Performance of Workstation With Offline and Integrated Metrology

One of the present developments in the semiconductor industry is integration of metrology tools in the main process tool, instead of measuring wafers stand-alone, i.e., at an offline metrology tool. One of the advantages expected from integrated metrology (IM) compared to offline metrology (OM) is an earlier detection of out-of-control situations in the process tool. Despite of the advantages expected from IM, manufacturers will only convert to IM if investments can be justified. The objective of this paper is to compare the effect of process failures in the processes on the performance of a workstation using OM or IM. Analytical relations for effective process time and flow time of a workstation, containing a single machine or a cascade machine, are derived and validated for both OM and IM. In the analysis, the performance of OM and IM practices are compared and the influence of process failures is investigated. It can be concluded that a workstation containing a single machine or a cascade machine with IM outperforms a workstation with OM with respect to flow time for lower levels of utilization. For higher levels of utilization the effective (maximal achievable) throughput of a workstation with IM is limited because of the increase of the process time by the quality measurement time. If the frequency of process failures is reduced by improving the workstation, a better performance of a workstation with OM is obtained with respect to flow time and throughput at higher utilization levels. In all situations, a workstation with OM and no transport time (inline metrology) has the best flow time performance. The opinion in industry that IM has a positive influence on flow time performance is not shared by the results of this paper. The results are obtained, e.g., by making no distinction between different metrology inspections or by neglecting the influence of a metrology module upon the reliability of the workstation. In practice, metrology configurations may not perf- orm equally so that a preference for one of them might be based on other considerations than flow time performance