Basics of ALD

Atomic Layer Deposition (ALD) is a unique vapor phase chemical process that yields ultra-thin film coatings with exceptional conformality on highly non-uniform and non-planar surfaces, often with sub-nanometer scale control of the coating thickness. While industry makes wide use of ALD in manufacturing of electronic devices, especially for advanced high-k dielectrics, there is growing interest in ALD and ALD-inspired processes for newer and more wide-ranging applications. For many materials, the ALD reaction chemistry proceeds readily at lower temperatures than chemical vapor deposition (CVD) processes, making ALD attractive, for example, for integration with biological or synthetic polymer structures.

The basic chemical mechanism active in atomic layer deposition involves two vapor phase reactive chemical species, typically a metal-organic precursor and a co-reactant as an oxygen source or as a reducing agent. The precursor and co-reactant species are transported sequentially into a heated reaction zone containing a receptive growth surface, resulting in two time-separated half-reaction steps. Time-separated exposure is ensured by purging the reactor with inert gas between reactant exposure steps. A typical ALD cycle for aluminum oxide is presented schematically in the adjacent figure. The first precursor exposure step leads to the first ALD half-reaction. In this step, the precursor chemically reacts and bonds to the surface without fully decomposing. The precursor also changes the dominant surface termination, leaving the surface ready to react with the co-reactant. The remaining vapor products are pumped or pushed out of the deposition zone using inert gas flow. For the second ALD half-reaction, the co-reactant is transported to the growth surface where the co-reactant reacts exothermally on the surface. The vapor products are flushed out, and the ALD cycle starts over again. In common thermally driven ALD processes, these half-reactions are driven by a favorable change in free energy, i.e. delta-G <0, and any activation barrier is easily traversed. Generally, the reaction enthalpy change is also <0, although a positive entropy change could also drive some reactions with delta-H >0 to be thermodynamically favorable. In plasma or other energetically enhanced ALD processes, different reactants are used which change the overall reaction thermodynamics. In this case, external energy is supplied during least one of the half-reaction steps to allow the entire reaction to proceed.