Monitoring for any type of contamination is essential to contamination control. Monitoring, specifically, for airborne molecular contamination (AMC) is important in industries where AMC can directly impact the product or processes. Implementing a multipoint AMC sampling system allows users to reap the benefits of online monitoring, while lowering the cost per sample point.

AMC is chemical contamination in the form of vapors or aerosols. These chemicals may be organic or inorganic in nature and can include acids, bases, polymer additives, organometallic compounds and dopants. AMC can cause many adverse effects, from corrosion on metal surfaces on the water to haze on wafers and optics to voltage shifts.

The main sources of AMC are building and cleanroom construction materials, general environment, process chemicals and operation personnel. Only continuous monitoring can ensure that facilities are performing properly and alert personnel when incidents occur. With multipoint online monitoring, incidents can be responded to almost instantly, instead of days or weeks after contamination. The most common chemicals to be monitored include ammonia, NMP, total amines, total acids and total sulfur, H2S, HF and HCL.

Implementing an online AMC monitoring system involves a number of distinct steps. These steps include determining the process to monitor, the chemicals to monitor and level at which monitoring will take place. In addition, it’s necessary to identify the instrumentation that will be most appropriate for the application. When evaluating different analyzers, the following factors should be considered:

• Target chemical: Some analyzers can evaluate multiple types of chemicals, while others are restricted to one. Generally, a detection limit of 1 ppb or less requires the use of an analyzer that targets a single chemical.

• Detection limits: The detection limit of the instrument should be, at most, half of what the maximum allowable limit is for the chemical being monitored. This will provide enough headroom to ensure the analyzer is delivering accurate data.

• Dynamic range: Minimum and maximum concentration levels that the analyzer can track determine its dynamic range. It’s advisable to use an analyzer with as large a dynamic range as possible so that the actual peak of any major events can be determined.

• Response time: This is the time that it takes to adjust from one fixed concentration level to another. Normally, an analyzer’s response time is listed as a percentage of the change in chemical concentration that the analyzer can display within a fixed amount of time.

• Zero and span drift: The analyzer’s zero and span drift are important to understand because, ultimately, they impact how frequently the instrument will need to be calibrated. For instance, if the control limit is 1 ppb and the analyzer purchased has a zero and/or span drift of 0.1 ppb per day, then, in the worst-case scenario, the analyzer must be calibrated every 10 days.

• Potential cross interferences: Most analyzers are susceptible to cross interference from other types of chemicals. The type of cross interference is based on the analyzing technology of the instrument and what steps the sensor manufacturer has taken to minimize such occurrences.

• Calibration method: It is ideal to purchase an analyzer that is easily calibrated without supplier support. Although some analyzers feature built-in calibration standards, most require external, fixed concentration gas samples for span calibration and N2 for calibrating the zero point.

• Operation cost: Analyzers generally require some type of monthly, quarterly or annual maintenance involving calibration and parts replacement. When purchasing an analyzer, it’s important to request a quote that includes the cost of regular maintenance to get an accurate idea of the total cost involved.

Successful implementation of an online AMC sampling system also involves determining the type of sampling system that will be used. The sampling system is what enables sampling from multiple locations using a single analyzer. Some important factors to consider when choosing a sampling unit include the number of locations to be sampled, the sample/purge flow, data accessibility, connectivity of the analyzer to the sampling system and connectivity to the sampling system to an external data logger or monitoring system.

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