Date of Award

Spring 1-1-2019

Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Steven M. George

Second Advisor

Michael Marshak

Third Advisor

Gordona Dukovich

Fourth Advisor

Charles Rodgers

Fifth Advisor

Neils Damrauer


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 oC. 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.