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&k's) 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&k's. However, n&k's 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&k's 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&k's have been analyzed.
Although comparable repeatability was obtained with either fixed or floated n&k's 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.