Takuya Urayama

Sterilization mechanism for Escherichia coli by plasma flow at atmospheric pressure

A mechanism for sterilizing Escherichia coli by a flowing postdischarge and UV radiation of argon plasma at atmospheric pressure was investigated by analyzing the surviving cells and the potassium leakage of cytoplasmic material and by morphological observation. Inactivation of E. coli results from the destruction of the cytoplasmic membrane and the outer membrane under plasma exposure and the destruction of nucleic acids by exposure to ultraviolet radiation from the plasma source.

Sterilization efficacy of a coaxial microwave plasma flow at atmospheric pressure

A coaxial cable for microwave transmission type of atmospheric low-temperature plasma source was developed. The plasma source has the advantages of portability, simple configuration, and ability to operate with a wide range of gases and mixtures of such gases. It mainly consists of a cavity, a quartz discharge tube, a coaxial cable, a microwave power source, and a gas supply system. Using this plasma source, the authors tried to clarify its sterilization efficacy against Geobacillus stearothermophilus. A log reduction number of G. stearothermophilus of at least 5 (10/sup -5/) was obtained under the following operating conditions: gas flow rate of argon of 14 SL/min, input power is 400 W, and substrate surface temperature of 353 K. Distributions of temperature and velocities of gas were also measured two-dimensionally by using an E-type thermocouple and a Pitot tube, respectively. The temperature distributions and the peak value of temperature were found to be influenced by the gas flow rate and the kinds of gas mixtures.

Inactivation of Escherichia Coli by a Coaxial Microwave Plasma Flow

An atmospheric low-temperature plasma flow generated by a microwave discharge utilizing a coaxial cable for microwave transmission was utilized for the inactivation of a bacteria. The employed device consists of a cavity, a quartz discharge tube, a coaxial cable, a microwave power source, and a gas supply system. Using this argon plasma source, we attempted to clarify the effects of exposure temperature, exposure time, exposure distance, input power, and gas flow rate on the number of surviving cells of Escherichia coli. A log reduction number of E. coli of at least 2 (10-2) was obtained at an exposure temperature of 353 K when the exposure time was 600 s with a gas flow rate of 5 Sl/min and an input power of 400 W. The number of surviving cells decreased with an increase of exposure time under any sterilization condition.

Electron energy control in an inductively coupled plasma at low pressures

A method of electron energy control was studied in an inductively coupled plasma that employed a multimode antenna, for high performance in device fabrication plasma processes. The electron energy was reduced by changing the azimuthal mode of a one-turn antenna from m=0 to 2 with no notable change in electron density. Langmuir probe measurements showed that the electron energy reduction was more pronounced in the higher modes and at lower pressures. The antenna size was also a critical parameter. These results were confirmed by optical emission spectroscopy. It was found to be essential for the energy control that the distance between the induction-field reverse points is shorter than the electron free path.

Generation and transport mechanisms of chemical species by a post-discharge flow for inactivation of bacteria

A post-discharge flow that is formed downstream of a microwave argon plasma in atmospheric air was investigated to clarify the generation and transport of chemical species, which are considered to result in the inactivation of bacteria. The flow, which is characterized by ultra-weak emission, can be visualized using an optical analysis system. This visualized jet-like flow forms downstream of the nozzle exit, and then, as the gas temperature is 877 K at the center of the nozzle exit, the main flow travels upstream around the quartz tube due to buoyancy, the reason being that the temperature decreases to room temperature at 30 mm downstream. It was clarified that excited argon atoms, molecular nitrogen (N2 second positive system) and OH radicals were generated in the post-discharge flow, subsequent to which NO2 and ions with a number density of 106 counts cm-3 were transported downstream below the main flow. These results imply that most of the heat and chemical species were transported by convective transport of the main flow, but that a small amount of chemically active species and ions might have been transported further downstream by diffusive transport, these species being considered to result in an inactivation effect on bacteria.

Sterilization characteristics by a coaxial microwave plasma flow at atmospheric pressure

A new type of atmospheric low temperature plasma source was developed utilizing a coaxial cable for microwave transmission rather than conventionally used waveguides. The plasma source has advantages such as portability, simple configuration, and ability to operate with wide range of gases and its mixtures. It mainly consists of a cavity, a quartz discharge tube, a coaxial cable, a microwave power source and a gas supply system. Using this plasma source, we tried to clarify sterilization characteristics of Bacillus subtilis. The log reduction numbers of Bacillus subtilis was obtained at least 5 (10/sup -5/) under the following operation conditions: gas flow rates of Ar (14 SLM) with mixing oxygen of 0.5 SLM and input power is 400 W. Distributions of temperature and gas velocity were also measured two-dimensionally by using E-type thermocouple and Pitot tube, respectively. The temperature distributions and the peak value of temperature were influenced by the gas flow rate and the kinds of mixing gas. The effective sterilization is obtained under the condition of the substrate surface temperature of 353 K.

Electron Energy Control in Inductively Coupled Plasma Employing Multimode Antenna

A method of electron energy control was studied in an inductively coupled plasma employing a multimode antenna, for high etching performance in ultra large-scale-integrated circuit (ULSI) fabrication processes. Electron energy was reduced by changing the azimuthal mode of the antenna from m=0 to m=1 with no notable change in electron density. The electron energy reduction at the m=1 mode was found by Langmuir probe measurement. This behavior was also confirmed by optical emission spectroscopy. Since the energy reduction was more remarkable at lower pressures, it was considered to be due to the reverse of the induction field with a smaller length in the higher mode antenna.

Thin Film Detection Employing Frequency Shift in Sheath Current Oscillation

For an end point detection of etching in ULSI fabrication processes, the behavior of the sheath oscillating current was theoretically and experimentally investigated under the radio frequency bias application. The oscillation of the displacement current in sheath was numerically calculated employing the fluid equations of plasma, and its frequency spectrum was analyzed in computational Fast Fourier Transformation. The results showed that the shift occurred mainly at lower regime than the center frequency under the condition of an relatively small bias voltage change. This behavior of the frequency shift was also confirmed by a simple experiment. A new method is proposed for detection technique of thin insulation film on substrate in process employing the frequency shift of this kind.

Sterilization of Escherichia coli by a coaxial microwave plasma flow

An atmospheric low temperature plasma flow generated by microwave discharge utilizing a coaxial cable for microwave transmission has some advantages such as portability, simple configuration, and low power operation. It mainly consists of a cavity, a quartz discharge tube, a coaxial cable, a microwave power source and a gas supply system. Using this argon plasma source, we investigated to clarify effects of exposure temperature, exposure time, exposure distance, input power and gas flow rate on the number of surviving cells of Escherichia coli. A log reduction number of E. coli of at least 3 (10/sup -3/) was obtained at exposure temperatures of 353 K when the exposure time was 600 seconds with a gas flow rate of 5 Sl/min and an input power of 400 W. Number of surviving cells decreases with increase of exposure time in any sterilization conditions.