JPS63184004A - Method and apparatus for setting gap between first and second objects to predetermined value - Google Patents

Abstract

PURPOSE:To prevent the interference of the reflected diffracted beam on the surface of the first object and detected diffracted beam and to set the gap between the first and second objects regardless of the positional shift between both objects, by adjusting the gap between the first and second objects corresponding to the intensity of the detected diffracted beam. CONSTITUTION:The beam emitted from a laser 17 is diffracted through the route of the first diffraction lattice 15 of a mask 13 the second diffraction lattice 16 of a wafer 12 the lattice 15 and the first and second lattices 15, 16 function as a double refraction lattice. Then, the diffracted beam guided to a photoelectric detector 16 in the diffracted beam is selected by adjusting the angle of inclination of a mirror 19. This diffracted light is converted to the electric signal corresponding to the intensity thereof by the detector 26 and said electric signal is processed by a signal processing circuit 20 through an amplifier 27 to emit a piezoelectric element driving signal. This signal is supplied to a piezoelectric element driving circuit 21 and a current is further supplied to a piezoelectric element 25 on the basis of said signal. By this method, the gap between the mask 13 and the wafer 12 is adjusted to be set to a predetermined value.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a method and apparatus for setting a gap between a first object and a second object to a predetermined value.

Specifically, the present invention relates to a method and apparatus for setting the gap between a mask and a wafer to a predetermined value when an image of a circuit pattern is transferred to a wafer in a semiconductor manufacturing process.

(Prior Art) In the process of manufacturing semiconductor devices such as VLSIs, a circuit pattern is generally transferred onto a wafer by an exposure device. In this apparatus, when a circuit pattern previously formed on a mask is irradiated with X-rays, an image of the circuit pattern is transferred onto a wafer. Before this circuit pattern is transferred, the gap between the mask and the wafer must be accurately set to a predetermined value.

By the way, an example of a method for setting the gap between a mask and a wafer is disclosed in Japanese Patent Laid-Open No. 116837/1983. This method uses a diffraction grating. That is, as shown in FIG. 1, a one-dimensional diffraction grating 2 is formed on the mask 1, and a reflective surface 4 is formed on the square evaporator 3. When the upper surface of the mask 1 is irradiated with laser light, the light that is diffracted and transmitted through the diffraction grating 2 of the mask 1 is reflected by the reflective surface 4 of the wafer 3, and is reflected again by the diffraction grating 2 of the mask 1.
diffracts at Of this diffracted light, the +1st-order diffracted light weight rL (
+1) and -1st-order diffracted light IrL (-1) are determined by /I11. The results of this measurement are shown in broken lines in FIG.

That is, the relationship between the intensity of the diffracted light and the gap is such that the period is P2.
/λ (P is the pitch of the diffraction grating 2,
λ is the wavelength of the laser beam). The gap between the mask 1 and the wafer 3 is adjusted so that the intensity IrL of the diffracted light is set to the peak value of the periodic function. Thereby, the gap between the mask 1 and the wafer 3 is set to a predetermined value.

(Problems to be Solved by the Invention) However, the diffraction grating of the mask 1 also works as a reflection type diffraction grating. Therefore, the ±1st-order diffracted light diffracted along the path from the mask 1 to the reflective surface 4 of the wafer 3 to the mask 1 may interfere with the first-order reflected diffraction light reflected from the upper surface of the mask. That is, assuming that the gap between the mask and the wafer is 2, the ±1st-order diffracted light has an optical path difference of 2z with respect to the first-order reflected diffracted light. When 2z-nλ (n is an integer), the ±1st-order diffraction light and the 1st-order reflected diffraction light interfere.

Therefore, as shown in 1g and 2, the relationship between the intensity of the diffracted light and the gap becomes a non-uniform periodic function with a period of λ/2. Therefore, there was a problem in that it was difficult to adjust the gap between the mask and the wafer.

An object of the present invention is to prevent the reflected diffracted light that is reflected and diffracted from the surface of the first object from interfering with the detected diffracted light, and regardless of the amount of positional deviation between the first object and the second object. An object of the present invention is to provide a method and apparatus for accurately setting the gap between a first object and a second object to a predetermined value.

[Structure of the Invention] (Means for Solving the Problems) A method and apparatus for setting the gap between a first object and a second object to a predetermined value based on the present invention are configured as follows. There is.

A first diffraction grating having a pattern of parallel stripes extending in a first direction is formed on the first object, the first diffraction grating being orthogonal to the first direction. A virtual surface is defined as a first virtual surface, and this first
A virtual surface in which the virtual surface of a one-dimensional diffraction grating having a pattern of parallel stripes, the stripes forming a second direction perpendicular to the first direction;
forming a second diffraction grating on a second object, the second diffraction grating extending in the direction of the first diffraction grating; the incident light is transmitted through and diffracted by the first diffraction grating to reveal the first diffracted light; the first diffracted light is transferred to the second diffraction grating; a step in which the first diffracted light is reflected and diffracted by a second diffraction grating to reveal a second diffracted light, and the second diffracted light is transferred to the first diffraction grating and the second diffracted light is is transmitted and diffracted by the first diffraction grating to reveal a third diffracted light, and some of the third diffracted light is migrated along a third virtual plane,
The other third diffracted light is transferred to a surface other than the third virtual plane, the other third diffracted light is detected, and the first diffracted light is transferred according to the intensity of the detected diffracted light. The method is characterized by comprising a step of adjusting the gap between the object and the second object and setting the gap to a predetermined value. Further, the second diffraction grating has a pattern of two sets of orthogonal stripes.
It may be a dimensional diffraction grating.

(Operation) The stripes of the first diffraction grating and the stripes of the second diffraction grating are orthogonal to each other. The first and second diffraction gratings act as a double diffraction grating.

Therefore, the third diffracted light diffracted from the first diffraction grating to the second diffraction grating to the first diffraction grating appears in a two-dimensional pattern. On the other hand, the reflected diffraction light reflected by the surface of the first diffraction grating is reflected only along the third virtual plane. In this invention, diffracted light that does not follow this third virtual plane is detected.

Therefore, there is no interference between the detected diffracted light and the reflected diffracted light reflected from the surface of the first object.

Further, according to the theoretical analysis (explained in the examples) in this invention, the gap between the first object and the second object remains at a predetermined value regardless of the amount of positional deviation between the first object and the second object. is set to

(Example) In the first to third examples, the case where the incident light is incident on the diffraction grating at a right angle is described, and the fourth example, absolutely, is the case where the incident light is incident on the diffraction grating at an angle. Let's talk about.

As shown in FIG. 3, a device for setting the gap between a mask and a wafer to a predetermined value is provided with a wafer table 11 that is movable in the X direction.

A wafer 12 is placed on the upper surface of this wafer table 11. A mask 13 is placed above the wafer 12. Between this mask 13 and wafer 12,
A predetermined interval is provided in two directions. Mask 13 is
It is supported by a holder 14. This holder 1
4 is supported by a piezoelectric element 25. By driving this piezoelectric element 25, the mask 13 is moved in two directions.

Furthermore, a transmission type first diffraction grating 15 is formed on the mask 13, as shown in FIG. Wafer 1
A reflective second diffraction grating 16 is formed on the upper surface of the diffraction grating 2 . These first and second diffraction gratings 15,16 are
are placed opposite each other. The first diffraction grating 15 is a one-dimensional diffraction grating with stripes extending in the X direction.

The second diffraction grating 16 is a one-dimensional diffraction grating with stripes extending in the X direction (alignment direction). That is,
The stripes of these gratings are orthogonal to each other.

This device further includes a laser 17 that emits coherent laser light, a photoelectric detector 26 that detects diffracted light and converts it into an electrical signal, and a signal processing circuit 20 that processes this electrical signal and generates a control signal. and a piezoelectric element drive circuit 21 that supplies current to the piezoelectric element 25 in accordance with a control signal.

With this device, the gap between the mask and the wafer is set to a predetermined value as follows.

The light emitted from the laser 17 is irradiated onto the mirror 18 . The light reflected by this mirror 18 passes through the first diffraction grating 1
5. Light diffracted by and transmitted through the first diffraction grating 15 is transferred to the second diffraction grating 16. The light diffracted and reflected by the second diffraction grating 16 is transferred to the first diffraction grating 15. The diffracted light that is diffracted by and transmitted through the first diffraction path 15 is transferred to the mirror 19 . These first and second diffraction gratings 15,16 thus act as a double diffraction grating. This mirror 19 allows only light in a specific direction out of the diffracted light diffracted by the diffraction gratings 15 and 16 to be transmitted to the photoelectric detector 26.
guided by. That is, by adjusting the inclination angle of the mirror 19, the diffracted light guided to the photoelectric detector 26 is selected. The diffracted light in a specific direction is converted by the photoelectric detector 26 into an electrical signal according to the intensity of the diffracted light. This electrical signal is supplied to the signal processing circuit 20 via the amplifier 27 and processed. This signal processing circuit 20 generates a piezoelectric element drive signal. This piezoelectric element drive signal force is supplied to the piezoelectric element drive circuit 21. The drive circuit 21 supplies current to the piezoelectric element 25 based on this signal. The piezoelectric element 25 is driven to adjust the gap between the mask 13 and the wafer 12. As a result, the mask 13 and the wafer 1
2 is set to a predetermined value. This setting device also includes a motor 22 that moves the wafer 12 in the X direction.
is provided.

By the way, when the laser beam is diffracted along the path of first diffraction grating 15 -> second diffraction grating 16 - first diffraction grating 15, as described above, the first and second diffraction gratings 15, 16
acts as a double diffraction grating. Therefore, among these diffracted lights, the 0th-order and ±1st-order diffracted lights appear in nine directions, as shown in FIG.

On the other hand, the laser beam may be reflected on the surface of the first diffraction grating 15. At this time, the reflected diffraction light reflected by the first diffraction grating 15 is reflected only on a plane that is perpendicular to the stripes of the first diffraction grating and includes the axis of the incident light. That is, if the optical axes of the incident light are two axes in FIG. 4 or FIG. 5, this reflected diffraction light is reflected within a plane including the X-axis and the two axes. On the other hand, 0
The second-order and ±1st-order diffracted lights appear in nine directions, as described above. Therefore, in this invention, the 0th and ±1st order diffracted lights that are not along the plane including the X-axis and the two axes are detected, and the detected 0th, 1st, and 1st-order diffracted lights do not interfere with the reflected diffracted light. .

Therefore, the gap between the mask and the wafer can be accurately adjusted using the detected diffracted light.

That is, in this invention, (0,1), (0,-1), (1,1), (1,-1), (-1,1
), (-1, -1), any one of the six diffracted lights is detected.

First, the case where (0, 1) or (0, -1) diffracted light is detected will be described. For this explanation, the principle of diffraction when laser light is diffracted by the diffraction gratings 15 and 16 will be explained.

As shown in FIG. 6, a typical diffraction grating having a pitch p and a light transmission width a is irradiated with coherent light having a wavelength λ at right angles. The diffraction pattern of the light beam diffracted by this diffraction grating is shown in FIG. Here, +0
The diffraction angle θ of the next diffracted light is 5ine −n #λ/p −
(1). The complex amplitude CrL of the n-th order diffracted light is a coefficient when the complex transmission function of the diffraction grating is expanded into a Fourier series as a periodic function. The amplitude Cn of this n-order diffracted light
is given by the following equation.

Here, the complex transmission function A (x) of the lattice is defined as A(x)−
0(-p/2≦X<-472)1 (-a/2≦x
<272) ... (3) 0 (a/2≦X
<p/2) and substitute it into equation (2), the amplitude Cr of the n-th order diffracted light becomes
L is Cn −1 sin (anπ/p) ) /nyr
−(4).

Next, a case will be described in which the stripes of the first diffraction grating 15 of the mask and the second diffraction grating 16 of the wafer are in the same direction. The optical model in this case is equivalent to the optical model shown in FIG.

When the first-order diffraction occurs in the mask, the m-th order in the wafer, and the r-th order in the mask, the transmitted light from the mask → wafer → mask is (j7 + m
+r) becomes the next diffracted light. The amplitude of this diffracted light wave is C
It is given by -C, ・C. This no
"The diffracted light is different from the light before it enters the mask. On the other hand, if the wafer is misaligned by ΔX with respect to the mask,
From equation (2), the amplitude of the light wave diffracted by the wafer is (X+Δx)ldx...(
5). When this is sorted out, Ca' −exp l−1(2yr tn/p>ΔX)
-exp l-i (2yr ta/P)Δx)・Cf
It becomes f1. ()-1-m+r) next passing diffracted light U(i
+m+r) is expressed by the following equation (7) as the incident light Ein.

U()+mar) -c -C' ・Cφexp[-1φxlEin)
m r −〇ノ・Cff1−Cr−exp[−1(φx+{2π
/P), mΔX)koEin
-(7) For example, the display of 0th-order diffracted light in the X direction is a combination of all diffracted lights that satisfy n+m+rmo.

Next, as shown in FIG. 4, a case where the stripes of the second diffraction grating 16 on the wafer are perpendicular to the stripes of the first diffraction grating 15 on the mask will be described. In this case, in the wafer, light is diffracted by a diffraction surface perpendicular to the diffraction surface of the mask, as shown in FIG. The diffraction order in the X direction is m, the diffraction order in the X direction is n1 The pitch in the X direction of the diffraction grating of the mask is P The pitch in the X direction of the diffraction grating of the 1 wafer is P The transmission of light in the X direction of the diffraction grating of the 5 mask If the width of the light transmission in the X direction of the diffraction grating of the wafer is a, the amplitude of the light wave diffracted by the wafer is given by the following equation.

In this equation (8), when a /P = 1/2, Cmn becomes: (1) C-1sin-nπl/nπ=19) n2. After all, the expression for the light U (l!+m+r,n) diffracted via the mask-wafer mask is U()+m
ar, n) -Cr-CIIIn-Cノ・exp[-i (φxy+
{2yr /P), lIlΔxl ]Ein
−(10). In Equation (10), φxy is the amount of phase shift of the diffracted light with respect to the light before entering the mask, and is given by:

As an example of this diffracted light, the intensity 1 (0,1) of the 0th-order diffracted light in the X direction and the 1st-order diffracted light in the Y direction is determined as follows. In this case, the combination of ノ+m+r,w01n-1 may be considered. However, here, since the influence of the combination of high-order diffracted lights on the amplitude is small, the combinations of diffracted lights from the 0th order to the 3rd order are considered. [1 (m.

n), rl, the following five combinations may be considered.

to, (0,1), 01 +1. (0,1), -11! -1, (0,1), -31 H, (0,1) , -31 1-3, (0, l), 3] 2-πλz/p2 From equation (10), the light wave The display formula for is U(0
,1) = (1/π) [(1/4)+2(1/π) 2・1
exp(-122)+ (1/9) ・exp(-i
18Z))] Bin , (11). The intensity of light I (0,1) is +{0,1)-10(
0,1) + 2 (12). Therefore, the intensity of the incident light is I. Then, the intensity of light 1 (0,1) is 1(0,1) = (1/π) 2・t(1/4)2+{1/π)
2cos 2Z+{1/9) CLlyr) 2・c
os18Z+4 C1/π) 4+{8/9) (1/
π) 4 ・cos16Z+{2/9)2 ・(1/π) 4
)・I.

becomes. As is clear from this equation, the intensity of the diffracted light is 1
(0, 1) is a function only of the gap 2 between the mask and the wafer, and is unrelated to the amount of positional deviation ΔX in the X direction between them. This is because m (the diffraction order in the X direction in the wafer's diffraction grating) in equation (10) is zero, so ΔX
This is because the term is not included in equation (13).

The relationship between the gap between the mask and the wafer and the light intensity in this case is shown in FIG. 8A. As is clear from this figure, the periodic function contains high frequency components.

For reference, a case will be described in which the diffraction orders in each grating of the mask and wafer and diffracted light within ±1st order are taken into consideration. In this case, the following three combinations are possible.

[1, (0,1), -11, [-1, (0,1), 11
, [0, (0, 1), 01 The display formula of light is U[O, l] - (1/π) 3- e + {1/
π) 3・e−12z12z +{1/2)2 ・ (1/π) −(1#r) 3e−12z+{1/2)2(1/π)
...(14) The light intensity is 1(0,1)-10(0,1) l 2-4(1/π)
6+{1/2)4 ・ (1/π)2+ (11π)
4cos2Z −(15) Then (1,
1), (1,-1), (-1°1), and (-1,-1) diffracted lights are detected. for example,(
1, 1) is detected, the combination of the diffracted lights diffracted via the mask → wafer mask is [1, (
It is sufficient to consider only two sets: 0°1), 0] and [0, (0, 1), 1]. Because [2, (0, l), -1
1゜[-1,(0,1),2],...[8,(
0,1), -7], [-7, (0,1), 8]...
...but what is the amplitude of the diffracted light at the mask? /
When p-172, Cn -5in(nπ/2)/nyr. Therefore, for even orders of n-2, 4, 6, 8, . . . , C becomes zero.

Therefore, based on equation (10), the expression for the light wave (1, 1) is LIo, 1l-(1/2) ・(1/π) ・(1
/π) −e−12z−e−1x+{11π)・(1
1π)・(1/2)e−1x−iX −12Z =(1/2)(1/π) 2・e 11+e
1...(16) becomes. However, Xyu2π/P φΔx, Z−πλ/PX
Z, P: pitch of mask grating, ΔX: positional deviation amount, 2
: Gap, λ: Laser light wavelength.

The light intensity is +{1,1)-1U[1,1] 12-(1/2)(1/π)4・(1+cos2Z)
...(17). The relationship between the gap between the mask and the wafer and the light intensity in this case is shown in FIG. 8B. In this case, the periodic function does not include high frequency components.

Therefore, to adjust the gap between the mask and the wafer, (0
, 1) It is more preferable to detect the (1°1) order diffracted light than to detect the next diffracted light.

As described above, according to the setting method of the present invention, diffracted light is detected that is a plane that is perpendicular to the stripes of the diffraction grating of the mask and that does not lie along a plane that includes the axis of the incident light. Therefore, the reflected diffracted light reflected by the diffraction grating of the mask,
There is no interference with the detected diffracted light. Moreover, (
As is clear from equation (13) or equation (17), the detected diffracted light depends only on the gap 2 and is not affected by the position in the X direction. Therefore, when setting the gap,
There is no need to strictly control the position in the X direction. Also,(
±1. ±1) When the next diffracted light is detected, the detection signal does not include a high frequency component.

Therefore, in this case, the gap is set to the predetermined value more accurately than when (0, ±1) order diffracted light is detected.

Furthermore, a second embodiment of the present invention will be described based on FIGS. 9 to 11.

In this embodiment, two sets of first and second diffraction gratings are provided. That is, 29 first diffraction gratings 31-1, 31-2 are formed on the mask 13, and the wafer 12
, 29 second diffraction gratings 32-1, 32-2 are formed. As shown in FIG. 10, the first diffraction grating 31-1, 31-2 has a grating pitch in the X direction of pxl.
, px2. The first diffraction gratings 31-1, 31-2 are arranged at a distance U from each other. The second diffraction grating 32-1, 32-2 is a one-dimensional diffraction grating whose grating pitches in the X direction are pyt and py2. The second diffraction gratings 32-1, 32-2 are arranged at a distance V from each other.

The 29 first diffraction gratings 31-1 and 31-2 of the mask each have a different grating pitch. Furthermore, the 29 second diffraction gratings 32-1, 32-2 on the wafer also have different grating pitches. Therefore, the diffracted light diffracted by one set of first and second diffraction gratings 31-1, 32-1 and the other set of first and second diffraction gratings 31-2.
The diffracted light diffracted by 32-2 appears separately.

Therefore, the setting device includes a pair of photoelectric detectors 2B-1 and 28-2 that separately detect the 29 diffracted lights, and a subtractor 28.
is provided. The diffracted light of 29 is detected by this photoelectric detector 2.
B-1 and 28-2 convert the signals into electrical signals separately.

These electrical signals are transmitted through amplifiers 27-1 and 27-2, respectively.
is supplied to the subtractor 28 via. This subtracter 28 calculates the difference between the 29 electrical signals. That is, the difference in the intensity of the 29 diffracted lights is calculated. This electrical signal difference is supplied to the signal processing circuit 20. Similar to the first embodiment, a piezoelectric element drive signal is generated from the signal processing circuit 20, and a current is supplied to the piezoelectric element 25 based on this signal. This adjusts the gap between the mask and the wafer.

(1,1) In the case of the following diffracted light, the intensity of the 29 diffracted light is 1
1 (1,1), I2 (1,1), the subtracter 28 calculates the following equation.

ΔI=11 (1,1) 12 (1,1) The relationship between the gap between the mask and the wafer and the light intensity in this case is shown in FIG. 11B. The periodic function does not contain high frequency components. Therefore, the set value is detected at zero point in the linear portion of the periodic function. Therefore, adjustment of the gap becomes easy. Note that u and v are appropriately determined.

This is because, as is clear from equation (17), the intensity of the diffracted light does not depend on the positions of the first and second diffraction gratings in the X direction.

Further, FIG. 11A shows the change in light intensity with respect to the gap between the mask and the wafer in the case of (0,1) order diffracted light. In this case, the periodic function contains high frequency components. However, the high frequency components are not so high as to prevent zero point detection.

In this example, the diffracted light to be detected is
) as well as I (0, -1), which is a plane orthogonal to the stripes of the first diffraction grating and does not follow the plane containing the axis of the incident light. For example, △I=It (0-,-1) I2 (0,1
)ΔI-・Il (0,1) 12 (0,1)
ΔI■11 (0,-1)-12(0,1) may be obtained.

Furthermore, a modification of this second embodiment will be explained.

In this modification, as shown in FIG. 12, 29 diffracted lights diffracted by two sets of first and second diffraction gratings are detected synchronously in order to calculate the difference in intensity of 29 diffracted lights. be done.

The pitch of the 29 first gratings on the mask is different, while the pitch of the 29 second gratings on the wafer may be the same or different. First diffraction grating 31
−1.31-2 is the same distance U as shown in FIG.
are placed on the mask spaced apart by A second diffraction grating is similarly placed on the wafer, spaced apart by a distance V.

Since the diffracted lights of 29 are detected synchronously, the oscillator 5
1, a vibrating mirror 41, and a synchronous detection circuit 29 are provided. This oscillator 51 generates a reference signal of a predetermined frequency. These reference signals are transmitted to the vibrating mirror 41 and
The signal is supplied to the synchronous detection circuit 29. This causes the vibrating mirror 41 to vibrate at a predetermined frequency. Therefore, the laser beam is divided into two directions alternately at predetermined time intervals by the vibrating mirror 41 and is incident on the two sets of diffraction gratings.

The 29 diffracted lights diffracted by the two sets of diffraction gratings are incident on the photoelectric detector 26 alternately at predetermined time intervals via the mirror 33, and are converted into 29 electrical signals. The electrical signals 29 are alternately supplied to the synchronous detection circuit 29 via the amplifier 27 at predetermined time intervals. In the synchronous detection circuit 29,
Twenty-nine electrical signals are synchronously detected based on a reference signal of a predetermined frequency. As a result, the intensities of the 29 diffracted lights are detected, and the difference between the intensities of the 29 diffracted lights is calculated. Of course, the detected diffracted light is (0, ±1) order diffracted light,
(±1.±1) order diffracted light may be used.

Furthermore, a third embodiment of the invention will be described. In the first and second embodiments, in the mask and the wafer,
In each case, a one-dimensional diffraction grating was formed, but in this third embodiment, a one-dimensional diffraction grating is formed on the mask, and a two-dimensional diffraction grating is formed on the wafer.

Also in this case, when the laser beam is diffracted along the path of first diffraction grating - second diffraction grating - first diffraction grating, the first and second diffraction gratings act as a double diffraction grating. . Therefore, the 0th-order and ±1st-order diffracted lights appear in nine directions, as shown in FIG. 13. . Further, the reflected light reflected by the first diffraction grating is reflected within a plane including the X-axis and two axes. Therefore, also in this case, zero-order and ±1st-order diffraction lights that do not follow the plane including the X-axis and the two axes are detected.

When (0, ±1) order diffracted light is detected, the intensity of the diffracted light is determined as follows.

is given by If high-order diffracted light of second order or higher is ignored, f-1, (1,1), 01 . 11, (-1,1), 01, f-1, (0
, 1), 11, (0, (-1, 1), 11, to, (0, 1), 01, [), (1
,1), -11, +1. It is sufficient to consider seven values: (0,1) and -111. In this case, the intensity of the diffracted light 1 (0,1) is +{0,1)−(L/π) 2 ・[(1/2)6
+{1/π) 4+{1/2)2 (1/π) 2
・cos2Z+4(1/π) 'cos2X+4
(1/π) 2+{1/2)3+{1/π)2) cos
X-cosZ]Io (18). Here, X
−(2π/px)ΔX.

Therefore, in this case, the gap is adjusted because ΔX is not maintained so that cosX=0.

As shown in FIG. 14, in this embodiment as well, 2
A set of first and second gratings may be provided.

That is, the mask 13 includes 29 first diffraction gratings 35-1.
．． 35-2 is formed, and 29 diffraction gratings 33-1 and 38-2 are formed on the wafer 12. 14th
As shown in the figure, the first diffraction grating 35-1.35-
2 is a one-dimensional diffraction grating with a grating pitch in the X direction of Px. The first diffraction gratings as=t, as-2 are arranged at a distance U from each other in the X direction. The second diffraction grating 36-1 is a two-dimensional diffraction grating in which the grating pitch in the X direction is P1, and the grating pitch in the X direction is Pl. The second diffraction grating 33-2 has a grating pitch in the X direction of P2. This is a two-dimensional diffraction grating. The second diffraction gratings 33-1 and 38-2 are arranged at a distance v(-u
+P/2).

In this third embodiment, similarly to the second embodiment,
The gap is adjusted by detecting the difference in the intensity of the 29 diffracted lights. The gap is therefore adjusted by the same device as shown in FIG. Therefore, a description of the device will be omitted.

When the (0,1) order diffracted light is detected, the difference in intensity ΔI
is Δ I=11 (0,1) 12 (0,1
). The relationship between the gap and the difference in diffracted light intensity in this case is shown in FIG. The periodic function shown in this graph is the result of considering high-order diffracted light of ±2 or more based on equation (18).

Therefore, the periodic function contains high frequency components.

In this example, the difference between U and V is Px/2. Therefore, the diffraction light obtained by the diffraction grating 35-1.38-1 and the diffraction light obtained by the diffraction grating 35-2.36-2 are out of phase by π in the X direction. Therefore, if the positional deviation amount ΔX in the X direction is maintained at a predetermined value, the difference between the 29 diffracted lights is detected, and the set value is detected as 0 point in the linear portion of the periodic function.

Furthermore, in this third embodiment, the gap is also adjusted using the detection method using synchronous detection shown in FIG. In this case, the X of the 29 second gratings of the wafer
The pitches in the directions may be the same or different. Furthermore, if the laser light is incident obliquely with respect to the X direction, there is an advantage that the detection optical system (for example, a mirror) does not block the exposure light for exposing the circuit pattern.

Next, a fourth embodiment of the invention will be described. In the embodiment described above, the incident light is perpendicular to the mask 13. However, the incident light is not necessarily the mask 1
It does not have to be perpendicular to 3.

In this fourth embodiment, a case where light is incident on the mask 13 obliquely will be explained.

As shown in FIG. 16, the first diffraction grating 15 formed on the mask 13 and the second diffraction grating 16 formed on the wafer 12 are arranged in the same manner as shown in FIG. ing. As shown in FIG. 16, a virtual plane perpendicular to the stripes of the first diffraction grating 15 is a first virtual plane 101.
It is stipulated that A virtual surface obtained by tilting the first virtual surface 101 at a predetermined angle (α) in the direction of the stripes is defined as a second virtual surface 102 . First virtual surface 101
A virtual surface that is symmetrical to the second virtual surface 102 with reference to is defined as a third virtual surface 103. Light emitted from the laser 17 is incident on the first diffraction grating 15. The optical axis 104 of this incident light is along the second virtual plane 102. Some of the incident light is reflected and diffracted by the upper surface of the first diffraction grating 15. This reflected diffraction light is transmitted to the third virtual surface 103
will be migrated only in accordance with

Most of the remaining incident light is transmitted and diffracted by the first diffraction grating 15 to reveal a first diffracted light, and this first diffracted light is transferred to the second diffraction grating 16 where it is subjected to second diffraction. A second diffracted light is reflected and diffracted by the grating, and this second diffracted light is transferred to the first diffraction grating 15 and transmitted through and diffracted by the first diffraction grating 15, thereby revealing a third diffracted light. Served. This third diffraction light appears as a two-dimensional diffraction pattern. This diffraction pattern is similar to the case where the incident light is orthogonal to the first diffraction grating 15.

However, the origin 1 (0, O) of the diffraction pattern overlaps with the case where the incident lights are orthogonal. That is, as shown in FIG. 16, the origin 1 (0, O') is a line (
2'). That is, when the incident light is orthogonal to the first diffraction grating 15 (α-Or z
' -z), the origin l (0°0) is on the optical axis of the incident light.

Some third diffracted light, i.e. I (0, O), ■ (±
1.0) The next diffracted light is transferred along the third virtual plane 103, and the other third diffracted light, namely ■ (0, ±1), I
(±1.±1) The next diffracted light is transferred to a location other than the third virtual surface 103. Therefore, the other third diffracted light does not interfere with the reflected diffracted light reflected and diffracted by the upper surface of the first diffraction grating 15.

Therefore, similarly to the above-described embodiment, the other third diffracted light, that is, the (0,±1) and I (±1°±1) order diffracted light is detected. The detection results are substantially the same as those shown in FIGS. 8A and 8B. Therefore, the incident light is the first
Even when the incident light is incident on the first diffraction grating 15 obliquely, it is accurately aligned in the same way as when the incident light is incident on the first diffraction grating 15 at right angles.

As shown in FIG. 17, the second diffraction grating may be a two-dimensional diffraction grating. In this case, the diffraction pattern when the incident light is obliquely incident on the first diffraction grating 15 is the same as when the second diffraction grating is one-dimensional. The detection results in this case are substantially the same as those for the 11th person, 11B.

[Effects of invention The third diffracted light appears in a two-dimensional pattern.

On the other hand, the reflected diffraction light reflected by the surface of the first diffraction grating is reflected only along the third virtual plane. In this invention, the diffracted light that does not follow this third virtual plane (that is, the other third diffracted light) is detected. Therefore, the detected diffraction light and the reflected diffraction light do not interfere, and the first object and the second object
The gap with the object is set accurately to a predetermined value.

Further, as theoretically analyzed in the embodiment, the gap between the first object and the second object is accurately set to a predetermined value regardless of the relative positions of the first object and the second object.

Furthermore, the present invention can also be applied to relative alignment between a mask and a wafer in an exposure apparatus.

Therefore, the first object may be a mask, and the second object may be a wafer.

[Brief explanation of the drawing]

FIG. 1 is a diagram schematically showing the principle of a method of setting the gap between the mask and Ueno 1 to a predetermined value based on the prior art. FIG. 2 is a diagram showing the detection obtained by the method shown in FIG. FIG. 3 is a graph showing the results and shows the relationship between the intensity of diffracted light and the gap; FIG. FIG. 4 is a perspective view showing the first and second diffraction gratings formed on the mask and the wafer, respectively, in the apparatus shown in FIG. 3; FIG. 5 is the mask FIG. 6 is a perspective view showing the pattern of diffracted light diffracted by the diffraction gratings of the wafer and the wafer, and FIG. 6 is a diagram showing the principle of diffraction based on the present invention. FIG. 7 is a diagram showing the principle of diffraction based on the present invention, in which incident light is transmitted from the first diffraction grating of the mask to the second diffraction grating of the wafer.
Figures 8A and 8B, which schematically show an optical model equivalent to the optical model when diffracted by the first diffraction grating of the mask, are based on the first embodiment of the present invention. FIG. 9 is a graph showing the relationship between the intensity of the diffracted light detected by the device and the gap between the mask and the wafer, based on the second embodiment of the present invention. FIG. 10 is a plan view schematically showing the apparatus shown in FIG. 9, and FIG. 11A and FIG. 11B are diagrams showing diffraction gratings formed on each of the mask and the wafer. is the difference in the intensity of light and light weight detected by the method based on the second embodiment of the present invention, and
A graph showing the relationship between the gap between the mask and the wafer, FIG. 12, schematically shows an apparatus for setting the gap between the mask and the wafer to a predetermined value based on a modification of the second embodiment of the present invention. FIG. 13 is a perspective view showing patterns of diffracted light diffracted by the diffraction gratings of the mask and wafer, respectively, in the third embodiment of the present invention. FIG. 15 is a plan view showing a diffraction grating used in the method of setting the gap between a mask and a wafer to a predetermined value based on the embodiment, and FIG. 15 shows the diffraction detected by the method based on the third embodiment of the present invention. 16 and 17, which are graphs showing the relationship between the difference in light intensity and the gap between the mask and the wafer, are graphs showing the relationship between the difference in light intensity and the gap between the mask and the wafer. FIG. 16 is a perspective view showing a pattern of diffracted light; FIG. 16 shows a case where the second diffraction grating is one-dimensional, and FIG. 17 shows a case where the second diffraction grating is two-dimensional. 13... First object (mask), 12... Second object (wafer), 15... First diffraction grating, 16...
Second diffraction grating, 17... Light source (laser), 19...
・Detection means (mirror), 26... detection means (photoelectric detector), 20... gap adjustment means (signal processing circuit),
22... Gap adjustment means (piezoelectric element drive circuit), 2
5... Gap adjustment means (piezoelectric element), 101...
First virtual surface, 102... Second virtual surface, 103...
- Third virtual surface, 104...optical axis of incident light. Applicant's Representative Patent Attorney Takehiko Suzue Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8A Figure 88 Figure 9 Figure 10 Figure 11A Figure 13- B; 20 = Figure 16 Figure 17

Claims (34)

[Claims] (1) A method for setting a gap between a first object and a second object, which are arranged opposite to each other, to a predetermined value, comprising: a one-dimensional diffraction grating having a pattern of parallel stripes; A first diffraction grating in which the stripes extend in a first direction is formed on a first object, a virtual plane orthogonal to the first direction is defined as a first virtual plane, and a first diffraction grating is formed on a first object.
A virtual surface in which the virtual surface of a one-dimensional diffraction grating having a pattern of parallel stripes, the stripes forming a second direction perpendicular to the first direction;
forming a second diffraction grating extending in the direction of the second object; and incident light emitted from the light source and having an optical axis along the second virtual plane and the first direction the first diffraction light is incident on the diffraction grating, the incident light is transmitted and diffracted by the first diffraction grating, and the first diffraction light is revealed; a step in which the diffracted light is reflected and diffracted by a second diffraction grating to reveal a second diffracted light, and the second diffracted light is transferred to the first diffraction grating and the second diffracted light is a step of being transmitted and diffracted by the first diffraction grating to reveal a third diffracted light, and some of the third diffracted light is transferred along a third virtual plane;
The other third diffracted light is transferred to a surface other than the third virtual plane, the other third diffracted light is detected, and the first diffracted light is transferred according to the intensity of the detected diffracted light. A method for setting a gap between a first object and a second object to a predetermined value, the method comprising the steps of adjusting the gap between the object and the second object and setting the gap to a predetermined value. . (2) The gap between the first object and the second object according to claim 1, wherein the first object is a mask and the second object is a wafer. How to set it to a given value. (3) A predetermined angle between the first virtual surface and the second virtual surface (
The method for setting the gap between the first object and the second object to a predetermined value according to claim 1, characterized in that α) is zero. (4) The second direction of the second diffraction grating is defined as the x direction, the first direction of the second diffraction grating is defined as the y direction, and is symmetrical to the optical axis of the incident light with respect to the first virtual plane. A point on a certain line is defined as an origin, and in the step of detecting the diffracted light, (0, ±1)-order light of the other third diffracted light is detected. A method of setting the gap between the first object and the second object to a predetermined value according to the first range. (5) In the step of detecting the diffracted light, (±1, ±1)-order light of the other third diffracted light is detected. A method of setting the gap between a first object and a second object to a predetermined value. (6) The gap between the first object and the second object according to claim 1, wherein two sets of the first diffraction grating and the second diffraction grating are provided. How to set to a given value. (7) The pitches of the two first diffraction gratings are different,
The two second diffraction gratings have different pitches, and in the step of detecting the diffracted light, the third diffraction light diffracted from the first and second diffraction gratings of one set and the other third diffraction light are detected. The other third diffracted light diffracted from the first and second diffraction gratings of the set is detected separately, the difference in intensity between these two diffracted lights is calculated, and the difference in intensity is calculated according to the difference in intensity. The method for setting the gap between the first object and the second object to a predetermined value according to claim 6, characterized in that the gap between the first object and the second object is adjusted. . (8) The pitches of the two first diffraction gratings are different, and the two sets of first and second diffraction gratings are alternately irradiated with light emitted from the light source at predetermined time intervals, and the diffracted light is In the detecting step, the other third diffracted light diffracted from the first and second diffraction gratings of one set and the other third diffracted light diffracted from the first and second diffraction gratings of the other set. The third diffracted light is detected alternately at predetermined time intervals, the difference in intensity between the two diffracted lights is calculated, and the gap between the first object and the second object is adjusted according to this difference in intensity. 7. A method for setting a gap between a first object and a second object to a predetermined value according to claim 6. (9) In a method of setting a gap between a first object and a second object, which are arranged opposite to each other, to a predetermined value, a one-dimensional diffraction grating having a pattern of parallel stripes, A first diffraction grating extending in a first direction is formed on the first object, a virtual surface perpendicular to the first direction is defined as a first virtual surface, and the first diffraction grating extends in a first direction.
A virtual surface in which the virtual surface of a step in which a second diffraction grating, which is a two-dimensional diffraction grating having a pattern of two orthogonal stripes, is formed on the second object; Incident light whose axis is along the second virtual plane and the first direction is incident on the first diffraction grating, the incident light is transmitted and diffracted by the first diffraction grating, and a first diffracted light appears. a step in which the first diffracted light is transferred to a second diffraction grating, the first diffracted light is reflected and diffracted by the second diffraction grating to reveal a second diffracted light; a step in which the diffracted light is transferred to the first diffraction grating, the second diffracted light is transmitted and diffracted by the first diffraction grating, and a third diffracted light appears; is migrated along a third virtual plane,
The other third diffracted light is transferred to a surface other than the third virtual plane, the other third diffracted light is detected, and the first diffracted light is transferred according to the intensity of the detected diffracted light. A method for setting a gap between a first object and a second object to a predetermined value, the method comprising the steps of adjusting the gap between the object and the second object and setting the gap to a predetermined value. . (10) The first object is a mask, and the second object is
A method for setting a gap between a first object and a second object to a predetermined value according to claim 9, wherein the object is a wafer. (11) The first object and the second object according to claim 9, wherein the predetermined angle (α) between the first virtual plane and the second virtual plane is zero. A method of setting the gap between the object and the object to a predetermined value. (12) The second direction of the second diffraction grating is defined as the x direction, the first direction of the second diffraction grating is defined as the y direction, and is symmetrical to the optical axis of the incident light with the first virtual plane as a reference. A point on a certain line is defined as an origin, and in the step of detecting the diffracted light, (0, ±1)-order light of the other third diffracted light is detected. A method of setting the gap between the first object and the second object to a predetermined value according to range item 9. (13) The gap between the first object and the second object according to claim 9, wherein two sets of the first diffraction grating and the second diffraction grating are provided. How to set to a given value. (14) The distance between the two first diffraction gratings is u, the distance between the two second diffraction gratings is v, the pitch of the first and second diffraction gratings in the x direction is p_x, and N is an arbitrary integer. When u and v are defined as v=u+{(2N+1)/2)}・p_x, in the step of detecting the diffracted light, the diffracted light from the first and second diffraction gratings of one set is and the other third diffracted light diffracted from the other set of first and second diffraction gratings are detected separately, and the intensities of these two diffracted lights are detected separately. The difference between the first object and the second object according to claim 13 is calculated, and the gap between the first object and the second object is adjusted according to this difference in intensity. A method of setting the gap with the second object to a predetermined value. (15) The pitches of the two first diffraction gratings are the same,
The two second diffraction gratings have different pitches in the y direction, and in the step of detecting the diffraction light, the other third diffraction light diffracted from the first and second diffraction gratings of one set is and the first and second of the other set, which have the same order as this,
Claim 14, characterized in that the other third diffracted light diffracted from the diffraction grating is detected separately.
A method of setting the gap between the first object and the second object to a predetermined value as described in . (16) The pitch of the two first diffraction gratings is the same, and the two sets of first and second diffraction gratings are alternately irradiated with light emitted from the light source at predetermined time intervals, and the said diffraction light is In the detecting step, the other third diffracted light diffracted from the first and second diffraction gratings of one set and the other third diffracted light diffracted from the first and second diffraction gratings of the other set. The third diffracted light is detected alternately at predetermined time intervals, the difference in intensity between the two diffracted lights is calculated, and the gap between the first object and the second object is adjusted according to this difference in intensity. A method for setting a gap between a first object and a second object to a predetermined value according to claim 13. (17) A first object and a second object placed opposite each other.
a one-dimensional diffraction grating having a pattern of parallel stripes, the stripes extending in a first direction and perpendicular to the first direction; A virtual surface is defined as a first virtual surface, a virtual surface obtained by tilting the first virtual surface at a predetermined angle (α) in the first direction is defined as a second virtual surface, and the first virtual surface is defined as a second virtual surface. A virtual plane symmetrical to the second virtual plane is defined as a third virtual plane as a reference; a first diffraction grating formed on the first object; and a one-dimensional diffraction grating having a pattern of parallel stripes. and this stripe is in a second direction orthogonal to the first direction.
a second diffraction grating formed on the second object and extending in the direction of; and an optical axis emitting incident light incident on the first diffraction grating along the second virtual plane and the first direction. the first light source and the incident light is the first
The first diffracted light is transmitted and diffracted by the diffraction grating of
This first diffraction light is transferred to a second diffraction grating, reflected and diffracted by the second diffraction grating to reveal a second diffraction light, and this second diffraction light is transferred to the first diffraction grating. The first diffraction grating transmits and diffracts the third diffracted light, and some of the third diffracted light is transferred along the third virtual plane, and the other third diffracted light is transmitted and diffracted by the first diffraction grating.
The diffracted light is transferred to a surface other than the third virtual surface, and
and a gap adjustment means that adjusts the gap between the first object and the second object according to the intensity of the detected diffracted light and sets the gap to a predetermined value. A device for setting a gap between a first object and a second object to a predetermined value. (18) The first object is a mask, and the second object is
18. The apparatus for setting the gap between the first object and the second object to a predetermined value according to claim 17, wherein the apparatus is a wafer. (19) The first object and the second object according to claim 17, wherein the predetermined angle (α) between the first virtual plane and the second virtual plane is zero. A device that sets the gap with an object to a predetermined value. (20) The second direction of the second diffraction grating is defined as the x direction, the first direction of the second diffraction grating is defined as the y direction, and is symmetrical to the optical axis of the incident light with respect to the first virtual plane. A point on a certain line is defined as the origin, and the means for detecting the diffracted light includes a detecting means for detecting (0, ±1) order light of the other third diffracted light. An apparatus for setting a gap between a first object and a second object to a predetermined value according to claim 17. (21) The means for detecting the diffracted light includes a detecting means for detecting (±1, ±1) order light of the other third diffracted light and a detecting means for detecting the diffracted light. An apparatus for setting a gap between a first object and a second object to a predetermined value according to claim 17. (22) The detection means for detecting the diffracted light includes a mirror arranged to reflect the diffracted light of a predetermined order of the other third diffracted light, and photoelectrically converts the diffracted light reflected by the mirror. and a photoelectric detector that uses this as a detection signal, and the gap adjustment means includes a signal processing circuit that processes the electrical signal and generates a control signal, and based on this control signal,
The first object and the second object according to claim 17, further comprising a mask moving means for moving the mask a predetermined distance and adjusting the gap between the mask and the wafer. A device that sets the gap between the (23) The gap between the first object and the second object according to claim 17, wherein two sets of the first diffraction grating and the second diffraction grating are provided. A device that sets the value to a predetermined value. (24) The pitches of the two first diffraction gratings are different, and the pitches of the two second diffraction gratings are different, and the detection means for detecting the diffracted light is connected to the first and second diffraction gratings of one set. Detecting means for separately detecting the other third diffracted light diffracted from the diffraction grating and the other third diffracted light diffracted from the other set of first and second diffraction gratings. and the first object according to claim 23, wherein the gap adjustment means includes calculation means for calculating the difference in intensity between the two diffracted lights. A device that sets a gap with a second object to a predetermined value. (25) The pitches of the two first diffraction gratings are different, and the detection means for detecting the diffracted light is configured such that the light emitted from the light source is transmitted to the two sets of first and second diffraction gratings at predetermined time intervals. When irradiated alternately, the other third diffracted light diffracted from the first and second diffraction gratings of one set and the third diffracted light diffracted from the first and second diffraction gratings of the other set. The gap adjustment means is equipped with a detection means that alternately detects the other third diffracted light at predetermined time intervals, and the gap adjustment means is equipped with a calculation means that calculates the difference in intensity between the two diffracted lights. An apparatus for setting a gap between a first object and a second object to a predetermined value according to claim 23. (26) A first object and a second object placed opposite each other.
a one-dimensional diffraction grating having a pattern of parallel stripes, the stripes extending in a first direction and perpendicular to the first direction; A virtual surface is defined as a first virtual surface, a virtual surface obtained by tilting the first virtual surface at a predetermined angle (α) in the first direction is defined as a second virtual surface, and the first virtual surface is defined as a second virtual surface. A virtual plane that is symmetrical to the second virtual plane as a reference is defined as a third virtual plane, and a two-dimensional structure having a first diffraction grating formed on the first object and two sets of orthogonal stripe patterns. a second diffraction grating formed on a second object, the optical axis of which emits incident light that is incident on the first diffraction grating along a second virtual plane and the first direction; the first light source and the incident light is the first
The first diffracted light is transmitted and diffracted by the diffraction grating of
This first diffraction light is transferred to a second diffraction grating, reflected and diffracted by the second diffraction grating to reveal a second diffraction light, and this second diffraction light is transferred to the first diffraction grating. The first diffraction grating transmits and diffracts the third diffracted light, and some of the third diffracted light is transferred along the third virtual plane, and the other third diffracted light is transmitted and diffracted by the first diffraction grating.
The diffracted light is transferred to a surface other than the third virtual surface, and
and a gap adjustment means that adjusts the gap between the first object and the second object according to the intensity of the detected diffracted light and sets the gap to a predetermined value. A device for setting a gap between a first object and a second object to a predetermined value. (27) The first object is a mask, and the second object is
27. The device for setting the gap between the first object and the second object to a predetermined value according to claim 26, wherein the device is a wafer. (28) The first object and the second object according to claim 26, wherein the predetermined angle (α) between the first virtual plane and the second virtual plane is zero. A device that sets the gap with an object to a predetermined value. (29) The second direction of the second diffraction grating is defined as the x direction, the first direction of the second diffraction grating is defined as the y direction, and is symmetrical to the optical axis of the incident light with the first virtual plane as a reference. A point on a certain line is defined as the origin, and the means for detecting the diffracted light includes a detecting means for detecting (0, ±1) order light of the other third diffracted light. An apparatus for setting a gap between a first object and a second object to a predetermined value according to claim 26. (30) The detection means for detecting the diffracted light includes a mirror arranged to reflect the diffracted light of a predetermined order of the other third diffracted light, and photoelectrically converts the diffracted light reflected by the mirror. and a photoelectric detector that uses this as a detection signal, and the gap adjustment means includes a signal processing circuit that processes the electrical signal and generates a control signal, and based on this control signal,
The first object and the second object according to claim 26, further comprising a mask moving means for moving the mask a predetermined distance and adjusting the gap between the mask and the wafer. A device that sets the gap to a predetermined value. (31) The gap between the first object and the second object according to claim 26, wherein two sets of the first diffraction grating and the second diffraction grating are provided. A device that sets the value to a predetermined value. (32) The distance between the two first diffraction gratings is u, the distance between the two second diffraction gratings is v, the pitch of the first and second diffraction gratings in the x direction is p_x, and N is an arbitrary integer. When u and v are defined as v=u+{(2N+1)/2}・p_x, the detection means for detecting the diffracted light is configured to detect the diffracted light from the first and second diffraction gratings of one set. and detecting means for separately detecting the other third diffracted light diffracted from the first and second diffraction gratings of the other origin, and the gap The adjustment means is provided with a calculation means for calculating the difference in intensity between the two diffracted lights, and adjusts the gap between the first object and the second object according to claim 31. A device that sets a predetermined value. (33) The pitches of the two first diffraction gratings are the same,
The two second diffraction gratings have different pitches in the y direction, and the detection means for detecting the diffraction light detects the other third diffraction light diffracted from the first and second diffraction gratings of one set. It is characterized by comprising a detection means for separately detecting the light and the other third diffracted light having the same order as this and diffracted from the first and second diffraction gratings of the other set. An apparatus for setting a gap between a first object and a second object to a predetermined value according to claim 32. (34) The pitch of the two first diffraction gratings is the same, and the means for detecting the diffracted light is such that the light emitted from the light source is alternately applied to the two sets of first and second diffraction gratings at predetermined time intervals. When the other third diffracted light is diffracted from the first and second diffraction gratings of one set, and the other third diffracted light is diffracted from the first and second diffraction gratings of the other set. and a third diffracted light at predetermined time intervals, and the gap adjustment means includes a calculation means for calculating the difference in intensity between the two diffracted lights. An apparatus for setting a gap between a first object and a second object to a predetermined value according to claim 31.