Y. Segui

Measure and analysis of potential decay in polypropylene films after negative corona charge deposition

Abstract Surface potential decay after negative corona charge deposition has been studied for polypropylene films 50 μm thick, over a wide range of decay times and charging voltage (500– 2800 V ). At low initial potential (500– 800 V ), the surface potentials were stable and did not decay. At high initial potential (1200– 2800 V ), the surface potential decay has been clearly observed with differences in the decay rate. The observed decay behavior has been computer simulated to elucidate the transport mechanism in polypropylene films. A good agreement between measured and calculated decay curves was obtained with a model in which charges are transported by hopping of injected carriers between localized trapping centers. The model is characterized by parameters such as the free carrier mobility, the mean free time of carrier between traps and the mean capture time in trap. The transport parameters have been determined by a fitting procedure.

Growth Modes of SiOx Films Deposited by Evaporation and Plasma-Enhanced Chemical Vapor Deposition on Polymeric Substrates

In order to study the very first stages of plasma-enhanced chemical vapor deposition (PECVD) of SiO2 on polymer substrates, we used a distributed electron cyclotron resonance (DECR) reactor, with the substrate placed (I) in the active glow zone, (II) downstream therefrom, and (III) downstream, but shielded from photon emission (e.g., VUV) from the plasma. For comparison, we also study films deposited by physical vapor ddposition (PVD, thermal evaporation). To characterize the ultra-thin deposits, we used oxygen plasma etching combined with scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectroscopy (RBS). We elucidate the roles of various energetic species (VUV photons, ions, atomic oxygen, and other radicals) in the plasma during the earliest growth phase, and the origin of the “interphase” which is present between the deposited SiO2 and the polymer substrate.

Growth and structure of hyperthin SiO2 coatings on polymers

Transparent inorganic oxide coatings on polymers are playing an increasingly important role in pharmaceutical, food, and beverage packaging, and more recently in encapsulation of organic, light-emitting display devices. Such coatings are being prepared by physical or by chemical vacuum-deposition methods. They possess barrier properties against permeation of gases or vapors when they are thicker than a certain critical thickness, dc; for d<dc, the “oxygen transmission rate” (in standard cm3/m2/day/bar), for example, is roughly the same as that of the uncoated polymer. This fact is commonly attributed in the literature to a “nucleation” phase of the coating’s growth, during which it is thought to present an island-like structure. In order to test this hypothesis, we have deposited hyperthin SiO2 coatings on various flexible polymeric substrates using plasma-enhanced chemical vapor deposition. The film thicknesses investigated here, well below dc (typically in the range 1–10 nm), were determined by Rutherfo...

Deposition of SiO2-Like Films by HMDSN/O2 Plasmas at Low Pressure in a MMP-DECR Reactor

The effects of process parameters such as O2/HMDSN (hexamethyldisilazane) ratio, microwave discharge power and deposition pressure on the growth rate, chemical bonding nature, and refractive index of thin films deposited by microwave plasma from HMDSN with oxygen, have been investigated. The plasma was created in a Microwave Multipolar reactor excited by Distributed Electron Cyclotron Resonance. The films were deposited at room temperature and characterized by Fourier Transform Infrared spectroscopy and ellipsometry. Growth rate increased with the discharge power P or the deposition pressure but decreased significantly with increasing O2/HMDSN ratio. A large change in the film composition was observed when the O2/HMDSN ratio was varied: films deposited with only HMDSN precursor are polymer-like but as the O2/HMDSN ratio increased, organic groups decreased. For relative pressure values over 70%, deposited films are SiO2-like with refractive index values close to those found for thermal silicon dioxide.

Investigations of SiOx-polymer “interphases” by glancing angle RBS with Li+ and Be+ ions

Abstract In order to better understand the formation of an “interphase”, which is a known feature between SiO 2 coatings deposited by plasma enhanced chemical vapor deposition (PECVD) and polymeric substrates, we have deposited such layers on Kapton® polyimide (PI). Various deposition routes have been used, namely thermal evaporation (physical vapor deposition, PVD), and distributed electron cyclotron resonance (DECR) PECVD, with the substrate placed within the active glow zone and downstream from it. The actual presence and the width of the interphase were investigated using glancing angle Rutherford backscattering spectroscopy (RBS), with a scattering angle of 92°. To maximize the depth resolution, the RBS experiments have been performed using “heavy” (Li + and Be + ) ions.

Characterization by RBS of hyper-thin SiO2 layers on various polymers

Abstract In order to investigate the growth mode of plasma-enhanced chemical vapor deposited (PECVD) SiO 2 gas barrier layers on various polymers (namely polyimide, PI; polyethyleneterephthalate, PET; polycarbonate, PC) we use Rutherford backscattering spectroscopy (RBS) in two different ion beam modification (IBM) geometries, with 1 and 1.5 MeV α particles (He 2+ ions). In the case of Kapton ® PI, a polymer which is damaged very little by the incident ion beam, we have followed the evolution of the surface concentration of silicon atoms versus PECVD growth duration: the relationship was found to be perfectly linear over a film thickness range 5⩽ d ⩽500 A. Using grazing incidence, we found a 7 nm thick “interphase” region between the SiO 2 coating and the substrate. However, for the case of the two other polymers (PET and PC), major artifacts were noted during RBS measurements, even under very mild beam conditions (1 nA, 8.0×10 12 ions/cm 2 ), because of the creation of abundant volatile molecular fragments by the incident particles below the surface of the organic substrate. Scanning electron micrographs confirm the damage caused by these volatile species, as they escape through the sample surface; this greatly limits the use of the RBS technique for most polymers, including PET and PC.

Simulation of surface potential decay of corona char ged polyimide

Surface potential decay (SPD) of corona charged polyimide (PI) films has been studied using different initial potentials Vo. It has been noticed that the rate of decay increases with initial charging levels. Surface potential profile over the surface of the PI films and return potential generated after charge neutralization have been also experimentally recorded. In order to analyse and explain the decay behavior, the recorded SPD in PI film has been computer simulated using a numerical model based on charge transport by hopping of injected carriers between localized trapping centers. Three parameters (transit time tT, trapping time τT and mean capture time before release τF) controlling the decay kinetic have been estimated by fitting procedure. The effects of the electric field strength on the transport parameters have been also discussed.

Investigation on the origin of dielectric constant evolution in films deposited from organosilicon molecules in microwave DECR plasma reactor

Abstract Low k dielectric thin films were elaborated in microwave distributed electron cyclotron resonance reactor by using organosilicon vapor and oxygen mixture as precursors. The dielectric constant evolution was studied as a function of the oxygen addition. Contribution of each permittivity component to the variation of the measured dielectric constant was carried out. Although the electronic and ionic polarizations have their influences on the value of the measured permittivity, they have not shown any effect on its evolution. A method to follow the dipolar polarization contribution to the dielectric constant is described. It suggests that both Si–O and Si–C polar groups present in the deposited films may have the predominant effect on the behavior of the dielectric constant.

New plasma deposition process of amorphous GaxAs1−x in an r.f. capacitively coupled diode system

Abstract A new process to obtain hydrogenated a-Ga x As 1− x (amorphous alloys) by glow discharge in an r.f. capacitively coupled system is described. These alloys are obtained from metallic gallium and arsine gas (AsH 3 ). A novel use of simultaneous sputtering and gas decomposition in the same system allows the control of the deposition. The composition of a-Ga x As 1− x is monitored by the gas phase composition. The conduction mechanisms are shown to be strongly dependent on gallium or arsenic in excess.

Determination of space‐charge dynamics and transport parameters in hydrogenated carbon layers by time‐resolved electroluminescence measurements

A new way is presented for deriving space‐charge dynamics and transport parameters in thin films sandwiched between two metallic electrodes. The measurement method uses the detection of electroluminescence excited by square voltage pulses. The space‐charge buildup time is derived from the electroluminescence time lag when the structure is excited by a single square voltage pulse. The relaxation time of the space charge is investigated by using a double pulse excitation. Carrier mobility is determined by an optical transit‐time technique. Measurements are carried out on hydrogenated amorphous carbon layers with different composition and electrical properties. The films display space‐charge limited conduction which is due to the existence of trap states. Their concentration is derived from the current–voltage characteristics and the mobility measurements. The trap concentration increases with a decreasing graphite content in the films. Film properties are weakly temperature dependent in graphite‐rich layers...