Abstract:
Apparatus for integrated capacitors and associated methods are disclosed. In one embodiment, an integrated capacitor includes a first plurality of metal members that are fabricated using a first plurality of metal layers, and are oriented in a first orientation. The integrated capacitor also includes a second plurality of metal members that are fabricated using a second plurality of metal layers. The second plurality of metal members are oriented transverse to the first orientation. The integrated capacitor further includes a third plurality of metal members, which are fabricated using a third plurality of metal layers, and are oriented in the first orientation.

Description:
TECHNICAL FIELD 
     The disclosed concepts relate generally to electronic circuitry and components and, more particularly, to capacitors with improved characteristics, circuits using such capacitors, and associated methods. 
     BACKGROUND 
     Advances in electronics has allowed increased levels of integration. The technology for fabrication of integrated semiconductor devices has contributed to those advances, and has provided a vehicle for integrating a relatively large number of circuits and devices. Along with active devices, passive devices have also been integrated in semiconductors structures.  FIGS. 1A-1B  illustrate a conventional integrated capacitor that uses metal segments  103  arranged in three metal layers (M 1 -M 3 ).  FIG. 1B  shows a cross section of the metal segments in each metal layer and their biasing pattern. Unlike the capacitor in  FIGS. 1A-1B , in another conventional integrated capacitor, vertically stacked metal segments  103  may be biased with a positive-negative-positive or negative-positive-negative pattern. 
       FIGS. 2A-2B  show another conventional integrated capacitor. The capacitor uses metal segments  103  (metal layers M 1  and M 3 ), which are arranged in a similar manner as the capacitor in  FIGS. 1A-1B . In addition, however, the capacitor in  FIGS. 2A-2B  includes segments members  106  (metal layer M 2 ). Metal segments  106  in metal layer M 2  are arranged perpendicularly to metal segments  103  in metal layers M 1  and M 3 , and biased as shown in  FIG. 2B . 
     SUMMARY 
     Apparatus for integrated capacitors and associated methods are disclosed. In one exemplary embodiment, an integrated capacitor includes a first plurality of metal members that are fabricated using a first plurality of metal layers, and are oriented in a first orientation. The integrated capacitor also includes a second plurality of metal members that are fabricated using a second plurality of metal layers. The second plurality of metal members are oriented transverse to the first orientation. The integrated capacitor further includes a third plurality of metal members, which are fabricated using a third plurality of metal layers, and are oriented in the first orientation. 
     In another exemplary embodiment, an apparatus includes a first set of metal members that are physically spaced apart from one another, and are oriented in a first orientation. The apparatus also includes a second set of metal members, which are physically spaced apart from one another, and are rotated with respect to the first orientation. The apparatus further includes a third set of metal members that are physically spaced apart from one another, and are oriented in the first orientation. 
     In yet another exemplary embodiment, a method of fabricating an integrated capacitor includes fabricating a first plurality of metal members in a first orientation. The first plurality of metal members are fabricated using a first plurality of metal layers. The method also includes fabricating a second plurality of metal members in an orientation that is transverse to the first orientation. The second plurality of metal members are fabricated using a second plurality of metal layers. The method further includes fabricating a third plurality of metal members in the first orientation. The third plurality of metal members are fabricated using a third plurality of metal layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The appended drawings illustrate only exemplary embodiments and therefore should not be considered as limiting its scope. Persons of ordinary skill in the art appreciate that the disclosed concepts lend themselves to other equally effective embodiments. In the drawings, the same numeral designators used in more than one drawing denote the same, similar, or equivalent functionality, components, or blocks. 
         FIGS. 1A-1B and 2A-2B  illustrate conventional integrated capacitors. 
         FIG. 3  depicts an isometric or three-dimensional (3D) view of an integrated capacitor according to an exemplary embodiment. 
         FIG. 4  shows a cross section view of an integrated capacitor according to an exemplary embodiment. 
         FIG. 5  depicts a biasing arrangement of an integrated capacitor according to an exemplary embodiment. 
         FIG. 6  illustrates a biasing arrangement of an integrated capacitor according to another exemplary embodiment. 
         FIG. 7  depicts an analog to digital converter (ADC) according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosed concepts relate generally to integrated capacitors. Integrated capacitors according to various embodiments provide a number of advantages. First, the disclosed capacitors provide relatively high capacitance density per unit area or volume. Second, the capacitors provide improved planar uniformity (e.g., chemical mechanical polish uniformity) of the layers used to fabricate the capacitors, which reduces or tends to reduce capacitance variations, and reduces capacitor mismatch. As a result, the disclosed capacitors may be matched to each other with a relatively low degree of mismatch. Consequently, the capacitors may be used in applications where good matching of capacitors is desired, as described below in detail. 
       FIG. 3  illustrates an isometric or 3D view of an integrated capacitor  200  according to an exemplary embodiment. In the embodiment shown, capacitor  200  uses metal members (fingers, lines, traces, etc.) fabricated using six metal layers, labeled M 1 -M 6 , respectively. The metal members are spaced apart from one another, both laterally or horizontally (with respect to metal members fabricated using a given metal layer) and vertically (metal members fabricated using one metal layer, and metal members fabricated using a metal layer above or below the first metal layer). The spacing between the metal members may be filled with dielectric, as described below in detail. 
       FIG. 3  also shows the biasing arrangement of the metal members. Note that other biasing arrangements may be used, such as reversing the polarity of the bias applied to the metal members in  FIG. 3 , etc. Furthermore, note that the final biasing polarity of the metal members may depend on how they (and, ultimately, the integrated capacitor) are coupled to other circuitry and/or sources of bias signals. 
     Metal layers M 1 -M 6  are progressively fabricated (or arranged or disposed) in a vertical direction, starting with metal layer M 1 . Thus, metal layer M 1  is fabricated (e.g., in or above a substrate, such as a silicon wafer), followed by a layer of dielectric (e.g., silicon dioxide). ( FIG. 3  does not show the dielectric layers for the sake of clarity of presentation.) Metal layer M 2  is then fabricated above the dielectric layer, followed by metal layer M 3 , another dielectric layer, metal layer M 4 , and so on. 
     Metal layers M 1 -M 6  may include a plurality of metal members, generally fabricated laterally (e.g., in parallel with respect to one another in the plane of the respective metal layer). Thus, as an example, metal members M 1 A-M 1 C are fabricated in a coplanar fashion, using metal layer M 1 . As another example, metal members M 3 A-M 3 C are fabricated in a coplanar fashion, using metal layer M 3 . 
     In the embodiment shown in  FIG. 3 , the metal members in metal layers M 1  and M 2  are fabricated in the same direction. In other words, the metal members in metal layers M 1  and M 2  are parallel to one another. The metal members in metal layers M 3  and M 4  are also arranged in the same direction (e.g., parallel with one another). 
     Moreover, the metal members in metal layers M 3  and M 4  are arranged a transverse direction relative to the metal members in layers M 1  and M 2 . Thus, in the embodiment shown, the metal members in metal layers M 3 -M 4  are arranged in a direction perpendicular to the direction of the metal members in metal layers M 1 -M 2 . 
     The metal members in metal layers M 5  and M 6  are arranged in the same direction (e.g., parallel with one another). Moreover, the metal members in metal layers M 5  and M 6  are arranged a transverse direction relative to the metal members in layers M 3  and M 4 . In the embodiment shown in  FIG. 3 , the metal members in metal layers M 5 -M 6  are arranged in a direction perpendicular to the direction of the metal members in metal layers M 3 -M 4 . 
     The features of the integrated capacitor (e.g., metal layers, metal members, and dielectric layers of the integrated capacitor) may be fabricated using a number of techniques, as persons of ordinary skill in the art understand. For example, in some embodiments, the features of the integrated capacitor may be fabricated using a masked technique, such as photolithography. As another example, in some embodiments, the features of the integrated capacitor may be fabricated using a maskless technique (e.g., laser ablation, punching, etc.). As yet another example, in some embodiments, the features of the integrated capacitor may be fabricated in part using a masked technique and in part using a maskless technique. 
       FIG. 4  shows a cross section view of the integrated capacitor of  FIG. 3 . The metal members of the metal layers are labeled with the metal layer&#39;s name and the respective metal member&#39;s label. Thus, metal layer M 1  includes metal members M 1 A, M 1 B, and M 1 C. As another example, metal layer M 6  includes metal members M 6 A, M 6 B, and M 6 C. In the view illustrated, metal members M 3 B, M 3 C, M 4 B, and M 4 C are not visible. 
     As noted above, metal members M 1 A-M 1 C are fabricated using metal layer M 1 . Dielectric layer D 1  is fabricated above metal members M 1 A-M 1 C. During the formation of dielectric layer D 1 , the space (or void) among metal members M 1 A-M 1 C is typically filled with dielectric. Thus, dielectric layer D 1  extends to and fills the space between metal members M 1 A-M 1 B. Similarly, dielectric layer D 1  extends to and fills the space between metal members M 1 B-M 1 C. 
     Metal members M 2 A-M 2 C are fabricated above dielectric layer D 1 , using metal layer M 2 . Dielectric layer D 2  is fabricated above metal members M 2 A-M 2 C, followed by metal members M 3 A-M 3 C, fabricated using metal layer M 3 . Dielectric layer D 2  extends to and fills the space between metal members M 2 A-M 2 B. Similarly, dielectric layer D 2  extends to and fills the space between metal members M 2 B-M 2 C. 
     Dielectric layer D 3  is fabricated above metal members M 3 A-M 3 C. Dielectric layer D 3  extends to and fills the space between metal members M 3 A-M 3 B. Similarly, dielectric layer D 3  extends to and fills the space between metal members M 3 B-M 3 C. Metal members M 4 A-M 4 C are fabricated above dielectric layer D 3 , using metal layer M 4 . 
     Dielectric layer D 4  is fabricated above metal members M 4 A-M 4 C. Dielectric layer D 4  extends to and fills the space between metal members M 4 A-M 4 B. Similarly, dielectric layer D 4  extends to and fills the space between metal members M 4 B-M 4 C. Metal members M 5 A-M 5 C are fabricated above dielectric layer D 4 , using metal layer M 5 . 
     Dielectric layer D 5  is fabricated above metal members M 5 A-M 5 C. Dielectric layer D 5  extends to and fills the space between metal members M 5 A-M 5 B. Similarly, dielectric layer D 5  extends to and fills the space between metal members M 5 B-M 5 C. Metal members M 6 A-M 6 C are fabricated above dielectric layer D 5 , using metal layer M 6 . 
     If desired, a dielectric layer (not shown) may be formed above metal members M 6 A-M 6 C (e.g., to fill the space between metal members M 6 A-M 6 B and between metal members M 6 B-M 6 C). Rather than a complete dielectric layer, some dielectric may be formed between metal members M 6 A and M 6 B and also between metal members M 6 B and M 6 B. In either case, the dielectric separating metal member M 6 A from metal member M 6 B, and metal member M 6 C from metal member M 6 B, causes the formation of capacitors, as described below in detail. 
     After the fabrication of one or more of dielectric layers D 1 -D 4 , a planarization step may be performed. For example, after the fabrication of dielectric layer D 1 , a planarization process may be performed to planarize the upper surface of dielectric D 1 . As another example, after the fabrication of dielectric layer D 4 , a planarization process may be performed to planarize the upper surface of dielectric D 4 . 
     The planarization process may use a variety of techniques, such as chemical mechanical polish (CMP). Fabricating metal members M 3 A-M 3 C and M 4 A-M 4 C in a direction or orientation that is rotated (e.g., transverse) with respect to the direction or orientation of metal members M 1 A-M 1 C and M 2 A-M 2 C, and/or metal members M 5 A-M 5 C and M 6 A-M 6 C (e.g., metal members M 3 A-M 3 C are rotated with respect to metal members M 2 A-M 2 C, or metal members M 4 A-M 4 C are rotated with respect to metal members M 2 A-M 2 C (or metal members M 1 A-M 1 C)). That particular arrangement of the metal members improves the planarization uniformity, which reduces or tends to reduce capacitance variations among two or more integrated capacitors fabricated using the disclosed techniques. 
     In some applications, reduced capacitance variations provides advantages. More specifically, in some circuit applications, the absolute value of the capacitance of two or more capacitors may affect the performance of the circuit. In such applications, a reduction in capacitance variations between the two or more capacitors results in improved performance of the circuit. 
     In other applications, the ratio of the capacitance of two or more capacitors may affect the performance of the circuit. In those applications, a reduction in capacitance variations between the two or more capacitors results in less variation in the ratio of the capacitors and, hence, improved performance of the circuit. 
       FIG. 5  depicts a biasing arrangement of an integrated capacitor according to an exemplary embodiment. Specifically,  FIG. 5  shows an example of how the metal members of the integrated capacitor of  FIG. 3  may be biased. Within a metal layer, the metal members are biased with alternate bias polarities. For example, metal member M 6 A is biased with a negative voltage, whereas neighboring metal member M 6 B is biased with a positive voltage. Similarly, metal member M 6 C is biased with a negative voltage, i.e., an alternate bias polarity with respect to metal member M 6 B. Thus, metal members fabricated using the same metal layer have differing or opposite bias polarities. 
     In the embodiment shown, the same or a similar biasing arrangement applies to the rotated or transverse metal layers, i.e., metal layers M 3  and M 4 . Thus, for example, metal member M 4 A is biased with a negative voltage, whereas neighboring metal member M 4 B is biased with a positive voltage. Similarly, metal member M 4 C is biased with a negative voltage, i.e., an alternate bias polarity with respect to metal member M 4 B. Thus, metal members fabricated using the same metal layer have differing or opposite bias polarities. 
     A similar biasing arrangement applies between metal members that have the same direction or orientation. More specifically, a metal member fabricated in a given metal layer has a differing (or alternate or opposite) bias polarity than do the corresponding metal members fabricated in the metal layer above (or below). For example, consider metal layers M 1  and M 2 . Metal member M 1 A (fabricated using metal layer M 1 ) is biased with a negative voltage, whereas metal member M 2 A (fabricated using metal layer M 2 , which is above metal layer M 1 ) has an opposite bias voltage (positive). 
     As another example, consider metal layers M 4  and M 3 . ( FIG. 3  shows an example of the biasing arrangement for metal layers M 3  and M 4 .) Metal member M 4 A (fabricated using metal layer M 4 ) is biased with a negative voltage, whereas metal member M 3 A (fabricated using metal layer M 3 , which is below metal layer M 4 ) has an opposite bias voltage (positive). 
       FIG. 5  also shows the capacitors formed between various metal members. As persons of ordinary skill in the art understand, two conductors, separated by a dielectric, form a capacitor. If the conductors are biased with opposite polarities (e.g., one conductor coupled to a voltage that is positive with respect to a voltage coupled to another conductor), the capacitor charges. By using the techniques described above, a number of capacitors are formed between metal members fabricated using a given metal layer. In addition, capacitors are formed between metal members fabricated using two respective metal layers. 
     For example, in  FIG. 5 , metal member M 6 A is separated by dielectric layer D 5  from metal member M 5 A ( FIG. 4  shows this feature explicitly). Furthermore, metal members M 6 A and M 5 A are biased, respectively, negative and positive. Thus, a capacitor  209  forms between metal members M 6 A and M 5 A. Similar capacitors form between metal fingers M 6 B and M 5 B (capacitor  212 ) and between metal fingers M 6 C and M 5 C (capacitor  215 ). 
     As another example, metal member M 6 A is separated by dielectric (not shown explicitly) from metal member M 6 B. Metal members M 6 A and M 6 B are biased, respectively, negative and positive. Consequently, a capacitor  203  forms between metal members M 6 A and M 6 B. Similar capacitors form between metal members M 6 B and M 6 C (capacitor  206 ), between metal members M 5 A and M 5 B, between metal members M 2 A and M 2 B, between metal members M 1 B and M 1 C, etc. 
     Note that metal members M 4 A and M 3 A have opposite bias voltages, and are separated by dielectric. Thus, capacitors  256  and  259  (or a capacitor that represents both) form between metal members M 4 A and M 3 A. Similar capacitors form between metal members M 4 A and M 5 A (capacitor  250 ), between metal members M 4 A and M 5 C (capacitor  253 ), between metal members M 3 A and M 2 B, etc. 
     Some metal members, even though separated by dielectric, have the same bias (e.g., same polarity voltage). A capacitor does not form between such metal members. For example, metal members M 5 B and M 4 A have the same bias (same polarity voltage). As another example, metal members M 3 A and M 2 A, or metal members M 3 A and M 2 C, have the same bias. 
     One group of the metal members that have the same bias (e.g., same polarity voltage) applied to them are typically coupled to one another, and form one electrode or terminal of the integrated capacitor. Another group of metal members that have the opposite bias (e.g., a differing voltage, an opposite polarity voltage) of the first group of metal members applied to them are typically coupled to one another, and form the other electrode or terminal of the integrated capacitor. 
     Referring to  FIG. 5 , for example, metal members M 1 A, M 1 C, M 2 B, M 4 A, M 5 B, M 6 A, and M 6 C have the same bias (negative) applied to them, and therefore form one electrode or terminal of the integrated capacitor. Furthermore, metal members M 1 B, M 2 A, M 2 C, M 3 A, M 5 A, M 5 C, and M 6 B have the same bias (positive, which is the opposite of the preceding group of metal members) applied to them, and therefore form one electrode or terminal of the integrated capacitor of  FIG. 5 . 
     Note that a variety of biasing schemes are possible, depending on factors such as desired capacitance, available metal layers, metal member density, etc. As merely one example, an alternative biasing scheme may be obtained by reversing the bias polarities of the metal members of the integrated capacitor in  FIG. 5 .  FIG. 6  shows an integrated capacitor having such as biasing scheme. Thus, M 6 A has a negative bias in  FIG. 5 , but a positive bias in  FIG. 6 ; M 6 B has a positive bias in  FIG. 5 , but a negative bias in  FIG. 6 ; and so on. 
     Although the exemplary embodiments illustrated and described have three metal members per metal layer, other numbers and arrangements of metal members may be used, as persons of ordinary skill in the art understand. The choice of the number of metal members depends on factors such as desired overall capacitance, available area (e.g., area on a semiconductor die that one may allocate to a given integrated capacitor), etc., as persons of ordinary skill in the art understand. Generally speaking, increasing the number of metal members per metal layer allows fabricating integrated capacitors with larger overall capacitance. 
     Similarly, although the exemplary embodiments illustrated and described are fabricated using six metal layers, other numbers and arrangements of metal layers may be used, as persons of ordinary skill in the art understand. The choice of number of metal layers depends on factors such as desired overall capacitance, available fabrication technology (e.g., how many overall metal layers are available), etc., as persons of ordinary skill in the art understand. 
     Referring to  FIG. 5 , generally speaking, in a given implementation, the horizontal or lateral spacing between the metal members, shown as H in  FIG. 5 , may be made smaller than the spacing between the metal layers, shown as V in the figure. In other words, metal members may be fabricated with a smaller spacing to each other (e.g., the spacing between metal members M 6 A and M 6 B) compared to the spacing between a metal layer and a metal layer above or below it. As a result, larger capacitance values may be more readily accommodated by increasing the number of metal members fabricated using a metal layer, rather than increasing the number of metal layers. 
     Note that, although the description of the exemplary embodiments refers to metal layers and metal members, other materials may be used instead of metal, as persons of ordinary skill in the art understand. For example, in some embodiments, semiconductor material, for example, doped silicon, or polysilicon may be used to fabricate the members for the integrated capacitors. 
     Furthermore, the exemplary embodiment of an integrated capacitor in  FIG. 3  shows metal members M 3 A-M 3 C and M 4 A-M 4 C as having an orientation or direction that is transverse or nearly or substantially transverse (e.g., a few degrees less than or more than 90 degrees because of, for example, fabrication tolerances) to the orientation or direction of metal members M 1 A-M 1 C, M 2 A-M 2 C, M 5 A-M 5 C, and M 6 A-M 6 C. As persons of ordinary skill in the art understand, however, other arrangements may be used. 
     Generally, metal members M 3 A-M 3 C and M 4 A-M 4 C are rotated with respect to metal members M 1 A-M 1 C, M 2 A-M 2 C, M 5 A-M 5 C, and M 6 A-M 6 C. As a result, an angle forms between the respective orientations of metal members M 3 A-M 3 C and M 4 A-M 4 C on the one hand, and metal members M 1 A-M 1 C, M 2 A-M 2 C, M 5 A-M 5 C, and M 6 A-M 6 C on the other hand. 
     In some embodiments, the angle, which corresponds to the degree of rotation of metal members M 3 A-M 3 C and M 4 A-M 4 C with respect to metal members M 1 A-M 1 C, M 2 A-M 2 C, M 5 A-M 5 C, and M 6 A-M 6 C, may have values other than 90 degrees (or nearly or substantially 90 degrees). Generally speaking, the angle may have values between zero and 90 degrees. For example, the angle may have values of, say, 25 degrees, 45 degrees, or 75 degrees in various exemplary embodiments. 
     As noted above, integrated capacitors according to exemplary embodiments provide improved capacitance uniformity and reduced capacitance variation. Such capacitors may prove advantageous in a variety of electronic devices, such as integrated circuits (IC), that include circuitry that may be sensitive to capacitance variations and/or may benefit from reduced capacitance variations. 
     Examples of such circuitry include ADCs, digital to analog converters (DACs), sample and hold circuits, filters (e.g., switched capacitor filters), charge coupled devices (CCDs), and the like. As merely one example,  FIG. 7  depicts an ADC  300  according to an exemplary embodiment. 
     The topology and operation of ADC  300  is well known by persons of ordinary skill in the art. Briefly, ADC  300  is a 14-bit pipelined ADC, which incorporates a sample and hold stage that uses a pair of matched capacitors  303 A- 303 B. The sample and hold stage is followed by 11 stages that include an arrangement of amplifiers and matched capacitors to provide the analog to digital conversion. 
     The sample and hold stage capacitor, i.e., capacitors  303 A- 303 B, have a value of 2 pF. The first stage following the sample and hold stage uses capacitors  306 A- 306 B, with a value of 0.5 pF. Assuming a one volt signal swing, the 14-bit resolution of ADC  300  results in a specified capacitance mismatch of ½ 14 , or 0.006%. For 0.5 pF capacitors, the specified mismatch is 0.03 fF. 
     Stage two following the sample and hold stage uses 0.25 pF capacitors. Stages three through 11 use 0.15 pF capacitors, which leads to even smaller capacitor mismatch values than for the first stage. By using integrated capacitors according to various embodiments, the capacitor mismatch or variation may be reduced or improved, which results in better operation of ADC  300 . Similar results may be accomplished in other circuits and devices, as persons of ordinary skill in the art understand. 
     Referring to the figures, persons of ordinary skill in the art will note that the various blocks shown might depict mainly the conceptual functions and signal flow. The actual circuit implementation might or might not contain separately identifiable hardware for the various functional blocks and might or might not use the particular circuitry shown. For example, one may combine the functionality of various blocks into one circuit block, as desired. Furthermore, one may realize the functionality of a single block in several circuit blocks, as desired. The choice of circuit implementation depends on various factors, such as particular design and performance specifications for a given implementation. Other modifications and alternative embodiments in addition to those described here will be apparent to persons of ordinary skill in the art. Accordingly, this description teaches those skilled in the art the manner of carrying out the disclosed concepts, and is to be construed as illustrative only. 
     The forms and embodiments shown and described should be taken as illustrative embodiments. Persons skilled in the art may make various changes in the shape, size and arrangement of parts without departing from the scope of the disclosed concepts in this document. For example, persons skilled in the art may substitute equivalent elements for the elements illustrated and described here. Moreover, persons skilled in the art may use certain features of the disclosed concepts independently of the use of other features, without departing from the scope of the disclosed concepts.