ALE: Gas-phase and/or Thermal ALE
Tuesday, July 23, 2019 8:00 AM in Room Regency Ballroom A-C
ALE1-TuM-1 Analyses of Hexafluoroacetylacetone (Hfac) Adsorbed on Transition Metal Surfaces
Tomoko Ito, Kazuhiro Karahashi, Satoshi Hamaguchi (Osaka University, Japan)
Transition metals are known as hard-to-etch materials for reactive ion etching (RIE) processes. Although Ar milling processes are now widely used for such metal etching, physical sputtering processes with high energy ions have problems of low selectivity and poorly controlled etched profiles. Surface damages induced by energetic ion bombardment may also cause degradation of the material surface properties. Therefore alternative transition-metal etching processes with high selectivity and low damage, rather than physical sputtering, have been sought after in the industry. In recent years, atomic layer etching (ALE) by the formations of volatile organic metal complexes has attracted much attention as a means to achieve atomically controlled and low damage etching. ALE reactions may be considered as reverse reactions of atomic layer deposition (ALD). For example, metal beta-ketoenolate complexes are often used as precursors for ALD, so stable adsorption of beta-ketones on a metal surface and the formation of metal beta-ketoenolates thereon are crucial steps for the development of corresponding ALE processes. In this study, we have examined surface reactions of transition metals or their oxides with hexafluoroacetylacetone (hfac)  and demonstrated that such reactions can be used to develop ALE processes for some transition metals. Experiments were performed in what we call the “Atomic-Layer-Process (ALP) Surface Analysis System,” which consists of ALP reaction chambers and an in-situ high-resolution X-ray photoelectron spectroscopy (XPS) system. In the reaction chamber, the substrate temperature can be controlled by a ceramic heater, which is installed on the back side of the sample. After exposure to reactive gases, the sample can be transferred from the reaction chamber to the XPS chamber without being exposed to ambient air for in- situ surface chemical analysis. Ion irradiation effects on an hfac adsorbed metal surface may be studied with the use of low energy Ar+ ion beam of the XPS system. We used Ni and Co substrates in this study. It has been found that hfac molecules adsorbed on a metal oxide surface are less likely to be decomposed at room temperature than those on a metal surface and, at an elevated temperature, a metal oxide surface is preferentially etched by hfac than a metal surface.
 H. L. Nigg and R. I. Masel, J. Vac. Sci. Technol. A, 17, 3477 (1999).
ALE1-TuM-3 Thermal Atomic Layer Etching of Silicon Nitride using an Oxidation and “Conversion-Etch” Mechanism
Aziz Abdulagatov, Steven M. George (University of Colorado - Boulder)
The thermal atomic layer etching (ALE) of silicon nitride was demonstrated using an oxidation and “conversion-etch” mechanism (see Supplemental Figure 1). In this process, the silicon nitride surface was oxidized to a silicon oxide layer using O2 or ozone. The silicon oxide layer was converted to an Al2O3 layer using trimethylaluminum (TMA). The Al2O3 layer was fluorinated by HF to an AlF3 layer prior to the removal of the AlF3 layer by ligand-exchange using TMA. Silicon nitride ALE was studied using Si3N4 films deposited using low pressure chemical vapor deposition (LPCVD). In situ spectroscopic ellipsometry was employed to monitor the thickness of both the Si3N4 and the silicon oxide layer during ALE. These studies observed that the silicon nitride film thickness decreased linearly with number of reaction cycles while the silicon oxide thickness remained constant.
Using an O2-HF-TMA reaction sequence, the Si3N4 ALE etch rate was 0.26 Å/cycle at 290°C. This etch rate was obtained using static reactant pressures of 250, 0.65 and 1.0 Torr, and exposure times of 10, 5 and 5 s, for O2, HF and TMA, respectively. Employing similar dosing conditions, the process using O3 yielded a higher Si3N4 etch rate of 0.47 Å/cycle (see Supplemental Figure 2). The Si3N4 etch rates remained the same for O3 pressures from 30-250 Torr. The order of the reactant sequence affected the Si3N4 etch rate. Changing the reactant sequence from O3-HF-TMA to O3-TMA-HF reduced the Si3N4 etch rate from 0.47 to 0.20 Å/cycle at 290°C. The Si3N4 ALE etch rate was also reduced at lower temperatures. Using the O3-HF-TMA reaction sequence, the Si3N4 etch rate was reduced from 0.47 Å/cycle at 290°C to 0.07 Å/cycle at 210°C.
Si3N4 ALE also decreased the roughness of the Si3N4 surface. The RMS roughness of the initial Si3N4 films was 4.7 Å measured using atomic force microscopy (AFM). The RMS roughness decreased to 3.1 Å after 80 ALE cycles. An SiO2 oxide thickness of ∼10-15 Å remained after Si3N4 ALE at 290°C. This oxide could be removed by 15 sequential TMA and HF exposures after the Si3N4 ALE. Thermal Si3N4 ALE should be useful in advanced semiconductor fabrication. Thermal Si3N4 ALE could also find applications in optoelectronics, photonics and MEMS fabrication.
ALE1-TuM-4 Thermal Dry Atomic Layer Etching of Cobalt with Sequential Exposure to Molecular Chlorine and Diketones
Mahsa Konh, Chuan He, Xi Lin (University of Delaware); Xiangyu Guo, Venkateswara Pallem (American Air Liquide); Robert Opila, Andrew Teplyakov, Zijian Wang, Bo Yuan (University of Delaware)
The mechanism of thermal dry etching of cobalt films is discussed for a thermal process and a sequential exposure to chlorine gas and a diketone (either 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hexafluoroacetylacetone, hfacH) or 2,4-pentanedione (acetylacetone, acacH)). The process can be optimized experimentally to approach atomic layer etching (ALE), and a sequential exposure to Cl2 and hfacH dry etchants at 140°C is shown to proceed efficiently. The use of acacH as a diketone does not result in ALE with chlorine even at 180°C; however, the decrease of surface chlorine concentration and chemical reduction of cobalt is noted. Thermal desorption analysis suggests that the reaction of chlorinated cobalt surface exposed to the ambient conditions (oxidized) with hfacH does produce volatile Co-containing products within the desired temperature range and the products contain Co3+. The effect of ligands on the energy required to remove surface cobalt atoms is evaluated using density functional theory and the findings are consistent with the experimental observation of surface smoothing during atomic layer etching.
ALE1-TuM-5 Spontaneous Etching of B2O3 and TiO2 by HF: Removal Reaction in WO3 ALE and TiN ALE
Austin Cano (University of Colorado - Boulder); Suresh Kondati Natarajan (Tyndall National Institute, Ireland); Joel Clancey (University of Colorado - Boulder); Simon Elliot (Schrödinger Inc); Steven M. George (University of Colorado - Boulder)
Thermal atomic layer etching is typically based on two sequential surface reactions. The first reaction activates the surface layer and the second reaction leads to material removal by the desorption of volatile etch products. The surface activation can be halogenation, conversion to a different material, or oxidation of the initial material. For example, BCl3 is able to convert the WO3 surface to a B2O3 surface layer during WO3 ALE. The B2O3 surface layer is then spontaneously removed by etching using HF. In another example, O3 is able to oxidize the TiN surface to a TiO2 surface layer during TiN ALE. The TiO2 surface layer is then spontaneously removed by etching using HF.
This study explored the spontaneous etching of B2O3 and TiO2 with HF using Fourier Transform Infrared (FTIR) spectroscopy and quadrupole mass spectrometry (QMS) analysis. The initial B2O3 films were grown using B2O3 ALD with BCl3 and H2O as the reactants. The initial TiO2 films were grown using TiO2 ALD with TiCl4 and H2O as the reactants. FTIR measurements observed the growth of the B2O3 films and TiO2 films by monitoring the absorbance of the B-O and Ti-O stretching vibrations, respectively, versus number of ALD cycles. FTIR experiments also observed the spontaneous etching of B2O3 and TiO2 with HF by measuring the loss of the absorbance of the B-O and Ti-O stretching vibrations, respectively (See Supplemental Figure 1).
QMS studies were also able to monitor the volatile etch products during the spontaneous etching of B2O3 nanopowder with HF. The expected reaction products are BF3 and H2O based on the reaction B2O3 + 6HF → 2BF3 + 3H2O. In comparison, the QMS detected B(OH)F2, BF3 and H2O as the main etch products (See Supplemental Figure 2). In addition, the QMS also revealed species at higher masses that were consistent with six-member ring species, such as B3O3F3.
The reaction of HF with B2O3 and TiO2 was also examined using a density functional theory (DFT) based computational approach. By comparing the thermodynamic free energy profiles of competing self-limiting surface and bulk reactions, the DFT calculations predicted the spontaneous etching of B2O3 by HF above -160°C and of TiO2 (but not TiN) above 90°C, in agreement with the experimental findings.
ALE1-TuM-6 Thermal Based Atomic Layer Etching of Aluminum Oxide and Titanium Nitride
Varun Sharma, Tom Blomberg, Marko Tuominen, Suvi Haukka (ASM, Finland)
Thermal based Atomic Layer Etching (th-ALEt) has opened a new horizon and triggered an increased interest in the Semiconductor Industry for the fabrication of sub-10 nm as well as complex 3D nano-devices. In the th-ALEt technique, a material is chemically etched from thermally activated surface by sequence of one or more reactants each separated by purge steps. Unlike the conventional anisotropic plasma etching, th-ALEt is isotropic, selective and its slow etch rate may possess excellent atomic-scale control. Most of the reported th-ALEt chemistries utilize hydrogen fluoride (HF from HF-pyridine) as one of the reactants. However, due to some safety concerns associated with the use and handling of HF, we have considered other fluorine donating compounds. In this work, we report niobium pentafluoride (NbF5) as an alternative to HF. Carbon tetrachloride (CCl4) is used as a co-reactant with NbF5 to etch aluminum oxide (Al2O3) as well as titanium nitride (TiN). The various attributes of the etching process like etch rates, selectivity and post-etch surface roughness were studied. It was found that NbF5 promotes the fluorination of Al2O3 and the fluorinated Al2O3 surface can be etched away by a subsequent exposure of CCl4 gas. TiN can be etched in continuous pulsed mode just by CCl4, while adding NbF5 to the process enables etch-rate control. The etch results and proposed reaction pathway for the etching of Al2O3 and TiN will be discussed in the presentation.
ALE1-TuM-7 Thermal Atomic Layer Etching of Amorphous and Crystalline Hafnium Oxide, Zirconium Oxide and Hafnium Zirconium Oxide
Jessica A. Murdzek, Steven M. George (University of Colorado - Boulder)
Thermal atomic layer etching (ALE) can be achieved with sequential surface reactions using the fluorination and ligand-exchange mechanism. For metal oxide ALE, fluorination converts the metal oxide to a metal fluoride. The ligand-exchange reaction then removes the metal fluoride by forming volatile products. Previous studies have demonstrated the thermal ALE of amorphous HfO2 and ZrO2 ALD films. No previous investigations have explored the differences between the thermal ALE of amorphous and crystalline films. The thermal ALE of crystalline films is important because amorphous films may not crystallize easily when they are too thin. Consequently, amorphous films may have to be grown thicker, crystallized, and then etched back to obtain the desired ultrathin crystalline film thickness.
This study explored the thermal ALE of amorphous and polycrystalline films of hafnium oxide, zirconium oxide, and hafnium zirconium oxide. HF was used as the fluorination reactant. Dimethylaluminum chloride (DMAC) or titanium tetrachloride was employed as the metal precursor for ligand-exchange. The amorphous films had a much higher etch rate per cycle than the crystalline films. The differences were most pronounced for hafnium oxide. At 250 °C with HF and DMAC as the reactants, the etch rate was 0.03-0.08 Å/cycle for crystalline HfO2 and 0.68 Å/cycle for amorphous HfO2 (See Supplemental Figure 1).
Under the same conditions at 250 °C with HF and DMAC as the reactants, the etch rate was 0.60-0.82 Å/cycle for crystalline ZrO2 and 1.11 Å/cycle for amorphous ZrO2. In comparison, the etch rate was 0.16-0.26 Å/cycle for crystalline HfZrO4 and 0.69 Å/cycle for amorphous HfZrO4. The etch rates for HfZrO4 were between HfO2 and ZrO2 for both the amorphous and crystalline films. When HF and TiCl4 were used as the reactants at 250 °C, the etch rates were smaller than the etch rates with HF and DMAC as the reactants for every material. The etch rates also increased with temperature for both the amorphous and crystalline films. The differences between amorphous and crystalline HfO2 are sufficient to obtain selective thermal ALE of amorphous HfO2.
ALE1-TuM-8 Isotropic Atomic Layer Etching of Cobalt with Smooth Etched Surfaces by using Cyclic Repetition of Plasma Oxidation and Organometallization
Sumiko Fujisaki (Hitachi R&D Group, Japan)
Isotropic atomic layer etching (ALE), which produces atomically precise, conformal removal, will have an important role in semiconductor manufacturing. This is because highly selective ALE has become necessary to deal with processing of new materials with the advances in minitualization of devices such as 3D structures. In the past several years, isotropic ALE of various materials has been reported which includes thermal ALE for metal oxides and thermal-cyclic ALE for nitride films . To meet the requirements concertning the variety of materials to be etched, isotropic ALE of cobalt must be developed. In 2018, thermal ALE of cobalt has been reported by using treatment with formic acid and ligands to produce volatile cobalt complexes . In this paper, the authors successfully demonstrate isotropic ALE of cobalt film with smooth etched surfaces, which is important issue in the development of ALE of cobalt.
The experimental apparatus used in this study is 300-mm ALE tool equipped with inductively-coupled plasma source and infrared lamps. The cyclic ALE process is composed of three step repetitions: oxidation of cobalt surface with oxygen plasma, organometallization of the cobalt oxide with a low acidity ligand vapor, and sublimation of the organemetallic cobalt by thermal annealing.
The etching depth of cobalt increased with increasing the number of repetitions of the cycle. For one cycle of etching, it was 1 nm high. The root-mean-square (RMS) roughness of etched cobalt surface was estimated to be 0.8 nm. It was found that formation of homogeneous CoO (II) was important because compound oxide such as Co3O4 (II & III) resulted in rough etched surfaces with columnar morphology. Furthermore, controllability of etching amount was substantially improved by using low reactivity ligands compared to high-reactivity ligands. These results implied that the combination of homogenious CoO formation, low-reactivity ligands, and sublimation was essential for achieving smooth etched surfaces and excellent controllability of etching amount.
In conclusion, we have obtained well-controlled etch front roughness and etching depth of cobalt, which can be applied to semiconductor process, by controlling reactions of both oxidation and metal complex formation using the 300-mm ALE apparatus.
 K. Shinoda et al., J. Phys. D: Appl. Phys. 50, 194001 (2017).
 C. Winter, AVS 65th, PS+EM+TF-ThM5 (2018).