a method of making an etch structure in a substrate involves the steps of providing a mask on a substrate with a pattern that leaves at least one opening leaving the substrate in direct contact with the ambient , performing an isotropic or quasi - isotropic etch through a mask to create a cavity under the mask , which mask is left behind as a suspended membrane above the cavity ; and performing a subsequent anisotropic etch that etches anisotropically the pattern of the mask in the bottom of the cavity .