Ground shield capacitor

In one embodiment, an apparatus includes a first reference voltage coupled to a first metal layer and a second reference voltage coupled to a second metal layer. A first finger type in the plurality of fingers is coupled to the first metal layer at a first area and coupled to the first metal layer and the second metal layer at a second area. A second finger type in the plurality of fingers is coupled to the second metal layer at the first area and coupled to the first metal layer and the second metal layer at the second area. Also, the first finger type and the second finger type alternately positioned next to each other.

BACKGROUND

Particular embodiments generally relate to ground shield capacitors.

For a passive component, such as an inductor or transformer, the area under the passive component in an integrated circuit (IC) chip is often left unused. This avoids the impact of the passive component on circuits under the passive component and the impact of the circuit on the passive component. The impacts include electric coupling (capacitive) and magnetic coupling (eddy currents).

A ground shield may be placed under the passive component to terminate electric fields resulting from electric coupling. Additionally, the performance of the passive component may be improved by the use of the ground shield. For example, the ground shield may increase an inductor's quality factor (Q). Also, the electric coupling between the passive component and a substrate or another structure under the passive component may be reduced. However, it is possible that ground shields will not block eddy currents, and thus, even when a ground shield is used, circuits are often not placed under the passive component.

Not having anything under the passive component may cause problems in chip fabrication. For example, it is better for chip fabrication to maintain the density of each metal layer between an upper limit and a lower limit. A passive component made with a high-level metal and nothing under the high-level metal layer violates density rules for lower-level metal. Workarounds exist that place metal fill around the passive component. However, the fill takes up additional area. Using a ground shield on a metal layer under the passive component will meet metal density rules without a guard ring of metal fill.

FIG. 1shows an example of a transformer102with a conventional ground shield104for an integrated circuit (IC) chip. Although transformer102is shown, another passive component may be used. Transformer102in this example includes two coils, a primary coil and a secondary coil.

Ground shield104is situated under transformer102and includes a plurality of fingers106. Fingers106include gaps in between them that do not allow a circle of current to flow around ground shield104, which avoids the adverse effects of eddy currents.

Each finger106is coupled to contacts108. This couples the fingers to a ground110. Also, fingers106are all coupled to the same layers of metal.

In addition to ground shield104, the chip may include a de-coupling capacitor. In some radio frequency circuits, a high frequency current is pulled from the supply. Bond wire inductance acts as a large impedance at high frequencies. So, an alternating current (AC) low impedance path to ground is required on the chip. Typically, a large de-coupling capacitor between supply and ground is used. These de-coupling capacitors require significant area on the chip.

One example of a de-coupling capacitor that may be used is a metal-oxide-metal (MOM) capacitor.FIG. 2shows an example of a conventional MOM capacitor200. MOM capacitor200includes a plurality of metal lines202. Odd metal lines202amay be connected to a first connection at the bottom, which may be connected to ground204. Even metal lines202bmay be connected to a second connection at the top, which may be connected to a supply206. Odd metal lines202aand even metal lines202balternate in MOM capacitor200. Capacitance between even metal lines202band odd metal lines202ais then formed.

Conventionally, ground shield104and MOM capacitor200are separate structures in different areas of the chip. Having separate structures may be an inefficient use of area on the chip.

SUMMARY

In one embodiment, an apparatus includes a first reference voltage coupled to a first metal layer and a second reference voltage coupled to a second metal layer. A first finger type in the plurality of fingers is coupled to the first metal layer at a first area and coupled to the first metal layer and the second metal layer at a second area. A second finger type in the plurality of fingers is coupled to the second metal layer at the first area and coupled to the first metal layer and the second metal layer at the second area. Also, the first finger type and the second finger type alternately positioned next to each other.

In one embodiment, the first finger type is coupled to a third metal layer at the first area and coupled to the third metal layer and a fourth metal layer at the second area. The second finger type is coupled to the fourth metal layer at the first area and coupled to the third metal layer and the fourth metal layer at the second area.

In one embodiment, the first finger type is coupled to a third finger type at the first metal layer at the first area. The second finger type is coupled to the third finger type at the second metal layer at the first area.

In one embodiment, a system includes a passive device, where the apparatus is included under the passive device.

In one embodiment, a method includes coupling a first reference voltage to a first finger type in the plurality of fingers on a first metal layer at a first area and coupling the first reference voltage to the first metal layer and a second metal layer at a second area. The method also includes coupling a second reference voltage to a second finger type in the plurality of fingers on the second metal layer at the first area and coupling the second reference voltage to the first metal layer and the second metal layer at the second area. The first finger type and the second finger type are alternately positioned next to each other.

In one embodiment, the method includes coupling the first reference voltage through a third finger type on the first metal layer and coupling the second reference voltage through the third finger type on the second metal layer. The first finger type is coupled to the first metal layer at the third finger type and the second finger type is coupled to the second metal layer at the third finger type.

The following detailed description and accompanying drawings provide a more detailed understanding of the nature and advantages of the present invention.

DETAILED DESCRIPTION

Described herein are techniques for a ground shield capacitor. In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. Particular embodiments as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include modifications and equivalents of the features and concepts described herein.

FIG. 3shows an example of a ground shield capacitor300according to one embodiment. Ground shield capacitor300provides a capacitor that is in the shape of a ground shield and offers features provided by both a ground shield and an AC decoupling capacitor. Ground shield capacitor300may be placed under the area of a passive component, such as a transformer, an inductor, or a balun in an IC chip.

Ground shield capacitor300includes a supply connection302that couples ground shield capacitor300to a first reference voltage, such as a supply voltage. A ground connection304couples ground shield capacitor300to a second reference voltage, such as ground. In one embodiment, supply connection302is on different metal layers from ground connection304. For example, supply connection302is on metal layers1and3and ground connection304is on metal layers2and4.

Ground shield capacitor300includes a plurality of fingers306. Fingers306may be conductive metal lines in the chip. Fingers306are arranged in a radial structure around ground shield capacitor300. For example, fingers306extend outwardly from a point and are arranged in a circular manner. Different radial structures may be used in which fingers306are arranged around ground shield capacitor300. Although radial structures are described, other non-radial structures may be used.

Fingers306may include primary fingers306aand secondary fingers306b. Although primary fingers306aand secondary fingers306bare described, other arrangements may be used. Primary fingers306amay provide connection points for secondary fingers306b. In one embodiment, a first secondary finger306bmay be considered a first finger type, a second secondary finger306bmay be considered a second finger type, and primary finger306amay be considered a third finger type. Primary fingers306amay carry supply and ground on alternating metal layers, i.e., the metal layers1and3of primary fingers306aare coupled to supply connection302and the metal layers2and4of primary fingers306aare coupled to ground connection304.

Certain secondary fingers306bare connected together by vias (not shown) to metal layers, such as metal layers1,2,3, and4, except at an inner edge that connects to primary fingers306a. Although metal layers1-4are mentioned, other metal layers may be used. As will be described in more detail below, alternating secondary fingers306bconnect with either supply connection302or ground connection304. For example, a first secondary finder306bis coupled to metal layers1and3and a second secondary finger306bis coupled to metal layers2and4.

Capacitors are formed between secondary fingers306bbecause the connections on alternating secondary fingers306bare to supply and then ground, which creates a potential difference across secondary fingers306b. The capacitors do not create paths for eddy currents (e.g., circular current paths) because the eddy currents see every capacitor that is formed by secondary fingers306bin series. The capacitors in series create a high impedance for eddy currents, which minimizes the eddy currents that can flow. Additionally, gaps308provide additional protection by adding high impedance for eddy currents by breaking the capacitors at certain points. However, the capacitors are in parallel from supply to ground creating a low impedance path from supply to ground, which is desirable.

FIG. 4shows a zoomed-in version of an area310shown inFIG. 3according to one embodiment. Other areas of ground shield capacitor300are similar. Primary fingers306aare shown along with secondary fingers306b. As discussed above, alternating secondary fingers306bconnect with either metal layers1and3or metal layers2and4. For example, at402a, a first connection with primary finger306afor an odd secondary finger306bis shown. First connection402amay be on metal layers1and3. At a second connection402b, an even secondary finger306bis connected to primary finger306a. Second connection402bmay be on metal layers2and4. The pattern continues as every other secondary finger306bconnects to either metal layers1and3or metal layers2and4in a first area at the inner edge of secondary fingers306b.

At a second area, vias404may be used to couple fingers306bto all four metal layers1,2,3, and4. This couples all four metal layers to ground or supply in an alternating manner. This structure creates capacitance between even fingers306band odd fingers306b.

A side view shows the connections of even fingers306band odd fingers306bat the first area and second area.FIG. 5shows an example of a side view of metal layers1,2,3, and4according to one embodiment. An odd finger306band an even finger306bare shown.

Odd finger306bis connected to metal layers2(M2) and4(M4). Metal layers2and4are coupled to ground at a first area402aat primary finger306a. Even finger306bis coupled to metal layers1(M1) and3(M3) at first area402aat primary finger306a. Metal layers1and3are coupled to the supply at primary finger306a.

Vias404couple the metal layers together at a second area402b. For example, via404couples metal layers1,2,3, and4together. For odd secondary finger306a, this couples all four metal layers to ground. For even secondary finger306b, via404couples all four metal layers to the supply. Although only one set of vias404are shown, vias404may be located at multiple points on secondary fingers306b.

Because odd secondary finger306band even secondary finger306bare next to each other and either coupled to ground or supply, capacitance is formed between them. However, because within a secondary finger306bmetal layers are coupled to either ground or supply, vertical capacitance does not occur.

Ground shield capacitor300terminates electric fields like a conventional ground shield. Also, ground shield capacitor300minimizes eddy currents because every capacitor is seen in series. However, parallel capacitance creates a low impedance from supply to ground. This couples the supply to AC ground. In some radio frequency (RF) circuits, high frequency current is pulled from the supply. The path from supply to ground sees a parallel capacitance that creates a low impedance from supply to ground. This provides the desired low impedance AC coupling from the supply to ground.

Other uses may also be possible for ground shield capacitor300.FIG. 6shows an example of an AC coupling ground shield600according to one embodiment. AC coupling ground shield600includes a first capacitor602aand a second capacitor602b. First capacitor602aand second capacitor602bare split between right and left sections of AC coupling ground shield600, respectively. AC coupling ground shield600acts as two AC coupling capacitors from a first stage to a second stage in the IC chip.

In this example, AC coupling ground shield600may be placed under an inductor604; however, other passive components may be used. The same structure as described above with ground shield capacitor600may be used for AC coupling ground shield600. However, the difference is that two separate capacitors are being formed by AC coupling ground shield600. For example, first AC coupling capacitor602ahas an input P606aand an output P608aand second AC coupling capacitor602bhas an input N606band an output N608b.

Another use for ground shield capacitor300is to provide differential tuning capacitance. Inductors and transformers may need some additional capacitance in parallel to be tuned to a desired frequency. In this case, ground shield capacitor300may be placed under the inductor or transformer. The inductor or transformer may then be tuned to resonate at the desired frequency using the capacitance of ground shield capacitor.

FIG. 7shows an example of a circuit diagram in which AC coupling ground shield600and a tuning ground shield capacitor700may be used. As shown, inductors702a/bare provided along with current sources704in a first stage. A second stage includes an amplifier706. AC coupling capacitors are needed in between the first stage and second stage. In one example, AC coupling ground shield capacitor600is placed under inductors702a/bin the chip. For example, referring toFIG. 6, the right section of AC coupling ground shield600may form a first AC coupling capacitor602aand the left side of AC coupling ground shield600forms a second capacitor602b. In this case, an input N606ais coupled to a node N1and an output N608ais coupled to a node N2. An input P606bis coupled to a node P1and an output P608bis coupled to a node P2in the circuit diagram. By placing AC coupling ground shield capacitor600under inductors702, the first and second stage may be moved closer together in the chip, which may use less area on the chip.

Tuning ground shield capacitor700is coupled in between first inductor702aand second inductor702b. If ground shield capacitor300is used, the supply connection may be coupled instead to a node N3and the ground connection may be coupled instead to node N1. Thus, connections to supply and ground are substituted for coupling a tuning capacitor across inductors702aand702btogether.

Accordingly, various implementations of a ground shield capacitor are provided. For example, ground shield capacitor300as described inFIG. 3is coupled from supply to ground. AC coupling ground shield capacitors600AC couple a first stage to a second stage. And finally, a tuning ground shield capacitor700couples across a passive component or pair of passive components to provide tuning capacitance. Other connections and implementations may also be used.

Accordingly, capacitors may be placed under passive components. This may allow the modeling of the capacitor and passive component separately. For AC coupling ground shield capacitors, separate modeling is allowed because small errors in the capacitance of the AC coupling ground shield capacitor do not impact the circuit as long as the capacitance is large enough. However, for a tuning ground shield capacitor, the passive component and capacitor may be modeled as a single unit to achieve the exact tuning of the desired frequency.

FIG. 8shows a flowchart800of a method for providing a ground shield capacitor according to one embodiment. At802, a first signal from a first reference voltage is coupled through odd secondary fingers306bon first and third metal layers at a first area. For example, a supply voltage may be coupled to metal layers1and3.

At804, the first signal through odd secondary fingers306bis coupled to the first, second, third, and fourth metal layers at a second area. At806, a second signal through even secondary fingers306bis coupled to second and fourth metal layers at the first area. At808, the second signal is coupled to the first, second, third, and fourth metal layers at the second area. Accordingly, at the second area, odd secondary fingers306bare coupled to the first reference voltage and even secondary fingers306bare coupled to the second reference voltage.

The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents may be employed without departing from the scope of the invention as defined by the claims.