Abstract:
Cross-coupled first and second helical inductors formed in an IC. The cross-coupled first and second helical inductors comprise a first helical conductor having a first portion and a second portion, and a second helical conductor having a first portion and a second portion. The second helical conductor is in close proximity to the first helical conductor. The first helical inductor is formed by the first portion of the first helical conductor and the second portion of the second helical conductor. The second helical inductor is formed by the second portion of the first helical conductor and the first portion of the second helical conductor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional application of U.S. application Ser. No. 11/210,989 filed on Aug. 24, 2005, which has been allowed but has not yet issued. This application claims the benefit of the filing date of U.S. application Ser. No. 11/210,989, which is incorporated herein by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   Differential resonant (i.e., inductor/capacitor (LC)-based) oscillators are increasingly being used to perform low-jitter frequency synthesis in integrated circuit (IC) systems. This trend has been made possible as a result of the relatively recent feasibility of implementing inductors monolithically with good quality factor, Q, using interconnect wiring metal layers. In qualitative terms, Q of a resonant system is the ratio of the total energy in a system to the energy lost per cycle.  FIG. 1  illustrates a block diagram of a resonant oscillator circuit  1  having a pair of resonant LC tanks  2  and  3  that consist of ideally identical inductors  4  and  5  and variable capacitors  6  and  7 . Each resonant tank oscillates differentially with respect to the other tank at a frequency, f=1/2π(LC) 0.5 , where L is the value of the inductance of the tank inductor and C is the value of the capacitance of the tank variable capacitor. Cross-coupled gain transistors  8  and  9  periodically replenish energy into the tanks  2  and  3  to sustain oscillations that would otherwise decay and disappear due to parasitic resistive losses in the inductors and capacitors. Tunable output frequencies are typically generated by modulating the capacitance of the variable capacitors  6  and  7  using some control voltage, V control . 
     FIG. 2  illustrates a perspective view of a known planar spiral inductor  11  formed in an IC using a single layer of interconnect metal to form, for example, three turns  12 ,  13  and  14 . The first turn  12  of the inductor  11  starts at end  15  and the third turn  14  ends at end  16 , which is interconnected to a feed  17  by vias  18  and  19  and underpass element  20 . Such inductors can be built to exhibit a relatively good quality Q due to the fact that the physical separation between the highest interconnect level, which is typically where the inductor is formed, and the semiconductor substrate below ensures that minimal energy will be dissipated as a result of eddy currents being magnetically induced in the substrate. However, because of weak mutual magnetic coupling between inductor windings, these inductors typically need to be extremely large in order to achieve a target self-inductance, and thus consume a large area in the IC, making implementation rather expensive. 
   It is known to create a differential resonant oscillator in an IC by using a pair of the planar spiral inductors shown in  FIG. 2  to achieve a circuit design of the type shown in  FIG. 1 . In differential resonant oscillators, the mutual inductive coupling between the two planar spiral inductors is usually tailored to provide very strong magnetic coupling between the two inductors. Strong magnetic coupling between the inductors mitigates problems that may occur due to asymmetries and mismatches between the left and right resonant tanks that occur during IC manufacturing. Strong coupling can also prevent undesirable nonlinear effects that may cause the left tank to behave in a non-differential fashion from the right tank. Without strong coupling, the two resonant tanks can oscillate independent of each other in a non-differential fashion. 
   The coupling of the two tanks through the negative impedance generator (i.e., the cross-coupled gain transistors  8  and  9 ) is typically insufficient to eliminate the effects caused by tank asymmetries. One such effect is the left tank oscillating with a different voltage amplitude and non-180° phase alignment from the right tank due to large voltage amplitude oscillations about typically very nonlinear capacitance-versus-control-voltage characteristics of the variable tuning capacitors  6  and  7 . Such instabilities can produce undesirable oscillator output jitter. 
   One known practical way of tightly coupling the two tanks is implementing strong magnetic coupling of the spiral planar inductors of a differential resonant oscillator through cross-coupling of the inductors.  FIG. 3  illustrates a perspective view of a cross-coupled pair  21  of planar spiral inductors  22  and  23 . For ease of illustration, each inductor is shown as having a single turn. Inductor  23  is cross-coupled with inductor  22  by vias  24  and  25  and cross-coupling element  26 . 
   Although tight mutual coupling can be achieved with the cross-coupled planar inductor pair  21  shown in  FIG. 3 , the inductor pair  21  consumes a relatively large amount of area on the die. Since die cost is commensurate with area, area can be a significant impediment to practical implementation of certain circuit architectures and applications. Moreover, the orientation of the turns of the inductors  22  and  23  is such that there is very strong negative magnetic coupling. When the differential nature of the left and right tank oscillations is taken into account, the polarity of this net negative coupling will be flipped to yield net additive magnetic linkage between the two inductors  22  and  23  and the substrate. This linkage can lead to energy being dissipated as a result of eddy currents being magnetically induced in the substrate and lower the inductor Q. 
   In addition, the resulting orientation of the two planar spiral inductors  22  and  23  creates another key drawback. When the right-hand rule is applied to determine the orientation of the magnetic flux lines, it becomes apparent that the magnetic fields from the differentially driven inductors are additive as they penetrate through the substrate and surrounding vicinity of the inductors. This will induce noise through eddy current generation, which can limit the number of resonant oscillators that can be monolithically integrated in a single IC die. 
   A need exists for an inductor pair formed on an IC that has strong mutual magnetic coupling between the inductors, that has low energy loss due to eddy currents being generated in the IC substrate, and that consumes a small amount of area on the IC die. 
   SUMMARY OF THE INVENTION 
   The invention provides a cross-coupled helical inductor configuration and method. The cross-coupled helical inductor configuration is formed in an IC and comprises a first helical inductor, a second helical inductor, a first coupling conductor, and a second coupling conductor. The first helical inductor comprises a first plurality of conductors, which are disposed in respective layers of the IC and are conductively connected to form a first helical pattern of conductors. The second helical inductor comprises a second plurality of conductors, which are disposed in respective layers of the IC and are conductively connected to form a second helical pattern of conductors. The first and second helical inductors are positioned in close proximity to one another in the IC. The first coupling conductor connects at least a first one of the first plurality of conductors to at least a first one of the second plurality of conductors. The second coupling conductor connects at least a second one of the second plurality of conductors to at least a second one of the first plurality of conductors. If a first electrical current is introduced into the first helical inductor, the connection between the first one of the first plurality of conductors and the first one of the second plurality of conductors causes a first electromagnetic field to exist in a first portion of the first helical inductor and a second electromagnetic field to exist in a second portion of the second helical inductor. The first and second electromagnetic fields are oppositely directed, and the oppositely directed electromagnetic fields result in reduced noise levels in a substrate of the IC. 
   The method comprises providing an IC having first and second helical inductors formed therein in close proximity to one another, and causing at least a first electrical current to travel in the first and second helical inductors. The first helical inductor comprises a first plurality of conductors disposed in respective layers of the IC and conductively connected to form a first helical pattern of conductors. The second helical inductor comprises a second plurality of conductors disposed in respective layers of the IC and conductively connected to form a second helical pattern of conductors. At least a first one of the first plurality of conductors is connected by a first coupling conductor to at least a first one of the second plurality of conductors, and at least a second one of the first plurality of conductors is connected by a second coupling conductor to at least a second one of the second plurality of conductors. Causing at least the first electrical current to travel in the first and second helical inductors causes a first electromagnetic field to exist in a first portion of said the first helical inductor and a second electromagnetic filed to exist in a second portion of the second helical inductor. The first and second electromagnetic fields are oppositely directed, and the oppositely directed electromagnetic fields result in reduced noise levels in a substrate of the IC. 
   These and other features and advantages of the invention will become apparent from the following description, drawings and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a block diagram of a resonant oscillator circuit that has a pair of resonant LC tanks, each having an inductor and a variable capacitor. 
       FIG. 2  illustrates a perspective view of a known planar spiral inductor formed in an IC using a single layer of interconnect metal to form three turns. 
       FIG. 3  illustrates a perspective view of a cross-coupled pair of planar spiral inductors. 
       FIG. 4  illustrates a perspective view of a known helical inductor formed in an IC. 
       FIG. 5  illustrates a perspective view of a cross-coupled helical inductor pair of the invention in accordance with one exemplary embodiment. 
       FIG. 6  illustrates a perspective view of a cross-coupled helical inductor pair of the invention in accordance with another exemplary embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention provides a pair of cross-coupled helical inductors formed in an IC that can be used in conjunction with other elements formed in the IC to produce a differential resonant oscillator circuit.  FIG. 4  illustrates a perspective view of a known helical inductor  27  formed in an IC. The helical inductor  27  has turns  28 A- 28 H that are formed in respective metal layers of the IC. The turns are interconnected by vias  29 A- 29 H. Turns  28 A and  28 B are shorted together by vias  29 A and  29 B, respectively, such that turns  28 A and  28 B function as a single turn. Therefore, in this example of a known helical inductor, the inductor has seven turns. Other configurations of helical inductors that have a greater or lesser number of turns are also known. 
   Although it is known to construct helical inductors in ICs, planar spiral inductors are by far the most common form of inductors used in ICs due to their high Q, which reduces jitter. Helical inductors have a lower Q than planar spiral inductors, and therefore generally are more susceptible to jitter. However, in performing circuit simulations with the cross-coupled helical inductors of the invention, it was observed that the strong mutual magnetic coupling that exists between the turns of cross-coupled helical inductors eliminated undesirable nonlinear effects, which can cause the left and right resonant tanks to oscillate in a non-differential fashion, i.e., with other than a 180° phase alignment between the tanks. This tradeoff between having a high Q and maintaining the 180° phase alignment is justified under certain circumstances. In other words, there are advantages to sacrificing some Q in order to ensure that the 180° phase alignment is maintained between the inductors. 
   In addition, the strong mutual coupling that exists between turns of cross-coupled helical inductors of the invention enables the amount of die area needed to implement the cross-coupled helical inductors to be reduced in comparison to the amount of die area needed to implement cross-coupled planar spiral pair inductors with similar mutual coupling strength. Furthermore, the juxtaposition of the differentially driven cross-coupled helical inductors of the invention and the orientation of the cross-coupled turns reduce net magnetic field penetration into the area surrounding the inductors, the underlying semiconductor substrate in particular. Consequently, there is subtractive, rather than additive, magnetic penetration into the surrounding vicinity of the inductors in the IC, which reduces the likelihood that eddy currents will be generated in the IC substrate that will result in energy loss. 
     FIG. 5  illustrates a perspective view of a cross-coupled helical inductor pair  30  of the invention in accordance with one exemplary embodiment. In accordance with this embodiment, the helical inductor pair  30  comprises a first inductor  40  and a second inductor  50 . The inductor pair  30  is formed in eight metal layers of the IC using an eight-metal-layer IC process. Of course, the invention is not limited to any particular IC process. The invention also is not limited with respect to the number of turns that the inductors have, or with respect to the number of turns that are cross-coupled. 
   In the exemplary embodiment shown in  FIG. 5 , the inductors  40  and  50  each have six turns, although the inductors  40  and  50  each are formed in eight layers of metal. From the lowermost layer (layer one) to the uppermost layer (layer eight) of inductor  40 , the layers are labeled  40 A- 40 H, respectively. Similarly, from the lowermost layer to the uppermost layer of inductor  50 , the layers are labeled  50 A- 50 H, respectively. The vias  43 - 49  and  51  interconnect the layers  40 A- 40 H of inductor  40 . The vias  53 - 59  and  61  interconnect the layers  50 A- 50 H of inductor  50 . 
   A T-junction  91  that is connected to the supply voltage, V DD , is formed in layer one. The currents, i 1  and i 2 , flow in the direction shown from the T-junction to each of the inductors  40  and  50 . Starting at layer one  40 A of inductor  40 , the current, i 1 , flows in the counterclockwise direction, as indicated by arrow  39 . The current flows through the turn  42 A formed by the combination of layers one and two  40 A and  40 B, which are short-circuited together by vias  43  and  44 . Short-circuiting layers  40 A and  40 B places them in parallel, which essentially halves the resistance of the turn  42 A in comparison to the resistance of each of the other turns  42 B- 42 F for higher Q. The current flowing through turn  42 A flows through vias  45  into layer three  40 C. The current flows through turn  42 B formed in layer three  40 C in the same counterclockwise direction. The current flowing through turn  42 B flows through vias  46  into turn  42 C formed in layer four  40 D. The current flows through turn  42 C formed in layer four  40 D in the same counterclockwise direction, as indicated by arrow  41 . 
   The current flowing through turn  42 C flows through vias  47  into cross-coupling element  60 , which cross-couples the current into inductor  50  from inductor  40 . The cross-coupling element  60  is connected by vias  58  to layer  50 F of inductor  50 . Layer  50 E is represented by dashed lines because it is not used to form a turn, but is used to form the cross-coupling elements. The current flows through turn  52 D formed in layer  50 F in the clockwise direction, as indicated by arrow  71 . The current flowing through turn  52 D flows through vias  59  into turn  52 E formed in layer  50 G of inductor  50 . The current flows through turn  52 E in the same clockwise direction and flows through vias  61  into turn  52 F formed in layer  50 H of inductor  50 . The current flows through turn  52 F formed in layer  50 H in the same clockwise direction, as indicated by arrow  72 . 
   The cross coupling of current from inductor  50  into inductor  40  will now be described. Starting at layer one  50 A of inductor  50 , the current, i 2 , flows in the counterclockwise direction, as indicated by arrow  62 . The current flows through turn  52 A formed by the combination of layers one and two  50 A and  50 B of inductor  50 , which are short-circuited together by vias  53  and  54 . As indicated above, short-circuiting layers  50 A and  50 B places them in parallel, which essentially halves the resistance of the turn  52 A in comparison to the resistance of each of the other turns  52 B- 52 F. The current flowing through turn  52 A flows through vias  55  into layer three  50 C. The current flows through turn  52 B formed in layer three  50 C in the same counterclockwise direction. The current flowing through turn  52 B flows through vias  56  into turn  52 C formed in layer four  50 D. The current flows through turn  52 C formed in layer four  50 D in the same counterclockwise direction, as indicated by arrow  63 . 
   The current flowing through turn  52 C flows through vias  57  into cross-coupling element  70 , which cross-couples the current into inductor  40  from inductor  50 . The cross-coupling element  70  is connected by vias  48  to layer  40 F of inductor  40 . Layer  40 E is represented by dashed lines because it is not used to form a turn, but is used to form the cross-coupling elements. The current flows through turn  42 D formed in layer  40 F in the clockwise direction, as indicated by arrow  81 . The current flowing through turn  42 D flows through vias  49  into turn  42 E formed in layer  40 G of inductor  40 . The current flows through turn  42 E in the same clockwise direction and flows through vias  51  into turn  42 F formed in layer  40 H of inductor  40 . The current flows through turn  42 F formed in layer  40 H in the same clockwise direction, as indicated by arrow  82 . 
   It can be seen that a total of three of the six turns from each of the inductors  40  and  50  are cross-coupled. This provides maximum coupling for this particular inductor pair  30 . As described below with reference to  FIG. 6 , a lesser number of turns may be cross-coupled if weaker magnetic coupling is desired, as may be the case under certain circumstances. One of the important aspects of the invention that can be noted from the above description of  FIG. 5  is that the magnetic field reverses polarity when the current is cross-coupled from inductor  40  into inductor  50 , and vice versa. For example, the current flows in the counterclockwise direction in turn  52 C formed in layer  50 D of inductor  50 , but flows in the clockwise direction in turn  42 D formed in layer  40 F of inductor  40 . Likewise, the current flows in the counterclockwise direction in turn  42 C formed in layer  40 D of inductor  40 , but flows in the clockwise direction in turn  52 D formed in layer  50 F of inductor  50 . In accordance with the invention, it has been determined that this orientation provides very strong mutual magnetic coupling that ensures that a 180° phase alignment is maintained between the inductors  40  and  50 , i.e., that the inductors will be differentially driven. In addition, the very strong magnetic coupling achieved enables the cross-coupled helical inductor pair to consume less area on the IC than a comparable planar spiral inductor pair. 
     FIG. 6  illustrates a cross-coupled helical inductor pair  110  in accordance with another exemplary embodiment of the invention. In accordance with this embodiment, a single turn from each inductor is cross-coupled to the other inductor. Inductor  120  is formed in eight metal layers  120 A- 120 H. Each of the layers  120 A and  120 C- 120 H has a turn formed in it  122 A- 122 G. The turns  122 A- 122 G are interconnected by vias  141 - 147 . Inductor  130  is formed in eight metal layers  130 A- 130 H. Each of the layers  130 A and  130 C- 130 H has a turn formed in it  132 A- 132 G. The turns  132 A- 132 G are interconnected by vias  151 - 157 . Layers  120 B and  130 B are not used to form turns, but are used instead to form the cross-coupling elements  140  and  150 . 
   Currents, i 1  and i 2 , flow in the directions indicated from the T-junction  161 , which is tied to V DD , to the turns  122 A and  132 A, respectively, of inductors  120  and  130 , respectively. In turn  122 A of inductor  120 , the current flows in the counterclockwise direction, as indicated by arrow  171 . The current flowing through turn  122 A flows through vias  141  and into cross-coupling element  150 . The current flowing through cross-coupling element  140  flows through vias  152  into turn  132 B formed in layer  130 C of inductor  130 . The current flows through turn  132 B in the clockwise direction, as indicated by arrow  182 . The current flows through each of the turns  132 C- 132 G in the same clockwise direction, as indicated by arrow  183 . 
   In turn  132 A of inductor  130 , the current flows in the counterclockwise direction, as indicated by arrow  181 . The current flowing through turn  132 A flows through vias  151  and into cross-coupling element  150 . The current flowing through cross-coupling element  150  flows through vias  142  into turn  122 B formed in layer  120 C of inductor  120 . The current flows through turn  122 B in the clockwise direction, as indicated by arrow  172 . The current flows through each of the turns  132 C- 132 G in the same clockwise direction, as indicated by arrow  173 . 
   The weaker coupling provided by the helical inductor pair  110  shown in  FIG. 6  is beneficial in circumstances where the self-resonance frequency of the inductors is a limitation. Each inductor reaches its self-resonance frequency when its inductive reactance is exactly cancelled by the net parasitic capacitance between coils and between the lowest coil and the substrate. Beyond this frequency, the inductor behaves as a capacitor and is no longer useful. To increase the self-resonance frequency of the inductor pair, the effective capacitive coupling between two adjacent turns and between the lowest turn and the substrate needs to be reduced. In understanding how to accomplish this goal, it is important to realize that there exists a gradual resistive voltage drop along the inductor. This gradual voltage drop translates into a potential difference between two adjacent turns that is only a small fraction of the total tank voltage amplitude. The effective capacitance then becomes that same fraction of the DC capacitance between these turns. The same principle can be applied to explain why the effective capacitance between the lowest turn and the substrate is also a small fraction of the DC capacitance. 
   Two construction details can be implemented in order to increase the self-resonance frequency. First, by reducing the number of cross-coupled turns, as in  FIG. 6 , the differential potential difference between the two turns from opposite inductors that face each other is reduced. In the example shown in  FIG. 6 , this would be capacitive coupling primarily between the turns in layers one and three. Much larger effective capacitive parasitic exists where more turns are cross-coupled, such as in  FIG. 5 . Second, tying the pair of inductors together at the lowest instead of highest metal level, as illustrated in  FIGS. 5 and 6 , significantly reduces the effective capacitive parasitic to the substrate (held at a fixed potential) because one end of each inductor is held at a fixed potential. 
   It should be noted that the invention has been described with reference to exemplary embodiments and that the invention is not limited to the embodiments described herein. Also, modifications can be made to the embodiments described herein and all such modifications are within the scope of the invention. Such modifications may include, for example, using a different number of turns, short-circuiting turns between adjacent metal layers, creating hybrids that incorporate planar spirals wired in a helical fashion using multiple levels of interconnect. Other modifications will be apparent to those skilled in the art in view of the description provided herein. 
   Also, the invention is not limited to resonant oscillators and may find application in other circuits, such as, for example, circuits that may benefit from tight mutual coupling. Examples of such circuits include RF blocks (low-noise amplifiers, mixers, and power amplifiers, etc.).