Numerical Simulation of Laser Induced Substrate Heating for Direct Write of Mesoscopic Integrated Conformal Electronics
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ABSTRACT: A numerical tool is developed to simulate the optical and thermal interactions of selected lasers with precursors and substrates in support of the emerging technology for the direct write of Mesoscopic Integrated Conformal Electronics (MICE). The code couples the Discrete Ordinate Method (DOM) radiation model with the multi-physics computation fluid dynamics code CFD-ACE to predict the conductive and radiative heat transport in the process. This paper provides a brief overview of the numerical model. Selected simulations are presented including comparison with empirical data. The capabilities, limitations, and potential applications of the model with respect to MICE are discussed. Future model enhancements are proposed.
INTRODUCTION Under DARPA sponsorship, advanced techniques are being developed to allow the direct write of Mesoscopic Integrated Conformal Electronics (MICE). Laser processing of selected precursors is one such technique being developed by CMS Technetronics to enable the direct deposition of electronic components onto diverse substrate materials. For complex circuits, controlling the temperature of the deposition process is critical and potentially difficult when a range of thermal and optical material properties is involved. Temperature control is critical because the temperature dictates the rate, extent and quality of the process. Simulation tools that can predict the temperature history of the deposited substances will help optimize this technology by preventing underheating or overheating of the materials.
THEORETICAL FORMULATION Conductive Heat Transport The foundation of the thermal simulation is the CFD code CFDACE which can solve for the conductive and convective heat transfer through disparate solids and fluids. The basic energy equation to solve is Energy: aph + V. puh = V. q +,r: Vu + dp (1) at
dt
"Pointof Contact: Sam Lowry (256) 726-4853 243 Mat. Res. Soc. Symp. Proc. Vol. 624 © 2000 Materials Research Society
where p, u, p,q, t, and h are density, velocity, pressure, heat flux, stress tensor, and enthalpy, respectively. Radiation Radiative heat transport in the laser process is currently modeled using the Discrete Ordinate Method (DOM) [1,21. The DOM method is capable of modeling the transmittance and absorption of non-gray radiation (i.e., with spectral effects) through semi-transparent materials. It also accounts for the reflection and absorption of radiation at opaque surfaces. The limitation of the DOM method for modeling laser radiation is that it is relatively dispersive as compared to the non-dispersive beam of a laser. This effect is minimized by introducing the laser into the solution domain within less than 100 microns of the substrate to reduce any numerically induced beam divergence prior to striking the surface. Laser Intensity The laser intensity distribution is introduced at the upper boundary of the solution domain assuming either a Gaussian or "Tophat" (square wave) distribution. The duration, pulse frequency, and power of the laser may also be varied.
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