Real-time and in situ monitoring of sputter deposition with RHEED for atomic layer controlled growth

Abstract

Sputter deposition is a widely used growth technique for a large range of important material systems. Epitaxial films of carbides, nitrides, metals, oxides and more can all be formed during the sputter process which offers the ability to deposit smooth and uniform films from the research level up to an industrial scale. This tunable kinematic deposition process excels in easily adapting for a large range of environments and growth procedures. Despite the vast advantages, there is a significant lack of in situ analysis options during sputtering. In particular, the area of real time atomic layer control is severely deficient. Atomic layer controlled growth of epitaxial thin films and artificially layered superlattices is critical for both understanding their emergent phenomena and engineering novel material systems and devices. Reflection high-energy electron diffraction (RHEED) is one of the most common in situ analysis techniques during thin film deposition that is rarely used during sputtering due to the effect of the strong permanent magnets in magnetron sputter sources on the RHEED electron beam. In this work we have solved this problem and designed a novel way to deter the effect of the magnets for a wide range of growth geometries and demonstrate the ability for the first time to have layer-by-layer control during sputter deposition by in situ RHEED. Atomic layer controlled growth of complex thin film heterostructures is essential for the understanding and engineering of their properties. Molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) techniques readily take advantage of reflection high-energy electron diffraction (RHEED) as an in situ diagnostic tool for determining the structure of the surface during deposition, enabling layer-by-layer control at the unit cell and sub unit cell level.1–5 However, this powerful in situ analysis technique is not commonly available in conjunction with sputter deposition despite the commonality of this technique. The strong permanent magnets present in the magnetron sputter sources have deterred the inclusion of RHEED as an in situ analysis technique during sputter grown epitaxial thin films. This has left much unknown about the growth mechanisms (e.g., Stranski-Krastanov, Frank-van der Merwe or Volmer-Weber modes,6,7 and layer-by-layer versus step flow) for many thin film systems deposited by the sputter technique. In order to achieve real time layer-by-layer controlled growth during sputtering this issue needs to be solved. The observation of intensity oscillations of the RHEED specular reflection in MBE growth of semiconductors has been exploited for many years to control stoichiometry and growth rate.1–3 The RHEED technique was then readily adapted to oxide MBE growth4 and subsequently, high-pressure RHEED was developed for PLD by Rijnders et al.5 and has been widely adopted for growing epitaxial oxide films and controlling complex interfaces by this technique. The development of in situ RHEED analysis for sputter growth would introduce similar advantages-for example, rapid optimization of growth parameters and control of growth rates, and enhance reproducibility of interface and superlattice growth-to this widely used and technologically important deposition technique. Through finite element modeling of different experimental setups we have been able to construct a general solution for implementing RHEED with magnetron sputtering by mitigating and creating a predictable uniaxial bending of the electron beam. Using this general guideline, the major detrimental effects from the magnets can be avoided in many common sputter geometries. In this work we choose one of the common sputter geometries that we have modeled and demonstrate digital control of magnetron sputter deposition using in situ high-pressure RHEED by applying this technique to the widely studied model oxide system, SrRuO3 (SRO). During 90° off-axis sputtering of SRO films we observed strong specular spot oscillations that extended to more than 20 unit cells. This allows us to identify the growth mode as layer-by-layer during sputter grown SRO which is in contrast to the commonly assumed growth mode of step flow. This establishes our ability to have unit cell control during sputter growth in real time. Similar results are observed during the growth of perovskites La0.7Sr0.3MnO3 (LSMO) and LaAlO3 (LAO), confirming that this approach can be universally applied to sputter deposition of other materials.