Abstract:
A computer system using 3D IC is cooled by using liquid coolants such as water, oil, and ionic liquid. Liquid coolant flows in a closed coolant conduit which is configured thermally to contact heat-generating components and a liquid-liquid heat exchanger. The heat generated in 3D IC chips is carried out by liquid coolant and dissipated to heat exchanger where cooling water dissipates heat to large water body. For economic stable operation, cooling water is pumped from large water body such as river to a water tower where water level is kept constant to ensure heat exchanger work at optimal condition. The simple approach for computer system cooling provided in this disclosure is a cost-effective data center efficiency solution.

Description:
FIELD 
     The embodiment of present invention is generally related to liquid cooling system for heat-generating components of computers. More specifically, the present invention relates liquid cooling system in computer systems having stacked three-dimensional (3D) integrated circuit (IC) used in data center. 
     BACKGROUND 
     Since it was invented in 1958, IC has been scaled to improve the performance. However, after the turn of the century, scaling resulted in short-channel effect and memory wall, which requires other approach to improve the IC performance more effectively. 
     3D IC offers a reasonable route to further improve IC performance. It improves IC performance by increasing device density, reducing the interconnect delay, and breaking memory wall with the application of 3D stacking using through silicon via (TSV). 3D IC also makes one chip package have more functional diversification than those enhanced only by shrinking the size of the features. The main advantages of 3D IC are the smaller form factor, low energy consumption, high speed, and functional diversification. It is predicted that 3D IC will be an enabler for improvement of data center performance and efficiency with positive consequences for global energy consumption and environment. 
     One of the biggest challenges in 3D IC stacking technology using TSV is thermal management owing to the high heat flux up to about 200 watts per square centimeter. Disclosure of this invention will provide a solution of 3D IC thermal management. 
     Data centers for internet and mobile devices are the most critical components in our information age. They serve industries, civil communications, military and defense applications, and transportations. Data centers consist of multiple computers usually called servers and switches. Both of them use many ICs. When a computer works, ICs will change status, or change the on-and-off status, which consumes electricity and generates significant heat. Even when computer system is at idle condition, it still consumes electricity due to the current leakage and circuit requirement. 
     Multiple servers are accommodated in a server rack at data center. Each computer consumes significant electricity. It is common for a server (computer) to consume over a hundred watts. In a server rack, i.e. a module of servers, there are multiple computers. Similarly, there are many server racks in a data center. Therefore, a data center consumes large amount of electricity and a large data center consumes the same amount of electricity as a small or medium size town. Among the contributions to the electricity consumption, most electricity is consumed by servers and their cooling systems. It is quite often that cooling system uses the same amount of electricity as the server computers. It is estimated that the date centers consume about two percent of total electricity generated worldwide. 
     Power usage effectiveness (PUE) is usually used to measure the efficiency of a data center. It is defined as a ratio of total energy used by facility to that used by information technology (IT) equipment. An ideal PUE is 1.0, but average PUE worldwide now is about 2.0 although some data center claims their PUE is significantly below 2.0. The average PUE value of 2.0 indicates the necessity to improve the data center cooling effectiveness. One approach to improve the cooling efficiency is to use water cooling to replace current air cooling. In the past, water cooling was used for large scale computers, but did not obtain large scale application for personal computers or servers in data center because of its limitation by the shape of heat-generating components and thus the complexity. 
     As the dimensions of integrated circuit components decrease, more components are compacted in a given area of a semiconductor integrated circuit (for example of 3D IC). Accordingly, more transistors are held on a given area and thus more heat is generated in the same area. In order to keep the IC temperature in allowed range for proper performance, heat generated has to be transferred out of integrated circuit effectively and economically. With the internet getting popular, more and more servers are installed and in service to support the internet function. The trend of applications of more mobile devices and cloudy technology will drive more electricity consumption at data centers in the future. 
     Current servers are located in an air-conditioner-regulated environment, usually in a specially designed building. The heat generated by microprocessors, memory chips, and power supply chips is released locally, which is like a large heater in a room cooled by air conditioner. Due to the low efficiency of air conditioner, the cooling system uses lots of electricity, occupies large footprints, and causes high costs. 
     Accordingly, it is very significant to provide an effective method to reduce cooling power and improve cooling efficiency for computer system, especially for the system with large number of computers such as data center. Cooling technology now becomes an enabler to improve data center efficiency. 
     Improving cooling system in data center not only saves energy consumption, but also benefits ecological and environmental systems. A few percent reduction of electricity consumption in data center cooling system will significantly decrease the emission of carbon dioxide amount, which equivalents to shut down multiple coal power plants with environmental benefit in addition to the cost reduction. 
     The heat generated in electronic devices in a data center has to be transferred outside the accommodating construction and dissipated to environment, which consumes tremendous electricity. In order to prevent the overheat of ICs, the surface of the ICs should be kept not very high, which means the temperature difference between high temperature source (IC surface) and low temperature environment will be significant low, resulting in the challenge in engineering realization and high costs in cooling system. 
     Traditionally, heat-generating components in computers are cooled by cold air supplied by air-conditioners. The air in server&#39;s building exchanges and dissipates heat on chiller&#39;s cold surface. By applying work, air conditioners transfer heat from a cold surface to a hot surface, and thus heat is dissipated to air outside the building by heat exchanging. This cooling method is accompanied with uses of lots of compressors and fans, and thus consumes significant electricity because of the low efficiency and high costs for air conditioning system. 
     In order to lower the cost of using air conditioner, cold air is used to directly cool the heat generating components in winter at north areas. However, the air humanity has to be controlled well and the application is also limited by weather and season. 
     Similarly, lots of power is used by fans in the server rack to dissipate heat from component surface to air by blowing air through the server rack, which also consumes significant energy, makes noise, and has low efficiency. 
     In order to overcome low efficient challenge in air cooling problems, water is used for cooling the heat-generating components. Current heat-generating components are mainly microprocessor unit (MPU), dynamic random-access memory (DRAM), and power chips. Microprocessor has a flat shape and it is relatively easy to use liquid cooling on a flat surface. However, it is difficult to use liquid cooling on DRAM dual in-line memory module (DIMM) due to the irregular shape although some attempts were tried. 
     In order to overcome the intrinsic problem mentioned above, liquid cooling was used by circulating liquid coolant on the surface of ICs to improve the efficiency. However, this method has to use chillers to cool the liquid, resulting in a low cooling efficiency. 
     In order to use natural water body for data center cooling, air cooling of server rack was combined with heat dissipation to large natural water bodies such as ocean, river, and lake. This approach may be the lowest data center operating cost and has the best potential for future application. However, there are lots of challenges for the realization of this method. Therefore, some novel method is disclosed in this invention for improving server cooling and data center efficiency. 
     SUMMARY 
     Methods for cooling a computer system using 3D ICs are provided herein. In some embodiments, a method of cooling multiple-computer systems includes: (a) circulating a first liquid coolant to dissipate heat from 3D IC heat-generating components such as microprocessors, memory chips, and power chips to the first liquid coolant; (b) heat-dissipating from the first coolant to a large water body such as river, reservoir, and ocean. 
     In one embodiment, there are a first coolant supply conduit and a first coolant return conduit on both sides of the IC package, the former supplies the first coolant to heat-generating components in servers, and the latter carries the heated first coolant out of heat-generating components in servers for heat exchange and thus dissipates heat to a second coolant in the heat exchanger so that the first coolant can be reused by circulation in a closed loop. 
     The most important thing for a reliable cooling performance is to keep the flow rate controllable in the cooling conduit on the heat-generating components. This is enabled by controlling the pressure in the supply conduit by using an in-line pump, large ratio of cross-sectional area of supply conduit to the sum of cooling conduit cross-sectional areas on the heat-generating components. The large cross-sectional area of supply conduit determines the constant pressure of first liquid coolant and then the controllable flow rates in cooling conduit on each heat-generating component, and then reliable cooling performance on every heat-generating component. 
     In one embodiment, liquid-liquid heat exchanger is used to dissipate heat finally to large water body. The water from large water body as a second liquid coolant needs to be pretreatment before used for cooling such as filtration to remove particles. After the pretreatment, the second coolant from the large water body will be pumped to a water tower where water surface level is maintained constant so that the water pressure on the outlet is kept constant, resulting in a constant delivery water pressure. After the second liquid coolant is used in heat exchanger, the only change is the little rise in temperature such as a few degrees. This discharge water is environmentally benign so that it can be returned to the large water body. For cooling performance controlling, valves are used on the conduit of the second liquid coolant so that the flow rate can be effectively controlled. For automatic control of the cooling performance, temperature sensors are disposed on the conduit of the second liquid coolant to feedback data for controlling the opening of the valves. 
     In winter season of north area, temperature is so low that water in the large water body may freeze. In order to avoid possible damage on conduits caused by freezing, the conduits of the second liquid coolant should have good protection such as underground arrangement. Such ideas are also applicable to other related parts like pumps. 
     Sucking of water by pump from the large water body is impacted by the water level elevation, especially when the large water body is a river. Special caution should be paid for adjustment of the relative conduit location and prevention of freeze in winter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  depict one embodiment of computer cooling system having 3D ICs in accordance with one embodiment of the invention; 
         FIG. 2  depicts a schematic view of a 3D IC cooling method that may be utilized to cool the computer in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention generally provide apparatus and methods for removing heat from a 3D IC computer system. Particularly, embodiments of the present invention provide methods and apparatus for removing heat from the 3D IC package directly in the computer system. In one embodiment, a cooling liquid is disposed contacting to the heat-generating 3D IC components. The heat is carried out of the electronic device by cooling liquid and dissipated to a large water body such as river, reservoir, or ocean. 
       FIG. 1  schematically illustrates a cooling system  100  in accordance with one embodiment of the present invention. The cooling system  100  generally comprises a building  102  configured to accommodate computers. The cooling system  100  further comprises a river  130  in connection with the building  102  via a cooling water tower  132 , liquid-liquid heat exchanger  142 , cooling water conduit  152 , drain conduit  126 , pump outlet conduit  144 , and pump inlet conduit  146 . 
     The building  102  generally comprises a left sidewall  104 , a front sidewall  106 , a right sidewall  108 , back sidewall  110 , and roof  140 . In one embodiment, the building  102  comprises first floor  134  and second floor  136 . 
     The cooling system  100  comprises server rack  116  and server rack  118  on first floor  134 . The cooling system  100  also includes server rack  112  and server rack  114  on second floor  136 . A server rack usually accommodates multiple servers. In one embodiment, server rack  114  accommodates server  120  and server  122 . 
     The cooling system  100  is configured to position a cooling liquid supply conduit  148  to flow cooling liquid  138  into server  120  and carry heat out of server  120  by flowing cooling liquid  138  out of server  120  in return conduit  150 . The cooling liquid supply conduit  148  and return conduit  150  are connected to a liquid-liquid heat exchanger  142 . The chip contact details will be further described below with references in  FIG. 2 . The heat exchanger  142  dissipates heat in the cooling liquid  138  to cooling water  154 . In one embodiment, one end of the liquid-liquid heat exchanger  142  is configured to be connected with cooling water tower  132  for taking cooling water  154  and the other end is connected to river for draining cooling water  154 . 
     During cooling process, the supply conduit  148  has a higher pressure compared with return conduit  150  to ensure the flow rate for cooling performance. The cooling liquid  138  in the supply conduit  148  has a lower temperature than the cooling liquid  138  in return conduit  150 . The cooling liquid  138  in return conduit  150  transfers heat out of server  120  to cooling water  154  in liquid-liquid heat exchanger  142 . During the cooling liquid  138  flowing through heat exchanger  142 , temperature of cooling liquid  138  keeps falling, and attains such a low temperature when flowing out of the heat exchanger  142  that the temperature meets the requirement for flowing into heat-generating components in server  120 . 
     The heat exchanger  142  can be configured for cooling of one server, or one server rack, or multiple server racks. When heat exchanger  142  is used for cooling of multiple servers, the constant pressures in supply conduit  148  and return conduit  150  should be kept well. The cooling liquid  138  should be stable and bubbles are not allowed in order to ensure the quality of cooling and heat exchanging. 
     The liquid-liquid heat exchanger  142  may have high heat exchange efficiency due to the high density of liquid. The temperature difference between supply conduit  148  and return conduit  150  is low to avoid high temperature variation in heat-generating components in computer system. Typical temperature difference between these two conduits is 10-30° C. The circulation of cooling liquid  138  is driven by a pump  156  in order to have acceptable heat exchanging rate on the surface of heat-exchanging components. 
     During cooling processing of one embodiment, cooling water  154  is sucked from the river  130 . For data center located in north cold area, the pump inlet conduit  146  should be well protected from freezing because it may damage the pipe system. In one embodiment, the pump inlet conduit  146  is laid underground to avoid freezing in winter. Similarly, pump  124 , tower  132 , conduits  144 ,  152 , and  126  should be protected well during winter for data center located in north area. 
     According to one embodiment of the invention, the elevation of cooling water  154  in cooling tower  132  should be automatically controlled the same all the time. This can be controlled by a continuous operation mode of cooling water pump  124 , or non-continuous operation mode, depending on the design. After data center facility is in operation, the cooling water flow rate is mainly determined by water level of the cooling water  154  in cooling water tower  132 . In one embodiment, a regulating valve  158  is used to adjust the flow rate of cooling water  154  in the liquid-liquid heat exchanger  142  by varying the opening. 
     In one embodiment, a grate and filter is used at one end of cooling water inlet conduit  146  to keep the contaminants out of the cooling system. In addition, the elevation of one end of cooling water conduit  146  for sucking water in the river  130  should be adjusted according to the level of river, especially in the north area where river water level changes with season significantly. 
     For convenience of operation, the building  102  should be located close to the river  130  to reduce the length of the conduits. To ensure the performance of cooling system  100 , the river current  128  should be high enough for cooling of a data center. Generally, the river stream  128  should have a discharge of 40 m 3 /s or higher for cooling of a large data center. 
     In one embodiment, the cooling liquid  138  is deionized water. In another embodiment, the cooling liquid  138  is oil or ionic liquid. 
       FIG. 2  schematically illustrates an enlarged view of the server  220  disposed in the server rack  114  of  FIG. 1 . The server  220  includes a microprocessor  222 , a memory  224 , a microprocessor liquid conduit  254  configured to flow a first liquid coolant  238  thermally contacting with the microprocessor  222 , a memory liquid conduit  252  configured to flow the first liquid coolant  238  thermally contacting the memory  224 . 
     The microprocessor  222  has a front side  246 , a back side  244 , a device layer  232 , a silicon layer  230 , and a plural of through silicon via (TSV)  234 . Electricity is mostly consumed at device layer, so that this layer becomes the main heat-generating component. In one embodiment, the first liquid coolant  238  flows in the microprocessor conduit  254  for carry out heat from the microprocessor  222 . 
     In one embodiment, the memory  224  has a memory front side  240 , a memory back side  242 , a memory device layer  226 , a memory silicon layer  228 , and a plural of memory TSV  236 . The memory back side  242  is bonded with the microprocessor back side  244  for mechanical and electric interconnection between the microprocessor  222  and the memory  224 . 
     The cross-sectional areas of liquid conduits impact cooling effectiveness. In one embodiment, the cross-sectional areas of the supply conduit  248  and the return conduit  250  are significantly larger than those of the MPU liquid conduit  254  and the memory liquid conduit  252 . 
     During cooling processing, the cooling liquid  238  is circulated in a closed loop shown in  FIG. 1 . Liquid conduits shown in  FIG. 2  are part of the total closed loop. In order to have effective heat exchanges between heat-generating components and the cooling liquid  238 , moderate flow rate in heat-generating components should be kept. Generally, the turbulent flow in MPU liquid conduit  254  and the memory liquid conduit  252  should be maintained. The pump  156  shown in  FIG. 1  drives the flow rate and ensures the effectiveness of heat dissipation. 
     Heat dissipation makes temperature in the return conduit  250  higher than that in the supply conduit  248 . The higher temperature difference between these two conduits means more energy carried out at a same flow rate. However, low temperature difference should be kept in order to have a more uniform temperature on the heat-generating components. The non-uniformity of temperature may introduce extra stress, resulting in reliability issues. Typical temperature difference between the supply conduit  248  and return conduit  250  is about 20° C. 
     MPUs consume most power in a computer system. Effective contact between the MPU liquid conduit  254  and the MPU  222  is the key to cool the MPU. The plane ship of the MPU  222  generally makes the realization of thermal contact easy. However, common memory is packaged in single in-line memory module (SIMM) or dual in-line memory module (DIMM), which has a non-plane shape, resulting in challenges in thermal contact effectiveness. 
     Recently, 3D IC stacked by using TSV provides effective way to make DRAM package have a plane geometry. In one embodiment of this disclosure, stacked DRAM as the memory  224  is used for the server  220 . Therefore, the memory  224  has a plane for obtaining effective thermal contact with the cooling liquid  238 . 
     In one embodiment, heat-generating components are modules, but there are some passive components which release small amount of heat. For dissipating this heat, a cooling conduit may be thermally contacted with the motherboard or interposer to dissipate heat. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.