A method for improving the surface irregularities of polyimide coatings

Abstract

The application belongs to the technical field of semiconductor manufacturing, and discloses a method for improving irregular bubbles on the surface of a polyimide coating. The method comprises the following steps: applying a polyimide resin solution to the surface of a substrate to form a wet film; placing the wet film in an environment with a temperature of 80-100 DEG C to perform low-temperature pre-baking treatment, so that the internal solvent and gas slowly escape in an unsealed state on the surface layer; and then transferring the wet film to an environment with a temperature of 120-140 DEG C to perform high-temperature curing, so as to complete imidization reaction. Through the segmented gradient heating, the coating surface is prevented from being sealed too early, the color irregular bubble defects generated in the existing one-step high-temperature curing process are fundamentally eliminated, the coating surface is smooth and clean, the compactness and insulation performance are significantly improved, the process is simple and controllable, and the existing equipment or resin formula does not need to be changed.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor manufacturing technology, and specifically relates to a method for improving irregular bubbles on the surface of polyimide (PI) coatings. Background Technology

[0002] Polyimide (PI) resin is widely used in electronics, aerospace, flexible displays, and semiconductor packaging due to its excellent high and low temperature resistance, insulation, mechanical properties, and chemical stability. In semiconductor manufacturing processes, polyimide is often used as a dielectric layer, passivation layer, or stress buffer layer, and its coating and curing quality directly affects the performance and reliability of the device.

[0003] When coating polyimide resin onto a substrate surface to form a functional coating, a heat curing step is usually required to remove the solvent and complete the imidization reaction. In existing technologies, to improve production efficiency, operators often use a one-step method to directly heat-cure the wet film at high temperatures. For example, the coated polyimide wet film is directly placed in an oven at 130°C for heat treatment.

[0004] However, in actual production, it was found that when using the above-mentioned one-step high-temperature direct heating curing process, a large number of irregularly distributed colored bubbles easily appear inside the coating. These bubbles vary in size and are randomly distributed, severely damaging the density and smoothness of the coating, resulting in decreased insulation performance, optical distortion, and a significant reduction in yield.

[0005] Analysis revealed that the primary cause of these bubbles was the rapid evaporation of the solvent on the wet film surface during the direct one-step high-temperature heating process, leading to the rapid curing of the surface polyimide resin into a dense, hard surface layer. Meanwhile, the solvent and low-molecular-weight volatiles deep within the coating attempted to escape upon heating but were blocked by the sealed hard surface layer. These trapped gases accumulated within the coating, forming bubbles. Due to the interference effect of light, these bubbles exhibit a colored appearance under a microscope.

[0006] In the prior art, patent document CN111061127A discloses a polyimide curing method that uses multi-stage heating for curing. However, this method must be performed after exposure and development, and is a post-curing scheme in conjunction with the photolithography process. Its purpose is to prevent bulging and peeling during subsequent acid and alkali treatment. Patent document CN110385241A discloses the preparation of a polyimide composite coating on an aluminum alloy surface, with two-step curing temperatures of 170~200℃ and 230~350℃, respectively. Patent document CN106928481A discloses an optimized preparation method for polyimide films. Although its heating program includes multi-stage heating, its purpose is to prepare PI films for OLED flexible substrates with low thermal expansion coefficients. The heating between each temperature segment is continuous, without setting up an independent low-temperature pre-baking stage, and it does not address the problem of bubble defects.

[0007] Therefore, how to effectively suppress the generation of bubbles in polyimide coatings while ensuring curing efficiency is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] The purpose of this invention is to provide a method for improving irregular bubbles on the surface of a polyimide coating, so as to solve at least one technical problem existing in the prior art.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for improving irregular bubbles on the surface of a polyimide coating includes the following steps: Coating step: Apply the polyimide resin solution to the substrate surface to form a wet film; Low-temperature pre-baking step: The coated substrate is placed in a first temperature environment for pre-baking treatment, so that the solvent and low molecular weight volatiles in the wet film can slowly escape in the state where the coating surface is not completely sealed. The first temperature is 80~100℃. High-temperature curing step: After the pre-baking treatment is completed, the substrate is transferred to a second temperature environment for high-temperature curing treatment, so that the polyimide resin completes the imidization reaction and forms a cured coating. The second temperature is 120~140℃.

[0010] Compared with the prior art, the beneficial effects of the present invention are as follows: By heating in stages, premature sealing of the coating surface is avoided, allowing solvents and gases to escape smoothly. This solves the problem of irregular colored bubbles in existing one-step high-temperature curing processes, resulting in a smooth and clean coating surface.

[0011] This invention only optimizes the heating process, changing the one-step method to a two-stage gradient heating method. It does not require changes to existing coating equipment or adjustments to the resin formulation, resulting in low modification costs and easy application on existing production lines.

[0012] This method has simple process steps, and the coating density, adhesion and insulation performance are significantly improved, which ensures the reliability of the product in high-end fields such as semiconductor devices. It has extremely high industrial practical value and industrialization prospects. Attached Figure Description

[0013] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0014] In the attached diagram: Figure 1 This is a microscopic photograph of the polyimide coating surface after treatment in Comparative Example 1 of the present invention.

[0015] Figure 2 This is a second microscopic photograph of the polyimide coating surface after treatment in Comparative Example 1 of the present invention.

[0016] Figure 3 This is a microscope image of the polyimide coating surface after coating treatment in Example 1 of the present invention. Detailed Implementation

[0017] In a first aspect, the present invention provides a method for improving irregular bubbles on the surface of a polyimide coating, comprising the following steps: Coating step: Apply the polyimide resin solution to the substrate surface to form a wet film; Low-temperature pre-baking step: The coated substrate is placed in a first temperature environment for pre-baking treatment, so that the solvent and low molecular weight volatiles in the wet film can slowly escape in the state where the coating surface is not completely sealed. The first temperature is 80~100℃. High-temperature curing step: After the pre-baking treatment is completed, the substrate is transferred to a second temperature environment for high-temperature curing treatment, so that the polyimide resin completes the imidization reaction and forms a cured coating. The second temperature is 120~140℃.

[0018] In a specific embodiment, the low-temperature pre-baking step involves placing the coated wet film in a first temperature environment for pre-baking treatment: (a) The first temperature is lower than the boiling point of the solvent commonly used in polyimide resin solutions, so the evaporation rate of the solvent is moderate at this temperature and bubbles will not be generated due to violent boiling of the solvent.

[0019] (b) The first temperature is lower than the imidization initiation temperature of the polyimide resin. Therefore, the resin will not cross-link and cure at this temperature, and the coating surface will maintain a certain degree of permeability and will not form a dense closed layer.

[0020] (c) At this temperature, the solvent in the wet film changes from a liquid to a gaseous state upon heating, and diffuses from the interior of the coating to the surface. Since the surface layer is not sealed, the gaseous solvent can easily escape from the surface. Over time, most of the solvent and low-molecular-weight volatiles in the wet film are expelled, and the coating changes from a wet state to a semi-dry state.

[0021] (d) The preferred range of the first temperature is 80~100℃, more preferably 85~95℃, and most preferably 90℃. At 80℃, the solvent removal in the PI is insufficient, which leads to problems such as incomplete coating curing and low adhesion in subsequent processes; at temperatures above 100℃, the coating surface hardens prematurely, and some internal gases are blocked to form bubbles, which cannot completely solve the problem of irregular bubbles.

[0022] In a specific embodiment, the processing time for the low-temperature pre-drying step is 60-120 seconds, more preferably 90 seconds. If the processing time is too short, the solvent will not be sufficiently removed; if the processing time is too long, it will affect production efficiency.

[0023] In a specific embodiment, the high-temperature curing step involves transferring the substrate, after low-temperature pre-baking, to a second temperature environment (120~140℃) for final curing. In this step: (a) Since the low-temperature pre-baking step has removed most of the solvent and gas from the wet film, there is no risk of gas accumulation inside the coating. Therefore, even if the surface is quickly sealed at high temperature, no bubbles will be generated.

[0024] (b) The second temperature is the imidization reaction temperature of the polyimide resin. At this temperature, polyamic acid (PAA) undergoes a dehydration cyclization reaction and is converted into polyimide. The coating changes from a semi-dry state to a fully cured hard coating.

[0025] (c) The preferred range of the second temperature is 120~140℃, more preferably 125~135℃, and most preferably 130℃.

[0026] (d) The processing time of the high-temperature curing step is 115~125s, preferably 120s.

[0027] In a specific embodiment, after the high-temperature curing step, a cooling step is further included: cooling the cured coating to room temperature. The cooling temperature is 22~25℃, and the cooling time is 30~90s, preferably 60s. The cooling method can be natural cooling or forced cooling.

[0028] In a specific embodiment, the coating method in the coating step is any one of spin coating, roll coating, or spray coating. In semiconductor wafer coating processes, spin coating is preferred, and a coating machine such as the TEL ACT-8 can be used.

[0029] In a specific embodiment, the solvent of the polyimide resin solution is at least one of N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAc), and dimethyl sulfoxide (DMSO).

[0030] In a specific embodiment, the substrate is any one of a semiconductor wafer, a metal substrate, or a flexible substrate.

[0031] Secondly, the present invention provides a polyimide coated article, which is prepared by the above method.

[0032] The polyimide coated products prepared by the method of the present invention have no colored bubble defects on the coating surface, have a smooth and clean surface, good density, strong adhesion, and excellent insulation properties. They can be widely used as dielectric layers, passivation layers or stress buffer layers in semiconductor devices.

[0033] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0034] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0035] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0036] In the following examples and comparative examples, the polyimide resin solutions used are commercially available, the coating equipment is a commonly used coating machine in the art, and the process temperature and time parameters can be precisely controlled by standard oven equipment.

[0037] Example 1 This embodiment provides a method for improving irregular bubbles on the surface of a polyimide coating, the specific steps of which are as follows: (1) Place the semiconductor wafer on the carrier platform of the TEL ACT-8 coating machine, and spin-coat the surface of the wafer with a polyimide resin solution. The viscosity of the polyimide resin solution is 2800cp, forming a uniform wet film. The solvent of the polyimide resin solution is N-methylpyrrolidone (NMP).

[0038] (2) The coated wafer was transferred into an oven and pre-baked at 90°C for 90 seconds. During this stage, the NMP solvent in the wet film evaporated slowly upon heating, diffusing from the inside of the coating to the surface and escaping. Since 90°C is much lower than the boiling point of NMP, the solvent evaporation rate was moderate, and bubbles were not generated due to violent boiling. At the same time, 90°C is lower than the imidization initiation temperature of polyimide resin, so the resin did not undergo cross-linking and curing, the coating surface remained transparent, and the internal solvent and gas could be smoothly discharged. No skin formation was observed on the coating surface, and the solvent evaporated smoothly.

[0039] (3) After pre-baking, the wafer is transferred to an oven at 130°C and kept at this temperature for 120 seconds to allow the polyimide resin to fully complete the imidization reaction and form a hard cured coating. Since most of the solvent and gas in the wet film has been removed in step (2), there is no risk of gas accumulation inside the coating. Therefore, even if the surface is quickly sealed at a high temperature of 130°C, no bubbles will be generated.

[0040] (4) Transfer the cured wafer to a cooling box at 23°C, maintain this temperature for 60 seconds, and remove it after cooling to below 23°C.

[0041] The coating surface was inspected using a microscope, such as... Figure 3 As shown, no colored bubble defects were detected on the wafer surface; under the microscope, there were no bubbles or interference patterns, and the coating surface was smooth and clean.

[0042] Example 2 The only difference between this embodiment and Embodiment 1 is that: The low-temperature pre-baking step is performed at a temperature of 85℃ for 90 seconds.

[0043] The remaining steps and parameters are the same as in Example 1.

[0044] After processing in this embodiment, no colored bubble defects were detected on the coating surface, the surface was smooth, and the coating quality was good.

[0045] Example 3 The only difference between this embodiment and Embodiment 1 is that: The low-temperature pre-baking step is performed at a temperature of 95°C for 90 seconds.

[0046] The remaining steps and parameters are the same as in Example 1.

[0047] After processing in this embodiment, no colored bubble defects were detected on the coating surface, the surface was smooth, and the coating quality was good.

[0048] Comparative Example 1 The only difference between this comparative example and Example 1 is that: The low-temperature pre-baking step was eliminated. After coating, the wafer was directly transferred to a 130°C oven for one-step curing, which took 120 seconds.

[0049] The remaining steps and parameters are the same as in Example 1.

[0050] After processing, the comparative sample was examined under a microscope, such as... Figure 1 and Figure 2 As shown, numerous irregularly distributed colored bubble defects were detected on the coating surface. The bubbles varied in size and severely damaged the coating's density and smoothness. This result indicates that in the one-step 130℃ direct curing process, the solvent on the wet film surface evaporates rapidly and solidifies quickly to form a dense surface layer, while the internal solvent and gas are blocked, forming bubbles, making bubble formation unavoidable.

[0051] Comparative Example 2 The only difference between this comparative example and Example 1 is that: The low-temperature pre-baking step is performed at a temperature of 80℃ for 90 seconds.

[0052] The remaining steps and parameters are the same as in Example 1.

[0053] After treatment in this comparative example, no obvious bubbles were detected on the coating surface. However, subsequent performance tests revealed that the coating was not fully cured and had low adhesion. The reason for this was that the pre-baking temperature of 80℃ was too low, resulting in insufficient solvent removal and residual solvent affecting the completeness of the imidization reaction.

[0054] Comparative Example 3 The only difference between this comparative example and Example 1 is that: The low-temperature pre-baking step is performed at 100℃ for 90 seconds.

[0055] The remaining steps and parameters are the same as in Example 1.

[0056] After treatment in this comparative example, a small number of minor bubble defects were detected on the coating surface. The reason for this was that the pre-baking temperature of 100℃ was too high, approaching the critical temperature for rapid solvent evaporation, resulting in a slight skin formation on the surface and the blockage of some internal gases, which formed bubbles.

[0057] The parameters and results of the summarized examples and comparative examples are shown in Table 1.

[0058] Table 1 As shown in Table 1, Example 1 achieved a bubble-free coating with excellent quality through a two-stage process of pre-baking at 90℃ and curing at 130℃. Comparative Example 1 demonstrates that a one-step method of direct curing at 130℃ produces numerous bubble defects. Comparative Example 2 demonstrates that a pre-baking temperature of 80℃ is too low, resulting in insufficient solvent removal. Comparative Example 3 demonstrates that a pre-baking temperature of 100℃ is too high, leading to the appearance of slight bubbles.

[0059] A comparison of Example 1 with Comparative Examples 1-3 clearly demonstrates that the two-stage gradient heating method of low-temperature pre-baking and high-temperature curing proposed in this invention, along with the precise control of the low-temperature pre-baking temperature (85-95°C), is key to solving the problem of irregular bubbles on the surface of polyimide coatings. Within this temperature range, the coating surface remains permeable, allowing internal solvents and gases to escape smoothly, fundamentally eliminating the conditions for bubble formation.

[0060] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for improving irregular bubbles on the surface of a polyimide coating, characterized in that, Includes the following steps: (1) Apply a polyimide resin solution to the surface of the substrate to form a wet film; (2) The coated substrate is placed in a first temperature environment for pre-baking treatment, the first temperature being 80~100℃; (3) After the pre-baking treatment is completed, the substrate is transferred to a second temperature environment for high-temperature curing treatment, the second temperature being 120~140℃.

2. The method according to claim 1, characterized in that, The first temperature is 85~95℃; the second temperature is 125~135℃.

3. The method according to claim 1, characterized in that, The processing time for the pre-drying step is 60~120s.

4. The method according to claim 1, characterized in that, The processing time for the high-temperature curing step is 115~125s.

5. The method according to claim 1, characterized in that, Following the high-temperature curing step, a cooling step is also included: cooling the cured coating to room temperature.

6. The method according to claim 5, characterized in that, Cool to 22~25℃ for 30~90 seconds.

7. The method according to claim 1, characterized in that, The coating method can be any one of spin coating, roller coating, or spray coating.

8. The method according to claim 1, characterized in that, The solvent for the polyimide resin solution is at least one of N-methylpyrrolidone, N,N-dimethylacetamide, and dimethyl sulfoxide.

9. The method according to any one of claims 1 to 8, characterized in that, The substrate can be any one of semiconductor wafers, metal substrates, or flexible substrates.

10. A polyimide-coated article, characterized in that, It is prepared by the method according to any one of claims 1 to 9.

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