Vertical Etching Research Articles

Optical emission spectroscopy as a method for evaluating the change in Si etching structures profile in ICP SF6/C4F8 plasma: Microstructures

In this work, a method for in situ diagnostics of the etching profile of silicon structures (etching window sizes 15–400 μm) using optical emission spectroscopy was proposed. To determine the relationship between the etching profile and plasma parameters, the influence of technological parameters on the etching characteristics (vertical and lateral etching rate, selectivity in relation to photoresist, and sidewall angle) was studied. As a general parameter, which reflects the changes in plasma characteristics depending on the selected technological parameters, the parameter X (C/F ratio in SF6/C4F8 plasma) was introduced. Based on the results obtained, a general pattern between the lateral etching rate, sidewall angle, and optical emission spectra was identified. Thus, ranges of X values, at which the lateral etching rate does not exceed 5 nm/min for 15–30 μm structures and 15 nm/min for 100 μm structures, were estimated: 0.38 ≤ X ≤ 0.77 and 0.28 ≤ X ≤ 0.46, respectively. For 250–400 μm structures, ranges of X values, at which the sidewall angle is acute, straight, and obtuse, were determined: 0.16 ≤ X < 0.29, 0.29 ≤ X ≤ 0.41, 0.41 < X ≤ 0.75, respectively.

One-Step Dry-Etching Fabrication of Tunable Two-Hierarchical Nanostructures.

Two-hierarchical nanostructures, characterized by two distinct configurations along the height direction, exhibit immense potential for applications in various fields due to their significantly enhanced controllable degree compared to single-order structures. However, due to the limitations imposed by planar technology, the realization of two-hierarchical nanostructures encounters huge challenges. In this work, we developed a one-step etching method based on inductively coupled plasma reactive ion etching for two-hierarchical nanostructures. Thanks to the shrinking effect of the Cr mask and the generation of a passivation layer during etching, the target materials experienced two different states from vertical etching to shrink etching. Consequently, the achieved two-hierarchical nanostructure configuration features a cross-section of an upper triangle and a lower rectangle, showing higher controllable degrees compared to the one-order ones. Both the mask pattern and etching parameters play crucial roles, by which two-hierarchical structures with diversiform shapes can be constructed controllably. This method for two-hierarchical nanostructures offers advantages including excellent control over structural properties, high processing efficiency, uniformity across large areas, and universality in materials. This developed strategy not only presents a simple and rapid nanofabrication platform for realizing optoelectronic devices, but also provides innovative ideas for designing the next generation of high-performance devices.

Effect of Triton X-100 surfactant and agitation on tetramethylammonium hydroxide wet etching for microneedle fabrication

Solid silicon (Si) microneedles have many applications such as skin pretreatment to form micrometer-sized holes in the skin surface in transdermal drug delivery systems. Wet etching based on tetramethylammonium hydroxide (TMAH) is an efficient method to fabricate solid microneedles. However, it is challenging to increase the density of microneedle arrays due to the faster lateral etching than the vertical etching that requires a large initial mask size. In this work, we used wet etching based on TMAH to fabricate solid Si microneedles. One kind of nonionic surfactant, Triton X-100, was introduced into the TMAH solution to suppress the lateral etching. When Triton X-100 was added into TMAH for a given etching condition, the maximum height (attained right before the mask fell off) of microneedles could reach ∼230 μm for 600 μm square-shaped mask size and 700 μm array period, compared to microneedles of maximum 152 μm height for the same mask size and period without surfactant addition. Correspondingly, when the target heights of microneedles were the same as ∼230 μm, denser (down to 700 μm period, 600 μm mask size) microneedle arrays were achieved with the help of Triton X-100, in comparison to arrays down to 900 μm period (800 μm mask size) without surfactant addition. Furthermore, agitation by a magnetic stirring bar is important for the fabrication of dense solid Si microneedle arrays based on TMAH. The microneedle structures were rhombic pyramid in shape with Triton X-100 and agitation. But microneedle structures obtained with Triton X-100 yet without agitation were octagonal pyramid in shape with a much less steep side surface.

Polarization Analysis of Vertically Etched Lithium Niobate-on-Insulator (LNOI) Devices

LNOI devices have emerged as prominent contributors to photonic integrated circuits (PICs), benefiting from their outstanding performance in electro-optics, acousto-optics, nonlinear optics, etc. Due to the physical properties and current etching technologies of LiNbO3, slanted sidewalls are typically formed in LNOI waveguides, causing polarization-related mode hybridization and crosstalk. Despite the low losses achieved with fabrication advancements in LNOI, such mode hybridization and crosstalk still significantly limit the device performance by introducing polarization-related losses. In this paper, we propose a vertically etched LNOI construction. By improving the geometrical symmetry in the waveguides, vertical sidewalls could adequately mitigate mode hybridization in common waveguide cross sections. Taking tapers and bends as representatives of PIC components, we then conducted theoretical modeling and simulations, which showed that vertical etching effectively exempts devices from polarization-related mode crosstalk. This not only improves the polarization purity and input mode transmittance but also enables lower polarization-related losses within more compact structures. As a demonstration of fabrication feasibility, we innovatively proposed a two-step fabrication technique, and successfully fabricated waveguides with vertical sidewalls. Such vertical etching technology facilitates the development of next-generation high-speed modulators, nonlinear optical devices, and other advanced photonic devices with lower losses and a smaller footprint, driving further innovations in both academic research and industrial applications.

Cryogenic DRIE processes for high-precision silicon etching in MEMS applications

Cryogenic deep reactive ion etching (Cryo DRIE) of silicon has become an enticing but challenging process utilized in front-end fabrication for the semiconductor industry. This method, compared to the Bosch process, yields vertical etch profiles with smoother sidewalls not subjected to scalloping, which are desired for many microelectromechanical systems (MEMS) applications. Smoother sidewalls enhance electrical contact by ensuring more conformal and uniform sidewall coverage, thereby increasing the effective contact area without altering contact dimensions. The versatility of the Cryo DRIE process allows for customization of the etch profiles by adjusting key process parameters such as table temperature, O2 percentage of the total gas flow rate (O2 + SF6), RF bias power and process pressure. In this work, we undertake a comprehensive study of the effects of Cryo DRIE process parameters on the trench profiles in the structures used to define cantilevers in MEMS devices. Experiments were performed with an Oxford PlasmaPro 100 Estrelas ICP-RIE system using positive photoresist SPR-955 as a mask material. Our findings demonstrate significant influences on the sidewall angle, etch rate and trench shape due to these parameter modifications. Varying the table temperature between −80 °C and −120 °C under a constant process pressure of 10 mTorr changes the etch rate from 3 to 4 μm min−1, while sidewall angle changes by ∼2°, from positive (<90° relative to the Si surface) to negative (>90° relative to the Si surface) tapering. Altering the O2 flow rate with constant SF6 flow results in a notable 10° shift in sidewall tapering. Furthermore, SPR-955 photoresist masks provide selectivity of 46:1 with respect to Si and facilitates the fabrication of MEMS devices with precise dimension control ranging from 1 to 100 μm for etching depths up to 42 μm using Cryo DRIE. Understanding the influence of each parameter is crucial for optimizing MEMS device fabrication.

Influence of electrolytic plasma spatial distribution on nanoporous structure etching on 4H-SiC surface

Single-crystal SiC nanoporous structure has important applications in the field of micro electromechanical systems, chemical sensors and biomedical devices. However, SiC exhibits a strong chemical inertness due to the short interatomic distance and high binding energy, making it significantly challenging to fabricate nanoporous structure. Electrolytic plasma-assisted chemical etching (EPACE) based on electrochemical anodic oxidation and high-activity electrolytic plasma etching is an ideal method for SiC nanoporous structure etching. In which, the spatial distribution of electrolytic plasma has an important impact on EPACE performance, such as etching efficiency and stability. In order to analyze the impact mechanism, this paper studies EPACE in tool electrode mounting posture of vertical and horizontal types by visual observation, real-time recording of voltage-current waveforms during processing, and the morphology analysis of SiC nanoporous structure after etching. It can be found that in the vertical etching type, EPACE can proceed stably because the workpiece immersion depth is greater than the bubble accumulation thickness. However, in the horizontal etching type, bubbles accumulate seriously between the workpiece and the liquid surface, which may cause bubble cavitation and abnormal discharge, leading to damage to the etching surface. The etching current in the vertical etching type is about 1.3 times that of the horizontal etching type, indicating that the bubble mass transfer in the vertical etching type is faster and the etching impedance is smaller, which is beneficial to improve the etching efficiency and obtain a uniform nanoporous structure. Finally, a SiC nanoporous layer with a thickness of 10.6 μm was prepared at etching time t = 50 min in vertical etching type, and the etching efficiency was 212 nm/min. In addition, the prepared nanoporous structure (nanofiber) was connected to the SiC substrate, indicating that it has a good bonding strength.

Plasma-based etching approach for GEM detector microfabrication at FBK for X-ray polarimetry in space

Gas Electron Multiplier (GEM) detectors are crucial for enabling high-resolution X-ray polarization of astrophysical sources when coupled to custom pixel readout ASIC in Gas Pixel Detectors (GPD), as in the Imaging X-ray Polarimetry Explorer (IXPE), the Polarlight cubesat pathfinder and the PFA telescope onboard the future large enhanced X-ray Timing and Polarimetry (eXTP) Chinese mission. The R&D efforts of the IXPE collaboration have resulted in mature GPD technology. However, limitations in the classical wet-etch or laser-drilled fabrication process of GEMs motivated our exploration of alternative methods. This work focuses on investigating a plasma-based etching approach for fabricating GEM patterns at Fondazione Bruno Kessler (FBK). The objective is to improve the aspect ratio of the GEM holes, to mitigate the charging of the GEM dielectric which generates rate-dependent gain changes. Unlike the traditional wet-etch process, Reactive Ion Etching (RIE) enables more vertical etching profiles and thus better aspect ratios. Moreover, the RIE process promises to overcome non-uniformities in the GEM hole patterns which are believed to cause systemic effects in the azimuthal response of GPDs equipped with either laser-drilled or wet-etch GEMs. We present a GEM geometry with 20 μm in diameter and 50 μm pitch, accompanied by extensive characterization (SEM and PFIB) of the structural features and aspect ratios. The collaboration with INFN Pisa and Turin enabled us to compare the electrical properties of these detectors and test their performance in their use as electron multipliers in GPDs. Although this R&D work is in its initial stages, it holds promise for enhancing the sensitivity of the IXPE mission in X-ray polarimetry measurements through GEM pattern with more vertical hole profiles. The outcomes of this study have the potential to advance the current technological platforms and improve the capabilities of future space-based X-ray polarimetry missions.

Low global warming C5F10O isomers for plasma atomic layer etching and reactive ion etching of SiO2 and Si3N4

AbstractPlasma atomic layer etching (ALE) processes for SiO2 and Si3N4 and reactive ion etching (RIE) processes for SiO2 with hole patterns were developed using C4F8 and the low global warming potential gases of perfluoroisopropyl vinyl ether (PIPVE) and perfluoropropyl vinyl ether (PPVE). The ALE windows of SiO2 and Si3N4 were in the range of 50.0–57.5 V for all precursors. Etch per cycle of SiO2 was determined to be 5.5 Å/cycle (C4F8), 3.3 Å/cycle (PIPVE), and 5.4 Å/cycle (PPVE), all lower than that of Si3N4. PPVE reduced global warming emissions by 49%, demonstrating better vertical etch profiles in RIE compared to C4F8.

Synergistic catalytic biomass-H2O gasification for H2 production and biochar etching mechanism: Experimental and DFT studies

Biomass-H2O gasification facilitated by multi-metal synergistic catalysis for H2 production. Oxygen transfer, carbon dissolution, lateral/vertical etching mechanism, and hydrogen production capacity of biomass self-contained K and added Ni catalytic gasification were studied through biomass loaded with K and Ni pyrolysis, biochar gasification experiments, and DFT calculations. The H2 yield from KNi catalytic gasification was 67.09 mmol/g (increased by 13.51%). Driven by *OH, K migrates and transforms from the inside of the carbon matrix to form active sites (CK), increasing carbon defects (40–50%) and reactivity. The vertical etching ability of Ni on biochar is enhanced from outside to inside (forming NiC and CK to reduce carbon dissolution energy barrier) and the gasification reaction rate is increased. The competition between the strong attraction of Ni on OH and the van der Waals force of K on OH leads to a 7.7% increase in the energy barrier of the rate-determining step (H transfer). The work enhances the understanding of the multi-metal catalytic gasification of rich H2 and provides a foundation for developing catalytic gasification technology.

Editors’ Choice—Atomic Layer Etching of Tungsten Disulfide Using Remote Plasma-Induced Oxidation and Wet Etching

WS2 is an emerging semiconductor with potential applications in next-generation device architecture owing to its excellent electrical and physical properties. However, the presence of inevitable surface contaminants and oxide layers limits the performance of WS2-based field-effect transistors (FETs); therefore, novel methods are required to restore the pristine WS2 surface. In this study, the thickness of a WS2 layer was adjusted and its surface was restored to a pristine state by fabricating a recessed-channel structure through a combination of self-limiting remote plasma oxidation and KOH solution etching processes. The reaction between the KOH solution and WOX enabled layer-by-layer thickness control as the topmost oxide layer was selectively removed during the wet-etching process. The thickness of the WS2 layer decreased linearly with the number of recess cycles, and the vertical etch rate was estimated to be approximately 0.65 nm cycle−1. Micro-Raman spectroscopy and high-resolution transmission electron microscopy revealed that the layer-by-layer etching process had a nominal effect on the crystallinity of the underlying WS2 channel. Finally, the pristine state was recovered by removing ambient molecules and oxide layers from the surface of the WS2 channel, which resulted in a high-performance FET with a current on/off ratio greater than 106. This method, which provides a facile approach to restoring the pristine surfaces of transition-metal dichalcogenide (TMDC) semiconductors with precise thickness control, has potential applications in various fields such as TMDC-based (opto)electronic and sensor devices.

Vernier Effect-Based Displacement Sensor Using In-Fiber Hollow Microsphere With a Hole

We demonstrate a compact subnanometer resolution displacement senor based on Vernier effect, which consists of a section of capillary fusion spliced with a single-mode fiber (SMF) and an in-fiber hollow microsphere with a hole (HMH). Two concave erbium-doped fibers (EDFs) obtained by chemical etching are fusion spliced for the hollow microsphere, which further has a hole by vertical polishing and chemical etching. The Newton’s rings phenomenon observed on the endface of EDF is the judgment condition of the hollow microsphere’s ultrathin wall via polishing, and then of the beginning of etching. Consequently, there is no concave area around the hole on the EDF endface, which significantly enhances the endface reflectivity. Combined with the lead-in SMF endface and the inner surface of the HMH, three-beam interference is generated from beams reflected from the three mirrors, which can produce two kinds of Vernier effect. They are analyzed theoretically and thus distinguishable in spectral patterns. Due to the low loss of light transmission in the hollow microsphere, the wider reflection spectra of identifiable interference pattern with Vernier effect are obtained, and the maximum displacement sensitivity can be further improved to 451.5 pm/nm with a subnanometer resolution of 0.044 nm. Plus the high robustness resulting from the in-fiber structure of the hollow microsphere, the displacement sensor shows prospects in health monitoring, surface topography, precision machining and other fields.

Elaboration of Silicon Nanostructures with Vapor-Phase Silver Assisted Chemical Etching: Correlation between Structural and Optical Properties.

Based on the widely used wet metal-assisted electroless etching, we develop in this work a novel vapor-phase silver-assisted chemical etching (VP-Ag-ACE) suitable for the elaboration of highly doped p-silicon (Si) nanostructures with strong, visible, and multi-peak photoluminescence (PL) emissions. The lateral and vertical etching rates (LER and VER) were discussed based on the etching mechanism of the VP-Ag-ACE. The antireflective suitability of the vapor-etched layer has been evaluated by a reflectivity measurement and exhibits reflectivity values lower than 3%. The PL emission at both room and low temperatures emissions were deeply discussed and correlated with the structural properties of the Si morphologies and their surface states based on the FTIR results.

Advanced dry etching of GaAs/AlGaAs multilayer wafer with InAs quantum dot for circular defect in photonic crystal laser

We previously proposed the circular defect in two-dimensional photonic crystal (CirD) laser based on a GaAs/AlGaAs multilayer wafer, which is fabricated by dry etching. This CirD laser uses InAs quantum dot (QD) layers as the gain medium, but this inhibits vertical dry etching. We improved the dry etching process by introducing three QD layers and three-step dry etching for fabricating the CirD laser. As a result, CirD structures with good etching profiles could be fabricated and excellent optical properties were obtained. We observed lasing by a CirD laser fabricated by deep etching under photoexcitation measurement for the first time.

Etching of molybdenum via a combination of low-temperature ozone oxidation and wet-chemical oxide dissolution

Etching of molybdenum was demonstrated in two steps. Mo was first oxidized in an ozone gas ambient to form molybdenum oxide. It is shown that comparable oxide thicknesses can be obtained in ozone and oxygen but at lower temperatures for the former. Initial oxide growth is fast but then considerably slows down due to its diffusion-limited character. The metal-oxide thickness can be controlled by temperature and defines the amount of metal etch per cycle (EPC). XPS analysis showed that the thermally grown oxide is MoO3. In the second, wet-chemical step, MoO3 was dissolved selectively toward the Mo metal using an aqueous solution. The dissolution rate of amorphous MoO3 formed in O3 at temperatures below ∼230 °C is fast, but the dissolution of MoO3 formed at Tox &amp;gt; 230 °C was shown to be incomplete. Cross-section TEM showed a matrix of amorphous oxide with crystallized MoO3 islands, the latter more difficult to dissolve. However, the crystalline phase could be completely and selectively removed using a more concentrated NH4OH solution at an elevated temperature (70 °C). The EPC was determined for temperatures between 150 and 290 °C. The etch rates increased with temperature from 1–2 nm/cycle at 150 °C to 5–6 nm/cycle at 290 °C. This hybrid thermal-wet etching sequence is well suited for vertical and lateral recess etching as it shows a controlled and isotropic dissolution of polycrystalline Mo at the nanoscale. Furthermore, the process shows a progressive surface smoothening upon increasing the number of etching cycles.

Effect of Mask Geometry Variation on Plasma Etching Profiles

It is becoming quite evident that, when it comes to the further scaling of advanced node transistors, increasing the flash memory storage capacity, and enabling the on-chip integration of multiple functionalities, "there's plenty of room at the top". The fabrication of vertical, three-dimensional features as enablers of these advanced technologies in semiconductor devices is commonly achieved using plasma etching. Of the available plasma chemistries, SF6/O2 is one of the most frequently applied. Therefore, having a predictive model for this process is indispensable in the design cycle of semiconductor devices. In this work, we implement a physical SF6/O2 plasma etching model which is based on Langmuir adsorption and is calibrated and validated to published equipment parameters. The model is implemented in a broadly applicable in-house process simulator ViennaPS, which includes Monte Carlo ray tracing and a level set-based surface description. We then use the model to study the impact of the mask geometry on the feature profile, when etching through circular and rectangular mask openings. The resulting dimensions of a cylindrical hole or trench can vary greatly due to variations in mask properties, such as its etch rate, taper angle, faceting, and thickness. The peak depth for both the etched cylindrical hole and trench occurs when the mask is tapered at about 0.5°, and this peak shifts towards higher angles in the case of high passivation effects during the etch. The minimum bowing occurs at the peak depth, and it increases with an increasing taper angle. For thin-mask faceting, it is observed that the maximum depth increases with an increasing taper angle, without a significant variation between thin masks. Bowing is observed to be at a maximum when the mask taper angle is between 15° and 20°. Finally, the mask etch rate variation, describing the etching of different mask materials, shows that, when a significant portion of the mask is etched away, there is a notable increase in vertical etching and a decrease in bowing. Ultimately, the implemented model and framework are useful for providing a guideline for mask design rules.

Silicon carbide nano-via arrays fabricated by double-sided metal-assisted photochemical etching

Silicon carbide (SiC) has gradually dominated the market of power semiconductor electronic devices because of its excellent electrical properties and chemical stability. However, fabricating micro/nano structures in SiC that are crucial for device functioning has been challenging. In this paper, a novel double-sided metal-assisted photochemical etching (double-sided MAPCE) was proposed to efficiently fabricate SiC nano-via arrays. The experimental results demonstrated that the vertical etching rate of double-sided MAPCE was ∼ 2.2 times higher than that of traditional MAPCE. The etching rate of SiC was found to be closely related to the type of noble metal, the concentration of oxidant (H2O2) and the UV light power density. The mechanism of double-sided MAPCE was disclosed as that the bottom noble metal layer catalyzed the reduction of H2O2, therefore, a built-in electric field was created which separated the photogenerated electron-hole pairs, thus, more photogenerated holes participated in oxidation of SiC resulting in improved SiC etching rate. The proposed method is possible to be used to process wide bandgap semiconductors.

GaN damage-free cyclic etching by sequential exposure to Cl2 plasma and Ar plasma with low Ar+-ion energy at substrate temperature of 400 °C

Damage-free atomic layer etching (ALE) of GaN was demonstrated using a cyclic process in which the chlorinated layer formed by Cl2 plasma exposure was removed by exposure to Ar plasma with energy-controlled ions when the substrate temperature was maintained at 400 °C. The layer chlorinated at 400 °C by Cl2 plasma exposure was found to be thinner than that chlorinated at 25 °C. Therefore, in the case of an Ar+-ion energy of 70 eV, the “ALE synergy” parameter, which quantifies the degree to which a process approaches the ideal ALE regime, decreased from 86% at a substrate temperature of 25 °C to 24% at a substrate temperature of 400 °C. A substrate temperature of 400 °C promoted etching even at the lower ion energy of 40 eV, thereby resulting in a higher ALE synergy of 62%. The vertical etching profile with no degradation of photoluminescence near the band edge on the etched surface was then observed. The proposed high-temperature ALE method is promising for realizing a vertical pattern profile via damage-free etching of GaN.

Ultrahigh Frequency Ultrasonic Transducers (150MHz) Based on Silicon Lenses.

Acoustic microscopes and acoustic tweezers have great value in the application of microparticle manipulation, biomedical research and non-destructive testing. Ultrahigh frequency (UHF) ultrasonic transducers act as the key component in acoustic microscopes, and acoustic tweezers and acoustic lenses are essential parts of UHF ultrasonic transducers. Therefore, the preparation of acoustic lenses is crucial. Silicon is a suitable material for preparing acoustic lenses because of its high acoustic velocity, low acoustic attenuation and excellent machinability. In previous research, silicon lenses were mainly prepared by etching. However, etching has some drawbacks. The etching of large sizes is complex, time-consuming and expensive. Furthermore, vertical etching is preferred to spherical etching. Thus, a new method of ultra-precision machining was introduced to prepare silicon lenses. In this paper, silicon lenses with an aperture of 892 μm and a depth of 252 μm were prepared. Then, UHF ultrasonic transducers with a center frequency of 157 MHz and a -6-dB bandwidth of 52% were successfully prepared based on silicon lenses. The focal distance of the transducers was 736 μm and the F-number was about 0.82. The transducers had a lateral resolution of 11 μm and could distinguish the 13 μm slots on silicon wafers clearly.

Asymmetric Interference Behavior in 3D NAND Cell and the Reverse Trend Induced by Undercut of Sacrificial Nitride Film

In this letter, the word line (WL) interference behavior in 3D NAND cell and the impact on program sequence of 3D NAND array operation are discussed. The WL interference induced by source-side neighboring cell is always smaller than that induced by drain-side neighboring cell. Due to this asymmetric characteristic, program sequence from top to bottom will have the advantage of suffering less operation window loss resulting from WL interference, over the traditional program sequence from bottom to top. This advantage can be clearly validated by the experimental results. However, at some specific WL’s of the array, the trend is reversed. The root cause of the reverse trend is then clarified to result from the unexpected undercut of sacrificial nitride film occurring during the imperfect vertical channel etching process for 3D NAND array formation.

Vertical and Lateral Etch Survey of Ferroelectric AlN/Al1-xScxN in Aqueous KOH Solutions.

Due to their favorable electromechanical properties, such as high sound velocity, low dielectric permittivity and high electromechanical coupling, Aluminum Nitride (AlN) and Aluminum Scandium Nitride (Al1−xScxN) thin films have achieved widespread application in radio frequency (RF) acoustic devices. The resistance to etching at high scandium alloying, however, has inhibited the realization of devices able to exploit the highest electromechanical coupling coefficients. In this work, we investigated the vertical and lateral etch rates of sputtered AlN and Al1−xScxN with Sc concentration x ranging from 0 to 0.42 in aqueous potassium hydroxide (KOH). Etch rates and the sidewall angles were reported at different temperatures and KOH concentrations. We found that the trends of the etch rate were unanimous: while the vertical etch rate decreases with increasing Sc alloying, the lateral etch rate exhibits a V-shaped transition with a minimum etch rate at x = 0.125. By performing an etch on an 800 nm thick Al0.875Sc0.125N film with 10 wt% KOH at 65 °C for 20 min, a vertical sidewall was formed by exploiting the ratio of the planes and planes etch rates. This method does not require preliminary processing and is potentially beneficial for the fabrication of lamb wave resonators (LWRs) or other microelectromechanical systems (MEMS) structures, laser mirrors and Ultraviolet Light-Emitting Diodes (UV-LEDs). It was demonstrated that the sidewall angle tracks the trajectory that follows the of the hexagonal crystal structure when different c/a ratios were considered for elevated Sc alloying levels, which may be used as a convenient tool for structure/composition analysis.