Coherent light generators – Particular active media – Gas
Reexamination Certificate
2000-05-23
2003-08-05
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Gas
C372S058000
Reexamination Certificate
active
06603787
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a discharge-pumped excimer laser device having a cross-flow fan whose rotatable shaft is rotatably supported by magnetic bearings, and more particularly to a discharge-pumped excimer laser device with an improved layout for magnetic bearings and improved protective bearings.
2. Description of the Related Art
FIG. 1
of the accompanying drawings schematically shows a conventional excimer laser device. As shown in
FIG. 1
, the conventional excimer laser device has a casing
101
filled with a laser gas, a preliminary ionizing electrode (not shown) disposed in the casing
101
for preliminarily ionizing the laser gas, and a pair of main discharge electrodes
102
disposed in the casing
101
for producing an electric discharge to make it possible to oscillate a laser beam. The casing
101
also houses therein a cross-flow fan
103
for producing a high-speed gas flow between the main discharge electrodes
102
.
The cross-flow fan
103
has a rotatable shaft
104
projecting from opposite ends thereof and rotatably supported in a non-contact manner by a plurality of radial magnetic bearings
106
,
107
disposed on opposite sides of the casing
101
and an axial magnetic bearing
108
disposed near the radial magnetic bearing
106
. The rotatable shaft
104
can be rotated by an induction motor
109
connected to an end thereof near the radial magnetic bearing
107
. The casing
101
has a pair of windows
105
on its opposite ends for emitting the laser beam out of the casing
101
.
When the radial magnetic bearings
106
,
107
are not in operation, the rotatable shaft
104
is supported by protective bearings
110
,
111
that are disposed respectively on the shaft end near the motor
109
and on the shaft end near the radial magnetic bearing
106
. The protective bearings
110
,
111
cannot use a general lubricant for the purpose of preventing the laser gas from being contaminated. The protective bearings
110
,
111
are in the form of rolling bearings each comprising special self-lubricated balls that are highly resistant to corrosion and inner and outer races of stainless steel.
When a high voltage is applied between the main discharge electrodes
102
, an electric discharge occurs therebetween to generate a laser beam. The generated laser beam is emitted through the windows
105
out of the casing
101
. When the electric discharge occurs, the laser gas between the main discharge electrodes
102
is deteriorated and its discharge characteristics are impaired to the extent that no repetitive discharge pumping will be possible. To avoid this shortcoming, the cross-flow fan
103
is operated to circulate the laser gas in the casing
101
to generate a high-speed laser gas flow between the main discharge electrodes
102
. Specifically, the laser gas between the main discharge electrodes
102
is replaced each time an electric discharge occurs therebetween for thereby performing stable repetitive pumping.
In the above conventional excimer laser device, however, the cross-flow fan
103
vibrates relatively largely during operation, tending to cause optical components (not shown) of the excimer laser device to have their optical axes displaced, imposing adverse effects on the properties of the laser beam. Specifically, when the excimer laser device is in operation, the laser gas in the casing
101
is pressurized to a pressure ranging from 1 to 3 kg/cm
2
while the cross-flow fan
103
is rotating. Therefore, the cross-flow fan
103
needs a large drive power, and hence the motor
109
is required to be large in size. The motor
109
applies a rotational drive power to the cross-flow fan, and produces a radial magnetic attractive force which produces vibrations because of an eccentric positional error between its rotor and stator due to assembling errors and machining errors. Inasmuch as the radial magnetic attractive force is greater in proportion to the surface area of the rotor of the motor
109
, the vibrations caused by the radial magnetic attractive force also become greater if the motor
109
is greater in size.
In recent years, there is a demand for discharge-pumped excimer laser devices to produce a high laser beam output by way of highly repetitive pumping. To achieve the highly repetitive pumping, the laser gas between the main discharge electrodes
102
needs to be replaced in a shorter period of time, and hence the laser gas flow generated by the cross-flow fan
103
needs to be higher in speed. The motor
109
needs to be large in size in order to rotate the cross-flow fan
103
at a higher speed. If the motor
109
becomes larger in size, the radial magnetic attractive force produced by the motor
109
is also larger in magnitude. Thus, the motor
109
produces larger vibrations, which make it difficult for the motor
109
to rotate at a higher speed. As a result, the discharge-pumped excimer laser device is unable to carry out stable highly repetitive pumping.
The protective bearings
110
,
111
are positioned on the shaft ends where dust particles produced in the casing
101
during operation find it difficult to reach because such dust particles would otherwise enter rolling surfaces of the protective bearings
110
,
111
to obstruct rotation thereof. With the protective bearings
110
,
111
positioned on the shaft ends, however, when the rotatable shaft
104
of the cross-flow fan
103
is supported by the protective bearings
110
,
111
while the radial magnetic bearings
106
,
107
are not in operation, e.g., while the discharge-pumped excimer laser device is not in operation or is being shipped, the inter-bearing span or distance between the protective bearings
110
,
111
is longer than when the rotatable shaft
104
is supported by the radial magnetic bearings
106
,
107
.
As a result, the static deflection of the rotatable shaft
104
supported by the protective bearings
110
,
111
increases. Consequently, an air gap around the rotatable shaft
104
needs to be increased so as to prevent the outer circumferential surfaces of the rotatable shaft
104
at the radial magnetic bearings
106
,
107
and the motor
109
from physically contacting inner casing surfaces. One problem with the increased air gap is that it reduces the operating forces of the radial magnetic bearings
106
,
107
. Specifically, as the air gap becomes greater, larger magnetic bearings are required. Since the operating forces of magnetic bearings are generally lowered in proportion to the square of the air gap, if the air gap is increased twice, then magnetic bearings that are four times greater in size will be required.
If the rotatable shaft
104
needs to be supported by the protective bearings
110
,
111
due to a failure of the radial magnetic bearings
106
,
107
, then the critical speed of the rotatable shaft
104
is reduced as the inter-bearing span becomes longer than when the rotatable shaft
104
is supported by the radial magnetic bearings
106
,
107
. When the rotatable shaft
104
is supported by the protective bearings
110
,
111
, therefore, it suffers violent vibrations upon rotation, displacing the optical axes of the optical components of the discharge-pumped excimer laser device. For restarting the discharge-pumped excimer laser device, therefore, the optical axes are required to be adjusted again. Accordingly, the discharge-pumped excimer laser device cannot quickly be restarted.
The self-lubricated balls of the protective bearings
110
,
111
have a relatively low allowable rotational speed and allowable load because they have a problem as to their mechanical strength. If the cross-flow fan
103
rotates at higher speeds and the motor
109
becomes larger in size and hence the rotatable shaft
104
becomes larger in size, then the protective bearings
110
,
111
cannot be used due to the insufficient mechanical strength thereof.
FIG. 2
of the accompanying drawings shows the conventional cross-flow fan
103
. As shown in
FIG. 2
, the conventional cross-flow f
Barada Toshimitsu
Nakazawa Toshiharu
Sekiguchi Shinichi
Shinozaki Hiroyuki
Armstrong Westerman & Hattori, LLP
Ebara Corporation
Ip Paul
Menefee James
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