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Introduction to mems
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  1. 1. Overview of MEMS and Microsystems 2/25/2022 1
  2. 2. What are MEMS? o MEMS is an acronym for Micro-Electro-Mechanical Systems. o The first M (micro) indicates the small size of MEMS devices. Any engineering system that performs electrical and mechanical functions with components in micrometers is a MEMS. (1 µm = 1/10 of human hair) o The E (electro) refers to electricity, often in the form of electrostatic forces. o The second M (mechanical) refers to the fact that these tiny devices have moving parts. o Lastly, S (systems) indicates that “electro” and “mechanical” go together, that the electricity and moving parts are integrated into a single system on a MEMS device. 2/25/2022 2 REVA UNIVERSITY
  3. 3. What are MEMS? o Micro Electro Mechanical Systems (MEMS ) are devices that have static or movable components with some dimensions on the scale of microns. o MEMS combine microelectronics and micromechanics, and sometimes micro-optics They are referred by different names in different countries o MEMS : USA o MicroSystemsTechnology (MST): EUROPE o Micromachines : JAPAN o Smart materials and Smart Structures: India 2/25/2022 3 REVA UNIVERSITY
  4. 4. What are MEMS? o Micro-electromechanical systems (MEMS) is a process technology used to create tiny integrated devices or systems that combine mechanical and electrical components. o They are fabricated using integrated circuit (IC) batch processing techniques and can range in size from a few micrometers to millimetres. o These devices (or systems) have the ability to sense, control and actuate on the micro scale, and generate effects on the macro scale. 2/25/2022 4 REVA UNIVERSITY
  5. 5. How are MEMS made? • Let us first consider creating a thin, flexible diaphragm that may ultimately be used as part of a MEMS pressure sensor. • We start with a thin silicon substrate, called a wafer, typically measuring 200-400 µm thick fig a. • A thin layer of silicon dioxide (SiO2) is then “grown” on the wafer by placing it in a furnace at an elevated temperature fig b. • Next, a thin layer of photosensitive material called photoresist, or simply resist, is deposited on the SiO2 layer in a process called spinning Fig c. 2/25/2022 5 REVA UNIVERSITY
  6. 6. • A transparent plate with selective opaque regions called a mask is then brought in close proximity to the wafer Fig.d. • Ultraviolet light is shown through the mask Fig e. • On the regions of the photoresist that make contact with the UV light, the resist • undergoes a photochemical process in which it hardens and becomes less soluble. (This is true for a negative resist. If a positive resist were used, then the exposed regions would become more soluble.) The unexposed resist is removed by using a chemical called a developer, leaving a portion of the SiO2 layer exposed Fig.f. 2/25/2022 6 REVA UNIVERSITY
  7. 7. 2/25/2022 REVA UNIVERSITY 7 Another example
  8. 8. 2/25/2022 8 REVA UNIVERSITY
  9. 9. MEMS and Microsystems MEMS as a Microsensor Microsensors (To sense and detect certain physical, chemical, biological and optical quantity and convert it into electrical output signal) 2/25/2022 9 REVA UNIVERSITY
  10. 10. MEMS and Microsystems 2/25/2022 10 REVA UNIVERSITY
  11. 11. MEMS and Microsystems Microactuators (to operate a device component, e.g., valves, pumps, electrical and optical relays and switches; grippers, tweezers and tongs; linear and rotary motors; micro gyroscopes, etc. MEMS as a Microatuator -Motor 2/25/2022 11 REVA UNIVERSITY
  12. 12. Components of Microsystem Microsystems = sensors + actuators + signal transduction 2/25/2022 12 REVA UNIVERSITY
  13. 13. Intelligent Microsystems –Micromechatronics systems 2/25/2022 13 REVA UNIVERSITY
  14. 14. MEMS and Microsystem Products 2/25/2022 14 REVA UNIVERSITY Micromotors • All three components Rotor Stator andTorque transmission gear made with Nickel. • The toothed rotor,Which has diameter 700µm. • Gear wheel 250µm. • The gap between the rotor and the Axle and between Rotor and the Stator is 4µm. • The Height of the unit 120µm
  15. 15. Microgears • Two-level gear made from ceramics. • Pitch of the gear is 100µm. 2/25/2022 REVA UNIVERSITY 15
  16. 16. Microturbines • The turbine is made by Nickel. • The rotor has a diameter of 130µm. • A gap between the axle and the rotor is 5µm. • The turbine height is 150µm. • The maximum rpm 150,000perminute with lifetime up to 100 million rotations. 2/25/2022 REVA UNIVERSITY 16
  17. 17. Micro- optical components • These components are used for high speed signal transmission in the telecommunication industry. • It is silicon based manufacturing process. 2/25/2022 REVA UNIVERSITY 17
  18. 18. Micro- optical components 2/25/2022 REVA UNIVERSITY 18
  19. 19. MEMS and Microsystem Products 2/25/2022 19 REVA UNIVERSITY
  20. 20. 2/25/2022 20 REVA UNIVERSITY
  21. 21. Evaluation of Microfabrication • The origin of modern Micro fabrication to the invention of transistors by W. Schockley, J. Bardeen, and W. H. Brattain in 1947. • The IC concept first evolved from the production of a monolithic circuit at RCA in 1955 after the invention of transistors. • The first IC was produced 3 years later by Jack Kilby of TI. 2/25/2022 21 REVA UNIVERSITY
  22. 22. Microelectronics 1. Stationary structures. 2. Transmit electricity for specific electrical functions. 3. IC die is protected from contacting media 4. Use single crystal silicon dies, silicon compounds, ceramics and plastic materials 5. Fewer components to be assembled Microsystems 1. May involve moving components 2. Perform a great variety of specific biological, chemical, electromechanical and optical functions 3. Delicate components are interfaced with working media 4. Use single crystal silicon dies and few other materials, e.g. GaAs, quartz, polymers, ceramics and metals 5. Many more components to be assembled 2/25/2022 REVA UNIVERSITY 22
  23. 23. Microelectronics 1. Complex patterns with high density of electrical circuitry over substrate 2. Mature IC design methodologies. 3. Large number of electrical feed-through and leads 4. Industrial standards available 5. Mass production. 6. Fabrication techniques are proven and well documented 7. Manufacturing techniques are proven and well documented Microsystems 1. Lack of engineering design methodology and standard. 2. Simpler patterns over substrates with simpler electrical circuitry. 3. Fewer electrical feed-through and leads. 4. No industrial standard to follow in design, material selections, fabrication processes and packaging. 5. Batch production, or on customer- need basis. 6. Many microfabrication techniques are used for production, but with no standard procedures. 7. Distinct manufacturing techniques. 2/25/2022 REVA UNIVERSITY 23
  24. 24. The Multidisciplinary Nature of Microsystem Design and Manufacture 2/25/2022 24 REVA UNIVERSITY
  25. 25. 2/25/2022 REVA UNIVERSITY 25
  26. 26. 2/25/2022 26 REVA UNIVERSITY
  27. 27. 2/25/2022 REVA UNIVERSITY 27
  28. 28. Miniaturization Makes Engineering Sense!!! • Small systems tend to move or stop more quickly due to low mechanical inertia. It is thus ideal for precision movements and for rapid actuation. • Miniaturized systems encounter less thermal distortion and mechanical vibration due to low mass. • Miniaturized devices are particularly suited for biomedical and aerospace applications due to their minute sizes and weight. • Small systems have higher dimensional stability at high temperature due to low thermal expansion. • Smaller size of the systems means less space requirements. This allows the packaging of more functional components in a single device. • Less material requirements mean low cost of production and transportation. • Ready mass production in batches. 2/25/2022 28 REVA UNIVERSITY
  29. 29. Applications of Microsystems in the Aautomotive Industry • 52 million vehicles produced worldwide in 1996There will be 65 million vehicle produced in 2005. • Principal areas of application of MEMS and microsystems. • Safety • Engine and power train • Comfort and convenience • Vehicle diagnostics and health monitoring • Telematics, e.g. GPS, etc 2/25/2022 29 REVA UNIVERSITY
  30. 30. 2/25/2022 REVA UNIVERSITY 30
  31. 31. Silicon Capacitive Manifold Absolute Pressure Sensor 2/25/2022 REVA UNIVERSITY 31
  32. 32. Application of MEMS and Microsystems in Biomedical Industry • Disposable blood pressure transducers: Lifetime 24 to 72 hours; annual production 20 million units/year, unit price $10 • Catheter tip pressure sensors • Sphygmomanometers • Respirators • Lung capacity meters • Barometric correction instrumentation • Medical process monitoring • Kidney dialysis equipment • Micro bio-analytic systems: bio-chips, capillary electrophoresis, etc. 2/25/2022 32 REVA UNIVERSITY
  33. 33. Application of MEMS and Microsystems in Aerospace Industry • Cockpit instrumentation. • Sensors and actuators for safety -e.g. seat ejection • Wind tunnel instrumentation • Sensors for fuel efficiency and safety • Microsattellites • Command and control systems with MEMtronics • Inertial guidance systems with microgyroscopes, accelerometers and fiber optic gyroscope. • Attitude determination and control systems with micro sun and Earth sensors. 2/25/2022 REVA UNIVERSITY 33
  34. 34. Application of MEMS and Microsystems in Aerospace Industry • Power systems with MEMtronicswitches for active solar cell array reconfiguration, and electric generators. • Propulsion systems with micro pressure sensors, chemical sensors for leak detection, arrays of single-shot thrustors, continuous microthrusters and pulsed microthrousters. • Thermal control systems with micro heat pipes, radiators and thermal switches. • Communications and radar systems with very high bandwidth, low-resistance radio-frequency switches, micromirrorsand optics for laser communications, and micro variable capacitors, inductors and oscillators 2/25/2022 REVA UNIVERSITY 34
  35. 35. Application of MEMS and Microsystems in Consumer Products • Scuba diving watches and computers • Bicycle computers • Sensors for fitness gears • Washers with water level controls • Sport shoes with automatic cushioning control • Digital tire pressure gages • Vacuum cleaning with automatic adjustment of brush beaters • Smart toys 2/25/2022 REVA UNIVERSITY 35
  36. 36. Application of MEMS and Microsystems in theTelecommunication Industry • Optical switching and fiber optic couplings • RF relays and switches • Tunable resonators 2/25/2022 REVA UNIVERSITY 36
  37. 37. Materials for MEMS and Microsystems
  38. 38. Substrates and Wafers ▪ The frequently used term substrate in microelectronics means a flat macroscopic object on which micro-fabrication. ▪ Substrate serves an additional purpose: It acts a signal transducer besides supporting other transducer that convert mechanical action to electrical out puts or vice versa. ▪ In semiconductor, the substrate is a single crystal cut in slices from a larger piece called wafer. ▪ Wafers can be of silicon or other single crystalline materiel such as a Quartz, or Gallium Arsenide.
  39. 39. Over view Materials for MEMS and Microsystems ▪ This chapter will cover the materials used in “silicon- based” MEMS and Microsystems. As such, silicon will be the principal material to be studied. ▪ Other materials to be dealt with are silicon compounds such as: SiO2, SiC, Si3N4 and polysilicon. ▪ Also will be covered are electrically conducting of silicon piezoresistors and piezoelectric crystals for electromechanical actuations and signal transductions. ▪ An overview of polymers, which are the “rising stars” to be used as MEMS and Microsystems substrate materials, will be studied too.
  40. 40. Why silicon is the most important in IC Industry? ▪ Silicon (Si) is the most abundant material on earth. It almost always exists in compounds with other elements. ▪ Single crystal silicon is the most widely used substrate material for MEMS and microsystems. ▪ The popularity of silicon for such application is primarily for the following reasons.
  41. 41. Why silicon is the most important in IC Industry? I. It is mechanically stable and it is feasible to be integrated into electronics on the same substrate (b/c it is a semiconducting material). II. Electronics for signal transduction such as the p or n-type piezoresistive can be readily integrated with the Sisubstrate-ideal for transistors. III. Silicon is almost an ideal structure material. It has about the same Young’s modulus as steel (∼2x105MPa), but is as light as aluminum with a density of about 2.3 g/cm3. IV. It has a melting point at 1400oC, which is about twice higher than that of aluminum. This high melting point makes silicon dimensionally stable even at elevated temperature.
  42. 42. V. Its thermal expansion coefficient is about 8 times smaller than that of steel, and is more than 10 times smaller than that of aluminium. VI. Silicon shows virtually no mechanical hysteresis. It is thus an ideal candidate material for sensors and actuators. VII. Silicon wafers are extremely flat for coatings and additional thin film layers for either being integral structural parts, or performing precise electromechanical functions. VIII.There is a greater flexibility in design and manufacture with silicon than with other substrate materials. Treatments and fabrication processes for silicon substrates are well established and documented.
  43. 43. Crystal Growing and Theory ❑ How the single crystal can be grown in practice. ❑ The starting point in any IC fabrication is we must have single crystal silicon wafer so must get the substrate material. ❑ In order to get substrate material you must grow single crystal silicon. ❑ If its found nature as SiO2, so first a silicon dioxide is reduced in order to obtain silicon its purified in order to get very high purity semiconductor grade silicon i.e. 99.9999% purity. ❑ Crystal growth can be broadly classified as 2 types 1. Bridgman Technique ( NOT IN SYLLABUS) 2. Czochralski Technique
  44. 44. Czochralski Method ❑ Czochralski (CZ) is also known as liquid solid mono component growth system. ❑ It has basically 4 subsystems in this CZ crystal growth system i.e. 1. Furnace 2. Crystal pulling mechanism 3. Ambient control and 4. Control system Scheme of the Bridgman technique. 1 crucible, 2 growing crystal, 3 seed, 4 furnace.
  45. 45. ❑ The most important part in this furnace is the crucible i.e. a cup in which charge is going to be placed and this is usually made up of quartz. ❑ Usually the crucible is made of quartz and it is a single, you can use it only once because after the crystal growth when ever cooling down the system thermal mismatch usually the quartz crucible is going to crack so you can not reuse the crucible. ❑ The quartz crucible is usually placed inside a graphite susceptor. A susceptor is you can view it outer jacket i.e. I have bigger cup of graphite in which I am going to place the quartz cup. ❑ Heating is done by usually by RF. • So inside the furnace ✓ quartz crucible ✓ graphite susceptor ✓ Heater ✓ Cooling for the outer quartz chamber
  46. 46. Crystal pulling mechanism ❑ A pull rod is pull up during the crystal growth but beware of Oxygen. ❑ I have the melt inside the crucible but you can remember the seed crystal was not in contact with the melt, it was held some ware up while the charge was below. ❑ After the charge is molten its uniformly in the liquid state then pull rod is gradually load. ❑ Very slowly pull rod is pull up so what will happen the melt is contact with the seed crystal will get solidified and pull up the solid crystal. ❑ But accurate control is necessary and the pull rate you should carefully adjusted.
  47. 47. Seed Single crystal silicon Quartz crucible Water cooled chamber Heat shield Carbon heater Graphite crucible Fig: Schematic of Crystal pulling mechanism
  48. 48. Fig: Crystal pulling mechanism Fig: Silicon Ingot
  49. 49. Silicon Compounds ▪ There are 3principal silicon compounds used in MEMS and microsystems: 1. Silicon dioxide (SiO2) 2. Silicon carbide (SiC) and 3. Silicon nitride (Si3N4) ▪ Each Has distinct characteristic and unique applications.
  50. 50. Silicon Dioxide(SiO2) ▪ It is least expensive material to offer good thermal and electrical insulation. ▪ Also used a low-cost material for “masks” in micro fabrication processes such as etching, deposition and diffusion. ▪ Used as sacrificial material in “surface micromachining”. ▪ Above all, it is very easy to produce: ✓ by dry heating of silicon: Si + O2→SiO2 ✓ by oxide silicon in wet steam: Si + 2H2O →SiO2+ 2H2
  51. 51. Silicon dioxide(SiO2) –cont’d
  52. 52. Silicon Carbide (SiC) ❑ The principle applications of SiC in Microsystems is its dimensional and chemical stability at high temperature. ❑ It has very strong resistance to oxidation even at very high temperature. ❑ Thin films of silicon carbide are often deposited over MEMS components to protect them from extreme temperature. ❑ Its very high melting point and resistance to chemical reactions make it ideal candidate material for being masks in micro fabrication processes. ❑ Using SiC in MEMS is that Dry etching with aluminium masks can easily pattern the thin SiC film.
  53. 53. Silicon Nitride (Si3N4) ▪ Produced by chemical reaction: 3SiCl2H2+ 4NH3→Si3N4+ 6HCL + 6H2 ▪ Used as excellent barrier to diffusion to water and ions. ▪ Its ultra strong resistance to oxidation and many etchants make it a superior material for masks in deep etching. ▪ Applications of silicon nitride include optical waveguides, encapsulants to prevent diffusion of water and other toxic fluids into the substrate. ▪ Also used as high strength electric insulators.
  54. 54. Polycrystalline Silicon ▪ It is usually called “Polysilicon”. ▪ It is an aggregation of pure silicon crystals with randomly orientations deposited on the top of silicon substrates:
  55. 55. Selected properties Si3N4 film sare as follows:
  56. 56. Polycrystalline silicon –cont’d • These polysilicon usually are highly doped silicon. • They are deposited to the substrate surfaces to produce localized “resistors” and “gates for transistors”. • Being randomly oriented, polysilicon is even stronger than single silicon crystals.
  57. 57. Polycrystalline silicon –cont’d
  58. 58. Silicon Piezoresistors
  59. 59. Silicon Piezoresistors–Cont’d
  60. 60. Silicon Piezoresistors–Cont’d
  61. 61. Silicon Piezoresistors–Cont’d
  62. 62. Gallium Arsenide (GaAs)
  63. 63. Gallium Arsenide (GaAs)-Cont’d
  64. 64. Quartz
  65. 65. Quartz-Cont’d ▪ Quartz is ideal material for sensors because of its extreme dimensional stability. ▪ It is used as piezoelectric material in many devices. ▪ It is also excellent material for microfluidics systems used in biomedical applications. ▪ It offers excellent electric insulation in microsystems. ▪ A major disadvantage is its hard in machining. It is usually etched in HF/NH4F into desired shapes. ▪ Quartz wafers up to 75 mm diameter by 100 µm thick are available commercially.
  66. 66. Piezoelectric Crystals
  67. 67. Piezoelectric Crystals –Cont’d
  68. 68. Piezoelectric Crystals –Cont’d
  69. 69. Polymers ❑ What is polymer? Polymers include: Plastics, adhesives, Plexi glass and Lucite. ❑ Principal applications of polymers in MEMS: Currently in biomedical applications and adhesive bonding. New applications involve using polymers as substrates with electric conductivity made possible by doping. ❑ Molecular structure of polymers: It is made up of long chains of organic (hydrocarbon) molecules. The molecules can be as long as a few hundred nm. ❑ Characteristics of polymers: Low melting point; Poor electric conductivity Thermoplastics and thermoset sare common industrial products Thermoplastics are easier to form into shapes. Thermosets have higher mechanical strength even at temperature up to 350oC
  70. 70. Polymers as industrial materials ❑ Polymers are popular materials used for many industrial products for the following advantages: ✓ Light weight ✓ Ease in processing ✓ Low cost of raw materials and processes for producing polymers ✓ High corrosion resistance ✓ High electrical resistance ✓ High flexibility in structures ✓ High dimensional stability
  71. 71. Polymers for MEMS and microsystems 1) Photo-resist polymers are used to produce masks for creating desired patterns on substrates by photolithography technique. 2) The same photoresistpolymers are used to produce the prime mold with desirable geometry of the MEMS components in a LIGA processin micro manufacturing. 3) Conductive polymers are used as “organic” substrates for MEMS and microsystems. 4) The ferroelectric polymersthat behave like piezoelectric crystals can be used as the source of actuation in micro devices such as in micro pumping. 5) The thin Langmuir-Blodgett (LB) film scan be used to produce multilayer microstructures. 6) Polymers with unique characteristics are used as coating substance to capillary tubes to facilitate effective electro-osmotic flow in microfluidics. 7) Thin polymer films are used as electric insulatorsin micro devices, and as dielectric substancein micro capacitors. 8) They are widely used for electromagnetic interference (EMI) and radio frequency interference (RFI) shielding in microsystems. 9) Polymers are ideal materials for encapsulation of micro sensors and the packaging of other microsystems.
  72. 72. Conductive Polymers ❑ Polymers are poor electric conducting materials by nature. ❑ A comparison of electric conductivity of selected materials are:
  73. 73. Conductive Polymers –Cont’d
  74. 74. Langmuir-Blodgett (LB) films ▪ The process was first introduced by Langmuir in 1917 and was later refined by Blodgett. That was why it is called Langmuir-Blodgett process, or LB films. ▪ The processin volves the spreading volatile solvent over the surface-active substrate materials. ▪ The LB process can produce more than one single monolayer by depositing films of various compositions onto a substrate to produce a multilayer structure. ▪ LB films are good candidate materials for exhibiting ferro(iron)-, pyro(heat)and piezoelectric properties. LB films may also be produced with controlled optical properties such as refractive index and anti reflections. ▪ They are thus ideal materials for micro sensors and optoelectronic devices.
  75. 75. Langmuir-Blodgett (LB) films –Cont’ ❑ Following are a few examples of LB film applications in microsystems: ❑ Langmuir-Blodgett (LB) films –Cont’d (1)Ferroelectric (magnetic) polymer thin films: ▪ The one in particular is the Poly-vinylidenefluoride (PVDF). ▪ Applications of this type of films include: - Sound transducers in air and water, - Tactile sensors, - Biomedical applications such as tissue compatibility, cardio- pulmonary sensors and implantable transducers and sensors for prosthetics and rehabilitation devices. ▪ As a piezoelectric source. The piezoelectric coefficient of PVDF is given in Table 7-14. (2) Coating materials with controllable optical properties: Broadband optical fibers that transmit light at various wavelengths.
  76. 76. Langmuir-Blodgett (LB) films –Cont’d (3) Microsensors: • Many electrically conducting polymeric materials are sensitive to the exposed gas and other environmental conditions. So they are suitable materials for micro sensors. • Its ability of detecting specific substances relies on the reversible and specific absorption of species of interest on the surface of thepolymer layer and the subsequent measurable change of conductivity of the polymer.