Cofired multi-layer ceramic structure incorporating a microwave component FIELD OF THE INVENTION The present invention relates generally to microelectronics, and more particularly to a cofired multi-layer ceramic structure incorporating a microwave component, in particular a circulator composed of a ferrite film. Specially, the invention concerns an integrated circuit comprising a cofired assembly of a plurality of layers of a dielectric carrier of circuit elements, a film including a material having ferroelectric and ferromagnetic properties incorporated in at least one of said dielectric carrier layer (s), which film is interiorly positioned within the cofired assembly.

BACKGROUND OF THE INVENTION Low temperature cofired ceramic (LTCC) process technologies are routinely performed to incorporate passive components in a multi-layer substrate that is sintered to form a cohesive structure. Ceramic tape is formed by mixing ceramic powders with organic binders and processing the mixture into a thin tape that is commonly referred to as"green tape"in its unfired state. Multiple sheets of this ceramic green tape are printed with passive components consisting of metalized circuit patterns, and these sheets are arranged in layers that are electrically vertically interconnected by conductive vias. The layers are subsequently assembled, laminated, and cofired at a relatively low temperature (less than 1000°C). Cofiring"sinters"the substrate layers thereby creating a monolithic integrated circuit. The low temperature sintering used in LTCC processes enables the incorporation of components made from highly conductive materials such as silver (Ag), gold (Au), copper (Cu), silver-palladium (Ag-Pd), and platinum-gold (Pt-Au), rather than the less conductive materials such as tungsten (W) and molybdenum (Mo) that must be used in high temperature cofired ceramic (HTCC) applications.

After the LTCC process is complete, the formerly multi-layer structure is a ceramic package that is relatively mechanically strong, hermetic, thermally conductive, and chemically inert.

In millimeter wave or microwave communications systems, e. g. a ceramic packaging is used for instance such as a Microwave Integrated Circuit (MIC).

In packaging MICs, electrical performance must often be weighed against physical constraints imposed by MIC manufacturing and component insertion methodologies.

A circulator is a multi-port component that permits radio frequency (RF) to flow in one direction between adjacent ports. This is one application for ferrite components. Other applications include phase shifters, limiters, duplexers, switches, modulators, T/R modules, attenuators and dampers. An example of a typical junction circulator 10 is shown in Figure 1, and consists of three ports 12 that form a symmetrical Y intersecting at a junction disk 14 that is coupled to a magnet 16. Microwave energy 18 can flow in one direction only, for example from port A to B, B to C, and from C to A. When one of the ports is terminated usually with a resistive load, the component is referred to as an isolator.

The magnet included in a typical circulator increases the thickness of the device substantially. Therefore, to append a circulator or isolator to a ceramic package, the typical circulator is"dropped in"and secured mechanically (by a flange or through-hole mounting process) after the sintering step. Electrical connections are established by soldering connecting means such as leads from the drop-in component to leads attached to previously printed metalized components such as conductors, resistors, and capacitors. However, drop-in components are expensive and procurement is somewhat time-consuming because the components are difficult to specify and suppliers typically require relatively long lead times. Furthermore, the drop-in process requires an extra trimming step that produces waste and slows processing time. Performance and yield are also suboptimal because soldering

electrical connections increases the risk of circuit failure, and increased spacing is required between components.

Constructing the circulator out of a magnetized ferrite material, thereby eliminating the step incorporating the sintering process, can reduce the thickness of the typical circulator. However, such circulators are still relatively large and heavy devices because direct current and an induction coil are required to generate a magnetic field. The component cannot survive temperatures above 130°C, so the reduced circulator must be dropped in after the do-firing utilizing microstrips or coplanar waveguide (CPW) launches and bonded (using an epoxy adhesive, and gold wire ribbon or tin lead solder bonds, for example) in order to append the reduced circulator to the MIC. However, if the application requires additional components to be directly connected to a microstrip circulator, the microstrip circulator must be bonded to an adjacent microstrip using connection tabs.

Alternatively, when ordering a microstrip component from a manufacturer, a user can specify a single substrate upon which additional components can be printed.

CPW launches are also widely used to integrate planar components in MIC designs.

CPW approaches disadvantageously require a high degree of mechanical complexity or an electrically large substrate size, making integration in broadband MIC applications difficult.

As may be appreciated from this description of current techniques for including such components in an integrated circuit package, there is a need for an improved, more efficient, and less expensive system and a better method of incorporating circulators and similar devices in monolithic circuit structures.

The prior art discloses less relevant techniques, such as US 5,065, 275, describing electrically isolated dielectrics to be used as integrated capacitors in densely packed PCBs. Moreover, US 5,277, 725 provides for production of ceramic housing incorporating material with low epsilon. Similar thoughts are also found in, e. g. EP 507 719 Al and WO 93/01928.

According to US 5,5532, 667, a ferromagnetic material in ink or tape form is sinterable using a same firing profile as and has approximately the same thermal shrinkage characteristics as low-temperature-cofired-ceramic (LTCC) tape, and is chemically non-reactive therewith. The ferromagnetic material is applied to the surfaces of LTCC tape sheets to form desired elements such as cores for inductors and transformers and magnetic shields. Ferromagnetic vertical interconnects (vias) can be formed by punching holes through tape sheets and filling them with ferromagnetic ink. The tape sheets and ferromagnetic elements are laminated together and cofired to form an integral structure. Ferromagnetic and non-magnetic components can be fabricated separately and inserted into cavities in tape sheets prior to cofiring. A multi-layer transformer includes primary and secondary coils, each being formed of vertically aligned, arcuate conductors which are printed on separate tape sheets and vertically interconnected at their ends to form continuous electrical paths therethrough. Thus, this invention relates to method of producing a transformer. A LTCC layer is provided with a conductive structure and enclosed between a ferromagnetik layers. Consequently, this invention differs from the present invention in structure and object.

A high-frequency use non-reciprocal circuit is disclosed in the European Patent Application No. 653 799. The circuit includes a sintered body, which is a high-frequency use magnetic body obtained by a ceramic lamination/integral firing technique, a plurality of central electrodes, which are arranged in the sintered body to be separated from each other through a magnetic layer while intersecting with each other at central portions. Electrodes are arranged for deriving impedance- matching capacitance formed in the vicinity of the intersecting portion to be in series with the central electrodes. The magnetic layer provided between the central electrodes serves as an insulating layer for electrically insulating the central electrodes from each other, a high-frequency use magnetic layer, and a material layer for deriving the impedance-matching capacitance. Thus, several layers provided with several conductors and characteristics are used to produce a component.

SUMMARY OF THE INVENTION The present invention fulfills the needs described above by providing a system and method of incorporating microwave components into a cofired or ferrit circuit structure during the sintering step thereby avoiding previously required separate steps to mechanically secure the component to the circuit structure. This also provides better performance, greater component yield, and a more compact and reliable construction.

This is achieved by means of an initially mentioned circuit, in which the film including a material having ferroelectric and ferromagnetic properties further comprises a metalized circuit pattern, being protectively encapsulated within the co- fired assembly.

More specifically, the present invention concerns integrating a circulator component consisting of a ferroelectric/ferromagnetic film upon which a circuit pattern is printed and that is incorporated as a layer, or portion of a layer, that is sintered in a process, such as an LTCC process, along with all other ceramic layers of an integrated circuit.

An exemplary embodiment of the present invention takes the form of a microwave integrated circuit (MIC) that includes a cofired assembly of multiple layers of a dielectric tape that serves as a carrier for circuit elements and that incorporates and interiorly positions a film including a material that has ferroelectric and ferromagnetic properties in at least one of the dielectric tape layers. Prior to cofiring, each layer is electrically connected to the other layer (s) by means of holes made through the layers, commonly referred to as"vias", which are subsequently filled with a conductive material. The ferrite film, at least partly, includes at least one circuit element that is configured to perform a faraday rotation of a microwave signal. The circuit element includes a metalized circuit pattern that is screen printed on the film. The circuit pattern, in combination with the properties of the ferrite material, can define several different types of components that rely upon a faraday

rotation to manipulate a microwave signal such as isolators, phase shifters, limiters, or dampers. Furthermore, several circulators can be joined together to increase the number of available ports. The dielectric layers form a multi-layer substrate, each layer of which may contain screen printed circuit components, in which at least one layer includes the ferrite film and thus at least one circuit element configured to perform a faraday rotation of a microwave signal. The ferrite film can make up a portion of a non-ferrite layer, or an entire layer of the substrate can be formed of the ferrite film. After assembly of the multi-layer substrate, the layers are cofired together in a process that sinters the layers to form a monolithic structure, thereby protectively encapsulating the ferrite film and metalized circuit pattern within the cofired assembly.

In an alternative embodiment, multiple layers of ferrite film are required to create a single circulator or similar device. In this embodiment, the multi-layer substrate, each layer composed of a dielectric material upon which circuit components are printed, the layers being configured to be assembled and cofired together, includes multiple adjacent ferrite layers that together form a circuit element that is configured to perform a faraday rotation of a microwave signal.

The present invention also includes a method of manufacturing an integrated circuit that consists of an assembly of a multiple layers of a dielectric carrier, in which each layer can be impregnated with circuit elements and electrically connected to the other layers by vias. At least one of the layers includes a film having ferroelectric and/or ferromagnetic properties. A circuit metallization pattern is screen printed on the ferrite film to form a circuit element and associated conductors that perform a faraday rotation of a microwave signal. The layers are then assembled and sintered together according to an LTCC process, thereby forming an integrated circuit. The ferrite film and circuit metallization pattern are protectively encapsulated within the cofired assembly. Any excess material is trimmed from the integrated circuit.

The system and method of the present invention can be implemented in a process, preferably a cofiring process and most preferably in LTCC, HTCC or similar processes. Thus, material used for non-ferrite layers may comprise any material suitable for carrying out the aforementioned process, such as ceramics or compositions thereof. The ferrite film preferably includes a grain-oriented ferrite, with crystal axes aligned predominately in the same direction so as to create a permanent ceramic magnet.

Additional objects, advantages and novel features of the invention will be set forth in part in the description and drawings which follow and in part will become more apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and form part of the specification, illustrate the present invention when viewed with reference to the description, wherein: Figure 1 is an illustration of a typical circulator; Figure 2 is an illustration of a layer of ferrite that is configured as at least one circulator according to an exemplary embodiment of the present invention; Figure 3 is an exploded view of the layers of an integrated circuit that includes a ferrite film configured as at least one circulator according to an exemplary embodiment of the present invention; Figure 4 is an illustration of the assembled layers of an integrated circuit that includes a ferrite film configured as at least one circulator, wherein the ferrite film composes an entire layer of the integrated circuit according to an exemplary embodiment of the present invention; Figure 5 is an illustration of the assembled layers of an integrated circuit that includes a ferrite film configured as at least one circulator, wherein the

ferrite film composes a partial layer of the integrated circuit according to an exemplary embodiment of the present invention; Figure 6 is an exploded view of the layers of an integrated circuit that includes multiple layers of ferrite film that together create at least one circulator according to an exemplary embodiment of the present invention; and Figure 7 is a schematic cross-sectional view of a manufacturing embodiment according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale ; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention.

Referring now in detail to an exemplary embodiment of the present invention, which is illustrated in the accompanying drawings in which like numerals designate like components, Figure 2 is an illustration of a ferrite layer 20 that is configured as a circulator 26 according to an exemplary embodiment of the present invention. The ferrite film 22 includes a ferrite material, which is a member of a group of nonmetallic, ceramic-like, usually ferromagnetic compounds of ferric oxide with other oxides, often characterized by extremely high electrical resistivity, and is selected for its dielectric and magnetic properties according to the application. The ferrite material selected must be magnetized, for example to approximately 3,000- 5,000 Gauss. In addition, a coercive field He exceeding any remanent magnetization (4nMR), a dielectric loss tangent that is less than 0.001, and uniaxial anisotropy (HA)

range of 10-30 kOe is desirable. An example of an acceptable material is Sr- hexaferrite ceramic (SrFel20l9). Such a ceramic can be polarized to achieve the desired magnetism.

The circulator 26 is configured to perform a faraday rotation of a microwave signal. The Faraday effect is proportional to the magnetic field strength, B, and to the distance the signal travels in the medium. The angle of rotation of the plane of polarization may be described by: OR = V-B/ where V is a constant (min/G-cm) that varies according to the composition of the ferrite film 22, B is the strength of the magnetic field (in Gauss) and/is the path length.

In the exemplary embodiment, a metallic pattern 24 is printed on the ferrite film 22 so as to create a circulator 26 component. Microwaves 16 can flow from one port 12 to an adjacent port 12 in a counter-clockwise direction. The metallic pattern 24 can be connected to additional metallic circuit elements including resistors, conductors, and other circulators.

After the metallic pattern 24 and other metalized circuit elements are completed, the ferrite layer 20 is collated with non-ferrite layers 32 of the MIC 30.

Figure 3 is an exploded view of the layers 32 of the MIC 30, where one layer is a ferrite film 22 configured as at least one circulator 26 according to an exemplary embodiment of the present invention. The collation can include multiple ferrite layers 20 as well as multiple non-ferrite layers 32, arranged in any order. Each ferrite layer 20 and each non-ferrite layer 32 can be laminated and punched with vias prior to collation.

According to one embodiment, after collation, and as illustrated in Figure 4, the non-ferrite layers 32 and the ferrite layers 20 are assembled and sintered together. The resulting MIC 30 fully incorporates and hermetically seals a ferrite film 22 configured to establish at least one circulator 26 within the MIC 30

package. The entire ferrite layer 20 can be composed of the ferrite film 22.

Alternatively, Figure 5 is an illustration of the assembled layers of an MIC 30 that includes a ferrite film 22 configured as a circulator 26, where the ferrite film 22 composes only a portion of a layer 32 of the MIC 30. The remainder of that layer 32 is a non-ferrite material.

One aspect of the present invention is that a single circulator 26 or similar device can be composed of multiple ferrite layers 20 made of ferrite film 22.

Figure 6 is an exploded view of the layers 32 of a MIC 30 that includes multiple ferrite layers 20 including a ferrite film 22, where combination of the ferrite layers 20 creates at least one circulator or similar device.

According to one aspect of the invention, as illustrated in figure 7, the ferrite portion 70 consists of a magnetizable material. In this case, the magnetizable material provided in one or several layers 72 of ceramic is arranged inside a static field 75 generated between electrodes 76 and 77. The magnetizable material is magnetized upon exposure to the field generated when a voltage is applied to the electrodes.

Although the present invention is described with respect to use in conjunction with LTCC processes, it will be understood by those skilled in the art that the present invention is also applicable to HTCC, PGA, chip carrier, or other hybrid substrate processes.

In view of the foregoing, it will be appreciated that the present invention provides a system and a method for creating integrated circuits that incorporate microwave components consisting of ferrite materials prior to cofiring according to an LTCC process. Still, it should be understood that the foregoing relates only to the exemplary embodiments of the present invention, and that numerous changes may be made thereto without departing from the spirit and scope of the invention as defined by the following claims.