JPS6173006A - Thickness measuring method of transmission attenuation type thickness gauge - Google Patents

Abstract

PURPOSE:To take a high accuracy measurement while reducing an error in quantization accompanying digital conversion by adjusting a measurement signal to a proper signal level in a forecasted thickness area, and inputting the signal to an A/D converter. CONSTITUTION:The detection signal of a photodetector 12 is passed through a linear DC amplifier 16 and a main amplifier 18 and adjusted to the proper level by the signal adjusting circuit 40, composed of an offset circuit 44, amplifying circuit 46, and two multiplexers 48 and 50 set so as to obtain a desired output for estimated thickness, and an input signal protecting circuit 42. In this case, the signal adjusting circuit 40 performs switching control over the multiplexers 48 and 50 under the command of a CPU32 to input a level signal of one of three stages to the A/D converter 24. Consequently, part of the measurement signal is amplified to the proper level to reduce an error in quantization, thereby measuring the thickness with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field to which the Invention Pertains] The present invention relates to a method for measuring the thickness of various sheets, films, etc. using an attenuated transmission thickness gauge using infrared rays, β-rays, or the like.

[Prior art and its problems]

Conventional optical thickness gauges, ie, transmission attenuation type thickness gauges, are known in analog type and digital type. Therefore, as a thickness measuring method of a conventional digital thickness gauge, for example, the method shown in FIGS. 2 and 3 has been proposed.

In the measurement method shown in FIG. 2, an object to be measured 14 is disposed between a light source 10 and a photodetector 12, and a detection signal detected by the photodetector 12 is transmitted to a primary DC amplifier 16 and a main amplifier 18. , logarithmic amplifier 20, and A/D via the zero/span adjustment circuit 22.
The thickness of the object to be measured 14 is measured by supplying it to a converter 24, and the measured value is displayed on a display 26 as appropriate. An automatic zero adjustment circuit 28 is connected to the main amplifier 18, and an X
Y recorders 30 are connected in parallel. Furthermore, the A
The CPU 32 uses the signal obtained by the /D converter 24 to calculate the extinction coefficient μ, the reflectance r, and the average value and deviation rate of the measured values of the Shinkin Bank, thereby expanding the function of the measurement system.

In addition, the measurement method shown in FIG. 3 includes a light source 10 and a photodetector 1.
2, and the detection signal detected by the photodetector 12 is transmitted to the primary DC amplifier 16 and the main amplifier 1.
8 to the A/D converter r: t24, and
The device is configured to measure the thickness of 4. In this case, A/D
The measurement signal obtained by the converter 24 is scaled by the CPU 32, and the measured value is displayed on peripheral devices 34 including a display and the like. Note that the CPU 32 is configured to be provided with an external memory 36 to perform scaling calibration as appropriate.

Further, the signal obtained by the A/D converter 24 is used to calculate the average value and deviation rate of the absorption coefficient μ, reflectance r, and the measured values of the credit union using peripheral equipment 30, and is used as a measurement system. Functions will be expanded.

However, the basic difference between the two measurement methods described above lies in the signal linearization process and the instrument calibration method. For example, in the former measurement method shown in FIG. 2, linearization processing is performed by a logarithmic amplifier 20, and instrument calibration is performed by hardware using one circuit 22 for zero/span. That is,
Open the optical path from the light source 10 to the photodetector 12, perform zero adjustment (display value = 0), then set a specific standard sample as the object to be measured and perform span adjustment (display value =
sample thickness). In this case, mainly the photodetector 1
Relatively large drift in part 2 (drift over time due to fogging or dirt on the lens or drift due to ambient temperature)
Therefore, it is necessary to periodically retreat the measurement head of the photodetector 12 to the reference position, measure the voltage when the optical path is open, and automatically perform drift correction using the automatic zero adjustment circuit 28.

Additionally, since the zero adjustment is performed in principle with the optical path open, the reflectance r cannot be processed and errors are likely to occur.On the other hand, the span adjustment is performed by adjusting the display value of the digital display 26 to the reference value of the standard sample. Since this is done by manually operating a dial or the like to match, there are drawbacks such as difficulty in accurate calibration using the average value of a large number of samples.

On the other hand, in the latter measurement method shown in FIG. 3, linearization processing and instrument calibration are performed by software in the CPU 32 based on a computer program. That is, in this measurement method, all calculations are performed by the program 6Q in the CPU 32, so compared to the former measurement method shown in FIG. Precise sample-based calibration is possible. In addition, in order to perform automatic zero adjustment, the upper limit of the voltage when the optical path is open is suppressed to below the rated voltage of the A/D converter, and in order to be faithful to Hair's equation. Since V=0 (cutoff), the digitization error accompanying digital conversion is not reduced.

As mentioned above, although the latter measurement method is clearly superior to the former measurement method, both have a common drawback in terms of resolution associated with A/D conversion. The voltage detected by the photodetector is converted into a digital signal by an A/D converter. In this case, in FIG. 4, the voltage resolution ΔV is determined by the following equation based on the rated voltage vp of the A/D converter and the number of bits n.

FS ΔV= −, , , , (11 2′″ −1 However, as is clear from FIG. 4, the thickness resolution Δt is not constant, and for the same material, the thicker it is, the worse it becomes. This tendency is a basic feature of transmission attenuation type thickness gauges.

Generally, if the measurement sensitivity coefficient is β, the resolution ΔL is obtained by the following equation.

■ES Δt ,= , , , , (
2) β (2f'-1) Therefore, β is Bayer and Lambert's equation 7 % (3): o: Measured voltage when the optical path is open μ: Absorption coefficient of the sample r: Reflectance of the sample t : Obtained from the thickness of the sample using the following formula.

)V β= =-μV,,,141 t For example, rated voltage V, q'= l OV (L OmV
), and when the number of bits n=12, the resolution Δt is calculated using the above formula (
Based on 2), it is as follows.

2.44 Δt −, , , (5) β Therefore, as is clear from the above equation (5), the resolution Δt of the thickness of the object to be measured deteriorates as the sensitivity coefficient β decreases (
rough)

However, if the thickness of the object to be measured is, for example, in the range t, ~t, as shown in FIG. 5, the measurement voltage will be V, ~t.
V2. At this time, if the rated voltage VEX of the A/D converter is IOV and the number of bits n is 12, the measurement is performed using almost a small area. That is, in the case of a film, the thinner the film, the higher the sensitivity coefficient β (=μV), so the absolute value of the resolution is quite small and usually does not pose a problem. However, the relative resolution worsens as the thickness becomes thinner, so quantization errors may become impossible to ignore. Therefore, when we calculated the decrease in relative resolution, especially in thin regions, for various films, we obtained the characteristics shown in FIGS. 6 and 7.

Figure 6 uses a polypropylene film as a sample, and measures it to a thickness of 10 μm using the conventional measurement method shown in Figure 3.
It measures around m, and in this case the absolute resolution is 0.
Although 62 μm is acceptable, the relative resolution reaches as high as 2%, which is shown to be a value that cannot be ignored in terms of measurement accuracy.

In addition, in Figure 7, a polyethylene film is used as a sample, and a precision ultra-thin film with a thickness of 5 μm or less is measured in the same manner as in Figure 6. Resolution has been shown to be an issue.

As an example, extinction coefficient μ=0. OO1, reflectance r=0.1
A polypropylene film of t, = 20 μm-L2
When measuring in the range of = 30 μm, the measurement voltage range is V, = 8.82V to V2 = 8.73V, which is only 9
Only the 0 mV region is used. At this time, the average resolution is 0.28 μm in absolute value, but exceeds 1% in relative value, which causes problems as the required accuracy becomes stricter.

[Purpose of the invention]

An object of the present invention is to provide a thickness measurement method using a conventional transmission attenuation type thickness gauge in which linearization processing of a measurement signal of a measured object and instrument calibration are performed using software based on a computer program, in which the measurement signal is set in advance. By adjusting the signal level to an appropriate level for the expected thickness region and inputting it to the A/D converter, the quantization error associated with digital conversion is reduced and measurement accuracy is improved. It is an object of the present invention to provide a thickness measuring method capable of measuring the thickness of a thin film, etc., which has a large sensitivity coefficient and poor relative resolution, to a higher degree of +a.

[Key points of the invention]

In order to achieve the above object, in the thickness measurement method using the transmission attenuation type thickness gauge of the present invention, the amount of light transmitted from the light source to the object to be measured is detected and measured, the obtained measurement signal is amplified, and In a measurement system using an optical transmission attenuation type thickness gauge that measures the thickness of a workpiece by performing scaling calculation processing after A/D conversion, measurement of the back part of the measurement signal input to the A/Dim device It is characterized by adjusting the level of signals within the region to obtain appropriate resolution.

In the thickness measurement method described above, when the measurement signal input to the A/D converter is a signal in an extremely thin area, the measurement signal is compared with a predetermined offset voltage to obtain an appropriate signal level. It is preferable that the signal be amplified at a predetermined amplification factor and selected by a changeover switch. In this case, an offset circuit and an amplifier circuit are provided to set the offset voltage, and the output of the offset circuit and the output of the amplifier circuit are manually controlled by a multiplexer according to the signal level of the measurement signal input to the A/D converter. Alternatively, it is preferable to automatically switch and selectively take out.

[Embodiments of the invention]

Next, an embodiment of the method for measuring thickness using a transmission attenuation type thickness needle according to the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a block circuit diagram showing an embodiment of a measurement circuit of a transmission attenuation type thickness gauge that implements the thickness measurement method of the present invention. The circuit configuration shown in FIG.
Since the circuit configuration is the same as that shown in the figure, the same reference numerals are given to the same components as those shown in FIG. 3, and detailed explanation thereof will be omitted. That is, the object to be measured 14 is disposed between the light source 10 and the photodetector 12, and the photodetector 12
The present invention is the same as the conventional system in that the detection signal detected in the above is configured to be supplied to the A/D converter 24 via the primary DC amplifier 16 and the main amplifier 18. Further, a CPU 32 is provided on the output side of the A/D converter 24, and the A/D converter 2
The measurement signal obtained in step 4 is scaled, and an external memory 36 is provided to calibrate the scaling.

However, in this embodiment, the main amplifier 18 and A
A signal adjustment circuit 40 and an input signal protection circuit 42 are provided between the /D converter 24 and the signal is adjusted to an appropriate level. That is, the signal adjustment circuit 40 includes an offset circuit 44, an amplifier circuit 46, and two multiplexers 48.5. In this case, the signal adjustment circuit 40 switches and controls the multiplexers 48 and 50 according to a control command from the CPU 32, and
The /D converter 24 is configured to receive a signal at one of three levels. The three level signals are placed as follows.

Vze: Output V2 when the output signal of the main amplifier 18 is not amplified at all. (= G −V, −): The output signal of the main amplifier 18 is sent to the amplifier circuit 46 without passing through the offset circuit 44.
Output V when amplified by (amplification factor G), (=Gl:Vl-V,s)): The output signal of the main amplifier 18 is passed through the offset circuit 44 (offset voltage V. Output when amplified by amplification factor G) Next, the basic operation of the circuit of this embodiment having the above configuration will be explained.

For example, assume that the thickness of various polypropylene films of 10 to 200 μm is to be measured. In this case, for a thickness of 10 μm to 50 μm, the multiplexer 48°50 is set to obtain the output of v3. Also, 50μ
Since the relative resolution is not much of a problem for m or more, the above v
multiplexer 48.50 to obtain an output of 2. Furthermore, when checking the basic signal of the open state or cut-off state of the optical path as necessary, the multiplexer 48 and 50 are set to directly obtain the output of the Vt". In this case, the main amplifier 18, the offset circuit 44 Each output characteristic v of the circuit 46 and the input signal protection circuit 42
v, , vJ, and v each have the characteristics shown in the lower part of FIG.

Next, the basic operation of the signal adjustment circuit 40, which is the main part of the circuit of this embodiment described above, will be explained in detail.

First, according to the basic formula (Beyer and Langhart's formula), the daily output Vbi of the main amplifier is as shown in the following formula, and its output characteristics are as shown in FIG.

vl,=v,(1-1)8-/-t609. (6) Next, this main amplifier 18 is input to the offset circuit 44 to limit the measurement range (1, to 1.) and set a predetermined offset voltage -VOS<this offset voltage in the thickness range 1.
．． −t2, which is the upper limit of t2 measured voltage). As a result, the output vL of the offset circuit 44 is as shown in the following equation, and its output characteristics are as shown in FIG.

v, 6= v, (1r) e2''...(
7) V1=V vas...(8
) In this way, the offset output V is amplified through the amplifier circuit 46. As a result, the output Vj of the amplifier circuit
is as shown in the following equation, and its output characteristics are as shown in FIG.

V3=G-V1=G (V, (1-r) e-"-V,
, )・・・・・(9) On the other hand, if the output v3 of the amplifier circuit is directly approximated between the measurement range t, ~t2, the output v3 is expressed by the following equation, and its output characteristics are as shown in Figure 11. It is.

8-・1-μt・・・α0)VJ=p
G Vo (1N) '+ G Vo (1r
) Vo5...(11) In the above equation (11), since everything except Vj is a constant, if this is transformed, it can be expressed as the following equation.

L=AV+B (12) That is, A and B are conversion coefficients for converting electrical signals into thickness, and can be determined by signal measurement (instrument calibration) of a plurality of standard samples.

Furthermore, errors caused by such linear approximation are generally extremely small compared to quantization errors. However, the measurement range t1~L
If 2 is set too wide, the error will become large, which may cause problems. Note that the offset voltage V was obtained by using the method of the present invention described above for a sample with an extinction coefficient ゛μ = 0.001 and a reflectance r-0.1. S=8.734 V, amplification factor G=
When an error analysis was performed when processing as 113.9, the results shown in Table 1 were obtained.

From the results in Table 1 above, the linearization error can be considered zero, and the resolution associated with A/D conversion is 1.1% of the conventional one.
It was confirmed that there was a significant decrease from 0.01% to 0.01%.

In the method of the present invention described above, noise components are also amplified as the signal level increases, but these can be processed through simple statistical processing (digital filtering) in the CPU 32. In other words, when calibrating the instrument, one standard sample is , for example, 100 times in a row, and the average value is calculated and used, so it is hardly affected by noise.

In this way, according to the present invention, even if the change in the output signal for measuring the thickness of the object to be measured is minute, the signal 2□□□ adjustment circuit 40 amplifies a part of the region to an appropriate level, It is possible to reduce quantization errors and achieve highly accurate thickness measurements. Note that the multiplexers 48 and 50 can be freely programmed by the CPU 32 to select which detection output, but as a general rule, it is important to obtain a signal of as high a level as possible without exceeding the rated voltage of the A/D converter 24. It is preferable to set it so that Further, although there is some drift in the signal adjustment circuit 40, it is extremely small compared to the drift in the photodetector 12, so it is sufficient to provide the automatic zero adjustment circuit 28 before the signal adjustment circuit 40.

〔Effect of the invention〕

As is clear from the embodiments described above, according to the present invention,
In response to changes in the signal detected due to changes in thickness, by appropriately adjusting the level of the measurement signal for thin films in particular, it is possible to reduce quantization errors and achieve highly accurate thickness measurements. can. Moreover, this signal level adjustment can be performed automatically by limiting the measurement range depending on the object to be measured and converting it to an appropriate level. In this case, the level adjustment can be configured to be performed manually, and the handling operation is extremely simple. Furthermore, when implementing the measuring method of the present invention, automatic zero adjustment and instrument calibration can be performed in exactly the same manner as in the past.

In the above-mentioned embodiment, a signal adjustment circuit was shown in which a signal level set in three levels was selected manually when inputting a measurement signal to an A/D converter. For example, without being limited to the specific embodiments, it is possible to change the setting voltage and amplification factor type of the offset circuit and amplifier circuit, change the connection circuit thereof, change the rated voltage of the A/D converter, etc. A similar effect can be obtained by configuring the circuit.

Therefore, it goes without saying that various design changes can be made without departing from the spirit of the invention.

[Brief explanation of the drawing]

FIG. 1 is a measurement circuit diagram showing an embodiment of a thickness measurement method in a transmission attenuation type thickness gauge according to the present invention, and FIGS.
The figures are measurement circuit diagrams of typical conventional transmission attenuation type thickness gauges, each showing a different measurement method.
Fig. 4 is a characteristic diagram showing the relationship between thickness resolution and quantization error in the A/D converter of a conventional measurement circuit, and Fig. 5 shows the change in measurement voltage in a predetermined thickness range of the object to be measured. FIGS. 6 and 7 are characteristic curve diagrams showing the relative resolution characteristics of a particularly thin object to be measured, and FIGS. 8 to 11 are diagrams for carrying out the method of the present invention shown in FIG. 1. FIG. 3 is a characteristic diagram showing the operating characteristics of the signal adjustment circuit. io, , , light source 12. , , photodetector 14 , , object to be measured 16. , , primary DC amplifier 18 , , main amplifier 20. , logarithmic amplifier 22), atmosphere/span adjustment circuit 24, A/D converter 26. ,,display unit 28,,
, automatic zero adjustment circuit 30. , XY recorder 32),
CPU, 34. ,,Peripheral devices 36,,,External memory 40. , , signal adjustment circuit 42) , , input signal protection circuit 44, , , offset circuit 46. ,,amplifier circuit 48.50. ,, Multiplexer, 111 Constant voltage V Measurement voltage V Resolution % Resolution % L O L

Claims (3)

[Claims] (1) An optical system that detects and measures the amount of light transmitted from a light source to the object to be measured, amplifies the obtained measurement signal, performs A/D conversion, and then performs scaling calculation processing to measure the thickness of the object to be measured. In a measurement system using a transmission attenuation type thickness gauge, the level of a signal within a part of the measurement area input to an A/D converter is adjusted so as to obtain an appropriate resolution. Thickness measurement method using a transmission attenuation type thickness gauge. (2) In the thickness measuring method according to claim 1, when the measurement signal input to the A/D converter is a signal in an extremely thin region, the signal level is adjusted to be appropriate. A method for measuring thickness in a transmission attenuation type thickness gauge, comprising comparing the measurement signal with a predetermined offset voltage, amplifying the signal with a predetermined amplification factor, and selecting the resultant signal using a changeover switch. (3) In the thickness measuring method according to claim 2, an offset circuit and an amplifier circuit are provided, and the output of the offset circuit and A thickness measurement method using a transmission attenuation type thickness gauge, which consists of selectively taking out the outputs of amplifier circuits by manually or automatically switching each output using a multiplexer.