A method for improving the strength of the metallization connection of n-type bismuth telluride-based thermoelectric elements based on anchoring effect

Abstract

The application discloses a method for improving the metalization connection strength of n-type Bi2Te3-based thermoelectric elements based on anchoring effect, and comprises the following steps: configuring a n-type Bi2Te3-based wafer surface treatment agent, the composition of which comprises 40-50% sulfuric acid (volume), 1-2% nitric acid (volume), and the rest is water; immersing a clean n-type Bi2Te3-based wafer into the surface treatment agent to perform surface etching; then immersing the etched n-type Bi2Te3-based wafer into ultrapure water to perform ultrasonic treatment; performing metalization connection on the ultrasonically treated n-type Bi2Te3-based wafer through electroplating or chemical plating, the plated metal is easy to form anchoring effect with the n-type Bi2Te3, the metalization connection strength is improved; and finally performing scribing and cutting to obtain the Bi2Te3-based thermoelectric elements with high metalization connection strength. The application makes the n-type Bi2Te3-based wafer have high surface roughness before electroplating, which is beneficial to forming anchoring effect with the plated metal such as nickel, so that the Bi2Te3-based thermoelectric elements with high metalization connection strength are obtained, and the thermoelectric elements are not physically damaged, and the product qualification rate can be improved.

Description

Technical Field

[0001] This invention relates to the field of thermoelectric devices, and more particularly to a method for improving the metallization connection strength of n-type Bi2Te3-based thermoelectric elements based on the anchoring effect. Background Technology

[0002] With the rapid development of human society, the demand for electricity is constantly increasing while fossil fuels are being consumed continuously. Energy depletion and environmental pollution have become two major problems facing the world. Thermoelectric materials can achieve the free conversion between thermal energy and electrical energy through the movement of internal charge carriers, while the process is characterized by being pollution-free, noiseless, and structurally simple, providing new ideas and means to solve energy and environmental problems. On the other hand, research in microelectronics technology is driving the rapid development of the information age, with the rise of cutting-edge technologies such as the Internet of Things, smart wearable devices, and 5G communication technology. Thermoelectric chips, with their advantages of flexibility, long-term effectiveness, and dispersion, have greater development potential in fields such as microelectromechanical systems, wireless sensing, and smart wearable devices. Therefore, promoting the miniaturization and micro-scale design of thermoelectric chips has become one of the important trends in thermoelectric research in recent years.

[0003] Although micro-thermoelectric semiconductor chips (micro-TEC chips) have great application value, their service reliability has always hindered their industrialization and commercialization. The most important factor is the poor metallization bond strength between the thermoelectric material surface and the barrier layer, resulting in high contact resistance and thermal resistance. Over long-term use, this can lead to material interface delamination and device performance degradation. Therefore, surface treatment is typically performed on the thermoelectric material before depositing the barrier layer. This removes surface oxides and increases surface roughness and activity, thereby ensuring that the processed metal layer has good metallization bond strength and thermoelectric properties.

[0004] Surface treatment of conventional n-type Bi₂Te₃-based wafers typically involves two steps: surface pretreatment and electroplating. Traditional surface treatment, due to the large wafer thickness, can employ sandblasting to increase roughness, followed by arc spraying to pre-deposit a layer of nickel to facilitate nickel plating adhesion. However, the thickness of conventional n-type Bi₂Te₃-based wafers is approximately 3 mm, which is 10 times that of n-type Bi₂Te₃-based wafers used in micro-devices. Using traditional methods to treat these wafers can lead to breakage, significantly increasing production costs. Therefore, there is an urgent need to find a new surface treatment method suitable for n-type Bi₂Te₃-based wafers used in micro-devices. Summary of the Invention

[0005] The technical problem this invention aims to solve is to provide a method for improving the metallization connection strength of n-type Bi₂Te₃-based thermoelectric elements, addressing the shortcomings of the existing technology. This method ensures that the n-type Bi₂Te₃-based wafer has a certain roughness and consistent orientation after processing, making it easier for the plating metal to anchor to the n-type Bi₂Te₃, thus facilitating higher plating metallization connection strength during electroplating. This method causes no physical damage to the wafer, improves the product yield of thermoelectric elements while significantly reducing production costs and increasing production efficiency.

[0006] The technical solution adopted by the present invention to solve the above-mentioned problems is as follows:

[0007] A method for improving the metallization connection strength of n-type Bi2Te3-based thermoelectric elements includes the following steps:

[0008] A surface treatment agent for n-type Bi2Te3-based wafers is prepared. The surface treatment agent consists of 40-50% concentrated sulfuric acid (by volume), 1-2% concentrated nitric acid (by volume), and the balance being water.

[0009] The clean n-type Bi2Te3-based wafer is immersed in a surface treatment agent for surface etching;

[0010] The etched n-type Bi2Te3-based wafer was then immersed in ultrapure water for ultrasonication.

[0011] After ultrasound treatment, n-type Bi2Te3-based wafers are directly metallized and connected by electroplating or chemical plating.

[0012] Finally, the obtained n-type Bi2Te3-based wafer is diced to obtain an n-type Bi2Te3-based thermoelectric element, thereby improving the metallization connection strength of the n-type Bi2Te3-based thermoelectric element.

[0013] According to the above scheme, the clean n-type Bi2Te3-based wafer is pre-washed with water and solvents such as acetone and anhydrous ethanol to remove surface grease and other dirt or impurities.

[0014] According to the above scheme, the mass percentage concentration of sulfuric acid is 80-90%; the mass percentage concentration of nitric acid is 60-70%.

[0015] According to the above scheme, the ultrasonic time for ultrapure water is 5 to 20 minutes.

[0016] According to the above scheme, the surface etching temperature is 40-50℃ and the time is 1-12 minutes.

[0017] According to the above scheme, the thickness range of the n-type Bi2Te3-based wafer is 200-500 μm; the n-type Bi2Te3-based thermoelectric element is a cubic particle with an edge length of 200-500 μm.

[0018] Based on the above content, without departing from the basic technical concept of the present invention, various modifications, substitutions or changes can be made to the content in various forms according to common technical knowledge and means in the field.

[0019] In micro thermoelectric devices, the metallization strength, contact resistance, and contact thermal resistance between the thermoelectric material and the metal coating affect the device's service performance and reliability. This invention employs chemical etching for surface pretreatment of an n-type Bi₂Te₃-based wafer, followed by direct electroplating or chemical plating to prepare the coating. The coating metal exhibits an anchoring effect with the n-type Bi₂Te₃. Subsequent dicing and cutting yields the n-type Bi₂Te₃-based thermoelectric element, thereby improving the metallization strength. Device performance testing revealed that this method also reduces interfacial contact resistance and interfacial contact thermal resistance, thus improving the device's thermoelectric performance and reducing power consumption.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] 1. This invention does not cause any physical damage to thermoelectric elements such as n-type Bi2Te3-based wafers. By utilizing the different reactivity and surface energy of different crystal planes, a specific surface treatment agent is used to target the (110) crystal plane of n-type Bi2Te3, breaking the interlayer van der Waals bonds. While ensuring a good surface treatment morphology and processing rate, it causes inter-plane peeling, thereby improving the surface roughness, surface activity, and surface orientation of the n-type thermoelectric material. Since the etched morphology grooves are crisscrossed, the nickel layer can easily penetrate into them to form an anchoring effect, which is beneficial to obtaining a high metallization connection strength with the nickel layer. Since the in-plane electrical and thermal conductivity of Bi2Te3 is greater than its out-of-plane electrical and thermal conductivity, the interface contact resistance and interface contact thermal resistance are reduced. The weakening of the thermal effect at the interface further optimizes the service life, cycle count, and performance of the thermoelectric device.

[0022] 2. Since n-type Bi2Te3-based wafers are mostly cut in air, this invention is easy to operate, can remove oxides remaining on the wafer surface, improve the product qualification rate of n-type Bi2Te3-based thermoelectric elements, and at the same time reduce the production links and processes of arc spraying, reduce production costs, and is suitable for large-scale production. Attached Figure Description

[0023] Figure 1 Images of n-type Bi2Te3-based wafers before and after surface pretreatment.

[0024] Figure 2 This is a photograph of a cut element from an n-type Bi2Te3-based wafer that has undergone Ni-gold plating.

[0025] Figure 3 The produced 2.0×9.0mm2 Miniature thermoelectric devices.

[0026] Figure 4 The basic morphology of n-type Bi2Te3-based wafers under different etching times is shown in the SEM images.

[0027] Figure 5 The cross-section of the n-type Bi2Te3-based wafer after Ni plating is characterized by SEM and EDS.

[0028] Figure 6 The diagram shows the contact resistance of the n-type Bi2Te3-Ni bonding surface after electroplating.

[0029] Figure 7 This is a diagram showing the metallization bonding strength of the n-type Bi2Te3-Ni bonding surface after electroplating.

[0030] Figure 8 The images show SEM images of the surface before electroplating and cross-cut test results of the n-type Bi2Te3-Ni bonding surface after electroplating, for comparative examples and embodiments.

[0031] Figure 9 The graph shows the mass change of reactions between different monoacids and n-type Bi2Te3-based wafers. Detailed Implementation

[0032] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the present invention is not limited to the following embodiments.

[0033] In the following examples, the nitric acid used was AR analytical grade with a concentration of 60-70%; the concentrated H2SO4 was 80-90% by mass.

[0034] In the following embodiments, the thickness of the Bi2Te3-based wafer ranges from 200 to 500 μm, and it is an n-type Bi2Te3-based crystal rod (specific chemical composition Bi2Te3) prepared by hot extrusion or zone melting. 3-x Se x The stoichiometric ratio of elements Bi, Te, and Se is 2:(3-x):x, where x is 0.2-0.8 (x is 0.6 in the example). The sample was obtained by inner circle cutting or wire cutting.

[0035] Example 1

[0036] A method for improving the metallization connection strength of n-type Bi2Te3-based thermoelectric elements, comprising the following steps:

[0037] 1) For circular n-type Bi2Te3-based wafers with a diameter of 30 mm and a thickness of 0.5 mm obtained by the inner circle cutting method, after washing with acetone to remove surface contaminants, dehydrating with anhydrous ethanol, and drying, they are immersed in a surface treatment agent for surface etching (the surface treatment agent consists of 45% sulfuric acid (volume), 1% nitric acid (volume), and the balance being water), the treatment temperature is 45℃, and the treatment time is 10 min;

[0038] 2) Transfer the n-type Bi2Te3-based wafer processed in step 1) into ultrapure water for ultrasonic treatment for 10 minutes;

[0039] 3) The n-type Bi2Te3-based wafer after ultrasonication in step 2) is pre-plated with nickel in an electroplating solution, followed by electroless nickel plating and gold plating.

[0040] The specific conditions for nickel electroplating are as follows: nickel sulfate 200 g / L; nickel chloride 45 g / L; boric acid 45 g / L; 8000#A semi-bright nickel plating bath starter 10 mL / L; 8000#B semi-bright nickel plating additive 0.8 mL / L; low-foaming wetting agent 1 mL / L; pH value 4.0; temperature 55℃; cathode current density 3 A / dm³. 2 3 minutes;

[0041] The specific conditions for electroless nickel plating are as follows: nickel sulfate 0.095 mol / L, sodium hypophosphite 0.227 mol / L, succinic acid 0.135 mol / L, malic acid 0.179 mol / L, pH 6, temperature 90℃, 40 min;

[0042] The specific conditions for gold electroplating are as follows: potassium gold cyanide 15g / L, citric acid 35g / L, potassium citrate 55g / L, pH 4.5, temperature 50℃, and cathode current density 1.3A / dm³. 2 10 minutes;

[0043] 4) The nickel-plated and gold-plated n-type Bi₂Te₃ substrate wafer obtained in step 3) is cut into several cubic thermoelectric elements with a side length of 380 μm. Using medium-temperature solder, the cubic thermoelectric elements are soldered to the electrodes between the upper and lower substrates via reflow soldering to obtain a 2.0 × 9.0 mm [element / unit]. 2 Miniature thermoelectric devices were developed, and their cooling performance was tested.

[0044] Figure 1 The image shows an n-type Bi2Te3 substrate wafer after surface pretreatment obtained in step 2) of Example 1, where the material is exposed on a fresh surface.

[0045] Figure 2 The image shows a component obtained by cutting the n-type Bi2Te3 substrate wafer after nickel and gold plating in step 3) of Example 1.

[0046] Figure 3 The micro thermoelectric device is produced in step 4) of Example 1.

[0047] Table 1 shows the cooling performance of the micro thermoelectric device produced in step 4) of Example 1, with a maximum cooling temperature difference of 62.8°C.

[0048] Table 2 shows the cooling performance of the micro thermoelectric devices fabricated using the traditional method. The traditional preparation process is sandblasting + arc spraying + electroplating nickel + electroless nickel plating + electroless gold plating. That is, the surface treatment in steps (1) and (2) of Example 1 is not used. Instead, a pretreatment method of sandblasting + arc spraying nickel is used (sandblasting uses 320-mesh spherical glass beads abrasive, pressure 0.05MPa; arc spraying process: Φ1.2mm nickel wire, working voltage 25V, working current 90A, air pressure 0.65MPa, spray gun voltage 9V), and the subsequent processing is the same.

[0049] Comparing Tables 1 and 2, it can be seen that the resistance of the device fabricated using the conventional method increases by 0.38Ω, mainly due to contact resistance, while the maximum cooling temperature difference decreases by 4.8℃ (from 58℃). Therefore, this invention effectively reduces the interfacial contact resistance without damaging the wafer integrity, indicating that the interfacial contact performance between the surface-treated n-type Bi2Te3-based wafer and the subsequent nickel layer is improved.

[0050] Table 1

[0051]

[0052] Table 2

[0053]

[0054]

[0055] Example 2

[0056] A method for improving the metallization connection strength of n-type Bi2Te3-based thermoelectric elements, comprising the following steps:

[0057] 1) For circular n-type Bi2Te3-based wafers with a diameter of 30 mm and a thickness of 0.4 mm obtained by wire cutting, after washing with acetone to remove surface contaminants and rinsing with water, the wafers are immersed in a surface treatment agent for surface etching. The surface treatment agent consists of 2% (volume) nitric acid, 50% (volume) H2SO4, and the balance being water. The treatment temperature is 40℃ and the treatment time is 12 min.

[0058] 2) Transfer the n-type Bi2Te3-based wafer processed in step 1) into ultrapure water and sonicate for 5 minutes;

[0059] 3) The n-type Bi2Te3-based wafer after ultrasonication in step 2) is pre-plated with nickel in an electroplating solution, followed by electroless nickel plating and gold plating (the specific conditions for nickel plating, electroless nickel plating, and gold plating are the same as in Example 1).

[0060] 4) The nickel-plated and gold-plated n-type Bi₂Te₃ substrate wafer obtained in step 3) is cut into several cubic thermoelectric elements with a side length of 290 μm. Using medium-temperature solder, the elements are soldered to the electrodes between the upper and lower substrates via reflow soldering to obtain a 4.7 × 4.9 mm [element / piece]. 2 Miniature thermoelectric devices were developed, and their cooling performance was tested.

[0061] Figure 4 The SEM surface morphology of the n-type Bi2Te3-based wafer used in step 1) of Example 2 shows obvious etching marks.

[0062] Figure 5 The image shows the SEM cross-sectional morphology of the n-type Bi2Te3 substrate wafer after step 3) in Example 2. It can be seen that the nickel layer penetrates deep into the n-type Bi2Te3, and the degree of metallization is high.

[0063] Figure 6 The image shows the contact resistance test results of the n-type Bi2Te3 substrate wafer and nickel interface after electroplating in step 3) of Example 2. It can be seen that it has a small contact resistance.

[0064] Figure 7 The image shows the test results of the n-type Bi2Te3 substrate wafer and nickel metallization connection strength after electroplating in step 3) of Example 2. It can be seen that it has a high metallization connection strength.

[0065] Table 3 shows the cooling performance of the micro thermoelectric device produced in step 4) of Example 2, with a maximum cooling temperature difference of up to 62°C.

[0066] Table 3

[0067]

[0068] Comparative Example 1

[0069] The difference from Example 1 is that the surface treatment agent consists of 15% phosphoric acid, 30% nitric acid, 45% acetic acid, 20% sulfuric acid, and the balance being water. Other conditions are the same as in Example 1.

[0070] In Comparative Example 1, the orientation of the n-type Bi₂Te₃-based wafer is not specific, resulting in a slower etching rate and a relatively flat wafer surface after etching, forming small mounds. Figure 8 As shown, although the intermetallic bonding strength of the n-type Bi2Te3-based wafer processed by this method is good, the large grain size of the nickel plating on the surface makes it prone to cracking during the cutting process.

[0071] Comparative Example 2

[0072] The difference from Example 1 is that the surface treatment agent includes a roughening solution and a descaling solution; the roughening solution consists of 10% (volume) hydrochloric acid, 40% (volume) hydrogen peroxide, 5% (volume) nitric acid, and the remainder is water; the descaling solution consists of 50% (volume) hydrofluoric acid, 10% (volume) hydrochloric acid, 5% (volume) nitric acid, and the remainder is water; the roughening temperature is 30°C and the roughening time is 5 min; the descaling temperature is 25°C and the descaling time is 5 min.

[0073] Comparative Example 2 exhibits a relatively fast etching rate that is difficult to control in actual operation, resulting in a powdery morphology after etching with no obvious grain shape. Figure 8 As shown, due to the large amount of powder adhering to the surface, a dust removal step is required after the etching step, and a corresponding dust removal agent is prepared. The dust removal agent also requires the use of hydrofluoric acid, which is highly dangerous. This increases the processing time and further damages the surface morphology of the wafer, causing the wafer with the electroplated nickel layer to fall off during the cross-cut test, indicating poor intermetallic bonding strength.

[0074] Comparative Example 3

[0075] The difference from Example 1 is that the surface treatment agent used is a single acid solution. The n-type Bi₂Te₃-based wafer was etched using 98% sulfuric acid, 68% nitric acid, 38% hydrochloric acid, 20% hydrofluoric acid, and 80% glacial acetic acid, respectively. The mass change of the n-type Bi₂Te₃-based wafer over time during the reaction is shown below. Figure 9 As shown in the figure. Experiments show that 68% nitric acid can uniformly etch n-type Bi2Te3-based wafers, and their mass decreases uniformly over time. 98% sulfuric acid will passivate the surface of the n-type Bi2Te3-based wafer, preventing further reaction. Other single acids do not react with it.

[0076] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A method for improving the metallization connection strength of n-type Bi2Te3-based thermoelectric elements, characterized in that, Includes the following steps: (1) A surface treatment agent for n-type Bi2Te3-based wafers is prepared, wherein the composition of the surface treatment agent by volume percentage includes: 40-50% sulfuric acid, 1-2% nitric acid, and the balance being water; wherein the mass percentage concentration of sulfuric acid is 80-90% and the mass percentage concentration of nitric acid is 60-70%; (2) Immerse the clean n-type Bi2Te3-based wafer in the surface treatment agent for surface etching; (3) The n-type Bi2Te3-based wafer after surface etching is immersed in ultrapure water for ultrasonication; (4) After ultrasound treatment, the n-type Bi2Te3-based wafers are directly metallized by electroplating or chemical plating. (5) The n-type Bi2Te3-based wafer obtained in step (4) is diced to obtain an n-type Bi2Te3-based thermoelectric element, which is used to prepare thermoelectric devices, thereby improving the metallization connection strength of the n-type Bi2Te3-based thermoelectric element.

2. The method for improving the metallization connection strength of n-type Bi2Te3-based thermoelectric elements according to claim 1, characterized in that, In step (2), the n-type Bi2Te3-based wafer is immersed in a surface treatment agent, the surface etching temperature is 40~50 ℃, and the treatment time is 1~12 min.

3. The method for improving the metallization connection strength of n-type Bi2Te3-based thermoelectric elements according to claim 1, characterized in that, In step (3), the time for ultrasonication of ultrapure water is 5~20 min.

4. The method for improving the metallization connection strength of n-type Bi2Te3-based thermoelectric elements according to claim 1, characterized in that, The thickness of the n-type Bi2Te3-based wafer ranges from 200 to 500 μm, and it is obtained by inner circle cutting or wire cutting of n-type Bi2Te3-based crystal rods prepared by hot extrusion or zone melting.

5. The method for improving the metallization connection strength of n-type Bi2Te3-based thermoelectric elements according to claim 1, characterized in that, The clean n-type Bi2Te3-based wafers were pre-washed with water and then with acetone and anhydrous ethanol.

6. The method for improving the metallization connection strength of n-type Bi2Te3-based thermoelectric elements according to claim 1, characterized in that, n-type Bi2Te3-based thermoelectric elements are cubic particles with an edge length of 200~500um.

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