The sintering behavior of primarily covalently bonded β-SiC, Si, and Si3N4 was studied using surface area and densification measurements as well as observations of microstructures developed during firing. The existence of highly dense, microscopic regions and large (≥100°) dihedral angles in fired compacts of β-SiC and Si which experience little macroscopic densification suggests that macroscopic densification is not intrinsically limited by the effects of surface energy. The mechanism proposed to explain the microstructure that develops in unsinterable covalent solids which do not undergo a phase change is based on the existence of a high ratio of surface and/or vapor-phase matter transport-to-volume and/or grain-boundary transport. The addition of boron to both β-SiC- and Si-containing carbon retards surface and/or vapor-phase transport and grain growth at lower temperatures, which results in enhanced densification at high temperatures. Macroscopic densification of β-SiC and α-Si3N4 can also be retarded by the formation of a continuous network of high-aspect-ratio grains of the polymorphic form that rigidities the sintering body. Finally, the sintering of pure Si depends sensitively on particle size in the submicron range. Nearly theoretical density is achieved in Si powder of ∼0.06-μm size. This result suggests that other pure covalently bonded solids can also be sintered to high density without applied pressure.

https://ceramics.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1151-2916.1976.tb09424.x

Sintering of Covalent Solids

The processing parameters of monolithic polycrystalline silicon resistors are examined, and the effect of grain size on the sensitivity of polysilicon resistivity versus doping concentration is studied theoretically and experimentally. Because existing models for polysilicon do not accurately predict resistivity dependence on doping concentration as grain size increases above 600 A, a modified trapping model for polysilicon with different grain sizes and under various applied biases is introduced. Good agreement between theory and experiments demonstrates that an increase in grain size from 230 to 1220 A drastically reduces the sensitivity of polysilicon resistivity to doping levels by two orders of magnitude. Such an increase is achieved by modifications of the integrated-circuit processes. Design criteria for the optimization of monolithic polysilicon resistors have also been established based on resistivity control, thermal properties, and device geometry.

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Modeling and optimization of monolithic polycrystalline silicon resistors

The diffusion of arsenic in polycrystalline silicon films has been studied over the temperature range 750–950 °C and for grain sizes 210–510 nm. Rutherford backscattering spectrometry was used to measure the concentration profiles of arsenic, initially introduced into the polysilicon by ion implantation, after various annealing steps. The concentration profiles were found to be determined by a combination of two factors—the low diffusivity in the bulk of the rains and the much higher diffusivity in the grain boundaries. The diffusivity of arsenic in the grain boundaries was independent of concentration, with an activation energy of 3.9 eV, very close to that of the low‐concentration arsenic diffusivity in single‐crystal silicon. However, the value of the diffusivity was 8.6×104 exp(−3.9/kT)cm2/s, four orders of magnitude higher than the single‐crystal value. The diffusivity in the interior of the grains was the same as that in single‐crystal silicon.

Diffusion of arsenic in polycrystalline silicon

Structure of chemically deposited polycrystalline-silicon films

An imaging guidewire for imaging tissues from inside a patient's body cavity. The imaging guidewire includes an elongated main body portion and an end portion. The end portion has a housing near to the guidewire's distal end, an ultrasonic beam emitting assembly having a pivotable part, and a driver for producing a pivotal motion on the pivotable part. The housing has a portion that is acoustically transparent. The pivotable part is movable and is operatively connected to the housing. The pivotable part can either have on it a transducer for emitting ultrasound or a reflector for reflecting ultrasound. In either case, when the pivotable part pivots it sweeps ultrasonic energy over a selected angle. The driver is located near to the transducer such that all driving motions for driving the pivotal motion occur near the distal end of the imaging guidewire.

Imaging guidewire with back and forth sweeping ultrasonic source

The diffusion of electrically active impurities into polycrystalline silicon deposited by the thermal decomposition of silane has been studied. Both boron and phosphorus have been found to diffuse more rapidly into polycrystalline silicon than into single‐crystal silicon. The diffusivity is greatest in films deposited at about 1000°C although this maximum is a function of the deposition rate, film thickness, and substrate material as well as the deposition temperature. The maximum diffusivity is associated with a structure which has the maximum {110} texture. Although most of the films have been deposited onto amorphous SiO2, a nucleating layer of polycrystalline silicon has been found to influence the diffusion of impurities into a thick layer of polycrystalline silicon deposited over the nucleating layer. An optimum structure is obtained with a nucleating layer containing maximum {110} texture. The enhanced diffusion is tentatively attributed to diffusion down defects in the polycrystalline silicon. The...

T. I. Kamins

Papers

Diffusion of Impurities in Polycrystalline Silicon