Abstract:
Application of a potential difference across a doped region formed on an integrated circuit allows management of thermal energy directly on the chip surface. Individual temperature control cells are formed by ion implanting N- and P-type dopant into adjacent regions, and then forming a metal bridge across the similarly positioned ends. Placing a potential drop across metal contacts of the cell changes the temperature of the contacts relative to that of the electrically conducting bridge. Fabrication of arrays of temperature control cells of various shapes and sizes permits extremely precise heating and cooling of specific regions of the integrated circuit. Management of thermal energy on the IC in accordance with the present invention may be enhanced by forming arrays of temperature control cells possessing multiple tiers.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of forming a temperature control structure for an integrated circuit, and in particular, to a temperature control structure fabricated on the surface of an integrated circuit that is capable of changing the heat distribution profile directly on the chip. 
     2. Description of the Related Art 
     As device sizes continue to shrink into the submicron region, packing densities have increased dramatically. One consequence of this heightened density is that the thermal energy generated by the operation of semiconducting devices is now a key factor that must be considered in designing a chip. Absent some form of heat management, heat from device operation could accumulate and degrade device performance. 
     FIG. 1 shows a schematic depiction of the Seebeck Effect. Due to this thermoelectric phenomenon, a Seebeck voltage (V S ) will be generated on aluminum contacts Al 1  and Al 2  when these contacts are connected to a second conductor of a different material (silicon), and when the Al 1  and Al 2  contacts are maintained at different temperatures T 1  and T 2  respectively. The Seebeck voltage generated is proportional to the temperature difference (T 1 −T 2 ), with 
       V   S =α( T   1   −T   2 ) 
     where α is the Seebeck coefficient. Room temperature values of α are typically in the range of −10.0 to +10.0 μV/° C. With Al/Si junctions, values of α may be as large as 1.4 mV/° C., which is the same order of magnitude as the temperature coefficient of the base-emitter voltage (V be ) of a bipolar transistor. 
     FIG. 2 shows a schematic depiction of the Peltier Effect. The Peltier Effect is simply the converse of the Seebeck Effect Under the Peltier Effect, current passed through junctions of dissimilar conductors (such as Al and Si) results in absorption of heat at one junction and the emission of heat at the other junction. Due to the frequent application of voltages to operate silicon type semiconductor devices having metal contacts, the Peltier Effect is perhaps the dominant thermal effect experienced by integrated circuits. 
     The Peltier and Seebeck Effects are related directly by            (       V   SA     -     V   SB       )         T   1          T   2         =       ∫     T   1       T   2                Π   ab     T                        T                                
     Since II ab  (Peltier) is defined per unit current flow, the power output (V*I) from a Seebeck junction pair with a small temperature difference ΔT is related to the Peltier heat transfer by a factor of the form ΔT/T. 
     In considering FIG. 2, it is important to note that the relative temperatures of T 1  and T 2  are directly related to the polarity of the voltage drop applied across the aluminum contacts. Therefore, reversing the polarity of the voltage source would make T 1  less than T 2 . 
     It is also important to note that changing the conductivity type of the silicon bridge in FIG. 2 will also have the effect of reversing the relationship between T 1  and T 2 . Thus, where the Si bridge is N-type rather than P-type as shown, T 1  would also be less than T 2 . 
     The Peltier Effect has previously been used to manage heat generated on IC structures. FIG. 3 shows such a conventional Peltier structure as used to cool an IC present on a chip. 
     Conventional Peltier structure  300  includes monolithic blocks of N- and P-type conductivity type  304  and  306 , respectively. N-type block  304  and P-type block  306  are typically composed of different Tellurium/Bismuth compounds. For example, BiSb 4 Te 7.5  exhibits P-type properties, while Bi 2 Te 2 Se is an N-type material. 
     Second end  304   b  of N-type block  304  and second end  306   b  of P-type block  306  are electrically connected by metal strip  308 . First end  304   a  of N-type block  304  and first end  306   a  of N-type block  306  are connected to metal contacts  310  and  312 , respectively. 
     As described in detail in connection with FIG. 2 above, application of a potential difference across contacts  310  and  312  results in a temperature change at metal strip  308 , thus and on chip  302  in contact therewith. Specifically, as shown in FIG. 3 application of a negative voltage difference across N block contact  310  and P block contact  312  results in a temperature increase at contacts  310  and  312 , and a corresponding temperature decrease at metal strip  308 . Conversely, application of a positive voltage difference across N block contact  310  and P block contact  312  would result in a fall in the temperature of contacts  310  and  312  and a corresponding rise in the temperature of metal strip  308 . 
     As is evident from the above description in connection with FIG. 3, application of a voltage difference to the conventional Peltier structure can result in temperature control of a chip. However, it is important to note that this conventional Peltier structure suffers from a number of serious disadvantages. 
     First, the temperature control exercised over the chip is crude and non-specific. In particular, the monolithic P- and N-type blocks making up the conventional Peltier structure are far larger than the individual semiconducting structures present on the chip. Application of a voltage to the conventional Peltier structure therefore necessitates a temperature change over the entire chip, and all component structures of an integrated circuit present thereon. Temperature control of specific structures of the integrated circuit, or even of selected regions of the chip, is not possible with the conventional Peltier structure. 
     Therefore, there is a need in the art for a temperature control device which enables highly specific and selective management of heat on an IC circuit present on a chip. 
     A second disadvantage associated with the conventional Peltier cell structure is the substantial power consumption of the device. A current in the range of between 3 and 5 Amp would typically be required to maintain a desired temperature change across the monolithic P and N type blocks shown. This is due in part to the sheer mass of the P- and N-type blocks of the Peltier cell, which dwarf any individual semiconducting structures present on the chip. 
     Therefore, there is a need in the art for a temperature control device which consumes relatively little power while effectively managing the temperature of an IC. 
     A third disadvantage associated with a conventional Peltier cell is the space consumed by the device. As shown in FIG. 3, transfer of thermal energy to and from the chip is accomplished in a direction out of the plane of the chip. This orientation consumes precious space present around the chip. This space is generally quite limited, due to the reduced size of portable applications such as laptop computers, as well as the necessary presence of nearby components such as circuit boards, chip packaging, buses, or other forms of communication structures. 
     Therefore, there is a need in the art for a temperature control device which occupies relatively little space and which can be incorporated into an IC device directly upon the surface of a semiconducting chip. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a temperature control structure for an IC that can be fabricated directly on the chip surface. The structures in accordance with the present invention utilize the thermoelectric properties of the Peltier Effect to accomplish the temperature control function. 
     Specifically, the heat management structure in accordance with the present invention incorporates an array of Peltier cells fabricated directly upon the surface of the chip. Selective application of voltage to the temperature control structures present in the array may serve to generate “cold” regions at particular chip regions, facilitating channeling and dissipation of thermal energy generated thereupon. 
     An apparatus for controlling temperature on an integrated circuit in accordance with one embodiment of the present invention comprises a first doped region of a first conductivity type formed within a semiconducting material and having a first end and a second end, a first contact connected to the first end and in electrical communication with a voltage source, and a second contact connected to the second end and in electrical communication with the voltage source, wherein application of a potential difference across the first and second contacts changes a temperature of the first contact relative to the second contact. 
     A process for forming a temperature control structure in accordance with the present invention comprises the steps of comprises the steps of forming a first doped region of a first conductivity type within a semiconducting material, the first doped region having a first end and a second end, forming a first contact connected to the first end, forming a second contact connected to the second end, and connecting the first contact and the second contact to a voltage source. 
     A method for controlling temperature on an integrated circuit comprises the steps of providing a doped region in the silicon having a first end and a second end, the first end in connection with a first metal contact and the second end in connection with a second metal contact, and applying a potential difference across the first metal contact and the second metal contact whereby a temperature of the first metal contact changes relative to a temperature of the second metal contact because of the Peltier Effect. 
     The features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic depiction of the Seebeck Effect. 
     FIG. 2 shows a schematic depiction of the Peltier Effect. 
     FIG. 3 shows a perspective view of a conventional Peltier cell structure as used to cool an IC structure on a silicon chip. 
     FIGS. 4A-4B show cross-sectional views of two embodiments of simple temperature control structures in accordance with the present invention. 
     FIG. 5 shows a top view of a square temperature control structure in accordance with a first alternative embodiment of the present invention. 
     FIG. 6 shows a top view of a temperature control structure utilizing multiple doped regions in accordance with a second alternative embodiment of the present invention. 
     FIG. 7 shows a top view of a circular array of Peltier cells having doped regions of the opposite conductivity type in accordance with a third alternative embodiment of the present invention. 
     FIG. 8 shows a top view of a silicon chip bearing cooling structures in accordance with the present invention. 
     FIG. 9 shows a perspective view of a hybrid chip bearing both conventional Peltier structures and arrays of Peltier cell structures in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention teaches temperature control structures for an integrated circuit utilizing the Peltier Effect which are formed directly on the surface of a chip. FIGS. 4A-4B show cross-sectional views of two simple embodiments of a temperature control structure in accordance with the present invention. 
     FIG. 4A shows temperature control structure  400   a  formed within silicon block  402   a.  Temperature control structure  400   a  includes silicon region  404   a  containing N-type dopant. N-type region  404   a  may be formed in a variety of ways, including ion-implantation or chemical vapor deposition followed by thermal diffusion. 
     N-type region  404   a  also includes metal contacts  406   a  and  408   a  formed over first end  410   a  and second end  412   a,  respectively. Metal contacts  406   a  and  408   a  may be formed by conventional metallization processing steps, as are commonly employed to create interconnect lines of an integrated circuit. 
     Metal contacts  406   a  and  408   a  of N-type region  404   a  are in electrical communication with other portions of the integrated circuit through metallization layer  420   a  by way of via  426   a.  Temperature control structure  400   a  is otherwise thermally and electrically isolated from metallization layer  420   a  by intervening dielectric layer  422   a.    
     As is evident from discussion of the Peltier Effect in connection with FIG. 2, application of a voltage potential across metal contacts  406   a  and  408   a  will create a temperature gradient across N-type doped region  404   a.  Specifically, application of a negative potential difference between metal contact  406   a  and metal contact  408   a  will create a high temperature region in the vicinity of the junction  414   a  between metal contact  406   a  and doped region  404   a,  and a corresponding low temperature region in the vicinity of the junction  416   a  between metal contact  408   a  and doped region  404   a.    
     In this manner, the temperature control structure is seen to control the distribution of thermal energy over highly specific regions of the silicon block. Changing the type and/or concentration of conductivity-altering dopant present in the doped region will affect the character of this temperature control. Changing the magnitude and/or polarity of the voltage drop applied across the metal contacts will also affect the character of the temperature control exerted. 
     FIG. 4B shows temperature control structure  400   b  that is formed within silicon block  402   b.  Temperature control structure  400   b  includes silicon region  404   b  containing N-type dopant. Metal contacts  400   b  and  408   b  are formed directly in silicon block  402   b  adjacent to first end  410   b  and second end  412   b  of doped silicon region  404   b.  This may be accomplished by etching holes into silicon block  402   b,  and then filling these holes with metal. 
     As with temperature control structure  400   a  depicted in FIG. 4, metal contacts  400   b  and  408   b  are in electrical communication with an overlying metallization layer  420   b  by way of via  426   b.  Temperature control structure  400   b  is otherwise thermally and electrically isolated from metallization layer  420   b  by intervening dielectric layer  422   b.    
     Because the metal contacts  400   b  and  408   b  of the embodiment shown in FIG. 4B are positioned directly in the silicon block  402   b,  the metal/silicon junctions  414   b  and  416   b  are oriented transverse to the direction of heat conduction through the silicon, thereby distributing the flowing heat uniformly through the depth of the silicon. This is somewhat different from the embodiment shown in FIG. 4A, where thermal energy is conducted upward to laterally oriented metal silicon junctions  414   a  and  414   b  positioned over the silicon. Despite this slight difference in spatial orientation, the embodiments shown in FIGS. 4A and 4B should operate with similar efficiencies. 
     FIG. 5 shows a top view of a square temperature control structure in accordance with a first alternative embodiment of the present invention. Square temperature control structure  500  includes a square doped region  502  having interior metal contact  504  and exterior metal contact  506 . Application of a potential drop across contacts  504  and  506  will give rise to a change of temperature at the junction between contacts  504  and  506  and square doped region  502 . This Peltier heating or cooling will affect the thermal environment of central region  508 . 
     FIG. 6 shows a top view of a temperature control structure utilizing an array of doped regions of the same conductivity type in accordance with a second alternative embodiment of the present invention. Temperature control structure  600  includes multiple doped regions  602  of the same conductivity type having first and second ends  602   a  and  602   b,  respectively. First ends  602   a  are oriented around the periphery of central region  608 . The first and second ends of successive doped regions  602  are electrically linked by metal bridges  604 . Application of a potential drop across contacts  605  and  606  will give rise to a change in temperature at the junction between the metal bridges  604  and doped regions  602 . This Peltier heating or cooling will affect the thermal environment of central region  608 . 
     One feature of temperature control structure  600  that enhances its effectiveness relative to the embodiments shown in FIGS. 4A,  4 B, and  5  is utilization of an array of multiple doped regions rather than a single doped region. This design offers a larger number of metal/silicon junctions at which temperature differences can arise and serve to conduct heat on the chip. 
     FIG. 7 shows a top view of a circular array of Peltier cells having doped regions of the opposite conductivity type comprising a cooling structure in accordance with a third alternate embodiment of the present invention. Cooling structure  700  consists of circular array  702  of Peltier cells inlaid within circular array  704  of Peltier cells. Both circular arrays  702  and  704  consist of Peltier cells of alternating N- and P-type silicon regions  706  and  708 , respectively, connected by an electrically conducting bridge structure  710 . External circular array  704  is connected to a voltage supply by way of contacts  712  and  714 . Internal circular array  702  is connected to a voltage supply by way of contacts  716  and  718 . 
     Due to the Peltier Effect described above, application of a potential difference across contacts  712  and  714  of external circular array  704 , and also across contacts  716  and  718  of internal circular array  702 , creates low temperature region  720  at the center of internal circular array  702 , with higher temperatures generated at region  722  lying along the periphery of external circular array  704 . 
     Semiconducting devices located at the center of internal array  702  will therefore be maintained at a constant, lower temperature than silicon at the periphery of external array  704 . The constant ambient temperature inside internal array  702  will ensure reliable operation of the semiconductor devices located therein. 
     Several key design features of temperature control structure  700  enhance its efficiency and effectiveness as compared to the embodiments shown in FIGS. 4A,  4 B, and  5 - 6 . First, utilization of paired doped regions of the opposite conductivity type simplifies construction of the array by permitting the same ends of the doped regions to be connected by the metal bridges. Second, the use of multiple consecutive tiers of arrays compounds the degree temperature control exerted over the integrated circuit. Finally, the circular shape of the array encompassing the central area maximizes convection of thermal energy to and from this smaller area of the array distributing heat within the larger peripheral regions of the array. 
     Although the invention has been described above in connection with one specific preferred embodiment, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Various other modifications and alterations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope of the present invention. 
     For example, while device  700  of FIG. 7 is described as a cooling structure, the present invention is not limited to that specific application. Heating of specific structures or regions of an integrated circuit could also be accomplished utilizing the principles in accordance with the present invention. 
     Furthermore, while the array shown in FIG. 7 is circular in shape and includes two separate tiers of Peltier cells, the present invention is not limited to this configuration. Arrays of various shapes containing any number of tiers of Peltier cells may be employed and still fall within the scope of the present invention. 
     Thus, FIG. 8 shows a top view of a silicon chip bearing cooling structures in accordance with the present invention. As shown in FIG. 8, rectangular arrays  800  and trapezoidal arrays  802  of Peltier cells are carefully positioned on the surface of chip  806  to conduct thermal energy away from active device area  810  to periphery  812  of chip  806 . In this manner, circuit components located within active device area  810  may be maintained at a constant (low) temperature. 
     Moreover, the temperature control device in accordance with the present invention may be used in conjunction with a conventional Peltier device. FIG. 9 shows a perspective view of a hybrid chip bearing both conventional Peltier structures and arrays of Peltier cell structures in accordance with the present invention. Peltier cell arrays  900  in accordance with the present invention conduct heat from active region  902  to the periphery  904  of chip  906 . Conventional Peltier structures  908  in contact with chip periphery  904  may then efficiently remove heat in a direction normal to the surface of the chip, as shown by the arrows in FIG.  9 . 
     Therefore, it is intended that the following claims define the scope of the present invention, and that the methods and structures within the scope of these claims and their equivalents be covered hereby. 
     The following co-pending, commonly assigned patent applications contain subject matter related to the present application, and are incorporated herein by reference: U.S. patent application Ser. No. 09/216,032, entitled “TEMPERATURE REGULATOR CIRCUIT AND PRECISION VOLTAGE REFERENCE FOR INTEGRATED CIRCUIT”.