Qi, Xuele (2009-12). Ultrafast Laser Induced Thermo-Elasto-Visco-Plastodynamics in Single Crystalline Silicon. Doctoral Dissertation. Thesis uri icon

abstract

  • A comprehensive model for describing the fundamental mechanism dictating the interaction of ultrafast laser pulse with single crystalline silicon wafer is formulated. The need for establishing the feasibility of employing lasers of subpicosecond pulse width in Laser Induced Stress Waves Thermometry (LISWT) for single crystalline silicon processing motivated the work. The model formulation developed is of a hyperbolic type capable of characterizing non-thermal melting and thermo-elastoviscoplastic deformation as functions of laser input parameters and ambient temperature. A plastic constitutive law is followed to describe the complex elasto-viscoplastic responses in silicon undergoing Rapid Thermal Processing (RTP) annealing at elevated temperatures. A system of nine first-order hyperbolic equations applicable to describing 3-D elasto-viscoplastic wave motions in silicon is developed. The group velocities of certain selected frequency components are shown to be viable thermal indicators, thus establishing the feasibility of exploiting nanosecond laser induced propagating stress waves for the high-resolution thermal profiling of silicon wafers. Femtosecond laser induced transport dynamics in silicon is formulated based on the relaxation-time approximation of the Boltzmann equation. Temperature-dependent multi-phonons, free-carrier absorptions, and the recombination and impact ionization processes governing the laser model and carrier numbers are considered using a set of balance equations. The balance equation of lattice energy and equations of motion of both parabolic and hyperbolic types are derived to describe the complex thermo-elastoplastodynamic behaviors of the material in response to ultrafast laser pulsing. The solution strategy implemented includes a multi-time scale axisymmetric model of finite geometry and a staggered-grid finite difference scheme that allows both velocity and stress be simultaneously determined without having to solve for displacements. Transport phenomena initiated by femtosecond pulses including the spatial and temporal evolutions of electron and lattice temperatures, along with electron-hole carrier density, are found to be functions of laser fluence and pulse width. The femtosecond laser heating model that admits hyperbolic energy transport is shown to remedy the dilemma that thermal disturbances propagate with infinite speed. Non-thermal melting fluence is examined favorably against published experimental data. That it is feasible to explore femtosecond laser induced displacement and stress components for 1K resolution thermal profiling is one of the conclusions reached.
  • A comprehensive model for describing the fundamental mechanism dictating the
    interaction of ultrafast laser pulse with single crystalline silicon wafer is formulated.
    The need for establishing the feasibility of employing lasers of subpicosecond pulse
    width in Laser Induced Stress Waves Thermometry (LISWT) for single crystalline
    silicon processing motivated the work. The model formulation developed is of a
    hyperbolic type capable of characterizing non-thermal melting and thermo-elastoviscoplastic
    deformation as functions of laser input parameters and ambient temperature.
    A plastic constitutive law is followed to describe the complex elasto-viscoplastic
    responses in silicon undergoing Rapid Thermal Processing (RTP) annealing at elevated
    temperatures. A system of nine first-order hyperbolic equations applicable to describing
    3-D elasto-viscoplastic wave motions in silicon is developed. The group velocities of
    certain selected frequency components are shown to be viable thermal indicators, thus
    establishing the feasibility of exploiting nanosecond laser induced propagating stress
    waves for the high-resolution thermal profiling of silicon wafers.
    Femtosecond laser induced transport dynamics in silicon is formulated based on
    the relaxation-time approximation of the Boltzmann equation. Temperature-dependent
    multi-phonons, free-carrier absorptions, and the recombination and impact ionization
    processes governing the laser model and carrier numbers are considered using a set of
    balance equations. The balance equation of lattice energy and equations of motion of
    both parabolic and hyperbolic types are derived to describe the complex thermo-elastoplastodynamic
    behaviors of the material in response to ultrafast laser pulsing. The
    solution strategy implemented includes a multi-time scale axisymmetric model of finite
    geometry and a staggered-grid finite difference scheme that allows both velocity and
    stress be simultaneously determined without having to solve for displacements.
    Transport phenomena initiated by femtosecond pulses including the spatial and temporal
    evolutions of electron and lattice temperatures, along with electron-hole carrier density,
    are found to be functions of laser fluence and pulse width. The femtosecond laser
    heating model that admits hyperbolic energy transport is shown to remedy the dilemma
    that thermal disturbances propagate with infinite speed. Non-thermal melting fluence is
    examined favorably against published experimental data. That it is feasible to explore
    femtosecond laser induced displacement and stress components for 1K resolution
    thermal profiling is one of the conclusions reached.

publication date

  • December 2009