Low viscosity photoresist thickening method

Abstract

The application discloses a low-viscosity photoresist thickening method, and belongs to the field of photoetching processes, which comprises the following steps: spin-coating a first layer of photoresist on a wafer on which a Ti-Cu thin film seed layer has been grown; performing first soft baking on the wafer; the temperature of the first soft baking is lower than the temperature specified in the technical manual of the photoresist, and the time of the first soft baking is shorter than the time specified in the technical manual of the photoresist; performing first cooling on the wafer; spin-coating a second layer of photoresist on the wafer; performing second soft baking on the wafer; performing second cooling on the wafer to obtain a resist layer. The method is simple and easy to implement, can break through the limit of the thickness of the photoresist, expand the process window of the thickness of the photoresist, make a high-thickness resist layer by using low-viscosity photoresist, is beneficial to reducing production cost, avoids the trouble of replacing the type of photoresist and the photoresin coating machine due to different requirements of the height of the convex points, and is beneficial to stabilizing product quality.

Description

Technical Field

[0001] This invention relates to a method for thickening low-viscosity photoresist, belonging to the field of photolithography. Background Technology

[0002] In the semiconductor packaging and testing industry, bumping technology is a process that connects the exposed metal leads of integrated circuits to metal bumps on a ceramic substrate, improving circuit reliability and performance. Bumping technology reduces intermediate steps, thus lowering production costs.

[0003] However, bumping technology also faces some challenges. The height of the metal bumps is determined by the package design requirements. Typically, to increase package reliability and support higher currents, copper bumps may need to be quite high. This places considerable demands on the thickness of the required resist layer, requiring not only high resolution, less solvent, and a high-viscosity, high-hardness resin to support the required thickness of the bumps, but also the fact that conventional coating equipment in the industry is unlikely to be capable of spraying such high-viscosity photoresist, and even if it were, the hardware cost would be extremely high. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this invention provides a method for thickening low-viscosity photoresist, which can break through the thickness limit of photoresist and make a thicker resist layer with lower viscosity photoresist.

[0005] The technical solution adopted by this invention to solve its technical problem is:

[0006] A method for thickening low-viscosity photoresist includes the following steps:

[0007] The first layer of photoresist is spin-coated onto the wafer on which the Ti-Cu thin film seed layer has been grown;

[0008] The wafer is subjected to a first soft bake; the temperature of the first soft bake is lower than the temperature specified in the technical guide of the photoresist, and the time of the first soft bake is shorter than the time specified in the technical guide of the photoresist.

[0009] The wafer is subjected to a first cooling process;

[0010] A second layer of photoresist is spin-coated onto the wafer;

[0011] The wafer is then subjected to a second soft bake;

[0012] The wafer is cooled a second time to obtain a resist layer.

[0013] The low-viscosity photoresist thickening method provided in this application effectively overcomes the limitations of single-layer photoresist thickness through two spin-coating and baking processes, achieving a thicker resist layer. This technology allows for the creation of thicker resist layers based on existing low-viscosity photoresists, not only reducing the equipment requirements for high-viscosity photoresists but also significantly reducing equipment investment and material testing time. Furthermore, the precise temperature and time control during the first soft bake ensures that the first photoresist layer exhibits good film quality and adhesion while evaporating solvents, laying the foundation for high-quality spin-coating of the subsequent second photoresist layer.

[0014] In a preferred embodiment, prior to the first soft baking step, there is a step of performing a first EBR treatment on the first layer of photoresist.

[0015] Incorporating an EBR (Edge Bead Removal) step helps improve pattern clarity in photolithography, preventing excess photoresist at the edges from affecting the accuracy of subsequent processes during spin coating. The first EBR treatment is performed after spin coating the photoresist, ensuring that excess photoresist at the edges is removed before the initial soft bake, thus avoiding interference with the subsequent high-quality spin coating of the second layer of photoresist. This contributes to improving the overall uniformity and processing quality of the resist layer.

[0016] In a preferred embodiment, the first EBR treatment employs a chemical edge washing method.

[0017] Using chemical edge washing for EBR processing allows for precise control of the washing effect and reduces the impact of residual photoresist on subsequent processes. Chemical methods are highly efficient at removing edge photoresist, minimizing physical damage that may occur with mechanical edge washing, ensuring the cleanliness of the photoresist film at the wafer edges, and further improving the edge stability of the photoresist coating.

[0018] In a preferred embodiment, the rotation speed of the first EBR process is lower than the rotation speed used when spin-coating the first layer of photoresist.

[0019] By reducing the rotation speed during EBR processing, the removal range of photoresist at the wafer edge can be better controlled, avoiding over-processing of the EBR due to excessively high rotation speed, which would reduce the effective thickness of the photoresist. Fine adjustment of the rotation speed helps improve the uniformity of the photoresist, especially in processes with high thickness requirements, ensuring the integrity of the thick film.

[0020] As a preferred embodiment, the coating machine used for spin-coating photoresist has a spraying capacity suitable for solutions with viscosities ranging from 10 cP to 800 cP. This ensures that the coating equipment can flexibly handle photoresists with different viscosity ranges, and this requirement is relatively low, as most coating machines can meet it. This effectively reduces the need for equipment hardware upgrades and avoids material replacement issues caused by equipment problems.

[0021] As a preferred embodiment, the temperature of the hot plate used for soft baking is adjustable within a range of 50°C to 150°C. This wide temperature adjustment range allows for flexible selection of the optimal soft baking parameters according to different photoresists and process requirements.

[0022] In a preferred embodiment, the viscosity of the photoresist used is greater than or equal to 400 cP and less than 800 cP. Photoresist with this viscosity can enhance the mechanical strength and precision tolerance of the photoresist when forming a thicker coating in a single spin coating, meeting the thick-film requirements of the copper pillar bump process. Furthermore, subsequent spin coating and soft baking can maintain the uniformity and stability of the resist layer while ensuring its thickness.

[0023] In a preferred embodiment, the temperature of the first soft baking is 100℃~120℃, and the time is 5min~10min. By appropriately reducing the soft baking temperature and time, the evaporation rate of the solvent in the photoresist can be effectively controlled, avoiding defects such as cracks or pinholes in the film layer caused by rapid evaporation. Based on photoresist with a viscosity greater than or equal to 400cP, the optimized temperature and time settings not only improve the surface quality of the photoresist film, but also ensure the adhesion between photoresist films in the subsequent spin coating process, thereby improving the overall thickness and performance of the final resist layer.

[0024] In a preferred embodiment, the first cooling temperature is room temperature, and the time is 30 seconds. 30 seconds of room temperature cooling removes heat from the photoresist surface, which is beneficial for the next spin coating. This short cooling time stabilizes the photoresist layer, avoids excessively rapid cooling, and does not significantly increase process time, thus improving production efficiency.

[0025] In a preferred embodiment, prior to the second soft baking step, a second EBR treatment is performed on the second photoresist layer. Performing the EBR treatment after the second spin coating further ensures clean edges for each photoresist layer, contributing to the uniformity of the stacked photoresist layers.

[0026] In a preferred embodiment, the second EBR treatment employs a chemical edge washing method.

[0027] In a preferred embodiment, the rotation speed of the second EBR process is lower than the rotation speed used when spin-coating the second layer of photoresist.

[0028] In a preferred embodiment, the second cooling is performed at room temperature for 30 seconds.

[0029] The beneficial effects of this invention are: the low viscosity photoresist thickening method of this invention is simple and easy to implement, can break through the limit of photoresist layer thickness, expand the applicable process window of photoresist thickness, and use low viscosity photoresist to make a high thickness resist layer, which is conducive to reducing production costs and avoiding the trouble of changing the type of photoresist and the coating machine due to different bump height requirements, which is conducive to product quality stability.

[0030] Other features and advantages of this application will be set forth in the following description and will be apparent in part from the description or may be learned by practicing the application. The objectives and other advantages of this application may be realized and obtained by means of the structures particularly pointed out in the written description and the accompanying drawings. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the existing technology for growing metal bumps on a wafer.

[0032] Figure 2 This is a schematic diagram showing a metal bump height requirement exceeding the photoresist thickness limit, resulting in plating defects.

[0033] Figure 3 This is a schematic diagram of the process of growing metal bumps on a wafer after applying this application.

[0034] Figure reference numerals: 1. Wafer; 21. Titanium layer; 22. Copper layer; 3. Resist layer; 31. First photoresist layer; 32. Second photoresist layer; 4. Metal bump. Detailed Implementation

[0035] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0036] The following disclosure provides many different embodiments or examples for implementing different structures of the invention. To simplify the disclosure, specific examples of components and arrangements are described below. Of course, these are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed.

[0037] Unless otherwise specified, all technical and scientific terms used herein have the standard meaning in the field to which the claimed subject matter pertains. Where multiple definitions exist for a term, the definition herein shall prevail.

[0038] Conventional processes for manufacturing metal bumps in the semiconductor industry include Figure 1 As shown, photoresist is spin-coated onto wafer 1 and cured to form resist layer 3. Then, metal bumps 4 are grown where there is no resist layer 3. Typically, the thickness of resist layer 3 is 20 μm, and the metal bumps 4 are copper bumps with a height of 15 μm. Currently, the height requirements for copper bumps are generally increasing, for example, 25 μm. However, the resist layer formed by low-viscosity photoresist can only reach a maximum thickness of 20 μm. Growing higher metal bumps at this thickness can easily lead to plating defects, such as… Figure 2 As shown. Poor plating can lead to cold solder joints and missing solder joints in the substrate packaging. If a higher viscosity photoresist is used, more expensive equipment is required. Due to the change in photoresist type and coating machine, other equipment and process parameters must be adjusted and tested multiple times.

[0039] To address the above problems, this application provides a method for thickening low-viscosity photoresist, comprising the following steps:

[0040] S1: Spin-coat the first layer of photoresist onto the wafer on which the Ti-Cu thin film seed layer has been grown.

[0041] S2: Perform the first soft bake on the wafer.

[0042] S3: Perform the first cooling on the wafer.

[0043] S4: Spin-coat a second layer of photoresist onto the wafer.

[0044] S5: Perform a second soft bake on the wafer.

[0045] S6: Perform a second cooling on the wafer to obtain the resist layer.

[0046] In step S2, the temperature of the first soft baking is lower than the temperature specified in the photoresist technical guide, and the time of the first soft baking is shorter than the time specified in the photoresist technical guide. The Ti-Cu thin film seed layer refers to the deposition of a thin titanium layer 21 and a relatively thin copper layer 22 on the wafer before it undergoes step S1.

[0047] This application's embodiments, through two spin-coating and baking processes, effectively overcome the limitations of single-layer photoresist thickness, achieving a thicker resist layer. This technology allows for the creation of thicker resist layers based on existing low-viscosity photoresists, not only reducing the equipment requirements for high-viscosity photoresists but also significantly decreasing equipment investment and material testing time. Furthermore, the precise temperature and time control during the first soft bake ensures that the first photoresist layer maintains good film quality and adhesion while evaporating solvents, laying the foundation for high-quality spin-coating of the subsequent second photoresist layer.

[0048] Using this method, a metal bump with a relatively large height is formed by the regeneration of the resist layer, which will be like... Figure 3As shown, for example, the thickness of the first photoresist layer 31 is 20 μm, and the thickness of the second photoresist layer 32 is 10 μm. After curing, a total of 30 μm resist layer 3 is formed. At this point, the 25 μm high copper bump will not protrude above the resist layer 3, and plating failure will not occur. Therefore, using this invention, high-quality metal bumps with a relatively large height can be produced based on existing materials and processes. Compared with methods such as changing the type of photoresist, it has minimal changes, minimal cost, and minimal risk, while meeting product process requirements.

[0049] In implementing this invention, appropriate coating speed, soft baking temperature, and time can be selected according to the specific photoresist material and photolithography process. The specific implementation method is a double-layer coating process: coating-soft baking-EBR edge washing-cooling-coating-soft baking-EBR edge washing-cooling. Conventional coating processes are single-step coatings, while this method uses two coatings of photoresist to stack the film thickness. Because the composition, properties, and molecular structure of the two photoresist coatings are consistent, the adhesion between the two layers is not significantly affected even with two overlapping coatings. Detailed explanation follows:

[0050] Before step S2, there is step S11: performing the first EBR treatment on the first layer of photoresist.

[0051] That is, the steps of this method become:

[0052] S1: Spin-coat the first layer of photoresist onto the wafer on which the Ti-Cu thin film seed layer has been grown.

[0053] S11: Perform the first EBR treatment on the first layer of photoresist.

[0054] S2: Perform the first soft bake on the wafer.

[0055] S3: Perform the first cooling on the wafer.

[0056] S4: Spin-coat a second layer of photoresist onto the wafer.

[0057] S5: Perform a second soft bake on the wafer.

[0058] S6: Perform a second cooling on the wafer to obtain the resist layer.

[0059] The first EBR process employs either a physical or chemical method. Chemical edge washing is preferred, and the rotation speed for the first EBR process is lower than that used when spin-coating the first layer of photoresist.

[0060] The first EBR process removes excess photoresist from the wafer edges, preventing distortion or irregularities at the edges of the first photoresist layer. Performing EBR before the first soft bake helps improve the edge uniformity of the first photoresist layer, ensuring the quality of subsequent spin coating and exposure processes for the second photoresist layer. It also helps the second photoresist layer to be spin-coated onto a flat surface and ensures a smooth top surface for the second photoresist layer itself.

[0061] Preferably, the first cooling temperature is room temperature, and the time is 30 seconds. 30 seconds of room temperature cooling removes heat from the photoresist surface, which is beneficial for the next spin coating. This short cooling time stabilizes the photoresist layer, avoids excessively rapid cooling, and does not significantly increase process time, thus improving production efficiency. Alternatively, the second cooling temperature can also be room temperature, and the time can be 30 seconds.

[0062] In a preferred embodiment, before step S5, there is a step S41: performing a second EBR process on the second layer of photoresist.

[0063] That is, the steps of this method become:

[0064] S1: Spin-coat the first layer of photoresist onto the wafer on which the Ti-Cu thin film seed layer has been grown.

[0065] S2: Perform the first soft bake on the wafer.

[0066] S3: Perform the first cooling on the wafer.

[0067] S4: Spin-coat a second layer of photoresist onto the wafer.

[0068] S41: Perform a second EBR process on the second layer of photoresist.

[0069] S5: Perform a second soft bake on the wafer.

[0070] S6: Perform a second cooling on the wafer to obtain the resist layer.

[0071] Or:

[0072] S1: Spin-coat the first layer of photoresist onto the wafer on which the Ti-Cu thin film seed layer has been grown.

[0073] S11: Perform the first EBR treatment on the first layer of photoresist.

[0074] S2: Perform the first soft bake on the wafer.

[0075] S3: Perform the first cooling on the wafer.

[0076] S4: Spin-coat a second layer of photoresist onto the wafer.

[0077] S41: Perform a second EBR process on the second layer of photoresist.

[0078] S5: Perform a second soft bake on the wafer.

[0079] S6: Perform a second cooling on the wafer to obtain the resist layer.

[0080] The second EBR process employs either physical or chemical methods. Chemical methods are preferred for edge washing, and the rotation speed during the second EBR process is lower than that used when spin-coating the second layer of photoresist.

[0081] The purpose of the second EBR process is to remove excess photoresist (often called edge beads) from the wafer edges to prevent particle contamination or other defects in subsequent processes. Ensuring clean wafer edges improves the reliability of subsequent processes.

[0082] The coating machine used for spin coating the photoresist in steps S1 and S4 has a spraying capacity suitable for solutions with viscosities ranging from 10 cP to 800 cP. This requirement is relatively low and can be met by most coating machines, effectively reducing the need for equipment hardware upgrades.

[0083] The temperature of the hot plate used in steps S2 and S5 of the soft baking process is adjustable from 50°C to 150°C. This wide temperature range allows the soft baking process to flexibly adapt to different types of photoresist materials. By precisely controlling the baking temperature, it is ensured that the solvent gradually evaporates at the appropriate temperature, avoiding premature hardening or cracking of the photoresist film due to excessive temperature. This ensures the uniformity and quality of the photoresist layer and reduces process defects.

[0084] In some embodiments, the viscosity of the photoresist used is greater than or equal to 400 cP and less than 800 cP. Accordingly, the temperature of the first soft bake is 100°C to 120°C, and the time is 5 min to 10 min. Appropriate control of the soft bake temperature and time helps the solvent in the photoresist to gradually evaporate, preventing the photoresist surface from hardening too quickly at high temperatures or causing stress cracks. This temperature range effectively maintains the uniformity of the photoresist layer and ensures interlayer adhesion during subsequent coating processes, ultimately forming a stable thick film structure.

[0085] Application Cases

[0086] For example, a conventional Class A photoresist with a viscosity of 500 cP can produce a resist layer with a maximum thickness of 15 μm. However, the current product process requires a resist layer thickness of 25 μm, which cannot be met by a single application of Class A photoresist. Switching to a high-viscosity Class B photoresist with a viscosity of 800 cP would increase costs and material testing time, and also carries the risk that the existing equipment might not be compatible with high-viscosity photoresist coating, potentially requiring the purchase of new equipment, leading to significant losses and waste in both cost and time. Using the coating method of this invention, the resist thickness can be accumulated. After two coatings, the maximum resist layer thickness is expected to reach 25 μm to 40 μm, meeting the product process requirements. Furthermore, using existing photoresist materials and coating equipment significantly improves risk control, reduces material costs, and minimizes time.

[0087] Therefore, the embodiments of this application can break through the original limits of photoresist thickness. Thick-film photoresist processes are commonly used in the semiconductor industry for products manufactured using bumping technology. This invention effectively solves the problem of insufficient viscosity of the photoresist itself or the inability of the equipment hardware to fabricate high-thickness resist layers. This method is simple, easy to implement, and highly effective, and can be widely applied in the semiconductor manufacturing field. By implementing this invention, the quality and stability of semiconductor products can be effectively improved, production costs reduced, and the sustainable development of the semiconductor industry promoted.

[0088] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0089] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A method for thickening low-viscosity photoresist, characterized in that, Includes the following steps: The first layer of photoresist is spin-coated onto the wafer on which the Ti-Cu thin film seed layer has been grown; The wafer is subjected to a first soft bake; The temperature of the first soft baking is lower than the temperature specified in the technical instructions for the photoresist, and the time of the first soft baking is shorter than the time specified in the technical instructions for the photoresist. The wafer is subjected to a first cooling process; A second layer of photoresist is spin-coated onto the wafer; The wafer is then subjected to a second soft bake; The wafer is subjected to a second cooling process to obtain a resist layer; The two coats of photoresist have the same composition, properties, and molecular structure.

2. The method for thickening low-viscosity photoresist according to claim 1, characterized in that, Before the first soft baking step, there is a step: performing a first EBR treatment on the first layer of photoresist.

3. The method for thickening low-viscosity photoresist according to claim 2, characterized in that, The first EBR treatment uses a chemical method for edge washing.

4. The method for thickening low-viscosity photoresist according to claim 2, characterized in that, The rotation speed of the first EBR process is lower than the rotation speed used when spin-coating the first layer of photoresist.

5. The method for thickening low-viscosity photoresist according to claim 1, characterized in that, The coating machine used for spin-coating photoresist has a spraying capability that can handle solutions with viscosities ranging from 10 cP to 800 cP.

6. The method for thickening low-viscosity photoresist according to claim 1, characterized in that, The temperature of the hot plate used for soft baking is adjustable from 50℃ to 150℃.

7. The method for thickening low-viscosity photoresist according to claim 1, characterized in that, The viscosity of the photoresist used is greater than or equal to 400 cP and less than 800 cP.

8. The method for thickening low-viscosity photoresist according to claim 1, characterized in that, The first cooling process was carried out at room temperature for 30 seconds.

9. The method for thickening low-viscosity photoresist according to claim 1, characterized in that, Before the second soft baking step, there is a step: performing a second EBR treatment on the second layer of photoresist.

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