SYSTEMS AND METHODS FOR REDUCING CIRCUIT AREA

Cross-Reference to Related Application

[0001] This application claims the benefit under 35 U.S.C. § 119(e) of United States Provisional Patent Application No. 60/684,496 filed May, 24 2005, which is hereby incorporated by reference herein in its entirety.

Field of the Invention

[0002] The present invention relates to the field of integrated circuit design. More particularly, the present invention relates to systems and methods for reducing circuit area.

Background of the Invention

[0003] An inductor is an electronic device used in circuits for its property of inductance. The behavior of inductors is related to phenomena associated with magnetic fields. When electric current flows through an inductor, magnetic fields may be created. Moreover, if the current varies with time, then the magnetic field will vary with time. This time- varying magnetic field induces a voltage in electrical conductors exposed to it. The circuit parameter of inductance relates the induced voltage to the current.

[0004] One property of an inductor is its quality factor, Q. The quality factor of an inductor is proportional to the ratio of its inductance to its resistance at a given frequency, and is a measure of the inductor's efficiency. The higher the quality factor, the more efficient the inductor is. The quality factor of an inductor may be influenced by several factors, one factor being eddy currents, which are the circulating flow of charges caused by a moving magnetic field within a nearby conductive material or device. The flow of eddy currents generates magnetic fields that oppose changes in external magnetic fields. Generation of eddy currents near an inductor degrades the inductor's quality factor, and thus if one is concerned with the quality factor of an inductor, it is usually desirable to avoid having eddy currents in devices near inductors.

[0005] On-chip inductors, despite being large, are often made of only two metal layers. A minimum metal density requirement for each metal layer over an entire wafer of an IC may be set to reduce topographical variations, increase uniformity, and target a certain yield. Therefore, it may be desired to increase the metal density count of a layer. Placing metal fills inside or near an inductor increases the metal density count, but has the possible drawback of decreasing the quality factor of the inductor through eddy current loss. Measurement results of inductors with metal fills have been reported before, but none have established a relationship between fill cell size and the inductor quality factor. This relationship is of great interest because it allows one to estimate the largest device that can be placed inside an inductor.

[0006] It should be noted that the topmost metal layer in a fabrication process is often referred as the thick top metal. The thick top metal layer is often several times thicker than the other metal layers, and thus has a lower resistance. This is beneficial for making inductors because a lower resistance improves the quality factor of an inductor. [0007] Spiral on-chip inductors are a type of inductor used in the design of, for example, radio frequency integrated circuits (RFICs). These spiral on-chip inductors often occupy more than half of the total chip area in RFICs. The region in and around an on-chip inductor is typically kept clear of active and passive devices to avoid the generation of eddy currents in the devices, which, as discussed, degrades the quality factor of the inductor. However, leaving the area near an inductor empty is a waste of space and increases chip size. This is a problem because reducing the area, and therefore the cost of circuits is a concern in circuit design. [0008] Therefore, it is beneficial to reduce restrictions on the spacing among devices on a circuit. Specifically, it would be of benefit to reduce spacing restrictions between inductors and other devices.

Summary of the Invention

[0009] Methods and systems are provided for reducing circuit area. Some embodiments provide electronic devices including an inductor formed from a path having two ends that loops substantially in a plane around a center area, wherein the path crosses itself at at least two points and wherein the path defines an outer boundary of the inductor; and a circuit that is located within the outer boundary of the inductor and substantially within or adjacent to the plane.

[0010] Other embodiments provide electronic devices including an inductor formed from a path having two ends that loops substantially in a plane around a center area, wherein the path defines an outer boundary of the inductor; and a circuit that is located, within the outer boundary of the inductor and substantially within or adjacent to the plane, and wherein the circuit comprises a signal path that is rake-shaped and crosses the path of the inductor at substantially perpendicular angles.

Brief Description of the Drawings

[0011] The above and other advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with accompanying drawings, in which like reference characters refer to like parts throughout, and in which:

[0012] FIG. 1 is an illustration of eddy current in metal fills inside a magnetic field;

[0013] FIG. 2 is a graph illustrating the relationship between inductor qualify factor and metal fill side length in accordance with the present invention;

[0014] FIG. 3 is a photograph that includes two inductors that may be used in accordance with certain embodiments of the present invention;

[0015] FIG. 4 contains two graphs relating to eddy current loss with respect to the presence of metal fill;

[0016] FIG. 5 is a schematic diagram of a voltage controlled oscillator that may be used in accordance with certain embodiments of the present invention;

[0017] FIG. 6a is a simplified illustration of a layout style for a voltage tuning input of a voltage controlled oscillator and associated current flows in accordance with certain embodiments of the present invention;

[0018] FIG. 6b is a simplified illustration of another layout style for a voltage tuning input of a voltage controlled oscillator and associated current flows in accordance with certain embodiments of the present invention;

[0019] FIG. 7a is a simplified illustration of rake-shaped metal wiring using for a voltage-tuning input in accordance with certain embodiments of the present invention;

[0020] FIG. 7b is a simplified illustration of a layout of a voltage controlled oscillator in accordance with certain embodiments of the present invention;

[0021] FIG. 8a is a graph illustrating the relationship between frequency and tune voltage in accordance with some embodiments of the present invention;

[0022] FIG. 8b is a graph illustrating the relationship between output power and tune voltage in accordance with some embodiments of the present invention;

[0023] FIG. 9a is a graph illustrating the relationship between phase noise and offset frequency in accordance with some embodiments the present invention;

[0024] FIG. 9b is another graph illustrating the relationship between phase noise and offset frequency in accordance with some embodiments of the present invention;

[0025] FIG. 10 is a graph illustrating the relationship between phase noise and tune voltage in accordance with some embodiments of the present invention; and

[0026] FIG. 11 is a photograph of two voltage controlled oscillators, one with varactors and active devices outside the inductor and one with varactors and active devices inside the inductor in accordance with some embodiments of the present invention.

Detailed Description of the Invention

[0027] One consideration in the design of integrated circuits is that, by keeping the sizes of devices in close proximity to an inductor small, the induced eddy current loops will be localized in small regions and thus the reduction of the inductor's quality factor will be lessened. A second consideration is that by carefully planning the current paths of devices, the magnetic coupling between the device currents and the inductor currents may be reduced. A further consideration is the relationship between the placement of metal fills in an around an inductor and the inductor's quality factor.

[0028] In accordance with some embodiments of the present invention, systems and methods for placing passive and active devices inside an inductor are provided. In particular, non-active devices, such as varactors, as well as active devices are placed inside an inductor to create a compact voltage controlled oscillator (VCO) that has equal performance to traditional voltage controlled oscillators while using significantly less area. It should be noted that the systems and methods of the present invention may also be applicable to other types of circuits. For example, the capacitors of a phase locked loop filter may be placed under a voltage controlled oscillator inductor. These systems and methods may result in the reduction of layout area and therefore the cost of circuits.

[0029] FIG. 1 illustrates eddy currents in metal fills inside a magnetic field. In this example, because the effect of metal fill structures with small dimensions is of particular interest, it may be assumed that the skin effect in the metal fill and the effect of the induced currents on the magnetic field can be neglected.

[0030] The circular metal fill 110 has a radius R ₀ 115, and occupies about the same amount of area as N ² smaller circular metal fills 120 with radii R _o /N 125. In this example, N has a value of 2. For mathematical convenience, circular metal fills are used to approximate square metal fills that are used in real layouts. Assuming a small fill cell size compared to the dimensions of the inductor, the magnetic field, B, is uniform over the area of the metal fill cell and is not a function of radius, r 135. Radius, r 135, is the radius that encloses the magnetic flux, φ , in a circular loop, where φ is given by:

(I) φ = B - dA = B - (πr ² ) .

Using Faraday's Law, dφ dB ₂

(II) V = = πr ² dt dt where V is the potential developed along any current path as a result of changing flux induced by AC current in the inductor. The negative sign indicates that the current, Ieddy 140, induced by V, will flow in such a direction as to oppose the flux that produced it. The resistance, R, of a thin cylindrical sheet of metal fill, bounded by the dashed and dotted lines 130 in FIG. 1, is equal to:

(DD R = -£-- (2πr) , h - dr where p is the resistivity of the fill metal, h is the height of the metal fill, and dr is the incremental thickness of the cylindrical sheet. From equation II and equation IH, the total power dissipated in a metal fill with radius, R ₀ , is:

Since a large metal fill can be replaced by N ² small metal fills, the power dissipation for the two cases are:

)

The power dissipated in the metal fills is an additional loss mechanism for the inductor and thus reduces its quality factor. However, equation V shows, the power dissipation is reduced as the fill cell sizes are reduced. Accordingly, in some embodiments of the systems and methods of the present invention, electric devices are divided into smaller electric devices to lessen adverse effects on other near-by devices.

[0031] Equation V provides guidelines on device sizing inside an inductor, but is not sufficiently accurate for quantitative estimates. For more accurate estimates, full-wave simulations on inductors with different fill cell sizes may be run using an electro-magnetic simulator such as ElectroMagnetic extractor (EMX) available from Integrand Software Inc. of Berkeley Heights, NJ. However, any suitable simulator may be used. FIG. 2 illustrates the results from EMX simulations and shows a simulated quality factor at 2 GHz of a five-turn, 4.5 nH differential inductor, L _d i _ff , for different fill cell sizes and resistivity. Inductor L^s 510 is used in the VCO of FIG. 5, which will be described later. The inductor may be constructed using the thick top metal, and have an outer diameter of 200 μ m, inner diameter of 80 μ m, trace width of 10 μ m, and trace spacing of 3 μ m. The metal fills may be placed in the center of the inductor and may be constructed by stacking all the available metal and polysilicon layers without vias in between. FIG. 2 illustrates the inductor quality factor versus square metal fill side length for multi-layer fills 210, metal-only fills 220, and polysilicon-only fills 230. A metal fill side length of zero represents the case were no metal fills are used. The quality factor degrades rapidly as the metal fill dimensions become large, as can been seen only with multi-layer fills 210 and metal-only fills 220. However, for the more resistive polysilicon fills 230, the eddy loss is much less, as predicted by equation V. It should be noted that the term "M6" refers to metal layer number six, and is the thick top metal layer of the fabrication process used in this embodiment of the present invention. However, the number of metal layers can be altered without departing

from the scope of the methods and systems of the present invention. It should also be noted that, in this embodiment, the muli-layer fills include metal layers M1-M6 as well as a polysilicon layer.

[0032] The accuracy of EM simulations may be compared against measurement data from test structures, for example, a 0.25 μ m BiCMOS process. A single-ended 2.3 nH inductor may be constructed using the thick top metal layer, with an outer diameter of 200 μ m, and inner diameter of 100 μ m, a trace width of 10 μ m, and a trace spacing of 3 μ m. A die photograph of such test structures is shown in FIG. 3. Inductor 310 is without metal fills, while the inductor 320 is with metal fills. In this example, the metal fills are 7 μ m by 7 μ m squares with spacing of 3 μ m, and are of the multi-layer type. The metal fills are placed in area 340, inside the inductor, and area 350, around the inductor.

[0033] FIG. 4 illustrates quality factor versus frequency, in graph 410, and the percent degradation in quality factor versus frequency, in graph 420. The measured quality factor of plain inductor 310 is shown with the line formed of "o's" 430. The measured quality factor of inductor 320 with metal fills is shown with the line formed of "*'s" 440. Lines 450 and 460 show the corresponding simulation results. An error of less than 5% in quality factor is observed between the simulated data, 450 and 460, and the measured data, 430 and 440, for a frequency below 3 GHz. A maximum quality factor degradation of approximately 10% occurs at its peak as shown in FIG. 4 at reference label 470. In contrast, the typical application range of an inductor in a tunable VCO is below its peak quality factor frequency since the varactors and the parasitics of the active devices add significantly to the tank capacitance. [0034] The phase noise of voltage controlled oscillators (VCOs) is very sensitive to the quality factor of inductors. Because of this, VCOs are useful in demonstrating the systems and methods of the present invention. Two identically designed VCOs having differed layouts may be used to demonstrate some of the advantages of placing a device inside an inductor according to systems and methods of the present invention. Specifially, VCOs may be placed inside an inductor for this purpose. As discussed above, however, placing transistors and varactors near the inductor will decreases the quality factor of the inductor due to the presence of eddy currents. However, by using a tank layout for VCOs in the inductor that places the transistors and

varactors under the inductor as described above, significant losses due to eddy currents may be avoided.

[0035] Referring to FIG. 5, the layout of an inductor L _d m 510 may be modified to allow placement of varactors 530 and 540 inside inductor 510. As show in FIG. 6, by folding the leads of inductor 510 "outside in," the cathodes of the varactors can be connected along the inner-most turn of the inductor. It should be noted that although this connection can result in a distributed effect, which is undesirable in a narrowband circuit, the actual effect on the circuit is small because the inner-most turn only contributes to a small fraction of the total inductance. The varactors 530 and 540 may be connected to the inductor 510 as illustrated in FIG. 5. It should be noted that the plurality of varactors 670 shown in FIG. 6 are collectively shown as varactors 530 and 540 in FIG. 5. It should also be noted that the buffer 550 and the peak detector 560 are auxiliary circuits to facilitate measurements of the circuit.

[0036] FIGS. 6a and 6b illustrate two layout options for a V _tune line 520, that connects the anodes of the varactors 530 and 540 together. In FIG. 6a the anodes of the varactors 530 and 540 are connected together on the outside of the inductor. For example, at reference label 660, the connection of the anodes of multiple varactors 670 is shown. A drawback of this configuration is that the current paths of V _tune line 520 are parallel to the flow of the inductor current, causing further unwanted magnetic coupling. The configuration illustrated in FIG. 6b distributes V _tUne line 520 from the center 650 of the inductor, thus keeping the wires with parallel current flow far apart.

[0037] FIG. 7a illustrates the details of rake-shaped metal wiring used to connect the anodes of the varactors 530 and 540. The shape of the wiring is similar to that of a patterned ground shield (PGS). The fingers of the rake-shaped wiring are oriented such that the current flow is perpendicular to the direction of the inductor current, thus reducing magnetic coupling between the two. Furthermore, since the V _tune node is a signal ground for differential signals, the rake- shaped wiring acts as a grounded PGS that absorbs stray electric filed from the inductor to the substrate, thus improving the quality factor of the inductor.

[0038] Another concern in the VCO tank layout is the resistance of V _tune line 520, since its series resistance adds thermal noise that is directly upconverted into phase noise. In order to address this, parasitic resistance extraction may be performed on the longest wire path from V _tun e pin 520 to the varactor anode. In order to lower the wire resistance, multiple metal layers may be

strapped together. Lowering the resistance of V _tun e line 520 worsens its eddy current effect on the inductor, as Equation 5 suggests. However, the rake-shaped wiring in FIG. 7a prevents eddy currents from circulating in large loops and thus reduces their effect.

[0039] FIG. 7b illustrates a layout of the VCO. Simulating the differential inductor, L _d iff 510, by itself using EMX finds a quality factor of approximately 7 at 2 GHz. Small varactors of dimensions 12 μ m by ll μ m, collectively shown as Dl 530 and D2 540, are connected to the differential inductor. In this case 82 varactor diodes are connected to the inductor, though any appropriate number may be used. The varactor diodes Dl 530 and D2 540 may be made from p+ base in the n-well and have a simulated quality factor of about 40 at 2 GHz. It should be noted that the positions and connections of varactors 770 of FIG. 7 substantially correspond to the positions and connections of varactors 670 in FIG. 6B. The location of transistors 580 is also show in FIG. 7b.

[0040] Unlike the previously discussed metal fills, varactors actively participate in the circuit operation and carry AC currents. Their effect on the inductor is thus potentially more complicated than just extra loss due to eddy currents. For ease of simulation, the varactors may be replaced with parallel plate capacitors with similar plate resistivity as the varactor diodes while keeping the wiring the same. Simulations show that quality factor degradation is minimal when the varactors are placed under the inductor traces instead of at the center of the inductor. Not only are they exposed to a smaller magnetic field there, but they also perform the role of a PGS by further isolating the inductor from the lossy substrate. The active devices, which include a cross-coupled pair, current source, and current mirror may be replaced at the bottom center of the inductor. An EMX simulation may be run on the entire VCO structure which includes the differential inductor, the rake-shaped multi-layer metal routing, the parallel plate capacitors used to model the varactors, and the active devices. The extracted inductor S-parameters, which include all the eddy current effects, is used to evaluate the VCO in circuit simulations. Simulation results confirm that the performance of the compact VCO is close to the performance of the VCO with a plain inductor.

[0041] Benefits of embodiments of the systems and methods of the present invention may be seen by constructing two VCOs, as discussed above, in accordance with the systems and methods of the present invention. Referring to FIG. 11, the first is a VCO with varactors and active devices inside the inductor (hereinafter, "VCO IN") 1110. The second is a VCO with

varactors and active devices outside the inductor, (hereinafter, "VCO OUT") 1120. Both VCO IN 1110 and VCO OUT 1120 may be implemented in a 0.25 μ m BiCMOS process with only the peak detector implemented in bipolar transistors. Both VCOs, shown in the die photograph of FIG. 11, consume 3.2mA from a 1.8V supply. The VCO output is connected to abuffer stage as well as a peak detector running off a 2.5V supply. It should be noted that the systems and methods of the present invention may be implemented in any suitable transistor technology, including, for example, bipolar, CMOS, BiCMOS, and GaAs.

[0042] Properties of VCO IN 1110 and VCO OUT 1120 are illustrated in FIGS. 8-10. The information in these figures was generated by characterizing sets of VCO DSf 1110 and VCO OUT 1120 with a Cascade RF probe station available from Cascade Microtech Inc. of Beaverton, Oregon and an Agilent E4446A spectrum analyzer available from Agilent Technologies Inc. of Palo Alto, California. As shown, the measured data for VCO IN 1110 and VCO OUT 1120 was consistent despite the smaller area occupied by VCO IN 1110.

[0043] FIG. 8a illustrates the tuning characteristics of VCO DSf 1110 and VCO OUT 1120. As illustrated, a wide tunning range of 520 MHz, or 26% of the center frequency, was achieved. FIG. 8b illustrates the output power for different tune voltages. Both VCO DSf 1110 and VCO OUT 1120 have similar output power except for at high tune voltages, which corresponds to low output frequencies.

[0044] FIG. 9 illustrates the noise spectrums of VCO IN 1110 and VCO OUT 1120 for a 2 GHz carrier frequency and a corner frequency of approximately 300 kHz, as respectively shown by lines 910 and 920. Line 930 has a slope of 1/f ² and line 940 has slope of 1/f ³ . [0045] Lastly, FIG. 10 illustrates the variation in phase noise for different tuning voltages at various offset frequencies. The phase noise level at 600 kHz offset is shown for VCO DSf 1110 and VCO OUT 1110 on lines 1020 and 1110 respectively. The phase noise level at 3 MHz offset is shown for VCO IN 1110 and VCO OUT 1120 on lines 1040 and 1130 respectively. As can be seen by reviewing FIGS. 8, 9, and 10, VCO DSf 1110 and VCO OUT 1120 have very similar performance. The die photograph of FIG. 11 of VCO DSf 1110 and VCO OUT 1120 illustrates that despite having the approximately the same performance, VCO OUT 1110 occupies 0.3 x 0.25 mm ² while VCO DSf 1120 only occupies 0.2 x 0.2 mm ² . This size difference results in an area savings of 47%. As discussed, this area savings results in lowering the cost of manufacturing circuits.

[0046] Although the invention has been described and illustrated in the foregoing illustrative embodiments, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the details of implementation of the invention can be made without departing from the spirit and scope of the invention. For example, although one example used was placing a VCO inside an inductor, the invention is not limited in this manner. Rather, according to various embodiments of the present invention, a phase locked loop filter or any other suitable circuit or combination of circuits may be placed under the VCO inductor. [0047] Therefore, other embodiments, extensions, and modifications of the ideas presented above are comprehended and should be within the reach of one versed in the art upon reviewing the present disclosure. Accordingly, the scope of the present invention in its various aspects should not be limited by the examples presented above. The individual aspects of the present invention, and the entirety of the invention should be regarded so as to allow for such design modifications and future developments within the scope of the present disclosure. The present invention is limited only by the claims that follow.