Dingemans, G

Goverde, JC

Hoogeland, D-J

Kessels, WMM

Keuning, W

Langereis, E

Potts, SE

Van de Sanden, MCM

Van den Elzen, LRJG

English

UCL Discovery

Low Temperature Plasma-Enhanced ALD of Metal Oxide Thin FilmsStephen E. Potts, L. R. J. G. van den Elzen,G. Dingemans, E. Langereis, W. Keuning,J. C. Goverde, D. Hoogeland,M. C. M. van de Sanden and W. M. M. Kessels216th ECS Meeting, Vienna, Austria6th October 2009The research leading to these results hasreceived funding from the EuropeanCommunity’s Seventh FrameworkProgramme (FP7/2007-2013) under grantagreement number CP-FP213996-1.1/ Applied Physics / Plasma & Materials ProcessingOutline• The ALD temperature window• Why low temperature plasma-enhancedALD?• ALD Reactors• Film characterisation• spectroscopic ellipsometry and Rutherford backscattering• Overview of low temperature plasma-enhanced ALD of metal oxides• Comparison with thermal procedures and literature• Growth per cycle• Atoms deposited per cycle• Film composition• ConclusionsAl2O3TiO2Ta2O52/ Applied Physics / Plasma & Materials ProcessingALD Temperature WindowSubstrate Temperature →Growth per Cycle →The ALD Temperature Window• Assumption: a sub-monolayer of material is deposited• Loss of surface groups with increasing temperatureABCDABCDA. CondensationB. Insufficient thermal energyC. CVDD. EvaporationOH OH OH2OΔTSubstrate/film surfaceT. Suntola, Mater. Sci. Rep., 4, 261 (1989).3/ Applied Physics / Plasma & Materials ProcessingWhy Low Temperature ALD?• Some applications require high film quality but the substrates required are temperature-sensitive.• Dense, defect-free films required.• Organic substrates• Moisture permeation barriers in OLEDs• Thin film transistors• Metals (or polymers) requiring acorrosion-resistant barrier layer• Higher temperatures can alter the metal’smechanical propertiesCoating metal substrates at TU/eOLEDs at TU/e4/ Applied Physics / Plasma & Materials ProcessingLow Temperature ALD in the LiteratureMaterial Metal Precursor Oxidant Lowest Ts (°C) ReferenceAl2O3[Al(CH3)3] H2O 33 Groner et al.[Al(CH3)3] O3 25 Kim et al.[Al(CH3)3] O2 plasma 25 van Hemmen et al.TiO2TiCl4 H2O 100 Aarik et al.TiCl4 H2O2 100 King et al.[Ti(OiPr)4] H2O 150 Ritala et al.[Ti(OiPr)4] H2O2 77 Liang et al.Ta2O5TaCl5 H2O 80 Kukli et al.[Ta(NMe2)5] H2O 150 Maeng et al.[Ta(NMe2)5] O2 plasma 100 Heil et al.PtOx[Pt(acac)2] O3 120 Hämäläinen et al.[Pt(CpMe)Me3] O2 plasma 100 Knoops et al.ZnO[Zn(CH2CH3)2] H2O 60 Guziewicz et al.[Zn(CH2CH3)2] H2O2 25 King et al.For full references, see S. E. Potts et al., ECS Trans., 25, 233 (2009).5/ Applied Physics / Plasma & Materials ProcessingWhy Plasma-Enhanced ALD?• Gas ionised by electrical energy• Ions• Electrons• Neutral species including radicals• Radicals react with surface groups• Ion energy and ion flux  surface ion bombardment• Can lead to denser films• Increased reactivity• Extension of temperature window down to room temperature?W. M. M. Kessels et al., ECS Trans., 3, 183 (2007).O2 plasma6/ Applied Physics / Plasma & Materials Processing• p-type Si{100} substrates• Diagnostics• Film thickness:− Spectroscopic ellipsometry (SE)• Film composition− RBS and ERD (H)Experimental Details (Plasma & Thermal ALD)Remote Plasma ALD ReactorsSpectroscopic ellipsometerICP plasma source (O2)Gate valveTurbo pumpLiquid/solid precursor lines with ALD valvesSubstrateholderALD-I(home-built)FlexALTM OpALTME. Langereis et al., J. Phys. D: Appl. Phys., 42, 073001 (2009).7/ Applied Physics / Plasma & Materials Processing1. [Al(CH3)3]TMA2. [Ta(NMe2)5]PDMAT3. [Ti(OiPr)4]TTIP4. [Ti(CpMe)(OiPr)3]5. [TiCp*(OCH3)3]Plasma-Enhanced ALD of Metal Oxides• Measured using in situ SE• No nucleation delay• Slope gives growth per cycle for the processTs = 100 °C0 100 200 300 400 50001020304050  Thickness (nm)ALD Cycles1. Al2O32. Ta2O5   Ta2O5 3. TiO24. TiO25. TiO2H3C AlCH3CH3Me2N TaNMe2NMe2NMe2NMe2TiiPrO OiPrOiPrTiiPrO OiPrOiPrOiPrCH3TiMeO OMeOMeH3CCH3CH3H3C CH38/ Applied Physics / Plasma & Materials ProcessingAl2O3: Growth per Cycle[], [] J. L. van Hemmen et al., J. Electrochem. Soc. 154, G165 (2007).[] M. D. Groner et al., Chem. Mater., 16, 639 (2004).[] S. K. Kim et al., J. Electrochem. Soc., 153, F69 (2006).Plasma-enhanced ALD gives the higher growths per cycle at low deposition temperatures.• Water processes: lower growths per cycle at low temperatures• Ozone process: many extra surface groups at Ts < 100 °C.• Reduction in growth per cycle with increasing Ts dehydroxylation.0 50 100 150 200 250 3000.000.050.100.150.20FlexALTM: Literature: Plasma-enhanced  Thermal - H2O Thermal - H2O  Thermal - O3  Growth per Cycle (nm)Substrate Temperature (°C)9/ Applied Physics / Plasma & Materials ProcessingAl2O3: Film Composition• [C] < 1 at.% in each case.• –OH is prominent at lower temperatures.• Leads to increasing mass density of the films with deposition temperature.• No significant composition difference between plasma and thermal ALD.0 50 100 150 200 250 3001.21.41.61.82.02.22.40 50 100 150 200 250 3000246810121416180 50 100 150 200 250 3002.02.22.42.62.83.03.2   FlexALTM: Plasma-enhanced Thermal - H2O[O]/[Al] RatioIdeal ratiofor Al2O3Bulk Al2O3  [H] (at.%)ERD detection limit  Mass Density (g cm-3)Substrate Temperature (°C)10/ Applied Physics / Plasma & Materials ProcessingAl2O3: Al Atoms Deposited• GPC decreases with increasing substrate temperature for plasma ALD.• This is less apparent when focussing on the Al atoms deposited per cycle.• Thermal H2O process deposits fewer atoms below optimisation temperature.H3C AlCH3CH3TMA0 50 100 150 200 250 300 350 4000.000.050.100.150.200 50 100 150 200 250 300 350 400024Plasma-enhanced: Thermal: OpALTM  OpALTM FlexALTM  FlexALTM  Growth per Cycle (nm)  Al Atoms Deposited(×1014 atoms/cycle)Substrate Temperature (°C)11/ Applied Physics / Plasma & Materials ProcessingWaterWater (TU/e)OzoneOxygen Plasma (TU/e)0 20 40 60 80 100 120 140 160 180 200  Time (s)1.  TMA Dose2.  TMA Purge3.  Oxidant Dose4.  Oxidant PurgeAl2O3: Cycle TimeLower deposition temperatures require a longer oxidant purge for thermal processes.[2][3][1][1][1] L.R. J. G. van den Elzen, G. Dingemans et al., work to be published (2009).[2] S. K. Kim et al., J. Electrochem. Soc., 153, F69 (2006).[3] M. D. Groner et al., Chem. Mater., 16, 639 (2004). OxidantTs = 25 °CTs = 33 °CTs = 100 °CTs = 177 °CTs = 33 °C12/ Applied Physics / Plasma & Materials ProcessingTiO2: Growth per CyclePlasma-enhanced ALD: higher growth per cycle• Dehydroxylation with increasing Ts.• Use of alkoxy-based precursors  no chlorine in final film.• [Ti(OiPr)4] + water process: very low growth per cycle• TiCl4 process: etching at Ts = 150-175 °C.[] W. Keuning et al., work to be published.[] E. Langereis et al., work to be published.[] S. E. Potts et al., work to be published.[] M. Ritala et al., Chem. Mater., 5, 1174 (1993).[] J. Aarik et al., J. Cryst. Growth, 220, 531 (2000).0 50 100 150 200 250 3000.000.020.040.060.08 [Ti(OiPr)4] [Ti(CpMe)(OiPr)3] [TiCp*(OMe)3] [Ti(OiPr)4] + H2O TiCl4 + H2O  Growth per Cycle (nm)Substrate Temperature (°C)13/ Applied Physics / Plasma & Materials Processing0 50 100 150 200 250 3000.000.020.040.060.080 50 100 150 200 250 3000.00.51.01.52.02.5 Growth per Cycle (nm)  [Ti(OiPr)4] [Ti(CpMe)(OiPr)3] [TiCp*(OMe)3]Ti Atoms Deposited(×1014 atoms/cycle)Substrate Temperature (°C)• All 3 Ti precursors: same film composition for Ts ≥ 50 °C • [O]/[Ti] = 2 (TiO2)• [C] < 1 at.%.• [H] below detection limit at Ts ≥ 50 °C (0.1-5%)• –OH groups only seen atTs = 25 °C for [Ti(OiPr)4]• Thermal route [H] ~0.3 at.%TiO2: Film Composition & Ti Atoms DepositedReduction in GPC but Ti atoms deposited per cycle are almost constant for Cp precursors.TiiPrO OiPrOiPrTiiPrO OiPrOiPrOiPrCH3TiMeO OMeOMeH3CCH3CH3H3C CH314/ Applied Physics / Plasma & Materials ProcessingTa2O5: Growth per Cycle[] S. B. S. Heil et al., J. Vac. Sci. Technol. A, 26, 472 (2008).[] S. E. Potts et al., work to be published.[] W. J. Maeng et al., J. Vac. Sci. Technol. B, 24, 2276 (2008).[] W. J. Maeng and H. Kim, Electrochem. Solid-State Lett., 9, G191 (2006).[] K. Kukli et al., Thin Solid Films, 260, 135 (1995).• [O]/[Ta] Ratio:• ALD-I = 2.5• FlexAL = 2.5-3• Lit. PDMAT = 2.6• Lit. TaCl5 = ~2 ± 0.1• [C] and [N] < 1 at.% in all cases for PDMAT• [H] detected but < 5 at.% (ALD-I)• From TaCl5 [Cl] up to 6 at.%• Difference in GPC due to different reactors?0 50 100 150 200 250 3000.000.050.100.150.200.250.30O2 Plasma: Thermal H2O: PDMAT (ALD-I)  PDMAT (lit.) PDMAT (FlexAL)  TaCl5 (lit.) PDMAT (lit.)  Growth per Cycle (nm)Substrate Temperature (°C)15/ Applied Physics / Plasma & Materials ProcessingTa2O5: Film Composition & Ta Atoms Deposited• Drop in growth per cycle with increasing temperature.• Number of Ta atoms deposited reasonably constant for Ts = 50-200 °C.• A big increase in Ta deposited at 250 °C• decomposition of PDMAT = CVD• H decreases with increasing substrate temperature.Me2N TaNMe2NMe2NMe2NMe20 50 100 150 200 2500.00.10.20.30 50 100 150 200 250012340 50 100 150 200 2500.00.51.01.52.02.53.03.5  Growth per Cycle (nm)  Ta Atoms Deposited(×1014 atoms/cycle) [O]/[Ta] Ratio [O]/[Ta] RatioSubstrate Temperature (°C)010203040506070 H Concentration [H] (at.%)Ideal ratio for Ta2O516/ Applied Physics / Plasma & Materials ProcessingConclusions• Benefits of plasma processes depend on the application.• Plasma processes can give high quality films at lowertemperatures.• Analysis of atoms/cycle gives additional insight into the ALD temperature window.Al2O3 from [Al(CH3)3]• Higher growth per cycle down to room temperature for the plasma process.• Higher quality films at low temperatures for the plasma process compared with the ozone thermal process.• Al2O3 processes more sensitive to surface dehydroxylation compared with TiO2 and Ta2O5.• Reduced cycle times for the plasma process.Ta2O5 from [Ta(NMe2)5]• Stoichiometric Ta2O5 down to 150 °C.• CVD at 250 °C.TiO2 from [Ti(OiPr)4], [Ti(CpMe)(OiPr)3] or [TiCp*(OMe)3]• Pure, stoichiometric TiO2 down to 50 °C.• Higher growth per cycle than for [Ti(OiPr)4] with water.

Low Temperature Plasma-Enhanced ALD of Metal Oxide Thin Films

Low Temperature Plasma-Enhanced ALD of Metal Oxide Thin FilmsS. E. Potts, L. R. J. G. van den Elzen,G. Dingemans, E. Langereis, W. Keuning,M. C. M. van de Sanden and W. M. M. KesselsBaltic ALD Conference, Uppsala, Sweden15th-16th June 20091/ Applied Physics / Plasma & Materials ProcessingOutline• The ALD temperature window• Why low temperature ALD?• Low temperature ALD in the literature• Why plasma-enhanced ALD?• Experimental details• Overview of low temperature plasma-enhanced ALD of metal oxides: comparison with thermal routes• Al2O3• TiO2• Ta2O5• Conclusions2/ Applied Physics / Plasma & Materials ProcessingALD Temperature WindowSubstrate Temperature →Growth per Cycle →The ALD Temperature Window• Assumption: a sub-monolayer of material is deposited• Loss of surface groups with increasing temperatureABCDABCDA. CondensationB. Insufficient thermal energyC. CVDD. EvaporationOH OH OH2OΔSubstrate/film surface3/ Applied Physics / Plasma & Materials ProcessingWhy Low Temperature ALD?• Some applications require high film quality but the substrates required are temperature-sensitive.• Organic substrates• Organic polymers or small organic molecules• Moisture permeation barriers in OLEDs• Thin film transistors• Metals (or polymers) requiring a corrosion-resistant barrier layer• Higher Ts can alter the metal’s mechanical properties• Dense, defect-free films required• High resistance to wear and/or chemical attack• Al2O3, TiO2, Ta2O5, combinations (stacks)Flexible OLED displaywww.science.eduwww.agcoauto.comCorrosion on gears4/ Applied Physics / Plasma & Materials ProcessingLow Temperature ALD in the LiteratureMaterial Metal Precursor Oxidant Lowest Ts (°C) ReferenceAl2O3[Al(CH3)3] H2O 33 Groner et al.[Al(CH3)3] O3 25 Kim et al.[Al(CH3)3] O2 plasma 25 van Hemmen et al.TiO2TiCl4 H2O 100 Aarik et al.TiCl4 H2O2 100 King et al.[Ti(OiPr)4] H2O 150 Ritala et al.[Ti(OiPr)4] H2O2 77 Liang et al.Ta2O5TaCl5 H2O 80 Kukli et al.[Ta(NMe2)5] H2O 150 Maeng et al.[Ta(NMe2)5] O2 plasma 100 Heil et al.PtOx[Pt(acac)2] O3 120 Hämäläinen et al.[Pt(CpMe)Me3] O2 plasma 100 Knoops et al.ZnO[Zn(CH2CH3)2] H2O 60 Guziewicz et al.[Zn(CH2CH3)2] H2O2 25 King et al.For full references, see S. E. Potts et al., ECS Trans., submitted (2009).5/ Applied Physics / Plasma & Materials ProcessingWhy Plasma-Enhanced ALD?• Gas ionised by electrical energy• Ions• Electrons• Neutral species• Which (re)combine to form radicals• Radicals react with surface groups• Ion energy and ion flux  surface ion bombardment• Can lead to denser films• Increased reactivity• Extension of temperature window down to room temperature?W. M. M. Kessels et al., ECS Trans., 3, 183 (2007).O2 plasma6/ Applied Physics / Plasma & Materials Processing• p-type Si{100} substrates• Diagnostics• Film thickness:− Spectroscopic ellipsometry (SE)• Film composition− RBS and ERD (H)Experimental Details (Plasma & Thermal ALD)Remote Plasma ALD ReactorsSpectroscopic ellipsometerICP plasma source (O2)Gate valveTurbo pumpLiquid/solid precursor lines with ALD valvesSubstrateholderALD-I(home-built)FlexALTM OpALTM7/ Applied Physics / Plasma & Materials ProcessingTMAPDMATTitanium isopropoxideMethylcyclopentadienyl-tris(isopropoxy)titanium1.2.3.4.0 100 200 300 400 50001020304050 1.  Al2O32.  Ta2O53.  TiO24.  TiO2  Thickness (nm)ALD CyclesPlasma-Enhanced ALD of Metal Oxides• Measured using in situ SE• No nucleation delay• Slope gives growth per cycle for the processH3C AlCH3CH3Me2N TaNMe2NMe2NMe2NMe2TiiPrO OiPrOiPrTiiPrO OiPrOiPrOiPrCH3Ts = 100 °C8/ Applied Physics / Plasma & Materials ProcessingAl2O3: Growth per Cycle[], [] J. L. van Hemmen et al., J. Electrochem. Soc. 154, G165 (2007).[] M. D. Groner et al., Chem. Mater., 16, 639 (2004).[] S. K. Kim et al., J. Electrochem. Soc., 153, F69 (2006).Plasma-enhanced ALD gives the higher growths per cycle at low deposition temperatures.0 50 100 150 200 250 3000.000.050.100.150.20FlexALTM: Literature: Plasma-enhanced  Thermal - H2O Thermal - H2O  Thermal - O3  Growth per Cycle (nm)Substrate Temperature (°C)• Water processes: lower growths per cycle at low temperatures• Ozone process: many extra surface groups at Ts < 100 °C.• Reduction in growth per cycle with increasing Ts dehydroxylation.9/ Applied Physics / Plasma & Materials ProcessingAl2O3: Film Composition• [C] < 1 at.% in each case.• –OH is prominent at lower temperatures.• Leads to increasing mass density of the films with deposition temperature.• No significant composition difference between plasma and thermal ALD.1.21.41.61.82.02.22.40246810121416180 50 100 150 200 250 3002.02.22.42.62.83.03.2  FlexALTM: Plasma-enhanced Thermal - H2O[O]/[Al] RatioIdeal ratiofor Al2O3Bulk Al2O3  [H] (at.%)ERD detection limit  Mass Density (g cm-3)Substrate Temperature (°C)10/ Applied Physics / Plasma & Materials ProcessingWaterWater (TU/e)OzoneOxygen Plasma (TU/e)0 20 40 60 80 100 120 140 160 180 200  Time (s)1.  TMA Dose2.  TMA Purge3.  Oxidant Dose4.  Oxidant PurgeAl2O3: Cycle TimeLower deposition temperatures require a longer oxidant purge.[2][3][1][1][1] J. L. van Hemmen et al., J. Electrochem. Soc., 154, G165 (2007).[2] S. K. Kim et al., J. Electrochem. Soc., 153, F69 (2006).[3] M. D. Groner et al., Chem. Mater., 16, 639 (2004). OxidantTs = 25 °CTs = 33 °CTs = 100 °CTs = 177 °CTs = 33 °C11/ Applied Physics / Plasma & Materials ProcessingTiO2: Growth per Cycle[] W. Keuning et al., work to be published.[] E. Langereis et al., work to be published.[] M. Ritala et al., Chem. Mater., 5, 1174 (1993).[] J. Aarik et al., J. Cryst. Growth, 220, 531 (2000).0 50 100 150 200 2500.000.020.040.060.08 [Ti(OiPr)4] [Ti(CpMe)(OiPr)3] [Ti(OiPr)4] + H2O TiCl4 + H2O  Growth per Cycle (nm)Substrate Temperature (°C)Plasma-enhanced ALD: higher growth per cycle• Dehydroxylation with increasing Ts.• Use of alkoxy-based precursors  no chlorine in final film.• [Ti(OiPr)4] + water process: very low growth per cycle• TiCl4 process: etching at Ts = 150-175 °C.12/ Applied Physics / Plasma & Materials ProcessingTiO2: Film Composition• Both precursors: same film composition• [C] < 1 at.%.• [H] below detection limit at Ts ≥ 50 °C• Thermal route [H] ~0.3 at.%0 50 100 150 200 250 3000.00.51.01.52.02.5 [O]/[Ti] RatioSubstrate Temperature (°C)[Ti(OiPr)4]: [Ti(CpMe)(OiPr)3]: [O]/[Ti]  [O]/[Ti] [H]ERD detection limit02468101214 [H] (at.%)Hydroxyl groups only seen at room temperature.13/ Applied Physics / Plasma & Materials ProcessingTa2O5[] S. B. S. Heil et al., J. Vac. Sci. Technol. A, 26, 472 (2008).[] W. J. Maeng et al., J. Vac. Sci. Technol. B, 24, 2276 (2008).[] W. J. Maeng and H. Kim, Electrochem. Solid-State Lett., 9, G191 (2006).[] K. Kukli et al., Thin Solid Films, 260, 135 (1995).• [O]/[Ta] Ratio:• Our films = 2.5• Lit. PDMAT = 2.6• Lit. TaCl5 = ~2 ± 0.1• [C] and [N] < 1 at.% in all cases for PDMAT• [H] detected but < 5 at.%• From TaCl5 [Cl] up to 6 at.%• Difference in growth per cycle due to different reactors?0 50 100 150 200 250 3000.000.050.100.15 [Ta(NMe2)5] + O2 plasma (ALD-I) [Ta(NMe2)5] + O2 plasma (literature) [Ta(NMe2)5] + H2O (literature) TaCl5 + H2O (literature)  Growth per Cycle (nm)Substrate Temperature (°C)14/ Applied Physics / Plasma & Materials ProcessingConclusions• Plasma-enhanced and thermal ALD routes compared• Advantages of plasma-enhanced ALD dependant on process:• Al2O3 from TMA• Higher growth per cycle down to room temperature than the thermal process with water• Higher quality films at low temperatures than the ozone process• Reduced cycle times• TiO2 from [Ti(OiPr)4] or [Ti(CpMe)(OiPr)3]• Pure, stoichiometric films down to 50 °C.• Higher growth per cycle than [Ti(OiPr)4] with water.• Ta2O5 from PDMAT• High purity, stoichiometric films down to 100 °C.

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Low Temperature Plasma-Enhanced ALD of Metal Oxide Thin Films

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