1.  Technical Field
The present invention relates generally to metrology tools, and more particularly, to combining reference measurement collections of at least three reference measurement systems into a weighted reference measurement collection. 
2.  Related Art
In the metrology industry, it is useful to know how well a given measurement system performs.  One way to accomplish this task is to identify a reliable, trusted reference measurement system (hereinafter “RMS”) and collect measurements of multiple samples (e.g., of wafers in the semiconductor industry) using the RMS to establish a respective reference measurement collection.  A measurement system to be evaluated (i.e., a “system-under-test”) can then be used to measure the same multiple samples.  The measurement values for each system can then be compared, for example, by plotting the system-under-test measured values on one axis and the reference measurement collection values of the same samples on another axis of a graph.  When these measurements encompass a range of values, a best-fit line can be drawn among them to determine how well the system-under-test matches the RMS. 
The reference measurement collection can include measurements from either a single measurement system or multiple reference measurement systems (called a “composite RMC”).  Composite RMCs are used in order to draw upon the advantages of each system, while avoiding or de-emphasizing the disadvantages of each system.  For example, a critical dimension scanning electron microscope (CDSEM) system can be used as a reference for a critical dimension (CD) (linewidth).  A CDSEM has the advantage that it is fast.  However, it has an inherent offset in its measurement.  However, if a limited number of samples are measured with the CDSEM and a cross-section scanning electron microscope (XSEM), the measurement using the XSEM can be used to determine this offset.  The XSEM is slow but has little offset.  In this example, the CDSEM and XSEM together form a composite RMC. 
Typically, multiple RMSs are used in a cascading effect, where each system is referenced to the next system.  One inherent problem with conventional RMS approaches (either single or multiple systems), however, is that they sometimes do not provide a good reference because every measurement, no matter how good, has some kind of error.  Reliance on such techniques is especially dangerous when the user does not realize the RMS that is considered reliable (or good enough) is, in fact, not reliable.  Another shortcoming of conventional multiple RMS approaches is that sometimes a user cannot determine which system should be the RMS, which is a common situation when dealing with leading-edge metrology systems. 
One approach for generating composite RMCs is to simply average measurements for each sample from the various RMSs.  Unfortunately, this approach does not penalize measurements that do not agree well with measurements from other systems of the same sample, which are referred to as “flyers.”  Another approach to composite RMSs is described in: M. Sendelbach and C. Archie, “Scatterometry measurement precision and accuracy below 70 nm,” Metrology, Inspection, and Process Control for Microlithography XVII, Proceedings of SPIE, Vol.  5038, pp.  224–238, 2003.  This approach also does not address the flyer issue. 
In view of the foregoing, there is a need in the art for improved techniques for generating a composite reference measurement collection.