Non-hydrolytic route to low-temperature processed sol-gel ZTO TFT

1. Non-hydrolytic route to low-temperature processed sol-gel ZTO TFT

2. Low temperature sol-gel High reactivity of precursor = Low temperature gelation 150°C low temperature Excess ammonia Zinc hydroxide precipitate Stable solution “Only condensation is required” after spin-coating → Minimize post-annealing temperature

3. Low temperature sol-gel High reactivity of precursor = Low temperature gelation High reactivity of precursor ↔ Solution moisture stability “Trade-off” This dilemma overcame by “Sol-gel on chip” process Sn δ+ Large positive charge → Susceptive to nucleophillic attack “High reactivity of alkoxide precursor”

4. Precursor reactivity Reactivity of alkoxide Electronegative alkoxy group = electropositive metal atom -> highly prone to nucleophilic attack Nucleophilic addition of the XOH group into the positively charged metal atom Proton transfer, within the transition state M(OR)n(XOH) from the entering molecule to the leaving alkoxy group Departure of the positively charged protonated species Metal atom M and the leaving group ROH have to be positively charged

5. Precursor reactivity Zinc tin oxide TFT <<Advantages>> “High DC bias stress stability” “Low sensitivity toward visible light” 400°C High processing temperature “Low reactivity of tin precursor” Hydrolysis rate of tin alkoxide precursor Less steric side group (isopropoxide) but trimer formation Tin tert-butoxide is best precursor for fast hydrolysis

6. Precursor reactivity Hydrolysis rate of tin alkoxide precursor Trimeric tin isopropoxide – “less reactive” (Metal atom coordination is occupied by molecular bonding) Monomeric tin tert-butoxide– “high reactivity” Indium oxo-cluster – ???

7. Precursor reactivity Condensation rate of tin alkoxide precursor High temperature is required to complete reaction: ≡Sn-OH + Sn-OH → ≡Sn-O-Sn≡ + H2O “Stannic acid formation” Limit of general hydrolytic sol-gel process for ZTO TFT

8. Non-hydrolytic route Hydrochloric acid elimination route to sol-gel SnO2 Tin chloride is generally used (Spin-coating,CVD,ALD) 0.5O2(g)+ SnCl2+ H2O(l)→ SnO2+ 2HCl(g) → 510°C is required to obtain SnO2 SnCl4(l)+ 2H2O(l)→ SnO2 + 4HCl(g) → 180°C is required to obtain SnO2 “High reactivity of tin(IV) chloride” Low-temperature approaches were usually based on organometallic precursor Precursors that are volatile, flammable → cannot be spin-cast

9. Non-hydrolytic route Ester elimination route to sol-gel SnO2 Tin tert-butoxide + acetic acid Sn(tBu)4+ HOAc(g) Low oxidative character of carboxyl acids Low temperature (from 75°C) High quality and purity ALD (at 200°C) -SnO2 film very low content of carbon residue →

10. Non-hydrolytic route Reaction between metal alkoxide compounds and carboxylic acids Modifying the reactivity of alkoxides 1) TEOS + HOAc Positively charged HOR group – good leaving group Si(Oac)4 hydrolyze faster then Si(Oet)4 Gelation time of TEOS – 1000h with HCl – 92h with HOAc – 72h

11. Non-hydrolytic route Reaction between metal alkoxide compounds and carboxylic acids Modifying the reactivity of metal alkoxide 2) Ti(OBun)4 Ti(OBun)4 + AcOH – Stabilization of solution i) Alkoxide substitution – positively charged alcohol will be removed ii) Hydrolysis of this new precursor Acetate groups are not immediately removed through hydrolysis or condensation → Functionality of precursor is decreased, slower gelation

12. Non-hydrolytic route Reaction mechanism Nucleophilic attack of an hydroxo group of carboxyl acid Which one will be attacked? Metal atom Alkoxyl group x≤1 : Chemical modification x≥2 : Hydrolysis (Atom with highest charge is attacked)

13. Non-hydrolytic route Reaction mechanism Chemical modification until x=2 Hydrolysis and ligand exchange

14. Non-hydrolytic route 3) Polymerization Intramolecular esterification Extramolecular esterification ROH + AcOH → ROAc + H2O (Excluded in ALD system)

15. Non-hydrolytic route ALD process can be regarded as a model system for the mechanistic study of the Lewis acid catalysis in the esterification reaction by metal salts Possible mechanisms A) ≡M-OR’ + RCOOH → M-OH + RCOOR’ - slow ≡M-OH + M-OR’ → ≡M-O-M≡ + R’OH - fast B) ≡M-OR’ + RCOOH → M-OOCR + R’OH - fast ≡M-OOCR + M-OR’ → ≡M-O-M≡ + RCOOR’ - slow in ALD process, Alkoxide adsorption speed <<<<<< Acetic acid adsorption speed → Supports B mechanism C) ≡M-OR + M-OR’ → ≡M-O-M≡ + R’OR (Ether elimination) Carboxyl acid is essential No ether was detected by NMR and gas chromatography Self-limiting growth was observed

16. Carboxyl acid effect ALD-HfO2 with non-hydrolytic sol-gel process Effect of carboxyl acid Formic acid Higher reactivity Lower temperature stability HCOOH → CO + H2O (from ~150°C) Acetic acid Lower reactivity Do not decompose until 1000°C Acetic acid is suitable at general post-annealing temperature

17. Solvent effect Solvent effect on tin butoxide 1) Tin butoxide + Acetic acid in pyridine ← Pyridine ligand occupy vacant coordination of alkoxide Lose reactivity and stabilization of solution 2) Tin butoxide + Acetic acid in toluene Fast gelation

18. Solvent effect For the successful ester elimination reaction, – A non-coordinating solvent – An electropositive metal alkoxide – Vacant coordination sites on the metal alkoxidecenter – A non-chelating carboxylate ligand “Easier nucleophillic attack”

19. Conclusions • SnO2 can be prepared at low-temperature by nonhydrolytic sol-gel route • Reaction mechanism: M-O-M bonding by esterification • Future work • Sol-gel on chip - Acetic acid addition in steam annealing • New aqueous sol – Sn(OAc)(OBu) precursor • Surface sol-gel – (Alkoxide adsorption – washing – Acetic acid) cycle