MEMS and Microsystems Unit-1 Introduction to MEMS & Microsystems: MEMS: MEMS stands for Micro-electromechanical systems. • • Micro - Small size, microfabricated structures Electro - Electrical signal ( In / Out ) • • Mechanical - Mechanical functionality (Out/ In ) Systems - Structures, Devices, Systems controls Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology. MEMS contains at least one physical dimension in micro meter level. Generally MEMS are made up of components between 1 to 100 micrometers in size. Microsystems: Engineering systems that could contain MEMS components that are design to perform specific engineering functions. Introduction to Microsensors: Sensor: The word sensor derived from a latin word "Sentire" which means to perceive. It is a device which gives information about the physical and chemical signals which could not otherwise be directly perceived by our senses. (or) It is a device that responds to a physical or chemical, some stimulus and is transmit a resulting impulse as per measurement. Transducer: The word transducer derived from a latin word "Transducere" which means to lead across. It is a device that converts energy from some system to another in the same or in different form. Technological aspects of sensors: Full scale input (Span): is basically the dynamic range over which a stimulus can be converted by a sensor. For broad span it is expressed in decibels. • • For power ratios 1dB = 10 log (P2/P1) For voltage ratios 1dB = 10 log (V2/V1). Full scale output: It is the algebraic difference between the end points of the output. Accuracy: the highest deviation of a value represented by the sensor reading from the ideal or true value. Calibration error: It is the inaccuracy permitted by the manufacturer when sensor is calibrated at the factory. Hysteresis: It is the maximum difference in output at any measurand value when the value is approached first with an increasing and decreasing mode. Resolution: It is the minimum change of the measurand value necessary to produce a detectable change in the output or smallest increments of stimulus that can be sensed. For analog sensors, expressed in terms of the units of the stimulus (eg., 0.50C) For digital sensors, expressed in terms of the no. of bits in the data word (eg., 8 bits) Drift: Changes in sensor performance within hours, day or years. Reliability: The ability of a sensor to perform a required function under stated conditions for a stated period. Offset: output of sensor with zero measurand applied at room temperature. Warmup time: Time between applying sensor power and the moment when the sensor can operate within its sensing accuracy. Application of MEMS: Automotive Electronics Medical Communications Defence Internal navigation sensors Air conditioning Compressor sensor Disk drive heads Blood pressure sensor Munitions guidance Inkjet printer Heads Brake force sensors & Suspension control Accelerometers Fuel level and vapour pressure sensors Projection screen televisions Muscle stimulators & drug delivery systems Implanted pressure sensors Fibre-optic network components RF Relays, switches and filters Earthquake Sensors Prosthetics Projection displays in portable Communications devices and Instrumentation Voltage controlled oscillators (VCOs) Surveillance Arming systems Embedded sensors Airbag sensors Avionics pressure sensors "Intelligent" tyres Mass data storage systems Miniature analytical instruments Pacemakers Splitters and Couplers Data storage Tuneable lasers Aircraft control Evolution of MEMS: Micromachining/MEMS/Microsystems Evolution: Discovery of strong piezoresistive effect in Si and Ge (1954) Photolithography and etching recognised as a tool for micromachining (1980) Silicon established as an excellent mechanical material (1982) Mushrooming gtowth of MEMS industry Evolution of Semiconductor Sensors: • • • • Discovery Phase (1947-1960) Basic technology development phase (1960-1970) Batch Process Phase (1970-1980) Micromachining Phase (1980-Present). Last ten years Next ten years "Decade of Microprocessor" "Decade of Microsensors". Chronology of MEMS Development: • • • 1975-1989 Micromachining/Silicon MEMS/Microsystems 1979(1993*) First MEMS accelerometer (commercial*) 1990- Surface Micromachining, Advanced MEMS Technology compatible with CMOS Process. MEMS Market: Characteristics of MEMS Market: • • few large volume applications CAGR(Compound Annual Growth Rate) : 25% Today Large Volume Applications: • • • Ink Jet Head Pressure and acceleration sensor forautomotive applications Pressure sensors for medical applications New Killer Applications expected: • • • RF MEMS for wireless applications. It will start with high end applications, like every time and not with big markets Opto MEMS for telecommunication. It may restart but not within the next 24 months. Bio-MEMS strong growth since 1997 for DNA chip and proteomic chip but very fragmented in terms of applications and technologies. Major growth areas of MEMS, they use a specific process to make devices for key customers that is the applications specific. MEMS Market growth: (Millions of US Dollars) Automotive Medical IT&Ind Military and Aerospace Total 1996 355 165 492 62 1074 2000 646 291 733 111 1781 2004 1172 716 1514 202 3604 Sales of MEMS Devices: Devices and applications Ink Jet Printers, microfluidics mass 1996 biolab chips: 400-500 3000-4450 Pressure Sensors: automotive, medical and industries 390-760 1100-2150 Accelerometers and Gyroscopes: Automotive and Aerospace 350-540 700-1400 Optical switches Communications and flow sensors, 2003 displays: Photonics and 25-40 440-950 Other devices such as micro relays, sensors, disk heads 510-1050 1230-2470 Total in MILLION (€) 1675-2890 6470-11420 MEMS Materials: Silicon: Silicon is the material used to create most integrated circuits used in consumer electronics in the modern world. It is also an attractive material for the production of MEMS, as it displays many advantageous mechanical and chemical properties: Single crystalline silicon is an almost perfect Hookean material. This means that when silicon is bent there is virtually no hysteresis and hence almost no energy loss. This property makes it to the ideal material, where many small motions and high reliability are demanded, as silicon displays very little fatigue and can achieve service lifetimes in the range of billions to trillions of cycles. 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 be produced. 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 orstereolithography and are especially well suited to micro fluidic applications such as disposable blood testing cartridges. 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, aluminium, copper, chromium, titanium, tungsten, platinum, and silver. Ceramics: A ceramic material is an inorganic, non-metallic, often crystalline oxide, nitride or carbide material. Some elements, such as carbon or silicon, may be considered ceramics. Material Properties: Mechanical Properties of materials:Young’s Modulus: Young ' s modulus = Y= longitudinal stress longitudinal strain F/A FL = L / L A L If higher the Y, lesser the material deforms. Poisson's ratio: Thermal expansion: Thermal expansion is the tendency of material to change in length, area, and volume in response to a change in temperature. Hardness: Hardness is defined as the ability of a material to resist plastic deformation. Creep: Permanent deformation of a material under constant load (or constant stress) as a function of time at high temperatures. Yield strength: The stress above which the material will permanently deforms. Tensile strength: The stress above which the material will completely break. Fatigue: Fatigue is the tendency of a member to fail at stress levels below yield stress when subject to cyclical loading. Toughness: Toughness is the ability of a material to absorb energy and plastically deform without fracturing. ELECTRONIC PROPERTIES OF MATERIALS: Mobility: It is the measure of how quickly an electron can move through a metal or semiconductor in presence of electrical field. Resistivity: It is a measure of the resisting power of a specified material to the flow of an electric current. The unit for resistivity is the ohm-metre. Piezoresistive effect: The piezoresistive effect is a change in the electrical resistivity of a semiconductor or metal when mechanical strain is applied. Piezoelectric effect: Piezoelectric Material will generate electric potential when subjected to some kind of mechanical stress. Compression Effect: Decrease in volume and it has a voltage with the same polarity as the material. Tension Effect: Increase in volume and it has a voltage with opposite polarity as the material. Polarity: It is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole Thermoresistivity: The property of resistance changes with change in temperature. Pyroelectricity: Pyroelectricity is the property of certain materials to generate a electrical potential or temporary voltage when they are heated or cooled. Eg: ZnO OPTICAL PROPERTIES: Photovoltaic effect: The generation of voltage across the PN junction in a semiconductor due to the absorption of light radiation is called photovoltaic effect. The Devices based on this effect is called photovoltaic device. The solar cells are based on the principles of photovoltaic effect Photoelectric effect: The photoelectric effect is the emission of electrons or other free carriers when light shines on a material. Electrons emitted in this manner can be called photo electrons. PIEZOELECTRIC MATERIAL: Piezoelectric material is one that posses the property converting mechanical energy into electrical energy and vice versa. Eg: Quartz (SiO2) APPLICATIONS: Power Generating Sidewalk: Gyms And Workplaces • • • Vibrations caused from machines in the gym. At workplaces piezoelectric crystal are laid in the chairs for storing energy. Utilizing the vibrations in the vehicle like clutches, gears etc. MOBILE KEYPADS & KEYBOARDS: For every key pressed vibrations are created. charging purposes. These vibrations can be used for FLOOR MATS AND PEOPLE POWERED DANCE CLUBS: • • • Series of crystals can be laid below the floor mats, tiles and carpets. One footstep can only provide enough electrical current to light two 60-watt bulbs for one second. [source: Christian Science Monitor]. When mob uses the dance floor, an enormous voltage is generated. This energy is used to power the equipment of nightclubs. OTHER APPLICATIONS: Electric cigarette lighter: Pressing the button of the lighter causes a spring-loaded hammer to hit a piezoelectric crystal, producing a sufficiently high voltage that electric current flows across a small spark gap, thus heating and igniting the gas. used in electronic drum pads to detect the impact of the drummer's sticks. Transient pressure measurement to study explosives, internal combustion engines (knock sensors), and any other vibrations, accelerations, or impacts. MEMS Material Properties: SILICON: Mechanical Properties of Silicon: Density = Aluminium and 1/3 of steel. Hardness = ½ of steel and > Iron, tungsten and Al Thermal expansion coefficient = 1/5 of steel Yield strength = 2 times > steel Youngs modulus = steel Thermal conductivity = 1.5 of steel High sensitivity to stress. This property used as piezoresistive sensors. • • • • • • • • • It is mechanically stable and it can be integrated into electronics on the same substrate. Silicon is almost an ideal structural material. It has about the same Young’s modulus as steel (about 2 × 105 MPa), but is as light as aluminum. It has a melting point at 1400 , which is a about twice as high as that of aluminum. This high melting point makes silicon dimensionally stable even at elevated temperature. Its thermal expansion coefficient is about 8 times smaller than that of steel, and is more than 10 times smaller than that of aluminum. Silicon is an elastic material with no plasticity or creep below 800℃. It shows virtually no fatigue under all conceivable circumstance. These unique characteristics make it an ideal material for sensing and actuating in microsystems. However, it is a brittle material. Therefore, undesirable brittle fracture behavior with weak resistance to impact loads needs to be considered in the design of such microsystems. Another disadvantage of silicon substrates is that they are anisotropic. This makes accurate stress analysis of silicon structures tedious, since directional mechanical property must be included. Single crystalline silicon is an almost perfect Hookean material. This means that when silicon is bent there is virtually no hysteresis and hence almost no energy loss. This property makes it to the ideal material METALS: • • • • • High strength Conduct electricity Conduct heat Shiny High melting point POLYMER: Structure: This type of material is made up of long chains of organic (mainly hydrocarbon) molecules. The combined molecules, i.e., polymer molecules, can be a few hundred nanometers long. Properties: Low mechanical strength, low melting point, and poor electrical conductivity characterize polymers. • • Thermoplastics-easily formed to the desired shape for the specific product Thermosets-have better mechanical strength and temperature resistance up to 350 degrees Applications of Polymers: 1. Plastics 2. Rubbers 3. Fibers 4. Surface finishesand protective coatings 5. Adhesives CERAMICS: • • • • • • • • • Low density compared to metals High melting point High hardness and very brittle Low toughness High electrical resistivity Low thermal conductivity High temperature wear resistance Thermal Shock resistance High corrosion resistance In crystalline ceramics the nitrides of silicon, aluminium and titanium as well as silicon carbide and other ceramics are increasingly applied in MEMS fabrication due to advantageous combinations of material properties. Silicon Dioxide: • Used as a thermal and electric insulator, • used as a mask in the etching of silicon substrates, • used as a sacrificial layer in surface micromachining. • Has much stronger resistance to most etchants than silicon. Production: Heating silicon in an oxidant such as oxygen with or without steam. Chemical reactions for such processes • • “dry” oxidation Si + O2 → SiO2 “wet” oxidation Si + 2H2O → SiO2 + 2H2 Silicon Carbide: • The high sublimation temperature of SiC (approximately 2700 °C) makes it useful for bearings and furnace parts. • Silicon carbide does not melt at any known temperature. • Silicon Carbide MEMS are suitable for harsh environment. • It is also highly inert chemically. There is currently much interest in its use as a semiconductor material in electronics, where its high thermal conductivity, high electric field breakdown strength and high maximum current density make it more promising than silicon for high-powered devices. • SiC also has a very low coefficient of thermal expansion (4.0 × 10−6/K) and experiences no phase transitions that would cause discontinuities in thermal expansion. Principal applications: • Its dimensional and chemical stability at high temperatures. • very strong resistance to oxidation even at very high temperatures. • Thin films of silicon carbide are often deposited over MEMS components to protect them from extreme temperature. Silicon Nitride: • It is a chemical compound of the elements silicon and nitrogen. Si3N4 is the most thermodynamically stable of the silicon nitrides. • It is a white, high-melting-point solid that is relatively chemically inert, being attacked by dilute HF and hot H2SO4. • It is very hard. • It has a high thermal stability. Automobile industry One of the major applications of sintered silicon nitride is in automobile industry as a material for engine parts, such as diesel engines precombustion chambers spark-ignition engines exhaust gas control valves. Electronics Silicon nitride is often used as an insulator and chemical barrier in manufacturing integrated circuits.
