Jacques Pelletier

Plasma Sterilization : Methods and Mechanisms

Utilizing a plasma to achieve sterilization is a possible alternative to conventional sterilization means as far as sterilization of heat-sensitive materials and innocuity of sterilizing agents are concerned. A major issue of plasma sterilization is the respective roles of ultraviolet (UV) photons and reactive species such as atomic and molecular radicals. At reduced gas pressure (£10 torr) and in mixtures containing oxygen, the UV photons dominate the inactivation process, with a significant contribution of oxygen atoms as an erosion agent. Actually, as erosion of the spore progresses, the number of UV photons successfully interacting with the genetic material increases. The different physicochemical processes at play during plasma sterilization are identified and analyzed, based on the specific characteristics of the spore survival curves.

The respective roles of UV photons and oxygen atoms in plasma sterilization at reduced gas pressure: the case of N/sub 2/-O/sub 2/ mixtures

In the reduced-pressure (/spl les/10 torr) afterglow stemming from discharges in O/sub 2/- containing mixtures such as N/sub 2/-O/sub 2/, the test-reference spores are ultimately inactivated by UV photons through destruction of their genetic material (DNA). To show this, we assume the inactivation to result from a sufficiently large number of successful hits of the DNA strands by UV photons. This implies that the higher the UV intensity, the shorter the time required to reach the lethal dose. Simultaneously, the increased erosion of the spores by the oxygen atoms as time elapses reduces the incident number of photons required to meet the lethal dose. Erosion, as observed by scanning electron microscopy, also increases with the O/sub 2/ percentage in the mixture. Actually, sterilization time is found to be the shortest when the O/sub 2/ percentage in the mixture is set to maximize the UV emission intensity, which occurs at O/sub 2/ percentages typically below 2%, where erosion is low. This proves the predominant role of UV radiation over erosion as far as spore inactivation is concerned. In any case, plasma sterilization always implies some erosion of the test spores, in contrast to what happens with conventional sterilization methods.

Plasma-based ion implantation and deposition: a review of physics, technology, and applications

After pioneering work in the 1980s, plasma-based ion implantation (PBII) and plasma-based ion implantation and deposition (PBIID) can now be considered mature technologies for surface modification and thin film deposition. This review starts by looking at the historical development and recalling the basic ideas of PBII. Advantages and disadvantages are compared to conventional ion beam implantation and physical vapor deposition for PBII and PBIID, respectively, followed by a summary of the physics of sheath dynamics, plasma and pulse specifications, plasma diagnostics, and process modeling. The review moves on to technology considerations for plasma sources and process reactors. PBII surface modification and PBIID coatings are applied in a wide range of situations. They include the by-now traditional tribological applications of reducing wear and corrosion through the formation of hard, tough, smooth, low-friction, and chemically inert phases and coatings, e.g., for engine components. PBII has become viable for the formation of shallow junctions and other applications in microelectronics. More recently, the rapidly growing field of biomaterial synthesis makes use of PBII and PBIID to alter surfaces of or produce coatings on surgical implants and other biomedical devices. With limitations, also nonconducting materials such as plastic sheets can be treated. The major interest in PBII processing originates from its flexibility in ion energy (from a few electron volts up to about 100 keV), and the capability to efficiently treat, or deposit on, large areas, and (within limits) to process nonflat, three-dimensional workpieces, including forming and modifying metastable phases and nanostructures.

Application of wide‐gap semiconductors to surface ionization: Work functions of AlN and SiC single crystals

For surface ionization purposes, a study of the work functions of SiC and AlN, both refractory wide‐gap semiconductors, has been undertaken. Work function measurements have been performed in the 300–1600‐K range using the Shelton retarding field method. Surface cleaning was carried out by heating in uhv to a high temperature using of a cw CO2 laser. Both n‐ and p‐type 6H SiC single crystals with extreme bulk doping levels were investigated. For each doping type, the work functions have been found to be temperature independent. They exhibit only a slight variation from the n to the p type (4.75 and 4.85 eV, respectively) giving evidence, as in the case of Ge and Si, of Fermi level pinning at the surface by the intrinsic surface states. For an n‐type AlN single crystal, the work function, measured in the high temperature range, remains constant as the temperature varies. The value of 5.35 eV obtained for AlN makes it a most attractive material for positive surface ionization applications.

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.

Distributed electron cyclotron resonance in silicon processing: Epitaxy and etching

The microwave multipolar plasma (MMP) is confined by a multipolar magnetic field and excited by a 2.45 GHz microwave electric field. The distributed electron cyclotron resonance (DECR) excitation uses the confinement magnets to obtain the resonant magnetic field necessary for ECR excitation. The major MMP asset for surface treatment is a substantial ion flux under controlled and low energy (down to a few eV). Unlike conventional ECR, the DECR ion flux onto the substrate is homogeneous and not influenced by magnetic fields. Silicon homoepitaxy by SiH4 DECR is achieved in the 400–800 °C range. The substrate is cleaned by an H2 or Ar plasma before deposition. Control of the ion energy during cleaning and deposition is paramount. Above 600 °C the defect density drops from 1010 to 105 cm−2. N‐type As doped layers are obtained by adding AsH3 to the plasma. The growth rate is almost independent of temperature and doping level. Abrupt As profiles are obtained when the ion impact energy is adjusted to 50 eV. Recent etching work concerns the study of etch rate and anisotropy in the etching of Si and SiO2 by MMP’s of fluorinated gases and halogen mixtures, and the etching of polymers in oxygen‐based MMPs.The microwave multipolar plasma (MMP) is confined by a multipolar magnetic field and excited by a 2.45 GHz microwave electric field. The distributed electron cyclotron resonance (DECR) excitation uses the confinement magnets to obtain the resonant magnetic field necessary for ECR excitation. The major MMP asset for surface treatment is a substantial ion flux under controlled and low energy (down to a few eV). Unlike conventional ECR, the DECR ion flux onto the substrate is homogeneous and not influenced by magnetic fields. Silicon homoepitaxy by SiH4 DECR is achieved in the 400–800 °C range. The substrate is cleaned by an H2 or Ar plasma before deposition. Control of the ion energy during cleaning and deposition is paramount. Above 600 °C the defect density drops from 1010 to 105 cm−2. N‐type As doped layers are obtained by adding AsH3 to the plasma. The growth rate is almost independent of temperature and doping level. Abrupt As profiles are obtained when the ion impact energy is adjusted to 50 eV. Recen...

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.