Abstract:
A method for producing an on-chip signal transforming device. 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.

Description:
This is a division of application Ser. No. 10/012,209, filed Nov. 5, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to integrated circuit devices, and, more particularly, to baluns and transformers manufactured on integrated circuit chips. 
     2. Description of Related Art 
     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. 
     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. 
     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. 
     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. 
     Thus, there is a need for baluns and transformers that provide improved symmetry and magnetic coupling. 
     SUMMARY OF THE INVENTION 
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 a  illustrates an electrical circuit representation of a balun; 
     FIG. 1 b  illustrates a balun according to one embodiment of the present invention; 
     FIG. 2 illustrates a balun according to yet another embodiment of the present invention; 
     FIG. 3 a  illustrates a balanced transformer according to one embodiment of the present invention; 
     FIG. 3 b  illustrates an electrical circuit representation of transformer  300 ; 
     FIG. 3 c  illustrates two amplifiers coupled by a balanced line transformer; 
     FIG. 4 a  illustrates 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 
     FIGS. 4 b ( 1 - 3 ), and  4   c  illustrate three layers used to make the inductors of the balun illustrated in FIG. 4 a.    
    
    
     DETAILED DESCRIPTION 
     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. 
     FIG. 1 a  illustrates 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. 
     FIG. 1 b  illustrates 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. 
     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 . 
     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. 
     FIG. 3 a  illustrates 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  308   a  and  308   b . Interleaved inductor  314  includes spiral inductors  314   a  and  314   b . The stacked interleaved structure provides good magnetic coupling between inductors  308  and  314  and symmetric parasitic characteristics between  308   a  and  308   b , as well as between  314   a  and  314   b . FIG. 3 b  illustrates 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. 3 c  illustrates two amplifiers coupled by a balanced line transformer. Because of the DC isolation between amplifier  320  and amplifier  324 , amplifer  320 &#39;s output can be set to a bias voltage V A  and amplifier  324 &#39;s input can be set to a different bias voltage V B . 
     FIG. 4 a  illustrates 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. 4 b   1 , 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  420   a - 1 , and  420   a - 2  were one inductor crosses over the other. In one embodiment illustrated in FIG. 4 b   2 , 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. 4 b   2 , inductor  408  makes two right angle turns at the cross over area  420   a - 1 , and although not shown in FIG. 4 b   2 , inductor  410  also makes two right angle turns at the cross over area  420   a - 2 . In the embodiment shown in FIG. 4 b   3 , inductor  410  crosses over inductor  408  at an angle, such as, for example, 45 degrees or some other angle. Further, as shown in FIG. 4 c  inductor  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 . 
     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 . 
     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.