Ellipsometry is a non-contacting, nondestructive technique for the measurement of surfaces and very thin films on surfaces using elliptically polarized light. Few measuring techniques are as direct or inherently as precise as ellipsometry. Quality ellipsometers can detect film and surface conditions less than an  Angstrom thick.

Ellipsometry is superior to alternate methods of thin film measurement such as reflectometry because two parameters (DELTA and PSI) instead of one (intensity) are independently determined in any single measurement. This fact permits the film refractive index to be measured in addition to the film thickness. Two independent parameters also place tighter constraints on models representing more complicated films. In addition, ellipsometric measurements are insensitive to intensity fluctuations of the source, temperature drifts of electronic components, and macroscopic roughness which can be a serious problem in reflectometry but not in ellipsometry, for which absolute intensity measurements are not required.

Transparent films from under one angstrom up to several microns and absorbing films less than 500 angstroms thick can be measured ellipsometrically. The surface upon which the film is measured can be a semiconductor, dielectric or metal. The film can be transparent or absorbing in a medium of transparent solid, liquid, gas or vacuum. The measuring polarized light can range from the ultra violet to the Infrared.

Gaertner Scientific has helped pioneer this exciting field since the late 30's when we constructed our first polarizing spectrometer. We are proud of our past involvement and look forward to supplying our customers with instruments of the finest quality.


applications of ellipsometry
Ellipsometers are used in a wide variety of surface and thin film studies - in microelectronics, solar cells, optical disks, X-ray films, biological layers and more recently, in superconductivity. Here are a few examples: 

Measurements of Silicon wafers in the Semiconductor industry
Measurements of dielectric and oxide films on metals and semiconductors
Detection and measurement of contamination on metal and glass
Monitoring vapor deposition of organic and inorganic films
Measurement of reflective and non-reflective coatings on glass
Measurement of anodized and very thin painted layers
Detection of contamination and impurities on surfaces and in thin films
Blood coagulation and immunological reactions
Monitoring protein growth
Crystal and mineral surface analysis
Epitaxial growth
Chemisorption on metal and other surfaces
Measurement of air flow birefringence
Temperature dependence of film thickness of liquid helium films
Plasma anodization
Antibody-antigen reaction in immunology
Boundary roughness effect of layered systems
Electron hole plasma generation Ge
AI2O3 amorphous film growth and crystal island development
Fluid interface analysis
Stress corrosion cracking
Sub-mono-layer molecular film
Pyrolytic film characterization
Plasma diagnostics
Polymer absorption
Thermal oxidation kinetics


brief theory
Ellipsometry is based on the polarization of a light beam being altered on reflection from a bare or film coated surface. An elliptically polarized light beam is defined by the angular position of the ellipse (azimuth), its shape (ellipticity), and the sense of rotation of the light vector. Two parameters determine the state of polarization; these are the amplitude ratio, PSI and the phase difference DELTA.

At the end of the 1800's, Paul Drude stated the relationship between thickness of the film and the optical constants of the surface (or substrate) upon which the film is deposited. This relationship is mathematically stated as the fundamental equation of ellipsometry:

The ellipsometer is used to measure PSI and DELTA, and thus with the known values of PSI, and DELTA the film thickness and index can be calculated. Since the equation contains complex quantities, its solution is done by computer.

While a number of different ellipsometry methods exist for obtaining PSI and DELTA the most common are the nulling, rotating analyzer, and exclusive to Gaertner the StokesMeterô method.

The oldest measurement method is manual nulling which requires rotating the polarizer and analyzer circles until the reflected beam from the sample is extinguished. When extinction is achieved (takes a few minutes), the polarizer and analyzer circles are read. These readings are then converted into film thickness and film refractive index by using tables, graphs, or personal computers. 

Measurement by automatic ellipsometers was introduced with the advent of the personal computer. Typically a rotating analyzer encodes the reflected beam intensity in less than one second, then a A/D converter digitizes the intensity and sends the data to a computer where the data is modeled for film thickness and film refractive index. 

Newer methods such as the StokesMeterô  polarimeter measure by reflecting the measuring light beam from 4 stationary detectors. The measuring laser light beam can be described by the normalized stokes parameters [1,S1,S2,S3]  which in turn yield PSI and DELTA. 

For a more thorough treatment of ellipsometry, the reader is referred to the publications page.

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