Heat sink comprising synthetic diamond material

Abstract

A heat sink comprising a heat spreader (2) made from synthetic diamond and having a front surface for mounting one or more components to be cooled like a laser disc (8) and a rear surface for direct fluid cooling (10). A plurality of ribs (4,7) is bonded to the rear surface of the heat spreader (2) to stiffen the heat spreader. Both the heat spreader and the plurality of ribs are formed of synthetic diamond material. The ribs (4,7) may be fixed to the heat spreader by braze bonds (6).

Claims

1. A heat sink comprising: a heat spreader having a front surface for mounting one or more components to be cooled and a rear surface for direct fluid cooling; and a plurality of ribs bonded to the rear surface of the heat spreader to stiffen the heat spreader, wherein both the heat spreader and the plurality of ribs are formed of synthetic diamond material; and wherein the rear surface of the heat spreader has a flatness of less than 10 micrometres at least over regions bonded to the ribs, and wherein each rib has an interface surface bonded to the rear surface of the heat spreader, the interface surface having a flatness of less than 10 micrometres.

2. The heat sink according to claim 1, wherein the plurality of ribs are bonded to the rear surface of the heat spreader by braze bonds.

3. The heat sink according to claim 1, wherein the heat spreader has a thickness in a range 0.8 mm to 4.5 mm.

4. The heat sink according to claim 1, wherein the plurality of ribs each have a thickness in a range 0.8 mm to 4.5 mm.

5. The heat sink according to claim 1, wherein the flatness of the rear surface of the heat spreader is less than 2 micrometres at least over regions bonded to the ribs.

6. The heat sink according to claim 1, wherein the flatness of the interface surface of each rib is less than 2 micrometres.

7. The heat sink according to claim 1, wherein the front surface of the heat spreader has a convex or concave curvature at least over a portion of the front surface.

8. The heat sink according to claim 1, wherein the front surface of the heat spreader comprises a reflective coating.

9. The heat sink according to claim 1, wherein the plurality of ribs are configured in an axially symmetric pattern over the rear surface of the heat spreader.

10. The heat sink according to claim 9, wherein the axially symmetric pattern of ribs over the rear surface of the heat spreader is configured to provide axially symmetric flexural stiffness to the heat sink.

11. A heat sink assembly comprising: a heat sink according to claim 1; a heat generating component bonded to the front surface of the heat spreader; and a fluid cooling system configured to deliver fluid to the rear surface of the heat spreader.

12. The heat sink assembly according to claim 11, wherein the heat generating component is a laser disc comprising a laser-active medium.

13. The heat sink assembly according to claim 11, wherein the heat generating component is a reflecting structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a better understanding of the present invention and to show how the same may be carried into effect, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

(2) FIGS. 1(a) and 1(b) show schematic illustrations of a heat sink according to an embodiment of the invention;

(3) FIG. 2 shows a schematic illustration of a heat sink according to another embodiment of the invention; and

(4) FIG. 3 shows a schematic illustration of a heat sink assembly comprising a heat generating component, a heat sink, and a fluid cooling system.

DETAILED DESCRIPTION

(5) As described in the summary of invention section, one aspect of the present invention is concerned with the fabrication of a heat sink comprising:

(6) a heat spreader having a front surface for mounting one or more components to be cooled and a rear surface for direct fluid cooling; and

(7) a plurality of ribs bonded to the rear surface of the heat spreader to stiffen the heat spreader, wherein both of the heat spreader and the plurality of ribs are formed of synthetic diamond material.

(8) To fabricate such a heat sink, the rear surface of the heat spreader can be processed to a flatness of less than 10 micrometres, preferably less than 2 micrometres, at least over regions to be bonded to the ribs. Each rib has an interface surface to be bonded to the rear surface of the heat spreader. These interface surfaces can also be processed to a flatness of less than 10 micrometres, preferably less than 2 micrometres. The plurality of ribs can then be bonded to the rear surface of the heat spreader by, for example, braze bonds or metal diffusion bonds. For example, an active carbide forming braze bond can be utilized such as those containing titanium or an alternative carbide forming component for bonding to diamond. High temperature braze bonds can be utilized as there is no thermal expansion coefficient mismatch if synthetic diamond is utilized for both the heat spreader and ribs.

(9) The heat spreader may have a thickness in a range 0.8 mm to 4.5 mm. Generally embodiments of the present invention will be applicable where relatively thick, stiff heat sinks are required but where cost or synthesis capability prohibits greater thicknesses. The ribs may also have a thickness in a range 0.8 mm to 4.5 mm. The ribs will generally have a thickness which does not significantly exceed that of the heat spreader to avoid the requirement to grown thicker pieces of diamond material which is challenging and expensive. Advantageously, the heat spreader and the ribs will have the same thickness and can thus be grown using the same synthesis process. Alternatively, each rib may comprise more than one layer of diamond material bonded together, e.g. via braze bonds. It is currently challenging to produce heat spreaders in a cost-effective manner with a high thickness (for example, over around 4.5 mm). Bonding ribs to a heat spreader gives the heat spreader a stiffness that is much greater than that of a heat spreader without ribs, and therefore makes the heat spreader suitable for higher power optical and semiconductor systems.

(10) As the lateral dimension of the ribs is significantly smaller than that of the heat spreader, more efficient use of the diamond material is achieved while retaining the desired rigidity. The lateral dimension of the ribs should be sufficiently large as to provide sufficient braze area and bonding strength. Furthermore, the spacing of the ribs should be sufficiently small to achieve the desired level of stiffness under thermal and cooling loads. For example, the spacing between the ribs may be approximately the same as the thickness of the heat spreader. For certain applications it is advantageous for the plurality of ribs to be configured in an axially symmetric pattern over the rear surface of the heat spreader. For example, this is advantageous in many optical applications so any optical distortions within, for example, a laser cavity are symmetric, and the flexural strength of the heat spreader is substantially uniform regardless of the axial direction in which it is measured.

(11) The front surface of the heat spreader may be processed to have a convex or concave curvature at least over a portion of the front surface. For example, a concave curvature may be provided for provision of a concave mirror component. Alternatively, a slightly convex curvature can be advantageous to ensure a good contact when bonded to an overlying component. The front surface of the heat spreader may also comprise a reflective coating for many optical applications.

(12) FIGS. 1(a) and 1(b) show schematic illustrations of a heat sink according to an embodiment of the invention. The heat sink comprises a synthetic diamond heat spreader 2 with synthetic diamond ribs 4 bonded to a rear surface of the synthetic diamond heat spreader as shown in plan view in FIG. 1(a). FIG. 1(b) shows a cross-sectional view of the heat sink showing the ribs 4 bonded to the heat spreader 2 via braze bonds 6.

(13) The embodiment illustrated in FIGS. 1(a) and 1(b) comprises six ribs in a radial distribution akin to spokes of a wheel. FIG. 2 shows a schematic illustration of a heat sink according to another embodiment of the invention. This embodiment also comprises a synthetic diamond heat spreader 2 with ribs bonded to a rear surface thereof via braze bond. Here, rather than using elongate ribs a plurality of smaller (shorter) pieces of synthetic diamond material 7 are distributed to form each major supporting rib. Such a configuration can be fabricated using off-cuts of diamond, e.g. from the process used to fabricate the heat spreader, which would otherwise be discarded. As such, this approach can utilize a larger fraction of diamond material, reducing waste and decreasing fabrication costs.

(14) A heat sink assembly can be constructed using the heat sink configuration as described herein. FIG. 3 shows a schematic illustration of an example of such a heat sink assembly. The heat sink assembly comprises a heat sink as described herein comprising a heat spreader 2 having a front surface and a rear and a plurality of ribs 4 bonded to the rear surface of the heat spreader via braze bonds 6 to stiffen the heat spreader 2. A heat generating component 8 is bonded to the front surface of the heat spreader. For example, the heat generating component 8 can be a laser disc comprising a laser-active medium or a reflecting structure (i.e. a mirror). A fluid cooling system 10 is configured to deliver fluid to the rear surface of the heat spreader.

(15) While preferably both the heat spreader and the plurality of ribs are each formed of monolithic synthetic diamond material to provide the best combination of thermal conductivity, stiffness, and thermal expansion coefficient matching between the ribs and the heat spreader, it is also envisaged that some benefits can be achieved by applying synthetic diamond stiffening ribs to a heat spreader formed of a different material. Alternatively, it is also envisaged that some benefits can be achieved by applying non-diamond stiffening ribs to a synthetic diamond heat spreader to stiffen the heat spreader if, for example, the synthetic diamond heat spreader is relatively thin. Alternative materials may include sapphire and zinc selenide. For example, a sapphire or zinc selenide substrate may be stiffened using diamond ribs.

(16) The term “stiffness” is used herein to refer to the resistance of a component to deflection under a given load. Stiffness of a component depends not only on the Young's Modulus of the component material, but also on how the component is loaded, and the dimensions and shape of the component. For the purposes of a heat spreader used in a high power optical or semiconductor application, the flexural stiffness is important. A heat spreader with a plurality of ribs bonded to it is stiffer than a heat spreader with no ribs bonded to it.

(17) The primary purpose of the ribs is to stiffen the heat spreader, although they also have the additional purpose of increasing the surface area to spread heat. However, the fins of a conventional heat spreader are designed to maximize the surface area to volume ratio in order to greatly increase the overall surface area and so maximize the heat spreading. In the present application, the ribs are required for stiffening, and so maximizing the surface area to volume ratio of the ribs would reduce the amount of stiffening required. The ribs therefore increase the overall surface of the rear surface of the heat spreader by no more than 100%, by no more than 75% by no more than 50% or by no more than 30%.

(18) By way of example, FIG. 1a shows a heat spreader 2 with a rear surface having a diameter of 90 mm, leading to a surface area of 6362 mm.sup.2. Six ribs 4 are brazed to the rear surface. Each rib has dimensions of 27×5×4.5 mm. The total exposed surface area of each ribs 4 is 423 mm.sup.2, leading to an additional surface area for six ribs of 2538 mm.sup.2. The increase in surface area is therefore less than 40%.

(19) Using the further example of FIG. 2, a heat spreader 2 is shown with a rear surface having a diameter of 90 mm, leading to a surface area of 6363 mm.sup.2. Twelve ribs 7 are brazed to the rear surface. Each rib has dimensions of 7×5×4.5 mm. The total exposed area of each ribs 7 is 143 mm.sup.2, leading to an additional surface area for six ribs of 1716 mm.sup.2. The increase in surface area is therefore less than 27%. By using ribs with a lower surface area to volume ratio than heat spreading fins have, the stiffening effect of the ribs is maximized.

(20) While this invention has been particularly shown and described with reference to embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.