Modeling of pyrolytic laser direct writing: Noncoherent structures and instabilities

Three-dimensional simulations of pyrolytic laser direct writing from gas-phase precursors are presented. They are based on a fast method for the calculation of temperature distributions induced by an energy beam in deposits of arbitrary shape. Analytical approximations, fast Fourier transform, and the multigrid technique are combined in the algorithm. Temperature dependences of the absorptivities and heat conductivities of the deposit and the substrate have been taken into account. Self-consistent modeling of the growth process allows one to explain oscillations in the height and width of lines caused by the feedback between the shape of the deposit, the temperature distribution, and the growth rate. For the deposition of W from an admixture of WCl6 + H-2 and a-SiO2 substrates, the oscillations originate from a sharp increase in the absorptivity of the deposit with temperature. With the deposition of Si from SiH4, or C from CH4, C2H2, and C2H4, onto a-SiO2, the oscillations are related to the large ratio of height/width of the deposit and the increase in temperature on its upper surface. This increase also explains the transition from line-type to fiber-type growth. The hysteresis of this transition with respect to laser power and scanning velocity is explained as well. The same algorithm can be used in the modeling of pyrolytic etching and e-beam microprocessing when the feedback between the temperature distributions and changes in the processing geometry is important. (C) 1997 American Institute of Physics.