High-pressure arc lamps. A typical arc lamp is filled with mercury (Hg) or mercury-xenon (Hg-Xe) mixtures at a pressure of 30-40 atm. The distribution of the emitted spectrum depends on the partial pressures of Hg and Xe as well as of the total pressure of the discharge plasma. The dominating wavelengths range in the UV domain. The most prominent ones are the G-line at 436 nm used for features sizes down to 0.8 m, and the I-line at 365 nm with a maximal resolution of 0.35 m. At the moment I-line printing is the state of the art technology in industry. Other wavelengths of the spectrum have to be filtered out. Thereby the beam intensity is reduced and the exposure time is increased.
Excimer lasers. For DUV lithography excimer lasers are the most powerful and brightest light sources. A noble gas, e.g., Krypton (Kr) or Argon (Ar), is excited and reacts with a molecule composed of two identical atoms like Fluorine (F).b Laser emission occurs through transitions from a metastable excited state to an unstable ground state. Excimers have a bandwidth of about 1 nm and emit strongly in a multimode fashion with relatively poor spatial coherence. This is a crucial advantage for lithography applications, because it avoids or at least relaxes the problem of ``speckle.'' Speckle means printing of a random pattern caused by phase variations of a narrow linewidth laser due to a nonideal optical system. The high-power pulses of excimer lasers allow extremely short exposure times (10-20 ns), which increases the throughput. The two most attractive wavelengths are 248 nm and 193 nm emitted from a KrF- and ArF-laser, respectively.