Variable Angle Spectroscopic Ellipsometry of Anodically Oxidized Tantalum Films Jovan Trujillo...

Variable Angle Spectroscopic Ellipsometry of Anodically Oxidized

Tantalum Films

Jovan TrujilloFlexible Display Center

10/06/06

Flexible Display Center at Arizona State University 06-Oct-06

-2-Copyright © 2006 Arizona State University

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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

Substrate

Gate Metal

Gate Dielectric

a-Si:H

IMD

n+ a-Si contactSource metal Drain metal



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Anatomy of a Pixel

transistor

capacitor



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Why Tantalum Oxide?

Material ProcessDielectric Constant

Problems

Silicon Nitride PE-CVD ~7

Step coverage, low-k,

low breakdown voltage.

Hafnium SilicateReactive sputtering

~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 )

Platinum CathodeTantalum Anode

Current change over time

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80

time (min)

Cu

rren

t (m

A)

60 mA ramp to 100 V

0.05% vol acetic acid

5.5 L water

room temp.

Hydrogen bubbles



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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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The Data

Franke, E.; M. Schubert; C.L. Trimble; M.J. DeVries; J.A. Woollam. Optical properties of amorphous and polycrystalline tantalum oxide thinFilms measured by spectroscopic ellipsometry from 0.03 to 8.5 eV. Thin Solid Films 2001, 388, 283-289.

Experimental Data

Photon Energy (eV)0.0 1.0 2.0 3.0 4.0 5.0 6.0

< 1

>

-20

0

20

40

60

80

100

Exp E 65°Exp E 67°Exp E 69°Exp E 71°Exp E 73°Exp E 75°

Anodic oxidation

Reactive sputtering



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The Data

Experimental Data

Photon Energy (eV)0.0 1.0 2.0 3.0 4.0 5.0 6.0

< 2

>

-60

-40

-20

0

20

40

60

Exp E 65°Exp E 67°Exp E 69°Exp E 71°Exp E 73°Exp E 75°

Franke, E.; M. Schubert; C.L. Trimble; M.J. DeVries; J.A. Woollam. Optical properties of amorphous and polycrystalline tantalum oxide thinFilms measured by spectroscopic ellipsometry from 0.03 to 8.5 eV. Thin Solid Films 2001, 388, 283-289.

Anodic oxidation

Reactive sputtering



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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.

Modeling Process



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The Gaussian Oscillator



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Experimental vs. ModelGenerated and Experimental

Photon Energy (eV)0.0 1.0 2.0 3.0 4.0 5.0 6.0

< 1

>

-20

0

20

40

60

80

100

Model Fit Exp E 65°Exp E 67°Exp E 69°Exp E 71°Exp E 73°Exp E 75°

Generated and Experimental

Photon Energy (eV)2.4 2.6 2.8 3.0 3.2 3.4 3.6

< 1

>

-10

0

10

20

30

40

50

60

Model Fit Exp E 65°Exp E 67°Exp E 69°Exp E 71°Exp E 73°Exp E 75°



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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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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 amorphousTantalum 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 RauppShawn O’RourkeCurtis D. MoyerDirk BotteschVirginia WoolfBarry O’BrienEdward BawolekMichael MarrsScott AgenoConsuelo RomeroDiane Carrillo

Engineers at J. A. Woollam Co., Inc.:

Neha Singh



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Correlation Matrix

E1Offset.2

PoleMag.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

Wafer 5 of FESEM experiments

Variable Angle Spectroscopic Ellipsometry of Anodically Oxidized Tantalum Films Jovan Trujillo...

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