Diagnostics of Pulsed Plasma Fluxes in Laser Deposition of Thin Films
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Mat. Res. Soc. Symp. Proc. Vol. 397 0 1996 Materials Research Society
Fig. 1: Block diagram of the experimental setup allowing simultaneous quantitative measurements of some plasma parameters the plasma consists only of single charged ions, the energy distribution of the ions can be calculated from time of flight measurements. The distance between target and ion probe as well as the measuring angle can be varied, thus allowing spatially resolved measurements of the plasma parameters. For determination of the degree of ionization it is necessary to know the relative content of the different ion species (single or multiple charged). For that purpose a modified quadrupole mass spectrometer (QMS) without ionization source was arranged above the target and directly near the ion probe. The QMS can detect mass/charge ratios of up to 400 atomic mass units. For a separation of the different charged particles in the quadrupole field it is necessary to keep the plasma density in the quadrupole field as low as possible to avoid shielding effects. Therefore a grid from a titanium foil (50 holes of 30 jim diameter) was mounted in front of the QMS. With a high sensitive and fast amplifier the signal at the Faraday cup of the QMS was directly displayed on an oscilloscope. Both the temporal shape and the total charge has been determined at a given ratio m/e. Beyond it from time of flight measurements with the QMS the energy distribution of the different ion species could be calculated. For the investigations two different lasers were used: a TEA-CO 2 laser (wavelength=l 0,6 jim, pulse duration (FWHM) 180 ns, pulse energy variable up to 4 J) and an excimer laser (wavelength=308 nm, pulse duration (FWHM) 50 ns, pulse energy variable up to 2 J). The pulsed plasma fluxes were produced by focusing the laser beam on a rotating single element target (Ti). The spot size was about 4 mm2 for the excimer and 5 mm2 for the CO 2 laser. The experiments were carried out in a high vacuum stainless steel apparatus at pressures below 10-6 mbar.
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3. MEASUREMENTS The described experimental technique has been used to investigate the parameters of pulsed plasma fluxes, produced by pulsed laser irradiation of the Ti target, in dependence on the irradiation conditions. In Fig 2. the temporal shape of the total ion current pulse in the plasma probe is shown for TEA-CO 2 and excimer laser radiation at similar laser flux densities on the target. In the case of CO 2 laser radiation the ions have a shorter flight time and the peak is relatively sharp, whereas in the plasma produced by excimer laser radiation the pulse has a more broadened, belllike shape and the integrated intensity of the ion current is higher.
Excimer TEA-CO
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Time of flight [Its] Fig. 2: Temporal shape of the ion current pulse in the plasma probe for CO 2 (power 2 2 density Q=6.8x 107 W/cm ) and excimer laser radiation (Q=7.4x 107 W/cm ), Ti target, distance to target L=9cm The measurements with the QMS shown, that the signals measured with the plasma probe consist of s
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