Monte-carlo Simulation of Thin Film Deposition in a Square Groove

1 ABSTRACT A Monte-Carlo programme has been developed to simulate thin film deposition in a square groove. It demonstrates that conformal coating requires (1) the mean free part to be greater than the feature size; (2) a low sticking coefficient for incoming reactant molecules. 2 INTRODUCTION The increasing use of anisotropic etching in the manufacture of integrated circuits is stimulating research into methods of coating steep-walled surface structures uniformly. The problem is particularly acute in the case of metal deposition, where thin portions are likely to lead to early failure. Technical solutions include planarization, bias sputtering, and chemical vapour deposition (CVD). Numerical and theoretical studies of both deposition and etching have been published which include evaporation step coverage 1 , sputtering uniformity 2 , general algorithms 3 and thermodynamic analysis 4. The most common approach has been to set up an initial geometry and to compute a new surface profile point by point by considering the relative orientations of source and surface element. Motion in the gas phase is assumed to be either diffusive or collision-free, a sticking coefficient of unity is used, and surface mobility is neglected in all cases but one 3. Where a 399 plasma or energetic beam is present the angular variaton of etch/deposition rate haB been considered for both physical and chemical processes. These analyses neglect two interacting cases: (1) The transition from collision-free to diffusion-dominated gas phase motion. (2) the effect of a non-zero sticking coefficient on step coverage. The mean free path X varies with gas pressure Xp-constant « 7xl0~ 3 m.Pa for air. (1) For atom clusters and large molecules the constant could be smaller by up to an order of magnitude, making X of order 10-6 m at pressures 10 3-10 4 Pa. In some reduced pressure CVD systems therefore, the reactants could be considered to move by gas flow and diffusion only several X from a stepped surface. Near the surface neither diffusion nor collision free motion hold good. The second case can be posed as a question: to what extent is the conformal coating often obtained by CVD due to a rate-limiting step on the surface? A small sticking coefficient means that the walls and floor of a groove act as sources for reactant molecules, so that conformal coating might be caused by the efficient redistribution of reactants rather than the slow decomposition of molecules on a fully loaded …