Three papers about HiPIMS and mass spectrometry


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Internal conference about three papers: Takagi 2006, Jouan 2010 and Greczynski 2010. High Power Impulse Magnetron Sputtering technique is analyzed. Conference developed at Institut des Matériaux Jean Rouxel de Nantes (France).

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Three papers about HiPIMS and mass spectrometry

  1. 1. Three papers about HiPIMS and Mass Spectrometry Javier García Molleja In collaboration with: OAxel Ferrec OPierre-Yves Jouan OJulien Keraudy
  2. 2. Generation processes of super-high- energy atoms and ions in magnetron sputtering plasma O Y. Takagi, Y. Sakashita, H. Toyoda, H. Sugai O Vacuum 80 (2006) 581–587
  3. 3. 1. IntroductionO It is essential to have a better understanding of a surface process of the sputtering target, growing films, and gas- phase processes in the magnetron plasma.O The surface qualities of sputtered films are influenced by incidence of particles having kinetic energies much higher than bond energies.
  4. 4. 2.ExperimentO A planar unbalanced magnetron discharge is made in a cubic metal vessel, 0.2 m in dimension, which is evacuated with a turbomolecular pump of 200 l/s.O Rare gas (Ar, Kr or Xe) is fed into the vessel through a variable leak valve at a pressure of 0.3–0.7 Pa.O The discharge was maintained by a DC power (VD~600 V, ID<0.2 A) applied to a 5-cm- diameter cathode with a grounded coaxial cylinder anode.
  5. 5. 2. ExperimentO A sputter target of tungsten or permalloy (80% Ni/20% Fe) was set on the cathode.O Surface magnetic flux densities were ~600 and ~800 G for target materials of permalloy and tungsten, respectively.O Ion EDF was measured at a substrate position, z = 0.1 m away from the target surface.O QMA with an energy filter of resolution ~0.5 eV.O By a movable spherical Langmuir probe 1 mm in diameter made from platinum 2D profile plasma density and electron temperature were obtained.
  6. 6. 2. ExperimentO Although the observed peak density was ~1016 m-3 at z~0.01 m, the maximum density may be much higher near the target (Te~10 eV).O Plasma density of 3·1014 m-3 at the position of the QMA orifice (z = 0.1 m). Electron temperature ~2 eV.O Maxima configure a ring pattern.
  7. 7. 2. Experiment O p=0.4 Pa, I=0.1 A. O Corrected taking account of ion acceleration by a sheath at the orifice and the difference in work functions.
  8. 8. 2. ExperimentO Ar (M = 39.9), Kr (M = 83.8), and Xe (M = 131.3).
  9. 9. 2. ExperimentO High kinetic energies often observed in EDF measurements are considered to originate from backscattering processes on the target.O The maximum kinetic energies Ebmax given by a two-body collision model in the first approximation (head-on collision):
  10. 10. 3. SimulationO Kinetic energies of fast Ar atoms backscattered from the target surface were simulated by the TRIM code.O 105 argon ions with energy 200–600 eV were perpendicularly injected onto the flat target (permalloy, Al, W).O Ion positive charge was assumed to be neutralized on the target metal surface.O Eventually a part of injected ions were ejected, as backscattered atoms, from the target surface whose kinetic energies and directions were calculated by the TRIM code and tabulated in advance.
  11. 11. 3. SimulationO Tungsten (M = 183.9), permalloy (M = 58.13), and aluminum (M = 26.98).O Ebmax coincided with the shoulder of the EDF curve.
  12. 12. 3. Simulation
  13. 13. 3. SimulationO The Ar+ energy distribution measured at the position of the QMA was simulated in Monte Carlo method using the EDF of backscattered Ar atom in the TRIM code.O Superhigh-energy ions were found at a position 0.1 m away from the target, which was attributed to backscattered fast argon atoms (Ar(fast)).O There are three ionization processes of backscattered Ar(fast).
  14. 14. 3. SimulationO Electron–atom ionization, Ar(fast)+e -> Ar(fast)++2e.O Charge exchange, Ar(fast)+Ar(slow)+ -> Ar(fast)++Ar(slow).O Atom–atom ionization, Ar(fast)+Ar(slow) -> Ar(fast)++Ar(slow)+e (a slow atom is also ionized with the same probability as the fast atom).O Ar+ ions are lost in gas phase through the charge exchange process: Ar(fast)++Ar(slow) -> Ar(fast)+Ar(slow)+.
  15. 15. 3. SimulationO In the experimental conditions (na = 1.0·1020 m-3, np = 4·1014 m-3, Te = 2 eV) and plotted as a function of the impact energy, assuming the slow particle to be at rest. For reference, the mean free path ael of fast Ar atom for momentum transfer collision with slow Ar atoms was indicated.O The dominant ionization mechanism corresponds to the shortest ionization free path.O Above the ionization threshold energy (15.75 eV), the fast Ar atom is dominantly ionized by atom–atom collisions. Below the threshold, however, the ionization occurs mainly by charge exchange collisions. Electron impact ionization is always negligible.
  16. 16. 3. Simulation
  17. 17. 3. SimulationO The high-energy tail caused by backscattered fast Ar atoms were simulated in the Monte Carlo method.O The box was filled with slow Ar atoms and a plasma composed of slow Ar+ ions and electrons.O When particles collided with the walls, they were absorbed.O We injected fast Ar atoms (backscattered Ar atoms) from the origin.O The ions and atoms arriving at a substrate (0.03 m X 0.03 m) on the wall at z = 0.1 m were counted memorizing the kinetic energies.O In the case of the momentum transfer collisions, the scattering angle was determined in the center of mass coordinate taking account of the angular dependence of differential cross section.O Velocities after collisions were calculated from energy and momentum conservation and were transformed to the laboratory frame.
  18. 18. 3. SimulationO If the high-energy particle bombardment should be relevant to sputter film damage, it would be caused by injection of fast atoms rather than fast ions.
  19. 19. 4. Comparison betweenexperiment and simulationO Fast Ar+ ion flux passing through the QMA orifice can be expressed.O The low-energy majority ion flux is approximately given by the Bohm flux.O Enables a quantitative comparison of the simulation results with the measured results on the high-energy tail of EDF.O Good agreement between the simulation results and the measured values was obtained not only in the relative shape of EDF but also in the quantitative values.O The population of fast Ar+ ion for tungsten target was about ten times higher than for the permalloy case.
  20. 20. 5. SummaryO High-energy particle incidence on a substrate significantly affects qualities of deposited films.O When the mass number of target material was larger than that of rare gas, a fast ion tail in the EDF was observed in a range of super-high energies (>100 eV), with a majority of slow ions.O According to the simulation, the backscattered rare gas atoms of super-high energies are converted to fast ions, 1% at most, by atom–atom collisions in the gas phase; however, the rest of them impinge on the substrate as fast atoms.O Guideline for reducing the high-energy particle bombardment on the substrate is proposed.
  21. 21. HiPIMS Ion Energy DistributionMeasurements in Reactive Mode O Pierre-Yves Jouan, Laurent Le Brizoual, Mihaï Ganciu, Christophe Cardinaud, Sylvain Tricot, and Mohamed-Abdou Djouadi. O IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 38, NO. 11, NOVEMBER 2010
  22. 22. 1. IntroductionO Aluminum nitride is of great interest due to its unique properties.O Its piezoelectric behavior and its high acoustic wave velocity permit excellent acoustic wave applications (SAW and BAW).O Parameters like the magnetron configuration (balanced or unbalanced), the magnetic field distribution, or even the substrate bias have not been so widely investigated.O Setting the substrate to its self-biased potential can change the energy of ions bombarding the substrate.O Due to the low ionization rate of the conventional magnetron systems, only few species (ions) are controlled, and therefore, the detrimental role of energetic neutrals cannot be avoided.
  23. 23. 2. ExperimentalO A 50-mm-diameter and 6-mm-thick aluminum target was mounted on this cathode.O The deposition of the AlN thin films is done in the nitrided mode of the target (N2/(Ar + N2) = 25% to 35%).O The target is first cleaned in a 100% argon atmosphere at 0.27 Pa for 15 min to remove any previous nitride or oxide layer on the Al target.O Pressure is adjusted down to 0.27 Pa. The target is nitrided for 15 min. The total gas mass flow rate is 40 sccm.
  24. 24. 2. ExperimentalO The power applied to the target in the dc reactive magnetron sputtering is 150 W.O The applied voltage in HiPIMS is −1 kV, the pulse width is 28 μs, and the frequency (repetition rate) is 1.6 kHz.O Mass spectrometer from Hiden (EQP 1000). The spectrometer is differentially pumped by a turbomolecular pump allowing a lower pressure limit of 3.7 × 10−6 Pa. The mass spectrometer aperture is located in front of the target at a distance of 30–100 mm.O The spectrometer is referenced to the ground (0 V). Nevertheless, in order to optimize the species collection, the extractor potential was varied.
  25. 25. 2. ExperimentalO The substrate is replaced by a conducting plate of 30 mm in diameter located in front of the spectrometer aperture and isolated from the ground potential. A 380-μm slit is located at the middle of this plate to allow plasma species to get through and reach the spectrometer aperture. When the plasma is ON, the plate reaches the floating potential (Vf).
  26. 26. 2. ExperimentalO Vext=0 V, Vaxis=-40 V, Vtr=3 V, Vdyn=-4000 V.O These values lead to a transit time of 89 μs for Ar+, 74 μs for N2+, 72 μs for Al+, and 52 μs for N+. The difference in the transit time for an ion at 1 and 100 eV ranges from 2.5 to 3.6 μs for the investigated ions. Therefore, the gate width for the acquisition was set to 4 μs.
  27. 27. 3. ResultsO The IED of a dc magnetron sputtering discharge was performed: power of 150 W and a pressure of 0.27 Pa in a mixture of 75% Ar + 25% N2 gas.O The spectrometer head was positioned in front of the target at a distance of 30 mm and was grounded.
  28. 28. 3. ResultsO Two ion populations are visible on the IED of Al+ and N+. The low-energy peak corresponds to low-energy ions coming from the gas plasma (Ar and N2). The second peak and the broad energy tail result from energetic sputtered neutrals ionized by the electron impact during their transport between the target and the mass spectrometer head.O However, no extraction potential is applied to attract ions inside the spectrometer (grounded configuration). When the extractor is referenced to a negative potential, more low- energy ions are attracted inside the spectrometer.O The low-energy part of the curve is enhanced, but the energetic tail remains unchanged.O It is worth to notice that, as only argon is used, the Al+ distribution is wider than that obtained with a mixture of Ar/N2.
  29. 29. 3. ResultsO The added plate with the slit was therefore grounded or let at a floating potential.O Curves are normalized to their area. The IED is broader, and the energetic tail slightly extends at higher energies in the floating case. In fact, when the substrate becomes floating from grounded, the difference between the plasma potential and the substrate potential increases from 2 to 30 V which leads to a 1.5 times thicker sheath.O A floating potential or a slight bias voltage (−10 to −20) improves significantly the crystalline quality of AlN films.O A conventional magnetron sputtering plasma implies that most of the incoming species at the substrate surface are neutrals. Energetic backscattered neutrals could not be controlled.
  30. 30. 3. ResultsO The HiPIMS produces such a high amount of ions in the plasma due to an ionization rate of 10% to 70% for power densities around 1–3 kW/cm2 avoiding energetic neutrals.O For the HiPIMS, the mean plasma potential value is between 3 and 4 eV for 35% nitrogen content but does not vary significantly when the percentage of nitrogen is changed.
  31. 31. 3. Results O Experimental conditions (pulsewidth = 28 μs, repetition rate = 1.6 kHz, and applied voltage = −1 kV). O There is a preionization voltage (−300 V) in between the pulses. O The first peak minimum in the absolute value is associated with the primary electrons generated at the beginning of the discharge. O The shoulder that follows this first peak is enhanced in the reactive Ar+ − N2 case. O Secondary electrons are emitted by the target are responsible for the observed shoulders. O The secondary electron emission yield is higher for AlN (0.3 electron/ion) than for a pure Al target (0.08 electron/ion) and they sustaing the high negative floating potential for a longer time.
  32. 32. 3. ResultsO For a floating substrate located at 30 mm from the target, ionized Al+, N+, N2+ , and Ar+ were detected, including low signal from Ar2+ and Al2+.O The overall signal of each ion investigated is higher than that observed in the dc mode, particularly at a high energy.O Target species (Al+ and N+) have a larger energy tail of up to 70 eV for aluminum ions and up to 50 eV for N+ ions.O The ratio of the Al+/Ar+ signal reaches 0.71.O Low-energy peak correspond to thermalized ions in the plasma and of a higher energy tail.O The high-energy tail originates from sputtered neutrals (by self-sputtering and Ar+ bombardement) which are ionized on their way to the substrate.
  33. 33. 3. Results
  34. 34. 3. ResultsO The floating potential, which is referenced to the ground, varies during the pulse.O It is worth to notice that ions are detected after at least 20 μs, and therefore, the floating potential seen by ions is around −20 V or even less.O Moreover, the integrated measurements are done during a larger time than the pulse duration: the floating potential is mainly around zero.O The time-resolved measurements were performed to investigate the plasma composition during the impulse sequence. The HiPIMS discharge is dominated by Al+ ions.O A high power density at the target causes the gas to undergo very strong rarefaction and could explain the lower detected signal of Ar+.
  35. 35. 3. ResultsO For each ion, the signal increases quickly, shortly after the end of the pulse (28 μs), and is the maximum for a delay of 50 μs.O The collected ion current at the substrate is delayed from the beginning of the pulse. This delay can be explained by geometrical consideration, magnetic field configuration, target–mass-spectrometer first electrode distance etc.O The diffusion rate of the plasma can be estimated to ≈ 0.6 cm·μs−1. Diffusion is dominated by elastic scattering.O Most of the ion flux reaches the substrate at the end of the pulse and during the OFF time of the impulse sequence.O Despite these large measured energies, AlN films have better crystallinity in the HiPIMS than in the dc mode.O The rocking curve measurements of the (0002) diffraction peak indicate an FWHM of 1.6º in the dc reactive magnetron sputtering and an FWHM of 1.1° in the HiPIMS.O The momentum transfer seems to be lower than that of the energetic backscattered neutrals.
  36. 36. 4. ConclusionO The floating substrate configuration leads to an energetic tail in the dc configuration.O This one is sustained to a high negative value for a longer time when the target is nitrided due to the high secondary electron emission yield of AlN.O The main ionic species detected is Al+ whose intensity is 150 times higher than that in the dc mode.O The HiPIMS plasma show that most of the ionic flux reaches the substrate during the pulse OFF time.O Large fraction of energetic ions coming from the target have been used for momentum transfer, and better crystallized AlN films were obtained.
  37. 37. Time and energy resolved ion mass spectroscopy studies of the ion flux duringhigh power pulsed magnetron sputtering of Cr in Ar and Ar/N2 atmospheres O G. Greczynski, L. Hultman O Vacuum 84 (2010) 1159–1170
  38. 38. 1. IntroductionO In HiPIMS the high temporal electron density in the plasma results in an effective ionization of sputtered material.O Since the influence of ion bombardment during film growth on properties of resulting coatings is well proven it is of primary importance to understand what factors affect the ion energy distribution function (IEDF) in the case of HIPIMS processing.O HiPIMS produces a high intensity peak of low energy ions and more energetic ions produce a high-energy tail, significantly broader than for the DC discharge.O Metal and gas ions could be detected in the after glow plasma on the ms time scales, thus long after the current pulse was turned off.
  39. 39. 2. ExperimentalO Industrial CC800/9 coating system manufactured by CemeCon AG in Germany upgraded with the HIPIMS technology.O The base pressure in the vacuum chamber after the overnight bake-out was 4·10-5 Pa.O Chromium target of dimensions 88x500 mm2 was sputtered in Ar or Ar/N2 atmosphere at the total pressure of 0.4 Pa.O The cathode was operated in the frequency range between 100 Hz and 300 Hz. The average power was between 500 W and 4500 W, and the pulse duration was 200 s.
  40. 40. 2. ExperimentalO The spectrometer was facing the center of the target from the distance of 21 cm.O Sputtered atoms with an average energy of 10–30 eV are expected to make several collisions (get thermalized).O IEDFs were measured both in time-averaged and time- resolved mode with commercially available mass spectrometer PSM003 (from Hiden Analytical, UK), mass discrimination up to 300 amu with a 0.01 amu resolution.O The orifice of the spectrometer was grounded and aligned along the target surface normal.O The IEDF were often recorded for more than one isotope. Data presented in this paper are scaled by the abundance values.O Due to the fact that quadrupole mass analyzers transmit more at low mass than at high mass, the 1/mass transmission function is often assumed.O Data quantification in the absolute terms is not attempted due to the fact that the energy-dependent transmission function of our mass spectrometer is not known.
  41. 41. 2. ExperimentalO Special care has been taken to account for a potential signal drop caused by sputter-deposited material on the spectrometer orifice.O The target was sputtered for 60 s in pure Ar prior to such control scans. Following that, Cr+ and Ar+ spectra were recorded. The total signal drop after performing all measurements did not exceed a factor of 2.O The scanned energy range for singly charged ions was between 0 eV and 100 eV in 0.5 eV steps.O The detector gate window was in this case set to 10 s and a delay time with respect to the onset of the voltage pulse to the cathode was varied from 20 s up to 240 s in 10 s intervals. The total acquisition time per data point was 1ms. For singly charged ions, the ion energy was scanned between 0 eV and 50 eV in 1 eV steps.O For ions in the energy range between 0 and 100 eV, the TOF does not vary more than 10%.
  42. 42. 3. Results and discussionO The exact shape of current and voltage waveforms is to a great extent determined by the size of the capacitor bank.O The voltage is not constant throughout the pulse, but rather drops from the peak value. The resulting target current is thus proportional to the rate at which the voltage drops.O Between 0 s and 100 s discharge operates at typical HIPIMS conditions with large dynamic changes in both current and voltage, that later stabilize in the second phase (100–200 s) to resemble that of DC sputtering.
  43. 43. 3. Results and discussionO Current levels achieved during the second phase constitute only a fraction of typical DC currents for a corresponding target voltage. This observation clearly indicates that this, DC-like discharge, operates under Ar- depleted conditions owing to the severe gas rarefaction.O Not even a 750 s long pulse-off time (after high current pulse) was enough to completely restore the initial conditions.
  44. 44. 3. Results and discussionO Film growth will be determined by ion flux generated during the HIPIMS phase.O Ar2+ to the total ion signal is below 2%, owing to the very high second ionization potential of 27.76 eV (for comparison: 2nd ionization potential for Cr is 16.57 eV).O IEDFs of singly and doubly charged Cr ions comprises low-energy peaks and very pronounced high-energy tails that completely dominate the ion energy spectrum for energies larger than 5 eV.O ~90% reduction of the original energy flux under the present conditions.
  45. 45. 3. Results and discussionO The high energy tail that develops with increasing Ep represents primarily the original distribution function of sputter-ejected metal atoms convoluted with (i) the probability function for electron-impact ionization, (ii) the probability function for collisions of metal ions with Ar neutrals, and finally (iii) the energy transmission function of the spectrometer.O In parallel, a contribution from the backreflected metal ions to the high-energy tail is also expected.O Cr ions that arrive at the target with sufficiently high energy can kick out another Cr atom (self-sputtering) or get neutralized and reflected.
  46. 46. 3. Results and discussionO The position of this low-energy peak reflects the value of plasma potential in the region where ions were created, averaged over the entire time period of the pulse.O Due to the fact that the plasma potential varies quite substantially during the relatively short voltage pulse and tends to saturate in the long post-discharge phase on the millisecond scale, the position of the low-energy peak in time averaged measurements is determined by the plasma potential in the post-discharge phase.O Typical value of Vpd decreases with increasing pulse energy.O Higher energy Ar+ ions are thought to originate either from momentum transfer in collisions with sputtered species or from Ar ions that after being accelerated towards the cathode got reflected as energetic neutrals to undergo post-ionization in the next stage.O The intensity of Ar+ signal is independent of the pulse energy (ions contributing to this spectral feature are created in the post- discharge phase).O Cr+ and Cr2+ ions in the high-energy portion of the IEDFs increases with increasing Ep. This effect may be ascribed to a higher probability of electron-impact ionization.
  47. 47. 3. Results and discussionO There is an increased amount of back-reflected metal ions.O A severe drop in the deposition rate is observed for increasing pulse energy.O The increase of temporal plasma density with pulse energy has only a small effect on the intensity of Ar+.O Low-energy peak of Cr+ IEDF although noticeable, is still small.O The uniform increase in the intensity of Cr2+ IEDF suggests that most of these ions are created during the most energetic phase of the discharge.
  48. 48. 3. Results and discussionO Metallic modeO In the Cr+ IEDF the first ions are detected 35 s after the beginning of the voltage pulse the intensity in the low-energy part of the energy spectrum (below 10 eV) is several times lower than in the high energy part.O In the next time interval (55–75 s) the Cr+ ions increase tremendously in intensity to a large extent preserving the original shape of IEDF.O Times longer than 150 s after the onset of the pulse, the IEDF consists of a single peak.The position of this low-energy peak does not coincide with a thermalized peak observed in time-averaged measurements (time-averaged data include also information about the plasma state after 200 s).O The difference in detected energy of thermalized ions is caused by the drop in plasma potential after the pulse is turned off.O The intensity of the low energy peak at 2 eV is significantly lower in the case of Cr2+, which is a consequence of a significantly lower probability for double-ionization events in the post-discharge plasma, but its behavior is the same than singly ionized Cr.O Ion spectrum is significantly faster for Cr2+ ions.
  49. 49. 3. Results and discussion
  50. 50. 3. Results and discussionO Ar+ ions exhibit high intensity already between 15 s and 35 s after the onset of the voltage pulse clearly preceding the Cr ions.O Between 35 s and 55 s, the IEDF increases in intensity and broadens up to 20 eV to collapse drastically in the next 20 s. After 75 s the IEDF gradually narrows down to a single peak.O A sudden drop observed after 55 s coincides with an equally dramatic increase in the number of Cr+ and Cr2+ ions.O The density of Ar gas is reduced in the vicinity of the cathode due to effective energy exchange with high temporal flux of sputter-ejected metal neutrals.O The observed drop in the intensity of Ar+ signal can also result from lower ionization probability.O With increasing pulse energy, all peaks (including the table current) move to the left on the time axis that is caused by the fact that the discharge was operated at 100 Hz.
  51. 51. 3. Results and discussionO With increasing frequency time between the pulses becomes short enough to ensure immediate ignition and no such effects are observed.O The first detected species are in all cases Ar+ ions. It is remarkable that the intensity of Ar+ signal increases up to the point when first Cr+ ions are detected.O Increasing intensity of Cr+ IEDF is then accompanied by a sudden drop in the Ar+ signal.O On the later stage Cr+ IEDF decays and the Ar+ IEDF slowly picks up. One reason for this could be rarefaction effects and quenching of electron density.O The high-energy tails drop off after the peak maximum is reached.O The maxima of the Cr2+ signal coincide with the rising portions of the Cr+ peaks meaning that doubly charged ions are most effectively produced during the time interval when single charged ions possess a very broad energy spectrum.O The rapid decrease of Cr2+ intensity may be indicative of a plasma cooling effect induced by the still-increasing concentration of metal atoms.
  52. 52. 3. Results and discussionO Reactive modeO The total gas pressure was kept constant at 0.4 Pa and the N2-to-Ar gas ratio, fN2/Ar, was varied between 0 (metallic mode) and 5 (sputtering in heavily poisoned mode).O The signal detected at m/e=15 is assumed to be entirely dominated by N+ ions; the contribution from N22+ ions is expected to be orders of magnitude lower due to its very high ionization potential.O The IEDF of singly charged Cr ions for increasing fN2/Ar comprises the low-energy peak and very pronounced high-energy tails. With increasing N2 content in the plasma the low-energy peak moves from 1.1 eV to 0.5 eV, indicating that the post-discharge, Vpd, decreases.O The IEDF of Ar+ shows a more or less uniform increase in intensity with increasing nitrogen content in the plasma up to fN2/Ar =0.2. Thereafter the count rate decreases, preserving the overall shape of IEDF, to end up at approximately half of the initial value at fN2/Ar =5.O The overall shape of N2+ IEDF is similar to that of Ar+, especially at higher nitrogen flows.
  53. 53. 3. Results and discussionO The IEDF of N+ is qualitatively different as it possesses the high-energy tail typical for Cr+.O This energetic stream of monoatomic nitrogen ions is created through the electron-impact ionization of nitrogen atoms originating from the target: either sputtered from the nitrided portion of the target or being the result of dissociation of back-attracted N2+ ions at the target surface.O IEDF of Cr+ it can be seen that the high-energy tail of the most energetic distributions (between 55 s and 95 s) falls off faster with increasing ion energy for fN2/Ar = 2 (15 J, 300 Hz) than in the case of sputtering in pure Ar.O Qualitatively similar changes with respect to the operation in metallic mode are also observed in the case of Ar+ despite the fact that Ar ions are by far less energetic than Cr ions.
  54. 54. 3. Results and discussionO Intensity and the maximum ion energy are greatly reduced when sputtering in the poisoned mode. This is, however, accompanied by the increase in intensity on the later stage (after 135 s) when only thermalized species are present.O The IEDF of N2+ ions is quite narrow with most energetic ions appearing between 35 s and 75 s, thus about the same time as for Ar+ ions.O IEDFs of N+ ions, bring yet another evidence that supports their origin from the target; energetic ions appear at the same time interval as most energetic Cr+ ions (55–95 s) and with a certain delay with respect to N2+ ions.O Independent of fN2/Ar the first ions detected are always Ar+ and N2+.O Both ion types gain intensity as time goes by up to the point where Cr+ and N+ signals start to take off, at around 50 s.O The increase of the Cr+ and N+ ion flux (that reach maximum at ca. 80 s and 70 s, respectively) is accompanied by a decrease in a number of Ar+ and N2+ ions detected. The situation is reversed in the following time interval.O Ar+ and N2+ features two characteristic peaks: one at early stage of discharge associated with more energetic ions and one broader bump composed of thermalized ions.
  55. 55. 3. Results and discussion
  56. 56. 3. Results and discussionO The intensity of Cr+ decays with a longer time constant than that of N+.O This phenomenon is related to the fact that after 100 s the character of the discharge changes from HIPIMS to DC-like.O Comparing DC discharge varying the nitrogen percentage N+ content is 4–10 times lower than in the case of the HIPIMS discharge operated at the same average power.
  57. 57. 3. Results and discussionO Very high temporal energy density on the target can enhance the dissociative sputtering of CrN, as well as, lead to more effective decomposition of back-reflected N2+ ions.O Similar to Ar ions, the number of N2+ ions is affected by the energetic flux of Cr atoms from the target.O The intensity of the Ar+ and Cr+ signals increases with decreasing Ar flow, at least for the lower values of fN2/Ar.O Varying gas composition together with related poisoning of the target surface gives a number of effects that can account for this unexpected result: (i) an increased volume density of species to be ionized, (ii) an increased probability for ionization event per each gas atom, (iii) the plasma volume probed in experiment may change.O Certain changes of the gas density in front of the target can not be excluded, as the primary cause of gas rarefaction.O The secondary electron emission from nitrided surfaces is believed to be higher than for metals.
  58. 58. 3. Results and discussionO Ions available during film growth with HIPIMS may constitute a major part of the particle flux incident on the substrate.O With increasing pulse energy, the emission from Cr+ ions increases faster than that from Cr neutrals.O For cases where formation of a compound is energetically favorable even without the presence of activated species, surface reactions may continue also during the pulse-off time and it has to be taken into account in any analysis of film growth processes.O If the extra energy is required to promote compound formation, film growth will proceed mainly during the high- energy pulse.O At Ep =3 J ion flux contributions are as follows: Cr+ – 32.3%, Ar+ – 65.3%, Cr2+ – 1.2% and Ar2+ – 1.2%.O At Ep =30 J, respectively: Cr+ – 52.2%, Ar+ – 41.1%, Cr2+ – 5.4% and Ar2+ – 1.3%.O For the pulse energy of 30 J, the energy contributions are as follows: Cr+ – 76%, Cr 2+ – 14%, Ar+ – 9% and Ar2+ – 1%.
  59. 59. 3. Results and discussionO Doubly charged ions upon application of typical bias voltages in the range 50–100 V is energetic enough to cause high lattice defect density that may eventually lead to disruption of epitaxial growth.O The films would then become denser, but exhibit high compressive internal stress and inert gas incorporation.O In the experimental set up used in our experiment, the application of a synchronized biasing pulse between 0 s and 40 s should affect the average energy of a significant portion of the Ar+ ions, without having a noticeable effect on the energy spectrum of the Cr+ and Cr2+ ions.O One may envision different biasing scenarios for growth of CrNx films since, in case of Cr, nitride formation is believed to take place mainly during the time when intense plasma is present. In particular, it would be interesting to evaluate separately the roles of low energy N2+ ions and energetic N+ ions in the nitride formation process.
  60. 60. 4. ConclusionsO Increasing the pulse energy during sputtering in the metallic mode leads to a rapid (linear) increase of the number of doubly charged Cr ions.O The composition (and energy) of the ion flux can be significantly altered by varying the pulse energy.O Low energy N2+ molecular ions and energetic N+ ions are present while sputtering in reactive mode in contrast to the reactive DC sputtering.O Time-resolved studies reveal a significant variation in the composition of the ion flux throughout the pulse.O The properties (composition and energy) of the ion flux incident on the substrate can be adjusted varying the pulse energy and with time variations in the composition of ion flux.O Time evolution of ion flux composition is proposed to be an inherent feature of the high power pulsed discharges.