JPH0613963B2 - Surface defect inspection method - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a surface defect inspection method particularly suitable for discriminating and inspecting minute unevenness defects and foreign matter defects on an optically transparent object to be inspected. It is a thing.

[Background of the Invention]

As a means for measuring minute irregularities on the surface of an optically transparent object, there is one that uses light interference. As an example of this, there is a so-called Michelson interferometer as shown in FIG. The Michelson interferometer is, for example, published by Iwanami Shoten (Showa
34) Hiroshi Kubota, "Applied Optics", pages 98 to 100. This principle will be explained with reference to FIG.

A light beam emitted from a coherent light source 1 such as a laser beam is divided into two light beams by a beam splitter 2, one light beam of which is projected on a surface of an object 4 to be inspected through an objective lens 3, and the other light beam is generally an objective lens 3. Is projected on the reference mirror 6 through the objective lens 5 having a magnification equal to.
The reference mirror 6 is a mirror surface having high plane accuracy in the field of view of the objective lens 5. The reflected lights from the object 4 to be inspected and the reference mirror 6 are condensed by the objective lenses 3 and 5, respectively, are combined by the beam splitter 2, and an interference image is formed on the image plane 7.

This interference principle in this case, the distance OQ ₁ point to Q ₁ on the point O and the object to be inspected 4 on the reflective surface of the beam splitter 2, the difference between the distance OQ ₂ points Q ₂ on the reference mirror 6 and a point O If Δx is an odd multiple of 1 / 4λ, where λ is the wavelength of the light beam used, the reflected light from the inspected object 4 and the reflected light from the reference mirror 6 weaken each other due to interference, while Δx If is an even multiple of 1 / 4λ, both reflected lights will strengthen each other. That is, when the position of the reference mirror 6 is moved in the optical axis direction, the image plane 7 repeats bright and dark, and the image plane 7 becomes the maximum value of the brightness every time Δx becomes an integral multiple of 1 / 4λ.

According to this principle, the unevenness on the inspected sample 4 becomes bright and dark with a period of 1 / 4λ according to the amount of the unevenness, and is observed on the image plane 7.

Such a method of detecting minute irregularities on the surface of an optically transparent object by using light interference is disclosed in, for example, JP-A-57-120.
It is described in Japanese Patent No. 805. In this known example, the reference mirror 6 of the Michelson interferometer is slightly tilted with respect to the optical axis to change the background interference condition so that one or more pairs of bright and dark interference fringes always occur in the detection range. The conditions are such that minute irregularities on the object to be inspected can be detected with good contrast.

However, the methods described in these publicly known examples only consider reflection from the object 4 to be inspected or the reference mirror 6, and the object 3 or the objective 5 inserted from the object inserted in the optical path. No consideration is given to reflected light.
In particular, it is usually necessary to use a combination lens such as a plan objective lens shown in FIG. 6 in which a plurality of lenses are combined in order to correct aberrations in the objective lenses 3 and 5, etc. Since various lights such as the reflected lights q ₁ , q ₂ , and q _n and the reflected light from the reference mirror 6 interfere with each other and are detected as background noise on the image plane 7, minute amounts on the inspected object 4 are detected. There is a problem that it is difficult to recognize an interference image on the image surface 7 due to such unevenness.

Further, when the reflectance of foreign matter such as dust on the inspected object 4 is high, the reflected light from the foreign matter and the reflected light from the inspected object in the same phase cannot be distinguished from each other. When the ratio is low, an interference image cannot be obtained on the image plane 7, but the image plane 7
If the background in the vicinity of the foreign matter is bright, the uneven defects on the inspected object 4 are observed as in the above case, so that there is a problem that only the uneven defects on the inspected object cannot be accurately detected.

[Object of the Invention]

The object of the present invention is to solve the above-mentioned problems of the prior art by discriminating, as an image signal, a minute unevenness defect existing on an inspected object that cannot be identified only by a phase contrast microscope optical system and a foreign matter defect such as dust. An object of the present invention is to provide a surface defect inspection method capable of inspecting.

[Outline of Invention]

In order to achieve the above object, the present invention arranges a phase-contrast microscope optical system and a microscope optical system with the same detection optical axis, and uses the phase-contrast microscope optical system to perform phase modulation by an object to be inspected. The diffracted light and the direct light amplitude-modulated by the inspected object interfere with each other through the phase plate and are received by the photoelectric conversion means, and at least a minute amount on the inspected object is received according to the phase difference amount from the inspected object. A first two-dimensional gray-scale image signal that reveals the uneven defect is detected, and based on the difference in reflectance or transmittance between the object to be inspected and the foreign matter defect on the object to be inspected by using the microscope optical system. The reflected light or the transmitted light from the inspected object is received by the photoelectric conversion means, and the second two-dimensional grayscale image signal in which the foreign matter defect on the inspected object is revealed according to the reflectance or the transmittance is detected. , On the same detection optical axis Then, the first two-dimensional gray image signal and the second two-dimensional gray image signal detected in the same coordinate system are compared with each other to discriminate between the minute irregularity defect and the foreign matter defect on the inspected object and inspect them. Is a surface defect inspection method.

Example of Invention

First, the principle of the present invention will be described with reference to FIGS. FIG. 2 shows an example of the optical system of the phase contrast microscope. Direct light 8 modulated by the transmittance of the object 14 to be inspected is shown.
As a background strength, the phase change is applied to the diffracted light 9 that is phase-modulated by the change in the refractive index or the change in the thickness of the object to be inspected so as to directly interfere with the light,
The phase change caused by 14 can be observed on the image plane 17 with a contrast of light and dark. Here, 10 is a light source, 11 is a collector lens, 12 is a ring slit, 13 is a condenser lens, and 15 is an objective lens. The direct light 8 having a large amplitude is attenuated by the absorption film 18 so that the direct light 8 and the diffracted light 9 interfere with each other. However, from the main use of the phase contrast microscope, the fidelity of the inspected object 14 is not greatly impaired. To
The absorption rate of the absorption film 18 is up to about 90%.

For the purpose of the present invention, it suffices to detect the presence or absence of a phase difference. Therefore, if the diffracted light 9 is mainly detected without further reducing the direct light 8 or letting the direct light 8 pass through at all, a low level due to the inspected object 14 is detected. The phase difference, that is, the minute unevenness of the inspected object 14 can be detected with high sensitivity.

Depending on how the direct light 8 and the diffracted light 9 are caused to interfere with each other by the phase plate 16, that is, when the phase relationship between both light beams is the same, the background contrast 19 due to the direct light 8 becomes bright,
It becomes dark at the opposite phase. By the way, when a foreign matter such as dust is present on the inspected object 14, the foreign matter is detected as a dark portion on the image plane 17, but when the background contrast 19 is dark, it cannot be detected, and on the contrary, the background contrast 19 is bright. When can be detected. However, it is difficult to identify the detected foreign matter as a minute unevenness defect on the inspected object 14.
On the other hand, for example, when normal transmitted illumination or epi-illumination is used, minute irregularity defects cannot be detected, but foreign matters can be easily detected.

Therefore, a phase contrast microscope optical system and, for example, a ring slit
Use with transmitted illumination (normal microscope) with 12 removed,
Of the detected defects 20 and 21 shown in (a) when the background contrast of the image plane 19 by the phase contrast microscope optical system is bright as shown in the figure, the defect 21 that is also detected in the normal microscope is a foreign matter, and is detected. The defect 20 that is not identified can be identified as a fine uneven defect on the inspected object 14. As shown in Fig. 4, when the background contrast of the image plane 19 by the phase contrast microscope optical system is dark
The detected defect 20 shown in (a) is a fine concavo-convex defect on the inspected object, and the defect usually detected by the microscope can be identified as a foreign substance. The hatched area in FIGS. 3 and 4 represents a dark area in the visual field.

Hereinafter, the present invention will be described in more detail using examples with reference to the drawings, but these are merely examples, and various modifications and improvements may be made without departing from the scope of the present invention. Of course.

Referring to FIG. 1, the transmitted illumination light 25 emitted from the transmitted illumination light source 10 passes through a collector lens 11, a ring slit 12 and a condenser lens 13, and an object 14 to be inspected (the magnetic bubble memory is Gd ₃ G ₉₅ O ₁₂ Y ₃ Fe ₅ O on an optically transparent wafer such as
A magnetic garnet transparent thin film such as ₁₂ is prepared by this liquid phase growth as a substrate. In order to secure the characteristics of the magnetic bubble memory, it is necessary to measure the unevenness of this transparent base. ). From the inspected object 14 by the transparent illumination light 25,
Direct light 29 that is amplitude-modulated mainly by the transmittance of the object to be inspected and diffracted light 26 that is phase-modulated mainly by the thickness or refractive index of the object to be inspected are emitted. Direct light 29
Undergoes a phase change of λ / 4 or −λ / 4 by the phase plate 16, is attenuated by the absorption film 18, and is converted so as to interfere with the diffracted light 26. This allows the TV camera
It is possible to detect a two-dimensional grayscale image signal (shown in FIG. 3 (a) or FIG. 4 (a)) according to the phase difference amount of the object to be inspected on the 23 image planes.

On the other hand, the epi-illumination light 28 from the epi-illumination light source 24 has its optical axis bent by the half mirror 22, passes through the objective lens 15, and
The inspected object 14 is illuminated by epi-illumination. Object to be inspected 14
The reflected light 27 given an amplitude according to the reflectance of the is detected by a television camera (having the same detection optical axis as the phase difference microscope optical system) 23 and is a two-dimensional grayscale image having the same position coordinates. A signal (shown in FIG. 3 (b) or FIG. 4 (b)) is obtained.

Illumination of the object 14 to be inspected is through illumination light source 10 and epi-illumination light source.
It is performed by switching 24, and a phase contrast microscope illumination optical system can be obtained when the transparent illumination light source 25 is used, and a normal microscope illumination method can be obtained when the epi-illumination light source 22 is used. .

Depending on how the phase plate 16 is selected, the television camera 23 can detect an image as shown in FIG. 3 with a bright background or as shown in FIG. 4 with a dark background. In the case of FIG. 3, by comparing FIG. 3 (a) obtained by the phase contrast microscope optical system with FIG. 3 (b) obtained by the illumination of a normal microscope, the singularity 21 existing in both is found. Singularity 2 which is a foreign substance such as dust and exists only in (a)
It can be judged that 0 is a minute unevenness defect. In the case of FIG. 4, the singular point (a) obtained by the phase contrast microscope optical system is a minute unevenness defect, and the singular point obtained by ordinary microscope illumination.
It can be determined that (b) is a foreign substance such as dust.

In the above embodiments, the phase contrast microscope illumination and the episcopic illumination of the ordinary microscope are combined, but the illumination method of the ordinary microscope combined with the phase difference microscope illumination may be another illumination method, for example, transmitted illumination or dark field illumination or differential interference. It may be combined with a microscope.

〔The invention's effect〕

As described above, according to the present invention, it is possible to discriminate a minute unevenness defect on an object to be inspected and a foreign matter defect such as dust which cannot be identified only by a phase contrast microscope optical system from a two-dimensional image signal as an image signal for inspection. There is an effect that can be.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an apparatus for carrying out a surface defect inspection method according to the present invention, FIG. 2 is a schematic diagram showing a configuration of a phase contrast microscope optical system, FIGS. 3 (a), (b) and 4 Figures (a) and (b) are images showing images observed with a phase contrast microscope optical system and a normal microscope, respectively. Figure 5 is a schematic diagram showing the structure of an interference optical system used in a conventional surface defect inspection method. 6 and 6 are schematic views of the objective lens used in the optical system shown in FIG. 10 ... Transmitted illumination light source, 11 ... Contact lens, 12 ... Ring slit, 13 ... Condenser lens, 14 ... Inspected object, 15
… Objective lens, 16… Phase plate, 17… Image plane, 18… Absorption film, 19
… Background contrast, 20… Image of minute unevenness defect, 21… Image of foreign matter such as dust, 22… Half mirror, 23… TV camera,
24 ... Epi-illumination light source, 25 ... Transmitted illumination light, 26 ... Diffracted light, 27 ...
Reflected light, 28 ... Epi-illumination light, 29 ... Direct light.

Claims (1)

[Claims] 1. A phase-contrast microscope optical system and a microscope optical system are arranged with the same detection optical axis, and diffracted light phase-modulated by an object to be inspected using the phase-contrast microscope optical system and the object to be detected. The direct light amplitude-modulated by the inspection object is made to interfere through the phase plate and is received by the photoelectric conversion means, and at least minute irregularity defects on the inspection object are revealed according to the phase difference amount from the inspection object. The two-dimensional gray-scale image signal of No. 1 is detected, and the reflection from the inspected object is performed based on the difference in the reflectance or the transmittance between the inspected object and the foreign matter defect on the inspected object using the microscope optical system. Light or transmitted light is received by a photoelectric conversion means, a second two-dimensional grayscale image signal in which a foreign matter defect on an object to be inspected is revealed according to the reflectance or the transmittance, and the same detection optical axis is detected. Based on the same coordinate system Comparing the first two-dimensional gray image signal with the second two-dimensional gray image signal, and discriminating the fine unevenness defect and the foreign matter defect on the inspected object and inspecting the surface defect inspection method. .