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High-k Dielectric for Flexible Displays using Anodically Oxidized Tantalum

High-k Dielectric for Flexible Displays using Anodically Oxidized Tantalum. Jovan Trujillo Flexible Display Center 3/2/07. Current state of development. Anatomy of a Field Effect Transistor. Source metal. n+ a-Si contact. Drain metal. IMD. a-Si:H. Gate Dielectric. Gate Metal.

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High-k Dielectric for Flexible Displays using Anodically Oxidized Tantalum

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  1. High-k Dielectric for Flexible Displays using Anodically Oxidized Tantalum Jovan Trujillo Flexible Display Center 3/2/07

  2. Current state of development 2-March-2007 -2-

  3. Anatomy of a Field Effect Transistor Source metal n+ a-Si contact Drain metal IMD a-Si:H Gate Dielectric Gate Metal Substrate 2-March-2007 -3-

  4. Transistors are Electrical Switches 2-March-2007 -4-

  5. Anatomy of a Pixel transistor capacitor 2-March-2007 -5-

  6. Goals of Tantalum Anodization Research • Fundamental • Understand relationship between process conditions and electrical characteristics • Develop spectroscopic ellipsometry techniques to characterize Ta2O5 film and interfaces. • Applied • Use Ta2O5 to improve pentacene based organic transistors • This work is in collaboration with Parul Dhagat • Identify problems with implementing in main 6” process line. • This work is in collaboration with entire process team • Developmental • Identify factors in quality control • Supply etch engineer with material • Propose approach for increasing production 2-March-2007 -6-

  7. Applied electric field causes opposing internal electric field. Charge builds up while under voltage. Defects in film cause charge to leak through. Current is released when voltage removed. How Dielectrics Work 2-March-2007 -7-

  8. Why Tantalum Oxide? 2-March-2007 -8-

  9. Anodic oxidation process( a self limiting reaction ) 0.05% vol acetic acid 5.5 L water room temp. 60 mA ramp to 100 V Hydrogen bubbles Tantalum Anode Platinum Cathode Final current < 0.40 mA giving current flux of 25 fA/m2 @ 100 V 2-March-2007 -9-

  10. Role of Acid • Purpose: • Increases conductivity of solution; create more ions for reaction. • Problems: • Negative ions from acid will contaminate the oxide. • Higher leakage current. • Lower breakdown voltage. • Why acetic acid? • Based on paper by Kalra, Katyal, and Singh, 1989. • Acetic acid caused highest breakdown voltage. • Effect of carbon contamination appears minimal. 2-March-2007 -10-

  11. Measuring Thickness with Ellipsometry(first paper) 2-March-2007 -11-

  12. Thickness and Index Uniformity Max-Min Thickness variation < 3 nm Max – Min Index variation < 0.02 11 wafer maps have been made with ellipsometry 2-March-2007 -12-

  13. Effect of Initial Current on Surface Roughness 60 mA process 20 mA process Roughness (rms) = 0.696 nm Roughness (mean) = 0.516 nm Roughness (peak-to-valley) = 7.22 nm Roughness (rms) = 0.564 nm Roughness (mean) = 0.476 nm Roughness (peak-to-valley) = 2.99 nm Sputtered Ta metal Roughness (rms) = 0.463 nm Roughness (mean) = 0.334 nm Roughness (peak-to-valley) = 3.36 nm Thank you Hanna Heikkinen 2-March-2007 -13-

  14. Electrical Characterization • Expected Dieletric Constant of ~ 28 • Paper by Kalra, Katyal, and Singh reported dielectric constant using 0.05 %v/v acetic acid. • Aluminum capacitors made by sputtering through stainless steel stencil using MRC-602 “King Kong” • Automated wafer maps of dielectric constant and leakage flux made using Electroglas 2001 “Famine”, LabView program, HP4284A LCR meter, and HP3457A multimeter. • 2022 1 mm2 and 1004 4 mm2 capacitors per wafer • Data processing done using VBA scripts and Minitab. • Outliers > 3  removed to normalize data • ANOVA and Tukey’s test used to compare wafers 2-March-2007 -14-

  15. First Capacitor BatchDielectric Constant 60 mA ramp to 100 V, 1:40 hours, final current 0.40 mA 2-March-2007 -15-

  16. First Capacitor BatchDielectric Constant 2-March-2007 -16-

  17. First Capacitor BatchDielectric Constant 2-March-2007 -17-

  18. Capacitor Area Variation 2-March-2007 -18-

  19. First Capacitor BatchLeakage flux fA/m2 2-March-2007 -19-

  20. First Capacitor BatchLeakage flux fA/m2 2-March-2007 -20-

  21. First Capacitor BatchLeakage flux fA/m2 2-March-2007 -21-

  22. Leakage Flux vs. Voltage 2-March-2007 -22-

  23. Initial Conclusions • Dielectric constant verified to be ~28 • Similar pattern in contour plots indicates systematic error caused by stencil. • Leakage flux variation caused by unknown factor. • Only one wafer showed acceptable leakage flux levels at 10 V. • Something is contaminating the films. 2-March-2007 -23-

  24. Summary of experiments in quality control 2-March-2007 -24-

  25. Summary of experiments in quality control 2-March-2007 -25-

  26. Final Conclusions • Doping level (10 ppb – 100 ppm?) contamination significantly affects leakage flux and dielectric constant. • Stay away from HCl vapors. • Coat pallet with tantalum before sputtering product. • Substrate quality affects dielectric constant. • Iron contamination increases leakage flux and creates uniformity issues. 2-March-2007 -26-

  27. First Attempt at Applying to Pentacene OTFT(transistor fabrication and characterization by Parul Dhagat) Pentacene on SiO2 Pentacene on Ta2O5 2-March-2007 -27-

  28. Interface characterization is important • Free valence shells and surface roughness reduce field effect mobility. • Interface treatment with OTS (octaldecytrichlorosilane) can improve pentacene transistor performance. • Ellipsometry can help. [Angst, David L.; Gary W. Simmons.Moisture Absorption Characteristics of Organosiloxane Self-Assembled Monolayers. Langmuir 1991, 7, 2236-2242.] 2-March-2007 -28-

  29. OTS on Ta2O5Model Based Approach • Literature reports high quality OTS monolayer to be ~25 Å thick. • Cauchy model used for monolayer. • Parameters for Ta2O5 held constant. • Reported thicknesses in Å 2-March-2007 -29-

  30. OTS on Ta2O5fingerprint approach RAS of OTS on SiO2 • RAS – Reflection Anisotropy Spectroscopy • Requires bulk to be optically isotropic • Requires interface to be optically anisotropic • Technique rotates the sample and measures change in relectance for s-polarized light. 2-March-2007 -30-

  31. Kinetics of Growth • Paper by Zhang, Macdonald, Sikora and Sikora, 1998 argues final current limited by barrier field at interface, not by thickness of film. • Used phosphoric acid, 99.95% pure Ta rods, not in clean room. High Field Model Point Defect Model Vs. Zhang, Lei; Digby D. Macdonald; Elzbieta Sikora; Janusz Sikora. On the Kinetics of Growth of Anodic Oxide Films. Journal of the Electrochemical Society 1998, 145, 3. 2-March-2007 -31-

  32. Pulsed Anodization • Assume anode acts like diode impeding current flow. • Use pulsed voltage to break interface barrier and pump more current. • Expect to increase film thickness and/or improve oxide stoichiometry. • Pulsed anodization controlled using relays and LabView program. 2-March-2007 -32-

  33. Research Plan • Papers in progress • Spectroscopic Ellipsometry of Anodized Tantalum • High Performance Pentacene Transistors using Optimized Anodized Tantalum Process (with Parul) • Reflectance Anisotropy Spectroscopy of OTS on Anodized Tantalum (with Parul) • Physical and Electrical Film Uniformity of Anodized Tantalum Films • Tantalum Pentoxide Capacitors using Pulsed Anodization Visit: http://www.public.asu.edu/~jtrujil1 2-March-2007 -33-

  34. Acknowledgements The FDC group: Dr. Gregory Raupp Shawn O’Rourke Curtis D. Moyer Dirk Bottesch Barry O’Brien Edward Bawolek Michael Marrs Scott Ageno Ke Long Consuelo Romero Diane Carrillo Virginia Woolf Susan Allen Marilyn Kyler Parul Dhagat Hanna Heikkinen Engineers at J. A. Woollam Co., Inc.: Neha Singh 2-March-2007 -34-

  35. Step Coverage 2-March-2007 -35-

  36. Capacitor Damage 2-March-2007 -36-

  37. More Displays 2-March-2007 -37-

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