Atomic Layer Deposition (ALD) is a well known method for depositing uniform and conformal thin-films over substrates of various shapes, even over complex 3D (three dimensional) structures. In ALD the coating is grown by alternately repeating, essentially self-limiting, surface reactions between a precursor and a surface to be coated. Therefore the growth mechanism in an ALD process is commonly not as sensitive as other coating methods to e.g. the flow dynamics inside a reaction chamber which may be a source for non-uniformity, especially in coating methods relying on gas-phase reactions. In an ALD process two or more different reactants (precursors) are introduced to the reaction chamber in a sequential, alternating, manner and the reactants adsorb on surfaces, e.g. on a substrate, inside the reaction chamber. The sequential, alternating, introduction of reactants is commonly called pulsing (of reactants).
In between each reactant pulse there is commonly a purging period during which a flow of inert gas, often called the carrier gas, purges the reaction chamber from e.g. surplus precursor and by-products resulting from the adsorption reactions of the previous precursor pulse. A film can be grown by an ALD process by repeating several times a pulsing sequence comprising the aforementioned reactant pulses and purging periods. The number of how many times this sequence called the “ALD cycle” is repeated depends on the targeted film, or coating, thickness.
A problem with known coating methods and apparatuses is the mechanical screening of the substrate object by a supporting structure. The fact, that the object to be coated commonly rests on a supporting frame, or on another supporting structure such as the bottom of a reaction chamber, almost inevitably leads to non-uniformities in the deposited coating. This problem is exacerbated when the coating is applied on small objects or powders comprising many small objects (i.e. small particles) that may be in contact with, or reside very close to, each other in addition to resting on a supporting structure. This may cause screening of the objects in many places of its surface.
Coating methods employed because of their potential for highly uniform and conformal coatings, such as ALD or other methods based on alternately repeating surface reactions, may loose some of their key benefit of conformality and uniformity due to the aforementioned screening problem. This occurs especially when a uniform and/or conformal coating is required all around an object/substrate.
Some methods have been proposed to alleviate this problem by causing the objects to become separated during the coating process. For example U.S. Pat. No. 7,132,697 and U.S. Pat. No. 7,396,862 disclose the formation of a fluidized bed of particles to be coated in an ALD process. The particles to be coated are fluidized by injecting a fluidizing gas through a bed of particles, which lifts the particles upwards, mechanically suspending them and dispersing them in the fluidizing gas. Patent application publication WO2006/135377 discloses an ALD coating method in which particles are fluidized by intermittently suspending them above a supporting surface. In practice the suspension is carried out by an ultrasonic source.
Some drawbacks of the discussed coating methods of the prior art are the limitation of the size, weight, number or total volume of objects that can be coated. E.g. an ultrasonic source is not able to efficiently separate larger or heavier objects from each other by fluidizing them. This is especially true for big numbers of these objects. Additionally it is problematic for the proposed arrangement in patent application publication WO2006/135377 to even handle big numbers or volumes of objects (particles) to be coated. Utilizing a high pressure gas stream, as in U.S. Pat. No. 7,132,697 and U.S. Pat. No. 7,396,862, for fluidization will not markedly alleviate these problems. Furthermore, using a gas stream for fluidizing the objects to be coated poses additional difficulties in suitably arranging precursor flows inside a reaction chamber. Arrangements of the prior art additionally require complex electrical and/or mechanical constructions to realize an ultrasonic or a high pressure gas assisted fluidization systems.