Methods for reducing residual stress in thin films

Abstract

The present invention provides a method for reducing residual stress of a film, comprising the following steps: S1, fixing a substrate on a column with a curved surface by means of high-temperature adhesive tape, and closely attaching the substrate to the surface of the column to form a pre-bending effect on the substrate; S2, moving the substrate to a vacuum coating chamber through a glove box filled with N2 gas, and growing a thin film on the substrate; S3, fixing the film and the substrate on a heating table by means of high-temperature adhesive tape for heat treatment to release the residual stress of the film. The method provided by the present invention does not change the thickness of the film, reduces the residual stress of the film by means of attachment and heat treatment, improves the packaging performance of the thin film package, simplifies the preparation process, and shortens the preparation time of the packaged film. The problem that the flexible substrate curls inward due to the influence of residual stress is solved, which is not conducive to the preparation of devices. The film prepared by the present invention has low residual stress and high water barrier performance, and can realize independent packaging of flexible optoelectronic devices.

Description

Technical Field

[0001] The invention relates to the field of electrical technology, and in particular to a method for reducing residual stress of a film. Background Art

[0002] There is growing interest in foldable, wearable, and flexible optoelectronic products for displays, mobile phones, and batteries. Typically, organic materials in flexible optoelectronic devices are susceptible to corrosion by moisture and oxygen. Thin-film encapsulation is an effective solution. Currently, chemical vapor deposition (CVD), atomic layer deposition (ALD), magnetron sputtering, and other coating processes are often used to prepare encapsulation films. In recent years, many researchers have made excellent achievements in the field of thin-film encapsulation and applied them to commercial electronic products. CVD is widely used because of its many advantages such as simple preparation process, short time, and easy etching. In order to be used in the field of flexible displays, the mechanical stability of the film is a key characteristic. They are often limited by the influence of the film's own residual stress. Stress will reduce the critical tensile pressure that the film can withstand, resulting in poor flexibility. Reducing residual stress can greatly improve the film's resistance to bending and stretching.

[0003] ALD is a method that can deposit materials layer by layer on the surface of a substrate in the form of a single atomic film. In addition to the advantages of precise thickness control, the main advantages of ALD include: low growth temperature, uniformity and conformality of thin films on high aspect ratio structures, and deposition of functional ultra-thin films. These characteristics and the diversity of precursor selection make ALD widely used in electronic equipment devices. Among them, the inorganic film ALD-Al2O3 has excellent barrier properties and is widely used in packaging devices, but due to the deposition process problems, there will be residual stress inside it, causing it to curl inward, and it is not easy to prepare devices. In addition, high temperature environments (above 80°C) will damage organic electronic devices, so low-temperature ALD is required to grow the packaging layer. The residual stress of the film grown in a low-temperature environment will increase, making it difficult to achieve high flexibility and stretchability. We take ALD-Al2O3 as an example. Therefore, technological advances in the display field are occurring rapidly, and in response to the needs of these industries, academia and industry have recently begun to actively study suitable packaging materials and structures. Among the many flexible packaging films nowadays, the more mature one is the organic-inorganic composite thin film packaging technology, which widely uses organic-inorganic stacked structures, regulation of neutral surfaces, positive reaction force offset and other processes to reduce stress or eliminate the negative effects of stress.

[0004] Multilayer thin film packaging technology, adjusted neutral plane engineering, and zero stress structure are widely used to solve the negative effects of residual stress.

[0005] Multilayer thin film packaging technology: Plasma enhanced chemical vapor deposition (PECVD) process deposition of SiN xThe Al2O3 film deposited by the thin film and spatially resolved atomic layer deposition (SR-ALD) process exhibits stress in opposite directions, and the stress of the Al2O3 film is eliminated through the stacked structure. The stress of the Al2O3 film is released by organic polymers (PI, Parylene, graphene), organic-inorganic hybrid alucone films or metals. The above methods will increase the thickness of the packaging layer, which is difficult to achieve a smaller bending radius (below 3mm).

[0006] Adjusting the neutral plane engineering: The device or Al2O3 film is designed on the neutral plane through the Young's modulus and thickness of the material. The neutral plane will not deform when bent, which will improve the bending resistance of the neutral plane material. However, this method will add many steps due to structural design, which not only increases the process time but also increases the economic cost.

[0007] Zero stress structure: By growing Al2O3 films of the same thickness on both sides of the substrate at the same time, the stresses on both sides interact with each other to achieve stress balance and obtain a flat low-packaging substrate. However, this method does not reduce the internal stress of the film, but only solves the problem of curling. Summary of the invention

[0008] In view of the above problems, the purpose of the present invention is to propose a method for reducing residual stress of a film. By adjusting the surface area, a low residual stress and flat surface low encapsulation substrate is obtained, which is easy to prepare optoelectronic devices, and a single-layer film can realize a flexible encapsulation function, thereby realizing a low residual stress encapsulation film and a flat surface low encapsulation substrate.

[0009] To achieve the above object, the present invention adopts the following specific technical solutions:

[0010] The present invention provides a method for reducing residual stress of a film, comprising the following steps:

[0011] S1. Fix the substrate on the column with a curved surface by means of a high-temperature adhesive tape, and closely attach the substrate to the surface of the column to form a pre-bending effect on the substrate;

[0012] S2, moving the substrate into a vacuum coating chamber through a glove box filled with N2 gas, and growing a thin film on the substrate;

[0013] S3. Fix the substrate with the thin film grown on it on a heating table by using high-temperature tape for heat treatment to release the residual stress inside the thin film.

[0014] Preferably, the substrate is a flexible PI substrate, a flexible PEI substrate, a flexible PEN substrate or a flexible parylene substrate, and the thickness of the substrate is 15-50 μm.

[0015] Preferably, after a thin film is grown on a substrate, the substrate drives the thin film to contract inward for the first time. When the substrate contracts to a threshold value, the residual stress inside the thin film drives the substrate to contract inward for the second time. During the second contraction, the substrate generates a reaction force to inhibit the contraction of the thin film. When the residual stress of the thin film is equal to the reaction force, the first equilibrium state is reached.

[0016] Preferably, the temperature for preparing the film in step S2 is 40-60° C., and the thickness of the film is 30-50 nm.

[0017] Preferably, the material of the thin film is ALD-Al2O3, CVD-SiO2, SiN2 or magnetron sputtered oxide film.

[0018] Preferably, after the substrate and the film are heat treated in step S3, the reaction force is reduced, and the residual stress of the film drives the substrate to shrink inward for a third time. During the third shrinkage process, the substrate generates a reaction force to inhibit the film from shrinking; when the residual stress of the film is equal to the reaction force again, a second equilibrium state is reached, and the residual stress of the film reaches a minimum value.

[0019] Preferably, the heat treatment time on the heating table is 10-50 min.

[0020] Preferably, the temperature of the heating stage is 60°C-100°C.

[0021] Preferably, the surface area difference between the film and the substrate is made zero through a pre-bending process.

[0022] Compared with the existing technology, the method provided by the present invention does not need to increase the thickness of the encapsulation film, and reduces the residual stress inside the film through process means. Compared with the organic-inorganic stacked structure widely used today, the process time is significantly reduced. The growth of the organic layer requires several hours, and the method of the present invention only requires 30 minutes of process time, which shortens the preparation time of the encapsulation film and reduces the economic cost. The present invention solves the current problem that the flexible substrate curls inward due to the influence of residual stress, which is not conducive to the preparation of devices, and the device will be under stress after the top package is prepared, shortening the life of the device. The present invention prepares a single-layer film with low residual stress and high water barrier performance, which can realize independent packaging of flexible optoelectronic devices, and provides a feasible method for the realization of encapsulation films with smaller radius. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a flow chart of a method for reducing residual stress of a thin film provided according to an embodiment of the present invention.

[0024] Figure 2 It is a schematic diagram of the preparation process of the low stress film provided according to an embodiment of the present invention.

[0025] Figure 3 It is a schematic diagram of the process of releasing the residual stress of the film for the first time in the pre-bending process provided according to an embodiment of the present invention.

[0026] Figure 4 It is a schematic diagram of a process of releasing residual stress of a thin film for the second time under a thermal expansion process provided in an embodiment of the present invention.

[0027] Figure 5 Schematic diagram of SEM of the film surface before and after the bending test according to an embodiment of the present invention.

[0028] Figure 6 Schematic diagram of the change of WVTR of a film before and after a folding test according to an embodiment of the present invention. DETAILED DESCRIPTION

[0029] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, the same modules are represented by the same reference numerals. In the case of the same reference numerals, their names and functions are also the same. Therefore, the detailed description thereof will not be repeated.

[0030] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not constitute a limitation of the present invention.

[0031] Figure 1 The flowchart of the method for reducing residual stress of a thin film provided according to an embodiment of the present invention is shown.

[0032] Figure 2 The figure shows a process for preparing a low stress film according to an embodiment of the present invention.

[0033] like Figure 1 and Figure 2 As shown, the method for reducing residual stress of a film according to an embodiment of the present invention comprises the following steps:

[0034] S1. Fix the substrate on the column with a curved surface by means of a high-temperature adhesive tape, and closely adhere the substrate to the surface of the column to produce a pre-bending effect on the substrate.

[0035] First, the substrate is fixed on the column with a curved surface by applying external force with a high-temperature tape. The material of the column with a curved surface is aluminum. The radius of the column with a curved surface can be any value. The commonly used radius in the experiment is 3mm, 5mm, 7mm or 9mm; and a film with a thickness of 30-50nm is grown on the substrate. The temperature of the growing film is 40-60℃.

[0036] The substrate is a flexible PI substrate, a flexible PEI substrate, a flexible PEN substrate or a flexible parylene substrate, and the thickness of the substrate is 15-50 μm.

[0037] S2. Move the substrate to a vacuum coating chamber through a glove box filled with N2 gas, and grow a thin film on the substrate.

[0038] The material of the film is ALD-Al2O3, CVD-SiO2, SiN2 or magnetron sputtering oxide film.

[0039] Figure 3 The diagram shows a process of releasing residual stress of a thin film for the first time in a pre-bending process according to an embodiment of the present invention.

[0040] like Figure 3 As shown in the figure, after the preparation is completed and the external force (F1) applied by the high-temperature tape on the substrate is eliminated, the substrate will first drive the film to shrink inward, and the film will limit the shrinkage of the substrate. After the substrate shrinks to the threshold, the residual stress inside the film will drive the substrate to shrink inward again, further reducing the surface area difference between the two. During the shrinkage process, the substrate will generate a reaction force to inhibit the shrinkage state of the film. When the reaction force (F2) is equal to the remaining residual stress (F0′), the substrate and the film reach a mechanical equilibrium steady state. The pre-bending process can increase the surface area difference between the substrate and the film and further promote the shrinkage of the substrate and release the residual stress.

[0041] S3. Fix the film and substrate on a heating table by high-temperature tape for heat treatment to release the residual stress inside the film.

[0042] A flat, low-stress film and substrate with equal surface area can be obtained through a pre-bending process.

[0043] The temperature range of the heating stage is 60°C-100°C, with the optimum being 80°C.

[0044] The heat treatment time on the heating table is 10-50 minutes, and the optimal time is 20 minutes.

[0045] Figure 4 The process of releasing the residual stress of the film for the second time in the thermal expansion process provided by an embodiment of the present invention is shown.

[0046] like Figure 4As shown in the figure, by heat treating the substrate at 60℃-100℃ (suitable storage temperature for optoelectronic devices) for 10-25min, thermal stress (F3) will be generated inside the substrate after returning to room temperature, breaking the mechanical equilibrium steady state at this stage and increasing the surface area of ​​the substrate. The reaction force (F2) will decrease, and the ALD-Al2O3 film will shrink inward for a second time to release the residual stress. When the reaction force (F2′) is equal to the remaining residual stress (F0′) again, the sample will reach a mechanical equilibrium steady state again, and there will be less residual stress in the film. At the same time, the surface area difference between the film and the substrate is adjusted by changing the pre-bending radius. When the surface area difference is zero, a flat low-stress packaging substrate can be obtained.

[0047] Figure 5 A SEM schematic diagram of the film surface before and after the bending test provided in an embodiment of the present invention is shown.

[0048] like Figure 5 As shown, the method provided by the present invention is not only applicable to ALD-Al2O3 films, but also to any films with residual stress. After measuring the mechanical stability of the film, there is no crack on the surface of the film after bending 5000 times at a bending radius of 3mm.

[0049] Figure 6 A schematic diagram showing the change in WVTR of a film before and after a folding test provided in an embodiment of the present invention is shown.

[0050] like Figure 6 As shown, the water vapor permeability of the film after being bent 5000 times at a bending radius of 3 mm was measured. The results show that the film prepared by the method provided by the present invention still has excellent packaging performance after the bending test.

[0051] The above scheme can be used to prepare a single-layer ALD-Al2O3 film with low residual stress and high water barrier performance, which can realize independent packaging of flexible optoelectronic devices and provide a feasible method for the realization of packaging films with smaller radius.

[0052] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

[0053] The above specific implementations of the present invention do not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for reducing residual stress in a film, characterized in that: The following steps are involved: S1. Fixing the substrate on the column with a curved surface by means of a high-temperature adhesive tape, and closely attaching the substrate to the surface of the column to produce a pre-bending effect on the substrate; S2, moving the substrate into a vacuum coating chamber through a glove box filled with N2 gas, and growing a thin film on the substrate; S3, fixing the substrate with the thin film grown thereon on a heating platform by means of the high temperature tape for heat treatment to release the residual stress inside the thin film.

2. The method for reducing residual stress in a thin film according to claim 1, characterized in that: The substrate is a flexible PI substrate, a flexible PEI substrate, a flexible PEN substrate or a flexible parylene substrate, and the thickness of the substrate is 15-50 μm.

3. The method for reducing residual stress in a thin film according to claim 2, characterized in that: After the thin film is grown on the substrate, the substrate drives the thin film to contract inward for the first time. When the substrate contracts to a threshold value, the residual stress inside the thin film drives the substrate to contract inward for the second time. During the second contraction, the substrate generates a reaction force to inhibit the contraction of the thin film. When the residual stress of the thin film is equal to the reaction force, the first equilibrium state is reached.

4. The method for reducing residual stress of a thin film according to claim 3, characterized in that: The temperature for preparing the film in step S2 is 40-60° C., and the thickness of the film is 30-50 nm.

5. The method for reducing residual stress of a thin film according to claim 4, characterized in that: The material of the film is ALD-Al2O3, CVD-SiO2, SiN2 or magnetron sputtering oxide film.

6. The method for reducing residual stress in a thin film according to claim 5, characterized in that: After the substrate and the film are heat treated in step S3, the reaction force decreases, and the residual stress of the film drives the substrate to shrink inward for the third time. During the third shrinkage process, the substrate generates a reaction force to inhibit the film from shrinking. When the residual stress of the film is equal to the reaction force again, a second equilibrium state is reached, and the residual stress of the film reaches a minimum value.

7. The method for reducing residual stress of a thin film according to claim 6, characterized in that: The heat treatment time on the heating table is 10-50 minutes.

8. The method for reducing residual stress of a thin film according to claim 7, characterized in that: The temperature of the heating stage is 60°C-100°C.

9. The method for reducing residual stress of a thin film according to claim 8, characterized in that: The surface area difference between the film and the substrate is made zero through a pre-bending process.

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