Graduate Thesis Or Dissertation
Atomic Layer Etching of Metal Films, Metal Nitrides, and Metal Oxides with Bcl3 and Xef2 Public Deposited
The continued miniaturization and increase in architectural complexity of transistor-based devices require new process methods. One such method is called atomic layer etching (ALE). ALE is based upon sequential self-limiting or pseudo-self-limiting reactions that remove materials with atomic level control. Until recently, ALE was based upon halogen adsorption followed by high energy ion bombardments for isotropic (directional) etching of materials. However, recently, new thermal based techniques have been developed for anisotropic (nondirectional) etching of semiconductor-based devices.
New approaches for thermal ALE have been demonstrated for crystalline AlN, W/WO₃, TaN/Ta₂O₅, MOCVD GaN and Ga₂O₃. Crystalline AlN was demonstrated to etch with Sn(acac)₂ and HF as the reactants. WO₃, Ta₂O₅ and Ga₂O₃ were demonstrated to etch with BCl₃ and HF. TaN and W were etched with O₂/O₃ in conjunction with BCl₃ and HF. MOCVD GaN was etched with BCl₃ and XeF₂.
All systems were analyzed with spectroscopic ellipsometry as the main technique for thickness analysis. Thermal ALE of all systems was shown to be possible in the general temperature range of 150 to 250°C. Investigations into the reaction pathway were conducted for each system. Ga₂O₃, MOCVD GaN, and AlN proceeded through a fluorination and ligand exchange. In this process, the surface is fluorinated, and the resulting fluoride is transferred for the ligand on Sn(acac)₂ or BCl₃ creating volatile complexes. In the case for WO₃ and Ta₂O₅, BCl₃ converts the surface to a B₂O₃ layer while forming volatile metal chlorides. B₂O₃ is then spontaneously removed by interaction with HF which does not fluorinate WO₃ or Ta₂O₅. This pathway is important when a fluorination step would create volatile fluorides leading to noncontrolled spontaneous etching. TaN and W are etched similarly, except that an oxidation step is required. The thermal ALE of other metal derivatives such as metal phosphides, sulfides, tellurides and elemental metals can be etched through these two pathways.
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