BACKGROUND OF INVENTION  
       [0001]     1. Technical Field  
         [0002]     The technical field is resistors.  
         [0003]     2. Related Art  
         [0004]     Resistors are typically specified as having a nominal resistance, a room temperature resistance variation, and temperature coefficient of resistance. The temperature coefficient of resistance describes the variation in resistance of a resistor as a function of temperature change.  
         [0005]     Integrated circuit performance depends upon the electrical characteristics of semiconductor devices in the circuit. One method for adjusting the electrical characteristics of a semiconductor device is by “trimming” one or more resistors in the device. Trimming of resistors can be done by chemical, mechanical, and electrical means. Conventional trimming methods involve heating a resistor to change electrical and physical properties of the resistor. Once a conventional resistor has been trimmed, however, its properties are fixed and cannot be changed once the semiconductor wafer carrying the circuit has been diced and packaged.  
       SUMMARY OF INVENTION  
       [0006]     According to a first embodiment, a method of operating a resistor comprises providing a tuner that is electrically coupled to the resistor, detecting a resistance of the resistor, and adjusting the temperature of the resistor when the resistance of the resistor is outside a nominal resistance range. The temperature of the resistor may therefore be controlled while the resistor is in use, such as when the resistor is in use as a resistive element in a semiconductor circuit.  
         [0007]     According to the first embodiment, the performance of a circuit incorporating the resistor is improved because the temperature, and therefore the resistance of the resistor is controlled.  
         [0008]     Those skilled in the art will appreciate the advantages and benefits of various embodiments of the invention upon reading the following detailed description of the embodiments with reference to the below-listed drawings.  
         [0009]     According to common practice, the various features of the drawings are not necessarily drawn to scale. Dimensions of various features may be expanded or reduced to more clearly illustrate the embodiments of the invention. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0010]     The detailed description will refer to the following drawings, wherein like numerals refer to like elements, and wherein:  
         [0011]      FIG. 1 . is a schematic illustration of a system having a tunable resistor;  
         [0012]      FIGS. 2A  is a top plan view of an embodiment of a tunable resistor;  
         [0013]      FIG. 2B  is a section view taken on line  2 B- 2 B in  FIG. 2A ;  
         [0014]      FIG. 3A  is a top plan view of an alternative embodiment of a tunable resistor;  
         [0015]      FIG. 3B  is a section view taken on line  3 B- 3 B in  FIG. 3A ;  
         [0016]      FIG. 4A  is a section view illustrating a tunable resistor encased in dielectric material;  
         [0017]      FIG. 4B  is a section view taken on line  4 B- 4 B in  FIG. 4A ;  
         [0018]      FIG. 4C  is a section view taken on line  4 C- 4 C in  FIG. 4A ;  
         [0019]      FIG. 4D  is a section view taken on line  4 D- 4 D in  FIG. 4A ;  
         [0020]      FIG. 5  is a plan view of alternative resistor configuration;  
         [0021]      FIGS. 6A-6D  illustrate a subtractive etch method for manufacturing a tunable resistor;  
         [0022]      FIGS. 7A-7E  illustrate a damascene process for manufacturing a tunable resistor;  
         [0023]      FIG. 8  illustrates a hybrid subtractive etch/damascene process for manufacturing a tunable resistor; and  
         [0024]      FIG. 9  illustrates an alternative tunable resistor embodiment. 
     
    
     DETAILED DESCRIPTION  
       [0025]      FIG. 1  is a schematic illustration of an embodiment of the invention. In  FIG. 1 , a resistor system  100  allows for tuning of a resistor  32 . Specifically, the resistor  32  can be actively tuned while in use. The resistor system  100  may be formed on a structure such as, for example, a semiconductor chip. Other environments are also suitable.  
         [0026]     The resistor system  100  comprises a heater driver circuit  10 , a tuner  20 , a resistor  32 , a dielectric  34 , and a heater  36 . In general operation, the tuner  20  senses the resistance of the resistor  32 , and determines the deviation of the sensed resistance from a nominal resistance or resistance range. The resistance range can be described as R±□R. If the resistance of the resistor  32  is outside the nominal resistance range, the tuner  20  then provides feedback to the heater driver circuit  10 , which adjusts the heat output of the heater  36 . The heater  36  thereby changes the temperature of the resistor  32  until the tuner  20  detects a resistance for the resistor  32  that falls within the nominal resistance range. For the purposes of this specification, a “nominal resistance” is a subset of a “nominal resistance range”, and the term “nominal resistance range” is used hereinafter to encompass both terms.  
         [0027]     In a preferred embodiment, the heater  36  maintains the resistor  32  at an elevated temperature T. The elevated temperature T is initially selected to place the resistor  36  within the target resistance range R+□R. The elevated temperature T should be selected to be sufficiently higher than an expected temperature of the environment, so that reducing the output of the heater  36  in effect results in cooling of the resistor  36 . For example, in one embodiment, a resistor  32  is utilized in an environment where the temperature may be expected to stay at about 50° C. The elevated temperature T may therefore be selected as 70° C. The temperature of, and therefore the resistance of the resistor  32 , can thus be adjusted either upwardly or downwardly by either increasing or decreasing the output of the heater  36 .  
         [0028]     The heater  36  can be a resistive element. Therefore, the heat output of the heater  36  can be easily regulated by changing a current I through the heater  36  during operation of the resistor  32 . When a change of the resistance of the resistor  32  is warranted, the current I can be reduced or increased accordingly.  
         [0029]     The resistance of the resistor  32  may be detected, for example, by including any known resistance sensing device in the tuner  20 . For example, one method of detecting the resistance of the resistor  32  is to pass a calibration current through the resistor  32 . The resistance can also be detected by detecting an oscillation frequency f in a device incorporating the resistor  32 . Other methods may also be used to detect resistance. As an alternative to detecting resistance, a temperature sensing device (not illustrated) can be connected to the resistor  32 , or generally located in the vicinity of the resistor  32 . The temperature sensing device can be used to determine what temperature correction is needed to place the resistor  32  within a nominal resistance range. The temperature sensing device may be, for example, a discrete or integrated thin film thermocouple.  
         [0030]     The resistor  32  may be an “on chip” resistor, and the resistor system  100  may be fabricated over the silicon area of a semiconductor chip. In contrast to conventional resistor trimming processes, the resistor  36  may be adjusted during operation of the chip. Conventional trimming processes occur before a chip is diced and packaged. The embodiments of the present invention allow for adjustment of resistance after a chip is diced and packaged.  FIGS. 2A-9  illustrate further aspects of the present invention, and are discussed in detail below.  
         [0031]      FIGS. 2A and 2B  illustrate an embodiment of a tunable resistor  40  and associated elements.  FIG. 2A  is a top plan view, and  FIG. 2B  is a section view taken on line  2 B- 2 B.  FIGS. 2A and 2B  generally illustrate an embodiment of the resistor  40  as formed by a damascene process. A dielectric  50  separates the resistor  40  from a heater  60 . In  FIGS. 2A and 2B , the resistor  40  is disposed within the dielectric  50 , which is in turn disposed within the heater  60 .  
         [0032]     The heater  60  is attached to conductive leads  62 ,  64  at contacts  66 ,  68 , respectively. The conductive leads  62 ,  64  may be electrically connected to a heater driver circuit (not illustrated), such as the circuit  10  illustrated in  FIG. 1 . The heater driver circuit provides a heating current to the heater  60 . The heater  60  may comprise, for example, a resistive element that generates heat when a current passes therethrough. The leads  62 ,  64  may extend “vertically” (a direction perpendicular to the page in  FIG. 2A ) to contact the heater  60 . Conductive leads  42 ,  44  may similarly extend vertically to contact the resistor  40 . Multiple contacts  46 ,  48  may be used. The leads  42 ,  44  may be connected to a tuner (not illustrated), such as the tuner  20  illustrated in  FIG. 1 . The nature of the leads  42 ,  44 ,  62 ,  64  is illustrated in  FIGS. 4A-4D  and is discussed in detail below.  
         [0033]      FIGS. 3A and 3B  illustrate another embodiment of a tunable resistor  90  and associated elements.  FIG. 3A  is a top plan view, and  FIG. 3B  is a section view taken on line  3 B- 3 B.  FIGS. 3A and 3B  generally illustrate an embodiment of the resistor  90  as formed by a subtractive etch process. The resistor  90  is disposed in a dielectric  80 , which separates the resistor  90  from a heater  70 .  
         [0034]     The heater  70  is attached to conductive leads  72 ,  74  at contacts  76 ,  78 , respectively. The conductive leads  72 ,  74  may be electrically connected to a heater driver circuit (not illustrated), such as the heater driver circuit  10  illustrated in  FIG. 1 . The heater driver circuit provides heating current to the heater  70 . The conductive leads  72 ,  74  may extend vertically (a direction into the page in  FIG. 3A ) to contact the heater  70 . Leads  92 ,  94  may similarly extend vertically to contact the resistor  90 . Multiple contacts  96 ,  98  may be connected to the leads  92 ,  94 . The leads  92 ,  94  may be connected to a tuner (not illustrated), such as the tuner  20  illustrated in  FIG. 1 . The nature of the leads  72 ,  74 ,  92 ,  94  is illustrated in  FIGS. 4A-4D  and is discussed in detail below.  
         [0035]     The above resistor embodiments can be partially or fully enclosed (not illustrated in  FIGS. 2A-3B ) within a dielectric material. An example of this configuration is illustrated in  FIGS. 4A-4D .  
         [0036]      FIGS. 4A-4D  illustrate an embodiment of a tunable resistor and associated elements enclosed in a dielectric material.  FIG. 4A  is a section view in front elevation, and  FIGS. 4B, 4C , and  4 D are section views taken on lines  4 B- 4 B,  4 C- 4 C, and  4 D- 4 D, respectively. In  FIG. 4A , a resistor  110  is disposed over a dielectric  120 , which is disposed over a heater  130 . The resistor  110 , the dielectric  120 , and the heater  130  are encased in dielectric material  150 . The dielectric material  150  can be formed from, for example, one or more layers or bodies of dielectric material.  
         [0037]     In  FIG. 4A , portions of conductors  111 ,  115  extend “vertically” downwardly through the dielectric material  150  to contact the resistor  110 . The conductors  111 ,  115 , may be connected to a tuner (not illustrated), such as the tuner  20  illustrated in  FIG. 1 . Portions of conductors  131 ,  135  extend upwardly through the material  150  and contact the heater  130 . The conductors  131 ,  135  supply heating current to the heater  130 . The conductors  131 ,  135  may be connected to a heater driver circuit (not illustrated), such as the heater driver circuit  10  illustrated in  FIG. 1 .  
         [0038]     The conductor  111  is illustrated as formed by a wire  112  and a via  113 . The via  113  can be formed in the dielectric material  150  by etching through the material  150  and subsequently metallizing the through hole. The wire  112  can be formed by, for example, damascene or subtractive etch processes. The conductor  115  is similarly comprised of a via  116  and a wire  117 . There may be two each of the vias  113 ,  116  (only one of each via is visible in  FIG. 4A ). The conductors  131 ,  135  may have similar configurations.  
         [0039]      FIG. 4B  illustrates the resistor  110  in plan view. As shown in  FIG. 4B , the vias  113 ,  116  (each one associated with a conductor) contact the resistor  110 . The vias  113 ,  116  can be used to connect the resistor  110  to a tuner, and/or to any number of additional components in an integrated circuit.  
         [0040]     As shown in  FIG. 4C , the dielectric  120  can have a plan view footprint that substantially conforms in shape and size to that of the heater  130 . The dielectric  120  can also be of any other shape, size or thickness that prevents electrical contact between the resistor  110  and the heater  130 .  
         [0041]     Referring to  FIG. 4D , the heater  130  may have a simple metallic strip configuration that conforms generally in shape to the resistor  110 . As is also shown in  FIG. 4A , the heater  130  is connected to the conductors  131 ,  135 .  
         [0042]     The dielectric material  150  may have both electrical and thermal insulation properties. Thermal insulative properties are desirable because the heater  130  and the resistor  110  will generate heat during use, which may affect the operation of components near to the resistor  110 . The use of dielectrics which are poor thermal conductors is particularly advantageous when the tunable resistor  110  is formed on a semiconductor chip. Examples of materials suitable for forming the dielectric material  150  include polyarylene ether (available under the trade name DOW CHEMICAL SILK), FLAIR manufactured by Honeywell, spin-on methyl silsexquoixane (MSQ), hygrogen silsexquoixane (HSQ), silica aerogels, SiC x O y H z , SiO 2 , and FSG.  
         [0043]     The dielectric material  150  may be formed by, for example, PECVD, HDP CVD, thermal CVD, spin-on processes, and lamination pressing of dielectric laminate layers. The embodiments illustrated in  FIGS. 2A-3B  may also be encased in dielectric material in a manner similar to the embodiment illustrated in  FIGS. 4A-4D .  
         [0044]     The dielectric  120  preferably has relatively high thermal conductivity. High thermal conductivity in the material  120  allows heat from the heater  130  to more effectively heat the resistor  110 . Examples of dielectric materials with suitable thermal conductivity properties include silicon dioxide (SiO 2 ) and alumina (Al 2 O 3 ). These materials are also suitable to form the dielectrics  34 ,  50  and  80  illustrated in  FIGS. 1-3B .  
         [0045]     The heaters discussed above may be resistive elements that produce heat when a current passes therethrough. The heaters may be formed from, for example, refractory metals such as tantalum, tantalum nitride, tungsten, tungsten nitride, TiAl 3  and TiN. The heaters may also be formed from thin layers of high conductivity materials such as copper and aluminum. The heaters may be formed by any suitable method, such as, for example, chemical vapor deposition (CVD) and sputtering.  
         [0046]     The resistors in the above embodiments can be formed from any materials that are suitable for forming resistors. Such materials include, for example, conductors and semiconductors. If a linear or approximately linear coefficient of resistance is desired, then metals, including the refractory metals discussed above, are desirable. If a nonlinear coefficient of resistance is desired, semiconductors such as, for example, doped silicon and germanium are suitable. Materials such as bismuth and antimony are also suitable semiconductors. The resistors can be formed by methods such as, for example, chemical vapor deposition and sputtering.  
         [0047]     The resistors illustrated in  FIGS. 2A-4D  are relatively simple in configuration. Other resistor types and configurations can be made tunable according to the principles of the present invention. For example,  FIG. 5  illustrates a serpentine resistor  210  in isolation. The serpentine resistor  210  can be placed proximate to a heater (not shown) in a manner similar to the embodiments discussed above.  
         [0048]     Tuners used in the above embodiments may be any device that is capable of sensing resistances. Analog devices are examples of suitable tuners. The tuner may also be a mechanical device. Heater driver circuits used in the above embodiments may be, for example, current sources. A tuner and a heater can be part of a single component or circuit, or located at different points in an integrated circuit.  
         [0049]     As discussed above, the resistors according to the embodiments of the present invention can be actively tuned while the resistors are in use. For example, the resistor may be coupled to other circuit components on a semiconductor chip and serve as a resistive element of the chip, while the temperature and therefore the resistance of the resistor are actively monitored and controlled. The performance of the circuit incorporating the resistor may therefore be improved because the resistance of the resistor may be maintained within a nominal range.  
         [0050]     The above methods discuss active tuning of resistors while the resistors are in use. In an alternative embodiment, a one-time adjustment can be made to a resistor after the chip is diced and packaged. This embodiment is described below using the schematic embodiment shown in  FIG. 1  as an example.  
         [0051]     Referring to  FIG. 1 , the resistance of the resistor  32  is sensed after a chip housing the resistor  32  is diced and packaged. The heater  36  can elevate the temperature of the resistor  32  by a certain value before the resistance is sensed.  
         [0052]     The tuner  20  then determines the deviation of the resistance of the resistor  32  from a nominal resistance range. At this time, the heater driver circuit  10  may be permanently adjusted so that it generates a current that will to bring the resistor  32  into the desired resistance range. The heater driver circuit  10  can be adjusted by, for example, blowing one or more fuses in the heater driver circuit  10  to establish a constant current from the heater driver circuit  10 . Examples of suitable fuses include laser fuses and anti-fuses.  
         [0053]      FIGS. 6A-6D  illustrate a subtractive etch method for manufacturing a tunable resistor.  FIG. 6A  is a sectional view of a first stage of manufacture. In  FIG. 6A , wires  610  and vias  612  are fabricated. The wires  610  and vias  612  can be formed in dielectric material  620  using known methods such as subtractive etch or damascene processes. The wires  610  can be formed from, for example, a refractory metal lined with a metal such as copper or aluminum.  
         [0054]     Referring to  FIG. 6B , three layers (not shown in  FIG. 6B ), including a heater conductor layer, a high thermal conductivity insulative material, and a resistive layer are deposited over the dielectric  620 . These layers are then patterned using lithography and etched to form a heater  630 , a dielectric  640 , and a resistor  650 .  
         [0055]     Referring to  FIG. 6C , additional dielectric material is formed over the article shown in  FIG. 6B , resulting in dielectric material  660 . Wires  670  and vias  672  are formed in the dielectric material  660 . The wires  610  and vias  612  form conductors  615 , and the wires  670  and vias  672  form conductors  675 . The dielectric  640  preferably has relatively high thermal conductivity, and the dielectric materials  620 ,  660  preferably have relatively low thermal conductivity, as discussed above. The dielectric materials  620 ,  660  can be formed from, for example, one or more layers or bodies of dielectric material.  
         [0056]      FIG. 6D  illustrates an alternative subtractive etch fabrication method. The embodiment illustrated in  FIG. 6D  is similar to that of  FIG. 6C , and like reference numbers indicate like elements. In  FIG. 6D , a heater  630 ′ and a dielectric  640 ′ are formed by patterning and etching, and a resistor  650 ′ is patterned afterwards. The resistive layer used to form the resistor  650 ′ can be deposited after forming the heater  630 ′ and the dielectric  640 ′.  
         [0057]      FIGS. 7A-7D  illustrate a damascene method for manufacturing a tunable resistor.  FIG. 7A  is a sectional view of a first stage of manufacture. In  FIG. 7A , wires  710  and vias  712  are fabricated. The wires  710  and the vias  712  can be formed in a dielectric material  720  using methods such as subtractive etch or damascene processes. The wires  710  can be formed from, for example, a refractory metal lined with a metal such as copper, aluminum, tungsten or doped silicon.  
         [0058]     Referring to  FIG. 7B , an intermetal dielectric (not shown in  FIG. 7B ), such as SiLK™is deposited. The intermetal dielectric is then patterned and etched to form an intermetal dielectric  760  having a trench  761 . Three layers, including a heater conductor layer  730 , a high thermal conductivity insulative material layer  740 , and a resistive layer  750  are deposited over the intermetal dielectric  760 .  
         [0059]     Referring to  FIG. 7C , excess material is then removed from the wafer surface, leaving a heater  732 , a dielectric  742 , and a resistor  752 . Referring to  FIG. 7D , wires  770  and vias  772  are formed in a dielectric material  780  to connect to the resistor  752 . The dielectric materials  720 ,  780  can be formed from, for example, one or more layers or bodies of dielectric material.  
         [0060]      FIG. 7E  illustrates an alternative embodiment in which conductors  772 ′ connect to the top of a resistor  752 ′, and conductors  712 ′ connect to the top of a heater  732 ′. A dielectric  742 ′ is disposed between the resistor  752 ′ and the heater  732 ′. The conductors  712 ′,  772 ′ are illustrated as comprising a via and wire portion.  
         [0061]      FIG. 8  illustrates a hybrid method involving damascene and subtractive etch processes. In  FIG. 8 , a heater  830  is formed using subtractive etch processes. A dielectric  840  and a resistor  850  are formed using damascene processes. Alternatively, a heater and a dielectric can be formed using subtractive etch processes, and a resistor formed using damascene processes (not shown).  
         [0062]     In another alternative embodiment, a heater can be formed using damascene processes and a dielectric and a resistor can be formed using subtractive etching (not shown). In yet another embodiment, the heater can be formed using damascene processes and the resistor can be fabricated using subtractive etching (not shown).  
         [0063]      FIG. 9  illustrates an embodiment in which a resistor  950  contacts wires  970 . The wires  970  can be formed using, for example, subtractive etch or damascene processes. A heater  930  is illustrated as connected to wires  910  by way of vias  912 .  
         [0064]     In the above embodiments, the heater and resistor locations may be switched.  
         [0065]     In the above embodiments, wires formed by damascene processes can be formed by single damascene processes (as shown) or by dual damascene processes.  
         [0066]     In this specification, the terms “vertically”, “downwardly” and “upwardly” are used to describe elements in relation to the drawing figures, and are not intended to impart any required orientation on any elements described herein.  
         [0067]     The resistors and the associated materials and circuitry discussed above may be formed in many environments. Examples of suitable environments include over the silicon area of semiconductor chips, by fabrication as thin films on glass quartz substrates, in semiconductor packages, on Al 2 O 3  substrates, and on sapphire substrates.  
         [0068]     The heaters and resistors in the above embodiments can be of any dimensions suitable for incorporation in the environment utilized. Examples of heater thicknesses include a range of about 0.1 micrometers to 5 micrometers, and heater widths may be in the range of about 0.1 micrometers to about 10 mm. In one embodiment, a resistor has a thickness of 0.5 micrometers and a width of 5 micrometers. Examples of resistor thicknesses include a range of about 2 nanometers (nm) to about 0.1 micrometers. One resistor embodiment has a thickness of 50 nm. Resistor width may be the same or similar to heater width.  
         [0069]     In an alternative embodiment, a separate heater is not required to heat a resistor. In this embodiment, a DC current is applied directly to the resistor in order to heat the resistor.  
         [0070]     The foregoing description of the invention illustrates and describes the present invention. Additionally, the disclosure shows and describes only selected preferred embodiments of the invention, but it is to be understood that the invention is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings, and/or within the skill or knowledge of the relevant art.  
         [0071]     The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments, not explicitly defined in the detailed description.
The present invention discloses a device having a resistor; a heater disposed proximate to the resistor and capable of raising the temperature of the resistor; a dielectric disposed between the heater and the resistor; and a tuner electrically coupled to the resistor, wherein the heater adjusts the resistance of the resistor in response to the tuner.