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  1. 1. 1 21ST CENTURY’S REVOLUTION :MEMS TECHNOLOGY Kaneria Dhaval1, Ekata Mehul2 1 Pursuing M.Tech., Embedded System, U.V.Patel college of Engineering and Technology, Kherva, Mehsana, India, 2 Head eiTRA - eInfochips Training and Research Academy, Ahmedabad Abstract— We are grateful in a revolution of microelectronics, which has dramatically reduced the cost and increased the capability of electronics. This has given much potential to prosper in the area of micro mechanics encompassing MEMS (Micro Electro Mechanical Systems). MEMS promises to revolutionize nearly every product category by bringing together silicon based microelectronics with micro machining technology, making possible the realization of complete systems on a chip. often referred to as micro systems technology, are fabricated using modified silicon and nonsilicon fabrication technology. It reduces cost and increases reliability of the system. MEMS is a process technology used to create tiny integrated devices or systems that combine mechanical and electrical componentsMEMS has been identified as one of the most promising technologies for the 21st Century and has the potential to revolutionize both industrial and consumer products by combining silicon based microelectronics with micromachining technology. Its techniques and microsystem based devices have the potential to dramatically effect of all of our lives and the way we live. If semiconductor micro fabrication was seen to be the first micro manufacturing revolution, MEMS is the second revolution. IndexTerms—Technology,Febrication,Packeging, Application in various field,future scope,revolution reliability). Furthermore, it is clear that current MEMS products are simply precursors to greater and more pervasive applications to come, including genetic and disease testing, guidance and navigation systems, power generation, RF devices( especially for cell phone technology), weapon systems, biological and chemical agent detection, and data storage. Micro mirror based optical switches have already proven their value; several start-up companies specializing in their development have already been sold to large network companies for hundreds of millions of dollars. The promise of MEMS is increasingly capturing the attention of new and old industrises alike, as more and more of their challenges are solved with MEMS. After extensive development, todays commercial MEMS – also known as Micro System Technologies (MST), Micro Machines (MM) have proven to be more manufacturable, reliable and accurate, dollar for dollar, than their conventional counterparts. However the technical hurdles to attain these accomplishments were often costly and time- consuming, and current advances in this technology introduce newer challenges still. Because this field is till in its infancy, very little data on design, manufacturing processes or liability are common or shared. II. FEBRICATION I. INTRODUCTION Micro electromechanical systems (MEMS) is a technology of miniaturization that has been largely adopted from the integrated circuit (IC) industry and applied to the miniaturization of all systems not only electrical systems but also mechanical, optical, fluid, magnetic, etc. Micro Electromechanical systems or MEMS, represent an extraordinary technology that promises to transform whole industries and drive the next technological revolution. These devices can replace bulky actuators and sensors with micron-scale equivalent that can be produced in large quantities by fabrication processes used in integrated circuits photolithography. This reduces cost, bulk, weight and power consumption while increasing performance, production volume, and functionality by orders of magnitude. For example, one well known MEMS device is the accelerometer (its now being manufactured using mems low cost, small size, more MEMS devices are fabricated using a number of materials, depending on the application requirements. One popular material is polycrystalline silicon, also called “polysilicon” or “poly”. This material is sculpted with techniques such as bulk or surface micro- machining, and Deep Reactive Ion Etching (DRIE), proving to be fairly durable for many mechanical operations. Another is nickel, which can be shaped by PMMA (a form of plexiglass) mask platng (LIGA), as well as by conventional photolithographic techniques. Other materials – such as diamond, aluminum, silicon carbide and gallium arsenide – are currently being evaluated for use in micro machines for their desirable properties; e.g., the hardness of diamond and silicon carbide. To create moveable parts, several layers are needed for structural and electrical interconnect (ground plane) purposes, with socalled “sacrificial” oxide layers in between. The current manufacturing record is five layers, making possible a variety of complex mechanical systems. These
  2. 2. 2 capabilities, developed over the last several years, are beginning to unlock the almost unlimited possibilities of MEMS applications. The methods used to integrate multiple patterned materials together to fabricate a completed MEMS device are just as important as the individual processes and materials themselves. Depending on the type of material used fabrication techniques are classified as: A. Silicon Micro fabrication: The two most general methods of MEMS integration are: Surface micro machining ,Bulk micro machining The two key capabilities that make bulk micromachining a viable technology are Anisotropic etchants of Si, such as ethylene-diamine and pyrocatechol (EDP), potassium hydroxide (KOH), and hydrazine (N2H4). These preferentially etch single crystal Si along given crystal planes.Etch masks and etch-stop techniques that can be used with Si anisotropic etchants to selectively prevent regions of Si from being etched. Good etch masks are provided by SiO2 and Si3N4, and some metallic thin films such as Cr and Au (gold). 1. Surface Micromachining Surface micromachining enables the fabrication of complex multicomponent integrated micromechanical structures that would not be possible with traditional bulk micromachining. This technique encases specific structural parts of a device in layers of a sacrificial material during the fabrication process. The substrate wafer is used primarily as a mechanical support on which multiple alternating layers of structural and sacrificial material are deposited and patterned to realize micromechanical structures. The sacrificial material is then dissolved in a chemical etchant that does not attack the structural parts. The most widely used surface micromachining technique, polysilicon surface micromachining, uses SiO2 as the sacrificial material and polysilicon as the structural material. Figure 2 Process flow of bulk micromachining B. Non-Silicon Micro fabrication: Figure 1 Process flow of surface micromachining  Advantages of surface micro machining a) Structures, especially thicknesses, can be smaller than 10 µm in size, b) The micro machined device footprint can often be much smaller than bulk wet-etched devices, c)It is easier to integrate electronics below surface microstructures, and d)Surface microstructures generally have superior tolerance compared to bulk wet-etched devices. The primary disadvantage is the fragility of surface microstructures to handling, particulates and condensation during manufacturing. Surface Micro machining is being used in commercial products such as accelerometers to trigger air bags in automobiles. 2. Bulk Micromachining and Wafer Bonding Bulk micromachining is an extension of IC technology for the fabrication of 3D structures. Bulk micromachining of Si uses wet- and dry-etching techniques in conjunction with etch masks and etch stops to sculpt micromechanical devices from the Si substrate. The development of MEMS has contributed significantly to the improvement of non-silicon micro fabrication techniques. Two prominent examples are LIGA and plastic molding from micro machined substrates. 1. LIGA LIGA is a German acronym standing for lithographie, galvanoformung (plating), and abformung (molding). However, in practice LIGA essentially stands for a process that combines extremely thick-film resists (often >1 mm) and x-ray lithography, which can pattern thick resists with high fidelity and results in vertical sidewalls. Although some applications may require only the tall patterned resist structures themselves, other applications benefit from using the thick resist structures as plating molds (i.e., material can be quickly deposited into the mold by electroplating). A drawback to LIGA is the need for high-energy x-ray sources that are very expensive and rare.
  3. 3. 3 IV. PACKAGING Figure 3 Process flow of LIGA The LIGA process exposes PMMA (poly methyl metha crylate) plastic with synchrotron radiation through a mask. This is shown at the top of the Figure 1. Exposed PMMA is then washed away, leaving vertical wall structures with spectacular accuracy. Structures a third of a millimeter high and many millimeters on a side are accurate to a few tenths of a micron. Metal is then plated into the structure, replacing the PMMA that was washed away. This metal piece can become the final part, or can be used as an injection mold for parts made out of a variety of plastics. As with micromachining processes, many MEMS sensor-packaging techniques are the same as, or derived from, those used in the semiconductor industry. However, the mechanical requirements for a sensor package are typically much more stringent than for purely microelectronic devices. Microelectronic packages are often generic with plastic, ceramic, or metal packages being suitable for the vast majority of IC applications. For example, small stresses and strains transmitted to a microelectronics die will be tolerable as long as they stay within acceptable limits and do not affect reliability. In the case of a MEMS physical sensor, however, such stresses and strains and other undesirable influences must be carefully controlled in order for the device to function correctly. Failure to do so, even when employing electronic compensation techniques, will reduce both the sensor performance and long-term stability. Standard IC Packages  Ceramic Packages  Plastic Packages  Metal Packages III. MEMS DESIGN PROCESS There are three basic building blocks in MEMS technology, which are,Deposition Process-the ability to deposit thin films of material on a substrate, Lithographyto apply a patterned mask on top of the films by photolithograpic imaging. Etching-to etch the films selectively to the mask. A MEMS process is usually a structured sequence of these operations to form actual devices. Figure 5 Standard IC packeges A. MEMS Mechanical Sensor Packaging A MEMS sensor packaging must meet several requirements : • Protect the sensor from external influences and environmental effects. Since MEMS inherently include some microscale mechanical components, the integrity of the device must be protected against physical damage arising from mechanical shocks, vibrations, temperature cycling, and particle contamination. The electrical aspects of the device, such as the bond wires and the electrical properties of the interconnects, must also be protected against these external influences and environmental effects • Protect the environment from the presence of the sensor. In addition protecting the sensor, the package must prevent the presence of the MEMS from reacting with or contaminating potentially sensitive environments. The Figure 4 MEMS design flow starting to end
  4. 4. 4 classic examples of this are medical devices that contain packaged sensors that can be implanted or used within the body; these must be biocompatible, nontoxic, and able to withstand sterilization. • Provide a controlled electrical, thermal, mechanical, and/or optical interface between the sensor, its associated components, and its environment. Not only must the package protect both the sensor and its environment, it must also provide a reliable and repeatable interface for all the coupling requirements of a particular application. In the case of mechanical sensors, the interface is of fundamental importance since, by its nature, specific mechanical coupling is essential but unwanted effects must be prevented. A simple example would be a pressure sensor where the device must be coupled in some manner to the pressure but isolated from, for example, thermally induced strains. The package must also provide reliable heat transfer to enable any heat generated to be transmitted away from the MEMS device to its environment. V.APPLICATIONS OF MEMS A.Communications: High frequency circuits will benefit considerably from advent of the RF-MEMS technology. Electrical components such as inductors and tunable capacitors can be improved significantly compared to their integrated counter parts if they are made using MEMS technology. If the integration of such components, the performance of communication circuits will improve, while the total circuit area, power consumption and cost will be reduced. In addition, the mechanical switch, as developed by several research groups, is a key component with huge potential in various micro wave circuits B. Biotechnology: MEMS enabling new discoveries in science and engineering such as the polymerase chain Reaction (PCR) Microsystems for DNA amplification and identification, micro machined scanning Tunneling microscopes (STMs), Biochips for detection of hazardous chemical and biological agents, and Microsystems for high-throughput drug screening and selection. C. Inertial sensors: Inertial sensors are mechanics sensors aiming at measuring accelerations, in the mechanics science definition. There are two categories of inertial sensors. They are, accelerometers which measures variation of rotational speed and gyroscopes which measures variation of rotational speed. D. Accelerometers: Figure 6 Capacitive accelerometer’s working diagram(reference from Figure 7 Schematic of micro accelerometer, ADXLseries, produced by Analog Device. Figure 8 Schematic of micro accelerometer with closerview On these diagrams, we can see a micro accelerometer device and the chip including associated electronics, made by Analog Device. This is a two axis micro accelerometer. This means it is able to measure accelerations in two directions at a time (in the directions of the plane). Micro accelerometers were the first MEMS device to flood the market. Micro accelerometers measure variation of translational speed. So acceleration, deceleration, even very high deceleration, like…shock! The sensor that detects a shock and launches the airbag is a micro accelerometer combined with a electronic circuit able to decide wether or not the shock was an accident or just your car passing a pothole. There are lots of applications, like navigation, micro accelerometers can help in increasing precision. There are more and more to say about micro accelerometers, they are still the spearhead of MEMS industry. E. Gyroscopes: Micro gyroscopes are newer in the market compared to micro accelerometers. Some devices have appeared on the market for navigation application. The key point in these devices is sensitivity. F. RF switches: RF switches have been under development for years, but the commercial applications just begin to appear. The reason is the difficulty to combine high efficiency, reproducibility and reliability. RF switches will be preferred to full electronic switches on applications where security, integration capabilities, power consumption and other parameters are critical. G. Consumer Market:
  5. 5. 5 Sports Training Devices,omputer Peripherals, Car and Personal Navigation Devices,Active Subwoofers etc H. Industrial Market: Earthquake Detection and Gas Shutoff,Machine Health, Shock and Tilt Sensing etc. I. Military: Tanks,Planes,Equipment for Soldier etc. Table I. Application of MEMS in various fields VI. THE FUTURE OF MEMS TECHNOLOGY A. Industry Challenges Some of the major challenges facing the MEMS industry include: 1. Access to Foundries. MEMS companies today have very limited access to MEMS fabrication facilities, or foundries, for prototype and device manufacture. In addition, the majority of the organizations expected to benefit from this technology currently do not have the required capabilities and competencies to support MEMS fabrication. For example, telecommunication companies do not currently maintain micromachining facilities for the fabrication of optical switches. Affordable and receptive access to MEMS fabrication facilities is crucial for the commercialization of MEMS. 2. Design, Simulation and Modelling. Due to the highly integrated and interdisciplinary nature of MEMS, it is difficult to separate device design from the complexities of fabrication. Consequently, a high level of manufacturing and fabrication knowledge is necessary to design a MEMS device. Furthermore, considerable time and expense is spent during this development and subsequent prototype stage. In order to increase innovation and creativity, and reduce unnecessary ‘time-to-market’ costs, an interface should be created to separate design and fabrication. As successful device development also necessitates modelling and simulation, it is important that MEMS designers have access to adequate analytical tools. 3. Packaging and Testing. The packaging and testing of devices is probably the greatest challenge facing the MEMS industry. As previously described, MEMS packaging presents unique problems compared to traditional IC packaging in that a MEMS package typically must provide protection from an operating environment as well as enable access to it. Currently, there is no generic MEMS packaging solution, with each device requiring a specialized format. Consequently, packaging is the most expensive fabrication step and often makes up 90% (or more) of the final cost of a MEMS device. 4. Standardization. Due to the relatively low number of commercial MEMS devices and the pace at which the current technology is developing, standardization has been very difficult. To date, high quality control and basic forms of standardization are generally only found at multi-million dollar (or billion dollar) investment facilities. However, in 2000, progress in industry communication and knowledge sharing was made through the formation of a MEMS trade organization. Based in Pittsburgh, USA, the MEMS industry group (MEMS-IG) with founding members including Xerox, Corning, Honeywell, Intel and JDS Uniphase, grew out of study teams sponsored by DARPA that identified a need for technology road mapping and a source for objective statistics about the MEMS industry. In addition, a MEMS industry roadmap, sponsored by the Semiconductor Equipment and Materials International organization (SEMI) 5. Education and Training. The complexity and interdisciplinary nature of MEMS require educated and well-trained scientists and engineers from a diversity of fields and backgrounds. The current numbers of qualified MEMS-specific personnel is relatively small and certainly lower than present industry demand. Education at graduate level is usually necessary and although the number of universities offering MEMSbased degrees is increasing, gaining knowledge is an expensive and time-consuming process. Therefore, in order to match the projected need for these MEMS scientists and engineers, an efficient and lower cost VII. CONCLUSIONS MEMS promises to revolutionize nearly every product category by bringing together silicon-based microelectronics with micromachining technology, making possible the realization of complete systems-on-achip.Future Work. MEMS will be the indispensable factor for advancing technology in the 21st century and it promises to create entirely new categories of products. The automotive industry, motivated by the need for more efficient safety systems and the desire for enhanced performance, is the largest consumer of MEMSbased technology. In addition to accelerometers and
  6. 6. 6 gyroscopes, micro-sized tire pressure systems are now standard issues in new vehicles, putting MEMS pressure sensors in high demand. Such micro-sized pressure sensors can be used by physicians and surgeons in a telemetry system to measure blood pressure at a stet, allowing early detection of hypertension and restenosis. Alternatively, the detection of bio molecules can benefit most from MEMSbased biosensors. Medical applications include the detection of DNA sequences and metabolites. MEMS biosensors can also monitor several chemicals simultaneously, making them perfect for detecting toxins in the environment. REFERENCES [1] Teymoori, M.M. ; Asadollahi, H.,‘’MEMS Based Medical Microsensors’’,Computer and Electrical Engineering, 2009. ICCEE '09. Second International Conference on Vol:1 Digital Object Identifier: 10.1109/ICCEE.2009.80 Publication Year: 2009,Page(s): 158- 162 [2] Sethuramalingam, T.K. ; Vimalajuliet, A. “Design of MEMS based capacitive accelerometer”,Mechanical and Electrical Technology (ICMET), 2010 2nd International Conference on Digital Object Identifier: 10.1109/ICMET.2010.5598424 Publication Year: 2010 , Page(s): 565- 568 [3] Lyshevski, S.E.“Micro-electromechanical systems: motion control of micro-actuators”,Decision and Control, 1998. Proceedings of the 37th IEEE Conference on Vol:4 Object Identifier: 10.1109/CDC.1998.761988 [4] Digital Publication Year: 1998 , Page(s): 4334- 4335 [5] MEMS: technology, design, CAD and applications [6] Lal, R. ; Apte, P.R. ; Bhat, K.N. ; Bose, G. ; Chandra, S. ; Sharma, D.K”MEMS: technology, design, CAD and applications” Design Automation Conference, 2002. Proceedings of ASPDAC 2002. 7th Asia and South Pacific and the 15th International Conference on VLSI Design. Proceedings.Digital Object Identifier: 10.1109/ASPDAC.2002.994879 Publication Year: 2002 , Page(s): 24- 25 [7] Fujita, H. “A decade of MEMS and its future”,Micro Electro Mechanical Systems, 1997. MEMS '97, Proceedings, IEEE., Tenth Annual International Workshop on Digital Object Identifier: 10.1109/MEMSYS.1997.581729 Publication Year: 1997 , Page(s): 1- 7 [8] O'Neal, C.B. ; Malshe, A.P. ; Singh, S.B. ; Brown, W.D. ; Eaton, W.P. ”Challenges in the packaging of MEMS”, Advanced Packaging Materials: Processes, Properties and Interfaces, 1999. Proceedings. International Symposium on Digital Object Identifier: 10.1109/ISAPM.1999.757284 Publication Year: 1999 , Page(s): 41- 47 [9] Petersen, K.,”MEMS in the coming decade”,Nano/Micro Engineered and Molecular Systems (NEMS), 2010 5th IEEE International Conference on Digital Object Identifier: 10.1109/NEMS.2010.5592523 Publication Year: 2010 , Page(s): 1- 9 [10] Mansour, R.R. ; Bakri-Kassem, M. ; Daneshmand, M. ; Messiha, N.,”RF MEMS devices”, MEMS, NANO and Smart Systems, 2003. Proceedings. International Conference on Digital Object Identifier: 10.1109/ICMENS.2003.1221974 Publication Year: 2003 , Page(s): 103- 107 [11] Tjerkstra, R. W., de Boer, M., Berenschot, E., Gardeniers, J.G.E., van der Berg, A., and Elwenspoek, M., “Etching Technology for Microchannels”,Proceedings of the 10th Annual Workshop of Micro Electro Mechanical Systems(MEMS ’97), Nagoya, Japan, Jan. 26-30, 1997, pp. 396-398. [12] Journal of Microelectromechanical Systems ( [13] Journal of Micromechanics and Microengineering ( [14] Berkeley Sensor and Actuator Center, http://bsac.eecs.berkeley. [15] University of Stanford, [16] Free scale semiconductor,