How to measure birefringence with ellipsometry.
Few realize that ellipsometry can also be used to measure the phase change in birefringent materials. Birefringent materials have a different refractive index for light traveling along different directions.
The refractive index also describes the phase velocity for light, so a different index ([n.sub.x] [not equal to] [n.sub.y]) creates a fast and slow axis. Light oscillating along the slow axis will be "delayed" relative to the light oscillating along the fast axis.
Common birefringent materials include crystals with non-symmetric atomic spacing (e.g. calcite, sapphire) and oriented polymer films. The birefringent effect can be very beneficial. Liquid crystals are a twisted birefringent material used in many laptop displays. Some thin polymer films have low in-plane birefringence with higher out-of-plane birefringence. SE data are acquired at different incidence angles through these films and can be used to characterize both the in-plane and out-of-plane birefringence for proper viewing-angle compensation.
Birefringence is not always welcome. Photolithography optics need low birefringence to clearly produce the small line-widths used in microelectronics. To produce smaller features, lithography is shifting toward shorter wavelengths.
At 157 nm, the optics are made from calcium fluoride (Ca[F.sub.2]) because other materials are absorbing. Ca[F.sub.2] is cubic, so it was believed to exhibit only stress-induced birefringence. Researchers have shown that Ca[F.sub.2] can exhibit birefringence levels far above the lithography tolerance at 157 nm when light travels along certain crystal directions.
Optics manufacturers have struggled to change designs to compensate for this characteristic. It is essential to measure birefringence at the operation wavelength to insure that the lithography tool meets its specification.
Spectroscopic Ellipsometry (SE) A common metrology technique for characterizing thin film thickness and refractive index (n and k). It operates on the basis of measuring a polarization change (both amplitude and phase) when light is reflected from a coating.
Web Resources for Ellipsometry:
* www.grc.nasa.gov/WWW/Optinstr/tuma/ ellipse.html
Hilfiker is an engineer at J.A. Woollam Co. Inc.
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|Title Annotation:||Vacuum Technology|
|Publication:||R & D|
|Date:||Jul 1, 2004|
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