Airborne Molecular Contamination (AMC) can be a “silent poisoner” of your process and product. AMC is any unwanted organic or inorganic molecule that is carried by the air to the surface of the product. The world of wafer fabrication categorizes AMC as acids, bases, condensables, and dopants.1 In a more global sense, AMC is productively thought of as any molecule carried by the air that masks or reacts with the surface.
If you are involved in semiconductors, nanodevices, or critical aerospace applications, AMC is an obvious concern. If you are involved in production that does not use a cleanroom, such as engineered coatings or consumeroriented products, consider how controlling AMC can improve your product yield and product performance.
Given the inherent scale of molecules, AMC is neither typically detected by particle monitors nor is it trapped by traditional filtration systems. In terms of airborne contamination, historically, the bulk of attention has been given to particulate contamination, in part probably because it is easier to detect. However, many critical product failures, both immediate and latent, have been traced to AMC.
AMC AND PRODUCT DAMAGE
AMC changes the nature of the surface of the product and therefore its properties. AMC may leave a condensable, non-reactive coating on the surface; in extreme cases, this residue forms a sticky, ointment-like residue. Typically, the residue is an exceedingly thin film that is not visible and is detected through laboratory analysis. AMC residue can prevent adherence of coating or doping material. Or the AMC layer may remain underneath a subsequent desired coating layer and react with either the substrate or the overlying coating. AMC may also contact the surface, react with and modify (damage) the surface, and then volatilize, leaving no detectable residue.
WHERE DOES AMC COME FROM?
AMC may be carried into the production or assembly area or generated within the production area via outgassing or volatilization of process chemicals or materials (see Table). Any volatile chemical in the assembly facility, even those with fairly low vapor pressure, can volatize enough to contaminate a critical product. This includes outgassing from plastics and from personnel.
AMC can also originate outside the cleanroom. Most air filters are designed to trap particles, not the much smaller molecules of volatile chemicals. Even if the cleanroom is equipped with AMC filters, volatile chemicals can enter via equipment, product, and people entering the room. That is why double or triple bagging is frequently employed to keep products entering the cleanroom from contaminating it—assuming that the products are sufficiently clean when packed into nonoutgassing bags.
In our experience, facilities in locales with poor air quality, with high Volatile Organic Compounds (VOCs), may have higher levels of AMC. The smaller the difference between the quality of the air between a room and the area immediately outside it, the easier it is to maintain appropriate filtration; this applies to filtration of particles,2 and molecules.
PREVENTION STRATEGY
Save time, money, and improve product yields; consider the potential impact of AMC. Explore all of the environments your product sees; review the assembly flow. Become aware of critical surfaces and consider how AMC can impact the surface. Review even seemingly non-critical process. This includes knowing the build and transport processes employed within the plant and by your supply chain. In the second installment, we will elaborate on AMC detection and prevention techniques that can help you to achieve higher yields and satisfied customers.
References
- "Classification of Air-borne Molecular Contaminant Levels in Clean Environments," SEMI F21-1102 Standard; available through ANSI
- B. Kanegsberg and E. Kanegsberg, “Contamination Control In and Out of the Cleanroom”, Controlled Environments Magazine, May, 2009.
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