CHAPTER-1
INTRODUCTION
Microelectromechanical systems (MEMS) are small integrated devices or systems that combine electrical and mechanical components. They range in size from the sub micrometer level to the millimeter level and there can be any number, from a few to millions, in a particular system. MEMS extend the fabrication techniques developed for the integrated circuit industry to add mechanical elements such as beams, gears, diaphragms, and springs to devices.
Examples of MEMS device applications include inkjet-printer cartridges, accelerometer, miniature robots, microengines, locks inertial sensors microtransmissions, micromirrors, micro actuator (Mechanisms for activating process control equipment by use of pneumatic, hydraulic, or electronic signals) optical scanners, fluid pumps, transducer, pressure and flow sensors. New applications are emerging as the existing technology is applied to the miniaturization and integration of conventional devices.
These systems can sense, control, and activate mechanical processes on the micro scale, and function individually or in arrays to generate effects on the macro scale. The micro fabrication technology enables fabrication of large arrays of devices, which individually perform simple tasks, but in combination can accomplish complicated functions.
MEMS are not about any one application or device, nor are they defined by a single fabrication process or limited to a few materials. They are a fabrication approach that conveys the advantages of miniaturization, multiple components, and microelectronics to the design and construction of integrated electromechanical systems. MEMS are not only about miniaturization of mechanical systems; they are also a new paradigm for designing mechanical devices and systems.

1.1 MEMS Technology
Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology. While the electronics are fabricated using integrated circuit (IC) process sequences, the micromechanical components are fabricated using compatible "micromachining" processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.

Fig-1 MEMS Device
Microelectronic integrated circuits can be thought of as the "brains" of a system and MEMS augments this decision-making capability with "eyes" and "arms", to allow microsystems to sense and control the environment. Sensors gather information from the environment through measuring mechanical, thermal, biological, chemical, optical, and magnetic phenomena. The electronics then process the information derived from the sensors and through some decision making capability direct the actuators to respond by moving, positioning, regulating, pumping, and filtering, thereby controlling the environment for some desired outcome or purpose. Because MEMS devices are manufactured using batch fabrication techniques similar to those used for integrated circuits, unprecedented levels of functionality, reliability, and sophistication can be placed on a small silicon chip at a relatively low cost.

1.2 MEMS / Microsystems
MEMS is an abbreviation for Micro Electro Mechanical Systems. This is a rapidly emerging technology combining electrical, electronic, mechanical, optical, material, chemical, and fluids engineering disciplines. As the smallest commercially produced "machines", MEMS devices are similar to traditional sensors and actuators although much, much smaller. E.g. Complete systems are typically a few millimeters across, with individual features devices of the order of 1-100 micrometers across.

Fig-2 Internal Pattern
MEMS devices are manufactured either using processes based on Integrated Circuit fabrication techniques and materials, or using new emerging fabrication technologies such as micro injection molding. These former processes involve building the device up layer by layer, involving several material depositions and etch steps. A typical MEMS fabrication technology may have a 5 step process. Due to the limitations of this "traditional IC" manufacturing process MEMS devices are substantially planar, having very low aspect ratios (typically 5 -10 micro meters thick). It is important to note that there are several evolving fabrication techniques that allow higher aspect ratios such as deep x-ray lithography, electrodeposition, and micro injection molding.
MEMS devices are typically fabricated onto a substrate (chip) that may also contain the electronics required to interact with the MEMS device. Due to the small size and mass of the devices, MEMS components can be actuated electrostatically (piezoelectric and bimetallic effects can also be used). The position of MEMS components can also be sensed capacitively. Hence the MEMS electronics include electrostatic drive power supplies, capacitance charge comparators, and signal conditioning circuitry. Connection with the macroscopic world is via wire bonding and encapsulation into familiar BGA, MCM, surface mount, or leaded IC packages. A common MEMS actuator is the "linear comb drive" (shown above) which consists of rows of interlocking teeth; half of the teeth are attached to a fixed "beam", the other half attach to a movable beam assembly. Both assemblies are electrically insulated. By applying the same polarity voltage to both parts the resultant electrostatic force repels the movable beam away from the fixed. Conversely, by applying opposite polarity the parts are attracted. In this manner the comb drive can be moved "in" or "out" and either DC or AC voltages can be applied. The small size of the parts (low inertial mass) means that the drive has a very fast response time compared to its macroscopic counterpart. The magnitude of electrostatic force is multiplied by the voltage or more commonly the surface area and number of teeth. Commercial comb drives have several thousand teeth, each tooth approximately 10 micro meters long. Drive voltages are CMOS levels.
The linear push / pull motion of a comb drive can be converted into rotational motion by coupling the drive to push rod and pinion on a wheel. In this manner the comb drive can rotate the wheel in the same way a steam engine functions.

CHAPTER-2
MEMS DESCRIPTION
MEMS technology can be implemented using a number of different materials and manufacturing techniques; the choice of which will depend on the device being created and the market sector in which it has to operate.
1.Silicon:- The economies of scale, ready availability of cheap high-quality materials and ability to incorporate electronic functionality make silicon attractive for a wide variety of MEMS applications. Silicon also has significant advantages engendered through its material properties. In single crystal form, silicon is an almost perfect Hookean material, meaning that when it is flexed there is virtually no hysteresis and hence almost no energy dissipation. The basic techniques for producing all silicon based MEMS devices are deposition of material layers, patterning of these layers by photolithography and then etching to produce the required shapes.
2.Polymers:- Even though the electronics industry provides an economy of scale for the silicon industry, crystalline silicon is still a complex and relatively expensive material to produce. Polymers on the other hand can be produced in huge volumes, with a great variety of material characteristics. MEMS devices can be made from polymers by processes such as injection moulding, embossing or stereolithography and are especially well suited to microfluidic applications such as disposable blood testing cartridges.
3.Metals:- Metals can also be used to create MEMS elements. While metals do not have some of the advantages displayed by silicon in terms of mechanical properties, when used within their limitations, metals can exhibit very high degrees of reliability. Metals can be deposited by electroplating, evaporation, and sputtering processes. Commonly used metals include gold, nickel, aluminum, chromium, titanium, tungsten, platinum, and silver

CHAPTER-3
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, Lithography-to apply a patterned mask on top of the films by photolithograpic imaging, and Etching-to etch the films selectively to the mask. A MEMS process is usually a structured sequence of these operations to form actual devices.

Fig-3 Micro Fabrication Process
3.1 Deposition Processes
One of the basic building blocks in MEMS processing is the ability to deposit thin films of material. In this text we assume a thin film to have a thickness anywhere between a few nanometers to about 100 micrometer
MEMS deposition technology can be classified in two groups:
Depositions that happen because of a chemical reaction is called Chemical Vapor Deposition (CVD)
1-Electrodeposition
2-Epitaxy
3-Thermal oxidation
These processes exploit the creation of solid materials directly from chemical reactions in gas and/or liquid compositions or with the substrate material. The solid material is usually not the only product formed by the reaction. Byproducts can include gases, liquids and even other solids.
Depositions that happen because of a physical reaction is called Physical Vapor Deposition (PVD)
1-Evaporation
2-Sputtering
3-Casting
Common for all these processes are that the material deposited is physically moved on to the substrate. In other words, there is no chemical reaction which forms the material on the substrate. This is not completely correct for casting processes, though it is more convenient to think of them that way.This is by no means an exhaustive list since technologies evolve continuously.
3.1.1 Chemical Vapor Deposition (CVD)
In this process, the substrate is placed inside a reactor to which a number of gases are supplied. The fundamental principle of the process is that a chemical reaction takes place between the source gases. The product of that reaction is a solid material with condenses on all surfaces inside the reactor.
The two most important CVD technologies in MEMS are the Low Pressure CVD (LPCVD) and Plasma Enhanced CVD (PECVD). The LPCVD process produces layers with excellent uniformity of thickness and material characteristics. The main problems with the process are the high deposition temperature (higher than 600°C) and the relatively slow deposition rate. The PECVD process can operate at lower temperatures (down to 300° C) thanks to the extra energy supplied to the gas molecules by the plasma in the reactor. However, the quality of the films tend to be inferior to processes running at higher temperatures. Secondly, most PECVD deposition systems can only deposit the material on one side of the wafers on 1 to 4 wafers at a time. LPCVD systems deposit films on both sides of at least 25 wafers at a time. A schematic diagram of a typical LPCVD reactor is shown in the figure below.

Fig -4: Typical hot-wall LPCVD reactor.
1-Electrodeposition
This process is also known as "electroplating" and is typically restricted to electrically conductive materials. There are basically two technologies for plating: Electroplating and Electroless plating. In the electroplating process the substrate is placed in a liquid solution (electrolyte). When an electrical potential is applied between a conducting area on the substrate and a counter electrode (usually platinum) in the liquid, a chemical redox process takes place resulting in the formation of a layer of material on the substrate and usually some gas generation at the counter electrode.
In the electroless plating process a more complex chemical solution is used, in which deposition happens spontaneously on any surface which forms a sufficiently high electrochemical potential with the solution. This process is desirable since it does not require any external electrical potential and contact to the substrate during processing. Unfortunately, it is also more difficult to control with regards to film thickness and uniformity. A schematic diagram of a typical setup for electroplating is shown in the figure below.

Fig -5 Typical setup for electrodeposition
2-Epitaxy
This technology is quite similar to what happens in CVD processes, however, if the substrate is an ordered semiconductor crystal (i.e. silicon, gallium arsenide), it is possible with this process to continue building on the substrate with the same crystallographic orientation with the substrate acting as a seed for the deposition. If an amorphous/polycrystalline substrate surface is used, the film will also be amorphous or polycrystalline.
There are several technologies for creating the conditions inside a reactor needed to support epitaxial growth, of which the most important is Vapor Phase Epitaxy (VPE). In this process, a number of gases are introduced in an induction heated reactor where only the substrate is heated. The temperature of the substrate typically must be at least 50% of the melting point of the material to be deposited. An advantage of epitaxy is the high growth rate of material, which allows the formation of films with considerable thickness (>100µm). Epitaxy is a widely used technology for producing silicon on insulator (SOI) substrates. The technology is primarily used for deposition of silicon. A schematic diagram of a typical vapor phase epitaxial reactor is shown in the figure below.

Fig-6: Typical cold-wall vapor phase epitaxial reactor
3-Thermal oxidation
This is one of the most basic deposition technologies. It is simply oxidation of the substrate surface in an oxygen rich atmosphere. The temperature is raised to 800° C-1100° C to speed up the process. This is also the only deposition technology which actually consumes some of the substrate as it proceeds. The growth of the film is spurned by diffusion of oxygen into the substrate, which means the film growth is actually downwards into the substrate. As the thickness of the oxidized layer increases, the diffusion of oxygen to the substrate becomes more difficult leading to a parabolic relationship between film thickness and oxidation time for films thicker than ~100nm. This process is naturally limited to materials that can be oxidized, and it can only form films that are oxides of that material. This is the classical process used to form silicon dioxide on a silicon substrate. A schematic diagram of a typical wafer oxidation furnace is shown in the figure below.

Fig -7: Typical wafer oxidation furnace

3.1.2 Physical Vapor Deposition (PVD)
PVD covers a number of deposition technologies in which material is released from a source and transferred to the substrate. The two most important technologies are evaporation and sputtering.
1-Evaporation
In evaporation the substrate is placed inside a vacuum chamber, in which a block (source) of the material to be deposited is also located. The source material is then heated to the point where it starts to boil and evaporate. The vacuum is required to allow the molecules to evaporate freely in the chamber, and they subsequently condense on all surfaces. This principle is the same for all evaporation technologies, only the method used to the heat (evaporate) the source material differs. There are two popular evaporation technologies, which are e-beam evaporation and resistive evaporation each referring to the heating method. In e-beam evaporation, an electron beam is aimed at the source material causing local heating and evaporation. In resistive evaporation, a tungsten boat, containing the source material, is heated electrically with a high current to make the material evaporate. Many materials are restrictive in terms of what evaporation method can be used (i.e. aluminum is quite difficult to evaporate using resistive heating), which typically relates to the phase transition properties of that material. A schematic diagram of a typical system for e-beam evaporation is shown in the figure below.

Fig -8: Typical system for e-beam evaporation of materials.
2-Sputtering
Sputtering is a technology in which the material is released from the source at much lower temperature than evaporation. The substrate is placed in a vacuum chamber with the source material, named a target, and an inert gas (such as argon) is introduced at low pressure. Gas plasma is struck using an RF power source, causing the gas to become ionized. The ions are accelerated towards the surface of the target, causing atoms of the source material to break off from the target in vapor form and condense on all surfaces including the substrate. As for evaporation, the basic principle of sputtering is the same for all sputtering technologies. The differences typically relate to the manor in which the ion bombardment of the target is realized. A schematic diagram of a typical RF sputtering system is shown in the figure below.

Fig-9: Typical RF sputtering system

3-Casting
In this process the material to be deposited is dissolved in liquid form in a solvent. The material can be applied to the substrate by spraying or spinning. Once the solvent is evaporated, a thin film of the material remains on the substrate. This is particularly useful for polymer materials, which may be easily dissolved in organic solvents, and it is the common method used to apply photoresist to substrates (in photolithography). The thicknesses that can be cast on a substrate range all the way from a single monolayer of molecules (adhesion promotion) to tens of micrometers. In recent years, the casting technology has also been applied to form films of glass materials on substrates. The spin casting process is illustrated in the figure below.

Fig-10: The spin casting process used for photoresist in photolithography

3.2 LITHOGRAPHY
3.2.1 Pattern Transfer
Lithography in the MEMS context is typically the transfer of a pattern to a photosensitive material by selective exposure to a radiation source such as light. A photosensitive material is a material that experiences a change in its physical properties when exposed to a radiation source. If we selectively expose a photosensitive material to radiation (e.g. by masking some of the radiation) the pattern of the radiation on the material is transferred to the material exposed, as the properties of the exposed and unexposed regions differs as shown in figure

Fig-11: Transfer of a pattern to a photosensitive material
This discussion will focus on optical lithography, which is simply lithography using a radiation source with wavelength(s) in the visible spectrum.
In lithography for micromachining, the photosensitive material used is typically a photoresist (also called resist, other photosensitive polymers are also used). When resist is exposed to a radiation source of a specific a wavelength, the chemical resistance of the resist to developer solution changes. If the resist is placed in a developer solution after selective exposure to a light source, it will etch away one of the two regions (exposed or unexposed). If the exposed material is etched away by the developer and the unexposed region is resilient, the material is considered to be a positive resist (shown in figure 12a). If the exposed material is resilient to the developer and the unexposed region is etched away, it is considered to be a negative resist (shown in figure 12b).

Fig-12: a) Pattern definition in positive resist, b) Pattern definition in negative resist
Lithography is the principal mechanism for pattern definition in micromachining. Photosensitive compounds are primarily organic, and do not encompass the spectrum of materials properties of interest to micro-machinists. However, as the technique is capable of producing fine features in an economic fashion, a photosensitive layer is often used as a temporary mask when etching an underlying layer, so that the pattern may be transferred to the underlying layer (shown in figure 13a). Photoresist may also be used as a template for patterning material deposited after lithography (shown in figure 13b). The resist is subsequently etched away, and the material deposited on the resist is "lifted off".
The deposition template (lift-off) approach for transferring a pattern from resist to another layer is less common than using the resist pattern as an etch mask. The reason for this is that resist is incompatible with most MEMS deposition processes, usually because it cannot withstand high temperatures and may act as a source of contamination. Once the pattern has been transferred to another layer, the resist is usually stripped. This is often necessary as the resist may be incompatible with further micromachining steps. It also makes the topography more dramatic, which may hamper further lithography steps

Fig-13: a) Pattern transfer from patterned photoresist to underlying layer by etching, b) Pattern transfer from patterned photoresist to overlying layer by lift-off.
.
3.2.2 Alignment
In order to make useful devices the patterns for different lithography steps that belong to a single structure must be aligned to one another. The first pattern transferred to a wafer usually includes a set of alignment marks, which are high precision features that are used as the reference when positioning subsequent patterns, to the first pattern. Often alignment marks are included in other patterns, as the original alignment marks may be obliterated as processing progresses. It is important for each alignment mark on the wafer to be labeled so it may be identified, and for each pattern to specify the alignment mark to which it should be aligned. Depending on the lithography equipment used, the feature on the mask used for registration of the mask may be transferred to the wafer. In this case, it may be important to locate the alignment marks such that they don't effect subsequent wafer processing or device performance. For example, the alignment mark will cease to exist after a through the wafer DRIE etch. Pattern transfer of the mask alignment features to the wafer may obliterate the alignment features on the wafer. In this case the alignment marks should be designed to minimize this effect, or alternately there should be multiple copies of the alignment marks on the wafer, so there will be alignment marks remaining for other masks to be registered to.

Fig-14: Transfer of mask registration feature to substrate during lithography (contact aligner)
Alignment marks may not necessarily be arbitrarily located on the wafer, as the equipment used to perform alignment may have limited travel and therefore only be able to align to features located within a certain region on the wafer. The region location geometry and size may also vary with the type of alignment, so the lithographic equipment and type of alignment to be used should be considered before locating alignment marks. Typically two alignment marks are used to align the mask and wafer, one alignment mark is sufficient to align the mask and wafer in x and y, but it requires two marks (preferably spaced far apart) to correct for fine offset in rotation.
As there is no pattern on the wafer for the first pattern to align to, the first pattern is typically aligned to the primary wafer flat. Depending on the lithography equipment used, this may be done automatically, or by manual alignment to an explicit wafer registration feature on the mask.

3.3 Etching Processes
In order to form a functional MEMS structure on a substrate, it is necessary to etch the thin films previously deposited and/or the substrate itself. In general, there are two classes of etching processes:
Wet etching where the material is dissolved when immersed in a chemical solution
Dry etching where the material is sputtered or dissolved using reactive ions or a vapor phase etchant
3.3.1 Wet etching
This is the simplest etching technology. All it requires is a container with a liquid solution that will dissolve the material in question. Unfortunately, there are complications since usually a mask is desired to selectively etch the material. One must find a mask that will not dissolve or at least etches much slower than the material to be patterned. Secondly, some single crystal materials, such as silicon, exhibit anisotropic etching in certain chemicals. Anisotropic etching in contrast to isotropic etching means different etches rates in different directions in the material. The classic example of this is the <111> crystal plane sidewalls that appear when etching a hole in a <100> silicon wafer in a chemical such as potassium hydroxide (KOH). The result is a pyramid shaped hole instead of a hole with rounded sidewalls with a isotropic etchant. The principle of anisotropic and isotropic wet etching is illustrated in the figure below.

Figure-15: Difference between anisotropic and isotropic wet etching.
3.3.2 Dry etching
The dry etching technology can split in three separate classes called reactive ion etching (RIE), sputter etching, and vapor phase etching.In RIE, the substrate is placed inside a reactor in which several gases are introduced. Plasma is struck in the gas mixture using an RF power source, breaking the gas molecules into ions. The ion is accelerated towards, and reacts at, the surface of the material being etched, forming another gaseous material. This is known as the chemical part of reactive ion etching. There is also a physical part which is similar in nature to the sputtering deposition process. If the ions have high enough energy, they can knock atoms out of the material to be etched without a chemical reaction. It is very complex tasks to develop dry etch processes that balance chemical and physical etching, since there are many parameters to adjust. By changing the balance it is possible to influence the anisotropy of the etching, since the chemical part is isotropic and the physical part highly anisotropic the combination can form sidewalls that have shapes from rounded to vertical. A schematic of a typical reactive ion etching system is shown in the figure below.
1-Sputter etching is essentially RIE without reactive ions. The systems used are very similar in principle to sputtering deposition systems. The big difference is that substrate is now subjected to the ion bombardment instead of the material target used in sputter deposition.
2-Vapor phase etching is another dry etching method, which can be done with simpler equipment than what RIE requires. In this process the wafer to be etched is placed inside a chamber, in which one or more gases are introduced. The material to be etched is dissolved at the surface in a chemical reaction with the gas molecules. The two most common vapor phase etching technologies are silicon dioxide etching using hydrogen fluoride (HF) and silicon etching using xenon diflouride (XeF2), both of which are isotropic in nature. Usually, care must be taken in the design of a vapor phase process to not have bi-products form in the chemical reaction that condense on the surface and interfere with the etching process.

Figure-16: Typical parallel-plate reactive ion etching system
CHAPTER-4
FABRICATION TECHNOLOGIES
The three characteristic features of MEMS fabrication technologies are miniaturization, multiplicity, and microelectronics. Miniaturization enables the production of compact, quick-response devices. Multiplicity refers to the batch fabrication inherent in semiconductor processing, which allows thousands or millions of components to be easily and concurrently fabricated. Microelectronics provides the intelligence to MEMS and allows the monolithic merger of sensors, actuators, and logic to build closed-loop feedback components and systems. The successful miniaturization and multiplicity of traditional electronics systems would not have been possible without IC fabrication technology. Therefore, IC fabrication technology, or microfabrication, has so far been the primary enabling technology for the development of MEMS. Microfabrication provides a powerful tool for batch processing and miniaturization of mechanical systems into a dimensional domain not accessible by conventional techniques. Furthermore, microfabrication provides an opportunity for integration of mechanical systems with electronics to develop high-performance closed-loop-controlled MEMS. Advances in IC technology in the last decade have brought about corresponding progress in MEMS fabrication processes. Manufacturing processes allow for the monolithic integration of microelectromechanical structures with driving, controlling, and signal-processing electronics. This integration promises to improve the performance of micromechanical devices as well as reduce the cost of manufacturing, packaging, and instrumenting these devices.
4.1 IC Fabrication
Any discussion of MEMS requires a basic understanding of IC fabrication technology, or microfabrication, the primary enabling technology for the development of MEMS. The major steps in IC fabrication technology are:
1-Film growth: Usually, a polished Si wafer is used as the substrate, on which a thin film is grown. The film, which may be epitaxial Si, SiO2, silicon nitride (Si3N4), polycrystalline Si, or metal, is used to build both active or passive components and interconnections between circuits.
2-Doping: To modulate the properties of the device layer, a low and controllable level of an atomic impurity may be introduced into the layer by thermal diffusion or ion implantation.
3-Lithography: A pattern on a mask is then transferred to the film by means of a photosensitive (i.e., light sensitive) chemical known as a photoresist. The process of pattern generation and transfer is called photolithography. A typical mask consists of a glass plate coated with a patterned chromium (Cr) film.
4-Etching: Next is the selective removal of unwanted regions of a film or substrate for pattern delineation. Wet chemical etching or dry etching may be used. Etch-mask materials are used at various stages in the removal process to selectively prevent those portions of the material from being etched. These materials include SiO2, Si3N4, and hard-baked photoresist.
5-Dicing: The finished wafer is sawed or machined into small squares, or dice, from which electronic components can be made.
6-Packaging: The individual sections are then packaged, a process that involves physically locating, connecting, and protecting a device or component. MEMS design is strongly coupled to the packaging requirements, which in turn are dictated by the application environment.

Fig-17 Packaging of MEMS Device
4.2 Micro Molding
In the micromolding process, microstructures are fabricated using molds to define the deposition of the structural layer. The structural material is deposited only in those areas constituting the microdevice structure, in contrast to bulk and surface micromachining, which feature blanket deposition of the structural material followed by etching to realize the final device geometry. After the structural layer deposition, the mold is dissolved in a chemical etchant that does not attack the structural material. One of the most prominent micromolding processes is the LIGA process. LIGA is a German acronym standing for lithographie, galvanoformung, und abformung (lithography, electroplating, and molding). This process can be used for the manufacture of high-aspect-ratio 3D microstructures in a wide variety of materials, such as metals, polymers, ceramics, and glasses. Photosensitive polyimides are also used for fabricating plating molds. The photolithography process is similar to conventional photolithography, except that polyimide works as a negative resist.
Example: An insulin pump fabricated by classic MEMS technology

1. Pumping membrane 2. Pumping chamber
3. Inlet 4. Outlet
5. Large mesa 6. Upper glass plate
7. Bottom glass plate 8. patterned thin layer

Fig-18 Micro Moulding

CHAPTER-5
Applications
1-Pressure Sensors
MEMS pressure microsensors typically have a flexible diaphragm that deforms in the presence of a pressure difference. The deformation is converted to an electrical signal appearing at the sensor output. A pressure sensor can be used to sense the absolute air pressure within the intake manifold of an automobile engine, so that the amount of fuel required for each engine cylinder can be computed.
2-Accelerometers
Accelerometers are acceleration sensors. An inertial mass suspended by springs is acted upon by acceleration forces that cause the mass to be deflected from its initial position. This deflection is converted to an electrical signal, which appears at the sensor output. The application of MEMS technology to accelerometers is a relatively new development.
Accelerometers in consumer electronics devices such as game controllers (Nintendo Wii), personal media players / cell phones (Apple iPhone ) and a number of Digital Cameras (various Canon Digital IXUS models). Also used in PCs to park the hard disk head when free-fall is detected, to prevent damage and data loss. iPod Touch: When the technology become sensitive. MEMS-based sensors are ideal for a wide array of applications in consumer, communication, automotive and industrial markets.

Fig-19 iPod with MEMS Technology
The consumer market has been a key driver for MEMS technology success. For example, in a mobile phone, MP3/MP4 player or PDA, these sensors offer a new intuitive motion-based approach to navigation within and between pages. In game controllers, MEMS sensors allow the player to play just moving the controller/pad; the sensor determines the motion.
3-Inertial Sensors
Inertial sensors are a type of accelerometer and are one of the principal commercial products that utilize surface micromachining. They are used as airbag-deployment sensors in automobiles, and as tilt or shock sensors. The application of these accelerometers to inertial measurement units is limited by the need to manually align and assemble them into three-axis systems, and by the resulting alignment tolerances, their lack of in-chip analog-to-digital conversion circuitry, and their lower limit of sensitivity
.

Figure-20 A Typical MEMS device
4-Microengines
A three-level polysilicon micromachining process has enabled the fabrication of devices with increased degrees of complexity. The process includes three movable levels of polysilicon, each separated by a sacrificial oxide layer, plus a stationary level. Microengines can be used to drive the wheels of microcombination locks. They can also be used in combination with a microtransmission to drive a pop-up mirror out of a plane. This device is known as a micromirror.

Some Other Commercial applications include: 1. Inkjet printers, which use piezoelectrics or thermal bubble ejection to deposit ink on paper. 2. Accelerometers in modern cars for a large number of purposes including airbag deployment in collisions. 3. MEMS gyroscopes used in modern cars and other applications to detect yaw; e.g. to deploy a roll over bar or trigger dynamic stability control. 4. Silicon pressure sensors e.g. car tire pressure sensors, and disposable blood pressure sensors. 5. Displays e.g. the DMD chip in a projector based on DLP technology has on its surface several hundred thousand micromirrors. 6. Optical switching technology which is used for switching technology and alignment for data communications. 7. Bio-MEMS applications in medical and health related technologies from Lab-On-Chip to MicroTotalAnalysis (biosensor, chemosensor). 8. Interferometric modulator display (IMOD) applications in consumer electronics. Used to create interferometric modulation - reflective display technology as found in mirasol displays. 9. MEMS IC fabrication technologies have also allowed the manufacture of advanced memory devices (nanochips/microchips).

Figccccc-21 MEMS based memory device
As a final example, MEMS technology has been used in fabricating vaporization microchambers for vaporizing liquid microthrusters for nanosatellites. The chamber is part of a microchannel with a height of 2-10 microns, made using silicon and glass substrates
CHAPTER-6
ADVANTAGES & DISADVANTAGES

Advantages of MEMS:- 1. Minimize energy and materials use in manufacturing 2. Cost/performance advantages 3. Improved reproducibility 4. Improved accuracy and reliability 5. Increased selectivity and sensitivity 6. Farm establishment requires huge investments

Disadvantages of MEMS:- 1. Micro-components are Costly compare to macro-components 2. Design includes very much complex procedures 3. Prior knowledge is needed to integrate MEMS devices

CHAPTER- 7
THE FUTURE
Each of the three basic microsystems technology processes we have seen, bulk micromachining, sacrificial surface micromachining, and micromolding/LIGA, employs a different set of capital and intellectual resources. MEMS manufacturing firms must choose which specific microsystems manufacturing techniques to invest in.
MEMS technology has the potential to change our daily lives as much as the computer has. However, the material needs of the MEMS field are at a preliminary stage. A thorough understanding of the properties of existing MEMS materials is just as important as the development of new MEMS materials.
Future MEMS applications will be driven by processes enabling greater functionality through higher levels of electronic-mechanical integration and greater numbers of mechanical components working alone or together to enable a complex action. Future MEMS products will demand higher levels of electrical-mechanical integration and more intimate interaction with the physical world. The high up-front investment costs for large-volume commercialization of MEMS will likely limit the initial involvement to larger companies in the IC industry. Advancing from their success as sensors, MEMS products will be embedded in larger non-MEMS systems, such as printers, automobiles, and biomedical diagnostic equipment, and will enable new and improved systems.
How the MEMS and Nano Exchange Can Help?

The MEMS and Nanotechnology Exchange provides services that can help with some of these problems.
We make a diverse catalog of processing capabilities available to our users, so our users can experiment with different fabrication technologies. Our users don't have to build their own fabrication facilities, and
Our web-based interface lets users assemble process sequences and submit them for review by the MEMS and Nanotechnology Exchange's engineers and fabrication sites.

CHAPTER-8 conclusion The automotive industry, motivated by the need for more efficient safety systems and the desire for enhanced performance, is the largest consumer of MEMS-based technology. In addition to accelerometers and 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 MEMS-based 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.
Lastly, the dynamic range of MEMS based silicon ultrasonic sensors have many advantages over existing piezoelectric sensors in non-destructive evaluation, proximity sensing and gas flow measurement. Silicon ultrasonic sensors are also very effective immersion sensors and provide improved performance in the areas of medical imaging and liquid level detection.
The medical, wireless technology, biotechnology, computer, automotive and aerospace industries are only a few that will benefit greatly from MEMS.
This enabling technology allowing the development of smart products, augmenting the computational ability of microelectronics with the perception and control capabilities of microsensors and microactuators and expanding the space of possible designs and applications.
MEMS devices are manufactured for unprecedented levels of functionality, reliability, and sophistication can be placed on a small silicon chip at a relatively low cost.
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-a-chip.
MEMS will be the indispensable factor for advancing technology in the 21st century and it promises to create entirely new categories of products.

List of Figure

S. No. | Fig. No. | Details | Page No. | 1 | 1 | MEMS Device | 2 | 2 | 2 | Internal Pattern | 3 | 3 | 3 | Micro Fabrication Processes | 6 | 4 | 4 | Typical hot-wall LPVCD reactor | 8 | 5 | 5 | Typical setup for Electrodeposition | 8 | 6 | 6 | Typical cold –wall vapour phase epitaxial reactor | 9 | 7 | 7 | Typical wafer oxidation furnace | 10 | 8 | 8 | Typical system for e-beam evaporation of materials | 11 | 9 | 9 | Typical RF sputtering system | 11 | 10 | 10 | The spin casting process as used for photoresist in Photolithography | 12 | 11 | 11 | Transfer of a pattern to a photosensitive material | 13 | 12 | 12 | a)Pattern definition in positive resist, b)Pattern definition in negative | 14 | 13 | 13 | a)Pattern transfer from patterned photoresist to underlying layer by Etching, b) Pattern transfer from patterned photoresist to overlying layer by lift –off | 15 | 14 | 14 | Transfer of mask registration feature to substrate during lithography (contact aligner) | 16 | 15 | 15 | Difference between anisotropic and isotropic wet etching | 17 | 16 | 16 | Typical parallel-plate reactive ion etching system | 18 | 17 | 17 | Packaging of MEMS Device | 20 | 18 | 18 | Micro Moulding | 21 | 19 | 19 | iPod with MEMS Technology | 22 | 20 | 20 | A typical MEMS device | 23 | 21 | 21 | MEMS based memory device | 24 |

REFERENCES [1]. Waldner, Jean-Baptiste (2008). Nanocomputers and Swarm Intelligence. London: ISTE John Wiley & Sons. p. 205. [2]. Electromechanical monolithic resonator, US patent 3614677, Filed April 29, 1966; Issued October 1971. [3]. Kovacs, G.T.A.; Maluf, N.I.; Petersen, K.E. (1998). "Bulk micromachining of silicon". Proceedings of the IEEE 86 (8): 1536. [4]. Chang, Floy I. (1995). Gas-phase silicon micromachining with xenon difluoride 2641. p. 117.. [5]. Chang, Floy I-Jung (1995). Xenon difluoride etching of silicon for MEMS (M.S.). Los Angeles: University of California.. [6]. Laermer, F.; Urban, A. (2005). Milestones in deep reactive ion etching 2. p. 1118. [7]. Bustillo, J. M.; Howe, R. T.; Muller, R. S. (August 1998). "Surface Micromachining for Microelectromechanical Systems". Proceedings of the IEEE 86 (8): 1552–1574. [8]. Johnson, R. Collin. There's more to MEMS than meets the iPhone, EE Times, (2007-07-09). Retrieved 2007-07-10. [9]. www.wikipedia.org/wiki. [10]. www.howstuffworks.com [11]. www.youtube.com [12]. MEMS Clearinghouse http://www.memsnet.org/ [13]. MEMS Exchange http://www.mems-exchange.org/ [14]. MEMS Industry Group http://www.memsindustrygroup.org/