Sandra G. Castro

Mechanisms of polymer film deposition from r.f. discharges of acetylene, nitrogen and helium mixtures☆

Mixtures of C2H2, He and N2 were polymerized in an r.f. discharge. Quantitative optical emission spectroscopy was used to determine relative concentrations of the key species CH and CN in the plasma as a function of the proportions of monomer gases in the feed, and to delineate the behavior of the electron mean energy and number density in the discharge. Transmission IR spectroscopy (IRS) and electron spectroscopy for chemical analysis (ESCA) of films deposited from discharges containing different proportions of nitrogen revealed the incorporation of both CH and CN in the deposited material. Deposition rates were also determined and their connection with deposition mechanisms via gas-phase and surface reactions is outlined.

Properties of titanium oxide films obtained by PECVD

Abstract Amorphous TiO 2 films containing carbon and excess oxygen were deposited from glow discharge plasmas of titanium tetraisopropoxide Ti(OC 3 H 7 ) 4 , helium, oxygen and argon mixtures. The discharge was generated in a stainless steel vacuum chamber by two parallel-plate electrodes connected to a 13.56 MHz power supply. A substrate holder, to which a d.c. bias voltage could be applied, was positioned outside the region between the two electrodes, i.e. in a region of lower plasma density. The substrate voltage bias, V B , and the ratio of the partial pressures of O 2 to Ti(OC 3 H 7 ) 4 [oxygen to precursor (OTP) ratio] in the chamber were adopted as the principal deposition parameters. Details of the molecular structure of the films were investigated by infrared spectroscopy and X-ray photoelectron spectroscopy. The latter was also used to determine the O/Ti and C/Ti atomic ratios at the film surface. The total (surface and bulk) O/Ti and C/Ti atomic ratios were determined using Rutherford backscattering spectroscopy. While neither the surface nor the total O/Ti ratio varied significantly with V B and the OTP ratio, both the surface and total C/Ti ratios varied strongly with these parameters. The refractive index, determined by ultraviolet-visible spectroscopy, and the electrical conductivity, measured by a two-point probe, were influenced by the polarity and magnitude of V B .

Amorphous oxygen‐containing hydrogenated carbon films formed by plasma enhanced chemical vapor deposition

Films were deposited from glow discharge plasmas of acetylene–oxygen–argon mixtures in a deposition system fed with radio frequency power. The principal variable was the proportion of oxygen in the gas feed, Xox. The chemical structure and elemental composition of the films were investigated by transmission infrared spectrophotometry and x‐ray photoelectron spectroscopy. Optical properties—refractive index, absorption coefficient, and optical gap—were determined from transmission ultraviolet‐visible spectroscopy data. The latter also allowed the determination of film thicknesses and hence deposition rates. It was found that the oxygen content of the films and, within limits, the refractive index are controllable by the selection of Xox.

Structural and optical properties of amorphous hydrogenated fluorinated carbon films produced by PECVD

Fluorinated films were deposited from radiofrequency discharges of toluene/trifluorotoluene and toluene/sulfur hexafluoride mixtures. Actinometric optical emission spectroscopy was used to determine trends in the concentrations of the plasma species H, CH, and F as a function of the partial pressure of trifluorotoluene or sulfur hexafluoride present (expressed as a percentage of the total chamber pressure) designated RT and Rs, respectively. Transmission infrared spectrophotometry and electron spectroscopy for chemical analysis revealed that the films contain various fluorine-containing functionalities and that the degree of fluorination increases as RT or Rs is increased. Ultraviolet-visible spectrophotometry of films deposited at various values of RT and of Rs allowed the determination of the absorption coefficients α of the films. From plots of α as a function of photon energy it was possible to calculate the optical gap (E04). An intriguing result was a decline and subsequent rise in E04 as Rs was increased. Molecular modelling using ZINDO/PM3 calculations provided a semi-quantitative explanation of the dependence of E04 on Rs.

Characterization of diamond fluorinated by glow discharge plasma treatment

Abstract The surface fluorination of diamond by treatment in glow discharge plasmas of CF4 for different times has been investigated. High quality diamond films were deposited onto silicon substrates using hot filament chemical vapor deposition (HFCVD). Subsequently, the films were exposed to a radiofrequency glow discharge plasma of CF4 for times ranging from 5 min to 1 h. The effects of the plasma treatment on the surface morphology, diamond quality and elemental composition were investigated using atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. Differences in film roughness caused by the plasma treatment were detected by AFM and confirmed by scanning electron microscopy (SEM). Raman spectroscopic analyses showed that the original diamond was of high quality and that the bulk of each film was unchanged by the plasma treatment. Analyses using XPS revealed increased surface fluorination of the films at longer treatment times. In addition, the density of free radicals in the films was probed using electron paramagnetic resonance spectroscopy (EPRS), revealing that untreated diamond possesses an appreciable density of free radicals (6×1012 g−1) which initially falls with treatment time in the CF4 plasma but increases for long treatment times.

Amorphous hydrogenated fluorinated carbon films produced by PECVD

Abstract films were deposited from discharges of acetylene—helium—carbon tetrafluoride mixtures. Actinometric optical emission spectrometry (AOES) revealed trends in the concentrations of the plasma species H, CH, F and CF 2 as a function of the proportion of CF 4 in the feed R c . The influence of plasma/polymer surface interactions on the plasma concentrations of such species was investigated using a dynamic form of AOES. Plasma H, CH and F species were possibly produced by the following mechanisms: the gas phase fragmentation of their parent molecules (C 2 H 2 and CF 4 ) or H- or F-containing species; the release of H, CH and F units from the deposited polymer surface upon interaction with energetic plasma particles (helium metastables or electrons). Film composition and structure were probed using transmission infra-red spectrophotometry (IRS) and X-ray photoelectron spectroscopy (XPS). The groups —CH, —CO, and —CF were present in each fluorinated film. As revealed by IRS and XPS, methyl groups in the films were steadily replaced by CF x groups ( x =1-3) as R c was increased. Similarly, as revealed by XPS and AOES, the degree of fluorination of the films was positively correlated with the plasma F concentration.

Nitrogenated diamond produced by introducing ammonia into the gas feed in hot-filament CVD

Abstract Nitrogenated diamond and carbon nitride films have been produced using ammnonia as the source of atomic nitrogen. A feed mixture of C 2 H 5 OH+NH 3 +H 2 was used in a hot filament CVD reactor. The deposition parameters were optimized for the growth of well-faceted crystals onto either SiO 2 or undoped diamond substrates. Film structure and morphology were studied using optical and atomic force microscopies, and Raman and photoluminescence spectroscopies. Three types of film, according to the deposition substrate used and the nitrogen doping, have been identified. All films presented semiconductor characteristics and their deep and shallow activation energies have been determined from resistance versus temperature measurements.

Structural and optical properties of plasma-deposited amorphous hydrogenated oxygenated carbon films

Amorphous hydrogenated oxygenated carbon (a-C:O:H) films were deposited from C6H6/O2/He/Ar mixtures in a deposition system fed rf power. The principal variable was the percentage of oxygen in the feed, Rox. Film structure and composition were investigated as a function of Rox using transmission infrared- and x-ray photoelectron spectroscopy. To some degree, greater values of Rox lead to greater incorporation of oxygen functionalities such as OH, C–O, and C=O into the deposited material. As revealed by ultraviolet-visible spectrophotometry, the optical gap E04 increased from ∼3.1 to ∼3.5 eV as Rox was increased from 0% to ∼50%. Semiempirical methods (PM3 and ZINDO-S/CI) allow modeling of the dependence of E04 on Rox. Broad agreement between the results of the experimental and theoretical analyses was obtained.

Fluorine-containing amorphous hydrogenated carbon films

Abstract Films were produced from trifluorotoluenc/hydrogen (TFT/H 2 ) and trifluorotoluene/tetrafluoromethane (TFT/CF 4 ) mixtures by plasma-enhanced chemical vapour deposition (PECVD) in a vacuum system fed with RF power. Actinometric optical emission spectroscopy was used to determine the trends in the plasma concentration [H] of the species H as a function of the proportion of the comonomer (H 2 or CF 4 ) in the plasma feed. Also, the plasma concentration of the species CF 2 , [CF 2 ], was measured as a function of the proportion of CF 4 in the feed. The observed rise in [H] with increasing H 2 flows, and a rise in [CF 2 ] with increasing CF 4 flows, are attributed to the increased supply of H and CF 2 units, respectively, to the discharge. On the other hand, an observed rise in [H] with increasing CF 4 flows implies increased gas-phase reactions involving the comonomer or plasma/polymer-surface reactions. X-ray photoelectron spectroscopy (XPS) of the films deposited at various proportions of H 2 and CF 4 indicates the presence of CH n , CH−CF, C=0, CF and CF 2 groups. The degree of fluorination of the films decreases as the proportion of H 2 in the feed is increased and rises with increasing proportions of CF 4 .

Nitrogenation of diamond by glow discharge plasma treatment

Abstract The effect of nitrogen incorporation on the electrical properties of diamond is currently under intense study. In this work, diamond grown by hot filament chemical vapor deposition was exposed to a radiofrequency (40 MHz) glow discharge of pure nitrogen for times of between 5 min and 1 h. The effect of this exposure on the chemical and elemental composition of the samples was assessed by Raman and X-ray photoelectron spectroscopy. Changes in the electrical resistance with increasing film nitrogenation and temperature were measured. The scope and limitations of this approach to nitrogenation are evaluated.