Silicon biporous wick for high heat flux heat spreaders

1. A heat spreader, comprising:

a wicking structure comprising a biporous wick comprising a plurality of first pores and a plurality of second pores, wherein the first pores are larger than the second pores;

an evaporator including the wicking structure or the wicking structure above the evaporator;

a vapor space on top of the wicking structure; and

a condenser on top of the vapor space, wherein:

the evaporator conducts heat from a heat source to a working fluid in the biporous wick,

the heat vaporizes the working fluid into vapor that escapes from the first pores,

the vapor is transported to the condenser above the first pores and the second pores through the vapor space, and

the condenser condenses the vapor into a condensate comprising the working fluid.

2. The heat spreader of claim 1 , wherein: the biporous wick comprises a plurality of structures formed in a semiconductor, at least some of the structures are between the plurality of the first pores, surfaces of each of the structures comprise a plurality of second pores,

voids are created in the first pores and the second pores as the vapor escapes through the first pores into the vapor space,

the condensate is collected in the wicking structure, and

the voids draw the condensate comprising the working fluid into the biporous wick under capillary action provided by the second pores.

3. The wicking structure of claim 1 , wherein:

the structures comprise pyramidal structures, and

the surfaces of the pyramidal structures comprise the second pores.

4. The wicking structure of claim 3 , wherein the pyramidal structures have a base and sidewalls that slope towards each other.

5. The wicking structure of claim 4 , wherein the pyramidal structures comprise silicon and surfaces of pyramidal structures comprise black silicon.

6. A heat spreader comprising the wicking structure of claim 1 , comprising:

the evaporator including the biporous wick, wherein the pyramidal structures are tall enough to span an entire height of the vapor space and provide structural support to the condenser.

7. The wicking structure of claim 2 , wherein the semiconductor is silicon.

8. The heat spreader of claim 1 , wherein the condenser has a surface area that is at least 50 times larger than a surface area of the heat source.

9. The heat spreader of claim 2 , comprising:

the semiconductor comprising silicon;

a first silicon wafer including the condenser; and

a second silicon wafer including the evaporator, wherein the silicon wafers are hermetically bonded together to form the vapor space between the condenser and the evaporator.

10. A monolithic silicon piece comprising the heat spreader of claim 1 .

11. The heat spreader of claim 1 , wherein first pores have a first diameter of 100 micrometers-400 micrometers and the second pores have a second diameter of 10 micrometers to 50 micrometers.

12. A method of fabricating a heat spreader comprising:

forming a wicking structure comprising a biporous wick comprising a plurality of first pores and a plurality of second pores, wherein the first pores are larger than the second pores;

providing an evaporator including the wicking structure or the wicking structure above the evaporator;

providing a vapor space on top of the wicking structure; and

providing a condenser on top of the vapor space, wherein:

the evaporator conducts heat from a heat source to a working fluid in the biporous wick,

the heat vaporizes the working fluid into vapor that escapes from the first pores,

the vapor is transported to the condenser above the first pores and the second pores through the vapor space, and

the condenser condenses the vapor into a condensate comprising the working fluid.

13. The method of claim 12 , further comprising:

modeling heat flux in a heat spreader comprising the evaporator including the biporous wick, the condenser, and a vapor space, wherein:

the biporous wick comprises a plurality of structures formed in a semiconductor, at least some of the structures are between the plurality of the first pores, surfaces of each of the structures comprise a plurality of second pores,

the condensate is collected in the wicking structure, and

the voids draw the condensate comprising the working fluid into the biporous wick under capillary action provided by the second pores.

14. The method of claim 12 , wherein:

the structures comprise pyramidal structures and

the forming further comprises forming the second pores into the surfaces of the pyramidal structures.

15. The method of claim 14 , wherein the pyramidal structures have a base and sidewalls that slope towards each other.

16. The method of claim 14 , wherein the pyramidal structures comprise silicon and surfaces of pyramidal structures comprise black silicon.

17. The method of claim 12 , wherein the semiconductor is silicon.

18. The method of claim 17 , wherein the modeling selects a volume of the vapor space that depends on a thickness of the biporous wick and sizes of the pores.

19. The method of claim 17 , wherein the condenser has a surface area that is at least 50 times larger than a surface area of the heat source.

20. The method of claim 12 , further comprising monolithically forming a silicon piece comprising the condenser, the evaporator, and the vapor space, wherein the vapor space is between the condenser and the evaporator and the evaporator comprises the biporous wick.

21. A wicking structure, comprising:

a biporous wick comprising a plurality of first pores and a plurality of second pores; and

a monolithic piece consisting essentially of silicon; wherein:

each of the first pores are larger than each of the second pores, and

the second pores are formed in the monolithic piece.

22. The heat spreader of claim 1 , wherein:

the vapor is transported vertically from the wicking structure to the condenser vertically above the first pores, the second pores, and the evaporator, and

the first pores and the second pores are in contact with the vapor space.