Coherent light generators – Particular active media – Gas
Reexamination Certificate
2000-11-22
2003-08-05
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular active media
Gas
C372S020000, C372S019000, C372S102000, C372S100000, C372S098000
Reexamination Certificate
active
06603788
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to line-narrowed excimer and molecular fluorine laser systems, and particularly to line-narrowing at an output coupling side of the laser resonator.
2. Discussion of the Related Art
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems operating around 248 nm, as well as the following generation of ArF-excimer laser systems operating around 193 nm. The ArF and KrF lasers have a broad characteristic bandwidth around 600 pm (FWHM). Vacuum UV (VUV) will use the F
2
-laser which characteristically emits two or three closely spaced lines around 157 nm.
It is important for their respective applications to the field of sub-quarter micron silicon processing that each of the above laser systems become capable of emitting a narrow spectral band of known bandwidth and around a very precisely determined and finely adjustable absolute wavelength.
Techniques for reducing bandwidths by special resonator designs to less than 100 pm (for ArF and KrF lasers) for use with all-reflective optical imaging systems, and for catadioptric imaging systems to less than 0.6 pm, are being continuously improved upon.
For the application of excimer lasers as light sources for steppers and/or scanners for photographic microlithography, it is desired to have laser emission within a range that is much small than the natural linewidth which is approximately 300 to 400 pm for ArF and KrF lasers. The extent of the desired line narrowing depends on the imaging optics of the stepper/scanner devices. The desired bandwidth for catadioptic systems is less than around 50 pm, and for refractive optics it is less than around 0.8 pm. Currently, used systems for the KrF laser emitting around 248 nm have a bandwidth around 0.6 pm. To improve the resolution of the projection optics, a narrower laser bandwidth is desired for excimer laser systems of high reliability and very small bandwidth of 0.4 pm or less.
A line-narrowed excimer or molecular fluorine laser used for microlithography provides an output beam with specified narrow spectral linewidth. It is desired that parameters of this output beam such as wavelength, linewidth, and energy and energy dose stabilty be reliable and consistent. Narrowing of the linewidth is generally achieved through the use of a linewidth narrowing and/or wavelength selection and wavelength tuning module (hereinafter “line-narrowing module”) consisting most commonly of prisms, diffraction gratings and, in some cases, optical etalons.
The line-narrowing module typically functions to disperse incoming light angularly such that light rays of the beam with different wavelengths are reflected at different angles. Only those rays fitting into a certain “acceptance” angle of the resonator undergo further amplification, and eventually contribute to the output of the laser system.
For the broadband excimer lasers such as the ArF and KrF lasers mentioned above, the central wavelength of the line-narrowed output beam may be tuned within their respective characteristic spectra. Tuning is typically achieved by rotating the grating or highly reflective (HR) mirror of the line-narrowing module. It is recognized in the present invention that it is also desirable to have tuning within a single selected line of the molecular fluorine laser for very narrow bandwidth, precise wavelength applications.
Line-narrowing is typically performed at the rear optics module of the laser resonator, while output coupling of the laser beam from the laser resonator is typically performed at the front optics module, or at the opposite side of the discharge chamber as the line-narrowing module.
FIG. 1
illustrates this principle and schematically shows a conventional line-narrowed laser resonator. The resonator includes a discharge chamber
1
having line-narrowing optics such as a line-selecting prism
4
and a highly reflective mirror
2
or a beam expander and a grating on one side of the discharge chamber
1
, and an output coupler
3
on the other side of the discharge chamber
1
. A beam separation box
5
is also shown in
FIG. 1
including beam splitters
6
and
7
and an energy detector
8
, for monitoring the energy of the output beam
9
. A disadvantage of these configurations is that radiation emanating from the discharge chamber toward the front optics module, and which is directly output coupled on a first pass, never traverses the line-narrowing optics in the rear optics module. This background radiation serves to limit the attainable degree of the spectral purity of the beam. It is recognized in the present invention that it would be desirable to suppress this background radiation to improve the spectral purity of the beam.
Excimer and molecular fluorine laser systems also typically include feedback wavelength and energy monitoring components, such as the energy monitoring components illustrated in FIG.
1
. In addition, beam steering optics are typically included for maintaining a proper alignment of the beam. Each of these components is disposed outside of the resonator at the output coupling end of the resonator. Thus, these laser systems include complex electro-optical units at both the front and rear of the discharge chamber. It is desired to simplify the laser resonators of excimer and molecular fluorine laser systems.
Another development goal for excimer and molecular fluorine laser systems is to have system that operate at higher and higher repetition rates. Greater throughput may be achieved in this way. However, optical components such as apertures within the resonator are more susceptible at higher repetition rate to heating and related distortions. It is recognized that at these higher repetition rates it is desirable to reduce or prevent thermal heating of apertures within the resonator.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an excimer or molecular fluorine laser system wherein background radiation originating from a single pass gain in the discharge chamber is suppressed to improve the spectral purity of the output beam.
It is a further object of the invention to provide a simplified laser resonator for an excimer or molecular fluorine laser system.
It is another object of the invention to reduce or prevent thermal heating of apertures that may part of a resonator of a laser operating at high repetition rates such as 2-8 kHz or more.
In accord with these objects, an excimer or molecular fluorine laser system is provided having a discharge chamber filled with a laser gas mixture at least including a halogen gas and a buffer gas, a plurality of electrodes within the discharge chamber connected to a power supply circuit for energizing the gas mixture, and a resonator for generating a laser beam including a line-narrowing module on one side of the discharge chamber for reducing a bandwidth of the laser beam. The laser beam is output coupled from the resonator on the same side of the discharge chamber as the line-narrowing module and after the beam is line-narrowed at the line-narrowing module. A substantially total intensity of the laser beam impinges upon a line-narrowing optical element of the line-narrowing module and is thereby line-narrowed. The line-narrowing optical element serves to disperse the beam such that only a selected portion of the spectral distribution of the beam incident at the line-narrowing optical element remains within a beam acceptance angle.
In further accord with the above objects, an excimer or molecular fluorine laser system is provided having a discharge chamber filled with a laser gas mixture at least including a halogen gas and a buffer gas, a plurality of electrodes within the discharge chamber connected to a power supply circuit for energizing the gas mixture, and a resonator for generating a laser beam including a line-narrowing module on one side of the laser chamber and at least one aperture for reducing a bandwidth of the laser beam. The laser beam is output coupled from the resona
Stamm Uwe
Vogler Klaus Wolfgang
Jr. Leon Scott
Lambda Physik AG
Sierra Patent Group Ltd.
Smith Andrew V.
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