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THIN FILMS ON GLASS Founded 1996

THIN FILMS ON GLASS Founded 1996. D.G. Ast N.W. Ashcroft. History. IRG 2 was formed in 1996 Members from Physics, Appl&Eng Physics, Chem Engineering, Theoretical&Applied Mechanics, Mat Sci&Eng. IRG Leaders: D. Ast, N. Ashcroft. D.Ast. Current, Future Applications.

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THIN FILMS ON GLASS Founded 1996

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  1. THIN FILMS ON GLASSFounded 1996 D.G. Ast N.W. Ashcroft

  2. History • IRG 2 was formed in 1996 • Members from Physics, Appl&Eng Physics, Chem Engineering, Theoretical&Applied Mechanics, Mat Sci&Eng. • IRG Leaders: D. Ast, N. Ashcroft D.Ast

  3. Current, Future Applications Active Flat Panel Displays Body sized ,digital, X-ray sensor Monolithic Fiber Optic Switching Network Large, Biological Sensor Arrays Basic Issues: Adhesion, Inter-diffusion, Limits of Patterning

  4. Our Goal To advance the understanding of the properties of thin film on glass Surface Characterization Thin Film Deposition Initial Atomic Configuration at Interface Time evolvement of Interface Configuration

  5. Cornell Members • N. Ashcroft, 2 GRA’s, Physics (Theory) • D. Ast, 1GRA, MS&E (Thin Film Electronics) • J. Blakely, 1 S. Res. Assoc., MS&E (Surface Science) • R. Dieckmann, 1GRA, MS&E (Diffusion) • H. Hui , ½ GRA, T&AM • S. Baker, ½ GRA, MS&E • J. Silcox, 1 Res. Assoc., A&EP, (Interface Analysis) } (Adhesion)

  6. Industrial Members * Industrial Connections • F. Fehlner (Corning Inc.) • G. Couillard (Corning Inc.) • Corning Inc. (Glass-Ceramics, TFTs, MEMs) • Eastman Kodak (Active Matrix OLEDs) • Xerox Rochester (Analysis: TOF-SIMS) University Connections • Penn State (TFTs), Alfred U (Theory), TU of • Clausthal (Experiment/Theory) * Those attending bi-weekly IRG 2 meetings

  7. Interactions • Surfaces: Blakely, Umbach, Cooper, Headrick, Fehlner, Couillard • Surface modifications: Blakely, Umbach, Headrick, Ast, Nemchuk, Fehlner, Couillard. • Thin film deposition: Engstrom, Ast, Couillard, Fehlner, Baker. • Interface Characterization: Silcox, Jiang, Neaton, Ashcroft • Interface Adhesion: Ashcroft, Neaton, Baker, Hui • Interface Diffusion: Dieckmann, Thompson, Hui, Fehlner • Bulk Diffusion: Dieckmann, Hui, Ashcroft • Thin Film Electronics: Ast, Nemchuk, Couillard (Corning), Kalal (Corning), Fehlner (Corning), Thompson; Malliaras, Ast, Tang (EK), Williams (EK)

  8. Outline • Research Strategy • Areas of concentration • Accomplishments • Future research plans • Summary

  9. Research Strategy • Concentrate on surface/thin film interaction • Study bulk to the extend required to understand surface and near surface phenomena. • Investigate both simple and complex glasses. • Couple, wherever possible, theory to experiment. • Develop new instrumentation and techniques to characterize glass-surfaces and thin films on glass

  10. Surface, Near Surface and Bulk Surface Near Surface Bulk • Cleaning or polishing glass, required in most experiments, introduces near surface modifications. • Near surface layer has properties different from bulk

  11. Glass Systems Selected by IRG 2 • SiO2 (Oxidized Si, fused silica, fused quartz) OURCONTROL • extremely stable  • transport properties sensitive to trace amounts of H2O  • lacks ‘glass like’ intermediate range order, high viscosity  • K2O  SiO2THEORY + UHV CLEAVE EXPERIMENTS • simple, binary composition  • not stable, reacts with moisture  • Corning 1737 METAL & S.C. FILMS • Uniquely stable, reproducible, surface  • complex composition 

  12. Structure of Oxide Glass Multi-oxides have meso-scale short range order lowering viscosity

  13. Simple versus Complex Glasses: 1737 Smooth, stable Surface Multi-oxide glass K2O x SiO2 Hydrophilic Surface Binary-oxide glass High viscosity SiO2 Single ‘molecule’ Technologically useful glasses are complex mixture adjusted empirically to exhibit both a low viscosity and chemical stability

  14. IRG 2 Accomplishments • Surface Modifications of Glass by Cleaning * • Mass transport mechanisms on glass surfaces • X-ray scattering from glass surfaces (CHESS) • Properties of low temperature oxide films. • Na-22 diffusion in quartz, silica, 1737 glass. * Of large interest to the glass community

  15. IRG 2 Accomplishments (II) • Na affinity of 1737, OA-2 and NA-35 glass • Properties of poly-Si films deposited on glass, silica, oxidized Si, LTO, SiNx. • Structure of the Cr/Glass Interface • Substrate - Thin Si Film Interaction • STM imaging and spectroscopy of glass surfaces

  16. Examples of IRG Research • The Cr/Glass Interface • Substrate - Thin Film interactions • STM imaging of Glass Surfaces

  17. CHROME ON GLASS

  18. Chrome on Glass • Cr is known to adhere well to glass • Cr conductivity is low, 12.9 -cm • Al, when sintered at 400 C adheres well to SiO2 • Al conductivity 2.7 -cm • Au conductivity 2.35 -cm • Cu conductivity 1.7 -cm • Ag conductivity 1.6 -cm Cr is frequently used to ‘anchor’ better conductors, such as Cu Low line resistance crucial in large displays (gate pulse distortion)

  19. Empiricism of Adhesion High Heat of Metal Oxide Formation Good Adhesion Ti3O5 = - 586 Kcal/mole TiO2 = - 228 Kcal/mol Al2O3 = - 410 Kcal/mol Al2O = - 14 Kcal/mole Cr2O3 = - 276 Kcal/mol CrO2 = - 142 Kcal/mole SiO2 = - 210 Kcal/mol SiO = - 21 Kcal/mole Cu 2O = - 40 Kcal/mole CuO = - 37 Kcal/mole Ag2O = - 8 Kcal/mole Ag2O2 = - 7 Kcal/mole Low Heat of Oxide Formation Poor Adhesion

  20. The Cr-Glass Interface Initial Interface? • Oxygen Plasma cleaned 1737 glass • Cr evaporated at room temperature • Analyzed by STEM • Sample 1 observed 48 hrs. after deposition; • Sample 2 observed 2 weeks after deposition. Cr film Glass Substrate Sample 1 Inter-diffusion? Cr film Glass Substrate Sample 2

  21. X EELS and Bright Field Detectors ElectronEnergy Loss Spectrometer Annular Dark Field detector 100 keV Incident Beam DE = 0.7eV Spatial Drift < 0.3 nm/min Energy Drift < 0.03 eV/min Windowless EDX (X-ray) Detector Y 1 atom wide (2.1 Å) beam is scanned across the sample in the x-y plane to form a 2-D image Cornell’s UHV STEM acquires both positional, chemical (EDX) and bonding information (EELS) with atomic resolution

  22. Cr/1737 Interface Kinetics Annular dark field (ADF) image of Sample 1. Interface is broken up. No signs of inter-diffusion TEM bright field image of Sample 2. A well defined, 5nm wide, inter-diffusion layer can be seen.

  23. Probing the Inter-Diffusion Layer Probing beam located in Cr, interface, 1737 glass

  24. Probing the Inter-Diffusion Layer Annular Dark Field (ADF) image of evaporated Cr film on glass. Relative composition changes across the diffusion layer. All data are integrated from EELS spectra.

  25. Probing the Inter-diffusion Layer EELS spectra of Cr-L, Si-L and O-Kedge across interface

  26. Probingthe Inter-diffusion Layer Ba segregates to Interface Ca segregates to Interface

  27. Proposed Mechanism Cr displaces alkaline earth metal, which moves to interface. Atom arrangements in glass deduced from ELNES.

  28. Probing the Bonding State of Cr Cr L3 and L2 absorption peaks are due to the excitations of the 2p electrons (2p3/2 and 2p1/2) to unoccupied bound 3d states. Cr L3,, L2 edge position shifts to higher energies in the inter-diffusion layer. For the interpretation of this finding visit our poster !

  29. Probing the State of Oxygen a Oxygen K-edge due to Cr b Oxygen K-edge due to Si

  30. Analyzing the State of Oxygen • SiO2 O K-edge (tetrahedrally) coordinated Si) strongly resembles 1737’s O K-edge • Hybridized Cr d-O p anti-bonding states (octahedrally coordinated Cr) • Corner-sharing tetrahedra having alternately Cr and Si at the centers DFT-LDA calculations of of Op-projected DOS of SiO2, Cr2SiO4, and Cr2O3 by Neaton and Ashcroft

  31. Substrate Thin Film Interactions

  32. Thin Film Electronic on Glass Poly-Si directly contacts glass - high potential for interaction

  33. Advanced Thin Film Electronics Ast, Lam, CNF Tang, Fleming, Williams Finished OLED Display Active Matrix: two 32x32 displays

  34. Preparing Glass for Si Deposition RCA cleaning process†: • Base solution of NH4OH:H2O2:H2O at 75 °C [removes organics] • H2O rinse to 6 Mcm • Acid solution of HCl:H2O2:H2O at 75 °C [removes metals & particles] • H2O rinse to 16 Mcm • All glass used for thin film deposition is cleaned prior to use • Physics of cleaning is very little understood • The most demanding applications use the ‘RCA Clean’† to clean glass † W. Kern and D. A. Poutinen, RCA Rev. 31 187 (1970).

  35. RCA Clean: Better TFTs - Why ? • TFT has lowest leakage current on 120 min RCA cleaned glass • TFT has highestleakage current on oxidized Si control wafer Ast, Couillard (CNF)

  36. Data X-ray Reflectivity of RCA Cleaned Glass Modulation at 0.1 Å-1 is due to interference from reduced density surface layer. Fit to 3-layer model Surface layer: 60 ± 5 Å thick 78 ± 3% bulk density Surface roughness unchanged X-ray Reflectivity: 60 min. RCA Clean Blakely, Umbach, Headrick CHESS

  37. STEM Image of Cleaned Glass Silcox, Jiang, STEM Facility Reduced electron density near surface layer, 60 Å thick

  38. Properties of Near Surface Layer • X-ray  electron density, thickness (Blakely, Umbach, Headrick - CHESS) • STEM  electron density, thickness, SiO2 rich (Silcox, Jiang - STEM Facility) • TOF SIMS, XPS, ICP  SiO2, B, Al, Sr, Mg, Na, Ca, B  (Fehlner, Couillard, Ast, Xerox) • I-V of TFTs  acts like deposited SiO2, must influence Na transport (Ast, Couillard, Fehlner)

  39. Novel method to evaluate Na - 22 diffusion across a a very thin layer by analyzing a much wider profile. Initial state: Sample 1 contains a well defined Na-22 distribution, sample 2 with with a near surface layer (“barrier” ) is tracer free. Experiment: Sample 1 and 2 are clamped together; annealed Analysis: Profile in sample 2 yields diffusivity of Na-22 across barrier.

  40. Na-22 Profile Diffusion of Na-22 tracer slows with increasing thickness of surface layer from RCA clean

  41. Analysis Residual radioactivity for different resistances (calculated, integrated across profile) Na-22 profiles for different interfacial resistances (calculated)

  42. Additional complication • To maintain electrical neutrality during the diffusion of Na+, an other charged species must move, such as H+ or OH-.

  43. H2O - Na-22 interaction (I) • D*Na in fused quartz, OH- < 5 ppm, is greater than in 1737 • D*Na in fused silica, OH- < 1000 ppm, is smaller than in 1737

  44. Na diffusion into dry, wet glass Residual radioactivity as a function of depth in 1733 glass in which Na-22 was diffused in wet air (80 C bubbler) Analysis Different D values in near surface (ns) and bulk (bu) regions Thickness of near surface layer increases with time as l = kpt

  45. Correlation with OH- by IR OH- IR Absorption Follows parabolic time dependence expected for diffusion controlled process. Estimated average OH concentration in near surface layer is 4060 ppm, compared to bulk concentration of 330 ppm

  46. Results D (650 °C) Na, RCA Layer = 3.3 10-15 cm2/s D (650 ° C) CVD SiO2 = 1.4  10-14 cm2/s

  47. 50 Å Resolving the Glass Surface UHV STM of Si(111) 7x7 reconstruction [imaged at Cornell] • JEOL JSPM 4500 (funded by CCMR) • First in United States • Atomic resolution STM • Atomic resolution non-contact AFM • Ultra-high vacuum • High temperature imaging (up to 500 C) • 10 µm scan range • Bolts on to existing analysis chamber UHV Contact AFM of sputter induced ripples on SiO2 [imaged at Cornell]

  48. Imaging Glass via STM Imparting Transient Conductivity: I) Electron beam deposits charge in trap states and conduction band II) Photoelectron is excited across metal-glass Schottky into conduction band

  49. Future Research Plans • Surface Science of binary glasses - UHV AFM/STM imaging of cleaved surfaces, coupled to theory of binary glasses. (Major Capital Investment) •Adhesion of metal films to glass: Quantitative measurements, coupled with ab initio theory. (Major Personnel Investment) • Interactions of thin films with glass substrates (Materials used by the thin film electronic industry)

  50. (I) Surface Science • In situ cleaving of glass in the AFM/STM - statistics of surface atom distribution, (Blakely) - calculation of electronic states of surface atoms (Ashcroft). • In situ evaporation of sub-mono, mono- and multi- mono layer metal films in the AFM/STM - adhesion, nucleation and initial growth of metal films on glass - STEM spectroscopy (Blakely, Silcox, Ashcroft).

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