Note: Descriptions are shown in the official language in which they were submitted.
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Specification
TITLE OF THE INVENTION
NARROW BAND EXCIMER LASER AND
WAVELENGTH DETECTING APPARATUS
TECHNOLOGICAL FIELD
This invention relates to a narrow band excimer laser
and a wavelength detection device, and more particularly, to
a narrow band excimer laser suitable for a light source of a
reduction projection aligner for use in manufacturing of
semiconductors.
_ACKGROUND ART
An attention has been paid to the use of an excimer
laser as a light source of reduction projection aligner
(hereinafter called a stepper) for manufacturing semicon-
ductor devices. This is because the excimer laser may
possibly extend the light exposure limit to be less than 0.5
~m since the wavelength of the excimer laser is short (for
example the wavelength of KrF laser is about 248.4 nm),
because with the same resolution, the focal depth is greater
-than a g line or an i line of a mercury lamp conventionally
used, because the numerical aperture (NA) of a lens can be
made small so that the exposure region can be enlarged and
large power can be obtained, and because many other advant-
ages can be expected.
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An excimer laser utilized as a light source of the
stepper is required to have a narrow bandwidth with a beam
width of less than 3pm as well as a large output power.
A technique of narrowing the bandwidth of the excimer
laser beam is known as the injection lock method. In the
injection lock method, wavelength selecting element (etalon,
diffraction grating, prism, etc.) are disposed in a cavity
of an oscillation stage so as to generate a single mode
oscillation by limiting the space mode by using a pin hole
and to injection synchronize the laser beam in an amplifi-
cation stage. With this method, however, although a rela-
tively large output power can be obtained, there are such
defects that a misshot occurs, that it is difficult to
obtain 100% the locking efficiency, and that the spectrum
purity degrades. Furthermore, in this method, the output
light beam has a high degree of coherency so that when the
output light beam is used as a light source of the reduction
type projection aligner, a speckle pattern generates.
Generally it is considered that the generation of speckle
pattern depends upon the number of space transverse modes.
When the number of space transverse modes contained in the
laser light is small, the speckle pattern becomes easy to
generate. Conversely, when the number of the space trans-
verse modes increases, the speckle pattern becomes difficult
to generate art. The injection lock method described above
is a technique for narrowing the bandwidth by greatly
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decreasing the number of space transverse modes. Since
generation of speckle pattern causes a serious problem, this
technique can not be adopted in the reduction projection
aligner.
Another projection technique for narrowing the band-
width o the excimer layer beam is a technique utilizing a
air gap etalon acting as a wavelength selective element. A
prior art technique utilizing the air gap etalon was devel-
oped by AT & T Bell Laboratory wherein an air gap etalon is
disposed between the front mirror and a laser chamber of an
excimer laser device so as to narrow the bandwidth of the
excimer laser. This system, however, cannot obtain a very
narrow spectral bandwidth. In addition there are problems
that the power loss is large due to the insertion of the air
gap etalon. Further it is impossible to greatly increase
the number of the space transverse modes. Furthermore, the
air gap etalon has a problem of poor durability.
Accordingly, an excimer laser device has been proposed
wherein a relatively high durable diffraction grating is
used as the wavelength selective element.
In the excimer laser having a diffractive grating which
acts as a wavelength selective element, a pin hole is
provided in a resonator (laser cavity) to reduce the spread
angle of the beam in the grating. Alternatively a beam
expander is provided to expand the laser beam incident to
the grating. For this beam expander, a prism expander
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utilizing a prism is typically been used.
The narrow band excimer laser used as the stepper must
not only have a narrow bandwidth with a line width of less
than 3 pm but also must produce a large output.
In the construction in which a pin hole is disposed in
a resonator, however, the output becomes greatly reduced and
the number of space transverse modes necessary for prevent-
ing generation of a speckle pattern decreases, so that such
construction can not be used.
In the construction in which a prism beam expander is
used, the expander must have a large magnifying power in
order to narrow the line width.
However, when the magnifying power of the prism beam
expander becomes large, the incident angle of the laser beam
to a prism of the prism expander becomes large or it becomes
necessary to increase the number of prisms, thereby increas-
ing the loss. As a result, a large output cannot be pro-
duced.
Furthermore, where a narrow band excimer laser is used
as the light source of a stepper, it is necessary to narrow
the bandwidth of the output laser beam and then to control
the wavelength of the output laser beam whose bandwidth has
been narrowed to a stable condition at a high accuracy.
A monitor etalon has been used for measuring the line
width of the output beam and for detecting the wavelength.
The monitor etalon is constituted by an air gap etalon
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wherein a pair of partial reflective mirrors are disposed to
confront each other with a predetermined air gap therebet-
ween. The transmissive wavelength of this air gap etalon is
expressed by the following equation
m~ = 2nd cos 3
where m represents the order, d the partial mirrors spacing,
n the refractive index of the medium the par ial reflective
mirrors, and ~ an angle between the normal of the etalon and
the axis of the incident light.
This equation shows that where n, d and m are constant,
as the wavelength varies, the angle 3 changes. The monitor
etalon detects the wavelength of the beam by utilizing this
characteristics. In the monitor etalon described above,
when the pressure in the air gap and the ambient temperature
vary, the angle ~ varies even when the wavelength is con-
stant. Accordingly, where the monitor etalon is used for
detectiny the wavelength, the pressure in the air gap and
the ambient temperature are controlled to be constant.
However, it is difficult to precisely control the
pressure in the air gap and the ambient temperature.
Therefore, it is impossible to detect the absolute wave-
length at a high accuracy.
For this reason, apparatus has been proposed wherein
the beam to be detected is inputted to the monitor etalon
together with a reference beam having a known wavelength,
and the wavelength of the beam is detected by detecting a
wavelength of the beam relative to the reference beam.
In this apparatus, a light beam transmitting through
the monitor etalon is directly inputted to a beam detector
such as CCD image sensor.
However, in this apparatus, since the output of the
monitor etalon is directly inputted to the beam detector,
the beam to be detected and the reference beam cannot be
inputted to the beam detector with a sufficient beam quan-
tity, and an interference fringe cannot be formed on the
beam detector.
Accordingly, it is an object of this invention to
provide a novel narrow band excimer laser of the type using
a prism beam expander and a diffraction grating as a band-
narrowing element, capable of preventing increase of loss
even when the magnifying power of the prism expander is
increased.
Another object of this invention is to provide a novel
wavelength detecting apparatus of a narrow band excimer
laser capable of inputting a reference beam and a beam to be
detected into a beam detector with a sufficient quantity and
capable of detecting the interference fringes of both beams
at a high accuracy.
DISCLOSURE OF THE INVENTION
According to one aspect of this invention, there is
provided a narrow band oscillation excimer laser comprising
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a prism beam expander and a diffractive grating which are
used as a bandwidth narrowing element, the ruling direction
o the grating and the beam spreading direction of the prism
beam expander being substantially coincided with each other;
and selective oscillation means for selectively oscillating
a linearly polarized wave which is substantially parallel
with the beam expanding direction of the prism beam ex-
pander. The selective oscillation means may be constructed
by a polarizing element disposed in the laser cavity.
The selective oscillation means may be constructed by a
window provided on a rear side or a front side of the laser
chamber in such a manner that the window is inclined with a
Brewster's angle with respect to the optical axis of the
laser beam in a plane containing the beam expanding direc-
tion of the prism expander and the optical axis of the laser
beam.
The selective oscillation means may include a prism
beam expander in which one surface of a prism is coated with
a reflection preventive film for selectively preventing a
reflection of a polarized component which is parallel with
the beam expanding direction of the prism beam expander.
By selectively oscillating a linearly polarized wave
which is substantially parallel with the beam expanding
direction of the prism beam expander, the loss can be
reduced even though the incident angle to the prism is large
or the number of the prisms is increased. This is because a
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linearly polarized wave parallel with the beam expanding
direction of the prism beam expander has a large transmis-
sivity even the incident angle to the prism islarge. With
this construction, the narrow ban laser device can generate
a large output with a small spectrum line width.
According to the other aspect of this invention there
is provided a wavelength detecting apparatus for use in a
narrow band oscillation excimer laser, comprising an etalon,
light incidence means for projecting a reference light
generated by a source of reference light and a laser beam to
be detècted upon the etalon; light condensing means for
condensing lights passed through the etalon; light detecting
means disposed in a rear side focal plane of the light
condensing means for detecting an interference fringe of
lights condensed by the light condensing means.
The reference light and the light to be detected are
passed through the light condensing means, and then focused
on the detection surface of the light detecting means
disposed on the focal plane of the light condensing means.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
Fig. l(a) is a plan view showing one embodiment of a
narrow band excimer laser according to this invention;
Fig. l(b) is a side view of the embodiment shown in
Fig. l(a);
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Fig. 2(a) is a plan view showing another embodiment of
the narrow band excimer laser according to this invention;
Fig. 2~b) is a side view showing the another embodiment
shown in Fig. 2(a);
Fig. 3~a) is a plan view showing a still another
embodiment of the narrow band excimer laser;
Fig. 3~b) is a side view of the embodiment shown in
Fig. 3~a);
Fig. 4 is a diagrammatic representation showing one
embodiment of the wavelength detecting apparatus for the
narrow band excimer laser of this invention;
Fig. S is a diagrammatic representation showing another
embodiment of the wavelength detecting apparatus for the
narrow band excimer laser using a beam condensing mirror;
Fig. 6 is a diagrammatic representation showing still
another embodiment of the wavelength detecting apparatus for
narrow band excimer laser using a converging type light
filter;
Fig. 7 is a diagrammatic representation showing a
further embodiment of the wavelength detecting apparatus for
the narrow band excimer laser using a lamp as a source of
reference light;
Fig. 8 is a diagrammatic representation showing a still
of the wavelength detecting apparatus using a lamp and an
optical fiber;
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Fig. 9 is a diagrammatic representation showing another
wavelength detecting apparatus for the narrow band excimer
laser in which a shutter and a filter are inserted;
Fig. 10 is a flow chart showing one example of the main
routine for detecting a wavelength when the wavelength
detecting apparatus shown in Fig. 9 is used;
Fig. 11 is a flow chart showing and example of a
reference light detecting subroutine; and
Fig. 12 is a flow chart showing one example of a light
detecting subroutine.
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BEST MODE FOR CARRYING OUT THE INVENTION
The embodiments--o`fthis invention will now be described
with reference to the accompanying drawings. The narrow
band excimer laser shown in Figs. l(a) and l(b) comprises a
front mirror 1, and a grating 6 acting as a rear mirror and
a wavelength selective element. Between the front mirror 1
and the grating 6, a laser chamber 3, a polarizing element
4, and two prisms 5a and Sb acting as a beam expander (prism
beam expander) are provided. -Thus, a laser cavity is
20 constructed between the front mirror 1 and the grating 6.
The laser chamber 3 is filled with a laser gas contain-
ing KrF, etc. which can circulate in the chamber. For the
purpose of exciting the laser gas, discharge electrodes, not
shown, are provided in the laser chamber 3. Windows 2a and
2b are provided at both ends of the laser chamber 3 at
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predetermined angles.
The purpose of the grating 6 is to select a beam having
a specific wavelength by utilizing diffraction of light beam.
The grating 6 is provided with a number of grooves directed
in the same direction. In this specification, the direction
perpendicular to these grooves is termed a ruling direction.
By changing the angle of grating 6 with respect to an
incident beam within a plane containing the ruling direction
of the grating 6, a beam having a specific wavelength can be
selected. In other words, the grating 6 reflects only a
specific diffracted light in a predetermined direction (in
this case, the direction of incident beam), the specific
diffracted light corresponding to the angle of the grating
with respect to the incident beam. As a consequence, a beam
having a specific wavelength can be selected.
The prism beam expander including prisms 5a and 5b is
disposed such that its beam expanding direction substan-
tially coincides with the ruling direction of the grating 6.
The grating 6 is irradiated by the laser beam expanded by the
prism beam expander.
The polarizing element 4 selectively transmit only the
polarized beam which is substantially parallel with the beam
expanding direction of prism beam expander made up of prisms
5a and 5b. The polarizing element 4 may be constructed, for
example, by a polarizing prism utilizing a birefringence
material (crystal, calcite, etc.), A brewster's dispersion
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prism, a glass substrate (quarts, CaF2 or MgF2), which is
arranged at a Brewster's angle or a glass substrate coated
such that it transmits a certain polarized light component~
With such construction, the apparatus shown in Figs.
l~a) and l(b) selectively oscillates a linearly polarized
light wave parallel to the beam spreading direction by the
prism beam expander made up of prisms Sa and Sb. The
linearly polarized light wave parallel with the beam expand-
ing direction of the prism beam expander has a large trans-
missivity even when the incident angle of the beam to the
prism is large. Therefore, even when the magnifying power
of the beam expander is made large for line-narrowing, the
loss does not increase greatly. In other words, according to
this invention, it is possible to construct a narrow band
excimer laser with a small loss.
The modified embodiment of this invention shown in
Figs. 2(a) and 2(b) is constructed such that a specific
linearly polarized light wave can be selected by a rear side
window of the laser chamber 3. In this embodiment, the rear
side window 2b of the laser chamber 3 makes substantially
the Brewster's angle ~ with respect to the beam expanding
direction of the prism beam expander constructed by pri~ms
Sa and 5b, in a plane including the beam expanding direction
of the beam expander and the optical axisofthe laser beam.
In the modification shown in Fig. 2, only the rear side
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window 2b is set to make the Brewster's angle. However, it
is possible to arrange both the rear side window 2b and the
front side window 2a can be set to make the Brewster's
angle. It is also possible to set only the front side
window 2a to make the Brewster's angle.
A still another embodiment shown in Figs. 3(a~ and 3(b)
is constructed such that by coating the prisms 5a and Sb
which constitute the prism beam expander with an anti-reflec-
tion coating film that selectively transmits only the polar-
ized light component parallel to the beam expanding directionof the prism ~eam expander. In Fig. 3(b), portions 5c and
5b shown by dotted lines indicate these coated portions.
The coating can be applied to one beam transmitting
surface of at least one prism. With this consturction, even
when the incident angle increases, the transmissivity is
higher than 99%.
Fig. 4 shows one embodiment of a wavelength detecting
apparatus for the narrow band excimer laser according to
this invention. In this embodiment, as the beam to be
detected, the output beam La of the narrow band excimer
laser 10 is used, and as a reference light source 20, a
He-Ne laser or an Ar laser or other types of laser is used.
The reference laser beam generated by the reference light
source 20 and the excimer laser beam hàve different wave-
length.
A part of the laser beam La outputted from the narrow
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band excimer laser 10 is sampled by a beam splitter 30, and
this sampled beam is inputted to a beam splitter 40. The
reference beam Lb generated by the reference light source 20
is inputted to the other surface of the beam splitter 40.
The beam splitter 40 transmits a part of the sampled
beam La outputted from the beam splitter 30 and reflects a
part of the reference beam Lb outputted by the reference
beam source 20, thus combining the sampled beam with the
reference beam. The combined beam outputted from the beam
splitter 40 is spreaded by a concave lens 50, and the spread
beam is inputted to an etalon 60.
The etalon 60 is constituted by two transparent plates
60a and 60b whose inner surfaces are made to act as partial
reflecting mirrors. The wavelength of the beam transmitted
through the etalon 60 varies corresponding to the angle of
incident light to the etalon. Thus, the reflecting films
are coated on the etalon plates in 2-wavelength coating so
as to partially reflect both the reference beam Lb and the
excimer laser beam La having different wavelengths from the
reference beain Lb.
The beams transmitted through the etalon 60 are input-
ted to a condenser lens 70. The condenser lens 70 may be an
achromatic lens to which correction for color aberration is
performed. When the laser beam transmits through the achro-
matic condenser lens 70, the color aberration is compensated
for.
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The beam detector 80 is disposed at the focal point of
the condenser lens 70 so that the light beam passed through
the condenser lens 70 is focused on the beam detector 80. On
the detecting surace of the beam detector 80, a first
interference fringe corresponding to the reference beam and
a second interference fringe corresponding to the beam to be
detected are formed. Base on these first and second inter-
ference fringes, the beam detector 80 detects the relative
wavelength of the beam to be detected with respect to the
wavelength of the reference beam. Thus, the beam detector
80 can detect the absolute wavelength of a beam to be
detected when the wavelength of the reference beam is known.
detected wavelength.
The beam detector 80 may be constituted by a one-
dimensional or two-dimensional image sensor, a diode array
or a position sensitive detector (PSD).
As above described, the beams are inputted to the
etalon 60 after being spread with the concave lens 50, and
the beams transmitted through the etalon 60 are focused on
the beam detector 80 with the condenser lens 70. Therefore,
a sufficient quantity of beams are inputted to the beam
detector 80 and the interference f~inges of both beams can
be clearly formed.
Fig. 5 illustrates another embodiment of the wavelength
detecting apparatus of the narrow band excimer laser accord-
ing to this invention. For convenience of description,
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elements doing the same performance are designated by the
same reference numerals and characters in Fig. 5 and sub-
sequent drawings.
In the embodiment shown in Fig. 5, instead of the
achromatic condenser lens 70, such condenser mirror 90 as a
concave mirror or an eccentric parabolic mirror is used.
More particularly, a reference beam Lb and an excimer laser
beam La are inputted to an etalon 60 through a concave lens
50, and the beam transmitted through the etalon 60 is
reflected by the condenser mirror 90. The reflected light
beam is inputted to the detecting surface of a beam detector
80 disposed at the focus of the condenser mirror 90. Since
the condenser mirror 90 having a reflecting surface is used,
there is no achromatic aberration so that it is possible to
focus the interference fringes of the excimer laser beam La
and the reference laser beam Lb at the same position, that
is, on the beam detector 80 disposed at the focal point of
the mirror 90.
In this embodiment shown in Fig. 5, it is possible to
detect at a high accuracy the interference fringes of a
sufficient light quantity by using the concave lens 50 and
the condenser mirror 90 as in the previous embodiment.
Fig. 6 shows a still another embodiment of the wave-
length detecting apparatus of the narrow band excimer laser
according to this invention.
In this embodiment, the excimer laser beam La and the
3~a
reference laser beam Lb are synthesized by using a synthe-
sizer type optical fiber 40a. More particularly, the
excimer laser beam La sampled by a beam splitter 30 is
applied to a beam-synthesizer 14 through a condenser lens
11, an optical fiber sleeve 12 and an optical fiber 13,
while the reference laser beam Lb generated by the reference
light source 20 is applied to the beam synthesizer 14 via
condenser lens 15, an optical fiber sleeve 16 and an optical
fiber 17. The beam synthesizer 14 synthesizes these two
beams La and Lb, and the beam thus synthesized is inputted
to an optical fiber 18, and the beam spread by a sleeve 19
is inputted to a monitor etalon 60. The beam transmitted
through the monitor etalon 60 is focused on the beam detec-
tor 80 through an achromatic condenser lens 70.
In the embodiment shown in Fig. 6, the position of the
interference fringe is not influenced by the position of the
synthesizing type optical fiber 10, but is solely determined
by the positional relation among the etalon 60, the con-
denser lens 70 and the beam detector 80. Therefore, there
is an advantage that the optical system can be adjusted
easily, in addition to the advantages of the previous
embodiments.
Although in the embodiment shown in Fig. 6, correction
of the color aberration is performed by using the achromatic
lens 70, this lens can be substitu~ed by the concave mirror
or the eccentric parabolic mirror 90 shown in Fig. 5.
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Fig. 7 shows further embodiment showing the wavelength
detecting apparatus of the narrow band excimer laser in
which a plane light source, that is a lamp 20a is used as
the reference light source. This lamp 20a may be a mercury
lamp generating a reference light having a wavelength of
253.7 nm, for example. More particularly, the excimer laser
beam La sampled by a beam splitter 30 is spread by a concave
lens 21 and then applied to a beam splitter 40 which synthe-
sizes the spread beam with the reference beam Lc from the
mercury lamp 20. The synthesized beam is inputted to an
etalon 60. The beam transmitted through the etalon 60 is
focused on the beam detector 80 through an achromatic
-~ condenser lens 70.
In the embodiment shown in Fig. 7, the color aberration
is corrected by using the achromatic condenser lens 70.
However, this lens can be substituted by the concave mirror
or the eccentric parabolic mirror 90 shown in Fig. 5.
Fig. 8 illustrates yet another embodiment of the
wavelength detecting apparatus of the narrow band excimer
laser of this invention in which a mercury lamp 20a is used
as the reference light source. In this embodiment, the
excimer laser beam La is guided to a beam splitter 40 by
using an optical fiber 23. More particularly, the excimer
laser beam sampled by a beam splitter 30 is inputted to a
sleeve 22 through a condenser lens 11. Thereafter, the beam
is outputted from a sleeve 24 through an op~ical fiber 23.
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The beam which is spread by passing through the sleeve 24 is
- applied to a beam splitter 40 where the beam is synthesized
with the reference light beam Lc from the mercury lamp 20
and is then inputted to an etalon 60. The light beam
transmitted through the etalon 60 is focused on a beam
detector 80 via an achromatic condenser lens 70.
In the embodiment shown in Fig. 8, the achromatic
condenser lens 70 may be substituted by a concave mirror or
an eccentric parabolic mirror 90 shown in Fig. S.
Fig. 9 shows another embodiment of the wavelength
detecting apparatus of the narrow band excimer laser. In
this embodiment, the achromatic condenser lens 70 in Fig. 8
is replaced with a condenser mirror 90 such as a concave
- mirror or an eccentric parabolic mirror. In addition, a
filter 41, shutters 42 and 43 are added.
More particularly, a filter 41 which selects only a
light beam having a predetermined wavelength among the
reference light beams Lc generated by the mercury lamp 20a
is provided between a shutter 42 and a beam splitter 40.
Thus, only the reference light beam having the predetermined
wavelength is inputted to beam splitter 40. For example,
where the apparatus is used as the wavelength detector of
the beam (having a wavelength of 248.4 nm) generated by a
KrF narrow band excimer laser, a mercury lamp is used as the
lamp 20a, and an interference filter for a beam having a
wavelength of 253.7 nm, which is close to that of the
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excimer laser, is used as the filter 16. Shutters 42 and 43
are provided for the purpose of independently detect the
reference light beam and the light beam to be detected (the
laser beam of excimer laser). For detecting the reference
light beam, the shutter 42 is opened and the shutter 43 is
closed. For detecting the light beam to be detected, the
shutter 43 is opened and the shutter 42 is closed.
In this embodiment, the filter 41 is disposed between
the beam splitter 40 and the shutter 42. However, the
filter 41 may be disposed between the lamp 20a and the
- shutter 42. Furthermore, where the wavelengths of the
reference light beam and of the beam to be detected are
close to each other, and where the beam to be detected
transmits through the filter 41, the filter 41 may be placed
at a suitable position in the light path between the beam
splitter 40 and the light beam detector 80.
The detecting operation of the reference light beam and
the beam to be detected of this embodiment shown in Fig. 9
will now be described with reference to the flow charts
shown in Figs. 10 - 12.
Fig. 10 shows the main routine for the wavelength
detection. First, the reference light beam detection
subroutine is executed at step 100. As shown in Fig. 11, in
this subroutine, the shutter 43 on the side of the beam to
be detected is closed while the shutter 42 on the side of
the reference light beam is opened so as to input only the
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reference light beam Lc to the beam detector 80 via the
etalon 60 at steps 200 and 210. Then, the radius Rs of the
interference fringe of the reference beam formed on the beam
detector 80 is detected and stored in a memory, not shown,
at step 220.
Upon completion of this reference beam detection
subroutine, the count value T of a timer, not shown, is
: cleared to zero at step 110. Then, at step 120, a judgement
is made as to whether the count value T is larger than a
predetermined preset time K (for example, several minutes).
Where T < K, the detection subroutine for detecting the beam
. to be detected is executed at step 140. In this detection
subroutine, as shown in Fig. 12, by closing the shutter 42
on the side of the reference light beam and opening the
shutter 43 on the side of the beam to be detected, only the
beam La to be detected is applied to the beam detector 80 at
steps 300 and 310. Then, the radius Re of the interference
fringe of the beam to be detected formed on the surface of
the beam detector 80 is detected at step 320.
Upon completion of the detection subroutine for detect-
ing the beam to be detected, the radius Re determined by
this subroutine is compared with the radius Rs which has
been determined and stored in the previous reference beam
subroutine so as to detect the absolute wavelength of the
beam to be detected at step 150. Thereafter, the executions
of steps 140 and 150 are repeated until a condition T ~ K is
obtained.
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When T ~ K at step 120, the reference beam detection
subroutine shown in Fig. 11 is executed again and the stored
data Rs is renewed by the radius Rs of the interference
fringe of the reference beam determined by the subroutine.
Then, after the count value T of the timer is cleared to
zero, the subroutine for detecting the beam to be detected
is executed again.
All processings described above are executed automatic-
ally.
More particularly, in the wavelength detection process-
ing described above, where the wavelengths of the reference
beam and the beam to be detected are close to each other, it
is difficult to simultaneously detect the interference
fringes of both beams. Therefore, the interference fringes
are independently detected by using the shutters 42 and 43.
; Since the reference beam is relatively stable, it is advan-
tageous to detect the interference fringe in a relatively
long preset period K. In a case other than detecting the
reference beam, it is designed that the beam to be detected
is always detected. In other words, the detection period
for the reference beam is set to be sufficiently longer than
the detection period for the beam to be detected.
In this embodiment, the radius of an interference
fringe is detected. However, the absolute wavelength of the
beam to be detected can also be obtained by detecting the
diameter or position of the interference fringe.
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In the embodiments shown in Figs. 4 - 9, when the light
quantities of the reference beam and the beam to be detected
are small so that the detection of the interference fringes
is difficult, a collimator lens may be disposed in front of
the etalon 60 so as to apply parallel light to the etalon,
thus increasing the light quantity.
In the embodiments shown in Figs. 4 - 9, the beam
splitter 40 is disposed such that the beam to be detected is
applied to the beam transmissive side while the reference
beam is applied to the opposite side. However, their
positional relationship may be reversed. Where the wave-
lengths of the reference beam and of the beam to be detected
are close to each other, a partial reflection mirror may be
used, whereas where the difference between wavelengths are
large, a dichroic mirror may be used.
The embodiments described above are constituted by
using an air gap etalon. However, it may be substituted by
a solid etalon.
In the above described embodiments, the focused
positions of the reference beam and of the beam to be
detected are made to coincide with each other by correcting
the color aberration of the condenser lens 70, or by using
the condenser mirror 90. However, it may be so constructed
that the condensing lens 70 or the beam detector 80 is
disposed movable in the direction of the optical axis so as
to correct the focused position.
- 23 ~
2f3é~3~
Where the waveleng~sof the reference beam and the
beam to be detected are close to each other in the embodi-
ments shown in Figs. 4, 6, 7 and 8, a condenser lens whose
color aberration is not corrected may be inserted between
the etalon 60 and the beam detector 80.
In the embodiments shown in Figs. 4, 5, and 7, the
laser beam La is spread by the concave lens 50 or 21 and the
spread laser beam is inputted to the etalon 60. However,
the concave lens may be substituted with a convex lens.
According to this modification the laser beam is condensed
at first by the convex lens and then spread. The spread
beam is inputted to the etalon 60. Furthermore, instead of
using the concave lens 50, adiffusion plate (e.g., a frosted
glass) may be used.
FIELD OF APPLICATION IN INDUSTRY
As above described, in the narrow band excimer laser
according to this invention, by substantially coinciding the
ruling direction of the grating with the beam expanding
direction of the prism beam expander and by providing selec-
tive oscillation means for selectively oscillating a linear-
ly polarized light wave substantially parallel with the beam
spreading direction of a prism beam expander, it becomes
possible to greatly decrease the loss in the prism beam
expander. For this reason, it becomes possible to narrow
the spectrum line width and to obtain a large output.
- 24 -
~3~,~0
Further, according to the wavelength detecting appara-
tus of the narrow band excimer laser of this invention, with
a simple construction of providing a light condensing device
on the rear side of an etalon, a sufficient quantity of
light can be applied to a light detector. As a result, it
becomes possible to form clear interference fringes on the
light detector, thus enabling to detect the absolute wave-
length of the beam to be detected at a high accuracy.
Especially, by using an achromatic lens or a light condens-
ing mirror as light condensing means, even when the wave-
lengths of the reference beam and that of the detected beam
are not equal, the absolute wavelength can be accurately
detected without constructing the optical system to be
movable.
The narrow band excimer laser and the wavelength
detecting apparatus of the excimer laser are especially
suitable to use as a light source of a reduction projection
aligner for use in manufacturing semiconductor devices.
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