JP4873489B2 - Thin-film thermophysical property measuring device - Google Patents

Description

  The present invention relates to an apparatus for measuring the thermal diffusivity and thermal conductivity of a thin film using a thermoreflectance method, and irradiates a measurement pulse with laser light for heating while irradiating the measurement object with continuous probe light. The present invention also relates to a high-speed pulse high-speed time response measuring apparatus that observes a temperature change by detecting the reflected light.

  The picosecond thermoreflectance method is one of the powerful methods for measuring the thermal diffusivity of thin films. When one side of the thin film is irradiated with ultrashort pulse light as heating pulse light, the temperature of the irradiated surface of the thin film instantaneously rises, and then heat diffuses into the thin film. In order to observe the temperature change of the thin film surface, the surface to be measured is irradiated with the temperature measuring pulse light, and the change in the thin film surface temperature is observed from the change in the reflectance of the probe light depending on the temperature change.

As an example of this measurement method, the picosecond thermoreflectance device described in Non-Patent Document 1 below uses pulse light repeatedly emitted from two lasers as heating pulse light and temperature measurement pulse light, respectively. By controlling the phase in synchronization with each other, the time difference between the arrival of the heating pulse light and the temperature measurement pulse light on the thin film can be changed. However, this measuring apparatus can be used only with a laser that has little jitter between pulses and has a special mechanism for synchronizing the pulse period with an external reference frequency. In addition, the following patent documents 1-4 exist as a technique relevant to the picosecond thermoreflectance method.
JP 2003-322628 A JP 2001-116711 A JP 2001-83113 A JP 2002-122559 A Naoyuki Taketoshi, Tetsuya Baba, and Akira Ono, REVIEW OF SCIENTIFIC INSTRUMENTS 76, (2005) 094903 Naoyuki Taketoshi, Tetsuya Baba and Akira Ono, Meas. Sci. Technol. 12 (2001) 2064-2073

  An object of the present invention is to solve the problems of the background art described above, and to use a pulsed laser that has jitter and does not have a special pulsed light emission time control mechanism as a heating pulsed light. An object of the present invention is to provide a thin film thermophysical property measuring apparatus capable of measuring.

In order to solve the above problem, the invention of claim 1 includes a heating pulse laser that emits a heating pulse light for instantaneously heating a thin film, and a plurality of emission ends for guiding the heating pulse light. An optical fiber having a refractive index n, means for measuring the emission time of the heating pulsed light emitted from the optical fiber, and temperature change of the thin film using the temperature dependence of the reflectance of the thin film surface In order to detect the temperature measurement laser, the temperature measurement laser that emits continuous temperature measurement laser light irradiated to the measurement unit of the thin film, the means for detecting the reflected light of the temperature measurement laser light, and the temperature measurement Means for measuring the intensity of the reflected laser light, means for recording the intensity of the reflected light as a function of the elapsed time from the emission time of the heating pulse light, and the recorded reflected light of the temperature measuring laser. Calculate thermophysical property value based on time change of intensity And a means for recording the intensity of the reflected light after the heating pulsed light has reached the means for measuring the emission time is actually reflecting the temperature measuring laser light. The distance from the incident end of the optical fiber to the means for measuring the emission time of the heating pulse light when the shortest time to start recording the intensity of light is ts and the speed of light in vacuum is c A distance L2 from the incident end to the exit end used for irradiating the thin film with respect to L1 is longer than c × t _s / n, and is configured as a thin film thermophysical property measuring apparatus.

  According to a second aspect of the present invention, in the first aspect of the invention, a heating pulse laser that repeatedly emits light is used.

According to a third aspect of the present invention, in the first or second aspect of the present invention, when the time interval of the pulse light repeated from the heating pulse laser is t _rep and the light velocity in vacuum is c, L2 of the optical fiber Is shorter than c × t _rep / n−L1.

A fourth aspect of the present invention, in the invention of any one of claims 1 to 3, reaches the means for one pulse light with a heating pulse light to emit light repeatedly measures the emission time, time of the t _s After the lapse of time, the reflected light intensity of the temperature measuring laser beam is recorded in a predetermined time range, and thereafter the same recording is repeated a predetermined number of times each time the next heating pulse light arrives, and the recording is averaged. It is in the structure made to do.

  The invention of claim 5 is the invention according to any one of claims 1 to 4, wherein the optical fiber irradiates the surface at the same position as the measurement part of the thin film, and the position opposite to the measurement part across the thin film. And an exit end for irradiating the back surface, and means for arbitrarily selecting either irradiation on the surface of the thin film or irradiation on the back surface of the thin film.

  According to the present invention, even when the heating pulse laser used to heat the thin film does not have a light emission time control mechanism or when the jitter of the pulse light emission period is large, the heating pulse light path has an appropriate length. By using an optical fiber whose output side is branched so as to have a difference in thickness, it is possible to measure the emission of the heating pulse light faster than a single heating pulse light is applied to the thin film. By starting the measurement based on the light emission time, the temperature change of the thin film can be recorded from the time before the thin film is irradiated with the heating pulse light. Further, by using a branched optical fiber, it is possible to simultaneously arrange the heating pulse light at the same position as the measurement unit and at the opposite position across the thin film without having any switching device.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a conceptual diagram of a thin-film thermophysical property measuring apparatus according to an embodiment of the present invention.

  A temperature measuring continuous laser beam 12 emitted from the temperature measuring laser 2 is irradiated to the measuring portion of the sample 1. The temperature measuring laser 2 is composed of, for example, a CW semiconductor laser having a wavelength of 780 nm and an output of 2 mW. The temperature measuring continuous light laser 12 reflected from the measuring section is converted into a voltage signal by the measuring photodetector 3 and input to the A / D converter 18.

  The heating pulsed light 11 is irradiated to the same position as the measurement unit (surface heating surface temperature measurement). On the other hand, the heating pulsed light 10 is irradiated to the opposite position across the measurement part and the thin film 1 (back surface heating surface temperature measurement). One of the heating pulse lights 10 and 11 is selected by the shutters 4 and 5. The heating pulse laser 6 is composed of, for example, an Nd: YAG laser having a wavelength of 1064 nm, a pulse width of 2 ns, a repetition frequency of 20 kHz, and a jitter of 10%.

  The heating pulse light 8 emitted from the heating pulse laser 6 enters from the incident end 14 of the optical fiber 7. In the optical fiber 7, for example, the distance from the entrance end 13 to the exit end 14 is 2 m, and the distance from the entrance end 13 to the exit end 15 and the distance from the entrance end 13 to the exit end 16 are both 31 m. It is composed of a split optical fiber coupler with a ratio of 1.4.

  A part of the heating pulsed light 8 incident from the incident end 13 becomes the heating pulsed light 9 and reaches the triggering photodetector 17 after about 9 ns. On the other hand, the remaining heating pulse lights 10 and 11 reach the thin film after about 145 ns.

  The heating pulse light 9 detected by the trigger detector 17 is converted into an electric pulse and input as a trigger of the A / D converter 18. The A / D converter 18 acquires an electrical signal from the measurement photodetector 3 for a specified number of samplings after the trigger is input, and incorporates a built-in memory with a change in reflection intensity of the measurement continuous laser beam 12 as a voltage change. To accumulate. This is regarded as one measurement, and each time a repeated trigger is input, it is similarly stored in the built-in memory, averaged after the specified number of measurements is completed, and recorded in the control computer 19.

  The averaging process may be performed after the data stored in the memory built in the A / D board is read out to the control computer 19, but when the A / D board 18 includes a circuit such as an FPGA. High-speed processing is possible by performing averaging processing on the A / D board 18.

  Recording of the reflection intensity change of the continuous laser light for measurement 12 by the A / D board 18 is started from the time before the heating pulse light 10 or the heating pulse light 11 reaches the thin film. The time change of the temperature of the measurement part is analyzed with the moment when the sample 1 is irradiated with the heating pulse light 12 as the time origin to obtain the thermophysical value of the thin film.

The relationship representing the temporal change in temperature due to pulse heating is described in, for example, Non-Patent Document 2 described above, and the temperature T _RF of the measurement unit in the configuration of the back surface heating surface temperature measurement is expressed by the following equation (1). .

Here, t is the elapsed time after the heating pulse light reaches the thin film, Q is the energy per unit area absorbed by the thin film for one heating pulse light, ρ is the density of the thin film, and Cp is the specific heat capacity of the thin film. , D is the thickness of the thin film, κ _f is the thermal diffusivity of the thin film, α is the absorption coefficient of the thin film, b _f is the thermal permeability of the thin film, and b _s is the thermal permeability of the substrate on which the thin film is formed. . Further, the temperature T _FF measurement part in the configuration of the measuring surface heating surface temperature is represented by the following formula (6).

FIG. 2 shows a temperature change of a titanium nitride thin film having a thickness of 600 nm formed on a glass substrate. In order to heat from the opposite position across the thin film of the measurement unit, the heating pulse light 10 is irradiated to the sample 1, and at this time, the heating pulse light 11 is shielded by the shutter 4. At time 0, the heating pulse light having a pulse width of 2 ns reaches the opposite side of the thin film measurement portion, and the temperature of the irradiated thin film surface is instantaneously heated. Thereafter, heat diffuses in the thin film toward the measurement unit, and the temperature of the measurement unit rises. The result of fitting by equation (1) is the analytical curve of FIG. 1, and the thermal diffusivity of this titanium nitride thin film is calculated to be 2.2 × 10 ⁻⁶ m ² / s.

  FIG. 3 shows temperature changes of a 400 nm thick titanium nitride thin film formed on a glass substrate. Both the case where the heating pulse light 10 was used (back surface heating surface temperature measurement) and the case where the heating pulse light 11 was used (surface heating surface temperature measurement) were shown. When the thin film is formed on an opaque substrate, the measurement can be performed with the configuration of surface heating surface temperature measurement.

  In FIG. 1, a continuous-temperature laser for temperature measurement is used to measure the temperature change of the measurement unit, but a radiation thermometer, a thermocouple, or the like may be used instead.

  As described above, according to the present invention, even when the heating pulse laser used for heating the thin film has no emission time control mechanism or when the jitter of the pulse emission period is large, the path of the heating pulse light By using an optical fiber whose output side is branched so as to have an appropriate difference in length, it is possible to measure the emission of the heating pulse light faster than a single heating pulse light is applied to the thin film, By starting the measurement based on the emission time of the heating pulse light, the temperature change of the thin film can be recorded from a time before the thin film is irradiated with the heating pulse light. Further, by using a branched optical fiber, it is possible to simultaneously arrange the heating pulse light at the same position as the measurement unit and at the opposite position across the thin film without having any switching device.

  The present invention can be directly used to measure thermophysical values of thin film materials widely used in advanced industries. The thermophysical property value of the thin film is used for the thermal design of products made up of thin films such as CPU, optical disk and memory.

The conceptual diagram which shows the structure of the thin film thermophysical property measuring apparatus of this invention It is a figure which shows the elapsed time and the temperature change of a thin film with the time when the heating pulse light was irradiated to the opposite surface of the measurement site | part of the thin film being set to 0. It is a figure which shows the elapsed time and the temperature change of a thin film when a heating pulse light is irradiated to the measurement site | part of a thin film (surface heating surface temperature measurement), and a case where it is irradiated to the opposite surface (back surface heating surface temperature measurement). .

Explanation of symbols

1: Thin film sample 2: Continuous laser for temperature measurement 3: Photodetector for measurement 4, 5: Shutter 6: Pulse laser for heating 7: Optical fiber 8, 9, 10, 11: Pulse light for heating 12: Temperature measurement Laser beam 13: incident end 14: exit end 1
15: Output end 2
16: Output end 3
17: Photodetector for trigger 18: A / D converter 19: Computer for control

Claims (5)

A heating pulse laser that emits heating pulse light for instantaneously heating the thin film; and
An incident end for entering the heating pulse light, an emission end used for measuring the emission time of the heating pulse light, and an emission end used for irradiating the thin film with the heating pulse light. An optical fiber having a refractive index n,
Means for measuring the emission time of the heating pulsed light emitted from the optical fiber;
In order to detect the temperature change of the thin film using the temperature dependence of the reflectance of the thin film surface, a temperature measuring laser that emits a continuous temperature temperature measuring laser beam irradiated to the measurement unit of the thin film;
Means for detecting reflected light of the temperature measuring laser beam;
Means for recording the intensity of the reflected light as a function of the elapsed time from the emission time of the heating pulse light;
Means for calculating a thermophysical value based on a temporal change in intensity of the recorded reflected light, and a thin-film thermophysical property measuring apparatus comprising:
After the heating pulse light reaches the means for measuring the emission time, until the means for recording the intensity of the reflected light actually starts recording the intensity of the reflected light of the temperature measuring laser light When the shortest time of ts is ts and the speed of light in vacuum is c,
Used to irradiate the thin film with the heating pulse light from the incident end with respect to the length L1 from the incident end of the optical fiber to the emission end used for measuring the emission time of the heating pulse light. exit end to a length that is L2, the thin film thermal physical property measurement apparatus, characterized in that c × t _s / n over long.   The thin film thermophysical property measuring apparatus according to claim 1, wherein the heating pulse laser uses repetitively pulsed light. 3. The length L <b> 2 of the optical fiber is shorter than c × t _rep / n−L <b> 1 when the time interval of pulse light repeated from the heating pulse laser is t _rep. The thin film thermophysical property measuring apparatus described. Repeat one pulse light with the heating pulse light emitted reaches to the means for measuring the light emission time, the t the measuring of temperature laser light reflected in a predetermined time range after lapse of time _s 4. The recording is then averaged, and the same recording is repeated a predetermined number of times each time the next heating pulse light arrives, and the recording is averaged. 5. The thin film thermophysical property measuring apparatus described.   The optical fiber has an emission end for irradiating the surface of the thin film with the heating pulse light at the same position as the measurement portion, and an emission end for irradiating the back surface opposite to the measurement portion with the thin film interposed therebetween. And an emission end used for measuring the emission time of the heating pulse light, and means for arbitrarily selecting either irradiation on the surface of the thin film or irradiation on the back surface of the thin film. The thin-film thermophysical property measuring apparatus according to any one of claims 1 to 4.

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