Coherent light generators – Particular resonant cavity – Specified cavity component
Reexamination Certificate
1999-12-22
2001-02-20
Sanghavi, Hemang (Department: 2874)
Coherent light generators
Particular resonant cavity
Specified cavity component
C372S020000, C372S098000, C372S100000, C372S102000, C372S107000, C372S057000
Reexamination Certificate
active
06192064
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
(typically comprising a partially reflecting mirror) 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 resonance cavity. The chamber is adjustably mounted within the laser frame so that it can be finely positioned manually within the defined resonance 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 embodimet, for example, a 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 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.
In prior art devices stepper motor
15
can be stepped in increments as small as 1 &mgr;m. A lever linkage de-magnifies these steps by a factor of 26 to reduce the size of the step to 38 nm. These linear steps provide pivot movement to tuning mirror
15
about pivot line
17
so that each minimum linear step of stepper motor produces a pivot action of mirror
14
of about 0.47 microradians. From experience a pivot of 0.47 microradian produces a change in the laser nominal wavelength of about 0.05 pm.
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 is controlled by very fine and rapid positioning of an R
MAX
mirror in a line narrowing module. Bandwidth is controller by adjusting the curvature of a grating in the line narrowing module. 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. In preferred embodiments, feedback signals from a wavelength monitor are used to position the R
MAX
mirror. In other preferred embodiments a separate laser beam reflected off the R
MAX
mirror on to a photodiode array is used to position the mirror.
REFERENCES:
patent: 4951285 (1990-08-01), Cole et al.
patent: 4991178 (1991-02-01), Wami et al.
patent: 5095492 (1992-03-01), Sandstrom
patent: 5249192 (1993-09-01), Kuizenga et al.
patent: 5856991 (1999-01-01), Ershov
Algots John M.
Buck Jesse D.
Das Palash P.
Erie Frederick G.
Ershov Alexander I.
Cymer Inc.
Ross, Esq. John R.
Sanghavi Hemang
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