AMC and filtration

Airborne Molecular Contamination (AMC) is a growing problem which has emerged in the world of cleanroom filtration over the last few years. Continued improvements in cleanroom environments have virtually eradicated the risk of particulate contamination, as filtration technology is capable of reducing hundreds of thousands of sub micron particles per litre to virtually nil per litre, through the use of the highest-grade ULPA filters. However, the problem of AMC remains because of the existence of chemicals in the air, such as benzene, and oxides of sulphur and nitrogen. Also, chemicals which are generated in the cleanroom process can return in re-circulated air – and the chemicals themselves can liberate others. This can in turn affect the quality and character of the product being made, therefore the control of AMC, such as benzene, is of prime importance in many cleanroom processes. If chemicals were allowed to corrupt the process, the end product would have to be discarded, at huge cost. So, whilst particle removal has previously been the priority, the elimination of AMC has become the focus for professionals involved in the improvement of process environments. Photronics of Manchester, manufacturers of photomasks, were experiencing such a problem with AMC within a new "Chemically Amplified Resist" (CAR) process, which has been developed for the 0.12 micron (and beyond) technology mode. As part of this process there is a requirement to reduce the risk of AMC, caused by ammonia and amines. Photronics were encountering levels of ammonia-based AMC, which they needed to bring below 1 ppb or less to prevent it interfering with the CAR process. This particular application involved the Air Handling Units (AHUs) serving the Lithography area of one of Photronics' cleanrooms, where they produce photomasks for semiconductor manufacturing. Therefore the removal of ammonia from the cleanroom atmosphere was of vital importance. McLeod Russel proposed the use of an AFP-based product, and using information based on a transfer of measurement data on the ammonia concentration within the cleanroom, McLeod Russel supplied a variant of their AFP carbon filters, specially developed for the removal of ammonia and other alkaline gases. It was necessary to deploy this specialist range of carbon filters to remove AMC, as conventional carbon filters found in bonded or loose fill filters are not acceptable due to dust liberation. These filters had to fit the existing AHUs, as this was the most sensible location for removing Ammonia AMC from the cleanroom airflow. Tony Oldham, Facilities Manager at Photronics, commented: "We were already working closely with another AMC filter manufacturer and the results obtained were excellent, i.e. Ammonia levels were <1 ppb. As part of the trial we were keen to look at alternative suppliers, and following discussions with representatives from McLeod Russell, we were convinced that their filters were potentially capable of meeting our requirements. In addition the design/construction of the filters is similar to their standard range of particulate filters, and hence they were compatible with the existing filter housings within our AHUs. "It was decided to target an isolated area within lithography, that was not influenced by the other existing AMC filters. Once again the results were excellent with resulting Ammonia levels < 1ppb. Obviously it is early days and the working life of the filters can only be projected at this time, but "so far, so good!" The company's range of AFP filters can be used for the removal of AMC in "clean" process air environments, designed for those cases in which the gas concentrations are relatively low. Such concentrations can reduce the yield of critical production processes in cleanrooms. This new generation of filters are fitted with a revolutionary new filter medium in which very small activated carbon particles are evenly suspended and bonded to the three dimensional network of synthetic fibres. This way, the carbon particles are brought in close contact with the air stream to be cleaned, without glue, perforated plates, dust emissions or leaks from the settlements of granular carbon beds. Compared to conventional filters, the AFP design offers lowest pressure drops, high specific adsorption capacities, optimised carbon deployment and a low filter weight.

Contact Bill Anderson on tel: +44(0)1282 413131