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MEMS — A Small World with Big Opportunities SwRI engineers take on the big challenge of working on the very small scale By Heather Hanson I n modern technology applications, sometimes smaller is better — and microscopic is better yet. Microelectromechanical systems (MEMS) have moved out of the basic research arena to become a useable technology. What are MEMS? The short answer is microscopic machines; however, they are much more diverse than that. The current driver in MEMS applications is the ability to make an existing device microscopically small, or to create a new device that would not work if it were inches in size but that works well at the micron scale. MEMS devices also potentially can be manufactured for low cost at high volumes, mirroring the semiconductor industry upon which they are based. MEMS are an enabling technology, a building block for solving problems in nearly every technical field. They often are used to make sensors, including the air bag sensor in most modern automobiles. In other applications they interact with their environment to change it in some way, such as an ion propulsion device that can move tiny satellites in space or an optical system that diverts light beams. Sometimes, they interact with themselves, such as the timed-locking mechanism on a nuclear warhead. Such a mechanism comprises 6 Technology Today • Winter 2004 D1M014463_1533 An SwRI-developed MEMS device undergoes testing in a probe station. The one-centimeter-square chip contains more than 60 different mechanical devices. The microprobe tips provide drive signals and sensing connections to the chip. MEMS methodologies D014463_1578 MEMS devices are fabricated using methods originally created to process the small features necessary for integrated circuits. These processes require a layerbased design — meaning that threedimensional structures are created by Heather Hanson, shown in front of a scanning electron microscope, is a senior research engineer in the Medical Systems Department in the Automation and Data Systems Research Division. Hanson has designed mechanical components, assemblies, packaging and fixtures for single unit, low volume and high volume production. Her work in microelectromechanical systems includes actuator and sensor design and development. RELIEVING STRESS MEMS technology detects crack growth Many engineering structures, such as pressure vessels, pipelines, bridges, boats and aircraft, are subjected to stress in a corrosive environment. Because these conditions can cause structural failures, experimental models have been developed to determine a structure’s susceptibility to stress corrosion cracking (SCC) and to measure the rate of crack propagation. One limitation of these models is that crack propagation may occur at a rate slower than the current limit for measurement. Rates below that level are too small to accurately predict whether SCC may lead to component failure during the structure’s expected life cycle, given current test methods. However, small, lightweight sensors installed in critical areas of an aircraft could alert maintenance crews to replace a part before failure occurs. Using MEMS technology, a miniature SCC test beam can be fabricated out of the structural material of interest and mounted on the MEMS chip. Such a test beam can detect crack growth based on either a resistive or capacitative measurement as a static stress load is applied to the test beam prior to installation, and either a static or a cyclic load is imparted to the beam Numerous cases of structural failures from during testing. stress corrosion cracking (SCC) have been reported in boilers, pressure vessels, oil and Engineers at SwRI have built gas production and transmission piping, a system that can detect the nuclear power generation components, small change in resistance that bridges, sea craft, aircraft and more. SwRI results from a MEMS-based SCC developed a MEMS device using structural sensor made of brass, operating engineering materials for sensitive crack growth in a corrosive environment. The rate measurements that facilitate SCC sensing system’s usefulness depends on and monitoring. A U.S. patent is pending. the structural materials available for the MEMS fabrication process. As a crack grows, the cross-sectional area of the test beam is decreased. This change in cross section affects the resistivity of the test beam, which can be detected by the sensor. Because the test beam is so small, a small crack has a relatively large effect. This method is useful provided that the structural material is electrically conductive. To make a capacitive measurement, a conductive plate is attached to one end of the test beam. This plate overlaps a series of stationary conductive plates on the chip. After a constant load is applied, the conductance can be measured between the test beam and the stationary plates to determine the position of the end of the test beam. As a crack begins to form, the test beam weakens and the applied load causes it to move more than it did initially. This additional motion is correlated with the crack growth to obtain a growth rate. Again, because the test beam is so small, a small crack has a relatively large effect on the beam’s motion under a given load. The MEMS fabrication process used by SwRI engineers is derived from the semiconductor industry and is material-dependent. The established processes are designed for conductive materials that have a minimized corrosion risk. The fabrication processes would have to be improved for materials such as aluminum and stainless steel before the sensor would be commercially viable. SwRI is pursuing funding to achieve this goal. Technology Today • Winter 2004 D013892 gears and linkages that can open a switch with the correct electrical input. Because they are micro-scale, they can be installed in small spaces. They also can interact with molecules, opening up a new realm of possible chemical, biological and medical applications. MEMS researchers at Southwest Research Institute (SwRI) outsource the production of MEMS, enabling them to select the process steps best suited to the design of a particular device. The selection of a fabrication facility is an early part of the design process. This process determines such requirements as layer thicknesses, number of layers, materials, minimum feature sizes, residual stress and final package size. As a multidisciplinary research and development organization, SwRI provides the wide range of technical expertise needed to design, develop and package MEMS devices for a variety of applications. 7 IN MOTION MEMS actuators fabricated for optical and mechanical applications D014584_9398 One of the key abilities of MEMS devices is that they can produce mechanical motion on a very small scale. This provides the means to create compact actuators with motions in the range of nanometers to millimeters and with the ability to generate forces up to the milli-newton range. MEMS actuators are generally based on either electrostatic or thermal operation, although piezoelectric, magnetic and even hydraulic types also have been demonstrated. An electrostatic actuator relies on the potential difference between a movable and a fixed surface and exerts a force on the movable surface. These actuators are very low power (microwatts) but require high voltage, sometimes in excess of 100 V, to generate a sufficient electric potential. Their response time is fast — tens to hundreds of microseconds — but they can be limited in their maximum displacement. Such actuators have a “pull-in” voltage limit whereby they fully deflect after moving a certain portion of their range, making this type of actuator most suitable for binary positioning. The well-known comb drive actuator is of this type. Thermal actuators, on the other hand, use Joule heating to produce thermal expansion. They have only a slightly slower response time than electrostatic actuators and can be operated over their entire range of motion. One of the first MEMS projects at SwRI was an internally funded development study of several of these micro-actuators and their use in switching applications. Actuators were designed based on both electrostatic and thermal methods, with a variety of force-displacement characteristics and the ability to produce both horizontal and vertical motion. Also developed were two novel actuators that have since been patented by SwRI — a singlematerial, in-plane, bi-directional thermal actuator and a vertical thermal actuator based on a bi-metal design. One switching device produced with the actuators from this project was an optical switch. It was designed to switch light between any of several input collimated or parallel optical fibers to any output fiber. The switch was based on a two-dimensional array of micromirrors mounted on the end of electrostatic out-of-plane actuators. The mirrors, which are smaller than the head of a pin, were fabricated flat, but were flipped and latched into position by a hinge mechanism on the end of the actuator arm. The switch was scaleable to allow additional input or output fiber channels. Such a switch can be used as a cross-connect in fiber optic communications systems or in optical scientific instrumentation. Several sponsored programs for development of unique microstructures and sensors have benefited from this internal research. Some of the actuator designs have been used directly in the development of the SwRI “materials lab on a chip.” The handling and testing techniques, design procedures and measurement tools developed under the internal research project laid the groundwork for many subsequent MEMS projects at SwRI. Although this MEMS device has more than 100 functioning mechanical devices, the entire chip can rest on a fingertip. MEMS devices are incredibly small yet highly functional. 8 Technology Today • Winter 2004 stacking layers of material(s). Initially the materials were polysilicon and metals, but the technology has progressed to include numerous other materials, including polymers. Typically, the layers range in thickness from 1 micron to 1,000 microns, depending on the fabrication process. The diameter of a human hair is around 90 microns, fitting easily in the realm of MEMS. The most common processes in use today involve photolithography, or etching a design into photosensitive materials. In this process, a structural material of constant thickness is deposited onto a substrate chip. A photoresistant material is applied onto the structural material in a particular pattern of interest. The structural layer is then etched according to the photoresist pattern, and the photoresist is removed. This process is repeated until the desired layers have been placed. Some of the materials deposited throughout this process are “sacrificial.” When the deposition and photolithography processes are complete, the entire chip is exposed to an etchant, which removes the sacrificial layer(s), thus making room for the other layers to move. There are many variations of this basic process, which can be categorized into bulk micromachining, LIGA (a process that makes parts with significant depths from metals, metal alloys, plastics or ceramics) and surface micromachining, to name a few. SwRI has extensive experience in designing MEMS devices. Outsourcing allows Institute staff members to custom design the devices and select the MEMS methodology best-suited for a particular project. must have time to cool between cycles. It can be used for discrete motions, similar to a mechanical relay switch. MEMS devices also can be activated in other ways. A magnetic field can be applied to attract a magnetic material. A Sacrificial Material Layer Substrate Methods of actuation Researchers can make small devices: but how do they work? MEMS devices employ several basic methods of actuation. The most common is the electrostatic method, which requires high voltage (more than 100 volts) but little current. A receptor plate is charged, which causes it to be attracted to a grounded plate. One or both of these plates can move toward the other. Sometimes these plates are bonded on one end, forming a cantilever beam or a series of interdigited comb fingers. In other cases, they can be spokes on a wheel that the electrostatic attraction causes to turn. This attraction produces the motion and is repeated to obtain a desired effect. This effect may be raising and lowering mirrors in an optical device at a particular frequency, pumping fluid through a microchannel or operating a micromotor. Joule resistive heating is another common actuation method that requires a more moderate voltage and current (about 5 volts, about 5 milliAmperes). This method can be used to produce devices containing either a bimetallic construct or a single material of varying widths. The bimetallic version requires two materials to be bonded together that have differing thermal expansion coefficients. When the current is applied, the two materials expand at different rates, causing a straight beam to bend in the same manner as its large-scale counterpart, the thermostat strip. A single-material version must use varying thicknesses or widths for the two beams that are attached to each other on one end. The difference in cross-sectional area causes the arms to heat at different rates, producing the same beam-bending effect. Unlike the electrostatic method, joule resistive heating cannot be used to resonate a structure because the structure voltage can be applied to a device made from a piezoelectric material to cause motion. In the area of microfluidics, a stationary actuator may cause the motion of the fluid. For example, an ultrasonic pump uses piezoelectric material to create a Patterned Photoresist After 1st Etching Sacrificial Layer Material Layer Patterned Photoresist Material Layer After 2nd Etching Sacrificial Layer Completed Micromachined Profile After Etching Sacrificial Layer Substrate The illustration depicts the steps used in the surface micromachining process. A layering process is used to create MEMS components and obtain air gaps between layers that allow the components to move. Technology Today • Winter 2004 9 Small size, big challenges The small size of MEMS devices creates design challenges. One of these is “stiction,” the result of two surfaces not having great enough stiffness to overcome surface adhesion. During processing, liquids can stay between layers rather than drying out, pulling the layers together via D014669 surface acoustic wave. This wave, also known as acoustic streaming, causes motion in the contacting fluid. Electrohydrodynamic pumps use a traveling electric field to pull a dielectric fluid along a desired channel path. This pump and similar electrokinetic pumps are used in biomedical applications for such things as separating DNA molecules. The Institute developed a bi-directional horizontal thermal actuator. This patented device can move in two directions upon the application of the electrical current. NEW METHODS SwRI techniques enhance quality, reliability in MEMS technology MEMS technology is entering a critical transition period from research to widespread commercialization. While several of the simpler devices are being mass-produced, such as inkjet printer heads and airbag sensors, the more complex designs of new MEMS applications present significant challenges for manufacturing. Although current semiconductor processes are well suited to reliability at high volumes, MEMS devices that require movement are affected by additional parameters, such as fatigue strength, failure strength and the elastic modulus. A material’s elastic modulus is determined by calculating the ratio of stress to strain or by determining the relative stiffness of a material within its elastic range. SwRI has developed techniques to assist MEMS designers and manufacturers in these areas. Designers need to validate that their designs can meet lifetime requirements, while manufacturers need to monitor important properties to ensure that their processes are producing high-quality components. The elastic modulus is one of the most important material properties required for MEMS design. It is used to set design limits on stress, strain and displacement. SwRI developed a technique that employs a resonating cantilever beam with an end mass on the MEMS chip. The beam is driven with a sinusoidal motion from a MEMS actuator comprised of interdigited fingers, or a comb-drive. As the drive signal frequency approaches the natural frequency of the beam, the amplitude of the vibration increases dramati- 10 capillary forces. Once the surfaces are held in contact in this manner, the devices are useless because the layers are not strong enough to overcome the capillary forces holding them together. Even room humidity can be the source of the liquid agent that causes this phenomenon. This can happen in varying degrees, and is most often seen as a type of friction that requires more force to overcome than had been planned. As described earlier, MEMS devices are designed and fabricated using layers. The 3-D structures are made from these layers. Therefore, devices that need to stand up off the chip, such as mirrors, must be made flat and be able to lift up on their own or by an actuator on the chip, because they are too small to be effectively handled manually. MEMS devices can be manually activated using a microscope and special probes, but it is only practical to do cally. The natural frequency is detected visually, or by using a laser-driven, optical system. Using the detailed computer model developed at SwRI, the elastic modulus can be determined. The test method was validated using atomic force microscopy. Using SwRI’s Displacement Mapping Software (DISMAP), three-point bending notch specimens are used to perform strain field measurements. A scratch drive actuator (SDA) MEMS device is used to apply a load to the beam. The beam deflects under the load and a digital image is captured before and after the load application. The digital images are imported into the DISMAP software to measure the motion of the grains on the surface of the beam and the strain field of the material. These parameters are important to understanding when a device will break under a given mechanical load. Fatigue strength is most important for MEMS devices that operate in resonance, such as those in telecommunications applications, where large numbers of operational cycles are quickly accumulated. The Institute designs notched beams, attached to comb-drives, to determine this parameter based on the research of Van Arsdell and Brown1. The devices are operated at their natural frequency until failure occurs. The test devices are combined with advanced digital signal processing to automatically measure the natural frequency and amplitude, and to count the elapsed fatigue cycles. Using probabilistic techniques developed by SwRI, parameters derived from these devices can be used for reliability assessment and failure prediction. 1 W.W. Van Arsdell and S.B. Brown, “Subcritical Crack Growth in Silicon MEMS,” IEEE Journal of Microelectromechanical Systems, Vol. 8, No. 3, Sept. 1999, pp. 319–327. Technology Today • Winter 2004 Technology Today • Winter 2004 11 D006708 this in a research laboratory setting, such as at SwRI. The manufacturing process causes another difficult challenge for MEMS. When a layer of material is deposited, all of the atoms in the layer are not in the same energy state. This results in a compressive or tensile stress inside the layer, known as residual stress. After the sacrificial layers are released from the chip and the structural layers can move, structures made with high residual stresses will bend upward, off the chip (tensile) or downward toward the chip (compressive). Devices can be designed to make use of residual stress, but researchers usually design around it. Once devices have been fabricated and assembled, they must be protected from their environment, yet in many cases they must be able Institute engineers developed vertical thermal actuators operated by a differential expansion between two layers of to interact with their envidissimilar materials in each arm. To obtain the initial upward curvature, the device takes advantage of residual ronment. Thus, the final stresses in the film layers. An electrostatic version with integrated flip-up mirrors was used to produce an optical packaging is typically indiswitch. SwRI holds U.S. patent No. 6,608,714. vidualized for the given device. For example, a chemical sensor must be in sensors and devices. They can interact workers often replace parts according to a contact with the chemical it is sensing. If with the environment at the molecular given schedule rather than first showing this chemical is in liquid form, it must be level to achieve new goals. ❖ that stress corrosion cracking has occurred injected into the MEMS chip in some manor may occur soon. MEMS fabrication ner. If the chemical is in gaseous form, the Comments about this article? techniques allow for smaller fabrication of MEMS device must be in contact with air Contact Hanson at (210) 522-5798 standard test structures that could be but not clogged by dust particles because, or [email protected]. employed in small locations. to a MEMS device, a dust particle is a large SwRI project teams can consist of a object. Of course, all devices must be provariety of experts across several technical tected from vibration and handling. Acknowledgments areas. For example, the SwRI team that The author gratefully acknowledges the contributions of the following SwRI staff designed an in vivo drug delivery system Solving industry problems with MEMS members to the preparation of this article, included experts in microencapsulation, as follow: chemistry, mechanical engineering, fluid The SwRI MEMS team seeks ways to “Relieving Stress,” Manager Dr. Sean Brossia dynamics, electronics, bioengineering apply MEMS technology to industry proband Research Engineer Andy Veit. and MEMS. lems. An example of this is a stress corro“In Motion,” Research Engineer sion-cracking sensor (see sidebar, p. 7) Joseph N. Mitchell. Conclusion being developed using internal research “New Methods,” Institute Scientist funds. Current methods available for in Dr. Steve Hudak, Principal Engineer MEMS have the ability to impact situ monitoring of stress corrosion crackDr. Dan Nicolella, Research Engineer almost every technical field. Their small ing are too cumbersome to install in the Joseph N. Mitchell and former SwRI staff member Rick Fess. size, high volume and low cost enable minimal spaces on an airplane, for examthe creation of a suite of disposable ple. Because of this, airplane maintenance