Integrated balun and transformer structures

[0002] This is a division of application Ser. No. 10/012,209, filed Nov. 5, 2001.

[0003] 1. Field of the Invention

[0004] The present invention relates generally to integrated circuit devices, and, more particularly, to baluns and transformers manufactured on integrated circuit chips.

[0005] 2. Description of Related Art

[0006] Increasingly, due to reliability, performance and cost considerations, devices that previously resided on a printed circuit board (PCB) are being integrated into an integrated circuit (IC) chip. Transformers, inductors, and baluns are examples of devices that have migrated to the IC chip.

[0007] Due to the relatively noisy environment on an IC chip, many signals on a chip are typically differential or double-ended signals. Differential signals offer good common-mode rejection of noise; noise typically affects both halves of the differential signal in the same manner, and since information is contained in the difference of both signal halves, the difference does not change appreciably despite the noise that has been added to both halves.

[0008] A balun is an example of a device that accepts a single-ended signal and transforms it into a differential signal and vice versa. The term balun suggests its function: conversion of balanced (differential) signals to unbalanced (single-ended) signals. Signals on PCBs are typically single-ended signals. Since IC chips use differential signals, baluns have been placed on PCBs to transform a single-ended signal into a differential signal. Baluns have also been placed on IC chips. However, present designs suffer from asymmetrical parasitic characteristics (e.g., asymmetric parasitic capacitances and resistances) and poor magnetic coupling. The asymmetrical parasitic characteristics cause a differential signal to be asymmetrical. Symmetry in a differential signal is very important to the proper functioning of many differential circuits. Furthermore, poor magnetic coupling results in inefficient energy transfer.

[0009] A transformer is another example of a device that has on-chip uses. For example, a transformer can be used to match impedances between amplifier stages while providing DC isolation between the stages. A transformer for differential signals also needs to offer good magnetic coupling as well as symmetrical parasitic characteristics. Present designs do not offer relatively good parasitic characteristics and good magnetic coupling.

[0010] Thus, there is a need for baluns and transformers that provide improved symmetry and magnetic coupling.

[0011] A method for producing an on-chip signal transforming device is described. The method includes providing a substrate and laying a first conductive layer above the substrate, wherein the first conductive layer has a plurality of interleaved inductors. The method then includes laying a second conductive layer above the substrate, wherein the second conductive layer has at least one inductor.

[0012] The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings in which like references denote similar elements, and in which:

[0013] FIG. 1aillustrates an electrical circuit representation of a balun;

[0014] FIG. 1billustrates a balun according to one embodiment of the present invention;

[0015] FIG. 2 illustrates a balun according to yet another embodiment of the present invention;

[0016] FIG. 3aillustrates a balanced transformer according to one embodiment of the present invention;

[0017] FIG. 3billustrates an electrical circuit representation of transformer 300;

[0018] FIG. 3cillustrates two amplifiers coupled by a balanced line transformer;

[0019] FIG. 4aillustrates another balun according to one embodiment of the present invention, which has the capability to transform a differential signal to a single ended signal and vice-versa; and

[0020] FIGS. 4b(1-3), and 4cillustrate three layers used to make the inductors of the balun illustrated in FIG. 4a.

[0021] Methods and apparatus for integrated transformers and baluns are described. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, one skilled in the art will appreciate that the present invention may be practiced in a variety of circuits, especially radio frequency circuits, without these specific details. In other instances, well-known operations, steps, functions and devices are not shown in order to avoid obscuring the invention. Repeated usage of the phrases “in one embodiment,” “an alternative embodiment,” or an “alternate embodiment” does not necessarily refer to the same embodiment, although it may.

[0022] FIG. 1aillustrates an electrical circuit representation of a balun. Balun 100 includes inductors 114 and inductors 108 and 110. Inductors 108 and 110 are dc (direct current) decoupled from and magnetically coupled with inductor 114 permitting the transfer of energy. Balun 100 can convert the signal of unbalanced transmission line 102 into to a signal of balanced transmission lines 104 and 106. Balun 100 can also convert a signal of balanced transmission lines 104 and 106 to a signal of unbalanced transmission line 102. One advantage of the balanced portion is that external noise affects the lines of the balanced transmission equally without appreciably affecting the potential difference in lines 104 and 106. While the electric circuit representation for a balun has been known for a long time, the designs for integrated baluns and transformers of the present invention have heretofore been unkown.

[0023] FIG. 1billustrates a balun according to one embodiment of the present invention. Balun 100 includes two interleaved inductors 108 and 110 in metal layer 108′ and vertically displaced spiral inductor 114 in metal layer 114′, all of them above substrate 116. Making inductors 108 and 110 interleaved causes each of the inductors to have substantially equivalent parasitic characteristics; in other words they are symmetrical. Furthermore, having the interleaved inductors stacked above inductor 114 provides for relatively better magnetic coupling in comparison to a lateral coupling arrangement in which interleaved inductors of a primary winding are placed adjacent to and in the same layer as the inductor of a secondary winding.

[0024] While in the above description, balun 100 has interleaved inductors 108 and 110 above spiral inductor 114, in an alternative embodiment spiral inductor 114 is above interleaved inductors 108 and 110.

[0025] FIG. 2 illustrates a balun according to yet another embodiment of the present invention. Balun 200 includes two interleaved inductors 208 and 210 in metal layer 208′ and vertically displaced spiral inductor 214 in metal layers 214′ and 214″, all of them above substrate 216. A first portion of inductor 214 is in layer 214′ and a second portion of inductor 214 is in layer 214″. Having an inductor which has twice the number of windings of inductor 114 split among two layers results in an inductor with a larger inductance, which can be desirable in some instances because it results in larger magnetic coupling.

[0026] FIG. 3aillustrates a transformer according to one embodiment of the present invention. Transformer 300 includes stacked interleaved inductors 308 and 314 in metal layers 308′ and 314′ respectively. Interleaved inductor 308 includes spiral inductors 308aand 308b. Interleaved inductor 314 includes spiral inductors 314aand 314b. The stacked interleaved structure provides good magnetic coupling between inductors 308 and 314 and symmetric parasitic characteristics between 308aand 308b, as well as between 314aand 314b. FIG. 3billustrates an electrical circuit representation of transformer 300. A transformer such as transformer 300 is desirable because it can be used, for example, to match impedances between amplifier stages while providing DC (direct current) isolation between the stages. FIG. 3cillustrates two amplifiers coupled by a balanced line transformer. Because of the DC isolation between amplifier 320 and amplifier 324, amplifer 320's output can be set to a bias voltage V_Aand amplifier 324's input can be set to a different bias voltage V_B.

[0027] FIG. 4aillustrates another balun according to one embodiment of the present invention, which has the capability to transform a differential signal to a single ended signal and vice-versa. Transformer 400 includes inductors 408, 410 and 414. As shown in FIG. 4b1, the inductors 408 and 410 are interleaved to provide good magnetic coupling between them. According to one embodiment, they are disposed in the same layer except at areas 420a-1, and 420a-2 were one inductor crosses over the other. In one embodiment illustrated in FIG. 4b2, inductor 408 is entirely disposed on metal layer 408′, with inductor 410 crossing inductor 408 using vias 422 that electrically connect to another metal layer 410′ above or below the metal layer 408.′ In the embodiment shown in FIG. 4b2, inductor 408 makes two right angle turns at the cross over area 420a-1, and although not shown in FIG. 4b2, inductor 410 also makes two right angle turns at the cross over area 420a-2. In the embodiment shown in FIG. 4b3, inductor 410 crosses over inductor 408 at an angle, such as, for example, 45 degrees or some other angle. Further, as shown in FIG. 4cinductor 414 is stacked on a different metal layer 414′, disposed adjacent to one of layers 408′ and 410′, and allows inductor 414 to couple to each of inductors 408 and 410.

[0028] In a modified embodiment, inductors 408 and 410 can be disposed in separate metal layers 408′ and 410′, respectively. In the embodiment, since the inductors are on separate layers, a cross over is not required, although turns within a layer can be made within a layer to equalize coupling between inductors 408 and 410.

[0029] Thus, methods and apparatus for integrated baluns and transformers have been described. Although the present invention has been described with reference to specific exemplary embodiments, it will be evident to one of ordinary skill in the art that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.