The source of radiation for photolithography has traditionally been a Hg or Hg-rare gas discharge lamp. The emission spectra from a high pressure mercury lamp is shown in Fig.1 . The equilibrium pressure of Hg and the rare gas determine the spectral distribution of the light output. This output is high in the near-UV region 350-450 nm and in the mid-UV region 300-350 nm, and lower in the DUV region 200-300 nm.