Channelized filters using semiconductor technology.

Abstract

An exemplary channelized filter implemented in semiconductor technology includes a dielectric substrate with metal traces fabricated from semiconductor on one side and input and output ports. Signal traces connected between the input and output ports carry the signal to be filtered. Filter traces are connected at intervals along the length of the signal traces to provide frequency-varying reactance. Ground traces provide a ground reference. A silicon housing with a cavity fabricated from semiconductor has a metal layer deposited on the housing. The periphery of the housing is sized to engage with corresponding ground traces on the periphery of the substrate. Separate cavity walls individually surround each of the filter traces, thereby providing electromagnetic field shielding. A metal-to-metal conductive bond is formed between the cavity walls that engage with the ground traces to establish a common ground reference. The filter traces are preferably serpentine to minimize the footprint area of ​​the substrate.

Description

Technical Field

[0001] Embodiments of the present invention relate to a channelized microstrip filter having an enclosure with a finely processed interior manufactured using semiconductor manufacturing technology, and provide manufacturability that improves the performance of the microstrip filter and yields reproducible performance results.

Background Art

[0002] Microstrip filters at high frequencies, i.e., frequencies above 1 GHz, have been manufactured using various materials and techniques. It is desirable to test subassemblies before mounting them in large circuit assemblies so that the assembly consists of known good devices. However, it has been demonstrated that it is very difficult to test high-frequency module-type microstrip filters and confirm that they have acceptable performance before mounting them in large assemblies. One reason for this is that the performance of the filter is highly susceptible to the influence of the housing in which it is incorporated. If, after being mounted as part of a large assembly, it is found that the module-type microstrip filter does not meet the performance specifications, the module-type microstrip filter may require cumbersome on-site adjustment work to obtain acceptable filter performance in the large assembly, or it may be necessary to remove the modular microstrip filter and replace it with another modular microstrip.

[0003] To electromagnetically isolate an RF filter, a housing such as sheet metal, machined metal, or cast metal can be used to enclose the microstrip filter. However, variations in the physical dimensions of metal housings often result in undesirable variations in electromagnetic coupling within the filter element, leading to variations in the performance of the filter.

[0004] As part of the overall demand for miniaturization of electronic circuits, there is a desire to minimize the area occupied by high-frequency filters. To reduce the overall area of ​​the filter, the individual internal elements of the filter need to be brought closer together. This increases the possibility of electromagnetic field interactions between elements, making it difficult to account for such interactions during filter design, and can lead to undesirable variations, negatively impacting the reproducible performance of each unit.

[0005] An improved high-frequency microstrip filter is needed to minimize these problems. [Overview of the Initiative]

[0006] An object of the embodiments of the present invention is to provide an improved modular microstrip filter that can minimize the problems described above.

[0007] An exemplary channelized filter implemented using semiconductor technology includes a dielectric substrate having a metal trace manufactured using semiconductor technology on one side, and input and output ports. A signal trace connected between the input and output ports carries the signal to be filtered. Filter traces are connected at intervals along the length of the signal trace and provide frequency-varying reactance. A ground trace provides a reference ground. A silicon housing having a cavity made of semiconductor material has a metal layer deposited thereon. The periphery of the housing is sized to engage with the ground trace corresponding to the periphery of the substrate. The walls of individual cavities individually surround each filter trace, thereby blocking the electromagnetic field. A metal-to-metal conductive bond is formed between the cavity walls engaging with the ground trace to establish a common reference ground. The filter traces are preferably meandering to minimize the substrate footprint area. [Brief explanation of the drawing]

[0008] The features of exemplary embodiments of the present invention will become apparent from the detailed description of the invention, the claims, and the accompanying drawings. [Figure 1] Figure 1 is a perspective view showing a high-frequency channel filter according to an embodiment of the present invention, which has a housing in an open position. [Figure 2] Figure 2 is a perspective view showing the channelized filter shown in Figure 1, with the housing in its final assembly position. [Figure 3] Figure 3 is a typical cross-sectional view showing an assembled channelized filter according to an embodiment of the present invention. [Figure 4] Figure 4 is a graph showing the performance characteristics of an exemplary channelization filter over a frequency range according to an embodiment of the present invention. [Figure 5] Figure 5 is a partial perspective view of an alternative embodiment of the channeling filter of the present invention, showing a function for testing the filter performance from the bottom surface of a planar substrate. [Figure 6] Figure 6 is a partially exploded top view showing the embodiment of Figure 5. [Modes for carrying out the invention]

[0009] One aspect of the present invention recognizes the difficulties associated with reproducibly manufacturing high-frequency channeled filters with consistent performance that do not require post-manufacturing adjustments, by minimizing the cross-coupling of electromagnetic fields between filter elements. Effective element shielding is crucial to minimize such cross-coupling, particularly between adjacent elements, and to minimize undesirable performance variations between units. Furthermore, effective element shielding, by using meandering filter elements, can minimize the total area footprint of the filter and make the filter elements more compact.

[0010] Another aspect of the present invention recognizes improvements to input / output coupling that facilitate the reliable testing of the performance of high-frequency modular filters before mounting them in larger electronic assemblies. In one embodiment, testing is performed from a port on the bottom surface of the substrate opposite to the substrate containing the filter elements, thereby facilitating access to the test probe and connection to the larger circuit assembly.

[0011] Figure 1 is a perspective view showing a high-frequency channelized filter 100, such as a microstrip filter, according to one embodiment of the present invention, comprising a substrate 105 and a housing 110 in an open position. In this specification, the term “channelized” is used to mean the use of conductive channels to isolate individual elements of a filter, as opposed to a single housing space covering multiple elements or the entire filter / circuit. A primary signal conductor (transmission line) 115 is located on the upper surface of the substrate 105 and is preferably made of a low-loss dielectric such as silicon carbide, alumina, InP, GaAs, or quartz, and extends between the input port 120 and the output port 125. A reference conductor 130, functioning as ground, extends across various regions of the upper surface of the substrate 105 and is interconnected by multiple vias 135 to a reference conductor / ground 140 located substantially across the entire bottom surface of the substrate 105. Multiple individual filter elements 145, 150, 155, and 160 are selected to provide frequency variations of inductance and / or capacitance at their respective connection points to the primary signal conductor 115. The passband and stop characteristics of the illustrated bandpass microstrip filter are defined by the synergistic effect of the selected inductance and capacitance values ​​of the filter elements positioned along the signal line.

[0012] The housing 110 substantially surrounds the upper outer periphery of the substrate 105 when in the closed position. The housing 110 is preferably made of silicon and has a plane 165 positioned to engage with the upper surface of the substrate 105. The housing 110 includes a plurality of cavities 170. The plurality of cavities 170 are preferably formed by deep reactive ion etching (DRIE) for micro-precision dimensions. Microfabrication of silicon allows for precise control of the shape of the electromagnetic cavities 170 of the channeled microstrip filter. The cavities 170 correspond to areas on the upper surface of the substrate 105 that help to individually seal the filter elements and signal lines. The surface portion of the plane 165 is conductive, i.e., preferably plated with a good conductor such as gold, and positioned to engage with the reference / ground region 130. The vertical side walls 175 and bottom 180 of the cavities formed within the cover are also preferably conductive, i.e., plated with a good conductor such as gold. Therefore, when the housing 110 is placed in the assembled position to engage with the top surface of the substrate 105, all surfaces of the housing facing the substrate are continuously conductive and connected to ground 130. The ends of the inner walls formed by the etched cavities 170 of the housing 110 correspond to and are positioned to engage with the corresponding ground regions 130, which include inner ground regions of the periphery that insulate the individual filter elements. Thus, each of the individual filter elements is sealed on the substrate in a separate, grounded volume / chamber, thereby providing isolation between the filter elements and essentially eliminating undesirable cross-coupling. The notches 190 in the end walls of the housing 110 provide openings for accessing the input 120 and output 125 signal lines. This not only facilitates connecting the input and output signal lines of the channeled microstrip filter to a larger circuit assembly, but also allows for the temporary mounting of probes for testing the filter before it is mounted on the larger circuit assembly.

[0013] Figure 2 shows a perspective view of the channelized filter 100, which includes a housing 110 in its final assembly position engaging with the substrate 105, providing a continuous outer periphery ground connection to the substrate ground 130, except for the notch 190. Each of the inner walls defined by the etched regions 170 within the housing 110 engages with the corresponding ground region 130 on the substrate 105. As can be seen with reference to Figure 1, each of the elements 145, 150, 155, and 160 is completely surrounded on the top and above of the substrate 105 by a metallic ground plane, with only a portion of each element forming a connection to the signal line 115 extending outside the individual metallic housing. Since the ground 140 extends substantially across the entire bottom surface of the substrate 105 and is connected to the top-side ground of the substrate 105 by multiple vias 135 distributed throughout the ground region, each of the filter elements is also surrounded by a metallic ground on the bottom surface of the substrate 105. The degree of sealing of the filter elements provides highly effective electromagnetic shielding, minimizing cross-coupling between elements and preventing coupling of the channeled microstrip filter to external circuits. In the high-frequency region in which the channeled microstrip filter operates, it is preferable that each of the top and bottom metals is connected by multiple vias spaced less than 0.1 wavelengths with respect to the highest operating frequency, in order to provide effective continuous electromagnetic grounding to the metal 130 on the top surface of the substrate and the metal 140 on the bottom surface of the substrate. Preferably, the conductive layers on the surface 165 of the housing 110 that engage with the conductive metal 130 on the substrate 105 are bonded to each other using gold-gold thermocompression bonding for micron-precision assembly.

[0014] The exemplary circuit elements are implemented by traces 145, 150, 155, and 160, as shown in Figure 1. However, those skilled in the art will understand that these filter elements are merely examples of various types and numbers of filter elements and layouts, and that other filter configurations and layouts may be used to provide desired frequency selectivity using the techniques of the embodiments of the present invention. For example, conductor 115 in different filter topologies may not be continuous between the input and output, nor may it be a DC short circuit. That is, conductor 115 can be discontinuous by elements such as a "pi-of-cap" that introduces a gap in tandem consisting of three capacitors, i.e., a shunt capacitor, a series capacitor, and a second shunt capacitor. Similarly, short-circuited shunt stubs may be open-circuited shunt stubs, coupled-line stubs, or higher-order subcircuits in which a capacitive segment follows an inductive segment.

[0015] To minimize the footprint area occupied by the filter, each filter element is meandering along its length. The signal line 115, the metal ground trace 130 on the upper surface of the substrate, and the filter elements are located in a common plane parallel to the plane of the substrate. The filter elements are meandering within this plane. As used herein, “meandering” means bending at one or more angles, preferably 45 degrees or more, within the same plane. Using 90 degrees as an example, the filter element 160 consists of a first segment 161 connected to the signal line 115, a second segment 162 connected to the end of segment 161 and perpendicular to segment 161, and a segment 163 connected to the end of segment 162 and perpendicular to segment 162 and parallel to segment 161. In contrast, conventional filter elements typically extend in a linear fashion, which would require a substrate that is significantly wider and / or longer than the substrate 105, resulting in a significantly larger footprint area for the substrate associated with the filter. In conventional "open-face" filters (which do not use channelized filter elements), a meandering approach to reduce the filter footprint presents serious problems. Such an approach significantly increases design effort because the interactive coupling from one filter element to another or to the signal line requires repeated electromagnetic simulations and trial-and-error experiments to resolve issues such as increased return loss, distortion of the falloff slope, and undesirable spikes in stopband rejection. Furthermore, the final design results of such an approach are susceptible to manufacturing tolerances and the height and width of the housing channel due to the cross-coupling of all elements passing through empty space. Ground segments 131, 132, and 133 are spaced apart from the segments of the filter element 160 and function to surround the entire meandering length of the filter element. These ground segments, in combination with the corresponding engagement walls of the associated cavity, provide an effective ground chamber for the entire meandering filter element 160, except for a small portion of the element 160 connected to the signal line 115.

[0016] To prevent undesirable cross-coupling between segments 163 and 161 of the filter element, a ground segment 132 is positioned between them, and together with the corresponding engaging walls of the associated cavity, provides insulation between these two segments. The ground segment 131 also functions to provide insulation between segment 161 of the filter element and the parallel portion of the signal line 115. The ground segment 133 provides insulation between segment 163 of the filter element and the adjacent filter element 155 having a portion parallel to segment 163. Of course, the associated cavity walls engaging with the ground segments 131 and 133 complete the corresponding chambers that provide insulation.

[0017] Figure 3 shows a typical cross-section of the channelized filter 100 assembled with the housing 110 engaged with the substrate 105. The gold-plated layer 310 covers the inner surface of the housing 110 and the surface 165 that engages with the metal ground 130 on the upper surface of the substrate 105. The housing is preferably fabricated from a microfabricated silicon wafer with a metal layer having a peak-to-valley roughness of less than 1 micron. Such smoothness contributes to consistent performance and loss reduction, especially at high frequencies.

[0018] Typical vias 135 provide conductivity between the upper and lower ground metallization regions on the substrate. In one embodiment, the total height 320 of the housing 110 is approximately 1 mm, the height 325 of the internal cavity is approximately 0.635 mm, and the width 330 of the cavity is approximately 0.800 mm. These dimensions are consistent with the response of the exemplary filter described with respect to Figure 4.

[0019] Figure 4 is a graph 400 showing the performance characteristics of an exemplary channeled filter 100 over the frequency range of 0 GHz to 5 GHz. The vertical axis represents decibels (dB), and the horizontal axis represents frequency in gigahertz. Curve 405 shows the transmission characteristics (S21) of the bandpass filter, showing a passband with relatively low signal loss from approximately 1.5 GHz to 3.5 GHz, with signal loss increasing below and above this range. Curve 410 shows the input reflection loss (S11), which begins to increase from approximately 1 GHz and returns to a relatively low value at approximately 4 GHz. The minimum input reflection loss within the bandpass filter range is 20 dB. These filter characteristics of the exemplary channeled microstrip filter 100 with compression / folding / serpentine filter elements occupy a substantially larger area than filter 100 and are superior to the characteristics of conventional microstrip filters using linear filter elements.

[0020] Figures 5 and 6 are partial perspective views of an alternative embodiment of a channeled filter 500, which includes a bottom access port on the bottom surface 505 of the planar substrate 510, providing a function for engaging a probe to test the performance of the filter. The differentiating feature of filter 500 over filter 100 is related to the input / output port accessible from the bottom surface 505 of the substrate 510, in contrast to the input / output port access located on the top surface of the substrate for filter 100; therefore, only one of the two exemplary input / output ports 515 is illustrated.

[0021] Figure 5 shows a partial view of the bottom surface 505 of the substrate 510, and Figure 6 shows a partial view of the top surface 520 of the substrate 510. Referring to Figure 5, a portion 525 of the bottom surface 505 is shown removed to indicate a row of vias connecting the metallized areas (grounds) of the top and bottom surfaces. The signal line 530 of the filter 500 is the same as the signal line 115 of the filter 100. As seen in Figure 6, the signal line 530 terminates in a metallized area 535 on the top surface 520 of the substrate and is sandwiched by a metallized ground area 540. This is the structure provided for the input / output ports for the filter 100. As seen in Figure 5, a region 545 on the bottom surface 505 of the substrate 510 does not have a metallized area. A via 550 connects a strip of metallized area 555 on the bottom surface 505 to a metallized area 535 located on the top surface 520. Via 550 conductively couples the signal on signal line 530 to a metal conductor 555 on the bottom surface of the substrate. An extended area 560 of the non-metallized area 545 on the bottom surface of the substrate provides impedance conversion to provide a 50Ω impedance to port 515. As shown in Figure 5, probe 565 includes a central signal conductor 570 and a pair of ground terminations 575 indicated to engage with one of the input and output ports 515 on the bottom surface of the substrate. The other lower port (not shown) is identical and provides access to the other input and output port. The ports on the bottom surface of the substrate simplify testing and allow easy access to and coupling of the probe (and connection points to other circuits in the larger assembly) without requiring a notch in the housing, as in filter 100. Thus, the housing of filter 500 does not require and does not have a notch in the end wall. This eliminates the opportunity for electromagnetic fields to enter and exit through the notch, improving shielding and reducing the complexity of housing manufacturing. Similarly, there is no opportunity for foreign matter to enter the filter cavity through the notch and cause harmful effects on filter performance such as detuning or short circuits. Shape 540 is understood to be actually connected to ground and manufactured as a single metallized region instead of two separate metallized regions. The difference is that the silicon (Si) coating covers the ground but not the 540.

[0022] The concept of a channelized filter is not limited to exemplary microstrip lines where the signal trace runs on the ground plane and the upper housing 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 the substrate is sandwiched between an upper ground housing and a lower ground housing, i.e., the upper channelized housing and the mirror-image lower channelized housing cooperate to surround and enclose individual filter elements within individual separate chambers.

[0023] Although exemplary implementations of the present invention have been illustrated and described in detail herein, it will be apparent to those skilled in the art that various modifications, additions, substitutions, etc. can be made without departing from the spirit of the present invention. For example, the cavities can be of different heights, and various bonding techniques including eutectic bonding such as indium-gold or gold-tin, or copper pillar bonding can be used to attach the housing to the ground metallization region on the substrate. The housing of the channelized filter can be bonded to a plurality of corresponding substrates manufactured on a single wafer rather than as an insulated substrate. The height of the cavity is only limited by the manufacturing capabilities of the silicon etching tool. A silicon housing with two different etching (cavity) depths is possible.

[0024] The scope of the present invention is defined in the following claims.

Claims

1. 1. A high frequency channelized filter implemented in semiconductor technology, comprising: a substantially planar dielectric substrate; input and output ports disposed on the substrate; lithographically produced metal traces disposed on one of the two major surfaces of the substrate, a first metal trace carrying a high frequency signal to be filtered between the input port and the output port; a plurality of second metal traces connected to the first metal trace at spaced intervals along the length of the first metal trace, each of the plurality of second metal traces providing a frequency-varying reactance to the first metal trace; metal traces including a third metal trace that serves as a reference ground for the first metal trace and the second metal trace; a micromachined silicon enclosure attached to the substrate, the silicon enclosure having a first plane, a cavity formed by semiconductor processing so as to extend from the first plane toward a back surface, and an interior wall defining the cavity; a layer of conductive metal deposited to cover all of the first plane, the cavity, and the interior wall before attaching the silicon enclosure to the substrate; Equipped with the silicon enclosure having a substantially continuous area of the first plane at a periphery of the silicon enclosure, the area being sized to engage a corresponding area of the third metal trace at a periphery of the substrate; each of the plurality of second metal traces having a corresponding respective cavity defined by a cavity wall that engages an adjacent one of the third metal traces, such that each of the plurality of second metal traces is individually surrounded by a conductive metal to provide electromagnetic field shielding for each of the plurality of second metal traces; a metal-to-metal conductive bond is formed between the third metallic trace and the mating first plane to establish a common ground reference; filter.

2. 10. The filter of claim 1, further comprising a corresponding cavity dimensioned with associated cavity walls that engage a third metal trace adjacent to the first metal trace to substantially surround the first metal trace with conductive metal between the input port and the output port to provide electromagnetic field shielding.

3. 2. The filter of claim 1, wherein the substrate is silicon carbide and the deposited metal coating is gold.

4. a metal deposited on the other major surface of the substrate; a plurality of closely spaced, contiguous through-hole conductive vias arranged to connect the third metal traces on the one major surface of the substrate to metal deposited on the other major surface of the substrate and define a common ground reference; The filter of claim 1 further comprising:

5. 10. The filter of claim 1, wherein the cavity is formed by micromachining a silicon wafer and depositing a metal having a peak-to-valley roughness of less than 2 microns.

6. 2. The filter of claim 1, wherein the input port and the output port are disposed on one of the main surfaces of the substrate, and the silicon housing has cutouts in its peripheral wall adjacent to the input port and the output port.

7. 10. The filter of claim 1, wherein at least one of the plurality of second metal traces serpentines along its length to minimize the area of the substrate required to support the second metal trace.

8. 8. The filter of claim 7, wherein at least 50% of the plurality of second metal traces each meander along their length to minimize the area of the substrate required to support the second metal traces.

9. 8. The filter of claim 7, wherein all of the plurality of second metal traces each meander along the length of the second metal trace to minimize the area of the substrate required to support the second metal trace.

10. The filter of claim 7 , wherein the first metal trace meanders between the input port and the output port.

11. a first via that connects a portion of the first metal trace on one main surface of the substrate near one end to an input metal that is disposed on the other main surface of the substrate; a second via connecting the first metal trace near the other end of the first metal trace on one major surface of the substrate to output metal disposed on the other major surface of the substrate; an input port and an output port connected to the input metal and the output metal, respectively, and disposed on the other main surface of the substrate; Furthermore, providing easy access to the input and output ports for testing the performance of the filter before implementing the filter in a larger circuit assembly; The filter of claim 1 .

12. 12. The filter of claim 11, wherein the silicon housing has a continuous area of the first planar surface surrounding an entire perimeter of the silicon housing, the continuous area of the first planar surface being sized to engage a corresponding continuous area of the third metal trace about an entire perimeter of the substrate such that there are no openings between an interior and an exterior of the silicon housing.

13. 12. The filter of claim 11, further comprising impedance matching means associated with the input and output metals on the other of the two major surfaces of the substrate, transforming the impedance at the input and output metals adjacent each of the first and second vias to substantially 50 ohms for testing and connection to external circuitry.

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