Cleaning, an essential step when operating on the micro and nanoscale, is used to remove contaminants from the surface of the substrate before it is used in another fabrication process. This sometimes means simply burning away material, often using an asher to do this. Ashers use heat to remove unwanted material, such as photoresist. They are often used to clean a wafer, although they can sometimes be used to selectively etch away material as well.