Instabilities and Structure Formation in Laser Processing
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D. BAUERLE, E. ARENHOLZ, N. ARNOLD, J. HEITZ, and P.B. KARGL Angewandte Physik, Johannes-Kepler-Universit~t Linz, A-4040 dieter. baeuerle@jk. uni-linz.ac.at
Linz,
Austria,
ABSTRACT This paper gives an overview on different types of instabilities and structure formation in various fields of laser processing. Among the examples discussed in detail are non-coherent structures observed in laser-induced chemical vapor deposition (LCVD), in laser-induced surface modifications, and in laser ablation of polymers. INTRODUCTION Structures that develop on solid or liquid surfaces under the action of laser light can be classified into coherent structures and non-coherent structures. Coherent structures such as the so-called ripples, are directly related to the coherence, the wavelength, and the polarization of the laser light. For non-coherent structures a direct relation to laser parameters is absent [1]. The ranges where different types of coherent and non-coherent structures are observed after UV-laser irradiation of PET (polyethylene-terephthalate) are shown in Fig.l.
60
S 9-
Figure 3non-coherent
Damag on
1
10
(ripples)
structures PET
after
and
observed KrF-laser
oirradiation. The threshold fluence for ablat'ion is 0th(PET,248nm) t 2 40mJ/cm
20
0
1: Coherent
100
1000
10000
NUMBER OF PULSES N 573
Mat. Res. Soc. Symp. Proc. Vol. 397 01996 Materials Research Society
INSTABILITIES IN DIRECT WRITING Non-coherent structures have been observed in different LCVD systems during pyrolytic direct writing. The oscillations shown in Fig.2a are neither related to the wavelength and polarization of the laser light nor to latent heat effects. Their period, A, has been found to increase with laser power, scanning velocity, the size of focus, and the pressure of the reactant gas. In the WC16 + H2 + (02 ) system, the oscillating behavior is closely related to changes in absorbed laser power due to changes in surface absorptivity. If the absorptivity increases with increasing temperature, there is a positive feedback which results in an increase in deposition rate. However, the increase in cross section of the stripe increases the heat loss due to conduction along the stripe, and the temperature drops again. Oscillations occur if in some temperature interval a sufficiently steep increase in absorptivity exists. This increase may be due to changes in surface chemistry and/or morphology. The theoretical analysis of this phenomenon is based on the fact that the heat conductivity of the deposit, KD is much higher than that of the substrate, Ks
i.e. e* = K/KS >> 1. In this case,
the heat flux is directed
mainly along the metal line and then gradually dissipates into the substrate. This allows to treat the problem in one dimension and to calculate the temperature distribution for the given parameters of the line. From the balance between the deposition rate and material removal from the reaction zone due to scanning one can derive simplified equations for the evolution of the height, h, of the stripe and the temperature
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