METHOD AND SYSTEM FOR TAILORING FLUIDIC RESONANT FREQUENCY IN A MEMS-BASED COOLING SYSTEM

A fluid flow system is described. The fluid flow system includes an actuator and a chamber having a feature therein. The actuator is configured to vibrate in response to a driving signal. The chamber is in communication with the actuator. The chamber is characterized by a fluidic resonant frequency. Vibration of the actuator tends to drive a fluid through the chamber. The feature is within the chamber and obstructs direct flow of the fluid within the chamber such that the fluidic resonant frequency is less than a nominal fluidic resonant frequency that would exist without the feature.

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 large 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 both mobile devices such as smartphones and larger devices such as laptops and desktop computers. Consequently, additional cooling solutions for computing devices are desired. Moreover, such cooling systems may be desired to be optimized to better provide the desired cooling for mobile and other devices.

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, notebook computers, virtual reality devices, and other computing devices such as servers 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 to 5G 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.

Although described in the context of a cooling system, the techniques and/or devices described herein may be used in other applications. For example, the actuator may be used in other devices and/or the cooling system may be used for other purposes. The devices are also described in the context of actuators (i.e. cooling elements) that are coupled to a support structure at a central region or at the edges. In other embodiments, the actuator could be coupled to (e.g. anchored to) a support structure in another manner. For example, the actuator may be attached to the support structure along an edge of the actuator. Various systems are described and particular features highlighted. Various characteristics of the systems may be combined in manners not explicitly depicted herein.

A fluid flow system is described. The fluid flow system includes an actuator and a chamber having a feature therein. The actuator is configured to vibrate in response to a driving signal. The chamber is in communication with the actuator. The chamber is characterized by a fluidic resonant frequency. Vibration of the actuator tends to drive a fluid through the chamber. The feature is within the chamber and obstructs direct flow of the fluid within the chamber such that the fluidic resonant frequency is less than a nominal fluidic resonant frequency that would exist without the feature. In some embodiments, the feature is configured to increase an effective length for the chamber. In some embodiments, the chamber includes an upper chamber and a lower chamber. The actuator is between the upper chamber and the lower chamber. The feature is in the upper chamber. The actuator directs the fluid from the upper chamber to the lower chamber in response to the driving signal.

The chamber may include an upper chamber having a top wall. In some such embodiments, the feature includes at least one mesa extending from the top wall. The mesa(s) may be configured such that at least seventy-five percent of the fluid is directed around the mesa and not more than twenty-five percent of the fluid travels between the mesa and the actuator. The mesa(s) may have footprint(s) configured to reduce fluidic vortices. In some embodiments, the footprint is selected from a triangle, a diamond, and a flattened diamond. In some embodiments, the upper chamber has a top wall having a split vent therein. The split vent includes a first aperture and a second aperture. The feature includes a divider wall separating the first aperture from the second aperture such that the first aperture is in fluid communication with a first portion of the chamber and the second aperture is in fluid communication with a second portion of the chamber. In some such embodiments, the chamber is characterized by a center line. The first aperture is offset from the center line in a first direction and the second aperture is offset from the center line in a second direction opposite to the first direction. The split vent may include a third aperture and a fourth aperture. The fluid flow system may further include a support structure. In such embodiments, the actuator includes a central region and a perimeter. The actuator is supported by the support structure at the central region. At least a portion of the perimeter is unpinned and vibrates in response to the driving signal.

A cooling system including a cooling element, a chamber, and a feature within the chamber are described. The cooling element vibrates in response to a driving signal. The chamber is in communication with the cooling element and is characterized by a fluidic resonant frequency. The chamber includes an orifice plate having at least one orifice therein. Vibration of the actuator tends to drive a fluid through the chamber and out the orifice(s). The feature obstructs direct flow of the fluid within the chamber such that the fluidic resonant frequency is less than a nominal fluidic resonant frequency that would exist without the feature. The feature may be configured to increase an effective length for the chamber.

The chamber may include an upper chamber having a top wall. The feature may include at least one mesa extending from the top wall. The mesa(s) are configured such that at least seventy-five percent of the fluid is directed around the mesa(s) and not more than twenty-five percent of the fluid travels between the mesa(s) and the cooling element. In some embodiments, the chamber includes an upper chamber having a top wall. The top wall may have a split vent therein. The split vent includes a first aperture and a second aperture. The feature includes a divider wall separating the first aperture from the second aperture such that the first aperture is in fluid communication with a first portion of the chamber and the second aperture is in fluid communication with a second portion of the chamber. In some embodiments, the chamber is characterized by a center line. In such embodiments, the first aperture is offset from the center line in a first direction, and the second aperture is offset from the center line in a second direction opposite to the first direction.

The cooling system may include a support structure. In such embodiments, the cooling element includes a central region and a perimeter. The cooling element is supported by the support structure at the central region. At least a portion of the perimeter is unpinned and vibrates in response to the driving signal.

A method includes driving an actuator configured to induce a vibrational motion at a frequency. The actuator is in communication with a chamber. The chamber is characterized by a fluidic resonant frequency. Vibration of the actuator tends to drive a fluid through the chamber. A feature resides within the chamber and obstructs direct flow of the fluid within the chamber such that the fluidic resonant frequency is less than a nominal fluidic resonant frequency that would exist without the feature. In some embodiments, the feature is configured to increase an effective length for the chamber.

FIGS.1A-1Hare diagrams depicting an exemplary embodiment of active MEMS cooling system100usable with heat-generating structure102and including a centrally anchored cooling element120or120′ (also referred to herein as actuator120or120′). Cooling element120and cooling element120′ are interchangeable. For clarity, only certain components are shown.FIGS.1A-1Hare not to scale. Although shown as symmetric, cooling system100need not be.

FIGS.1A and1Bdepict cross-sectional and top views of cooling system100. Cooling system100includes top plate110having vent112therein, actuator (or cooling element)120, orifice plate130having orifices132therein, support structure (or “anchor”)160and chambers140and150(collectively chamber140/150) formed therein. The top wall of flow/chamber140/150is formed by the bottom surface of top plate110in the embodiment shown. Flow chamber140/150may thus be considered to be formed between top plate110and orifice plate130. The top wall of flow chamber140/150has features170thereon. Actuator120is supported at its central region by anchor160. InFIG.1B, actuator120is shown by a dashed line and anchor160is shown by a dotted/dashed line. For simplicity, orifices132are not depicted inFIG.1B. Regions of actuator120closer to and including portions of the actuator's perimeter (e.g. tip121) vibrate when actuated. Regions of actuator120closer to and including portions of the actuator's perimeter (e.g. tip121) vibrate when actuated. In some embodiments, tip121of actuator120includes a portion of the perimeter furthest from anchor160and undergoes the largest deflection during actuation of actuator120. For clarity, only one tip121of actuator120is 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, actuator120is shown as substantially flat. For in-phase operation, actuator120is driven to vibrate between positions shown inFIGS.1B and1C. 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 actuator120toward heat-generating structure102at a speed of at least forty-five meters per second. In some embodiments, the fluid is driven toward heat-generating structure102by actuator120at 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 actuator120.

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 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).

Actuator120can be considered to divide the interior of active MEMS cooling system100into top chamber140and bottom chamber150. Top chamber140is formed by actuator120, the sides, and top plate110. Bottom chamber150is formed by orifice plate130, the sides, actuator120and anchor160. Top chamber140and bottom chamber150are connected at the periphery of actuator120and 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, actuator120motion, and the frequency of operation. Top chamber140has a height, hl. 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 actuator120does 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.

Top plate110also includes features170that extend from top plate110. InFIG.1B, features170are depicted by dotted lines. Features170may be considered to be mesas or pedestals that protrude from top plate110. Thus, cavities may be considered to be formed around features170. For example, cavities may be considered to be formed between features170and the outer wall of top chamber140. Although shown as having a particular shape (i.e. diamond shaped), features170may have another shape including but not limited to rectangular, triangular, oval, circular, and/or another shape. Although features170are shown as symmetric and having the same shape, in some embodiments, features170may have different shapes and/or may be asymmetric. Although shown as located closer to vent112in top chamber110, features170may be located elsewhere. Features170may have a height, u, of at least one hundred and fifty micrometers and not more than four hundred and fifty micrometers (e.g. at least fifty percent and not more than ninety percent). In some embodiments, features170may have a height of at least two hundred and fifty micrometers (e.g. at least seventy percent of the upper chamber height in some embodiments). However, the height of features170is also desired to be sufficiently small that cantilevered arms123do not strike features170(as well as orifice plate130and the remainder of top plate110). The length, v1, of features170may be at least ten percent and not more than ninety percent of the length of a free portion of actuator120). The width of features170, v2, may be at ten percent of the width, D, of actuator120and not more than ninety percent the width of upper chamber. Because of the presence of features170, top plate110may be viewed as having a varying thickness, top chamber140(and flow chamber140/150) may be viewed as having a varying height, and flow chamber140/150may be viewed as having a top surface with features170protruding therefrom (or, conversely, a top surface having cavities therein).

Bottom chamber150has a height, h2. In some embodiments, the height of bottom chamber150is sufficient to accommodate the motion of actuator120. Thus, no portion of actuator120contacts 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 actuator120plus at least five micrometers and not more than ten micrometers. In some embodiments, the deflection of actuator120(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 actuator120is at least ten micrometers and not more than sixty micrometers. However, the amplitude of deflection of actuator120depends 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 actuator120at the central portion of actuator120. Thus, at least part of the perimeter of actuator120is unpinned and free to vibrate. In some embodiments, anchor160extends along a central axis of actuator120(e.g. perpendicular to the page inFIGS.1A-1E). In such embodiments, portions of actuator120that vibrate (e.g. including tip121) move in a cantilevered fashion. Thus, portions of actuator120may 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 actuator120that 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 actuator120. In such embodiments, all portions of the perimeter of actuator120are free to vibrate (e.g. analogous to a jellyfish). In the embodiment shown, anchor160supports actuator120from the bottom of actuator120. In other embodiments, anchor160may support actuator120in another manner. For example, anchor160may support actuator120from the top (e.g. actuator120hangs 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 actuator120.

Actuator120has a first side distal from heat-generating structure102and a second side proximate to heat-generating structure102. In the embodiment shown inFIGS.1A-1G, the first side of actuator120is the top of actuator120(closer to top plate110) and the second side is the bottom of actuator120(closer to orifice plate130). Actuator120is actuated to undergo vibrational motion as shown inFIGS.1A-1G. The vibrational motion of actuator120drives fluid from the first side of actuator120distal from heat-generating structure102(e.g. from top chamber140) to a second side of actuator120proximate to heat-generating structure102(e.g. to bottom chamber150). The vibrational motion of actuator120also draws fluid through vent112and into top chamber140; forces fluid from top chamber140to bottom chamber150; and drives fluid from bottom chamber150through orifices132of orifice plate130. Although described in the context of a single, continuous actuator, in some embodiments, actuator120may be formed by two (or more) actuators. Each of the actuators as one portion pinned (e.g. supported by support structure160) and an opposite portion unpinned. Thus, a single, centrally supported actuator120may be formed by a combination of multiple actuators supported at an edge.

Actuator120has a length, L, that depends upon the frequency at which actuator120is desired to vibrate. In some embodiments, the length of actuator120is at least four millimeters and not more than ten millimeters. In some such embodiments, actuator120has a length of at least six millimeters and not more than eight millimeters. The depth of actuator120(e.g. perpendicular to the plane shown inFIGS.1A-1E) may vary from one fourth of L through twice L. For example, actuator120may have the same depth as length. The thickness, t, of actuator120may vary based upon the configuration of actuator120and/or the frequency at which actuator120is desired to be actuated. In some embodiments, the actuator thickness is at least two hundred micrometers and not more than three hundred and fifty micrometers for actuator120having a length of eight millimeters and driven at a frequency of at least twenty kilohertz and not more than twenty-five kilohertz. In some embodiments, actuator120is driven at a frequency of at least twenty-two kilohertz and not more than twenty four kilohertz. The length, C of chamber140/150is close to the length, L, of actuator120. For example, in some embodiments, the distance, d, between the edge of actuator120and 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. This distance, d, may be termed the edge vent.

In the embodiment shown, actuator120is supported (held in place) by anchor160along the central axis (out of the plane of the page inFIG.1A) at central portion122(hereinafter anchored region122). Thus, cantilevered arms123(denoted inFIG.1Bonly) that are actuated to vibrate are to the right and left of anchor160. In some embodiments, actuator120is a continuous structure having two portions which are free and actuated (e.g. the cantilevered arms123). In some embodiments, actuator120includes separate cantilevered portions each of which is attached to the anchor160and actuated. Cantilevered arms123of actuator120may be driven to vibrate in a manner analogous to the wings of a butterfly (in-phase) or to a seesaw (out-of-phase).

Although not shown inFIGS.1A-1Gactuator120may include one or more piezoelectric layer(s). Thus, actuator120may be driven by a piezoelectric that is mounted on or integrated into actuator120. In some embodiments, actuator120is driven in another manner including but not limited to providing a piezoelectric on another structure in cooling system100. In some embodiments, it is possible that a mechanism other than a piezoelectric may be used to drive actuator120. In some embodiments, piezoelectric may be located only on or in cantilevered arms123of actuator120. In some embodiments, piezoelectric may be on or in all of actuator120. Thus, actuator120may be a multilayer actuator in which the piezoelectric is integrated into actuator120. For example, actuator120may include 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 a Ti (e.g. a Ti alloy such as Ti6Al-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 actuator120also 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 actuator. Although described in the context of a piezoelectric, another mechanism for actuating actuator120can be utilized. Such other mechanisms may be on (e.g. affixed to) actuator120, integrated into actuator120or may be located elsewhere (e.g. on anchor160).

In the embodiment shown inFIG.1B, anchor160extends most but not all of the depth, D, of actuator120. The entire perimeter of actuator120is free. However, anchor160still holds in place the central, anchored region122of actuator120. Thus, anchor160need not extend the entire length of the central axis in order for cantilevered arms123to vibrate as desired. In some embodiments, anchor160extends along the central axis to the perimeter of actuator120. In some such embodiments, anchor160has a depth of at least D.

Although actuator120is depicted as rectangular, actuators may have another shape. In some embodiments, corners of actuator120may be rounded. In some embodiments, the entire cantilevered arm123might be rounded. Other shapes are possible. For example, in some embodiments, the anchor may be limited to a region near the center of the actuator. In some such embodiments, the actuator may be symmetric around the anchor. For example, anchor160and actuator120may have a circular footprint. Such an actuator may be configured to vibrate in a manner analogous to a jellyfish or similar to the opening/closing of an umbrella. In some embodiments, the entire perimeter of such an actuator vibrates in-phase (e.g. all move up or down together). In other embodiments, portions of the perimeter of such an actuator vibrate out-of-phase.

Actuator120may 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 actuator120. The portion of actuator120undergoing vibrational motion (e.g. each cantilevered arm123having a length (L−a)/2)) is driven at or near resonance (the “structural resonance”) of actuator120. This portion of actuator120undergoing vibration may be cantilevered section123in some embodiments. The frequency of vibration for structural resonance is termed the structural resonant frequency. Use of the structural resonant frequency in driving actuator120reduces the power consumption of cooling system100. Actuator120and 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. The nominal acoustic resonant frequency (the resonant frequency for cooling system100in the absence of features170) can be calculated as follows. 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 actuator120and near the connection between top chamber140and bottom chamber150). The distance between these two regions is C/2. In the absence of features170, the distance between these two regions (roughly C/2) is the distance the fluid would travel in top chamber140. Thus, C/2=nλ/4, where X is the acoustic wavelength for the fluid and n is odd (e.g. n=1, 3, 5, etc.) for cooling system100in the absence of features170. The nominal acoustic resonant frequency for fluid chambers140/150is given by the speed of sound in the fluid divided by λ, or the speed of sound in the fluid divided by 2C/n. For the lowest order mode, C=X/2. Because the length of chamber140(e.g. C) is close to the length of actuator120, in some embodiments, it is also approximately true that L/2=nλ/4, where X is the acoustic wavelength for the fluid and n is odd. Thus, the frequency at which actuator120is driven, v, is at or near the structural resonant frequency for actuator120. The frequency v is also at or near the nominal acoustic resonant frequency for at least top chamber140in the absence of features170. The acoustic resonant frequency of top chamber140generally varies less dramatically with parameters such as temperature and size than the structural resonant frequency of actuator120. Consequently, in some embodiments, actuator120may 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 actuator120(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 actuator120also 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 actuator120. 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 actuator120(e.g. r1≥200 μm). In some such embodiments, orifices132are at least three hundred micrometers from tip121of actuator120(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 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-1G. 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, actuator120has been actuated so that its tip121moves away from top plate110.FIG.1Ccan thus be considered to depict the end of a down stroke of actuator120. Because of the vibrational motion of actuator120, 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 actuator120is 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.

Actuator120is also actuated so that tip121moves away from heat-generating structure102and toward top plate110. FIG. D can thus be considered to depict the end of an up stroke of actuator120. Because of the motion of actuator120, 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 actuator120is 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 actuator120moves 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 actuator120and 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 actuator120contacting 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, actuator120undergoes vibrational motion indicated inFIGS.1C-1D, 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, actuator120is driven to vibrate at or near the structural resonant frequency of actuator120. Further, the structural resonant frequency of actuator120is 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 actuator120may be at frequencies from 15 kHz through 30 kHz. In some embodiments, actuator120vibrates 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 actuator120is within ten percent of the acoustic resonant frequency of cooling system100. In some embodiments, the structural resonant frequency of actuator120is within five percent of the acoustic resonant frequency of cooling system100. In some embodiments, the structural resonant frequency of actuator120is 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 actuator120. As a result, heat-generating structure102may be cooled.

FIGS.1E-1Fdepict an embodiment of active MEMS cooling system100including centrally anchored actuator120in which the actuator is driven out-of-phase. More specifically, sections of actuator120on opposite sides of anchor160(and thus on opposite sides of the central region of actuator120that is supported by anchor160) are driven to vibrate out-of-phase. In some embodiments, sections of actuator120on opposite sides of anchor160are driven at or near one hundred and eighty degrees out-of-phase. Thus, one section of actuator120vibrates toward top plate110, while the other section of actuator120vibrates toward orifice plate130/heat-generating structure102. Movement of a section of actuator120toward top plate110(an upstroke) drives fluid in top chamber140to bottom chamber150on that side of anchor160. Movement of a section of actuator120toward 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, actuator120undergoes vibrational motion indicated inFIGS.1A,1E, and1F, alternately drawing fluid through vent112from the distal side of top plate110into top chamber140for each side of actuator120; 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, actuator120is driven to vibrate at or near the structural resonant frequency of actuator120. Further, the structural resonant frequency of actuator120is 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 actuator120may be at the frequencies described for in-phase vibration. The structural resonant frequency of actuator120is within ten percent of the acoustic resonant frequency of cooling system100. In some embodiments, the structural resonant frequency of actuator120is within five percent of the acoustic resonant frequency of cooling system100. In some embodiments, the structural resonant frequency of actuator120is 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 actuator120. As a result, heat-generating structure102may be cooled.

As discussed above, actuator120may 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 actuator120. At acoustic resonance, an antinode in pressure occurs near the periphery of cooling system100. Although the node in pressure occurs near vent112, the exact location of the node may differ based upon various factors. Differences in the location of the node and/or antinode result in different acoustic resonant frequencies. For example, depending upon whether cantilevered arms123vibrate in-phase (FIGS.1C-1D) or out-of-phase (FIGS.1E-1F) the location of the node differs. If cantilevered arms123of actuator120are driven in-phase, the in-phase motion of cantilevered arms123may increase the pressure near vent112(e.g. near the center of cooling system100). Thus, the location of the pressure node may be outside of vent112/outside of top chamber140. The precise location of the node may depend upon the characteristics of the fluid outside of vent112(i.e. the fluid reservoir). If cantilevered arms123of actuator120are driven out-of-phase, the in phase motion of cantilevered arms123may not change the pressure near vent112. Thus, the node may remain at or near vent112. The location of the pressure node may thus be independent of the characteristics of the fluid reservoir. As a result, the distance between the node and the antinode in pressure for actuator120being driven in-phase is larger than for the actuator being driven out-of-phase. Consequently, the acoustic resonant frequency for cantilevered arms123being driven out-of-phase is higher than for cantilevered arms123being driven in-phase. For example, for acoustic and structural resonant frequencies being in the range of 20 kHz-25 kHz, the acoustic resonant frequency for out-of-phase vibration of cantilevered arms123may be 3 kHz-4 kHz (e.g. nominally 3.5 kHz) higher than for in-phase vibration. Thus, the acoustic resonant frequency may be desired to be further tailed, for example to more closely match the structural resonant frequency and/or to improve fluid flow.

Features170in flow chamber140/150may allow for tailoring of the acoustic resonant frequency. Features170obstruct the flow of fluid in top chamber140. Because the features170protrude from top plate110, the distance between features170and actuator120is smaller than for remaining portions of top plate110. As a result, fluid tends to flow around features170. For example, in some embodiments, features170are sufficiently tall that at least sixty percent of the fluid flows around features170instead of between features170and actuator120. In some embodiments, at least seventy-five percent of the fluid flows around features170. In some embodiments, at least eighty percent of the fluid flows around features170. In some embodiments, not more than ninety-five percent of the fluid flows around features170. For example, at least eighty and not more than eighty five percent of the fluid flows around features170. This may occur, for example, for a top cavity height hl of three hundred micrometers and a feature height u of two hundred and fifty micrometers (e.g. a gap between features170and actuator120of fifty micrometers). Other heights and/or other fractions are possible.

This fluid flow may be seen inFIG.1G. The unlabeled arrows ofFIG.1Gindicate the direction of flow of the majority (e.g. eighty to eighty-five percent of the fluid) while actuator120is driven. Because of the presence of features170, the fluid does not follow a straight path from vent112to tip121of actuator120and into bottom chamber150. Features170thus obstruct the flow of fluid in top chamber140. The path taken by the fluid around features170is different from the length of the portion of top chamber140(C/2) and different from the length of the portion of actuator120(L/2). Features170thus increase an effective length for top chamber140. Consequently, the effective length of the path traveled by the fluid between the node (e.g. at or near vent112) and the antinode (near the edge of chamber140/150) has been increased by the presence of features170. The increase in the effective length of top chamber140modifies the wavelength of the standing pressure wave in cooling system100and, therefore, the acoustic (or fluidic) resonant frequency. Thus, the presence of features170may reduce the fluidic resonant frequency to be less than a nominal fluidic resonant frequency of top chamber140in the absence of features170. Further, the combination of the length (v1) and width (v2) of features170may be selected to provide the desired effective path of the fluid being driven. Thus, desired acoustic resonant frequency may be obtained. Although the presence of features170may restrict flow somewhat, in some embodiments, features170are configured such that flow is not significantly changed. For example, in various embodiments, the flow rate may drop by not more than five percent, ten percent, fifteen percent or twenty percent. Thus, adequate flow for cooling may be maintained while tailoring the frequency.

Although shown in the context of a uniform actuator inFIGS.1A-1G, cooling system100may utilize actuators having different shapes.FIG.1Hdepicts an embodiment of engineered actuator120′ having a tailored geometry and usable in a cooling system such as cooling system100. Actuator120′ 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 actuator120′ being actuated. Each cantilevered arm123includes step region124, extension region126and outer region128. In the embodiment shown inFIG.1H, 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 actuator120, and/or another mechanism.

Use of engineered actuator120′ may further improve efficiency of cooling system100. Extension region126is thinner than step region124and outer region128. This results in a cavity in the bottom of actuator120′ 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 actuator120′ to provide a valve preventing backflow through orifices132may be improved. Thus, performance of cooling system100employing actuator120′ may be improved.

Further, cooling elements used in cooling system100may have different structures and/or be mounted differently than depicted inFIGS.1A-1H. 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-1H, 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.

Using the cooling system100actuated for in-phase vibration or out-of-phase vibration of actuator120and/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 actuator120/120′ may be vibrated at frequencies of 15 kHz or more, users may not hear any noise associated with actuation of actuators. If driven at or near structural and/or acoustic resonant frequencies, the power used in operating cooling systems may be significantly reduced. Actuator120/120′ does not physically contact top plate110or orifice plate130during vibration. Thus, resonance of actuator120/120′ may be more readily maintained. More specifically, physical contact between actuator120/120′ and other structures disturbs the resonance conditions for actuator120/120′. Disturbing these conditions may drive actuator120/120′ out of resonance. Thus, additional power would need to be used to maintain actuation of actuator120/120′. Further, the flow of fluid driven by actuator120/120′ 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 actuator120/120′ allows the position of the center of mass of actuator100to remain more stable. Although a torque is exerted on actuator120/120′, the force due to the motion of the center of mass is reduced or eliminated. As a result, vibrations due to the motion of actuator120/120′ may be reduced. Moreover, efficiency of cooling system100may be improved through the use of out-of-phase vibrational motion for the two sides of actuator120/120′. 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.

In addition to improved cooling performance, cooling system100has an acoustic (or fluidic) resonant frequency that can be tailored. The shape, height, footprint (e.g. length and width) of features170may be selected to provide the desired increase in distance traveled by fluid driven by vibration of actuator120. As a result, the acoustic resonant frequency of cooling system100may be tuned. For example, the acoustic resonant frequency may be tailored to better match the structural resonant frequency of actuator120, to provide improved thermal dissipation by the fluid driven by actuator120, and/or for other purposes.

FIG.2depicts an embodiment of active MEMS cooling system200including a centrally anchored, engineered actuator.FIG.2is not to scale. For simplicity, only portions of cooling system200are shown. 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 structure102. Cooling system200includes top plate210having vent212, actuator220, orifice plate230including orifices232, top chamber240having a gap, bottom chamber250having a gap, flow chamber240/250, anchor (i.e. support structure)260, pedestal290, and features270that are analogous to top plate110having vent112, actuator120, orifice plate130including orifices132, top chamber140having gap142, bottom chamber150having gap152, flow chamber140/150, anchor (i.e. support structure)160, pedestal190, and features170, respectively. Thus, actuator220is centrally supported by anchor260such that at least a portion of the perimeter of actuator220is free to vibrate. Actuator220includes an anchored region222, cantilevered arms223, and tips221that are analogous to anchored region122, cantilevered arms123, and tips121. Actuator220also includes step region224, extension region226, and outer region228that are analogous to step region124, extension region126, and outer region128, respectively, of actuator120′. Thus, cooling system200expressly integrates engineered actuator220.

In addition, active cooling system200includes features270, which protrude from the surface of top plate210into top chamber240. Features270are analogous to features170of cooling system100. Thus, features270obstruct the direct flow of the fluid within top chamber240and increase the effective length for top chamber240. Thus, the acoustic resonant frequency for cooling system200is less than a nominal fluidic resonant frequency that would exist for top chamber240in the absence of features270. Moreover, the surface of features closest to actuator220is not parallel to the remaining surface of top plate210facing actuator220. Thus, the height of features270may vary while still allowing for tuning of the acoustic resonant frequency of cooling system200.

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. As for cooling system100, the presence of features270allows for tailoring of the acoustic resonant frequency. Thus, the performance may be improved.

FIG.3depicts an embodiment of active MEMS cooling system300. For simplicity, only a top view of cooling system300is shown.FIG.3is not to scale. For simplicity, only portions of cooling system300are shown. Cooling system300is analogous to cooling system(s)100and/or200. Consequently, analogous components have similar labels. For example, cooling system300is used in conjunction with a heat-generating structure (not shown), which is analogous to heat-generating structure102. Cooling system300includes top plate310having vent312, actuator320, orifice plate (not shown) including orifices (not shown), top chamber (not shown) having a gap, bottom chamber (not shown) having a gap, flow chamber (not shown), anchor (i.e. support structure)360, and features370that are analogous to top plate110having vent112, actuator120, orifice plate130including orifices132, top chamber140having gap142, bottom chamber150having gap152, flow chamber140/150, anchor (i.e. support structure)160, and features170, respectively. Thus, actuator320is centrally supported by anchor360such that at least a portion of the perimeter of actuator320is free to vibrate. Actuator320includes an anchored region322, cantilevered arms323, and tips321that are analogous to anchored region122, cantilevered arms123, and tips121. Although not indicated inFIG.3, actuator320may be an engineered actuator analogous to actuator120′ and/or220.

Features370protrude from the surface of top plate310into top chamber340. Features270are analogous to features170of cooling system100. Instead of being configured as diamonds, features370have a triangular footprint. Although shown with sharp corners, features370have rounded corners in some embodiments. The use of rounder corners may reduce or prevent the occurrence of vortices in the fluid flow. Thus, features370obstruct the direct flow of the fluid within the top chamber and increase the effective length for the top chamber. Thus, the acoustic resonant frequency for cooling system300is less than a nominal fluidic resonant frequency that would exist for cooling system300in the absence of features370.

Cooling system300operates in an analogous manner to cooling system100. Cooling system300thus shares the benefits of cooling system100. Thus, performance of a device employing cooling system300may be improved. As in cooling system100, the presence of features370allows for tailoring of the acoustic resonant frequency. Thus, the performance may be improved.

FIG.4depicts an embodiment of active MEMS cooling system400. For simplicity, only a top view of cooling system400is shown.FIG.4is not to scale. For simplicity, only portions of cooling system400are shown. Cooling system400is analogous to cooling system(s)100,200and/or300. Consequently, analogous components have similar labels. For example, cooling system400is used in conjunction with a heat-generating structure (not shown), which is analogous to heat-generating structure102. Cooling system400includes top plate410having vent412, actuator420, orifice plate (not shown) including orifices (not shown), top chamber (not shown) having a gap, bottom chamber (not shown) having a gap, flow chamber (not shown), anchor (i.e. support structure)460, and features470that are analogous to top plate110having vent112, actuator120, orifice plate130including orifices132, top chamber140having gap142, bottom chamber150having gap152, flow chamber140/150, anchor (i.e. support structure)160, and features170, respectively. Thus, actuator420is centrally supported by anchor460such that at least a portion of the perimeter of actuator420is free to vibrate. Actuator420includes an anchored region422, cantilevered arms423, and tips321that are analogous to anchored region122, cantilevered arms123, and tips121. Although not indicated inFIG.4, actuator420may be an engineered actuator analogous to actuator120′ and/or220.

Features470protrude from the surface of top plate410into top chamber440. Features470are analogous to features170of cooling system100. Features470include those with diamond and triangular footprints. Although shown with sharp corners, features470have rounded corners in some embodiments. The use of rounder corners may reduce or prevent the occurrence of vortices in the fluid flow. Thus, features470obstruct the direct flow of the fluid within the top chamber and increase the effective length for the top chamber. Thus, the acoustic resonant frequency for cooling system400is less than a nominal fluidic resonant frequency that would exist for cooling system400in the absence of features470.

Cooling system400operates in an analogous manner to cooling system100. Cooling system400thus shares the benefits of cooling system100. Thus, performance of a device employing cooling system400may be improved. Further, the presence of features470allows for tailoring of the acoustic resonant frequency. Thus, the performance may be improved.

FIG.5depicts an embodiment of active MEMS cooling system500. For simplicity, only a top view of cooling system400is shown.FIG.5is not to scale. For simplicity, only portions of cooling system500are shown. Cooling system500is analogous to cooling system(s)100,200,300and/or400. Consequently, analogous components have similar labels. For example, cooling system500is used in conjunction with a heat-generating structure (not shown), which is analogous to heat-generating structure102. Cooling system500includes top plate510having vent512, actuator520, orifice plate (not shown) including orifices (not shown), top chamber (not shown) having a gap, bottom chamber (not shown) having a gap, flow chamber (not shown), anchor (i.e. support structure)560, and features570that are analogous to top plate110having vent112, actuator120, orifice plate130including orifices132, top chamber140having gap142, bottom chamber150having gap152, flow chamber140/150, anchor (i.e. support structure)160, and features170, respectively. Thus, actuator520is centrally supported by anchor560such that at least a portion of the perimeter of actuator520is free to vibrate. Actuator520includes an anchored region522, cantilevered arms523, and tips521that are analogous to anchored region122, cantilevered arms123, and tips121. Although not indicated inFIG.5, actuator520may be an engineered actuator analogous to actuator120′ and/or220.

Features570protrude from the surface of top plate510into top chamber540. Features570are analogous to features170of cooling system100. Features570are configured to further restrict the path taken by fluid through cooling system520. Although shown with sharp corners, features570have rounded corners in some embodiments. The use of rounder corners may reduce or prevent the occurrence of vortices in the fluid flow. Thus, features570obstruct the direct flow of the fluid within the top chamber and increase the effective length for the top chamber. In particular, fluid takes a path between features570. Thus, the acoustic resonant frequency for cooling system500is less than a nominal fluidic resonant frequency that would exist for cooling system500in the absence of features570.

Cooling system500operates in an analogous manner to cooling system100. Cooling system500thus shares the benefits of cooling system100. Thus, performance of a device employing cooling system500may be improved. Further, the presence of features570allows for tailoring of the acoustic resonant frequency. Thus, the performance may be improved.

FIGS.6A-6Bdepict an embodiment of active MEMS cooling system600.FIG.6Adepicts a side view, whileFIG.6Bdepicts a top view.FIGS.6A-6Bare not to scale. For simplicity, only portions of cooling system600are shown. Cooling system600is analogous to cooling system(s)100,200,300,400and/or500. Consequently, analogous components have similar labels. For example, cooling system600is used in conjunction with a heat-generating structure (not shown), which is analogous to heat-generating structure102. Cooling system600includes top plate610, actuator620, orifice plate630including orifices632, top chamber640having a gap, bottom chamber650having a gap, flow chamber640/650, anchor (i.e. support structure)660, pedestal690, and feature670that are analogous to top plate110, actuator120, orifice plate130including orifices132, top chamber140having gap142, bottom chamber150having gap152, flow chamber140/150, anchor (i.e. support structure)160, pedestal190, and features170, respectively. Thus, actuator620is centrally supported by anchor660such that at least a portion of the perimeter of actuator620is free to vibrate. Actuator620includes an anchored region622, cantilevered arms623, and tips621that are analogous to anchored region122, cantilevered arms123, and tips121. Although not indicated inFIGS.6A-6B, actuator620may be an engineered actuator analogous to actuator120′ and/or220.

Cooling system600includes a vents612-1and612-2(collectively split vent612) and feature670. Thus, split vent612provides an inlet for fluid into top chamber640. Feature670is a divider wall that protrudes from the surface of top plate610into top chamber640. Feature670is analogous to features170of cooling system100in that feature670is configured to further restrict the path taken by fluid through cooling system620. Although shown with sharp corners, feature670has rounded corners in some embodiments. The use of rounder corners may reduce or prevent the occurrence of vortices in the fluid flow. Feature (or wall)670separates opposite sides of chamber640. Feature670is not shown as extending to the top of central portion622of actuator620. However, in some embodiments, feature670extends to actuator620. Feature670mostly or completely prevents fluid from vent612-1from reaching the opposing side of chamber640. Feature670mostly or completely prevents fluid from vent612-2from reaching the opposing side of chamber640. Thus, feature670obstructs the direct flow of the fluid within top chamber640.

During operation, fluid enters top chamber640through each vent612-1and612-2. Because of the presence of divider wall670, the fluid moves generally toward the edge of top chamber640on the same side of feature670as the corresponding vent612-1and612-2. As fluid is driven further from split vent612and past feature670, the fluid spreads to occupy more of top chamber640. Thus, the fluid may be viewed as transitioning from two-dimensional flow to three-dimensional flow. This change in fluid flow may be considered to increase the effective length of top chamber640. Thus, the acoustic (i.e. fluidic) resonant frequency of cooling system600is reduced. Depending upon the size, location, and geometry of split vent612and feature670, the acoustic resonant frequency of cooling system600may be tailored.

Cooling system600operates in an analogous manner to cooling system100. Cooling system600thus shares the benefits of cooling system100. Thus, performance of a device employing cooling system600may be improved. Further, the presence of feature670allows for tailoring of the acoustic resonant frequency. Thus, the performance may be improved.

FIG.7depicts an embodiment of active MEMS cooling system700. For simplicity, only a top view of cooling system700is shown.FIG.7is not to scale. For simplicity, only portions of cooling system700are shown. Cooling system700is analogous to cooling system(s)100,200,300,400,500and/or600. Consequently, analogous components have similar labels. For example, cooling system700is used in conjunction with a heat-generating structure (not shown), which is analogous to heat-generating structure102. Cooling system700includes top plate710having vents712-1and712-2(collectively split vent712), actuator720, orifice plate (not shown) including orifices (not shown), top chamber (not shown) having a gap, bottom chamber (not shown) having a gap, flow chamber (not shown), anchor (i.e. support structure)760, and feature770that are analogous to top plate110having vent112, actuator120, orifice plate130including orifices132, top chamber140having gap142, bottom chamber150having gap152, flow chamber140/150, anchor (i.e. support structure)160, and features170, respectively. Thus, actuator720is centrally supported by anchor760such that at least a portion of the perimeter of actuator720is free to vibrate. Actuator720includes an anchored region722, cantilevered arms723, and tips721that are analogous to anchored region122, cantilevered arms123, and tips121. Although not indicated inFIG.7, actuator720may be an engineered actuator analogous to actuator120′ and/or220.

Cooling system700is most analogous to cooling system600. Thus, split vent712and feature (divider wall)770are analogous to split vent612and feature670. In addition, vents712-1and712-2are offset. Fluid is driven through cooling system700in an analogous manner to cooling system600. Thus, fluid transitions from a one-dimensional flow to a two-dimensional flow. This may increase the effective length of the top chamber and reduce the acoustic resonant frequency of cooling system700to below a nominal acoustic resonant frequency that would be present in the absence of feature770. Further, because vents712-1and712-2are offset, fluid traveling through cooling system700has an even longer path than if vents712-1and712-2were aligned. Thus, the acoustic resonant frequency may be further reduced.

Cooling system700operates in an analogous manner to cooling system600. Cooling system700thus shares the benefits of cooling system600. Thus, performance of a device employing cooling system700may be improved. Further, the presence of feature770allows for tailoring of the acoustic resonant frequency. Thus, the performance may be improved.

FIG.8depicts an embodiment of active MEMS cooling system800. For simplicity, only a top view of cooling system800is shown.FIG.8is not to scale. For simplicity, only portions of cooling system800are shown. Cooling system800is analogous to cooling system(s)100,200,300,400,500,600and/or700. Consequently, analogous components have similar labels. For example, cooling system800is used in conjunction with a heat-generating structure (not shown), which is analogous to heat-generating structure102. Cooling system800includes top plate810having vents812-1,812-2,812-3, and812-4(collectively split vent812), actuator820, orifice plate (not shown) including orifices (not shown), top chamber (not shown) having a gap, bottom chamber (not shown) having a gap, flow chamber (not shown), anchor (i.e. support structure)860, and feature870that are analogous to top plate110having vent112, actuator120, orifice plate130including orifices132, top chamber140having gap142, bottom chamber150having gap152, flow chamber140/150, anchor (i.e. support structure)160, and features170, respectively. Thus, actuator820is centrally supported by anchor860such that at least a portion of the perimeter of actuator820is free to vibrate. Actuator820includes an anchored region822, cantilevered arms823, and tips821that are analogous to anchored region122, cantilevered arms123, and tips121. Although not indicated inFIG.8, actuator820may be an engineered actuator analogous to actuator120′ and/or220.

Cooling system800is most analogous to cooling system(s)600and/or700. Thus, split vent812and feature (divider wall)870are analogous to split vent612and feature670. In addition, a larger number of vents812-1,812-2,812-3, and812-4(i.e. four instead of two) are present. Fluid is driven through cooling system800in an analogous manner to cooling system800. Thus, fluid transitions from a one-dimensional flow to a two-dimensional flow. This may increase the effective length of the top chamber and reduce the acoustic resonant frequency of cooling system800to below a nominal acoustic resonant frequency that would be present in the absence of feature870. Thus, the acoustic resonant frequency may be reduced.

Cooling system800operates in an analogous manner to cooling systems600and700. Cooling system800thus shares the benefits of cooling system(s)600and/or700. Thus, performance of a device employing cooling system800may be improved. Further, the presence of feature870allows for tailoring of the acoustic resonant frequency. Thus, the performance may be improved.

FIGS.9A and9Bdepict embodiments of active MEMS cooling systems900A and900B, respectively. For simplicity, only top views of cooling systems900A and900B are shown.FIGS.9A and9Bare not to scale. For simplicity, only portions of cooling systems900A and900B are shown. Cooling systems900A and900B are analogous to cooling system(s)100,200,300,400,500,600,700and/or800. Consequently, analogous components have similar labels. For example, cooling systems900A and900B are used in conjunction with a heat-generating structure (not shown), which is analogous to heat-generating structure102. Cooling systems900A and900B each includes top plate910having vent912, actuator920, orifice plate (not shown) including orifices (not shown), top chamber (not shown) having a gap, bottom chamber (not shown) having a gap, flow chamber (not shown), and anchor (i.e. support structure)960that are analogous to top plate110having vent112, actuator120, orifice plate130including orifices132, top chamber140having gap142, bottom chamber150having gap152, flow chamber140/150, and anchor (i.e. support structure)160, respectively. Thus, actuator920is centrally supported by anchor960such that at least a portion of the perimeter of actuator920is free to vibrate. Actuator920includes an anchored region922, cantilevered arms923, and tips921that are analogous to anchored region122, cantilevered arms123, and tips121. Although not indicated inFIGS.9A and9B, actuator920may be an engineered actuator analogous to actuator120′ and/or220. In some embodiments, cooling system(s)900A and/or900B may include features analogous to feature(s)170,270,370,470,570,670,770, and/or870. In some such embodiments, vent912may be a split vent.

In cooling systems900A and900B, cooling element920is driven by piezoelectrics925A and925B, respectively. As can be seen inFIGS.9A and9B, piezoelectrics925A and925B occupy a different fraction of cantilevered arms923. For example, piezoelectric925A occupies substantially all of cantilevered arm923. Piezoelectric925B occupies approximately half of cantilevered arm923. For cooling systems900A and900B, the maximum flow rate for a given amplitude of deflection of tip921of actuator920occurs at a lower frequency for piezoelectric925B that occupies less of cantilevered arm923. Thus, in addition to tailoring the acoustic resonant frequency, cooling systems may also tailor the frequency at which the maximum flow rate occurs.

FIGS.10A-10Bdepict an embodiment of active MEMS cooling system1000including a top centrally anchored cooling element.FIG.10Adepicts a side view of cooling system1000in a neutral position.FIG.10Bdepicts a top view of cooling system1000.FIGS.10A-10Bare not to scale. For simplicity, only portions of cooling system1000are shown. Referring toFIGS.10A-10B, cooling system1000is analogous to cooling system100. Consequently, analogous components have similar labels. For example, cooling system1000is used in conjunction with heat-generating structure1002, which is analogous to heat-generating structure102.

Cooling system1000includes top plate1010having vents1012, cooling element1020having tip1021, orifice plate1030including orifices1032, top chamber1040having a gap, bottom chamber1050having a gap, flow chamber1040/1050, anchor (i.e. support structure)1060, pedestal1090, and features1070that are analogous to top plate110having vent112, cooling element120having tip121, orifice plate130including orifices132, top chamber140having gap142, bottom chamber150having gap152, flow chamber140/150, anchor (i.e. support structure)160, pedestal190, and features170, respectively. Thus, cooling element1020is centrally supported by anchor1060such that at least a portion of the perimeter of cooling element1020is free to vibrate. In some embodiments, anchor1060extends along the axis of cooling element1020(e.g. in a manner analogous to anchor260A and/or260B). In other embodiments, anchor1060is only near the center portion of cooling element1020(e.g. analogous to anchor1060C and/or1060D). Although not explicitly labeled inFIGS.10A and10B, cooling element1020includes 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 element1020are driven in-phase. In some embodiments, cantilevered arms of cooling element1020are driven out-of-phase. In some embodiments, a simple cooling element, such as cooling element120, may be used.

Anchor1060supports cooling element1020from above. Thus, cooling element1020is suspended from anchor1060. Anchor1060is suspended from top plate1010. Top plate1010includes vent1013. Vents1012on the sides of anchor1060provide a path for fluid to flow into sides of chamber1040.

As discussed above with respect to cooling system100, cooling element1020may be driven to vibrate at or near the structural resonant frequency of cooling element1020. Further, the structural resonant frequency of cooling element1020may be configured to align with the acoustic resonance of the chamber1040/1050. Moreover, features1070may be used to tailor the acoustic resonant frequency of chamber1040/1050. The structural and acoustic resonant frequencies are generally chosen to be in the ultrasonic range. For example, the vibrational motion of cooling element1020may 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 system1000operates in an analogous manner to cooling system100. Cooling system1000thus shares the benefits of cooling system100. Thus, performance of a device employing cooling system1000may be improved. In addition, suspending cooling element1020from anchor1060may further enhance performance. In particular, vibrations in cooling system1000that may affect other cooling cells (not shown), may be reduced. For example, less vibration may be induced in top plate1010due to the motion of cooling element1020. Consequently, cross talk between cooling system1000and other cooling systems (e.g. other cells) or other portions of the device incorporating cooling system1000may be reduced. Thus, performance may be further enhanced.

FIGS.11A-11Cdepict an embodiment of active MEMS cooling system1100including multiple cooling cells configured as a module termed a tile, or array.FIG.11Adepicts a perspective view, whileFIGS.11B-11Cdepict side views.FIGS.11A-11Care not to scale. Cooling system1100includes four cooling cells1101A,1101B,1101C and1101D (collectively or generically1101), which are analogous to one or more of cooling systems described herein. More specifically, cooling cells1101are analogous to cooling system100and/or400. Tile1100thus includes four cooling cells1101(i.e. four MEMS jets). Although four cooling cells1101in a2x2configuration are shown, in some embodiments another number and/or another configuration of cooling cells1101might be employed. In the embodiment shown, cooling cells1101include shared top plate1110having apertures1112, cooling elements1120, shared orifice plate1130including orifices1132, top chambers1140, bottom chambers1150, anchors (support structures)1160, and features1170that are analogous to top plate110having apertures112, cooling element120, orifice plate130having orifices132, top chamber140, bottom chamber150, anchor160, and features170, respectively. In some embodiments, cooling cells1101may be fabricated together and separated, for example by cutting through top plate1110, side walls between cooling cells1101, and orifice plate1130. Thus, although described in the context of a shared top plate1110and shared orifice plate1130, after fabrication cooling cells1101may be separated. In some embodiments, tabs (not shown) and/or other structures such as anchors1160, may connect cooling cells1101. Further, tile1100includes heat-generating structure (termed a heat spreader hereinafter)1102(e.g. a heat sink, a heat spreader, integrated circuit, or other structure) that also has sidewalls, or fencing, in the embodiment shown. Cover plate1106is also shown. Heat spreader1102and cover plate1106may be part of an integrated tile1100as shown or may be separate from tile1100in other embodiments. Heat spreader1102and cover plate1106may direct fluid flow outside of cooling cells1101, provide mechanical stability, and/or provide protection. Electrical connection to cooling cells1101is provided via flex connector1180(not shown inFIGS.11B-11C) which may house drive electronics1185. Cooling elements1120are driven out-of-phase (i.e. in a manner analogous to a seesaw). Further, as can be seen inFIGS.11B-11Ccooling element1120in one cell is driven out-of-phase with cooling element(s)1120in adjacent cell(s). By driving cooling elements1120out-of-phase, vibrations in cooling system1100may be reduced.

Cooling cells1101of cooling system1100functions in an analogous manner to cooling system(s)100,400, and/or an analogous cooling system. Consequently, the benefits described herein may be shared by cooling system1100. Because cooling elements in nearby cells are driven out-of-phase, vibrations in cooling system1100may be reduced. Because multiple cooling cells1101are used, cooling system1100may enjoy enhanced cooling capabilities. Further, multiples of individual cooling cells1101and/or cooling system1100may be combined in various fashions to obtain the desired footprint of cooling cells.

FIG.12is a flow chart depicting an exemplary embodiment of method1200for operating a cooling system. Method1200may include steps that are not depicted for simplicity. Method1200is described in the context of piezoelectric cooling system100. However, method1200may be used with other cooling systems including but not limited to systems and cells described herein.

One or more of the actuator(s) in a cooling system is actuated to vibrate, at1202. At1202, an electrical signal having the desired frequency is used to drive the actuator(s). In some embodiments, the actuators are driven at or near structural and/or acoustic resonant frequencies at1202. The driving frequency may be15kHz or higher. If multiple actuators are driven at1202, the cooling actuators may be driven out-of-phase. In some embodiments, the actuators are driven substantially at one hundred and eighty degrees out of phase. Further, in some embodiments, individual actuators are driven out-of-phase. For example, different portions of an actuator may be driven to vibrate in opposite directions (i.e. analogous to a seesaw). In some embodiments, individual actuators may be driven in-phase (i.e. analogous to a butterfly). In addition, the drive signal may be provided to the anchor(s), the actuator(s), or both the anchor(s) and the actuator(s). Further, the anchor may be driven to bend and/or translate.

Feedback from the piezoelectric actuator(s) is used to adjust the driving current, at1204. In some embodiments, the adjustment is used to maintain the frequency at or near the acoustic and/or structural resonant frequency/frequencies of the actuator(s) and/or cooling system. Resonant frequency of a particular actuator may drift, for example due to changes in temperature. Adjustments made at1204allow the drift in resonant frequency to be accounted for.

For example, piezoelectric actuator120may be driven at its structural resonant frequency/frequencies, at1202. 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. At1204, feedback is used to maintain actuator120at resonance and, in some embodiments in which multiple actuators are driven, one hundred and eighty degrees out of phase. Thus, the efficiency of actuator120in driving fluid flow through cooling system100and onto heat-generating structure102may be maintained. In some embodiments,1204includes sampling the current through cooling element120and/or the current through anchor160and adjusting the current to maintain resonance and low input power.

Consequently, actuators, such as actuator(s)120,220,320,420,520,620,720,820,920,1020and/or1120may operate as described herein. Method1200thus provides for use of piezoelectric cooling systems described herein. Further, because of the presence of features170,270,370,470,570,670,770,870,1070, and/or1170, the acoustic resonant frequency corresponding to the frequency at which the actuator is driven may be tailored. Thus, piezoelectric cooling systems may more efficiently and quietly cool semiconductor devices at lower power.