Patent Grant
9673504
The present invention relates to microwave integrated circuits, and more specifically to miniaturized, broadband microwave directional couplers, specifically quadrature directional couplers that use spiral broad side coupled lines.
Existing quadrature couplers for matching and combining of monolithic microwave integrated circuit (MMIC) power amplifiers are limited in bandwidth to about 3:1, due to fundamental limits, and are furthermore limited in size by the necessity of the structure being approximately a quarter wavelength at the center frequency of operation. There is therefore a need for ultra-small and wider bandwidth directional couplers useful in a number of applications. For instance, quadrature couplers are used in mixers and matching other nonlinear components such as limiters. Thus, microwave directional couplers are important and versatile components used in a large variety of applications, including mixers, power splitters and combiners, test equipment, and many others.
Directional couplers are four-port circuits which, in the simplest instance, comprise a pair of coupled lines with an electromagnetic coupling between the lines. The wave propagation down these lines can be described in terms of two modes: an even mode and an odd mode. These waves may also propagate down the lines with different velocities. These types of couplers may include edge coupled, multiple edge coupled, broadside coupled, and spiral edge coupled circuits. In general, tighter coupling is needed for broader bandwidth operation. As will be appreciated, coupling strength is determined in part by how close together the lines are situated. However, there is a practical limitation on how close together the lines can be made.
One common arrangement for coupled lines has long rectangular strips of metal placed side by side on a flat or planar dielectric material in the so-called edge coupled configuration. The coupling strength in this case may be severely limited, but can be increased by using multiple, appropriately interconnected, pairs of these lines. One such arrangement is called the Lange coupler. Arranging the conducting strips so that one is stacked on top of the other, in a so-called ‘broad side coupled configuration’, can further increase the available coupling. To be of practical use, the dimensions and configuration of the two lines must be such that the coupling is the desired strength, and that the structure is well matched to each of the four connecting ports, a typical value being 50 ohms. Simultaneously fulfilling these criteria may not be possible with achievable dimensions.
With current planar technology, it is a relatively straightforward task to achieve a matched impedance for the four ports in a single section style coupler. However, a single section coupler, theoretically, has a band limit, which is typically a tradeoff with the amount of allowed over-coupling. For instance, a peak-to-peak over coupling of 1 dB has a maximum theoretical bandwidth of about 2.4:1. The Lange coupler utilizes an edge coupling technique and seeks to increase the bandwidth by tightening the coupling utilizing interdigitated interconnections between planar transmission lines. However, this technique can only be extended so far. A single Lange coupler section is limited to <2 dB of over coupling. The edge coupled Lange coupler is also relatively large due to the requirement to be approximately one quarter wavelength long at the center frequency of operation.
By using multiple quarter-wave coupled line sections it is possible to greatly increase the bandwidth of operation. However, this further complicates miniaturization for integrated circuit applications. For instance, while single section quadrature couplers can only theoretically have a maximum bandwidth of 2.4:1, one can increase the bandwidth of the directional coupler by adding sections. Typically in a multi-section coupler, there are at least three sections, with the center section requiring the most tightly coupled lines. When trying to increase the bandwidth of the aforementioned Lange coupler, the size of the center section is approximately as large as the outer two sections, which dramatically increases the size of the directional coupler.
Therefore, there is a need in a multi-section coupler to minimize the size of the center section while at the same time providing it with a tight coupling characteristic.
Embodiments of the present disclosure provide a system and method for a miniaturized multi-section directional coupler using a multi-layer MMIC process. Briefly described, in architecture, one embodiment of the system, among others, can be implemented as follows. A miniaturized directional coupler comprises a broadside coupled stacked pair of spirals.
The present disclosure can also be viewed as providing a miniaturized, multi-sectioned, directional coupler using a multi-layer monolithic microwave integrated circuit (MMIC) process. Briefly described, in architecture, one embodiment of the coupler, among others, can be implemented as follows. The coupler has a monolithic microwave integrated circuit, having a central section with a tight coupling surrounded by sections of lighter coupling, the tight coupling being comprised of a pair of broadside coupled spiral lines, and the lighter coupling being comprised of meandered edge coupled lines.
The present disclosure can also be viewed as providing an apparatus, which briefly described, in one embodiment of the coupler, among others, can be implemented as miniaturized broadside coupled spirals used as a component in one of: directional couplers, baluns, microwave frequency transformers, filters, and mixers.
The present disclosure can also be viewed as providing a method for improving the bandwidth of a multi-section directional coupler having a center section. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following step: providing the center section with a pair of broadside coupled stacked spirals.
Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims.
Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.
Prior to further describing the subject invention in detail, it will be appreciated that quadrature directional couplers are extraordinarily useful devices in microwave circuitry with many kinds of implementations possible, most commonly ones used on planar circuits. Planar circuits are typically those that utilize quartz, alumina or, in the case of gallium arsenide integrated circuits, a gallium arsenide substrate onto which patterns are formed utilizing a conductor such as gold or copper, which is typically a quarter wavelength long at the center frequency of operation. These structures include interconnections at four points. These points are positioned in close proximity so that there is a strong electromagnetic coupling between the lines. One can adjust the coupling between the lines and the characteristic impedances of the structure so that one can end up with a signal input on one of the four ports and whereas the two remaining ports have signals coming out with equal amplitude and 90° phase differences, there being nothing coming out of the fourth port. These type of couplers are typically referred to as 3 dB couplers which refers to the fact that the amplitude of the 90° phase shifted output signals at the two output ports are one half the amplitude of the input signal.
In microwave circuits these structures tend to be quarter wavelength in size, having a useful bandwidth of operation that is approximately 2:1 or 3:1 depending on the intended usage and design parameters that are used. However, there are many cases in which one may wish to have a bandwidth which is much greater than 2:1 or 3:1. According to circuit theory, one cannot obtain this greater bandwidth with only a single section. It is noted that these structures can be made more compact by meandering or winding the lines back and forth on themselves. However, if the meander lines are too close together then undesired coupling occurs which can decrease the effective coupling. It may also be necessary to further increase the length of the meandered section in this case.
One of the reasons that this quadrature coupler is particularly useful is that equally split powers but 90° phase difference signals can be achieved. Given an identical pair of amplifiers which individually have poor input or output match, the additional use of an ideal quadrature coupler enables perfect match of the combination. For example, consider what happens when the amplifiers are each placed at one of the equally split ports. A signal that enters the coupler is split in two equal parts. Equal portions of the signal enter each amplifier and equal portions are reflected back into the quadrature coupler. The reflected portions acquire an additional 90° phase difference, for a total of 180°, and are therefore canceled at the input. This reflected power is absorbed in a termination placed at the fourth port of the coupler. Likewise, a second, identical coupler is placed at the output of the two amplifiers, providing similar matching to the output as well as recombining the amplified signals in phase. The result is that the entire structure is perfectly matched to 50 ohms.
In addition to amplifiers, this circuit can be utilized for limiters, mixers, and various types of circuits. However, these microwave circuits may still be limited to a bandwidth of 2:1 or 3:1. In order to increase this bandwidth, multiple coupled lines sections may be utilized. In one instance, three coupled lines sections are used for a bandwidth of 4:1 or 5:1. These individual sections are more difficult to implement in planar thin-film circuitry than conventional sections. In fact, the tightness of coupling required for the center of these three sections is so tight it is almost completely impractical to implement on a thin film circuit.
Rather than the edge coupled designs used in the past or simple versions of broad side coupling not involving spirals, in the subject invention a tiny coupler useful as the center section of a multi-section directional coupler uses miniature spiral broadside coupled lines. The use of these broadside coupled spirals increases the over-coupling of the broadside coupling structure through the mutual coupling of the turns of the spiral and thus provides a tight enough coupling such that the tiny coupler can be utilized as the center section of a multi-section directional coupler. Having provided a miniaturized three section directional coupler with this technique, it can be shown that the bandwidth of such a coupler is increased to 5.1:1 or better.
The over coupling and the concomitant increased bandwidth may be achieved in a structure that can be much smaller than the elongated quarter wavelength sections described previously. This ability to achieve this structure is due to the coiling of the lines in a tiny spiral. With this structure, it can be shown that over coupling of greater than 7 dB can be achieved in a single section utilizing a back-to-back broadside coupled spiral structure and that this over coupling can be used in a multi-section directional coupler to achieve greater than 4:1 bandwidth.
The coupling of the spiral broad side structure may be so strong that it can behave in a manner that is almost an ideal transformer. Coupling factors of 0.95 are easily achieved, in which 1.0 is ideal. In addition to its usefulness by itself as a miniature coupler and in the critical middle section of a directional coupler, it is also useful as a transformer in improved filters and extremely wide bandwidth baluns.
Moreover, since there is even mode and odd mode wave propagation in the coupler, the velocities along the two spirals are different if there are differences in the dielectric utilized between the two spirals, such as the relative proximity of the ground plane below or air above the structure. Twisting the lines in the spirals can equalize these waves by making the electrical paths equal. If the electrical paths are not equal, one can have degraded performance at high frequencies.
In a preferred embodiment of a multi-section directional coupler, a center section with the tightly coupled spirals is flanked by two under-coupled outer meander line sections whose characteristics can be tuned with capacitive pads and resistors for impedance matching purposes.
As to the ability to provide such a spiral structure with requisite characteristics, providing the subject spiral structure with the appropriate impedances and other characteristics is not possible utilizing conventional planar microwave processing techniques. The reasons are that for conventional planar microwave processing, high dielectric constant insulating material is utilized between the metallized layers, and the dielectric layers are typically exceedingly thin. This precludes the ability to set electrical parameters in the tiny structures required for the miniaturized couplers, and especially for miniaturized spiral couplers.
It has been found that the use of a three layer production technology called 3MI facilitates parameter control to permit fabrication of these spirals with the appropriate dimensions and properties to tailor the impedance of the spiral structures. The 3MI process is successful primarily because in 3MI a low dielectric constant material is utilized between the layers, in one embodiment polyimide, and in which the thickness of the dielectric layer is relatively thick and on the order of 4μ. In place of the polyimide, other materials, such a bisbenzocyclobutene (BCB) electronic resin sold under the tradename CYCLOTENE may be used. The use of this 3MI process permits control of the electrical characteristics and parameters necessary to fabricate the miniaturized spiral structure, with the term 3MI referring to a layered structure with three metal layers.
As previously noted, it is extremely difficult to build a structure with sufficiently tight coupling for the center section of, for instance, a three section directional coupler to get a bandwidth wider than 4:1-5:1. As one goes to more and more sections for even wider bandwidths the structure of the center section gets even more difficult. There is also a problem with the outer sections in that the lines in the outer sections need to be weakly coupled, i.e. under-coupled. This typically requires lines that are both very large and far apart from each other, thus further increasing size. Even with meandering, the outer sections are considerably larger than the single center section meandered embodiment. By applying the MMIC Multi-layer interconnect (3MI) process it is possible, with the use of previously unavailable components, to design a multi-section coupler in the same small space, with the subject technology expanding the bandwidth to >4:1. Future expansion of the approach enables bandwidths of up to 10:1 in a space compatible with MMIC integration.
It is noted that the innermost section of a multi-section coupler must have extremely tight coupling, to levels that are not practical with existing thin film technology. A spiral broad side coupler enabled by the new processing can have coupling to very high levels, and the approaches developed allow tuning of the coupling levels. Furthermore, the outer coupling sections can also be improved with appropriate use of the 3MI capability. The solution is equally applicable to GaAs and GaN implementations. In the following, a quadrature coupler is defined to have four ports, and ideally has an equal split between the direct and coupled ports at a 90 degree phase difference, while no power exits the isolated port.
The following information and table presents a comparison of available peak coupling from the range of structures under consideration showing the superiority of spiral broadside coupled lines.
Starting with a single edge coupled pair of lines, examples of available peak coupling have been determined at approximately 8 GHz and with approximately 50 ohms impedance for each of the four ports. There is over coupling if the peak of the coupling is >0 dB, and under coupling if the peak is <0 dB. Only the single edge coupled pair is under-coupled. The approximate value of the normalized impedance for the even mode is also shown in the following table:
As understood from Table 1, the Lange coupler implementation for planar circuitry is limited to less than 2 dB. This limitation is insufficient for use in the wider bandwidth, three section implementation. As further understood from Table 1, edge coupling increases from the single pair to the 4 pair versions over a range of −5 dB (under-coupled) to less than +2 dB of over coupling.
A pair of planar, edge coupled (non-broadside) spiral coupled lines was tested and was no more tightly coupled than 0 dB for the stated conditions. However, as to spiral broadside coupled lines, a single pair of spiral broadside coupled lines, matched to 50 ohms, can be over coupled to +3.9 dB, when fabricated using the “3MI process”, which has 3 independent metal interconnect layers and will be described hereinafter. Note, a pair of back to back spiral broadside coupled lines was found to have a peak coupling of +7.5 dB in a 50 ohm matched condition. In either the case of a single pair of broadside coupled lines, or back to back pairs, the amount of strong over coupling was significant and enables use in a multi-section directional coupler to give it a wide bandwidth.
The result is a wide range of applications. It should be noted, that by adding a fourth metal layer, it is possible to further increase the over coupling by an additional 2 dB.
As can be seen, for multi-section directional couplers, with appropriate use of a specialized 3MI multilayered technology one can make extremely tightly coupled inner sections and also be able to design outer sections that are reduced in size. This 3MI technique permits stacking tightly wound spirals which serve as the center section of a three section coupler, with the tightly wound and tightly coupled spirals permitting increased bandwidth for the three section coupler in an extremely small package size. The net result is to achieve miniaturized couplers in which the tight couplings result in increased bandwidth made possible by utilizing spiral broadside coupled lines and a specialized fabrication technique to create the MMIC structures.
More particularly, it has been found that it may be theoretically impossible to obtain the desired bandwidth utilizing single section tightly coupled meander line technology. The result is that one has to go to at least a three section or a multi-section version of a directional coupler. However, in order to adjust the parameters for the coupling, one needs a specialized manufacturing technology, such as the 3MI technology, to be able to adjust the circuit parameters in the small microwave circuits. This technology utilizes a low dielectric constant material between metal layers. It is common practice to use a thin high dielectric material placed in between layers for use in structures like capacitors. This structure is inappropriate for use in coupled structures because the corresponding required line widths become extremely small, which also makes the lines very resistive and lossy. This approach severely limits design flexibility. Thus, prior techniques cannot offer the design flexibility that is achievable with the 3MI technology that uses low dielectric material between the metal layers.
Using the 3MI process, one can build structures that were not previously realizable. With the process described above, one is now able to make a miniaturized version of a multi-section coupler with very good performance. These couplers have a wider bandwidth than heretofore possible while also achieving the same accuracy of the magnitude of the split, namely the 90° phase split.
In particular with the 3MI technology one can provide a unique spiral structure with multiple metallization layers with a low dielectric constant insulator that is thick enough to give one a large degree of control over the coupling parameters and impedance parameters of the transmission lines. The 3MI process utilizes an additional set of processing steps added to standard MMIC fabrication techniques as follows:
The final metal layer from the finished MMIC process becomes the first layer of the three layer metal interconnect (3MI). A layer of dielectric material such as polyimide is deposited over the surface of the MMIC and holes are patterned in the polyimide where via connections will be needed subsequently. In one embodiment, a layer of gold is deposited and patterned on the top of this first polyimide layer. This comprises the second metal interconnect layer. The polyimide deposition and patterning, and metal deposition and patterning steps are then repeated to complete the third metal interconnect layer. This process could then be repeated additional times for additional interconnect layers. A BCB electronic resin or similar material can be used in place of the polyimide.
In one embodiment, the basic process ends up with transistors, resistors, capacitors, and a layer of interconnect metallization on the surface of gallium arsenide. The gallium arsenide also has via connections to a back side metallized ground plane. Then a layer of polyimide is put down as a 4μ thick layer of plastic. Thereafter, particular locations are provided with through-holes through the plastic that are photolithographically defined. Then, using standard semiconductor processes, through-holes are made. This allows the via connections in between the layers. Then a patterned layer of metallization is placed on top of the first layer of plastic, in one embodiment using an electroplating process used to build up a total of 2 microns of metallization. Then one provides a layer of plastic and another layer of metal interconnect is followed by making more through-holes in the polyimide and then plating up an additional metal layer on top of what has been provided, with the top layer dielectric being 4μ in thickness.
The net result is that by utilizing this process one can make very small coupled spirals for the center of a three section coupler and to adjust the properties and offsets of these spirals in such a way that one can achieve the required increased bandwidth. In one embodiment, it was also found that rather than superimposing identical spirals on top of each other, which would result in a coupling a bit too tight, the two layers could be slightly offset to be able to tune the coupling between the spirals.
By reducing the amount of overlap between metal layers, the coupling is reduced in a manner that can be tightly controlled, enabling fine tuning of the overall performance. In addition, there are interconnections between layers that provide a twisted structure that serves to equalize the signals traveling on adjacent lines. The current and corresponding fields on the lower layer couple more strongly to the substrate material, which tends to have a higher dielectric constant, and to the ground plane. Likewise, the currents and corresponding fields on the upper layer are much less affected by surrounding dielectric material. If unchecked, the differences in propagation would accumulate and degrade performance, especially at higher frequencies in the band. Instead, the currents are periodically switched between layers with an interlayer via connection. This switching, in effect, causes a half twist at each of the interlayer connections. Therefore, on average, the currents and corresponding fields and waves are subjected to similar loading. In this manner, the signal paths are equalized which tends to improve overall performance, especially at higher frequencies.
The number of twists per turn can vary. Empirically, one or two twists per turn of the spiral are sufficient. By employing low dielectric constant material between the broadside coupled metal layers in the spirals, a very wide range of coupling and impedance levels is realizable, enabling the design of miniaturized, high performance couplers with wide bandwidth when using a multi-section design. In general, it is possible for a coupler with a given number of coupled line sections to trade off ripple and bandwidth, so long as the coupling and impedance is realizable with the fabrication approach. A larger number of sections enables increasingly wide bandwidths with decreasing amounts of ripple, but nonetheless requires increasingly strong coupling in the center section. This structure is achieved using a multilayer processing scheme such as the 3MI used by BAE Systems.
According to one embodiment, a miniaturized multi-sectioned, directional coupler using a multi-layered MMIC process comprises a monolithic microwave integrated circuit, having a central section with a relatively tight coupling, surrounded by sections of lighter coupling, the relatively tight coupling being comprised of a pair of spiral coupled lines, and the lighter coupling using meandered edge coupled lines with capacitive loading of the lines in several places. Moreover, the circuit includes a plurality of metallization layers, set on the top of the monolithic microwave integrated circuit, and vertically oriented structures, situated below the corners of the meandered lines of the lighter coupling. In one embodiment, the vertically oriented structures have square pad structures to capacitively tune the coupling and impedance of the lines of the meandered edge coupler section.
The following features may be included. The coupler may have three or more metallization layers set on the top of the monolithic microwave integrated circuit, with a coupler utilizing a plurality of low dielectric constant insulation layers between the metallization layers. The low dielectric constant insulation layer in one embodiment is polyimide. In another embodiment, the low dielectric constant insulation layer may be a BCB electronic resin.
In summary, a miniaturized quadrature microwave integrated circuit is provided with an increased bandwidth due to the use of spiral broadside coupled lines and the utilization of fabrication technology that involves the utilization of a low dielectric constant material between metallization layers and an improved MMIC fabrication methodology.
In the ideal case, it is sufficient to describe the behavior of this circuit in terms of even odd mode impedances and the propagation characteristics along the transmission lines. It is noted that the even mode travels down the lines with a particular velocity assuming a gallium arsenide substrate and a polyimide dielectric, with air on top. The odd mode has its own propagation velocity. The result is that the even and odd modes propagate down the lines with different velocities. This becomes a frequency limiting factor.
However, ignoring the difference in velocities, the closer the two lines are together, the more tightly the lines are coupled. In general the even and odd mode impedances are dependent on the actual geometry in a complex fashion. The odd mode impedance tends to relate to how close the lines are to each other, and the even mode impedance tends to relate to how close the lines are to the underlying ground plane. For instance, going from gallium arsenide to polyimide, one can more readily adjust the electrical distance to the ground plane and thus the impedances. However, the lines can only be placed so close together due to the physical limit on how close together the lines can be placed.
When utilizing typical thin-film processing technology in order to obtain the appropriate impedances, extremely narrow lines should be used. These lines will have a higher resistance which becomes a practical limit to the impedances that can be achieved because of the small size. Typically, the characteristic impedance for the structure is on the order of 50 ohms. The 50 ohms is the square root of the product of Zoe and Zoo. Being able to create the appropriate impedances for the coupler utilizing conventional thin-film technology is challenging because instead of a single equation of the voltage equaling some trigonometric function of distance, one has two sets of equations and thus many more parameters to deal with.
Current planar edge coupled technology offers a reasonably straightforward method to achieve a matched impedance for a single section coupler. However, this single section coupler theoretically has a bandwidth limit.
Instead of measuring voltages and currents in a microwave circuit, since one cannot usually do so, one measures power. The power in each of the lines is measured in terms of S parameters with the S parameter indicating the ratio of power in and power out of the respective ports.
With respect to
It will be appreciated that this is a three metallized layer structure, with the M1 layer corresponding to the interconnects to the first spiral and with M2 and M3 referring to the first and second spirals stacked on top of one another. As mentioned previously, it is the use of a low dielectric constant dielectric layer, such as available with polyimide, and the relative thickness of this polyimide layer that provides for the flexibility in designing of the miniaturized spirals described above.
In short, what is provided is a multi-section directional coupler having a central section with very tight coupling, surrounded by sections of lighter coupling. The characteristics of the middle and outer sections allow design tradeoffs in bandwidth and pass band ripple. The design approach is illustrated in one embodiment by a three section coupler that operates over the 3 to 13 GHz band with approximately 1 dB of coupling variation over the band. The coupling for the inner spirals and the outer lines is approximately +4.3 dB and −14 dB respectively. This design is only slightly larger than a single section version using the conventional approach, but is much wider in bandwidth.
While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications or additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.
This application claims benefit of U.S. Provisional Application Ser. No. 62/040,447 entitled, “MINIATURIZED MULTI-SECTION DIRECTIONAL COUPLER USING MULTI-LAYER MMIC PROCESS” filed Aug. 22, 2014, the entire disclosure of which is incorporated herein by reference.
This invention was made with United States Government support under Contract No. N00019-10-C-0070 awarded by the US Department of the Navy. The United States Government has certain rights in this invention.
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| Number | Date | Country | |
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| 20160056521 A1 | Feb 2016 | US |
| Number | Date | Country | |
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| 62040447 | Aug 2014 | US |
