Effect of fab scale, process diversity and setup on semiconductor wafer processing cost

Semiconductor business requires not only high-volume, low-cost fabs but also high-mix, flexible fabs that accommodate many kinds of products such as System-on-Chip. The effect of process diversity on fab design and size requirements is still a problem that has received little attention in the modeling literature. This paper presents a general model of fabs running multiple process flows that explicitly considers the effect of fab capacity, process diversity, and setup on wafer processing cost. Cost is classified into fixed cost and variable cost from the cost behavior patterns perspective. Our model then classifies fixed cost into capacity-independent cost and capacity-dependent cost. The effects of fab capacity, process diversity and setup on cost are evaluated using our model. The model predicts the following. (1) The capacity independent cost model predicts lower processing costs for larger fabs at utilization. (2) As a fab´s processes become more diverse (i.e., there are more types of equipment required) the minimum efficient size of the fab increases. However, simply changing the product mix ratio will affect wafer cost but will not affect the minimum efficient size. (3) Setup requires not only equipment cost due to setup time but also direct costs associated with setup. The size of the fab determines whether or not dedicating equipment to reduce setups will reduce wafer cost