JPH0158442B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of inspecting the presence or absence of minute irregularities on the surface of an object to be inspected using optical interference.

FIG. 1 shows an example of an interference optical system for obtaining an interference image of the surface of an object to be inspected. A beam splitter 2 transmits a beam emitted from a coherent light source 1 such as a laser beam.
One beam is projected onto the surface of the object to be inspected 4 through an objective lens 3, and the other beam is projected onto a reference mirror 6 through an objective lens 5 (generally with the same magnification as the objective lens 3). . Reference mirror 6
is a mirror surface with high planar accuracy within the field of view of the objective lens 5. The reflected lights from the object to be inspected 4 and the reference mirror 6 are focused by objective lenses 3 and 5, respectively, and combined by a beam splitter 2.
An image is formed on the image plane 7 as an interference image.

The principle of interference in this case will be explained by folding back the figure at the reflecting surface of the beam splitter 2 and superimposing the virtual image of the reference mirror 6 formed by the beam splitter 2 on the surface of the object 4 to be inspected.

A virtual image 9 of the folded reference mirror surface is shown in FIG. 2 by a broken line. Here, it is assumed that the surface of the object to be inspected 8 and the virtual image 9 of the reference mirror surface are perfectly parallel. Then, if the interval 10 is an odd multiple of 1/4λ, where λ is the wavelength of the light beam used, the reflected lights weaken each other due to interference. Therefore, a dark image 20 with uniform interference intensity is formed on the image plane 7.
On the other hand, if the interval 10 is an even number multiple of 1/4λ, the reflected lights will strengthen each other, and the image plane 7 will become a bright image with uniform interference intensity. In other words, when the position of the reference mirror is moved in the optical axis direction, the image plane repeats brightness and darkness, and the interval 10 becomes 1/2λ.
The image plane 7 has a maximum brightness every time the brightness increases by an integral multiple of .

This relationship becomes as shown in FIG. 3 when the horizontal axis represents the change in interval 21 and the vertical axis represents the interference intensity 22.
23 is a brightness change.

Considering the characteristics of such interference, the case where it is applied to the inspection for the presence or absence of minute defects will be considered in FIG. 4. Assume that there is a minute defect 25 in the background 24 that is recessed from the background 24 on the surface 8 of the object to be inspected. Assuming that the depth 26 of this defect 25 is 1/8λ, the interference intensity between the background 24 and the defect 25 will be as shown in the graph of FIG. In other words, the brightness change 27 of the background 24 with respect to the interval change 21 reaches a maximum value at 1/2nλ (n: a positive integer) as shown by the solid line (this is exactly the same as in Fig. 3), but the defect The brightness change 28 of the background 24 changes with a phase delayed by 1/8λ from the brightness change 27 of the background 24, as shown by the broken line.

In order to be able to recognize the presence or absence of defect 25, background 2 is required.
4 and the defect 25, that is, the contrast of the defect 25 against the background 24, is required, but this graph shows that this contrast changes with a change in the spacing 21, which is a condition for interference. ing. Here, the condition where the difference in brightness between the background 24 and the defect 25 increases, that is, the contrast of the defect 25 becomes maximum, is 2 in the interval 29, 30.
However, at the interval 29, as shown in FIG.
A dark interference image 33 of No. 5 appears, and although not shown at interval 30, the brightness and darkness of the two are reversed. Under such conditions, the interference image 33 of the defect 25 with sufficient contrast moves within the image plane 7 due to movement of the object to be inspected or scanning of the irradiation light or interference image (hereinafter referred to as movement, etc.). etc. direction 3
5, so detection is easy.

However, on the other hand, under the conditions of the intervals 31 and 32, there is no difference in brightness between the background 24 and the defect 25, and for example, at the interval 31, as shown in FIG. 7, the interference image 33 of the defect 25 is Interference image 3 of background 24
4, the defect 25 cannot be detected. This is an essential drawback of the interference method, and can occur for all defects whose unevenness relative to the background is smaller than 1/4λ. In other words, even if it is possible to completely eliminate vibrations that occur in the reference mirror and the object to be inspected, and to follow the slight waviness of the surface of the object to be inspected and control the spacing to the order of several tenths of the wavelength, the defects that still exist Unless the size of the irregularities is constant, there is no optimal value for the spacing, and oversights are unavoidable.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the conventional methods described above and to provide a method for inspecting minute unevenness defects without overlooking them using optical interference.

The key point of the present invention is to consider the active use of interference fringes. In other words, one or more pairs of bright and dark interference fringes are always generated within the detection range, and the interference conditions are varied within this range.

An embodiment of the present invention will be described below.

As is known, interference fringes are generally generated by tilting a reference mirror. That is, as shown in FIG. 8, the direction of movement of the object to be inspected is set to x, and the reference mirror is set parallel to the surface 8 of the object to be inspected in the y direction.
Tilt only in the x direction at an angle 36 as shown. In this way, the light reflected from the surface 8 of the object to be inspected and the light reflected from the virtual image 9 of the reference mirror surface weaken each other due to interference, and dark interference fringes 44 are formed at the position of the interval 41.
occurs. Interference fringes 45 also occur at a distance 42 which is different from the distance 41 by a distance 43 of 1/2λ.

Under these conditions, it is assumed that the defect 25 crosses the image plane 7 in the x direction, that is, from left to right in the figure, due to movement of the object to be inspected or the like. Interference image 3 of defect 25
3 reaches a position 47 where the distance 46 between the bottom surface of the defect 25 and the virtual image 9 on the reference mirror surface is equal to the distance 41, the brightness is the same as that of the interference fringes 44, that is, the darkest state is reached. At this position 47, if the depth 26 of the defect 25 is 1/8λ, the two interference fringes 44, 45
Since the interval 48 indicates a height difference of 1/2λ,
The position is located to the right of the position of the interference fringes 44 by a distance 49 that is 1/4 of this interval 48.

As the defect 25 moves, as can be expected from FIG. 5, the contrast of the defect 25 gradually decreases until it reaches zero contrast at position 50, making it unrecognizable. However, as we move further, the contrast gradually increases until it reaches a maximum again at position 51. The contrast at this time is at position 4.
7, the defect 25 portion is brighter than the surrounding area.

In this way, by changing the interference conditions along the direction of movement of the object to be inspected, it is possible to provide contrast to defects depending on the location. The location where good contrast is obtained is determined by the amount of unevenness of the defect, but within the range that can be detected by a detector (in the image plane if detected visually)
If at least one pair of bright and dark interference fringes is generated in each area, there will always be one or two locations where the contrast is maximum within that range, making it possible to detect defects.

Note that at the same time, there is no problem even if the interference conditions are different in the direction perpendicular to the direction of movement, etc., and interference fringes are generated obliquely. Such a situation occurs when the reference mirror is tilted in the y direction or when there is a tilt in the y direction due to undulations on the surface of the object to be inspected. Even in this case, there is no problem if the object to be inspected is moved, etc., and the reference mirror is adjusted in advance so that one or more pairs of interference fringes are generated in the moving direction or scanning direction over the entire inspection range.

As described above, by using this method, it is possible to prevent overlooking due to lack of contrast, which was an essential drawback, and to perform reliable defect detection while maintaining the high detection sensitivity of interferometry.

[Brief explanation of drawings]

Figure 1 shows an example of an interference optical system, Figure 2 shows the principle of interference, Figure 3 shows interference intensity, and Figures 4 to 7 show interference images used for defect detection. FIG. 8 is a diagram showing the defect detection principle using interference according to the present invention. 1: coherent light source, 4: object to be inspected, 2
5: Defect, 33: Interference image, 44, 45: Interference fringes.

Claims (1)

[Claims] 1. A coherent light source, an interference optical system for obtaining an interference image of the surface of an object to be inspected using the light rays emitted from this light source, a photoelectric detector placed on the image plane, and movement of the object to be inspected for inspection. or means for scanning the object to be inspected with irradiation light or scanning the interference image on a detector, and in a method for inspecting the presence or absence of defects such as irregularities on the surface of the object to be inspected from the interference image, the object to be inspected. A surface defect inspection method characterized by generating at least one pair of bright and dark interference fringes in the moving direction of the object or in the scanning direction of the irradiation light or the interference image, and performing an inspection using the interference image of the surface of the object to be inspected. .