Uniformity Based on the formula of deposition rate calculated in the previous page, we can calculate further the film thickness at different positions on the substrate, in order to get an image about the uniformity of the film. Considering a large wafer as in the picture above, we calculate the ratio d/d0 between the thicknesses measured in two different points of the substrate, situated at the distance x. The thickness ratio is equal with the ratio between the deposition rates in these points. If one point is situated in the center of the wafer, and the wafer is placed in relation to the source as shown below, then based on the calculated expression for deposition rate: It is observable that film uniformity is a problem for evaporation processes. The larger the wafer (x), the worse the uniformity. Several solutions were found to improve this uniformity.
Improving uniformity
One method to improve uniformity is to move the substrate in front of the source, modifying in this way the distance x.
Another method is placing rotating shutters (can block the evaporating flux to the substrate) between the source and substrate.
The ideal configuration in terms of better uniformity was found to be placing the substrates and source on the circumference of a sphere, like in the picture:
Substrate preparation
