Patent Application
20140151009
This invention relates generally to an article including a thermal interface element, and to a method of assembling the article.
Over the last decades, power densities of high performance semiconductor devices have been constantly on the rise. Thermal management of these electronics, more specifically the performance of Thermal Interface Materials (TIMs), has not advanced at the same rate as the semiconductor devices. Hence, today's high performance semiconductors are either run at only a fraction of their capacity, or are prone to thermal related failures. Although TIMs have progressed during the last decade, they are still the thermal bottleneck in most high power applications.
TIMs play a key role in the thermal management of electronic systems by providing a path of low thermal resistance between the heat generating devices and the heat spreader/sink. Typical TIMs include adhesives, greases, gels, phase change materials, pads, and solder alloys. Most traditional TIMs consist of a polymer matrix, such as an epoxy or silicone resin, and thermally conductive fillers, such as boron nitride, alumina, aluminum, zinc oxide, solders including indium and silver. However, these traditional TIM systems have either high thermal resistance or low fatigue life when stressed in high temperature cycles. The primary mode of failure is delamination and cracking, induced by stresses caused by CTE mismatches in the adjoining silicon (2.5 ppm/K) and copper heat spreader (25 ppm/K).
An ideal TIM is expected to exhibit high thermal conductivity, e.g., as in a metal such as indium, along with high fatigue life during thermal cycling. These properties can sometimes be difficult to balance. Other desirable properties of a TIM include low bulk and interface thermal resistances, sufficient compliance to absorb thermally induced strain without causing early fatigue failure and silicon die fracture, sufficient conformability to accommodate warpage and surface roughness of the die and heat-sink surfaces, processibility at relatively low temperatures, robustness during storage and operation, and reworkability.
In many instances, it is also important that the TIM be compatible with the present standard of electronics assembly processes. Therefore, a relatively easy method for incorporating the TIM into the electronics assembly, without multiple and long winding steps of assembling, and without an additional cost of any particular assembling aid, is desirable. Therefore, there is a need for providing a compatible, highly heat dissipating material, along with designs and processes for conducting heat away from the heat-producing components in electronic devices.
In one embodiment of the present invention, a method of assembling an article is disclosed. The method includes positioning a pre-fabricated thermal interface element between a heat source and a heat-sink. The pre-fabricated thermal interface element includes an indium substrate and a plurality of nanosprings disposed on the indium substrate.
Another embodiment of the present invention is a thermal interface element. The thermal interface element includes a pre-formed structure of a plurality of free-standing inorganic nanosprings disposed on an indium substrate.
Another embodiment of the present invention is also directed to a thermal interface element. The thermal interface element consists essentially of an indium substrate and a plurality of free-standing inorganic nanosprings disposed on the substrate.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, in which like characters represent like parts throughout the drawings, wherein:
Embodiments of the present invention describe an article including a heat source, heat-sink, and a thermal interface element and an associated method of preparation of the article.
In the following specification and the claims that follow, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.
One embodiment of the present invention describes an article having a thermal interface element, which typically facilitates heat transfer among article components and/or out of the article altogether.
In one embodiment, the heat-sink 14 is a component acting as a heat spreader. A heat spreader can absorb heat from one source and spread the heat to the surroundings, or to the selected heat-sinks. A heat-sink is capable of drawing heat from a heat source. In one embodiment, the heat-sink 14 has a thermal conductivity greater than about 1 watt/mK. In another embodiment, the heat-sink 14 has a thermal conductivity greater than about 10 watt/mK. The article 10 further comprises at least one thermal interface element 16 disposed in thermal communication with the heat-sink 14 and the heat source 12. The thermal interface element includes a substrate 18, and a plurality of nanosprings 20 as shown in
The thermal interface element 16 is an element that transmits heat from a heat source 12. In one embodiment, the thermal interface element 16 is an element to transmit heat from a heat source 12 to one or more heat-sinks. In the article of the present embodiment, the thermal interface element 16 primarily transmits heat from the heat source 12 to the heat-sink 14. In another embodiment, the thermal interface element 16 further transmits heat to the surroundings (not shown), along with transmitting heat to the heat-sink 14.
One embodiment of the present invention is a method of assembling the article 10. In one embodiment, the thermal interface element 16 is pre-fabricated and then assembled into the article 10. The pre-fabricated thermal interface element 16 includes the plurality of nanosprings 20 disposed on the substrate 18.
In one embodiment, the thermal interface element consists essentially of an indium substrate, and a plurality of free-standing inorganic nanosprings disposed on the substrate. In this embodiment, the plurality of free-standing inorganic nanosprings are directly disposed on the indium substrate, without the presence of any intervening layers or any other layers supporting the indium substrate. The absence of any other support layer can sometimes be beneficial, since such a layer (or layers) may materially affect the stability or thermal conductivity of the indium substrate as a part of the thermal interface element.
In one embodiment, the thermal interface element may include a capping layer 26, as shown in
In one embodiment, the pre-fabricated thermal interface element comprises a metallic cap layer 26 on at least 50% of the nanosprings. Thus, in one embodiment, the cap layer is in contact with an upper region 30 of at least 50% of the nanosprings as shown in
According to one embodiment, the plurality of nanosprings 20 is free-standing. As used herein, the term “free-standing” means “without a supporting solid or liquid matrix filling the void space between individual springs”. Thus, apart from the substrate 18 and an optional capping layer 26 (
The method of assembling the article 10 includes positioning a pre-fabricated thermal interface element 16 between a heat source 12 and a heat-sink 14. As used herein, “positioning a pre-fabricated thermal interface element 16 between a heat source 12 and a heat-sink 14” means that the thermal interface element 16 is disposed in between the heat source 12 and the heat-sink 14 so as to be in thermal communication with both the heat source 12 and the heat-sink 14. The “thermal communication” as used herein denotes an efficient thermal conductivity (and decreased thermal impedance) between the heat source 12 and the heat-sink 14 through the thermal interface element 16. The thermal impedance between the heat source 12 and the heat-sink 14 is considered to be less if the temperature rise per unit of the heat dissipation from the heat source 12 to the surroundings of the plurality of nanosprings 20 is less than 10% of the temperature rise due to unit heat transferred to the heat-sink 14 through the thermal interface element 16.
As used herein, the “between a heat source 12 and a heat-sink 14” or “thermal communication” as used herein need not indicate a direct mechanical contact between the thermal interface element 16 and the heat source 12 or the heat-sink 14. There may be intervening layers between the heat source 12 and the thermal interface element, and further between the thermal interface element and the heat-sink 14. For example, a bottom layer 22, a top layer 24, or both, may be positioned in the article 10 (as shown in
In one embodiment, the bottom layer 22 has an accommodative coefficient of thermal expansion (CTE) with the heat source and the substrate 18. “Accommodative CTE” as used herein is a CTE within 20% of the CTE of the substrate, the heat source, or both. In one embodiment, the top layer 24 has an accommodative coefficient of thermal expansion (CTE) with the heat-sink and the material of the nanosprings 20 or the material of the capping layer 26, if present. As used herein, the “top” and “bottom” are used for the convenience of description and are not to be limiting for their interchange in any article 10 for any application.
The positioning of pre-fabricated thermal interface element 16 within the article during assembly of the article 10 may include different methods. In one embodiment, the thermal interface element 16 is simply inserted between the required layers, such as, for example, the heat source 12 and the heat-sink 14, or the bottom layer 22 and the top layer 24. In one embodiment, the thermal interface element 16 is assembled into the article 10 using an adhesive layer. In one embodiment, an adhesive layer functions as a bottom layer 22 or a top layer 24. In another embodiment, there is an adhesive layer (not shown) as well as a bottom or top layer 22, 24 between the heat source 12 and the heat-sink 14.
In one embodiment, the thermal interface material is positioned within the article through a solder-free incorporation. That is, the thermal interface element is incorporated into the article without soldering any part of the thermal interface element to any layers of the article 10. For example, considering a thermal interface element 16 with an indium substrate 18 and a plurality of nanosprings 20, the indium substrate 18 is bonded to the bottom layer 22 of the article 10 without soldering the indium substrate 18 to the bottom layer 22. As used herein “bonded” implies mechanical proximity and thermal conductivity between the substrate 18 and the heat-sink 24, and does not necessarily imply a metallic bonding.
In one embodiment, the thermal interface element 16 is bonded to at least one layer of the article 10 without any metallic bonding between the thermal interface element 16 and the layers of the article 10. In one embodiment, the thermal interface element 16 is bonded to at least two layers of the article 10 without any metallic bonding between the thermal interface element 16 and the layers of the article 10. In one example of an article 10, a thermal interface element 16 includes an indium substrate 18; a plurality of nanosprings 20; and a capping layer 26. The capping layer 26 is bonded to the heat-sink 14; and the indium substrate 18 is bonded to the bottom layer 22 of the article 10, without soldering the capping layer 26 to the heat -sink 14, or soldering the indium substrate 18 to the bottom layer 22.
As used herein, “soldering” means joining any two or more surfaces by the action of melting and re-solidification of a low-melting point material between at least a portion of the surfaces. The low-melting point material may be termed as a solder material. The solder materials may be composed of organic, inorganic or composite materials. In one embodiment, the solder material is a low melting point metal or alloy of a low melting point alloy. Non-limiting examples for the solder materials include indium metal and alloys of indium metal.
In one embodiment, preparing a pre-fabricated thermal interface element includes the steps of disposing a plurality of nanosprings on at least one side of the substrate 18. For example, in one embodiment, the plurality of free-standing nanosprings 20 is disposed on an indium substrate 18 to form a pre-fabricated thermal interface element. Further, in one embodiment, a thermal interface element 16 (having a structure of a substrate 18, a plurality of nanosprings 20 disposed on the substrate 18, and a capping layer 26 disposed on the plurality of nanosprings 20) is formed by disposing the plurality of nano springs 20 on the indium substrate 18, and further disposing the capping layer 26 on the plurality of nanosprings 20.
The plurality of nanosprings 20 or the capping layer 26 may be disposed by any method selected from the group consisting of glancing angle deposition (GLAD), chemical vapor deposition (CVD), physical vapor deposition (PVD), electro deposition, plasma deposition, sol-gel, micromachining, laser ablation, rapid prototyping, sputtering, or any combination of these methods. In one embodiment, the plurality of nanosprings is disposed using a CVD method. In one embodiment, GLAD of a material at an oblique angle produces nanowire growth due to local shadowing caused by surface morphology. Rotation during PVD deposition produces helical nanowire structures.
In one embodiment, the plurality of nanosprings 20 and the capping layer 26 are deposited using the GLAD method. The GLAD method is particularly useful in controlling the shapes, dimensions, and density of the nanosprings 20 and the capping layers 26. In one embodiment, the capping layers 26 have a layer density greater than about 60%. In a further embodiment, the capping layers 26 are more than about 95% dense.
In one embodiment, the plurality of nanosprings 20 is deposited using the GLAD method with a glancing angle in the range of about 78° to about 87°. In a further embodiment, the glancing angle used for the deposition of the nanosprings is in the range of about 80° to about 85°. In one embodiment, the capping layers 26 are deposited using the GLAD method with a glancing angle in the range of about 0° to about 10°. In a further embodiment, the glancing angle used for the capping layers 26 deposition is in the range of about 3° to about 8°.
In one embodiment, the plurality of nanosprings 20 are inorganic nanosprings. As used herein, the “inorganic nanosprings” means that the material constituting the nanosprings comprises an inorganic material. In one embodiment, the capping layer 26 is formed of an inorganic material. The inorganic material is any material (or combination of materials) that does not comprise a carbon-to-carbon bond. In one embodiment, the inorganic material comprises a metallic material, an alloy, a ceramic, or a composite. In one embodiment, the inorganic material comprises copper, aluminum, silver, gold, platinum, tungsten, silicon, zinc oxide, silicon nitride, titanium, molybdenum, tantalum, or any combinations of these materials. In one embodiment, the inorganic material of the capping layer includes an alloy of copper, aluminum, silver, gold, platinum, tungsten, silicon, zinc oxide, silicon nitride, titanium, or any combinations of these materials.
In one embodiment, pre-fabricating the thermal interface element 16 includes the steps of disposing a plurality of free-standing nanosprings 20 on at least two sides of the substrate 18. For example, using a desired material, the plurality of nanosprings 20 can be disposed on two sides of a rectangular substrate 18 of a finite thickness by GLAD, with an oblique angle deposition on one side of the substrate 18, and an oblique angle deposition on the other side of the substrate 18, or simultaneously on both sides of the substrate 18.
In one embodiment, the plurality of nanosprings 20, substrate 18, and the capping layer 26 includes one or more organic materials along with inorganic materials. The combination of the organic and inorganic materials may be in the form of a composite. In another embodiment, the nanosprings 20, substrate 18, and the capping layer 26 consist essentially of inorganic materials. In this embodiment, any organic material that may be present is in the form of impurities. In one particular embodiment, the inorganic material consists essentially of highly thermally conductive metals such as, for example, indium, copper, silver, gold, and platinum.
In one embodiment, more than about 50% of the nanosprings 20 of the thermal interface element 16 have a thermal conductivity greater than about 1 watt/mK per nanospring, and in certain embodiments this percentage is more than about 75%. In another embodiment, more than about 50% of the nanosprings 20 have a thermal conductivity greater than about 10 watt/mK per nanospring; and in certain embodiments this percentage is more than about 75%. In yet another embodiment, more than about 75% of the nanosprings 20 have a thermal conductivity greater than about 100 watt/mK per nanospring.
In one embodiment, the capping layer 26 has a thermal conductivity greater than about 10 watt/mK. In a further embodiment, the capping layer 26 has a thermal conductivity greater than about 100 watt/mK. In a specific embodiment, the thermal interface element 16 has a thermal conductivity greater than about 100 watt/mK.
In one embodiment, the nanosprings 20, as a group, have a median spring diameter less than about 2 micrometers. The “spring diameter” used herein is not indicative of any structure of the nanosprings, but is indicative of the cross sectional width of the nanosprings. The “cross sectional width” refers to the largest dimension in a cross section of nanosprings 20 in a direction perpendicular to the length of the nanospring at any given location on the nanosprings 20. For example, if a nanospring of a certain length is of a regular rectangular shape all throughout the length of the nanospring, the cross sectional width is the diagonal dimension of the rectangle in a direction perpendicular to the length of the nanospring. In an example with cylindrical nanosprings 20 of different diameters through the length, the cross sectional width is the largest diameter of the nanosprings 20 in a direction perpendicular to the length of the nano springs. In a further embodiment, the median spring diameter of the plurality of nanosprings 20 is in a range from about 10 nm to about 2 μm. In a specific embodiment, the plurality of nanosprings 20 has a median spring cross sectional width in a range from about 100 nm to about 1 μm.
In one embodiment, there can be a number of nanosprings 20 in thermal communication at a particular area of capping layer 26 or substrate 18. In general, as the number of nanosprings having physical contact with a surface in a particular area increases, the thermal conductivity between the nanosprings and the surface also increases. In one embodiment of the present invention, the thermal interface element 16 comprises at least about 105 nanosprings in 1 cm2 of area. In a further embodiment, the thermal interface element 16 comprises at least about 107 nanosprings in 1 cm2 of area. In a further, specific embodiment, the thermal interface element 16 comprises at least about 108 nanosprings in 1 cm2 of area.
In one embodiment, the bond between the nanosprings 20, the substrate 18, and the capping layer 26, is strong. In some instances, the bonding force between the plurality of nanosprings 20 and the substrate 18 or the capping layer 26 is at least about 10 N/cm2. In another embodiment, the plurality of nanosprings 20 has at least about 100 N/m2 of bond strength with the substrate 18 and the capping layer 26. In another particular embodiment, the plurality of nanosprings 20 has at least about 400 N/m2 of bond strength with the substrate 18 and the capping layer 26.
In one embodiment, the capping layer 16 and the substrate 18 act as a bond line for the plurality of nanosprings 20 with the heat source 12 and the heat-sink 14, respectively. As used herein, a “bond line” is a joining layer that serves as an interface for the two layers to join. In one embodiment, the thicknesses of the capping layer 26 and the substrate 18 acting as bond lines are (independently) less than about 1 mm. In a further embodiment, the thicknesses of the capping layers 26 and the substrate 18 are less than about 100 μm. In one specific embodiment, the thicknesses of these layers are less than about 30 μm. In one embodiment, the substrate 18 and the capping layer 26 have different thicknesses. Relatively thin bond lines (bond layers) can function to lower the thermal resistance values between the heat source 12 and heat-sink 14. However, thin bond lines can sometimes reduce reliability, due to thermal strain when placed in between materials of dissimilar CTE. The compliance of the nanospring structures 16 reduces stress in the bond layer to maintain a longer fatigue life. The compliant spring structures move independently from one another during differential thermal expansion of the two materials being bonded together and relieve stress on the bond layer material. The collective spring constant values of the nanosprings create a structure resistant to deflection under shear or normal loading. Generally, the relatively small size of the nanosprings and the high spring density contribute to a comparatively high overall stiffness of the structure. during both normal and shear loading.
The following example illustrates methods, materials and results, in accordance with specific embodiments, and as such should not be construed as imposing limitations upon the claims. All components are commercially available from common chemical suppliers.
In operation, an indium substrate 118, having a surface 120 upon which a coating (nanospring, or a combination of nanospring and capping layer) is to be applied, was positioned at an angle θ with respect to target 106. In one example of copper nanospring growth, the angle θ was 6.5°, however a range of angles between 3° and 12° may be equally applicable, depending on other combinations of settings and parameters applied during the coating process. This configuration of substrate relative to target during deposition is also called Glancing Angle Deposition (GLAD). The glancing angle α in GLAD growth is defined as the arrival angle of a vapor flux to the normal position of a depositing surface. Since α is equal to 90°-θ, the corresponding GLAD glancing angle is in the range from about 78° to about 87°.
Prior to deposition, chamber 100 was pumped to a vacuum below 10−5 torr. Substrate 118 was rotated at 1 rpm rate during the process. Electron gun 102 was configured to emit an electron beam of 1.2-1.4 A, having a 18 kV accelerating voltage to scan target 106. During the growth of the Cu nanosprings, surface 120 of the indium substrate 118 was maintained at below 400° C., and was maintained at an angle θ of approximately 6.5° with respect to target 106. After the growth of nanosprings to a desired height, a capping layer can also be formed atop the nanosprings in the same system. If a capping layer is desired at the conclusion of the formation of the nanosprings, the substrate can be moved to increase the θ angle gradually to 45°, while rotating at a higher speed (e.g. 20 RPM). If a dense capping layer is desired, the electrode 116 can be powered with approximately 100 A at 30 V, so that a highly ionized vapor flux will arrive at the substrate surface, to densify the cap layer. The substrate 118 may be pre-patterned with a periodic array of small protrusions or seeds (e.g., by block copolymer template), to intercept vapor flux, and to control the nucleation location, so that a uniform nanospring pattern with controlled spacing between features may be fabricated.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
This invention was made with Government support under contract number N66001-09-C-2014 awarded by The Defense Advanced Research Projects Agency. The Government has certain rights in the invention.
