CLEANROOM NEWS
Process Spotlight: Spectroscopic Ellipsometry, Part I—How to approach measurements for different materials
In the November 2007 newsletter, we discussed spectroscopic ellipsometry (SE) as a tool for characterizing thin films, specifically optical constants (n,k) and thickness measurement. SE involves measurements of the change in polarization state that occurs with reflection from a surface. If light of a known polarization state is used to illuminate a sample, the resulting (typically elliptical) polarization state of the reflected light can be measured. SE is nondestructive, and does not require a “step” edge to measure thickness, as profilometry. By taking data at multiple wavelengths and several different angles of incidence, a wealth of information can be generated to fit optical constants and thickness with high accuracy. Ellipsometry is effective in measuring thin films from a few nanometers thick to several microns thick.
Fig 1. Typical ellipsometry measurement setup (source: Wikipedia Commons)
A great advantage of SE is that it measures a ratio of two values, as described by the following equation: r = tan Y e i
D
= R p
/ R s
Tan Y is the ratio of amplitudes upon reflection, and D measures change in phase. By measuring a ratio instead of an absolute value, we eliminate the need for a reference beam, thus simplifying measurement and calculation.
In most cases, the equations that govern ellipsometry do not yield to direct solution. In simplest terms, we cannot determine the actual physical structures and phenomena directly from the data. Instead, we build a model of what we believe is present, and use Fresnel’s equations to calculate model data from some initial guesses of material parameters and thicknesses. We then refine our model to better match the experimental data, and continue to do this until we achieve a mean-squared error (MSE) that we deem acceptable.

Fig. 2. Ellipsometry flowchart, from JA Woollam Co website
For an in depth look at the theory of ellipsometry, the reader is referred to the JA Woollam Co website
( http://www.jawoollam.com/ ) or the text references at the end of this article.
From a practical standpoint, it is easy to be overwhelmed by the amount of information and choices presented by SE. What angles should I use? What wavelength range (VUV to IR are possible) and step
(10nm? 1nm?)? Can I trust tabulated optical constants? It is very easy to end up taking more data than is necessary to solve the problem. If you are working with a “white light” ellipsometer such as Woollam’s
M2000, this is not as critical, as measurements are taken very quickly. If however, you are using a tool which scans wavelengths with a monochromator (such as the VASE tool at the LCDRF), this can mean the difference between measurements of several minutes and several hours per sample.
There are several important distinctions to make that in large part determine your approach to measurement. The first is to balance the number of variables you are fitting with the number that you are measuring. In general, we measure two variables, psi and delta, at each wavelength with SE. If we are fitting n and k, that is two variables at each wavelength to fit. If we also wish to measure thickness, then we have more fit variables than measured, giving us an underdetermined system. By adding a second angle, we can supply enough data to solve this problem. Taking additional angles can help to overdetermine the system and supply some confidence that our solution is a good one.
TYPES OF SAMPLES
Another important distinction is the type of sample being measured. Is it transparent? Materials such as
SiO2, Al2O3, and even Si in the IR are transparent. This allows us to consider k to be zero in these areas, and eliminates the need to fit for it. Materials that are truly transparent can be fit using a Cauchy relation.
The Cauchy equation insures that the n values thus determined are physically real (monotonically decreasing with wavelength). Materials such as ITO present additional challenge because they are absorbing at all wavelengths (though we consider them “transparent” conductors).
Is it absorbing? Metals and other highly absorbing films can be fit easily using a direct inversion to find the pseudo optical constants. Since we are not measuring a thickness (if it is opaque), we have a one to one correspondence (psi and delta, n and k) of variables and can fit for n and k easily.

Fig. 3. TiO2 film showing absorption in infrared (<0.1eV) and UV (>3eV), but transparency ( e
2
~ 0) in visible wavelength range. (JA Woollam Co)
The most challenging films to model are those that are anisotropic. Materials which are anisotropic include stretched polymers (most plastic substrates, compensation films, etc.) and liquid crystals. These materials require more complex models.
Additional material properties that can be incorporated into models include grading (which often occurs in films such as TiO2), surface roughness and oxidation (common in semiconductors).
CHOICE OF ANGLES
Fig. 4. Reflection from a TiO2 surface for p and s polarizations vs. angle of incidence.
In general, we would like to look at angles of incidence where we have maximum difference in reflectivity between p and s polarization states. Angles near Brewster’s angle are optimal for this.
Brewster’s angle is determined by the index of the top surface, and is generally 55-60 degrees for low index dielectrics, and near 75 degrees for semiconductors.
Early ellipsometers did not have the sensitivity of today’s models, and it was critical to determine this angle first, then choose angles near this for SE measurements. Today, most ellipsometers can effectively operate anywhere from 40-85 degrees angle of incidence.

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Figure 5. Suggested angles for SE measurements for various films (JA Woollam Co)
Next month we will continue to look at ellipsometry measurements of real materials, and evaluation of
MSE.
The LCDRF has a JA Woollam Co variable angle spectroscopic ellipsometer (VASE) which is available for usage by our Industrial Partners. This tool is configured for measurement at wavelengths from UV (193nm) to the IR (2.2um). If you are interested in using this tool or finding out more about it’s capabilities, please contact Doug Bryant.
References
•
H. G. Tompkins, A Users's Guide to Ellipsometry , Academic Press Inc, London (1993), ISBN 0-12-
693950-0
•
H. G. Tompkins and W. A. McGahan, Spectroscopic Ellipsometry and Reflectometry , John Wiley &
Sons Inc (1999) ISBN 0-471-18172-2
•
H. Fujiwara, Spectroscopic Ellipsometry: Principles and Applications , John Wiley & Sons Inc (2007),
ISBN 0-470-01608-6
•
JA Woollam Company Standard Ellipsometry Short Course, Vanderbilt Univ. June 16-19, 2008
IPP NEWS
LCI Welcomes Assistant Professor Chanjoong Kim
On July 1, Dr. Chanjoong Kim joined the Liquid Crystal Institute/Chemical Physics Interdisciplinary
Program as an assistant professor. Kim comes to Kent State after holding a postdoctoral fellowship in the
Department of Physics and SEAS at Harvard University. His main area of interest is soft matter physics.
Kim earned his Ph.D. in Physical Chemistry from the University of Wisconsin, Madison in 2000. His doctoral work focused on Langmuir monolayers. He is familiar with the Northeast Ohio area as he previously attended the Department of Polymer Engineering at the University of Akron before transferring to the University of Wisconsin. His undergraduate and masters degrees in polymer science were earned in Korea at Seoul National University.
Kim is excited to be joining the LCI and helping to expand its research of soft matter. Because of his background in Physical Chemistry, he feels that the LCI is a perfect fit for him. Initially, his research will focus on three main areas: (1) the study of the microscopic dynamics of colloidal suspensions with construction of a confocal rheometer, (2) the microscopic study of dynamic response of biological systems to mechanical stresses, and (3) the study of dynamics of lipid monolayer systems on biopolymers.
He plans to hire a postdoctoral fellow and advise a CPIP student within the year and is currently organizing a lab space in Room 356 of the LCM building. He hopes to begin teaching classes in soft matter with a focus on biomaterials and related biology in the 2009-2010 academic year.

His office at the Liquid Crystal Institute is located in Room 307. He can be reached by phone at (330)
672-9319 or Email: ckim@lci.kent.edu.
Kent State Receives Multi-Millions in Grant Awards from State of Ohio
State Funding to Support University’s Projects, Partnerships and Student Scholarships
Ohio state administration, education and development officials this week announced numerous grants for
Kent State strategic scientific programs and innovative economic development projects.
The state awards announced included:
Bioterrorism Detection Device Program
Kent State also was part of a $3 million Ohio Third Frontier grant, with the Northeastern Ohio
Universities College of Medicine (NEOUCOM), announced today to support the development of a technology company incubated at Kent State’s Centennial Research Park. The grant supports future expansion of Pathogen Systems, Inc., which licenses biosensor technology developed by a Kent State and
NEOUCOM researchers.
Alpha Micron Project
Kent State also is a partner in a $5 million state grant received by Kent-based Alpha Micron Inc. The funding, also a Third Frontier grant from the engineering and physical sciences research and commercialization program, is a three-year award for adaptive window technology and involves Kent
State and NASA.
Bailey receives national research associateship
Congratulations to Chris Bailey, a CPIP Ph.D. student planning to graduate this August, for receiving a
National Research Council (NRC) research associateship. This associateship will provide postdoctoral funding for his work at the Air Force Research Lab (AFRL) in Dayton, Ohio.
This fellowship (only available to national laboratories) required a 3000 word proposal of his proposed research at the government laboratory titled, "Controlling the surface alignment and phase separation of photoactive discotic liquid crystal mixtures for the development of more efficient organic photovoltaic devices". He plans to use mixtures of different organic molecules that form columnar liquid crystal phases to improve performance of organic photovoltaics.
He’ll be working with his advisor, Dr. Michael Durstock and Dr. Timothy Bunning. The funding is guaranteed for two years with a possible extension of a third year.
More information can be found at http://www7.nationalacademies.org/rap/
New Faces at LCI
Manoj Mathews is a postdoctoral fellow working in Dr. Quan Li’s Chemical Synthesis lab for one year.
He previously worked at the Nanotechnology Research Institute (Japan) after earning his doctorate from
Mangalore University, Center for Liquid Crystal Research (Mangalore, India).
Oscar Baldovino-Partaleon, National Astrophysics Institute, Puebla, Mexico will be visiting with
Professor Peter Palffy-Muhoray for three months.
Lorenzo Marrucci, Dipartimento di Scienze Fisiche, Complesso Universitario di Monte S. Angelo, via
Cintia, Napoli, Italy will be visiting with Professor Peter Palffy-Muhoray for four months.
Recent LCI Seminars (videos available on IPP web)

July 7, 2008
Prof. Sir John Ball, Mathematical Institute, University of Oxford, UK, "Remarks on Q-tensor theory"
Note: This is a joint seminar with Department of Mathematical Sciences
June 11, 2008
Prof. Yuriy Reznikov, Institute of Physics, Kiev, Ukraine, "Ferroelectric colloids in nematic liquid crystals"
Upcoming LCI Seminars
November 5
Prof. Peixuan Guo, Dane and Mary Louise Miller Endowed Chair in Biomedical Engineering, University of Cincinnati, Title: T.B.A.
December 10
Prof. Nader Engheta, H. Nedwill Ramsey Professor of Electrical and Systems Engineering, and Professor of Bioengineering, University of Pennsylvania, Title: T.B.A.
Phil Bos pbos@lci.kent.edu
330-672-2511