Introduction of substrates into vacuum environment is accomplish by gradually reducing the number of substrates being transferred simultaneously as the clean and evacuated environment is progressed. Cassettes are maintained in clean atmospheric environment and do not enter the vacuum environment. Several vacuum locks are linearly staggered so as to introduce progressively higher level of vacuum environment. The number of substrates transported through this arrangement is a portion of the number of substrates present in each cassette. The staggered vacuum locks lead to a series of processing chambers, wherein a yet smaller number of substrates, e.g., one or two, are transported.