A double-layer positive photoresist stripping method for preparing metal bumps

Abstract

This invention discloses a method for removing a double-layer positive photoresist for preparing metal bumps: S1, cleaning the epitaxial wafer; S2, spin-coating a first-type positive photoresist as the bottom layer onto the epitaxial wafer; S3, soft baking to cure the first-type positive photoresist; S4, spin-coating a second-type positive photoresist as the top layer onto the first-type positive photoresist; S5, soft baking to cure the second-type positive photoresist; S6, exposure using a photolithography machine to expose the double-layer photoresist; S7, post-baking; S8, development using an alkaline developer to develop the double-layer photoresist; S9, metal deposition using a metal deposition apparatus; S10, removing the double-layer photoresist using a photoresist stripping solution; S11, preparing metal bumps on the surface of the epitaxial wafer. This invention achieves a lift-off process using a double-layer positive photoresist, solving the problem of difficult preparation of small-sized metal bumps in Micro-LED micro-display devices, and enabling the preparation of metal bumps with small spacing and high thickness.

Description

Technical Field

[0001] This invention relates to the field of advanced semiconductor packaging technology, and in particular to a method for removing a double-layer positive photoresist for preparing metal bumps. Background Technology

[0002] Metal bumps are a crucial component in advanced semiconductor packaging technology. For example, the packaging of Micro-LED microdisplay devices requires metal bumps, which are typically fabricated using a lift-off process. The basic steps of the lift-off process are: first, spin-coating one or more layers of photoresist onto a clean epitaxial wafer surface; then, after photolithography processes such as soft baking, exposure, post-baking, and development, resulting in an "undercut" profile of the photoresist pattern; next, obtaining a discontinuous metal film on the epitaxial wafer surface through methods such as metal deposition; and finally, using a photoresist stripper to remove the photoresist and the metal film on it, leaving the metal bumps in direct contact with the epitaxial wafer intact. As the electrode size of Micro-LED chips continues to shrink, the metal bumps that need to be fabricated on the electrodes are also becoming smaller and more spaced, with dimensions ranging from several micrometers to sub-micrometers. This places even higher demands on the lift-off process.

[0003] The key to the lift-off process lies in the different cross-sectional shapes of the photoresist pattern. If the photoresist pattern has an "undercut" cross-section, no metal film will deposit on the photoresist sidewalls during metal deposition. This allows the photoresist to dissolve quickly during the stripping process, making it easy to remove the metal film. Conversely, if the photoresist pattern has a "topcut" cross-section, a large amount of metal will deposit on the photoresist sidewalls during metal deposition, encasing them and preventing the stripping solution from contacting the photoresist, making it difficult to remove. Generally, positive photoresist produces a "topcut" pattern after development, while negative photoresist produces an "undercut" pattern. Therefore, negative photoresist is more suitable for the stripping process than positive photoresist. However, negative photoresist tends to absorb developer during development, resulting in "swelling," which limits its resolution and makes it difficult to use in lift-off processes for fabricating small metal bumps.

[0004] One solution is to use a double-layer photoresist stripping method. The principle is as follows: a double-layer photoresist, where the top layer is typically a high-resolution positive photoresist used to control the size of the metal bumps; the bottom layer is typically a non-photosensitive release retardant (LOR). The development rate of the bottom LOR is more than 1.5 times that of the top high-resolution positive photoresist. Therefore, by controlling the development time, the cross-section of the double-layer photoresist pattern can be formed into an "undercut" shape. Currently, this double-layer photoresist stripping method is commonly used to fabricate small-sized metal bumps. However, this method also has shortcomings: when the fabricated metal bumps are very dense, that is, the spacing between the metal bumps is very small, the corresponding spacing of the photolithographic pattern is also very small. Since the development rate of the bottom photoresist is faster than that of the top photoresist, when fabricating such small-spacing photolithographic patterns, it is easy to cause over-development of the bottom release retardant, leading to a "collapse" phenomenon in the top photoresist, making it difficult to meet the stripping requirements.

[0005] Existing patents, such as Chinese Patent Publication No. CN115542687A, published on December 30, 2022, disclose a double-layer resist stripping method for KrF photolithography. Based on 248nm photoresist, by selecting two positive photoresists with different development rates, a bottom-cut stepped photoresist profile can be obtained after baking and development. Then, metal is deposited using electron beam evaporation to obtain a discontinuous metal layer on the substrate surface. Finally, a suitable stripping solution is used to remove the mask layer and its metal layer, ultimately obtaining metal lines of 0.45μm-0.25μm. However, this method has the following shortcomings: 1. The photoresist used in this method consists of two layers of 248nm positive photoresist, which carries the risk of fusion; 2. This method requires very precise control of the development rate, easily leading to underdevelopment or overdevelopment; 3. The bottom resist layer used in this method is relatively thin, making it impossible to fabricate thick metal bumps.

[0006] In summary, there is currently no mature and stable method for removing double-layer photoresist for fabricating metal bumps with small spacing and high thickness. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a method for lifting off a double-layer positive photoresist for fabricating metal bumps. Unlike existing solutions, this invention utilizes a double-layer positive photoresist with different resolutions to achieve a lift-off process, enabling the fabrication of metal bumps with small spacing.

[0008] This invention is achieved through the following scheme:

[0009] A method for removing a double-layer positive photoresist for fabricating metal bumps includes the following steps:

[0010] S1, epitaxial wafer cleaning;

[0011] S2, a first type of positive photoresist is spin-coated onto the epitaxial wafer;

[0012] S3, soft baking, to cure the first type of positive photoresist;

[0013] S4, spin-coating the top layer of the second type of positive photoresist onto the surface of the first type of positive photoresist;

[0014] S5, soft baking, to cure the second type of positive photoresist;

[0015] S6, Exposure, using a lithography machine to expose the double-layer photoresist;

[0016] S7, post-baking, reduces the "standing wave" effect of the photoresist;

[0017] S8, Development: Develop the double-layer photoresist using an alkaline developer.

[0018] S9, Metal deposition, depositing a metal film on the developed epitaxial wafer using a metal deposition equipment;

[0019] S10, Use stripping solution to strip the double-layer photoresist;

[0020] S11, metal bumps are prepared on the surface of the epitaxial wafer;

[0021] The resolution of the first type of positive photoresist is lower than that of the second type of positive photoresist. In step S6, the photochemical reaction area of ​​the first type of positive photoresist is larger than that of the second type of positive photoresist. After development in step S8, a double-layer photoresist cross-sectional pattern in the form of an "undercut" is obtained.

[0022] Furthermore, the non-exposed areas of the first type of positive photoresist are soluble in alkaline developing solution; the non-exposed areas of the second type of positive photoresist are insoluble in alkaline developing solution.

[0023] Preferably, the development time of the double-layer positive photoresist using alkaline developer in step S8 is 80s-180s.

[0024] Furthermore, in step S3, the thickness of the cured first type of positive photoresist is 1μm-14μm to ensure that thick metal bumps can be peeled off subsequently.

[0025] Furthermore, the film-forming resin of the first type of positive photoresist is different from that of the second type of positive photoresist, and the glass transition temperature (Tg) of the film-forming resin in the first type of positive photoresist is higher than that of the film-forming resin in the second type of positive photoresist, so as to ensure that the first type of positive photoresist and the second type of positive photoresist will not fuse during the baking process.

[0026] Preferably, the film-forming resin of the first type of positive photoresist is cyclobutylimide resin, and the film-forming resin of the second type of positive photoresist is phenolic resin.

[0027] In the above method, since the resolution of the first type of positive photoresist is lower than that of the second type of positive photoresist, after the double-layer photoresist is exposed, the photochemical reaction area of ​​the first type of positive photoresist is larger than that of the second type of positive photoresist. After sufficient development, the pattern area of ​​the first type of positive photoresist is larger than that of the second type of positive photoresist, thus obtaining a photoresist cross-sectional pattern with an "undercut" shape. Then, through metal deposition, a discontinuous metal film is obtained on the surface of the epitaxial wafer. Finally, the photoresist and the metal film on the photoresist are stripped off using a photoresist stripping solution, ultimately obtaining metal bumps.

[0028] Furthermore, the non-exposed areas of Type I positive photoresist are soluble in alkaline developer, while the non-exposed areas of Type II positive photoresist are insoluble in alkaline developer. As the development time increases, photoresist cross-sectional patterns with different undercut sizes can be obtained, making stripping easier and more effective. This increases the controllable range of development time and enhances process controllability.

[0029] The double-layer positive photoresist stripping method for fabricating metal bumps provided by this invention can solve the problem of difficult fabrication of small-sized metal bumps in Micro-LED micro-display devices, and can achieve the fabrication of metal bumps with small spacing and high thickness. Compared with existing double-layer photoresist stripping schemes, this method does not require the use of expensive non-photosensitive underlying stripping adhesive, resulting in lower cost, more reliable implementation, and no problem of difficult photoresist removal. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the double-layer positive photoresist lift-off process in the method provided by the present invention.

[0031] Figure 2 This is a schematic diagram of the epitaxial wafer structure in the method provided by the present invention.

[0032] Figure 3 This is a schematic diagram of the structure after spin-coating two layers of positive photoresist in the method provided by the present invention.

[0033] Figure 4 This is a schematic diagram of the structure in which a double-layer positive photoresist forms an "undercut" shape in the method provided by the present invention.

[0034] Figure 5 This is a schematic diagram of the structure for obtaining a discontinuous metal film using the method provided by the present invention.

[0035] Figure 6This is a schematic diagram of the structure for obtaining metal bumps in the method provided by the present invention.

[0036] Appendix Figure 2-6 The structures represented by each label are listed below:

[0037] Illustration: 1. Substrate; 2. Semiconductor epitaxial layer; 3. Type I positive photoresist; 4. Type II positive photoresist; 5. Metal film; 6. Metal bump. Implementation

[0038] The method provided by the present invention will be briefly described below with reference to some accompanying drawings, so that those skilled in the art can understand the technical scenario of the embodiments of this application.

[0039] In the method provided by this invention, in step S2, the thickness of the first type of positive photoresist spin-coated is between 1 μm and 14 μm. The selection of the film thickness can be determined according to the thickness of the metal bumps to be prepared. In order to achieve a better lift-off effect, the film thickness of the first type of positive photoresist is generally more than 1.25 times the thickness of the metal bumps to be prepared.

[0040] In step S3, the softening temperature of the first type of positive photoresist is 100℃-130℃. If the softening temperature is too high, the photoresist will be denatured; if the softening temperature is too low, the photoresist will not cure sufficiently. The softening time is 1min-5min. If the softening time is too long, the photoresist will be denatured; if the softening time is too short, the photoresist will not cure sufficiently.

[0041] In step S4, a second type of positive photoresist is spin-coated. The thickness of the second type of positive photoresist is between 0.35 μm and 2 μm. The second type of photoresist is used to control the size of the metal bumps, so the thickness of the second type of photoresist can be made as thin as possible.

[0042] In step S5, the softening temperature of the second type of positive photoresist is 95℃-105℃. If the softening temperature is too high, the photoresist will be denatured; if the softening temperature is too low, the photoresist will not cure sufficiently. The softening time is 50s-70s. If the softening time is too long, the photoresist will be denatured; if the softening time is too short, the photoresist will not cure sufficiently.

[0043] In step S6, exposure is performed based on the exposure amount of the second type of positive photoresist, and the first type of positive photoresist and the second type of positive photoresist are exposed simultaneously; where, excessive exposure will cause the subsequent photolithographic pattern to become larger, while insufficient exposure will cause the subsequent photolithographic pattern to fail to develop fully.

[0044] In step S7, the post-exposure baking temperature is 100℃-115℃. If the post-exposure baking temperature is too high, the photoresist will deform. If the post-exposure baking temperature is too low, the "standing wave" effect of the photoresist after exposure cannot be effectively reduced. The post-exposure baking time is 50s-70s. If the post-exposure baking time is too long, the photoresist will deform. If the post-exposure baking time is too short, the "standing wave" effect of the photoresist after exposure cannot be effectively reduced.

[0045] In step S8, an alkaline developer is used to develop the double-layer photoresist, such as 2.38% TMAH, for a development time of 80s-180s. If the development time is too long, the second type of photoresist will "collapse" or crack, and if the development time is too short, the photoresist cross-sectional pattern with an "undercut" shape will not be obtained.

[0046] In step S9, a metal film is deposited using a metal deposition device. The thickness of the metal film is less than or equal to 10 μm, which can be set according to the requirements of the device. The metal film can be a single layer of metal or a multilayer of metal.

[0047] In step S10, a photoresist stripping solution is used for stripping, such as a photoresist stripping solution containing N-methylpyrrolidone as the active ingredient; the stripping time is 10 min to 30 min.

[0048] In step S11, metal bumps are prepared, and the spacing between the metal bumps is greater than or equal to 1 μm. Example 1

[0049] A method for removing a double-layer positive photoresist for fabricating metal bumps, the process flow is as follows: Figure 1 As shown, it includes the following steps:

[0050] S1, cleaning the epitaxial wafer using a standard cleaning procedure. Figure 2 This is a schematic diagram of the epitaxial wafer structure, which is obtained by growing an epitaxial layer 2 on a substrate 1.

[0051] S2, spin-coating the first type of positive photoresist 3, using a spin coater to spin-coat the surface of the epitaxial layer 2 with a positive photosensitive polyimide photoresist of model ZKPI-5510L, resulting in a photoresist film thickness of 2μm;

[0052] S3, soft baking, use a heating stage to bake the epitaxial wafer at a baking temperature of 120℃ for 2 minutes to cure the first type of positive photoresist 3;

[0053] S4, spin-coating the second type of positive photoresist 4 on the top layer, using a spin coater to spin-coat the surface of the first type of positive photoresist 3 with a resolution of up to 0.35 μm, specifically model RZJ-5312-12, resulting in a photoresist film thickness of 5000 Å;

[0054] S5, soft baking: The epitaxial wafer is baked using a heated stage at 95°C for 60 seconds to cure the type II positive photoresist 4, resulting in the structure shown below. Figure 3 As shown: from bottom to top, the layers are substrate 1, epitaxial layer 2, first type positive photoresist 3, and second type positive photoresist 4;

[0055] S6, Exposure: Expose the double-layer photoresist using a photolithography machine at an exposure dose of 50 mJ / cm². 2 ;

[0056] S7, post-baking, using a heating stage to bake the epitaxial wafer at a temperature of 115℃ for 60 seconds, in order to reduce the "standing wave" effect of the photoresist and make the photolithographic lines more delicate;

[0057] S8, development, using 2.38% TMAH developer for 120 seconds, the resulting structure is as follows. Figure 4 As shown, the structures of the first type of positive photoresist 3 and the second type of positive photoresist 4 are in an "undercut" shape;

[0058] S9, Metal Deposition: A metal film is deposited using a metal deposition apparatus. The metal film material is Cr / Pt / Au, with corresponding thicknesses of 300 Å / 300 Å / 5000 Å. The resulting structure is as follows. Figure 5 As shown, a discontinuous metal film 5 covers the surface of the epitaxial layer 2 and the second type of positive photoresist 4;

[0059] S10, use ASTP-130 photoresist stripping solution to strip the metal film 5 on the surface of the first type positive photoresist 3, the second type positive photoresist 4 and the second type positive photoresist 4, the stripping time is 10 min;

[0060] S11, square metal bumps 6 with dimensions of 0.5μm × 0.5μm were prepared, with a spacing of 2μm between adjacent metal bumps, forming a structure as shown in Figure 6. Figure 6 As shown. Example 2

[0061] A method for peeling off a double-layer positive photoresist for fabricating metal bumps, such as Figure 1 As shown, it includes the following steps:

[0062] S1, cleaning the epitaxial wafer using a standard cleaning procedure. Figure 2 This is a schematic diagram of the epitaxial wafer structure, which is obtained by growing an epitaxial layer 2 on a substrate 1.

[0063] S2, spin-coating the first type of positive photoresist 3, using a spin coater to spin-coat the surface of the epitaxial layer 2 with a positive photosensitive polyimide photoresist of model ZKPI-5510L, resulting in a photoresist film thickness of 14μm;

[0064] S3, soft baking, use an oven to bake the epitaxial wafer at a temperature of 120℃ for 3 minutes to cure the first type of positive photoresist 3;

[0065] S4, spin-coating the second type of positive photoresist 4 as the top layer, using a spin coater to spin-coat the surface of the first type of positive photoresist 3 with a positive photoresist of model RZJ-390PG-50, resulting in a photoresist film thickness of 2μm;

[0066] S5, Soft baking: The epitaxial wafer is baked using a heated stage at 100℃ for 120 seconds to cure the type II positive photoresist 4, resulting in the structure shown below. Figure 3 As shown: from bottom to top, the layers are substrate 1, epitaxial layer 2, first type positive photoresist 3, and second type positive photoresist 4;

[0067] S6, Exposure: Expose the double-layer photoresist using a photolithography machine at an exposure dose of 120 mJ / cm². 2 ;

[0068] S7, development, using 2.38% TMAH developer for 160 seconds, the resulting structure is as follows. Figure 4 As shown, the structures of the first type of positive photoresist 3 and the second type of positive photoresist 4 are in an "undercut" shape;

[0069] S9, Metal Deposition: A metal film is deposited using a metal deposition apparatus. The metal film material is Cr / Au, with a corresponding thickness of 300 Å / 20000 Å. The resulting structure is as follows. Figure 5 As shown, a discontinuous metal film 5 covers the surface of the epitaxial layer 2 and the second type of positive photoresist 4;

[0070] S10, using ASTP-130 photoresist stripping solution to strip the metal film 5 on the surface of the first type positive photoresist 3, the second type positive photoresist 4, and the second type positive photoresist 4, the stripping time is 20 min;

[0071] S11, a rectangular metal bump 6 with a size of 2μm × 4μm is prepared. The short sides of adjacent metal bumps are spaced 4μm apart, and the long sides are spaced 4μm apart, forming a structure as shown in Figure 6. Figure 6 As shown. Example 3

[0072] A method for peeling off a double-layer positive photoresist for fabricating metal bumps, such as Figure 1 As shown, it includes the following steps:

[0073] S1, cleaning the epitaxial wafer using a standard cleaning procedure. Figure 2This is a schematic diagram of the epitaxial wafer structure, which is obtained by growing an epitaxial layer 2 on a substrate 1.

[0074] S2, spin-coating the first type of positive photoresist 3, using a spin coater to spin-coat the surface of the epitaxial layer 2 with a positive photosensitive polyimide photoresist of model WPR-5100, resulting in a photoresist film thickness of 3μm;

[0075] S3, soft baking, use a heating stage to bake the epitaxial wafer at a baking temperature of 110℃ for 5 minutes to cure the first type of positive photoresist 3;

[0076] S4, spin-coating the second type of positive photoresist 4 as the top layer, using a spin coater to spin-coat the surface of the first type of positive photoresist 3 with positive photoresist of model RZJ-5312-20, resulting in a photoresist film thickness of 1μm;

[0077] S5, soft baking: The epitaxial wafer is baked using a heated stage at 100℃ for 60 seconds to cure the type II positive photoresist 4, resulting in the structure shown below. Figure 3 As shown: from bottom to top, the layers are substrate 1, epitaxial layer 2, first type positive photoresist 3, and second type positive photoresist 4;

[0078] S6, Exposure: Expose the double-layer photoresist using a photolithography machine at an exposure dose of 70 mJ / cm². 2 ;

[0079] S7, post-baking: The epitaxial wafer is baked using a heating stage at a temperature of 110℃ for 60 seconds to reduce the "standing wave" effect of the photoresist and make the photolithographic lines more delicate.

[0080] S8, development, using 2.38% TMAH developer for 140 seconds, the resulting structure is as follows. Figure 4 As shown, the structures of the first type of positive photoresist 3 and the second type of positive photoresist 4 are in an "undercut" shape;

[0081] S9, Metal Deposition: A metal film is deposited using a metal deposition apparatus. The metal film material is Cr / Pt / Au, with corresponding thicknesses of 300 Å / 1000 Å / 10000 Å. The resulting structure is as follows. Figure 5 As shown, a discontinuous metal film 5 covers the surface of the epitaxial layer 2 and the second type of positive photoresist 4;

[0082] S10, using ASTP-130 photoresist stripping solution to strip the metal film 5 on the surface of the first type positive photoresist 3, the second type positive photoresist 4, and the second type positive photoresist 4, the stripping time is 15 min;

[0083] S11, circular metal bumps 6 with a radius of 5 μm were prepared, with a spacing of 10 μm between adjacent metal bumps, forming a structure as shown. Figure 6 As shown.

[0084] The above embodiments are only used to illustrate the design concept and features of the method of the present invention, and their purpose is to enable those skilled in the art to understand the content of the method of the present invention and implement it accordingly. The protection scope of the method of the present invention is not limited to the above embodiments. Therefore, all equivalent changes or modifications made based on the principles and design ideas disclosed in the present invention are within the protection scope of the present invention.

Claims

1. A method for stripping a double-layer positive photoresist for fabricating metal bumps, characterized in that, Includes the following steps: S1, epitaxial wafer cleaning; S2, a first type of positive photoresist is spin-coated onto the epitaxial wafer; S3, soft baking, to cure the first type of positive photoresist; S4, spin-coating the top layer of the second type of positive photoresist onto the surface of the first type of positive photoresist; S5, soft baking, to cure the second type of positive photoresist; S6, Exposure, using a lithography machine to expose the double-layer photoresist; S7, post-baking, reduces the "standing wave" effect of the photoresist; S8, Development: Develop the double-layer photoresist using an alkaline developer. S9, Metal deposition, depositing a metal film on the developed epitaxial wafer using a metal deposition equipment; S10, Use stripping solution to strip the double-layer photoresist; S11, metal bumps are prepared on the surface of the epitaxial wafer; In this process, the resolution of the first type of positive photoresist is lower than that of the second type of positive photoresist. In step S6, the photochemical reaction area of ​​the first type of positive photoresist is larger than that of the second type of positive photoresist. After development in step S8, a double-layer photoresist cross-sectional pattern in the form of an "undercut" is obtained.

2. The method for removing a double-layer positive photoresist for preparing metal bumps according to claim 1, characterized in that, In step S8, the alkaline developer is used to develop the double-layer photoresist for 80s-180s.

3. The method for removing a double-layer positive photoresist for preparing metal bumps according to claim 1, characterized in that, In step S3, the thickness of the cured first type of positive photoresist is 1μm-14μm.

4. The method for removing a double-layer positive photoresist for preparing metal bumps according to claim 1, characterized in that, The film-forming resin of the first type of positive photoresist is different from that of the second type of positive photoresist, and the glass transition temperature of the film-forming resin in the first type of positive photoresist is higher than that of the film-forming resin in the second type of positive photoresist.

5. The method for removing a double-layer positive photoresist for preparing metal bumps according to claim 4, wherein the film-forming resin of the first type of positive photoresist is cyclobutylimide resin, and the film-forming resin of the second type of positive photoresist is phenolic resin.

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