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- Herausgeber: GRIN Verlag
- Sprache: Englisch
Diploma Thesis from the year 2005 in the subject Electrotechnology, grade: Master 9.8/10, , language: English, abstract: Microelectromechanical systems (MEMS) are collection of microsensors and actuators that have the ability to sense its environment and react to changes in that environment with the use of a microcircuit control. They also include the conventional microelectronics packaging, integrating antenna structures for command signals into microelectromechanical structures for desired sensing and actuating functions. The system may also need micropower supply, microrelay, and microsignal processing units. Microcomponents make the system faster, more reliable, cheaper, and capable of incorporating more complex functions. In the beginning of 1990s, MEMS appeared with the aid of the development of integrated circuit fabrication processes, in which sensors, actuators, and control functions are co-fabricated in silicon [1]. Since then, remarkable research progresses have been achieved in MEMS under the strong promotions from both government and industries. In addition to the commercialization of some less integrated MEMS devices, such as microaccelerometers, inkjet printer head, micromirrors for projection, etc., the concepts and feasibility of more complex MEMS devices have been proposed and demonstrated for the applications in such varied fields as microfluidics, aerospace, biomedical, chemical analysis, wireless communications, data storage, display, optics, etc. Some branches of MEMS, appearing as microoptoelectromechanical systems (MOEMS), micro total analysis systems, etc., have attracted a great research since their potential applications’ market.
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Veröffentlichungsjahr: 2009
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1.1 Overview of MEMS
Microelectromechanical systems (MEMS) are collection of microsensors and actuators that have the ability to sense its environment and react to changes in that environment with the use of a microcircuit control. They also include the conventional microelectronics packaging, integrating antenna structures for command signals into microelectromechanical structures for desired sensing and actuating functions. The system may also need micropower supply, microrelay, and microsignal processing units. Microcomponents make the system faster, more reliable, cheaper, and capable of incorporating more complex functions.
In the beginning of 1990s, MEMS appeared with the aid of the development of integrated circuit fabrication processes, in which sensors, actuators, and control functions are co-fabricated in silicon [1]. Since then, remarkable research progresses have been achieved in MEMS under the strong promotions from both government and industries. In addition to the commercialization of some less integrated MEMS devices, such as microaccelerometers, inkjet printer head, micromirrors for projection, etc., the concepts and feasibility of more complex MEMS devices have been proposed and demonstrated for the applications in such varied fields as microfluidics, aerospace, biomedical, chemical analysis, wireless communications, data storage, display, optics, etc. Some branches of MEMS, appearing as microoptoelectromechanical systems (MOEMS), micro total analysis systems, etc., have attracted a great research since their potential applications’ market.
At the end of 1990s, most of MEMS devices with various sensing or actuating mechanisms were fabricated using silicon bulk micromachining, surface
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micromachining, and lithography, galvanoforming, moulding (LIGA) processes [2]. Three-dimensional microfabrication processes incorporating more materials were presented for MEMS recently because of specific application requirements (e.g., biomedical devices) and higher output power microactuators.
Micromachining has become the fundamental technology for the fabrication of MEMS devices and, in particular, miniaturized sensors and actuators. Silicon micromachining is the most advanced of the micromachining technologies, and it allows for the fabrication of MEMS that have dimensions in the submillimeter range. It refers to fashioning microscopic mechanical parts out of silicon substrate or on a silicon substrate, making the structures three dimensional and bringing new principles to the designers. Employing materials such as crystalline silicon, polycrystalline silicon, silicon nitride, etc., a variety of mechanical microstructures including beams, diaphragms, grooves, orifices, springs, gears, suspensions, and a great diversity of other complex mechanical structures have been conceived.
In some applications, stresses and strains to which the structure is subjected to may pose a problem for conventional cabling. In others, environmental effects may affect system performance. Advances in ultra flat antenna technology coupled with MEMS sensors and actuators seem to be an efficient solution. The integration of micromachining and microelectronics on one chip results in so-called smart sensors [3]. In smart sensors, small sensor signals are amplified, conditioned, and transformed into a standard output format. They may include microcontroller, digital signal processor, application-specific integrated circuit (ASIC), self-test, self-calibration, and bus interface circuits simplifying their use and making them more accurate and reliable.
Silicon micromachining has been a key factor for the vast progress of MEMS in the last decade. This refers to the fashioning of microscopic mechanical parts out of silicon substrates and, more recently, other materials. It is used to fabricate such features as clamped beams, membranes, cantilevers, grooves, orifices, springs, gears, suspensions, etc. These can be assembled to create a variety of sensors. Bulk micromachining is the commonly used method, but it is being replaced by surface micromachining that offers the attractive possibility of integrating the machined
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device with microelectronics that can be patterned and assembled on the same wafer. Thus power supply circuitry and signal processing using ASICs can be incorporated. It is the efficiency of creating several such complete packages using existing technology that makes this an attractive approach.
1.2 Silicon Micro Accelerometers
Micromachined inertial sensors, consisting of acceleration and angular rate sensors are produced in large quantities mainly for automotive applications [4], where they are used to activate safety systems, including air bags, and to implement vehicle stability systems and electronic suspensions. Besides these automotive applications accelerometers are used in many other applications where low cost and small size are important, e.g. in biomedical applications for activity monitoring and in consumer applications such as the active stabilization of camcorder pictures. Miniaturized acceleration sensors are also of interest to the air and space industries and for many other applications.
Silicon acceleration sensors generally consist of a proof mass which is suspended to a reference frame by a spring element. Accelerations cause a displacement of the proof mass, which is proportional to the acceleration. This displacement can be measured in several ways, e.g. capacitively by measuring a change in capacitance between the proof mass and an additional electrode or piezoresistively by integrating strain gauges in the spring element [3]. To obtain large sensitivity and low noise a large proof mass is needed, which suggests the use of bulk micromachined techniques. For less demanding applications surface micromachined devices seem to be more attractive because of the easy integration with electronic circuits and the fact that bulk micromachining requires the use of wafer bonding step [5]. Recently, some designs have been presented which combine bulk and surface micromachining to realize a large proof mass in a single wafer process.
The technology can be classified in a number of ways, such as mechanical or electrical, active or passive, deflection or null-balance accelerometers, etc.
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This thesis reviewed following type of the accelerometers:
¾Electromechanical
¾Piezoelectric
¾Piezoresistive
¾Capacitive and electrostatic force balance
¾Resonant accelerometer
Depending on the principles of operations, these accelerometers have their own subclasses.
1.2.1 Electromechanical Accelerometers
Electromechanical accelerometers [6], essentially servo or null-balance types, rely on the principle of feedback. In these instruments, an accelerationsensitive mass is kept very close to a neutral position or zero displacement point by sensing the displacement and feeding back the effect of this displacement. A proportional magnetic force is generated to oppose the motion of the mass displaced from the neutral position, thus restoring this position just as a mechanical spring in a conventional accelerometer would do. The advantages of this approach are better linearity and elimination of hysteresis effects, as compared to the mechanical springs. Also, in some cases, electrical damping can be provided, which is much less sensitive to temperature variations. One very important feature of electromechanical accelerometers is the capability of testing the static and dynamic performances of the devices by introducing electrically excited test forces into the system. This remote self-checking feature can be quite convenient in complex and expensive tests where accuracy is essential. These instruments are also useful in acceleration control systems, since the reference value of acceleration can be introduced by means of a proportional current from an external source. They are used for general-purpose motion measurements and monitoring low-frequency vibrations. There are a number of different electromechanical accelerometers: coil-and-magnetic types, induction types, etc.
