Patent Publication Number: US-7719112-B2

Title: On-chip magnetic components

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to copending U.S. provisional application entitled, “On-Chip Magnetic Components,” having Ser. No. 60/836,010, filed Aug. 7, 2006, which is entirely incorporated herein by reference. 
    
    
     BACKGROUND 
     Various electronic devices comprise power supplies that convert electricity from one form into another. For example, most computing devices, such as desktop computers, laptop computers, and personal digital assistants comprise power supplies that convert voltage into a form that can be used by the various components of the devices, such as central processing units (CPUs) or other processors. Conversion may comprise alternating current (AC) to direct current (DC) conversion, as well as DC to DC conversion. In the former case, AC voltage, for example from a wall outlet, is converted into DC voltage, which is used by the internal components of the electronic device. In the latter case, the DC voltage is reduced to a level required by the internal components. 
     Current power supplies comprise various discrete components, such as inductors and capacitors, that are separately manufactured and then mounted, for example, on a circuit board. Due to the aggregation of the various discrete components, such power supplies are relatively large and heavy. Although not necessarily a critical concern in terms of larger electronic devices such as desktop computers, the size and heft of conventional power supplies can be disadvantageous for some applications, such as mobile electronic devices. 
     System-on-chip (SOC) is an emerging trend of integrating all components of an electronic system including digital, analog, mixed-signal, communication, and sensor functions into a single integrated circuit. The proliferation of the SOC concept has generated interest in integrating power management into integrated circuit chips. Power SOCs that monolithically integrate all active and passive components using low-cost semiconductor manufacturing processes would provide an extremely attractive solution with significant improvement in performance, as well as an unprecedented reduction in board space, parts count, and time to market. Unfortunately, the development of power SOCs is seriously hindered by a few major technical barriers. One such barrier is development of a cost effective means of integrating inductors and transformers onto a silicon chip while achieving adequate performance in terms of inductance, DC series resistance, maximum saturation current, and quality factor (Q factor), which is the ratio of reactive impedance to equivalent series resistance (ESR), and therefore provides a measure of inductor performance. 
     Current research on integrated magnetics for power SOCs has predominantly focused on utilizing micro-electro-mechanical-system (MEMS) micromachining technology as a post-processing step after the completion of a chip (e.g., a complementary metal-oxide-semiconductor (CMOS) chip) containing all power switching devices and control circuitry. The high DC resistance (typically 0.5 to 5 ohms (Ω)) and poor Q factor (typically 3 to 8) of the MEMS inductors/transformers, however, severely limit the current handling capability and efficiency of the power SOC. Furthermore, the large increase of fabrication complexity and cost associated with the MEMS post-processing approach raises questions as to the feasibility of large scale commercialization of the power SOC. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosed systems, methods, and apparatus can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale. 
         FIG. 1  is a perspective view of an example conventional electrical connection between an integrated circuit chip and various leads. 
         FIG. 2  is a perspective view of a first embodiment of a chip having bond wires to which magnetic material has been applied. 
         FIG. 3  is a side view of the embodiment illustrated in  FIG. 2 . 
         FIG. 4  is a perspective view of a second embodiment of a chip having bond wires to which magnetic material has been applied. 
         FIG. 5  is a perspective view of a third embodiment of a chip having bond wires to which magnetic material has been applied. 
         FIG. 6  is a perspective view of a fourth embodiment of a chip having bond wires to which magnetic material has been applied. 
         FIG. 7  is a simplified circuit model of a bond wire inductor. 
         FIG. 8  is a graph that compares measured inductance for various bond wire inductors. 
         FIG. 9  is a graph that compares measured inductance for various bond wire inductors. 
         FIG. 10  is a graph that compares measured inductance of MEMS micro-inductors, wire-wound inductors, and bond wire inductors. 
         FIG. 11  is a graph that compares determined Q factor of MEMS micro-inductors, wire-wound inductors, and bond wire inductors. 
         FIG. 12  is a graph that identifies measured inductance for bond wire inductors over a high frequency range. 
         FIG. 13  is a perspective view of a fifth embodiment of a chip having bond wires to which magnetic material has been applied. 
     
    
    
     DETAILED DESCRIPTION 
     From the above, it can be understood that it would be desirable to have power supplies that are smaller in size and/or weight. In some cases, it would be particularly desirable to have power systems-on-chips (power SOCs). As described in the following, power SOCs that integrate active and passive components using low-cost semiconductor manufacturing processes can be manufactured by forming magnetic components on bond wires of the chip. In some embodiments, the magnetic components comprise magnetic material that is held to the bond wires with a suitable binder. 
     Turning to the figures, in which like numerals identify corresponding components,  FIG. 1  shows an example of conventional electrical connection of an integrated circuit (IC) chip  10  to various leads  12 , for example provided in a lead frame of an electronic package (partially shown). Various contacts of the chip  10  are connected to the circuit board leads  12  using bond wires  14 . Such bond wires  14  may provide power to the chip  10  or may be used to transmit signals to and from the chip. Given that the bond wires  14  are made of electrically conductive materials, such as gold or aluminum, and since current flows through the bond wires, the bond wires act as natural inductors. 
     Although the bond wires  14  act as natural inductors, the inductance created by the bond wires is typically insufficient for power converter applications. By way of example, the bond wires may exhibit an inductance of only a few nano-Henrys (nH) (e.g., approximately 3-4 nH), whereas higher inductance, for example in the range of several tens of nH, may be required for power SOCs. 
     Increased inductance can be achieved by adding magnetic material to the bond wires. For example, powdered ferrite can be sintered on the bond wires to increase their inductance. Unfortunately, such sintering requires very high temperatures and/or chemical processing techniques that are inconsistent with the processes used to fabricate semiconductor (e.g., silicon) chips. More particularly, the temperatures and/or chemicals currently used can damage the chip. 
     The above difficulties can be avoided by adding magnetic material to the bond wires using a fabrication process that is compatible with chip fabrication. For example, particles of a magnetic metal, such as ferrite, can be mixed with an appropriate binder material, such as an epoxy, and the solution can be applied to the bond wires to increase their inductance.  FIGS. 2 and 3  illustrate an embodiment of such application. As indicated in those figures, a first bond wire  16  extends upwardly from the plane of a top surface of a power supply chip  18  to a separate lead  20 . By way of example, both the chip  18  and the lead  20  are provided on a package lead frame  21  ( FIG. 3 ). In addition, a second bond wire  22  connects two discrete points on the chip  18  and also extends upwardly from the plane of the top surface of the chip. As is further shown in the figures, each bond wire  16 ,  22  is provided with a mass  24 ,  26  of magnetic material, such as powdered ferrite suspended in epoxy, on a portion of the bond wire that is beyond the plane of the top surface of the chip  18 . In the embodiment of  FIGS. 2 and 3 , the masses  24  and  26  each comprise a bead of material. By way of example, the beads can have a thickness (e.g., central diameter) of approximately 20-100 microns (μm) and can be provided on the bond wires  16 ,  22  by brushing, squeegeeing, dipping, dripping, inking, or other viable techniques. In some embodiments, the chip  18  can be flipped (i.e., inverted) and the bond wires  16 ,  22  dipped in a solution comprising the magnetic material. In other embodiments, the beads can be provided on the bond wires  16 ,  22  using robotics that apply the beads to the bond wires with a high degree of precision. Irrespective of the method used to apply the magnetic material, once the magnetic material has been applied, it can be dried and, if necessary, cured at a temperature below 200° C. 
       FIG. 4  illustrates a further embodiment. In that embodiment, larger portions of the bond wires  16 ,  22  are coated with a mass  28 ,  30  of magnetic material. Specifically, substantially the entire lengths of the bond wires  16 ,  22  are coated with the magnetic material. Generally speaking, the greater the amount of magnetic material that is applied to the bond wires, the greater the increase in inductance that can be achieved given that increased mass yields reduced magnetic reluctance and greater magnetic flux. By way of example, the application of magnetic material can increase the natural inductance of the bond wires by at least a factor of 5-10. 
       FIG. 5  illustrates yet another embodiment. In that embodiment, a single mass  32  of magnetic material is applied to multiple bond wires, in particular a primary bond wire  34  and a secondary bond wire  36 , to form a transformer. 
     The above-described power SOCs enable various advantages that are difficult to achieve with existing technologies, such as MEMS inductors. For example, each of the control circuitry, power switches, gate drivers, feedback compensation networks, and the like can be fabricated with standard silicon processing technology, eliminating the need for costly post-CMOS MEMS processing steps. In addition, the on-chip bond wire inductors and transformers can be integrated into the power SOC packaging process with minimal changes, thereby facilitating cost-effective, high current, high efficiency power SOCs. Furthermore, aluminum and gold bond wires, due to their relatively large diameters, are much more conductive than the thin metal films in MEMS inductors. Therefore, a much lower DC resistance and higher Q factor can be expected for the bond wire inductors. Moreover, the electromagnetic field of a bond wire inductor is mainly distributed outside the silicon substrate. Therefore, Eddy power loss in the silicon substrate at high frequency, which is a major concern in MEMS magnetics, can be reduced or minimized. 
     Experimentation 
     On-chip bond wire inductors were investigated using two types of ferrite epoxy composite materials. The first material was a custom formulated magnetic epoxy comprised of manganese-zinc (MgZn) ferrite powder with an average particle size of 10 μm, a thermoplastic resin, and a solvent from the Methode Development Corporation. The manganese-zinc ferrite loading powder is commercially available (Steward 73300). The average surface area of the powder was 1.4 square meters per gram (m 2 /g). The saturation moment of the bulk powder was 79.4 electromagnetic units per gram (emu/g). The cured ferrite composite (no solvent) comprised 96% by mass ferrite with the balance consisting of polymer. The effective permeability was between 12 and 16. The second material was a ferrite nanocomposite from the Inframat Corporation that was comprised of very fine (NiZn)Fe 2 O 4  nanoparticles with an average size of 5-15 nanometers (nm) and a commercial epoxy. 
     Testing was conducted on standalone copper wires. The copper wires had fixed lengths of 20 mm and respective diameters of 250 μm (10 mil) and 500 μm (20 mil) to emulate bond wires in IC packages. The ferrite epoxy materials were manually brushed onto the copper bond wires to form a ferrite beads. Curing comprised a thermal treatment of the ferrite beads in an oven at 140° C. for thirty minutes for both ferrite epoxy materials. The ferrite-polymer composites displayed negligible conductivity and therefore were electrically self-isolated from the bare copper bond wires. 
     Testing was also conducted on aluminum bond wires that were bonded onto a printed circuit board (PCB) substrate.  FIG. 6  depicts the aluminum bond wires  38  with and without a ferrite epoxy bead  40  on the PCB substrate  42 . The wirebonding was conducted on a Electrodyne M20 wirebonder using 5 mil aluminum bond wires. 
     Each of the bond wire inductors were characterized with HP 4284A high precision LCR meters in a low frequency range up to 1 MHz. DC resistance was measured with an Instek 801H milli-ohm meter. High frequency measurement was performed using an Agilent 8753 S-parameter network analyzer. The Q factor and inductance were then determined using the following equations: 
                     L   eff     =     -     1     ω   ⁢           ⁢     Im   ⁡     (     y   11     )                     [     Equation   ⁢           ⁢   1     ]               Q   =     -       Im   ⁡     (     y   11     )         Re   (     y   11     )                 [     Equation   ⁢           ⁢   2     ]               
where L eff  is inductance, Q is the Q factor, w is the frequency, and y 11  is the y parameter reading from the network analyzer, with Im and Re being the imaginary and real components of y 11 , respectfully.
 
       FIG. 7  illustrates the simplified equivalent model of the bond wire inductor that was used in electrical parameter extraction, with a capacitor, C s , connected in parallel with an inductor, L s , and a resistor, R s .  FIGS. 8 and 9  show the measured inductance for the 10 mil and 20 mil bond wire inductors with no ferrite bead, the ferrite polymer bead, and the ferrite nanocomposite bead, respectively. The inductance of the bare copper wires was increased by a factor of 2.8 to 3.5 with the addition of the ferrite epoxy beads. The DC resistance of the 10 mil and 20 mil bond wire inductors was measured as 7.1 mΩ and 1.7 mΩ, respectively, using an Instek 801H milli-ohm meter. The 10 mil bond wire inductor demonstrated an inductance of 38 nH and a DC resistance of 7.1 mΩ. 
       FIG. 10  compares the state-of-the-art MEMS micro-inductors, commercial wire-wound inductors, and the bond wire inductors evaluated in the experimentation (i.e., “in this work”) in terms of inductance and DC winding resistance achieved. As is apparent from the results shown in  FIG. 10 , the bond wire inductors offer a solution for high current power SOC applications in which the MEMS micro-inductors fall short. 
       FIG. 11  compares the Q factor of the three evaluated approaches. The bond wire inductors demonstrated a Q factor of 30-40 in a frequency range of 2 to 20 MHz, which is similar to that of the commercial wire-wound inductors but 3-30 times higher than that of the MEMS micro-inductors. 
       FIG. 12  shows the measured inductance of the bond wire inductors over a high frequency range, indicating a self-resonant frequency between 700 and 800 MHz. 
       FIG. 13  illustrates yet another embodiment illustrating use of magnetic material. In the embodiment of  FIG. 13 , a plurality of bond wires  44  are connected in series with each other on a semiconductor chip  46  and the bond wires are at least partially encapsulated by magnetic material  48 . 
     In the foregoing, powdered ferrite and ferrite nanoparticles have been identified as magnetic materials that can be used to increase the inductance of the bond wires. It is noted, however, that other magnetic materials may be used. Furthermore, the magnetic material need not be in powdered form. For example, a solid magnetic mass can, in some embodiments, be adhered or otherwise attached to the bond wires. Furthermore, although epoxy and thermoplastic resin have been identified as binder materials in cases in which the magnetic material is suspended in a binder material, other binder materials, such as polymeric and/or organic materials, can be used. Moreover, a binder material or adhesive may not be necessary. For example, magnetic material, such as magnetic metal, can, in some embodiments, be applied to the bond wires in a molten state. 
     It is further noted that the inductance provided to the power SOC is “free” given that the natural inductance of the bond wires is leveraged to form the magnetic component, thereby providing a convenient and inexpensive solution to power management. Furthermore, it is noted that the inductance is provided without a concomitant increase of parasitic resistance, resulting in high Q factors. Moreover, although the addition of the magnetic material to bond wires has been described herein for power SOC applications, it is noted that the present techniques can be used to form a magnetic component in association with substantially any chip.