Langmuir probe characterization of laser ablation plasmas

Abstract

For laser ablation plumes that are significantly ionized, Langmuir probes have proved to be a useful tool for measuring the plume shape, ion energy distribution, and electron temperature. Typically in laser ablation plasmas the flow velocity is supersonic, which complicates the interpretation of the current-voltage probe characteristic. In this paper we describe some recent developments on the application of Langmuir probes for laser ablation plume diagnosis. We have investigated the behavior of the probe when it is orientated perpendicular, and parallel, to the plasma flow, and show how an analytical model developed for plasma immersion ion implantation, can quantitatively describe the variation of the ion current with probe bias for the case when the plasma flow is along the probe surface. The ion signal recorded by a probe in the parallel position is proportional to the ion density and the square root of the bias voltage. It is shown that the current varies as m i −1/2 so that by comparing the ion signals from the parallel and perpendicular positions it is possible to estimate the mass of the ions detected. We have also determined the temporal variation of electron temperature. A planar probe oriented parallel to the plasma flow, where the ion current due to the plasma flow is eliminated, gives a more reliable measurement of T e ͑Ͻ0.6 eV͒. The measured T e is consistent with the measured ion current, which is dependent on T e when the time taken for an element of plasma to traverse the probe is longer than the time taken for the matrix ion sheath extraction phase.

Heating and compression of a laser produced plasma in a pulsed magnetic field

“…At 600 ns, in the absence of the magnetic field, the ablation plume is approximately semi-ellipsoidal in shape, in agreement with the adiabatic and isentropic model of plume expansion. 13,16 The leading edge of the plume has just reached the coil which is in accordance with the ion signal in Fig. 2(a).…”

“…Fitting the deposition profile yields a value of $2.3 for the aspect ratio of the ablation plume. 16 Using this value, and the deposition rate of 2 Â 10 À3 nm per laser shot in the center of the profile, gives a value of $7.5 Â 10 13 for the number of atoms deposited per pulse, 13,16 if the plume was not skimmed by the coil aperture. The deposition with the field (Fig.…”

“…13 The visible light emission was recorded using a fast, intensified charge coupled device (ICCD) camera (380-700 nm) and with a time-and space-resolving spectrometer operating in the wavelength range of 488-532 nm. Both the ICCD camera and the spectrometer used 200 ns gate widths triggered at a controllable delay after the laser pulse.…”

The effect of relative plasma plume delay on the properties of complex oxide films grown by multi-laser, multi-target combinatorial pulsed laser deposition

“…Further analysis of the plasma plumes was carried out using a planar Langmuir probe [8], held in position in place of the substrate. The probe was biased at -30 V and measurements were taken across a 39 Ω load resistor, with two parallel 0.66 µF capacitors used to prevent charge saturation.…”

Langmuir probe measurements and mass spectrometry of plasma plumes generated by laser ablation of La0.4Ca0.6MnO3

“…Langmuir probe measurement is a relatively simple and direct plasma diagnostic technique. [4][5][6][7][8][9][10] A small metal probe with variable bias voltage is placed in the flowing ablation plasma, and, depending on the polarity of the bias, an electron or positive ion current is collected as the plasma flows past the probe leading to the time-of-arrival (TOA) distribution. The TOA distributions usually show the presence of a smooth current signal for both elemental and multi-component target.…”