J Benedikt

Thin film deposition by means of atmospheric pressure microplasma jet

An RF microplasma jet working at atmospheric pressure has been developed for thin film deposition application. It consists of a capillary coaxially inserted in the ceramic tube. The capillary is excited by an RF frequency of 13.56 MHz at rms voltages of around 200–250 V. The plasma is generated in a plasma forming gas (helium or argon) in the annular space between the capillary and the ceramic tube. By adjusting the flows, the flow pattern prevents the deposition inside the source and mixing of the reactive species with the ambient air in the discharge and deposition region, so that no traces of air are found even when the microplasma is operated in an air atmosphere. All these properties make our microplasma design of great interest for applications such as thin film growth or surface treatment. The discharge operates probably in a γ -mode as indicated by high electron densities of around 8×10 20 m −3 measured using optical emission spectroscopy. The gas temperature stays below 400 K and is close to room temperature in the deposition region in the case of argon plasma. Deposition of hydrogenated amorphous carbon films and silicon oxide films has been tested using Ar/C2H2 and Ar/hexamethyldisiloxane/O2 mixtures, respectively. In the latter case, good control of the film properties by adjusting the source parameters has been achieved with the possibility of depositing carbon free SiOx films even without the addition of oxygen. Preliminary results regarding permeation barrier properties of deposited films are also given.

Origin of the energetic ions at the substrate generated during high power pulsed magnetron sputtering of titanium

High power impulse magnetron sputtering (HiPIMS) plasmas generate energetic metal ions at the substrate as a major difference to conventional direct current magnetron sputtering (dcMS). The origin of these very energetic ions in HiPIMS is still an open issue, which is unravelled using two fast diagnostics: time-resolved mass spectrometry with a temporal resolution of 2 µs and phase resolved optical emission spectroscopy with a temporal resolution of 1 µs. A power scan from dcMS-like to HiPIMS plasmas was performed, with a 2 inch magnetron and a titanium target as sputter source and argon as working gas. Clear differences in the transport as well as the energetic properties of Ar + ,A r 2+ ,T i + and Ti 2+ were observed. For discharges with highest peak power densities a high energetic group of Ti + and Ti 2+ could be identified with energies of approximately 25 eV and of 50 eV, respectively. A cold group of ions was always present. It is found that hot ions are observed only when the plasma enters the spokes regime, which can be monitored by oscillations in the IV characteristics in the MHz range that are picked up by the used VI probes. These oscillations are correlated with the spokes phenomenon and are explained as an amplification of the Hall current inside the spokes as hot ionization zones. To explain the presence of energetic ions, we propose a double layer (DL) confining the hot plasma inside a spoke: if an atom becomes ionized inside the spokes region it is accelerated because of the DL to higher energies whereas its energy remains unchanged if it is ionized outside. In applying this DL model to our measurements the observed phenomena as well as several measurements from other groups can be explained. Only if spokes and a DL are present can the confined particles gain enough energy to leave the magnetic trap. We conclude from our findings that the spoke phenomenon represents the essence of HiPIMS plasmas, explaining their good performance for material synthesis applications.

Initial polymerization reactions in particle-forming Ar/He/C2H2 plasmas studied via quantitative mass spectrometry.

The initial polymerization reactions in particle forming Ar/He/C 2H 2 plasmas are studied using molecular beam mass spectrometry (MBMS). The measured mass spectra are disentangled and quantified with the help of Bayesian probability theory. This approach uses the measured mass spectra and the cracking patterns (CPs) of the species that are formed in the plasma as the main input parameter. The CPs are either taken from calibration measurements or the NIST database or estimated based on a comparison to CPs of similar molecules. These estimated CPs are then modified by Bayesian analysis to fit the measured data. The CPs of C 6H 2, C 6H 4, and C 8H 2, which are not available in the NIST database, are determined in this way and can serve as good estimation until precise data is published. The temporal evolution after plasma ignition of the densities of in total 22 species (hydrocarbons, noble gases, and impurities) are quantified and expressed as partial pressures. The most abundant products in our plasma are C 4H 2 and C 6H 2 molecules with maximum partial pressures of 0.1 and 0.013 Pa, respectively. Our quantitative data can be used to validate plasma chemistry models. First comparison is made to a plasma chemistry model of similar C 2H 2 plasma already available in the literature. The comparison indicates that dissociative electron attachment to C 2 n H 2 ( n > 1) molecules is a dominant source of negative ions in C 2H 2 plasmas. Additionally, the C 2H 4 has been identified as a precursor for C n H 4 molecules.

Dynamic of the growth flux at the substrate during high-power pulsed magnetron sputtering (HiPIMS) of titanium

The temporal distribution of the incident fluxes of argon and titanium ions on the substrate during an argon HiPIMS pulse to sputter titanium with pulse lengths between 50 to 400xa0µs and peak powers up to 6xa0kW are measured by energy-resolved ion mass spectrometry with a temporal resolution of 2xa0µs. The data are correlated with time-resolved growth rates and with phase-resolved optical emission spectra. Four ion contributions impinging on the substrate at different times and energies are identified: (i) an initial argon ion burst after ignition, (ii) a titanium and argon ion flux in phase with the plasma current due to ionized neutrals in front of the target, (iii) a small energetic burst of ions after plasma shut off, and (iv) cold ions impinging on the substrate in the late afterglow showing a pronounced maximum in current. The last contribution originates from ions generated during the plasma current maximum at 50xa0µs after ignition in the magnetic trap in front of the target. They require long transport times of a few 100xa0µs to reach the substrate. All energy distributions can be very well fitted with a shifted Maxwellian indicating an efficient thermalization of the energetic species on their travel from target to substrate. The energy of titanium is higher than that of argon, because they originate from energetic neutrals of the sputter process. The determination of the temporal sequence of species, energies and fluxes in HiPIMS may lead to design rules for the targeted generation of these discharges and for synchronized biasing concepts to further improve the capabilities of high-power impulse magnetron sputtering (HiPIMS) processes.

Time-resolved measurement of film growth during high-power pulsed magnetron sputtering (HIPIMS) of titanium

The growth rate during high-power pulsed magnetron sputtering (HIPIMS) of titanium is measured with a temporal resolution of up to 25xa0µs using a rotating shutter concept. According to that concept a 200xa0µm slit is rotated in front of the substrate synchronous with the HIPIMS pulses. Thereby, the growth flux is laterally distributed over the substrate. By measuring the resulting deposition profile with profilometry, the temporal variation of the growth flux per pulse is deduced. The time-resolved growth rates are measured for 0.25, 0.5 and 1xa0Pa with pulse lengths of 50, 200 and 400xa0µs for an average power of 100xa0W. We can clearly identify, the individual phases of a HIPIMS pulse consisting of ignition, current rise, gas rarefaction, plateau/self-sputtering, and afterglow as described in the literature. In addition, the maximum film growth is only reached after gas rarefaction, indicating a dynamic change in local transport properties. After the end of the HIPIMS pulse, the growth rate decays following two time constants of 100xa0µs and of ∼ms, respectively. The first is consistent with the decay of the ion flux in the afterglow; the second with a decay of reactive neutrals. The absolute comparison of growth rates indicates that a reduction of the efficiency to 30% for very short pulses is typical for a true HIPIMS plasma.

High power impulse sputtering of chromium: correlation between the energy distribution of chromium ions and spoke formation

High power magnetron sputtering (HiPIMS) discharges generate ions with high kinetic energies in comparison to conventional dc magnetron sputtering. The peculiar shape of the ion energy distribution function (IEDF) is correlated to the formation of localized ionization zones (IZ) in the racetrack of a HiPIMS discharge, so called spokes. This is explained by a local maximum of the electrical potential inside these localized IZ. By using ion energy mass spectrometry, probe experiments and plasma spectroscopy the connection between IZ and IEDFs is evaluated with high temporal resolution. The data of a floating probe next to the target is used to directly monitor the movement of the spokes in the direction. Chromium is used as target material, because the plasma undergoes a sequence from stochastic spoke formation, to regular spoke pattern rotating in the direction to a homogeneous plasma torus with increasing plasma power. In particular, the analysis of the transition from the regular spoke pattern to the homogeneous plasma torus at very high plasma powers shows that the high energy part of the IEDF is not affected and only the low energy part is modified. Consequently, one could consider the homogenous plasma torus at very high plasma powers as a a single ionization zone localized over the complete torus, which is formed by merging individual spokes with increasing power. Details and consequences of that model are discussed.

Time-resolved measurement of film growth during reactive high power pulsed magnetron sputtering (HIPIMS) of titanium nitride

The growth rate during reactive high power pulsed magnetron sputtering (HIPIMS) of titanium nitride is an inherent time-dependent process. By using a rotating shutter setup it is possible to gain an insight into its variation with a temporal resolution of up to 25xa0µs. In this apparatus a 200xa0µm slit is rotated in front of the substrate synchronous with the HIPIMS pulses. This ensures that the incoming growth flux is laterally distributed over the substrate. By measuring the resulting deposition profile with profilometry and x-ray photoelectron spectroscopy, the temporal variation of the titanium and nitrogen growth flux per pulse is deduced. The analysis reveals that film growth occurs mainly during an HIPIMS pulse, with the growth rate following the HIPIMS phases ignition, current rise, gas rarefaction, plateau and afterglow. The growth fluxes of titanium and nitrogen follow slightly different behaviours with titanium dominating at the beginning of the HIPIMS pulse and nitrogen at the end of the pulse. This is explained by the gas rarefaction effect resulting in a dense initial metal plasma and metal films which are subsequently nitrified.

Time-resolved measurement of film growth during high-power pulsed magnetron sputtering (HPPMS) of titanium: the rotating shutter concept

The growth rate during high-power pulsed magnetron sputtering (HPPMS) of titanium is measured with a temporal resolution of up to 54xa0µs using a rotating shutter concept. According to that concept a 200xa0µm slit is rotated in front of the substrate synchronous with the HPPMS pulses. Thereby, the growth flux is laterally distributed over the substrate. By measuring the resulting deposition profile with profilometry, the temporal variation of the growth flux per pulse is deduced. The analysis reveals that film growth occurs mainly during a HPPMS pulse, with the growth rate slowly increasing during the pulse and decaying afterwards with a decay time of 100xa0µs. The maximum of film deposition shifts to earlier times in the pulse with increasing peak power.

The search for growth precursors in reactive plasmas : from nanoparticles to microplasmas

The identification of growth precursors in reactive plasmas is of major interest for any optimization of plasma processes. Various diagnostics might be employed to detect important radicals in plasma chemistry. However, the species which produce the highest signal in a specific discharge might not be relevant for film growth since they are not efficiently pumped by the substrate surfaces. This paper describes concepts to characterize the plasma chemistry using reactor parameters, which might be compared with diagnostics such as mass spectrometry. In addition, relevant methods to study surface processes such as the roughness evolution during thin film growth are illustrated. The search for growth precursors and material synthesis is illustrated for two examples: (i) the formation of nanoparticles in low pressure reactive plasmas; (ii) the generation of higher hydrocarbons in microplasmas operated at atmospheric pressure.

The effect of surface reactions of O, O3 and N on film properties during the growth of silica-like films

The effect of surface reactions of O, O3 and N radicals during the growth of silica-like (SiOxCyHz) films on film properties is investigated. A SiOxCyHz film is deposited from a He/Hexamethyldisiloxan (HMDSO) cold atmospheric plasma on a rotating substrate. The surface of this film is, during the growth, treated on the opposite site of the substrate by a second cold atmospheric plasma with helium and an addition of O2 or N2. A reactor with four separated cells and gas curtains between them is used to avoid cross-contamination of the ambient atmosphere in each cell. The changes in film composition after the deposition with and without a treatment by O, O3 and N are investigated by Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy. Additionally, the effect of each species on the deposition rate is also presented and discussed.