A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. The controlled synthesis of materials as thin films is a fundamental step in many applications. A familiar example is the household mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface. The process of silvering was once commonly used to produce mirrors, while more recently the metal layer is deposited using techniques such as sputtering. Advances in thin film deposition techniques during the 20th century have enabled a wide range of technological breakthroughs in areas such as magnetic recording media, electronic semiconductor devices, integrated passive devices, LEDs, optical coatings , hard coatings on cutting tools, and for both energy generation and storage . It is also being applied to pharmaceuticals, via thin-film drug delivery. A stack of thin films is called a multilayer.