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 (<i>n&k</i>'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 <i>n&k</i>'s. However, <i>n&k</i>'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 <i>n&k</i>'s variations was generated. The low-k dielectric
stack has the refractive index (<i>n</i>) 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 <i>n&k</i>'s have been analyzed.
Although comparable repeatability was obtained with either fixed or floated <i>n&k</i>'s model, the correlation
(R<sup>2</sup>) to CD-SEM result has been improved by floating <i>n&k</i> in the model in comparison to that of fixed <i>n&k</i>
model. In this paper, we also discuss some differences in applying various optical models (i.e, EMA and
Cauchy) in scatterometry measurements.
A series of experiments were performed to determine if Tokyo Electron's (TEL) Optical Digital Profilometry (ODP) scatterometry technology could meet the requirements for CD (Critical Dimension) control in lithography applications. ODP technology, using broadband optical spectra taken from grating structures, is non-destructive, and in addition to CD, provides sidewall angle (SWA), profile and film thickness information in a single measurement. ODP's output was compared to CD-SEM (Critical Dimension-Scanning Electron Microscopy) to develop the correlation of different metrology techniques. ODP was able to demonstrate excellent correlation to CD-SEM and provide robust uniformity measurement with high repeatability. ODP measurements were also used to optimize within-wafer CD uniformity by controlling the scanner process parameters. The high resolution of CD and SWA data taken by ODP demonstrated high sensitivity to scanner process control and a significant reduction in CD variation. From the experimental results, the ODP technology was well qualified for CD control in lithography applications.