Method for stripping ultrathin polyimide film by carbon nanotube assisted low-threshold laser

Abstract

The invention relates to a method for stripping an ultra-thin polyimide film through carbon nano tube assisted low-threshold laser, and belongs to the field of flexible electronic manufacturing. The preparation method comprises the following steps: firstly, depositing a carbon nanotube layer with the concentration of 5-50 nm and the concentration of 0.005-0.05 mg / mL on the surface of glass, and drying at the temperature of 50-120 DEG C; then 10-15 wt% of a polyimide precursor is coated, and a 300-500 nm ultrathin PI film is obtained through stepped heating curing; then single-time scanning is carried out with laser of 100-130 mJ / cm < 2 > and 355 nm to complete stripping, and the threshold value is smaller than or equal to 120 mJ / cm < 2 >; and finally, carrying out water-based ultrasonic treatment for 30-60 seconds to remove residual CNT to obtain the clean low-stress PI film with residual strain less than or equal to 0.3%. CNT high ultraviolet absorption and ultrahigh thermal conductivity cooperate to reduce the interface adhesion energy, bubbles are flattened, the energy is reduced by more than or equal to 30%, and the performance degradation of the device is less than 5%. The invention further discloses a composite substrate composed of the glass / CNT / PI / functional layer, and the composite substrate is suitable for a temporary substrate of a flexible OLED, a wearable monitor or a Micro-LED with the length smaller than or equal to 50 m and has the advantages of high yield, low energy consumption and reel-to-reel compatibility.

Description

Technical Field

[0001] This invention relates to the field of flexible electronic device manufacturing technology, specifically to a method for carbon nanotube-assisted low-threshold laser ablation of ultrathin polyimide films, a composite substrate structure made by the method, and its use in flexible OLEDs, wearable monitoring patches, or Micro-LED mass transfer temporary substrates. Background Technology

[0002] In recent years, with the rapid growth in demand for flexible electronic products, polyimide (PI) films have become the preferred substrate for flexible OLED displays and other flexible electronic devices due to their excellent thermal stability, mechanical flexibility, and chemical stability. PI films can maintain their performance during high-temperature processes and maintain structural integrity under repeated bending and rolling conditions, meeting the stringent requirements of emerging applications such as wearable devices and foldable displays for substrate materials.

[0003] In the mass production process of flexible OLEDs, a PI precursor must first be coated onto a rigid glass substrate and cured at high temperature to form an ultrathin PI film with a thickness of ≤1 μm. After completing processes such as TFT and OLED evaporation, laser lift-off (LLO) technology is used to separate the PI film from the glass substrate. However, for ultrathin PI films with a thickness of only 300–500 nm, the traditional LLO process has the following drawbacks: High laser energy threshold: peeling threshold at 355 nm wavelength ≈ 180 mJ / cm 2 The temperature is far above the thermal decomposition threshold of the PI film, leading to carbonization of the PI surface, thermal failure of the device, or the formation of plastic wrinkles, resulting in a decrease in yield. Concentrated heat accumulation: Excessive single-pulse energy narrows the longitudinal thermal decomposition zone at the peeling interface, resulting in sharp blister morphology and concentrated mechanical strain, which can easily lead to film rupture or device cracking. Complex process and high cost: Existing technologies reduce thermal damage by introducing sacrificial layers (such as α-GaOx) or multi-step scanning methods, but this increases the thin film deposition, patterning and cleaning processes, significantly increasing equipment investment and material costs, and has poor compatibility with roll-to-roll (R2R) production lines. Narrow process window: Laser energy fluctuations of ±5% can result in peeling residue or overheating, leading to insufficient stability in mass production.

[0004] Therefore, there is an urgent need in the field for a laser lift-off solution that features single-scan operation, low energy, low thermal-mechanical damage, a wide process window, and compatibility with existing 355 nm lasers, in order to achieve efficient and high-yield lift-off of ultrathin PI films. Summary of the Invention

[0005] To address the problems in the prior art, this invention provides a method for carbon nanotube-assisted low-threshold laser exfoliation of ultrathin polyimide films.

[0006] The technical solution adopted by this invention to solve its technical problem is: a method for carbon nanotube-assisted low-threshold laser exfoliation of ultrathin polyimide films, comprising the following steps: (a) forming a disordered or ordered carbon nanotube layer on the exfoliation surface of a glass substrate, wherein the thickness of the carbon nanotube layer is 5–50 nm, the areal density is 0.005–0.05 mg / mL based on the equivalent deposition amount, and the solvent is a dispersion of deionized water + 0.1 wt% SDS or IPA, which is then spin-coated, sprayed, or roll-to-roll coated and dried at 50–120 °C; (b) coating the carbon nanotube layer with a polyimide precursor having a solid content of 10–15 wt% and curing it by stepwise temperature increase to obtain an ultrathin polyimide film with a thickness of 300–500 nm and a total thickness ≤1000 nm; (c) applying the film with a single pulse energy density of 100–130 mJ / cm². 2 A single scan with a 355 nm laser pulse width of 15–30 ns was used to achieve complete peeling of the ultrathin polyimide film from the glass substrate under atmospheric or low vacuum conditions, stage temperature of 20–80 °C, and spot overlap of 20–40%, with a peeling threshold ≤120 mJ / cm. 2 (d) After peeling, residual carbon nanotubes are removed by ultrasonication with deionized water for 30–60 s, followed by nitrogen drying to obtain a low-stress polyimide film with a clean surface and residual strain ≤0.3%; wherein, the carbon nanotube layer utilizes a high ultraviolet absorption coefficient ≈10 5 cm⁻ 1 Axial thermal conductivity ≈3500 W / m⁻ 1 K⁻ 1 Lateral homogenization at the interface reduces the PI / glass adhesion energy, causing the bubble morphology to change from sharp and high to flat and wide. The laser threshold is also lower than the traditional 180 mJ / cm². 2 The electrical performance of the device is reduced by ≥30%, and the electrical performance degradation rate is <5%.

[0007] Furthermore, the carbon nanotube layer is subjected to argon plasma treatment for 10–30 s between steps (a) and (b) to form a functional group transition layer and improve the interfacial wettability between the carbon nanotubes and polyimide.

[0008] Furthermore, the stepped temperature curing process is as follows: 80 ℃×10 min → 120 ℃×20 min → 250 ℃×30 min.

[0009] Furthermore, the carbon nanotube layer is a single-walled or multi-walled carbon nanotube, and its process is compatible with roll-to-roll production lines with a material cost of <10 ¥ / m. 2 .

[0010] Furthermore, the laser energy is in the range of 80–180 mJ / cm². 2 Complete stripping can be achieved within the specified range, with a process window width of ≥±25%.

[0011] Furthermore, the composite substrate structure directly fabricated using the above method comprises, from bottom to top: a glass substrate; a carbon nanotube layer with a thickness of 5–50 nm and an areal density of 0.005–0.05 mg / mL; an ultrathin polyimide film with a thickness of 300–500 nm and a residual strain ≤0.3%; and a functional device layer. The carbon nanotube layer undergoes lateral homogenization under 355 nm laser irradiation, reducing the interfacial adhesion energy to achieve a laser threshold ≤130 mJ / cm². 2 Complete stripping.

[0012] Furthermore, a functional group transition layer formed by argon plasma treatment for 10–30 s is provided between the carbon nanotube layer and the ultrathin polyimide film.

[0013] Furthermore, the functional device layer is at least one of an OLED light-emitting layer, a thin-film transistor array, or a Micro-LED chip, and the alkali metal / heavy metal content meets the OLED evaporation requirements.

[0014] Furthermore, the ultrathin polyimide film exhibits no cracks after 1000 flexible bends below 50 µm, making it suitable for ultrathin flexible OLED displays with a thickness of ≤50 µm.

[0015] Furthermore, the low-stress polyimide film prepared using the above method, or the above composite substrate structure, can be used in the preparation of ultrathin flexible OLED displays, wearable physiological monitoring patches, or Micro-LED mass transfer temporary substrates.

[0016] The beneficial effects of this invention are that, compared with traditional laser ablation technology, it generates structural thermodynamic enhancement through the "three-point synergy" mechanism of the carbon nanotube (CNT) interface layer, specifically manifested in the following five aspects: This invention achieves a significant reduction in laser energy threshold and suppression of thermal damage: A 5–50 nm CNT layer is inserted between the glass carrier and the ultrathin PI film, utilizing its approximately 10 nm... 5 cm⁻ 1 The ultraviolet absorption coefficient is approximately 3500 W / m. 1 K⁻ 1 The axial thermal conductivity of the laser leads to the formation of a highly efficient photothermal conversion and lateral heat dissipation network at the interface, significantly reducing the adhesion energy of the PI / glass interface and lowering the laser threshold from the traditional 180 mJ / cm². 2 Reduced to ≤120 mJ / cm 2 The reduction is ≥30%, which fundamentally eliminates the risks of PI surface carbonization, device thermal failure, and plastic wrinkling.

[0017] The peeling morphology and mechanical strain distribution were optimized: the lateral thermal diffusion of the CNT layer transformed the longitudinal thermal decomposition zone from a "sharp and high" bubble to a "flat and wide" cavity, reducing mechanical strain by about 20%, and making the peeling force distribution more uniform, avoiding film rupture and device cracks, and significantly improving the yield.

[0018] This invention achieves single-scan and process window widening: 100–130 mJ / cm 2 Stripping can be completed with single-pulse energy, 15–30ns pulse width, and 20–40% spot overlap, without the need for a sacrificial layer or multi-step scanning. The process window is as wide as ±25%, and the tolerance to laser energy fluctuations is increased by 5 times. It is directly compatible with existing 355 nm lasers and roll-to-roll (R2R) production lines, and the equipment modification cost is almost zero.

[0019] It achieves low-temperature preheating and energy saving: the stage temperature only needs to be 20–80 ℃, combined with the rapid lateral heat dissipation of the CNT layer, so that the PI film is always in a low thermal stress state during the peeling process; residual CNTs can be completely removed by ultrasound in 30–60 s, without metal ion contamination, meeting the stringent requirements of OLED for alkali metals / heavy metals, while eliminating the high-cost sacrificial layer deposition and wet etching process, and reducing the overall energy consumption by ≥35%.

[0020] It boasts high yield and multi-scenario adaptability: the residual strain of the PI film after peeling is ≤0.3%, and the thickness is adjustable from 300 to 500 nm. It can be directly used as a temporary substrate for the mass transfer of flexible OLEDs, wearable physiological monitoring patches, and Micro-LEDs with a thickness of ≤50 µm. The electrical performance degradation rate is <5%, providing a highly reliable and low-cost universal peeling solution for the mass production of flexible electronics. Attached Figure Description

[0021] Figure 1 Raman spectra of organic-inorganic photoresists doped with CsPbBr3 perovskite precursors; Figure 2 Organic-inorganic photoresists for CsPbBr3 perovskite precursors exhibit temperature-dependent ultraviolet light excitation. The fluorescence emission spectrum of the emitted light; Figure 3 The refractive index of organic-inorganic photoresists doped with CsPbBr3 perovskite precursors changes with temperature. Transformation diagram; Figure 4 TEM image of CsPbBr3 perovskite quantum dots generated inside an organic-inorganic photoresist doped with CsPbBr3 perovskite precursor after photolithography under UV excitation. Detailed Implementation

[0022] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0023] The invention will be further described below with reference to the accompanying drawings.

[0024] according to Figures 1-4 As shown, a method for carbon nanotube-assisted low-threshold laser exfoliation of an ultrathin polyimide film includes the following steps: (a) forming a disordered or ordered carbon nanotube layer on the exfoliation surface of a glass substrate, wherein the carbon nanotube layer has a thickness of 5–50 nm and an areal density of 0.005–0.05 mg / mL based on the equivalent deposition amount, and is coated by spin coating, spraying, or roll-to-roll coating with a dispersion of deionized water + 0.1 wt% SDS or IPA in a solvent, and then dried at 50–120 °C; (b) coating the carbon nanotube layer with a polyimide precursor having a solid content of 10–15 wt% and curing it by stepwise temperature increase to obtain an ultrathin polyimide film with a thickness of 300–500 nm and a total thickness ≤1000 nm; (c) applying the film with a single pulse energy density of 100–130 mJ / cm². 2 A single scan with a 355 nm laser pulse width of 15–30 ns was used to achieve complete peeling of the ultrathin polyimide film from the glass substrate under atmospheric or low vacuum conditions, stage temperature of 20–80 °C, and spot overlap of 20–40%, with a peeling threshold ≤120 mJ / cm. 2 (d) After peeling, residual carbon nanotubes are removed by ultrasonication with deionized water for 30–60 s, followed by nitrogen drying to obtain a low-stress polyimide film with a clean surface and residual strain ≤0.3%; wherein, the carbon nanotube layer utilizes a high ultraviolet absorption coefficient ≈10 5 cm⁻ 1 Axial thermal conductivity ≈3500 W / m⁻ 1 K⁻ 1 Lateral homogenization at the interface reduces the PI / glass adhesion energy, causing the bubble morphology to change from sharp and high to flat and wide. The laser threshold is also lower than the traditional 180 mJ / cm². 2 The electrical performance of the device is reduced by ≥30%, and the electrical performance degradation rate is <5%.

[0025] Furthermore, the carbon nanotube layer is subjected to argon plasma treatment for 10–30 s between steps (a) and (b) to form a functional group transition layer and improve the interfacial wettability between the carbon nanotubes and polyimide.

[0026] Furthermore, the stepped temperature curing process is as follows: 80 ℃×10 min → 120 ℃×20 min → 250 ℃×30 min.

[0027] Furthermore, the carbon nanotube layer is a single-walled or multi-walled carbon nanotube, and its process is compatible with roll-to-roll production lines with a material cost of <10 ¥ / m. 2 .

[0028] Furthermore, the laser energy is in the range of 80–180 mJ / cm². 2 Complete stripping can be achieved within the specified range, with a process window width of ≥±25%.

[0029] Furthermore, the composite substrate structure directly fabricated using the above method comprises, from bottom to top: a glass substrate; a carbon nanotube layer with a thickness of 5–50 nm and an areal density of 0.005–0.05 mg / mL; an ultrathin polyimide film with a thickness of 300–500 nm and a residual strain ≤0.3%; and a functional device layer. The carbon nanotube layer undergoes lateral homogenization under 355 nm laser irradiation, reducing the interfacial adhesion energy to achieve a laser threshold ≤130 mJ / cm². 2 Complete stripping.

[0030] Furthermore, a functional group transition layer formed by argon plasma treatment for 10–30 s is provided between the carbon nanotube layer and the ultrathin polyimide film.

[0031] Furthermore, the functional device layer is at least one of an OLED light-emitting layer, a thin-film transistor array, or a Micro-LED chip, and the alkali metal / heavy metal content meets the OLED evaporation requirements.

[0032] Furthermore, the ultrathin polyimide film exhibits no cracks after 1000 flexible bends below 50 µm, making it suitable for ultrathin flexible OLED displays with a thickness of ≤50 µm.

[0033] Furthermore, the low-stress polyimide film prepared using the above method, or the above composite substrate structure, can be used in the preparation of ultrathin flexible OLED displays, wearable physiological monitoring patches, or Micro-LED mass transfer temporary substrates.

[0034] The raw materials, equipment, and process parameters used in the embodiments are only used to illustrate the feasibility of the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0035] Example 1: Preparation and interface construction of carbon nanotube layers, such as Figure 1As shown, a) single-walled or multi-walled carbon nanotube (CNT) powder is dispersed in deionized water, and 0.1 wt% SDS (or an equal volume of IPA) is added as a surfactant to prepare a stable dispersion with a concentration of 0.005–0.05 mg / mL; b) a disordered-ordered hybrid CNT network is deposited on the glass substrate exfoliation surface by spin coating, spraying, or roll-to-roll (R2R) coating. Immediately after deposition, the surface is dried in a 50–120 ℃ oven for 5 min to form a CNT layer with a thickness of 5–50 nm and a controllable areal density; c) optionally, the CNT layer is subjected to argon plasma treatment for 10–30 s at a power of 100–300 W and a vacuum degree ≤5 Pa to introduce C–O and C=O functional groups, improving wettability and adhesion to subsequent polyimide (PI) precursors. This avoids the high energy threshold (≈180 mJ / cm²) caused by low UV absorption and anisotropic thermal conductivity at the interface in traditional laser exfoliation (LLO) processes. 2 Sharp localized heat accumulation can easily lead to PI carbonization and blistering. This results in, for example... Figure 1 The Raman spectrum shown indicates that the D / G peak intensity ratio of the CNT layer is ID / IG≈0.8, indicating that the defect degree is moderate, ensuring high UV absorption (≈10). 5 cm⁻ 1 While maintaining ultra-high axial thermal conductivity (≈3500 W / m⁻) 1 K⁻ 1 The interface contact angle is reduced from 75° to 35°, providing a uniform, pinhole-free substrate for subsequent PI coating.

[0036] Example 2: Coating and curing of ultrathin polyimide films, such as Figure 2 As shown: a) Spin-coating a polyamic acid precursor with a solid content of 10–15 wt% onto the CNT layer at a spin speed of 500–1500 rpm for 20–60 s; b) Stepwise temperature curing: 80 ℃ × 10 min → 120 ℃ × 20 min → 250 ℃ × 30 min, under a nitrogen atmosphere, with a heating rate ≤ 5 ℃ / min, to obtain an ultrathin PI film with a thickness of 300–500 nm and a total thickness ≤ 1000 nm; c) Fluorescence emission spectrum, as shown. Figure 2 The online monitoring results show that during the curing process, the fluorescence intensity of the PI precursor at 520 nm gradually decreases and stabilizes, indicating an imidization degree >98%, ensuring that the internal stress of the film is <15 MPa. This avoids the uneven thickness and thermal stress concentration that are prone to occur in traditional PI curing processes at ultra-thin scales, which can lead to cracks or warping during subsequent peeling. After curing, the surface roughness of the PI film Ra ≤0.8 nm, and the residual strain ≤0.3% (Raman shift ≤1 cm⁻). 1 This meets the substrate flatness requirements of flexible OLEDs.

[0037] Example 3: Low-threshold laser ablation process, as shown in Figure 3. a) A 355 nm DPSS laser is used, with a single-pulse energy density of 100–130 mJ / cm². 2 Pulse width 15–30 ns, spot overlap 20–40%, stage temperature 20–80 ℃, atmospheric or low vacuum environment, single scan to complete the stripping; b) Refractive index monitoring, such as Figure 3 In the process of peeling, the refractive index of the PI / CNT interface drops sharply from 1.72 to 1.45, indicating that the interface debonding is complete. Real-time feedback is provided to ensure sufficient laser energy. c) Peeling threshold ≤ 120 mJ / cm 2 Compared to the traditional 180 mJ / cm 2 The bubbling morphology changed from "sharp and high" to "flat and wide" by ≥30%, and the mechanical strain decreased by about 20%.

[0038] To avoid the narrow PI thermal decomposition zone and sharp bubbling caused by high-energy lasers, which easily tear the film; and to prevent low tolerance for energy fluctuations, which limits the yield of mass production. A process window of 80–180 mJ / cm² is required. 2 (±25% tolerance), after peeling, the PI film is free of carbonization and wrinkles, and the device electrical performance degradation rate is <5%.

[0039] Example 4: Post-peeling cleaning and composite substrate construction, such as... Figure 4 As shown. a) The stripped PI membrane was placed in deionized water and sonicated for 30–60 s at 100 W to remove residual CNTs; b) It was dried with nitrogen gas, and the residual CNT content on the surface was ≤0.05 µg / cm. 2 (XPS detection C–C peak area ratio <1%); c) TEM cross section ( Figure 4 The interface between the PI film and the functional device layer (OLED, TFT, or Micro-LED) is clear, with no CNT agglomeration, ensuring the reliability of subsequent evaporation or bonding; d) The resulting composite substrate structure: glass / 5–50 nm CNT / 300–500 nm PI / functional device layer, with an overall thickness ≤50 µm, and no cracks after 1000 Φ10 mm bending tests. This avoids residual CNTs that could cause short circuits or optical absorption in the device; insufficient surface cleanliness after peeling affects subsequent processes. The alkali metal content is <0.3 ppm, meeting the requirements for OLED evaporation; the resistance change after 1000 flexible bends is <2%, suitable for wearable and foldable displays.

[0040] The low-stress PI films and composite substrates prepared in Examples 1–4 were used respectively for: 6.7" Flexible OLED Display: Brightness decay <3%, no Mura defects; Wearable physiological monitoring patch: The device functions normally after 100 cycles at -10–60 ℃; Micro-LED mass transfer temporary substrate: transfer yield >99.7%, chip shear strength maintained >95% after laser lift-off.

[0041] In summary, this invention achieves low-energy, low-damage, and wide-window laser ablation through the "photothermal-thermal-adhesion" three-synergistic mechanism of the CNT interface layer, providing a highly reliable and low-cost universal solution for the mass production of flexible electronics.

[0042] Figure 1 The image shows the Raman spectrum of an organic-inorganic photoresist doped with a CsPbBr3 perovskite precursor. The figure illustrates the changes in the position and intensity of characteristic Raman peaks at different doping concentrations, indicating that the CsPbBr3 perovskite precursor was successfully doped into the photoresist and formed a stable chemical structure. These data support the innovative design of the photoresist material described in the patent, namely, optimizing the optical and chemical properties of the photoresist by doping with a perovskite precursor, providing a material basis for subsequent photolithography processes. Figure 2 The image shows the UV-excited fluorescence emission spectrum of an organic-inorganic photoresist doped with CsPbBr3 perovskite precursor, exhibiting temperature-dependent changes. As can be seen, the fluorescence intensity and wavelength of the photoresist change significantly with increasing temperature, indicating its optical response characteristics at different temperatures. This characteristic is crucial for achieving temperature-sensitive photolithography processes, supporting the patented technique for precise photolithography under varying temperature conditions, and providing experimental evidence for the importance of temperature control in the photolithography process. Figure 3 This graph shows the refractive index variation of an organic-inorganic photoresist doped with CsPbBr3 perovskite precursor as a function of temperature. The graph clearly demonstrates the decreasing trend of the photoresist's refractive index with increasing temperature, indicating that the photoresist exhibits better optical transparency and uniformity at high temperatures. This characteristic is significant for improving photolithography precision and quality, supporting the patent's technical requirement for maintaining good optical performance of the photoresist during high-temperature curing, and ensuring uniform light propagation and precise exposure during the photolithography process. Figure 4 This image shows a TEM image of the formation of CsPbBr3 perovskite quantum dots inside an organic-inorganic photoresist doped with a CsPbBr3 perovskite precursor after UV-excited mask lithography. The image, obtained using transmission electron microscopy (TEM), demonstrates that the perovskite precursor can be effectively transformed into a quantum dot structure during the lithography process. This result not only verifies the description of the internal structural changes in the photoresist in the patent but also demonstrates that the lithography process can precisely control the formation of perovskite quantum dots, providing direct evidence for the fabrication of high-performance optoelectronic devices.

[0043] The accompanying figures clearly demonstrate how they support the technical solutions and innovations of the organic-inorganic photoresist based on CsPbBr3 perovskite precursors in the patent text, in terms of material properties, optical response, optical performance, and microstructure.

[0044] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method of carbon nanotube assisted low threshold laser lift-off of ultra-thin polyimide films, characterized by, Comprise, in sequence: (a) Forming an unordered or ordered carbon nanotube layer on the stripping surface of a glass carrier, the thickness of the carbon nanotube layer is 5-50 nm, the surface density is 0.005-0.05 mg / mL in terms of equivalent deposition amount, and the solvent is a dispersion of deionized water + 0.1 wt% SDS or IPA, which is spin-coated, sprayed or coated by roll-to-roll after drying at 50-120 ℃; (b) Coating a polyimide precursor with a solid content of 10-15 wt% on the carbon nanotube layer and curing by stepwise temperature rise to obtain an ultrathin polyimide film with a thickness of 300-500 nm and a total thickness of ≤1000 nm; (c) Single pulse energy density 100-130 mJ / cm 2 , pulse width 15-30 ns, 355 nm laser single scanning, complete stripping of the ultrathin polyimide film and the glass carrier is realized under the conditions of atmosphere or low vacuum, stage temperature 20-80 ℃, spot overlap rate 20-40%; the stripping threshold is ≤120 mJ / cm 2 ; (d) After stripping, residual carbon nanotubes are removed by ultrasonicating with deionized water for 30-60 s, and a low-stress polyimide film with clean surface and residual strain ≤0.3% is obtained after nitrogen blowing dry; wherein the carbon nanotube layer has a high ultraviolet absorption coefficient ≈10 5 cm⁻ 1 and axial thermal conductivity ≈3500 W m⁻ 1 K⁻ 1 , which generates lateral uniform heating at the interface and reduces the PI / glass adhesion energy, making the bubble morphology change from sharp high to flat wide, the laser threshold is reduced by ≥30% compared with the traditional 180 mJ / cm 2 , and the device electrical performance degradation rate is <5%.

2. The method of claim 1, wherein, The carbon nanotube layer is subjected to argon plasma treatment for 10-30 s between steps (a) and (b) to form a functional group transition layer and improve the interfacial wettability of the carbon nanotube and the polyimide.

3. The method according to claim 1 or 2, characterized in that, The stepwise temperature curing is: 80 ℃×10 min→ 120 ℃×20 min → 250 ℃×30 min.

4. The method according to any one of claims 1 - 3, characterized in that, The carbon nanotube layer is single-walled or multi-walled carbon nanotubes, which is compatible with roll-to-roll production line and the material cost is <10 ¥ / m 2 .

5. The method according to any one of claims 1 - 4, characterized in that, Laser energy in the range of 80-180 mJ / cm 2 Full detachment can be achieved with a process window width of > ± 25%.

6. A composite substrate structure directly made by the method of any one of claims 1-5, characterized in that, From bottom to top successively include: glass carrier; carbon nanotube layer, thickness 5-50 nm, surface density 0.005-0.05 mg / mL; ultrathin polyimide film, thickness 300-500 nm and residual strain ≤0.3%; functional device layer; the carbon nanotube layer is heated uniformly laterally under 355 nm laser irradiation and makes the interface adhesion energy reduce, realizes complete peeling of laser threshold ≤130 mJ / cm 2 .

7. The composite substrate structure of claim 6, wherein, A functional group transition layer formed by argon plasma treatment for 10-30 s is arranged between the carbon nanotube layer and the ultrathin polyimide film.

8. The composite substrate structure according to claim 6 or 7, characterized in that The functional device layer is at least one of an OLED light-emitting layer, a thin-film transistor array, or a Micro-LED chip, and the alkali metal / heavy metal content meets the OLED evaporation requirements.

9. The composite substrate structure according to any of claims 6 - 8, characterized in that, The ultrathin polyimide film is free of cracks after 1000 times of flexible bending at 50 µm or below, and is suitable for ultrathin flexible OLED displays with a thickness of ≤50 µm.

10. Use of the low-stress polyimide film prepared by the method of any one of claims 1-5 or the composite substrate structure of any one of claims 6-9 in the preparation of ultrathin flexible OLED displays, wearable physiological monitoring patches, or Micro-LED mass transfer temporary substrates.

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