HIGH FREQUENCY MICROELECTRONICS PACKAGE

Abstract: A high frequency microelectronic package suitable for high-frequency microelectronic devices includes a base (102) which is at least partially conductive and attached to an RF substrate (106) with a cavity (120) formed at its center. The base may be metal or ceramic with a metal layer deposited thereon. A pattern of conductive paths (122) for providing interconnection from the inside to the outside of the package are formed on the surface of the RF substrate. These conductive patterns are designed to have a constant impedance when uncovered, regardless of the dielectric property of the material used to cover the conductive patterns. Namely, a sealing cap or a lid (114), made from a variety of dielectric materials, may be attached to the RF substrate by a non-conductive adhesive (112), such as a polymer adhesive or low temperature seal glass, to seal the package once the microelectronic device has been mounted inside.

Description: HIGH FREQUENCY MICROELECTRONICS PACKAGE This application is a continuation-in-part of application Serial No. 08/526,535, which was a continuation of application Serial No. 08/1 34,269, now issued as Patent No. 5,465,008. This application is also related to application Serial No. 08/231 ,492, a divisional of Serial No. 08/1 34,269, now issued as Patent No. 5,448,826. BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a microelectronic package suitable for high frequency devices, using a minimum of conductive materials and a process for making the electronic package. More particularly, the invention relates to a microelectronic package design that eliminates superfluous electrical conductors and can be used as an electronic interconnect housing for high frequency electronic devices and components. Description of Related Art A key requirement for the packaging of a microelectronic device is that signals move through the package's conductive interconnects in such a way that the electrical interconnection causes minimal change in the signals. It is difficult, however, to fabricate microelectronic packages to achieve minimal signal change at higher frequencies, i.e., greater than 20 Gigahertz (GHz) . A conventional microelectronic package design achieves transfer of signals in and out of the package for frequencies as high as 23 GHz. This package has a metal cover, which must be maintained at the same electrical potential as the electrical circuit ground to achieve such performance. In order to ground the cover, however, internal vias and external side metallization are required. This added metal, in relatively close proximity to the internal circuit and electrical conductors, debases and limits the ultimate performance of the device. Moreover, conventional packages have limited frequency range, excessive transmitted and reflective loss, limited input/output isolation, high cost, and limited reliability, resulting in a lack of general applicability. Therefore, a need exists for a microelectronic package that eliminates superfluous electrical conductors, provides structural packaging members fabricated from non-conductors and having a relatively uniform dielectric constant, has a high frequency range, and is low in cost without sacrificing reliability. SUMMARY OF THE INVENTION Accordingly, the present invention is directed to a microwave electronic package that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the apparatus and method particularly pointed out in the written description and claims hereof, as well as the appended drawings. To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention is a microelectronic package suitable for high-frequency electronic devices. The package comprises a base having at least partial conductivity which establishes the electrical ground plane and having a top and a bottom; an RF substrate made of ceramic and having a first cavity and a plurality of conductive patterns deposited on the surface of the ceramic RF substrate; means for attaching the ceramic RF substrate to the top of the base; a ceramic seal ring substrate having a second cavity larger than the first cavity; second means for attaching the ceramic seal ring substrate to the RF substrate, the second attaching means generally matching the dimensions of the seal ring; and a ceramic lid attached to the ceramic seal ring substrate by a non-conductive third attaching means. In another aspect, the present invention is a process for assembling a ceramic microelectronic package having a base, a RF substrate made of ceramic, and a ceramic seal ring substrate, each of the base, the RF substrate, and the ceramic seal ring substrate having a top surface and a bottom surface. The process comprises screen printing a conductive paste on the top surface of the RF substrate; drying and firing the conductive paste; patterning conductive paths using the conductive paste on the top surface of the RF substrate; screen printing a first seal glass layer on the top surface of the RF substrate; drying and glazing the first seal glass layer; screen printing a second seal glass layer on the bottom surface of the ceramic seal ring substrate; drying and glazing the second seal glass layer; subassembling the RF substrate and the ceramic seal ring substrate; and attaching the top surface of the base to the bottom surface of the RF substrate. The step of subassembling the RF substrate and the ceramic seal ring substrate includes the following substeps: abutting the top surface of the RF substrate to the bottom surface of the ceramic seal ring substrate to form a subassembly, heating the subassembly, screen printing a metallic material on the bottom surface of the RF substrate, and drying and firing the metallic material. The step of attaching the top surface of the base to the bottom surface of the RF substrate includes the following substeps: applying an adhesive material in the alternative to the top surface of the base or to the bottom surface of the RF substrate, abutting the top surface of the base to the bottom surface of the RF substrate to form an assembly, and curing the assembly with heat, pressure, or other means appropriate to the sealing material used. In a first alternative embodiment of the present invention, the ceramic seal ring, third attaching means, and ceramic lid may be replaced by a sealing cap. This sealing cap is attached directly to the top of the RF substrate by the second means for attaching. The process for assembling the ceramic microelectronic package which incorporates the sealing cap includes screen printing a conductive paste on the top surface of the RF substrate; drying and firing the conductive paste; patterning conductive paths using the conductive paste on the top surface of the RF substrate; screen printing a layer of adhesive on the top surface of the RF substrate; drying and glazing the first seal glass layer; screen printing a metallic material on the bottom surface of the RF substrate, and drying and firing the metallic material; and attaching the top surface of the base to the bottom surface of the RF substrate. The step of attaching the top surface of the base to the bottom surface of the RF substrate includes the following substeps: applying an adhesive material in the alternative to the top surface of the base or to the bottom surface of the RF substrate, abutting the top surface of the base to the bottom surface of the RF substrate to form an assembly; and heating the assembly. In a second alternative embodiment of the present invention, leads may be electrically attached to the conductive paths formed on the RF substrate. These leads are shaped to extend outwardly from the RF substrate to facilitate making the necessary electrical connections to the package. In yet a third alternative embodiment of the present invention, the conductive patterns which are deposited on the RF substrate may be formed without transitions, i.e., eliminating the compensation in impedance which is used in the transition from the microstrip line inside the package to the embedded microstrip line immediately underneath the seal ring or cap sidewall to the microstrip line outside of the package. As a result of eliminating the impedance compensation, though contrary to conventional wisdom within the art, the conductor patterns exhibit improved insertion loss and return loss responses in high frequency applications ( > 1 GHz), even as high as 30 - 45 GHz. Thus, the third alternative embodiment is capable of housing a high frequency microelectronic device, without having to incorporate transition compensations in the conductor patterns in the embedded microstrip at the feedthrough under the seal ring or sealing cap. The process for assembling the third alternative embodiment may follow the processes for either the preferred embodiment which incorporates the ceramic seal ring and lid, or the first alternative embodiment which incorporates the sealing cap. BRIEF DESCRIPTION OF THE DRAWINGS Understanding of the present invention will be facilitated by consideration of the following detailed description of a preferred embodiment of the present invention taken in conjunction with the accompanying drawings, in which like numerals refer to like parts and in which: Figure 1 is a diagrammatical representation of a ceramic microelectronic package in accordance with the present invention; Figure 2A is a top view of the package of the present invention with the lid removed; Figure 2B is a cross-sectional view of the inventive package with the lid removed, taken along line 2B--2B in Figure 2A; Figure 3 is a diagrammatical representation of a cavity in a microelectronic package, in which the electromagnetic wave is propagating in the z-direction; Figure 4 is a plot of cutoff frequencies for LSM n and TE 10 modes in an empty rectangular waveguide; Figure 5 is a plot of electrical transmission loss with frequency; Figure 6 is a plot of the reflected loss with frequency; Figure 7 is a diagrammatical representation of a microwave electronic package in accordance with a first alternative embodiment of the present invention which includes a sealing cap; Figure 8A is a top view of the package of a first alternative embodiment of the present invention with the sealing cap installed; Figure 8B is a cross-sectional view of the package of a first alternative embodiment with the sealing cap installed, taken along line 8B--8B in Figure 8A; Figure 9 is a diagrammatical representation of a microwave electronic package in accordance with a second alternative embodiment of the present invention which includes leads extending outward from the package. Figure 10A is a diagrammatical representation of a microelectronic package in accordance with a third alternative embodiment of the present invention with its sealing cap removed, showing electrical conductors lacking feedthrough impedance compensation; Figure 10B is a cross-sectional view of the package of a third embodiment, taken along line 10B-10B in Figure 1 0A; Figure 1 1 is a plot of electrical insertion loss as a function of frequency for both the preferred embodiment and the third alternative embodiment of the present invention; and Figure 1 2 is a plot of the electrical return loss as a function of frequency for both the preferred embodiment, and the third alternative embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION An exemplary embodiment of the apparatus of the present invention is shown in Figure 1 and is designated generally by reference numeral 1 00. As embodied and shown in Figure 1 , the present invention includes a base 102, attaching means 104, a RF substrate 1 06 with conductive traces 1 22 patterned thereupon, second attaching means 1 08, a ceramic seal ring substrate 1 10, non-conducting third attachment means 1 1 2, and a ceramic lid 1 14. The package 1 00 is used as an electronic interconnect housing for high frequency (i.e., beyond 20 GHz) devices and components. The details of this package are described below. The base 102 has multiple uses, including providing a mechanical mount for the package 100 onto a circuit board or carrier, a thermal and electrical mount for the microelectronic devices and other electrical components 1 1 6 to be housed in the package, and as a ground reference, or "ground plane", for the high frequency conductors and other signal traces used as the interconnect. As embodied herein, the base can be made from either of two different types of materials. First, the base can be made of a conductive metal, such as Kovar ® , Invar ® , copper, copper-tungsten, copper- molybdenum, or molybdenum. Each metal offers advantages in electrical and thermal conductivity, structural strength, low coefficient of thermal expansion, and compatibility to electroplated conductors (for example, nickel, silver, silver-platinum, silver-palladium, or gold) . The choice of these metals is only exemplary, and one skilled in the art will recognize that other metals can be used for the base 102. The base can alternatively be made from a ceramic material, compatible with the remainder of the materials used in the package 100. Use of such a ceramic base, however, requires some form of metallization to be applied to certain areas, such as the die attach area for device mounting, for the ground connection, or the bottom of the base 1 02 for attaching the package to a circuit board or carrier. Examples of such ceramic material are aluminum oxide, aluminum nitride, beryllium oxide, fosterite, cordierite, quartz, fused silica, or other ceramics that would have a composition making them usable as a packaging material. Other ceramic materials not enumerated herein, but providing acceptable electrical and physical properties, can be used and would be known to those skilled in the art. The attaching means 104 comprises some form of adhesive material which will differ depending on whether the base 102 is metal or ceramic. For a metal base, a metallic solder made from some composition providing good adhesion between metals would be applied either to the top of the base 1 02 or the bottom of the RF substrate 106. An example of such a composition would be gold-germanium or gold-tin composite, although other suitable compositions will be apparent to those skilled in the art. For a ceramic base, either of two types of attaching material may be used. If metallization is applied to the top of the base 102 and the bottom of the RF substrate 106, a metal solder would be applied as described above in the case of a metal base. Alternatively, a glass material, e.g. , seal glass, would be applied between the base and the RF substrate 106 in order to adhere the base to the RF substrate. Those skilled in the art will recognize that other materials not enumerated herein, but providing acceptable electrical and physical properties, can also be used. The RF substrate 106 is made of a ceramic material, such as one of the ceramic materials enumerated above with respect to the base 1 02. The RF substrate 106 has a cavity area 1 20 cut out of it such that, when attached to the base 102, the RF substrate and base together provide a planar interconnect for a device 1 1 6 or devices mounted on the base. Moreover, the RF substrate 1 06 has a number of conductive patterns 1 22 deposited on its surface. The conductive patterns 1 22 deposited on the RF substrate have a specific shape which is designed to maintain a uniform characteristic impedance from end to end of the conductor. The design is accomplished through the use of various electrical modeling and simulation software tools, as well as through experimentation. As described above with respect to the attaching means 104, depending on the material chosen for the base 102, the bottom side of the RF substrate may have metallization applied to it in order to attach the RF substrate to the base. The second attaching means 108 is a glass material (seal glass) deposited on either or both the RF substrate and the seal ring substrate. If seal glass is to be deposited on the RF substrate, it is formed so that the dimensions of the glass deposited on the RF substrate will substantially match those of the seal ring substrate. Seal ring substrate 1 10 is made of the same or similar material as that used on the RF substrate 106. As described above for the second attaching means 108, if seal glass is to be deposited on the seal ring 1 1 0, it is applied to the bottom of the seal ring substrate layer, which, when fused to the RF substrate or to glass deposited on the RF substrate, creates a hermetic seal between the two substrates. To attain the proper hermeticity and lamination of the seal ring substrate and the RF substrate, when fusing the two substrates together by firing the seal glass, they can be weighted. A cavity 1 24 is formed in the seal ring substrate which is larger than the cavity formed in the RF substrate. This can be seen in the overhead view provided by Figure 2A, and the cross-sectional view provided in Figure 2B. The seal ring substrate cavity 1 24 is made larger so that a portion of each of the conductors 1 22 is exposed inside the cavity 1 24, as shown in Figure 2A. The outer dimensions of the seal ring substrate 1 1 0 will vary in order to expose a portion of each of the conductors 1 22 on the outside of the package 100, as is also shown in Figure 2A. These exposed portions of the RF substrate conductors 1 22 are used as the connection point for the electronic device or devices 1 1 6 inside the package to the next component outside the package. Referring again to Figure 1 , the ceramic lid 1 1 4 may be made of the same or a similar material as that used for the RF substrate 106 and/or the seal ring substrate 1 10. The third attaching means, a non-conductive polymer adhesive, e.g., epoxy 1 1 2, is affixed to the bottom of the ceramic lid in a window frame fashion in order to adhere the ceramic lid 1 14 to the seal ring substrate 1 10. Alternatively, the third attaching means may be a low temperature seal glass. Either the bottom or the top of the ceramic lid 1 14 may be coated with a material that reduces cavity resonances. The package 100 can transmit and receive electrical signals with minimal loss and uses a minimal amount of conductive and non-conductive materials. The package also eliminates the need for a stripline type of transmission line through a ceramic wall. The construction of the package is not limited to a single material combination, but can use a variety of materials, both conductive and dielectric, to produce a package suitable for high frequency electronic devices. The inventive package 100 satisfies the requirements for a microelectronics package suitable for high frequency devices, using a minimum of conductive materials. First, the package satisfies the structural requirements of resistance to thermal and mechanical shock, moisture, salt atmosphere, vibration, and acceleration, as well as having the characteristic of solderability. The package also satisfies several electrical requirements, including low parasitic effects (i.e., inductance and capacitance), minimal discontinuity reactances, low dissipation loss, and minimal interaction with surrounding devices and environment. In accordance with the conventional wisdom in the art, in order to meet the requirements for a high frequency feedthrough, an impedance match low-loss interconnect for the device to the outside of the package is required. Such an interconnect minimizes or eliminates the need for tuning the device for optimal performance. To meet this requirement, a package must have low insertion loss (less than 0.02 f(GHz) dB and high return loss (greater than 1 5 dB) over the operating frequency band. To meet the requirements for high frequency feedthrough, the feedthrough structure should be planar, provide a good match for the electrical and magnetic fields at all interfaces, have a minimum number of transitions along the signal path, have a minimum number of discontinuities in the ground path, have a minimum feedthrough length (but long enough to reduce interaction between discontinuities), and have minimum ground inductance. Historically, a feedthrough structure consisted of a planar transition medium of a microstrip to stripline to microstrip. A microstrip transmission line consists of a metallized conductor above a ground plane, separated by a dielectric material. As is known in the art, the position and width of the microstrip conductor to the lower ground conductors, as well as the ground electrical lengths, determine the characteristic impedance. A stripline transmission line consists of a metallized conductor between two ground planes, separated by dielectric materials. The stripline region in a feedthrough is constructed in one of two ways: ( 1 ) by inserting the feedthrough into an aperture of a metal housing, creating a ground environment above and below the planar conductor, or (2) by connecting the metallization above the conductor electrically to metallization below the conductor by a vertical metallized via and/or external metallization. The position and width of the stripline conductor to the upper and lower ground conductors, as well as the ground electrical lengths, determine the characteristic impedance. The width to position of the microstrip connector to the ground plane determines its characteristic impedance. In providing metallization only at selected locations, the package of the present invention avoids certain disadvantages of conventional, fully- metallized high frequency microelectronic packages. For a package that is fully metallized, the cavity within the package (such as the cavity formed by the base 102, the RF substrate 106, the seal ring substrate 1 1 0, and the lid 1 14) must house the electronic device without degrading its performance and must accommodate high frequency feedthroughs and DC-lines. The design of the cavity should preclude propagation of waveguide modes and cavity resonances. In previous approaches using either metallized packages or multi-layer ceramic packages with metallized walls and lids, the ground potential of the lid 1 14 was a concern, because a grounded lid helps improve isolation between the feedthroughs and shield the device from any environmental effects. In the present invention, the cavity of the package is designed so that the cutoff frequency for the dominant TE 10 wave guide mode is beyond the maximum operating frequency. If this condition is not met, there is a possibility that the energy injected into the package, which is carried by the TEM mode, may couple to the TE 10 mode or any other modes supported by the cavity. At higher frequencies this mode conversion may influence the insertion loss in the package more than the dissipated loss or the impedance mismatch in the feedthrough. Cavity size that does not support waveguide modes additionally improves the isolation between feedthroughs and prevents spurious mode propagation. As illustrated in Figure 3, a package cavity 302 can be treated as an H-plane dielectric slab-loaded structure 300, where the electromagnetic wave is propagating in the z-direction. The dominant waveguide mode in the cavity can be either quasi-TE 10 or LSM„ (longitudinal-section magnetic), depending on the dielectric constant value and waveguide dimensions. As illustrated in Figure 4, the cutoff frequencies (designated f cd ) for both the quasi-TE 10 and the LSM n cases, as normalized with respect to the TE 10 mode cutoff frequency (£ = c/2W, W being the x-dimension) in an empty rectangular waveguide, are determined by the following equation: As shown in Figure 3, for equation (1 ), h, H and ε r are the substrate thickness, cover height, and substrate dielectric constant respectively. In order to suppress waveguide cutoff modes W/H should be less than 0.5, and h/H should be less than 0. 1 . For heavier loading of the package, the TE 10 mode becomes dominant, and significantly lower cutoff frequencies are observed. Thus, care must be taken during design of the cavity and feedthroughs to avoid exciting these modes. For waveguide dimensions H < W < L, the lowest order resonant mode is LSM 11 V or TE 101 . The electric field plane for such dimensions is parallel to the x-y plane and is maximum at the center, falling sinusoidally to zero at the walls of the waveguide. The magnetic field lines are closed contours parallel to the x-z plane, and the field strength is maximum at the walls and minimum at the center of the waveguide. The resonant frequency for this configuration is given by the following equation: I where W is in centimeters and f 0 is in GHz. As can be seen in equation (2), the lowest resonant frequency occurs when L is very large as compared to W and is given by the cutoff frequency for the dominant mode. Resonance can also occur when the largest dimension equals half the waveguide wavelength. Figure 5 illustrates the measurement and modeling results of the electrical transmission loss incurred through the package 100 of the present invention, displaying the loss as a function of frequency. Figure 5 also shows the modeled, or predicted, results of the single feedthrough of the package. A first curve 500 and a second curve 502 (identified respectively by an "X" and a box) are the measured and modeled results, respectively, of the package 1 00. The measurement was made of two package feedthroughs, two ribbon bonds, and a microstrip transmission line mounted in the cavity area formed by the RF substrate 106, the seal ring substrate 1 10, and the ceramic lid 1 14 across the full length of that cavity to connect both feedthroughs. Creating an electrical model to represent all of the physical components, and adjusting the model to obtain a fit of the measured response, permits analysis of individual components of interests. In the example depicted in Figure 5, the complete package 100, a microstrip, and ribbon bonds were analyzed, the feedthrough being analyzed independently. The modeled result of the independent feedthrough is identified by a third curve 504 (identified by a diamond) . This approach, called de-embedding, is commonly used in the electronics industry. However, the third embodiment of this invention successfully avoids this practice entirely. Figure 6 displays the amount of reflected loss a signal experiences when propagating from one medium to another. As in Figure 5, a first curve 600 and a second curve 602 (identified respectively by "X" and a box) for the measured and modeled results, respectively, of the package 100. A third curve 604 (identified by a diamond) is the modeled result of a single feedthrough. Fabrication Process - First Embodiment Another aspect of the present invention is a process for assembling a microelectronic package. The process includes the following steps: Screen printing a pattern of conductive paste; drying and firing the conductive paste; etching to further define the pattern of conductive paths in the conductive paste; screen printing a first seal glass layer on the top of the RF substrate; drying and glazing the first seal glass layer; screen printing a second seal glass layer on the bottom of the seal ring substrate; drying and glazing the second seal glass layer; assembling the RF substrate to the ceramic seal ring substrate; and attaching the top surface of the base to the bottom surface of the RF substrate. Before beginning the process, fully fired (hardened) ceramics are selected for the RF substrate 106, the ceramic seal ring substrate 1 1 0, and the ceramic lid 1 14. Preferably, the same ceramic is selected for each component. As for the base 102, if it is made of a ceramic, it could be the same fully fired ceramic selected for the other components, or it could be a different ceramic material which is compatible with the RF substrate but may have either better electrical or thermal properties. Any of the ceramic substrates may be alumina (Al 2 0 3 ) of various purities (e.g. , 96%, 99.6%), berrylia (BeO) , Barium Titanate (BaTiO 3 ), fused silica (SiQ ), or aluminum nitride (AIN), the material being selected to meet the specialized requirements of the product to be assembled. For example, aluminum nitride and berrylia are desirable for high power, heat dissipative applications. Fully fired (hardened) ceramics are commercially available from ceramic vendors. A fully fired ceramic substrate is typically purchased in one inch to 4.5 inch blanks, with the size selected according to the product to be assembled and the quantity of product desired. The RF substrate 106, preferably 96% pure alumina, is fabricated as follows. The RF substrate is cleaned ultrasonically using a detergent suitable for electronic applications and rinsed with deionized water, then fired to burn out any residues from the detergent. Preferably, an Alconox detergent is used, although those skilled in the art will recognize that other detergents can be used. Next, a conductive paste is screen printed over the top surface of the RF substrate 1 06. A first method of defining the conductive traces is to provide a preliminary conductive pattern which has significantly larger dimensions than the desired final dimensions of conductive pattern 1 22. Those skilled in the art will recognize that thin film techniques may also be used to deposit the conductive pattern. The photolithography/etch sequence described below will then be used to define the final dimensions of the conductive traces. A second possible method is to screen print the conductive paste using a screen that provides the actual final dimensions of the conductive pattern 122. If the second method is selected, no etch step is required. After the conductive paste is printed over the substrate, it is dried. The RF substrate 106 is then fired according to the specifications provided by the manufacturer of the conductive paste. With respect to the first method of defining the conductive pattern 1 22, the method of forming the preliminary conductive pattern uses thick film screen printing techniques, as are known in the art. Depending on the application and the type of conductor used, it may be desirable to repeat the printing sequence at least once to attain greater thickness and more uniform density of material, with each printing step being followed by drying and firing steps. The firing temperature, time, and conditions depend on the type of conductive material used, the appropriate parameters being provided by the supplier of the conductive paste. Preferably, the printing, drying, and firing steps will be performed twice in the present invention. The second method for defining the conductive pattern 1 22 requires similar processing after the conductive paste is printed onto the RF substrate. A large number of conductive pastes are available, and the selection of such a paste will depend upon the product being fabricated. Many pastes are combinations of gold and glass, with variations in the mixtures providing various levels of hermeticity, wire bondability, solderability, etchability, and adhesion. Other possible pastes include silver or copper. Selection of the appropriate paste for the desired product quality falls within the level of skill in the art. Preferably, however, an etchable gold conductor is used in the present invention, and the preferred embodiment is described accordingly. When a preliminary conductive pattern with dimensions larger than the desired final dimensions is in place on the RF substrate 1 06, a photolithographic process is used to more precisely define the dimensions of the conductors after which an etch is performed to remove the excess conductive material. This patterning step follows the process as is known in thin film technology in which a photoresist (PR) layer is spun or otherwise coated onto the surface of the preliminary conductive pattern; the PR is exposed to ultraviolet light modulated by a mask bearing the desired pattern; and the unexposed PR is rinsed away using a developer, leaving the areas to be etched exposed. The etch solution which is used for gold conductors is a mixture of potassium-iodine and iodine. After etching, the PR is stripped, and a clean/fire step is performed to burn away any chemical or organic residues remaining after the etching step. This technique is used to define the ultimate dimensions of the conductive traces 1 22 to assure that all of the conductor's dimensions, including thickness, are uniform and within the tolerances desired for reliable operation. After the clean/fire step, a seal glass is screen printed on the top surface of the RF substrate to form second attaching means 108. Note that seal glass may be deposited on either or both of the seal ring substrate and the RF substrate. If seal glass is to be deposited on the top of the RF substrate, it will substantially match the dimensions of the seal ring substrate 1 10, which has a larger cavity than that of the RF substrate. Thus, the seal glass will not completely cover the top surface of the RF substrate. Multiple printings of seal glass may be done in order to obtain the desired total thickness, with the preferred embodiment including three such printing steps. After each printing step, the seal glass paste is dried and glazed. Because the RF substrate 106 is also to be joined to the base 102, a seal glass may be printed onto the bottom surface of the RF substrate if the base is ceramic. As described above, depending on the material chosen for the base 102, the bottom side of the RF substrate 106 may be metallized or may have a seal glass layer placed on it. The glazing temperature is selected to be high enough that volatile materials (organics) within the glass are burned off, but not so high that the conductor on the substrates will melt or flow. The temperature depends on the type of material used, and appropriate temperature ranges are provided by the glass manufacturer. Some slight adjustments in temperature may be necessary due to variations between different types of ovens. Such adjustments are within the level of skill in the art. The selection of the seal glass is dominated by the type of product to be fabricated. An important feature of the present invention is that the seal glass be selected to have a coefficient of thermal expansion (CTE) and dielectric constant that match as close as possible the CTE and dielectric constant of the ceramic selected for the substrates. Matching of the CTEs eliminates differential thermal stress between each layer of a multilayer structure. Separate from the fabrication of the RF substrate 106, the ceramic seal ring substrate 1 10 is fabricated. As with the RF substrate, a ceramic material is selected which is fully fired and thus already hardened, the preferred ceramic being 96% pure alumina. The ceramic is the same as that selected for the RF substrate. In the preferred embodiment, the seal ring substrate is cut to create a cavity 1 24 at its center. This machining is generally performed by laser ablation using a C0 2 laser, which is the industry standard machining technique. Other machining techniques that may be used are ultrasonic machining or wire cutting. Other types of lasers may be used as well. The cavity cut into the seal ring is larger than the corresponding cavity that will be cut in the RF substrate, as is apparent from the drawings. The resulting ceramic seal ring substrate may then be mechanically scrubbed to remove any laser slag that may have built up on the substrate. After removing the laser slag (if necessary), the seal ring substrate 1 10 is cleaned ultrasonically using a detergent, preferably Alconox, and fired to burn off any residues from the detergent. As previously mentioned, since the second attaching means 108 may be formed by deposition of seal glass on one or both of the RF substrate and seal ring substrate, the following step will be incorporated only if seal glass is to be deposited on the bottom of the seal ring. A seal glass layer is applied to the bottom of the seal ring substrate. As with the RF substrate 106, application of the seal glass layer requires screen printing, drying and glazing, which steps may be performed a number of times, depending on the materials used, to attain the desired thickness. Upon fabricating the RF substrate 106 and the seal ring substrate 1 10, the two substrates are subassembled, requiring several steps. First, the substrates 106, 1 10 are aligned and mated for proper assembly, then the subassembly is fired, during which it may be weighted or clamped together to promote proper lamination and hermeticity between the two substrates. After firing, it may be desirable to check the hermeticity of the seal. The subassembly may be cleaned and inspected, then is laser machined to create the center cavity 1 20 in RF substrate 106, after which it is inspected again. The laser slag is removed by scrubbing . Second, the subassembly is cleaned ultrasonically using a detergent, preferably 1 1 1 - trichloroethylene, to remove any grease and is rinsed with deionized water. The subassembly is then fired to burn away any residues left from the detergent. Third, a conductive paste is applied to the bottom surface of the subassembly (i.e. , the bottom surface of the RF substrate 1 06), and the screen printed conductive paste is then dried and fired, as described above. Again, the preferred conductive paste is a gold paste, and the print-dry-fire step can be performed multiple times to achieve the desired thickness. After applying the conductive paste, the subassembly may be singulated by sawing; that is, each independent subassembly may be separated from the blank. After completing the subassembly, final assembly of the package is performed. First, an appropriate base must be selected. In the preferred embodiment the base is made from Kovar ® . A hermeticity check may be performed on the subassembly before final assembly; and the subassembly may be cleaned and fired as necessary. Before attaching the base 102 to the subassembly, the bottom surface of the RF substrate may be smoothed in order to prepare the subassembly for soldering. Preparation techniques may vary, depending on the type of solder used. These techniques may affect the amount of solder flow, hermeticity, and visual quality of the assembly. Next, the base 102 and the subassembly are fitted together with attaching. means 104 therebetween, and the fitted assembly is fired. After firing, a final hermeticity check may be run; the assembly may be cleaned using an acetone or other detergent to remove grease and other residues; and electrical testing may be performed on sample packages. After completion of the package, the microelectronic circuit to be packaged therein is attached using die attach techniques as are known in the art. The ceramic lid is then attached to the top of seal ring 1 10 by a non- conductive polymer adhesive 1 1 2, such as epoxy, or a seal glass which can be fired and glazed at a sufficiently low temperature to avoid damaging the microelectronic circuit. The process for using a non-conductive polymer adhesive 1 12 comprises screen printing the polymer onto the bottom side of lid 1 14, then pre-baking the lid 1 14 to partially cure the adhesive. Once the microelectronic circuit is mounted inside the package, the lid 1 14 is aligned with the ceramic seal ring 1 10 then clamped in place, after which a final cure is performed at a temperature that is low enough to avoid damage to the microelectronic circuit. First Alternative Embodiment Referring now to Figure 7, a first alternative embodiment of the present invention is shown and designated generally by reference numeral 700. As embodied and shown in Figure 7, the present invention includes a base 1 02, attaching means 104, a RF substrate 1 06 with conductive traces 122 patterned thereupon, second attaching means 108, and a sealing cap 1 26. The package 700, like the package 100 discussed above, is used as an electronic interconnect housing for high frequency (i.e., beyond 1 GHz) devices and components. The details of this package 700 are described below. The base 102 has multiple uses, including providing a mechanical mount for the package 100 onto a circuit board or carrier, a thermal and electrical mount for the microelectronic devices and other electrical components 1 16 to be housed in the package, and as a ground reference for the high frequency conductors 1 22 and other signal traces used as the interconnect. As embodied herein, the base 102 can be made from either of two different types of materials. First, the base can be made of a conductive metal, such as Kovar ® , Invar ® , copper, cold rolled steel, copper- tungsten, copper-molybdenum, and molybdenum. Each metal offers advantages in electrical and thermal conductivity, structural strength, low coefficient of thermal expansion, and compatibility to electroplated conductors (for example, nickel, silver, silver-platinum, silver-palladium, or gold). In addition to the metals listed above, the base 102 may be made of several low expansion iron-nickel alloys, having a nickel content ranging from 42-52%, with the balance being predominantly iron. One such alloy is identified as Alloy 46, which consists of a combination of 46% nickel with the balance predominantly iron, and is available in commercial quantities from several sources, including National Eiectronic Alloys of Oakland, New Jersey. Specifications for the metallurgical properties of Alloy 46 are found in American Society for Test Methods (ASTM) Publication F30. Alloy 46 is a prime material for a low cost package 700, ideal for lower power devices where thermal dissipation is not a significant requirement. The choice of these metals is only exemplary, and one skilled in the art will recognize that a variety of other metals can be used for the base 102. The base 102 can alternatively be made from a ceramic material, compatible with the remainder of the materials used in the package 700. Use of such a ceramic base, however, requires some form of metallization to be applied to certain areas, such as the die attach area for device mounting, for ground connection, or the bottom of the base 1 02 for attaching the package 700 to a circuit board or carrier. Examples of such ceramic material are aluminum oxide, aluminum nitride, beryllium oxide, fosterite, cordierite, quartz, fused silica, or other ceramics that would have a composition making them usable as a packaging material. Other ceramic materials not enumerated herein, but providing acceptable electrical and physical properties, can be used and would be known to those skilled in the art. The attaching means 104 comprises some form of adhesive material. These adhesive materials may differ depending on whether the base 1 02 is metal or ceramic. For a metal base, a metallic solder made from some composition providing good adhesion between metals would be applied either to the top of the base 102 or the bottom of the RF substrate 106. An example of such a composition would be copper-silver joining alloy, although other suitable composites will be apparent to those skilled in the art. For a ceramic base, a variety of attaching materials may be used. Specifically, the ceramic base may be attached with the same options set out above for use with a metal base. Any preparation of the ceramic base for attachment is obvious to those skilled in the art. For example, if the ceramic base is to be attached with metal solder, the base must be metallized so that the solder will adhere. As an alternative to solder, a glass material, e.g., seal glass, could be applied between the base and the RF substrate 106 in order to adhere the base to the RF substrate. Another material suitable for either a metal or ceramic base 102 is an organic adhesive. Organic adhesives are an ideal low cost alternative as its use eliminates several process steps and many expensive materials listed above. For example, even if metallization of the ceramic base is required for electrical considerations, use of the organic adhesives eliminate the need to prepare the bottom side of RF substrate 1 06 for attachment to the base. These organic adhesives may include metal filled adhesives such as conductive polymers for higher frequency applications, polyimide, polyvinyl acetate, polyvinyl alcohol, acrylic, phenolic, phenol-resorcinol, epoxy, urea formaldehyde, melamine, alkyd, phenolic-vinyl, phenolic-polyvinyl butyrate, phenolic-nylon, and phenolic neoprene. Those skilled in the art will recognize that the use of a polymer adhesive will permit use of a polymer or a ceramic- filled polymer for base 102. It is to be appreciated, however, that those skilled in the art will recognize that any adhesive providing acceptable electrical and physical properties can also be used. The RF substrate 106 may be made of a ceramic material, such as one of the ceramic materials enumerated above with respect to the base 102. Additionally, the RF substrate 106 may also be made from a number of other dielectrics including glass, low cost electronic grade ceramic, and several plastics, depending on the electrical requirements. More specifically, the glass dielectric materials suitable for the RF substrate 106 include soda lime (float or lamp bulb), alumina silicate, borosilicate (low loss electrical) 99.5% silica for high temperatures. Further, the plastic dielectric materials may be filled or unfilled. The filler could be in the form of a glass matt (for example, G 1 0 printed circuit board material). Also, the ceramics listed above could be used in powdered form as an amorphous filler in the plastic. The plastics could include any of those commonly used in electrical applications, such as epoxy molding compounds and thermoset molding compounds. Specific choices for such plastics include nylon, resins, novalacks, phenolics, polysulfones, polyetherimides, and liquid crystal plastics. It is to be appreciated, however, that those skilled in the art will recognize that any plastic which provides acceptable electrical and physical properties can also be used. The RF substrate 106 has a cavity area 1 20 cut out of it such that, when attached to the base 102, the RF substrate and base together provide a planar interconnect for a device 1 1 6 or devices mounted on the base. Moreover, the RF substrate 1 06 has a number of conductive patterns 1 22 deposited on its surface. The RF conductors 122 deposited on the RF substrate have a specific shape, which is designed to maintain a uniform characteristic impedance from end to end of the conductor. This is done through the use of various electrical modeling and simulation software tools, as well as through experimentation. As described above with respect to the attaching means 104, depending on the material chosen for the base 102, the bottom side of the RF substrate may have metallization applied to it in order to attach the RF substrate to the base. The second attaching means 108 for the first alternative embodiment is an epoxy deposited on either or both the RF substrate 106 and the sealing cap 126. If epoxy is to be deposited on the RF substrate, it is formed so that the dimensions of the epoxy deposited on the RF substrate will substantially match those of the sealing cap. Sealing cap 126 may be made of the same or similar material as that used on the RF substrate 106. More specifically, the sealing cap 1 26 can be made from a number of dielectric materials including glass, low cost electronic grade ceramic, and several plastics, depending on the electrical requirements. The glass dielectric materials may include soda lime (float or lamp bulb), alumina silicate, borosilicate (low loss electrical) 99.5% silica for high temperatures. Further, the plastic dielectric materials may be filled or unfilled . The filler could be in the form of a glass matt (for example G10 printed circuit board material). Also, the ceramics listed above could be used in powdered form as an amorphous filler in the plastic. The plastics could include any of those commonly used in electrical applications, such as epoxy molding compounds and thermoset molding compounds. Specific choices for such plastics include nylon, resins, novalacks, phenohcs, polysulfones, polyetherimides, and liquid crystal plastics. It is to be appreciated, however, that those skilled in the art will recognize that any plastic which provides acceptable electrical and physical properties can also be used. As described above for the second attaching means 108, if seal glass is to be deposited on the sealing cap 1 26, it is applied to the bottom of the sealing cap, which, when fused to the RF substrate layer or to glass deposited on the RF substrate layer, creates a hermetic seal between the RF substrate 1 06 and the sealing cap 1 26. To attain the proper hermeticity and lamination of the sealing cap and the RF substrate, when fusing the two surfaces together by firing the seal glass, they can be weighted. The outer dimensions of the sealing cap 126, like the seal ring substrate 1 10 as shown in Figure 1 , will vary in order to expose a portion of each of the conductors 1 22 on the outside of the package 700. As shown in Figure 8A, these exposed portions of the RF substrate conductors 1 22 are used as the connection point for the electronic device or devices 1 1 6 inside the package to the next component located outside the package 700. As shown in Figure 8B, the sealing cap 126 is formed with a chamber 128 which has a larger diameter than the cavity formed in the RF substrate. This allows the placement of the sealing cap directly against the upper surface of the RF substrate while still allowing a portion of each of the conductors 1 22 to be exposed inside the chamber 1 28. The process for assembling a microelectronic package in accordance with the first alternative embodiment of the present invention includes the following steps: screen printing a pattern of conductive paths 1 22 on the RF substrate 106; screen printing a first attaching means 104 on the top of the base 102; and attaching the top surface of the base to the bottom surface of the RF substrate. Before beginning the assembly process for the first alternative embodiment, the material for the RF substrate 106 is selected. If the RF substrate is made of a ceramic material, it could be the same fully fired ceramic which may be selected for the other components. Any of the ceramic substrates may be alumina (AI 2 O 3 ) of various purities (e.g., 96%, 99.6%), berrylia (BeO), barium titanate (BaTi0 3 ) , fused silica (Siφ ), or aluminum nitride (AIN), the material being selected to meet the specialized requirements of the product to be assembled, which will be within the level of skill in the art. For example, aluminum nitride and berrylia are desirable for high power, heat dissipative applications. In addition to ceramic, the RF substrate may be made from a number of dielectrics including glass, low cost electronic grade ceramic, and several plastics, depending on the electrical requirements. These dielectric materials include soda lime (float or lamp bulb), alumina silicate, borosilicate, and filled or unfilled plastics including nylon, resins, novalacks, phenolics, polysulfones, polyetherimides, and liquid crystal plastics. Fabrication Process - First Alternative Embodiment The RF substrate 106 of the first alternative embodiment is fabricated as follows. The RF substrate is cleaned ultrasonically using a detergent suitable for electronic applications and rinsed with deionized water, then fired to burn out any residues from the detergent. Preferably, an Alconox detergent is used, although those skilled in the art will recognize that other detergents can be used. Next, a conductive trace pattern is formed on the top surface of the RF substrate 106. Such conductive pattern may be formed using a variety of methods. One method of defining the conductive traces is to screen print a preliminary conductive pattern using thick film screen printing techniques. This pattern would has significantly larger dimensions than the desired final dimensions of conductive pattern 1 22. Depending on the application and the type of conductor used, it may be desirable to repeat this printing sequence at least once to attain greater thickness and more uniform density of the printed material, with each printing step being followed by drying and firing steps. When a preliminary conductive pattern with dimensions larger than the desired final dimensions is in place on the RF substrate 106, a photolithographic process is used to more precisely define the dimensions of the conductors after which an etch is performed to remove the excess conductive material. This patterning step follows the process as is known in thin film technology in which a photoresist (PR) layer is spun or otherwise coated onto the surface of the preliminary conductive pattern; the PR is exposed to ultraviolet light modulated by a mask bearing the desired pattern; and the unexposed PR is rinsed away using a developer, leaving the areas to be etched exposed. The RF substrate is then immersed in an etch solution for etching. This etch solution, which is typically used for gold conductors, is a mixture of potassium-iodine and iodine. After etching, the PR is stripped, and a clean/fire step is performed to burn away any chemical or organic residues remaining after the etching step. This technique is used to define the ultimate dimensions of the conductive traces 1 22 to assure that all of the conductor's dimensions, including thickness, are uniform and within the tolerances desired for reliable operation. Another possible method of defining the conductive traces is to screen print the conductive paste using a screen that provides the actual final dimensions of the conductive pattern 1 22. This method does not require the use of an etching process. After the conductive paste is printed over the substrate, it is dried. The RF substrate 106 is then fired according to the specifications provided by the manufacturer of the conductive paste. Yet another possible method of defining the conductive traces when ceramic is selected for the RF substrate involves the application of metal foil directly on the surface of the ceramic. For example, copper foil may be directly bonded to alumina and other ceramics through a process known as direct bond copper. After bonding the copper foil to the ceramic, the copper foil may be etched using the photolithography/etch sequence as described below to define the final dimensions of the conductive traces. Still another possible method of defining the conductive traces, particularly useful for low cost packages, involves the implementation of the printed wiring board industry's method of printing a polymer thick film on a substrate. Using this approach, it is possible to incorporate a filled polymer substrate material, instead of alumina ceramic. This allows a large number of RF substrates to be simultaneously formed from a single sheet of polymer. These RF substrates may then be printed and/or etched with a conductor pattern for subsequent separation into single RF substrates. In order to establish a sufficient conductor thickness, the thick film which is printed on the polymer substrate may be repeatedly plated using electroplate or electrodeless plate techniques. For example, a high volume approach is to print a silver polymer thick film on a polymer substrate, followed by plated nickel, and plated gold. Next, a second attaching means 108 is screen printed on the top surface of the RF substrate. Recall from above that the second attaching means 108 for the first alternative embodiment consists of a non-conductive polymer adhesive, such as epoxy. Note that the epoxy may be deposited on either or both of the sealing cap and the RF substrate If epoxy is to be deposited on the top of the RF substrate, it will substantially match the dimensions of the sealing cap 1 26. Multiple printings of epoxy may be done in order to obtain the desired total thickness. Separate from the fabrication of the RF substrate 106, sealing cap 1 26 is fabricated. As discussed above, the sealing cap may be made from a number of dielectric materials including glass, low cost electronic grade ceramic, and several plastics, depending on the electrical requirements. The glass dielectric materials include soda lime (float or lamp bulb), alumina silicate, borosilicate (low loss electrical) 99.5% silica for high temperatures. Further, the plastic dielectric materials may be filled or unfilled. The filler could be in the form of a glass matt (for example, G 1 0 printed circuit board material). Also, the ceramics listed above could be used in powdered form as an amorphous filler in the plastic. The plastics could include any of those commonly used in electrical applications, such as epoxy molding compounds and thermoset molding compounds Specific choices for such plastics include Nylon, resins, novalacks, phenohcs, polysulfones, polyetheπmides, and liquid crystal plastics. It is to be appreciated, however, that those skilled in the art will recognize that any plastic which provides acceptable electrical and physical properties can also be used. After completing the RF substrate, the final assembly of the package is performed. First, an appropriate base 1 02 must be selected from the materials listed above. In the first alternative embodiment, the base 102 is made of Alloy 52 A hermeticity check may be performed on the subassembly before final assembly, and the subassembly may be cleaned and fired as necessary. Before attaching the base 102 to the bottom surface of the RF substrate 106, the bottom of the RF substrate may be smoothed to facilitate the attachment. Preparation techniques may vary, depending on the type of base material used. Next, the base 102 and the RF substrate 106 are fitted together with attaching means 104 therebetween. Namely, the attaching means 1 04 for the first alternative embodiment is an organic adhesive. Such adhesive is, perhaps, a more simple method of attaching the RF substrate 1 06 to the base 1 02. It is to be appreciated, however, that any of the adhesives discussed above will operate to retain the RF substrate and the base 102 together. After completion of the package, the microelectronic circuit to be packaged therein is attached to the RF substrate 106 using die attach techniques as are known in the art. The sealing cap 1 26 is then sealed to the upper surface of the RF substrate 106 by a non-conductive polymer adhesive, such as epoxy 108. The process for using a non-conductive polymer adhesive 1 1 2 comprises screen printing the adhesive onto the bottom side of sealing cap 1 26, then pre-baking the sealing cap to partially cure the adhesive. Once the microelectronic circuit is mounted inside the package 700, the sealing cap 126 is aligned with the RF substrate 106. The sealing cap is then clamped in place on the surface of the RF substrate, after which a final cure is performed at a temperature that is low enough to avoid damage to the microelectronic circuit. The above-described process provides a significant advantage over prior art process sequences in that the number of steps is reduced by more than 30%. In addition, the components used can be formed of less expensive materials, making the package more attractive for both its processing, and the material required. Second Alternative Embodiment In a second preferred embodiment, the device of the present invention is shown in Figure 9 and generally designated 900. As shown in Figure 9, device 900 is shown with a number of leads 1 30 extending outwardly from the RF substrate 106. These leads are made of Kovar ® , and are attached to the RF substrate at the ends of the conductors 1 22. Such attachment may be made by soldering, brazing, or other suitable methods known in the art. It is to be appreciated, however, that those skilled in the art will recognize that many lead alloys, including nickel, iron-nickel, and copper alloys which provide acceptable electrical and physical properties can be used. The process for assembling the second alternative embodiment of the present invention mimics the process for assembling the first alternative embodiment. However, following the attachment of the RF substrate 106 to the base 102, the combination is loaded into a lead fixture. This lead fixture maintains a particular orientation between the RF substrate 106 and the leads 130 which facilitates attaching the leads 1 30 to the ends of the conductors 1 22. After the attachment of the leads to the RF substrate, the microelectronic circuit to be packaged therein is attached using die attach techniques as are known in the art. Then, as in the first alternative embodiment, the sealing cap 126 is attached to the upper surface of the RF substrate 106 by a non-conductive polymer adhesive, such as epoxy 108. Third Alternative Embodiment Referring now to Figure 10A, a third alternative embodiment of the present invention is shown and designated generally by reference numeral 1000. Similar to the first embodiment as shown in Figure 1 , package 1000 includes a base 102, a first attaching means 104, an RF substrate 106 with conductive traces 1 32 patterned thereon, a second attaching means 108, a seal ring substrate 1 10, a third attaching means 1 1 2, and a lid 1 14 (not shown in this figure) . Figure 10B shows the third alternative embodiment of the present invention in cross-section. Like Figure 2B, Figure 10B illustrates the cooperation of the base 102, the RF substrate 106, and the conductive trace 132. Namely, the conductive trace 1 32 is separated from the base 1 02 by an RF substrate 106. As in the previously discussed embodiments, this RF substrate has a certain dielectric constant which is used in the determination of the shape of the conductive trace 1 32. While most of the components of the package 1000 are analogous to the components of the package 100, the conductive traces 1 32 are significantly different than the conductive traces 122. More specifically, the conductive traces 132 are formed without any physical transitions for impedance compensation at the feedthrough, at which point the conductive trace becomes, effectively, an embedded microstrip. In other words, a transition segment 1 20 consisting of portions of the conductive trace 1 32 on both sides of the feedthrough plus the feedthrough have a constant impedance. Here, the constant impedance is illustrated as a constant linewidth, however, as is known in the art, impedance is a function of material conductivity, thickness and linewidth, and will also be effected by substrate material and dielectric constant. Ordinarily, as discussed above in conjunction with the preferred embodiment, a conductor which passes through areas having different dielectric surroundings, commonly referred to as a feedthrough, would have to be "tuned" for the particular frequency which the conductor carried. This "tuning" was intended to minimize the insertion loss, and maximize the return loss of the individual conductor. In general, such "tuning" was accomplished by varying the width of the conductor to compensate for the differences between the impedance of the conductor in two different dielectric environments. Referring momentarily back to Figure 2A, typical impedance compensated conductive traces 122 are shown. Each conductive trace 1 22 is attached to the top surface of the RF substrate 106 and has a narrowed section which passes under the seal ring substrate. This seal ring substrate, due primarily to both its proximity to the conductive trace and its dielectric constant, changes the impedance of the conductive trace by causing it to behave as an embedded microstrip. As just discussed above, it is common to narrow the covered section of the conductive trace 1 32 to compensate for the changed impedance along the length of the conductive trace, despite the presence of the seal ring substrate. Referring again to Figure 10A, the conductive trace 1 32 is positioned on the surface of the RF substrate 106, and passes under the seal ring substrate 1 10 at the feedthrough. These conductive traces 1 32, unlike the conductive traces 122 shown in Figure 2A, do not have narrowed portions designed to compensate for the difference in electrical behavior at high frequency between a microstrip line and the embedded microstrip at the feedthrough. It is to be appreciated, however, that if the conductive trace 1 32 is maintained at a constant impedance, the field match of the entire conductive trace 132 is improved. Thus, while failure to compensate for the embedded microstrip would ordinarily be regarded as a poor design, it has been discovered, through modelling and experimentation, that the derogation in the electrical signal on the conductive trace 1 32 due to the lack of impedance compensation is greatly outweighed by the improved field match. In order to show the benefits of having a constant impedance ^ conductive trace 1 32 over an impedance-compensated conductive trace 1 22 feedthroughs, a series of tests were conducted. Specifically, a package 1 00 in accordance with the first embodiment of the present invention, having impedance-matched conductive traces 122, was tested to determine both its electrical insertion loss and its electrical return loss for electrical signals having frequencies between 30 and 45 GHz. Also, a package 1000 in accordance with the third alternative embodiment of the present invention, having field-matched conductive traces 1 32 with constant impedance, was tested to determine both its electrical insertion loss and its electrical return loss for electrical signals having frequencies between 30 and 45 GHz. The results of these tests are shown in Figures 1 1 and 1 2. Referring now to Figure 1 1 , a plot of the electrical insertion loss is shown for packages of both the first embodiment and a third alternative embodiment. More specifically, in Figure 1 1 the electrical insertion loss of a package of the first embodiment is designated 1 1 02, and the electrical insertion loss of a package of the third alternative embodiment is designated 1 1 04. By comparing the two curves 1 102, 1 104, it is to be appreciated that the electrical insertion loss of the third alternative embodiment is greatly improved over the preferred embodiment which incorporates a feedthrough having a compensated impedance. In fact, between approximately 34.5 GHz and 37.5 GHz the package 100 of the first embodiment has an insertion loss nearly 1 dB greater than the package 1000 of the third alternative embodiment. In Figure 1 2, a plot of the electrical return loss for both the first embodiment and a third alternative embodiment are shown. Within Figure 1 2, curve 1 202 represents the return loss of the package of the first embodiment and curve 1204 represents the return loss of the package of the third alternative embodiment. By comparing these curves 1 202, 1 204 it becomes apparent that the return loss of the package 1000 is consistently greater than the return loss of the package 1 00. In fact, around 34 and 43 GHz the return loss of the package 1 000 is over 10 dB higher. In light of Figures 1 1 and 1 2, It is to be appreciated that despite being contrary to conventional wisdom within the art, in higher frequency applications, a conductive trace 1 32 is more effective if no impedance compensation techniques are employed and the conductive trace has a constant impedance. Additionally, it has also been discovered that by designing a conductive trace 132 for use in an uncovered environment, e.g. with an air dielectric having a dielectric constant of 1 , the placement of a short dielectric cover has very little effect on the electrical function of that conductive trace. More specifically, it has been found that conductive traces 1 32, which have been designed for operation in an uncovered environment, are only slightly effected by placement of the seal ring substrate 1 1 0 over a portion of the conductive trace. This indicates that although the seal ring substrate, with a dielectric constant of 9.5, is placed directly over the conductive trace, the performance of the conductive trace 1 32 is better if the impedance is constant along the length of the line. It is to be appreciated that while the conductive traces 1 32 of the third alternative embodiment have been discussed in conjunction with the package 1000, any of the previously discussed embodiments may incorporate conductive traces 1 32. More specifically, package 1 00 of the first embodiment, package 700 of the first alternative embodiment, and package 900 of the second alternative embodiment may incorporate conductive traces 132. In essence, because conductive traces 1 32 do not contain impedance-compensated transitions, it is no longer necessary to precisely control the dielectric surroundings of the conductive trace 1 32. Thus, it is to be appreciated that either the seal ring substrate 1 10 and lid 1 14, or the sealing cap 1 26, may be used in conjunction with conductive traces 132. Further, the materials from which the seal ring substrate and lid, or the sealing cap, may be selected can have a variety of dielectric constants. As a result of the use of constant impedance conductive traces 1 32, the materials from which the package 1000 may be created are less critical. Specifically, any of the materials discussed above in conjunction with other preferred or alternative embodiments may be used for package 1 000. In fact, combinations of materials which are not specifically enumerated above, yet are known in the art as having the necessary electrical and/or physical characteristics, are suitable for making package 1 000. Another significant advantage of the constant impedance conductive traces 1 32 is the ease of impedance matching between the input and output loads. For example, a commonly used impedance is 50 ohms, for both high frequency integrated circuits to the housed in the package and external devices. The absence of required impedance compensation at the feedthrough allows the conductive traces 1 32 to be formed with an impedance of 50 ohms, or at whatever the user's impedance requirements are specified to be. Any additional impedance matching for termination requirements, e.g. , additional wire bonds or leads, can be easily accomplished using conventional techniques, such as stubs, at locations of the conduction trace away from the feedthrough and transition segment, without regard to any effects on feedthrough compensation. Fabrication Process - Third Alternative Embodiment The fabrication process of the third alternative embodiment of the present invention depends on the selection of the materials for the various components. A discussion of the fabrication process, which could incorporate combinations of materials and structures from any of the previously discussed embodiments, would be lengthy and repetitive. Consequently, a detailed fabrication process for a package 1000 will not be discussed here. It is to be appreciated, however, that any of the previously discussed fabrication processes may be used, depending on the materials and structure chosen. The third alternative embodiment described above is a significant improvement over the prior art packages in that the design of the, conductive trace 1 32 expands the useful frequency range of the package. Additionally, the design of the conductive traces 1 32 is simplified by the omission of impedance-compensating transitions. Moreover, because each conductive trace is not designed to only work with a specific dielectric cover, a variety of dielectric materials may be used. It will be apparent to those skilled in the art that various modifications and variations can be made in the apparatus and process of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modification and variations of this invention provided they come within the scope of the appended claims and their equivalence. WE CLAIM:

Claims: CLAIMS 1 . A microelectronic package suitable for high-frequency electronic devices, comprising: a base having a top and a bottom, at least a portion of each of said top and said bottom of said base being conductive; an RF substrate having a top surface, a bottom surface, and a first cavity; a plurality of conductive patterns deposited on said top surface of said RF substrate, each said conductive pattern having a transition segment having a predetermined constant impedance when uncovered; first means for attaching said RF substrate to said top of said base; a ceramic seal ring substrate having a second cavity larger than said first cavity; second means for attaching said ceramic seal ring substrate to said ceramic circuit substrate, said second attaching means being non-conductive; a ceramic lid attached to said ceramic seal ring substrate; and third means for attaching said ceramic seal ring substrate to said ceramic lid; wherein said base acts as a single ground plane and said ceramic lid is electrically isolated from said single ground plane. 2. The microelectronic package recited in claim 1 wherein the base comprises a conductive metal. 3. The microelectronic package recited in claim 1 wherein the base comprises a ceramic material and said top of said base is partially metallized. 4. The microelectronic package recited in claim 1 wherein the base comprises a polymer material and said top of said base is partially metallized. 5. The microelectronic package recited in claim 1 wherein the predetermined constant impedance is selected to match termination impedances of the high-frequency electronic devices. 6. The microelectronic package recited in claim 1 , further comprising a plurality of conductive leads, each said lead electrically attached to a respective said conductive pattern deposited on said top surface of said RF substrate, wherein each said lead extends away from said package for connection to another electrical device. 7. A microelectronic package suitable for high-frequency electronic devices, comprising: a base having a top and a bottom, at least a portion of said base being electrically conductive; an RF substrate having a top surface, a bottom surface, and a first cavity; a first means for attaching said bottom surface of said RF substrate to said top of said base; a plurality of conductive patterns deposited on said top surface of said RF substrate, each said conductive pattern having a transition segment having a predetermined constant impedance when uncovered; a sealing cap having a chamber, said chamber being larger than said first cavity; a second means for attaching said sealing cap to said RF substrate, said second attaching means being non-conductive; wherein said base acts as a single ground plane and said sealing cap is electrically isolated from said single ground plane. 8. The microelectronic package recited in claim 7 wherein the base comprises a conductive metal. 9. The microelectronic package recited in claim 7 wherein the base comprises a ceramic material and said top of said base is partially metallized. 10. The microelectronic package recited in claim 7 wherein the base comprises a polymer material and said top of said base is partially metallized. 1 1 . The microelectronic package recited in claim 7 wherein the sealing cap is formed such that the plurality of conductive patterns deposited on said top surface of the RF substrate are partially exposed. 12. The microelectronic package recited in claim 7, further comprising a plurality of conductive leads, each said lead electrically attached to a respective said conductive pattern deposited on said top surface of said RF substrate, wherein each said lead extends away from said package for connection to another electrical device. 1 3. The microelectronic package recited in claim 7 wherein the sealing cap is made of a ceramic material. 1 4. The microelectronic package recited in claim 7 wherein the sealing cap is made of a plastic material. 1 5. The microelectronic package recited in claim 7 wherein the RF substrate is made of a plastic material. 1 6. The microelectronic package recited in claim 1 5, wherein said plastic material is filled. 1 7. The microelectronic package recited in claim 7, further comprising a plurality of conductive leads, each said lead electrically attached to a respective said conductive pattern deposited on a surface of said RF substrate, wherein each said lead extends away from said package for connection to another electrical device. 1 8. The microelectronic package recited in claim 7 wherein the predetermined constant impedance is selected to match termination impedances of the high-frequency electronic devices. 19. A process for assembling a microelectronic package suitable for retaining a high frequency integrated circuit, said package having a base and an RF substrate, each of said base and said RF substrate having a top surface and a bottom surface, said process comprising: forming a conductive pattern on said top surface of said RF substrate for providing means for electrical connection between said high frequency integrated circuit and an external device, said conductive pattern defining a plurality of conductive traces, each said conductive trace having a transition segment, wherein at least said transition segment of said conductive trace exhibits a predetermined constant impedance when uncovered; cutting a cavity in said RF substrate, said cavity being larger than said integrated circuit; printing a conductive layer on said bottom surface of said RF substrate; attaching said top surface of said base to said bottom surface of said RF substrate to form an assembly; and firing said assembly. 20. The process recited in claim 1 9 wherein the step of forming a conductive pattern comprises screen printing a conductive paste on said top surface of said RF substrate. 21 . The process recited in claim 20 wherein the step of printing a conductive pattern further comprises: selecting said predetermined constant impedance to match an impedance of at least one of said high frequency integrated circuit and said external device. 22. In a microelectronic package suitable for high-frequency electronic devices, said package comprising a base having a top and an bottom, at least a portion of each of said top and said bottom of said base being conductive, an RF substrate having a top surface, a bottom surface, and a first cavity, a plurality of conductive patterns deposited on said top surface of said RF substrate a first means for attaching said RF substrate to said top of said base, a ceramic seal ring substrate having a second cavity larger than said first cavity, a second means for attaching said ceramic seal ring substrate to said ceramic circuit substrate, said second attaching means being non-conductive, a ceramic lid attached to said ceramic seal ring substrate; and a third means for attaching said ceramic seal ring substrate to said ceramic lid, wherein said base acts as a single ground plane and said ceramic lid is electrically isolated from said single ground plane, the improvement comprising: each said conductive pattern having a transition segment including a feedthrough where said ceramic seal ring contacts said conductive pattern, said transition segment having a predetermined constant impedance matched to an impedance of at least one of a high frequency integrated circuit disposed within said package and a high frequency device disposed external to the package. 23. In a microelectronic package suitable for high-frequency electronic devices, said package comprising a base having a top and a bottom, at least a portion of said base being electrically conductive, an RF substrate having a top surface, a bottom surface, and a first cavity, a first means for attaching said bottom surface of said RF substrate to said top of said base, a plurality of conductive patterns deposited on said top surface of said RF substrate, a sealing cap having a chamber, said chamber being larger than said first cavity, a second means for attaching said sealing cap to said RF substrate, said second attaching means being nonconductive, wherein said base acts as a single ground plane and said sealing cap is electrically isolated from said single ground plane, the improvement comprising: each said conductive pattern having a transition segment including a feedthrough where said sealing cap contacts said conductive pattern, said transition segment having a predetermined constant impedance matched to an impedance of at least one of a high frequency integrated circuit disposed within said package and a high frequency device disposed external to the package.

Inventors: ANDERSON PAUL M, BABIARZ JOSEPH, GOETZ MARTIN, GOING TIMOTHY, LINDNER ALAN W, WEIN DEBORAH S, ANDERSON, PAUL, M., BABIARZ, JOSEPH, GOETZ, MARTIN, GOING, TIMOTHY, LINDNER, ALAN, W., WEIN, DEBORAH, S.
Applicants: STRATEDGE CORP, STRATEDGE CORPORATION
Application year: 1997
Main classification: H01L23
IPC codes: H01L23