There has been considerable interest in the development of isotropic atomic layer etching (ALE) for the conformal removal of thin films. Material selectivity is crucial for the development of isotropic ALE because the next generation of semiconductor devices will be constructed with miniaturized 3D structures using a variety of very thin films. We developed plasma-assisted thermal-cyclic ALE, which is a repetition of surface modification by plasma exposure and removal of the modified surface by infrared heating. We developed a 300-mm tool, namely, dry chemical removal (DCR), which is equipped with an inductively coupled plasma (ICP) source and infrared lamps, to facilitate rapid thermal desorption of the modified surface. An important feature of the plasma-assisted thermal-cyclic ALE is that it has more tuning knobs than that of conventional ALE because it uses two temperatures: a low temperature for surface modification and an elevated temperature for the removal of the modified surface. This paper presents the selective ALE of various materials, i.e., Si3N4, TiN, W, and SiGe using the developed tool. The mechanisms of the selectivity are divided into two categories: formation of an ammonium salt-based modified layer and selectivity control by adjusting the infrared heating time. This paper reviews the selective ALE mechanisms, focusing on the results of in situ analysis of surface reactions, and presents some of the latest findings.
Selective etching of several hard-to-etch materials is achieved by cyclic repetition of conversion into volatile organometallics followed by volatilization. A feature of this etching technology is the processes of adding volatility to these materials by converting the surface of the materials into intermediatory generated organometallics with thermal stability; one key point is stabilizing materials for the organometallics, and the other is a reaction pathway via inherently stable organometallics. In layers of Co metal, one hard-to-etch material, a specific oxidation state of Co in the Co oxidizing first step avoids the reaction pathways that generate a mixture of multiple organo-cobalt complexes in the following step. For La2O3, another hard-to-etch material, an organo-lanthanum complex generated in the ligand adsorbing first step is immediately stabilized by a stabilizer. The surface-modified layer composed of the resultant stabilized organo-lanthanum complex prevents the ligand species from diffusing deeply and from increasing the modified layer thickness. The following step, in both cases, is rapid thermal annealing by infrared (IR) irradiation to remove the surface modified layer without decomposition. The etched amount increases as the number of cycle repetitions increases with high selectivity.
A selective, rapid thermal-cyclic atomic-level etching (ALE) of tungsten is developed. The first step of this process is exposing the surface of tungsten with hydrofluorocarbon plasma at −22°C to form a tungsten fluoride-based surface modified layer on the tungsten surface. The second step is rapid thermal annealing with infrared (IR) irradiation to remove the surface modified layer. Tungsten 4f peaks and a fluorine 1s peak, which were assigned to tungsten fluoride, were observed by in-situ x-ray photoelectron spectroscopy immediately after plasma exposure. The peaks that originated from tungsten fluoride disappeared after the samples were annealed. Cyclic etching tests were carried out by repeating plasma exposure and IR irradiation with a 300-mm ALE tool. Films of tungsten, TiN, and SiO2 were used as sample materials. The amount of etched tungsten increased as the number of cycle repetitions increased. The etched amount per cycle for tungsten was 0.8 nm. In comparison, etching of TiN and SiO2 was not detected. Conformal etching profiles of patterned samples after 60 cycles were obtained. Furthermore, the etched amount per cycle showed saturation behavior with regard to plasma exposure time. Selective, rapid thermal cyclic ALE of tungsten was thus successfully demonstrated.
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