Introduction to (plasma-enhanced) atomic layer deposition

Film growth by the atomic layer deposition (ALD) method relies on alternate pulsing of the precursor gases and vapors into a vacuum chamber and their subsequent chemisorption on the substrate surface (Fig. 1) [1,2]. The different steps in the process are saturative such that ALD film growth is self-limiting yielding one submonolayer of film per deposition cycle. ALD has some unique characteristics making the method technologically very relevant: (1) ultimate control of film thickness; (2) excellent conformality on very high aspect ratio structures; (3) good uniformity on large substrates; and (4) straightforward to apply to produce multilayer structures. The process was developed more than 3 decades ago but only in the late 1990s the interest increased highly, especially due to the continuously decreasing device dimensions in the semiconductor industry. Currently, it is expected that in some specific cases only ALD can meet the extremely high demands on conformality and process control. It is therefore expected that for an increasing amount of applications ALD will take over from the traditional film growth techniques such as (PE)CVD and PVD. In the last few years, research has provided several ALD processes of which some are currently implemented in industry. However, the materials that can be deposited by the strictly chemical method (thermal ALD) are limited by the availability of precursors and by (thermally-driven) surface reactions. By the introduction of a low-temperature plasma step in the ALD reaction cycle, it is possible to deliver additional reactivity to the surface in the form of plasma-produced species (Fig. 2). This opens up a processing parameter space that is unattainable by the strictly thermally-driven process [3]. Consequently, the plasma-enhanced ALD (PE-ALD) technique has a bright prospect for a large variety of applications also outside the typical use in semiconductor devices. In this opening presentation of the special session on plasma-enhanced ALD, the method of ALD will be presented and the merits of the technique will be addressed. Subsequently the method of plasma-enhanced ALD will be introduced as well as its unique processing benefits.