JPS601562B2 - Film thickness measurement method - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a method for measuring the thickness of a film by a non-contact method.
More specifically, in the opto-acoustic effect, the amplitude of the pressure wave in the sealed container induced by the standing temperature wave that occurs in the sample is measured at two different modulation frequencies of the incident electromagnetic wave, and the film thickness is measured from the ratio of the amplitudes. This relates to a non-contact film thickness measurement method.

The opto-acoustic effect is when a substance placed in a sealed container filled with gas is irradiated with modulated electromagnetic waves that the substance absorbs, producing pressure waves with the same frequency as the electromagnetic wave modulation frequency within the sealed container. refers to a phenomenon.

Conventional film thickness measurement methods include a stylus method, a repeated reflection method, an ellipsometry method, an electrical resistance method, a capacitance method, an ultrasonic method, an eddy current method, and the like.

However, in the case of the stylus method, the target material is limited to hard materials;
In the case of the repeated reflection method, it is a destructive test, and the ellipsometric method cannot uniquely determine the film thickness and cannot be applied to opaque materials, and other electromagnetic measurement methods cannot be applied due to the electromagnetic properties of the material. It has drawbacks such as limited range. Furthermore, a non-contact film thickness measurement method using the opto-acoustic effect has been known, but in this method, a double layer consisting of a layer to be measured and a thin absorption layer that efficiently absorbs incident electromagnetic waves is used as a sample. This sample was placed in a sealed container filled with gas (hereinafter referred to as background gas) so that the layer to be measured was in contact with the background gas and formed part of the wall of the sealed container, and the sample was modulated. The ratio of the amplitude of the pressure wave generated at that time by irradiating the electromagnetic wave to the amplitude of the pressure wave similarly measured only in the absorbing layer is determined, and the film thickness is calculated from the slope of the logarithm of this ratio with respect to the modulation frequency of the electromagnetic wave. seek.

According to this method, since the amplitude of the temperature fluctuation on the sample surface is proportional to the amplitude of the pressure wave, there is no need to directly measure the temperature on the sample surface, and therefore the film thickness can be measured without contact. However, it has the following drawbacks. In other words, to determine the film thickness, the frequency dependence of pressure waves must be measured twice: once for the absorbing layer only, and once for the double layer consisting of the absorbing layer and the layer to be measured, which requires time and effort. .
It is an object of the present invention to provide a film thickness measuring method capable of eliminating the above-mentioned drawbacks and accurately and quickly measuring the film thickness of films of a wide range of substances.

In the film thickness measurement method according to the present invention, a double layer consisting of a layer to be measured and a thin absorption layer having an appropriate absorption coefficient and thickness for incident electromagnetic waves is used as a sample, and the sample is placed in a sealed container containing a background gas. The layer to be measured is placed in contact with the background gas and forms part of the wall of the sealed container, and the sample is irradiated with modulated electromagnetic waves.The amplitude of the pressure wave generated at that time is determined by the modulation frequency. It is characterized by measuring at two modulation frequencies by changing the amplitude, and determining the film thickness from the ratio of the two amplitudes.

Therefore, this measurement method is non-contact and does not destroy the sample at all, and does not depend on the transparency or electromagnetic properties of the sample, and has a much wider range of application than conventional film thickness measurement methods. This is a thickness measurement method.

Embodiments of the present invention will be described below based on the drawings.

FIG. 1 shows an embodiment of the present invention, in which 1 is a sample;
Reference numeral 2 denotes a sealed container, in which an electromagnetic wave entrance window 3 is formed on one wall surface, and a microphone 5 as a sound pressure detection element is mounted on the other wall surface so as to maintain airtightness.

6 is a light source (for example, a xenon lamp, a tungsten lamp, or various lasers); 7 is a chopper for modulating electromagnetic waves (the modulation frequency is 10 to 2000 Hz); 8
is an amplifier that amplifies the output of the microphone 5; 9 receives the reference signal 71 from the chopper 7 and the output of the amplifier 8;
This amplifier 9 is a so-called twin-in amplifier that measures the amplitude and phase of a signal, and a recorder (not shown) is connected to this amplifier 9.

Note that 4 is a circle for keeping the 3 parts of the entrance window airtight. Further, the sealed container 2 is filled with a background gas. The sample 1 consists of a layer to be measured la and an absorption layer lb as shown in FIG. It is attached to the entrance window 3 by, for example, pasting with adhesive tape so as to constitute a part of the wall surface, or it is attached to the wall surface facing the entrance window 3 as shown in FIG. The absorption layer lb absorbs the modulated electromagnetic wave beam, generates heat, and has the role of causing periodic temperature changes on the surface of the layer la to be measured that is in contact with the absorption layer lb. This absorbent layer 1b' is formed, for example, by applying a solution of carbon black binder resin. Next, the operation of the above measuring device will be explained. Modulated electromagnetic wave beam 61
When sample 1 is irradiated with , the electromagnetic wave is absorbed by the absorption layer lb,
The energy is converted into thermal energy within the absorbing layer lb, causing periodic temperature changes in the absorbing layer lb. Since the layer to be measured la and the absorption layer lb are in contact, the layer to be measured la
The surface temperature on the absorption layer lb side also changes with the same period and phase. This temperature wave propagates within the layer la to be measured, heats the background gas on the opposite surface, and generates a pressure wave within the sealed container 2. The amplitude of this pressure wave (hereinafter referred to as sound pressure) is measured with a microphone 5. The sound pressure Q measured in this way satisfies the following relationship. Q2 Q〇 1 eXp {- (Kijisa) 2
s is the thermal diffusivity of the measured value la, s is a constant determined by the magnitude of the optical absorption coefficient and thermal diffusivity of the absorption layer lb, and x is the thickness of the measured layer la.

If a constant is entered and the thermal diffusivity Qs of the layer to be measured la is known, the sound pressure Q at two appropriate modulation angular frequencies, , 2,
, Q2, the thickness x of the layer to be measured can be determined from the following relationship. QI=(2÷)input eXp{
-(Makiimo〆x}Misaki(2Q2 We will explain how to find the constant input in the I order.

When the sample 1 consists of only the absorbing layer lb, the sound pressure Q is given by the following equation. Q: One piece of Q〇...
[31] Therefore, it is possible to measure Q as a function of the modulation angular frequency and find a constant value from the relationship between logQ and log.

For the sake of convenience in finding x using the formula '2}, it is effective to obtain an absorbing layer lb in which the input is 1. It will be shown below that an absorbing layer lb having an input of 1 can actually be obtained. That is, 35% carbon black is dissolved in an appropriate amount of ethyl alcohol together with 65% polyvinyl butyral binder resin, and a paint that has been polished in a ball mill for more than 5 hours is applied to the electromagnetic wave entrance window 3, and the film thickness is 5 mounds and 10 mcm, respectively. Absorbent layers of 13 mm, 13 mm and 30 mm were obtained. FIG. 5 shows the results of measuring the dependence of Q on the electromagnetic wave modulation angular frequency for each of these absorption layers. As shown in Figure 5, the relationships between log and logQ are all linear relationships,
The constant value can be calculated from the slope. The value of the constant obtained in this way is 1 when the thickness of the absorption layer is 5 peaks.
, and all values greater than 1 were obtained for absorbent layers of 5 or more. Therefore, in the case of the paint used in this case, it was experimentally confirmed that the constant value is 1 for the absorption layer having a film thickness of 5A. Next, the measurement results of the polymer film according to the present invention will be described.

That is, the above-mentioned coating material consisting of carbon and binder resin polyvinyl butyral is applied to a polyvinyl chloride resin having a thickness of 6 r whose film thickness was previously determined by the eddy current method, and the carbon-butyral resin layer is formed into an absorbing layer lb, A double-layer sample was prepared with a polyvinyl chloride resin layer as the layer to be measured la, and this was set up as shown in Figure 4. White light was incident using a xenon lamp as a light source, and the modulation angular frequency was measured. =1
The sound pressure ratio Q, /Q2 at 0000, ■2 = 1600 was found to be 4.8×10-2. Note that the thickness of the absorbent layer lb was 5 mounds. Using this value, the value of the thermal diffusivity of polyvinyl chloride resin obtained separately, 1.07×10-3・se〆1, and input=1, the film thickness is calculated from the formula '2}: 6.0 Very good agreement was observed with the value obtained by the eddy current method. In this example of measuring the film thickness of a polymer film, a film with carbon dispersed in resin was used as the absorption layer, but when measuring the thickness of a thin film on a substrate such as a metal, etc. , the substrate itself may be used as the absorption layer.

Further, according to the present invention, it is also possible to measure the thickness of a liquid film on a solid, such as the thickness of an oil film, which has been difficult to measure in the past. As described above, the present invention has advantages over various conventional film thickness measurement methods, such as non-contact and non-destructive testing and a significantly wider range of application.

[Brief explanation of the drawing]

Figures 1 to 5 show an embodiment of the present invention, in which Figure 1 is a block diagram of the measurement system, Figure 2 is a side view showing an example of the structure of a sample, and Figures 3 and 4. FIG. 5 is a cross-sectional view for explaining how a sample is attached to a sealed container, and FIG. 5 is a characteristic diagram showing the relationship between sound pressure and modulation angular frequency. 1...sample, la...layer to be measured, lb...
Absorption layer, 2... sealed container, 3... electromagnetic wave incidence window, 4... 0 ring, 5... microphone, 6... light source, 7... Chobba, 8... Amplifier, 9... Lock-in amplifier. Note that the same reference numerals in the figures indicate the same or corresponding parts. 1st
Figure 2 Figure 3 Figure 4 Figure 5

Claims (1)

[Claims] 1. A double layer consisting of a layer to be measured and an absorbing layer that efficiently absorbs incident electromagnetic waves is used as a sample, and the sample is placed in a sealed container containing a required gas, and the layer to be measured is in contact with the gas, and the layer in the sealed container is The sample is installed so as to form part of a wall surface, and the sample is irradiated with electromagnetic waves having different modulation frequencies, and the film thickness is determined from the ratio of the amplitudes of the respective pressure waves generated. Film thickness measurement method.