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. Sulfuric 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 similarly to 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 3D microfabrication Nanofabrication Semiconductor fabrication References Further reading Journals Journal of Microelectromechanical Systems (J.MEMS) Sensors and Actuators A: Physical Sensors and Actuators B: Chemical Journal of Micromechanics and Microengineering 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 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), Nitaigour Premchand Mahalik (2006) "Micromanufacturing and Nanotechnology", Springer, D. Widmann, H. Mader, H. Friedrich: Technology 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