Introduction to MEMS
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1.1 Micro Electro-Mechanical System (MEMS)
Micro electro-mechanical structures and systems are miniature devices that enable the operation of complex systems. They exist today in many environments, especially automotive, medical, consumer, industrial, and aerospace. Their potential for future penetration into a broad range of applications is real, supported by strong developmental activities at many companies and institutions. The technology consists of a large portfolio of design and fabrication processes (a toolbox), many borrowed from the integrated circuit industry. The development of MEMS is inherently interdisciplinary, necessitating an understanding of the toolbox as well as the end application.
In the United States, the technology is known as micro electro mechanical systems (MEMS); in Europe it is called micro systems technology (MST). A question asking for a more specific definition is certain to generate a broad collection of replies, with few common characteristics other than “miniature.” But such apparent divergence in the responses merely reflects the diversity of applications this technology enables, rather than a lack of commonality. MEMS is simultaneously a toolbox, a physical product, and a methodology all in one.
• It is a portfolio of techniques and processes to design and create miniature systems.
• It is a physical product often specialized and unique to a final application-one can seldom buy a generic MEMS product at the neighborhood electronics store.
• “MEMS is a way of making things,” reports the Microsystems Technology Office of the United States Defense Advanced Research Program Agency (DARPA). These “things” merge the functions of sensing and actuation with computation and communication to locally control physical parameters at the micro scale, yet cause effects at much grander scales.
Although a universal definition is lacking, MEMS products possess a number of distinctive features. They are miniature embedded systems involving one or many micro-machined components or structures.
Table 1: Examples of present and future applications of MEMS
• Invasive and noninvasive biomedical sensors
• Miniature biochemical analytical instruments
• Cardiac management systems (e.g. pacemakers, catheters)
• Drug delivery systems (e.g. insulin, analgesics)
• Neurological disorders (e.g. neurostimulation)
• Engine and propulsion control
• Automotive safety, braking and suspension systems.
• Mass data storage systems.
• Electromechanical signal processing.
• Distributed control of aerodynamic and hydrodynamic systems.
• Telecommunication optical fiber
• Inertial systems for munitions guidance and personal navigation.
• Distributed unattended sensors for asset tracking, environmental and security surveillance.
• Weapons safing, arming, and fuzing
• Integrated micro-opto-mechanical components for identify- friend-or foe systems
• Head- and night-display system.
• Low power, high density mass data storage devices.
• Embedded sensors and actuators for condition- based maintenance.
• Active conformable surfaces for distributed aerodynamic control of aircraft.
• Integrated fluidic systems for miniature propellant and combustion control.
• Miniature fluidic systems for early detection of biochemical warfare.
They enable higher level functions, although in and of themselves their utility may be limited (a micro machined pressure sensor in one’s hand is useless, but under the hood it controls the fuel-air mixture of the car engine. They often integrate smaller functions into one package for greater utility) for example, merging an acceleration sensor with electronic circuits for self-diagnostics. They can also bring cost benefits, directly through low unit pricing, or indirectly by cutting service and maintenance costs. Although the vast majority of today’s MEMS products are best categorized as components or subsystems, the emphasis in MEMS technology is on the “systems” aspect. True Microsystems may still be a few years away, but their development and evolution rely on the success of today’s components, especially as these components are integrated to perform functions ever increasing in complexity.
Building micro-systems is an evolutionary process. One notable example is the evolution of crash sensors for airbag safety systems. Early sensors were merely mechanical switches. They later evolved into micro-mechanical sensors that directly measured acceleration. The current generation of devices integrates electronic circuitry with a micro-mechanical sensor to provide self-diagnostics and a digital output. It is anticipated that the next generation of devices will also incorporate the entire airbag deployment circuitry that decides whether to inflate the airbag. As the technology matures, the airbag crash sensor may be integrated one day with micro-machined yaw-rate and other inertial sensors to form a complete micro-system responsible for passenger safety and vehicle stability. Examples of future micro-systems are not limited to automotive applications. Efforts to develop micro machined components for the control of fluids are just beginning to bear fruit. These could lead one day to the integration of micro-pumps with micro-valves and reservoirs to build new miniature drug delivery systems.
2. MATERIALS FOR MEMS
If we view micromachining technology as a set of generic tools, then there is no reason to limit its use to one material. Indeed, micromachining was demonstrated in silicon, glass, ceramics, polymers, and compound semiconductors made of group III and V elements, as well as a wide variety of metals including titanium and tungsten. However, silicon remains the primary material of choice for micro-electro-mechanical systems. Unquestionably, this popularity arises from the large momentum of the electronic integrated circuit industry and the derived economic benefits, not the least of which is the extensive industrial infrastructure. Anyway the type of application and economics determines the final choice of materials.
2.1 Silicon material system
The silicon material system encompasses, in addition to silicon itself, a host of materials commonly used in the semiconductor integrated circuit industry. Normally deposited as thin films, they include silicon oxides, nitrides, and carbides as well as metals such as aluminum, titanium, tungsten, and copper.
Silicon is one of very few materials that can be economically manufactured in single crystal substrates. This crystalline nature provides significant electrical and mechanical advantages. The precise modulation of silicon’s electrical conductivity using impurity doping lies at the very core of the operation of electronic semiconductor devices. Mechanically, silicon is an elastic and robust material whose characteristics have been very well studied and documented. It becomes evident that silicon is a suitable material platform on which electronic, mechanical, thermal, optical, and even fluid flow functions can be integrated. Silicon, as an element exists in any of three forms:
Silicon has a diamond lattice crystal structure that can be regarded as simple cubic. In other words, the primitive unit—the smallest repeating block—of the crystal lattice resembles a cube. Silicon is a good thermal conductor with conductivity approximately one hundred times larger than that of glass. In complex integrated systems, the silicon substrate can be used as an efficient heat sink. Silicon is also known to retain its mechanical integrity at temperatures up to about 500° C. At higher temperatures, silicon softens appreciably and plastic deformation sets in.
While the mechanical and thermal properties of polysilicon are similar to those of single crystal while the mechanical and thermal properties of polysilicon are similar to those of single crystal silicon, polysilicon experiences slow-stress-annealing effects at temperatures above 250° C, making its operation at elevated temperatures subject to long-term instabilities, drift, and hysteresis effects. The interactions of silicon with gases, chemicals, biological fluids, and enzymes remain the subject of many research studies, but for the most part, silicon is considered stable and resistant to many elements and chemicals typical of daily applications. For example, experiments have shown that silicon remains intact in the presence of Freon gases as well as corrosive automotive fluids such as brake fluids. Silicon has also proven to be a suitable material for applications involving the delivery of ultra-high-purity gases.
Table2: Properties of selected list of materials
SiC AlN 92% Al2O3
Relative permeability (ε0) 11.8 3.8 4 3.75 9.7 8.5 9
(V/cm×106) 3 5-10 5-10 25-40 4 13 11.6
Electron mobility (cm2/V.s) 1500 - - - 1000 - -
Hole mobility (cm2/V.s) 400 - - - 40 - -
Young’s modulus (GPa) 160 73 323 107 450 340 275
Yield strength (GPa) 7 8.4 14 9 21 16 15..4
Poisson’s ratio 0.22 0.17 0.25 0.16 0.14 0.31 0.31
Density (g/cm3) 2.4 2.3 3.1 2.65 3.2 3.26 3.62
Coefficient of thermal expansion (10-6/0C) 2.6 0.55 2.8 0.55 4.2 4.0 6.57
Thermal conductivity at
300 K (W/cm.K) 1.57 0.014 0.19 0.0138 5 1.60 0.36
Specific heat (J/g.K) 0.7 1.0 0.7 0.787 0.8 0.71 0.8
Melting temperature (0C) 1415 1700 1800 1610 2830 2470 1800
In medicine and biology, studies are ongoing to evaluate silicon for chronic medical implants. Preliminary medical evidence indicates that silicon is benign in the body and does not release toxic substances when it comes in contact with biological fluids. Unfortunately, silicon is not an active optical material—silicon-based lasers do not exist. Because of the particular interactions between the crystal atoms and the conduction electrons, silicon is effective only in detecting light; emission of light is very difficult to achieve. At infrared wavelengths above 1.1 mm silicon is transparent, but at wavelengths shorter than 0.4 mm (in the blue and ultraviolet portions of the visible spectrum), it reflects over 60% of the incident light.
Crystalline silicon is a hard and brittle material deforming elastically until it reaches its yield strength, at which point it breaks. Its tensile yield strength is 7 GPa equivalent to a 700-kg (1500 lb.) weight suspended from a 1-mm2 area. Its Young’s modulus is dependent on crystal orientation with an average value of 160 GPa, near that of stainless steel. Crystalline nature, mechanical properties are uniform across wafer lots, and wafers are free of intrinsic stresses. This helps to minimize the number of design iterations for silicon transducers that rely on stable mechanical properties for their operation. Bulk mechanical properties of crystalline silicon are largely independent of impurity doping, but stresses tend to rise when dopant concentrations reach high levels (~1020cm-3).
2.1.2 Silicon oxide and nitride
It is often argued that silicon is such a successful material because it has a stable oxide that is electrically insulating, unlike germanium whose oxide is soluble in water, or gallium arsenide whose oxide cannot be grown appreciably. Various forms of silicon oxides (SiO2, SiOx, silicate glass) are widely used in micromachining, due to their excellent electrical and thermal insulating properties. They are also used as sacrificial layers in surface micromachining processes because they can be preferentially etched in hydro fluoric acid (HF) with high selectivity to silicon.
Silicon nitride (SixNy) is also a widely used insulating thin film and is effective as a barrier against mobile ion diffusion, in particular, sodium and potassium ions found in biological environments. Its Young’s modulus is higher than that of silicon and its intrinsic stresses can be controlled by the specifics of the deposition process. Silicon nitride is an effective masking material in many alkaline etches solutions.
Polymers, in the form of polyimides or photoresist, can be deposited with varying thicknesses from a few nanometers to hundreds of microns. Standard photoresist is spin-coated to a thickness of 1 to10 mm, but special photoresists such as the epoxy-based SU8 can form layers up to 100-mm-thick. Hardening of the resist under ultraviolet light produces rigid structures. Spin-on organic polymers are generally limited in their application because they shrink substantially after the solvent evaporates, and because they cannot sustain temperatures above 200° C. Because of their unique absorption and adsorption properties, polymers have gained acceptance in the sensing of chemical gases and humidity.
2.1.4 Thin metal films
The choice of a thin metal film depends greatly on the nature of the final application. Thin metal films are normally deposited either by sputtering, evaporation, or chemical vapor deposition and gold, nickel and Permalloy (NixFey) can also be electroplated.
For basic electrical interconnections, aluminum is the most common and is relatively easy to deposit by sputtering, but its operation is limited to non-corrosive environments and to temperatures below 300º C. For higher temperatures and harsher environments, gold, titanium, and tungsten are excellent substitutes. Aluminum tends to anneal over time with temperature causing changes in its intrinsic stresses. As a result, it is typically located away from stress or strain sensing elements. Aluminum is a good light reflector in the visible, and gold excels in the infrared. Platinum and palladium are two very stable materials for electro chemistry, though their fabrication entails some added complexity. Gold, platinum, and iridium are good choices for microelectrodes used in electrochemistry and in sensing bio potentials. Silver is also useful in electrochemistry. Chromium, titanium, and titanium-tungsten are frequently used as very thin (10–100 nm) adhesion layers for highly stressed metals with a tendency to peel off, such as sputtered or evaporated tungsten, nickel, platinum, or palladium. Metal bi-layers consisting of an adhesion layer (e.g., chromium) and an intermediate nickel or platinum layer are normally used to solder with silver-tin or tin-lead alloys. For applications requiring transparent electrodes, such as liquid crystal displays, indium-tin-oxide (ITO) meets the requirements. Finally, Permalloy has been explored as a material for thin magnetic cores.
2.2 Other materials and substrates
Over the years, micromachining methods were applied to a variety of substrates to fabricate passive microstructures and transducers. Fabrication processes for glass and quartz are mature and well established but for other materials, such as silicon carbide, new techniques are being explored and developed. In the process, these activities add breadth to micromachining technology and enrich the inventory of available tools.
1) Glass and quartz substrates
2) Silicon carbide and diamond
3) Gallium arsenide and other group III-V compound semiconductors
4) Shape-memory alloys
3. PROCESS FOR MICROMACHINING
Micro machining is the set of design and fabrication tools that precisely machine and form structures and elements at a scale well below the limits of our human perceptive faculties-the micro scale. Micromachining is the underlying foundation of MEMS fabrication; it is the toolbox of MEMS. Arguably, the birth of the first micro-machined components dates back many decades, but it was the well-established integrated circuit industry that indirectly played an indispensable role in fostering an environment suitable for the development and growth of micromachining technologies.
Micromachining is a parallel (batch) process in which hundreds or possibly thousands of identical elements are fabricated simultaneously on the same wafer. Moreover, the minimum feature dimension is on the order of one micrometer, about a factor of 25 times smaller than what can be achieved using conventional machining. Fundamentally, silicon micromachining combines adding layers of material over a silicon wafer with etching (in the sense of selectively removing material) precise patterns in these layers or the underlying substrate. The implementation is based on a broad portfolio of fabrication processes including material deposition, patterning, and etching techniques. Lithography plays a significant role in the delineation of accurate and precise patterns. We divide the toolbox into two major categories
Basic process tools
Advanced process tools
The basic process tools are well-established methods and are usually available at major foundry facilities; the advanced process tools are unique in their nature, and are normally limited to a few specialized facilities.
3.1 Basic process tools
Epitaxy, sputtering, evaporation, chemical vapor deposition, and spin-on methods are common techniques used to deposit uniform layers of silicon, metals, insulators, or polymers. Lithography is a photographic process for printing images onto a layer of photosensitive polymer (photoresist) that is subsequently used as a protective mask against etching.
Wet and dry etchings, including deep reactive ion etching, form the essential process base to selectively remove material. The following sections describe the fundamentals of each of the basic process tools.
Figure: 1 Illustration of basic process in micro-machining
Epitaxy is a common deposition method to grow a crystalline silicon layer over a silicon wafer, but with a differing dopant type and concentration. The epitaxial layer is typically 1 to 20mmthick. It exhibits the same crystal orientation as the underlying crystalline substrate, except when grown over an amorphous material. For example a layer of silicon dioxide, it is polycrystalline. Epitaxy is a widely used step in the fabrication of CMOS circuits, and has proven efficient in forming wafer-scale p-n junctions for controlled electrochemical etching. The growth occurs in a vapor-phase chemical deposition reactor from the dissociation at high temperature (> 800º C) of a silicon source gas. Common silicon sources are silane (SiH4), silicon dichlorosilane (SiH2Cl2), or silicon tetrachloride (SiCl4). Nominal growth rates vary between 0.2 and 1.5 mm/min depending on the source gas and the growth temperature.
High-quality silicon dioxide is obtained by oxidizing silicon in either dry oxygen or in steam at elevated temperatures (850–1150º C). Oxidation mechanisms have been extensively studied and are well understood. Charts showing final oxide thickness as a function of temperature, oxidizing environment, and time are widely available
3.1.3 Sputter deposition
In sputter deposition, a target object made of a material to be deposited is physically bombarded by a flux of inert ions (e.g., argon, helium) in a vacuum chamber. Material particles from the target are ejected and deposited on the wafer. There are three general classes of sputter tools differing by the ion excitation mechanism.
Evaporation involves the local heating of a target material to a sufficiently high temperature in order to generate a vapor that condenses on a substrate. Nearly any material (e.g., Al, Si, Ti, Mo, glass, Al2O3 … and so on), including many high melting point refractory metals (W, Au, Cr, Pd, Pt), can be evaporated provided it has a vapor pressure above the background pressure (0.1–1 Pa), and that the carrier in which the target is contained is itself not evaporated—the carrier is usually made of tungsten.
3.1.5 Chemical vapor deposition
Chemical vapor deposition (CVD) works on the principle of initiating a chemical reaction in a vacuum chamber, resulting in the deposition of a reacted species on a heated substrate. In contrast to sputtering, CVD is a high-temperature process with typical deposition temperatures above 300º C. The field of CVD has grown substantially, driven by the demand within the semiconductor industry for high-quality thin dielectric and metal films for multi layer electrical interconnects. Common thin films deposited by CVD include polysilicon, silicon oxides and nitrides, tungsten, titanium, and tantalum as well as their nitrides, and most recently, copper and low permittivity dielectric insulators (r < 3). The latter two are becoming workhorse materials for very high-speed electrical interconnects in integrated circuits. The deposition of polysilicon, and silicon oxides and nitrides is routine within the MEMS industry.
3.1.6 Spin-on methods
Spin-on is a simple process to put down layers of dielectric insulators and organic materials. Unlike the methods described earlier, the equipment is simple requiring a variable speed-spinning table with appropriate safety screens. A nozzle dispenses the material as a liquid solution in the center of the wafer. Spinning the substrate at high speeds (500 to 5000 rpm) rapidly spreads the material in a uniform manner. Photoresist and polyimides are common organic materials that can be spun on a wafer with thickness typically between 0.5 and 20 mm. The organic polymer is normally in suspension in a solvent solution. Subsequent baking or exposure to ultraviolet radiation causes the solvent to evaporate, and cures the film.
Lithography involves three sequential steps:
1. Application of photoresist (or resist), which is a photosensitive emulsion layer.
2. Optical exposure to print an image of the mask onto the resist.
3. Immersion in an aqueous developer solution to dissolve the exposed resist and render visible the latent image.
The mask itself consists of a patterned opaque chromium layer on a transparent glass substrate. The pattern layout is generated using a computer-aided design (CAD) tool, and transferred into the thin chromium layer at a specialized mask-making facility. A complete micro fabrication process frequently involves several lithographic operations.
Positive photoresist is an organic resin material containing a “sensitizer.” It is spin-coated on the wafer with a typical thickness between 0.5 mm and 10 mm. As mentioned earlier, special types of resists can be spun to thickness of up to 100 mm, but the large thickness poses significant challenges to exposing and defining features below 25 mm in size. The sensitizer prevents the dissolution of unexposed resist during immersion in the developer solution. Exposure to light in the 200-to 450 nm range (ultraviolet to blue) breaks down the sensitizer, causing exposed regions to immediately dissolve in developer solution.
The exact opposite process happens in negative resists exposed areas remain and unexposed areas dissolve in the developer. Optical exposure can be accomplished in one of three different modes: contact, proximity, or project and implimentationion. In contact lithography, the mask touches the wafer. This normally shortens the life of the mask, and leaves undesired residue on the wafer and the mask. In proximity mode, the mask is brought to within 25–50 mm of the resist surface. In contrast, project and implimentationion lithography project and implimentations an image of the mask onto the wafer through complex optics.
Figure: 2 Illustration of proximity and project and implimentationion lithography
The objective is to selectively remove material using imaged photoresist as a masking template. The pattern can be etched directly into the silicon substrate, or into a thin film, which in turn can be used as a mask for subsequent etches. For a successful etch, there must be sufficient selectivity between the masking material and the material being etched.
Etching thin films is relatively easier than etching bulk silicon. Etching of silicon lies at the core of what is often termed “bulk micro-machining.” No ideal silicon etch method exists, leaving process engineers with a number of techniques, each suitable for some applications but not others. Distinctions are made on the basis of isotropy, etch medium, and selectivity of the etch to other materials. Isotropic etchants etch uniformly in all directions, resulting in rounded cross-sectional features. In contrast, anisotropic etchants etch in one direction preferentially over others, resulting in trenches or cavities delineated by flat and well-defined surfaces; these need not be perpendicular to the surface of the wafer. The etch medium (wet vs. dry) plays a role in selecting a suitable method. Wet etchants in aqueous solution offer the advantage of low-cost batch fabrication—usually 20 to 25 wafers can be etched simultaneously—and can be either of the isotropic or anisotropic type. Dry etching involves the use of reactant gases in low-pressure plasma. The equipment is specialized, and requires the plumbing of ultra-clean pipes to bring high-purity reactant gases into the vacuum chamber.
Figure: 3 Schematic illustrations of cross-sectional trench profiles resulting from four different types of etch methods.
3.2 Advanced process tools
3.2.1 Anodic bonding
Anodic bonding is a simple process that joins together a bare silicon wafer and a sodium-containing glass substrate. It is fundamental to the manufacture of a wide variety of sensors, including pressure sensors, because it provides a rigid support, in the form of a glass substrate, for the rather fragile silicon wafer. The bonding is performed at a temperature between 200° and 500° C in a vacuum, air, or an inert gas environment. The application of a large voltage (500–1500 V) across the two substrates, with the glass held at the negative potential, causes mobile positive ions (mostly Na+) in the glass to migrate away from the silicon-glass interface towards the cathode, leaving behind fixed negative charges The bonding is complete when the ion current (measured externally as an electron current) vanishes, indicating that all mobile ions have reached the cathode. The electrostatic attraction between the fixed negative charges in the glass and positive charges in the silicon holds the two substrates together, and facilitates the chemical bonding of glass to silicon. This is the reason anodic bonding is also known as electrostatic bonding.
3.2.2 Silicon-fusion bonding
Silicon fusion bonding, also known as direct wafer bonding, is a process capable of securely joining two silicon substrates. It emerged as an important step in the development of silicon-on-insulator (SOI) technology during the 1980s for high frequency and radiation-hardened CMOS applications. SOI wafers made by silicon fusion bonding are commercially available today from many vendors. The concept was quickly extended to the manufacture of pressure sensors and accelerometers in the late 1980’s, and is now widely accepted as an important technique in the MEMS toolbox.
The bonding can be between two bare silicon surfaces, or with an intermediate silicon dioxide layer (SOI-type). The bonding mechanism is not well understood, but it is widely believed that it occurs at the molecular level between silicon and oxygen atoms at the interface. Both wafers are first cleaned in sulfuric acid, followed by hydrochloric acid to remove organic and metal contaminants.
Surface cleanliness is necessary to ensure a uniform and void-free bond. The two wafers are then immersed in an ammonium hydroxide solution at approximately 100° C. This “hydration” step serves to provide hydroxyl (OH) groups on the bond surfaces to make them hydrophilic. The bond surfaces are then carefully brought into contact and held together by Van-der-Waals forces. Poor bonding and separation occur when using bowed or non-planar wafers. A temperature anneal at 800° to 1100° C promotes and strengthens the bond according to the reaction:
Si-O-H…. H-O-Si Si-O-Si + H2O.
A thin polysilicon film can be fusion-bonded to a silicon wafer or to a silicon dioxide layer if it exhibits a very smooth and planar surface. This can be achieved by using chemo-mechanical polishing.
3.2.3 Sol-gel deposition methods
A sol-gel process is a chemical reaction between solid particles in colloidal suspension within a fluid (a sol) to form a gelatinous network (a gel) that can be transformed to solid phase upon removal of the solvent. Sol-gel is not a unique process, but rather represents a broad type of processing capable of forming glasses and ceramics in a multitude of shapes, starting from basic chemical precursors
Figure 4.Basic flow of a sol-gel process
A widespread application of sol-gel processing is in the coating of surfaces with optical absorption or index-graded antireflective materials. It has been used in research laboratories to deposit thick piezoelectric films on silicon substrates A sol-gel process starts by dissolving appropriate chemical precursors in a liquid to form a sol. Taking the sol through its gel-point transforms it to a gel. This is the point in the phase diagram where the sol undergoes polymerization, and abruptly changes from a viscous liquid state to a gelatinous network. Both sol formation and gelation are low temperature steps. The gel is then formed into a solid shape (e.g., fiber or lens), or applied as a film coating on a substrate by spinning, dipping, or spraying. For example, tetraethoxysilane (TEOS) in water can be converted into a silica gel by hydrolysis and condensation using hydrochloric acid as a catalyst. Drying and sintering at an elevated temperature (200–600° C) results in the transition of the gel to glass, and then densification to silicon dioxide. Silicon nitride, alumina, and piezoelectric lead-zirconium-titanate (PZT) can also be deposited by sol-gel methods.
3.3 Combining the tools
The sequence in which various tools from the toolbox are combined determines a unique micro-fabrication process. It may be specific to a particular design, or may be sufficiently general that it can be used to fabricate a broad range of different designs.
Surface micro-machining builds a stack of polysilicon thin films with alternating layers of sacrificial silicon oxide. A typical stack contains a total of four or five layers, but may be more complex. For example, the process at Sandia National Laboratories stacks up to five polysilicon and five oxide layers. The polysilicon films form the structural elements, and are normally deposited using LPCVD followed by a high temperature anneal (> 900ºC) for relieving mechanical stresses. The silicon dioxide layer is deposited using CVD. The polysilicon and silicon dioxide layers are each 2-mm-thick; however, Robert Bosch uses a process with 10-mm-thick polysilicon grown by epitaxy over silicon dioxide. Each of the layers in the stack is lithographically patterned and etched before the next layer is deposited in order to form the appropriate shapes, and to make provisions for anchor points to the substrate.
The final “release” step consists of etching the silicon dioxide (hence the sacrificial term) to free the polysilicon plates and beams, thus allowing motion in the plane of and perpendicular to the substrate
Figure 5.Schematic illustration of the basic process steps in surface micro- machining.
4. COMMERCIAL MEMS STRUCTURES AND SYSTEMS
Examining various types of micro-electromechanical (MEM) structures and systems. It is apparent that with a vast and diverse set of fabrication tools, creativity abounds. Indeed, the list of MEM structures and devices continues to grow daily as more applications prove to benefit from miniaturization. But just as necessity is the mother of all inventions, it is economics that ultimately determines the commercial success of a particular design or technology. Demonstrations of micro-machined devices are innumerable, but the successful products are few.
Three general categories form the total extent of MEMS: sensors, actuators, and passive structures. Sensors are transducers that convert mechanical, thermal, or other forms of energy into electrical energy; actuators do exactly the opposite. Passive structures are devices in which no transducing occurs.
4.1 General design methodology
The design process begins with the identification of the general operating principles and overall structural elements, then proceeds onto analysis and simulation, and finally onto outlining of the individual steps in the fabrication process. This is often an iterative process involving continuous adjustments to the shape, structure, and fabrication steps. The layout of the lithographic masks is the final step before fabrication, and is completed using specialized computer-aided design (CAD) tools to define the two-dimensional patterns. Early design considerations include the identification of the general sensing or actuation mechanisms based on performance requirements. For instance, output force requirement of a mechanical micro-actuator may favor thermal or piezoelectric methods and preclude electrostatic ones. Similarly, the choice of piezoresistive sensing is significantly different from capacitive or piezoelectric sensing. The interdisciplinary nature of the field brings together considerations from a broad range of specialties including mechanics, optics, fluid dynamics, materials science, electronics, chemistry, and even biological sciences. On occasion, determining a particular approach may rely on economic considerations or ease of manufacture rather than performance.
4.2 Techniques for sensing and actuation
4.2.1 Common sensing methods
Sensing is by no means a modern invention. There are numerous historical accounts describing the measurement of physical parameters most notably, distance, weight, time, and temperature. The objective of modern sensing is the transducing of a specific physical parameter, to the exclusion of other interfering parameters, into electrical energy. Occasionally, an intermediate conversion step takes place. For example, pressure or acceleration is converted into mechanical stress, which is then converted to electricity. Perhaps the most common of all modern sensing techniques is temperature measurement using the dependence of various material properties on temperature. This effect is pronounced in the electrical resistance of metals—the temperature coefficient of resistance (TCR) of most metals ranges between 10 and 100 parts per million per degree centigrade.
Table 3. The Relative Merits of Piezoresistive, Capacitive, and Electromagnetic Sensing Methods
Piezoresistivity and piezoelectricity are two sensing techniques used in MEMS. Impurity-doped silicon exhibits a piezoresistive behavior, which lies at the core of many pressure and acceleration sensor designs. Measuring the change in resistance and amplifying the corresponding output signal tend to be rather simple, requiring a basic knowledge of analog circuit design. A drawback of silicon piezoresistivity is its strong dependence on temperature that must be compensated for with external electronics.
In contrast, capacitive sensing relies on an external physical parameter changing either the spacing or the relative dielectric constant between the two plates of a capacitor. For instance, an applied acceleration pushes one plate closer to the other. Or in the example of relative humidity sensors, the dielectric is an organic material whose permittivity is a function of moisture content. The advantages of capacitive sensing are very low power consumption and relative stability with temperature. Additionally, the approach offers the possibility of electrostatic actuation to perform closed-loop feedback. Naturally, capacitive sensing requires external electronics to convert changes in capacitance into an output voltage. Unlike measuring resistance, these circuits can be substantially intricate if the change in capacitance is small. This is frequently the case in MEMS where capacitance values are on the order of 1 pF and less. Yet another sensing approach utilizes electromagnetic signals to detect and measure a physical parameter. Magneto-resistive sensors on the read heads of high-density computer disk drives measure the change in conductivity of a material slab in response to the magnetic field of the storage bit. In Hall effect devices, a magnetic field induces a voltage in a direction orthogonal to current flow. Hall effect sensors are extremely inexpensive to manufacture, and make excellent candidates to measure wheel velocity in vehicles. Another form of electromagnetic transducing uses Faraday’s law to detect the motion of a current-carrying conductor through a magnetic field. The control electronics for magnetic sensors can be readily implemented using modern CMOS technology. But generating magnetic fields often necessitates the presence of a permanent magnet or a solenoid
4.2.2 Common actuation methods
A complete shift in paradigm becomes necessary to think of actuation on a miniature scale—a four-stroke engine is not scalable. The next five schemes illustrate the diversity and the myriad of actuation options available in MEMS. They are
Electrostatic actuation relies on the attractive force between two plates or elements carrying opposite charges. A moment of thought quickly reveals that the charges on two objects with an externally applied potential between them can only be of opposite polarities. Therefore, an applied voltage, regardless of its polarity, always results in an attractive electrostatic force.
Piezoelectric actuation can provide significantly large forces, especially if thick piezoelectric films are used. Commercially available piezo-ceramic cylinders can provide up to a few newtons of force with applied potentials on the order of a few hundred volts. However, thin-film (< 5 mm) piezoelectric actuators can only provide a few milli- newtons. Both piezoelectric and electrostatic methods offer the advantage of low power consumption since the electric current is very small.
Thermal actuation consumes more power than electrostatic or piezoelectric actuation, but can provide, despite its gross inefficiencies, actuation forces on the order of hundreds of millinewtons or higher. In another approach known as thermo-pneumatic actuation, a liquid is heated inside a sealed cavity. Pressure from expansion or evaporation exerts a force on the cavity walls, which can bend if made sufficiently compliant. This method also depends on the absolute temperature of the actuator.
Lorentz forces form the dominant mechanism in magnetic actuation on a miniaturized scale. This is largely due to the difficulty in depositing permanently magnetized thin films. Electrical current in a conductive element that is located within a magnetic field gives rise to an electromagnetic force—the Lorentz force—in a direction perpendicular to the current and magnetic field. This force is proportional to the current, magnetic field, and length of the element. A conductor 1 mm in length carrying 10 mA in a 1-T magnetic field is subject to a force of 10 mN. Lorentz forces are useful for closed-loop feedback in systems employing electromagnetic sensing. Two yaw-rate sensors described later make use of this method.
Phase recovery using shape-memory alloys.
Shape-memory alloys undoubtedly offer the highest energy density available for actuation effect. They can provide very large forces when the temperature of the material rises above the critical temperature, typically around 100º C. The challenge with shape-memory alloys lies in the difficulty of integrating their fabrication with conventional silicon manufacturing processes.
Anyway the choice of actuation depends on the nature of the application, ease of integration with the fabrication process, and economic justification.
Table 4. Comparison of Various Actuation Methods on the Basis of Maximum Energy Density
Physical & material parameters
½ ε0 E 2
E = electric field
ε0= dielectric permittivity 5 V/mm
α = coefficient of expansion
∆T = temperature rise
Y = Young’s modulus 3 × 10-6/º C
100 GPa ~ 5
½ B 2/ μ0
B = magnetic field
μ0= magnetic permeability
100 GPa ~ 4
E = electric field
Y = Young’s modulus
d33 = piezoelectric constant
2 × 10-12 C/N
Critical temperature ~ 10 [from
reports in literature]
(Actual energy output may be substantially lower depending on the overall efficiency of the system.)
A primary application for micro-pumps is likely to be in the automated handling of fluids for chemical analysis and drug-delivery systems. Stand-alone micro-ump units face significant competition from traditional solenoid or stepper motor-actuated pumps. For instance, The Lee Company, Westbrook, Connecticut, manufactures a family of pumps measuring approximately 51 mm × 12.7 mm × 19 mm (2 in. × 0.5 in. × 0.75 in.) and weighing, fully packaged, a mere 50 g. They can dispense up to 6 mL/min with a power consumption of 2 W from a 12-V DC supply. But micro-machined pumps can have a significant advantage if they can be readily integrated along with other fluid-handling components, such as valves, into one completely automated miniature system.
The basic structure of the micro-pump is rather simple, consisting of a stack of four wafers. The bottom two wafers define two check valves at the inlet and outlet. The top two wafers form the electrostatic actuation unit. The application of a voltage between the top two wafers actuates the pump diaphragm, thus expanding the volume of the pump inner chamber. This draws liquid through the inlet-check-valve to fill the additional chamber volume. When the applied AC voltage goes through its null point, the diaphragm relaxes and pushes the drawn liquid out through the outlet check valve. Each of the check valves comprises a flap that can move only in a single direction: The flap of the inlet-check-valve moves only as liquid enters to fill the pump inner chamber; the opposite is true for the outlet-check-valve
The novelty of the design is in its ability to pump fluid in either a forward or reverse direction—hence its bi-directionality. At first glance, it appears that such a scenario is impossible because of the geometry of the two check valves. This is true as long as the pump diaphragm displaces liquid at a frequency lower than the natural frequencies of the two check valve flaps. But at higher actuation frequencies—above the natural frequencies of the flap—the response of the two flaps lags the actuation drive. In other words, when the pump diaphragm actuates to draw liquid into the chamber, the inlet-valve flap cannot respond instantaneously to this action and remains closed for a moment longer. The outlet-check valve is still open from the previous cycle and does not respond quickly to closing.
In this instance, the outlet-check-valve is open and the inlet- check-valve is closed, which draws liquid into the chamber through the outlet rather than the inlet. Hence, the pump reverses its direction. Clearly, for this to happen the response of the check valves must lag the actuation by at least one quarter of a cycle—the phase difference between the check valves and the actuation must exceed 90º. This occurs at frequencies above the natural frequency of the flap.
Figure 6.Illustration of a cut-out of a silicon micro-pump
If the drive frequency is further increased, then the displacement of the flaps becomes sufficiently small that the check valves do not respond to actuation. The pump rate initially rises with frequency and reaches a peak flow rate of 800 mL/min at 1 kHz. As the frequency continues to increase, the time lag between the actuation and the check valve becomes noticeable. At exactly the natural frequency of the flaps (1.6 kHz), the pump rate precipitously drops to zero. At this frequency, the phase difference is precisely 180º, meaning that both check valves are simultaneously open—hence no flow. The pump then reverses direction with further increase in frequency, reaching a peak backwards flow rate of –200 mL/min at 2.5 kHz. At about 10 kHz, the actuation is much faster than the response of the check valves, and the flow rate is zero.
For this particular device, the separation between the diaphragm and the fixed electrode is 5 mm, the peak actuation voltage is 200 V, and the power dissipation is less than 1 mW. The peak hydrostatic backpressure developed by the pump at zero flow is 31 kPa (4.5 psi) in the forward direction, and 7 kPa (1 psi) in the reverse direction.
Figure 7. Fabrication process for an electrostatically actuated micro-pump.
The fabrication is rather complex involving etching many cavities separately in each wafer, and then bonding the individual substrates together to form the stack. Etching using any of the alkali hydroxides is sufficient to define the cavities. The final bonding can be done either by gluing the different parts or using silicon-fusion bonding.
Today MEMS and associated product concepts generate plenty of excitement, but not without skepticism. Companies exploring for the first time the incorporation of MEMS solutions into their systems do so with trepidation, until an internal “MEMS technology champion” emerges to educate the company and raise the confidence level. With many micro-machined silicon sensors embedded in every car and in numerous critical medical instruments, and with additional MEMS products finding their way into our daily lives and the height of the hidden psychological barrier appears to be declining.
The future promises to bring innovative and novel MEMS solutions to a wide variety of applications, ranging from biochemical analysis to wireless and optical systems. Most of these devices and systems remain in the research and development phase, but show significant potential for becoming commercial products.
1) An Introduction to micro-electromechanical Systems Engineering
By Nadim Maluf, Artech House, Boston, London
2)iitb.ac.in Website of IIT Bombay.
3) Journal of Micromechanics and Microengineering
Scientific journal published by the Institute of Physics, Bristol, United Kingdom
4) Fundamentals of Microfabrication
By Madou, M Boca Raton, FL: CRC Press, 1997.
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Micro Electro Mechanical Systems(MEMS)
MEMS Presentation1.ppt (Size: 172 KB / Downloads: 150)
MEMS : A Technology from Lilliput
MEMS - acronym for Micro Electro-Mechanical Systems.
It is a portfolio of techniques and processes to design and create miniature systems.
That means- it is simultaneously a toolbox, a physical product and a methodology all in one
It is at it’s developing stage- main application are in automobile and medical field.
Example- 1. Air bag safety system
2. Pace makers etc…
The Sandbox: Materials for MEMS
MEMS mainly uses Silicon material system.
Silicon material system consists of
2) silicon oxide and nitrides
MEMS also uses
polymers –in the form of polyimides or photoresist
thin metal films - nickel, permalloy, titanium, aluminium etc…
Other materials and substrates used in MEMS are
1) Glass and quartz substrates.
2) Silicon carbide and diamond.
3) Gallium arsenide and other group III-V compound semiconductors.
4) Shape-memory alloys.
The Toolbox: Process for MEMS
The set of design and fabrication tools that precisely machine from structures and elements at a micro scale.
It is a parallel (batch) process.
Mainly tools for this can be divided into two
1) Basic process tools
2) Advanced process tools
Actual surface micro machining is a combination of any of these tools, which may depend on the type of application.
Illustration of the basic process tools
Commercial MEMS Structures and Systems
General design methodology
Techniques for sensing and actuation
i) common sensing methods
ii) common actuation methods
Common sensing methods
Common actuation methods
Electrostatic, Thermal, Magnetic, Piezoresistive, phase recovery using shape-memory alloys etc…
Relative merits of sensing methods
Illustration of MEMS: Micro pump