ALD2019 Session ALE-SuP: Atomic Layer Etching Poster Session

Atomic layer etching of cobalt and iron has a number of important applications. The mechanisms of thermal dry etching of thin films of these metals were investigated using temperature programmed desorption (TPD) to understand surface chemistry involved in each reaction step. X-ray photoelectron spectroscopy (XPS) and microscopic investigations were used to characterize the surfaces obtained as a result of the etching process. Diketones, such as 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hfacH) and 2,4-pentanedione (acacH), were tested as etchants. It was determined that in order for the volatile etching products to be formed, the films had to be oxidized or chlorinated, since clean surfaces resulted in decomposition of the diketonates. The oxidized surfaces were shown to evolve volatile transition metal-containing products at temperatures much higher compared to those on surfaces pre-exposed to Cl₂. However, the mechanism of the etching process appeared to be more complicated on chlorine-exposed surfaces. For example, a number of products of a general formula of Co(hfac)_xCl_y were followed for hfacH reaction with cobalt films, and Co³⁺ was shown to participate in the process.

In semiconductor devices, the low electrical resistivity of tungsten coupled with its high resistance to electromigration have driven its use in contacts between transistor source/drains and higher layer interconnects. However, due to the diminishing dimensions of such devices, the need for precision controlled monolayer etch processes have become a necessity so as to enable current and future device architectures. Accordingly, Atomic Layer Etch (ALE) functions as a complementary process technique to well-established Atomic Layer Deposition (ALD) methodologies, such that sequential self-limiting etch processes can be targeted with the desired monolayer control for ultra-thin film material removal. Recently, thermal ALE processes for W have been reported by Parsons and co-workers^1,2 in which the W is first oxidized by a pulse of O₂ or O₃ gas and then the oxidized material is subsequently removed via gas phase pulses of WF₆ or WCl₆.

Herein, we present a first principles based computational analysis of this thermal ALE process for W metal using an oxidation step followed by introduction of WCl₆ as a co-reactant. We have investigated oxidants for the first pulse in the ALE sequence including O₂, N₂O and H₂O₂. It is shown that bulk oxidation of W is not spontaneous, but coverage of the W surface is dependent and subject to thermodynamic barriers of approximately 2 eV. The energetics associated with the removal of possible volatile etch species in the second ALE step such as WOCl₄ and WO₂Cl₂ will be presented. Additionally, the use of Cl₂ gas as an alternative to WCl₆ in the second ALE step has also been explored. A full reaction mechanism for these thermal atomic layer etch processes will be discussed.

1. Xie, W., Lemaire, P. C., Parsons, G. N. Thermally Driven Self-Limiting Atomic Layer Etching of Metallic Tungsten Using WF₆ and O₂. ACS Appl. Mater. & Interfaces, 2018, 10, 9147–9154.

2. Xie, W., Lemaire, P., Parsons, G.N. Self-Limiting Thermal Atomic Layer Etching of Tungsten Metal Using O₂ Oxidation and WCl₆ or WF₆: Role of Halogen Species in Temperature Dependence of ALE Reaction Rate. AVS ALE Workshop, 2018, Incheon, South Korea.

The task of controlled synthesis of semiconductor self-organizing nanostructures - quantum dots, filamentous nanocrystals, metallic nanodroplets - is extremely important, first of all, to create effective sources of single and entangled photons - the basis of quantum cryptography systems, as well as functional elements based on filamentary nanocrystals, single quantum dots and / or their complexes with a given topology, on the basis of which cellular automata, memory elements, integrated photonics and functional blocks quantum computing systems [1-3].

At the same time, drip epitaxy, based on separate deposition of components of groups III and V, allows not only to significantly expand the range of structures formed (quantum dots, rings, disks and complex, hybrid structures based on them), but also to realize independent control of the density and size of quantum dots, as well as to use for their creation virtually any A3B5 system, which is inaccessible to techniques based on the Stranski – Krastanov mechanism.

The use of structured GaAs and Si substrates and the features of droplet epitaxy techniques will effectively localize epitaxial growth at given points on the surface, thereby ensuring precise positioning and control of the parameters of synthesized nanostructures — metal nanoscale droplets (catalytic centers) and quantum dots based on them (in the case of GaAs).

The structuring of the substrates was carried out by a combination of methods of focused ion beams and plasma-chemical etching. However, after obtaining substrates with nanoscale relief, a broken layer was formed on the surface in the depressions caused by plasma and penetration of Ga ions after exposure to focused beams. To remove damaged layers, the best method is layer-by-layer etching of the GaAs surface [4].

To determine the penetration depth of gallium ions, a simulation was carried out and it was revealed that after treatment in chlorine plasma, 12 atomic disturbed layers remain, which were later removed using the atomic layer etching method. When using the “soft” etching mode, the angle of inclination of the nanoscale structures changed and the growth of quantum dots did not occur.

At the end of the experimental studies, samples were obtained with nanoscale surface profiling, a combination of methods of focused ion beams, plasma chemical etching, atomic layer etching, in which GsAs quantum dots were obtained by dropping epitaxy.

Silicon removal with true atomic fidelity has been shown to require fine control of reactant species concentrations, ion energies, and chamber conditioning. Such fine control of these process parameters is not straightforward to achieve, nor to verify, on legacy process equipment. This work highlights progress toward achieving atomic layer etching (ALE) of silicon wafers and packaged die in standard Cl₂/Ar plasma chemistry for failure analysis applications. Wafer-scale etch experiments show an etch rate decrease to below 1 nm/cycle with decreasing substrate temperature and Cl₂ surface modification step time, however the etch rate increases with the number of etch cycles due to insufficient control over Cl₂ reactant supply; this is especially problematic for silicon removal depths above a few hundred nm, required for failure analysis application. We will use time-resolved optical emission spectroscopy to demonstrate best-case reactant control in this system and suggest methods for achieving ALE-like etching. Furthermore, we observe that the D.C. voltage bias generated during the surface modification and sputter steps is significantly above target values during the first few seconds after plasma ignition for both steps. We have developed multi-step ignition schemes for both stages to ensure smooth transitions where the voltage bias remains very near zero during the Cl₂ plasma generation step and below the sputter threshold of the modified surface layer(s) during the sputter desorption step. For packaged die processing, we show that silicon removal is strongly dependent on both the carrier substrate type as well as die mounting scheme. Using the best developed method, we will present cross-section microscope images showing back-side handle silicon removal to within a few hundred nanometers of the still-functional active device area.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.This paper describes objective technical results and analysis. Any subjective views or opinions that might beexpressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Near-field scanning optical microscopy (NSOM) provides us with eyes for the nanoworld by combining the potentials of scanning probe technology with the power of optical microscopy. To acquire optical images beyond the diffraction limit, NSOM probes require a sub-wavelength optical aperture with wide cone angle of the probe for efficiently channeling the illumination light to the tip apex. Between the two NSOM aperture probe fabrication methods, chemical etching creates tips with wider cone angles of the probe. To determine the mechanism of probe formation and optimize the cone angle with the chemical etching method, different etching times were studied. Additionally, the NSOM probe surface was modified with different fluorescent compounds for high-resolution, fluorescence-based chemical sensing applications.

As the sizes of semiconductor devices continue to diminish, atomically controlled damage-less selective etching processes are absolutely crucial for the fabrication of such devices. Ligand-exchange processes of organic compounds deposited on metal surfaces, and low-energy ion or cluster beam processes are candidates for such highly selective precise etching processes. For the control of these processes, it is important to understand and control surface reactions of organic compounds and low-energy ions/clusters. Molecular beam experiments provide an understanding of the dynamics and kinetics of chemical interactions of gas molecules with solid surfaces. In this study, a new surface-reaction analysis system with molecular beams has been developed for the analyses of etching reactions. The system has differentially-pumped beam sources for low-energy ions, thermal-molecular and metastable radicals/clusters that independently irradiate the sample surface set in an ultra-high-vacuum (UHV) chamber. To study surface reactions, we detected the scattered species and desorbed products with a differentially pumped quadrupole mass spectrometer (QMS) and measured adsorbed chemical states on the surface during various beam irradiation by X-ray photoelectron spectroscopy (XPS). The QMS provided time-resolved measurements and could be synchronized with an ion or molecular beam. The system can experimentally simulate an atomically controlled process such as atomic layer etching (ALE) or atomic layer deposition (ALD). Also presented as sample experimental data obtained in this system are desorbed species, and surface chemical states during beam irradiation of transition metal (Ni, Cu etc.) surfaces with halogen and organic molecules (such as diketone).

Nanometer and angstrom scale etching is getting more critical as the critical dimension of semiconductor devices shrinks down to 10nm. Atomic layer etching (ALE) processes are being developed and studied to control etch depth in nanoscale and atomic scale by limiting the amounts of chemical reactants available on the surface with self-limited reactions. [1] With cyclic ALE processes, low surface roughness, high uniformity and high selectivity can be achieved with low damage to devices. [2-3]

In this work, cyclic plasma etching for SiO₂ and Si₃N₄ was developed and characterized with surface modification in an inductively coupled plasma (ICP) reactor with fluorocarbon, hydrofluorocarbon, and fluoro-ether plasmas. The process consists of two steps of surface modification and removal step. In the first step, thin fluoro- or hydrofluro-carbon layers are deposited on SiO₂ and Si₃N₄ surface with fluorocarbon, hydrofluorocarbon, and fluoroether plasmas. In the second step, the modified layers are removed with ions or radicals generated from Ar or O₂ plasmas. At the bias voltage increased, the incomplete etched region, the self-limiting etched region, and the sputtering region appeared, and lower etching rate was obtained using O₂ plasma than Ar plasma. Etching rate were compared at various conditions of reaction gases and plasma power and the rate could be controlled under 10 Å/cycle. Etching rate dependences are investigated on ion energy, etching time, FC film deposition, and precursor selection. Self-limited etching rate was obtained and higher selectivities of SiO₂/Si and Si₃N₄/Si etch rate were obtained with fluoroether.

References

[1] C. Li, D. Metzler, C. S. Lai, E. A. Hudson, and G. S. Oehrlein, J. Vac. Sci. Technol., A, 34, 041307 (2016)

[2] Y. Ishii, K. Okuma, T. Saldana, K. Maeda, N. Negishi, and J. Manos, Jpn. J. Appl. Phys., 56, 06HB07 (2017)

[3] K. Koh, Y. Kim, C.Kim, and H. Chae, J. Vac. Sci. Technol. A, 36, 01B106 (2018)

Copper has been used as the interconnects in the semiconductor memory devices because it has many advantages such as low resistance and low diffusivity. In addition, the electromigration phenomenon which causes wire deformation and breakage occurs less on copper [1]. Currently, copper has been etched through a damascene process because direct dry etching process has not been developed. However, the damascene process reveals some limitations in achieving fine patterns of several nanometers [2]. To solve this issue regarding the damascene process, the intense studies on copper patterning are being performed using conventional dry etching.Cyclic etching, as another approach to etch the copper films, can be a prospective etching technique. Cyclic etching including surface modification and its removal can provide the good etching performance of copper films by effectively inducing surface reaction and precisely controlling the etch depth. These results are attributed to the nature of self-limiting process and the removal of the film by layer-by-layer. There are possible various gas combinations in cyclic etching of copper films. In this study, cyclic etching with two sequential steps of surface modification and ion bombardment was performed. The surface modification and etch depth (etch rate) of copper film were confirmed using surface profilometer, scanning probe microscopy, and field emission scanning electron microscopy (FESEM) as a function of various parameters such as the time of surface modification (plasma exposure) and the bombardment energy of ions. Besides, the resultant etch profile and etch mechanism of copper film in the cyclic etching have been investigated by FESEM, X-ray photoelectron spectroscopy, and Raman spectroscopy.

Acknowledgments This research was supported by the MOTIE(Ministry of Trade, Industry & Energy (10080450) and KSRC(Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device.

References[1] A. Strandjord, S. Popelar, C. Jauernig, Microelectron. Reliab. 42, 265–283(2002) [2] H. Helneder, H. Ko¨rner, A. Mitchell, M. Schwerd, U. Seidel, Microelectron. Eng. 55, 257–268 (2001)

Atomic Layer Etching (ALE) has been developed almost 40 years ago, but has gained interest these last years for micro and nanoelectronic processes where high precision patterning is required. ALE consists in a sequential process relying on the self-limited adsorption of precursor radicals on the first monolayer(s) of material to be etched. Then, under a low energy ion bombardment, the etch products form and desorb until the adsorbed layer is depleted. This self-limited reaction removes a few monolayers. By repeating the cycle, the material is etched a few monolayers by a few monolayers.

ALE of SiO₂ has been achieved with a 3-step process and reported in the literature [1]. The first step is an Ar/C₄F₈ plasma without any ion bombardment. The oxide surface is then coated with a very thin fluorocarbon layer. The second step is a pure Ar plasma still with no RF self-bias voltage where C₄F₈ is pumped out of the chamber. The third and final step is an Ar plasma with about 10V RF self-bias voltage. The ion energy must be kept below the sputtering threshold. If the thickness of the FC layer is accurately controlled, the etch step can be self-limited.

Although quasi-ALE of SiO₂ has been demonstrated with this process, fluorocarbon contamination of chamber walls is an issue. This affects the reproducibility of ALE processes and hence chamber cleaning is required. Cryogenic Atomic Layer Etching (Cryo-ALE) is proposed as a potential solution. In this new process, the substrate is cooled to very low temperature by liquid nitrogen. The plasma phase deposition step is replaced by a physisorption step consisting in exposing the cooled substrate to a fluorocarbon gas flow. Under such conditions, species are adsorbed only at the cooled surface and hence wall pollution is mainly suppressed. Cryo-ALE of SiO₂ has been shown to be effective at -120°C. The details regarding the overall process are presented in G. Antoun’s abstract.

If the temperatures is raised of a few degrees (for instance to -110°C), no etching is observed since C₄F₈ does not significantly physisorb at such temperature. Therefore, understanding physisorption conditions as well as activation of etching using a physisorbed layer is relevant for a better control of the process. Desorption mass spectrometry experiments will be fully presented and discussed in this paper.

[1] Metzler et al., J. Vac. Sci. Technol. A 32(2), 020603-1 2014

Acknowlegment: The authors thank S. Tahara from Tokyo Electron Miyagi for helpful discussions.

While the Si-based electronics whose basis materials consisting of silicon substrate and silicon dioxide, have played a leading role in the semiconductor industry, germanium has also been receiving steady attention as a new channel material for its high carrier mobility. In particular, the studies on germanium oxide films of which the most are discussing mainly the Ge oxidation process on Ge substrates, have been conducted for several decades. However, the formation of GeO₂ film through the oxidation process restricts its own potential for utilization because it is only applicable on Ge substrates. Oxidation process conducted on the SiGe layer causes Ge condensation at its interface where Ge atoms remain nonbonded to oxygen due to the less negative Gibbs free energy to form the GeO₂ compared to SiO₂. In this context, the growth of GeO₂ film by the atomic layer deposition (ALD) enables its utilization to be enlarged even for use on Si substrate. For example, a literature examined the applicability of ALD GeO₂ film on Si substrate as a secure memory device employing its unusual film property of dissolving in water. However, the GeO₂ film inevitably exhibits unstable nature under the wet etching process, which means that the fundamental study on the GeO₂ film needs further explorations including dry etch characteristics.