Heat spreader with high heat flux and high thermal conductivity

Abstract

A system and method is disclosed for fabricating a heat spreader system, including providing a plurality of bottom microporous wicks recessed in a bottom substrate, bonding a center substrate to the bottom substrate, and bonding a top substrate having a top chamber portion to the center substrate to establish a first vapor chamber with said plurality of bottom microporous wicks.

Claims

1. A heat spreader apparatus, comprising: a bottom substrate having a first plurality of recessed microporous wicks; a center substrate bonded to said bottom substrate; a top substrate bonded to said center substrate, said top substrate having a top chamber portion to establish a first vapor chamber with said first plurality of recessed microporous wicks; and a first vapor chamber side port in vapor communication with the first vapor chamber.

2. The apparatus of claim 1, wherein said bottom substrate comprises: at least one bottom substrate pillar recessed in said bottom substrate.

3. The apparatus of claim 2, wherein said top substrate comprises: at least one top substrate pillar recessed in said top substrate.

4. The apparatus of claim 3, wherein said at least one bottom substrate pillar is bonded to said at least one top substrate pillar.

5. The apparatus of claim 3, further comprising at least one center substrate pillar bonded between said at least one bottom substrate pillar and said at least one top substrate pillar.

6. The apparatus of claim 3, wherein said center substrate is bonded between said at least one bottom substrate pillar and said at least one top substrate pillar.

7. The apparatus of claim 1, further comprising: a second bottom substrate having a second plurality of recessed microporous wicks; a second center substrate bonded to said bottom substrate; a second top substrate bonded to said second center substrate, said second top substrate having a second top chamber portion to establish a second vapor chamber with said second plurality of recessed microporous wicks; and a second vapor chamber side port in vapor communication with the second vapor chamber.

8. The apparatus of claim 7, wherein said first and second bottom substrates are coupled together and said first and second top substrates are coupled together with said first and second vapor chamber side ports in complementary opposition to enable vapor communication between said first and second vapor chambers.

9. The apparatus of claim 7, further comprising: a wafer matrix having a plurality of hexcell traps to seat said first and second bottom substrates for proper alignment of said first and second bottom substrates for bonding.

10. The apparatus of claim 1, wherein said center substrate comprises glass.

11. The apparatus of claim 1, wherein said center substrate comprises silicon.

12. A heat spreader apparatus, comprising: a bottom wafer having a first plurality of recessed microporous wicks; a center wafer bonded to said bottom wafer; a top wafer bonded to said center wafer, said top wafer having a top chamber portion to establish a first vapor chamber with said first plurality of recessed microporous wicks; and a first vapor chamber side port in vapor communication with the first vapor chamber.

13. The apparatus of claim 12, wherein said bottom wafer comprises: at least one bottom substrate pillar recessed in said bottom wafer.

14. The apparatus of claim 13, wherein said top wafer comprises: at least one top substrate pillar recessed in said top wafer.

15. The apparatus of claim 14, wherein said at least one bottom substrate pillar is bonded to said at least one top substrate pillar through a center wafer cut-through portion.

16. The apparatus of claim 12, wherein the center wafer consists of borosilicate glass.

17. The apparatus of claim 16, wherein the center wafer is bonded to said top wafer using glass frit.

18. The apparatus of claim 12, wherein said center wafer comprises silicon.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view depicting a heat spreader constructed according to the invention.

(2) FIG. 2 is a cross sectional, enlarged view of a portion of the cavity depicted in the heat spreader of FIG. 1, along the line 2-2.

(3) FIG. 3 is a plan view of the portion of the cavity shown in FIG. 2.

(4) FIG. 4 is a perspective view showing a support structure, for the heat spreader of the invention, made up of interconnecting cells.

(5) FIG. 5 is a perspective view showing another embodiment of a support structure, for the heat spreader of the invention, made of up interconnecting cells.

(6) FIG. 6 is a perspective view showing one embodiment of a silicon wafer matrix for facilitating fabrication of the support structure illustrated in FIG. 5.

(7) FIG. 7 is a cross section view showing a triple-stack heat spreader in accordance with one embodiment of the invention.

(8) FIG. 8 is a cut-away plan view of the triple-stack heat spreader illustrated in FIG. 7.

(9) FIGS. 9-13 illustrate fabrication steps for one embodiment of a triple-stack heat spreader.

(10) FIG. 14 is a cross section view showing one embodiment of a triple-stack heat spreader that has a glass center layer.

(11) FIG. 15 is a cut-away plan view of the glass center layer illustrated in FIG. 14.

DETAILED DESCRIPTION OF THE INVENTION

(12) FIG. 1 is a perspective view depicting a heat spreader constructed according to this invention. The heat spreader 100 transfers heat from a heat source, such as the microelectronic circuit components 102, 104, 106, 108, 110, and 112, to a heat sink 114, using a phase change coolant, which is contained, in both vapor and liquid forms, in a cavity 116.

(13) As depicted by FIG. 2, which is a cross sectional enlarged view of a portion of the heat spreader 100, and by FIG. 3, which is a plan view of the portion of the heat spreader shown in FIG. 2, surrounding the cavity 116 of the heat spreader, which is the primary location for flow of the coolant in vapor form, multiple microporous wicks, such as, for example, the wicks 118, 120, and 122, and the wicks 124, 126, and 128, support flows of the coolant in the liquid phase, via capillary action, from the heat sink to the source.

(14) In addition, the cavity includes multiple macroporous wicks, such as, for example, the wicks 130, 132, and 134, to support flows of the coolant, in both the liquid and vapor phases, including liquid/vapor mixtures, from the source to the heat sink.

(15) In one embodiment, the microporous wicks are microporous nanotube wicks and, in particular, may be microporous carbon nanotube wicks. Carbon nanotube wicks are typically individually grown in the spreader in areas near the heat source or attached to the macrowicks in such areas. Moreover, as depicted in FIG. 1, in a typical application, the heat spreader will be configured to be positioned between a substantially planar surface 136 of the heat source (See 102, 104, 106, 108, 110, 112) and a substantially planar surface 138 of the heat sink 114, with the heat source and heat sink surfaces being substantially parallel to each other.

(16) The nanotube wicks may be oriented substantially perpendicular to the planar surfaces, as depicted by the wicks 118, 120, and 122, or the wicks may be oriented substantially parallel to the planar surfaces, as depicted by the wicks 124, 126, and 128. Alternatively, the wicks may include, as in the embodiment depicted in FIGS. 2 and 3, both perpendicular and parallel wicks.

(17) In more particular embodiments of the heat spreader, the effective pore size of the microporous wicks is very small, with a high flow resistance, and will range between approximately 10 nm and 1,000 nm in radius. This provides a high capillary pressure for liquid pumping. Microporous nanotube wicks, when grown on an internal surface of the heat spreader, will typically range in height from approximately 100 to 2,000 microns. The microwicks will preferably be connected to the macrowicks to provide a continuous supply route for liquid coolant. When the microwicks are attached to the macrowicks, the microwicks will typically range in height from 1 to 1,000 microns. The pore size of the macroporous wicks will range between approximately 1 and 500 microns.

(18) The heat spreader may include, in addition, support structure for positioning the spreader between substantially planar surfaces of the heat source and the heat sink. This embodiment is depicted in FIG. 4, which is a perspective view showing a support structure made up of interconnecting cells, with cells 136 and 138 shown. In one embodiment, this support structure is fabricated out of silicon, or can be made from metal materials. Each cell includes multiple macroporous wicks, such as the wicks 140 and 142 in cell 136, as well as the wicks 144, 146, and 148 in cell 138.

(19) Each cell made of silicon or metal materials may include, in one approach to fabrication, an upper piece and a lower piece, symmetrical in geometry. Both the upper and lower pieces could be gold bonded, then reinforced by epoxy poured into a pre-etched cavity. The heat spreader structure can be, for example, a non-metallic material, such as silicon, SiC or SiNa, or a metallic material, such as copper, aluminum or silver. For a non-metallic structure, the fabrication process would typically use a dry or wet etch MEMS (microelectromechanical system) process. For a metallic structure, fabrication process would typically employ the sintering of metal particles.

(20) The macroporous wicks establish passageways that extend through the cellular support structure in a direction substantially parallel to the planar surfaces. Although the scale of FIG. 4 is too small to properly represent them, the interior surfaces of the cells 136 and 138 also contain microporous wicks, similar to the microporous wicks depicted in FIGS. 2 and 3.

(21) As shown in FIG. 4, in one embodiment the cells making up the support structure are hexagonal in cross section, although as those skilled in the pertinent art will appreciate, other geometric shapes for the cells, such as, for example, a triangular cross section, may be possible and desirable for particular applications of the heat spreader. In this two phase cell design, each cell is coated with bi-wick structures made of both macroparticles and nanoparticles.

(22) Only a very small amount of liquid coolant is needed, to cover the wick structure. The cavity is primarily occupied by saturated coolant vapor. The macroparticles incorporate relatively large pores, to reduce pressure losses in the liquid flow attributable to viscosity, while the microwicks generate large capillary forces to circulate the liquid coolant within the spreader, without the need for an external pump.

(23) The phase change involves the absorption and release of a large amount of latent heat at the evaporation and condensation regions of the spreader. With the proper sizing of components, this allows the heat spreader of this invention to operate with no net rise in temperature. This mechanism, which is the cornerstone of modern heat pipe technology, is very efficient for heat transfer. The incorporation of nanotechnology in this invention allows heat pipe technology to advance to a new level of performance and to be integrated into a multifunctional structural material, making possible a significant increase in the thermal mass of composite structures.

(24) FIG. 5 is a perspective view of one embodiment of a support structure of a heat spreader for use between substantial planer surfaces of the heat source and the heat sink. The heat spreader 500, illustrated in FIG. 5 the form of multiple hexcells or cells, has vapor chamber side ports 502 etched into top and bottom substrates 504, 506 and center substrates 508. The vapor chamber side ports 502 enable vapor communication between adjacent coupled cells 510. In one embodiment, exterior side surfaces 512 of each vapor chamber side port 502 are metalized for introduction of a solder (or glass frit) and for a subsequent heating step to bond adjacent cells 510. A bonding gap 514 is provided between each cell 510 to enable introduction of the solder during assembly. For purposes of this description, heat spreader 500 is illustrated with vapor side ports 502 exposed along its outer perimeter, although in actual use vapor chamber side ports would only be formed between coupled cells 510 to maintain a closed-system heater spreader.

(25) In one embodiment of a wafer matrix used in the fabrication process of the multi-cell heat spreader, FIG. 6 illustrates a plurality of hexcell traps 602 that are etched or otherwise provided into a wafer, such as a silicon wafer, to position respective hexcells 510 for further fabrication steps. The hexcell traps 602 are preferably etched adjacent one another and recessed into the wafer 600 to partially seat each cell 510, with each cell 510 spaced apart by a bonding gap distance 604 to enable introduction of solder between adjacent cells 510 and in support of a heating step to bond adjacent cells 510. Although illustrated as hexcells, the hexcell traps 602 are configured to receive the shape and dimensions of the cells 510 that may be fabricated in other predetermined shapes, such as squares or rectangles, such as the heat spreader 100 illustrated in FIG. 1.

(26) In one embodiment of a square heat spreader 700 illustrated in FIG. 7, a center substrate 702 is bonded between top and bottom substrates (704, 706). Each of the top and bottom substrates preferably has a plurality of recessed microporous wicks, such as etched microporous silicon or microporous parallel nanotube wicks (708, 710). The top substrate 704 has a top chamber portion in complementary opposition to a bottom chamber portion 714 in the bottom substrate 706 to collectively define a vapor chamber 716. A top substrate pillar 718 is preferably formed from the top substrate 704 and is bonded to a lower substrate pillar 720 extending from the lower substrate 706 to provide structural support for the heat spreader 700. In an alternative embodiment, top and bottom substrate pillars (720, 722) have a center substrate pillar 724 bonded between them. Although illustrated as a separate structure from the center substrate 702, the center substrate pillar is preferably formed from the center substrate 702 as described more fully, below. The center substrate 702 and center substrate pillar 724 are preferably glass, such as borosilicate glass, bonded between top and bottom substrates (704, 706) using an anodic-bonding technique. However, the center substrate may be silicon and bonded to top and bottom substrates using a glass frit bonding process.

(27) A fill port 726 is preferably etched through the top substrate 704 to enable charging of the vapor chamber with a fluid. In an alternative embodiment, the fill port 726 may be etched through at an outer perimeter of the top substrate 704, with the fill port extending into the center substrate 702 that has been previously etched through to the vapor chamber 716 for presentation of a fluid to the interior of the heat spreader for charging.

(28) FIG. 8 is a cross-sectional view of the heat spreader illustrated in FIG. 7, taken through the center substrate 702. The top substrate pillars 718 are formed at a distance D away from an interior edge of the center substrate 702, preferably by 0.75 cm, although 0.1-2.0 cm may be used depending on the total width W.sub.VC of the vapor chamber. The center substrate pillar 724 may be formed in the center of the vapor chamber 716 and bonded between the top substrate pillar 720 and bottom substrate pillar 722. The interior width W.sub.VC is preferably 3.0 cm, with the outer width of the vapor chamber W.sub.VC being 4.0 cm, although the described system is scalable to other dimensions using common wafer fabrication techniques once the overall system configuration is known. Although FIG. 8 illustrates a pattern of equally-spaced top substrate pillars 718, in an alternative embodiment the pillars are formed in a hexagonal, triangular or other pattern to provide appropriate structural support for the heat spreader between top and bottom substrates.

(29) FIGS. 9-13 illustrate one embodiment of a fabrication process for forming a heat spreader system. Top and bottom substrates (900, 902), alternatively referred to as top and bottom wafers, are each provided with a plurality of microporous wicks in top and bottom substrate recesses (908, 910). In a preferred embodiment, the microporous parallel nanotube wicks are carbon nanotube wicks (904, 906) formed by synthesizing and having a diameter of 1.5 mm and height of 250 um carbon nanotube clusters. Or, other wicks may be used, such as etched porous silicon on both top and bottom substrates. In one embodiment, a silicon center substrate 912 may be glass frit bonded, preferably using the glass frit described in Table 1, between the bottom substrate 902 and the top substrate 900 to establish a vapor chamber. In a preferred embodiment such as illustrated in FIG. 10, a silicon center substrate 1000 having a 1.5 mm thickness is preferably first provided with a center substrate post 1002, such as by etching, and a channel 1003 is etched around an outer perimeter of the center substrate 1000 to define a first center substrate bonding surface 1004. A glass frit, preferably the glass frit described in Table 1 (below), is screen printed on the bottom wafer 902 on the first center substrate bonding surface 1004, dried in an oven and fired in a furnace to burn the matrix and pre-fuse the glass patterns.

(30) In FIG. 11, the center substrate 1000 has been flipped in orientation and bonded to bottom wafer 902 at the first center substrate bonding surface 1004 and between the center substrate post 1002 and the bottom substrate pillar 915 by applying a suitable temperature-time-pressure cycle under vacuum as understood by one of ordinary skill in the art. In FIG. 12, the center substrate 1000 is etched again but through a second surface 1200 of the center wafer 1000 to free the center substrate post 1002 by removing the remainder of the center wafer 1202 from the heat spreader. In FIG. 13, the top substrate 900 screen printed with the glass frit, dried and fired to remove the matrix and pre-fuse the glass patterns. The top substrate 900 is then is then bonded to the center substrate 1000 and center substrate posts 1002 through the top substrate pillar through suitable application of temperature-time pressure cycle under vacuum.

(31) In one heat spreader designed for use with a glass center substrate (preferably borosilicate glass) and silicon top and bottom substrates, the Frit has the compositions described in Table 1.

(32) TABLE-US-00001 TABLE 1 PbO MgO Al.sub.2O.sub.3 SiO.sub.2 ZnO B2O.sub.3 Matrix 77 1 2 6 3 11 Glass 20 11 25 44

(33) FIGS. 14 and 15 illustrate another embodiment of a heat spreader that has a glass center substrate rather than a silicon center substrate. Top and bottom substrates (1402, 1404) have a glass center layer 1406 coupled between them, preferably with coupled with a glass frit bonding technique. In this embodiment, the glass layer has been precut such as through the use of an ultrasonic knife to have cut through portions 1502 to enable vapor transport between an upper and lower plurality of microporous wicks such as microporous parallel nanotube wicks (1408, 1410). The center substrate glass layer 1406 is also bonded between top and bottom substrate pillars (1412, 1414) extending from the top substrate and bottom substrate, respectively (1402, 1404). In alternative embodiments, there may be more or fewer cut-through portions 1502. Also, although only single top and bottom substrate pillars (1412, 1414) are illustrated, the heat spreader may be provided with a plurality of substrate pillars with the center substrate glass layer 1406 coupled between them. In an alternative embodiment, the center layer may be a third silicon substrate layer, with or without etched cut-through portions. In such a case, diffusion bonding may be used to bond the center substrate between top and bottom substrates.

(34) The preferred embodiments of this invention have been illustrated and described above. Modifications and additional embodiments, however, will undoubtedly be apparent to those skilled in the art. Furthermore, equivalent elements may be substituted for those illustrated and described herein, parts or connections might be reversed or otherwise interchanged, and certain features of the invention may be utilized independently of other features. Consequently, the exemplary embodiments should be considered illustrative, rather than inclusive, while the appended claims are more indicative of the full scope of the invention.