Kouichirou Honda

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.

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.

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.

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.

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.

Behavior of Fe Impurity during HCl Oxidation

Behavior of Fe impurity at the Si-SiO 2 interface of a metal-oxide-semiconductor (MOS) made through HCl oxidation was studied with electrical measurements and transmission electron microscopy. Fe impurity was introduced in silicon wafers by ion implantation to the doses of 1.0 x 10 15 and 1.0 x 10 14 cm -2 . The wafers were then oxidized in HCl/O 2 ambient. The Fe impurity either nucleated (1.0 x 10 15 cm -2 ) to precipitates in the Si substrate near the Si-SiO 2 interface or scattered uniformly in the SiO 2 (1.0 x 10 15 and 1.0 x 10 14 cm -2 ). The precipitates were identified as metallic α-FeSi 2 or semimetallic FeSi. The precipitate is supposed to form a weak spot in the silicon oxide, where the electric field is strengthened ; however, the dielectric effect of uniformly scattered Fe impurity in the SiO 2 film is relatively small. Fe precipitates were gradually included into the SiO 2 film and finally dissolved during HCl oxidation. As a result, Fe impurity was scattered in the middle area of the SiO 2 film. The gettering ability of HCl oxidation is attributed to its enhancement of dissolution of Fe-silicides in the SiO 2 film.

Microdefects distribution in Czochralski‐grown silicon crystals

Striated microdefect distribution (swirl defects) are formed by the heat treatment in Czochralski‐grown silicon crystals. The striation observed by etching has two period components 1–2 mm and 300–500 μm along growth direction. The corresponding microdistribution of oxygen was examined by the scanning infrared absorption method with the collimated beam of 100 μm in diameter. The results show that the distribution with the period of 1–2 mm was only observed, but not the microdistribution with the period of 300–500 μm. It is proposed from the results that microdefects are originated at a high oxygen‐concentration region from the nuclei which were introduced in a striated pattern with the period of 300–500 μm during crystal growth. The nucleus is not interstitial oxygen itself.

Silicon surface roughness—Structural observation by reflection electron microscopy

Surface roughness of polished silicon wafers was observed by reflection electron microscopy. Small steps were clearly resolved as fringe pattern, and rather rough steps of 1.2–1.6 nm in height and 200–500 nm in interval were observed as dark and bright bands. This is the first direct visualization of polished wafer surface roughness.

Instrument measuring temperature dependence of minority‐carrier lifetime without contact

An instrument measuring the temperature dependence of minority‐carrier lifetimes without contacts is described. The temperature range is 100 to 420 K, and the shortest decay time observed is about 2 μs. The instrument utilizes the photo decay of microwave power reflected from the sample, therefore, it is a contactless method. As an example, the instrument was applied to high‐quality silicon crystals used in very large scale integrated circuit fabrication. The measured temperature dependence could be explained by the Shockley–Read–Hall theory by assuming a level for the recombination centers at 0.18 eV from the valence band.