Ellipsometry
It can be used to characterize composition, roughness, thickness (depth), crystalline nature, doping concentration, electrical conductivity and other material properties.
The term "spectroscopic" relates to the fact that the information gained is a function of the light's wavelength or energy (spectra).
[2][non-primary source needed] The measured signal is the change in polarization as the incident radiation (in a known state) interacts with the material structure of interest (reflected, absorbed, scattered, or transmitted).
Ellipsometry can probe the complex refractive index or dielectric function tensor, which gives access to fundamental physical parameters like those listed above.
It is commonly used to characterize film thickness for single layers or complex multilayer stacks ranging from a few angstroms or tenths of a nanometer to several micrometers with an excellent accuracy.
The exact nature of the polarization change is determined by the sample's properties (thickness, complex refractive index or dielectric function tensor).
Although optical techniques are inherently diffraction-limited, ellipsometry exploits phase information (polarization state), and can achieve sub-nanometer resolution.
Most models assume the sample is composed of a small number of discrete, well-defined layers that are optically homogeneous and isotropic.
Methods of immersion or multiangular ellipsometry are applied to find the optical constants of the material with rough sample surface or presence of inhomogeneous media.
New methodological approaches allow the use of reflection ellipsometry to measure physical and technical characteristics of gradient elements in case the surface layer of the optical detail is inhomogeneous.
For instance, it is relatively insensitive to scatter and fluctuations and requires no standard sample or reference beam.
Models can be physically based on energy transitions or simply free parameters used to fit the data.
Using an iterative procedure (least-squares minimization) unknown optical constants and/or thickness parameters are varied, and
values which match the experimental data best provide the optical constants and thickness parameters of the sample.
Modern ellipsometers are complex instruments that incorporate a wide variety of radiation sources, detectors, digital electronics and software.
By that the complex refractive index or the dielectric function tensor in the corresponding spectral region can be obtained, which gives access to a large number of fundamental physical properties.
Infrared spectroscopic ellipsometry (IRSE) can probe lattice vibrational (phonon) and free charge carrier (plasmon) properties.
Since only intensity of light measurements are needed, almost any type of camera can be implemented as the CCD, which is useful if building an ellipsometer from parts.
Typically, imaging ellipsometers are configured in such a way so that the laser (L) fires a beam of light which immediately passes through a linear polarizer (P).
[7] This elliptically polarized light then reflects off the sample (S), passes through the analyzer (A) and is imaged onto a CCD camera by a long working distance objective.
For simplification of future calculations, the compensator can be fixed at a 45 degree angle relative to the plane of incidence of the laser beam.
Due to the fact that the imaging is done at an angle, only a small line of the entire field of view is actually in focus.
This process can be used to study, for instance, the growth of a thin film,[8] including calcium phosphate mineralization at the air-liquid interface,[9] etching or cleaning of a sample.
By in situ ellipsometry measurements it is possible to determine fundamental process parameters, such as, growth or etch rates, variation of optical properties with time.
Therefore, the mechanical setup has to be adjusted, which can include additional optical elements (mirrors, prisms, or lenses) for redirecting or focusing the light beam.
Because the environmental conditions during the process can be harsh, the sensitive optical elements of the ellipsometry setup must be separated from the hot zone.
In the simplest case this is done by optical view ports, though strain induced birefringence of the (glass-) windows has to be taken into account or minimized.
Despite all these problems, in situ ellipsometry becomes more and more important as process control technique for thin film deposition and modification tools.
Ellipsometry is a very sensitive measurement technique and provides unequaled capabilities for thin film metrology.
Because the incident radiation can be focused, small sample sizes can be imaged and desired characteristics can be mapped over a larger area (m2).
