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MICROFABRICATION TECHNOLOGIES

MICROFABRICATION TECHNOLOGIES. Microsystem Products Microfabrication Processes. Relative Sizes in Microtechnology and Nanotechnology. Design Trend and Terminology. Miniaturization of products and parts, with features sizes measured in microns (10 -6 m) or smaller

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MICROFABRICATION TECHNOLOGIES

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  1. MICROFABRICATION TECHNOLOGIES • Microsystem Products • Microfabrication Processes ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  2. Relative Sizes in Microtechnology and Nanotechnology ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  3. Design Trend and Terminology • Miniaturization of products and parts, with features sizes measured in microns (10-6 m) or smaller • Microelectromechanicalsystems (MEMS) - miniature systems consisting of both electronic and mechanical components • Microsystem technology (MST) - refers to the products as well as the fabrication technologies • Nanotechnology - even smaller entities whose dimensions are measured in nanometers (10-9 m) ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  4. Advantages of Microsystem Products • Less material usage • Lower power requirements • Greater functionality per unit space • Accessibility to regions that are forbidden to larger products • In most cases, smaller products should mean lower prices because less material is used ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  5. Types of Microsystem Devices • Microsensors • Microactuators • Microstructures and microcomponents • Microsystems and micro-instruments ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  6. Microsensors A sensor is a device that detects or measures some physical phenomenon such as heat or pressure • Most microsensors are fabricated on a silicon substrate using the same processing technologies as those used for integrated circuits • Microsensors have been developed to measure force, pressure, position, speed, acceleration, temperature, flow, and various optical, chemical, environmental, and biological variables ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  7. Microactuators An actuator converts a physical variable of one type into another type, and the converted variable usually involves some mechanical action • An actuator causes a change in position or the application of force • Examples of microactuators: valves, positioners, switches, pumps, and rotational and linear motors ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  8. Microstructures and Microcomponents Micro-sized parts that are not sensors or actuators • Examples: microscopic lenses, mirrors, nozzles, gears, and beams • These items must be combined with other components in order to provide a useful function ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  9. Microscopic Gear and Human Hair Image by scanning electron microscope - gear is high-density polyethylene molded by a process similar to LIGA (photo courtesy of W. Hung, Texas A&M U., and M. Ali, Nanyang Tech. U). ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  10. Microsystems and micro-instruments Integration of several of the preceding components with the appropriate electronics package into a miniature system or instrument • Products tend to be very application-specific • Examples: microlasers, optical chemical analyzers, and microspectrometers • The economics of manufacturing these kinds of systems have made commercialization difficult ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  11. Industrial Applications of Microsystems • Ink-jet printing heads • Thin-film magnetic heads • Compact disks • Automotive components • Medical applications • Chemical and environmental applications • Other applications ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  12. Ink-Jet Printing Heads • Currently one of the largest applications of MST • A typical ink-jet printer uses up several cartridges each year • Today’s ink-jet printers have resolutions of 1200 dots per inch (dpi) • This resolution converts to a nozzle separation of only about 21 m • Certainly in the microsystem range ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  13. Ink-Jet Printer Head • Resistance heater boils ink to create plume that forces drop to be expelled onto paper ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  14. Thin-Film Magnetic Heads • Read-write heads are key components in magnetic storage devices • Reading and writing of magnetic media with higher bit densities limited by the size of the read-write head • Development of thin-film magnetic heads was an important breakthrough not only in digital storage technology but microfabrication technologies as well • Thin-film read-write heads are produced annually in hundreds of millions of units, with a market of several billion dollars per year ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  15. Thin-Film Magnetic Read-Write Head ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  16. Compact Disks (CDs) and DVDs • Important commercial products - storage media for audio, video, and computer software • Molded of polycarbonate (ideal optical and mechanical properties for the application) • Diameter D = 120 mm and thickness = 1.2 mm • Data consists of small pits (depressions) in a helical track that begins at D = 46 mm and ends at D = 117 mm • Tracks separated by 1.6 m • Pits are 0.5 m wide and ~ 0.8 m to 3.5 m long ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  17. Molds for CDs • A master for the mold is made from a smooth thin layer of photoresist on a glass plate (D = 300 mm) • Photoresist is exposed to a laser beam that writes data into surface while glass plate is rotated and moved slowly to create spiral track • Exposed regions are removed; they will correspond to pits in the CD track • A thin layer of nickel is deposited onto surface by PVD and electroforming builds up Ni thickness ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  18. Molds for CDs (continued) • This negative impression of the master is called the father • Several impressions of the father are made (called mothers), whose surfaces are identical to the original master • Finally, the mothers are used to create the actual mold impressions (called stampers) • The stampers will be used to mass-produce the CDs ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  19. Molding and Further Processing of CD • Once molded, the pitted side of the polycarbonate disk is coated with aluminum by PVD to create a mirror surface • To protect this layer, a thin coating of polymer is deposited on the metal • Thus, the final CD is a sandwich • Thick polycarbonate substrate on one side • Thin polymer layer on the other side • Very thin layer of Aluminum in between ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  20. Reading the Compact Disk • In operation, the laser beam of a CD player reads through the polycarbonate substrate onto the reflective surface • The reflected beam is interpreted as a sequence of binary digits ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  21. Automotive Components • Micro-sensors and other micro-devices are widely used in modern automobiles • Between 20 and 100 sensors are installed in a modern automobile • Functions include cruise control, anti-lock braking systems, air bag deployment, automatic transmission control, power steering, all-wheel drive, automatic stability control, and remote locking and unlocking • In 1970 there were virtually no on-board sensors ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  22. Medical Applications • A driving force for microscopic devices is the principle of minimal-invasive therapy • Small incisions or even available body orifices to access the medical problem • Standard medical practice today is to use endoscopic examination accompanied by laparoscopic surgery for hernia repair and removal of gall bladder and appendix • Similar procedures are used in brain surgery, operating through small holes drilled in skull ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  23. Microfabrication Processes • Many MST products are based on silicon - Why? • Microdevices often include electronic circuits, so both the circuit and the device can be made on the same substrate • Silicon has good mechanical properties: • High strength and elasticity, good hardness, and relatively low density • Techniques to process silicon are well established from processing of ICs ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  24. Other Materials and MST Processing • MST often requires other materials in addition to silicon to obtain a particular microdevice • Example: microactuators often consist of several components made of different materials • Thus, microfabrication techniques consist of more than just silicon processing: • LIGA process • Other conventional and nontraditional processes performed on microscopic scale ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  25. Silicon Layer Processes • First application of silicon in MST was in the fabrication of piezoresistive sensors to measure stress, strain, and pressure in the early 1960s • Silicon is now widely used in MST to produce sensors, actuators, and other microdevices • The basic processing technologies are those used to produce integrated circuits • However, there are certain differences between processing of ICs and fabrication of microdevices ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  26. Microfabrication vs. IC Fabrication • Aspect ratios (height-to-width ratio of the features) in microfabrication are generally much greater than in IC fabrication • The device sizes in microfabrication are often much larger than in IC processing • The structures produced in microfabrication often include cantilevers and bridges and other shapes requiring gaps between layers • These features are not found in integrated circuits ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  27. Aspect Ratio • Aspect ratio (height-to-width ratio) typical in (a) fabrication of integrated circuits and (b) microfabricated components ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  28. 3D Features in Microfabrication • Chemical wet etching of polycrystalline silicon is isotropic, with the formation of cavities under the edges of the resist • However, in single-crystal Si, etching rate depends on orientation of the lattice structure • 3-D features can be produced in single-crystal silicon by wet etching, provided the crystal structure is oriented to allow the etching process to proceed anisotropically ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  29. Crystal Faces in Cubic Lattice Structure • Three crystal faces in silicon cubic lattice structure: (a) (100) crystal face, (b) (110) crystal face, and (c) (111) crystal face ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  30. Bulk Micromachining • Certain etching solutions, such as potassium hydroxide (KOH), have a very low etching rate in the direction of the (111) crystal face • This permits formation of distinct geometric structures with sharp edges in single-crystal Si • Bulk micromachining - relatively deep wet etching process on single-crystal silicon substrate • Surface micromachining - planar structuring of the substrate surface, using much more shallow etching ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  31. Bulk Micromachining • Several structures that can be formed in single-crystal silicon substrate by: (a) (110) silicon and (b) (100) silicon ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  32. Bulk Micromachining of Thin Membrane in Silicon Substrate • (1) Si substrate is doped with boron, (2) a thick layer of Si is applied on doped layer by epitaxial deposition, (3) both sides are thermally oxidized to form a SiO2 resist on the surfaces, (4) resist is patterned by lithography, and (5) anisotropic etching removes the Si except in the boron doped layer ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  33. Cantilevers and Similar Structures • Surface micromachining can be used to construct cantilevers, overhangs, and similar structures on a silicon substrate • The cantilevered beams are parallel to but separated by a gap from the silicon surface • Gap size and beam thickness are in the micron range ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  34. Micromachining to Form Cantilever • (1) SiO2 layer formed on Si substrate - thickness will set the gap size for the cantilever; (2) portions of the SiO2 layer are etched using lithography; (3) poly-Si layer is applied; (4) portions of the poly-Si layer are etched using lithography; and (5) SiO2 layer beneath the cantilevers is selectively etched ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  35. Lift-Off Technique in Microfabrication • To pattern metals such as platinum on a substrate for use in certain chemical sensors • Process: (1) resist is applied to substrate and structured by lithography, (2) platinum is deposited, (3) resist is removed, lifting Pt but leaving desired Pt microstructure ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  36. LIGA Process • An important technology of MST • Developed in Germany in the early 1980s • LIGA stands for the German words • LIthographie (in particular X-ray lithography) • Galvanoformung (translated electrodeposition or electroforming) • Abformtechnik (plastic molding) • The letters also indicate the LIGA process sequence ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  37. Processing Steps in LIGA • (1) Apply resist, X-ray exposure through mask, (2) remove exposed portions of resist, (3) electrodeposition to fill openings in resist, (4) strip resist for (a) mold or (b) metal part ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  38. Advantages of LIGA • LIGA is a versatile process – it can produce parts by several different methods • High aspect ratios are possible (large height-to-width ratios in the fabricated part) • Wide range of part sizes is feasible - heights ranging from micrometers to centimeters • Close tolerances are possible ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  39. Disadvantages of LIGA • LIGA is a very expensive process • Large quantities of parts are usually required to justify its application • LIGA uses X-ray exposure • Human health hazard ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  40. Other Microfabrication Processes • Soft lithography • Nontraditional and traditional processes and rapid prototyping adapted for microfabrication • Photochemical machining • Electroplating, electroforming, electroless plating • Electric discharge machining • Electron beam machining • Ultrasonic machining • Microstereolithography ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  41. Soft Lithography Term for processes that use an elastomeric flat mold to create a pattern on a substrate surface • A master pattern is fabricated on a silicon surface using lithography • This pattern is then used to produce a flat mold of polydimethylsiloxane (PDMS) • Two processes: • Micro-imprint lithography • Micro-contact printing ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  42. Soft Lithography • Fabricating the elastomeric mold (like a rubber stamp): (1) master pattern is made by traditional lithography, (2) PDMS mold is cast from pattern, (3) cured flat mold is peeled off pattern for use ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  43. Micro-Imprint Lithography • (1) Mold is positioned above and (2) pressed into resist, (3) mold is lifted, and (4) remaining resist is removed from substrate surface in defined regions ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  44. Microstereolithography (MSTL) • MSTL layer thickness t = 10 to 20 m typically • In conventional STL, t = 75 m to 500 m • MSTL spot size is as small as 1 or 2 m • Laser spot size diameter in STL ~ 250 m • MSTL materials not limited to photosensitive polymer • Researchers report fabricating 3-D ceramic and metallic microstructures • Starting material is a powder rather than a liquid ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  45. Ultra-High Precision Machining • Trends in conventional machining include taking smaller and smaller cut sizes • Enabling technologies include: • Single-crystal diamond cutting tools • Position control with resolutions ~ 0.01 m • Applications: computer hard discs, photocopier drums, mold inserts for compact disk reader heads, high-definition TV projection lenses ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  46. Ultra-High Precision Machining • One reported application: milling of grooves in aluminum foil using a single-point diamond fly-cutter • The aluminum foil is 100 m thick • The grooves are 85 m wide and 70 m deep ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

  47. Ultra-High Precision Machining • Ultra-high precision milling of grooves in aluminum foil ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

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