Atomic layer Deposition

Atomic layer deposition (ALD) is a technique capable of depositing a variety of thin film materials from the vapor phase. ALD has shown great promise in emerging semiconductor and energy conversion technologies. This review is intended to introduce the As device requirements push toward smaller and more spatially demanding structures, ALD has demonstrated potential advantages over alternative deposition methods, such as chemical vapor deposition (CVD) and various physical vapor deposition (PVD) techniques, due to its conformality and control over materials thickness and composition. These desirable characteristics originate from the cyclic, self-saturating nature of ALD processes.
reader to the basics of ALD and highlight current applications pertaining to microelectronics and energy that were selected because of their importance in either industry or research. For a more comprehensive summary of ALD and its many applications, the reader is referred to existing reviews on the topic [1–10].
ALD was popularly introduced as atomic layer epitaxy (ALE) by Suntola and Antson in 1977, depositing ZnS for flat panel displays [11]. As further ALE processes were developed to incorporate metals and metal oxides, many materials were deposited non-epitaxially and the more general name of ALD was adopted to reflect this [1]. It should be noted, too, that many ALD procedures were developed from a variety of CVD processes. In contrast to their CVD analogs, the ALD procedures feature alternating exposure of chemical precursors to react to form the desired material, often at significantly lower temperatures [12].A general ALD process is illustrated in Fig. 1. It consists of sequential alternating pulses of gaseous chemical precursors that react with the substrate. These individual gas-surface reactions are called ‘half-reactions’ and appropriately make up only part of the materials synthesis. During each half-reaction, the precursor is pulsed into a chamber under vacuum (<1 Torr) for a designated amount of time to allow the precursor to fully react with the substrate surface through a self-limiting process that leaves no more than one monolayer at the surface. Subsequently, the chamber is purged with an inert carrier gas (typically N2 or Ar) to remove any unreacted precursor or reaction by-products. This is then followed by the counter-reactant precursor pulse and purge, creating up to one layer of the desired material. This process is then cycled until the appropriate film thickness is achieved. Typically, ALD processes are conducted at modest temperatures (<350 °C). The temperature range where the growth is saturated depends on the specific ALD process and is referred to as the ‘ALD temperature window’. Temperatures outside of the window generally result in poor growth rates and non-ALD type deposition due to effects such as slow reaction kinetics or precursor condensation (at low temperature) and thermal decomposition or rapid desorption of the precursor (at high temperature). In order to benefit from the many advantages of ALD, it is desirable to operate within the designated ALD window for each deposition process.