High-frequency electromagnetic bandgap device and method for making same

A high-frequency Electromagnetic Bandgap (EBG) device, and a method for making the device are provided. The device includes a first substrate including multiple conducting vias forming a periodic lattice. The vias of the first substrate extend from the lower surface of the first substrate to the upper surface of the first substrate. The device also includes a second substrate having multiple conducting vias forming a periodic lattice. The vias of the second substrate extend from the lower surface of the second substrate to the upper surface of the second substrate. The second substrate is positioned adjacent to, and overlapping, the first substrate, such that the lower surface of the second substrate is in contact with the upper surface of the first substrate, and such that a plurality of vias of the second substrate are in contact with a corresponding plurality of vias of the first substrate.

TECHNICAL FIELD

The present invention generally relates to Electromagnetic Bandgap (EBG) devices, and more particularly, to EBG devices having high bandgap and resonant frequencies.

BACKGROUND OF THE INVENTION

EBG devices are devices generally having an ability to suppress and filter electromagnetic energy. EBG devices are often used to help suppress switching noise and electromagnetic radiation in printed circuit boards (PCBs) and packages containing electronic devices. Such devices are also sometimes used to improve the performance of planar antennas by reducing cross-coupling between antenna array elements through surface waves, and by suppressing and directing radiation. EBG devices can be useful in other active and passive devices and applications such as oscillators, waveguides, transmission lines, amplifiers, filters, power combining circuits, phased arrays, mixers, and microwave components and devices.

A typical EBG device generally has a periodic structure, such as for example, a lattice, that is made up of periodic perturbations. These periodic perturbations, also known as vias, can take the form of holes or dielectric or metal rods or posts. Often an EBG device takes the form of a uniform substrate material with metal on both sides creating a parallel plate. The substrate between the parallel plates is typically loaded with metal or dielectric rods or patches that form the periodic perturbations.

FIG. 1Aprovides an example of a conventional EBG device50located in a printed circuit board (PCB)62.FIG. 1Bprovides an enlarged view of the EBG device50. As shown, EBG device50has a dielectric layer52positioned between two ground planes54and54a. Embedded in dielectric layer52are conductive vias56in a regular periodic pattern. Conductive vias56are typically formed from metal or a metal alloy. EBG device50is also shown having a coplanar waveguide input58, and a coplanar waveguide output60. In operation, the periodic pattern of conductive vias56acts to filter the coplanar waveguide input58before the signal is output at the coplanar waveguide output60.

A typical EBG device50functions to block or suppress the propagation of electromagnetic radiation that falls within a certain defined frequency band known as a stopband or bandgap. The EBG device50can be characterized by its stopband/bandgap characteristics. These can include the width of the stopband/bandgap and the location in the frequency spectrum of the stopband/bandgap. For a given EBG device50, the characteristics of the stopband/bandgap are generally determined by the physical characteristics and location of the periodic perturbations or conductive vias56in the device. The overall effect of the conductive vias56in an EBG device50is to create a filter that blocks electromagnetic energy in a certain frequency range from propagating in the substrate and on the surface of the substrate. Characteristics of the perturbations, or conductive vias56, that can determine the bandgap characteristics include the spacing of the perturbations, the size of the perturbations, and the material used to create the perturbations. By choosing certain materials, sizes, and locations, the width and frequency location of the bandgap can be selected.FIG. 1Cgenerally illustrates the transmission characteristics associated with the conventional EBG device50. As can be seen, the conventional EBG device50will typically pass certain frequency ranges (those above and below the bandgap), and will attenuate frequencies that fall within the bandgap. As seen inFIG. 1C, the bandgap is bounded on the high end by an upper bandgap frequency above which signals are not significantly attenuated.

Conventional EBG devices discussed above can also be formed to allow some frequencies of electromagnetic energy within the bandgap to propagate. This is commonly accomplished by including defects, called defect resonators, in the EBG structure when it is manufactured. These defect resonators are interruptions or defects in the symmetry of the otherwise regular pattern of periodic perturbations56in the EBG device50. For example, in an EBG device50including a periodic pattern of perturbations that are conductive vias56, a defect could be formed by not including one of the conductive vias in the periodic pattern when the EBG device is manufactured. In another example involving a single substrate plane with a periodic pattern of via apertures filled with a dielectric material, a defect could be formed by not filling one of the via apertures.

In operation, a defect resonator in an EBG device50typically creates an area of resonance in the EBG device50by localizing energy within the structure, allowing transmission of a narrow frequency within the stopband or bandgap of the EBG device50. In effect, an EBG device50formed with a defect resonator typically acts as a high-Q filter, suppressing frequencies within the bandgap except for those resonated by defects.FIG. 1Dprovides a general illustration of the frequency characteristics of the conventional EBG device50having a defect resonator. As can be seen, an EBG device50having a defect resonator will typically allow some frequencies within the bandgap to pass through the EBG device without being significantly attenuated. The frequencies within the bandgap at which signals pass through the EBG device50having a defect resonator without being significantly attenuated are referred to as resonant frequencies.

Although characteristics of EBG devices with and without defect resonators can be selected prior to the manufacturing of the structures, manufacturing process imprecision, process tolerance limitations, and manufacturing cost tradeoffs can make it difficult to manufacture EBG devices having high upper band gap frequencies and high resonant frequencies to provide for desired performance in high-frequency applications. It is therefore desirable to provide for a bandgap devices, and methods for producing such devices, that can achieve higher upper bandgap frequencies and resonant frequencies without requiring the use of atypical, expensive manufacturing processes.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a high-frequency Electromagnetic Bandgap (EBG) device is provided. The device includes a first substrate including multiple conducting vias forming a periodic lattice in the first substrate. The vias of the first substrate extend from the lower surface of the first substrate to the upper surface of the first substrate. The device also includes a second substrate having multiple conducting vias forming a periodic lattice in the second substrate. The vias of the second substrate extend from the lower surface of the second substrate to the upper surface of the second substrate. The second substrate is positioned adjacent to, and overlapping, the first substrate, such that the lower surface of the second substrate is in contact with the upper surface of the first substrate, and such that a plurality of vias of the second substrate are in contact with a corresponding plurality of vias of the first substrate.

According to another aspect of the present invention, a high-frequency Electromagnetic Bandgap (EBG) device is provided. The device includes a first substrate made of material including a low-temperature co-fired ceramic. The first substrate includes a periodic lattice of conducting rods having a first diameter. The rods extend from the lower surface of the first substrate to the upper surface of the first substrate. The device also includes a second substrate made of material including low-temperature co-fired ceramic. The second substrate includes a periodic lattice of conducting rods having a second diameter. The rods extend from the lower surface of the second substrate to the upper surface of the second substrate. The second substrate is positioned adjacent to, and overlapping, the first substrate, such that the lower surface of the second substrate is in contact with the upper surface of the first substrate. The location of the conducting rods in the first substrate corresponds to the location of the conducting rods in the second substrate. Lower exposed surfaces of the conducting rods of the second substrate are in contact with upper exposed surfaces of the conducting rods of the first substrate. A ground plane at least partially covers the upper surface of the second substrate, and is in contact with upper exposed surfaces of the conducting rods of the second substrate.

In accordance with yet another aspect of the present invention, a method for fabricating an Electromagnetic Bandgap (EBG) device is provided. The method includes the steps of providing a first substrate and arranging a periodic lattice of conducting vias in the first substrate such that the vias of the first substrate have upper surfaces having a first cross-sectional area exposed on the upper surface of the first substrate. The method further includes the steps of providing a second substrate and arranging a periodic lattice of conducting vias in the second substrate such that the location of the vias of the second substrate correspond to the location of vias in the first substrate, and such that the vias of the second substrate have lower surfaces having a second cross-sectional area exposed on the lower surface of the second substrate. The method further includes the step of positioning the second substrate adjacent the first substrate such that the lower surface of the second substrate overlaps the upper surface of the first substrate, and such that lower surfaces of the vias of the second substrate are in contact with upper surfaces of corresponding vias of the first substrate. The conducting vias of the first and second substrates are formed such that the second cross-sectional area is less than the first cross-sectional area.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 2, an Electromagnetic Bandgap (EBG) device70is shown including a first planar substrate72. As shown, first planar substrate72includes a periodic lattice of vias74embedded in first planar substrate72. In the present embodiment, first planar substrate72is made of low-temperature co-fired ceramic (LTCC), and the periodic lattice of vias74are conductive vias formed in the shape of columns or rods. First planar substrate72has a lower surface and an upper surface, and conductive vias74, formed in first planar substrate72, extend from the lower surface of first planar substrate72to the upper surface of first planar substrate72. More specifically, lower surfaces of the conducting vias74are exposed on the lower surface of first planar substrate72, and upper surfaces of conductive vias74are exposed on the upper surface of first planar substrate72. As shown, the conductive vias74are in the form of cylindrical columns that have a height equal to the thickness of first planar substrate72and a diameter. It should be appreciated that because in the present embodiment the conductive vias74are in the shape of columns that the exposed upper surfaces and lower surfaces of the conductive vias74take the form of circles in the upper and lower surfaces, respectively, of first planar substrate72.

In alternate embodiments, first planar substrate72may be formed from FR4, or other materials used to form printed circuit boards (PCBs), or from other dielectric material. It should also be appreciated that in alternate embodiments, the conductive vias74formed in first planar substrate72may be in shapes other than columns or rods, and may be formed of material other than conducting material, such as, for example, a dielectric material.

EBG device70is also shown including a second planar substrate76that includes a periodic lattice of vias78formed within the second planar substrate76. As shown, the vias78extend from the lower surface of the second planar substrate76through second planar substrate76to the upper surface of second planar substrate76. It should be appreciated that both the upper and lower surfaces of the vias78are exposed on the upper and lower surfaces, respectively, of second planar substrate76.

In the present embodiment, second planar substrate76is formed of LTCC, and the vias78are formed of a conducting material, such as, for example, a metal or metal alloy, and are in the form of cylindrical columns or rods extending from the lower surface of second planar substrate76to the upper surface of second planar substrate76. In the present embodiment, the conductive columns78have a height equal to the thickness of second planar substrate76and have a diameter. It should be appreciated that because in the present embodiment the conductive vias78are in the shape of columns or rods, that the exposed upper and lower surfaces of the conductive vias78take the form of circles in the upper and lower surfaces, respectively, of second planar substrate76. As shown inFIG. 2, the diameter of the conductive columns78, formed in second planar substrate76, and therefore the diameter of the exposed upper and lower surfaces of the conductive columns78, is less than the diameter of the conductive columns74formed in first planar substrate72, and is therefore also less than the diameter of the exposed upper and lower surfaces of the conductive columns74. In an alternate embodiment, the total area in second planar substrate76occupied by each of the conductive volumes78is less than the total area in first planar substrate72occupied by each of the corresponding conductive columns74.

Although in the present embodiment, second planar substrate76is formed of LTCC, it should be appreciated that in alternate embodiments, second planar substrate76may be formed from FR4, or other materials used to form PCBs, or from other dielectric material. Although in the present embodiment, the vias78formed in second planar substrate76are columns or rods formed of conducting material, it should be appreciated that in alternate embodiments, vias78may have a shape other than a cylindrical column or rod shape, and may be formed from material other than material that is conducting, such as, for example, dielectric material. In embodiments in which the vias74and/or78have shapes other than cylindrical columns or rods, the width and/or surface area of the surfaces of vias78exposed on the surface of substrate76is less than the width and or surface area of the surfaces of vias74exposed on the surface of substrate72.

Continuing withFIG. 2, the periodic lattice of conductive vias78formed in the second planar substrate76has the same period and spacing as the periodic lattice of conductive vias74formed in the first planar substrate72. In other words, the periodic lattice of conductive vias78and74formed in the second planar substrate76and in the first planar substrate72, respectively, are formed such that when first planar substrate72and second planar substrate76are positioned properly with respect to each other, the conductive vias78and74formed in the first planar substrate72and the second planar substrate76, respectively, overlap, and are in contact with, each other.

As shown inFIG. 2, second planar substrate76is positioned adjacent to first planar substrate72, such that the lower surface of second planar substrate76is in contact with the upper surface of first planar substrate72. In addition, second planar substrate76is positioned relative to first planar substrate72, such that the conductive vias78formed in second planar substrate76overlap with corresponding conductive vias74formed in first planar substrate72. In addition, because the lower surface of second planar substrate76is adjacent to the upper surface of first planar substrate72, and because the lower surfaces of conductive vias78are exposed in the lower surfaces of second planar substrate76, and the upper surfaces of conductive vias74are exposed in the upper surfaces of first planar substrate72, it should be appreciated that the lower surfaces of conductive vias78are in physical contact with the upper surfaces of conductive vias74.

In the present embodiment, the overall result is a conductive path from the lower surfaces of the conductive vias74exposed on the lower surfaces of first planar substrate72through the conductive vias74exposed on the upper surface of first planar substrate72, on to the upper surfaces of conductive vias74to the lower surfaces of conductive vias78exposed on the lower surfaces of second planar substrate76, through conductive vias78, and on to the exposed upper surfaces of the conductive vias78on the upper surface of second planar substrate76.

The EBG device70also includes a lower ground plane80having upper and lower surfaces, and having its upper surface positioned adjacent to, and in contact with, the lower surface of first planar substrate72. It should be appreciated that the lower exposed conductive surfaces of conductive vias74are in contact with the upper surface of lower ground plane80. EBG device70further includes an upper ground plane82positioned adjacent to the upper surface of second planar substrate76, such that the lower surface of upper ground plane82is in contact with the upper surface of second planar substrate76and the upper conducting surfaces of conductive vias78exposed in the upper surface of second planar substrate76.

In the present embodiment, upper ground plane82also includes a coplanar waveguide formed in the upper ground plane82, and having a coplanar waveguide input84and a coplanar waveguide output86. Coplanar waveguide input84and coplanar waveguide output86are positioned, such that they are not in electrical contact with the upper surfaces of conductive vias78. The resulting EBG device70will have a bandgap with respect to signals provided at the coplanar waveguide input84. More specifically, frequencies of a signal provided at coplanar waveguide input84that fall within the frequency range of the bandgap of EBG device70will be attenuated as they pass through EBG device70from input84to output86.

As shown inFIG. 2, the resulting EBG device70has a periodic matrix of stacked conductive vias79formed of upper conductive vias78formed in second planar substrate76and stacked on top of lower conductive vias74formed in first planar substrate72. The upper conductive vias78of the stacked conductive via79have a diameter that is less than the diameter of the lower conductive vias74of the stacked conductive vias79. Because the diameter of the conductive vias78is smaller than the diameter of the lower conductive vias74, the stacked conductive vias79of EBG device70may be spaced closer together than typical conductive vias without having the upper exposed conducting surfaces of the upper conductive vias78in contact with the input84and/or output86of the coplanar waveguide formed in the upper ground plane82. By allowing for less distance between the stacked conductive vias79formed in the EBG device70, EBG device70is enabled to exhibit upper bandgap frequencies higher than conventional EBG devices.

Referring toFIG. 3, an EBG device90having a defect resonator88is provided. The EBG device90generally illustrated inFIG. 3is identical to the EBG device70of the embodiment ofFIG. 2, with the exception that a defect resonator88is present in the EBG device90. More specifically, as shown, the periodic lattice or matrix of conductive vias74formed in the first planar substrate72is interrupted by the absence of a conductive via74in the middle of the periodic matrix of conductive vias74. In addition, the periodic matrix of conductive vias78formed in the second planar substrate76is interrupted by the absence of a conductive via78in the middle of the periodic matrix of conductive vias78. Because the periodic lattice of vias formed in the first planar substrate72and second planar substrate76is interrupted by the absence of the vias noted above, the EBG device90exhibits a resonant frequency within the bandgap. The discontinuity in the periodic matrix of conductive vias is referred to as a defect resonator88. The resonant frequency of EBG device90caused by defect resonator88is in part determined by the location and physical characteristics of the defect resonator88. Because, as noted above with respect to the embodiment generally illustrated inFIG. 2, the conductive vias78and74formed in second planar substrate76and first planar substrate72, respectively, may be located closer together because of the smaller diameter of the conductive vias78, EBG device90can achieve a higher defect resonant frequency than typical EBG devices.

Although the defect resonator88of EBG device90in the present embodiment is formed by the absence of conductive vias78and74in both second planar substrate76and first planar substrate72, respectively, it should be appreciated that in alternate embodiments, because a defect resonator88may be formed by changing the physical characteristics of the defect resonator88and/or the location of the defect resonator88, EBG device90may have defect resonators88formed by a lack of conductive vias in either second planar substrate76or first planar substrate72, or both second planar substrate76and first planar substrate72. Multiple defect resonators88may also be formed by having multiple absences of conductive vias in the periodic matrices formed in second planar substrate76and/or first planar substrate72. In addition, it should be appreciated that a defect resonator88may be formed by altering the shape and/or size of conductive vias formed in second planar substrate76and/or first planar substrate72relative to the shape and/or size of vias of regular, periodic matrices of vias formed in second planar substrate76and first planar substrate72.

In one specific alternate embodiment, an EBG device90is formed with coplanar waveguide input84and coplanar waveguide output86each having a width of 4 mils, the spaces between coplanar waveguide input84and upper ground plane82having a width of 4 mils, and the spaces between output86and upper ground plane82having a width of 4 mils. In this alternate embodiment, the conductive rods78formed in the second planar substrate76have a diameter of 4 mils, and the conductive rods74formed in first planar substrate72have a diameter of 8 mils. In this embodiment, the EBG device90exhibits an upper bandgap of greater than approximately 76.5 GHz and a resonant frequency of greater than approximately 76.5 GHz.

In an yet another alternate embodiment of EBG device90, the widths of coplanar waveguide input84and coplanar waveguide output86, the spacing between coplanar waveguide input84and upper ground plane82, the spacing between coplanar waveguide output86and upper ground plane82, the diameter of conductive vias78, and the diameter of conductive vias74are selected, such that the EBG device90exhibits a resonant frequency and upper bandgap of greater than approximately 65 GHz.

Referring toFIG. 4, a method100for making a high-frequency EBG device is provided. In a first step102of the method, a first substrate is provided. In the present embodiment, the substrate is made of low-temperature co-fired ceramic. In an alternate embodiment, the substrate is made of FR4 or other materials used to fabricate printed circuit boards (PCBs), or other dielectric material. In a second step104of the method, conducting vias are arranged in the first substrate in a regular periodic matrix or lattice. In an alternate embodiment, the vias are made of a dielectric material. In still another alternate embodiment, the periodic matrix of vias is interrupted by at least one defect or discontinuity in the periodic matrix. In a third step106of the method, a second substrate is positioned such that it overlaps, and is in contact with, the first substrate. In the present embodiment, the second substrate is made of low-temperature co-fired ceramic. In an alternate embodiment, the substrate may be made of FR4 or other materials used to fabricate PCBs, or other dielectric material.

In a fourth step108of the method, conducting vias that are smaller than the conducting vias arranged in the first substrate are arranged in the second substrate in a regular periodic matrix or lattice. The vias and the second substrate are arranged such that the lower surfaces of the vias arranged in the second substrate overlap, and are in contact with, upper surfaces of the vias arranged in the first substrate. In an alternate embodiment, the vias are made of a dielectric material. In still another alternate embodiment, the periodic matrix of vias in the second substrate is interrupted by at least one defect or discontinuity in the periodic matrix. In still another alternate embodiment, both the first substrate and second substrate have discontinuities in their respective matrices of conductive vias, and the location of the discontinuities in the first substrate correspond to the location of discontinuities in the second substrate. In a fifth step110of the method, a ground plane is provided on the upper exposed surface of the second substrate such that the ground plane is in contact with exposed upper surfaces of the conducting vias of the second substrate. In a sixth step112of the method, a coplanar waveguide is formed in the ground plane and positioned relative to the conducting vias of the second substrate such that the upper bandgap of the resulting structure is greater than approximately 65 GHz. In an alternate embodiment in which discontinuities are present in either the first or second substrates, the coplanar waveguide is formed and positioned relative to the conductive vias of the substrates such that the resulting structure has a resonant frequency of greater than approximately 65 GHz.

As described above, the invention advantageously provides for EBG devices with resonant frequencies and upper bandgap frequencies of greater than 65 GHz without requiring the use of atypical and expensive processing method. The invention advantageously permits the spacing between vias and a periodic lattice of vias to be decreased to achieve higher resonant and upper bandgap frequencies without causing the periodic vias to interfere with the input and output of coplanar waveguides formed in the EBG device.