Alignment system

An alignment system for aligning a first object, having a first alignment mark, and a second object having a second alignment mark, disposed at a position which is approximately conjugate with the first object with respect to a projection optical system. The alignment system includes a detecting device for detecting light from the first and second alignment marks with light irradiated by light. The detecting device detects the light from the second alignment mark as passed through the projection optical system and the first objects. An adjusting device adjusts the spacing between the first object and the second object in the direction of an optical axis of the projection optical system. The adjusting device is arranged to change the optical path length for the light from the second alignment mark, between the first and second objects, by the adjustment of that spacing, control device controls the adjusting device. The detecting device can produce a first signal based on the light from the first and second alignment marks at a first distance, and a second signal based on the light from the first and second alignment marks at a second distance. The control device is operable to determine a positional deviation between the first and second objects on the basis of the first and second signals, and the relative position of the first and second objects can be adjusted to correct the positional deviation.

FIELD OF THE INVENTION AND RELATED ART 
This invention relates to an alignment system for bringing two objects into 
a predetermined positional relation and, more particularly, to an 
alignment system usable in an exposure apparatus, for use in the 
manufacture of semiconductor microcircuits, for detecting alignment marks 
provided on a mask or reticle and a wafer and for bringing the mask or 
reticle and the wafer into a predetermined positional relation on the 
basis of the detected positional relationship between the marks. 
In the field of exposure apparatuses for photoprinting a circuit pattern 
upon a semiconductor wafer, for the manufacture of semiconductor 
microcircuits, step-and-repeat type projection exposure apparatuses, each 
called a "stepper", are used prevalently in consideration of recent 
further miniaturization of circuit patterns and the increasing diameter of 
wafers. Many varieties of alignment systems have been proposed and 
developed with respect to the alignment process to be made in such 
exposure apparatuses, as may be representd by a stepper, for aligning an 
original such as a reticle with a member to be exposed, such as a wafer 
whose surface is coated with a resist material. Typical examples are 
disclosed in U.S. Pat. Nos. 4,251,129 and 4,406,546 both assigned to the 
same assignee of the subject application. According to these proposals, 
alignment marks formed on a reticle and a wafer are observed through an 
optical system and, on the basis of the observation, manipulation is made 
to bring these alignment marks into a predetermined positional relation to 
thereby align the reticle and the wafer with each other. 
In the typical examples described above, a laser source which functions as 
an alignment light source, as well as photodetecting means are disposed on 
one side of a reticle which is remote from a projection optical system and 
a wafer, and the reticle and the wafer are scanned at a constant speed 
with the laser beam. Reflectively diffracted lights from the marks of the 
reticle and the wafer are received by the photo-detecting means, such that 
the positional relation between these marks and, therefore, any relative 
positional deviation between the reticle and the wafer, is detected. 
SUMMARY OF THE INVENTION 
The inventors of the subject application have found some inconveniences 
peculiar to the conventional alignment systems. One is multireflection of 
light which occurs between the reticle surface and the wafer surface. 
Particulars are as follows: 
Upon the alignment operation, light passes through a reticle and is 
projected upon a wafer. The light is reflected by the wafer and the 
reflected light passes again through the reticle. At this time, a portion 
of the light is reflected by a glass surface of the reticle, so that there 
occurs a reflected light of about 4% in intensity, directed back to the 
wafer. Such reflected light impinges again on the wafer and is reflected 
thereby again. Such twice-reflected light goes back to the reticle and 
passes again through the reticle. Also, at this time, there occurs a 
reflected light of about 4% in intensity, directed back to the wafer. In 
this manner, in addition to the light reflected once by the wafer 
(hereinafter, such light will be referred to also as a "signal light W1"), 
which light is a signal light that should be actually detected, to detect 
the position of the alignment mark of the wafer, there occur 
multireflection lights such as a light reflected twice by the wafer 
(hereinafter, such light will be referred to as a "signal light W2"), a 
light reflected three times by the wafer (hereinafter, such light will be 
referred to as a "signal light W3") and so on. All of these lights are 
received by the photodetecting means. Hereinafter, the composition of such 
multireflected lights will be referred to as a "signal light W.sub.T ". 
Since the reflected light from the wafer goes through a projection optical 
system to the photodetecting means, the precision of detection at the 
photodetecting means is affected by the aberration of the projection 
optical system. The multireflected lights described above are such lights 
as having passed through the projection optical system by plural times. 
Therefore, the aberration of the projection optical system has a large 
effect upon the multireflected lights. Accordingly, when the 
photodetecting means receives a light in which the signal light W.sub.T is 
superimposed upon the signal light W1, the detecting precision may be 
deteriorated. 
Another problem is that, where a single-wavelength laser is used as an 
alignment light source, there is a possibility of interference between the 
reflected light from the wafer and the multireflected lights described 
above. The state of interference fringe upon a light-receiving surface of 
the photodetecting means, which a fringe is formed as a result of the 
interference, is determined by an optical path difference between the 
optical path concerning the signal light W1 and each of the optical paths 
concerning the signal lights W2, W3, W4, ..., and such optical path 
difference is determined by the distance between the reticle and the wafer 
in the direction of an optical axis of the projection optical system. 
Actually, such an optical path difference is variable due to the variation 
in temperature or pressure within a projection exposure apparatus or due 
to the effect of the flatness of a wafer used. More specifically, the 
optical path difference changes at different moments and/or at different 
sites of different shot areas on the wafer. As a result, the state of the 
interference fringe upon the photodetecting means changes accordingly. 
The measured value concerning the position of an alignment mark on a wafer, 
namely the site of the wafer, is changeable with the change in the state 
of the interference fringe described above, such that the measured value 
includes inaccuracies within a certain range of variation. This is a bar 
to the attainment of the detection of the positional relation between a 
reticle and a wafer, at a currently required precision of an order not 
greater than 0.1 micron. Also, this makes it difficult to accurately align 
a reticle and a wafer with each other. 
Accordingly, it is a primary object of the present invention to provide an 
alignment system by which a reticle or photomask and a wafer can be 
accurately aligned with each other. 
Briefly, in accordance with an aspect of the present invention, to achieve 
the above object, there is provided an alignment system, comprising: 
illumination means for illuminating, with a light beam, alignment marks 
provided on a photomask and a wafer; 
light receiving means for receiving lights from the alignment marks and for 
producing signals corresponding to the positions of the alignment marks; 
interval changing means for changing the interval between the photomask and 
the wafer, from a first interval to a second interval; 
detecting means for detecting the positional relation between the alignment 
marks on the basis of the output signals produced from said light 
receiving means as the photomask and the wafer are spaced by the first 
interval and the second interval, respectively; and 
driving means for displacing at least one of the photomask and the wafer in 
accordance with the detection by said detecting means to thereby align the 
photomask and the wafer. 
The interval changing means of the present invention is effective to change 
the optical path length of a light beam between the photomask and the 
wafer. Accordingly, when the alignment system of the present invention is 
incorporated into a projection exposure apparatus for projecting a circuit 
pattern of a photomask upon a wafer by use of a reduction projection lens 
system, the interval between the photomask and the wafer can be changed in 
any of various methods such as, for example, a method wherein a wafer 
stage is displaced in a direction of an optical axis of a projection lens 
system; a method wherein a photomask stage is displaced in a direction of 
an optical axis of a projection lens system; a method wherein at least one 
of lenses constituting a projection lens system is displaced in a 
direction of an optical axis of the projection lens system; and a method 
wherein the refracting power of an air lens between predetermined lenses 
of a projection lens system is changed. 
These and other objects, features and advantages of the present invention 
will become more apparent upon a consideration of the following 
description of the preferred embodiments of the present invention taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawing, there is shown an alignment system according to 
one embodiment of the present invention, which is incorporated into a 
projection exposure apparatus, called a "stepper". 
In the drawing, denoted at 1 is a reticle (photomask) having formed thereon 
a circuit pattern (not shown) and alignment marks (not shown); at 2, a 
reduction projection lens system; and at 3, a wafer whose surface is 
coated with a resist material. When the circuit pattern formed on the 
reticle 1 is illuminatd by a photoprinting light supplied from an 
illumination system 10, the circuit pattern is projected by the projection 
lens system 3, having an optical axis AX, and is photoprinted on the wafer 
3. Wafer stage 4 is adapted to hold the wafer 3 thereon and also is 
adapted to be moved, by a driving system 9, in each of an X-axis 
direction, a Y-axis direction, a Z-axis direction and a .theta. 
(rotational) direction, as illustrated. The position of the wafer stage 4 
can be monitored by a well-known laser interferometer system, not shown. 
The reticle 1 is held by a reticle stage 12 which is adapted to be 
displaced, by a driving system (not shown), in each of the X, Y and 
.theta. directions. The reticle stage 12 may be arranged to be moved also 
in the Z-axis direction, as desired. It will be appreciated that the X, Y 
and .theta. directions are contained in a plane perpendicular to the 
optical axis AX of the projection lens system, while the Z direction is 
parallel to the optical axis AX. 
The projection lens system 3 used in the present embodiment provides an 
imaging system which is of a what is called "dual telecentric type" 
wherein it is telecentric both on the reticle 1 side (object side) and on 
the wafer 3 side (image side), and it forms in a reduced scale an image of 
a circuit pattern of the reticle 1 on the wafer 3. 
The illumination system 10 comprises a well-known type light source such 
as, for example, an Xe-Hg lamp or an excimer laser, as well as an optical 
system for directing the light from the light source to the reticle 1. 
This optical system may comprise an optical integrator such as disclosed 
in U.S. Pat. No. 4,497,013 assigned to the same assignee of the subject 
application, which integrator can function to illuminate the reticle 1 
surface with uniform illuminance. 
Light source device 5 operates to project a light beam inclinedly to the 
wafer 3, from between the projection lens system 2 and the wafer 3, as 
illustrated. The light reflected at this time inclinedly from the wafer 3 
surface is extracted from between the projection lens system 2 and the 
wafer 3 and is received by a position sensor 6. The position sensor 6 
produces an output signal which is changeable in accordance with the 
position of incidence of the reflected light upon a light receiving 
surface thereof. The light source device 5 and the position sensor 6 of 
the present embodiment cooperate to provide a detecting system for 
detecting the surface position of the wafer 3 in the Z direction. The 
surface position detecting system of the present embodiment may comprise a 
specific arrangement as disclosed in Japanese Laid-Open Patent 
Application, Laid-Open No. Sho62-140418. However, this is not restrictive 
but any other arrangement such as disclosed in U.S. Pat. Nos. 4,558,949, 
4,395,117 and 4,600,282, for example, may be used. 
Piezoelectric device 7 is provided in the wafer stage 4 and forms a driving 
mechanism for moving the wafer stage 4 in the Z direction along the 
optical axis of the projection lens system. This Z-axis driving mechanism 
is controlled by a controller 8 and the driving system 9, with the 
controller 8 processing the outputs of the surface position detecting 
system described above. Thus, the Z-axis driving mechanism operates to 
move the wafer 3 in the Z direction so as to place the wafer 3 just upon 
the focal surface of the projection lens system 2. 
There is provided an alignment mark position detecting system 11 which 
functions to detect the positional relation between an alignment mark on 
the reticle 1 and an alignment mark on the wafer 3. This detecting system 
comprises a laser light source (not shown) and a photodetector (not 
shown). The laser beam emanating from the laser light source irradiates a 
mirror 12, the reticle 1, the projection lens system 2 and the wafer 3 in 
the named order and, at the same time, scans the alignment marks of the 
reticle 1 and the wafer 3. The reflectively diffracted light from the 
alignment mark of the reticle 1 and the reflectively diffracted light from 
the alignment mark of the wafer 3, both caused at that time, is detected 
by the photodetector (not shown) with the aid of the projection lens 
system 2 and the mirror 12. The output signals of the photodetector are 
processed by the controller 8, whereby, on the basis of calculations, any 
relative positional deviation (comprising components .DELTA.X, .DELTA.Y 
and .DELTA..theta.) between the reticle 1 and the wafer 3 with respect to 
the X, Y and .theta. directions is detected. On the basis of the thus 
detected positional deviation, the driving system 9 operates to displace 
the wafer stage 4 in the X, Y and .theta. directions to thereby align the 
reticle 1 and the wafer 3 with each other. 
While only one set of an alignment mark position detecting system is 
illustrated at 11 in the drawing, actually the alignment system of the 
present embodiment includes another set of alignment mark position 
detecting system which is disposed symmetrically with the illustrated 
alignment mark position detecting system, with respect to the optical axis 
AX of the projection lens system 2. 
The alignment mark position detecting system 11 may comprise a specific 
arrangement such as disclosed in, for example, the aforementioned U.S. 
Pat. Nos. 4,251,129 and 4,406,546. 
As discussed in these patents, a quarter waveplate may be provided within 
the projection lens system 2 of the present embodiment while, on the other 
hand, a laser beam from the unshown laser light source may comprise a 
particular linearly polarized light. By doing so, and when the reticle 1 
and the wafer 3 are irradiated with such a laser beam, the light reflected 
from the reticle 1 and the light reflected from the wafer 3 can be 
separated from each other so that they can be detected by separate 
detecting systems. 
The alignment mark position detecting system 11 of the present embodiment 
utilizes what is called a "dark-field detection method" wherein 
diffraction light caused by an alignment mark of a reticle or a wafer is 
detected, as described hereinbefore. However, this is not restrictive. For 
example, a detection method wherein an image of an alignment mark of a 
reticle or a wafer is picked up by use of an image pickup device, such as 
a TV camera or otherwise, may be used. 
When, in operation of the alignment system of the present embodiment, any 
relative positional deviation between a reticle 1 and a wafer 3 is going 
to be detected, the wafer stage 4 is displaced in the direction of the 
optical axis AX of the projection lens system 2 (i.e. in the Z-axis 
direction) by a certain or predetermined amount .DELTA.Z, while taking the 
focus position of the projection lens system 2 as a reference position. 
Namely, the interval (relative distance) between the reticle 1 and the 
wafer 3 is enlarged or reduced by an amount .DELTA.Z. During such 
displacement, the position detecting operation by use of the alignment 
mark position detecting system 11 with regard to the alignment marks of 
the reticle 1 and the wafer 3 is repeatedly carried out through N cycles. 
More particularly, such repeated position detecting operations are 
executed at uniform or regular intervals, within the range of the 
displacement .DELTA.Z of the wafer stage 4. To assure this, an 
autofocusing mechanism which comprises the surface position detecting 
system (namely, the light source 5, the position sensor 6, the 
piezoelectric device 7, the controller 8 and the driving system 9) 
operates to control the position of the wafer stage 4 during the repeated 
position detecting operations. 
More specifically, one cycle of position detecting operation is made at a 
displacement .DELTA.Z/N from the focus position of the projection lens 
system 2; a second cycle of the position detecting operation is made at a 
displacement 2.DELTA.Z/N. In this manner, until the end of the 
displacement .DELTA.Z, the positional relation (deviation) between the 
alignment marks of the reticle 1 and the wafer 3 is measured repeatedly at 
different positions of the wafer stage 4. The measured values are 
sequentially and successively stored into a memory (not shown) within the 
controller 8. Then, the controller 8 operates to calculate an average of 
the measured values, obtained by the N cycles of position detecting 
operations, to thereby determine the positional relation between the 
alignment marks of the reticle 1 and the wafer 3. By this, relative 
positional deviation (.DELTA.X, .DELTA.Y and .DELTA..theta.) between the 
reticle 1 and the wafer 3 is determined and, on the basis of this, the 
driving system 9 actuates to adjust the position of the wafer stage 4. 
The alignment system of the present embodiment is set so as to satisfy the 
following relation, where the wavelength of the single-wavelength laser 
beam from the alignment mark position detecting system 11 is denoted by 
.lambda. and the amount of displacement of the wafer stage 4 is denoted by 
.DELTA.Z, as described above: 
EQU .DELTA.Z=n(.lambda./2) 
wherein n=1, 2, 3, ... 
Particulars will be described while taking an example where 
.DELTA.Z=.lambda./2. 
As described hereinbefore, the change in the state of interference fringe 
upon the light receiving surface of a photodetector, which change is a 
cause of the incorrect detection of the alignment mark position, results 
from the change in the difference, in the optical path length, between the 
light reflected once by a wafer and a multireflected light from the wafer. 
Usually, it is not possible to maintain the wafer stage 4 and the reticle 1 
completely stationary, and the relative distance between the reticle and 
the wafer is continuously changing by a minute amount. Accordingly, the 
optical path difference which determines the state of the interference 
fringe is also changing. 
Even where the interval between the reticle 1 and the wafer 3 is set at 
such distance that satisfies the optimum exposure conditions, there is no 
assurance that exactly the same distance is maintained at each of 
different sites of different shot areas on a wafer 3 if the wafer is used 
in a stepper by which the different shot areas thereof are exposed 
sequentially. This is because of the possible effect of the flatness of 
the wafer. Additionally, due to the variation in pressure or temperature, 
it is difficult to assure that a reticle 1 and a wafer 3 are continuously 
retained exactly at the same distance or interval. As a result, the state 
of the interference fringe is not constant but variable. 
In accordance with the alignment system of the present embodiment, in 
consideration thereof, the state of the interference fringe is positively 
changed so as to allow that the relative position of a reticle and a wafer 
is measured continually under the same interference condition. The 
principle is as follows: 
Assuming now that a signal light W.sub.i (a light reflected by "i" times) 
and a signal light W.sub.i+1 (a light reflected by "i+1" times), such as 
the aforementioned signal light W1 and the signal light W2, in an occasion 
where a reticle 1 and a wafer 3 are placed at an interval that satisfies 
the optimum exposure condition, have an optical path difference .DELTA.l 
and if the interval (optical path length) between the reticle and the 
wafer is enlarged by an amount .DELTA.Z=.lambda./2, then the difference in 
the optical path length between the signal light W.sub.i and the signal 
light W.sub.i+1 becomes equal to .DELTA.l' (=.DELTA.l+.lambda.). Thus, the 
state of the interference fringe is substantially the same as that 
established under the initially set condition. From this, it is seen that 
the state of interference fringe upon the light receiving surface of the 
photodetector, where it is considered as a periodic function, approximates 
to a function having a period of .DELTA.Z=.lambda./2. 
Actually, while changing the interval between a reticle 1 and a wafer 3 at 
a predetermined pitch, during one cycle of that periodicity, the 
measurement was made by N times (N.gtoreq.2) and an average of the 
measured values was obtained to determine the relative positional 
deviation .DELTA.X between the reticle 1 and the wafer 3. From the 
results, it has been confirmed that the detected deviation .DELTA.X 
constantly corresponds to a numerical value which can be obtained by 
measurement where the state of interference fringe is maintained constant. 
Also, it has been confirmed that the measured value changes periodically 
substantially along a sine curve, with a cycle .lambda./2. 
As described hereinbefore, the present embodiment utilizes the phenomenon 
of a sine curve change in the measured value which the inventors of the 
subject application have first found. More specifically, the wafer stage 4 
is displaced by an amount .DELTA.Z (=.lambda./2) and, within this range of 
.DELTA.Z=.lambda./2, the displacement is uniformly divided into portions 
of a number N so that the measurement is repeatedly carried out through N 
cycles and, thereafter, an average of measured values is determined. By 
doing so, the relative positional deviation .DELTA.X between the reticle 
and the wafer can be accurately detected, independently of the state of 
the interference fringe at the time prior to the initiation of the 
displacement of the wafer stage 4. 
By selecting, as the displacement .DELTA.Z (the amount of change in the 
optical path length), such a value that is within the range of the depth 
of focus of the alignment mark position detecting system, there is no 
possibility of deterioration in the mark position detecting accuracy due 
to the displacement of the stage. 
Also, where the displacement .DELTA.Z of the wafer stage 4 is set to 
satisfy ".DELTA.Z=n(.lambda./2)" and the number N of the measurement 
cycles is suitably set to satisfy "N&gt;n", the positional deviation .DELTA.X 
between the reticle 1 and the wafer 3 can be detected at higher precision. 
Further, the relationship between the speed of displacement of the wafer 
stage 4 and the timing of measurement by the alignment mark position 
detecting system 11 may be determined, for example, in the following 
manner: 
Assuming now that, with a particular shot area on a wafer 3, the wafer 3 
and a reticle 1 are set at an optimum interval by which the wafer 3 is 
placed at the focus position of a projection lens system 1 and if the 
wafer stage 4 is thereafter to be displaced upwardly or downwardly at a 
constant speed V through a distance .DELTA.Z=n(.lambda./2), then the 
actuation of the piezoelectric device for the movement of the wafer stage 
4 may be controlled so that the speed V satisfies the following relation: 
EQU V=n(.lambda./2)(1/N.DELTA.t) 
wherein N is the number of measurements and .DELTA.t is a required time 
interval between successive measurements. By controlling the displacement 
while monitoring the position of the wafer 3 by use of the above-described 
autofocusing mechanism, the measurements of the number N can be made 
exactly. Accordingly, high-accuracy alignment is attainable. 
One important feature of the present invention lies in the detection of an 
alignment mark while changing the interval between a reticle and a wafer. 
Within this scope of the present invention, the described embdiment may be 
modified in various ways. For example, the displacement of the wafer stage 
4 in the described embodiment may be replaced by displacement of the 
reticle stage 12 in the direction of the optical axis or, alternatively, 
may be replaced by the combination of the reticle stage 12 displacement 
and the wafer stage 4 displacement. Substantially the same effect is 
attainable in these cases. 
Further, the foregoing description has been made to a case where 
measurements of a number N are made while moving the wafer stage 4 at a 
constant speed, this is not restrictive. A possible alternative is that a 
measured value X.sub.1 obtained at a certain reticle-to-wafer interval and 
another measured value X.sub.2 obtained at another reticle-to-wafer 
interval which is established upon completion of the displacement of the 
reticle or wafer through a distance .DELTA.Z=P.lambda./4 (where P is an 
odd number), are averaged to determine the positional deviation .DELTA.X 
between the reticle and the wafer. Even by doing so, the deviation can be 
correctly measured, independently of the state of interference fringe. 
Accordingly, high-accuracy alignment is attainable, also in this case. 
Moreover, it is not always necessary to change the reticle-to-wafer 
interval at a constant speed by the constant-speed movement of the wafer 
stage 4 or otherwise. Namely, for example, while monitoring the current 
position of the wafer stage 4 by use of the above-described autofocusing 
mechanism and by controlling the displacement of the wafer stage 4, an 
"m-th" measurement among the measurements of a number N (wherein N&gt;m) may 
be executed at the displacement of m(.DELTA.Z/N). 
In accordance with the present invention, as has hitherto been described, 
the positional relation between a first object such as a reticle or 
photomask and a second object such as a wafer is detected by repeatedly 
executing the measurement of the position of alignment marks while 
changing the interval (optical path length) between the first and second 
objects. By doing so, the positional relation (deviation) between the 
alignment marks of the first and second objects can be measured 
accurately, without being affected by multireflection of light caused 
between the first and second objects. Accordingly, the first and second 
objects can be aligned very accurately. 
While the invention has been described with reference to the structures 
disclosed herein, it is not confined to the details set forth and this 
application is intended to cover such modifications or changes as may come 
within the purposes of the improvements or the scope of the following 
claims.