Patent Publication Number: US-7898370-B2

Title: Hybrid surface mountable packages for very high speed integrated circuits

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
     The present application claims priority from U.S. Provisional Patent Application Ser. No. 61/015,542, filed Dec. 20, 2007 and entitled “VERY-HIGH-SPEED SURFACE MOUNTABLE PACKAGES FOR MULTIPLE MICROWAVE INTEGRATED CIRCUITS,” which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Surface mountable packages with grid array technology have been widely used for high-speed integrated circuits. Most grid array technologies, such as land grid arrays (LGAs), are generally only applied to 10 Gbps integrated circuits because of bandwidth limitations of the interconnections between the grid arrays and the integrated circuits. However, very-high-speed integrated circuits, also known as microwave integrated circuits (MICs), require very-high-speed interconnects, which are defined herein as interconnects capable of speeds higher than about 25 Gbps. 
     For some applications, multiple co-packaged MIC chips are required due to the difficulty, loss of performance, and cost entailed in integrating all required functions into a single MIC chip. However, communication between the co-packaged MIC chips has proven problematic due to unfavorable RF/microwave performance and cavity resonances, and spurious modes in the operating frequency. 
     BRIEF SUMMARY OF SOME EXAMPLE EMBODIMENTS 
     In general, example embodiments of the present invention relate to hybrid surface mountable packages. The example hybrid surface mountable packages each include multiple co-packaged microwave integrated circuits (MICs) that are connected with very-high-speed interconnects that exhibit favorable RF/microwave performance and cavity resonances and few or no spurious modes in the operating frequency. 
     In one example embodiment, a hybrid surface mountable package includes a housing at least partially defining a sealed cavity, two MIC chips positioned inside the sealed cavity, and a very-high-speed interconnect connecting the two MIC chips to each other. The very-high-speed interconnect includes strong coupling co-planar waveguide (CPWG) transmission lines. 
     In another example embodiment, a hybrid surface mountable package includes a housing at least partially defining a sealed cavity, two MIC chips positioned inside the sealed cavity, two microwave connectors positioned outside the cavity, and a very-high-speed interconnect connecting the MIC chips to the microwave connectors. The very-high-speed interconnect includes a strong coupling wall feed thru extending through the housing, first strong coupling CPWG transmission lines connecting the feed thru to the microwave connectors, and second strong coupling CPWG transmission lines connecting the feed thru to the MIC chips. 
     In yet another example embodiment, a hybrid surface mountable package includes a first housing at least partially defining a first sealed cavity, a second housing at least partially defining a second sealed cavity, first MIC chips positioned inside the first sealed cavity, second MIC chips positioned inside the second sealed cavity, two microwave connectors positioned outside the cavity, and a very-high-speed interconnect connecting the MIC chips to the microwave connectors. The very-high-speed interconnect includes a first strong coupling wall feed thru connecting the first MIC chips and the second MIC chips through the first housing and the second housing, a second strong coupling wall feed thru extending through the second housing, first strong coupling CPWG transmission lines connecting the second feed thru to the microwave connectors, and second strong coupling CPWG transmission lines connecting the second feed thru to the second MIC chips. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To further clarify certain aspects of example embodiments of the invention, a more particular description of the invention will be rendered by reference to example embodiments thereof which are disclosed in the appended drawings. It is appreciated that these drawings depict only example embodiments of the invention and are therefore not to be considered limiting of its scope nor are they necessarily drawn to scale. Aspects of example embodiments of the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  is a perspective view of an example transponder; 
         FIG. 2  is a perspective view of an example hybrid surface mountable package that can be included within the example transponder of  FIG. 1 ; 
         FIG. 3  is a section view of a portion of the example hybrid surface mountable package of  FIG. 2 ; 
         FIGS. 4A-4C  are various views of an example strong coupling co-planar waveguide (CPWG) transmission line; 
         FIG. 5A  is a chart of S-parameters test results on a simulation of the strong coupling CPWG transmission line of  FIGS. 4A-4C ; 
         FIG. 5B  is a chart of field distribution test results on a simulation of the strong coupling CPWG transmission line of  FIGS. 4A-4C ; 
         FIGS. 6A-6C  are various views of an example strong coupling wall feed thru transmission line; 
         FIG. 7A  is a chart of S-parameters test results on a simulation of the strong coupling feed thru transmission line of  FIGS. 6A-6C ; 
         FIG. 7B  is a chart of field distribution test results on a simulation of the strong coupling feed thru transmission line of  FIGS. 6A-6C ; 
         FIG. 8A  is a top view of a transmission line having a ground-signal-ground (GSG) structure; 
         FIG. 8B  is a top view of a transmission line having a ground-signal-signal-ground (GSSG) structure; and 
         FIG. 8C  is a top view of a transmission line having a ground-signal-ground-signal-ground (GSGSG) structure; 
         FIG. 9  is a section view of a portion of another example hybrid surface mountable package; 
         FIG. 10A  is a chart of S 21  responses of feed thrus having various gaps; and 
         FIG. 10B  is a chart of spurious mode frequencies of feed thru having various gaps. 
     
    
    
     DETAILED DESCRIPTION OF SOME EXAMPLE EMBODIMENTS 
     Example embodiments of the present invention relate to hybrid surface mountable packages. The example hybrid surface mountable packages each include multiple co-packaged microwave integrated circuits (MICs) that are connected to each other with very-high-speed interconnects that exhibit favorable RF/microwave performance and cavity resonances and few or no spurious modes in the operating frequency. The term “high-speed” as used herein refers to speeds below about 25 Gbps, such as 10 Gbps. The term “very-high-speed” as used herein refers to speeds of about 25 Gbps or above, such as 40 Gbps. The term “co-packaged” as used herein refers to multiple integrated circuit chips being packaged in the same sealed cavity. 
     I. Example Environment 
     With reference first to  FIG. 1 , an example transponder  100  is disclosed. The example transponder  100  is one environment in which the example hybrid surface mountable packages disclosed herein can be employed. The example transponder  100  includes a housing  102 , a transmit port  104  defined in the housing  102 , and a receive port  106  defined in the housing  102 . As disclosed in  FIG. 1 , fiber optic cables  108  and  10  can be received into port  104  and  106 , respectively. 
     The example transponder is substantially compliant with the 40 G 300 pin MSA. It is noted, however, that the example hybrid surface mountable packages disclosed herein are not limited to employment in high-speed or very-high-speed transponders, but can also be employed in any environment where hybrid surface mountable packages with multiple co-packaged MICs would be beneficial. For example, any other transceiver or transponder that operates at about 40 Gbps or above can employ the example hybrid surface mountable packages disclosed herein. 
     II. First Example Hybrid Surface Mountable Package 
     With reference now to  FIG. 2 , an example hybrid surface mountable package  200  is disclosed. As disclosed in  FIG. 2 , the example hybrid surface mountable package  200  includes a board  202  and a grid array  204 , a housing  206 , a transmit microwave connector  208 , and a receive microwave connector  210  all mounted to the board  202 . 
     With reference now to  FIG. 3 , additional aspects of the example hybrid surface mountable package  200  are disclosed. As disclosed in  FIG. 3 , the housing  206  of the hybrid surface mountable package  200  at least partially defines a sealed cavity  212 . The housing  206  may include a ceramic interposer  207  that sits on top of the board  202 , and a metal seal ring  209  is brazed to the ceramic interposer  207 . The metal seal ring  209  allows for a metal lid  211  to be brazed and form a housing  206  that at least partially defines a hermetically sealed cavity  212 . The housing  206  may also provide electromagnetic radiation shielding and protection from damage. 
     Two MIC chips  214  and  216  are positioned inside the sealed cavity  212 . The MIC chips  214  and  216  are thus co-packaged in a single sealed cavity  212 , as disclosed in  FIG. 3 . The very-high-speed interconnects of  FIG. 3  may enable the MICs  214  and  216  to operate at data rates at least as high as 40 Gbps. For example, very-high-speed signals, such as 40 Gbps signals, can travel between the MIC chip  214  and the MIC chip  216 , and from MIC chip  214  to microwave connectors  208  and  210  (see  FIG. 2 ) via a very-high-speed interconnect  218 . 
     The very-high-speed interconnect  218  includes first strong coupling co-planar waveguide (CPWG) transmission lines  220  which connect the two MIC chips  214  and  216  to each other. The very-high-speed interconnect  218  also includes a strong coupling wall feed thru  222  extending through the housing  206 , second strong coupling CPWG transmission lines  224  connecting the feed thru  222  to the microwave connectors  208  and  210  (see  FIG. 2 ), and third strong coupling CPWG transmission lines  226  connecting the feed thru  222  to the MIC chip  214 . The feed thru  222 , which may include a buried strip line under the ceramic interposer  207 , may substantially prevent radiation from emanating from the feed thru  222 . The strong coupling CPWG transmission lines  220 ,  224 , and  226  may help confine electromagnetic radiation near the signal plane and also help to eliminate spurious modes at the operating frequency range of the MIC chips  214  and  216 . 
     Also disclosed in  FIG. 3  is the grid array  204  positioned outside the sealed cavity  212 , and high-speed interconnects  228  between the grid array  204  and the MIC chips  214  and  216 . While the very-high-speed interconnect  218  is capable of data rates at least as high as 40 Gbps, the high-speed interconnects  228 , which are routed through multiple layers of the board  202 , are only configured to carry data signals at data rates that are less than about 25 Gbps. The high-speed interconnects  228  may also be configured to carry power, ground, and other DC signals. The difference in physical features between the high-speed interconnects  118  and the high-speed interconnects  228  will be discussed below in connection with  FIGS. 4A-7B . 
     Also disclosed in  FIG. 3  are additional aspects of the board  202 . The very-high-speed interconnects  218  and the high-speed interconnects  228  are integrated into the board  202 , which may be a multilayer ceramic board. The multiple ceramic layers of the board  202  may be produced from ceramic materials and processing such as high temperature co-fired ceramic (HTCC) or low temperature co-fired ceramic (LTCC) materials, although these layers may be produced from other materials and/or other processes. 
       FIGS. 4A-4C  disclose aspects of an example strong coupling CPWG transmission line  400 . The example CPWG transmission line  400  uses about 500 um thick HTCC material as a dielectric layer  402  with a dielectric constant of about 9.2 and tangent loss of about 0.00015. It is understood that this configuration of the dielectric layer  402  is only one example configuration. Other dielectric layers with various configurations can instead be employed in connection with the CPWG transmission line  400 . 
     The dielectric layer  402  also includes ground vias  404  positioned along the transmission line. The ground vias  404  can help to confine the electric field, maintain fundamental mode, and eliminate the spurious modes. The ground vias  404  may be connected to the side grounds and bottom grounds in the CPWG transmission line  400 . The ground vias  404  can be formed using a drilling process and may be gold plated. The RF performance of the CPWG transmission line  400  can be improved by optimizing the locations and sizes of the ground vias  404 . For example, in some example embodiments the ground vias  404  may be positioned in double rows on each side of the transmission line with about 0.4 mm of space between each via in each row, and between the rows. 0.4 mm is about 1/10 of the shortest wavelength of highest operating frequency 40 GHz. 
       FIG. 5A  discloses S-parameters test results  500  on a simulation of the strong coupling CPWG transmission line of  FIGS. 4A-4C . The S-parameters test is used to characterize scattering parameters in high-frequency circuits.  FIG. 5B  discloses field distribution test results  550  on a simulation of the strong coupling CPWG transmission line of  FIGS. 4A-4C . These results  500  and  550  show that a strong coupling has the electromagnetic field confined near the signal plane without radiation, thus eliminating spurious modes and cavity resonances below about 40 GHz. In this example, the strong coupling is achieved with a trace width of about 0.2 mm with an about 0.095 mm gap between traces. 
       FIGS. 6A-6C  disclose aspects of an example strong coupling wall feed thru transmission line  600 . The example strong coupling wall feed thru transmission line  600  uses about 500 um thick HTCC material as a dielectric layer  602  with a dielectric constant of about 9.2 and tangent loss of about 0.00015. The example strong coupling wall feed thru transmission line  600  also uses a ceramic interposer  604  that sits on top of the dielectric layer  602  and under which the example strong coupling wall feed thru transmission line  600  extends. It is understood that this configuration of the dielectric layer  602  is only one example. Other dielectric layers with various configurations can instead be employed in connection with the strong coupling wall feed thru transmission line  600 . The dielectric layer  602  also includes ground vias  606  positioned along the transmission line. The ground vias  606  may be similar to the ground vias  404  of  FIGS. 4A-4C . 
       FIG. 7A  discloses S-parameters test results  700  on a simulation of the strong coupling wall feed thru transmission line of  FIGS. 6A-6C .  FIG. 7B  discloses field distribution test results  750  on a simulation of the strong coupling wall feed thru transmission line of  FIGS. 6A-6C . These results show that a strong coupling has the electromagnetic field confined near signal plane without radiation, thus eliminating spurious modes and cavity resonances below about 40 GHz. In this example, a strong coupling is achieved with a trace width of about 0.2 mm with an about 0.095 mm gap between traces, and a trace width at strip line of about 0.09 mm. 
     With reference now to  FIGS. 8A-8C , aspects are disclosed of example transmission lines  800 ,  820 , and  840 . The example transmission line  800  has a ground-signal-ground (GSG) structure with ground lines  802  and  804  surrounding signal line  806 , The example transmission line  820  has a ground-signal-signal-ground (GSSG) structure with ground lines  822  and  824  surrounding signal lines  826  and  828 . The example transmission line  840  has a ground-signal-ground-signal-ground (GSGSG) structure with ground lines  842 ,  844 , and  846  interleaved with signal lines  848  and  850 . 
     The example transmission lines for very-high-speed interconnects disclosed herein may be configured for single-ended signals with a GSG structure. However, the very-high-speed interconnects disclosed herein may also be configured for differential pair signals with a GSSG structure or GSGSG structure. A strong coupling for GSG, GSSG, or GSGSG structures can be designed to minimize radiation and eliminate cavity resonances and spurious modes that may occur due to a large cavity dimension or long transmission lines. In order to achieve a strong coupling in the GSSG structure of the example transmission line  820 , a relatively small gap, such as an about 0.095 mm gap, is required between signal traces  822  and  824  and ground traces  826  and  828  and also between the positive signal trace  826  and the negative signal trace  828 . 
     III. Second Example Hybrid Surface Mountable Package 
     With reference now to  FIG. 9 , another example hybrid surface mountable package  200 ′ is disclosed. As disclosed in  FIG. 9 , the example hybrid surface mountable package  200 ′ is similar to the example hybrid surface mountable package  200 , except that the example hybrid surface mountable package  200 ′ additionally includes a second housing  206 ′ at least partially defining a second sealed cavity  212 ′, respectively. The second sealed cavity  212 ′ has two MIC chips  214 ′ and  216 ′ positioned inside the second sealed cavity  212 ′. In addition, the example hybrid surface mountable package  200 ′ includes a strong coupling wall feed thru  230  connecting the MIC chip  216 ′ and the MIC chip  214 ′. 
     IV. Example Strong Coupling Configurations 
     In general, the operation frequency range for a feed thru is limited by spurious modes, which cause the loss dips at their resonance frequencies. The spurious modes in a feed thru can be generated due to the mode transition from a CPWG transmission line and a strip line under a wall. A feed thru with a strong coupling can be to eliminate the spurious modes in the operating frequency range. A strong coupling is achieved by using relatively small gaps and trace widths and by using relatively dense ground vias. 
     For example, in a CPWG transmission line, the smaller the gap and trace width, the stronger the coupling between signal trace and side ground to make the electric field concentrated near the gap. For a given dielectric material with a given dielectric constant and thickness, the gap map be determined by trace width for impedance match such as 50 ohm for a single-ended transmission line. 
     As disclosed in  FIG. 10A , the S 21  responses of feed thrus vary as the gaps of the feed thrus vary. For example, the thickest line represents a feed thru with a gap of about 0.35 mm, the medium thickness line represents a feed thru with a gap of about 0.18 mm, and the thinnest line represents a feed thru with a gap of about 0.095 mm. In the chart  1000  of  FIG. 10A , the dips in the lines are caused by spurious modes. The structure of each feed thru is similar to the example strong coupling wall feed thru transmission line  600  of  FIG. 6 . 
     As disclosed in the chart  1050  of  FIG. 10B , the spurious mode frequencies of feed thrus vary as the gaps of the feed thrus vary. As disclosed in  FIG. 10B , the smaller the gap, the stronger the coupling between signal trace and side grounds and thus the higher the spurious mode frequency. Therefore, for HTCC material with an about 9.2 dielectric constant and about 500 um thickness, for an about 40 Gb/s fiber optic link application, a strong coupling can be achieved by configuring a feed thru with a gap of about 0.18 mm or less, and a trace of about 0.3 mm or less. In some example embodiments using the same HTCC material, a strong coupling can be achieved by configuring a feed thru with a gap between about 0.095 mm and about 0.18 mm and a trace between about 0.2 mm and about 0.3 mm.