Patent Publication Number: US-7902953-B1

Title: Method and apparatus for improving inductor performance using multiple strands with transposition

Description:
TECHNICAL FIELD 
     Embodiments of the present invention relate to inductor-based circuits. More specifically, embodiments of the present invention relate to a method and apparatus for improving inductor performance using multiple strands with transposition. 
     BACKGROUND 
     Spiral inductors are important components of many high-frequency circuits such as voltage controlled oscillators, low noise amplifiers, mixers, and other components. Generally, a high quality factor, Q, is desirable for high performance. A high quality factor translates to lower phase noise in voltage controlled oscillators and a lower noise figure in low noise amplifiers. 
     The quality factor is a function of energy stored, which may be defined by the difference between peak magnetic energy and peak electric energy, and the energy loss in one oscillation cycle. The quality factor is limited by energy stored in parasitic capacitance, ohmic loss in the inductor windings, and energy loss in the substrate. Resistance in the inductor windings increases with frequency due to skin effect and proximity effect which causes significant degradation in the inductor&#39;s quality factor. 
     The skin effect causes AC current to crowd towards a surface of an inductor winding due to self induction. The proximity effect causes AC current to crowd towards the outer edges of parallel lines due to mutual induction. The skin effect and proximity effect are caused by eddy currents which are induced by time-varying magnetic field. Eddy currents flow in a direction that produces an opposing magnetic field. Eddy currents combine with applied currents and result in current crowding on edges of lines. 
     Prior approaches of increasing the width of an inductor winding had limited benefits since most AC current flows along the sides of the winding due to the lateral skin effect and proximity effect. Also, the utilization of multiple metal levels for winding had limited benefit because most AC current flows through the top surface of the top metal and the bottom surface of the bottom metal in an inductor metal stack. 
     SUMMARY 
     According to an embodiment of the present invention, an inductor winding is split into multiple strands. The strands may be split laterally and/or vertically. According to one aspect of the present invention, the strands are transposed along the length of the winding. According to another aspect of the present invention, the strands are transposed such that all possible positions in the winding cross section are occupied equally or approximately the same distance along the length of the winding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present invention are illustrated by way of example and are by no means intended to limit the scope of the present invention to the particular embodiments shown. 
         FIG. 1  illustrates a target device in which a spiral inductor may be implemented according to an exemplary embodiment of the present invention. 
         FIG. 2   a - d  illustrates a spiral inductor with vertical transposition according to an exemplary embodiment of the present invention. 
         FIG. 3  illustrates a layout view of a multi-stranded magnetic component with vertical transposition according to an exemplary embodiment of the present invention. 
         FIGS. 4   a - 4   c  illustrate cross-sectional views at positions of the multi-stranded magnetic component of  FIG. 3  according to an embodiment of the present invention. 
         FIG. 5  illustrates a layout view of a multi-stranded magnetic component with vertical and lateral transposition according to an exemplary embodiment of the present invention. 
         FIG. 6   a - 6   c  illustrate cross-sectional views at positions of the multi-stranded magnetic component of  FIG. 5  according to an embodiment of the present invention. 
         FIG. 7  illustrates a layout view of a multi-stranded magnetic component according to an alternate embodiment of the present invention. 
         FIG. 8  illustrates a transposition sequence cross-section of the multi-stranded magnetic component illustrated in  FIG. 7  according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation, specific nomenclature is set forth to provide a thorough understanding of embodiments of the present invention. It will be apparent to one skilled in the art that specific details in the description may not be required to practice the embodiments of the present invention. In other instances, well-known circuits, devices, and programs are shown in block diagram form to avoid obscuring embodiments of the present invention unnecessarily. Additionally, some embodiments of the invention are described in the context of field programmable gate arrays (“FPGA”), but the invention is applicable to other contexts as well, including other semiconductor devices such as programmable logic devices, complex programmable logic devices, application specific integrated circuits, processors, controllers and memory devices. 
       FIG. 1  illustrates a device  100  which implements spiral inductors according to an exemplary embodiment of the present invention. In this example, the device  100  is a target device such as an FPGA which a system may be implemented on. The target device  100  may be a semiconductor device having a hierarchical structure that may take advantage of wiring locality properties of circuits formed therein. 
     The target device  100  includes a plurality of logic-array blocks (LABs). Each LAB may be formed from a plurality of logic blocks, carry chains, LAB control signals, (lookup table) LUT chain, and register chain connection lines. A logic block is a small unit of logic providing efficient implementation of user logic functions. A logic block includes one or more combinational cells, where each combinational cell has a single output, and registers. According to one embodiment of the present invention, the logic block may operate similarly to a logic element (LE), such as those found in Stratix® devices manufactured by Altera Corporation, or a combinational logic block (CLB) such as those found in Virtex devices manufactured by Xilinx Inc. In this embodiment, the logic block may include a four input lookup table (LUT) with a configurable register. According to an alternate embodiment of the present invention, the logic block may operate similarly to an adaptive logic module (ALM), such as those found in Stratix devices manufactured by Altera Corporation. LABs are grouped into rows and columns across the target device  100 . Columns of LABs are shown as  111 - 116 . It should be appreciated that the logic block may include additional or alternate components. 
     The target device  100  includes memory blocks. The memory blocks may be, for example, dual port random access memory (RAM) blocks that provide dedicated true dual-port, simple dual-port, or single port memory up to various bits wide at up to various frequencies. The memory blocks may be grouped into columns across the target device in between selected LABs or located individually or in pairs within the target device  100 . Columns of memory blocks are shown as  121 - 124 . 
     The target device  100  includes digital signal processing (DSP) blocks. The DSP blocks may be used to implement multipliers of various configurations with add or subtract features. The DSP blocks include shift registers, multipliers, adders, and accumulators. The DSP blocks may be grouped into columns across the target device  100  and are shown as  131 . 
     The target device  100  includes a plurality of input/output elements (IOEs)  140 . Each IOE feeds an I/O pin (not shown) on the target device  100 . The IOEs are located at the end of LAB rows and columns around the periphery of the target device  100 . According to an embodiment of the present invention, the IOEs  140  are high speed serial IOEs. Each IOE includes a bidirectional I/O buffer and a plurality of registers for registering input, output, and output-enable signals. When used with dedicated clocks, the registers provide performance and interface support with external memory devices. According to an embodiment of the present invention, each of the IOEs  140  may also include a voltage controlled oscillator having a spiral inductor. The spiral inductor may be a multi-stranded spiral inductor utilizing vertical and/or lateral transposition. 
     The target device  100  may include routing resources such as LAB local interconnect lines, row interconnect lines (“H-type wires”), and column interconnect lines (“V-type wires”) (not shown) to route signals between components on the target device. 
       FIG. 1  illustrates an exemplary embodiment of a target device. It should be appreciated that a system may include a plurality of target devices, such as that illustrated in  FIG. 1 , cascaded together. It should also be appreciated that, as indicated above, the target device may include the same or different semiconductor devices arranged in a different manner. A target device may also include FPGA resources other than those described in reference to the target device  100 . Thus, while the invention described herein may be utilized on the architecture described in  FIG. 1 , it should be appreciated that it may also be utilized on different architectures and on different semiconductor devices. 
       FIGS. 2   a - d  illustrate a layout view of a four layer spiral inductor with vertical transposition according to an exemplary embodiment of the present invention. Layer  201  shown in  FIG. 2   a  represents a first metal layer (metal n). Layer  202  shown in  FIG. 2   b  represents a second metal layer (metal n+1). Layer  203  shown in  FIG. 2   c  represents a third metal layer (metal n+2). Layer  204  shown in  FIG. 2   d  represents a fourth metal layer (metal n+3). The metal layers  201 - 204  each include a track  205 - 208 . Each track spans the distance of a winding of the spiral inductor and supports segments of strands of the winding. As shown, track  205  supports strand segments  210 ,  221 ,  243 ,  234 , and  216 . Track  206  supports strand segments  220 ,  211 ,  242 ,  222 ,  233 ,  244 ,  215 , and  235 . Track  207  supports strand segments  230 ,  241 ,  212 ,  232 ,  223 ,  214 ,  245 , and  225 . Track  208  supports strand segments  240 ,  231 ,  213 ,  224 , and  246 . 
       FIG. 2   a  illustrates a first strand coupled to terminal  218  and  219  on track  205 . The first strand includes strand segment  210  which resides on track  205 , strand segment  211  which resides on track  206 , strand segment  212  which resides on track  207 , strand segment  213  which resides on track  208 , strand segment  214  which resides on track  207 , strand segment  215  which resides on track  206 , and strand segment  216  which resides on track  205 . As illustrated, crossing segment  251  transposes the first strand from track  205  on layer  201  to track  206  on layer  202 . Crossing segment  252  transposes the first strand from track  206  on layer  202  to track  207  on layer  203 . Crossing segment  253  transposes the first strand from track  207  on layer  203  to track  208  on layer  204 . Crossing segment  254  transposes the first strand from track  208  on layer  204  to track  207  on layer  203 . Crossing segment  255  transposes the first strand from track  207  on layer  203  to track  206  on layer  202 . Crossing segment  256  transposes the first strand from track  206  on layer  202  to track  205  on layer  201 . 
     A second strand is coupled to terminals  228  and  229  which resides on track  206 . The second strand includes strand segment  220  which resides on track  206 , strand segment  221  which resides on track  205 , strand segment  222  which resides on track  206 , strand segment  223  which resides on track  207 , strand segment  224  which resides on track  208 , and strand segment  225  which resides on track  207 . As illustrated, crossing segment  261  transposes the second strand from track  206  on layer  202  to track  205  on layer  201 . Crossing segment  262  transposes the second strand from track  205  on layer  201  to track  206  on layer  202 . Crossing segment  263  transposes the second strand from track  206  on layer  202  to track  207  on layer  203 . Crossing segment  264  transposes the second strand from track  207  on layer  203  to track  208  on layer  204 . Crossing segment  265  transposes the second strand from track  208  on layer  204  to track  207  on layer  203 . 
     A third strand is coupled to terminals  238  and  239  on track  207 . The third strand includes strand segment  230  which resides on track  207 , strand segment  231  which resides on track  208 , strand segment  232  which resides on track  207 , strand segment  233  which resides on track  206 , strand segment  234  which resides on track  205 , and strand segment  235  which resides on track  206 . As illustrated, crossing segment  271  transposes the third strand from track  207  on layer  203  to track  208  on layer  204 . Crossing segment  272  transposes the third strand from track  208  on layer  204  to track  207  on layer  203 . Crossing segment  273  transposes the third strand from track  207  on layer  203  to track  206  on layer  202 . Crossing segment  274  transposes the third strand from track  206  on layer  202  to track  205  on layer  201 . Crossing segment  275  transposes the third strand from track  205  on layer  201  to track  206  on layer  202 . 
     A fourth strand is coupled to terminals  248  and  249  on track  208 . The fourth strand includes strand segment  240  which resides on track  208 , strand segment  241  which resides on track  207 , strand segment  242  which resides on track  206 , strand segment  243  which resides on track  205 , strand segment  244  which resides on track  206 , strand segment  245  which resides on track  207 , and strand segment  246  which resides on track  208 . As illustrated, crossing segment  281  transposes the fourth strand from track  208  on layer  204  to track  207  on layer  203 . Crossing segment  282  transposes the fourth strand from track  207  on layer  203  to track  206  on layer  202 . Crossing segment  283  transposes the fourth strand from track  206  on layer  202  to track  205  on layer  201 . Crossing segment  284  transposes the fourth strand from track  205  on layer  201  to track  206  on layer  202 . Crossing segment  285  transposes the fourth strand from track  206  on layer  202  to track  207  on layer  203 . Crossing segment  286  transposes the fourth strand from track  207  on layer  203  to track  208  on layer  204 . 
       FIG. 3  illustrates a layout view of a section of a multi-stranded magnetic component  300  with vertical transposition according to an exemplary embodiment of the present invention. The layout view illustrates four layers of the magnetic component  300 , metal n, metal n+1, metal n+2, and metal n+3. A first track  301  resides on a first layer (metal n) of the magnetic component  300 . A second track  302  resides on a second layer (metal n+1) of the magnetic component  300 . A third track  303  resides on a third layer (metal n+2) of the magnetic component. A fourth track  304  resides on a fourth layer (metal n+3) of the magnetic component. 
     Strands A, B, C, and D compose the winding of the magnetic component  300  and are vertically transposed through each of the tracks on magnetic component. Crossing segment  311  transposes strand A from track  301  to track  302 . Crossing segment  312  transposes strand A from track  302  to track  303 . Crossing segment  313  transposes strand A from track  303  to track  304 . Crossing segment  314  transposes strand A from track  304  to track  303 . Crossing segment  315  transposes strand A from track  303  to track  302 . Crossing segment  316  transposes strand A from track  302  to track  301 . 
     Crossing segment  321  transposes strand B from track  301  to track  302 . Crossing segment  322  transposes strand B from track  302  to track  303 . Crossing segment  323  transposes strand B from track  303  to track  304 . Crossing segment  324  transposes strand B from track  304  to track  303 . Crossing segment  325  transposes strand B from track  303  to track  302 . 
     Crossing segment  331  transposes strand C from track  303  to track  304 . Crossing segment  332  transposes strand C from track  304  to track  303 . Crossing segment  333  transposes strand C from track  303  to track  302 . Crossing segment  334  transposes strand C from track  302  to track  301 . Crossing segment  335  transposes strand C from track  301  to track  302 . Crossing segment  336  transposes strand C from track  302  to track  303 . 
     Crossing segment  341  transposes strand D from track  304  to track  303 . Crossing segment  342  transposes strand C from track  303  to track  302 . Crossing segment  343  transposes strand D from track  302  to track  301 . Crossing segment  344  transposes strand D from track  301  to track  302 . Crossing segment  345  transposes strand D from track  302  to track  303 . Crossing segment  346  transposes strand D from track  303  to track  304 . In this example, 4 strands are supported by tracks  301 - 304 . These strands are unique and electrically isolated from one another. According to an embodiment of the present invention, the electrical isolation provides that there is no DC current path between any two strands except at the terminals. 
       FIGS. 4   a - 4   c  illustrates cross-sectional views at positions of the multi-stranded magnetic component of  FIG. 3  according to an embodiment of the present invention.  FIG. 4   a  illustrates a cross-sectional view of the multi-stranded magnetic component  300  (shown in  FIG. 3 ) at line X-X′. A strand segment for strand C and a strand segment for strand D are shown at the metal n+3 layer. A via is shown to connect the strand segment for strand C at the metal n+3 layer with the strand segment for strand C at the metal n+2 layer. A via is shown to connect the strand segment for strand D at the metal n+3 layer with the strand segment for strand D at the metal n+2 layer. A strand segment for strand A and a strand segment for strand B are shown at the metal n+1 layer. A via is shown to connect the strand segment for strand A at the metal n+1 layer with the strand segment for strand A at the metal n layer. A via is shown to connect the strand segment for strand B at the metal n+1 layer with the strand segment for strand B at the metal n layer. 
       FIG. 4   b  illustrates a cross-sectional view of the multi-stranded magnetic component  300  (shown in  FIG. 3 ) at line Y-Y′. A strand segment for strand C is shown at the metal n+3 layer. A strand segment for strand D is shown at the metal n+2 layer. A strand segment for strand A is shown at the metal n+1 layer. A strand segment for strand B is shown at the metal n layer. 
       FIG. 4   c  illustrates a cross-sectional view of the multi-stranded magnetic component  300  (shown in  FIG. 3 ) at line Z-Z′. A strand segment for strand C is shown at the metal n+3 layer. A strand segment for strand D and a strand segment for strand A are shown at the metal n+2 layer. A via is shown to connect the strand segment for strand D at the metal n+2 layer with the strand segment for strand D at the metal n+1 layer. A via is shown to connect the strand segment for strand A at the metal n+2 layer with the strand segment for strand A at the metal n+1 layer. A strand segment for strand B is shown at the metal n layer. 
       FIG. 5  illustrates a layout view of a section of a multi-stranded magnetic component  500  with vertical and lateral transposition according to an exemplary embodiment of the present invention. The layout view illustrates four layers of the magnetic component  500 , metal n, metal n+1, metal n+2, and metal n+3. A first plurality of tracks  501 - 504  resides on a first layer (metal n) of the magnetic component  500 . The plurality of tracks  501 - 504  are laterally adjacent to one another. A second plurality of tracks  505 - 508  resides on a second layer (metal n+1) of the magnetic component  500 . The plurality of tracks  505 - 508  are laterally adjacent to one another. A third plurality of tracks  509 - 512  resides on a third layer (metal n+2) of the magnetic component  500 . The plurality of tracks  509 - 512  are laterally adjacent to one another. A fourth plurality of tracks  513 - 516  resides on a fourth layer (metal n+3) of the magnetic component  500 . The plurality of tracks  513 - 516  are laterally adjacent to one another. 
     The magnetic component  500  includes a plurality of strands which compose the winding of the magnetic component. Each of the strands span the length of the winding and are both are vertically and laterally transposed through each of the tracks on the magnetic component. Crossing segment  511  transposes strand  510  from track  501  to track  505 . Crossing segment  512  transposes strand  510  from track  505  to track  506 . 
     Crossing segment  521  transposes strand  520  from track  502  to track  501 . Crossing segment  522  transposes strand  520  from track  501  to track  505 . Crossing segment  523  transposes strand  520  from track  505  to track  506 . Crossing segment  531  transposes strand  530  from track  503  to track  502 . Crossing segment  532  transposes strand  530  from track  502  to track  501 . Crossing segment  533  transposes strand  530  from track  501  to  505 . Crossing segment  541  transposes strand  540  from track  504  to track  503 . Crossing segment  542  transposes strand  540  from track  503  to track  502 . Crossing segment  543  transposes strand  540  from track  502  to track  501 . 
     Other crossing segments and strands are also illustrated in  FIG. 5 . These strands are similarly transposed both vertically and laterally. The section of the layout labeled “Repeating Unit Cell  550 ” may be repeated (along the length of the winding) to further vertically and laterally transpose the strands in the winding. According to an embodiment of the present invention each of the strands in the magnetic component are transposed to occupy each of the tracks  501 - 516 . In one embodiment, each strand occupies each of the plurality of tracks for a distance that is approximately equal. In the example illustrated in  FIG. 5 ,  15  strands are supported by tracks  501 - 516 . These strands are unique and electrically isolated from one another. 
       FIGS. 6   a - 6   c  illustrate cross-sectional views at positions of the multi-stranded magnetic component of  FIG. 5  according to an embodiment of the present invention.  FIG. 6   a  illustrates a cross-sectional view of the multi-stranded magnetic component  500  (shown in  FIG. 5 ) at line X-X′. At level metal n, no strand is shown at track  502  as strand  530  is crossing over from track  503  by way of crossing segment  531 . 
       FIG. 6   b  illustrates a cross-sectional view of the multi-stranded magnetic component  500  (shown in  FIG. 5 ) at line Y-Y′. Crossing segment  551  is shown to vertically transpose strand  550  from track  504  at layer metal n to track  508  at layer metal n+1. 
       FIG. 6   c  illustrates a cross-sectional view of the multi-stranded magnetic component  500  (shown in  FIG. 5 ) at line Z-Z′. Crossing segment  561  is shown to horizontally transpose strand  560  from track  515  to track  516 . 
     The crossing segments illustrated in  FIGS. 2   a - 2   c ,  3 , and  4   a - 4   c  are vias which vertically transpose a strand segment from one layer to another. Single vias have been illustrated to transpose a strand segment vertically. It should be appreciated, however, that more than one via may be used to transpose a strand segment vertically. In addition to vias, crossing segments may also include cross-over, cross-under, and cross-around segments, and other types of bridging components to vertically and horizontally transpose strands to laterally or horizontally adjacent tracks. In  FIGS. 5 and 6   c , cross-around segments are illustrated as crossing segments for horizontal transposition. 
     The multi-stranded magnetic component illustrated in  FIGS. 3 ,  4   a - 4   c ,  5 , and  6   a - 6   c  may be used to implement the spiral inductor illustrated in  FIGS. 2   a - 2   d  or a transformer, balun, or other magnetic component with winding. The transposition pattern illustrated in  FIGS. 3 ,  4   a - 4   c ,  5 , and  6   a - 6   c  may be transformed into an octagonal foot print to resemble the spiral inductor illustrated in  FIG. 2 . Alternatively, the transposition pattern may be transformed into a square, circular, or other shaped foot print. It should also be appreciated that instead of forming a single loop pattern as shown in  FIG. 2 , the transposition patterns may be used to form multi-loop patterns. 
       FIG. 7  illustrates a layout view of a section of a multi-stranded magnetic component  700  with vertical and lateral transposition according to an alternate embodiment of the present invention. The layout view illustrates four layers of the magnetic component  700 , metal n (M n ), metal n+1 (M n+1 ), metal n+2 (M n+2 ), and metal n+3 (M n+3 ). A first plurality of tracks  701 - 704  resides on a first layer (M n+3 ) of the magnetic component  700 . The plurality of tracks  701 - 704  are laterally adjacent to one another. A second plurality of tracks  705 - 708  resides on a second layer (M n+2 ) of the magnetic component  700 . The plurality of tracks  705 - 708  are laterally adjacent to one another. A third plurality of tracks  709 - 712  resides on a third layer (M n+3 ) of the magnetic component  700 . The plurality of tracks  709 - 712  are laterally adjacent to one another. A fourth plurality of tracks  713 - 716  resides on a fourth layer (M n ) of the magnetic component  700 . The plurality of tracks  713 - 716  are laterally adjacent to one another  700 . 
     The magnetic component  700  includes a plurality of strands which compose the winding of the magnetic component. Each of the strands span the length of the winding and are both are vertically and laterally transposed through each of the tracks on the magnetic component. The black squares on the strand segments represent vias or via arrays to metal levels below. 
       FIG. 8  illustrates a transposition sequence cross-section of the multi-stranded magnetic component illustrated in  FIG. 7  according to an embodiment of the present invention.  FIG. 8  illustrates the transposition scheme for the 4 level magnetic component winding which has been divided into 15 strands. Transposition would occur 15 times or a multiple of 15 times along the winding to optimize current uniformity. In one embodiment, for S strands the transposition should occur a multiple of S times along the winding such that each strand occupies all positions (tracks) equally. 
     According to embodiments of the present invention, magnetic component winding is split into multiple strands to create a planar litz wire. The strands are kept electrically isolated except at the end of windings. The strands are transposed (twisted, woven, braided) along a length of the winding. In one embodiment, all possible positions (tracks) in the winding cross section are occupied equally. Transposition may occur laterally and/or vertically for multi-level metal implementations. The technique disclosed for magnetic component winding may used for inductors, transformers, balun, or other magnetic components which experiences skin and proximity effects. The transposition scheme may be applied to single turn or multi-turn magnetic components. Embodiments of the present invention may improve the high-frequency Q of spiral inductors by reducing skin effect and proximity effect. Higher Q spiral inductors enable lower phase noise of voltage controlled oscillators and improved performance of other inductor-based circuits. 
     In the foregoing specification embodiments of the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the embodiments of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense.