Dimitrios Mataras

Staphylococcus epidermidis adhesion to He, He/O2 plasma treated PET films and aged materials: Contributions of surface free energy and shear rate

Adhesion studies of bacteria (Staphylococcus epidermidis) to plasma modified PET films were conducted in order to determine the role of the surface free energy under static and dynamic conditions. In particular, we investigated the effect of the ageing time on the physicochemical surface properties of helium (He) and 20% of oxygen in helium (He/O(2)) plasma treated polyethylene terephthalate (PET) as well as on the bacterial adhesion. Treatment conditions especially known to result in ageing sensitive hydrophilicity (hydrophobic recovery) were intentionally chosen in an effort to obtain the widest possible range of surface energy specimens and also to avoid strong changes in the morphological properties of the surface. Both plasma treatments are shown to significantly reduce bacterial adhesion in comparison to the untreated PET. However, the ageing effect and the subsequent decrease in the surface free energy of the substratum surfaces with time - especially in the case of He treated samples - seem to favor bacterial adhesion and aggregation. The dispersion-polar and the Lifshitz-van der Waals (LW) acid-base (AB) thermodynamic approaches were applied to calculate the Gibbs free energy changes of adhesion (DeltaG(adh)) of S. epidermidis interacting with the substrates. There was a strong correlation between the thermodynamic predictions and the measured values of bacterial adhesion, when adhesion was performed under static conditions. By decoupling the (DeltaG(adh)) values into their components, we observed that polar/acid-base interactions dominated the interactions of bacteria with the substrates in aqueous media. However, under flow conditions, the increase in the shear rate restricted the predictability of the thermodynamic models.

Combined effect of electrode gap and radio frequency on power deposition and film growth kinetics in SiH4/H2 discharges

The combined effect of the variation of the interelectrode gap (1.3–2.5 cm) and radio frequency (13.56–50 MHz) on the properties of highly diluted silane in hydrogen discharges used for the deposition of microcrystalline silicon thin films is presented. The investigation included electrical and optical discharge measurements as well as the in situ determination of the film growth rate. In the lower frequencies regime, the increase of the interelectrode gap for the same applied voltage results in higher current flows and higher total power dissipation. On the other hand, at 50 MHz the variation of the interelectrode space has only a slight effect on the total power dissipation, due to the low excitation voltage. However, at all frequencies, the increase of the interelectrode space results in a drop of the power dissipation per discharge volume. This is related to the less effective energy transfer to the electrons that is due to the enhancement of the bulk relative to the sheath ohmic heating. The variatio...

Practical silicon deposition rules derived from silane monitoring during plasma-enhanced chemical vapor deposition

We clarify the difference between the SiH4 consumption efficiency η and the SiH4 depletion fraction D, as measured in the pumping line and the actual reactor of an industrial plasma-enhanced chemical vapor deposition system. In the absence of significant polysilane and powder formation, η is proportional to the film growth rate. Above a certain powder formation threshold, any additional amount of SiH4 consumed translates into increased powder formation rather than into a faster growing Si film. In order to discuss a zero-dimensional analytical model and a two-dimensional numerical model, we measure η as a function of the radio frequency (RF) power density coupled into the plasma, the total gas flow rate, the input SiH4 concentration, and the reactor pressure. The adjunction of a small trimethylboron flow rate increases η and reduces the formation of powder, while the adjunction of a small disilane flow rate decreases η and favors the formation of powder. Unlike η, D is a location-dependent quantity. It is...

Disilane as a growth rate catalyst of plasma deposited microcrystalline silicon thin films

The effect of small disilane addition on the gas phase properties of silane-hydrogen plasmas and the microcrystalline silicon thin films growth is presented. The investigation was conducted in the high pressure regime and for constant power dissipation in the discharge with the support of plasma diagnostics, thin film studies and calculations of discharge microscopic parameters and gas dissociation rates. The experimental data and the calculations show a strong effect of disilane on the electrical properties of the discharge in the pressure window from 2 to 3 Torr that is followed by significant raise of the electron number density and the drop of the sheaths electric field intensity. Deposition rate measurements show an important four to six times increase even for disilane mole fractions as low as 0.3 %. The deposition rate enhancement was followed by a drop of the material crystalline volume fraction but films with crystallinity above 40 % were deposited with different combinations of total gas pressur...

Glancing Angle Deposition Effect on Structure and Light-Induced Wettability of RF-Sputtered TiO2 Thin Films

Crystalline TiO2 films were prepared on unheated glass substrates by radio frequency (RF) reactive magnetron sputtering at normal angle of incidence (a = 0°) and at glancing angle (a = 87°). The effect of the glancing angle on the structure, microstructure, and wetting properties of the films was investigated. The inclination of the substrate led to phase transformation of the deposited films from rutile to either rutile/anatase or anatase, depending on the working pressure. Extreme shadowing at 87° results in a remarkable increase of the films’ porosity and surface roughness. The mechanism of the glancing-angle-induced crystalline phase formation is thoroughly discussed based on the thermodynamic, kinetic, and geometrical aspects of the nucleation and is related with the microstructural changes. Both crystalline phase and microstructure significantly affect the wetting properties of the TiO2 films. Glancing-angle-deposited anatase TiO2 exhibits a high degree of porosity and roughness, a high rate of UV-induced wettability conversion, and a long-term highly hydrophilic nature in dark. Therefore, anatase TiO2 is potentially a good candidate for applications as dye-sensitized solar cells (DSSC)/perovskite solar cells, microfluidic devices, and self-cleaning surfaces prepared on thermosensitive substrates.

Disilane addition versus silane-hydrogen flow rate effect on the PECVD of silicon thin films

The effect of small disilane addition to the silane/hydrogen mixture and of the total silane/hydrogen flow rate on the silicon thin film growth rate and crystallinity were investigated. The study was performed by using simplified gas phase chemistry model along with plasma diagnostics such as electrical and deposition rate measurements. The results showed that even small disilane addition induces an increase in the electron density, silane electron-induced dissociation rate, and film growth rate. The increase in the total flow rate caused a linear increase in the film growth rate despite the negligible effect on the discharge microscopic parameters. Similar deposition rates and crystallinities were achieved with both disilane addition and increase in the flow rate, but the deposition efficiency was much higher in the case of disilane addition. The simplified gas phase chemistry model indicated an increase in the silyl production rate either with the disilane addition or the increase in the flow rate. Almo...

ECWR plasma enhanced chemical vapour deposition of microcrystalline silicon thin films

An ECWR plasma source was used in order to deposit microcrystalline silicon thin films. The effect of input power and silane content on the deposition rate and the materials properties was investigated. Deposition rates, up to 25A/sec, and high crystallinity degree were achieved using high silane content which is in contrast to the conventional CCP method. The amorphous to crystalline transition zone was determined revealing that in ECWR discharges microcrystalline silicon growth is favoured in a wide range of experimental conditions.

Plasma modelling and diagnostics for the prediction of structural changes in silicon thin film deposition

In the present work, we apply a 2D self consistent model of SiH4/H2 discharges in order to investigate the transition from microcrystalline to amorphous silicon growth. The model is used to examine the relative importance of ions and atomic hydrogen in the film growth process. This examination also involves monitoring the changes that take place in the electrical properties of the plasma, the gas phase as well as the surface chemistry near the transition between the two growth regimes. Based on these results, the discussion is extended to the mechanism of a-Si:H to μc- Si:H growth transition and the use of plasma diagnostics that can be used to monitor this transition. Keywords: Micro Crystalline Si, deposition, PECVD, thin film