Variable Angle Spectroscopic
Ellipsometry of Anodically Oxidized
Tantalum Films
Jovan Trujillo
Flexible Display Center
10/06/06
Current state of development
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Current Problems with Dielectric Materials




Voltages approaching 60 V are needed to drive display.
 Dielectric materials break down at such high voltages.
 High voltages due to mobility of a-Si:H and dielectric constant of a-Si:N:H.
 Breakdown due to low breakdown voltage of a-Si:N:H.
 Anodically oxidized tantalum can be grown withstand 100 V.
Color displays will require smaller pixels.
 Design engineers report that a-Si:N:H will not have enough capacitance for smaller
pixels.
 Anodically oxidized tantalum has a dielectric constant 4x of a-Si:N:H.
Step coverage.
 Low temperatures reduce surface diffusion of deposited materials, causing
“breadloafing”
 Poor adhesion to steps and edges cause open and short circuits.
 Anodic oxidation grows from steps and edges, eliminating the “breadloafing” problem.
Organic transistors need high-k materials.
 Current organic transistors have very low drive current, possibly due to silicon oxide
dielectric.
 Literature has reported successful application of tantalum oxide to pentacene based
transistors.
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Anatomy of a Field Effect Transistor
Source metal
n+ a-Si contact
Drain metal
IMD
a-Si:H
Gate Dielectric
Gate Metal
Substrate
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Anatomy of a Pixel
transistor
capacitor
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Why Tantalum Oxide?
Material
Silicon Nitride
Hafnium Silicate
Process
PE-CVD
Reactive
sputtering
Dielectric
Constant
Problems
~7
Step coverage,
low-k,
low breakdown
voltage.
~12
worse step
coverage,
stoichiometry
problems,
slow deposition
rate
Aluminum Oxide
Reactive
sputtering
~9
same as hafnium
silicate
Tantalum Oxide
Anodic oxidation
~ 28
etch selectivity,
mask changes
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Anodic oxidation process
( a self limiting reaction )
60 mA ramp to 100 V
Hydrogen bubbles
Current change over time
80
70
0.05% vol acetic acid
5.5 L water
Current (mA)
60
50
40
30
20
room temp.
10
0
0
10
20
30
40
50
60
70
80
time (min)
Tantalum Anode
Platinum Cathode
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Things we need to know …
 What is the effect of starting current?
 Does a high initial current cause interface roughness?
 Does it create a porous film?
 What is the thickness of the oxide?
 Needed to study etch chemistries.
 Needed to study growth mechanism.
 Needed to calculate metal consumption.
 What is the index of refraction?
 Index of refraction is related to film stoichiometry, crystallinity.
 Changes in this parameter give qualitative information about
changes in film.
 Currently used to catch changes in silicon nitride film.
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Spectroscopic Ellipsometry ( SE )
 No papers have been published on SE for anodically oxidized
tantalum.
 All previous work has been with reactively sputtered tantalum
oxide.
 Need SE model to track changes in thickness, interfaces, and
material quality.
 A simple Cauchy model does not work near band gap.
 Provides qualitative information on changing stoichiometry and
crystallinity.
 Provides information on interface formation.
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How it works…
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How it works…
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The Data
Reactive sputtering
Anodic oxidation
Experimental Data
100
80
Exp E 65°
Exp E 67°
Exp E 69°
Exp E 71°
Exp E 73°
Exp E 75°
< 1 >
60
40
20
0
-20
0.0
1.0
2.0
3.0
Photon Energy (eV)
4.0
5.0
6.0
Franke, E.; M. Schubert; C.L. Trimble; M.J. DeVries; J.A. Woollam. Optical properties of amorphous and polycrystalline tantalum oxide thin
Films measured by spectroscopic ellipsometry from 0.03 to 8.5 eV. Thin Solid Films 2001, 388, 283-289.
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The Data
Reactive sputtering
Anodic oxidation
Experimental Data
60
40
< 2 >
20
0
Exp E 65°
Exp E 67°
Exp E 69°
Exp E 71°
Exp E 73°
Exp E 75°
-20
-40
-60
0.0
1.0
2.0
3.0
Photon Energy (eV)
4.0
5.0
6.0
Franke, E.; M. Schubert; C.L. Trimble; M.J. DeVries; J.A. Woollam. Optical properties of amorphous and polycrystalline tantalum oxide thin
Films measured by spectroscopic ellipsometry from 0.03 to 8.5 eV. Thin Solid Films 2001, 388, 283-289.
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Modeling Process


Find optical functions for tantalum metal using data transform model.
Fit transparent region (600 – 1700 nm) of oxide to Cauchy function to
find thickness.

Fit entire spectra with Cauchy function to find optical functions on a
point by point basis.
 Film thickness is now a constant.
 This is only an approximation to the real optical functions

Fit more complicated oscillator model to optical functions.
 This helps with creating a good initial guess for parameters.
 All fits use Levenberg-Marquadt to minimize error. A good initial guess helps
avoid local minima.
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The Gaussian Oscillator
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Experimental vs. Model
Generated and Experimental
100
Model Fit
Exp E 65°
Exp E 67°
Exp E 69°
Exp E 71°
Exp E 73°
Exp E 75°
80
40
20
0
-20
0.0
Generated and Experimental
1.0
2.0
3.0
Photon Energy (eV)
4.0
5.0
6.0
60
50
Model Fit
Exp E 65°
Exp E 67°
Exp E 69°
Exp E 71°
Exp E 73°
Exp E 75°
40
< 1 >
< 1 >
60
30
20
10
0
-10
2.4
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2.6
2.8
3.0
Photon Energy (eV)
3.2
3.4
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3.6
Results and Analysis
 Using Gaussian function
 oxide thickness = 1860.52 ± 0.977 Å
 MSE = 35.04
 Refractive index = 2.2143
 Using Gaussian function with porous interfacial layer between
metal and oxide.
 oxide thickness = 1857.85 ± 1.1 Å
 MSE = 21.78
 Refractive index = 2.2100
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Thickness verification with FESEM
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Comparison of Refractive Index
Compare with n = 2.2100 @ 550 nm for anodic oxidation
Franke, Eva; C. L. Trimble; M. J. DeVries; J. A. Woollam; M. Schubert; F. Frost. Dielectric function of amorphous
Tantalum oxide from the far infrared to the deep ultraviolet spectral region measured by spectroscopic ellipsometry.
Journal of Applied Physics 2000, 88, 9.
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Future work
 Understand why Tauc-Lorentz and Cody-Lorentz models are
giving poor results.
 Further develop the fitting process so that more accurate
information about the interfaces can be obtained.
 Verify the kinetics of growth for anodic oxidation.
 Use ellipsometry to calculate etch rates of various receipes.
 Work with Dr. Jabbour’s student on evaluating tantalum oxide
for organic transistors.
 Evaluate the use of VASE for studying interface treatments
between dielectric materials and a-Si:H.
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Acknowledgements
The FDC group:
Dr. Gregory Raupp
Shawn O’Rourke
Curtis D. Moyer
Dirk Bottesch
Virginia Woolf
Barry O’Brien
Edward Bawolek
Michael Marrs
Scott Ageno
Consuelo Romero
Diane Carrillo
Engineers at J. A. Woollam Co., Inc.:
Neha Singh
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Correlation Matrix
Wafer 5 of FESEM experiments
E1Offse
t.2
PoleMa
g.2
Amp1
.2
En1.2
Br1.2
Thick.
2
Thick.
1
EMA2
.1
E1Offset.2
1
-0.949
0.799
0.675
0.534
0.389
-0.425
0.353
PoleMag.2
-0.949
1
-0.862
-0.825
-0.667
-0.462
0.392
-0.355
Amp1.2
0.799
-0.862
1
0.744
0.436
0.239
-0.269
0.238
En1.2
0.675
-0.825
0.744
1
0.898
0.187
-0.186
0.167
Br1.2
0.534
-0.667
0.436
0.898
1
0.059
-0.038
0.022
Thick.2
0.389
-0.462
0.239
0.187
0.059
1
-0.793
0.826
Thick.1
-0.425
0.392
-0.269
-0.186
-0.038
-0.793
1
-0.949
EMA2.1
0.353
-0.355
0.238
0.167
0.022
0.826
-0.949
1
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Step Coverage
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Capacitor Damage
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More Displays
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