Applied Science and Convergence Technology 2020; 29(3): 41-49
Published online May 30, 2020
Copyright © The Korean Vacuum Society.
Doo San Kima, Ju Eun Kima, You Jung Gilla, Yun Jong Janga, Ye Eun Kima, Kyong Nam Kimb, Geun Young Yeoma,c, and Dong Woo Kima,*
aSchool of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
bSchool of Advanced Materials Science and Engineering, Daejeon University, Daejeon 34520, Republic of Korea
cSKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University, Suwon 16419, Republic of Korea
Correspondence to:E-mail: firstname.lastname@example.org
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To determine a suitable etching method for the fabrication of semiconductors with a few nm or less thickness, many atomic layer etching (ALE) techniques have been studied. Previously, ALE studies on silicon-based materials have been reported; however, recently, the number of ALE studies on metals have also been increasing. Metals are applied to semiconductor devices as electrodes and hard mask materials, thus, there is an increasing need for precise etching using ALE techniques. Therefore, in this brief review, recently reported ALE studies on metals will be summarized, and the ALE process results for various metals will be described for two ALE methods, namely, anisotropic ALE and isotropic ALE.
Keywords: Atomic layer etching, Metal, Anisotropic, Isotropic, Precise etch, Low damage
In the semiconductor industry, the integration of semiconductors by miniaturization is still underway, in accordance with Moore’s law. However, as the integration further progresses, it is difficult to realize high integration of semiconductors with conventional process technology. To fabricate a semiconductor with a line width of less than a few nanometers, a new precision etching technology is required. This technology should be able to ensure precision etching thickness control, no physical and chemical damage caused by plasma, no loading effect, etc., which cannot be ensured by conventional etching technology [1-4]. In addition, next generation device structures are being built as 3-dimensional (3D) complex structures, such as gate all around (GAA) field effect transistors (FETs) and vertical FETs. Therefore, a new precision etching technology is required, not only for anisotropic etching, but also for isotropic etching. To address this issue, atomic layer etching (ALE) has been actively investigated as one of the precision etching technologies [5-7]. Recently, ALE studies on various materials, such as insulators, 2-dimensional materials, metals, and semiconductor materials, have been reported. In particular, the number of ALE studies on metals have been increasing recently [8-11]. ALE of metals, such as Cu, W, and Co, can be applicable to the etching of electrodes for semiconductor devices and the etching of mask materials used for photolithography, such as Cr [12-16]. In addition, ALE research is being conducted on materials such as Co, Fe, Ni, and Pt, used in the ferromagnetic material layer of next generation nonvolatile memory devices, such as magnetoresistive random access memory devices . Moreover, ALE has been studied on metal nitrides, such as TiN, used for diffusion barriers and complementary metal-oxide-semiconductor gate electrodes and AlN, applicable to micro-electro-mechanical systems [18,19].
Conventional reactive ion etching (RIE) and ALE are compared in Fig. 1. In the case of RIE, reactive gases used for etching are dissociated and ionized and materials are rapidly etched through a combination of chemical and physical reactions. However, after the etching by reactive ions, chemical and physical damage remains on the surface, and it is difficult to precisely control the etch depth. ALE is a cyclic etch process, consisting of a reactive gas adsorption (chemisorption) step, which forms a chemically modified layer on the material surface, and a desorption step, which removes the modified layer formed during the adsorption step anisotropically or isotropically; the purge steps between the adsorption and desorption steps remove the gases that remain in the chamber. ALE is a method of modifying and etching the surface layer of the material through the self-limited reaction, which allows the precise thickness control of etching on the angstrom scale. It has the advantages of low physical and chemicaletch damage, high etch selectivity, excellent etch uniformity, and an improved loading effect, compared to RIE [20-22]. The following sections briefly introduce anisotropic/isotropic ALE for metals.
ALE can be divided into anisotropic ALE and isotropic ALE; anisotropic ALE primarily uses plasma. First, the surface is modified by adsorbing a reactive gas or radicals on the surface of the material to be etched, for the formation of a chemically modified layer. The surface modified layer is then removed, preferentially by ions with a controlled energy using plasma, and the residual gases that remain during the adsorption step and the desorption step are then purged during a purge step, between adsorption and desorption steps. Therefore, it is possible to remove materials vertically, layer by layer. This method has advantages, such as low ion damage, high etch selectivity, low surface roughness, and a precise etch depth/cycle.
For the adsorption step, reactive gas molecules/radicals can be adsorbed on the substrate surface by three different adsorption states, such as reversible saturation, irreversible non-saturation, and irreversible saturation, as shown in (a)–(c), respectively. Reversible saturation, is a physisorption state in which the reactive gas molecules/radicals are adsorbed with a weak bond of van der Waals force on the substrate surface. For the adsorption by reversible saturation, when the adsorption gas flow is stopped, the amount of the adsorbed species on the substrate surface is gradually decreased by the desorption of the adsorbed gas molecules/radicals. Irreversible non-saturation is another physisorption state having multilayer deposition of the reactive gas species, without saturation on the substrate surface. In this case, even though the reactive gas flow is stopped, no desorption of adsorbed species is observed. Irreversible saturation is obtained when the reactive gas molecules/radicals form strong chemical bonds with the atoms on the substrate surface. The reactive gas molecules/radicals are saturated by one monolayer on the substrate surface and, even though the reactive gas flow is stopped, the chemisorbed reactive gas molecules/radicals on the surface remain without desorption. For ALE, similar to atomic layer deposition (ALD), only one layer of the substrate surface needs to be chemisorbed uniformly without desorption or spontaneous etching; therefore, as the adsorption condition, Fig. 2(c) is required. (Table I shows the characteristics of physical adsorption and chemical adsorption.)
When reactive gas molecules/radicals are chemically adsorbed on the surface of the substrate, the binding energy of the surface atoms are changed. As the reactive gas molecules/radicals are adsorbed on the surface, as shown in Fig. 3(a), the surface atoms in the substrate form strong chemical bonds with the reactive gas molecules/radicals with the binding energy (Ea). Due to the strong binding of surface atoms in the substrate with the reactive gas molecules/radicals, the binding energy between the surface atoms and the second layer atoms are weakened to a lower binding energy (Es) from the binding energy between bulk atoms (Eb). Because Es is lower than Ea and Eb (Ea is generally higher than Eb), by using an ion energy higher than Es and lower than Ea during the desorption step, it is possible to remove one monolayer per cycle during the ALE process. In general, highly reactive halogen gas-based radicals, such as fluorine-based (CF4, CHF3, etc.), chlorine-based (Cl2, BCl3, etc.), and bromine-based (HBr) radicals, are used for adsorption, and these radicals form a strong bond with the atoms on the substrate surface. Therefore, the binding energy between the atoms of the second layer and those of the first layer bonded to halogen atoms is weakened from Eb to Es [23-25].
Figure 3(b) shows the etch amount per cycle, measured as a function of the average ion energy used during the desorption step after the adsorption in the ALE process for a fixed desorption time. It can be divided into three sections, and, in the region where the energy of the ions is low (in general, the energy of the ion has an energy distribution and, in this case, some ions have an energy higher than Es and the rest have an energy lower than Es, and, with the increase in ion energy, the percentage of ions having Es is increased until the ALE window is reached), only some of the atoms on the surface layer are removed, owing to the insufficient energy of the ions to remove the modified (chemisorbed) layer on the surface (and insufficient removal of surface atoms for the ions with an energy higher than Es during the fixed desorption time). With an increase in the average ion energy, all of the chemisorbed layer can be removed within the desorption time, without noticeable sputtering of the materials, and it can continue until the ion energy is higher than Eb. Therefore, the etch depth per cycle at this period stays the same until the ion energy reaches the energy for noticeable sputtering. However, if the ion energy is higher than Eb (the sputter threshold energy), the etch depth per cycle is noticeably increased with the increase in ion energy by sputter, etching the atoms located under the chemisorbed layer after the removal of the top chemisorbed layer. Therefore, there is an appropriate energy range for ions for ALE and it is important to control it precisely. For the anisotropic ALE, inert gas ions, such as He+, Ne+, and Ar+, are typically used as the desorption ion and the ion energy is given by biasing the substrate for the conventional etch system; for the ion beam source, the ion energy is given by accelerating the ion using the grid system in the ion source.
In addition to the ion energy, obtaining a saturated etch depth per cycle, which corresponds to one monolayer per cycle, requires the full coverage of the surface by chemisorption by supplying a sufficient adsorption gas molecule/radical dose to the substrate surface. The chemisorbed species must be fully removed by supplying a sufficient desorption ion dose with adequate energy to the surface. The effect of the adsorption species dose and desorption species dose on the etch per cycle are shown in (c) and 3(d), respectively. The saturated etch per cycle is obtained only when a sufficient adsorption species dose and ion dose are supplied during the adsorption step and desorption step, respectively. When an insufficient dose is supplied during the adsorption step and/or desorption step, a saturated etch per cycle behavior is still observed; however, in this case, it is not only the etch per cycle that can be varied by changing the insufficient dose during the adsorption step and/or the desorption step, but an increased surface roughness is also observed.
During the chemical adsorption step of the reactive gas molecules/ radicals on the substrate surface, spontaneous etching can occur. During the desorption step by ion bombardment, in addition to the desorption of the chemisorbed layer (a compound of reactive adsorption species with first layer atoms on the substrate), the atoms exposed after the removal of the chemisorbed layer can be sputtered if the energy of the ions is higher than Eb (in the ion energy distribution, even though the average ion energy is lower than Eb, some of ions can have an energy higher than Eb). Spontaneous etching (α) and sputter etching (β) can deteriorate the ideal saturation behavior of the ALE, which may be called the “ALE ideality %.” 
For the desorption ion species, rather than using inert gas ions for the momentum transfer to remove the chemisorbed layer formed during the adsorption step, reactive ions can be used to form more volatile compounds with the chemisorbed layer on the substrate. Park
Figure 6 shows the etch depth and EPC measured as a function of etch cycles for the physical method (O/Cl radical adsorption and Ar+ ion desorption) and the chemical method (O radical adsorption and Cl+ ion desorption); both these methods are shown in Fig. 5. The EPCs for the two methods were not the same, at ~1.5 and ~1.1 Å/cycle for the physical method and chemical method, respectively. However, the etch depth increased linearly with the number of etch cycles for both cases; therefore, the Cr etch depth could be precisely controlled in the angstrom scale using both ALE methods. The higher EPC for the physical method could be related to the non-ideality factor α caused by possible spontaneous etching during the adsorption period.
Unlike anisotropic ALE, thermal isotropic ALE is a method in which reactant A is reacted on the surface to form a modified layer and reactant B is then reacted to remove the modified layer by converting it into a volatile compound. Figure 10 shows the process configuration of the isotropic ALE. In each step, the whole surface layer reacts thermally through a self-limiting reaction and because there are no directional ions involved in the reaction process or desorption process, the ALE occurs isotropically. Thermal ALE is applicable to manufacturing various next generation 3D devices and area selective ALD, for the selective and precise removal of materials deposited on unwanted areas during the selective ALD. Depending on how the reactant reacts with metal during the thermal ALE process, it is possible to etch the atomic layer of metal through various surface modification methods, such as “oxidation-conversion-fluorination,” “oxidation-fluorination,” and “condensation” reactions, which are shown in Fig. 11. Generally, for the thermal ALE of metal, the oxidation of the metal surface is required before the next step of the desorption process.
In this process, ∆G is negative, so the reaction occurs spontaneously. WO3 is then converted to B2O3 by BCl3 and the corresponding reaction is as follows:
Because this process also has a negative value of ∆G, the reaction occurs spontaneously and the WOxCly that is formed after the reaction is removed in a gas state. The B2O3 layer, formed by conversion, is removed by HF and the corresponding reaction is as follows:
In this process, the B2O3 layer is removed and the increased film thickness is reduced. By repeating this process, it was confirmed that the thickness of W decreased linearly as the number of cycles increased and the EPC was confirmed to be ~2.44 Å/cycle. Among the three steps, the first oxidation step forming the WOx is not a self-limiting step and the WOx thickness increases with an increase in the oxidation time, even though the other two steps are self-limited. Therefore, the WOx formed during the first oxidation step needs to be carefully controlled so as to not have a thick WOx layer on the W surface during the ALE.
Lee et al. has reported the thermal ALE of TiN through the “oxidation-fluorination” method . The thermal ALE process of TiN is shown in Fig. 13. First, when oxidation is performed using O3 on the TiN surface, TiO2 is formed. Even though the formation of TiO2 from TiN is also not a self-limiting step, it appears that the thickness of TiO2 is approximately controlled by the diffusion of O into TiN. The corresponding reaction is as follows:
NO and O2 are removed in the gas state and TiO2 is formed on the surface.
In the next step, the TiO2 formed on the surface is converted to TiF4 by the fluorination step, via the reaction with HF. The corresponding reaction is as follows:
Volatile TiF4 is removed and TiN is etched. Figure 14 shows the etch depth by repeating this process and comparing the etch depth with other materials. The ALE process was performed at 250 ℃ and it can be observed that the TiN thickness decreases linearly as the number of cycles increase; however, the thicknesses of SiO2 and Si3N4 remain constant, even as the number of cycles increased, confirming that highly selective TiN etching can be achieved. Besides TiN, other metal nitrides can also be etched by the “oxidation-fluorination” thermal ALE method, which is also applicable to metal carbides, metal sulfides, and elemental metals.
Finally, the “condensation” thermal ALE is a method investigated by Chen
The anisotropic ALE and isotropic ALE studies of metals that are currently being investigated are briefly summarized in Table II, detailing the adsorption and desorption chemistry, process temperature, and etch per cycle for each material.
Next generation semiconductor devices require precise etching, ultra-high selective etching, etching with no damage, etc. Owing to these requirements, ALE, which can remove a self-limited atomic layer per etch cycle by a cyclic etch method, is being widely investigated. This technology is required to etch the materials used for GAA FETs, vertical FETs, etc., requiring Å scale precision in etching, with extremely high etch selectivity. ALE methods are divided into anisotropic ALE, using directional ions during the adsorption step or plasma during the desorption step, and isotropic ALE, using chemical reactants for reactions and vaporization of the compounds formed during the previous steps at a heated state. For metals, it has been found that ALE with a few Å per cycle is also possible without increasing the surface roughness. However, the investigation of metal ALE is still limited and further investigation is required. For devices using metallic compounds, such as magnetic devices, particularly spin-transfer torque magnetoresistive random access memory and phase change random access memory, ALE techniques for various metals may be required.
Moreover, there are some challenges in applying the metal ALE process directly to the industry. The biggest challenge is the long process time resulting in poor throughput, similar to the ALE of other materials. In addition, it is difficult to precisely control ion energy in the case of anisotropic ALE using plasma, which makes it difficult to obtain ultrahigh selective etching, and in the case of thermal ALE, a high process temperature and the chemicals used in the etching may damage the surface after the ALE. Therefore, it is not only the processing techniques, but also the tools for metal ALE that need to be further investigated for the successful application of metal ALE to next generation semiconductor devices.
This work is supported by the MOTIE (Ministry of Trade, Industry & Energy (20003588) and KSRC (Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device and the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (2018 R1A2A3074950).