JPH0333221B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a method for measuring the thermal conductivity of a substance, and in particular to a method for measuring the thermal conductivity of a thin film, which is necessary for optimally designing thin film electronic devices such as integrated circuits. The purpose is to measure the thermal diffusivity or thermal diffusivity with high accuracy, simply, and quickly.

[Background of the invention]

Conventionally, as a method for evaluating the thermal conductivity of a substance, a method has been used in which a sample is irradiated with a continuous wave laser beam as a heat source and the temperature change in that area is measured with a thermocouple. In this method, the change in temperature difference between the laser irradiated area and the non-irradiated area is substituted into a theoretical equation for thermal diffusion, and the thermal conductivity included in the equation as a constant specific to the material is determined. According to this method, sufficiently thick samples can be measured with good accuracy, but for thin film samples formed on a substrate, the measurement results are strongly influenced by the thermal conductivity of the substrate, so it is not accurate. The drawback was that measurements could not be expected.

For example, the substrate thickness is 1mm and the thin film thickness is 1μ.
Since the thin film is in contact with a substrate 1000 times thicker when m temperature difference). That is, the fact that the thin film to be measured is much thinner than the substrate makes it difficult to apply this method. In addition, with conventional methods, it is troublesome to have to install a thermocouple on the sample to measure temperature.Furthermore, the heat capacity of this thermocouple cannot be ignored relative to the heat capacity of the sample, so the accuracy of measurement is reduced. The drawback was that it got even worse.

To deal with this, thermal conductivity measurement of thin films is carried out by attaching a film of a substance with a known melting point to the surface of the thin film to be measured, and then observing the melting and deformation of the material by irradiation with pulsed laser light to determine the temperature. A law was proposed (Japanese Patent Application No. 55-148209). That is, the thermal conductivity of the thin film to be measured is calculated from the energy value of the laser beam required for a substance with a known melting point attached to the surface of the thin film to be measured to reach its melting point.
However, in this proposed method, it is necessary to observe the state of the sample surface with a microscope each time the sample is irradiated with pulsed laser light, which takes time for measurement.
In addition, there is considerable variation in the recognition of whether or not a substance with a known melting point undergoes melting deformation.
There was a problem in that measurement errors were likely to occur because they were directly linked to errors in the energy threshold of the pulsed laser beam for producing melting deformation.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems in the prior art and to provide a thermal conductivity measurement method that is simpler, easier, and has a temperature identification method with better measurement accuracy.

[Summary of the invention]

In the present invention, in order to achieve the above object, firstly, in the first invention, a substance whose temperature at which the optical properties change is known is attached in a thin film form to the surface of the measurement object, and The surface of the object to be measured is heated by absorption of the laser beam, and when it reaches a specific temperature is detected from the change in the optical properties of the thin film material.The energy value of the laser beam at that time is determined and the temperature In the method of calculating conductivity, a pulsed laser beam with an adjustable pulse width is used as a heating laser beam, and a continuous wave laser beam with a weak output is emitted before and after the pulsed laser beam.
Calculate the thermal conductivity by determining the intensity difference between the reflected light or transmitted light from the thin film material of the continuous wave laser beam before and after laser beam irradiation, and by determining the pulse width of the pulse and laser beam when the intensity difference occurs. This is the method to do so. Further, in the second invention, a pulsed laser beam with adjustable pulse width is used as the heating laser beam, and a waveform change of the reflected laser beam or transmitted laser beam from the thin film material is used as the change in optical properties, The method is to calculate the thermal conductivity by determining the laser beam irradiation time up to the point at which this waveform change occurs.

That is, in the thermal conductivity measurement according to the present invention, a substance whose temperature at which the optical properties change is known is adhered to the surface of the object to be measured in the form of a film, and this is used as a measurement sample.
By irradiating this with laser light and detecting changes in optical properties due to laser heating, the temperature of the surface of the object to be measured is determined. Here, the change in optical properties refers to a change in optical reflectance due to melt perforation of the attached film-like material, or a change in optical reflectance due to crystallization or structural change. If the temperature at which such a change in optical properties occurs is known,
The temperature of the surface of the object to be measured is determined from optical measurements such as reflectance. In particular, since the reflected light intensity is usually measured by converting it into an electrical signal, the measurement is simpler, faster, more accurate, and easier to automate than recognizing melt deformation through microscopic observation. .

Once the relationship between the energy value of the pulsed laser beam irradiated to the sample and the temperature of the surface of the object to be measured is obtained, the thermal conductivity K can be calculated using the thermal diffusion equation K∂ ² T/∂x, as in the conventional method. ² −cρ∂T/∂t=F ………(1) Calculated from. Here, T is the temperature, x is the position in the depth direction of the laser beam irradiation part, c is the specific heat, ρ is the density, t is the time, and F is the power density of the absorbed laser beam. Here, the film-like substance attached to the surface of the object to be measured is extremely thin, with a thickness of several hundred angstroms, and plays the role of an absorber for pulsed laser light and a detector for temperature identification. There is. When the pulse width of the pulsed laser beam is several tens to hundreds of nanoseconds, the temperature change due to heat conduction can be limited to the vicinity of the surface, so if the thickness of the object to be measured is several thousand Å or more, the The conductivity K is approximately calculated as K=t ₀ /πcρ(2F/T ₀ ) ² …(2). Here, t ₀ is the pulse width of the pulsed laser beam, and T ₀ is the temperature at which a change in optical properties occurs in the film-like material. When measuring changes in the optical properties of a film-like substance attached to the surface of an object to be measured,
Before and after irradiating the sample with pulsed laser light, the sample may be irradiated with low-output continuous wave laser light and the difference in reflected light intensity may be measured. Observing the light waveform with an oscilloscope is faster and has higher measurement accuracy.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.
First, FIG. 1 corresponds to a method in which a continuous wave laser beam is used to measure the change in the optical properties described above, and a difference in the intensity of the reflected light is detected. Here, a GaAlAs semiconductor laser 1 was used as a laser light source. This semiconductor laser is small and inexpensive, and is capable of continuous oscillation or pulse oscillation using a laser driving power source 7, and its pulse width can also be freely changed by changing the pulse current from the power source 7. The light emitted from this laser passes through the half mirror 4 and is irradiated onto the surface of the sample 5. The sample 5 can be moved with an accuracy of 3 μm by operating the manipulator 6, and the laser irradiation position can be changed. A sample 5 is irradiated with pulsed laser light 2 from a laser 1 several times with different pulse widths. Before and after that, the laser 1 is operated in continuous wave mode, and the optical path of the reflected light of the continuous wave laser beam 3 from the sample 5 is changed by the half mirror 4 and detected by the photodetector 8. An ammeter 9 is connected to the photodetector, and from this current, the sample 5
The intensity of the reflected light can be read. By determining the difference in intensity of reflected light before and after irradiation with pulsed light, it is possible to determine whether or not a change has occurred in the film-like substance on the surface of the sample. In other words, the thermal conductivity of the sample, that is, the substance to be measured, is calculated from the temperature at which the phenomenon that causes a difference in the intensity of reflected light from the film-like substance occurs and the pulse width of the pulsed light when the difference occurs. .

Next, FIG. 2 corresponds to a method of observing the waveform of the reflected light of the pulsed light itself in measuring changes in optical properties. The configuration of the measuring device in this case differs from that in Fig. 1 because the reflected light 10 of the pulsed laser beam 2 from the sample 5 itself is detected by the photodetector 8 after its optical path is changed by the half mirror 4. It is at the point where it is done. A high-speed oscilloscope 11 is connected to the photodetector, and the waveform of the reflected light is drawn with respect to the time axis. By observing this reflected waveform, the time at which an optical change occurs in the film-like substance on the sample surface is determined. The device in Figure 2 only needs to irradiate a single, relatively long pulse, instead of multiple pulses with different pulse widths as in the device in Figure 1, and the pulse oscillation Since there is no need to measure the reflectance of the sample each time using a continuous wave laser beam, the measurement is simple and quick. Furthermore, by observing the reflected waveform in detail, it is possible to determine the laser irradiation time required to cause a change in the optical properties of the film-like substance on the sample surface, making it possible to perform highly accurate measurements.

Specific examples will be described below.

Example 1 The object to be measured was a polymethyl methacrylate film with a thickness of 1 μm coated on a glass substrate using a spinner coating method. A film was formed by high frequency sputtering method and used as a sample. This sample was placed in a thermal conductivity measuring device having the configuration shown in FIG. Then, a GaAlAs semiconductor laser 1 was used to irradiate the sample with pulsed laser light having a diameter of 2 mm and an output of 6 mV with varying pulse widths. Then, before and after each pulsed laser beam, the output of the semiconductor laser was lowered to 0.2 mV, continuous wave laser beam was emitted, and the intensity of reflected light from each sample was measured. The results are shown in FIG.
In Figure 3, the horizontal axis is the pulse width of the 6 mV pulsed laser beam, and the vertical axis is the reflected light intensity of the 0.2 mV laser beam.
This shows the difference before and after irradiation with 6mV pulsed laser light. Here, the pulse width is 50nsec,
The reflected light intensity is starting to decrease. Therefore, it can be seen that the Te film reached its melting point by irradiation with laser light with a pulse width of 50 nsec, and melt perforation was performed. From this result, a value of 7×10 ⁻⁴ cal/sec·cm·deg was obtained as the thermal conductivity of the thin film to be measured using the above-mentioned formula (2). (However, the values below are not the incident energy, but are corrected and converted to the energy actually absorbed by Te.) This is the literature value of the thermal conductivity of polymethacrylate, 5×10 ^-4 cal/sec・cm・The value is close to deg.

Example 2 A sample prepared under the same conditions as in Example 1 was placed in an apparatus having the configuration shown in FIG. Irradiating the sample with a pulsed laser beam with an output of 6 mV while changing the pulse width is the same as in Example 1, but in Example 2 shown in Figure 2, the reflected light of the pulsed laser beam is measured at the same time as the irradiation. Then, the waveform was drawn on the screen of a high-speed oscilloscope. Figure 4 shows the results when the pulse width was 150 nsec. The reflected waveform has an irradiation time of 40nsec.
It has two inflection points of 70nsec. At the first inflection point, the Te film has reached its melting point and started melting, so the reflectance begins to decrease, and at the second inflection point, the Te film has sufficiently melted and the perforation is formed. It has been previously confirmed that the reflectance decreases further due to the onset of Therefore, of the two inflection points of the reflected waveform, the shorter irradiation time, that is, 40 nsec, is the time when the surface reaches the melting point of Te, 450°C. The thermal conductivity of the thin film was determined to be 5.5×10 ^-4 cal/sec・cm・deg. This is even closer to the literature value than the result of Example 1, which means that highly accurate measured values were obtained.

In this way, by measuring the reflected waveform of the pulsed laser beam irradiated onto the sample with an oscilloscope and determining the time at which the reflected waveform changes, it is possible to easily, quickly, and accurately obtain the thermal conductivity of a thin film. becomes.

In Examples 1 and 2 described above, a half-circle was installed between the laser and the sample stage as a thermal conductivity measuring device.
A mirror has been installed. This is provided to guide the light reflected from the sample to the photodetector. However, if the laser light that is incident on the sample is not incident perpendicularly to the sample but is incident at an angle, the reflected light will be Light can be directly received by the photodetector without intervening a half mirror. The apparatus in this case has a configuration as shown in FIG. When we performed measurements similar to those in the example using this device, we were able to obtain thermal conductivity with almost the same accuracy by simply correcting for the change in beam diameter due to the laser beam being incident on the sample at an angle. was completed.

In all of the measurement examples described so far, the intensity of reflected light from the sample is measured. However, if the sample itself is transparent, thermal conductivity can be evaluated by measuring the intensity of transmitted light instead of reflected light. In this case, the apparatus has the configuration shown in FIG. If the optical properties of the film-like substance attached to the sample surface change due to melt perforation or structural changes, not only the reflectance but also the optical transmittance will change, so the sample itself to be measured will transmit light. Measuring the intensity of transmitted light is sufficient to evaluate the thermal conductivity of the material. In Example 2,
Since the object to be measured was a translucent polymethyl methacrylate film coated on a glass substrate, an attempt was made to measure the same sample using an apparatus having the configuration shown in FIG. As a result, the pulse width of the laser light required to change the transmitted light intensity is exactly the same as the pulse width of the laser light when the reflected light intensity changes as shown in Example 2, and the resulting heat conduction The ratio was also the same as in Example 2.

In these examples, the pulse width of the pulsed laser beam until the film-like material Te on the surface reaches its melting point is determined. As a film-like substance, not only Te but also
Any low melting point metal film such as Bi, In, Pb, Sn, etc. can be used. Furthermore, the temperature can be determined using a film-like substance as long as it causes a clear change in optical properties when heated to a certain temperature.
Not limited to melting. For example, if the film of a film-like substance attached to the surface of the measurement object is amorphous, the reflectance increases due to heating to exceed the crystallization temperature, or if the film is a magnetic substance that has a magnetic effect,
In principle, it is also possible to identify the temperature by detecting the change in the polarization plane of the reflected light due to the magnetic phase transition temperature being exceeded by heating.

Furthermore, in the two embodiments described above, a thin film with a thickness of about 1 μm is selected as the object to be measured. As stated in Japanese Patent Application No. 55-148209, if a pulsed laser beam is used as a light source, the thermal conductivity of even a thin film with a thickness of 10 μm or less can be accurately evaluated. However, even if the object to be measured is not a thin film but has a sufficient thickness, it is of course possible to measure thermal conductivity with high precision using the method of the present invention, considering the measurement principle. In this case, there is no need for detailed operations such as the installation of thermocouples, which were required in the conventional method, making the measurement simple and quick. Further, the laser light does not have to be an extremely short pulsed laser beam, but may be a relatively long laser pulsed beam or a continuous wave laser beam.

Furthermore, in FIGS. 1 and 2, if the temperature of the sample is variably adjusted by placing the measurement sample in an electric furnace, attaching a heater, or placing it in a cooling tank,
It is possible to determine the temperature change in thermal conductivity by performing measurements similar to those in the above example at each temperature and processing the data to take the difference.

〔Effect of the invention〕

As explained above, according to the present invention, conventionally,
It is now possible to easily, quickly, and highly accurately evaluate the thermal conductivity of thin films with thicknesses of several thousand Å to several micrometers, which has been considered difficult. The equipment used in this case consists of semiconductor lasers, high-speed oscilloscopes, etc. Something,
It is relatively easily available and has a simple configuration. In particular, in recent years, the measurement of thermal conductivity of thin film materials has become important in the optimal design of thin film electronic devices and the design of heat transfer bodies for heat dissipation in integrated circuit devices. The conductivity measurement method is very effective and its effects are extremely large. Moreover, the present invention,
It is possible to measure the thermal conductivity of not only thin films but also sufficiently thick materials, and in this case, it has the advantage of being able to evaluate thermal conductivity more quickly, simply, and with higher precision than conventional measurement methods.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of an embodiment of the present invention used in a method for determining temperature by detecting changes in the reflectance of a sample before and after irradiation with pulsed laser light, and Figure 2 shows the reflection of pulsed laser light from the sample. FIG. 3 is a diagram showing the configuration of an embodiment of the present invention using a method for determining temperature from a waveform. FIG.
The figure is a schematic diagram of the waveform of the pulsed laser beam reflected from the sample according to the Figure 2 method, Figure 5 is a configuration diagram of another embodiment of the Figure 1 method, and Figure 6 is another example of the Figure 2 method. FIG. Explanation of symbols 1... Semiconductor laser, 2... Pulse/
Laser light, 3...Continuous wave laser light, 4...Half・
Mirror, 5... Sample, 6... Manipulator, 7... Laser drive power supply, 8... Photo detector, 9... Ammeter, 10... Reflected pulse laser beam, 11... High speed oscilloscope.

Claims (1)

[Claims] 1. A thin film of a substance whose temperature at which the optical properties change is known is attached to the surface of the object to be measured, and the object to be measured is irradiated with a laser beam, and the surface of the object to be measured is changed by absorption of the laser beam. detecting from a change in the optical properties of the thin film material that the film has been heated to a specific temperature;
In the method of calculating the thermal conductivity by determining the energy value of the laser beam at that time, a pulsed laser beam with an adjustable pulse width and a weak output continuous wave laser beam emitted before and after the heating laser beam are used. , find the intensity difference between the reflected light or the transmitted light from the thin film material of the continuous wave laser light before and after the pulsed laser light irradiation, and find the pulse width of the pulsed laser light when the intensity difference occurs, and calculate the temperature. A thermal conductivity measuring method characterized by calculating conductivity. 2 A substance whose temperature at which the optical properties change is known is attached in the form of a thin film to the surface of the object to be measured, and the object to be measured is irradiated with laser light, and the surface of the object to be measured is heated by absorption of the laser light until it reaches a specific temperature. Detecting that this has been reached from a change in the optical properties of the thin film material,
In the method of calculating thermal conductivity by determining the energy value of the laser beam at that time, a pulsed laser beam with adjustable pulse width is used as the heating laser beam,
Using a waveform change of reflected or transmitted laser light from the thin film material as a change in optical properties,
A thermal conductivity measuring method characterized by calculating thermal conductivity by determining the laser beam irradiation time up to the point at which this waveform change occurs.