Specifically AMC

Airborne molecular contamination (AMC) control has become fully integrated into the cleanroom environmental management requirements of high-tech manufacturing facilities. The optimum control of AMC involves three steps: 1. Assessment of the air quality both outside and inside the facility to identify target contaminants as well as those that could affect the performance of the AMC control system. 2. Selection and qualification of an AMC control system. 3. Ongoing monitoring of both the controlled environment and the performance of the AMC control system1.

Some of the many applications that require AMC control include cleaning the outside air being used for ventilation and pressurisation, removing tramp and fugitive emissions from recirculation airstreams, and cleaning process emission and exhaust streams. And just as there are a wide variety of AMC control applications in the cleanroom, there seems to be an equal number of control options available to contamination control personnel. With what may seem to be an overwhelming number of items to be considered, how is the contamination control engineer supposed to make the proper choice for a particular application? Here are the key issues to understand and steps you need to take as you prepare to establish a successful AMC control program.

AMC Classifications The first consideration in the selection of an AMC control system should be an assessment for the type and quantities of AMC to be controlled. This can be done by direct gas monitoring or with passive or real-time reactivity monitoring2. There can be dozens of contaminants present in a makeup or recirculation airstream, but only relatively few may require close control. Contaminant classifications can help to group similar contaminant types and help make the selection of the proper control system more straightforward. SEMI Standard F-21-11023 classifies AMC in cleanrooms by chemical properties – acids, bases, condensables or dopants – providing a way to characterise the environment by groups of materials that could have similar effects on an exposed wafer. The purpose of this standard is to classify cleanrooms with respect to their molecular (non-particulate) contaminant levels. The classifications are defined as: 1. Acid: a corrosive material whose chemical reaction is that of an electron acceptor 2. Base: a corrosive material whose chemical reaction is that of an electron donor 3. Condensable: a chemical substance capable of condensation on a clean surface (excluding water) 4. Dopant: a chemical element that modifies the electrical properties of a semiconductive material.

Another way to characterise AMC is based on potential effects to exposed personnel. Looking at it in this light, we could generally classify AMC as toxic, corrosive, irritating or odourous in order of decreasing severity. Given the recent publicity that personnel exposure to potentially toxic and hazardous materials in semiconductor manufacturing has received, AMC control systems must also be designed to protect people as well as products. Definitions related to personnel protection are: 1. Toxic: substances which can cause damage to living tissue, impairment of the central nervous system or in extreme cases, death 2. Corrosive: substances which are likely to cause deterioration or damage to the interior of a building or its contents 3. Irritant: substances which can cause discomfort, and potentially permanent damage, to an exposed person 4. Odourous: substances which primarily affect the sense of smell.

Although none of these classifications are definitive, an environmental assessment can at least provide the AMC control system designer a starting point with an indication of contaminant types that may be present and levels to which they must be controlled. But even with a classification system in place, it does not always guarantee a quick and easy selection of the most effective and economical AMC control system. What it does do is help determine if absolute contaminant control is required or whether a system with a fractional efficiency could be employed.

Specifying a control system Manufacturers have become much more sophisticated in their knowledge and understanding of AMC and its effects in the cleanroom. They have a better general understanding of where AMC control should be applied and why. Additionally, as their knowledge of AMC-related problems has increased, so too have their expectations for an AMC control system. In fact, some manufacturers' concerns about the proper selection of a control system have become so acute that it is being reflected in their control specifications. One manufacturer may call for a minimum of 90% removal of target contaminants, while another will set AMC control limits at 1ppb or less. Still another may require that the system must last a minimum of one year between filter changeouts. As strict as these may seem, there are current specifications that require all three of the above criteria to be met. Compounding the situation, some manufacturers insist on trying to find a "one filter fits all" solution for AMC control, demanding that a single filter should meet all of their control criteria for all contaminants of concern. But chlorine requires one type of filter, ammonia another, and organic compounds still another. The type of filters/systems used for toxic gases would not be the same for odour control. Are you looking for efficiency or service life? Do you require absolute control of one contaminant or relative control of a group of contaminants? These are only some of the questions that an AMC control system designer must consider, even though his customer may not understand the implications of not taking all of this into consideration. The removal efficiency of an AMC control system can be considered as the fraction of a single contaminant or group of contaminants that are removed either by physical or chemical means. Many manufacturers are able to provide test data for their systems that show removal efficiency over time. However, this testing has been performed almost exclusively under accelerated conditions using high contaminant challenge concentrations that can be up to three to four orders of magnitude higher than what would be expected under actual use conditions. Although realistic extrapolations of filter efficiency can be provided, many manufacturers are now requiring low-level gas challenge testing for efficiency ratings of the filters (see Fig. 1)4. This is the best way to gauge filter performance; however, testing of this sort is more complicated, takes a long time to complete, and is much more expensive to perform. One of the main problems in trying to provide an efficiency rating for a particular contaminant/filter combination is that any rating has to have a time component specified along with it. If a specification calls for a 90% minimum removal efficiency, the end user is expecting that to hold true over the life of the filter.

Specification by limits Most tests, however, are only run long enough to provide an "initial" efficiency, and the test is stopped after one hour, eight hours, 24 hours and so on. All this does is provide information on how well the filter may work under a specific contaminant load, but does not provide or predict an indication of performance beyond the stated test period. An examination of the efficiency curves can provide some additional indication of future performance but it still cannot guarantee performance under actual-use conditions. Further, a filter that performs well against a single contaminant, e.g. ammonia, can perform poorly when gas mixtures are considered. Some manufacturers have investigated specific AMC-related process problems well enough that they have been able to set specific control levels for one or more contaminants. Common control specifications call for less than 1ppb of ammonia in lithography bays or less than 1ppb of chlorine or hydrogen fluoride (HF) in metallisation processes. Regardless of the ambient levels of ammonia, chlorine or HF, the AMC control system is expected to keep the controlled environment at or less than 1ppb over the service life of the system. Depending on ambient levels, this could require a minimum working filter efficiency of 50%, 80%, 95%, and so on. If transient high-level episodes are probable, filter efficiencies greater than 99% may be required to maintain a 1ppb level coming out of the AMC control system. Cost-of-ownership is always a major consideration when applying AMC control and can unfortunately be the primary factor when the final purchasing decision is made. Budget cycles, production schedules, capital expenditures, or simply a customer's preconceived notion of how long a filter should function before it must be replaced can be the basis for specifying filter life. Testing to determine working removal capacities of a certain AMC control product for a specific contaminant follows the same basic protocol as for efficiency testing. Using a known contaminant concentration at a specified airflow (typically the filter's maximum rated airflow), and running to a specific efficiency endpoint, one can calculate the amount of contaminant that has been removed. The media's removal capacity is reported as a volumetric capacity (g/cc) or as a weight percentage. This can then be used to estimate media consumption rates under a given set of conditions and provide an estimate of service life for a specific filter type. Capacities have to be determined for each contaminant of concern in order to provide a total consumption rate for a particular application. This is not practical, however, due to the uncertainty of the total contaminant load, the nature of the contaminants in question and any associated safety concerns as well as the time and costs involved. Further, the effects other contaminants in the cleanroom may have on the AMC control system have to be accounted for. Using these observed removal capacities are useful in providing estimates of service life, and if a conservative approach is taken, these estimates can be valid tools for the determination of filter service life (see Table 1)5. Ultimately, filter changeout schedules will come from experience. Actual-use conditions often dictate that different media types in individual AMC control systems will need to be replaced at different intervals to maintain optimum performance. This is not the answer that manufacturers want to hear because they want to be able to set maintenance schedules and budgets. However, the reality is that one cannot predict, much less guarantee, filter service life from testing on one or two target contaminants while not knowing the total contaminant load the system will actually see.

Final considerations Although the specification of an AMC control system is a difficult undertaking, given the proper considerations, one can be successful in specifying an effective and economical solution for most applications. An end user cannot go into this process with preconceived notions about how a particular system will perform under a given set of conditions; rather, users should determine what they ultimately want to achieve by the installation of such a system. These five points should always be considered in the design and specification of an AMC control system: 1. Does AMC pose an immediate health threat to cleanroom personnel or is it primarily an odour control issue? Personnel protection requires 100% control of the offending contaminants whereas odour control could be achieved with a system operating at an overall efficiency of 50% or less. Process protection requires very high performance, but does not necessarily mean absolute control. Many specifications today call for minimum removal efficiencies of 90% and this is realistic to expect. An assessment can determine the types and concentrations of AMC present. 2. Accept filter performance testing for what it truly is Efficiency or capacity test data for a single contaminant cannot be used as an absolute predictor of system performance. It can, however, provide an indication of the relative performance differences between competitive systems. 3. Request performance data for contaminant concentrations to be generated at or near expected use conditions if at all possible If system performance and/or filter life estimates are based on accelerated test results, consider limiting gas challenge concentrations to no more than two orders of magnitude above what would be expected under actual use conditions. Require real-time monitoring data for any testing performed. 4. Filter service life should not be a part of a performance specification Filter life estimates should be required for all systems being considered, but again, these should be used as relative comparisons and not absolute values. There is no way to provide service life estimates without having performance data for the total contaminant load and not just a one or two target contaminants. Monitoring of filter performance using direct gas monitoring, reactivity monitoring or passive samplers should be started along with system start-up and should continue as long as the AMC control system is in place. 5. Use a multiple stage AMC control system AMC control often calls for different media types based on the various contaminant types expected. Just as particulate control in cleanrooms requires several stages of filtration to achieve the desired cleanliness level in the protected space, the same should be considered for an AMC control system. A 'prefilter' stage should be used to remove as much of the 'junk' AMC as possible. This would help protect and preserve the final filter's ability to remove those primary contaminants of concern with good efficiency and capacity. A properly designed, installed and maintained AMC control system can fairly easily achieve removal efficiencies greater than 90% for specific target contaminants. How long a system will meet specific performance criteria depends on the average and peak values of all contaminants present, which must be considered in the final design of the AMC control system. The optimum system will contain multiple stages of filtration to provide high initial and average removal efficiencies as well as an acceptable service life. It will involve higher front-end costs, but will ultimately lead to lower operating costs and a lower likelihood of AMC making its way into the cleanroom and into critical process areas.

T: + 1 770 662 8545 chris_muller@purafil.com