JP2718165B2 - Interval measuring device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an interval measuring apparatus for measuring an interval between two objects with high accuracy, for example, in a semiconductor manufacturing apparatus, measuring an interval between a mask and a wafer, This is suitable for controlling to a predetermined value.

2. Description of the Related Art Conventionally, in a semiconductor manufacturing apparatus, the distance between a mask and a wafer is measured by an interval measuring device or the like, and controlled so as to be a predetermined distance, and then the pattern on the mask surface is exposed on the wafer surface. Transcribed. As a result, highly accurate exposure transfer is performed.

The present applicant disposes a first object such as a mask provided with a physical optical system element and a second object such as a wafer in a previous Japanese Patent Application No. 63-33206 in opposition to each other, and attaches a physical optical element on the first object to the first object. A light beam is made incident, the light deflected in a predetermined direction by the physical optical element is reflected by the second object surface, and then guided onto the light receiving means surface, and the light (spot) incident position on the light receiving means surface A distance measuring device that determines the distance between the first object and the second object by detecting the distance is proposed.

In this method, no problem occurs when the projected light beam accurately irradiates only the physical optical element for the incident light beam.However, if the light beam diameter is large or the position to this element is misaligned, the physical property for the emitted light beam is not increased. When irradiating the optical element, counter-incident light is generated. In this case, the counter-incident light of the projected light beam passes through the physical optical element for the output light beam, changes in a direction defined by the same, is reflected by the wafer, and then transmits through the physical optical element for the incident light beam. It may be emitted.

This so-called counter-incident light beam enters the vicinity of the normal signal light beam incident position on the light receiving means surface.

In this case, the position of the center of gravity obtained by integrating the spots of the two adjacent light beams is detected. As for the normal signal light beam, as shown in the proposal, the spot movement amount of the light beam on the light receiving means surface is theoretically linear with respect to the change in the interval between the two measured surfaces. This is shown by the straight line in Fig. 10.
Shown as S1. In this case, it is extremely easy to calculate the interval between the surfaces to be measured from the position of the spot.

On the other hand, when the back-incident light beam passes through the physical optical element for the outgoing light beam, is reflected by the second object, and then passes through the physical optical element for the incident light beam, vignetting of the light beam generally occurs. In addition, since the vignetting state changes depending on the distance between the surfaces to be measured, the spot movement amount on the light receiving means surface becomes non-linear with respect to the change in the surface distance. This is shown as a curve S2 in FIG.

In this case, the change in the overall center of gravity of the spot of the normal signal light beam and the spot of the reverse incident light beam is also non-linear with respect to the change in the measured surface interval. Therefore, the process of obtaining the surface interval from the spot movement amount becomes complicated. Therefore, further improvement is required when the light flux is large or when there is a possibility that the incident position on the element may fluctuate.

(Problems to be Solved by the Invention) The present invention is an improved invention of the prior application, and when a first object and a second object corresponding to a mask and a wafer are arranged to face each other and the distance between them is measured, By appropriately setting the components, the so-called back-incident light beam can be effectively prevented from being incident on the light receiving means surface, and the first object and the second object can be accurately determined without a complicated signal processing circuit or the like. It is an object of the present invention to provide an interval measuring device capable of measuring an interval of the object.

(Means for Solving the Problems) A first object and a second object, which are partially provided with a first physical optical element and a second physical optical element, are arranged to face each other, and a first object on the first object plane is provided. The luminous flux from the light projecting means is incident on the physical optical element,
After diffracted light of a predetermined order from the first physical optical element is reflected by the second object plane, the diffracted light is incident on a second physical optical element on the first object plane, and the predetermined light from the second physical optical element is reflected by the second physical optical element. By guiding the diffracted light of the order onto the light receiving means surface and detecting the incident position of the diffracted light on the light receiving means surface, the distance between the first object and the second object is determined. The light beam from the light projecting means is directly incident on a region where the principal ray is incident on the surface of the second physical optical element, and the light beam of a predetermined order diffracted from the second physical optical element is reflected on the second object surface. Thereafter, each element is set so as not to enter the first physical optical element.

(Embodiment) FIG. 1 is a schematic diagram of an optical system according to an embodiment in which the present invention is applied to an apparatus for measuring a distance between a mask and a wafer in a semiconductor manufacturing apparatus.

Light beam from the light source 101 of 1 such as for example H _e -N _e laser or a semiconductor laser in the figure, 2 first object, for example a mask, 3 is a second object, for example a wafer, the mask 2 and the wafer 3, for example It is oppositely disposed at an interval d ₀ as shown in FIG. 2 (a). Reference numerals 4 and 5 denote first and second physical optical elements for incidence and emission, respectively, provided on a part of the mask 2 surface.
These physical optical elements 4 and 5 are composed of, for example, linear diffraction gratings of equal pitch.

8 is a light receiving means, which is composed of a line sensor, a PSD, etc.
The position of the center of gravity of the incident light beam is detected. Reference numeral 9 denotes a signal processing circuit which obtains the position of the center of gravity of the light beam incident on the surface of the light receiving means 8 using the signal from the light receiving means 8 and calculates the distance between the mask 2 and the wafer 3 as described later. .

Reference numeral 10 denotes an optical probe, which includes a light receiving means 8 and, if necessary, a signal processing circuit 9;
Is relatively movable.

In this embodiment, the distance measurement direction is the z direction, the direction perpendicular to the z direction, the direction parallel to the mask surface 2 is the y direction, and the direction perpendicular to the z, y directions is the x direction. Then, as shown in FIG. 2 (B), a plane parallel to the x direction and perpendicular to the mask surface 2 is defined as P,
A plane parallel to the y-direction and perpendicular to the mask plane 2 is defined as P ′, and the incident light 1 on the mask plane 2 is projected onto the P plane and the P ′ plane,
Handle the optical path when disassembled in two directions.
(Note that FIG. 1 shows an optical path on the P surface.) In this embodiment, a light beam 101 (wavelength λ = 830 nm) from a light source 101, for example, a semiconductor laser, is applied to a first light beam on a mask 2 surface.
The light is made to enter the point A on the surface of the diffraction grating 4 from an oblique direction.
The diffraction light of a predetermined order diffracted at an angle θ1 from the first diffraction grating 4 is reflected at a point B (C) on the surface of the wafer 3. The reflected light 31 is the reflected light when the wafer 3 is located at the position P1 near the mask 2 and the reflected light 32 is the reflected light 32
There is a reflected light when displaced from the position P1 by a distance d _G.

Next, the reflected light from the wafer 3 is converted to the second light on the first object 2 surface.
At the point D (E) on the surface of the diffraction grating 5.

The second diffraction grating 5 has an optical function of emitting diffracted light having a constant emission angle regardless of the incident position of the incident light beam.

Then, the diffracted light (61, 62) of a predetermined order diffracted at an angle θ2 from the second diffraction grating 5 is guided to the surface of the light receiving means 8 via the condenser lens 7.

Then, the distance between the mask 2 and the wafer 3 is calculated using the position of the center of gravity of the incident light beam (61, 62) on the surface of the light receiving means 8 at this time.

In the present embodiment, the first and second diffraction gratings 4 and 5 provided on the surface of the mask 2 are formed at a predetermined known pitch, and a predetermined order (for example, ± 1 order) of the light beam incident thereon.
Are obtained in advance.

FIG. 3 is an explanatory diagram showing the functions of the first and second diffraction gratings 4 and 5 on the surface of the mask 2 and the relationship between the distance between the mask 2 and the wafer 3.

FIG. 3A shows the first and second physical optical elements 4 on the mask 2 surface.
5 (B) is an optical path on the P plane in FIG. 2 (B), and FIG. 2 (C) is an optical path on the P ′ plane in FIG. 2 (B). FIG.

In the present embodiment, the first and second diffraction gratings 4 simply have a function of bending incident light, but may have a converging or diverging effect.

In the figures (A), (B) and (C), for example, the point 11 is the mask 2
As the distance between the mask 2 and the wafer 3 increases at the transmission point of the center of gravity of the emitted light beam when the distance between the wafer and the wafer 3 is 100 μm, the transmission point of the emitted light beam moves rightward in FIG. Is set so that point 12 is transmitted.

The diffraction grating pattern may be provided with convergence and divergence powers in the direction A in FIG. 2A to adjust the spread of the light beam.

In FIG. 3, when the distance measurement range between the mask 2 and the wafer 3 is set to, for example, 100 μm to 200 μm, the first and second diffraction gratings are correspondingly considered in consideration of the condition of the reverse incidence line described later. What is necessary is just to set the area of 4,5.

Next, a method for determining the distance between the mask 2 and the wafer 3 will be described with reference to FIG.

When as shown in FIG. _{1, AD = 2d 0 tanθ1, AE} = 2 (d 0 + d G) tanθ1, a _{∴d M = DE = AE-AD} = 2d G tanθ1 ...... (1). The amount of movement S of the incident light on the surface of the light receiving means 8 is as follows: S = d _M = 2d _G tan θ1 (2) The shift amount ΔS of the incident light beam on the surface of the light receiving means 8 with respect to the change in the unit gap between the mask 2 and the wafer 3 That is, the sensitivity ΔS is Becomes

In this embodiment, by detecting the position S of the incident light beam on the surface of the light receiving means 8, the distance d _G is obtained from the equation (2), and the distance d from the predetermined position between the mask 2 and the wafer 3 is obtained from this value d _G. The amount of deviation is determined.

Mask 41 and the wafer 42 is opposed at a distance d ₀ as the first reference. The value of d ₀ when this other example TM-23
Measure with 0N (trade name: manufactured by Canon Inc.) or the like.
At this time, d _G is obtained from the value of d ₀ and the incident position of the light beam by the equation (2).

Next, as shown in FIG. 2 (B), the second physics of the counter-incident light beam 13 incident on the second physical optical element 5 at the same incident angle as the light beam 1 incident on the first physical optical element 4 on the mask surface 2 After the diffracted light from the optical element 5 is diffracted by the first physical optical element 4 of the light beam 1, the diffracted light of the same order as the diffracted light incident on the light receiving means 8 is reflected on the surface of the wafer 3 as shown by a dotted line. The conditions for preventing the light from being incident on the first physical optical element 4 back and diffracted, and then incident on the light receiving means 8 and becoming a noise will be described.

FIG. 4 is an explanatory view showing the arrangement of the first and second physical optical elements 43 and 44 and the optical path of a normal signal light beam 48. For convenience of explanation,
Each physical optical element is a linear diffraction grating having only a light beam deflecting function, and the principal ray of each light beam is on the plane shown in FIG.

In the figure, 41 is a mask, 42 is a wafer, 43 is a first physical optical element as an incident diffraction grating, and 4 is a second physical optical element as an outgoing diffraction grating. Reference numeral 48 denotes a principal ray of a light beam projected from the light source, which is incident on a point P on the incident diffraction grating 43.

Now, the mask 1 and the spacing between the wafer 2 alpha is changed to the maximum value d ₂ from the minimum value d ₁ distance measurement range, the point of incidence position from the point to Q ₁ on the exit diffraction grating 44 of the principal ray 48 Q ₂ Move between. Further, the incident angle and the outgoing angle of the chief ray 48 are θ
Assuming that ₁ , θ ₂ , θ ₃ (the angle is positive in the counterclockwise direction from the surface normal), there is the following relationship.

Here, λ is the wavelength, and P ₁ and P ₂ are the pitches m ₁ and m ₂ of the incident diffraction grating and the output diffraction grating, respectively, are the diffraction orders.

Next, the state of the optical path of the reverse incident light beam is shown in FIG. The counter-incident light beam 49 enters the output diffraction grating 44 at the same incident angle θ ₁ as the normal light beam 48, exits at the diffraction angle θ ₄ , reflects off the wafer 2, enters the incident diffraction grating 43, and It is emitted at θ _5. At this time, there is the following relationship.

From equations (5) and (7) From equations (4) and (6) Becomes Therefore, sin θ ₃ = sin θ _{5, that} is, the reverse incident light beam 49 has the same angle as the normal light beam 48 at the mask 41.
Is emitted. For this reason, as described above, it is generally difficult for the two light beam spots to overlap and separate on the surface of the light receiving means 8, which has an adverse effect on the measurement of the surface distance.

Next, conditions for preventing the reverse incident light beam will be considered. Of course, it is most desirable that the back-incident light beam be completely removed. However, in practice, if the ratio of the amount of light to the normal signal light beam is small to some extent, adverse effects can be substantially avoided.

In general, as a result of various investigations, the counter-incident light beam that enters the point where the principal ray of the normal signal light beam enters the output diffraction grating 44 is reflected by the wafer 42 over the entire interval measurement range, and then reflected by the incidence diffraction grating 43. If it is not reached, substantially good detection can be performed.

A light ray passing through the center of the signal light flux incident area on the surface of the diffraction grating 44 for emission is defined as a principal light ray of the signal light flux.

That is, in FIG. 5, after the light rays incident on the line segments Q ₁ and Q ₂ are reflected by the wafer 42, the light is prevented from being incident on the incident diffraction grating 43 within the range of d ₁ ≦ d ≦ d _2. Better detection is possible.

After reflection this light group in the wafer 42, the region that is incident on the mask 41 again, a line segment S _1, S ₂ For the case of d = d ₁ of the line segment _{R 1, R 2, d =} d 2 . Considering only the point S _1, as is clear from FIG. 5, which may be determined the conditions that do not enter the incident diffraction grating 43. If the size of the incident diffraction grating 43 is a, the condition at this time is: That is, Becomes

Incidentally, theta ₄ is (5), represented as _{(6) sinθ 4 = sinθ 1} -sinθ 2 -sinθ 3 ...... from the equation (9).

In addition, the measurement range of the distance between the mask and the wafer is such that when the signal light beam incident on the incident diffraction grating is reflected on the wafer and then re-enters the mask, the entire light beam is incident on the output diffraction grating. It is defined as the range of the mask-wafer interval that satisfies the condition that no occurrence occurs. At this time, assuming that the size of the output diffraction grating is b and the interval between the incident diffraction grating and the output diffraction grating is c, 2d ₁ tan θ ₂ = a + c 2d ₂ tan θ ₂ = b + c Therefore, the minimum measurement interval d ₁ and the maximum measurement interval d ₂ It is expressed.

If tan θ ₄ > 0, that is, sin θ ₁ −sin θ ₂ −sin θ ₃ > 0 (10), it is possible to prevent back-incident light at an arbitrary surface interval and when the incident diffraction grating has an arbitrary size. Can be.

As described above, if the arrangement and the grating pitch of the incident diffraction grating 43 and the exit diffraction grating 44 are set so as to satisfy the condition of the expression (8) or (10), the adverse effect due to the reverse incident light is substantially eliminated. can do. In the present embodiment, there is no problem with respect to diffracted light having a degree other than the predetermined orders m ₁ and m ₂ , since the exit angle is different from that of a normal signal light flux.

Next, a specific first embodiment will be described with reference to FIGS.

The present embodiment is an example in which the incident diffraction grating 43 and the exit diffraction grating 44 are rectangular and arranged adjacent to a scribe line, for example, as shown in FIG. 6, installed on an IC pattern scribe line. ing.

FIG. 7A is a view of the mask 41 from above (xy plane),
FIG. 1B is an xz sectional view, and FIG. 1C is a yz sectional view.
However, the direction connecting the centers of both diffraction gratings in the mask plane is x,
The direction orthogonal to this is y, and the direction perpendicular to the mask plane is z. The wavelength λ of the light beam is λ = 0.83 μm, the x and y components of the incident angle of the normal incident light beam 48 are 25 ° and -10 °, the x and y components of the output angle are 5 ° and -5 °, respectively, and the mask 41 The measurement range of the distance between the wafer and the wafer 42 is 50 μm to 100 μm.

The size of the first physical optical element 43 on the incident side is 30 μm in the x direction.
The dimensions of the second physical optical element 44 on the emission side are 50 μm in the x direction and 50 μm in the y direction.

First, an optical path in the xz plane will be described with reference to FIG.

The light beam transmitted through the incidence diffraction grating 43 is to set the diffraction angle theta _x of the ordinary ray to be incident on the adjacent regions 44a on the incident diffraction grating 43 in the case of minimum spacing 50 [mu] m. That is, θ _x = tan ⁻¹ (15/50) = 16.7 ° At this time, the x component P _1x of the grating pitch of the incident diffraction grating 43 is (sinusoidal diffraction is considered hereinafter) sin16.7 ゜ −sin25 ゜ = 0.83 / P _1x P _1x = −6.136 (μm) Here, the x (y) component of the grating pitch is a vector whose direction is orthogonal to the diffraction grating and whose size is equal to the reciprocal of the grating pitch. And the reciprocal of the x (y) component. Therefore, the sign of the x (y) component of the grating pitch corresponds to the direction of the grating.

The x component P _2x of the diffraction pitch of the output diffraction grating 44 is sin (−16.7 °) −sin5 ° = 0.83 / P _2x P _2x = −2.216 (μm) The output diffraction grating 44 has the same angle 25 as that of a normal light beam. 1 diffraction angle phi _x-order diffracted light (dotted line in the figure) opposite the incident light beam 49 that ° incident sinφ _x -sin25 ° = 0.83 / - so is positive (2.216) phi _{x =} 2.76 ° phi _x, xz plane Inside, the reverse incident light beam cannot enter the diffraction grating 43 for incidence. This is also apparent from the fact that the expression (10) representing the condition for preventing back-incident light is satisfied, that is, sin25 ゜ −sin16.7 ゜ −sin5 ゜> 0.

Next, light in the yz plane will be described with reference to FIG. 7 (C).

The normal light beam 48 enters the incident diffraction grating 43 at −10 °, is diffracted in the vertical direction, is reflected by the wafer 42, and is diffracted and emitted from the output diffraction grating 44 in the −5 ° direction. At this time, the y components P _1y and P _2y of the grating pitch of both diffraction gratings are P _1y = 4.780 (μm) and P _2y = 9.523 (μm). At this time, the diffraction angle φ _y of the back-incident light beam 49 is sin φ _y −sin (−10 °) = 0.83 / 9.523 φ _y = −4.96 ° Here, considering the light paths in the xz plane and the yz plane, the emission side When the distance between the mask and the wafer is 50 μm, the reversely incident light beam 49 incident on the diffraction grating 44 passes through the region 50 indicated by the two-dot chain line shown in FIG.
In the case of μm, the light passes through the region 51 indicated by the two-dot chain line, and as a result, does not enter the incident-side diffraction grating 43. Naturally, when the distance between the mask and the wafer is between 50 and 100 μm, the light passes through the region between the region 50 and the region 51.

As described above, in this embodiment, the condition (10) is satisfied in the xz plane.
Each element is set so as to satisfy the following condition, so that a reverse incident light beam incident on the output side diffraction grating does not enter the incident side diffraction grating.

 Next, a second embodiment will be specifically described with reference to FIG.

In the present embodiment, the incidence diffraction grating 43 and the emission diffraction grating 44 are arranged at an interval from each other. FIG. 8 is a schematic diagram showing the arrangement on the xz plane and the optical path. In the figure, an incident diffraction grating 43 having a size of 30 μm and an output diffraction grating 44 having a size of 70 μm are arranged at intervals of 10 μm, and other conditions are the same as those of the first embodiment as shown in FIG. It is.

Diffraction angle theta _x at this time a normal light beam 48 is ^{_{θ x = tan -1 (20/50)}} = 21.8 ° Thus, the diffraction angle phi _x reverse incident light beam 49 (9) sinφ _{x =} sin25 ° from the equation -Sin21. 8 ゜ −sin5 ゜ φ _x = −2.06 ゜, which does not satisfy the condition (10). However, the condition (8) satisfies the following condition: −2 × 100 × tan (−2.06 °) <2 × 50 × tan21.8 ° −15 7.19 <25.
In the range of μm, reverse incident light can be prevented.

Next, FIG. 9 shows a third embodiment in which rectangular incidence diffraction gratings and emission diffraction gratings are arranged obliquely to each other.

FIG. 9 (A) is a view of the mask viewed from above, (xy plane),
FIG. 1B shows an optical path in the xz plane, and FIG. 1C shows an optical path in the yz plane.

The wavelength of the light beam is 0.83 μm, the x component and the y component of the incident angle of the normal optical path 48 are 5 ° and −5 °, respectively, and the x component and the y of the output angle.
The components are −10 ° and −20 °, respectively, and the measurement range of the distance between the mask and the wafer is 50 μm to 100 μm.
Is 30 μm in the x direction and 20 μm in the y direction, and the size of the output side diffraction grating 44 is 60 μm in the x direction and 40 μm in the y direction. Both diffraction gratings 43 and 44 are obliquely shifted as shown in FIG. 9 (A). It is arranged.

Diffraction angle theta _x of the ordinary ray 48 in FIG. 9 (B) becomes ^{_{θ x = tan -1 (15/50)}} = 16.70 °.

Therefore, the diffraction angle phi _x reverse incident light 49 satisfies the sinφ from (9) _x = Sin5 ° -sin16.7 ° -sin (-10 °) φ _x = -1.52 ° This condition (8) .

In FIG. 9C, the diffraction angle θ _y θ _y = tan ^-1 (10/50) = 11.31 of the normal ray 48 is obtained. From the equation (9), the diffraction angle φ _y of the counter-incident light beam 49 is given by sin φ _y = sin (-5 ゜) -sin 11.31 ゜ -sin (-20
゜) φ _y = 3.37 ゜This satisfies condition (10).

In FIG. 9 (A), 90 and 91 have plane distances of 50 μm and 100 μm, respectively.
This is the return area of the reversely incident light beam when m.

In general, the arrangement of the light projecting means and the light receiving means has restrictions such as that the exposure area must not be blocked. Therefore, in setting the incident angle and the output angle of the signal light beam, this must be considered in addition to the conditions for preventing the counter-incident light described above.

When the diffraction gratings on the incidence side and the emission side are arranged obliquely on the scribe line as in the present embodiment,
The degree of freedom in setting the emission angle is large, and the first embodiment (the sixth embodiment)
It is generally preferable to arrange both diffraction gratings side by side in the scribe line direction as shown in FIG.

In each of the above embodiments, the first and second
1, the case where a linear diffraction grating is used as the second physical optical element has been described, but this is another physical optical element, such as a physical optical element having a converging and diverging effect like a lens, or a cylindrical lens, like the lens. A physical optical element having a refractive power in only one direction may be used.

In this case, a vector representing the grating pitch and direction of the diffraction grating Varies depending on the position on the physical optical element surface. Ie vector Is a function of x and y. Basically, each element can be set in the same manner as described above in consideration of the condition for preventing back-incident light.

It is needless to say that the diffraction order to be considered is not limited to the first order described here.

(Effect of the Invention) According to the present invention, the relative position between the object to be measured and the optical probe is slightly changed by using the diffracted light from the first and second physical optical elements provided on the first object plane. Even in the case of a change, it is possible to always perform high-precision interval measurement, and to perform high-precision measurement effectively eliminating unnecessary light due to back-incident light other than the detection light from entering the detection means. A suitable distance measuring device can be achieved.

[Brief description of the drawings]

FIG. 1 is a schematic view of an optical system according to an embodiment of the present invention, FIGS. 2 (A) and 2 (B) are explanatory diagrams of light beams incident on the mask and the wafer of FIG. 1, and FIGS. (B) and (C) are explanatory diagrams showing the function of the physical optical element on the mask surface, and FIGS. 4 and 5 are explanatory diagrams of optical paths of normal rays and counter-incident rays incident on the mask in the present invention. 6 and 7 are explanatory diagrams of a first embodiment of the present invention, FIG. 8 is an explanatory diagram of a second embodiment of the present invention, FIG. 9 is an explanatory diagram of a third embodiment of the present invention, and FIG. The figure is an explanatory diagram showing the relationship between the surface interval and the light spot position on the light receiving means surface. In the figure, 1 is a light beam, 2, 41 is a mask, 3, 42 is a wafer, 4, 43 is a first physical optical element, 5, 44 is a second physical optical element, 61, 62 are diffracted lights, and 7 is a condensed light. A lens, 8 is a light receiving means, 9 is a signal processing circuit, 10 is an optical probe, 48 is normal light, and 49 is reverse incident light.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Shigeyuki Suda, Inventor 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Jun Hattori 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon (56) References JP-A-2-74815 (JP, A) JP-A-2-1512 (JP, A)

Claims (3)

(57) [Claims] A first object and a second object partially provided with a first physical optical element and a second physical optical element are arranged to face each other, and projected onto the first physical optical element on the first object plane. A light beam from an optical unit is made incident, and after diffracted light of a predetermined order from the first physical optical element is reflected on the second object surface, a second light beam on the first object surface is reflected.
By making the light incident on the physical optical element, guiding the diffracted light of a predetermined order from the second physical optical element onto the light receiving means surface, and detecting the incident position of the diffracted light on the light receiving means surface, the first When calculating the distance between the object and the second object, the light beam from the light projecting means directly enters a region where the principal ray of the light beam is incident on the surface of the second physical optical element. An interval measuring device, wherein each element is set so that a light beam of a predetermined order to be diffracted is reflected by the second object surface and does not enter the first physical optical element. A first physical optical element and a second physical optical element arranged on a scribe line on a mask surface at an angle to the scribe line direction, and the mask and the wafer are arranged to face each other; A light beam is made incident on the first physical optical element on the surface from the light projecting means, diffracted light of a predetermined order from the first physical optical element is reflected on the wafer surface, and then made incident on the second physical optical element. Determining the distance between the mask and the wafer by guiding the diffracted light of a predetermined order from the second physical optical element onto the light receiving means surface and detecting the incident position of the diffracted light on the light receiving means surface; An interval measuring device characterized by the above-mentioned. 3. A predetermined beam diffracted from the second physical optical element, wherein the light beam from the light projecting means is directly incident on an area where the principal ray of the reflected light beam from the wafer is incident on the second physical optical element. 3. The distance measuring apparatus according to claim 2, wherein each element is set so that a light beam of the order does not enter the first physical optical element after being reflected by the wafer.