Jacky Mathias

Plasma sputtering deposition of platinum into porous fuel cell electrodes

Platinum is deposited into porous carbon materials relevant for fuel cell electrodes using plasma sputtering techniques. The resulting platinum concentration profile extends up to 2 µm into the porous carbon and is well fitted by a generalized stretched Gaussian function, which displays the non-thermal nature of the penetration process. Platinum deposits are observed to grow as clusters. On the outermost carbon particles, platinum nano-cluster sizes of 3.5 nm have been measured. In tests using actual PEM fuel cells, current densities as high as 1000 mA cm−2 have been obtained at 400 mV with 25 cm2 plasma electrodes. This compares favourably with commercially available electrodes but the present electrodes have a platinum density 4.5 times lower and hence can be considered to be significantly more efficient.

Diagnostic system for plasma/surface energy transfer characterization

The knowledge of the effective energy deposited onto a surface by the reactive particles (ions, electrons, metastables, photons, etc.) in plasma processes such as thin-film deposition, sputtering, etching, etc., is of high interest to understand the basic mechanisms of energy transfer. In this article, a diagnostic is developed to directly measure the global energy transferred to surfaces (reactor walls, substrates, material to be modified, etc.) immerged in low-pressure plasmas. The diagnostic is based on a commercial HFM7-Vattel® microsensor, confined in a temperature-controlled substrate holder. The manufacturer calibration specifications are only given for atmospheric pressure. They cannot be used in low-pressure plasma conditions (typically 0.1–20Pa). Thus, for this particular application, a calibration of the microsensor is required. It is performed at various pressures, between vacuum and the ambient, according to the NIST protocol and using a homemade blackbody (BB). It is shown that only curves o...

Measuring the energy flux at the substrate position during magnetron sputter deposition processes

In this work, the energetic conditions at the substrate were investigated in dc magnetron sputtering (DCMS), pulsed dc magnetron sputtering (pDCMS), and high power impulse magnetron sputtering (HiPIMS) discharges by means of an energy flux diagnostic based on a thermopile sensor, the probe being set at the substrate position. Measurements were performed in front of a titanium target for a highly unbalanced magnetic field configuration. The average power was always kept to 400 W and the probe was at the floating potential. Variation of the energy flux against the pulse peak power in HiPIMS was first investigated. It was demonstrated that the energy per deposited titanium atom is the highest for short pulses (5 μs) high pulse peak power (39 kW), as in this case, the ion production is efficient and the deposition rate is reduced by self-sputtering. As the argon pressure is increased, the energy deposition is reduced as the probability of scattering in the gas phase is increased. In the case of the HiPIMS dis...

Direct measurements of the energy flux due to chemical reactions at the surface of a silicon sample interacting with a SF6 plasma

Energy exchanges due to chemical reactions between a silicon surface and a SF6 plasma were directly measured using a heat flux microsensor (HFM). The energy flux evolution was compared with those obtained when only few reactions occur at the surface to show the part of chemical reactions. At 800 W, the measured energy flux due to chemical reactions is estimated at about 7 W cm−2 against 0.4 W cm−2 for ion bombardment and other contributions. Time evolution of the HFM signal is also studied. The molar enthalpy of the reaction giving SiF4 molecules was evaluated and is consistent with values given in literature.

Electrical and thermal characterization of carbon nanotube films

The remarkable electrical and thermal properties of carbon nanotubes (CNTs) make them attractive for microelectronics applications and, in particular, for interconnects. A multilayer device was designed in order to measure electrical and thermal properties of CNT films. This device is composed of an iron catalyst thin film deposited by pulsed laser ablation upon which a dense multi-walled carbon nanotube (MWCNT) film was grown by radio frequency plasma enhanced chemical vapor deposition. Finally a thin metallic layer was deposited over all by physical vapor deposition. Scanning electron microscopy images were intensively used to check the length (several tens of micrometers) and diameter (10 to 30 nm) of the nanotubes and to adjust the different steps of the process to get the desired film morphology (dense and vertically aligned). The CNT structure was investigated by high-resolution transmission electron microscopy and Raman spectrometry. The MWCNT carpet showed an ohmic behavior during current-voltage ...

Thermal conductivity measurement of porous silicon by the pulsed-photothermal method

Thermal properties of two types of porous silicon are studied using the pulsed-photothermal method (PPT). This method is based on a pulsed-laser source in the nanosecond regime. A 1D analytical model is coupled with the PPT technique in order to determine thermal properties of the studied samples (thermal conductivity and volumetric heat capacity).At first, a bulk single crystal silicon sample and a titanium thin film deposited on a single crystal silicon substrate are studied in order to validate the PPT method. Porous silicon samples are elaborated with two different techniques, the sintering technique for macroporous silicon and the electrochemical etching method for mesoporous silicon. Metallic thin films are deposited on these two substrates by magnetron sputtering. Finally, the thermal properties of macroporous (30% of porosity and pores diameter between 100 and 1000 nm) and mesoporous silicon (30% and 15% of porosity and pores diameter between 5 and 10 nm) are determined in this work and it is found that thermal conductivity of macroporous (73 W m−1 K−1) and mesoporous (between 80 and 50 W m−1 K−1) silicon is two times lower than the single crystal silicon (140 W m−1 K−1).

On the measurement of energy fluxes in plasmas using a calorimetric probe and a thermopile sensor

Two different diagnostics for the determination of the energy influx in plasma processes were used to characterize an ion beam source and an asymmetric RF discharge. The related energy fluxes were measured in dependence on the ion energy and on the RF power, respectively. The first sensor, called HFM (Heat Flux Microsensor) is a thermopile which allows for direct energy flux measurements. With the second sensor, a calorimetric probe, the energy influx has been calculated from the temporal temperature evolution preliminarily registered. Although the working principle of both sensors is different, the obtained results are in good agreement. In the ion beam (<1.5 keV)) rather high energy influxes are achieved (up to 700 mW cm−2), whereas the values measured in the asymmetric RF discharge were lower than 50 mW cm−2 for discharge powers in the range 10–100 W. The performances and limitations of both sensors are compared and discussed.

Highly sensitive measurements of the energy transferred during plasma sputter deposition of metals

This work reports results obtained from heat flux measurements performed during the deposition of metallic thin films by low-pressure plasma sputtering. It introduces a sensitive diagnostic, which allows us to perform such measurements directly during the process and to follow in real-time mechanisms involved in the plasma/surface interaction. Although quantitative results are provided and discussed, the main scope of this paper is a qualitative study of the sputter-deposition process via the energy flux transfers. The diagnostic developed for energy flux measurements is presented and the versatility of the experimental apparatus is described. Results on the study of the deposition of Pt (and Fe) thin films demonstrate a good reproducibility of the measurements and the ability to separate the energetic contribution of the main plasma (~300?mW?cm?2) from the deposition process contribution (2 to 23?mW?cm?2). The influence of gas pressure, plasma power and target bias voltage on the energy transferred to the silicon substrate is also studied.

Reactive pulsed laser deposition assisted by rf discharge plasma

AlN nitride films are grown by reactive pulsed laser ablation of aluminum target in N2 atmosphere. The influence of process parameters such as N2 pressure and laser fluence is investigated. Films are characterized by Rutherford Backscattering Spectroscopy, Nuclear Reaction Analysis, X Ray Diffraction and X Ray Photoelectron Spectroscopy. O contamination appears in the film and its origin is discussed. To enhance N2 dissociation, a RF discharge device is coupled to the deposition chamber. Its effect on thin film composition is studied. Emission spectroscopy is performed in order to find the best RF working point for N2 molecule dissociation and to understand species transport from the target towards the substrate as a function of process parameters. Thin film with a stoichiometry near to Al1N1 can be obtained with low O contamination working with 6 J/cm2 laser fluence, 0.01 mbar N2 with RF discharge added.

Aluminum nitride growth by reactive pulsed laser deposition

The growth of aluminum nitride films by reactive laser ablation has been studied. The influence of process parameters such as laser energy density, nitrogen pressure on the composition, chemical nature and structure of the films has been investigated. Rutherford backscattering spectrometry, nuclear reaction analysis, x-ray diffraction were used to characterize the films. The main problem in AlN film growth was the oxygen incorporation. The origin of this contamination and the mechanisms of incorporation were studied, and the crucial parameter was found to be the residual pressure during ablation. Due to the difference in chemical reactivity between oxygen and nitrogen atomic species, it is necessary to increase the density of atomic nitrogen to obtain pure AlN films. Thus, ar radio-frequency discharge device was added allowing a better nitrogen molecule dissociation. Finally, despite 10 percent O composition deviations, the AlN phase was obtained in the laser deposited films.