Coherent light generators – Particular active media – Gas
Reexamination Certificate
2001-07-12
2004-04-13
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular active media
Gas
C372S057000
Reexamination Certificate
active
06721345
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods and apparatus for stabilizing output beam parameters of a gas discharge laser. More particularly, the present invention relates to a gas discharge laser system which includes components for monitoring a corona discharge ignition voltage at an electrostatic dust precipitator, to provide gas mixture status information, which is preferably for guiding gas control actions for maintaining an optimal gas mixture composition over long periods.
2. Description of the Related Art
Pulsed gas discharge lasers such as excimer and molecular flourine 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-containing species 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.
Efficiencies of excitation of the gas mixtures and various parameters of output beams of these lasers vary sensitively with the compositions of their gas mixtures. An illustrative gas mixture composition for a KrF laser may have gas mixture component ratios around ~0.1% F
2
/~1.0% Kr/~98.9% Ne (see U.S. Pat. No. 4,393,505, which is assigned to the same assignee and is hereby incorporated by reference). For an ArF laser, an around 1.0% concentration of argon would be used instead of the around 1.0% krypton of the KrF laser. A F
2
laser may have a gas component ratio around ~0.1% F
2
/~99.9% He and/or Ne (see U.S. Pat. No. 6,157,662, which is assigned to the same assignee as the present application and is hereby incorporated by reference). Small amounts of a gas additive, e.g., Xe, may be added to any of these gas mixtures for improving energy stability or overshoot control, for example. (see U.S. patent application Ser. No. 09/513,025, which is assigned to the same assignee as the present application and is hereby incorporated by reference; see also R. S. Taylor and K. E. Leopold, Transmission Properties of Spark Preionization Radiation in Rare-Gas Halide 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 lasers typically results 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-containing species, e.g., F
2
or HCL, in the gas mixture.
FIG. 1
shows laser output efficiency versus fluorine concentration for a KrF-excimer laser, illustrating a decreasing output efficiency away from a central maximum.
FIG. 2
shows the dependence of output energy on driving voltage (i.e., applied by the discharge circuit to the gas mixture at electrodes within a discharge chamber of the laser).
FIG. 3A
illustrates effects of gas mixture aging on laser output energy.
FIG. 3B
further illustrates how the slope of the curve for energy output vs. driving voltage also decreases with aging of the gas mixture.
For industrial applications, it is recognized in the present invention that it would be 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 fluorine 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. For example, uptimes of greater than 95% or even 98% wold be advantageous and may be achieved if precise control and stabilization of output beam parameters, including precise control of the composition of the gas mixture were provided with these laser systems.
Unfortunately, gas contamination occurs during operation of excimer and molecular fluorine lasers due to the aggressive nature of the fluorine or chlorine-containing species 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 impurities in the chamber. Moreover, the reactions take place and the gas mixture degrades whether the laser is operating (discharging) or not. The passive gas 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 reduce the rate of 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)). Gas purification systems, such as cryogenic gas filters (see U.S. Pat. Nos. 4,534,034, 5,136,605, 5,430,752, 5,111,473 and 5,001,721, which are hereby incorporated by reference) or electrostatic particle filters (see U.S. Pat. Nos. 4,534,034 and 5,586,134, which are hereby incorporated by reference) may also be used to extend excimer and molecular fluorine laser gas lifetimes to, e.g., 100 million shots before a new fill of the gas mixture into the laser tube may become advisable.
It is not easy to directly measure the halogen concentration within the laser tube for making rapid online adjustments (for example, see U.S. Pat. No. 5,149,659, disclosing monitoring chemical reactions in the gas mixture, which is hereby incorporated by reference). A more preferable approach may be to indirectly monitor the halogen concentration by monitoring a parameter that varies with a know relationship to the halogen concentration. In such a method, precise values of the parameter would be directly measured, and the F
2
concentration would be calculated from those values or pulled from tables stored in a memory accessible by a control processor of the laser system. In this way, the F
2
concentration may be indirectly monitored (see U.S. patent application Ser. No. 09/734,459, which is assigned to the same assignee as the present application and is hereby incorporated by reference, disclosing indirect monitoring of the composition of the gas mixture by monitoring laser input and/or output beam parameters).
Some methods have been disclosed for such indirect monitoring of halogen depletion in a narrow band excimer laser by monitoring beam profile (see U.S. Pat. No. 5,642,374, hereby incorporated by reference) and spectral (band) width (see U.S. Pat. No. 5,450,436, hereby incorporated by reference). However, beam profile and spectral width are each influenced by various other operational conditions such as repetition rate, tuning accuracy, thermal conditions and aging of the laser tube. Thus, the same spectral width can be generated by different gas compositions depending on these other operating conditions.
Another way of stabilizing, during operation, a gas mixture with a gas composition initially provided within a discharge chamber of an excimer or molecular fluorine gas discharge laser is described in U.S. Pat. No. 6,243,405, which is assigned to the same assignee as the present application and is hereby incorporated by reference
Ahlborn Gerhard
Bragin Igor
Kleinschmidt Juergen
Jr. Leon Scott
Lambda Physik AG
Stallman & Pollock LLP
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