Abstract:
A capacitive device is disclosed, including a first conductor formed on a lower metal layer and coupled to a first terminal. A second conductor is formed on an upper metal layer and a plurality of wires is partitioned into groups, each group including one wire from a respective metal layer. First and second wires of each group are coupled to a second terminal. A third wire of each group, adjacent to the first wire, is coupled to the first conductor. A fourth wire of each group, adjacent to the second wire, is coupled to the second conductor. Fifth wires of a first subset of the groups are coupled to the second conductor and fifth wires of a second subset of the groups are coupled to the first conductor. The fifth wire of each group is adjacent to the first wire and the second wire.

Description:
PRIORITY CLAIM 
       [0001]    The present application claims benefit of priority to provisional application No. 61/953,905 titled “PRECISION HALF CELL FOR SUB-FEMTO UNIT CAP AND CAPACITIVE DAC ARCHITECTURE IN SAR ADC” and filed on Mar. 16, 2014. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    Embodiments described herein are related to the field of capacitors used in semiconductor devices. More particularly, these embodiments relate to methods for implementing capacitors within an analog-to-digital converter circuit. 
         [0004]    2. Description of the Related Art 
         [0005]    An array of capacitors may be used as part of a digital-to-analog converter (DAC) circuit. DAC circuits are used in some analog-to-digital converter (ADC) architectures, such as successive approximation register (SAR) ADCs, for example. Small, accurately matched, capacitors are desirable for a DAC used in a SAR ADC to produce an accurate digital value representing a given analog signal input. SAR ADCs are used in some integrated circuits (ICs) designs, such as some system-on-a-chip (SoC) designs. 
         [0006]    Some ICs are manufactured in a semiconductor fabrication process that includes multiple layers of metal interconnects which are used to connect various circuit devices to each other to create various functional blocks that may be found in a given IC, including DACs and ADCs. The metal layers are separated from each other by a non-conductive layer, such as silicon dioxide (i.e., glass, or referred to herein as an “oxide layer” or simply “oxide”). Capacitors may be constructed from these multiple metal layers by processing each metal layer in a given region into specific shapes, such as wires and plates, and then connecting the various wires and plates to form a capacitor. 
         [0007]    To use capacitors in an IC design, a basic building block may be utilized, referred to herein as a unit capacitor cell. A unit capacitor cell in a given IC design may have a unit value of capacitance and building capacitors with a capacitance greater than the unit value requires combining two or more unit capacitor cells, allowing capacitors to be designed with capacitance values equal to an integer multiple of the unit value. 
         [0008]    In some IC designs, however, some circuits may benefit from a capacitor whose value that is not an integer multiple of a unit value of a unit capacitor cell. A method of designing and replicating a capacitor cell with capacitance less than one unit value is desired. 
       SUMMARY OF THE EMBODIMENTS 
       [0009]    Various embodiments of a capacitive device are disclosed. Broadly speaking, a capacitive device includes a first conductor formed on a lower metal wiring layer of a plurality of metal wiring layers, wherein the first conductor is coupled to a first terminal. The capacitive device also includes a second conductor formed on an upper metal wiring layer of the plurality of metal wiring layers, and a plurality of parallel wires partitioned into a plurality of groups, wherein parallel wires included in each group of the plurality of groups are formed on a respective one of a subset of the plurality of metal wiring layers, wherein the subset of the plurality of metal wiring layers is between the upper metal wiring layer and the lower metal wiring layer. A first parallel wire and a second parallel wire of each group of the plurality of groups are coupled to a second terminal. A third parallel wire of each group of the plurality of groups is coupled to the first conductor, wherein the third parallel wire is adjacent to the first parallel wire. A fourth parallel wire of each group of the plurality of groups is coupled to the second conductor, wherein the fourth parallel wire is adjacent to the second parallel wire. A fifth parallel wire of each group of a first subset of the plurality groups is coupled to the second conductor, and wherein the fifth parallel wire of each group of a second subset of the plurality of groups is coupled to the first conductor. Also, the fifth parallel wire of each group of the plurality of groups is adjacent to the first parallel wire and the second parallel wire. 
         [0010]    In a further embodiment, the first subset of the plurality of groups is mutually exclusive to the second subset of the plurality of groups. In a still further embodiment, a number of groups included in the first subset of the plurality of groups is equal to a number of groups included in the second subset of the plurality of groups. 
         [0011]    In another embodiment, an edge of the third parallel wire included in a given group of the plurality of groups is parallel to an edge of the third parallel wire included in any other given group of the plurality of groups. In one embodiment, a dielectric material is included between each adjacent parallel wire in a given group of the plurality of groups. 
         [0012]    In a given embodiment, the second conductor is coupled to a third terminal. In a further embodiment, the third terminal is coupled to a ground voltage reference. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The following detailed description makes reference to the accompanying drawings, which are now briefly described. 
           [0014]      FIG. 1  illustrates a block diagram of an embodiment of an analog-to-digital converter. 
           [0015]      FIG. 2  shows a first embodiment of a structure for a unit capacitor cell. 
           [0016]      FIG. 3  illustrates an embodiment of a structure for a sub-unit capacitor cell. 
           [0017]      FIG. 4  illustrates an alternate embodiment of a structure for a sub-unit capacitor cell. 
           [0018]      FIG. 5  illustrates a cross section of an embodiment of a single stack of wires used in an integrated circuit. 
           [0019]      FIG. 6A  shows a 2-dimensional representation of an array of capacitor cell structures. 
           [0020]      FIG. 6B  illustrates a circuit diagram representing the array of capacitor cell structures. 
           [0021]      FIG. 7  illustrates a flowchart of a method for operating an embodiment of an analog-to-digital converter. 
           [0022]      FIG. 8  shows an embodiment of an array of capacitive cells including a row of border cells. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    To construct a desired capacitor value in an IC design, such as in an analog-to-digital converter (ADC) unit, an array of unit capacitor cells may be utilized. Employing such an array, may require connecting terminals of two or more unit capacitor cells in parallel, allowing capacitors to be designed with capacitance values equal to an integer multiple of the unit value. To build a capacitor with a capacitance value that is not an integer multiple of a unit value of a capacitor cell, a sub-unit capacitor cell with capacitance less than one unit value may be employed. As used herein, a sub-unit capacitor cell refers to a capacitor cell in which the capacitance value is less than one unit value of capacitance. For example, the capacitance of a unit capacitor cell may be 10 femtofarads, making the capacitance value of a sub-unit capacitor anything less than 10 femtofarads. 
         [0024]    In  FIG. 1 , a block diagram of an embodiment of an analog-to-digital converter is illustrated. ADC  100  is an embodiment of a successive approximation register (SAR) ADC which may be included in an SoC device. ADC  100  includes SAR control unit  101 , digital-to-analog converter (DAC)  103 , and comparator circuit  105 . ADC  100  receives input signal  110  to be measured, reference signal  112 , and communicates with other portions of SoC via system bus  114 . 
         [0025]    SAR control unit  101  may correspond to a state machine or other suitable processing unit designed to adjust and route signals to DAC  103  and comparator  105  in order to determine a digital value corresponding to a voltage level of an input signal. In operation, SAR control unit  101  may receive a command via system bus  114  to begin a measurement of the voltage level of input signal  110 . In response to receiving the command, SAR control unit adjusts switches in DAC  103  to couple input signal  110  to a first terminal of each of a plurality of capacitors  107  within DAC  103  and adjust switches to couple a second terminal of each of capacitors  107  to a ground signal. Each of capacitors  107  will begin charging and SAR control unit  101  allows the various capacitors to charge to a voltage level equal to the voltage level of input signal  110 , at which point SAR control unit  101  decouples the first terminal from input signal  110 . This process is sometimes referred to as “sampling the input.” 
         [0026]    DAC  103  is implemented as a capacitive DAC, i.e., an array of capacitors are used rather than an array of resistors such as used in resistive DACs. DAC  103  may receive a series of digital signals from SAR control unit  101  and, in response, output a corresponding voltage level. DAC  103  includes capacitors  107  and a plurality of switches that enable the first terminal of each of capacitors  107  to be independently coupled to either input signal  110  or reference signal  112 . Capacitors  107  are designed such that a first capacitor has a first capacitance value and each additional capacitor has a capacitance value equal to one half of the capacitance of the prior capacitor. For example, if the first capacitor has a capacitance of ‘C,’ then the second capacitor would have a capacitance of ½C, the third would have a capacitance of ¼C, then ⅛C, and so on. 
         [0027]    Once input signal  110  has been sampled, then SAR control unit  101  couples the first terminal of the first capacitor to reference signal  112 , then couples the second terminal of each of the plurality of capacitors to a first input of comparator circuit  105 . An output of the comparator corresponds to the most significant bit (MSB) of a value corresponding to the voltage level of input signal  110 . SAR control circuit  101  decouples the second terminal of the capacitors from comparator circuit  105  and then couples the first terminal of the second capacitor to reference signal  112  and then again couples the second terminal of each capacitor to the first input of comparator circuit  105 . The updated output of the comparator corresponds to the second MSB of the value corresponding to the voltage level of input signal  110 . This process repeats until all bits of the value corresponding to the voltage level of input signal  110  have been determined. In various embodiments, the result may be stored in a register within SAR control unit  101  or may be output onto system bus  114 . 
         [0028]    A total number of capacitors required for DAC  103  is dependent upon a resolution of ADC  100 , i.e., a number of bits of the value representing the voltage of input signal  110  (i.e., the result). At least one capacitor is needed for each bit. In some embodiments, additional capacitors may be required for sampling input signal  110 , for stabilizing or adjusting reference signal  112 , for general noise reduction, etc. Accuracy for ADC  100  is dependent on the relative capacitance values of each capacitor for each bit of the result. As mentioned above, if the capacitance for the first capacitor corresponding to the MSB is ‘C’, then the capacitance for the second capacitor corresponding to the second MSB needs to be ½C for the best possible accuracy. The more the capacitance of the second capacitor deviates from ½C, the less accurate the measurement for the second MSB will be. The same principal applies to the remaining capacitors of the plurality of capacitors. Therefore, it may be desirable that design of DAC  103  include capacitor designs that can be adjusted to a fine resolution of capacitance. 
         [0029]    The total capacitor value is dictated by the performance requirement. The total size of the capacitor array is, however, determined by the smallest unit cell that can be generated to satisfy the matching requirement. In some embodiments, when compared to other DAC designs, such as, for example, a resistive ladder DAC, a capacitive DAC such as DAC  103  may have advantages including compact area and low power. 
         [0030]    It is noted that ADC  100  of  FIG. 1  is merely an example for demonstration of disclosed concepts. Some functional components and some operational details have been omitted to focus on the disclosed subject matter. In other embodiments, additional functional units may be included and operation may deviate from the description above. 
         [0031]    Turning to  FIG. 2 , an embodiment of a structure for the ADC unit capacitor cell is shown. Unit capacitor cell  200  may be one structure used in a capacitive DAC in an SoC, such as, e.g., DAC  103 , to create capacitors of various capacitances by linking to other unit capacitor cells. Unit capacitor cell  200  is a three dimensional structure employing a plurality of metal layers formed during manufacture of the SoC. Generally speaking, metal layers in a semiconductor manufacturing process may be referred to in the order each layer is deposited onto circuits of an IC, from the first layer (metal-1) to the last layer (metal-6 as illustrated). Unit capacitor cell  200  includes top plate  202  and bottom plate  204  which correspond to a first terminal and a second terminal of a capacitor encompassing unit capacitor cell  200 . Top plate  202  and bottom plate  204  each include respective portions of a plurality of wires (wire  214   a  through wire  222   d ) created in metal-2 through metal-5, with the wires all running parallel to each other within unit capacitor cell  200 . Unit capacitor cell  200  also includes conductors  210  and  212 . 
         [0032]    Top plate  202  includes conductors  210  and  212  formed in metal-1 (conductor  210 ) and metal-6 (conductor  212 ). Between conductor  210  and conductor  212 , multiple layers of wires (wire  214   a  through wire  222   d ) are created in metal-2 through metal-5. A first portion of the wires (wires  214   a - d ,  216   a - d , and  218   a - d ) are connected through metal vias (examples of which are highlighted as vias  230 ) to each other and to conductors  210  and  212  to form top plate  202 . Bottom plate  204  includes wires  220   a - d  and wires  222   a - d  in metal layers 2-5. Wires  220   a - d  are connected to each other by vias as are wires  222   a - d . Wires  220   a - d  may be connected to wires  222   a - d  external by one or more of metal layers 2-5 external to the structure of unit capacitor cell  200 . 
         [0033]    The space around the wires  214 - 222  and conductors  210  and  212  are filled by a dielectric material (not illustrated), such as, for example silicon dioxide. Silicon dioxide is a commonly used dielectric to form capacitors in ICs and is also referred to herein as “oxide.” Capacitance is created in unit capacitor cell  200  due to effects of electric fields surrounding wires  220   a - d  on wires  214   a - d  and  216   a - d  and electric fields surrounding wires  222   a - d  on wires  216   a - d  and wires  218   a - d . Capacitance may also be created from coupling of wires  220   d  and  222   d  to conductor  210  and from coupling of wires  220   a  and  222   a  to conductor  212 . The amount of capacitance of unit capacitor cell  200  is determined by the length and size of the wires  214 - 222 , the spacing between the wires, and the properties of the dielectric used between wires of top plate  202  and wires of bottom plate  204 . 
         [0034]    Bottom plate  204  may be a sensitive node and parasitic stray capacitances from other circuits near unit capacitor cell  200  may couple unwanted signals to bottom plate  204 . In various embodiments, it may be advantageous to minimize such coupling to bottom plate  204 . In the architecture of unit capacitor cell  200 , bottom plate  204  is covered by the top plate  202  on four sides. Conductors  210  and  212 , as well as wires  214   a - d  and wires  218   a - d , protect bottom plate  204  from external disturbances that may be generated due to other circuits near the capacitor cell. 
         [0035]    Unit capacitor cell  200  may be designed towards a goal of achieving a uniform distribution of metal and oxide in order to create as consistent as possible capacitance value for all unit capacitor cells used in DAC  103 . The structure of unit capacitor cell  200  includes arranging wires  214   a - 222   d  in an array, leading to a symmetrical layout which may achieve a uniform density. This may allow the manufacturing to be precise and accurate such that matching can be achieved between individual unit capacitor cells used in DAC  103 . 
         [0036]    It is noted that unit capacitor cell  200  in  FIG. 2  is merely an example. In other embodiments, a number of metal layers included in the capacitor cell may differ. In addition, more than the illustrated five stacks of wires may be utilized. Structures other than those illustrated may be included as part of a given unit capacitor cell design. 
         [0037]    Moving to  FIG. 3  an embodiment of a structure for a sub-unit capacitor cell is illustrated. Sub-unit capacitor cell  300  may be another structure used in DAC  103  in  FIG. 1  to create capacitors of various values. Sub-unit capacitor cell  300 , similar to unit capacitor cell  200  in  FIG. 2 , is a three dimensional structure formed in a plurality of metal layers, and is designed to have a capacitance that is less than that of unit capacitor cell  200 . Also similar to unit capacitor cell  200 , sub-unit capacitor cell  300  includes top plate  302  and bottom plate  304 , corresponding to a first terminal and a second terminal of a capacitor encompassing sub-unit capacitor cell  300 . Also included in sub-unit capacitor cell  300  is third plate  306 , corresponding to a third terminal of the capacitor encompassing sub-unit capacitor cell  300 . Top plate  302 , bottom plate  304 , and third plate  306  each include respective portions of a plurality of wires (wire  314   a  through wire  322   d ) created in metal-2 through metal-5, with the wires all running parallel to each other within sub-unit capacitor cell  300 . Sub-unit capacitor cell  300  also includes conductors  310  and  312 . 
         [0038]    Two capacitors may be formed in the single cell structure of sub-unit capacitor cell  300 , with the capacitance of each capacitor less than that of unit capacitor cell  200 . A first capacitor is formed between top plate  302  and bottom plate  304  while the second capacitor is formed between bottom plate  304  and third plate  306 . Bottom plate  304  is created similarly to bottom plate  204  of unit capacitor cell  200 , and includes wires  320   a - d  and  322   a - d . Top plate  302 , however, includes fewer wires than top plate  202  of unit capacitor cell  200 . Only wires  314   a - d  and  316   c - d  are connected to conductor  310  through metal vias to form top plate  302 . Wires  316   a - b  and  318   a - d  are connected to conductor  312  through vias to form third plate  306 . In other words, the wires connected to conductor  310  are mutually exclusive to the wires connected to conductor  312 . 
         [0039]    Compared to unit capacitor cell  200  in  FIG. 2 , it may be seen how sub-unit capacitor cell  300  is created from a similar cell structure. By omitting a row of vias, wire  314   a  is not connected to conductor  312  the way that wire  214   a  is connected to conductor  212 . Additionally, a row of vias may be omitted between wires  316   b  and  316   c  as well as between wire  318   d  and conductor  310 . By omitting these three rows of vias, top plate  302  is disconnected from third plate  306 . It is noted that, in the illustrated embodiment, top plate  302  and third plate  312  each include one conductor ( 310  and  312 , respectively) and six wires ( 314   a - d  and  316   c - d  in top plate  302  and  316   a - b  and  318   a - d  in third plate  306 ). In essence, top plate  202  of unit capacitor cell  200  is split in half to form top plate  302  and third plate  306  of sub-unit capacitor cell  300 . In some embodiments, this equal distribution of wires and conductors between top plate  302  and third plate  306  may cause the capacitance of each of the two capacitors in sub-unit capacitor cell  300  to be one-half of the capacitance of unit capacitor cell  200 . It may be observed that top plate  302  and bottom plate  306 , in addition to including equal distributions of wires and conductors also forms an equivalent, symmetrical shape. The symmetry of the two plates may contribute, in some embodiments, to an accuracy of the capacitance of each of the two capacitors in sub-unit capacitor cell  300 . By using top plate  302  and bottom plate  304  in place of top plate  202  and bottom plate  204 , half as much capacitance may be added to a circuit. In such an embodiment, third plate  306  may be coupled to a ground reference signal or a power supply signal to be used as shielding for a sensitive signal coupled to bottom plate  304 . 
         [0040]    It is noted that sub-unit capacitor cell  300  is an example intended to demonstrate disclosed subject matter. In other embodiments, the structure of  FIG. 3  may differ from the structure illustrated. For example, the various vias used to connect the wires and conductors may be repositioned or a number of rows or columns of wires may be different. 
         [0041]    Turning now to  FIG. 4 , an alternate embodiment of a structure for a sub-unit capacitor cell is shown. Sub-unit capacitor cell  400  may be another structure used in DAC  103  in  FIG. 1  to create capacitors of various values. Sub-unit capacitor cell  400 , similar to unit capacitor cell  200  in  FIG. 2  and sub-unit capacitor cell  300  in  FIG. 3 , is a three dimensional structure formed in a plurality of metal layers. Also, similar to sub-unit capacitor cell  300 , sub-unit capacitor cell  400  includes top plate  402 , bottom plate  404 , and third plate  406 . Top plate  402 , bottom plate  404 , and third plate  406  each include respective portions of a plurality of parallel wires (wire  414   a  through wire  422   d ) created in metal-2 through metal-5. Sub-unit capacitor cell  400  also includes conductors  410  and  412 . Like sub-unit capacitor cell  300 , sub-unit capacitor cell  400  may be used to create a capacitor cell with two capacitors, each capacitor with less capacitance than unit capacitor cell  200 . 
         [0042]    Compared to sub-unit capacitor cell  300  in  FIG. 3 , it may be seen how sub-unit capacitor cell  400  is created from the similar cell structure. In the embodiment illustrated in  FIG. 3 , four wires on one side of the cell structure,  314   a - d , are connected to conductor  310  and four wires on the opposite side of the cell structure,  318   a - d , are connected to conductor  312 . In contrast, the embodiment illustrated in  FIG. 4  has two wires on each of these two sides of the cell structure, which are connected to each conductor. Wires  414   c - d  and  418   c - d  are connected to conductor  410 . Similarly, wires  414   a - b  and  418   a - b  are connected to conductor  412 . It is noted that the wires in the middle of the cell structure,  416   a - d , do not change connections, with  416   a - b  connected to conductor  412  and  416   c - d  connected to conductor  410 . 
         [0043]    It is noted that the design of sub-unit capacitor cell  400  creates top plate  402  out of metal-1 (conductor  410 ), metal-2 (wires  414   d ,  416   d , and  418   d ) and metal-3 (wires  414   c ,  416   c , and  418   c ) only, while third plate  406  includes only metal-6 (conductor  412 ), metal-5 (wires  414   a ,  416   a , and  418   a ), and metal-4 (wires  414   b ,  416   b , and  418   b ). Similar to sub-unit capacitor cell  300 , each of top plate  402  and third plate  406  include six wires and one conductor, and consequently, the two capacitors are designed to each have a capacitance equal to one-half of the capacitance of unit capacitor cell  200 . Since top plate  402  and third plate  406  do not include wires from a same metal layer, if a given metal layer, from metal-2 through metal-5, experiences non-conformities during manufacturing, the capacitor that includes the non-conforming metal layer may have a different capacitance from the other capacitor in the same cell structure. For example, if metal-4 is over etched during manufacturing, then wires  414   b ,  416   b ,  418   b ,  420   b , and  422   b  may be thinner than the corresponding wires in the other metal layers. This difference may cause the capacitor from bottom plate  404  to third plate  406  to be lower than the capacitor from top plate  402  and bottom plate  404 . If these capacitors are used in a design such as DAC  103  in  FIG. 1 , the accuracy of DAC may be negatively impacted. Metal layer non-conformities are addressed in further detail below. 
         [0044]    It is noted that sub-unit capacitor cell  400  is merely an example. In other embodiments, the structure of  FIG. 4  may differ from the structure illustrated. Dimensions may differ as well the number of metal layers used. 
         [0045]    Moving now to  FIG. 5 , a cross section of an embodiment of a single stack of wires used in an integrated circuit is shown. The wires may correspond to any vertical stack of wires shown in  FIGS. 2-4 , such as, for example, wires  314   a - d  in  FIG. 3  or  414   a - d  in  FIG. 4 . Wires  514   a - d  are shown as a cross section and, to correspond to  FIGS. 2-4 , are created in metal-2 ( 514   d ) through metal-5 ( 514   a ). 
         [0046]    In the design of a unit or sub-unit capacitor cell, wires  514   a - d  may be drawn (either by hand or by IC design software) to be aligned vertically and to be of equal line widths and heights. In some semiconductor manufacturing processes, however, wires created in the various metal layers may have different dimensions despite being designed to be the same dimensions. In some cases, dimensional variations may be due to non-conformities as mentioned above in regards to  FIG. 4 . “Non-conformities” as used herein may refer to various processing steps during IC manufacturing that deviate from expected parameters. For example, non-conformities may include lithographic variations which may result in over-etching or under etching of a metal layer, resulting wires of differing widths. Non-conformities may also include depositing too much or too little metal or having an uneven deposition of a layer of metal, which may lead to wires that are taller or shorter than other wires in the same cell structure. 
         [0047]    In some semiconductor manufacturing processes, a certain amount of dimensional variation may be expected. For example, some processes may include different design rules for each layer of metal to compensate for changes in thermal properties for each additional metal layer. A progressive widening of metal wires from metal-1 to metal-6 (and beyond) may, therefore, be an intentional design strategy in some processes. 
         [0048]    In the illustrated example, wires  514   a - d  are shown to have varying line widths (the smallest labeled ‘X’) and varying wire heights (the smallest labeled ‘Y’). As can be seen, the metal-2 wire ( 514   d ) is the thinnest and the metal-5 wire ( 514   a ) is the thickest, with the metal-3 ( 514   c ) and metal-4 ( 514   b ) layers increasing at each layer. In addition, the metal-2 wire ( 514   d ) is the tallest and the metal-5 wire ( 514   a ) is the shortest, with the metal-3 ( 514   c ) and metal-4 ( 514   b ) layers decreasing at each layer. 
         [0049]    In many circuits, these dimensional variations of the metal layers may not have a significant impact on performance of the circuits. When these metal layers are used to create capacitors, however, the dimensional variations may impact the capacitance of capacitor cells such as those shown in  FIGS. 2-4 . Referring back to  FIG. 4 , if wires  414   a - 422   d  have relative wire thicknesses for each metal layer corresponding to  FIG. 5 , then the wires in metal-5 will be closer to each other than the wires of metal-1. As a result, the capacitance between top plate  402  and bottom plate  404  will be less than the capacitance between bottom plate  404  and third plate  406 . In the design of DAC  103 , sub-unit capacitor cell  400  may be used provide a capacitance value that is one half of the capacitance value of unit capacitor cell  200 . Due to the wire thickness variations however, a capacitor created using top plate  402  and bottom plate  404  will have a capacitance that is less than one half of the capacitance of unit capacitor cell  200 . 
         [0050]    Sub-unit capacitor cell  300  of  FIG. 3  may be used to mitigate at least a portion of the effects of wire thickness variations. If wires  314   a - 322   d  have relative wire thicknesses for each metal layer corresponding to  FIG. 5 , then the wires in metal-5 will be closer to each other than the wires of metal-1, just as in the  FIG. 4  example. In sub-unit capacitor cell  300 , however, since both top plate  302  and third plate  306  include wires from metal-2 through metal-5, the dimensional variations are mitigated to some degree. Wire  314   a  in metal-5 will be thicker than wire  318   d  in metal-2. Since wire  314   a  is included in top plate  302  and wire  318   d  is included in third plate  306 , the variations between metal layers may be at least partially averaged out. The capacitance value between top plate  302  and bottom plate  304  may be closer to one half the capacitance of unit capacitor cell  200  than the capacitance value between top plate  402  and bottom plate  404  when created from similar cell structures. 
         [0051]    It is noted that  FIG. 5  is merely an example for demonstrating the disclosed subject matter. In other embodiments, dimensional variations between wires created in different metal layers may have different relative properties. For example, relative wire heights may not gradually decrease in each metal layer as shown on  FIG. 5 . Likewise, relative wire widths may not gradually increase in each metal layer as shown. 
         [0052]      FIG. 6A  illustrates a 2-dimensional representation of an embodiment of an array of capacitor cell structures. Capacitor array  600  may correspond to a capacitor array used in a capacitive DAC circuit, such as, for example, DAC  103  in  FIG. 1 . Capacitor array  600  includes a plurality of cell structures,  611   a  through  614   n , each of which can be designed as either a unit capacitor cell  200  of  FIG. 2  or a sub-unit capacitor cell  300  of  FIG. 3 . The view of  FIG. 6  is looking down at the top of the array of cell structures with the parallel wires (e.g.  314   a - 322   d ) running vertically from the top of the array to the bottom. 
         [0053]    In the illustrated embodiment, bottom plates  604  of capacitor array  600  are connected to a common node. Wires  620   a - n  correspond to wires  220   a  or wires  320   a  in  FIGS. 2 and 3 , respectively, while wires  622   a - n , likewise, correspond to wires  222   a  or wires  322   a . While only wires created in metal-5 (i.e.,  222   a  or  322   a ) may be visible in the figure, other wires aligned to  222   a  and  322   a  run underneath (e.g.,  222   b - d  or  322   b - d ). By running the wires of the bottom plates of each cell structure  611   a - 614   n  through each neighboring cell structure, the bottom plates of each cell structure can be connected to the same node at the edge of the array in any of metal layers 2-5. 
         [0054]    Top plates  602  are connected in various groups to create a plurality of capacitors, each with a capacitance value determined by a number of cell structures connected and by whether each of the connected cell structures is designed as a unit capacitor cell  200  or a sub-unit capacitor cell  300 . In various embodiments, the plurality of capacitors may have a same capacitance value, a different capacitance value or any suitable combination thereof. In the present embodiment, four capacitors are shown,  630 - 633 . Capacitor  630  includes cell structures  611   a ,  611   b , and  611   c . Capacitor  631  includes cell structures  612   a ,  612   b ,  613   b , and  613   c . Capacitor  632  includes cell structure  613   a  and capacitor  633  includes cell structures  614   a ,  614   b , and  614   c . Individual cell structures for each capacitor are joined in metal-1 by wires connected between each connector  210  or connector  310 . As shown by capacitor  631 , a capacitor can be created from cell structures from more than one row or column. 
         [0055]    Moving to  FIG. 6B , an equivalent circuit for the capacitors  630 - 633  of  FIG. 6A  is illustrated. As described above, the bottom plates of each capacitor  630 - 633  are connected to a common node. In contrast, the top plates of each capacitor  630 - 633  are not connected and may each be coupled to a different signal. 
         [0056]    To determine a capacitance value for each capacitor  630 - 633 , a sum is calculated for the capacitance of each cell structure included in each capacitor. For example, if the capacitance of each unit capacitor cell  200  is one femtofarads (fF) then the capacitance of each sub-unit capacitor cell is one-half of that of a unit capacitor cell, i.e., 0.5 fF. Assuming that the ‘a’ column of cell structures ( 611   a - 614   a ) are designed as sub-unit capacitor cells  300  and the other columns ( 611   b - 614   d ) are designed as unit capacitor cells  200 , then the capacitance of capacitor  631  is 0.5 fF ( 612   a ) plus 1 fF ( 612   b ) plus 1 fF ( 613   c ) plus 1 fF ( 613   d ) for a total of 3.5 fF. Capacitance values for the other capacitors will be 2.5 fF for capacitor  630 , 0.5 fF for capacitor  632 , and 2.5 fF for capacitor  633 . 
         [0057]    It is noted that  FIG. 6A  and  FIG. 6B  are merely examples. In other embodiments, capacitor array  600  may include various numbers of rows and columns. Capacitors may be created using any suitable number of cell structures and each cell structure may individually be designed as a unit capacitor cell or a sub-unit capacitor cell. The bottom plates may not be connected together as illustrated and one or more top plates may be connected together. 
         [0058]    Turning to  FIG. 7 , a flowchart of a method for operating an embodiment of an analog-to-digital converter (ADC) is shown. Method  700  may be used to operate a SAR ADC, such as, for example, ADC  100  in  FIG. 1 . ADC  100  may further include a capacitor array such as, for example, capacitor array  600  in  FIG. 6 . Referring collectively to  FIG. 1 ,  FIG. 6 , and  FIG. 7 , the method may begin in block  701 . 
         [0059]    ADC  100  receives an input signal (block  702 ). The input signal corresponds to a signal for which a voltage level is to be measured. The input signal may have a slowly changing (relative to the conversion speed of ADC  100 ) voltage level. For example, an output of a temperature sensor in an enclosure may change some number of millivolts per second. In other embodiments, the voltage level of the input signal may change more rapidly, such as, e.g., an output of a microphone, which may rise and fall by a volt in less than one microsecond. In some embodiments, in particular to measure a fast transitioning input signal, ADC  100  may sample the input signal for a predetermined period of time to capture the voltage level at a particular point in time. 
         [0060]    ADC  100  connects the input signal to a first terminal of a plurality of capacitors in capacitor array  600  (block  702 ). DAC may include a plurality of switching circuits (e.g., analog multiplexors, transmission gates, etc.) to couple the input signal or other reference signals to each of the plurality of capacitors. In some embodiments, the plurality of capacitors may include all capacitors in capacitor array  600  while in other embodiments, a proper subset of capacitors in array  600  may be included in the plurality of capacitors. SAR control logic  101  adjusts the switching circuits to couple the input signal to the first terminal of each of the plurality of capacitors. The first terminal may correspond to top plates  602  of capacitors  630 - 633 . 
         [0061]    SAR control logic  101  adjusts switching circuits to couple bottom plates  604  to a ground reference voltage while top plates  602  are coupled to the input signal (block  706 ). This adjustment allows capacitors  630 - 633  to charge to the current voltage level of the input signal. Charging the capacitors to the voltage level of the input signal may be referred to as “sampling” the input signal. It is noted that if the voltage level across capacitors  630 - 633  begins higher than the voltage level of the input signal, then capacitors  630 - 633  will discharge rather than charge to reach the voltage level of the input signal. 
         [0062]    The further actions of the method may depend on the voltage level across each capacitor of the plurality of capacitors (block  708 ). In some embodiments, SAR control logic may keep top plates  602  coupled to the input signal and bottom plates  604  coupled to the ground reference for a predetermined amount of time long enough to ensure the voltage level across capacitors  630 - 633  is equal to the voltage level of the input signal. In other embodiments, comparator  105  may be used to determine that capacitors  630 - 633  have charged to the voltage level of the input signal. In either embodiment, if the voltage level across capacitors  630 - 633  is not equal to the voltage level of the input signal, then the method may remain in block  708 . Otherwise, the method may move to block  710 . 
         [0063]    One capacitor of the plurality of capacitors may be selected and charged to the voltage level of a reference signal (block  710 ). Each capacitor of the plurality of capacitors may correspond to one bit of a digital result determined by ADC  100 . For example, if ADC  100  includes a twelve-bit result register, then the plurality of capacitors includes at least twelve capacitors, one for each bit of the result (additional capacitors may also be included for signal conditioning or other purposes). The capacitor corresponding to the most significant bit (MSB) of the digital result has the largest capacitance value of the twelve capacitors. The capacitance value for each subsequent capacitor corresponding to the next most significant bit is one-half of the capacitance of the previous capacitor. A value of each of the capacitors in array  600  in DAC  103  is critical to the accuracy of ADC  100 . Capacitors in array  600  may be sensitive to parasitic capacitances from surrounding circuits and from mechanical stress due to temperature changes or physical pressure on the IC. Any mismatch in the capacitances may cause non-linearity issues in the ADC performance, resulting in less accurate results. Use of sub-unit capacitor cell  300  in capacitor array  600  may help to achieve a high degree of capacitance matching between the capacitors in array  600 , resulting in accurate performance of ADC  100 , even in the presence of mechanical and temperature induced stress. To determine the digital result, each capacitor is selected, one at a time beginning with the MSB capacitor, and the top plate of the selected capacitor is coupled to a first reference voltage signal. 
         [0064]    The further actions of the method may again depend on the voltage level across each capacitor of the plurality of capacitors (block  712 ). After the selected capacitor has been coupled to the reference voltage, the bottom plates  604  of the plurality of capacitors, including the selected capacitor, are coupled to comparator  105  and the voltage level at the bottom plates  604  is compared to a second reference voltage level. The value of the bit corresponding to the selected capacitor is determined by the output of comparator  105 . 
         [0065]    If the output of comparator  105  is a logic low, then the value of the bit corresponding to the selected capacitor is a ‘0’ (block  714 ). A logic low from comparator  105  may correspond to the voltage at the bottom plates  604  being less than the second reference voltage. 
         [0066]    If the output of comparator  105  is a logic high, then the value of the bit corresponding to the selected capacitor is a ‘1’ (block  716 ). A logic high from comparator  105  may correspond to the voltage at the bottom plates  604  being greater than the second reference voltage. 
         [0067]    Further actions of the method may depend on a number of capacitors selected (block  718 ). If all capacitors corresponding to a bit of the digital result have not been selected and coupled to the second reference voltage, then the method may return to block  710  to select the next capacitor. Otherwise, method  700  may be complete and end in block  720 . 
         [0068]    It is noted that, in regards to a Complementary Metal-Oxide-Semiconductor Field-Effect Transistor (or Complementary MOSFET, or simply CMOS) circuit design, “logic 1”, “high”, “high state”, or “high level” refers to a voltage sufficiently large to turn on a n-channel MOSFET and turn off a p-channel MOSFET, while “logic 0”, “low”, “low state”, or “low level” refers to a voltage that is sufficiently small enough to do the opposite. In other embodiments, different technology may result in different voltage levels for “low” and “high.” 
         [0069]    It is also noted that method  700  is an example method for operating an embodiment of a SAR ADC. Many embodiments of SAR ADCs are known and methods for operating other embodiments may differ from the operations disclosed in method  700 . A different number of operations may be performed and some operations illustrated to occur in series may be performed in parallel. 
         [0070]    Moving now to  FIG. 8 , an embodiment of an array of capacitive cells including a row of border cells is presented. Capacitive array  800  includes multiple capacitive unit/sub-unit cells  803  including top plates, third plates and bottom plates. Along an edge of capacitive array  800  is a row of border cells  805 . Between capacitive cells  803  and border cells  805  are input signal wires  807  which connect one or more input signals to top plates of one or more capacitive cells  803 . Capacitive array may be used, for example, to create capacitors  107  in DAC  103  of SAR ADC  100  in  FIG. 1 . 
         [0071]    Capacitive cells  803  includes multiple cells which are a mix of unit cells such as, e.g., capacitive unit cell  200  in  FIG. 2  and capacitive sub-unit cell  300  in  FIG. 3 . Although eight capacitive cells  803  are illustrated, any suitable number may be included with any suitable mix of unit and sub-unit cells. Each capacitive cell  803  may be connected to one or more other capacitive cells  803  to form one or more capacitors with various capacitances. Referring to the example of DAC  103 , several capacitors are created in which successive capacitors have capacitances equal to one-half of a previous capacitor. Input signal wires  807  connect signals, such as, for example, analog input signals from SAR control unit  101 , to the capacitors created in capacitive array  800 . In the present embodiment, four input signal wires  807   a - d  connect input signals to four capacitors in the array. Between border cells  805  and capacitive cells  803 , each input signal wire  807   a - d  includes a stack of wires created in the same metal layers as the capacitive cells. This structure of stacked wires may, in some embodiments, create a more uniform coupling to each capacitor, helping to create a capacitive DAC  103  with capacitors  107  that have more accurate and predictable capacitances thereby allowing for an accurate DAC  103  and consequently an accurate SAR ADC  100 . 
         [0072]    To further the accuracy of the capacitors created in capacitive array  800 , border cells  805  are created next to the wire stacks of input signal wires  807 , opposite of capacitive cells  803 . Border cells  805  are created in the same metal layers as capacitive cells  803  and are created with a similar structure. For example, conductors are created in the top and bottom layers of the structure and a plurality of parallel wires run in between the top and bottom conductors. Using a similar structure for border cells  805  as for capacitive cells  803  may provide a balance to both electrical coupling of the input signal wires  807  to capacitive cells  803  as well as mechanical stresses due to temperature changes and physical forces on the chip. This balancing of the coupling of the input signals may further improve accuracy of the capacitors in the capacitive array and therefore to SAR ADC  100 . 
         [0073]    It is noted that the structure of  FIG. 8  is merely an example. Other embodiments may include any suitable number of input signals and any number of capacitive cells and border cells. Although three metal layers are shown in  FIG. 8 , any suitable number of metal layers may be used in other embodiments. 
         [0074]    Although specific embodiments have been described above, these embodiments are not intended to limit the scope of the present disclosure, even where only a single embodiment is described with respect to a particular feature. Examples of features provided in the disclosure are intended to be illustrative rather than restrictive unless stated otherwise. The above description is intended to cover such alternatives, modifications, and equivalents as would be apparent to a person skilled in the art having the benefit of this disclosure. 
         [0075]    The scope of the present disclosure includes any feature or combination of features disclosed herein (either explicitly or implicitly), or any generalization thereof, whether or not it mitigates any or all of the problems addressed herein. Accordingly, new claims may be formulated during prosecution of this application (or an application claiming priority thereto) to any such combination of features. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in the specific combinations enumerated in the appended claims.