(Tutorial) Thermal Atomic Layer Etching

Abstract

Thermal atomic layer etching (ALE) can be performed using sequential self-limiting reactions that yield controlled etching at the atomic level. Thermal ALE is the opposite of atomic layer deposition (ALD). The sequential reactions during thermal ALE are based on surface modification followed by volatile release of the modified surface layer. Surface modification typically results from fluorination. Volatile release can be accomplished using metal precursors that undergo ligand-exchange reactions with the fluorinated surface layer. This tutorial will first concentrate on thermal Al2O3 ALE as a model system. The tutorial will then survey the thermal ALE of other materials and selectivity in thermal ALE.Thermal Al2O3 ALE is a model system that illustrates the basics of thermal ALE. The surface modification reaction in thermal Al2O3 ALE results from the fluorination of Al2O3 to AlF3 using HF. Al2O3 fluorination by HF is a self-limiting process that forms a thin stable AlF3 layer on the Al2O3 surface with a thickness of several Angstroms. The AlF3 layer can then be removed by ligand-exchange reactions. During the ligand-exchange reactions, a molecular precursor transfers a ligand to the AlF3 layer and the AlF3 layer transfers a F atom to the molecular precursor. This ligand-exchange breaks the strong F bridge-bonding in the AlF3 layer. A typical molecular precursor for ligand-exchange is Al(CH3)3 (trimethylaluminum, TMA).The ligand-exchange reaction with TMA adds CH3 ligands to the AlF3 layer. In addition, the TMA molecular precursor forms AlF(CH3)2 (dimethylaluminum fluoride (DMAF)) after ligand-exchange. The DMAF ligand-exchange product is volatile at the etching temperatures. Progressive ligand-exchange reactions between TMA and the AlF3 layer continue to break F bridge-bonding and eventually form volatile DMAF etch products. DMAF is monitored as the etch product by mass spectrometry investigations. However, these studies reveal that dimers of DMAF with itself (DMAF/DMAF) and dimers of DMAF with TMA (DMAF/TMA) have the largest signal intensities.Thermal Al2O3 ALE using HF and TMA as the reactants occurs with a temperature-dependent etch rate that varies from 0.14 Å/cycle at 250°C to 0.75 Å/cycle at 325°C. The etch rate can also be dependent on the HF pressure because thicker fluoride layers are formed on Al2O3 at higher HF pressure. There is also competition between thermal Al2O3 ALE and AlF3 ALD. At lower temperatures, the TMA and HF reactants can produce AlF3 ALD instead of Al2O3 ALE. The presence of HF coverage on the surface at lower temperatures is believed to lead to AlF3 ALD.In addition to thermal Al2O3 ALE, a variety of other metal oxides and metal oxides, including HfO2, ZrO2, Ga2O3, VO2, AlN and GaN, can be etched using sequential fluorination and ligand-exchange reactions. Various fluorination reactants can be employed such as HF, SF4 and XeF2. A wide range of molecular precursors are also effective for ligand-exchange such as AlCl(CH3)2, Sn(acac)2, TiCl4, SiCl4 and BCl3. In addition, a variety of other materials, such as ZnO, Si, SiO2, Si3N4, TiO2 and WO3 can be etched using conversion reactions that first convert the surface of the original material to a new surface layer that has a pathway for etching. Thermal ALE pathways are also available for etching elemental metals, such as Ni and Co, using chlorination and ligand-addition reactions.Thermal ALE yields isotropic etching that will be valuable for advanced semiconductor fabrication. Isotropic thermal ALE will be necessary to provide lateral etching in three-dimensional structures such as nanowire or nanosheet transistors. The ligand-exchange reactions during thermal ALE are also extremely selective depending on the stability and volatility of the etch products. This selectivity can be tuned by using different molecular precursors for ligand-exchange. The selectivity during ligand-exchange can provide new pathways for area-selective etching.