Voids in dust clouds suspended in the plasma sheath

Voids in dusty plasma are a new phenomenon, which is still not understood. In this work we have studied experimentally for first time voids in the sheath of a radio-frequency (RF) dusty plasma. Injecting big dust particles into the plasma, we form a dust cloud in the sheath. The behaviour of the cloud as a function of RF power and gas pressure is investigated using video imaging. Both dependencies show a threshold for the void formation. This threshold is characterised by a sudden decrease in the inter-particle distance, while in the non-void mode the distance increases with power and pressure. We have performed Langmuir probe measurements of the floating potential in the bulk plasma close to the sheath in order to estimate the form of the potential well trapping the dust grains

The chemistry of a CCl2F2 radio frequency discharge

A systematic study of the chemistry of stable molecules and radicals in a low pressure CCl2F2 radio frequency discharge for dry Si etching has been performed. Various particle densities have been measured and modeled. The electron density, needed as an input parameter to model the CCl2F2 dissociation, is measured by a microwave cavity method. The densities of stable molecules, like CClF3, CF4, 1,2-C2Cl2F4 and the etch product SiF4, are measured by Fourier transform absorption spectroscopy. The density of the CF2 radical is measured by means of absorption spectroscopy with a tunable diode laser. Its density is in the order of 1019 m-3. All density measurements are presented as a function of various plasma parameters. Moreover, optical emission intensities of Cl and F have been recorded as a function of plasma parameters. It appears that the feed gas (CCl2F2) is substantially dissociated (about 70%–90%) in the discharge. Based on the obtained data the dissociation rates of several molecules have been estimated. The measured total dissociation rate of CCl2F2 is 8×10-15 m3¿s-1. For this molecule the dissociation rate is substantially higher than the dissociative attachment rate (10-15 m3¿s-1). The dissociation rate for CClF3 is 2×10-15 m3¿s-1 and for CF4 about 3×10-16 m3¿s-1. The total dissociation rate of C2Cl2F4 is higher than 5 ×10-15 m3¿s-1, of which 2.5±0.5 × 10-15 m3¿s-1 is due to dissociative attachment. Furthermore it has been found that the presence of a silicon wafer strongly affects the plasma chemistry. Optical emission measurements show that the densities of halogen radicals are significantly depleted in presence of Si. Moreover, the densities of several halocarbon molecules display a negative correlation with the density of the etch product SiF4. © 1995 American Vacuum Societ

Polymerisation in low-pressure plasmas

Plasma polymerisation, both gas-phase and surface induced, is a grand issue in the chemistry of active processing plasmas. This paper reviews several aspects of plasma polymerisation under low-pressure conditions. The impact of plasma polymerisation on plasma and surface chemistry is thoroughly discussed, with emphasis on the links between surface and gas phase polymerisation. The consequences of polymerisation, including surface charging and surface reactions, etching performance of the plasma, and in situ formation of dust particles are highlighted using examples from fluorocarbon and silane chemistries. Further, polymerisation schemes involving various precursors are presented. The final part of th is review is devoted to experimental methods to study polymerisation processes; several techniques like infrared absorption and mass spectrometry are discussed in more detail

Koude plasma's in biomedische technologie

No abstract

In situ powder diagnostics in low temperature plasmas

We present a survey of diagnostics developed for dust particles, present in low pressure, low temperature plasmas. Formation of microscopic particles is an extensively studied phenomenon, and its complexity requires specific experimental methods. The most valuable information about the properties and behaviour of powders is obtained in situ in the plasma. Various interactions of photons with dust particles offer a variety of in situ diagnostic techniques, which allow characterising a wide category of particles. Here we shall concentrate on classification of photoninduced processes in dusty plasmas and demonstrate their diagnostic possibilities. Methods to determine physical and chemical properties of particles, dust density and size from nanometer to micrometer scale, will be described and illustrated with examples

Electron attachment mass spectrometry as a diagnostics for electronegative gases and plasmas

Electron attachment mass spectrometry (EAMS) has been developed to study mixtures of electronegative gases and plasmas. A quadrupole mass spectrometer (QMS) has been used to detect negative ions, formed from sampled species by attachment of low energy electrons. Varying the electron energy allows to collect the attachment cross section of the considered species. EAMS appears to be a very powerful technique to study the chemistry of electronegative gases. Unlike ionization mass spectrometry, where cross sections are low at the threshold and rather flat over a broad range of electron energies, attachment resonances are sharp and distinct. Also very limited fragmentation of the parent negative ion occurs, so a given molecule yields only a few different negative ions. This facilitates identification of components in a gas mixture. It is particularly advantageous for detection of large, fragile molecules, which break up after ionization, but can be easily transformed into large negative ions. Moreover, sensitive detection of active species is possible due to their relatively high attachment cross sections. A particularly important application of EAMS is the determination of an effective attachment cross section in a plasma. Recording this cross section allows to decide on the actual negative ion formation mechanism in the plasma environment, where active products of plasma conversion can significantly alter the negative ion production channels and consequently the whole balance of charged particles. Examples of EAMS applications to fluorocarbon gases and low-pressure radio-frequency plasmas are discussed. In a CF4 discharge conversion of the parent gas into species like C2F6 and C3F8 is easily visualized. The dominant mechanism of negative ion formation in the plasma is electron attachment to these minority species and not to the parent gas. Also larger polymers are readily formed in fluorocarbon plasmas. In a C2F6 discharge molecules with up to ten carbon atoms (the mass limit of our apparatus) have been detected using EAMS