Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
2000-10-31
2002-12-10
Leung, Quyen (Department: 2828)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S057000, C372S099000
Reexamination Certificate
active
06493374
ABSTRACT:
BACKGROUND OF THE INVENTION
In many laser applications precise control of beam output is desired. One such application for such lasers is the light source for integrated circuit lithography. Currently the KrF excimer laser is the choice light source for state of the art integrated circuit lithography devices. Specifications for the light source are becoming tighter as efforts are made to increase production and produce finer integrated circuit patterns.
Typical specifications for a 248 nm KrF laser call for bandwidths of about 0.6 pm full width half maximum, wavelength stability within 0.1 pm of the specified wavelength and energy dose stability of about ±0.5 percent. In addition, control of beam cross section intensity values are important.
FIG. 1
shows some of the features of a prior art KrF excimer laser system used for IC lithography. The system includes a laser frame structure
5
within which is mounted a laser chamber
3
containing two elongated electrodes (not shown) between which is a gain medium, a line narrowing module (referred to as a “line narrowing package” or LNP)
7
shown disproportionately large and an output coupler
4
. The LNP portion of
FIG. 1
represents a top view of the LNP. The beam cross section is generally rectangular, typically about 3.5 mm wide and about 15 mm high. In prior art devices each of the line narrowing module
7
and the output coupler module
4
comprise frames which are fixedly mounted to laser frame structure
5
. Optical components within the frames of the output coupler module and the line narrowing module are adjusted manually to define the laser resonant cavity. The chamber is adjustably mounted within the laser frame so that it can be finely positioned manually within the defined resonant cavity from time to time in the direction of the beam width as shown by arrows
3
A on FIG.
1
. These adjustments permit a laser technician to align the resonance cavity with the gain medium in order to achieve optimum beam output parameters. In this prior art for example, a three prism beam expander
18
is comprised of prisms
8
,
10
and
12
mounted on prism plate
13
. In the prior art device, prism plate
13
can be manually adjusted in the direction of arrows
13
A as an alignment technique. The prior art device also includes a manual adjustment of the curvature of the surface of grating
16
into an increasingly or decreasingly concave shape by expanding or contracting bending mechanism
20
to place larger or smaller compressive forces on legs
17
A and
17
B. The adjustment is done primarily to control bandwidth of the output beam. Another prior art technique for forcing a concave shape on the grating surface is described in U.S. Pat. No. 5,095,492.
Typical prior art lithography excimer lasers now in use incorporate two automatic feedback controls to regulate pulse energy and nominal wavelength. Pulse energy is controlled in a feedback system by measuring the output pulse energy with a beam output monitor
22
as shown in FIG.
1
and then using these measurements with a computer controller
24
to control the high voltage applied between the electrodes in order to regulate pulse energy within desired limits. The beam output monitor
22
(also called a wavemeter) also measures the nominal or center wavelength and bandwidth of the pulsed output beam. Computer controller
24
adjusts the pivot position of tuning mirror
14
using stepper motor
15
in order to control the nominal wavelength of the beam to within desired limits.
What is needed are improvements which will provide easier, faster and more precise control of laser beam output parameters.
SUMMARY OF THE INVENTION
The present invention provides a smart laser having automatic computer control of pulse energy, wavelength and bandwidth using feedback signals from a wavemeter. Pulse energy is controlled by controlling discharge voltage, wavelength by controlling the position of an R
MAX
mirror and bandwidth is controller by adjusting the curvature of a grating to shapes more complicated than simple convex or simple concave. A preferred embodiment provides seven piezoelectric driven pressure-tension locations on the back side of the grating at 5 horizontal locations to produce shapes such as S shapes, W shapes and twisted shapes. Preferred embodiments include automatic feedback control of horizontal and vertical beam profile by automatic adjustment of a prism plate on which beam expander prisms are located and automatic adjustment of the R
MAX
tilt. Other preferred embodiments include automatic adjustment of the horizontal position of the laser chamber within the resonance cavity.
REFERENCES:
patent: 6212217 (2001-04-01), Erie et al.
Buck Jesse D.
Das Palash P.
Erie Frederick G.
Fomenkov Igor V.
Cymer Inc.
Leung Quyen
Monbleau Davienne
Ross John R.
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