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Both reflectometry and ellipsometry are optical, non-contact and non-destructive methods. In general, reflectometer is used to acquire reflection spectrum over a wavelength range. If there is film or coating on some kind of substrate, the film or coating thickness can also be figured out from the measured  reflection  spectrum. Ellipsometry measurement is performed at non normal incident angles. The two ellipsometry parameters, Psi and Del, give more information than reflectance itself. Therefore, more information can be accessible through ellipsometry technique, such as multiple layer analysis, dielectric constants calculation, surface or interface roughness, inhomogeneity behavior etc. Of course, those information is always derived from an optical model, which leads to some difficulties  to use this technique although it is much more powerful than reflectometry technique.

For thickness measurement, both techniques rely on modeling. In general, ellipsometry gives better accuracy than reflectometry in thin thickness range such as below micron level. For a typical ellipsometer configuration, the maximum thickness measurable is below 10 microns. However, reflectometer can measure up to hundreds microns of non absorbing thick coatings.

How thick and how thin I can measure for thickness?

These optical methods need to have light to penetrate through film and reflect back from film/substrate interface. Without meeting such condition, the film thickness can not be figured out because of lacking necessary phase information. One example is a sample with thick metal films on it. Because metal has high absorption in visible and Near infrared range, light only can penetrate metal film with a depth of less than 1000 Angstroms. For such film with a thickness above 1000 Angstroms, it is impossible to measure its thickness by reflectometry or ellipsometry although reflection spectrum can be obtained with reflectometer and optical properties for metal film can be obtained with ellipsometry. Roughly, the measurable thickness for various films can be estimated from penetration depth if knowing its extinction coefficient or absorption coefficients.

You can find following applications with ellipsometry.

• Optical constants (refractive index n and extinction coefficient k) for thin films, coatings and bulk substrate

• Accurate nondestructive thickness determination for thin films

• Alloy concentration determination for various thin films such as Ge in SiGe alloy, Al in AlGaN films

• Band gap determination for GaN, SiC, AlN, AlGaN, etc.

• Porosity measurement in low-K films

• Relative volume fraction determination for each component in nano-composite

• Physical thickness and optical properties for each layer in a multiple layer stack or periodic structure such as quantum well structure

• Thickness and optical properties uniformity information

• Inhomogeneous film analysis in physical density or alloy concentration

• Optical properties for high-k films

• Nondestructive measurement for electrical conductivity of metal films, metallic compounds (such as WN, TiN, TaN, etc.), doped semiconductor epi layers (thickness can be also determined at the same time), other compound oxides such as ITO films

• Nondestructive measurement for doping concentration in doped semiconductors (active dopant! not total concentration as given by destructive SIMS analysis)

Almost all materials can be characterized with ellipsometry, such as Metals, polymers, ceramics, glasses, semiconductors and their compounds, composites etc.

Here is a partial list of application fields for ellipsometry:

1. Semiconductor Industry:

   • Photoresist

   • Gate dielectrics

   • Semiconductors and their alloys or compounds such as, SiGe, InGaAs

2. Photonics

   • Optical coatings

   • Semiconductor compounds

   • Functional films in Optical MEMS

3. Data Storage

   • Diamond-like carbon (DLC)

   • Magnetic films

4. Flat Panel Display (FPD)

   • Thin film transistors (TFT) stack

   • Conductive oxide: Indium Tin Oxide (ITO)

Any Advanced Applications for ellipsometry?

Yes. Here are some examples for advanced applications for ellipsometry.

1. Ultra-thin gate dielectric films

   • Porous structure

   • Deep trench profile

   • Graded structure

   • Composites

   • Corrosion & protection of metals and alloys

2. Real time process monitoring for deposition rates, etching rates, etc.