Using queueing curve approximations in a fab to determine productivity improvements

Fabs are under great pressure to produce more output with the same equipment set. However, if throughput is increased without productivity improvements, cycle time will naturally rise. Texas Instruments Incorporated is using a queueing curve approximation in order to determine a particular machine set´s productivity improvements over time regardless of fab loadings. By measuring the machine set´s monthly average throughput and cycle time, a fab can determine if true improvements have been made and also determine how much cycle time would increase or decrease with loadings. In order to have widespread use in fabs, a particular formula should be relatively simple and easy to use. We will discuss a queueing curve approximation and how it can be used in the fabs. We will also discuss its accuracy both under ideal conditions and conditions of data that are insufficient and/or rough approximations. Other formulation problems such as machine or lot dedication (one queue per machine versus one queue to all machines), various arrival/service time distributions and percentage improvement determination are also addressed