A high-sensitivity micro-atomic layer thermopile heat flow sensor and its preparation method

Abstract

The present invention belongs to the technical field of heat flow sensors, and specifically relates to a high-sensitivity micro atomic layer thermopile heat flow sensor and a preparation method thereof. The present invention adopts two thermopile lines with opposite polarities of thermoelectric potential signals to prepare a multi-line structure, so as to avoid the use of a conductive gold film required for preparing a multi-line structure with the same ALTP material, and further significantly improves the effective length of the ALTP film on this basis; the size of the sensor is reduced while ensuring the sensitivity of the ALTP heat flow sensor, thereby reducing the influence of the sensor on the flow field.

Description

Technical Field

[0001] The present invention belongs to the technical field of heat flow sensors, and in particular relates to a high-sensitivity micro-atomic layer thermopile heat flow sensor and a preparation method thereof. Background Art

[0002] Atomic layer thermopile (ALTP) refers to a material with lateral Seebeck effect, that is, when there is a temperature difference between the upper and lower surfaces of the inclined epitaxial film, a thermoelectric potential is generated perpendicular to the temperature gradient due to the anisotropy of the material's carrier transport. Since the material is anisotropic, when the inclined epitaxial film is under the action of the temperature gradient, the carrier conduction is like that in two materials, so it is called an atomic layer thermopile.

[0003] It is generally believed that the output potential direction of ALTP is determined by the conductivity type of the material. If the majority carriers of the material are holes, the output potential points to one end of the tilt direction (Kouhei Takahashi et al., "Influence of interband transition on the laser-induced voltage in thermoelectric Ca x CoO2thin films》); if the majority carriers of the material are electrons, the output potential is opposite. At present, ALTP heat flow sensor has become the main means of capturing high-frequency pulsating heat flow signals due to its fast response characteristics, and has great application prospects in wind tunnel tests. In order to minimize the interference of the sensor to the flow field, its size should be as small as possible. However, the sensitivity of the ALTP heat flow sensor is proportional to the length of the ALTP film, so the reduction in the size of the sensor will inevitably lead to a decrease in sensitivity.

[0004] Based on this, researchers have proposed a method to improve the sensitivity of ALTP heat flow sensors using a multi-line structure (CN202110066961). However, when this method uses ALTP thin film to prepare a multi-line structure, since the polarity of the thermoelectric potential of each line is the same, a conductive film must be used to connect the two adjacent lines end to end to achieve effective superposition of the thermoelectric potential signal. The conductive film will not contribute to the size of the thermoelectric potential, but will seriously reduce the effective utilization rate of the substrate area. Summary of the invention

[0005] In view of the above-mentioned problems or shortcomings, in order to solve the shortcomings of the existing ALTP heat flow sensor in terms of both miniaturization and sensitivity, the present invention provides a high-sensitivity micro atomic layer thermopile heat flow sensor and a preparation method thereof.

[0006] A high-sensitivity micro atomic layer thermopile heat flow sensor comprises: a tilted single crystal substrate, two thermopile lines, a lead gold film and an external wire.

[0007] The two thermopile lines are made of two materials with opposite polarities of thermoelectric potential signals, namely n-type and p-type ALTP thin film materials. The two thermopile lines are arranged on an inclined single crystal substrate and are alternately arranged in series along the inclined direction of the single crystal substrate.

[0008] The lead gold film has two locations, which are respectively connected to the first and last thermopile lines after being connected in series, and then are respectively connected to external wires.

[0009] There are two external connecting wires, which are respectively connected to two lead gold films and serve as the terminals for external connection of the whole sensor.

[0010] Furthermore, the inclined single crystal substrate is a LaAlO3, SrTiO3 or MgO single crystal substrate inclined at 5°-45° along the c-axis.

[0011] Furthermore, the two thermopile lines are made of the same material and modulated into n-type and p-type respectively, so as to optimize the problem of reduced device stability due to different thermal stability or thermal expansion coefficients of different materials at high temperatures.

[0012] like Figure 1 As shown in the figure, when there is a temperature gradient on the surface of the ALTP film, due to the lateral Seebeck effect, each ALTP line will generate a lateral thermoelectric potential perpendicular to the temperature gradient direction. Since the conductivity types of the two adjacent lines are different, the thermoelectric potential V generated by ALTP line 2 and ALTP line 3 is 1- The direction is opposite to V2. On the one hand, ALTP line 3 is equivalent to a conductive film that connects the thermoelectric potentials of ALTP line 2 in series. On the other hand, the thermoelectric potential generated by ALTP line 3 itself is also connected in series through ALTP line 2. Therefore, the potential output of the entire sensor is V2 = mV1 + (m-1) V2. Compared with multi-line devices made of the same ALTP material, using ALTP material with the opposite conductivity type to replace the conductive film can not only achieve the superposition of output signals, but also contribute part of the potential output. Therefore, under the requirement of the same sensitivity coefficient, combined with micromachining technology, the size of the sensor can be further reduced, laying a solid foundation for the miniaturization research of ALTP thermal flow sensors.

[0013] like Figure 2 As shown, the preparation process of the above-mentioned high-sensitivity micro-atomic layer thermopile heat flow sensor includes the following steps:

[0014] Step 1: epitaxially grow an n-type or p-type ALTP thin film on a tilted single crystal substrate by sputtering, metal organic chemical vapor deposition or pulsed laser deposition.

[0015] Step 2: Etch the n-type or p-type ALTP film into N mutually non-intersecting thermopile lines through a first photolithography process, where N≥2.

[0016] Step 3: Prepare N-1 p-type or n-type thermopile lines by a second photolithography and metal organic deposition; the N-1 p-type or n-type thermopile lines are respectively distributed between adjacent n-type or p-type thermopile lines to realize the sequential series connection of N n-type or p-type thermopile lines.

[0017] Step 4: Prepare connected lead gold films on the first and last thermopile lines after the series connection, for a total of two locations; then connect two external wires to the two lead gold films respectively, and connect to the outside through the external wires (such as for external testing of sensors).

[0018] The high-sensitivity micro atomic layer thermopile heat flow sensor provided by the present invention prepares a multi-line structure by using two thermopile lines with opposite polarities of thermoelectric potential signals, thereby avoiding the use of a conductive gold film required for preparing a multi-line structure using the same ALTP material, and further significantly improves the effective length of the ALTP film on this basis, and finally obtains an ALTP heat flow sensor with extremely high sensitivity. Based on this, the sensor size can be reduced in combination with micromachining technology while ensuring the sensitivity of the ALTP heat flow sensor, thereby reducing the impact of the sensor on the flow field. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the structure and principle of the present invention;

[0020] Figure 2 is a preparation flow chart of the present invention;

[0021] Figure 3 It is a structural example diagram of the sample of the embodiment;

[0022] Figure 4 2θ spectrum and ω scanning curve of the sample of the embodiment;

[0023] Figure 5 It is the heat flow test curve of the sample of the embodiment;

[0024] Reference numerals: 1-single crystal substrate, 2-n-type LaCaMnO3 line, 3-p-type YBa2Cu3O 7-δ Lines, 4-lead gold film, 5-external wire. DETAILED DESCRIPTION

[0025] The technical solution of the present invention is described in detail below in conjunction with the accompanying drawings and embodiments.

[0026] Example:

[0027] Step 1: epitaxially grow a LaCaMnO3 functional layer film using metal organic vapor deposition.

[0028] First, La source, Ca source and Mn source were dissolved in tetrahydrofuran at concentrations of 0.0138 mol / L, 0.0320 mol / L and 0.0344 mol / L, and the precursor solution was obtained after sufficient stirring. The surface-cleaned strontium titanate substrate was placed in the deposition chamber, vacuumed to 2 Pa, the substrate heating temperature was adjusted to 830 ° C, the oxygen flow rate was 500 mL / min, the argon flow rate was 700 mL / min, the nitrous oxide flow rate was 500 mL / min, and the liquid volume was 50 ml. Metal organic vapor phase deposition was used to epitaxially grow a 250 nm LaCaMnO3 functional layer film.

[0029] Step 2: Use the first photolithography process to achieve LaCaMnO3 film patterning.

[0030] First, positive photoresist was spin-coated at a speed of 1000 r / s for 10 seconds, and then the high-speed mode was turned on at 3000 r / s for 30 seconds to evenly cover the surface of the film with photoresist. The sample was placed on a heating table at 100°C and forward-baked for 1 minute to improve the adhesion between the photoresist and the sample surface. Contact exposure was performed using a mask for 3 seconds. The sample was immersed in a developer and gently shaken to dissolve the photoresist on the exposed part, followed by cleaning and drying. Argon ions were used to bombard the sample surface to etch away the film without photoresist protection, and acetone was used to remove the remaining photoresist on the substrate surface to form a LaCaMnO3 multi-line pattern.

[0031] Step 3: Use the second photolithography process and metal organic deposition to realize YBa2Cu3O 7-δ Patterning preparation of thin films.

[0032] First, spin-coat negative photoresist, adjust the rotation speed to 1000r / s, run for 10s, then turn on the high-speed mode of 3000r / s and run for 30s to evenly spin-coat the photoresist on the surface of the film; place the substrate on a heating table at 100℃ and bake for 1min to improve the adhesion of the photoresist to the sample surface; use a mask to perform contact exposure for 3s; place the substrate on a heating table at 120℃ and bake for 90s, then flood expose for 45s; immerse the sample in the developer and shake it gently to dissolve the unexposed part of the photoresist, then clean and dry it.

[0033] YBa2Cu3O 7-δ Metal organic deposition process of thin films:

[0034] (1) Preparation of precursor solution: First, yttrium acetate, barium acetate, and copper acetate are mixed in deionized water according to the stoichiometric ratio of metal ions of 1:2:3 to dissolve; trifluoroacetic acid is added. Since copper trifluoroacetate is easy to sublimate at high temperatures, an excess of 1% copper acetate is added when the raw materials are mixed. During the preparation of the solution, a chemical reaction occurs to eventually form yttrium trifluoroacetate, barium trifluoroacetate, copper trifluoroacetate, and acetic acid. The precursor solution is then purified by distilling methanol multiple times, and finally methanol is used as a solvent to prepare a precursor solution;

[0035] (2) Spin coating: Then spin-coat the precursor liquid, adjust the speed to 1000 r / s, run for 10 seconds, then turn on the high-speed mode to 4000 r / s, run for 120 seconds, so that the precursor liquid is evenly spin-coated on the surface of the film. The solvent evaporates during rotation, leaving a layer of glue on the substrate;

[0036] (3) Low temperature pretreatment: After the coating, the sample is placed in a quartz tube for low temperature pretreatment. Before the low temperature pretreatment stage, the air in the quartz tube is first removed with dry oxygen to prevent the gel film from absorbing water and losing its integrity. When the temperature in the tube is greater than 70°C, moist oxygen (P H2O =2.1%), the heating rate after 200°C should be very slow (about 10 hours) to allow the gas produced by the reaction to slowly overflow the surface of the film. In order to exhaust the carbon-containing residue, the temperature is raised to 300°C and then quickly raised to 400°C (about 20 minutes), then cooled with the furnace, and then acetone is used to remove the remaining photoresist on the substrate surface;

[0037] (4) High temperature heat treatment: The precursor film is also subjected to a high temperature crystallization treatment at 830°C to make it into a YBa2Cu3O film with good orientation and crystallization. 7-δ During the high temperature heat treatment, a humid Ar / O2 mixed gas (1.2 L / min O2, P H2O =2.1%)

[0038] (5) Post-annealing treatment: The film was placed in an oxygen atmosphere and annealed at 500°C (30 min) to make YBa2Cu3O 7-δ Realizing the transition from tetragonal phase to orthorhombic phase, YBa2Cu3O 7-δ The film is about 250nm.

[0039] Step 4: Prepare the lead gold film using the third photolithography process.

[0040] First, spin-coat the negative photoresist, adjust the speed to 1000r / s, run for 10s, then turn on the high-speed mode to 3000r / s, run for 30s, so that the photoresist is evenly spin-coated on the surface of the film; place the substrate on a heating table at 100℃ and bake for 1min to improve the adhesion of the photoresist to the sample surface; use a mask to perform contact exposure for 3s; place the substrate on a heating table at 120℃ and bake for 90s, then flood-expose for 45s; immerse the sample in the developer and gently shake it to dissolve the unexposed part of the photoresist, then clean and dry it. Then put the sample into the chamber and evacuate it to 2.13×10 -3 Pa, adjust the oxygen partial pressure to 0.5Pa, the sputtering current to 0.15A, and the timing to 20s; then turn off the oxygen, and after the gas pressure drops and stabilizes, adjust the argon partial pressure to 0.7Pa, the sputtering current to 0.2A, and the timing to 3.5min; finally, use acetone to remove the remaining photoresist on the surface of the substrate to form a lead gold film.

[0041] Step 5: Use welding technology to connect the copper wire (external wire) to the gold electrodes (lead gold film) at both ends. At this point, the atomic layer thermopile heat flow sensor with high sensitivity is completed.

[0042] The samples prepared in this example were subjected to heat flow test and XRD test, and the results are as follows ( Figure 4 shown):

[0043] Figure 4 (a) shows YBa2Cu3O 7-δ The 2θ spectrum of the film and the ω scanning curve of the (005) diffraction peak, YBa2Cu3O 7-δ The films are all c-axis oriented, the (005) diffraction peak position is 38.49°, the half-height width of the ω scanning curve of the (005) crystal plane is 0.39°, and the peak position is 31.43°. Therefore, YBa2Cu3O 7-δ The film grows along the substrate normal at an angle of 11.75° (31.43°-19.25°=12.18°), and the epitaxial quality is high. Figure 5 As shown in (b), the LaCaMnO3 films are also c-axis oriented, with the (002) diffraction peak at 48.7°, the half-height width of the ω scanning curve of the (002) crystal plane at 0.51°, and the peak position at 11.68°. Therefore, the LaCaMnO3 film grows along the substrate normal at an angle of 11.75° (24.35°-12.68°=11.67°), which is consistent with the YBa2Cu3O 7-δ The epitaxial growth is consistent.

[0044] Figure 5 (a) shows the LaCaMnO3 single line at 39.71kW / m 2Response curve under heat flow, potential output is about -42μV, YBa2Cu3O 7-δ Single line at 39.71kW / m 2 The potential output under heat flow is about 410μV. 7-δ / LaCaMnO3 double line at 39.71kW / m 2 The potential output under heat flow is about 450μV, which basically meets the requirements of YBa2Cu3O 7-δ The absolute values ​​of the output potentials of the lines and the LaCaMnO3 lines are superimposed. Therefore, this embodiment proves that the sensitivity can be further enhanced by replacing the conductive lines with ALTP materials of different conductivity types in a multi-line structure.

[0045] It can be seen from the above embodiments that the present invention is based on a multi-line structure and uses ALTP materials of different conductive types to replace the conductive lines. On the basis of the original multi-line structure, the effective length of the ALTP film can be further increased, thereby significantly improving the sensitivity of the ALTP heat flow sensor, so that the resolution of the sensor is also correspondingly improved, and can be used for lower heat flow testing. At the same time, under the premise of ensuring sensitivity, combined with micro-machining technology, the size of the sensor can be further reduced, the influence of the sensor on the flow field can be reduced, and the test accuracy can be improved.

Claims

1. A high-sensitivity micro-atomic layer thermopile heat flow sensor, characterized in that: It includes a tilted single crystal substrate, two types of thermopile lines, lead gold film and external wires; The two thermopile lines are two materials with opposite polarities of thermoelectric potential signals, namely n-type and p-type ALTP thin film materials. The two thermopile lines are arranged on the inclined single crystal substrate and are alternately arranged in series along the inclined direction of the single crystal substrate. The lead gold film has two locations, which are respectively connected to the first and last two thermopile lines after being connected in series, and then are respectively connected to external wires; There are two external wires, which are respectively connected to two lead gold films and serve as the terminals for external connection of the overall sensor.

2. The high-sensitivity micro-atomic layer thermopile heat flow sensor according to claim 1, characterized in that: The inclined single crystal substrate is a LaAlO3, SrTiO3 or MgO single crystal substrate inclined by 5°-45° along the c-axis.

3. The high-sensitivity micro-atomic layer thermopile heat flow sensor according to claim 1, characterized in that: The two thermopile lines are made of the same material and modulated into n-type and p-type respectively.

4. The method for preparing the high-sensitivity micro-atomic layer thermopile heat flow sensor according to claim 1, characterized in that: The following steps are involved: Step 1, epitaxially growing an n-type or p-type ALTP thin film on a tilted single crystal substrate by sputtering, metal organic chemical vapor deposition or pulsed laser deposition; Step 2: Etching the n-type or p-type ALTP film into N mutually non-intersecting thermopile lines through the first photolithography process, where N ≥ 2; Step 3, preparing N-1 p-type or n-type thermopile lines by a second photolithography and metal organic deposition; the N-1 p-type or n-type thermopile lines are respectively distributed between adjacent n-type or p-type thermopile lines, so as to realize the sequential series connection of N n-type or p-type thermopile lines; Step 4: Prepare connected lead gold films at two locations on the first and last thermopile lines after they are connected in series, and then connect two external wires to the two lead gold films respectively.

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