CN117316902A - Copper-based nano-structure interface material and preparation method thereof - Google Patents

Abstract

The application discloses a copper-based nano-structure interface material, which is formed by copper grains with the transverse dimension of 500nm-100 mu m in a spaced distribution manner; wherein the thickness of the interface material is 10-100 mu m; the spacing distance of the copper grains is 500nm-100 mu m. The heat transfer between the chip and the radiator can be realized quickly, and meanwhile, the heat stress caused by the difference of the thermal expansion coefficients of the chip (silicon) and the radiator (copper) can be borne, so that the bottleneck of weak links such as large thermal resistance and easiness in cracking of the current thermal interface material is solved.

Description

Copper-based nano-structure interface material and preparation method thereof

Technical Field

The application relates to the technical field of thermal interface materials, in particular to a copper-based nanostructured interface material and a preparation method thereof.

Background

Along with the continuous increase of miniaturization, integration and densification of electronic products, the power and the heating value of the electronic products are also increased sharply. The high temperature can reduce the stability, reliability and service life of the electronic equipment. It is counted that 55% of failures in electronic products are due to the operating temperature of the product exceeding its upper limit. In order to ensure the efficient and stable operation of the equipment, how to quickly and effectively discharge heat is a problem to be solved. On the heat dissipation path, the most thermally resistive point is often the interface of the heat source and the heat sink. The current effective solution is to fasten the heat source and the radiator, and to conduct out the heat by the contact between them. But the heat conduction capacity of the radiator is limited due to the existence of interface thermal resistance. The reason for the thermal resistance of the interface is that the contact interface between the two is not possible to be completely flat, so that most of the interface between the heat source and the radiator is separated by an air gap, and the heat dissipation effect is greatly reduced. The method for solving the problem is to place a thermal interface material with high thermal conductivity between a heating source and a radiator to form an efficient heat transfer channel so as to greatly reduce contact thermal resistance and improve the efficiency of the radiator. This requires the interface material to have high heat conduction and good compressibility.

The technical bottleneck of thermal interface materials must solve two key problems: firstly, the heat-transfer capacity is fast, and secondly, the heat-transfer capacity can be matched with the thermal stress caused by the difference of the thermal expansion coefficients of the heat source and the radiator. Thus, an excellent thermal interface material should have both high thermal conductivity (low thermal resistance) and high flexibility (easy deformation), and be a thermal-force community. The ideal thermal interface material is required to have high thermal conductivity, low thermal expansion coefficient and easy deformation. The high heat conduction can reduce the self heat resistance of the thermal interface material and improve the heat transfer efficiency; the better the deformability, the better the interface material will be in engagement with the contact surface at lower mounting pressures, and the largest possible filling of the voids in the contact surface will be possible, reducing the contact thermal resistance between the contact surfaces. The most commonly used thermal interface materials today can be divided into polymer-based and metal-based. The greatest advantage of polymer-based thermal interface materials is their high flexibility, but their limited ability to conduct heat. The high-conductivity filler is filled along the heat conduction path to effectively improve the heat conductivity, but excessive filler addition can increase the viscosity and rigidity and reduce the flexibility. Metal-based thermal interface materials can provide high thermal conductivity and low thermal interface resistance, but have poor interface compliance, and are subject to large thermal stresses due to Coefficient of Thermal Expansion (CTE) mismatch, and are prone to stress failure.

Currently, a common consensus in the industry is that it is desirable to further improve the performance of thermal management systems, depending critically on the thermal conductivity of the thermal interface material. The development of interface materials with higher thermal conductivity, high thermal stability and excellent mechanical properties has important significance in promoting the rapid development of modern electronic equipment.

Disclosure of Invention

In order to solve the above-mentioned shortcomings in the art, the present application aims to provide a copper-based nanostructured interface material and a preparation method thereof. The microstructure of the nano metal copper crystal grains is optimized, and the strain of the separately arranged nano copper crystal grains is utilized to absorb the thermal stress caused by the difference of the thermal expansion coefficients of the heat source and the radiator, so that the interface material with low thermal resistance and high flexibility is obtained.

According to an aspect of the present application, there is provided a copper-based nanostructured interface material formed by copper grains having a lateral dimension of 500nm to 100 μm being spaced apart;

wherein the thickness of the interface material is 10-100 mu m;

the spacing distance of the copper grains is 500nm-100 mu m.

According to some embodiments of the present application, the copper grains are spaced apart by a distance of 1-30 μm.

According to some embodiments of the present application, the copper grains have a lateral dimension of 1-30 μm.

According to some embodiments of the application, the interface material has a thickness of 10-50 μm.

According to some embodiments of the present application, the shape of the copper grains includes: columnar, helical or saw-tooth.

According to some embodiments of the present application, the copper grains are spiral in shape;

preferably, the helical copper grains are periodic along the axial direction.

According to some embodiments of the present application, the copper-based nanostructured interface material is surface-coated with an oxidation preventing layer.

According to some embodiments of the application, the oxidation preventing layer is a nickel layer.

According to another aspect of the present application, there is provided a method for preparing a copper-based nanostructured interface material, comprising:

presetting a nucleation point on a copper radiating substrate by adopting a micro-nano processing stripping process;

and preparing the nano copper structure on the substrate with the preset nuclear point by adopting a rotary glancing method.

According to some embodiments of the present application, the method further comprises coating the surface of the nano copper structure with an oxidation preventing layer.

Compared with the prior art, the application at least comprises the following beneficial effects:

the application provides a copper-based nanostructure interface material which is an advanced new material integrating nanostructure and material, is used for a heat conduction interface between a chip of an electronic product and a radiator, and is connected with the chip and the radiator in a brazing mode. The heat transfer between the chip and the radiator can be realized quickly, and meanwhile, the heat stress caused by the difference of the thermal expansion coefficients of the chip (silicon) and the radiator (copper) can be borne, so that the bottleneck of weak links such as large thermal resistance and easiness in cracking of the current thermal interface material is solved.

The method for preparing the copper-based nano-structure interface material by combining vapor deposition and micro-nano processing is adopted to realize loose micro-nano structure and controllable preparation of parameters such as morphology, size, density and the like of nano copper grains, and finally the required microstructure is obtained.

Drawings

Fig. 1 is a schematic diagram of a five-layer interface system (left) and three spiral-characteristic nano-copper grains (right) of a copper-based nanostructured interface material preparation according to an example embodiment of the present application.

Fig. 2 is a diagram showing a conventional solder layer as a thermal interface material model.

Fig. 3 is a schematic view of a glancing method.

Fig. 4 is a graph of interface morphology and deformation of nano-copper structures according to example embodiments and comparative examples of the present application.

Fig. 5 is a laser direct write exposure pattern according to an example embodiment of the present application.

FIG. 6 is a graph of nucleation site morphology remaining on a substrate after a lift-off process according to an exemplary embodiment of the present application.

Detailed Description

The following description of the embodiments of the present application will be made clearly and fully with reference to the embodiments of the present application, and it is apparent that the described embodiments are some, but not all, of the embodiments of the present application. All other embodiments, which can be made by those skilled in the art based on the embodiments herein without making any inventive effort, are intended to be within the scope of the present application.

It is particularly pointed out that similar substitutions and modifications made in relation to the present application will be apparent to a person skilled in the art and are all considered to be included in the present application. It will be apparent to those skilled in the relevant art that modifications and variations can be made in the methods and applications described herein, or in the appropriate variations and combinations, without departing from the spirit and scope of the application. It will be apparent that the described embodiments are only some, but not all, of the embodiments of the present application.

The application is carried out according to the conventional conditions or the conditions suggested by manufacturers if the specific conditions are not noted, and the raw materials or auxiliary materials and the reagents or the equipment are conventional products which can be obtained commercially if the manufacturers are not noted.

The present application is described in detail below.

The thermal interface material is positioned between the heat source and the heat sink and functions to transfer heat. The technical bottleneck must solve two key problems: firstly, the heat-transfer capacity is fast, and secondly, the heat-transfer capacity can be matched with the thermal stress caused by the difference of the thermal expansion coefficients of the heat source and the radiator. Therefore, an excellent thermal interface material should have both high thermal conductivity (low thermal resistance) and high flexibility (easy deformation).

There is a need for improved thermal conductivity of polymer-based interface materials. The polymer-based interface material mainly comprises: heat conductive silicone grease, heat conductive silicone rubber, heat dissipation gaskets, heat conductive glue and the like. Is suitable for the requirements of heat conductivity coefficient, high flexibility and goodGood electrical insulation properties, etc. The method has the characteristics of low price, simple process, impact resistance and the like, and has important roles in modern electronic packaging application. Current polymeric thermally conductive interface materials are statistically more than 80% of the market. However, the thermal conductivity of the polymer material is very poor (0.2W/mK), and the heat dissipation requirement of the next-generation electronic equipment cannot be met. There are generally two approaches to improving the heat conducting property: one is to change the molecular orientation of the polymer material to improve the heat conducting property of the material; the other method is to fill the material with high heat conduction property into the polymer material to prepare the composite heat conduction polymer material, but the highest heat conduction coefficient (5W/m.K) reported at present can not meet the high-power heat dissipation requirement. How to prepare the material with the heat conductivity coefficient exceeding 20W/m.K and the interface thermal resistance lower than 1mm in the civil field ² K/W polymer-based thermal interface materials remain a significant challenge.

The metallic solder interface material needs to address the application bottleneck of flexibility. The metal material has a large amount of free electrons, so that rapid heat transfer can be realized, and the metal material is widely applied to heat sinks (such as copper and aluminum) and electronic packaging materials (such as indium and tin). However, since the thermal expansion coefficient of metal is significantly different from that of semiconductor material, a large thermal stress is generated at the interface during multiple temperature cycles or thermal shock, and interface damage is caused. In order to improve the performance, researchers develop metal matrix composite materials, integrate the excellent performance of metal materials and reinforcing phases, realize performance optimization, and are popularized and applied in the field of thermal management. Taking common metal Cu as an example, the thermal expansion coefficient of Cu is 16.6X10 ^-6 K ^-1 Much higher than the thermal expansion coefficient of the semiconductor die material, which makes the device susceptible to failure due to excessive thermal stress. The addition of alloying elements to form Kovar alloys, invar alloys, have coefficients of thermal expansion that match the semiconductor chip material, but have reduced thermal conductivity (less than 20W/m-K) and reduced compressibility. With the increasing power of electronic devices, thermal stress-induced failure issues are increasingly pronounced. Metals such as low melting point solder indium and tin are commonly used for soldering packaging of electronic devices, and can also be used for thermal interface materials. Introduction of thermal stress to resist differences in thermal expansion coefficients of heat sources and heat sinksResulting in cracking of the interfacial layer, the indium layer thickness needs to be greater than 200 microns. However, the increased thickness reduces the heat conductivity of the indium layer, so that the thickness and the heat conductivity of the solder interface layer seem to be contradictory objects which are difficult to coordinate. Since the metal-based interface has intrinsic high thermal conductivity and can be optimized in mechanical flexibility through various ways such as alloying, filler addition, microstructure design and the like, the metal-based interface is still a preferred scheme in the next generation of advanced thermal interface materials and has been verified in practical application.

The copper-based nanostructured interface material is a metallic thin film material that is structurally designed on a hundred nanometer scale. The thickness of the metal film is between tens of micrometers, and the metal film is formed by copper grains with transverse dimensions of hundreds of nanometers to several micrometers in a spaced mode.

The shape characteristics, lateral dimensions, spacing distances, periods, etc. of copper grains are the main factors affecting the elastic and plastic deformability. The shape characteristics of the copper crystal grains comprise columnar, spiral, saw tooth and the like; the lateral size and spacing of copper grains, which are between tens of nanometers and hundreds of nanometers, can affect their stress-strain behavior.

The nano-copper interface material of the present application may be directly deposited on the surface of a heat source (e.g., a silicon-based chip) or a heat sink (e.g., copper). If deposited on the copper-based heat spreader surface, the copper heat spreader surface is in direct contact with the discrete nano-copper grains, and no solder connection is required between the two. At this point it is necessary to deposit solder on the other surface of the discrete nano-copper grains. If deposited on the surface of a silicon-based chip, the influence of high temperature on the chip needs to be considered due to the high temperature during deposition. Direct deposition of nano-copper grains on the surface of temperature sensitive chips is generally avoided. The heat spreader covered with discrete nano-copper grains/solder is contacted with a heat source, and a certain pressure is applied and heated. After the temperature exceeds the melting point, the solder melts, and the soldering connection between the heat source and the radiator is completed.

The nano-copper interface material may also be formed in a self-supporting foil morphology. Firstly, a solder layer is deposited on the surface of a substrate, then a discrete nano copper grain film is deposited, and finally, a solder layer is deposited. A sandwich foil structure of solder/discrete nano-copper/solder is formed. And finally, stripping the sandwich foil structure from the substrate to form the self-supporting foil material. When the foil is used for connecting the heat source and the radiator, the foil is placed between the heat source and the radiator, and is pressed with a certain pressure and heated. After the temperature exceeds the melting point, the solder melts, completing the braze joint of the heat source-interface material-heat sink.

The brazing connection of the chip and the radiator is carried out based on the nano copper-based interface material, and the brazing connection is represented by a module shown in fig. 1. The module is a stacked structure consisting of five layers, and the stacked structure sequentially comprises the following components along the direction of heat flow: silicon-based chip (heat source) -tin solder layer-nano copper layer-tin solder layer-copper radiator. The deformation of the discrete nano-copper grains can release stress caused by the difference in thermal expansion coefficient, and thus the thickness of the solder layer can be reduced by several tens micrometers. This facilitates heat transfer and increases the thermal conductivity of the interface material.

As shown in fig. 2, a stacked structure model is formed by three layers of a single solder layer as a thermal interface material, and the stacked structure model sequentially comprises: silicon-based chip (heat source) -tin alloy solder layer-copper radiator. In the three-layer stacked structure, the solder layers are susceptible to thermal creep fatigue failure and bending deformation, and are weak links most susceptible to module failure. According to the design theory of the project, thermal stress caused by the difference of the thermal expansion coefficients of materials in the multilayer structure is released by the nano copper interface layer at the central position of the module through strain.

The method for preparing the nano copper interface material by combining vapor deposition and micro-nano processing is adopted.

The linear motion of vapor deposition particles is adopted, so that the shielding effect of nucleation points on subsequent particles is caused, a mechanism of loose columnar crystals is formed, the gradient of the direction of deposited particles is increased, the shielding effect is aggravated, and finally a loose columnar structure growing obliquely can be obtained, namely, the glancing incidence deposition method.

The nucleation sites in this approach are randomly formed and thus the final columnar crystalline structure is also uncontrollable. However, if the nucleation point is preset in advance, the particles which are subsequently reached continuously grow at the preset point, and the position and the number of the final columnar crystals can be adjusted. This opens a window for us to adjust the microstructure of the film.

Specifically, nucleation points can be prepared on the surface of the substrate in advance by a micro-nano processing method, the positions and the densities of the nucleation points are adjusted according to a design scheme, and then a thin film is grown by a glancing incidence deposition method, as shown in fig. 3.

In order to obtain columnar, spiral, saw-tooth crystal grain forms, a substrate can be prepared and rotated at a certain speed in a mode while glancing and depositing, so that controllable preparation of parameters such as the form, the size, the density and the like of nano copper crystal grains is realized, and finally, a required microstructure is obtained (as shown in fig. 4).

To avoid the natural oxidation and thermal cycling oxidation of the nanostructured copper grains, an oxidation-resistant layer is required to cover, and metallic nickel may be selected. The metallic nickel is deposited by evaporation or sputtering without breaking vacuum in the same chamber. In order to reduce thermal stress caused by the difference in thermal expansion coefficients, a base material having a smaller difference in thermal expansion coefficient from copper is selected as much as possible.

Example 1

(1) After ultrasonic cleaning of copper radiator substrate with acetone and isopropanol, photoresist LOR5A (3000 r/min, time 1min, baking at 200 ℃ C. For 10 min) and S1813 (1500 r/min, time 1min, baking at 115 ℃ C. For 2 min) were spin coated sequentially on the substrate.

(2) Laser direct writing exposure pattern is adopted, MF319 developer is used for soaking 35s, the photoresist is hardened at 90 ℃ for 2min, and plasma photoresist removal is adopted for 10min (figure 5) for removing the residual photoresist after exposure.

(3) And (5) depositing copper by magnetron sputtering. Background vacuum degree is less than 10 ^-5 Pa, vacuum degree is 2Pa, power is 150W when a film is deposited, and a copper layer is deposited at 150nm.

(4) The substrate (figure 6) with the surface regularly arranged nucleation points, which are 20 μm apart and 10 μm in diameter, is obtained by immersing in NMP solution and removing the participating photoresist by a stripping process.

(5) And placing the substrate with the preset nucleation point into a dip angle rotary table in the vacuum chamber. The included angle θ=16° between the substrate plane and the source beam, and the substrate rotation speed r=10r/min. The power of the magnetron sputtering copper source is 100W, the vacuum degree is 5Pa, the distance between the source and the substrate is 10cm, and a copper nano layer with the thickness of 10 mu m is deposited.

(6) The substrate plane forms an angle of 90 degrees with the source beam, and the substrate rotation speed R=10r/min. And starting a tin source, and depositing a nickel layer with the thickness of 30 mu m to obtain a copper heat dissipation/nano copper grain layer/tin layer structure.

(7) And (3) attaching one side covered with the copper grain layer and the tin layer to the silicon-based chip, placing the silicon-based chip in a vacuum heat treatment furnace for heating, and forming a structure of firmly combining the copper radiator, the interface material and the heat source after the temperature rises above the melting point of tin.

Example 2

This example prepares an independently supported interfacial heat transfer foil material.

(1) And depositing a copper sacrificial layer on the surface of the substrate, and depositing a copper film with the thickness of 200 nanometers on the surface of the substrate by adopting a magnetron sputtering technology.

(2) And depositing a tin solder layer on the surface of the copper sacrificial layer, and depositing the tin solder layer with the thickness of 50 microns on the surface of the copper sacrificial layer by adopting a magnetron sputtering technology.

(3) And preparing a discrete copper grain layer on the surface of the tin solder layer.

(4) The substrate covered with the copper sacrificial layer/tin solder layer was placed into a dip turntable within a vacuum chamber. The angle θ=20° between the substrate plane and the source beam, and the substrate rotation speed r=6 rpm. The power of the magnetron sputtering copper source is 100W, the vacuum degree is 5Pa, the distance between the source and the substrate is 10cm, and a copper nano layer with the thickness of 10 microns is deposited.

(5) The substrate plane is at 90 ° to the source beam, and the substrate rotation speed r=10 rpm. The tin source was turned on and a 50 μm thick nickel layer was deposited. And obtaining the structure of the copper sacrificial layer/the tin layer/the nano copper grain layer/the tin layer.

(6) Soaking the above structure in distilled water diluted nitric acid (65% HNO concentration) ₃ ) Etching the copper sacrificial layer in the solution at room temperature to obtain the independently supported tin layer/nano copper grain layer/tin layer foil.

(7) The brazing filler metal welds the heat source and the radiator, the foil is placed between the heat source (silicon-based chip) and the radiator (copper), the foil is placed in a vacuum heat treatment furnace for heating, and after the temperature rises above the melting point of tin, the structure of firmly combining the copper radiator, the interface material and the heat source is formed.

Comparative example

In order to distinguish the structural differences between the rotating glancing method and the conventional deposition method for preparing copper-containing nano-films, the present example shows the structure of the conventional deposition method for preparing films (fig. 4 a).

In this example the substrate plane is at 90 ° to the source beam and the substrate rotation speed r=10 rpm (where the substrate rotation only plays a role in the uniformity of the film thickness). A copper film was deposited to a thickness of about 10 um. The deposition conditions are as follows: the power of the magnetron sputtering copper source is 100W, the vacuum degree is 5Pa, the distance between the source and the substrate is 10cm, and a copper nano layer with the thickness of 10 microns is deposited. From the cross-sectional morphology, it can be seen that the thin film grows into columnar crystals which are densely arranged, and the growth direction of the columnar crystals still has a certain included angle (small angle) with the vertical direction of the interface due to the shielding effect. The grain nucleation sites of the film are randomly formed. The deformation behavior of the film under the action of external force is tested by adopting a nano-indentation method (10 mN load), so that the film can be seen to have higher hardness, and the plastic deformation is mainly performed under the action of external force (figure 4 c). This is significantly different from the case of example 1 in which elastic deformation is the main factor due to external force (fig. 4 d). The hardness of example 1 is significantly less than that of the comparative example and the elastic recovery is significantly better than that of the comparative example. Therefore, example 1 not only has better high thermal conductivity, low thermal resistance (thermal), but also has better elastic deformation (mechanical).

Test examples

The interface materials prepared in the examples and comparative examples were tested for properties such as thermal conductivity.

The test steps are as follows:

the thermal conductivity of the material was tested using a physical property test system and a fourier transform thermal analysis system. The physical property test system is mainly used for measuring the heat conductivity of the material based on a steady-state heat flow principle; the Fourier change analysis system is used for measuring the thermal diffusivity of the composite material based on the instantaneous heat flow principle, and measuring the specific heat of the nano copper interface material by combining a differential scanning calorimeter, and obtaining the thermal conductivity of the material according to the thermal diffusivity, the density and the specific heat. The device adopts MicReD T3Ster equipment to test thermal resistance, the equipment can collect transient temperature response curves of the device in real time based on a static test method, and can analyze thermal properties of related structures of a heat transfer path of the device to construct an equivalent thermal model of the device.

The test results are shown in the following table:

the above description of embodiments is only for aiding in the understanding of the method of the present application and its core ideas. It should be noted that it would be obvious to those skilled in the art that various improvements and modifications can be made to the present application without departing from the principles of the present application, and such improvements and modifications fall within the scope of the claims of the present application.

Claims (10)

1. The copper-based nano-structure interface material is characterized by being formed by copper grains with the transverse dimension of 500nm-100 mu m in a spaced mode;

wherein the thickness of the interface material is 10-100 mu m;

the spacing distance of the copper grains is 500nm-100 mu m.

2. The interface material of claim 1, wherein the copper grains are spaced apart by a distance of 1-30 μm.

3. The interface material of claim 1, wherein the copper grains have a lateral dimension of 1-30 μm.

4. The interface material of claim 1, wherein the interface material has a thickness of 10-50 μm.

5. The interface material of claim 1, wherein the shape of the copper grains comprises: columnar, helical or saw-tooth.

6. The interface material of claim 4, wherein the copper grains are helical in shape;

preferably, the helical copper grains are periodic along the axial direction.

7. The interface material of claim 1, wherein the copper-based nanostructured interface material surface is covered with an oxidation preventing layer.

8. The interface material of claim 7, wherein the oxidation preventing layer is a nickel layer.

9. A method of preparing a copper-based nanostructured interface material according to any of claims 1 to 8, comprising:

presetting a nucleation point on a copper radiating substrate by adopting a micro-nano processing stripping process;

and preparing the nano copper structure on the substrate with the preset nuclear point by adopting a rotary glancing method.

10. The method of claim 9, further comprising coating the surface of the nano-copper structure with an oxidation preventing layer.