Potassium hydroxide (KOH) etching of a patterned [100] oriented silicon wafer produces V-shaped etch pits. We demonstrate that the remaining thickness of silicon at the tip of the etch pit can be reduced to approximately 5 microm using an appropriately sized etch mask and optical feedback. Starting from such an etched chip, we have developed two different routes for fabricating 100 nm scale slits that penetrate through the macroscopic silicon chip (the slits are approximately 850 microm wide at one face of the chip and gradually narrow to approximately 100-200 nm wide at the opposite face of the chip). In the first process, the etched chips are sonicated to break the thin silicon at the tip of the etch pit and then further KOH etched to form a narrow slit. In the second process, focused ion beam milling is used to etch through the thin silicon at the tip of the etch pit. The first method has the advantage that it uses only low-resolution technology while the second method offers more control over the length and width of the slit. Our slits can be used for preparing mechanically stable, transmission electron microscopy samples compatible with electrical transport measurements or as nanostencils for depositing nanowires seamlessly connected to their contact pads.