Ellipsometry

Ellipsometry is a versatile and powerful optical technique for the investigation of the dielectric properties (complex refractive index or dielectric function) of thin films.

It has applications in many different fields, from semiconductor physics to microelectronics and biology, from basic research to industrial applications. Ellipsometry is a very sensitive measurement technique and provides unequalled capabilities for thin film metrology. As an optical technique, spectroscopic ellipsometry is non-destructive and contactless.

Upon the analysis of the change of polarization of light, which is reflected off a sample, ellipsometry can yield information about layers that are thinner than the wavelength of the probing light itself, even down to a single atomic layer. Ellipsometry can probe the complex refractive index or dielectric function tensor, which gives access to fundamental physical parameters and is related to a variety of sample properties, including morphology, crystal quality, chemical composition, or electrical conductivity. 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 name "ellipsometry" stems from the fact that the most general state of polarization is elliptic. The technique has been known for almost a century, and has many standard applications today. However, ellipsometry is also becoming more interesting to researchers in other disciplines such as biology and medicine. These areas pose new challenges to the technique, such as measurements on unstable liquid surfaces and microscopic imaging.

Basic principles

Ellipsometry measures the change of polarization upon reflection or transmission. Typically, ellipsometry is done only in the reflection setup. 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 and the polarization state of light, and can achieve angstrom resolution. In its simplest form, the technique is applicable to thin films with thickness less than a nanometer to several micrometers. The sample must be composed of a small number of discrete, well-defined layers that are optically homogeneous and isotropic. Violation of these assumptions will invalidate the standard ellipsometric modeling procedure, and more advanced variants of the technique must be applied (see below).

Experimental details

Experimental setup

Electromagnetic radiation is emitted by a light source and linearly polarized by a polarizer. It can pass through an optional compensator (retarder, quarter wave plate) and falls onto the sample. After reflection the radiation passes a compensator (optional) and a second polarizer, which is called an analyzer, and falls into the detector. Instead of the compensators some ellipsometers use a phase-modulator in the path of the incident light beam. Ellipsometry is a specular optical technique (the angle of incidence equals the angle of reflection). The incident and the reflected beam span the plane of incidence. Light which is polarized parallel to this plane is named p-polarized (p-polarised). A polarization direction perpendicular is called s-polarized (s-polarised), accordingly. The "s" is contributed from the German "senkrecht" (perpendicular).
(See also Fresnel equations)

Data acquisition

Ellipsometry measures the complex reflectance ratio, $\rho$ , of a system, which may be parametrized by $\Psi$ and $\Delta$ . The polarization state of the light incident upon the sample may be decomposed into an s and a p component (the s component is oscillating perpendicular to the plane of incidence and parallel to the sample surface, and the p component is oscillating parallel to the plane of incidence). The amplitudes of the s and p components, after reflection and normalized to their initial value, are denoted by $r_{s}$ and $r_{p}$ , respectively. Ellipsometry measures the complex reflectance ratio, $\rho$ (a complex quantity), which is the ratio of $r_{p}$ over $r_{s}$ :

\rho ={\frac {r_{p}}{r_{s}}}=\tan(\Psi )e^{i\Delta }

Thus, $\tan(\Psi )$ is the amplitude ratio upon reflection, and $\Delta$ is the phase shift (difference). (Note that the right hand side of the equation is simply another way to represent a complex number.) Since ellipsometry is measuring the ratio (or difference) of two values (rather than the absolute value of either), it is very robust, accurate, and reproducible. For instance, it is relatively insensitive to scatter and fluctuations, and requires no standard sample or reference beam.

Data analysis

Ellipsometry is an indirect method, i.e. in general the measured $\Psi$ and $\Delta$ cannot be converted directly into the optical constants of the sample. Normally, a model analysis must be performed. Direct inversion of $\Psi$ and $\Delta$ is only possible in very simple cases of isotropic, homogeneous and infinitely thick films. In all other cases a layer model must be established, which considers the optical constants (refractive index or dielectric function tensor) and thickness parameters of all individual layers of the sample including the correct layer sequence. Using an iterative procedure (least-squares minimization) unknown optical constants and/or thickness parameters are varied, and $\Psi$ and $\Delta$ values are calculated using the Fresnel equations. The calculated $\Psi$ and $\Delta$ values which match the experimental data best provide the optical constants and thickness parameters of the sample.

Definitions

Single-wavelength vs. spectroscopic ellipsometry

Single-wavelength ellipsometry employs a monochromatic light source. This is usually a laser in the visible spectral region, for instance, a HeNe laser with a wavelength of 632.8 nm. Therefore, single-wavelength ellipsometry is also called laser ellipsometry. The advantage of laser ellipsometry is that laser beams can be focused on a small spot size. Furthermore, lasers have a higher power than broad band light sources. Therefore, laser ellipsometry can be used for imaging (see below). However, the experimental output is restricted to one set of $\Psi$ and $\Delta$ values per measurement. Spectroscopic ellipsometry (SE) employs broad band light sources, which cover a certain spectral range in the infrared, visible or ultraviolet spectral region. 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. Spectroscopic ellipsometry in the near infrared, visible up to ultraviolet spectral region studies the refractive index in the transparency or below-band-gap region and electronic properties, for instance, band-to-band transitions or excitons.

Standard vs. generalized ellipsometry (anisotropy)

Standard ellipsometry (or just short 'ellipsometry') is applied, when no s polarized light is converted into p polarized light nor vice versa. This is the case for optically isotropic samples, for instance, amorphous materials or crystalline materials with a cubic crystal structure. Standard ellipsometry is also sufficient for optically uniaxial samples in the special case, when the optical axis is aligned parallel to the surface normal. In all other cases, when s polarized light is converted into p polarized light and/or vice versa, the generalized ellipsometry approach must be applied. Examples are arbitrarily aligned, optically uniaxial samples, or optically biaxial samples.

Jones matrix vs. Mueller matrix formalism (Depolarization)

There are typically two different ways of describing mathematically, how an electromagnetic wave interacts with the elements within an ellipsometer (including the sample), the Jones matrix and the Mueller matrix formalism. In the Jones matrix formalism the electromagnetic wave is described by a Jones vector with two orthogonal complex-valued entries for the electric field (typically $E_{x}$ and $E_{y}$ ), and the effect that an optical element (or sample) has on it is described by the complex-valued 2x2 Jones matrix. In the Mueller matrix formalism the electromagnetic wave is described by Stokes vectors with four real-valued entries, and their transformation is described by the real-valued 4x4 Mueller matrix. When no depolarization occurs both formalisms are fully consistent. Therefore, for non-depolarizing samples the simpler Jones matrix formalism is sufficient. If the sample is depolarizing the Mueller matrix formalism should be used, because it gives additionally access to the amount of depolarization. Reasons for depolarization are, for instance, thickness non-uniformity or backside-reflections from a transparent substrate.

Advanced experimental approaches

Imaging ellipsometry

Ellipsometry can also be done as imaging ellipsometry by using a CCD camera as a detector. This provides a real time contrast image of the sample, which provides information about film thickness and refractive index. Advanced imaging ellipsometer technology operates on the principle of classical null ellipsometry and real-time ellipsometric contrast imaging, using a single-wavelength ellipsometer setup with a laser as light source. The laser beam gets elliptically polarized after passing a linear polarizer (P) and a quarter-wave plate (C). The elliptically polarized light is reflected off the sample (S), passes an analyzer (A) and is imaged onto a CCD camera by a long working distance objective. In this PCSA configuration, the orientation of the angles of P and C is chosen in such a way that the elliptically polarized light is completely linearly polarized after it is reflected off the sample. The ellipsometric null condition is obtained when A is perpendicular with respect to the polarization axis of the reflected light achieving complete destructive interference, i.e., the state at which the absolute minimum of light flux is detected at the CCD camera. The angles of P, C, and A that obtained the null condition are related to the optical properties of the sample. Analysis of the measured data with computerized optical modeling leads to a deduction of spatially resolved film thickness and complex refractive index values.

In situ ellipsometry

In situ ellipsometry refers to dynamic measurements during the modification process of a sample. This process can be, for instance, the growth of a thin film, 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. In situ ellipsometry measurements require a number of additional considerations: The sample spot is usually not as easily accessible as for ex situ measurements outside the process chamber. 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. Furthermore, the samples can be at elevated temperatures, which implies different optical properties compared to samples at room temperature. Despite all these problems, in situ ellipsometry becomes more and more important as process control technique for thin film deposition and modification tools. In situ ellipsometers can be of single-wavelength or spectroscopic type. Spectroscopic in situ ellipsometers use multichannel detectors, for instance CCD detectors, which measure the ellipsometric parameters for all wavelength in the studied spectral range simultaneously.

Ellipsometric Porosimetry

Ellipsometric porosimetry measures the change of the optical properties and thickness of the materials during adsorption and desorption of a volatile species at atmospheric pressure or under reduced pressure depending on the application. The EP technique is unique in its ability to measure porosity of very thin films down to 10 nm, its reproducibility and speed of measurement. Compared to traditional porosimeters, Ellipsometer porosimeters are well suited to very thin film pore size and pore size distribution measurement. Film porosity is a key factor in silicon based technology using low-k materials, organic industry (encapsulated organic light-emitting diodes) as well as in the coating industry using sol gel techniques.

Magneto-optic generalized ellipsometry

Magneto-optic generalized ellipsometry (MOGE) is an advanced infrared spectroscopic ellipsometry technique for studying free charge carrier properties in conducting samples. By applying an external magnetic field it is possible to determine independently the density, the optical mobility parameter and the effective mass parameter of free charge carriers. Without the magnetic field only two out of the three free charge carrier parameters can be extracted independently.

Advantages

Ellipsometry has a number of advantages compared to standard reflection intensity measurements:

Ellipsometry measures at least two parameters at each wavelength of the spectrum. If generalized ellipsometry is applied up to 16 parameters can be measured at each wavelength.
Ellipsometry measures an intensity ratio instead of pure intensities. Therefore, ellipsometry is less affected by intensity instabilities of the light source or atmospheric absorption.
No reference measurement is necessary.
Both real and imaginary part of the dielectric function (or complex refractive index) can be extracted without the necessity to perform a Kramers–Kronig analysis.

Ellipsometry is especially superior to reflectivity measurements when studying anisotropic samples.

References

R. M. A. Azzam and N. M. Bashara, Ellipsometry and Polarized Light, Elsevier Science Pub Co (1987) ISBN 0-444-87016-4
A. Roeseler, Infrared Spectroscopic Ellipsometry, Akademie-Verlag, Berlin (1990), ISBN 3-05-500623-2
H. G. Tompkins, A Users's Guide to Ellipsometry, Academic Press Inc, London (1993), ISBN 0-12-693950-0
H. G. Tompkins and W. A. McGahan, Spectroscopic Ellipsometry and Reflectometry, John Wiley & Sons Inc (1999) ISBN 0-471-18172-2
I. Ohlidal and D. Franta, Ellipsometry of Thin Film Systems, in Progress in Optics, vol. 41, ed. E. Wolf, Elsevier, Amsterdam, 2000, pp. 181–282
M. Schubert, Infrared Ellipsometry on semiconductor layer structures: Phonons, Plasmons, and Polaritons, Series: Springer Tracts in Modern Physics, Vol. 209, Springer (2004), ISBN 3-540-23249-4
H. G. Tompkins and E. A. Irene (Editors), Handbook of Ellipsometry William Andrews Publications, Norwich, NY (2005), ISBN 0-8155-1499-9
H. Fujiwara, Spectroscopic Ellipsometry: Principles and Applications, John Wiley & Sons Inc (2007), ISBN 0-470-01608-6