Polished Silicon Carbide Research Articles

S1303-2-4 SiC単結晶の酸化剤援用研磨とそのメカニズム([S1303-2]先端材料と加工(2))

S1303-2-4 SiC単結晶の酸化剤援用研磨とそのメカニズム([S1303-2]先端材料と加工(2))

Effect of Annealing Temperature on SiC Wafer Bow and Warp

ABSTRACTSingle-side polished silicon carbide wafers could exhibit large bow and warp due to the presence of mechanical damage on the unpolished surface. In this study, we investigated the effect of thermal annealing on the wafer bow. Two commercial-grade, double-side polished 3¡¨ 6H SiC wafers with the bow less than 5 μm were lapped using 12 μm diamond grit. One wafer was lapped on the C-face and another on the Si-face. The lapped wafers were subjected to annealing in vacuum at temperatures between 500¢XC and 2040¢XC. The wafer bow and x-ray rocking curves were measured prior to annealing and after each annealing step in order to evaluate the extent of the surface damage and degree of healing. Thermal annealing led to a decrease in the wafer bow and sharpening of the x-ray rocking curves. However, the wafer bow generated by lapping was not completely removed until annealing temperature reached ∼2000°C.

The Effect of Mixed Abrasive Slurry on CMP of 6H-SiC Substrate

Silicon carbide (SiC) is a wide band gap semiconductor being developed for high temperature, high power, and high frequency device applications. For the manufacturing of SiC to semiconductor substrate, many researchers have studied on the subject of SiC polishing. However, SiC faces many challenges for wafer preparation prior to epitaxial growth due to its high hardness and remarkable chemical inertness. A smooth and defect free substrate surface is important for obtaining good epitaxial layers. Therefore, hybrid process, chemical mechanical polishing (CMP) has been proposed to achieve epi-ready surface. In this paper, the material removal rate (MRR) is investigated to recognize how long the CMP process continues to remove a damaged layer by mechanical polishing using 100 nm sized diamond, and the authors tried to find the dependency of mechanical factors such as pressure, velocity and abrasive concentration using single abrasive slurry (SAS). Especially, the authors tried to get an epi-ready surface with mixed abrasive slurry (MAS). The addition of the 25nm sized diamond in MAS provided strong synergy between mechanical and chemical effects resulting in low subsurface damage. Through experiments with SAS and MAS, it was found that chemical effect (KOH based) was essential and atomic-bit mechanical removal was efficient to remove residual scratches in MAS. This paper concluded that SiC CMP mechanism was quite different from that of relatively soft material to achieve both of high quality surface and MRR.

High Rate Silicon Carbide Polishing to Ultra-smooth Surfaces

ABSTRACTSilicon Carbide has a unique combination of properties that include a nearly diamond-like hardness, intrinsic electrical semiconductivity and a high thermal conductivity. This combination of properties has led to it's use in a number of applications including substrates for Light Emitting Diodes (LEDs), power, RF (radio frequency) and other electronic devices in addition to mirror substrates and optical devices as well as stop layers in integrated circuit chip manufacture. In addition, the chemical inertness and high hardness of SiC has historically resulted in low removal rates during chemical mechanical Planarization (CMP). Recent efforts in our labs have led to being able to polish single crystal silicon carbide at removal rates up to 400 nm/hr yielding a root mean squared roughness on the order of a nanometer as determined by AFM and interferometry. The high rates and smoothness obtained are expected to translate to other types of silicon carbide. Fundamental studies by FTIR, streaming potential and ESCA have been done to elucidate the mechanism of silicon carbide polishing.

Microtensile bond strength of different components of core veneered all-ceramic restorations

The present study aims to evaluate the core-veneer bond strength and the cohesive strength of the components of three commercial layered all-ceramic systems. Two surface treatments for the core surface finish and different veneering ceramics with different thermal expansion coefficients (TEC) were applied. The selected systems were two CAD-CAM ceramics; Cercon and Vita Mark II and one pressable system; (IPS)Empress 2 for layering technique. Standardized core specimens were fabricated according to the manufacturer's instructions, or polished with 1200 siliconcarbide polishing paper. The core specimens were veneered with either its manufacturer's veneer or an experimental veneer with higher TEC. The obtained micro-bars were subjected to the microtensile bond strength test. The obtained data were analyzed using one and two-way ANOVA. A finite element analysis (FEA) model of the test setup was analyzed. Scanning Electron Microscopy (SEM) was carried out at the fracture surface. The core materials were significantly stronger than the veneering materials and the layered core-veneer specimens of which the results were statistically comparable. Polishing the core surfaces did not have an effect on the core-veneer bond strength. Experimental veneer with higher TEC resulted in massive fractures in both the core and veneering material. SEM and FEA demonstrated fracture pattern and mechanism of failure. The core-veneer bond strength is one of the weakest links of layered all-ceramic restorations and has a significant role in their success. To exploit fully the high strength of zirconium oxide cores, further research work is needed to improve its bond with its corresponding veneering material.

Electro-chemical mechanical polishing of silicon carbide

In an effort to improve the silicon carbide (SiC) substrate surface, a new electro-chemical mechanical polishing (ECMP) technique was developed. This work focused on the Si-terminated 4H-SiC (0001) substrates cut 8° off-axis toward 〈1120〉. Hydrogen peroxide (H2O2) and potassium nitrate (KNO3) were used as the electrolytes while using colloidal silica slurry as the polishing medium for removal of the oxide. The current density during the polishing was varied from 10 µA/cm2 to over 20 mA/cm2. Even though a high polishing rate can be achieved using high current density, the oxidation rate and the oxide removal rate need to be properly balanced to get a smooth surface after polishing. A two-step ECMP process was developed, which allows us to separately control the anodic oxidation and removal of formed oxide. The optimum surface can be achieved by properly controlling the anodic oxidation current as well as the polishing rate. At higher current flow (>20 mA/cm2), the final surface was rough, whereas a smoother surface was obtained when the current density was in the vicinity of 1 mA/cm2. The surface morphology of the as-received wafer, fine diamond slurry (0.1 µm) polished wafer, and EMCP polished wafer were studied by high-resolution atomic force microscopy (AFM).

Effect of surface preparation on Ni Ohmic contact to 3C-SiC

The effect of roughness and chemical treatment of 3C-SiC film surface on Ni ohmic contact was studied in this work. 3C-SiC(1 1 1) film was grown on Si(1 1 1) in a chemical vapor deposition reactor. The 3C-SiC surface was polished using a chemical mechanical polishing (CMP) technique to get a smooth and flat surface. The polished surface was oxidized and then was etched in BHF solution to remove subsurface damages formed during the CMP process. The morphology of thus prepared silicon carbide (SiC) surfaces was investigated using SEM and AFM. Ni contact resistance to the 3C-SiC films was evaluated using linear transmission line method pattern. The formation of good ohmic contact characteristics was observed from Ni contact to all the tested SiC samples. After the CMP process, it was found that the RMS roughness of 3C-SiC surface apparently reduces and the specific contact resistance to 3C-SiC decreases as well, in proportion to the SiC surface roughness. The sacrificial oxidation and etching of the polished SiC surface abruptly decrease the contact resistance to be 3.7×10 −4 Ω cm 2. It was shown that the surface morphology and subsurface damage concentration of 3C-SiC films are important factors to give a great effect on the contact characteristic of the 3C-SiC films. However, it was considered that the reduction of subsurface damage concentration is essential to get better contact resistance to 3C-SiC surface.

Tribochemical polishing of silicon carbide in oxidant solution

Tribochemical polishing (TCP) has been applied to finish polycrystalline silicon carbide samples. With this technique, material is removed without intentionally using abrasives, by chemical dissolution stimulated by friction in a suitable reactive fluid. For the latter, oxidant solutions such as CrO 3, H 2O 2 and KMnO 4 were used. A smooth (Ra=1 nm), defect-free SiC surface is achieved when a SiC sample is polished in a 3 wt.% CrO 3 solution at speeds of 4–6 cm/s and loads of 1.96 N–9.8 N. The polishing rate is 3–7×10 −6 mm 3/N m when rubbing against a Si 3N 4 tool. Plane samples with surface dimensions of 2 cm×2 cm were polished by rubbing against cast iron. The surface roughness was less than 1 nm, analyzed by atomic force microscopy. The polishing rate is 0.2–0.4 μm/h, which is comparable to that of the finishing step in ceramic lapping. The polishing mechanism of SiC is discussed in terms of interaction between mechanics and chemistry.

On chemo-mechanical polishing (CMP) of silicon nitride (Si3N4) workmaterial with various abrasives

Chemo-mechanical polishing (CMP) studies were conducted using various abrasives [boron carbide (B4C), silicon carbide (SiC), aluminium oxide (Al2O2), chromium oxide (Cr2O3), zirconium oxide (ZrO2), silicon oxide (SiO2), cerium oxide (CeO2), iron oxide (Fe2O3), yttrium oxide (Y2O3), copper oxide (CuO), and molybdenum oxide (Mo2O3)] to investigate their relative effectiveness in the finishing of uniaxially pressed Si3N3 bearing balls by magnetic float polishing (MFP) technique. CMP depends both on the chemical and the mechanical effectiveness of the abrasive and the environment with respect to the workmaterial. Among the abrasives investigated for CMP of Si3N4 balls, CeO2 and ZrO2 were found to be most effective followed by Fe2O3 and Cr2O3. Extremely smooth and damage-free Si4N3 bearing ball surfaces with a finish R4a of ≈ 4 nm and R1 of ≈ 40 nm were obtained after polishing with either CeO2 or ZrO2. Thermodynamic analysis (Gibb's free energy of formation) indicated the feasibility of the formation of SiO2 layer on the surface of the Si3N4 balls with these abrasives. This is particularly so in a water environment which facilitates chemo-mechanical interaction between abrasive and workmaterial by participating directly in the chemical reaction leading to the formation of a softer SiO2 layer. Since the hardness of some of the abrasives which were found to be most effective in CMP, namely. CeO2, ZrO2, and Fe2O2 is closer to that of SiO2 layer but significantly lower than the hardness of the Si3N4 workmaterial, removal of the SiO2 reaction layer effectively without scratching and/or damaging the Si3N3 substrate is facilitated by the subsequent mechanical action of the abrasives. The chemical reaction would proceed on a continuing basis only if the passivating layers are removed continously by subsequently mechanical action. It was found that the CMP ability in an oil-based polishing environment to be rather limited. A mechanism similar to the CMP of Si4N4 may be applicable to the polishing of silicon wafers, various glasses, and SiC due to similarities in the material removal processes.

Micro/nanotribological studies of polysilicon and SiC films for MEMS applications

Microelectromechanical (MEMS) devices are currently made from single-crystal silicon, various polysilicon films and other ceramic materials. Silicon carbide (SiC) film has recently been pursued as a material for use in MEMS devices owing to its excellent mechanical properties and high-temperature capabilities. Since surface properties, friction and wear are important issues in such small-scale devices, it is essential that the materials used in MEMS have good micro/nonotribological properties. using atomic force/friction force microscopy (AFM/FFM), surface roughness, microscale friction and microscale scratch/wear resistance of 3CSiC (cubic SiC) films, as well as undoped single-crystal Si(100) and doped and undoped polysilicon films for comparison were measured. Nanohardness and modulus of elasticity were measured using a Nanoindenter. Surface roughness values showed that the as-deposited films of SiC and undoped polysilicon were rougher than the doped polysilicon film and Si(100). Polishing of the as-deposited samples resulted in comparable values of roughness between SiC and undoped and doped polysilicon films, while Si(100) still remained the smoothest and the doped polysilicon film, the roughest. It was found that the polished SiC and doped polysilicon films showed the lowest friction followed by undoped polysilicon film, while Si(100) showed high friction. Microscale scratch/wear studies clearly showed that SiC film was much more scratch/wear resistant than the other materials, which showed comparable resistance levels. SiC also showed higher hardness and modulus of elasticity compared to the other materials. These results show that 3CSiC film posessess desirable micro/nanotribological characteristics that make it an ideal material for use in MEMS devices.

Adhesion of silver films to ion-bombarded silicon carbide: ion-dose dependence and Weibull statistical analysis of pull-test results

Silver films were deposited by evaporation onto polished silicon carbide substrates that had been sputtered for various lengths of time with 500 eV ions. The adhesion strength of the films was measured as a function of the ion bombardment time and species with a pull-type adhesion tester. Adhesion of Ag to argon-ion-sputtered surfaces was low except at the highest dose, in which case very good adhesion was measured. In contrast, oxygen/argon-ion sputtering produced a rapid rise in adhesion strength, but adhesion was limited to less than 60 MPa, even for lengthy bombardment. The applicability of Weibull statistical analysis to the distribution of pull-test failure strengths was investigated. A good fit of the data to the Weibull expression was observed. A significant Weibull threshold stress was found and the Weibull modulus was low.

Pulsed laser damage characteristics of vapor-deposited copper mirrors on silicon carbide substrates

Pulsed 100-nsec 10.6-microm laser damage characteristics of composite bare copper mirrors were determined. The mirrors were prepared by vapor deposition of a copper film on polished silicon carbide substrates formed by chemical vapor deposition, Comparisons are made with a bulk oxygen-free high-conductivity (OFHC) copper mirror prepared by precision diamond turning. Vapor-deposited Cu on SiC was found to be inherently superior in resisting surface damage related to defects and to thermal stress. However, melt thresholds are comparatively lower. The overheating problem was analyzed by a thermal modeling of thresholds measured on mirrors having different film thicknesses. Results are consistent with reduced thermal conductivity in the SiC substrate as compared with bulk Cu. The effect of the interface was evaluated from samples that had been ion polished prior to deposition and on others where an intermediate chromium layer was deposited. The scattering and figure of the mirrors are also reported.