Non-destructive fluid self-assembly method for micro-nano devices and applications thereof

Abstract

This invention provides a non-destructive fluid self-assembly method for micro / nano devices and its application. The fluid self-assembly method includes: providing an assembly substrate bonded with solder; and performing fluid self-assembly on the solder-bonded assembly substrate using a liquid with an oxygen content below 1 ppm and a neutral pH as the assembly solution. The method provided by this invention is a non-destructive fluid self-assembly method. By controlling oxygen and using a pH-neutral assembly solution, it can solve the problem of device damage caused by acidic assembly solutions in existing technologies, while also achieving a higher assembly yield.

Description

Technical Field

[0001] This invention belongs to the field of micro-nano manufacturing technology, specifically relating to a non-destructive fluid self-assembly method for micro-nano devices and its application. Background Technology

[0002] Integrating micrometer-scale components onto patterned substrates is fundamental to the manufacturing of advanced electronic systems, a process crucial for achieving high-performance, low-power, and miniaturized electronic devices. With the continuous development of electronic technology, especially in fields such as high-performance computing, optical communication, semiconductors, biosensing, microLED (micro-light-emitting diode) display technology, flexible electronics, and microelectromechanical systems (MEMS), traditional integration methods are increasingly unable to meet the ever-demanding requirements. Against the backdrop of ever-increasing demands for miniaturization and high precision, how to efficiently and precisely assemble millions or even tens of millions of micrometer-scale devices onto a substrate has become one of the key challenges in achieving high integration and high performance. For example, microLEDs present significant technical challenges in micro- and nano-array assembly applications. MicroLEDs consist of regular arrays of micrometer-scale (typically no more than 50 μm) inorganic LED pixels, exhibiting significant advantages in image quality, lifespan, and luminous efficiency. During mass transfer, micrometer-scale chips need to be precisely transferred from the original growth wafer (such as sapphire, silicon carbide, etc.) to the final display backplane. MicroLED displays have extremely high requirements for manufacturing precision and reliability, and the mass transfer must meet extremely stringent standards, including assembling tens of millions of micrometer-sized chips on the display panel with ultra-high yield, precision and throughput.

[0003] Fluidic self-assembly (FSA) technology offers a high-throughput, high-precision, and cost-effective method for assembling large-area, high-density microdevices. This method utilizes long-range forces (typically fluid flow or surface tension) to guide micro-components to desired sites on a substrate and leverages localized forces to achieve permanent connections between components. Through these microscopic interactions, FSA enables parallel, high-precision self-alignment and self-assembly of micro-components, offering high transfer throughput and ease of operation, making it an effective method suitable for electronic device integration.

[0004] Currently, fluid self-assembly technology typically utilizes a dip soldering process based on the different wettability of low-melting-point solder with metals (such as Au, Cu, etc.) and substrates. Solder is selectively deposited uniformly onto metal pads, and then the dip-soldered substrate is placed into an assembly bottle filled with microelectronic components. Acid is added to remove oxides from the solder surface, and heating melts the solder. The flow of the solution drives the collision and bonding between the microelectronic components and the molten solder. Subsequently, the minimization of the surface energy of the droplet-shaped molten solder drives the self-alignment of the microelectronic components' position and angle. After the solder cools and solidifies, electrical interconnects are formed.

[0005] Current fluid self-assembly methods typically use acidic solutions as the fluid medium to prevent oxidation of low-melting-point solder alloys during assembly. The pH of these acidic solutions is usually between 2 and 3, and mild weak acids (such as acetic acid) are often chosen to prepare the assembly solution to minimize corrosion of the solder alloy. However, even with these acidic solutions, corrosion of the solder alloy is unavoidable during actual assembly. As assembly time increases, the volume of deposited solder alloy gradually decreases, affecting the self-alignment process after assembly. The solder alloy not only provides the electrical interconnection function of the final device, but its volume uniformity directly affects the conductivity uniformity and overall performance of the device. Furthermore, acidic conditions limit the extension of fluid self-assembly time and affect the assembly yield of extremely small electronic components. Summary of the Invention

[0006] To solve all or part of the above-mentioned technical problems, the present invention provides the following technical solutions: A first aspect of the present invention provides a non-destructive fluid self-assembly method for micro / nano devices, comprising: Provides an assembly substrate with solder incorporated; A liquid with an oxygen content of less than 1 ppm and a neutral pH value is used as the assembly fluid for fluid self-assembly on the assembly substrate with solder.

[0007] The method provided by this invention can avoid the oxidation of solder alloys by strictly controlling the oxygen content in the assembly solution; and by using a pH-neutral liquid as the assembly solution, it can solve the problem of corrosion of solder alloys by acidic solutions in the prior art. Therefore, the fluid self-assembly method provided by this invention can improve the assembly yield by extending the assembly time, and has significant advantages when assembling micro-components for extended periods, avoiding the problem in the prior art where the volume of the solder alloy gradually decreases with prolonged assembly time caused by acidic assembly solutions. Low-melting-point solder alloys can provide electrical interconnectivity for assembled devices. Based on the method of this invention, the morphology of the solder alloy can maintain integrity during long-term assembly, resulting in higher placement accuracy and better conductivity uniformity of the assembled electronic devices.

[0008] In some embodiments, the assembly solution is deoxygenated water.

[0009] In some embodiments, the method for preparing deoxygenated water includes: freezing deionized water and slowly thawing it under a low-pressure environment below -0.1 MPa to remove dissolved oxygen.

[0010] In some embodiments, the method for preparing the deoxygenated water includes: freezing deionized water and slowly thawing it in a low-oxygen environment with an oxygen content of less than 1 ppm to remove the dissolved oxygen.

[0011] In some embodiments, freezing deionized water specifically includes freezing deionized water at a temperature below 0°C until it is completely solidified.

[0012] In some embodiments, the slow melting includes melting frozen deionized water at room temperature and low pressure below -0.1 MPa for more than 12 hours.

[0013] In some embodiments, fluid self-assembly is performed using the assembly fluid in an environment with an oxygen content of less than 1 ppm.

[0014] In some embodiments, the solder is selectively bonded to the solder joints of the assembly substrate in an environment with an oxygen content of less than 1 ppm. Specific methods employed include, for example, dip soldering or coating (e.g., scraping), with dip soldering being preferred as it allows for uniform solder deposition and is highly efficient. The solder used in this invention can be any material known in the art, such as low-melting-point solder alloys, like solder paste.

[0015] In some embodiments, the thickness of the metal layer on the solder joints of the assembled substrate is 200 nm or more, preferably 200 nm to 300 nm, so that the solder deposited on the solder joints has excellent mechanical stability.

[0016] Oxygen-controlled dip soldering improves yield primarily because the yield of the dip soldering process is affected by the dewetting of the molten solder. Specifically, the contact angle of the molten solder is smaller (e.g., about 30°) in the substrate metal pad area and larger (e.g., about 150°) in the non-solder joint area. This high contact angle difference between the solder joint and non-solder joint areas is beneficial to improving dip soldering yield. In existing technologies, dip soldering in an oxygen-rich environment followed by the removal of solder oxides using an acidic solution can negatively impact the dewetting of the molten solder due to the difference in contact angle between the solder joint and non-solder joint areas. Furthermore, in an acidic environment, the acidic solution on the top surface of the molten solder can corrode the solder pulled out after dip soldering, leading to uneven solder deposition. Under oxygen-free conditions, the slippage of the molten solder along the contact line on the substrate surface is more controllable, and corrosion of the solder joint by the acidic solution can also be avoided.

[0017] In a typical embodiment, the fluid self-assembly method specifically includes: S1: The dip soldering method is used to selectively bond the solder to the solder joints of the assembly substrate; S2: Place the assembly substrate with solder on it in the assembly container, add the assembly liquid and the component to be assembled into the assembly container, heat the temperature to the temperature that melts the solder, and under disturbance conditions, align the component to be assembled with the solder joints on the assembly substrate and assemble them to obtain the assembled device. Steps S1 and S2 are both carried out in an environment with an oxygen content of less than 1 ppm.

[0018] For example, steps S1 and S2 can be performed in a glove box (e.g., a glove box under a nitrogen atmosphere) or a vacuum environment, or in a low-oxygen assembly environment where the oxygen content meets the above conditions. Methods for controlling the low oxygen content of the assembly environment can employ freeze-melt vacuum degassing, or other chemical deoxygenation methods, degassing membranes, and electrochemical deoxygenation methods.

[0019] In some embodiments, the assembly yield is improved by extending the assembly time of step S2.

[0020] In some embodiments, the assembly time in step S2 is more than 10 minutes, at which point the assembly yield is high.

[0021] In existing technologies, to minimize the corrosion of solder by acidic solutions, assembly time is typically less than 1 minute, resulting in low assembly yield. The method provided by this invention, however, does not have the problem of corrosive assembly solutions, and therefore can improve assembly yield by extending the assembly time.

[0022] In some embodiments, the non-destructive fluid self-assembly method further includes cleaning the assembly substrate. For example, the assembly substrate can be cleaned using ultraviolet ozone and / or plasma.

[0023] In some embodiments, the non-destructive fluid self-assembly method further includes cleaning the solder to at least remove oxides from the solder surface. Methods for cleaning the solder may include, for example, using flux or mechanical polishing to remove oxides from the solder surface. Fluxes include, for example, rosin flux, organic acid flux, and inorganic acid flux; organic acid fluxes include, for example, succinic acid and glutaric acid.

[0024] In some embodiments, the non-destructive fluid self-assembly method further includes: after assembly, using an ammonia solution to remove non-specifically adhered components to the assembled device. For example, the assembled device is placed in the aforementioned ammonia solution and shaken, which causes the non-specifically adhered components to detach. The pH value of the ammonia solution can be 11-12.

[0025] In some embodiments, the fluid self-assembly method further includes: encapsulating the assembled device, for example, using materials such as polyimide for encapsulation.

[0026] In some embodiments, the component to be assembled may be a silicon nitride, gallium nitride, or other similar material, or an electronic component such as a microLED.

[0027] For example, the fluid self-assembly method may include: (1) The substrate with patterned metal electrodes (i.e., the assembly substrate) and the low-melting-point solder alloy are cleaned, and then the substrate and solder alloy are placed in an environment with an oxygen content of less than 1 ppm (e.g., in a glove box with a nitrogen atmosphere). On the cleaned assembly substrate, the array of metal solder joint areas has similar surface energies, the low-melting-point solder has good wettability in the solder joint area, and there are no oxide residues on the surface of the cleaned solder alloy.

[0028] (2) After freezing the deionized water, it is slowly thawed at room temperature and low pressure to remove dissolved oxygen. This step is repeated several times to obtain deoxygenated water with an oxygen content of less than 1 ppm.

[0029] (3) In a glove box with an oxygen content of less than 1 ppm, the cleaned solder alloy is melted by heating, and the assembly substrate is placed in the molten solder for processing so that the solder is selectively and uniformly deposited on the solder joints of the assembly substrate. Then the assembly substrate is slowly removed from the molten solder.

[0030] (4) In an environment with an oxygen content of less than 1 ppm, the above-mentioned dip-soldered assembly substrate is placed in a container, deoxygenated water and the parts to be assembled are added to the container, and the temperature is heated to a temperature that can melt the solder alloy; the parts to be assembled are aligned with the solder joints on the assembly substrate by physical movement or external force and then assembled to obtain the assembled device.

[0031] (5) After assembly, remove the assembled device and place it in an ammonia solution and shake it to remove non-specific adhesions at non-solder joints by electrostatic repulsion. Finally, rinse with deionized water.

[0032] (6) Spin-coating polyimide onto the resulting assembled device for encapsulation.

[0033] A second aspect of the present invention provides the application of the fluid self-assembly method described in any of the technical solutions in microLED assembly.

[0034] Compared with the prior art, the present invention has at least some or all of the following beneficial effects: The fluid self-assembly method provided by the present invention is a non-destructive method. By strictly controlling the oxygen content and using a pH-neutral assembly solution, it solves the problem of device damage caused by acidic assembly solutions in the prior art, and can also improve the assembly yield by extending the assembly time. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a schematic flowchart of the non-destructive fluid self-assembly method according to Embodiment 1 of the present invention; Figure 2 These are assembly diagrams and microscope images of Embodiment 1 of the present invention at assembly times of 2 min, 4 min, and 10 min, respectively. Figure 3 The images are microscopic images (microscopic image scale bar is 200 μm) and SEM images (SEM image scale bar is 20 μm) of the low melting point solder alloy in Comparative Example 1 during assembly in a conventional acidic environment for 0-20 min. Figure 4 This is a microscope image of the assembly of Comparative Example 2 at an assembly time of 10 min (scale bar: 200 μm). Figure 5 These are microscope images (scale bar 200 μm) of Example 2 at assembly times of 1 min, 5 min, and 10 min. Figure 6 The images are microscope images of the substrate assembled in Example 1 before (left) and after treatment with an ammonia solution at pH 12 (right). Detailed Implementation

[0037] The invention will be more fully understood through the following detailed description, which should be read in conjunction with the accompanying drawings. Detailed embodiments of the invention are disclosed herein; however, it should be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, the specific functional details disclosed herein should not be construed as limiting, but rather as the basis for the claims and as intended to teach those skilled in the art to employ the representative basis of the invention in different ways in any suitable detailed embodiment.

[0038] In addition, unless otherwise specified, all raw materials used in the following embodiments can be purchased from the market or other sources, and all production and testing equipment used are known in the art, as are the testing methods used.

[0039] Example 1 This embodiment provides a non-destructive fluid self-assembly method for micro / nano devices. Figure 1This is a flowchart illustrating the method of this embodiment, which specifically includes the following steps: (1) The silicon substrate with patterned gold-plated solder joints (i.e., the assembly substrate, the thickness of the Au layer on the solder joints is 300nm) is cleaned. The cleaning steps include: placing the substrate in an acetone solution and sonicating for 5 min, then rinsing the substrate with a large amount of deionized water, then immersing the substrate in a piranha solution (the volume ratio of H2SO4 to H2O2 is 3:1), cleaning at 80°C for 5 min, and finally rinsing it clean with deionized water and drying it with a nitrogen gun.

[0040] (2) The low-melting-point solder used in this embodiment has the following composition: 32.5 wt% Bi, 16.5 wt% Sn, and 51 wt% In. The low-melting-point solder was cleaned by heating it to 90°C to melt it, and then adding a hydrochloric acid solution with a pH of 3 to rinse the surface of the molten solder. After the above cleaning steps, the metal solder joint area array on the substrate had similar surface energies, the low-melting-point solder had good wettability in the solder joint area, and no oxide residue remained on the cleaned solder surface.

[0041] (3) Place the cleaned substrate and solder in a glove box environment with a nitrogen atmosphere containing less than 1 ppm of oxygen, and perform dip soldering on the assembly substrate using the above-mentioned solder. Specifically, this includes: heating the above-mentioned solder to a molten state, placing the assembly substrate in the molten solder so that the solder selectively adheres to the solder joints of the assembly substrate, and then slowly pulling the substrate out of the molten solder.

[0042] (4) Freeze the deionized water at a temperature below 0 °C until it is completely solidified, and then maintain it at a low pressure below -0.1 MPa and room temperature for 12 h until the deoxygenated water is completely melted. During the slow melting process, the dissolved oxygen is discharged. Repeat this step 3 times to obtain deoxygenated water with an oxygen content of less than 1 ppm.

[0043] (5) In a glove box with a nitrogen atmosphere containing less than 1 ppm of oxygen, the assembly substrate treated in step (3) is placed in a container, and the silicon nitride chip to be assembled and the deoxygenated water prepared in step (4) are added to it. The mixture is heated to 90°C to melt the solder. Mechanical disturbance is applied to ensure that the silicon nitride chip and the position on the substrate with solder are in full contact, thereby completing the alignment and assembly to obtain the assembled device. The entire assembly time in this embodiment is 10 min.

[0044] (6) After assembly, remove the assembled device and shake it in an ammonia solution with pH=12 to remove non-specific adhesions at non-solder joints by using electrostatic repulsion. Finally, rinse with deionized water.

[0045] (6) Spin-coating polyimide onto the resulting assembled device for encapsulation.

[0046] Figure 2 This embodiment shows assembly diagrams and microscope images at assembly times of 2 min, 4 min, and 10 min (the fluid assembly result is shown as a disk-shaped microchip with a diameter of 45 μm and a thickness of 5 μm; the scale bar of the microscope image is 100 μm). Figure 2 As shown, in this embodiment of the invention, dip soldering is performed under oxygen-controlled conditions, and self-assembly is performed in an oxygen-controlled and pH-neutral environment. As the assembly time is extended, the number of effective collisions between electronic components and solder joints increases, thereby improving the yield to approximately 100%.

[0047] Figure 6 These are microscope images (left) and (right) of the substrate assembled in Example 1 before and after treatment with an ammonia solution at pH 12. Systematic research revealed that in the fluid assembly process based on simple perturbations, SiN… x The chip, being the main component, tends to adhere to non-solder joint areas of the substrate, a phenomenon known as non-specific adhesion. Smaller chips are more prone to non-specific adhesion. Even after assembly, rinsing with deionized water and isopropanol cannot remove all non-specifically adhered chips. This is because the attraction between the chip and the silicon substrate primarily stems from van der Waals forces and electrostatic attraction, with electrostatic attraction being dominant. Under neutral and weakly acidic conditions, the substrate and chip surfaces carry opposite charges; under the influence of electrostatic attraction, the chip adheres to the non-solder joint areas of the silicon wafer. By utilizing the fact that silicon oxide and silicon nitride carry the same surface charge under alkaline conditions, placing the assembled, not-completely-dried substrate (to reduce the influence of van der Waals forces) in an ammonia solution and simply stirring and shaking can remove non-specific adhesion.

[0048] Example 2 The only difference between Example 2 and Example 1 is that the Au layer thickness on the solder joints of the silicon substrate used in Example 2 is 100 nm. The rest is the same as in Example 1 and will not be described again here.

[0049] Besides the corrosion of deposited solder by acidic environments, which limits the extension of assembly time, maintaining the morphological stability of the solder during assembly is also crucial for extending assembly time and improving assembly yield. The property of solder to maintain a constant volume of deposited solder during prolonged mechanical shaking is called the mechanical stability of the solder. Systematic research has revealed that the thickness of the metal layer on the substrate plays a significant role in the mechanical stability of the solder. This invention found that when the Au layer on the substrate is 200-300 nm thick, the mechanical stability of the solder is good. However, when the Au layer thickness on the substrate is lower (e.g., in Example 2), the mechanical stability of the solder decreases compared to Example 1. Figure 5 These are microscope images from Example 2 taken during an assembly time of 1-10 minutes. Figure 5As shown, although neutral deoxygenated water was used as the assembly solution in Example 2, after extending the assembly time to 10 minutes, a large area of ​​solder deposited on the pads detached, affecting the solder's self-alignment capability and limiting the improvement of assembly yield. Therefore, in some preferred embodiments, the metal layer on the substrate's solder joints is made to be 200 nm or larger to ensure that the solder has good mechanical stability, thereby facilitating the improvement of assembly yield by extending the assembly time.

[0050] Comparative Example 1 The only difference between Comparative Example 1 and Example 1 is that Comparative Example 1 uses a hydrochloric acid solution with a pH of 3 as the assembly solution.

[0051] In Comparative Example 1, the acidic assembly solution caused severe corrosion to the solder, making it impossible to extend the assembly time to more than 10 minutes. Figure 3 The images shown are microscopic images (scale bar 200 μm) and SEM images (scale bar 20 μm) of the deposited solder alloy corroded in the acidic assembly solution during the assembly time of Comparative Example 1 from 0 to 20 min. It was found that the surface morphology of the solder deposited on pads with a size of 30 μm changed significantly after only immersion in the acidic environment for 5 min, adversely affecting the assembly results.

[0052] Comparative Example 2 The only difference between Comparative Example 2 and Example 1 is that Comparative Example 2 uses deionized water that has not undergone deoxygenation treatment as the assembly solution. The rest of the procedures are the same as in Example 1, and will not be repeated here.

[0053] Figure 4 Microscopic images (scale bar 200 μm) of the assembly process in Comparative Example 2 at an assembly time of 10 min are shown. The assembly yield of Comparative Example 2 at 10 min was only 6.4%. Comparative Example 2 did not strictly control the oxygen content in the assembly environment, which led to the oxidation of the solder, thus significantly reducing the assembly yield under the same assembly time.

[0054] In summary, the fluid self-assembly method provided by this invention is a non-destructive method. By strictly controlling the oxygen content (including the oxygen content of the assembly solution, the oxygen content during the dip soldering process, and the oxygen content during the self-assembly process) and using a pH-neutral assembly solution, it not only solves the problem of device damage caused by acidic assembly solutions in the prior art, but also improves the assembly yield.

[0055] In addition, the inventors of this case also conducted experiments with other raw materials, process operations, and process conditions described in this specification, referring to the aforementioned embodiments, and obtained relatively ideal results in all cases.

[0056] All aspects, embodiments, features, and examples of this invention should be considered illustrative and used to explain and illustrate the invention, but not to limit the invention. The scope of the invention is defined only by the claims.

[0057] Although the invention has been described with reference to illustrative embodiments, those skilled in the art will understand that various other changes, omissions, and / or additions can be made without departing from the spirit and scope of the invention, and that elements of the described embodiments can be substituted with substantially equivalents. Furthermore, many modifications can be made without departing from the scope of the invention to adapt particular situations or materials to the teachings of the invention. Therefore, this invention is not intended to be limited to the specific embodiments disclosed for carrying out the invention, but rather is intended to encompass all embodiments falling within the scope of the appended claims.

Claims

1. A non-destructive fluid self-assembly method for micro / nano devices, characterized in that, include: Provides an assembly substrate with solder incorporated; A liquid with an oxygen content of less than 1 ppm and a neutral pH value is used as the assembly fluid for fluid self-assembly on the assembly substrate with solder.

2. The non-destructive fluid self-assembly method according to claim 1, characterized in that: The assembly solution is deoxygenated water.

3. The non-destructive fluid self-assembly method according to claim 2, characterized in that, The method for preparing deoxygenated water includes: freezing deionized water and slowly thawing it under a low pressure environment below -0.1 MPa to remove dissolved oxygen.

4. The fluid self-assembly method according to claim 3, characterized in that, Freezing deionized water specifically includes freezing deionized water at a temperature below 0°C until it is completely solidified. And / or, the slow melting includes: melting frozen deionized water at room temperature and low pressure below -0.1 MPa for more than 12 hours.

5. The non-destructive fluid self-assembly method according to any one of claims 1-4, characterized in that: Fluid self-assembly is performed using the assembly fluid in an environment with an oxygen content of less than 1 ppm.

6. The non-destructive fluid self-assembly method according to any one of claims 1-4, characterized in that: In an environment with an oxygen content of less than 1 ppm, the solder is selectively bonded to the solder joints of the assembly substrate. Preferably, the thickness of the metal layer on the solder joints of the assembled substrate is 200 nm or more, and more preferably 200 nm to 300 nm.

7. The non-destructive fluid self-assembly method according to any one of claims 1-4, characterized in that, Specifically, it includes: S1: The dip soldering method is used to selectively bond the solder to the solder joints of the assembly substrate; S2: Place the assembly substrate with solder on it in the assembly container, add the assembly liquid and the component to be assembled into the assembly container, heat the temperature to the temperature that melts the solder, and under disturbance conditions, align the component to be assembled with the solder joints on the assembly substrate and assemble them to obtain the assembled device. Steps S1 and S2 are both carried out in an environment with an oxygen content of less than 1 ppm.

8. The non-destructive fluid self-assembly method according to claim 7, characterized in that: The assembly yield can be improved by extending the assembly time of step S2. Preferably, the assembly time of step S2 is 10 minutes or more.

9. The non-destructive fluid self-assembly method according to claim 7, characterized in that, Also includes: The assembly substrate and the solder are cleaned; And / or, the non-destructive fluid self-assembly method further includes: after assembly, using an ammonia solution to remove non-specifically adhered parts to be assembled on the assembled device. And / or, the non-destructive fluid self-assembly method further includes: encapsulating the assembled device.

10. The application of the non-destructive fluid self-assembly method according to any one of claims 1-9 in microLED assembly.

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