An Optimized Procedure for Process Equipment and Minienvironment Specification with PWP Measurement

To characterize the cleanliness of minienvironments and process tools, PWP measurements are used. The PWP (particles per wafer pass) value is obtained from the difference between the measuring of particle sizes and counts on an unstructured wafer surface before and wafer handling in any kind of process equipment. For higher requirements, PWP-counts of e.g. 0.005 - 0.001 have to be obtained. Not only is the proof of these values are not economically viable due to of the large number of samples require, but also that these low particle additions on the wafer surface must be identified within the detection limit of the existing wafer measuring system. The previous qualification procedure with so many samples involved can not be used economically. Based on statistical considerations and the analysis of experiments, a cost-optimized procedure needs to be developed and proved. To fulfil this aim, an experimental set-up must also be installed. With the combination of statistical analysis and experimental investigations the expected PWP values and the qualification costs. This paper presents a procedure which allows an economic cleanliness qualification of process equipment with PWP measurements. With this procedure it is possible to calculate the cost of equipment qualification in advance based on given accuracy and PWP values.