JP2009204627A - Film thickness measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of easily and surely measuring a film thickness even with a detection waveform with a bottom surface reflection wave superposed on an interface reflection wave. <P>SOLUTION: In the method of measuring the thickness of an object to be measured, the amplitude of the bottom surface reflection wave of an ultrasonic wave transmitted from an opposed side surface to the thin film of a thin film support body includes a larger characteristic than that of the interface reflection wave thereof. The method includes the steps of: adjusting the levels of a peak at a rise time and a maximum peak of a received composite reflection wave to predetermined values, respectively; measuring each threshold value exceeding time in a rising area and a climbing area to a maximum peak with regard to the predetermined threshold values in the state above, respectively; evaluating a measurement value of the film thickness from a time difference between the two threshold value exceeding times; and correcting the evaluated measurement value from a predetermined correction formula to determine the film thickness. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention is particularly applicable to very thin films or layers (hereinafter referred to as “thin films”) such as plating applied to the inner surface of pipes of power transmission towers, communication towers, etc., and scales attached to the inner surface of boiler piping. Is generally referred to as a "."

  In the description of the present invention, “maximum peak” refers to a peak indicating the maximum value of the measured ultrasonic waveform, and “antiphase peak” refers to a peak that is in an antiphase relationship with respect to the maximum peak. Say. The “rising region” is a region where the reference point or zero point is shifted to the peak or the antiphase peak, and the “rising region” is a region where the measured ultrasonic waveform is shifted to the first peak. Say.

  There are many methods for measuring the thickness of a thin film attached to the inner surface of a pipe for a power transmission tower, a communication tower, a boiler pipe, etc. (hereinafter collectively referred to as “thin film support”) using ultrasonic waves. It has been proposed and put into practical use. However, the measurement methods that have been proposed so far measure the boundary reflected wave reflected at the interface between the thin film support and the thin film and the bottom reflected wave reflected at the outer surface of the thin film in a temporally separated manner. (For example, refer to Patent Document 1). For this reason, when the thickness of the thin film is very thin, the bottom surface reflected wave cannot be measured by being superimposed on the boundary surface reflected wave.

  As a solution when the bottom surface reflected wave is superimposed on the boundary surface reflected wave, the time difference between the rising edge of the boundary surface reflected wave and the time when the amplitude of the boundary surface reflected wave and the bottom surface reflected wave is overlapped is zero. A method for measuring the film thickness by measurement has been proposed (see, for example, Patent Document 2). However, in this method, it is essential to be able to predict the shape of the reflected wave in advance in order to determine where the superimposed portion is, but in the actual measurement, the reflected wave has a shape different from the waveform of the transmitted ultrasonic wave. This is not practical because it is extremely difficult to determine where the overlap portion is, and in some cases there may be no amplitude zero.

JP-A-8-159742 JP 2001-227933 A

  In the present invention, there are many common combinations of a thin film support and a thin film to be measured, such as iron, steel, zinc, and scale, in which the bottom surface reflected wave exhibits a larger amplitude than the boundary surface reflected wave. The present invention has been conceived by focusing on the above, and it is intended to provide a method capable of easily and reliably measuring the film thickness even with a detection waveform in which a bottom surface reflected wave and a boundary surface reflected wave are superimposed.

  The film thickness measuring method according to the present invention comprises a thin film support and a thin film attached to one side of the thin film support, and a bottom surface reflected wave of an ultrasonic wave transmitted from the surface of the thin film support facing the thin film. This is a method for measuring the thickness of the thin film of the measurement object having a characteristic that the amplitude of the measurement object has a characteristic larger than the amplitude of the boundary surface reflected wave. The method of the present invention is started by transmitting ultrasonic waves from the surface of the thin film support opposite to the thin film using a conventional ultrasonic flaw detector as in the prior art, and the boundary surface of the transmitted ultrasonic waves. A combined reflected wave composed of the reflected wave and the bottom reflected wave is received by the ultrasonic flaw detector, and the received combined reflected wave rises to the first peak and the opposite phase peak (hereinafter referred to as “rising region”). Is the time difference from the time when the threshold value of the peak is passed through a threshold value of a predetermined percentage (hereinafter referred to as “when the threshold value is passed”), or the time difference from when the threshold value is passed in the rising region to the maximum peak. The measurement value of the film thickness is obtained by measurement, and the thickness of the thin film is determined by calculating the correction value from the obtained measurement value based on a predetermined correction formula.

The earliest part of the waveform of the received composite reflected wave is basically the boundary reflected wave component because the boundary reflected wave returns earlier than the bottom reflected wave, and the maximum peak part of the waveform is Since the bottom surface reflected wave is larger than the boundary surface reflected wave, it can be basically regarded as a component of the bottom surface reflected wave. For this reason, the measurement value of the film thickness is obtained with the rising point of the synthetic reflected wave as the measurement point of the boundary reflection wave, the maximum peak time as the measurement point of the bottom reflection wave, and the above-mentioned correction is performed to obtain the film thickness. Is possible. In the present invention, the basic idea is to capture the first wave at the rising edge as the boundary reflection wave and the wave at the maximum peak as the bottom reflection wave. The time of passage is the measurement point of the boundary surface reflected wave, and the time of passage of the threshold in the maximum peak rise region is the measurement point of the bottom surface reflected wave. At least at the beginning of the rising region, as described above, the boundary surface reflected wave returns earlier than the bottom surface reflected wave, so that it can be processed substantially as a waveform of the boundary surface reflected wave. On the other hand, the curve of the maximum peak rising region is changed depending on the degree of superposition of the boundary surface reflected wave and the bottom surface reflected wave. In the present invention, basically, this curve change amount is compared with an actual measurement value of a thin film measured with an optical microscope, and a correction formula is obtained.
Vc = (Vm−b) / a
Where Vm: measured value of thin film thickness
Vc: Correction value of thin film thickness
a, b: The coefficient set by the material of the thin film and the thin film support is derived.

The time when these threshold values are passed is taken as the measurement point depending on the degree of superposition of the boundary surface reflected wave and the bottom surface reflected wave, the frequency of the ultrasonic wave to be used, the magnitude of the detected amplitude of the synthesized reflected wave, and the threshold value. This includes the possibility that the target value will change. For this reason, the correction equation is obtained with these fluctuation factors fixed. As one of the application forms of the present invention, when the thin film support is iron or steel and the thin film is zinc, the thin film measuring method of the present invention amplifies the rising peak and the maximum peak to a predetermined value, respectively. Attenuating and setting the ratio of a predetermined ratio to the peak value at that time as the threshold value in the rising region and the threshold value in the maximum peak rising region, respectively, the coefficients a and b of the correction equation are
a = 1.798
b = 151.74
By doing so, the value of the film thickness having practically sufficient accuracy can be obtained.

When the thin film support is iron or steel and the thin film is aluminum,
a = 1.8232
b = 238.36
By doing so, the value of the film thickness having practically sufficient accuracy can be obtained in the same manner as described above.

  By the way, although it depends on the measurement conditions such as the ultrasonic flaw detector to be used and the ultrasonic frequency, when the thickness of the thin film is relatively thick or the combination of the thin film support and the material of the thin film, At least one peak appears between the maximum peak and the first peak. For this reason, when the above-mentioned maximum peak is amplified or attenuated to a predetermined value and a value of a predetermined ratio with respect to the peak value at that time is read, if this intervening peak is larger than the threshold value, the maximum peak threshold value passing There is a possibility that the value at the time of passing through the threshold value of the peak intervening instead of the time may be erroneously read. Examples of such relatively large peaks intervening include a combination of an aluminum thin film support and a nickel or chromium thin film, or a combination of a copper thin film support and a tin thin film. In particular, when reading is automatically performed, it is necessary to have a more complicated program for automatically recognizing whether the peak value is an intermediate peak value or the maximum peak value.

  In order to avoid such erroneous readings, in another application of the present invention, a relatively large inverse that results in an erroneous reading between the immediately preceding antiphase peak leading to the maximum peak and the first peak. Focusing on the fact that no phase peak is present, instead of the above-described threshold passing in the maximum peak rising region, the threshold passing time in the rising region to the opposite phase peak immediately before the maximum peak is used.

As is obvious, the time between the threshold passing in the rising region and the threshold passing in the rising region to the antiphase peak is the same as when the threshold is passed in the rising region and the maximum peak when the thin film of the same thickness is measured. And the values a and b in the correction formula are similarly different. The coefficients a and b in the correction formula in this application form are respectively
When the thin film support is aluminum and the thin film is nickel:
a = 0.3682
b = 103.36
When the thin film support is aluminum and the thin film is chromium:
a = 0.7669
b = 118.97
When the thin film support is copper and the thin film is tin:
a = 1.4523
b = 66.614
By doing so, the value of the film thickness having practically sufficient accuracy can be obtained in the same manner as described above.

Here, it should be noted that the method based on the rising region and the rising region of the antiphase peak can be used as it is even when another peak is not interposed between the peak at the rising time and the maximum peak. In this case, the coefficients a and b of the above-described correction formula in the above combination are set as follows.
When the thin film support is iron or steel and the thin film is zinc:
a = 1.115
b = 130.06
When the thin film support is iron or steel and the thin film is aluminum:
a = 1.7771
b = 84.347

On the other hand, although the explanation is mixed, if the thin film is thin, another peak may not be interposed between the peak at the time of rising and the maximum peak, and the above-mentioned thin film support: aluminum / thin film: nickel, thin film support: aluminum / Thin film: Chromium, Thin film support: Copper / Thin film: Tin can be measured by measuring the rising threshold region passing through the threshold and the maximum peak rising region passing through the threshold. The coefficients a and b in these cases are as follows.
Thin film support: Aluminum / Thin film: Nickel
a = 0.614
b = 232.97
Thin film support: Aluminum / Thin film: Chromium
a = 1.3772
b = 212.88
Thin film support: Copper / Thin film: Tin
a = 1.3592
b = 162.91

  According to the present invention, the time difference at the time of passing the threshold value in the rising region of the synthetic reflected wave and the rising region to the maximum peak, or the threshold value in the rising region to the antiphase peak immediately before the rising region of the synthetic reflected wave and the maximum peak. The film thickness is obtained by measuring the time difference during passage, and the film thickness can be obtained as a practical value by correcting the value using a predetermined correction formula, whereby the conventional waveform is superimposed. Accordingly, it is possible to measure a very thin film thickness that could not be measured, and to measure the thickness of the thin film regardless of whether the boundary surface reflected wave and the bottom surface reflected wave are separated.

It is a block diagram which shows the example of the ultrasonic flaw detector which can be used with the method of this invention. It is typical sectional drawing for demonstrating the production | generation process of a reflected wave. It is a figure which shows the example of the waveform of the synthetic | combination reflected wave obtained in the thickness measurement using an ultrasonic wave. It is a figure which shows the example of the synthetic | combination reflected wave for demonstrating the setting of a threshold value. It is a graph which shows the relationship between the measured value at the time of using galvanization as a thin film, and an actual value. It is a graph which shows the relationship between the measured value at the time of using aluminum plating as a thin film, and a measured value. FIG. 5 is a diagram similar to FIG. 4, illustrating an example of a composite reflected wave for explaining another threshold setting method.

  The film thickness measurement method according to the embodiment of the present invention is performed using a known ultrasonic flaw detector 1 as illustrated in FIG. Since the signal processing and the like of the ultrasonic flaw detector 1 are out of the scope of the present invention, the details thereof will be omitted, but are outlined as follows.

  In the ultrasonic flaw detector 1, the ultrasonic pulse P generated by the pulsar 2 is transmitted from the outer surface side of the measurement object 4 by the ultrasonic transducer 3. The measurement object 4 is composed of a thin film support 4a and a thin film 4b attached to the inner surface of the thin film support 4a, as shown schematically in FIG. A portion of the boundary surface 4c between the support 4a and the thin film 4b is reflected as a boundary surface reflected wave P1, and the ultrasonic energy transmitted through the boundary surface 4c is substantially totally reflected as a bottom surface reflected wave P2 at the outer surface 4d of the thin film 4b. Is done. The boundary surface reflected wave P1 and the bottom surface reflected wave P2 reflected by the boundary surface 4c and the outer surface 4d, respectively, are received by the ultrasonic transducer 3, amplified by the signal amplifying unit 5, and synthesized by the display device 6. It is displayed as a reflected wave Pc (see FIG. 3).

  Here, the combination of the thin film support 4a and the thin film 4b handled in the present invention has a characteristic that the amplitude of the bottom surface reflected wave P2 of the ultrasonic wave transmitted from the outer surface of the thin film support 4a is larger than the amplitude of the boundary surface reflected wave P1. It is a combination on condition of having. Therefore, a typical example of the combination of the thin film support 4a and the thin film 4b handled in the present invention is a combination of a thin film support made of iron or steel and a thin film of zinc, tin, aluminum or scale. it can.

  When the thickness of the thin film 4b is relatively large, the boundary surface reflected wave P1 that is the first reflected wave and the bottom surface reflected wave P2 that is the second reflected wave are as shown in FIG. The boundary surface reflected wave P1 and the bottom surface reflected wave P2 are detected as a combined reflected wave Pc separated from each other, and the boundary surface reflected wave P1 and the bottom surface reflected wave P2 approach each other as the thickness of the thin film 4b decreases (FIG. 3 (B)), finally, the boundary surface reflected wave P1 and the bottom surface reflected wave P2 cannot be separated from each other (see FIG. 3C).

  Since the method of converting the measured time difference value of the reflected wave into the thickness of the thin film is outside the scope of the present invention, a description thereof will be omitted, but as shown in FIG. When the boundary surface reflected wave P1 and the bottom surface reflected wave P2 are completely separated, when the boundary surface reflected wave P1 and the bottom surface reflected wave P2 rise between Ti1 and Ti2, or when passing through a threshold (Lx), between Tt1 and Tt2, Alternatively, the thickness of the thin film 4b is calculated by measuring the time difference dT0 between the maximum peak times Tm1 and Tm2 and calculating it based on a predetermined time difference-thickness conversion formula corresponding to the measurement conditions. Further, as shown in FIG. 3B, when the boundary surface reflected wave P1 and the bottom surface reflected wave P2 are not completely separated but partially overlapped, In the illustrated case, it is possible to extract Tt1 and Tt2 or maximum peak times Tm1 and Tm2 when the boundary surface reflected wave P1 and bottom surface reflected wave P2 pass through the threshold value (Ly). The thickness of the thin film 4b can be measured by measuring the time difference dT0 between the times Tt1 and Tt2 or between the maximum peak times Tm1 and Tm2.

  In contrast to these cases, as shown in FIG. 3C, when the bottom surface reflected wave P2 is superimposed on the boundary surface reflected wave P1 and cannot be separated, the above-described method cannot be used. . For this reason, in the present invention, the time difference dT1 between the threshold passing time Ta in the rising region Ra and the threshold passing time Tb in the maximum peak rising region Rb of the combined reflected wave Pc in which the bottom surface reflected wave P2 is superimposed on the boundary surface reflected wave P1 is calculated. The thickness of the thin film 4b is obtained by measuring.

  By the way, as to which time point of the boundary surface reflected wave P1 and the bottom surface reflected wave P2 is used as the measurement point, at the time of rising (Ti1, Ti2), at the time of passing the threshold (Tt1, Tt2), and at the time of the maximum peak (Tm1, Tm2) However, at the time of rising, there is a tendency to change depending on the degree of amplification of the signal amplifying unit 5, and at the maximum peak, there is a case where it cannot be clearly read from the signal waveform. For this reason, the present invention makes these fluctuation factors substantially constant by taking the following measures.

  As for the frequency of the ultrasonic transducer to be used, when the frequency is low, the resolution is lowered, and when the frequency is high, the reflected wave cannot be detected because the attenuation rate during propagation of the thin film support is large. For this reason, the frequency of the ultrasonic transducer to be used is appropriately selected according to the measurement object.

  As shown in FIGS. 4A and 4B, the level of the peak W1 and the level of the maximum peak W2 at the rising edge of the combined reflected wave are varied with respect to fluctuations due to the detected amplitude of the combined reflected wave. This is achieved by adjusting the amplification degree of the signal amplifying unit 5 so as to be a predetermined value, and setting the predetermined values in the state as the threshold values La and Lb in the rising region Ra and the maximum level rising region Rb, respectively. This means that Ta and Tb can be made constant when the threshold is passed by making the waveforms of the composite reflected wave in the rising region Ra and the maximum level rising region Rb substantially constant. In addition, the signal intensity of the reflected wave from the bottom surface is detected at an almost constant ratio depending on the material of the thin film support and the thin film. It can be detected as a change in time value.

[Experimental Example 1]
The steel thin film support 4a was galvanized to form a thin film 4b to prepare a sample. The signal amplifying unit transmits ultrasonic waves using the ultrasonic transducer 3 of 25 MHz, and the amplitude of the peak W1 at the rising edge of the received composite reflected wave Pc is 75% on the tube scale of the display 6. 5, the threshold value La is set to a level of 25%, and the value of Ta when passing through the threshold value in the rising region is measured (see FIG. 4A), and the maximum peak W2 of the combined reflected wave Pc is measured. The signal amplification unit 5 adjusts the level so that the amplitude is 75% on the tube scale of the display 6, and the threshold value Lb is set to a level of 25%. Was measured (see FIG. 4B). The measured value (Vm) of the film thickness was calculated from the time difference dT1 between the two threshold passage times Ta and Tb obtained in this way. The result is shown in FIG. The correction formula obtained from the approximate formula of this graph is as follows.
Vc = (Vm-151.74) /1.2798 --- (1)
(Vm: measurement value of thin film thickness, Vc: correction value of thin film thickness)

  In order to verify the validity of this correction formula, the measured value (Vr) of the thickness of the thin film 4b of the sample subjected to the measurement was measured using an optical microscope, and each measured value (Vm) obtained in the above-described experiment was measured. ) Using the above correction equation (1) to calculate the correction value (Vc) and compare it with the actual measurement value Vr. The error between the correction value Vc and the actual measurement value Vr is 6 μm on average and the maximum However, it was 11 μm, and it was found that it was in a practically usable range.

[Experiment 2]
As a sample, a thin film support 4a made of steel was subjected to aluminum plating to form a thin film 4b, and an experiment was conducted in the same manner as in the above experimental example. It has been found that the relationship between the measured value Vm and the actually measured value Vr is as shown in FIG. The correction formula obtained from the approximate formula of this graph is as follows.
Vc = (Vm-238.36) /1.8232 --- (2)

  As in Experimental Example 1, in order to verify the validity of this correction formula, the measured value Vr of the thickness of the thin film 4b of the sample was measured using an optical microscope, and each measured value Vm obtained in the above experiment was measured. When the correction value Vc is calculated from the above correction equation (2) and compared with the actual measurement value Vr, the error between the correction value Vc and the actual measurement value Vr is 9 μm on average and 20 μm at maximum. Yes, it was found that it was in a practically usable range.

When the thin film support was aluminum and the thin film was nickel, the thin film support was aluminum, the thin film was chromium, the thin film support was copper, and the thin film was tin. . As a result, it was found that the correction formula can be used by setting as follows.
When the thin film support is aluminum and the thin film is nickel:
Vc = (Vm−103.36) /0.3682
When the thin film support is aluminum and the thin film is chromium:
Vc = (Vm-118.97) /0.7669
When the thin film support is copper and the thin film is tin:
Vc = (Vm−66.614) /1.4523

  In the case of a combination of an aluminum thin film support 4a and a nickel plating or chrome plating thin film 4b, a combination of a copper thin film support 4a and a tin plating thin film 4b, or a case where the thickness of the thin film is relatively thin As illustrated in FIG. 7, the peak W3 located between the peak W1 and the maximum peak W2 at the time of rising shows a value close to the value of the maximum peak W2. When the value of the intermediate peak W3 is close to the value of the maximum peak W2, the above-described means for avoiding the fluctuation due to the amplitude of the combined reflected wave, that is, the peak level is adjusted to a predetermined value. In the method of counting the threshold passing time at that time, the waveforms of the maximum peak W2 and the intermediate peak W3 are displayed across the horizontal line corresponding to the threshold as shown by the broken line in FIG. And the intermediate peak W3 may be confused. In particular, when the reading is automated, the intermediate peak W3 is positioned before the maximum peak W2, and therefore there is a possibility that the value at the time of passing through the threshold value of the intermediate peak W3 may be read instead of the maximum peak W2.

  In order to eliminate such erroneous reading, in another embodiment of the present invention, the threshold value passing time Ta in the rising region and the threshold value passing time Tc in the rising region to the antiphase peak W4 immediately before reaching the maximum peak W2 This is done by measuring the time between. Here, it should be noted that the measurement method using the antiphase peak can be similarly used for a waveform having no peak between the rising peak W1 and the maximum peak W2 shown in the above description.

[Experimental Example 3]
Samples consisting of the following combinations were prepared.
Sample a thin film support: aluminum / thin film 4b: nickel sample b thin film support: aluminum / thin film 4b: chromium sample c thin film support: copper / thin film: tin sample d thin film support: iron / thin film: zinc sample e thin film support Body: Iron / Thin film: Aluminum Similar to the above experimental example, the ultrasonic wave is transmitted using the ultrasonic transducer 3 of 25 MHz, and the amplitude of the peak W1 at the rising edge of the received composite reflected wave Pc is the tube of the display 6 The level is adjusted by the signal amplifying unit 5 so as to be at a position of 75% on the surface scale, the threshold value La is set to a level of 25%, and the value of Ta when passing through the threshold value in the rising region is measured, and the maximum peak W2 is reached. The signal amplifying unit 5 adjusts the level so that the amplitude of the antiphase peak W4 immediately before the signal is 75% of the tube scale of the display 6 and scales the threshold value Lc. The value of Tc at the time of passing the threshold in the maximum peak rise region was measured by setting the level to 25%. The measured value (Vm) of the film thickness was calculated from the time difference dT2 between the two threshold values Ta and Tc thus obtained. The results were graphed, and the correction formulas obtained from the approximate formulas of this graph were as follows.
Sample a: Vc = (Vm−103.36) /0.3682
Sample b: Vc = (Vm−118.97) /0.7669
Sample c: Vc = (Vm−66.614) /1.4523
Sample d: Vc = (Vm−130.06) /1.1015
Sample e: Vc = (Vm−84.347) /1.7771
(Vm: measurement value of thin film thickness, Vc: correction value of thin film thickness)

  The error between the correction value Vc obtained by this correction equation and the optically measured actual value Vr is 6.7 μm on average and 12.3 μm at the maximum, which is in a range that can be used practically. It turned out to be in place.

  In the above description, when the boundary surface reflected wave P1 and the bottom surface reflected wave P2 are completely separated or partially overlapped, the thickness of the thin film 4b is obtained by the conventional technique. However, even in these cases, the method according to the present invention, that is, the time difference between the threshold value passing in the rising region and the threshold value passing in the rising region to the maximum peak is measured, and the measured value of the film thickness is calculated from this time difference. However, it should be noted that the thickness of the thin film can be obtained by correcting the calculated measurement value using the correction formula described above.

  In the above description, the threshold values La, Lb and Lc serving as measurement points are respectively set to the level of the peak W1 at the time of rising, the level of the maximum peak W2 and the level of the antiphase peak W4 immediately before reaching the maximum peak W2. Although it has been described that the level is a predetermined ratio, the predetermined ratio may be 0% to 100%.

  The present invention obtains a correction formula for a combination of a thin film support and a thin film used in a pipe for a power transmission tower, a communication tower, a boiler pipe, etc., and determines a coefficient. Basically, other combinations can be used.

1 Ultrasonic flaw detector
2 Pulsar
3 Ultrasonic transducer
4 measurement object 4a thin film support 4b thin film 4c interface 4d outer surface
5 Signal amplifier
6 Display dT0, dT1, dT2 Time difference La, Lb, Lc, Lx Threshold
P Ultrasonic pulse Pc Synthetic reflected wave P1 Interface reflected wave P2 Bottom reflected wave Ra Rising area Rb Maximum peak rising area Rc Antiphase peak rising area Ti1, Ti2 Rising Tm1, Tm2 Maximum peak Tt1, Tt2, Ta, Tb, Tc When the threshold value is passed Vm Measured value Vc Correction value Vr Measured value W1 Rise peak W2 Maximum peak W3 Reverse phase peak

Claims (1)

It consists of a thin film support and a thin film attached to one side of the thin film support, and the amplitude of the ultrasonic wave reflected from the bottom surface of the thin film support opposite to the thin film is the amplitude of the boundary reflection wave. A method for measuring a thickness of a thin film of an object having a larger characteristic,
Receives the composite reflected wave consisting of the boundary reflected wave and bottom reflected wave of the transmitted ultrasonic wave,
The amplification level of the signal amplifying unit is adjusted so that the peak level at the rising edge of the composite reflected wave becomes a predetermined value, and when the threshold value passes through the rising region with a predetermined value as a threshold value with respect to the predetermined value. In addition to measuring, the amplification level of the signal amplification unit is adjusted so that the maximum peak level of the combined reflected wave becomes the same value as the predetermined value, and the threshold value passes through the maximum level increasing region using the same value as the threshold value as a threshold value. Measure the time
Measure the time difference between the two measured thresholds to determine the film thickness measurement,
A predetermined correction formula from the obtained measurement value,
Vc = (Vm−b) / a
Where Vm is a measured value of the thin film thickness, Vc is a corrected value of the thin film thickness,
a, b: A film thickness measuring method for determining a thickness of a thin film by calculating a correction value based on a coefficient set by the material of the thin film and the thin film support.

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