Two-step femtosecond laser etching for bulk micromachining of 4H–SiC membrane applied in pressure sensing

A method of two-step laser etching for bulk micromachining of 4H–SiC membranes through a femtosecond (fs) laser with a wavelength of 532 nm, a pulse width of 290 fs and a repetition rate of 100 kHz is present in this paper. Using a control variable method, the first step of fs-laser etching for rapid material removal and the second step of fs-laser etching mainly for improving surface morphology are studied by changing the laser fluence, scanning spacing, number of scans and scanning speed. The average surface roughness of the membrane bottom after the first step of laser etching is 691 nm, which is reduced to 237 nm after the second step. Compared with one-step laser etching, two-step laser etching obtains a better surface morphology, and it only takes 38% of the processing time of one-step laser etching. The finite element analysis shows that the sensitivity of the SiC pressure sensor drops with the decrease of the membrane roughness. The sensor sensitivity of the membrane fabricated by two-step laser etching is only 3.0% lower than that of the ideal smooth model. Furthermore, a 4H–SiC laser-prepared membrane with a diameter of 1.2 mm and a thickness of 75 μm applied in pressure sensor is tested, showing good linearity and repeatability at a load of up to 10 MPa.     

Femtosecond laser micromachining in combination with ICP etching for 4H–SiC pressure sensor membranes

4H–SiC is one of the most promising materials for pressure sensing in harsh environments. A Yb:KGW femtosecond laser was employed to fabricate 4H–SiC sensor membranes with size of Φ1200 × 80 μm. The optimal parameter combination under 15 μJ single pulse energy was obtained with the laps of 16, the scanning speed of 130 mm/s, the scanning line interval of 2 μm and the repetition rate of 100 kHz. High size accuracy (±1%) and steep sidewall (87.4°) were achieved. Wet cleaning and inductively coupled plasma (ICP) etching can obviously improve the membrane bottom surface morphology. The surface roughness Ra in X direction was reduced from 0.82 μm to 0.15 μm, and that in Y direction was reduced from 1.32 μm to 0.16 μm. Pinhole defect was related to the nonuniform distribution of laser fluence. This defect can be avoided by reducing the laser spot overlap ratio. Energy-Dispersive X-ray Spectroscopy (EDS) and Raman spectrum were adopted to analyse the changes of material properties after laser processing. The analysis indicated that the crystal properties of the membrane bottom and the thin epitaxial layers on the front side of membrane are not damaged by the integrating micromachining. The results indicate the potential of utilizing the femtosecond laser combined with ICP etching to fabricate 4H–SiC sensor membranes.     

Investigation of structural transformation and residual stress under single femtosecond laser pulse irradiation of 4H–SiC

Silicon carbide (SiC) is an attractive semiconductor material for devices that operate under extreme conditions. However, its high hardness and chemical stability hinder micromachining. Femtosecond laser has been proposed extensively as an effective micromachining tool for SiC. However, the fundamental mechanisms of laser-material interaction during femtosecond laser irradiation process remain elusive. This paper presents a comprehensive study of the structural transformation and residual stress induced by irradiating a 4H–SiC target with a single-pulse femtosecond laser. The energy dependence of the structural characteristics at the spot center and the spatial distribution across the laser spot were determined using optical microscopy and micro-Raman spectroscopy. The effect of the laser fluence on the residual stress was discussed. The obtained results showed that no structural changes occurred at low energy, whereas bond breaking occurred among crystalline SiC (c-SiC) at high energy. A central disk with no structural transformation and no residual stress was formed for the fluence of 22.2 J/cm², owing to phase explosion-induced spluttering. Meanwhile, an inhomogeneous distribution of the structural transformation was generated from the spot centre to the edge. Based on this, threshold fluences for modification and structural transformation were proposed and calculated. The fundamental mechanisms for different laser-fluence regimes are discussed, and some suggestions for improving the surface quality are put forward. This study provides deep insights into the laser-material interaction mechanisms and is beneficial for optimising the utilisation of femtosecond laser for 4H–SiC micromachining.     

Surface damage and threshold determination of Ge–As–Se glasses in femtosecond pulsed laser micromachining

Ge–As–Se chalcogenide glasses were prepared and the surface damage characteristics under femtosecond laser irradiation were experimentally investigated. Femtosecond laser beams with different power intensities, focusing depths, pulse repetition rates, and pulse numbers were employed to determine the damage thresholds of the samples. The surface morphologies and feature sizes of the damage craters were studied with regard to laser energy density to evaluate the damage circumstances of the glasses. The generation of the surface damage was also recorded by using different numbers of pulses. Results showed that the damage area increased monotonically with the laser power at low energy density and tended to saturation when reaching a critical value. According to the linear circumstances of laser energy density on the damage crater area, the average damage thresholds of the Ge–As–Se glasses with different repetition rates and pulse numbers were obtained. Results showed that the damage threshold decreased with the increase in pulse repetition rate and number of pulse.     

Effect of nitrogen impurity on the stabilization of 3C–SiC polytype during heteroepitaxial growth by vapor–liquid–solid mechanism on 6H–SiC substrates

This work reports on the stabilization of 3C–SiC polytype during heteroepitaxial growth by vapor–liquid–solid (VLS) on on-axis and 2° off-axis 6H–SiC(0001) substrates using Si–Ge as liquid phase. It was found that, depending on growth conditions (mainly temperature or nitrogen amount in the reactor), the deposit could be either a complete 3C or 6H–SiC layer or even a mixture of both polytypes. The proportion of 3C inside the deposit increases when 1) nitrogen amount in the reactor increases or 2) temperature is decreased. Though the effect of temperature could be explained in terms of 3C–SiC initial island dissolution, the influence of nitrogen is less obvious but it is shown to be effective at the early stage of growth. Several hypotheses are proposed such as SiC lattice modification by N incorporation or surface effects during the early stage of growth.     

Ohmic contact formation mechanisms of TiN film on 4H–SiC

The atomic structure, interfacial charge distribution, bonding nature, and interfacial electronic states of a 4H–SiC/TiN interface are systematically investigated to understand the Ohmic contact formation mechanisms of TiN to 4H–SiC. The experiment results clearly demonstrate that the well-arranged TiN (111)-oriented lattice planes are parallel to the (0001) SiC-oriented substrate, which is in line with the XRD results. In addition, the interface is coherent without any secondary phase layers, amorphous layers, or transition regions, which confirms the direct contact of TiN to SiC at the atomic scale, exhibiting a linear current–voltage relationship. Quantitatively, first-principle calculations reveal that the Schottky barrier height (SBH) is as low as 0.03 eV and that the band gap nearly vanishes at the interface, indicating an excellent Ohmic contact of TiN to 4H–SiC. Furthermore, the SBH is significantly reduced through the interfacial charge polarization effect and strong coupling of interfacial electronic states, enhancing the quantum electron transport. The present results provide insight into the complicated electronic effects of the Ohmic contact interface and indicate that TiN is a promising SiC Ohmic contact material for advanced next-generation power device applications.     

Mechanistic difference between Si-face and C-face polishing of 4H–SiC substrates in aqueous and non-aqueous slurries

The traditional aqueous-based polishing slurries have been extensively used in the ultra-precision machining process of SiC substrates, but their processing efficiency remains a major challenge in making SiC wafers with high surface quality. SiC polishing slurries based on non-aqueous solvents have been explored and reported, however, the mechanism for the accelerated SiC material removal rate (MRR) remains unknown. In this work, the Si-face and C-face of the SiC wafer were polished with water and methanol as polishing liquid carriers, respectively. The MRR of Si-face using the methanol-based slurry, can reach 260.9 nm/h, and the polished Si-face surface roughness Ra reduces to 0.150 nm. In contrast, the MRR of Si-face by using the aqueous-based slurry, is 66.8 nm/h, the polished Si-face surface roughness Ra is 0.691 nm. However, the results of MRR and Ra for C-face are opposite. The reaction between the polishing liquid carriers and the atomic structures of Si-face and C-face lead to differences of the MRRs by analyzing contact angle, XPS, and molecular dynamics (MD) simulation results. The newly revealed polishing mechanisms shined light for speeding up the development of SiC polishing slurries based on the specific aspects of the polishing surface of SiC.     

Growth of 3C–SiC films on Si substrates by vapor–liquid–solid tri-phase epitaxy

Cubic SiC films (3C–SiC) were deposited on (111) Si substrates by a vapor–liquid–solid tri-phase growth method. In such a process a thin copper layer, which was evaporated on the Si substrate prior to the growth, was melted at high temperature as the flux and then methane (carbon source) was diffused into the liquid layer to react with Si, leading to the growth of SiC on the substrate. Copper showed some good properties as the flux, including high silicon and carbon solubility, low growth temperature and low volatility. Suitable growth parameters to go with the copper flux were identified, under which (111) textured 3C–SiC films were grown. Small numbers of (220) grains were observed to embed in the (111) films, which were difficult to avoid completely. Etching pits of the Cu melt on the substrate surface may act as the preferred sites for the growth of (220) grains.     

Effects of SiC on the microstructures and mechanical properties of B4C–SiC–rGO composites prepared using spark plasma sintering

In this study, B₄C–SiC–rGO composites with different SiC contents were prepared by spark plasma sintering at 1800 °C for 5 min under a uniaxial pressure of 50 MPa. The effects of SiC on the microstructures and mechanical properties of the B₄C–SiC–rGO composites were investigated. The optimal values for flexural strength (545.25 ± 23 MPa) and fracture toughness (5.72 ± 0.13 MPa·m^1/2) were obtained simultaneously when 15 wt.% SiC was added to 5 wt.%–GO reinforced B₄C composites (BS15G5). It was found that SiC and rGO inhibited the grain growth of B₄C and improved the mechanical properties of the B₄C–SiC–rGO composites. The clear and narrow grain boundaries of rGO–B₄C and rGO–SiC, as well as the semi-coherent B₄C–SiC interface, indicated strong interface compatibility. The twin structures of SiC and B₄C observed in the composites improved their fracture toughness. Crack deflection and crack bridging caused by the SiC grains as well as rGO bridging and rGO pull-out were observed on the crack propagation path.     

Effect of heat treatment on the mechanical properties of PIP–SiC/SiC composites fabricated with a consolidation process

Continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) have been considered as candidates for heat resistant and nuclear materials. Three-dimensional (3D) SiC/SiC composites were fabricated by the polymer impregnation and pyrolysis (PIP) method with a consolidation process, mechanical properties of the composites were found to be significantly improved by the consolidation process. The SiC/SiC composites were then heat treated at 1400 °C, 1600 °C and 1800 °C in an inert atmosphere for 1 h, respectively. The effect of heat treatment temperature on the mechanical properties of the composites was investigated, the mechanical properties of the SiC/SiC composites were improved after heat treatment at 1400 °C, and conversely decreased with increased heat treatment temperature. Furthermore, the effect of heat treatment duration on the properties of the SiC/SiC composites was studied, the composites exhibited excellent thermal stability after heat treatment at 1400 °C within 3 h.     

Quality improvement of single crystal 4H SiC grown with a purified β-SiC powder source

In the processing of single crystal SiC using the PVT method, defects such as micropipes and dislocations occur due to various reasons, including growth rate, temperature gradient, seed quality, pressure change and the SiC source powder. Among these factors, the SiC source powder was investigated to reduce defects in single crystal SiC. β-SiC powder was used to reduce the growth temperature and change basic properties of the particle, including microstructure, particle size and chemical composition, through the purification process. The structure of the purified β-SiC particle was changed into a spherical structure and its particle size expanded. Chemical analysis revealed reduced free carbon, oxide phases such as silica (SiO₂), silicon oxycarbide and metallic impurities. Purified β-SiC powder showed increased particle size of 37 µm and showed improved purity. With this, we grew single crystal 4H SiC and compared the micropipe and dislocation density to that of single crystal 4H SiC grown with non-purified β-SiC powder. The experimental results confirmed that the 4H SiC wafer grown by purified β-SiC powder exhibited improved quality.     

Preparation of SiO2–SiC composites with a precursor method

SiO₂–SiC composite particles were prepared through a hybrid sol–gel precursor process. Compacts were prepared by using a conventional sintering process. The techniques of DSC–TG, SEM and XRD were use to characterize the composite particles and the sintered compacts. It was found that a core–shell structure was constructed in the composite particles with cores of SiC and shells of amorphous SiO₂. Nucleation of SiO₂ occurred at about 1200 °C. The optimized sintering temperature for 30SiO₂–70SiC (vol.%) composites was about 1400 °C with a relatively homogeneous microstructure. The maximum density was about 2.03 g cm^?3.     

Structural and compositional characterization of 6H–SiC implanted with N+ and Al+ ions using optical methods

Using plane-view and cross-sectional Raman spectroscopy, polarized infra-red spectroscopy and photothermal spectroscopy, the structure, composition and internal stress of 6H–SiC crystal implanted sequentially with N⁺ and Al⁺ ions to form a (SiC)_1−x(AlN)_x solid solution were studied non-destructively and self-consistently. The optimum implantation temperature for the synthesis of a (SiC)_1−x(AlN)_x solid solution with a 6H structure was found to be 600 °C.     

Influence of annealing on the mechanical properties of SiC–Si composites with sub-micron SiC microstructures

The influence of annealing temperature (1000, 1100 and 1200°C) on the mechanical properties of SiC–Si composites has been evaluated. Three SiC powders with particle sizes in the range of 0.24 to 0.7 μm were used to produce the composites. Before application the SiC powders were treated with hydrofluoric acid to remove the extent of SiO₂. With this treatment a successful infiltration of green-bodies especially produced of SiC powder with a mean particle size of 0.24 μm was possible. The bending strength decreased with decreasing SiC starting particle size as well as with increasing annealing temperature. However, the fracture toughness was independent on SiC starting particle size and annealing temperature. XRD diffraction analysis showed that internal stress, expressed by broadening of XRD peaks, is low and had no effects on the mechanical properties of the composites.     

Porous SiC–Si3N4 composite ceramics with excellent EMW absorption property prepared by gelcasting using DMAA

The porous SiC–Si₃N₄ composite ceramics with good EMW absorption properties were prepared by combination of gelcasting and carbothermal reduction. The pre-oxidation of Si₃N₄ powders significantly improved the rheological properties of slurries (0.06 Pa s at 103.92 s⁻¹) and also suppressed the generation of NH₃ and N₂ from Si₃N₄ hydrolysis and reaction between Si₃N₄ and initiator APS, thereby reducing the pore defects in green bodies and enhancing mechanical properties with a maximum value of 42.88 MPa. With the extension of oxidation time from 0 h to 10 h, the porosity and pore size of porous SiC–Si₃N₄ composite ceramics increased from approximately 41.86% and 1.0–1.5 μm to 46.33% and ~200 μm due to the production of CO, N₂ and gaseous SiO, while the sintering shrinkage decreased from 16.24% to 10.50%. With oxidation time of 2 h, the Si₂N₂O fibers formed in situ by the reaction of Si₃N₄ and amorphous SiO₂ effectively enhanced the mechanical properties, achieving the highest flexural strength of 129.37 MPa and fracture toughness of 4.25 MPa m^1/2. Compared with monolithic Si₃N₄ ceramics, the electrical conductivity, relative permittivity and dielectric loss were significantly improved by the in-situ introduced PyC from the pyrolysis of three-dimensional network DMAA-MBAM gel in green bodies and the SiC from the carbothermal reduction reaction between PyC and SiO₂ and Si₃N₄. The porous SiC–Si₃N₄ composite ceramics prepared by the unoxidized Si₃N₄ powders demonstrated the optimal EMW absorption properties with reflection loss of −22.35 dB at 8.37 GHz and 2 mm thickness, corresponding to the effective bandwidth of 8.20–9.29 GHz, displaying great application potential in EMW absorption fields.     

Brazing SiC ceramic using novel B4C reinforced Ag–Cu–Ti composite filler

The development of new composite fillers is crucial for joining ceramics or ceramics to metals because the composite fillers exhibit more advantages than traditional brazing filler metal. In this research, novel B₄C reinforced Ag–Cu–Ti composite filler was developed to braze SiC ceramics. The interfacial microstructure of the joints was characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effect of B₄C addition and brazing temperature on the microstructure evolution and mechanical properties of the joints was analyzed. The results revealed that TiB whisker and TiC particles were simultaneously synthesized in the Ag-based solid solution and Cu-based solid solution due to the addition of B₄C particles. As the brazing temperature increased, the thickness of Ti₃SiC₂+Ti₅Si₃ layers adjacent to SiC ceramic increased. Desirable microstructure similar to the metal matrix reinforced by TiB whisker and TiC particles could be obtained at brazing temperature of 950 °C. The maximum bending strength of 140 MPa was reached when the joints brazed at 950 °C for 10 min, which was 48 MPa (~52%) higher than that of the joints brazed using Ag–Cu–Ti filler.     

Fretting wear and electrochemical corrosion of well-adhered CVD diamond films deposited on steel substrates with a WC–Co interlayer

Diamond films have been grown on carbon steel substrates by hot-filament chemical vapour deposition methods. A Co-containing tungsten-carbide (WC–Co) coating prepared by high velocity oxy-fuel spraying was used as an intermediate layer on the steel substrates to minimize the early formation of graphite (and thus growth of low quality diamond films) and to enhance the diamond film adhesion. The effects of the WC–Co interlayer on nucleation, quality, adhesion, tribological behaviour and electrochemical corrosion of the diamond film were investigated. The diamond films exhibit excellent adhesion under Rockwell indentation testing (1500 N load) and when subjected to high-speed, high-load, long-time reciprocating dry sliding ball-on-flat wear tests against a Si₃N₄ counterface in ambient air (500 rpm, 200 N, 300,000 cycles). A WC–Co interlayer with appropriate chemical pretreatment is shown to play an important role in improving the nucleation, quality and adhesion of the diamond film, relative to that shown by substrates without such pretreatment.     

Mechanism of Hardening of a Pressure-Sintered Material Based on a SiC–C Solid Solution

The kinetics of sintering of a powdered solid solution of carbon in silicon carbide sintered under a high pressure is considered. The role of diamond-like carbon clusters in the structure of silicon carbide in the process of structure formation is determined. It is shown that the parameters of the microstructure of sintered ceramics based on a SiC–C solid solution are correlated with its hardness.     

Modification of electrophoretically deposited nano-hydroxyapatite coatings by wire brushing on Ti–6Al–4V substrates

In the present study, after successful synthesis of nano-HA powders by chemical precipitation method, wire-brushing (WB) treatment was effectively employed on Ti–6Al–4V substrates for the modification of electrophoretically deposited nano-hydroxyapatite coatings. The precipitated nano-HA particles were characterized by XRD, FT-IR, and DLS analyses. The morphology and particle size of synthesized nano-HA particles was observed by FE-SEM. The Ti–6Al–4V plates were initially wire brushed at different times and rotational speeds. Microhardness profile and AFM analysis of substrates were then studied. It was found that WB treatment at 16,000 rpm for 60 s leads to a maximum enhancement in the hardness and roughness of surface. Then, the electrophoretic deposition of nano-HA particles was carried out at constant voltage of 30 V and after 60 s. The results of adhesion test and potentiodynamic polarization measurements showed that WB treatment on Ti–6Al–4V substrates could efficiently increase the bonding strength between coating and substrate as well as corrosion resistance.     

Thermal effects of a laser beam on the electrodeposition of Ni–P alloys

The paper deals with the effects of an incident laser beam on electrodeposition of Ni–P alloys from dilute acetate solutions. The kinetics of separate reductions of Ni2+ and H2PO?2 species were first investigated by linear sweep voltammetry, varying the hypophosphite concentration and the solution temperature: comparison of the kinetically limited current densities of the two reductions suggested that increasing temperature might reduce the significance of P codeposition. This tendency was confirmed by deposition runs carried out at controlled current. Deposition performance was discussed in terms of faradaic yield and deposit properties, namely P content together with the aspect and the structure of the alloys. Use of a continuous or pulsed laser beam was shown to reduce the P content in the deposit at high current densities; in some cases, amorphous structures were replaced by more crystalline forms with assistance of a laser beam.