Silicon Light Machines, Fitel Technologies (Furukawa), Ilotron, Integrated Micro Machines, Ioλon, JDS Uniphase / Cronos, LightConnect, Lucent, Luxcore (Synchordia), MemLink, MEMSCAP S.A., Nanovation, Nortel / Xros / CoreTek, Onix Microsystem, Optical Micro Machines, PHS MEMS, Siemens, Standard MEMS, Zygo TeraOptix.
Of late, the advances in the area of Micro-Opto-Electro-Mechanical Systems (MOEMS) based, devices are utilized widely for various applications in the area of biomedical applications.
MOEMS devices in wearables, allow the measurement of things like oxygen, CO2, and glucose levels in the blood, pulse rate, and so on. It’s possible they could enable a small device in the home which will tell you whether you should go to see a doctor.
The glucose concentration can be determined by analyzing the optical signal changes in wavelength, polarization or intensity. Non-invasive optical measurement of glucose is performed by focusing a beam of light onto the body. The light is modified by the tissue after transmission through the target area. An optical signature or fingerprint of the tissue content is produced by the diffuse light that escapes the tissue has penetrates
Some MOEMS approaches use optical components other than mirrors. For example, startup Aeponyx of Montreal uses electrostatically actuated MEMS and silicon nitride waveguides. In response to changing voltages, these waveguides bend and move, redirecting light. Aeponyx is bringing a product based on this approach to market in 2019 with a micro-optical switch intended for use in telecom access networks and data center applications.
“For the switch itself, our key advantage is our switching speed compared to 3D MEMS,” said Philippe Babin, CEO of Aeponyx. He added that their approach also offers a tenfold reduction in cost and a smaller size for their solution.
The technique requires the use of stress compensation and management in the MOEMS structures, which consist of silicon nitride in a silicon dioxide sandwich sitting atop a silicon-on-insulator wafer. The arrangement leads to stress in the films — this must be accounted for.
MOEMS play an important role in shaping the future of display technologies, in general, and micro-displays in particular. In general, these applications can be classified as follows: Embedded Direct View Systems, where the image on the display is directly viewed by the
observer, Front Projection And Rear Projection Systems, where a real image is projected onto a screen and viewed from front or rear side, and Near Eye Applications, where a virtual image is projected internally within the eye.
The following MEMS technologies are currently being developed for possible micro-display applications: Micro-mirror arrays (“MMAs”), Grated Light Valves, Liquid Crystal on Silicon (LCOS), and Field Emission Displays (“ThinCRT”).
Micromirror arrays are monolithically integrated MEMS structures fabricated over a CMOS control circuit. The MEMS structure consists of a silicon array of aluminized mirrors that can be rotated between two angles( TI approach: ± 10°). When the mirror is in its on-state, light from a projection source is directed towards a projection lens to appear as a pixel on a projection screen. In the off-position, the light is directed away from the lens and the pixel appears dark.
Micromirror is a kind of optical—Microelectromechanical systems (MEMS) device that has a vast array of applications due to its high performance, low production cost, small size, and low power consumption. It has been widely used in laser projection, barcode scanners, confocal microscopy, finger printing sensing, head-up displays, optical coherence tomography (OCT) and Lidar.
For the application of laser projection, the displays are created by using a micromirror to scan a modulated light beam to cover the desired field of view. The micromirror can be a single dual-axis mirror or two separate, orthogonal single-axis mirrors that include a fast scan mirror and slow scan mirror. The light consists of three laser colors, red, green and blue (RGB). RGB lasers need to be modulated simultaneously, in
order to create the proper color mix for each pixel.
A scanning laser projector is an example of a resonant system. It needs a mirror which is resonant in at least one dimension. In order to keep the micromirror at a resonance frequency and phase, it is necessary to have a feedback signal to conduct closed-loop control. The common
feedback methods include optical, electrostatic, magnetic and piezoresistive.
A 12- × 6- × 6-in. footprint micromirror MOEMS-based portable TV system can project a 70- or 100-in. picture on the wall, making it possible to avoid the problem presented by large flat panels without sacrificing visual quality. “The TV looks just as good as a flat-panel display or even better,” Mignardi said. “It’s a laser display so the colors are really good.”
LIDAR (Light Detection and Ranging) Systems
One of the most promising application spaces for Optical MEMS technology is LIDAR Systems, which employ light in the form of a pulsed laser to measure ranges (variable distances) to an object of interest. Traditional LIDAR systems have been used for applications like geo-spatial mapping, to sense and record data about the topography of the earth and major landscape features in a given region. More recently, novel and exciting applications have emerged in areas like Autonomous Driving. As automakers vie with one another to develop the self-driving car, many of them are relying on cutting-edge MEMS LIDAR systems with 2D MEMS Scanning at their core. AGM’s Optical MEMS technology provides viable solutions for this functionality.
A team at MIT’s Photonic Microsystems Group have integrated LIDAR systems onto a single microchip that can be mass produced in commercial CMOS foundries yielding a potential on-chip LIDAR system cost of about $10 each. Instead of a mechanical rotation system, optical phased arrays with multiple phase controlled antennas emitting arbitrary beam patterns may make devices more robust.
Our device is a 0.5 mm x 6 mm silicon photonic chip with steerable transmitting and receiving phased arrays and on-chip germanium photodetectors. The laser itself is not part of these particular chips, but our group and others have demonstrated on-chip lasers that can be integrated in the future. In order to steer the laser beam to detect objects across the LIDAR’s entire field of view, the phase of each antenna must be controlled.
In this device iteration, thermal phase shifters directly heat the waveguides through which the laser propagates. The index of refraction of silicon depends on its temperature, which changes the speed and phase of the light that passes through it. As the laser passes through the waveguide, it encounters a notch fabricated in the silicon, which acts as an antenna, scattering the light out of the waveguide and into free space. Each antenna has its own emission pattern, and where all of the emission patterns constructively interfere, a focused beam is created without a need for lenses.
Virtual Reality / Augmented Reality
The highly-charged, developing field of Virtual Reality / Augmented Reality brings together a number of cutting edge technologies to deliver a singular user experience that is unlike any that had ever been possible before.
By gathering sensory and spatial data from the user’s surroundings, the VR / AR system is able to generate a virtual model of the user’s environment that can be manipulated, diminished, augmented, or entirely replaced. For the Virtual environment to function properly, advanced sensing and continuous tracking of movement and orientation is required. The augmented reality uses involve projecting data onto surfaces, objects, or even into the eye. This allows data to be overlaid on top of a scene, thereby presenting the user with information about nearby objects of interest.
Optical MEMS devices using Micromirrors and related technology play a crucial role in enabling such functionality and AGM is well positioned to meet this challenge. Bosch Sensortec of Reutlingen, Germany, has its own MOEMS mirror technology, with possible applications in virtual user interfaces and augmented reality, according to Lucas Ginzinger, head of the Optical MEMS product area. The company’s technology, he said, makes it possible “to project a user interface to almost every surface you can imagine, even if curved or graded. Our interactivity solution enables a wide range of possible input, from classic touch to gesture control.”
Optical MEMS Micromirrors Could Be a Driving Force in “Smart Glass”
Between residential and commercial infrastructures, buildings accounted for 40% of the total U.S. energy consumption in 2019. Of this 40%, the majority of the energy is spent on heating and cooling the building. In an effort to conserve this energy, smart glass—glass with light transmission properties that can be dynamically changed—has become a hot research topic in recent years. While the technology is nowhere near perfect, recent research from the University of Kassel shows how micro-opto-electromechanical system (MOEMS) mirror arrays may open doors for smart glass technology.
Smart glass refers to glass, often for windows, that can change its light transmission properties based on its environment. For example, on a winter day, smart glass allows more light to enter a room to naturally heat it. On a summer day, the glass may allow less light to filter through, keeping the room cool. There are currently many techniques used to accomplish this feat. Techniques are either active (requiring electrical stimulation) or passive. Among active techniques, polymer dispersed liquid crystal (PDLC) glass, suspended particle device (SPD) glass, and electrochromic (EC) glass, are the most popular.
Another technique commonly investigated is micro-opto-electromechanical (MOEMS) mirrors, also known as optical MEMS. Optical MEMS is a technology that uses microscopic, electronically-controlled mechanical devices to redirect light. Built on a semiconductor substrate, the technology uses control circuitry to produce currents that generate magnetic forces to manipulate the mechanical part as desired.
Micromirror Arrays Sandwiched in Window Panes
In a paper published in the Journal of Optical Microsystems, researchers from the University of Kassel have unveiled their findings on a new form of smart glass that leverages millions MOEMS invisible to the naked eye. The micromirror array is placed in between window panes, where the orientation of the mirrors is controlled by the voltage between respective electrodes. The system also relies on motion sensors in the room to detect the number and position of people in the room, steering light accordingly.
Researchers claim this technique yields high actuation speeds (<1ms), 40 times lower power consumption than PDLC displays, and overall energy savings of 35% for buildings in daylight. The researchers also claim that their technique allows for a 30% reduction in a building’s CO2 production.
Grating Light Valve
This technology was invented at Stanford University and is proprietary to Silicon Light Machines, CA. A Grating Light Valve pixel consists of an array of small ribbons, which can be moved up or down over a small distance by electrostatic forces. The ribbons are fabricated by surface machining MEMS techniques. These “ribbons” are arranged such that each element can either reflect or diffract light, hence, a beam of
light can be switched between two directions at a very high speed. At present, only linear arrays are available. A microdisplay consisting of a grating light valve will require an additional linear scanner. The principle relies on the availability of cheap RGB laser sources.
Liquid Crystal On Silicon (LCOS)
Microdisplays based on LCOS combine two mature technologies: IC and LC. Displays are constructed from a CMOS backplane (driving electronics) and a Liquid Crystal array. The devices are assembled in wafer-scale, allowing for a “fabless” business model, where design and integration companies, CMOS foundries and LC foundries operate together.
Microdisplays are realised by two approaches: transmissive LCOS, where the light passes through the chip, and reflective LCOS (rLCOS) as shown below. Reflective – rLCOS – offer several advantages including higher brightness, virtually no pixelation, and the possibility of higher electronics integration. As LCOS combines electronics, optical and micromachined functions (e.g. spacers), these structures are to be considered as microsystems.
Field Emission Displays (“ThinCRT”)
This emerging technology is a combination of the familiar screen technology of a cathode ray tube (CRT) with a low-power, miniaturised cold cathode array to form a display that is only a few millimeters thick. Each pixel consists of thousands of emitter tips of approximately 0.15 µ in diameter. These tips emit electrons that are accelerated towards a phosphor screen on the faceplate to appear as a bright spot to the
viewer. In contrast to the previous three technologies, which are spatial light modulators and require an illumination source as well as an optical projection system, Field Emission Displays are direct view systems.
3D Imaging based on Optical MEMS Technology
A number of novel techniques for generating high-quality 3D images make use of Optical MEMS Technology. Structured light is generated using Optical MEMS devices and projected onto an object of interest. The light reflected from the object is then captured with a camera. Through the application of advanced image processing algorithms, a detailed 3D profile of the object is inferred. In this way, users are able to generate precise 3D images of objects of various sizes including some that are too small to see with the naked eye.
Three-dimensional sensing, for instance, could be an application that would benefit from the ability to steer light in precise patterns to enable structured illumination. Measuring the distortion of a known light pattern, such as beams that intersect to form squares, can yield 3D information about a surface or object. Machine vision is also a possible application.
MOEMS accelerometers combine optical measurements with the micro-electronic mechanical system (MEMS) technology. MOEMS technology has gained increasing attention in the scientific community due to its wide variety of advantages, such as immunity to electromagnetic interference, electrical insulation, corrosion resistance, remote sensing, high sensitivity, and multiplexing ability. A wide range of applications exists for of this type of accelerometers, including inertial navigation with high accuracy, vibration sensing of vehicles, seismic sensing, and oil-field applications.
In recent decades, many schemes using different optical techniques have been introduced, including grating interferometry, Fabry–Perot cavity, fiber Bragg grating, photonic crystal nano-cavity, light force, an evanescent wave (EW) coupling. Among these above-mentioned methods, interferometry accelerometer designs using micro-gratings have the potential to achieve more compact size and a higher sensitivity compared with other types,
Interferometers based on diffraction gratings have been widely used in displacement measurement. Since the low thermal expansion coefficient of the grating on a quartz substrate, grating interferometers have the features of high reliability, high sensitivity, and
small zero deviation drift. In order to further improve the performance of the grating accelerometer, the acceleration and displacement sensing element of the elastic coefficient, the mass block, and the cross-axis suppression ratio needs to be further optimized. The displacement sensor system needs to be more precisely modeled, making the sensors work at the best working point. In addition, a low noise
interface circuit should be applied, and noise from the detectors and laser, such as 1/f noise, thermal noise, and relative intensity noise (RIN) should be carefully considered.
Military applications and challenges
Military applications include Internal navigation units on a chip for munitions guidance and personal navigation. Weapons sating, arming, and fuzing to replace current warhead, systems and improve safety and reliability, embedded sensors and actuators for condition-based maintenance of machines and vehicles, on-demand amplified structural strength in lower weight weapons systems disaster-resistant buildings. Distributed unattended sensors for asset tracking, border patrol, environmental monitoring surveillance, and
Whilst the important role of MEMS is confirmed for future military platforms, further developments in the design and performance of these devices is, however, necessary in order to satisfy the stringent requirements set for military applications. More specifically (and typically):
Military specifications (including aircraft, missiles and munitions) are particularly demanding (for example):
- Vibration: 20 to 3,000 Hz (for 5g to 20g)
- Structural Resonance: > 3,000 Hz
- Temperature: -65oC to > +125oC
- Mechanical shock: up to 100g for fighter aircraft up to 300g for missiles more than 15,000g for gun launched munitions
- Angular Acceleration: >500,000 rad/S2 (spinning gun launched munitions)
Other, more generic, challenges will also need to be addressed, namely:
- Military MEMS will depend, heavily, on the commercial/civil MEMS developments as low volumes, for the military markets, will attract high costs.
- Military product life cycles exceed those for commercial/consumer products where both process availability and product obsolescence become a major concern.
- Access to military-specific MEMS developments by the civil markets may have security implications.
- Repair of MEMS is not, normally, feasible and diagnostics is difficult.
In spite of these hurdles, there is little doubt that microsystems will proliferate within military platforms providing intelligent functionality and enhanced performance.
MOEMS promise better systems and solutions, but there is a need for further improvement and innovation to address integration and cost.
In conventional bulk and surface micromachining technologies, MOEMS, MEMS, or NEMS devices are processed in bulk silicon substrates or are deposited and processed on top of silicon substrates. However, many crystalline photonic materials and other high-performance MEMS and NEMS materials cannot be deposited directly on silicon substrates and thus cannot be integrated in this way.
“However, both volume increase and innovative technologies, like WLP, or wafer-level packaging, will drive the cost down for a wider market diffusion,” he said.
Wafer-level heterogeneous integration technologies for microoptoelectromechanical systems (MOEMS), microelectromechanical systems (MEMS), and nanoelectromechanical systems (NEMS) enable the combination of dissimilar classes of materials and components into single systems. Thus, high-performance materials and subsystems can be combined in ways that would otherwise not be possible, and thereby forming complex and highly integrated micro- or nanosystems. Examples include the integration of high-performance optical, electrical or mechanical materials such as monocrystalline silicon, graphene or III-V materials with integrated electronic circuits.
Eric Mounier, senior technology and market analyst for MEMS and Sensors at Yole Développement, predicts a bright future for the technology. “The MOEMS market was $865 million in 2016, with more than eight million units shipped,” Mounier said. “In 2022, Yole Développement forecasts market value will be $1249 million with almost 30 million units shipped. It is almost five percent CAGR [compound annual growth rate] in value.”
Most of today’s market is in projection, with micromirror MOEMS devices the bulk of current and forecasted future shipments. Today, micromirror devices hold a commanding position in such applications as cinema. Future growth prospects for this and other types of MOEMS lie elsewhere, including automotive applications, and also with ultrashort throw laser TV (or mobile TV), where a small, portable projector is used to create a large viewing screen on a nearby surface.
The Miniaturized spectrometers segment is growing faster than the overall molecular spectrometer systems market: the total molecular spectroscopy market will experience a 7% annual growth rate while the market of mini and micro spectrometers is expected to grow at a CAGR2015-2021 of 11%, to reach almost $ 300 M in 2021. This report from Tematys, specifically studies the market and the applications of miniature and micro spectrometers.
To reach these applications, technological breakthroughs are necessary. Recent progresses take advantage of new micro-technologies such as MEMS (Microelectromechanical Systems), MOEMS (Micro-Opto-Electro-Mechanical Systems), micro-mirror arrays, Linear Variable Filters or integrated Photonics to reduce cost and size of spectrometers while allowing good performance, improved robustness and high volume manufacturability.