Single Crystal 6H Silicon Carbide Research Articles

Mechanism of Unstable Material Removal Modes in Micro Cutting of Silicon Carbide.

This study conducts large-scale molecular dynamics (MD) simulations of micro cutting of single crystal 6H silicon carbide (SiC) with up to 19 million atoms to investigate the mechanism of unstable material removal modes within the transitional range of undeformed chip thickness in which either brittle or ductile mode of cutting might occur. Under this transitional range, cracks are always formed in the cutting zone, but the stress states cannot guarantee their propagation. The cutting mode is brittle when the cracks can propagate and otherwise ductile mode cutting happens. Plunge cutting experiment is conducted to produce a taper groove on a 6H SiC wafer. There is a transitional zone between the brittle-cut and ductile-cut regions, which has a mostly smooth surface with a few brittle craters on it. This study contributes to the understanding of the detailed process of brittle-ductile cutting mode transition (BDCMT) as it shows that a transitional range can occur even for single crystals without internal defects and provides guidance for the determination of tcritical from taper grooves made by various techniques, e.g., to adopt larger tcritical around the end of the transitional range to increase machining efficiency for grinding or turning as long as the cracks do not extend below the machined surface.

Investigation of Ga ion implantation-induced damage in single-crystal 6H-SiC

In order to investigate the damage in single-crystal 6H-silicon carbide (SiC) in dependence on ion implantation dose, ion implantation experiments were performed using the focused ion beam technique. Raman spectroscopy and electron backscatter diffraction were used to characterize the 6H-SiC sample before and after ion implantation. Monte Carlo simulations were applied to verify the characterization results. Surface morphology of the implantation area was characterized by the scanning electron microscope (SEM) and atomic force microscope (AFM). The ‘swelling effect’ induced by the low-dose ion implantation of 1014−1015 ions cm−2 was investigated by AFM. The typical Raman bands of single-crystal 6H-SiC were analysed before and after implantation. The study revealed that the thickness of the amorphous damage layer was increased and then became saturated with increasing ion implantation dose. The critical dose threshold (2.81 × 1014−3.26 × 1014 ions cm−2) and saturated dose threshold (˜5.31 × 1016 ions cm−2) for amorphization were determined. Damage formation mechanisms were discussed, and a schematic model was proposed to explain the damage formation.

The mechanism of ductile deformation in ductile regime machining of 6H SiC

Silicon Carbide (SiC) is a material with excellent mechanical and electrical properties. Much attention has been paid to the ductile regime machining of SiC in recent years, but there is still controversy regarding the mechanism of its ductile response in ultraprecision machining. While some researchers proposed the high pressure phase transformation (HPPT) theory from experimental studies on 6H SiC, later molecular dynamics (MD) studies disapproving the HPPT theory adopted 3C SiC as the workpiece. This study conducts MD simulations to investigate the atomicscale details of ductile deformation in the machining of 6H SiC for the first time, using an interaction potential that can reproduce the HPPT of SiC. The HPPT in the machining of 6H SiC are visualized for the first time by MD simulations, although the quantity of HPPT is very small. The dislocation structures in the machining of 6H SiC are also visualized by MD for the first time. Frank partial dislocations and basal plane edge dislocations are identified to be primarily responsible for the ductile deformation. Taper cutting experiment on a single crystal 6H SiC wafer produces a ductile-cut surface, and micro Raman spectroscopy on the machined surface finds no peaks for amorphous SiC, which agrees with the MD result that the quantity of HPPT is very small. These results indicate that the origin of ductile response for 6H SiC is a combination of HPPT and dislocation activities, while dislocation plasticity plays a major role.

Microtrenching effect of SiC ICP etching in SF6/O2 plasma

Inductively coupled plasma (ICP) etching of single crystal 6H-silicon carbide (SiC) is investigated using oxygen (O2)-added sulfur hexafluoride (SF6) plasmas. The relations between the microtrenching effect and ICP coil power, the composition of the etch gases and different bias voltages are discussed. Experimental results show that the microtrench is caused by the formation of a SiFxOy layer, which has a greater tendency to charge than SiC, after the addition of O2. The microtrenching effect tends to increase as the ICP coil power and bias voltage increase. In addition, the angular distribution of the incident ions and radicals also affects the shape of the microtrench.

Fracture toughness characterisation of electronic materials and interfaces

We present a technique for measuring the fracture toughness of nanoscale specimens of electronic materials and interfaces, in-situ inside electron microscopes. The technique employs a custom micro-machined mechanical testing device on which an edge-cracked tension specimen is integrated. The specimen is fabricated using a focused ion beam, which allows for testing of both single phase and interfacial materials, and virtually eliminates any restrictions on the type of materials that can be tested. We demonstrate the technique with single crystal 6H silicon carbide specimens, which are 200?500nm thick, 2?5?m wide and approximately 10?m long. The measured values of Young's modulus and fracture toughness are 590GPa and 4.82MPa-m1/2 respectively, which compare favourably to the experimental results on bulk and thin film silicon carbide in literature.

Temperature-dependent optical properties of silicon carbide for wireless temperature sensors

The temperature-dependent optical properties of materials, such as refractive index and reflectivity, can be used for remote sensing of temperatures using a laser beam. Wireless sensing mechanism eliminates the need for external electrical contacts to the sensor, typically required in electrical property-based sensors. Such contacts tend to melt at elevated temperatures. This paper investigates the optical response of silicon carbide to temperature changes, indicating the potential for realizing silicon carbide-based high temperature wireless sensors by using a helium–neon laser of wavelength 632.8 nm. The laser power reflected by single crystal 6H–silicon carbide substrates has been measured at different temperatures up to 750 °C. The reflected powers exhibited oscillatory patterns caused by multiple beam interference for which the substrate itself acts as a Fabry–Pérot etalon. These results were used to calculate the refractive index, reflectivity and thermo-optic coefficients of silicon carbide as a function of temperature. Two samples with different dopant profiles were investigated to examine the effects of dopants on the optical response of silicon carbide substrates. With proper modification of the microstructure in the substrate, the temperature dependence of reflectivity can be used directly to measure temperature remotely.

Detection of Residual Stress in Silicon Carbide MEMS by μ-Raman Spectroscopy

AbstractMicro-Raman (μ-Raman) spectroscopy is used to measure residual stress in single-crystal, 6H-silicon carbide (SiC) used in micro-electro-mechanical-systems (MEMS) devices. These structures are bulk micro-machined by back etching a 250-μm-thick, single-crystal 6HSiC wafer (p-type, 7 Ω-cm, 3.5° off-axis) to form a 50-μm thick diaphragm. A Wheatstone bridge, patterned of piezoresistive elements, is formed across the membrane from a 5-μm, 6HSiC (n-type, doped 3.8 × 1018 cm-3) epilayer; the output of the bridge is proportional to the flexure of the MEMS diaphragm. For these samples, the μ-Raman spectroscopy is performed using a Renishaw InVia Raman spectrometer with an argon-ion excitation source (λ = 514.5 nm, hν = 2.41 eV) with an approximate 1-μm2 spot size through the 50X objective. By employing an incorporated piezoelectric stage with submicron positioning capabilities along with the Raman spectral acquisition, spatial scans are performed to reveal areas in the MEMS structures that contain residual stress. Shifts in the transverse optical (TO) Stokes peaks of up to 2 cm-1 along the edge of the diaphragm and through the piezoresistors indicate significant material strain induced by the MEMS fabrication process.The phonon deformation potential is measured to quantify the material stress as a function of the shift in the Raman peak position. The line center of the TO Stokes peak is shifted by applying a uniform stress to the sample and monitoring the applied stress using a strain gauge, while the μ-Raman spectrum is being measured. A spectral analysis code tracks the shift of the Raman peak position with respect to the line center of the Rayleigh peak to account for any thermal drift of the spectrometer during the time of the area scan.

Studies of 6H–SiC devices

Silicon carbide (SiC) is recently receiving increased attention due to its unique electrical and thermal properties. It has been regarded as the most appropriate semiconductor material for high power, high frequency, high temperature, and radiation hard microelectronic devices. The fabrication processes and characterization of basic device on 6H–SiC were systematically studied. The main works are summarized as follows: The homoepitaxial growth on the commercially available single-crystal 6H–SiC wafers was performed in a modified gas source molecular beam epitaxy system. The mesa structured p + n junction diodes on the material were fabricated and characterized. The diodes showed a high breakdown voltage of 800 V at room temperature. They operated with good rectification characteristics from room temperature to 673 K. Using thermal evaporation, Ti/6H–SiC Schottky barrier diodes were fabricated. They showed good rectification characteristics from room temperature to 473 K. Using neon implantation to form the edge termination, the breakdown voltage was improved to be 800 V. n-Type 6H–SiC MOS capacitors were fabricated and characterized. Under the same growing conditions, the quality of poly-silicon gate capacitors was better than Al. In addition, SiC MOS capacitors had good tolerance to γ rays.