©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e MICROFABRICATION TECHNOLOGIES 1.Microsystem Products 2.Microfabrication Processes

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

©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e Design Trend and Terminology  Miniaturization of products and parts, with features sizes measured in microns (10 -6 m) or smaller  Microelectromechanical systems (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 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 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 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 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 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 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 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 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 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  Resistance heater boils ink to create plume that forces drop to be expelled onto paper Ink-Jet Printer Head

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

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

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

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

©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e  (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 SiO 2 resist on the surfaces, (4) resist is patterned by lithography, and (5) anisotropic etching removes the Si except in the boron doped layer Bulk Micromachining of Thin Membrane in Silicon Substrate

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

©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e 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 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  (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 Processing Steps in LIGA

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