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- What difference between reflectometry and ellipsometry?
Both reflectometry and ellipsometry are optical, non-contact
and non-destructive methods. In general, reflectometer is
used to acquire reflection spectrum over a wavelength range.
If there is film or coating on some kind of substrate, the
film or coating thickness can also be figured out from the
measured reflection spectrum. Ellipsometry
measurement is performed at non normal incident angles. The
two ellipsometry parameters, Psi and Del, give more information
than reflectance itself. Therefore, more information can be
accessible through ellipsometry technique, such as multiple
layer analysis, dielectric constants calculation, surface or
interface roughness, inhomogeneity behavior etc. Of course,
those information is always derived from an optical model,
which leads to some difficulties to use this technique
although it is much more powerful than reflectometry
- Which one should I choose for thickness measurement then
For thickness measurement, both techniques rely on
modeling. In general, ellipsometry gives better accuracy
than reflectometry in thin thickness range such as below
micron level. For a typical ellipsometer configuration, the
maximum thickness measurable is below 10 microns. However,
reflectometer can measure up to hundreds microns of non
absorbing thick coatings.
How thick and how thin I can measure for thickness?
These optical methods need to have light to
penetrate through film and reflect back from film/substrate
interface. Without meeting such condition, the film
thickness can not be figured out because of lacking
necessary phase information. One example is a sample with
thick metal films on it. Because metal has high absorption in
visible and Near infrared range, light only can penetrate
metal film with a depth of less than 1000 Angstroms. For
such film with a thickness above 1000 Angstroms, it is
impossible to measure its thickness by reflectometry
or ellipsometry although reflection spectrum can be obtained
with reflectometer and optical properties for metal film can
be obtained with ellipsometry. Roughly, the measurable
thickness for various films can be estimated from
penetration depth if knowing its extinction coefficient or
- Can you give some application examples with
spectroscopic ellipsometry measurement?
You can find following applications with
constants (refractive index n and extinction coefficient
k) for thin films, coatings and bulk substrate
nondestructive thickness determination for thin films
Alloy concentration determination for various thin films such as Ge in SiGe alloy, Al in AlGaN films
Band gap determination for GaN, SiC, AlN, AlGaN, etc.
Porosity measurement in low-K films
Relative volume fraction determination for each component in nano-composite
Physical thickness and optical properties for each layer in a multiple layer stack or periodic structure such as quantum well structure
and optical properties uniformity information
Inhomogeneous film analysis in physical density or alloy concentration
Optical properties for high-k films
Nondestructive measurement for electrical conductivity of metal films, metallic compounds (such as WN, TiN, TaN, etc.), doped semiconductor epi layers (thickness can be also determined at the same time), other compound oxides such as ITO films
Nondestructive measurement for doping concentration in doped semiconductors (active dopant! not total concentration as given by destructive SIMS analysis)
FAQ on Reflectometry
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What is the accuracy of the system?
In general, there two different cases. One is direct
technique and the other one is indirect. For reflection and
transmission measurement, they are direct measurement
although they need background dark measurement and reference
measurement. These measurements basically are light level
measurement. Therefore the accuracy for these measurement
purely depends on hardware setup.
For film thickness measurement, it is indirect measurement
comparing transmission measurement. A model is always needed
to get thickness information. therefore, thickness accuracy
not only relies on hardware but also data's mathematically
handling which is the second step involved in thickness
measurement. From theory section, it has been learnt that
thickness is obtained from phase thickness in the layer
which is related to wavelength, physical thickness and its
optical properties such as refractive index and extinction
coefficient. Therefore, thickness accuracy also relies on
accuracy of optical properties used in the layer stack.
What if I can not find optical constants in the
database for our films?
Optical constants are not constants. They vary at different
wavelength and by different processing technique. Processing
techniques affect packing density, materials crystalinity
etc. therefore affect optical properties. It is recommended
that user should evaluate what optical constants sets are
proper for his films. If user can not find optical constants
in database, we can provide service to determine optical
constants precisely with spectroscopic ellipsometry
technique and then provide to user for use.
- What if the film is not homogeneous?
User can use average optical properties for that film.
Why measured thickness depends on initial input or
To speed up calculation, program always tries to start from
the input thickness to minimize the sigma between measured
and calculated spectra. Depends on the range, it is possible
to miss true minima (global) and then will give out
thickness obtained at local minima. There is trade off here.
Wide range provides high possibility to find global minima
but take more time to do the job.
- What if the detector is saturated?
You need to start over all measurement again including
measuring dark background after you reduced integrations
When should I use small pixel group?
Pixel group function provides average signal over boxed
pixels. If you measure very thick films which will display
high dense fringes in the reflection spectrum, it is
recommended that use small pixel grouping.
What assumptions used in film thickness measurement?
Like all other optical model based technique, there are
following assumptions in film thickness measurement:
a. Layer stack generally describes the real structure of
b. Film and substrate are homogeneous
c. Over sampling beam area, film properties are uniform
d. Sample surface are flat and interface between film and
substrate are sharp
e. Sample is flat
f. For reflection measurement, sample surface and reference
surface are adjusted to the same level (height)
g. Nothing changed or moved such as fiber bending or power
h. Light source is stable
- How to improve measurement accuracy?
Any improvement in above question will lead to a better
FAQ on Ellipsometry
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- What wavelength range does your
ellipsometer system cover?
We cover the range from
190 nm to 30 µm.
- Is one measurement for whole range?
No. There are
several detectors and different light sources. Therefore, it needs at least two different measurements.
- What are the incident angles for ellipsometry measurement?
They depend on your substrate and desired precision. The incident angles could vary from 30 to 89 degree automatically
- Do you use multiple angle measurement?
Yes or no. It depends on your specific samples.
- How long will it take for one measurement?
Measurement time varies from sample to sample and also model
to model. For example, one full measurement at one incident
angle or one mapping site only takes 1 second with our array
based ellipsometer tool. While one measurement may take
minutes or even hours with scanned single element detector
- How big samples can you handle?
TFProbe SE under standard configuration, can handle samples up to 300mm in diameter.
- How small samples you can take?
We can take samples at a minimum size of
1mm in diameter.
- How about the sampling area on sample?
The parallel beam size is adjustable and it is between 1 to 5mm in diameter. For small samples, we use focused beam that has a beam size about 100µm only.
- Can you check uniformity information over a wafer?
Yes, we can run as many point measurements as you want over a maximum 300mm wafer.
- Will your tools come with turn-key
Yes. There is full recipe setup sp user can
click button once to get final results.
- Can you measure Ge concentration in SiGe alloy?
Yes, we can measure Ge concentration in both strained and relaxed SiGe films as the same principle for Al concentration in AlGaN films.
- Can you determine optical band gap for GaN, AlGaN, or SiC, etc.?
Yes, we do run band gap calculations based on the obtained optical constants from the ellipsometry modeling.
- Can you measure multiple layer stack?
Yes. A spectroscopic ellipsometry run at variable angles will help to get accurate results for each layer.
- Can you measure embedded ITO layer conductivity with ellipsometry technique?
Yes, with near infrared or middle infrared ellipsometer, Drude dispersion could be used to obtain electrical properties of conducting films.
- What materials ellipsometer can work with?
materials can be characterized with ellipsometry, such as
Metals, polymers, ceramics, glasses, semiconductors and
their compounds, composites etc.
- How about application fields for ellipsometry?
a partial list of application fields for ellipsometry:
Flat Panel Display (FPD)
Any Advanced Applications for ellipsometry?
are some examples for advanced applications for
Ultra-thin gate dielectric films
Real time process monitoring for deposition rates, etching rates, etc.
- Other questions?
Background on Ellipsometry
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Spectroscopic ellipsometry (SE) is a powerful technique to
precisely measure thin film thickness, determine
investigate surface and interface phenomenon and many other
physical, chemical and optical properties of materials. We provide high quality spectroscopic ellipsometer system for various applications. Besides ellipsometer system itself, the advanced analysis
software is essential to extract the desired information as above-mentioned, such as thickness, roughness, alloy concentration and dielectric
constants. TFProbe™ 3.0 from us offers powerful analysis functions for ellipsometry sensitivity study, photometry / ellipsometry simulation and
data regression. Unique but configurable mode allows different users to access different level and suitable for both R&D and production
quality control purpose.
There are many techniques for characterizing materials, each having its own advantages and disadvantages and each being uniquely able to
reveal material properties that other techniques can't access. Spectroscopic ellipsometry (SE) is an optical technique that is particularly
flexible in that it can be used to determine the optical and physical properties of a wide variety of thin-film materials. Its ability to do
this without contact or damage to the material of interest has seen it become routinely used in R&D laboratories and within manufacturing
facilities for monitoring thin film growth and deposition processes.
SE relies on the determination of the polarization state of a beam of polarized light reflected from the sample under characterization. When
performing SE measurements, the polarization state is determined at many discrete wavelengths over a broad wavelength range. The change in the
polarization state can be traced to the physical properties of the thin film by means of a model. Characteristics such as layer thickness,
surface roughness, refractive index (n) and extinction coefficient (k) of the materials can be determined with excellent precision through
The instrument determines two ellipsometry angles Ψ and Δ, which describe the change in the polarization state of the beam upon
reflection from the sample. The ratio of the amplitude of the polarization within the plane of incidence (P) to the amplitude of the
polarization perpendicular to the plane of incidence (S) is represented by Ψ. The phase retardation between the two polarization
vectors P and S is represented by Δ. Changes in Δ and Ψ essentially depend upon the optical constants, n and k, of the
layer materials and substrate, physical thickness of the individual layers and surface roughness. A regression analysis allows the
determination of these parameters.
SE data for Δ and Ψ are obtained at a number of incident angles in a plane normal to the sample surface and typically at 100-200
different wavelengths for each angle. SE instruments use a white light source and individual wavelengths are selected for detection by either
a motor driven monochromator, or a multi-channel detector that can detect many wavelengths simultaneously. Increasing the number of angles and
wavelengths at which data are acquired improves analysis precision, especially for complicated epitaxial structures.