Sidewall Etching Research Articles

Enhanced Selectivity and Reduced Arde in Nanoscale Si Trench Etching: The Effects of Asynchronously Pulsed Plasmas, Bias Parameters, and Fluorocarbon Gas Additives

The continuous miniaturization of semiconductor devices made it increasingly difficult to achieve high aspect ratio (HAR) features with precise control over the profile due to low etch selectivity over mask and aspect ratio dependent etching (ARDE). In this study, we investigated the effects of pulsed plasma modes, bias pulsing parameters, and additive gases(CF4 and C4F8) on the etching characteristics of nanoscale silicon (Si) trench using inductively coupled plasmas (ICPs) with Cl2/Ar gas mixtures. Compared to CW plasmas, synchronously pulsed plasmas exhibit improved Si etch profiles and reduced ARDE effects. Furthermore, asynchronously pulsed plasmas showed further decreased in the ARDE effect compared to synchronously pulsed plasmas. The additive gases changed the Si trench sidewall etch profiles by protecting Si trench sidewalls during etching using asynchronously pulsed plasmas. To understand the etch mechanism of pulsed plasmas, plasmas have been characterized with high voltage probes, time resolved optical emission spectroscopy (OES), a residual gas analyzer (RGA), and a retarding field energy analyzer (RFEA). Also, the XPS measurement has been performed to understand chemical reactions on the etched material surfaces. Experimental results demonstrated that, by controlling bias pulsing parameters and utilizing additive gases, nanoscale Si trench etch characteristics can be more precisely managed. This technique showed a promise for advanced etching applications requiring HAR features, such as nano-through silicon vias (TSVs), shallow trench isolation (STI), etc., making it a valuable method for next-generation semiconductor device fabrication.

Inductively Coupled Plasma Reactive Ion Etching of (−201) β‐Ga2O3 Nano‐Fins

This study reports the effect of inductively coupled plasma reactive ion etching using a BCl3/Ar gas mixture on the sidewall taper angle (STA), etch rate, and surface morphology of (−201) β‐Ga2O3 substrates, as well as (−201)‐oriented β‐Ga2O3 films on sapphire, for nano‐fins of ≈200 nm width. Various etch parameters are systematically investigated for the β‐Ga2O3 films on sapphire, revealing that the STA is lower at high plasma density when chemical component increases, and the ion scattering decreases; achieving a STA of 1.5° ± 0.7°, accompanied by an etch rate of 114 nm min−1 and a reduction in surface roughness compared to before etching. Additionally, a strong positive correlation between the STA and the crystal orientation in the β‐Ga2O3 substrates is observed at lower plasma density, while a weak correlation is found between the STA and the crystal orientation at high plasma density. This study provides valuable insights for the fabrication of β‐Ga2O3 devices.

Self-catalyzed growth of sub-25-nm-diameter InAs nanowire arrays on Si patterned substrate

The feature size of advanced Si-based chips is approaching its physical limit, and it is difficult to continue Moore's Law by simply pursuing the technical route of miniaturizing device size to increase the integrated density. III-V nanowire devices represented by InAs material have much higher mobility than Si based devices, but they have not been successfully applied to CMOS nanowire devices and have not shown excellent performance. The reason is that the hole mobility of III-V materials is much lower than their electron mobility. If the nanowire diameter becomes ultrafine, the quantum confinement effect becomes more prominent, the light hole band will reverse above the heavy hole band, and the hole mobility will be greatly increased. Aiming at the integrability of III-V high mobility CMOS devices on Si, how to prepare ultrafine InAs nanowire arrays on Si/SiO2 patterned substrates has been studied in this work. New method in this work has solved the problem that the nanohole will enlarge due to etching side wall of nanohole by HF process before growth. A technology and mechanism of ultrafine InAs nanowire arrays grown by this method was developed, and the smallest diameter of nanowire in array has reached only 25 nm.

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.

Dodecagonal GaN Microrods: Chemical Stability of a‐, m‐, and c‐Plane Walls and Their Application to Water‐Splitting

AbstractPhotoelectrolysis of water is a sustainable option for the production of hydrogen fuel. GaN nano‐ or microstructures are considered for water‐splitting due to their general high chemical stability and high surface‐to‐volume ratio enhancing the process effectiveness. In this study GaN structures with dodecagonal microrods are used as a working electrode for the water‐splitting process. Microrods are grown using a plasma‐assisted molecular beam epitaxy process allowing tailoring of microrod height and density. Their unique property is the of presence twelve sidewalls with alternating a‐ and m‐plane orientations. This enables a simultaneous study of the chemical stability of c‐, a‐, and m‐plane walls of GaN. The water‐splitting process is performed using a 1 mol l−1 NaOH electrolyte solution. Non‐zero current measured at zero bias under illumination indicates that the process takes place. A degradation of the GaN structure is observed after a prolonged process time. In short‐term exposures, etching of microrod sidewalls is observed. Roughening of the a‐plane walls studied by transmission electron microscopy indicates that this orientation is etched with a fastest rate. The internal crystalline structure is not influenced by the etching and remains stable as shown by the X‐ray absorption spectroscopy study.

Enhanced Second-Harmonic Generation in Thin-Film Lithium Niobate Circular Bragg Nanocavity.

Second-order nonlinearity gives rise to many distinctive physical phenomena, e.g., second-harmonic generation, which play an important role in fundamental science and various applications. Lithium niobate, one of the most widely used nonlinear crystals, exhibits strong second-order nonlinear effects and electro-optic properties. However, its moderate refractive index and etching sidewall angle limit its capability in confining light into nanoscales, thereby restricting its application in nanophotonics. Here, we exploit nanocavities formed by second-order circular Bragg gratings, which support resonant anapole modes, to achieve a 42 000-fold enhanced second-harmonic generation in thin-film lithium niobate. The nanocavity exhibits a record-high normalized conversion efficiency of 1.21 × 10-2 cm2/GW under the pump intensity of 1.9 MW/cm2. Besides, we also show s- and p-polarization-independent second-harmonic generation in elliptical Bragg nanocavities. This work could inspire the study of nonlinear optics at the nanoscale on thin-film lithium niobate, as well as other novel photonic platforms.

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.

Comparative analysis of microstructure, electrical and optical performance in sidewall etching process for GaN-based green micro-LED

Micro-LEDs show the size-dependent external quantum efficiency (EQE) reduction problem, mainly owing to increased non-radiative recombination loss at the sidewall for smaller chip size. In this work, the evolution of microstructure, surface potential and optical performance of the green micro-LED sidewall was investigated comparatively after inductively coupled plasma (ICP) and tetramethylammonium hydroxide (TMAH) etching through transmission electron microscopy (TEM), Kelvin probe force microscope (KPFM), cathodoluminescence (CL) and time-resolved photoluminescence (TRPL). As confirmed by TEM and geometric phase analysis (GPA), ICP etching causes sidewalls to form atomically rough semi-polar surfaces and increases 25% compressive strain at the sidewall compared to the inside. TMAH solution introduces new sidewall defects due to excessive etching of three atomic layers of InGaN. Holes accumulate at the surface because of build-in electric field as showed by KPFM. The sidewall defects lead to a decrease in carrier lifetime resulting in uneven luminescence of micro-LED mesa. TMAH treatment removes the damaged layer and reduces the non-radiative recombination rate. ICP causes damage to the nanoscale structure, however the influence of sidewall defects on the carrier behavior is in the micron range due to unavoidable surface dangling bonds and surface lattice relaxation. A non-radiative recombination mechanism is proposed based on strain relaxation.

Performance Enhancement of Multiple Quantum Shell Nanowire‐Based Micro‐Light Emitting Diodes with Underlying GaInN/GaN Superlattices

GaInN/GaN multiple quantum shell (MQS) nanowires (NWs) are of great interest as high‐efficiency micro‐light emitting diodes (micro‐LEDs), mainly due to their quantum confined Stark effect suppression and dry etching insensitivity features. Herein, morphological and device properties corresponding to NW‐LEDs with different numbers of GaInN/GaN superlattices (SLs) are evaluated. The scanning electron microscopy measurements revealed that the polar‐plane MQS are shrunken, while the semipolar‐plane underwent expansion upon the introduction of the SLs. The current density–voltage–light output analysis at low current density indicates that samples with a greater number of SL pairs exhibit higher light output. Electroluminescence spectra show that NWs lacking SLs exhibit an emission wavelength of 700 nm, which is derived from indium‐rich clusters in polar‐plane MQS, whereas those with SLs emit at a significantly shorter wavelength of 560 nm. The reduction in the polar‐plane MQS coupled with the enhancement in MQS quality resulting from the SLs is identified as the primary contributing factor. Additionally, the external quantum efficiency factor for NW‐LEDs, which remained consistent even as the emission area decreased, is assessed. These findings suggest that NW‐LEDs with SLs possess the potential to mitigate the emission degradation associated with sidewall etching and realize high‐efficiency micro‐LEDs.

Investigation of emission plane control in GaInN/GaN multiple-quantum shells for efficient nanowire-based LEDs.

Significant attention has been directed toward core-shell GaInN/GaN multiple-quantum shell (MQS) nanowires (NWs) in the context of high-efficiency micro light-emitting diodes (micro-LEDs). These independent three-dimensional NWs offer the advantage of reducing the impact of sidewall etching regions. Furthermore, the emitting plane on the sidewalls demonstrates either nonpolar or semipolar orientation, while the dislocation density is exceptionally low. In this study, we assessed how changes in the NW morphology are affected by GaInN/GaN superlattice (SL) structures grown at varying growth temperatures, as well as control of the emission plane via the p-GaN shell and emission sizes. The SL growth rate was enhanced at elevated growth temperatures, accompanied by the shrinkage of the (0001)-plane and expansion of the (11̄01)-plane on the NWs. The samples exhibited a higher light output when the SLs were grown at elevated temperatures compared to those grown with lower temperatures. A similar trend was observed for the samples with a gradual temperature transition during the growth. These findings indicate that the dimensions of the (0001)-plane can be controlled through SL growth, which in turn influences the emission properties of NW-LEDs. In addition, the emission properties of NW-LEDs with different growth time p-GaN shells and different emission sizes were investigated. Based on the NW-LED characteristics, it was revealed that the weak emission of the (0001)-plane was the dominant factor for the limited light output, and the most effective way to realize high efficiency devices is to suppress current injection into the apex or minimize the grown (0001)-plane region. Overall, it is one promising way to control the emission planes of NWs, which holds significant relevance for the potential application of NW-LEDs in the realm of micro-LEDs.

2D Analysis and simulation of quartz crystal etch penetration by revisiting a previous geometric method

This paper presents a quartz wet etching simulation procedure tailored for microelectromechanical systems (MEMS) fabrication. It utilizes geometric methods and is primarily founded on modifying some aspects of the traditional slowness vector approach. Through a theoretical analysis of the crystal etching trajectory, we provide a macroscopic elucidation for the formation of 2D etch profiles and optimize wafer etch penetration during double-sided wet etching convergence of upper and lower surfaces. Based on the proposed theoretical model, we develop a CAD-like quartz crystal etch simulator that allows for simulations of various quartz cut types and etch penetration. To validate the accuracy of the simulator, we quantitatively compare the simulation with experimental results using the angle between adjacent etched sidewall profiles as a metric. The comparison reveals a minimum error of only 2.2° and a maximum error of 11.7° between the simulation and experimental results. The developed simulator is able to predict the profiles of different etching profiles well, and it has certain engineering application capability.

Exploring oxide-nitride-oxide scalloping behavior with small gap structure and chemical analysis after fluorocarbon or hydrofluorocarbon plasma processing

The scalloping of oxide-nitride-oxide (ONO) stacked layers on vertical sidewalls during high-aspect-ratio contact etch is commonly seen and characterized by the horizontal etching of oxide and nitride layers at different etch rates. To understand the mechanisms of ONO scalloping in complex plasma chemistry, it is crucial to examine the surface chemistry of silicon dioxide and silicon nitride processed with single fluorocarbon (FC) or hydrofluorocarbon (HFC) gases. To simulate the isotropic etching of SiO2 and Si3N4 sidewalls, we use a horizontal trench structure to study the effect of neutral radicals produced by FC (Ar/C4F8), HFC (Ar/CH3F, CH2F2, or CH3F), FC/HFC (Ar/C4F8/CH2F2), or FC/H2 (Ar/C4F8/H2), plasma for aspect-ratio (AR) up to 25. To eliminate the effect of ions, oxide and nitride trench structures were treated by inductively coupled plasma. The changes in the film thickness as a function of AR were probed by ellipsometry. Additionally, x-ray photoelectron spectroscopy (XPS) measurements on oxide and nitride substrates processed by Ar/C4F8 and Ar/CH2F2 plasma were performed at various locations: outside of the trench structure, near the trench entrance (AR = 4.3), and deeper in the trench (AR = 12.9). We find a variety of responses of the trench sidewalls including both FC deposition and spontaneous etching which reflect (1) the nature of the FC and HFC gases, (2) the nature of the surfaces being exposed, and (3) the position relative to the trench entrance. Overall, both the etching and deposition patterns varied systematically depending on the precursor gas. We found that the ONO scalloping at different ARs is plasma chemistry dependent. Oxide showed a binary sidewall profile, with either all deposition inside of the trench (with FC and FC/H2 processing) or etching (HFC and FC/HFC). Both profiles showed a steady attenuation of either the deposition or etching at higher AR. On the nitride substrate, etching was observed near the entrance for HFC precursors, and maximum net etching occurred at higher AR for high F:C ratio HFC precursors like CHF3. XPS measurements performed with Ar/C4F8 and Ar/CH2F2 treated surfaces showed that Ar/C4F8 overall deposited a fluorine-rich film outside and inside of the trench, while Ar/CH2F2 mostly deposited a cross-linked film (except near the trench entrance) with an especially thin graphitic-like film deep inside the trench.

Etched Silicon Planar CRL Optics for the High-Energy X-ray Diffraction Beamlines 11-ID-B and 11-ID-C at the APS

The focusing of high-energy X-rays is most commonly achieved by commercially available compound refractive lenses (CRL), which are added together in larger structures. There is a need for high-energy X-ray beamlines to enable focusing in only one dimension down to a few micrometers to study layered materials and thin films. High-energy X-rays above 50 keV require often 100 lenses or more, therefore it is more efficient to etch the complete arrays of lenses into a monolithic substrate like a silicon wafer. Custom lens arrays were fabricated for the high-energy x-ray beamlines 11-ID-B, and 11-ID-C at the Argonne National Laboratory. The fabrication required tuning parameters of the Bosch process to optimize the etching of deep vertical sidewalls at a reasonable speed without overheating. The beamline 11-ID-B is equipped with lens arrays of 85-110 lenses for 59 keV and 190-250 lenses for 87 keV photon energy, focusing the beam vertically below 5.0 µm at the sample positions between 1600 mm to 2100 mm. 11-ID-C has received arrays of 435 lenses that focus 106 keV X-rays to a vertical height of 2.0 µm at the sample position. In both cases, the flux density gain in the focal spot is larger than a factor of 20, relative to using slits to cut the beam size. The lens setup is permanently installed and can be moved in and out of the beam during the experiment without further alignment. The lenses have enabled operando diffraction studies on interfaces in batteries and the development of pair distribution function (PDF) analysis using grazing incidence geometry.

ICP etching of GaN microstructures in a Cl2–Ar plasma with subnanometer-scale sidewall surface roughness

Substrate temperature, RF power, and ICP power were investigated for their effects on GaN micropillar sidewall roughness and etch characteristics. Elevated substrate temperature was shown to improve the sidewall etch morphology at low RF powers (reduced physical bombardment) and low ICP powers (lower plasma densities). Increased lateral etching is observed with both increased ICP power and substrate temperature, which both act to increase the chemical component of the etch. Etch conditions with a high chemical driving force resulted in faceting along the a-plane on the sidewalls. This faceting produced extremely smooth surfaces with root-mean-square roughness (Rq) as low as 0.20 nm which is comparable to typical epitaxy-ready surfaces and smaller than the a-plane lattice spacing of 0.3186 nm. The smooth surfaces produced in this study enable possibilities for laser facets or for new device structures that require high quality surfaces for GaN regrowth.

Application of femtosecond laser etching in the fabrication of bulk SiC accelerometer

Silicon carbide (SiC) is a promising material to fabricate MEMS accelerometers used in extreme environments for vibration monitoring. However, with ultra-high hardness and corrosion resistance, it is difficult to etch bulk SiC by traditional MEMS processing methods. Here, we presented a bulk SiC accelerometer structure and its femtosecond laser deep etching method. Combined with the sensor design and femtosecond laser etching method, the sensitivity of the sensor could be improved and the processing difficulty could be reduced. The sensitive beams and mass block of the designed accelerometer were fabricated by femtosecond laser and high size accuracy with the maximum size error of about 3.1% was achieved. A fillet transition on the sensitive beam can eliminate crack propagation during laser etching and improve the quality of the etched gap. The cross-section of the etched sensitive beam can be regarded as an isosceles trapezoid with a steepness of 84.76°. The laser etching sidewall surface of the beam is smooth with a surface roughness (Sa) of 0.255 μm. The piezoresistors on the beam of the accelerometer remain stable before and after laser etching. Generally, the results show that femtosecond laser etching is a feasible method to fabricate bulk SiC accelerometers.

Design, fabrication, and characterisation of a silicon microneedle array for transdermal therapeutic delivery using a single step wet etch process

The fabrication of silicon in-plane microneedle arrays from a simple single wet etch step is presented. The characteristic 54.7° sidewall etch angle obtained via KOH etching of (100) orientation silicon wafers has been used to create a novel microneedle design. The KOH simultaneously etches both the front and back sides of the wafer to produce V shaped grooves, that intersect to form a sharp pyramidal six-sided microneedle tip. This method allows fabrication of solid microneedles with different geometries to determine the optimal microneedle length and width for effective penetration and minimally invasive drug delivery. A modified grooved microneedle design can also be used to create a hollow microneedle, via bonding of two grooved microneedles together, creating an enclosed hollow channel. The microneedle arrays developed, effectively penetrate the skin without significant indentation, thereby enabling effective delivery of active ingredients via either a poke and patch application using solid microneedles or direct injection using hollow microneedles. This simple, scalable and cost effective method utilises KOH to etch the silicon wafer in-plane, allowing microneedles with variable length of several mm to be fabricated, as opposed to out-of-plane MNs, which are geometrically restricted to dimensions less than the thickness of the wafer. These microneedle arrays have been used to demonstrate effective delivery of insulin and hyaluronic acid into the skin.

Vertical silicon etching by using an automatically and fast-controlled frequency tunable rf plasma source

An automatically and fast-controlled frequency tunable radiofrequency (rf) system is employed to a plasma etching device, where the rf system contains two rf amplifiers operational in 37 MHz–43 MHz for a plasma source and a wafer stage. Both impedance matching circuits for the source and the wafer stage have no variable capacitors, leading a compact design of the rf system; the power reflection can be minimized by adjusting the frequencies. The rf frequency and the output power are automatically controlled so as to minimize a reflection coefficient and to maintain a constant net power corresponding to a forward power minus a reflected power for both the source and the stage. The source is operated with SF6 and C4F8 gases for silicon etching and passivation in the Bosch process, respectively. For both the cases, the impedance tuning can be accomplished within several ms and the net power is maintained at a constant level. By alternatively switching the SF6 and C4F8 plasmas with pulse widths of 5 s and 2 s, respectively, a vertical silicon etching is performed, where a scallop structure is clearly formed on the etching side wall. By shortening the pulse widths down to 1 s and 0.4 s for the SF6 and C4F8 plasmas, the size of the scallop structure is significantly reduced; the usability of the automatically and fast-controlled rf plasma source for the Bosch process is demonstrated.

Sidewall chemical analysis of plasma-etched nano-patterns using tilted X-ray photoelectron spectroscopy combined with in-situ ion sputtering

A novel experimental technique for sidewall analysis of plasma-etched nano-patterns using tilted X-ray photoelectron spectroscopy (XPS) combined with in-situ ion sputtering was investigated. Complete removal of the residue layer (on the mask surface) and the mask layer using in-situ ion sputtering in the XPS chamber was conducted to reveal the sidewall information of the plasma-etched hole in SiO2 in addition to the trench in Si, after anisotropic plasma etching. The influence of the measured profile depth changes after adjusting the take-off angle in the sidewall chemistry of the plasma-etched hole in SiO2 and the trench in Si was examined. To demonstrate the analysis results, the sidewall compositions of the plasma-etched hole in SiO2 with various oxygen contents in the plasma were comparatively analyzed using XPS and Auger electron spectroscopy. It was observed that fluorinated carbon polymer (C-Fx, x ≤ 3) and SixOy (x ≤ 2, y ≤ 3) residue layers were formed on the hole in SiO2 and trench in Si sidewalls after C4F8/CH2F2/O2/Ar and HBr/Cl2 plasma etching, respectively. Thus, changes in the chemical characteristics of these sidewall etching residues were realized, revealing their dependency on the pattern size and type, plasma gas chemistry, and mask material. In addition, effectiveness of the dry cleaning process was evaluated using the tilted XPS combined with in-situ Ar ion sputtering.

The Role of Oxygen on Anisotropy in Chromium Oxide Hard Mask Etching for Sub-Micron Fabrication

Chromium and its oxides have been playing a vital role in the fabrication of micro- and nano-scale structures in numerous applications for several decades. Controllable, robust and anisotropically dry-etched hard masks and their optimal etch recipes are required in state-of-the-art device fabrication techniques. In terms of manufacturability and repeatability, a mechanistic understanding of the plasma-etching process of chromium oxide (Cr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) is necessary for its adoption as a hard mask. We present a systematic investigation of plasma etching of chromium oxide films via an inductively coupled plasma-reactive ion etching (ICP-RIE) system in nanoscale. The effects of plasma composition, ICP source power and HF platen power on the etch rate, sidewall profile, surface morphology, and dc-bias have been methodically investigated. We paid particular attention to studying how oxygen content can be used to control the etch profile of nano trenches using chlorine/oxygen gas mixtures, including extremes of very low and very high oxygen content. It was found that chromium oxide etch mechanisms are dependent strongly on the oxygen level. We achieved desirable vertical sidewalls with reasonable etch rates when the oxygen content is in the range 10-40% in the plasma. Oxygen content below 10% resulted in positively tapered etch profiles with low etch rates. On the other hand, bowl-like etch profiles with undercut formation was observed at high oxygen content above 40%, caused by re-emission of the reactive species at this regime. As a hard mask material, patterning Cr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> films compared to Cr metal is advantageous in terms of etch uniformity and reproducibility. Contrary to Cr, Cr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> is not as sensitive to chamber wall conditions.

Erbium-ytterbium co-doped aluminium oxide waveguide amplifiers fabricated by reactive co-sputtering and wet chemical etching.

We report on the fabrication and optical characterization of erbium-ytterbium co-doped aluminum oxide (Al2O3:Er3+:Yb3+) waveguides using low-cost, low-temperature deposition and etching steps. We deposited Al2O3:Er3+:Yb3+ films using reactive co-sputtering, with Er3+ and Yb3+ ion concentrations ranging from 1.4-1.6 × 1020 and 0.9-2.1 × 1020 ions/cm3, respectively. We etched ridge waveguides in 85% pure phosphoric acid at 60°C, allowing for structures with minimal polarization sensitivity and acceptable bend radius suitable for optical amplifiers and avoiding alternative etching chemistries which use hazardous gases. Scanning-electron-microscopy (SEM) and profilometry were used to assess the etch depth, sidewall roughness, and facet profile of the waveguides. The Al2O3:Er3+:Yb3+ films exhibit a background loss as low as 0.2 ± 0.1 dB/cm and the waveguide loss after structuring is determined to be 0.5 ± 0.3 dB/cm at 1640 nm. Internal net gain of 4.3 ± 0.9 dB is demonstrated at 1533 nm for a 3.0 cm long waveguide when pumped at 970 nm. The material system is promising moving forward for compact Er-Yb co-doped waveguide amplifiers and lasers on a low-cost silicon wafer-scale platform.