Patent Publication Number: US-11658136-B2

Title: Channelized filter using semiconductor fabrication

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of U.S. patent application Ser. No. 16/916,644, filed Jul. 17, 2020, entitled “CHANNELIZED FILTER USING SEMICONDUCTOR FABRICATION” the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Embodiments of the invention relate to channelized microstrip filters made using semiconductor fabrication technology with an enclosure composed of micromachined interiors that enhance the performance of the microstrip filters and provide manufacturability that yields repeatable performance results. 
     High-frequency, i.e. frequencies of 1 GHz and higher, microstrip filters have been constructed using a variety of materials and techniques. It is preferable to test subassemblies prior to being installed in a larger circuit assembly so that the assembly is built of known good devices. However, the ability to test a high-frequency modular microstrip filter to confirm that it has acceptable performance prior to installation in the larger assembly has proven to be a substantial challenge, in part because the filter performance is sensitive to the housing in which it is assembled. If it is discovered that the modular microstrip filter does not meet performance specifications after being installed as part of the larger assembly, the modular microstrip filter may require a tedious in-place tuning process or may require removal and replacement with another modular microstrip in order to yield acceptable filter performance in the larger assembly. 
     In order to provide electromagnetic isolation an RF filter, a housing, such as of sheet metal, machined metal, or a casted metal, may be used to enclose the microstrip filter. However, variations in the physical dimensions of the metal housing often results in undesired variations in electromagnetic coupling within the filter elements and leads to variations in the performance of the filter. 
     There is a desire to minimize the area occupied by high-frequency filters as part of an overall desire towards miniaturization of electronic circuits. Reducing the overall area of the filter requires that individual internal elements of the filter be increasingly closer together. This increases the likelihood of electromagnetic field interaction between the elements which leads to increased challenges in being able to account for such interactions during the design of the filter and provides an undesired variable that adversely impacts reproducible performance from unit to unit. 
     There exists a need for an improved high-frequency microstrip filter that minimizes these difficulties. 
     SUMMARY 
     It is an object of embodiments of the present invention to provide improved modular microstrip filters that minimize such difficulties. 
     A semiconductor technology implemented high-frequency channelized filter includes a dielectric substrate with metal traces disposed on one of two major surfaces of the substrate. An input and output port disposed on the substrate and one of the metal traces carrying a high-frequency signal to be filtered between the input and output port. Other of the metal traces are connected to the one metal trace at intervals along the length of the one metal trace each providing a reactance to the high-frequency signal where the reactance varies with frequency and additional traces of the metal traces serving as a reference ground for the one metal trace and the other metal traces. A silicon enclosure mounted to the substrate with a first planar surface with cavities in the enclosure that extend through the first surface, and internal walls within the silicon enclosure defining the cavities. A layer of conductive metal covers the first planar surface, cavities and the internal walls. The silicon enclosure having substantially continuous areas of metal on the first planar surface about the periphery of the silicon enclosure that engage corresponding areas of the additional traces about the periphery of the substrate. The cavities surround the respective other metal traces with the internal cavity walls engaging the additional traces adjacent the respective other metal traces to individually surround each of the other metal traces with a conductive metal thereby providing electromagnetic field isolation between each of the other metal traces. Conductive engagement is formed between the first planar surfaces and the additional traces to establish a common reference ground therebetween. 
     A high-frequency filter having a substantially planar dielectric substrate and metal traces that are lithographically fabricated on one of two major surfaces of the substrate is also provided in which an input and output port are disposed on the substrate. One of the metal traces carrying a high-frequency signal is to be filtered between the input and output port. Other of the metal traces, connected to the one metal trace at intervals along the length of the one metal trace, each providing a reactance to the high-frequency signal where the reactance varies with frequency. Additional traces of the metal traces serve as a reference ground for the one metal trace and the other metal traces. A micro-machined silicon enclosure is mounted to the substrate with a first planar surface with cavities in the enclosure that extend through the first surface, and internal walls within the silicon enclosure define the cavities. A layer of conductive metal covers the first planar surface, cavities and internal walls. The silicon enclosure has substantially continuous areas of metal on the first planar surface about the periphery of the silicon enclosure that engage corresponding areas of the additional traces about the periphery of the substrate. Each cavity surround at least one of the other metal traces with the internal cavity walls engaging the additional traces adjacent the respective other metal traces to individually surround the at least one of the other metal traces with a conductive metal thereby providing electromagnetic field isolation between cavities. Conductive engagement is formed between the first planar surfaces and the additional traces to establish a common reference ground therebetween. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Features of exemplary embodiments of the invention will become apparent from the description, the claims, and the accompanying drawings in which: 
         FIG.  1    shows a perspective view of a high-frequency channelized filter in accordance with an embodiment of the present invention with an enclosure in an open position; 
         FIG.  2    shows a perspective view of the channelized filter as shown in  FIG.  1    with the enclosure in final assembled position; 
         FIG.  3    shows a representative cross-section of the assembled channelized filter in accordance with the embodiment of the present invention; 
         FIG.  4    shows a graph illustrating performance characteristics of the exemplary channelized filter over a frequency range in accordance with an embodiment of the present invention; 
         FIG.  5    is a partial perspective view of an alternative embodiment of a channelized filter of the present invention that shows the ability to test the performance of the filter from the bottom surface of the planar substrate; 
         FIG.  6    shows a partial, exploded top view of the embodiment of  FIG.  5   . 
     
    
    
     DETAILED DESCRIPTION 
     One aspect of the present invention resides in the recognition of the difficulties associated with repeatably manufacturing high-frequency channelized filters with consistent performance that does not require post-manufacture tuning by minimizing cross-coupling of electromagnetic fields between filter elements. Effective element shielding is important to minimize such cross-coupling, especially between adjacent elements, in order to minimize undesired unit to unit performance variations. Effective element shielding also allows filter elements to be compacted by using meandering filter elements to minimize the total area footprint of the filter. 
     Another aspect of the present invention resides in the recognition of an improvement in input/output coupling that facilitates the ability to reliably test the performance of a high-frequency modular filter prior to installation of the filter in a larger electronic assembly. In one embodiment, testing from ports on the bottom surface of the substrate opposite to that containing the filter elements provides ease of access for test probes and for connection in a larger circuit assembly. 
       FIG.  1    shows a perspective view of a high-frequency channelized filter  100 , e.g. a microstrip filter, in accordance with an embodiment of the present invention with a substrate  105  and an enclosure  110  in an open position. The term “channelized” is used herein to refer to the use of conductive channels that separate individual elements of the filter as opposed to a single enclosure space that covers multiple elements or an entire filter/circuit. A primary signal conductor (transmission line)  115  is disposed on the top side of substrate  105 , which is preferably made of a low-loss dielectric such as silicon carbide, alumina, InP, GaAs, quartz, and extends between input and output ports,  120 ,  125 . A reference conductor  130 , which acts as ground, extends across various regions on the top of substrate  105  and is interconnected by a plurality of vias  135  to a reference conductor/ground  140  located across substantially the entire bottom side of substrate  105 . A plurality of individual filter elements  145 ,  150 ,  155 , and  160  are selected to provide frequency varying values of inductance and/or capacitance to the respective points of connection to the primary signal conductor  115 . The combined effect of the selected inductance and capacitance values of the filter elements as disposed at locations along the signal line define the passband and rejection characteristics of the illustrative bandpass microstrip filter. 
     The enclosure  110 , when in the closed position, substantially surrounds the top peripheral surface of the substrate  105 . Enclosure  110  is preferably made of silicon with a planar surface  165  disposed to engage the top surface of substrate  105 . The enclosure  110  contains a plurality of cavities  170 , preferably formed by deep reactive ion etching (DRIE) for micro-precision dimensions. The silicon micro-machining enables tightly controlled geometries of the electromagnetic cavities  170  of the channelized microstrip filter. The cavities  170  correspond to those areas on the top surface of substrate  105  that will help individually encapsulate the filter elements and the signal line. The surface portions of planar surface  165  are conductive, i.e. preferably plated with a good conductor such as gold, and are disposed to engage the reference/ground areas  130 . The vertical sidewalls  175  and the bottom  180  of the cavities formed in the cover are also conductive, i.e. also preferably plated with a good conductor such as gold. Thus, when the enclosure  110  is placed in the assembled position to engage the top surface of substrate  105 , all surfaces of the enclosure facing the substrate are continuously conductive and connected to the ground  130 . The ends of the interior walls formed by the etched cavities  170  of the enclosure  110  correspond to and are disposed to engage corresponding ground areas  130 , including those ground areas interior of the periphery that separate the individual filter elements. Therefore, each of the individual filter elements are enclosed above the substrate within a separate volume/chamber that is grounded which provides isolation and basically eliminates undesired cross-coupling between filter elements. Cut-out portions  190  in the end walls of the enclosure  110  provide an opening for access to the input  120  and output  125  signal lines. This facilitates not only connection of the input and output signal line of the channelized microstrip filter to a larger circuit assembly but also allows for the temporary engagement of probes to test the filter prior to installation in the larger circuit assembly. 
       FIG.  2    shows a perspective view of the channelized filter  100  with the enclosure  110  in a final assembled position engaging the substrate  105  to provide a continuous peripheral ground connection with the ground  130  of the substrate except for the cut-out portions  190 . Also, each of the interior walls defined by the etched areas  170  in the enclosure  110  engage corresponding ground areas  130  on the substrate  105 . As will be seen by referring to  FIG.  1   , each of the elements  145 ,  150 ,  155 ,  160  will be totally surrounded on and above the top of the substrate  105  by a metallic ground surface with only portions of each of the elements that form a connection with the signal line  115  extending outside of the individual metallic enclosures. Because the ground  140  extends substantially over the entire bottom side of the substrate  105  and is connected by the plurality of vias  135  distributed throughout the ground area to the ground on the top side of the substrate  105 , each of the filter elements is also surrounded on the bottom of the substrate  105  by a metallic ground. The degree of encapsulation of the filter elements provides a very effective electromagnetic shield that minimizes cross coupling between elements as well as preventing coupling to circuitry outside of the channelized microstrip filter. In order to provide an effective continuous electromagnetic grounding of the metal  130  on the top side of the substrate and the metal  140  on the bottom side of the substrate at the high-frequencies of operation of the channelized microstrip filter, each of the top and bottom metals are connected by a plurality of vias that preferably have a spacing not more than 0.1 wavelengths for the highest frequencies of operation. Preferably, the conductive layers on surface  165  of enclosure  110  that engage conductive metal  130  on substrate  105  are coupled together using gold-to-gold thermocompression bonding for micron precision assembly. 
     Exemplary circuit elements are implemented by the traces  145 ,  150 ,  155  and  160  as shown in  FIG.  1   . However, those skilled in the art will understand that these filter elements are merely exemplary of various types and numbers of filter elements and layouts, and that other filter configurations and layouts can be used to provide desired frequency selectivity using the techniques of the embodiments of this invention. For example, the conductor  115  in a different filter topology may not be continuous or a DC short-circuit between the input and the output; it could be made non-continuous by an element such as “pi-of-cap” that introduces gaps consisting of three capacitors in tandem—a shunt capacitor, a series capacitor, and a second shunt capacitor. Similarly, the short-circuited shunt stubs could be open-circuited shunt stubs, coupled-line stub, or even higher order sub-circuit such as inductive segment followed by a capacitive segment. 
     In order to minimize the footprint area occupied by the filter, the individual filter elements meander over the length of the filter elements. The signal line  115 , metallic ground traces  130  on the top surface of the substrate, and the filter elements reside in a common plane parallel to the plane of the substrate. The filter elements meander within this plane. As used herein, “meander” means to turn at one or more angles, preferably at 45 degrees or more within the same plane. Using 90 degrees as an example, filter element  160  consists of a first segment  161  that is connected to the signal line  115 , a second segment  162  coupled to the end of segment  161  and being perpendicular to segment  161 , and segment  163  coupled to the end of segment  162  and being perpendicular to segment  162  and parallel to segment  161 . In comparison, a conventional filter element would typically extend in a straight line which would require a substantially wider and/or longer substrate than the substrate  105  and cause the associated substrate of the filter to have a substantially larger footprint area. A meandering approach to reduce the filter footprint in a conventional “open-face” filter (without channelized filter elements) brings serious challenges. Such an approach would require substantially increased design effort because the interactive coupling from one filter element to another filter element or the signal line would require repeated electromagnetic simulations and trial-and-error experiments to resolve issues of raised return loss, skewed slope of fall-off, undesired spikes in the stop band rejection etc. And yet the final design outcome of such an approach is sensitive to manufacturing tolerance and the housing channel height and width due to the cross coupling of all elements through an empty space. The ground segments  131 ,  132  and  133  are spaced apart from the segments of filter element  160  and function to surround the entire meandering length of the filter element. These ground segments in combination with the corresponding engaged walls of the associated cavities provide an effective grounded chamber for the entirety of the meandering filter element  160  except for the small portion of  160  that connects to the signal line  115 . 
     In order to prevent undesired cross coupling of segment  163  of the filter element with segment  161 , the ground segment  132  is disposed therebetween which together with the corresponding engaged wall of the associated cavity provides isolation between these two segments. Ground segment  131  also serves to provide isolation between segment  161  of the filter element and a parallel portion of the signal line  115 . Ground segment  133  provides isolation between segment  163  of the filter element and adjacent filter element  155  which has a portion parallel to segment  163 . Of course, the walls of the associated cavities that engage the ground segments  131  and  133  complete the corresponding chambers that provide isolation. 
       FIG.  3    shows a representative cross-section of the channelized filter  100  as assembled with the enclosure  110  engaging the substrate  105 . A layer of gold plating  310  covers the interior surfaces of enclosure  110  and surfaces  165  that engage the metallic grounds  130  on the top surface of the substrate  105 . The enclosure is preferably made of a micromachined silicon wafer on which a layer of metal is deposited having a peak to valley roughness of less than 1 microns. Such smoothness contributes to consistent performance and reduced losses especially at higher frequencies. 
     Representative vias  135  provide continuity between the top and bottom ground metallization on the substrate. In one example of an embodiment, the overall height  320  of the enclosure  110  is approximately 1 mm, the internal cavity height  325  is approximately 0.635 mm, and the width  330  of the cavity is approximately 0.800 mm. These dimensions coincide with the response of the exemplary filter discussed with regard to  FIG.  4   . 
       FIG.  4    is a graph  400  illustrating performance characteristics of the exemplary channelized filter  100  over a frequency range of 0 GHz to 5 GHz. The vertical axis represents decibels (dB) and the horizontal axis shows frequency in gigahertz. Curve  405  shows the bandpass filter transmission characteristics (S 21 ) showing the passband with relatively low signal loss from about 1.5 GHz to 3.5 GHz with increasing signal loss below and above this range. Curve  410  shows the input reflection loss (S 11 ) that begins to increase starting about 1 GHz and returns to relatively low values about 4 GHz. Input reflection loss within the bandpass filter range is a minimum of 20 dB. These filter characteristics for the exemplary channelized microstrip filter  100  with compressed/folded/meandering filter elements compare favorably to the characteristics for a conventional microstrip filter that occupies a substantially larger area than filter  100  and uses straight line filter elements. 
       FIGS.  5  and  6    are a partial perspective views of an alternative embodiment  500  of a channelized filter that includes bottom access ports on the bottom surface  505  of the planar substrate  510  that provide the ability to engage probes and test the performance of the filter. Since the differentiating features of filter  500  relative to filter  100  relate to input/output ports accessible from the bottom  505  of the substrate  510  as opposed to access to the input/output ports disposed on the top side of the substrate for filter  100 , only one of two exemplary input/output port  515  is shown. 
       FIG.  5    shows a partial view of the bottom  505  of substrate  510  while  FIG.  6    shows a partial view of the top  520  of substrate  510 . Referring to  FIG.  5   , a section  525  of the bottom  505  is shown removed to show the rows of vias connecting the top and bottom surface metallization (ground). The signal line  530  for filter  500  is the same as signal line  115  for filter  100 . As seen in  FIG.  6   , signal line  530  terminates at metallization  535  on the top side  520  of the substrate and is flanked by metallic ground regions  540 . This is the structure as provided for the input/output ports for filter  100 . As seen in  FIG.  5   , a region  545  on the bottom  505  of substrate  510  has no metallization. A via  550  connects a strip of metallization  555  on the bottom  505  with metallization  535  disposed on the top surface  520 . The via  550  provides a conductive coupling of the signal on the signal line  530  to the metal conductor  555  on the bottom of the substrate. The expanded areas  560  of region  545  without metallization on the bottom of the substrate provide an impedance transformation to provide a 50 ohm impedance for the port  515 . As seen in  FIG.  5   , a probe  565  includes a center signal conductor  570  and a pair of ground terminations  575  shown engaging one of the input and output ports  515  on the bottom of the substrate. The other bottom port (not shown) is identical and provides access to the other of the input and output ports. Ports on the bottom surface of the substrate simplify testing and allow probes (and connection points to other circuitry in the larger assembly) easy access and coupling without requiring a cutout of the enclosure as in filter  100 . Thus, the enclosure of filter  500  does not require and does not have a cutout in the end walls. This provides increased shielding as there is no opportunity for electromagnetic fields to enter/exit through the cutout areas and reduces manufacturing complexity of the enclosure. Similarly, there is no opportunity for foreign objects to enter into the filter cavity through the cutouts and cause detrimental effect such as de-tuning and short-circuiting on filter performance. It is understood that the shapes  540  are actually connected to the ground and manufactured as one metallization instead of two separate metallizations. The difference is that Si cover lands on the ground but not on  540 . 
     The concept of a channelized filter is not limited to the exemplary microstrip line where the signal trace runs on top of a ground plane and a top enclosure forms individual chambers around each filter element. It is also applicable to other types of transmission lines such as stripline where the signal trace on a substrate is captivated between a top ground enclosure and a bottom ground enclosure, i.e. where a top channelized enclosure and a mirror image bottom channelized enclosure cooperate to surround and sandwich the individual respective filter elements within individual separate chambers. 
     Although exemplary implementations of the invention have been depicted and described in detail herein, it will be apparent to those skilled in the art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention. For example, the cavities can be different heights and various bonding techniques including eutectic bonding such as indium-gold or gold-tin, or copper pillar bonding could be used to attach the enclosure to the ground metallization on the substrate. The enclosures of the channelized filters can be bonded to a plurality of corresponding substrates manufactured on a single wafer rather than as separated substrates. The cavity height is only limited by the fabrication capability of the silicon etching tool. A silicon enclosure with two different etch (cavity) depths is possible. 
     The scope of the invention is defined in the following claims.