Etching Of Sapphire Research Articles

In situ monitoring of sapphire nanostructure etching using optical emission spectroscopy

Fabrication of nanostructures on sapphire surfaces can enable unique applications in nanophotonics, optoelectronics, and functional transparent ceramics. However, the high chemical stability and mechanical hardness of sapphire make the fabrication of high density, high aspect ratio structures in sapphire challenging. In this study, we propose the use of optical emission spectroscopy (OES) to investigate the sapphire etching mechanism and for endpoint detection. The proposed process employs nanopillars composed of polymer and polysilicon as an etch mask, which allows the fabrication of large-area sapphire nanostructures. The results show that one can identify the emission wavelengths of key elements Al, O, Br, Cl, and H using squared loadings of the primary principal component obtained from principal component analysis of OES readings without the need of domain knowledge or user experience. By further examining the OES signal of Al and O at 395.6 nm, an empirical first-order model can be used to find a predicted endpoint at around 170 s, indicating the moment when the mask is completely removed, and the sapphire substrate is fully exposed. The fabrication results show that the highest aspect ratio of sapphire nanostructures that can be achieved is 2.07, with a width of 242 nm and a height of 500 nm. The demonstrated fabrication approach can create high sapphire nanostructures without using a metal mask to enhance the sapphire etch selectivity.

Etching conditions effect on anisotropic properties of sapphire

By patterning sapphire substrate (PSS) and then growing gallium nitride (GaN) film through wet etching process, the generation of dislocation and lattice defects of LED substrate can be significantly reduced, and the luminous efficiency and stability of the prepared led beads can be greatly improved. Due to the current anisotropic etching mechanism of sapphire has not been fully revealed, the etching evolution process and structure morphology cannot be accurately predicted and controlled, and ultimately it is difficult to ensure the structural quality of the patterned substrate. In this study, the etching rates of all crystal planes of sapphire in 98%H2SO4:85%H3PO4 = 3:1 etching solution were obtained through experiments, and the microstructure etching process and surface morphology were analyzed in detail. In addition, the influence of etching conditions on sapphire etching microstructure and surface morphology was also analyzed through experiments. The experimental results show that: 1) increasing the temperature can speed up the etching reaction, but it will make the surface quality worse; 2) Phosphoric acid as an etching buffer can improve the quality of etched structural planes; 3) The optimal etching rate and surface quality can be obtained by etching in 98%H2SO4:85%H3PO4 = 3:1 solution.

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.

Evolutionary Kinetic Monte Carlo Simulation of Anisotropic Wet Etching of Sapphire at Different Concentrations and Temperatures

Evolutionary Kinetic Monte Carlo Simulation of Anisotropic Wet Etching of Sapphire at Different Concentrations and Temperatures

Analysis of Etch Profiles on C-Plane Wafers in Wet Etching of Sapphire Based on Undercutting Rate Distributions in Mixture of H2SO4 and H3PO4 at 236 °C

In this paper, the formation and evolution of etch profiles with different mask shapes and sizes on C-plane wafers of sapphire are analyzed based on undercutting rate distributions of characteristic planes. The effect of concentrations of etchants on etch profiles is further analyzed. Firstly, etch rate distributions under different experimental conditions (236 °C, H2SO4:H3PO4 = 3:1 and 6:1) are obtained by wet etching experiments of sapphire hemispheres. Undercutting rate distributions of characteristic planes on C-plane wafers under different experimental conditions are obtained by the maximum positive curvature (MPC) recognition method. Then, the effect of different mask shapes and sizes on the formation and evolution of etch profiles of complex cavities and islands on C-plane wafers are analyzed based on the undercutting rate distribution under the experimental condition (236 °C, H2SO4:H3PO4 = 3:1). Finally, characteristic differences of etch profiles on C-plane wafers at different concentrations are explained based on undercutting rate distributions of characteristic planes at the corresponding concentrations (236 °C, H2SO4:H3PO4 = 3:1 and 6:1). These provide a basis for the study of the anisotropic wet etching mechanism of sapphire.

Cathodoluminescent Analysis of Sapphire Surface Etching Processes in a Medium-Energy Electron Beam.

Sapphire crystals are widely used in optics and optoelectronics. In this regard, it is important to study the stability of crystals under external influence and the possibility of modifying their surfaces by external influence. This work presents the results of studying the processes of the action of an electron beam with an average energy of 70 keV or less under vacuum conditions on the surfaces of sapphire substrates of various orientations. The effect of etching a sapphire surface by an electron beam in vacuum at room temperature was discovered. The highest etching rate was observed for A-plane sapphire (the average pit etching rate was 10−6 µm3/s). It was shown that the rate of etching of a sapphire surface increased many times over when gold is deposited. An in situ method for studying the process of etching a sapphire surface using cathodoluminescence analysis was considered. Possible mechanisms of sapphire etching by a beam of bombarding electrons were considered. The results obtained could be important in solving the problem of the stability of sapphire windows used in various conditions, including outer space. In addition, the proposed method of metal-stimulated etching of a sapphire surface can be widely used in patterned sapphire substrate (PSS) technology and further forming low-dislocation light-emitting structures on them.

Substrate Modification during Chemical Vapor Deposition of hBN on Sapphire.

A comparison of hexagonal boron nitride (hBN) layers grown by chemical vapor deposition on C-plane (0001) versus A-plane (112̅0) sapphire (α-Al2O3) substrate is reported. The high deposition temperature (>1200 °C) and hydrogen ambient used for hBN deposition on sapphire substantially alters the C-plane sapphire surface chemistry and leaves the top layer(s) oxygen deficient. The resulting surface morphology due to H2 etching of C-plane sapphire is inhomogeneous with increased surface roughness which causes non-uniform residual stress in the deposited hBN film. In contrast to C-plane, the A-plane of sapphire does not alter substantially under a similar high temperature H2 environment, thus providing a more stable alternative substrate for high quality hBN growth. The E2g Raman mode full width at half-maximum (FWHM) for hBN deposited on C-plane sapphire is 24.5 ± 2.1 cm-1 while for hBN on A-plane sapphire is 24.5 ± 0.7 cm-1. The lesser FWHM standard deviation on A-plane sapphire indicates uniform stress distribution across the film due to reduced undulations on the surface. The photoluminescence spectra of the hBN films at 300 and 3 K, obtained on C-plane and A-plane sapphire exhibit similar characteristics with peaks at 4.1 and 5.3 eV reported to be signature peaks associated with defects for hBN films deposited under lower V/III ratios. The dielectric breakdown field of hBN deposited on A-plane sapphire was measured to be 5 MV cm-1, agreeing well with reports on mechanically exfoliated hBN flakes. Thus, under the typical growth conditions required for high crystalline quality hBN growth, A-plane sapphire provides a more chemically stable substrate.

Application of the evolutionary kinetic Monte Carlo method for the simulation of anisotropic wet etching of sapphire

In this paper, the simple, rejection-based kinetic Monte Carlo simulation method is applied for the approximate simulation of the etch rates and three-dimensional etch structures during anisotropic wet etching of sapphire. Based on the analysis of the composition of the atomic structure of sapphire, a model of the simplified atomic structure of sapphire is proposed, which reduces the difficulty of classifying types of surface atoms on the different crystallographic planes. This enables adopting a previously proposed six-index classification method (to differentiate effectively between the various surface atoms of sapphire) and a previously proposed removal probability function (RPF) (to link the removal probability of the surface atom with the configuration of its neighbors). By reducing errors between simulated rates and experimental rates of 11 typical crystallographic planes in the <−1 1 0 0> and <1 1 −2 0> crystallographic zones continuously with an evolutionary algorithm, reasonable values of nine energy parameters of the RPF are obtained. The simulated results describe approximately the experimental counterparts.

Continuum level-set model for anisotropic wet etching of patterned sapphire substrates

We present a continuum modeling approach to simulate anisotropic wet etching of single-crystal sapphire employing mixtures of sulfuric acid and phosphoric acid. Wet etching of sapphire leads to the formation of crystal facets with high Miller–Bravais indices. The resulting complex three-dimensional topographies can be exploited to optimize the patterning of sapphire substrates which are employed for gallium nitride based light-emitting diodes. Due to the strong impact of the three-dimensional sapphire topography on the light extraction efficiency of the final device, precise control over the wet etching process, in particular etchant mixture, etch time, and temperature, is highly important. We present our model in the context of process technology computer-aided design, where we use the level-set method to track the evolution of the three-dimensional etch profile over time. In order to describe the intricate anisotropy of wet etched sapphire, we propose a flexible interpolation method for the etch rate distribution, which incorporates experimentally characterized crystal facets and deduces local extrema in the distribution based on local convexity/concavity considerations. The developed model has been calibrated and evaluated based on scanning electron microscopy and atomic force microscopy characterizations from the literature. Our model enables accurate sapphire etching simulations, where the emerging and disappearing crystal facets show a good agreement with experimental observations for several etchant mixtures and temperatures.

Research on Sapphire Deep Cavity Corrosion and Mask Selection Technology.

Aimed at the problem of the small wet etching depth in sapphire microstructure processing technology, a multilayer composite mask layer is proposed. The thickness of the mask layer is studied, combined with the corrosion rate of different materials on sapphire in the sapphire etching solution, different mask layers are selected for the corrosion test on the sapphire sheet, and then the corrosion experiment is carried out. The results show that at 250 °C, the choice is relatively high when PECVD (Plasma Enhanced Chemical Vapor Deposition) is used to make a double-layer composite film of silicon dioxide and silicon nitride. When the temperature rises to 300 °C, the selection ratio of the silicon dioxide layer grown by PECVD is much greater than that of the silicon nitride layer. Therefore, under high temperature conditions, a certain thickness of silicon dioxide can be used as a mask layer for deep cavity corrosion.

Sapphire-supported nanopores for low-noise DNA sensing

Solid-state nanopores have broad applications from single-molecule biosensing to diagnostics and sequencing. The high capacitive noise from conventionally used conductive silicon substrates, however, has seriously limited both their sensing accuracy and recording speed. A new approach is proposed here for forming nanopore membranes on insulating sapphire wafers to promote low-noise nanopore sensing. Anisotropic wet etching of sapphire through micro-patterned triangular masks is used to demonstrate the feasibility of scalable formation of small (<25 μm) membranes with a size deviation of less than 7 μm over two 2-inch wafers. For validation, a sapphire-supported (SaS) nanopore chip with a 100 times larger membrane area than conventional nanopores was tested, which showed 130 times smaller capacitance (10 pF) and 2.6 times smaller root-mean-square (RMS) noise current (18–21 pA over 100 kHz bandwidth, with 50–150 mV bias) when compared to a silicon-supported (SiS) nanopore (~1.3 nF, and 46–51 pA RMS noise). Tested with 1k base-pair double-stranded DNA, the SaS nanopore enabled sensing at microsecond speed with a signal-to-noise ratio of 21, compared to 11 from a SiS nanopore. This SaS nanopore presents a manufacturable nanoelectronic platform feasible for high-speed and low-noise sensing of a variety of biomolecules.

From nanoparticles generation to nanostructures diversity at thermoplasmonics laser-induced backside wet etching of sapphire

The nanoparticles and nanostructures formed during the thermoplasmonics laser-induced backside wet etching of sapphire were studied using electron microscopy and X-Ray energy dispersive analysis. It is shown that the generation and subsequent modification of nanoparticles result in the formation of various plasmonic structures, such as crystalline silver nanoaggregates and hybrid structures obtained due to the interaction of the sapphire decomposition products with silver nanoparticles, such as randomly mixed nanoalloys and Al/O nanoaglomerates surrounded by silver nanoparticles. Possible mechanisms for the creation and modification of nanomaterials depending on the localization of the thermoplasmonics process in space and time are discussed.

A Nanoscale Modification of Materials at Thermoplasmonic Laser-Induced Backside Wet Etching of Sapphire

Laser-induced backside wet etching using water solution of plasmon precursor (AgNO3) as an absorbing medium provides effective structuring of such a complex material as sapphire. At the same time, this process is accompanied by the formation of silver nanoparticles and a significant modification of a surface layer of sapphire. Hybrid plasmon structures are formed on the surface of sapphire. In this paper, the structure and phase composition of surface layer formed during the transformation of the materials in the process of etching have been studied using electron microscopy methods, electron diffraction, and EDX analysis; possibilities of creating of new plasmon nanostructures have been analyzed.

Improved etching technique for creating sapphire nanostructures

Researchers experimented with multi-level etching masks and achieved a nine-fold increase in sapphire etching depth, which may lead to new applications in photonics, optoelectronics and other multifunctional materials.

Efficiency of laser-induced backside wet microstructuring of sapphire increases with pressure

The study is directed to the development of the complex machining of hard transparent materials such as quartz, sapphire, and diamond using pulsed laser light (5 ns pulse duration, 532 nm wavelength, 1 kHz repetition rate). The deepening rate of the laser-induced backside etching of sapphire in a water solution of silver nitrate was studied as a function of liquid pressure in the range of 0.1–25 MPa. Its linear increase of about 1.5–2 times with pressure was demonstrated, which associated with the stronger microbubbles cavitation impact on hard material and chemical etching by sub- and super-critical water. The increase in the etching efficiency of sapphire at a higher pressure for this particular case can be generalized to other types of laser-induced backside wet etching and other materials.

Thermoplasmonic laser-induced backside wet etching of sapphire

The technology of thermoplasmonic laser-induced backside wet etching is proposed to microstructure such solid and difficult-to-process materials as sapphire. In this technology, the laser absorbing medium is silver nanoparticles obtained from a precursor (AgNO3) and providing significant laser radiation absorption due to the presence of plasmonic absorption with the formation of a substantially localised region with extremely high temperatures and pressures. This approach makes it possible to process sapphire both in the ‘gentle’ regime with the formation of nanometre structures and in the ‘strong’ regime to allow for the formation of ‘deep’ structures with etching rates up to several micrometers per pulse and high aspect ratio. The possibility of formation of periodic structures in sapphire in the above-threshold regime is shown.

Morphology of single picosecond pulse subsurface laser-induced modifications of sapphire and subsequent selective etching

The effect of 1030nm single picosecond pulsed laser-induced modification of the bulk of crystalline sapphire using a combined process of laser amorphization and selective wet chemical etching is studied. Pulse durations of more than 1 picosecond are not commonly used for this subsurface process. We examine the effect of 7 picosecond pulses on the morphology of the unetched, as well as etched, single pulse modifications, showing the variation of shape and size when varying the pulse energy and the depth of processing. In addition, a qualitative analysis of the material transformation after irradiation is provided as well as an analysis of cracking phenomena. Finally, a calculated laser intensity profile inside sapphire, using the Point Spread Function (PSF), is compared to the shape of the modifications. This comparison is employed to calculate the intensity threshold leading to amorphization, which equals 2.5⋅1014 ± 0.4⋅1014 W/cm2.

Incubation effect during laser-induced backside wet etching of sapphire using high-repetition-rate near-infrared nanosecond lasers

Laser-induced backside wet etching (LIBWE) has been extensively studied in recent years due to its high-quality processing of transparent materials. Toxic organic solutions and ultraviolet (UV) lasers with low repetition rates are necessary in conventional LIBWE, which limits the processing efficiency and results in environmental problems. Some studies have found that near-infrared lasers can also be employed in LIBWE when aqueous solutions containing Cu2+ are used as the absorbing liquids. These new findings show dramatic enhancements in processing efficiency, but the underlying mechanisms have not been clearly identified. In this study, we found that the etching rate of LIBWE using near-infrared lasers could be increased greatly and that the processing quality deteriorated appreciably when using high-repetition-rate laser pulses. By observing the evolution of laser-induced cavitation bubbles, we identified two etching mechanisms during LIBWE when high-repetition-rate laser pulses were used and determined that the incubation effect drove the transformation between the two mechanisms. Our findings demonstrate that LIBWE using a near-infrared nanosecond laser can be used not only to obtain high-quality micro-grooves on sapphire but also to cut sapphire substrates efficiently by utilizing the incubation effect.

Etching of Sapphire in Supercritical Water at Ultrahigh Temperatures and Pressures under the Conditions of Pulsed Laser Thermoplasmonics

The method of thermoplasmonic laser-induced backside wet etching (TPLIBWE) is applied for effective and well-controlled microstructuring of sapphire. The method is based on the generation of highly absorbing silver nanoparticles in the course of the pulsed-periodic laser irradiation. The silver nanoparticles are formed as a result of the reduction of a water-dissolved precursor, AgNO3. The process of sapphire etching occurs via the formation of supercritical water at ultrahigh temperatures and pressures (which significantly exceed the critical values for water) and the formation of silver nanoparticles at the sapphire/water interface as a result of the absorption of laser radiation. The mechanism of TPLIBWE is considered and the etching rate, which reaches ~100 nm/pulse, is determined. The formation of aluminum nanoparticles, which indicates a deep destruction of Al2O3 as a result of TPLIBWE, is observed.

Study on sapphire microstructure processing technology based on wet etching

Aiming at the problem that sapphire surface roughness is quite large after wet etching in sapphire microstructure processing technology, we optimize the wet etching process parameters, study on the influences of concentration and temperature of etching solution and etching time on the sapphire surface roughness and etching rate, choose different process parameters for the experiment and test and analyze the sapphire results after wet etching. Aiming at test results, we also optimize the process parameters and do experiment. Experimental results show that, after optimizing the parameters of etching solution, surface roughness of etched sapphire is 0.39 nm, effectively with reduced surface roughness, improved light extraction efficiency and meeting the production requirements of high-precision optical pressure sensor.