Abstract:
An RF microcircuit package and interconnection device is disclosed which minimizes impedance mismatch between circuit elements. Multiple signal vie and close proximity ground vies as well as tuned wire bonds are disclosed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a “package” for the assembly of microelectronic circuitry and more particularly to methods, techniques and devices for routing and coupling RF signals between discrete components and substrates within the package. 
     BACKGROUND OF THE INVENTION 
     Microcircuit electronic products are typically assembled from individual components assembled into “packages”. In practice individual components are “picked” and “placed ” on a substrate. Substrates may be flexible polymer films or rigid ceramic pieces. Substrates may have multiple layers of circuit traces with interconnections between layers made in the “Z” direction through vies. Some components can be directly electrically attached to pads on the substrate with solder or adhesive or the like. Other components have connection pads that do not lie in the plane of the surface of the substrate. To accommodate the “height” of these pads, a technique such as wire bonding is used to route signals between the pads that lie in different planes. 
     For example it has been common practice to glue or solder integrated circuits to a ceramic substrate and then to make circuit interconnections with “wire bonds” between pads on the integrated circuit (IC) and a pad on the substrate. The “wire bond” is a small loop of wire that leaves the pad on the IC and loops over the edge of the IC and drops to the surface of the substrate. Wire bonds are rapidly stitched onto the IC during production and a typical throughput is  5  wire bonds per second. Both vie and wire bonds form acceptable signal paths for the package as long as the signal of interest is “low frequency”. 
     However, wire bonds both attenuate and radiate signals in the low gigahertz range. The wire itself has a parasitic inductance, which becomes a significant circuit element in the gigahertz range. The conventional solutions to reducing the attenuation have included both “wedge” bonding and “ribbon bonding”. Both of these techniques are efforts to reduce the length of the wire and therefore to reduce the inductance of the “wire”. 
     In the wedge technique, the wire is routed at an acute angle off the pads to minimize the total length of the connection. Such connections can be made at a rate of approximately 2-3 bonds per second. Ribbon bonds substitute a flat ribbon wire for the conventional circular cross section wire of wire bonding. The ribbon has a parasitic capacitance, which is large with respect to the inductance, which minimizes attenuation. However successful ribbon wire bonding is critically dependent on component placement. This process is also extremely slow. 
     A similar signal routing problem arises on the substrate itself. In a multi-layered substrate linear signal traces typically lie close to a ground plane. As a consequence signal paths can be designed with a constant nominal impedance using well known “stripline” techniques for RF signals. However if the signal path departs from the stripline path and passes in the Z direction then the asymmetry creates an impedance mismatch with an associated attenuation and reflection. Ceramic substrates are particularly prone to this problem because they are brittle and additional thickness in the z-axis is required for mechanical strength. The increased thickness and high dielectric constant exacerbates the impedance mismatch problems. 
     Consequently there is a need for improved interconnection devices and methods for production of high performance high frequency circuitry “packages”. 
     SUMMARY OF THE INVENTION 
     In contrast to the prior art, the present invention teaches a solution to the impedance mismatch problem within the substrate and at the surface of the substrate. In accordance with one principle of the invention, a capacitor is formed very near the bonding pad of the substrate. This capacitance forms a resonant circuit with the wire bond “wire”. This circuit topology behaves like a transmission line with nominal characteristic impedance to couple signals with reduced attenuation and suppression of reflected and radiated energy. 
     In a second teaching of the invention two or more vie are supplied for the transmission of Z direction RF signals. In operation the mutual capacitance of the parallel vies form a transmission line for the signal. In an alternate form a ground via is placed near the signal via to form a transmission line like connection. These various principles may be used alone or combined to create high performance packages. 
     Several benefits flow from package construction that incorporates the techniques. Low loss connections allows for repartitioning of RF signal processing elements to achieve lower cost or higher performance. For example, it may be possible to use discrete circuit elements rather than integrated circuit elements in some instances where the absence of “good” interconnection technology would force the use of lower performance integrated circuit elements. The connection methodology is also tolerant of component placement which implies lower production costs. These techniques also allow the production of high performance RF packages using lower cost conventional production equipment, which implies lower costs as well. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Throughout the several views of the drawings the invention is shown in several illustrative embodiments to depict various aspects and features of the invention wherein: 
     FIG. 1 is a cross section view of a package using aspects of the invention; 
     FIG. 2 is a plan view of a package using certain aspects of the invention; 
     FIG. 3 is a graph showing return loss test results taken for prototype packages; 
     FIG. 4 is a graph showing insertion loss test results taken for prototype packages; and 
     FIG. 5 is a graph showing performance of a package using laminate substrates. 
    
    
     DETAILED DESCRIPTION 
     The disclosure is based on performance testing and a brief discussion of the testing methods is offered to clarify the disclosure. In general, a device under test is provided with an RF input port and an RF output port. The amount of radio frequency power injected into the circuit is taken as unity and the amount of power extracted is compared to the applied power to measure an “insertion loss”. A low insertion loss corresponds to “better” performance. Reflected energy due to impedance mismatch also contributes to reduced performance. Return loss is a measure of this parameter. Return loss is conventionally measured as a negative number and so a higher value of return loss corresponds to better performance. 
     The measurements of return loss and insertion loss are functions of frequency and they are usually plotted against frequency. Broadband performance is highly desirable especially for newer digital phone technology where broad bandwidth is available. 
     FIG. 1 shows a package  30  in cross section. The arrow  9  indicates the Z-axis in this figure. This package includes a substrate  16 , which may be ceramic, or polymer and will typically be laminated with several layers of “circuits”. For clarity only a one-layer structure is shown as substrate  16 . As is typical of microwave practice the lower surface of the substrate is a ground plane  18  formed by a copper layer on the substrate  16 . A single signal trace is developed on this substrate  16  by the transmission line formed by trace  20  and the ground plane  18 . For any given substrate material and dielectric constant the proper width of the trace  20  can be easily computed, as is well known in this art. Trace  20  passes under and is bonded to a trace  28  on the lower surface of substrate  24 . In a typical construction a ceramic substrate  24  may carry several integrated circuits or other discrete components which are interconnected to a ceramic or polymer laminate substrate  16  to make up a product. 
     In the construction shown in FIG. 1, the substrate  24  has many vias or vies that carry the ground plane to the underside of the integrated circuit  22 . A first signal via  14  and a second via  15  (seen in FIG. 2) together form a signal path in the Z direction to take the signal from the transmission line  20  to the bonding area  12  on the upper surface of substrate  24 . In contrast to prior art structures the RF signal is split between two vias seen as via  14  and via  15  in FIG.  1  and FIG.  2 . The first signal via  14  and the second signal via are positioned near the ground via  17 . The two signal vias and the companion ground vias taken together form a constant impedance connection in the Z-axis between the transmission line  20  and the bonding pad  12 . It should be apparent that more than one additional ground via or signal via can be used to achieve the impedance match characteristic. 
     In FIG. 1 the wire  10  bond makes the electrical signal path connection from the bonding pad  12  to the port  2  connection to complete the description of the test package  30 . 
     FIG. 2 shows the arrangement of FIG. 1 in plan view. The arrow  11  identifies the X-Y axis for this configuration. The signal from port  1  enters the test package  30  through the transmission line connection formed by trace  20  and the ground plane  18  (not seen). This transmission line  20  is connected to trace  28 . The connection trace  28  is shown in phantom view in this figure. This connection trace  28  couples with the first signal via  14  and via  15  is shown as separation distance  32 . The magnitude of this separation distance can be used to “tune” the transmission line characteristics for the signal via. 
     On the upper surface of the substrate  24  the bonding pad  12  is shown with an exaggerated width labeled as  36  on FIG.  2 . This extended bonding pad has substantial capacitance with respect to the ground planes of substrate  22  and substrate  24 . It is the capacitance of the pad that “tunes” the inductance of the wire  10  to produce a characteristic design impedance. Although the pad is shown extended in the Y direction forming a “T” shaped connection it should be appreciated that other shapes are possible and potentially more desirable in particular situations. It is a characteristic of many RF integrated circuits that the density of interconnections is low and as a consequence the periphery of the die is available for such structures. It is also noted that the T shape takes advantage of the close ground plane a base or lower side of most integrated circuits. The equivalent circuit is a capacitor from the inductance of the wire to ground forming a pass band filter in the signal path. 
     With respect to the positioning distance  32  of the signal via  15  and the signal via  14 , it should be understood that the exact spacing to create the desire characteristic impedance depends on the dielectric constant of the substrates  24  and  16  and the proximity of other ground features. Consequently, more than one signal via may be desirable given a specific package design. The shape and location of the vie should be considered illustrative of the invention rather than limiting. In a similar fashion, the ground via  17  which is a complement of the signal vie may be positioned to “tune” the connection. No exact relationship exists because of the proximity of other stray capacitance and proximity of other ground structures. 
     In the various tests, the RF signal is injected into port I and extracted from port  2 . The power available at port  2  depends on the quality of the various connections between port  1  and port  2  and the tests measure the “return loss” and “insertion loss” between the ports. In general the two topologies shown in FIG.  1  and FIG. 2 were explored on composite test packages and the data is averaged for these tests. The use of Teflon filled ceramic substrates are preferred. It is believed that the increased dielectric constant of such material improves the performance of the techniques of the invention. 
     FIG. 3 shows the measured return loss of the package as line  26 . This performance is compared to a similar package that does not practice the invention labeled  27  in the figure. The −15 dB line  29  is taken as the performance limit for comparison purposes. In general if the return loss is above −15 dB the signals are so degraded as to render the package impractical. The package of the invention does not reach the −15 dB point until approximately 36-gigahertz. By contrast the conventional technology reaches the −15 dB point at 13 GHz. Thus the package of the invention is capable of higher frequency operation. It is also important to note that the frequency range is very large which is a requirement for broadband operation. 
     FIG. 4 sows the insertion loss characteristic for the package. The line  33  is taken with the test packages incorporating the invention while line  31  represents the results of the conventional technique. Line  33  lies above line  31  until about the 40 GHz point demonstrating lower insertion loss for the device. It is important to note that the figure shows that millimeter wavelength packages of the present invention attenuates the out of band noise more than the conventional package. This low insertion loss in the passband coupled with increased attenuation for out of passband energy is especially useful in high performance communication products such as cellular phones and the like. 
     FIG. 5 shows the result of a test package made with laminate substrates and Teflon filled ceramic material for a substrate. In this figure the package represented by line  35  offers superior performance in the millimeter wavelength range of 50 gigahertz. 
     Various embodiments of the invention are show together to make up a high performance RF package. However each concept may be used alone or combined with other techniques without departing from the scope of the invention. It should also be apparent that other modifications may be made without departing from the scope of the invention.