Kazuhiko Kashima

Oxygen precipitation in Czochralski‐grown silicon wafers during hydrogen annealing

In studying the effect of the ramping process on oxygen precipitation in Czochralski‐grown silicon wafers in hydrogen annealing, we have found that the oxygen precipitate density in the bulk region depends on the ramping‐up rate at temperatures between 900 and 1200 °C. Few oxygen defects are observed when the ramping‐up rate is 30 °C/min or more. Decreasing the ramping‐up rate exponentially increases the oxygen precipitate density. The nucleation for oxygen precipitates can therefore be controlled by adjusting the ramping‐up rate during hydrogen annealing.

Intrinsic Gettering in Nitrogen-Doped and Hydrogen-Annealed Czochralski-Grown Silicon Wafers

The properties of nitrogen-doped and hydrogen-annealed Czochralski-grown silicon (NHA-CZ-Si) wafers were investigated in this study. The quality of the subsurface was investigated by monitoring the generation lifetime of minority carriers, as measured by the capacitance-time measurements of a metal oxide silicon capacitor (MOS C-t). The intrinsic gettering (IG) ability was investigated by determining the nickel concentration on the surface and in the subsurface as measured by graphite furnace atomic absorption spectrometry (GFAAS) after the wafer was deliberately contaminated with nickel. From the results obtained, the generation lifetimes of these NHA-CZ-Si wafers were determined to be almost the same as, or a little longer than those of epitaxial wafers, and the IG ability was proportional to the total volume of oxygen precipitates [i.e., bulk micro defects (BMDs)], which was influenced by the oxygen and nitrogen concentrations in the wafers. Therefore, it is suggested that the subsurface of the NHA-CZ-Si wafers is of good quality and the IG capacity is controllable by the nitrogen and oxygen concentrations in the wafers.

Effect of Sn atoms on incorporation of vacancies in epitaxial Ge1−xSnx film grown at low temperature

The anomalous increase and decrease in the S-parameters of Doppler broadening spectroscopy in positron annihilation spectroscopy in a narrow range of Sn atom content were detected in a Ge1−xSnx thin film grown by MBE at low temperatures. The increase can be explained in terms of vacancies when the target content of 1.7% Sn atoms is incorporated in a Ge matrix, owing to the binding nature between them. However, the S-parameters were markedly decreased when the target content of Sn atoms in the film grown at the same temperature was 0.1%. These changes in the S-parameters correspond to the carrier concentrations obtained by Hall measurements.

Neutron transmuted magnetic Czochralski grown silicon wafer for power device

The authors examine the preparation of low-cost Si wafers for power devices with diameters of 125 mm and above, using a transversal-superconductive-magnetic-field Czochralski pulling growth method and neutron transmuted doping. The puller has shown a very high pulling rate and low cost. The oxygen concentration in the growing crystal can be kept below 5*10/sup 17/ atoms/cm/sup 3/, as this level of oxygen concentration does not affect the recovery of electrical characteristics and actually makes the silicon wafer more resilient to the high-temperature device production processes. Dislocation loop formation in crystals densely irradiated by fast neutrons and then subjected to high-temperature heat treatment is caused by small grains with a direction slightly different to that of the grand mass crystals. Methods for preventing the formation of loops are described.<<ETX>>

Strong Quadrupole-Strain Interaction of Vacancy Orbital in Boron-Doped Czochralski Silicon

We have carried out ultrasonic measurements of a boron-doped silicon ingot grown by the Czochralski method in order to determine the quadrupole-strain interaction constant of a vacancy orbital. The low-temperature softening of the elastic constant \(C_{44}\) shows a remarkable variation depending on positions of the ingot, which reflects the distribution of vacancy concentration \(N\) in the ingot. An infrared laser scattering tomograph was employed to measure the density and size of voids in the silicon wafers by determining the vacancy concentration \(N_{\text{cons}}\) consumed in void formation. Using a combination of laser scattering tomography and low-temperature softening, we have found a sum rule in which the initially created vacancy concentration \(N_{\text{total}}\) corresponds to the sum of the residual vacancy concentration \(N\) and the consumed vacancy concentration \(N_{\text{cons}}\) as \(N_{\text{total}} = N + N_{\text{cons}}\). Taking account of the sum rule, we deduce the interaction co...

Determination of silicon evaporation rate at 1200 °C in hydrogen

Separation by implanted oxygen silicon wafers have been investigated with cross‐sectional transmission electron microscopy after annealing at 1200 °C in argon as well as in hydrogen. It is observed that the buried oxide has experienced little change in the thickness, which is ascribed to the low hydrogen solubility in the crystal (about 3×1015 cm−3) and provides a natural mark to measure the thickness variation of the top silicon directly. The evaporation rate as determined in this method is less than 0.1 nm/min with an accuracy of ±0.1 nm/min, at least two orders of magnitude lower than reported in previous investigations.

Elastic Softening of Surface Acoustic Wave Caused by Vacancy Orbital in Silicon Wafer

We have performed surface acoustic wave (SAW) measurements to examine vacancies in a surface layer of a boron-doped silicon wafer currently used in semiconductor industry. A SAW with a frequency of fs = 517 MHz was optimally generated by an interdigital transducer with a comb gap of \(w=2.5\) µm on a piezoelectric ZnO film deposited on the (001) silicon surface. The SAW propagating along the [100] axis with a velocity of \(v_{\text{s}}=4.967\) km/s is in agreement with the Rayleigh wave, which shows an ellipsoidal trajectory motion in the displacement components ux and uz within a penetration depth of λp = 3.5 µm. The elastic constant Cs of the SAW revealed the softening of ΔCs/Cs = 1.9 × 10−4 below 2 K down to 23 mK. Applied magnetic fields of up to 2 T completely suppress the softening. The quadrupole susceptibilities based on the coupling between the electric quadrupoles Ou, Ov, and Ozx of the vacancy orbital consisting of Γ8–Γ7 states and the symmetry strains eu, ev, and ezx associated with the SAW ac...

pH-responsive silicon wafer prepared by hydrogen annealing

We found that silicon wafers prepared by hydrogen gas annealing at high temperatures exhibit good pH-response while those without hydrogen annealing do not. When the pH-responsive silicon wafer is heated in air at a high temperature, e.g. 600°C, so that oxide layer forms on the surface, the pH-responsive properties disappear. It is conceived that a small amount of mobile protons are present in the pH-responsive silicon wafer. The silicon wafers have a potential use for pH-electrode materials which are applicable to pH-measurement even in HF solution.

Precise Fabrication of Silicon Wafers Using Gas Cluster Ion Beams

Precise surface processing of a silicon wafer was studied by using a gas cluster ion beam (GCIB). The damage caused to the silicon surface was strongly dependent on irradiation parameters. The extent of damage varied with the species of source gas and the acceleration voltage (Va) of cluster ions. It also varied with the cluster size and residual gas pressure. The influence of electron acceleration voltage (Ve) used for ionization of a neutral cluster was also investigated. The irradiation damage, such as an amorphous silicon (a‐Si) layer, a mixed layer of a‐Si and c‐Si (transition layer), and surface roughness, was increased with Ve. It is suggested that the increase in the amount of energy per atom was induced by high Ve, because of variation of the cluster size and/or cluster charge. An undamaged smooth surface can be produced by Ar‐GCIB irradiation at low Ve and Va.

Cross‐sectional TEM Observations of Si Wafers Irradiated With Gas Cluster Ion Beams

Irradiation by a Gas Cluster Ion Beam (GCIB) is a promising technique for precise surface etching and planarization of Si wafers. However, it is very important to understand the crystalline structure of Si wafers after GCIB irradiation. In this study, the near surface structure of a Si (100) wafer was analyzed after GCIB irradiation, using a cross‐sectional transmission electron microscope (XTEM). Ar‐GCIB, that physically sputters Si atoms, and SF6‐GCIB, that chemically etches the Si surface, were both used. After GCIB irradiation, high temperature annealing was performed in a hydrogen atmosphere. From XTEM observations, the surface of a virgin Si wafer exhibited completely crystalline structures, but the existence of an amorphous Si and a transition layer was confirmed after GCIB irradiation. The thickness of amorphous layer was about 30 nm after Ar‐GCIB irradiation at 30 keV. However, a very thin (< 5 nm) layer was observed when 30 keV SF6‐GCIB was used. The thickness of the transition layer was the sam...