Paul M. Bryant

Role of positive ions in determining the deposition rate and film chemistry of continuous wave hexamethyl disiloxane plasmas.

New data shed light on the mechanisms of film growth from low power, low pressure plasmas of organic compounds. These data rebalance the widely held view that plasma polymer formation is due to radical/neutral reactions only and that ions play no direct role in contributing mass at the surface. Ion reactions are shown to play an important role in both the plasma phase and at the surface. The mass deposition rate and ion flux in continuous wave hexamethyl disiloxane (HMDSO) plasmas have been studied as a function of pressure and applied RF power. Both the deposition rate and ion flux were shown to increase with applied power; however, the deposition rate increased with pressure while the ion flux decreased. Positive ion mass spectrometry of the plasma phase demonstrates that the dominant ionic species is the (HMDSO-CH(3))(+) ion at m/z 147, but significant fragmentation and subsequent oligomerization was also observed. Chemical analysis of the deposits by X-ray photoelectron spectroscopy and secondary ion mass spectrometry show that the deposits were consistent with deposits reported by previous workers grown from plasma and hyperthermal (HMDSO-CH(3))(+) ions. Increasing coordination of silicon with oxygen in the plasma deposits reveals the role of ions in the growth of plasma polymers. Comparing the calculated film thicknesses after a fixed total fluence of 1.5 × 10(19) ions/m(2) to results for hyperthermal ions shows that ions can contribute significantly to the total absorbed mass in the deposits.

Negative ion density measurements in a reactive dc magnetron using the eclipse photodetachment method

The density of negative oxygen ions in the bulk plasma of a reactive dc magnetron has been determined for the first time using a combination of laser photodetachment and Langmuir probing. Experimental results are obtained for various O2/Ar gas mixtures (0?100%), applied powers (50?600?W) and total discharge pressures (2?25?mTorr). The measurements reveal that the O? ion dominates over with the latter less than 2% of the total observed. Variation of the operating parameters showed clear trends in the negative ion densities with maxima observed at particular powers (200?W) and oxygen partial pressures (10% O2). The negative ion density was found to increase with the chamber pressure and the main loss reaction for O? was determined to be ion?ion recombination with O+, and Ar+.In this study, the maximum negative ion density obtained was found to be 7.7 ? 1015?m?3 at 200?W applied power, 25?mTorr total pressure and 50% oxygen partial pressure, giving the ratio of the negative ion to electron density, ? = 1.4, indicating that the plasma is moderately electronegative. These new results show that significant concentrations of negative ions are present in the bulk magnetron plasma when operated in Ar/O2 gas mixtures during dc sputtering. The influence of these ions on thin film growth is briefly discussed.

Time-resolved triple probe investigations of a pulsed magnetron discharge

Time-resolved measurements of the electron temperature Te and density ne at the centerline of a bipolar pulsed dc magnetron argon discharge were obtained using a triple probe. Two electron temperature spikes at the pulse transients were observed and are interpreted as being due to the presence of energetic electrons generated during these periods. During the off time the observed rapid decay of Te and gradual decay of ne are shown to be a consequence of enhanced plasma retention due to the magnetized electrons. The rapid rise in ne during the on time was observed to reach a maximum, coinciding with a minimum in Te (with Te decaying rapidly), probably due to enhanced ionization by the energetic electrons. Throughout the rest of the pulse period Te increased slightly whereas ne decreased due to global collisional heating of electrons with an additional energetic electron group formed during the on time. The results also show that the electron temperature and plasma density increase with decreasing duty cycl...

Temporal evolution of an electron-free afterglow in the pulsed plasma polymerisation of acrylic acid.

By use of time and energy-resolved mass spectrometry, negative ions with masses ranging from m/z = 1-287 amu have been observed in the afterglow of a low-pressure (10 mTorr) pulsed acrylic acid polymerizing plasma. The most intense peaks, seen at m/z = 71, 143, 215, and 287, are assigned to the dehydrogenated oligomer of the form [nM-H](-) for n = 1, 2, 3, and 4, respectively. The results strongly suggest that both m/z = 71 and 143 ions are produced in the on period of the pulse cycle (0.1 ms duration), with higher masses m/z = 215 and 287 being produced by neutral ion chemistry in the off period (up to 40 ms in duration). The increase in the intensity of the [3M-H](-) and [4M-H](-) peaks in the off period is accompanied by a rapid fall in the concentration of [M-H]- ions and electrons, the latter decreasing from approximately 10(15) m(-3) to zero within 150 micros. Deep into the afterglow, Langmuir probe measurements show that the charge species only consist of positive and negative ions, present at equal concentrations in excess of approximately 10(14) m(-3) even after 10 ms that is, the plasma is wholly electron free. To describe the growth of large negative ions a number of possible ion-neutral chemical pathways have been postulated, and a calculation of the ambipolar diffusion rates to the walls suggests that, in the off period, the positive and negative ion contribution to the deposition rate is small ( approximately 1%) compared to the net total deposition rate. However, the observations do indicate that it may be necessary to update models of film growth in the pulsed plasma polymerization of acrylic acid to account for negative ions.

Surface protein gradients generated in sealed microchannels using spatially varying helium microplasma

Spatially varied surface treatment of a fluorescently labeled Bovine Serum Albumin (BSA) protein, on the walls of a closed (sealed) microchannel is achieved via a well-defined gradient in plasma intensity. The microchips comprised a microchannel positioned in-between two microelectrodes (embedded in the chip) with a variable electrode separation along the length of the channel. The channel and electrodes were 50 μm and 100 μm wide, respectively, 50 μm deep, and adjacent to the channel for a length of 18 mm. The electrode separation distance was varied linearly from 50 μm at one end of the channel to a maximum distance of 150, 300, 500, or 1000 μm to generate a gradient in helium plasma intensity. Plasma ignition was achieved at a helium flow rate of 2.5 ml/min, 8.5 kVpk-pk, and 10 kHz. It is shown that the plasma intensity decreases with increasing electrode separation and is directly related to the residual amount of BSA left after the treatment. The plasma intensity and surface protein gradient, for the different electrode gradients studied, collapse onto master curves when plotted against electrode separation. This precise spatial control is expected to enable the surface protein gradient to be tuned for a range of applications, including high-throughput screening and cell-biomolecule-biomaterial interactions.

Time resolved 2-D optical imaging of a pulsed unbalanced magnetron plasma

Using wavelength filtered two dimensional (2-D) optical imaging, the temporal and spatial evolution of selected excited species in a pulsed magnetron discharge has been studied. A titanium target was sputtered at a pulse frequency of 100 kHz, in an argon atmosphere, at an operating pressure of 0.27 Pa. The radial information of the emissivity was determined using the Abel inversion technique. The results show strong excitation of the observed species above the racetrack in the on-time, and the possible development of an ion–acoustic wave, initiated after the off–on transition. The on–off transition is accompanied by a burst of light from the plasma bulk consistent with the transient plasma potential reaching about +200 V. During this phase, we argue that there is a release of secondary electrons from the grounded substrate and walls due to ion bombardment, as well as an increased confinement of energetic plasma electrons. The characteristic decay times of the selected transitions at 750.4, 751.5, 810.4 and 811.5 nm (ArI), present within the bandpass width of our filters, is briefly discussed in terms of the production of fast electrons in the system.

Discharge and Plasma Bullet Formation in a Capillary DBD Atmospheric-Pressure Microplasma Jet

Time-resolved ICCD images of discharge and plasma bullet formation in a capillary dielectric barrier discharge at atmospheric pressure have been obtained across the whole period in the voltage cycle. The images clearly show that the capillary discharge ignites at the inner edges of both electrodes, when each electrode is working as a cathode, and this leads to the formation of afterglow plasma bullets traveling upstream and downstream.

Theory of cylindrical Langmuir probes in weakly ionized, non-thermal, stationary and moderately collisional plasmas

A finite length cylindrical Langmuir probe is modelled as an ellipsoid of revolution with spheroidal equipotential surfaces and confocal orthogonal hyperboloidal electric field lines. The theory is applicable in the transition regime of probe operation between the collisionless and fully collisional limits. The plasma is assumed to be weakly ionized, non-thermal and stationary, being characterized by frozen reactions and constant temperatures. It is further assumed that in an isotropic plasma the cold ions follow the field lines, as a result of ion–neutral collisions, in the presheath and sheath regions with collisionless Maxwellian electrons. The governing system of equations is derived and solved numerically with the results presented of the presheath and sheath solutions in collisionless and collisional regimes. These show convergence to the respective collisionless and collisional radial motion limits for spherical and cylindrical probes. Analytical approximations are also obtained for the sheath width (defined as the point where the ions reach the Bohm speed) and the Bohm potential over a wide range of collisionality. The collisional presheath drop according to the perturbation theory of Shih and Levi, as applied to cylindrical probes, is shown to significantly underestimate the numerical results. These are in better agreement with the collisional presheath drop for spheres even for long probes. Application of the theory to experimentally derived probe characteristics is also discussed.

Time-Resolved Imaging of a Silicon Micro-Cavity Discharge Array

Time-resolved imaging of an array of silicon based micro-cavity discharge devices, operating in helium at 700 Torr, is presented. The array is shown to ignite four times per cycle, with the brightest emission corresponding to a distinct current peak in the negative phase of the applied sinusoidal voltage. The cavities do not ignite simultaneously and the instantaneous intensity across the array is non-uniform. Before the brightest emission peak is reached, optical imaging shows some of the cavities to be ignited. During the emission peak, neighboring cavities are successively ignited, resulting in two emission regions which propagate across the array with wavefront velocities of 3 and 6 km/s.

Optimum circuit design for the detection of laser photodetachment signals

Laser photodetachment is a widely used diagnostic technique for the detection of negative ions in low-temperature plasma sources. In one common method electrons, photodetached from the negative ions by an incident laser beam, are collected by a Langmuir probe biased above the local plasma potential. The photodetachment current signal, transmitted through the Langmuir probe feedthrough and interconnecting cables (transmission circuit), is converted into a voltage signal by a detection circuit. The negative ion density and temperature (using the two-pulse technique) are obtained from signal processing. Circuit analysis of the signal acquisition circuit, both transmission and detection parts, is used to obtain the design criteria for the undistorted transmission of the photodetachment signal. The effects of changing the circuit parameters from their optimum values are investigated experimentally in a conventional low-pressure magnetron system. The results show that an inappropriate choice of the circuit parameters can lead to significant distortion of the temporal photodetachment signal, leading to large measurement errors of the negative ion density and temperature.