Spectroscopic Ellipsometry (Sopra GES 5) - Login | Nanolab, UCLA

SE Sopra GES 5 UCLA Nanoelectronic Facility
Superusers:
Spectroscopic Ellipsometry
(Sopra GES 5)
PREPARED BY: You-Sheng (Wilson) Lin, Nanolab Staff
10-10-2004
Steve Franz X68923
Tom Lee X64641
You-Sheng Lin X68923
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I. Introduction
A. Principle
Spectroscopic ellipsometry (SE) is a non-contact, non-destructive optical technique that
enables the determination of material refractive indices and layer thicknesses by
measuring the change in polarization of a probing light beam upon reflection from a
sample. When linearly polarized light reflects from a surface, elliptically polarized light
is generated under certain conditions. The amount of induced ellipticity depends on the
surface properties (refractive index, bulk or layered sample…). Ellipsometry technique
measures the phase and amplitude relationships between two orthogonal polarizations (p
and s waves) upon reflection. When p and s waves are reflected, they experience a phase
shift and an amplitude reduction (not necessarily the same for both waves). The
ellipsometric parameter Delta is defined as d1 - d2 where d1 and d2 denotes the induced
phase shift difference between p and s waves, respectively while the ellipsometric
parameter tan(Psi) is defined as the ratio of the complex amplitude of the total reflection
coefficient of the p and s waves ( |RP| / |RS| ). Ellipsometry is said to be self-referencing
because measurements do not require any reference sample and are largely insensitive to
variations in the beam intensity and ambient environment, making this technique highly
accurate and reproducible.
B. Instrumentation
The Sopra GES 5 spectroscopic ellipsometer allows for variable-angle broadband
spectroscopic ellipsometry. It covers a spectral range from 210 nm to 900 nm. And
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upgradable for wider spectal range in DUV, NIR, and IR ranges. The UV/Visible/NIR
source is a Xenon lamp whose maximum of intensity is located at 450 nm. Two
attenuators are available corresponding to an attenuation of 17 % and 7 % respectively
and the system can be set to automatically remove them during a measurement when the
intensity is very low in order to improve the signal to noise ratio. Our configuration in the
UV-Visible-NIR spectrum is rotating polarizer - fixed analyzer ellipsometry. When the
polarizer rotates, the incident beam is linearly polarized with a direction that rotates in
time. After reflection on the sample, the light is elliptically polarized with its axis
rotating in time. After the analyzer, the light is again linearly polarized with a fixed
direction and a time dependent amplitude. The advantage of this technique is that the
measurement is not affected by the polarization sensitivity of the detector (fixed
polarization) but it requires a perfectly randomly polarized incident beam. The beam size
is typically between 1 mm 2 and 10 mm 2 depending on the aperture. The variable angles
are from 7º to 90º with variable +/- 0.01º.
C. Analysis
The ellipsometric parameters are fitted using the Levenberg-Marquardt regression
method. Different models are available to describe the dispersion relation of the refractive
index of the measured materials including the Gaussian and Lorentz oscillator models,
the Cauchy and Sellmeir laws, the Kramers-Kronig relations, the Forouhi interband
model, the model dielectric function, the Drude model... Point to point calculation (i.e.
calculation of the n and k values from the data at each wavelength for a single layer of
known thickness) is also possible. In addition, whenever the sample is bulk or can be
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treated as such because of its strong absorption, the dispersion relation is simply obtained
by direct calculation of n and k at all wavelengths without any assumption.
II. Operating Procedure
A. Define the spectral range for the measurement
This procedure will help users to define measurement parameters before starting a
measurement.
1) Definition of the spectral range
• Open Winelli
• Go to File and select Simulation.
The Simulation Parameters Window appears.
• Select Model to define the structure that you will simulate
a. In the folder “Structure”:
• Make sure the Layer type is material
• Add or subtract desire layers
• Click the each layer, and select the material data from the files (by click on the
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blank space) for the appropriate N.K file. Also, enter the composition and
concentrations if applicable.
• Enter the thickness of the film and by checking the box next to the thickness
will try to fit the thickness close to the estimate thickness.
b. In the folder “Boundaries”:
• Make sure the WaveLength is Variable and Unit is µm.
• Define the first, the last wavelengths and the number of points for the
measurements. Initiate with the visible spectrum (0.3 µm- 0.8 µm) and 200 points
to describe precisely the sample.
• Enter the incidence angle that you will use during the measurement (use 75º).
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c. In the folder “Special”:
• Check the microspot box (the aperture typically equals 2.0). If you don’t want to
use microspot, please ask staff or superusers for assistant.
• Then press Run. You will come back to the Simulation Parameters Window.
• Press Simulation to plot tan(ψ) and cos(∆) (for the structure you defined in
‘Model’)
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• Go back to the Simulation Parameters Window. Change the Output (in the right
bottom corner) from graphic to spreadsheet.
• Press Simulation one more time to obtain the values of tan (ψ) and cos (∆) in an
array.
Go to Analysis and select Adjustment. This mode gives you the possibility to change
manually the parameters of your model.
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• The ‘structure’ screen appears. Check the parameters that you want to vary manually (Thickness,
concentration, …).
• Press Run
• Now, you can adjust manually by using the slide or typing a number, every parameter
checked in the structure.
• So you are able to define the spectral range where the measurements are the most
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sensitive to parameters variations.
• Then save and exit. (A window should appear concerning the regression. Don’t take
care of it, and select the Simulation Parameters Window)
2) Definition of the optimum number of wavelengths for the measurement
B. Sample alignment Procedure
• Open Gespack by double click the Gespack icon
User: Engineer
Password: Sopra
1. Place the wafer on the sample stage
2. Make sure the stage is low enough for 75º by lowering the Z
stage.
3. Click on the Show
4. Change the parameter values :
a. λ : the most intense wavelength 0.45 micron
b. Angle of analyzer : 45°
c. Plan of incidence: 75°
5. Press Run (Wait for 1 minute, and the count, signal.......)
6. Adjust the Z position to obtain maximum of signal
7. Attenuators On as required to keep signal around 10 millions
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8. If the signal is below 5 millions counts, open the slit (right hand side under the table)
to get 10 millions.
9. Adjust the tilt of the sample holder (with both screws) to obtain the maximum signal
and to obtain a symmetry between -1 and +1 if needed.
10. Turn the Attenuators ON as required to keep signal around 10 millions
11. Adjust the Z position again to obtain maximum signal
12. Turn the Attenuators ON as required to keep signal around 10 millions
13. Adjust the slit of the spectrometer to obtain 10 millions.
14. If the slit is fully opened and you don’t have 10 millions, the let the slit fully opened
15. Stop and Exit
• Parameters for the measurement
Analyzer Position Mode (Please make sure that all the following parameters are correct
and don’t change anything unless you discuss with superuser or staff)
In the “Polarisation/Geometry folder”,
Previous tracking: Adjust the analyzer position at ψ where tan ψ is the first
ellipsometric parameter of the previous measurement. More usual mode to
make measurements. You need to put a starting position different than zero
(45 is good). Does not work for very thick samples and a low number of
points (when tan ψ is closed to 0, ψ is closed to 0, so the analyzer position
will be at 0)
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Incidence angle
Fixed: at the indicated position all along the measurement
Use of calibrated angel: Yes
Theory Angel: 75 and Mea. 75.2
In the “Time and wavelength” folder,
Integration time
Variable: The integration time can change depending on the number of
counts on the detector. The measurement stop is the number of counts
exceed a given value (generally 4 000 000) of a given time (generally 10s or
5s)
Spectrometer position
Scanned: for spectroscopic measurement. Enter the values obtained in step 1
Unit: µm
From the simulated result, put correct set to get right measurement duration.
(Ex: From 0.25 to 0.85; step: 0.003 Number of points = 200)
Background noise
In the “Other” folder,
Meas/Scan: measured at the beginning of each scan
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Signal correction
Special
Non Linearity + polarization: makes correction of the non linearity of the
detector and of the residual polarization using the values calibrated in the
GETEST software (Mostly used)
Use attenuators: Checked when you adjust the signal on the detector with
both attenuators (Used for optimized the measurement and reduce the noise)
Microspots
Press Run/Accept
C. Start the Measurement
1. By click the Run bottom
2. Depending on the number of data points, it will take up to 20 minutes.
3. At the end of the measurement, save it and analyze it with the software WINELLI
III. Analysis of the data
A. Thickness measurement analysis
1. Open Winelli software
2. Open last measurement data (filename.dat)
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3. Enter in Analysis menu and Select Regression.
4. Define the structure of your film mixed with void (void.nk)
5. Fit on the thickness of the film and on the concentration of void. Click Run the
regression using function “cos(2psi) and sin(2psi)*cos(delta)” (if you have film
> 500 A) mode.
6. Examine the curve
7. Save and print the regression results.
8. You can fine the thickness data from the table.
B. Analysis using a Cauchy dispersion law (for better fitting, optional)
1. Go back to the Structure screen (Right next to Run on the top)
2. Change the “layer type” into dispersion law and click on “Law parameters”.
3. Select Standard Dielectric Function, UV term, and Cauchy (n,k). Ex: For oxide
films, enter as started values 1.45 for A, 0.01 for B and fit on the three parameters
A,B,C.
4. Fit on the thickness for films. Run the regression using “cos(2psi) and
sin(2psi)*cos(delta)” mode.
5. Examine the curve and fit.
6. Save and Print the regression.
7. Bo back to the structure screen and in law parameter screen. Save the refractive
index as Cauchy.nk.
8. Exit the regression.
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Appendix:
In the Analyzer
In the “Polarisation/Geometry folder”,
Previous tracking: Adjust the analyzer position at ψ where tan ψ is the first
ellipsometric parameter of the previous measurement. More usual mode to
make measurements. You need to put a starting position different than zero
(45 is good). Does not work for very thick samples and a low number of
points (when tan ψ is closed to 0, ψ is closed to 0, so the analyzer position
will be at 0)
Current tracking: At each step make a first measurement to found tan ψ and
then adjust the analyzer position to ψ. Best accuracy but the measurement is
increased.
Limited tracking: Same mode as previous tracking but the minimum angle
will be 5 degrees. Avoid to have an analyzer position to 0.
Fixed: at the indicated position all along the measurement (not optimized for
accuracy)
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Incidence angle
Scanned: The angle of incidence is scanned during the measurement.
Fixed: at the indicated position all along the measurement
Use of calibrated angel: Yes
Theory Angel: 75 and Mea. 75.2
In the “Time and wavelength” folder,
Integration time
Scanned: You can repeat the same measurement with a given time step (only
interesting for non stable samples).
Fixed: at the indicated value all along the measurement (non optimized)
Variable: The integration time can change depending on the number of
counts on the detector. The measurement stop is the number of counts
exceed a given value (generally 4 000 000) of a given time (generally 10s or
5s)
SNR: You must indicate the integration time for one measurement, a target
value for the SNR, and a maximum number of iterations
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Spectrometer position
Scanned: for spectroscopic measurement. Enter the values obtained in step 1
Unit: µm
From the simulated result, put correct set to get right measurement duration.
(Ex: From 0.25 to 0.85; step: 0.003 Number of points = 200)
Fixed: at the indicated position all along the measurement (variable angle
measurement for example)
In both case, the measurement can be done in eV, µm, nm or cm -1 .
Background noise
In the “Other” folder,
Default: fixed at the indicated value (counts/s).
Meas/Scan: measured at the beginning of each scan
Meas/Int: measured before each measurement
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Signal correction
Special
No correction: no correction for the measured signal (no optimized)
Non Linearity: makes correction of the non linearity of the detector using
the values calibrated in the GETEST software
Non Linearity + polarization: makes correction of the non linearity of the
detector and of the residual polarization using the values calibrated in the
GETEST software (Mostly used)
Use attenuators: Checked when you adjust the signal on the detector with
both attenuators (Used for optimized the measurement and reduce the noise)
Use Microspots
Mirror analyzer: make twice the measurement at A and –A for the analyzer
position (used only for the calibration)
Init Spectrometer: makes an initialization of the spectrometer before each
scanning measurement
Init Goniometer: makes an initialization of the goniometer each scanning
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measurement
Compensator: only available if compensator option is checked in the setup
of the instrument
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