G. Cicala

Deposition of super-hydrophobic fluorocarbon coatings in modulated RF glow discharges

Superhydrophobic coatings were deposited in modulated RF glow discharges fed with tetrafluorothylene. Such coatings are characterized by a high fluorination degree, ribbon-like randomly distributed surface microstructures, and a certain crystallinity. Combined high fluorination degree and surface texture/roughness leads to the super hydrophobic behaviour, as attested by water contact angle values of 150° and more. The coatings were characterized by means of XPS, FT-IR and X-ray diffraction, while time resolved optical emission spectroscopy was utilized to investigate the plasma phase.

Morphological and structural study of plasma deposited fluorocarbon films at different thicknesses

The growth of fluorocarbon thin films has been obtained by continuous and modulated r.f. plasmas fed with C2F4. The kinetics surface morphology of the coatings has been investigated and analyzed on films of various thicknesses. Such a study has allowed the evolution of structures in size and shape to be followed. Specifically, it has been observed that modulated plasmas with the proper duty cycles (⩽7%) lead to the formation of micrometer-long ribbon-shaped nanostructures. The precursors of the ribbons are nuclei that align into a spiral-mimic. Highly nanostructured films with an optimal thickness and ribbon density exhibit a super-water repellent surface with contact angles up to 170°. The surface roughness and smoothing of fluorocarbon films are strongly affected by the modulated and continuous plasmas, respectively.

Rf glow discharge of SiF4-H2 mixtures : diagnostics and modeling of the a-Si plasma deposition process

Hydrogenated and fluorinated amorphous silicon films (a‐Si:H,F) have been deposited in a parallel‐plate rf plasma reactor fed with SiF4‐H2 mixtures. The plasma phase characterization has been performed by optical emission spectroscopy for the analysis of the emitting species (SiF*3,SiF*2,SiF*,H*, and H*2), mass spectrometry for the analysis of stable species, and Langmuir electrical probes for the evaluation of electron density (ne) and temperature (kTe). The deposition rate (rD) has been monitored by laser interferometer. The effect of the rf power, gas composition, total pressure, and dopant (B2H6, PH3) addition on the plasma phase composition and on the film growth rate has been studied. The data have been discussed on the basis of a chemical model where the chemisorption of SiF4 and SiF2 and the subsequent interaction with H atoms are the determinant steps also from the kinetic point of view. It has been found that the rate equation rD ∝ [H][SiF2] is able to fit the experimental results.

Anatomy of μc-Si thin films by plasma enhanced chemical vapor deposition: An investigation by spectroscopic ellipsometry

A detailed analysis of the anatomy of microcrystalline (μc-Si) films deposited by plasma enhanced chemical vapor deposition from both SiF4–H2 and SiH4–H2 mixtures is performed by spectroscopic ellipsometry (SE). Specifically, the μc-Si film anatomy consists of an interface layer at the substrate/μc-Si bulk layer, a bulk μc-Si layer, and a surface porous layer. All these layers have their own microstructures, which need to be highlighted, since it is this overall anatomy which determines the optical properties of μc-Si films. The ability of SE to discriminate the complex microstructure of μc-Si thin films is emphasized also by the comparison with the x-ray diffraction data which cannot provide unambiguous information regarding the distribution of the crystalline and the amorphous phases along the μc-Si film thickness. Through the description of the μc-Si film anatomy, information on the effect of the growth precursors (SiF4 or SiH4) and of the substrate (c-Si or Corning glass) on the growth dynamics can be...

Mechanism of silicon film deposition in the RF plasma reduction of silicon tetrachloride

Plasma-chemical reduction of SiCl4 in mixtures with H2 and Ar has been studied by optical emission spectroscopy (OES) and laser interferometry techniques. It has been found that the Ar:H2 ratio strongly affects the plasma composition as well as the deposition (rD) and etch (rE) rates of Si: H, Cl films and that the electron impact dissociation is the most important channel for the production of SiClx species, which are the precursors of the film growth. Chemisorption of SiClx and the reactive surface reaction SiClx+H→−SiCl(x−1)0+HCl are important steps in the deposition process. The suggested deposition model givesrD ∞ [SiClx][H], in agreement with the experimental data. Etching of Si: H, Cl films occurs at high Ar: H2 ratio when Cl atoms in the gas phase become appreciable and increases with increasing Cl concentration. The etch rate is controlled by the Cl atom chemisorption step.

Modulated rf discharges as an effective tool for selecting excited species

Modulated NH3-fed rf glow discharges have been investigated; the emissions of the active species NH* and N2* have been collected during the modulation period (time on+time off). In this study, modulated discharges have been characterized by a constant time on of 7 ms and a tunable time off in the range of 0–1000 ms. It has been found that the power modulation represents an effective tool for selecting excited species: this procedure may be exploited in surface grafting treatments of polymers where high selectivity of chemical groups is requested, e.g., for preferentially grafting −NH2 groups onto polyethylene with respect to all other N-containing functionalities.

Deposition of silicon films from SiCl4 glow discharges: A kinetic model of the surface process

The deposition of amorphous silicon films from plasma‐chemical reduction of silicon tetrachloride has been studied using optical emission spectroscopy, mass spectrometry, and laser interferometry as diagnostic techniques. The experimental results have been examined using a kinetic model of the surface processes. In the model, silicon tetrachloride (SiCl4) and silicon dichloride (SiCl2), chemisorbed on the growing surface, interact with hydrogen atoms to give free bond silicon species (−SiClx), which are reactive intermediates for the silicon‐to‐silicon bond formation on the surface. The temperature dependence of the deposition rate in the kinetic region has been also investigated. It has been established that the apparent activation energy for the silicon plasma deposition depends on the electronic character of the material (i.e., undoped, n, and p doped). As the surface becomes boron or phosphorus doped, the value of the apparent activation energy changes with respect to the undoped surface, as a result ...

Plasma deposition and characterization of photoluminescent fluorinated nanocrystalline silicon films

Fluorinated nanocrystalline silicon films, nc‐Si:H,F, have been deposited from SiF4–SiH4–H2 mixtures by means of the plasma enhanced chemical vapor deposition technique. The presence of fluorine atoms, which are effective etchant species, promotes selective etching giving nanocrystalline films. These materials, with grain size of 100–200 A, show a room temperature photoluminescence centered at 1.62 eV. Also, the widening of the optical energy gap (Eg=2.12 eV) is mainly due to the presence of nanocrystals rather than to the H content of 4.5 at. %.

Plasma deposition of amorphous SiC:H,F alloys from SiF4‐CH4‐H2 mixtures under modulated conditions

Fluorinated and hydrogenated amorphous silicon‐carbon alloys (a‐SiC:H,F) are produced by glow discharge decomposition of SiF4‐CH4‐H2 mixture. Small amount of CH4 in SiF4‐H2 mixture are enough to produce silicon carbon alloys having optical gap ranging between 1.8 and 2.6 eV. Materials, having 1.95 eV band gap and exhibiting optoelectronic properties typical of state of art a‐SiC:H, are deposited under plasma modulation conditions.

Plasma deposition of a –Si, Ge: H, F thin films from SiF 4 –GeH 4 –H 2 mixtures

The deposition of hydrogenated and fluorinated silicon-germanium alloys (Si 1− x Ge x :H.F) by glow discharge decomposition of silicon tetrafluoride (SiF 4 ) and germane (GeH 4 ) mixtures has been studied. Optical emission spectroscopy (OES), for the analysis of the emitting species in plasma phase, and mass spectroscopy (MS) for the analysis of the stable species, are used for the plasma diagnostics. In addition, in situ measurements of the deposition rate by laser interferometry are performed. A series of alloys with germanium content ranging from 0 to 55% has been prepared by varying the gas compositional ratio. Data on the optical gap ( E c ), sub-gap absorption, and photo-to-dark conductivity ratio (δσ/σ) are used to evaluate the quality of the materials. An alloy a –Si 0.75 Ge 0.25 : H, F having E c δ 1.5 eV and δσ/σ = 10 4 has been prepared by adding 1% of GeH 4 to SiF 4 in the feed.