Abstract:
RF feedthroughs for use with monolithic microwave integrated circuits (MMIC) are installed in environmentally protective or hermetically sealed packages that provide electromagnetic shielding. A feedthrough for an MMIC package has a dielectric substrate, a microstrip or transmission line formed on the substrate for transmitting high frequency electronic signals and a wall disposed above the transmission line and the substrate. The wall of the feedthrough has a varying thickness so that the narrowest portion of the wall is disposed on the transmission line substantially perpendicular to the substrate. The transmission line also has a varying width so that the narrowest width portion of the transmission line crosses the narrowest portion of the wall. The narrowest portion of the wall may be created by placing two oppositely facing concaved surfaces on each side of the wall. To reduce parasitic capacitance, the substrate and the wall may each have an air cavity embedded in respective bodies.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to packaging of microwave circuits and more particularly concerns packaging that provides an ultra-low insertion loss feedthrough with particular interests in the high microwave frequencies and in applications using hermetically sealed packages for monolithic microwave integrated circuits. 
     2. Description of Related Art 
     Ultra high speed monolithic microwave integrated circuits (MMIC), microwave integrated circuits (MIC), other integrated circuits and hybrid circuit dies are mounted in environmentally protective or hermetically sealed packages that provide electromagnetic shielding and easy handling. Known manufacturing techniques include cofired ceramic enclosures using thick or thin film metallization, glass or quartz seals, ceramic enclosures using thin-film metallization, metal enclosures having ceramic feedthroughs, and metal enclosures having glass feedthroughs. 
     In cofired ceramic packages generally available for MMIC&#39;s, the main contributor to poor microwave performance is the feedthrough. In conventional designs, discontinuity for poor performance exists due to the lead attachment, to passage of the conductor into and out from the ceramic wall, to changes in the conductor width and to coupling of RF signals to the lid and lid sealing ring. These discontinuities introduce higher-order modes and reflection as a result of impedance mismatch and contribute to overall poor feedthrough performance by having higher insertion loss. An MMIC package capable of good performance in the microwave range should have low insertion loss per feedthrough. 
     The insertion loss of a coaxial line or stripline formed on the feedthrough through a hermetically sealed ceramic wall increases with higher frequency, which results in a diminished signal strength. High insertion loss degrades MIC performance in many ways such as increased noise figure of small signal devices and reduced output power and efficiency of a power amplifier. 
     FIGS. 1-3 show a conventional feedthrough. The conventional feedthrough  200  has a substrate  210  (see FIG. 1) which is typically 15 mils thick and a block wall  220  mounted on the top surface of the substrate, as seen in FIGS. 1 and 2. The feedthrough  200  has a conductive microstrip line  240  traced on the top surface for transmitting RF signals. As seen in FIG. 1, the microstrip line  240  has three sections: an outer section  242 , a middle  244  and an inner section  246 . In general, the middle section  244  has a narrower width than that of the other sections. 
     As shown in FIG. 1, the block wall  220  covers substantially all of the middle section  244  of the microstrip line  240 . The middle section  244  is formed on the substrate  210  using a tungsten layer which forms an hermetically sealed joint to the block wall  220 . The outer and inner sections  242  and  246  are gold plated to at least 100 micro inches thick, but the middle section  244  is not and cannot be plated with gold, since it is already sealed with the block wall  220 . As a result, the middle section  244  increases insertion loss. Moreover, the trace width of the middle section  244  is reduced to compensate for the additional parasitic capacitance due to the block wall  220 , further adding to the series resistance for the middle section  244 . 
     The bottom  249 , (see FIG.  2 ), sides  247  and  247 ′ (see FIG. 1) and top  248  (see FIG. 2) surfaces of the feedthrough are also metallized and plated with nickel so that the feedthrough  200  can be brazed in place between the side wall and the base flange of the package making a hermetically sealed package. 
     At higher microwave frequencies, including millimeter wave, the thickness of the substrate  210  must be reduced to eliminate higher order transmission modes. The reduction of substrate thickness necessitates reduction in strip width of microstrip line  240 , including all three sections  242 ,  244 , and  246 . This further increases the insertion loss due to the block wall  220  section by compensating for the parasitic capacitance arising from the block wall  220 . 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a high-frequency low-loss hermetic feedthrough which has an impedance compensated thin hermetic wall to provide high frequency packages and modules with low insertion loss. 
     It is another object of the present invention to provide an RF feedthrough suppressing surface modes. 
     Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     According to one embodiment of the present invention, a feedthrough for an MMIC package has a substrate, a microstrip transmission line formed on the substrate for transmitting high frequency electronic signals and a wall disposed above the transmission line and the substrate. The wall of the feedthrough has varying thickness so that the narrowest width portion of the transmission line substantially crosses the narrowest portion of the wall. The narrowest portion of the wall may be created by placing two oppositely facing concave surfaces on each side of the wall. To reduce parasitic capacitance, the substrate and the wall may each have an air cavity imbedded in respective bodies near the vicinity of the transmission line. 
     According to one feature of the invention, the substrate of the feedthrough is made of dielectric ceramic material cofired to form an hermetic feedthrough. 
     According to other features of the invention, the transmission line has a first section, a second section and a middle section formed between the first and second sections. The middle section has a narrower width than that of the first and second sections. The first section of the transmission line extends to the narrower middle section by forming a transition edge of about a 90 degree angle. To minimize the area of the transmission line being crossed over by the wall, the narrow portion of the wall is preferably disposed above the middle section of the transmission line which also has the narrowest width. According to the present invention, the length of the middle section of the transmission line may be made longer or shorter than the width of the narrow portion of the wall, or none at all depending on the wall thickness of the narrowest width. This notch in the transmission line depends on the degree of compensation desired for increased parasitic capacitance due to the hermetic wall. 
     According to another embodiment of the present invention, a feedthrough for a MMIC package includes a substrate, a transmission line formed on the substrate for conducting an electronic signal and a wall having an inverse pyramid shape. In other words, the wall has a layered construction with a narrower lower portion and a wider upper portion. The narrower portion is preferably disposed substantially above and crosses the transmission line. The wall is made of a dielectric material and is disposed on the transmission line. 
     According to one feature of the invention, the substrate has an air cavity formed substantially above the transmission line. The wall comprises a plurality of layers constructed to have gradually increasing width from the narrower lower portion to the wider upper portion. These and other aspects, features and advantages of the present invention will be better understood by studying the detailed description in conjunction with the drawings and the accompanying claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A detailed description of embodiments of the invention will be made with reference to the accompanying drawings, wherein like numerals designate corresponding parts in the several figures. 
     FIG. 1 is a top plan view of a prior art feedthrough for a microwave package. 
     FIG. 2 is a side elevational view of FIG. 1; 
     FIG. 3 is a cross-sectional view of FIG. 1 along the line  3 - 3  thereof; 
     FIG. 4 is a perspective view of an RF feedthrough according to a preferred embodiment of the present invention; 
     FIG. 5 is a top plan view of FIG. 4; 
     FIG. 6 is a cross-sectional view of FIG. 4 along the line  6 - 6  of FIG. 5; 
     FIG. 7 is an exploded view of an MMIC packaging using RF feedthroughs of the present invention; 
     FIG. 8 is an assembled representation of the MMIC packaging of FIG. 7; 
     FIG. 9 is a top plan view of an RF feedthrough according to a second embodiment; 
     FIG. 10 is a side elevational view of FIG. 9; and 
     FIG. 11 is a cross-sectional view of FIG. 9 along the line  11 - 11  thereof. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Low noise or high power integrated circuit packaging according to a preferred embodiment of the present invention is shown in the drawings for the purposes of illustration. The present invention is suited for applications in which low insertion loss and hermeticity are required to protect semiconductors and other internal elements. Generally, insertion loss is the combination of energy lost due to signal dissipation and signal reflection resulting from the transmission of an RF signal. The loss is measured between two detection points, such as from an input lead to an output lead. One of the main properties affecting the insertion loss is the electrical conductivity of the metals used for transmitting the RF signal. The other component causing an increase in insertion loss is due to the impedance mismatch loss. 
     FIG. 4 illustrates a perspective view of the low loss feedthrough  10  constructed in accordance with the preferred embodiment of the present invention for use in hermetic microwave packages. The feedthrough  10  has a substrate  20 , a wall  30  mounted on the substrate  20  and a microstrip line  40  (also referred to as a microstrip transmission line disposed between the wall  30  and the substrate  20 ). A main feature of the low loss feedthrough  10  is a unique structure of the wall  30  and the shape of the transmission line formed around the hermetically sealed wall  30 , for housing the MMICs in an hermetic package. 
     The feedthrough  10  according to the present invention provides the microstrip line  40 , also known as a transmission line, which conducts a signal, preferably in microwave through millimeter-wave frequency, between the exterior of an electronics package, such as an MMIC package, and integrated circuit chips located inside such package. The integrated circuit chips or MIC circuits operate at frequencies generally greater than 1 GHz. 
     A dielectric substrate  20  shown in FIG.4 is formed of a suitable dielectric material,such as an aluminum oxide or silicon dioxide. the wall  30 , which hermetically seals the integrated circuits contained inside a package, is hermetically sealed on top of tungsten patterned with a microstrip line on substrate  20  as well as the cofired ceramic wall  30  and the substrate  20 . For higher frequency packages,the dielectric substrate  20  has thinner thickness to minimize undesirable effects caused by surface modes. A conventional base substrate is approximately 15 mils thick. To maintain a constant characteristic impedance in connection with a microstrip line  40  deposited on the substrate  20 , the allowable width of the microstrip line  40  is related to the thickness of the substracte  20  to form a 50 ohm impedance. The dielectric substrate  20  has a width of about 40 mils, length of 60 mils, and thickness of about 8 mils. The transmission line would have a width of about 8 mils for the sections  42  and  46  and perhaps 5 mils in section  44 . At higher frequencies, these dimensions are expected to be further reduced. 
     The microstrip line  40  shown in FIGS.4 and 5 is a metallization formed on the dielectric substrate  20 . As seen in FIG. 5, by using a printing method in particular, the microstrip line  40  is divided into three different segments: an outer section  42 , a middle section  44  and inner section  46 . The outer, middle and inner sections  42 ,  44 , and  46  contiguously form a single piece microstrip line  40 . The outer section  42  and the inner section  46  are separated by the wall  30 . The wall is projected from the substrate  20 , preferably crossing the middle section  44  of the microstrip line  40 . The microstrip line  40  is made with a suitable metallization process using conductive materials, such as tungsten. Alternatively, any suitable material deemed proper by one of ordinary skill in the art may be used for the microstrip line  40 . The process of forming the microstrip line  40  will be described later. 
     The buried section  45  (cross-hatched) of the middle section  44  which is a printed tungsten film directly under the wall  30 , forms an hermetically sealed RF feedthrough wall for the package. The microstrip line  40 , except the buried section  45 , is gold plated to reduce its sheet resistance, and enable bonding during assembly. However, the buried section  45  cannot be plated, and thus it remains as tungsten film behaving as a series resistance between the outer and inner sections  42  and  46 . By minimizing the wall thickness, two beneficial results occur: First, the resistance value between the hermetically sealed buried section  45  under the ceramic wall  30  is minimized. Second, the amount of parasitic capacitance due to the wall  30  is reduced resulting in less impedance compensation required to maintain low mismatch loss. 
     Impedance compensation requires a combination of reduction of transmission line width and length of line reduced as seen by the middle section  44  of the microstrip line  40 . When it is properly compensated, low mismatch loss is achieved over a broad frequency range. In general, the greater the reduction of transmission line width and the shorter the middle section  44 , the higher the frequency that can be achieved. This compensated section should be physically close to the vertical wall  30 . Preferably, the length of the buried section  45  should be minimized. For a given design requirement, the designer may select a shorter buried section  45 , and may choose to design a different middle section  44  to reduce mismatch loss. All of these variations are covered by this invention including the fact that for a very thin section of the wall  30 , no impedance compensation for the buried section  44  may be required. 
     In the preferred embodiment, as seen in FIG. 4, the top  51 , side  52  and bottom  53  surfaces of the feedthrough  10  are metallized and plated with nickel so that the feedthrough  10  can be brazed in place between the side wall and the base flange of the package making a hermetically sealed package. 
     In the preferred embodiment, the length of the buried section  45  covered under the wall  30  is reduced to less than about  8  mils. Preferably the middle section  45  covered under the wall  30  should be as short as practically possible, but still maintaining structural integrity and hermeticity. The ultimate for this structure is the inverse pyramid structure for the wall which is described as a second embodiment of the invention. Such minimization of the buried section covered by the wall  30  reduces insertion loss due to its associated series resistance with the tungsten film and reduced mismatch loss compensation requirements. 
     The feedthrough  10  according to the present invention has the wall  30  to form a hermetic structure which protects the critical elements inside the microwave package. Unlike the conventional wall, the feedthrough  10  has a unique shape and design to reduce the insertion loss, while maintaining the hermeticity of the MMIC package. The wall  30  is configured so that only a small width of the ceramic is over the microstrip line  40 . For simple structures, there are two opposite facing recesses or concaved surfaces  32  and  34  on each side and middle portion of the wall  30 , as seen in FIGS. 4 and 5. When the feedthrough  10  is installed in an MMIC package, the outer concave surface  32  forms an outer surface of the package, whereas the inner concaved surface  34  forms an inner surface. Preferably, both have a radius of about 6.7 mils. As an alternative embodiment, the wall  30  may be equipped with only one recess or concaved, grooved or indented surface. 
     As best seen in FIG. 5, the wall  30  is positioned substantially in the middle of the substrate  20  to evenly divide the microstrip line  40  into two exposed segments. In particular, the wall  30  is placed on the middle section  44  of the microstrip line  40 . The wall is made of a dielectric material. In that regard, the wall  30  and substrate  20  may be made with the same dielectric material, so it makes a hermetically sealed joint between the wall  30  and the substrate  20 . 
     The wall  30  has a width of about 18 to 20 mils, a lenght of about 40 mils and a thickness of about 20 mils. As a result of having two opposite facing concaved surfaces  32  and  34  in the wall  30 , the narrowest width region of the wall, having the width of about 5 to 8 mils, covers the middle section  44  of the microstrip line  40  effectively reducing the effect of parasitic capacitance and the length of unplated tungsten film under the wall  30 , thus reducing the parasitic series resistance. Alternatively, in lieu of the concaved surfaces  32  and  34 , the wall  30  may be equipped with other types of recesses, such as grooves or indentations. 
     FIG. 5 illustrates a top plan view of the low loss feedthrough  10 . As shown, the concaved surfaces  32  and  34  of the wall  30  cross the middle section  44  of the microstrip line  40  to minimize the microstrip line region covered by the wall  30 . In addition, the middle section  44  of the microstrip line  40  has a narrower width and length than the outer and inner sections  42  and  46  to optimize impedance matching by minimizing the mismatch loss. As frequency increases, the length and width need to be shorter. 
     The area directly under the concaved surface of wall  30 , the buried section  45 , is the bare tungsten film area which has higher series resistance than the other part of the middle section  44 . The buried section  45  is the source of parasitic resistance and capacitance that are being reduced by this invention. When the parasitic capacitance is reduced, then the required impedance compensation to minimize the mismatch loss is also reduced. Thus, the ultimate limit for the narrowest width region of the wall depends on the structural strength required for the wall and the hermeticity requirements. 
     FIG. 6 illustrates a side elevational view of the low loss feedthrough  10 . Depending on specific design requirements, the dimensions of each component of the feedthrough  10  may be varied. The wall structure  30 , described above and shown in FIG. 6, reduces stress of manufacturing and requires the use of special ceramic forming techniques to make the small geometry required for the electrical performance. The assembly operations on materials are all optimized to reduce stresses through the assembly to maintain the integrity of the small geometry. As shown in FIG. 6, the wall structure  30  is disposed over the microstrip line  40  formed on and extending across the top surface of the substrate  20 . 
     The manufacturing process of the feedthrough  10  according to the present invention shall now be described. First, to make a wall  30  of the feedthrough  10 , an alumina ceramic tape having a thickness of about 20 mils is taped to a metal frame. The alumina ceramic tape is punched with a hard punch tool having a shape of the concaved surface formed in the middle portion of the wall  30  to individually punch out the concave portion. A thick film of tungsten paste is then screen printed on the alumina ceramic tape, including the top and bottom surfaces, the top of the wall  30  and the bottom of substrate  20 . The resulting metallized trace forms a microstrip/stripline transmission line. The wall  30  created by the above process is then laminated on the substrate  20  and cofired in a furnace at a temperature between 1,500 to 1,600 degrees C, thereby bonding the two ceramic layers to one another and sintering the alumina ceramic tape applied between the tungsten paste and the substrate  20 . In this process, the tungsten traces are bonded to and between the dielectric substrate  20  and ceramic wall  30 . 
     Once fired, the two vertical surfaces parallel to the transmission line need to be metallized, if not already metallized during the cofired process. This metallization with brazing or soldering will make the hermetic seal possible. This can be done in a variety of methods. One such method is to separate each feedthrough and fixture it on its side for another printing and firing step. This firing should occur with a conductor with a lower firing temperature than the cofiring step. This step may be either repeated for the opposite side, or if fixtured properly, both sides may be printed at the same time and fired once. Once metallized, the feedthrough is plated with a metal, either as a base coat for soldering, or for brazing into a package housing. This plating is typically performed using nickel. 
     The present invention can be equally implemented, employing either high or low temperature fired materials and processes known to one of ordinary skill in the art. 
     FIG. 7 illustrates an exploded view of an MMIC package  70  using the feedthrough  10  of the present invention. FIG. 8 illustrates a representative MMIC package  70  incorporating two RF feedthroughs and four DC feedthroughs, with like reference numerals designating like elements. As seen in FIG. 7, the MMIC package  70  includes a flange  90  and a side wall  80  preferably made of an alloy of iron, cobalt and nickel or an alloy whose composition gives a thermal expansion that closely matches the ceramic material. The side wall  80  has receptacles for receiving the RF feedthroughs  10  of the present invention and further receiving DC feedthroughs  75 . 
     FIG. 9 illustrates a top plan view of the low-loss feedthrough  100  constructed in accordance with a second embodiment of the present invention for use in hermetic MMIC packages. The main feature of the low-loss feedthrough is a step-up multilayer wall structure for housing the MMIC package and embedded air cavity above the microstrip line. 
     In the feedthrough  100 , the microstrip line  120  conducts a signal between the exterior of an electronic package and semiconductor devices located inside such a package. A dielectric substrate  112  shown in FIG. 9 is formed of a suitable dielectric material, such as an aluminum oxide or silicon dioxide. A multilayered ceramic wall  130  is positioned on the substrate surface. The two ceramic layers (i.e., the dielectric substrate  112  and the ceramic wall  130 ) are then pressed together and fired at a temperature between 1,500 to 1,600 degrees C, thereby bonding the two ceramic layers to one another and vaporizing the organic binder material that binds the aluminum oxide and the tungsten paste which becomes the microstrip line. In this process, the tungsten traces are bonded to and between the dielectric substrate  112  and the ceramic wall  130 . The present invention can be equally implemented employing either high or low temperature fired materials and processes. 
     The construction of the multilayered ceramic wall  130  is now described with respect to FIGS. 10 and 11. The wall  130  is multilayered and stacked in an inverse pyramid shape to minimize the covering of the microstrip line  120  and the wall  130 . In particular, the ceramic wall  130  includes a first ceramic layer  132 , a second ceramic layer  134  disposed on top of the first ceramic layer  132 , a third ceramic layer  136  disposed on top of the second ceramic layer  134  and a fourth ceramic layer  138  disposed on top of the third ceramic layer  136  to form a step-like cross-sectional shape as shown in FIGS. 10 and 11. The upper layers of the wall  130  are wider than a lower layer to allow sufficient area for seam-sealing a package lid. 
     The first ceramic layer  132  preferably has a length of about 5 mils and a width of about 15 mils. The second ceramic layer  134  preferably has a length of about 15 mils and a width of about 25 mils. The third ceramic layer  136  preferably has a length of about 25 mils and a width of about 25 mils and a width of about 30 mils. 
     Between the third ceramic layer  136  and the fourth ceramic layer  138 , there is preferably an air cavity  140  formed substantially above the middle section  124  (see FIGS. 9,  10 ) of the microstrip line  120 . The air cavity  140  is approximately 15 mils by 25 mils and maintains hermeticity through the ceramic wall layers  132 ,  134 ,  136 ,  138 , at the same time reducing the parasitic capacitance of the feedthrough  120 . In addition, the air cavity  140  reduces the resistance of the first transmission line section  122  (see FIGS. 9,  10 ) which operates as an input line. 
     The embedded air cavity  140  is designed to isolate the effect of the top layer ground on the transmission line. This allows the predominant electric field pattern to remain concentrated in the lower substrate, matching the field pattern of the microstrip bonding pad area connected to the second transmission line section  126  (see FIGS. 9,  10 ). 
     One benefit of having a multilayered wall  130  having the narrowest wall as the first ceramic wall  132  is its reduced susceptibility to misalignment of the ceramic wall. In general the ceramic wall alignment has a tolerance of ±5 mils. By extending the narrow portion of the microstrip line  120  well inside the ceramic wall region, the misalignment does not expose the narrow trace width outside the ceramic wall. 
     While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. 
     The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore in tended to be embraced therein.