Spectroscopic ellipsometry is non-invasive, non-destructive, non-contact, and can allow a user to determine several film properties simultaneously. The technique is fast and requires no sample preparation. It is also precise, reproducible, very sensitive to thin films below 10 nm, and it covers a wide spectral range usually from 190-2100 nm. Spectroscopic ellipsometry is applicable to almost any thin film material and it is ideal for in-situ applications.
Materials suitable for spectroscopic ellipsometry include semiconductors, dielectrics, polymers, organics, and metals. Ellipsometry can also be used to study solid-liquid or liquidliquid interfaces.
Depending on the type of material, ellipsometry can measure thickness from a few A to tens of microns. How far into the material the light can travel depends on the material’s absorption coefficient. It is important to keep in mind that for optically opaque samples, such as metal films greater than about 50 nm, ellipsometry can only determine optical properties and NOT thickness. This is because of the strong absorbing nature of metals and thus a very shallow light penetration depth. For transparent or semi-transparent samples, on the other hand, the maximum thickness that’s measurable by an ellipsometer depends on the spectral resolution of the instrument.
Depending on the instrument, the spectral range can be anywhere from 145 nm-2100 nm. For example, the Auto-SE and Smart-SE instruments have a fixed spectral range from 450 nm to 1000 nm, the UVISEL PLUS system offers two different spectral ranges: 190 nm-920 nm, and 190 nm-2100 nm, and the UVISEL 2 spectral range is from 190 nm-1000 nm with an optional NIR extension up to 2100 nm. We also offer the UVISEL 2-VUV instrument which has a spectral range from 145 nm-880 nm, also with an optional NIR extension up to 2100 nm.
The NIR region is useful for measuring the thickness of materials absorbing in the visible range, as well as thick samples. The NIR region can also be helpful if you need to know optical properties in the NIR region.
The Far Ultra Violet (FUV) region is useful for determining the band gap, crystallinity, composition, and absorption of dielectrics and semiconductors. It also helps measure ultra-thin films and films with a low index contrast. And, measurements over the FUV range are, of course, necessary for applications where you need to know the material’s optical properties over that region, e.g., photolithography.
A reflectometer measures an intensity ratio of light, whereas spectroscopic ellipsometry measures the change in the polarization state of light.
Both spectroscopic ellipsometry and reflectometry are non-contact optical techniques, and both require modeling to obtain a result. A reflectometer, however, measures an intensity ratio of light, whereas spectroscopic ellipsometry measures the change in the polarization state of light (i.e. electric field vector).
Reflectometry for a native oxide layer on c-Si.
Reflectometry is not sensitive to small changes in thin film thickness, so it is generally used on thicker (>100 nm) samples, whereas ellipsometry is very sensitive to ultrathin films. The figures on right show reflectometry (top) and the parameter Δ, measured by ellipsometry (bottom) for a native oxide layer on c-Si.
The parameter Δ, measured by ellipsometry for a native oxide layer on c-Si.
Clearly, reflectometry is not sensitive to the ultrathin layer, while ellipsometry is. Also, since reflectometry is intensity-based, things like changes in the lamp intensity can alter the results. Ellipsometry is polarization-based, so lamp intensity is only important to obtain good signal to noise. An ellipsometer can measure reflectance and transmittance, as well.
Reflectometry is capable of measuring single and thick films (>100 nm), but it does not carry as much sensitivity as ellipsometry does for handling thin films (<100nm). Also, reflectometry is generally used for single layered transparent films. It is not useful for certain samples, such as those that are multilayered, anisotropic, absorbing and/or graded.
The application of spectroscopic ellipsometry is useful anywhere there are thin films, including: photovoltaics, microelectronics and semiconductors, flat panel displays, optoelectronics, metallurgy, optical coatings, biochemistry, nanotechnology, polymers, and organic materials. Some of the materials measured with ellipsometry and used in the above applications include c-Si, a-Si, p-Si, mc-Si, CdTe, CIGS, CdS, SiN, SiC, GaAs, AlGaAs, AlN, InGaN, SnO2, SiO2, PET, PEN, ZnO, PbS, PbSe, TiO2, Al, Ag, Au, and carbon nanotubes, just to name a few. If you are unsure whether your application is suitable for spectroscopic ellipsometry, please contact us.
