Akira Ohsawa

Breakdown in silicon oxides (II)—correlation with Fe precipitates

Thin oxides grown on silicon substrate in which Cu+ ions had been implanted before oxidation were studied by transmission electron microscope (TEM) and scanning TEM imaging methods. Cu precipitates, stacking faults, and dislocations appeared at the SiO2/Si interface on the degraded specimens. The Cu precipitates reduce the breakdown strength by local thinning of the oxide thickness. Stacking faults and dislocations, however, do not reduce the breakdown strength.

Fundamental properties of intrinsic gettering of iron in a silicon wafer

Properties of intrinsic gettering of Fe were studied by measuring Fe‐B complex concentration and interstitial Fe concentration in a denuded zone after isochronal or isothermal annealing followed by quenching using deep level transient spectroscopy. We calculated the Fe concentration as the Fe‐B complex concentration plus the interstitial Fe concentration. Silicon wafers were contaminated with a surface Fe concentration of 4.2×1011 to 3.2×1013 cm−2 to show the relation between Fe concentration in the wafer and the temperature at which gettering occurs. Supersaturation of Fe impurities was found necessary for intrinsic gettering of Fe in the contamination range of 4.0×1012 to 3.5×1014 cm−3. Therefore, the gettering temperature is lower for low‐level Fe contamination than for high‐level contamination. The reduction of Fe concentration saturated with annealing time, which shows that the oxygen precipitates in the bulk defect region do not work as an infinite gettering sink. We found that the saturated Fe conc...

Catastrophic breakdown in silicon oxides: The effect of Fe impurities at the SiO2‐Si interface

The behaviors of Fe impurities at the SiO2‐Si interface of metal‐oxide‐semiconductor (MOS) capacitors was studied with electrical measurements and transmission electron microscopy. The MOS capacitors were fabricated on silicon wafers which had been intentionally contaminated by Fe+ ion implantation. It is confirmed that Fe impurities either scattered uniformly, or nucleated at the interface of SiO2‐Si. Uniformly scattered Fe impurities lower the barrier height of the SiO2‐Si interface. The nucleated Fe precipitates are in a metallic α‐FeSi2 phase, penetrating both the silicon oxide and the silicon substrate. They degrade the MOS capacitors not only by reducing the barrier height of the SiO2‐Si interface, but also by inducing weak spots in the silicon oxide where the electric field is strengthened and local tunneling currents are enhanced.

Degradation of metal‐oxide‐semiconductor devices caused by iron impurities on the silicon wafer surface

A reliable method of controlling iron impurities on the silicon wafer surface at low levels has been developed by using an iron‐contaminated HNO3 solution. Iron ions are thought to react with the silicon native oxide to form an Fe(III)‐O complex in proportion to the iron concentration in the solution. Using this method, we have quantitatively investigated the influence of iron impurities on metal‐oxide‐semiconductor device characteristics. The drastic degradation of generation lifetime, surface generation velocity, and dielectric breakdown strength of SiO2 have been observed above the surface iron concentration of 1×1012, 5×1012, and 1×1013 cm−2, respectively.

Influence of carbon and oxygen on donor formation at 700 °C in Czochralski‐grown silicon

Influence of oxygen and carbon on donor formation by annealing at 700 °C was studied by infrared absorption, resistivity, and spreading resistance measurements in Czochralski‐grown silicon. Donor concentration correlates with both oxygen and carbon reductions by the annealing at 700 °C. Donor distribution corresponds to etch pit distribution observed after the annealing. The donor is proposed to be oxygen precipitate nucleated at carbon site. The influence of the donor generation on the device parameter in metal‐oxide‐ silicon memory field‐effect transistors is discussed.

Determination of oxygen concentration profiles in silicon crystals observed by scanning IR absorption using semiconductor laser

Concentration profiles of interstitial oxygen in silicon crystals were determined by scanning IR absorption using PbTe‐Pb0.82Sn0.18Te semiconductor laser with the wavelength of 9.04±0.02 μm. IR beam was collimated to be 200 μm in diameter. Czochralski‐grown crystals showed a local concentration fluctuation of about 1.8×1017 cm−3 from the average of 1.1×1018 cm−3. This fluctuation corresponds to an oxygen striation in the silicon crystal.

Mechanical strength of silicon crystals with oxygen and/or germanium impurities

Using an indentation test at 900 °C, we studied the mechanical strength of epitaxial and Czochralski (CZ) silicon single crystals with germanium concentrations between 1018 and 1021 cm−3. The strength of CZ crystals is found to increase for germanium concentrations above 6×1019 cm−3 and that of epitaxial crystals independent of germanium concentrations below 3×1020 cm−3. Yamada‐Kaneta, Kaneta, and Ogawa [16th International Conference on Defects in Semiconductors (Lehigh University, Bethlehem, Pennsylvania, 1991), p. 286] report that in CZ crystals alloyed with germanium in excess of 6×1019 cm−3, oxygen atoms occupy the interstitial sites next to the germanium atoms. A model compatible with the observations is that the increased strength is due to dislocation immobilization by oxygen atoms adjacent to germanium atoms.

Ultraclean Technique for Silicon Wafer Surfaces with HNO3-HF Systems

We have developed a wafer cleaning technique called the slight etch (SE) using an HNO3 and trace HF mixture. A 30 nm surface removal by the SE reduces the surface Fe concentration by one tenth, compared to conventional RCA and all measured elements below a concentration of 1010 cm-2, without roughness degradation. The ultraclean surface results in a significant improvement in the C-t retention time and defect density of SiO2. Since the etch selectivity of silicon for the oxide is more than 10, this cleaning is also ideal for wafers with patterned oxides.

Intrinsic Gettering of Iron Impurities in Silicon Wafers

We present a new experimental approach to using intrinsic gettering to remove Fe impurities. We annealed samples, followed by quenching, and measured the Fe concentration near the surface using deep-level transient spectroscopy. The supersaturation of Fe impurities is necessary for the intrinsic gettering of Fe. However, for a higher supersaturation, Fe impurities precipitate faster than gettering. The optimum degree of supersaturation is one order of magnitude. Gettering is limited by the reaction of Fe with the oxygen precipitates in the defect region, rather than by Fe diffusion to the defect region.

Oxygen striation and thermally induced microdefects in Czochralski‐grown silicon crystals

Oxygen striations in Czochralski‐grown silicon crystals have been directly observed by applying the scanning IR absorption technique to longitudinal sections. The oxygen striations were found to correlate clearly with thermally‐induced‐microdefect distributions. The results show that oxygen plays a very important role in microdefect formation. A critical oxygen concentration for microdefect introduction was estimated to be (7–8)×1017 cm−3 for 96‐h, 600 °C heat treatment.