A flexible circuit multilayer film stress control method

Abstract

The application provides a flexible circuit multilayer film stress control method, a curing medium layer adopts stepwise temperature rising, in a first heating section, a heating temperature is raised from room temperature to a first temperature at a first temperature rising rate, and is kept at the first temperature for a first time length; in a second heating section, the heating temperature is raised from the first temperature to a second temperature at a second temperature rising rate, and is kept at the second temperature for a second time length; in a third heating section, the heating temperature is raised from the second temperature to a third temperature at a third temperature rising rate, and is kept at the third temperature for a third time length; in a cooling section, a cooling temperature is lowered from the third temperature to room temperature at a first temperature lowering rate. The application has the beneficial effects of reducing the warping amount and film stress of the multilayer film structure, effectively reducing cracks and delamination caused by stress, improving the reliability of the multilayer film structure under thermal cycling, and improving the electrical performance of the flexible circuit.

Description

Technical Field

[0001] This invention belongs to the field of flexible electronics technology, and in particular relates to a method for stress control of multilayer thin films in flexible circuits. Background Technology

[0002] Electroluminescence detection technology is a commonly used testing method. When testing micro-light-emitting diode (Micro-LED) chips, flexible probes are required to avoid damaging the chips. Flexible circuits are a key component of flexible probes, consisting of multiple thin films, including alternating dielectric and metal layers. Due to the mismatch in the coefficients of thermal expansion (CTE) of these multilayer films, high internal stress can occur, leading to delamination or warping, which affects the electrical performance of the flexible circuit. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention provides a method for stress control of multilayer thin films in flexible circuits, which effectively solves the technical problem of delamination or warping in multilayer thin films of flexible circuits and overcomes the shortcomings of existing technologies.

[0004] The technical solution adopted in this invention is: a method for stress control of multilayer thin films in flexible circuits, wherein the multilayer thin film includes alternating dielectric layers and metal layers, and the dielectric layers are cured by stepped temperature increase, including:

[0005] In the first heating section, the heating temperature is increased from room temperature to a first temperature at a first heating rate, and the first temperature is maintained for a first duration.

[0006] In the second heating section, the heating temperature is increased from the first temperature to the second temperature at a second heating rate, and the second temperature is maintained for a second duration.

[0007] In the third heating section, the heating temperature is increased from the second temperature to the third temperature at a third heating rate, and the third temperature is maintained for a third duration.

[0008] In the cooling section, the cooling temperature is reduced from the third temperature to room temperature at a first cooling rate.

[0009] Furthermore, the second heating rate is greater than the first heating rate and the third heating rate, and the first cooling rate is less than the first heating rate, the second heating rate, and the third heating rate.

[0010] Furthermore, the first heating rate is 1-2℃ / min, the first temperature is 60-100℃, and the first duration is 10-20min.

[0011] Furthermore, the second heating rate is 2-3℃ / min, the second temperature is 130-170℃, and the second duration is 10-25min.

[0012] Furthermore, the third heating rate is 1-2℃ / min, the third temperature is 220-280℃, and the third duration is 20-60min.

[0013] Furthermore, the first cooling rate is 0.5–1 °C / min.

[0014] Furthermore, the difference in the coefficients of thermal expansion between the dielectric layer and the metal layer, ΔCTE, is less than ±50 ppm / ℃.

[0015] Furthermore, the thickness ratio of the dielectric layer to the metal layer is set to 1: (0.05~80), preferably, the thickness ratio is set to 1:0.5~2.

[0016] Furthermore, the dielectric layer material is one or more of polyimide, photosensitive polyimide, benzocyclobutene, parylene, or organic-inorganic hybrid materials.

[0017] Furthermore, the protective atmosphere is nitrogen, argon, vacuum, or a hydrogen-containing mixture.

[0018] The advantages and positive effects of this invention are as follows: by adopting the above technical solution, by optimizing the thickness ratio of the dielectric layer to the metal layer, and by using stepped heating during the dielectric layer heating and curing process, the warpage and film stress of the multilayer thin film structure are reduced, effectively reducing stress-induced cracks and delamination, improving the reliability of the multilayer thin film structure under thermal cycling, and enhancing the electrical performance of the flexible circuit. Detailed Implementation

[0019] This invention provides a method for stress control of multilayer thin films in flexible circuits. The embodiments of this invention are described below.

[0020] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "set" and "connection" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0021] Flexible circuits, as key components of flexible probes, comprise multiple alternating dielectric and metal layers. For example, a first dielectric layer is first coated on a substrate, followed by a first metal layer deposited on the first dielectric layer using physical vapor deposition. Next, a second dielectric layer is coated on the first metal layer, followed by another second metal layer, then a third dielectric layer is coated on the second metal layer, and finally a metal oxide layer is deposited on the third dielectric layer, forming a multilayer thin-film structure. In existing technologies, the significant differences in the coefficients of thermal expansion of the multilayer thin films lead to high internal stresses between the films, making the multilayer thin-film structure prone to delamination or warping, thus affecting the electrical performance of the flexible circuit.

[0022] This invention discloses a method for stress control of a flexible circuit multilayer thin film. After the dielectric layer is coated, it needs to be heated and cured. During the heating and curing process, a stepped heating stage is adopted, including a first heating stage, a second heating stage, a third heating stage, and a cooling stage. In the first heating stage, the heating temperature is increased from room temperature to a first temperature at a first heating rate, and held at the first temperature for a first duration. In the second heating stage, the heating temperature is increased from the first temperature to a second temperature at a second heating rate, and held at the second temperature for a second duration. In the third heating stage, the heating temperature is increased from the second temperature to a third temperature at a third heating rate, and held at the third temperature for a third duration. In the cooling stage, the cooling temperature is reduced from the third temperature to room temperature at a first cooling rate. The second heating rate is greater than the first heating rate and the third heating rate, and the first cooling rate is less than the first heating rate, the second heating rate, and the third heating rate. The first heating stage is a low-temperature stage with a relatively small heating rate to prevent rapid evaporation of the dielectric layer solvent, which could lead to blistering or cracking of the film layer. The second heating section is a medium-temperature section with a relatively high heating rate to accelerate curing and provide time for molecular chain rearrangement, reducing internal stress between film layers. The third heating section is a high-temperature section with a relatively low heating rate to avoid excessive internal stress caused by overly rapid curing. The cooling section uses a low cooling rate to further reduce thermal shock stress caused by CTE differences.

[0023] Specifically, the first heating rate is 1–2 °C / min, the first temperature is 60–100 °C, and the first duration is 10–20 min; the second heating rate is 2–3 °C / min, the second temperature is 130–170 °C, and the second duration is 10–25 min; the third heating rate is 1–2 °C / min, the third temperature is 220–280 °C, and the third duration is 20–60 min; and the first cooling rate is 0.5–1 °C / min.

[0024] Specifically, the difference in thermal expansion coefficients between the dielectric layer and the metal layer, ΔCTE, is <±50 ppm / ℃. The dielectric layer material is one or more of polyimide, photosensitive polyimide, benzocyclobutene, parylene, or organic-inorganic hybrid materials. The metal layer includes an adhesion layer and a conductive layer. The adhesion layer is made of Cr, Ti, Ta, W, NiCr, or TiW, and the conductive layer is made of Au, Cu, Ag, Al, Pt, or Pd.

[0025] Preferably, the dielectric layer material is polyimide. When coating the dielectric layer, the coating material is polyamic acid. The polyamic acid undergoes a dehydration and cyclization reaction of its molecular chains through curing heating, forming a stable and rigid five-membered imide ring structure, thereby generating the polyimide, i.e., the dielectric layer. The first heating stage is a solvent evaporation stage, slowly removing the NMP solvent to prevent rapid solvent evaporation from causing bubbles or cracks in the film layer; the second heating stage is a pre-imidization stage, allowing time for molecular chain rearrangement to promote partial imidization and release initial stress; the third heating stage is a complete amination stage, completing the cyclization reaction and forming a stable dielectric layer structure. The cooling stage is a stress relaxation stage, using natural cooling to slowly lower the temperature and reduce thermal stress accumulation.

[0026] The medium layer can be configured as a nanocomposite layer, wherein the volume fraction of inorganic nanoparticles is 10% to 50%; or a gradient composition layer, wherein the content of inorganic fillers varies continuously along the thickness direction; or a porous structure layer with a porosity of 5% to 30%.

[0027] Specifically, the thickness ratio of the dielectric layer to the metal layer is set to 1:(0.05~80), preferably 1:(0.5~2). Through COMSOL multiphysics simulation, this optimized thickness ratio makes the maximum principal stress <100MPa.

[0028] According to Stoney's formula, the radius of curvature R of wafer warpage caused by thin film deposition on the substrate is related to the thin film stress σ. f The relationship is:

[0029]

[0030] Among them, E s V s t s These represent the elastic modulus, Poisson's ratio, and thickness of the substrate, t. f The thickness is the thin film thickness.

[0031] For a multilayer thin film structure, the overall bending moment equilibrium condition is:

[0032]

[0033] Where, σ i t i zi denoted as stress, thickness, and neutral axis distance of the i-th layer, respectively.

[0034] By controlling the thickness ratio of the dielectric layer to the metal layer, the total bending moment of the multilayer structure can be brought close to zero, thereby controlling the warpage of the wafer.

[0035] Preferably, the thickness of the dielectric layer is set to 0.5–50 μm, the thickness of the metal layer is set to 11–1100 nm, the thickness of the adhesion layer is 1–100 nm, and the thickness of the conductive layer is 10–1000 nm.

[0036] If the dielectric layer is too thin (<0.5μm), pinhole defects are difficult to avoid; if the dielectric layer is too thick (>50μm), the volume shrinkage and thermal stress during the curing process increase significantly, resulting in excessive warpage and easy delamination at the interface. If the metal layer is too thin (<11nm), its continuity and conductivity are insufficient to meet the high-frequency signal transmission requirements of the probe; if the metal layer is too thick (>1.1μm), the accumulation of internal compressive or tensile stress leads to film cracking, especially after thermal cycling, the crack density increases significantly.

[0037] Specifically, the protective atmosphere for heat curing is nitrogen, argon, vacuum, or a hydrogen-containing mixture.

[0038] Example 1: A method for stress control of a flexible circuit multilayer thin film, wherein the multilayer thin film includes alternating dielectric and metal layers. The dielectric layer is made of polyimide and has a thickness of 1 μm. The metal layer includes an adhesion layer (Cr) and a conductive layer (Au). The adhesion layer has a thickness of 50 nm, the conductive layer has a thickness of 450 nm, and the total thickness of the metal layer is 500 nm. The thickness ratio of the dielectric layer to the metal layer is 1:0.5. The coefficient of thermal expansion of polyimide is 20–50 ppm / ℃, that of Cr is 6 ppm / ℃, and that of Au is 14 ppm / ℃. The protective atmosphere is nitrogen.

[0039] During the curing of the dielectric layer, a stepped heating process is employed, comprising a first heating stage, a second heating stage, a third heating stage, and a cooling stage. In the first heating stage, the heating temperature is increased from room temperature to a first temperature of 60°C at a first heating rate of 1°C / min, and held at the first temperature for a first duration of 10 minutes. In the second heating stage, the heating temperature is increased from the first temperature to a second temperature of 130°C at a second heating rate of 2°C / min, and held at the second temperature for a second duration of 10 minutes. In the third heating stage, the heating temperature is increased from the second temperature to a third temperature of 220°C at a third heating rate of 1°C / min, and held at the third temperature for a third duration of 20 minutes. In the cooling stage, the cooling temperature is reduced from the third temperature to room temperature at a first cooling rate of 0.5°C / min.

[0040] Implementation 2: A method for stress control of a flexible circuit multilayer thin film, wherein the multilayer thin film includes alternating dielectric and metal layers. The dielectric layer is made of polyimide and has a thickness of 0.5 μm. The metal layer includes an adhesion layer (Cr) and a conductive layer (Au). The adhesion layer has a thickness of 100 nm, the conductive layer has a thickness of 900 nm, and the total thickness of the metal layer is 1000 nm. The thickness ratio of the dielectric layer to the metal layer is 1:2. The coefficient of thermal expansion of polyimide is 20–50 ppm / ℃, that of Cr is 6 ppm / ℃, and that of Au is 14 ppm / ℃. The protective atmosphere is nitrogen.

[0041] During the curing of the dielectric layer, a stepped heating process is employed, comprising a first heating stage, a second heating stage, a third heating stage, and a cooling stage. In the first heating stage, the heating temperature is increased from room temperature to a first temperature of 80°C at a first heating rate of 1.5°C / min, and held at the first temperature for a first duration of 12 minutes. In the second heating stage, the heating temperature is increased from the first temperature to a second temperature of 150°C at a second heating rate of 2.5°C / min, and held at the second temperature for a second duration of 20 minutes. In the third heating stage, the heating temperature is increased from the second temperature to a third temperature of 250°C at a third heating rate of 1.5°C / min, and held at the third temperature for a third duration of 40 minutes. In the cooling stage, the cooling temperature is reduced from the third temperature to room temperature at a first cooling rate of 0.7°C / min.

[0042] Implementation 3: A method for stress control of a flexible circuit multilayer thin film, wherein the multilayer thin film includes alternating dielectric and metal layers. The dielectric layer is made of polyimide containing 10% SiO2 particles to form a nanocomposite layer with a thickness of 1 μm. The metal layer includes an adhesion layer (Cr) and a conductive layer (Au). The adhesion layer has a thickness of 50 nm, the conductive layer has a thickness of 450 nm, and the total thickness of the metal layer is 500 nm. The thickness ratio of the dielectric layer to the metal layer is 1:0.5. The coefficient of thermal expansion of polyimide is 20–50 ppm / ℃, that of Cr is 6 ppm / ℃, and that of Au is 14 ppm / ℃. The protective atmosphere is nitrogen.

[0043] During the curing of the dielectric layer, a stepped heating process is employed, comprising a first heating stage, a second heating stage, a third heating stage, and a cooling stage. In the first heating stage, the heating temperature is increased from room temperature to a first temperature of 100°C at a first heating rate of 1.5°C / min, and held at the first temperature for a first duration of 20 minutes. In the second heating stage, the heating temperature is increased from the first temperature to a second temperature of 170°C at a second heating rate of 2.5°C / min, and held at the second temperature for a second duration of 25 minutes. In the third heating stage, the heating temperature is increased from the second temperature to a third temperature of 280°C at a third heating rate of 1.5°C / min, and held at the third temperature for a third duration of 60 minutes. In the cooling stage, the cooling temperature is reduced from the third temperature to room temperature at a first cooling rate of 0.7°C / min.

[0044] Comparative Example 1: A method for stress control of a flexible circuit multilayer thin film. The structure of the multilayer thin film is the same as that in Example 1. During the heating and curing of the dielectric layer, constant temperature curing is adopted, with a heating temperature of 280°C and a holding time of 30 minutes.

[0045] Comparative Example 2: A method for stress control of a flexible circuit multilayer thin film. The structure of the multilayer thin film is the same as that in Example 1. When the dielectric layer is heated and cured, a rapid heating method is adopted, with the heating temperature rising from room temperature to 280 degrees Celsius and holding for 30 minutes. The heating rate is 10 degrees Celsius / min.

[0046] Comparative Example 3: A method for stress control of a flexible circuit multilayer thin film, wherein the multilayer thin film includes alternating dielectric layers and metal layers. The dielectric layer is made of polyimide and has a thickness of 10 μm. The metal layer includes an adhesion layer Cr and a conductive layer Au. The adhesion layer has a thickness of 10 nm, the conductive layer has a thickness of 90 nm, and the total thickness of the metal layer is 100 nm. The thickness ratio of the dielectric layer to the metal layer is 1:0.01.

[0047] In the fabrication of flexible circuits, the control methods of Examples 1, 2, and 3, and Comparative Examples 1, 2, and 3 were used to control the stress between the multilayer thin films. According to the standard SEMI MF534, "Test Methods for Warpage and Bending of Silicon Wafers," a thin film stress meter was used to detect the warpage and stress of the multilayer thin film structure. The test results are shown in the table below:

[0048] Example 1 Step heating 1:0.5 8.2 +35 (La) 99.1 98.5 Example 2 Step heating 1:2 5.6 -12 (pressure) 99.5 99.0 Example 3 Stepped heating + nanocomposite 1:0.5 4.1 +8 (pull) 99.8 99.3 Comparative Example 1 Constant temperature curing 1:0.5 42.3 +186 (La) 85.2 76.4 Comparative Example 2 Rapid heating 1:0.5 35.7 -142 (pressure) 88.6 81.2 Comparative Example 3 Step heating 1:0.01 68.9 +312 (La) 62.3 54.1

[0049] As can be seen from the table above, in Examples 1, 2, and 3, due to the use of stepped heating and curing and the adoption of an optimized thickness ratio, the warpage and film stress were significantly reduced compared to Comparative Examples 1, 2, and 3. The warpage was less than 10 μm, which effectively improved the delamination yield and electrical yield.

[0050] The advantages and positive effects of this invention are:

[0051] By optimizing the thickness ratio of the dielectric layer to the metal layer and using stepped heating during the dielectric layer curing process, the warpage and film stress of the multilayer thin film structure were reduced, effectively reducing stress-induced cracks and delamination, improving the reliability of the multilayer thin film structure under thermal cycling, and enhancing the electrical performance of the flexible circuit.

[0052] This concludes the detailed description of the embodiments disclosed herein. The definitions of the components described above are not limited to the specific structures, shapes, or methods mentioned in the embodiments, and those skilled in the art can easily modify or substitute them.

[0053] The embodiments of the present invention have been described in detail above, but the content described is only a preferred embodiment of the present invention and should not be considered as limiting the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the patent coverage of the present invention.

Claims

1. A method for stress control of a flexible circuit multilayer thin film, wherein the multilayer thin film comprises alternating dielectric layers and metal layers, characterized in that, The curing of the dielectric layer employs a stepped temperature increase, including: In the first heating section, the heating temperature is increased from room temperature to a first temperature at a first heating rate, and the first temperature is maintained for a first duration. In the second heating section, the heating temperature is increased from the first temperature to the second temperature at a second heating rate, and the second temperature is maintained for a second duration. In the third heating section, the heating temperature is increased from the second temperature to the third temperature at a third heating rate, and the third temperature is maintained for a third duration. In the cooling section, the cooling temperature is reduced from the third temperature to room temperature at a first cooling rate.

2. The method for stress control of flexible circuit multilayer thin films according to claim 1, characterized in that: The second heating rate is greater than the first heating rate and the third heating rate, and the first cooling rate is less than the first heating rate, the second heating rate, and the third heating rate.

3. The method for stress control of multilayer thin films in flexible circuits according to claim 2, characterized in that: The first heating rate is 1-2℃ / min, the first temperature is 60-100℃, and the first duration is 10-20min.

4. The method for stress control of flexible circuit multilayer thin films according to claim 2, characterized in that: The second heating rate is 2-3℃ / min, the second temperature is 130-170℃, and the second duration is 10-25min.

5. The method for stress control of a flexible circuit multilayer thin film according to claim 2, characterized in that: The third heating rate is 1-2℃ / min, the third temperature is 220-280℃, and the third duration is 20-60min.

6. The method for stress control of a flexible circuit multilayer thin film according to claim 2, characterized in that: The first cooling rate is 0.5 to 1 °C / min.

7. A method for stress control of flexible circuit multilayer thin films according to any one of claims 1-6, characterized in that: The difference in the coefficients of thermal expansion between the dielectric layer and the metal layer is ΔCTE < ±50 ppm / ℃.

8. A method for stress control of flexible circuit multilayer thin films according to any one of claims 1-6, characterized in that: The thickness ratio of the dielectric layer to the metal layer is set to 1:(0.05~80), preferably 1:(0.5~2).

9. A method for stress control of flexible circuit multilayer thin films according to any one of claims 1-6, characterized in that: The dielectric layer material is one or more of polyimide, photosensitive polyimide, benzocyclobutene, parylene, or organic-inorganic hybrid materials.

10. A method for stress control of a flexible circuit multilayer thin film according to any one of claims 1-6, characterized in that: The protective atmosphere is nitrogen, argon, vacuum, or a hydrogen-containing mixture.

Images