Coherent light generators – Particular active media – Gas
Reexamination Certificate
2001-10-17
2004-07-20
Porta, David (Department: 2878)
Coherent light generators
Particular active media
Gas
C372S055000, C372S057000
Reexamination Certificate
active
06765946
ABSTRACT:
BACKGROUND OF THE INVENTION
Gas discharge lasers such as excimer lasers are well known light sources useful for integrated circuit lithography. These lasers typically comprise two elongated electrodes (for example, about 50 cm in length) separated by about ¾ inch. A high voltage pulse power source provides high voltage electrical pulses to produce discharges between the electrodes to create a gain region in a circulating laser gas. A tangential fan is provided to produce sufficient laser gas flow to remove from the discharge region substantially all debris produced by each discharge prior to the next succeeding discharge. The following four patents assigned to Applicants' employer describe prior art tangential fan designs: U.S. Pat. Nos. 6,034,984, 6,061,376, 6,195,378 B1 and 6,144,686. These patents are incorporated herein by reference.
FIGS. 1
a
and
1
b
are cross-sectional end and side views respectively showing the inner structure of a laser chamber
100
in a conventional excimer laser (see Akins et al., U.S. Pat. No. 4,959,840, issued Sep. 25, 1990, and incorporated herein by reference in its entirety). A laser enclosure
102
provides isolation between a laser chamber interior
105
and the exterior
110
. Typically enclosure
102
is formed by a pair of half enclosure members
112
and
114
(see
FIG. 1
a
), which are coupled together and sealed using an o-ring seal
116
, extending along a perimeter of enclosure
102
. Laser chamber interior
105
is filled to a predetermined pressure with a lasing gas
108
. A pulsed gas discharge is generated in a discharge region
122
by a high voltage pulse applied between a cathode assembly
118
and an anode assembly
120
. The pulsed gas discharge typically produces excited argon fluoride, krypton fluoride, or fluorine molecules, which generate laser pulse output energy. The pulse output energy propagates from discharge region
122
through an optical output window assembly
162
(see
FIG. 1
b
). Cathode assembly
118
and anode assembly
120
, defining discharge region
122
, extend parallel to one another along the length of laser chamber
100
.
Recirculation of lasing gas
108
is provided by a tangential fan
140
, which rotates about an axis
142
and includes a plurality of substantially parallel straight blade members
144
extending along the length of laser chamber
100
between hub members
146
. A typical rotation rate for current tangential fans is on the order of approximately 3800 revolutions per minute (rpm). As shown by arrows in
FIG. 1
a
, the flow of gas
108
is upward through tangential fan
140
and transversely across discharge region
122
as directed by a vane member
152
. Lasing gas
108
that has flowed through discharge region
122
becomes dissociated and heated considerably by the pulsed gas discharge. A gas-to-liquid heat exchanger
158
(not shown in
FIG. 1
b
) extending along the length of laser chamber
100
is positioned in the gas recirculation path to cool the heated gas. Other vane members, e.g. vane members
160
, direct the flow of gas
108
through heat exchanger
158
and elsewhere along the gas recirculation path. Recirculation cools and recombines lasing gas
108
, thereby allowing repetitively pulsed laser operation at rates in the range of 2,000 to 4,000 Hz.
Since lasing gas
108
is recirculated and reused, it is important to maintain cleanliness and to prevent contamination of the gas environment within laser chamber interior
105
, in order to maximize the pulse energy performance, stability, and working life of lasing gas
108
.
In excimer lasers of the type referred to in there patents, the electric discharge produces acoustic waves which can reflect back to the discharge region at a time coincident with a succeeding pulse, especially the next succeeding pulse. These returning shock waves can disturb the gain region produced by the succeeding pulse and adversely affect the quantity of the laser pulse produced in the gain region. One of the important improvements provided by the fans described in the above patents was to vary the circumferential position of the blades to reduce cylindrical symmetry of the fan blades.
Thus, in one of the designs a helical blade pattern was provided. In another design, the fan was segmented and the blades in each segment were offset from each other. Designs were also proposed to improve the stiffness of the fan blade structure. These and other improvements were described in these patents to avoid aerodynamic buffeting effects which could reduce bearing life and adversely affect beam quality.
The improvements described in the above patents provided substantial improvements in laser performance including beam quality. However, Applicants have discovered that even after incorporating the above described improvements, laser beam quality can be adversely affected when the laser is operating at several combinations of specific discharge repetition rates and specific fan rotation rates.
What is needed is a better tangential fan.
SUMMARY OF THE INVENTION
The present invention provides an electric discharge laser apparatus having a laser chamber containing a laser gas and two longitudinal electrodes defining a discharge region for producing electrical discharges and a tangential fan for circulating the laser gas and having blade members configured to minimize adverse effects of reflections of electric discharge generated shock waves back to the discharge region simultaneously with a subsequent discharge.
In preferred embodiments, the blades are arranged in segments in a pattern simulating a double helix. In other preferred embodiments, the blades are arranged in segments with the blades in each segment positioned in an asymmetrical pattern. In other preferred embodiments, the blades in each segment are arranged to simulate a double helix and also arranged asymmetrically with respect to blades in other segments.
In another embodiment, the blades are arranged to simulate the double helix pattern and in addition are arranged in an asymmetrical pattern with respect to the blades in the same segment and/or with respect to blades in other segments.
REFERENCES:
patent: 4686680 (1987-08-01), Hoag et al.
patent: 4923364 (1990-05-01), Nilsen et al.
patent: 4975925 (1990-12-01), Derrickson
patent: 5611667 (1997-03-01), Nagamori et al.
patent: 5770933 (1998-06-01), Larson et al.
patent: 5848089 (1998-12-01), Sarkar et al.
patent: 5870420 (1999-02-01), Webb
patent: 6026103 (2000-02-01), Oliver et al.
patent: 6034984 (2000-03-01), Hofmann et al.
patent: 6321448 (2001-11-01), Makino et al.
Partlo William N.
Rettig Curtis L.
Cray William
Cymer Inc.
Monbleau Davienne
Porta David
LandOfFree
Fan for gas discharge laser does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Fan for gas discharge laser, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fan for gas discharge laser will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3208538