Efficient new vuv light source
01/01/1998
Efficient new VUV light source
The search for short wavelength radiation sources for lithography has recently concentrated on pulsed lasers and laser-produced plasmas. Now, researchers at Rutgers University and Munich Technical University have demonstrated an efficient and bright UV and VUV excimer (CW) light source, based on continuous e-beam excitation. The enabling technology for this source (see figure) is an e-beam window made of SiNx, which is both thinner and stronger than conventional titanium foil windows. The windows are manufactured using standard semiconductor-industry methods. High gas pressure on one side facilitates the formation of brightly-emitting excimers, while the low energy absorption in the window allows the use of continuous low-voltage (10-20 keV) electron beams.
The low energy absorption in the foil and the use of a rugged 20 keV electron gun make this source remarkably reliable. According to researcher H. Dahi, it has been operated continuously for hours without any degradation in performance. Conversion efficiencies from electron kinetic energy to radiation as high as 35% have been measured with pure argon and xenon excimers. The actual volume of the light source is a few cubic centimeters. Radiative power densities as high as 35 W/cm2 (per unit area of foil) have been measured.
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The VUV source consists of an electron gun in the vacuum region, a silicon nitride e-beam window and a high pressure region that emits radiation transmitted though the magnesium fluoride window.
The second excimer continua of argon, krypton, and xenon with peak intensities at 125, 145, and 172 nm, respectively are observed for pure gas pressures above 100 mbar. Similar results would be expected for neon (83 nm) and helium (60 nm). Adding specific impurities to the gas causes the dominant wavelength to shift. For example, the familiar ArF excimer band at 193 nm appears when a high-pressure argon-fluorine gas mixture is used. Since the resulting emission is CW, the pulse stability problems familiar to the designers of laser-based step-and-scan systems can be avoided. Multiphoton absorption-initiated damage to optics should also cease to be a concern. However, the excimer bands are generally too broad for lithography with systems having chromatic aberration. To narrow the emission spectrum, one can add a species that emits preferentially on a narrow atomic line. Adding hydrogen to neon, for example, transfers 10% of the electron beam energy to the Lyman-a line at 121.6 nm, with an extremely narrow linewidth.
The brightness of this VUV source results from highly efficient rare gas excimer production, which requires: a) high excitation energy, b) low gas temperature, c) high gas pressure, and d) low impurity levels, among other things. Condition (a) creates excited atoms. Conditions (b) and (c) allow effective molecule (e.g., Ar2*) formation and minimize the transfer of pump energy into other channels. Condition (d) is important to prevent the quenching of the excited atoms and molecules by unwanted impurities. - K.D.