How can ALD throughput be increased? How can ALD throughput be increased? Mikhail Erdmanis

Outline 1.ALD reactor types 2.What defines ALD throughput 3.Saturation character of ALD growth 4.The ways to increase the grows rate 5.Problems which appear while increasing the growth rate 6.ALD coating of high aspect ratio structures 7.Batch ALD reactors 8.How can ALD throughput be increased 9.Conclusion

1. ALD reactor types Diffusion reactor (t cycle = 1 min) The reactants are exposing without using a carrier gas. After the exposures, the reactants are removed by opening up completely to the pump and evacuating the reactor. Because of the long residence times in the reactor, these exposures can utilize reactants very efficiently. The evacuation times for these ALD reactors is slow in the absence of a purge gas. Viscous flow (constant) reactor (t cycle = 1-2 sec) The reactants are exposed with a carrier gas flowing through the reactor The carrier gas is in viscous flow and flows continuously to the pump

Constant flow reactors are divided into: Pressure inside constant flow reactors: The optimum carrier gas pressure is around ~ 1 Torr, which is a trade-off between gas interdiffusion and entrainment Cross-flow reactor Showerhead reactor

2. What defines ALD throughput: The growth rate of certain deposited material The surface area which is processed simultaneously

3. Saturation character of ALD growth Growth rate: Is a product of GPC (growth per cycle) and cycle time Is defined by the saturation point of surface terminated chemical reaction

4. The ways to increase growth rate: Decrease deposition time of each reactant (improve deposition rate) Make purge operation faster

5. Problems which appear while increasing the growth rate When reducing the cycle time film quality gets worse, especially if one needs to cover the substrates of complicated shape reliability of the pump system degenerates The price of such a speeding is: High pressure makes significantly difficult rapid removal of molecular precursors out from the chamber (longer purge time) Chemical usage efficiency (which is inversely proportional to the flow rate) sufficiently reduces Increasing the deposition rate requires: the flux of chemicals into the ALD reactor should be maximized to minimize the time that an ALD reaction needs to reach self-termination

ALD temperature window The substrate temperature must be higher than T1: to prevent condensation of any of the reactants in several types of surface reactions there is an activation energy that has to be exceeded The substrate temperature must be lower than T2: if the temperature is too high an undesirable decomposition of a reactant will happen re-evaporation is more intensive at higher temperatures and will result in a decreasing growth rate versus temperature.

6. ALD coating of high aspect ratio structures Extremely demanding to chemical injection system and chamber design Step coverage in deep-trench structures is a function of precursor residence time Long cycles and high pressure to avoid the defect formation The use of very-high-concentration doses of precursor with high sticking coefficients could cause more than a single-monolayer ALD reaction on the wafer surface Using low-vapor-pressure precursors to meet the step-coverage requirements for structures with aspect ratios of >50:1 requires pulse times lasting several seconds. For single-wafer ALD systems, such long pulse times are impractical for manufacturing

7. Batch ALD reactor Batch ALD systems can afford longer pulse times than single-wafer systems by processing many wafers at a time so that full surface saturation is achieved Reduce the cost of ownership of the inherently slow traditional ALD process By running the process on a batch instead of a single-wafer tool, chemical consumption decreased significantly Brings new opportunities…

Batch ALD reactor The larger the amount of wafers to be coated, the more complicated ALD installation is needed Larger-volume process chamber demands the extremely good gas flow dynamics: uniform precursor injection and evacuation in the process chamber The use of liquid-injectors positioned at the wafer edges to provide true cross-flow gas dynamics required to achieve uniform within- wafer and wafer-to-wafer film thickness, film composition and conformal step coverage Pulse times must be increased enough to satisfy the residence times required for the precursor to penetrate to the bottoms of deep trenches (in HAR cases) Temperature uniformity of the substrates should be provided Careful monitoring of temperature, pressure and precursor flow …and new requirements

8. How can ALD throughput be increased To go to batch for those films that can be processed with batch Better chemical injection systems and gas flow dynamics to provide the uniform precursor injection and evacuation Minimum chamber volume allows excellent temperature control, fast warm-up times, higher throughout and reduced precursor usage Precursor separation in space could bring new possibilities in increasing the deposition rate (e.g. ALD system with rotating stage)

And some more practical issues... Multiple showerhead gas delivery to insure uniform gas distribution Smart load lock system for faster wafer exchange Adequate cycle time and precursor pressure Even batch ALD installation should still be easy to clean Pumping system reliability and maintainability

9. Conclusion Multiple wafer processing is a key to increase throughput Modelling of gas flow dynamics can be utilized to optimize the gas flow, reducing the areas available for defect formation Optimization of all process parameters such as temperature, pressure and cycle time. Precursor separation in space, not in time Minimizing ALD process chamber volume in order to increase throughput and efficiency of precursor utilization

References Steven M. George, Atomic Layer Deposition: An Overview, Chem. Rev. 2010, 110, 111–131 Riikka L. Puurunen, Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process, JOURNAL OF APPLIED PHYSICS 97, Ernst Granneman, Batch ALD: Characteristics, comparison with single wafer ALD, and examples, ScienceDirect, Surface & Coatings Technology 201 (2007) 8899– consulting.com/Atomic_Layer_Deposition_ALD_Manufacturing_Aviza_Technology.html

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