![(a) A MEMS silicon motor together with a strand of human hair , and
(b) the legs of a spider mite standing on gears from a micro-engine
[Reference: Sandia National Labs, SUMMiT *Technology,
http://mems.sandia.gov].](https://image.slidesharecdn.com/chapter2-210512115848/85/MEMS-Chapter-2-17-320.jpg)
MEMS Chapter 2
- 2. Books 1.G. K. Ananthasuresh, K. J. Vinoy, S. Gopalkrishnan K. N. Bhat, V. K. Aatre, Micro and Smart Systems, Wiley India, 2012. 2. S. E.Lyshevski, Nano-and Micro-Electromechanical systems: Fundamentals of Nano-and Microengineering (Vol. 8). CRC press, (2005). 3. S. D. Senturia, Microsystem Design, Kluwer Academic Publishers, 2001. 4. M. Madou, Fundamentals of Microfabrication, CRC Press, 1997. 5. G. Kovacs, Micromachined Transducers Sourcebook, McGraw-Hill, Boston, 1998. 6. M.H. Bao, Micromechanical Transducers: Pressure sensors, accelerometers, and Gyroscopes, Elsevier, New York, 2000.
- 4. CHAPTER I INTRODUCTION TO MEMS (MICRO ELECTRO MECHANICAL SYSTEMS)
- 5. Course Objective To introduce :- Basic terms and definitions of MEMS. MEMS processes and transducer mechanisms. Course Outcomes The students will be able to :- List and define basic terminology used in MEMS processes and transducers. Describe transducer working mechanism.
- 6. Chapter 1: Introduction to MEMS Introduction History Concepts of MEMS: Principles, application and design, Scaling Properties/Issues Micromachining Processes Substrates Lithography Wet/Dry etching processes Deposition processes Film stress, exotic processes Mechanical Transducer Transduction methods Accelerometers, gyroscopes, pressure sensors, MEMS microphones, mechanical structures, actuators.
- 10. Micro-electromechanical systems (MEMS) is a process technology used to create tiny integrated devices or systems that combine mechanical and electrical components. They are fabricated using integrated circuit (IC) batch processing techniques and can range in size from a few micrometers to millimetres.
- 11. MEMS devices (or systems) have the ability to sense, control and actuate on the micro scale, and generate effects on the macro scale. MEMS, an acronym, originated in the United States Microsystems Technology (MST) - Europe and Micromachines in - Japan. Regardless of terminology, the uniting factor of a MEMS device is in the way it is made.
- 12. The device electronics are fabricated using ‘computer chip’ IC technology. The micromechanical components are fabricated by sophisticated manipulations of silicon and other substrates using micromachining processes. Processes such as bulk and surface micromachining, as well as high-aspect-ratio micromachining (HARM) selectively remove parts of the silicon or add additional structural layers to form the mechanical and electromechanical components.
- 13. While integrated circuits are designed to exploit the electrical properties of silicon. MEMS takes advantage of either silicon’s mechanical properties or both its electrical and mechanical properties.
- 14. MEMS can be found in systems ranging across automotive, medical, electronic, Communication and defence applications. Current MEMS devices include accelerometers for airbag sensors, inkjet printer heads, computer disk drive read/write heads, projection display chips, blood pressure sensors, optical switches,microvalves, biosensors.
- 15. Schematic illustration of MEMS Components
- 16. MEMS devices are very small; their components are usually microscopic. Levers, gears, pistons, as well as motors and even steam engines have all been fabricated by MEMS
- 17. (a) A MEMS silicon motor together with a strand of human hair , and (b) the legs of a spider mite standing on gears from a micro-engine [Reference: Sandia National Labs, SUMMiT *Technology, http://mems.sandia.gov].
- 18. The DENSO Micro-Car is a miniature version of Toyota’s first passenger car. Fabricated using MEMS, at 1/1000th the size of the original, it consists of a 0.67 mm magnetic-type working motor and when supplied with 3 V 20 mA of alternating current through a 18 μm copper wire, the engine runs at 600 rpm equivalent to 5-6 mm/s
- 20. Lecture No:-02
- 21. History The history of MEMS is useful to illustrate its diversity, challenges and applications. The following list summarizes some of the key MEMS milestones
- 22. 1950’s 1958 Silicon strain gauges commercially available. 1959 “There’s Plenty of Room at the Bottom” – Richard Feynman gives a milestone presentation at California Institute of Technology. He issues a public challenge by offering $1000 to the first person to create an electrical motor smaller than 1/64th of an inch.
- 23. 1960’s 1961 First silicon pressure sensor demonstrated 1967 Invention of surface micromachining. Westinghouse creates the Resonant Gate Field Effect Transistor, (RGT). Description of use of sacrificial material to free micromechanical devices from the silicon substrate.
- 24. 1970’s 1970 First silicon accelerometer demonstrated 1979 First micromachined inkjet nozzle
- 25. 1980’s Early 1980’s: first experiments in surface micromachined silicon. Late 1980’s:Micromachining leverages microelectronics industry and widespread experimentation and documentation increases public interest. 1982 Disposable blood pressure transducer 1982 “Silicon as a Mechanical Material”. Instrumental paper to entice the scientific community – reference for material properties and etching data for silicon. 1982 LIGA Process 1988 First MEMS conference
- 26. 1990’s Methods of micromachining aimed towards improving sensors. 1992 MCNC starts the Multi-User MEMS Process (MUMPS) sponsored by Defense Advanced Research Projects Agency (DARPA) 1992 First micromachined hinge 1993 First surface micromachined accelerometer sold (Analog Devices, ADXL50) 1994 Deep Reactive Ion Etching is patented 1995 BioMEMS rapidly develops 2000 MEMS optical-networking components become big business
- 27. Principles MEMS - mechanical microstructures, microsensors, microactuators and microelectronics, all integrated onto the same silicon chip. The designer applies the Classical Lagrangian Newtonian mechanics Electromagnetics (Maxwell’s equations) to study conventional electromechanical systems and MEMS.
- 28. Microsensors detect changes in the system’s environment by measuring mechanical thermal magnetic chemical or electromagnetic information or phenomena. Microelectronics process this information and signal the microactuators to react and create some form of changes to the Environment.
- 29. However, MEMS is not just about the miniaturization of mechanical components or making things out of silicon. MEMS is a manufacturing technology A paradigm(way of seeing things) for designing and creating complex mechanical devices and systems as well as their integrated electronics using batch fabrication techniques.
- 30. Micro-optoelectromechanical systems (MOEMS) is also a subset of MST and together with MEMS forms the specialized technology fields using miniaturized combinations of optics, electronics and mechanics.
- 31. Applications
- 32. MEMS has several distinct advantages as a manufacturing technology. Interdisciplinary nature of MEMS technology and its micromachining techniques Batch fabrication increased performance reliability, reduced physical size, volume, weight and cost.
- 33. (a)The first commercial accelerometer from Analog Devices (1990);its size is less than 1 cm2 (left) , and (b) capacitive sense plates, 60 microns deep (right) .
- 35. The accelerometer is essentially a capacitive or piezoresistive device consisting of a suspended pendulum proof mass/plate assembly. As acceleration acts on the proof mass, micro machined capacitive or piezoresistive plates sense a change in acceleration from deflection of the plates.
- 36. Scaling Properties/Issues Scaling theory is a valuable guide to what may work and what will not. By understanding how phenomena behave and change as their scale size changes. We can gain some insight and better understand the profitable approaches.
- 37. Why are we now interested in miniature versions of devices and systems? Reductions in cost, weight, and Power consumption might be some of the reasons for miniaturization. But this might not be the case always. Sometimes, a particular principle would not even work if the mircosystems. Certain micro-opto-mechanical devices do not work at the macro level. In most other cases, miniaturization is preferred because scaling leads to several advantages
- 38. When miniaturizing any device or system, it is critical to have a good understanding of the scaling properties of the transduction mechanism, the overall design, and the material and the fabrication processes involved. The scaling properties of any one of these components could present a great challenge. Since MEMS devices can be thousands of times smaller than their macroscale counterparts, we cannot expect that the macroscale phenomena and designs will transfer directly to the microscale.
- 39. MEMS performance is inversely related to size The raw sensitivity of most sensors decreases, however the frequency response improves.. The fundamental limit of most MEMS sensor system is thermal noise. Temperature, the vibration of molecules, causes all mechanical and electrical devices to jitter around with an average kinetic energy of a few thousands of a billionth of a billionth of a Joule. While objects on the macroscale are virtually unaffected by this small amount of energy, MEMS devices that are built on the
- 40. SCALING IN THE MECHANICAL DOMAIN A fixed–fixed beam deformed under its own weight.
- 41. Inertial effects are due to various accelerations experienced by a body. These include acceleration due to gravity, centrifugal and centripetal accelerations, etc. We cannot always neglect inertial effects. This is because things move faster at small scales and, hence, have significantly large inertial forces. We note that the natural frequency of a beam is inversely proportional to its size. So, as the size decreases, the natural frequency of free vibration goes up, indicating that small beams vibrate at a much greater rate than large beams of same proportions.
- 42. SCALING IN THE ELECTROSTATIC DOMAIN Electrostatic force is widely used in microsystems, but at the macroscale we hardly use it. What makes electrostatic force so attractive at the microscale? The answer is the scaling effect
- 43. SCALING IN THE THERMAL DOMAIN Did you ever wonder why elephants have large ears ? It has to do with scaling effects. Metabolic activity in living creatures produces heat, and warm- blooded animals must maintain a certain temperature. The heat produced is proportional to the volume (cube of the size) of the animal. This heat is generally lost through the skin, that is, the surface of the animal. Because of scaling, we can say that large animals produce much more heat than their surfaces can lose by convection as compared with small animals. This is because large animals have relatively more volume than surface area. In order not to overheat, large animals need special appendages for increasing surface area. Large ears on elephants are such appendages.
- 44. SCALING IN DIFFUSION Diffusion is the phenomenon of spreading of a species due to its concentration gradients without the help of an external force. Heat and electric currents flow because of diffusion. Diffusion happens when two different gases, liquids, or solids come into contact with each other. The rate of diffusion depends on the diffusion coefficient
- 45. Micromachining Processes Micromachining Processes MEMS processes fall into three general classifications 1.Bulk micromachining 2. Surface micromachining 3.High-aspect-ratio micromachining
- 46. 1. BULK MICROMACHINING Bulk micromachining involves the removal of part of the bulk substrate. It is a subtractive process that uses wet anisotropic etching or a dry etching method such as reactive ion etching (RIE), to create large pits, grooves and channels. Materials typically used for wet etching include silicon and quartz while dry etching is typically used with silicon, metals, plastics and ceramics.
- 47. 2.SURFACE MICROMACHINING Surface micromachining involves processing above the substrate, mainly using it as a foundation layer on which to build. It was initiated in the 1980’s Material is added to the substrate in the form of layers of thin films on the surface of the substrate (typically a silicon wafer). These layers can either by structural layers or act as spacers, later to be removed, when they are known as sacrificial layers. Hence the process usually involves films of two different materials: A) A structural material out of which the free standing structure is made (generally polycrystalline silicon or polysilicon, silicon nitride and aluminium) and B) A sacrificial material, deposited wherever either an open area or a free standing mechanical structure is required (usually an oxide).
- 49. (HARM), which includes technology such as LIGA (a German acronym from Lithographie, Galvanoformung, Abformung translated as lithography, electroforming and moulding). High-aspect-ratio micromachining (HARM) is a process that involves micromachining as a tooling step followed by injection moulding or embossing and, if required, by electroforming to replicate microstructures in metal from moulded parts. It is one of the most attractive technologies for replicating microstructures at a high performance-to-cost ratio and includes techniques known as LIGA. Products micromachined with this technique include highaspect- ratio fluidic structures such as moulded nozzle plates for inkjet printing and microchannel plates for disposable microtitreplates in medical diagnostic applications. The materials that can be used are electroformable metals and plastics, including acrylate, polycarbonate, polyimide and styrene.
- 50. Lecture No:-04
- 51. Substrates The most common substrate material for micromachining is SILICON Other crystalline semiconductors germanium (Ge) and gallium arsenide (GaAs)
- 52. successful in the microelectronics industry and will continue to be in areas of miniaturization for several reasons: i) silicon is abundant, ii)inexpensive, and iii)can be processed to unparalleled purity silicon’s ability to be deposited in thin films is very amenable to MEMS high definition and reproduction of silicon device shapes using photolithography are perfect for high levels of MEMS precision silicon microelectronics circuits are batch fabricated (a silicon wafer contains hundreds of identical chips not just one)
- 53. Lithography There are processes for creating material layers required in the fabrication of microsystems. The successful development of microsystems involves successive steps of deposition and patterning of various material layers. One of the key steps in patterning is the process of transferring a geometrical pattern on a mask to a radiation sensitive material called a resist. This process is known as lithography.
- 54. Wet/Dry etching processes Wet Etching Wet etching describes the removal of material through the immersion of a material (typically a silicon wafer) in a liquid bath of a chemical etchant. These etchants can be isotropic or anisotropic.
- 55. Isotropic etchants etch the material at the same rate in all directions, and consequently remove material under the etch masks at the same rate as they etch through the material.This is also known as undercutting The most common form of isotropic silicon etch is HNA, which comprises a mixture of hydrofluoric acid (HF), nitric acid (HNO3) and acetic acid
- 56. Anisotropic etchants etch faster in a preferred direction. Potassium hydroxide (KOH) -most common anisotropic etchant -relatively safe to use. Structures formed in the substrate are dependent on the crystal orientation of the substrate or wafer. Most such anisotropic etchants progress rapidly in the crystal direction perpendicular to the (110) plane and less rapidly in the direction perpendicular to the (100) plane. The direction perpendicular to the (111) plane etches very slowly if at all. Figures 19c and 19d shows examples of anisotropic etching in (100) and (110) silicon. Silicon wafers, originally cut from a large ingot of silicon grown from single seed silicon, are cut according to the crystallographic plane. They can be supplied in terms of the orientation of the surface plane.
- 57. Deposition Processes Evaporation, sputtering, CVD, etc 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 nanometer to about 100 micrometer. MEMS deposition technology can be classified in two groups:
- 58. Depositions that happen because of a chemical reaction: ◦ Chemical Vapour Deposition (CVD) ◦ Electrodeposition ◦ Epitaxy ◦ Thermal oxidation Depositions that happen because of a physical reaction: ◦ Physical Vapor Deposition (PVD) ◦ Casting
- 59. Film stress, exotic processes Stress is defined microscopically as the force per unit area acting on the surface of a differential volume element of a solid body. Mechanical stress in thin films is an important reliability issue in microelectronic devices and systems. The presence of large stresses can lead to the formation of defects that can cause device failure. The ability to control the magnitude of stress during film formation is, therefore, crucial to the fabrication of defect-free and reliable electronic devices and systems. However, the origin of stress in thin films is still a subject of intense debate. The development of a detailed understanding of the origin of stress hinges on our ability to make accurate stress measurements during and after film deposition. To this end, two novel MEMS structures were developed to measure the stress of thin films deposited using chemical vapor deposition (CVD).
- 60. Deposited films will often be under mechanical stress. A further factor limiting film thickness and the structures that can be created is mechanical stress in the deposited films. Too much stress will lead to the structure buckling or the films’ wrinkling or cracking. (Buckling is the sudden change in shape of a structural component under load such as the bowing of a column under compression or the wrinkling of a plate under shear) High-temperature nitride films have a particular problem: they exhibit high tensile stresses within the film and cannot be deposited directly onto silicon. A stress-relieving layer of oxide is required.
- 62. It is possible to control the stress in films by altering the deposition parameters or the composition of the resulting film (using PECVD to deposit a hydrogen-contaminated silicon oxynitride layer, for instance). The mechanical, electrical, and chemical (etching) properties of the film will all be affected by the deposition parameters used, so it is necessary to carefully characterize and monitor each process (a demanding and time-consuming job) or seek out a foundry that has experience with the processes required for the device under development. Nitride films have higher stress but are mechanically harder and chemically more resilient (to attach to and with respect to diffusion of ions or moisture). Signs of Stress: Stress in films may cause one of the following several problems: Cracking or wrinkling of the film Strings peeling off from sharp corners Twisting or buckling of structures (particularly, cantilever beams) Buckling of the silicon wafer (in extreme cases)
- 63. Mechanical Transducers There is a tremendous variety of direct mechanical sensors that have been or could be micromachined depending on their sensing mechanism (usually piezoresistive, piezoelectric or capacitive) and the parameters sensed (typically strain, force and displacement).
- 64. Piezoresistive sensors As a result of the piezoresistive effect (defined as the change in resistivity of the material with applied strain), changes in gauge dimension result in proportional changes in resistance in the sensor. The piezoresistive effect in semiconductors is considerably higher than in traditional metals, making silicon an excellent strain sensor. MEMS piezoresistors are readily manufactured using bulk silicon doped with p-type or n-type impurities.
- 65. Piezoelectric sensors utilize the piezoelectric effect in which an applied strain (or force) on a piezoelectric crystal results in a potential difference across the crystal. Similarly, if the crystal is subjected to a potential difference, a displacement, or strain, is produced. The effect can be used to sense mechanical stress (i.e. displacement) and as an actuation mechanism, although displacements are small even for large voltages. Common piezoelectric materials used for MEMS applications include quartz, lead zirconate titanate (PZT), polyvinylidene fluoride (PVDF) and ZnO, PVDF and ZnO being the most common. Silicon is not piezoelectric; hence a thin film of a suitable material must be deposited on the devices.
- 66. Capacitive sensors Capacitive (or electrostatic) sensing is one of the most important (and widely used) precision sensing mechanisms and includes one or more fixed conducting plates with one or more moving conducting plates. Capacitive sensing relies on the basic parallel- plate capacitor equation shown below. As capacitance is inversely proportional to the distance between the plates, sensing of very small displacements is extremely accurate.
- 67. Resonant sensors MEMS resonant sensors consist of micromachined beams or bridges which are driven to vibrate at their resonant frequency. They can be attached to membranes or designed to adhere to a particular substance (as in the case of a biosensor). Movement of the membrane or increased build-up of the binding substance will affect the resonant frequency and can be monitored using implanted piezoresistors.
- 68. a) Strain gauge - a strain gauge is a conductor or semiconductor that is fabricated on or bonded directly to the surface to be measured. An example of a polysilicon strain sensor unable to be fabricated by any other method than MEMS is an implantable piezoresistive strain gauge to measure forces in heart and brain tissue. b) Accelerometer - accelerometers sense acceleration by using a suspended proof mass on which external acceleration can act (Figure 7). Upon acceleration (or deceleration), a force (F=ma) is generated on the proof mass resulting in displacement. The force or displacement is usually measured by piezoresistive and capacitive methods.
- 69. b) Accelerometer - accelerometers sense acceleration by using a suspended proof mass on which external acceleration can act (Figure 7). Upon acceleration (or deceleration), a force (F=ma) is generated on the proof mass resulting in displacement. The force or displacement is usually measured by piezoresistive and capacitive methods.
- 70. Types of mechanical sensor include: https://youtu.be/eqZgxR6eRjo
- 71. c) Gyroscope – a gyroscope is a device that measures the rotation rate and detects inertial angular motion. As a result it can be found, for example, in transportation, navigation and missile guidance applications. It relies on measuring the influence of the Coriolis force on a body in a rotating frame. MEMS gyroscopes typically use vibrating structures because of the difficulty of micromachining rotating parts with sufficient useful mass.
- 72. d) Pressure sensor - MEMS pressure sensors are usually based around thin membranes with sealed gas or vacuum-filled cavities on one side of the membrane and the pressure to be measured on the other side. Piezoresistive and capacitive membrane deflection measurement techniques are most commonly used in commercial pressure sensors.
- 73. Transduction methods The process of converting one form of energy to another is known as transduction. A transducer is a device that converts energy from one form to another.
- 74. Accelerometers An accelerometer is a device that measures proper acceleration. Proper acceleration, being the acceleration (or rate of change of velocity) of a body in its own instantaneous rest frame.
- 75. Highly sensitive accelerometers are components of inertial navigation systems for aircraft and missiles. Accelerometers are used to detect and monitor vibration in rotating machinery. Accelerometers are used in tablet computers and digital cameras so that images on screens are always displayed upright. Accelerometers are used in drones for flight stabilization. Coordinated accelerometers can be used to measure differences in proper acceleration, particularly gravity, over their separation in space; i.e., gradient of the gravitational field.
- 76. Conceptually, an accelerometer behaves as a damped mass on a spring. When the accelerometer experiences an acceleration, the mass is displaced to the point that the spring is able to accelerate the mass at the same rate as the casing. The displacement is then measured to give the acceleration.
- 77. Gyroscope Gyroscope is a device that measures the rotation rate and detects inertial angular motion. As a result it can be found, for example, in transportation, navigation and missile guidance applications. It relies on measuring the influence of the Coriolis force on a body in a rotating frame. MEMS gyroscopes typically use vibrating structures because of the difficulty of micromachining rotating parts with sufficient useful mass.
- 78. Pressure sensor - MEMS pressure sensors are usually based around thin membranes with sealed gas or vacuum-filled cavities on one side of the membrane and the pressure to be measured on the other side. Piezoresistive and capacitive membrane deflection measurement techniques are most commonly used in commercial pressure sensors.
- 79. MEMS microphones The application of MEMS (microelectro- mechanical systems) technology to microphones has led to the development of small microphones with very high performance. MEMS microphones offer high SNR, low power consumption, good sensitivity, and are available in very small packages that are fully compatible with surface mount assembly processes. MEMS microphones exhibit almost no change in performance after reflow soldering and have excellent temperature characteristics.
- 80. Mechanical structures: These are non-moving structures, such as microbeams and microchannels. Miniature mechanical structures showing (a) polymer mesopump; (b) silicon nano tip fabricated using bulk micromachining.
- 81. Actuators Sensors are transducers that convert mechanical, thermal, or other forms of energy into electrical energy, actuators do the exact opposite.
- 82. An actuator is a device that is responsible for moving or controlling a mechanism or system. It is controlled by a signal from a control system or manual control. It is operated by a source of energy, which can be mechanical force, electrical current, hydraulic fluid pressure, or pneumatic pressure, and converts that energy into motion. An actuator is the mechanism by which a control system acts upon an environment. The control system can be simple (a fixed mechanical or electronic system), software- based (e.g. a printer driver, robot control system), a human, or any other input.