High-yield thermocompression welding method based on direct laser mass transfer process

Abstract

The invention relates to the technical field of display screen preparation, in particular to a high-yield thermal compression welding method based on a direct laser mass transfer process, which comprises the following steps: pasting a resin bearing adhesive film on a TFT (Thin Film Transistor) back plate, presetting a metal welding spot on a Micro-LED (Light Emitting Diode) chip, transferring the Micro-LED chip from a sapphire substrate to the resin bearing adhesive film of the TFT back plate by using the laser mass transfer process, iCP etching is carried out, a flexible lamination is arranged on one side of the sapphire pressing sheet and is aligned with the Micro-LED chip, and a graphite sheet is arranged on one side, far away from the Micro-LED chip, of the TFT backboard; and after welding is finished, cooling is conducted to the room temperature, soaking is conducted, and the Micro-LED display screen is obtained. By the adoption of the steps, the problems that in the prior art, a Micro-LED chip deviates, residues are left after welding, and welding fails due to warping of a back plate are solved, and therefore the high-yield Micro-LED display screen is obtained.

Description

Technical Field

[0001] This invention relates to the field of display screen manufacturing technology, and in particular to a high-yield hot-press welding method based on direct laser mass transfer technology. Background Technology

[0002] Micro-LED, with its advantages of self-emission, high brightness, high contrast, long lifespan, and stable operation in high-temperature and high-humidity environments, is considered the most promising next-generation display technology after LCD and OLED, and has broad market prospects in AR / VR, wearable devices, automotive displays, and ultra-large-size splicing screens. However, the commercialization of Micro-LED is still limited by the mass transfer process: a single display screen requires the picking, alignment, and welding of tens of millions of light-emitting chips, and places extremely high demands on positional accuracy, yield, and cost control.

[0003] Currently, the industry commonly employs a multi-step indirect transfer route using laser lift-off (LLO) and laser-induced forward transfer (LIFT). This route first uses a laser to lift the Micro-LED chip (Chips on Wafer, CoW) from the sapphire substrate to a first temporary carrier; then, a laser is used again to transfer the chip from the temporary carrier to a second temporary carrier; finally, electrical connections are achieved through thermopressing or laser welding. This multi-step transfer not only increases equipment and time investment but also introduces problems such as multiple accumulations of alignment errors and chip flipping / missing, severely hindering large-scale mass production.

[0004] In theory, the traditional transfer process can be compressed into a single step, significantly reducing cycle time and chip damage. However, most existing research remains at the proof-of-concept stage, with major bottlenecks concentrated in three areas: First, the laser energy window is narrow; insufficient energy prevents the chip from completely detaching, while excessive energy can easily cause cracks or metal electrode ablation. Second, the chip and backplane electrodes require subsequent welding to form a reliable electrical connection, but large-area backplanes warp at high temperatures due to the mismatch in thermal expansion coefficients of the various layers, leading to some electrodes having poor solder joints or open circuits. Therefore, achieving high-yield, high-precision transfer and high-flatness, high-reliability welding on large-size TFT backplanes remains a core challenge that must be overcome before Micro-LED displays can achieve mass production. Summary of the Invention

[0005] The purpose of this invention is to provide a high-yield hot-press welding method based on direct laser mass transfer technology, which solves the problems of Micro-LED chip misalignment, post-weld residue, and welding failure caused by backplane warping in the prior art, thereby achieving high-yield, high-resolution Micro-LED display manufacturing.

[0006] To achieve the above objectives, the present invention provides a high-yield hot-press welding method based on direct laser mass transfer technology, comprising the following steps: S1. Clean the sapphire substrate with Micro-LED chips and the TFT backplane, and dry them with a nitrogen gun to obtain a clean sapphire substrate with Micro-LED chips and a clean TFT backplane for later use. S2. A resin adhesive film is attached to the clean TFT backplane obtained in S1. Metal solder joints are pre-set on the clean Micro-LED chip obtained in S1. The Micro-LED chip is transferred from the sapphire substrate to the resin adhesive film of the TFT backplane using a laser mass transfer process to obtain a TFT backplane with a Micro-LED chip. S3. Perform ICP etching on the TFT backplate with Micro-LED chip obtained in S2 to remove the resin adhesive film outside the area where the Micro-LED chip is located, and obtain the etched TFT backplate. S4. A flexible stack and a sapphire sheet are sequentially disposed on the side of the TFT backplate etched in S3 that is close to the Micro-LED chip. A graphite sheet is disposed on the side of the TFT backplate etched in S3 that is far away from the Micro-LED chip, thus obtaining the assembled structure to be welded. S5. Place the structure to be welded in S4 into a wafer bonding machine, evacuate, pressurize and heat, cool to room temperature after welding, remove the sapphire wafer and graphite sheet to obtain the display screen, soak the display screen in ethyl acetate to obtain a clean Micro-LED display screen.

[0007] Preferably, in S1, the cleaning is performed using acetone and ethanol in sequence.

[0008] Preferably, in S2, the thickness of the resin adhesive film is 2-5 μm.

[0009] Preferably, in S2, the laser energy density of the laser mass transfer process is 900-1200 mJ / cm². 2 .

[0010] Preferably, in S2, the distance between the Micro-LED chip and the TFT backplane before transfer is 50-100μm.

[0011] Preferably, in S3, the etching gas for ICP etching is Ar and O2 in a volume ratio of 4:1.

[0012] Preferably, in S4, the flexible stack includes an adhesive layer, an elastic layer, and an insulating layer arranged sequentially along the direction away from the sapphire sheet.

[0013] Preferably, the adhesive layer is a polydimethylsiloxane alkyl agent; the elastic layer is a polyimide adhesive; and the barrier layer is a Teflon film.

[0014] Preferably, in step S5, the vacuum is evacuated to below 1×10⁻⁶. -3 Apply pressure to 4000-6000N and maintain for 8-15 minutes, then heat to 250-300℃ and maintain for 8-15 minutes.

[0015] Preferably, in S5, the sample is soaked in ethyl acetate for 15-20 minutes.

[0016] Therefore, the high-yield hot-press welding method based on direct laser mass transfer technology, which employs the above steps, has the following advantages: 1. This invention simplifies the multi-step transfer process of traditional transfer technology into a single step by using direct laser transfer technology, greatly simplifying the transfer process, improving transfer efficiency, and enabling Micro-LED chip transfer accuracy to reach ±1.5μm, achieving high-precision transfer and fully meeting current industrialization needs; 2. This invention improves the warping problem caused by the mismatch of thermal expansion coefficients of the materials in each layer under the high temperature of hot-press welding of large-area backplates through ICP etching process and the introduction of flexible stacked structure, thereby improving the welding yield.

[0017] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of a high-yield hot-press welding method based on direct laser mass transfer technology according to the present invention, wherein... Figure 1 (a) is a schematic diagram of the resin bonding film applied to the TFT backplane; Figure 1 (b) is a schematic diagram of the Micro-LED chip being transferred from the sapphire substrate to the TFT backplane; Figure 1 (c) in the diagram is a schematic diagram of the ICP etching resin substrate; Figure 1 (d) in the diagram is a schematic of thermo-press welding; Figure 2 This is a schematic diagram of the welding structure in Embodiment 1 of the present invention; Figure 3 This is a schematic diagram of the combined structure after transfer in S2 of Embodiment 1 of the present invention, wherein... Figure 3 (a) in the image is the overall top view after the transfer. Figure 3 (b) in the middle is Figure 3 A magnified view of (a) in the image. Figure 3 (c) in the figure is a side cross-sectional view of a single Micro-LED chip after transfer; Figure 4This is a topographic image of the ICP etching process in S3 of Embodiment 1 of the present invention; Figure 5 This is a schematic diagram of the combination of Micro-LED displays in Embodiment 1 and Comparative Examples 1-2 of the present invention, wherein... Figure 5 Image (a) in the diagram is an enlarged view of the Micro-LED display screen in Comparative Example 1. Figure 5 Image (b) is an enlarged view of the Micro-LED display screen in Comparative Example 2. Figure 5 (c) in the figure is a magnified view of the Micro-LED display screen in Example 1. Detailed Implementation

[0019] The present invention will be further described below with reference to the accompanying drawings and embodiments. Unless otherwise defined, the technical or scientific terms used in this invention should be understood in their ordinary sense by those skilled in the art. The features mentioned above or in the specific examples mentioned in this invention can be combined arbitrarily, and these specific embodiments are only used to illustrate the invention and are not intended to limit the scope of the invention.

[0020] This invention provides a high-yield hot pressing welding method based on direct laser mass transfer technology, comprising the following steps: S1. Clean the sapphire substrate with Micro-LED chips and the TFT backplane, and dry them with a nitrogen gun to obtain a clean sapphire substrate with Micro-LED chips and a clean TFT backplane for later use.

[0021] S2. A resin adhesive film is attached to the clean TFT backplane obtained in S1. Metal solder joints are pre-set on the clean Micro-LED chip obtained in S1. The Micro-LED chip is transferred from the sapphire substrate to the resin adhesive film of the TFT backplane using a laser mass transfer process, thereby obtaining a TFT backplane with a Micro-LED chip.

[0022] S3. Perform ICP etching on the TFT backplate with Micro-LED chip obtained in S2 to remove the resin adhesive film outside the area where the Micro-LED chip is located, and obtain the etched TFT backplate.

[0023] S4. A flexible laminate and a sapphire sheet are sequentially disposed on the side of the TFT backplane closest to the Micro-LED chip after S3 etching. A graphite sheet is disposed on the side of the TFT backplane furthest from the Micro-LED chip after S3 etching, resulting in the assembled structure to be soldered. The graphite sheet has high thermal conductivity, which can quickly disperse heat in the soldering area, reduce local thermal gradients, and further suppress warping. The sapphire sheet is rigid, and when combined with the flexible laminate, it can evenly distribute pressure across the entire area, avoiding excessive or insufficient local pressure.

[0024] S5. Place the structure to be welded in S4 into a wafer bonding machine, evacuate, pressurize and heat, cool to room temperature after welding, remove the sapphire wafer and graphite sheet to obtain the display screen, soak the display screen in ethyl acetate to obtain a clean Micro-LED display screen.

[0025] In some embodiments of the present invention, in S1, acetone and ethanol are used sequentially for cleaning to remove organic contaminants and particulate matter from the surface.

[0026] In some embodiments of the present invention, in step S2, the thickness of the resin receiving film is 2-5 μm. The resin receiving film has high adhesion and energy absorption capacity, effectively receiving the high-speed incoming Micro-LED chip, preventing it from bouncing, shifting, or flipping, and absorbing impact to avoid damage to the Micro-LED chip. If the resin receiving film is too thick, it will affect the subsequent soldering yield, making it difficult for the Micro-LED chip electrodes to pierce the resin receiving film and contact the TFT backplane electrodes; if it is too thin, it will affect the receiving effect during transfer.

[0027] In some embodiments of the present invention, in S2, the laser energy density of the laser mass transfer process is 900-1200 mJ / cm². 2 The laser mass transfer process utilizes the Coherent UV Transfer system, with a core light source of a 248nm KrF excimer laser. The pulse width is set to 20ns and the maximum frequency to 20Hz. This system is equipped with a beam shaping module, mask, and projection lens to ensure uniform laser spot coverage of the target chip area. The GaN-layer Micro-LED chip operates at 900mJ / cm². 2 It will detach and release on both sides, 1200mJ / cm 2 This represents the upper limit of the system's energy density.

[0028] Laser mass transfer technology utilizes the characteristic that the energy of 248nm laser photons lies between the band gaps of GaN and sapphire to achieve selective photothermal absorption. The GaN layer selectively absorbs the laser energy, causing the GaN-sapphire interface to instantly heat up to over 1000℃. GaN decomposes into liquid Ga and N2 gas, and the resulting high-pressure nitrogen gas creates a recoil pressure. This recoil pressure exceeds the van der Waals force between the Micro-LED chip and the sapphire substrate, pushing the Micro-LED chip detach from the sapphire substrate and directly onto the TFT backplane, where it is captured by the resin adhesive film, completing the one-step transfer. This process enables the simultaneous transfer of multiple Micro-LED chips, improving mass transfer efficiency.

[0029] In some embodiments of the present invention, in step S2, the distance between the Micro-LED chip and the TFT backplane before transfer is 50-100 μm. This distance setting facilitates improved positioning accuracy during mass transfer, ensuring precise alignment between the Sn solder joints on the Micro-LED chip and the pads on the TFT backplane.

[0030] In some embodiments of the present invention, in S3, the etching gas for ICP etching is Ar and O2 in a volume ratio of 4:1. ICP etching utilizes inductively coupled plasma to generate highly reactive oxygen free radicals (from O2), which react chemically with the resin film to etch it away. Ar acts as a physical bombardment, breaking chemical bonds and removing reaction products. The ICP etching operation only etches away the resin substrate not obscured by the Micro-LED chip, thereby reducing the overall thickness of the resin substrate while retaining the necessary resin substrate as an adhesion layer beneath the Micro-LED chip.

[0031] In some embodiments of the present invention, in step S4, the flexible stack includes an adhesive layer, an elastic layer, and an insulating layer sequentially disposed along the direction away from the sapphire sheet. The flexible stack adaptively deforms under pressure, ensuring that even if the underlying TFT backplane is warped, it can still conform to the minute irregularities on the surface of the Micro-LED chip. This ensures that the Sn solder joints of each Micro-LED chip receive uniform pressure and temperature, reducing the breakage rate of the Micro-LED chip during the soldering process and ensuring smooth peeling of the sapphire sheet after soldering. This fundamentally solves the problems of poor soldering and open circuits caused by poor contact.

[0032] In some embodiments of the present invention, the adhesive layer is a polydimethylsiloxane (PDMS) base agent. The adhesive layer is firmly bonded to the sapphire sheet, ensuring that the flexible laminate does not detach.

[0033] The elastic layer is made of polyimide (PI) adhesive, which has excellent elastic recovery and high temperature resistance. Its function is to buffer pressure and absorb and compensate for the stress caused by the mismatch of thermal expansion coefficients between the TFT backplane and the Micro-LED chip, so that the pressure is evenly transmitted to each Micro-LED chip and avoids local overload.

[0034] The insulating layer is a PTFE Teflon film, which has extremely low surface energy, prevents adhesion, is resistant to high temperatures and chemically inert. It can effectively block air from entering the solder joint area to prevent solder joint oxidation, and does not react with any materials, ensuring the cleanliness of the Micro-LED chip and solder joint, while preventing the rest of the flexible stack from contaminating the Micro-LED chip.

[0035] In some embodiments of the present invention, in step S5, the vacuum is evacuated to below 1 × 10⁻⁶. -3To prevent oxidation of the Sn solder joints at high temperatures, which would severely hinder soldering and lead to poor soldering, pressure is applied to 4000-6000N and held for 8-15 minutes, followed by heating to 250-300℃ and holding for another 8-15 minutes. The 4000-6000N pressure ensures tight contact between the Sn solder joints of the Micro-LED chip and the TFT backplane, breaking the surface oxide film and promoting metal atom diffusion. The 8-15 minute holding time provides sufficient time for atomic diffusion, forming a stable metallic bond. Heating above the melting point of the Sn solder joints melts them and forms a stable intermetallic compound with the TFT backplane under pressure, achieving reliable ohmic contact and mechanical connection. This reduces slippage of the Micro-LED chip during high-temperature soldering, improving soldering yield.

[0036] In some embodiments of the present invention, in step S5, ethyl acetate is used to soak the resin-coated film for 15-20 minutes to decompose and remove it.

[0037] Example 1 S1. The sapphire substrate with Micro-LED chips and the TFT backplane are cleaned with acetone and ethanol, and then dried with a nitrogen gun to obtain a clean sapphire substrate with Micro-LED chips and a clean TFT backplane for later use.

[0038] S2, such as Figure 1 As shown in (a), a 3.5μm resin adhesive film is applied to the clean TFT backplane obtained in S1, and Sn metal solder joints are pre-set on the clean Micro-LED chip obtained in S1. A Coherent UV Transfer system is used, with a 248nm KrF excimer laser as the core light source, a pulse width of 20ns, and a maximum frequency of 20Hz. The system is equipped with a beam shaping module, a mask, and a projection lens to ensure that the laser spot uniformly covers the target chip area. Figure 1 As shown in (b), a laser mass transfer process is used to transfer the Micro-LED chip from the sapphire substrate onto the resin adhesive film of the TFT backplane. The laser energy density is 1150 mJ / cm². 2 Before the transfer, the distance between the Micro-LED chip and the TFT backplane was 100μm, resulting in a TFT backplane with a Micro-LED chip, as shown below. Figure 3 As shown.

[0039] S3, such as Figure 1 As shown in (c), the TFT backplane with Micro-LED chips obtained in S2 is subjected to ICP etching. The etching gases are O2 and Ar in a volume ratio of 1:4. The resin adhesive film outside the area where the Micro-LED chips are located is removed to obtain the etched TFT backplane, as shown in (c). Figure 4 As shown.

[0040] S4, such as Figure 2 As shown, a flexible stack and a sapphire sheet are sequentially arranged on the side of the TFT backplate after S3 etching close to the Micro-LED chip, and a graphite sheet is arranged on the side of the TFT backplate after S3 etching away from the Micro-LED chip, resulting in an assembled structure to be soldered.

[0041] The flexible laminate comprises an adhesive layer, an elastic layer, and a barrier layer sequentially arranged along the direction away from the sapphire substrate. The adhesive layer is a PDMS base; the elastic layer is a PI adhesive; and the barrier layer is a Teflon film.

[0042] S5, such as Figure 1 As shown in (d), the structure to be welded in S4 is placed in the wafer bonding machine and the vacuum is evacuated to below 1×10⁻⁶. -3 The pressure was increased to 5000 N and held for 10 minutes, followed by heating to 260°C and holding for 10 minutes. After soldering, the surface was cooled to room temperature, and the sapphire wafer and graphite sheet were removed to obtain the display screen. The display screen was then immersed in ethyl acetate for 15-20 minutes to obtain a clean Micro-LED display screen. Figure 5 As shown.

[0043] Comparative Example 1 The difference between this comparative example and Example 1 is that the pressurization followed by heating in S4 and S5 is not performed; instead, simultaneous heating and pressurization are performed. All other steps are the same as in Example 1, resulting in a Micro-LED display screen.

[0044] Comparative Example 2 The difference between this comparative example and Example 1 is that S4 was not performed; all other steps are the same as in Example 1, resulting in a Micro-LED display screen.

[0045] Test case The Micro-LED displays obtained in Example 1 and Comparative Examples 1-2 are as follows: Figure 5 As shown, it can be seen that, compared with Comparative Example 1, Comparative Example 2 has a better light-emitting effect in the central area, but there are still many unlit Micro-LED chips in the edge area. This indicates that the bonding strategy of applying pressure first and then heating can further improve the bonding effect. Compared with Comparative Example 2, the display screen welded by the strategy of applying pressure first and then heating and using flexible stacking in Example 1 has the most uniform illumination pattern and the best light-emitting effect. This shows that under these conditions, the welding defects caused by warping can be effectively improved, ensuring the normal light-emitting and display performance of the display screen.

[0046] Therefore, the present invention provides a high-yield hot-press welding method based on direct laser mass transfer technology using the above steps. By using direct laser transfer technology, the multi-step transfer process of traditional transfer technology is simplified to a one-step transfer, which greatly simplifies the transfer process, improves the transfer efficiency, and enables chip transfer accuracy to reach ±1.5μm, achieving high-precision transfer and fully meeting the current industrialization needs.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A high-yield hot pressing welding method based on direct laser mass transfer technology, characterized in that: Includes the following steps: S1. Clean the sapphire substrate with Micro-LED chips and the TFT backplane, and dry them with a nitrogen gun to obtain a clean sapphire substrate with Micro-LED chips and a clean TFT backplane for later use. S2. A resin adhesive film is attached to the clean TFT backplane obtained in S1. Metal solder joints are pre-set on the clean Micro-LED chip obtained in S1. The Micro-LED chip is transferred from the sapphire substrate to the resin adhesive film of the TFT backplane using a laser mass transfer process to obtain a TFT backplane with a Micro-LED chip. S3. Perform ICP etching on the TFT backplate with Micro-LED chip obtained in S2 to remove the resin adhesive film outside the area where the Micro-LED chip is located, and obtain the etched TFT backplate. S4. A flexible stack and a sapphire sheet are sequentially disposed on the side of the TFT backplate etched in S3 that is close to the Micro-LED chip. A graphite sheet is disposed on the side of the TFT backplate etched in S3 that is far away from the Micro-LED chip, thus obtaining the assembled structure to be welded. S5. Place the structure to be welded in S4 into a wafer bonding machine, evacuate, pressurize and heat, cool to room temperature after welding, remove the sapphire wafer and graphite sheet to obtain the display screen, soak the display screen in ethyl acetate to obtain a clean Micro-LED display screen.

2. The high-yield hot pressing welding method based on direct laser mass transfer technology according to claim 1, characterized in that: In S1, acetone and ethanol are used for cleaning in sequence.

3. The high-yield hot pressing welding method based on direct laser mass transfer technology according to claim 1, characterized in that: In S2, the thickness of the resin adhesive film is 2-5 μm.

4. The high-yield hot pressing welding method based on direct laser mass transfer technology according to claim 1, characterized in that: In S2, the laser energy density of the laser mass transfer process is 900-1200 mJ / cm². 2 .

5. The high-yield hot pressing welding method based on direct laser mass transfer technology according to claim 1, characterized in that: In S2, the distance between the Micro-LED chip and the TFT backplane before transfer is 50-100μm.

6. The high-yield hot pressing welding method based on direct laser mass transfer technology according to claim 1, characterized in that: In S3, the etching gas for ICP etching is Ar and O2 in a volume ratio of 4:

1.

7. A high-yield hot-press welding method based on direct laser mass transfer technology according to claim 1, characterized in that: In S4, the flexible stack includes an adhesive layer, an elastic layer, and an insulating layer arranged sequentially along the direction away from the sapphire sheet.

8. A high-yield hot-press welding method based on direct laser mass transfer technology according to claim 7, characterized in that: The adhesive layer is a polydimethylsiloxane alkyl agent; the elastic layer is a polyimide adhesive; and the barrier layer is a Teflon film.

9. A high-yield hot pressing welding method based on direct laser mass transfer technology according to claim 1, characterized in that: In S5, the vacuum is evacuated to below 1×10⁻⁶. -3 Apply pressure to 4000-6000N and maintain for 8-15 minutes, then heat to 250-300℃ and maintain for 8-15 minutes.

10. A high-yield hot pressing welding method based on direct laser mass transfer technology according to claim 1, characterized in that: In S5, soak in ethyl acetate for 15-20 minutes.

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