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Non-hydrolytic sol-gel process for thin film electronics

Non-hydrolytic sol-gel process for thin film electronics. Outline. Sol-gel chemistry. 1) Alkoxide sol-gel M-OR (R= Me,Et,Pr,Bu ,…) M-OR + H 2 O → M-OH + ROH (hyd.) M-OH + M-OH → M-O-M (con.) 2) Metal salt sol-gel M-X (X=Cl,CH 3 COO,NO 3 ) M-X + H 2 O → M-OH + HX (hyd.)

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Non-hydrolytic sol-gel process for thin film electronics

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  1. Non-hydrolytic sol-gel process for thin film electronics

  2. Outline Sol-gel chemistry 1) Alkoxide sol-gel M-OR (R=Me,Et,Pr,Bu,…) M-OR + H2O → M-OH + ROH (hyd.) M-OH + M-OH → M-O-M (con.) 2) Metal salt sol-gel M-X (X=Cl,CH3COO,NO3) M-X + H2O → M-OH + HX (hyd.) M-OH + M-OH → M-O-M (con.)

  3. Metal salt route Metal salt route – anion decomposition Metal salts in solution Complex with water 2) Complex with solvent 3) Complex with stabilizer (Chelation) As-deposited film is composed of metal cation complexes, hydroxides, and anions. Anions should be thermally decomposed - chemical bonds form between the cation and oxygen H H H H H H H H H H H H Et Et Et Et Et Et H H H H H H O O O O O O O O O O O O Zn2+ Zn2+

  4. Metal salt chemistry Metal salts are not hydrolyzed in alcohol/water Unlike alkoxides (Hydrolysis can be initiated in basic solution) Journal of Colloid and Interface Science 325 (2008) 459-463 • ex) Spray pyrolysis of aqueous zinc nitrate solution • ← Decrease in mass by 400ºC annealing • Change in molecular weight: • Anhydrous zinc nitrate (189) → Zinc oxide (81) • 57% decrease • 20 min after 400ºC annealing • Film became dark brown (color of N2O4 gas)

  5. Metal salt chemistry Deposition from metal salt solution Forms metal salt thin film By annealing, Dehydration → Decomposition to oxide Hydrolysis can be incorporate into decomposition process or not Hydrolysis of zinc nitrate? Zn(NO3)2 + 2H2O → Zn(OH)2 + 2HNO3 ∆G = 370 + 2·237 - 554 - 2·111 = 68 kJ/mol Then how to hydrolyze zinc nitrate? Zn(NO3)2 + 2NaOH → Zn(OH)2 + 2NaNO3 ∆G = 370 + 2·419 – 554 - 2·372 = -90 kJ/mol

  6. Metal salt route Metal salt route – anion decomposition Anion species determines decomposition temperature – nitrate anion is more volatile - Chloride-based IGO - Nitrate-based IGO

  7. Metal salt chemistry Acid condition TG-DSC : ~250°C – evaporation of the residual solvent 340°C – vaporization/decomposition of the organic component total weight loss 79% FT-IR : 1580,1420,1120,954 cm-1 – carboxylate of zinc acetate In the acetic acid solution (pH 4.7) CH3COOH → H+ + CH3COO- Zn(H2O)62+ + CH3COO-→ Zn(CH3COO)(H2O)5+ Zn(CH3COO)2·xH2O forms during spin-coating as the water evaporates Decomposition of dehydrated zinc acetate At least 450°C annealing is required for TFT operation Acetic acid 0.19cm2/Vs

  8. Metal salt chemistry Base condition TG-DSC : 134°C – evaporation of water, ammonia and dehydration 249°C – No explanation total weight loss 1.4% FT-IR : 1613, 1470 cm-1 – N-H bonds In the ammonia solution (pH >10) Deprotonation : Zn(H2O)62+→ Zn(OH)42- Ammine-hydroxo zinc complex: Zn(OH)x(NH3)y(2-x)+ Decomposition of ammine-hydroxo zinc complex Only 150°C annealing is required for TFT operation Ammonia 2.95cm2/Vs

  9. Metal salt chemistry Combustion process

  10. Alkoxide sol-gel Alkoxide sol-gel – hydrolysis & condensation Electronegativity of metal Silicon is generally less electropositive Less susceptive to nucleophilic attack δ(Si) in Si(OEt)4 = +0.32 δ(Ti) in Ti(OEt)4 = +0.63 δ(Zr) in Zr(OEt)4 = +0.65 δ(M): partial positive charge Hydrolysis is initiated with attack of nucleophile and coordination expansion Acid-catalyzed hydrolysis Base-catalyzed hydrolysis

  11. Alkoxide sol-gel Alkoxide sol-gel – hydrolysis & condensation Hydrolysis: Steric and inductive effects Steric effects Increased length or branch of alkoxy groups → lowered hydrolysis rate Oligomer formation Short branches tend to make alkoxides to form trimer or higher oligomer – reduce reactivity

  12. Alkoxide sol-gel Alkoxide“sol-gel onchip” High reactivity of alkoxide enables “in situ hydrolysis”

  13. Sol-gel on chip Solution stability and Spin-coating Reactivity of alkoxides >> Reactivity of metal salts H2O Alkoxideprecursors with transition metal atom: react with water vapor in air (even in alcohol solution) Spin-coating will result poor coating (Solution contains many oligomers or pricipitates) M(OR)x M(OR)x(OH)y Metal salt precursors have lower reactivity than alkoxide precursors Precursor solution is stable in air (with solution stabilizer) M(OR)x(OH)y

  14. Sol-gel on chip Dilemma • Precursor with high reactivity • Enables low-temperature process • Unstable solution • Precursor with low reactivity • Stable solution • Requires high temperature annealing

  15. Sol-gel on cnip Solution Separation of reactants (precursor and water) ‘Sol-gel on chip’ process: Reactive alkoxide is separated from water until thin film is formed Reaction is initiated after spin-coating Dry ambient Wet ambient, low-temperature annealing

  16. Non-hydrolytic sol-gel Zinc tin oxide Tin oxide has high dehydration temperature, ‘Sol-gel on chip’ process couldn’t obtain highperformances Tin(IV) tert-butoxide

  17. Non-hydrolytic sol-gel Zinc tin oxide Mechanical stability 4) Visible light insensitivity Chemical stability Electrical stability

  18. Non-hydrolytic sol-gel Chemical approach Metal alkoxide sol-gel Metal salt sol-gel Difference in ‘reactivity with water’ influences hydrolysis, but dehydration remains untouched Non-hydrolytic sol-gel process Directly condensate between precursors 1. Ester elimination reaction M-OR + M-CH3COO → M-O-M + R-CH3COO 2. Ether elimination reaction M-OR + M-OR → M-O-M + R-O-R 3. Metal halide elimination reaction M-OR + M-X → M-O-M + R-X

  19. Non-hydrolytic sol-gel Advantages Traditional sol-gel – works well for silica (Slow reaction rate) Transition metal oxide – Fast reaction leads poor control over the size and shape of oxide To control metal oxide growth, 1) Alternative water source 2) Chemical modification of precursor Advantage of non-aqueous route high reproducibility Better composition control Crystal growth control without ligands

  20. Non-hydrolytic sol-gel NHSG for ALD thin-film NHSG can be applied to ALD for deposition temperature reduction Deposition temperature for SnO2: 180~700 °C With ester elimination: 75~200 °C System: Sn(OtBu)4 + CH3COOH Coating on CNT (with non-oxidizing environment)

  21. Experimental Reaction temperature for given system ex) TiO2 deposition Metal oxide Precursor A Precursor B Temperature Condensation step Ref. → Usually ester elimination is advantageous for deposition temperature reduction

  22. Non-hydrolytic sol-gel Zinc tin oxide TFT by nonhydrolytic sol-gel Precursors should be separated to prevent reaction between them Precursor layers were stacked and reacted by annealing

  23. Non-hydrolytic sol-gel Zinc tin oxide TFT by nonhydrolytic sol-gel Performances were achieved with low-temperature annealing nonhydrolytic sol-gel

  24. Electron Devices Meeting (IEDM), 2011 IEEE International ZrO2 and interface Current working ZrO2 dielectric for higher mobility (Processing temperature: 450°C) SnO2 TFT on SiO2– 5.0 cm2/Vs SnO2 TFT on ZrO2– 103.0 cm2/Vs

  25. Electron Devices Meeting (IEDM), 2011 IEEE International ZrO2 and interface • Current working • Low-temperature CdS TFT case Direct condensation between thioacetic ligands: Enables 300 °C process ZrO2 also processed by alkyl halide elimination process Mobility on ZrO2: 48 cm2/Vs Mobility on PECVD SiO2: 5 cm2/Vs “ZrO2thin films induce the more effective charge accumulation at the channel region of the CdS”

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