UBM structure, brain-computer interface electrode and method

Abstract

The invention provides a UBM structure, a brain-computer interface electrode and a method. The UBM structure is used for a brain-computer interface electrode, the brain-computer interface electrode comprises a flexible electrode layer and a flexible electrode substrate wrapping the flexible electrode layer, the flexible electrode substrate is provided with an opening exposing the flexible electrode layer, the UBM structure is arranged in the opening, and the UBM structure comprises a first adhesion layer, a second adhesion layer and a third adhesion layer, the flexible electrode layer is attached to the flexible electrode layer exposed at the bottom of the opening; the wetting layer is attached to the adhesion layer; the second adhesion layer is adhered to the wetting layer; the surface protection layer is attached to the second adhesion layer; wherein the total thickness of the UBM structure is smaller than or equal to 1 micrometer, 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 protection layer is made of gold or platinum. According to the technical scheme, multiple improvements of biocompatibility, electrical properties, mechanical reliability and process feasibility are achieved.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of microelectronic packaging and biomedical engineering, in particular to a UBM structure, a brain-machine interface electrode and a method. BACKGROUND

[0002] Brain-Machine Interface (BMI; or, Brain-Computer Interface, BCI) electrodes require high-density interconnects at the micron scale, ensuring long-term stable electrochemical performance and biocompatibility. Existing electrode-chip interconnect methods include Wire Bonding and Flip-Chip Bonding.

[0003] Wire Bonding is simple, but occupies a large area, and the connection reliability is limited under long-term mechanical stress and liquid environment. Flip-Chip Bonding can achieve high-density, miniaturization and low parasitic inductance packaging, suitable for the miniaturization needs of brain-machine interface systems. However, Flip-Chip Bonding requires the introduction of an Under Bump Metallization (UBM) layer between the electrode and the chip solder 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 been optimized for the special requirements of flexible, implantable brain-machine interface electrodes (such as biocompatibility, low stress, long-term stability). Therefore, it is necessary to provide a new type of UBM structure and process method to better support the Flip-Chip Bonding of brain-machine interface electrodes. SUMMARY

[0005] The present disclosure aims to provide a UBM structure, a brain-machine interface electrode and a method to better support the Flip-Chip Bonding of brain-machine interface electrodes.

[0006] In a first aspect, the present 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 in the opening, the UBM structure comprising: a first adhesion layer attached to the flexible electrode layer exposed at a bottom of the opening; a wetting layer attached to the adhesion layer; a second adhesion layer attached to the wetting layer; and a surface protection layer attached to the second adhesion layer; wherein a total thickness of the UBM structure is less than or equal to 1 micrometer, the first and second adhesion layers are titanium, the wetting layer is copper or gold, and the surface protection layer is gold or platinum.

[0007] In some optional embodiments, the first and second adhesion layers have a thickness of 5 nanometers to 50 nanometers, the wetting layer has a thickness of 200 nanometers to 500 nanometers, and the surface protection layer has a thickness of 50 nanometers to 500 nanometers.

[0008] In some optional embodiments, the total thickness of the UBM structure is less than a depth of the opening.

[0009] In some optional embodiments, the UBM structure has a horizontal width of 200 micrometers to 300 micrometers.

[0010] In a second aspect, the present disclosure provides a brain-computer interface electrode, the brain-computer interface electrode being a flexible 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 opening having a UBM structure disposed therein, the UBM structure being the UBM structure of the first aspect.

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

[0012] In some optional embodiments, the UBM structure has a horizontal width of 200 micrometers to 300 micrometers, and a center-to-center spacing of 200 micrometers to 300 micrometers.

[0013] In a third aspect, the present disclosure provides a method of manufacturing the UBM structure of the first aspect, comprising the following steps: disposing a photoresist on the brain-computer interface electrode, and forming an opening in the flexible electrode substrate by a photolithography process, the opening exposing the flexible electrode layer; depositing a first adhesion layer, a wetting layer, a second adhesion layer and a surface protection layer on the flexible electrode layer in the opening in sequence, wherein the first adhesion layer and the second adhesion layer are titanium, the wetting layer is copper or gold, and the surface protection layer is gold or platinum; removing the photoresist 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.

[0014] In some optional embodiments, the first adhesion layer, the wetting layer, the second adhesion layer and the surface protection layer are deposited by sputtering or thermal evaporation.

[0015] In a third aspect, the present disclosure provides a bonding method of the brain-computer interface electrode according to the second aspect, comprising the following steps: applying a photoresist on the brain-computer interface electrode, and forming an opening on the flexible electrode substrate by a photolithography process, the opening exposing the flexible electrode layer; depositing a first adhesion layer, a wetting layer, a second adhesion layer and a surface protection layer on the flexible electrode layer in the opening in sequence, wherein the first adhesion layer and the second adhesion layer are titanium, the wetting layer is copper or gold, and the surface protection layer is gold or platinum; removing the photoresist 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. applying a solder bump on the UBM structure; flip-chip setting the brain-computer interface electrode provided with the solder bump on a substrate, so that the solder bump is aligned with a pad on the substrate; melting and solidifying the solder bump by a reflow soldering process, so as to bond the UBM structure and the pad on the substrate as a whole.

[0016] As described above, in order to overcome the many deficiencies and technical biases of the existing UBM technology in the special application field of brain-computer interface electrodes, and better support the flip-chip bonding of brain-computer interface electrodes and substrates, the present disclosure provides a UBM structure, a brain-computer interface electrode and a method. The UBM structure of the present disclosure is designed with a four-layer structure, and through thickness optimization and material optimization, the technical effects obtained include but are not limited to: (1) Low stress: The UBM structure provided by the present disclosure can effectively buffer and release thermal mechanical stress, and can prevent interface delamination or cracking in the process (such as reflow soldering) and thermal cycling in the use environment. Generally, if the coefficients of thermal expansion (CTE) of the brain-computer interface electrode, the solder bump, the UBM layer and the substrate do not match, internal stress will be generated when the temperature changes, and for micron-level high-density electrode sites, this stress concentration effect is particularly significant, which can easily lead to connection failure. The UBM structure of the present disclosure reduces the difference in the coefficients of thermal expansion of the brain-computer interface electrode, the solder bump and the substrate through multi-layer material design and thickness optimization, and plays a role as a "stress buffer", which can reduce internal stress and improve mechanical stability and reliability.

[0017] (2) Long-term stability The UBM structure of the present disclosure has a surface protection layer made of gold or platinum material on the surface, which is stable in performance, reduces the risk of corrosion, and improves reliability and long-term stability.

[0018] (3) Biocompatibility The UBM structure of the present disclosure has a surface protection layer made of gold or platinum material on the surface, which has good biocompatibility.

[0019] (4) Flexible matching The UBM structure of the present disclosure has a total thickness limited to 1 micron or less, which can provide good flexible support and is better suitable for flexible brain-computer interface electrodes.

[0020] In summary, the present disclosure provides a UBM technical solution for the field of brain-computer interface, which is highly compatible with standard semiconductor manufacturing processes, suitable for wafer-level batch processing, and can accurately control micron-level pattern size. The present disclosure makes collaborative innovation from the material system, multi-layer structure and process method, and finally realizes the fourfold improvement of biocompatibility, electrical performance, mechanical reliability and process feasibility, which provides key technical support for the realization of the next generation of high-performance, high-density and long-term reliable implantable brain-computer interface devices. BRIEF DESCRIPTION OF DRAWINGS

[0021] Other features, objects and advantages of the present disclosure will become more apparent from the following detailed description of non-limiting embodiments made with reference to the accompanying drawings. The drawings are for the purpose of illustrating specific embodiments and are not to be considered as limiting the present disclosure. In the drawings: Figure 1 is a longitudinal cross-sectional structure schematic diagram of a brain-computer interface electrode according to an embodiment of the present disclosure; Figure 2 is a longitudinal cross-sectional structure schematic diagram of a semiconductor package structure formed after the brain-computer interface electrode and the substrate are joined according to an embodiment of the present disclosure; Figure 3This is a schematic flowchart of a method for preparing a UBM structure according to an embodiment of the present disclosure; 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

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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: (1) The metal layer of UBM is easily corroded in an electrolytic environment, resulting in insufficient reliability; (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; (3) Lack of UBM structure that balances electrical performance and biocompatibility.

[0028] Therefore, a novel UBM structure and manufacturing process are needed to better support the flip-chip bonding of brain-computer interface electrodes.

[0029] 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.

[0030] Here, the UBM structure 20 includes: The first adhesive layer 21 is attached to the flexible electrode layer 11 exposed at the bottom of the opening 13; Wetting layer 22 is attached to adhesive layer 21; The second adhesive layer 23 is attached to the wetting layer 22; Surface protective layer 24 is attached to the second adhesive layer 23; 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).

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] In some alternative implementations, the total thickness of the UBM structure 20 is less than the depth of the opening 13.

[0036] 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.

[0037] 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: Brain-computer interface electrode 10, such as Figure 1 As shown; The substrate 30 is disposed below the brain-computer interface electrode 10, and the surface of the substrate 30 has pads 301. Solder bumps 40 are disposed between the brain-computer interface electrode 10 and the substrate 30; 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.

[0038] 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).

[0039] 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.

[0040] 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.

[0041] refer to Figure 3 This disclosure provides a method for preparing a UBM structure, comprising the following steps: Step S1: Refer to 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] Step S3: Reference Figure 1 The 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.

[0047] refer to Figure 4 This disclosure provides a method for bonding brain-computer interface electrodes, the method comprising the following steps: Steps S1 to S3: with Figure 3 Steps S1 to S3 are the same.

[0048] Step S4: Reference Figure 2Solder bumps 40 are provided on the UBM structure 20.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] Ultimately, a reliable flip-chip bonding interconnect structure was obtained, in which implantable flexible brain-computer interface electrodes are bonded to the substrate.

[0053] 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.

[0054] 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. An UBM structure, characterized in that, A brain-computer interface electrode includes 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 in the opening, the UBM structure including: 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; a surface protection layer attached to the second adhesion layer; wherein the total thickness of the UBM structure is less than or equal to 1 micrometer, the first and second adhesion layers are titanium, the wetting layer is copper or gold, and the surface protection layer is gold or platinum.

2. The UBM structure of claim 1, wherein, The thickness of the first and second adhesion 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 protection layer is between 50 nanometers and 500 nanometers.

3. The UBM structure of claim 2, wherein, The total thickness of the UBM structure is less than the depth of the opening.

4. The UBM structure of claim 1, wherein, The horizontal width of the UBM structure is between 200 micrometers and 300 micrometers.

5. A brain-computer interface electrode, characterized by The brain-computer interface electrode is a flexible electrode including 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 opening having a UBM structure disposed therein, the UBM structure being as claimed in any one of claims 1-4.

6. The brain-machine interface electrode of claim 5, wherein, The flexible electrode layer is gold, and the flexible electrode substrate is polyimide or epoxy-based negative photoresist.

7. The brain-machine interface electrode of claim 5, wherein, 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.

8. A method of forming the UBM structure of claim 1, wherein, The method includes the following steps: disposing photoresist on a brain-computer interface electrode, forming an opening in the flexible electrode substrate by a photolithography process, the opening exposing the flexible electrode layer; sequentially depositing a first adhesion layer, a wetting layer, a second adhesion layer, and a surface protection layer on the flexible electrode layer in the opening, wherein the first and second adhesion layers are titanium, the wetting layer is copper or gold, and the surface protection layer is gold or platinum; removing the photoresist by a lift-off process to obtain a UBM structure in the opening, the total thickness of the UBM structure being less than or equal to 1 micrometer.

9. The method of claim 8, wherein, The first adhesion layer, the wetting layer, the second adhesion layer, and the surface protection layer are deposited by sputtering or thermal evaporation.

10. The method of bonding a brain-machine interface electrode as defined in claim 5, wherein, The method includes the following steps: disposing photoresist on a brain-computer interface electrode, forming an opening in the flexible electrode substrate by a photolithography process, the opening exposing the flexible electrode layer; sequentially depositing a first adhesion layer, a wetting layer, a second adhesion layer, and a surface protection layer on the flexible electrode layer in the opening, wherein the first and second adhesion layers are titanium, the wetting layer is copper or gold, and the surface protection layer is gold or platinum; removing the photoresist by a lift-off process to obtain a UBM structure in the opening, the total thickness of the UBM structure being less than or equal to 1 micrometer; disposing a solder bump on the UBM structure; The brain-computer interface electrode provided with the solder bump is flip-chip arranged on a substrate, and the solder bump is aligned with a pad on the substrate; Through a reflow soldering process, the solder bump is melted and solidified, so as to integrate the UBM structure and the pad on the substrate.

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