JPS6219965Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an apparatus for inspecting defects by photoelectrically scanning the inspection surface of a sheet or the like with an imaging device, and particularly relates to an apparatus using a CCD (charge-coupled device) image sensor.

For example, as shown in FIG. 1, in a defect inspection device in which three imaging devices 1, 2, and 3 are arranged side by side to scan the entire width of a sheet-like object 4, in many cases, the imaging devices 1 and 3 at both ends are The edge of the object 4 to be inspected is included in the inspection range (scanning range), and the imaging devices 1 and 3 also scan the background portion outside the object 4 to be inspected.

In a reflected light type device where the light source for illuminating the inspected object 4 is on the surface side of the inspected object 4 as shown by the solid line 5 in the figure, the background of the inspected object 4 becomes dark, and in this case, the image pickup device 1 or Video signal output from 3
VS has a waveform including a low-level background signal as shown in FIG. Note that in an imaging device using a CCD image sensor, the video signal is output in the form of a high-frequency pulse synchronized with a scanning clock signal, and the video signal VS in Figure 2 is expressed as an envelope of the high-frequency pulse. There is.

In FIG. 2, BL is the blank period between each scan, and BG is the background signal mentioned above, whose level E ₁ is very small corresponding to the dark background. SE is a test signal obtained from the reflected light of the test object 4, and the average level (ground level) of the test signal SE is high corresponding to the reflectance of the test object 4. Dark defects such as black stains are caused by the formation level.
It appears as a defect signal n whose level decreases in a pulse-like manner from _E2 .

When discriminating the defect signal n from the video signal VS as described above, basically, the video signal VS is processed by an appropriate integrating circuit (envelope detection circuit) and level shift circuit, etc., and the ground level E is determined. Create a threshold signal _SH1 that is slightly lower than _E2 and follows gradual fluctuations in the ground level _E2 , and
Defect signal n is discriminated by comparing SH ₁ and video signal VS. However, this alone cannot distinguish between the background signal BG at a low level _E1 and the defect signal n, so a threshold signal _SH2 for background discrimination is set at a constant level slightly higher than the background level _E1 , and this By comparing the threshold signal SH ₂ and the video signal VS, the background processing signal Sb shown in the figure is obtained.
This signal is used to detect only the true defect signal. For example, background processing signal
A simple gate method that enables the defect detection signal only during the high level "H" period of Sb, and a threshold signal for defect discrimination during the low level "L" period of the signal Sb.
Various processing circuits are known that utilize the background processing signal Sb, such as holding SH ₁ to improve the accuracy of defect discrimination.

In addition, in a transmitted light type apparatus in which a light source is placed on the back side of the object to be inspected 4 as shown by the dotted line 5' in FIG. Since the same problem as above occurs regarding the background portion away from the edge of object 4, background processing similar to that in the above case is performed as shown in FIG. That is, the background signal in this case
The level E ₁ of BG is extremely higher than the average level E ₂ of the inspection signal SE, and the bright defect signal n is also at the ground level E ₂
Appears as a higher pulse. Therefore, in addition to the threshold signal SH ₁ for defect discrimination, a threshold signal for background discrimination at a constant level slightly lower than the background level E ₁ is used.
Set SH ₂ , create a background processing signal Sb by comparing the threshold signal SH ₂ and the video signal VS,
This is used to perform processing such as invalidating the period corresponding to the background.

By the way, as mentioned above, in conventional defect inspection equipment, the threshold value _SH2 for background discrimination is at a fixed level. There was an inconvenience that when the VS level fluctuated greatly, the desired background processing could not be performed. In other words, in the reflected light type shown in Figure 2, the background level E ₁ is the threshold value.
(This is mainly caused by an increase in dark current, and the temperature dependence of dark current is particularly large in CCD image sensors.) In the case of the transmitted light type shown in Fig. 3 _, the background level E ₁ When the threshold value SH2 is _below (this is mainly caused by extreme fluctuations in the light source), the background signal BG is not detected, and as a result, correct inspection point detection cannot be performed.

This idea was made to solve the above-mentioned disadvantages of the conventional device. Instead of setting the threshold for background discrimination at a fixed level, it uses a dummy output (described later) of the CCD image sensor to eliminate fluctuations in dark current and A defect inspection device that detects level fluctuations in the video signal caused by fluctuations in incident light and changes the threshold for background discrimination in accordance with the fluctuations, ensuring that correct background processing is always performed. This is what we provide. Hereinafter, embodiments of this invention will be described in detail based on the drawings.

First, the dummy output of the CCD image sensor will be explained. FIG. 4a is a structural explanatory diagram of a general CCD one-dimensional image sensor. As is well known, three rows of unit elements are formed in parallel on the surface of a semiconductor chip, and the central row of unit elements A ₁ to _{A o} is a photosensitive row,
A light-shielding layer is formed on the planes of the unit element arrays B ₁ -B _o and C _{1 -Co} in order to use the unit element arrays B ₁ -B _o and C _{1 -Co} on both sides as transfer columns (shift registers), respectively. However, at the same time, several elements at both ends of the unit elements A ₁ to _{A o} are also shielded from light. In FIG. 4a, the dotted area is the area where the light-shielding layer is formed, and the area from the unit elements A ₄ to A _o-3 that is not shaded is the imaging area 20. However, the unit element of the imaging area 20
A ₄ to _{A o-3} and the light-shielded unit element A ₁ at both ends
〜A ₃ and A _o-2 〜A _o have the same element structure, and upon receiving a predetermined transfer signal, the unit element array A ₁ 〜
Charges accumulated in _the odd numbered _elements A ₁ , _{A 3 , A 5 .} The charges in the shift registers B _{1 -B} o and C 1 -C _o are transferred to the shift registers C 1 -C o one by one, respectively, and then the charges in the shift registers B ₁ -B _o and C ₁ -C _o are transferred to the buffer amplifier 21 in response to a predetermined shift clock signal (scanning clock signal). are output alternately through the This is the video signal VS, that is, a signal in which the accumulated charges of the unit element arrays A ₁ to _{A o} are sequentially output in the form of high-frequency pulses.

Therefore, when the image sensor surface shown in FIG. 4a is uniformly irradiated with light, the video signal VS
is as shown in Figure 4b. In other words, the imaging area 20
An effective signal corresponding to the amount of irradiation light is obtained from the unit elements A ₄ to _{A o-3} , but as shown by the symbol DS before and after this effective signal, the unit elements A ₁ that are shielded from light at both ends of the photosensitive array Very low level invalid signals from ~A ₃ and A _o-2 ~ _{A o} are output. This invalid signal DS is called a dummy output or dummy signal. Note that this dummy signal DS is included in the blank period BL in FIGS. 2 and 3.

By the way, the above dummy signal DS is in the imaging area 20.
Since the output is from the same element as the element in , changes in dark current due to temperature changes also appear as changes in the level of the dummy signal DS. In addition, the above-mentioned light-shielding layer is usually extremely thin and does not have perfect light-shielding properties, so when extremely strong light is irradiated, the shielded element
A ₁ to _{A 3} and A _o-2 to _{A o} are also slightly sensitive to light. Therefore, when there is strong incident light, the level of the dummy signal DS also changes due to fluctuations in the incident light.

This invention focuses on the characteristics of the dummy signal DS described above, and changes the threshold for background discrimination in response to changes in the level of the dummy signal DS. Fifth
The figure shows an embodiment of the background processing signal generating circuit portion according to this invention, and FIG. 6 shows the waveforms of each part.

In FIG. 5, an imaging device 22 using the above-mentioned CCD image sensor operates in response to a signal from a drive circuit 23, and outputs a video signal from it.
VS is supplied to gate circuit 25 and comparator 24. The video signal VS shown in Figure 6 includes a background signal of low level E ₁ corresponding to a dark background as in Figure 2.
BG, and also the above-mentioned dummy signals before and after the valid signals of the background signal BG and test signal SE.
DS exists.

As shown in FIG. 6a, the drive circuit 23 supplies the gate signal a to the gate circuit 25 for extracting the dummy signal DS that is present immediately before the valid video signal. signal
The logical product of VS is taken, and only the dummy signal DS is extracted from the gate circuit 25 as shown in FIG. 6b. Note that the gate signal a is transmitted to the imaging device 22.
It can be easily generated based on the drive signal given to the

The dummy signal DS extracted by the gate circuit 25 is input to the peak hold circuit 26. The gate signal a is also supplied to this peak hold circuit 26 as a synchronizing signal, and it holds and outputs the peak level of the dummy signal DS from the gate circuit 25 for each scanning period. Figure 6c shows peak hold circuit 2
6 output is shown.

The output of the peak hold circuit 26 is amplified by α times by an amplifier 27, and further amplified by a constant voltage by an adder 28.
V ₁ is added, and the output of this adder 28 is supplied to the comparator 24 as a threshold signal SH ₂ for background discrimination. Here, the amplification factor α and addition voltage V ₁ are set appropriately, and the threshold signal SH ₂ is set to the background level E ₁
Make it slightly higher. Then, comparator 2
4 outputs a high-frequency pulse train in which the blank period BL and the background signal BG have been eliminated, and the waveform shaping circuit 29 shapes this pulse train into a continuous square wave to obtain the background processed signal Sb.

Now, in the above configuration, due to the influence of temperature etc.
Even if the background level _E1 changes due to a change in the dark current of the CCD image sensor, this change in dark current also appears as a level change in the dummy signal DS, and the threshold signal _SH2 for background discrimination is based on this dummy signal. The threshold signal changes in response to DS level fluctuations.
The level difference between SH ₂ and the background level E ₁ is maintained within an appropriate range. Therefore, the background level E ₁ is the threshold
SH ₂ or higher, or formation level E ₂ is the threshold
Malfunctions such as _SH2 or lower are extremely rare, and accurate background processing can be performed at all times.

In addition, as shown in Fig. 3, in a transmitted light type device that includes a background signal BG of an extremely high level _E1 due to direct light from a light source, the amplification factor α and addition voltage _V1 should be set appropriately. , threshold signal SH ₂
is set to be somewhat lower than the background level _E1 .
In this configuration, fluctuations in the light source cause the background level to
Even if _E1 changes, changes in the strong direct light from the light source will also appear as level changes in the dummy signal DS,
Since the threshold signal SH ₂ changes in accordance with the level fluctuation of this dummy signal DS, the threshold signal SH ₂
The level difference between this and the background level _E1 is kept within an appropriate range. Therefore, the background level E ₁ is the threshold SH ₂
Malfunctions in which the background level E ₂ becomes lower than the threshold value SH 2 or the ground level E ₂ exceeds the threshold value SH 2 are extremely rare, and accurate background processing can be performed at all times.

Note that in the above embodiment, only the dummy signal output immediately before the valid video signal is extracted;
Depending on the case, a dummy signal output immediately after the valid video signal may be extracted, or dummy signals before and after the effective video signal may be extracted.

As explained in detail above, the defect inspection device of this invention generates a threshold signal for background discrimination that changes in accordance with level fluctuations of the dummy signal based on the dummy signal of the CCD image sensor,
Since the background processing signal is obtained by comparing this threshold signal with the video signal, accurate background discrimination can always be performed without being affected by changes in dark current or strong incident light, eliminating malfunctions. This is possible.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing an overview of the defect inspection device, Figs. 2 and 3 are waveform diagrams showing conventional problems,
Figure 4a is an explanatory diagram of the structure of the CCD image sensor.
FIG. 4b is a waveform diagram showing a dummy signal, FIG. 5 is a block diagram showing an embodiment of the background processing signal generating section of the defect inspection device according to this invention, and FIG. 6 is a waveform diagram of each part in the same block diagram. It is. 22...imaging device, 23...drive device, 24...
... Comparator, 25 ... Gate circuit, 26 ... Peak hold circuit, 27 ... Amplifier, 28 ... Adder, VS ... Video signal, BL ... Blank period,
BG...Background signal, SE...Test signal, DS...Dummy signal, _SH2 ...Threshold signal for background discrimination.

Claims (1)

[Scope of Claim for Utility Model Registration] In an apparatus for photoelectrically scanning a surface to be inspected with an imaging device using a CCD image sensor and inspecting defects on the surface to be inspected from the obtained video signal, one of the above CCD image sensors. an imaging device having a unit element whose portion is shielded from light; a drive circuit for driving the imaging device; and a circuit for extracting a dummy signal from the unit element of the imaging device whose portion is shielded from light using a gate signal of the drive circuit. a gate circuit, a peak hold circuit that holds the peak of the dummy signal from the gate circuit from the gate signal of the drive circuit, an amplifier for amplifying the output from the peak hold circuit, and an amplifier for amplifying the output from the amplifier. an adder that adds a predetermined voltage to produce a threshold signal; a comparator that compares the signal from the adder with the signal from the imaging device; and a waveform that shapes the waveform of the signal from the comparator. A defect inspection device characterized in that a background processing signal for distinguishing the inspected surface from its background portion is obtained from a shaping circuit.