Focus detecting method and apparatus

A method of detecting a focus position of a projection optical system at a high throughput and with high accuracy. A sensor pattern (SP) which is provided on a wafer stage (WS) as a light-receiving part of a photoelectric sensor (PES) is moved in the direction of an optical axis (AX) of a projection optical system (PL), and at the same time, it is moved in a direction (X) perpendicular to the optical axis (AX). While doing so, a reticle pattern (RP) which is provided on a reticle (R) is illuminated by illuminating light (IL) for exposure, and an image of the reticle pattern (RP) is formed on the sensor pattern (SP) through the projection optical system (PL). Light passing through the sensor pattern (SP) is received by the photoelectric sensor (PES), and a focus position is detected on the basis of the intensity of the transmitted light.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of detecting a focus of a 
projection optical system. More particularly, the present invention 
relates to a focus detecting method which may be suitably applied to the 
detection of a focus position of a projection optical system which is 
attached to a projection exposure apparatus used to produce a 
semiconductor integrated circuit, a liquid crystal panel, an imaging 
device (CCD), a thin-film magnetic head, or a magneto-optical disc. 
2. Description of the Related Art 
A projection exposure apparatus which is used to produce a semiconductor 
integrated circuit or a liquid crystal panel, for example, needs to 
strictly align a photosensitive substrate with the imagery position (focus 
position) of a projection optical system. To comply with the need, there 
have heretofore been proposed various methods in which an image of a mask 
pattern is received with a photoelectric sensor, and a focus position is 
obtained on the basis of an output signal from the photoelectric sensor. 
The conventional methods may be roughly classified into two types. The 
first type of conventional focus position detecting method will be 
explained below with reference to FIGS. 9(a) and 9(b). 
FIGS. 9(a) and 9(b) show one example of conventional focus position 
detecting methods. FIG. 9(a) schematically shows the arrangement of a 
projection exposure apparatus, and FIG. 9(b) is a graph showing the 
waveform of an output signal from a photoelectric detector (photoelectric 
sensor). As shown in FIG. 9(a), a reticle R1, which serves as a mask, has 
a pattern (reticle pattern) RPA having a light-transmitting portion. The 
reticle pattern RPA is illuminated with a bundle of illuminating light 
rays IL1 from an illumination system (not shown). An image of the pattern 
RPA is projected through a projection optical system PL1 onto a 
light-transmitting sensor pattern SPA which is provided on a wafer stage 
WS1, on which a photosensitive substrate is mounted. A ray bundle passing 
through the sensor pattern SPA is incident on a photoelectric sensor PES1 
which is disposed directly below the sensor pattern SPA. In this example, 
the constituent elements have previously been positioned so that the image 
of the reticle pattern RPA which is projected by the projection optical 
system PL1 will be superimposed on the sensor pattern SPA. In addition, 
the constituent elements have been designed so that the projected image of 
the reticle pattern RPA and the sensor pattern SPA will substantially 
match each other in terms of shape and size. Here, a Z-axis is taken in a 
direction parallel to the optical axis of the projection optical system 
PL1, and an X-axis is taken in a direction parallel to the plane of FIG. 
9(a) within a plane perpendicular to the Z-axis. Further, a Y-axis is 
taken in a direction perpendicular to the plane of FIG. 9(a). 
In the above-described arrangement, if an output signal I from the 
photoelectric sensor PES1 is monitored with the wafer stage WS1 moved in 
the optical axis direction (Z-axis direction) of the projection optical 
system PL1, an output that is shown by the curve L1 in FIG. 9(b) is 
obtained. It should be noted that, in FIG. 9(b), the abscissa axis 
represents the position z in the direction Z, and the ordinate axis 
represents the output signal I of the photoelectric sensor PES1. When the 
projected image of the reticle pattern RPA and the sensor pattern SPA come 
into imagery relation to each other, that is, when the sensor pattern SPA 
comes at the focus position, almost all the ray bundle passing through the 
reticle pattern RPA passes through the sensor pattern SPA, and thus the 
output signal I of the photoelectric sensor PES1 reaches a maximum. As the 
distance from the focus position increases, the image of the reticle 
pattern RPA spreads on the sensor pattern SPA, resulting in a decrease in 
the quantity of light passing through the sensor pattern SPA. Accordingly, 
the output signal I from the photoelectric sensor PES1 decreases. Thus, 
the position BF1 in the optical direction of the sensor pattern SPA at 
which the output signal I reaches a maximum is detected as being a focus 
position. 
The second type of conventional focus position detecting method is 
disclosed, for example, in U.S. Pat. No. 4,629,313, or Japanese Patent 
Unexamined Publication (KOKAI) No. 4-211110. The second method will be 
explained below with reference to FIGS. 10(a) to 10(e). 
FIGS. 10(a) to 10(e) show another example of conventional focus position 
detecting methods. FIG. 10(a) schematically shows the arrangement of a 
projection exposure apparatus. FIGS. 10(b) to 10(d) respectively show the 
waveforms of outputs signals from a photoelectric sensor which are 
obtained when the position of the wafer stage WS1 in the optical axis 
direction is at three different points z.sub.1, z.sub.2 and z.sub.3. The 
projection exposure apparatus shown in FIG. 10(a) has an arrangement 
approximately similar to that of the projection exposure apparatus shown 
in FIG. 9(a). However, the arrangement in FIG. 10(a) differs from the 
arrangement in FIG. 9(a) in that the wafer stage WS1 is moved not in the 
direction Z but in a direction (direction X) which is perpendicular to the 
optical axis of the projection optical system PL1, and which is parallel 
to the plane of FIG. 10(a), and while doing so, the output signal I of the 
photoelectric sensor PES1 is monitored, and this operation is repeated 
with the position of the wafer stage WS1 in the optical axis direction 
varied to carry out measurement, thereby detecting a focus position. It 
should be noted that, in FIGS. 10(b) to 10(d), the abscissa axis 
represents the position x in the direction X, and the ordinate axis 
represents the output signal I of the photoelectric sensor PES1. 
As shown in FIGS. 10(b) to 10(d), at the positions z.sub.1 and z.sub.3 in 
the direction Z, the deviation from the focus position is relatively 
large, and the image of the reticle pattern RPA is out of the focus 
position. Therefore, the waveform of the output signal I is relatively 
gentle, as shown by the curves L2 and L4. On the other hand, the position 
z.sub.2 in the direction Z is close to the focus position, and the image 
of the reticle pattern RPA is therefore sharp at the position z.sub.2. 
Accordingly, the waveform of the output signal I draws a curve of good 
contrast, as shown in by the curve L3 in FIG. 10(c). Differences between 
the maximum and minimum values of the output signals in FIGS. 10(b) to 
10(d) are assumed to be C.sub.1 to C.sub.3, respectively. 
FIG. 10(e) is a graph showing the relationship between the position z in 
the optical axis direction and the differences C.sub.1 to C.sub.3 defined 
as contrast C. The abscissa axis represents the position z in the 
direction Z, and the ordinate axis represents the contrast C. As shown by 
the curve L5 in FIG. 10(e), the contrast C draws a gentle curve which 
reaches a maximum at the position BF2 in the direction Z. In this case, 
the position BF2 at which the contrast is the highest is detected as being 
a focus position. It should be noted that there is another method in which 
comparison is made in terms of the angle at which each of the curves L2 to 
L4 rises in place of the contrast C.sub.1 to C.sub.3. 
The above-described first method suffers, however, from some problems 
described below. If the image of the reticle pattern RPA is not closely 
superimposed on the sensor pattern SPA, the quantity of light entering the 
photoelectric sensor PES1 decreases, and thus the SN ratio of the signal 
deteriorates. Consequently, the reproducibility of detection of the focus 
position becomes degraded. In general, the signal becomes more sensitive 
as the reticle pattern or the sensor pattern becomes thinner, and 
moreover, it is desirable for the reticle pattern to have the same line 
width as that of the actual circuit pattern. For this reason, the reticle 
pattern is generally designed to have a small size which is close to the 
resolution of the projection optical system. More specifically, in an 
exposure apparatus for a semiconductor integrated circuit, for example, 
the line width of the reticle pattern is generally not larger than 1 
.mu.m. Therefore, even if the pattern alignment has been strictly 
effected, the pattern position shifts with passage of time owing to the 
drift of the apparatus or other reason. Further, replacement of the 
reticle also causes the pattern position to shift because of errors in the 
reticle positioning accuracy or the pattern drawing accuracy. Therefore, 
pattern alignment is needed almost every time detection of a focus 
position is carried out. Accordingly, it takes a great deal of time to 
detect a focus position, causing the throughput (productivity) of the 
exposure process to be deteriorated. 
In general, alignment is carried out by a method in which the 
light-receiving part of a photoelectric sensor is moved in a plane 
perpendicular to the projection optical system to search for a point at 
which the output of the photoelectric sensor reaches a maximum. Since the 
alignment is carried out before the detection of a focus position, the 
light-receiving part of the photoelectric sensor is not necessarily 
coincident with the image-forming plane. When the light-receiving part of 
the photoelectric sensor does not coincide with the image-forming plane, 
the image of the reticle pattern is not sharp, and the focus position 
cannot accurately be obtained. 
In the second method, no strict alignment is required before the detection 
of a focus position, but it is necessary to effect scanning many times 
with the position of the wafer stage WS1 changed in the direction Z. 
Accordingly, the second method suffers from the problem that the 
throughput deteriorates. 
In recent years, circuit patterns have become increasingly small and fine, 
and it has become necessary to align the photosensitive substrate with the 
focus position (i.e., to effect focusing) with a higher degree of 
accuracy. Projection exposure apparatuses have heretofore been adapted to 
automatically correct the focus position by arithmetically predicting a 
change of the focus position which may be caused, for example, by a change 
in the atmospheric pressure, absorption of exposure light by the 
projection optical system, or a change in the illumination method (e.g., a 
change of .sigma. value, which is a coherence factor, annular 
illumination, etc.). However, it is also necessary in order to effect even 
more strict focusing to detect a focus position by actual measurement. 
With the achievement of fine circuit patterns, the focus detecting 
operation by actual measurement must be carried out more frequently than 
in the past. Therefore, it is impossible to meet the demand with a focus 
position detecting method of low measuring efficiency. 
An object of the present invention is to provide a method of detecting a 
focus of a projection optical system, which requires no strict alignment, 
and which enables a reduction in the time required for measurement. 
SUMMARY OF THE INVENTION 
The present invention provides a method of detecting a focus position of a 
projection optical system which projects an image of a pattern on a first 
surface onto a second surface. The method employs a pattern for detection 
which is disposed on one of the first and second surfaces, and a 
photoelectric detector having a light-receiving part of predetermined 
configuration which is disposed on the other of the first and second 
surfaces. With the pattern for detection illuminated, the pattern for 
detection and the light-receiving part of the photoelectric detector are 
moved relative to each other in a direction parallel to an optical axis of 
the projection optical system, and at the same time, the pattern for 
detection and the light-receiving part of the photoelectric detector are 
moved relative to each other in a direction (direction X) perpendicular to 
the optical axis. An output signal from the photoelectric detector which 
is obtained when the light-receiving part crosses a bundle of light rays 
forming an image of the pattern for detection is taken in, and a focus 
position of the projection optical system is obtained on the basis of the 
output signal thus taken in. 
In this case, the pattern for detection and the light-receiving part of the 
photoelectric detector may be moved in such a manner that either one of 
the two is moved parallel to the optical axis, and at the same time, the 
other is moved in a direction perpendicular to the optical axis. 
One example of the pattern for detection is one or a plurality of 
slit-shaped patterns arranged in a predetermined direction (direction X), 
as shown in FIG. 5(a). One example of the light-receiving part of the 
photoelectric detector, which corresponds to the above pattern for 
detection, has an edge extending in a direction (direction Y) 
perpendicular to the predetermined direction, as shown in FIG. 5(b). 
Further, when the projection optical system is used to project an image of 
a mask pattern illuminated by an illumination optical system onto a 
photosensitive substrate, the pattern for detection may be disposed on the 
mask pattern side and illuminated by the illumination optical system. 
According to the above-described focus detecting method of the present 
invention, the pattern for detection and the light-receiving part of the 
photoelectric detector are moved relative to each other in a direction 
perpendicular to the optical axis of the projection optical system, and at 
the same time, they are moved relative to each other in the optical axis 
direction, thereby detecting a focus position. Accordingly, alignment in a 
plane perpendicular to the optical axis of the projection optical system 
need not be so strict as in the first type of conventional method, but it 
is only necessary that there should be a position at which the image of 
the pattern for detection and the light-receiving part of the 
photoelectric detector coincide with each other in the range within which 
the two elements move in the direction X during the detection of a focus 
position, provided that the position in the direction Y is accurate. 
Accordingly, it is not necessary to effect alignment strictly every time 
the detection of a focus position is carried out. Thus, the detection of a 
focus position can be rapidly effected. 
Further, since there is no need for repeated movement in a plane 
perpendicular to the projection optical system as is needed in the second 
type of conventional method, the time required for measurement can be 
shortened. 
In a case where either the pattern for detection or the light-receiving 
part of the photoelectric detector is moved parallel to the optical axis, 
and at the same time, the other is moved in a direction perpendicular to 
the optical axis, the control operation is facilitated because the pattern 
for detection and the light-receiving part are each moved only in one 
direction. 
In a case where the pattern for detection is one or a plurality of 
slit-shaped patterns arranged in a predetermined direction (direction X), 
and the light-receiving part of the photoelectric detector, which 
corresponds to the above pattern for detection, is a pattern having an 
edge which extends in a direction (direction Y) perpendicular to the 
predetermined direction, the light-receiving area of the light-receiving 
part can be enlarged, and it is therefore easy to form a pattern 
constituting the light-receiving part. In addition, the pattern 
configuration of the light-receiving part of the photoelectric detector 
need not be changed according to the measurement line width of the pattern 
for detection; this is another advantage of the present invention. 
Further, in a case where the projection optical system is used to project 
an image of a mask pattern illuminated by an illumination optical system 
onto a photosensitive substrate, and the pattern for detection is disposed 
at the mask pattern side and illuminated by the illumination optical 
system, there is no need for providing a special illumination system for 
the detection of a focus position.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
One embodiment of the focus detecting method according to the present 
invention will be described below with reference to FIGS. 1 to 3(b). 
FIG. 1 schematically shows the arrangement of a step-and-repeat projection 
exposure apparatus to which the focus detecting method of this embodiment 
may be suitably applied. Referring to FIG. 1, a light source system AL has 
a mercury-vapor lamp as a light source for exposure, an elliptic mirror, a 
collimator lens, an interference filter, etc. Illuminating light IL 
emitted from the light source system AL passes through an optical 
integrator (fly-eye lens) 11, an aperture stop (.sigma. stop) 12 and a 
condenser lens 13 to enter a dichroic mirror 14 for bending the optical 
path. After being bent at approximately right angles by the dichroic 
mirror 14, the illuminating light IL illuminates a reticle R having a 
circuit pattern MP drawn thereon at an approximately uniform illuminance. 
As a result, an image of the circuit pattern MP on the reticle R is 
projected onto a wafer W through a projection optical system PL. It should 
be noted that the wafer W is not at the exposure position in FIG. 1 
because the figure shows a state of the apparatus during the detection of 
a focus position. 
It should be noted that, as the illuminating light IL for exposure, it is 
possible to use laser light, e.g., excimer laser (KrF excimer laser, ArF 
excimer laser, etc.), or metal vapor laser or YAG laser harmonic, in 
addition to emission lines from a mercury-vapor lamp or other light 
source. The light source system AL is further provided with a shutter for 
cutting off exposure light, or an illumination condition switching device 
for changing characteristics of light rays illuminating the reticle R, 
which has been disclosed in U.S. Pat. No. 5,335,044, in addition to the 
above-described optical elements. Further, a relay optical system (not 
shown) and a variable field stop (not shown) for limiting the illumination 
area on the reticle R are provided in front of the condenser lens 13. 
Here, a Z-axis is taken in a direction parallel to an optical axis AX of 
the projection optical system PL, and a Y-axis is taken in a direction 
perpendicular to the plane of FIG. 1 within a plane perpendicular to the 
Z-axis. Further, an X-axis is taken in a direction parallel to the plane 
of FIG. 1. 
The reticle R is held by vacuum on a reticle stage 1 which is slightly 
movable in a direction (direction Z) parallel to the optical axis AX and 
also two-dimensionally movable and slightly rotatable in a plane 
(XY-plane) perpendicular to the optical axis AX by the action of a driving 
system (not shown). The reticle R is formed with the above-described 
circuit pattern MP and a reticle pattern RP for focus position detection, 
which is disposed at the periphery of the circuit pattern MP. Further, the 
position of the reticle stage 1 in each of the directions X and Y is 
always detected at a resolution of about 0.01 .mu.m, for example, by a 
laser interferometer (not shown) which is disposed at the periphery of the 
reticle stage 1. The laser interferometer is disclosed in U.S. Pat. No. 
5,243,195. 
The wafer W is held by vacuum on a wafer holder 2 which is provided on the 
wafer stage WS. The wafer stage WS is movable in a plane (XY-plane) 
perpendicular to the optical axis AX of the projection optical system PL 
by the action of a driving motor 7. By moving the wafer stage WS according 
to the step-and-repeat method, the circuit pattern MP on the reticle R is 
transferred onto the wafer W by exposure. Further, the wafer stage WS has 
a Z-stage which is movable in the direction (direction Z) of the optical 
axis AX of the projection optical system PL. The Z-stage enables the wafer 
W to move so that the surface of the wafer W coincides with the image 
surface of the projection optical system PL. 
In addition, a glass substrate 15 is installed on the Z-stage of the wafer 
stage WS at a position which is in close proximity to the wafer holder 2. 
The glass substrate 15 has been formed with a sensor pattern SP having a 
predetermined configuration to receive light passing through the reticle 
pattern RP and forming an image of the reticle pattern RP through the 
projection optical system PL. A photoelectric sensor PES for receiving the 
image-forming light passing through the sensor pattern SP is disposed 
below the sensor pattern SP. 
The position of the wafer stage WS in the XY-plane is measured with high 
accuracy by a combination of a laser interferometer 6 which is disposed at 
the periphery of the wafer stage WS, and a moving mirror 5 which is 
provided on an end portion of the wafer stage WS so as to reflect laser 
light from the laser interferometer 6. It should be noted that, in FIG. 1, 
only the laser interferometer for the direction X is shown. 
The position of the wafer W (or the sensor pattern SP) in the direction Z 
is measured by an oblique incidence focus position detecting system which 
comprises a light-projecting system 3 and a light-receiving system 4 
(hereinafter, the combination of the light-projecting and -receiving 
systems 3 and 4 will also be referred to as "oblique incidence optical 
system 3 and 4"). The focus position detecting system has been disclosed 
in U.S. Pat. No. 4,650,983. Light rays emitted from the light-projecting 
system 3 are those in a wavelength band in which the photosensitive 
material on the wafer W will not be affected by exposure light. The light 
rays are projected onto the wafer W obliquely to the optical axis AX in 
the form of a pinhole or slit image. The light-receiving system 4 has been 
designed so that when the surface of the wafer W coincides with the image 
surface of the projection optical system PL, the position of the reflected 
image from the wafer W coincides with a pinhole or slit provided inside 
the light-receiving system 4. A signal I.sub.z from the light-receiving 
system 4 which corresponds to the position of the wafer W in the direction 
Z is sent to a stage controller 10. The stage controller 10 controls the 
Z-stage so that the surface of the wafer W coincides with the image 
surface on the basis of the signal I.sub.z. Further, a plane-parallel 
plate (not shown) for shifting light rays is provided inside the 
light-receiving system 4. The angle of the plane-parallel plate is 
adjusted so that the reflected light from the wafer W always coincides 
with the pinhole or slit at the image surface of the light-receiving 
system 4 even if the image surface of the projection optical system PL 
varies. 
The oblique incidence optical system 3 and 4 is one example of a sensor for 
detection in the direction Z. There are other systems having the same 
function as that of the oblique incidence optical system 3 and 4, for 
example, a system in which the position of reflected slit image-forming 
light is detected by a line sensor. Such a system may also be used for 
measurement. The projection exposure apparatus in this embodiment is 
provided with a mechanism for alignment and other mechanisms in addition 
to the above-described mechanisms. However, since such mechanisms are not 
directly related to this embodiment, description thereof is herein 
omitted. 
Next, the mechanism for detecting a focus position of the projection 
optical system PL in this embodiment will be explained in detail. 
As has been described above, the reticle pattern RP for focus position 
detection is provided at the periphery of the circuit pattern MP on the 
reticle R. The reticle pattern RP is usually provided, for example, in a 
street line around or inside the circuit pattern MP so that the reticle 
pattern RP will not interfere with the essential circuit pattern MP. 
Further, the sensor pattern SP having a predetermined configuration is 
formed on the wafer stage WS, and the photoelectric sensor PES, which 
receives a bundle of light rays passing through the sensor pattern SP, is 
provided below the sensor pattern SP, as described above. The sensor 
pattern SP may be regarded as being a light-receiving part of the 
photoelectric sensor PES. 
FIG. 3(a) shows the reticle pattern RP on the reticle R, and FIG. 3(b) 
shows the sensor pattern SP on the wafer stage WS. As shown in FIG. 3(a), 
the reticle pattern RP is a line-and-space pattern comprising a plurality 
of slit-shaped light-transmitting portions 30a to 30f which are formed in 
a light-blocking area 31. The light-transmitting portions 30a to 30f are 
arranged at equal spaces in the direction of scan, which is performed 
during the detection of a focus position, and elongate in a direction 
perpendicular to the scan direction. As has been described above, it is 
preferable for the line-and-space pattern to be as thin as possible when 
the detecting resolution is taken into consideration. Moreover, as the 
dimension of the line-and-space pattern becomes closer to the dimension of 
the actual circuit pattern MP, the optical path in the projection optical 
system PL of diffracted light from the circuit pattern MP and that of 
diffracted light from the reticle pattern RP coincide with each other more 
accurately, and thus the detection is less affected by the optical path 
difference in the projection optical system PL between the circuit pattern 
MP and the reticle pattern RP. Accordingly, the space between each pair of 
adjacent slit-shaped light-transmitting portions of the reticle pattern RP 
is set as close to the resolution of the projection optical system PL as 
possible. The image of the reticle pattern RP is formed on the sensor 
pattern SP, which is provided on the wafer stage WS, through the 
projection optical system PL. 
As shown in FIG. 3(b), the sensor pattern SP is a single slit-shaped 
light-transmitting portion 32 which is formed in a light-blocking area 33 
so as to be elongate in a direction perpendicular to the scan direction. 
The sensor pattern SP is usually designed to be approximately equal in 
size to the image of the reticle pattern RP which is projected onto the 
sensor pattern SP. The photoelectric sensor PES is provided below the 
sensor pattern SP. Thus, light rays passing through the sensor pattern SP 
are received by the photoelectric sensor PES. 
It should be noted that a condenser lens may be provided between the sensor 
pattern SP and the photoelectric sensor PES. Further, when there is no 
sufficient space for installing a photoelectric sensor on the wafer stage 
WS, the arrangement may be such that the sensor pattern SP alone is 
provided on the wafer stage WS, and light passing through the sensor 
pattern SP is led to a photoelectric sensor provided in another place 
(outside the wafer stage WS) through an optical fiber bundle or the like. 
An output signal I from the photoelectric sensor PES is sent to a focus 
position detecting system 9, shown in FIG. 1, together with the output 
signal I.sub.z from the light-receiving system 4, and subjected to 
arithmetic processing in a unit (CPU) provided in the focus position 
detecting system 9. On the basis of the result of the arithmetic 
processing, a focus position is detected. It should be noted that both the 
focus position detecting system 9 and the stage controller 10 are 
controlled by a main control system 8. 
Next, an example of a focus position detecting operation in this embodiment 
will be explained. 
First, in FIG. 1, the wafer stage WS is moved to a measurement start point 
on the basis of a command from the stage controller 10. A scanning start 
and end points in the direction Z of the wafer stage WS are determined so 
that all the projected image of the reticle pattern RP passes through the 
area of the sensor pattern SP during the measurement, and that the center 
of the scanning range in the direction Z coincides with the design focus 
position of the projection lens by using the laser interferometers 6, 
which measure the position of the wafer stage WS in the directions X and 
Y, and the oblique incidence optical system 3 and 4, which measures the 
position of the wafer stage WS in the direction Z. Then, the wafer stage 
WS is moved in the direction Z. It should be noted that since the oblique 
incidence optical system 3 and 4, which performs measurement in the 
direction Z, is adapted to detect the center of the optical axis of the 
projection optical system PL, if the surface of the sensor pattern SP is 
not parallel to the XY-plane, a difference in height is produced between 
the light-receiving position of the sensor pattern SP and the position of 
the wafer W at the point of detection by the oblique incidence optical 
system 3 and 4 in the direction Z. Therefore, the positional difference in 
the direction Z between the two points has previously been measured by the 
oblique incidence optical system 3 and 4, and a focus position is obtained 
from the result of the measurement by taking the difference into 
consideration. 
After the wafer stage WS has reached the measurement start point, the 
shutter which has been closed to cut off illuminating light for exposure 
is opened, thereby illuminating the reticle pattern RP with illuminating 
light IL for circuit pattern exposure. At this time, it is desirable to 
limit the exposure range by the variable field stop so that no light rays 
will pass through a region other than the reticle pattern RP and 
unnecessarily strike on and heat the wafer stage WS or the projection 
optical system PL. After the shutter has been opened, the wafer stage WS 
is moved in the direction X and, at the same time, the wafer stage WS 
(Z-stage) is moved in the direction Z on the basis of a command from the 
stage controller 10. At this time, the output signal I from the 
photoelectric sensor PES and the output signal I.sub.z from the 
light-receiving system 4 are sent in parallel to the focus position 
detecting system 9. 
FIG. 2 shows the waveform of the output signal I from the photoelectric 
sensor PES. The abscissa axis represents the position x in the direction X 
and the position z in the direction Z of the sensor pattern SP, and the 
ordinate axis represents the output signal I. In FIG. 2, the solid-line 
curve 21 shows the output signal I from the photoelectric sensor PES. As 
the wafer stage WS comes toward the focus position, the amplitude of the 
waveform of the output signal I increases, whereas, as the wafer stage WS 
comes away from the focus position, the amplitude of the output signal I 
decreases and converges to zero. 
The broken-line curve 22 shows the output signal I obtained when the sensor 
pattern SP moves in the direction Z with the image of a light-transmitting 
portion of the reticle pattern RP kept coincident with the 
light-transmitting portion 32 of the sensor pattern SP (i.e., without the 
light-transmitting portion image and the light-transmitting portion 32 
moving in the direction X). The curve 22 draws the same waveform that is 
shown in FIG. 9(b), which has been explained in connection with the 
conventional technique. The curve 23 shown by the one-dot chain line shows 
the output signal I obtained when the sensor pattern SP moves in the 
direction Z when the projected image of the light-blocking portion 31 
between a pair of adjacent light-transmitting portions of the reticle 
pattern RP is kept coincident with the sensor pattern SP. In this case, 
when the image of the reticle pattern RP lies at the focus position, no 
light reaches the photoelectric sensor PES because of the presence of the 
light-blocking portion. However, as the image of the reticle pattern RP 
comes away from the focus position, the image spreads. Therefore, light 
begins to leak from the periphery of the light-blocking portion, and thus 
the quantity of light passing through the sensor pattern SP increases. The 
curves 22 and 23 are estimated by the focus position detecting system 9, 
for example, from envelopes connecting the peak points of the output 
signal I, and a point at which the difference between the curves 22 and 23 
reaches a maximum is determined as a focus position BF. At this time, the 
focus position detecting system 9 subjects the peak points of the output 
signal I to statistical processing (e.g., least square approximation), 
thereby approximately obtaining functions expressing the envelopes (curves 
22 and 23). Then, a point at which the difference between the curves 22 
and 23 reaches a maximum is calculated on the basis of the two approximate 
functions, and the calculated point is determined as a focus position BF. 
The wafer stage WS (Z-stage) is controlled by the stage controller 10 using 
the oblique incidence optical system 3 and 4 so that the surface of the 
wafer W comes at the focus position BF obtained as described above. It 
should be noted that when a predetermined offset occurs with respect to 
the measured value BF owing to the thickness of the photosensitive 
material, characteristics thereof, etc., or when an offset (deviation in 
the direction Z) is produced between a plane on which the projected image 
of the oblique incidence optical system 3 and 4 is formed and a plane 
which is to be aligned with the focus position because of a step on the 
wafer (shot region) W, the wafer stage WS is controlled so that the 
surface of the wafer W comes to a position determined by taking the offset 
into consideration, for example, by giving an optical or electrical offset 
to the oblique incidence optical system 3 and 4. 
As will be clear from FIG. 2, the method of this embodiment makes it 
possible to obtain a signal which has a high rate of change and a 
favorable SN ratio in comparison to the first conventional method (the 
method shown in FIGS. 9(a) and 9(b)), without requiring strict alignment 
in the direction X. This is one of advantages obtained by the method of 
this embodiment. In comparison to the second conventional method, shown in 
FIGS. 10(a) to 10(e), the contrasts C.sub.1, C.sub.2 and C.sub.3 at the 
respective points in the second conventional method match the differences 
between the curves 22 and 23 in FIG. 2 at the positions z.sub.1, z.sub.2 
and z.sub.3 in the optical axis direction (direction Z). That is, when the 
method of this embodiment and the second conventional method are compared 
to each other, signals obtained by the two methods are the same in terms 
of the rate of change. In addition, the method of this embodiment makes it 
possible to obtain information which is equivalent to that shown in FIG. 
10(e), which is obtained by the second conventional method, by a single 
scan. 
Although in this embodiment exposure light for a circuit pattern is used as 
the illuminating light IL, and a light-transmitting pattern is used as the 
reticle pattern RP, it should be noted that a reflecting pattern may also 
be used as the reticle pattern RP. In such a case, as shown, for example, 
in U.S. Pat. No. 5,241,188, illuminating light for exposure is led to the 
wafer stage (WS) side by a light guide or the like to illuminate the 
reticle pattern RP through the sensor pattern SP and the projection 
optical system PL, and reflected light from the reticle pattern RP is 
focused on the sensor pattern SP through the projection optical system PL. 
Then, a bundle of light rays passing through the sensor pattern SP is 
received with the photo-electric sensor PES to obtain a focus position. 
Although a basic method of this embodiment has been described above, when 
it is desired to obtain a focus position even more accurately, it is 
possible to employ a method in which the above-described measurement is 
carried out a plurality of times, and an average of the measured data is 
determined to obtain a focus position. It is also possible to employ a 
method in which data is sampled in more detail only in the vicinity of the 
focus position BF. More specifically, the speed of movement of the wafer 
stage WS in the direction Z is slowed down in the neighborhood of the 
focus position BF, and data sampling intervals in the direction Z are 
reduced, thereby sampling data at a higher density. 
Further, since the travel in the direction X is very small, it can be 
ignored in the actual practice. However, in a case where there may be an 
error in measurement by the oblique incidence optical system 3 and 4, 
which carries out measurement in the direction Z during the movement, due 
to inclination of the sensor pattern SP with respect to the XY-plane, it 
is preferable to previously measure a difference in height between the 
light-receiving position of the sensor pattern SP and the position of the 
wafer W at the point of detection by the oblique incidence optical system 
3 and 4 by the oblique incidence optical system 3 and 4 and to correct the 
error in measurement. 
It is also possible to prepare a reticle pattern RP adapted for a plurality 
of directions by taking into consideration a difference (astigmatism) in 
focus position which may occur according to the pattern direction. The 
arrangement may also be such that a plurality of reticle patterns RP are 
provided on the reticle R, and an average image surface is obtained on the 
basis of results of measurement carried out at a plurality of points in 
the entire exposure area on the reticle R, and then the wafer stage WS is 
controlled so that the surface of the wafer W coincides with the average 
image surface. For such an arrangement, it is preferable to use, for 
example, a special reticle for image surface measurement which has reticle 
patterns RP over the entire area thereof. It is also possible to provide a 
plurality of detecting devices each comprising a combination of a sensor 
pattern SP and a photoelectric sensor PES, and to measure an image surface 
by using the plurality of detecting devices at the same time. 
Next, other examples of the combination of a reticle pattern RP and a 
sensor pattern SP will be explained with reference to FIGS. 4(a) to 7(b). 
It should be noted that in the following examples, the size of slit-shaped 
marks constituting reticle and sensor patterns or the space between each 
pair of adjacent slit-shaped marks are set as close to the resolution of 
the projection optical system PL as possible, in the same way as in the 
case of the relationship between the above-described reticle pattern RP 
and sensor pattern SP, which are shown in FIGS. 3(a) and 3(b). 
FIGS. 4(a) and 4(b) show another example of the combination of a reticle 
pattern and a sensor pattern. FIG. 4(a) shows a reticle pattern, and FIG. 
4(b) shows a sensor pattern. Referring to FIG. 4(a), the reticle pattern 
RP1 comprises a plurality of slit-shaped light-transmitting portions 34a 
to 34f which are arranged in a light-blocking area 35 at equal spaces in 
the scan direction and elongate in a direction perpendicular to the scan 
direction. As shown in FIG. 4(b), the sensor pattern SP1 also comprises a 
plurality of slit-shaped light-transmitting portions 36a to 36f which are 
arranged in a light-blocking area 37 at equal spaces in the scan direction 
and elongate in a direction perpendicular to the scan direction. 
In this example, both the reticle pattern RP1 and the sensor pattern SP1 
comprise a plurality of light-transmitting portions in order to improve 
the SN ratio. In this case, however, the number of light-transmitting 
portions of the reticle and sensor patterns RP1 and SP1 which overlap each 
other differs according to the position in the direction X, and hence the 
quantity of light entering the photoelectric sensor PES varies. Therefore, 
it is necessary to correct the measured data for the variation of the 
light quantity. 
FIGS. 5(a) and 5(b) show still another example of the combination of a 
reticle pattern and a sensor pattern. FIG. 5(a) shows a reticle pattern, 
and FIG. 5(b) shows a sensor pattern. Referring to FIG. 5(a), the reticle 
pattern RP2 comprises a plurality of slit-shaped light transmitting 
portions 38a to 38f which are arranged in a light-blocking area 39 at 
equal spaces in the scan direction and elongate in a direction 
perpendicular to the scan direction. As shown in FIG. 5(b), the sensor 
pattern SP2 comprises a light-transmitting portion 40 of a rectangular cut 
portion which is formed in a light-blocking area 41, and which has 
approximately the same width in a direction perpendicular to the scan 
direction as that of one of the projected images of the slit-shaped 
light-transmitting portions constituting the reticle pattern RP2, and 
further has a straight-line edge 42 at an end thereof in the scan 
direction. The width in the scan direction of the light-transmitting 
portion 40 is sufficiently wider than the width in the scan direction of 
the projected image of the reticle pattern RP2. 
In this method, the edge 42 of the sensor pattern SP2 is utilized. If the 
sensor pattern SP2 is scanned in the direction X using the reticle pattern 
RP2 and the sensor pattern SP2, the quantity of light entering the 
photoelectric sensor PES is measured as being an integrated quantity. This 
will be explained below with reference to FIGS. 8(a) and 8(b). 
FIG. 8(a) shows the waveform of the output signal I from the photoelectric 
sensor PES, and FIG. 8(b) shows the waveform of a signal dI/dx obtained by 
differentiating the output signal I shown in FIG. 8(a) with respect to the 
position x in the direction X. It should be noted that, in FIGS. 8(a) and 
8(b), the abscissa axis represents the position z in the direction Z and 
the position x in the direction X. The curve 24 in FIG. 8(a) shows the 
output signal I from the photoelectric sensor PES. As the wafer stage WS 
moves, the output signal I shown by the curve 24 stepwisely increases, and 
as the wafer stage WS approaches the focus position, the difference in 
height between a pair of adjacent steps increases, whereas, as the wafer 
stage WS comes away from the focus position, the height difference 
decreases and converges to zero. 
That is, since the photoelectric sensor PES outputs a signal in such a form 
that a waveform equivalent to the curve 21 in FIG. 2 is integrated, the 
signal must be differentiated to obtain an output signal equivalent to the 
curve 21 in FIG. 2. The curve 25 in FIG. 8(b) is obtained by 
differentiating the curve 24 in FIG. 8(a) with respect to the position x 
of the wafer stage WS. Thus, an output signal equivalent to the curve 21 
in FIG. 2 is obtained. Accordingly, a focus position is calculated by a 
method similar to that described in connection with FIG. 2, and the wafer 
stage WS is moved to the focus position in the direction Z. More 
specifically, in a case where the sensor pattern SP2 shown in FIG. 5(b) is 
used, the focus position detecting system 9 differentiates the output 
signal from the photoelectric sensor PES, statistically processes the 
differential signal in the same way as in the above-described embodiment 
to obtain approximate functions expressing envelopes, and calculates a 
focus position BF on the basis of the approximate functions. It should be 
noted that the differential signal may be obtained in the focus position 
detecting system 9 in a software manner or by using a differentiating 
circuit. 
According to the method of this example, reticle patterns having various 
line widths can be measured with the configuration of the sensor pattern 
SP kept constant, and the sensor pattern SP can be readily formed, 
advantageously, although the arithmetic processing becomes somewhat 
complicated. 
FIGS. 6(a) and 6(b) show a further example of the combination of a reticle 
pattern and a sensor pattern. FIG. 6(a) shows a reticle pattern, and FIG. 
6(b) shows a sensor pattern. Referring to FIG. 6(a), the reticle pattern 
RP3 comprises a single slit-shaped light-transmitting portion 43 which is 
formed in a light-blocking area 44 and elongate in a direction 
perpendicular to the scan direction. As shown in FIG. 6(b), the sensor 
pattern SP3 also comprises a single slit-shaped light-transmitting portion 
45 which is formed in a light-blocking area 46 and elongate in a direction 
perpendicular to the scan direction. 
The feature of this example resides in the method of scanning the wafer 
stage WS. That is, while being reciprocatively moved in the direction X, 
the wafer stage WS is continuously moved in the direction Z to thereby 
obtain a waveform equivalent to the curve 21 shown in FIG. 2. 
FIGS. 7(a) and 7(b) show a still further example of the combination of a 
reticle pattern and a sensor pattern. FIG. 7(a) shows a reticle pattern, 
and FIG. 7(b) shows a sensor pattern. Referring to FIG. 7(a), the reticle 
pattern RP4 comprises a single slit-shaped light-transmitting portion 47 
which is formed in a light-blocking area 48 and elongate in a direction 
perpendicular to the scan direction. As shown in FIG. 7(b), the sensor 
pattern SP4 comprises a plurality of slit-shaped light-transmitting 
portions 49a to 49f which are arranged in a light-blocking area 50 at 
equal spaces in the scan direction and elongate in a direction 
perpendicular to the scan direction. In this example, the combination of 
reticle and sensor patterns is in reverse relation to that shown in FIGS. 
3(a) and 3(b). 
Although in the above-described examples the sensor pattern SP is moved in 
the directions X and Z, the same result can be obtained by moving the 
reticle pattern RP instead. It is also possible to move both the reticle 
pattern RP and the sensor pattern SP. In such a case, for example, while 
the reticle pattern RP is being moved in the direction X, the sensor 
pattern SP is moved in the direction Z. It is also possible to effect 
scanning by moving the reticle pattern RP or the sensor pattern SP in the 
three directions X, Y and Z at the same time. 
Thus, the focus detecting method according to the described embodiment 
enables a focus position to be detected in a reduced period of time 
without requiring strict positioning accuracy. A projection exposure 
apparatus or other similar apparatus is provided with a mechanism which 
measures an atmospheric pressure change and a temperature change, 
calculates a change of the focus position on the basis of parameters 
previously obtained, and automatically adjusts the angle of a 
plane-parallel plate of a light-receiving system in an oblique incidence 
optical system so that the system will not shift from the focus position, 
as shown in U.S. Pat. No. 4,687,322. The projection exposure apparatus is 
further provided with a function whereby a change of the focus position 
caused by absorption of exposure light by a projection optical system of 
the apparatus is calculated on the basis of the energy quantity of 
exposure light entering the projection optical system, as shown in U.S. 
Pat. No. 4,801,977. 
However, these functions are based on predictive calculation, and hence 
cannot cope with a factor which cannot be predicted. Therefore, if the 
functions are combined with the method of detecting a focus position by 
actual measurement according to the above-described embodiment to thereby 
compensate for the shortcomings of the functions, the advantageous effects 
of the functions can be exhibited even more favorably. Particularly, a 
change of the focus position caused by absorption of exposure light is a 
relatively rapid change, unlike the change in the atmospheric pressure, 
and therefore, the detection of a focus position must be carried out 
frequently (e.g., every 10 minutes). However, the method of the 
above-described embodiment enables a focus position to be detected at high 
speed. Accordingly, by using the focus position detecting method in 
combination with the projection exposure apparatus, it is possible to 
improve not only the throughput of the exposure process but also the 
accuracy in the entire apparatus. 
Although in the above-described embodiment illumination is effected from 
above the reticle R, the present invention is not necessarily limited 
thereto. For example, the arrangement may be such that illuminating light 
for exposure is led into the wafer stage WS through a light guide so as to 
illuminate the sensor pattern SP from below it, and light passing through 
the sensor pattern SP is passed through the projection optical system PL 
to illuminate the reticle pattern RP and received at the upper side 
thereof. The arrangement may also be such that a reflecting pattern is 
used as the reticle pattern RP, and reflected light from the reticle 
pattern RP is received by a photoelectric sensor through the projection 
optical system PL, the sensor pattern SP, and a beam splitter (not shown). 
It should be noted that the focus detecting method of the present invention 
may be applied not only to a step-and-repeat exposure apparatus but also 
to a step-and-scan exposure apparatus in which exposure is carried out 
with the reticle and the wafer being scanned relative to each other, 
without a need of change. 
Thus, the present invention is not necessarily limited to the 
above-described embodiment, but may adopt various arrangements without 
departing from the gist of the present invention. 
According to the focus detecting method of the present invention, an 
imagery position (focus position) of a projection optical system can be 
detected simply by scanning a pattern for detection and a light-receiving 
part relative to each other once in two directions. Therefore, it is 
possible to detect a focus position with high accuracy in a short time 
without requiring strict alignment. Accordingly, if the focus detecting 
method is applied to an exposure apparatus, the throughput is 
advantageously improved. The focus detecting method is particularly useful 
in a case where the focus position of the projection optical system varies 
with a change in exposure conditions or illuminating conditions, and the 
focus position must be frequently measured. 
In a case where either the pattern for detection or the light-receiving 
part of the photoelectric detector is moved parallel to the optical axis, 
and at the same time, the other of the two is moved in a direction 
perpendicular to the optical axis, the control of the stage system is 
facilitated, and the structure of the stage system can be advantageously 
simplified. 
In a case where the pattern for detection is one or a plurality of 
slit-shaped patterns arranged in a predetermined direction, and the 
light-receiving part of the photoelectric detector, which corresponds to 
the above pattern for detection, is a pattern having an edge which extends 
in a direction perpendicular to the predetermined direction, the 
light-receiving area of the light-receiving part can be enlarged, and it 
is therefore unnecessary to change the pattern configuration of the 
light-receiving part of the photoelectric detector according to the 
measurement line width of the pattern for detection. Moreover, it is easy 
to form a pattern constituting the light-receiving part; this is another 
advantage of the present invention. 
Further, in a case where the projection optical system is used to project 
an image of a mask pattern illuminated by an illumination optical system 
onto a photosensitive substrate, and the pattern for detection is disposed 
at the mask pattern side and illuminated by the illumination optical 
system, there is no need for providing a special illumination system for 
the detection of a focus position.