Patent Document

TECHNICAL FIELD 
       [0001]    The invention relates generally to an integrated circuit (IC) package and, more particularly, to a resonance matching IC package. 
       BACKGROUND 
       [0002]    Turning to  FIGS. 1-3 , an example of packaged IC  100  with an air gap package is shown. In this example, the IC  102  has an active region  306  formed with an epitaxial or epi layer  304  that is formed over an IC substrate  302 . The active region  306  can include active elements and bond pads  308 . For example, a laterally diffused MOS (LDMOS) transistor can be formed in epi layer  304  with bond pads  308  being coupled to the gate and drain of the LDMOS transistor. This IC  102  is then secured to a package housing. In this example, the package housing includes a source flange  104 , a package substrate  106 , a leadframe (which can generally comprise the drain flange  116  and gate flange  110  and which can be electrically isolated from the source flange  104  by way of package substrate  106 ), and lid  114 . For example, the source flange  104  can be electrically coupled to the source of an LDMOS transistor formed in the active region  306  of the IC  102  by securing the IC to the source flange  104 , while the drain flange  116  and gate flange  110  can be electrically coupled to the drain and gate of an LDMOS transistor formed in the active region  306  of the IC  102  by bond wires  108 . Alternatively, the leadframe (e.g., the drain flange  116  and gate flange  110 ) can extend over the IC  102  and be secured to the IC  102  with solder bumps. The bond wires  108  or solder bumps can also be generally referred to bonding fixtures. The lid  114  can then be secured to the leadframe (e.g., the drain flange  116  and gate flange  110 ) with a bond  112  (e.g., epoxy) so as to allow an air cavity to be formed over IC  102 . 
         [0003]    The air gap package used with packaged IC  100  provides a mechanically stable package with very good performance characteristics, but there are some drawbacks. The biggest drawback being the cost. Therefore, there is a need for a lower cost package with comparable performance characteristics and mechanical stability. 
       SUMMARY 
       [0004]    In accordance with an embodiment of the present invention, an apparatus is provided. The apparatus comprises a package housing; an integrated circuit (IC) that is secured to the package housing, wherein the IC has: an IC substrate; an epitaxial layer formed over the substrate and having an active region and an upper surface, wherein the upper surface is substantially exposed; and a plurality of bond pads formed over the epitaxial layer; a plurality of bond fixtures, wherein each bond fixture is secured to and in electrical contact with at least one of the bond pads and with the package housing; and a fill formed over at least a portion of the epitaxial layer so as to substantially encapsulate the active region, wherein the fill has a dielectric constant that is substantially equivalent to the dielectric constant of air, and wherein the fill has a thickness, and wherein the thickness is sufficiently large enough to confine parasitics of the active region at the upper surface of the epitaxial layer. 
         [0005]    In accordance with an embodiment of the present invention, the package housing further comprises a leadframe that is secured to and in electrical contact with the plurality of bond fixtures. 
         [0006]    In accordance with an embodiment of the present invention, the leadframe extends over at least a portion of the IC, and wherein the plurality of bond fixtures further comprises a plurality of solder bumps. 
         [0007]    In accordance with an embodiment of the present invention, the plurality of bond fixtures further comprises a plurality of bond wires. 
         [0008]    In accordance with an embodiment of the present invention, the package housing further comprises: a flange that is secured to and in electrical contact with the IC; and a package substrate that is secured to the flange and the leadframe. 
         [0009]    In accordance with an embodiment of the present invention, the flange further comprises a first flange, and wherein the leadframe further comprises: a second flange that is electrically coupled to at least one bond pad; and a third flange that is electrically coupled to at least one of the bond pads. 
         [0010]    In accordance with an embodiment of the present invention, the active region further comprises a laterally diffused MOS (LDMOS) transistor that is coupled to the first flange at its source, the second flange at its drain, and the third flange at its gate. 
         [0011]    In accordance with an embodiment of the present invention, a molding compound is applied to encapsulate the fill. 
         [0012]    In accordance with an embodiment of the present invention, an apparatus is provided. The apparatus comprises a package housing having: a first flange; a package substrate secured to the first flange; a second flange secured to the package substrate; and a third flange secured to the package substrate; an IC having: an IC substrate that is secured to the first flange; an epitaxial layer formed over the substrate and having an LDMOS transistor in active region and an upper surface, wherein the upper surface is substantially exposed, and wherein the source of the LDMOS transistor is coupled to the first flange; and a first set of bond pads formed over the epitaxial layer, wherein the first set of bond pads are coupled to the drain of the LDMOS transistor; a second set of bond pads formed over the epitaxial layer, wherein the second set of bond pads are coupled to the drain of the LDMOS transistor; a first set of bond wires, wherein each bond wire from the first set of bond wires is secured to and in electrical contact with at least one of the bond pads from the first second of bond pad and with the second flange; a second set of bond wires, wherein each bond wire from the second set of bond wires is secured to and in electrical contact with at least one of the bond pads from the second of bond pads and with the third flange; and a fill formed over at least a portion of the epitaxial layer so as to substantially encapsulate the active region, wherein the fill has a dielectric constant that is substantially equivalent to the dielectric constant of air, and wherein the fill has a thickness, and wherein the thickness is sufficiently large enough to confine parasitics of the active region at the upper surface of the epitaxial layer. 
         [0013]    In accordance with an embodiment of the present invention, the thickness is greater than 10 μm. 
         [0014]    In accordance with an embodiment of the present invention, a molding compound is applied to encapsulate the fill. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0016]      FIG. 1  is a cross-sectional view of packaged IC that employs an air gap package; 
           [0017]      FIG. 2  is a isometric view of the packaged IC of  FIG. 1  with the lid removed; 
           [0018]      FIG. 3  is a cross-sectional view of the IC of  FIG. 1 ; 
           [0019]      FIG. 4  is a cross-sectional view of a packaged IC employing a lower cost package; and 
           [0020]      FIGS. 5-16  are charts illustrating performance characteristics of the packaged ICs of 
           [0021]      FIGS. 1 and 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
         [0023]    When looking to migrate from an air gap package (e.g., packaged IC  100 ), conventional thought is generally insufficient. Some lower cost packages would be an epoxy mold compound (EMC) package and a green mold compound package, where, in each case, a mold compound is deposited directly onto the IC (e.g., IC  102 ), but conventional thought, though, would dismiss these types of packages as a replacement for air gap packages because empirical evidence shows significant performance degradation. 
         [0024]    One reason for this is that most of literature related to EMC packages centers on the reliability, thermal, and mechanical performance. Almost no data can be found regarding the effects of mold compounds on the electrical performance of microelectronic devices. In other words, conventional thought ignored the impact of a mold compound on the electrical or electromagnetic performance. While this may appear to be intuitive, it is not, because most ICs do not include active components or an active region (e.g., active region  306 ) at the surface of an IC. This type of assembly is relatively uncommon, and techniques that are suitable for other conventional applications (e.g., microprocessors) may not be applicable for ICs that include active components or an active region (e.g., active region  306 ) at the surface. 
         [0025]    For ICs that include active components or an active region (e.g., active region  306 ) at the surface, it can be said that when an active or passive device is encapsulated with a mold compound, the electromagnetic fields in the volume around the die and interconnects that carry the signals in and out the package are affected by the change of the dielectric constant (ε) and dissipation factor (tan δ) of the mold compound, namely for high frequency applications. Typical values of dielectric constant for mold compounds are around  4 , and the dissipation factor is typically between about 0.001 and about 0.01. This means that, compared to air cavity packages (e.g., packaged IC  100 ), the electrical performance can be affected, mainly, in two aspects. The first aspect is related to the high dielectric constant of the mold compound that can increase the capacitive coupling between the structures in the surface of the die and also between the interconnects. The second aspect is related to an increment of losses due to the increase in the dissipation factor caused by the mold compound. This effect can cause degradation in the efficiency of the devices and generation of heat that should be dissipated in order to keep the temperature under specifications. 
         [0026]    Turning to  FIG. 1 , an example of a packaged IC  200  with a lower cost package can be seen. In this example, IC  402  is similar to IC  102  in construction in that it includes an active region or active components at its surface and will typically include an epi layer formed over a substrate. For example, IC  402  can be an LDMOS high power high frequency transistor that can be used in the power amplifiers for the wireless infrastructure. This type of example device is able to deliver 90W of continuous RF power at frequencies of 2 GHz, and conventional packages are air cavity type (i.e., as shown in the example of  FIG. 1 ). In this example and similar to packaged IC  100 , there can be two dies (e.g., IC  402 ) coupled in parallel with wire bonds (e.g.,  108 ) to the flanges (e.g.,  110  and  116 ). The dies (e.g., IC  402 ) can have an area of about 5 mm 2  with a thickness of about 50 μm, and the package can be made of a copper-tungsten (CuW) alloy that can be designed to match the coefficient of thermal expansion of a Alumina cover (e.g., lid). The metal base (e.g.,  104 ) can be plated with Au to be able to use an gold silicide (AuSi) eutectic die attach process. 
         [0027]    The difference between packaged ICs  100  and  400  lies in the “cover.” With the example packaged IC  400 , the lid  104  has been replaced with a fill  406  formed over the IC  402  and a mold compound  404  formed over the fill  406 . Alternatively, the mold compound  404  can be omitted and a thicker fill  406  can be applied to the region illustrated as the mold compound  404 . Typically, the fill  406  should encapsulate the upper or top surface of the IC  402  and be of sufficient thickness (e.g., greater than about 10 μm) to confine electromagnetic fields at the surface of the IC  402 . 
         [0028]    A reason for using fill  406  to confine electromagnetic fields at the surface of IC  402  relates to the change in the output resonant frequency of the parts imparted by the fill  406  itself Because IC  402  includes an active region at the surface, the parasitic capacitances and resistances and the inductance of the wires can form a resistor-inductor-capacitor (RLC) circuit. In order to maximize the transference of RF power to the load, the resonance frequency of the equivalent RLC circuit of the IC  402  (e.g., LDMOS transistor) should be matched to the specified frequency of operation of the amplifier. By filling the air cavity of the package with a mold compound  404 , for example, the dielectric constant of the cavity increases from 1 (e.g., dry air) to approximately 4. The increase of the dielectric constant of the cavity affects the previously described RLC equivalent circuit in two ways. The first is due to an increase of the parasitic capacitances because the active devices (e.g., in the active region of IC  402 ) at the surface coupled through a media of a higher dielectric constant. For the second way, the fill  406  affects the resonant frequency due to the increased capacitive coupling between wire bonds. 
         [0029]    In order to account for the effect of the fill  406  on the radio frequency (RF), samples were prepared (labeled Groups A and B in  FIG. 5 ), with both groups having substantially the same output resonant frequency. Group A functioned as the control group, which employed an air cavity similar to that shown in  FIG. 1 . Group B included the wire bond length adjustments to obtain substantially the same resonant frequency as Group A (air cavity packages). The reduction in length of the wires reduces the inductance value of the RLC circuit and compensates for the increase of the parasitic capacitance explained above. The parasitic resistance, as shown in this example, is minimally affected and the final output resonant frequency of Group B, after application of fill  406 , matches the output resonant frequency of Group A. 
         [0030]    Once the output resonance of both Groups A and B have been substantially matched, a relatively accurate comparison of the performance can be accomplished by employing continuous wave RF measurements. In  FIG. 6 , the results of the measurement of the P1dB versus frequency for the center and edges of the Wideband Code Division Multiple Access (WCDMA) band. The one decibel compression point P1dB is generally defined as the power where the curve output RF power versus input RF power falls one decibel below the asymptotic linear characteristic. This parameter can be important in that it can define the maximum power a RF transistor can deliver at the linear regime. The effect of the fill  406  on P1dB is evident from the data that shows a clear deterioration of 6.5% in the maximum linear power the LDMOS transistor can deliver for the example shown in  FIG. 6 . 
         [0031]    In  FIG. 7 , an example of a continuous wave measurement of maximum gain versus frequency for Groups A and B measured at the center and edges of WCDMA band can be seen. This parameter can be important as it measures the maximum gain the transistor can deliver in the linear regime. The curves show a worsening of 0.35 dB in maximum gain for the group that has a fill  406 . 
         [0032]      FIG. 8  shows an example of two typical curves of gain versus power out. This comparison shows that the fill  406  degrades the gain performance, not only at the peak of the curve (as shown in the example of  FIG. 7 ), but in the entire range of output power. From the curves, it can be seen that there is a generally constant gain reduction of approximately 0.35 dB for the linear range. It can also be observed that the gain curve of the device with fill  406  falls in the nonlinear region at lower output power confirming that P1dB can be seriously affected. 
         [0033]    In  FIG. 9 , an example diagram of the drain efficiency versus RF output power can be seen. For output power up to 48 dBm in this example, it can be observed that there is no difference between the devices with fill  206  and the air cavity control parts. However, for output power above 48 dBm, there is a clear degradation of the drain efficiency. This degradation is likely due to loss tangent of material used for fill  406 . 
         [0034]    In order to understand the effects of fill  406  on the performance of the RF transistor while operating in a real application, a measurement employing both groups of transistors exciting them with a WCDMA modulation standard was made.  FIG. 10  shows an example of the third order modulation distortion (IMD 3 ) for control group A and group B as a function of frequency. IMD 3  is generally defined as the ratio of the power in one of the third-order tones to that in one of the main tones. As one can observe, the fill  406  can increase the adjacent channel power degrading the linearity of the transistor. Additionally,  FIG. 11  shows an example of two typical curves of IMD 3  versus power out. In this plot, a degradation in the linearity of the devices due to the fill  406  can also be seen. 
         [0035]    In the load pull measurements, the impedance seen can be varied by the output of the transistor to other than 50Ω in order to measure performance parameters. In the case of power transistors, a load pull power bench is used to evaluate large signal parameters such as compression characteristics, saturated power, efficiency and linearity as the output load is varied across the Smith chart.  FIG. 12  shows an example of the load pull results obtained for P1dB as a function of frequency. It can be seen that there is a degradation of P1dB in the order of 5 to 6% when the fill  406  is employed.  FIG. 13  shows an example of the results of the maximum gain as a function of frequency for Groups A and B measured using a load pull setup at the center and edges of the WCDMA band frequencies. 
         [0036]    As it was explained above, when fill  406  is used, the change of the dielectric constant above the surface of IC  402  can change the capacitive coupling between different structures of the device (e.g., gate, drain, and source of an LDMOS transistor).  FIGS. 14-16  show examples of the measurements of the parasitic capacitances. It can be seen that, on average, ICs having fill  406  can have a 4% higher values of gate-drain capacitance, a 10% higher drain-source capacitance, and a 20% higher gate-drain capacitance. 
         [0037]    Therefore, in order to reduce the impact of the encapsulating material on the performance of the devices, it is necessary to use a low dielectric constant, low loss material to cover the die, probably using a glob-top technique. Preferably, the dielectric constant of the fill  406  should be approximately equal to that of dry air to achieve desirable results. 
         [0038]    Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.

Technology Category: h