Plume dynamics of laser-produced aluminum plasma in ambient nitrogen

https://doi.org/10.1016/j.apsusc.2004.09.093Get rights and content

Abstract

We report on the plume dynamics of pulsed laser ablated aluminum in ambient nitrogen pressure varying from 0.01 to 70 Torr using ICCD images of the expanding plasma plume. At pressures of 0.01 to 1 Torr plasma expansion followed shock model whereas at 10 and 70 Torr plasma expansion followed drag model. The difference in the values of vapor and shock temperatures calculated at 0.1 and 70 Torr has been used to understand the formation of AlN in vapor phase. At pressures ≥ 1 Torr and later times plasma–gas interface showed instability in the leading edge of the expanding plume attributed to Rayleigh–Taylor instability.

Introduction

Interaction of laser light with a target results in the formation of plasma provided the intensity of incident laser radiation is above the ablation threshold of the target material. The recombining plasma thus formed has been characterized in terms of plasma density and temperature, velocity of various plasma species, ground state density, three-body recombination rate, etc. using various diagnostic techniques, viz. optical emission spectroscopy (OES) [1], [2], [3], optical time-of-flight measurements (TOF) [4], laser-induced fluorescence (LIF) [5], and Langmuir probe [6]. The expanding plasma has routinely been used for the deposition of thin films of various materials including superconductors [7], semiconductors [8], polymers [9], and superlattices [10]. The quality of the deposited thin films involves optimizing various parameters, viz. laser wavelength and fluence on a target, choice of a proper substrate and its temperature, ambient gas and its pressure, and target–substrate distance. The effect of these parameters on the properties of thin films has been studied using x ray diffraction, atomic force microscopy, scanning electron microscopy, photoluminescence, etc.

In the last few years extensive work has been done to understand the propagation of plasma in ambient gases using fast photography. An intensified charge-coupled device (ICCD) is used to obtain two-dimensional images of the expanding plasma plume. These images are used to calculate the position of the plume front, its velocity, and plume length of the expanding plasma. Solving for conservation of mass, momentum, and energy equations vapor density, vapor pressure, and vapor temperature can be calculated [11]. The plasma expansion dynamics depends on the ambient pressures and shows characteristic dynamics variation at different pressures [12]. In vacuum plasma expands freely and therefore a linear variation is expected between the delay time and plume front position. However, in presence of an ambient gas the collisions between the driven gas (ambient gas) and the driver gas (ejecta from the target) results in the deceleration of expanding plume and formation of shock waves. Therefore, the plasma dynamics has been discussed based on shock wave and drag models [13]. Gas phase nanoparticle synthesis has been reported in the literature [14] which provides a novel way to understand the transportation and subsequent deposition of nanoparticles on a substrate, and to control the particle size. Plume splitting and Rayleigh–Taylor (RT) instability are the other interesting features that have been observed during the course of plasma expansion in presence of an external magnetic field [15]. In an expanding plasma presence of large temperature and density gradients as well as pressure gradients at the plasma front contribute to self-generated magnetic fields and have been investigated in detail [16], [17]. Polarized emission in laser-produced plasmas has been recently demonstrated using OES and possible cause for such an emission has been discussed [12], [18].

In the present work we report on the ablation characteristics of expanding aluminum plasma in nitrogen ambient. Plume dynamics has been discussed in terms of shock and drag models at various ambient nitrogen pressures. The hydrodynamic relations are used to estimate vapor temperature and velocity just behind the shock front. The plume length is measured experimentally and compared with theoretical predictions. The distortions observed in the ICCD images of the expanding plume front are quantified as instability, RT in the laser-ablated plasma.

Section snippets

Experimental details

We used a Q-switched Nd:YAG (DCR-4G, Spectra Physics) laser with a pulse width of 8 ns at full width half maximum (FWHM) at a repetition rate of 10 Hz, operating in the fundamental mode (λ = 1.064 μm) for creating plasma of aluminum in vacuum and in presence of nitrogen gas. The laser beam was focused on the aluminum target in a vacuum chamber to a spot size of ∼260 μm. The target was continuously rotated so that the laser beam falls on the fresh surface. The vacuum chamber was evacuated to a

Plume images

Fig. 1 shows the ICCD images of the expanding aluminum plasma at different pressures of ambient nitrogen at the incident laser energy of 88 mJ. We infer the following information from these images. The plume expansion is observed up to 120 ns in vacuum but as the pressure is increased the plume is observed to survive for longer time (140 ns at 0.01 Torr, 580 ns at 0.1 Torr, 980 ns at 1 Torr, 1180 ns at 10 Torr, and beyond 1400 ns at 70 Torr, respectively). Increase in ambient pressure results in the

Conclusions

In the present work we showed that at low pressures plasma expansion followed shock model whereas at high pressures it followed drag model. Lower limit of the shock wave validity condition agrees well with the recorded ICCD images of the expanding plume. Comparison of experimentally and theoretically obtained plume length showed reasonable agreement at high pressures. AlN formation at 70 Torr is discussed in terms of vapor and shock temperature calculated using ICCD images. Perturbations

Acknowledgements

Work is partly supported by Department of Science and Technology, New Delhi.

References (28)

  • V. Narayanan et al.

    Appl. Surf. Sci.

    (2004)
  • A. De Giacomo

    Spectrochim. Acta B

    (2003)
  • X.T. Wang et al.

    J. Appl. Phys.

    (1996)
  • J.T. Knudston et al.

    J. Appl. Phys.

    (1987)
  • C. Dutouquet et al.

    J. Phys. D: Appl. Phys.

    (2001)
    T. Ikegami et al.

    J. Vac. Sci. Technol. A

    (2001)
  • R.K. Dwivedi et al.

    Int. J. Mod. Phys. B

    (1998)
  • C.M. Gilmore et al.

    Appl. Phys. A

    (2002)
  • S. Im et al.

    J. Appl. Phys.

    (2000)
  • S. Nishio et al.

    J. Appl. Phys.

    (1996)
  • V. Pankove et al.

    J. Appl. Phys.

    (2002)
  • Y.B. Zeldovich et al.

    Physics of Shock Waves and High-temperature Hydrodynamic Phenomena

    (1966)
  • A.K. Sharma et al.

    Appl. Phys. Lett.

    (2004)
  • A. Misra et al.

    IEEE Trans. Plasma Sci.

    (1999)
  • D.B. Geohegan et al.

    Appl. Phys. Lett.

    (1998)
  • Cited by (0)

    View full text