Yoshinori Takao

A miniature electrothermal thruster using microwave-excited microplasmas: Thrust measurement and its comparison with numerical analysis

A microplasma thruster has been developed, consisting of a cylindrical microplasma source 10mm long and 1.5mm in inner diameter and a conical micronozzle 1.0–1.4mm long with a throat of 0.12–0.2mm in diameter. The feed or propellant gas employed is Ar at pressures of 10–100kPa, and the surface-wave-excited plasma is established by 4.0GHz microwaves at powers of <10W. The thrust has been measured by a combination of target and pendulum methods, exhibiting the performance improved by discharging the plasma. The thrust obtained is 1.4mN at an Ar gas flow rate of 60SCCM (1.8mg∕s) and a microwave power of 6W, giving a specific impulse of 79s and a thrust efficiency of 8.7%. The thrust and specific impulse are 0.9mN and 51s, respectively, in cold-gas operation. A comparison with numerical analysis indicates that the pressure thrust contributes significantly to the total thrust at low gas flow rates, and that the micronozzle tends to have an isothermal wall rather than an adiabatic.A microplasma thruster has been developed, consisting of a cylindrical microplasma source 10mm long and 1.5mm in inner diameter and a conical micronozzle 1.0–1.4mm long with a throat of 0.12–0.2mm in diameter. The feed or propellant gas employed is Ar at pressures of 10–100kPa, and the surface-wave-excited plasma is established by 4.0GHz microwaves at powers of <10W. The thrust has been measured by a combination of target and pendulum methods, exhibiting the performance improved by discharging the plasma. The thrust obtained is 1.4mN at an Ar gas flow rate of 60SCCM (1.8mg∕s) and a microwave power of 6W, giving a specific impulse of 79s and a thrust efficiency of 8.7%. The thrust and specific impulse are 0.9mN and 51s, respectively, in cold-gas operation. A comparison with numerical analysis indicates that the pressure thrust contributes significantly to the total thrust at low gas flow rates, and that the micronozzle tends to have an isothermal wall rather than an adiabatic.

A miniature electrothermal thruster using microwave-excited plasmas: a numerical design consideration

A miniature electrothermal thruster has been proposed using azimuthally symmetric microwave-excited plasmas, and numerical investigations have been conducted for design consideration. The microthruster consists of a microplasma source and a micronozzle. The former, made of a dielectric chamber 1 mm in radius and 10 mm long covered with a grounded metal, produces high temperature plasmas in Ar at around atmospheric pressures. The latter converts such high thermal energy into directional kinetic energy through supersonic nozzle expansion to obtain the thrust required. The numerical model consists of three modules: a global model and an electromagnetic model for microplasma sources and a fluid model for micronozzle flows. Simulation was conducted separately for the plasma source and nozzle flow. The numerical results indicated that the microwave power absorbed in plasmas increases with increasing microwave frequency and relative permittivity of dielectrics, to achieve plasma density in the range 1019–1022 m−3, electron temperature in the order of 104 K and heavy particle temperature in the range 103–104 K at a microwave input power of ≤ 10 W; in practice, surface waves tend to be established in the microplasma source at high frequencies and permittivities. A certain combination of frequency and permittivity was found to significantly enhance the power absorption, enabling the microplasma source to absorb almost all microwave input powers. Moreover, the micronozzle flow was found to be very lossy because of high viscosity in thick boundary layers, implying that shortening the nozzle length with increasing half-cone angles suppresses the effects of viscous loss and thus enhances the thrust performance. A thrust of 2.5–3.5 mN and a specific impulse of 130–180 s were obtained for a given microwave power range of interest, which is applicable to a station-keeping manoeuvre for microspacecraft less than 10 kg.

Plasma-Induced Defect-Site Generation in Si Substrate and Its Impact on Performance Degradation in Scaled MOSFETs

Plasma-induced ion-bombardment damage was studied in terms of defect sites created underneath the exposed Si surface. From the shift of capacitance-voltage (C- V) curves, the defect sites were found to capture carriers (being negatively charged in the case of an Ar plasma exposure). This results in a change of the effective impurity-doping density and the profile. We also report that the defect density depends on the energy of ions from plasma. A simplified and quantitative model is proposed for the drain-current degradation induced by the series-resistance increase by the damage. The relationship derived between the defect density and the drain-current degradation is verified by device simulations. The proposed model is useful to predict the device performance change from plasma process parameters.

Microplasma thruster for ultra-small satellites: Plasma chemical and aerodynamical aspects*

A microplasma thruster has been developed of electrothermal type using azimuthally symmetric microwave-excited microplasmas. The microplasma source was ~2 mm in diameter and ~10 mm long, being operated at around atmospheric pressures; the micronozzle was a converging-diverging type, having a throat ~0.2 mm in diameter and ~1 mm long. Numerical and experimental results with Ar as a working gas demonstrated that this miniature electrothermal thruster gives a thrust of >1 mN, a specific impulse of ~100 s, and a thrust efficiency of ~10 % at a microwave power of <10 W, making it applicable to attitude-control and station-keeping maneuver for a microspacecraft of <10 kg.

Optical and Electrical Characterization of Hydrogen-Plasma-Damaged Silicon Surface Structures and Its Impact on In-line Monitoring

Si surface damage induced by H2 plasmas was studied in detail by optical and electrical analyses. Spectroscopic ellipsometry (SE) revealed a decrease in the pseudo-extinction coefficient in the region of photon energy higher than ~3.4 eV upon H2-plasma exposure, which is attributed to the disordering of crystalline silicon (c-Si). The increase in in the lower energy region indicates the presence of trap sites for photogenerated carriers in the energy band gap in the E–k space of Si. The current–voltage (I–V) measurement of metal-contacted structures was performed, revealing the following characteristic structures: thinner surface (SiO2) and thicker interface (SiO2:c-Si) layers on the Si substrate in the case of H2-plasma exposure than those with Ar- and/or O2-plasma exposure. The structure assigned on the basis of both SE and I–V was further analyzed by a layer-by-layer wet-etching technique focusing on the removability of SiO2 and its etch rate. The residual damage-layer thickness for the H2-plasma process was thicker (~10 nm) than those for other plasma processes (<2 nm). Since the interface layer plays an important role in the optical assessment of the plasma-damage layer, the present findings imply that a conventional two-layer (SiO2/Si) optical model should be revised for in-line monitoring of H2-plasma damage.

Model for Bias Frequency Effects on Plasma-Damaged Layer Formation in Si Substrates

Bias frequency effects on damaged-layer formation during plasma processing were investigated. High-energy ion bombardment on Si substrates and subsequent damaged-layer formation are modeled on the basis of range theory. We propose a simplified model introducing a stopping power Sd(Eion) with a power-law dependence on the energy of incident ions (Eion). We applied this model to damaged-layer formation in plasma with an rf bias, where various energies of incident ions are expected. The ion energy distribution function (IEDF) was considered, and the distribution profile of defect sites was estimated. We found that, owing to the characteristic ion-energy-dependent stopping power Sd(Eion) and the straggling, the bias frequency effect was subject to suppression, i.e., the thickness of the damaged layer is a weak function of bias frequency. These predicted features were compared with experimental data on the damage created using an inductively coupled plasma reactor with two different bias frequencies; 13.56 MHz and 400 kHz. The model prediction showed good agreement with experimental observations of the samples exposed to plasmas with various bias configurations.

Numerical and experimental study of microwave-excited microplasma and micronozzle flow for a microplasma thruster

Plasma and aerodynamic features have been investigated for a microplasma thruster of electrothermal type using azimuthally symmetric microwave-excited microplasmas. The thruster developed consisted of a microplasma source 1.5 mm in diameter, 10 mm long with a rod antenna on axis, and a converging-diverging micronozzle 1 mm long with a throat 0.2 mm in diameter. The feed or propellant gas employed was Ar at pressures of 10–50 kPa with flow rates of 10–70 SCCM (SCCM denotes standard cubic centimeter per minute at STP) and the surface wave-excited plasmas were established by 4.0 GHz microwaves at powers of ≤6 W. Numerical analysis was made for the plasma and flow properties by developing a self-consistent, two-dimensional model, where a two-temperature fluid model was applied to the entire region through the microplasma source to the micronozzle (or through subsonic to supersonic); in the former, an electromagnetic model based on the finite difference time-domain approximation was also employed for analysis ...

Structural and electrical characterization of HBr/O2 plasma damage to Si substrate

Silicon substrate damage caused by HBr/O2 plasma exposure was investigated by spectroscopic ellipsometry (SE), high-resolution Rutherford backscattering spectroscopy, and transmission electron microscopy. The damage caused by H2, Ar, and O2 plasma exposure was also compared to clarify the ion-species dependence. Although the damage basically consists of a surface oxidized layer and underlying dislocated Si, the damage structure strongly depends on the incident ion species, ion energy, and oxidation during air and plasma exposure. In the case of HBr/O2 plasma exposure, hydrogen generated the deep damaged layer (∼10 nm), whereas ion-enhanced diffusion of oxygen, supplied simultaneously by the plasma, caused the thick surface oxidation. In-line monitoring of damage thicknesses by SE, developed with an optimized optical model, showed that the SE can be used to precisely monitor damage thicknesses in mass production. Capacitance–voltage (C–V) characteristics of a damaged layer were studied before and after dil...

Plasma chemical behaviour of reactants and reaction products during inductively coupled CF4 plasma etching of SiO2

A two-dimensional fluid model has been developed to study plasma chemical behaviour of etch products as well as reactants during inductively coupled CF4 plasma etching of SiO2. The plasma fluid model consisted of Maxwells equations, continuity equations for neutral and charged species including gas-phase and surface reactions and an energy balance equation for electrons. The surface reaction model assumed Langmuir adsorption kinetics with the coverage of fluorine atoms, fluorocarbon radicals and polymers on SiO2 surfaces. Numerical results indicated that etch product species occupy a significant fraction of reactive ions as well as neutrals in the reactor chamber during etching, which in turn leads to a change in plasma and surface chemistry underlying the processing. In practice, the density of SiF4 was typically about 10% of that of the feedstock CF4, being comparable to that of the most abundant fluorocarbon radical CF2; moreover, the density of was typically about 5% of that of the most abundant fluorocarbon ion . The density and the distribution of such product species in the reactor chamber were changed by varying the ion bombardment energy on the substrate surfaces, gas pressure, mass flow rate and coil configuration, which arises in part from gas-phase reactions depending on plasma electron density and temperature. Surface reactions on the chamber walls and on the substrate also affect the product density and distribution in the reactor; in particular, the surface reactions on the SiO2 dielectric window as well as substrate surfaces were found to largely affect the product density and distribution.

Two-dimensional particle-in-cell Monte Carlo simulation of a miniature inductively coupled plasma source

Two-dimensional axisymmetric particle-in-cell simulations with Monte Carlo collision calculations (PIC-MCC) have been conducted to investigate argon microplasma characteristics of a miniature inductively coupled plasma source with a 5-mm-diameter planar coil, where the radius and length are 5 mm and 6 mm, respectively. Coupling the rf-electromagnetic fields to the plasma is carried out based on a collisional model and a kinetic model. The former employs the cold-electron approximation and the latter incorporates warm-electron effects. The numerical analysis has been performed for pressures in the range 370–770 mTorr and at 450 MHz rf powers below 3.5 W, and then the PIC-MCC results are compared with available experimental data and fluid simulation results. The results show that a considerably thick sheath structure can be seen compared with the plasma reactor size and the electron energy distribution is non-Maxwellian over the entire plasma region. As a result, the distribution of the electron temperature...