Patent Publication Number: US-11640964-B2

Title: Integrated capacitors in an integrated circuit

Description:
FIELD 
     The present invention relates to integrated capacitors which are integrated into integrated circuits (ICs). 
     BACKGROUND 
     Modern day integrated circuits increasingly incorporate so-called passive components—that is to say capacitors, inductors, and resistors—integrated with active components such as transistors. Whereas it has long been recognised that so-called parasitics (that is to say capacitances, inductances and resistances which are an unavoidable result of the design of the active components) affect circuit performance, more recently circuits have been designed to deliberately introduce well-defined passive components into the integrated circuits. The benefits are self-evident—for example, fewer discrete passive components are required alongside the IC which simplifies circuit-board level assembly. However, since such integrated passive components occupy space on the integrated circuit chip, resulting in larger devices, they do increase the cost of each device. 
     Some types of integrated circuit present greater challenges than others for integrating passive components, and in particular as far as regards this disclosure is concerned, for integrating capacitors. For example, analogue circuits, in which currents are generally larger than for equivalent digital circuits present increased challenges since capacitances have to be correspondingly larger. Furthermore, the challenges are greater for high voltage circuits such as may be used for instance in automotive applications, since the higher operating voltages require either thicker, or higher quality, dielectric to avoid leakage within the capacitors. 
     Whereas a variety of different forms of integrated capacitances are known, and even some combinations thereof, in general combining integrated capacitances heretofore has been challenging. 
     SUMMARY 
     According the present disclosure, there is provided a silicon-on-insulator, SOI, integrated circuit, IC, comprising: a semiconductor substrate having first and second major surface; a buried oxide, BOX, layer on the first surface; a plurality of semiconductor layers over the BOX layer and comprising a plurality of regions and including at least an n-type buried layer (NBL) and a n-well (DPN); an insulating layer (STI) over the plurality of semiconductor layers; a patterned polysilicon layer (poly1) over the insulating layer; a metallisation stack comprising a plurality of patterned metal layers having layers of insulating material therebetween, the layers of insulating material each having a plurality of conducting vias therethrough connecting the patterned metal layers; a lateral isolation structure (DTI) comprising a plug of polysilicon material in a trench extending through the plurality of semiconductor layers to the first major surface and having an oxide lining; wherein the integrated circuit comprises an integrated capacitor comprising a parallel arrangement of a metal-insulator-metal, MIM, capacitor, a second capacitor, a third capacitor, and a fourth capacitor: wherein the MIM capacitor is comprised within the metallisation stack; the second capacitor comprises as plates the substrate and an n-type one of the plurality of semiconductor layers, and comprises the buried oxide layer as dielectric; the third capacitor comprises as plates the polysilicon layer and a further n-type one of the plurality of semiconductor layers and comprises the insulating layer as dielectric; and the fourth capacitor comprises as plates the polysilicon plug and at least one of the plurality of semiconductor layers and comprises the oxide-lining as dielectric. 
     The capacitance-per unit area may thereby be significantly increased, over known integrated capacitors, with acceptable linearity. 
     In one or more embodiments the MIM comprises a grid of conductive fingers separated by insulating material and is comprised in the metallisation stack, such that each patterned metal layer comprises a respective pair of comb-structured electrodes each having interdigitated fingers approximately perpendicular to a backbone, and each being galvanically coupled to a horizontally offset comb-structured electrode in a neighbouring layer, and wherein a one of the comb-structured electrodes on an upper metal layer comprises a first electrode of the integrated capacitor, and a one of the comb-structured electrodes on a lower-most metal layer is galvanically connected to the second capacitor. In one or more embodiments, the MIM capacitor has four metal layers. 
     In one or more embodiments the one of the comb-structured electrodes on an upper metal layer is the uppermost layer. Substantially all of the metal layers within the metallisation stack may be available for inclusion in the MIM capacitor. 
     In one or more embodiments, a thickness of one or more upper metal layers of the metallisation stack is greater than a thickness of one of more lower metal layers. Thus, with respect to layer thicknesses the conventional design rules for the backend design processing for standard semiconductors may be followed. 
     In one or more embodiments the lateral isolation structure (DTI) forms a perimeter of a region of the IC and the second capacitor, the third capacitor and the fourth capacitor each contained within the perimeter. 
     In one or more embodiments the SOI IC further comprises a transistor outside a perimeter of the integrated capacitor. The integrator capacitor may be used in conjunction with this or other transistors. 
     In one or more embodiments the plurality of semiconductors layers comprises an n-well layer having an N-type doping concentration of at least 5×10{circumflex over ( )}18 cm −2 . Furthermore, the plurality of semiconductors layers may comprise a buried n-well layer having an N-type doping concentration of at least 5×10{circumflex over ( )}18 cm −2 . In particular, the doping levels required for the remainder of the IC may be used without modification when designing the capacitances. The limitation in degrees of freedom may be countered by using area as a design parameter. 
     In one or more embodiments the n-well layer and the buried n-well layer fill the region of the IC defined by the perimeter. In one or more embodiments at least one of the buried oxide layer, the trench liner, and the insulation layer has a thickness in a range of 300-450 nm. The trench of the lateral isolation structure (DTI) may extends into the substrate. In one or more embodiments the semiconductor substrate underneath the polysilicon material in the trench has a higher doping level than a doping level of a rest of the semiconductor substrate underneath the MIM capacitor. This may assist in providing a particularly good conductivity for the electrode of the trench capacitor. 
     In one or more embodiments the horizontal offset between each comb-structured electrode and the comb-structured electrode in a neighbouring layer is equal to a finger spacing, such that oppositely galvanically connected fingers are vertically aligned. This may provide for a particularly efficient use of volume for the MIM capacitor. 
     These and other aspects of the invention will be apparent from, and elucidated with reference to, the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments will be described, by way of example only, with reference to the drawings, in which 
         FIGS.  1   a ,  1   b  and  1   c    show different perspectives of a known MIM (metal-insulator-metal) capacitor integrated into the metal stack of an integrated circuit; 
         FIGS.  2   a ,  2   b  and  2   c    show different aspects of a 3-level MIM capacitor similar to that illustrated in  FIG.  1   , assembled onto a semiconductor-on-insulator (SOI) integrated circuit (IC); 
         FIG.  3    shows, schematically, an IC comprising a multi-source capacitor comprising a three-level MIM together with three other sources of capacitance, according to one or more embodiments; 
         FIG.  4    shows a simplified circuit diagram of the integrated capacitor of  FIG.  3   ; 
         FIG.  5    shows, schematically, an IC comprising a multi-source capacitor comprising a four-level MIM together with other three sources of capacitance, according to one or more embodiments; 
         FIG.  6   a    shows the top-metal view, and a first partial cross-section of an integrated capacitor such as that shown in  FIG.  3   ; 
         FIG.  6   b    shows an exploded view of various layers in an integrated capacitor such as that shown in  FIG.  3   ; 
         FIG.  6   c    shows the top metal view as shown in  FIG.  6   a   , together with a second partial cross-section of an IC comprising an integrated capacitor such that shown in  FIG.  3   ; 
         FIG.  7   a    shows the top-metal view, and a first partial cross-section of an integrated capacitor such as that shown in  FIG.  4   ; 
         FIG.  7   b    shows an exploded view of various layers in an integrated capacitor such as that shown in  FIG.  4   ; 
         FIG.  7   c    shows the top metal view as shown in  FIG.  7   a   , together with a second partial cross-section of an IC comprising an integrated capacitor such that shown in  FIG.  4   ; 
         FIG.  8    shows a schematic view of an integrated circuit showing an integrated capacitor along with an active component; 
         FIG.  9    shows a schematic view of an integrated circuit showing an integrated capacitor along with a further active component; and 
         FIG.  10    shows capacitance-voltage (CV) curves for integrated capacitors according to 2 embodiments, compared with corresponding CV curves for stand-alone MIM capacitors. 
     
    
    
     It should be noted that the Figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these Figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar features in modified and different embodiments. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIGS.  1   a ,  1   b  and  1   c    show different perspectives of a known MIM (metal-insulator-metal) capacitor integrated into the metallisation stack of an integrated circuit.  FIG.  1   a    shows a perspective view of a matrix of rods or fingers,  110 ,  120  to form a MIM capacitor. One half of the rods, labelled A,  110 , are connected together and these form the first electrode of the capacitor; the other half of the rods, labelled B,  120  are connected together to form the second electrode of the capacitor. The space  130  between the two sets of electrodes forms the dielectric. In a conventional MIM capacitor this dielectric is formed of the insulating layers (typically silicon oxide) between each of the metal layers in the metallisation stack of the IC. 
       FIG.  1 B  shows a vertical cross-section through such an MIM capacitor structure indicating the electric field lines  140 . As can be seen, the field lines are predominately contained within the overall area of the capacitor. 
       FIG.  10    shows, schematically, the arrangement of such an MIM capacitor on an integrated circuit. One half of the fingers are connected together and to the first contact  150  of the MIM capacitor; the other half of the fingers are similarly connected together and to a ground  160 .  FIG.  1 C  shows the connectedness of the electrodes along both diagonal directions through the stack, which results in individual capacitors elements between each pair of neighbouring fingers both in a vertical direction and in a horizontal direction.  FIG.  1   c    further shows the component layers of the integrated circuit underneath the metallisation stack. The integrated circuit is based on a substrate  170 , which is electrically isolated from parts of the active device by means of a buried oxide layer, BOX,  175 . On top of the BOX lies a plurality of semiconductor layers  180 . These may generally be referred to as epi layers or epitaxial layers. They may typically include a p-type epi layer  182 . Overlying the p-type epitaxial (p-epi) layer may be a p-doped well (p-well) layer  184 . The plurality of semiconductor layers may include patterned layers having different doping types of different doping concentrations (not shown), which may be formed by processes such as diffusion and ion-implantation, and provide for the manufacture of fabrication of active semiconductor devices, such as will be familiar to the skilled person. Overlying the plurality of semiconductor layers is shown an insulating layer  185 . This may typically take the form of a so-called shallow trench isolation (STI) layer. For the parts of the integrated circuit shown, the STI layer  185  shields the capacitor in the metallisation stack from the rest of the semiconductor layers. 
     Over the STI layer is a plurality of patterned metal layers M 1 , M 2 , . . .  190  separated by insulating layers  195  as mentioned above. As shown in  FIG.  10    the plurality of metal layers form the electrodes of the MIM capacitor; elsewhere in the semiconductor integrated circuit, the plurality of metal layers provide connections to and between the various electrodes, such as gates, drains, sources etc. of the active components of the integrated circuit, again as will be familiar to the skilled person. Hereinbelow the series of metal layers separated by insulating layers will be described as the “metallisation stack”. The metal layers may have the same thickness, or may have different thicknesses, as provided for in standard “back end of the line” processing. In one non-limiting example, the metal layers M 1  and M 2  may have a thickness of 250 nm, metal layer M 3  and M 4  may have thickness of 350 nm. In this example there may be an aluminium overlayer (sometimes designated AlCap) having a thickness of 2 μm, and a metal underlayer (sometimes designated ULM) having a thickness of 3.7 μm. 
       FIGS.  2   a ,  2   b  and  2   c    show different aspects of a 3-level MIM capacitor similar to that illustrated in  FIG.  1   , assembled onto a semiconductor-on-insulator (SOI) integrated circuit (IC) in more detail. 
     The upper part of  FIG.  2   a    shows a view of the top patterned metal layer—M 3  in this case. The metal layer M 3  is patterned so as to provide two sets of fingers or digits  210  and  220 . The two sets of fingers are interdigitated or interleaved. Each set of fingers is connected together by means of a backbone, being backbone  215  in the case of fingers  210  and backbone  225  in the case of fingers  220 . Along each of the backbones is a series of vias  230 ,  235  which extends down from the top metal layer (and thus are not directly visible). 
     The lower part of  FIG.  2   a    shows a schematic cross-section along line AA′, which cuts across the interdigitated fingers. This view is similar to that shown in  FIG.  1    and shows the substrate  270 , buried oxide layer  275 , plurality of semiconductor layer  280  and shallow trench isolation STI layer  285  underneath the metallisation stack formed of metal layers M 1 , M 2  and M 3   290  separated by insulating layer  295 . The STI layer  285  has “gaps” in it, shown at  206 , corresponding to parts of “active area” AA, i.e. semiconductor layers. The first group of electrodes are shown connected to a ground  260  and the second group are connected to a capacitor contact  250 . This lower part of the figure also depicts the diagonal interconnectedness  202  between the metal fingers and the individual capacitance elements  204 . 
       FIG.  2   b    shows, schematically, the three metal layers M 1 , M 2 , and M 3  in more detail. As shown, M 3  is patterned to provide the top layer  210   a ,  215   a , of the first electrode  210 ,  215 , together with the top layer  220   a    215   a  of the second electrodes  220 ,  225 ; M 2  is patterned to provide the middle layer  210   b ,  215   b , of the first electrode  210 ,  215 , together with the middle layer  220   b ,  215   b  of the second electrodes  220 ,  225 ; similarly, M 1  is patterned to provide the middle layer  210   c ,  215   c , of the first electrode  210 ,  215 , together with the middle layer  220   c ,  215   c  of the second electrodes  220 ,  225 . Finally,  FIG.  2   b    also shows the layer  206 , corresponding to an active area (AA). 
     The top part of  FIG.  2   c    is a copy of the top part of  FIG.  2   a   , and shows the position of the cross-section BB′ depicted in the lower half of the figure. As shown, this cross-section is along the backbone  215  of the first electrode. Below the metallisation stack, the cross-section is again similar to that shown in  FIG.  1   , and shows the substrate  270 , buried oxide layer  275 , plurality of semiconductor layer  280  and shallow trench isolation STI layer  285 . However, in this case the insulating layers  292  between the metal layers M 1 , M 2  and M 3   290  are explicitly shown. Moreover, in this cross-section, the patterning of the metal layers is not visible since the backbone is shown. However, this view does show the vias  230 ,  235  connecting neighbouring pairs M 1  &amp; M 2 , and M 2  &amp; M 3  of the three metal layers M 1 , M 2  and M 3 . 
     In addition, the lower part of  FIG.  2   c    shows the contact region  294  in the top metal and above the layers of the metallisation stack forming the capacitor, and which forms the non-grounded contact  250  of the MIM capacitor. 
     Although MIM capacitors such as that described above can achieve relatively high capacitances with good linearity, there are some applications in which higher capacitance is required. Moreover, for some applications accurate linearity of the capacitor (that is to say the extent to which the voltage across the capacitor is directly proportional to the potential difference between the two plates or electrodes) may be required. 
       FIG.  3    shows, schematically, an IC comprising a multi-source capacitor comprising a three-level MIM together with three other sources of capacitance, according to one or more embodiments. Although one or more of these other sources of capacitance may be familiar to the skilled person, the inventors of the present disclosure have made the realisation that all of them may be combined without significantly degrading the linearity of the combined capacitance. This is particularly the case, when, as will be discussed in more detail hereinbelow, n-doped semiconductors layers (such as an n-well DPN or an n-type buried layer NBL) are used rather than p-type layers (such as p-epi of p-well). 
       FIG.  3    shows an integrated capacitor  300 , comprising a three-level, also referred to as a three-layer, MIM capacitor  310  built into a metallisation stack of a SOI IC, along with three other designed-in capacitances. As discussed above, the MIM capacitor has electrodes formed of a stack of pairs of interdigitated fingers  390 , in which the fingers of each one  390 ′,  390 ″ of the pair are connected together by a backbone, the stack being offset so that neighbouring fingers form part of the opposite electrodes both in a vertical (perpendicular to the IC surface) and a horizontal (parallel to the IC surface) direction. The patterned metal layers forming electrodes are separated by insulating layers  395 , which together with the insulating material between neighbouring fingers within a metal layer formed the dielectric of the MIM capacitor. 
     The metallisation stack comprising the MIM capacitor has as its lowest level an insulating level layer  395   a . The metallisation stack is fabricated in conventional and standard fabrication processes which will be well known to the person skilled in the art, during the latter parts of the manufacturer of the SOI integrated circuit (sometimes called “back-end”—BE, or “back-end-of-the-line”—BEOL—processing). The other three capacitive elements are designed into the early stages of the IC manufacture (sometimes called front-end—FE—processing), again using conventional and standard fabrication processes. The capacitive elements each make use of insulating layers which are fabricated during conventional device manufacturer, to form respective dielectric for the capacitive elements. They each have a commonly connected first electrode, being the substrate of the SOI IC. The second electrode of each of the three are also commonly galvanically connected to an I/O terminal or connection, to provide the integrated capacitor. It will be appreciated that in this context I/O refers to input/output of the integrated capacitor; it may or may not be an external I/O pin of the integrated circuit itself, and typically, in order to provide full benefits of integrated capacitor, will be directly connected to another part of the circuit of the IC. Considering the MIM capacitor in the metallisation stack as being the first element, the second element of the multisource integrated capacitor may be loosely termed a “BOX capacitor”; the third element of the multisource integrated capacitor may be loosely termed a “gate-poly capacitor”, and the fourth element of the multisource integrated capacitor may loosely be termed a “DTI liner” capacitor. These will now each briefly be described in turn. 
     Firstly, considering the “BOX capacitor”. This is illustrated pictorially on  FIG.  3    at  320 . The first electrode of this capacitor element is the conductive substrate which is, typically, for a SOI IC an N-type silicon substrate. The second electrode is the lowermost epitaxial layer, or group of layers,  382 , on the buried oxide layer, and the dielectric of this capacitive element is the buried oxide layer  375  itself. The layer or group of layers  382  is typically defined within the semiconductor processing as the “NBL” layer or N-type buried layer. Conventionally this layer is provided by over-doping the one or more epitaxial p-type layers grown on to the oxide in the SOI wafer manufacture, to provide a high doped N-type layer. The doping level of this layer is typically in a range of 10{circumflex over ( )}18-10{circumflex over ( )}20 cm −2 , and may have a preferred range of 10{circumflex over ( )}19-20{circumflex over ( )}20 cm −2 , or a preferred value of 5×10{circumflex over ( )}19 cm −2 . This level of doping is sufficient for it to act as an “M” or metal-like electrode or plate of this parallel plate MOS capacitive element. The thickness of the buried oxide layer (the “O” layer of the MOS) may be chosen to suit the application, but the inventors have determined that a particularly useful range for the buried oxide layer is within the range 100-600 nm, and more particular between 300-400 nm. 350 nm has been, in an embodiment, found to produce effective results. 
     As mentioned above, the SOI IC includes a plurality of semiconductor layers  380  over the buried oxide layer  375 . Some of the semiconductor layers may be patterned and/or have a range of different doping types and concentrations provided by conventional semiconductor processing steps including diffusion and iron implantation. The processing steps are chosen so as to provide the desired active devices in the IC. One of the processing steps which will be well known to the skilled person is to provide an insulating layer, which typically may be an oxide layer, or a stack of insulating layers including oxide layers and nitride layers, which overlies some of the active layers and provides isolation between the active layers and the later fabricated metallisation stack. It also typically provides isolation between the gate contact of the active device and the gate itself. This layer is conventionally known as the shallow trench isolation or STI layer, and is shown in  FIG.  3    as layer  385 . Since the STI layer is used in the active part of a SOI IC to provide isolation for the later deposited gate contact structure, which typically includes a poly silicon layer  365 , it underlies the deposited (or grown) polysilicon layer  365 . 
     A lateral portion of the STI layer  385  is designed to be the dielectric for the “gate-poly capacitor”. And the first electrode of this gate-poly capacitor is the polysilicon layer  365  (used in the gate structure of active devices elsewhere on the IC). The other electrode of this capacitive element is the topmost layer or group of layers of the plurality of semiconductor layer  380 . This layer is typically heavily doped N-type and is chosen as a DN well (DPN) or high voltage well (NHV)  394 . The doping level of this layer is typically in a range of 10{circumflex over ( )}18-10{circumflex over ( )}20 cm −2 , and may have a preferred range of 10{circumflex over ( )}19-20{circumflex over ( )}20 cm −2 , or a preferred value of 5×10{circumflex over ( )}19 cm −2 . Once again, one or other of these layers are formed by conventional device processing which is used to provide active devices elsewhere in the IC. In order for this capacitive element to form a part of the multisource capacitor, one electrode should be grounded and the other electrode connected to the common I/O electrode. In this instance the gate poly layer  365  is grounded to the substrate  370 . There only remains to connect the N-well to the capacitor I/O electrode. This may achieved by a galvanic connection  388  through the STI, through an isolated part of the polysilicon layer  365 , and a (typically metal) via  386  to the lowest metal layer in the MIM, and in particular to the comb structure in the lowest metal layer which is connected to the I/O terminal. This is illustrated in  FIG.  3    schematically as connection  388 , which may be centrally located. However, in practical devices, this connection is typically provided at the periphery of the integrated capacitor, such as will be discussed in more detail with respect to  FIG.  6   . The thickness of the STI oxide layer acting as dielectric is typically in the range of 250-650 nm or 500-600 nm. The inventors have found that a thickness of 400 nm provides a particularly effective capacitor contribution. 
     It will be appreciated that both the second capacitive elements (BOX capacitor) and the third capacitive element (gate-poly capacitor) take the form of parallel plate capacitance is with a horizontal (oxide) dielectric layer arranged between two horizontal plates electrodes. These capacitances thus scale directly with that area of the IC chip which is allocated as the integrated capacitor. Similarly, the MIM capacitor also scales with the area. In contrast, the fourth capacitive element scales with the perimeter of that area of the IC which is allocated as the integrated capacitor. 
     The fourth capacitive element, loosely termed as the “DTI-liner” capacitor, makes use of the construction of an isolation feature provided on some SOI ICs. This isolation feature is typically used to surround one or more active elements within the IC and in this case is used to surround the area of the integrated capacitor. The isolation is provided by etching a trench through the semiconductor layers  380  grown or deposited over the buried oxide and into the buried oxide  375  itself. The trench may thus be described as a deep trench, and the isolation feature as a deep trench isolation, DTI. In contrast with some other forms of deep trench isolation, in this case the DTI structure extends through the buried oxide layer, in order to make contact with the underlying substrate  370 . In some embodiments the trench extends into the substrate. Typically, additional dopant may be introduced into the exposed substrate at the bottom of the trench either by diffusion or ion implantation, as shown at  372  in  FIG.  3   . This more heavily doped layer of the substrate may have a doping level which is typically in a range of 10{circumflex over ( )}18-10{circumflex over ( )}20 cm −2 , and may be in a preferred range of 10{circumflex over ( )}19-10{circumflex over ( )}20 cm −2 . In contrast, the substrate itself may have a much lower doping level, and typically has an n-type doping in a range of 2×10{circumflex over ( )}14-10{circumflex over ( )}16 cm −2 , and may have a preferred value of 10{circumflex over ( )}15 cm −2 . The trench is then lined using an insulating material  374 . This may comprise deposited oxide or grown oxide. Conductive material, typically polysilicon, is then deposited into the central region of the trench  376 . Since the trench surrounds the area of the integrated capacitor, it provides lateral isolation from the remainder of the IC. Furthermore, since the conductive material  376  is grounded to the semiconductor substrate  370 , it may be used as one plate of an additional capacitor element. The lining of the trench forms the dielectric of the capacitor, and the edges of the semiconductor layers  870  which are in contact with the lining form the other plate or electrode. Typically, the oxide liner has a lateral thickness of between 200 and 600 nm or between 300 and 400 nm. The inventors have found that a thickness of 300 nm is particularly effective. As already mentioned, the magnitude of this capacitive element scales with the perimeter of the area of the integrated capacitor. The total area of the plates of this capacitor is the perimeter multiplied by the total thickness of the semiconductor layers and the DTI which typically equals approximately 5 μm. The doping level of the semiconductor layers in contact with the (inner) lining of the DTI trench are relatively high: they are typically in a range of 10{circumflex over ( )}18-10{circumflex over ( )}20 cm −2 , and may have a preferred range of 10{circumflex over ( )}19-10{circumflex over ( )}20 cm −2 , or a preferred value of 5×10{circumflex over ( )}19 cm −2 , as already mentioned for the DPN layer. 
       FIG.  4    shows an integrated capacitor  400  comprising a four-level MIM capacitor  410  built into a metallisation stack of a SOI IC, along with three other designed-in capacitances. This embodiment is broadly similar to that shown in  FIG.  3   , with the exception that in this case the MIM capacitor has four metal levels or layers  490 , each separated by dielectric material  495 , together with an additional dielectric or insulating layer  495   a , between the lowest metal layer M 1  and the semiconductor layers. Since the galvanically-connected comb structures in each layer are diagonally offset from one another, in this case the ground-connected comb structure on the topmost of the four metal layers is directly over the I/O terminal-connected comb structure of the lowest layer which is galvanically connected to the semiconductor layer (typically “DPN layer”) which forms the non-grounded electrode of the STI capacitor element. In contrast, in the three layer MIM capacitor element shown in  FIG.  3   , the I/O terminal-connected comb structure on the topmost of the—in that case three-metal layers is directly over the I/O terminal-connected comb structure of the lowest layer which is galvanically connected to the semiconductor layer  384  (typically “DPN layer”) which forms the non-grounded electrode of the STI capacitor element. 
       FIG.  5    shows a simplified equivalent circuit of an integrated capacitor according to one or more embodiments. The integrated capacitor  500  has a first terminal or electrode  510  which may also be described as a I/O electrode. The integrated capacitor has a second terminal  520  which is a ground. The capacitor comprises four, parallel connected, capacitors being the MIM capacitor  310 , the BOX capacitor  320 , the STI capacitor  330 , and the DTI capacitor  340 . As discussed above, the MIM capacitor has as an electrodes or plate, a one of the comb-structures in an upper metal layer (typically “M 3 ”, or “M 4 ” using conventional semiconductor processing terms, wherein M 1  is the lowermost metal layer in the metallisation stack). 
       FIGS.  6   a ,  6   b  and  6   c    show a variety of views of an integrated capacitor comprising a three-level MIM and three other parallel capacitor elements, such as that shown in  FIGS.  3   a ,  3   b    and  3   c.    
       FIG.  6   a    shows, at the top, the topmost metal layer of the MIM having two comb-structures, one of which has fingers  210  connected to a backbone  215 , and the other has fingers  220  connected to a backbone  225 . The bottom half of  FIG.  6   a    shows a schematic cross-section through AA′, and is the same as  FIG.  3   . 
       FIG.  6   b    shows an exploded view of the various layers associated with the embodiment described in  FIG.  6   a   . The top three layers are the MIM metal layers, as seen in  FIG.  2   b   : thus, as shown, M 3  is patterned to provide the top layer  210   a ,  215   a , of the first electrode  210 ,  215 , together with the top layer  220   a    225   a  of the second electrodes  220 ,  225 ; M 2  is patterned to provide the middle layer  210   b ,  215   b , of the first electrode  210 ,  215 , together with the middle layer  220   b    225   b  of the second electrodes  220 ,  225 ; similarly, M 1  is patterned to provide the middle layer  210   c ,  215   c , of the first electrode  210 ,  215 , together with the middle layer  220   c    225   c  of the second electrodes  220 ,  225 .  FIG.  6   b    also shows the polysilicon layer  365  as shown in  FIG.  3   , extending over the area of the integrated capacitor. The polysilicon layer is connected to one of the comb-structures in each of the metal layers, and thereby grounded, by vias which, as shown at  630 , may be aligned to the vias  230 . 
     Also shown in  FIG.  6   b    in plan view are various layers associated with the integrated capacitor, including the DTI polysilicon  376 , the buried N-well layer NBL  382 , the deep N-well DPN  384 , and the metal contact  666  for connecting the metallisation stack to the underlying semiconductor. 
     It will be appreciated that the galvanic connection  386  and  388  between the top semiconductor layer  384  and the metal layers which connects to the I/O terminal and is visible towards the centre of  FIG.  6   a    is not shown in  FIG.  6   b    (although an associated gap, or hole, in the polysilicon layer  365  is shown in  FIG.  6   b   ). In practical implementations and embodiments of this connection is not made in the centre of the device—the depiction in  FIG.  6   a    is for illustrative purposes only. Rather, typically, the connection between the layer  384  and the I/O terminal is made at the periphery of the integrated capacitor, as shown schematically by CONT  666 . In general, then the gap or hole shown towards the centre of the polysilicon layer would then not be needed so would not be present. 
       FIG.  6   c    shows, in the top half of the figure, another copy of the view of the topmost metal layer of the MIM having two comb-structures, and shows the position BB′ of the schematic cross-section shown in the lower half of the figure. 
     The cross-section shown the lower part of the figure is along the back-bone of the comb-structures in the metal layers, and shows the back-bones  215   c ,  225   b  and  215   a  of the three metal layers M 1 , M 2  and M 3  respectively, along with the dielectric layers  292  in between the metal layers. The vias  235  connecting the metal layers are also shown. This figure also shows a “top metal” layer  294 . The figure also shows the lower dielectric layer  292  between the M 1  backbone  215 C and the poly layer  265 , along with STI  285 , buried oxide layer  275  and substrate  270 . 
       FIGS.  7   a ,  7   b  and  7   c    show a variety of views of an integrated capacitor comprising a four-level MIM and three other parallel capacitor elements, such as that shown in  FIGS.  3   a ,  3   b  and  3   c   . As is immediately apparent, apart from the fact that there are four metal layers rather than the three metal layer shown in  FIGS.  6   a ,  6   b  and  6   c   , embodiments illustrated by this figure are similar to those shown and described in  FIGS.  6   a ,  6   b  and  6   c   , so will not be discussed in detail. However, it will be noticed that the thicknesses of the metal layers M 1  and M 2 , as shown, may be different from the thicknesses of the upper layers M 3  and M 4 . The person skilled in the art of SMART-MOS, or similar, integrated circuits will be aware that the design rules for such integrated circuits can provide for different thicknesses of the various metal layers within a metallisation stack. Thus embodiments of the present disclosure include metal layers which are the same thickness, and extends to metal layers having different thicknesses. Shown in the figure is a non-limiting example in which M 3  and M 4  have a greater thickness than M 1  and M 2 . 
     Turning now to  FIGS.  8  and  9   , these two figures illustrate, schematically, parts of an integrated circuit including an integrated capacitor according to embodiments of the present disclosure. Considering first  FIG.  8   , the integrated capacitor includes a three level MIM. The integrated capacitor  300  is the same as that depicted in  FIG.  3    so will not be described in more detail; however the schematic also depicts an active device, in this case an NPN transistor, which illustrates how the semiconductor, metallic and insulating layers used to fabricate integrated capacitor are used in conventional process flow, for the fabrication of other component on the same integrated circuit. As already mentioned, in this particular example the illustrative other component is an active component being an NPN transistor, having collector  820 , base  830  and emitter  840  metal contacts in at least the metal layer M 1  (corresponding to the lowest metal layer in the metallisation stack of the device). The lowest metal layer in the metallisation stack is separated from the active device by the dielectric layer  395 , which may also be called an inter-layer dielectric layer (and commonly referred to as ILD0). As can be seen from  FIG.  8   , the active device may be surrounded by a deep trench isolation structure, which may be the same as that for the integrated capacitor  300 , and may be formed using the same process steps. 
     Thus the DTI structure may extend through the buried oxide layer, in order to achieve contact with the underlying substrate  370 . In some embodiments the trench extends into the substrate. Typically, additional dopant is introduced into the exposed substrate at the bottom of the trench either by diffusion or ion implantation, as shown at  372  in  FIG.  8   . This more heavily doped layer of the substrate may have a doping level which is typically in a range of 10{circumflex over ( )}18-10{circumflex over ( )}20 cm −2 , and may be in a preferred range of 10{circumflex over ( )}19-10{circumflex over ( )}20 cm −2 . 
     The trench is lined using an insulating material  374 . Conductive material, typically polysilicon, is then deposited into the central region of the trench  376 . The trench may provide lateral isolation for the active device, or a group of active ices, from the remainder of the IC and in particular from the integrated capacitor  300 . Typically, the oxide liner has a lateral thickness of between 200 and 600 nm or between 300 and 400 nm. 
     The active NPN transistor  810  the device is fabricated on the substrate  370 , over which is the buried oxide layer  375 , and includes a n-type buried layer NBL over the buried oxide, over which lies a deep n-type layer DPN, which is connected to a collector contact  822  by means of a more heavily doped N region  824 . Confined by this deep n-type layer DPN is a p-type region  832 , within which is a more heavily doped p-type region LVP  834 , which itself is connected to the base contacts  836  by a yet more heavily doped p-type region  838 . Finally, within the LVP  834  is a lightly doped n-type layer NLD  842 , which is connected to an emitter contact  844  by means of a heavily doped n+ doped contact regions  846 . As shown, each of the heavily doped regions  824 ,  838 ,  846  providing contact to the collector metal layer  820 , base metal layer  830  and emitter metal layer  840  extend through the shallow trench isolation, STI layer  385 . 
     From the above discussion, it will be appreciated that the layers used to form the three additional capacitances of an integrated capacitor according to the present disclosure, can be provided during standard manufacturing processes of the integrated circuit, and thus do not add additional complexity to the fabrication of the integrated circuit. 
       FIG.  9    illustrates, schematically, another part of an integrated circuit including an integrated capacitor according to embodiments of the present disclosure. In this case, the integrated capacitor is adjacent a PNP transistor having a floating base. Once again, similarly to the example shown in  FIG.  8   , the layers forming each of the additional capacitances of the integrated capacitor are deployed in the fabrication of the active device: the device is fabricated on the substrate  370 , over which is a buried oxide layer  375 . Over the buried oxide layer is a p-type epi layer  182 , into which is formed a P-type well  184 . Over the P-type well is shown a part of the STI layer  385 . The PNP transistor is formed within the active semiconductor layers, and may be isolated from the remainder of the integrated circuit, by means of a different isolation structure DTI  372 ,  374 ,  376  as described above. The transistor includes an anode metal contact (shown in the first metal layer M 1  as  920 ) and a cathode metal contact (shown in the first metal layer M 1  as  930 ); the anode contacts  922  and cathode contact  932  are connected to respective p-type layers  924 ,  934  by means of heavily p-doped contact regions  926  and  936  respectively. The p-type layers  924 ,  934  are located within the p-type epi region  182 , and isolated by parts of the deep n-well DPN  384 . 
     Although not shown in  FIGS.  8  and  9   , the skilled person will appreciate that the metallisation stack over the active devices such as transistor  810 ,  910  will generally include additional layers metal layers M 2 , M 3  etc.  390  with interlayer dielectric  395  therebetween in region  940  over the active devices, with at least three metal layers included in the case of a three layer MIM capacitor or with at least four metal layers included in the case of a four layer MIM capacitor. The associated layout, design rules et cetera of these additional, not shown, metal layers are entirely conventional. However it will be noted that in any individual metal layer the separation x,  945 , between the outermost fingers of the MIM capacitor and metal tracks connecting active devices or other parts of the integrated circuit will typically be kept at or above a predetermined minimum separation. This minimum separation may follow conventional design rules for any particular integrated circuit design and in particular for any particular “node” of semiconductor technology. For example, for one particular CMOS 12 nm node, the minimum separation may be 800 nm. 
       FIG.  10    shows 4 plots of modelled capacitance on the ordinate or Y-axis against voltage bias on the abscissa or X-axis. The upper two plots  1010 ,  1020  show the capacitance of an integrated capacitor according to embodiments of the present disclosure and including, respectively, a four level MIM and a three level MIM, together with the three other—DTI capacitor, STI capacitor, and BOX capacitor—elements. The lower two plots  1030 ,  1040 , show corresponding capacitance of an integrated capacitor which consists solely of the corresponding four level MIM and three level MIM respectively. As can be seen, the integrated capacitors according to embodiments have capacitance levels which are typically a factor of approximately 1.8 times higher than the stand-alone MIM integrated capacitors. For example, under a positive bias voltage the three-level MIM capacitance is increased from approximately 3.1 pF to 6 pF, and the four level MIM capacitance is increased from approximately 4.4 pF to approximately 7.1 pF. The improvements under negative bias is slightly less but still at least a 1.7 times improvement in capacitance. The figure also shows a —relatively small—nonlinearity around −5 to +2 V; the skilled person will appreciate that this nonlinearity is not so significant as to create design impediments. 
     From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of integrated capacitors, and which may be used instead of, or in addition to, features already described herein. 
     By now it will have been appreciated that the present disclosure extends to a silicon-on-insulator, SOI, integrated circuit, IC, comprising integrated capacitor, wherein the integrated capacitor comprises a parallel arrangement of a metal-insulator-metal, MIM, capacitor, a second capacitor, a third capacitor, and a fourth capacitor: wherein the MIM capacitor is comprised in a metallisation stack of the SOI IC; wherein the second capacitor comprises a substrate of the SOI 
     IC, an n-type doped one of a plurality of semiconductor layers underneath the metallisation stack, and the buried oxide layer therebetween as dielectric; the third capacitor comprises a polysilicon layer between the metallisation stack and the plurality of semiconductor layers, and a further n-typed doped one of the plurality of semiconductor layers, and the an insulating layer therebetween as dielectric; and wherein the fourth capacitor comprises a polysilicon plug in a deep trench isolation structure and at least one of the plurality of semiconductor layers, and comprises the oxide-lining of the deep trench isolation structure as dielectric. 
     In embodiments, the lateral isolation structure (DTI) forms a perimeter of a region of the IC and the second capacitor, the third capacitor and the fourth capacitor each contained within the perimeter. In embodiments, the SOI further comprises a transistor outside a perimeter of the integrated capacitor. In embodiments, the plurality of semiconductors layers comprises an n-well layer having an N-type doping concentration of at least 5×10{circumflex over ( )}18 cm −2 . In embodiments, the plurality of semiconductors layers comprises a buried n-well layer having an N-type doping concentration of at least 5×10{circumflex over ( )}18 cm −2 . In embodiments, the n-well layer and the buried n-well layer fill the region of the IC defined by the perimeter. 
     Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. 
     Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom. 
     For the sake of completeness it is also stated that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality and reference signs in the claims shall not be construed as limiting the scope of the claims.