The second component of DCM is measuring the OVL target correctly. Even for the same OVL target, different OVL values can be reported by different measurement conditions or different targets.1–6 For example, different color filter selection can report different OVL values, specifically when the OVL target exhibits asymmetry (for either current or previous layer). Therefore, measurement conditions which are defined in the measurement recipe need to be optimized to provide the correct OVL. The reader might want to refer to the elements of Fig. 2 in the following discussion. In the past, a low total measurement uncertainty (TMU) was one of the key elements for such selection, however, in advanced metrology tools, with a TMU performance below 0.5 nm, TMU is no longer a key indicator for accuracy or a component of inaccuracy. Model residuals are often used for determining the most accurate recipe or target. However, they also have systematic issues; for example, a measurement with a low sensitivity to OVL will produce low OVL values, potentially with low residuals. Another way to evaluate the accuracy is the Qmerit algorithm, described last year and this year in SPIE papers.1–4 This algorithm produces a quantitative measure of the asymmetry impact. The Archer Self Calibration algorithm, described in a poster in this year’s SPIE,3 allows correcting for that asymmetry and improving the OVL accuracy. The OVL linearity arrays and programmed OVL shifts in different fields are used to verify the target sensitivity to OVL and have some correlation to accuracy, but they are not immune to systematic OVL shifts. Another way to select the optimal recipe condition is by ADI to AEI matching, however, in some cases this is disturbed by an OVL bias introduced by the etch process. In this case and others, the CD-SEM target embedded in the hybrid target allows independent verification of the measurement accuracy. We can see in this table that all accuracy measurement methods with an HVM acceptable ease of use (methods 1 to 6) are not accurate enough. Using a CD-SEM hybrid target is both accurate and acceptable for HVM recipe verification. Obviously, during the development stage, the ease of use is less a factor and then different methods (methods 7 to 10) come into play. They will be discussed in the next section.