Layer Etching Research Articles

(Invited) Volatile Products during Thermal Atomic Layer Etching of Metal Oxides and Elemental Metals

Atomic layer etching (ALE) utilizes sequential, self-limiting reactions to remove material with atomic level control. The first reaction modifies the surface and the second reaction volatilizes the modified surface layer. Thermal ALE removes the modified surface layer using thermal reactions. Thermal Al2O3 ALE was first reported based on fluorination and ligand-exchange reactions [1]. HF was used to fluorinate Al2O3 and Sn(acac)2 was employed to remove fluorinated Al2O3 by ligand-exchange reactions [1]. Subsequent studies revealed that Al2O3 could also be etched with other metal precursors via ligand-exchange such as trimethylaluminum (TMA) [2]. Many other oxide and nitride materials have also been etched using the fluorination and ligand-exchange mechanism [3].Successful thermal ALE requires that the etch products be stable and volatile. However, what are the volatile products? The most direct way to observe volatile etch products is to employ quadrupole mass spectrometry (QMS). Recently, QMS was used to study the thermal ALE of Al2O3 using HF for fluorination and TMA for ligand-exchange [4]. Gas-phase species were monitored after each reactant exposure. The volatile etch products were consistent with fluorination and ligand-exchange reactions. The ligand-exchange product was AlF(CH3)2 (dimethylaluminum fluoride (DMAF)). DMAF was observed as a dimer with itself (DMAF-DMAF) or with TMA (DMAF-TMA) [4].The QMS signals in these previous studies of thermal Al2O3 ALE required integration over time. To measure the signals in real time, a new QMS reactor was designed with several improvements. First, the etch products in a N2 carrier gas were allowed to form a molecular beam through expansion. This molecular beam entrained the volatile etch products and directed the etch products with line-of-sight to the QMS ionizer. A skimmer also ensured that the QMS ionizer could be maintained at low pressure in a differentially pumped chamber. This new QMS reactor was then employed to examine different thermal ALE reactions.Recent work using quartz crystal microbalance (QCM) has shown that thermal Ni ALE can be performed using chlorination and ligand-addition reactions [5]. The Ni was first chlorinated using SO2Cl2(sulfuryl chloride). The chlorinated Ni surface was then volatilized by ligand-addition using P(CH3)3 (trimethyl phosphine). To reveal the volatile etch products, NiCl2 powder was used to mimic the chlorinated Ni. NiCl2was exposed to P(CH3)3 and QMS was used to identify the volatile etch products. These studies revealed that the major volatile product was NiCl2(P(CH3)3)2. The isotopic abundance of Cl and Ni helped to confirm this identification. NiCl2(P(CH3)3)2 was observed with peak intensity at 92°C.Motivated by the successful identification of NiCl2(P(CH3)3)2, other metals were examined such as Pd and Pt. PdCl2 and PtCl2 were again used to model chlorination of Pd and Pt using SO2Cl2. As expected from a ligand-addition mechanism, exposure of P(CH3)3 to PdCl2 and PtCl2 led to the observation of PdCl2(P(CH3)3)2 and PtCl2(P(CH3)3)2, respectively. PdCl2(P(CH3)3)2 and PtCl2(P(CH3)3)2 were observed with peak intensities at 150°C and 198°C, respectively. The observation of these metal complexes suggests that Pd and Pt could both be etched using chlorination and ligand-addition reactions.Thermodynamic calculations were also performed to understand the ligand-addition reactions. These calculations revealed that the ligand-addition of P(CH3)3 to NiCl2, PdCl2 and PtCl2 was favorable. The addition of the first P(CH3)3 to NiCl2, PdCl2 and PtCl2 led to increased stability by 2.3, 1.7 and 2.9 eV, respectively. The addition of the second P(CH3)3 to NiCl2P(CH3)3, PdCl2P(CH3)3, and PtCl2P(CH3)3 further increased the stability by 1.9, 1.5 and 1.3 eV, respectively. The formation of metal complexes with Cl and P(CH3)3 ligands is also predicted by the 16 or 18 electron rule for inorganic complexes. The Cl and P(CH3)3ligands add electrons to the outer shell of the metals to form complexes with 16 or 18 electrons. In the case of NiCl2(P(CH3)3)2, PdCl2(P(CH3)3)2 and PtCl2(P(CH3)3)2, the metal complexes all have 16 electrons in their outer shells. Y. Lee and S.M. George, “Atomic Layer Etching of Al2O3 Using Sequential, Self-Limiting Thermal Reactions with Sn(acac)2 and HF”, ACS Nano 9, 2061 (2015). Y. Lee, J.W. DuMont and S.M. George, “Trimethylaluminum as the Metal Precursor for the Atomic Layer Etching of Al2O3 Using Sequential, Self-Limiting Thermal Reactions”, Chem. Mater. 28, 2994 (2016). S.M. George, “Mechanisms of Thermal Atomic Layer Etching”, Acc. Chem. Res. 53, 1151 (2020). J.W. Clancey, A.S. Cavanagh, J.E.T. Smith, S. Sharma and S.M. George, “Volatile Etch Species Produced During Thermal Al2O3 Atomic Layer Etching”, J. Phys. Chem. C 124, 287 (2020).J.A. Murdzek and S.M. George, “Thermal Atomic Layer Etching of Nickel Using SO2Cl2 and P(CH3)3”, Presentation at AVS ALE2020 Workshop, June 29-30, 2020.

(Invited) Precise Control of Nanoscale Cu Etching via Gas-Phase Oxidation and Chemical Complexation

We present a cyclic process for selective and anisotropic atomic layer etching of copper: an oxygen plasma modulates the depth and directionality of the oxidized layer while formic acid vapor selectively removes the copper oxide scale from the metallic copper. Via density functional theory, with finite temperature and pressure free energy corrections, we evaluate the feasibility of formation of gas-phase Cu(II) and Cu(I) complexes with formate, water, formic acid, and combinations thereof as ligands. These complexes result from the neutralization reaction between copper oxide (CuO and Cu2O) and formic acid, with and without water. We identified and evaluated the formation free energies of formato, formic acid, aquahydroxo, and aquaformato complexes of Cu(II) and Cu(I). Under relevant experimental pressures, we find the water-free dimeric tetra(μ-formato)dicopper(II) “paddlewheel” complex (Cu2(HCOO)4) to be the most favorable etching product, with its formation reaching equilibrium conditions from CuO. The most likely precursor for the dimer is the diformatodi(formic acid)copper(II) monomer, which favorably dimerizes under the same water-lean condition at which the dimer persists. Stabilization of gas-phase Cu (oxide) derivatives thus can be achieved through complexation, enabling gas-phase etching of Cu. This work provides complementary experimental and theoretical studies that illuminate the nature of highly controlled etching with formic acid of nanoscopic CuO(s) layers covering Cu nanoarchitectures, which is relevant for the fabrication of next-generation integrated circuits.

Investigation on Ge0.8Si0.2-Selective Atomic Layer Wet-Etching of Ge for Vertical Gate-All-Around Nanodevice.

For the formation of nano-scale Ge channels in vertical Gate-all-around field-effect transistors (vGAAFETs), the selective isotropic etching of Ge selective to Ge0.8Si0.2 was considered. In this work, a dual-selective atomic layer etching (ALE), including Ge0.8Si0.2-selective etching of Ge and crystal-orientation selectivity of Ge oxidation, has been developed to control the etch rate and the size of the Ge nanowires. The ALE of Ge in p+-Ge0.8Si0.2/Ge stacks with 70% HNO3 as oxidizer and deionized (DI) water as oxide-removal was investigated in detail. The saturated relative etched amount per cycle (REPC) and selectivity at different HNO3 temperatures between Ge and p+-Ge0.8Si0.2 were obtained. In p+-Ge0.8Si0.2/Ge stacks with (110) sidewalls, the REPC of Ge was 3.1 nm and the saturated etching selectivity was 6.5 at HNO3 temperature of 20 °C. The etch rate and the selectivity were affected by HNO3 temperatures. As the HNO3 temperature decreased to 10 °C, the REPC of Ge was decreased to 2 nm and the selectivity remained at about 7.4. Finally, the application of ALE in the formation of Ge nanowires in vGAAFETs was demonstrated where the preliminary Id–Vds output characteristic curves of Ge vGAAFET were provided.

First Demonstration of Novel Vertical Gate-All-Around Field-Effect-Transistors Featured by Self-Aligned and Replaced High-κ Metal Gates.

A novel n-type nanowire/nanosheet (NW/NS) vertical sandwich gate-all-around field-effect-transistor (nVSAFET) with self-aligned and replaced high-κ metal gates (HKMGs) is presented for the first time, aiming at a 3 nm technology node and beyond. The nVSAFETs were fabricated by an integration flow of Si/SiGe epitaxy, quasi-atomic layer etching (qALE) of SiGe selective to Si, formation of SiGe/Si core/shell NS/NW structure, building of nitride dummy gate, and replacement of the dummy gate. This fabrication method is complementary metal oxide semiconductor (CMOS)-compatible, simple, and reproducible, and NWs with a diameter of 17 nm and NSs with a thickness of 20 nm were obtained. Excellent control of short-channel-effects was presented. The device performance was also investigated and discussed. The proposed integration scheme has great potential for applications in chip manufacturing, especially with vertical channel devices.

Removal of GaN film over AlGaN with inductively coupled BCl3/Ar atomic layer etch

Atomic layer etching (ALE) of thin film GaN (0001) is reported in detail using sequential surface modification by BCl3 adsorption and removal of the modified surface layer by low energy Ar plasma exposure in a reactive ion etching system. The estimated etching rate of GaN is ∼ 0.74 nm/cycle. The GaN is removed from the surface of AlGaN after 135 cycles. To study the mechanism of the etching, the detailed characterization and analyses are carried out, including scanning electron microscope (SEM), x-ray photoelectron spectroscopy (XPS), and atomic force microscope (AFM). It is found that in the presence of GaCl x after surface modification by BCl3, the GaCl x disappears after having exposed to low energy Ar plasma, which effectively exhibits the mechanism of atomic layer etch. This technique enables a uniform and reproducible fabrication process for enhancement-mode high electron mobility transistors with a p-GaN gate.

An attempt to generate mechanical white etching layer on rail surface on a new rolling contact test bench

This study presents a successful attempt to reproduce White Etching Layer (WEL) under pure mechanical conditions, with the use of a new experimental test bench with tests conditions representative of the wheel-rail contact. The effect of the presence of a run-in or corrosive fuse layer at the rail surface has a great effect on the microstructure evolution. First, a tribological analysis of the effect of the surface preparation on material flows in the contact, hardness evolution, and formation of WEL is presented. Then a multiscale characterization approach combining optical microscopy, scanning electron microscopy and near surface EBSD characterization at the sub-μm level has been carried out to understand the formation mechanism of such mechanically formed WEL.The different scenarios of the wear behavior and microstructural transformation were drawn. The influence of the conditions of preparation of the sample surfaces, in particular of the presence of a tribological fuse layer (either a run-in layer or a corrosive layer) on the wear fatigue competition and the WEL formation, were also studied.

Gas-phase surface functionalization of SiNx with benzaldehyde to increase SiO2 to SiNx etch selectivity in atomic layer etching

We show that the functionalization of a SiNx surface with benzaldehyde can be used to increase the overall SiO2 to SiNx etch selectivity during atomic layer etching (ALE). The surface reactions, composition, as well as film thickness during ALE are monitored using in situ surface infrared spectroscopy and in situ four-wavelength ellipsometry. Prior to ALE, we show that benzaldehyde can selectively populate a plasma-deposited SiNx surface with benzene rings through a self-limiting reaction with surface —NHx (x = 1, 2) groups, while no reaction occurs with —OH groups on a plasma-deposited SiO2 surface. Using alternating cycles of a C4F8/Ar and an rf-biased Ar plasma, ALE is performed on bare and benzaldehyde-exposed SiNx and SiO2. Over the first 16 ALE cycles, the SiO2 to SiNx etch selectivity increases from ∼2.1 to ∼4.5 due to the selective functionalization of the SiNx surface with benzaldehyde. A detailed analysis of the infrared spectra of the bare and benzaldehyde-functionalized SiNx surfaces shows that benzaldehyde promotes the formation of a more graphitic hydrofluorocarbon film on the SiNx surface, which inhibits etching.

Selective atomic layer reaction between GaN and SiN in HBr neutral beam etching

We investigated higher selective etching between SiN and GaN using an HBr neutral beam (NB) and found that it exhibited a more selective reaction compared to Cl2 NB. The etching rate of GaN mainly depended on the desorption rate of the etching product (GaClx or GaBrx) assisted by the bombardment of NB. As a result, in the case of the HBr neutral beam, the GaN etching rate was drastically decreased at the atomic layer level because the vapor pressure of the etching product, GaBrx, was much lower than that of GaClx. On the other hand, the dominant etching progression of SiN was mainly caused by neutral beam bombardment energy, and the desorption of the etching products (SiClx or SiBrx) was almost the same in both cases because the vapor pressures of the etching product, SiClx and SiBrx. In particular, in the case of the HBr neutral beam, the neutral beam energy dependence of the surface reaction layer of SiN was larger than that of Cl2 NB. As a result, the etching selectivity between SiN and GaN in HBr NB improved to 2.1 at 10 W of bias power, whereas that with Cl2 NB was saturated at 0.41 at more than 10 W of bias power. We found that the surface atomic layer etching reaction could precisely control the desorption rate of the etching product by optimizing the neutral beam bombardment energy and gas chemistry. The HBr neutral beam process can achieve atomic layer level selective reactions on the SiN/GaN structure.

Surface modification of PS polymer by oxygen-atom treatment from remote plasma: Initial kinetics of functional groups formation

Self-limiting surface reactions are promising for atomic layer etching of materials. Classical treatments with plasmas do not allow for self-limiting functionalization but cause the formation of several functional groups and degradation. To avoid this, the polystyrene was oxidized in the plasma afterglow with the O-atom density 7 × 1017 m−3 and dissociation fraction 7%. X-ray Photoelectron Spectroscopy was used to study the formation of various oxygen groups versus the O-atom dose. The surface was saturated with the OH groups already after the dose of 1 × 1019 m−2, indicating a large initial sticking. The functionalization remained almost intact over an order of magnitude larger doses. Measurable quantities of the CO groups appeared simultaneously with the benzene ring's degradation at a dose of 1 × 1020 m−2. The ring degradation was accomplished with a significant increase in the OH concentration what was explained by the formation of a thicker oxygen-rich film. OCO and O(CO)O groups appeared after the dose of 1 × 1021 m−2, i.e., a 100-times larger dose than required for the initial saturation with the OH. The observations are sound with the predictions from the literature obtained through the density functional theory of oxygen adsorption on polystyrene.

Temperature field and material response as a function of rail grinding parameters

Rail grinding is widely used as a technique to both reprofile the railhead in case of wear and to remove damage. However, grinding may lead to surface burning and the formation of a white etching layer (WEL). Taking into account the rail head position, the study established an analytical thermal model based on a non-uniformly distributed heat source to predict the temperature field during grinding. The grinding temperature during a rail grinding experiment is measured through thermocouples to validate the model. In addition, the rail material response in terms of surface burn and white etching layer is analyzed in detail. Results indicate that with a grinding temperature of around 400 °C, a WEL starts to appear on the rail surface. Retained austenite is found on the ground rail surfaces, indicating the existence of martensite, resulting from the coupling effect of thermal stress and mechanical stress. A diagram is developed to describe the relationship between the grinding temperature, surface burn, and WEL.

Vertical Sandwich GAA FETs With Self-Aligned High-k Metal Gate Made by Quasi Atomic Layer Etching Process

We presented and demonstrated a new type of vertical nanowire (NW) and nanosheet (NS) field-effect transistors (FETs), named vertical sandwich gate-all-around FETs or VSAFETs, which were formed with the process compatible in the main stream industry. The VSAFETs with self-aligned high- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${k}$ </tex-math></inline-formula> metal gates (HKMGs) were fabricated with epitaxy of Si/SiGe/Si sandwich structure, an isotropic quasi-atomic-layer-etch (qALE) process, and gate replacement process. The gate-length of VSAFETs is mainly determined by the thickness of SiGe film grown by epitaxy. The NW diameter and NS thickness could be obtained by the isotropic qALE method, which can be used to the Si-selective etching of SiGe. As a result, p-type NS and NW VSAFETs with good device characteristics were fabricated. Device performance and the influence of silicide, Ge fraction, Si cap, and high thermal process were investigated; threshold voltage tuning and reliability were also discussed.

Doped or Quantum-Dot Layers as In Situ Etch-Stop Indicators for III/V Semiconductor Reactive Ion Etching (RIE) Using Reflectance Anisotropy Spectroscopy (RAS).

Reflectance anisotropy spectroscopy (RAS), which was originally invented to monitor epitaxial growth, can—as we have previously shown—also be used to monitor the reactive ion etching of III/V semiconductor samples in situ and in real time, as long as the etching rate is not too high and the abrasion at the etch front is not totally chaotic. Moreover, we have proven that—using RAS equipment and optical Fabry‒Perot oscillations due to the ever-shrinking thickness of the uppermost etched layer—the in situ etch-depth resolution can be as good as ±0.8 nm, employing a Vernier-scale type measurement and evaluation procedure. Nominally, this amounts to ±1.3 lattice constants in our exemplary material system, AlGaAsSb, on a GaAs or GaSb substrate. In this contribution, we show that resolutions of about ±5.6 nm can be reliably achieved without a Vernier scale protocol by employing thin doped layers or sharp interfaces between differently doped layers or quantum-dot (QD) layers as etch-stop indicators. These indicator layers can either be added to the device layer design on purpose or be part of it incidentally due to the functionality of the device. For typical etch rates in the range of 0.7 to 1.3 nm/s (that is, about 40 to 80 nm/min), the RAS spectrum will show a distinct change even for very thin indicator layers, which allows for the precise termination of the etch run.

General Synthesis of Hierarchically Macro/Mesoporous Fe,Ni-Doped CoSe/N-Doped Carbon Nanoshells for Enhanced Electrocatalytic Oxygen Evolution.

Constructing hierarchical porosity and designing rational hybrid composition are effective strategies for enhancing the electrocatalytic performance of hybrid catalysts for electrochemical energy conversion. Here, we develop a multistep "molecule/ion-exchange" strategy toward the synthesis of hierarchically macro/mesoporous Fe,Ni-doped CoSe/N-doped carbon nanoshells with tunable pore structures and compositions. Polystyrene (PS)@Co-based amorphous coordination polymer (Co-CP) core-shell particles with hierarchically macro/mesoporous nanoshells are first prepared by ligand-molecule-exchange etching of the outer layers in PS@Co-based metal-organic framework precursors. Afterward, a liquid-solid dual-ion-exchange reaction of PS@Co-CP particles with [Fe(CN)6]3- and [Ni(CN)4]2- ions leads to the formation of PS@Co-CP/Co-Fe Prussian blue analogue (PBA)/Co-Ni PBA particles, which are further transformed into hierarchically macro/mesoporous Fe,Ni-doped CoSe/N-doped carbon particles via a vapor-solid selenization reaction. Moreover, this approach could be extended to synthesize different hierarchically porous core-shell composites with various morphologies and tailored compositions. Because of their unique hierarchically porous nanoarchitecture, these Fe,Ni-doped CoSe/N-doped carbon particles with optimized composition show enhanced performance for electrocatalytic oxygen evolution.

Investigations of Sidewall Passivation Technology on the Optical Performance for Smaller Size GaN-Based Micro-LEDs

Micro-light emitting diodes (Micro-LEDs) based on III-nitride semiconductors have become a research hotspot in the field of high-resolution display due to its unique advantages. However, the edge effect caused by inductively coupled plasma (ICP) dry etching in Micro-LEDs become significant with respect to the decreased chip size, resulting in a great reduction in device performance. In this article, sector-shaped GaN-based blue Micro-LEDs are designed and fabricated. Additionally, the device performance of different size Micro-LEDs with passivation are investigated with respect to those without passivation. Several methods have been applied to minimize the etching damage near the edge, including acid-base wet etching and SiO2 passivation layer growth. The room temperature photoluminescence (PL) results demonstrate that the light emission intensity of Micro-LEDs can be significantly enhanced by optimized passivation process. PL mapping images show that the overall luminescence of properly passivated Micro-LEDs is enhanced, the uniformity is improved, and the effective luminescence area is increased. The recombination lifetime of carriers in Micro-LEDs are increased by the usage of passivation process, which proves the reduction in non-radiative recombination centers in Micro-LEDs and improved luminescence efficiency. As a result, the internal quantum efficiency (IQE) is improved from 14.9% to 37.6% for 10 μm Micro-LEDs, and from 18.3% to 26.9% for 5 μm Micro-LEDs.

Patterning of transition metal dichalcogenides catalyzed by surface plasmons with atomic precision

Summary Plasmon-induced charge transfer has drawn considerable interests and spurred rapid developments of photovoltaics and photocatalysis. Various plasmonic metal/transition metal dichalcogenide (TMD) heterostructures have been developed to promote charge separation. For plasmon-induced catalysis, the transformation of TMDs is rare, and the erosion of TMDs has not yet been reported. Here, we report the etching of two-dimensional TMDs by planar surface plasmon polaritons (SPPs). We found that SPPs can etch various TMDs into desired layers and lateral size through controlling the light power and incident direction. In aqueous media, the strong plasmonic coupling at Au/MoS2 interface generates holes in the valence band of MoS2 that can weaken its interlayer interaction. As a synergistic effect, the plasmonic hot electrons produce oxidizing H2O2 to dissolve MoS2 from the top layers. Our results provide a new perspective for understanding the plasmonic coupling at metal-TMD interfaces and a reliable route toward fabricating well-defined TMDs.

Thermal Atomic Layer Etching of Aluminum Oxide (Al2O3) Using Sequential Exposures of Niobium Pentafluoride (NbF5) and Carbon Tetrachloride (CCl4): A Combined Experimental and Density Functional Theory Study of the Etch Mechanism

Thermal atomic layer etching (ALEt) of amorphous Al2O3 was performed by alternate exposures of niobium pentafluoride (NbF5) and carbon tetrachloride (CCl4). The ALEt of Al2O3 is observed at tempera...

Green Manufacturing of Silyl-Phosphate for Use in 3D NAND Flash Memory Fabrication

Three-dimensional NAND flash memory featuring dozens of vertically stacked memory cells is the state-of-the-art technology for most storage platforms. To fabricate three-dimensional (3D) NAND memory, lateral etching of the Si₃N₄ layer over SiO₂ is an essential step that is conducted through a wet etching process using a phosphoric acid-based etchant. Silyl-phosphate or highly selective nitride serves as an etching solution additive to control the SiO₂ layer dissolution rate. However, silyl-phosphate is prepared with an expensive monomeric silica precursor and at high reaction temperatures and generates environmentally harmful byproduct gases, such as HCl, HF, and CH₃OH. This study demonstrates that silyl-phosphate can be prepared using low-cost polymeric silica under a mild reaction temperature by changing the characteristic acidity of phosphoric acid. The possibility of tuning the phosphoric acid acidity was first studied by molecular dynamics simulations, and phosphoric acids with stronger acidity were prepared by the evaporation of water from H₃PO₄ (85%). The concentrated phosphoric acid enabled a fast reaction of polymeric silica and phosphate at a low reaction temperature (80 °C). The obtained silyl-phosphate lowered the SiO₂ layer dissolution rate, thereby yielding a Si₃N₄/SiO₂ layer etching ratio of up to 940. The proposed method offers an environmentally friendly production process for special chemicals used in 3D NAND flash memory fabrication.

Molecular Mechanism of Thermal Dry Etching of Iron in a Two-Step Atomic Layer Etching Process: Chlorination Followed by Exposure to Acetylacetone

Controlling thickness and morphology of transition-metal thin films used in magnetic random-access memory fabrication is a major challenge. Thermal dry etching has emerged as a method of choice for preparing the desired films; however, the mechanisms of the underlying surface processes are very difficult to assess. In this work, thermal dry etching of metallic iron by using sequential exposure to chlorine gas and 2,4-pentanedione (acetylacetone, acacH) was investigated. The mechanism of reacting acacH with chlorine-modified iron surface was studied by following the fragments desorbing from the surface in temperature-programmed desorption experiments in half-cycle processes that compared the chlorinated, oxidized, mixed (Cl and O), and clean (sputtered) iron films. In situ Auger electron spectroscopy and ex situ X-ray photoelectron spectroscopy of each sample after each etching step confirmed that the surface is activated by chlorine and then the chlorine-containing iron species are removed from the top layer of the sample, resulting in a metallic iron surface. No etching of clean (sputtered) surface was observed with acacH. The complete mechanism of acacH reaction with chlorinated iron samples is complicated, and etch products can contain both Fe2+ and Fe3+. However, a number of major conclusions, including the formation of surface intermediates and the final products of etching, primarily removal of Fe(acac)xCly, are inferred by comparing the results of these experiments with the computational investigation of selected surface processes using density functional theory calculations.

Topographical selective deposition: A comparison between plasma-enhanced atomic layer deposition/sputtering and plasma-enhanced atomic layer deposition/quasi-atomic layer etching approaches

In this work, we focus on the development of topographically selective deposition (TSD) leading to local deposition on the vertical sidewalls of 3D structures. A proof of concept is provided for the TSD of Ta2O5. The TSD process relies on plasma-enhanced atomic layer deposition (PEALD) alternating with quasi-atomic layer etching (ALE). Quasi-ALE involves a fluorination treatment followed by a directional Ar+ sputtering step. We show that the fluorination treatment allows a significant decrease in the incident kinetic energy of the subsequent directional Ar+ sputtering step. Conversely, when no fluorination step is carried out, TSD requires high incident kinetic energies during the directional Ar+ sputtering step, which, in turn, leads to detrimental plasma-induced damage on horizontal surfaces, such as roughness, also promoting by-product redeposition. The benefits and shortcomings of these two TSD approaches—PEALD/quasi-ALE and PEALD/energetic Ar+ sputtering—are compared in light of potential bottom-up technological developments.

Broadband piston mode operation of solidly mounted resonator employing A1 Lamb mode on lithium niobate

This paper discusses the impact of etched holes given to the solidly mounted resonator (SMR)-supported A1 Lamb mode resonators. The authors pointed out that the etched holes are effective at suppressing the lateral leakage and the transverse mode resonances for free-standing A1 Lamb mode resonators, and the technique was named the broadband piston mode (BPM). First, the SMR structure is designed for an A1 Lamb mode resonator, and it is shown that the release windows are effective in the suppression of transverse mode resonances even when the SMR is added. Detailed analysis is given to spurious responses that newly appeared by applying the SMR. Then, the discussion is extended to the impact of partial etching of the SMR layers underneath the release windows. The result indicates that the etching of the top SMR layer is enough for recovery of the BPM function.