Advanced process technologies: Plasma, direct-write, atmospheric pressure, and roll-to-roll ALD
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Introduction Although it all started in the 1970s with thin-film electroluminescent displays,1 it was only recently that atomic layer deposition (ALD) found several mainstream applications, mostly in the semiconductor and related industries.2 For example, ALD is used in the manufacturing of magnetic read-write recording heads,3 in high-κ dielectric deposition in 3D capacitors for dynamic random-access memory (DRAM) technology,4 and in gate-stack formation of logic devices.5 For these applications, ALD processes use conventional metal oxide thin-film materials (particularly Al2O3, HfO2, and ZrO2-based) and are based on the thermally driven ALD surface chemistry of halide and metal-organic precursors with H2O. For practical applications, the processes are initially also mostly explored and introduced as single-wafer processes, although multiple-wafer batch processes have been implemented for DRAM technology. Among the many novel applications that could benefit from thin films prepared by ALD, several will depend on new and even disruptive process technologies to extend the capabilities of ALD in order to make them technologically feasible and economically viable. New process technologies can be required for a large variety of reasons, such as the need for low-temperature processing, the preference for bottom-up patterning capability, the use of flexible, large-area or other non-wafer-like substrates, and the necessity for fast and/or high-throughput processing. These novel applications are likely to be seen first within the
fields of nanoelectronics, photovoltaics, and flexible electronics. They constitute large, expanding markets in which cost reduction is a main driver and with each of them having its particular challenges (i.e., miniaturization, high-throughput processing, and substrate material handling, respectively). In this article, some recent developments are presented in which ALD processes and applications expand beyond singlewafer semiconductor processing with conventional thermal ALD. As advanced process technologies rely heavily on innovative, new reactor, and processing concepts that are often not reported in the open scientific literature due to business reasons, this overview is not intended to be historical and exhaustive. The aim is merely to illustrate how innovations within the field of ALD can contribute to the integration of viable manufacturing schemes with the benefits of ALD thin-film processing.
Beyond single-wafer thermal ALD The most common type of ALD reactor is a single-wafer thermal ALD reactor in which ALD cycles are carried out in the time domain. This means that precursor and reactant (here the term “reactant” instead of “second precursor” is used for the agent that leads to oxidation or reduction of the growth surface) pulsing as well as the intermediate purge steps are carried out subsequently in time. For a well-optimized reactor and process, the cycle time can be below 1 s. Single-wafer ALD reactors are generally used for processes requiring extremely tight process
W.M.M. (Erwin) Kessels
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