Smoothly changing illuminant

A local variation in porosity can be produced by an inhomogeneous illumination intensity. However, any image projected on the backside of the wafer generates a smoothed-out current density distribution on the frontside, because of random diffusion of the charge carriers in the bulk. This problem can be reduced if thin wafers or illumination from the frontside is used. However, sharp lateral changes in porosity cannot be achieved. 

Figure 12.1 Consider a processing element that is located at a position where the illuminant changes linearly from left to right. The plot in (b) shows the smooth linear curve of the illuminant. Local space average color computed by the element on the left will be a little lower and local space average color computed by the element on the right will be a little higher. Both effects cancel each other and space average color will be in equilibrium, which will result in vertical bands. The positive and negative contributions cancel each other.

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