MEMS-BASED SYSTEM FOR COOLING A VAPOR CHAMBER

A server system is described. The server system includes a vapor chamber in thermal communication with a plurality of heat sources and an array of microelectromechanical system (MEMS) jets arranged to cause a fluid to impinge on a surface of the vapor chamber.

BACKGROUND OF THE INVENTION

As computing devices grow in speed and computing power, the heat generated by the computing devices also increases. Various mechanisms have been proposed to address the generation of heat. Active devices, such as fans, may be used to drive air through larger computing devices, such as laptop computers or desktop computers. Passive cooling devices, such as heat spreaders, may be used in smaller, mobile computing devices, such as smartphones, virtual reality devices and tablet computers. However, such active and passive devices may be unable to adequately cool not only mobile devices and larger devices, but may also be inadequate for high power computing systems, such as server systems. Server systems utilize multiple high power processors. In addition, servers are typically housed in racks that carry multiple servers systems. Consequently, high power systems may be desired to be placed in proximity to other high power systems while maintaining their heat dissipation. Consequently, additional cooling solutions for computing devices, particularly high power dissipation computing devices, are desired.

DETAILED DESCRIPTION

As semiconductor devices become increasingly powerful, the heat generated during operations also grows. For example, processors for mobile devices such as smartphones, tablet computers, notebooks, and virtual reality devices can operate at high clock speeds, but produce a significant amount of heat. Because of the quantity of heat produced, processors may run at full speed only for a relatively short period of time. After this time expires, throttling (e.g. slowing of the processor's clock speed) occurs. Although throttling can reduce heat generation, it also adversely affects processor speed and, therefore, the performance of devices using the processors. As technology moves to5G and beyond, this issue is expected to be exacerbated.

Larger devices, such as laptop or desktop computers include electric fans that have rotating blades. The fan that can be energized in response to an increase in temperature of internal components. The fans drive air through the larger devices to cool internal components. However, such fans are typically too large for mobile devices such as smartphones or for thinner devices such as tablet computers. Fans also may have limited efficacy because of the boundary layer of air existing at the surface of the components, provide a limited airspeed for air flow across the hot surface desired to be cooled and may generate an excessive amount of noise. Passive cooling solutions may include components such as a heat spreader and a heat pipe or vapor chamber to transfer heat to a heat exchanger. Although a heat spreader somewhat mitigates the temperature increase at hot spots, the amount of heat produced in current and future devices may not be adequately addressed. Similarly, a heat pipe or vapor chamber may provide an insufficient amount of heat transfer to remove excessive heat generated.

Furthermore, high power computing systems, such as server systems, are desired to be cooled. Server systems utilize multiple high power processors. For example, some server systems use four processors for which at least two hundred Watts per processor (eight hundred Watts per server) is desired to be dissipated. Some server systems are desired to dissipate four hundred Watts per processor (one thousand six hundred Watts per server). Future generations of servers may use higher power and/or more processors for which more power is desired to be dissipated. In addition, servers are typically housed in racks that carry multiple servers systems. Consequently, high power systems may be desired to be placed in proximity to other high power systems while maintaining their heat dissipation. Thus, to achieve optimum performance in such systems, high power dissipation is desired.

Current server systems typically use fans or liquid cooling for power dissipation. Fans are limited in their ability to dissipate heat. For example, the volume and speed of the air flow from a set of fans (e.g. five) may be insufficient to provide more than approximately eight hundred Watts of heat dissipation. Thus, fans may not be used in higher power processor systems. Further, fans are generally tall. For example, a server utilizing fans capable of dissipating eight hundred Watts may use a fan system that is at least forty four millimeters in height. Other fan systems may be sixty millimeters to eighty millimeters in height. Thus, the server system employing fans may be larger than desired. Liquid cooling provides higher efficiency heat dissipation. However, the use of liquid in connection with electrical systems, such as server systems, may be less desirable. Further, heated fluid is generally routed to an external chiller and then returned to the server system. Consequently, components outside of the server system (e.g. external ducting to and from the chiller) may be required. Thus, other techniques for providing heat dissipation for high power systems are still desired.

A server system includes a vapor chamber and an array of microelectromechanical system (MEMS) jets. The vapor chamber is in thermal communication with a plurality of heat sources. The array of MEMS jets is arranged to cause a fluid to impinge on a surface of the vapor chamber. Each MEMS jet in the array of MEMS jets may have a height of not more than 1.5 millimeter. In some embodiments, the array of MEMS jets includes at least 720 jets and dissipates at least 1400 W. The fluid may be air. The vapor chamber may include fins having at least a portion of the surface. Each of the fins may be parallel another of the fins. The fins may be oriented parallel to a heat source surface or perpendicular to the heat source surface.

In some embodiments, the array of MEMS jets includes cooling cells. Each cooling cell includes a cooling element and an orifice plate including orifices therein. The cooling element is configured to drive the fluid through the plurality of orifices, forming a plurality of fluid jets. In some embodiments, the jets have a velocity of greater than 30 meters per second. The vapor chamber has a first surface and a second surface opposite to the first surface in some embodiments. The array of MEMS jets is configured to cause the fluid to impinge on the first surface and the second surface. In some embodiments, the system further includes a duct system configured to direct the fluid from outside of the server system to the array of MEMS jets and to direct heated fluid from the vapor chamber to the outside of the server system.

A system including a vapor chamber and an array of cooling elements is described. The vapor chamber is in thermal communication with a plurality of heat sources. The array of cooling elements is configured to undergo vibrational motion when actuated to drive a fluid to impinge on a surface of the vapor chamber. The array of cooling elements is configured to dissipate at least 800 Watts when actuated. The array of cooling elements may be configured to dissipate at least 1600 Watts when actuated. In some embodiments, the array of cooling elements is configured to drive the fluid through orifices in a least one orifice plate when actuated. Thus, fluid jets having a velocity of at greater than 30 meters per second may be formed. In some embodiments, the array of cooling elements has a height of not more than 1.5 millimeter.

In some embodiments, the vapor chamber has a first surface and a second surface opposite to the first surface. The array of cooling elements is configured to cause the fluid to impinge on the first surface and the second surface. The vapor chamber may include fins having at least a portion of the surface the fluid impinges on. The system may further include a duct system. The duct system is configured to direct the fluid from outside of the server system to the array of MEMS jets and to direct heated fluid from the vapor chamber to the outside of the server system. The system may include at least 720 cooling elements, include at least 720 cooling elements, and dissipates at least 1400 W.

A method for providing a cooling system is described. The method includes providing a vapor chamber in thermal communication with a plurality of heat sources. The method also includes providing an array of MEMS jets coupled with the vapor chamber and arranged to cause a fluid to impinge on a surface of the vapor chamber. In some embodiments, providing the array of MEMS jets includes providing cooling cells. Each cooling cell includes a cooling element and an orifice plate including a plurality of orifices therein. The cooling element is configured to drive the fluid through the orifices, forming fluid jets.

FIGS.1A-1Gare diagrams depicting an exemplary embodiment of active MEMS cooling system100usable with heat-generating structure102and including a centrally anchored cooling element120or120′. Cooling element120is shown inFIGS.1A-1Fand cooling element120′ is shown inFIG.1G. For clarity, only certain components are shown.FIGS.1A-1Gare not to scale.FIGS.1A and1Bdepict cross-sectional and top views of cooling system100in a neutral position.FIGS.1C-1Ddepict cooling system100during actuation for in-phase vibrational motion.FIGS.1E-1Fdepict cooling system100during actuation for out-of-phase vibrational motion. Although shown as symmetric, cooling system100need not be.

Cooling system100includes top plate110having vent112therein, cooling element120, orifice plate130having orifices132therein, support structure (or “anchor”)160and chambers140and150(collectively chamber140/150) formed therein. Cooling element120is supported at its central region by anchor160. Regions of cooling element120closer to and including portions of the cooling element's perimeter (e.g. tip121) vibrate when actuated. In some embodiments, tip121of cooling element120includes a portion of the perimeter furthest from anchor160and undergoes the largest deflection during actuation of cooling element120. For clarity, only one tip121of cooling element120is labeled inFIG.1A. Also shown is pedestal190that connects orifice plate130to and offsets orifice plate130from heat-generating structure102. In some embodiments, pedestal190also thermally couples orifice plate130to heat-generating structure102.

FIG.1Adepicts cooling system100in a neutral position. Thus, cooling element120is shown as substantially flat. For in-phase operation, cooling element120is driven to vibrate between positions shown inFIGS.1C and1D. This vibrational motion draws fluid (e.g. air) into vent112, through chambers140and150and out orifices132at high speed and/or flow rates. For example, the speed at which the fluid impinges on heat-generating structure102may be at least thirty meters per second. In some embodiments, the fluid is driven by cooling element120toward heat-generating structure102at a speed of at least forty-five meters per second. In some embodiments, the fluid is driven toward heat-generating structure102by cooling element120at speeds of at least sixty meters per second. Other speeds may be possible in some embodiments. Cooling system100is also configured so that little or no fluid is drawn back into chamber140/150through orifices132by the vibrational motion of cooling element120.

Heat-generating structure102is desired to be cooled by cooling system100. In some embodiments, heat-generating structure102generates heat. For example, heat-generating structure may be an integrated circuit. In some embodiments, heat-generating structure102is desired to be cooled but does not generate heat itself. Heat-generating structure102may conduct heat (e.g. from a nearby object that generates heat). For example, heat-generating structure102might be a heat spreader or a vapor chamber. Thus, heat-generating structure102may include semiconductor component(s) including individual integrated circuit components such as processors, other integrated circuit(s) and/or chip package(s); sensor(s); optical device(s); one or more batteries; other component(s) of an electronic device such as a computing device; heat spreaders; heat pipes; other electronic component(s) and/or other device(s) desired to be cooled. In some embodiments, heat-generating structure102may be a thermally conductive part of a module containing cooling system100. For example, cooling system100may be affixed to heat-generating structure102, which may be coupled to another heat sink, vapor chamber, integrated circuit, or other separate structure desired to be cooled.

The devices in which cooling system100is desired to be used may also have limited space in which to place a cooling system. For example, cooling system100may be used in computing devices. Such computing devices may include but are not limited to smartphones, tablet computers, laptop computers, tablets, two-in-one laptops, hand held gaming systems, digital cameras, virtual reality headsets, augmented reality headsets, mixed reality headsets and other devices that are thin. Cooling system100may be a micro-electro-mechanical system (MEMS) cooling system capable of residing within mobile computing devices and/or other devices having limited space in at least one dimension. For example, the total height, h3, of cooling system100(from the top of heat-generating structure102to the top of top plate110) may be less than 2 millimeters. In some embodiments, the total height of cooling system100is not more than 1.5 millimeters. In some embodiments, this total height is not more than 1.1 millimeters. In some embodiments, the total height does not exceed one millimeter. In some embodiments, the total height does not exceed two hundred and fifty micrometers. Similarly, the distance between the bottom of orifice plate130and the top of heat-generating structure102, y, may be small. In some embodiments, y is at least two hundred micrometers and not more than 1.2 millimeter. For example, y may be at least two hundred and fifty micrometers and not more than three hundred micrometers. In some embodiments, y is at least five hundred micrometers and not more than one millimeter. In some embodiments, y is at least two hundred micrometers and not more than three hundred micrometers. Thus, cooling system100is usable in computing devices and/or other devices having limited space in at least one dimension. However, nothing prevents the use of cooling system100in devices having fewer limitations on space and/or for purposes other than cooling. Although one cooling system100is shown (e.g. one cooling cell), multiple cooling systems100might be used in connection with heat-generating structure102. For example, a one or two-dimensional array of cooling cells might be utilized.

Cooling system100is in communication with a fluid used to cool heat-generating structure102. The fluid may be a gas or a liquid. For example, the fluid may be air. In some embodiments, the fluid includes fluid from outside of the device in which cooling system100resides (e.g. provided through external vents in the device). In some embodiments, the fluid circulates within the device in which cooling system resides (e.g. in an enclosed device).

Cooling element120can be considered to divide the interior of active MEMS cooling system100into top chamber140and bottom chamber150. Top chamber140is formed by cooling element120, the sides, and top plate110. Bottom chamber150is formed by orifice plate130, the sides, cooling element120and anchor160. Top chamber140and bottom chamber150are connected at the periphery of cooling element120and together form chamber140/150(e.g. an interior chamber of cooling system100).

The size and configuration of top chamber140may be a function of the cell (cooling system100) dimensions, cooling element120motion, and the frequency of operation. Top chamber140has a height, h1. The height of top chamber140may be selected to provide sufficient pressure to drive the fluid to bottom chamber150and through orifices132at the desired flow rate and/or speed. Top chamber140is also sufficiently tall that cooling element120does not contact top plate110when actuated. In some embodiments, the height of top chamber140is at least fifty micrometers and not more than five hundred micrometers. In some embodiments, top chamber140has a height of at least two hundred and not more than three hundred micrometers.

Bottom chamber150has a height, h2. In some embodiments, the height of bottom chamber150is sufficient to accommodate the motion of cooling element120. Thus, no portion of cooling element120contacts orifice plate130during normal operation. Bottom chamber150is generally smaller than top chamber140and may aid in reducing the backflow of fluid into orifices132. In some embodiments, the height of bottom chamber150is the maximum deflection of cooling element120plus at least five micrometers and not more than ten micrometers. In some embodiments, the deflection of cooling element120(e.g. the deflection of tip121), z, has an amplitude of at least ten micrometers and not more than one hundred micrometers. In some such embodiments, the amplitude of deflection of cooling element120is at least ten micrometers and not more than sixty micrometers. However, the amplitude of deflection of cooling element120depends on factors such as the desired flow rate through cooling system100and the configuration of cooling system100. Thus, the height of bottom chamber150generally depends on the flow rate through and other components of cooling system100.

Top plate110includes vent112through which fluid may be drawn into cooling system100. Top vent112may have a size chosen based on the desired acoustic pressure in chamber140. For example, in some embodiments, the width, w, of vent112is at least five hundred micrometers and not more than one thousand micrometers. In some embodiments, the width of vent112is at least two hundred fifty micrometers and not more than two thousand micrometers. In the embodiment shown, vent112is a centrally located aperture in top plate110. In other embodiments, vent112may be located elsewhere. For example, vent112may be closer to one of the edges of top plate110. Vent112may have a circular, rectangular or other shaped footprint. Although a single vent112is shown, multiple vents might be used. For example, vents may be offset toward the edges of top chamber140or be located on the side(s) of top chamber140. Although top plate110is shown as substantially flat, in some embodiments trenches and/or other structures may be provided in top plate110to modify the configuration of top chamber140and/or the region above top plate110.

Anchor (support structure)160supports cooling element120at the central portion of cooling element120. Thus, at least part of the perimeter of cooling element120is unpinned and free to vibrate. In some embodiments, anchor160extends along a central axis of cooling element120(e.g. perpendicular to the page inFIGS.1A-1F). In such embodiments, portions of cooling element120that vibrate (e.g. including tip121) move in a cantilevered fashion. Thus, portions of cooling element120may move in a manner analogous to the wings of a butterfly (i.e. in phase) and/or analogous to a seesaw (i.e. out of phase). Thus, the portions of cooling element120that vibrate in a cantilevered fashion do so in phase in some embodiments and out of phase in other embodiments. In some embodiments, anchor160does not extend along an axis of cooling element120. In such embodiments, all portions of the perimeter of cooling element120are free to vibrate (e.g. analogous to a jellyfish). In the embodiment shown, anchor160supports cooling element120from the bottom of cooling element120. In other embodiments, anchor160may support cooling element120in another manner. For example, anchor160may support cooling element120from the top (e.g. cooling element120hangs from anchor160). In some embodiments, the width, a, of anchor160is at least 0.5 millimeters and not more than four millimeters. In some embodiments, the width of anchor160is at least two millimeters and not more than 2.5 millimeters. Anchor160may occupy at least ten percent and not more than fifty percent of cooling element120.

Cooling element120has a first side distal from heat-generating structure102and a second side proximate to heat-generating structure102. In the embodiment shown inFIGS.1A-1F, the first side of cooling element120is the top of cooling element120(closer to top plate110) and the second side is the bottom of cooling element120(closer to orifice plate130). Cooling element120is actuated to undergo vibrational motion as shown inFIGS.1A-1F. The vibrational motion of cooling element120drives fluid from the first side of cooling element120distal from heat-generating structure102(e.g. from top chamber140) to a second side of cooling element120proximate to heat-generating structure102(e.g. to bottom chamber150). The vibrational motion of cooling element120also draws fluid through vent112and into top chamber140; forces fluid from top chamber140to bottom chamber150; and drives fluid from bottom chamber150through orifices132of orifice plate130. Thus, cooling element120may be viewed as an actuator. Although described in the context of a single, continuous cooling element, in some embodiments, cooling element120may be formed by two (or more) cooling elements. Each of the cooling elements as one portion pinned (e.g. supported by support structure160) and an opposite portion unpinned. Thus, a single, centrally supported cooling element120may be formed by a combination of multiple cooling elements supported at an edge.

Cooling element120has a length, L, that depends upon the frequency at which cooling element120is desired to vibrate. In some embodiments, the length of cooling element120is at least four millimeters and not more than ten millimeters. In some such embodiments, cooling element120has a length of at least six millimeters and not more than eight millimeters. The depth of cooling element120(e.g. perpendicular to the plane shown inFIGS.1A-1F) may vary from one fourth of L through twice L. For example, cooling element120may have the same depth as length. The thickness, t, of cooling element120may vary based upon the configuration of cooling element120and/or the frequency at which cooling element120is desired to be actuated. In some embodiments, the cooling element thickness is at least two hundred micrometers and not more than three hundred and fifty micrometers for cooling element120having a length of eight millimeters and driven at a frequency of at least twenty kilohertz and not more than twenty-five kilohertz. The length, C of chamber140/150is close to the length, L, of cooling element120. For example, in some embodiments, the distance, d, between the edge of cooling element120and the wall of chamber140/150is at least one hundred micrometers and not more than five hundred micrometers. In some embodiments, d is at least two hundred micrometers and not more than three hundred micrometers.

Cooling element120may be driven at a frequency that is at or near both the resonant frequency for an acoustic resonance of a pressure wave of the fluid in top chamber140and the resonant frequency for a structural resonance of cooling element120. The portion of cooling element120undergoing vibrational motion is driven at or near resonance (the “structural resonance”) of cooling element120. This portion of cooling element120undergoing vibration may be a cantilevered section in some embodiments. The frequency of vibration for structural resonance is termed the structural resonant frequency. Use of the structural resonant frequency in driving cooling element120reduces the power consumption of cooling system100. Cooling element120and top chamber140may also be configured such that this structural resonant frequency corresponds to a resonance in a pressure wave in the fluid being driven through top chamber140(the acoustic resonance of top chamber140). The frequency of such a pressure wave is termed the acoustic resonant frequency. At acoustic resonance, a node in pressure occurs near vent112and an antinode in pressure occurs near the periphery of cooling system100(e.g. near tip121of cooling element120and near the connection between top chamber140and bottom chamber150). The distance between these two regions is C/2. Thus, C/2=nλ/4, where λ is the acoustic wavelength for the fluid and n is odd (e.g. n=1, 3, 5, etc.). For the lowest order mode, C=λ/2. Because the length of chamber140(e.g. C) is close to the length of cooling element120, in some embodiments, it is also approximately true that L/2=nλ/4, where λ is the acoustic wavelength for the fluid and n is odd. Thus, the frequency at which cooling element120is driven, v, is at or near the structural resonant frequency for cooling element120. The frequency v is also at or near the acoustic resonant frequency for at least top chamber140. The acoustic resonant frequency of top chamber140generally varies less dramatically with parameters such as temperature and size than the structural resonant frequency of cooling element120. Consequently, in some embodiments, cooling element120may be driven at (or closer to) a structural resonant frequency than to the acoustic resonant frequency.

Orifice plate130has orifices132therein. Although a particular number and distribution of orifices132are shown, another number and/or another distribution may be used. A single orifice plate130is used for a single cooling system100. In other embodiments, multiple cooling systems100may share an orifice plate. For example, multiple cells100may be provided together in a desired configuration. In such embodiments, the cells100may be the same size and configuration or different size(s) and/or configuration(s). Orifices132are shown as having an axis oriented normal to a surface of heat-generating structure102. In other embodiments, the axis of one or more orifices132may be at another angle. For example, the angle of the axis may be selected from substantially zero degrees and a nonzero acute angle. Orifices132also have sidewalls that are substantially parallel to the normal to the surface of orifice plate130. In some embodiments, orifices may have sidewalls at a nonzero angle to the normal to the surface of orifice plate130. For example, orifices132may be cone-shaped. Further, although orifice place130is shown as substantially flat, in some embodiments, trenches and/or other structures may be provided in orifice plate130to modify the configuration of bottom chamber150and/or the region between orifice plate130and heat-generating structure102.

The size, distribution and locations of orifices132are chosen to control the flow rate of fluid driven to the surface of heat-generating structure102. The locations and configurations of orifices132may be configured to increase/maximize the fluid flow from bottom chamber150through orifices132to the jet channel (the region between the bottom of orifice plate130and the top of heat-generating structure102). The locations and configurations of orifices132may also be selected to reduce/minimize the suction flow (e.g. back flow) from the jet channel through orifices132. For example, the locations of orifices are desired to be sufficiently far from tip121that suction in the upstroke of cooling element120(tip121moves away from orifice plate13) that would pull fluid into bottom chamber150through orifices132is reduced. The locations of orifices are also desired to be sufficiently close to tip121that suction in the upstroke of cooling element120also allows a higher pressure from top chamber140to push fluid from top chamber140into bottom chamber150. In some embodiments, the ratio of the flow rate from top chamber140into bottom chamber150to the flow rate from the jet channel through orifices132in the upstroke (the “net flow ratio”) is greater than 2:1. In some embodiments, the net flow ratio is at least 85:15. In some embodiments, the net flow ratio is at least 90:10. In order to provide the desired pressure, flow rate, suction, and net flow ratio, orifices132are desired to be at least a distance, r1, from tip121and not more than a distance, r2, from tip121of cooling element120. In some embodiments r1is at least one hundred micrometers (e.g. r1≥100 μm) and r2is not more than one millimeter (e.g. r2≤1000 μm). In some embodiments, orifices132are at least two hundred micrometers from tip121of cooling element120(e.g. r1≥200 μm). In some such embodiments, orifices132are at least three hundred micrometers from tip121of cooling element120(e.g. r1≥300 μm). In some embodiments, orifices132have a width, o, of at least one hundred micrometers and not more than five hundred micrometers. In some embodiments, orifices132have a width of at least two hundred micrometers and not more than three hundred micrometers. In some embodiments, the orifice separation, s, is at least one hundred micrometers and not more than one millimeter. In some such embodiments, the orifice separation is at least four hundred micrometers and not more than six hundred micrometers. In some embodiments, orifices132are also desired to occupy a particular fraction of the area of orifice plate130. For example, orifices132may cover at least five percent and not more than fifteen percent of the footprint of orifice plate130in order to achieve a desired flow rate of fluid through orifices132. In some embodiments, orifices132cover at least eight percent and not more than twelve percent of the footprint of orifice plate130.

In some embodiments, cooling element120is actuated using a piezoelectric. Thus, cooling element120may be a piezoelectric cooling element. Cooling element120may be driven by a piezoelectric that is mounted on or integrated into cooling element120. In some embodiments, cooling element120is driven in another manner including but not limited to providing a piezoelectric on another structure in cooling system100. Cooling element120and analogous cooling elements are referred to hereinafter as piezoelectric cooling element though it is possible that a mechanism other than a piezoelectric might be used to drive the cooling element. In some embodiments, cooling element120includes a piezoelectric layer on substrate. The substrate may include or consist of stainless steel, a Ni alloy, Hastelloy, Al (e.g. an Al alloy), and/or Ti (e.g. a Ti alloy such as Ti6A1-4V). For example, in some embodiments, the substrate may include or consist of grade 2 Ti. Orifice plate130may be formed of the same material as the substrate. For example, orifice plate130may include or consist of grade 2 Ti. Top plate110and surrounding structures such as the frame and structures306and396depicted inFIGS.3A-3Gmay be formed of a stainless steel such as SUS430. SUS430 or an analogous material may be selected to better match the coefficient of thermal expansion of the substrate and/or orifice plate120. In some embodiments, orifice plate130is diffusion bonded to the substrate and/or anchor160. In some embodiments, piezoelectric layer includes multiple sublayers formed as thin films on the substrate. In other embodiments, the piezoelectric layer may be a bulk layer affixed to the substrate. Such a piezoelectric cooling element120also includes electrodes used to activate the piezoelectric. The substrate functions as an electrode in some embodiments. In other embodiments, a bottom electrode may be provided between the substrate and the piezoelectric layer. Other layers including but not limited to seed, capping, passivation or other layers might be included in piezoelectric cooling element. Thus, cooling element120may be actuated using a piezoelectric.

In some embodiments, cooling system100includes chimneys (not shown) or other ducting. Such ducting provides a path for heated fluid to flow away from heat-generating structure102. In some embodiments, ducting returns fluid to the side of top plate110distal from heat-generating structure102. In some embodiments, ducting may instead direct fluid away from heat-generating structure102in a direction parallel to heat-generating structure102or perpendicular to heat-generating structure102but in the opposite direction (e.g. toward the bottom of the page). For a device in which fluid external to the device is used in cooling system100, the ducting may channel the heated fluid to a vent. In such embodiments, additional fluid may be provided from an inlet vent. In embodiments, in which the device is enclosed, the ducting may provide a circuitous path back to the region near vent112and distal from heat-generating structure102. Such a path allows for the fluid to dissipate heat before being reused to cool heat-generating structure102. In other embodiments, ducting may be omitted or configured in another manner. Thus, the fluid is allowed to carry away heat from heat-generating structure102.

Operation of cooling system100is described in the context ofFIGS.1A-1F. Although described in the context of particular pressures, gap sizes, and timing of flow, operation of cooling system100is not dependent upon the explanation herein.FIGS.1C-1Ddepict in-phase operation of cooling system100. Referring toFIG.1C, cooling element120has been actuated so that its tip121moves away from top plate110.FIG.1Ccan thus be considered to depict the end of a down stroke of cooling element120. Because of the vibrational motion of cooling element120, gap152for bottom chamber150has decreased in size and is shown as gap152B. Conversely, gap142for top chamber140has increased in size and is shown as gap142B. During the down stroke, a lower (e.g. minimum) pressure is developed at the periphery when cooling element120is at the neutral position. As the down stroke continues, bottom chamber150decreases in size and top chamber140increases in size as shown inFIG.1C. Thus, fluid is driven out of orifices132in a direction that is at or near perpendicular to the surface of orifice plate130and/or the top surface of heat-generating structure102. The fluid is driven from orifices132toward heat-generating structure102at a high speed, for example in excess of thirty-five meters per second. In some embodiments, the fluid then travels along the surface of heat-generating structure102and toward the periphery of heat-generating structure102, where the pressure is lower than near orifices132. Also in the down stroke, top chamber140increases in size and a lower pressure is present in top chamber140. As a result, fluid is drawn into top chamber140through vent112. The motion of the fluid into vent112, through orifices132, and along the surface of heat-generating structure102is shown by unlabeled arrows inFIG.1C.

Cooling element120is also actuated so that tip121moves away from heat-generating structure102and toward top plate110.FIG.1Dcan thus be considered to depict the end of an up stroke of cooling element120. Because of the motion of cooling element120, gap142has decreased in size and is shown as gap142C. Gap152has increased in size and is shown as gap152C. During the upstroke, a higher (e.g. maximum) pressure is developed at the periphery when cooling element120is at the neutral position. As the upstroke continues, bottom chamber150increases in size and top chamber140decreases in size as shown inFIG.1D. Thus, the fluid is driven from top chamber140(e.g. the periphery of chamber140/150) to bottom chamber150. Thus, when tip121of cooling element120moves up, top chamber140serves as a nozzle for the entering fluid to speed up and be driven towards bottom chamber150. The motion of the fluid into bottom chamber150is shown by unlabeled arrows inFIG.1D. The location and configuration of cooling element120and orifices132are selected to reduce suction and, therefore, back flow of fluid from the jet channel (between heat-generating structure102and orifice plate130) into orifices132during the upstroke. Thus, cooling system100is able to drive fluid from top chamber140to bottom chamber150without an undue amount of backflow of heated fluid from the jet channel entering bottom chamber140. Moreover, cooling system100may operate such that fluid is drawn in through vent112and driven out through orifices132without cooling element120contacting top plate110or orifice plate130. Thus, pressures are developed within chambers140and150that effectively open and close vent112and orifices132such that fluid is driven through cooling system100as described herein.

The motion between the positions shown inFIGS.1C and1Dis repeated. Thus, cooling element120undergoes vibrational motion indicated inFIGS.1A,1B, and1D, drawing fluid through vent112from the distal side of top plate110into top chamber140; transferring fluid from top chamber140to bottom chamber150; and pushing the fluid through orifices132and toward heat-generating structure102. As discussed above, cooling element120is driven to vibrate at or near the structural resonant frequency of cooling element120. Further, the structural resonant frequency of cooling element120is configured to align with the acoustic resonance of the chamber140/150. The structural and acoustic resonant frequencies are generally chosen to be in the ultrasonic range. For example, the vibrational motion of cooling element120may be at frequencies from 15 kHz through 30 kHz. In some embodiments, cooling element120vibrates at a frequency/frequencies of at least 20 kHz and not more than 30 kHz (e.g. 23 kHz-25 kHz). The structural resonant frequency of cooling element120is within ten percent of the acoustic resonant frequency of cooling system100. In some embodiments, the structural resonant frequency of cooling element120is within five percent of the acoustic resonant frequency of cooling system100. In some embodiments, the structural resonant frequency of cooling element120is within three percent of the acoustic resonant frequency of cooling system100. Consequently, efficiency and flow rate may be enhanced. However, other frequencies may be used.

Fluid driven toward heat-generating structure102may move substantially normal (perpendicular) to the top surface of heat-generating structure102. In some embodiments, the fluid motion may have a nonzero acute angle with respect to the normal to the top surface of heat-generating structure102. In either case, the fluid may thin and/or form apertures in the boundary layer of fluid at heat-generating structure102. As a result, transfer of heat from heat-generating structure102may be improved. The fluid deflects off of heat-generating structure102, traveling along the surface of heat-generating structure102. In some embodiments, the fluid moves in a direction substantially parallel to the top of heat-generating structure102. Thus, heat from heat-generating structure102may be extracted by the fluid. The fluid may exit the region between orifice plate130and heat-generating structure102at the edges of cooling system100. Chimneys or other ducting (not shown) at the edges of cooling system100allow fluid to be carried away from heat-generating structure102. In other embodiments, heated fluid may be transferred further from heat-generating structure102in another manner. The fluid may exchange the heat transferred from heat-generating structure102to another structure or to the ambient environment. Thus, fluid at the distal side of top plate110may remain relatively cool, allowing for the additional extraction of heat. In some embodiments, fluid is circulated, returning to distal side of top plate110after cooling. In other embodiments, heated fluid is carried away and replaced by new fluid at the distal side of cooling element120. As a result, heat-generating structure102may be cooled.

FIGS.1E-1Fdepict an embodiment of active MEMS cooling system100including centrally anchored cooling element120in which the cooling element is driven out-of-phase. More specifically, sections of cooling element120on opposite sides of anchor160(and thus on opposite sides of the central region of cooling element120that is supported by anchor160) are driven to vibrate out-of-phase. In some embodiments, sections of cooling element120on opposite sides of anchor160are driven at or near one hundred and eighty degrees out-of-phase. Thus, one section of cooling element120vibrates toward top plate110, while the other section of cooling element120vibrates toward orifice plate130/heat-generating structure102. Movement of a section of cooling element120toward top plate110(an upstroke) drives fluid in top chamber140to bottom chamber150on that side of anchor160. Movement of a section of cooling element120toward orifice plate130drives fluid through orifices132and toward heat-generating structure102. Thus, fluid traveling at high speeds (e.g. speeds described with respect to in-phase operation) is alternately driven out of orifices132on opposing sides of anchor160. Because fluid is driven through orifices132at high speeds, cooling system100may be viewed as a MEMs jet. The movement of fluid is shown by unlabeled arrows inFIGS.1E and1F. The motion between the positions shown inFIGS.1E and1Fis repeated. Thus, cooling element120undergoes vibrational motion indicated inFIGS.1A,1E, and1F, alternately drawing fluid through vent112from the distal side of top plate110into top chamber140for each side of cooling element120; transferring fluid from each side of top chamber140to the corresponding side of bottom chamber150; and pushing the fluid through orifices132on each side of anchor160and toward heat-generating structure102. As discussed above, cooling element120is driven to vibrate at or near the structural resonant frequency of cooling element120. Further, the structural resonant frequency of cooling element120is configured to align with the acoustic resonance of the chamber140/150. The structural and acoustic resonant frequencies are generally chosen to be in the ultrasonic range. For example, the vibrational motion of cooling element120may be at the frequencies described for in-phase vibration. The structural resonant frequency of cooling element120is within ten percent of the acoustic resonant frequency of cooling system100. In some embodiments, the structural resonant frequency of cooling element120is within five percent of the acoustic resonant frequency of cooling system100. In some embodiments, the structural resonant frequency of cooling element120is within three percent of the acoustic resonant frequency of cooling system100. Consequently, efficiency and flow rate may be enhanced. However, other frequencies may be used.

Fluid driven toward heat-generating structure102for out-of-phase vibration may move substantially normal (perpendicular) to the top surface of heat-generating structure102, in a manner analogous to that described above for in-phase operation. Similarly, chimneys or other ducting (not shown) at the edges of cooling system100allow fluid to be carried away from heat-generating structure102. In other embodiments, heated fluid may be transferred further from heat-generating structure102in another manner. The fluid may exchange the heat transferred from heat-generating structure102to another structure or to the ambient environment. Thus, fluid at the distal side of top plate110may remain relatively cool, allowing for the additional extraction of heat. In some embodiments, fluid is circulated, returning to distal side of top plate110after cooling. In other embodiments, heated fluid is carried away and replaced by new fluid at the distal side of cooling element120. As a result, heat-generating structure102may be cooled.

Although shown in the context of a uniform cooling element inFIGS.1A-1F, cooling system100may utilize cooling elements having different shapes.FIG.1Gdepicts an embodiment of engineered cooling element120′ having a tailored geometry and usable in a cooling system such as cooling system100. Cooling element120′ includes an anchored region122and cantilevered arms123. Anchored region122is supported (e.g. held in place) in cooling system100by anchor160. Cantilevered arms123undergo vibrational motion in response to cooling element120′ being actuated. Each cantilevered arm123includes step region124, extension region126and outer region128. In the embodiment shown inFIG.1G, anchored region122is centrally located. Step region124extends outward from anchored region122. Extension region126extends outward from step region124. Outer region128extends outward from extension region126. In other embodiments, anchored region122may be at one edge of the actuator and outer region128at the opposing edge. In such embodiments, the actuator is edge anchored.

Extension region126has a thickness (extension thickness) that is less than the thickness of step region124(step thickness) and less than the thickness of outer region128(outer thickness). Thus, extension region126may be viewed as recessed. Extension region126may also be seen as providing a larger bottom chamber150. In some embodiments, the outer thickness of outer region128is the same as the step thickness of step region124. In some embodiments, the outer thickness of outer region128is different from the step thickness of step region124. In some embodiments, outer region128and step region124each have a thickness of at least three hundred twenty micrometers and not more than three hundred and sixty micrometers. In some embodiments, the outer thickness is at least fifty micrometers and not more than two hundred micrometers thicker than the extension thickness. Stated differently, the step (difference in step thickness and extension thickness) is at least fifty micrometers and not more than two hundred micrometers. In some embodiments, the outer step (difference in outer thickness and extension thickness) is at least fifty micrometers and not more than two hundred micrometers. Outer region128may have a width, o, of at least one hundred micrometers and not more than three hundred micrometers. Extension region has a length, e, extending outward from the step region of at least 0.5 millimeter and not more than 1.5 millimeters in some embodiments. In some embodiments, outer region128has a higher mass per unit length in the direction from anchored region122than extension region126. This difference in mass may be due to the larger size of outer region128, a difference in density between portions of cooling element120, and/or another mechanism.

Use of engineered cooling element120′ may further improve efficiency of cooling system100. Extension region126is thinner than step region124and outer region128. This results in a cavity in the bottom of cooling element120′ corresponding to extension region126. The presence of this cavity aids in improving the efficiency of cooling system100. Each cantilevered arm123vibrates towards top plate110in an upstroke and away from top plate110in a downstroke. When a cantilevered arm123moves toward top plate110, higher pressure fluid in top chamber140resists the motion of cantilevered arm123. Furthermore, suction in bottom chamber150also resists the upward motion of cantilevered arm123during the upstroke. In the downstroke of cantilevered arm123, increased pressure in the bottom chamber150and suction in top chamber140resist the downward motion of cantilevered arm123. However, the presence of the cavity in cantilevered arm123corresponding to extension region126mitigates the suction in bottom chamber150during an upstroke. The cavity also reduces the increase in pressure in bottom chamber150during a downstroke. Because the suction and pressure increase are reduced in magnitude, cantilevered arms123may more readily move through the fluid. This may be achieved while substantially maintaining a higher pressure in top chamber140, which drives the fluid flow through cooling system100. Moreover, the presence of outer region128may improve the ability of cantilevered arm123to move through the fluid being driven through cooling system100. Outer region128has a higher mass per unit length and thus a higher momentum. Consequently, outer region128may improve the ability of cantilevered arms123to move through the fluid being driven through cooling system100. The magnitude of the deflection of cantilevered arm123may also be increased. These benefits may be achieved while maintaining the stiffness of cantilevered arms123through the use of thicker step region124. Further, the larger thickness of outer region128may aid in pinching off flow at the bottom of a downstroke. Thus, the ability of cooling element120′ to provide a valve preventing backflow through orifices132may be improved. Thus, performance of cooling system100employing cooling element120′ may be improved.

Using the cooling system100actuated for in-phase vibration or out-of-phase vibration of cooling element120and/or120′, fluid drawn in through vent112and driven through orifices132may efficiently dissipate heat from heat-generating structure102. Because fluid impinges upon the heat-generating structure with sufficient speed (e.g. at least thirty meters per second) and in some embodiments substantially normal to the heat-generating structure, the boundary layer of fluid at the heat-generating structure may be thinned and/or partially removed. Consequently, heat transfer between heat-generating structure102and the moving fluid is improved. Because the heat-generating structure is more efficiently cooled, the corresponding integrated circuit may be run at higher speed and/or power for longer times. For example, if the heat-generating structure corresponds to a high-speed processor, such a processor may be run for longer times before throttling. Thus, performance of a device utilizing cooling system100may be improved. Further, cooling system100may be a MEMS device. Consequently, cooling systems100may be suitable for use in smaller and/or mobile devices, such as smart phones, other mobile phones, virtual reality headsets, tablets, two-in-one computers, wearables and handheld games, in which limited space is available. Performance of such devices may thus be improved. Because cooling element120/120′ may be vibrated at frequencies of 15 kHz or more, users may not hear any noise associated with actuation of cooling elements. If driven at or near structural and/or acoustic resonant frequencies, the power used in operating cooling systems may be significantly reduced. Cooling element120/120′ does not physically contact top plate110or orifice plate130during vibration. Thus, resonance of cooling element120/120′ may be more readily maintained. More specifically, physical contact between cooling element120/120′ and other structures disturbs the resonance conditions for cooling element120/120′. Disturbing these conditions may drive cooling element120/120′ out of resonance. Thus, additional power would need to be used to maintain actuation of cooling element120/120′. Further, the flow of fluid driven by cooling element120/120′ may decrease. These issues are avoided through the use of pressure differentials and fluid flow as discussed above. The benefits of improved, quiet cooling may be achieved with limited additional power. Further, out-of-phase vibration of cooling element120/120′ allows the position of the center of mass of cooling element100to remain more stable. Although a torque is exerted on cooling element120/120′, the force due to the motion of the center of mass is reduced or eliminated. As a result, vibrations due to the motion of cooling element120/120′ may be reduced. Moreover, efficiency of cooling system100may be improved through the use of out-of-phase vibrational motion for the two sides of cooling element120/120′. Consequently, performance of devices incorporating the cooling system100may be improved. Further, cooling system100may be usable in other applications (e.g. with or without heat-generating structure102) in which high fluid flows and/or velocities are desired.

Further, cooling elements used in cooling system100may have different structures and/or be mounted differently than depicted inFIGS.1A-1G. In some embodiments, the cooling element may have rounded corners and/or rounded ends but still be anchored along a central axis such that cantilevered arms vibrate. The cooling element may be anchored only at its central region such that the regions surrounding the anchor vibrate in a manner analogous to a jellyfish or the opening/closing of an umbrella. In some such embodiments, the cooling element may be circular or elliptical in shape. In some embodiments, the anchor may include apertures through which fluid may flow. Such an anchor may be utilized for the cooling element being anchored at its top (e.g. to the top plate). Although not indicated inFIGS.1A-1G, the piezoelectric utilized in driving the cooling element may have various locations and/or configurations. For example, the piezoelectric may be embedded in the cooling element, affixed to one side of the cooling element (or cantilevered arm(s)), may occupy some or all of the cantilevered arms, and/or may have a location that is close to or distal from the anchored region. In some embodiments, cooling elements that are not centrally anchored may be used. For example, a pair of cooling elements that have offset apertures, that are anchored at their ends (or all edges), and which vibrate out of phase may be used. Thus, various additional configurations of cooling element120and/or120′, anchor160, and/or other portions of cooling system100may be used.

FIGS.2A-2Bdepict plan an embodiment of active MEMS cooling system200including a top centrally anchored cooling element.FIG.2Adepicts a side view of cooling system200in a neutral position.FIG.2Bdepicts a top view of cooling system200.FIGS.2A-2Bare not to scale. For simplicity, only portions of cooling system200are shown. Referring toFIGS.2A-2B, cooling system200is analogous to cooling system100. Consequently, analogous components have similar labels. For example, cooling system200is used in conjunction with heat-generating structure202, which is analogous to heat-generating structure202.

Cooling system200includes top plate210having vents212, cooling element220having tip221, orifice plate230including orifices232, top chamber240having a gap, bottom chamber250having a gap, flow chamber240/250, and anchor (i.e. support structure)260that are analogous to top plate110having vent112, cooling element120having tip121, orifice plate130including orifices132, top chamber140having gap142, bottom chamber150having gap152, flow chamber140/150, and anchor (i.e. support structure)160, respectively. Also shown is pedestal290that is analogous to pedestal190. Thus, cooling element220is centrally supported by anchor260such that at least a portion of the perimeter of cooling element220is free to vibrate. In some embodiments, anchor260extends along the axis of cooling element220. In other embodiments, anchor260is only near the center portion of cooling element220. Although not explicitly labeled inFIGS.2A and2B, cooling element220includes an anchored region and cantilevered arms including step region, extension region and outer regions analogous to anchored region122, cantilevered arms123, step region124, extension region126and outer region128of cooling element120′. In some embodiments, cantilevered arms of cooling element220are driven in-phase. In some embodiments, cantilevered arms of cooling element220are driven out-of-phase. In some embodiments, a simple cooling element, such as cooling element120, may be used.

Anchor260supports cooling element220from above. Thus, cooling element220is suspended from anchor260. Anchor260is suspended from top plate210. Top plate210includes vent213. Vents212on the sides of anchor260provide a path for fluid to flow into sides of chamber240.

As discussed above with respect to cooling system100, cooling element220may be driven to vibrate at or near the structural resonant frequency of cooling element220. Further, the structural resonant frequency of cooling element220may be configured to align with the acoustic resonance of the chamber240/250. The structural and acoustic resonant frequencies are generally chosen to be in the ultrasonic range. For example, the vibrational motion of cooling element220may be at the frequencies described with respect to cooling system100. Consequently, efficiency and flow rate may be enhanced. However, other frequencies may be used.

Cooling system200operates in an analogous manner to cooling system100. Cooling system200thus shares the benefits of cooling system100. Thus, performance of a device employing cooling system200may be improved. In addition, suspending cooling element220from anchor260may further enhance performance. In particular, vibrations in cooling system200that may affect other cooling cells (not shown), may be reduced. For example, less vibration may be induced in top plate210due to the motion of cooling element220. Consequently, cross talk between cooling system200and other cooling systems (e.g. other cells) or other portions of the device incorporating cooling system200may be reduced. Thus, performance may be further enhanced.

FIGS.3A-3Fdepict an embodiment of active MEMS cooling system300including multiple cooling cells configured as a module termed a tile, or array.FIG.3Adepicts a perspective view, whileFIGS.3B-3Edepict side views.FIG.3Fdepicts module395including multiple cooling systems300.FIGS.3A-3Fare not to scale. Cooling system300includes four cooling cells301A,301B,301C and301D (collectively or generically301), which are analogous to one or more of cooling systems described herein. More specifically, cooling cells301are analogous to cooling system100and/or200. Tile300thus includes four cooling cells301(i.e. four MEMS jets). Although four cooling cells301in a 2×2 configuration are shown, in some embodiments another number and/or another configuration of cooling cells301might be employed. In the embodiment shown, cooling cells301include shared top plate310having apertures312, cooling elements320, shared orifice plate330including orifices332, top chambers340, bottom chambers350, anchors (support structures)360, and pedestals390that are analogous to top plate110having apertures112, cooling element120, orifice plate130having orifices132, top chamber140, bottom chamber150, anchor160, and pedestal190. In some embodiments, cooling cells301may be fabricated together and separated, for example by cutting through top plate310, side walls between cooling cells301, and orifice plate330. Thus, although described in the context of a shared top plate310and shared orifice plate330, after fabrication cooling cells301may be separated. In some embodiments, tabs (not shown) and/or other structures such as anchors360, may connect cooling cells301. Further, tile300includes heat-generating structure (termed a heat spreader hereinafter)302(e.g. a heat sink, a heat spreader, and/or other structure) that also has sidewalls, or fencing, in the embodiment shown. Cover plate306having apertures therein is also shown. Heat spreader302and cover plate306may be part of an integrated tile300as shown or may be separate from tile300in other embodiments. Heat spreader302and cover plate306may direct fluid flow outside of cooling cells301, provide mechanical stability, and/or provide protection. Electrical connection to cooling cells301is provided via flex connector380(not shown inFIGS.3B-3F) which may house drive electronics385. Cooling elements320are driven out-of-phase (i.e. in a manner analogous to a seesaw). Further, as can be seen inFIGS.3B-3CandFIGS.3D-3Ecooling element320in one cell is driven out-of-phase with cooling element(s)320in adjacent cell(s). InFIGS.3B-3C, cooling elements320in a row are driven out-of-phase. Thus, cooling element320in cell301A is out-of-phase with cooling element320in cell301B. Similarly, cooling element320in cell301C is out-of-phase with cooling element320in cell301D. InFIGS.3D-3E, cooling elements320in a column are driven out-of-phase. Thus, cooling element320in cell301A is out-of-phase with cooling element320in cell301C. Similarly, cooling element320in cell301B is out-of-phase with cooling element320in cell301D. By driving cooling elements320out-of-phase, vibrations in cooling system300may be reduced. Cooling elements320may be driven in another manner in some embodiments.

In some embodiments, two sets of four cooling cells301may be combined and integrated in a manner analogous to system300.FIG.3Fis an exploded view of module395including two cooling systems300and, therefore, eight cooling cells301. Cooling system300are enclosed in copper heat spreader392and cover396having vents397therein. Also shown is connector396that may house drive electronics analogous to drive electronics385. Although not shown, vents397may have a dust cover that reduces or prevents the flow of dust (e.g. carried by the fluid flowing into vents397) from reaching the internal portion of module395Such standardized modules395may facilitate incorporation into devices. Aperture398through which flow exits module395is between cover396and heat spreader392. In some embodiments, aperture398occupies most or all of the side of module395. In some embodiments, module395may be approximately forty to sixty millimeters on a side (e.g. forty-five millimeters by fifty-five millimeters) and not more than three millimeters thick. Module395may be capable of dissipating 10 W of power (while consuming not more than approximately 3 W of power). Direct flow through module395may be at least 0.3 cfm (e.g. on the order of 0.35 cfm) and entrained flow may be at least 0.5 cfm (e.g. 0.7 cfm or approximately twice the direct flow). Thus, the entrained airflow achieved using module395is at least the same as the direct airflow. In some embodiments, the entrained airflow is at least 1.5 multiplied by the direct airflow through module395. In some embodiments, the entrained airflow may be twice the direct airflow through module395. At such flows, the back pressure for module395may be not more than 2 kPa-2.2 kPa. Further, module395may have a top surface temperature that is significantly lower than the heat spreader (not shown inFIG.3F) to which module395is coupled or heat spreader392. This occurs because the active heat dissipation of module395starts from the region the fluid enters vents397opposite to heat spreader392. Consequently, during operation the top surface of module395may be at least ten degrees Celsius cooler than a heat spreader392or other component to which module395is thermally coupled via heat spreader392. In some embodiments, the top surface of module395is at least fifteen degrees Celsius cooler than heat spreader392during operation. The thin form factor (e.g. less than three millimeters thick), high back pressure and flow, little to no noise (e.g. less than 27 dBA) and low top surface temperature may facilitate use of module395in devices including but not limited to notebook computers.

Cooling cells301of cooling system300and module395functions in an analogous manner to cooling system(s)100,200, and/or an analogous cooling system. Consequently, the benefits described herein may be shared by cooling system300and module395. Because cooling elements in nearby cells are driven out-of-phase, vibrations in cooling system300may be reduced. Because multiple cooling cells301are used, cooling system300may enjoy enhanced cooling capabilities. Further, multiples of individual cooling cells301and/or cooling system300may be combined in various fashions to obtain the desired footprint of cooling cells.

FIGS.4A and4Bdepict perspective views of embodiments of active MEMS cooling systems600A and600B in which tiles601A and601B are incorporated.FIGS.4A-4Bare not to scale and not all components are shown. Cooling system600A includes three tiles601A, which are analogous to tiles300. Thus, each tile601A includes four cooling cells (i.e. four MEMS jets) in the embodiment shown. Also shown are heat-generating structure602A (e.g. a heat spreader), flex connector680A and electronics685A that are analogous to heat-generating structure302, flex connector380, and electronics385. Drive electronics685can be located on the flex connector or on a separate board. MEMS cooling system600A may be used in a computing device such as a laptop. Thus, cooling system600A shares the benefits of cooling system100,400, and500. Further, the addition of more tiles601A allows MEMS cooling system600A to provide additional cooling power.

Cooling system600B is a perspective view of an embodiment of a cooling system that may be used in high power dissipation applications. For example, cooling system600B may be utilized in a server system and/or other high power computing device. Cooling system600B may be desired to dissipate at least 300 Watts, 800 Watts, 1600 Watts, 2400 Watts, 3200 Watts, or more. Cooling system600B includes multiple tiles601B (of which only three are labeled), each of which may be analogous to tile500. The cover plates of tiles601B are shown. Each tile601B includes four cooling cells (i.e. four MEMS jets) in the embodiment shown. In other embodiments, each tile may include another number of cooling cells and/or another number of tiles601B may be used. Also shown are heat-generating structure602B, flex connector680B and electronics685B that are analogous to heat-generating structure502, flex connector580, and electronics585. However, because cooling system600B is desired to be utilized for high power dissipation applications, heat-spreading structure602B may be a vapor chamber or analogous device (hereinafter vapor chamber). Vapor chamber602B is, therefore, in thermal communication with a heat sources (not shown), such as high power processors utilized in a server system. Vapor chamber602B may be used in lieu of a heat spreader in order to better spread heat across a larger surface and reduce the occurrence of hot spots. Thus, use of a vapor chamber602B in combination with cooling system600B may provide more efficient cooling for the structures (not shown) for which heat is desired to be dissipated. In some embodiments, heat-generating structure602B is a heat spreader or other thermally conductive structure that is in thermally coupled with a vapor chamber that is part of a device desired to be cooled. However, in other embodiments, a heat spreader may be used. Vapor chamber602B includes a high thermal conductivity material, such as copper. Also shown are ducts603B surrounding tiles601B. Ducting603B is used to direct heated fluid (e.g. air) driven by tiles601B.

Tiles601B are arranged in an array. Although a rectangular array is shown, in some embodiments the array may have another shape. Cooling system600B may be considered to include an array of MEMS jets arranged to cause a fluid to impinge on a surface of vapor chamber602B. Cooling system600includes one hundred and ninety two tiles601B and thus over seven hundred and twenty (i.e. seven hundred and sixty eight) MEMS jets. As discussed with respect toFIGS.1A-1F, MEMs jets of cooling system600B use vibrational motion of an actuator (i.e. a cooling element) to drive fluid (e.g. air) to impinge on vapor chamber602B at high speed such as those described herein. For example, cooling system600B may be capable of dissipating at least 1400 W (e.g. during steady state operation). In some embodiments, cooling systems having (the same or) another number of MEMS jets may be capable of dissipating other powers (e.g. at least 800 W, at least 1600 W, at least 2400 W, at least 3200 W, at least 3600 W and/or another power). Further, the profile of cooling system600B may be low. As indicated previously, the thickness of tiles601B including MEMS jets is less than 1.5 millimeter. For example, the thickness of tiles601B may be 1-1.3 millimeter in some embodiments. In some embodiments, tiles601B may be on the order of 1-1.5 mm or less. Vapor chamber602B may be nominally five millimeters in some embodiments. Ducting603B may be nominally five millimeters thick. Consequently, cooling system600B may be not more than fifteen millimeters thick in some embodiments. In some embodiments, cooling system600B may be not more than twenty-five millimeters thick.

In operation, cooling elements in tiles601B are driven in a manner analogous to that described for cooling system100. Thus, tiles601B use vibrational motion of cooling elements therein to drive fluid (e.g. air) toward vapor chamber602B at high speed. For example, the jets have a velocity of greater than 30 meters per second. The MEMS jets of tiles601B drive the fluid to impinge on the surface of vapor chamber602B. The fluid cools vapor chamber602B and is directed to an outlet or other cooling mechanism by ducting system603B. Thus, cool fluid is directed toward the inlets in tiles601B and heated fluid used to cool vapor chamber602B is carried away from cooling system600B.

Thus, cooling system600B shares the benefits of cooling systems100,200,300, and395. In addition, cooling system600B has enhanced cooling capabilities. Cooling system600B may be used to cool systems requiring high power dissipation, such as servers. This is indicated inFIG.5, depicts a graph of pressure versus flow and that indicates the performance of an active MEMS cooling system versus that of fans. Line650indicates the performance of active MEMS cooling system, while line652indicates the performance of a set of fans. In some embodiments, cooling system600B may dissipate 1400 W of heat drawing on the order of 163 W of power with an air flow of nominally sixty-five cubic feet per minute (CFM) with an air velocity of on the order two hundred kilometers per hour. An analogous traditional cooling system using five fans and drawing approximately the same power (e.g. 164 W) requires a flow of approximately two hundred and sixty CFM with an air velocity of nominally twenty kilometers per hour and dissipates only eight hundred watts of heat. MEMS cooling system600B also provides greater cooling than conventional systems employing fans at a smaller profile. For example, in some embodiments, cooling system600B may be not more than half of the height of a cooling system employing fans. For example, as discussed above, cooling system600B may have a height of not more than thirty millimeters in some embodiments. In some such embodiments, cooling system has a height of not more than twenty millimeters. For example, cooling system600B may be nominally not more than fifteen millimeters tall. In contrast, a traditional 5-fan system may be forty-five millimeters in height or taller. Thus, more server systems and cooling systems600B may be provided in a particular server rack. In addition, MEMS cooling system600B need not use liquids for cooling. Consequently, reliability and safety issues that may occur when using fluids for cooling electronics may be avoided. Further, an external chiller may be unnecessary. Instead, cooling system600B may draw fluid (e.g. air) from the cool aisle and exhaust heated fluid to the hot aisle in a data center. Thus, performance, compactness, efficiency, and reliability may be improved for high power dissipation applications such as servers. Cooling system600B may also entrain fluid that does not travel through tiles601B. Such entrained fluid may be blended with fluid carrying heat from vapor chamber602B. As a result, fluid exhausted by cooling system600B may have a moderate temperature.

FIGS.6A-6Cdepict an embodiment of MEMS cooling system700within a chassis710of a server that may be used in a data center.FIGS.6A-6Bare perspective views, whileFIG.6Cis a top view indicating a heat map. For clarity,FIGS.6A-6Care not to scale and not all components are shown. For example, processors712being cooled by cooling system700are shown. However, memory and other components are omitted. MEMS cooling system700includes cooling tiles701(of which only one is labeled), ducting703, and vapor chamber702. In the embodiment shown, each tile701includes four cooling cells (not explicitly labeled inFIGS.6A-6C). In some embodiments, approximately seven hundred and forty cooling cells are present and cooling system700may be not more than 420 mm wide, 500 mm long, and 10 mm high. Thus, cooling system700is compact. Tiles701are also arranged in an array that is not rectangular. Vapor chamber702may be approximately five millimeters thick. Thus, the cooling system700may have a small profile analogous to that described in the context of cooling system600. As indicated by the heat map inFIG.6C, cooler fluid (e.g. air) is drawn into chassis710, driven by MEMS cooling system700via vibrational motion, and used to cool processors712. The fluid carries off the heat generated by processors712.

Cooling system700shares the benefits of cooling systems100,200,300,395,600A and/or600B. Cooling system700may be used to cool server system710, which requires high power dissipation. In some embodiments, cooling system700may dissipate at least 1400 W of heat while occupying less space than a traditional fan system. For example, cooling system700may have a height of not more than thirty millimeters in some embodiments. In some such embodiments, cooling system700has a height of not more than twenty-six millimeters. For example, cooling system700may be nominally not more than fifteen millimeters tall. Thus, more server systems and cooling systems700may be provided in a particular server rack. In addition, MEMS cooling system700need not use liquids for cooling. Consequently, reliability and safety issues that may occur when using fluids for cooling electronics may be avoided. Further, an external chiller may be unnecessary. Instead, cooling system700may draw fluid (e.g. air) from the cool aisle and exhaust heated fluid to the hot aisle in a data center. Thus, performance, compactness, efficiency, and reliability may be improved for high power dissipation applications such as servers. Cooling system700may also entrain fluid and use the entrained fluid to reduce the temperature of heated fluid exiting chassis710.

FIGS.7A-7Ddepict an embodiment of active MEMS cooling system800(including cooling systems800-1,800-2,800-3, and800-4) within a chassis810of a server that may be used in a data center.FIGS.7A and7Bare front and side views, respectively.FIGS.7C and7Dare perspective views of system800.FIG.7Dindicates fluid flow and temperature. For clarity,FIGS.7A-7Dare not to scale and not all components are shown. For example, processors (or switch components)812on printed circuit board (PCB) being cooled by cooling system800are shown. However, memory and other components are omitted. Each active MEMS cooling system800includes cooling tiles801(of which only one is labeled), ducting803, and vapor chamber802. In the embodiment shown, each tile801includes four cooling cells (not explicitly labeled inFIGS.7A-7D). Vapor chamber802includes multiple interconnected tiers (which may also be considered horizontal fins). In the embodiment shown, the tiers are parallel to the PCB, but the tiers may have another orientation in some embodiments. Cooling system800drives fluid (e.g. air) on multiple surfaces of each tier. This is indicated by the arrows inFIGS.7A and7B. Thus, cooling systems800-4and800-3drive fluid onto opposing surfaces of the first tier of cooling vapor chamber802. Cooling systems800-2and800-1drive fluid onto opposing surfaces of the second tier of vapor chamber802. The tiers are connected via a central, vertical portion of vapor chamber802. In some embodiments, another number of tiers may be present. For example, vapor chamber802may include a single tier or may include three or more tiers. Each tier may have corresponding cooling system(s) driving fluid onto one or more surfaces of the vapor chamber. Although multiple tiers are present, the total height of vapor chamber802and cooling systems800may be relatively small. For example, the total height may be not more than thirty millimeters and/or may be less than that of a traditional fan system, while cooling systems800may still provide significantly higher cooling power.

In operation, cooler air may be drawn into chassis810from the cool aisle. The cooler fluid is used by cooling systems800to dissipate heat from vapor chamber802, and thus processors812. The heated fluid (e.g. carrying heat generated by processors812) is exhausted to the hot aisle. As indicated by the fluid flow inFIG.7D, cooler fluid is drawn into chassis810, driven by MEMS cooling system800via vibrational motion can cool vapor chamber802, which is thermally connected to processors812. The fluid carries off the heat generated by processors812. In some embodiments, up to 200 CFM (or more) may be driven by cooling systems800.

Cooling system800shares the benefits of cooling systems100,200,300,395,600A,600B, and/or700. Cooling system800may be used to cool server system810, which requires high power dissipation. In some embodiments, cooling system800may dissipate at least 1400 W, 2400 W, 3200 W, 3600 W or more while occupying less space than a traditional fan system. In addition, MEMS cooling system800need not use liquids for cooling. Consequently, reliability and safety issues that may occur when using fluids for cooling electronics may be avoided. Further, an external chiller may be unnecessary. Instead, cooling system800may draw fluid (e.g. air) from the cool aisle and exhaust heated fluid to the hot aisle in a data center. Thus, performance, compactness, efficiency, and reliability may be improved for high power dissipation applications such as servers. Further, cooling system800may entrain fluid that does not travel through tiles801. Such entrained fluid may not be used to directly cool processors812. Instead, the entrained fluid may be blended with heated fluid carrying heat from vapor chamber802. As a result, fluid leaving system810may have a moderate temperature.

FIGS.8A-8Ddepict an embodiment of active MEMS cooling system900(including cooling systems900-1,900-2,900-3, and900-4) that may be used for a data center network hub.FIGS.8A and8Bare perspective and top views, respectively.FIGS.8C and8Dare perspective views.FIGS.8B,8C, and8Dinclude heat maps. For clarity,FIGS.8A-8Dare not to scale and not all components are shown. Each active MEMS cooling system900includes cooling tiles901(of which only one is labeled), ducting903, and vapor chamber902. In the embodiment shown, each tile901includes four cooling cells (not explicitly labeled inFIGS.8A-8D). Vapor chamber902includes multiple interconnected tiers (which may also be termed fins). Further cooling systems900drive fluid (e.g. air) on multiple surfaces of each tier. Thus, cooling systems900are analogous to cooling systems800. In operation, cooler air may be drawn in from the cool aisle and driven by cooling tiles901. The cooler fluid is used by cooling systems900to dissipate heat from vapor chamber902, and thus from network hub912. The heated fluid is exhausted to the hot aisle. As indicated by the heat maps inFIGS.8B-8D, cooler fluid drawn in and driven by MEMS cooling system900via vibrational motion can cool hub912. The fluid carries off the heat generated by hub912.

Cooling system900shares the benefits of cooling systems100,200,300,395,600A,600B,700, and/or800. Cooling system900may be used to cool network hub, which requires high power dissipation. In some embodiments, cooling system900may dissipate at least 2400 W, 3200 W, 3600 W or more while occupying less space than a traditional fan system. In addition, MEMS cooling system900need not use liquids for cooling. Consequently, reliability and safety issues that may occur when using fluids for cooling electronics may be avoided. Further, an external chiller may be unnecessary. Instead, cooling system900may draw fluid (e.g. air) from the cool aisle and exhaust heated fluid to the hot aisle in a data center. Thus, performance, compactness, efficiency, and reliability may be improved for high power dissipation applications such as servers. Cooling system900may also entrain fluid that does not travel through tiles901. Such entrained fluid may be blended with fluid carrying heat from vapor chamber902. As a result, fluid exhausted by cooling system900may have a moderate temperature.

FIGS.9A-9Ddepict perspective views of an embodiment of active MEMS cooling system1000(including cooling systems1000-1,1000-2,1000-3,1000-4,1000-5, and1000-6). Thus,FIGS.9A-9Ddepict a three tier cooling system.FIGS.9B-9Ddepict the temperature of the vapor chamber and flow. For clarity,FIGS.9A-9Dare not to scale and not all components are shown. Each active MEMS cooling system1000includes cooling tiles1001(of which only one is labeled), ducting1003, and vapor chamber1002. In the embodiment shown, each tile1001includes four cooling cells (not explicitly labeled inFIGS.9A-9D). Vapor chamber1002includes three interconnected tiers. Further cooling systems1000drive fluid (e.g. air) on multiple surfaces of each tier. Thus, cooling systems1000have a structure and function analogous to cooling systems800and/or900.

Cooling system1000shares the benefits of cooling systems100,200,300,395,600A,600B,700,800and/or900. Cooling system1000may be used to cool high power dissipation systems. In some embodiments, cooling system1000may dissipate at least 2400 W, 3200 W, 3600 W or more while occupying less space than a traditional fan system. In addition, MEMS cooling system1000need not use liquids for cooling. Consequently, reliability and safety issues that may occur when using fluids for cooling electronics may be avoided. Further, an external chiller may be unnecessary. Thus, performance, compactness, efficiency, and reliability may be improved for high power dissipation applications such as servers. Cooling system1000may also entrain fluid that does not travel through tiles1001and blend the entrained fluid with heated fluid. As a result, fluid exhausted by cooling system1000may have a moderate temperature.

FIGS.10A-10Ddepict an embodiment of active MEMS cooling system1100(including systems1100-1,1100-2,1100-3,1100-4,1100-5,1100-6,1100-7,1100-8,1100-9, and1100-10that are referred to collectively or generally as1100).FIGS.10A-10Ddepict a cooling system using multiple vertical tiers. For clarity,FIGS.10A-10Dare not to scale and not all components are shown. Each active MEMS cooling system1100includes cooling tiles1101(of which only one is labeled), ducting (not explicitly labeled), and vapor chamber1102having multiple vertical fins. In the embodiment shown, each tile1101includes four cooling cells (not explicitly labeled inFIGS.10A-10D). Vapor chamber1102includes five interconnected vertical fins. Further cooling systems1100drive fluid (e.g. air) on multiple surfaces of each fin. Thus, cooling system1100has a structure and function analogous to cooling systems800,900, and/or1000. Vapor chamber1102is thermally connected to processor1112, which may be a GPU in the embodiment shown. As indicated inFIG.10A, in some embodiments, the height (zl) of such a system may be nominally ninety millimeters and the width (wl) may be nominally one hundred millimeters. Other heights and widths are possible. In addition, as shown inFIG.10B, active MEMS cooling system1100may intake fluid (i.e. air) from the cold aisle and exhaust heated fluid to the hot aisle.

Cooling system1100shares the benefits of cooling systems100,200,300,395,600A,600B,700,800,900and/or1000. Cooling system1100may be used to cool high power dissipation systems. In some embodiments, cooling system1100may dissipate at least 2400 W, 3200 W, 3600 W or more while occupying less space than a traditional fan system. For example, cooling system1100and vapor chamber1102may have a height of approximately ninety millimeters or less and a width of not more than one hundred millimeters. Other sizes and/or other numbers of fins, tiles, and/or cooling cells are possible. In addition, MEMS cooling system1100need not use liquids for cooling. Consequently, reliability and safety issues that may occur when using fluids for cooling electronics may be avoided. Further, an external chiller may be unnecessary. Thus, performance, compactness, efficiency, and reliability may be improved for high power dissipation applications such as servers. Cooling system1100may also entrain fluid that does not travel through tiles1101and blend the entrained fluid with heated fluid. As a result, fluid exhausted by cooling system1100may have a moderate temperature.

FIGS.11A-11Bdepict an embodiment of active MEMS cooling system1200including vapor chamber1202and cooling systems1200-land1200-2. For clarity,FIGS.11A-11Bare not to scale and not all components are shown. Each active MEMS cooling system1200-1and1200-2includes cooling tiles1201(of which only one is labeled) and ducting1203(of which only one is labeled). Cooling systems1200-1and1200-2are thermally coupled to vapor chamber1202. More specifically, cooling system1200uses multiple surfaces of vapor chamber1202. Thus, in the embodiment shown, cooling system1200-1is on the opposite side of vapor chamber1202from cooling system1200-2. In the embodiment shown, cooling systems1200-1and1200-1each includes thirty-six tiles and each tile1201includes four cooling cells (not explicitly labeled inFIGS.11A-11B). Thus, cooling system1200has a structure and function analogous to cooling systems800,900,1000, and/or1100.

In the embodiment shown, vapor chamber1202is thermally coupled with processor1212, which is connected to circuit board1213. Vapor chamber1202includes a module connector1202-1and a wider horizontal fin1202-2. Cooling systems1200-1and1200-1thus drive a fluid onto at least the surface of horizontal fin1202-2. Also shown is driving board1215, which may be integrated into or adjacent to circuit board1213. In some embodiments, driving board1215includes drive electronics for cooling system1200and may thus be considered part of cooling system1200.

Cooling system1200allows the cooling systems to be stacked to enable scaling and higher cooling performance. Stated differently, cooling system1200may be considered a single cooling module1200of a modular cooling system.FIGS.11A-11Bdepict a single tier, or module, cooling system. Such a module1200may be considered the basic building block of a multiple module cooling system. Module connector1202-1of vapor chamber1202allows an analogous vapor chamber for another cooling module (not shown) to be placed on top of cooling module1200and to connect (e.g. thermally and physically connect) with vapor chamber1202(the bottom tier cooling module). The number of tiers in such a modular cooling system depends on the power dissipated by each module1200and the available track height. Each module1200is fifteen millimeters high in some embodiments. Such a module includes a single vapor chamber (e.g. analogous to vapor chamber1202) with the module connector (e.g. analogous to module connector1202-1), the MEMS active tiles (e.g. analogous to tiles1201) arranged in an array and duct work (e.g. analogous to ducts1203) to collect the hot air for evacuation into the hot aisle. The vapor chamber module connector1202-1can be used to connect the upper module to the module below it in the stack. On the lowest rung of the multi-tier stack, the module connector is in thermal contact with the component(s) that are desired to be cooled. For example, in the embodiment shown inFIGS.11A-11B, the component1213being cooled is a server processor (e.g. an Intel Xeon).

Although the modular nature of the cooling system is discussed in the context of cooling system1200, other systems described herein may be modular in nature. For example, cooling systems800,900,1000, and/or1100may be reconfigured in a modular fashion. In such embodiments, cooling systems may include apertures in which module connectors analogous to module connector1202-1may be provided to connect vapor chambers of different modules. Thus, cooling systems may be built out vertically or horizontally in order to satisfy the cooling needs in the space available.

Cooling system1200shares the benefits of cooling systems100,200,300,395,600A,600B,700,800,900,1000and/or1100. Cooling system1200may be used to cool high power dissipation systems. In some embodiments, cooling system1200may dissipate at least 300 W, 800 W. 2400 W, 3200 W, 3600 W or more while occupying less space than a traditional fan system. In addition, MEMS cooling system1200need not use liquids for cooling. Consequently, reliability and safety issues that may occur when using fluids for cooling electronics may be avoided. Further, an external chiller may be unnecessary. Moreover, cooling system1200is modular in nature. This allows increased flexibility in providing cooling solutions to multiple applications. Thus, performance, compactness, efficiency, and reliability may be improved for high power dissipation applications such as servers. Cooling system1200may also entrain fluid that does not travel through tiles1201and blend the entrained fluid with heated fluid. As a result, fluid exhausted by cooling system1200may have a moderate temperature.

FIG.12is a flow chart depicting an embodiment of method1300for driving flow using a MEMS cooling system. Method1300may include steps that are not depicted for simplicity. Method1300is described in the context of piezoelectric cooling systems100and600B. However, method1300may be used with other cooling systems including but not limited to systems and cells described herein.

Some portion of the cooling elements in one or more MEMS cooling system(s) is actuated to vibrate, at1302. Stated differently, one or more cooling cells are activated at1302. The number of cooling elements driven at1302may depend upon the temperature of the heat-generating structure, the power drawn, or another parameter. In some embodiments, therefore, the number of cooling cells driven may be adjustable. In other embodiments, all of the cooling cells are driven. Also at1302, an electrical signal having the desired frequency is used to drive the cooling element(s). In some embodiments, the cooling elements are driven at or near structural and/or acoustic resonant frequencies. The driving frequency may be 15 kHz or higher. If multiple cooling elements are driven at1302, the cooling elements may be driven out-of-phase. In some embodiments, the cooling elements are driven substantially at one hundred and eighty degrees out-of-phase. Further, in some embodiments, individual cooling elements are driven out-of-phase. For example, different portions of a cooling element may be driven to vibrate in opposite directions (i.e. analogous to a seesaw). In some embodiments, individual cooling elements may be driven in-phase (i.e. analogous to a butterfly). In addition, the drive signal may be provided to the anchor(s), the cooling element(s), or both the anchor(s) and the cooling elements(s). Further, the anchor may be driven to bend and/or translate.

Feedback from the cooling element(s) is used to adjust the driving current, at1304. In some embodiments, the adjustment is used to maintain the frequency at or near the acoustic and/or structural resonant frequency/frequencies of the cooling element(s) and/or cooling system. Resonant frequency of a particular cooling element may drift, for example due to changes in temperature. Adjustments made at1304allow the drift in resonant frequency to be accounted for.

For example, one or more cooling elements120of cooling system600B may be driven at their structural resonant frequency/frequencies, at1302. The number of cooling elements driven may be selected to efficiently cool the computing device. This resonant frequency may also be at or near the acoustic resonant frequency for top chamber140. This may be achieved by driving piezoelectric layer(s) in anchor160(not shown inFIGS.1A-1F) and/or piezoelectric layer(s) in actuator120. At1304, feedback is used to maintain actuator120at resonance and, in embodiments in which multiple actuators are driven, one hundred and eighty degrees out of phase. Thus, the efficiency of cooling element120in driving fluid flow through cooling system100and onto heat-generating structure102may be maintained. For similar reasons, the efficiency of cooling system600B utilizing such cooling elements is also maintained. In some embodiments,1304includes sampling the current through cooling element120and/or the current through anchor160and adjusting the current to maintain resonance and low input power.

Consequently, cooling systems, such as cooling systems100,200,300,395,600A,600B,700,800,900,1000, and/or1200may operate as described herein. Method1300thus provides for use of piezoelectric cooling systems described herein. Thus, piezoelectric cooling systems may more efficiently and quietly cool high power computing devices.