Yoshiki Morikawa

High-quality SiO2 film formation by highly concentrated ozone gas at below 600°C

Highly concentrated (>93 vol %) ozone (O3) gas was used to oxidize silicon for obtaining high-quality SiO2 film at low temperature. Compared to O2 oxidation, more than 500 °C lower temperature oxidation (i.e., from 830 to 330 °C) has been enabled for achieving the same SiO2 growth rate. A 6 nm SiO2 film, for example, could be grown at 600 °C within 3 min at 900 Pa O3 atmosphere. The temperature dependence of the oxidation rate is relatively low, giving an activation energy for the parabolic rate constant of 0.32 eV. Furthermore, a 400 °C grown SiO2 film was found to have satisfactory electrical properties with a small interface trap density (5×1010 cm−2/eV) and large breakdown field (14 MV/cm).

Development of a continuous generation/supply system of highly concentrated ozone gas for low-temperature oxidation process

A system is described which can continuously generate/supply highly concentrated (HC) ozone gas to satisfy the future need for practical low-temperature oxidation. This system comprises four ozone vessels, each with independent temperature control. The system can supply a constant flow of HC ozone gas by allocating one of four modes of operation, i.e., accumulation/storage, vaporization (supply), evacuation, and cooling, to each of the ozone vessels so that all the modes can be simultaneously addressed. The maximum flow rate is 60 sccm with a flux stability of ±1.1%, and an ozone concentration of over 99.5 vol % can be achieved at the system outlet. The system was applied to the formation of an ultrathin SiO2 film on a 4 in. diameter silicon wafer substrate.

Reactive oxygen beam generation system using pulsed laser evaporation of highly concentrated solid ozone

A reactive oxygen beam generation system is described for the formation of high-quality and high-precision films. This system utilizes pulsed laser evaporation of highly concentrated solidified ozone (O3). The equipment for safely generating and handling a large amount of high-purity liquid and solid O3 was also developed for this purpose. The beam is characterized by its high concentration of oxygen atoms in an excited state [O(1D)], constant flux per laser shot (4×1017 molecules cm−2 shot−1), appropriate level of kinetic energy (KE) for enhancing the surface reaction (mean KE of 0.4 eV, maximum KE of 2 eV) and small angular spread (6°). These characteristics enabled us to precisely control the SiO2 film thickness by the number of laser shots, and achieve an enhanced Si oxidation rate and new local oxidation process.

High Quality Gate Dielectric Film on Poly-Silicon Grown at Room Temperature using UV Light Excited Ozone

We have grown SiO 2 films on polycrystalline Si using excited ozone produced by ultraviolet light irradiation of ozone, and characterized their electrical properties in the metal-insulator-semiconductor capacitor configuration. SiO 2 films of ∼8.5 nm thickness on poly-Si layers were grown in 60 min even at room temperature. The leakage current density across the SiO 2 film fitted well the Fowler-Nordheim tunnel current behavior and breakdown occurred at above 12 MV/cm, showing that the film was of device quality. The rate of Si oxidation by excited ozone was similar for both Si(100) and Si(lll) wafers, as was the interface trap density (D it ). These results indicate that excited ozone can form a homogenous SiO 2 film on poly-silicon. We conclude that excited ozone is one of the most efficient reactive species for SiO 2 film formation on poly-Si at room temperature.

Rapid Oxidation of Silicon Using UV-Light Irradiation in Low-Pressure, Highly Concentrated Ozone Gas below 300 °C

A low-temperature, damage-free process for growing ultrathin (<6 nm) silicon dioxide (SiO2) films was successfully developed. The excitation of low-pressure, highly concentrated O3 gas using photons with energies less than 5.6 eV led to rapid growth rates of 2 and 3 nm within 1 and 5 min, respectively, even when the process temperature was as low as 200 °C. The enhanced oxidation rate was due to an increased supply of O(1D) atoms at the Si surface. Transmission electron microscope images revealed that the SiO2 film formed with a uniform thickness and a smooth, distinct SiO2/Si interface. Capacitance–voltage and current–voltage measurements showed that 200 and 300 °C as-grown films had a satisfactorily low density of mobile ions and trap charges as well as ideal insulating properties.

Sloping lifetime control by electron irradiation for 4.5 kV PT-SIThs

This paper describes the possibility of realizing sloping lifetime control by electron irradiation. 4500 V class SIThs which have the wider n-base region are irradiated with 1 MeV electron beams on the cathode side or the anode side. These devices have sloping lifetime distributions. As a result, in the case of devices with cathode-side irradiation, we successfully improved the tradeoff relationship.

Enhanced oxidation of silicon using a collimated hyperthermal ozone beam

Silicon was oxidized by a collimated hyperthermal ozone beam produced by pulsed-laser ablation of solid ozone to increase the controllability of the silicon dioxide film thickness and to achieve low-temperature oxidation. The oxidation rate could be accurately controlled by the number of laser shots to which the number of supplied ozone molecules was proportional. Ozone molecules with a translational energy of around 1 eV obtained by laser ablation produced an initially rapid oxidized region with no temperature dependence in which a 0.6 nm silicon dioxide film could be synthesized at room temperature with only 200 laser shots. Higher-efficiency oxidation was also achieved in comparison with that by using a spray of ozone with thermal energy.

Advantage of Highly Concentrated (≥90%) Ozone for Chemical Vapor Deposition SiO2 Grown under 200 °C Using Hexamethyldisilazane and Ultraviolet Light Excited Ozone

We have compared the UV-light-excited ozone chemical vapor deposition (CVD) process conditions and the film quality for the cases where either highly concentrated (≥90%: HC) or 7% ozone and either hexamethyldisilazane (HMDS) or tetraethoxysilane (TEOS) are used. The SiO2 film deposited using HMDS and UV-excited HC ozone with an optimized flow rate has the highest quality in terms of leakage current density, etching rate, and deposition rate which are comparable or superior to those of the conventional thermal TEOS SiO2 grown at 620 °C. These results lead to a conclusion that it is preferable to use HC ozone for UV-light-excited-ozone CVD to deposit the high quality SiO2 films at a practical rate at a temperature as low as 200 °C.

Hyperthermal O3 Beam Produced by Laser Ablation of Solid-Ozone Film

In order to obtain a highly concentrated hyperthermal ozone beam for more effective Si oxidation, we performed laser ablation of solid-ozone. A KrF pulse laser was irradiated onto solidified ozone on a sapphire substrate cooled to 30 to 60 K using a cryocooler. A mixture of ozone, molecular oxygen and atomic oxygen was detected using a time-of-flight method through a quadrupole mass filter. The velocity distribution of ablated ozone molecules was almost the same as the distribution at thermal equilibrium. An ozone beam with a fitted temperature of 2500 K and maximum translational energy of 3 eV was obtained under optimum laser conditions.

Rapid and Uniform SiO2 Film Growth on 4 inch Si Wafer Using 100%-O3 Gas

We have developed a lamp-heated cold-wall chamber that can process a large Si wafer using a highly concentrated (>90 vol.%) ozone gas to achieve rapid and uniform oxidation at a lower temperature than that used in conventional thermal oxidation. Uniform SiO2 formation with a film thickness uniformity of within 0.2 nm was achieved. The SiO2 growth rate, however, was not markedly accelerated compared with that achieved using conventional low (i.e., 10 vol.%)-concentration O3 gas. This was considered to originate from the decomposition of O3 gas in the gas phase before arriving at a heated surface as determined from the local ozone concentration measurements we performed. By increasing gas flow velocity so as to reduce the area of the thermal boundary layer on the heated surface in which decomposition of O3 to molecular oxygen is enhanced, SiO2 growth rate was actually improved.