Thermal etching of metal oxides can occur spontaneously or by sequential surface reactions during atomic layer etching (ALE). This thermal etching does not require the use of plasmas or energetic species to remove material. Thermal etching can be studied by measuring the decrease of film thickness. However, the mechanisms of thermal etching still require the identification of volatile etch products. To address this issue, we have used quadrupole mass spectrometry (QMS) to detect volatile etch products during thermal etching. A custom reactor was built to measure the etch products with high sensitivity. This reactor uses molecular beam techniques to direct the etch products with line-of-sight to the QMS ionizer. Figure 1 shows a schematic of the QMS reactor. Nested inlet lines allow two reactants to be transported to the sample holder.

This talk will explore the thermal etching of metal oxides using three different approaches: (1) chlorination followed by ligand addition; (2) exposure to acetylacetone; and (3) fluorination followed by ligand exchange or conversion. For the first approach, chlorination followed by ligand addition will be utilized for CoO thermal ALE with SO₂Cl₂ as the chlorination reactant and tetramethylethylenediamine (TMEDA) as the ligand-addition reactant at 250°C. The proposed mechanism for CoO thermal ALE is shown in Figure 2. SO₂Cl₂ converts the CoO surface to a CoCl₂ layer and yields SO₃. Then the TMEDA adds to CoCl₂ and forms volatile CoCl₂(TMEDA) etch products. QMS studies were able to confirm the formation of CoCl₂(TMEDA) etch products. Figure 3 displays the CoCl₂(TMEDA) etch products observed for the 5^th CoO ALE cycle during the TMEDA exposure at 250°C. The natural abundance of the Cl isotopes affirmed that CoCl₂(TMEDA) was the volatile etch product.

Additional time-resolved experiments were able to monitor the reactants and volatile etch products. Figure 4 displays the ion intensities of C₃N₈N⁺ (largest crack of TMEDA) and CoCl(TMEDA)⁺ (largest crack of CoCl₂TMEDA) during the TMEDA exposure at 250°C. The self-limiting nature of the ligand-addition reaction is revealed by the decrease of the CoCl(TMEDA)⁺ ion intensity with time while the TMEDA ion intensity remains constant. Chlorination followed by ligand addition was also utilized for the thermal ALE of other first row transition metal oxides (Fe₂O₃, NiO, and ZnO). M(TMEDA)Cl₂ was observed as the volatile etch product for M= Fe and Ni. Zn(TMEDA)Cl was observed for ZnO.

For the second approach, acetylacetone (Hacac) and hexafluoroacetylacetone (Hhfac) have been known to spontaneously etch a variety of metal oxides including ZnO, CuO_x and Fe₂O₃. In addition, some thermal ALE procedures have been developed using Hacac together with an oxidation reactant to clean the metal oxide surface after the Hacac exposure. This talk will highlight results for ZnO etching using sequential Hacac and O₃ exposures and also consecutive Hacac exposures. Zn(acac)₂ was observed by the QMS studies during the alternating Hacac and O₃ exposures on ZnO throughout the Hacac exposures at 250°C. Additional experiments using Hacac exposures with no O₃ exposures also observed the continuous yield of Zn(acac)₂. These experiments provide evidence for the spontaneous etching of ZnO by Hacac according to the reaction: ZnO + 2Hacac -> Zn(acac)₂ +H₂O. This reaction is believed to proceed spontaneously through a ligand substitution/hydrogen transfer mechanism. Other metal oxides were also etched using sequential Hacac and O₃ exposures including V₂O₅, Fe₂O₃, CoO and CuO at 250°C. M(acac)₂ species were observed for M=Co, Fe and Cu. VO(acac)₂ was observed for V₂O₅.

Lastly, for the third approach, Al₂O₃ thermal ALE was explored using sequential BCl₃ and HF exposures at 300°C. This thermal ALE process could occur by (A) conversion of Al₂O₃ to B₂O₃ by BCl₃ followed by the spontaneous etching of B₂O₃ by HF or (B) fluorination of Al₂O₃ to AlF₃ followed by the ligand exchange of AlCl₃ with BCl₃ to produce volatile AlCl₃. QMS analysis revealed that initial exposures of BCl₃ on Al₂O₃ led to a conversion reaction forming B₂O₃ and volatile AlCl₃. Subsequently, the HF exposure on B₂O₃ released BF₃ and H₂O. After removing the B₂O₃ conversion layer, the HF proceeded to fluorinate Al₂O₃ to form a AlF₃ layer and volatile H₂O. Additional BCl₃ exposures then underwent ligand exchange with the AlF₃ layer to form BFCl₂ and AlCl₃. The BCl₃ continued to convert the underlying Al₂O₃ to B₂O₃. Moreover, the BCl₃ exposures were also observed to spontaneously etch the B₂O₃ conversion layer. QMS studies revealed that the reaction of BCl₃ with the B₂O₃ conversion layer or initial B₂O₃ substrates produced B₃O₃Cl₃, a boroxine ring species. BCl₃ has the unusual ability to convert Al₂O₃ to B₂O₃ and then spontaneously etch the B₂O₃.