Atmospheric Pressure Atomic layer deposition (AP – ALD) Course: Thin Film Technology (Fall 2014) Name: René Kirrbach

Outline Introduction AP-ALD Process Conclusion Questions References

1. Introduction ALD can provide precise coatings with high conformity But: Conventional ALD is very slow & requires low pressure (0.1-2.0 Tor) Atmospheric Pressure – ALD don’t need vacuum Process becomes faster, cheaper & can easier integrated New, cheaper applications like flexible electronics, organic solar cells printing, smart textiles [1] - ALD can operate at low temperatures This figure is modified and was taken from [1]

1. Introduction Challenges: Common deposition materials: Al2O3, ZnO, … Precursors must delivered to a system with a higher pressure than the vapor pressure Mixing of precursors must be avoided Not much knowledge about: design parameters like gas flow rates, reactor geometry, … How effects the system pressure surface reaction processes Common deposition materials: Al2O3, ZnO, … - For instance, ZnO is used as anti-dirt coating / get a hydrophobic surface

2. Process Flow Basic process flow is the same as in conventional ALD processes (see picture)  precursors are sequentially transported to the reaction zone, separated by purging steps  half-reactions are self-limiting Several approaches reactor type: Flow tube reactor (ordinary ALD reactor) Spatial ALD - Conventional ALD: precursor and co-reactant species transported sequentially into reaction zone (heated) containing the substrate leads to two time-separated halfreaction Steps; remaining vapor products are pumped or pushed out of the deposition zone using inert gas flow; halfreactions are self-limiting Exemplary process Flow for Al2O3. This was taken from [4].

Roll-to-Roll deposition device 2. Process Flow Pre-cursor 1 Purge (Inert gas) Substrate Pre- cursor 2 Roll-to-Roll deposition device Spatial ALD: Either the substrate is moving (a), or the deposition device, or both (b) Size of the Unit cells (+ barriers) and the relative velocity determines the time of cycle High process speed, because no chamber has to be evacuated (a)The figure shows the principle structure of an spatial ALD approach. Deposition device has to be in close proximity to substrate http://books.google.fi/books?id=9WyXTae58DgC&pg=PA436&lpg=PA436&dq=close+proximity+ald&source=bl&ots=Vl2RhBt7PC&sig=GbHkCRto-MZiDFc1aOOKH5WcP00&hl=de&sa=X&ei=2EVvVJKlGMniywPM1ILgAg&ved=0CCQQ6AEwAQ#v=onepage&q=close%20proximity%20ald&f=false (b) The figure shows a circular structured deposition device in a Roll-to-Roll system.

2. AP-ALD Process Process properties: Temperatures between 50- 250°C, typically 100°C for ZnO Growth per cycle typically higher at higher pressure (factor ~1.5-2 with Al2O3) due to a change of absorption/desorption kinetics of precursor and reactant on samples surface for instance excess of water on the sample resulting a CVD component in film growth Temperature dependence of the growth per cycle for deposition of Al2O3 and ZnO for a normal ALD (blue) and an AP-ALD (red) process. The graph was taken from [5]. General: temperature must be low enough so that the precursor doesn’t decompose during surface adsorption, but high enough to thermally activate the reaction [2]  “temperature window” Viscosity increases with temperature  Reynolds number is decreasing ( go for a laminar gas flow) In the near of the substrate diffusive transport dominates; diffusion decreases linear pressure; described by the Peclet-number; At high pressure in the middle of the flowing gas, transport relies on convection, not anymore on diffusion Al2O3 is a factor of ∼1.5-2 larger than at low pressure. [1]  Possible explanation: High TMA exposure may result a change in he adsorption and desorption kinetics of the precursor and reactant on the sample surface (for instance higher enthalpy for water desorption from AL-OH—H20 versus Zn-OH—H2O will leave excess water on the surface  resulting a higher than expected growth rate)  and slow water desorption may result in a CVD component to film growth; other possibility: ALD “temperature window” may depend on deposition pressure ALD / CVD growth depend on purge time, pressure and gas flow rate of purge gas in a flow-tube reactor (no spatial ALD) [5].

3. Conclusion AP-ALD is cheap & fast, because of the lack of vacuum  ALD becomes interesting for cheap applications (e.g. smart textiles & printed electronics) Spatial ALD is a realization approach for ALD that allows fast processing, especially for Roll-to-Roll systems Based on relative movement of samples surface & periodically structured depositions device (Precursor 1 – Inert gas – Precursor 2 – Inert gas - …) High pressure can lead to CVD component, that increases film growth  Optimization of process parameters (e.g. gas flow, purge time, …) necessary

4. Questions?

5. References [1] G. N. Parsons, J. S. Jur, “Continuous Atmospheric Pressure Atomic Layer Deposition Process for Controlled Nanoscale Thin Film Coatings”, ACS Appl. Mater. Interfaces, 2011, 3 (2), poster, pp. 299- 308. [2] G. N. Parsons, S. M. George, M. Knez, “Progress and future directions for atomic layer deposition and ALD-based chemistry”, MRS Bulletin, Vol. 36, Nov. 2011. [3] Q. P., Parsons, G.N. et al. Nano Letters, 7, 719 (2007) [4] “ROLL-TO-ROLL SPATIAL ALD”, online, 23.11.2014, available at: http://tmcporch.com/ [5] J. S. Jur, G. N. Parsons, “Atomic Layer Deposition of Al2O3 and ZnO at Atmospheric Pressure in a Flow Tube Reactor”, ACS Appl. Mater. Interfaces, 2011, pp. 299–308.