Educational & Professional | 5 Chapters
Author: Dr. P. Elamurugan
This book aims to fortify the student clear knowledge an understanding of the fundamentals of Electrical, Electronics and their applications in Instrumentation Engineering. The book will also serve as a prerequisite for post graduate and specialized studies and research. This prologue to micro integrated system accentuate on applications in Electrical Electronics and Instrumentation Engineering. The book travel around the liaison between fundamen....
Intrinsic Characteristics of MEMS – Energy Domains and Transducers- Sensors and Actuators Introduction to Micro fabrication - Silicon based MEMS processes – New Materials – Review of Electrical and Mechanical concepts in MEMS – Semiconductor devices – Stress and strain analysis Flexural beam bending- Torsional deflection.
THE INTRINSIC CHARACTERISTICS OF MEMS
There is no doubt that MEMS will continue to find major new applications in the future. The reason for technology development and commercialization may vary by case. Nevertheless, there are three generic and distinct merits for MEMS devices and micro fabrication technologies: Miniaturization, Microelectronics Integration, and Parallel fabrication with high precision. MEMS products will compete in the market place on the grounds of functional richness, small sizes, unique performance characteristics (e.g., fast speed), and/or low cost. For advanced students of MEMS, it is important to realize that the three merits will not automatically lead to product and market advantages. One must understand the complex interplay between these elements to fully unleash the power of MEMS technology.
The length scale of typical MEMS devices generally ranges from 1 mm to 1 cm. (However, a large array of MEMS devices or an entire system may occupy much bigger footprint or volume.) Small dimensions give rise to many operational advantages, such as soft springs, high resonance frequency, greater sensitivity, and low thermal mass. For example, the heat transfer to and from a micro machined device is generally fast. A case in mind is the ink jet printer nozzle, with the time constant of droplet ejection being on the order of 20 ms. Small size allows MEMS devices to be less intrusive in biomedical applications (e.g., neuron probes). Being small also means that MEMS devices can be integrated nonintrusively in crucial systems, such as portable electronics, medical instruments, and implants (e.g., capsule endoscopes). From a practical point of view, smaller device footprint leads to more devices per wafer and greater economy of scale. Hence the cost of MEMS devices generally scales favorably with miniaturization. However, all things do not work better when miniaturized. Some physical phenomena do not scale favorably when the dimensions are reduced, while certain physical phenomena that work poorly at the macroscale suddenly becomes very practical and attractive at the microscale. Scaling laws are observation about how physics work at different sizes. A well-known example is that fleas can jump dozens of times its own height whereas elephants cannot jump at all, even though an elephant has far more muscle mass than a flea. A rigorous scaling-law analysis starts with the identification of a characteristics length scale (denoted L) for a device of interest. For example, the length of a cantilever or the diameter of a circular membrane may be denoted as L of the respective element. The remaining pertinent physical dimensions are assumed to scale linearly with the characteristics length scale, with locked ratios. A performance merit of interest (e.g., stiffness of a cantilever or resonant frequency of a membrane) is expressed as a function of L, with dimension terms other than the characteristic length scale term expressed as a fraction or multiples of L. The expression is then simplified to extract the overall effect of L.
Circuits are used to process sensor signals, provide power and control, improve the signal qualities, or interface with control/computer electronics. MEMS products today are increasingly being embedded with computing, networking, and decision-making capabilities. By integrating micromechanical devices with electronics circuitry and offering the combined system as a product, significant advantages can be produced in a competitive market place. The ability to seamlessly integrate mechanical sensors and actuators with electronics processors and controllers at the single wafer level is one of the most unique characteristics of MEMS. This process paradigm is referred to as monolithic integration—fabrication of various components on a single substrate in an unbroken, wafer-level process flow. (The word “monolithic” means “one stone”. Hence “monolithic fabrication” means fabrication on one piece of wafer.) Though not all MEMS devices should adhere or have adhered to the monolithic integration format, it is observed that silicon circuits that are monolithically integrated with mechanical elements have been involved in several successful commercial MEMS applications, such as Analog Devices accelerometers, digital light processors, and ink jet printer heads. Monolithic processes do not involve hybrid assembly methods such as robotics pick-and-place or any manual attachment of individual parts. Dimensions and precision of placement are guaranteed by lithography. Monolithic integration improves the quality of signals by reducing the length of signal paths and noise. Monolithic integration with circuits is arguably the only way by which a large and dense array of sensors or actuators can be addressed. In the case of DLP, for example, each mirror is controlled by a CMOS logic circuit that is buried directly underneath.Without the integration of circuits, it is impossible to address individual mirrors in such a large and dense array.
Parallel Fabrication with Precision
MEMS technology can realize two- or three-dimensional features with small dimensions and precision that cannot be reproducibly, efficiently, or profitably made with traditional machining tools. Combined with photolithography, MEMS technology can be used to realize unique threedimensional features such as inverted pyramid cavities, high aspect ratio trenches, throughwafer holes, cantilevers, and membranes.To make these features using traditional machining or manufacturing methods is prohibitively difficult and inefficient. MEMS and Microelectronics are also different from traditional machining, in that multiple copies of identical dies are made on a same wafer. This practice can contribute to lowering the cost of individual units. Modern lithography systems and techniques provide not only finely defined features, but also uniformity across wafers and batches.
Energy Domains and Transducers
MEMS technology enables revolutionary sensors and actuators. In general terms, sensors are devices that detect and monitor physical or chemical phenomenon, whereas actuators are ones that produce mechanical motion, force, or torque. Sensing can be broadly defined as energy transduction processes that result in perception, whereas actuation is energy transduction processes that produce actions. Sensors and actuators are collectively referred to as transducers, which serve the function of transforming signals or power from one energy domain to another. There are six major energy domains of interests: (1) electrical domain (denoted E); (2) mechanical domain (Mec); (3) chemical domain (C); (4) radiative domain (R); (5) magnetic domain (Mag); and (6) thermal domain (T). These energy domains and commonly encountered parameters within them are summarized in Figure.The total energy within a system can coexist in several domains and can shift among various domains under right circumstances.
Sensors generally transform stimulus signals in various energy domains to one that is detectable by humans or into the electrical domain for interfacing with electronics controllers, recorders, or computers. For example, a thermal-couple temperature sensor transforms a thermal signal, temperature, into an electrical signal (e.g., voltage) that can be read electronically. Often, more than one sensing principles can be used for a transduction task.
Temperature variation can be perceived via such phenomenon as resistance changes, volume expansion of fluids, increased radiation power of an object, color change of engineered dyes, shifted resonance frequency of resonant beams, or greater chemical reactivity. Energy transduction pathways for particular sensor and actuation tasks do not have to involve only two domains. Rather, the transduction process may incorporate multiple domains. Direct transduction pathways that involve the minimal number of domains do not necessarily translate into simpler device, lower cost, or better performances.
Energy and signal transduction is a vast space of research and development and a continuing source of innovation. The desire to discover and implement efficient, sensitive, and lowcost sensing principles transcends the boundary of scientific and technological disciplines. Because many sensing tasks can be achieved in more than one ways, either directly (from one energy domain to another) or indirectly (hopping through intermediate energy domains), there is essentially unlimited number of transduction pathways for achieving one sensor or actuator need. Each transduction pathway entails different sensing material, fabrication method, design, sensitivity, responsivity, temperature stability, cross-sensitivity, and cost, among others.A tradeoff study must be conducted, taking account of= performance, cost, manufacturing ease, robustness, and, increasingly more important these days, intellectual property rights. The development of sensors and actuators is a rich and rewarding research experience.To invent a new sensor principle for a particular application involves selecting or inventing the energy transduction paths, device designs, and fabrication methods that yield simple transduction materials, high performance, and low-cost fabrication. I will discuss a few specific examples of sensors to illustrate the richness of this field and to exemplify the excitement involved with research and development activities. In many cases, new sensing methods resulted in new device capabilities and industrialization opportunities.
Acceleration sensing (Mec--E transduction). Acceleration can be sensed in many different ways. A micromachined proof mass suspended by cantilevers will experience an inertial force under an applied acceleration. The force will cause movement of the suspended proof mass.The movement can be picked up using piezoresistors, resistor elements whose resistance change under applied stress (Mec--E). The displacement can also be sensed with a capacitor (Mec--E). This is the principle of Analog Device accelerometers. These two methods involve moving mechanical mass. Can one build accelerometers without moving parts? The answer is yes. I will illustrate one example in the following. Inertial force can also move a heated mass, whose ensuing displacement can be picked up by temperature sensors (Mec--T--E).Thermal sensing does not provide as good a performance as capacitive sensing of moving air mass, but the fabrication is readily compatible with integrated circuits. This is the principle of a lowcost acceleration sensor (manufactured by MEMSIC Corporation) designed for low-sensitivity applications. No moving mass is required, eliminating concerns of mechanical reliability. Since no moving mass is needed, the device is compatible with mass batch microelectronics foundries, reducing the time to market significantly.
Olfactory sensing (C--E transduction). Information about the presence and concentration of certain molecules responsible for smell or pertaining to environmental monitoring can be obtained using a number of strategies.A carbon-based material can be designed to specifically absorb certain molecules and alter the electrical resistivity (C→E direct transduction).The absorbance of certain molecules in the path of surface acoustic wave devices can alter mechanical properties such as frequency of surface acoustic wave transmission (C→M→E). These methods generally involve sophisticated electronics or algorithms. Can one build olfactory sensors that are simpler and more intuitive? I will illustrate one example below.The binding of chemical molecules can also alter the color of a specially designed chemical compound, which can be detected using low-cost optoelectronics diodes (C→R→E transduction) or directly by human beings without electronics (C→R). Sensors based on this strategy are being made by ChemSensing Corporation. DNA sequence identification (C→E transduction). DNA molecules consist of a chain of base pairs, each with four possible varieties—A,C,G, or T.The sequence of base pairs in a DNA chain determines the code of synthesizing proteins. The ability to decipher base pair sequences of DNA molecules rapidly, accurately, and inexpensively is of critical importance for pharmaceutical and medical applications.There are a wide variety of innovative methods for detection of DNA sequence through their telltale binding (hybridization) events. Certain DNA molecules may be chemically modified to incorporate (tagged) fluorescence reporters that lights or dims upon binding with another DNA strand. In the most widely practiced case today, chemical binding events are turned into optical signals first before transduced to the electrical domain (C→R→E).The fluorescent image is captured using high power fluorescent microscopes. However, fluorescent imaging requires sophisticated microscope and is not suitable for portable, field applications.DNA molecules attached to gold nanoparticles can report the event of hybridization through aggregation of gold particles, which can result in changes of optical reflectance (C→R→E)  or electrical resistivity (C→E) . The detection method with gold nanoparticles provides better sensitivity and selectivity compared with fluorescence methods while eliminating the need of cumbersome fluorescent imaging instruments. It is, therefore, amendable for miniaturization and remote deployment.This principle is the technological basis of Nanosphere Corporation.