Multi-level hierarchical hybrid structures to replace single-level wicks in next generation vapor chambers

Abstract

Improved vapor chambers are provided using monolithic wick structures having deep features (≥150 um) and two or more different feature heights above the substrate. Such monolithic multi-level wick structures provide improved performance in vapor chambers by alleviating the tradeoff between fluid transport (which favors tall pin-fins) and heat transfer (which favors short pin-fins).

Claims

1. A passive wicking-based microfluidic heat spreader comprising: a monolithically microfabricated array of wicking features, wherein the monolithically microfabricated array of wicking features includes a substrate and features having two or more different vertical feature heights above the substrate; wherein the monolithically microfabricated array of wicking features does not include any wafer-to-wafer bonds; wherein the monolithically microfabricated array of wicking features includes features having a vertical feature height of 150 microns or more.

2. The passive wicking-based microfluidic heat spreader of claim 1, wherein the wicking features include one or more pins that rise vertically from the substrate surface.

3. The passive wicking-based microfluidic heat spreader of claim 2, wherein vertical heights of the one or more pins are configured to provide a vertical height gradient in the monolithically microfabricated array of wicking features.

4. The passive wicking-based microfluidic heat spreader of claim 2, wherein one or more of the pins is a multilevel pin having two or more pin features with different vertical heights above the substrate surface.

5. The passive wicking-based microfluidic heat spreader of claim 1, further comprising one or more fluid passages in the substrate.

6. The passive wicking-based microfluidic heat spreader of claim 1, wherein at least one of the fluid passages is configured as a hole passing vertically though the substrate.

7. The passive wicking-based microfluidic heat spreader of claim 1, further comprising one or more vertical vias through the substrate.

8. The passive wicking-based microfluidic heat spreader of claim 7, wherein a height/width aspect ratio of at least one of the vertical vias is 10 or more.

9. A vapor chamber comprising: a passive wicking-based microfluidic heat spreader according to claim 1; a capping layer disposed to form an enclosure with the passive wicking-based microfluidic heat spreader; an evaporative coolant disposed in the enclosure.

10. A vapor chamber comprising: a first passive wicking-based microfluidic heat spreader according to claim 1; and a second passive wicking-based microfluidic heat spreader according to claim 1; wherein the first and second passive wicking-based microfluidic heat spreaders are disposed to form an enclosure; and an evaporative coolant disposed in the enclosure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG. 1 shows an exemplary embodiment of the invention.

[0038] FIGS. 2A-B show two views of an exemplary monolithically microfabricated array of wicking features.

[0039] FIGS. 2C-D shows examples of the use of the monolithically microfabricated array of wicking features of FIGS. 2A-B in a vapor chamber.

[0040] FIG. 3 shows examples of multi-level pins.

[0041] FIG. 4 shows examples of pins having holes and/or roughness.

[0042] FIG. 5 shows an exemplary pin array having a gradient of pin height.

DETAILED DESCRIPTION

A) General Principles

[0043] In this work we provide multi-level post (pillar) type structures (note that multi-level pin fin type structures have not been reported in any previous study) with the following characteristics — [0044] 1. Will have two or more levels of pillars (conventional methods can also make more than one level structures) [0045] 2. The maximum height difference in the multi-level structure is more than 150 um (conventional grayscale lithography technique has only demonstrated 3D structures with a maximum height difference of 100 um). We can easily push this to more than 150 um and this is a major advantage of our approach. This is especially useful in many applications since structures associated with microfluidics and microfluidic cooling technologies (here we can mention passive heat spreaders, like vapor chamber of heat pipe) operate in the micro-meso scale. [0046] 3. The resolution of steps achievable is also pretty high in our method, 2-3 um as compared to conventional chip stacking (chip stacking has a resolution of 30-50 um for the in-between middle layers)

[0047] We provide improvement (higher thermal performance and being able to scale up the technology) in passive cooling devices by having multi-level microstructures of different heights. The performance of most conventional passive cooling devices (vapor chambers, heat pipes) is almost solely determined by the microstructure pore size on the evaporator wick. A smaller microstructure pore size helps in fluid retention over the hot-spots, reduces conduction resistance of the thin film of fluid and enhances heat transfer area during device operation. Although, the full potential of these small pored structures are not utilized as smaller pore sizes are also accompanied by other problems. Smaller pore sizes simultaneously reduce the total amount of fluid that can be successfully wicked back from the condenser to the evaporator thus putting a transport-based limit (called, capillary limit). These issues lead to two more issues that are the primary hurdles to widespread use and commercialization—low critical heat flux (CHF) that can be dissipated from the hotspot and device cannot be scaled up to dissipate heat from larger areas. To mitigate these problems, truly 3D structures can be made monolithically out of a single wafer (e.g., a silicon wafer) as described herein.

[0048] These devices could have a combination of features—taller pin-fins, channels, arteries wherever fluid transport is desired and have smaller pored structures over and near the hot-spots to maintain low resistance and good thermal performance. The standardization of the new method (which can be done with great ease) into processing flows in industry and academia will significantly expand the design space available to us in terms of structure types and topologies we can make monolithically. Additionally, the vapor chamber can also have some much taller pin fins interspersed in the heater zone. These are structural pins acting as bonding sites with the other layer, to provide mechanical support to the overall device and sustain a higher pressure before bursting. Moreover, these pins also provide shorter pathways for liquid return from the condenser to the evaporator, thus increasing capillary transport limited CHF.

[0049] FIG. 1 shows an exemplary embodiment of the invention that is a passive wicking-based microfluidic heat spreader 102 including: [0050] a monolithically microfabricated array of wicking features (e.g., pins 108, 110, 112), where the monolithically microfabricated array of wicking features includes a substrate 106 and features having two or more different vertical feature heights above the substrate (e.g., features 108, 110, 112 have three different heights above substrate 106). The monolithically microfabricated array of wicking features does not include any wafer-to-wafer bonds, and it includes features having a vertical feature height of 150 microns or more. As indicated above, such deeply etched wicking structures are not possible to make with conventional fabrication methods, and thus, to the best of our knowledge, have not been previously reported.

[0051] Here a monolithically microfabricated array of wicking features is an array of wicking features fabricated by processing a single wafer (as opposed to processing two or more wafers and then bonding them together). As a result, a monolithically microfabricated array of wicking features has the structural feature of not including any wafer-to-wafer bonds.

[0052] The wicking features can include one or more pins that rise vertically from the substrate surface. Vertical heights of the one or more pins can be configured to provide a vertical height gradient (FIG. 5) in the monolithically microfabricated array of wicking features. One or more of the pins can be a multilevel pin (FIG. 3) having two or more pin features with different vertical heights above the substrate surface.

[0053] One or more fluid passages (e.g., 212 on FIG. 2A) can be present in the substrate. At least one of the fluid passages can be configured as a hole passing vertically though the substrate (e.g., 212 on FIG. 2A).

[0054] One or more vertical vias (e.g., 214 on FIG. 2A) can pass through the substrate. A height/width aspect ratio of at least one of the vertical vias can be 10 or more.

[0055] A vapor chamber can include a first passive wicking-based microfluidic heat spreader as above (e.g., 202a on FIG. 2C, and a second passive wicking-based microfluidic heat spreader as above (e.g., 202b on FIG. 2C), where the first and second passive wicking-based microfluidic heat spreaders are disposed to form an enclosure. In this example, an evaporative coolant (e.g., 230 on FIG. 2C is disposed in the enclosure.

[0056] A vapor chamber can include a passive wicking-based microfluidic heat spreader as above (e.g., 202 on FIG. 2D), a capping layer (e.g., 220 on FIG. 2D) disposed to form an enclosure with the passive wicking-based microfluidic heat spreader, and an evaporative coolant (e.g., 230 on FIG. 2D) disposed in the enclosure.

B) Examples

[0057] FIGS. 2A-B show another exemplary embodiment. Here passive wicking-based microfluidic heat spreader 202 includes substrate 204, tall pins 206, intermediate pins 208 and short pins 210. It also includes fluid inlet/outlet ports (e.g., port 212) through the substrate and vertical via(s) (e.g., 214). As indicated above, pins of different height have different functions. Short pins 210 are disposed near hotspots for better thermal performance, intermediate pins 208 provide better fluid transport from device edge to center, and tall pins 206 can act as mechanical support pillars.

[0058] Vertical vias are often desirable for establishing multi-layer multifunctional chips. Our approach enables easy creation of high aspect ratio vertical vias, that are expected to enable next generation 3D electronic vertically expanded chiplets. Vertical vias and other through holes (for fluid charging or flow) can be simultaneously fabricated with ease during wick formation because of the one shot etching employed by this process.

[0059] The fluid ports are typically much larger in lateral dimension than the vias to accommodate flow, so their aspect ratio is lower than that of the vias. These are easy to make, a variety of other methods can be used—laser cutting, water jet cutting, micromachining, drilling. Our method enables simultaneous creation of all these different features (active wick microstructures, other steps in silicon for integration, roughness, holes, vertical vias, through ports) monolithically out of a single substrate.

[0060] As indicated above, a vapor chamber can be formed by making an enclosure that includes wick structures as described herein. FIG. 2C shows a first example, where wick structures 202a and 202b form an enclosure in which evaporative coolant 230 is disposed. Here wick structure 202b can be the condenser and wick structure 202a can be the evaporator (as shown), or vice versa. FIG. 2D shows a second example, where wick structure 202 and capping layer 220 form an enclosure in which evaporative coolant 230 is disposed. Here it is preferred that wick structure 202 be the evaporator, as shown.

[0061] Single features, e.g., a single pin, can individually be multi-level. FIG. 3 shows some examples. Here pin 302 includes features 302a, 302b, 302c, 302d at different heights. Similarly, pin 304 includes features 304a, 304b, 304c, 304d, 304e at different heights. In a passive heat spreader, these individual pin features can be used for increasing surface area for heat transfer, increasing capillarity of the wick, modulating porosity with wick height for easier vapor venting etc. This capability is enabled by the lack of a limitation on the number of lithography rounds that can be reliably performed on the oxide, so different pins can be designed to also have multiple levels.

[0062] Another capability provided by this technology is well-controlled porosity and/or roughness of individual pin features, as in the examples of FIG. 4. Here pin 402 has holes 402a and controlled roughness 402b, 402c. Similarly, pin 404 has holes 404a, 404b and controlled microroughness 404c. This approach is a reliable way to introduce well controlled multi-height pillared roughness to the base of any microstructure instead of relying on other methods (UV laser rastering, hydrothermal synthesis of nanotube, nanowire etc.) which are stochastic and thus provide less control over the roughness elements and parameters (porosity, element width and height, pitch, density). Base structuring is usually beneficial since it enhances mass transport and typically improves heat transfer performance as well.

[0063] FIG. 5 shows an example of a gradient of pin height across the whole array. Here 502 is a monolithic multi-layer wick structure, and the pin array 504 has a height gradient from center to edge. This example is a wick with micro-pillar heights decreasing monotonically as we move towards the center of the device. Such devices, which have wick permeability monotonically increasing as we move to the device periphery can be an attractive solution to the problems associated with the massive liquid-to-vapor volume expansion. During device operation, the expanding vapor often gets trapped in monoporous wicks—unable to escape, they increase vapor pressure within the vapor chamber, which suppresses further phase change (thus reducing thermal performance). The monoporous wick also restricts lateral vapor spreading, which slows vapor transport to the condenser and worsens transport related issues arising in the device. Having taller pins as we move to the device periphery will help reroute the expanding vapor efficiently and quickly away from the hot-spots, thus preventing issues of vapor clogging and accumulation near the hot-spots, thus helping maintain the same high levels of performance at all vapor qualities. A 2.5D version of this gradient idea (where the peripheral pins have a higher pitch instead of taller height, thereby making a pitch gradient) has already been fabricated and demonstrated to work better than a corresponding monoporous counterpart in the literature. The device of FIG. 5 is expected to further improve the performance.