Alignment device and method with focus detection system

An alignment device which may include an objective optical system for detecting the light from a first mark formed, for alignment, in a first mark area on a substrate and the light from a second mark formed, for alignment, in a second mark area, a first detection optical system having a first detection area within the viewing field of the objective optical system and adapted to detect the light from the first mark through the objective optical system, a second detection optical system having a second detection area, different from the first detection area, within the viewing field of the objective optical system and adapted to detect the light from the second mark through the objective optical system, and a focus detection system for detecting the deviation of the first mark area with respect to the focal plane of the first detection optical system and the deviation of the second mark area with respect to the focal plane of the second detection optical system, by irradiating the first and second detection areas respective with light beams and receiving the reflected lights therefrom.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an alignment device and method adapted to 
be used in various manufacturing or inspecting apparatus such as an 
exposure apparatus for use in the manufacturing process of semiconductor 
devices or liquid crystal display devices, or a coordinate measuring 
apparatus, and more particularly such alignment device provided with a 
focusing mechanism. 
2. Related Background Art 
In the manufacturing process of the semiconductor devices and the liquid 
crystal display devices, there has been employed an alignment device for 
precisely positioning a wafer, on which the semiconductor devices are 
formed, or a glass plate on which the liquid crystal display devices are 
formed, in a desired position by detecting an alignment mark (wafer mark) 
provided on such wafer or glass plate. In an example of semiconductor 
device manufacture to be explained in the following, the wafer has to be 
subjected to 10 to 20 superposed exposures on an exposure apparatus such 
as an aligner or a stepper, and it has been required to improve the 
precision of such superposed exposures. The alignment device is composed 
of an alignment sensor for detecting the positions of the wafer marks and 
a control system for determining the target position of the wafer, based 
on thus detected positions. 
In such process, it is difficult to detect the positions of all the wafer 
marks with a single alignment sensor, because the surface coarseness 
varies depending on the exposures and the subsequent processes, and also 
because a step difference may exist between the wafer mark and the 
surrounding surface of the wafer, depending on the layer structure 
thereon. For this reason, following an alignment sensors are used, 
depending on the purpose: 
1) LSA (laser step alignment) sensor: an alignment sensor system for 
irradiating the wafer mark with laser beam and measuring the position of 
the wafer mark by the diffracted or scattered light; utilized widely in 
various wafer processes; 
2) FIA (field image alignment) sensor: a sensor for position measurement by 
processing the image of a wafer mark, formed by illumination with light of 
a wide wavelength band obtained for example from a halogen lamp; effective 
for the measurement of an asymmetrical mark on an aluminum layer or on the 
wafer surface; and 
3) LIA (laser interferometric alignment) sensor: an alignment sensor system 
for irradiating a wafer mark formed as a diffraction grating with laser 
beams of slightly different frequencies from two directions and detecting 
the position of the wafer mark from the phase of the interference light 
obtained by interference of two diffracted lights generated from said 
wafer mark; effective for a wafer mark of a small step difference or for a 
wafer with significant surface coarseness. 
These alignment sensors have been selectively utilized depending on the 
applications. 
The optical systems are generally provided with auto focusing mechanisms, 
and the alignment sensor is also provided with an auto focusing mechanism 
for maintaining the inspected surface within a predetermined range with 
respect to the alignment sensor (such function being also called 
focusing). Such auto focusing mechanism is composed of an auto focusing 
sensor for irradiating the object wafer mark with detecting light beam and 
detecting the axial (focus) position of the inspected surface from the 
reflected light, and a drive mechanism for setting said focus position at 
a predetermined (in-focus) position. However, even if alignment sensors of 
plural kinds are provided, there is provided only one auto focusing 
sensor, for irradiating the predetermined measuring point on the wafer 
with a light beam for focus position measurement, for common use by these 
alignment sensors. 
Because of the recent increase of the sensors of various kinds incorporated 
in the exposure apparatus, an efficient arrangement is desired for the 
accessory devices of the exposure apparatus. For this reason, there has 
been recently employed an alignment system which incorporates, as 
disclosed in the Japanese Patent Application Laid-Open No. 5-291109, 
plural different alignment sensors in a common objective lens. 
In such alignment system with plural alignment sensors utilizing a common 
objective lens, the range of height of the inspected surface (focusing 
range), in which the position detection can be achieved most precisely, 
differs slightly among the plural alignment sensors. Consequently, when 
the wafer surface is set at a height by irradiating a predetermined 
portion of the wafer with a focus position detecting light beam and 
utilizing the reflected light in the conventional manner, the optimum 
surface height for a certain alignment sensor may not be optimum for 
another alignment sensor, so that some alignment sensors may become 
incapable of exact position detection. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an alignment device and 
method capable of exact focusing for alignment sensors, to be used in 
optical detection of the position of an alignment mark on an object to be 
aligned. 
The above-mentioned object can be attained, according to the present 
invention, by an alignment device comprising: 
an objective optical system for detecting the light from a 1st alignment 
mark formed in a 1st mark area on a substrate and the light from a 2nd 
alignment mark formed in a 2nd mark area; 
a 1st detection optical system having a 1st detection area in the viewing 
field of said objective optical system and adapted to detect the light 
from said 1st mark through said objective optical system; 
a 2nd detection optical system having a 2nd detection area, different from 
said 1st detection area, in the viewing field of said objective optical 
system and adapted to detect the light from said 2nd mark through said 
objective optical system; and 
a focus detection system for irradiating said 1st and 2nd detection areas 
respectively with light beams and respectively receiving reflected lights, 
thereby detecting the deviation of said 1st mark area with respect to the 
focal plane of said 1st detection optical system and the deviation of said 
2nd mark area with respect to the focal plane of said 2nd detection 
optical system. 
The above-mentioned configuration allows to respectively detect the 
deviation of the 1st mark area with respect to the focal plane of the 1st 
detection optical system and the deviation of the 2nd mark area with 
respect to the focal plane of the 2nd detection optical system, since a 
focus detection system is provided for projecting focus detecting light 
beams respectively on the detection areas of the 1st and 2nd detection 
optical systems utilizing a common objective optical system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the following there will be explained, with reference to the attached 
drawings, an embodiment of the alignment device of the present invention. 
The present embodiment is applied to an alignment system adapted for use 
in a projection exposure apparatus and having two different alignment 
sensors which utilize in common an off-axis objective lens. 
FIG. 1 schematically shows the configuration of a projection exposure 
apparatus of the present embodiment, wherein a reticle R, to be subjected 
to exposure operation, is supported on a reticle holder 1 and the pattern 
of said reticle R is transferred, through a projection optical system PL, 
onto each of shot areas on a wafer W, by means of exposing light from an 
unrepresented illumination optical system. The Z-axis is defined parallel 
to the optical axis AX of the projection optical system PL, and X- and 
Y-axes are defined, with a plane perpendicular to the Z-axis, respectively 
in directions parallel and perpendicular to the plane of FIG. 1. The wafer 
W, coated with photoresist, is placed, by a wafer holder 2, on a Z-stage 
3, which is in turn placed on an XY-stage 4. The Z-stage 3 effects fine 
adjustment of the position of the wafer W in the Z-direction, and the 
XY-stage 4 effects positioning of the wafer W in the X- and Y-directions. 
At an end of the wafer holder 2 there is fixed an L-shaped movable mirror 
7, which, in cooperation with a laser interferometer 8, constantly 
measures the X- and Y-coordinates of the wafer W. The measured coordinates 
are supplied to a stage control unit 9, which controls the drive amount of 
the Z-stage 3 based on the focus position information from an alignment 
control unit 15 to be explained later, thereby focusing the wafer W to 
each of the alignment sensors. Also the stage control unit 9 controls the 
function of a drive system 10 for stepped drive of the XY-stage 4, based 
on the position information of wafer marks (alignment marks) detected by 
the alignment control unit 15, thereby aligning the center of each shot 
area, in which the pattern of the reticle R is to be transferred, of the 
wafer W to the optical axis AX of the projection optical system PL. 
On said wafer holder 2 there is provided a reference mark plate 6 bearing 
various alignment reference marks thereon. Above the reticle R there is 
provided a reticle alignment microscope 5. Determination of the center 
position of the projected image of the reticle R (so-called reticle 
alignment) is achieved by simultaneously observing the alignment mark on 
the reticle R and a predetermined reference mark on the reference mark 
plate 6 through said alignment microscope 5 and maintaining the positional 
deviations of said marks with a predetermined range through the positional 
adjustment of the reticle R. Subsequently there is measured the baseline 
amount, which is the amount of deviation between the center of detection 
of each alignment sensor and the center of the projected image of the 
reticle R (i.e. the optical axis AX of the projection optical system PL), 
by detecting the positions of various reference marks on the reference 
mark plate 6 with the corresponding alignment sensor. Each shot area can 
be exactly aligned to the exposure position by controlling the X- and 
Y-coordinates of the wafer W, based on a value obtained by adding said 
baseline amount to the wafer mark position measured by each alignment 
sensor. 
In the present embodiment, there is provided, at the side of the projection 
optical system PL, an off-axis alignment system 11 having two alignment 
sensors, i.e. an FIA (field image alignment) sensor and an LIA (laser 
interferometric alignment) sensor. The alignment system 11, receiving the 
illuminating light from an external halogen lamp 13 through optical fibers 
14, irradiates the wafer W with different light beams through a prism 
mirror 12, and the reflected lights from the wafer W return to the 
alignment system 11 through the prism mirror 12. Various detection signals 
from the alignment system 11 are supplied to the alignment control unit 
15. 
FIG. 2 shows the detailed configuration of the alignment system 11 in FIG. 
1, wherein the optical fibers 14 emit, for the FIA sensor, illuminating 
light L3 of a broad band (bandwidth 270 nm or larger), non-actinic to the 
photoresist on the wafer W. Said illuminating light L3 is transmitted 
through a condenser lens 16 and illuminates a field diaphragm plate 17 
with a uniform intensity. The illuminating light limited by the field 
diaphragm plate 17 is reflected by a dichroic mirror 18 and enters a beam 
splitter 20 through a lens system 19. The illuminating light reflected and 
split by said beam splitter is reflected by a prism mirror 21 and enters a 
beam splitter 22, and the illuminating light reflected by said beam 
splitter 22 is guided through an objective lens 23 and the prism mirror 12 
and irradiates a predetermined area on the wafer W. 
In the above-explained wafer illuminating optical path, the field diaphragm 
plate 17 is conjugate with the wafer (i.e. in imaging relationship) with 
respect to a synthesized system consisting of the lens system 19 and the 
objective lens 23. Also the illuminating area of the FIA system on the 
wafer W is uniquely determined by the shape and size of an aperture formed 
on the field diaphragm plate 17. 
Within the illuminating light from the optical fibers 14, the light 
reflected by the wafer (normal reflected light, scattered light etc.) is 
guided through the prism mirror 12, objective lens 23, beam splitter 22 
and prism mirror 21 and returns to the beam splitter 20, and the light 
transmitted by said beam splitter 20 (about 1/2 of the returning light) is 
transmitted by a dichroic mirror 25 and proceeds to a detection system 51 
of the FIA sensor. In said detection system 51, the light coming from the 
dichroic mirror 25 is guided by a mirror 26 and forms an image of the 
wafer mark on an index plate 27 bearing index marks. The light emerging 
from said index plate 27 is guided through a phototaking relay lens 28, a 
mirror 29, a wavelength filter 30 (for intercepting a specified spectral 
band), a relay lens 31 and a beam splitter 32, thereby forming images of 
the wafer mark and the index mark respectively on the light-receiving 
faces of a two-dimensional image sensor 33X for the X-axis and a 
two-dimensional image sensor 33Y for the Y-axis, respectively composed for 
example of two-dimensional CCD's. The wavelength filter 30 is used for 
intercepting the light of a wavelength region same as that of the strong 
light reflected from the wafer W when the LIA sensor, to be explained 
later, is in use. 
The index plate 27 is so positioned as to be conjugate with the exposed 
surface of the wafer W with respect to a synthesized system of the 
objective lens 23 and the lens system 24. Also the index plate 27 is so 
positioned as to be conjugate with the light-receiving faces of the image 
sensors 33X, 33Y, with respect to the relay lens systems 28, 31. Said 
index plate 27 is composed of a transparent substrate on which the index 
marks are formed for example by a chromium layer, and the images of the 
wafer marks are formed in transparent portions. The index marks are 
composed of those for the X-axis and those for the Y-axis. The image 
sensors 33X, 33Y take the images of the wafer marks for the FIA system and 
those of the index marks for the X- and Y-axes. The X-coordinate of the 
wafer mark for the X-axis and the Y-coordinate of the wafer mark for the 
Y-axis can be determined by processing the image signals from the image 
sensors 33X, 33Y, thereby detecting the positional relationship between 
the images of the wafer marks and the index marks. Though not illustrated 
in FIG. 2, an illumination system is provided independently for 
illuminating the index plate 27. 
In the above-explained configuration, the objective lens 23 and the prism 
mirror 12 are called the common objective system, which is used in common, 
in the present embodiment, for the FIA sensor, the LIA sensor and the auto 
focusing sensor. The lenses of the common objective system and those of 
the detection system 51 of the FIA sensor after the mirror 26 are 
positioned coaxially. 
Again referring to FIG. 2, the reflected light (including scattered light 
and diffracted light) from the wafer mark formed on the wafer W returns to 
the beam splitter 22 through the prism mirror 12 and the objective lens 
23, and the light transmitted by said beam splitter 22 enters a detection 
system 34 of the LIA sensor. Said detection system 34 of the LIA sensor 
provides the beam splitter 22 with a pair of laser beams to each of the X- 
and Y-axes, and the two pairs of laser beams transmitted by said beam 
splitter are guided through the objective lens 23 and the prism mirror 12 
and respectively irradiate the wafer marks of a diffraction grating 
pattern for the X-axis and those for the Y-axis provided on the wafer W. 
Thus, as exemplified in FIG. 5, a wafer mark 45X of diffraction grating 
pattern is irradiated with the laser beams L1, L2 of different 
frequencies, from two symmetrical directions. 
The wafer marks of the diffraction grating pattern generate, by the 
irradiation with said two laser beams, two diffracted lights Li(+1), 
L2(-1) in a same direction. The two diffracted lights from said wafer mark 
mutually interfere, and the resulting interference light enters the 
detection system 34 of the LIA system, which effects photoelectric 
conversion on the interference light and detects the phase of thus 
obtained beat signal. This phase information is supplied, as the 
positional information of the alignment mark in the X-direction, to the 
alignment control unit 15. Similar operations are conducted for the 
Y-axis. Thus the detection system 34 of the LIA sensor is provided with a 
light emitting system for irradiating the wafer, under predetermined 
optical conditions, with two laser beams for the X-axis and two laser 
beams for the Y-axis, or four laser beams in total, through the objective 
lens 23, and a light receiving system for photoelectrically detecting, 
independently, the interference light from the wafer mark for the X-axis 
and that for the Y-axis. The details of such LIA sensor are disclosed in 
the U.S. Pat. No. 4,710,026 and the U.S. patent application Ser. No. 
418,260 filed on Oct. 6, 1989. 
In the following there will be given an explanation on the auto focusing 
sensor of the present embodiment. Detecting light M, emitted from a light 
source 35 for auto focusing, such as an LED or a laser diode, enters a 
slit plate 37 bearing a focus detecting pattern, through a condenser lens 
36. The detecting light M transmitted by the focus detecting pattern of 
the slit plate 37 is guided through a dichroic mirror 18 and a lens system 
19 to a beam splitter 20. Said detecting light M is so selected that the 
wavelength range thereof is non-actinic to the photoresist on the wafer W, 
for example in a wavelength region from red to near infrared. The dichroic 
mirror 18 has such wavelength selectivity as to reflect, within the 
illuminating light L3 from the optical fibers 14, the light of a 
wavelength region used for position detection and to transmit, within the 
detecting light M, the light of a wavelength region used for focus 
detection. Thus the position detecting light and the focus detecting 
light, irradiating the wafer W, have mutually different wavelength regions 
in order to avoid undesirable effect. 
The focus detecting light reflected by the beam splitter 20 is guided 
through the prism mirror 21, beam splitter 22, objective lens 23 and prism 
mirror 12, and irradiates the wafer W. The slit plate 37 is substantially 
conjugate with the exposed surface of the wafer W with respect to the lens 
system 19 and the objective lens 23, whereby an image or a defocused image 
of the focus detecting pattern 40 of the slit plate 37 is projected onto 
the exposed surface of the wafer W. The light reflected by the exposed 
surface of the wafer W is guided through the prism mirror 12, objective 
lens 23, beam splitter 22 and prism mirror 21 and returns to the beam 
splitter 20, and the light transmitted by the beam splitter 20 proceeds to 
a dichroic mirror 25 through a lens system 24. 
Said dichroic mirror 25 has such wavelength selectivity, contrary to that 
of the dichroic mirror 18, as to transmit the position detecting light 
emitted from the optical fibers 14 and to reflect the focus detecting 
light emitted from the light source 35. Consequently the focus detecting 
light reflected by the dichroic mirror 25 enters a line sensor 39, 
composed for example of a one-dimensional CCD, through the outside of a 
pupil-limiting light shield plate 38 provided for destructing the 
telecentricity at the image side. On said line sensor 39 there is formed 
an image (or a defocused image) of the focus detecting pattern projected 
onto the wafer W. Stated differently, the exposed surface of the wafer W 
and the light-receiving face of the line sensor 39 are mutually 
substantially conjugate with respect to the objective lens 23 and the lens 
system 24. In order to avoid entry of stray laser light in the use of the 
LIA sensor, a wavelength filter plate 30 is preferably provided between 
the dichroic mirror 25 and the lens system 24. 
In the following there will be explained, with reference to FIG. 3, the 
auto focusing sensor of the present embodiment. FIG. 3 illustrates the 
auto focus sensor only, in the configuration shown in FIG. 2, however 
without the optical path folding mirrors and the like shown in FIG. 2. 
Referring to FIG. 3, the slit plate 37 bears five focus detecting patterns 
40A, 40B, . . . of which only two are illustrated in FIG. 3. Images 41A, 
41B . . . of said focus detecting patterns 40A, 40B, . . . are projected 
onto the wafer W through the lens system 19 and the objective lens 23, and 
the reflected light from the wafer W enters the line sensor 39 through the 
objective lens 23, lens system 24, dichroic mirror 25 and light shield 
plate 38, whereby images 42A, 42B, . . . are projected onto said line 
sensor 39. 
On the line sensor 39, the images 42A, 42B, . . . are projected in 
respectively different positions along a predetermined direction (defined 
as direction U). The pupil-limiting light shield plate 38 serves to shield 
a half area lower than the optical axis AXa, in the direction U. The 
above-explained optical system is telecentric at the wafer side from the 
objective lens 23, but is non-telecentric at the line sensor side from the 
lens system 24, because of the function of the light shield plate 38. 
Consequently, if the wafer W is displaced parallel to the optical axis AXa 
(in the Z-direction), the positions of the images 41A, 41B, . . . of the 
focus detecting patterns do not change on the wafer W (though said images 
being defocused), but the images 42A, 42B . . . on the line sensor 39 are 
displaced along the direction U as indicated by an arrow. Thus the 
position of a measured point on the wafer W in the Z-direction (i.e. focus 
position) can be detected from the amount of lateral displacement of the 
image in the direction U on the line sensor 39. 
Now there will be explained, with reference to FIG. 4, an example of the 
viewing fields on the wafer W, to be respectively detected by the FIA 
sensor and the LIA sensor through the objective lens 23. In FIG. 4, a 
circular area represents the viewing field 43 of the objective lens 23 on 
the wafer, and, within said viewing field 43 there are defined five 
detection areas 44A to 44E. 
Detection areas 44A and 44E are the detection areas for the LIA sensor, and 
the area 44A is irradiated with the two light beams of the LIA system, for 
detection in the X-direction, while the area 44E is irradiated with the 
two light beams of the LIA system, for detection in the Y-direction. 
Detection areas 44B, 44C and 44D are those for the FIA sensor. A 
rectangular area 48 in the viewing field 43 represents the image taking 
area of the image sensors 33X, 33Y. The detection area 44C is subjected to 
image analysis by the image sensor 33X, while the detection area 44D is 
subjected to image analysis by the image sensor 33Y. Also the detection 
area 44B is subjected to image analysis by the image sensors 33X and 33Y, 
for measuring the wafer registration. 
In the detection areas 44A to 44E in FIG. 4, there are tentatively shown 
wafer marks to be respectively detected in said areas. 
More specifically, in the detection area 44A, there is detected a wafer 
mark 45X of a diffraction grating pattern (hereinafter called diffraction 
grating mark) having a predetermined pitch in the X-direction, for the LIA 
sensor, and, in the detection area 44E, there is detected a diffraction 
grating mark 45Y having a predetermined pitch in the Y-direction. In the 
detection area 44C there is detected a line-and-space (L/S) pattern 47X 
having a predetermined pitch in the X-direction, and, in the detection 
area 44D there is detected an L/S pattern 47Y having a predetermined pitch 
in the Y-direction. Also in the detection area 44B, a box-in-box wafer 
mark 46 is detected. 
The above-mentioned diffraction grating marks 45X, 45Y, L/S patterns 47X, 
47Y and mark 46 are respectively formed, for example as shown in FIG. 8, 
in mark areas MAa, MAb positioned between the plural shot areas SA on the 
wafer. 
The registration measurement can be achieved, based on the image signals 
from the image sensors 33X. 33Y (see FIG. 2) of the FIA sensor, by 
detecting the positional relationship between the images of the outer box 
46a and the inner box 46b and determining the positional deviation between 
the outer box 46a and the inner box 46b. 
FIG. 6 shows the images 41A to 41E of the focus detecting patterns, 
respectively projected in the detection areas 44A to 44E in the viewing 
field 43. As illustrated in FIG. 6, the focus detecting pattern images 41A 
to 41E, projected on the detection areas 44A to 44E, are light-and-dark 
patterns (multiple patterns) having a predetermined pitch in a direction 
of 45.degree. to the X-axis. 
Since these patterns are so formed as to cross the wafer marks to be 
detected in these detection areas, the influence of said wafer marks on 
the focus detection can be alleviated. The focus detecting pattern can 
also be composed of a single slit pattern. 
FIG. 7 illustrates the focus detecting pattern images 42A to 42E 
reprojected on the light-receiving face 39a of the line sensor 39 (see 
FIG. 3), corresponding respectively to the focus detecting pattern images 
shown in FIG. 6. In FIG. 7, photosensor elements are arranged, along the 
direction U, on said light-receiving face 39a of the line sensor 39, and 
the images 42A to 42E of the focus detecting patterns are projected in 
respectively different positions along the direction U. As each of said 
images 42A to 42E is composed of a light-and-dark pattern having a 
predetermined pitch in the direction U, the positions of said images 42A 
to 42E along said direction U can be determined by processing the image 
signals, obtained from the line sensor 39, in the alignment control unit 
15 (see FIG. 1). 
The amount of positional deviation, in the Z-direction, of a wafer area 
bearing the diffraction grating mark 45X (see FIG. 4) can be detected by 
determining the amount of positional deviation uA of the image 42A with 
respect to a reference position, in the direction U. Also the amount of 
positional deviation, in the Z-direction, of a wafer area bearing the 
wafer mark 46 (see FIG. 4) can be detected by determining the amount of 
positional deviation uB of the image 42B with respect to the reference 
position in the direction U. Furthermore, the amount of positional 
deviation, in the Z-direction, of a wafer area bearing the L/S pattern 47X 
(see FIG. 4) can be detected by determining the amount of positional 
deviation uC of the image 42C with respect to the reference position in 
the direction U. The amount of positional deviation, in the Z-direction, 
of a wafer area bearing the L/S pattern 47Y (see FIG. 4) can be detected 
by determining the amount of positional deviation uD of the image 42D with 
respect to the reference position in the direction U. The amount of 
positional deviation, in the Z-direction, of a wafer area bearing the 
diffraction grating mark 45Y (see FIG. 4) can be detected by determining 
the amount of position deviation uE of the image 42E with respect to the 
reference position in the direction U. 
Now there will be briefly explained an example of the method of 
determination of the reference position on the line sensor 39. 
In case of the FIA sensor, the reference mark of the reference mark plate 6 
(cf. FIG. 1) is positioned for example in the detection area 44C, and said 
reference mark is taken by the image sensor 33X while said reference mark 
plate 6 is moved in the Z-direction. Then the image signal from said image 
sensor 33X is analyzed and the position of the reference mark plate 6 in 
the Z-direction is so determined that the contrast of the image of said 
reference mark becomes highest. Then the image 41C of the focus detecting 
pattern is projected on the reference mark plate 6, and the pattern image 
42C is projected on the line sensor 39. The position of said pattern image 
42C in this state on the line sensor 39 is taken as the reference position 
for the pattern image 42C. The reference positions can be determined in a 
similar manner for the pattern images 42B and 42D. 
In case of the LIA sensor, the reference mark of the reference mark plate 6 
is positioned for example in the detection area 44A, and the reference 
mark is irradiated with the two laser beams while the reference mark plate 
6 is moved in the Z-direction. Then the interference light from the 
reference mark is detected, and the position of the reference mark plate 
in the Z-direction is so determined that the beat signal of the 
interference light has the largest amplitude. Then the image 41A of the 
focus detecting pattern is projected on the reference mark plate 6, and 
the image 42A is projected on the line sensor 39. The position of the 
pattern image 42A in this state on the line sensor 39 is defined as the 
reference position of the pattern image 42A. The reference position can be 
determined in a similar manner for the pattern image 42E. 
The line sensor 39 may also be replaced by a two-dimensional image sensor, 
having pixels of plural lines along the direction U. In such case the 
position of the pattern image on the sensor can be determined by adding 
the image signals of the pixels of said plural lines. 
It is also possible to place, in front of the light-receiving face of the 
line sensor 39, a cylindrical lens having light-condensing function in a 
direction perpendicular to the direction U shown in FIG. 7. 
The amounts of positional deviation of the mark bearing areas on the wafer, 
from the in-focus position, are represented by 1/.beta..sup.2 of the 
lateral deviations uA to uE shown in FIG. 7, wherein .beta. is the 
(lateral) image magnification of the synthesized system, consisting of the 
objective lens 23 and the lens system 24, from the wafer W to the line 
sensor 39. Consequently, for detecting the X- and Y-coordinates of the 
wafer mark on the wafer, the position detection is executed after the 
position of the Z-stage 3, shown in FIG. 1, is adjusted for each alignment 
sensor to be used. In this manner the position of the wafer mark can be 
detected in highly precise manner, in an accurately focused state, for 
each of the FIA sensor and the LIA sensor. Also for improving the 
precision of focusing, auto focusing by a servo system may be conducted 
also during the position measurement by the alignment sensor. Besides, in 
case wafer marks (for example 45X and 45Y) are present at the same time in 
plural detection areas (for example 44A and 44E) in the viewing field of 
the objective optical system, the focus positions can be simultaneously 
detected for said marks, so that the detection time required for the 
detection of the positions of the wafer marks can be shortened. 
If it is only required, for example in FIG. 4, to detect the focus position 
in the detection area 44A, the focus detecting patterns may be shielded by 
an unrepresented shutter, except for the pattern 40A shown in FIG. 3. This 
method enables exact focus detection without the influence of the stray 
light from the projected images of other focus detecting patterns. Also in 
place for such shutter, there may be provided light sources for 
respectively illuminating the focus detecting patterns 40A, 40B . . . and 
these light sources may be suitably controlled to only illuminate a 
specified focus detecting pattern. 
Furthermore, it is also possible to replace, as shown in FIG. 2, the slit 
plate 37 with a liquid crystal display panel 52 for generating the focus 
detecting patterns. This method enables easy optimization of the shape of 
the focus detecting patterns according to the shape of the wafer marks to 
be detected, and allows to erase unnecessary focus detecting patterns. 
Furthermore, instead of the auto focusing sensor shown in FIG. 3, for 
projecting the images of the focus detecting patterns onto the wafer W 
through the objective lens 23, there may be provided a light emitting 
system, different from the objective lens 23, for projecting plural images 
41A, 41B, . . . of the focus detecting patterns obliquely to the optical 
axis AXa. In such case, the focus positions of the corresponding 
mark-bearing areas can be detected by receiving the light reflected from 
the wafer W with a light-receiving system positioned substantially 
symmetrically to said light-emitting system to re-focus the images of the 
focus detecting patterns and determining the amounts of lateral deviations 
of thus re-focused images. 
In the configuration shown in FIG. 2, the image sensors 33X, 33Y are so 
designed as to take the image of a same area within the viewing field of 
the objective lens, and to effect image analysis in the different portions 
in said image taking area, but said image sensors 33X, 33Y may have 
mutually different image taking areas. 
Furthermore, the foregoing explanation has explained a case in which the 
FIA sensor and the LIA sensor are united, but the present invention is 
naturally applicable also to a case in which united are plural alignment 
sensors selected from the FIA sensor, the LIA sensor and other alignment 
sensors (for example LSA sensor). Furthermore, the present invention is 
applicable in case an alignment sensor of TTL (through-the-lens) system or 
an alignment sensor of TTR (through-the-reticle) system is united.