WhitePaper: Atomic Layer Deposition at RhySearch
Atomic Layer Deposition (ALD) is a well-established coating technique in the semiconductor industry and is now becoming an increasingly attractive option in the field of optics for coating of components with complex geometries. In this WhitePaper we outline some fundamentals of ALD and the benefits and challenges it offers for optical designs. In addition we describe the deposition system and ALD process development at RhySearch.
Introduction
Atomic Layer Deposition is a chemical vapour technique to deposit thin films via alternating cycles of a material precursor and a co-reactant, enabling the deposition of a wide range of materials for different applications. The technique was developed in Finland in the 1970s and first applied commercially for components used in large displays [1]. After its adoption for DRAM in the early 2000s and subsequent use in 2007 as a gate material in CMOS processes, it is now well established as a standard process in the semiconductor industry. The surge of interest in the technique also drove an increased effort in precursor development to expand the range of available materials [2]. Since the early 2000s there has also been a significant increase in the number of manufacturers worldwide which are offering commercial ALD systems. For the optics community the potential of this technique is of growing interest as component geometries increase in complexity and given the potential of utilizing ALD for conformal deposition of such structures.
A typical ALD cycle consists of four steps, illustrated in Figure 1. First a gaseous precursor is injected to the deposition chamber (1), typically from a heated metal-organic compound. After a self-limiting surface reaction is complete any excess precursor and reaction by-products are removed in a purge step (2). A co-reactant is then used in step (3) to form the intended material. H2O or O3 are used for thermal ALD formation of oxides. Various plasma processes (O2, N2, or SF6) can be used to form oxides, fluorides or nitrides via plasma enhanced ALD (PEALD). A final purge step (4) is used to remove by-products and the entire cycle is repeated to build up layers of the desired material.
[1] Puurunen et al., Chemical Vapour Deposition, 20 10-11-12, p332, (2014)
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