Preventing or identifying the source of silicone contamination requires an holistic approach. You have to be aware of the entire operation, not just your own part of the process. This implies awareness of oils, lubricants, and other materials used not only in the product but also in support materials such as vacuum pumps, transfer lines, automation, as well as materials handling equipment and even storage containers. Storage containers can be a contamination source; silicone is commonly associated with surface agents used during manufacture of plastic items.
Researchers at the Stanford linear accelerator (SLAC), investigating aging of wires used for gas detectors, found aging to be related to contamination from auxiliary equipment, a gas bubbler.1 Silicone oil from the bubbler, at levels of 18 ppb in the gas phase, was sufficient to produce aging. Non-silicone oil did not affect aging.
People are also a potential source of silicone contamination. Many creams, cosmetics, hair care products, antiperspirants and some eye-glass cleaning tissues contain silicones. Careful education and training helps; production personnel need to understand why procedures and constraints are in place.
The source of airborne silicone contamination may be remote from the fabrication facility. It is prudent to visually observe the plant layout as well as neighboring operations. In one instance, exterior rooftop outlet vents located near air intake vents, directed contaminated air from a non-critical process back into the cleanroom.2 Even if your facility has somehow excluded silicones, the coating facility down the block might impact your operation.
It may not be desirable or feasible to exclude silicone from all manufacturing processes. If silicones are anticipated, contamination can be minimized during equipment and process design. Masks or sleeves may be needed to protect critical components from silicones during and after a manufacturing process. In electronic contact manufacturing, sometimes a conductive polymer can be applied during manufacture that will act as a barrier to deposition of silicone.
Prevention is the ideal. Once contamination is strongly suspected, through visual or performance indication, a number of analytical tools can be used for confirmation. Some analytical instruments are available as compact, portable systems. This takes detection from the analytical laboratory to the fabrication facility.
If the exact silicone compound does not have to be identified, the presence of silicon can be determined relatively rapidly by elemental analysis, e.g. atomic adsorption or inductively coupled plasma (ICP). To localize the contaminant, a two dimensional technique such as scanning electron microscopy with energy dispersive X-ray analysis (SEM/EDX) might be used. The SEM provides a characteristic visual observation of a very small area (e.g. a coating defect or a particle). EDX elemental analysis of that area identifying carbon, oxygen, and silicon can be used as evidence that some sort of silicone is the culprit.
Silicone can be detected by Fourier transform infrared spectroscopy (FTIR), with some degree of molecular identification by comparison of the suspect contaminant with a known material. The technique can be performed on specific areas of the surface, such as suspect particles. Especially if mixed soils may be present, it is advisable to have a sample of the suspect silicone-based contaminant to run for comparison.
Other techniques provide more definitive identification of silicone contaminants. One is electron spectroscopy for chemical analysis (ESCA), which is also referred to as X-Ray photoelectron spectroscopy (XPS). Another is secondary ion mass spectrometer (SIMS).3
For volatile silicone contaminants, thermal desorption gas chromatography mass spectroscopy (TD-GC-MS) can be used. The technique was used to detect outgassing of silicones materials used in a hydrocarbon sensor.4 It might be pointed out that silicones have value in GC instrumentation. Gas chromatography is actually gas-liquid chromatography, and the liquid portion is often a silicone.
Where contamination of a complex object is suspected, extractive techniques are preferable. In most cases, the sample will be shipped to an analytical facility. It is important to take care that the extraction process itself does not introduce spurious indications of silicone contamination (through cosmetics, perhaps). It is helpful to prepare, ship and analyze an additional sample where silicone contamination is known to be absent.
Once identification of the specific silicone contaminant has been made, the question of how to remove the contaminant arises. Some answers will be provided in our next column.
1 J. Va’vra. SLAC-Pub-5207 (1990).
2 B. Kanegsberg, M. Chawla. M. A2C2 Magazine (March, 2001).
3 B. Kanegsberg, E. Kanegsberg. A2C2 Magazine (December, 2003).
4 E. Butrym. “Scientific Instrument Services application note,” http://www.sisweb.com/referenc/applnote/app-88.htm