ALD2019 Session ALE2-WeM: Materials Selective ALE

TiN ALE was achieved with Rapid Thermal Sources using O₂ plasma and Cl₂. TiN surface was oxidized to TiO₂ and the following Cl₂ flow absorbed Cl on the oxidized surface. This modified layer can be thermally removed under intense light source irradiation. Full removal of few nm blank ALD TiN layer is demonstrated. XPS observed surface composition change after each step. Surface smoothen was also observed after etching. The ultra-short thermal pulses enabled fine control of surface reactions in each ALE steps. Thermal ALE of Ge under similar approach will also be discussed.

Etching processes with atomic level precision are important in order to provide next-generation of semiconductor devices that have densely arrayed high-aspect-ratio structures. There is thus considerable interest in the development of isotropic atomic layer etching (ALE) for a variety of materials used in semiconductor manufacturing. One approach for isotropic ALE is rapid thermal-cyclic ALE, which consists of cyclic repetitions of plasma exposure at lower temperature and infrared (IR) lamp annealing. The plasma exposure produces self-limiting modified layers on the surface of the target materials, and the IR lamp annealing removes the modified layer by thermal desorption. The authors demonstrated rapid thermal-cyclic ALE of a variety of materials such as Si₃N₄, SiO₂, TiN, and W over the last several years.

Rapid thermal-cyclic ALE processes were originally developed on the basis of an understanding of plasma-surface reactions using in-situ analysis. As for Si₃N₄, in-situ x-ray photoelectron spectroscopy (XPS) analysis revealed that hydrogen and fluorine containing plasmas produce a self-limiting layer of ammonium hexafluorosilicate on the surface of Si₃N₄. Thermal desorption spectroscopy (TDS) analysis showed that lamp annealing decomposes the ammonium hexafluosilicate into SiF₄, HF, and NH₃. The result of TDS is consistent with the result of thermodynamic calculation of the decomposition of ammonium hexafluorosilicate. This technology is also applicable to other nitride films besides Si₃N₄. The authors have demonstrated self-limiting isotropic ALE of TiN using this technology.

A 300-mm ALE apparatus that consists of an inductively coupled plasma (ICP) source for downflow radicals, IR lamps for rapid thermal annealing, and in-situ ellipsometry for thickness monitoring was developed for rapid thermal-cyclic ALE. A self-limiting nature in both the plasma exposure step and the lamp annealing step has previously been demonstrated for Si₃N₄, TiN, and W. Conformal, highly selective etching for patterned samples was confirmed by transmission electron microscopy (TEM). Layer by layer ALE for Si₃N₄ of more than 100 cycles was confirmed by in-situ ellipsometry. Moreover, it was demonstrated that etching selectivity between different materials could be switched from infinitely selective to nonselective by adjusting the lamp annealing time. In this talk, applications of rapid thermal-cyclic ALE for a variety of dielectric and metal films will be presented.

Atomic layer etching (ALE) applies sequential deposition, reactant purge and etching steps with a short processing step length to establish self-limited material removal and atomic scale precision. The reactants during the ALE deposition steps may exhibit material-selective deposition based on the chemical affinity of precursor gases to the substrate material and nature of interfacial bonding. Integrating the feature of material -selective deposition with an etching step opens a new processing window for selective ALE. In this work, we evaluated the deposition behaviors of different hydrofluorocarbon (HFC) precursors, i.e. mixtures of methane (CH₄) with trifluoromethane (CHF₃) and mixtures of methane with octafluorocyclobutane (C₄F₈), on Si and HfO₂ surfaces, respectively. This is followed by the investigation of substrate-dependent selective deposition using a mixture of HFC precursors to achieve HfO₂ etching and etching selectivity relative to Si. Our results show that during the purge step of ALE sequences using CH₄/CHF₃ selectively deposit a fraction of a nm thick FC layers on Si surface while self-desorption is observed on the HfO₂ surface . In contrast, mixtures of CH₄ with C₄F₈ etchant do not show this self-desorption behavior for HfO₂. By utilizing the selective deposition behavior seen for CH₄/CHF₃-based ALE in conjunction with low energy Ar ion bombardment, we were able to remove the top HfO₂ layer while simultaneously forming a FC passivation layer on the underlying Si surface. In order to confirm the etching performance of CH₄/CHF₃-based ALE of HfO₂, X-ray photoelectron spectroscopy (XPS) was used to study whether any HfO₂ remains on the sample at the end of the processing cycle where the in-situ ellipsometry indicated a depletion of the HfO₂ layer . The XPS results show that after the ALE processing the Hf-O peaks no longer exist in the Hf4f and O1s spectra. Instead, a weak intensity of the fluorinated Hf peaks are observed, suggesting the HfO₂ layer with an initial thickness of 2.9 nm was removed and a few hafnium etching byproducts were left on the substrate. These results support the concept that gas pulsing of complex HFC precursors during ALE sequences provides the opportunity to achieve material-selective deposition and enable ALE selectivity of HfO₂ relative to Si.

Stringent processing windows are required for the fabrication of sub‑7‑nm semiconductor devices, which in turn places severe constraints on conventional plasma­‑assisted etching. Atomic layer etching (ALE) is a promising etching technique that can provide high etch fidelity, directionality, layer‑by‑layer removal, and selectivity to meet the stringent processing demands. Plasma‑assisted ALE of SiO₂ and SiN_x typically consists of two sequential half‑cycles: fluorocarbon (CF_x) deposition from a fluorocarbon plasma followed by an Ar plasma activation step. Typically, selectivity is achieved through manipulating the plasma and processing parameters. Recently, we proposed a methodology to further increase etch selectivity by selective prefunctionalization of the SiO₂ or SiN_x surface with hydrocarbons. We show that the abundance of hydrocarbon on the prefunctionalized surface promotes the formation of an etch inhibiting graphitic carbon film after just a few ALE cycles.

In this study, we used in situ attenuated total reflection Fourier transform infrared (ATR‑FTIR) spectroscopy and in situ 4‑wavelength ellipsometry during ALE to monitor the surface reactions, film composition, and net film thickness. We show that cyclic azasilanes can be used to selectively functionalize SiO₂ over SiN_x. Figure 1 shows the infrared spectra after 5 ALE cycles of SiO₂ etching with (blue) and without (black) surface prefunctionalization. For the prefunctionalized surface, we observe an increase in absorbance from ~1500 – 1800 cm^‑1, assigned to a graphitic hydrofluorocarbon film. After just 4 ALE cycles, this graphitic hydrofluorocarbon film reaches a thickness that acts as an etch stop layer. As a result, after 10 cycles, the etched thickness of SiO₂ was ~23% of the case where there was no surface functionalization. This methodology can therefore be used to enhance overall etch selectivity for SiN_x over SiO₂. Further, we will discuss the role of Ar⁺ ion energy during the Ar plasma activation step on graphitic hydrofluorocarbon film formation. The length and structure of the hydrocarbon chain on the prefunctionalized surface will also be addressed.