Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2001-06-29
2002-12-17
Sanghavi, Hemang (Department: 2874)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S020000, C372S057000, C372S099000, C385S037000
Reexamination Certificate
active
06496528
ABSTRACT:
BACKGROUND OF THE INVENTION
Narrow Band Gas Discharge Lasers
Gas discharge ultraviolet lasers used as light sources for integrated circuit lithography typically are line narrowed. A preferred prior art line narrowing technique is to use a grating based line narrowing unit along with an output coupler to form the laser resonance cavity. The gain medium within this cavity is produced by electrical discharges into a circulating laser gas such as krypton, fluorine and neon (for a KrF laser); argon, fluorine and neon (for an ArF laser); or fluorine and helium and/or neon (for an F
2
laser).
Line Narrowing Packages
A sketch of such a prior art system is shown in
FIG. 1
which is extracted from Japan Patent No. 2,696,285. The system shown includes output coupler (or front mirror)
4
, laser chamber
3
, chamber windows
11
, and a grating based line narrowing unit
7
. The line narrowing unit
7
is typically provided on a lithography laser system as an easily replaceable unit and is sometimes called a “line narrowing package” or “LNP” for short. This prior art unit includes two beam expanding prisms
27
and
29
and a grating
16
disposed in a Litrow configuration. Gratings used in these systems are extremely sensitive optical devices. A typical grating surface may have 10,000 grooves per inch created in an aluminum layer or layers on a thick glass substrate. These gratings and techniques for fabricating them are described in U.S. Pat. No. 5,999,318 which is incorporated herein by reference. A prior art technique for avoiding distortion of the grating surface is to mount the grating on a metal grating mount made of a material having a small co-efficient of thermal expansion closely matched to the thermal expansion co-efficient of the grating glass substrate. The gratings deteriorate rapidly under ultraviolet radiation in the presence of oxygen in standard air. For this reason, the optical components of line narrowing units for lithography lasers are typically purged continuously during operation with nitrogen.
FIG. 2
is a sketch showing a prior art line narrowing unit fabricated by Applicants' employer, Cymer, Inc., as a part of a prior art line narrowed lithography KrF laser system incorporating such a device. The unit includes three beam expanding prisms
8
,
10
and
12
, a tuning mirror
14
and a grating
16
. Note that the nitrogen purge from bottle
44
enters the unit on the back side of the tuning mirror
46
to avoid purge flow. directly on the grating face. In this system the wavelength of the laser beam
6
is controlled in a feedback arrangement in which the wavelength of the beam is measured by monitor
22
and computer controller
24
uses the wavelength information to adjust the angular position of tuning mirror
14
to control the wavelength to a desired value. The bandwidth control device
20
is used to mechanically bend grating
16
to make it slightly concave, for example. This device is described in detail in U.S. Pat. No. 5,095,492 assigned to Cymer. Use of this device permits reduction of the bandwidth somewhat, but it still goes out of specification when the laser is run at high duty cycle.
For many years, designers for line narrowed lasers have believed that distortions of the laser beam could be caused by gas flow near the face of the grating. Therefore, laser designers in the past have made special efforts to keep the purge nitrogen from flowing directly on the face of the grating. Several examples of these efforts are described in the Japan Patent 2,696,285 referred to above. In the example shown in extracted
FIG. 1
, the purge flow is directed from N
2
gas bottle
44
toward the back side of grating
16
through port
46
.
Increased Repetition Rates
Line narrowed ultraviolet laser light sources currently in use in the integrated circuit industry typically produce about 10 mJ per pulse at repetition rates of about 1000 Hz and duty factors of about 20 percent. Increased integrated circuit production can be achieved at higher repetition rates and greater duty cycles. Applicants' employer is currently selling a 2000 Hz gas discharge lithography laser and Applicants and their fellow workers have designed a 4000 Hz gas discharge lithography laser. Applicants have experienced difficulties maintaining consistent narrow bandwidths at these higher repetition rates and duty cycles.
A need exists for reliable line narrowing devices and techniques for high repetition rate, high duty cycle gas discharge lasers.
SUMMARY OF THE INVENTION
The present invention provides a grating based line narrowing device for line narrowing lasers producing high energy laser beams. Techniques are provided for minimizing adverse effects of heat produced by the laser beam inside the line narrowing device.
A flexural grating mount is provided which virtually eliminates stress on the grating caused by differential thermal expansion between the grating and the LNP housing structure. In a preferred embodiment the grating which is comprised of a very thin lined aluminum surface on a thick ultra low expansion glass substrate is attached to an aluminum housing structure using a flexural grating mount. At least one flexure joint is provided in the grating mount which permits thermal expansion and contraction of the aluminum housing without producing undesirable mechanical stresses in the glass substrate of the grating. In some embodiments the mount comprises a metal plate and the flexure joint is a H-Flex joint which is machined into the metal plate. In another embodiment, two H-Flex joints are provided. In other embodiments, the flexure joint is a dovetail joint permitting one end of the mount to slip relative to the other.
In another preferred embodiments a stream of gas is directed across the face of the grating. In other embodiments the effect of a hot gas layer on the face of the grating is reduced with the use of helium as a purge gas and in other embodiments the purge gas pressure is reduced to reduce the optical effects of the hot gas layer.
REFERENCES:
patent: 3845409 (1974-10-01), Wada et al.
patent: 4977563 (1990-12-01), Nakatami et al.
patent: 5095492 (1992-03-01), Sandstrom
patent: 6147341 (2000-11-01), Lemaire et al.
patent: 6212217 (2001-04-01), Erie et al.
patent: 6295399 (2001-09-01), Engelberth
patent: 04314374 (1991-04-01), None
patent: 2696285 (1991-12-01), None
patent: 05167172 (1991-12-01), None
patent: 2631607 (1992-10-01), None
Alvarez, et al., “Exposure of Inert Gas Purifers to Air,”Journal of the IEST: 41(6):26-33, (1998).
Press, et al., “Numerical Recipes, The art of Scientific Computing,” Cambridge University Press, pp. 274-277, 289-293 and 312-321, (1990).
Ridgeway, et al., “Use of Atmspheric Pressure Ionization Mass Spectrometry for Evaluating Point-of-Use Perifiers: Intercomparison of Resin Based and Getter Type Purifiers Under Challenge Conditions,”.
Algots John M.
Cybulski Raymond F.
Hulburd William G.
Lysik Michael S.
Titus Clay C.
Cymer Inc.
Ross John R.
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