A method for measuring thermal conductivity of thin films based on frequency domain photothermal radiation

Abstract

The present invention discloses a method and device for measuring the thermal conductivity of a thin film based on frequency domain photothermal radiation. The measuring method comprises the following steps: firstly, a thin film to be measured is prepared on a substrate, and a heating layer is prepared thereon to obtain a sample to be measured, and a heating layer is prepared on the substrate to obtain a reference sample; a measuring device is used to measure the sample to be measured and the reference sample; the collected signal is Fourier transformed and the spectrum amplitude with a frequency greater than 0 is taken, and the spectrum amplitude of the sample to be measured is divided by the spectrum amplitude of the reference sample to obtain dimensionless PTR data; the PTR data is fitted to obtain coefficients a and b; the thickness of each film layer and the substrate is measured, and the thermal conductivity of the film to be measured is calculated. The present invention uses a non-contact measurement method to avoid damage to the surface of the film to be measured, and the thermal physical parameters of the film to be measured can be calculated with only one measurement. By adjusting the duty cycle and period of the rectangular pulse, a measurement signal with a better signal-to-noise ratio is obtained, which reduces the difficulty of infrared signal detection.

Description

Technical Field

[0001] The invention relates to the technical field of contactless optical measurement, and in particular to a method for measuring thin film thermal conductivity based on frequency domain photothermal radiation. Background Art

[0002] With the continuous improvement of the integration of optoelectronic devices, the application of semiconductors, metals, organics and nano-doped films is becoming more and more extensive. The thermal properties of various films with nanometer thickness determine the thermal conduction and heat dissipation of the device. The technology for accurately measuring the thermal properties of thin films has attracted much attention in recent years. However, the commonly used nano-film thermal property measurement technology still has some shortcomings. For example, when using the 3ω method to measure thermal conductivity, it is necessary to make a patterned metal electrode on the surface of the sample to be measured. At this time, the photolithography process often damages the organic film; in addition, the preparation process of the metal electrode is relatively complicated and the cost is high.

[0003] Photothermal radiometry (PTR) is a non-contact, non-destructive measurement technique first proposed by Nordal and Kanstada in 1979. This technique is based on the photothermal effect, that is, the surface infrared radiation intensity increases when the sample absorbs the incident light and the temperature rises. Therefore, when the sample is heated by light with periodic intensity modulation, a periodic change in the surface temperature of the sample will be formed, thereby radiating a periodic infrared signal. The infrared radiation signal contains the thermal physical property information of the sample. By comparing the radiation signal collected by the infrared detector with the amplitude or phase of the original modulation signal, the thermal characteristic parameters of the sample can be obtained by inversion calculation. The PTR method has been proven to be an effective thermal property measurement technology.

[0004] However, existing PTR measurement devices all use a sinusoidally modulated light source in the time domain to heat the surface of the sample under test, and the modulation signal is a single frequency signal. In order to obtain infrared radiation data under heating by light sources with different modulation frequencies, it is necessary to change the frequency of the sinusoidal modulation signal within a certain frequency range and at a certain step size, and repeatedly measure it. Through data processing, a curve of the relationship between the amplitude / phase difference and the modulation frequency is drawn, and then the thermal physical parameters of the sample under test are obtained by curve fitting and inversion. In order to ensure the accuracy of the measurement of thermal physical parameters, the step size of the frequency modulation frequency cannot be too large; this results in a large amount of data required for measurement, and the experimental process is relatively cumbersome. In addition, when the thermal conductivity of the sample under test is high, the light source needs to be modulated at high frequency. When using external modulation technology, high requirements are placed on the bandwidth of the modulator. At the same time, under the irradiation of a high-frequency modulated light source, the temperature rise on the surface of the sample under test is often low, resulting in a weak infrared radiation signal, and a phase-locked amplification technology is required to detect the weak signal. The above problems have greatly limited the application of the PTR measurement method. Summary of the invention

[0005] The purpose of the present invention is to provide a method for measuring the thermal conductivity of a thin film based on frequency domain photothermal radiation. A non-contact measurement method is used to avoid damage to the surface of the thin film to be measured. A rectangular pulse heating light is used in combination with a frequency domain signal processing technology. The thermal physical properties of the thin film to be measured can be calculated with only one measurement. By adjusting the duty cycle and period of the rectangular pulse, a measurement signal with a better signal-to-noise ratio is obtained, thereby reducing the difficulty of infrared signal detection.

[0006] To achieve the above object, the present invention provides a method for measuring thin film thermal conductivity based on frequency domain photothermal radiation, comprising the following steps:

[0007] S1. First, prepare a sample to be tested and a reference sample: prepare a thin film to be tested on a substrate, and prepare a heating layer thereon to obtain a sample to be tested, and then prepare a heating layer on the substrate to obtain a reference sample;

[0008] S2. Using a measuring device to measure the sample to be tested and the reference sample;

[0009] S3, performing Fourier transform on the collected signal and taking the spectrum amplitude with frequency greater than 0, dividing the spectrum amplitude of the tested sample by the spectrum amplitude of the reference sample to obtain dimensionless PTR data;

[0010] S4, fitting the PTR data to obtain coefficients a and b;

[0011] S5. Measure the thickness of each film layer and the substrate, and calculate the thermal conductivity of the film being measured.

[0012] Preferably, the dimensionless PTR data obtained in step S3 is specifically obtained according to the measurement principle of PTR, when a rectangular pulse light source is used to irradiate the surface of the sample to be measured, after the sample to be measured absorbs photon energy, the surface temperature of the sample changes through the photothermal effect, and radiates energy outward, at this time, a thermal radiation signal is detected by an infrared detector to obtain the temperature change information of the surface of the sample to be measured;

[0013] According to the characteristics of thermal conduction, if the spot of the heating light is uniform and smaller than the thermal diffusion length, and the radiation signal collected by the detector comes from the central area of ​​the heating light, the thermal conduction of the sample is described by a one-dimensional model:

[0014]

[0015] When the film to be tested is prepared on a substrate and a heating layer is prepared on the surface, in the above formula, k eq is the equivalent thermal conductivity of the sample being tested, (ρCp) eq is the product of the equivalent heat capacity density of the sample being tested, d is the total thickness of the sample being tested, T is the temperature rise of the sample being tested, when the sample is composed of three materials with thicknesses of d1, d2, and d3 respectively, the equivalent thermal conductivity of the sample being tested is:

[0016]

[0017] Among them, R 12 , R 23 is the interface thermal resistance between different materials, and the total thickness of the tested sample is:

[0018] d=d1+d2+d3 (3);

[0019] When the heating light is a rectangular pulse, in a heating cycle, the heating light is regarded as an external heat source described by a piecewise function, thereby introducing a piecewise boundary condition, which is defined as a heating section and a heat dissipation section. The initial condition and boundary condition of formula (1) in these two time periods can be expressed as:

[0020] The heating section is

[0021] The heat dissipation section is

[0022] q is the heating power density, which is related to the heating light power density, the light absorption coefficient of the heating layer, the heating spot area and the thickness of the heating layer; τ is the duration of the rectangular pulse; τ0 is the rectangular pulse period. The temperature rise of the sample under test can be obtained by formula (1) and the above boundary conditions. The equivalent parameter k in the formula below is eq and (ρCp) eq Substituting k and ρCp:

[0023] Heating section 0≤t≤τ;

[0024]

[0025] Heat dissipation section τ≤t≤τ0;

[0026]

[0027] For the heating section, the relationship between the temperature rise and time at the sample surface, i.e., at z = d, is obtained by formula (4):

[0028]

[0029] Get the radiation intensity on the surface of the sample being tested:

[0030]

[0031] Where T0 is the initial temperature of the sample, ε is the surface emissivity of the sample under test, and σ is the Stefan-Boltzmann constant. When the surface temperature rise T(t) is T(t)+T0≈T0, the above formula is simplified to:

[0032]

[0033] Let η0 be the detection coefficient of the infrared optical system:

[0034]

[0035] Where D is the aperture of the optical system, f is the focal length of the optical system, η is the transmittance of the optical system, S is the photosensitive surface area of ​​the detector, and β is the vertical axis magnification of the optical system. The output voltage of the infrared detector is obtained as:

[0036]

[0037] Among them, R is the responsivity of the detector, and the Fourier transform is:

[0038]

[0039] The output electrical signal is related to multiple parameters. In order to eliminate the influence of the errors of these parameters on the measurement results, a reference sample consisting of a substrate and a heating layer is prepared in the actual measurement. The spectrum amplitude with circular frequency ω>0 is taken, and the spectrum amplitude of the measured sample is divided by the spectrum amplitude of the reference sample to obtain a dimensionless quantity PTR:

[0040]

[0041] Where A is a constant coefficient, which is the ratio of the thickness of the sample being tested to that of the reference sample.

[0042] Preferably, the fitting of the PTR data to obtain the coefficients a and b in step S4 is specifically to use formula (12) to perform curve fitting on the measurement results that have been processed in the same way, that is, to obtain the coefficients a and b, and a and b are respectively:

[0043]

[0044] The subscripts s and r represent the tested sample and the reference sample, respectively.

[0045] Preferably, in step S5, the thickness of each film layer and the substrate is measured, and the thermal conductivity of the film to be measured is calculated as the equivalent thermal diffusion efficiency and coefficient of the sample to be measured and the reference sample:

[0046]

[0047] When the thickness of the substrate, the film to be measured and the heating layer are known, the equivalent thermal diffusion coefficient α can be calculated based on the fitted a and b. s and α r ;

[0048] In the one-dimensional model, the equivalent heat capacity density product (ρCp) eq It is expressed as:

[0049]

[0050] When the interfacial thermal resistance between the layers of the sample being tested is ignored, the thermal conductivity k2 of the film being tested can be calculated according to formula (2) and the known thickness of each layer.

[0051] A device for measuring thermal conductivity of a thin film based on frequency domain photothermal radiation, the device comprising a heating light source, an acousto-optic modulator arranged at the rear end of the heating light source, a heating light converging optical system arranged at the rear end of the acousto-optic modulator, a sample stage arranged at the rear end of the heating light converging optical system, an infrared optical system arranged at the rear end of the sample stage, an infrared detector arranged at the rear end of the infrared optical system, and a signal acquisition and processing module arranged at the rear end of the infrared detector, wherein the acousto-optic modulator is connected to a signal generator.

[0052] Therefore, the present invention adopts the above-mentioned thin film thermal conductivity measurement method based on frequency domain photothermal radiation, which has the following beneficial effects:

[0053] (1) A light source with a rectangular pulse waveform in the time domain is used to illuminate the surface of the film to be tested. After detecting the infrared radiation signal on the sample surface under the excitation of the rectangular light pulse, the data is Fourier transformed. Since the rectangular pulse signal can be regarded as a linear combination of sinusoidal signals of different frequencies, and after Fourier transformation, the amplitude / phase value of the radiation signal in the frequency domain is the amplitude / phase of the output signal after the different frequency components pass through the sample (which can be regarded as a system), only one measurement is needed to obtain the frequency domain information of the response of the film to be tested under heating by light sources of different frequencies, thereby directly obtaining the curve of the change of amplitude / phase with frequency.

[0054] (2) The measurement results are fitted with the theoretical expression of the system response in the frequency domain to obtain the thermal physical parameters of the sample under test. In addition, since the duty cycle and period of the rectangular pulse are controllable, a more appropriate surface temperature rise can be obtained by adjusting these two parameters, thereby avoiding detection difficulties caused by a small signal-to-noise ratio of the radiation signal.

[0055] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 A schematic diagram of a test sample used in the invention;

[0057] Figure 2 It is a schematic diagram of the measuring device in the invention;

[0058] Figure 3 Schematic diagram of the measuring device in step S2 of the embodiment;

[0059] Figure 4: is a graph showing the change in sample surface temperature in operation step S3 in the embodiment, wherein (a) is the sample to be tested and (b) is the reference sample;

[0060] Figure 5 Dimensionless PTR data and fitting results in operation step S3 in the embodiment. DETAILED DESCRIPTION

[0061] The present invention provides a method for measuring thermal conductivity of a thin film based on frequency domain photothermal radiation, comprising the following steps:

[0062] S1. First, prepare a sample to be tested and a reference sample: prepare a thin film to be tested on a substrate, and prepare a heating layer thereon to obtain a sample to be tested, and then prepare a heating layer on the substrate to obtain a reference sample.

[0063] S2. Use a measuring device to measure the sample to be tested and the reference sample.

[0064] S3, perform Fourier transform on the collected signal and take the spectrum amplitude with frequency greater than 0, divide the spectrum amplitude of the sample under test by the spectrum amplitude of the reference sample to obtain dimensionless PTR data, according to the measurement principle of PTR, when a rectangular pulse light source is used to irradiate the surface of the sample under test, after the sample under test absorbs photon energy, the surface temperature of the sample changes through the photothermal effect, and radiates energy outward, at this time, a thermal radiation signal is detected by an infrared detector, and the temperature change information of the surface of the sample under test is obtained;

[0065] According to the characteristics of thermal conduction, if the spot of the heating light is uniform and smaller than the thermal diffusion length, and the radiation signal collected by the detector comes from the central area of ​​the heating light, the thermal conduction of the sample is described by a one-dimensional model:

[0066]

[0067] When the film to be tested is prepared on a substrate and a heating layer is prepared on the surface, in the above formula, k eq is the equivalent thermal conductivity of the sample being tested, (ρCp) eq is the product of the equivalent heat capacity density of the sample being tested, d is the total thickness of the sample being tested, T is the temperature rise of the sample being tested, when the sample is composed of three materials with thicknesses of d1, d2, and d3 respectively, the equivalent thermal conductivity of the sample being tested is:

[0068]

[0069] Among them, R 12 , R 23 is the interface thermal resistance between different materials, and the total thickness of the tested sample is:

[0070] d=d1+d2+d3 (3);

[0071] When the heating light is a rectangular pulse, in a heating cycle, the heating light is regarded as an external heat source described by a piecewise function, thereby introducing a piecewise boundary condition, which is defined as a heating section and a heat dissipation section. The initial condition and boundary condition of formula (1) in these two time periods can be expressed as:

[0072] The heating section is

[0073] The heat dissipation section is

[0074] q is the heating power density, which is related to the heating light power density, the light absorption coefficient of the heating layer, the heating spot area and the thickness of the heating layer; τ is the duration of the rectangular pulse; τ0 is the rectangular pulse period. The temperature rise of the sample under test can be obtained by formula (1) and the above boundary conditions. The equivalent parameter k in the formula below is eq and (ρCp) eq Substituting k and ρCp:

[0075] Heating section 0≤t≤τ;

[0076]

[0077] Heat dissipation section τ≤t≤τ0;

[0078]

[0079] For the heating section, the relationship between the temperature rise and time at the sample surface, i.e., at z = d, is obtained by formula (4):

[0080]

[0081] Get the radiation intensity on the surface of the sample being tested:

[0082]

[0083] Where T0 is the initial temperature of the sample, ε is the surface emissivity of the sample under test, and σ is the Stefan-Boltzmann constant. When the surface temperature rise T(t) is T(t) = T0 ≈ T0, the above formula is simplified to:

[0084]

[0085] Let η0 be the detection coefficient of the infrared optical system:

[0086]

[0087] Where D is the aperture of the optical system, f is the focal length of the optical system, η is the transmittance of the optical system, S is the photosensitive surface area of ​​the detector, and β is the vertical axis magnification of the optical system. The output voltage of the infrared detector is obtained as:

[0088]

[0089] Among them, R is the responsivity of the detector, and the Fourier transform is:

[0090]

[0091] The output electrical signal is related to multiple parameters. In order to eliminate the influence of the errors of these parameters on the measurement results, a reference sample consisting of a substrate and a heating layer is prepared in the actual measurement. The spectrum amplitude with circular frequency ω>0 is taken, and the spectrum amplitude of the measured sample is divided by the spectrum amplitude of the reference sample to obtain a dimensionless quantity PTR:

[0092]

[0093] Where A is a constant coefficient, which is the ratio of the thickness of the sample being tested to that of the reference sample.

[0094] S4. Fit the PTR data to obtain coefficients a and b. Use equation (12) to fit the measurement results that have been processed in the same way to obtain coefficients a and b, where a and b are:

[0095]

[0096] The subscripts s and r represent the tested sample and the reference sample, respectively.

[0097] S5. Measure the thickness of each film layer and substrate, calculate the thermal conductivity of the film being tested, and the equivalent thermal diffusion efficiency and coefficient of the tested sample and the reference sample:

[0098]

[0099] When the thickness of the substrate, the film to be measured and the heating layer are known, the equivalent thermal diffusion coefficient α can be calculated based on the fitted a and b. s and α r ;

[0100] In the one-dimensional model, the equivalent heat capacity density product (ρCp) eq It is expressed as:

[0101]

[0102] When the interfacial thermal resistance between the layers of the sample being tested is ignored, the thermal conductivity k2 of the film being tested can be calculated according to formula (2) and the known thickness of each layer.

[0103] Figure 1The schematic diagram of the structure of the sample to be tested, in (a), "1" represents the heating layer, "2" represents the film to be tested, and "3" represents the substrate; in (b), "1" represents the heating layer, and "3" represents the substrate. The function of the heating layer "1" is to absorb the incident heating light as a heating body and radiate infrared signals with a certain infrared emissivity. In order to reduce the impact on the thermal conductivity measurement, the heating layer is required to have a thermal conductivity much greater than that of the film to be tested.

[0104] like Figure 2 As shown, a device for measuring thermal conductivity of a thin film based on frequency domain photothermal radiation comprises a heating light source 1, an acousto-optic modulator 2 arranged at the rear end of the heating light source 1, a heating light converging optical system 3 arranged at the rear end of the acousto-optic modulator 2, a sample stage 4 arranged at the rear end of the heating light converging optical system 3, an infrared optical system 5 arranged at the rear end of the sample stage 4, an infrared detector 6 arranged at the rear end of the infrared optical system 5, and a signal acquisition and processing module 7 arranged at the rear end of the infrared detector 6. The acousto-optic modulator 2 is connected to a signal generator 8. First, a signal generator is used to generate a rectangular pulse signal. The generated rectangular pulse signal is used by a modulator to modulate the output light of the heating light source to obtain a rectangular pulse heating light. The rectangular pulse heating light reaches the sample stage through the heating light converging optical system (the sample is placed in the sample stage, and the sample stage is evacuated to reduce the influence of convective heat dissipation on the measurement results). The heating layer on the surface of the sample absorbs the incident light and generates a temperature rise, thereby heating the lower layer of the film to be measured and radiating an infrared signal outward. The radiated infrared signal is received by the infrared optical system and converged to the photosensitive surface of the infrared detector. The infrared detector converts the received infrared radiation signal into an electrical signal, which is finally collected by the signal acquisition module and transmitted to the signal processing module to process and calculate the measurement results, thereby obtaining the thermal conductivity of the film to be measured.

[0105] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0106] Example

[0107] The specific steps for measuring the thermal conductivity of polyimide (PI) film are as follows:

[0108] Step 1: Sample diagram as shown Figure 1 As shown, a PI film "2" whose thermal conductivity is to be measured is spin-coated on a quartz substrate "3", and a Cr metal layer "1" is prepared on the PI film layer "2" by magnetron sputtering technology to obtain a measured sample; a Cr metal layer "1" is sputtered on a quartz substrate "3" to obtain a reference sample; wherein, the layer thickness of the quartz substrate "3" is 400μm, and the thermal density product is 730*2200J / (kg*m 3 ), the thickness of PI film "2" is 200nm, and the heat capacity density product is 1100*1300J / (kg*m 3), the thickness of Cr metal layer "1" is 100nm, and the heat capacity density product is 448*7150J / (kg*m 3 ), with an emissivity of 0.31.

[0109] Step 2: Thermal conductivity measurement device such as Figure 3 As shown, the heating light source is a 532nm continuous output laser 11, the laser output power is 2W, and the output spot radius is 1mm. The heating light convergence optical system consists of lenses 12 and 14 with a focal length of 100mm. Lens 12 is placed 50mm behind the laser 11 to converge the laser into the acousto-optic modulator 13. The signal generator 16 generates a rectangular pulse signal with a period of 100ms and a duty cycle of 0.1 to control the acousto-optic modulator 13 to modulate the 532nm laser and obtain a rectangular pulse heating light (heating time is 10ms). Lens 14 is placed after the acousto-optic modulator 13, and the distance between lens 14 and lens 12 is 200mm. The light beam is collimated to make the spot radius 1mm. The collimated heating light is incident on the sample stage 18 through the reflector 15 to heat the surface of the sample 17. Afterwards, an infrared optical system consisting of two parabolic metal reflectors 19 and 20 with a focal length of 50.8 mm, a reflectivity of 96% and an aperture of 50.8 mm (the center distance between the parabolic metal reflector 19 and the surface of the sample 17 is 50 mm, and the distance between the parabolic metal reflector 20 and the infrared detector 21 is 50 mm) is used to collect the infrared radiation from the heating area of ​​the sample 17 and reach the photosensitive surface of the infrared detector 21 (the radius of the photosensitive surface is 0.5 mm). The infrared detector 21 (the detector response is 40000 V / W) converts the received infrared radiation signal into an electrical signal, which is collected by the signal acquisition card 22 and transmitted to the computer 23 for subsequent processing.

[0110] Step 3: The collected data is as follows: Figure 4 As shown in the figure (where the horizontal axis represents time in ms, the vertical axis represents voltage in V, where (a) is the sample under test and (b) is the reference sample), data is collected multiple times and the average value is taken to reduce the influence of noise; the average value data is Fourier transformed and the spectrum amplitude with a frequency greater than 0 is taken, and the dimensionless PTR data is obtained by dividing the spectrum amplitude of the sample under test by the spectrum amplitude of the reference sample, as shown in the figure. Figure 5 As shown in (a) (where the horizontal axis is frequency in Hz and the vertical axis is signal amplitude), it is observed that the curve has a peak value, and the corresponding frequency is 29270 Hz.

[0111] Step 4: Use formula (12) to fit the PTR data, as follows: Figure 5 As shown in (b), the parameters A = 1.001, a = 13.3461, b = 13.4388 are obtained;

[0112] Step 5: Substitute the calculated a and b into formulas (13)-(16), combine the thickness of the substrate, PI and metal Cr layer and the product of the heat capacity density, and when the interface thermal resistance is not considered, the thermal conductivity k of the PI film to be tested can be calculated to be 0.1086.

[0113] Therefore, the present invention adopts the above-mentioned method and device for measuring thermal conductivity of thin films based on frequency domain photothermal radiation, uses a non-contact measurement method to avoid damage to the surface of the film to be measured, and uses rectangular pulse heating light combined with frequency domain signal processing technology. The thermal physical properties of the film to be measured can be calculated with only one measurement. By adjusting the duty cycle and period of the rectangular pulse, a measurement signal with a better signal-to-noise ratio is obtained, reducing the difficulty of infrared signal detection.

[0114] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for measuring thin film thermal conductivity based on frequency domain photothermal radiation, characterized in that: The following steps are involved: S1. First, prepare a sample to be tested and a reference sample: prepare a thin film to be tested on a substrate, and prepare a heating layer thereon to obtain a sample to be tested, and then prepare a heating layer on the substrate to obtain a reference sample; S2. Using a measuring device to measure the sample to be tested and the reference sample; S3, performing Fourier transform on the collected signal and taking the spectrum amplitude with frequency greater than 0, dividing the spectrum amplitude of the tested sample by the spectrum amplitude of the reference sample to obtain dimensionless PTR data; The dimensionless PTR data obtained in step S3 is specifically obtained according to the measurement principle of PTR. When a rectangular pulse light source is used to irradiate the surface of the sample under test, the sample under test absorbs photon energy, and the surface temperature of the sample changes through the photothermal effect, and radiates energy outward. At this time, a thermal radiation signal is detected by an infrared detector to obtain temperature change information of the surface of the sample under test; According to the characteristics of thermal conduction, if the spot of the heating light is uniform and smaller than the thermal diffusion length, and the radiation signal collected by the detector comes from the central area of ​​the heating light, the thermal conduction of the sample is described by a one-dimensional model: When the film to be tested is prepared on a substrate and a heating layer is prepared on the surface, in the above formula, k eq is the equivalent thermal conductivity of the sample being tested, (ρCp) eq is the product of the equivalent heat capacity density of the sample being tested, d is the total thickness of the sample being tested, T is the temperature rise of the sample being tested, when the sample is composed of three materials with thicknesses of d1, d2, and d3 respectively, the equivalent thermal conductivity of the sample being tested is: Among them, R 12 , R 23 is the interface thermal resistance between different materials, and the total thickness of the tested sample is: d=d1+d2+d3 (3); When the heating light is a rectangular pulse, in a heating cycle, the heating light is regarded as an external heat source described by a piecewise function, thereby introducing a piecewise boundary condition, which is defined as a heating section and a heat dissipation section. The initial condition and boundary condition of formula (1) in these two time periods can be expressed as: The heating section is The heat dissipation section is q is the heating power density, which is related to the heating light power density, the light absorption coefficient of the heating layer, the heating spot area and the thickness of the heating layer; τ is the duration of the rectangular pulse; τ0 is the rectangular pulse period. The temperature rise of the sample under test can be obtained by formula (1) and the above boundary conditions. The equivalent parameter k in the formula below is eq and (ρCp) eq Substituting k and ρCp: Heating section 0≤t≤τ; Heat dissipation section τ≤t≤τ0; For the heating section, the relationship between the temperature rise and time at the sample surface, i.e., at z = d, is obtained by formula (4): Get the radiation intensity on the surface of the sample being tested: Where T0 is the initial temperature of the sample, ε is the surface emissivity of the sample under test, and σ is the Stefan-Boltzmann constant. When the surface temperature rise T(t) is T(t)+T0≈T0, the above formula is simplified to: Let η0 be the detection coefficient of the infrared optical system: Where D is the aperture of the optical system, f is the focal length of the optical system, η is the transmittance of the optical system, S is the photosensitive surface area of ​​the detector, and β is the vertical axis magnification of the optical system. The output voltage of the infrared detector is obtained as: Among them, R is the responsivity of the detector, and the Fourier transform is: The output electrical signal is related to multiple parameters. In order to eliminate the influence of the errors of these parameters on the measurement results, a reference sample consisting of a substrate and a heating layer is prepared in the actual measurement. The spectrum amplitude with circular frequency ω>0 is taken, and the spectrum amplitude of the measured sample is divided by the spectrum amplitude of the reference sample to obtain a dimensionless quantity PTR: Where A is the constant coefficient, that is, the ratio of the thickness of the sample under test to that of the reference sample; S4, fitting the PTR data to obtain coefficients a and b; In step S4, the coefficients a and b are obtained by fitting the PTR data using equation (12) to perform curve fitting on the measurement results that have been processed in the same way, i.e., obtaining the coefficients a and b, where a and b are respectively: Among them, the subscripts s and r represent the tested sample and the reference sample, respectively; S5, measuring the thickness of each film layer and the substrate, and calculating the thermal conductivity of the film being measured; In step S5, the thickness of each film layer and the substrate is measured, and the thermal conductivity of the film to be measured is calculated, specifically the equivalent thermal diffusion efficiency and coefficient of the sample to be measured and the reference sample: When the thickness of the substrate, the film to be measured and the heating layer are known, the equivalent thermal diffusion coefficient α can be calculated based on the fitted a and b. s and α r ; In the one-dimensional model, the equivalent heat capacity density product (ρCp) eq It is expressed as: When the interfacial thermal resistance between the layers of the sample being tested is ignored, the thermal conductivity k2 of the film being tested can be calculated according to formula (2) and the known thickness of each layer.

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