Abstract:
An integrated circuit package is described that includes a capacitor structure having a pair of plates separated by a dielectric material. An integrated circuit (IC) die is carried by a top surface of the first capacitor plate. The die carried by the capacitor structure is electrically connected to a multiplicity of contacts. A protective encapsulant covers the die and a portion of the capacitor structure while leaving a surface of the second capacitor plate at least partially exposed. In some embodiments, one of the capacitor plates (typically the lower capacitor plate) is formed from the same lead frame sheet as the contacts. In LLP implementations, the lower capacitor plate is substantially co-planar with the contacts. Depending on the implementation the capacitor structure can be electrical connected in a variety of different manners. One or both of the plates can be electrically connected to either (or both of) selected bond pads on the die or selected leads or contacts.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to integrated circuit (IC) packages. More particularly, the invention relates to IC packages with embedded capacitive components. 
     BACKGROUND OF THE INVENTION 
     In many types of circuits, it is desirable to provide discrete components such as capacitors and inductors in combination with integrated circuits. In some circumstances, efforts have been made to incorporate a discrete component, such as a capacitor, into the package that protects the integrated circuit. By way of example, U.S. Pat. No. 6,091,144 describes a package which has a capacitor structure formed on a die pad. Other packages with integrated capacitor structures include U.S. Pat. Nos. 5,498,901 and 5,629,559. These capacitors are typically used to reduce power supply noise delivered to the integrated circuit. Although such structures likely work well, there are continuing efforts to improve the manufacturability of the packages and to improve their electrical performance. 
     One relatively recently developed package is a leadless leadframe style package (LLP). A LLP is a type of surface mounted integrated circuit package that uses a metal (typically copper) leadframe substrate to form a chip scale package (CSP). As illustrated in FIGS. 1 a, b , and  c , in known leadless leadframe packages, a copper leadframe strip or panel  101  is patterned, typically by stamping or etching, to define a plurality of arrays  103  of device areas  105 . Each device area  105  includes a die attach pad  107  and a plurality of contacts  109  disposed about associated die attach pad  107 . Very fine tie bars  111  are used to support the die attach pads  107  and contacts  109  during manufacturing. 
     During assembly, IC dice are attached to respective die attach pads  107  and conventional wire bonding is used to electrically couple bond pads on each die to associated contacts  109  within the same device area  105 . After the wire bonding, a plastic cap is molded over the top surface of each device area  105  or over the entire array  103 . The capped dice are then cut from the array and tested using known sawing and testing techniques. 
     FIG. 2 provides a cross-section of a known LLP. Die attach pad  107  supports die  120 , often attached by a non-conductive resin  160 . Die  120  is electrically connected to its associated contacts  109  by bonding wires  122 . A molded plastic cap  125  encapsulates die  120  and bonding wires  122  and fills the gaps between die attach pad  107  and contacts  109 , holding the contacts in place. During singulation, tie bars  111  are cut. The resulting packaged chip can then be surface mounted on a printed circuit board (PCB) or other substrate using known techniques. Since LLPs are growing in popularity, it would be desirable to provide simple techniques for integrating capacitor structures into such packages. It would also be desirable if the improved techniques are also applicable to some other package designs. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing and other objects of the invention, an integrated circuit package is described that includes a capacitor structure having a pair of plates separated by a dielectric material. An integrated circuit (IC) die is carried by a top surface of the first capacitor plate. The die carried by the capacitor structure is electrically connected to a multiplicity of contacts. A protective encapsulant covers the die and a portion of the capacitor structure while leaving a surface of the second capacitor plate at least partially exposed. 
     In some embodiments, one of the capacitor plates (typically the lower capacitor plate) is formed from the same lead frame sheet as the contacts. In LLP implementations, the lower capacitor plate is substantially co-planar with the contacts. Depending on the implementation the capacitor structure can be electrically connected in a variety of different manners. One or both of the plates can be electrically connected to either (or both of) selected bond pads on the die or selected leads or contacts. 
     Other features, advantages, and objects of the present invention will become more apparent and be more readily understood from the following detailed description, which should be read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
     FIG. 1 a  is a diagrammatic top view of a known lead frame strip suitable for use in forming LLPs, with FIGS. 1 b  and  1   c  showing successively more detailed views of the elements in the strip. FIG. 1 c  is a single LLP element consisting of a substrate and contacts; 
     FIG. 2 is a diagrammatic cross sectional side view of a known LLP; 
     FIGS. 3 a  and  3   b  depict, respectively, a cross-section and top view of a first embodiment of the present invention wherein the package lead contacts are coplanar with an exposed capacitor forming plate; 
     FIG. 4 is a top view of a second embodiment of the present invention with alternative connections that use the capacitor plates as an internal power bus; 
     FIG. 5 is a top view of an alternative embodiment of the present invention with yet other alternative connections where the capacitor may be used for circuits in the IC die; 
     FIG. 6 illustrates process steps suitable to form an LLP embodiment of the present invention; 
     FIG. 7 a  is a diagrammatic top view of a lead frame strip suitable for use in forming the leadless packages of FIGS.  3 ( a ) &amp; ( b ). FIGS. 7 b  and  7   c  show successively more detailed views of the elements in the strip. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One of the goals of the present invention is to provide an inexpensive structural capacitor that can be incorporated into an integrated circuit (IC) package. FIG. 3 a  illustrates a side cross-section of a first embodiment of the present invention, and FIG. 3 b  the respective top view of the first embodiment. Referring to FIG. 3 a , in the first described embodiment, a capacitor  121  is formed in a leadless leadframe style package (LLP)  100  that is mounted on a printed circuit board (PCB)  390 . Capacitor  121  is formed by sandwiching a dielectric material  380  between a die attach plate  308  and a substrate plate  107 . Die attach plate  308  can optionally be made slightly smaller than substrate plate  107  such that down-bonding to the top surface of substrate plate  107  is possible. Substrate plate  107  has a bottom surface that is exposed outside the package, and is available for electrical connection directly to the PCB. As will be appreciated by those familiar with capacitor design, the capacitance of the capacitor structure can be designed to a desired value within a relatively broad range by varying such factors as the distance between the plates, the choice and thickness of the dielectric material, and the size and/or configuration of the plates. 
     A wide variety of materials can be used to form dielectric film  380 ; for example, adhesive tapes such as polyimide or epoxy resins can be used. Known dielectrics provide the desired characteristics of good thermal conductivity, uniform thickness, and rigid adherence to plates  107  and  308 . An IC die  120  may be secured directly or indirectly to die attachment plate  308  using any suitable die attachment technique. By way of example, a wide variety of suitable die attachment adhesives  160  may be used to secure the die  120  to the attachment plate  308 . One suitable adhesive is an electrically non-conductive epoxy resin. Alternatively, if desirable for a particular application (such as when a contact is formed on the back surface of the die), an electrically conductive adhesive or solder could be used. Substrate plate  107  and coplanar contacts  109  are suitable for surface mounting on the PCB  390 . When mounting the package  100 , PCB pads  330  are electrically bonded to IC package contacts  109  thereby electrically connecting PCB circuits to associated contacts  109 . The contacts  109 , in turn are electrically connected to die terminals (bond pads)  340  on the die  120  and/or one of the capacitor plates by conventional techniques such as wire bonding using bonding wires  122 . 
     The substrate plate  107  is physically attached and electrically connected to PCB substrate pad  331  by conventional techniques such as soldering or adhesive bonding with an electrically conductive adhesive. When the package&#39;s substrate plate  107  is intended to be grounded, the PCB substrate pad  331  is electrically (and thermally) connected to the PCB&#39;s ground plane  317  using conventional PCB techniques. For example, when the PCB is a multi-layered printed circuit board, PCB vias  332  may be provided to electrically connect the substrate pad to the ground plane  317 . With this arrangement, vias  332  dissipate heat into ground plane  317 . In alternative embodiments where the package&#39;s substrate plate  107  is intended to have an electrical potential, the substrate pad  331  may be electrically connected to a power plane. 
     System designers may improve the LLP&#39;s electrical and thermal performance by increasing the number of vias  332  to decrease the thermal resistance and electrical impedance path between substrate plate  107  to the PCB power or ground plane  317 . For example, in current designs, each via  332  typically has only about 0.7 nH of inductance. Thus, a large number of via  332  connections acting in parallel, can reduce the effective impedance to the power or ground plane  317  to negligible levels. This multitude of contacts additionally provides for a beneficial increase in the thermal dissipation from die  120  into PCB  390 . By way of example, in this LLP embodiment of the present invention, vertical thermal conductance J C  is roughly 1.6° C./W, as compared to about 1° C./W in prior-art. A potentially more significant contributor to thermal resistance is capacitor dielectric  380 . It should be appropriately chosen by the designer to include thermal conductance properties necessary to meet the required J C  for the application. Alternatively, instead of attaching part of the exposed surface of substrate plate  107  to solid PCB substrate pad  331 , the system designer may choose to eliminate substrate pad  331 , and directly attach the exposed surface of substrate plate  107  to grounded vias  332  that protrude through the PCB surface. It should be appreciated that this could significantly decrease electrical and thermal performance. This alternative implementation may be desired if the decreased electrical and thermal performance is acceptable, and the area below substrate plate  107  is needed for routing PCB traces. 
     The discrete capacitor  121  can be used in many different ways including functioning as a power supply filter, or as a capacitor available for use by circuits internal (or possibly even external) to IC package  100 . Because capacitor  121  is very close to die  120 , it has significantly lower parasitic components than does an externally placed capacitor. When used as a power supply filter, the lower parasitic components of capacitor  121  enables it to better reduce power supply noise than do capacitors placed further away from the die. Additionally, if circuits in die  120  require a capacitor that is too large to form in the die, system designers can significantly improve performance and reduce cost by connecting capacitor  121  to circuits in die  120 . 
     Still referring to FIG. 3 a , a cross-sectional view of the connections to capacitor  121  is shown. The die attach plate  308 , which also is the upper capacitor plate, can be connected to die terminals  340  by suitable techniques such as down-bonding with bond wire  351 . This plate can also be connected to IC package contact  109  by bond wire  371  for external PCB connection. 
     A top view of FIG. 3 a  is illustrated in FIG. 3 b . In this application, the plates of capacitor  121  are coupled to opposite power supply polarities to reduce noise in power supplied to die  120 . By way of example, die attach plate  308  is charged to positive supply potential  370  via bond wire  371 , and substrate plate  107  is grounded through the PCB substrate pad (shown in FIG. 3 a ). Alternatively, the polarities of capacitor plates  308  and  107  could be reversed. The power connection to die  120  is completed by wire  351  from die attach plate  308  to die terminal  340 . Thus, a power connection from die  120  to external power source  370  is made. Usually, the current practice is to make an IC power supply connection by bonding wire  372  directly from die terminal  340  to the corresponding IC package contact  109 . This practice can result in longer bond wire connections than if the capacitor plate is used as an intermediate path. It should be appreciated that the effective bond wire parasitic length of traditional power connection  372 , made from the die to the IC package contact, can be reduced because die attach plate  308  has less impedance than a bond wire. Moreover, a parallel multiplicity of connections to die attach plate  308 , either from IC contacts  109  or from the die terminals  340 , can further decrease power supply parasitic components. 
     In a second embodiment, illustrated in FIG. 4, substrate plate  107  or die attach plate  308  provide a low impedance PCB power supply or ground connection to die  120  by down-bonding bond wires  350  and  365  from die  120  to die attach plate  308  and substrate plate  107 , respectively. In this mode, in addition to filtering power supply noise, both die attach plate  308  and substrate plate  107  act as an internal power bus for die  120 . In this embodiment, bond wire parasitic components are significantly reduced in two ways. First, the down-bonded bond wire length is shorter than the wire that must be connected to package contacts  109  for power; resulting in reduced electrical impedance to the ground or power plane  317 . In current technology, the longer bond wires  372  that connect directly from the die terminal  340  to package power contact  373  have about 3 nH of inductance, and down-bonded bond wires, such as  365  and  350 , only about 1 nH; a one third reduction in overall parasitic impedance. Second, to get power to die  120  a multiplicity of additional die-to-plate down-bonded connections, such as  362  or  367 , are possible without requiring additional package power contacts  373 . In this case, the effective package parasitic component is equal to the individual down-bonded bond wire impedance values taken in parallel. For example, if bond wires  365  and  367  were both down-bonded from die terminal  340  to substrate plate  107 , their effective impedance to ground would be reduced by one half. In many current implementations, each connection between die bond pad  340  to a power source requires a separate package power contact  373  that is connected externally to power source  370 . 
     FIG. 5 illustrates another alternative embodiment of the present invention. In this embodiment, the capacitor is connected to circuits in the IC die. To connect the capacitor to circuits internal to die  120 , die terminals  340  are directly down-bonded to the capacitor plates as exemplified by wires  361  and  367 . It should be appreciated that the capacitor can instead be used externally, whereby the capacitor plates would be bonded to the IC package contacts for external connection. 
     The capacitor formed by each of the foregoing described structures has several degrees of freedom to adjust its effective capacitance value to a desired target. For example, the designer can reduce or increase the size of the plates to proportionally change the capacitance value of the integrated capacitor. Other variables to adjust in the parallel plate capacitor equation are the dielectric thickness and dielectric constant k. For current LLP configurations, a capacitance value of at least 0.001 μF is achievable and desirable. 
     In the embodiments described above, the lower capacitor plate (substrate plate  107 ), is formed from the same sheet (lead frame) as the contacts  109 . However, it should be appreciated that in alternative embodiments, the upper capacitor plate may be a die attach pad formed from the lead frame. In leadless configurations, the contacts may be down-set relative to the die attach pad in order to permit the exposed lower capacitor plate to be substantially co-planar with the contacts. Alternatively, the exposed lower capacitor plate may be arranged to protrude below the surface of the contacts. 
     The principles of the present invention may also be applied to leaded package configurations having an exposed lower capacitor plate. By way of example, in a leaded configuration, a metal slug may be used as the exposed lower capacitor plate. The leads and the upper capacitor plate may then be formed from the lead frame. Such leaded packages may take any appropriate packaging configuration including QFP (quad flat packages), DIP (dual in-line packages), etc. Like the previously described embodiments, the capacitor structures in these devices may be wired in any suitable manner. 
     It should be appreciated that the described lead frame based capacitor structures can be created in strip or panel form and later singulated. One suitable panel based manufacturing approach suitable for forming the packages illustrated in FIG. 3 will be briefly described with reference to FIGS. 6 and 7. In this embodiment, a lead frame panel is patterned in a conventional manner as described above with reference to FIGS. 1 a-c . (Step  705 ). The lead frame panel includes a plurality of two dimensional arrays of device areas that each include a substrate plate  107  and a plurality of contacts  109 . A second metal sheet is patterned as a capacitor plate sheet that has a plurality of two dimensional arrays of die attach plates thereon. The die attach plates  308  are sized and positioned such that they will align with the substrate plates  107  on the lead frame panel. (Step  707 ). In Step  715  the dielectric is applied, by any appropriate technique, to one of the capacitor plates. By way of example, the dielectric could be a resin screened on by a mask, or a stamped adhesive tape. The capacitor plate sheet is then attached to the lead frame in step  720  with the die attach plates  308  aligned over their associated substrate plates  107 , thus forming several arrays of capacitors. 
     After the capacitor structures have been formed, dice may be attached to the respective die attach plates (step  725 ) using conventional die attach techniques. Once the dice are in place they are electrically connected to the contacts  109  and the capacitor plates (step  730 ) using conventional techniques such as wire bonding. As discussed above, the electrical connections of the capacitor may vary significantly based on the particular design. 
     After the dice have been electrically connected, a cap is molded over each array. Step  735 . In alternative embodiments, the dice can be individually molded. Thereafter the dice may be tested and singulated in step  740  and  745  respectively. It should be appreciated that the described steps are exemplary and that in several circumstances, the order of the various steps may be changed, some steps eliminated or combined and others added. 
     Although only a few embodiments of the present invention has been described in detail, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, the exposed lead frame based capacitor structure can be applied to a variety of designs other than LLPs. For example, leaded packages such a quad flat packs (QFPs) and dual inline packages (DIPs) may incorporate the described capacitor structure. The materials used to form the capacitors as well as the sizes and configurations of the plates may be widely varied. In the illustrated embodiments, the die attach platform has been shown as having a smaller footprint than the substrate plate  107 . This permits wire bonding to both capacitor plates (assuming the die attach platform is also sufficiently larger than the die). However, when there is no need to wire bond to the lower capacitor plate, the plates may be the same size, or if desired, the exposed plate may even have a smaller footprint. Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.