Method and an apparatus for aligning first and second objects with each other

A distance between a mask and a wafer is set such that exposure light beams emerged from the mask are converged by the projection lens to be focused on the wafer. According to the present invention, two mask marks of diffraction gratings are formed on the mask and spaced at a predetermined distance from each other. When the alignment light beams are applied to the mask marks, two diffracted light beams of predetermined order emerge individually from the mask marks in such a manner that the respective optical axes of the two diffracted light beams, which are directed opposite to the advancing direction of the diffracted light beams, intersect each other one the first point. Thus, the diffracted light beams advance as if the diffracted light beams were the two light beams emerging from the first point. Therefore, the two diffracted light beams can be focused on the wafer or neighborhood of it. The diffracted light beams can be incident on a wafer mark which is formed on the wafer and is diffraction grating. Thus rediffracted light beams emerge from the wafer mark and are detected, so that the mask and the wafer are aligned with each other. Accordingly, the alignment can be performed, despite a great diffraction between the wave-lengths of the exposure light beam and the alignment light beam.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and an apparatus for aligning 
first and second objects with each other, and more particularly, to a 
method and an apparatus for aligning a mask and a wafer with each other 
during a projection/exposure process in the manufacture of semiconductor 
devices. 
2. Description of the Related Art 
In a projection/exposure process in the manufacture of a semiconductor 
device, an exposure light beam emitted from light source 1 is applied to a 
circuit pattern previously formed on mask 2, as shown in FIG. 1. An image 
of the circuit pattern is projected on wafer 4 after being reduced in size 
by means of projection lens 3. Thereupon, a resist of wafer 4 is exposed, 
so that the pattern image is transferred to wafer 4. 
In order to transfer the image of the circuit pattern accurately to a 
predetermined portion of the wafer, the mask and wafer must be aligned 
with each other before the exposure light beam is applied to the mask. The 
TTL (through the lens) method is a major aligning method for this purpose. 
This method is characterized in that an alignment light beam, which has a 
wavelength different from that of the exposure light beam, is transmitted 
through projection lens 3. A method using two diffraction gratings is 
stated in some documents (by G. Dubroeucq, 1980, ME; W. R. Trutna Jr., 
1984, SPIE), as an example of the TTL method. As shown in FIG. 2, 
diffraction gratings 5 and 6 are formed on mask 2 and wafer 4, 
respectively. An alignment light beam emitted from alignment light source 
(laser light source) 7 is diffracted along a path from diffraction grating 
6 of the wafer to diffraction grating 5 of the mask. The intensity of the 
diffracted light beam is detected by means of detector 8. Since the 
diffracted light beam carries information on dislocation between the mask 
and wafer, the position of the wafer relative to the mask is detected as 
the intensity of the diffracted light beam changes. 
It is to be desired that the wire of the circuit pattern should be as thin 
as possible, that is, resolution R=.varies. .lambda./NA should be 
minimized (.lambda.: wavelength of the exposure light; NA=sin.alpha., 
where .alpha. is half the angle at which the exposure light beam is 
converged on the wafer). Resolution R can be lessened by widening angle 
.alpha. or reducing .lambda.. Due to structural restrictions on the 
projection lens, however, half-angle .alpha. cannot be unlimitedly 
increased. It is advisable, therefore, to reduce wavelength .lambda. of 
the exposure light beam. Presently, a g-line light beam (436 nm) is 
utilized as the exposure light beam. For a thinner circuit pattern wire, 
however, an i-line light beam (365 nm) or Krf excimer laser beam (248 nm) 
is expected to be used as the exposure light beam in the future. 
The resist of wafer 4 is sensitive to a light beam with a wavelength of 500 
nm or less. Accordingly, a light beam with a wavelength exceeding 500 nm 
is used as the alignment light beam, in order to avoid affecting the 
resist. Currently, an He-Ne laser beam of 633-nm wavelength is the most 
prevalent light beam for the purpose. Even at present, therefore, the 
exposure light beam and the alignment light beam have different 
wavelengths. The difference between the two wavelengths, however, is 
expected to be increased in the future. 
Meanwhile, the image of the circuit pattern should be formed focused on the 
wafer for accurate exposure thereon. Thus, the distance between the mask 
and wafer is set so that the exposure light beam from the mask can be 
converged by the projection lens to be focused on the wafer. In other 
words, the aberration of the projection lens is adjusted so as to be 
minimized only for the exposure light beam, that is, the projection lens 
has chromatic aberration for light beams of any other wavelengths than 
that of the exposure light beam. 
In aligning the mask and wafer with each other, therefore, the diffracted 
alignment light beam from the mask cannot be focused on the wafer, and 
instead, is focused on a point at distance d from the wafer, as shown in 
FIG. 2. If a g-line beam (436 nm) is used as the exposure light beam, the 
distance between the mask and wafer ranges from about 600 mm to 800 mm, 
while distance d is only scores of millimeters. 
Conventionally, ordinary engineers believes that the sensitivity of 
diffracted light beams to be detected is too low for a mask and a wafer to 
be aligned accurately with each other, unless the diffracted alignment 
light beam is focused on a mask mark. Therefore, prior art aligning 
apparatuses are provided with means for correcting the length of the 
optical length of the diffracted alignment light beam, as shown in FIG. 2. 
More specifically, return mirrors 9 are disposed in the middle of the path 
of the diffracted alignment light beam. The optical path of the diffracted 
alignment light beam is extended by the distance for which the diffracted 
alignment light beam passes between mirrors 9, so that the diffracted 
alignment light beam from the mask can be focused on the wafer. If the 
aligning apparatus is provided with such correction means, however, the 
apparatus will inevitably be complicated in construction. 
If a Krf excimer laser beam, whose wavelength is extremely short (248 nm), 
is used as the exposure light beam, moreover, the difference between the 
wavelengths of the exposure light beam and the alignment light beam is 
very large. Therefore, the diffracted alignment light beam is focused on a 
point at distance D (several thousands of millimeters) from the wafer, as 
shown in FIG. 2. In this case, the return mirrors must be positively 
increased in size or complicated in construction, in order to correct the 
length of the optical path of the alignment light beam. Practically, 
therefore, it is impossible to correct to the optical path length by means 
of the return mirrors. Thus, if the wavelength of the exposure light beam 
is very short (i.e., if there is a great difference between the 
wavelengths of the exposure light beam and the alignment light beam), the 
mask and wafer conventionally cannot be aligned with each other. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a method and an apparatus 
for highly accurately aligning first and second objects with each other by 
means of a simple arrangement, in a system such that a projection lens is 
interposed between the first and second objects, whereby a light beam with 
a wavelength different from that of an alignment light beam is transmitted 
through the projection lens to be incident upon the second object, thereby 
forming an image of the first object thereon. 
More specifically, the object of the invention is to provide a method and 
an apparatus capable of accurately aligning a mask and a wafer with each 
other by means of a simple arrangement, despite a great difference between 
the wavelengths of an exposure light beam and an alignment light beam. 
According to the present invention, there is provided a method for aligning 
first and second objects with each other, which objects are moved relative 
to each other and in parallel, so as to be aligned, a projection lens 
being disposed between the first and second objects, a first mark formed 
on the first object, the first mark including a diffraction grating region 
having two diffraction points, each capable of diffracting a light beam 
applied thereto, the two diffraction points spaced at a predetermined 
distance from each other, a second mark formed on the second objects, the 
second mark including a diffraction grating region, 
the method comprising steps of: 
directing an alignment light beam emitted from a light source to the first 
mark, the alignment light beam diffracted by the two diffraction points of 
the first mark, so that two diffracted light beams or predetermined orders 
emerge individually from the two diffraction points in such a manner that 
the respective optical axes of the two predetermined-order diffracted 
light beams, which are directed opposite to the advancing direction of the 
diffracted light beams, intersect each other on a first intersection point 
at a predetermined distance from the first mark; 
transferring the two predetermined-order diffracted light beams through the 
projection lens toward the second mark, so that the two diffracted light 
beams are converged by the projection lens and are incident on the 
diffraction grating region of the second mark in such a manner that the 
respective optical axes of the two diffracted light beams, which are 
directed to the advancing direction of the diffracted light beams, 
intersect each other on a second intersection point at a predetermined 
distance (=d.sub.1 .gtoreq.0) from the second mark, whereby the two 
diffracted light beams are diffracted by the diffraction grating region of 
the second mark, and two re-diffracted light beams of predetermined orders 
emerge from the diffraction grating region of the second objects; 
detecting the predetermined-order re-diffracted light beams and generating 
a detection signal; and 
adjusting the first and second objects relative to each other in response 
to the detection signal, thereby aligning the first and second objects 
with each other.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It has conventionally been believed that a diffracted alignment light beam 
emitted from one alignment mark must be focused on the other alignment 
mark. However, the inventors hereof have found that this is not 
indispensable. Prior to a description of the present invention, a 
preliminary invention based on this finding will be explained. 
As shown in FIG. 3, an alignment light beam emitted from laser 23 is 
applied to wafer mark (diffraction grating) 22 via mirror 25 and 
projection lens 13. As a result, n-order diffracted light beams emerge 
from mark 22. If the exposure light beam is a g-line (436 nm), two 
.+-.1-order diffracted light beams of the n-order diffracted light beams 
are converged by lens 13 to be focused on a point at distance d (several 
tens of millimeters) from mask mark (diffraction grating) 21. These light 
beams are spaced at a predetermined distance when they are incident upon 
mark 21. The two .+-.1-order diffracted light beams are individually 
transmitted through mark 21 to be diffracted thereby, so that two 
.+-.1-order re-diffracted light beams emerge. These re-diffracted light 
beams are transferred to detector 26 via mirror 27 and prism 28. In the 
meantime, the diffracted light beams are superposed to interfere with each 
other, thus forming an interference light beam, that is, an interference 
fringe. The intensity change of the interference light beam is detected by 
means of detector 26. The .+-.1-order diffracted light beams diffracted by 
wafer mark 22 carry position information based on the their phase change. 
The .+-.1-order re-diffracted light beams diffracted by mask mark 21 carry 
information on the respective positions a mask and a wafer, based on their 
phase change. Accordingly, the interference light beam carries information 
on the mask and wafer positions. Thus, the relative positions of the mask 
and wafer can be determined by detecting the intensity change of the 
interference light beam. The mask and wafer are aligned with each other in 
accordance with the result of the detection. 
Even though the .+-.1-order diffracted light beams diffracted by wafer mark 
22 are not focused on mask mark 21, therefore, the mask and wafer can be 
aligned with each other. 
When using a Krf excimer laser (248 nm) as the exposure light beam, 
however, its wavelength is considerably different from that of the 
alignment light beam (633 nm). As shown in FIG. 3, therefore, the 
alignment light beam is focused on point b at distance D (several 
thousands of millimeters) from mask mark 21. Accordingly, the two 
.+-.1-order diffracted light beams diffracted by wafer mark 22 are spaced 
at so long a distance from each other, at the position corresponding to 
mask mark 21, that they cannot be incident upon mark 21. If the mask mark 
is increased in size, however, the detection of the diffracted light beams 
is liable to err. Thus, even according to the preliminary invention by the 
inventors hereof, the mask and wafer cannot be aligned with each other if 
the wavelength of the exposure light beam is extremely short. 
Thereupon, the inventors hereof have completed the present invention as 
described below. According to this invention, the mask and wafer can be 
aligned with high accuracy even if there is a great difference between the 
respective wavelengths of the exposure light beam and the alignment light 
beam. 
Referring now to FIGS. 4 and 5, a first embodiment of the present invention 
will be described. 
An aligning apparatus is constructed as follows. Two mask marks 41-1 and 
41-2, each composed of a diffraction grating, is formed on mask 11. These 
marks are spaced at a shown predetermined distance from each other. Each 
mask mark may be a one- or two-dimensional or checkered diffraction 
grating. One wafer mark 42, composed of a diffraction grating, is formed 
on wafer 12. Mark 42 may also be a one- or two-dimensional or checkered 
diffraction grating. 
In the aligning apparatus according to this embodiment, in contrast with 
the case of the conventional apparatus, the optical path of the alignment 
light beam extends from laser 43 to detector 47 via mask marks 41-1 and 
41-2, projection lens 13, and wafer mark 42, in the order named. Thus, the 
alignment light beam emitted from laser 43 is split into two light beams 
by mirror 44-1 and prism 44-2, and the split beams are transferred to 
marks 41-1 and 41-2, individually. Thereupon, the two light beams are 
transmitted individually through mask marks 41-1 and 41-2 to be diffracted 
thereby, so that two n-order (n=0, .+-.1, . . . ) diffracted light beams 
emerge. The two diffracted light beams of the predetermined orders are 
transferred to wafer mark 42 through projection lens 13. Then, these light 
beams are individually reflected by mark 42 to be diffracted thereby, so 
that two n-order re-diffracted light beams emerge. The re-diffracted light 
beams of the predetermined orders are transferred to detector 47 via 
mirror 45, lens 46, mirror 53-1 and prism 53-2. Then, re-diffracted light 
beams are converted into detection signals by detector 47. These detection 
signals are processed by means of signal processing unit 48. In response 
to an output signal from unit 48, the position of the mask or the wafer is 
adjusted by means of position adjusting unit 49. 
Comparing FIGS. 3, 4 and 5, the principle of the present invention will now 
be described. 
If the exposure light beam is a Krf excimer laser, as shown in FIG. 3, the 
diffracted light beams from wafer mark 22 are focused on point b at 
distance D (several thousands of millimeters) from the mask, as mentioned 
before. When two light beams having the same wavelength as the alignment 
light beam emerge from point c at distance d.sub.1 (several tens of 
millimeters) from the wafer, therefore, they are focused on point b at 
distance d.sub.2 
##EQU1## 
from the mask, as shown in FIG. 5 (.beta. is the inverse magnification of 
the projection lens for the alignment light beam). 
Here let it be supposed that imaginary mask 11-1 is disposed at point b. If 
two light beams emerge from point b on mask 11-1, they are focused on 
point c. In the present invention, two mask marks 41-1 and 41-2 are 
situated on the respective optical paths of these light beams, 
individually. Thus, marks 41-1 and 41-2 are spaced at a predetermined 
distance so that the two light beams pass them separately. Therefore, if 
the respective optical axes of the two diffracted light beams of the 
predetermined orders, diffracted by mask marks 41-1 and 41-2, are set so 
as to intersect each other on point b on imaginary mask 11-1, the 
diffracted light beams are transferred along the optical paths of the 
light beams from point b to wafer mark 42. The two predetermined-order 
diffracted light beams include a -1-order diffracted light beam diffracted 
by mask mark 41-1 and a .+-.1-order diffracted light beam diffracted by 
mask mark 41-2. 
Wafer mark 42 is also situated on the respective optical paths of the two 
light beams emerging from point b. Accordingly, the two .+-.1-order 
diffracted light beams from the mask marks are spaced at a predetermined 
distance when they are incident upon mark 42. These diffracted light beams 
are reflected by the wafer mark to be diffracted thereby, so that two 
n-order re-diffracted light beams emerge. Thus, .+-.1-order re-diffracted 
light beams of two n-order re-diffracted light beams emerge at right 
angles to the wafer mark. The re-diffracted light beams are converged by 
projection lens 13, are reflected by mirror 45, and then advance in 
parallel by means of lens 46. The re-diffracted light beams are 
transferred to detector 47 via mirror 53-1 and prism 53-2. As in the case 
of FIG. 3, the diffracted light beams interfere with each other, thus 
forming an interference light beam, that is, an interference fringe. The 
intensity change of the interference light beam is detected by means of 
detector 47. This intensity corresponds to dislocation between the mask 
and wafer. Based on the detection result of the intensity change of the 
interference light beam, therefore, position adjusting unit 49 adjusts the 
position of the mask or wafer. 
Thus, according to the first embodiment, the two .+-.1-order diffracted 
light beams diffracted by the two mask marks are supposed to be light 
beams emerging from imaginary mask 11-1. Accordingly, the diffracted light 
beams from mask marks are focused on point c at a relatively short 
distance from a focal surface of the wafer, so that they can be incident 
upon the wafer mark. Even if the wavelength of the exposure light beam is 
extremely short, as in the case of the Krf excimer laser, for example, the 
mask and wafer can be aligned with each other, based on the 
above-described principle of the present invention. Unlike the case of the 
conventional arrangement, moreover, return mirrors for correcting the 
optical paths of the diffracted light beams need not be disposed between 
the mask and wafer. Thus, the arrangement between the mask and wafer is 
simplified. 
Referring now to FIG. 6, a second embodiment of the present invention will 
be described. 
As shown in FIG. 6, the second embodiment resembles the first embodiment in 
that two .+-.1-order diffracted light beams from two mask marks are 
transferred to wafer mark 42 as if they were light beams emerging from 
imaginary mask 11-1. The second embodiment, however, differs from the 
first embodiment in that the .+-.1-order diffracted light beams from the 
mask marks are focused on the wafer mark, that is, they are converged on 
one point on the wafer mark. To meet with this, imaginary mask 11-1 is 
situated at distance D 
##EQU2## 
from mask 11. 
More specifically, two alignment light beams, in the form of spherical 
waves, are illuminated to two mask marks 41-1 and 41-2, individually, in a 
manner such that they are collected in an incidence pupil of projection 
lens 13. The alignment light beams are diffracted by the two mask marks, 
and two n-order diffracted light beams emerge to carry mask position 
information based on their phase change. Two 0-order diffracted light 
beams enter the incidence pupil of lens 13. A -1-order diffracted light 
beam from mask mark 41-1 and a +1-order diffracted light beam from mask 
mark 41-2 are transferred to lens 13 as if they were light beams emerging 
from b point of imaginary mask 11-1. The two .+-.1-order diffracted light 
beams take the form of plane waves after they are transmitted through the 
projection lens. These two diffracted light beams, in the form of plane 
waves, are converged on one point on wafer mark 42, whereupon they 
interfere with each other, thus forming an interference fringe. This 
interference fringe is a moire pattern, a periodic pattern, which depends 
on angle .theta.. Here .theta. is an angle value half that of the angle 
formed between the .+-.1-order diffracted light beams converged on the 
wafer mark. 
The two interfering .+-.1-order diffracted light beams are reflected by 
wafer mark 42 to be diffracted thereby again, and .+-.1-order 
re-diffracted light beams emerge. This .+-.1-order re-diffracted light 
beams superposed with each other, are reflected perpendicularly by mark 
42, and are then applied to detector 47 via mirror 51. Since the 
.+-.1-order re-diffracted light beams are interference light beams, 
information on the respective positions of the mask and wafer can be 
obtained by detecting their intensity change. Thereafter, the mask and 
wafer are aligned with each other in the same manner as in the first 
embodiment. 
Thus, also in the second embodiment, the mask and wafer can be aligned with 
each other even if the wavelength of the exposure light beam is extremely 
short, that is, even though the respective wavelengths of the exposure 
light beam and the alignment light beam are considerably different. In 
contrast with the case of the conventional arrangement, moreover, return 
mirrors for correcting the optical paths of the diffracted light beams 
need not be disposed between the mask and wafer. Thus, the arrangement 
between the mask and wafer is simplified. 
Further, according to this embodiment, the two .+-.1-order order diffracted 
light beams from the mask marks are converged on one point on wafer mark 
42, where they interfere with each other. In contrast with the case of the 
first embodiment, therefore, the re-diffracted light beams from the wafer 
mark need not be superposed. Thus, there is no need of means for 
superposing the re-diffracted light beams. 
In the first embodiment, furthermore, the wafer mark sometimes may be 
skewed, failing to be set horizontally, if the wafer is distorted. Since 
the two .+-.1-order diffracted light beams are separate from each other 
when they are incident upon the wafer mark, the re-diffracted light beams 
from the wafer mark may possibly be reflected slantly, not vertically. 
Therefore, the alignment between the mask and wafer may be subject to an 
error. According to the second embodiment, however, the two .+-.1-order 
diffracted light beams are converged on one point of the wafer mark, so 
that there is no possibility of such an error. In the case of the first 
embodiment, moreover, the re-diffracted light beams may be changed in 
phase if there is a difference between the ambient temperatures around the 
-1- and +1-order re-diffracted light beams. Thus, the alignment may 
possibly be subject to an error. According to the second embodiment, 
however, the re-diffracted light beams cannot be changed in phase. 
The following is a description of the ways of setting distance D between 
imaginary mask 11-1 and mask 11, the distance between mask marks 41-1 and 
41-2, and the pitch of the mask marks. 
Let it be supposed that the inverse magnification of the projection lens 
for the alignment light beam, indicative of the relation between the 
respective positions of the imaginary mask and the wafer mark, is .beta., 
the angle formed between the light beam emerging from point b on the 
imaginary mask and the optical axis of the projection lens is 81' and the 
angle formed between the alignment light beam incident upon mask marks 
41-1 and 41-2 and the optical axis of the projection lens is 
.theta..sub.R. 
Thereupon, distance D between imaginary mask 11-1 and mask 11 and distance 
2r between two mask marks 41-1 and 41-2 have a correlation as follows: 
EQU r=Dtan.theta..sub.1 =Dtan.beta..theta.. (1) 
Pitch P of mask marks 41-1 and 41-2 are set as follows: 
EQU n.lambda./P=sin.theta..sub.R +sin.theta..sub.1 =sin.theta..sub.R 
+sin.beta..theta., 
where n is the order number of the diffracted light beams from the mask 
marks, and .lambda. is the wavelength of the alignment light beam. Thus, 
pitch P of the mask marks is set as if the .+-.1-order diffracted light 
beams from the mask marks were ones emerging from the imaginary mask. 
In the second embodiment, moreover, the two .+-.1-order order diffracted 
light beams from the mask marks are focused on the wafer mark. However, 
these diffracted light beams need not be exactly focused, but only be 
focused within the depth of focusing. Also in this case, the .+-.1-order 
diffracted light beams interfere with each other, thereby forming an 
interference fringe. Thus, the distance between the mask and wafer may be 
somewhat varied. 
Specifically, signal processing unit 48 may be arranged as follows. For 
example, it may be designed so that the mask or wafer 12 is oscillated 
horizontally at a predetermined frequency to modulate the alignment light 
beam, whereby the detection signals are synchronously demodulated in 
accordance with the predetermined frequency. Alternatively, unit 48 may be 
arranged so that the phase (or frequency) of the wavelength of the 
alignment light beam is modulated by means of phase shift mechanism 52, 
whereby the detection signals are synchronously demodulated in accordance 
with the predetermined frequency. 
Referring now to FIG. 7, a third embodiment of the present invention will 
be described. 
This embodiment is a more specific version of the second embodiment. 
In the third embodiment, two alignment light beams are applied to mask 
marks 41-1 and 41-2 via lens 61 and condenser lens 62. In doing this, the 
alignment light beams are incident upon the mask marks so as to be 
directed toward the center of the incidence pupil of projection lens 13 
(as indicated by broken line). Two .+-.1-order diffracted light beams from 
marks 41-1 and 41-2 are transmitted through the incidence pupil to be 
converged on wafer mark 42. As mentioned before, wafer mark 42 may be a 
one- (FIG. 8A) or two-dimensional diffraction grating (FIG. 8B) or 
checkered diffraction grating (FIG. 8C). The best selection depends on the 
operating conditions of a projection/exposure unit. 
During transfer of a circuit pattern, for example, a mask and a wafer 
sometimes may be aligned with each other by means of an alignment light 
beam. In this case, a two-dimensional diffraction grating (FIG. 8B) or a 
checkered diffraction grating (FIG. 8C) may be used as a wafer mark. In 
the case of the embodiment shown in FIG. 6, the intensity of the 
re-diffracted light beams is more improved by employing the checkered 
diffraction grating for the wafer mark. Thus, re-diffracted light beams 
from the wafer mark are distributed in a two-dimensional manner. Among 
these re-diffracted light beams of the two-dimensional distribution, 
re-diffracted light beams of predetermined orders can be situated off the 
optical path of the exposure light beam. If mirror 51 is located so that 
the predetermined-order re-diffracted light beams can be detected, 
therefore, the exposure light beam cannot be intercepted by the mirror. 
Whether wafer mark 42 is illuminated by the exposure light beam so that a 
resist of mark 42 is separated, or whether the wafer mark is not 
illuminated by the exposure light beam so that the resist remains 
unseparated, depends on a user's selection. In other words, the user can 
select the separation or retention of wafer mark 42 by forming a 
chromium-free window at point e of mask 11 which is conjugate to the wafer 
mark, or by depositing chromium to point e to intercept the exposure light 
beam. 
FIG. 9 shows modifications of the detecting means and the signal processing 
means. 
In FIG. 9, mask 11 has four continuously arranged marks A, B, C and D, as 
mask marks 41-1, and four continuously arranged marks a, b, c and d, as 
mask marks 41-2. It is necessary only that marks A to D and a to d be 
arranged on a dicing line outside a circuit pattern. Marks A, B, C and D 
are spaced at a predetermined distance from marks a, b, c and d, 
respectively. In wafer 12, on the other, four marks Aa, Bb, Cc and Dd are 
continuously arranged as wafer marks. Thus, when the alignment light beam 
is applied to marks A and a of the wafer, diffracted light beams from 
these marks are transferred to mark Aa of the wafer to be diffracted 
thereby. Likewise, when the alignment light beam is applied to marks B and 
b (or C and c, or D and d), the diffracted light beams are transferred to 
mark Bb (or Cc or Dd) of the wafer. 
The following three methods are available as signal processing methods 
using the marks described above. 
According to the first method, the alignment beams incident on marks A and 
a are slightly oscillated in the alignment direction. Since the diffracted 
light reflected by the wafer also oscillates, a modulated detection signal 
is obtained as a result of the oscillation. This technique is similar to 
that used in the signal processings performed in an ordinary photoelectric 
microscope. 
According to a second method, the alignment light beam is applied 
alternately to marks A and a and to marks B and b. As a result, 
re-diffracted light beams emerge alternately from marks Aa and Bb of the 
wafer. The pitch of mark Aa of the wafer is deviated from that of mark Bb 
by a quarter pitch. Therefore, the re-diffracted light beams from marks Aa 
and Bb are different in diffraction angle, so that they are detected 
independently of each other, or alternately. Then, the difference between 
these two re-diffracted light beams is calculated, and the mask and wafer 
are aligned so that the difference is zero. 
According to the third method, the alignment beams are not oscillated. 
Instead, the right and left alignment beams are made to have a frequency 
difference of .omega. by means of frequency modulator 52, as is shown in 
FIG. 6. Since the re-interfered light coming from wafer mark 42 produces a 
beat signal resulting from frequency difference .omega., the position 
adjustment can be performed by measuring a phase change in the beat 
signal. (This method is generally referred to as an optical heterodyne 
method.) 
When manufacturing a semiconductor device, a plurality of circuit patterns 
are transferred successively to one wafer. Accordingly, the mask and wafer 
must be aligned with each other every tie a pattern is transferred. Thus, 
a number of alignment marks should be formed on the wafer. As shown in 
FIG. 9, for example, marks Aa and Bb of the wafer are used for the 
alignment for the transfer of a first layer, and marks Cc and Dd are used 
for the alignment for the transfer of a second layer. Let us suppose, 
however, that the alignment light beam is applied to marks A and a and 
marks B and b of the mask at the time of the transfer of the first layer. 
In this case, individual .+-.1-order diffracted light beams from marks A, 
a, B and b are transferred to marks Aa and Bb of the wafer. Possibly, 
however, 0-order diffracted light beams from marks A, a B and b may be 
illuminated to mark Cc or Dd of the wafer. In such a case, the 0-order 
diffracted light beams are reflected by mark Cc or Dd to be incident upon 
detector 47, so that the alignment is subject to an error. This error can 
be prevented by setting distance 2r between the mask marks relatively 
large so that angle .theta. at which the diffracted light beams from the 
mask marks are incident upon the wafer can be set relatively wide. 
In the embodiments described above, the mirror is disposed between the mask 
and the wafer, whereby the re-diffracted light beams from the wafer mark 
are guided to the detector. Alternatively, however, as shown in FIG. 3 the 
mirror may be disposed in a manner such that the re-diffracted light beams 
from the wafer mark are guided to the detector after they are transferred 
to above the mask. Moreover, two wafer marks and one mask mark may be 
arranged on the wafer and the mask, respectively. As shown in FIG. 7, 
furthermore, the two mask marks may be in the form of one diffraction 
grating which extends relatively long.