E. Amanatides

High pressure regime of plasma enhanced deposition of microcrystalline silicon

An investigation of the effect of the total gas pressure on the deposition of microcrystalline thin films form highly diluted silane in hydrogen discharges was carried out at two different frequencies. The study was performed in conditions of constant power dissipation and constant silane partial pressure in the discharge while using a series of plasma diagnostics as electrical, optical, mass spectrometric, and in situ deposition rate measurements together with a simulator of the gas phase and the surface chemistry of SiH4∕H2 discharges. The results show that both the electron density and energy are affected by the change of the total pressure and the frequency. This in turn influences the rate of high energy electron–SiH4 dissociative processes and the total SiH4 consumption, which are favored by the frequency increase for most of the pressures. Furthermore, frequency was found to have the weakest effect on the deposition rate that was enhanced at 27.12MHz only for the lowest pressure of 1Torr. On the ot...

Frequency variation under constant power conditions in hydrogen radio frequency discharges

The effect of driving frequency (13.56–50 MHz) on the electrical characteristics and the optical properties of hydrogen discharges has been studied, under constant power conditions. The determination of the discharge power and impedance was based on current and voltage wave form measurements, while at the same time spatially resolved Hα emission profiles were recorded. As frequency is increased, the rf voltage required for maintaining a constant power level is reduced, while the discharge current increases and the impedance decreases. Concurrently the overall Hα emission intensity decreases and its spatial distribution becomes more uniform. Further analysis of these measurements through a theoretical model reveals that frequency influences the motion of charged species as well as the electron energy and the electric field, resulting in a modification of their spatial distribution. Moreover, the loss rate of charged species is reduced, leading to an increase of the plasma density and to a decrease of the e...

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.

Gas phase and surface kinetics in plasma enhanced chemical vapor deposition of microcrystalline silicon: The combined effect of rf power and hydrogen dilution

A gas phase and surface simulator of highly diluted silane in hydrogen rf discharges used for the deposition of microcrystalline silicon has been developed. The model uses the spatial density distribution of SiH (X 2Π) radicals measured using laser induced fluorescence and the total silane consumption for estimating the primary electron induced silane dissociation, thus avoiding fluid or statistical approaches commonly used for the prediction of electron impact rate coefficients. A critical analysis is made for the relative importance of all the parameters involved either in the gas phase chemistry or in the surface processes. The model results are compared to experimental data concerning disilane production and film growth rate over a wide range of rf power densities in 2% and 6% SiH4 in H2 discharges. The good agreement between experimental and model results allows for the extension of the discussion to the composition of the radical flux reaching the substrate, the relative contribution of each of the ...

Effect of frequency in the deposition of microcrystalline silicon from silane discharges

The influence of frequency in the range from 13.56 to 50 MHz, on the properties of 2% silane in hydrogen 0.5 Torr discharges used for the deposition of microcrystalline silicon thin films, has been investigated. The experiments were carried out under constant power conditions as determined through Fourier transform voltage and current measurements. The increase of frequency leads to a decrease of the rf field, an extension of the bulk, and a marked increase of the electron density and the amount of power consumed by electrons. These changes induce a decrease of the rate of high-energy electron–molecule collision processes (>10.5 eV) at higher frequencies and an enhancement of lower energy processes. Thus, there is a significant increase in the hydrogen flux toward surfaces, which can explain the beneficial effect of frequency to the crystallinity of μc-Si:H thin films. At the same time, SiH4 electron impact dissociation is enhanced mainly due to the increase of electron density. On the contrary, ionizatio...

Covalent immobilization of liposomes on plasma functionalized metallic surfaces.

A method was developed to functionalize biomedical metals with liposomes. The novelty of the method includes the plasma-functionalization of the metal surface with proper chemical groups to be used as anchor sites for the covalent immobilization of the liposomes. Stainless steel (SS-316) disks were processed in radiofrequency glow discharges fed with vapors of acrylic acid to coat them with thin adherent films characterized by surface carboxylic groups, where liposomes were covalently bound through the formation of amide bonds. For this, liposomes decorated with polyethylene glycol molecules bearing terminal amine-groups were prepared. After ensuring that the liposomes remain intact, under the conditions applying for immobilization; different attachment conditions were evaluated (incubation time, concentration of liposome dispersion) for optimization of the technique. Immobilization of calcein-entrapping liposomes was evaluated by monitoring the percent of calcein attached on the surfaces. Best results were obtained when liposome dispersions with 5mg/ml (liposomal lipid) concentration were incubated on each disk for 24h at 37°C. The method is proposed for developing drug-eluting biomedical materials or devices by using liposomes that have appropriate membrane compositions and are loaded with drugs or other bioactive agents.

Deposition rate optimization in SiH4/H2 PECVD of hydrogenated microcrystalline silicon

Abstract Intrinsic hydrogenated microcrystalline silicon films have been deposited by Plasma Enhanced Chemical Vapor Deposition using highly diluted SiH 4 in H 2 discharges, aiming at the increase of the deposition rate. Following a systematic optimization of the main process parameters, an increase of the film growth rate up to 7.5 A/s has been achieved, from 1 Torr 6% SiH 4 in H 2 dust-free discharges at a frequency of 30 MHz. The experimental results are combined to a mass transfer model that can very well predict the deposition rate, for revealing the main reasons leading to the fast growth of μc-Si:H.

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...

Development of a hollow cathode plasma source for microcrystalline silicon thin films deposition

The development of a hollow cathode plasma source and its implementation in an already existing plasma reactor used for the deposition of μc-Si:H thin films is presented. Electrical measurements of hydrogen discharges at 13.56 MHz were carried for two different in geometry hollow electrodes and the electrical parameters were compared with a previous Capacitively Coupled source installed in the same reactor. The results show a significant enhancement of discharge current and higher power dissipation for the same applied voltage. At reduced gas pressures that are necessary to minimize the dust formation, the hollow electrode with the larger cavity geometry showed better performance, achieving high electron densities of above 109 cm−3. Microcrystalline silicon thin films were deposited from SiH4/H2 discharges and an enhancement of the growth rate was found for the hollow cathode source as compared with the CCP source.