1. Field of the Invention
This invention relates generally to semiconductor device manufacturing and, more particularly, to a method and apparatus for prioritizing production lots based on grade estimates and output requirements.
2. Description of the Related Art
There is a constant drive within the semiconductor industry to increase the quality, reliability and throughput of integrated circuit devices, e.g., microprocessors, memory devices, and the like. This drive is fueled by consumer demands for higher quality computers and electronic devices that operate more reliably. These demands have resulted in a continual improvement in the manufacture of semiconductor devices, e.g., transistors, as well as in the manufacture of integrated circuit devices incorporating such transistors. Additionally, reducing the defects in the manufacture of the components of a typical transistor also lowers the overall cost per transistor as well as the cost of integrated circuit devices incorporating such transistors.
Generally, a set of processing steps is performed on a wafer using a variety of processing tools, including photolithography steppers, etch tools, deposition tools, polishing tools, rapid thermal processing tools, implantation tools, etc. During the fabrication process various events may take place that affect the performance of the devices being fabricated. That is, variations in the fabrication process steps result in device performance variations. Factors, such as feature critical dimensions, doping levels, contact resistance, particle contamination, etc., all may potentially affect the end performance of the device.
In a typical semiconductor fabrication facility, wafers are processed in groups, referred to as lots. The wafers in a particular lot generally experience the same processing environment. In some tools, all of the wafers in a lot are processed simultaneously, while in other tools the wafers are processed individually, but under similar conditions (e.g., using the same operating recipe). Typically, a lot of wafers is assigned a priority in the beginning of its processing cycle. Priority may be assigned on the basis of the number of wafers in the lot or its status as a test or experimental lot, for example.
At a particular processing step, the relative assigned priorities of all the lots ready for processing are compared. Various rules are applied to determine which of the eligible lots is selected for processing. For example, for two lots with the same priority, the older of the lots is often selected for subsequent processing. In the case of a test lot of wafers (i.e., generally including a reduced number of wafers), the lot is subjected to one or more experimental processing steps or recipe adjustments in an attempt to improve the performance of the process or the performance of the resultant devices. Before commencing production of regular production lots using the experimental parameters, it is useful to first test the effectiveness of the changes based on the resulting characteristics of the wafers in the test lot. Hence, a test lot would be assigned a relatively high priority over other production lots, such that its processing is completed more quickly. Regardless of the particular priority assignments made, the rules are essentially static and predetermined. The priority of a particular lot does not typically change during its processing cycle, unless its status changes from being a production lot to a test lot, for example.
After fabrication of the devices is complete, each device is subjected to numerous functional tests to determine performance characteristics such as its maximum operating speed. Based on the results of the performance tests each device is assigned a grade, which effectively determines its market value. In general, the higher a device is graded, the more valuable the device. However, some applications do not require high-end devices. Accordingly, maximizing the output of the fabrication facility may correspond to maximizing the profitability of the facility.
Because of the variation in performance of the end product devices, it is difficult to predict the throughput of the processing line for devices of a particular grade. Hence, the supply of completed devices may not match the current output demands for the fabrication facility. For example, if a large number of high performing devices (i.e., more expensive devices) have been produced, but the current demand is for lower cost devices (i.e., slower), orders may not be able to be filled with the desired grade device. As a result, the manufacturer may be forced to sell devices of a higher grade at a lower price to fill the order. If the demand is for higher grade devices, and the supply of higher grade devices is diminished, the manufacturer may be unable to fill the order at all. Either situation results in lost profits for the manufacturer.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
One aspect of the present invention is seen in a method for prioritizing production flow. The method includes processing a plurality of manufactured items in a process flow; measuring characteristics of a plurality of manufactured items in the process flow; estimating performance grades for the plurality of manufactured items based on the measured characteristics; grouping the manufactured items with like estimated performance grades; assigning priorities to groups of manufactured items with like estimated performance grades; and directing the plurality of manufactured items through the process flow based on the assigned priorities.
Another aspect of the present invention is seen in a manufacturing system including a plurality of processing tools adapted to process a plurality of manufactured items in a process flow, a metrology tool, and a process control server. The metrology tool is adapted to measure characteristics of a plurality of manufactured items in the process flow. The process control server is adapted to estimate performance grades for the plurality of manufactured items based on the measured characteristics, group the manufactured items with like estimated performance grades, assign priorities to groups of manufactured items with like estimated performance grades, and direct the plurality of manufactured items through the process flow based on the assigned priorities.