Measured Electron Energy Distribution Research Articles

Temporal evolution of electron energy distribution function and plasma parameters in the afterglow of drifting magnetron plasma

The temporal behaviour of the electron energy distribution function (EEDF) and the plasma parameters such as electron density, electron temperature and plasma and floating potentials in a mid-frequency pulsed dc magnetron plasma are investigated using time-resolved probe measurements. A negative-voltage dc pulse with an average power of 160 W during the pulse-on period, a repetition frequency of 20 kHz and a duty cycle of 50% is applied to the cathode of a planar unbalanced magnetron discharge with a grounded substrate. The measured electron energy distribution is found to exhibit a bi-Maxwellian distribution, which can be resolved with the low-energy electron group and the high-energy tail part during the pulse-on period, and a Maxwellian distribution only with low-energy electrons as a consequence of initially rapid decay of the high-energy tail part during the pulse-off period. This characteristic evolution of the EEDF is reflected in the decay characteristics of the electron density and temperature in the afterglow. These parameters exhibit twofold decay represented by two characteristic decay times of an initial fast decay time τ1, and a subsequent slower decay time τ2 in the afterglow when approximated with a bi-exponential function. While the initial fast decay times are of the order of 1 µs (τT1 ∼ 0.99 µs and τN1 ∼ 1.5 µs), the slower decay times are of the order of a few tens of microseconds (τT2 ∼ 7 µs and τN2 ∼ 40 µs). The temporal evolution of the plasma parameters are qualitatively explained by considering the formation mechanism of the bi-Maxwellian electron distribution function and the electron transports of these electron groups in bulk plasma.

Effect of duty cycle on plasma parameters in the pulsed dc magnetron argon discharge

The time-resolved probe measurements of the plasma parameters and the electron energy distribution function are carried out in a unipolar pulsed dc magnetron argon discharge. The cathode target is driven by the 20kHz midfrequency unipolar dc pulses at three operating modes, such as constant voltage, constant power, and constant current with the duty cycles ranging from 10% to 90%. It is observed that as the duty cycle is reduced, the electron temperature averaged during the pulse-on period rapidly increases irrespective of the operating mode although the average electron density strongly depends on the operating mode. The comparison of the measured electron energy distribution functions shows that the electron heating during the pulse-on period becomes efficient in the pulse operation with short duty cycle, which is closely related to the deep penetration of the high-voltage sheath into the bulk during the pulse-on period.

Characterization of pulsed plasma in unbalanced magnetron argon discharge

Time-resolved probe measurements have been conducted in a unipolar pulsed dc magnetron system to investigate the temporal evolution of plasma parameters, such as the electron density and the electron temperature, and to determine how the driving frequency and the duty cycle of the dc pulse affect these parameters. A new probe measurement system was employed to perform the time-resolved measurements of I-V characteristic curves and their second derivatives with a maximum time resolution of 100 ns. The measurements were performed in a constant voltage mode, constant current mode, and constant power mode, with various pulse frequencies ranging from 5 kHz to 50 kHz and duty cycles ranging from 10% to 90%, to investigate the detailed temporal evolutions of the electron energy distribution function and the plasma parameters. The results show that as the pulse frequency increases, the electron density and the electron temperature exhibit insignificant changes. However, the reduction of the duty cycles results in a significant increase of the electron temperature, irrespective of the operating mode. A comparison of the measured electron energy distribution functions shows that the increase in the electron temperature is caused by a decrease in the population of trapped low-energy electrons and∕or by an increase in the population of drifting high-energy electrons. This result can be explained by considering the electron heating due to the deep-penetrating cathode sheath.

An investigation of an industrial coating environment with planar probe technology

A novel electrostatic probe method has been applied to the plasma characterization of an industrial magnetron-based ion plating plasma system. The probe consists of a small, 7-mm diameter, disk fitted with a guard ring and mounted near the substrate position. A negative DC self-bias is achieved on the probe surface relative to the plasma by application of a radio frequency (RF) burst across an in-line capacitor. Following the termination of the RF burst a current–voltage characteristic is obtained as the probe potential returns to a floating condition through the arrival of positive ions from the plasma. Use of capacitively coupled RF biasing permits operation of the probe in a deposition environment. The guard ring maintains sheath planarity at the probe surface and eliminates edge effects. Total positive ion flux and the energetic tail of the electron distribution function are determined from the probe's current–voltage characteristic. Results are reported for ion flux and electron energy distribution measurements in DC and pulsed DC plasmas in closed field unbalanced magnetron sputter ion plating (CFUBMSIP) industrial coating equipment. Argon and combined argon and reactive gas atmospheres at below 0.5 Pa were used. Measurements are correlated with the deposition system's configuration and deposition parameters, including magnetron and substrate voltages, bias currents, and pulsing conditions. Potential applications and limitations of the planar probe technique for routine process control in an industrial coating environment are discussed.

The high-resolution electron–ion collision facility at TSR

A new electron beam facility at the storage ring TSR is presented which makes electron–ion collision experiments possible without using the existing electron cooler device as an electron target. A long adiabatic acceleration section combined with magnetic adiabatic expansion (factor up to ≈90) is used to reduce the longitudinal and transverse electron temperatures. The facility can be operated with either a thermionic or a photocathode. Results from two-dimensional electron beam profile measurements and longitudinal electron energy distribution measurements are discussed.

Electron capture dissociation Fourier transform ion cyclotron resonance mass spectrometry in the electron energy range 0-50 eV.

Electron capture dissociation (ECD) of polypeptide cations was obtained with pencil and hollow electron beams for both sidekick and gas-assisted dynamic ion trapping (GADT) using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) with an electrostatic ion transfer line. Increasing the number of trapped ions by multiple ICR trap loads using GADT improved the ECD sensitivity in comparison with sidekick ion trapping and ECD efficiency in comparison with single ion trap load by GADT. Furthermore, enhanced sensitivity made it possible to observe ECD in a wide range of electron energies (0-50 eV). The degree, rate and fragmentation characteristics of ECD FTICR-MS were investigated as functions of electron energy, electron irradiation time, electron flux and ion trapping parameters for this broad energy range. The results obtained show that the rate of ECD is higher for more energetic (>1 eV) electrons. Long electron irradiation time with energetic electrons reduces average fragment ion mass and decreases efficiency of formation of c- and z-type ions. The obtained dependencies suggest that the average fragment ion mass and the ECD efficiency are functions of the total fluence of the electron beam (electron energy multiplied by irradiation time). The measured electron energy distributions in low-energy ECD and hot ECD regimes are about 1 eV at full width half maximum in employed experimental configurations.

Asymmetric emission of high-energy electrons in the two-dimensional hydrodynamic expansion of large xenon clusters irradiated by intense laser fields

Energy spectra and angular distributions have been measured of electrons that are emitted upon disassembly of ${\mathrm{Xe}}_{n}$ clusters $(n=20000--150000)$ following irradiation by intense ${(10}^{15}--{10}^{16}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2})$ laser pulses whose durations are varied over the 100--2200 fs range. The cluster explosion dynamics occur in the hydrodynamic regime. For the smaller clusters in the range that we have studied, a single-electron temperature adequately describes the measured electron energy distribution; in the case of larger clusters, a two-temperature fit becomes necessary. The total electron emission is found to be unexpectedly asymmetric and exhibits a resonance when the laser-pulse duration is $\ensuremath{\sim}1\mathrm{ps}.$ These results are rationalized by extending the hydrodynamic model to also take into account the force that the light field exerts on the polarization charge that is induced on the surface of the cluster. We show that the magnitude of this electrostrictive force is comparable to those of the Coulombic and hydrodynamic forces, and it exhibits resonance behavior. Contrary to the findings of the only other earlier report, we find that the low-energy component in the electron energy distribution is connected to the resonance in energy absorption by the cluster. The high-energy component seems to be produced by a mechanism that is not so strongly influenced by the resonance.

Probe measurements of ion energy distribution in magnetized plasmas

A probe method for measurement of the ion energy distribution in magnetized plasmas is proposed. A cylindrical probe with end plugs is oriented parallel to the magnetic field and used to reduce the electron contribution to the total probe current. The contribution of ions to the second derivative of the total current is identified by experiments with different probe orientation and strength of the magnetic field. The ion density obtained from the measured ion energy distribution is in good agreement with the electron density obtained from measured electron energy distribution.

Reducing influence of ion current on measurements of electron energy distributions in collisional plasmas

A method for reducing the influence of ion current on probe measurements of electron velocity distributions in plasmas has been developed and tested in noble gas afterglow plasmas. It is valid for diffusive transport of ions to the probe. Experiments have demonstrated high accuracy of the proposed method.

Pressure and helium mixing effects on plasma parameters in temperature control using a grid system

Pressure and He mixing effects on plasma parameters in electron temperature control using a grid system are investigated. Electron temperature is higher in lower pressure, when the electron temperature is high and not controlled. Electron density can be increased by about three times by decreasing the source gas pressure from 20 to 1 mTorr, and by about two times by He mixing in the temperature controlled region (diffusion region), while the electron density is decreased in the source region. This electron density increase is mainly due to the increase of the high energy electron population, and the measured electron energy distribution functions clearly show this.

In situ diagnostics in plasmas of electronic-ground-state hydrogen molecules in high vibrational and rotational states by laser-induced fluorescence with vacuum-ultraviolet radiation

Detection of excited electronic-ground-state hydrogen molecules with v(") up to 13 in a magnetic multipole plasma source was performed for the first time by laser-induced fluorescence with vacuum-ultraviolet radiation. The measurements are taken after fast shutoff of the discharge current. The rovibrationally excited molecules live longer than the plasma background light so that the fluorescence light can be detected with good signal-to-noise ratio. Absolute level populations are measured as well as decay times. The theoretically predicted suprathermal population of the vibrational distribution is clearly identified. The H- density is calculated on the basis of the measured populations and the measured electron energy distribution function. It is in excellent agreement with the H- density measured by photodetachment.

The finite size effect in a planar inductively coupled plasma

Plasma parameters from the measured electron energy distribution are obtained with changing the chamber height at low pressure 2 mTorr. It is observed that electron density has a local peak at a certain chamber height while electron temperature decreases monotonously with increasing chamber height. The chamber height with the maximum electron density is shifted according to the bounce resonance condition when the driving frequency is changed. The electron kinetic model well agrees with the experiment. This shows that the electron density peak against the plasma size is due to the electron bounce resonance that has been theoretically discussed.

Dissociation of CO and formation of carbonate on a stepped, K-modified Cu(115) surface

It is demonstrated that CO dissociates on a potassium-modified Cu(115) surface at 125 K. The dissociation process is conditioned by both the presence of steps and an alkali metal coverage in the range of 0.4< θ K <1 ML. Carbonate is formed at 220 K via the Boudouard process CO+CO→CO 2+C and reaction with oxygen. Further annealing to 470 K results in decomposition of the carbonate with graphitic C and O atoms left on the surface. The evidence comes from synchrotron radiation-based measurements of the valence band electron energy distributions and high-resolution measurements of the K, C, and O core levels.

Field emission properties of carbon nanotubes

We have investigated the field emission properties of nanotube thin films deposited by a plasma enhanced chemical vapor deposition process from 2% CH4 in H2 atmosphere. Depending on the deposition of the metallic catalyst [Fe(NO3)3 in an ethanol solution or sputtered Ni] the nanotube films showed a nested or continuous dense distribution of tubes. The films consisted of multiwalled nanotubes (MWNTs) with diameters ranging from 40 down to 5 nm, with a large fraction of the tubes having open ends. The nanotube thin film emitters showed a turn-on field of less than 2 V μm−1 for an emission current of 1 nA. An emission site density of 10 000 emitters per cm−2 is achieved at fields around 4 V μm−1. The emission spots, observed on a phosphorous screen, show various irregular structures, which we attribute to open ended tubes. A combined measurement of the field emitted electron energy distribution (FEED) and the current-voltage characteristic allowed us to determine the work function at the field emission site. In the case of the MWNT thin films and arc discharge grown MWNTs we found work function values around 5 eV, which agree well with the global work function of 4.85 eV we determined by photoelectron spectroscopy. From the shape of the FEED peaks we can conclude that the field emission originates from continuum states at the Fermi energy, indicating the metallic character of the emission site. In the case of single-walled nanotubes we found significantly lower work function values of around 3.7 eV compared to those of MWNTs. We attribute this to a size dependent electrostatic effect of the image potential, which lowers the work function for small (&amp;lt;5 nm) structures.

Experimental Investigations on Ne/CF4 Inductively Coupled Discharges

A Langmuir probe measurement was applied to an inductively-coupled RF (13.56 MHz) Ne/CF4 discharge over a pressure range from 20 mTorr to 60 mTorr while changing the CF4 content from 5 to 20%, and keeping the power injected into the plasma at about 100 W. The measured electron energy distribution function (EEDF) under any condition deviated from the Maxwellian distribution in the energy region higher than the lowest excitation threshold energy of Ne. The average electron energy, electron density and dissociation rate coefficient estimated using the measured EEDF decreased slightly with increasing CF4 content. The EEDF and the dissociation rate coefficient in Ar/CF4 and Ne/CF4 discharges under an equally injected power were compared. The number of electrons with energy higher than the dissociation threshold energy in Ne/CF4 were much larger than that in Ar/CF4, yielding a difference in dissociation rate coefficient of one order of magnitude.

Field emission properties of nanocrystalline chemically vapor deposited-diamond films

We have deposited nanocrystalline diamond films on p-type Si(100) substrates using plasma enhanced chemical vapor deposition (CVD). The diamond films were deposited at substrate temperatures between 950 and 980 °C using a high methane concentration of 5% in H2. The films obtained showed good field emission properties with threshold fields of around 5 V μm−1 (for 1 nA emission current). X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy showed that the nanocrystalline films still exhibit the basic electronic features of diamond with a band gap of 5.5 eV and a negative electron affinity when the surface is hydrogen plasma treated. The Fermi level position in these films is found to be 1±0.2 eV above the valence band maximum. The energy resolved field emission measurements show the typical asymmetric peak shape of Fowler-Nordheim (FN) tunneling through a surface potential barrier. The electrons emitted originate from a continuum of electronic states at the Fermi energy of the emitter. From a combined measurement of the field emitted electron energy distribution and the field emission I–V characteristic of an emitter we could independently determine the work function and the local electric field present at the emission site. In the case of nanocrystalline CVD diamond emitters we determined work function values around 5.7 eV and local fields in the range of 2000–3000 V μm−1 (for emission currents of 10–1000 pA). The corresponding field enhancement factors can range from 250 to 1700. Simultaneous field and photoelectron emission spectroscopy showed no indication of field penetration. Deviations from the FN law in the high current regime of the I–V plots may be related to an internal resistance of the emitter.

Structure observed in measured electron energy distribution functions in capacitively coupled radio frequency hydrogen plasmas

Electron energy probability functions measured with a passively compensated Langmuir probe in asymmetric capacitively coupled hydrogen and deuterium plasmas exhibit structure. The otherwise relatively continuous distribution appears to have an abrupt peak in electron density near 5 eV. This structure occurs at a higher energy in deuterium than hydrogen and there is a correlation between floating potential and the voltage at which the structure is observed in the second derivative of the I(V) characteristic. While the cause of the structure has yet to be clarified, spectroscopic observations and computer-based hydrogen models indicate that the high energy tail of the distribution is strongly modulated during the radio frequency cycle. The effect of this modulation on plasma properties and probe measurements has yet to be explored.

Imaging energy analyzer for RHEED: energy filtered diffraction patterns and in situ electron energy loss spectroscopy

An imaging energy analyzer for the filtering of diffraction diagrams and for the measurement of electron energy distribution has been developed. It allows the separation of the inelastic and elastic contribution of the scattered electrons in the diffraction pattern, which are energy filtered and monitored at with video speed using a CCD camera. The inelastic diffuse background can be eliminated which is helpful for accurate structure determination. The energy resolution is excellent (Δ E/ E≈10 −4) and energy loss distributions are measured providing meaningful chemical information about the interface chemical composition as electron energy losses are sensitive to the chemical state of the surface. The capabilities of the new energy analyzer are illustrated through the analysis of SrTiO 3, GaAs and InP surfaces. In the case of the III–V substrates, it is demonstrated that the surface oxide desorption can be monitored by in situ RHEED associated EELS.

Cold emission characterization using secondary electron emission spectroscopy

Secondary electron emission spectroscopy is used to study the electron transport and emission properties of wide bandgap materials (e.g., diamond, BeO). The secondary electron distribution generated in diamond is found to be dominated by a high concentration of very low-energy electrons, and the emission characteristics of these low-energy electrons is used to deduce information about the cold emission properties of the material. In particular, analysis of the extremely high yields measured from diamond samples indicates that the low-energy electrons have long escape depths in the material. Furthermore, electron energy distribution measurements provide insight into the emission process at various surfaces. The electron transport and emission model inferred from the diamond measurements suggests that diamond and other wide bandgap materials may be promising cold emitter materials, in which case secondary electron emission spectroscopy can be a useful characterization tool.

Probe measurements of electron energy distributions in a strongly magnetized low-pressure helium plasma

Electron energy distributions (EED) in strongly magnetized (∼0.3 T), low-pressure (∼0.2 Pa) helium plasmas of the toroidal device “Blaamann” [K. Rypdal et al., Plasma Phys. Controlled Fusion 36, 1099 (1994)] have been measured. In the analysis of measurements, one applies a simplified expression for the limit of a strongly magnetized plasma relating the electron energy distribution to the first derivative of electron probe current with respect to the probe potential. It is shown that for the conditions investigated this approximation gives the same electron densities and slightly lower temperatures (up to 10%) as the kinetic theory for arbitrary magnetic field strength. Cylindrical probes, which are oriented along and perpendicular to the magnetic field, are used in the measurements. It is shown that these probes give nearly identical results. However, the probe that is oriented perpendicular to the magnetic field can give the electron energy distribution in a wider energy range and with better accuracy. The cross-field diffusion coefficient of electrons near the probe is estimated and shown to be classical.