Abstract:
Embodiments of circuits, apparatuses, and systems for a flip-chip power amplifier and impedance matching network are disclosed. Other embodiments may be described and claimed.

Description:
FIELD 
       [0001]    Embodiments of the present disclosure relate generally to the field of circuits, and more particularly to a flip-chip power amplifier and impedance matching network. 
       BACKGROUND 
       [0002]    Impedance matching networks with large transformation ratios are required on an output of a power amplifier given practical supply voltages and antenna impedances. These transformation ratios typically exceed 12:1. Such an impedance matching network is implemented by a combination of surface mounted devices (SMDs), e.g., capacitors, and conductive elements, e.g., inductors, in a laminate carrier. The SMDs on the laminate carrier and the conductive elements in the laminate carrier have significant variability in production and occupy a significant portion of the power amplifier&#39;s footprint. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    Embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
           [0004]      FIG. 1  illustrates a cross-sectional view of a radio frequency power amplifier module in accordance with some embodiments. 
           [0005]      FIG. 2  illustrates a top view of a radio frequency power amplifier module in accordance with some embodiments. 
           [0006]      FIG. 3  illustrates a top view of a radio frequency power amplifier module in accordance with some embodiments. 
           [0007]      FIG. 4  is a circuit diagram of radio frequency power amplifier module in accordance with some embodiments. 
           [0008]      FIG. 5  is a flowchart depicting a process of assembling a radio frequency power amplifier module in accordance with some embodiments. 
           [0009]      FIG. 6  is an exemplary wireless communication device in accordance with some embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Various aspects of the illustrative embodiments will be described using terms commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art. However, it will be apparent to those skilled in the art that alternate embodiments may be practiced with only some of the described aspects. For purposes of explanation, specific devices and configurations are set forth in order to provide a thorough understanding of the illustrative embodiments. However, it will be apparent to one skilled in the art that alternate embodiments may be practiced without the specific details. In other instances, well-known features are omitted or simplified in order not to obscure the illustrative embodiments. 
         [0011]    Further, various operations will be described as multiple discrete operations, in turn, in a manner that is most helpful in understanding the present disclosure; however, the order of description should not be construed as to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation. 
         [0012]    The phrase “in one embodiment” is used repeatedly. The phrase generally does not refer to the same embodiment; however, it may. The terms “comprising,” “having,” and “including” are synonymous, unless the context dictates otherwise. 
         [0013]    In providing some clarifying context to language that may be used in connection with various embodiments, the phrases “A/B” and “A and/or B” mean (A), (B), or (A and B); and the phrase “A, B, and/or C” means (A), (B), (C), (A and B), (A and C), (B and C) or (A, B and C). 
         [0014]    The term “coupled with,” along with its derivatives, may be used herein. “Coupled” may mean one or more of the following. “Coupled” may mean that two or more elements are in direct physical or electrical contact. However, “coupled” may also mean that two or more elements indirectly contact each other, but yet still cooperate or interact with each other, and may mean that one or more other elements are coupled or connected between the elements that are said to be coupled to each other. 
         [0015]      FIG. 1  illustrates a cross-sectional view of a radio frequency (RF) power amplifier (PA) module  100  in accordance with various embodiments. The RF PA module  100  includes an active die  104  and a passive die  108  coupled with a carrier substrate  112 . An active die, as used herein, may refer to a die that includes one or more integrated active components. An active component is a component capable of providing some power gain, such as a transistor. The active components of the active die  104  may form an RF power amplifier  116  that is designed to amplify an RF signal received at an input  118  of the RF PA module  100 . The active die  104  may be realized on, for example, a silicon or gallium arsenide (GaAs) substrate. 
         [0016]    A passive die, as used herein, may refer to a die that strictly includes integrated passive components. A passive component is a reactive component that is not capable of providing a power gain, such as inductors, capacitors, resistors, and/or transmission-line components. The passive components of the passive die  108  may form an impedance matching network  120  that implements at least a majority of impedance matching between the RF power amplifier  116  and an output  122  of the RF PA module  100 . In some embodiments, one or more of the passive components of the impedance matching network  120  may be disposed in the carrier substrate  112 , with the remaining passive components disposed in the passive die  108 . The passive die  108  may be realized on, for example, a low-loss substrate such as high-resistivity silicon, glass, or mechanical GaAs substrate. 
         [0017]    Both the active die  104  and the passive die  108  may be flip-chip coupled with the carrier substrate  112  through an array of metal posts  124  and solder caps  128 . In some embodiments, the metal posts  124  may be copper posts and the solder caps  128  may be tin and/or silver caps. The metal posts  124  and solder caps  128  may mechanically and electrically couple the active die  104  and passive die  108  with the carrier substrate  112 . The carrier substrate  112  may include traces  132  that electrically couple the RF power amplifier  116  with the impedance matching network  120 . The traces  132  may also electrically couple the RF power amplifier  116  with the input  118 ; and the impedance matching network  120  with the output  122 . The carrier substrate  112  may be a laminate carrier, e.g., a printed circuit board (PCB). In some embodiments, the carrier substrate  112  may be one or more lead frames that are attached to another, larger substrate (e.g., a PCB). 
         [0018]    All of the metal posts  124  may have an equal height of at least approximately 50 micrometers (μm), for example. Such a height may provide a desired electrical isolation between the dies, e.g., active die  104  and passive die  108 , and the carrier substrate  112 . Without realizing this desired electrical isolation, the electrical fields of the circuits in the dies may be adversely affected by a ground plane  134  in the carrier substrate  112 . 
         [0019]    A height of at least approximately  50  pm may also facilitate flow of an epoxy and filler particles  136  around and between the metal posts  124 . The epoxy and filler particles  136  may be injected into a mold so that it covers and protects the dies from moisture and/or mechanical stress. If the metal posts  124  are less than approximately  50  μm, flow of the epoxy and filler particles  136  may be restricted between the dies and the carrier substrate  112  due to sizes of the particles within the epoxy and filler particles  136 . 
         [0020]    Implementing the impedance matching network  120  in the passive die  108  and flip-chip coupling both the passive die  108  and the active die  104  to the carrier substrate  112  may provide a number of advantages. One such advantage is the realization of relatively low parasitic resistances in an electrical path from the RF power amplifier  116  to the impedance matching network  120  through the carrier substrate  112 , as compared to a prior art RF PA module. 
         [0021]    A prior art RF PA module may have an active die coupled with an off-die impedance matching network through wire bonds. The wire bonds coupling the active die to the off-die impedance matching network will have variable loop lengths that add parasitic resistance to the electrical paths therebetween and increase manufacturing variability. Due to the low impedance at an output of an RF power amplifier, e.g., 2 ohms, excessive parasitic resistance in the electrical paths is associated with a significant performance cost. 
         [0022]    The flip-chip coupling of the dies in the present disclosure, on the other hand, may be done with very high die-placement accuracy. This high die-placement accuracy, along with the low resistance and inductance of the metal posts  124 , may result in the low parasitic resistance of the electrical paths between the RF power amplifier  116  and the impedance matching network  120 . This, in turn, facilitates implementation of the impedance matching network  120  in the passive die  108 , even with the relatively low output impedance of the RF power amplifier  116  of the active die  104 . Manufacturing yields are also improved by the reduced variability in the assembly process. 
         [0023]    Implementing both inductors and capacitors of the impedance matching network  120  in the passive die  108 , rather than relying on SMDs, may also decrease the need for interconnect paths and mounting pads. This may reduce routing loss and overall footprint of the RF PA module  100 . 
         [0024]    Furthermore, the RF PA module  100 , by avoiding the incorporation of critical magnetic or transmission line structures in the carrier substrate  112 , avoids the significant variability in production and large critical dimensions associated with batch processes. Instead, the integrated passive components of the impedance matching network  120  may be reliably constructed using photolithographically-controlled processes. 
         [0025]    Integrating passive components in the passive die  108  may also provide a significant cost advantage compared to providing passive components in either the active die  104 , the carrier substrate  112 , or as SMDs attached to the surface of the carrier substrate  112 . 
         [0026]    Integrating passive components in the passive die  108  may still further provide a performance advantage due to component variations that track each other on the passive die  108  (e.g., capacitance of all capacitors move in the same direction). This leads to higher yields than if one component is at the high end of its tolerance range and another component is at the low end, which frequently occurs with SMDs. 
         [0027]      FIG. 2  is a top view of the RF PA module  100  in accordance with some embodiments. The RF PA module  100  is shown in  FIG. 2  without the epoxy and filler particles  136 . In addition to the active die  104  and the passive die  108  the RF PA module  100  may include a number of bypass capacitors  204 . The bypass capacitors  204  may be set across power lines and may operate to reduce noise that may be present in a power delivery system. 
         [0028]    While the RF PA module  100  is shown with one RF power amplifier, i.e., RF power amplifier  116 , coupled with one impedance matching network, i.e., impedance matching network  120 , other embodiments may have other numbers of RF power amplifiers and/or impedance matching networks included in an RF PA module.  FIG. 3  illustrates one such example. 
         [0029]      FIG. 3  is a top view of an RF PA module  300  in accordance with some embodiments. The RF PA module  300  may be similar to RF PA module  100 , with like-named components being substantially interchangeable. However, the RF PA module  300  may include two active dies, e.g., active die  304  and active die  308 , and two passive dies, e.g., passive die  312  and passive die  316 . The RF PA module  300  may be a dual-band RF PA module having a first RF power amplifier  320 , implemented in active die  304 , for operation in a first band of frequencies, e.g., a relatively high band of frequencies. The RF PA module  300  may also include a second RF power amplifier  324 , implemented in active die  308 , for operation in a second band of frequencies, e.g., a relatively low band of frequencies. The first RF power amplifier  320  may be electrically coupled with a first input  328 , while the second RF power amplifier  324  may be electrically coupled with a second input  332 . 
         [0030]    The first RF power amplifier  320  may also be electrically coupled with a first impedance matching network  336  implemented in the passive die  312 . Similarly, the second RF power amplifier  324  may also be electrically coupled with a second impedance matching network  340  implemented in the passive die  316 . The first impedance matching network  336  may also be electrically coupled with a first output  344  and the second impedance matching network  340  may also be electrically coupled with a second output  348 . 
         [0031]    The RF PA module  300  may also include one or more bypass capacitors  352 , similar to RF PA module  100 . 
         [0032]    While  FIG. 3  shows that each impedance matching network is implemented in its own passive die other embodiments may include more than one impedance matching network implemented in one passive die. Similarly, while  FIG. 3  shows that each RF power amplifier is implemented in its own active die, other embodiments may include more than one RF power amplifier implemented in one active die. 
         [0033]    In some embodiments, the architecture of the impedance matching network may be selected in a manner to facilitate implementation through use of integrated passive components on a passive die. For example, a lattice matching network may provide a compact architecture that is particularly suitable for implementation on a passive die. 
         [0034]      FIG. 4  is a circuit diagram of an RF PA module  400  in accordance with various embodiments. The RF PA module  400  may be similar to, and substantially interchangeable with, RF PA module  100  and/or RF PA module  300 . The RF PA module  400  includes a quadrature RF power amplifier  404  having a first PA  408  and a second PA  412  operating in quadrature, i.e., with a 90 degree phase delta. The first PA  408  and the second PA  412  may be implemented in an active die  416 . 
         [0035]    The RF PA module  400  may also include a quadrature lattice matching network  420  electrically coupled with the quadrature RF power amplifier  404 . The quadrature lattice matching network  420  may be implemented in a passive die  422  and may provide quadrature phase combining and impedance matching in a three-port reactive network. The quadrature lattice matching network  420  may include a first path  424 , having a series inductor  428  and a shunt inductor  432 , and a second path  436  having a series capacitor  440  and a shunt capacitor  444 . The outputs of the two parallel paths  424  and  436  may be combined to a single-ended output at the output node  448  as illustrated. Resistor  452  may represent an output load. The compact architecture of the quadrature lattice matching network  420  may be amenable to full implementation on the passive die  422  while still providing a number of desirable impedance matching characteristics such as load-insensitivity, low insertion loss, low cost, and reduced voltage standing wave ratio (VSWR) on the output node  448 . 
         [0036]    While  FIG. 4  shows an architecture of a lattice matching network that may be particularly effective in an embodiment of this disclosure, i.e., quadrature lattice matching network  420 , other embodiments may use other lattice matching networks, such as any of those shown and described in U.S. patent application Ser. No. 13/070,424, titled “QUADRATURE LATTICE MATCHING NETWORK,” filed Mar. 23, 2011, which is hereby incorporated by reference in its entirety. In other embodiments, impedance matching networks other than lattice matching networks may be employed. 
         [0037]      FIG. 5  is a flowchart  500  depicting a process of assembling an RF PA module in accordance with various embodiments. In block  504 , “Attaching metal posts to the semiconductor wafer,” the assembly process may involve attaching metal posts to the semiconductor wafer. 
         [0038]    Attachment of one component to another component, as used herein, could be achieved by any of a number of possible microfabrication processes. A particular microfabrication process may be selected in light of the materials to be attached and other process variables. Such microfabrication processes may involve techniques such as, but not limited to, deposition (or growth), patterning, and etching. 
         [0039]    At block  508 , “Attaching solder caps to metal posts,” the assembly process may involve attaching a solder cap to each of the metal posts. 
         [0040]    At block  512 , “Thinning the semiconductor wafer,” the assembly process may involve reducing the thickness of the semiconductor wafer. Prior to block  512 , it may be desirable for the semiconductor wafer to have a certain thickness to increase mechanical stability and avoid warping during high temperature process steps. In some embodiments, the thickness of the silicon wafer may be approximately 750 μm for these process steps. However, the dimensions of the final package may be substantially smaller and the thickness of the semiconductor wafer may, therefore, be reduced at block  512 . In some embodiments, the thickness of the semiconductor wafer may be reduced to less than 250 μm. 
         [0041]    At block  516 , “Separating the dies from the semiconductor wafer,” the assembly process may involve separation of the dies, which may include active and/or passive dies, from the semiconductor wafer. In some embodiments, the semiconductor wafer may be mounted on a dicing tape that has a sticky backing to hold the dies in place once separated. The separating of the dies may be performed by scribing and breaking, dicing with a dicing saw, or cutting with a laser. 
         [0042]    At block  520 , “Flip-chip coupling the dies with carrier substrate,” the assembly process may involve flip-chip coupling of the dies, i.e., an active and a passive die, with the carrier substrate. The dies, with metal posts and solder caps attached, may be placed in the appropriate position on the substrate carrier. Placement of the dies may be tightly controlled with very high accuracy. As discussed above, the accurate placement of the dies may contribute to increased performance of the RF PA module as compared to the prior art RF PA modules that rely on wire bonding and/or SMDs. 
         [0043]    Once the dies are placed, the carrier substrate and the dies may be heated to a temperature that is at least a reflow temperature associated with the solder caps and less than a reflow temperature associated with the metal posts. The solder caps will then reflow to mechanically and electrically couple the dies with the carrier substrate. 
         [0044]    At block  524 , “Overmolding the attached dies,” one or more molds may be placed over the dies and epoxy and filler particles may be inserted into the mold(s). The epoxy may cure and the mold(s) may be removed. As discussed above, the cured epoxy may serve to protect the dies on the carrier substrate from moisture and mechanical stress. 
         [0045]    A block diagram of an exemplary wireless communication device  600  incorporating an RF PA module  604 , which may be similar to RF PA modules  100 ,  300 , and/or  400 , is illustrated in  FIG. 6  in accordance with some embodiments. In addition to the RF PA module  604 , the wireless communication device  600  may have an antenna structure  614 , a duplexer  618 , a transceiver  622 , a main processor  626 , and a memory  630  coupled with each other at least as shown. While the wireless communication device  600  is shown with transmitting and receiving capabilities, other embodiments may include devices with only transmitting or only receiving capabilities. 
         [0046]    In various embodiments, the wireless communication device  600  may be, but is not limited to, a mobile telephone, a paging device, a personal digital assistant, a text-messaging device, a portable computer, a desktop computer, a base station, a subscriber station, an access point, a radar, a satellite communication device, or any other device capable of wirelessly transmitting/receiving RF signals. 
         [0047]    The main processor  626  may execute a basic operating system program, stored in the memory  630 , in order to control the overall operation of the wireless communication device  600 . For example, the main processor  626  may control the reception of signals and the transmission of signals by transceiver  622 . The main processor  626  may be capable of executing other processes and programs resident in the memory  630  and may move data into or out of memory  630 , as desired by an executing process. 
         [0048]    The transceiver  622  may receive outgoing data (e.g., voice data, web data, e-mail, signaling data, etc.) from the main processor  626 , may generate the RF in  signal(s) to represent the outgoing data, and provide the RF in  signal(s) to the RF PA module  604 . The transceiver  622  may also control the RF PA module  604  to operate in selected bands and in either full-power or backoff-power modes. 
         [0049]    The RF PA module  604  may amplify the RF in  signal(s) to provide RF out  signal(s) as described herein. The RF out  signal(s) may be forwarded to the duplexer  618  and then to the antenna structure  614  for an over-the-air (OTA) transmission. 
         [0050]    In a similar manner, the transceiver  622  may receive an incoming OTA signal from the antenna structure  614  through the duplexer  618 . The transceiver  622  may process and send the incoming signal to the main processor  626  for further processing. 
         [0051]    In various embodiments, the antenna structure  614  may include one or more directional and/or omnidirectional antennas, including, e.g., a dipole antenna, a monopole antenna, a patch antenna, a loop antenna, a microstrip antenna or any other type of antenna suitable for OTA transmission/reception of RF signals. 
         [0052]    Those skilled in the art will recognize that the wireless communication device  600  is given by way of example and that, for simplicity and clarity, only so much of the construction and operation of the wireless communication device  600  as is necessary for an understanding of the embodiments is shown and described. Various embodiments contemplate any suitable component or combination of components performing any suitable tasks in association with wireless communication device  600 , according to particular needs. Moreover, it is understood that the wireless communication device  600  should not be construed to limit the types of devices in which embodiments may be implemented. 
         [0053]    Although the present disclosure has been described in terms of the above-illustrated embodiments, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations calculated to achieve the same purposes may be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. Those with skill in the art will readily appreciate that the teachings of the present disclosure may be implemented in a wide variety of embodiments. This description is intended to be regarded as illustrative instead of restrictive.