Any unwanted particles in the environment, such as organic and inorganic substances, dust, micro organisms, human skin, or vapours are regarded as contaminants. Humidity and temperature control is also maintained in a clean room. Contamination in a clean room can also occur from unclean surfaces. Walls, ceilings, or even materials used during experiments or fabrication in a clean room can be a source.
It is therefore important that all material and substrates to be used are cleaned prior to their use. There are several ways to clean substrates depending on the material and the type of contamination to be removed. Larger particles, such as glass fragments, can be cleaned through washing. Gentle application of chemicals such as Decon 90 can also used to release any larger particles like dust and grease stuck on the surface. Then, to remove relatively smaller particles, the slides can be placed in an ultrasonic bath with a 20% solution of Decon90 in ultra pure water. Subsequently, 20% solution of Ethanolamine in ultra pure water is used to reactively clean the substrate by removing dirt which is chemically bonded to it. Once this is completed, the substrates are submerged in pure ethanol to get rid of water itself from the surface which is replaced by ethanol. The substrate is then dried on a hot plate or through hot air and to be used within minutes before it absorbs any moisture from air.
The process described above is just one way of cleaning the substrate. As mentioned earlier, there are several methods available which can be used to suit the material and contaminant. For example, Martinez-Duarte et al (2010: p. 195) mention a host of possibilities. Among wet immersion techniques, diluted hydrofluoric acid, RCA, a process that uses ammonium hydroxide, hydrogen peroxide, water, hydrofluoric and hydrochloric acid, or supercritical cleaning can be used followed by rinsing in a solvent depending on application (Martinez-Duarte et al, 2010: p. 195).