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.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2009-12-7393 |
| Date | 2009 December 1900 |
| Creators | Qi, Xuele |
| Contributors | Suh, Chii-Der |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | application/pdf |
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