Yves Arnal

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.

PBII processing of dielectric layers: physical aspects limitations and experimental results

Processing of dielectric layers using a plasma-based ion implantation (PBII) technique has general implications in terms of plasma specifications and pulse characteristics. In particular, the different aspects of the processing of dielectric layers are discussed as functions of plasma density, pulse duration, and layer characteristics (thickness and permittivity). Clearly, severe limitations (true implantation energy, arcing) may appear for high-density plasmas as well as for long pulse durations when processing dielectric layers with thicknesses in the millimeter range. Typical examples of ion implantation in dielectric materials are presented, e.g. oxygen ion implantation in polymer sheets (for hydrophilic or adhesion treatments) and nitrogen implantation of polysilicon films on glass. The experimental results demonstrate the feasibility of processing dielectric layers with the PBII technique, but with severe limitations resulting from the process itself.

Etching mechanisms of polymers in oxygen microwave multipolar plasmas

A parametric study of polymer etching in an oxygen microwave multipolar plasma with independent rf wafer biasing is reported. The etch rate evolution as a function of atomic oxygen concentration, measured by actinometry, indicates a monolayer adsorption kinetics for the photoresist/oxygen system. Furthermore, a step‐like variation in the etch rate with ion bombardment energy is observed. In the low‐energy range, where sputtering effects are negligible, ion‐induced chemical etching is the main etching component. In the high‐energy range, an additional etching which exhibits sputtering behavior arises.

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.

Ion implantation based on the uniform distributed plasma

Abstract For over a decade, our group has been working on the development of distributed electron cyclotron resonance (DECR) plasma sources. The uniform distributed plasma (UDP) is the latest outgrowth of DECR, which has proved to be a flexible concept, leading to task-adapted plasma sources. Our plasma-based ion implantation (PBII) reactor is a 60 cm diameter, 70 cm high cylinder. The inside of the cylinder wall is covered with an array of 24 tubular magnets, 2.45 GHz microwave power feeds, and wave propagators. This peripheral plasma source of a good square meter produces a uniform distributed plasma (UDP), suitable for the treatment of wafers, pipes or objects of arbitrary form. Initial PBII plans concern the nitridation of silicon wafers. At 1 mTorr pressure and 1.3 kW input power, the N 2 plasma has a density of 2 × 10 10 ions cm −3 and an electron temperature of 1.2 eV. The N + /N 2 + ratio of 7/3 in the N 2 UDP plasma, determined by quadrupole mass spectrometry, is favorable for PBII applications. A 45 kV pulsed power supply is available for initial tests, but should be supplanted by a more powerful source for meaningful experiments.

New line of high voltage high current pulse generators for plasma-based ion implantation

The two general specifications required for plasma-based ion implantation are low pressure, large size plasmas and high voltage high current pulse generators. In addition, pulses with rise and fall times of the order of the inverse ion plasma frequency and with much longer durations than those of the inverse ion plasma frequency are most often required. To fulfill these requirements, a new type of high voltage generator using a pulse transformer has been developed. A “mettglass”® magnetic core is used as step-up pulse transformer. Voltage at the primary is provided by transistor switches which can achieve rise and fall times of less than 1 μs and maximum pulse currents of 100 A. The primary of the transformer consists of 96 turns wired in parallel and the secondary of 96 turns wired in series. The performances reported with this pulse generator were obtained on a test resistor and then on a substrate immersed in a plasma.

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.

Parametric study of the etching of SiO2 in SF6 plasmas: Modeling of the etching kinetics and validation

The uniform distributed electron cyclotron resonance plasma of SF6, excited at either 2.45 or 5.85 GHz, has been applied to study the etching of SiO2 by F atoms as a function of the three relevant plasma parameters: neutral F-atom flux, ion flux, and ion energy. Three saturation effects are observed. At constant ion current density, the etch rate at first increases linearly with F-atom flux, but then it reaches a plateau, which rises when one raises the ion current density. Second, at constant F-atom flux, initially the etch rate also climbs linearly with ion current density, and again, levels out at larger ion current density, and is higher at larger F-atom flux; however, the initial increase is independent of the F-atom flux. Third, the etch rate evolves similarly as a function of bias voltage for constant F-atom flux and ion current density. These results are first interpreted by a simple mechanism of F-atom adsorption on the SiO2 surface, followed by SiF4 formation at, and desorption from the surface,...

New trends in PBII technology: industrial perspectives and limitations

Abstract The two general specifications required for plasma-based ion implantation are low pressure large size plasmas and high voltage high current pulse generators. Due to the wide ion sheath expansion (up to a few tens of cm), large volumes of plasma are mandatory around the substrate. Multipolar discharges, which produce a peripheral ionization facing the substrate and can be easily scaled up, are well suited to PBII processing and begin to be widely used. However, hot filaments to sustain plasmas of reactive gases in multipolar magnetic field structures must be ruled out in favor of distributed electron cyclotron resonance (DECR) plasma sources. In order to produce the high voltage high current pulses necessary for PBII processing, generators using pulse transformers, where the voltage at the primary is provided by transistor switches and where the energy is stored at a low voltage level, appear particularly well-adapted to fulfill most of the PBII requirements in terms of reliability, compactness, cost and safety. At the industrial level, a very great advantage of PBII over ion beam implantation lies in achieving sequential processing in the same reactor, such as cleaning, etching and deposition prior to, during, or after the implantation process. As examples, thermochemical processing can be performed via PBII with or without external independent heating. More generally, the combination of DECR plasmas and magnetron discharges in the same reactor opens new possibilities for complex treatments such as PBII/CVD (chemical vapor deposition) in DECR plasmas or PBII/PVD (physical vapor deposition) in hybrid DECR-magnetron reactors. However, the transfer of processes from the laboratory to industry is mainly limited to very specific and low energy applications. In fact, mass production using high voltage PBII processing requires production tools still under development. Due to huge secondary electron emission and sheath thickness above 100 kV pulse voltages, large volume reactors (a few cubic meters) on one hand, high power pulse supplies (100 kV–1000 A/100 MW) on the other hand, are mandatory for the rise of PBII at the industrial scale.