Spectroscopic Ellipsometry
University of Texas at El PasoLynn Santiago
Dr. Elizabeth GardnerChem 5369
“[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
Ellipsometry – An Essential Tool for Characterizing Nanomaterials
Outline
Spectroscopic Ellipsometry Introduction How it works Setup Light Source Components and Functions Equation Advantages
Single Wavelength Ellipsometry Setup Components and Functions Advantages/Disadvantages
Imaging Ellipsometry 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
1. Light Source
2. Linear Polarizer
3. Compensator
4. Analyzer
5. Detector
Sample
Unpolarized Light
Elliptically Polarized Light
Linearly Polarized Light
Multiple Wavelengths
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
rp
rs
Rp = |rp|2
Rs = |rs|2
What are Rp and Rs components?
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
1. Light Source
2. Linear Polarizer
3. Compensator
4. Analyzer
5. Detector
Sample
Unpolarized Light
Elliptically Polarized Light
Linearly Polarized Light
One Wavelength
SWE Light Source
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/
This is not from an ellipsometer but shows what
a HeNe laser looks like.
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
Two New Components
Imaging Ellipsometry Setup
Laser Light Source
Linear Polarizer
Compensator
Analyzer
Objective
CCD Camera
Sample
Unpolarized Light
Elliptically Polarized Light
Linearly Polarized Light
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/Ellipsometry_principle__150dpi_s.pdf
http://www.wikipedia.org