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Normalized intensity of the Ti I 453.324 nm emission line in the direction parallel to the target surface against distance to the target, at the beginning and the end of the discharge pulse. Dark grey bars indicate the magnetron (left) and the anode cover (right). The light grey areas indicate data points affected by vignetting due to the extent of the field of view. Data points are connected by lines to guide the eye. Dotted lines are exponential decay fits to the data points not affected by vignetting.
Source publication
The velocity distribution function of titanium neutrals in the target region of a high power impulse magnetron sputtering discharge was investigated by optical emission spectroscopy. A high-resolution plane grating spectrograph combined with a fast, gated, intensified CCD camera was used to study the shape of selected optical emission lines. Dopple...
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Citations
... The peak power in the HiPIMS discharge was 25 kW with a peak power density of 1.25 kW cm −2 and an averaged power density of 2.4 W cm −2 . Within the accuracy of the electrical measurements, the current and voltage time traces are identical to the ones obtained without the Cr insert, published in a previous article [48] (590 V case). The DCMS discharge was operated at 290 V and 65 mA resulting in a power of 19 W and a power density of about 1 W cm −2 . ...
... Measurements of the rotational ion velocity by optical emission spectroscopy were performed, as described previously [15,48]. The light emission of the plasma was coupled into an optical fiber (diameter of 880 µm) using a lens (focal length of 150 mm). ...
... The chromium neutral line was used to determine the instrumental profile of the spectrometer and the reference wavelength from the HCL emission. The determination of the velocity distribution function (VDF) from the emission lines was performed, as described by Held et al [48]. The emission lines in HiPIMS discharges are mainly broadened by Doppler broadening and instrumental broadening. ...
The transport of sputtered species from the target of a magnetron plasma to a collecting surface at the circumference of the plasma is analyzed using a particle tracer technique. A small chromium insert at the racetrack position inside a titanium target is used as the source of tracer particles, which are redeposited on the collecting surface. The azimuthal velocity of the ions along the racetrack above the target is determined from the Doppler shift of the optical emission lines of titanium and chromium. The trajectories are reconstructed from an analysis of the transport physics leading to the measured deposition profiles. It is shown that a simple direct-line-of sight re-deposition model can explain the data for low power plasmas (DCMS) and for pulsed high power impulse magnetron plasmas (HiPIMS) by using the Thompson velocity distribution from the sputter process as starting condition. In the case of a HiPIMS plasma, the drag force exerted on the ions and neutrals by the electron Hall current has to be included causing an azimuthal displacement in \ExB direction. Nevertheless, the Thompson sputter distribution remains preserved for 50\% of the re-deposited growth flux. The implications for the understanding of transport processes in magnetron plasmas are discussed.
... The discharge was monitored by current and voltage measurements with commercial probes (Tektronix TCP A400, Tektronix P6015A) attached to the connection cable between the power supply and the magnetron assembly. Discharge conditions were selected to be the same as in earlier publications [20,36]. The applied voltage was −590 V with a repetition frequency of 40 Hz and a pulse length of 100 µs. ...
... Ti and peak power densities of 1.1 kW cm −2 . The corresponding voltage and current waveforms can be found in a previous publication [36]. ...
... The determination of the VDF from the emission lines was performed as described by [36]. The method is based on the analysis of the two dominant line broadening mechanisms, Doppler broadening and instrumental broadening. ...
Magnetron sputtering discharges feature complex magnetic field configurations to confine the electrons close to the cathode surface. This magnetic field configuration gives rise to a strong electron drift in azimuthal direction, with typical drift velocities on the order of \SI{100}{\kilo\meter\per\second}. In high power impulse magnetron sputtering (HiPIMS) plasmas, the ions have also been observed to follow the movement of electrons with velocities of a few \si{\kilo\meter\per\second}, despite being unmagnetized. In this work, we report on measurements of the azimuthal ion velocity using spatially resolved optical emission spectroscopy, allowing for a more direct measurement compared to experiments performed using mass spectrometry. The azimuthal ion velocities increase with target distance, peaking at about \SI{1.55}{\kilo\meter\per\second} for argon ions and \SI{1.25}{\kilo\meter\per\second} for titanium ions. Titanium neutrals are also found to follow the azimuthal ion movement which is explained with resonant charge exchange collisions. The experiments are then compared to a simple test-particle simulation of the titanium ion movement, yielding good agreement to the experiments when only considering the momentum transfer from electrons to ions via Coulomb collisions as the only source of acceleration in azimuthal direction. Based on these results, we propose this momentum transfer as the primary source for ion acceleration in azimuthal direction.
... At linear plasma devices, such as PSI-2, or in magnetron sputtering discharges, high-resolution emission spectroscopy, laser absorption spectroscopy and laser-induced fluorescence resolve the shape of the radiative transitions by sputtered atoms [26][27][28][29]. Also, the measurements of polarization properties of metallic mirrors using emission properties of the backscattered atoms rely on the theoretical description of the measured line shapes [30]. ...
High-resolution emission spectroscopy provides valuable information on the physical sputtering process during plasma-wall interaction. Up to now, analyzing the observed spectral lines during sputtering did not account for the finite size of the targets. It becomes crucial if the size of the target becomes comparable with the distance the sputtered atoms travel before emitting the photons. So, for example, the generally used standard emission model based on an infinite target or the point source approximation breaks for observations using two lines of sight: parallel and perpendicular to the normal of the target. It is impossible to achieve consistent results for energy and angular distribution of sputtered atoms.
The new space-resolved emission model for finite-size targets developed in this work removes this gap. It incorporates the space-velocity transformation for the distribution function and includes the finite lifetime of excited states. The model was validated using emission spectra of sputtered atoms from a polycrystalline tungsten sample bombarded by monoenergetic Ar+ with kinetic energies of 100 eV to 140 eV at normal incidence in the linear plasma device PSI-2. Using the new model enables the simultaneous fitting of the line shapes of sputtered tungsten for both observation angles. The optimization process is performed using the standard Thompson distribution by separating the energy-dependent parameter and the angular distribution.
... where a and b are the fitting parameters, E msb is the modified surface binding energy, E i is the ion energy and Λ is the energy transfer factor. The above equation was selected as a fitting relation between the average atom energy and ion energy because it is obtained from the Sigmund distribution when evaluating the average energy of sputtered atoms (see Appendix of [41]). The Equation is discussed in more detail in Section 4. We introduce two fitting parameters, a and b, to fit the average energies of sputtered atoms. ...
... The integration is performed up to the maximum atom energy (i.e., E max = ΛE i ). Detailed evaluation of the integrals can be found in the Appendix of Ref. [41]. With several reasonable simplifications, the following equation is obtained for the average energy of sputtered atoms: ...
The energy of the sputtered atoms is important to control the microstructure and physical properties of thin films. In this work, we used the SRIM program to simulate the energy of sputtered atoms. We analyzed the energy distribution functions (EDFs) and the average energies of the atoms in different spatial directions for a range of target materials and Ar ion energies. The results were compared to the analytical equations for EDFs derived by Sigmund and Thompson and with experimental data from the literature. The SRIM simulations give realistic EDFs for transition metals, but not for elements lighter than Si. All EDFs show a low-energy peak positioned close to one-half of the surface binding energy and a high-energy tail decreasing as approximately E−2. We analyzed the characteristics of EDFs, specifically, the position of low- and high-energy peaks, FWHM, and the energy tail, with respect to the ion energy and position of the element in the periodic table. The low-energy peak increases with atomic number for elements within each group in the periodic table. Similar changes were observed for FWHM. For the period 5 and 6 elements, additional broad high-energy peaks were observed at emission angles above 45° when sputtered by Ar ions with 300 eV and also in some heavier elements when bombarded by 600 eV and 1200 eV ions. The transition metals in groups 4, 5, and 6 in periods 5 and 6 have the highest average energies, while the lowest average energies have elements in group 11. The results of simulations show that the average energies of sputtered atoms were inversely proportional to the sputtering yield, i.e., the higher the sputtering yield, the lower the average energy of sputtered atoms. We established an empirical equation for transition metals to estimate the average energy of sputtered atoms from the sputtering yield. The angular distribution of the average atom energy depends on the atomic number. Transition metals with 22 < Z < 72 have an anisotropic energy distribution, with the highest average energies in the 40°–70° range. For the elements in group 11, the angular distribution of the average energies is more isotropic.
... From the ionization frequency, we can calculate the ionization time τ iz = 1/ν iz , which describes how long freshly sputtered neutrals will travel through the plasma before being ionized. For titanium, we find a time of τ iz = 0.5 µs, which corresponds to about 0.75 mm, when assuming a mean velocity of 1500 m s −1 [54]. This is in excellent agreement to the length of 0.5 mm that we estimated from the optical measurements, especially considering that we measured the electron density in 8 mm distance from the target and we expect larger densities (and, thus, a smaller τ iz ) closer to the target surface, as recently observed by Dubois et al [55]. ...
The ionization of sputtered species in high power impulse magnetron sputtering (HiPIMS) of titanium, chromium, and aluminium targets is analyzed using Abel-inverted spectroscopic imaging to locate the position of ionization. From the spatial emission of neutrals, it is deduced that most of the sputtered titanium particles become ionized within 0.5 mm distance from the target, whereas sputtered aluminum or chromium can travel much further through the discharge before ionization occurs. Probe measurements reveal the reason for this difference to be the unusually high electron temperature of around 4.5 eV for titanium compared to 2.6 eV and 1.5 eV for aluminum and chromium as the target material, respectively. These probe measurements are then compared to a global model derived from the ionization region model (IRM). Excellent agreement between model and measurements can be reached, but only if the transport physics for the confinement of the species is adjusted. Using the model, the difference between the three discharges can be traced back to be mostly caused by the sputter yield. Thus, we propose that ionization in discharges with low-yield materials should generally be expected to occur closer to the target surface, leading the ions to be affected more strongly by the electric field across the magnetic trap region, resulting in a more severe deposition rate loss compared to high-yield materials.
... The peak power in the HiPIMS discharge was 25 kW with a peak power density of 1.25 kW/cm 2 and an averaged power density of 2.4 W/cm 2 . Within the accuracy of the electrical measurements, the current and voltage time traces are identical to the ones obtained without the Cr insert, published in a previous article [39] (590 V case). The DCMS discharge was operated at 290 V and 65 mA resulting in a power of 19 W and a power density of about 1 W/cm 2 . ...
... Measurements of the rotational ion velocity by optical emission spectroscopy were performed, as described previously [13,39]. The light emission of the plasma was coupled into an optical fiber (diameter of 880 µm) using a lens (focal length of 150 mm). ...
... The determination of the velocity distribution function (VDF) from the emission lines was performed, as described in [39]. The emission lines in HiPIMS discharges are mainly broadened by Doppler broadening and instrumental broadening. ...
The transport of sputtered species from the target of a magnetron plasma to a collecting surface at the circumference of the plasma is analyzed using a particle tracer technique. A small chromium insert at the racetrack position inside a titanium target is used as the source of tracer particles, which are redeposited on the collecting surface. The azimuthal velocity of the ions along the racetrack above the target is determined from the Doppler shift of the optical emission lines of titanium and chromium. The trajectories are reconstructed from an analysis of the transport physics leading to the measured deposition profiles. It is shown that a simple direct-line-of sight re-deposition model can explain the data for low power plasmas (DCMS) and for pulsed high power impulse magnetron plasmas (HiPIMS) by using the Thompson velocity distribution from the sputter process as starting condition. In the case of a HiPIMS plasma, the drag force exerted on the ions and neutrals by the electron Hall current has to be included causing an azimuthal displacement in \ExB direction. Nevertheless, the Thompson sputter distribution remains preserved for 50\% of the re-deposited growth flux. The implications for the understanding of transport processes in magnetron plasmas are discussed.
... The discharge was monitored by current and voltage measurements with commercial probes (Tektronix TCP A400, Tektronix P6015A) attached to the connection cable between the power supply and the magnetron assembly. Discharge conditions were selected to be the same as in earlier publications [36,20]. The applied voltage was -590 V with a repetition frequency of 40 Hz and a pulse length of 100 µs. ...
... Using titanium targets, these values result in peak currents of 50 A, peak target-area-normalized current densities of 2.5 Acm −2 and peak power densities of 1.1 kWcm −2 . The corresponding voltage and current waveforms can be found in a previous publication [36]. ...
... The determination of the velocity distribution function (VDF) from the emission lines was performed as described in [36]. The method is based on the analysis of the two dominant line broadening mechanisms, Doppler broadening and instrumental broadening. ...
Magnetron sputtering discharges feature complex magnetic field configurations to confine the electrons close to the cathode surface. This magnetic field configuration gives rise to a strong electron drift in azimuthal direction, with typical drift velocities on the order of \SI{100}{\kilo\meter\per\second}. In high power impulse magnetron sputtering (HiPIMS) plasmas, the ions have also been observed to follow the movement of electrons with velocities of a few \si{\kilo\meter\per\second}, despite being unmagnetized. In this work, we report on measurements of the azimuthal ion velocity using spatially resolved optical emission spectroscopy, allowing for a more direct measurement compared to experiments performed using mass spectrometry. The azimuthal ion velocities increase with target distance, peaking at about \SI{1.55}{\kilo\meter\per\second} for argon ions and \SI{1.25}{\kilo\meter\per\second} for titanium ions. Titanium neutrals are also found to follow the azimuthal ion movement which is explained with resonant charge exchange collisions. The experiments are then compared to a simple test-particle simulation of the titanium ion movement, yielding good agreement to the experiments when only considering the momentum transfer from electrons to ions via Coulomb collisions as the only source of acceleration in azimuthal direction. Based on these results, we propose this momentum transfer as the primary source for ion acceleration in azimuthal direction.
... Moreover, these measurements have numerous advantages compared to other optical diagnostics: (a) they allow probing of the ground state of the sputtered atoms in dcMS, (b) they are highly space and spectrally resolved, and (c) they allow measurement of the flux and energy of sputtered atoms at the substrate position in front of the target. Other optical plasma diagnostics can be found in the literature, allowing the sputtered atom velocity distribution function (AVDFs) description [17][18][19]. ...
In this study, the energy flux of sputtered atoms on a substrate was correlated to the properties of titanium nitride (TiN) films deposited using direct current magnetron sputtering (dcMS) under mixed Ar and N2 atmospheres. The neutral titanium sputtered atoms velocity distribution func tions (AVDFs) were measured by tunable diode-laser induced fluorescence (TD-LIF), and the flux of particles and their energy were derived. Mass spectrometry was used to characterize the energy-resolved flux of the ions. It was found that the neutral sputtered atoms flux and deposi tion rate were in good agreement, indicating that the flux of the neutral titanium ground state represents the number of deposited atoms. Moreover, TiN films were deposited at different gas pressures and at various Ar/N2 gas mixtures close to the conditions where stoichiometric TiN was formed, without bias voltage and heating of the substrates. The energy flux of the sputtered neutral Ti into the substrate was calculated from TD-LIF measurements. At a relatively low magnetron discharge pressure of 0.4 Pa, we demonstrated that the energy of sputtered neutral Ti impinging on the substrate is higher than the energy flux of ionized particles corresponding mainly to Ar +. Thus, the influence of the energy flux of the sputtered atoms on the texture and microstructure of the films is revealed. The (200) texture was obtained at 0.4 Pa when the energy flux of the sputtered atoms was higher than the ion energy flux. At 1.3 Pa where the sputtered atoms energy flux is one order lower compared to 0.4 Pa the (111) texture was obtained. The high-energy flux of the ground state of Ti sputtered atoms seems to allow stress removal in the films. 28 Keywords: dcMS, diode laser-induced fluorescence spectroscopy, TiN microstructure and texture.
... Such challenges are likely to persist at some distances outside the magnetic trap, as measurements [13] at the separatrix seem to imply. Similar problems can be anticipated when measuring the ion movement using optical emission spectroscopy or laser induced fluorescence [51,52]. ...
Spokes are long wavelength oscillations observed in the magnetized region of direct current magnetron sputtering (DCMS), high power impulse magnetron sputtering (HiPIMS), as well as other E x B discharges. Spokes rotate in front of the cathode with velocities between about 2 km/s and 15 km/s, making it difficult to perform quantitative measurements. This is overcome by synchronizing Langmuir probe measurements to the movement of spokes in DCMS to obtain the probe current-voltage (I-V) characteristic without averaging out the spoke influence. The I-V curves are then evaluated using magnetized probe theory, revealing the strong plasma parameter modulations, caused by the spokes. The plasma density was found to oscillate between 2.5 × 10 ¹⁶ m ⁻³ and 1.7 × 10 ¹⁷ m ⁻³ , which corresponds to a modulation strength of more than 70 % or an almost seven times increase of density. In good agreement with previous work, a plasma potential minimum of −55 V is found ahead of the spoke followed by a sudden increase to about 2 V inside the spoke. The electron temperature was found to oscillate between 3 eV and 7 eV. On top of that oscillation, electrons experience a sudden energy increase as they move inside the spoke, crossing the potential jump at the leading edge for the spoke. On basis of these observations a model is presented to explain spokes in DCMS. These results are then compared to HiPIMS spokes under otherwise similar conditions. The plasma parameter modulation found for HiPIMS is much weaker than for DCMS, which is explained by the higher collision frequency for electrons in HiPIMS plasmas.
... However, this should play no significant role here. In high power magnetron sputtering discharges (HIPMS) where clearly higher magnetic field strengths (up to 180 mT) occurs, Zeeman pattern was estimated to be still negligible [32]. Thus, under the assumption of almost similar Landé-g factor, Zeeman pattern is also negligible here. ...
The electric field is a fundamental parameter for plasma sources and devices. Its knowledge is a dominant setscrew for many processes such as controllable fluxes and energies of charged particles onto surfaces and for the electron energy distribution function. However, experimental data of electric field strengths in micro-structured surface dielectric barrier discharges are rare. Due to geometric configurations and dimensions in micrometer scale, probe-based investigations are challenging. To tackle these issues, we exploit the optical access into micro cavities of a plasma array to use the Stark effect of the allowed 492.19 nm (1D ?→1P o) and forbidden 492.06 nm helium line (1F o ?→1P o). Based on it, we present spatially-integrated and time-resolved electric field strengths in a range between 20 kV cm-1and 60 kV cm-1depending on various parameters such as cavity diameters in 100 µm range and excitation properties. The obtained electric fields can be controlled just by bipolarity of applied voltage and show a good agreement to previous simulated field strengths in pore and silicon-based devices. As expected from simulation dealing with discharges in pores, a smaller cavity dimension yields higher electric field strengths. Due to these high electric fields and the option of this plasma source to easily integrate a catalyst in the discharge volume, this micro cavity plasma array promises further insights into plasma-enhanced catalysis.
