Abstract:
An integrated transformer structure is disclosed. In one embodiment, a circuit is disclosed for a transformer circuit including two metallization layers and other layers, a conducting spiral, a gapped spiral, two radial conductors and various short bridging conductors and vias. Superior transformer price/performance is obtained without the use of more than two metallization layers. The invention may be embodied in GaAs and/or other technologies.

Description:
RELATED APPLICATIONS  
       [0001]     None.  
       FIELD OF THE INVENTION  
       [0002]     The invention generally relates to electronic circuits. The invention more particularly relates to inductors, especially transformers and autotransformers embodied in semiconductor based integrated circuits.  
       BACKGROUND OF THE INVENTION  
       [0003]     Modern designs for analog electronic circuits are embodied as ICs (integrated circuits) and face considerable design challenges. As compared with other electrical elements (diodes, transistors, resistors, capacitors, etc.), inductors are more difficult to form in integrated circuits with good cost/performance. Inductors present particular difficulties because they may require relatively large areas and/or numbers of layers to achieve desired values of inductance. The traditional (discrete component) three-dimensional helical inductor shape is more or less impossible to realize within integrated circuits that are essentially two-dimensional in character.  
         [0004]     It is desirable to keep the number of metallization layers to a minimum, especially in GaAs processes where often only two metallization layers are economically available.  
         [0005]     The disclosed improved circuit designs for superior inductive circuit elements are capable superior tradeoffs between circuit performance and cost.  
       SUMMARY  
       [0006]     Accordingly, the invention provides transformer and other circuits with superior performance and which may be implemented as an IC (integrated circuit) with semiconductor technologies such as GaAs (Gallium Arsenide) or InP (Indium Phosphate). Other semiconductor devices or totally different technologies may also be used.  
         [0007]     A new topology/layout for designing spiral transformers (or mutually coupled spiral inductors) in integrated circuits (IC) processes which require as few as two metal/interconnect layers is disclosed.  
         [0008]     According to an aspect of the invention, a circuit is disclosed for a transformer circuit including two metallization layers and other layers, a conducting spiral, a gapped spiral, two radial conductors and various short bridging conductors and vias.  
         [0009]     Several variants of these aspects are also discussed together with alternative exemplary embodiments.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention:  
         [0011]      FIGS. 1A, 1B  and  1 C show views of a prior art planar transformer.  
         [0012]      FIG. 2  shows a plan view of a one layer of metallization of a transformer according to an embodiment of the invention.  
         [0013]      FIG. 3  shows a plan view of a second layer of metallization of a transformer according to an embodiment of the invention.  
         [0014]      FIG. 4  shows a perspective view, from an upper viewpoint of an exemplary transformer according to an embodiment of the invention.  
         [0015]      FIG. 5  shows a further perspective view, from lower viewpoint of the exemplary transformer of  FIG. 4 . 
     
    
       [0016]     For convenience in description, identical components have been given the same reference numbers in the various drawings.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     In the following description, for purposes of clarity and conciseness of the description, not all of the numerous components shown in the schematics and/or drawings are described. The numerous components are shown in the drawings to provide a person of ordinary skill in the art a thorough, enabling disclosure of the present invention. The operation of many of the components would be understood and apparent to one skilled in the art.  
         [0018]      FIGS. 1A, 1B  and  1 C show a prior art planar transformer. The planar transformer of  FIGS. 1A, 1B  and  1 C uses one metal layer (such as a top metal layer) for routing of both primary and secondary windings of the transformer. The primary and secondary windings are formed as two spiral inductors, more particularly as two intertwined spiral inductors. An exemplary top metallization layer is shown as  FIG. 1A .  
         [0019]     In the transformer of  FIG. 1 , each turn of the secondary winding spiral inductor is placed in between two adjacent turns of the primary winding spiral inductor and visa versa except for the first outermost turn of the primary winding and the last innermost turn of the secondary winding. This typically makes it necessary to use a conductor in a third dimension, such as using a second metallization layer to make electrical contact to the innermost ends of the respective spiral inductors.  FIG. 1B  shows the use of a lower metallization layer to access the inner port contact.  FIG. 1C  shows a cross section view of the same exemplary transformer.  
         [0020]     A potential shortcoming of circuits having a topology similar to that of  FIGS. 1A, 1B ,  1 C is that the self inductance of each inductor is relatively smaller due to the increased separation between two adjacent turns of each inductor. The closer the two adjacent turns of an inductor are the higher the self-inductance will be proportionately. Another potential disadvantage of circuits having a topology similar to that of  FIGS. 1A, 1B ,  1 C is the inherent asymmetry of the design, which results in very unequal self-inductances of the primary winding versus the secondary.  
         [0021]     Each inductor in the example shown in  FIGS. 1A, 1B ,  1 C may, for example, have four full turns and an inner port contact. Referring to  FIGS. 1A, 1B ,  1 C, one spiral conductor  1  makes up most of the primary winding and another spiral conductor  3  makes up most of the secondary winding. Radial conductors  21 ,  23  form part of the primary and secondary windings respectively. Via  11  electrically connects primary spiral  1  to radial conductor  21  to form the primary circuit. Via  13  electrically connects secondary spiral  3  to radial conductor  23  to form the secondary circuit.  
         [0022]     An improved transformer conforming to the invention may be embodied without exceeding the desirable limit of two metallization layers.  FIG. 2  shows a plan view of a one layer of metallization of a transformer according to an embodiment of the invention.  FIG. 3  shows a plan view of a second layer of metallization of a transformer according to an embodiment of the invention. The transformer of  FIGS. 2 and 3  may be fabricated using as few as two metallization layers, together with the a number of interconnects which may typically be fabricated as metallic vias or other means well known in the art.  
         [0023]     Referring to  FIG. 2 , a spiral conductor  201  having an endpoint  201 E in the upper the metallization layer  200  is shown and forms most of the primary winding of the transformer. Primary via  251  connects the spiral conductor  201  to the lower metallization layer which is shown in  FIG. 3 .  
         [0024]     Still referring to  FIG. 2 , also formed in the upper metallization layer  200  are a number of short conductors that form bridges  202 , each having, typically, two secondary vias  252 . The short conductors  202  and secondary vias  252  form part of the secondary winding.  
         [0025]     Referring to  FIG. 3 , a radial conductor  351  is formed in the lower metallization layer  300  and forms part of the primary winding of the transformer. The radial conductor  351  is connected to the primary via  251 .  
         [0026]     Several other structures in the lower metallization layer  300  are collectively arranged as a gapped spiral  363 . Another radial conductor  362  connects to the gapped spiral  363 , thus forming most of the secondary winding of the transformer. Portions of the gapped spiral  363  connect to the secondary vias  252  and hence to the upper metallization layer.  
         [0027]     Thus, referring both to  FIGS. 2 and 3 , the primary winding of the transformer may be composed of spiral conductor  201 , the primary via  251 , and radial conductor  351 . Similarly, the secondary winding of the transformer may be composed of gapped spiral  363 , radial conductor  362 , bridges (short conductors)  202 , and second vias  252 . The bridges serve to electrically interconnect the parts of the gapped spiral by bridging across the radial conductors.  
         [0028]      FIG. 4  shows a perspective view, from an upper viewpoint of an exemplary transformer according to an embodiment of the invention.  
         [0029]      FIG. 5  shows a further perspective view, from lower viewpoint of the exemplary transformer of  FIG. 4 .  
         [0030]     Transformers created as embodiments of the invention such as those disclosed above have a number of advantages, over previously developed solutions. For example, the separation between adjacent turns of the inductors is less than with transformers embodied using previously developed solutions. This allows a higher self inductance for a given area or, alternatively, a small real-estate and hence a lower total cost to fabricate a given self inductance. Primary and secondary windings may be interchanged within the general scope of the invention as with most transformer designs.  
         [0031]     Also, primary and secondary windings may have the same area and may be placed physically aligned when one on top of the other. This allows a higher degree of coupling between the windings than may be found in transformers according to previously developed solutions. Indeed the use of a thin substrate, such as a GaAs (Gallium Arsenide) substrate, may allow a very high degree of coupling notwithstanding these substantial capacitance so formed. Using GaAs technologies transformers may readily be achieved with total thicknesses of about 0.3 to 10 microns and with overall dimensions of about 50 to 500 microns. Generally speaking, capacitance between the layers is highly predictable and readily taken account of in circuit simulation using techniques well known in the art.  
         [0032]     Two particular transformer designs having similar outer dimensions, one of them according to the prior art design of  FIGS. 1A, 1B ,  1 C and the other according to the inventive design of  FIGS. 2 and 3  were simulated and compared. In the inventive transformer, the self inductance of the primary winding was 5.96 nH (nanoHenries) and the self inductance of secondary winding was 5.93 nH, the coupling coefficient of two windings was approximately 0.9. In the prior art transformer, the self inductance of primary winding was 2.57 nH (nanoHenries) and the self inductance of secondary windings was 2.08 nH, the coupling coefficient of two windings was approximately 0.7. Thus, using the invention, a coupling coefficient of at least 0.8 is readily obtainable. The advantages in the inventive transformer of higher self inductance higher coupling coefficient and more equal self inductances are readily apparent.  
         [0033]     The invention may be particularly valuable in GaAs integrated circuits since the provision of multiple metallization layers may be particularly expensive in GaAs integrated circuits, and in many cases the number of metallization layers available and is limited to just two.  
         [0034]     Typical embodiments, fabricated in GaAs, may have spirals with diameters of about 50 to 300 microns. Also in GaAs embodiments, the two metallization layers together with an interposed insulating layer may have a total thickness or separation of about 0.3 to 10 microns. Using such techniques coupling coefficients of 0.9 or better are readily achieved.  
         [0035]     Other topologies devices could also be used to construct embodiments of the invention using the appropriate fabrication arrangements. For example, the invention is not limited to square shape spirals but other shapes such as circles or polygons may be used in their place.  
         [0036]     The embodiments described above are exemplary rather than limiting and the bounds of the invention should be determined from the claims. Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.