JPS601561B2 - 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, the film thickness is measured using the fact that the amplitude of the pressure wave inside the sealed container, which is exceeded by the standing wave of temperature that occurs inside the sample in the opto-acoustic effect, depends on the modulation frequency of the incident electromagnetic wave. It concerns the measurement method.

The opto-acoustic effect is a phenomenon in which when a substance placed in a sealed container filled with gas is irradiated with modulated electromagnetic waves that the substance absorbs, pressure waves at the electromagnetic wave modulation frequency are generated inside the sealed container. say.

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, the stylus method is a destructive test, the ellipsometric method cannot unambiguously determine the film thickness and cannot be applied to opaque materials, and other electromagnetic measurement methods measure the electromagnetic properties of the material. However, it has disadvantages such as the limited scope of application. In summary, conventional film thickness measurement methods generally require destructive testing or have difficulty measuring non-electrically conductive thin films on non-electrically conductive substrates, which significantly limits the scope of application. have. It is an object of the present invention to provide a film thickness measuring method capable of eliminating the above drawbacks and non-destructively and accurately measuring the film thickness 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 that efficiently absorbs incident electromagnetic waves is used as a sample, and this sample is placed in a sealed container filled with gas (hereinafter referred to as background gas). The layer to be measured is placed in contact with the background gas and forms part of the panel surface of a sealed container, and the sample is irradiated with modulated electromagnetic waves to measure the amplitude of the pressure wave generated at that time and the absorption layer. The method is characterized in that the ratio of the amplitudes of pressure waves similarly measured using a chisel is taken, and the film thickness is determined from the slope of the logarithm of this value with respect to the square root of the modulation frequency of the electromagnetic wave. Therefore, the Hongo method is a non-contact method, there is no risk of destroying the sample, and it does not depend on the transparency or electromagnetic properties of the sample, so it has a much wider range of application than conventional film thickness measurement methods. This is a film thickness measurement method. Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, in which 1 is a sample;
Reference numeral 2 denotes a sealed container having an electromagnetic wave entrance window 3 formed on its striking surface, and a microphone 5 as a sound pressure detection element 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 200 Hz); 8 is an amplifier for amplifying the output of the microphone 5; is a so-called lock-in amplifier that receives the reference signal from the chopper 7 and the output of the amplifier 8, and measures the amplitude and phase of the signal.
Although not shown, a recorder 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 inner surface of 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 absorption layer lb is formed, for example, by applying a carbon black binder resin solution. Next, the operation of the above measurement system will be explained.

When the sample 1 is irradiated with the modulated electromagnetic wave beam 6a, the electromagnetic wave is absorbed by the absorbing layer lb, and the energy is converted into thermal energy within the absorbing layer lb, increasing the temperature of the absorbing layer lb. Now, if sample 1 is composed of only the absorbing layer lb, the above thermal energy will be absorbed by the absorbing layer lb.
The heat is transferred by conduction to the background gas in contact with the gas, causing a periodic temperature rise in the gas.

In this case, pressure waves are generated due to the constant volume of the background gas. Assuming that the incident electromagnetic wave is modulated by the angular frequency, the temperature fluctuation component 8(t) at the surface of the absorbing layer lb in contact with the background gas is 8(t)d80COS(t-g), . ．． 、． ．． 、． ．． It is represented by m.

However, 8o is a value determined by the thermal conductivity of the absorption layer lb itself and geothermal heat if the absorption coefficient of the absorption layer lb is sufficiently large, and 8o is the initial phase angle. In addition, the amplitude oo of the fluctuation component of the absorption layer surface temperature and the amplitude Q of the pressure fluctuation inside the sealed container 2
Q=rP between.

8.

No2 1gagT. ......{21 There is a relationship. However, Po and To are the pressure and temperature inside the sealed container 2,
r and ag are the ratio of the constant pressure specific heat to the constant volume specific heat of the background gas and the thermal diffusivity, and 1g is the length of the sealed container 2 in the electromagnetic wave incident direction. From this equation '2), it can be seen that when the conditions inside the sealed container 2 are determined, there is a proportional relationship between the sample surface temperature and the amplitude of each pressure wave. Let us consider the case where the sample [1} consists of two layers: a layer to be measured la and an absorbing layer lb.

Since the layer la to be measured and the absorbing layer lb are in contact, the surface temperature of the layer la to be measured on the side of the absorbing layer lb is also expressed by the equation '1}. 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 in the same way as when the sample 1 consists of only the absorbing layer lb. At this time, the temperature 8s (x, t) of the surface of the layer to be measured la in contact with the background gas is the layer to be measured l
If the thickness of a is x, and the thermodynamic diffusion power is as, then 8S(X,
t) Ni808Xp (1aS sentence) COS (t-aSX-
) ……………Represented in the form of {3}.

In addition, as is thermal diffusivity Qs, as=(■/2Qs)
Defined as 1/2. From this, by comparing equations '1) and '3', the logarithm of the ratio of the amplitudes of 8s(x, t) and h(t), R=OS(x, t)/8(t), is hR=A-aSX=A-(Tama)1/2X-. -. -
- Indicated by (4).

Here A=・n{mgoodwill≠云書竺;)△} It is.

Therefore, the 1/2 relationship with lnR is a linear relationship, and the film thickness x can be determined from the slope of the straight line using the known value of the thermal diffusivity Qs.

Next, the measurement results of the polymer film according to the present invention will be described.

That is, 35% carbon black was dissolved in an appropriate amount of ethyl alcohol together with 65% polyvinyl butyral binder resin, and the coating was milled in a ball mill for 5 hours, and the film thickness was determined in advance by the eddy current method (6 Yamakawa) Polyvinyl chloride resin. The carbon-butyral resin layer is applied to the absorbent layer lb.
A double-layer sample was prepared with a polyvinyl chloride resin layer as the layer to be measured la, and this was installed as shown in Figure 4. White light was incident on the light source 6 using a xenon lamp, and pressure waves were detected. The dependence of amplitude on film thickness was measured using frequency as a parameter. Note that the thickness of the absorption layer lb is 5 to loAm. FIG. 5 shows the dependence of the R value on the square root of frequency, which was obtained by dividing the value thus measured by the amplitude previously obtained with the reference sample of only the absorbing layer lb.

From FIG. 5, there is a direct relationship between the R value and the square root of the modulation angular frequency, and the value is 0.013. This value and the separately determined thermal diffusivity value of polyvinyl chloride resin 1.07×10
The film thickness was found to be 6.0 μm using sec-1 of -3, which was in very good agreement with the value determined by the eddy current method. In this example of measuring the film thickness of a polymer film, a film in which carbon was dispersed in a resin was used as the absorption layer. However, when measuring the thickness of a thin film existing on a substrate such as a metal, for example, 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. FIG. 6 shows another embodiment of the present invention, in which the '4' formula is used in principle as in the above embodiment, but a reference sample consisting only of an electromagnetic wave absorption layer and a 2. Samples consisting of an absorbing layer and a layer to be measured are separately attached to the material attachment parts of two sealed containers of the same shape and size equipped with sound pressure detectors of the same performance, and electromagnetic waves are irradiated to them. The logarithm of the ratio of the output signal intensities of the two sound pressure detectors is measured.

In Figure 7, 1,101 is a sample, 2,102 is a sealed container, 3,103 is an electromagnetic wave entrance window, 4,104 is a circle, 5,105 is a microphone, 6 is a light source, 7 is a chopper, 8,108 is an amplifier, 9.109 is a lock-in amplifier, 10 and 15 are arithmetic circuits, 11 is an X-Y recorder, 1
2 is a beam splitter, 13 and 113 are concave mirrors, and 14 is a scanner. The electromagnetic wave beam 6a modulated by the chopper 7 is split by the beam splitter 12 into light beams 6b and 6c, and the concave mirrors 13 and 113 separate the electromagnetic wave beam 6a into a container. The light is focused on the entrance window 3,103 of 2,102.

The sample 1 consists of an absorbing layer lb and a layer to be measured la, and the sample 101 consists only of an absorbing layer lb. The output signal of the microphone 5, 105 as a sound pressure detector is transmitted to the amplifier 8, 1.
After being amplified in step 08, only signals having the same frequency component as the electromagnetic wave modulation frequency are detected and amplified by lock-in amplifiers 9 and 109.

Thereafter, it becomes an input to the arithmetic circuit 10, where the logarithm value of the ratio of the output signal intensities of the two microphones 5 and 105 is obtained, and the arithmetic output 10a becomes the Y-axis input to the x-Y recorder 11. On the other hand, the modulation frequency of the chopper 7 is continuously swept as a function of time by the scanner 14, and the reference signal 7a becomes an input to the lock-in amplifier 9,109.

Further, the voltage output of the scanner 14 is converted into its square root value by the arithmetic circuit 15, and then the output signal 15a is
This becomes the X-axis input of the Y recorder 11. X-Y recorder 1
1 operates according to the input to the x and Y axes, and the relationship of the '41 formula is recorded. By actually measuring the slope of this recorded straight line, the concentration can be immediately determined. Further, the sealed container 2, 102 may be made of a pressure-electricity converting material so that it also serves as a blue pressure detector.

As described above, according to the present invention, an absorption layer is provided on one surface of the layer to be measured, a modulated electromagnetic wave is irradiated to this absorption layer, and as a result, heat is generated, and this temperature wave is propagated to the other surface. Since the magnitude of the amplitude attenuation that occurs when the wave propagates depends on the thermal diffusivity and the distance of wave propagation, that is, the thickness of the layer to be measured and the frequency, the thickness of the layer to be measured can be determined by changing the frequency.

Temperature can also be measured by heating this gas by bringing the surface to be measured into contact with a background gas in a sealed container and measuring the resulting pressure fluctuations, thus allowing non-contact and non-destructive measurement of the membrane. Thickness can be measured. Furthermore, it has the advantage of being able to accurately measure film thicknesses over a wide range of applications, from single films to films on solids and liquid films.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a measurement system showing an embodiment of the thermal diffusivity measuring device according to the present invention, Fig. 2 is a side view showing the structure of the sample, and Figs. 3 and 4 are the samples placed in a sealed container. Figure 5 is a cross-sectional view to explain the installation state, Figure 5 is a characteristic diagram showing the relationship between the ratio of the signal strength from the absorption layer to the layer to be measured and the signal strength of the absorption layer only, and the modulation angular frequency. FIG. 3 is a block diagram of a measurement system showing another embodiment of the invention. 1,101...sample, la...layer to be measured, lb...absorption layer, 2,102...
Sealed container, 3,103... Electromagnetic wave incidence window, 5,
105...Microphone, 6...Light source, 7...Chopper, 8,108...Amplifier, 9,109...Lock-in amplifier , 10
, 15... Arithmetic circuit, 11... X-Y recorder, 12... Beam splitter, 13, 113... Concave mirror. Note that the same reference numerals in the figures indicate the same or corresponding parts. 1st
Figure 2 Figure 3 Figure 4 Figure 5 Figure 6

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 the wall surface, and the sample is irradiated with modulated electromagnetic waves, and the ratio of the amplitude of the generated pressure wave to the amplitude of the pressure wave similarly measured only on the absorption layer is calculated. , a film thickness measurement method characterized by determining the film thickness from the slope of the logarithm value of this value with respect to the square root of the modulation frequency of the electromagnetic wave.