Microfabrication processes and apparatuses for fabricating microstructures on a substrate are disclosed. The substrate has a current diffraction grating pattern formed by current surface modulations over at least a portion of the substrate's surface that exhibit a substantially uniform grating linewidth over the surface portion. An immersion depth of the substrate in a fluid for patterning the substrate is gradually changed so that different points on the surface portion are immersed for different immersion times. The fluid changes the linewidth of the surface modulations at each immersed point on the surface portion by an amount determined by the immersion time of that point, thereby changing the current diffraction grating pattern to a new diffraction grating pattern formed by new surface modulations over the surface portion that exhibit a spatially varying grating linewidth that varies over the surface portion.