A method for preparing high-indium pillars with small aperture

Abstract

This application relates to the field of semiconductor manufacturing, and in particular to a method for fabricating high indium pillars with small apertures. The method specifically includes the following steps: homogenizing a photoresist layer on the surface of the device to be formed into an indium pillar array, with a photoresist thickness of 10-12 μm; pre-baking at 100°C for 2 minutes; exposure for 3.8-4.2 seconds to control the photoresist angle at 133-137°; development and fixing to form openings with apertures less than 7 μm; hard baking at 110°C for 1 minute; evaporation deposition of the indium pillars; and photoresist stripping to form high-height indium pillars with small apertures, thereby improving the quality of the indium pillars.

Description

Technical Field

[0001] This application relates to the field of semiconductor manufacturing, and in particular to a method for preparing high-indium pillars with small apertures. Background Technology

[0002] Indium pillars are used to connect devices, such as large-area infrared chips, and corresponding signal processing circuits. For related information, please refer to the background technology in the paper "Preparation method of high-density micro indium pillar end face flattening for infrared focal plane devices" (publication number CN103413814A). The core point is the formation of high indium pillars under small aperture openings in photoresist.

[0003] To obtain high-quality indium pillars, the photoresist openings need to have an undercut structure, that is, the bottom of the opening is wide and the top is narrow. This makes it difficult for the bottom side of the opening to adhere to the bottom of the wider indium pillar, so that the morphology of the indium pillar is not easily affected after the photoresist is peeled off. For this purpose, a double-layer adhesive process is required. For details, please refer to the indium pillar and its preparation method published in CN112645276B.

[0004] The double-layer photoresist process is relatively complex, but the 4620 photoresist has low viscosity and thinness when used as a single layer, so the double-layer photoresist process can only be used. Therefore, the 8000 negative photoresist needs to be used for single-layer coating, and it can also have an undercut structure during development, which is more suitable for the preparation of indium pillars with high-quality appearance. You can refer to the indium pillar preparation method of infrared focal plane chip published in CN106024982A.

[0005] In the process of preparing indium pillars using the 8000 photoresist, the photoresist thickness is 7-9 μm and the exposure time is 10 seconds. This results in a thin photoresist thickness and a large photoresist angle, which has a significant impact on the height of the indium pillar when the aperture is small. In particular, when the aperture is less than 7 μm, the average height of the indium pillar is only 4 μm, resulting in low indium pillar quality. Summary of the Invention

[0006] In order to prepare high-quality indium pillars with smaller apertures, this application provides a method for preparing high-indium pillars with small apertures.

[0007] The method for preparing a high-indium pillar with a small aperture provided in this application adopts the following technical solution.

[0008] A method for preparing a high-indium pillar with a small aperture includes the following steps.

[0009] S1. Apply a thin layer of adhesive to the surface of the indium pillar array to be formed, with a thickness of 10-12 μm.

[0010] S2. Perform pre-baking at 100℃ for 2 minutes;

[0011] S3, perform an exposure of 3.8-4.2s to control the photoresist angle at 133-137°;

[0012] S4. Develop and fix to form openings with a pore size of less than 7μm;

[0013] S5. Perform a hard film baking at 110°C for 1 minute.

[0014] S6. Perform indium column evaporation deposition;

[0015] S7. Remove the photoresist.

[0016] By adopting the above technical solution, the photoresist thickness is increased, thereby increasing the corresponding aperture depth. This provides sufficient height for the formation of taller indium pillars. Furthermore, the exposure time is shortened under the premise of successful development, so that the photoresist angle is not too large, making the height of the deposited indium pillars less restricted. Consequently, the height of the indium pillars can be successfully increased under small apertures, which in turn helps to improve the morphology of the indium pillars.

[0017] Optionally, in step S6, a mixture of surfactant and argon gas is injected into the vacuum evaporation chamber.

[0018] By adopting the above technical solution, the surfactant can reduce the surface tension of the evaporated indium metal droplets, increase the wetting angle of the indium pillar, reduce the lateral growth of the indium pillar, promote the vertical growth of the indium pillar, and thus effectively increase the height of the indium pillar under small aperture.

[0019] Optionally, the surfactant may include perfluorooctane sulfonic acid.

[0020] By adopting the above technical solution, perfluorooctane sulfonic acid, as an anionic surfactant, is significantly more efficient than other types of surfactants in reducing the surface tension of indium metal droplets. It also maintains excellent stability at a high temperature of 200℃ and has good compatibility with indium.

[0021] Optionally, the surfactant has a volume percentage of 0.1-0.5% in argon gas.

[0022] By adopting the above technical solution, too little surfactant will result in an insignificant increase in the height of the indium pillar, while too much surfactant will cause the excess fluoride to decompose under vacuum heat, resulting in gaps or holes on the surface of the indium pillar, affecting the morphology of the indium pillar. Furthermore, the surface of the indium pillar is easily covered with a thin layer of fluoride, which affects the bonding strength between the device and the pad.

[0023] Optionally, in step S6, the evaporation source is activated and a mixed gas is injected.

[0024] By adopting the above technical solution, the surfactant cannot be adsorbed on the unactivated solid indium source, which can reduce the waste of surfactant and ensure sufficient phase contact between indium vapor and surfactant, so as to better control the deposition and formation of indium pillars.

[0025] Optionally, the mixed gas is injected in a pulsed manner.

[0026] Optionally, the pulse period of the mixed gas is 1-2 injections per second, the duration of each injection is 5-10 ms, and the injection flow rate is 0.2-0.4 L / min.

[0027] By adopting the above technical solution, the amount of surfactant injected can be better controlled, ensuring that the amount of surfactant remains appropriate.

[0028] Optionally, after deposition is completed in S6, a two-step annealing process is performed.

[0029] By adopting the above technical solution, it is helpful to stabilize the morphology of the indium pillar.

[0030] Optionally, in the two-step annealing, the first step is to solidify at 80-100℃ for 180s in an inert atmosphere, and the second step is to maintain the morphology at 150-180℃ for 90s in a reducing atmosphere.

[0031] Optionally, the inert atmosphere includes nitrogen, and the reducing atmosphere includes hydrogen and argon in a volume ratio of 1:10.

[0032] By adopting the above technical solution, the first annealing step volatilizes the residual trace solvent and surfactant, and reduces the internal stress of the indium pillar, thereby stabilizing the morphology of the indium pillar. The second annealing step uses hydrogen to reduce the oxide layer on the surface of the indium pillar, which improves the subsequent bonding strength. At this time, the temperature is close to the melting point of indium, which causes a nanoscale liquid film to appear on the surface of the indium pillar. This allows the edges and grain boundary grooves of the indium pillar to migrate towards the plane and the top of the pillar, improving the morphology of the indium pillar. It can also further reduce the internal stress of the indium pillar and improve the mechanical properties of the indium pillar.

[0033] In summary, this application includes at least the following beneficial effects.

[0034] 1. Increasing the photoresist thickness increases the corresponding aperture depth, providing sufficient height for the formation of taller indium pillars. Furthermore, shortening the exposure time while ensuring successful development prevents the photoresist angle from becoming too large, making the height of the deposited indium pillars less restricted. Consequently, the height of the indium pillars can be successfully increased even with small apertures, which in turn helps improve the morphology of the indium pillars.

[0035] 2. Surfactants can reduce the surface tension of evaporated indium metal droplets, increase the wetting angle of indium pillars, reduce the lateral growth of indium pillars, promote the vertical growth of indium pillars, and thus effectively increase the height of indium pillars under small apertures. Attached Figure Description

[0036] Figure 1 This is a flowchart of the main steps of this application;

[0037] Figure 2 (a) is a scanning electron microscope (SEM) image of the indium column of Example 1, and (b) is a scanning electron microscope (SEM) image of the indium column of Comparative Example 1. Detailed Implementation

[0038] The present application will be further described in detail below with reference to the accompanying drawings.

[0039] This application discloses a method for preparing a high-indium column with a small aperture, referring to... Figure 1 Specifically, it includes the following steps.

[0040] S1. Apply a uniform adhesive to the surface of the device to be formed, which can be a large-area infrared chip with an adhesive thickness of 10-12 μm.

[0041] S2. Perform pre-baking at 100℃ for 2 minutes.

[0042] S3. Perform an exposure of 3.8-4.2s to control the photoresist angle at 134-136°.

[0043] S4. Develop in RD6 developer for 20 seconds, then fix in deionized water for 30 seconds to form openings with a pore size of less than 7 μm. Here, pore size refers to the top of the opening, i.e., the minimum pore size.

[0044] S5. Perform a hard film baking at 110°C for 1 minute.

[0045] S6. Perform indium column evaporation deposition.

[0046] When the evaporation source is activated in S6, i.e. when the indium source is heated to 200°C, a mixture of surfactant and argon is injected 1 cm above the indium source. The surfactant is preferably perfluorooctane sulfonic acid, and the surfactant accounts for 0.1-0.5% of the volume of the argon. The surfactant is injected in a pulsed manner, with the pulse period of the mixed gas being 1-2 injections per second, the duration of each injection being 5-10 ms, and the injection flow rate being 0.2-0.4 L / min.

[0047] In addition, before proceeding to S7 after S6, a two-step annealing process is performed. The first step of annealing is carried out at 80-100℃ for 180s in an inert atmosphere, which can be nitrogen, to solidify the structure. The second step of annealing is carried out at 150-180℃ for 90s in a reducing atmosphere to stabilize the morphology. The reducing atmosphere can be hydrogen and argon in a volume ratio of 1:10.

[0048] S7. Place the device in acetone to strip the photoresist.

[0049] The following detailed description uses specific embodiments and comparative examples.

[0050] Example 1:

[0051] S1. Apply a uniform adhesive to the surface of the indium pillar array to be formed. The device can be a large-area infrared chip with an adhesive thickness of 10μm.

[0052] S2. Perform pre-baking at 100℃ for 2 minutes.

[0053] S3. Perform an exposure of 3.8 seconds to control the photoresist angle at 134°.

[0054] S4. Develop in RD6 developer for 20 seconds, then fix in deionized water for 30 seconds to form an opening with a pore size of 5μm.

[0055] S5. Perform a hard film baking at 110°C for 1 minute.

[0056] S6. Perform indium column evaporation deposition.

[0057] When the evaporation source is activated in S6, i.e. when the indium source is heated to 200°C, a mixture of surfactant and argon is injected 1 cm above the indium source. The surfactant is preferably perfluorooctane sulfonic acid, and the surfactant accounts for 0.1% of the volume of the argon. The surfactant is injected in a pulsed manner, with the pulse period of the mixed gas being once per second, the duration of each injection being 5 ms, and the injection flow rate being 0.2 L / min.

[0058] In addition, before proceeding to S7 after S6, a two-step annealing process is performed. The first step of annealing is carried out at 80°C for 180 seconds in an inert atmosphere, which can be nitrogen. The second step of annealing is carried out at 150°C for 90 seconds in a reducing atmosphere to stabilize the appearance. The reducing atmosphere can be hydrogen and argon in a volume ratio of 1:10.

[0059] S7. Place the device in acetone to strip the photoresist.

[0060] Example 2:

[0061] S1. Apply a uniform adhesive to the surface of the indium pillar array to be formed. The device can be a large-area infrared chip with an adhesive thickness of 12μm.

[0062] S2. Perform pre-baking at 100℃ for 2 minutes.

[0063] S3. Perform an exposure of 4.2s to control the photoresist angle at 137°.

[0064] S4. Develop in RD6 developer for 20 seconds, then fix in deionized water for 30 seconds to form an opening with a pore size of 5μm.

[0065] S5. Perform a hard film baking at 110°C for 1 minute.

[0066] S6. Perform indium column evaporation deposition.

[0067] When the evaporation source is activated in S6, i.e. when the indium source is heated to 200°C, a mixture of surfactant and argon is injected 1 cm above the indium source. The surfactant is preferably perfluorooctane sulfonic acid, and the surfactant accounts for 0.5% of the volume of the argon. The surfactant is injected in a pulsed manner, with the pulse period of the mixed gas being 2 injections per second, each injection lasting 10 ms, and the injection flow rate being 0.4 L / min.

[0068] In addition, before proceeding to S7 after S6, a two-step annealing process is performed. The first step of annealing is carried out at 100°C for 180 seconds in an inert atmosphere, which can be nitrogen, to solidify the structure. The second step of annealing is carried out at 180°C for 90 seconds in a reducing atmosphere to stabilize the morphology. The reducing atmosphere can be hydrogen and argon in a volume ratio of 1:10.

[0069] S7. Place the device in acetone to strip the photoresist.

[0070] Comparative Example 1:

[0071] The difference from Example 1 is that the exposure time is 10s, and the mixture of surfactant and argon gas is not injected in S6, and no two-step annealing is performed after S6.

[0072] Comparative Example 2:

[0073] The difference from Example 1 is that no two-step annealing was performed after S6.

[0074] The indium pillars on the devices of the above embodiments and comparative examples were subjected to the following tests.

[0075] The height of the indium pillars is measured by SEM. Five to eight indium pillars on a device can be selected, and the height of each indium pillar can be counted and the average height calculated.

[0076] Shear strength was determined using a push-pull force tester, such as the solder ball shearing module of the Dage 4000, by vertically shearing an indium pillar at a speed of 4 μm / s with a shearing tool height of 3 μm. The breaking force of the indium pillar was recorded, and the shear strength was calculated in conjunction with the base area of ​​the indium pillar.

[0077] Pull-out strength was determined using a universal testing machine, such as the Instron 5943, with micron-level clamps. The clamps were bonded to the top of the indium pillar with epoxy resin and then vertically removed at a rate of 2 μm / s. The maximum tensile force was recorded during the separation process between the indium pillar and the device, and the pull-out strength was calculated in combination with the base area of ​​the indium pillar.

[0078] The specific results are shown in the table below. Figure 2 Since the indium pillar heights of Examples 1, 2 and Comparative Example 2 are similar, scanning electron microscope images of Examples 1 and 1 are used for illustration.

[0079]

[0080] Combining Examples 1 and 2 and Comparative Example 1, and simultaneously combining Figure 2 It is known that increasing the film thickness, extending the exposure time, and adding surfactants can effectively increase the height of the indium pillar under small aperture, which helps to improve the quality of the indium pillar under small aperture.

[0081] As can be seen from Examples 1 and 2 and Comparative Example 2, two-step annealing can effectively improve the mechanical properties of indium pillars, enabling them to have a longer lifespan during subsequent device soldering and contributing to the quality improvement of indium pillars with small apertures.

[0082] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A method for preparing a high-indium column with a small aperture, characterized in that: Specifically, the following steps are included: S1. Apply a thin layer of adhesive to the surface of the indium pillar array to be formed, with a thickness of 10-12 μm. S2. Perform pre-baking at 100℃ for 2 minutes; S3, perform an exposure of 3.8-4.2s to control the photoresist angle at 133-137°; S4. Develop and fix to form openings with a pore size of less than 7μm; S5. Perform a hard film baking at 110°C for 1 minute. S6. Perform indium column evaporation deposition; S7. Remove the photoresist.

2. The method for preparing a high-indium pillar with a small aperture according to claim 1, characterized in that: In step S6, a mixture of surfactant and argon gas is injected into the vacuum evaporation chamber.

3. The method for preparing a high-indium pillar with a small aperture according to claim 2, characterized in that: The surfactant includes perfluorooctane sulfonic acid.

4. The method for preparing a high-indium pillar with a small aperture according to claim 3, characterized in that: The surfactant has a volume percentage of 0.1-0.5% in argon gas.

5. The method for preparing a high-indium pillar with a small aperture according to claim 2, characterized in that: In step S6, the evaporation source is activated to inject mixed gas.

6. The method for preparing a high-indium pillar with a small aperture according to claim 2, characterized in that: The mixed gas is injected in a pulsed manner.

7. The method for preparing a high-indium pillar with a small aperture according to claim 6, characterized in that: The pulse period of the mixed gas is 1-2 injections per second, with each injection lasting 5-10 ms and an injection flow rate of 0.2-0.4 L / min.

8. The method for preparing a high-indium pillar with a small aperture according to claim 1, characterized in that: After deposition is completed, S6 undergoes a two-step annealing process.

9. The method for preparing a high-indium pillar with a small aperture according to claim 8, characterized in that: In the two-step annealing process, the first step involves consolidation at 80-100℃ for 180 seconds in an inert atmosphere, and the second step involves maintaining the morphology stable at 150-180℃ for 90 seconds in a reducing atmosphere.

10. The method for preparing a high-indium pillar with a small aperture according to claim 9, characterized in that: The inert atmosphere includes nitrogen, and the reducing atmosphere includes hydrogen and argon in a volume ratio of 1:10.

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