Double-sided nanowire array film

Abstract

Disclosed herein is a nanostructured composite thermal interface and method for fabricating said interface, which comprises a metal foil having an array of metal nanowires disposed thereon, and an adhesive polymer interspersed within the nanowires in each array, which acts as a binder to adhere said interface to the surfaces of target objects, e.g., CPUs and heat sinks.

Description

[Technical field]

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 341,758, filed May 13, 2022, the contents of which are incorporated herein in their entirety. [Background technology]

[0002] Efficient heat dissipation is important for the energy conservation and safe operation of energy-dense devices and systems. Existing data centers consume approximately 200 terawatt-hours (TWh) each year, and this will continue to grow with the development of cloud computing and artificial intelligence. Cooling accounts for up to 33-40% of a data center's energy usage and consumes billions of tons of water per year. The heat flux of high-power electronics, such as solid-state lasers, wide bandgap transistors, and phased array radars, is unprecedented, at approximately 1kW / cm. 2 , which, without effective thermal management, can lead to rapid device performance and lifetime degradation.

[0003] With the increasingly inevitable and common integration of different elements in electrical devices and systems, thermal interface resistance, which may constitute more than 50% of the total thermal resistance, has become a bottleneck in thermal management. Thermal interface materials (e.g., greases, compounds, pads, tapes, solders, and epoxies) have been used to thermally bridge and mechanically couple the interfaces. Summary of the Invention [Problem to be solved by the invention]

[0004] However, developing high performance thermal interface materials remains a significant challenge as they need to be both thermally conductive and flexible to simultaneously reduce thermal interface resistance and accommodate thermal stresses.

[0005] Thermal greases, compounds, and epoxies are highly compatible, but they have the drawback of low thermal conductivity.Solders are highly thermally conductive, but have the drawback of low mechanical compatibility, require high temperature soldering processes, and cannot be applied to large cross-section connections, significantly hindering their use as universal connections.

[0006] Aligned or ordered parallel nanostructures, including carbon nanotubes (CNTs), nanowires, graphene, nanosheets and nanofibers, are promising for fabricating thermal interface materials with thermal conductivity and mechanical flexibility. However, the fabrication of these nanostructures is usually costly and cannot be mass-produced. In addition, most of these nanostructures are not adhesive, cannot be soldered and mainly rely on pure dry thermal contact with the substrate, which greatly impairs their performance. [Means for solving the problem]

[0007] Summary of the Invention Disclosed herein is a novel nanostructured composite thermal interface (NCTI) and a method for fabricating the NCTI. In one embodiment, the NCTI consists of a free-standing, double-sided copper nanowire (CuNW) array integrated with an adhesive polymer to create a double-sided copper nanowire-polymer composite film. The film can be used as an adhesive, high-performance, paper-like thermal and electrical interface material. The copper nanowire array is grown bottom-up on two sides of a thin copper foil and then coated with an adhesive polymer.

[0008] When used to connect two objects, the nanowires penetrate the coated flexible polymer layer and directly contact the surface morphology of the objects with light pressure, acting as efficient thermal and electrical channels, while the intermediate highly conductive copper foil acts as a thermal / electrical regulator and the adhesive polymer provides strong adhesion to connect the interfaces.

[0009] By way of example, specific exemplary embodiments of the systems and methods of the present disclosure will now be described with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0010] [Figure 1] Schematic showing the formation of NCTIs where a double-sided nanowire array film acts as a conductive and compliant scaffold and the glue strongly bonds the NCTI to the mating surface, maintaining its structural integrity after curing. [Diagram 2] 1 shows a schematic of a triple-layer NCTI bonded joint and an SEM image of a cross section of the actual interface. [Diagram 3] The compatibility of the nanowires with the surface morphology of the target object, which is in direct contact with the target object to form efficient vertical heat flow channels across the interface, is shown diagrammatically, and the SEM image shows a top view of the nanowires penetrating the adhesive polymer layer and in direct contact with the substrate surface. [Figure 4A] 1 illustrates a schematic diagram of a manufacturing process for an NCTI according to the present disclosure. [Figure 4B] 1 illustrates a schematic diagram of a manufacturing process for an NCTI according to the present disclosure. [Figure 4C] 1 illustrates a schematic diagram of a manufacturing process for an NCTI according to the present disclosure. [Figure 4D] 1 illustrates a schematic diagram of a manufacturing process for an NCTI according to the present disclosure. [Diagram 5] 13 is an SEM image of a double-sided nanowire array without an adhesive polymer layer. [Figure 6] 1 illustrates the placement of an NCTI between two target objects. [Figure 7] FIG. 13 is an SEM image showing a cross-section of a double-sided nanowire array without a polymer layer. [Figure 8] FIG. 13 is an SEM image showing a cross-section of a resin-embedded, double-sided nanostructured composite film. [Figure 9A]The thermal and mechanical properties of NCTI are illustrated. (A) Schematic of half sample for frequency domain thermoreflectance (FDTR) measurements and thermal resistance configuration. (B) Regular FDTR data plot and best least squares fitting to heat transfer model, inset shows multiple test spots. (C) Summary histogram of RNW-resin / object and kNW-resin extracted from FDTR measurements at different test spots. (D) Histogram showing the composition of total thermal resistance dominated (75.3%) by CuNW / resin composite layer. (E) Graph showing stiffness of isolated CuNW bundle using in situ cylindrical flat punch indenter indentation monitored by SEM, inset shows SEM image of nanoindentation process. (F) Graph showing tensile strength test of two copper parts bonded by NCTI (blue) and pure AB resin (orange), respectively, showing comparable adhesive strength, inset shows image of test sample. [Figure 9B] The thermal and mechanical properties of NCTI are illustrated. (A) Schematic of half sample for frequency domain thermoreflectance (FDTR) measurements and thermal resistance configuration. (B) Regular FDTR data plot and best least squares fitting to heat transfer model, inset shows multiple test spots. (C) Summary histogram of RNW-resin / object and kNW-resin extracted from FDTR measurements at different test spots. (D) Histogram showing the composition of total thermal resistance dominated (75.3%) by CuNW / resin composite layer. (E) Graph showing stiffness of isolated CuNW bundle using in situ cylindrical flat punch indenter indentation monitored by SEM, inset shows SEM image of nanoindentation process. (F) Graph showing tensile strength test of two copper parts bonded by NCTI (blue) and pure AB resin (orange), respectively, showing comparable adhesive strength, inset shows image of test sample. [Figure 9C]The thermal and mechanical properties of NCTI are illustrated. (A) Schematic of half sample for frequency domain thermoreflectance (FDTR) measurements and thermal resistance configuration. (B) Regular FDTR data plot and best least squares fitting to heat transfer model, inset shows multiple test spots. (C) Summary histogram of RNW-resin / object and kNW-resin extracted from FDTR measurements at different test spots. (D) Histogram showing the composition of total thermal resistance dominated (75.3%) by CuNW / resin composite layer. (E) Graph showing stiffness of isolated CuNW bundle using in situ cylindrical flat punch indenter indentation monitored by SEM, inset shows SEM image of nanoindentation process. (F) Graph showing tensile strength test of two copper parts bonded by NCTI (blue) and pure AB resin (orange), respectively, showing comparable adhesive strength, inset shows image of test sample. [Figure 9D] The thermal and mechanical properties of NCTI are illustrated. (A) Schematic of half sample for frequency domain thermoreflectance (FDTR) measurements and thermal resistance configuration. (B) Regular FDTR data plot and best least squares fitting to heat transfer model, inset shows multiple test spots. (C) Summary histogram of RNW-resin / object and kNW-resin extracted from FDTR measurements at different test spots. (D) Histogram showing the composition of total thermal resistance dominated (75.3%) by CuNW / resin composite layer. (E) Graph showing stiffness of isolated CuNW bundle using in situ cylindrical flat punch indenter indentation monitored by SEM, inset shows SEM image of nanoindentation process. (F) Graph showing tensile strength test of two copper parts bonded by NCTI (blue) and pure AB resin (orange), respectively, showing comparable adhesive strength, inset shows image of test sample. [Figure 9E]The thermal and mechanical properties of NCTI are illustrated. (A) Schematic of half sample for frequency domain thermoreflectance (FDTR) measurements and thermal resistance configuration. (B) Regular FDTR data plot and best least squares fitting to heat transfer model, inset shows multiple test spots. (C) Summary histogram of RNW-resin / object and kNW-resin extracted from FDTR measurements at different test spots. (D) Histogram showing the composition of total thermal resistance dominated (75.3%) by CuNW / resin composite layer. (E) Graph showing stiffness of isolated CuNW bundle using in situ cylindrical flat punch indenter indentation monitored by SEM, inset shows SEM image of nanoindentation process. (F) Graph showing tensile strength test of two copper parts bonded by NCTI (blue) and pure AB resin (orange), respectively, showing comparable adhesive strength, inset shows image of test sample. [Figure 9F] The thermal and mechanical properties of NCTI are illustrated. (A) Schematic of half sample for frequency domain thermoreflectance (FDTR) measurements and thermal resistance configuration. (B) Regular FDTR data plot and best least squares fitting to heat transfer model, inset shows multiple test spots. (C) Summary histogram of RNW-resin / object and kNW-resin extracted from FDTR measurements at different test spots. (D) Histogram showing the composition of total thermal resistance dominated (75.3%) by CuNW / resin composite layer. (E) Graph showing stiffness of isolated CuNW bundle using in situ cylindrical flat punch indenter indentation monitored by SEM, inset shows SEM image of nanoindentation process. (F) Graph showing tensile strength test of two copper parts bonded by NCTI (blue) and pure AB resin (orange), respectively, showing comparable adhesive strength, inset shows image of test sample. [Figure 10A]1 shows a schematic of NCTI applied to a computer CPU and cooler. [Figure 10B] 10B is a graph comparing the operating temperature of the CPU of FIG. 10A using NCTI versus thermal paste as a thermal interface material. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] A manufacturable, low-cost nanostructured composite thermal interface (NCTI) is disclosed and, in one embodiment, is fabricated by integrating free-standing, double-sided copper nanowire (CuNW) arrays with an adhesive polymer.

[0012] When used to connect two objects, light pressure allows the nanowires to penetrate the coated flexible polymer layer and directly contact the surface of the target object, ensuring the surface morphology of the target object, allowing the NCTI to act as an efficient thermal and electrical channel, while the highly conductive copper foil between the nanowire arrays acts as a thermal / electrical regulator, and the adhesive polymer provides strong adhesion to connect the interface.

[0013] Based on a penetrable metal nanowire scaffold, NCTI exhibits extremely high thermal conductivity of over 100 W / mK, two orders of magnitude higher than those of composite epoxies (~1 W / mK) and one order of magnitude higher than those of solders (~10 W / mK). NCTI is also empirically found to be highly electrically conductive, like a metal, and approximately one order of magnitude more conductive than commercially available double-sided copper tape.

[0014] In addition to high thermal and electrical conductivity, NCTI also has high mechanical compatibility: both the high aspect ratio copper nanowire arrays and the adhesive polymer are flexible to accommodate thermal stresses arising from possible mismatch in the thermal expansion coefficients of the joined objects.

[0015] The adhesive, high performance, paper-like interface film can be applied directly through light pressure at room temperature to bond two target objects together, providing excellent heat dissipation and electrical conductivity without scale limitations.

[0016] Figure 1 shows a schematic of the formation of an NCTI where a double-sided nanowire array acts as a conductive, compliant scaffold and an adhesive polymer strongly bonds the NCTI to the mating surface of the target object, maintaining structural integrity after curing. Figure 2 shows a schematic of a perspective view of the three-layer structure of the joint with the NCTI bonded, along with an SEM image showing a cross-section of the actual interface. Figure 3 shows a side view of the same interface, along with an SEM image showing a top view of the NCTI showing the metal nanowires threading through the adhesive polymer layer for direct contact with the surface of the target object.

[0017] 4 shows one possible process for fabricating the NCTIs described herein. In step (a), templates 404a, 404b are bonded to either side of a metal foil 402. Templates 404a, 404b are preferably commercially available sheets having pores as defined herein within which the nanowires are grown. The templates may be made of polycarbonate material, as one example.

[0018] In a preferred embodiment, the metal foil 402 is a copper foil that may be about 1 μm to 100 μm thick, preferably about 10 μm thick, although various other metals can be used. In step (b), the nanowires 406 are preferably deposited by electrochemical deposition, although other deposition methods can also be used. The deposition results in vertically aligned nanowire arrays 408 grown on both sides of the metal foil 402. In a preferred embodiment, the nanowires are composed of copper, although various other metals can also be used.

[0019] In some embodiments, nanowires may be grown by immersing the foil in an electrolytic bath, the electrolyte including, for example, copper sulfate. Other electrolytes may be used to grow nanowires composed of materials other than copper. In situations where the surface of the target object is irregular or curved, conformal NCTIs may be produced according to the methods disclosed in co-pending patent application PCT / US2023 / 021452, entitled "Growth of Vertically-Aligned Nanowires on Conductive Surfaces," the contents of which are incorporated herein in their entirety.

[0020] In various embodiments, the diameter of the nanowires varies from 10 nm to 30 μm, preferably in the range of 50 to 1000 nm. In various embodiments, the height of the nanowires varies from approximately 5 μm to 100 μm, preferably in the range of 20 μm to 50 μm. In various embodiments, the nanowires cover 5% to 50% of the surface area of ​​the metal foil base. Other diameters, heights, diameter / height ratios and packing fractions may be used depending on the application.

[0021] In process step (c), templates 404a, 404b are removed by chemical dissolution. In process step (d), the top of the nanowire array 408 is coated with an adhesive polymer layer 410. FIG. 5 is an image showing the double-sided nanowire array before application of the adhesive polymer. Preferably, the adhesive polymer has a viscosity of 10,000 cP or less. In one embodiment, the adhesive polymer can be a 100:12 mixture of EpoxySet #145-20005 and Hardener #145-20010 from Allied High Tech Products Inc. of Cerritos, Calif., with a viscosity of 300 cP. Other equivalent products can also be used.

[0022] FIG. 6 illustrates the process of mating the NCTI with the target object 602a, 602b. The NCTI 604 is placed on the surface 602a, 602b of the target object and light pressure is applied. The pressure forces the adhesive polymer 410 between the nanowires 406 of the nanowire array 408. The applied pressure forces the nanowires 406 through the adhesive polymer layer and into contact with the target object surfaces 602a, 602b, which significantly contributes to the high thermal conductivity and electrical performance of the material. Preferably, the pressure required is less than 1000 psi.

[0023] Figure 2 shows a second image of an actual NCTI according to the present disclosure, showing the adhesive polymer coating the nanowires. This view is similar to the schematic shown in the inset of Figure 6. The inset of Figure 3 shows that the top of the nanowires penetrates the adhesive polymer layer to allow contact with the surface of the target object (not shown).

[0024] In some embodiments, the double-sided nanowire array 408 connected to the intermediate Cu-based layer 402 acts as a conductive and compliant scaffold due to the high thermal conductivity of copper and the high aspect ratio (>100) of the nanowires 406. In some embodiments, the adhesive polymer layer can be a low viscosity glue (e.g., resin, epoxy, superglue, etc.) whose application is to generate strong capillary forces resulting from the high surface energy of the nanowire array 408 by the adhesive polymer 410 to strongly bond the nanostructured scaffold to the mating surface of the target object, as described above. Of note, one advantage of NCTI is that it allows room temperature bonding compared to soldering processes that occur at much higher temperatures (typically >200° C.). This allows for a wide range of applications in various environments and materials.

[0025] In some embodiments, prior to glue curing, regularity of the nanowire array is important to ensure that the nanowires conform to the substrate morphology of the target object's surface and directly contact the surface to form efficient vertical heat flow channels throughout the interface. In some embodiments, the highly thermally conductive base layer 402 (i.e., copper foil in the preferred embodiment) between the nanowire arrays 408 acts as a lateral thermal regulator to collect and spread heat, increasing the uniformity of the heat flux, as shown in FIG. 3. In some embodiments, NCTI can synergistically regulate heat flow along both the out-of-plane and in-plane directions of the interface, which is a significant advantage over vertically aligned CNTs, nanowires, and nanofibers. In some embodiments, in contrast to contact in a purely dry state, after glue curing, the contact between the nanowires and the mating substrate is fixed and secured without any external pressure, providing strong interface adhesion and resilience to thermal cycling and vibration.

[0026] FIG. 7 is an SEM image showing the cross-section of a double-sided nanowire array without polymer, while FIG. 8 is an SEM image showing the cross-section after polymer has been applied.

[0027] For practical applications, it is important to measure both the heat transfer properties of the NCTI and the thermal interface resistance between the NCTI and the substrate. The frequency domain thermoreflectance (FDTR) technique can be used to measure the heat transfer properties of the NCTI. As shown in FIG. 9A, the FDTR technique is a pump-probe optical technique in which a modulated pump beam excites the sample while a probe beam measures the temperature-dependent change in reflectance. The phase delay between the pump and probe lasers can be measured as a function of the modulation frequency, and the thermal properties including thermal conductivity and thermal interface resistance in and across the surface of the sample can be extracted by fitting the measured data to a 2D thermal conduction model of multilayer thin films. Due to the symmetry of the NCTI, half of the thermal interface can be characterized as depicted in FIG. 9A, and the sample can be fabricated by bonding the nanowires to a Cu substrate using a resin as an adhesive polymer.

[0028] In Figure 9 (B-D), FDTR measurements can be performed at nine different locations on the sample from both sides (half of the NCTI). When using resin as the adhesive polymer, the average thermal conductivity of the 25 μm thick CuNW / resin composite can be measured to be 71.6 ± 14.1 W / mK, which is 0.349 ± 0.069 mm. 2 This corresponds to a thermal resistance of 1.0 ± 0.025 mm2 / W. In comparison, the measured thermal conductivity of a 10 μm Cu-based layer in the middle of the NCTI can be 376.6 ± 37.7 Wm K. The thermal interface resistances of the CuNW-resin / substrate and CuNW-resin / Cu-based interfaces are 0.075 ± 0.013 mm2, respectively. 2 K / W and 0.027±0.005 mm 2 Based on these FDTR measurements, the total thermal resistance (including thermal interface resistance) of the NCTI in a fully bonded sample is estimated to be 0.928 ± 0.140 mm2 by assuming that the NCTI can be symmetrically bonded between two Cu substrates. 2 It can be estimated that the kinetic energy is K / W.

[0029] As shown in Figure 9D, the CuNW / resin composite layer can increase the total thermal resistance by a large amount (75.3%). In Figure 9E, the Young's modulus of the nanowire array (d = 200 nm) is characterized to be about 2.5 GPa through in situ cylindrical flat punch indenter (d = 10 μm) indentation, which is two orders of magnitude smaller than that of bulk copper. Such a flexible structure allows for strong morphological conformity under light external pressure during the bonding process. The adhesion strength of CuNW-resin can be measured and compared with pure resin, as shown in Figure 9F. The maximum load capacity of CuNW-resin can be measured to be 207.6 N, which is larger than that of bulk copper. 2 It exhibits a strong adhesive strength (255.5N) comparable to that of pure resin with a bonding area of ​​1.

[0030] NCTI was also tested with a CPU in a practical cooling test, as shown diagrammatically in FIG. 10A, which shows the NCTI 604 placed between the CPU 1102 and a heat sink 1104 with a cooling fan 1106. An open desktop with a 65W standard office-level CPU 1102 and an air cooler was assembled as the test platform. The cooling performance of the NCTI is shown graphically in FIG. 10B. The performance of the NCTI was evaluated by monitoring the temperature of the CPU 1102 at full load using its built-in temperature sensor and benchmarking it with a commercial thermal paste of the same thickness of 60 μm. As can be seen from the graph in FIG. 10B, the thermal paste enables stable full load operation of the CPU 1102 at a measured temperature of about 72° C. and an energy consumption of approximately 70.8 W. In contrast, with the NCTI, the operating temperature of the CPU 1102 is significantly reduced to about 58° C., demonstrating the better thermal performance of this novel material.

[0031] It is noteworthy that the energy consumption of the CPU drops to only about 66.6W, with a substantial saving of roughly 4.2W (about 5.9%). In some embodiments, based on a simple, low-barrier replacement using NCTI, a more powerful CPU with significantly improved performance can be installed in existing smart devices without increasing the operating temperature. Thus, NCTI presents a low-cost upgrade solution beyond hardware upgrades for a whole group of electronic devices and systems, including but not limited to data centers and portable devices. On the other hand, even if all the hardware remains intact, NCTI can benefit the electronic device or system by allowing it to run cooler, saving energy by reducing waste heat generation. This is expected to impact a wide range of semiconductor devices, saving a significant amount of energy.

[0032] As can be realized by one skilled in the art, various modifications of the implementations discussed herein are possible that fall within the scope of the invention. For example, the methods may use different materials for the metal foil base and the nanowires. Various embodiments may have nanowires with different heights, diameters, height to diameter ratios, and packing fractions. The density of the nanowires may vary depending on the template used. The length of time for growing the nanowires may also vary depending on the application. Finally, various embodiments may use different materials as the adhesive polymer. Various modifications of both the fabricated NCTI arrays and the fabrication process are possible and are contemplated to be within the scope of the invention.

[0033] Moreover, it is to be understood that the features of the various embodiments of the present disclosure are not mutually exclusive and may exist in various combinations and permutations without departing from the spirit and scope of the present invention, even if such combinations or permutations are not expressly stated herein. Accordingly, the exemplary apparatus and methods of the present disclosure should be taken as illustrative, rather than limiting, of the present invention. The scope of the present invention is defined by the claims that follow.

Claims

1. A conductive film or conductive foil, An array of conductive nanowires arranged on both sides of the conductive film or conductive foil, wherein the array comprises a plurality of nanowires extending from the film or foil, each nanowire having a space defined between them, and An adhesive polymer that at least partially fills the space between the nanowires in each arrangement to form a composite film, A nanostructured interface, including...

2. The nanostructured interface according to claim 1, wherein the nanowires come into contact with an object, and the adhesive polymer adheres the nanowires to the surface of the object.

3. The nanostructured interface according to claim 1, wherein the metal foil and the nanowire are made of copper.

4. The nanostructured interface according to claim 1, wherein the diameter of the nanowire is 10 nm to 30,000 nm.

5. The nanostructured interface according to claim 1, wherein the height of the nanowire is 5 μm to 100 μm.

6. The nanostructured interface according to claim 1, wherein the nanowires cover 5% to 50% of each side of the metal foil.

7. The nanostructured interface according to claim 1, wherein the thickness of the metal foil is 1 μm to 100 μm.

8. The nanostructured interface according to claim 1, wherein the adhesive polymer is a low-viscosity resin, glue, or epoxy.

9. The nanostructured interface according to claim 1, wherein the composite film adheres to the surface of a target object after the adhesive polymer has cured.

10. The process of preparing conductive foil, A process of joining templates to both sides of a metal foil, wherein each of the templates defines a plurality of voids within it, A step of growing thermally conductive nanowires through the voids in the template to create a nanowire array on both sides of the foil, The process of dissolving the template and A step of applying an adhesive polymer to each arrangement to fill at least partially the space between the nanowires and form a composite film. Methods that include...

11. The process involves placing the composite film between the surfaces of two target objects, A step of applying pressure to fill the spaces between the nanowires in each nanowire arrangement with the adhesive polymer, A step of curing the adhesive polymer to adhere the composite film to the surface of the target object. The method according to claim 10, further comprising:

12. The method according to claim 11, wherein the applied pressure brings the tip of the metal nanowire into contact with the surface of the target object.

13. The method according to claim 10, wherein the metal foil and the nanowire are composed of copper.

14. The method according to claim 10, wherein the adhesive polymer is a low-viscosity resin, glue, or epoxy.

15. The method according to claim 10, wherein the diameter of the nanowire is 10 nm to 30,000 nm.

16. The method according to claim 10, wherein the height of the nanowire is 5 μm to 100 μm.

17. The method according to claim 10, wherein the nanowires cover 5% to 50% of each side of the conductive foil.

18. The method according to claim 10, wherein the thickness of the conductive foil is 1 μm to 100 μm.

19. The method according to claim 10, wherein the adhesive polymer has a viscosity of 10,000 cP or less.

20. The method according to claim 11, wherein the applied pressure is 1000 psi or less.

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