Anisotropic Wet Etching Research Articles

Investigation of etch rate distribution by micro-hemisphere for the anisotropic wet etching of Ga-face gallium nitride crystal

The Ga-face gallium nitride (GaN) crystal has recently attracted significant research interest in the micro/nano device due to its exceptional properties. Therefore, exploring low-cost and high-efficiency anisotropic wet etching processes for GaN material is essential. This research investigates the measurement of the overall anisotropic etching rates by introducing a novel micrometer-level hemispherical structure. For the first time, we report the full anisotropic etching rate distribution for Ga-face GaN crystal planes based on the etched results of the micro hemisphere and wagon-wheel structure specimen. The etching experiments were carried out in 85 %wt H₃PO₄ etchant at 130°C and 150°C. The etching result of Ga-face GaN crystal orientations displays a hexagonal symmetry pattern. In addition, the overall etch rates measurement enables the identification of the crystal planes with maximum and minimum etch rates at different temperatures, and the experiment also shows the etch rate of primary crystallographic orientations, such as +c-plane, m-plane, and a-plane. Finally, the atomic structures of GaN crystallographic planes are analyzed to explain the anisotropy in the etching process on different crystal zones by the step-flow mechanism. The resulting etch rate database allows the numerical simulation of the etch front and helps understand the etching mechanism of the GaN crystal.

Vertical silicon nanowedge formation by repetitive dry and wet anisotropic etching combined with 3D self-aligned sidewall nanopatterning

Three-dimensional (3D) stacking of nano-devices is an effective method for increasing areal density, especially as downscaling of lateral device dimensions becomes impractical. This stacking is mainly achieved through plasma processing of stacked layers on top of a silicon (Si) substrate, which offers process flexibility but poses challenges in obtaining vertical sidewalls without plasma induced damage. A novel wafer-scale fabrication method is presented for realizing sub-200 nm vertically stacked Si nanowedges at the wafer scale, using iterative dry etching, wet anisotropic etching, and thermal oxidation. This approach forms nanowedges by the slow etching {111} Si planes, resulting in smooth surfaces at well-defined angles. A silicon nitride (Si3N4) hard mask is used in an iterative (etch-and-deposit) process, with its thickness determining the number of process iterations. By optimizing etch selectivity during dry etching and/or increasing the initial Si3N4 thickness, the number of process iterations can be increased. The periodicity of the nanowedges can be adjusted by varying the etch time of both dry and wet anisotropic etching. A thin silicon dioxide (SiO2) layer (∼6 nm) is grown on the nanowedges during each iteration. 3D sidewall patterning at the sub-20 nm scale is achieved using corner lithography and local oxidation of Si to selectively open the concave corners. Rhombus-shaped structures are formed at each concave corner after wet anisotropic etching of Si. This novel technology platform will allow for the 3D fabrication of high-density nanodevices for electronic, fluidic, plasmonic, and other applications.

Quantifying squeeze film damping in four-leaf clover-coupled micro-resonators: A comprehensive study under variable vacuum degrees

The squeeze film effect has a significant impact on the performance of micro-resonators when utilizing the electrostatic excitation and capacitive detection technique, due to the narrow air gaps present. It becomes the most critical damping effect that needs to be addressed. However, when micro-resonators are encapsulated in vacuum or rarefied air, squeeze film damping analysis can become considerably complex due to high-quality factors. In this study, we systematically investigate the squeeze film damping of a widely-used four-leaf clover-coupled micro-resonator under different vacuum degrees. The characteristic frequencies of micro-resonators are determined by applying the principle of mode superposition. Subsequently, the energy transfer theory is utilized to derive the theoretical formula for the quality factor caused by squeeze film damping, for varying levels of vacuum. Finite element analysis is then conducted to simulate the characteristic behavior and squeeze film damping of the resonator, confirming the accuracy of the proposed theory. In the experiment, a novel four-leaf silicon micro-resonator is fabricated using bulk anisotropic wet etching and other micromachining technologies. An experimental platform is established to study the squeeze film damping of the micro-resonator under various vacuum degrees, where the micro-resonator is excited electrostatically and detected capacitively. The frequency response and quality factor of the resonator under different vacuum degrees are measured using a house-made modal test circuit board. The experimental results demonstrate excellent agreement with the theoretical and simulated results, indicating that the proposed method can be a reliable and precise guide for understanding the squeeze film damping effect in advance, once the primary model of a micro-resonator has been designed.

Silver Dendrite‐Coated Silicon Pyramid Substrate as Surface‐Enhanced Raman Scattering Probe for Detecting Glucose

Raman spectroscopy offers a rapid and nondestructive method for qualitatively and quantitatively analyzing the molecular structure of substances. Herein, a facile and cost‐effective approach for the preparation of three‐dimensional (3D) surface‐enhanced Raman scattering (SERS) substrates is proposed, in which pyramid structures are fabricated using silicon anisotropic wet etching and silver dendrites are decorated on these silicon pyramid (PSi) substrates by immersing them in a mixture of hydrofluoric acid and silver nitrate for several minutes. The 3D SERS substrates, based on PSi coated with Ag dendrites, reveal high‐performance SERS enhancement (with an analytical enhancement factor reaching 3.54 × 1010) and can detect rhodamine 6G (R6G) in the presence of chemical enhancer (lithium chloride) at low concentrations down to 10−11 m. For glucose detection, 2‐thienylboronic acid is employed to faciliate glucose attachment to the substrate surface, achieving a detection limit as low as 10−6 m. The substrate exhibits good repeatability with a relative standard deviation of 11.223%, as well as reusability and long‐term stability for detection. The results also highlight the excellent sensitivity of the PSi‐based SERS substrate, which is expected to be instrumental in biochemical analysis and holds significant potential for developing noninvasive glucose sensor for diabetic patients using saliva samples.

Development of broadband high efficiency Mid-IR gratings for high-energy ultrafast lasers

Broadband high-efficiency diffraction gratings play a crucial role in the pulse stretcher and compressor of high-energy ultrafast lasers. Nevertheless, conventional grating manufacturing techniques, including mechanical ruling and holographic recording, face challenges in creating accurate blazed groove profiles necessary for the fabrication of broadband, high-efficiency mid-infrared gratings. In this work, we utilized combined electron-beam lithography and anisotropic wet etching technology to fabricate nearly perfect blazed grooves, producing high efficiency broadband mid-infrared (IR) grating for 4.3 µm 100 femtosecond laser. Global optimization was performed to achieve a design of > 90% efficiency over spectral range of 3.6 µm – 6.6 µm. Hybrid metal-dielectric coating (Au-Al2O3) is employed and optimized to minimize absorption and to enhance diffraction efficiency and laser-induced damage threshold (LIDT). Prototype gratings undergo testing at a desired application wavelengths of 4.3 µm in a tunable range of 0.2 µm, revealing that the optimal sample achieves a diffraction efficiency of 92%, closely approaching the theoretical value of 94.2%

Silicon flow stop frame for encapsulation of CMOS microsensor chips by wet anisotropic etching in KOH

Bulk micromachining in silicon is governed by the etching process where anisotropic (wet) etching in KOH can yield complex structures beyond that achievable with isotropic (dry) etching techniques. One example is the miniaturised frame reported herein with an area of 2.9 to 7.5 mm2, walls that are 1/10 mm thick, a height of 525 μm equipped with sloping walls that takes advantage of the 54.7° angle of the (111) planes to the horizontal (100) top surface of the wafer. Convex corners liable to damage are protected by sacrificial bridge structures which are etched thin to a point where the frame can be easily removed from the bulk substrate material. Frames made from isotropic (dry) etching processes have been made for comparison. Although the frame structure has different applications in microfabrication, the intended use is a flow stop barrier preventing liquid resins from entering the active area of a CMOS chemical sensor chip during encapsulation for use in aqueous or gaseous media. Beyond this specific proof-of-concept, the strategy is expected to be of general interest for all who treasures KOH etching and wants to explore new avenues based on this process.

Exploiting graded triangular gratings for optimal nano-focusing: A novel approach to enhance SERS efficiency

Plasmonic graded nano-gratings enable rainbow trapping of multiple resonant modes over a wide wavelength spectrum, useful for multi-channel Surface Enhanced Raman Spectroscopy (SERS) of molecular species. However, rectangular nano-gratings have limitations in achieving efficient rainbow trapping and localizing a wide spectrum of plasmonic modes due to their stepwise geometry, which induces high dissipation of surface plasmon polaritons into the substrate. An alternative platform of graded triangular nano-gratings enables increased localization and more efficient adiabatic transformation between neighboring grooves. Varying groove angles, depths, and periods in the tapered geometry allow for smooth adjustment of the surface plasmon polariton propagation constant, reducing losses and maximizing nano-focusing inside the groove tips. To overcome the limitation of low aspect ratio in wet-etching silicon, we employed a multi-step process of reactive ion etching of a SiO2 barrier layer to generate aperture width, followed by anisotropic wet-etching. The resulting graded triangular nano-gratings showed excellent SERS enhancement along three laser wavelength excitations. The enhancement factors of 638 and 785 nm wavelengths are 8.5 × 109 and 9 × 108, respectively, for the detection of 1 µM Rhodamine 6G. In addition, graded triangular nano-gratings show similar enhancement factors for other species, specifically the lipid DPEE-PEG, at the 532 nm laser excitation wavelength with an excellent SERS enhancement factor of 1.5 × 109. Owing to the ability of the graded triangular gratings to elicit pronounced SERS responses across three distinct laser excitations, they unequivocally qualify as “rainbow trapping” structures. Wider apertures, lower ohmic losses, and the ability to tune the groove angle beyond conventional etching methods bode well for graded triangular gratings as a superior platform for miniature sensors.

Metalorganic Chemical Vapor Deposition of AlN on High Degree Roughness Vertical Surfaces for MEMS Fabrication

AbstractAluminum nitride (AlN) grown on vertical surfaces can be utilized for the fabrication of advanced piezoelectric microelectromechanical systems (MEMS). The in‐plane motion of the parts of a MEMS element is possible when AlN is deposited on the vertical surfaces of the moving structure. For the best device performance, AlN must have high crystal quality, uniform coverage of the vertical sidewalls, and c‐axis crystalline orientation perpendicular to the plane of a vertical surface. The impact of the surface roughness (Rq) of the vertical sidewalls formed in Si using wet and dry etching methods on the crystal quality, crystallographic orientation, and uniformity of the metalorganic chemical vapor deposited (MOCVD) AlN thin films is studied in this paper. In both cases, AlN films demonstrated full sidewall coverage and grew crystalline in the c‐axis direction. AlN films grown on vertical Si surfaces achieved using anisotropic wet etching are highly crystalline and oriented in [0001] direction, while the films grown on vertical surfaces achieved using dry etching displayed a lower level of alignment with the Si sidewalls.

Wet‐Etching‐Boosted Charge Storage in 1D Nitride‐Based Systems for Imitating Biological Synaptic Behaviors

AbstractNitride materials for memristors are benefited from their high response speed and high power density. The memristive effects on 1D nitride structures has not yet been elucidated. Hence, the activation of the memristive capability of nanowire (NW)‐based nitride memristors using an uncomplicated fabrication as single‐step anisotropic wet etching is proposed. Among nitrides, gallium nitride, a third‐generation semiconductor, exhibits properties potentially suitable for neuromorphic applications. The wet etchant considerably alters the chemisorbed molecules and dangling bonds associated with the surface states of the nanowires. A device based on such NWs which exhibits low power consumption with no required compliance current and forming voltage for operation is demonstrated. It can integrate all memristive capabilities, including multiple state switching, nonvolatile bipolar memory, and Ca2+ dynamics‐imitating synaptic actions. The examination of the memristive process also highlights the significance of altering surfaces in the devices, in addition to the shared principles that underlie biological and artificial synapses. The operating mode of the nitride‐nanowire devices can be controlled by controlling the formation/dissolution of the oxygen‐conductive path along the nanowires. Thus, the study realizes nanowire memristors based on a nitride material framework, that is promising for application in the 1D–1D system downsizing required for the bio‐inspired artificial synapse.

Advanced low blaze angle x-ray gratings via nanoimprint replication and plasma etch.

We developed a new method of making ultra-low blaze angle diffraction gratings for x-ray applications. The method is based on reduction of the blaze angle of a master grating by nanoimprint replication followed by a plasma etch. A master blazed grating with a relatively large blaze angle is fabricated by anisotropic wet etching of a Si single crystal substrate. The surface of the master grating is replicated by a polymer material on top of a quartz substrate by nanoimprinting. Then a second nanoimprinting is performed using the 1st replica as a mold to replicate the saw-tooth surface into a resist layer on top of a Si grating substrate. A reactive ion etch is used to transfer the grating grooves into the Si substrate. The plasma etch provides reduction of the groove depth by a factor defined by the ratio of the etch rates for the resist and Si. We demonstrate reduction of the blaze angle of a master grating by a factor of 5 during fabrication of a 200 lines/mm blazed grating with a blaze angle of 0.2°. We investigated the quality and performance of the fabricated low blaze angle gratings and evaluate process accuracy and reproducibility. The new blaze angle reduction method preserves the planarity of the optical surface of the grating substrate and at the same time provides improvement in the grating groove quality during the reduction process.

Unraveling Anisotropic and Pulsating Etching of ZnO Nanorods in Hydrochloric Acid via Correlative Electron Microscopy.

Despite much technical progress achieved so far, the exact surface and shape evolution during wet chemical etching is less unraveled, especially in ionically bonded ceramics. Herein, by using in situ liquid cell transmission electron microscopy, a repeated two-stage anisotropic and pulsating periodic etching dynamic is discovered during the pencil shape evolution of a single crystal ZnO nanorod in aqueous hydrochloric acid. Specifically, the nanopencil tip shrinks at a slower rate along [0001̅] than that along the ⟨101̅0⟩ directions, resulting in a sharper ZnO pencil tip. Afterward, rapid tip dissolution happens due to accelerated etching rates along various crystal directions. Concurrently, the vicinal base region of the original nanopencil tip emerges as a new tip followed by the repeated sequence of tip shrinking and removal. The high-index surfaces, such as {101̅m} (m = 0, 1, 2, or 3) and {21̅ 1̅n} (n = 0, 1, 2, or 3), are found to preferentially expose in different ratios. Our 3D electron tomography, convergent beam electron diffraction, middle-angle bright-field STEM, and XPS results indicate the dissociative Cl- species were bound to the Zn-terminated tip surfaces. Furthermore, DFT calculation suggests the preferential Cl- passivation over the {101̅1} and (0001) surfaces of lower energy than others, leading to preferential surface exposures and the oscillatory variation of different facet etching rates. The boosted reactivity due to high-index nanoscale surface exposures is confirmed by comparatively enhanced chemical sensing and CO2 hydrogenation activity. These findings provide an in-depth understanding of anisotropic wet chemical etching of ionic nanocrystals and offer a design strategy for advanced functional materials.

Integrated Fabrication Process of Si Microcantilever Using TMAH Solution With Planar Molybdenum Mask

The anisotropic etching release of a silicon microcantilever structure becomes challenging when it has embedded metal layers. We integrated the anisotropic wet silicon etching and etch-back method for the microcantilever releasing process in a silicon-on-insulator (SOI) wafer using a planar molybdenum (Mo)-based mask. Mo mask, which has good stability on morphology in tetramethylammonium hydroxide (TMAH) etchant as well as electrical characteristics, was employed to protect the metal resistor on the cantilever surface from TMAH etchant during silicon etching. To obtain a planar topography for the Mo mask, we applied a sequence of three etch-back processes using ZEP 520A resist, which has a similar etching rate to the non-doped silicate glass (NSG) layer in the plasma CF <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{4}$</tex-math> </inline-formula> :H <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> gas (40:1). After surface planarization, the angle of the indentation region (metal edge boundary area) was about 180 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> (almost flat), thus avoiding cracks in the Mo mask layer. As a result, the cantilever with a metal resistor was successfully released from the silicon substrate by our proposed method. Therefore, this process promises a silicon microcantilever fabrication method without backside alignment masks, relatively low temperature, and complementary metal-oxide semiconductor (CMOS) compatibility for chemical and bio-sensor applications.

Monodisperse Micro-Droplet Generation in Microfluidic Channel with Asymmetric Cross-Sectional Shape.

Micro-droplets are widely used in the fields of chemical and biological research, such as drug delivery, material synthesis, point-of-care diagnostics, and digital PCR. Droplet-based microfluidics has many advantages, such as small reagent consumption, fast reaction time, and independent control of each droplet. Therefore, various micro-droplet generation methods have been proposed, including T-junction breakup, capillary flow-focusing, planar flow-focusing, step emulsification, and high aspect (height-to-width) ratio confinement. In this study, we propose a microfluidic device for generating monodisperse micro-droplets, the microfluidic channel of which has an asymmetric cross-sectional shape and high hypotenuse-to-width ratio (HTWR). It was fabricated using basic MEMS processes, such as photolithography, anisotropic wet etching of Si, and polydimethylsiloxane (PDMS) molding. Due to the geometric similarity of a Si channel and a PDMS mold, both of which were created through the anisotropic etching process of a single crystal Si, the microfluidic channel with the asymmetric cross-sectional shape and high HTWR was easily realized. The effects of HTWR of channels on the size and uniformity of generated micro-droplets were investigated. The monodisperse micro-droplets were generated as the HTWR of the asymmetric channel was over 3.5. In addition, it was found that the flow direction of the oil solution (continuous phase) affected the size of micro-droplets due to the asymmetric channel structures. Two kinds of monodisperse droplets with different sizes were successfully generated for a wider range of flow rates using the asymmetric channel structure in the developed microfluidic device.

Investigation of the Dependence of the Silicon Needle Shape on the KOH Solution Concentration during Anisotropic Wet Etching

Investigation of the Dependence of the Silicon Needle Shape on the KOH Solution Concentration during Anisotropic Wet Etching

Wafer-scale synthesis of a morphologically controllable silicon ordered array as a platform and its SERS performance.

In this study, we fabricated four different structures using single crystal silicon wafers for surface-enhanced Raman spectroscopy (SERS) applications. For single crystal silicon, different crystal orientations exhibit different physical and chemical properties. In chemical etching, the etching speed of different crystal planes also exhibits significant differences. We first used reactive ion etching (RIE) to process the surface of the substrate, and subsequently used KOH anisotropic wet etching technology to modify the surface of silicon wafers with different crystal orientations and produced four different results. In the RIE stage in an O2 atmosphere, the (110) silicon wafer formed a hexagonal hole structure, and the (100) silicon wafer formed an inverted pyramid hole structure; however, in the RIE-treated substrates in O2 and SF6 atmosphere, the (110) silicon wafer formed a pyramid with a diamond-shaped base, and the (100) silicon wafer showed a columnar structure with a "straw hat" at the top. The formation mechanisms of these four structures were elucidated. We also performed structure-related SERS characterizations of the four different structures and compared their performance differences.

Micro-gas sensor with a suspended micro-heater for ammonia gas detection

In this research, the ammonia micro-gas sensor is developed by micro-electromechanical systems (MEMS) technology. A suspended micro-heater is also integrated on the same chip to provide the optimal operating temperature during sensing. The main processing steps of the implemented micro-sensor in this study involve five photolithographic and seven thin-film deposition processes. In addition, in order to reduce the heat loss of the heater, this study uses anisotropic wet etching of MEMS technology to create a suspension structure that can reduce the heat conduction of silicon. Finally, tungsten trioxide is used as a sensing film to measure the variation of resistance. The chip size of the proposed micro ammonia sensor developed in this paper is 5 mm × 5 mm. At the concentration of 5 ppm ammonia, the sensing response can reach 252%, and the response time is 30 s. The lowest detection limit reaches 40 ppb. In summary, the micro-ammonia gas sensor with a micro-heater developed in this paper has the advantages of high response value, low detection limit and small size.

Numerical and Experimental Investigation of Nanostructure-Based Asymmetric Light Transmission Interfaces for Solar Concentrator Applications

Research in asymmetric light transmission interfaces has been recently gaining traction. While traditionally considered for optical circuitry applications, there is a new interest to use these interfaces in luminescent solar concentrators. Previous studies have shown that applying them to the top surface of a concentrator could mitigate surface losses. This paper presents experimental results for proof-of-concept asymmetric light transmission interfaces that may have potential applications in luminescent solar concentrators. The interfaces and the underneath substrate were created in a single step from polydimethylsiloxane using silicon molds fabricated on &lt;100&gt; wafers via anisotropic wet etching. The resulting structures were pyramidal in shape. Large surface areas of nanostructures repeating at 800 nm, 900 nm, and 1000 nm were tested for backward and forward transmission using a spectrometer. Results showed a 21%, 10%, and 0% average transmissivity difference between the forward and backward directions for each periodicity, respectively. The trends seen experimentally were confirmed numerically via COMSOL simulations.

Wet bulk micromachining characteristics of Si{110} in NaOH-based solution

Silicon wet bulk micromachining is an extensively used technique in microelectromechanical systems (MEMS) to fabricate variety of microstructures. It utilizes low-cost etchants and suitable for batch process that made it popular for industrial production. The etch rate and the undercutting at convex corner significantly affect the productivity. In wet anisotropic etching-based micromachining, Si{110} wafer is employed to fabricate unique shape geometries such as the microstructures with vertical sidewalls. In this research, we have investigated the etching characteristics of Si{110} in 10 M sodium hydroxide without and with addition of hydroxylamine (NH2OH). The main objective of the present work is to improve the etch rate and the undercutting at convex corners. Average surface roughness (R a), etch depth, and undercutting length are measured using a 3D scanning laser microscope. Surface morphology of the etched Si{110} surface is examined using a scanning electron microscope. The incorporation of NH2OH significantly improves the etch rate and the corner undercutting, which are useful to enhance the productivity. Additionally, the effect of etchant age on the etch rate and other etching characteristics are investigated. The etch rate of silicon and the undercutting at convex corners decrease with etchant aging. The results presented in this paper are very useful to scientists and engineers who use silicon wet anisotropic etching to fabricate MEMS structures using bulk micromachining. Moreover, it has great potential to promote the application of wet etching in MEMS.

Fabrication of microstructures in the bulk and on the surface of sapphire by anisotropic selective wet etching of laser-affected volumes

In this paper a processing technique for sapphire is presented which combines laser-induced amorphization and subsequent selective wet etching of amorphized sapphire as well as anisotropic wet etching of single-crystalline sapphire (α-Al2O3). Using this technique, microstructures can be realized on the surface and in the bulk of sapphire substrates. By focusing ultra-short laser pulses inside sapphire, its structure can be transformed from crystalline into amorphous. The modified material can be selectively removed using etchants, such as hydrofluoric acid or potassium hydroxide (KOH), solely dissolving the amorphized part. In this work, however, an etchant consisting of a standard solution of sulphuric acid and phosphoric acid (96 vol% H2SO4: 85 vol% H3PO4, 3:1 vol%) at 180 °C is utilized. This method allows the realization of structures which are impossible to achieve when using conventional etchants which solely dissolve the amorphized sapphire. Ultrashort pulsed laser irradiation (230 fs) is used in this study as starting point for the subsequent anisotropic etching to form microstructures on the surface or in the bulk of sapphire that are terminated by characteristic crystal planes. In particular, the appearance of etching-induced patterns formed by stacks of rhombohedra is shown for structures below the surface, whereas triangular pits are achieved in surface processing.

Etch and growth rates of GaN for surface orientations in the crystallographic zone: Step flow and terrace erosion/filling via the Continuous Cellular Automaton

Compared to other methods, we present the benefits of the Continuous Cellular Automaton (CCA) to describe in a simple and flexible manner anisotropic wet chemical etching of GaN as a combination of step flow and terrace erosion. In fact, the simplicity of the approach enables accounting for epitaxial growth of GaN based on the equivalent perspective of step flow and terrace filling (or build-up). A key ingredient is the direct removal/deposition of GaN groups of atoms, which directly enables describing etching/growth via the removal/deposition of a small number of different crystallographic cell types, each with a different removal/deposition rate. We focus on the derivation of mathematical expressions for the etch rates of the surface orientations contained in the <0001> crystallographic zone as a function of the removal rates of the various cell types, then describing their use also for growth. A least squares approach is used to determine optimal values for the removal/deposition rates of the various cell types. For the first time, Focused Ion Beam (FIB) etching is used to manufacture a well-known, vertically micromachined wagon-wheel structure for subsequent use in wet etching, thus enabling a comparison between the CCA-derived etch rates and the experimental values. For growth, the comparison is performed against data from the literature. The combination of average step flow and average terrace erosion/filling, as inherently contained in the CCA method, explains well the anisotropy of both etching and growth of GaN in the <0001> crystallographic zone.