Silicon Carbide Membrane Research Articles

Direct Wafer Bonding of SiC-SiC by SAB for Monolithic Integration of SiC MEMS and Electronics

Although the monolithic integration of silicon carbide (SiC) Micro-Electro-Mechanical Systems (MEMS) and SiC electronics is very promising, it is still very challenging due to the absence of suitable bulk machining technology of SiC. In this research, wafer bonding was proposed to assist the monolithic integration of SiC MEMS and SiC electronics by the formation of a suspended epitaxial SiC membrane. However, currently, SiC-SiC wafer bonding is still very difficult, especially for its direct wafer bonding. Surface activated bonding (SAB) method was applied to realize the direct wafer bonding of SiC-SiC at room temperature. The bonding energy of ∼1.4 J/m2 was obtained without orientation dependence. Correspondingly, the tensile strength of bonding interface is ∼12.2 MPa and could be improved by rapid thermal annealing to the values higher than 21.6 MPa. The bonding mechanisms were investigated through Monte Carlo simulation, interface analysis by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX), and in-situ analysis on the activated SiC surfaces by X-ray photoelectron spectroscopy (XPS).

Ion beam evaluation of silicon carbide membrane structures intended for particle detectors

Thin ion transmission detectors can be used as a part of a telescope detector for mass and energy identification but also as a pre-cell detector in a microbeam system for studies of biological effects from single ion hits on individual living cells. We investigated a structure of graphene on silicon carbide (SiC) with the purpose to explore a thin transmission detector with a very low noise level and having mechanical strength to act as a vacuum window. In order to reach very deep cavities in the SiC wafers for the preparation of the membrane in the detector, we have studied the Inductive Coupled Plasma technique to etch deep circular cavities in 325μm prototype samples. By a special high temperature process the outermost layers of the etched SiC wafers were converted into a highly conductive graphitic layer. The produced cavities were characterized by electron microscopy, optical microscopy and proton energy loss measurements. The average membrane thickness was found to be less than 40μm, however, with a slightly curved profile. Small spots representing much thinner membrane were also observed and might have an origin in crystal defects or impurities. Proton energy loss measurement (also called Scanning Transmission Ion Microscopy, STIM) is a well suited technique for this thickness range. This work presents the first steps of fabricating a membrane structure of SiC and graphene which may be an attractive approach as a detector due to the combined properties of SiC and graphene in a monolithic materials structure.

Rejection of submicron sized particles from swimming pool water by a monolithic SiC microfiltration membrane: Relevance of steric and electrostatic interactions

The rejection of submicron sized particles from swimming pool water by a ceramic silicon carbide (SiC) microfiltration membrane in monolithic configuration was investigated and mechanisms elucidated.Physicochemical properties showed that the surrogates used in challenge tests (i.e. MS2 bacteriophages and 50–500nm fluorescent microspheres) are adequate surrogates for viruses commonly found in pool water.The log-removal value (LRV) of the SiC membrane strongly depended on the size and electrostatic properties of the surrogates. Experiments with swimming pool water showed that organic matter and salts present in the pool water decreased the surface potential of the surrogates which in turn increased the LRV of the SiC membrane.Long-term experiments revealed, that the removal effectivity of the SiC membrane, over the course of a filtration cycle (~150min), increased for large surrogates (500nm microspheres), while the removal effectivity remained unchanged for small surrogates (MS2 phages). This effect was explained by progressive and permanent blockage of large mesopores (>500nm) in the active membrane layer by particles present in the pool water.

Large-area SiC membrane produced by plasma enhanced chemical vapor deposition at relatively high temperature

Advances in the growth of silicon carbide (SiC) thin films with outstanding thermal and mechanical properties have received considerable attention. However, the fabrication of large-area free-standing SiC membrane still remains a challenge. Here, the authors report a plasma enhanced chemical vapor deposition process at a relatively high temperature to improve the free-standing SiC membrane area. A systematic study on the microstructural, mechanical, and optical properties of hydrogenated polycrystalline silicon carbide (poly-SiCx:H) thin films deposited at 600 °C with different annealing temperatures has been performed. In the as-deposited state, SiCx:H thin films show a polycrystalline structure. The crystallinity degree can be further improved with the increase of the postdeposition annealing temperature. The resulting process produced free-standing 2-μm-thick SiC membranes up to 70 mm in diameter with root mean square roughness of 3.384 nm and optical transparency of about 70% at 632.8 nm wavelength. The large-area SiC membranes made out of poly-SiCx:H thin films deposited at a relatively high temperature can be beneficial for a wide variety of applications, such as x-ray diffractive optical elements, optical and mechanical filtering, lithography mask, lightweight space telescopes, etc.

Spatially Resolved Measurement of the Stress Tensor in Thin Membranes Using Bending Waves

The mode shape of bending waves in thin silicon and silicon carbide membranes is measured as a function of space and time, using a phase shift interferometer with stroboscopic light. The mode shapes hold information about all the relevant mechanical parameters of the samples, including the spatial distribution of static prestress. We present a simple algorithm to obtain a map of the lateral tensor components of the prestress, with a spatial resolution much better than the wavelength of the bending waves. The method is not limited to measuring the stress of bending waves. It is applicable in almost any situation, where the fields determining the state of the system can be measured as a function of space and time. I. INTRODUCTION Mechanical waves are a powerful tool to study material properties over a broad range of length scales. The applications range from seismic-wave methods to studying the structure of the Earth [1], SONAR for underwater location and navigation [2], ultrasonic testing of materials in engineering [3,4] and medical ultrasonography [3] ,d own to methods applicable to micro- and nanostructures like acoustic microscopy [3], Brillouin scattering [5], and picosecond pump-probe spectroscopy [6,7]. For all these methods, the length scale of the inhomogeneities of the material properties is supposed to be larger than the wavelength. In a previous work [8], we show how to extract the dispersion relation of bending waves of prestressed thin plates from measured mode shapes. Stress and bending stiffness of the membrane are determined as fit parameters using a theoretical model of the dispersion relation. Since the existence of a well-defined dispersion relation requires a homogeneous system, this method has the same shortcomings as the methods listed above. Here, we present an approach using full knowledge of the wave function to determine the mechanical properties of the sample. We measure the shape of a bending-wave mode in direct space and compare it to the equation of motion. The parameters found in the equation are the components of the stress tensor divided by the density of the membrane σij=ρ ,a s well as a constant proportional to the bending stiffness. With our simple algorithm, these parameters are obtained as a function of space using a linear fit. Since the full mode shape is available and we are not limited to far-field information, the wavelength of the bending waves is no limit for the spatial resolution. For the measurement of the stress-tensor components of semiconductors, all optical methods can be applied with spatial resolution in the order of a few micrometers [9,10] to roughly 100 μm [11], depending on the respective methods. However, these methods make use of particular optical properties and cannot straightforwardly be generalized to insulators or metals.

Combined slag flow model for entrained flow gasification

A slag flow and heat transfer model coupled with a particle capture sub-model and 3-D gasifier model was developed to describe the slag characteristics and gasification process in entrained flow gasifier. A criterion for particle capture was used to evaluate the interaction of particles colliding with the wall. Two kinds of gasifiers were simulated using the developed model: the membrane wall gasifier and the refractory wall gasifier. The model was proved reliable by comparing the simulation with industrial results. Sensitivity of the model was analyzed. The model predicted the local thickness of the solid and liquid slag layers as well as the temperature distribution across the slag layer. Further study investigated the influence of particle behavior on slagging in the gasifier. In addition, a comparison was made between the membrane wall gasifier and the refractory wall gasifier. The results indicated that temperature of critical viscosity and the thermal conductivity of the slag were crucial factors in determining the accuracy of the combined model. The slag of the membrane wall consisted of both a solid and liquid slag layer. The internal surface temperature of the steel was lower than 540K, which decreases the occurrence of thermal corrosion. The larger particle was beneficial to the capture efficiency and formation of slag layer, while the smaller one favors high carbon conversion. For the membrane wall gasifier, the thermal resistance of the solid slag layer contributed to the protection of the silicon carbide layer and membrane wall. For the refractory gasifier, thermal resistance of the refractory lining and environment convection were the major parts.

Impact of Thermal Treatment on the Residual Stress and Young's Modulus of thin a-SiC:H Membranes Applying Bulge Testing

Silicon carbide (SiC) as a wide bandgap semiconductor features outstanding electrical and mechanical properties as well as enhanced inertness against a large variety of chemical substances and a high temperature stability. Therefore, it is the material of choice for high performance microelectromechanical systems. This study presents the impact of thermal treatment on a 500nm thin hydrogenated amorphous SiC membrane. Layers were synthesized using an inductively-coupled plasma-enhanced chemical vapour deposition equipment and processed to obtain circular diaphragms. Bulge testing is performed with a uniformly applied pressure load and the bending characteristics as well as the flexural Young's modulus and residual tensile stress are determined. With increasing annealing temperature a massive increase of membrane stress and only a slight increase of Young's modulus occurs while the bending characteristic of the diaphragm is a nearly perfect membrane type independent of thermal loading.

Highly permeable and mechanically robust silicon carbide hollow fiber membranes

Silicon carbide (SiC) membranes have shown large potential for applications in water treatment. Being able to make these membranes in a hollow fiber geometry allows for higher surface-to-volume ratios. In this study, we present a thermal treatment procedure that is tuned to produce porous silicon carbide hollow fiber membranes with sufficient mechanical strength. Thermal treatments up to 1500°C in either nitrogen or argon resulted in relatively strong fibers, that were still contaminated with residual carbon from the polymer binder. After treatment at a higher temperature of 1790°C, the mechanical strength had decreased as a result of carbon removal, but after treatments at even higher temperature of 2075°C the SiC-particles sinter together, resulting in fibers with mechanical strengths of 30–40MPa and exceptionally high water permeabilities of 50,000Lm−2h−1bar−1. Combined with the unique chemical and thermal resistance of silicon carbide, these properties make the fibers suitable microfiltration membranes or as a membrane support for application under demanding conditions.

Toward a process-based molecular model of SiC membranes: III. Prediction of transport and separation of binary gaseous mixtures based on the atomistic reactive force field

The atomistic model of amorphous silicon-carbide membrane that was developed in Parts I and II of this series is utilized in nonequilibrium molecular dynamics (MD) simulations to study transport and separation of equimolar gaseous mixtures H2/CO2 and H2/CH4 in the membrane at high temperatures. We simulated membranes with up to about 39nm in thickness, containing up to 170,000 atoms, and up to 100ns to obtain reliable statistics. The effect of parameters such as the temperature, the applied pressure drop across the system, and the membrane׳s thickness on the separation properties was studied. The trends in the dependence of the separation factor on the membrane׳s thickness are consistent with experiments, namely the separation factor increases with the thickness up to an optimal value, beyond which it remains constant, or may even decrease. The dependence of the computed separation factor on the membrane׳s thickness is used in conjunction with the experimental values of the separation factor to estimate the thickness of actual membranes. The results are in agreement with the experimental data, demonstrating the value of MD simulations with a reactive force field for characterizing the properties of complex amorphous films, and flow and transport therein.

Study of sintering temperature on the structure of silicon carbide membrane

Study of the microstructure of silicon carbide (SiC) membrane as a function of sintering temperature and the percentage amount of additive kaolin is the outcome of the experimental fabrications presented in this paper. The SEM micrographs are used to investigate the impact of above parameters on the porosity of membrane. The experimental results show that the rise in the temperature causes more sintering of powder particles, growing granules, augmentation of the number of pores and consequently increasing the total porosity of membrane. Using XRD analyses, it is found that SiC amorphous phase is highly sensitive to the temperature and its crystallization physically grows with temperature increase.

세라믹 멤브레인 활용 직접 에탄올 연료전지

>> Direct ethanol fuel cell has been fabricated with ceramic membrane. A porous silicon carbide (SiC) membrane having approximately 30% porosity has been applied for a direct ethanol proton exchange membrane (DE-PEM) fuel cell. A horizontal type cell having Pt (18mg/cm) catalyst layer on both side of the ceramic membrane was used for the demonstration test. The ethanol oxidation based-fuel cell stack showed very high voltage (1.289V) and measurable current level (68mA) even though at room temperature.

Fabrication and Characterization of CeO<sub>2</sub>-NiO/SiC Membranes for Hydrogen Permeation

In order to enhance the hydrogen permeation, Silicon carbide (SiC) was prepared with CeO2-NiO mixed oxides via sol-gel process and the SiC membranes with various contents of χ wt% mixed oxides (χ = 20, 40, 60 and 80) were fabricated by hot-press sintering (HPS) at 1173 K under vacuum condition. The effect of the CeO2-NiO mixed oxides was studied in terms of properties on hydrogen permeation. All the membranes were characterized by XRD, FE-SEM and BET instrument. The hydrogen permeation test was carried out at various temperatures under 0.1 MPa. Hydrogen permeation flux of the 80 wt% CeO2-NiO/SiC membrane, which has the highest value, was obtained at 2.92×10-6mol/m2sPa, 323 K. The reaction enthalpy (ΔH°) was calculated as - 4.95 J/mol by Arrhenius’s plot.

Switchable wetting and flexible SiC thin film with nanostructures for microfluidic surface-enhanced Raman scattering sensors

In this paper, we present the fabrication of the flexible surface-nanostructured silicon carbide (SiC) membrane, whose wettability can be simply tuned from super-hydrophobicity to super-hydrophilicity via the combination of an improved deep reactive ion etching (DRIE) process and the KOH wet etching process. The thermal annealing is utilized to reduce the stress of SiC membrane produced by the plasma enhanced chemical vapor deposition (PECVD) process. High-dense SiC nano-balls are fabricated by the improved DRIE process via the self-mask effect of the enhanced passivation step. Resulting from both the minimized liquid–solid contact area and the fluorocarbon layer atop deposited during the DRIE process, the SiC film with nano-balls shows an outstanding superhydrophobicity. Then, after the removal of silicon substrate by KOH wet etching, the flexible SiC membrane with nano-tips is realized, and the wetting behavior switches to superhydrophilicity. Correspondingly, the static contact angle (CA) remarkably achieves more than 160° and less than 1°, respectively. The wetting stabilities of these super-hydrophobic and super-hydrophilic samples have been demonstrated by the impact tests. The water droplet bounced 14 times on the super-hydrophobic SiC nano-ball surface, while it spread completely within 8s on the super-hydrophilic SiC nano-tip surface. This switchable wetting behavior of SiC film was systematically investigated from both physical and chemical mechanisms by using SEM, AFM, EDX and FTIR. Furthermore, this novel surface-nanostructured SiC material shows the considerable surface-enhanced Raman scattering (i.e., SERS) property. The Raman spectrum of Rhodamine B (RhB) is significantly enhanced by a factor of 3.4×104, which is the first experimental demonstration of SiC-based SERS materials. In the meantime, the effects of the nanostructure morphology on both of the wetting behavior and the Raman scattering enhancement have been investigated by using the atomic force microscope (AFM). This flexible SiC SERS material with tunable wetting behavior has attractive application potential in micro/nano systems, especially in bio-medical fields.

Artificial Dielectric Layer Based on PECVD Silicon Carbide for Terahertz Sensing Applications

The refractive index of a conventional dielectric layer can be enhanced using an Artificial Dielectric Layer (ADL). Here we present the fabrication of low temperature PECVD Silicon Carbide (SiC) membranes with very high refractive index (up to 5 at 1 THz) in the terahertz frequency range. The SiC deposition parameters have been tuned to reach a very low residual stress level (-25 to 100MPa). The excellent mechanical properties of this material allowed the fabrication of very large square membranes (5mm side) with a thickness from 5μm to 10μm. Using silicon carbide as host material, ADL with a refractive index of 9.9 at 1 THz can be effectively realized.

On the Use of Porous and Nonporous Fillers in the Fabrication of Silicon Carbide Membranes

Hydrogen-permselective asymmetric silicon carbide (SiC) membranes were prepared in this study by incorporating highly porous SiC nanofibers into the membrane structure using a sacrificial interlayer-based technique. The resulting membranes were characterized with permeation studies, nitrogen adsorption experiments, and electron microscopy analysis. Their characteristics were compared with those of SiC membranes prepared with the same approach but making use, instead, of fine, nonporous SiC powders as fillers.

Toward a Process-Based Molecular Model of SiC Membranes. 1. Development of a Reactive Force Field

A broad class of important materials, such as carbon molecular sieves, silicon carbide (SiC), and silicon nitride, are fabricated by temperature-controlled pyrolysis of preceramic polymers. In particular, the fabrication of SiC membranes by pyrolysis of a polymer precursor that contains Si is quite attractive for separation of hydrogen from other gases. It has been quite difficult to extract atomistic-scale information about such SiC membranes since they are amorphous. In principle, ab initio quantum mechanics (QM) can provide information about the structure of the amorphous systems. However, to determine the structure of the SiC membrane layer one should capture in the simulations the various reactive processes involved in forming the layer. This requires QM simulations on systems with about 3000 atoms per cell at temperature of 1200 K for microseconds, which are far beyond the current QM capabilities. Instead, this paper extends the ReaxFF reactive force field, validated for high temperature reactions of other materials, to describe the processes involved in the thermal decomposition of hydridopolycarbosilane (HPCS) to form SiC nanoporous membranes. First, we carry out QM calculations on models meant to capture important reaction steps and structures. Then, we develop a model of the HPCS polymer and utilize ReaxFF to describe the thermal degradation and decomposition of the polymer as the system is heated in the molecular dynamics (MD) simulations. Analysis of the pyrolysis studies and their results leads to various quantities that can be compared with experimental data. Good agreement is found between the data and the results of the MD simulations.

Morphological Study on Porous Silicon Carbide Membrane Fabricated by Double-Step Electrochemical Etching

Morphological Study on Porous Silicon Carbide Membrane Fabricated by Double-Step Electrochemical Etching

Morphological Study on Porous Silicon Carbide Membrane Fabricated by Double-Step Electrochemical Etching

The structure of porous silicon carbide membranes that peeled off spontaneously during electrochemical etching was studied. They were fabricated from n-type 6H SiC(0001) wafers by a double-step electrochemical etching process in a hydrofluoric electrolyte. Nanoporous membranes were obtained after double-step etching with current densities of 10–20 and 60–100 mA/cm2 in the first and second steps, respectively. Microporous membranes were also fabricated after double-step etching with current densities of 100 and 200 mA/cm2. It was found that the pore diameter is influenced by the etching current in step 1, and that a higher current is required in step 2 when the current in step 1 is increased. During the etching processes in steps 1 and 2, vertical nanopore and lateral crack formations proceed, respectively. The influx pathway of hydrofluoric solution, expansion of generated gases, and transfer limitation of positive holes to the pore surface are the key factors in the peeling-off mechanism of the membrane.

Amorphous Silicon Carbide (<i>α</i>-SiC) Thin Square Membranes for Resonant Micromechanical Devices

We report an initial experimental exploration of engineering very thin, suspended amorphous silicon carbide (a-SiC) membranes into vibrating micromechanical devices. We show that micromachined a-SiC thin square membranes can make interesting multiple-mode flexural resonators, with frequency spectra exhibiting many measurable resonant modes over a wide frequency range (100kHz–10MHz) in the low radio frequency (RF) bands. Initial demonstration and preliminary data suggest interesting and rich dynamical, nonlinear, and dissipative properties in these micromechanical resonances. Specifically, for instance, at room temperature (T≈300K) and in moderate vacuum (e.g., ~20mTorr), resonant modes of an a-SiC square membrane (thickness: t≈1.5µm, size: 1mm×1mm) are observed in the ~100kHz–5MHz range, with measured quality factors (Q’s) in the range of ~2,500–9,000.

Graded Porous Silicon Carbide for High Temperature Membrane Applications

ABSTRACTDue to its excellent thermal shock resistance, mechanical and chemical stability at both room and elevated temperatures, silicon carbide (SiC) is an attractive material for environmental protection and energy production applications such as catalyst supports, molten metal filters and gas separation membranes. Precise pore size control and high porosity are the key deciding factors for such applications. In this study, we demonstrated the fabrication of bi-layered SiC membranes with a graded porosity, consisting of porous nano-SiC layer on the surface of a porous coarse-grained SiC support layer. Nano-SiC powders utilized for this study were synthesized using a novel process based on mechanical activation of silica fume and graphite mixtures, resulting in particle sizes as small as 30 nm. The effects of sintering temperature were investigated to control the pore size, particle size and overall density of the bi-layered membrane.