JPH01239403A - Film thickness measuring method - Google Patents

Abstract

PURPOSE:To accurately measure plating thickness by inducing an eddy current with a relatively high frequency current when the plating thickness is small or with a low frequency current when large. CONSTITUTION:A coil 1 is fitted to a substrate 1 where a plating film 3 is to be formed in a plating tank 4, and an impedance measuring instrument 5 having an oscillation source which generates two kinds of frequency is connected thereto. Then when the plating thickness is small, the eddy current is induced with the relatively high frequency current of, for example, 8MHz and the film thickness is measured, so that the impedance value varies greatly even in response to even slight film thickness variation. Further, when a 4MHz low frequency is used after the plating thickness increases, the entry of the impedance value into a saturation state can be evaded. Consequently, an accurate measurement is taken by varying the frequency with the film thickness.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for measuring the thickness of a plating film using an electromagnetic induction method.

[Conventional technology]

For plated products, the thickness of the coating is often more important than quality control aspects such as decoration and functionality.

In the case of electroplating, the main factors that affect the thickness of the film are the composition of the plating bath, PI (, stirring conditions, bath temperature,
There are current density distribution, plating time, etc., and if these main factors can be kept constant, it is theoretically possible to form a plating film with a constant thickness.

However, in practice, it is almost impossible to keep the above factors strictly constant, so conventionally, the film thickness is measured by electromagnetic induction method etc. after plating, and the plating conditions are modified to obtain the desired film thickness. A method has been adopted to do so. Film thickness measurement using the electromagnetic induction method utilizes the fact that when a high-frequency current is passed through a coil facing the plating film to induce eddy currents in the °C plating film, the coil exhibits an impedance value that corresponds to the plating thickness. It is.

[Problem to be solved by the invention]

However, the conventional electric V! However, with the film thickness measurement method using the induction method, it was not possible to accurately determine the film thickness during the plating process. For this reason, in the case of mass-produced plating, there is a problem in that a large number of defective products whose film thickness does not meet specifications are produced at the end of plating. Figure 3 shows the correlation between plating thickness and impedance value in film thickness measurement using the electromagnetic induction method, with plating thickness (μm) on the horizontal axis and impedance value (μm) on the vertical axis.
This is a graph expressed using an arbitrary scale. As shown in the figure, when the frequency of the high-frequency current used is high, for example 8Mt (z) (A in the figure), if the plating thickness becomes thicker than a certain value, the impedance value of the coil will change even if the plating thickness becomes thicker. The so-called saturation phenomenon is occurring.

Therefore, the film thickness measured by applying a high frequency current after saturation is not reliable. This is the first reason why film thickness could not be accurately determined in the past.

On the other hand, when the frequency of the high-frequency current used is low, for example, 4 MHz (B in the figure), the change in impedance value due to the change in film thickness is smaller than when the frequency is high. Therefore, when the plating thickness is thin, measuring the film thickness by passing a high-frequency current with a low frequency leads to a decrease in measurement accuracy. This is the second reason why film thickness could not be determined accurately in the past.

As is clear from the above reasons, the problem with conventional methods of poor measurement accuracy could be solved by changing the frequency of the high-frequency current used for measurement depending on the film thickness, that is, the progress of film formation. It is.

°Also, to put what I said in the first reason, if the frequency of the high-frequency current used is the same, the saturation values of the impedance of the coils will be the same for plating films with the same electrical characteristics. . Therefore, the difference between the impedance saturation value of the coil determined in advance for a sample with the same electrical characteristics as the plating film using a high-frequency current at the frequency used in the actual measurement, and the impedance saturation value determined for the actual measurement target is It can be said that this is due to error factors. Therefore, by using this difference, the measured value of film thickness can be corrected during plating, and the subsequent plating conditions such as plating time can be appropriately adjusted, and the final plating film with the desired thickness can be obtained. It is possible to obtain with high accuracy.

The present invention was made based on the above findings, and
By changing the frequency to be used according to the plating thickness, the plating thickness can be determined with high accuracy.In addition, the frequency of the high-frequency current is selected so that the impedance value of the coil is saturated during the plating, and the measurement is performed during the plating. By comparing the measured impedance saturation value with a predetermined standard impedance saturation value and correcting the film thickness value measured during plating, accurate film thickness measurement can be achieved and plating with the desired film thickness can be achieved. It is an object of the present invention to provide a method for measuring film thickness using an electrofli induction method, which can obtain a film.

[Means to solve the problem]

In order to achieve the above object, the invention according to claim 1 provides a method for inducing eddy current in a plating film and measuring the film thickness by the impedance value of a coil facing the eddy current. It is characterized by inducing an eddy current with a high frequency current at a relatively high frequency, and after the plating thickness becomes thick, a high frequency current at a relatively low frequency.
The invention according to claim 2 provides a method for inducing eddy currents in a plating film and measuring the film thickness by the impedance value of a coil placed in front of the eddy current, in which a high-frequency wave at a frequency at which the impedance value of the coil is saturated during plating is provided. The impedance saturation value E obtained by inducing eddy current in the plating film with an electric current, and the impedance saturation value ESt of the coil previously obtained using a high-frequency current of the same frequency for a standard sample having the same electromagnetic characteristics as the plating film. From, the formula:
The present invention is characterized in that the film thickness T determined by the impedance value of the coil is corrected based on Tc-T (Est/E) (where Tc is a film thickness correction value).

[Function] As described above, in the invention according to claim 1, since high frequency currents with different frequencies, that is, high frequency currents with high frequencies are passed while the plating thickness is thin, impedance changes even with a slight change in film thickness. After the plating becomes thicker and the plating thickness becomes thicker, a high-frequency current with a lower frequency is passed, so the impedance never becomes saturated. Further, in the invention according to claim 2, the measured value of the film thickness is corrected by comparing the impedance saturation value E measured during the plating with a standard impedance saturation value B it of the coil determined in advance. Therefore, it is possible to finally obtain a plating film with a desired thickness.

(Principle of invention) The principle of the present invention will be explained below.

The alternating current magnetic field is attenuated within a conductor such as a plating film, and in the case of a plane wave, the strength H of the magnetic field after the attenuation is expressed by the following equation.

H−H,exp(X/δ)exp(j X/δ”
) - (1) Here, Ho: Strength of magnetic field on the surface of the plating film, δ = [2/(ωμσ)]”, Penetration depth of the magnetic field, ω = 2πf: Angular frequency of alternating current, μ: Plating film magnetic permeability, σ: electrical conductivity of the plating film, f: frequency of alternating current. While the film thickness is thin, measurement accuracy can be improved by increasing the frequency because the change in impedance value due to the change in film thickness is larger than when the frequency is low due to the skin effect, but Equation (1) As is clearer, the higher the frequency, the greater the attenuation of the magnetic field within a conductor. Therefore,
When a high-frequency current with a relatively high frequency is used, as film formation progresses and the film thickness exceeds a certain level, a saturation phenomenon occurs in which no difference appears in the impedance value even if there is a difference in film thickness.

On the other hand, when using a high-frequency current with a relatively low frequency,
If the thickness of the plating film is not so thick that the electromagnetic induction method can be used, there will be no saturation state in the measurement, which is advantageous, but an increase in the film thickness will result in a slight increase in impedance value. It only appears as. For this reason, using it in film thickness measurement at the initial stage of film formation will reduce measurement accuracy.

Therefore, in the early stages of plating when the plating film is thin, measurement accuracy can be improved by determining the film formation rate or film thickness using a high-frequency current with a relatively high frequency. In the middle to late stages of plating, when the plating becomes thick, saturation of the impedance value of the coil can be avoided by determining the film thickness using a high-frequency current with a relatively low frequency. The above is the principle of the invention according to claim 1.

By the way, according to equation (1), the magnetic field strength H always attenuates equally if the factors in the equation are equal. Therefore, the frequency at which the impedance value of the coil is saturated during plating is selected, and the standard saturation value E of the impedance of the coil is selected for the sample whose magnetic permeability μ and conductivity σ are equal to those of the plated film.
If St is determined in advance, Tc = T (E, , /E) ... - (2) (where Tc is the correction value for the film thickness), for example, due to changes in measurement conditions during plating. The accompanying measurement value T of the film thickness can be corrected. The reason why the measured value T can be corrected using the above equation is as follows.

That is, as is clear from equation (1), the same frequency r
The saturation value E of the impedance of the coil obtained for a plating film whose electromagnetic properties, that is, magnetic permeability μ and conductivity σ of the plating film are the same as the standard sample at a high frequency current of , is the impedance standard of the same coil obtained previously. Saturation value E
If St and both measurement conditions are the same, the values should originally be the same. This is because, as shown in FIG. 2, the standard saturation value of impedance is not affected by the thickness of the film to be measured after saturation. However, the saturated output value E is the standard saturated output value E! It usually does not match t.

This is the separation distance (lift-off) between the coil and the plating film.
This is because the measurement conditions such as the temperature of the plating film are different.

If the measurement conditions (standard conditions) when measuring the standard saturation value Est, which is used as the measurement standard, differ from the measurement conditions in the actual measurement, the film thickness determined from the correlation between the impedance value and the film thickness determined in advance under the standard conditions may differ. There will be an error in the thickness.

However, according to the inventor's knowledge, since the ratio of the true film thickness to the film thickness with an error is equal to the ratio of the standard saturation value Est to the saturation value E, the impedance value of the coil can be calculated to that ratio. The principle of the invention described in claim 2 is that the correction can be made by multiplying the film thickness by the film thickness.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram of an apparatus for carrying out the present invention. In the figure, 4 is a plating tank, and the plating tank 4 has a support stand 7 for supporting a substrate 2 to be coated with a plating film. is placed with its supporting surface horizontal. A coil 1 is housed inside the support stand 7 with its coil axis perpendicular to the substrate 2, and the coil 1 is connected to an impedance measuring device 5. The impedance measuring device 5 includes an oscillation source that emits at least two types of frequencies. The plating surface of the substrate 2 is immersed in a plating bath 6, and a plating film 3 is formed by electroplating means including a plating electrode (not shown).

In such an apparatus, a high frequency current with a frequency of 8 MHz is applied to the coil 1 during the initial stage of plating when the plating thickness is thin. Then, even if the plating thickness changes slightly, the impedance measuring device 5 will show a relatively large increase in impedance. Next, after the plating thickness becomes thicker, the frequency is increased to 4.
A high frequency current of MHz is applied to the coil 1. In this case, the impedance displayed by the impedance measuring device 5 does not show a saturated state.

As described above, the film thickness can be measured with high accuracy.

Further, after switching to a high frequency current of 4 MHz, a high frequency current of a frequency of 8 MHz is applied alternately therewith. Then, when 8MHz current is applied, the impedance displayed by the impedance measuring device 5 does not change during plating.Then, read the saturation value E of the impedance when this change stops, and use the standard value determined in advance using the standard sample. Find the ratio between the saturation value Est and the saturation value E, and multiply this by the film thickness T determined by a high-frequency current with a frequency of 4 MHz.
The film thickness Tc is obtained by correcting errors in film thickness measurement due to variations in measurement conditions.

Note that the time required to complete plating can be calculated by dividing the corrected film thickness by the time required up to that point to determine the film formation rate. When the film formation rate was measured by the method of the present invention and the plating completion time was predicted based on this, the error in the film thickness after the plating was completed was about ±0.1 μm.

In the embodiment described above, two types of alternating current with different frequencies are applied to the coils separately, but the present invention is not limited to such an energization method, and the coils are energized with a mixture of two types of alternating current with different frequencies. It doesn't matter if it's a method.

〔Effect of the invention〕

In the invention according to claim 1, a high frequency current having a different frequency, that is, a high frequency current having a relatively high frequency while the plating thickness is thin, is passed, so that the impedance value changes greatly even with a slight change in the film thickness. Then, after the plating thickness becomes thick, a high-frequency current with a low frequency is applied to avoid the impedance value from reaching a saturated state, so that the plating thickness can be determined with high accuracy. In the invention, since the film thickness is corrected by passing a high frequency current that saturates the impedance value during plating, it is possible to finally obtain a plating film having a desired thickness with high precision.

[Brief explanation of the drawing]

FIG. 1 is a plating apparatus for carrying out the present invention, and FIG. 2 is a graph showing the correlation between film thickness and impedance. 1... Coil, 2... Substrate, 3... Plating film,
4... Plating tank, 5... Impedance measuring device, 6
...Plating bath, 7... Supporter Hsu Applicant: Sumitomo Metal Industries Co., Ltd. Agent Patent Attorney Noboru Kono Large box I Oral membrane A II'"rman 2 Y

Claims (1)

[Claims] 1. In the method of inducing eddy current in the plating film and measuring the film thickness by the impedance value of a coil facing the eddy current, while the plating thickness is thin, a high-frequency current with a relatively high frequency is used. , and a film thickness measuring method characterized in that, after the plating thickness has increased, eddy currents are induced using a high-frequency current with a relatively low frequency. 2. In the method of inducing eddy currents in the plating film and measuring the film thickness using the impedance value of a coil placed in front of it, eddy currents are induced in the plating film with a high-frequency current at a frequency that indicates the impedance value of the coil is saturated during plating. From the saturation value E of the impedance obtained by inducing a current and the saturation value E_s_t of the impedance of the coil previously obtained using a high-frequency current of the same frequency and having the same electromagnetic characteristics as the plating film, the formula: T_c =T(E_s_t/E) (where T_c is a film thickness correction value) A film thickness measuring method characterized in that the film thickness T determined by the impedance value of the coil is corrected based on the following formula.