Exposure apparatus using excimer laser source

An exposure apparatus comprises a projection optical system, to project the image of an object on a first plane onto a second plane, a laser means for outputting a light beam to the projection optical system for projection, a measuring device for detecting a fluctuation of the optical characteristic of the projection optical system attributable to deviation of the wavelength of the light beam output from the laser and a control device responsive to the measuring device to adjust the wavelength of the light beam.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an exposure apparatus for printing the pattern of 
a mask on a semi-conductive wafer, for example, in the manufacturing 
process of semiconductor integrated circuits, and in particular to a 
projection exposure apparatus utilizing an excimer laser. 
2. Related Background Art 
A projection exposure apparatus utilizing the output of an excimer laser to 
transfer the pattern of a mask or reticle (hereinafter referred to as the 
mask) onto a semicondcutive wafer is known from U.S. Pat. No. 4,458,994. 
It is known that the output wavelength of an excimer laser source can be 
determined variously by the selection of excimer gas, and it is suggested 
in the aforementioned U.S. patent that when using the output of an excimer 
laser source as the exposure energy in a lithography system utilizing a 
projection optical system, it is relatively easy to design the projection 
optical system so as to be corrected in chromatic aberration relative to 
the bandwidth of the output wavelength thereof. That is, where the 
projection optical system is constructed by the use of a plurality of 
optical elements differing in refractive index, it is possible to correct 
chromatic aberration relative to a wavelength of a limited bandwidth. 
However, in order to enhance the transmittance of the projection optical 
system, it is preferable that all elements constituting the refracting 
system of the optical system be formed of quartz. In that case, it is 
difficult to correct chromatic aberration completely relative to a 
plurality of wavelengths and therefore, only a single wavelength 
determined in design must be used as the exposure wavelength. 
Accordingly, for such a reason, in an exposure apparatus provided with an 
excimer laser source, the stability of the wavelength of the laser beam 
output from the laser source (the stability of the central wavelength and 
the wavelength range) becomes an important matter in respect of the 
fluctuation of the projection magnification, the fluctuation of the focus 
position, etc. Even slight deviation of the wavelength causes significant 
fluctuation of the projection magnification, fluctuation of the focus 
position, etc. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an apparatus for 
stabilizing the output wavelength of an excimer laser source. 
It is another object of the present invention to provide an exposure 
apparatus provided with an excimer laser source of high stability. 
The exposure apparatus of the present invention includes measuring means 
for detecting the amount of fluctuation of an optical characteristic of a 
projection optical system, and control means for controlling the output 
wavelength of the excimer laser source on the basis of the detected amount 
of fluctuation of the optical characteristic. 
Further, means for stabilizing the output wavelength of the excimer laser 
source is provided. This stabilizing means operates to stabilize the 
wavelength of the laser beam conforming to the measuring means after said 
wavelength of the laser beam is controlled by the control means. 
In a preferred embodiment of the present invention, the excimer laser 
source is provided with a Fabry-Perot etalon for output wavelength 
selection, and the stabilizing means is provided with a Fabry-Perot etalon 
for monitoring the output wavelength. 
In the present invention, when the deviation of the output wavelength of 
the excimer laser source occurs at integer times the free spectral range 
of the monitoring Fabry-Perot etalon, the stabilizing means 
feedback-controls the angle of inclination of the Fabry-Perot etalon for 
wavelength selection so that the output wavelength is controlled on the 
basis of the amount of fluctuation of the optical characteristic of the 
projection optical system detected by the measuring means and thereafter 
this output wavelength is stabilized. 
As a result, the central wavelength of the output of the excimer laser 
source is made to accurately coincide with the set wavelength at the 
starting of the exposure apparatus or even when a long time has elapsed 
after the exposure apparatus has been started, and the amount of 
wavelength deviation is held within a predetermined value (.+-.0.001 nm).

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, an excimer laser source 100 has a laser resonating portion 102 
installed in a cavity 101. Also provided in the excimer laser source are a 
front mirror 103, a rear mirror 105 and a wavelength selecting element 104 
constituted by a Fabry-Perot etalon or a grating. The laser beam 
reciprocally travels by reflection between the front mirror 103 and the 
rear mirror 105, and the wavelength range thereof is narrowed by the 
action of the wavelength selecting element 104 disposed between the two 
mirrors. By this narrowing, the spectral width of the laser beam using KrF 
as a laser medium which was of the order of 4 nm becomes 0.005 nm or less. 
This narrowed laser beam is designed to oscillate through the front mirror 
103. 
As the wavelength selecting element 104, use is made of a Fabry-Perot 
etalon of the type in which the free spectral range (FSR) is appropriately 
determined and the mirror surface spacing is fixed. 
The laser beam thus oscillated from the laser source 100 enters a beam 
splitter 201, where a part of the laser beam is taken out. This taken-out 
laser beam is expanded by a diffusing plate (or a lens for expanding the 
laser beam) 202, whereby a wave number spectrum corresponding to the 
wavelength is obtained by a Fabry-Perot etalon 203 for monitoring. 
As the Fabry-Perot etalon 203 for monitoring, use is made of a Fabry-Perot 
etalon of the type in which the free spectral range (FSR) is determined 
appropriately (0.01-0.02 nm) and like the Fabry-Perot etalon constituting 
the wavelength selecting element 104, the mirror surface spacing is fixed. 
From this, the wave number spectrum of the oscillated laser beam can be 
taken out. The wave number spectrum thus obtained is transmitted through a 
lens 204 and enters a spectral detector 205 such as a linear array 
detector comprising a CCD, a PCD (plasma coupled device) or the like, 
whereby an interference fringe determined by the spectrum is detected. 
The spectrum (interference fringe distribution) detected in the spectral 
detector 205 is input to a wavelength monitor 206, in which the spectral 
characteristic in the wavelength selecting element 104 is observed. The 
components from the diffusing plate 202 to the wavelength monitor 206 
constitute a wavelength monitoring device 200. 
The spectrum signal output from the detector 205 is also input to a 
controller 20. The controller 20 finds the amount of fluctuation of the 
wavelength from the set wavelength of the pre-input laser beam and the 
spectrum of the laser beam detected by the wavelength monitoring device 
200, and suitably feedback-controls the angle of inclination of the etalon 
104 on the basis of said amount of fluctuation of the wavelength through 
an actuator 21. The wavelength monitoring device 200, the controller 20 
and the actuator 21 together constitute a wavelength stabilizing device so 
as to accurately narrow the wavelength of the laser beam and stabilize the 
central wavelength thereof and the wavelength range. 
The laser beam transmitted through the beam splitter 201 enters an 
illuminating optical system 302 via a reflecting mirror 301. The laser 
beam having emerged from the illuminating optical system 302 irradiates a 
mask 303, and a pattern on the mask 303 is reduction-projected onto a 
wafer 305 placed on a wafer stage 306, through a projection lens 304 which 
is telecentric at both sides or at one side. In the present embodiment, 
use is made of an optical system which is non-telecentric at the mask 303 
side and telecentric at the wafer 305 side. 
An optical characteristic control device 307 corrects the fluctuation of an 
optical characteristic of the projection lens 304 caused by the influences 
of a variation in the atmospheric pressure, a variation in the atmospheric 
temperature and the irradiation energy of the exposure light, i.e., the 
fluctuation of the projection magnification or of the focus position. The 
projection lens 304 and the optical characteristic control device 307 are 
disclosed, for example, in U.S. Pat. No. 4,666,273 and therefore, need not 
be described in detail herein. Stated briefly, a plurality of portions of 
the lens space in the projection lens 304 are made into air chambers 
shielded from the atmosphere and the pressure in these air chambers is 
adjusted to correct the optical characteristic of the projection lens 304 
itself. The fluctuation of the performance of this projection lens itself 
is prevented by the optical characteristic control device 307 and 
therefore, if the wavelength of the laser beam is stable, that is, if the 
central wavelength coincides with the set wavelength (absolute value) and 
the amount of fluctuation of this wavelength is within a predetermined 
value (.+-.0.001 nm), a stable projection magnification and a stable focus 
position can always be obtained. 
So, in order to obtain the stable projection magnification and stable focus 
position of the projection lens, the wavelength stabilizing device is used 
to feedback-control the wavelength of the laser beam as described above. 
However, such feedback control alone may cause serious malfunctioning for 
the reason set forth below. 
The wavelength of the laser beam output from the excimer laser source 100 
fluctuates to a certain degree at the start of the oscillation of the 
laser due to the influences of the angle of inclination of the etalon 104, 
the atmospheric temperature, the atmospheric pressure, etc. Malfunctioning 
occurs when the then deviation of the wavelength is more than one half of 
the free spectral rang of the etalon 203 for monitoring. 
That is, from the viewpoint of the characteristic of the Fabry-Perot 
etalon, interference fringes of the same multiplex interference are 
obtained for a wavelength deviation integer times as great as the free 
spectral range. For example, where the free spectral range of the etalon 
203 for monitoring is set to 0.02 nm, the spectrum detected when the 
wavelength has changed by integer times 0.02 nm is entirely similar to the 
spectrum detected when the central wavelength of the oscillated laser beam 
is exactly coincident with the set wavelength. 
Thus, when at the start of the oscillation of the laser, the wavelength of 
the laser beam changes by more than one half of the free spectral range 
(in this case, 0.01 nm), even if the above-described feedback control is 
effected, the controller 20 effects the feedback control of the angle of 
inclination of the wavelength selecting etalon 104 through the actuator 21 
so that the central wavelength of the laser beam is stabilized to a 
wavelength differing from the set wavelength by 0.2 nm (or integer times 
0.02 nm). 
So, in order to prevent malfunctioning, the present embodiment is designed 
such that the fluctuation of the wavelength of the laser beam is detected 
by the use of the characteristic measuring device of the projection 
optical system provided in the exposure apparatus, the wavelength of the 
laser beam is controlled by the controller 10, and after this control, the 
wavelength of the laser beam is feedback-controlled by the wavelength 
monitoring device 200 or the like. This characteristic measuring device of 
the projection optical system is disclosed in U.S. Pat. No. 4,629,313 and 
therefore, only the function regarding the present embodiment will be 
described briefly herein. 
Two light-transmitting cruciform fiducial marks M1 and M2 are provided on 
the underside of a mask 303 at predetermined locations. The laser beam 
transmitted through the marks M1 and M2 enters the projection lens 304. 
The light beam having emerged from the projection lens 304 is imaged on a 
slit member on the two-dimensionally movable wafer stage 306. This slit 
member has a surface formed with a pair of slits S1 and S2, and said 
surface, like the wafer, is conjugate with the pattern surface of the mask 
with respect to the projection lens. The spacing between the pair of slits 
S1 and S2 is set equal to the spacing between the projected images of the 
marks M1 and M2 of the mask by the projection lens. Further, below the 
slit member, a pair of photoelectric detectors 308 are provided so as to 
receive the lights transmitted through the corresponding slits S1 and S2. 
Thus, by moving the wafer stage 306, the pair of slits S1 and, S2 scan the 
projected images of the marks M1 and M2 and detection signals output from 
the photoelectric detectors 308 are input to the controller 10. The 
controller 10 determines the relative positional relation between the 
projected images of the marks Ml, M2 and the slits S1, S2 on the basis of 
the detection signals and the position information of a laser 
interferometer 309, and stores it in a register. Further, the controller 
10 determines the amount of fluctuation of the projection magnification 
from the spacing between the projected images of the marks Ml and M2 and 
the design spacing between the projected images pre-input to the register, 
and stores it therein. 
Accordingly, assuming that the optical characteristic control device 307 is 
sequentially correcting the fluctuation of the optical characteristic of 
the projection lens 304 caused by a variation in the atmospheric pressure, 
a variation in the atmospheric temperature, the irradiation energy of the 
exposure light, etc., the measured fluctuation of the projection 
magnification may be said to have been caused by the aberration due to the 
wavelength deviation of the laser beam output from the laser source 100. 
In the exposure apparatus of this type, calibration, i.e., the set-up of 
the offset measurement of the alignment system of the exposure apparatus, 
the focus position adjustment, the measurement of the fluctuation of the 
projection magnification, etc., is effected without fail at the starting 
of the apparatus. Accordingly, the fluctuation of the projection 
magnification caused by the wavelength deviation of the laser beam is 
adjusted as follows during the calibration effected at the starting of the 
exposure apparatus. 
At a point of time whereat the excimer laser source 100 is oscillated and 
the central wavelength of the laser beam thereof becomes stable to a 
certain degree, the controller 10 finds the amount of fluctuation of the 
projection magnification as described above. On the basis of this amount 
of fluctuation, the controller 10 determines the amount of wavelength 
deviation of the laser beam and stores this determined value therein. 
In the case of the projection lens 304 in the present embodiment, if the 
wavelength of the laser beam output from the laser source 100 deviates by 
0.001 nm, the projected image shifts by about 0.02 .mu.m at the image 
height 10 mm of the projected image projected onto the wafer. Therefore 
the controller 10 repetitively effects the above-described position 
measurement, and thus measures the position of the projected image at an 
accuracy of the order of 0.05 .mu.m and finds the amount of wavelength 
deviation of the laser beam at an accuracy of .+-.0.0025 nm. 
Subsequently, the controller 10 determines the amount of variation in the 
angle of inclination of the wavelength selecting Fabry-Perot etalon 104 
necessary to adjust the thus found wavelength deviation of the laser beam. 
In accordance with this determination by the controller 10, the controller 
20 controls the angle of inclination of the etalon 104 through the 
actuator 21. As a result, the amount of wavelength deviation of the laser 
beam is held within .+-.0.0025 nm. Further, after this control has been 
effected, the controller 20 finds the amount of wavelength deviation from 
the set wavelength of the pre-input laser beam and the spectrum of the 
laser beam detected by the wavelength monitoring device 200, in order to 
adjust the wavelength deviation of the laser beam. On the basis of this 
amount of wavelength deviation, the controller 20 suitably 
feedback-controls the angle of inclination of the etalon 104 through the 
actuator 21. 
Thus, the central wavelength of the laser beam output from the laser source 
100 coincides with the set wavelength and the amount of wavelength 
deviation is held within a predetermined value (.+-.0.001 nm). 
A second embodiment of the present invention will now be described with 
reference to FIG. 2. The second embodiment is designed such that on the 
basis of the result of the measurement of the output wavelength deviation 
of the laser source, the position of the spectral detector of the 
wavelength monitoring device is moved relative to the interference fringe. 
As in the first embodiment, on the basis of the amount of fluctuation of 
the projection magnification determined at a point of time whereat the 
excimer laser source 100 is oscillated and the central wavelength of the 
laser beam thereof becomes stable to a certain degree, a controller 410 
determines the amount of wavelength deviation of the laser beam and stores 
it therein. 
Here, in order to accurately detect the spectrum (interference fringe 
distribution) of the laser beam by the wavelength monitoring device 200, 
the controller 410 corrects the position of the spectral detector 205 
relative to the interference fringes. For this purpose, as in the first 
embodiment, the measurement of the positions of the projected images of 
the fiducial marks M1 and M2 between two points spaced apart from each 
other by about 20 mm (.+-.10 mm) at the full angle of view of the 
projection lens is effected repetitively, whereby the positions of the 
projected images are measured at an accuracy of 0.04 .mu.m, and the amount 
of wavelength deviation of the laser beam is found at an accuracy of 
.+-.0.001 nm. 
Next, the controller 410 determines the necessary amount of movement of the 
detector 205 from the thus found amount of wavelength deviation, and on 
the basis of this determined value, it moves the detector 205 through an 
actuator 411 by a necessary amount in the direction of arrange men of the 
interference fringes. 
Subsequently, the controller 420 finds the amount of wavelength deviation 
from the set wavelength the pre-input laser beam and the spectrum of the 
laser beam detected by the wavelength monitoring device 200, in order to 
adjust the wavelength deviation of the laser beam. On the basis of this 
amount of wavelength deviation, the controller 420 suitably 
feedback-controls the angle of inclination of the etalon 104 through the 
actuator 21. 
Accordingly, the central wavelength of the laser beam output from the laser 
source 100 coincides with the set wavelength and the amount of wavelength 
deviation is held within a predetermined value (.+-.0.001 nm). 
When the necessary amount of movement determined by the controller 410 is 
greater relative to the range of movement of the detector 205, the 
necessary amount of movement can be made small if a program is set such 
that the controller 420 preliminarily adjusts the angle of inclination of 
the etalon 104 through the actuator 21 to a certain degree. 
After the position of the detector 205 is moved, the controller 410 again 
finds the amount of fluctuation of the projection magnification. On the 
basis of this amount of fluctuation of the projection magnification, the 
controller 410 judges whether the detector 205 accurately detects the 
spectrum of the laser beam. As a result, when the detector 205 accurately 
detects the spectrum, the controller 420 compares the spectrum of the 
laser beam detected by the wavelength monitoring device 200 with the 
design wavelength in the same manner as the above-described operation and 
finds the amount of wavelength deviation, and feedback-controls the angle 
of inclination of the etalon 104. Thereby the wavelength deviation of the 
laser beam is held within .+-.0.001 nm. 
Also, when the detector 205 does not detect the spectrum, the controller 
410 again effects the above-described preliminary adjustment. 
In each of the above-described embodiments, by measuring the amount of 
fluctuation of the focus position of the projection optical system and 
determining the amount of wavelength deviation on the basis of this amount 
of fluctuation, a similar effect can be obtained. 
Further, even if a device of such a construction as described below is used 
instead of the characteristic measuring device of the projection optical 
system used in each of the above-described embodiments, the fluctuation of 
the projection magnification can be found as in the above-described 
embodiments. 
A slit plate having a plurality of rectangular slits is provided on a wafer 
stage so as to be conjugate with the pattern surface of a mask with 
respect to the projection lens. Further, an illuminating system having a 
light source supplying light of the same wavelength as the exposure light 
which illuminates the slit plate from the inside thereof is provided. The 
design is such that light transmitted through the slit plate is 
transmitted through the projection lens, the mask and the illuminating 
optical system and the light reflected by a reflecting mirror passes 
through a relay optical system, etc. to a photoelectric detector. This 
photoelectric detector is provided at a location substantially optically 
conjugate with the pupil of the projection lens and is set so as to have a 
light-receiving surface substantially equal to the image of the pupil. The 
wafer stage is then moved and a fiducial mark provided on the mask is 
scanned by the image of a light-emitting slit. The position in which the 
mark of the mask and the light-emitting slit overlap each other at this 
time can be found in advance from the design data and therefore, if 
distortion exists in the projection lens, there occurs a deviation between 
the mark of the mask and the light-emitting slit, or a spacing deviation 
between the projected images of a plurality of marks of the mask. This 
deviation is detected by a photo-electric detector and a laser 
interferometer and further, on the basis of the detection signal, the 
controller finds the fluctuation of the projection magnification. 
Alternatively, after the printing of the pattern of a test mask onto the 
wafer is effected, the coordinates value of the wafer stage is set on the 
basis of the design data of the pattern of the test mask, and then the 
test mask is exposed again and superposition-printed. On the basis of a 
mark (or a latent image) thus formed by the superposition printing, the 
amount of deviation between marks is measured by the use of the alignment 
optical system of the apparatus, whereby the fluctuation of the projection 
magnification can be found. 
Where, as the wavelength selecting Fabry-Perot etalon, use is made of a 
Fabry-Perot etalon of the air gap type, i.e., the type which is provided 
with a retractile member such as a piezo element and in which this 
retractile member is expanded or contracted through a driving portion to 
thereby finely adjust the air gap, it is apparent that a similar effect 
can also be obtained if the air gap is controlled in conformity with the 
amount of wavelength deviation.