Plasma-on Time Research Articles

Plasma modification and deposition on inner tube faces by pulsed DC discharges

The modification of inner surfaces of tubes becomes increasingly important in many fluid transport applications. However, long tubes require special plasma techniques for a surface functionalization on the nanometer scale without damaging the bulk. This work focuses on pulsed DC glow discharges in inert argon, reactive oxygen, nitrogen and air plasma as well as polymerizing acetylene plasma revealing strong influences to the wetting behavior by the total duration of the plasma discharge time (plasma on-time). While the investigated polymers polyamide, polycarbonate, polyethylene terephtalate, and thermoplastic polyurethane react very differently on the inert and reactive etching (activation) plasma, the deposition of plasma polymer films (polymer-like carbon with decreasing cross-linking at lower plasma intensity and >50% hydrogen content) suppresses all substrate influences.

Ion density increase in high power twin-cathode magnetron system

In contrast to conventional plasma deposition methods, High Power Impulse Magnetron Sputtering (HIPIMS) utilizes extremely high power inputs in short pulses, providing for the discharge current densities up to several A/cm 2. High ion densities are observed not only during the plasma on-time, but also within the afterglow period. Once ions are generated, they can contribute to the peak ion density of the next pulse if the off-time between the pulses is sufficiently small. When the HIPIMS cycle contains two pulses, separated by a short plasma off-time and followed by a long afterglow period, the peak ion density for the second pulse can be 5–7 percents higher than for the first pulse. With a dual cathode system and time-shifted pulsing with distinct magnetron heads it was possible to increase ion density gain up to 40% for Ti targets.

The temporal evolution of the negative ion density in a low duty cycle pulsed reactive magnetron discharge

Time-resolved O− density measurements have been made in the bulk plasma of a unipolar pulsed-DC magnetron using laser photodetachment. The magnetron was operated with a titanium target at a total pressure of 1.3Pa and a fixed oxygen-to-argon partial pressure ratio of 10%. The duty cycle was maintained at 5% but the peak on-time discharge power was varied from 120 to 720 W.For all discharge powers, both the electron (ne) and negative ion (n−) densities increase during the plasma on-time, with the electron density reaching a maximum at the end of this phase. In the off-time, the electron density initially decreases at a rapid rate (characteristic decay time ~25μs) for the first 50μs, followed by a slower rate (~150μs) for the remainder of the off-time, however, the negative ion density continues to increase in this phase, reaching a maximum at about ~150μs after the termination of the discharge power. Both the electron and negative ion densities increase with discharge power. The maximum negative ion density was 2.5×1016m−3 for a peak power of 720W, corresponding to a negative ion-to-electron density ratio n−/ne (α) of about 3. In the long afterglow, this ratio reaches a maximum value of 12 as the electron density decreases faster than the negative ion density. This shows that in the afterglow the plasma is highly electronegative.

A study of the plasma electronegativity in an argon–oxygen pulsed-dc sputter magnetron

Using Langmuir probe-assisted laser photodetachment, the temporal evolution of the O− density was determined in the bulk plasma of a unipolar pulsed-dc magnetron. The source was operated in reactive mode, at a fixed nominal on-time power of 100 W, sputtering Ti in argon–oxygen atmospheres at 1.3 Pa pressure, but over a variation of duty cycles from 5% to 50% and oxygen partial pressures of 10% and 50% of the total pressure.In the plasma on-time, for all duty cycles the negative ion density (n−) rises marginally reaching values typically less than 2 × 1015 m−3 with negative ion-to-electron density ratios, α < 1. However, immediately after the transition from pulse on-to-off, n− falls by about 20–30% as fast O− species created at the cathode exit the system. This is followed by a rapid rise in n− to values at least 2 or 3 times that in the on-time. The rate of rise of n− and its maximum value both increase with decreasing duty cycle. In the off-time, the electron density falls rapidly (initial decay rates of several tens of μs), and therefore the afterglow plasma becomes highly electronegative, with α reaching 4.6 and 14.4 for 10% and 50% oxygen partial pressure, respectively. The rapid rise in n− in the afterglow (in which the electron temperature falls from about 5 to 0.5 eV) is attributed to the dissociative attachment of highly excited oxygen metastables, which themselves are created in the pulse on-time. At the lowest duty of 5%, the long-term O− decay times are several hundred μs.Langmuir probe characteristics show the clear signature that negative ions dominate over the electrons in the off-time. From the ion and electron saturation current ratios, α has been estimated in some chosen cases and found to agree within a factor between 2 and 10 with those obtained more directly from the photodetachment method.

Generation of size and structure controlled Si nanoparticles using pulse plasma for energy devices

Size and structure controlled, non-agglomerated nanoparticles are essential for many applications, such as electronics and energy storage. In this study, inductively coupled and square wave modulated RF (13.56 MHz) plasma was used to control the nanoparticle size, and a heater was installed inside the substrate to change the structure of Si nanoparticles from amorphous to crystalline. We found that the size of Si nanoparticles can be controlled by the plasma on-time. The nanoparticles were not agglomerated and grown mainly by surface deposition. The Si nanoparticles generated can be used in energy devices, such as secondary batteries, supercapacitors and solar cells.

Effect of pulse modulation on particle growth during SiH4 plasma process

The effects of pulse modulation on particle growth by coagulation between particles in a pulsed SiH4 plas- ma reactor were analyzed by using a discrete-sectional method. At the start of the plasma discharge, there is high con- centration of small-sized particles, and, later, the large-sized particles appear and grow by coagulation between small- sized particles. During plasma-off, the monomer generation stops and the particle concentration decreases with time by the effects of particle coagulation and fluid flow. As the pulse frequency decreases or as the duty ratio increases, the large-sized particles grow faster because more monomer particles are generated during longer plasma-on time. These results show that the pulse modulation, the changes of pulse frequency and duty ratio, can play a key role in suppressing the particle growth in the pulsed plasma process efficiently. This study proves that the pulsed plasma process can be applied to reduce the particle contamination in the plasma process for preparation of high quality thin films.

Numerical simulation on silane plasma chemistry in pulsed plasma process to prepare a-Si :H thin films

We numerically calculated the effects of pulse modulation (plasma-on and -off times) on the concentration changes of the chemical species (SiH4, SiHx, SiHx+ and polymerized negative ions) and also the growth rate of a-Si : H thin films in the pulsed SiH4 plasmas. During the plasma-on, SiHx is generated quickly by a fast dissociative reaction of SiH4, but, during plasma-off, SiHx disappears rapidly by a reaction with hydrogen and also by the deposition onto the reactor wall. During the plasma-on, the negative ions are polymerized by the reactions with SiH4, but, during the plasma-off, they disappear by neutralization reactions with positive ions. As the plasma-on time increases or as the plasma-off time decreases, the time-averaged concentrations of SiHx and negative ions and also the time-averaged film growth rate increase. This study shows quantitatively that polymerized negative ions, which are not considered to be preferred precursors for the high-quality thin films, can be efficiently reduced by the pulsed plasma process.

Preparation of magnetic nanoparticles by pulsed plasma chemical vapor synthesis

FePt nanoparticle is expected as a candidate for the magnetic material of the high density recording media. We attempted to synthesize FePt alloy nanoparticles using 13.56 MHz glow discharge plasma with the pulse operation of a square-wave on/off cycle of plasma discharge to control the size of nanoparticles. Vapors of metal organics, Biscyclopentadienyl iron (ferrocene) for Fe and (Methylcyclopentadienyl) trimethyl platinum for Pt, were introduced into the capacitively coupled flow-through plasma chamber, which consisted of shower head RF electrode and grounded mesh electrode. Synthesis experiments were conducted at room temperature under the conditions of pressure 0.27 Pa, source gas concentration 0.005 Pa, gas residence time 0.5 s and plasma powers 60 watts. Pulse width for plasma duration was chosen from 0.5 to 30 s and plasma off period was 4 s to each pulse operation. Visual observations during the particle growth showed plasma emission in the bulk region was increased with the particle growth. These were theoretically explained by using the model for both transient particle charging in the plasma and single particle behavior in the stationary plasma as well as assuming the similarity between the negative charged particle and negative gas containing plasma. Synthesized nanoparticles were directly collected onto TEM grid, which was placed just below the grounded mesh electrode in the plasma reactor downstream. TEM pictures showed two kinds of particles in size, one of which was nanometer size and isolated with crystal structures and the other appeared agglomerate of nanometer size particles. The size of agglomerated particle was controlled in the 10–120 nm range by varying the plasma-on time from 0.5 to 30 s, although the nanometer size particles did not change. The composition of FePt alloy particles could be altered by adjusting the source gas feed ratio. Also magnetization of FePt nanoparticles was measured by use of SQUID (superconducting quantum interference device) magnetometry measurements. As-synthesized FePt nanoparticles did not exhibit loop-shape characteristic, which indicated superpamagnetic property. Annealed nanoparticles with the composition of Fe58Pt42 at 650°C in atmospheric hydrogen showed clear hysterisis loop with the coercivity as large as 10 KOe.

Numerical analysis on particle coating by the pulsed plasma process

The a-Si thin-film growth on particles in the rotating pulsed SiH 4 plasma process was analyzed numerically. The evolutions of chemical concentrations ( SiH 4 , SiH x , SiH x + and polymerized negative ions) in the pulsed plasmas have been shown during the plasma-on and -off. During plasma-on, SiH 4 is consumed by the electron impact dissociative reactions, but, during plasma-off, the disappearance reaction of SiH 4 stops because the electrons disappear in the plasma reactor. During plasma-on, SiH x and SiH x + are generated quickly by a fast dissociative reaction of SiH 4 , but, during plasma-off, SiH x disappears rapidly by a reaction with hydrogen and also by the deposition onto the reactor wall and particles, and SiH x + is consumed quickly by fast neutralization reactions with the negative ions. The negative ions are polymerized by the reactions with SiH 4 during plasma-on, but, disappear by neutralization reactions during plasma-off. The growth rate of the film thickness profile depends on the SiH x concentration because the particles grow with the SiH x deposition. As the plasma-on time increases or as the plasma-off time decreases, the thin film thickness on the particles increases more quickly with faster SiH x deposition onto them. A fraction of the particles falling down in the gas phase ( W FP ) increases as the rotation speed of the plasma reactor increases. As W FP increases, as the particle concentration decreases, or as the particle diameter decreases, the film thickness on the particles increases more quickly because the flux of SiH x toward the particles increases. The pulsed plasma process can efficiently reduce the growth of polymerized negative ions and particles, both of which are not good for high-quality thin films. We showed that the high-quality thin films on the particles can be prepared successfully by deposition of low mass chemical precursors by pulsed plasma processes.

Quantitative Analysis on the Growth of Negative Ions in Pulse-Modulated SiH4 Plasmas

The evolutions of chemical species (SiH4, SiHx, SiHx+, and polymerized negative ions) in the pulse-modulated SiH4 plasmas during the plasma-on and -off times were analyzed by solving the model equations of chemical species. During the plasma-on time, the SiHx concentration increases with time, mainly by the generation reaction from SiH4, but during the plasma-off time, it decreases because of the reaction with H2. During the plasma-on time, the concentrations of negative ions increase with time by the polymerization reactions of negative ions, and those become almost zero in the sheath regions because of the electrostatic repulsion. During the plasma-off time, the concentrations of negative ions decrease with time by the neutralization reactions with positive ions, and some negative ions can diffuse toward the sheath regions because there is no electric field inside the reactor. Our theoretical analysis shows quantitatively that the pulse-modulation plasma technique can be an efficient method to reduce the polymerized negative ion clusters, which are not good precursors for a high-quality thin film and which can also be the sources of particle contamination.

Sn-containing composite thin films by plasma deposition of tetramethyltin

The time scale of the plasma on- and off-time is important for the control of the film composition in pulsed plasma process. Using low plasma on-times for the preparation of Sn-containing thin films with tetramethyltin (TMT) as precursor, significantly different results about the film chemistry and surface morphology were obtained, in contrast to the findings obtained at high plasma on-times in the literature. Upon changing the plasma on-times from 10 to 1 ms with the off-times below 520 ms and holding the other parameters constant, the surface morphology of the deposited films appeared from round particles of 100 nm at 10/330 through 40 nm at 10/510 in diameter to a tight smooth surface at 1/321 by atomic force microscope (AFM), while a stable particle size was reported at 10 ms on-time and 500–2000 ms off-times by other author. Detailed analysis of Fourier Transform Infrared Spectroscopy (FTIR) revealed that the content of the metal element Sn increased with plasma duty cycle or plasma on-time, in accord to the ‘usual’ plasma deposition mechanism. X-ray reflectometry was used to detect the aging effects under ambient condition on the film smoothness, density and thickness, while changes in film composition were observed by FTIR. In agreement with the morphology observation by AFM, a low roughness was found for the films deposited at low duty cycles. The film thickness increased by 3.6% after exposure to air, while a slight decrease in density was found. Higher density of the deposited substance attributed to higher Sn content was detected under higher plasma duty cycle, which was consistent with the FTIR results. Different deposition/ablation balance and weak Sn–C bond were attributed to the discrepancies.

The effect of frequency and duty cycle of a pulsed microwave plasma on the chemical vapor deposition of diamond

We examine the effect of a pulsed microwave discharge on the deposition rate of polycrystalline diamond by varying the pulse repetition rate and duty cycle. A simple model of the dynamic plasma chemistry is developed in order to explain the increase in growth rate with frequency for the same average power. Changing the duty cycle while keeping the total plasma on-time constant resulted in the same film thickness for all duty cycles. One possible implication of this is that growth takes place when the pulse is on.

Synthesis of nanosize Si–C–N powder in low pressure plasmas

Multicomponent powders based on the B–C–N–Si system are of great interest as starting materials for sintering advanced ceramics and composites with improved properties. The square-wave modulation of the electrical power supplied to low pressure (<100 Pa) radio frequency (rf) plasmas has been shown as a suitable method to produce high purity silicon and binary powders, such as SiC, SiN and BN. The present study investigates the synthesis of silicon carbonitride (SiCN) nanometric powder in a plasma-enhanced chemical vapour deposition reactor, by rf glow discharge decomposition of CH 4, SiH 4 and NH 3 gases at room temperature. The output of the rf source (13.56 MHz) was square-wave modulated at a period of 20 s with plasma-on times between 0.05 and 5 s. Transmission electron microscopy showed that the SiCN nanopowder was amorphous and that the average particle size increased from 9 to 100 nm as the plasma-on period increased. The chemical composition of the powder was analyzed by X-ray photoelectron spectroscopy and elemental analysis. Infrared spectroscopy revealed the presence of Si–C, Si–N, C–N and hydrogenated bonds. The infrared absorptions of hydrogenated bonds decreased as the plasma-on time increased, volume ratio with the increase in particle size.

Synthesis of silicon nitride particles in pulsed radio frequency plasmas

Silicon nitride (hydrogenated) particles are synthesized using a pulsed 13.56 MHz glow discharge. The plasma is modulated with a square-wave on/off cycle of varying period to study the growth kinetics. In situ laser light scattering and ex situ particle analysis are used to study the nucleation and growth. For SiH4/Ar and SiH4/NH 3 plasmas, an initial very rapid growth phase is followed by slower growth, approaching the rate of thin film deposition on adjacent flat surfaces. The average particle size can be controlled in the 10–100 nm range by adjusting the plasma-on time. The size dispersion of the particles is large and is consistent with a process of continuous nucleation during the plasma-on period. The large polydispersity is also reported for silicon particles from silane and differs from that reported in other laboratories. The silicon nitride particle morphology is compared to that of silicon and silicon carbide particles generated by the same technique. Whereas Si particles appear as rough clusters of smaller subunits, the SiC particles are smooth spheres, and the Si 3N4 particles are smooth but nonspherical. Postplasma oxidation kinetics of the particles are studied with Fourier transform infrared spectra and are consistent with a hydrolysis mechanism proposed in earlier work with continuous plasmas. Heat treatment of the powder in an ammonia atmosphere results in the elimination of hydrogen, rendering the silicon nitride resistant to atmospheric oxidation.