Spectroscopic
Ellipsometry
University of Texas at El Paso
Lynn Santiago
Dr. Elizabeth Gardner
Chem 5369
Ellipsometry – An Essential Tool for
Characterizing Nanomaterials
“[The ellipsometry] methods are
the workhorse analyses
of a laboratory, as they are used on
almost every project involving
surface chemistry, whether it be a
silicon surface or a metal surface.”
James, D.K., Tour, J.M.. Analytica Chimica Acta 568 (2006) 2-19
Outline
Spectroscopic Single Wavelength
Ellipsometry Ellipsometry
Introduction Setup
How it works Components and Functions
Setup Advantages/Disadvantages
Light Source
Components and Functions
Equation Imaging Ellipsometry
Advantages Setup
Components and Functions
Advantages/Disadvantages
Introduction to Spectroscopic Ellipsometry
It is used for a variety of measurements:
Thickness of films.
Optical properties.
Modeling of surface roughness.
Ellipsometry is:
well known
non-destructive
precise
accurate
analytical technique
Using Ellipsometry to Characterize
Nano-electronic-based Materials
The technique is used for the determination of physical
properties of organic molecular electronic-based
devices.
It is commonly used for the characterization of self-
assembled monolayers (SAMS), substrates, polymers
and thin layers.
It can probe molecular assemblies such as SAMS.
Doesn’t change their physical characteristics.
Determines whether you have single or multiple layers
assembled on a surface.
How does ellipsometry work?
1. Light is shined from a light source.
2. The light is polarized by passing through a linear polarizer.
3. The light is then elliptically polarized by passing through a
compensator.
4. The light hits the sample, is reflected and is linearly
polarized.
5. The analyzer detects the change of polarization.
6. The detector catches the light and send it to the computer
to process the data.
7. The measured data combined with computerized optical
modeling gives information of the film thickness and
refractive index values of a sample.
Spectroscopic Ellipsometry Setup
Multiple Wavelengths
Unpolarized Light
5. Detector
1. Light Source Linearly Polarized Light
4. Analyzer
2. Linear Polarizer
3. Compensator
Sample
Elliptically Polarized Light
Light Source
1. The light source consists of wavelengths in the
following regions
Ultraviolet
185nm – 260nm
Visible
0.4nm – 0.7nm
Infrared
0.7nm – 1.1μm
http://www.flame-detection.net/flame_detector/flame_detection_school/flame_spectrum.htm
SWE Components and Functions
2. Polarizer - produces light in a special state of
polarization at the output
3. Compensator - used to shift the phase of one
component of the incident light
Depending on orientation, it transforms the ellipse
of polarization
Linearly polarized light into elliptically polarized light
when set to 45° in respect to the linear polarization
axis.
4. Analyzer – second polarizer that detects the
linearly polarized light reflected off the sample
5. Detector
http://www.nanofilm.de/fileadmin/cnt_pdf/technology/Ellipsometry_principle__150dpi_s.pdf
Calculating Change in Polarization
This is the equation used to calculate the change
in polarization.
Ρ = Rp/Rs = tan(Ψ)eiΔ
Ρ = change in polarization
Rp = component oscillating in the plane of incidence
Rs = component is oscillating perpendicular to the plane
of incidence
Tan Ψ = amplitude ratio of reflection
Δ = phase shift
What are Rp and Rs
components?
rp
Rp = |rp|2
Rs = |rs|2
rs
SE Advantages
No contact with the films is required for the
analysis of films
Technique does not require a reference or
standards
It provides both the phase and amplitude ratio of
a sample
Analysis is less sensitive to the fluctuations of
light intensity
Concentrating the Light Source
We have seen that spectroscopic ellipsometry
uses a range of wavelengths to analyze a sample.
Now we will see an instrument that uses the
same concept but uses one particular
wavelength of light to analyze a sample.
Single Wavelength Ellipsometry
Also known as Laser Ellipsometry
Used in Imaging Ellipsometry
Uses a light source with a specific wavelength
http://www.eas.asu.edu/nanofab/capabilities/metrology.html
Single Wavelength Ellipsometry
Setup
One Wavelength
Unpolarized Light
5. Detector
1. Light Source Linearly Polarized Light
4. Analyzer
2. Linear Polarizer
3. Compensator
Sample
Elliptically Polarized Light
SWE Light Source
This is not from an
ellipsometer but shows what
a HeNe laser looks like.
Light Source – This is a laser with a specific
wavelength
Commonly a HeNe laser with the wavelength of
632.8 nm
http://www.technology.niagarac.on.ca/courses/phtn1333/
Pros and Cons of SWE
Advantages:
Laser can focus on a specific spot
Lasers have a higher power than broad
band light sources
Disadvantage:
Experimental output is restricted to one
set of Ψ and Δ values per measurement
Taking it a Step Further
Now there exists the technology to use
ellipsometry and view a sample while it is
being analyzed.
Imaging Ellipsometry
Combines SWE with
Microscopy
High Lateral Resolution
Possible to see tiny samples
High contrast imaging
capabilities to detect various
properties of samples
surface defects
Inhomogenities
Provides spatial resolution
for a variety of areas
Microanalysis
Microelectronics
Bio-analysis
http://www.soem.ecu.edu.au/physics/physics_facilities.htm
Imaging Ellipsometry Setup
Unpolarized Light
CCD Camera
Linearly Polarized Light
Laser Light Source
Analyzer
Linear Polarizer
Compensator Objective
Sample
Two New
Elliptically Polarized Light Components
Imaging Components and
Functions
Objective – images the illuminated
area of the sample onto the camera
CCD Camera - a camera with an
image sensor that is an integrated
circuit made with light sensitive
capacitors
http://www.nanofilm.de/fileadmin/cnt_pdf/technology/Ellipsometry_principle__150dpi_s.pdf
Pros and Cons of Imaging Ellipsometry
Advantages:
Provides film thickness and refractive index
Provides a real time contrast image of the sample
Ability to restrict ellipsometric analysis to a
particular region of interest within the field-of-view
The signal provided is spatially resolved to show
the details of the sample
Disadvantages:
The inclined observation angle
Only a limited area of the image appears to be
well-focused when using conventional optics
Acknowledgements
David Echevarría – Torres
Dr. Elizabeth Gardner
References
James, D.K., Tour, J.M.. Analytica Chimica Acta 568 (2006) 2-19.
Goncalves, D., Irene, E.A.. Quim. Nova, Vol. 25, No. 5, 794-800.
Nanofilm Surface Analysis
http://www.nanofilm.de/fileadmin/cnt_pdf/technology/Ellipsom
etry_principle__150dpi_s.pdf
http://www.wikipedia.org
