Modern lithography systems used in integrated circuit (IC) fabrication employ an optical projection printing that operates almost at the diﬀraction limit. The mask of each IC layout layer is projected onto the wafer substrate, which has been coated with a photoresist material, also called resist. The structure of the resist is altered by exposure to light so that, after the development, a silicon pattern can emerge. The next step can consist of implanting the dopant ions. Note that the small feature, which can be printed in this way, is about the wavelength of the light used.