Atomic layer deposition, or ALD, is a vapor phase process that can be used to create thin films of a wide range of materials in a single step. Due to its sequential, self-limiting reaction mechanism, ALD may achieve excellent conformality on high-aspect-ratio structures, allow for precise thickness control down to the Angstrom level, and produce films with tunable compositions. Due to these advantages, ALD has evolved as a potent tool for a wide range of industrial and scientific applications.
Cu(In, Ga)Se2 solar cell devices, high-k transistors, and solid oxide fuel cells are examples of advanced technologies. This selection of samples is intended to demonstrate the wide range of technologies that ALD influences, the wide range of materials that ALD can deposit – from metal oxides such as Zn1xSnxOy, ZrO2, and Y2O3 to noble metals such as Pt – and the way in which the unique characteristics of ALD can enable new levels of performance and deeper fundamental understanding to be achieved.
Atomic layer deposition (ALD) is a technique for depositing thin-film materials from the vapor phase capable of depositing a wide range of thin-film materials. When it comes to new semiconductor and energy conversion technologies, ALD has shown considerable promise thus far.
The sequential, self-saturating, gas-surface reaction control of the deposition process is the source of all of ALD’s fundamental advantages derived from this control. ALD is frequently preferred over rival deposition processes such as CVD or sputtering due to the conformality of the ALD-deposited films, which is the first of these factors to be discussed. Because of its self-limiting property, which limits the reaction at the surface to no more than one layer of precursor, the conformality of high aspect ratio and three-dimensionally structured materials is made possible. When the precursor pulse times are long enough, the precursor can disperse into deep trenches, allowing for complete reaction with the surface on the entire surface. In contrast, CVD and PVD may suffer from non-uniformity due to faster surface reactions and shadowing effects on high aspect ratio structures due to more immediate surface reactions and shadowing effects.
While ALD has many intriguing characteristics, it also has slow deposition rates. Most ALD rates are in the 100–300 nm/h range due to the high cycle periods needed in pulsing and purging precursors and the layer-by-layer nature of the deposition. This rate, however, is highly dependent on the reactor design and the aspect ratio of the substrate. The time required for pulsing and purging increases as the surface area and volume of an ALD reactor increase. Longer pulse and purge periods are also needed for high aspect ratio substrates to allow the precursor gas to distribute into trenches and other three-dimensional structures. To address this problem, spatial ALD has emerged as a promising technology that has the potential to increase throughput considerably.
Spatial ALD works by substituting the typical pulse/purge chamber with a spatially-resolved head that exposes the substrate to a different gas precursor dependent on its location. In one arrangement, when the head moves around the substrate, the exposed precursor changes, resulting in film development. Alternatively, spatial ALD has been demonstrated in which the substrate moves past stationary precursor nozzles that are positioned so that passing by them results in precursor cycling and film growth. Overall, spatial ALD approaches allow for deposition rates of roughly 3600 nm/h.
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