Topography processing is one source of statistical variation. The geometrical dimensions of the manufactured devices are subject to these variations and thus some of the device's characteristics will scatter, too. Numerous sources can be identified as a cause for this uncertainty. Lithography will only be able to transfer an approximate image of the masks onto the wafer which especially holds true for extremely small features. For another thing, the images prescribed on the wafer's surface by lithography need to be transferred permanently to the wafer itself by etching and deposition operations which cause additional variations. Furthermore, devices will differ due to inhomogeneities of the processing conditions across a wafer.
In order to control and enhance the yield of a fabrication process, its statistical characteristics need to be well known. Furthermore, fabrication processes need to be rendered immune against statistical variations as far as possible.