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Optical Design & Engineering

Riding the wavelength

Spectroscopic ellipsometry improves the measuring process for thin films.

From oemagazine June 2001
31 June 2001, SPIE Newsroom. DOI: 10.1117/2.5200106.0008

Spectroscopic ellipsometry is a noncontact, nondestructive metrology technique for thin-film characterization. A spectral ellipsometer (SE) reflects a light beam from the surface of a sample and measures the changes in polarization that occur. The technique is primarily used to measure film thickness and optical constants for both single- and multiple-layer coatings. The technique also can be used to investigate other material properties such as crystallinity, alloy ratio, doping concentration, uniformity, anisotropy, and more.

Microlithography is a common application area for ellipsometry. Measurement speed often is an important aspect when choosing a spectral ellipsometer. For example, if you will be routinely measuring a large number of coated surfaces for quality control or would like to map many points across a coated surface, then consider array-based systems. They can measure hundreds of wavelengths simultaneously to provide single-point results in a few seconds. If your application does not demand high speed, you may be better served with a high-accuracy, research-grade SE. Such a system will be slower, but its high-sensitivity performance may make it more flexible for a careful study of different materials. Typical measurement times will fall between five minutes and 60 minutes for a single point.

Another important consideration is the wavelength range. Commercial SEs can cover wavelengths from the vacuum ultraviolet (UV) to the infrared (IR) spectral regions, but not generally in the same instrument. Different applications may require a specific wavelength range. Other applications may benefit from an optimum range that provides higher sensitivity to the material property in which you are interested. For instance, silicon-germanium (SiGe) coatings are best measured from the UV to the near-IR spectral regions. The wavelengths from 250 to 650 nm cover the electronic transitions of SiGe and are best for determining the Si-to-Ge concentration. Light at these wavelengths will be absorbed, however. The thickness of a film can only be determined from spectral ellipsometry if the light beam is able to penetrate the material and return to the surface without being completely absorbed, so a system operating at these wavelengths would not be able to measure film thickness (see figure). Thus, SiGe applications also may require an SE that can measure above 650 nm.

Figure 1. The blue light is absorbed by the film, while the red light is not. A system operating at blue wavelengths could not measure thickness for this film.

In general, bigger is better when it comes to spectral range. It is safer to choose a wide range or to look for an SE that can be upgraded to extend its wavelength range in the future. This will provide flexibility for future applications.

One common question about ellipsometers is whether the illumination will affect photosensitive coatings. The answer depends on the configuration of the instrument—if an intense light beam shines on the sample, then it can trigger a reaction. Many SE designs limit the measurement beam to a very low-intensity output at a single wavelength before it reaches the sample. The source is then scanned to collect measurements at all wavelengths of interest. To be safe, it is best to send the vendor a photosensitive sample for testing before you purchase equipment for the application in question.

Spectral ellipsometry is used in microlithography for everything from verifying the optical properties and quality of coatings on projection optics to monitoring the thickness of the photoresists on the wafers. Again, the most general application is film thickness and refractive index measurements.

Lithographic films typically need to be characterized at the exposure wavelength, so it's best to choose an SE that covers those wavelengths (typically 157 nm, 193 nm, or 248 nm). Unfortunately, most materials absorb strongly in this spectral region, so it is important to make sure the SE also extends to sufficiently long wavelengths to perform good thickness measurements on the films of interest.

Properly selected and used, an ellipsometer is a powerful instrument that can yield a wide range of information about thin films of all types. oe

James Hilfiker

James Hilfiker is an applications engineer at the J. A. Woollam Co., Lincoln, NE.