Redefining critical in critical dimension metrology

Critical dimension (CD) metrology as practiced in semiconductor industry displays characteristics not observed in other metrology disciplines. This paper will present some of the unusual aspects of CD metrology and attempt to elucidate the causes for the observed behavior. Through an examination of the characteristics of measurement accuracy, it is possible to observe these situations where CD metrology departs from the ideal. The typical process for achieving accuracy involves the use of certified standards in a well-defined calibration procedure. However, calibrating CD instruments with linewidth standards will not necessarily guarantee sufficient accuracy of subsequent measurements of production samples. This well-known result follows from lack of physical models to relate the detected signal to sample shape in combination with the many-to-one nature of the mathematical mapping that describes the process of obtaining CD from feature shape. Despite this limitation, monitoring tools such as CD-SEM systems have demonstrated to be useful for process control and are extensively used in semiconductor manufacturing. The requisites for a well-behaved measurement process will be described in detail. The unusual characteristics of CD metrology will be identified, as will the underlying reasons for the behavior. These results will be examined in the light of common process control techniques to explain how CD-SEM measurements still add value despite the flaws. In conclusion the role and value of certified standards in feature shape determination will be placed in the context of CD metrology. Reference Measurement Systems in conjunction with calibration standards are recommended to characterize process variations and determine feature shapes across a variety of samples. In order to ensure that the high throughput monitoring metrology tools flag process excursions for not meeting specifications, feature shapes must be quantified with additional metrics besides a single number CD.