Rf Argon Discharge Research Articles

Probe diagnostics of non-Maxwellian plasmas

Various probe diagnostic methods have been applied to rf plasmas with non-Maxwellian electron energy distribution functions (EEDF) and the results of these diagnostic methods have been compared. Plasma density and electron temperature were obtained using standard procedures from the electron retardation region (classic Langmuir method), the ion saturation region, and the electron saturation region of the measured probe I/V characteristic. Measurements were made in a 13.56-MHz capacitive argon rf discharge at two gas pressures: p=0.03 Torr, where stochastic electron heating is dominant, and p=0.3 Torr, where collisional electron heating dominates. Thus, the measured EEDF at each gas pressure manifests a distinct departure from thermodynamic equilibrium being bi-Maxwellian at 0.03 Torr and Druyvesteyn-like at 0.3 Torr. Considerable differences in electron density and temperature were obtained from the different parts of the probe characteristic and these values differ dramatically in many cases from those found from integration of the measured EEDF’s, thus demonstrating that using standard procedures in non-Maxwellian plasma can give misleading results.

Evolution of the electron-energy-distribution function during rf discharge transition to the high-voltage mode.

The electron-energy-distribution function (EEDF) in low-pressure argon and helium rf discharges has been measured during the discharge transition from the low-voltage to the high-voltage mode using a recently developed high-resolution Langmuir probe technique. When the transition occurs, the EEDF becomes Maxwellian because of increased electron-electron collisions arising from a sharp rise in electron density and a steep fall in electron temperature caused by \ensuremath{\gamma}-electron injection.

Calculation of the ionization rate and electron transport coefficients in an argon rf discharge

The behavior of an rf discharge can be modeled by using a fluid approach. For this approach, the values of the mobility and diffusion coefficients as well as the ionization rate are necessary. These values are often obtained by extrapolating the data of dc Townsend discharges. To check whether this is justified we computed the coefficients for electrons in an rf discharge by using a kinetic model based on a two-term approximation of the electron energy distribution function. The calculated electron mobility and electron diffusion coefficients agree reasonably well with the extrapolated Townsend values. Significant deviations were found between the extrapolated Townsend ionization rate and the computed rf ionization rate as a function of the reduced electric field.

Langmuir Probe Determination of Charged Particles Density in an rf Discharge

AbstractThe article presents results of probe measurements of the electron and ion densities in an argon rf discharge. The electron density is determined from the electron probe current at the space potential which is estimated by extrapolation of the linear part of the probe electron current dependence to the plasma potential. The plasma potential is calculated from the floating potential. The positive ion density is determined from the probe ion saturated current according to collisional KOPICZYNSKI and ZAKREWSKI model [1]. The agreement between the electron and ion densities is better than in the work of HOPKINS and GRAHAM [2], where the collisionless Lafram‐boise theory is used under similar conditions.

7368. Design and use of a gridded probe in a low-pressure rf argon discharge: S G Ingram et al, Rev Scient Instrum, 61, 1990, 1883–1891

7368. Design and use of a gridded probe in a low-pressure rf argon discharge: S G Ingram et al, Rev Scient Instrum, 61, 1990, 1883–1891

Probe measurements of the space potential in a radio frequency discharge

The dc and radio frequency (rf) components (and harmonics) of the probe potential have been measured in the midplane of a 13.56-MHz parallel-plane rf discharge in argon over a wide range of discharge voltages and at gas pressures between 10 mTorr and 1.0 Torr. rf potential measurements were made with different input capacitances to determine the true magnitude of the rf plasma potential and the probe capacitance. For a symmetrically driven rf discharge, the rf plasma potential Vrf is mainly composed of the second harmonic of the driving voltage Vdr over a wide range of gas pressures. For high values of Vdr, Vrf ≊0.1 Vdr while the dc probe potential Vdc is about 0.4 Vdr. These results are in good agreement with corresponding theoretical predictions found in the literature. For an asymmetrically driven rf discharge with equal electrode area, the rf plasma potential has an additional fundamental harmonic component equal to half the rf driving voltage. Values of rf plasma potential and probe capacitance given here allow us to specify the requirements on probe circuitry for different kinds of probe measurements in rf discharges.

Abnormally low electron energy and heating-mode transition in a low-pressure argon rf discharge at 13.56 MHz.

The electron energy distribution function measured with improved energy resolution revealed a large number (\ensuremath{\approxeq}90%) of low-energy electrons having an abnormally low electron temperature (T\ensuremath{\approxeq}0.3 V) resulting in a considerably lower mean electron energy than found in all published probe measurements in argon rf discharges at 13.56 MHz. With increasing gas pressure an abrupt transition to a high-temperature mode was found. The low-temperature mode and the observed transition are attributed to a change from stochastic to collisional electron heating enhanced by the Ramsauer effect.

Design and use of a gridded probe in a low-pressure rf argon discharge

Low-pressure rf discharges can be investigated experimentally using many diagnostic techniques. In many cases the simple Langmuir probe can be used to make reliable measurements of basic plasma parameters. In the work reported here, this technique has been extended by employing a gridded Langmuir probe in a parallel-plate rf discharge for the first time. Design considerations and construction of the probe are discussed. It was found that optimum performance could be achieved using a construction based on two independent grids with a nonzero potential difference between them. The probe was used to measure plasma parameters in a low-pressure argon discharge excited at 13.56 MHz between two planar parallel electrodes. Results obtained for different probe orientations along the discharge axis show the electron energy distribution to be approximately Maxwellian in the plasma bulk. However, there is evidence for anisotropy in the electron energy distribution in the region of the plasma nearest to the driven electrode. In fact the high-energy tail observed can be attributed to secondary electron emission from the electrode surface.

Ion and electron energy analysis at a surface in an RF discharge

A retarding field analyser has been used to examine ion and electron energies at the grounded electrode of a capacitively coupled RF discharge in argon. The onset of collisional scattering of the ion distribution was observed at 0.05 mbar. A Tonks-Langmuir-type model has been used to generate a theoretical collision-free energy distribution which compares well with results at low pressure. The energies of electrons incident on the electrode were not consistent with the high-energy tail of a simple Maxwellian distribution in the plasma, and are better modelled by a two-temperature distribution.

Time-resolved plasma parameter measurements in a low-frequency rf glow discharge

A new approach to the use of Langmuir probes in a rf driven plasma is presented. The periodic nature of the rf is utilized to overcome the distortion of the probe characteristics caused by averaging over many rf cycles. Time-resolved measurements of the electron density, electron temperature, plasma potential, and floating potential in the negative portion of the rf cycle are obtained. The technique is used to characterize a low-pressure 100-kHz capacitively coupled rf argon discharge. The measured electron temperature is found to be approximately 0.5 eV.

Thin Cu xS sputtered films in Ar/H 2 atmospheres

The effect of hydrogen added to the rf argon discharge on the physical properties of thin Cu x S films produced by rf sputtering from a Cu 2S target is studied. Hydrogen content ranged from 0 to 5% of the total pressure. Electrical, structural and optical properties of films are analysed as a function of the hydrogen percentage and correlated with different processes taking place in the target and in the plasma which are monitored by glow discharge optical and mass spectroscopies (GDOS and GDMS). The produced films show resistivity values between 10 −4 and 10 2 Ω · cm, depending on the hydrogen content during the growth, and they are correlated with X-ray diffraction measurements.

Orifice probe for plasma diagnostics: II. Multi-parameter analysis

A multi-grid orifice probe and analysis technique are presented which permit the measurement of the following plasma parameters: the electron number density ne, the electron temperature Te, the space potential Vs, and an estimate of the metastable number density nm. Simultaneously, the analysis allows the quantitative determination of instrumental parameters such as the reflection coefficient for slow (<30 eV) electrons from the grids Re(G) and roughened (gold-black) collector Re(C), the secondary electron emission coefficient for fast (> 30 eV) electrons γe(G) from the grids, and the effective transparencies of the grids for electrons τe(G), and for metastables τm(G). Experimental ion, electron and total current characteristics are presented for a low-pressure (1·8×10−2 Torr) RF discharge in argon. The major part of the analysis method involves the use of ten `plateau values' obtained from the characteristics to determine values for most of these parameters. Te and Vs are determined separately. The values obtained are: ne=2·74×109 cm−3±18%, kTe=1·92 eV±5%, Vs=11·7±0·1 V, nm similar, equals 3·3×109 cm−3±50%, Re(G)=0·65±0·10, Re(C)=0·22±0·10, γe(G)=0·935±0·010, τe(G)=0·830±0·002, and τm(G)=0·850±0·002. The transparencies τe(G) and τm(G) are within 4% of the geometrical transparency τ(G)=0·860. Approximately equal numbers of atoms (1 in 2×105) are in metastable and ionized states.

Level populations in plasmas RF discharges and sonic channel

An absolute intensity calibration has been made to previously reported spectrographic measurements in an extensively studied argon rf discharge downstream of the exciting coil, and sonic afterglow. This calibration allows calculation of neutral argon atom density and of individual level population. Neutral atom densities were calculated from the electron density, the electron temperature, and Saha's equation, assuming that the electron temperatures were equal to the excitation temperatures. The densities calculated by these methods were far higher than those obtained from the perfect gas law, the gas temperature, and the pressure. The observed under- population of the ground level is shown to correspond to the “terminal non-equilibrium” defined by PARK. (3) The present data are compared with two existing experiments to show that agreement exists among seemingly inconsistent and confusing reports, provided the non-equilibrium phenomena are properly accounted for.

Propagation of Ion Waves in Weakly Ionized Gases

Theoretical and experimental results are presented on the propagation of longitudinal ion waves in weakly ionized gases in a frequency range extending from considerably below to well above the ion plasma frequency. The theory describes propagation in a uniform plasma on the basis of kinetic equations. First, a dispersion relation is derived and solved for the complex propagation constant; this is appropriate if the damping is weak. Second, the spatial dependence of the phase and amplitude of the disturbance excited by a pair of grids is calculated; when the damping is not weak, the amplitude does not decay exponentially with distance. The measurements are performed with grid-excited ion waves at frequencies between 0.1 and 10 MHz in hydrogen, nitrogen, argon, and krypton rf discharges with charged-particle densities of about ${10}^{9}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and electron temperatures of the order of 50 000\ifmmode^\circ\else\textdegree\fi{}K. At frequencies well below the ion plasma frequency ${\ensuremath{\omega}}_{i}$, the phase velocity is found to be frequency-independent and is given by the Tonks-Langmuir speed. At these frequencies, the attentuation is proportional to the neutral gas pressure and is therefore primarily caused by ion-neutral collisions. At frequencies approaching ${\ensuremath{\omega}}_{i}$, the attentuation is higher than expected from collisions, and the excess is attributed to ion Landau damping. Theory and experiment agree well at $\ensuremath{\omega}$ smaller than ${\ensuremath{\omega}}_{i}$. For frequencies greater than ${\ensuremath{\omega}}_{i}$, the phase velocity increases with frequency, but not as much as expected on the basis of Maxwellian velocity distribution of the ion gas. Moreover, the ratio of imaginary to real part of the propagation constant, which is found to decrease slightly with frequency, is much smaller than expected from the theory. Better agreement between experiment and theory in this frequency range can be obtained by assuming the ionic velocity distribution to decrease more rapidly at high velocities than a Maxwellian distribution. Non-Maxwellian velocity distributions of this kind are actually expected under the conditions of the experiment. The data reported in this paper furnish the first experimental evidence of ion-wave propagation at frequencies greater than ${\ensuremath{\omega}}_{i}$.