JP3106521B2 - Optical inspection equipment for transparent substrates - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical inspection apparatus for inspecting the surface of a transparent substrate by scanning a light spot.

[0002]

[Prior art]
A reticle or mask provided with a pattern for photolithography in a semiconductor manufacturing process needs to be used for exposure in an extremely clean state, and therefore it is necessary to completely remove dust and dust on the surface. For this reason, as an inspection device for dust and dust, a device that irradiates a laser spot obliquely on the surface of a test object and scans the surface to detect light scattered by dust and dust is used.

[0003]

However, in the conventional apparatus, since the reticle (mask) is transparent, scattered light due to a predetermined pattern formed on the back surface is detected as stray light when inspecting the front surface. , Which causes erroneous detection and lowers the detection accuracy.

Accordingly, the present invention has been made in view of such problems, and has a high detection accuracy by minimizing the influence of scattered light on the back surface when inspecting the surface of a transparent substrate such as a reticle or a mask. It is an object to provide an inspection device.

[0005]

In order to solve the above-mentioned problems, according to the present invention, a light spot is basically formed on a transparent substrate 50 to be detected in a predetermined direction as shown in a sectional optical path diagram of FIG. (Figure 1
Irradiating optical system 10 for scanning in the direction perpendicular to the paper)
And an optical inspection apparatus having a detection optical system 20 for detecting reflected and scattered light from the upper surface of the test transparent substrate 50, wherein the detection optical system 20 includes a light spot on the test transparent substrate 50. A light-receiving objective lens 21 for forming an image, a light-shielding plate 23 having a slit opening 24 conjugate with the scanning locus of the light spot on the image plane of the light-receiving objective lens 21,
Light detection means for detecting light passing through 23 slit openings 24
And 60 is provided, in which is inclined to the optical axis Ax ₂₀ of the detecting optical system 20 in the same direction as the optical axis Ax ₁₀ of the illumination optical system 10 in a subject the transparent substrate 50 on. Here, the scattered light from the surface of the test transparent substrate 50 is collected by the light receiving objective lens 22 and detected through the slit opening 24. The inclination angle A of the optical axis of the irradiation optical system 10, the inclination angle B of the optical axis of the detection optical system 20, the magnification of the light receiving objective lens 21, and the width of the slit 24 are determined by scattering light from the rear surface of the transparent substrate to be measured. It is configured so that it does not enter the slit.

In the above basic configuration, the detection optical system 20 has two light-receiving objective lenses eccentrically arranged in the scanning direction of the light spot with the optical axis of the irradiation optical system 10 interposed therebetween. It is preferable to have a configuration.

[0007]

[Action] As shown in Figure 1, the optical axis Ax ₁₀ of the illumination optical system 10 and the optical axis Ax ₂₀ of the detecting optical system 20 in a subject the transparent substrate 50 on
A specific configuration in the configuration inclined in the same direction as the above will be described with reference to FIG. FIG. 2 is an enlarged schematic cross-sectional view showing a state of an irradiation beam 100 and a reception beam 200 around a light spot formed on the surface 50a of the transparent substrate 50 to be inspected by the irradiation optical system. Upper surface 50a of test transparent substrate 50
The light spot condensed on the upper side is refracted in the test transparent substrate 50, reaches the back surface 50b, and scattered light is generated from the pattern surface formed here. Of the scattered light, the light which may be incident on the detection optical system giving the greatest effect is substantially parallel to the test transparent substrate 50 surface 50a to the optical axis Ax ₂₀ of the detection optical system
Ray 30 reaching the position closest to the received beam 200 above
It is.

As shown in FIG. 2, the beam irradiation angle of the irradiation optical system 10, that is, the angle A between the optical axis of the irradiation optical system and the normal to the surface to be inspected, and the beam receiving angle of the detection optical system 20, that is, the detection optical system Regarding the angle B between the system optical axis and the normal to the surface to be inspected, the magnification of the light receiving objective lens 21 and the width of the slit 24, the back surface of the transparent substrate 50 to be inspected
The conditions for blocking stray light generated from 50b will be described.

The condition under which the scattered light on the back surface of the test transparent substrate 50 does not become stray light is a condition under which the light cannot reach the slit opening 24 of the light receiving optical system 20, and the scattered light 30 is emitted from the test transparent substrate 50 into the air. Distance between the light spot position and the light spot position
Is larger than the value obtained by multiplying the slit width δ by the magnification M by the light receiving objective lens (the magnification at which the slit is projected on the transparent object to be examined, contrary to the actual direction of the light beam). At this time, it is desirable that the slit width δ is determined in consideration of the spread of the spot due to the aberration of the light receiving objective lens.

Here, the slit aperture of the light receiving objective lens 22
The width of the slit opening on the transparent substrate surface 50a corresponds to Mδ, with respect to the magnification M between the sample 24 and the transparent substrate surface 50a and the slit width δ. In FIG. 2, the numerical aperture of the converging spot by the irradiation optical system 10, that is, the numerical aperture N of the converging objective lens 11,
A ₁ , numerical aperture NA _{2 of} light receiving objective lens 22 on the side of transparent substrate 50 to be tested
, The following relationship holds.

Α = A−sin ⁻¹ NA ₁ β = B + sin ⁻¹ NA ₂ where α and β are the maximum incident angle of light in the irradiation beam and the maximum angle of light in the receiving beam, respectively. The light emission angles are α ′ and β ′ are the refraction angles in the respective transparent substrates 50 to be tested. Therefore, the following relationship holds: nsin α ′ = sin α nsin β ′ = sin β, and the distance Δ between the position when the scattered light 30 exits from the transparent substrate 50 into the air and the light spot position
Is given by Δ = d (tan α′−tan β ′).

Here, assuming that the emission angle of the scattered light beam 30 that may cause the greatest influence is β, the distance from the irradiation spot viewed from the light receiving optical system is Δcosβ. Therefore, the condition that the scattered light on the back surface of the test transparent substrate 50 does not become stray light, that is, the condition that the scattered light cannot reach the slit opening 24 of the light receiving optical system 20 is Δcosβ ≧ Mδ.

Now, rewriting using the equation, α (tan α′−tan β ′) cos β ≧ Mδ, and this equation gives a condition for eliminating stray light. In the above formula, cos β is replaced by cos β ′, and d (tan α′−tan β ′) cos β ′> d (tan α′−tan β ′) cos β ≧ Mδ tan α′cos β ′ −sin β ′ ≧ Mδ / d

[0014]

(Equation 2)

Here, by rewriting using the equation, the condition to be satisfied is:

[0016]

(Equation 3)

## EQU1 ## In order to reduce the stray light from the pattern, it is desirable to set the exit angle B of the detection optical system on the same side as the beam incident side of the irradiation optical system and make it as large as possible. For that purpose, it is sufficient to reduce Mδ under the condition of α, n, d being constant according to the equation.

[0018]

Embodiments of the present invention will be described below. The basic configuration of the preferred embodiment is as shown in the cross-sectional optical path diagram of FIG. 1 described above, and the laser light from the light source means 40 is collected on the test transparent substrate 50 via the objective lens 11 and the reflecting mirror 12. Glow. The scattered light from the test transparent substrate 50 is condensed on the spot opening 24 of the light shielding plate 23 by the light receiving objective lens 22 via the reflecting mirror 21, and is photoelectrically detected by the detecting means 60 via the field lens 25.

FIG. 3 is a perspective view showing the configuration of the embodiment of the present invention. A beam supplied from a laser light source 41 is deflected by a vibrating mirror 42 and is an fθ lens as an irradiation objective lens.
By 11, a laser spot is formed on the surface 50 a on the test reticle 50 as the test transparent substrate, and the trajectory 51 is drawn with the vibration of the vibrating mirror 42. The reflecting mirrors 12 and 13 have a function of irradiating a beam at a predetermined irradiation angle A by bending the optical path. Then, a pair of detection optical systems are arranged in parallel and symmetrically with the optical axis Ax _20b of the irradiation optical system interposed therebetween. Each detection optical system has the same basic configuration as that shown in FIG. Specifically, scattered light 200 from a foreign substance such as dust or dust on the surface 50a of the test reticle 50 is reflected by the reflecting mirror 21.
After the light is reflected by the light receiving objective lenses 22a and 22b,
The light is condensed on slit openings 24a and 24b of a and 23b, and is photoelectrically detected by photoelectric detectors 60a and 60b through field lenses 25a and 25b. Here, the reflection mirror 22 of the detection optical system is disposed at an appropriate angle so that the light receiving angle B of the detection optical system satisfies the above-described condition. Also in this configuration, it goes without saying that the slit openings 24a and 24b of each light shielding plate are conjugate with the laser spot locus 51 on the surface 50a of the transparent substrate to be measured.

In order to prevent the scattered light on the back surface of the test transparent substrate 50 from becoming stray light, Mδ may be reduced under the condition of α, n and d from the above equation. Is determined by the spread due to the aberration of the light receiving objective lens, and it is difficult to reduce the value. For this reason,
What is necessary is to reduce the imaging magnification M by the light receiving objective lens. However, since the length of the slit is long, practically, from the relationship between the scanning locus and the size of the light receiving objective lens,
It is preferable that M ≧ 2.5.

That is, in such a relatively small magnification range, when the numerical aperture on the reticle side is fixed,
Although the numerical aperture on the slit side becomes smaller and the slit length becomes longer, the spread of the spot due to aberration can be made smaller, so the slit width can be made smaller, and stray light can be more effectively removed. ,
This produces a synergistic effect.

Preferred examples of numerical values obtained from such a viewpoint are as follows. A = 45 °, NA ₁ = 0.0357 (corresponding to F / 14), d = 2.3,
When n = 1.5, M = 3.5 and δ = 0.2, β ′ ≒ 11.3 ° is obtained. Accordingly, the maximum angle β in the received light beam is 17.1 °, and the exit angle of the beam to be collected by the detection optical system, that is, the inclination angle of the optical axis of the detection optical system is B = 14.2 °. These almost satisfy the above-mentioned conditions.

In general, of the scattered light from the back surface 50b of the transparent substrate to be inspected, the intensity of the scattered light in the direction perpendicular to the scanning trajectory of the beam spot tends to be strong and likely to become stray light. Has two sets of detection optical systems as a configuration having two light-receiving objective lenses eccentrically arranged in parallel in the scanning direction of the light spot with the optical axis of the irradiation optical system 10 interposed therebetween. Thereby, the detection accuracy can be improved. That is, since the scattered light from foreign substances such as dust and dust on the transparent substrate 50 to be inspected is substantially uniform, the signals of both signals are taken, such as the AND of the detection signals by the two sets of detection optical systems arranged in parallel. , It is easy to remove noise due to stray light.

In the configuration of the present embodiment, when the thickness of the reticle as the test transparent substrate 50 changes, focusing is performed by moving the light receiving objective lens in the optical axis direction, and the light shielding plate is moved. It is possible to cope by adjusting the position of the slit opening by eccentric movement.

[0025]

As described above, according to the inspection apparatus of the present invention, when inspecting the surface of a reticle or the like as a transparent object to be inspected, stray light generated from the pattern formed on the back surface can be satisfactorily shielded. Therefore, an inspection with a high S / N and a small number of erroneous detections can be performed.

[Brief description of the drawings]

FIG. 1 is a sectional optical path diagram showing a basic configuration of an inspection device according to the present invention.

FIG. 2 is an enlarged optical path diagram for explaining conditions in the present invention.

FIG. 3 is a perspective view showing the configuration of an embodiment according to the present invention.

[Description of Signs of Main Parts]

 10 ... irradiation optical system 20 ... detection optical system 50 ... testing transparent substrate 51 ... spot scanning locus 22 ... light receiving objective lens 24 ... slit aperture 25 ... field lens 60 ... detection means

Claims (5)

(57) [Claims] An irradiation optical system for forming a light spot on a test transparent substrate and scanning in a predetermined direction, and a detection optical system for detecting scattered light reflected from the surface of the test transparent substrate. In the optical inspection device having, the detection optical system, the light receiving objective lens for forming an image of the light spot on the test transparent substrate, and the light spot on the image plane of the light receiving objective lens A light-shielding plate having a slit opening conjugate with the scanning trajectory; and light detecting means for detecting light passing through the slit opening of the light-shielding plate through a field lens, and light of the detection optical system on the test transparent substrate. The axis is inclined in the same direction as the optical axis of the irradiation optical system, the beam irradiation angle of the irradiation optical system, the beam incidence angle of the detection optical system, the magnification of the light receiving objective lens of the light receiving optical system, and the width of the slit. Is The scattered light from the back surface of the inspection transparent substrate is configured not to enter the slit, the incident angle of the light spot by the irradiation optical system on the surface of the test transparent substrate is A, The numerical aperture on the substrate side is NA1, the light receiving angle of the detection optical system is B, the numerical aperture of the light receiving objective lens on the side of the transparent substrate to be inspected is NA2, the width of the slit is δ, and the slit opening by the light receiving optical system is When the imaging magnification with the transparent substrate to be inspected is M, the thickness of the transparent substrate to be inspected is d, and the refractive index is n, An inspection apparatus for a transparent substrate, characterized by satisfying the following conditions. 2. The light receiving optical system according to claim 1, wherein said light receiving optical system includes two light receiving objective lenses eccentrically arranged in a scanning direction of said light spot with respect to an optical axis of said irradiation optical system. Item 2. An optical inspection device for a transparent substrate according to Item 1. 3. The transparent substrate according to claim 1, wherein an imaging magnification M between the slit opening and the test transparent substrate by the light receiving optical system is M ≧ 2.5. Optical inspection equipment. 4. The optical inspection apparatus for a transparent substrate according to claim 1, wherein the transparent substrate is focused by moving the light receiving objective lens in the optical axis direction. 5. The transparent substrate according to claim 1, wherein the transparent substrate is focused by adjusting the position of the slit opening by eccentrically moving the light shielding plate. Optical inspection equipment for substrates.