Microfabrication

Microfabrication or micromanufacturing are the terms to describe processes of fabrication of miniature structures, of sizes measured in microns and smaller. Historically the earliest micromanufacturing was used for semiconductor devices in integrated circuit fabrication and these processes have been covered by the term "semiconductor device fabrication," "semiconductor manufacturing," etc. Practical advances in microelectromechanical systems (MEMS) and other nanotechnology, where the technologies from IC fabrication are being re-used, adapted or extended have led to the extension of the scope and techniques of microfabrication.^[1]

Miniaturization of various devices presents challenges in many areas of science and engineering: physics, chemistry, material science, computer science, ultra-precision engineering, fabrication processes, and equipment design. It is also giving rise to various kinds of interdisciplinary research.^[1]

The major concepts and principles of micromanufacturing are laser technology, microlithography, micromechatronics, micromachining and microfinishing (nanofinishing). Among typical manufactured structures are integrated circuits, MEMS, MOEMS, nanotubes, fuel cells, biosensors, nanosensors, micropropulsion, etc.^[1]

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Origins

Microfabrication technologies originate from the microelectronics industry, and the devices are usually made on silicon wafers even though glass, plastics and many other substrate are in use. Microfabricated devices include integrated circuits (“microchips”), microsensors (e.g. air bag sensors), inkjet nozzles, flat panel displays, laser diodes, and hundreds of others. Micromachining, semiconductor processing, microelectronic fabrication, semiconductor fabrication, MEMS fabrication and integrated circuit technology are terms used instead of microfabrication, but microfabrication is the broad general term.

Traditional machining techniques such as electro-discharge machining, spark erosion machining, and laser drilling have been scaled from the millimeter size range to micrometer range, but they do not share the main idea of microelectronics-originated microfabrication: replication and parallel fabrication of hundreds or millions of identical structures. This parallelism is present in various imprint, casting and molding techniques which have successfully been applied in the microregime. For example, injection moulding of compact discs involves fabrication of micrometer-sized spots on the disc.

Microfabrication processes

Microfabrication is actually a collection of technologies which are utilized in making microdevices. Some of them have very old origins, not connected to manufacturing, like lithography or etching. Polishing was borrowed from optics manufacturing, and many of the vacuum techniques come from 19th century physics research. Electroplating is also a 19th century technique adapted to produce micrometre scale structures, as are various stamping and embossing techniques.

To fabricate a microdevice, many processes must be performed, one after the other, many times repeatedly. In memory chip fabrication some 30 lithography steps, 10 oxidation steps, 20 etching steps, 10 doping steps, and many others are performed.

Typical microfabrication processes include:

• Photolithography
• Etching (microfabrication), such as RIE (Reactive-ion etching) or DRIE (Deep reactive-ion etching)
• Thin film deposition, such as sputtering, chemical vapor deposition (CVD), evaporative deposition, or epitaxy
• Thermal oxidation
• Doping by either thermal diffusion or ion implantation
• Wire bonding
• Chemical-mechanical planarization (CMP)
• Wafer cleaning, also known as "surface preparation" (see below)

The complexity of microfabrication processes can be described by their mask count. This is the number of different pattern layers that constitute the final device. Modern microprocessors are made with 30 masks while a few masks suffice for a microfluidic device or a laser diode. Microfabrication resembles multiple exposure photography, with many patterns aligned to each other to create the final structure.

Cleanliness in wafer fabrication

Microfabrication is carried out in cleanrooms, where air has been filtered of particle contamination and temperature, humidity, vibrations and electrical disturbances are under stringent control. Smoke, dust, bacteria and cells are micrometers in size, and their presence will destroy the functionality of a microfabricated device.

Cleanrooms provide passive cleanliness but the wafers are also actively cleaned before every critical step. RCA-1 clean in ammonia-peroxide solution removes organic contamination and particles; RCA-2 cleaning in hydrogen chloride-peroxide mixture removes metallic impurities. Sulphuric acid-peroxide mixture (a.k.a. Piranha) removes organics. Hydrogen fluoride removes native oxide from silicon surface. These are all wet cleaning steps in solutions. Dry cleaning methods include oxygen and argon plasma treatments to remove unwanted surface layers, or hydrogen bake at elevated temperature to remove native oxide before epitaxy. Pre-gate cleaning is the most critical cleaning step in CMOS fabrication: it ensures that the ca. 2 nm thick oxide of a MOS transistor can be grown in an orderly fashion. Oxidation, and all high temperature steps are very sensitive to contamination, and cleaning steps must precede high temperature steps.

Surface preparation is just a different viewpoint, all the steps are the same as described above: it is about leaving the wafer surface in a controlled and well known state before you start processing. Wafers are contaminated by previous process steps (e.g. metals bombarded from chamber walls by energetic ions during ion implantation), or they may have gathered polymers from wafer boxes, and this might be different depending on wait time.

Wafer cleaning and surface preparation work a little bit like the machines in a bowling alley: first they remove all unwanted bits and pieces, and then they reconstruct the desired pattern so that the game can go on.

See also

• Semiconductor fabrication
• Integrated circuit
• Microelectronics
• Sensors
• Actuators
• Transducers
• Microfluidics
• 3D microfabrication

References

1. ^ ^{a b c} Nitaigour Premchand Mahalik (2006) "Micromanufacturing and Nanotechnology", Springer, ISBN 3540253777

• Journal of Microelectromechanical Systems (J.MEMS)
• Sensors and Actuators A: Physical
• Sensors and Actuators B: Chemical
• Journal of Micromechanics and Microengineering (JMM)
• Lab on a Chip
• IEEE Transactions of Electron Devices,
• Journal of Vacuum Science and Technology A: Vacuum, Surfaces, Films
• Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures: Processing, Measurement, and Phenomena

Books about microfabrication

• (2004)Geschke, Klank & Telleman, eds.: Microsystem Engineering of Lab-on-a-chip Devices, 1st ed, John Wiley & Sons. ISBN 3-527-30733-8.
• "Introduction to Microfabrication" (2004) by S. Franssila
• "Fundamentals of Microfabrication" (2nd ed, 2002) by M. Madou
• Micromachined Transducers Sourcebook by Gregory Kovacs(1998)
• Brodie & Murray: The Physics of Microfabrication (1982),
• D. Widmann, H. Mader, H. Friedrich: Tehnology of Integrated Circuits (2000),
• J. Plummer, M.Deal, P.Griffin: Silicon VLSI Technology (2000),
• G.S. May & S.S. Sze: Fundamentals of Semiconductor Processing (2003),
• P. van Zant: Microchip Fabrication (2000, 5th ed),
• R.C. Jaeger: Introduction to Microelectronic Fabrication (2001, 2nd ed),
• S. Wolf & R.N. Tauber: Silicon Processing for the VLSI Era, Vol 1: Process technology (1999, 2nd ed),
• S.A. Campbell: The Science and Engineering of Microelectronic Fabrication (2001, 2nd ed)
• T. Hattori: Ultraclean Surface Processing of Silicon Wafers : Secrets of VLSI Manufacturing