Coherent light generators – Particular active media – Gas
Reexamination Certificate
1999-11-23
2002-12-03
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Gas
C372S055000, C372S057000, C372S060000
Reexamination Certificate
active
06490307
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for stabilizing output beam parameters of a gas discharge laser. More particularly, the present invention relates to maintaining an optimal gas mixture composition over long, continuous operating or static periods using very small gas injections.
2. Discussion of the Related Art
Pulsed gas discharge lasers such as excimer and molecular lasers emitting in the deep ultraviolet (DUV) or vacuum ultraviolet (VUV) have become very important for industrial applications such as photolithography. Such lasers generally include a discharge chamber containing two or more gases such as a halogen and one or two rare gases. KrF (248 nm), ArF (193 nm), XeF (350 nm), KrCl (222 nm), XeCl (308 nm), and F
2
(157 nm) lasers are examples.
The efficiencies of excitation of the gas mixtures and various parameters of the output beams of these lasers vary sensitively with the compositions of their gas mixtures. An optimal gas mixture composition for a KrF laser has preferred gas mixture component ratios around 0.1% F
2
/1% Kr/98,9% Ne (see U.S. Pat. No. 4,393,505, which is assigned to the same assignee as the present application and is hereby incorporated by reference into the present application). A F
2
laser may have a gas component ratio around 0.1% F
2
/99.9% Ne (see U.S. patent application Ser. No. 09/317,526, which is assigned to the same assignee as the present application and is hereby incorporated by reference into the present application). Small amounts of Xe may be added to rare gas halide gas mixtures, as well (see R. S. Taylor and K. E. Leopold,
Transmission Properties of Spark Preionization Radiation in Rare
-
Gas Hailde Laser Gas Mixes
, IEEE Journal of Quantum Electronics, pp. 2195-2207, vol. 31, no. 12 (December. 1995). Any deviation from the optimum gas compositions of these or other excimer or molecular lasers would typically result in instabilities or reductions from optimal of one or more output beam parameters such as beam energy, energy stability, temporal pulse width, temporal coherence, spatial coherence, discharge width, bandwidth, and long and short axial beam profiles and divergences.
Especially important in this regard is the concentration (or partial pressure) of the halogen, e.g., F
2
, in the gas mixture. The depletion of the rare gases, e.g., Kr and Ne for a KrF laser, is low in comparison to that for the F
2
.
FIG. 1
shows laser output efficiency versus F
2
concentration for a KrF laser, showing a decreasing output efficiency away from a central maximum.
FIG. 2
shows how the temporal pulse width (pulse length or duration) of KrF laser pulses decrease with increasing F
2
concentration.
FIGS. 3-4
show the dependence of output energy on driving voltage (i.e., of the discharge circuit) for various F
2
concentrations of a F
2
laser. It is observed from
FIGS. 3-4
that for any given driving voltage, the pulse energy decreases with decreasing F
2
concentration. In
FIG. 3
, for example, at 1.9 kV, the pulse energies are around 13 mJ, 11 mJ and 10 mJ for F
2
partial pressures of 3.46 mbar, 3.16 mbar and 2.86 mbar, respectively. The legend in
FIG. 3
indicates the partial pressures of two premixes, i.e., premix A and premix B, that are filled into the discharge chamber of a KrF laser. Premix A comprised substantially 1% F
2
and 99% Ne, and premix B comprised substantially 1% Kr and 99% Ne. Therefore, for the graph indicated by triangular data points, a partial pressure of 346 mbar for premix A indicates that the gas mixture had substantially 3.46 mbar of F
2
and a partial pressure of 3200 mbar for premix B indicates that the gas mixture had substantially 32 mbar of Kr, the remainder of the gas mixture being the buffer gas Ne.
FIG. 5
shows a steadily increasing bandwidth of a KrF laser with increasing F
2
concentration.
In industrial applications, it is advantageous to have an excimer or molecular fluorine laser capable of operating continuously for long periods of time, i.e., having minimal downtime. It is desired to have an excimer or molecular laser capable of running non-stop year round, or at least having a minimal number and duration of down time periods for scheduled maintenance, while maintaining constant output beam parameters. Uptimes of, e.g., greater than 98% require precise control and stabilization of output beam parameters, which in turn require precise control of the composition of the gas mixture.
Unfortunately, gas contamination occurs during operation of excimer and molecular fluorine lasers due to the aggressive nature of the fluorine or chlorine in the gas mixture. The halogen gas is highly reactive and its concentration in the gas mixture decreases as it reacts, leaving traces of contaminants. The halogen gas reacts with materials of the discharge chamber or tube as well as with other gases in the mixture. Moreover, the reactions take place and the gas mixture degrades whether the laser is operating (discharging) or not. The passive or static gas (i.e., when the laser is not discharging, or operating) lifetime is about one week for a typical KrF-laser.
During operation of a KrF-excimer laser, such contaminants as HF, CF
4
, COF
2
, SiF
4
have been observed to increase in concentration rapidly (see G. M. Jurisch et al.,
Gas Contaminant Effects in Discharge
-
Excited KrF Lasers
, Applied Optics, Vol. 31, No. 12, pp. 1975-1981 (Apr. 20, 1992)) For a static KrF laser gas mixture, i.e., with no discharge running, increases in the concentrations of HF, O
2
, CO
2
and SiF
4
have been observed (see Jurisch et al., above).
One way to effectively reduce this gas degradation is by reducing or eliminating contamination sources within the laser discharge chamber. With this in mind, an all metal, ceramic laser tube has been disclosed (see D. Basting et al.,
Laserrohr für halogenhaltige Gasentladungslaser
” G 295 20 280.1, Jan. 25, 1995/Apr. 18, 1996 (disclosing the Lambda Physik Novatube, and hereby incorporated by reference into the present application)).
FIG. 6
qualitatively illustrates how using a tube comprising materials that are more resistant to halogen erosion (plot B) can slow the reduction of F
2
concentration in the gas mixture compared to using a tube which is not resistant to halogen erosion (plot A). The F
2
concentration is shown in plot A to decrease to about 60% of its initial value after about 70 million pulses, whereas the F
2
concentration is shown in plot B to decrease only to about 80% of its initial value after the same number of pulses. Gas purification systems, such as cryogenic gas filters (see U.S. Pat. No. 4,534,034) or electrostatic particle filters (see U.S. Pat. No. 5,586,134) are also being used to extend KrF laser gas lifetimes to 100 million shots before a new fill is advisable.
It is not easy to directly measure the halogen concentration within the laser tube for making rapid online adjustments (see U.S. Pat. No. 5,149,659 (disclosing monitoring chemical reactions in the gas mixture)). Therefore, it is recognized in the present invention that an advantageous method applicable to industrial laser systems includes using a known relationship between F
2
concentration and a laser parameter, such as one of the F
2
concentration dependent output beam parameters mentioned above. In such a method, precise values of the parameter would be directly measured, and the F
2
concentration would be calculated from those values. In this way, the F
2
concentration may be indirectly monitored.
Methods have been disclosed for indirectly monitoring halogen depletion in a narrow band excimer laser by monitoring beam profile (see U.S. Pat. No. 5,642,374) and spectral (band) width (see U.S. Pat. No. 5,450,436). Neither of these methods is particularly reliable, however, since beam profile and bandwidth are each influenced by various other operation conditions such as repetition rate, tuning accuracy, thermal conditions and aging of the laser tube. That is, the same bandwidth can be generated by different gas compos
de Mos Bruno Becker
Stamm Uwe
Vogler Klaus
Ip Paul
Jackson Cornelius H
Lambda Physik AG
Sierra Patent Group Ltd.
Smith Andrew V.
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