Vuong Vi

OCD metrology by floating n/k

In this paper, one of the major contributions to the OCD metrology error, resulting from within-wafer variation of the refractive index/extinction coefficient (n/k) of the substrate, is identified and quantified. To meet the required metrology accuracy for the 65-nm node and beyond, it is suggested that n/k should be floating when performing the regression for OCD modeling. A feasible way of performing such regression is proposed and verified. As shown in the presented example, the measured CDU (3σ) with n/k fixed and n/k floating is 1.94 nm and 1.42 nm, respectively. That is, the metrology error of CDU committed by assuming n/k fixed is more than 35% of the total CDU.

Low-k n&k variation impact on CD accuracy of scatterometry

Scatterometry is one of the advanced optical metrology techniques has been implemented in semiconductor manufacturing for monitoring and controlling critical dimensions, sidewall angle and grating heights as well as thicknesses of underlying films, due to its non-destructive nature, high measurement precision and speed. In traditional scatterometry approach, the optical properties (n&ks) of film stack have been used as fixed inputs in a scatterometry model, therefore, the process engineers have to assume that there is no significant impact on measurement results by small deviation from pre-extracted n&ks. However, n&ks of actual production wafers will always vary from the fixed values used in the model. The magnitude of the variations and its impact on the accuracy of scatterometry measurements has not been well-characterized yet. In this study, a low-k dielectric stack with noticeable n&ks variations was generated. The low-k dielectric stack has the refractive index (n) variation around 0.01 @ 633nm within a wafer, and is under two layers of patterned PR and BARC. Different scatterometry models with fixed and floated n&ks have been analyzed. Although comparable repeatability was obtained with either fixed or floated n&ks model, the correlation (R2) to CD-SEM result has been improved by floating n&k in the model in comparison to that of fixed n&k model. In this paper, we also discuss some differences in applying various optical models (i.e, EMA and Cauchy) in scatterometry measurements.