Thierry Lagarde

Multi-dipolar plasmas for uniform processing: physics, design and performance

The scaling up of conventional distributed electron cyclotron resonance plasmas presents limitations in terms of plasma density, limited to the critical density, and of uniformity, due to the difficulty of achieving constant amplitude standing wave patterns along linear microwave applicators in the metre range. The alternative solution presented in this study is the extension of the concept of distribution from one- to two-dimensional networks of elementary plasma sources sustained at electron cyclotron resonance (ECR). With the so-called multi-dipolar plasmas, large size and uniform low-pressure plasmas are produced from a two-dimensional network of elementary, independent plasma sources sustained at ECR. Each elementary plasma source consists of a permanent magnet on which microwaves are applied via an independent coaxial line. The plasma is produced by the electrons accelerated at ECR and trapped in the dipolar magnetic field of the magnet acting as a tri-dimensional magnetron structure. Large-size uniform plasmas can be obtained by assembling as many such elementary plasma sources as necessary, without any physical or technical limitations. Examples of two-dimensional networks are described and the performances in terms of density and uniformity of such plasma sources are presented. The interesting characteristics and advantages of multi-dipolar plasmas over distributed ECR plasmas are listed and the perspectives for plasma processing emphasized.

Influence of the multipolar magnetic field configuration on the density of distributed electron cyclotron resonance plasmas

In distributed electron cyclotron resonance plasma sources, the acceleration of electrons is produced by microwave electric fields, applied and distributed close to a multipolar magnetic field structure, providing along the magnets the condition for electron cyclotron resonance. The ensuing fast electrons are trapped in the multipolar magnetic field and drift along the magnets, hence the interest of a closed magnetic configuration to avoid losses at the boundaries of the confinement structure. The performances of two cylindrical reactors fed with microwave power through eight linear applicators and surrounded by either eight magnet bars or eight racetracks (magnetron-like magnetic structures) are measured and compared. In both cases plasma density saturates at the critical density, but in the case of the closed magnetic configuration the saturation is reached for a microwave input power a factor of ten lower than with the open magnetic configuration. This result confirms that the confinement effect of the multipolar magnetic field mainly applies to the fast electrons which generate the plasma.

Determination of the EEDF by Langmuir probe diagnostics in a plasma excited at ECR above a multipolar magnetic field

In order to better understand the mechanisms of plasma production above multipolar magnetic fields via electron cyclotron resonance, the electron energy distribution function (EEDF) of an argon plasma in the magnetic field of a planar magnetron-like structure is determined by using optical emission spectroscopy and a cylindrical Langmuir probe. After a brief recall of the validity conditions for probe measurements in a magnetic field, probe characteristics generally allow the determination of the whole EEDF while emission spectroscopy can only provide the integral of the distribution function above the threshold energy of the selected optical transitions. The probe results show that the EEDF in fact appears as the sum of two Maxwellian electron populations. The first one is the population of fast electrons, accelerated at electron cyclotron resonance and which produces the plasma, and the second one corresponds to the cold, plasma electrons produced by the fast electrons. The variations in the parameters which characterize these two electron populations, i.e. density and electron temperature, as a function of the position in the multipolar magnetic field clearly demonstrate that the fast electrons remain trapped in the magnetic field close to the multipolar structure while the population of the slow, cold plasma electrons diffuses away from the magnets with a nearly constant electron temperature. The variations as a function of external parameters, gas pressure, microwave power, microwave frequency or magnetic field configuration are also discussed. In all cases, the maximum of optical emission corresponds to the region in the magnetic field where the fast electrons, accelerated at electron cyclotron resonance, are trapped and oscillate within two field lines between two mirror points in front of two adjacent poles of opposite polarity. Finally, simulation of the plasma production, as deduced from the experimental values, allows the determination of the ionization frequency of fast electrons. The results are perfectly consistent with literature data on magnetron plasmas.

New trends in DECR plasma technology: applications to novel duplex treatments and process combinations with extreme plasma specifications

After a brief review of the plasma production and diffusion mechanisms above multipolar magnetic field structures, the possible means of sustaining magnetron discharges are listed. In particular, various designs of distributed electron cyclotron resonance (DECR) plasma reactors are described. At the industrial level, besides gas and pumping distribution, the control of plasma uniformity (up to square meters) is a necessary condition to obtain the desired uniformity of reactive species. Another process parameter, controlled via independent substrate biasing, is the energy distribution function of ions onto surfaces. Plasma-based ion implantation (PBII) in the 100-keV range requires large volumes (cubic meters) of very low-pressure plasma (below 10−4 torr), whereas ion bombardment in the very low energy range (a few eV) can only be obtained with quiescent, magnetic field-free plasmas exhibiting a very low electron temperature. In contrast, increasing the electron temperature of the diffusion plasma can enhance ionization of metallic vapors present in the central volume of a DECR reactor. The control of plasma parameters in magnetron-like plasmas opens new possibilities for complex treatments associating successive processes with extreme plasma specifications. As examples, thermochemical processing at low ion bombardment energy can be performed in DECR plasmas. Results on plasma nitriding of stainless steel for industrial applications are reported. More generally, plasma-assisted deposition (PAD) or PBII and deposition (PBIID), using chemical (CVD) or physical vapor deposition (PVD), can be operated with magnetron-like discharges: PACVD and PBIICVD in DECR plasmas, PAPVD and PBIIPVD in hybrid DECR magnetron reactors.

Influence of the applied field frequency on the characteristics of Ar and diffusion plasmas sustained at electron cyclotron resonance above multipolar magnetic field structures

In argon plasmas excited at electron cyclotron resonance above multipolar magnetic field structures, ion density increases linearly with microwave input power but saturates as it gets near the critical density. This behaviour is observed at the three microwave frequencies investigated, namely 960 MHz, 2.45 GHz, and 5.85 GHz, as well as for different magnetic field configurations. The saturation density is independent of the atomic or molecular nature of the gas, as shown with Ar and . Expectedly, the ion density saturation value varies as the square of the excitation frequency, while the microwave input power required to reach saturation is proportional to the critical density. For a given multipolar magnetic field confinement, the electron temperature is shown to decrease with increasing excitation frequency. This result stems from the confinement of the fast electrons, which generate the plasma. The evolution of the F-atom concentration in discharges, as measured by actinometry, is observed to saturate with microwave input power at values depending on gas pressures at both the 2.45 GHz and 5.85 GHz excitation frequencies. Ion density and F-atom concentration saturate at distinct microwave input powers.

Compact Waveguide-Based Power Divider Feeding Independently Any Number of Coaxial Lines

The device described in this paper has been designed to enable the feeding of many individual plasma sources from a single microwave generator, providing a noninterfering and constant supply of power to each coaxial line driving these plasma sources. The power coming from the generator flows through a waveguide under standing-wave conditions provided by the presence of a conducting plane located at the waveguide end opposite that linked to the generator. Power is extracted from the waveguide, at the maximum of intensity of the E-field standing wave, by a waveguide-to-coaxial-line transition designated as a probe. One or two probes can be set at each such maximum of field intensity (and this on both sides of the waveguide wide wall), yielding a compact power divider. Each coaxial line feeds a microwave field applicator, sustaining plasma, through a matching circuit comprising a tuning means and a ferrite isolator (circulator with a matched load), the latter ensuring that whatever happens to the plasma source, the other feeding lines are not affected. The conditions required for a perfect match of the microwave generator to the power divider are elaborated and examples of actual designs are presented

Multipolar magnetic field structures for the scaling-up of high density plasmas excited in the d.c. to microwave frequency range and applicable to sputtering and chemical processing

Multipolar magnetic fields, used for the production of large areas of low pressure, high density plasmas at electron cyclotron resonance, can also be employed favourably to scale-up plasma sources at any given excitation frequency. Since fast electrons responsible for plasma excitation undergo a drift motion perpendicular to the static magnetic field of a multipolar structure, plasma production along such a structure will be uniform when the amplitude of the electric field, which accelerates the electrons, is constant along the structure and the loss of energetic electrons at the extremities of the magnets is avoided by closing the magnetic structures onto themselves according to magnetron-like configurations. The limitations to the scaling-up of sources are derived in terms of the wave propagation, wavelength, source dimensions and mean free path of the fast electrons. Other requirements in the process scale-up concern the uniform distribution of process parameters, such as gas feeding, pumping, substrate biasing and substrate heating. The use of three-dimensional magnetron structures satisfies the explicit requirements, in particular the distribution of gas supply and pumping through the excitation structure. Two examples of novel reactor configuration, designed for chemical processing and sputtering, illustrate the new concepts developed in this work.

Plasma Production above Multipolar Magnetic Field Structures: From D.C. Magnetrons to Distributed ECR

Multipolar magnetic fields, currently used for the confinement and the production of low pressure plasmas, are particularly suitable for the scaling-up of plasma sources. In such magnetic field configuration, the fast electrons, responsible for plasma excitation, oscillate within two field lines between two adjacent, opposite magnetic poles. They also undergo a drift motion perpandicular to the magnetic field, hence the interest of closing the magnetic structures onto themselves according to magnetron-like configurations. The fast electrons can be produced: i) by electron emission from negatively biased filaments; ii) by applying r.f. or negative d.c. voltages on the magnetron structure; iii) at ECR by applying microwaves in the magnetic field region. Then, the ions and the slow electrons produced along the itinerary of the fast electrons diffuse perpandicularly to the magnetic field lines under the influence of the density gradients.