JPH061178B2 - Defect inspection method and apparatus - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a defect inspection method for an optically transparent object (hereinafter referred to as “inspected object”), and more particularly, to a micro unevenness defect on the surface of the inspected object. The present invention relates to a defect inspection method and device suitable for detection.

[Background of the Invention]

The light transmitted through the inspection object undergoes a phase change according to the thickness of the inspection object. By measuring the amount of this phase change, the phase contrast microscope can observe transparent cells such as live cells and bacteria or minute irregularities on a transparent substrate, and objects with little difference in color tone (references). : Kunio Yoshihara, "Physical Optics," Kyoritsu Shuppan, 1966, pp. 131-135).

FIG. 7 is a principle diagram of the optical system of the phase contrast microscope. The 0th-order diffracted light 1 modulated by the transmittance of the inspected object 7 is used as the background intensity, and the change in the refractive index or the thickness of the inspected object 7 is described. The phase change of the 1st or higher-order diffracted light 2 modulated according to the phase difference amount of the change of 1 The surface 10 allows the phase change due to the object 7 to be inspected to be observed with a light-dark contrast. In FIG. 7, 3 is a light source, 4 is a collector lens, 6 is a condenser lens, and 8 is a condenser lens.
Is an objective lens.

In order to cause the 0th-order diffracted light 1 and the high-order diffracted light 2 to interfere with each other, the 0th-order diffracted light having a large amplitude is attenuated by the ring slit 5 and the absorption film 11. Of these, the absorptivity of the absorption film 11 greatly impairs the fidelity of the object 7 to be inspected (similarity to an image observed with a normal optical microscope) from the main application of a phase contrast microscope such as biological observation. It is set to about 90% so as not to break it. For this reason, there is a drawback that only a weak contrast can be obtained with respect to the minute unevenness of the inspected object 7.

When the absorptance of the absorption film 11 is increased, an optical ring 12 called a halo is generated around a portion having a large amount of phase difference, that is, an unevenness amount of the object 7 to be inspected. This halo becomes larger as the amount of absorption of the absorption film 11 is larger and as the size of the detection object (fine irregularities) is smaller. FIG. 8 shows that the detected object B having a large amount of phase difference has a larger halo 12 than the detected object A having a small amount of phase difference.

When, for example, drawing a circuit pattern on a transparent substrate, the circuit becomes a defective product if there are minute irregularities. Therefore, it is effective to examine the halo in order to confirm the presence / absence of minute unevenness. However, since the halo is usually generated at a level opposite to the detection signal level 13 with respect to the background signal level 14, if a single threshold is set for the detection video signal and signal binarization is performed, the detection signal level is Both 13 and halo 12 cannot be separated from the background signal level 14.

As a method of separating both the detection signal level 13 and the halo 12 from the background signal level 14, as shown in FIG. 9, the detection signal is tri-valued by using two thresholds, a high level threshold 18 and a low level threshold 19. There is a method to make it. However, when this ternarization is used, both the detection signal level 13 and the halo 12 can be separated from the background signal level 14, but the background signal level is usually called shading as shown in FIG. Since it is affected by uneven lighting, it is difficult to set a threshold level that does not generate noise or the like.

On the other hand, IEE Transaction Communication SIO-M (IEE
E. Trans.Comm., Com) -26 (5), 611 (1978) by T. Fukinuki, "Notchless Bi-Level Quantizer-For-Facsimile and It's Effect on Coding Efficiency (Notchless Bi -l
evel Qnantizer for Facamile and Its Effect on Codi
ng Efficieney) ”is used in the literature. However, binarization by this floating threshold can eliminate the effects of shading, etc., as shown in FIG. However, there is a drawback in that the background signal level 14 of both the detection signal level 13 and the halo 12 cannot be separated as they are.

[Object of the Invention]

The object of the present invention is to visualize both the large and small fine unevenness in a plane existing on the surface of an optically transparent inspected object that cannot be identified by an ordinary optical microscope by using a phase contrast microscope. Moreover, it is an object of the present invention to provide a defect inspection method and an apparatus thereof capable of detecting with high sensitivity without being affected by shading and the like.

[Outline of Invention]

In order to achieve the above object, the present invention illuminates an object to be inspected and, among diffracted light obtained by being modulated according to the amount of phase difference based on the thickness or refractive index of the object to be inspected, the 0th order diffracted light Is almost completely shielded by an absorption film, and a high-order diffracted light is given a phase change by a phase plate to receive an interference optical image, which is received by a photoelectric conversion means to obtain a phase difference based on the thickness or refractive index of the object to be inspected. A phase difference grayscale image signal in which a large minute unevenness and a small minute unevenness are exposed in a plane according to the amount is detected, and the averaged signal processing means partially averages the detected phase difference grayscale image signal. The first and second threshold signals generated by applying predetermined positive and negative thresholds to the smooth grayscale image signal indicating the background obtained by performing the processing are formed, and the formed first and second threshold signals are formed. First and second
The defect inspection method is characterized by performing threshold processing on the phase difference grayscale image signal with the threshold signal of to detect both large and small fine unevenness defects in a plane. Further, according to the present invention, of the diffracted light obtained by illuminating the object to be inspected and being modulated according to the phase difference amount based on the thickness or the refractive index of the object to be inspected, the 0th-order diffracted light is almost completely absorbed. It shields and receives a high-order diffracted light by changing the phase by a phase plate to receive the interfering light image by the photoelectric changing means, and planarly according to the phase difference amount based on the thickness or the refractive index of the object to be inspected. A phase-contrast microscope optical system for detecting a phase-difference gray-scale image signal in which large minute irregularities and small minute irregularities are exposed, and a phase-difference gray-scale image signal partially detected by photoelectric conversion means of the phase-contrast microscope optical system. An averaging signal processing unit that obtains a smooth grayscale image signal indicating a background by performing averaging processing is provided, and a predetermined positive and negative values are applied to the smooth grayscale image signal indicating the background obtained by the averaging signal processing unit. Give a negative threshold Thresholding the phase difference grayscale image signal detected by the photoelectric conversion means of the phase contrast microscope optical system by the first and second threshold value signals formed by the first and second threshold value signal forming means Then, the defect inspecting apparatus is provided with a minute unevenness defect detecting means for detecting both a large minute unevenness defect and a small minute unevenness defect in a plan view. That is, in the present invention, when a minute unevenness defect is present on the surface of the transparent object to be inspected, the grayscale image due to this defect is made clear so that the defect detection signal and the halo can be easily separated from the background signal level. Furthermore, it is made less susceptible to the effects of shading and the like.
Therefore, in order to make clear the light-dark contrast of the image due to the minute unevenness defect, the 0th-order diffracted light of the diffracted light obtained by transmitting and illuminating the inspected object is attenuated or completely shielded. Then, an image of the object to be inspected is obtained by at least the first-order or higher-order diffracted light, and the defect is detected from this image.

As an embodiment of the present invention, the detected image is photoelectrically converted to separate the defect signal from the background. For example, as shown in FIG. 1, two thresholds are created, one of which is a threshold level corresponding to the background level and the other is a threshold level shifted to a slightly higher level, and a threshold level shifted to a slightly lower level. If the generated electric signal is separated into the background and the defect, the separation can be performed with high accuracy without noise. The determination for this separation may be performed by determining whether the detection signal level is higher than the high level threshold or lower than the low level threshold.

Example of Invention

An embodiment of the present invention will be described below with reference to FIGS. 2 to 6.

FIG. 2 is a block diagram of the detection optical system. This detection optical system 30
In, the transmitted illumination light 33 emitted from the transmitted illumination light source 22
Illuminates the object to be inspected 26 through the collector lens 23, the ring slit 24 and the condenser lens 25. Transmitted illumination light
According to 33, the 0th-order diffracted light 37 whose amplitude is mainly modulated based on the transmittance of the object to be inspected 26 and the amplitude modulation according to the phase difference amount of the sample based on the thickness or the refractive index of the object to be inspected 26. Higher order diffraction 34 and is emitted from the inspected object 26. The high-order diffracted light 34 undergoes a phase change of 1/4 wavelength or -1/4 wavelength by the phase plate 28, and the 0th-order diffracted light 37 is significantly attenuated or completely shielded by the absorption film 29.
As a result, a grayscale image mainly corresponding to the phase difference amount of the inspected object 26 is obtained on the image plane of the photoelectric conversion element 31, and the minute unevenness defect on the inspected object 26 can be photoelectrically converted.

The photoelectrically converted signal 33 is separated into a background and a defect by the binarization circuit 33 and output as a detection output 34.

FIG. 3 shows an example of the binarization circuit 32. This binarization circuit 32
The photoelectric conversion signal 33 is partially integrated in order, and an integration circuit 40 that outputs the average value of the signal 33 and a comparison that compares the sum of the output value of the integration circuit 40 and the threshold level V _H with the photoelectric conversion signal 33. A circuit 41, a comparison circuit 42 that compares the value obtained by subtracting the threshold level V _L from the output value of the integration circuit 40 with the photoelectric conversion signal 33 and outputs the inverted value of the result, and outputs of both comparison circuits 41 and 42 It is composed of a logical sum circuit 43 which takes a logical sum and outputs a detection output 34.

As shown in FIG. 4, the comparator circuit 41 detects a defect detection signal when there is an input photoelectric conversion signal equal to or higher than a high level threshold value set to a value higher by the level V _H than the average value of the signal 33. Output.

The comparator circuit 42 outputs a detection signal indicating that there is a defect when there is an input photoelectric conversion signal equal to or lower than the low level threshold value set to a value lower by the level V _L than the average value of the signal 33.

Therefore, the presence or absence of a defect is determined by taking the logical sum of the outputs of both comparison circuits 41 and 42.

FIG. 5 shows a second embodiment circuit of the binarizing circuit 32 shown in FIG.

The binarization circuit 32 in this embodiment includes a first defect detection circuit 47 composed of a comparison circuit 45, a delay circuit 46 and the like, a comparison circuit 48,
Second defect detection circuit composed of delay circuit 49, inverting circuit 50, etc.
51 and an OR circuit 52.

In the first defect detection circuit 47, the photoelectric conversion signal 33 is supplied to the input terminal and the input terminal of the comparison circuit 45, and the output of the comparison circuit 45 is fed back to the input terminal via the delay circuit 46.
V _H <0 level is further added to the input terminal.

Here, the input voltage and the output voltage of the first defect detection circuit 47 are V _{i (n)} and V _{0 (n)} , respectively. Since the voltage at the point A in FIG. 5 is V _{0 (n-1)} , if the gain on the input side of the comparison circuit 47 is adjusted to 1/2 and the gain on the input side is adjusted to 1, Becomes The first term is the partial average value of the input signal 33,
The second term is a value obtained by subtracting the level V _H from the input signal 33. FIG. 6 shows the signal level and the figure. Therefore,
The first defect detection circuit 47 can detect a halo in the example of FIG. The operation of the second defect detection circuit 51 is similar to that of the first defect circuit 47.

Therefore, in the present embodiment, by adjusting the gains on the respective sides of the comparison circuits 45 and 48, minute irregularities on the surface of the inspected object 26 can be accurately detected.

〔The invention's effect〕

ADVANTAGE OF THE INVENTION According to the present invention, both of the planarly large fine irregularities and small fine irregularities present on the surface of an optically transparent inspected object that cannot be identified by a normal optical microscope are made visible by using a phase contrast microscope. Both the detected halo (optical ring) and the detected signal level are detected separately from the background signal level,
It is possible to detect with high sensitivity without being affected by shading or the like.

[Brief description of drawings]

1 is an explanatory view of the method of the present invention, FIG. 2 is a configuration diagram of a detection optical system to which an embodiment of the present invention is applied, FIG. 3 is a block circuit diagram of a binarizing circuit shown in FIG. 4 is an operation explanatory view of the circuit shown in FIG. 3, FIG. 5 is a block circuit diagram of another embodiment of the binarization circuit, FIG. 6 is an operation explanatory view of the circuit shown in FIG. 5, and FIG. Optical system configuration diagram of general phase contrast microscope, 8th
Figure is an illustration of halo, Figure 9 is an illustration of ternarization, 10th
FIG. 11 is an explanatory diagram of shading, and FIG. 11 is an explanatory diagram of binarization of a floating threshold. 26 ... Inspected object, 29 ... Absorption film, 31 ... Photoelectric conversion element, 32 ...
Binarization circuit, 34 ... High-order diffracted light, 37 ... 0-order diffracted light.

Claims (2)

[Claims] 1. A diffracted light obtained by illuminating an object to be inspected and modulated according to a phase difference amount based on the thickness or refractive index of the object to be inspected is almost completely absorbed by a 0-th order diffracted light. It shields and receives a high-order diffracted light by changing the phase by a phase plate to receive the interfered optical image by photoelectric conversion means, and planarly according to the amount of phase difference based on the thickness or refractive index of the object to be inspected. It is obtained by detecting a phase-difference gray-scale image signal in which large minute unevenness and small minute unevenness are revealed, and partially averaging the detected phase-difference gray-scale image signal by the averaging signal processing means. Forming a first and second threshold signal generated by applying predetermined positive and negative threshold values to a smooth grayscale image signal indicating the background,
A defect inspection characterized by performing threshold processing on the phase difference gray-scale image signal by the formed first and second threshold signals to detect both a large fine unevenness defect and a small fine unevenness defect in a plane. Method. 2. A diffracted light obtained by illuminating an object to be inspected and modulated according to a phase difference amount based on a thickness or a refractive index of the object to be inspected is almost completely absorbed by a zero-order diffracted light. It shields and receives a high-order diffracted light by changing the phase by a phase plate to receive the interfered optical image by photoelectric conversion means, and planarly according to the amount of phase difference based on the thickness or refractive index of the object to be inspected. A phase-contrast microscope optical system for detecting a phase-difference gray-scale image signal in which large minute irregularities and small minute irregularities are exposed, and a phase-difference gray-scale image signal partially detected by photoelectric conversion means of the phase-contrast microscope optical system. An averaging signal processing means for performing an averaging process to obtain a smooth grayscale image signal indicating the background is provided, and a predetermined grayscale image signal indicating the background obtained by the averaging signal processing means is predetermined. Give positive and negative thresholds Threshold signal forming means for forming the generated first and second threshold signals, and the first and second threshold signals formed by the threshold signal forming means are detected by the photoelectric conversion means of the phase contrast microscope optical system. A defect inspecting apparatus, comprising: a minute unevenness defect detection unit that performs threshold processing on a phase difference grayscale image signal to detect both large small unevenness defects and small small unevenness defects on average.