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