This document provides an overview of atomic layer deposition (ALD), including its applications, deposition process, materials used, benefits, and characteristics. ALD involves sequential, self-limiting surface reactions to deposit thin, conformal films one atomic layer at a time. It can be used to deposit a wide variety of materials, such as oxides, nitrides, and metals. ALD enables highly uniform coatings on high aspect ratio structures down to the nanoscale.

1. Cambridge NanoTech ALD TutorialJuly 2011

2. ALD ApplicationsOther applicationsRoll to rollInternal tube linersNano-glueBiocompatibleMagneticChemicalCatalysisFuel cellsSemi / NanoelectronicsFlexible electronicsGate dielectricsGate electrodesMetal InterconnectsDiffusion barriersDRAMMultilayer-capacitorsRead headsMEMSEtch resistanceHydrophobic / antistictionOpticalAntireflectionOptical filtersOLED layersPhotonic crystalsTransparent conductorsElectroluminescenceSolar cellsLasersIntegrated opticsUV blockingColored coatingsNanostructuresInside poresNanotubesAround particlesAFM tipsWear resistantBlade edgesMolds and diesSolid lubricantsAnti corrosion

3. ALD Films- ALD films deposited with digital control of thickness; “built layer-by layer” - Each film has a characteristic growth rate for a particular temperatureCommon ALD MaterialsALD Deposition Rates at 250°COxidesAl2O3, HfO2, La2O3, SiO2, TiO2, ZnO, ZrO2, Ta2O5, In2O3, SnO2, ITO, FeOx, NiO2, MnOx, Nb2O5, MgO, NiO, Er2O3NitridesWN, Hf3N4, Zr3N4, AIN, TiN, TaN, NbNxMetalsRu, Pt, W, Ni, CoSulphidesZnS1.26 Å1.08 Å0.38 Å

4. Benefits of ALDPerfect filmsDigital control of film thicknessExcellent repeatability100% film densityAmorphous or crystalline filmsConformal CoatingExcellent 3D conformalityUltra high aspect ratio (>2,000:1)Large area thickness uniformityAtomically flat and smooth coatingChallenging SubstratesGentle deposition process for sensitive substratesLow temperature and low stressExcellent adhesionCoats challenging substrates – even teflon

5. ALD Reaction SequenceALD is based on the spatial separation of precursorsA single ALD cycle consists of the following steps:1) Exposure of the first precursor2) Purge or evacuation of the reaction chamber to remove the non-reacted precursors and the gaseous reaction by-products3) Exposure of the second precursor – or another treatment to activate the surface again for the reaction of the first precursor4) Purge or evacuation of the reaction chamberSingle CyclePrecursor APurgePrecursor BPurgeTime

6. ALD Example Cycle for Al2O3 Deposition Tri-methylaluminumAl(CH3)3(g)Methyl group(CH3)AlHCHHHOSubstrate surface (e.g. Si)In air H2O vapor is adsorbed on most surfaces, forming a hydroxyl group. With silicon this forms: Si-O-H (s)After placing the substrate in the reactor, Trimethyl Aluminum (TMA) is pulsed into the reaction chamber.

7. Methane reactionproduct CH4HReaction ofTMA with OHHCHHHHCCHHHAlOSubstrate surface (e.g. Si)Al(CH3)3 (g) + : Si-O-H (s) :Si-O-Al(CH3)2(s) + CH4ALD Cycle for Al2O3Trimethylaluminum(TMA) reacts with the adsorbed hydroxyl groups,producing methane as the reaction product

8. ALD Cycle for Al2O3Methane reactionproduct CH4Excess TMAHHCCHHAlOSubstrate surface (e.g. Si)Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl groups,until the surface is passivated. TMA does not react with itself, terminating the reaction to one layer. This causes the perfect uniformity of ALD.The excess TMA is pumped away with the methane reaction product.

9. ALD Cycle for Al2O3H2OOHHHHCCHHAlOAfter the TMA and methane reaction product is pumped away, water vapor (H2O) is pulsed into the reaction chamber.

10. 2 H2O (g) + :Si-O-Al(CH3)2(s) :Si-O-Al(OH)2(s) + 2 CH4ALD Cycle for Al2O3Methane reaction productNew hydroxyl groupMethane reaction productOxygen bridgesHOOAlAlAlOH2O reacts with the dangling methyl groups on the new surface forming aluminum-oxygen (Al-O) bridges and hydroxyl surface groups, waiting for a new TMA pulse. Again metane is the reaction product.

11. ALD Cycle for Al2O3HOOOAlAlAlOThe reaction product methane is pumped away. Excess H2O vapor does not react with the hydroxyl surface groups, again causing perfect passivation to one atomic layer.

12. HHHOOOOOAlAlAlOOOOOAlAlAlOOOOOAlAlAlOOOAl(CH3)3 (g) + :Al-O-H (s) :Al-O-Al(CH3)2(s) + CH4ALD Cycle for Al2O3One TMA and one H2O vapor pulse form one cycle. Here three cycles are shown, with approximately 1 Angstrom per cycle. Two reaction steps in each cycle: 2 H2O (g) + :O-Al(CH3)2(s) :Al-O-Al(OH)2(s) + 2 CH4

13. ALD Deposition CharacteristicsALD is insensitive to dose after saturation is achievedDeposition rate remains unchanged with increasing doseMgO Saturation Curve at 250°CLinear MgO Deposition

14. ALD “Window”Each ALD process has an ideal process “window” in which growth is saturated

15. Process parameters inside the ALD window allowfor reliable and repeatable results

16. The ALD window is defined by the precursor volatility / stabilityDecomposition limitedCondensation limitedGrowth RateÅ/cycleALDWindowSaturationLevelTemperatureDesorption limitedActivation energy limited

17. ALD Reaction TemperaturesALD is a chemistry driven processBased on precursor volatility/reactivityMost ALD ProcessesReactor Temp>400°C250°C300°C150°C150°CRoom T200°C100°C350°CHigh precursor volatility, lower thermal stability of precursorsLower precursor volatility, Slow desorption of precursors

18. High Aspect Ratio CoatingsALD is uniquely suited to coat ultrahigh aspect ratio structures enabling precise control of the coatings thickness and composition. Cambridge NanoTech’s research systems offer deposition modes for ultra high aspect ratio (>2,000:1)“Capillary tube”Cross Sectional SEMAAO template**Image courtesy of the University of Maryland

19. Compositional UniformityRefractive Index – EllipsometryCross sectional EDX 2.1042.1032.1012.1012.1052.1022.1042.1042.1012.1032.1032.1032.0992.1012.104Al2O3 Silica aerogel foamTa2O5 - 500Å film

20. ALD Precursors Good ALD precursors need to have the following characteristics: Volatility Vapor pressure (> 0.1Torr at T < 200°C) without decomposition Stability No thermal decomposition in the reactor or on the substrate Reactivity Able to quickly react with substrate in a self-limiting fashion (most precursors are air-sensitive) Byproducts Should not etch growing film and/or compete for surface sites Availability Precursor cylinders

21. Plasma Enhanced (PE)ALDRemote Plasma as a reactant

22. Expands ALD window for materials by decreasing activation energy

23. Lower temperature possible: avoids precursor decomposition