Riccardo d’Agostino

Spectroscopic diagnostics of CF4‐O2 plasmas during Si and SiO2 etching processes

The emission intensities in the range of 2000–8000 A of CF, CF2, O, F, CO, CO+, and CO2 produced in CF4/O2 radio‐frequency discharges, operated at 1 Torr of pressure and 50 W of power input, have been used to follow the etching process of Si and SiO2, as a function of the CF4/O2 feed composition. The addition of small amounts of nitrogen or argon to the plasma mixtures has permitted the determination of the effect of the oxygen addition to the gas feed on the electron densities of the plasma for a wide range of electron energies. The relative concentration profiles of F and O, as well as of CO and CO2, have been determined with this technique, as a function of the oxygen content in the feed. The important role played by atomic F as active etchant for both Si and SiO2 substrates has been confirmed.

Plasma treatments and plasma deposition of polymers for biomedical applications

The aim of this review is to describe how biomaterial engineering processes, particularly in the field of polymer modifications for enhanced blood-compatibility, can take advantage of low-pressure plasma techniques. It will also be emphasized how a diagnostic approach on both plasma and surface reactions can lead to understand and control the chemistry of plasma processes.

Plasma etching of Si and SiO2 in SF6–O2 mixtures

The products of reaction and etch rates of Si and SiO2 in SF6‐O2 plasmas have been studied as a function of feed composition in an alumina tube reactor at 27 mHz, 45 W, and 1 Torr. There is a broad chemical analogy with CF4‐02 plasmas. As in CF4‐02 mixtures, the rate of Si etching and 703.7‐nm emission from electronically excited F atoms each exhibit distinct maxima as a function of feed gas composition; these data support a model in which fluorine atoms, the etching species, compete with oxygen atoms for chemisorption on the Si surface. Without oxygen in the feed or Si in the reactor, no stable products could be detected. With an SF6‐O2 mixture in the absence of silicon, the final reaction products are F2, SOF4, and SO2F2. The product distribution was unaffected by small SiO2 substrates. When Si is etched, SiF4 is the only stable silicon‐containing etch product and SOF2 is formed in oxygen‐poor mixtures. Rapid etch rates (≳104 A/min for Si) can be obtained with a high selectivity in favor of silicon (Si:...

Mechanisms of etching and polymerization in radiofrequency discharges of CF4–H2, CF4–C2F4, C2F6–H2, C3F8–H2

Some results obtained during the etching of Si or the deposition of fluorocarbon films over Si substrates uncoupled from ground in rf plasmas fed with CF4–H2, C2F6–H2, C3F8–H2 and CF4–C2F4 mixtures are presented. The polymerization process is explained on the basis of a mechanism which involves CF and/or CF2 radicals as building blocks as well as an activation of the polymer surface by means of charged particle bombardment. It is definitively proved that the etch rate of Si depends only on the F‐atom concentration, independent of conditions in the various feeds.

Mechanisms of deposition and etching of thin films of plasma‐polymerized fluorinated monomers in radio frequency discharges fed with C2F6‐H2 and C2F6‐O2 mixtures

Deposition and etching of thin plasma‐polymerized fluorinated monomers have been studied in discharges fed with C2F6‐H2 and C2F6‐O2 mixtures, respectively. A parallel plate reactor with thermostated electrodes has been utilized for the present study. The experiments have allowed us to ascertain the role of positive ions and of radicals, as well as the effect of pressure and substrate temperature, on the mechanism of deposition. The mechanism of etching has also been investigated, and it has been found that both O and F atoms contribute to the process through overall first‐order kinetics. Conditions for a selective etching of noncross‐linked films with respect to cross‐linked ones have also been found.

FLUORINATED DIAMONDLIKE CARBON FILMS DEPOSITED FROM RADIO-FREQUENCY GLOW DISCHARGE IN A TRIODE REACTOR

Diamondlike carbon thin films have been deposited by H2–C2F6 fed rf‐glow discharges in a triode reactor. Raman spectroscopy and electron spectroscopy for chemical analysis have been utilized as diagnostic tools to investigate structural properties and chemical composition of deposited films. Transitions in film properties have been observed, from those typical of hydrogenated polymers to those of diamondlike and fluorinated polymers, by changing feed composition, substrate bias, and substrate temperature. In particular, it has been shown that the energy of the ions bombarding the growing films, as well as the fluorine contents in the films is the most critical parameter to obtain materials with diamondlike structure.

Ion‐beam sputtering deposition of fluoropolymer thin films

It is shown that it is possible to deposit thin films with various CFx composition (1.26≤x≤1.83) by ion‐beam sputtering. These materials with ‘‘teflon‐like’’ composition have been deposited at room temperature by Ar ion‐beam sputtering of a teflon target; the film chemical composition has been determined by electron spectroscopy for chemical analysis. The fluorine‐to‐carbon ratio of the films, as well as their crosslinking degree, is shown to depend on the energy of the ions impinging on the target.

Cell Adhesion on Nanotextured Slippery Superhydrophobic Substrates

In this work, the response of Saos2 cells to polymeric surfaces with different roughness/density of nanometric dots produced by a tailored plasma-etching process has been studied. Topographical features have been evaluated by atomic force microscopy, while wetting behavior, in terms of water-surface adhesion energy, has been evaluated by measurements of drop sliding angle. Saos2 cytocompatibility has been investigated by scanning electron microscopy, fluorescent microscopy, and optical microscopy. The similarity in outer chemical composition has allowed isolation of the impact of the topographical features on cellular behavior. The results indicate that Saos2 cells respond differently to surfaces with different nanoscale topographical features, clearly showing a certain inhibition in cell adhesion when the nanoscale is particularly small. This effect appears to be attenuated in surfaces with relatively bigger nanofeatures, though these express a more pronounced slippery/dry wetting character.

Dry etching of Ti in chlorine containing feeds

The reactivity of titanium has been studied in a chlorine containing environment with and without plasma. It has been found that chlorine (atoms or molecules) is not able to remove the native surface oxide unless drastic conditions are realized. Once the oxide is removed, titanium does not appreciably react with molecular chlorine without plasma unless fluorine is present on its surface. Fluorine has a catalytic effect and makes the titanium surface more reactive towards chlorine. Plasma superimposition does not change the reaction pattern, it acts only by increasing the substrate temperature and supplying energy by means of low energy ion bombardment.

Long-Lasting Antifog Plasma Modification of Transparent Plastics

Antifog surfaces are necessary for any application requiring optical efficiency of transparent materials. Surface modification methods aimed toward increasing solid surface energy, even when supposed to be permanent, in fact result in a nondurable effect due to the instability in air of highly hydrophilic surfaces. We propose the strategy of combining a hydrophilic chemistry with a nanotextured topography, to tailor a long-lasting antifog modification on commercial transparent plastics. In particular, we investigated a two-step process consisting of self-masked plasma etching followed by plasma deposition of a silicon-based film. We show that the deposition of the silicon-based coatings on the flat (pristine) substrates allows a continuous variation of wettability from hydrophobic to superhydrophilic, due to a continuous reduction of carbon-containing groups, as assessed by Fourier transform infrared and X-ray photoelectron spectroscopies. By depositing these different coatings on previously nanotextured substrates, the surface wettability behavior is changed consistently, as well as the condensation phenomenon in terms of microdroplets/liquid film appearance. This variation is correlated with advancing and receding water contact angle features of the surfaces. More importantly, in the case of the superhydrophilic coating, though its surface energy decreases with time, when a nanotextured surface underlies it, the wetting behavior is maintained durably superhydrophilic, thus durably antifog.