Job configuration for semiconductor manufacturing is the technique in which silicon chips are assigned to sites on wafers, wafers are assigned to jobs, and jobs are released into the manufacturing line, so as to meet the demand for chips. The process is complicated by yield uncertainties of the line and the configuration constraints imposed by the technology of the line. Yield losses can occur at a job level, where every chip within a job is bad, at a wafer level, where every chip on a wafer is bad, or at the site level, where one or more individual chips are bad.
In the static job configuration problem, the configuration is determined in an open loop fashion from data on the demanded numbers of chips and the yield uncertainties. A decision is made on the configuration, and the configuration is released into the line; neither cycle time nor the status of the jobs within the line are considered.
There are several measures of performance for meeting the demand. The semiconductor supplier may be measured on its "volume serviceability," the percent of the volume of the demand that is met. In a more stringent measure of performance called "mix serviceability," the supplier is measured in terms of the volume of demand supplied, but is not credited for shipping parts in excess of their demand. "Set serviceability" is another measure of performance. A customer may order a specified number of sets of components, for example, the set of chips needed for a printed circuit board or module. In set serviceability, the supplier is measured in terms of the demand for complete sets that is supplied. Set serviceability is more difficult to meet than mix or volume serviceability.
In meeting set serviceability requirements for the static job configuration problem, it is desirable that the configurations achieve the serviceability with the fewest number of wafers. Minimizing the number of wafers reduces the costs for the raw materials and reduces the number of wafers flowing through the line.
The technologies used to expose wafers (optical, e-beam and retical) and test equipment limitations may impose constraints on the possible job configurations. Some semiconductor manufacturing lines process single part jobs, where every chip within a job is identical. The job configuration required for such a line is referred to as a single part job or SPJ configuration scenario. Other manufacturing lines have more flexibility and allow for multiple part jobs having single part wafers, where every chip within a wafer is identical, but wafers within a job may be different. The job configuration required for such a line is referred to as a single part wafer or SPW configuration scenario. Some manufacturing lines process multiple part jobs having multiple part wafers, where a wafer (and therefore a job) may be made up of a plurality of different chips. The job configuration required for such a line is referred to as a multiple part wafer or MPW configuration scenario.
The logistics systems controlling the flow of jobs through the line may impose additional constraints on the allowable job configurations. For example, these systems may require that a job configuration having multiple part jobs with single part wafers be composed of several copies of the same standard job. In the multiple part jobs with multiple part wafers case, the systems may require that every multiple part wafer be identical, that is, there be a standard wafer, and every job within the configuration contains the standard wafer.
A conventional approach to semiconductor job configuration is known as ground start ratio. According to ground start ratio, the number of chip starts is the ratio of the demand for the chip type to the overall yield of the chip type. This ratio is computed independently for each chip in the set.
Because the job configuration data for each of the chip types is computed without reference to demand data, yield data or configuration data of any other chip type, the computation required for the ground start ratio is straightforward and does not require an excessive amount of time to compute. However, the survival rates of the chip types which make up a set are not independent of one another in determining the survival rate of the entire set. Consequently, the ground start ratio approach is ill-suited to suppliers measured by set serviceability.
Furthermore, because the ground start ratio is computed independently for each chip type, it cannot be used to take advantage of the substantial potential serviceability increases gained by using single part wafer and multiple part wafer configurations.
Therefore, what is needed is an apparatus and method for manufacturing sets of chips that achieves the desired serviceability using a reasonable number of wafers.