Abstract:
The claimed invention relates to arrangements of inductors and integrated circuit dice. One embodiment pertains to an integrated circuit die that has an inductor formed thereon. The inductor includes an inductor winding having a winding input and a winding output. The inductor also comprises an inductor core array having at least first and second sets of inductor core elements that are magnetically coupled with the inductor winding. Each inductor core element in the first set of inductor core elements is formed from a first metallic material. Each inductor core element in the second set of inductor core elements is formed from a second metallic material that has a different magnetic coercivity than the first magnetic material. The inductor further comprises a set of spacers that electrically isolate the inductor core elements. Some embodiments involve multiple inductor windings and/or multiple inductor core elements that magnetically interact in various ways. Particular embodiments involve core elements having different compositions and/or sizes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional application claiming priority to U.S. patent application Ser. No. 11/621,424 (Attorney Docket No. NSC1P362), entitled “Apparatus and Method for Wafer Level Fabrication of High Value Inductors on Semiconductor Integrated Circuits.” 
     
    
     BACKGROUND  
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to wafer level fabrication of high value inductors on semiconductor integrated circuits, and more particularly, to the optimization of power inductor arrays on integrated circuits for switching regulator applications. 
         [0004]    2. Background of the Invention 
         [0005]    Inductors are commonly used in the electronics industry for storing magnetic energy. Providing an electric current though a metal conductor, such as a metal plate or bar, typically creates an inductor. The current passing though the metal conductor creates a magnetic field or flux around the conductor. The amount of inductance is measured in terms of Henries. In the semiconductor industry, it is known to form inductors on integrated circuits. The inductors are typically created by fabricating what is commonly called an “air coil” inductor on the chip. The air coil inductor is usually either aluminum or some other metal patterned in a helical, toroidal or a “watch spring” coil shape. By applying a current through the inductor, the magnetic flux is created. 
         [0006]    Inductors are used on chips for a number of applications. Perhaps the most common application is DC-to-DC switching regulators. In many situations, however, on chip inductors do not generate enough flux or energy for a particular application. When this occurs, very often an off-chip discrete inductor is used. 
         [0007]    There are a number of problems in using off-chip inductors. Foremost, they tend to be expensive. With advances in semiconductor process technology, millions upon millions of transistors can be fabricated onto a single chip. With all these transistors, designers have been able to cram a tremendous amount of functionality onto a single chip and an entire system on just one or a handful of chips. Providing an off-chip inductor can therefore be relatively expensive compared to the overall cost of the system. Off-chip inductors can also be problematic in situations where space is at a premium. In a cell phone or personal digital assistant (PDA) for example, it may be difficult to squeeze a discrete inductor into a compact package. As a result, the consumer product may not be as small or compact as desired. 
         [0008]    An apparatus including an integrated circuit die with an inductor formed thereon is therefore needed. 
       SUMMARY OF THE INVENTION 
       [0009]    The claimed invention relates to arrangements of inductors and integrated circuit dice. One embodiment pertains to an integrated circuit die that has an inductor formed thereon. The inductor includes an inductor winding having a winding input and a winding output. The inductor also comprises an inductor core array having at least first and second sets of inductor core elements that are magnetically coupled with the inductor winding. Each inductor core element in the first set of inductor core elements is formed from a first metallic material. Each inductor core element in the second set of inductor core elements is formed from a second metallic material that has a different magnetic coercivity than the first magnetic material. The inductor further comprises a set of spacers that electrically isolate the inductor core elements. 
         [0010]    In another embodiment, an integrated die with an inductor formed thereon includes multiple inductor windings and multiple core elements. Each of the core elements in the inductor substantially surrounds and is magnetically coupled with at least one inductor winding. Some of the inductor core elements substantially surround and are magnetically coupled with more than one of the inductor windings. Some of the core elements are formed from a different metallic material than other core elements. The different metallic materials may have different magnetic coercivities. In some embodiments, the metallic materials have different compositions. Particular embodiments involve inductor core elements whose lengths vary such that not all of the core elements interact with all of the inductor windings and at least some of the inductor core elements are magnetically coupled to multiple inductor windings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0011]      FIG. 1A  is a block diagram of power regulator system. 
           [0012]      FIG. 1B  is a circuit diagram of a power regulator circuit. 
           [0013]      FIGS. 2A  is a plot illustrating the relationship between flux density versus magnetic field intensity in the inductor and core of the power regulator circuit of  FIG. 1B . 
           [0014]      FIG. 2B , a plot illustrating the relationship between the inductance and the current in the coil of the power regulator circuit of  FIG. 1B . 
           [0015]      FIG. 3  is a block diagram of a semiconductor chip having a power regulator circuit fabricated thereon according to the present invention. 
           [0016]      FIGS. 4A through 4D  illustrate various embodiments of a core array of the power regulator circuit according to the present invention. 
           [0017]      FIG. 5  illustrates a cross section of a core element of the core array according to the present invention. 
           [0018]      FIG. 6A  is a block diagram of the phase control circuit used in the regulator circuit of the present invention. 
           [0019]      FIG. 6B  is a diagram showing an output signal at the output node of the regulator circuit of the present invention. 
           [0020]      FIGS. 7A-7H  are a sequence of cross sections of a semiconductor chip illustrating the sequence of fabricating core elements of the core array used in the regulator circuit of the present invention. 
       
    
    
       [0021]    Like elements are designated by like reference numbers in the Figures. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    Referring to  FIG. 1A , a block diagram of a common power regulator system is shown. The system  10  includes a regulator circuit  12  and a controller  14  coupled between a power supply  16  and a device  18 , such as micro-controller, that requires a steady direct current (DC) voltage. The regulator circuit  12  includes an inductor (L) and a core (both not illustrated). The input voltage Vin is typically a pulsed input signal from the power supply  16  having a frequency (f) and an amplitude equal to Vin. With each positive and negative pulse transition, the inductor is cyclically energized and then de-energized, causing the flux in the core to increase and then decrease respectively. The output Vout of the regulator circuit  12  is coupled to the device  18 . Ideally, the output voltage is steadily maintained at the desired output voltage. If the output voltage strays, the controller  14  causes the frequency (i.e., sometimes referred to as the duty cycle) of the pulses of the input voltage Vin to either increase or decrease as needed to maintain a steady output voltage. 
         [0023]    ΔV is a measure of the input voltage Vin minus the output voltage Vout. This relationship can be expressed by equation (1) below: 
         [0000]      ΔV=Vin−Vout   (1) 
         [0000]    ΔV can also be expressed as the rate of change of current over time through the inductor (L). This relationship is expressed by equation (2) below: 
         [0000]      ΔV=L di/dt   (2) 
         [0000]    ΔV can further be expressed in terms of the switching frequency (f) of the input voltage Vin. This relationship is expressed by equation (3) below: 
         [0000]      ΔV=L/f   (3) 
         [0024]    Referring to  FIG. 1B , a circuit diagram of the regulator  12  is shown. The circuit  12  includes two transistors T 1  and T 2  with their channels coupled in series between Vcc and ground. A first capacitor C 1  is also coupled between Vcc and ground in parallel with the channels of transistors T 1  and T 2 . One end of the inductor coil L is coupled between the two transistors Ti and T 2  and the opposite end is coupled to the output node Vout. A core  20  is provided adjacent the inductor L. A second capacitor C 2  is coupled between the output node Vout and ground. The gate of transistor T 1  is coupled to receive the pulsed input signal Vin. The gate of transistor T 2  is coupled to receive the complement of signal Vin. The instantaneous energy E in the inductor L is defined by equation (4) below: 
         [0000]      E=∫BHdh   (4) 
         [0000]    Where B is the flux density or Webers per cm2; and 
         [0025]    H is the magnetic field intensity 
         [0026]    The regulator circuit  12  operates in alternating high and low phases. During the high phase, a positive voltage pulse of Vin is applied to the gate of transistor T 1 , while a complementary or negative pulse is applied to the gate of transistor T 2 . This causes transistor T 1  to turn on and transistor T 2  to turn off. With transistor T 1  on, current is pulled through transistor T 1  from Vcc to energize the coil of inductor L, creating a flux in the core  20 . The low phase occurs when the input pulse transitions low at the gate of transistor T 1  and high at the gate of transistor T 2 . When this occurs, the transistor T 1  turns off and transistor T 2  turns on. As a result, the inductor L pulls current from ground through transistor T 2 , causing the inductor to de-energize and the flux in the core  20  to collapse. The aforementioned cycle is repeated with each pulse of Vin and the complement is applied to transistors T 1  and T 2 . The output of the circuit  12  is ideally a steady voltage. Due to the cyclical increases and decreases of the energy in the inductor L and the flux in the core  20 , the output voltage Vout will typically have a ripple. The output capacitor C 2  is provided to smooth out the ripple. 
         [0027]    Referring to  FIG. 2A , a plot illustrating the relationship between the flux density as measured in Webers per cm 2  (B) versus the magnetic field intensity as measured in ampere turns per meter (H) in the inductor L and core  20  is shown. As evident in the figure, as the magnetic field H increases, the flux density B increases, until the saturation point “Bsat”. Once Bsat is reached, the magnetic flux B remains generally constant, even with an increase in the magnetic field density. The slope of the plot or curve, calculated by B/H, defines the permeability (i.e., the propensity) of the material of the core  20  to become magnetized. Bsat is thus defined as the point where the maximum state of magnetization or flux of the material of the core  20  is achieved. In other words, the point where the curve rolls off to a minimum slope represents the flux density saturation point (Bsat). Bsat varies from material to material. For example, Iron has a high level of permeability, whereas other materials such as FeNi (permalloy) have a relatively low permeability. The higher the permeability slope the greater the ability of the system to store magnetic flux, and hence energy for a given inducing current, or magnetic field. 
         [0028]    Referring to  FIG. 2B , a plot illustrating the relationship between the inductance (L) and the current (I) in the coil L is shown. With a relatively small current, the inductance L is high. As the current increases, the inductance L drops off until the saturation point Lsat is reached.  FIG. 2B  represents the derivative of H and B, and is plotting the slope of  FIG. 2A . Inductance then, relates to the derivative of B and H or in other words is proportional to the permeability. The inductance rolls off at the point following the magnetic saturation of the core material. 
         [0029]    The issue with common regulator circuits is that it has been difficult to fabricate cores  20  of sufficient size on an integrated circuit. The solution in the past has typically been to use an off-chip or discrete core. With the present invention, however, the core is fabricated on chip as described in detail below. 
         [0030]    Referring to  FIG. 3 , a block diagram of an integrated circuit formed on a semiconductor chip having a power regulator circuit fabricated thereon according to the present invention is shown. The integrated circuit  30  includes a plurality of regulator circuits  32  each having an input node  34  configured to receive complementary pulsed input signals  36  respectively (for the sake of simplicity, only the positive pulsed signal is shown). A plurality of inductor core windings  38  is associated with each of the plurality of regulator circuits  32 . The regulator circuits  32  are each identical to that illustrated in  FIG. 1B  with the exception of the core  20 . Rather than a specific core  20  associated with each circuit  32 , an engineered distributed core array  40  is provided for all of the regulator circuits  32 . The core array  40 , including a plurality of core elements (not illustrated), is positioned adjacent to and is magnetically coupled to each of the inductor windings  38 . An output node  42  is electrically coupled to the plurality of inductor windings  42 . A phase control circuit, connected between the output node  42  and the regulator circuits  32 , is provided to control the phase of each of the input signals  36 . 
         [0031]    The general principle of the present invention is the combined use of phased multiple driver circuits  32 , each driving one or more core elements of the core array  40 . The greater the degree of the sharing among the core elements of the array  40  by the phased driver circuits, the higher the overall level of saturation of the core  40  can be achieved. By engineering the length, width, and types of materials used to fabricate the core elements of the core  40  and the windings  38 , energy storage can be maximized while minimizing core losses. 
         [0032]    It should be noted  FIG. 3  as illustrated is figurative in the sense that it shows the regulator circuit of the present invention occupying virtually all of the area on the surface of the integrated circuit chip. It should be understood, however, that it is intended that the regulator circuitry of the present invention be fabricated on a chip along with other circuitry. In various embodiments, the other circuitry can include a wide variety of functions, such as a microprocessor or microcontroller, digital signal processing, memory or just about any other analog or digital circuitry commonly found on semiconductor integrated circuits. In other words, the power regulator of the present invention may be fabricated on and used on virtually any semiconductor integrated circuit. 
         [0033]    Referring to  FIGS. 4A through 4D , various embodiments of the engineered core array  40  are shown. 
         [0034]    Referring to  FIG. 4A , an embodiment of the core array  40  is shown. Core array  40  includes a plurality of core elements  50 , which are separated by spacers  52 . The purpose of the spacer  52  is to prevent eddy currents between the elements  50 . As illustrated in the figure, each of the elements  50  is positioned adjacent to and is magnetically coupled to one or more of the inductor windings  38 , which are electrically coupled between the plurality of regulator circuits  32  and the output node  42  respectively. In this embodiment, each of the core elements  50  is of the same width. The length of the core elements  50 , however, varies. In the embodiment illustrated, the three left most core elements  50  each have a length of six times (6×) a predetermined unit length and are in a non-staggered pattern with respect to one another. The remaining core elements  50 , on the right side of the array, are of different lengths and arranged in a staggered pattern. In the specific embodiment shown, the core elements  50  are four, three or two times (4×, 3×, and 2×) the predetermined length. The length of the elements  50  becomes shorter as a function of the length of the winding  38 . In an alternative embodiment, the length of the elements  50  can become longer as a function of the length of the windings. 
         [0035]    In various embodiments, the predetermined length of the elements may range from 1 um to 10 mm and the core elements  50  may range from two to 100 times the predetermined length. In yet another embodiment, all of the core elements  50  of the core array  40  can be of various lengths and arranged in a staggered pattern. 
         [0036]    Referring to  FIG. 4B , an embodiment of the core array  40  with a plurality of the core elements  50  is shown. The plurality of the core elements  50  are made from two different types of metals, M 1  and M 2 , which have different coercivities. In this embodiment, each of the core elements  50  are of the same length and width and are arranged in a parallel, non-staggered pattern. A spacer  52  separates the core elements  50 . In the embodiment shown, the core elements  50  are divided into first and second subsets. The first subset of core elements  50  are made from a first metal M 1 , such as nickel-iron permalloy, or any ferromagnetic material with a relatively low coercivity (Hc). The second subset of core elements  50  are made from a second metal M 2  having a higher coercivity, such as colbalt-nickel-iron or materials rich in Fe. The individual cores  50  of the first and second subsets M 1  and M 2  are arranged in an alternating pattern. The metal arrangement of M 1  and M 2 , separated by spacers  52 , is distributed along the length of the inductors. 
         [0037]    Referring to  FIG. 4C , an embodiment of the core array  40  including a plurality of the core elements  50  made from the same metal is shown. In this embodiment, each of the core elements  50  are the same length and width and are arranged in a parallel, non-staggered pattern. Spacers  52  also separate each of the core elements  50 . 
         [0038]    Referring to  FIG. 4D , another embodiment of the core array  40  including a plurality of the core elements  50  made from two different types of metals M 1  and M 2  of different coercivity is shown. In this embodiment, each of the core elements  50  is of the same length, but different widths. The core elements  50  are also arranged in a parallel, non-staggered, pattern. A spacer  52  separates the core elements  50 . With this arrangement, a majority of the first subset of core elements  50  made from the first metal M 1  with a lower coercivity are located near the regulator circuits  32  and minority is located near the output node  42 . A majority of the second subset of core elements  50  made from the second metal M 2  with a higher coercivity are located near the output node  42  and a minority are located near the regulator circuits  32 . With this embodiment, the benefits of the two metals M 1  and M 2  with their different B/H curves are exploited. For example, at the input of the core array  40 , there is a relatively large alternating current component on the input signals provided onto the inductor windings  38 . It is therefore advantageous to use a low coercivity metal, which provides a lower level of energy storage. On the other hand, the signal on the inductor windings  38  near the output node  42  has a relatively higher direct current component. The use of a metal capable of a higher degree of energy storage, such as cobalt-nickel-iron, is therefore beneficial. 
         [0039]    At the switching node or input node  34  near the transistors T 1  and T 2  along the windings  38 , there is a relatively large level of ripple. In one embodiment, the core  40  is therefore engineered to be weighted with high resistivity (to minimize eddy currents) and low coercivity (to minimize hysterisis losses) elements  50  near the driver circuits  32 . The trade-off of this arrangement, however, is reduced permeability and Bsat. At the output node  42  on the other hand, there is less ripple voltage. Consequently, the elements  50  of the core  40  near the output can be weighted with elements  50  having a higher Bsat and cooercivity material, while trading off higher conductivity and coercivity. 
         [0040]    It should be noted again that the embodiments shown in  FIGS. 4A through 4D  are merely exemplary. The number, length, width, pattern (staggered, non-staggered, or a combination thereof), types of metals, and specific arrangement of the individual core elements  50  can be selected in a wide combination of different designs, depending on a specific application. In no way should the specific arrangements as illustrated be construed as limiting the invention. 
         [0041]    Referring to  FIG. 5 , a cross section of a core element  50  of the core array  40  according to the present invention is shown. Each of the core elements  50  includes a first core member  50   a  positioned adjacent to and above the windings  38  and a second member  50   b  positioned adjacent to and below the core windings  38 . Together, the first core member  50   a  and the second core member  50   b  substantially form a loop around the core windings  38 . A gap  54  is provided between the first and second members  50   a  and  50   b . The gap allows a drop in the magnetic field between the two members  50   a  and  50   b . In the particular embodiment shown, the core element  50  has a length that spans three windings  38 . With this arrangement, each of the three windings  38  is magnetically coupled by the one core element  50 . It should be understood, however, that this length is arbitrary and that the two members  50   a  and  50   b  of each core element  50  may be fabricated to span any number of windings  38 . 
         [0042]    Referring to  FIG. 6A , a block diagram of the phase control circuit  46  of the present invention is shown. The phase control circuit  46  controls the phase of the plurality of pulsed input signals so that each is 360 degrees/N out of phase with respect to one another, where N equals the number of the regulator circuits  32 . For example, if there are one hundred regulator circuits  32  (N=100), then the phase control circuit  46  controls the input signals  36  so that they are each 3.6 degrees out of phase with respect to one another. With this arrangement, any voltage ripple on the individual windings  38  tend to destructively interfere with one another, substantially cancelling each other out in the aggregate. As a result, a generally steady output voltage at the output node  42  is generated, which is the sum of the instantaneous voltage on each of the plurality of inductor windings respectively. Since the output signal at node  42  is relatively ripple-free and steady, the smoothing capacitor C 2  can be either eliminated altogether or the size of the input capacitor C 1  and, to some degree, the output capacitor C 2  can be significantly reduced. 
         [0043]    Referring to  FIG. 6B , a diagram plotting the output voltage at the output node  42  over time is shown. The diagram illustrates a number of signals  58 , each of which is representative of the voltage on the individual windings  38 . As can be seen in the figure, the individual signals  58  are out of phase with respect to one another. As a result, the voltage ripple on the individual windings  38  tend to cancel each other out under low load conditions. Similarly the referred ripple to the input power supply are cancelled to a significant degree. The sum of the instantaneous voltage  58  on each of the plurality of inductor windings  38 , however, is a relatively constant with the cancellation effect. The net result is a steady output signal or voltage  59  at the output node  42 , represented by the thick black line in  FIG. 6B . 
         [0044]    Furthermore, with the arrangement the core elements  50  each spanning one or more windings  38 , the magnetic coupling tends to average out or be substantially evenly shared or distributed across the core array  40 . As a net result, the amount of magnetization “ripple” across the windings  38  is further minimized, resulting in a more steady output voltage signal at the node  42 . Without this distributed coupling across the array  40 , the individual core elements  50  would experience a greater level of magnetization ripple, leading to a higher level of hysteresis, radiative and eddy current related loss factors. As a result, the size of smoothing capacitor at the output node and input node  42  may be significantly reduced or eliminated all together. 
         [0045]    The phase control circuit  46  also optionally includes a modulation circuit  56 . The modulation circuit is configured to modulate the phase differences between the plurality of pulsed input signals  36  to either increase or decrease the transient current demands at the output node  42 . Meeting high speed transient demand is often a challenge from a design perspective. By modulating the phase difference, typically for short periods of time, a transient surge in energy demand can be achieved. This phase modulation or short-term frequency modulation scheme thus enables spontaneous maximization of the energy transfer from the core to the output node. 
         [0046]    Referring to  FIGS. 7A-7H , a sequence of cross sections of a semiconductor substrate (e.g., a wafer) illustrating how the core elements  50  of the core array  40  are fabricated is shown. 
         [0047]    In  FIG. 7A , a seed layer  60  is formed over a substrate  62  in the location where the core elements  50  of the array  40  are to be formed. According to one embodiment, the seed layer  60  actually includes three layers of metal, for example, a Ti—Cu—Ti or Ti—FeNi—Ti, formed over the substrate surface by either sputtering or physical vapor deposition. 
         [0048]    In the next step as illustrated in  FIG. 7B , a blanket layer of molding material  64 , such as photoresist or BCB, is applied over the seed layer  60 . 
         [0049]    The molding material  64  is then patterned using conventional lithography to form recess regions  66 , as illustrated in  FIG. 7C . Note the seed layer  66  is exposed at the bottom of the recess regions  66 . 
         [0050]    In the next step, the substrate is immersed in an electroplating bath. In the bath, the metal in the plating solution is plated onto the seed layer  62 , forming the metal regions M as illustrated in  FIG. 7D . 
         [0051]    As illustrated in  FIG. 7E , optionally, a nitride layer  68  is formed over the remaining molding material  64  and the metal M in the recess regions  66 . A “lift off” gap  70  is provided under the nitride layers  68  on the molding material  64 . In the next step, the molding material  64  is completely removed by exposure to a solvent through the lift off gaps  70 . 
         [0052]    As illustrated in  FIG. 7F , a layer of low temperature oxide, nitride or a combination of the two is formed across the surface of the substrate  62 , covering the top and side surfaces of the metal regions M. 
         [0053]    As illustrated in  FIG. 7G , a reactive ion anisotropic etch is then performed, which removes the oxide layer  72  and nitride layer  68  on the top surface of the metal regions M. The remaining oxide (or oxide/nitride) on the sidewalls of the metal regions M forms the spacers  52  described above. According to various embodiments, the thickness of the spacers may range from 500 Angstroms to 10 microns. The creation of the spacers  52  thus uses a novel processing scheme to electroplate the core elements in two stages on either side of the spacers, with the core elements sharing the electroplating seed layer. 
         [0054]    In a final step as illustrated in  FIG. 7H , the substrate undergoes a second plating bath, forming additional metal regions M, each separated by a spacer  52 . The second metal regions M are thus self-aligned with the original metal regions M 1  by the spacer  52 . 
         [0055]    The aforementioned process can be used to create the core elements  50  of the core array  40  as illustrated in  FIGS. 4A through 4D . In other words, the process can be used to make core elements  50  of different or the same lengths and widths, in a uniform or staggered pattern, or of different metals (i.e., M 1  and M 2 ). In embodiments using two different metals, the core elements  50  of the first metal M 1  are plated during the first electroplating operation and the core elements  50  of the second metal M 2  are plated during the second plating operation. Similarly, the core members  50   a  and  50   b  can be formed using the same technique. After the lower members  50   b  are fabricated on the substrate, the windings  38  are formed by a copper metallization step. Thereafter, the above process is again repeated to form the upper members  50   a . A dielectric layer is typically provided between the core members  50   a ,  50   b  and the winding  38  so that they are each electrically isolated from one another respectively. In various embodiments, the metals M 1  and M 2  may include a nickel-iron permalloy (80:20) or Orthonol (50:50); ZrCo, FeNiSi, FeNiCu, FeCo, CoMnZnFeNi, FeNiCo, or doped and non-doped combinations thereof. 
         [0056]    While this invention has been described in terms of several preferred embodiments, there are alteration, permutations, and equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. For example, the steps of the present invention may be used to form a plurality of high value inductors  10  across many die on a semiconductor wafer. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.