KEYWORDS: Semiconducting wafers, Critical dimension metrology, Metrology, Particles, Time metrology, Control systems, Metals, Scanning electron microscopy, Tin, Process control
As the semiconductor industry continues the transition to 300mm wafer factories, not only does the cost per wafer increase dramatically, but the number of eligible die (assuming equal die size) more than doubles. Given the parallel transition to design rules of 90nm and below, both the cost of production and the potential revenue from a 300mm wafer are vastly higher than that of a current 200mm wafer. For this reason alone, it is essential that wafer jeopardy, or the
number of wafers processed between metrology events, be reduced dramatically from the levels in a typical 200mm wafer line. The most promising method for achieving this is process tool-integrated metrology. Such systems allow rapid (in some cases near instantaneous) feedback on the process. Such a data stream, as input to an Advanced Process Control (APC) system, provides a volume of data and feedback lag time unparalleled by standalone metrology. In this case, critical dimension (CD) metrology is provided by a scatterometer integrated on a 200mm TEL CLEAN TRACK - ACT 8. The data, available on a wafer-by-wafer basis, is uploaded to the factory host where the APC application can update its state estimation before the entire lot has even completed processing.
Scatterometry is gaining acceptance as a technique for critical dimension (CD) metrology that complements the more established scanning electron microscopy (SEM) techniques. Scatterometry determines the dimensions of the submicron structures by inverse diffraction calculations. SEM and scatterometry are complementary in many respects. Therefore, they are likely to coexist in the foreseeable future. Scatterometry and CD-SEM instruments can be integrated to take advantage of the complementary nature of the two techniques. To explore the joint use of scatterometry and SEM measurements, we measured a set of photoresist grating samples with CDs ranging from 240 nm through 40 nm by scatterometer (Sensys CD-i) and cross-section SEM (Hitachi S-4700). Although a cross-section SEM was used as an absolute standard for comparison of profiles and CDs, our conclusions range to include CD-SEM techniques. It was found that for measurements of profiles that were patterned with high uniformity within the measurement area, scatterometry was very effective, and correlated best with SEM measurements. However, in cases of substantial line-to-line profile variations, or for isolated or non-periodic lines, SEM is the more appropriate measurement method.
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