Atomic layer deposition (ALD) has emerged over the past decade as a viable alternative to physical and chemical vapor deposition (PVD, CVD) techniques for a range of strategic thin film coatings. This is due to the improved control of thin film properties including the incorporation of extrinsic dopants, reduction of contaminants, and the formation of intrinsic defects during film growth through a low temperature, non-vacuum process. These benefits result from the time-sequenced introduction of precursors in the deposition zone where selective and self-limiting half reactions occur on the surface. In this manner, thin-film growth is determined by surface kinetics, and is able to avoid parasitic gas-phase reactions. As such, film properties can be controlled by variation of temperature and reactant partial pressure. However, conventional time-sequenced ALD is not compatible with many scaled industrial applications as film growth rates (~0.01 nm/s) are rather slow due to the need to purge reactants during each growth half-cycle.
In order to circumvent this limitation and enable industrial scale thin-film ALD processes, spatially separated ALD techniques have been developed. In this approach, the reactant precursors are introduced in different zones of the reactor, and the substrate is sequentially exposed to these zones as it moves beneath each zone. In this manner, the reactants can be continuously exhausted thus there is no need for the purge step during each reactant half-cycle, thereby enabling a substantial increase in the overall growth rate capability. Utilizing Spatial ALD, Illiberi et. al. recently investigated the properties of zinc oxide (ZnO) thin films deposited at high growth rates on glass and silicon substrates. Using diethylzinc (DEZ) and water precursors, the authors used a circular reactor head configuration with multiple heads for each reactant in order to provide continuous film growth as the substrate rotates beneath on a circular platen. ZnO films properties were investigated for variations in process parameters including rotation speed, reactant partial pressure, and temperature. In terms of growth rate, the authors were able to optimize reaction kinetics to achieve rates as high as 0.9 nm/sec, almost 2 orders of magnitude higher than conventional time-sequenced ALD, and comparable to industrial scale deposition methods such as sputtering and plasma-enhanced CVD.
Investigating the electronic properties of the ZnO thin-films, it was further demonstrated that variations in reactant partial pressure could control the film stoichiometry, which in turn controlled the density of intrinsic defects. By controlling these parameters, the authors demonstrated the growth of 250 nm thick, n-type ZnO films with conductivity ranging from ¨~4-150 mΩ-cm with corresponding carrier mobility in the 10-30 cm2/V-s range. With associated optical transparency of ~85%, these ZnO thin films were suitable for a range of applications requiring transparent conductors or active transistors over large areas, for example photovoltaics and large area display backplanes. Thus a versatile materials growth technique has recently been scaled to a large area, high throughput process that is adaptable to a range of manufacturing platforms. Such a tool represents a disruptive manufacturing capability impacting both semiconductor and flexible electronics industries.
Reviewed by Jeff Morse, PhD, National Nanomanufacturing Network
- Illiberi A, Roozeboom F, Poodt P. 2011. Spatial atomic layer deposition of zinc oxide thin films. ACS Applied Materials & Interfaces. 4(1): 268-272. http://dx.doi.org/10.1021/am2013097
Figure reprinted with permission from Illiberi A, Roozeboom F, Poodt P. 2011. Spatial atomic layer deposition of zinc oxide thin films. ACS Applied Materials & Interfaces. 4(1): 268-272. Copyright 2011 American Chemical Society.