Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2000-08-28
2004-09-21
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S065000, C030S329000, C074S424710
Reexamination Certificate
active
06795483
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a driving device which changes the orientation angle of optical components in laser apparatus.
2. Description of the Related Art
When an excimer laser or molecular fluorine F
2
laser is used as a stepper light source, the oscillated laser light must be subject to line-narrowing. Also, in order to prevent the central wavelength of the spectrum of the oscillated laser light with narrowed linewidth from being deviated from the target value during exposure, stability control must be performed precisely.
FIG.
4
(
a
) shows a construction surrounding a line-narrowing unit
9
mounted on a conventional laser apparatus, Alternate long and short dash lines in FIG.
4
(
a
) show optical axes of laser light L. FIG.
4
(
b
) is a cross-sectional view taken along line IV—IV in FIG.
4
(
a
).
The linewidth narrowing effect is achieved by reflecting the laser light at a certain fixed angle of incidence (angle of reflection) &phgr; relative to a reflector-type wavelength selecting element, or grating
2
, installed inside the line-narrowing unit
9
. The reflector-type wavelength selecting element
2
is an optical component that selects a wavelength by reflecting the laser light L. Reflector-type wavelength selecting element
2
select a wavelength that corresponds to the angle of incidence (angle of reflection) &phgr;. The angle of incidence &phgr; is determined by the orientation angle &thgr; of the reflection mirror
3
which totally reflects the incident laser light L and directs it unto the reflector-type wavelength selecting element
2
. The orientation angle &thgr; of reflection mirror
3
is changed by linear movement of a feed screw
16
a
(male screw) of the feed screw unit
16
in the directions of arrows B. With rotational movement of a control motor
8
as shown by arrow C, the feed screw
16
a
moves linearly.
During exposure the control motor
8
is drive controlled so as to make the deviation of the central wavelength of the oscillated laser spectrum to be extremely small. Specifically, clockwise or anticlockwise rotation of the drive shaft
8
a
of the control motor
8
, as shown by arrows C, makes the feed screw
16
a
to rotate in the same direction C as the drive shaft
8
a
. Rotation of the feed screw
16
a
results in its reciprocal movement in the direction of arrow B, thus determining the feed position of the feed screw
16
a
. This, in turn, determines the orientation angle &thgr; of the reflection mirror
3
and the angle of incidence &phgr; of the light relative to the reflector-type wavelength selecting element
2
, ad the central wavelength of the laser light L spectrum is thereby fixed at the target wavelength.
Here, a so-called high-precision screw is used for the feed screw unit
16
. The high-precision feed screw is machined so as to make the clearances between the feed screw
16
a
(male screw) and the nut
16
b
(female screw) to be extremely small. Specifically, each pair of feed screw
16
b
and nut
16
b
is managed by spot goods control. To reduce the contact friction between the feed screw
16
a
and the nut
16
b
, a lubricant (grease) is applied in abundant quantity onto a part D where the feed screw
16
a
and the nut
16
b
are contact with each other.
However, even if the thread clearances in the feed screw unit
16
can be made very small by spot goods control, it is structurally impossible to make them extremely small, and very small thread clearances inevitably exist In other words, the backlash in the feed screw unit
16
cannot be completely eliminated. Therefore, when the control motor
8
rotates back and forth in the direction of arrows B, and the feed screw
16
a
moves reciprocally in the directions of arrows B, precision of positioning the reflection mirror
3
is impaired, and if the precision of positioning the reflection mirror
3
is impaired, precision of controlling wavelength of laser light T, to be a target value is impaired, resulting in impaired wavelength stability. In other words, there is a problem of decreased precision in controlling wavelength and decreased wavelength stability.
Another drawback of the conventional feed screw unit
16
is that due to very small thread clearances, significant friction resistance exists during operation of the feed screw unit
16
. This leads to increased rotation torque of control motor
8
that is necessary for driving the feed screw unit
16
. This results in poor linear movement response of the feed screw
16
a
to the drive command to the control motor
8
, and a considerable time is required to change the wavelength of the laser light
1
, to the target wavelength. Therefore, there is a problem of decreased speed of wavelength control.
In order to decrease the friction resistance during operation of the feed screw unit, it is necessary to apply an abundant quantity of a lubricant on contact section T). However, contact section D communicates with the interior of the line-narrowing unit
9
. The excimer laser light L of deep ultraviolet wavelength zone scatters within the line-narrowing unit
9
. This deep ultraviolet light irradiates the lubricant and may promote the chemical decomposition reaction of the lubricant. If the decomposition reaction of the lubricant proceeds, the impurities thereby produced will pollute optical components located inside the line-narrowing unit
9
, and the performance of the laser apparatus may be impaired. Therefore, there is a problem of poor performance of the laser apparatus resulting from pollution of optical components.
OBJECTS AND SUMMARY OF THE INVENTION
With the foregoing in view, it is an object of the present invention to improve precision of wavelength control and wavelength stability, to increase speed of wavelength control and to prevent optical components from being polluted.
To achieve this object, the present invention provides an optical component driving device provided in a laser apparatus, comprising an optical component (
3
) that changes wavelength of the laser light (L) in accordance with an orientation angle (&thgr;)thereof, and a feed screw mechanism (
6
) that convert rotational movement of a rotary actuator (
8
) into linear movement of a feed screw (
6
a
), the orientation angle (&thgr;) of the optical component (
3
) being changed in accordance with the near movement of the feed screw (
6
a
) of the feed screw mechanism (
6
), characterized in that the feed screw (
6
a
) of the feed screw mechanism (
6
) is a ball screw (
6
a
).
The present invention will be described with reference to FIGS.
1
(
a
) and
1
(
b
).
The ball screw unit
6
is a feed screw mechanism that is used by previously applying a load (pre-load) between a ball screw
6
a
and a nut
6
b
and in which the ball screw
6
a
moves linearly by the sliding of the ball. This construction permits to make the mechanical screw clearances to be very small, thus making it possible to eliminate the backlash of the ball screw unit
6
, or to make it non-backlash. Since there is no screw clearance, it is possible to improve the precision of positioning the reflection mirror
3
when the control motor
8
rotates clockwise and anticlockwise as shown by arrows C, and the screw
16
a
moves reciprocally a shown by the arrows B. Better precision of positioning of the reflection mirror
3
results in improvement in precision of controlling the wavelength of the laser light L to the target wavelength, and improvement in wavelength stability
Since the ball screw
6
a
is caused to move linearly by the sliding of the balls, friction resistance during operation of the ball screw unit
6
is low. Therefore, smaller torque of the control motor
8
is required to drive the ball screw unit
6
. As a result, linear movement response of the ball screw
6
a
to the drive command to the control motor
8
can be increased, while the time required for adjustment of the wavelength of the laser light T, to the target value can be reduced.
Furthermore, due to better wavelength control precision and red
Bushida Satoru
Nakane Motoharu
Jr. Leon Scott
Komatsu Ltd.
Welsh & Katz Ltd.
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