Alignment apparatus utilizing a plurality of wavelengths

Two-colored illumination light emitted from first and second laser beam sources illuminates a reticle mark and a wafer mark. Diffraction light from the reticle mark and the wafer mark is received by two photoelectric detection elements, respectively. The one photoelectric element receives single-colored diffraction light from light of the first light source through a color filter to generate a reticle beat signal. The other photoelectric element receives two-colored light to generate a wafer beat signal. A phase difference between the reticle beat signal and the wafer beat signal when shutting off the second laser beam source is aligned with a phase difference between the two signals produced when turning on the second laser beam source and decreasing the power of the first laser light source.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an alignment apparatus used for a 
projection exposure apparatus including a projection optical system for 
transferring a pattern formed on a mask to a photosensitive substrate, and 
more particularly to an alignment apparatus of a heterodyne interference 
method for relative alignment of a mask with a photosensitive substrate by 
emitting alignment light having a plurality of wavelengths to the 
photosensitive substrate. 
2. Related Background Art 
In a projection exposure apparatus used for producing semiconductors, 
liquid crystal display elements, and so on, multilayer circuit patterns 
are formed on a wafer (or a glass plate, etc.). It is necessary for the 
multilayer circuit patterns to register with each other with high 
accuracy. In this case, it is very important to align a reticle (or a 
photo mask and the like) with the wafer with high accuracy, particularly 
when a pattern formed on the reticle is to be transferred to a second or 
subsequent layer on the wafer. Therefore an alignment system is necessary 
for detecting the relation between a reticle position and a wafer position 
with high accuracy. Thus, there have been used an alignment system of a 
TTL (Through-The-Lens) type for detecting a relatively positional relation 
between the reticle and the wafer through a projection optical system. 
It is necessary to use light having a different wavelength from exposure 
light as alignment light for use in an alignment system in order not to 
expose a photoresist coated on the wafer. 
However, employment of alignment light having a wavelength different from a 
wavelength of exposure light causes chromatic aberration in projection 
optical systems. U.S. Pat. Nos. 5,100,237 and 4,492,459 propose ideas to 
solve the problem of chromatic aberration. 
In a process disclosed in U.S. Pat. No. 5,100,237, arrangement of a 
chromatic aberration correcting lens about an optical axis of a projection 
optical system at an entrance pupil position enables the chromatic 
aberration between the wavelength of exposure light and that of alignment 
light to be corrected, thereby detecting (.+-.) primary diffracted light 
beams from a wafer mark to perform an alignment between a reticle and a 
wafer. 
In a process disclosed in U.S. Pat. No. 4,492,459, arrangement of a 
correcting optical system outside or inside an exposure optical path 
between a reticle and a projection optical system enables chromatic 
aberration caused by alignment light passing through a projection optical 
system to be corrected. A reticle mark image and a wafer mark image formed 
on a wafer are detected through the projection optical system before 
performing an alignment. 
The applicant of this application has proposed an alignment apparatus of a 
heterodyne interference method with respect to correction of chromatic 
aberration relative to alignment light. In the alignment apparatus of a 
heterodyne interference method, arrangement of chromatic aberration 
controlling members such as phase gratings to a position adjacent to a 
pupil plane of a projection optical system permits alignment light to be 
deflected, thereby performing alignment of a reticle and a wafer with each 
other with high accuracy. 
In addition to the problem of the chromatic aberration, since detection of 
a position of a wafer mark by a monochromatic light weakens a reflected 
light depending on a photoresist thickness, unevenness of a wafer mark, 
and so on, it is often difficult to detect the wafer mark. Therefore, the 
applicant of this application also has proposed an apparatus in which 
light beams having a plurality of wavelengths (colors) are used as 
alignment light beams and phase gratings for chromatic aberration are 
arranged relative to the light beams of wavelengths, respectively to 
thereby perform alignment with more high accuracy. 
As mentioned above, according to the apparatus in which light having a 
plurality of wavelengths is employed as alignment light and the phase 
gratings are arranged adjacent to a pupil plane of a projection optical 
system and in facing relation to the pupil plane, since chromatic 
aberration by the projection optical system itself offsets that by the 
phase gratings disposed adjacent to the pupil plane of the projection 
optical system, resulting chromatic aberration is negligible in a 
plurality of wavelengths of alignment light. Strictly speaking, however, 
since some chromatic aberration remains, plurally colored interference 
fringes can not be aligned with each other simultaneously and precisely on 
both the reticle and the wafer. 
Movement of a viewing system (or a microscope) of an alignment apparatus 
within an exposure area causes an amount of deviation between plurally 
colored interference fringes relative to the alignment light having a 
plurality of the wavelengths to be changed because of the characteristics 
of a projection optical system. Multilayer circuit patterns are 
transferred to a wafer by exposing it. Each of the transferred multiple 
layers has a different reflectance with respect to a wavelength. For 
example, a layer does not greatly reflect light having one wavelength, 
another layer does not reflect light having another wavelength. In this 
case, changing the amount of deviation between the two-colored 
interference fringes as mentioned above causes an alignment detecting 
position to be deviated by the change of the amount of deviation between 
the two-colored interference fringes in comparison with a layer uniformly 
reflecting the two light beams having the difference wave-lengths. In 
order to solve the problem, a process (or a contrast method) can be used 
for adjusting an optical system to maximize a contrast of a beat signal 
(or a wafer signal) obtained from a wafer mark to decrease the deviation 
between the two-colored interference fringes. However, it is necessary for 
the amount of deviation between the two-colored interference fringes to be 
controlled within a range of from several "nm" to several tens "nm" in 
order to restrict the deviation therebetween to the extent that it can be 
made negligible. Thus, the contrast method is not sufficient. 
Even if deviation between the two-colored interference fringes relative to 
the two-colored light on the wafer is adjusted to be decreased, adjustment 
thereof automatically causes deviation between the two-colored 
interference fringes to be generated on a reticle. For example, when a 
ratio of light quantity of the two-colored, light is 1:1 and the 
one-colored interference fringe is deviated from the other colored 
interference fringe by a half pitch, a beat signal (or a reticle signal) 
is not generated from a reticle mark. 
SUMMARY OF THE INVENTION: 
An object of the present invention is to provide an alignment apparatus of 
a heterodyne interference method in which deviation between the 
two-colored interference fringes relative to alignment light of two 
wavelengths can be measured and adjusted with high accuracy, and in which 
a reticle signal does not receive any influence by adjusting the deviation 
between the two-colored interference fringes on a wafer. 
The present invention provides an alignment apparatus adapted to be 
provided on a projection exposure apparatus for projecting through a 
projection optical system a transferring pattern on a mask onto a 
photosensitive substrate rested on a two-dimensionally movable stage and 
for detecting a position of the substrate on the basis of a diffraction 
grating-shaped mark formed on the substrate comprising: 
a reference optical member; 
a first illumination optical system for illuminating the reference optical 
member with a first pair of light beams having different frequencies and a 
first wavelength; 
a first photoelectric detection device for photoelectrically transforming a 
first heterodyne beam generated by diffraction of the first light beams by 
the reference optical member into a first beat signal; 
a second illumination optical system for illuminating the diffraction 
grating-shaped mark through the projection optical system with a second 
pair of light beams having different frequencies and the first wavelength; 
a third illumination optical system for illuminating the diffraction 
grating-shaped mark through the projection optical system with a third 
pair of light beams having different frequencies and a second wavelength 
different from the first wavelength; 
a second photoelectric detection device for photoelectrically transforming 
second and third heterodyne beams respectively generated by diffraction of 
the second and third light beams by the diffraction grating-shaped mark 
into a second beat signal, the second and heterodyne beam having the first 
wavelength, the third heterodyne beam having the second wavelength, the 
second and third heterodyne beams being returned through the projection 
optical system; 
a phase comparing device for detecting a phase difference between the first 
and second beat signals; 
a relative position adjusting device for adjusting a relative position in a 
diffraction grating-shaped mark measuring direction between a position on 
the diffraction grating-shaped mark illuminated by the second light beams 
and that thereon illuminated by the third light beams; and 
a control device for controlling the relative position adjusting device on 
the basis of a first phase difference detected by the phase comparing 
device when light intensity of the third light beams on the substrate is 
decreased below a predetermined level and a second phase difference 
detected by the phase comparing device when light intensity of the first 
light beams on the reference optical member is decreased within the range 
in which the second heterodyne beam can be detected. 
In one preferred embodiment, the reference optical member is formed on the 
mask, and the reference optical member includes a diffraction 
grating-shaped mark. 
In another preferred embodiment, an illumination light controlling element 
for controlling chromatic aberration of the projection optical system 
relative to the light beams of the first wavelength so that the chromatic 
aberration thereof becomes a predetermined value is provided in an area on 
one of a Fourier transform plane relative to the mask in the projection 
optical system and a plane adjacent to the Fourier transform plane through 
which the light beams of the first wavelength pass, and another 
illumination light controlling element for controlling chromatic 
aberration of the projection optical system relative to the light beams of 
the second wavelength so that the chromatic aberration thereof becomes a 
predetermined value is provided in an area on one of a Fourier transform 
plane relative to the mask in the projection optical system and a plane 
adjacent to the Fourier transform plane through which the light beams of 
the second wavelength pass. 
The present invention also provides a method for detecting a position of a 
substrate onto which a transferring pattern on a mask is projected through 
a projection optical system on the basis of a diffraction grating-shaped 
mark formed on the substrate comprising: 
providing a reference optical member; 
illuminating the reference optical member with a first pair of light beams 
having different frequencies and a first wavelength; 
photoelectrically transforming a first heterodyne beam generated by 
diffraction of the first light beams by the reference optical member into 
a first beat signal; 
illuminating the diffraction grating-shaped mark through the projection 
optical system with a second pair of light beams having different 
frequencies and the first wavelength; 
illuminating the diffraction grating-shaped mark through the projection 
optical system with a third pair of light beams having different 
frequencies and a second wavelength different from the first wavelength; 
photoelectrically transforming second and third heterodyne beams 
respectively generated by diffraction of the second and third light beams 
by the diffraction grating-shaped mark into a second beat signal, the 
second and heterodyne beam having the first wavelength, the third 
heterodyne beam having the second wavelength, the second and third 
heterodyne beams being returned through the projection optical system; 
detecting a phase difference between the first and second beat signals; 
adjusting a relative position in a diffraction grating-shaped mark 
measuring direction between a position on the diffraction grating-shaped 
mark illuminated by the second light beams and that thereon illuminated by 
the third light beams; and 
controlling the relative position on the basis of a first phase difference 
detected when light intensity of the third light beams on the substrate is 
decreased below a predetermined level and a second phase difference 
detected when light intensity of the first light beams on the reference 
optical member is decreased within the range in which the second 
heterodyne beam can be detected. 
In an alignment apparatus according to the present invention, when the 
reference grating is aligned with the diffraction grating-shaped mark 
formed on the photosensitive substrate by using the two kinds of light 
beams having the different wavelengths (or colors), the reference grating 
diffracts the first two light beams having the different frequencies and 
the first wavelength to generate the first heterodyne beam comprising, for 
example, (.+-.) primary diffracted light beams, and the first 
photoelectric detection device receives the first heterodyne beam to 
generate a first beat signal. The diffraction grating-shaped mark formed 
on the substrate diffracts the second two light beams having the different 
frequencies and the first wavelength to generate the second heterodyne 
beams comprising, for example, (.+-.) primary diffracted light beams. The 
diffraction grating-shaped mark also diffracts the third two light beams 
having the different frequencies and the second wavelength to generate the 
third heterodyne beams comprising, for example, (.+-.) primary diffracted 
light beams. The second photoelectric detection device receives the second 
and third heterodyne beams to generate a second beat signal. 
First, with the light intensity of the light beams having the second 
wavelength being decreased, the first phase difference between the first 
and second beat signals is measured. Then while the light intensity of the 
light beams having the first wavelength is decreased within the range in 
which the first beat signal can be detected, the second phase difference 
between the first and second beat signals is detected. The relative 
position adjusting device adjusts the relative position between the 
position on the substrate illuminated by the second light beams and that 
thereon illuminated by the third light beams on the basis of the two phase 
differences. This allows the phase differences to be corrected, thereby 
enabling the amount of deviation between the two-colored interference 
fringes to be adjusted. 
When performing the alignment after the adjustment is accomplished, the 
two-colored light beams illuminates the diffraction grating-shaped mark 
formed on the substrate to produce the second beat signal. Thus, even if 
one of the two light beams is decreased in light intensity due to the 
influence of a reflectance quality of the substrate, unevenness of the 
mark and so on, a position of the substrate can be detected with accuracy. 
On the other hand, with respect to the reference grating, even if the 
positions on the substrate to which the two-colored light beams are 
incident are shifted, the contrast of the first beat signal regarding the 
reference grating does not change. Generally, silica glass is coated with 
a chrome film and the reference grating is formed on the chrome film by 
patterning. Thus, it appears that the reference grating is ideally made, 
and it is not necessary to use two-colored light beams in order to perform 
alignment. Such an idea is employed in the present invention. 
The relative position adjusting device synchronizes phases of the two beat 
signals on the basis of a command from the control device. In this case, 
decreasing an intensity of the first wavelength light enables the second 
beat signal relative to the diffraction grating-shaped mark formed on the 
substrate to be obtained with intensity of the light beam of the second 
wavelength being sufficient without receiving the influence of the first 
wavelength light, thereby performing a phase synchronization with 
accuracy. 
In a case where the reference grating is formed on the mask and the 
reference grating includes a diffraction grating-shaped mark, an 
additional element on which the reference grating is provided is not 
necessary, and an amount of positional deviation between the mask and the 
substrate can be directly detected. 
In a case where an illumination light controlling element for controlling 
chromatic aberration of a projection optical system relative to the light 
beams of the first wavelength so that the chromatic aberration becomes a 
predetermined value is provided in an area on a Fourier transform plane 
relative to the mask in the projection optical system or a plane adjacent 
to the Fourier transform plane through which the light beams of the first 
wavelength pass, and where another illumination light controlling element 
for controlling chromatic aberration of a projection optical system 
relative to the light beams of the second wavelength so that the chromatic 
aberration becomes a predetermined value is provided in an area on a 
Fourier transform plane relative to the mask in the projection optical 
system or a plane adjacent to the Fourier transform plane through which 
the light beams of the second wavelength pass, since the chromatic 
aberration relative to the two-colored alignment light beams which is 
caused by the projection optical system are corrected, an amount of 
positional deviation between the mask and the substrate can be detected by 
a TTR (Through-The-Reticle) type. 
The present invention provides an alignment apparatus for detecting a 
position of a photosensitive substrate on the basis of a diffraction 
grating-shaped mark formed on the substrate comprising: 
an illumination optical system for illuminating the diffraction 
grating-shaped mark with a first pair of light beams having different 
frequencies and a first wavelength and a second pair of light beams having 
different frequencies and a second wavelength different from the first 
wavelength; 
a photoelectric detection device for photoelectrically transforming a first 
heterodyne beam having the first wavelength generated by diffraction of 
the first light beams by the diffraction grating-shaped mark and a second 
heterodyne beam having the second wavelength generated by diffraction of 
the second light beams by the diffraction grating-shaped mark into a beat 
signal; 
a reference signal generating device for electrically generating a 
reference signal having a frequency corresponding to each of a frequency 
difference between the first light beams and a frequency difference 
between the second light beams; 
a phase comparing device for detecting a phase difference between the 
reference signal generated from the reference signal generating device and 
the beat signal supplied from the photoelectric detection device; 
a relative position adjusting device for adjusting a relative position in a 
diffraction grating-shaped mark measuring direction between a position on 
the diffraction grating-shaped mark illuminated by the first light beams 
and that thereon illuminated by the second light beams; and 
a control device for controlling the relative position adjusting device on 
the basis of a first phase difference detected by the phase comparing 
device when light intensity of the first light beams on the substrate is 
decreased below a predetermined level and a second phase difference 
detected by the phase comparing device when light intensity of the second 
light beams on the substrate is decreased below a predetermined level. 
In one preferred embodiment, the illumination optical system comprises a 
heterodyne beam generating system including an acousto-optic modulator 
driven by a drive signal having a predetermined frequency difference, the 
acousto-optics modulator generates the first and second pairs of light 
beams, and the reference signal generating device generates the reference 
signal on the basis of the drive signal. 
In another preferred embodiment, the alignment apparatus is provided on a 
projection exposure apparatus for transferring through a projection 
optical system a transferring pattern on a mask onto a photosensitive 
substrate, 
the illumination optical system illuminates the diffraction grating-shaped 
mark formed on the substrate with the first and second pairs of light 
beams through the projection optical system, and 
the photoelectric detection device receives the heterodyne beams generated 
by diffraction of the light beams by the diffraction grating-shaped mark 
through the projection optical system. 
The present inventions also provides an alignment apparatus adapted to be 
provided on a projection exposure apparatus for projecting a transferring 
pattern image of a mask through a projection optical system onto a 
photosensitive substrate rested on a two-dimensionally movable stage and 
for performing an alignment between the mask and the substrate on the 
basis of a diffraction grating-shaped mask mark formed on the mask and a 
diffraction grating-shaped substrate mark formed on the substrate 
comprising: 
a first illumination optical system for illuminating the mask mark with at 
least one of a first pair of light beams having different frequencies and 
a first wavelength and a second pair of light beams having different 
frequencies and a second wavelength different from the first wavelength; 
a second illumination optical system for allowing the first light beams to 
pass through the projection optical system to generate a third pair of 
light beams having different frequencies and the first wavelength and 
allowing the second light beams to pass through the projection optical 
system to generate a fourth pair of light beams having different 
frequencies and the second wavelength to illuminate the substrate mark 
with the third and fourth light beams; 
a second photoelectric detection device for photoelectrically transforming 
third and fourth heterodyne beams respectively generated by diffraction of 
the third and fourth light beams by the substrate mark into a beat signal, 
the third heterodyne beams having the first wavelength, the fourth 
heterodyne beams having the second wavelength, the third and fourth 
heterodyne beams being respectively returned through the projection 
optical system; 
a reference signal generating device for electrically generating a 
reference signal having a frequency corresponding to each of a frequency 
difference between the first light beams and a frequency difference 
between the second light beams; 
a phase comparing device for detecting a phase difference between the 
reference signal generated from the reference signal generating device and 
the beat signal generated from the second photoelectric detection device; 
a relative position adjusting device for adjusting a relative position in a 
substrate mark measuring direction between a position on the substrate 
illuminated by the third light beams and that thereon illuminated by the 
fourth light beams; and 
a control device for controlling the relative position adjusting device on 
the basis of a first phase difference detected by the phase comparing 
device when light intensity of the third light beams on the substrate is 
decreased below a predetermined level and a second phase difference 
detected by the phase comparing device when light intensity of the fourth 
light beams on the substrate is decreased below another predetermined 
level. 
In another preferred embodiment, the first illumination optical system 
illuminates the mask mark only with the first light beams. 
In another preferred embodiment, the first illumination optical system 
comprises a heterodyne beam generating system for generating the first and 
second light beams, the heterodyne beam generating system includes an 
acousto-optic modulator which is driven by a drive signal having a 
predetermined frequency difference to generate the first and second light 
beams, and the reference signal generating device generates the reference 
signal on the basis of the drive signal. 
In another preferred embodiment, the alignment apparatus further comprises 
a first photoelectric detection device for photoelectrically transforming 
a heterodyne beam generated by diffraction of one of the first and second 
light beams by the mask mark into a beat signal, a phase difference 
comparing device for detecting a phase difference between the beat signal 
generated from the first photoelectric detection device and the beat 
signal generated from the second photoelectric detection device, and a 
control device for controlling a relative position between the mask and 
the stage on the basis of the phase difference. 
The present invention further provides an alignment apparatus for detecting 
a position of a photosensitive substrate on the basis of a diffraction 
grating-shaped mark formed on the substrate comprising: 
an illumination optical system for illuminating the diffraction 
grating-shaped mark with a first pair of light beams having different 
frequencies and a first wavelength and a second pair of light beams having 
different frequencies and a second wavelength different from the first 
wavelength; 
a photoelectric detection device for photoelectrically transforming a first 
heterodyne beam having the first wavelength generated by diffraction of 
the first light beams by the diffraction grating-shaped mark and a second 
heterodyne beam having the second wavelength generated by diffraction of 
the second light beams by the diffraction grating-shaped mark into a beat 
signal; 
a phase comparing device for detecting a phase difference between a 
predetermined reference signal and the beat signal supplied from the 
photoelectric detection device; 
a relative position adjusting device for adjusting a relative position in a 
diffraction grating-shaped mark measuring direction between a position on 
the diffraction grating-shaped mark illuminated by the first light beams 
and that thereon illuminated by the second light beams; and 
a control device for controlling the relative position adjusting device on 
the basis of a first phase difference detected by the phase comparing 
device when light intensity of the first light beams on the substrate is 
decreased below a predetermined level and a second phase difference 
detected by the phase comparing device when light intensity of the second 
light beams on the substrate is decreased below a predetermined level. 
The present invention further provides a method for detecting a position of 
a photosensitive substrate on the basis of a diffraction grating-shaped 
mark formed on the substrate comprising: 
illuminating the diffraction grating-shaped mark with a first pair of light 
beams having different frequencies and a first wavelength and a second 
pair of light beams having different frequencies and a second wavelength 
different from the first wavelength; 
photoelectrically transforming a first heterodyne beam having the first 
wavelength generated by diffraction of the first light beams by the 
diffraction grating-shaped mark and a second heterodyne beam having the 
second wavelength generated by diffraction of the second light beams by 
the diffraction grating-shaped mark into a beat signal; 
electrically generating a reference signal having a frequency corresponding 
to each of a frequency difference between the first light beams and a 
frequency difference between the second light beams; 
detecting a phase difference between the reference signal and the beat 
signal; 
adjusting a relative position in a diffraction grating-shaped mark 
measuring direction between a position on the diffraction grating-shaped 
mark illuminated by the first light beams and that thereon illuminated by 
the second light beams; and 
controlling the relative position on the basis of a first phase difference 
detected when light intensity of the first light beams on the substrate is 
decreased below a predetermined level and a second phase difference 
detected when light intensity of the second light beams on the substrate 
is decreased below a predetermined level. 
The present invention further provides a method adapted to apply to a 
projection exposure apparatus for projecting a transferring pattern image 
of a mask through a projection optical system onto a photosensitive 
substrate rested on a two-dimensionally movable stage and for performing 
an alignment between the mask and the substrate on the basis of a 
diffraction grating-shaped mask mark formed on the mask and a diffraction 
grating-shaped substrate mark formed on the substrate comprising: 
illuminating the mask mark with at least one of a first pair of light beams 
having different frequencies and a first wavelength and a second pair of 
light beams having different frequencies and a second wavelength different 
from the first wavelength; 
allowing the first light beams to pass through the projection optical 
system to generate a third pair of light beams having different 
frequencies and the first wavelength and allowing the second light beams 
to pass through the projection optical system to generate a fourth pair of 
light beams having different frequencies and the second wavelength to 
illuminate the substrate mark with the third and fourth light beams; 
photoelectrically transforming third and fourth heterodyne beams 
respectively generated by diffraction of the third and fourth light beams 
by the substrate mark into a beat signal, the third heterodyne beams 
having the first wavelength, the fourth heterodyne beams having the second 
wavelength, the third and fourth heterodyne beams being respectively 
returned through the projection optical system; 
electrically generating a reference signal having a frequency corresponding 
to each of a frequency difference between the first light beams and a 
frequency difference between the second light beams; 
detecting a phase difference between the reference signal and the beat 
signal; 
adjusting a relative position in a substrate mark measuring direction 
between a position on the substrate illuminated by the third light beams 
and that thereon illuminated by the fourth light beams; and 
controlling the relative position on the basis of a first phase difference 
detected when light intensity of the third light beams on the substrate is 
decreased below a predetermined level and a second phase difference 
detected when light intensity of the fourth light beams on the substrate 
is decreased below another predetermined level. 
The present invention further provides a method for detecting a position of 
a photosensitive substrate on the basis of a diffraction grating-shaped 
mark formed on the substrate comprising: 
illuminating the diffraction grating-shaped mark with a first pair of light 
beams having different frequencies and a first wavelength and a second 
pair of light beams having different frequencies and a second wavelength 
different from the first wavelength; 
photoelectrically transforming a first heterodyne beam having the first 
wavelength generated by diffraction of the first light beams by the 
diffraction grating-shaped mark and a second heterodyne beam having the 
second wavelength generated by diffraction of the second light beams by 
the diffraction grating-shaped mark into a beat signal; 
detecting a phase difference between a predetermined reference signal and 
the beat signal; 
adjusting a relative position in a diffraction grating-shaped mark 
measuring direction between a position on the diffraction grating-shaped 
mark illuminated by the first light beams and that thereon illuminated by 
the second light beams; and 
controlling the relative position adjusting device on the basis of a first 
phase difference detected when light intensity of the first light beams on 
the substrate is decreased below a predetermined level and a second phase 
difference detected when light intensity of the second light beams on the 
substrate is decreased below a predetermined level. 
According to the alignment apparatus of the present invention, since light 
beams having two wavelengths (two colors) are used which comprise a pair 
of light beams having the first wavelength and a pair of light beams 
having the second wavelength, a position of the substrate can be detected 
with high accuracy without receiving any influence of thin film 
interference and the like on the substrate. 
In addition, since deviation between an interference fringe on the 
substrate provided by a pair of light beams having the first wavelength 
and another interference fringe on the substrate provided by a pair of 
light beams having the second wavelength is adjusted by the reference 
signal having a frequency corresponding to each of a frequency difference 
between the light beams having the first wavelength and a frequency 
difference between the light beams having the second wavelength, the 
adjustment of the two-colored interference fringes can be performed 
without receiving any optical influence. 
Additionally, in an alignment apparatus of the present invention, when the 
heterodyne beam generating system generates the first and second pairs of 
light beams by the acousto-optic modulator driven by the drive signal 
having a predetermined frequency difference, the reference signal 
generating device easily can generate the reference signal on the basis of 
the drive signal. The alignment apparatus of the present invention also 
enables a frequency of the beat signal from the photoelectric detection 
device to be in integral multiple relation to that of the reference signal 
to easily and precisely detect a phase difference between the beat signal 
and the reference signal to align the two-colored interference fringes 
with each other with high accuracy. 
In a case where the alignment apparatus of the present invention is 
provided on a projection exposure apparatus for transferring through a 
projection optical system a transferring pattern on a mask onto a 
photosensitive substrate, where the illumination optical system 
illuminates the diffraction grating-shaped mark formed on the substrate 
with the first and second pairs of light beams through the projection 
optical system, and where the photoelectric detection device receives the 
heterodyne beams generated by diffraction of the light beams by the 
diffraction grating-shaped mark through the projection optical system, a 
TTL type can be employed in order to align interference fringes on the 
substrate produced by the two-colored light beams with each other with 
high accuracy. 
According to the alignment apparatus of the present invention, since 
two-colored light beams comprising the third pair of light beams having 
the first wavelength and the fourth pair of light beams having the second 
wavelength are employed, alignment between the mask and the substrate can 
be performed with high accuracy without receiving any influence of thin 
film interference and the like on the substrate. 
In addition, since deviation between an interference fringe provided on the 
substrate by a pair of light beams having the first wavelength and another 
interference fringe provided on the substrate by a pair of light beams 
having the second wavelength is adjusted by the reference signal having a 
frequency corresponding to each of a frequency difference between the 
third light beams and a frequency difference between the fourth light 
beams, the adjustment of the two-colored interference fringes can be 
performed without receiving any optical influence. 
Additionally, in a case where the first illumination optical system 
illuminates the mask mark only with the first pair of light beams, since 
the mask mark is observed only with the single-colored light beam, there 
is no influence of deviation between the two-colored interference fringes 
on the mask mark, thereby enabling the first photoelectric detection 
device to stably supply the beat signal. If the mask mark is observed with 
two-colored illumination light as the substrate is done, adjustment of 
two-colored interference fringes on the substrate causes deviation between 
two-colored interference fringes to be generated on the mask mark. For 
example, in a case where the two-colored interference fringes are 
identical in the intensity thereof and where one of the two-colored 
interference fringes is deviated from the other colored interference 
fringe in its phase by a half pitch, the beat signal in the side of the 
mask disappears. In order to avoid such a happening, in the present 
invention, the mask mark is observed only with the single-colored light 
beam. 
In this case, when adjusting the deviation between the two-colored 
interference fringes caused by the two-colored illumination light beams, 
employment of the following processing causes disappearance of the beat 
signal form the side of the mask. 
First, the one-colored illumination light and the other colored 
illumination light alternately illuminate the substrate. Then, the second 
photoelectric detection device receives the heterodyne beams from the 
substrate mark to generate the beat signal. On the other hand, the first 
photoelectric detection device receives the heterodyne beams from the mask 
mark to generate the other beat signal. Comparison of phases of the two 
beat signals from the first and second photoelectric detection devices is 
performed. In such a processing, since the mask mark is not observed with 
a light beam of the second wavelength, adjustment of the interference 
fringe having the second wavelength causes disappearance of the beat 
signal form the side of the mask. 
In the present invention, the reference signal generating device generates 
an electrical signal having a frequency which is identical to each of a 
frequency difference between the heterodyne beams from the substrate mark 
and a frequency difference between the heterodyne beams from the mask 
mark. The beat signal from the substrate mark is also produced. A phase 
difference between the two beat signals is detected. The relative position 
adjusting device adjusts a relative position between a portion of the 
substrate illuminated by the light beams having the first wavelength and a 
portion of the substrate illuminated by the light beams having the second 
wavelength. Thus, even if the first light beams of the first wavelength 
for observing the mask mark disappears, the deviation between the 
two-colored interference fringes can be adjusted. 
Generally, a silica glass is coated with chrome film, and mask marks are 
formed on the chrome film by patterning. Thus, it appears that the mask 
marks are ideally made in comparison with the substrate marks. Therefore, 
it is not necessary to use two-colored illumination light beams having 
difference wavelengths in order to perform alignment. Such an idea is 
employed in the present invention. Namely, in the present invention, since 
the beat signal from the mask mark is produced from only the light beams 
having the first wavelength, performing of the adjustment by the relative 
position adjusting device enables the beat signal from the mask mark to 
stably be provided. 
In the alignment apparatus of the present invention, in a case where the 
heterodyne beam generating system generates a first pair of light beams 
and a second pair of light beams by using the acousto-optic modulator 
which is driven by a drive signal having a predetermined frequency 
difference, the reference signal generating device can easily generate the 
reference signal on the basis of the drive signal. Since a frequency of 
the beat signal from the photoelectric detection device is in integral 
multiple relation to that of the reference signal, a phase difference 
between the beat signal and the reference signal can be easily and 
precisely detected to align the two-colored interference fringes with each 
other with high accuracy.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An alignment according to a preferred embodiment of the preset invention 
will be explained with reference to the drawings. In this embodiment, the 
present invention is applied to an alignment apparatus of a TTR type and 
of a heterodyne interference method which is employed in a projection 
exposure apparatus for transferring a pattern formed on a reticle to each 
of shot areas on a wafer through a projection optical system. 
FIG. 1 schematically shows a general construction of the projection 
exposure apparatus used in the embodiment. In FIG. 1, a reticle 4 is held 
on a reticle stage 9. A wafer 6 is held on a wafer stage 61. When 
performing exposure, an exposing illumination light of a wavelength 
.lambda..sub.0 from an exposure illumination system 67 illuminates the 
reticle 4 through a dichroic mirror 3. By the illumination light, a 
pattern image of the reticle 4 is transferred to each of shot areas of the 
wafer 6 coated with a photoresist through a projection optical system 5. 
The reticle stage 9 and the wafer stage 61 respectively position the 
reticle 4 and the wafer 6 on respective planes which are perpendicular to 
the optical axis of the projection optical system 5. Two-dimensional 
coordinates of the reticle 9 and the wafer stage 61 are detected by a 
reticle-side interferometer 62 and a wafer-side interferometer 63, 
respectively. The detected results are supplied to a central control 
system 64. On the basis of the detected results, the central control 
system 64 controls operations of the reticle stage 9 and the wafer stage 
61 through a reticle stage control system 65 and a wafer stage control 
system 66, respectively. A reference mark member 41 having reference marks 
formed thereon is fixedly mounted on the wafer stage 61. When performing 
reticle alignment, the reference mark member 41 is moved into an exposing 
field of the projection optical system 5, and the reference marks are 
illuminated from the bottom side thereof with illumination light of the 
wavelength .lambda..sub.0 which is identical to that of the exposing 
illumination light. The illumination light beam passing through the 
reference mark passes through the projection optical system 5, and is 
incident to an alignment mark formed on the lower surface of the reticle 4 
to afford the image of the reference mark thereon. Reticle alignment 
microscopes 39 and 40 are provided above the reticle 4. The microscopes 39 
and 40 pick up the image of the alignment mark and the reference mark, 
respectively, and supply image signals to the central control system 64. 
The central control system 64 processes the supplied image signals to 
obtain an amount of deviation of the alignment mark relative to the 
reference mark. 
In order to perform fine alignment for the wafer 6, an alignment system of 
a TTR type and of a heterodyne interference method is further provided. 
The alignment system includes a laser control system 69, an alignment 
light sending system 1a, a beam splitter 26, an objective lens 2, an 
alignment light receiving system 1b, an alignment signal processing system 
70, and so on. When performing alignment, the central control system 64 
controls two laser beam sources (stated hereinafter) provided in the 
alignment light sending system la through the laser control system 69 so 
that the two laser beam sources emit laser beams. The laser beams emitted 
from the two laser beam sources are modulated by a predetermined 
frequency, and are then directed to the beam splitter 26 as alignment 
light beams. The laser beams reflected by the beam splitter 26 pass 
through the objective lens 2 and the dichroic mirror 3, and is incident to 
the reticle 4. The laser beams passing through the reticle 4 are applied 
to the reference mark member 41 (or wafer mark). 
Heterodyne beams are generated by the laser beams being diffracted by the 
reference mark member 41 (or wafer mark). The other heterodyne beams are 
generated by the laser beams being diffracted by the reticle mark. The 
heterodyne beams from the reference mark member 41 (or wafer mark) and the 
reticle mark are incident to the alignment light receiving system 1b 
through the dichroic mirror 3, the objective lens 2, and the beam splitter 
26. In the alignment light receiving system 1b, two beat signals are 
generated. The beat signals are supplied to the alignment signal 
processing system 70, in which a phase difference between the two beat 
signals is detected. The detected phase difference is supplied to the 
central control system 64. The central control system 64 determines a 
target phase difference for alignment or performs final alignment on the 
basis of the detected phase difference. 
Next, the reticle alignment microscope and the alignment system of a 
heterodyne method will be specifically explained. 
FIGS. 2(a) and 2(b) show stage and alignment optical systems of the 
projection exposure system in this embodiment. In FIGS. 2(a) and 2(b), a 
Z-axis is taken to be parallel to the optical axis AX of the projection 
optical system 5. A X-axis and a Y-axis are defined by an orthogonal 
coordinate system which is set on a plane perpendicular to the Z-axis. 
FIG. 2(a) is a front view of the projection exposure apparatus as viewed 
in a Y-direction. FIG. 2(b) is a side view of FIG. 2(a). An alignment 
optical system 1 includes the alignment light sending system 1a, the beam 
splitter 26, and the alignment light receiving system 1b which are shown 
in FIG. 1. The wafer stage 61 in FIG. 1 includes a X-stage 8X for 
positioning the wafer 6 in a X-direction and a Y-stage 8Y for positioning 
the wafer 6 in the Y-direction. In FIGS. 2(a) and 2(b), a wafer holder 7 
for holding the wafer 6 is provided between the wafer stage and the wafer 
6. In actual practice, a Z-stage (not shown) for positioning the wafer 6 
in a Z-direction is rested on the X-stage 8X, and the reference mark 
member 41 is provided on the Z-stage. 
The alignment optical system 1 emits reticle alignment illumination light 
beams RB.sub.1 and RB.sub.2, wafer alignment illumination light beams 
WB.sub.1 and WB.sub.2, reticle alignment illumination light beams RB.sub.3 
and RB.sub.4, and wafer alignment illumination light beams WB.sub.3 and 
WB.sub.4. The reticle alignment beams RB.sub.1 and RB.sub.2 respectively 
have an average wavelength .lambda..sub.1 which is different from the 
wavelength .lambda..sub.0 of exposure light, and have different 
frequencies. The frequency difference between the beams RB.sub.1 and 
RB.sub.2 is .DELTA.f (=50 kHz). The wafer alignment beams WB.sub.1 and 
WB.sub.2 also respectively have the average wavelength .lambda.1, and have 
different frequencies. The frequency difference between the beams WB.sub.1 
and WB.sub.2 is .DELTA.f (=50 kHz). The reticle alignment beams RB.sub.3 
and RB.sub.4 respectively have an average wavelength .lambda..sub.2 which 
is close to the wavelength .lambda..sub.1, and have different frequencies. 
The frequency difference between the beams RB.sub.3 and RB.sub.4 is 
.DELTA.f (=50 kHz). The wafer alignment beams WB.sub.3 and WB.sub.4 also 
respectively have the average wavelength .lambda..sub.2, and have 
different frequencies. The frequency difference between the beams WB.sub.3 
and WB.sub.4 is .DELTA.f (=50 kHz). 
As shown in FIG. 2(a), the reticle alignment illumination light beams 
RB.sub.1 and RB.sub.2 is condensed on the reticle 4 by the objective lens 
2, and are incident to a diffraction grating-shaped reticle mark 35A on 
the lower side of the reticle 4 at respective incident angles and 
+.theta..sub.R1. 
FIG. 3 shows a pattern arrangement of the reticle 4 in this embodiment. In 
FIG. 3, the reticle 4 has a pattern area 32 in the center thereof. A light 
blocking zone 33 is provided around the central pattern area 32 of the 
reticle 4. Reticle marks 36A and 36B for the Y-axis are respectively 
provided in opposite sides of the light blocking zone 33 extending in the 
Y-direction. Each of the reticle marks 36A and 36B comprises a diffraction 
grating including a plurality of grating elements which are spaced at a 
pitch P.sub.R in the Y-direction. Reticle marks 35A and 35B for the X-axis 
are respectively provided in opposite sides of the light blocking zone 33 
extending in the X-direction. Each of the X-axis reticle marks 35A and 35B 
comprises a diffraction grating including a plurality of grating elements 
which are spaced at a pitch P.sub.R in the X-direction. Transmissive 
windows (referred to as "reticle windows" hereinafter) 37A and 37B for 
allowing the alignment light directed toward the wafer to pass 
therethrough are provided inside of the reticle marks 35A and 35B, 
respectively. Reticle windows 38A and 38B for allowing the alignment light 
directed toward the wafer to pass therethrough are provided inside of the 
reticle marks 36A and 36B, respectively. The reticle 4 further has 
cross-shaped alignment marks 34A and 34B which are formed on opposite 
sides of the reticle 4 in the X-direction outside of the light blocking 
zone 33. 
FIG. 9 is an enlarged view of the reticle mark 35A and the reticle window 
37A which are shown in FIG. 3. The reticle mark 35A is illuminated with 
light beams 50 comprising the illumination light beams RB.sub.1, RB.sub.2, 
RB.sub.3, and RB.sub.4. Light beams 51 comprising the illumination light 
beams WB.sub.1, WB.sub.2, WB.sub.3, and WB.sub.4 pass through the reticle 
window 37A. 
Referring back to FIGS. 2(a) and 2(b), the relation between the incident 
angles -.theta..sub.R1 and +.theta..sub.R1 and the grating pitch P.sub.R 
of the reticle mark 35A is indicated by the following numerical expression 
: 
EQU sin (.theta..sub.R1)=.lambda..sub.1 /P.sub.R (1) 
A (+) primary diffracted light beam RB.sub.1.sup.+1 derived from the 
illumination light beam RB.sub.1 and a (-) primary diffracted light beam 
RB.sub.2.sup.-1 derived from the illumination light beam RB.sub.2 are 
emitted upwardly in a vertical direction relative to the reticle 4, and 
return to the alignment optical system 1 through the objective lens 2 as 
alignment detection light beams (or heterodyne beams). 
Similarly, the reticle alignment beams RB.sub.3 and RB.sub.4 are condensed 
on the reticle 4 by the objective lens 2, and are incident to the reticle 
mark 35A on the reticle 4 at incident angles -.theta..sub.R2 and 
+.theta..sub.R2. A (+) primary diffracted light beam RB.sub.3.sup.+1 
derived from the illumination light beam RB.sub.3 and a (-) primary 
diffracted light beam RB.sub.4.sup.-1 derived from the illumination light 
beam RB.sub.4 are emitted upwardly in a vertical direction relative to the 
reticle 4, and return to the alignment optical system 1 through the 
objective lens 2. 
On the other hand, the wafer alignment illumination light beams WB.sub.1 
and WB.sub.2 pass through the reticle window 37A of the reticle 4, and 
reach a chromatic aberration control plate 10 in the projection optical 
system 5. Diffraction grating-shaped longitudinal chromatic aberration 
control elements G1.sub.A and G1.sub.B are provided in portions of the 
chromatic aberration control plate 10 through which the wafer alignment 
illumination beams WB.sub.1 and WB.sub.2 pass, respectively (see FIG. 8). 
The wafer alignment illumination light beams WB.sub.1 and WB.sub.2 are 
bent by the control elements G1.sub.A and G1.sub.B by angles 
-.theta..sub.G1 and +.theta..sub.G1, respectively, and are incident to a 
diffraction grating-shaped wafer mark 48A at respective incident angles 
-.theta..sub.W1 and +.theta..sub.W1 relative thereto. 
FIG. 4 shows a part of the shot areas layout on the wafer 6. In FIG. 4, 
wafer marks 49A and 49B for the Y-axis are provided at opposite sides of 
an exposed shot area 47 in the X-direction outside of thereof, 
respectively. Each of the wafer marks 49A and 49B comprises a diffraction 
grating including grating elements which are spaced at a pitch P.sub.W in 
the Y-direction. Wafer marks 48A and 48B for the X-axis are provided at 
opposite sides of the exposed shot area 47 in the Y-direction outside of 
thereof, respectively. Each of the X-axis wafer marks 48A and 48B 
comprises a diffraction grating including grating elements which are 
spaced at the pitch P.sub.W in the X-direction. Wafer marks are also 
provided around the other shot areas. 
FIG. 10 is an enlarged view of the wafer mark 48A shown in FIG. 4. In FIG. 
10, the wafer mark 48A is illuminated with light beams 51 comprising the 
illumination light beams WB.sub.1, WB.sub.2, WB.sub.3, and WB.sub.4. 
Referring back to FIG. 2(a), the relation between the incident angles 
-.theta..sub.W1 and +.theta..sub.W1 and the grating pitch P.sub.W of the 
wafer mark 48A is indicated by the following numerical expression: 
EQU sin (.theta..sub.W1)=.lambda..sub.1 /P.sub.W (2) 
A (+) primary diffracted light beam WB.sub.1.sup.+1 derived from the 
illumination light beam WB.sub.1 and a (-) primary diffracted light beam 
WB.sub.2.sup.-1 derived from the illumination light beam WB.sub.2 are 
emitted upwardly in a vertical direction relative to the wafer mark 48A, 
and the two diffracted light beams serve as alignment detection light 
beams (or heterodyne beams). 
Similarly, since the wafer alignment beams WB.sub.3 and WB.sub.4 are close 
to the illumination light beams WB.sub.1 and WB.sub.2 in their wavelength, 
it can be taken that the wafer alignment beams WB.sub.3 and WB.sub.4 
substantially pass through the longitudinal chromatic aberration control 
elements G1.sub.A and G1.sub.B, respectively. Thus, the illumination light 
beams WB.sub.3 and WB.sub.4 are bent by the control elements G1.sub.A and 
G1.sub.B by angles -.theta..sub.G2 and +.theta..sub.G2, respectively, and 
are incident to the wafer mark 48A at incident angles -.theta..sub.W2 and 
+.theta..sub.W2 relative thereto, respectively. A (+) primary diffracted 
light beam WB.sub.3.sup.+1 derived from the illumination light beam 
WB.sub.3 and a (-) primary diffracted light beam WB.sub.4.sup.-1 derived 
from the illumination light beam WB.sub.4 are emitted upwardly in a 
vertical direction relative to the wafer mark 48A, and are used as 
alignment detection light beams. 
In this case, as shown in FIG. 2(b), the wafer alignment illumination light 
beams are inclined by an angle .theta..sub.m relative to the wafer 6 in 
the non-measuring direction (or the Y-direction) by means of deflection 
function of the chromatic aberration control plate 10, and is incident to 
the wafer 6. Thus, a position on the chromatic aberration control plate 10 
through which each of the alignment detection light beams passes is 
different from a position thereon through which each of the illumination 
light beams passes. The alignment detection light beams from the wafer 
mark 48A pass through a longitudinal chromatic aberration control element 
G1.sub.c on the chromatic aberration control plate 10 (see FIG. 8), 
thereby enabling chromatic aberration of the alignment detection light 
beams to be corrected in a lateral direction before directed to the 
reticle window 37A. Thereafter, each of the detection light beams returns 
to the alignment optical system 1 through the reticle window 37A and the 
objective lens 2. The wafer alignment illumination light beams illuminate 
a position on the surface of the wafer 6 which is deviated by 
.DELTA..beta. in the Y-direction from a position thereon which would be 
illuminated if the control plate 10 is not arranged. 
FIG. 8 shows the chromatic aberration control plate 10. The chromatic 
aberration control plate 10 includes a transmissive glass substrate. 
Twelve longitudinal chromatic aberration control elements are arranged on 
the glass substrate. The three longitudinal chromatic aberration control 
elements G1.sub.A, G1.sub.B, and G1.sub.c are used for deflecting the 
wafer alignment illumination light beams and the alignment detection light 
beams in the alignment optical system 1 as shown in FIGS. 2(a) and 2(b). 
In actual practice, since there are the other three alignment optical 
systems for three axes, a total of twelve (=3 * 4) longitudinal chromatic 
aberration control elements G1.sub.A, G1.sub.B, and G1.sub.c . . . 
G4.sub.A, G4.sub.B, and G4.sub.c are provided on the chromatic aberration 
control plate 10. 
With reference to FIGS. 6(a) and 6(b) and 7, the alignment optical system 1 
will be explained more specifically. FIG. 6(b) is a view as the alignment 
optical system 1 is viewed in the same direction as in FIG. 2. FIG. 6(a) 
is a view as the alignment optical system 1 is viewed in the same 
direction as in FIG. 2(a). FIG. 6(c) is a bottom view of the arrangement 
shown in FIG. 6(a). FIG. 7 is a perspective view showing a front portion 
of the alignment optical system. 
In FIG. 6, a beam combining prism 13 combines a laser beam of the 
wavelength .lambda..sub.1 emitted from a first laser beam source 11 such 
as a laser diode and a laser beam of the wavelength .lambda..sub.2 emitted 
from a second laser beam source 12 such as a He-Ne laser beam source into 
an illumination light B. The illumination light B is incident to an 
acousto-optic modulator 14 (referred to as "AOM" hereinafter) driven at a 
frequency F.sub.1. Most of the AOMs are of a type (so-called Bragg type 
AOM) in which light beams are made incident at a Bragg angle in order to 
enhance intensity of a (+) primary diffracted light beam. In this 
embodiment, the AOM 14 is of a Raman-Nath type in which light beams are 
made incident vertically so that (.+-.) primary diffracted light beams are 
uniformly provided by Raman-Nath diffraction. Since the wavelengths of the 
two laser beams of the illumination light B are somewhat different, 
diffracted light beams derived from the two laser beams of the 
illumination light B are emitted at somewhat different angles by the AOM 
14. FIG. 7 shows diffracted light beams moving in different directions 
depending on a wavelength by using a solid line and a broken line. The 
frequency difference between the illumination light B and the (+) primary 
diffracted light beams is +F.sub.1. The frequency difference between the 
illumination light B and the (-) primary diffracted light beams is 
-F.sub.1. 
In FIG. 7, the laser beams emitted from the AOM 14 pass through a lens 15, 
and are incident to a spatial filter 16. Only the (.+-.) primary 
diffracted light beams are selected from the laser beams by the spatial 
filter 16. The selected diffracted light beams pass through a lens 17, and 
are incident to a AOM 18 to cross with respect to each other. The AOM 18 
is located to be turned by 45 degrees relative to the AOM 14 and is driven 
with a drive signal of a frequency F.sub.2. The AOM 18 is that of a Bragg 
type. An incident angle is set to be 1/sin 45.degree. of Bragg angle. 
Thus, as viewed from a plane turned by 45 degrees about an optical axis, 
the incident angle of the diffracted light beams is Bragg angle. In this 
case, the laser beams are subjected to frequency modulation of +F.sub.1 in 
the AOM 14, and pass through the AOM 18. In the AOM 18, (-) primary 
diffracted light beams strong in intensity are generated, and the (-) 
primary diffracted light beams are subjected to frequency modulation of 
-F.sub.2. Thus, the laser beams emitted from the laser beam sources 11 and 
12 are subjected to frequency modulation of +(F.sub.1 -F.sub.2) before 
being emitted as alignment beams B.sub.1 and B.sub.3. Similarly, the laser 
beams are subjected to frequency modulation of -F.sub.1 in the AOM 14, and 
pass through the AOM 18. In the AOM 18, (+) primary diffracted light beams 
strong in intensity are generated, and the (+) primary diffracted light 
beams are subjected to frequency modulation of +F.sub.2. The laser beams 
emitted from the laser beam sources 11 and 12 are subjected to frequency 
modulation of -(F.sub.1 -F.sub.2) before being emitted as alignment beams 
B.sub.2 and B.sub.4. As a result, the frequency difference .DELTA.f 
between the alignment beam B.sub.1 and the alignment beam B.sub.2 is shown 
as 2(F.sub.1 -F.sub.2), and the frequency difference .DELTA.f between the 
alignment beam B.sub.3 and the alignment beams B.sub.4 is also shown as 
2(F.sub.1 -F.sub.2). In this embodiment, the frequency difference .DELTA.f 
is set to be 50 kHz. The laser beams passing through the AOM 18 are 
incident to a spatial filter 20. The alignment beams B.sub.1, B.sub.2, 
B.sub.3, and B.sub.4 are selected from the laser beams by the spatial 
filter 20 to be directed to systems arranged next thereto. 
As shown in FIG. 6, the alignment beams B.sub.1, B.sub.2, B.sub.3, and 
B.sub.4 are condensed on a filed stop 22 by a lens 21. Configuration of 
the alignment beams on the reticle or the wafer is determined by the filed 
stop 22 before the alignment beams B.sub.1, B.sub.2, B.sub.3, and B.sub.4 
are separated into the reticle alignment illumination light beams 
RB.sub.1, RB.sub.2, RB.sub.3, and RB.sub.4 and the wafer alignment 
illumination beams WB.sub.1, WB.sub.2, WB.sub.3, and WB.sub.4 by a reticle 
and wafer beams separating prism 23. Thereafter, the alignment 
illumination light beams pass through a lens 24, and reach a direct-vision 
prism 25. The direct-vision prism 25 is rotatable about the optical axis, 
and is driven by a motor 80 in line with instructions from the central 
control system 64 shown in FIG. 1. The rotation of the direct-vision prism 
25 allows the relative angle between the two-colored illumination light 
beams having the wavelengths .lambda..sub.1 and .lambda..sub.2 to be 
changed, thereby separating the alignment illumination light beams 
RB.sub.1, RB.sub.2, RB.sub.3, RB.sub.4, WB.sub.1, WB.sub.2, WB.sub.3, and 
WB.sub.4 into the illumination light beams RB.sub.1, WB.sub.1, RB.sub.2, 
and WB.sub.2 having the wavelength .lambda..sub.1 and the illumination 
light beams RB.sub.3, WB.sub.3, RB.sub.4, and WB.sub.4 having the 
wavelength .lambda..sub.2. The illumination light beams having the 
wavelengths .lambda..sub.1 and .lambda..sub.2 the relative angle of which 
is changed thus pass through the beam splitter 26, and are directed to the 
objective lens 2 shown in FIG. 2. Changing of the relative angle between 
the illumination light beams having the wavelengths .lambda..sub.1 and 
.lambda..sub.2 allows a relative relation between a position on the 
reticle which the one-colored illumination light beams having the 
wavelength .lambda..sub.1 illuminate and a position on the reticle which 
the other colored illumination light beams having the wavelength 
.lambda..sub.2 illuminate to be changed. Similarly, changing thereof 
allows a position on the wafer which the one-colored beams illuminate and 
a position on the reticle which the other colored beams illuminate to be 
changed. 
On the other hand, when the alignment detection light beams from the 
reticle mark 37A and the wafer mark 48A return to the alignment optical 
system 1 (see FIG. 6), the detection light beams are reflected by the beam 
splitter 26. The reflected light beams pass through a lens 27, and are 
separated into reticle detection light beams and wafer detection light 
beams by a detection light separating prism 28 which is disposed in 
conjugate relation to both the reticle and the wafer. The reticle 
detection light beams RB.sub.1.sup.+1, RB.sub.2.sup.-1, RB.sub.3.sup.+1, 
and RB.sub.4.sup.-1 pass through the detection light separating prism 28, 
and enter a color filter 29 for allowing light beams having only the 
wavelength .lambda..sub.1 to pass therethrough. The color filter 29 
selects only the reticle detection light beams RB.sub.1.sup.+1 and 
RB.sub.2.sup.-1 therefrom, and a photoelectric detection element 30 
receives the selected beams RB.sub.1.sup.+1 and RB.sub.2.sup.-1. The wafer 
detection light beams WB.sub.1.sup.+1, WB.sub.2.sup.-1, WB.sub.3.sup.+1, 
and WB.sub.4.sup.-1 are reflected by the detection light separating prism 
28, and are received by a photoelectric detection element 31. The 
photoelectric detection element 30 generates a reticle beat signal S.sub.R 
corresponding to a position of the reticle mark. The photoelectric 
detection element 31 generates a wafer beat signal S.sub.W corresponding 
to a position of the wafer mark. 
The reticle beat signal S.sub.R is a sinusoidal beat signal of frequency 
.DELTA.f formed from the detection light beams RB.sub.1.sup.+1 and 
RB.sub.2.sup.-1. The wafer beat signal S.sub.W is a sinusoidal beat signal 
of frequency .DELTA.f formed from the detection light beams 
WB.sub.1.sup.+1, WB.sub.2.sup.-1, WB.sub.3.sup.+1, and WB.sub.4.sup.-1. 
The phase difference .DELTA..phi. [rad] between the two beat signals is 
changed by the movement of the reticle 4 relative to the wafer 6 in the 
X-direction. Similarly, the phase difference .DELTA..phi. is changed by 
the movement of the wafer 6 relative to the reticle 4 in the X-direction. 
The amount of relative movement .DELTA.x thereof is indicated by the 
following expressions: 
EQU .DELTA.X (on the reticle)=P.sub.R .multidot..DELTA..phi./(4.pi.)(3) 
EQU .DELTA.X (on the wafer)=P.sub.W .multidot..DELTA..phi./(4.pi.)(4) 
It should be noted that by the alignment system for the Y-direction, beat 
signals corresponding to a reticle mark and a wafer mark for the Y-axis 
are obtained, and a phase difference between the beat signals is 
determined. 
It should be noted that although in FIGS. 2(a) and 2(b) the wafer mark is 
positioned under the projection optical system 5, when the alignment 
optical system 1 is to be adjusted (or calibrated), the reference mark 
member 41 is moved to a position under the projection optical system 5, 
and a grating mark on the reference mark member 41 is illuminated with the 
wafer alignment illumination light beams. 
FIG. 5 is a plan view showing the arrangement on the reference mark member 
41. In FIG. 5, the reference mark member 41 is formed from a transmissive 
glass substrate or the like. The reference mark member 41 includes on the 
upper surface thereof frame-shaped reference marks 44A and 44B which are 
spaced from each other in the X-direction. In the space between the 
reference marks 44A and 44B, diffraction grating-shaped reference 
diffraction grating marks 46A, 46B, 45A, and 45B are formed. Each of the 
reference diffraction grating marks 46A and 46B comprises a plurality of 
grating elements which are spaced by a pre-determined pitch in the 
Y-direction. Each of the reference diffraction grating marks 45A and 45B 
comprises a plurality of grating elements which are spaced by a 
predetermined pitch in the X-direction. The pitch of the reference 
diffraction grating marks 45A to 46B is equal to the pitch of the wafer 
marks 48A to 49B shown in FIG. 4. 
As described above, when reticle alignment is to be carried out, the 
reference mark member 41 is moved into the exposure field of the 
projection optical system 5, and the reference marks 44A and 44B are 
illuminated with illumination light from the bottom side thereof. The 
illumination light from the bottom side of the reference marks 44A and 44B 
has the wavelength .lambda..sub.0 which is the same as that of the 
illumination light for exposure. When the calibration for alignment 
optical system 1 is to be performed, the reference mark member 41 is also 
moved into the exposure field of the projection optical system 5, and the 
grating mark 45A on the reference mark member 41 is illuminated with the 
wafer alignment illumination light in the same way as the wafer mark. 
Thus, in the alignment optical system 1 employed in the projection exposure 
apparatus according to this embodiment, two-colored illumination light 
beams having the wavelengths .lambda..sub.1 and .lambda..sub.2 are used 
for the wafer marks in order to decrease the influence of the thin film 
interference, etc. In addition, the color filter 29 for allowing only 
light beams of the wavelength .lambda..sub.1 to pass therethrough is 
provided at a position upstream of the photoelectric detection element 30 
for the reticle mark to block the light beams of the wavelength 
.lambda..sub.2 so that the photoelectric detection element 30 generates 
the reticle beat signal S.sub.R formed from only the light beams of the 
wavelength .lambda..sub.1, thereby eliminating the influence of deviation 
between the two-colored interference fringes on the reticle marks. The 
additional provision of the rotatable direct-vision prism 25 in the 
alignment optical system 1 allows the relative position between portions 
of the wafer which are illuminated with the two-colored illumination light 
beams having wavelengths .lambda..sub.1 and .lambda..sub.2 to be changed. 
Next, the operation of alignment according to the embodiment will be 
specifically explained mainly referring to FIG. 1. The operation of 
alignment comprises a first step for calibration of the alignment optical 
system 1 and a second step for exposure of the wafer 6. It should be noted 
that though the following is explanation for positioning of the reticle 
and the wafer in the X-direction, positioning thereof in the Y-direction 
is performed in the same way as in the X-direction. 
First, the first step for calibration of the alignment optical system 1 
will be explained. The reticle 4 is transferred on the reticle stage 9 by 
a reticle auto-loader (not shown). Reticle alignment for the reticle 4 is 
then carried out. By using the exposure light of the wavelength 
.lambda..sub.0 as illumination light, the reticle marks 34A and 34B and 
the reference marks 44A and 44B are observed with the reticle alignment 
microscopes 39 and 40 to perform the reticle alignment. As shown in FIG. 
3, the reticle marks 34A and 34B are formed on the reticle 4. As shown in 
FIG. 5, the reference marks 44A and 44B are formed on the reference mark 
member 41 mounted on the wafer stage 61. When the reticle alignment is 
finished, the reference diffraction grating mark 45A (see FIG. 5) on the 
reference mark member 41 is illuminated with the wafer alignment 
illumination light beams (or the laser beams) emitted from the alignment 
light sending system 1a. 
The wafer alignment illumination light beams applied to the reference 
diffraction grating mark 45A are diffracted by the grating mark 45A. The 
diffracted light beams from the reference diffraction grating mark 45A are 
received by the alignment light receiving system 1b. A beat signal (which 
is also referred to as "a wafer beat signal S.sub.W ") is generated from 
the photoelectric detection element 31. The beat signal is supplied to the 
central control system 64 through the alignment signal processing system 
70. The central control system 64 adjusts the rotation angle of the 
direct-vision prism 25 provided within the alignment light sending system 
1a through the motor 80 shown in FIG. 6(a) so that the beat signal S.sub.W 
is maximized in its amplitude. By the adjustment thereof, pre-alignment 
(or coarse adjustment) is finished. 
After the coarse adjustment is finished, fine adjustment process is carried 
out. The fine adjustment process comprises a first fine adjustment step 
and a second fine adjustment step. 
In the first fine adjustment step, in the line with the instructions from 
the central control system 64, the laser control system 69 shuts off the 
second laser beam source 12 emitting the laser beam of the wavelength 
.lambda..sub.2 shown in FIGS. 6(a) and 6(b). In this case, the relative 
position between the reticle 4 and the reference mark member 41 is 
detected by only the laser beam of the wavelength .lambda..sub.1 from the 
first laser beam source 11. 
FIG. 11 shows the reticle beat signal S.sub.R corresponding to the reticle 
mark 35A and the wafer beat signal S.sub.W corresponding to the reference 
diffraction grating mark 45A. The reticle and wafer beat signals S.sub.R 
and S.sub.W are supplied to the alignment signal processing system 70 as 
alignment signals in the first fine adjustment step. In FIG. 11, a 
horizontal axis shows time (t), and a vertical axis shows amplitude of the 
signals S.sub.R and S.sub.W. In this case, the alignment signal processing 
system 70 determines a phase difference .DELTA..phi..sub.1 between the 
reticle and wafer beat signals S.sub.R and S.sub.W. It is preferable that 
the phase difference .DELTA..phi..sub.1 is close to zero (0). However, it 
is not always necessary for the phase difference .DELTA..phi..sub.1 to be 
zero. 
Next, in the second fine adjustment step, in line with the instructions 
from the central control system 64, the laser control system 69 turns on 
the second laser beam source 12 shown in FIG. 6(a), and decreases the 
power of the first laser beam source 11 emitting the laser beam of the 
wavelength .lambda..sub.1 up to one-tenth of the power thereof in the 
first fine adjustment step. Since the color filter 29 blocks the light 
beams of the wavelength .lambda..sub.2, the photoelectric detection 
element 30 for the reticle mark 35A receives only the diffracted light 
beams of the wavelength .lambda..sub.1 to generate the reticle beat signal 
S.sub.R. The photoelectric detection element 31 for the reference 
diffraction grating mark 45A receives the two-colored diffracted light 
beams having the wavelengths .lambda..sub.1 and .lambda..sub.2 to generate 
the wafer beat signal S.sub.W. It should be noted that though according to 
the embodiment the power of the first laser beam source 11 emitting the 
laser beam of the wavelength .lambda..sub.1 in the second fine adjustment 
step is set to be one-tenth of the power thereof in the first fine 
adjustment step, it is not always necessary to do so. It is necessary to 
decrease the power of the first laser beam source 11 to the extent that 
the light beams of the wavelength .lambda..sub.1 do not influence the 
level of the wafer beat signal S.sub.W obtained from the light beams of 
the wavelength .lambda..sub.2. In other words, it is necessary to decrease 
the power of the first laser beam source 11 to the extent that the 
detection light beams WB.sub.3.sup.+1 and WB.sub.4.sup.-1 (shown in FIG. 
6) can be received by photoelectric detection element 31, which detection 
light beams are generated by diffraction of the light beams of the 
wavelength .lambda..sub.2 by the reference diffraction grating mark 45A. 
FIG. 12 shows in the same way as FIG. 11 the reticle beat signal S.sub.R 
and the wafer beat signal S.sub.W obtained through the second fine 
adjustment step. In a case where a ration of the first laser beam source 
11 to the second laser beam source 12 is set to be 1:1 in light intensity, 
in the second fine adjustment step relative to the first fine adjustment 
step, the reticle beat signal S.sub.R is decreased in its intensity to 
one-tenth and the wafer beat signal S.sub.W is increased in its intensity 
up to eleven-tenths. In FIG. 12, however, in order to clarify the relation 
between the reticle and wafer beat signals S.sub.R and S.sub.W, the two 
signal are enlarged in the vertical direction of FIG. 12 with holding of 
the relative ration between the two signals. From a result indicated by 
FIG. 12, the alignment signal processing system 70 determines the phase 
difference .DELTA..phi..sub.2 between the reticle beat signal S.sub.R and 
the wafer beat signal S.sub.W. 
The central control system 64 receives the phase differences 
.DELTA..phi..sub.1 and .DELTA..phi..sub.2 computed by the alignment signal 
processing system 70, and computes the relative difference .DELTA..phi. 
(=.DELTA..phi..sub.1 -.DELTA..phi..sub.2). Making the relative difference 
.DELTA..phi. zero is advantageous. However, it is permissible if the 
relative difference .DELTA..phi. is below a predetermined value. If the 
relative difference .DELTA..phi. is over the permissible level, in line 
with the instructions of the central control system 64, the rotation angle 
of the direct-vision prism 25 provided in the alignment light sending 
system 1a is adjusted according to the relative difference .DELTA..phi.. 
Thereafter, the phase differences .DELTA..phi..sub.1 and 
.DELTA..phi..sub.2 are measured again in the same way, and the same 
operation is repeated until the relative difference .DELTA..phi. is below 
the permissible level. The permissible level of the relative difference 
.DELTA..phi. depends on alignment accuracy to be required. As LSIs are 
highly integrated, high accuracy of a relative difference .DELTA..phi. 
less than or equal to 10 nm will be required as measured by converting the 
relative difference .DELTA..phi. to a length on the wafer stage 61. 
By the above process, the two-colored interference fringes provided on the 
reference mark member 41 by the two-colored illumination light beams 
having the different wavelengths .lambda..sub.1 and .lambda..sub.2 are 
aligned with each other with accuracy. That is, by the above calibration, 
the phase difference between the two-colored illumination light beams 
having the wavelengths .lambda..sub.1 and .lambda..sub.2 falls within a 
permissible range, and thereby enabling signals the contrast of which is 
good to be obtained when performing the final alignment. Next, the 
two-colored illumination light beams are emitted gain, a phase difference 
.DELTA..phi..sub.0 between the two beat signals obtained from the reticle 
4 and the reference mark member 41 is measured. 
Next, the second step for exposure of the wafer 6 will be explained. The 
wafer 6 is transferred onto the wafer stage 61 by the wafer auto-loader 
(not shown). The shot area 47 (see FIG. 4) of wafer 6 is positioned at an 
exposure position under the exposure optical system 5 with accuracy of 
better than .+-.1/4 of the pitch P.sub.W of the wafer marks 48A and 49A on 
the basis of a measured result by a wafer pre-alignment system (not 
shown). After positioning thereof, the reticle mark 35A and the wafer mark 
48A is illuminated by the alignment illumination light emitted from the 
alignment light sending system la, thereby performing final alignment (or 
fine alignment) between the reticle 4 and the shot are 47 of the wafer 6. 
In this fine alignment, the reticle stage 9 or the wafer stage 61 is 
controlled, so that a phase difference between the reticle beat signal 
S.sub.R and the wafer beat signal S.sub.w coincides with the phase 
difference .DELTA..phi..sub.0 measured in final portion of the first step. 
When the positions of the reticle 4 and the wafer 6 relative to each other 
are aligned and the accomplishment of the alignment therebetween is 
transferred to the central control system 64 through the alignment signal 
processing system, exposure light is emitted to thereby transfer the 
pattern image on the reticle 4 to the shot area 47. Thus, each of the shot 
areas on the wafer 6 is sequentially exposed. When exposure of all shot 
areas are finished, the exposed wafer 6 is replaced with a new wafer, and 
alignment and exposure for the new wafer is carried out in the same way as 
for the wafer 6. 
As mentioned above, in the embodiment, when performing alignment, the 
reticle marks on the reticle 4 are illuminated with only the illumination 
light of the wavelength .lambda..sub.1. Generally, silica glass is coated 
with chrome film and the reticle marks are formed on the chrome film by 
patterning. Thus, it appears that the reticle marks are ideally made in 
comparison with the wafer marks. Therefore it is not necessary to use 
two-colored illumination light beams in order to perform alignment. The 
embodiment is that uses the above idea very well. Since the reticle beat 
signal S.sub.R is formed from only the illumination light of the 
wavelength .lambda..sub.1, even if the direct-vision prism 25 (see FIG. 6) 
is rotated, the reticle beat signal S.sub.R in stable condition can be 
obtained constantly. 
In order to decrease influence of the thin film interference and the like, 
the two-colored illumination light beams having the wavelengths 
.lambda..sub.1 and .lambda..sub.2 are used for the wafer mark 48A. In this 
case, in order to align the two-colored interference fringes of the 
wavelengths .lambda..sub.1 and .lambda..sub.2 with each other, which are 
formed by the two-colored illumination light beams, with reference to the 
reticle beat signal S.sub.R, the laser beam source 11 emits the laser beam 
of the wavelength .lambda..sub.1 with low power, thereby enabling 
deviation between the two-colored interference fringes of the two 
wavelengths to be eliminated. 
In the above embodiment, a laser diode as the first laser beam source 11 
and a He-Ne laser beam source 12 as the second laser beam source are 
employed. Thus, a shutter for blocking a laser beam may be employed to 
shut off the second laser beam source. In a case where two laser diodes 
are used as the first and second laser beam sources, all that is needed to 
shut off the second laser beam source is to turn off the power therefor. 
In order to decrease the output of the laser diode to for example 
one-tenth of the original output thereof, it is necessary only to control 
the electric power thereof. In order to decrease the output of a gas laser 
such as the He-Ne laser to for example one-tenth of the original output 
thereof, it is desirable to use a filter having transmittance of 
one-tenth. 
In this embodiment, though the two-colored illumination light of the 
wavelengths .lambda..sub.1 and .lambda..sub.2 is used as the illumination 
light for alignment, illumination light having colors more than or equal 
to three colors may be used. In this embodiment, though the direct-vision 
prism 25 for adjusting interference fringes is used, a diffraction grating 
can be employed to bring about the same function as the direct-vision 
prism. 
A second embodiment according to the present invention will be explained 
with reference to FIGS. 13 to 16. In the second embodiment, elements which 
are identical to those shown in the first embodiment have been given the 
same reference numerals, and explanation of the same elements is omitted. 
The following is explanation of points of the second embodiment generally 
different from the first embodiment. 
As shown in FIG. 13, an alignment apparatus according to the second 
embodiment further includes a phase comparing circuit 71, an AOM driver 
72, an AOM driver 73, a mixing circuit 74, a BPF (band-pass filter) 75, a 
frequency multiplication circuit 76, a phase comparing circuit 77, and a 
selecting portion 78. 
The reticle beat signal S.sub.R and the wafer beat signal S.sub.W are sent 
to the phase comparing circuit 71 in which a phase difference between both 
the signals is detected. 
The AOM 14 is driven by a drive signal of frequency F.sub.1 which is 
supplied from the AOM driver 72. The AOM 18 is driven by a drive signal of 
frequency F.sub.2 which is supplied from the AOM driver 73. The drive 
signals respectively branch off in the AOM drivers 72 and 73 to be sent to 
the mixing circuit 74. The mixing circuit 74 mixes the drive signals of 
the frequencies F.sub.1 and F.sub.2 to produce electric signals having 
various frequencies such as a frequency of the difference between the two 
frequencies (F.sub.1 -F.sub.2), a frequency of the sum of the two 
frequencies (F.sub.1 +F.sub.2), and so on. The electric signals are 
supplied to the BPF 75 in which only an electric signal having the 
frequency of the difference between the two frequencies (F.sub.1 -F.sub.2) 
is obtained. The electric signal of the frequency (F.sub.1 -F.sub.2) is 
supplied to the frequency multiplication circuit 76 in which it is 
converted into an electric reference signal S.sub.E having a frequency of 
2 (F.sub.1 -F.sub.2) obtained by doubling the frequency of the input 
signal. 
When adjusting interference fringes formed on the reference diffraction 
grating mark 45A (see FIG. 5) by the two-colored illumination light having 
difference wavelengths, the electric reference signal S.sub.E having the 
frequency of 2 (F.sub.1 -F.sub.2) is sent to the phase comparing circuit 
77 in which the electric reference signal S.sub.E and the wafer beat 
signal S.sub.W are compared in phase. The compared result is supplied to 
the selection portion 78. The phase difference between the reticle beat 
signal S.sub.R and the wafer beat signal S.sub.W is also supplied from the 
phase comparing circuit 71 to the selection portion 78. The selection 
portion 78 supplies either of the two phase differences to the central 
control system 64. On the basis of the detected phase difference, the 
central control system 64 determines a target phase difference or carries 
out the final alignment. In this second embodiment, the phase comparing 
circuit 71, the mixing circuit 74, the BPF 75, the frequency 
multiplication circuit 76, the phase comparing circuit 77, and a selecting 
portion 78 are provided in the alignment signal processing system 70 shown 
in FIG. 1. The AOM drivers 72 and 73 are provided in the laser control 
system 69 shown in FIG. 1. 
As stated above, in the second embodiment, a reference signal generation 
system is provided which comprises the AOM drivers 72 and 73 for 
electrically driving the AOM 14 and the AOM 18, the mixing circuit 74, the 
BPF 75, and the frequency multiplication circuit 76. The reference signal 
generation system produces the electrical reference signal S.sub.E of a 
frequency which is identical to each of a frequency difference between a 
pair of the illumination light beams for illuminating the reticle marks 
and a frequency difference between a pair of the illumination light beams 
for illuminating the wafer marks. By detecting a phase difference between 
the electrical reference signal S.sub.E and the wafer beat signal S.sub.W 
when performing the calibration process followed by the exposure process, 
adjustment of deviation between the two-colored interference fringes 
produced on the reference diffraction grating mark 45A of the wafer 6 by 
the two-colored illumination light having the difference wavelengths 
.lambda..sub.1 and .lambda..sub.2 can be carried out without using the 
reticle beat signal S.sub.R. 
Next, the operation of the alignment according to the second embodiment 
will be explained. 
First, calibration of the alignment optical system 1 as the first step is 
carried out. 
In the calibration process, pre-alignment (or coarse adjustment) is 
performed in the same way as in the first embodiment, before carrying out 
a first fine alignment step and a second fine alignment step. 
In the first fine alignment step, in line with instructions from the 
central control system 64, the laser control system 69 shuts off (or stops 
the operation of) the second laser beam source 12 emitting the laser beam 
of the wavelength .lambda..sub.2 shown in FIGS. 13(a) and 13(b). The phase 
comparing circuit 77 shown in FIG. 13(a) then detects a phase difference 
between the wafer beat signal S.sub.W and the electric reference signal 
S.sub.E. 
FIG. 14 shows the electric reference signal S.sub.E and the wafer beat 
signal S.sub.W corresponding to the reference diffraction grating mark 
45A. The electric reference signal S.sub.E and the wafer beat signal 
S.sub.W are supplied to the phase comparing circuit 77 as alignment 
signals in the first fine adjustment step. In FIG. 14, a horizontal axis 
shows time (t), and a vertical axis shows signals S.sub.E and S.sub.W. In 
this case, the phase comparing circuit 77 determines a phase difference 
.DELTA..phi..sub.1 between the electrical reference signal S.sub.E and the 
wafer beat signals S.sub.W. The determined phase difference value is 
supplied to the central control system 64 through the selecting portion 
78. It is preferable that the phase difference .DELTA..phi..sub.1 is close 
to zero (0). However, it is not always necessary for the phase difference 
.DELTA..phi..sub.1 to be zero. 
Next, in the second fine adjustment step, in line with the instructions 
from the central control system 64, the laser control system 69 turns on 
(or works) the second laser beam source 12 shown in FIG. 13(a), and turns 
off (or stops the operation of) the first laser beam source 11 emitting 
the laser beam of the wavelength .lambda..sub.1. 
FIG. 15 shows the electrical reference signal S.sub.E and the wafer beat 
signal S.sub.W obtained through the second fine adjustment step in the 
same way as FIG. 14. From the result indicated by FIG. 15, the phase 
comparing circuit 77 determines the phase difference .DELTA..phi..sub.2 
between the electrical reference signal S.sub.E and the wafer beat signal 
S.sub.W. The result determined therein is supplied to the central control 
system 64 through the selecting portion 78. The central control system 64 
computes the relative difference .DELTA..phi. (=.DELTA..phi..sub.1 
-.DELTA..phi..sub.2). Making the relative difference .DELTA..phi. zero is 
advantageous. However, it is permissible if the relative difference 
.DELTA..phi. is within a predetermined value. If the relative difference 
.DELTA..phi. is over the permissible level, in line with the instructions 
of the central control system 64, the rotation angle of the direct-vision 
prism 25 provided in the alignment light sending system 1a is adjusted 
according to the relative difference .DELTA..phi. through the motor 80 
shown in FIG. 13(a). After that, the phase differences .lambda..sub.1 and 
.lambda..sub.2 are measured again by the same way, and the same operation 
is repeated until the relative difference .DELTA..phi. is within the 
permissible level. The permissible level of the relative difference 
.DELTA..phi. depends on alignment accuracy to be required. 
By the above process, the two-colored interference fringes provided on the 
reference mark member 41 by the two-colored illumination light beams 
having the different wavelengths .lambda..sub.1 and .lambda..sub.2 are 
aligned with each other with accuracy. That is, the phase difference 
between the two-colored illumination light beams having the wavelengths 
.lambda..sub.1 and .lambda..sub.2 falls within permissible range by the 
calibration, and thereby signal the contrast of which is good can be 
obtained when performing the final alignment. Next, the two-colored 
illumination light beams are emitted gain, and the reticle beat signal 
S.sub.R obtained from the reticle 4 and the wafer beat signal S.sub.W 
obtained from the reference mark member 41 are detected. One example each 
of the two signals is shown in FIG. 16. The reticle beat signal S.sub.R 
and the wafer beat signal S.sub.W are supplied to the phase comparing 
circuit 71. The phase comparing circuit 71 detects a phase difference 
.DELTA..phi..sub.0 between the two beat signals S.sub.R and S.sub.W, and 
supplies the detected result to the central control system 64 through the 
selecting portion 78. 
Next, the second step for exposure of the wafer 6 is carried out. In FIG. 
1, the wafer 6 is transferred onto the wafer stage 61 by the wafer 
auto-loader (not shown). The shot area 47 (see FIG. 4) of wafer 6 is 
positioned at an exposure position under the exposure optical system 5 
with accuracy of within .+-.1/4 of the pitch P.sub.W of the wafer marks 
48A and 49A on the basis of a value measured by a wafer pre-alignment 
system (not shown). After positioning of the shot area 47 of wafer 6 at 
the exposure position, the reticle mark 35A and the wafer mark 48A is 
illuminated with the alignment illumination light emitted from the 
alignment light sending system 1a, thereby performing final alignment (or 
fine alignment) between the reticle 4 and the shot are 47 of the wafer 6. 
In this fine alignment, the central control system 64 controls the reticle 
stage 9 or the wafer stage 61, so that a phase difference between the 
reticle beat signal S.sub.R and the wafer beat signal S.sub.W coincides 
with the phase difference .DELTA..phi..sub.0 measured in final portion of 
the second fine adjustment step. When the positions of the reticle 4 and 
the wafer 6 relative to each other is aligned and the accomplishment of 
the alignment therebetween is determined by the central control system 64, 
exposure light is emitted to thereby transfer the pattern image on the 
reticle 4 to the shot area 47. Thus, each of the shot areas on the wafer 6 
is sequentially exposed. When exposure of all shot areas on the wafer 6 
are finished, the exposed wafer 6 is replaced with a new wafer, and 
alignment and exposure for the new wafer is carried out in the same way as 
for the wafer 6. 
In the first and second fine adjustment steps, the laser beam source 11 of 
the wavelength .lambda..sub.1 and the laser beam source 12 of the 
wavelength .lambda..sub.2 are alternately shut off. The output or power of 
the laser beam sources 11 and 12 may be alternately decreased within a 
predetermined value. 
Thus, in the second embodiment, the photoelectric detection element 30 
receives the heterodyne beams having only the wavelength .lambda..sub.1 
from the reticle mark 35A of the reticle 4. That is, it results in the 
reticle mark being illuminated with only the illumination light of the 
wavelength .lambda..sub.1. If the reticle mark 35A is observed with 
two-colored illumination light having the different wavelengths as the 
reference diffraction grating mark 45A of the wafer 6 is done, chromatic 
aberration of the projection optical system 5 causes deviation between 
two-colored interference fringes formed on the reticle mark 35A by the 
two-colored illumination light when deviation between two-colored 
interference fringes on the wafer 6 is adjusted. For example, in a case 
where the two-colored illumination light beams are identical in intensity 
thereof and one of the two-colored interference fringes is deviated in the 
phase thereof by a half pitch from the other colored interference fringe, 
the reticle beat signal disappears. In order to avoid such a happening, 
the reticle mark is observed only with the single-colored light beam. 
When adjusting the deviation between the two-colored interference fringes 
having the different wavelengths, employment of the following processing 
causes disappearance of the reticle beat signal. 
First, the one-colored illumination light and the other colored 
illumination light alternately illuminate the wafer 6. Then, the 
photoelectric detection element 31 receives the heterodyne beams 
comprising, for example, (.+-.) primary diffracted light beams from the 
reference diffraction grating mark 45A to generate the wafer beat signal 
S.sub.w. On the other hand, the photoelectric detection element 30 
receives the heterodyne beams comprising, for example, (.+-.) primary 
diffraction light beams from the reticle mark 35A to generate the reticle 
beat signal S.sub.R. Next, comparison of phases of the two beat signals is 
performed. In such a processing, since the reticle mark 35A is not 
observed with the illumination light of the wavelength .lambda..sub.2, 
adjustment of deviation between the two-colored interference fringes 
causes disappearance of the reticle beat signal S.sub.R not to compare a 
phase difference between the two beat signals S.sub.R and S.sub.W. 
As opposed to that, according to the second embodiment, the drive signals 
from the AOM drivers 72 and 73 for electrically driving the AOM 14 and the 
AOM 18 are processed to generates the electrical reference signal S.sub.E 
having the frequency which is identical to each of the frequency 
difference between a pair of the illumination light beams for illuminating 
the reticle 4 and the frequency difference between a pair of the 
illumination light beams for illuminating the wafer 6. A phase difference 
between the electrical reference signal S.sub.E and the wafer beat signal 
S.sub.W is detected. On the basis of the detected phase difference, the 
direct-vision prism 25 adjusts deviation between a position on the wafer 6 
which the one-colored illumination light illuminates and a position on the 
wafer 6 which the other colored illumination light illuminates. That is, 
the electrical reference signal S.sub.E serves as a reference signal for 
adjusting the two-colored interference fringes having the wavelengths 
.lambda..sub.1 and .lambda..sub.2 produced on the reference diffraction 
grating mark 45A, thereby enabling the adjustment of deviation between the 
two-colored interference fringes without producing the reticle beat signal 
S.sub.R. Thus, when performing alignment, the illumination light of the 
wavelength .lambda..sub.1 can be made zero or lowered in its intensity. 
In the second embodiment, a frequency of the electrical reference signal 
S.sub.E is identical to that of the wafer beat signal S.sub.W. However, 
for example, a frequency of the electrical reference signal S.sub.E may be 
1/N (N is an integral more than or equal to 2) of that of the wafer beat 
signal S.sub.W. In this case, a phase difference therebetween can be 
determined. Similarly, when a phase difference between the electrical 
reference signal S.sub.E and the wafer beat signal S.sub.W is within a 
predetermined range, a frequency of the electrical reference signal 
S.sub.E may be increased by N times of that of the wafer beat signal 
S.sub.W. 
In the second embodiment, though the two-colored illumination light of the 
wavelengths .lambda..sub.1 and .lambda..sub.2 is used as illumination 
light for alignment, illumination light having colors more than or equal 
to three may be used. Additionally, in the second embodiment, though the 
direct-vision prism 25 is used for adjusting the interference fringes, a 
diffraction grating may employed in place of the direct-vision prism 25. 
The employment of a diffraction grating can bring about the same technical 
advantage as that of the direct-vision prism 25. In the second embodiment, 
the present invention is applied to an alignment apparatus of a TTR type. 
In addition to that, the present invention can be applied to an alignment 
apparatus of a multicolored heterodyne interference method with a TTL 
(Through-The-Lens) type or an off-axis type. 
According to the alignment apparatus of the present invention, when 
detecting a position of a substrate through a projection optical system by 
using illumination light having two wavelengths, since positions to be 
illuminated with light beams having first and second wavelengths are 
adjusted on the basis of both a phase difference between two beat signals 
which are obtained by the light beams of the second wavelength being 
decreased in their intensity and a phase difference between two beat 
signals which are obtained by decreasing the intensity of the light beams 
of the first wavelength to the extent that heterodyne beams can be 
detected, an amount of deviation between an interference fringe formed on 
the mark on the substrate by the one-colored light beams and an 
interference fringe formed on the mark thereon by the other colored light 
beams can be decreased. This allows alignment to be carried out with 
accuracy, even if reflectance of the substrate can be greatly changed 
depending on wavelength. Employment of single-colored alignment light 
beams for a reference grating prevents contrast of the beat signal (or the 
reticle beat signal) with respect to the reference grating from being 
lowered and disappearing, when a position on the substrate which the 
one-colored alignment light beams illuminate is aligned with a position on 
the substrate which the other colored alignment light beams illuminate. 
In a case where the reference grating is formed on the mask and the 
reference grating is composed of a diffraction grating-shaped mark, it is 
not necessary to employ an additional element for providing the reference 
grating thereon, and an amount of positional deviation between the mask 
and the substrate can be directly detected. 
In a case where an illumination light controlling element for controlling 
chromatic aberration of the projection optical system relative to the 
light beams of the first wavelength so that the chromatic aberration 
thereof becomes a predetermined value is provided in an area on a Fourier 
transform plane relative to the mask in the projection optical system or a 
plane adjacent to the Fourier transform plane through which the light 
beams of the first wavelength pass, and where another illumination light 
controlling element for controlling chromatic aberration of the projection 
optical system relative to the light beams of the second wavelength so 
that the chromatic aberration thereof becomes a predetermined value is 
provided in an area on a Fourier transform plane relative to the mask in 
the projection optical system or a plane adjacent to the Fourier transform 
plane through which the light beams of the second wavelength pass, since 
the mask is easily conjugate with the substrate with respect to the 
alignment light, an optical system for the mask and the substrate can be 
shared, thereby enabling alignment to be performed with a TTR system. 
According to the alignment apparatus of the present invention, since a 
phase difference between a reference signal and a beat signal is detected 
and alignment is performed on the basis of the detected result, when 
detecting a position of a substrate by using illumination light having two 
wavelengths, an amount of deviation between two-colored interference 
fringes on diffraction grating-shaped mark produced by two-colored light 
beams can be adjusted with high accuracy. Thus, this allows alignment to 
be carried out with accuracy, even if reflectance of the substrate can be 
greatly changed depending on a wavelength. 
In the alignment apparatus of the present invention, in a case where a 
heterodyne beam generating system generates the first and second pairs of 
light beams by a acousto-optic modulator driven by a drive signal having a 
predetermined frequency difference, and where a reference signal 
generating device generates a reference signal (or an electrical reference 
signal) on the basis of the drive signal, a frequency of a beat signal (or 
a wafer beat signal) from a photoelectric detection device can be in 
integral multiple relation to that of the reference signal to easily and 
precisely detect a phase difference between the beat signal and the 
reference signal to align two-colored interference fringes with each other 
with high accuracy. 
In a case where the alignment apparatus of the present invention is 
provided on a projection exposure apparatus for transferring through a 
projection optical system a transferring pattern on a mask (or a reticle) 
onto a photosensitive substrate (or a wafer), where an illumination 
optical system illuminates a diffraction grating-shaped mark on the 
substrate with the first and second pairs of light beams through the 
projection optical system, and where the photoelectric detection device 
receives heterodyne beams generated by diffraction of the light beams by 
the diffraction grating-shaped mark through the projection optical system, 
a TTL type can be employed in order to align interference fringes on the 
substrate produced by the two-colored light beams with high accuracy. 
According to the alignment apparatus of the present invention, since a 
third pair of light beams having a first wavelength and a fourth pair of 
light beams having a second wavelength are employed, alignment between the 
mask and the substrate can be performed with high accuracy without 
receiving the influence of thin film interference and the like on the 
substrate. 
In addition, since deviation between an interference fringe on the 
substrate provided by a pair of light beams of the first wavelength and 
another interference fringe on the substrate provided by a pair of light 
beams of the second wavelength is adjusted by a reference signal having a 
frequency corresponding to each of a frequency difference 5 between the 
third light beams and a frequency difference between the fourth light 
beams, the adjustment of deviation between the two-colored interference 
fringes can be performed without receiving any optical influence. 
Additionally, in a case where a first illumination optical system 
illuminates the mask mark only with the first pair of light beams, it 
results in the mask mark being observed only with the single-colored light 
beam. This can prevent contrast of the beat signal (or the reticle beat 
signal) with respect to the mask mark from being lowered and disappearing, 
when positions on the substrate which the two-colored alignment light 
beams illuminate are appropriately aligned with each other. 
According to the alignment apparatus of the present invention, in a case 
where the heterodyne beam generating system generates the first and second 
pairs of light beams by the acousto-optic modulator driven by the drive 
signal having a predetermined frequency difference, and where the 
reference signal generating device generates the reference signal on the 
basis of the drive signal, a phase difference between the beat signal and 
the reference signal can be easily and precisely detected, thereby 
enabling the two-colored interference fringes to be aligned with each 
other with high accuracy. 
Thus, the present invention is not necessarily limited to the foregoing 
embodiments, various changes and modifications can be imparted thereto 
without departing from the gist of the present invention.