Optical Grating Scatterometry
In the past few years, scatterometry has emerged as a method for
performing linewidth and line profile metrology, especially by
the semiconductor industry. The method uses a
periodic target containing repetitive lines whose profile, i.e., its
width, height, and shape, is to be determined. The technique relies
on the complicated behavior that the grating structure has on the
diffracted light as a function of incident angle and wavelength, and
the sensitivity to small changes in the grating profile. By comparing
measurements with a library of calculated model results, the profile
can be inferred. While the method claims extreme sub-nanometer-level
sensitivity to the line profile, there are a number of outstanding
issues that the user community has regarding the technique's absolute accuracy.
Some of these issues are outlined below:
- The method relies upon
rigorous coupled wave analysis (RCWA) of the diffracted light. RCWA
provides an accurate solution to Maxwell's equations for periodic
structures, but does not address the issues of finite structures,
finitely illuminated structures, or non-periodic structures.
Questions thus arise about the accuracy of the method for realistic
structures.
- The method relies on having accurate values for the
optical properties of the materials in the structure. Issues, such as
the presence of thin oxide films or near-surface optical properties,
are usually overlooked.
- Standards are needed in order to validate
the method. These structures would be characterized by a variety of
metrologies and would provide well-characterized profiles.
- The
limits of scatterometry, in terms of how long it will remain a viable
solution to the semiconductor industry's needs as device dimensions
are reduced, are unknown. It is essential that these limits be
determined so that the industry can develop alternative solutions.
Any advances that would extend those limits would be welcomed.
- Extensions to the scatterometry technique need to be explored. For
example, the method is primarily used to assess two-dimensional
profiles. Extending scatterometry to three-dimensional profiles may
reduce the need for non-functioning targets, or allow measurement of
line-edge and line-width roughness.
This program aims to address these and other issues both theoretically
and experimentally.
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