Temperature Characterization of Pulsed Laser Annealing of Semiconductors
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TEMPERATURE CHARACTERIZATION OF PULSED LASER ANNEALING OF SEMICONDUCTORS
D.L. KWONC* AND D.M. KIM** * Dept. of Electrical Engineering,
University of Notre Dame,
Notre Dame, Indiana 46556 **Dept. of Electrical Engineering,
Rice University,
Houston,
Texas 77251
ABSTRACT The dynamic characteristics of lattice temperature, T attained in Si under the influence of a high power laser beam irradiation are examined analytically over a wide range of laser wavelengths, pulse intensities and durations. The strongly temperature-dependent material parameters, such as optical absorption coefficient a(T) and thermal diffusivity D(T) are incorporated in heat diffusion equation, and their nonlinear coupling effect on the ensuing temperature in laser-processed semiconductors are examined. Specifically, the threshold pulse energy for surface melting is characterized as a function of both material and annealing laser beam parameters. This analytic description of pulsed laser heating of semiconductors should provide considerable insight into the transient heating phenomena and localized material modifications.
INTRODUCTION The unique characteristics of laser beam processing technique in fabrication of semiconductor devices could prove to be essential for the realiza4 tion of VLSI and/or VHSI'- . Full utilization of its inherent advantages is contingent upon the quantitative understanding of the underlying physical mechanisms. A quantity of interest is the transient depth profile of lattice temperature, T which ultimately determines the electrical characteristics of laser-processed semiconductors. The transient depth profile of T generated in Si annealed by a high-power laser beam is mainly determined by two parameters (i) the energy deposition depth, a-l(T) and (ii) the heat diffusion 1 2 length, (DT) / , where a(T) is the optical absorption coefficient, D(T) is the thermal diffusivity, and T is the laser heating time. For the case of 5 6 Si, these two parameters drastically decrease with increasing T 9 . Hence the deposition depth of the laser energy is reduced rapidly during laser pulse, and the heat conduction into the bulk of the sample is concomitantly slowed down. In this paper, the dynamics of T attained in Si during pulsed laser annealing is analytically characterized over a wide range of laser wavelengths, pulse intensities and durations, incorporating both temperature-dependent a(T) and D(T).
LASER HEATING OF SILICON For typical nonosecond laser annealing, the screening effect of phonon emission does not play a significant role in the coupling of laser energy to 7 0 the lattice ,1 . As a result, the absorption depth of the laser beam mainly determines the heating profile and the temperature, T grows in time Mat. Res.
Soc. Symp. Proc. Vol.
13 (1983)
tElsevier Science Publishing Co., Inc.
112 according to the following time-dependent, equation:
aT -
at
aT
a T
=
ax
D(T)
T
nonlinear heat diffusion
(I - R)
+
ax
Ia(T)e-a(T)x
cp
(i)
Here, since the specific heat, c(T) and mass density, p(T) are weakly de
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