Coherent light generators – Particular active media – Gas
Reexamination Certificate
2002-03-28
2004-09-14
Wong, Don (Department: 2828)
Coherent light generators
Particular active media
Gas
C372S059000
Reexamination Certificate
active
06792023
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to excimer and molecular fluorine laser systems, and particularly to line-narrowed systems for producing high spectral purity laser beams.
2. Discussion of the Related Art
Efficient transport of sub-200 nm radiation is complicated by strong absorption by photoabsorbing species such as water vapor and oxygen that are abundant in ambient air. That is, a sub-200 nm beam of radiation will propagate a certain distance along an beam path before it is substantially extinguished by absorptive losses due to any photoabsorbing species such as water, oxygen and hydrocarbons located along the beam path. For example, an extinction length (1/e) for 157 nm radiation emitted by the F
2
-laser is less than a millimeter in ambient air.
High intracavity losses also occur for lasers operating at wavelengths below 200 nm, again due particularly to characteristic absorption by oxygen and water, but also due to scattering in gases and all optical elements. As with the absorption, the short wavelength (less than 200 nm) is responsible for high scattering losses due to the wavelength dependence of the photon scattering cross section.
These complications from absorption and scattering are much less of a problem for lithography systems employing 248 nm light, such as is emitted by the KrF-excimer laser. Species such as oxygen and water in the cavity and atmosphere which absorb strongly below 200 nm, and specifically very strongly around 157 nm for the F
2
laser, and which also absorb radiation around 193 nm for the ArF laser, exhibit negligible absorption at 248 nm. The extinction length in ambient air for 248 nm light is substantially more than ten meters. Also, photon scattering in gases and optical elements is reduced at 248 nm compared with that occurring at shorter wavelengths. In addition, output beam characteristics are more sensitive to temperature-induced variations effecting the production of smaller structures lithographically at short wavelengths such as 157 nm, than those for longer wavelength lithography at 248 nm. Clearly, KrF excimer lasers do not have the same level of problems since the 248 nm light scatters less and experiences less absorption. Due to their continued importance for industrial applications, it is still desired to achieve improved spectral purity 248 nm beams, in addition to improved spectral purity sub-200 nm beams, such as 193 nm and 157 nm beams.
One way to deal with the absorption problems of the 157 nm emission of the F
2
laser, and generally for sub-200 nm radiation, is sealing the beam path with a housing or enclosure and purging the beam path with an inert gas. However, if a flow of purge gas is used in this technique to remove absorbing species from the beam enclosure, then turbulence and gas density fluctuations can tend to distort the wavefront and reduce the spectral purity. In addition, it may also be necessary to perform this purging technique with a very clean inert gas, e.g., containing less than 1 ppm of absorbing species such as water and oxygen. Commercial ultra high purity (UHP) grade gases may be obtained to satisfy these purity requirements at increased cost. Overall, this purging approach is expensive and inconvenient, and may still not provide the degree of spectral purity that is desired.
Another consideration is the energy stability. It is desired to maintain laser energy dose variations, and/or energy moving average variations, to less than, e.g., 0.5% or better. If residual oxygen or water vapor partial pressures fluctuate by 0.5% to 1.0%, e.g., then fluctuations in the absorption of the beam by these species could cause the energy dose stability to fall below desired or even tolerable levels. It is recognized in the present invention that a first step of lowering the partial pressures of photoabsorbing species along the laser beam path would serve to lower the % absorption fluctuation and increase the energy dose stability, even if the % concentrations of these species fluctuate at the same % value. It is desired, then, to have a technique for preparing the beam path of a DUV/VUV laser such that absorption and absorption fluctuations of the beam along the beam path are low enough to meet energy dose stability criteria, e.g., of <0.5%. It is desired to lower the residual pressure of the absorbing species substantially below 100 milliTorr to reduce both optical losses, e.g., to less than around 1% per meter of optical path length, and optical loss fluctuations.
Purge gases (e.g., N
2
, He, Ar, etc.) may be used in industrial lasers such as ArF and F
2
lasers for reducing the level of photoabsorbing species. Use of purge gases in optical modules of these laser resonators as well as KrF laser resonators, such as line-narrowing modules, may be used further to reduce ozone formation in these line-narrowing modules. Even though use of purge gases may be used for these purposes, it is recognized in the present invention that there still exists a comparatively fast effect that is not satisfactorily compensated by use of purge gases in line-narrowing modules, even at low purge flow rates. That effect relates to refractive index fluctuations due to gas density fluctuations in the purge gas inside of the line-narrowing module. It is desired to have a better way to reduce ozone formation in line-narrowing modules for industrial lasers such as KrF lasers, and that preferably also serves to reduce concentrations and concentration fluctuations of photoabsorbing species particularly for sub-200 nm systems such as ArF and F
2
lasers, wherein gas density fluctuations producing refractive index fluctuations are also below levels produced by systems that would use purge gas flows in the line-narrowing modules.
SUMMARY OF THE INVENTION
An excimer or molecular fluorine laser system includes a laser chamber filled with a gas mixture at least including a halogen-containing species and a buffer gas, multiple electrodes, including a pair of main discharge electrodes and at least one preionization electrode, within the laser chamber and connected to a discharge circuit for energizing the gas mixture, and a resonator including a line-narrowing and/or selection module for generating a laser beam at high spectral purity. The line-narrowing module includes one or more line-narrowing and/or selection optics within a sealed module coupled to vacuum equipment through a vacuum port for reducing the pressure within the module,
According to one embodiment, the pressure is maintained to not greater than 80 mbar such that refractive index fluctuations due to density fluctuations of gases in the module are substantially suppressed and for suppressing absorption by contaminant species in the beam path. The one or more line-narrowing optics may include one or more of a grating, beam expander, e.g., including one or more prisms for expanding the beam, an interferometric device, and/or one or more apertures on either side of the laser chamber. The vacuum port may be continuously open to the line-narrowing module to maintain the pressure not higher than 80 mbar, and/or the line-narrowing module may include a vacuum-tight block.
According to another embodiment, the pressure is maintained to not greater than 5 mbar such that refractive index fluctuations due to density fluctuations of gases in the module are substantially suppressed and for suppressing absorption by contaminant species in the beam path. The one or more line-narrowing and/or selection optics may include a grating, beam expander, interferometric device and/or wavefront compensation optic. The interferometric device may be an etalon or a device having non-plane-parallel plates. Refractive index fluctuations are preferably maintained less than 10
−6
.
According to another embodiment, the pressure is maintained to not greater than 10
−3
mbar such that refractive index fluctuations due to density fluctuations of gases in the module are substantially suppressed and for suppressing absorption by contaminant species in the beam path. A
Aab Konstantin
Kleinschmidt Juergen
Lokai Peter
Lambda Physik AG
Nguyen Tuan N.
Stallman & Pollock LLP
Wong Don
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