Abstract:
An apparatus for cooling selected elements within an integrated circuit, such as active transistors or passive circuit elements used in a radio frequency integrated circuit is provided. In one embodiment, the cooling apparatus includes a cold plate thermally coupled to the region proximate the integrated circuit element, a thermoelectric cooler thermally coupled to the cold plate; and a hot plate thermally coupled to the thermoelectric cooler. Heat is removed from the integrated circuit element through the cold plate and transmitted to the hot plate through the thermoelectric cooler. In one form, the hot plate is located or coupled to an exterior surface of an integrated circuit, such that heat transmitted to the ambient from the integrated circuit element is dissipated into the atmosphere surrounding the integrated circuit. In another form, the hot plate is embedded in the integrated circuit substrate to locally cool elements of the integrated circuit while dumping the heat into the substrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention is generally related to the field of integrated circuits and, more particularly, to a method and apparatus for cooling integrated circuits. 
     2. Description of Related Art 
     The use of radio frequencies (RF) and microwave frequencies have been utilized for most of the 20 th  century to provide communications. Early uses of RF and microwave technologies involved radio communications, both broadcast and two-way communication, and radar for detecting incoming aircraft. Much of this early technology was developed the 1940&#39;s to help in fighting World War II. 
     After the war, RF and microwave technologies were extended into other communication areas. Telephone companies used microwave technologies to carry voice communications across areas in which it was impractical to build transmission lines, such as, for example, in vary mountainous terrain. RF frequencies were also used by the emerging television industry to carry television broadcasts to peoples&#39; homes where their television sets received the broadcast signal. 
     More recently, RF transmissions have been used to carry satellite signals, both for military and commercial use as well as, more recently, for delivering television content to subscriber&#39;s homes as well as access to the Internet. RF and microwave frequencies are also used to provide wireless (cellular) telephone services, these services include analog, digital and personal communication services (PCS). 
     The transmission capacity of an electronic communications through RF transmissions is determined by the range of the frequency signals (bandwidth), and the number of channels in the bandwidth. It is expressed in bits per second, bytes per second or in Hertz (cycles per second). As more and more information is being transmitted through RF circuits, a need for greater bandwidth has developed to handle this increase in information transmittal. However, the bandwidths and channel capacity of RF, cellular, and microwave systems are limited by the signal-to-noise (S/N) ratios of the amplification and filtering process within the system. One important method to increase the S/N ratios is to reduce the thermal noise by lowering the operating temperature of the circuits. Therefore, it would be desirable to have an apparatus, system, and method for cooling RF circuits such that the bandwidths and channel capacity of the RF circuits could be increased. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus for cooling an integrated circuit component, such as a field effect transistor circuit used in a radio frequency transistor or receiver. In one embodiment, the cooling apparatus includes a cold plate thermally coupled to the integrated circuit component, a thermoelectric cooler thermally coupled to the cold plate; and a hot plate thermally coupled to the thermoelectric cooler. Heat is removed from the integrated circuit component through the cold plate and transmitted to the hot plate through the thermoelectric cooler. The hot plate is located at a surface of an integrated circuit such that heat transmitted to it from the integrated circuit component is dissipated into the atmosphere surrounding the integrated circuit chip. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIGS. 1A-1E depict circuit diagrams of examples of typical radio frequency (RF) circuits that benefit from cool operation; 
     FIG. 2 depicts a graph of a typical temperature dependency of the quality factor of on-chip spiral inductors; 
     FIG. 3 depicts a high-level block diagram of a Thermoelectric Cooling (TEC) device in accordance with the present invention; 
     FIG. 4 depicts a top planar view of direct coupled coolers for cooling IC RF circuits in accordance with the present invention; 
     FIG. 5 depicts a current-controlled thermoelectric cooler (TEC) circuit in accordance with the present invention; 
     FIGS. 6A-6B depict top cut-away planar and cross-sectional views of a patterned cold plate for cooling RF IC circuits in accordance with the present invention; 
     FIGS. 7A-7B depicts top cut-away planar and cross-sectional views illustrating direct thermal coupling of a cooler with the LNA/PA and body/substrate levels of an integrated circuit (IC) in accordance with the present invention; 
     FIG. 8 depicts a cross sectional view of an exemplary thermoelectric spot cooler fabricated over an RF CMOS IC in accordance with the present invention; and 
     FIG. 9 depicts a cross sectional view of an exemplary RF spiral inductor circuit wherein the thermoelectric cooler is incorporated in the passive inductor and the heat is rejected into the bulk substrate in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference now to the figures and, in particular, with reference to FIGS. 1A-1E, circuit diagrams of examples of typical radio frequency (RF) circuits that benefit from cool operation are depicted. FIG. 1A depicts an example of a passive antenna system. FIG. 1B depicts an example of input low noise amplifiers (LNAs). FIG. 1C depicts an example of the mixer stages in an RF circuit. FIG. 1D depicts an example of a quadrature oscillator. FIG. 1E depicts an example of a power amplifier (PA) at the output. The channel selectivity of these circuits and the filters employed in the signal path are determined by the quality factor of the passive inductors and capacitors, and the thermal noise voltages in the transistors. Both the quality factor and thermal noise voltages are strongly dependent on the operating temperature. 
     With reference now to FIG. 2, a graph of a typical temperature dependency of the quality factor of on-chip spiral inductors is depicted. The graph depicted in FIG. 2 relates the quality factor of a 150×150 μm 2  3.1 nanoHenry (nH) spiral inductor coil implemented in the clock generator of a CMOS test chip versus the frequency of operation in gigahertz. (GHz) for three temperatures. As shown in FIG. 2, the quality factor for the spiral inductor coil rises continuously as the temperature of the inductor is decreased for all frequencies of operation. For an inductor temperature of 100 degrees Celsius, the quality factor of the inductor coils is approximately in the range of 2-3 over the frequency range of 1.0 to 10.0 GHz. As the temperature of the inductor coils is decreased to 25 degrees Celsius, the quality factor increases to approximately 5.0 for the same frequency range. As the temperature of the inductor coils is further decreased to −123 degrees Celsius, the quality factor increases even further to approximately 15.0 to 18.0 over the same range of frequencies. Thus, a significant benefit is achieved by reducing the operating temperature of the inductive coils. Similar benefits in temperature reduction or achieved with other RF circuits. 
     The phase noise, L, of the oscillators are also directly affected by the operating temperature of the circuit. The temperature dependence of the phase noise of the oscillators are given by the following equation:          L                   {     Δ                 ω     }       =       k                   T   ·   R   ·   F   ·       (       ω   0       Δ                 ω       )     2           P   signal                              
     where 
     R=effective resistance of the (LC) tank (temperature dependent) 
     ω 0 =center frequency of oscillation 
     Δω=frequency offset 
     F=term related to noise from active devices P signal =power level of oscillation 
     T=the absolute operating temperature in kelvins 
     From this equation, it is evident that phase noise increases as the temperature of the oscillators increase. Therefore, it is beneficial to have an oscillator operating at lower temperatures to decrease the amount of phase noise. 
     With reference to FIG. 3, a high-level block diagram of a Thermoelectric Cooling (TEC) device  300  is depicted in accordance with the present invention. TEC device  300  is preferably connected to the integrated circuit device near the temperature sensitive element. Thermoelectric cooling, a well known principle, is based on the Peltier Effect, by which DC current from power source  302  is applied across two dissimilar materials causing heat to be absorbed at the junction of the two dissimilar materials. A typical thermoelectric cooling device utilizes p-type semiconductor  304  and n-type semiconductor  306  sandwiched between poor electrical conductors  308  that have good heat conducting properties. 
     As electrons move from p-type semiconductor  304  to n-type semiconductor  306  via electrical conductor  310 , the energy state of the electrons is raised due to heat energy absorbed from heat source  312 . This process has the effect of transferring heat energy from heat source  312  via electron flow through p-type semiconductor  304  and electrical conductor  310  to heat sink  316 . The electrons drop to a lower energy state in the electrical conductor  310  and release the heat energy. 
     With reference now to FIG. 4, a top planar view of direct coupled coolers for cooling IC RF circuits is depicted in accordance with the present invention. Integrated circuit  400  includes two coolers  404  and  406  thermally coupled to passive spiral coil  402 . Coolers  404  and  406  may be implemented as, for example, TEC device  300  in FIG.  3 . In this embodiment, the cold plate of cooler  406  is coupled directly to one end of the passive spiral coils  402  using via structures  408  and  410 . Via structures  408  and  410  and lower level interconnect  414  are preferably thermally and electrically conductive copper composition. The cold plate of cooler  404  is directly thermally coupled to the other end  420  of spiral coil  402 , preferably also of a copper composition. 
     Portions of coolers  404  and  406  as well as the spiral coil  402  are constructed within the same layer of the integrated circuit  400 . The interconnect  414  is constructed in a lower layer of the integrated circuit  400  from that of the spiral coil  402 . Although depicted using two coolers  404 - 406  to cool spiral coil  402 , a single cooler could be utilized as well. However, the two coolers working in tandem provide greater cooling of the spiral coil  402  than would a single cooler and help reduce any thermal gradient between different sections of the spiral coil  402 . 
     Electrical isolation between cooler  406  and passive spiral coil  402  may be achieved by using current-mode circuits or by using ultra-thin dielectric passivation layers such as chemical vapor deposition (CVD) silicon dioxide or anodized aluminum. Anodization of aluminum is preferable to CVD silicon dioxide because 1-10 nanometer (nm) dielectric layers can be easily formed, and the thermal conductivity of alumina (aluminum oxide) is better than that of silicon dioxide. 
     With reference now to FIG. 5, a current-controlled thermoelectric cooler (TEC) circuit is depicted in accordance with the present invention. Current-controlled TEC circuit  500  is an example of a current-mode circuit which may be used in conjunction with direct-coupled coolers  400  in order to maintain electrical isolation of the coolers  404 - 406  from passive spiral coil  402 . Current-controlled TEC circuit  500  includes p-channel field effect transistors  502 - 506 , n-channel transistors  508 , inverter  510 , and  512 - 514 , and TEC  516 . TEC  516  has a hot end  518  for dissipating heat and a cold end  520  which is thermally coupled to the device to be cooled. 
     The gate of transistor  508  is coupled to a bias control voltage V bc  as well as to the input of inverter  510 . The output of inverter  510  is coupled to the gate of transistor  506 . The drain of transistor  506  and the drain of transistor  508  are coupled to the source of transistor  512  and to the gates of transistors  512 - 514 , so that transistors  512  and  514  are in a current mirror configuration. The drains of transistors  512 - 514  are coupled to ground G nd . The source of transistor  514  is coupled to a second end of TEC  516 . Thus, current-controlled TEC circuit  500  maintains a constant current flow I 0  through TEC  516  based upon bias voltage V bc . Even if the cold end  520  of the TEC  516  is electrically connected to the device, by Kirchoff&#39;s law, there is no current flowing between the TEC  516  and the device. Thus the current-mode bias circuit  500  ensures electrical isolation for the TEC  516 . 
     With reference now to FIGS. 6A and 6B, FIG. 6A depicts a top planar view of a patterned cold plate in an integrated circuit chip for cooling RF IC circuits and FIG. 6B depicts a cross-sectional view of the section of the integrated circuit chip in accordance with the present invention. In this embodiment, as an alternative to using direct-coupled coolers as depicted in FIG. 4, a cold plate  602  is placed underneath the RF circuit  650 , such as, for example, one of the RF circuits depicted in FIGS. 1-5. By placing cold plate  602  under the RF circuits  650 , large areas of inductors and capacitors within the RF circuit  650  are cooled. However, cold plate  602  is not physically in contact with any of the circuits within RF circuits  650  but is separated by an dielectric material  604 . Cold plate  602  is thermally coupled to the thermoelectric cooler  606  by via thermal conductor  608 . 
     If cold plate  602  is constructed from metal and is used under the inductors within the RF circuit  650 , then cold plate  602  is patterned to avoid the inducement of circulating eddy currents in the metal layer resulting from magnetic coupling with the inductors. 
     The integrated chip  600  may contain other areas other than the RF circuits  650  that do not generate an excessive amount of heat and do not need to be cooled. Thus, an efficiency in power savings is achieved by the present invention by spot cooling only the portions (i.e. RF circuits  650 ) of the integrated circuit  600  that generate significant heat and need to be cooled. 
     With reference now to FIGS. 7A and 7B, FIG. 7A depicts a top cut-away planar view illustrating direct thermal coupling of a TEC cooler through the body/substrate levels of an integrated circuit (IC) and FIG. 7B depicts a cross-sectional view along cut  750  of the direct thermal coupling of the TEC cooler through the body/substrate levels of an integrated circuit (IC) in accordance with the present invention. Vias  702 - 712  thermally couple a cold plate  762  of the IC  700  to the body/substrate level  752  of IC  700 . Body/substrate level  752  may contain low-noise amplifier circuits. The cold plate  762  of TEC cooler  714  is separated from the body/substrate level  752  of IC  700  by intervening metalization and/or oxide layers  754 . 
     An electrical conductor  760  couples the p-type impurity thermoelement  758  to the n-type impurity thermoelement  756  thus allowing current to flow from electrical conductor  768  through thermoelements  756  and  758  and out through electrical conductor  766 . An electrically isolating, thermally conducting hot plate  764  is in physical contact with electrical conductors  766 - 768  allowing heat to flow from thermoelements  756 - 758  into hot plate  764 , where the heat may then be dissipated. 
     With reference now to FIG. 8, a cross sectional view of an exemplary thermoelectric spot cooler fabricated over an RF CMOS IC is depicted in accordance with the present invention. In this exemplary embodiment, integrated circuit (IC) chip  800  includes a low noise amplifier (LNA) transistor  808  which is formed as a silicon-on-insulator (SOI) transistor in buried oxide  894  that lies above a silicon substrate  890 . A thermoelectric cooler (TEC)  832  is placed above LNA transistor  808  for cooling LNA transistor  808 . A second transistor  806  to provide a current source for TEC  832  is also formed as an SOI transistor in buried oxide  894 . A conductive via structure  810  through oxide layers  816  couples the drain  826  of transistor  806  to TEC  832  to provide current to the p-type  838  and n-type  840  semiconductor material of TEC  832 . P-type  838  and n-type  840  semiconductor areas provide a similar function as p-type semiconductor  304  and n-type semiconductor  306  in FIG.  3 . 
     The heat spreader  830 , which acts as a heat sink, such as, for example, heat sink  316  in FIG. 3, for dissipating heat is thermally but not electrically coupled to the hot side element of TEC  832  through layer  834 . Layer  834  may be constructed, for example, from ultra-thin oxide or alumina. Heat spreader  830  could be coupled to layer  834  by solder. 
     N-type semiconductor  840  is thermally coupled to cold plate  828  through thin layer  836 . Layer  836  may also be constructed, for example, from ultra-thin oxide or alumina. 
     Cold plate  828  is thermally coupled to both the drain  824  and source  822  of transistor  808  through oxide layers  816  by using vias  814  and  812  respectively. Vias  812  and  814 , as well as via  810  are typically constructed from metal, such as, for example, copper (Cu) or tungsten (W), and are both good electrical and thermal conductors. Via  814  is thermally coupled to drain  824  through diffused region  818 , of an impurity type opposite drain diffusion  824 , which provides a thermal connection while maintaining electrical isolation of via  814  and cold plate  828  from drain  824 . Via  812  is thermally coupled to source  822  through a similarly diffused region  820  which provides a thermal connection while maintaining electrical isolation of via  812  and cold plate  828  from source  822 . 
     Thus, as heat is built up in transistor  808  by RF operation, the heat is carried away through vias  812  and  814  to cold plate  828  of TEC  832 . The heat is then transferred from cold plate  828  to heat spreader  830  where it may be dissipated away from the IC chip  800 . 
     Optionally, a reactive ion etch (RIE) etch of section  844  can be performed. The RIE etch forms a trench in section  844  which aids in ensuring further thermal isolation of cold plate  828  from via  810 , which is connected to hot plate  838 . 
     The structure depicted in FIG. 8 is given as an example of a thermoelectric spot cooler directly coupled to an RF IC device and is not intended to limit the present invention. For example, more or fewer metallization layers M 1 -M 5 , and LM may be utilized between the RF device, such as, for example, transistor  808  and cold plate  828 . Furthermore, transistor  808  may be any single or composite temperature sensitive device without departing from the scope and spirit of the present invention. Also, it should be noted that the present invention is not limited to RF transistors constructed as SOI transistors, but may be applied to bulk transistors and event to RF devices other than transistors. Furthermore, the elements of IC chip  802  may be constructed from other substances and compounds than those depicted. 
     With reference now to FIG. 9, a cross sectional view of an exemplary RF spiral inductor circuit wherein the thermoelectric cooler is incorporated in the passive inductor and the heat is rejected into the bulk substrate is depicted in accordance with the present invention. IC chip  900  includes spiral inductors having components  908  and  910  visible in the depicted view. Spiral inductor components  908  and  910  are formed from an electrically conductive material such as, for example, copper (Cu). Spiral inductor is formed in the cold end  904  and the inductor leads  908  and  910  of the inductor components are thermally coupled to cold end  904  which in turn is supported in part above the surface  930  of IC chip  900  by photoresist (PR) support  912 . 
     Thermoelectric cooler  902  includes a thin electrically but not thermally conducting layer  906  to couple cold end  904  to the cold ends of p-type element  914  and n-type element  916  of the TEC. Current to drive the TEC is provided through conductor  932 , which in the depicted example, lies in the second metallization layer M 2 . Thermoelectric cooler  902  also includes a second thin thermally but not electrically conductive layer  918  to provide a thermal coupling to via  920 . Via  920  then provides a thermal connection through oxide layers  922  to hot end  924  at substrate  926 . As heat is generated in the spiral inductor, it is transported by TEC  902  from cold end  904  to hot end  924  and into the bulk silicon substrate  926 , thus cooling the spiral inductor. 
     Although the present invention has been described primarily with reference to dissipating the heat either into the bulk substrate or into the atmosphere surrounding the integrated circuit via a hot plate located the surface of the integrated circuit, the heat may also be dissipated by other means. For example, the heat may be rejected via heat pipes rather than directly in air. Furthermore, the thermoelectric coolers are not limited to a single type of thermoelectric cooler, but may be implemented as any one of several different types of thermoelectric coolers, such as, for example, quantum point coolers. 
     It should also be noted that the present invention allows metal structures with photoresist or dielectric supports to be easily incorporated in the cooling process. Furthermore, it should also be noted that the present invention is not limited by the exemplary structure depicted and that there are a large number of alternative structures which may be utilized without departing from the scope and spirit of the present invention. 
     The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.