ABSTRACT
Extreme ultraviolet (EUV) lithography extends conventional optical lithography to higher resolution because it utilizes an exceedingly short imaging wavelength (λ = 13.5 nm) and provides a larger depth of focus because it employs a small numerical aperture (NA = 0.25) imaging system. Although it has a number of similarities with visible, deep ultraviolet (DUV), and 193 nm lithography, EUV lithography (EUVL) presents several unique technical challenges. For example, because EUV radiation is not transmitted through ambient air, an EUVL tool must be operated in a vacuum environment. Since all solid materials strongly absorb EUV radiation, conventional refractive optics is not an option and re°ective reticles and optics must be employed. Since no naturally occurring material provides more than about 1% re°ectivity at normal incidence in the EUV spectral region, EUV mirrors must be coated with special multilayer (ML) re°ective coatings. Since EUV sources are ineµcient, a high-temperature plasma source requiring extremely high input power and careful management of waste heat must be used. Furthermore, because hydrocarbons and water vapor are cracked by EUV radiation, depositing carbon on mirror surfaces and oxidizing their re°ective coatings, hydrocarbon and water vapor levels in an EUV exposure tool must be carefully controlled. While the development of EUVL has been and continues to be challenging, the successful insertion of EUVL into semiconductor manufacturing will bring many beneœts, among which are a much wider process window and an ability to print linewidths at 22 nm and smaller.
