A Mathematical Model for the Determination of Step Coverage

Step coverage of metal stripes over high angle steps on integrated circuit wafers is-necessary to produce reliable multi-level metal interconnect circuits. In order to determine the optimum deposition geometry for obtaining good step coverage a mathematical model has been developed which applies to all deposition geometries using straight line deposition. The model has been programmed to give a profile of the metal across steps or troughs. It takes into account the oblique incidence shadowing and gives the thickness uniformity across the wafer holder. Assumptions include a sticking coefficient of unity, pressure in the molecular flow range, and negligible mobility of depositing atoms on the surface of the substrate. Various evaporation and sputtering systems using point sources (filaments) and small area sources (boats) have been evaluated using the model. Step angles from 60° to 90° with various orientations on the substrate have been theoretically investigated. Profiles were plotted from the computer data for 90° steps and will be presented and compared with some actual cross sections representative of the assumed conditions for applicability of the model. The major problem areas defined by this work are self-shadowing at the edge of steps due to the oblique incidence necessary to obtain good step coverage and the necessity for the use of rather complex planetary motion type systems in order to obtain good step coverage.