Usually, silicon wafers are used as substrates for bulk micromachining, as they can be anisotropically wet etched, forming highly regular structures.
Such an approach is often used with very specific crystallographic orientations in the raw silicon to produce V-shaped grooves.
The surface of these grooves can be atomically smooth if the etch is carried out correctly, and the dimensions and angles can be precisely defined.
Bulk micromachining starts with a silicon wafer or other substrates which is selectively etched, using photolithography to transfer a pattern from a mask to the surface.
This etch takes advantage of the fact that silicon has a crystal structure, which means its atoms are all arranged periodically in lines and planes.