Lean randomization and exception handling
In recent years, randomization of process order as well as randomization of the wafer map in the cassettes delivering the wafers was increasingly used to be able to correlate yield variations with specific segments of the process flow (see [1] and [2]), as well as to reduce yield influences on specific wafer positions such as top and bottom of the wafer cassette. Many randomization efforts relied on wafer sorter based randomization, which costs cycle time and wafer sorter capacity. In this paper, we describe how randomization of both process sequence as well as wafer map changes can be implemented on process equipment as part of normal processing. This kind of randomization does not add cycle time and does not require wafer sorters and will therefore be called lean randomization. We will also describe some of the most important exceptions that can occur when implementing lean randomization and we will discuss how a fab can deal with those exceptions.