Contamination Control In and Out of the Cleanroom: Airborne Molecular Contamination, Part 2: Detecting AMC

“How do I know if I have AMC?” Hopefully, you are not suspicious of airborne molecular contamination because of poor yield or customer complaints. Last month, we discussed how AMC can be a “silent poisoner” of your process and product; and we discussed sources of AMC. Instead of suspecting that you might have AMC, be proactive — head it off at the pass. Detect AMC before it ruins the product; avoid damage to your bottom line. There are two basic approaches to monitoring

• AMC on surfaces
• Contamination in the air

Many monitoring techniques have been proposed or commercially developed. We discuss a select sample of them.

DETECTION ON THE COMPONENT—WITNESS SAMPLES
Collection
Thin film contamination can be detected on the product surface. Of course, by then you already have AMC; the secret is to detect it at a very low level. Periodically, a component can be removed from the build process and be analyzed for surface contamination. However, it may be easier and more cost effective to use witness samples. A witness sample is material that duplicates or emulates the surface of the product. If the product is metal, use a piece of the same metal; if plastic, use the same plastic so that the witness sample will have the same affinity for AMC.

To be most effective, a witness sample should be proximal to the products under construction. It should also undergo any cleaning steps that the product has experienced up to the point of test. In other words, you want a surface as similar as possible to that of your product.

A witness sample should be exposed to the cleanroom atmosphere for at least the longest period of time that the component would be exposed. Longer exposure times and/or larger exposure areas are even better and provide greater sensitivity, but a trade-off is a longer time before an analysis is performed. Then the witness sample should be appropriately packaged and transported for analysis. Include a control — a witness sample that has been prepared in the same manner but not exposed to the potential AMC. We cannot emphasize enough the importance of controls to clarify results and to distinguish contamination due to cleanroom exposure from other sources of contamination.

Analysis
Analysis can indicate: Is there contamination? How much? What is it?

Witness samples can be analyzed in situ or after extraction. In situ measurements involve analyzing the surface of the sample. Some techniques (such as FTIR, XPS, Auger, OSEE, and TOF-SIMS) have been discussed previously in our column.

Contact angle determination¹ is relatively inexpensive but provides limited information. It provides an indication of overall contamination but it does not speciate — meaning that the specific molecule or family of molecules cannot be identified. A high contact angle is consistent with organic contamination. However, a low contact angle does not necessarily indicate the absence of significant contamination. For example, salts and surfactants can lower the contact angle and impart a false sense of security.

Extraction, discussed in this column recently,² provides another method to detect contamination. AMC can be measured gravimetrically and then further speciated in a similar manner to in situ measurement. Extraction concentrates the residue, allowing very sensitive monitoring. This allows contamination levels, perhaps too low to be detected in situ but high enough to impact the quality of the component, to be detected. In addition, if limits for non-specific contamination are not exceeded, more costly speciation can be avoided.

AMC WHILE IT’S AIRBORNE
A witness sample or test component reveals AMC only after contamination has adhered to a surface. Consider adding AMC air monitoring. Air monitoring can help pin down the source of AMC.

“Hit and run” AMC may require an airborne approach. Reactive gas components, like acids or bases, may change the chemical composition of the surface but leave no directly detectable residue.

It helps to know what you are looking for. Testing for rapid response may need to be weighed against sensitivity. Testing for bases or acids may use different detectors or analytical methods than testing for volatile organics (VOCs).

In order to monitor trace contaminants in air, contaminants can be concentrated by using impingers or sorption tubes. An impinger is a water-filled tube through which air is bubbled. Airborne contaminants accumulate in the water; the water is then analyzed. Sorption tubes are packed with an absorbent material with large surface area. Sorption materials may be chosen based on affinity for a specific compound or group of compounds. As with witness samples, analysis of impinger and sorption tubes yields information about the average level of contamination over the collection time. In contrast, real time analysis indicates contamination spikes and emerging contamination issues.

Techniques for AMC Monitoring in Air
CDRS and IMS are short response time detection; they can be used for continuous monitoring. Cavity Ring Down Spectroscopy (CDRS) has been developed as a sensitive, rapid detector for ammonia. Ammonia levels of less than 1 ppb can be measured within seconds to minutes. CDRS measures light absorption in a mirrored cavity that effectively lengthens the light path, like being in a room with mirrors on opposite walls. Ion mobility spectrometry (IMS)³ detects both organic and inorganic compounds with a short response time but with limited ability to speciate.

Chromatography and mass spectrometry can provide detailed analysis of contamination species. However, they are not real time techniques. Thermal desorption (TD) coupled with gas chromatography mass spectrometry (GCMS) detects and speciates volatile organics but not inorganic acids or bases. With TD-GCMS, sorption tubes are heated to volatilize collected organics which are then analyzed by GCMS.

BE PROACTIVE
In a cleanroom environment, particle monitoring receives the lion’s share of attention. Particles, from whatever source, interfere with product performance; and air monitors can readily count and size particles in real time. Certainly, speciation of particles is important in determining the source of contamination; but we know that we can “fix” particle contamination, particularly in the air, by the use of appropriate filters. Initially, simply tracking the particles can be enough of an alert. Air monitoring equipment is widely available and relatively affordable.

AMC is more complicated. For air monitoring, equipment tends to be more complex and specialized in comparison with particle counters, particularly if a single chemical or class of chemicals is to be monitored. In addition, analysis time is often longer.

Look at your process. Whether the build and assembly is performed in a cleanroom, mini-environment, or in an uncontrolled area, determine the risks of AMC to your product. It is preferable to detect, and then take steps to eliminate, sources of contamination before it has time to damage the product surface. Determine steps that can be taken to test for the presence of AMC. Work with advisors and analysis lab personnel.

In the last of this three part series, we outline practical steps to prevent or reduce AMC.

References

1. B. Kanegsberg and M. Chawla, “Preventing and Measuring Contamination In and Out of the Cleanroom: Measuring Thin Film Surface Contamination.” A2C2 Magazine, September 2001.
2. B. Kanegsberg and E. Kanegsberg, “Contamination Control In and Out of the Cleanroom: The Right Extraction.” Controlled Environments Magazine, April 2009.
3. B. Kanegsberg and E. Kanegsberg, “Contamination Control In and Out of the Cleanroom: Are we there yet? Monitoring Contamination with IMS.” Controlled Environments Magazine, October 2008.