Displacement detecting system, an expose apparatus, and a device manufacturing method employing a scale whose displacement is detected by a selected detection head

A displacement detecting system includes a scale provided on a surface of a movable object and having a diffraction grating formed along a predetermined direction, a head unit disposed above the surface of the movable object and having a plurality of detection heads, for detecting displacement of the scale in the predetermined direction, the detection heads being disposed along a direction different from the predetermined direction, and a selecting device for selecting at least one detection head out of the detection heads, for detection of a displacement of the scale in the predetermined direction.

FIELD OF THE INVENTION AND RELATED ART 
This invention relates to a displacement detecting system for detecting the 
amount of displacement of a movable member along a two-dimensional plane. 
According to another aspect; the invention is concerned with an exposure 
apparatus having such displacement detecting system, and with a device 
manufacturing method which uses such exposure apparatus. 
The manufacture of semiconductor devices such as ICs or liquid crystal 
displays includes a process of transferring a circuit pattern formed on an 
original, such as a reticle or photomask, onto a resist coated wafer by 
means of a semiconductor exposure apparatus. 
As such exposure apparatus, step-and-repeat type exposure apparatuses or 
step-and-scan type exposure apparatuses having a printing chip size as 
.sqroot.2 times larger, are well known. 
FIG. 1 is a schematic side view of a known type step-and-repeat 
semiconductor exposure apparatus. As illustrated in FIG. 1, the apparatus 
comprises a moving mechanism having an X stage 1007 and a Y stage 1006. 
The moving mechanism is provided with a mirror support 1021 for supporting 
a measurement light path mirror 1023, to be described below. The mirror 
supporting member 1021 supports a bearing means 1010 through a tilt 
mechanism 1008. Mounted on the bearing means 1010 is a wafer chuck 1005 
for holding a wafer 1004 on its top surface. The wafer chuck 1005 is 
supported movably along a Z axis, through a fine Z motion mechanism 1009. 
Thus, the wafer chuck 1005 is movable along the X and Y directions, and 
also its position with respect to the Z direction as well as its tilt with 
respect to the X-Y plane can be adjusted minutely. 
Disposed above the wafer chuck 1005 are an original 1003 having a circuit 
pattern formed thereon placed opposed to the wafer 1004, a light source 
1001 for projecting the circuit pattern of the original 1003 onto the 
wafer 1004, and a barrel 1002 having a reduction projection optical 
system, for projecting an optical image, formed by the passage of light 
from the light source 1001 through the original 1003, onto the wafer 1004 
in a reduced scale. 
Laser distance measuring device 1020 is a component of a displacement 
detecting system for detecting the relative positional relationship 
between the mirror support 1021 and the barrel 1002 along the X-Y plane. 
The laser distance measuring device 1020 includes a measurement light path 
mirror 1023 supported by the mirror support 1021, a laser light source 
1022 for projecting laser light to the measurement light path mirror 1023, 
a reference light path mirror 1024 mounted on the barrel 1022, a half 
mirror 1025 for dividing the laser light from the laser light source 1022 
into measurement light and reference light, and a light path vending 
mirror for deflecting the reference light divided by the half mirror 1025 
toward the reference light path mirror 1024. 
The laser light projected by the laser light source 1022 is divided by the 
half mirror 1025 into the reference light and the measurement light, which 
are then reflected by the reference light path mirror 1024 and the 
measurement light path mirror 1023, respectively, back to the laser light 
source 1022. By comparing the phases of these lights, the position of the 
mirror support 1021 relative to the barrel 1022 is detected. 
For performing the exposure process, first a wafer 1004 coated with a 
resist is fixed to the wafer chuck 1005, and, by means of the tilt 
mechanism 1008 and/or the fine Z motion mechanism 1009, the inclination of 
the wafer surface and/or the position thereof with respect to the Z 
direction is adjusted. Then, the light source 1001 is turned on such that 
the circuit pattern as a whole of the original 1003 is projected at once 
in a reduced scale onto the wafer 1004, whereby the circuit pattern is 
printed on the wafer. Subsequently, the X stage 1007 and the Y stage 1006 
are moved stepwise by a predetermined distance to move and change the 
exposure region, while measuring the position of the mirror support 1021 
through the laser distance measuring device 1022. Then, the light source 
1001 is turned on again, and the exposure operation is performed again. 
This process is repeated, and finally, circuit patterns are printed on the 
whole surface of the wafer 1004. 
As compared therewith, the step-and-scan process is a process in which one 
circuit pattern is divided into plural blocks which are scanned 
sequentially in a timed relation with a wafer for exposure of it, rather 
than exposing the whole circuit pattern at once. To this end, a plate 
member having a slit formed therein is disposed between a light source and 
an original, and the original is made movable along a direction 
perpendicular to the lengthwise direction of the slit. Only a slit-like 
portion of the circuit pattern as illuminated is projected on a wafer and, 
simultaneously therewith, the original and the wafer are scanningly moved 
in a timed relation in a direction perpendicular to the lengthwise 
direction of the slit, such that the circuit pattern as a whole, formed on 
the original, is printed on the wafer. After this, the stage is moved 
stepwise by a predetermined distance to move and change the exposure 
region, and the above-described exposure operation is repeated. This is 
essentially the same as with the step-and-repeat exposure process. 
A large scaled integrated circuit (LSI) as represented by a DRAM, for 
example, is manufactured generally with processes of a number not less 
than twenty (20). In these processes, a technique is necessary for 
overlaying a circuit pattern of a current process upon a circuit pattern 
formed through a preceding process. An automatic alignment process is a 
process to this end, for automatically aligning an optical image of an 
original formed by a reduction optical system with a circuit pattern 
formed on a wafer. 
In the first process, as a circuit pattern is printed on a resist of a 
wafer, alignment marks to be used for the alignment operation are printed 
on the wafer together with the circuit pattern, in an outside (finally 
unnecessary) area of the circuit pattern, which area is called a scribe 
line. Then, through chemical and heat treatments, a predetermined 
structure is produced thereat. This is the process to be performed 
initially. Subsequently, a resist is applied again to this wafer, and the 
wafer is placed on a wafer stage. Then, while moving the stage, the 
position of an alignment mark is detected by means of an alignment mark 
detecting system having a TV camera, for example, and the stage is 
positioned. The position of the mark is then measured by means of a laser 
distance measuring device for measuring the position of the wafer stage. 
This operation is performed with respect to each alignment mark. The 
results are then statistically processed, and distortion of the wafer is 
calculated and the position to which the wafer stage is to be moved is 
determined. Then, while relying on the amount of movement of the laser 
distance measuring device, the wafer stage is moved to the thus determined 
position, for the position for exposure of a next circuit pattern, and 
then the exposure operation is performed. As described, it is very 
important for exposure of a wafer to detect the wafer position accurately. 
On the other hand, as a displacement detecting system for detecting the 
amount of movement of a movable member very precisely, there is a 
displacement detecting system based on the principle of diffraction 
grating interference. An example is disclosed in Japanese Laid-Open Patent 
Application, Laid-Open No. 215515/1993, which will be explained below with 
reference to FIG. 2. 
In FIG. 2, a semiconductor laser 211 is disposed at a predetermined 
distance from a movable member 222 having a diffraction grating 216 is 
provided on its surface. There are two mirrors 214a and 214b for 
deflecting coherent light, produced from an active layer of the 
semiconductor laser 211, toward one point in a space (on the diffraction 
grating 216), as well as detecting means 215 for receiving diffraction 
light from the diffraction grating 216. The surface of the active layer 
212 is placed substantially in parallel to the surface of the diffraction 
grating 216, and the light receiving surface of the detecting means 215 is 
placed substantially in parallel to the surface of the active layer 212. 
These optical components are disposed so that two laser lights 231a and 
231b from the mirrors 214a and 214b intersect with each other within a 
plane perpendicular to the surface of the active layer 212. 
When in the structure described above the active layer 212 of the 
semiconductor laser 211 is excited, photons are produced and some of them 
repeatedly go back and forth within the active layer 212. During this 
process, light amplification occurs and a portion goes through a resonator 
213a and emits light as laser light. The laser light is emitted from the 
opposite sides of the active layer 212, and emitted rays are reflected by 
the mirrors 214a and 214b toward a single point in space where the 
diffraction grating 216 is disposed. Since the surface of the active layer 
212 is placed parallel to the diffraction grating 216, the occupied space 
in the direction perpendicular to the plane on which the light is incident 
is small. Here, the light expands largely in a plane perpendicular to the 
thickness of the active layer, that is, in a plane parallel to the sheet 
of the drawing. As a result, the light is not incident on the outside of 
the diffraction grating 216 but is efficiently incident on the diffraction 
grating 216 and is diffracted thereby. Consequently, the diffraction light 
(signal light) increases, which in turn leads to an increase in light 
impinging on the light receiving means. Thus, the light reception 
efficiency increases and the signal-to-noise ratio (S/N ratio) increases, 
such that the detection precision is enhanced. 
As the light reflected by the mirror 214a or 214b is projected on the 
diffraction grating 216, the reflected light impinges on the diffraction 
grating so that m-th order diffraction light (m is an integer) from the 
diffraction grating 216, to be detected, is reflected therefrom 
substantially perpendicularly to the diffraction grating 216. Namely, if 
the pitch of the diffraction grating 216 is P, the wavelength of the 
coherent light is .lambda., and the incidence angle of the coherent light 
upon the diffraction grating 216 is .theta..sub.m, then the light is 
projected on the diffraction grating to satisfy the following relation: 
EQU .theta..sub.m .apprxeq.sin.sup.-1 (m.lambda./P) 
Also, two diffraction light beam of positive and negative m-th orders, 
being substantially perpendicularly projected from the diffraction grating 
216, are superposed one upon another and are projected on the light 
receiving means 215. The detecting means 215 detects the thus interfering 
lights. More specifically, the detecting means 215 detects the number of 
brightness/darkness patterns (pulses) of the interference fringe which 
corresponds to the movement state of the diffraction grating 216. Here, 
the light receiving surface of the detecting means 215 is placed 
perpendicularly to the light impinging thereon, to enhance the light 
reception efficiency. 
Higher and higher precision has been required for the alignment between an 
optical image of an original and a wafer, in consideration of further 
increases in the density and integration of each semiconductor device. The 
currently required alignment precision is 0.05 micron, for example. On the 
other hand, since a laser distance measuring device is based on the 
wavelength of a laser propagated within an air, there is a problem of 
instability of a measured value of about 0.03 micron due to any 
environmental disturbance, such as the fluctuation of air or the 
non-uniformness of temperature or pressure, for example. 
There might be an exposure apparatus having a displacement detecting system 
such as shown in FIG. 2 for measuring the amount of movement of a stage 
without using a laser distance measuring device. More specifically, as 
shown in FIG. 3, a two-dimensional grating scale 1122 comprising an X-axis 
and Y-axis two-dimensional diffraction grating may be provided on the 
surface of a wafer chuck 1105 and, also, four heads 1121 each having a 
semiconductor laser similar to that used in the example of FIG. 2 are 
mounted on the lower end portion of a barrel 1102, a pair being set in the 
X direction and another pair being set in the Y direction, so as to detect 
the amount of displacement of the wafer chuck 1105 along the X-Y plane. In 
this structure, the distance along which the laser goes through the air 
becomes shorter. Therefore, with this structure, the effect of any 
disturbance of the air may be reduced and stable measurement may be 
assured. 
However, if such a displacement detecting system is used in an exposure 
apparatus, for detecting displacement of a wafer, it will be necessary 
that in consideration of the movement amount of a wafer or the movement 
precision thereof, a two-dimensional grating scale has a size of about 150 
mm.times.150 mm (X- and Y-direction lengths) and a diffraction grating 
pitch of 2 microns both in the X direction and the Y direction. It is 
practically very difficult to prepare such a large size two-dimensional 
grating scale. Also, a large size two-dimensional grating will be fragile, 
and will be difficult to handle. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide a 
displacement detecting system capable of detecting the amount of movement 
along two-dimensional directions correctly, without use of a laser 
distance measuring device or a two-dimensional diffraction grating. 
It is another object of the present invention to provide an exposure 
apparatus in which, by use of such a displacement detecting system, a 
wafer can be positioned very precisely. 
It is a further object of the present invention to provide a device 
manufacturing method which uses such an exposure apparatus. 
In accordance with an aspect of the present invention, to achieve at least 
one of these objects, there is provided a displacement detecting system, 
comprising: a scale provided on a surface of a movable object and having a 
diffraction grating formed along a predetermined direction; a head unit 
disposed above the surface of the movable object and having a plurality of 
detection heads, for detecting displacement of the scale in the 
predetermined direction, the detection heads being disposed along a 
direction different from said predetermined direction; and selecting means 
for selecting at least one detection head out of the detection heads, for 
detection of a displacement of the scale in the predetermined direction. 
In one preferred form of this aspect of the present invention, the width of 
the diffraction grating in the direction along which the detection heads 
are disposed, may correspond to the width opposed by at least two 
detection heads of the detection heads of the head unit. 
In this or another preferred form of this aspect of the present invention, 
each of the detection heads may comprise a light source for providing a 
coherent light, projecting means for projecting the coherent light from 
the light source onto the diffraction grating, and detecting means for 
detecting interference light produced as a result of the interference of 
diffraction light of predetermined orders having been diffracted by said 
diffraction grating. 
In accordance with another aspect of the present invention, there is 
provided an exposure apparatus for printing circuit patterns on a wafer 
sequentially while moving the wafer, the apparatus comprising: a movable 
stage on which the wafer is to be placed; a scale provided in a portion of 
a surface of the movable stage, other than a portion on which the wafer is 
to be placed, the scale having a diffraction grating formed along a 
predetermined direction; a head unit disposed above the movable stage and 
having a plurality of detection heads, for detecting displacement of the 
scale in the predetermined direction, the detection heads being disposed 
along a direction different from the predetermined direction; and 
selecting means for selecting at least one detection head out of the 
detection heads, for detection of a displacement of the scale in the 
predetermined direction. 
In one preferred form of this aspect of the present invention, the width of 
the diffraction grating in the direction along which the detection heads 
are disposed, may correspond to the width opposed by at least two 
detection heads of the detection heads of the head unit. 
In this or another preferred form of this aspect of the present invention, 
each of the detection heads may comprise a light source for providing a 
coherent light, projecting means for projecting the coherent light from 
the light source onto the diffraction grating, and detecting means for 
detecting interference light produced as a result of the interference of 
diffraction light of predetermined orders having been diffracted by the 
diffraction grating. 
In accordance with a further aspect of the present invention, there is 
provided a device manufacturing method wherein circuit patterns are 
printed on a wafer, coated with a photosensitive material, sequentially 
while moving the wafer, the method comprising the steps of: placing the 
wafer on a movable stage; and detecting displacement of a scale in a 
predetermined direction; wherein the scale is formed on the movable stage 
at a position different from the wafer on the movable stage, and has a 
diffraction grating formed along the predetermined direction; and wherein 
the detecting step includes selecting at least one of detection heads of a 
head unit, disposed above the movable stage, for detecting displacement of 
the scale in the predetermined direction, the detection heads being 
disposed along a direction different from the predetermined direction. 
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 
Preferred embodiments of the present invention will be described with 
reference to the drawings. 
[First Embodiment] 
FIG. 4 is a schematic side view of a displacement detecting system 
according to a first embodiment of the present invention, which is 
incorporated into an exposure apparatus. FIG. 5 is a perspective view, 
schematically showing a main portion of the exposure apparatus of FIG. 4. 
The exposure apparatus of this embodiment is of step-and-scan type, and it 
comprises a movement mechanism having an X stage 7 movable in the X 
direction and a Y stage 6 movable in the Y direction. Also, it has a 
bearing means 10 which is supported by a tilt mechanism 8 for tilt 
adjustment with respect to the X-Y plane. Wafer chuck 5 for holding a 
wafer 4 thereon is supported by this bearing means 10 through a fine Z 
motion mechanism 9 which is expandable and contractible in the Z 
direction. Thus, the wafer chuck 5 provides a movable member whose 
position with respect to the X, Y and Z directions as well as its tilting 
with respect to the X-Y plane are adjustable. 
Disposed above the wafer 4 held by the wafer chuck 4 is an original 3 
having a circuit pattern formed thereon, which pattern is to be 
transferred to the wafer 4. The original 3 is held by an original holder 
11 which is movable in the Y direction. Disposed above the original 3 is a 
light source 1. Disposed between the original 3 and the wafer chuck 5 is a 
barrel (stationary member) 2 including a reduction optical system for 
projecting an optical image of the circuit pattern of the original 3, as 
formed by the passage of light from the light source 1 through the 
original 3, onto the wafer 4 at a reduced scale. 
Original scales 24 each having a one-dimensional diffraction grating are 
fixedly mounted onto the opposite end faces of the original holder 11 
which faces are parallel to the Y direction. Also, fixedly mounted at the 
positions on the top end face of the barrel 2 opposed to the original 
scales 24, are original heads 23 each for detecting the amount of movement 
of the original holder 11 relative to the barrel 2 on the basis of the 
interference of light at the diffraction grating of corresponding one of a 
the original scales 24. The structure of the original scales 24 and the 
original heads 23 as well as the principle of displacement detection by 
them are similar to those of the displacement detecting system of FIG. 2. 
Therefore, an explanation of them will be omitted here. 
Disposed between the light source 1 and the original 3 is a plate member 18 
having a slit 18a extending along the X direction. 
Mounted on the bottom face of the barrel 2 are four displacement sensors 
25. These displacement sensors 25 are operable to measure the distance 
between the wafer 4 as held by the wafer chuck 5 and the lower end of the 
barrel 2 in accordance with the principle of triangular measurement to 
thereby determine the position of the wafer 4 in the Z direction. Here, 
the details of the displacement sensor 25 will be explained with reference 
to FIGS. 6 and 7. 
As best seen in FIG. 6, on a base 251 made of silicone, for example, there 
are a detecting means 252 comprising a CCD line sensor, a light source 
means 255 comprising a semiconductor laser, an optical member 254 which 
serves to collect a divergent light beam 255a (see FIG. 7) emitted from 
the light source means 255, with respect to the Z direction and which also 
serves to diverge in the same manner in the Y direction so that it 
provides a linear light beam elongated in the Y direction, and a mirror 
253 for reflecting the light from the optical member 254 and for 
projecting the same obliquely upon the wafer 4 to be measured (see FIG. 
7), with a predetermined angle, as a light spot. These components are 
formed on the base 251 integrally therewith. 
In the structure described above, as shown in FIG. 7, the divergent light 
255a emitted by the light source means 255 is transformed by the optical 
member 254 into a linear light beam elongating in the Y direction. This 
light is reflected by the mirror 253, and the reflected light impinges on 
the wafer 4. The light is then reflected by the wafer 4, and the reflected 
light impinges on the detecting means 252. The detecting means 252 serves 
to detect the light quantity gravity center position of the incident light 
255c, and to determine the position coordinates of the incidence position 
of the light upon the detecting means 252. The optical arrangement is so 
set that, when the spacing between the wafer 4 and the position sensor 25 
is at a predetermined level, the light impinges on a predetermined 
position upon the detecting means 252. Thus, by detecting the position 
coordinates of the incidence position of the light upon the detecting 
means 252, the spacing to the wafer 4 is detected. 
Referring back to FIGS. 4 and 5, four wafer head units 21a, 21b, 21c and 
21d are mounted on the outer circumferential surface at the lower end of 
the barrel 2. Fixedly mounted on the top face of the wafer chuck 5, around 
the wafer 4, are corresponding four wafer scales 22a, 22b, 22c and 22d. 
More specifically, as best seen in FIG. 8A, of the wafer head units 21a, 
21b, 21c and 21d, two wafer head units 21a and 21b have their elongating 
directions parallel to the X direction, and they are disposed 
diametrically opposed to each other with respect to the central axis of 
the barrel 2. The remaining two wafer head units 21c and 21d have their 
elongating directions parallel to the Y direction, and they are disposed 
diametrically opposed to each other with respect to the central axis of 
the barrel 2. 
Also, as best seen in FIG. 8B, each of the wafer scales 22a, 22b, 22c and 
22d has a size of about 150 mm.times.150 mm (in length and width), and 
each wafer scale has a one-dimensional diffraction grating which is 
defined by forming, on a quartz substrate, short straight nickel chrome 
patterns of 1-micron widths, arrayed in the scale lengthwise direction at 
a pitch of 2 microns. Of these wafer scales 22a, 22b, 22c and 22d, two 
wafer scales 22a and 22b correspond to the wafer head units 21a and 21b 
having their elongating directions parallel to the X direction, and these 
wafer scales 22a and 22b are fixed so that their elongating directions are 
parallel to the Y direction. The other two one-dimensional wafer scales 
22c and 22d correspond to the wafer head units 21c and 21d having 
elongating directions parallel to the Y direction, and these wafer scales 
22c and 22d are fixed so that their elongating directions are parallel to 
the X direction. Also, each wafer scale has its surface placed at 
substantially the same level as the surface of the wafer or the surface of 
the wafer chuck with respect to the Z direction. 
Details of the wafer head units 21a, 21b, 21c and 21d will be explained, 
while taking one wafer head unit (21a) as an example. 
FIG. 9 is a perspective view of the wafer head unit 21a, as viewed from 
below. FIG. 9 also depicts the relation of the wafer head unit 21a with 
the corresponding wafer scale 22a. The wafer head unit 21a comprises, as 
shown in FIG. 8A or 8B, a number of element heads 210 being juxtaposed and 
adjoined in the lengthwise direction of the wafer head unit. At least two 
of the element heads 210 face to the wafer scale 21a, constantly. In other 
words, the wafer scale 22a has such a width that it is opposed to at least 
two element heads 210. Each of the element heads 210 has a similar 
structure as the semiconductor laser having been described with reference 
to FIG. 2. Also, the principle of displacement detection by using these 
element heads 210 is similar to that having been described with reference 
to FIG. 2. Therefore, a description of them will be omitted here. 
Two wafer head units 21a and 21b having lengthwise directions parallel to 
the X direction, serve to detect the amount of movement of the wafer chuck 
5 in the Y direction. The other two head units 21c and 21d having 
lengthwise directions parallel to the Y direction, serve to detect the 
amount of movement of the wafer chuck 5 in the X direction. It is to be 
noted that, while in this embodiment these two pairs of wafer head units 
(and thus the two pairs of wafer scales) are elongated along orthogonal 
directions (X and Y directions), this is not always necessary. What is 
necessary is that the two pairs of the wafer head units (or wafer scales) 
extend along different directions. 
FIG. 10 illustrates the structure of control means 30 for controlling the 
wafer head unit 21a. As described hereinbefore, the wafer head unit 21a 
comprises a plurality of element heads 210, and the outputs of these 
element heads 210 are applied through an array relay switch 31 to a head 
controller 32. The head controller 32 serves to discriminate one or those 
of the element heads 210 which is or are operable to perform the 
detection, on the basis of the light quantity of the reflected light, of 
the light directed to the one-dimensional diffraction grating of the wafer 
scale 22a (see FIG. 8A or 8B). Here, the head controller 210 signals a 
relay switch controller 33 so as to fetch outputs of at least two element 
heads 210 simultaneously. In response to the signal from the head 
controller 32, the relay switch controller 33 operates to connect the 
relays for at least two of the element heads 210 which are to be made 
active, and supplies to the head controller 32 a dual phase interference 
signal, which includes the direction. 
It will be understood from the foregoing description that the displacement 
detecting system of this embodiment of the present invention is provided 
by the wafer head units 21a, 21b, 21c and 21d, the wafer scales 22a, 22b, 
22c and 22d, and the control means 30. 
Next, the operation of the exposure apparatus of according to this 
embodiment will be explained. 
First, a wafer 4 coated with a resist is placed and fixed onto the wafer 
chuck 5, and by using the displacement sensors 25 the position of the 
wafer 4 in the Z direction is detected. Here, the position or tilt of the 
wafer 4 deviates from a predetermined position, and the tilt mechanism 8 
and/or the fine Z motion mechanism 9 is used to adjust the position of the 
wafer chuck 5, toward the predetermined position. 
Subsequently, a circuit pattern of an original 3 is projected onto the 
wafer 4 at a reduced scale, so as to print the circuit pattern on the 
wafer. Here, the light from the light source 3 is projected onto the 
original 3 through the slit 18a and, therefore, a portion of the circuit 
pattern of the original 3 is projected, in a slit-like form, onto the 
wafer. Simultaneously with this exposure of the wafer 4, the original 3 
and the wafer 4 are moved in a timed relation with a speed difference 
corresponding to the reduction magnification of the reduction optical 
system of the barrel 2, by which the whole of one original 3 is printed on 
the wafer 4, in a region corresponding to one chip. Thereafter, the X 
stage 7 and the Y stage 8 are moved stepwise through a predetermined 
distance to shift the exposure region on the wafer 4, and the 
above-described exposure operation is repeated. This is essentially the 
same as that of the step-and-repeat exposure process. 
The movement of the original 3 is performed by moving the original holder 
11 while detecting diffraction light from the diffraction grating of the 
original scale 24 by use of the original head 23. As regards the scan of 
the wafer 4, although details will be explained later, the principle 
thereof is basically the same as the scan of the original 3. 
FIG. 11 is a perspective view for explaining the positional relationship 
between the wafer head units and the wafer scales during the exposure 
operation of the exposure apparatus shown in FIG. 4. 
In FIG. 11, as the wafer chuck 5 moves in the negative Y direction, the 
wafer scales 22a, 22b, 22c and 22d also move in the negative Y direction. 
Here, of the wafer head units 21a, 21b, 21c and 22d, those (head units 21a 
and 21d) which are overlying associated wafer scales operate to read the 
amount of movement of these wafer scales 22a and 21d. Here, the element 
heads of one wafer head unit 21a operate to detect the amount of movement 
of the wafer scale 22a along the Y direction. As regards the other wafer 
head unit 21d, since the wafer scale 22d moves along the Y direction, it 
operates so that, when one element head and an adjacent element head 
provides detected values at the same time, the detected values with 
respect to the X direction are held while connecting the detected values 
sequentially, to prevent getting off the scale and to avoid the resultant 
discontinuity of detected values. 
As the wafer chuck moves further in the negative Y direction, the wafer 
head units 21b and 21c overlie the wafer scales 22c and 22d, while on the 
other hand the wafer head units 21a and 21b go out of overlying the wafer 
scales 22a and 22d. Thus, when the detected value of the wafer head unit 
21c and the detected value of the wafer head unit 21c (having operated 
hitherto) take simultaneous values, these detected values are connected, 
such that a wide detection stroke of the stage is assured. 
The amount of movement of the wafer chuck in the X and/or Y direction is 
detected on the basis of the principle of diffraction grating 
interference, as described hereinbefore. Therefore, instability of a 
detected value due to any environmental change as in the case where a 
laser distance measuring device is used for the detection, is avoided. 
Thus, the position of the wafer chuck 5 is detected with good precision. 
Also, since a used diffraction grating is a one-dimensional diffraction 
grating, the manufacture of the same and the handling of the same are 
quite easy. 
While the foregoing description has been made with reference to a 
step-and-scan type exposure apparatus, the invention is applicable also to 
a step-and-repeat type exposure apparatus: in such case the plate member 
18, the original head 23 and the original scale 24 are not provided, and 
the original 3 is held fixed such that the circuit pattern of the original 
3 is printed at once. 
[Second Embodiment] 
FIG. 12 is a schematic perspective view of a second embodiment, wherein a 
displacement detecting system of the present invention is incorporated 
into an exposure apparatus. FIG. 13 is a schematic view for explaining the 
positional relation among wafer scales of the exposure apparatus of FIG. 
12 as well as the positional relation among wafer head units of the 
exposure apparatus. 
In this embodiment, a plate member is fixedly attached to the lower end 
portion of a barrel 52, and four wafer scales 72a, 72b, 72c and 72d are 
fixedly mounted on this plate member. Also, fixedly mounted on the top 
surface of a wafer chuck 55, around a wafer 54, are four wafer head units 
71a, 71b, 71c and 71d. The structure and disposition of these wafer scales 
72a, 72b, 72c and 72d and of these wafer head units 71a, 71b, 71c and 71d 
are similar to those of the first embodiment. Further, the structure of 
the remaining portion of the exposure apparatus is similar to that of the 
first embodiment. Therefore, a description of them will be omitted here. 
In this embodiment as described above, the wafer head units 71a, 71b, 71c 
and 71d are provided on the wafer chuck 55. On the other hand, the wafer 
scales 72a, 72b, 72c and 72d which are light as compared with the wafer 
head units 71a, 71b, 71c and 71d, are provided on the barrel 52. This 
allows simplification of the holding mechanism to the barrel 52, and it 
enables stable detection. 
Next, an embodiment of device manufacturing method which uses any one of 
the exposure apparatuses described hereinbefore, will be explained. 
FIG. 14 is a flow chart of the sequence of manufacturing a semiconductor 
device such as a semiconductor chip (e.g. IC or LSI), a liquid crystal 
panel or a CCD, for example. Step 171 is a design process for designing 
the circuit of a semiconductor device. Step 172 is a process for 
manufacturing a mask on the basis of the circuit pattern design. Step 173 
is a process for manufacturing a wafer by using a material such as 
silicon. 
Step 174 is a wafer process which is called a pre-process wherein, by using 
the so prepared mask and wafer, circuits are practically formed on the 
wafer through lithography. Step 175 subsequent to this is an assembling 
step which is called a post-process wherein the wafer processed by step 
174 is formed into semiconductor chips. This step includes assembling 
(dicing and bonding) and packaging (chip sealing). Step 176 is an 
inspection step wherein an operability check, a durability check and so on 
of the semiconductor devices produced by step 175 are carried out. With 
these processes, semiconductor devices are finished and they are shipped 
(step 177). 
FIG. 15 is a flow chart showing details of the wafer process. Step 181 is 
an oxidation process for oxidizing the surface of a wafer. Step 182 is a 
CVD process for forming an insulating film on the wafer surface. Step 183 
is an electrode forming process for forming electrodes on the wafer by 
vapor deposition. Step 184 is an ion implanting process for implanting 
ions to the wafer. Step 185 is a resist process for applying a resist 
(photosensitive material) to the wafer. Step 186 is an exposure process 
for printing, by exposure, the circuit pattern of the mask on the wafer 
through the exposure apparatus described above. Step 187 is a developing 
process for developing the exposed wafer. Step 188 is an etching process 
for removing portions other than the developed resist image. Step 189 is a 
resist separation process for separating the resist material remaining on 
the wafer after being subjected to the etching process. By repeating these 
processes, circuit patterns are superposedly formed on the wafer. 
The foregoing description has been made to embodiments wherein a 
displacement detecting system is incorporated into an exposure apparatus. 
However, the invention is not limited to this, but it is applicable also 
to a movable stage in a mechanical working machine such as a high 
precision milling machine, a lathing machine or an electron beam working 
machine, for example. 
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.