1. Field of the Invention
The invention relates to the field of semiconductor metrology, and in particular, to a method and system for efficiently dealing with process issues indicated by metrology data.
2. Related Art
As the dimensions of semiconductor devices continue to shrink, accurate and efficient characterization of the components forming those devices becomes more critical. Tools used in such characterization efforts are commonly described as metrology tools. For example, ellipsometry tools, scatterometry tools, x-ray fluorescence tools, x-ray reflectometry tools, and electron microprobe analysis tools are all types of metrology tools used to evaluate the properties of the semiconductor, dielectric, and metal layers that form semiconductor devices.
A metrology tool is typically controlled by an instruction set (called a “recipe”) that defines measurement parameters for that tool. Those measurement parameters can include the particular test wafers to be measured (e.g., wafers from slots 3, 7, and 12 of a wafer cassette), what type of measurement to perform (e.g., refractive index, film thickness), what type of film to expect, what locations on a wafer to measure, and any other information related to the operation of the tool.
Conventional recipes specify a static set of metrology parameters that are applied to all test wafers in a particular test group (typically one or more cassettes of wafers). Therefore, all the test wafers are measured in the same manner. The results of this testing are then reviewed by an operator to determine the appropriate response.
The goal of a metrology tool is to monitor the performance of a process tool (or set of process tools) by evaluating structures on a test wafer processed by the process tool. Specifically, the output of a metrology tool is used to detect process excursions (i.e., process results that are outside the acceptable output range), so that appropriate corrective measures can be taken.
However, because of the static nature of conventional recipes, the detection of process excursions can sometimes occur too late for optimal response. For example, if one or more of the test wafers from a cassette exhibit process excursions, it would be desirable to perform additional metrology operations on those wafers (for example, to confirm the problem, to determine the extent of the problem, or to precisely characterize the problem). Unfortunately, because of the lag between the metrology operation and the manual review of the results, the test wafers on which the process excursions have been detected have often moved on to the next process step before any additional metrology can be performed. Consequently, any opportunity to “debug” the process is lost.
Furthermore, even if the problematic wafers are caught before any further processing is performed, performing the additional metrology operations to determine the scope of the problem can result in significant (expensive) production delays. Once a problem is detected, a new static recipe must be loaded into the metrology tool and the new metrology operation must be performed. The production line is typically shut down during this reconfiguration and restarting of the process tool, thereby resulting in significantly reduced fab output even if the problem is ultimately found to be of no consequence.
Accordingly, it is desirable to provide a system and method for detecting and evaluating process excursions without significantly delaying the overall process flow.