Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2000-12-18
2003-02-04
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S029011, C372S029010, C372S029014, C372S029021
Reexamination Certificate
active
06516013
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to excimer and molecular fluorine lasers, and particularly to an apparatus and method for monitoring the energy of output laser pulses in a feedback arrangement and providing enhanced energy stability.
DISCUSSION OF THE RELATED ART
For many industrial and laboratory applications, excimer lasers are used in an operating mode wherein active stabilization of the output power of the laser is important. The active stabilization of the output energy of excimer lasers typically involves an energy detector indirectly connected to a control component of the high voltage drive for the discharge in the laser tube and, accordingly, actively adjusting the drive voltage to stabilize the output energy. This is possible because, as the output energy or output power of the excimer laser is selected to be maintained in a certain range, it is known that this output energy value depends upon the input high voltage drive, as is illustrated at FIG.
1
. Thus, a variation of output energy may be compensated by adjusting the high voltage drive. See U.S. patent application Ser. Nos. 09/379,034, 60/123,928 and 60/124,785 (describing techniques for compensating output energy variation based on halogen depletion including gas replenishment, as well as high voltage adjustments over limited voltage ranges), each of which is assigned to the same assignee as the present invention and which is hereby incorporated by reference into the present application.
The present invention relates to the field of industrial excimer and molecular fluorine lasers and the application of these lasers in optical lithography, annealing, micro machining, and others. Excimer lasers used for these applications are mainly XeCl lasers (308 nm), KrF lasers (248 nm), and ArF lasers (193 mn). Molecular fluorine (F2) lasers (157 nm) are also used as well.
In these applications, optical imaging systems are used in combination with the laser. These imaging systems usually transform the output beam of the laser prior to illumination of a mask with the transformed laser beam. The mask is than imaged to the sample (wafer, workpiece) to form the desired light pattern for exposure.
A clear aperture is typically used in the optical imaging system causing the spatial beam size of the incident laser beam to be smaller after the aperture. The aperture improves homogeneity of illumination at the sample, as well as changes or varies the laser beam size, profile and/or divergence over the gas, laser tube, or other component lifetimes. For any particular application and a given sample or workpiece to be processed, the illumination energy density at the workpiece has been determined during process development or is determined with some calibration routines before or during processing.
For processing, the excimer or molecular fluorine laser is operated in an energy-stabilized or power-stabilized mode. An internal energy or power meter measures the output energy/power of the laser. A feedback circuit compares the actual energy/power with a desired, preset value. The high voltage from the laser power supply is set accordingly to an appropriate higher or lower value, depending on the result of the comparison. The internal energy/power meter module is usually designed to measure the total energy in the beam, and thus typically averages the spatial inhomogeneities of the beam profile (i.e., the spatial distribution of energy in the laser beam).
Typically, as illustrated at
FIG. 2
, a beam splitter
2
external to the laser resonator reflects a diagnostic beam portion
3
of the laser output beam
4
to the energy detector
6
, e.g., a photodiode, photomultiplier tube, CCD array, PSD, pyroelectric sensor, etc. A working beam portion
8
of the output beam traverses the beam splitter
2
as it proceeds towards a workpiece. The energy detector
6
then integrates the energy or power of the entire diagnostic beam portion
3
split off by the beam splitter
2
to provide a measure for the total output energy or power of the working beam portion
8
.
For industrial applications such as microlithography or TFT annealing, the excimer laser beam profile of the output beam at the workpiece/wafer will typically differ from the profile of the output beam at the point that it impinges upon the beam splitter. As illustrated at
FIG. 3A
, an optical component such as the input aperture
10
of an illumination system including a collimating lens
12
may be positioned along the optical path of the working beam portion
8
to produce a collimated beam
14
and to provide a selected beam profile.
FIG. 3B
illustrates the beam profile
16
of the working excimer laser beam
8
before it traverses the lens aperture combination
12
/
10
of FIG.
3
A.
FIG. 3C
illustrates the beam profile
18
of the collimated beam
14
that results from the working beam
8
traversing the lens aperture combination
12
/
10
. The spatial homogeneity of the collimated beam
14
after the aperture
10
is improved over that of the beam
8
incident at the aperture
10
. Referring back to
FIG. 3A
, the collimated beam
14
whose profile is shown at
FIG. 3C
then continues along the beam path towards beam shaping optics
20
. Therefore, that portion of the energy of the incident beam
8
which is cut off at the aperture
10
never reaches the workpiece, and it is at the workpiece that the value of the energy dose is significant. Moreover, that portion of the incident beam that is not cut off at the aperture
10
and does traverse the lens
12
will undergo some induced absorption by the material of the lens
12
, further differentiating the apertured and focused beam
14
from the incident beam
8
.
FIGS. 4A-4B
show alternative conventional optical arrangements
20
and
22
, respectively, that the diagnostic beam portion
3
traverses before reaching the energy detector
6
. The optical arrangement
20
of
FIG. 4A
includes an attenuator
24
for reducing the magnitude of the laser energy that will ultimately impinge upon the detector
6
in order to protect the detector
6
, and a pair of diffuser plates
26
,
28
disposed before the detector
6
. As shown, the beam is so diffused after the second diffuser
28
that some fraction of beam energy escapes around the outside dimensions of the detector
6
and is not included in the measurement of the energy of the beam
3
.
The optical arrangement
22
shown in
FIG. 4B
includes an attenuator
30
and a converging lens
32
. In contrast with the arrangement shown at
FIG. 4A
, there is no portion of the beam
3
that is diffused outside of the dimensions of the detector
6
. Some induced absorption, however, will occur at the lens
32
so the detector
6
will basically measure the energy of the split off beam portion
3
multiplied by the attenuation fraction of the attenuator
30
, minus the induced absorption at the lens
32
.
It is recognized that the measured energies at the detector
6
after the beam
3
traverses either of the arrangements
20
,
22
shown in
FIGS. 4A and 4B
, respectively, are inaccurate measures of the actual energy dose that is incident at the workpiece. Further, the beam profile at the detector
6
does not match the beam profile incident at the work piece. For example, referring to
FIG. 3A
, the effects of blocking the outer portion of beam
8
by the aperture
10
and the induced absorption by the lens
12
are not measured by the detector
6
in conventional energy monitoring configurations. It is also recognized that it is the energy delivered to the workpiece, and not the energy of the beam
8
just after the beam
4
traverses the beam splitter
2
, that should be stabilized by the feedback arrangement. Thus, it is desired to have an energy monitoring apparatus that achieves a more accurate measure of the energy and profile of the beam that is incident at the workpiece, so that the energy stability of the portion of the beam that is applied to the workpiece may be improved.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an energy stabiliza
Patzel Rainer
Stamm Uwe
Ip Paul
Lambda Physik AG
Rodriguez Armando
Sierrra Patent Group, Ltd.
Smith Andrew V.
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