Patent Publication Number: US-8983676-B2

Title: System for cooling a heat-generating device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of non-provisional application Ser. No. 11/829,716, filed Jul. 27 2007, now U.S. Pat. No. 8,046,113 the contents of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     With the advent of semiconductor devices having increasingly large component densities, the removal of heat generated by the devices has become an increasingly challenging technical issue. In the past, the low power dissipation of most chips accommodated the use of low cost, air-cooled or and liquid-convection heat sinks. However, many modern higher power-dissipation semiconductor chips now require substantially greater heat dissipation than heat sinks can reasonably provide. 
     A number of cooling methods currently exist that can provide high heat transfer rates that are desirable for cooling the higher-dissipation devices. Some of the more efficient cooling methods are the direct-fluid cooling methods, such as micro-channel cooling, spray-cooling, and jet impingement cooling, wherein a cooling fluid is introduced directly on the device to cool off the device. A direct-fluid method typically involves multi-phase cooling, because it involves a transformation of an amount of the applying cooling fluid from a liquid phase to a vapor/gas phase once it absorbs the heat generated from the applied device. 
     Thermal management of heat-generating devices involves a stable balance between the heat flux in such devices and the heat dissipated by the cooling mechanism, such as cooling fluid. Although multi-phase cooling provides high heat transfer rates, they are unstable at such high rates. That is because such high heat dissipation rates cannot be sustained for long periods of time due to an onset of unstable equilibrium. This is especially true for direct-phase cooling, such as direct-fluid cooling wherein the phase-changing fluid is applied directly to the surface of heat-generating devices and thus highly dependent on surface conditions of such devices. Thus, such a cooling method is forced to operate at lower performance to provide stability. 
     Accordingly, it is beneficial to have the ability to properly manage a direct-phase cooling method, such as a direct-fluid cooling method, for high heat dissipation in a heat-generating device so as to provide optimum performance and surface thermal conditions for such a device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments are illustrated by way of example and not limited in the following figure(s), in which like numerals indicate like elements, in which: 
         FIG. 1  illustrates one view of a layout of a cooling system for cooling a heat-generating device, in accordance with one embodiment. 
         FIG. 2  illustrates another view of a layout of a cooling system for cooling a heat-generating device, in accordance with one embodiment. 
         FIG. 3  illustrates a controller for controlling a cooling system in accordance with one embodiment. 
         FIG. 4  illustrates a method for thermal management of a heat-generating device in a cooling system, in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     For simplicity and illustrative purposes, the principles of the embodiments are described by referring mainly to examples thereof. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments. It will be apparent however, to one of ordinary skill in the art, that the embodiments may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the embodiments. 
       FIGS. 1-2  illustrate a layout of a spray-cooling system  100  for cooling a heat-generating device, in accordance with one embodiment. It should be understood that the following description of the system  100  is but one manner of a variety of different manners in which such a spray-cooling system may be configured. The spray-cooling system  100  is an example of a direct-fluid cooling system, for which one or more embodiments for cooling management as described herein may be applied. Thus, it should be understood that discussions of such cooling management embodiments are also applicable to other direct-fluid cooling systems, such as micro-channeling cooling systems, spray-cooling methods, and jet impingement methods. 
     Referring to  FIG. 1 , the cooling system  100  is configured for cooling one or more heat-generating devices, such as a semiconductor chip  101 . The cooling system  100  includes a body forming a cooling cap  103 , and one or more spray mechanisms  105 . The cap  103  and spray mechanisms  105  are, for example, integrated into a single cooling assembly. The cap  103  is configured to form a spray chamber  107  in which cooling fluid can be sprayed into thermal contact with, and preferably directly onto, the chip  101 . A grid of temperature sensors (not shown), such as thermocouple devices or capacitance sensors, may be placed on the top surface of the chip  101  and embedded within the chip  101  to measure temperature. The heat rate is measured using temperature gradients across the surface and beneath the surface (with embedded sensors). Temperature and heat rate are then received, calculated, and used by a controller (not shown) to control the rate of fluid delivery to each region of the thermally-managed surface of the chip  101 . The spray mechanism includes an inlet  109  for receiving cooling fluid, preferably in a liquid state. An outlet  111  for liquid and/or gaseous cooling fluid operably extends from the spray chamber. 
     In one example, the cap  103  includes a concave portion having surfaces  113  that form a cavity configured to conformingly adjoin to one or more surfaces of a component substrate  115  that is both electrically connected to and carrying the chip  101 . Here, the component substrate and chips form a package with which the cap is associated. The package is preferably in the conventional form of an integrated circuit component that has not received an encapsulant or lid. When the cavity of the cap  103  conformingly adjoins the component substrate  115 , the spray chamber  107  is formed, containing at least the portions of the semiconductor devices to be spray cooled. The cap  103  is preferably made of a material having a thermal coefficient of expansion substantially matching that of the component substrate. In one example, the cap  103  is retained against the component substrate  115  by an adhesive, a clamping mechanism, fasteners, or other attachment-type mechanisms, and a seal is formed such that liquid and vapor cooling fluid do not escape the spray chamber other than through designed orifices. The component substrate  115  is preferably configured to be vapor and liquid tight, but could be configured with a designated orifice, such as an outlet. The cap  103  forms a package-level cooling system to be affixed to the package and thereby form a cooled package. 
     The spray mechanism  105  is configured to spray cooling fluid onto the one or more chips  101 , which heat and vaporize some or all of the cooling fluid. The cooling fluid vapor that forms during the cooling process is retained in an enclosed spray chamber around the chips. In one example, the spray mechanism  105  is an incremental sprayer configured to eject an incremental amount of the cooling fluid on the chip  101 . The cooling fluid is typically sprayed in response to energizing control signals received via electrical contacts  121 , which are mounted on the cap  103  and provide inputs to the spray mechanism  105 . In one example, the control signals are sent to the spray mechanism  105  by the same controller noted earlier, which receives feedbacks from the temperature sensors mounted on or within the chip  101 . The quantity of liquid sprayed from an incremental sprayer may be highly controllable, such as by controlling the rate at which incremental amounts of cooling fluid are ejected. For example, by increasing or decreasing the frequency that an incremental sprayer is energized, the flow rate can be accurately adjusted. Additionally, because such a sprayer can be configured to deliver very small quantities of cooling fluid, and because a large number of sprayers can be fit into a small area, the heat distribution over that area can be accurately controlled by energizing some of the sprayers at a rate greater than that of other sprayers. These features provide for accurate delivery of cooling fluid at precise and controllable rates. Furthermore, incremental sprayers can be modular, offering quickly and easily replaceable units. An example of a type of incremental sprayer for the spray mechanism  105  is an inkjet-type sprayer, such as a thermal inkjet sprayer (TIJ sprayer). 
     Although a cooling fluid is used to describe the cooling system  100 , it should be understood that the cooling fluid may be replaced with any cooling phase change material (PCM) that is capable of deforming and spreading out across a surface of the heat-generating device in a manner similar to a fluid to absorb heat from such a device. As referred herein, a PCM is configured to change phase, for example, from a liquid to a gas at a predetermined temperature. Thus, a cooling PCM generally operates to dissipate heat by changing its phase to absorb the heat. 
     The operation of cooling system  100  is controlled via a software or hardware controller, as noted earlier. The controller can be separate from the cooling system  100 , whereby both may act as subsystems that may be combined together to form an overall integrated cooling system. A single controller may control the operation of one or a plurality of cooling systems  100 . Alternatively, the controller can be mechanical in nature, or incorporated within a chip that is being cooled by the system.  FIG. 3  illustrates an example of a controller  300  that is operable to control the operation of a cooling system  100 . It should be understood that the following description of the controller  300  is but one manner of a variety of different manners in which such a spray-cooling system may be configured. 
     The controller  300  provides an input section that includes an analog multiplexer (mux)  310 , a mux selector  312 , and a thermocouple converter  314  for receiving temperature measurements from the temperature sensors mounted on and within the chip  101 . An integrated circuit (IC)  318  acts as a processing unit to execute software or firmware stored therein (e.g., in a computer readable medium such as a read-only-memory or ROM chip or the like) to control the input section to select and receive temperature measurements, such as thermocouple measurements, from the different temperature sensors, such as thermocouples, mounted to the chip  101  in a manner understood in the art. The IC  318  may be an application specific integrated circuit (ASIC) or a general-purpose computer processor, such as one manufactured by Intel, AMD, or Cyrix. The IC  318  also executes software or firmware stored therein to implement a digital closed-loop or feedback control algorithm to achieve stable high heat flux and optimum surface thermal conditions of the chip  101 . Based on the implemented control algorithm, the IC  318  calculates and outputs the necessary parameters to an output section of the controller  300  so as to control the operations of the spray mechanism  105  in the cooling system  100 . In one example, the controller&#39;s output section includes an oscillator clock  320 , a multipurpose analog and digital (A/D) IC  330 , a power buffer  334 , an analog flip/flop buffer  236 , and a Darlington transistor  338 . The ND IC  330 , such as a PIC microcontroller from Microchip Technology of Chandler, Ariz., includes analog and digital channels to provide multipurpose functions. Here, the ND IC  330  used the calculated parameters provided by the main controller IC  318  to calculate or compute the actual frequency or delays so as to output energizing control signals (in the form of low level commands through the channels on the IC  330 ) to switch the firing pattern of the spray mechanism(s)  105  (represented by the TIJ atomizer output), in a manner understood in the art for spray cooling, at which fixed incremental amounts of cooling fluid are ejected from the spray mechanism  105  onto the chip  101 . As illustrated, the controller  300  further includes interfaces, such as the transistor-transistor logic (TTL) to RS-232 terminals  316  and  332 , throughout the controller to allow connection of external devices (such as computers) thereto. These interfaces enable information about the controller  300  at various stages to be passed on to external devices as desired by a user for any purposes, such as diagnostics or maintenance of controller  300 , the cooling system  100 , or both. 
       FIG. 4  illustrates a thermal management method  400  for a thermal-management controller, such as the controller  300 , to implement a closed-loop control of the cooling system  100  to control the fluid (or other PCM) delivery to the surface of a heat-generating device, such as the semiconductor chip  101 , to ensure optimum fluid film conditions and achieve stable high heat flux and optimum surface thermal conditions of the device. This method  400  continuously run during the operation of the cooling system  100 . For illustrative purposes only and not to be limiting thereof, the method  400  is discussed in the context of the controller  300  ( FIG. 3 ) and the cooling system  100  ( FIGS. 1-2 ). 
     At  410 , the method  400  starts by setting a reference surface temperature, T REF  for the chip  101  in the IC  218  of the controller  300 . T REF  may be set by IC  218  as desired by the chip designer or any interested system user based on, for example, design specifications of the chip  101 , including the processing speed and power consumption of the chip  101  and corresponding surface temperature range that would maintain the chip  101  in operation. This step is performed prior to the operation of the controller  300 . 
     At  412 , additional constants are also set or initialized for use with a desired closed-loop control algorithm implemented in the IC  218 . For example, if the desired closed-loop control algorithm includes a proportional/integral/derivative (PID) type linear controller, the constants K P , K I , and/or K D  are provided by the chip designer or any interested system user based on the desired control algorithm. Other set constants include the tolerance range or buffer for dissipated heat from the chip  101 , the maximum and minimum frequencies (or rates), f MAX  and f MIN , at which fixed incremental amounts of cooling fluid are to be ejected from the spray mechanism  105 , the incremental amount df for increasing the frequency or rate of fluid ejection from the spray mechanism  105 , and the max and minimum temperatures, T MAX  and T MIN  that may be used to filter the temperatures detected from the chip  101 . 
     At  414 , the controller  300  measures the temperature of the chip  101  continuously by receiving temperature measurements from temperature sensors on and within the chip  101 , and averaging (or filtering with T MAX  and T MIN ) such temperature measurements to come up with the surface temperature T M  of the chip  101  at each particular measurement time instance and the temperature gradient ΔT of the chip  101 . ΔT serves a measurement proxy for the heat flux in the chip  101 . In one example, the temperature gradient ΔT provides a difference in temperatures at different locations of the chip  101  at each particular measurement time instance, such as on the chip surface and within the chip to provide a temperature gradient substantially perpendicular or orthogonal to the chip surface. 
     At  416 , once the IC  218  receives (and calculates as needed) the measured T M  and ΔT, it proceeds to digitally implement the desired closed-loop control algorithm (i.e., discretized control algorithm) to provide thermal management of the chip  101 . Any suitable closed-loop control algorithm that employs T REF  as an input and T M  as a feedback may be used here, including linear control algorithms such as PID control, non-linear control algorithms such as non-linear optimal controls, and non-classical algorithms such as neural networks and fuzzy logic.  FIG. 4  illustrates the closed-loop control algorithm used in step  416  as a discretized PID control algorithm. Based on the implementation of the closed-loop control algorithm, the controller  300  outputs a discretized or digital energizing control signal f(k), where k represents the digital sampling, to the spray mechanism  105  to indicate the desired frequency at which the incremental amounts of cooling fluid are to be ejected from the spray mechanism  105  for application to the surface of the chip  101 . 
     At  418  and  420 , the output control frequency f(k) is compared against the predefined f MAX  and f MIN . 
     At  422 , if f(k)&gt;f MAX , then f(k) is set to f MAX . At  424 , if f(k)&lt;f MIN , then f(k) is set to f MIN . Of course, if f(k) is within the operating range between f MAX  and f MIN , it is passed through. 
     At  426 , the value f(k) is used to compute or calculate the volumetric flux, or rate of volume flow across the chip  101 , of the cooling fluid. In one example, the volumetric flux, or the rate of volume flow across a unit area, is computed based on past empirical data obtained on the cooling system  100  and its spray mechanism(s)  105  that provides a look-up table of how much fluid the spray mechanism  105  is able to deliver at specific frequencies f(k) (with interpolation and extrapolation of available look-up values as needed). Also, from the measured ΔT, the dissipated heat q of the chip  101  is computed or calculated, for example, from the one-dimensional heat conduction Fourier&#39;s law of the form, 
               q   =       -   K     ⁢       Δ   ⁢           ⁢   T       d   ⁢           ⁢   x           ,         
where q is the dissipated heat (units W/m2), K is the thermal conductivity of the hot body (the body being cooled) (units W/(m*K), i.e., the chip  101 , and dx=x 2 −x 1  is the distance between the two temperature sensors (units m) that provides the measured ΔT.
 
     At  428 , once the volumetric flux is know, the possible critical heat flux (CHF) with this volumetric flux is estimated or derived by using empirical or experimental data previously obtained (e.g., stored in the IC  218  as a lookup table). The CHF describes a local maximum heat dissipation level within a reasonable excess temperature range, beyond which the efficiency of heat transfer is decreased, thus causing localized overheating of the heating surface. 
     At  430 , the computed dissipated heat q plus some tolerance range or buffer (previously set at  412 ), i.e., q+buffer, is compared against the estimated CHF. 
     At  432 , if the computed dissipated heat q plus some tolerance range or buffer (previously set at  412 ), i.e., q+buffer, is less than the estimated CHF, then f(k) is passed through as F(k) and converted to an analog command signal for output at  436  to the spray mechanism  105  in the cooling system  100  to control its rate of fluid ejection. 
     At  434 , however, if q+buffer is equal to or larger than the estimated CHF, then the control frequency f(k) is increased by a predefined df (also previously set at  412 ), or F(k)=f(k)+df, and F(k) is passed through and converted to an analog command signal for output at  436  to the spray mechanism  105  in the cooling system  100  to control its rate of fluid ejection. 
     Accordingly, the systems and methods as described herein combine surface temperature and heat flux monitoring for precise thermal management. That is, any degradation of the cooling process relating to surface conditions (e.g., conditions of the cooling liquid film on the chip  101 ), fluid supply (e.g., from the cooling system  100  and its spray mechanism  105  therein), or any other factors may be captured by the monitoring schemes described above (e.g., using measured temperature gradient as a proxy for the heat flux) to thereby enable the control schemes as also described above to adapt accordingly. Furthermore, although embodiments have been described in the context of multi-phase cooling, such embodiments may be used for single or multi-phase thermal management solutions to improve the performance of the thermal management solution by increasing the heat transfer performance. 
     What has been described and illustrated herein is an embodiment along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the subject matter, which is intended to be defined by the following claims—and their equivalents—in which all terms are meant in their broadest reasonable sense unless otherwise indicated.