Positive Ion Density Research Articles

Local and fast relaxation phenomena after laser-induced photodetachment in a strongly electronegative rf discharge.

A one-dimensional self-consistent particle in cell/Monte Carlo method is used to study the local (at the laser impact region) and fast relaxation phenomena after laser-induced photodetachment in a strongly electronegative SiH(4)/H(2) rf discharge. The relaxation process of the local densities of the charged plasma species has been studied in association with the time evolution of the local electric field. The phenomena predicted theoretically about the relaxation processes, such as the potential well, the electrostatic oscillation, the long lasting potential structure, the distortion of the early potential perturbation on the measurement of negative ion temperature, and the depression in the positive ion density profile at the edges of the laser impact region, have been confirmed by our simulation results. Compared to the relaxation in weakly electronegative discharges, the local and even the global electric field in strongly electronegative discharges, has been weakened strongly after photodetachment. The relaxation of the local electric field lasts 100 rf cycles with the recovery of the local electron density and the local electron energy. The electrostatic oscillation exhibited as the deviation in quasineutrality, is very strong and continues over several rf cycles in our case. The large dip in the center of the positive ion density profile, observed in the experiment, is also reproduced by our model.

Optical actinometry of Cl2, Cl, Cl+, and Ar+ densities in inductively coupled Cl2–Ar plasmas

Optical emission (OE) actinometry has been used to measure the absolute densities of Cl2, Cl, Cl+, and Ar+ in a high-density inductively coupled (ICP) Cl2–Ar plasma at 18 mTorr as a function of the 13.56 MHz radio frequency (rf) power and Ar fraction. The fractional dissociation of Cl2 to Cl increases with rf power, with the dissociated fraction increasing from 78% to 96% at 600 W (10.6 W cm−2) as the Ar fraction increases from 1% to 78% due to an increase in electron temperature. Emission from Cl+* and Ar+* originates primarily from electron excitation of Cl+ and Ar+ (and not excitation of Cl and Ar), making actinometric determination of Cl+ and Ar+ densities feasible. For powers exceeding 600 W, the neutral (Cl2 and Cl) to ion (Cl+ and Ar+) flux ratio is found to be strongly dependent on Ar fraction, decreasing by a factor of ∼3.0 as the latter is increased from 13% to 78%. This dependence can be attributed mostly to the decrease in Cl density and relatively little to the small decrease in the total positive ion density from 1.8×1011 to 1.4×1011 cm−3, over the same range. OE spectroscopy is also used to estimate the rate constant for the dissociative excitation of Cl2 to the Cl (4p2D0J′=3/2,5/2) excited state with emission at 822.2 nm, yielding ∼10−13 cm3 s−1.

Diagnostics of inductively coupled chlorine plasmas: Measurement of electron and total positive ion densities

This work is part of a broader study that creates a set of experimentally measured chlorine plasma parameters (electron and gas temperatures, electron energy distribution function, electron density, densities of ion fractions, and total ion density). This set is obtained over a broad range of operating conditions (1–20 mTorr, 5–1000 W) of an inductively coupled plasma. In this part, we present the electron (ne) and total positive ion (ni+=nCl2++nCl+) densities. ne and ni+ were measured with a Langmuir probe across the diameter or in the middle of the reactor. Line integrated values of ne were independently obtained with microwave interferometry, and converted into the spatially resolved data using the Langmuir probe ne profiles. Finally, a method is presented for measuring relative positive ion densities, based on optical emission at 7504 Å from trace amounts of Ar.

Modelling of particle charging in the polar summer mesosphere: Part 1—General results

Rocketborne measurements of electron and positive ion number densities in the vicinity of noctilucent clouds and/or polar mesosphere summer echoes frequently give evidence of pronounced disturbances relative to a smooth background profile. A model is applied to study the disturbances in terms of diffusional charging of aerosol particles with special regard to the background plasma conditions, i.e. the electron/ion production rate and the coefficient of dissociative recombination which depends on the positive ion composition. It is demonstrated that the disturbances in electron and positive ion profiles are a complex function of the aerosol radius, the aerosol number density, the production rate, and the recombination coefficient. However, if the latter two are known, the model allows the determination of aerosol radii and number densities from the measurement of positive ion and electron number densities. Furthermore, we show that nearly all combinations of electron biteouts, positive ion biteouts, and positive ion enhancements can exist depending on the properties of the aerosol particles and the background plasma conditions. Concerning electrons the presence of aerosol particles will always lead to a depletion. The magnitude of this depletion increases with increasing aerosol size and number density. Concerning positive ions both depletions and enhancements can occur. While small electron/ion production rates favour deep depletions in both electrons and positive ions, enhancements in positive ions are most likely created if the recombination coefficient is large implying large positive cluster ions. However, enhancements of electrons cannot be created by the variation of the above mentioned parameters. Thus observations of electron enhancements are a strong indication of a charging mechanism different from the diffusional charging discussed in this paper, e.g. photo emission. We have applied the charging model to in situ observations of electrons and positive ions in the vicinity of noctilucent clouds and polar mesosphere summer echoes. These results are presented in a companion paper by Lübken and Rapp (J. Atmos. Sol. Terr. Phys. (2001), 63, 771–780).

Modelling of particle charging in the polar summer mesosphere: Part 2—Application to measurements

A model describing the charging of aerosol particles in the vicinity of the polar summer mesosphere is applied to in situ measurements of electron and ion number densities. More general results from this model are presented in a companion paper (Rapp and Lübken, 2001). Here we apply this model to various in situ measurements. Charging of the aerosols causes disturbances in the electron and ion number density profiles which depend on various parameters, such as the electron/ion production rate Q, the electron/ion recombination coefficient α, the aerosol size, r A, and the aerosol number density N A. We have analyzed measurements from rocket flights where at least two of these parameters are known (in most cases Q and α). Under most circumstances the disturbance in the electron density profile Δ n e (‘biteout’) depends on r A and N A in a different way than the disturbance in the positive ion density profile Δ n i. In many cases this allows to unambiguously determine r A and N A from measurements of Δ n e and Δ n i. From a total of 11 flights found in the literature we have analyzed in detail four cases which are characterized by simultaneous observations of noctilucent clouds (NLC) and/or polar mesosphere summer echoes (PMSE) by ground based or in situ techniques. In all cases our model successfully explains the observed plasma disturbances in terms of aerosol charging. During NLC conditions we arrive at particle radii of 32– 60 nm and number densities of 110– 850 cm −3 . Assuming that the particles consist of water ice the amount of ice in the particles corresponds to gas phase water vapor concentrations of 4– 14 ppm v which is rather large compared to ‘standard’ model values. These large values support the idea that water vapor is collected by the aerosols in a larger horizontal area and/or extended altitude range when they sediment from the mesopause region (88 km) to typical NLC altitudes (82 km) . In one flight we arrive at much smaller (1 nm) and much more abundant (several 100,000 cm −3 ) aerosol particles.

Peculiarities of plasma decay in the afterglow of a low-pressure pulsed discharge in oxygen

Weakly ionized plasma of a pulsed-discharge afterglow in oxygen at low pressures (0.05–0.15 torr) is investigated using probe diagnostics. The plasma conductivity is measured by supplying an additional probing current pulse at a certain instant during the afterglow. The spectral line intensities are also measured to additionally monitor the densities of charged particles. The measurements of the time behavior of the electron density in an oxygen afterglow plasma confirm the previous conclusion that the electrons escape due to enhanced diffusion, which results in the formation of an ion-ion plasma. The possibility of realizing the opposite ultimate case—the detachment decay regime with an increase in the electron density to the density of positive ions in the first stage and the transition to the electron-ion plasma in the second stage—is demonstrated.

Kinetic modeling of relaxation phenomena after photodetachment in a rf electronegative SiH4 discharge.

The global relaxation process after pulsed laser induced photodetachment in a rf electronegative SiH4 discharge is studied by a self-consistent kinetic one-dimensional particle-in-cell-Monte Carlo model. Our results reveal a comprehensive physical picture of the relaxation process, including the main plasma variables, after a perturbation up to the full recovery of the steady state. A strong influence of the photodetachment on the discharge is found, which results from an increase of the electron density, leading to a weaker bulk field, and hence to a drop in the high energy tail of the electron energy distribution function (EEDF), a reduction of the reaction rates of electron impact attachment and ionization, and a subsequent decrease of the positive and negative ion densities. All the plasma quantities related to electrons recover synchronously. The recovery time of the ion densities is about 1-2 orders of magnitude longer than that of the electrons due to different recovery mechanisms. The modeled behavior of all the charged particles agrees very well with experimental results from the literature. In addition, our work clarifies some unclear processes assumed in the literature, such as the relaxation of the EEDF, the evolution of the electric field, and the recovery of negative ions.

K-1712 酸素/アルゴン系高周波マグネトロン放電におけるイオンの奇妙な振舞い(J08-3 熱流体現象の原子・分子論的アプローチ(3))(J08 熱流体現象の原子・分子論的アプローチ)

The characteristics of rf planar magnetron discharges of an O_2/Ar mixture are clarified using the particle-in-cell/Monte Carlo (PIC/MC) method. The simulation is carried out for axisymmetrical magnetic fields. The spatialand temporal behavior of magnetron discharge is examined in detail. The spatial distribution of positive ion density shows a double peak structure. The position of one peak corresponds to the peak point of electron density, while the other peak is located at the peak point of negative ion density. The mechanism of this phenomenon is clearly explained.

The electrostatic interaction of charged, dust-particle pairs in plasmas

We report the results of a study of the electrostatic interaction between negatively charged particles in a plasma. The goal of the study was to investigate the possibility of an attractive interaction which would make possible the formation of “molecules” of particles. For all approximations relating the positive ion density to the local electrostatic potential that we examined, we find that the interaction is repulsive for all particle separations.

Measurements of negative ion density in fluorocarbon ECR plasma

The parameters of an electron-cyclotron resonance (ECR) C 4F 8/Ar gas plasma were measured using both a heated Langmuir probe and an 8-mm microwave interferometer. An attempt to estimate the negative ion density was made with three methods in the case where C 4F 8 and Ar gas mixture rate was less than 25%; the first method is based on the analysis of the probe characteristics, and the others on the charge neutrality condition. It was found that the ratio of the negative ion density to the positive ion density was 20–50%, depending on the assumed positive ion mass. Furthermore, the self-bias effect in C 4F 8/Ar ECR plasma was discussed.

Laser-induced photodetachment in high-density low-pressure SF6 magnetoplasmas

Using laser-induced photodetachment (LIPD), we investigate in some detail how different discharge parameters affect the negative ion fraction in high-density low-pressure SF6 magnetoplasmas sustained by the propagation of electromagnetic surface waves. A plane electrostatic probe is used for collection of the photodetached electrons. Careful testing of the LIPD technique itself is carried out prior to systematic measurements and adequate laser fluence conditions are determined. Negative ions are found to outnumber electrons several times, even at mTorr and submTorr pressures, indicating the important electronegative character of the discharge. The dependence of the negative ion fraction on gas pressure, argon admixture, microwave power, and axial and radial position in the reactor is interpreted on the basis of different negative ion formation and loss mechanisms. The negative ion fraction is found to be maximum in conditions and regions of minimal electron temperature and positive ion density.

Diagnostics of inductively coupled chlorine plasmas: Measurement of Cl2+ and Cl+ densities

The absolute densities of positive ions (Cl2+ and Cl+) are obtained over a 2–20 mTorr pressure range and 5–1000 W input radio-frequency rf power range in a transformer-coupled Cl2 plasma. The relative number density of Cl2+ is measured by laser-induced fluorescence. These laser-induced fluorescence data are calibrated by Langmuir-probe measurements of total positive-ion density at low powers to yield absolute values for nCl2+ and are corrected for changes in rotational temperature with rf power. In turn, the nCl2+ data are used to determine the effective-mass correction for refined Langmuir-probe measurements of the total positive-ion density. The density of Cl+ is then the difference between the total positive-ion and Cl2+ densities. For all the pressures, Cl2+ is found to be the dominant ion in the capacitively coupled regime (input powers below 100 W), while Cl+ is the dominant ion at higher powers (>300 W) of the inductively coupled regime. Experimental results are compared to those from a simple global model. This work is a continuation of a study that provides a complete set of experimentally measured plasma parameters over a broad range of conditions in a chlorine plasma, produced with a commercial, inductively coupled rf source.

Charged species profiles in oxygen plasma

The charged species profiles have been studied in oxygen plasma of asymmetrically driven capacitively coupled rf discharge. The electron (ne), negative (n), and positive (p) ion densities have been measured by Langmuir probe in a wide range of pressures (3–30 Pa) and interelectrode distances (2–10 cm). At small powers (ne<1015 m−3) it was obtained that the electronegativity is high (n/ne∼10) and the electron density profile is flat in plasma bulk. The negative ion density profile is nearly flat in the ion–ion core, where n≫ne, and falls off towards the plasma-sheath boundary with n≈0 with the spatial scale πDn/(2γd), determined by the negative ion diffusion coefficient Dn and detachment frequency γd. As the pressure decreases, this spatial scale exceeds half of the interelectrode distance and the negative ion density profile tends to a parabolic one. The obtained spatial distributions of charged particles are compared with the results of self-consistent modeling by use of a fluid approach for ions, and kinetic equation for electrons. Numerical simulation results agree well with both theory predictions and measurement data.

Dynamics of inductively-coupled pulsed chlorine plasmas in the presenceof continuous substrate bias

We report an analysis of the dynamics of a pulsed power,inductively-coupled chlorine plasma operated with continuous radio frequency(rf) bias applied to the substrate stage. A comparison of pulsed plasmasoperated with and without 12.5 MHz rf bias is performed through aninvestigation of the time dependences of electron (ne) and positive ion(ni+) densities and electron temperatures (Te), measured with aLangmuir probe in a 10 mTorr Cl2 plasma. There is no significant differencein the plasma characteristics with or without bias during the on portion ofthe power modulation of the source. Once the source power is turned off, Teinitially decreases rapidly, and ne and ni+ decay slowly, independentof the presence of the rf bias. About 25 µs into this decay, when Te isabout 0.5 eV (and would continue to decay in the absence of the rf bias), thepresence of the rf bias (70-100 W) causes Te to increase rapidly and reachvalues higher than those recorded at the end of the on portion. Meanwhile,ne and ni+ continue to decay, independent of substrate bias. Onlymuch later in the afterglow are charge densities affected by bias. About50 µs into the off period, ne with added substrate bias is lowerthan with no bias. If the off period is sufficiently long, the plasma in thepresence of the rf bias transforms into a so-called reactive ion etching (RIE)mode that is generated and sustained solely by the capacitively-coupled biaspower. This occurs rather abruptly when ni+ decays to the level ofpositive ion density for RIE operation (i.e. with the stage powered and thesource power off). Since ne at this instant has decayed well below theelectron density during RIE operation, it rapidly increases when thistransition occurs. The behaviour of Te in the off portion of the pulsedplasma indicates that the presence of rf bias prevents the sheath collapsethat occurs without bias. Since this will prevent negative ions (Cl-)from reaching the wafer, the reduced plasma-induced damage (with respect tocontinuous source operation) reported for pulsed plasma operation withhigh-frequency (e.g. ~13 MHz) bias cannot be caused by the dischargingof the wafer by negative ions.

The growth of cubic boron nitride films by RF reactive sputter

Boron nitride (BN) films were deposited by RF (radio frequency) sputtering. Hot-pressed hexagonal boron nitride (h-BN) of 4 N purity was used as sputtering target. The substrates were p-type Si(100) wafers with the resistance of (8–15) Ω cm, which were biased by negative DC voltage during deposition. The sputtering gas was the mixture of nitrogen and argon. Boron nitride films were characterized by Fourier transform infrared (FTIR) spectroscopy. It was found that the cubic boron nitride (c-BN) films could be formed at different radio frequency powers and different DC negative substrate bias. Furthermore, the c-BN films with almost pure cubic phase were successfully obtained under an appropriate radio frequency power and DC substrate bias voltage. Our results indicate that appropriate energy flow density of positive ions impinging on substrate for formation of c-BN should be necessary.

On the plasma parameters of a planar inductive oxygen discharge

Langmuir probes and a quadrupolemass spectrometer were used to determine the plasma parameters ofan oxygen plasma in a planar inductive discharge. The electron density,effective electron temperature, the dc plasma potential and the electronenergy probability function (EEPF) in the discharge centre plane were investigatedas functions of power, gas pressure and radial position. The ion energy distribution and relative density of positive ions at theradial sheath edge were investigated as functions of power and pressure.A volume-averaged global model of the electronegative oxygen discharge isdeveloped. The model uses a power balance equation to account forenergy deposited into the plasma and lost via collisions and particleflux. The particle densities are modelled via rate equations estimatedfrom collision cross sections assuming Maxwellian electron energy distributionfunctions. The volume-averagedmodel is shown to predict the experimental trends over a range of processconditions.

Diagnostic studies of aluminum etching in an inductively coupled plasma system: Determination of electron temperatures and connections to plasma-induced damage

Using trace rare gases-optical emission spectroscopy (TRG-OES) and Langmuir probe measurements, electron temperatures (Te) were obtained in Cl2/BCl3/N2 plasmas in an inductively coupled plasma system, under typical processing conditions for metal etching. A small amount (1.7% each) of the five rare gases was added to the plasma and emission spectra were recorded. TRG-OES Tes corresponding to the high-energy tail of the electron energy distribution function were derived from the best match between the observed and computed rare gas emission intensities. Te was determined as a function of total pressure, source power, fraction of BCl3 added to Cl2 and substrate material (SiO2, Al, and photoresist). Positive ion densities and relative electron densities were also measured for some of these conditions. At source and bias powers of 1000 and 100 W, TRG-OES Tes in Cl2/BCl3/N2/rare gas plasmas increased from 1.4 eV at 40 mTorr to 2.3 eV at 3 mTorr, about 15% lower than values computed from a global model and ∼1.4 times lower than those measured with a Langmuir probe. Reduced plasma induced damage to the gate oxide at higher pressures (18 vs 10 mTorr) correlates with a drop in both Te (1.7 vs 1.9 eV) and plasma density (1.0×1011 vs 1.3×1011 cm−3), but is due mostly to the lower Te.

Plasma density over Svalbard during the ISBJØRN campaign

Abstract. In 1997, reliable operation of the EISCAT Svalbard Radar (ESR) was achieved and a rocket launching facility at Ny Ålesund on Svalbard (79°N, 12°E) (SVALRAK) was established. On 20 November, 1977, the first instrumented payload was launched from SVALRAK. Although the payload configuration had been flown many times previously from Andøya Rocket Range on the Norwegian mainland, this presented an unprecedented in situ determination of positive ion density over Svalbard. Simultaneously, ESR measured similar density profiles but in a higher altitude regime. We have combined the ESR measurements with ionosonde data to establish a calibration and subsequently combined the ground-based and in situ determined profiles to give a composite positive ion density profile from the mesosphere to the thermosphere.Key words: Ionosphere (polar ionosphere; instruments and techniques)

Dynamics of pulsed-power chlorine plasmas

Characteristics of chlorine, transformer-coupled pulsed plasmas are reported. Time dependencies of electron (ne), positive ion (ni+), and negative ion (ni−) densities and electron temperatures (Te) were measured with a Langmuir probe and microwave interferometry at 240 and 500 W input powers, and pressures between 3 and 20 mTorr. During the OFF portion of the power modulation, ne decreases rapidly as Cl− is formed by dissociative attachment of Cl2. The formation of Cl− is accelerated at high Cl2 densities (at high pressures and low powers). At 10 mTorr and higher pressures, an ion–ion plasma forms near the end of the OFF portion of the cycle, the sheath collapses, and Cl− reaches the wafer. Te decays rapidly in the OFF period and increases with a similar time constant at the beginning of the ON cycle if electrons are present at a sufficiently high level. If ne is very low at the beginning of the ON cycle, such as at high pressure (10 mTorr), then Te exhibits a spike at the beginning of the ON period. In a comparison study, plasma induced damage is reduced when aluminum is etched under similar source power modulation conditions.

Chemistry of the galactic cosmic ray induced ionosphere of Titan

Titan's lower ionosphere (from 1 to 400 km) has been studied with a one‐dimensional ion‐neutral model. In this region of the atmosphere, galactic cosmic rays (GCRs) are the main ionization source. They penetrate to the deeper atmosphere and ionize the neutral constituents of Titan's atmosphere (mainly N2, CH4, Ar, H2, and CO) to produce N2+, N+, Ar+, CH4+, CH3+, CH2+, H2+, H+, and CO+. Fast reactions with the neutrals convert these ions into ions such as CH5+, C2H5+, and N2H+. Different pathways are proposed to obtain the ion and electron densities. The most abundant ions are cluster ions, like CH5+·CH4, HCO+·H2, and HCNH+·C2H4, and long chain hydrocarbon ions. In atmospheres very rich in N2, such as Titan's, ions like H4C7N+ and CH3CNH+ also represent an important contribution to the total positive ion density. Three‐body reactions may play an important role in the dense atmosphere of Titan, and special attention is devoted to them. The calculated electron density in the lower atmosphere reaches a peak of ≈ 2150 cm−3 at an altitude of 90 km.