JPH0370178B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an apparatus for projecting a laser beam onto a thin plate-shaped object to be inspected and detecting the transmitted light to inspect the presence or absence of surface defects and foreign matter adhesion. It is something.

[Background of the invention]

This type of laser inspection device is used, for example, to inspect the surface of a mask substrate in the manufacturing process of semiconductor components.

Figures 2 and 3 are charts showing the signal voltage when the surface to be inspected is scanned with a laser beam and the laser light emitted from the front side is detected and converted into an electrical signal, and the horizontal axis is is the time axis, and the vertical axis is the voltage.

Time t ₁ shown on the time axis indicates the time when the scanning spot passes through the pinhole, and time t ₂ indicates the time when the scanning spot passes through a minute adhered foreign object.

Curve 1 shown in Figure 2 is a detection curve for transmitted light in a narrow sense (excluding front scattered light).When passing through a pinhole, the light becomes strong at _t1 , and when passing through a foreign object at t ₂ shows that the light becomes weaker.

Curve 3 in FIG. 3 is a total output curve of curves 1 and 2 described above.

When surface inspection is performed in this manner, the following problems occur.

(i) When the diameter of the pinhole is minute, the spread angle due to diffraction of the laser beam by the pinhole becomes large, and the S/N ratio becomes small compared to a defect-free portion.

(ii) When the diameter of the pinhole is large, the spread angle of diffraction due to the pinhole becomes small, and the light transmitted through the pinhole becomes extremely strong, resulting in signal saturation.

[Purpose of the invention]

The present invention was made to solve the above-mentioned problems, and can detect even minute pinholes with high sensitivity.
Moreover, the present invention aims to provide a laser-type transmitted light type surface inspection device in which the signal is not saturated even by large pinholes (that is, the dynamic range is large).

[Summary of the invention]

The basic principle of the present invention created to achieve the above object will be briefly described below.

As mentioned above, when the pinhole is small, the spread angle of diffraction is large, so it is necessary to collect diffused light over a wide range.

Furthermore, when the pinhole is large, the spread angle of diffraction is small, so in order to prevent signal saturation, passing of the laser beam may be restricted or attenuated at a location close to the optical axis.

In order to achieve the above object based on the above-mentioned principle, the transmitted light type inspection device of the present invention includes a mirror and a condenser lens on the optical axis after passing through the object to be inspected and reaching the laser beam detection means. and divide the reflective surface of the mirror into (a) a nearby area including the point where it intersects with the optical axis, and (b) an area other than that, and The structure is characterized in that it is configured to reduce the reflectance of a nearby area including the area.

[Embodiments of the invention]

Next, an embodiment of the present invention will be described with reference to FIG.

A laser beam 5 is focused and scanned on the object 4 to be inspected.

The transmitted light 6 in a narrow sense has a small beam diameter, as shown by the chain line, and the forward scattered light 7 is diffused relatively widely, as shown by the broken line.

A mirror 8 is provided to bend the transmitted light 6 and scattered light 7 at approximately right angles, and the reflected light is collected by a lens 9 and detected by a light receiving element 10.

The surface of the mirror 8 is defined as (a) a nearby area 8a (shown with parallel hatching) including a point intersecting the optical axis;
(b) It is divided into other areas 8b and 8c, and the reflectance of the area 8a in the vicinity including the above-mentioned optical axis is lowered to about a fraction of that of the other areas 8b and 8c. When carrying out the present invention, good results are likely to be obtained if the content is 30% or less.

FIG. 4 shows a transmitted light detection curve 1' and a scattered light detection curve 2' in the inspection apparatus constructed as described above (FIG. 1).

As can be easily understood from Fig. 1, the transmitted light 6
is reflected in the area 8a of low reflectance, so the transmitted light detection curve 1' in this example is generally lower at a constant rate compared to the transmitted light detection curve 1 (FIG. 2) described above. There is.

Furthermore, as shown in FIG. 1, the scattered light 7 is diffused relatively widely and is reflected not only by the low reflectance area 8a but also by the high reflectance areas 8b and 8c. The scattered light detection curve 2' in this example is not much lower than the previously described scattered light detection curve 2 (FIG. 2). More specifically, the size of the pinhole is determined as follows.

When the pinhole is relatively large, the spread angle of diffraction is small, so the portion reflected by the high reflectance areas 8b and 8c is relatively small, and the majority is reflected by the low reflectance area 8a, so the signal is There is no risk of saturation.

When the pinhole is relatively small, the spread of diffraction is large, so most of the areas 8b and 8 have high reflectance.
It is reflected at c. Therefore, detection sensitivity is high.

Figure 5 shows the transmitted light detection curve 1' in this example.
A total output curve 3' of this and the scattered light detection curve 2' is shown.

〔Effect of the invention〕

As detailed above, by applying the present invention, even minute pinholes can be detected with high sensitivity.
In addition, it is possible to construct a laser-type transmitted light type surface inspection apparatus in which the signal is not saturated even by a large pinhole (that is, the dynamic range is large), which is an excellent practical effect.

[Brief explanation of drawings]

FIG. 1 is a perspective view schematically depicting an embodiment of the inspection apparatus of the present invention. FIGS. 2 and 3 are charts showing detection signal voltages in a conventional laser type surface inspection apparatus. FIGS. 4 and 5 are charts showing the detection signal voltage in the embodiment. 1, 1'...Detection curve of transmitted light in a narrow sense, 2,
2'...Detection curve of scattered light, 3,3'...Total output curve, 4...Object to be inspected, 5...Laser beam, 6...Transmitted light in a narrow sense, 7...Forward scattered light, 8...Mirror, 8a
...nearby area including the point intersecting the optical axis, 8b, 8c
...Area other than the above, 9...Lens for condensing light, 10...
Light receiving element.

Claims (1)

[Claims] 1 Project a laser beam onto a thin plate-shaped object to be inspected,
In a device that detects the transmitted light to inspect the surface defects and foreign matter adhesion of the inspected object, a mirror and a condensing lens are installed on the optical axis between the point where the transmitted light passes through the inspected object and the laser beam reaches the detection means. And, the reflective surface of the mirror is divided into (a) an area in the vicinity including the point intersecting with the optical axis, and (b) an area other than that, and the area in the vicinity including the point intersecting with the optical axis. 1. A laser-type transmitted light type surface inspection device, characterized in that it is configured to reduce the reflectance in the area.