Ubm structures, brain-computer interface electrodes, and methods

Abstract

This disclosure presents a UBM structure, a brain-computer interface electrode, and a method. The UBM structure is used for a brain-computer interface electrode, which includes a flexible electrode layer and a flexible electrode substrate covering the flexible electrode layer. The flexible electrode substrate has an opening exposing the flexible electrode layer, and the UBM structure is disposed within the opening. The UBM structure includes: a first adhesion layer attached to the flexible electrode layer exposed at the bottom of the opening; a wetting layer attached to the adhesion layer; a second adhesion layer attached to the wetting layer; and a surface protective layer attached to the second adhesion layer. The total thickness of the UBM structure is less than or equal to 1 micrometer. The first and second adhesion layers are made of titanium, the wetting layer is made of copper or gold, and the surface protective layer is made of gold or platinum. The technical solution of this disclosure achieves multiple improvements in biocompatibility, electrical performance, mechanical reliability, and process feasibility.

Description

Technical Field

[0001] This disclosure relates to the interdisciplinary field of microelectronic packaging and biomedical engineering, specifically to a UBM structure, brain-computer interface electrodes, and methods. Background Technology

[0002] Brain-Machine Interface (BMI) electrodes require high-density interconnects at the micrometer scale to ensure long-term stable electrochemical performance and biocompatibility. Existing electrode-chip interconnect methods include wire bonding and flip-chip bonding.

[0003] Wire bonding is a simple process, but it occupies a large area, and the reliability of the connection is limited under long-term mechanical stress and liquid environments. Flip bonding can achieve high-density, miniaturized, and low parasitic inductance packaging, making it suitable for the miniaturization requirements of brain-computer interface systems. However, flip bonding requires the introduction of an under-bump metallization (UBM) layer between the electrode and the chip bonding bump to improve wettability, enhance mechanical bonding strength, and prevent metal interdiffusion.

[0004] However, current UBM structures and processes are mostly designed for CMOS (Complementary Metal-Oxide-Semiconductor) chips and have not yet been optimized for the specific requirements of flexible, implantable brain-computer interface electrodes (such as biocompatibility, low stress, and long-term stability). Therefore, a novel UBM structure and process method are needed to better support the flip-chip bonding of brain-computer interface electrodes. Summary of the Invention

[0005] The purpose of this disclosure is to provide a UBM structure, brain-computer interface electrode, and method to better support the flip-chip bonding of brain-computer interface electrodes.

[0006] In a first aspect, this disclosure provides a UBM structure for a brain-computer interface electrode, the brain-computer interface electrode comprising a flexible electrode layer and a flexible electrode substrate covering the flexible electrode layer, the flexible electrode substrate having an opening exposing the flexible electrode layer, the UBM structure being disposed within the opening, the UBM structure comprising: a first adhesive layer attached to the flexible electrode layer exposed at the bottom of the opening; a wetting layer attached to the adhesive layer; a second adhesive layer attached to the wetting layer; and a surface protective layer attached to the second adhesive layer; wherein the total thickness of the UBM structure is less than or equal to 1 micrometer, the first adhesive layer and the second adhesive layer are made of titanium, the wetting layer is made of copper or gold, and the surface protective layer is made of gold or platinum.

[0007] In some alternative embodiments, the thickness of the first and second adhesive layers is between 5 nanometers and 50 nanometers, the thickness of the wetting layer is between 200 nanometers and 500 nanometers, and the thickness of the surface protective layer is between 50 nanometers and 500 nanometers.

[0008] In some alternative implementations, the total thickness of the UBM structure is less than the depth of the opening.

[0009] In some alternative implementations, the horizontal width of the UBM structure is between 200 micrometers and 300 micrometers.

[0010] In a second aspect, this disclosure provides a brain-computer interface electrode, which is a flexible electrode, comprising a flexible electrode layer and a flexible electrode substrate covering the flexible electrode layer. The flexible electrode substrate has an opening that exposes the flexible electrode layer, and a UBM structure is disposed within the opening. The UBM structure is the UBM structure described in the first aspect.

[0011] In some alternative embodiments, the flexible electrode layer is gold, and the flexible electrode substrate is polyimide or epoxy-negative photoresist.

[0012] In some alternative embodiments, the horizontal width of the UBM structure is between 200 micrometers and 300 micrometers, and the center-to-center spacing of the UBM structure is between 200 micrometers and 300 micrometers.

[0013] Thirdly, this disclosure provides a method for preparing the UBM structure as described in the first aspect, comprising the following steps:

[0014] Photoresist is applied to the brain-computer interface electrode, and an opening is formed on the flexible electrode substrate by photolithography, the opening exposing the flexible electrode layer;

[0015] A first adhesion layer, a wetting layer, a second adhesion layer, and a surface protective layer are sequentially deposited on the flexible electrode layer within the opening, wherein the first adhesion layer and the second adhesion layer are made of titanium, the wetting layer is made of copper or gold, and the surface protective layer is made of gold or platinum;

[0016] The photoresist is removed by a stripping process to obtain a UBM structure within the opening, the total thickness of the UBM structure being less than or equal to 1 micrometer.

[0017] In some alternative embodiments, the first adhesion layer, the wetting layer, the second adhesion layer, and the surface protective layer are deposited by sputtering or thermal evaporation.

[0018] Thirdly, this disclosure provides a method for bonding brain-computer interface electrodes as described in the second aspect, comprising the following steps:

[0019] Photoresist is applied to the brain-computer interface electrode, and an opening is formed on the flexible electrode substrate by photolithography, the opening exposing the flexible electrode layer;

[0020] A first adhesion layer, a wetting layer, a second adhesion layer, and a surface protective layer are sequentially deposited on the flexible electrode layer within the opening, wherein the first adhesion layer and the second adhesion layer are made of titanium, the wetting layer is made of copper or gold, and the surface protective layer is made of gold or platinum;

[0021] The photoresist is removed by a stripping process to obtain a UBM structure in the opening, and the total thickness of the UBM structure is less than or equal to 1 micrometer.

[0022] Solder bumps are provided on the UBM structure;

[0023] The brain-computer interface electrode with the solder bumps is flip-mounted on the substrate, so that the solder bumps are aligned with the pads on the substrate;

[0024] The solder bumps are melted and solidified by reflow soldering, thereby bonding the UBM structure to the pads on the substrate.

[0025] As mentioned above, to overcome the numerous shortcomings and technical biases of existing UBM technology in the specific application field of brain-computer interface electrodes, and to better support flip-chip bonding of brain-computer interface electrodes to substrates, this disclosure proposes a UBM structure, brain-computer interface electrodes, and a method. The UBM structure of this disclosure is designed with a four-layer structure. Through thickness and material optimization, the technical effects achieved include, but are not limited to:

[0026] (1) Low stress:

[0027] The UBM structure disclosed herein effectively buffers and releases thermomechanical stress, preventing interface delamination or cracking during process (such as reflow soldering) and thermal cycling in the operating environment. Typically, mismatches in the coefficients of thermal expansion (CTE) of brain-computer interface electrodes, solder bumps, the UBM layer, and the substrate can generate internal stress with temperature changes. This stress concentration effect is particularly pronounced for micron-level high-density electrode sites, easily leading to connection failure. The UBM structure of this disclosure, through multilayer material design and thickness optimization, reduces the difference in CTE between the UBM structure and the brain-computer interface electrodes, solder bumps, and the substrate, acting as a "stress buffer" to reduce internal stress and improve mechanical stability and reliability.

[0028] (2) Long-term stability

[0029] The UBM structure disclosed herein has a surface protective layer made of gold or platinum, which provides stable performance, reduces the risk of corrosion, and improves reliability and long-term stability.

[0030] (3) Biocompatibility

[0031] The UBM structure disclosed herein has a surface protective layer made of gold or platinum, which has good biocompatibility.

[0032] (4) Flexible matching

[0033] The UBM structure disclosed herein has a total thickness limited to 1 micrometer or less, which can provide good flexibility support and is well-suited for flexible brain-computer interface electrodes.

[0034] In summary, this disclosure provides a UBM (Ultra-Brain Interface) technology solution for the brain-computer interface field that is highly compatible with standard semiconductor manufacturing processes, suitable for wafer-level batch processing, and capable of precisely controlling micron-level pattern dimensions. This disclosure achieves a synergistic innovation in material systems, multilayer structures, and process methods, ultimately realizing a quadruple improvement in biocompatibility, electrical performance, mechanical reliability, and process feasibility. This provides crucial technical support for the realization of next-generation high-performance, high-density, and long-term reliable implantable brain-computer interface devices. Attached Figure Description

[0035] Other features, objects, and advantages of this disclosure will become more apparent from the following detailed description of non-limiting embodiments, taken with reference to the accompanying drawings. The drawings are for illustrative purposes only and are not intended to limit the scope of this disclosure. In the drawings:

[0036] Figure 1 This is a schematic diagram of the longitudinal cross-sectional structure of a brain-computer interface electrode according to an embodiment of the present disclosure;

[0037] Figure 2This is a longitudinal cross-sectional schematic diagram of a semiconductor packaging structure formed after the brain-computer interface electrode and the substrate are joined according to an embodiment of the present disclosure.

[0038] Figure 3 This is a schematic flowchart of a method for preparing a UBM structure according to an embodiment of the present disclosure;

[0039] Figure 4 This is a schematic flowchart of a method for bonding brain-computer interface electrodes according to an embodiment of the present disclosure. Detailed Implementation

[0040] The present disclosure will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, only the parts relevant to the invention are shown in the accompanying drawings.

[0041] It should be readily understood that the meanings of “on,” “above,” and “on top of” in this disclosure should be interpreted in the broadest sense, such that “on” means not only “directly on something,” but also “on something” including intermediate components or layers existing between the two.

[0042] Furthermore, for ease of description, spatial relative terms such as “below,” “under,” “lower,” “above,” and “upper” may be used herein to describe the relationship of one component or part to another component or part shown in the accompanying drawings. In addition to the orientations described in the figures, spatial relative terms are also intended to cover different orientations of the device during use or operation. The device may be oriented in other ways (rotated 90° or otherwise), and the spatial relative descriptive terms used herein may be interpreted accordingly.

[0043] It should be noted that the structures, proportions, sizes, etc., depicted in the accompanying drawings are merely for illustrative purposes to aid those skilled in the art in understanding and reading the content described herein, and are not intended to limit the implementation conditions of this disclosure. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in proportions, or adjustments to the size, without affecting the effectiveness and purpose of this disclosure, should still fall within the scope of the technical content disclosed herein. Furthermore, the terms such as "above," "first," "second," and "a" used in this specification are merely for clarity of description and are not intended to limit the scope of this disclosure. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of this disclosure's implementation.

[0044] It should be noted that, unless otherwise specified, the embodiments and features described in this disclosure can be combined with each other. This disclosure will now be described in detail with reference to the accompanying drawings and embodiments.

[0045] Current UBM processes are mostly designed for CMOS chips and have not yet been optimized for the specific requirements of flexible, implantable brain-computer interface electrodes (such as biocompatibility, low stress, and long-term stability), leading to the following problems:

[0046] (1) The metal layer of UBM is easily corroded in an electrolytic environment, resulting in insufficient reliability;

[0047] (2) Flexible electrode substrates, such as polyimide (PI) and epoxy negative photoresist (Su8), have a large difference in thermal expansion coefficients compared with traditional UBM metal layers, leading to bonding stress concentration;

[0048] (3) Lack of UBM structure that balances electrical performance and biocompatibility.

[0049] Therefore, a novel UBM structure and manufacturing process are needed to better support the flip-chip bonding of brain-computer interface electrodes.

[0050] refer to Figure 1 , Figure 1 This is a schematic diagram of the longitudinal cross-sectional structure of the brain-computer interface electrode 10 according to an embodiment of this disclosure. Figure 1 As shown, the brain-computer interface electrode 10 of this disclosure includes a flexible electrode layer 11 and a flexible electrode substrate 12 covering the flexible electrode layer 11. The flexible electrode substrate 12 has an opening 13 that exposes the flexible electrode layer 11, and a UBM structure 20 is disposed in the opening 13.

[0051] Here, the UBM structure 20 includes:

[0052] The first adhesive layer 21 is attached to the flexible electrode layer 11 exposed at the bottom of the opening 13;

[0053] Wetting layer 22 is attached to adhesive layer 21;

[0054] The second adhesive layer 23 is attached to the wetting layer 22;

[0055] Surface protective layer 24 is attached to the second adhesive layer 23;

[0056] The total thickness of the UBM structure 20 is less than or equal to 1 micrometer, the first adhesion layer 21 and the second adhesion layer 23 are titanium (Ti), the wetting layer 22 is copper (Cu) or gold (Au), and the surface protective layer 24 is a gold (Au) layer or platinum (Pt).

[0057] Here, by limiting the thickness of the UBM structure 20 to less than 1 micrometer, the brain-computer interface electrodes can be better supported to maintain good flexibility.

[0058] Here, the thickness of the first adhesion layer 21 can be between 5 nanometers and 50 nanometers, preferably between 10 nanometers and 20 nanometers; the thickness of the second adhesion layer 23 can be between 5 nanometers and 50 nanometers, preferably between 10 nanometers and 20 nanometers; the thickness of the wetting layer 22 can be between 200 nanometers and 500 nanometers, preferably between 300 nanometers and 500 nanometers; and the thickness of the surface protective layer 24 can be between 50 nanometers and 500 nanometers, preferably between 200 nanometers and 500 nanometers.

[0059] Here, the first adhesive layer 21 and the second adhesive layer 23 mainly serve to bond, ensuring that the materials of each layer achieve a firm mechanical connection and a good electrical connection; the wetting layer 22 has good wettability with the solder, which can ensure a good weld with the solder bump; the surface protective layer 24 is made of gold or platinum, which has excellent biocompatibility, extremely high corrosion resistance and excellent conductivity, and can be used to ensure the stability of long-term implantation.

[0060] Here, the flexible electrode layer 11 can be gold, and the flexible electrode substrate 12 can be polyimide (PI) or epoxy-based negative photoresist (SU8). The surface of the flexible electrode substrate 12 can be subjected to plasma activation treatment to further enhance its adhesion to the metal material of the flexible electrode layer 11.

[0061] In some alternative implementations, the total thickness of the UBM structure 20 is less than the depth of the opening 13.

[0062] In some alternative implementations, the horizontal width of the UBM structure 20 can be between 200 micrometers and 300 micrometers, and the center-to-center spacing of the UBM structure 20 can be between 200 micrometers and 300 micrometers.

[0063] refer to Figure 2 , Figure 2 This is a schematic diagram of the longitudinal cross-sectional structure of the semiconductor package structure formed after the brain-computer interface electrode 10 and the substrate 30 are bonded together according to an embodiment of this disclosure. Figure 2 As shown, the semiconductor package structure includes:

[0064] Brain-computer interface electrode 10, such as Figure 1 As shown;

[0065] The substrate 30 is disposed below the brain-computer interface electrode 10, and the surface of the substrate 30 has pads 301.

[0066] Solder bumps 40 are disposed between the brain-computer interface electrode 10 and the substrate 30;

[0067] The UBM structure 20 on the brain-computer interface electrode 10 and the pad 301 on the substrate 30 are electrically connected by solder bumps 40.

[0068] Here, substrate 30 can be a substrate formed of conductive and dielectric materials, including but not limited to printed circuit boards (PCBs) or flexible circuit boards (FPCs).

[0069] Here, the solder bump 40 includes, but is not limited to, tin-based solder, for example, a tin-silver-copper (SnAgCu) alloy or a gold-tin (AuSn) alloy.

[0070] In some alternative implementations, a silicon (Si) substrate 50 can be provided during the manufacturing process, and the flexible brain-computer interface electrode 10 can be disposed on the silicon substrate 50. The silicon substrate 50 can support and carry the flexible brain-computer interface electrode 10, ensuring the accuracy and yield of the manufacturing process.

[0071] refer to Figure 3 This disclosure provides a method for preparing a UBM structure, comprising the following steps:

[0072] Step S1: Reference Figure 1 Photoresist is applied to the brain-computer interface electrode 10, and an opening 13 is formed on the flexible electrode substrate 12 by photolithography, exposing the flexible electrode layer 11.

[0073] For example, a double-layer photoresist process can be used to pattern the brain-computer interface electrode 10, and then the patterned photoresist can be used to form an opening 13 on the flexible electrode substrate 12 by a process such as chemical etching.

[0074] Step S2: Reference Figure 1 A first adhesion layer 21, a wetting layer 22, a second adhesion layer 23 and a surface protective layer 24 are sequentially deposited on the flexible electrode layer 11 within the opening 13. The first adhesion layer 21 and the second adhesion layer 23 are made of titanium, the wetting layer 22 is made of copper or gold, and the surface protective layer 24 is made of gold or platinum.

[0075] For example, the first adhesion layer 21, the wetting layer 22, the second adhesion layer 23 and the surface protective layer 24 can be deposited by means of sputtering or thermal evaporation.

[0076] For example, titanium with a thickness between 5 nm and 20 nm can be deposited as a first adhesion layer 21, then copper with a thickness greater than or equal to 300 nm can be deposited as a wetting layer 22, then titanium with a thickness between 5 nm and 20 nm can be deposited as a second adhesion layer 23, and finally gold with a thickness greater than or equal to 100 nm can be deposited as a surface protective layer 24.

[0077] Step S3: Reference Figure 1The photoresist is removed by a lift-off process, and a UBM structure 20 is formed in the opening 13. The total thickness of the UBM structure 20 is less than or equal to 1 micrometer.

[0078] refer to Figure 4 This disclosure provides a method for bonding brain-computer interface electrodes, the method comprising the following steps:

[0079] Steps S1 to S3: with Figure 3 Steps S1 to S3 are the same.

[0080] Step S4: Reference Figure 2 Solder bumps 40 are provided on the UBM structure 20.

[0081] Step S5: The brain-computer interface electrode 10 with solder bumps 40 is flipped onto the substrate 30, so that the solder bumps 40 are aligned with the pads 301 on the substrate 30.

[0082] Step S6: The solder bumps 40 are melted and solidified by reflow soldering, thereby bonding the UBM structure 20 to the pads 301 on the substrate 30. Here, reflow soldering flip-chip bonding is used, and the bonding temperature can be controlled within the solder reflow temperature profile to reduce thermal stress on the flexible electrode substrate.

[0083] Optionally, after reflow soldering, a filler material such as low-modulus, insulating epoxy resin can be applied to the gap between the brain-computer interface electrode 10 and the substrate 30 to enhance mechanical reliability and to cover and protect the solder bumps 40.

[0084] Ultimately, a reliable flip-chip bonding interconnect structure was obtained, in which implantable flexible brain-computer interface electrodes are bonded to the substrate.

[0085] The above provides a brief description of the UBM structure, brain-computer interface electrode, and method according to embodiments of this disclosure. The brain-computer interface electrode of this disclosure features a UBM structure that improves the electrical interconnect reliability between the brain-computer interface electrode and the substrate through multilayer material design and thickness optimization; by using gold or platinum as a surface protective layer, it enhances the metal's corrosion resistance and biocompatibility under long-term implantation conditions; by improving CTE matching through material design and increasing flexibility through thickness optimization, it reduces mechanical stress in the bonding area, thereby improving the durability of the flexible electrode.

[0086] The above description is merely a preferred embodiment of this disclosure and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of this disclosure is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the above-described concept. For example, technical solutions formed by substituting the above features with (but not limited to) technical features disclosed in this disclosure that have similar functions.

Claims

1. A UBM structure, characterized in that, A brain-computer interface electrode, the brain-computer interface electrode comprising a flexible electrode layer and a flexible electrode substrate covering the flexible electrode layer, the flexible electrode substrate having an opening exposing the flexible electrode layer, and a UBM structure disposed within the opening, the UBM structure comprising: A first adhesive layer is attached to the flexible electrode layer exposed at the bottom of the opening; A wetting layer is attached to the first adhesive layer; The second adhesion layer is attached to the wetting layer; A surface protective layer is attached to the second adhesive layer; The total thickness of the UBM structure is less than or equal to 1 micrometer, the first and second adhesion layers are made of titanium, the wetting layer is made of copper or gold, and the surface protective layer is made of gold or platinum.

2. The UBM structure according to claim 1, characterized in that, The thickness of the first adhesion layer and the second adhesion layer is between 5 nanometers and 50 nanometers, the thickness of the wetting layer is between 200 nanometers and 500 nanometers, and the thickness of the surface protective layer is between 50 nanometers and 500 nanometers.

3. The UBM structure according to claim 2, characterized in that, The total thickness of the UBM structure is less than the depth of the opening.

4. The UBM structure according to claim 1, characterized in that, The horizontal width of the UBM structure is between 200 and 300 micrometers.

5. A brain-computer interface electrode, characterized in that, The brain-computer interface electrode is a flexible electrode, comprising a flexible electrode layer and a flexible electrode substrate covering the flexible electrode layer. The flexible electrode substrate has an opening that exposes the flexible electrode layer, and a UBM structure is disposed within the opening. The UBM structure is the UBM structure as described in any one of claims 1-4.

6. The brain-computer interface electrode according to claim 5, characterized in that, The flexible electrode layer is gold, and the flexible electrode substrate is polyimide or epoxy-negative photoresist.

7. The brain-computer interface electrode according to claim 5, characterized in that, The horizontal width of the UBM structure is between 200 and 300 micrometers, and the center-to-center spacing of the UBM structure is between 200 and 300 micrometers.

8. A method for preparing the UBM structure as described in claim 1, characterized in that, Includes the following steps: Photoresist is applied to the brain-computer interface electrode, and an opening is formed on the flexible electrode substrate by photolithography, the opening exposing the flexible electrode layer; A first adhesion layer, a wetting layer, a second adhesion layer, and a surface protective layer are sequentially deposited on the flexible electrode layer within the opening, wherein the first adhesion layer and the second adhesion layer are made of titanium, the wetting layer is made of copper or gold, and the surface protective layer is made of gold or platinum; The photoresist is removed by a stripping process to obtain a UBM structure within the opening, the total thickness of the UBM structure being less than or equal to 1 micrometer.

9. The method for preparing the UBM structure according to claim 8, characterized in that, The first adhesion layer, the wetting layer, the second adhesion layer, and the surface protective layer are deposited by sputtering or thermal evaporation.

10. A method for bonding brain-computer interface electrodes, characterized in that, The brain-computer interface electrode is a flexible electrode, comprising a flexible electrode layer and a flexible electrode substrate covering the flexible electrode layer, and the bonding method includes the following steps: Photoresist is applied to the brain-computer interface electrode, and an opening is formed on the flexible electrode substrate by photolithography, the opening exposing the flexible electrode layer; A first adhesion layer, a wetting layer, a second adhesion layer, and a surface protective layer are sequentially deposited on the flexible electrode layer within the opening, wherein the first adhesion layer and the second adhesion layer are made of titanium, the wetting layer is made of copper or gold, and the surface protective layer is made of gold or platinum; The photoresist is removed by a stripping process to obtain the UBM structure as described in claim 1 in the opening, wherein the total thickness of the UBM structure is less than or equal to 1 micrometer. Solder bumps are provided on the UBM structure; The brain-computer interface electrode with the solder bumps is flip-mounted on the substrate, so that the solder bumps are aligned with the pads on the substrate; The solder bumps are melted and solidified by reflow soldering, thereby bonding the UBM structure to the pads on the substrate.

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