Excursion Yield Loss and Cycle Time Reduction in Semiconductor Manufacturing

The importance of cycle time reduction is well known to the semiconductor manufacturing industry in the sense of reduced inventory costs and faster response to the market. Less emphasized is the fact that the overall die yield is also closely related to cycle time. In particular, some yield losses are due to "excursions," when process or equipment shift out of specifications. While some and perhaps most excursions are detected by in-line inspections, some are not detected until the wafers are tested in the probing area after fabrication. A long production cycle time will expose significant amounts of wafers in production to defective processing by such excursions. This paper introduces analytical formulas to quantify the revenue losses due to excursions not detected until end-of-line testing as a function of manufacturing cycle time, excursion probabilities and kill rates. The formulas provide a means to evaluate the revenue gains due to cycle time reduction, based on the assumption that the average selling prices of semiconductor products are declining steadily at predictable rates. Note to Practitioners-Formulas are provided to compute the expected total die losses over product lifetimes due to excursions in process and equipment that are not detected until end-of-line testing, given the process step, probability of occurrence and kill rate for each potential excursion, given the manufacturing cycle times, and given assumptions about baseline yield and yield learning rate. Formulas also are provided to compute the expected lifetime revenue losses from such excursions, given an assumed rate of price decline for die output. The impact of cycle time reduction on die output and revenue may be readily assessed using the formulas.