Resonance hairpin and Langmuir probe-assisted laser photodetachment measurements of the negative ion density in a pulsed dc magnetron discharge

Abstract

The time-resolved negative oxygen ion density n− close to the center line in a reactive pulsed dc magnetron discharge (10 kHz and 50% duty cycle) has been determined for the first time using a combination of laser photodetachment and resonance hairpin probing. The discharge was operated at a power of 50 W in 70% argon and 30% oxygen gas mixtures at 1.3 Pa pressure. The results show that the O− density remains pretty constant during the driven phase of the discharge at values typically below 5×1014 m−3; however, in the off-time, the O− density grows reaching values several times those in the on-time. This leads to the negative ion fraction (or degree of electronegativity) α=n−/ne being higher in the off phase (maximum value α∼1) than in the on phase (α=0.05–0.3). The authors also see higher values of α at positions close to the magnetic null than in the more magnetized region of the plasma. This fractional increase in negative ion density during the off-phase is attributed to the enhanced dissociative electron attachment of highly excited oxygen molecules in the cooling plasma. The results show that close to the magnetic null the photodetached electron density decays quickly after the laser pulse, followed by a slow decay over a few microseconds governed by the negative ion temperature. However, in the magnetized regions of the plasma, this decay is more gradual. This is attributed to the different cross-field transport rates for electrons in these two regions. The resonance hairpin probe measurements of the photoelectron densities are compared directly to photoelectron currents obtained using a conventional Langmuir probe. There is good agreement in the general trends, particularly in the off-time.

Inferring plasma parameters from the sheath characteristics of a dc biased hairpin probe

“…Its immense popularity began during the period 2005-2006, following its reincarnation as the hairpin probe (HP) by Peijak et al [2] in 2004. The simplified probe design and the resonance frequency detection technique based on reflectometry have stimulated enormous interest in the scientific domain, evinced by the overwhelming contributions by various authors [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Kim et al [3] applied this technique to measure spatial/temporal variation in electron density inside an industrial plasma reactor.…”

“…One of the significant steps taken was the design of a floating HP by Karkari et al [4,5] and his colleagues in Dublin City University, Ireland; along with this, a technique for measuring time-/phase-resolved electron density in industrial processing plasma was developed [6,7]. This technique has been regularly applied in a wide range of plasma applications such as (i) in dual frequency CCP (capacitively coupled plasma)-discharges [8,9], (ii) for comparing the electron density evolution with optical emission data in pulse inductively coupled plasmas using reactive gases [10] and (iii) for determining negative ions using pulsed laser photo-detachment in electronegative discharges [11][12][13].…”

“…Furthermore, in the thesis by Sirse, the dc biased HP was applied to determine sheath width variation in argon discharge [27]. In the same study, the sheath thickness around the HP had been compared with the help of a step front sheath and the Child-Langmuir model, while the negative ion concentration was determined by laser-assisted photo-detachment in conjunction with the HP for detecting the photo-detached electrons [11].…”

Electron and negative ion dynamics in a pulsed 100 MHz capacitive discharge produced in an O2 and Ar/O2/C4F8 gas mixture

“…The combination of an MRP for measuring excess electron density produced during photo-detachment was also reported [23][24][25]. This combination was previously applied to measure both CW [26] and time-resolved negative ion density [27] in varieties of gas mixtures and discharges. A technique based on pulse bias MRP was also reported for measuring negative ion density without using a laser [28].…”

Equilibrium properties of inhomogeneous partially-magnetized plasma containing negative ions

“…Though this concept is simple, the saturation currents for a cylindrical probe can be highly error-prone due to a number of factors such as contamination on the probe surface, uncertainty in the current collection area, and external factors such as magnetic fields, which can drastically reduce the electron saturation current to the probe. Recently, a few novel concepts to determine the negative ion parameters have been introduced, which are based on DC and pulsed-bias hairpin probes [38][39][40][41][42]. The DC bias hairpin has been used to estimate the sheath radius with the application of negative bias applied to the cylindrical pins [39,42].…”