Selective Wet Etching Process Research Articles

Selective Wet and Dry Etching of NiO over β-Ga2O3

In this study, a reaction-limited chemical wet etching process of NiO by an HNO3-based solution, with a complete selectivity over β-Ga2O3 was investigated. The threshold ion energies for inductively coupled plasmas (ICP) dry etching of NiO in BCl3 discharges, its selectivity to β-Ga2O3, and the cleaning effect for chlorine-based residues remaining on the surface after plasma exposure was also examined. One of the major drawbacks of β-Ga2O3 device design is the lack of p-type dopants. An alternative approach is to use a p-type oxide, NiO, to form heterojunctions with n-type Ga2O3. Therefore, a selective wet and dry etching process development is in demand for the optimized pattern transfer process. A solution of 1 HNO3: 4 H2O exhibited measurable etch rates for NiO above 40°C and wet etching activation energy of 172.9 kJ.mol-1 (41.3 kCal.mol-1, 1.8 eV/atom), which is firmly in the reaction-limited regime. The selectivity over β-Ga2O3 was infinite for temperatures up to 55 °C. The strong negative enthalpy for producing the etch product Ga(OH)4 suggests that HNO3-based wet etching of NiO occurs via the formation and dissolution of hydroxides. The BCl3/Ar ICP etching produced maximum etch rates up to 300 Å.min-1 for NiO and 800 Å.min-1 for β-Ga2O3 under moderate plasma power conditions. The selectivity for NiO: Ga2O3 was < 1 under all conditions. The ion energy threshold for the initiation of etching NiO was between 35-60 eV, depending on the plasma power conditions. And its etching mechanism was ion-driven, as determined by the linear dependence between etch rate and the square root of ion energy incident on the surface. By sharp contrast, the etching of Ga2O3 had a stronger chemical component without a well-defined ion energy threshold. After being exposed to BCl3 plasmas, both NiO and Ga2O3 surfaces show chlorine residues, which can both be removed by the standard 1NH4OH: 10H2O or 1HCl: 10H2O solution used for native oxide removal. In addition, according to the location of the Cl 2p3/2 peak in the XPS analysis on the surface, the chlorine particles were ionically bonded. Figure 1

Selective wet etching in fabricating SiGe nanowires with TMAH solution for gate-all-around MOSFETs

In this work, a selective wet etching process of Si to Si0.7Ge0.3 with TMAH solution to fabricate SiGe nanowires is systematically investigated. Initially, the 2.3% TMAH solution at 20 °C is applied for the as grown Si0.7Ge0.3/Si multilayers stack and a “rectangular” profile is achieved at the Si0.7Ge0.3 extremity due to its high selectivity. However, the reduction of Ge concentration due to the Ge interdiffusion caused by the high temperature anneal treatment will attain a reduced selectivity of Si to Si0.7Ge0.3 and a “rounding” Si0.7Ge0.3 extremity profile. Moreover, the profile of the upper Si0.7Ge0.3 extremity is clearly worse than the bottom one. This is because the Ge concentration of upper Si0.7Ge0.3 is 1.5% lower and the Ge interdiffusion of the upper Si0.7Ge0.3 is slightly worse. After increasing the TMAH concentration to 25%, both the upper and bottom Si0.7Ge0.3 extremity can achieve a “rectangular” extremity profile with an almost the same Si0.7Ge0.3 loss ~ 1.5 nm per side. Compared with 2.3% TMAH concentration, the Ge concentration of the etching off SiGe film can be reduced from lower than 29% to lower than 24% using the 25% TMAH solution at 20 °C. Therefore, the 25% TMAH solution at 20 °C is chosen as the optimal selective etching condition for the thermal treated Si0.7Ge0.3/Si multilayers stack samples. Finally, a vertical stacked double SiGe nanowire structure is successfully prepared by utilizing this optimal process condition. This indicates that it is a practicable technique for the selective wet etching of Si in fabricating SiGe nanowires.

Scalable Synthesis of Ti3C2Tx MXene

Scaling the production of synthetic 2D materials to industrial quantities has faced significant challenges due to synthesis bottlenecks whereby few have been produced in large volumes. These challenges typically stem from bottom‐up approaches limiting the production to the substrate size or precursor availability for chemical synthesis and/or exfoliation. In contrast, MXenes, a large class of 2D transition metal carbides and/or nitrides, are produced via a top‐down synthesis approach. The selective wet etching process does not have similar synthesis constraints as some other 2D materials. The reaction occurs in the whole volume; therefore, the process can be readily scaled with reactor volume. Herein, the synthesis of 2D titanium carbide MXene (Ti3C2Tx) is studied in two batch sizes, 1 and 50 g, to determine if large‐volume synthesis affects the resultant structure or composition of MXene flakes. Characterization of the morphology and properties of the produced MXene using scanning electron microscopy, X‐ray diffraction, dynamic light scattering, Raman spectroscopy, X‐ray photoelectron spectroscopy, UV–visible spectroscopy, and conductivity measurements show that the materials produced in both batch sizes are essentially identical. This illustrates that MXenes experience no change in structure or properties when scaling synthesis, making them viable for further scale‐up and commercialization.

MEMS-based thermal conductivity sensor for hydrogen gas detection in automotive applications

Accurate detection of hydrogen gas in vehicle interiors is very important for the future of a fuel cell car. Since this type of gas is highly volatile and flammable, the measurement methods have to be very reliable and precise due to safety reasons. In this paper a thermal conductivity sensor for hydrogen gas detection is presented, exhibiting a lower detection limit of 2000 ppm hydrogen in laboratory air. The sensor element is realized by micro-fabrication techniques on silicon wafers. The heated filament is exposed by a selective wet etching process creating a micro-hotplate on a thin membrane. In order to minimize power consumption, the sensor is operated in pulsed mode. Hydrogen gas detection was carried out using a synthetic gas testbench. Measurements of hydrogen contents ranging from 0% to 4% with an increment of 0.5% were successfully performed for ambient gas temperatures between −15°C and 84°C. Including humidity, high moisture contents have the greatest influence on thermal conductivity. This was predicted in theoretical investigations and confirmed in experiments. For evaluation, both the change in resistance ΔR as well as the time constant τ were taken as sensor output. For both quantities, the previously established theoretical relationship with thermal conductivity could be confirmed.

High-Sensitive TM Modes in Photonic Crystal Nanobeam Cavity with Horizontal Air Gap for Refractive Index Sensing

We propose a new type of refractive index sensing based on the transverse magnetic (TM) modes in the photonic crystal (PhC) nanobeam (NB) cavity with a horizontal air gap. The electric field of the resonant TM mode is strongly confined within the horizontal air gap present at the PhC NB cavity. In order to increase the quality (Q) factor and the sensitivity (S) of the refractive index change in the air simultaneously, the cavity structure is fully optimized. Because of the trade-off between the Q-factor and S of the TM mode in the PhC NB cavity with an air gap, there is an optimal thickness of the air gap in the dielectric slot. From the numerical simulation results, S can exceed 1000 nm/RIU with Q &gt; 40,000. When the dielectric slot becomes thin, S could be higher than 1200 nm/RIU. For practical applications, we suggest an Si-based PhC NB cavity with a horizontal SiO2 slot structure which can also provide high S with a high Q-factor after a very fine selective wet etching process. This new type of TM resonant mode in the PhC NB cavity can be an ideal platform for compact sensors in photonic integrated circuits for TM waveguide systems.

Selective Wet Etching in Fabricating SiGe and Ge Nanowires for Gate-all-Around MOSFETs

A selective wet etching process for fabricating SiGe and Ge nanowires for gate all around transistors is introduced in this paper. Two formulated proprietary chemical mixtures with highly selective etching properties (Si vs. SiGe and SiGe vs. Ge) can effectively dissolve the sacrificial layers with minimal damage to the interstitial nanowire materials. The Auger Electron Spectroscopy (AES) surface characterization indicates that no chemical contamination is left after the wet etching process.

Selective wet-etch processing of optically transparent flexible InGaZnO thin-film transistors

Flexible InGaZnO (IGZO) thin-film transistors (TFTs) having more than 85% optical transparency in the visible regime were fabricated on polyethylene naphthalate substrates using a selective wet etching process at a maximum temperature of 150 °C. An etch selectivity of &amp;gt;15 was used to pattern Al-doped ZnO S/D contacts on IGZO channel layers in a back-channel etch process. Flexible IGZO TFTs exhibited a field-effect mobility of ∼9.5 cm2/V s, threshold voltage of ∼4 V, subthreshold swing of 0.5 V/decade, and an Ion/Ioff ratio of ∼107. Similar I-V characteristics were measured for flexible TFTs under flat, tensile and compressive strain while a 3.8 V threshold voltage shift was observed after application of a dc gate-bias for 3600 s with the TFTs operating in the linear regime.

Optically Transparent Flexible IGZO TFTs Fabricated with a Selective Wet-Etch Process

The emerging transparent thin-film transistor (TFTs) technology has recently received great attention in the active-matrix organic light-emitting diode (AMOLED) flexible displays. This TFT technology also has the potential to enable enhanced functionality in conventional large-area electronics applications ranging from smart windows and heads-up displays to flexible sensor arrays. During the past few years, amorphous transition-metal oxides have been suggested to be one of the best candidates in this application. Amorphous InGaZnO (IGZO) TFTs are the most promising transparent-metal oxide semiconductor due to its high optical transparency and field-effect mobility (μFE) in TFT devices compared to conventional amorphous silicon (a-Si:H). The amorphous structure is more suitable for large-area processes than other polycrystalline metal-oxide TFTs. These unique advantages have resulted in the rapid development of IGZO TFT technology. Given the relatively poor selectivity to common semiconductor etchants, IGZO TFTs are typically fabricated using lift-off processes to define the electrical contacts. However, the success of the emerging transition-metal oxide materials system for flexible electronics requires compatibility with conventional TFT microfabrication manufacturing processes. Flexible IGZO TFTs having 85% optical transparency in the visible regime were fabricated on polyethylene napthalate substrates using a selective wet etching process to eliminate the need for lift-off processing. The devices were processed directly onto the plastic platform at a maximum temperature of 150°C. The TFT structure consisted of a sputtered 100 nm aluminum doped zinc oxide (AZO) gate electrode, a 300nm SiNx gate dielectric deposited using plasma enhanced chemical vapor deposition (PECVD) method at 150ºC, and a 50nm IGZO layer deposited using RF sputtering at room temperature. A 150 nm AZO source/drain (S/D) electrodes were deposited by using RF sputtering at 150ºC and patterned wet etching process in diluted HCl solution having a selectivity of 11 between AZO and IGZO. The channel width and length of the studied TFTs were 200 and 100 µm, respectively. The fabricated TFTs exhibited a field-effect mobility of ~10 cm2/V.sec, threshold voltage of ~5.0 V, a sub-threshold swing of 0.8 V/decade, and an Ion/Ioff ratio of 107. No current crowding behavior was observed for the TFTs at the low drain-source voltage (VD) regime. The I-V characteristics showed excellent ohmic contacts can be fabricated using AZO S/D electrodes through a wet etching process and promises a more conventional approach to large-area flexible electronics fabrication. The highly selective etching process provides a means to fabricate IGZO based circuits with low processing complexity that will enable system-on-“plastic” integration of flexible transparent flexible displays. In addition, the effect of long-term electrical bias on the electrical stability of the devices will also be presented.

Optically Transparent Flexible IGZO TFTs Fabricated with a Selective Wet-Etch Process

Given the relatively poor selectivity to common semiconductor etchants, IGZO (InGaZnO) thin film transistors (TFTs) are typically fabricated using lift-off processes to define the electrical contacts. However, the success of the emerging transition-metal oxide materials system for flexible electronics requires compatibility with conventional TFT fabrication processes. In this research, flexible IGZO TFTs having 85% optical transparency in the visible regime were fabricated on polyethylene napthalate (PEN) substrates using a selective wet etching process to eliminate the need for lift-off processing. The devices were processed directly onto the plastic platform at the maximum temperature of 150°C. The fabricated TFTs exhibited a field-effect mobility of ~12.5 cm2/V.sec, threshold voltage of ~5.0 V, and an Ion/Ioff ratio of <106. No current crowding behavior was observed for the TFTs at the low drain-source voltage (VD) regime. The highly selective etching process provides a means to fabricate IGZO based circuits with low processing complexity that will enable system-on-“plastic” integration of flexible transparent flexible displays.

Graphical based characterisation and modelling of material gradients in porosified LTCC

The chemical composition of low temperature cofired ceramics (LTCC) allows to locally embed air into sintered substrates by a selective wet chemical etch process. Therefore, LTCC substrate with areas of low permittivity can be created without material combination. The presented graphical method of material component contrast images enables the evaluation of their most important material properties which are their component distribution, their porosification gradient and their residual bearing surface. The graphical method, including focused ion beam and scanning electron microscopy analyses, is applied to different commercially available LTCC types having two porosification states each. Derived mathematical models, which are suitable for finite element method implementation, allow the characterisation of the effective permittivity reduction while keeping a maximum residual surface area for, e.g., metallisation purposes. The shape of the optimum material distribution function features an ‘air pocket’ of small width and a depth being dependent on the application specific operating frequency.

Wet chemical etching process for wafer scale isolation and interconnection of GaSb based device layers grown on GaAs substrates

An optimized selective wet chemical etching process using potassium sodium tartrate solutions and citric acid solutions has been reported for wafer scale isolation and interconnection of GaSb based device structures grown on GaAs substrates. Uniformly etched surfaces with vertical side walls with minimal under-cutting and edge rounding, and smooth surface morphology have been achieved over a 2 in. (50 mm) diameter wafer using an optimized etching conditions, etchant preparation process, and specific ratios of the etchant ingredients. Repeatable etch rates for GaSb of approximately 330 nm/min and 12 nm/s for GaAs have been obtained. Using the developed process, wafer scale monolithically interconnected GaSb devices as well as modules on semi-insulating GaAs substrates can be fabricated for a variety of optoelectronic applications.

Record-low propagation losses of 154 dB/cm for substrate-type W1 photonic crystal waveguides by means of hole shape engineering

Large propagation losses in the order 600-1000 dB/cm were reported in the past for planar line-defect photonic crystal waveguides with a weak vertical refractive index contrast (substrate-type). Loss-relevant factors are a well-suited design of the photonic crystal pattern and a high-quality fabrication technology. Here, the focus is on the latter. Two methods—a thermally driven mass transport process and an ultra-slow, selective wet-etching process—are assessed with respect to their capability of enabling low-loss waveguides. With the resulting hole shape, we experimentally demonstrate record-low propagation losses of 154 dB/cm for W1 photonic crystal waveguides in the InP/InGaAsP system.

Enhanced luminescence of GaN-based light-emitting diodes by selective wet etching of GaN/sapphire interface using direct heteroepitaxy laterally overgrowth technique

We introduce a method to enhance the luminescence of GaN-based LEDs by combining the direct heteroepitaxy laterally overgrowth (DHELO) technique with selective wet etching process. The epitaxial overgrowth of GaN layers on sapphire substrate with SiO 2 micro-rods array exhibited a reduced dislocation density and improved the crystal quality. The EL intensity of LEDs with SiO 2 micro-rods array was 6.5% higher than conventional LEDs at 20 mA. The selective wet etching process was then used to texture the LED sidewalls into inverted pyramid shape. Finally, the EL intensity could be further enhanced about 12.5% as compared with LEDs with SiO 2 micro-rods array when adopting the textured sidewalls.

Electrical, optical and surface properties of P 2S 5/(NH 4) 2S x+ Se-treated doped-channel field-effect transistors with double etch-stop layers

This work describes doped-channel field-effect transistors (DCFETs), featuring both low–high doped-channels and double AlAs etch-stop layers used in a selective etch recessed-gate process. A developed highly selective wet etching process is applied as a gate-recess technique to fabricate DCFETs. Selective wet etching using citric acid/H 2O 2/NH 4OH/H 2O solutions in conjunction with double thin AlAs etch-stop layers is a reasonably simple, safe, and reliable process for gate recessing in the fabrication of the DCFETs herein. Surface passivation using P 2S 5/(NH 4) 2S x+ Se on GaAs Schottky barrier diodes, formed by Pt/Au contacts, is examined for the first time and the results are compared with those of unpassivated devices. For the passivated Pt/Au gate device, the two-terminal gate-drain breakdown (source floating) at − 1 mA/mm is 17 V, and the device provides an excellent combination of transconductance and output current. The X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and photoluminescence results are highly consistent with the Schottky barrier measurements and the device performance.

Selective Wet Etching Process for Ni-InGaAs Contact Formation in InGaAs N-MOSFETs with Self-Aligned Source and Drain

Etch rates of Ni-InGaAs metallic film and Ni in HCl, HNO3, HF and SPM etch chemistries were determined using sheet resistance method, step height measurements, and cross sectional TEM characterization. HCl and HNO3 based chemistries were studied in detail. The etch selectivity of Ni over Ni-InGaAs in various etchants was studied. Concentrated HCl was found to have the highest selectivity of approximately 15.6. HNO3 (1:10) and HNO3 (1:20) solutions provides good selectivities of 4.3 and 4.5 respectively. The results of this work could be useful for the formation of Ni-InGaAs self-aligned source and drain in InGaAs N-MOSFETs.

The Highly Selective Wet Etching Process of SiO2 to TiSix

The relatively higher etch rate of titanium silicide to silicon oxide in HF-based solution was ascribed to the strong interaction of the positively charged hydrogen with titanium, which has very low electron affinity relative to other metals. Based on the electron-transfer mechanism of dissolution, we developed the highly selective etching material (T-Block) mainly composed of fluoride salt and low dielectric constant solvent. To overcome the self-limiting nature of the T-Block material in the etch process, we devised the iterative treatment process, which is more suitable for single-wafer processing equipment. In contrast to the normal etching solution such as buffered oxide etchant or HF, T-Block did not show any kind of failure regarding dissolution even when the underlying bit-line contact was disclosed to T-Block in the wet etch process. Therefore, if T-Block is applied in a fabrication process with the 6F2 cell design, a significant enhancement of productivity through process simplification as well as production yield is expected.

Single cell detection using a glass-based optofluidic device fabricated by femtosecond laser pulses

We demonstrate the fabrication of integrated three-dimensional microchannel and optical waveguide structures inside fused silica for the interrogation and processing of single cells. The microchannels are fabricated by scanning femtosecond laser pulses (523 nm) and subsequent selective wet etching process. Optical waveguides are additionally integrated with the fabricated microchannels by scanning the laser pulse train inside the glass specimen. Single red blood cells (RBC) in diluted human blood inside of the manufactured microchannel were detected by two optical schemes. The first involved sensing the intensity change of waveguide-delivered He-Ne laser light (632.8 nm) induced by the refractive index difference of a cell flowing in the channel. The other approach was via detection of fluorescence emission from dyed RBC excited by Ar laser light (488 nm) delivered by the optical waveguide. The proposed device was tested to detect 23 fluorescent particles per second by increasing the flow rate up to 0.5 microl min(-1). The optical cell detection experiments support potential implementation of a new generation of glass-based optofluidic biochip devices in various single cell treatment processes including laser based cell processing and sensing.

Uniform InGaAs quantum dot arrays fabricated using nanosphere lithography

We demonstrate the fabrication of optically active uniform InGaAs quantum dot arrays by combining nanosphere lithography and bromine ion-beam-assisted etching on a single InGaAs/GaAs quantum well. A wide range of lateral dot sizes was achieved from an oxygen plasma nanosphere resizing process. The increased lateral confinement of carriers in the dots results in low temperature photoluminescence blueshifts from 0.5 to 11 meV. Additional quantization was achieved using a selective wet-etch process. Our model suggests the presence of a 70 nm dead layer in the outer InGaAs radial edge, which we believe to be a result of defects and dislocations introduced during the dry-etch process.

CMOS compatible dual metal gate integration with successful Vth adjustment on high- k HfTaON by high-temperature metal intermixing

In this paper, we report for the first time a novel dual metal gate (MG) integration process for gate-first CMOS platform by utilizing the intermixing (InM) of laminated ultra-thin metal layers during high-temperature annealing at 1000 °C. In this process, an ultra-thin (∼2 nm) TaN film is first deposited on gate dielectric as a buffer layer. Preferable laminated metal stacks for NMOS and PMOS are then formed on a same wafer through a selective wet-etching process in which the gate dielectric is protected by the TaN buffer layer. Dual work function for CMOS can finally be achieved by the intermixing of the laminated metal films during the S/D activation annealing. To demonstrate this process, prototype metal stacks of TaN/Tb/TaN (NMOS) and TaN/Ti/HfN (PMOS) has been integrated on a single wafer, with WF of 4.15 and 4.72 eV achieved, respectively. Threshold voltage ( V th) adjustment and transistor characteristics on high- k HfTaON dielectric are also studied.

Surface texturing of p‐GaN layer for efficient GaN LED by maskless selective etching

The maskless selective wet etching of p-GaN layer with KOH in ethylene glycol (KE) and H3PO4/H2SO4 (HH) acids was developed for the highly efficient light-emitting diodes (LEDs). The p-GaN surfaces textured by the selective wet etching process without using etch mask showed hexagonal and stripe shapes in the KE and HH solutions, respectively. The current-voltage (I-V) characteristics of the LED textured by KE and HH solutions showed improved electrical properties compared to the non-etched LED. This result could be attributed to a reduced contact resistance due to an increased contact area between the metal electrode and p-GaN layer. In addition, the light-output power of the LED textured by KE and HH solutions was improved by 29.4% and 36.8% relative to that of the non-etched LED. This result was attributed to the increase in probability of escaping photons from the LED and the reduction of surface defects by the maskless selective wet etching process. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)