Spectroscopic Ellipsometry University of Texas at El Paso Lynn Santiago Dr. Elizabeth Gardner Chem 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

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

Calculating Change in Polarization This is the equation used to calculate the change in polarization. Ρ = R p /R s = tan(Ψ)e iΔ Ρ = change in polarization R p = component oscillating in the plane of incidence R s = component is oscillating perpendicular to the plane of incidence Tan Ψ = amplitude ratio of reflection Δ = phase shift

rprp rsrs R p = |r p | 2 R s = |r s | 2 What are R p and R s 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

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 nm 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

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

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) Goncalves, D., Irene, E.A.. Quim. Nova, Vol. 25, No. 5, Nanofilm Surface Analysis etry_principle__150dpi_s.pdf