(Student Award, 1st Place, Invited) Thermal Etching of Metal Oxides: Mechanisms Revealed By Quadrupole Mass Spectrometry
Authors: Partridge Jonathan Lawrence , George Steven M 
Published: November 2022

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 5th 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, CuOx 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.

Figure 1