Lilliputian Machines Set To Revolutionize RF, Optoelectronics, and Biomedical Applications

Charles Small

TechOnline (02/21/2002 12:00 AM ET)

MEMS (micro-electromechanical-systems) technology is set to explode into a number of new and different areas, including RF, optoelectronics, and biomedical applications. MEMS, however, are not new. MEMS research and development has been going for decades throughout the world. During the early 1970s, bulk-etched silicon wafers were being used to commercially produce pressure sensors and in the early 1980's experiments began using surface-micromachined polysilicon actuators for disc drive heads. Analog Devices began shipping the first fully integrated, single-chip MEMS accelerometers for automotive airbags in 1991.

While MEMS manufacturing leverages IC manufacturing, the principal users of MEMS are expected not to be limited to the traditional electronics and computer companies. Instead system integrators and commercial or defense product manufacturers who make automobiles, scientific analytical instruments, consumer goods, medical devices, aerospace navigational systems, and similar products will benefit from MEMS technology.

Boom Markets Predicted
According to Cahners In-Stat Group MEMS devices accounted for $4 billion in revenue in 2000 and by 2005 the MEMS market will quadruple to $11 billion. In addition, MEMS-based photonic switches are predicted be the first MEMS device to surpass $1 billion by 2004. In the past year, 30 new optical-switch companies have emerged, garnering $1.2 billion in funding, according to consultant Jeff D. Montgomery of ElectroniCast. Consultant Roger Grace estimates that over 600 university, government, and industry labs are currently working on MEMS.

Unlimited Applications
One of the first MEMS applications was a tire pressure sensor. MEMS are now used in many more automotive applications ranging from airbag accelerometer sensors to fuel sensors, engine and brake controls, and noise cancellation. Another innovation is "intelligent tires" that alert the driver to a flat.

Multi-axis accelerometers, inkjet printheads, scanners, gyros, force and displacement transducers, and a wide range of electromagnetic devices such as solenoids can be made using MEMS techniques. Tiny inertial sensors fabricated out of silicon, which can withstand more than 15,000 g, show promise of converting "dumb" artillery shells into "smart" guided munitions. A wide range of MEMS devices for RF applications include switches and relays, inductors, transmission lines, filters, antennas, and transformers. MEMS devices for optoelectronics include 2-D and 3-D micromirrors, optical attenuators, and fiber alignment and packaging aids. MEMS will be used in microscopic assembly lines to build the next generation of optical communications equipment, biotechnology, and micro-electronic and micro-mechanical devices. Microfluidic devices with miniature pumps and valves could serve as small refrigerators that could remove heat from electronics.

The pharmaceutical industry is adopting MEMS devices rapidly for testing new drugs. Another potential medical application is blood-screening sensors that can perform complete lab tests at bedside. Biomedical applications include networks of channels, pumps, valves, and mixers for analytical devices. MEMS can also be used as molds for plastic microfluidic parts. MEMS devices are envisioned for miniature surgical tools, fluid dispensing heads, and drug delivery, and implantable sensors.

Government-Sponsored Research and Development
During the 1990s the U.S. government began sponsoring MEMS projects. The Air Force Office of Scientific Research (AFOSR) was supporting basic research in materials and the Defense Advanced Research Projects Agency (DARPA) started its foundry service in 1993. Additionally, the National Institute of Science and Technology (NIST) began supporting commercial foundries for CMOS and MEMS devices. Government interest in MEMS continues, with significant ongoing funding through agencies such as DARPA. The Japanese and European Governments have increased their support of MEMS R&D to a combined level of over $70 to $100 million a year. The combined European and Japanese industrial annual investments in MEMS R&D are estimated to be over $200 million and growing.

MEMS Produced by a Variety of Means
MEMS manufacturing takes advantage of high volume batch processing based on semiconductor techniques. There are many different processes currently being used to manufacture MEMS. They include surface micromachining, bulk micromachining, electro-discharge micromachining (EDM), and high-aspect-ratio micromachining (HARM) technologies such as LIGA (a German acronym for Lithographie, Galvanoformung, Abformung or lithography, electroplating, and molding).

Silicon surface micromachining uses the same equipment and processes as IC manufacturing, so it was one of the first processes adapted to MEMS. Applications for surface micromachining include actuators and electrostatic motors as well as mirrors and accelerometers. Polycrystalline silicon is a good material from which to make MEMS. Polysilicon has a strength of 2 to 3 GPa, depending on surface flaws, while steel has a strength of 200 MPa to 1 GPa, depending on the process parameters. Polysilicon is extremely flexible; its maximum strain before fracture is ~0.5% and it does not readily fatigue.

Figure 1: This microphotograph shows a MEMS Precision Instruments HEXSIL polysilicon tweezers gripping a polysilicon gear. The tweezers can manipulate micron-scale objects.

In the LIGA process, polymethyl methacrylate (PMMA) plastic is selectively exposed to radiation through a mask. This allows some of the PMMA to be washed away, leaving structures that are then electroplated with metal. The structures can be the actual MEMS device or can be used as molds. The LIGA technique has been used to produce electrostatic motors and gears.

Figure 2: The Sandia National Laboratories' SUMMIT four-level poly process allows many gear stages to be chained into a transmission to give the desired reduction. The output gear engages the rack on the side of the tensile tester frame.

The TiNi Alloy Company is developing heat actuated shape-memory alloy (SMA) thin film microdevices. The initial application of these microactuators is in miniature valves, but other potential applications include miniature connectors, switches, and end effectors for microrobotic manipulators. SMA film actuators can produce large forces and displacements within small spaces at voltages compatible with electronics.

Argonne National Laboratory researchers have developed a process for growing diamond film that promises to bring the superior mechanical, tribological, and thermal properties of diamond to MEMS. The properties of silicon are not suitable for some potential MEMS applications. This is especially true for devices that require extensive sliding and rolling contact, such as micromotors for aerospace applications, because silicon wears too quickly.

MEMS Miniaturize RF Components
The wireless industry is faced with a number of tough design challenges. A 3G "smart" phone, PDA, or base station, could require as many as five radios for TDMA, CDMA, 3G, Bluetooth, and GSM. Such additional functions entail an increase in component count. Yet at the same time the industry must meet consumer demand for smaller form factors, lower costs, and reduced power consumption.

RF MEMS offer reductions in size, power, and signal loss thereby extending battery life and reducing weight. Applications for RF MEMS include RF front ends, microwave filters, antenna arrays, phase shifters, transmit/receive switches, inductors, resonators, transmission lines, and waveguides.

Analog Devices and Cronos now make MEMS relay/switch devices that combine the low-loss and signal-fidelity benefits of electromechanical relays with the small size and low power consumption of solid-state devices. Recent measurements have demonstrated less than 0.5 dB of loss and greater than 30 dB of isolation for signal frequencies up to 10 GHz.

MEMSCAP's MEMS-based RF switch uses the company's proprietary membrane process to create a low-loss, low-power device. The RF switch is composed of a movable metallic membrane. When an electrostatic force is applied, the membrane is pulled down to complete the circuit. The "above-IC" configuration can offer higher isolation and lower power consumption than electronic switch technology. MEMSCAP's MEMS-based tunable capacitor also uses the membrane process to create a device offering an improved high Q value and tuning range compared to traditional technology.

Miniature passive RF devices are becoming available too. MEMSCAP's MEMS-based "above-IC" high-Q inductor designs achieve cost reduction, high integration density, and performance improvement using IC-compatible processes. These inductors eliminate discrete inductors from the circuit board and place them on top of the chip, helping to shrink form factors.

MEMS in Optical Communications
Significant progress has been made in increasing the amount of data that an optical fiber can carry with the amount of data that can be transferred doubling every eight months. This data increase sprung from advances in DWDM (Dense Wavelength Division Multiplexing) technology. DWDM enables the transmission of a large number of wavelengths over a single fiber"currently up to 160 discrete channels, with 1000 channels under development"and increased transmission speed from OC48 (2.5 Gbps) through OC192 (10 Gbps) to the emerging OC768 (40 Gbps) and planned 100 Gbps.

Switching this data is traditionally done by converting the optical signals to electrical signals, switching them electronically, and then converting them back to optical signals (optical-electronic-optical or OEO). Today's OEO electronic switches throttle the wide bandwidth advantage of the optical fiber. All optical switches eliminate this problem.

MEMS is an enabling technology for fiber-optic applications frequently referred to as micro opto electro-mechanical systems or MOEMS. MEMS-based mirrors, shutters, and fiber aligners are used in optical switches, tunable filters, tunable lasers and variable attenuators. Some examples of the network architectures that can utilize optical add/drop multiplexers (OADMs) are: linear add-drops for backbone DWDM networks, hubbed rings in metro access networks, and a logical mesh ring that allows dynamic path reconfiguration. Large port-count optical cross-connects (OXCs) will be used in central offices for dynamic remote network provisioning.

Umachines has introduced a 2-D 1x2 MEMS fiber optic switch. The MEMS 1x2 switch measures 38 x 16 x 11 mm and provides channel selection between one input fiber and two output fibers for a number of network functions, including protection, Reconfigurable Optical Add/Drop Multiplexing (ROADM), monitoring, and provisioning. This MEMS switch avoids both stiction and frictional wear, resulting in a switch that has successfully cycled over 100 million times without any detectable change in insertion loss. The device features low insertion loss (<0.4 dB typical, 0.6 dB maximum), low polarization dispersion loss (<0.1 dB maximum) and low power consumption (6 mW).

Whenever an optical signal is transmitted through free air, it runs the risk of being distorted when it moves through turbulence in the atmosphere. If uncorrected, this turbulence limits the useful range of the signal as the strength of the signal degrades with distance. A CMDM (Continuous Membrane Deformable Mirror) from MEMS Optical can correct for this turbulence in real time greatly extending the useful range of the signal.

MEMS Optical also makes moving mirrors for tunable lasers. When put inside the laser cavity, this mirror can be moved by up to 20 µm. Actuated by MEMS Optical's patented vertical comb drive, this allows a simple and effective way of changing the path length inside the laser cavity.

Micralyne makes Silicon V-Groove Chips that are used to align fiber optic cable with either an active laser device or arrayed waveguide (AWG). Micralyne offers standard chip products for 2, 4, 8, 16, and 32 fiber arrays and manufactures customized chips based on customer specifications.

Silicon Light Machines, a wholly owned subsidiary of Cypress Semiconductor, has developed Grating Light Valve (GLV) technology which is used for creating a high-performance spatial light modulator on the surface of a silicon chip.

MEMSCAP's variable optical attenuators (VOAs) are based on the company's switch technology. They use a miniaturized shutter mechanism located between optical waveguides or fibers that mechanically blocks the light path, reducing the amplitude of the signal without distorting the waveform. Further, MEMSCAP's tunable filters, expected to ship in the second half of 2002, are based on the principle of having one of two mirrors fixed to an actuator, enabling variation of the distance between the two mirrors forming the optical cavity. These filters are expected to have a steep cut-off to avoid crosstalk from adjacent channels. In addition, they are expected to be thermally stable with low insertion loss.

Texas Instruments' DLP (digital light processor) was originally developed for displays but is being adapted to optical-communications switches. It is based on a MEMS device called the Digital Micromirror Device (DMD). TI's DMD is a fast, reflective digital light switch that is fabricated by CMOS-like processes over a CMOS memory. Each light switch has an aluminum mirror that can reflect light in one of two directions depending on the state of the underlying memory cell. CiDRA has demonstrated its AgileWave dynamic spectral equalizer that uses TI's DLP technology.

MEMS in Biomedical Applications
Microfluidics, a MEMS technology, enables the fabrication of networks of channels, chambers, and valves to control the flow of liquids in amounts as minute as one picoliter. These systems have few moving parts and require little assembly. They offer the potential to miniaturize analytical equipment that uses expensive chemicals and DNA samples. They take advantage of physical phenomena such as electro-osmosis, dielectro-phoresis, and surface interaction effects.

Figure 3: Micralyne uses MEMS techniques to make channels in a substrate for microfluidics applications.

Electrokinetic flow is generated when electrodes attached to computer-driven power supplies are placed in the reservoirs at each end of a channel and activated to generate electrical current through the channel. Under these conditions, fluids of the appropriate type will move by a process known as electro-osmosis. Typical flow rates within the channel are about a millimeter per second and the flow rate can be controlled with a high degree of precision. Another electrokinetic phenomenon known as electrophoresis occurs in the microchannels. This is the movement of charged molecules or particles in an electric field. Electrophoresis can be used to move molecules in solution, or to separate molecules with very subtle differences. Pressure can also be used to move fluid in the channels. On the microfluidic scale, small amounts of pressure produce highly predictable and reproducible fluid flow.

Another way to connect to the molecular world is through control by light. When a molecular design is at the scale of the wavelength of light, interesting quantum behaviors emerge"for example, quantum dot lasers that emit light and bandgap crystals that switch light. Arryx fabricates 10,000 independently controllable "tweezers" that can manipulate molecular objects in 3D (move, rotate, cut, place), all from one laser source passing through an adaptive hologram. With thousands of miniature robot arms, Arryx can sort cells and proteins as well as manipulate the organelles and DNA inside a living cell. Arryx's first product is the BioRyx 200 system. Future products will include dynamically configurable biochips, cell sorters, purification equipment and optical switch/router components.

Micralyne makes the Microfluidic Tool Kit, a user-configurable instrument that is being used in corporate and academic research laboratories for customized bio-analytical applications in protein, DNA, and cellular analysis.

Verimetra has introduced the "Data Knife," a suite of surgical tools that incorporate sensing and measuring devices. The Data Knife combines sensing and data-gathering capabilities on the edges of various surgical tools. These instruments are capable of distinguishing tissues, such as cartilage, bone, muscle, and vascular, and also of measuring tissue properties, including density, temperature, pressure, and electrical impulses.

Figure 4: This Zyvex fully released X/Y stage is pushed or pulled around by the connectors on its four sides. After moving it to the desired location, the connectors can be moved out of contact with the moveable portion of the stage.

Advances in Processing
Eliminating the need for a conventional semiconductor clean-room fab, MEMGen offers a single tool called an EFAB that rapidly creates 3-D micromachines from a variety of materials. EFAB (Electrochemical Fabrication) uses a patterning technology called Instant Masking to generate microstructures quickly and without the need for photolithography. Instant Masking makes it possible to rapidly deposit an unlimited number of independently patterned layers. Together, these layers form virtually arbitrary, complex 3-D shapes, overcoming the geometrical limitations of conventional microfabrication. MEMGen has used its EFAB technology to fabricate a micron-scale device having 38 metal layers.

Things to Come
Improved simulation and modeling tools for MEMS design are urgently needed because currently MEMS design is often a trial-and-error process, requiring deep knowledge of the micromachining process employed. Also MEMS packaging presents unique challenges compared to IC packaging. Where conventional IC packaging serves to isolate the chip from the environment, MEMS devices often must be in continuous and intimate contact with their environment. Consequently a new and specialized package must be developed for each new device. In fact, packaging is often the single most expensive and time consuming task in a MEMS product development program. Packaging simulation tools need to be developed as well as standardized MEMS packaging.

About the Author Charles H. Small is a technical editor based in Waltham, MA.