W.J. Everson

Gadolinium calcium oxyborate piezoelectric single crystals for ultrahigh temperature (>1000 °C) applications

ReCa4O(BO3)3 oxyborate crystals (ReCOB, where Re is a rare earth element such as Gd) were grown using the Czochralski pulling technique. The crystals belong to Cm space group and the relationships of the as-grown crystal morphology with crystallographic and physical coordinates were determined. The optimum length extensional and thickness shear vibrations of GdCOB were found for (ZYl)40° and (YXt)33° cuts, with electromechanical coupling factors k32 and k26, being on the order of 17.5% and 25% and piezoelectric coefficients d32 and d26 around −4.5 and 11.7 pC/N, respectively. Of particular significance is the nearly temperature independent behavior up to >1000 °C. Together with its high resistivity (∼5×106 Ω cm at 1000 °C) and high mechanical quality factor (∼4000 at 1000 °C) make GdCOB and/or ReCOB crystals promising candidates for the next generation sensing applications at ultrahigh temperature (>1000 °C).

Transmission electron microscopy analysis of mechanical polishing-related damage in silicon carbide wafers

The subsurface damage generated by mechanical polishing of silicon carbide wafers was investigated and quantified by plan view transmission electron microscopy (TEM) and atomic force microscopy (AFM). Damage generated during polishing using diamond abrasives with 0.5 µm particle size consists of dislocation loops with length up to 400 nm from the scratches. The total dislocation density was estimated at 5 × 1010 dislocations cm−2. TEM analysis of the Burgers vectors indicates that the initial perfect dislocations have a Burgers vector of b = a/3 11–20-type with many dislocation dissociated into two partials with b = a/3 1–100. The depth of damage was estimated to be up to 50 nm. 4H–SiC homoepitaxial layers grown on mechanically polished substrates without further surface treatment exhibit threading dislocation density along scratches in the order of 105 cm−1.

Chemi-Mechanical Polishing of On-Axis Semi-Insulating SiC Substrates

Conventional colloidal silica (CS) chemi-mechanical polishing (CMP) of Si-face SiC substrates typically results in a low material removal rate (MRR<0.1μm/h). In this study, the chemical surface oxidation and mechanical removal of the oxide layer during CMP of on-axis semi-insulating (SI) 6H SiC substrates, and the effect on material removal rate and surface roughness were investigated separately by addition of (a) compatible oxidizers, (b) abrasives and (c) both oxidizers and abrasives to the CS slurry. Neither oxidizer nor soft abrasive addition individually resulted in a significant MRR increase, yet both increased the substrate surface roughness. The addition of nano-size diamond abrasive (0.1μm grain size) to the CS slurry resulted in a MRR of 0.60μm/h, a 10x increase over CS CMP, and produced substrates with a Ra surface roughness of 5.5Å. The addition of 0.1μm diamond abrasive and sodium hypochlorite oxidizer to the CS slurry resulted in a MRR increase to 0.92μm/h and produced substrates with a Ra surface roughness of 5.2Å.

Growth of SiC Boules with Low Boron Concentration

The effects of growth conditions, diffusion barrier coatings, and hot zone materials on B incorporation in 6H-SiC crystals grown by physical vapor transport (PVT) were evaluated. Development of high purity source material with a B concentration less than 1.8x1015 atoms/cm3, was critical to the growth of boules with a B concentration less than 3.0x1016 atoms/cm3. Application of refractory metal carbide coatings to commercial graphite to serve as boron diffusion barriers and the use of very high purity pyrolytic graphite components ultimately led to the growth of SiC boules with boron concentrations as low as 2.4x1015 atoms/cm3. The effect of growth temperature and pressure were closely examined over a range from 2100°C to 2300°C and 5 to 13.5 Torr. This range of growth conditions and growth rates had no effect on B incorporation. Attempts to alter the gas phase stoichiometry through addition of hydrogen gas to the growth environment also had no impact on B incorporation. These results are explained by considering site competition effects and the ability of B to diffuse through the graphite growth cell components.

Novel reactive chemical mechanical polishing technology for fabrication of SiC mirrors

A new reactive chemical mechanical polishing process has been developed and optimized for polishing CVD SiC mirror samples. The studies show that the abrasives, chemical nature of the slurry, and other additives play an important role in the material removal rate and surface finish of the SiC mirror. The use of different abrasive types and sizes resulted in differing roughness and removal rates. The smaller abrasives created surface defectivity or higher roughness. This can be explained by different polishing rates of different orientations of SiC grains, resulting in the grain enhancement. Under optimal conditions with appropriate abrasive particles, roughness RMS as low as 0.2 nm was achieved on CVD SiC samples. The process also did not show any scratch-like features in the optical interferometry measurements.

Selectivity and Residual Damage of Colloidal Silica Chemi-Mechanical Polishing of Silicon Carbide

The selectivity, material removal rate, and the residual subsurface damage of colloidal silica (CS) chemi-mechanical polishing (CMP) of silicon carbide substrates was investigated using atomic force microscopy (AFM) and plan view transmission electron microscopy (TEM). Silica CMP, in most process conditions, was selective. In the damage region surrounding remnant scratches, the vertical material removal rate exceeded the planar material removal rate, which resulted in an enhancement of the scratches over the duration of the polishing process. The material removal rate was low, about 20 nm / hr. In addition, the selectivity leads to a slow removal of residual subsurface damage from mechanical polishing. The silica CMP polished surface exhibits significant subsurface damage observed by plan view TEM even after prolonged polishing of 16 hours.

Microwave Dielectric Loss Characterization of Silicon Carbide Wafers

Semi-insulating silicon carbide (SiC) wafers are important as substrates for high frequency devices such as AlGaN-GaN HEMT’s. A nondestructive characterization technique has been developed to measure the dielectric properties of SiC wafers in the GHz frequency range where the devices will operate in order to validate wafers for high yield working devices. The dielectric loss is measured at approximately 16 GHz in a split microwave cavity. Initial results show a correlation where the dielectric loss decreases as the resistivity increases, where the resistivity was measured using a Contactless Resistivity Mapping system (COREMA). The uniformity of dielectric loss across SiC wafers was evaluated using a split post dielectric resonator cavity fixed at 5.5GHz to measure the dielectric loss at five points on a wafer. Dielectric loss as a function of temperature from room temperature to 400°C was also studied.

Preparation and evaluation of damage free surfaces on silicon carbide

A new chemical mechanical polishing process (ACMP) has been developed by the Penn State University Electro-Optics Center for producing damage free surfaces on silicon carbide substrates. This process is applicable to the silicon face of semi-insulating, conductive, 4H, 6H, onaxis and off-axis substrates. The process has been optimized to eliminate polishing induced selectivity and to obtain material removal rates in excess of 150nm/hour. The wafer surfaces and resultant subsurface damage generated by the process were evaluated by white light interferometery, Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), and epitaxial layer growth. Residual surface damage induced by the polishing process that propagates into the epitaxial layer has been significantly reduced. Total dislocation densities measured on the ACMP processed wafers are on the order of the densities reported for the best as grown silicon carbide crystals [1]. Characterization of high electron mobility transistors (HEMTs) grown on these substrates indicates that the electrical performance of the substrates met or exceeded current requirements [2].