Variable Angle Spectroscopic Ellipsometry of Anodically Oxidized Tantalum Films Jovan Trujillo Flexible Display Center 10/06/06

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved Current state of development

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved 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.

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved Anatomy of a Field Effect Transistor Substrate Gate Metal Gate Dielectric a-Si:H IMD n+ a-Si contact Source metal Drain metal

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved Anatomy of a Pixel transistor capacitor

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved Why Tantalum Oxide? MaterialProcess Dielectric Constant Problems Silicon NitridePE-CVD~7 Step coverage, low-k, low breakdown voltage. Hafnium Silicate Reactive sputtering ~12 worse step coverage, stoichiometry problems, slow deposition rate Aluminum OxideReactive sputtering ~ 9same as hafnium silicate Tantalum OxideAnodic oxidation~ 28etch selectivity, mask changes

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved Anodic oxidation process ( a self limiting reaction ) Platinum CathodeTantalum Anode 60 mA ramp to 100 V 0.05% vol acetic acid 5.5 L water room temp. Hydrogen bubbles

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved 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.

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved 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.

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved How it works…

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved How it works…

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved The Data 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, Anodic oxidation Reactive sputtering

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved The Data 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, Anodic oxidation Reactive sputtering

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved  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

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved The Gaussian Oscillator

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved Experimental vs. Model

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved Results and Analysis  Using Gaussian function  oxide thickness = ± Å  MSE =  Refractive index =  Using Gaussian function with porous interfacial layer between metal and oxide.  oxide thickness = ± 1.1 Å  MSE =  Refractive index =

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved Thickness verification with FESEM

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved Comparison of Refractive Index Compare with n = 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.

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved 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.

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved 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

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved Correlation Matrix E1Offse t.2 PoleMa g.2 Amp1.2En1.2Br1.2 Thick. 2 Thick. 1 EMA2.1 E1Offset PoleMag Amp En Br Thick Thick EMA Wafer 5 of FESEM experiments

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved Step Coverage

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved Capacitor Damage

Flexible Display Center at Arizona State University06-Oct Copyright © 2006 Arizona State University All Rights Reserved More Displays