The temporal evolution of plasma density and electron temperature in HiPIMS discharges has been measured using the Langmuir probe and laser Thomson scattering techniques. Measurements were performed (non-simultaneously) at two positions within the plasma; in the low magnetic field strength region on the discharge axis and in the high magnetic field strength region of the magnetic trap, for peak power densities of 450 Wcm −2 and 900 Wcm −2 respectively. The maximum plasma densities and temperatures were 6.9 × 10 19 m −3 and 3.7 eV in the pulse-on time, and values decayed to 4.5 × 10 17 m −3 and 0.1 eV at times up to 250 µs into the afterglow. The results indicate that although intrusive, the Langmuir probe can provide a good indication of electron properties in regions of different electron magnetization in the discharge.High power impulse magnetron sputtering (HiPIMS) is a novel ionized physical vapour deposition (IPVD) technique 1 in which high metal ionization fractions (∼ 70% in Kouznetsov et al 2 ) are obtained through the creation of a dense plasma (∼ 10 19 m −3 ). This necessitates high target power densities (0.5-10 kWcm −2 ) and these are generated using short high-voltage pulses (width ∼ 10 − 100 µs) with low duty cycle (∼ 1%) to prevent target melting. The main advantage of IPVD is that the energy and directionality of the film forming ions can be controlled by biasing the substrate, in contrast to line of sight deposition using neutrals. 3 This results in HiPIMS producing films with superior properties (e.g. denser, harder and smoother) compared to DC magnetron operation. [4][5][6] A schematic of an unbalanced planar magnetron is shown in figure 1. Electrons perform a closed E × B drift inside the last closed flux surface boundary, which confines electrons and produces a dense plasma. This magnetic trap region produces the most intense ionization in the discharge, predominately through the process of electron impact ionization. 7 There are also open flux surfaces that guide electrons which have escaped the trap (and consequently ions through ambipolar diffusion) from the target vicinity to the substrate position. A detailed understanding of the plasma dynamics inside the magnetic trap and at the substrate is required in order to optimize the deposition process. The most fundamental plasma properties for any physics investigation are electron density (n e ) and temperature (T e ).Unfortunately HiPIMS poses a difficult environment for diagnostic operation due to the high sputter flux; the relative abundance of species has a dynamic evolution; n e between pulses spans several orders of magnitude and T e at least an order of magnitude; and the plasma is spatially heterogeneous. These effects are most pronounced inside the magnetic trap. Non-intrusive diagnostics are obviously desirable and examples of such techniques applied to HiPIMS include optical emission spectroscopy 8-10 and recently THz time-domain a) Electronic mail: ryanp@liverpool.ac.uk FIG. 1. Schematic of the magnetic field from an unbalanced p...
Filamentous fungi have a sturdy cell wall which is resistant to the usual DNA extraction procedures. We determined the DNA extraction procedure with the greatest yield of high quality fungal DNA and the least predilection for cross‐contamination of equipment between specimens. Each of six extraction methods was performed using Aspergillus fumigatus hyphae. The six methods were: (1) glass bead pulverization with vortexing; (2) grinding with mortar and pestle followed by glass bead pulverization; (3) glass bead pulverization using 1% hydroxyacetyl trimethyl ammonium bromide (CTAB) buffer in a water bath sonicator; (4) water bath sonication in CTAB buffer; (5) grinding followed by incubation with CTAB; and (6) lyticase enzymatic cell lysis. Genomic DNA yields were measured by spectrophotometry and by visual reading of 2% agarose gels, with shearing assessed by the migration of the DNA on the gel. Genomic fungal DNA yields were highest for Method 1, followed by Methods 5≅2>3≅4≅6. Methods 2 and 5, both of which involved grinding with mortar and pestle, led to shearing of the genomic DNA in one of two trials each. We conclude that the use of glass beads with extended vortexing is optimal for extraction of microgramme amounts of DNA from filamentous fungal cultures.
The temporal evolution of the electron temperature T e and density n e has been measured at two positions on the centre-line of an asymmetrically pulsed bi-polar HiPIMS plasma using incoherent laser Thomson scattering (LTS). The magnetron was operated with a tungsten target in argon atmospheres. The results show that in the plasma afterglow when positive voltage pulses are applied (above a threshold of at least 200 V) significant heating of the electrons can occur in which T e can rise to values comparable to the those measured in HiPIMS on-time. The on-set of the rises in T e are significantly delayed relative to the start of the positive pulse, with the delay time decreasing with the magnitude of the positive voltage. The delay is only weakly dependent on the operating pressure. The presence of large positive pulses can also affect the local electron density with n e seen to decay significantly more quickly in the afterglow than for the corresponding unipolar pulsing case, in which no positive pulse is applied. The LTS measurements were complemented by a time-resolved study of the plasma optical emission (neutral argon and tungsten lines). With increasing positive potentials applied in the afterglow the Ar(I) line intensities grow consistent with increasing T e. Interestingly, W(I) line intensities are detected in the afterglow with positive voltages >200 V despite the termination of all target sputtering, suggesting that tungsten is being re-sputtered from the vessel walls. With the aid of emissive probe measurements of the spatial and temporal evolution of plasma potential profile along the centre-line we discuss the phenomena of plasma electron heating and wall sputtering in the positive pulse. This is done in terms of the existence of a non-sustained reverse discharge, in which the vessel walls become an effective cathode.
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