JP3107026B2 - Transmission part shape inspection device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmission part shape inspection apparatus, and more particularly to a transmission part shape inspection apparatus for inspecting the shape of a transmission part such as a mask pattern by irradiating light. The present invention is suitably used, for example, for inspecting the edge shape of a light transmitting portion of a reticle used in a photolithography process in IC manufacturing. Of course, the present invention can be favorably applied to the shape inspection of a mask pattern for masking a photosensitive resin or the like in a photo edging technique for IC manufacturing, not limited to a reticle. Further, the present invention can be used not only for IC manufacturing but also for shape inspection of a mask member for masking light transmission. Further, in addition to the mask member, the present invention can be suitably used for an object that can be inspected depending on whether or not light is transmitted, such as an inspection of a shape of a cut surface of the member.

[0002]

2. Description of the Related Art Conventionally, the shape of an inspection object has been inspected by comparing an image representing the inspection object with a reference image. As the reference image, an image of a non-defective inspection object or an image generated from CAD data or the like is used. Both images were compared after each binarization. For example, JP-A-5-46747 discloses a technique for obtaining a binarized image of a wiring pattern on a printed circuit board.

However, in comparison of binary images, it is not possible to satisfactorily inspect an edge portion of an inspection object. On the other hand, the reference image and the image representing the inspection object
If the comparison is performed with multi-values such as six gradations, the rising or falling state of the edge portion can be compared, so that a good shape inspection can be performed.

Further, in order to perform inspection by a CPU and a program, light emitted from a light source and transmitted through an inspection object is photoelectrically converted by a CCD sensor or the like. In this case, the capabilities of light sources and sensors, such as laser light sources, change over time. Then, it becomes impossible to perform a good comparison with the reference image over a long period of use. Therefore, by calculating the ratio or difference between the light transmitted through the inspection object and the light not transmitted through the inspection object by dispersing the laser light source, the laser light source is not affected by the change of the laser light source. Can be.

[0005]

However, even if the conventional example of calculating the ratio of the light transmitted through the inspection object to the light not transmitted through the inspection object by spectrally dispersing the laser light source is used, the sensor and the sensor cannot be used. The measurement error of the A / D converter cannot be removed. Also, it is not possible to eliminate the influence of noise that may occur in an analog circuit after the sensor.

In an example of inspecting an edge portion of a pattern in a state of a rising or falling gradation change, it is desirable that the gradation does not change in a portion where the pattern does not change. However, due to the shot noise of the sensor, the measurement error of the A / D converter, and the noise of the analog circuit, a change in gradation occurs in an image where there is no change in the pattern. Then, the accuracy of processing such as comparison with the reference image is reduced.

That is, in the conventional example, when the level of light transmitted through the pattern is set to the maximum value and the level of light not transmitted through the pattern is set to the minimum value, the maximum value portion and the minimum value portion become unstable. was there.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an apparatus for inspecting the shape of a transparent portion which can improve the inconvenience of the prior art and, in particular, can inspect a shape having a transparent portion such as a mask pattern. With that purpose.

[0009]

Therefore, according to the present invention, there is provided a light source for irradiating an inspection object having a transmission area for transmitting light and a blocking area for blocking the light with inspection light, and a light source irradiating the inspection light with the light source. A first sensor that photoelectrically converts light transmitted from a transmission portion of the object, a correction unit that corrects an output of the first sensor, and an A / A that converts the sensor output corrected by the correction unit into digital data. A D converter,
And an arithmetic unit for inspecting the shape of the inspection object based on the digital data converted by the A / D converter. In addition, the correction means deforms the signal waveform of the first sensor output to change the maximum value of the first sensor output to A / D.
The value exceeds the upper limit of the measurement of the transducer and the first
And a gain correction means for setting the lowest value of the sensor output of the A / D converter to a value lower than the lower limit of the measurement of the A / D converter. This aims to achieve the above-mentioned object.

When the light source emits light, the light is transmitted or blocked according to the pattern of the inspection object. The first sensor photoelectrically converts the transmitted or blocked light. The signal corresponding to the edge portion of the shape of the transmitted pattern rises and falls with an inclination according to the illuminance distribution and the spot diameter of the light source. When the rising or falling state is compared with a predetermined reference image, it is possible to satisfactorily extract the shape of the inspection object, particularly, a defect at an edge portion. However, if the first sensor output is converted by the A / D converter, a high frequency noise component added by an analog circuit or a sensor is included depending on the capability of the light source and the sensitivity of the sensor. In particular, if a high-frequency noise component is included in a flat portion after rising or falling, the accuracy of defect inspection is reduced. For this reason, the gain correction means deforms the signal waveform of the first sensor output so that the maximum value of the first sensor output becomes a value exceeding the upper limit of the measurement of the A / D converter and the first sensor output is changed. The minimum value of the sensor output is set to a value lower than the lower limit of the measurement of the A / D converter. Then, the maximum value portion and the minimum value portion of the first sensor output exceed the capability of the A / D converter, to the extent that the slope of the rising and falling portions becomes slightly steep. Therefore, the data becomes monotonous digital data. For areas where transparent parts that are not particularly important in shape inspection and areas where non-transmissive parts continue are digital maximum or minimum values, a pattern sampling image that can be easily processed can be obtained, and the accuracy of shape inspection can be obtained. And the high-speed processing becomes possible.

The present invention can be favorably applied to a configuration in which noise contained in a light source is removed by taking the ratio of two sensors. Also, in the example of taking the ratio of these two sensors, the reference value of each sensor output is experimentally obtained in advance, and
If the other sensor outputs are corrected in accordance with the ratio with this reference value, the above-described pattern-sampling image that can be easily processed can be obtained.

[0012]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing a configuration of a transparent part shape inspection apparatus according to the present invention. The transmission part shape inspection apparatus includes a light source 1 that irradiates inspection light to an inspection target 5 having a transmission area that transmits light and a blocking area that blocks the light.
A first sensor 6 for photoelectrically converting light emitted from the light source 1 and transmitted from a transmission portion of the inspection object 5;
Correction means 8 for correcting the output of the sensor 6, the A / D converter 13 for converting the sensor output corrected by the correction means 8 into digital data, and the digital data converted by the A / D converter 13. Computing means (CPU) for inspecting the shape of the inspection object based on the information.

Here, the inspection object 5 generally includes a mask pattern 3 and a transparent material 4 such as glass.
The transmissive area of the inspection object refers to a notch (hole) of the mask pattern 3, and the cutoff area refers to an area covered by the mask pattern.

Further, the correcting means 8 transforms the signal waveform of the first sensor output so that the maximum value of the first sensor output becomes a value exceeding the upper limit of the measurement of the A / D converter, and There is provided a gain correction means 8A for setting the lowest value of the first sensor output to a value lower than the lower limit value of the measurement of the A / D converter. Since the gain correction means 8A amplifies the sensor output so that the maximum value and the minimum value of the sensor output exceed the upper and lower limits of the measurement of the A / D converter, the accuracy of the edge portion of the hole such as the mask pattern is reduced. Instead, it is possible to set the upper end or lower end of the A / D converter for a portion where the transmission region or the cutoff region is continuous, whereby it is possible to satisfactorily remove noise due to high frequency components that overlap the signal. it can.

In the example shown in FIG. 1, two sensors are used. That is, a first sensor 6 that photoelectrically converts light emitted from the light source 1 and transmitted from the transmission part of the inspection object 5 and a second sensor 7 that photoelectrically converts light emitted from the light source 1 are provided. ing. Then, the correction means 8
The output of the first sensor 6 is corrected based on the output of the second sensor 7.

Further, the correction means 8 comprises a ratio signal output means 9 for outputting a ratio signal obtained by taking a ratio of the second sensor output to the first sensor output, and a maximum value and a minimum value of the amplitude of the ratio signal. Amplifying means 10 for amplifying the signal waveform so that the values exceed the upper and lower limits of the measurement of the A / D converter, respectively. In this example, the ratio signal output means 9 and the amplification means 10 constitute a gain correction means 8A.

Further, in order to remove the high frequency component, the amplification width of the signal by the amplifying means 10 may be determined according to the amplitude of the high frequency component. That is, the ratio signal is amplified so that the maximum value and the minimum value of the ratio signal exceed the upper limit value and the lower limit value of the measurement of the A / D converter by a predetermined magnitude, respectively. This predetermined size is determined as follows. That is, the amplitude of the high-frequency noise component is measured according to the configuration of the analog circuit or the sensor to be implemented,
The amplification factor of the waveform is calculated so that this component exceeds the saturation level of the A / D converter 13 and the zero level.

Further, the correcting means 8 is provided for detecting sensor sensitivity and glass.
4 may be provided with a configuration to cope with a change in transmittance.
No. In this case, the transmittance of the inspection object changes and the first cell is changed.
If the sensor value changes, correct the value of the second sensor.
Can be. Therefore, when the ratio between the first sensor and the second sensor is calculated, pattern sampling can be performed without being affected by the sensitivity of the sensor or the transmittance of the inspection object.

[0020]

In this embodiment, the correction is performed so that the sensor output signal for detecting light passing through the pattern is saturated to a constant value by a signal in which the gain of the sensor output signal for detecting laser light is intentionally changed. When the laser beam does not pass through the pattern, the signal is offset to make the level at which no light enters "0".

FIG. 2 is a block diagram showing the configuration of one embodiment of the present invention. Here, a laser light source 1 that emits spot light is used as a light source. The spectroscopy unit 2 divides the laser light emitted by the laser light source 1 into two. One is a laser beam for scanning the pattern 5 of the inspection object, and the other is a second laser beam for monitoring the power of the laser beam.
The laser beam is directed to the sensor 7 of FIG. Pattern 5
The laser beam transmitted through is received by the first sensor 6. On the other hand, the power of the laser beam is detected by the second sensor 7. C
The PU 14 takes in the output of the sensor 5 that has been converted to digital by the A / D converter 8.

In this embodiment, as the amplifying means 10 shown in FIG. 1, a second A / D converter 24 for converting the output of the second sensor 7 into digital data,
Comparing means for comparing the output value of the second sensor 7 output from the / D converter 24 with a predetermined reference value, and when the output value of the second sensor 7 is larger than the reference value. A gain setting means 21 is provided for reducing the second sensor output and increasing the second sensor output in the opposite case. More specifically, the CPU 14 functions as a part of the gain setting unit 21 and a comparison unit.

That is, the CPU 14 controls the second sensor 7
A command is sent to the gain setting means 21 so that a certain value is obtained. The gain setting means 21 gives a gain to the second sensor 7 based on a command from the CPU 14 and outputs the gain. The gain setting means 21 includes a digitally settable variable amplifier and a programmable attenuator.

The correction circuit 20 outputs the ratio between the output of the second sensor 7 for which the gain has been set by the gain setting means 21 and the output of the first sensor 6. Thereby, the low frequency components of the laser light source 1 are removed. Further, by outputting the ratio, the CPU 14 always obtains a constant value even when the power of the laser light fluctuates. Specifically, the output of the correction circuit 20 is A
The data is converted into a digital value by the / D converter 13 and stored in the memory 26. By looking at the value in the memory 26, the CPU 14 can recognize the pattern shape which is not affected by the fluctuation of the laser beam.

In this embodiment, the amplifying means 10 sets the gain by the setting means and raises or lowers the ratio signal output from the ratio signal output means in the amplitude direction according to the set amount of the setting means. And an offset setting means 23 for offsetting the offset. The offset setting means 23 is C
An offset value is given to the offset setting circuit 22 according to a command from the PU 14. The offset setting circuit 22 adds the output value of the offset setting means 23 to the output value of the correction circuit 20. The offset setting means 23 includes a setting by a D / A converter and a voltage setting by a programmable resistor.

The A / D converter 13 A / D converts the output value of the offset setting circuit 22 according to the timing from the A / D conversion control unit 14 and stores the result in the memory 26. CPU1
4 can recognize the pattern image from the contents of the memory 26.

Next, details of the correction circuit will be described. The correction circuit 6 is usually a division circuit of the following equation.

[Output of correction circuit 20] = [Output of first sensor 6] / [Output of second sensor 7]

For example, when the output of the second sensor 7 is "a" at first, the output of the correction circuit 6 which assumes that the output of the first sensor 6 is "b" is b / a. If the output of the laser beam drops for some reason and the output of the second sensor 7 drops to a / 2, the output of the first sensor 6 also becomes b / 2, but the output of the correction circuit 6 becomes b / Same as a. This indicates that a constant value can always be obtained even if the output of the laser light fluctuates. In this embodiment, the following equation (1) is further employed.

[Output of correction circuit 20] = [Output of first sensor 6] / [[Output of second sensor 7] × k] (Equation 1) k: Gain setting by gain setting means 21 value

According to the equation (1), the detection value of the transmitted light of the pattern 5 is saturated at a constant value, and the A / D converter 11
Saturated above the upper limit of the measured value. Thereby, the measurement data of the transmitted light of the pattern 5 is set as the MAX value of the A / D converter 13. The output of the correction circuit 20 is shifted to a negative level by the offset setting circuit 22. This is because when the light does not pass through the pattern 5, the measurement data is converted to the A / D converter 1 by setting the measured value of the A / D converter 13 to the lower limit or less.
The MIN value is 3. By using the equation (1), stable pattern sampling data can be obtained even if there is a change in laser light or a change with time of the sensor.

Next, the operation of this embodiment will be described with reference to actual waveforms. FIG. 3 shows how the laser beam scans the pattern. The inspection object includes a non-transmissive area 3a,
And a transmission region 3b. The beam spot 1a is scanned with respect to this inspection object. FIG. 4 is a waveform diagram showing a waveform obtained by photoelectrically converting the scanning of the beam spot by the second sensor 7. In FIG. 4 and subsequent figures, "0" means
The 0 level of the A / D converter 13 is shown, and “saturation” indicates the saturation level of the A / D converter 13. In the example shown in FIG. 4, low-frequency noise is included.
When D-converted, if the value is larger than the saturation point, A / D
The converted value becomes the maximum value, and portions below the saturation point become multivalued values.

FIG. 5 shows the light transmitted through the inspection object shown in FIG. 3 photoelectrically converted by the first sensor 6. Since the light sources are the same, a low frequency component is added to the transmitted light in the example shown in FIG. Here, the waveforms in FIGS. 4 and 5 are indicated by thick lines, which means that high-frequency components are added within the range of the thick lines. The rising and falling portions in FIG. 5 are caused by the laser light being a circular spot. That is, a part of the spot first overlaps with the edge of the inspection object, and sequentially reaches the saturation level from the 0 level. By comparing this oblique rising or falling as multivalued digital data with digital data of a non-defective product, it is possible to inspect the edge portion for defects with extremely high accuracy.

The correction circuit 20 performs the division of “waveform b shown in FIG. 5 / waveform a shown in FIG. 4”. Then, the waveform shown in FIG. 6 can be obtained. In FIG. 6, the high-frequency components are shown by bold lines without being omitted. By taking such a ratio, noise included in the light source itself and a noise component common to both sensors can be removed. However, as shown in FIG. 6, high-frequency components remain over the saturation level or the zero level. I will.

For this reason, in this embodiment, this high-frequency component is removed by adding a gain to the waveform shown in FIG. First, the gain of the gain setting means 21 is set to 1. Second
If the output of the sensor 7 is a and the output of the first sensor 6 is b, the output of the correction circuit 20 is given by the following equation (2), and has the waveform shown in FIG.

[Output of Correction Circuit 20] = b / a (Equation 2)

In FIG. 6, the saturation level is the upper limit of measurement of the A / D converter 13, and the 0 level is the lower limit of measurement of the A / D converter. In this case, the measurement value in FIG. 3 generally includes a measurement error of the A / D converter 13, a shot noise of a sensor, and a noise of an analog circuit.

Therefore, when the output of the first sensor 6 is b,
When the output of the second sensor 7 is changed to 0.8a by the gain setting means 21, the output of the correction circuit 20 is given by the following equation (3). This waveform is shown in FIG.

[Output of Correction Circuit 20] = b / 0.8a = b / a + ΔV1 (Equation 3)

Next, an offset [-ΔV3] is set in the offset setting means 23. Offset setting circuit 22
Adds an offset [−ΔV3] to the output of the correction circuit 20. FIG. 8 is a waveform diagram showing an output waveform of the offset setting circuit 22. This waveform is converted into a digital signal by the A / D converter 13 and stored in the memory 26. The stored data at this time is A / D when the signal exceeds the saturation level.
The measured MAX value of the converter 13 is a MIN value of the A / D converter when the signal is equal to or lower than the 0 level.

Therefore, in FIG. 8, ΔV2 is equal to or greater than the MAX value of the A / D converter 13, and ΔV3 is the MI value of the A / D converter.
The CPU 14 sets ΔV1 and ΔV3 so as to be equal to or less than the N value, so that the CPU 14 outputs a pattern signal that does not include the measurement error of the A / D converter 13, the shot noise of the sensor, or the noise of the analog circuit. Obtainable. That is, since ΔV1 = ΔV2 + ΔV3,
It is preferable to measure the amplitude of the high-frequency component experimentally and add a gain to the waveform at a ratio that is twice the amplitude.

Further, the sensor output becomes 4 due to the secular change of the output of the first sensor 6 or the change of the transmittance of the glass 4 of the object to be measured.
/ 5 times, that is, 0.8b. This can be detected as follows. First, the value of the transmitted light portion output from the first sensor is converted into digital data by the A / D converter 13 and stored in the memory 26. Then, CP
U14 compares an output value of the first sensor with a reference value determined in advance and stored in the memory 26 or the like. From this ratio, a change in the output of the first sensor can be detected.

When the output of the first sensor 6 changes to, for example, 4/5, the CPU 14 also sets the output signal of the second sensor 7 to 4/5 by the gain setting means 21. In such a case, the output of the correction circuit 20 becomes a value represented by the following equation (3), and the waveform of FIG. 6 can be stably obtained even if the sensitivity of the sensor changes due to aging or a change in the transmittance of the pattern.

[Output of Correction Circuit 20] = (0.8b) /0.8a=b/a (Equation 4)

As described above, according to the present embodiment, the first
Even if the sensor output or the analog circuit changes over time, or the transmittance of the pattern changes, the detected value of the transmitted light of the pattern 5 can be kept constant and captured by the CPU.
Second, in detecting the transmitted light of the pattern 5, the CPU can capture a detection value that does not include the measurement error of the A / D converter 13, the shot noise of the sensor, or the noise of the analog circuit. . Third, in the case where the laser light does not pass through the pattern 5, the CPU may capture a detection value that does not include the measurement error of the A / D converter 13, the shot noise of the sensor, or the noise of the analog circuit. It is possible.

Thus, when the pattern is sampled by the laser beam, the CPU can obtain a constant and clearer pattern image. Then, it is possible to perform a pattern inspection of a pattern having a more configuration as compared with the related art, and since the gradation value of the transmission portion and the non-transmission portion becomes the upper limit value or the lower limit value of the A / D converter, Inspection processing can be performed stably at higher speed.

[0047]

Since the present invention is constructed and functions as described above, according to this, the gain correcting means deforms the signal waveform of the first sensor output to change the maximum value of the first sensor output to A. In order to set the value higher than the upper limit of the measurement of the / D converter and to set the minimum value of the first sensor output to be lower than the lower limit of the measurement of the A / D converter, the slope of the rising and falling portions is slightly To the extent that it becomes steep, the maximum value portion and the minimum value portion of the first sensor output can be set to values exceeding the capability of the A / D converter, and the data after A / D conversion becomes monotonous digital data. Become. Therefore, in a region where a transparent portion which is not particularly important in the shape inspection is continuous or a region where a non-transmissive portion is continuous, a digital maximum value or a minimum value is obtained, and a pattern sampling image which is easy to perform image processing can be obtained. Accuracy, and high-speed processing can be performed. In addition, even when sensor sensitivity or the like changes due to long-term use, a pattern sampling image can be obtained stably and well, and a shape inspection can be performed. It is possible to provide an unprecedented excellent transmission part shape inspection apparatus.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of one embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a state in which an inspection object is irradiated with spot light.

4 is a waveform diagram showing a state where the spot light shown in FIG. 3 is photoelectrically converted by the second sensor shown in FIG. 2;

5 is a waveform diagram showing a state in which light transmitted through an inspection object is photoelectrically converted by the first sensor shown in FIG. 2;

6 is a waveform chart showing a waveform obtained by taking a ratio between the waveform shown in FIG. 5 and the waveform shown in FIG. 4;

7 is a waveform chart showing a waveform obtained by adding gain to the waveform shown in FIG. 6 and amplifying the waveform.

8 is a waveform chart showing a state where the waveform shown in FIG. 7 is shifted downward.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source 2 Spectroscope 5 Inspection object 6 First sensor 7 Second sensor 8 Correction means 9 Ratio signal output means 10 Amplification means 11 Ratio calculation means 12 Correction means 13 A / D converter 14 CPU

Claims (6)

(57) [Claims] 1. A light source for irradiating an inspection object with an inspection light having a transmission area for transmitting light and a blocking area for blocking the light, and light irradiated by the light source and transmitted from a transmission portion of the inspection object. Sensor for photoelectrically converting the data, correction means for correcting the output of the first sensor, A / D converter for converting the sensor output corrected by the correction means into digital data, and A / D converter Calculating means for inspecting the shape of the inspection object based on the digital data converted by the converter, wherein the correcting means deforms a signal waveform of the first sensor output to output the first sensor output Is set to a value exceeding the upper limit value of the measurement of the A / D converter, and
And a gain correcting means for setting the lowest value of the sensor output to be lower than the lower limit value of the measurement of the A / D converter. 2. A light source for irradiating an inspection object with an inspection light having a transmission area for transmitting light and a blocking area for blocking the light, and light irradiated by the light source and transmitted from a transmission portion of the inspection object. A first sensor for photoelectrically converting the light, a second sensor for photoelectrically converting the light emitted by the light source, and a correction unit for correcting the output of the first sensor based on the output of the second sensor. An A / D converter for converting the sensor output corrected by the correction means into digital data, and an arithmetic means for inspecting the shape of the inspection object based on the digital data converted by the A / D converter. Ratio signal output means for outputting a ratio signal that is a ratio of the second sensor output to the first sensor output, and the maximum value and the minimum value of the amplitude of the ratio signal are respectively A / D Transmitted portion shape inspection apparatus characterized by comprising an amplification means for amplifying the signal waveform to exceed the upper limit value and a lower value of the measurement of the exchanger. 3. The amplifying means according to claim 1, wherein said ratio signal has a maximum value and a minimum value which are respectively larger than upper and lower limits of measurement of said A / D converter by predetermined magnitudes. 3. The inspection apparatus according to claim 2, further comprising an amplifying unit. 4. A second A / D converter, wherein the amplifying means converts an output of the second sensor into digital data,
Comparing means for comparing the output value of the second sensor output from the second A / D converter with a predetermined reference value; and the comparing means sets the output value of the second sensor to the reference value. Setting means for reducing the output of the second sensor when the output value is larger than the reference value and increasing the output of the second sensor when the output value of the second sensor is smaller than the reference value. 3. The apparatus for inspecting the shape of a transparent part according to claim 2, wherein: 5. An offset setting means for amplifying means for setting a gain by the setting means and offsetting a ratio signal output from the ratio signal output means upward or downward in an amplitude direction according to a set amount of the setting means. 5. The apparatus for inspecting the shape of a transparent part according to claim 4, further comprising means. 6. A light source for irradiating an inspection object with an inspection light having a transmission area for transmitting light and a blocking area for blocking the light, and light irradiated by the light source and transmitted from a transmission portion of the inspection object. A first sensor for photoelectrically converting the light, a second sensor for photoelectrically converting the light emitted by the light source, and a correction unit for correcting the output of the first sensor based on the output of the second sensor. An A / D converter for converting the sensor output corrected by the correction means into digital data, and an arithmetic means for inspecting the shape of the inspection object based on the digital data converted by the A / D converter. The transmitted light portion output from the first sensor.
Calculate the ratio by comparing the minute value with a predetermined reference value
CPU and the output signal of the second sensor at the ratio.
Gain setting means for correcting the error, and the second sensor
Output a ratio signal that is a ratio of the output of the first sensor to the output of the first sensor.
Output ratio signal output means, and the maximum value of the amplitude of the ratio signal.
The minimum value is the upper limit value of the measurement of the A / D converter, and
Amplification that amplifies the signal waveform to exceed the lower limit
Means for inspecting the shape of a transparent portion.