Method for manufacturing piezoresistor, vacuum sputtering device and piezoresistor thereof

Abstract

The application discloses a preparation method of a piezoresistor, a vacuum sputtering film equipment and the piezoresistor, and comprises at least two composite film coating chambers and a first cooling chamber arranged between two adjacent composite film coating chambers; the first cooling chamber is used for reducing the temperature of an electrode layer between the two adjacent composite film coating chambers to below a temperature threshold. The vacuum film coating equipment provided by the application can prevent the piezoceramic base from being reduced due to high temperature in sputtering in multiple composite film coating chambers by arranging a cooling chamber between the composite film coating chambers, and can seal and coat the electrode layer of the piezoresistor in the vacuum film coating equipment at one time, so that the obtained electrode layer has excellent conductive performance.

Description

Technical Field

[0001] This invention relates to the fabrication technology of sputtered electrodes for varistors, specifically to a method for fabricating varistors, a vacuum sputtering coating device, and a varistor thereof. Background Technology

[0002] The current manufacturing process for zinc oxide varistors typically involves screen printing silver or copper paste, followed by baking and sintering to form effective ohmic contact electrodes. Arc spraying is also used. For high-performance varistors requiring high surge capacity per unit area, the method using printed silver paste followed by baking and sintering offers the most stable performance. Varistors are primarily used for surge absorption in circuits. Given the limited installation size and the need for high current carrying capacity per unit area in zinc oxide varistors, the manufacturing process of the conductive layer in the electrode layer is particularly important. Current methods involve increasing the thickness of the conductive layer and reducing the ohmic contact resistance.

[0003] Sputtering technology is a revolutionary advancement in the fabrication of varistor electrodes. It involves applying a nickel-chromium or titanium base coat followed by sputtering a conductive layer. Compared to processes like printing, electroplating, and hot-dip galvanizing, the conductive layer formed by sputtering exhibits superior density and conductivity, making it ideal for varistors. However, in current technologies, the metal conductive layer of varistors directly fabricated using sputtering is often quite thin, typically only reaching around 1 μm. The primary reason for this thinness in existing industrial sputtering techniques is that excessively thick sputtering (due to prolonged single sputtering times) leads to temperature increases. Under vacuum conditions, excessively high temperatures cause zinc oxide deoxidation and reduction. Achieving a higher thickness through slow sputtering in the same or multiple chambers requires a long time. Furthermore, the heat generated during sputtering under vacuum cannot be effectively released, making it difficult to lower the temperature to the product's optimal operating temperature, ultimately resulting in substandard varistors.

[0004] Several existing technologies have focused on how to supplement the conductive layer (the structural layer directly connected to the electrode leads) of sputtered varistors, with a thickness of 1. One approach is to combine other processes with sputtering to prepare the varistor electrode layer, thereby improving its conductivity. For example, Chinese utility model patent application number 201420644271 uses electroplated aluminum spraying as the underlayer followed by vacuum sputtering of a copper conductive layer to overcome the thinness of the metal layer formed solely by vacuum sputtering. Similarly, Chinese invention patent application number 2018105051972 uses vacuum sputtering as the underlayer followed by hot-dip treatment to form the conductive layer, overcoming the thinness of the metal layer formed solely by vacuum sputtering and the poor uniformity of electroplating. However, these processes all combine other processes with sputtering to supplement the thickness of the sputtered conductive layer, resulting in a less dense conductive layer compared to a conductive layer formed directly by sputtering. 2. Adopting a composite layer structure design to improve the strength and thickness of the varistor electrode layer. For example, Chinese utility model patent application number 2013205912226 discloses a zinc oxide varistor with a composite electrode structure. It extends the safe sputterable thickness of the conductive copper electrode by sputtering a multilayer composite impact structure (chromium-copper-nickel alloy-chromium-copper-nickel alloy) to increase the density between metals, and finally can sputter to form a conductive copper electrode with a thickness of 0.5 to 1.5 μm.

[0005] In addition, existing vacuum sputtering coating equipment has been equipped with multiple sputtering chambers for continuous coating, thereby increasing the thickness of the electrode layer through multiple sputterings. For example, Chinese Patent Application No. 2010102983273 discloses a continuous multi-chamber sputtering coating chamber atmosphere isolation system including at least two sputtering chambers, comprising: adding a transition chamber between the first sputtering chamber and the second sputtering chamber, the transition chamber being connected to a vacuum generator, the vacuum degree of the transition chamber being greater than that of the first sputtering chamber, and the vacuum degree of the transition chamber being greater than that of the second sputtering chamber; so as to prevent the protective gas in the two sputtering chambers from mixing and causing contamination through the high vacuum degree of the transition chamber. However, in continuous vacuum coating equipment, protection against airflow contamination is usually only required when different types of metals (with different protective gases) are sputtered in adjacent sputtering chambers. On the other hand, the higher the vacuum level of the environment, the less favorable it is for heat dissipation. In this case, the sputtering thickness, time, and number of consecutive sputterings are required, which will increase the temperature in the entire vacuum system. When the temperature of a single sputtering chamber reaches above 350 degrees Celsius, the main material of the zinc oxide varistor, zinc oxide, will be deoxidized and reduced, and a conductive layer will form on the surface of the ceramic sheet. The resulting electrode layer will have poor electrical performance, with low varistor voltage and leakage current exceeding the specified value. Therefore, the sputtering time of a single chamber in the existing continuous vacuum coating system is not long enough to mass-produce qualified varistor products. Summary of the Invention

[0006] The purpose of this invention is to overcome the problem in the prior art that it is impossible to directly form varistors with sufficient conductivity through sputtering. This invention provides a method for preparing varistors, a vacuum sputtering coating equipment, and a varistor thereof. The vacuum sputtering coating equipment and method provided by this invention can deposit a conductive layer of more than 5μm (a structural layer in the electrode layer that is directly connected to the electrode leads) of the varistor in one step, which can improve product performance and enable mass production.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0008] A varistor made of zinc oxide includes: an electrode layer, wherein the electrode layer includes: a conductive layer, wherein the conductive layer is formed by a sputtering process and the thickness of the conductive layer is d, and d∈(5,14]μm.

[0009] According to one specific embodiment, in the above-mentioned varistor, the electrode layer further includes: a bottom transition layer, which is formed by a sputtering process and has a thickness of no more than 0.5 μm.

[0010] The bottom transition layer is a structural layer used to increase the adsorption force of the electrode layer, and the conductive layer is a structural layer directly connected to the electrode leads. In this invention, the corresponding conductive layer can already meet the high-performance conductivity requirements. Therefore, the thickness of the suitable bottom transition layer does not need to be too thick to meet the adsorption force requirements while further reducing production costs.

[0011] According to one specific embodiment, in the above-mentioned varistor, the thickness of the conductive layer is 6μm to 10μm.

[0012] According to one specific embodiment, the varistor described above includes: two electrode layers, a zinc oxide varistor ceramic substrate, and electrode leads. The electrode layers are respectively disposed on the upper and lower surfaces of the zinc oxide varistor ceramic substrate, and electrode leads are respectively disposed on the two electrode layers. The conductive layer in the electrode layer is a structural layer directly connected to the electrode leads.

[0013] According to one specific embodiment, in the above-mentioned varistor, the bottom transition layer material includes a first metal or its alloy, the first metal including nickel, chromium or titanium, and the conductive layer material includes a second metal or its alloy, the second metal including copper, silver or nickel.

[0014] The bottom transition layer material can be selected from the first metal or the metal alloy corresponding to the first metal, preferably a nickel-chromium alloy; the conductive layer material can be selected from the second metal or the metal alloy corresponding to the second metal; preferably copper (with a small amount of nickel added).

[0015] In a further embodiment of the present invention, a method for manufacturing a varistor made of zinc oxide is also provided, comprising: sealing and sputtering the electrode layer of the varistor in a vacuum environment;

[0016] The electrode layer comprises a bottom transition layer and a conductive layer; the thickness of the conductive layer is d, and d∈(5,14]μm.

[0017] According to one specific embodiment, in the above-mentioned method for manufacturing a varistor made of zinc oxide, the thickness of the bottom transition layer does not exceed 0.5 μm.

[0018] According to a specific embodiment, the method for manufacturing the zinc oxide varistor includes: performing a sputtering step N times in a vacuum environment or an inert gas environment to make the conductive layer of the varistor reach a thickness d, wherein N≥2;

[0019] A cooling step is added between two adjacent conductive layer sputtering steps to cool the temperature of the varistor to below the temperature threshold.

[0020] According to one specific embodiment, in the above-mentioned method for manufacturing a varistor made of zinc oxide, the cooling step employs molecular flow cooling technology to cool the temperature of the varistor below a temperature threshold.

[0021] According to one specific embodiment, in the above-mentioned method for manufacturing a varistor made of zinc oxide, the molecular flow is an inert gas.

[0022] According to one specific embodiment, in the above-mentioned method for manufacturing a varistor made of zinc oxide, the temperature threshold is 80℃~90℃.

[0023] According to one specific embodiment, in the above-mentioned method for manufacturing a varistor made of zinc oxide, the conductive layer material includes a second metal or its alloy, wherein the second metal includes copper, silver or nickel.

[0024] In a further embodiment of the present invention, a vacuum sputtering coating apparatus is also provided, comprising: at least two composite coating chambers; a first cooling chamber is provided between two adjacent composite coating chambers for reducing the temperature of the pressure-sensitive workpiece between the two adjacent composite coating chambers to below a temperature threshold; wherein the first cooling chamber is sealed to the two adjacent composite coating chambers.

[0025] According to one specific embodiment, in the above-mentioned vacuum sputtering coating equipment, the temperature threshold is 80℃~90℃.

[0026] According to one specific embodiment, in the above-mentioned vacuum sputtering coating equipment, the composite coating chamber is used to sputter the conductive layer of the varistor;

[0027] Furthermore, the temperature of all the composite coating chambers is below the safe temperature threshold, which is 280℃~350℃.

[0028] Specifically, the conductive layer material can be one of copper, silver, or nickel or their alloys, with copper being preferred.

[0029] According to one specific embodiment, in the above-mentioned vacuum sputtering coating equipment, the first cooling chamber is configured to use molecular flow cooling technology to reduce the temperature of the varistor between two adjacent composite coating chambers to below a temperature threshold.

[0030] According to one specific embodiment, in the above-mentioned vacuum sputtering coating equipment, the composite coating chamber is a single-sided sputtering chamber or a double-sided sputtering chamber, and a closed-loop magnetic field is provided in the composite coating chamber.

[0031] According to one specific embodiment, in the above-mentioned vacuum sputtering coating equipment, there are two composite coating chambers.

[0032] The vacuum sputtering coating equipment includes, in sequence: a pre-extraction chamber, a pre-sputtering chamber, a first composite coating chamber, a first cooling chamber, a second composite coating chamber, a post-transition chamber, and a second cooling chamber.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] 1. The varistor provided by this invention has a conductive layer in its electrode layer formed by sputtering. This electrode layer has a much higher density than that obtained by printing, and its thickness is greater than 5μm, exhibiting excellent conductivity. Furthermore, in terms of material selection, low-cost materials can be chosen for the conductive layer, reducing the manufacturing cost of the varistor electrode. According to the varistor performance test results, the varistor provided by this invention, with a 6μm thick copper conductive layer, has the equivalent conductivity of a 13μm thick silver conductive layer obtained by existing printing processes; it is a varistor that meets high-performance requirements, with performance indicators improved to 7KA / cm. 2 above.

[0035] 2. The vacuum sputtering coating equipment provided by this invention, based on the setting of multiple composite coating chambers, reduces the temperature of the varistor workpiece between adjacent composite coating chambers by setting a first cooling chamber between two adjacent composite coating chambers. While increasing the thickness of the electrode layer in a single vacuum sputtering, the reduction of electrode temperature prevents the electrode from being reduced due to excessive temperature during sputtering in multiple composite coating chambers. The resulting vacuum copper electrode sputtering preparation device provided by this invention can deposit an electrode layer of more than 5μm in one go. Experimental data shows that the vacuum sputtering coating equipment provided by this invention can deposit 3 to 4 vehicles of high-thickness varistor electrode layers per hour (each vehicle can carry about 5000 varistors with a diameter of 14mm); it can realize the mass production of varistors.

[0036] 3. Setting up a closed-loop magnetic field in the composite coating chamber increases the deposition rate, shortens the sputtering time, and improves processing efficiency. Attached Figure Description

[0037] Figure 1 This refers to the vacuum sputtering coating equipment described in the embodiments of the present invention. Detailed Implementation

[0038] The present invention will be further described in detail below with reference to experimental examples and specific embodiments. However, this should not be construed as limiting the scope of the above-mentioned subject matter of the present invention to the following embodiments; all technologies implemented based on the content of the present invention fall within the scope of the present invention.

[0039] Example 1

[0040] Figure 1 A vacuum sputtering coating apparatus according to an exemplary embodiment of the present invention includes: at least two composite coating chambers, with a first cooling chamber disposed between two adjacent composite coating chambers for reducing the temperature of a pressure-sensitive workpiece between the two adjacent composite coating chambers to below a temperature threshold; wherein the first cooling chamber is sealed to the two adjacent composite coating chambers. The temperature threshold is set to 80°C to 90°C.

[0041] Specifically, the composite coating chamber and the first cooling chamber have a channel through which the workpiece to be coated (in this embodiment, a varistor; the semi-finished varistor produced in the equipment is also called a varistor workpiece) can continuously pass. The first cooling chamber can employ one of various existing cooling technologies. Based on the characteristics of the sputtering process, this invention preferably configures the first cooling chamber to use molecular flow cooling technology (inert gas cooling technology, preferably nitrogen) to lower the temperature of the varistor workpiece below the temperature threshold. Furthermore, the first cooling chamber is connected to a vacuum generator. After the temperature of the varistor workpiece is lowered below the temperature threshold, the vacuum generator reduces the ambient vacuum level to the same level as, or slightly lower than, the vacuum level of the composite coating chamber. Once the vacuum level meets the requirements, the varistor workpiece is transferred from the corresponding channel of the first cooling chamber to the next composite coating chamber, thus achieving continuous sputtering. Further, in this embodiment, the composite coating chamber is used to sputter the conductive layer of the varistor; and the temperature of all the composite coating chambers is below a safe temperature threshold, which is 280℃~350℃. The composite coating chamber can be a single-sided sputtering chamber or a double-sided sputtering chamber; and a closed-loop magnetic field is provided in the composite coating chamber to increase the deposition rate, thereby shortening the sputtering time and improving processing efficiency. Considering that adding more sputtering chambers would increase the cost of the equipment, it is preferable to configure two composite coating chambers, as per reference. Figure 1 The vacuum sputtering coating equipment comprises, in sequence: a pre-evacuation chamber, a pre-sputtering chamber, a first composite coating chamber, a first cooling chamber, a second composite coating chamber, a post-transition chamber, and a second cooling chamber. All chambers in the vacuum sputtering coating equipment (except the composite coating chamber and the first cooling chamber) are connected using conventional sealed connections found in existing sputtering equipment. The pressure-sensitive workpiece, after being cooled in the second cooling chamber, can leave the vacuum environment and enter the air environment. The second cooling chamber is used to cool the pressure-sensitive workpiece to its oxidation protection temperature, which is typically set to 50°C to save time. In this embodiment, the second cooling chamber also employs inert gas molecular flow cooling technology.

[0042] Example 2

[0043] In a further embodiment of the present invention, a method for manufacturing a zinc oxide varistor is also provided, comprising: sealing and sputtering the electrode layer of the varistor in a vacuum environment; that is, processing the electrode layer of the varistor in the vacuum sputtering coating equipment provided in Embodiment 1; wherein the electrode layer comprises: a bottom transition layer and a conductive layer; the thickness of the conductive layer is d, and d∈(5,14]μm.

[0044] Specifically, in a vacuum or inert gas environment, the sputtering process is performed N times to achieve a conductive layer thickness d for the varistor, where N ≥ 2. A cooling step is added between adjacent conductive layer sputtering steps to cool the varistor to below a temperature threshold. The cooling step employs molecular flow cooling technology to lower the varistor temperature below the temperature threshold; the molecular flow is an inert gas, preferably nitrogen. After the varistor temperature drops below the temperature threshold, the vacuum level is reduced before proceeding to the next conductive layer sputtering step.

[0045] Table 1

[0046]

[0047] As shown in Table 1, the present invention employs seven different sets of process parameters to seal and sputter the electrode layer of the varistor in a vacuum environment to test the performance of the varistor produced by the method provided by the present invention. In processes 1 to 7, the pre-vacuum chamber of the vacuum sputtering coating equipment provided by the present invention is set to a pressure of 15 Pa, the vacuum chamber pressure is set to 5 Pa, the sputtering chamber (the chamber used for sputtering the underlying transition layer) is equipped with two nickel-chromium targets, a voltage of 400-500 V, and argon atom bombardment of the target material; the composite coating chamber is equipped with six pairs of copper targets, a voltage of 400-500 V, and argon atom bombardment of the target material; the second composite coating chamber is equipped with six pairs of copper targets, a voltage of 400-500 V, and nitrogen gas is used to cool the molecular flow.

[0048] Furthermore, a comparative experiment was conducted using the MYN15-621K zinc oxide varistor ceramic chip. This product has a chip diameter of approximately 13.5 mm and a conductive layer diameter of 11.2 mm. After soldering and encapsulation, the test products underwent initial electrical performance testing and were assessed for their ability to withstand 8 / 20 µs current surges. The 8 / 20 µs current surge was tested at 7 kA / cm², while the standard requirement for conventional products is generally 6 kA / cm². The initial leakage current was required to be no more than 20 µA, the varistor voltage change rate after the current surge should not exceed 10%, and there should be no visible damage. Comparative experimental data for the varistor products produced by processes 1 to 7 in Table 1 are shown in Tables 2 to 8 below:

[0049] Table 2:

[0050]

[0051] Table 3:

[0052]

[0053] Table 4:

[0054]

[0055] Table 5:

[0056]

[0057] Table 6:

[0058]

[0059] Table 7:

[0060]

[0061] Table 8:

[0062]

[0063]

[0064] The test results of the comparative groups in Tables 2 to 8 show that:

[0065] 1. When the thickness of the bottom transition layer exceeds 0.5µm, the product's impact resistance decreases. As shown in Table 4, after the impact, the encapsulation layer (the outermost epoxy resin encapsulation layer for moisture protection on the varistor) completely explodes, resulting in all products failing the appearance test. This is mainly due to the weak conductivity of the bottom nickel-chromium layer, leading to increased ohmic contact resistance and higher heat generation during high-current impacts.

[0066] 2. After the first cooling chamber reaches 160 degrees or above, the initial leakage current of the sputtered product is relatively large under the same process conditions, as shown in Table 3, which is basically greater than 20uA.

[0067] 3. When the temperature in the second cooling chamber exceeds 60 degrees Celsius, the product cools under natural conditions, causing rapid oxidation of the conductive layer surface, resulting in decreased conductivity and poor solderability. Table 8 shows that some products failed to meet appearance standards after impact.

[0068] 4. When increasing the coating thickness of a single chamber, the deposition rate needs to be reduced and the temperature rise slowed down. Sputtering thickness and time are not directly proportional. When the thickness of a single chamber reaches 5µm, the time for sputtering another 1µm thickness needs to be extended by at least 20 minutes. Some sputtered products have leakage current defects, as shown in Tables 6 and 7. The single product has stronger impact resistance, mainly because the resistance value decreases after the electrode layer thickness increases, and the product has a smaller change rate after current impact.

[0069] In summary, the varistor produced by process 1 has the highest efficiency and best product performance. Furthermore, it is preferable to have a base layer thickness of less than 0.5µm, a single-chamber sputtering thickness of less than 5µm, a cooling chamber temperature of less than 80 degrees Celsius, a second cooling chamber temperature of less than 55 degrees Celsius, and a conductive layer thickness of approximately 6–10µm.

[0070] Example 3

[0071] In a further embodiment of the present invention, a varistor manufactured using the zinc oxide material varistor manufacturing method described in Example 2 is also provided. The varistor includes an electrode layer, which comprises a conductive layer and a bottom transition layer. The bottom transition layer and the conductive layer are formed by a sputtering process, and the thickness of the conductive layer is 5 μm to 14 μm. The bottom transition layer is less than 0.5 μm and is made of nickel or chromium or their alloys. The conductive layer is made of copper.

[0072] Furthermore, the varistor includes: two electrode layers, a zinc oxide varistor ceramic substrate, and electrode leads. The electrode layers are respectively disposed on the upper and lower surfaces of the zinc oxide varistor ceramic substrate, and electrode leads are respectively disposed on the two electrode layers. Except for the electrode layers, the structure of the varistor is manufactured using conventional methods.

[0073] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for manufacturing a varistor made of zinc oxide material, characterized in that, include: The electrode layer of the varistor is sealed and sputtered in a vacuum environment; The electrode layer comprises: a bottom transition layer and a conductive layer; the thickness of the conductive layer is d, where d is 6μm to 14μm. In a vacuum environment, the sputtering process is performed N times to make the conductive layer of the varistor reach a thickness d, where N≥2; A cooling step is also included between two adjacent conductive layer sputtering steps, through which the temperature of the varistor is cooled to below the temperature threshold. The first sputtering step has a sputtering thickness of 3.1μm to 5μm, and the first cooling step cools the temperature of the varistor to below 80℃; the second sputtering step has a sputtering thickness of 3.1μm to 5μm, and the second cooling step cools the temperature of the varistor to below 55℃.

2. The method for manufacturing a varistor as described in claim 1, characterized in that, The thickness of the underlying transition layer does not exceed 0.5 μm.

3. The method for manufacturing a varistor as described in claim 2, characterized in that, The cooling step employs molecular flow cooling technology to cool the varistor to a temperature threshold or below that threshold.

4. The method for manufacturing a varistor as described in claim 3, characterized in that, The molecular flow is an inert gas.

5. The method for manufacturing a varistor as described in claim 4, characterized in that, The temperature threshold is 80℃~90℃.

6. The method for manufacturing a varistor as described in any one of claims 1 to 5, characterized in that, The varistor includes: two electrode layers, a zinc oxide varistor ceramic substrate, and electrode leads. The electrode layers are respectively disposed on the upper and lower surfaces of the zinc oxide varistor ceramic substrate, and electrode leads are respectively disposed on the two electrode layers.

7. The method for manufacturing a varistor as described in claim 6, characterized in that, The underlying transition layer material includes a first metal or its alloy, wherein the first metal includes nickel, chromium, or titanium. The material of the conductive layer includes a second metal or an alloy thereof, wherein the second metal includes copper, silver or nickel.

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