A method of forming a slot in a substrate comprises growing an oxide layer on a first side of a substrate, patterning and etching the oxide layer to form an opening, forming a material overlying the opening and the oxide layer, removing substrate material through a second side to a first distance from the first side, and anisotropic etching the substrate to create a substrate opening at the first side which is aligned with the opening in the oxide layer during anisotropic etching. The material overlying the opening and the oxide layer is selected so that an anisotropic etch rate of the substrate at an interface of the material and the substrate is greater than an anisotropic etch rate of the substrate at an interface of the oxide layer and the substrate.