Symmetrical MIMCAP capacitor design

Abstract

Semiconductor chip capacitance circuits and methods are provided comprising at least two capacitors mounted close to a substrate, wherein each capacitor has a lateral lower conductive plate mounted near enough to the substrate to have extrinsic capacitance greater than an upper plate extrinsic capacitance. One half of lower plates and one half of upper plates are connected to a first port, and a remaining one half of upper plates and lower plates are connected to a second port, the first and second port having about equal extrinsic capacitance from the lower plates. In one aspect, the substrate comprises a front-end-of-line capacitor defining a substrate footprint, and the at least two capacitors are back-end-of-line Metal-Insulator-Metal Capacitors disposed above the footprint. In another aspect, the at least two capacitors are at least four capacitors arrayed in a rectangular array generally parallel to the substrate.

Description

TECHNICAL FIELD

This invention relates generally to capacitors and, more particularly, to the methods and systems for capacitor structures with symmetrical polarity characteristics.

BACKGROUND ART

On-chip capacitors are critical components of integrated circuits that are fabricated on silicon semiconductors. These capacitors are used for a variety of purposes: illustrative examples include bypass and capacitive matching as well as coupling and decoupling. The design and implementation of capacitor structures on silicon semiconductor chips may be dependent upon one or more symmetrical structural, target circuit quality and low parasitic resistance performance characteristics.

More particularly, capacitor structures may be categorized as being formed in one of two regions: the Front End Of (production) Line (FEOL), or the Back End Of the Line (BEOL). In integrated-circuit fabrication lines, FEOL conventionally refers to earlier process stages that directly modify the semiconductor substrate or the immediate contacts to it; for example, dopant diffusion and implantation, sputtering of gate films, oxidations, and the patterning steps associated with these. In contradistinction, the BEOL is metalization (PVD) for interconnects and vias (vertical interconnects between planar interconnects) and associated non-conducting depositions and growths (for example, polymers, glasses, oxides, nitrides, and oxinitrides) for electrical isolation, dielectrics (for capacitance), diffusion barriers, and mechanical passivation (in particular, to prevent failure of interconnects by electromigration and stress migration). FEOL and BEOL are used in transferred sense to refer to the levels of an IC fabricated in the corresponding stages. BEOL is the metalization layers (say between four and ten) and associated insulating layers, and FEOL everything below that—mostly transistors.

It is known to use a metal oxide silicon (MOS) capacitor, or MOSCAP, for semiconductor chip capacitor elements formed on the chip substrate in the FEOL. However, MOSCAP capacitors generally require large chip area footprints in integrated circuits (IC). Accordingly, design requirements typically result in requiring large semiconductor chip footprint areas or real estate for MOSCAP capacitor structures relative to their circuit capacitance properties, resulting in high production costs and reduced semiconductor chip area availability for other circuit structures. Moreover, current leakage during a semiconductor circuit's idle mode is known to result in increased power consumption. Silicon semiconductor chip capacitor structures thus usually require large MOSCAP capacitor structures in order to avoid current leakage problems.

As the production cost of an IC is generally proportional to the real estate required, it is desired to reduce IC chip costs by reducing the footprint required for a MOSCAP structure. Accordingly, one possible technique for reducing FEOL MOSCAP footprints is to form additional capacitor structures in the BEOL in circuit communication with the FEOL MOSCAP, preferably increasing the capacitance of the total FEOL/BEOL capacitor structure while resulting in a relatively smaller FEOL MOSCAP footprint.

Two types of capacitors commonly utilized in the BEOL are a Metal-Insulator-Metal Capacitor (MIMCAP) 100 schematically illustrated in FIG. 1 with respect to a chip substrate 114, and a Vertical Native Capacitor (VNCAP) 200 schematically illustrated in FIG. 2. (FEOL structures are omitted for simplicity of illustration.) The MIMCAP 100 comprises a first plate 110 and a second plate 112, each having a connector or port 116, 118, respectively, with a dielectric material 120 placed between the plates 110, 112 to complete the capacitive structure. The VNCAP 200 also comprises a first plate 210 and a second plate 212, each having a connector or port 216, 218, respectively, with a dielectric material 220 placed between the plates 210, 212 to complete the capacitive structure. What is significant is that the lateral arrangement of the MIMCAP 100 plates 110, 112 above the substrate footprint 130 results in asymmetrical parasitic capacitances of the respective plates 110, 112, whereas the vertical arrangement of the generally parallel VNCAP 200 plates 210, 212, projecting parallel plate footprints 230, 232, respectively, results in symmetrical parasitic plate 210, 212 capacitance properties.

The MIMCAP 100 and VNCAP 200 each offer distinctive circuit behaviors and, in some BEOL applications, combinations of one or more MIMCAP's 100 with one or more VNCAP's 200 may be preferred. However, the asymmetrical parasitic capacitances of the MIMCAP 100 plates 110, 112 produce a polarity for the port terminals 116, 118. In one respect, a circuit using port 116 as an input port and port 118 as an output port results in different equivalent circuit behavior. In another respect, the polarity difference may render the MIMCAP 100 a unidirectional device. And incorrect polarity usage may cause circuit performance degradation. Accounting for such polarity issues results in circuit design inefficiencies as additional design time must be expended to distinguish between input and output polarities.

In many instances, multiple MIMCAPS capacitors are required on a single chip substrate, with each having the same intrinsic capacitance value. In configurations wherein the capacitors are close to the substrate, the variable extrinsic capacitances between the bottom plates closest to the substrate cannot be adequately controlled for in circuit design, as the value of the extrinsic capacitances may not be precisely predicted. Therefore, in conventional prior art practices wherein all of the plates closest to the substrate are connected together, and all of the plates farthest from the substrate are connected together, divergent capacitance values are effectively created in the otherwise individually equivalent capacitors.

Additional problems arise for high-density on-chip BEOL capacitor structures incorporating both MIMCAP's 100 and VNCAP's 200, since parallel connections between the VNCAP 200 and MIMCAP 100 components must be provided to accommodate the divergent polarities of the port terminals 116, 118, and forming such parallel connections presents structural limitations on the resultant composite MIMCAP 100/VNCAP 200 BEOL structure that diminishes possible chip real estate efficiencies. It also presents other difficulties in providing a symmetrical BEOL capacitor structure created from multiple VNCAP's and MIMCAP's.

What is needed is a system and method for enabling the efficient incorporation of lateral MIMCAP capacitors in BEOL applications. Accordingly, it is necessary to develop a technique to provide each of a plurality of MIMCAP capacitors on a substrate with symmetry with respect to the substrate.

SUMMARY OF THE INVENTION

In one aspect, a capacitance circuit assembly mounted on a semiconductor chip, and methods for making the same, are provided comprising at least two capacitors mounted close to a substrate, wherein each capacitor has a lateral lower conductive plate mounted near enough to the substrate to have extrinsic capacitance greater than an upper plate extrinsic capacitance. One half of lower plates and one half of upper plates are connected to a first port, and a remaining one half of upper plates and lower plates are connected to a second port, the first and second port having about equal extrinsic capacitance from the lower plates.

In another aspect, the at least two capacitors are Metal-Insulator-Metal Capacitors, and the capacitance circuit assembly is located in a back-end-of-line semiconductor capacitor circuit.

In another aspect, the substrate further comprises a front-end-of-line capacitor defining a substrate footprint, and the at least two capacitors are electrically connected to the front-end-of-line capacitor and disposed above the substrate within the front-end-of-line capacitor footprint.

In another aspect, the at least two capacitors are at least four capacitors. In a further aspect, the at least four capacitors are arrayed in a rectangular array generally parallel to the substrate.

In another aspect, a Vertical Native Capacitor is electrically connected to at least two capacitors and disposed above the substrate within the front-end-of-line capacitor footprint.

In another aspect, the first and second plates are formed of the same material. In a further aspect, the plates are a metal or polysilicon, and/or the dielectric material has a permeability value greater than about 4 (er >4).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic perspective depictions of two techniques for mounting capacitors on a substrate;

FIG. 3 depicts a schematic perspective of a MIM capacitor in relation to a substrate;

FIG. 4 depicts a schematic perspective of a conventional prior art connection of two MIM capacitors relative to a substrate;

FIG. 5 depicts a schematic perspective of a connection of two MIM capacitors relative to a substrate according to this invention;

FIG. 6 depicts a schematic perspective of a conventional prior art connection of four MIM capacitors relative to a substrate;

FIG. 7 depicts a schematic perspective of a connection of four MIM capacitors relative to a substrate according to this invention.

FIG. 8 is perspective view of a VNCAP element;

FIG. 9 depicts a schematic perspective of a connection of two MIM capacitors to a VNCAP according to this invention;

FIG. 10 depicts a schematic perspective of a conventional prior art connection of four MIM capacitors relative to a substrate; and

FIG. 11 depicts a top plan view of a symmetrical capacitor structure relative to a substrate according to this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 3 illustrates an example of a single asymmetrical BEOL MIMCAP 300 appropriate for use with the present invention, having a top plate 310 and a bottom plate 320 arrayed laterally with respect to a substrate 314 and having connectors or ports 316, 318, respectively. A dielectric material 315 placed between the plates 310, 320 completes a capacitive structure in a lateral plate arrangement with respect to a FEOL substrate 314. (Other FEOL structures are omitted for simplicity of illustration.)

The substrate 314 conventionally is formed of silicon which is dielectric. Preferably, the dielectric material 315 has a permeability value greater than about 4 (er >4). It is to be understood that the plates 310, 320 can be formed of the same material, e.g. polysilicon or copper or other conductive material, or different materials which can be used conventionally for capacitors, depending upon the need and processes.

The two conductive capacitive plates 310, 320 are mounted close enough to the substrate 314 to have an extrinsic or parasitic capacitance, represented diagrammatically by the extrinsic capacitance values 324, 322, respectively. The extrinsic capacitive values 322 between bottom plate 320 and substrate 314 defined within bottom plate footprint 340 are greater than the extrinsic capacitive values 324 between plate 310 and substrate 314 defined within top plate footprint 350, this difference resulting in differing port 316, 318 polarities as described above.

Referring now to FIG. 4, a pair of MIMCAP's 408, 409 are shown in a conventional parallel circuit structure 400 between two port terminals, wherein PORT 1 410 is connected to the bottom plates 432, 434 at connectors 402, 403, respectively; and PORT 2 420 is connected to the top plates 442, 444 at connectors 405, 406, respectively. The bottom plates 432, 434 form parasitic extrinsic capacitors 452, 454, respectively, with a FEOL substrate disposed below (not shown for clarity of illustration, but as described above with respect to FIGS. 1 and 3). As the schematic representation of FIG. 4(b) illustrates, the parallel circuit structure 400 is asymmetrical as both of the parasitic extrinsic capacitors 452, 454 are accordingly connected to PORT 1 410, and no parasitic extrinsic capacitors are connected to PORT 2 420.

In one aspect, a symmetrical multi-MIMCAP capacitor design is provided that eliminates the extrinsic/parasitic capacitance differences of individual asymmetrical MIMCAP's between their laterally-oriented top and bottom plates with respect to a FEOL chip substrate. For example, FIG. 5 illustrates the pair of MIMCAP's 408, 409 in a novel cross-coupled parallel circuit structure 500 between the two port terminals, wherein PORT 1 410 is connected to the MIMCAP 408 bottom plate 432 at connector 512 and to the MIMCAP 409 top plate 444 at connector 513; and PORT 2 420 is connected to the MIMCAP 408 top plate 442 at connector 515 and to the MIMCAP 409 bottom plate 434 at connector 516. Again, the bottom plates 432, 434 form equivalent parasitic extrinsic capacitors 452, 454, respectively, with a FEOL substrate disposed below (not shown for clarity of illustration, but as described above with respect to FIGS. 1, 3 and 4). As the schematic representation of FIG. 5(b) illustrates, the cross-coupled parallel circuit structure 500 is symmetrical as parasitic extrinsic capacitor 452 is accordingly connected to PORT 1 410, and parasitic extrinsic capacitor 454 is connected to PORT 2 620.

In another aspect, more than two MIMCAP's may be arranged in a cross-coupled parallel circuit structure to provide symmetrical BEOL MIM structures; what is important is that parasitic extrinsic capacitors created through substrate proximity are allocated evenly between the two circuit ports in order to prevent port polarity. For example, FIG. 6 illustrates another conventional multi-capacitor MIM structure 600, wherein four MIMCAP's 624 are arranged in a parallel circuit structure between a first port 630 and a second port 632, wherein all of the upper plates 610 are connected together by connector 630, and all of the bottom plates 612 more proximate to a chip substrate 614 are connected together by connector 632. (It is to be understood that the substrate 614 may be one continuous substrate element, and that the substrate is shown in discrete rectangular sections 614 in order to simplify and clarify the illustration.) Thus, there is no provision made for variations in the extrinsic capacitances of each of the MIMCAP's 624, and polarity results between the terminals 630 and 632.

In another aspect, FIG. 7 depicts a schematic perspective of an alternative circuit structure with the four MIM capacitors 624 relative to the substrate 614 according to this invention. More particularly, half of the upper plates 610 are connected to half of the bottom plates 612 by first port connectors 736, and the other half of the top plates 610 are connected by second port connectors 738 to the other half of the bottom plates 612. This results in a symmetrical capacitor circuit 700 design with respect to the substrate 614, whereby parasitic capacitance is allocated evenly to each of the first port 736 and second port 738, as also discussed with respect to FIG. 5 above. Thus, the present invention enables a composite symmetrical BEOL circuit 700 with multiple MIMCAP 624's, which offers improved Q-factor performance over a single asymmetrical MIMCAP BEOL circuit (such as MIMCAP 300 of FIG. 3) with a similar overall footprint.

In another aspect, the present invention also has application to multiple MIMCAP structures incorporating other types of capacitors. For example, it is desirable to incorporate VNCAP's in BEOL chip applications. FIG. 8 provides a perspective view of a VNCAP 800 illustrating a parallel metal plate and composite capacitance structure that is desirable in some BEOL capacitor applications. The VNCAP 800 is defined by three groups of progressively larger metal layers. A first bottom group 860 of four metal layers (M1 through M4) are each separated by an insulator (or dielectric) material layer (V1 through V3), generally the first metal layer M1 in circuit connection with FEOL circuit structures, illustratively including MOSCAP structures (not shown). A second middle group of larger metal layers 862 (M5 and M6, respectively, the fifth and sixth metal layers) are mounted on the first group of layers 860 and separated by a dielectric material layer V4 from each other. Lastly, a third largest top group 864 of metal layers (M7 and M8, respectively, the seventh and eighth metal layers) are mounted atop the second metal layer group 862 and separated by a dielectric material layer V7 from each other.

In another aspect, each of the three VNCAP metal levels 860, 862 and 864 further comprise parallel “−” signed and “+” signed metal plates. More particularly, the VNCAP first level 860 metal layers M1 through M4 further each comprise a plurality of “+” signed metal plates 820 in an alternative horizontal parallel relationship with a plurality of “−” signed metal plates 822. The VNCAP second middle level 862 metal layers M5 and M6 further each comprise a plurality of “+” signed metal plates 830 in an alternative horizontal parallel relationship with a plurality of “−” signed metal plates 832. And the VNCAP third top level 864 metal layers M7 and M8 further each comprise a plurality of “+” signed metal plates 840 in an alternative horizontal parallel relationship with a plurality of “−” signed metal plates 842.

VNCAP's may offer superior capacitance capabilities in BEOL applications over smaller footprints than may be practiced with other capacitor structures. In another aspect, the three divergently sized VNCAP 800 bottom 860, middle 862 and top 864 metal layers each define a capacitor region having discrete capacitance values Q1(C1), Q2(C2) and Q3(C3), respectively. Thus, the VNCAP 800 also offers additional advantages in BEOL applications for improving Q-factor performance in the overall FEOL/BEOL circuit structure by enabling multiple discrete Q elements within a small footprint, as is apparent to one skilled in the art.

Accordingly, in another aspect of the present invention, FIG. 9 depicts a schematic perspective of a symmetrical multi-capacitor BEOL circuit structure 900 according to this invention. More particularly, first and second MIM capacitors 920, 924 are in a cross-coupled parallel circuit connection with the VNCAP 800 of FIG. 8. (For clarity, the VNCAP 800 middle metal layers 862 are omitted from the view shown in FIG. 9). A PORT 1 terminal 901 is thus in circuit connection 909 with the first MIMCAP 920 upper plate 902, in circuit connection 922 with the second MIMCAP 924 bottom plate 904 and with the positive “+” VNCAP 800 capacitor plates through terminal 802 (as described above with respect to FIG. 8). PORT 2 terminal 902 is in circuit connection 907 with the second MIMCAP 924 upper plate 903, in circuit connection 920 with the first MIMCAP 920 bottom plate 901 and with the negative “−” VNCAP 800 capacitor plates through terminal 801.

Although the present VNCAP example is described with respect to specified numbers of metal layers within designated capacitor groupings, as well as overall metal layer totals, it is to be understood that the inventions described herein are not restricted to the specific exemplary embodiments. It will be readily apparent that more or less metal layers may be practiced within VNCAP's within the teachings herein, and one skilled in the art may readily form alternative embodiments with different metal layer numbers and combinations.

In another aspect, the present invention may also be practiced with other multi-MIMCAP structures. FIG. 10 depicts a schematic perspective of a conventional rectangular array multi-capacitor MIM structure 1000, wherein four MIMCAP's 1030 having upper plates 1010, lower plates 1012 and dielectric layer 1020 therebetween are arranged in a parallel circuit structure between a first port 1002 and a second port 1004. This type of array may offer advantages, including providing improved Q-factor values relative to other single MIMCAP or multi-MIMCAP arrays in BEOL applications. However, as discussed above, the four MIMCAP's 1030 each have greater parasitic capacitance values relative to the substrate 1014 at their bottom plates 1012 than at their upper plates 1010. In this conventional circuit structure, all of the upper plates 1010 are connected to the first port 1002, and all of the bottom plates 1012 more proximate to the chip substrate 1014 are connected to the second port 1004. (Again, it is to be understood that the substrate 1014 may be one continuous substrate element, and that the substrate is shown in discrete rectangular sections 1014 in order to simplify and clarify the illustration.) Thus, there is no provision made for variations in the extrinsic capacitances of each of the MIMCAP's 1030, and polarity results between the terminals 1002 and 1004 as discussed above. Correcting for the polarity, or taking the polarity into account in circuit design, poses many disadvantages over structures not presenting polarity between the ports 1002, 1004.

Accordingly, in another aspect, FIG. 11 provides a top plan view of symmetrical rectangular array multi-capacitor MIM structure 1100 according to the present invention. The four MIMCAP's 1030 arranged in a parallel circuit structure between a first port 1132 and a second port 1138, wherein half of the upper plates 1010 are connected to half of the bottom plates 1012 in adjacent MIMCAP's 1030 by port connectors 110 and port circuit wiring 1112, and the other half of the top plates 1010 are connected to the other half of the bottom plates 1012 in adjacent MIMCAP's 1030 by port connectors 110 and port circuit wiring 1112. This results in a symmetrical capacitor circuit 1100 design with respect to the substrate 1014, whereby parasitic capacitance is allocated evenly to each of the first port 1136 and second port 1138, as discussed above.

While preferred embodiments of the invention have been described herein, variations in the design may be made, and such variations will be apparent to those skilled in the art of capacitors, as well as to those skilled in other arts. For example, it will be understood that the present invention is not limited to the specific numbers and arrangements of MIMCAP's and VNCAP's described thus far, and the invention can work with circuit structures comprising more that four MIM capacitors.

Claims

1. A capacitance circuit assembly mounted on a semiconductor chip comprising:

a chip substrate;

at least two capacitors mounted close to the substrate;

wherein each capacitor has first and second conductive plates separated by a dielectric material;

wherein each second conductive plate is mounted flat near enough to said substrate to have a second plate extrinsic capacitance with said substrate greater than an extrinsic capacitance of each first conductive plate;

one half of said first plates and one half of said second plates connected to a first port by a first port circuitry, wherein the first port has a first port composite extrinsic capacitance from the one half of said second plates; and

a remaining one half of said first plates and a remaining one half of said second plates connected to a second port by a second port circuitry, wherein the second port has a second port composite extrinsic capacitance from the remaining one half of said second plates, the second port composite extrinsic capacitance about equal to the first port composite extrinsic capacitance.

2. The structure of claim 1 wherein the at least two capacitors are Metal-Insulator-Metal Capacitors, and the capacitance circuit assembly is located in a back-end-of-line semiconductor capacitor circuit.

3. The structure of claim 2, wherein the substrate further comprises a front-end-of-line capacitor with first and second terminals, the front-end-of-line capacitor defining a substrate footprint;

wherein the first port is electrically connected to the front-end-of-line capacitor structure first terminal and the second port is electrically connected to the front-end-of-line capacitor structure second terminal; and

wherein the at least two capacitors are disposed above the substrate within the front-end-of-line capacitor footprint.

4. The structure of claim 3 wherein the at least two capacitors are at least four capacitors.

5. The structure of claim 4 wherein the at least four capacitors are arrayed in a rectangular array generally parallel to the substrate.

6. The structure of claim 3, further comprising a Vertical Native Capacitor disposed above the substrate within the front-end-of-line capacitor footprint and having first and second terminals, and wherein the first port is electrically connected to the Vertical Native Capacitor first terminal and the second port is electrically connected to the Vertical Native Capacitor second terminal.

7. The structure of claim 1 wherein each of the first and second plates are formed of the same material.

8. The structure of claim 7 wherein the plates are a metal or polysilicon.

9. The structure of claim 7 wherein the dielectric material has a permeability value greater than about 4 (er >4).

10. A method for forming a semiconductor chip capacitance circuit, comprising the steps of:

forming a front end of line substrate structure;

mounting at least two capacitors close to the substrate, wherein each capacitor has first and second conductive plates separated by a dielectric material, and wherein each second conductive plate is mounted flat near enough to said substrate to have a second plate extrinsic capacitance with said substrate greater than an extrinsic capacitance of each first conductive plate;

connecting one half of said first plates and one half of said second plates to a first port by a first port circuitry, wherein the first port has a first port composite extrinsic capacitance from the one half of said second plates; and

connecting a remaining one half of said first plates and a remaining one half of said second plates to a second port by a second port circuitry, wherein the second port has a second port composite extrinsic capacitance from the remaining one half of said second plates, the second port composite extrinsic capacitance about equal to the first port composite extrinsic capacitance.

11. The method of claim 10 wherein the at least two capacitors are Metal-Insulator-Metal Capacitors, further comprising the step of locating the at least two capacitors in a back-end-of-line semiconductor capacitor circuit.

12. The method of claim 11, further comprising the steps of:

providing a front-end-of-line capacitor with first and second terminals in the substrate;

the front-end-of-line capacitor defining a substrate footprint;

connecting the first port electrically to the front-end-of-line capacitor structure first terminal;

connecting the second port electrically to the front-end-of-line capacitor structure second terminal; and

disposing the at least two capacitors above the substrate within the front-end-of-line capacitor footprint.

13. The method of claim 12 wherein the at least two capacitors are at least four capacitors.

14. The method of claim 13, further comprising the step of disposing the at least four capacitors in a rectangular array generally parallel to the substrate.

15. The method of claim 12, further comprising the steps of:

disposing a Vertical Native Capacitor above the substrate within the front-end-of-line capacitor footprint, the Vertical Native Capacitor having first and second terminals;

connecting the first port electrically to the Vertical Native Capacitor first terminal; and

connecting the second port electrically to the Vertical Native Capacitor second terminal.

16. The method of claim 10 wherein each of the first and second plates are formed of the same material.

17. The method of claim 16 wherein the plates are a metal or polysilicon.

18. The method of claim 16 wherein the dielectric material has a permeability value greater than about 4 (er >4).

19. A semiconductor circuit structure comprising:

a chip substrate comprising a front-end-of-line metal oxide silicon capacitor with first and second terminals, the metal oxide silicon capacitor defining a front-end-of-line capacitor footprint;

at least two back-end-of-line Metal-Insulator-Metal Capacitors electrically connected to the front-end-of-line capacitor structure and mounted close to the substrate and above the front-end-of-line capacitor footprint, wherein Metal-Insulator-Metal bottom conductive plates are mounted flat near enough to said substrate to have a bottom plate extrinsic capacitances with said substrate greater than Metal-Insulator-Metal top conductive plate extrinsic capacitances;

one half of said top plates and one half of said bottom plates connected to a first port by a first port circuitry, wherein the first port has a first port composite extrinsic capacitance from the one half of said bottom plates; and

a remaining one half of said top plates and a remaining one half of said bottom plates connected to a second port by a second port circuitry, wherein the second port has a second port composite extrinsic capacitance from the remaining one half of said bottom plates, the second port composite extrinsic capacitance about equal to the first port composite extrinsic capacitance.