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2.2.7 Spectroscopic Ellipsometry
Principle
Ellipsometry is a non-destructive optical technique used to analyze the dielectric
properties of a material, such as the complex refractive index (ñ =n − ik), and
thickness. This technique is applicable to thin lms with thicknesses ranging between
few nanometers to about 50µm, depending upon which the accuracy diers.
The electric eld E of the incident light is resolved into two components, one
parallel to the incident plane and the other perpendicular, denoted as E ip and E is
respectively. Similarly the reected light components are represented as E rp and
E sp . These components of the incident and reected light are related using Fresnel's
reection coecients expressed as,
tel-00916300, version 1 - 10 Dec 2013
r p = E rp /E ip ; r s =E rs /E is Eqn (2.9)
The ratio (r) of these reection coecients is conventionally written in the following
form,
r = r p /r s = tan Y. exp (iD) Eqn (2.10)
This ratio is a complex number which can be expressed using the ellipsometric
angles Y and D that decribes the amplitude component and the phase shift component
of the reection coecients, respectively. Since the Fresnel coecients are
dependent on multiple variables such as the complex refractive index, thickness,
wavelength and angle of incidence, from an analysis of the ellipsometric spectra the
optical constants of the dielectric sample under study can be obtained.
Experimental set-up and working
The reection coecients are recorded using a Jobin-Yvon ellipsometer (UVISEL).
The ellipsometry set-up (Fig. 2.16) consists of,
ˆ an unpolarized light source with a broad spectral range (Xenon lamp)
ˆ a polarizer
ˆ a phase modulator which dephases the parallel and perpendicular components
of the electric eld
ˆ the sample holder
ˆ an analyzer and a detector consisting of a monochromator and a photomultiplier
tube.
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