Atomic Layer Etching of Al₂O₃ Using Sequential, Self-Limiting Thermal Reactions with Sn(acac)₂ and Hydrogen Fluoride

Abstract

The atomic layer etching (ALE) of Al2O3 was demonstrated using sequential, self-limiting thermal reactions with tin(II) acetylacetonate (Sn(acac)2) and hydrogen fluoride (HF) as the reactants. The Al2O3 samples were Al2O3 atomic layer deposition (ALD) films grown using trimethylaluminum and H2O. The HF source was HF-pyridine. Al2O3 was etched linearly with atomic level precision versus number of reactant cycles. The Al2O3 ALE was monitored at temperatures from 150 to 250 °C. Quartz crystal microbalance (QCM) studies revealed that the sequential Sn(acac)2 and HF reactions were self-limiting versus reactant exposure. QCM measurements also determined that the mass change per cycle (MCPC) increased with temperature from -4.1 ng/(cm(2) cycle) at 150 °C to -18.3 ng/(cm(2) cycle) at 250 °C. These MCPC values correspond to etch rates from 0.14 Å/cycle at 150 °C to 0.61 Å/cycle at 250 °C based on the Al2O3 ALD film density of 3.0 g/cm(3). X-ray reflectivity (XRR) analysis confirmed the linear removal of Al2O3 and measured an Al2O3 ALE etch rate of 0.27 Å/cycle at 200 °C. The XRR measurements also indicated that the Al2O3 films were smoothed by Al2O3 ALE. The overall etching reaction is believed to follow the reaction Al2O3 + 6Sn(acac)2 + 6HF → 2Al(acac)3 + 6SnF(acac) + 3H2O. In the proposed reaction mechanism, the Sn(acac)2 reactant donates acac to the substrate to produce Al(acac)3. The HF reactant allows SnF(acac) and H2O to leave as reaction products. The thermal ALE of many other metal oxides using Sn(acac)2 or other metal β-diketonates, together with HF, should be possible by a similar mechanism. This thermal ALE mechanism may also be applicable to other materials such as metal nitrides, metal phosphides, metal sulfides and metal arsenides.

Atomic Layer Etching: What Can We Learn from Atomic Layer Deposition?

“…However, many processes relying on other chemistries can also be potentially developed, e.g., as demonstrated in a recent article by Lee and George who developed a thermally driven "reverse ALD" process. 49 For both ALEt and ALD, the self-limiting behavior of the surface chemistry is key and should be verified by saturation curves. In case of ALD, one needs to verify whether or not an atomic layer of material has been added to the film independent of the dosing times as long as these are sufficiently long.…”

“…A true "reverse ALD" process is probably more suited for isotropic etching, e.g., an ALEt process which is purely thermally driven. 49 Another aspect that is different for etch and deposition processes is that etch processes are characterized by an additional parameter which, by definition, does not have an equivalent for deposition processes. This parameter is the selectivity of the etch process, which is defined as the ratio between the rate of the layer being etched relative to that of a masking or underlying layer.…”

Atomic Layer Etching of HfO2Using Sequential, Self-Limiting Thermal Reactions with Sn(acac)2and HF

“…[21][22][23] The ALEt of Al 2 O 3 was recently reported using sequential, selflimiting thermal reactions with Sn(acac) 2 and HF as the reactants. 24 The linear removal of Al 2 O 3 was observed at temperatures from 150-250…”

“…In addition, the Al 2 O 3 films were smoothed by Al 2 O 3 ALEt cycles. 24 The overall Al 2 O 3 etching reaction was proposed to be: Al 2 O 3 + 6Sn(acac) 2 + 6HF → 2Al(acac) 3 + 6SnF(acac) + 3H 2 O. …”

“…• C. 24 The Sn(acac) 2 and HF thermal reactions were both self-limiting versus reactant exposure. In addition, the Al 2 O 3 films were smoothed by Al 2 O 3 ALEt cycles.…”