Nobuo Toyokura

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.

Properties of Molybdenum Silicide Film Deposited by Chemical Vapor Deposition

films used as gate electrodes and interconnects were deposited on oxidized Si substrates via reactions between and . At high temperatures, generated by reactions involving , , and reacts with Si to form volatile , , , and ; thus Si is etched away and is not available to form , resulting in deposited film consisting of only metallic Mo. At low temperatures, the deposited film consists of which is thermodynamically stable. The deposited films show characteristics (resistivity, crystal structure, and so forth) similar to those of films deposited by sputtering. Chlorine in the deposited films has a gettering effect for mobile ions such as Na+.

Phosphorus‐Doped Molybdenum Silicide Films for LSI Applications

The properties of phosphorus‐doped molybdenum silicide as a gate electrode, interconnecting material, and impurity diffusion source for LSIs have been studied. The phosphorus‐doped molybdenum silicide films were deposited by co‐sputtering using specially designed Mo and Si targets in a atmosphere. decomposed to P and during sputtering and the phosphorus combined with Mo and Si. Typically, a phosphorus concentration of in the films was used. As‐deposited films had amorphous structure and high resistivity. After the films were annealed at temperatures above 800°C, they became polycrystalline and resistivity was decreased. The resistivity of the film with a Mo/Si ratio of 2 to 1 was with annealing at 1000°C in . Phosphorus was able to diffuse from the doped molybdenum silicide films to the Si substrate during annealing in . After high temperature annealing up to 1100°C, the contact resistance between the molybdenum silicide and the silicon substrate was below . The reliability of doped molybdenum silicide gate MOSFETs is as good as polysilicon gates and the threshold voltage shift is within ±20 mV under stress conditions of 2.5 MV/cm for 1000 hr at 150°C.

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.

Donor‐related microdefects at surface of denuded zone in Czochralski‐grown silicon

Microdefects related to donors in the denuded zone formed after (1200–700 °C) annealing were investigated using spreading resistance, etching, and transmission electron microscope methods. It is found that donors are generated about 1×1014 cm−3 even at the surface of the denuded zone and are closely related to small oxygen precipitates rather than dislocation dipoles corresponding to etch pits. Donor formation at 700 °C annealing is concluded to be concerned with small oxygen precipitates nucleated at embryo sites in addition to carbon sites.