Method of forming low temperature cofired composite ceramic devices for high frequency applications and compositions used therein
The invention relates to the use of and method of forming Low Temperature Cofired Ceramic (LTCC) circuits for high frequency applications. Furthermore, the invention relates to the novel LTCC thick film compositions and the structure itself.
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The invention relates to the use of and method of forming Low Temperature Cofired Ceramic (LTCC) circuits for high frequency applications. Furthermore, the invention relates to the novel LTCC thick film compositions and the structure itself.
TECHNICAL BACKGROUND OF THE INVENTIONAn interconnect circuit board is a physical realization of electronic circuits or subsystems made from a number of extremely small circuit elements that are electrically and mechanically interconnected. It is frequently desirable to combine these diverse type electronic components in an arrangement so that they can be physically isolated and mounted adjacent to one another in a single compact package and electrically connected to each other and/or to common connections extending from the package.
Complex electronic circuits generally require that the circuit be constructed of several layers of conductors separated by insulating dielectric layers. The conductive layers are interconnected between levels by electrically conductive pathways, called vias, through a dielectric layer. Such a multilayer structure allows a circuit to be more compact.
Another useful dielectric tape composition is disclosed in U.S. Pat. No. 6,147,019 to Donohue et al. The Donohue et al. dielectric tape composition achieves a dielectric constant in the range of 7-8 and is not suitable as a low k material for electronic packaging signal processing applications.
A further useful dielectric tape composition is commercially available Product No. 951 (commercially available from E.I. du Pont de Nemours and Company). Once again, this dielectric tape composition achieves a dielectric constant in the range of 7-8 and is not suitable as a low k material for electronic packaging signal processing applications.
Most prior art LTCC thick film materials do not achieve a sufficiently low k to allow for use as the low k portion of an electronic package for signal processing applications. A typical use of thick film dielectric layers with a dielectric constant (k) of (prior art noted above details a k of greater than 6) is in buried passive component applications. In these LTCC buried passive component applications, dielectric thick films are common. However, in beamforming, filters, couplers, baluns, and other Radio Frequency (RF) signal processing applications which prefer lower k materials than k of 7-8, so the typical materials that are used are not LTCC materials, rather they are poly-tetra-fluoro-ethylene (PTFE) materials, such as Teflon® commercially available from E.I. du Pont de Nemours and Company.
These PTFE materials can achieve a dielectric constant (k) of approximately 3-4. This dielectric constant of 3-4 allows for a wider line width and creates the ability to maintain 50 ohms and to achieve lower dielectric loss of the circuit and lower tolerance effects from the screen patterning the lines. Today, low k PTFE dielectrics are used in nearly all RF modules above 30 GHz due to wavelengths in the dielectric media being smaller.
Antennas and phased arrays are similarly designed utilizing PTFE materials. Antennas and phased array modules from 1 MHz up to, and including, mm wavelengths are used in a wide range of communication and radar applications, such as cellular telephone base stations, mobile tracking communication system, GPS, commercial broadcasting linear arrays and planar-rectangular, planar-circular radar arrays. Additionally, new cellular base station technology of smart antennas is used to improve overall communication system capacity and performance.
Military electronic intercept and related RF intelligence gathering systems use “beamformers” to precisely locate signal sources. They are typically broadband to detect emissions in the range of interest.
“Beamformers” work by carefully controlling the amplitude and phase of RF energy conveyed to the radiating elements of an antenna array. Elements commonly used to make “beamformers” are quadrature couplers, hybrid junctions, phase shifters and power dividers.
When used in conjunction with specialized receivers, “beamformer” networks can identify the location of an RF energy source.
When interfaced with suitable transducers, beamformers can be used in acoustic source location devices related to sonar. Thus, beamformers are used in many direction finding systems.
U.S. Pat. No. 5,757,611 to Gurkovich et al. discloses an electronic package having a buried passive component such as a capacitor therein, and a method for fabricating the same. The electronic package includes a passive component portion which includes a plurality of layers of high k dielectric material, a signal processing portion which includes a plurality of layers of low k dielectric material, and at least one buffer layer interposed between the passive component portion and the signal processing portion. Gurkovich et al. does not disclose an LTCC structure which allows for the absence of a buffer layer between the low k and high k regions. Furthermore, Gurkovich et al. discloses a method of fabrication which utilizes pressure assisted lamination. Gurkovich et al. discloses the use of passive component portions in conjunction with signal processing and does not disclose the ability for passive component portions and signal processing as stand-alone features. Additionally, Gurkovic et al. discloses the use of capture pads along all vertical vias between all layers.
Additionally, presently available dielectric LTCC tapes typically have an X-Y shrinkage during processing on the order of 9-13% when formed into a multilayer circuit for high frequency applications. To minimize the shrinkage, designers utilize constraining tapes either internally as “non-functional layers” and/or externally. Internally used constraining tapes have a high dielectric constant in the order 15-25, which results in an increase in the dielectric constant of package/device overall. Externally constraining tapes require removal from the device because they are non-functional and the circuit surfaces are needed to add other functional characteristics, such as conductors, resistors etc. Most constraining tapes are alumina or silica-based and they do not react with standard thick film dielectric tapes, if used externally. Thus, allowing for removal.
SUMMARY OF THE INVENTIONThe present invention provides a low k thick film dielectric composition comprising, based on weight percent total inorganic composition: (1) 40-80 percent glass frit with a log viscosity range of 2-6 Poise; (2) 20-60 percent ceramic oxide selected from the group consisting essentially of silica, silicates, and mixtures thereof, wherein said ceramic oxide has a dielectric constant in the range of 2 to 5 k.
In one embodiment, the low k thick film dielectric composition above further comprises up to 5 weight percent inorganic oxides selected from the group consisting of copper oxide, silicon dioxide, aluminum oxides, mixed oxides and various other such oxides. Also present may be such products of mixed oxides such as aluminum silicate.
In a further embodiment, the present invention provides a method of using a low k thick film in the formation of a low temperature cofired ceramic structure for high frequency applications comprising the steps:
providing two or more layers of a low k thick film dielectric tape, having dielectric constant in the range of 2 to 5 and comprising, based on solids: (a) 40-80 weight percent glass composition; (b) 20-60 weight percent ceramic oxide; dispersed in a solution of (c) organic polymeric binder;
providing two or more layers of a high k thick film dielectric tape having a dielectric constant in the range of 5 to 8;
collating the layers of low k and high k thick film dielectric tapes wherein said dielectric tapes are not separated by a buffer layer;
laminating the layers of low k thick film and high k thick film to form an assembly; and
processing the assembly to form a low temperature cofired ceramic structure.
In a further embodiment, the present invention provides the method above wherein the glass composition consists essentially of, based on mole percent, 50-56% B2O3, 0.5-5.5% P2O5, SiO2 and mixtures thereof, 20-50% CaO, 2-15% Ln2O3 where Ln is selected from the group consisting of rare earth elements and mixtures thereof, 0-6% MI2O where MI is selected from the group consisting of alkali elements; and 0-10% Al2O3, with the proviso that the composition is water millable.
BRIEF DESCRIPTION OF THE DRAWINGS
The numbered items in
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- (10) Surface Metalization for wirebonding, soldering, brazing, and other post process applications as well as external RF lines for interconnect to the Stripline section(s)
- (20) 951 LTCC
- (30) Interposer
- (40) Signal Vias which connect the surface devices such is SMT's, IC's, packaged devices and other signal processing components to the internal microwave circuits on the internal Low K layers which form the stripline circuits.
- (50) Vias connecting the two stripline grounds in the LowK region for “via fencing” for microwave designs to improve circuit performance.
- (60) Solid, Gridded, or partial Grounds to form the grounds for the Stripline Sections
- (70) Thru-All Cavities to access baseplate from surface. Cavities from the top to place IC's or components or other devices which would benefit from being recessed planar to the surface of the LTCC.
- (80) Stripline, Buried Microstrip, Covered GCPW, laminated waveguide, and other methods for guiding propagated RF, microwave, or mmWave Signals or using for purposes of signal Lines for RF functions (Beamformer, Filters, antennas, couplers, etc.
- (90) Stripline section Low LTCC
- (100)Baseplate for thermal dissipation and/or mechanical strength which can be soldered, epoxied, or brazed.
The present invention utilizes both high k thick film dielectric tape compositions and low k thick film dielectric tape compositions in the formation of LTCC circuits for use in high frequency/signal processing applications. In particular, the present invention provides novel low k tape compositions for use in the manufacture of LTCC circuits for use in high frequency/signal processing applications. The present invention provides novel compositions and methods of using and making these circuits.
In a typical case of beamformer circuits, the difference of using PTFE and LTCC-based material are listed below in Table 1.
**“Microwave Laminate Material Considerations for Multilayer Military Applications”, R. Hornung & J. Frankosky, RF Globalnet Newsletter, 2006, Arlon Inc.
Two important components of phased array antennas are phase shifters and feed networks. With Low K LTCC Phase shifters, performance can be improved including power handling, losses, and bandwidth of the phase shifters. Feed networks including series, parallel, and space can also be implemented. Filters using low k LTCC can now be improved upon by designing in lower impedance allowing wider lines, which allows for overall better insertion loss, return loss, achieving rejection points, bandwidth In the case of Antenna Arrays path loss 88 db at 60 GHz is pushing the circuits to its limit. The only way to make up for extra loss at the higher frequencies is with the use of higher gain antenna Arrays can be implemented in LTCC, but a major drawback is that most LTCC systems have a dielectric constant>6, which lowers the gain and bandwidth of the antenna. A lower k-based LTCC (k=3 or 4) with lower dielectric loss material has the antenna array allowing for higher gain and improved bandwidth and other improved metrics. In summary, use of low k dielectric thick film materials either by themselves or in combination with other LTCC systems designs can be improved in the case of Phase Shifters, Feed Networks, Frequency Scanning Arrays, Wideband Arrays, Radar Phased Arrays, Beam formers, Filters, Couplers, Baluns, Power Dividers, Quadrature Couplers, Hybrid Junctions and others. Through the use of LTCC technology and the newly available low k dielectric thick film materials, layers of low k thick film dielectric materials may be placed in specified z locations in the electronic package stackup which allows for more degrees of freedom for the designer for RF, microwave, and mmwave signal processing.
As used herein, the terms “thick film” and “thick film paste” refer to dispersions of finely divided solids in an organic medium, which are of paste consistency or tape castable slurry and have a rheology suitable for screen printing and spray, dip, ink jet or roll-coating. As used herein, the term “thick film” means a suspension of powders in screen printing vehicles or tape castable slurry, which upon processing forms a film with a thickness of several microns or greater. The powders typically comprise functional phases, glass and other additives for adhesion to the substrate, etc. The vehicles typically comprise organic resins, solvents and additives for rheological reasons. The organic media for such pastes are ordinarily comprised of liquid binder polymer and various rheological agents dissolved in a solvent, all of which are completely pyrolyzable during the firing process. Such pastes can be either resistive or conductive and, in some instances, may even be dielectric in nature. The thick film compositions of the present invention contain an inorganic binder as the functional solids are required to be sintered during firing. A more detailed discussion of suitable organic media materials can be found in U.S. Pat. No. 4,536,535 to Usala, herein incorporated by reference. In some embodiments, fired dielectric thick film layers are on the order of 3-300 microns for a single print or tape layer, and all ranges contained therein. In further embodiments, the thickness of the fired dielectric thick film layer is in the range of 3-5 microns, 5-10 microns, 10-15 microns, 30-250 microns.
I. High k Dielectric Tape Composition(s)
The present invention utilizes commercially available dielectric thick film tape compositions as a constraining tape, either externally or internally. These commercially available high k dielectric thick film tapes comprise crystallizable glass-based systems such as borate-, borosilicate, or boro-phospho-silicate glass networks, as used in commercially available tape Nos. 951, 943 or tapes described in U.S. patent application Ser. No. 11/543,742, herein incorporated by reference (commercially available from E.I. du Pont de Nemours and Company). These commercially available high k tapes are particularly useful in the present invention. As used herein, “high k” tapes are in the range of 6 to 8 k. The dielectric tapes noted immediately above are not standard constraining tapes. Standard constraining tapes, and will react with the functional tape layers and cannot be removed from the LTCC device, without damaging the circuits.
The high k tapes useful in the present invention are typically very reactive and would likely react with standard constraining tapes noted above, if used internally or externally; and therefore, high frequency properties such as dielectric loss and dielectric constant may degrade. Therefore, standard constraining tapes are not useful for use in conjunction with these borate-based, low loss dielectric tapes used in high frequency applications.
The present inventors have developed low k dielectric thick film compositions and methods for their use which provide (1) low dielectric loss tape for high frequency application with lower dielectric constant than the presently available k 6-8 (2) a lower shrinkage value than that presently available shrinkage value of 7-12% without using a constraining tape and (3) in some embodiments, a low k tape which provides the added property of constraining the high k dielectric tape (Commercially available Product Nos. 943, 951 and commercially available tape disclosed in U.S. patent application Ser. No. 11/543,742), and finally (4) the low k dielectric tape reacts with the high k dielectric tape and upon firing results in a continuous structure without delamination and which allows the circuit designers to incorporate several k-value tapes at appropriate locations in the z-direction of the circuits to control the functional property of the circuits at the appropriate locations. In some embodiments, upon firing the low k and high k tapes, a homogeneous structure (i.e., a structure in which the individual tape layers are indistinguishable) results.
The present inventors have developed a novel low k and low dielectric loss tape. Furthermore the low k tape described in this invention is compatible with the commercially available dielectric thick film tapes and could be used in specific layers of the LTCC structure. The novel low k tape has lower shrinkage than any commercially available functional LTCC tapes with an additional property of constraining presently available other functional green tapes (for example the high k dielectric tapes disclosed above), if used in conjunction.
Typically, a LTCC tape is formed by casting a slurry of inorganic solids, organic solids and a fugitive solvent on a removable polymeric film. The slurry consists of glass powder(s) and ceramic oxide filler materials and an organic based resin-solvent system (medium) formulated and processed to a fluid containing dispersed, suspended solids. The tape is made by coating the surface of a removable polymeric film with the slurry, so as to form a uniform thickness and width of coating.
In one embodiment, LTCC tape materials available for use as a dielectric tape layer in high frequency LTCC applications are disclosed in U.S. patent application Ser. No. 11/543,742, the parent application of the present invention to which the present invention claims priority. Furthermore, some embodiments of the dielectric thick film tape composition of U.S. patent application Ser. No. 11/543,742 are useful in the present invention as the high k thick film tape layer. This dielectric tape is designed to eliminate potentially toxic constituents and exhibits a uniform and relatively low dielectric constant in the range of 6-8. Additionally, the dielectric tape has a low dielectric loss performance over a broad range of frequency up to 90 GHz or sometimes higher depending on the metal loading.
II. Low k Thick Film Dielectric Tape Composition(s)
The low k tape has a very low shrinkage compared to commercially available “LTCC circuit functional tapes” and in addition, it constrains other commercially available tapes if used in the z-direction of the LTCC structure and does not require removal after processing. The low k tape exhibits processing and materials compatibility with conductors and passive electronic materials when used to build high density, LTCC circuits. The low k tape system or “low k tape-based composite system” with other commercially available low loss tapes provides low dielectric loss over frequencies up to 90 GHz or higher, more circuit design freedom than PTFE structures, superior X-Y constraining effect and good bonding between the low k tape and high k tape without delamination of layers under the standard processing conditions of LTCC system described in the invention. No buffer layer is required between the thick film dielectric tape layers of the present invention.
Overall, the present invention provides a self constrained LTCC system which allows higher integration of RF, Microwave, and/or mm wave signal processing capability into one module, package, or board. There is no LTCC or multilayer ceramic system that exists which allows use of multiple high and low k dielectric layers to be used together in one composite module, package, or board (i.e., structure) which is self constrained in the X-Y direction and which also has low k and low loss. This invention will use combinations of layers consisting of various high k and low k values, thicknesses, and loss values into one LTCC structure.
Commercially available dielectric green tapes useful for LTCC devices have a lowest dielectric constant of approximately 6-7. Circuit designers are looking for a k value that is much lower than the commercially available dielectric thick film LTCC tapes. The low K dielectrics are used in nearly all RF modules above 30 GHz. Being able to place layers of K lower than 6-8 in certain z locations in the stack-up allows more degrees of freedom for the circuit designer.
Antennas are now similar to those designed in PTFE due to the use of lower K dielectrics on the external layers of the module. Using lower K allows wider RF lines to maintain a resistance of 50 ohms. This has a two-fold impact on the designs: (1) wider lines have higher yields because the line width tolerance has a smaller effect than does a narrower line and (2) wider lines give better performance (i.e. attenuation is lower) than narrower lines.
All green tapes shrink during the LTCC processing. The shrinkage is a function of many parameters: including particle size and particle size distribution of inorganic oxide present in the tape; ratio of organic to inorganic materials; kinetics of “Un-zipping” and depolymerization of polymers and “burn-out” of carbonaceous species; kinetics of glass-softening; interaction of glass components to inorganic filler materials present in the tape, if any; nucleation and growth of crystals, if the glass is crystallizable. Even though shrinkage is a three dimensional phenomenon, the most important aspect for LTCC circuit designers is X-Y shrinkage. Preferred crystal growth has less impact on the design. However elongated crystal growth could produce surface roughness, unwanted property variations. Zero X-Y shrinkage and/or control of shrinkage to a lowest possible level is a desirable. Ceramicists have developed materials and tapes to control and constrain the LTCC tapes. These constrain tapes are based on least sinterable ceramic materials such as alumina and silica at the LTCC processing temperature. Other requirements for these constraining tapes is that it should be easily removable from the surfaces of the LTCC circuits after the sintering of the circuits; i.e., least reactive to the functional tape layers. Some constraining tapes are used internally by inserting in the Z-direction of composite layers of the functional tapes and they will become part of the circuit and will not be removed unlike tapes used to constrain externally. All the available internally constraining such tapes have high dielectric constants compared to the dielectric constant needed for high frequency applications. The LTCC tapes used in high frequency applications and described in the earlier section is borate or boro-silicate, or boro-phospho-silicate glass based system which will crystallize at the LTCC processing conditions, leaving behind a low viscosity “remanent glass”. The externally constraining tapes react with the “remanent glass” so it is difficult to remove after processing without damaging the circuits, and/or leaving behind residues. Presence of such residues on the surface makes it impossible to add functional units on the surface. So an ideal solution is to develop a LTCC tape with a lower shrinkage value closer to zero or lowest acceptable shrinkage for design requirements without using a constraining tape.
The applications described in 1 of this section, also need the new tape that should be compatible with other commercially available low loss tapes to mix and match several different tapes with different dielectric properties that circuit designers' need. Furthermore the new tape should have a property to constrain other high shrinkage tapes commercially available, if used in conjunction with high frequency circuits. For example, U.S. patent application 943 Green tape described in U.S. Pat. No. 6,147,019 and EL-0518 has a shrinkage of 9.5% when used in LTCC devices. Without constraining, it is difficult to incorporate tapes with different shrinkages into a composite for the complex electronic functions listed earlier.
Constraining the tape internally and/or externally and reducing the shrinkage of the composite to a minimum are the essential needs for the future device requirements. Standard constraining tapes cannot be used in conjunction with these tape chemistries because they react together and cannot be removed after processing, thus degrading dielectric properties: increased K, dielectric loss, and circuit surface damage if constraining tape is removed mechanically.
The present invention is directed to a borate, boro-silicate, or a boro-phospho-silicate crystallizable glass-based tape with ceramic oxide filler components to control the crystallization of the glass, control the viscosity of the “remanent” glass, and lower the dielectric constant of the fired composite. Furthermore, the new tape may be compatible with other, commercially available low dielectric loss tapes so that they can integrated together for several property functions.
Furthermore the new tape may be incorporated into any LTCC composite system with compatable chemistry if the circuit designer so desires to incorporate specific layers of lower K and low loss dielectric properties in the circuits. Such incorporation may introduce alternations in the functional property of other tape layers in the system.
The materials are characterized by their freedom from toxic metal oxides such as oxides of lead cadmium. The materials are designed to process at about 850-875 oC useful in current tape dielectric materials. The processing conditions can be adjusted for a particular LTCC circuit. The tape is designed to cofire with conductors, buried capacitors and other passive electrical components applied by screen printing or tape casting or other similar processing conditions.
A. Ceramic Oxide(s)
The compositions described, that have small SiO2 additions, have shown significant improvement in the compatibility with Ag based conductor lines. The tendency to interact in proximity to Ag conductor lines is suppressed in the tape compositions tested that were made from glasses that give high viscosity “remanent glass”. The dielectric loss properties reported unexpectedly shows that the addition of small amount of SiO2 in the composition do not alter significantly the dielectric characteristics of the tape dielectric. The low addition levels of SiO2 addition to glass shown in this case was not reported in Donohue et al. U.S. Pat. No. 6,147,019. The addition of SiO2 was indicated as not beneficial to dielectric loss.
In the present invention, significantly higher amount of silica is added as second crystalline phase filler to borate or boro-silicate or boro-phospho-silicate glasses to reduce the dielectric constant and green tape shrinkage to satisfy the low k needs of LTCC designers.
Therefore, the present invention provides a low k thick film dielectric tape composition comprising, based on wt % total inorganic composition: (1) 40-80%, preferably 45-55% borate-based or, boro-silicate, or boro-phospho-silicate-based glass composition such as glass chemistry described in U.S. Pat. No. 6,147,019; EL-0518; DUPONT glass used in Green Tape 951, or similar crystallizable glasses with a log viscosity range at the peak firing temperature 2-6 Poise (2) 20-60%, preferably 30-50% ceramic oxide or mixed oxide fillers such as silica, silicates compatible to glass chemistry (3) 0-5% other inorganic oxides and compounds such as copper oxide and others with similar chemistries.
B. Glass Frit
In the formulation of tape compositions, the amount of glass relative to the amount of ceramic material is important. A filler range of 20-60% by weight is considered desirable in that the sufficient densification is achieved. If the filler concentration exceeds 60% by wt., the fired structure is not sufficiently densified and is too porous. Within the desirable glass to filler ratio, it will be apparent that, during firing, the filler phase will become saturated with liquid glass. The glass-filler ratio variation is also depends on the viscosity of the glass at the softening point, viscosity of the “remanent glass”, and the nature of the filler to the so-called glass “net-work formers”
For the purpose of obtaining higher densification of the composition upon firing, it is important that the inorganic solids have small particle sizes. In particular, substantially all of the particles should not exceed 15 um and preferably not exceed 10 um. Subject to these maximum size limitations, it is preferred that at least 50% of the particles, both glass and ceramic filler, be greater than 1 um and less than 6 um.
One embodiment of the glass composition used in this invention is a boro-phospho-silicate glass network consisting essentially of, based on mole percent, 50-56% B2O3, 0.5-5.5% P2O5, SiO2 and mixtures thereof, 20-50% CaO, 2-15% Ln2O3 where Ln is selected from the group consisting of rare earth elements and mixtures thereof, 0-6% MI2O where MI is selected from the group consisting of alkali elements; and 0-10% Al2O3, with the proviso that the composition is water millable. Another glass used in this invention has been described in Donahue and others in Hang et al. The inorganic filler used in this invention is silica powder has the surface area of 0.5-15.0 m2/gm preferably 7.0-13.0 m2/gm. Other mixed ceramic oxides and/or mixtures of ceramic oxides compatible to the wetting characteristics of the crystallizable glasses that have a log viscosity range 2-6 Poise at the maximum firing temperature of 850 oC.
C. Organic Medium
The organic medium in which the glass and ceramic inorganic solids are dispersed is comprised of an organic polymeric binder which is dissolved in a volatile organic solvent and, optionally, other dissolved materials such as plasticizers, release agents, dispersing agents, stripping agents, antifoaming agents, stabilizing agents and wetting agents.
To obtain better binding efficiency, it is preferred to use at least 5% wt. polymer binder for 90% wt. solids (which includes glass and ceramic filler), based on total composition. However, it is more preferred to use no more than 30% wt. polymer binder and other low volatility modifiers such as plasticizer and a minimum of 70% inorganic solids. Within these limits, it is desirable to use the least possible amount of binder and other low volatility organic modifiers, in order to reduce the amount of organics which must be removed by pyrolysis, and to obtain better particle packing which facilitates full densification upon firing.
In the past, various polymeric materials have been employed as the binder for green tapes, e.g., poly(vinyl butyral), poly(vinyl acetate), poly(vinyl alcohol), cellulosic polymers such as methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, methylhydroxyethyl cellulose, atactic polypropylene, polyethylene, silicon polymers such as poly(methyl siloxane), poly(methylphenyl siloxane), polystyrene, butadiene/styrene copolymer, polystyrene, poly(vinyl pyrollidone), polyamides, high molecular weight polyethers, copolymers of ethylene oxide and propylene oxide, polyacrylamides, and various acrylic polymers such as sodium polyacrylate, poly(lower alkyl acrylates), poly(lower alkyl methacrylates) and various copolymers and multipolymers of lower alkyl acrylates and methacrylates. Copolymers of ethyl methacrylate and methyl acrylate and terpolymers of ethyl acrylate, methyl methacrylate and methacrylic acid have been previously used as binders for slip casting materials.
U.S. Pat. No. 4,536,535 to Usala, issued Aug. 20, 1985, has disclosed an organic binder which is a mixture of compatible multipolymers of 0-100% wt. C1-8 alkyl methacrylate, 100-0% wt. C1-8 alkyl acrylate and 0-5% wt. ethylenically unsaturated carboxylic acid of amine. Because the above polymers can be used in minimum quantity with a maximum quantity of dielectric solids, they are preferably selected to produce the dielectric compositions of this invention. For this reason, the disclosure of the above-referred Usala application is incorporated by reference herein.
Frequently, the polymeric binder will also contain a small amount, relative to the binder polymer, of a plasticizer that serves to lower the glass transition temperature (Tg) of the binder polymer. The choice of plasticizers, of course, is determined primarily by the polymer that needs to be modified. Among the plasticizers which have been used in various binder systems are diethyl phthalate, dibutyl phthalate, dioctyl phthalate, butyl benzyl phthalate, alkyl phosphates, polyalkylene glycols, glycerol, poly(ethylene oxides), hydroxyethylated alkyl phenol, dialkyldithiophosphonate and poly(isobutylene). Of these, butyl benzyl phthalate is most frequently used in acrylic polymer systems because it can be used effectively in relatively small concentrations.
The solvent component of the casting solution is chosen so as to obtain complete dissolution of the polymer and sufficiently high volatility to enable the solvent to be evaporated from the dispersion by the application of relatively low levels of heat at atmospheric pressure. In addition, the solvent must boil well below the boiling point or the decomposition temperature of any other additives contained in the organic medium. Thus, solvents having atmospheric boiling points below 150° C. are used most frequently. Such solvents include acetone, xylene, methanol, ethanol, isopropanol, methyl ethyl ketone, ethyl acetate, 1,1,1-trichloroethane, tetrachloroethylene, amyl acetate, 2,2,4-triethyl pentanediol-1,3-monoisobutyrate, toluene, methylene chloride and fluorocarbons. Individual solvents mentioned above may not completely dissolve the binder polymers. Yet, when blended with other solvent(s), they function satisfactorily. This is well within the skill of those in the art. A particularly preferred solvent is ethyl acetate since it avoids the use of environmentally hazardous chlorocarbons.
In addition to the solvent and polymer, a plasticizer is used to prevent tape cracking and provide wider latitude of as-coated tape handling ability such as blanking, printing, and lamination. A preferred plasticizer is BENZOFLEX® 400 manufactured by Rohm and Haas Co., which is a polypropylene glycol dibenzoate.
ApplicationA green tape is formed by casting a thin layer of a slurry dispersion of the glass, ceramic filler, polymeric binder and solvent(s) as described above onto a flexible substrate, heating the cast layer to remove the volatile solvent. This forms a solvent-free tape layer. The tape is then blanked into sheets or collected in a roll form. The green tape is typically used as a dielectric or insulating material for multilayer electronic circuits. A sheet of green tape is blanked with registration holes in each corner to a size somewhat larger than the actual dimensions of the circuit. To connect various layers of the multilayer circuit, via holes are formed in the green tape. This is typically done by mechanical punching. However, a sharply focused laser or other method(s) can be used to volatilize and form via holes in the green tape. Typical via hole sizes range from 0.004″ to 0.25″. The interconnections between layers are formed by filling the via holes with a thick film conductive ink. This ink is usually applied by standard screen printing techniques. Each layer of circuitry is completed by screen printing conductor tracks. Also, resistor inks or high dielectric constant inks can be printed on selected layer(s) to form resistive or capacitive circuit elements. Furthermore, specially formulated high dielectric constant green tapes similar to those used in the multilayer capacitor industry can be incorporated as part of the multilayer circuitry.
After each layer of the circuit is completed, the individual layers are collated and laminated. A confined uniaxial or isostatic pressing die is used to insure precise alignment between layers. The laminate assemblies are trimmed with a hot stage cutter. Firing is typically carried out in a standard thick film conveyor belt furnace or in a box furnace with a programmed heating cycle. This method will, also, allow top and/or bottom conductors to be co-fired as part of the constrained sintered structure without the need for using a conventional release tape as the top and bottom layer, and the removal, and cleaning of the release tape after firing.
The dielectric properties of the fired tape (or film) of the present invention depend on the quantity and/or quality of total crystals and glasses present and other factors. The low temperature co-fired ceramic (LTCC) device dielectric properties also depend on the conductor used. The interaction of conductor with the dielectric tape may, in some embodiments, alter the chemistry of the dielectric portion of the device. By adjusting the heating profile and/or changing the quality and/or quantity of the filler in the tape and/or chemistry of the conductor, one skilled in the art could accomplish varying dielectric constant and/or dielectric loss values.
As used herein, the term “firing” means heating the assembly in an oxidizing atmosphere such as air to a temperature, and for a time sufficient to volatilize (burn-out) all of the organic material in the layers of the assemblage to sinter any glass, metal or dielectric material in the layers and thus density the entire assembly.
It will be recognized by those skilled in the art that in each of the laminating steps the layers must be accurate in registration so that the vias are properly connected to the appropriate conductive path of the adjacent functional layer.
The term “functional layer” refers to the printed green tape, which has conductive, resistive or capacitive functionality. Thus, as indicated above, a typical green tape layer may have printed thereon one or more resistor circuits and/or capacitors as well as conductive circuits.
It should also be recognized that in multilayer laminates having greater than 10 layers typically require that the firing cycle may exceed 20 hours to provide adequate time for organic thermal decomposition.
The use of the composition(s) of the present invention may be used in the formation of electronic articles including multilayer circuits, in general, and to form microwave and other high frequency circuit components including but not limited to: high frequency sensors, multi-mode radar modules, telecommunications components and modules, and antennas. The system described in the present invention allows higher integration of microwave functions into one module, package, or board. Other Major Significance is that no other LTCC or multilayer ceramic system that exists which allows use of multiple dielectric layers to be used together in one composite module, package, or board. This invention will use combinations of layers consisting of various K values, thicknesses, loss values into one composite structure.
Multilayer Circuit Formation
The present invention further provides a method of forming a multilayer circuit comprising the steps:
wherein, said circuit achieves a x,y-shrinkage in the range of 0-5% and wherein said low k constraining tape layer has a k value in the range of 2-5, and wherein said tapes allow more degrees of freedom for high-frequency LTCC circuit designers to mix and match several tapes with the tapes described in this invention for specific circuit requirements
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- 10 Surface Metalization for wirebonding, soldering, brazing, and other post process applications as well as external RF lines for interconnect to the Stripline section(s)
- 20 High k thick film tape
- 30 Interposer
- 40 Signal Vias which connect the surface devices such is SMT's, IC's, packaged devices and other signal processing components to the internal microwave circuits on the internal Low K layers which form the stripline circuits.
- 50 Vias connecting the two stripline grounds in the LowK region for “via fencing” for microwave designs to improve circuit performance.
- 60. Solid, Gridded, or partial Grounds to form the grounds for the Stripline Sections
- 70 Thru-All Cavities to access baseplate from surface. Cavities from the top to place IC's or components or other devices which would benefit from being recessed planar to the surface of the LTCC.
- 80 Stripline, Buried Microstrip, Covered GCPW, laminated waveguide, and other methods for guiding propagated RF, microwave, or mmWave Signals or using for purposes of signal Lines for RF functions (Beamformer, Filters, antennas, couplers, etc.
- 90 Stripline Section of low k LTCC
- 100 Baseplate for thermal dissipation and/or mechanical strength which can be soldered, epoxied, or brazed.
These multilayer circuits require that the circuit be constructed of several layers of conductors separated by insulating dielectric layers. The insulating dielectric layer may be made up of one or more layers of the tape of the present invention. The conductive layers are interconnected between levels by electrically conductive pathways through a dielectric layer. Upon firing, the multilayer structure, made-up of dielectric and conductive layers, a composite is formed which allows for a functioning circuit (i.e. an electrically functional composite structure is formed). The composite as defined herein is a structural material composed of distinct parts resulting from the firing of the multilayer structure which results in an electrically functioning circuit.
Another circuit design by mix and match with other commercially available tapes and tape of this invention is shown below
EXAMPLES Tape compositions used in the examples were prepared by ball milling the fine inorganic powders and binders in a volatile solvent or mixtures thereof. To optimize the lamination, the ability to pattern circuits, the tape burnout properties and the fired microstructure development, the following volume % formulation of slip was found to provide advantages. The formulation of typical slip compositions is also shown in weight percentage, as a practical reference. The inorganic phase is assumed to have a specific density of 3.5 g/cc for glass and 2.2 g/cc for silica and the organic vehicle is assumed to have a specific density of 1.1 g/cc. The weight % composition changes accordingly when using other glasses and oxides other than silica as the specific density may be different than those assumed in this example.
The above weight % slip composition may vary dependent on the desirable quantity of the organic solvent and/or solvent blend to obtain an effective slip milling and coating performance. More specifically, the composition for the slip must include sufficient solvent to lower the viscosity to less than 10,000 centipoise; typical viscosity ranges are 1,000 to 4,000 centipoise. An example of a slip composition is provided in Table 2. Depending on the chosen slip viscosity, higher viscosity slip prolongs the dispersion stability for a longer period of time (normally several weeks). A stable dispersion of tape constituents is usually preserved in the as-coated tape.
If needed, a preferred inorganic pigment at weight % of 0.1 to 1.0 may be added to the above slip composition before the milling process.
Glass powder used in this composition is a phospho-boro-silicate glass described in commonly assigned patent application Ser. No. 11/543,742. Silica in the composition #1 and #2 has a PSD ˜1.5 (D50) and silica in composition #3 is finer with surface area ˜8-12 m2/gm.
Property Measurements: Dielectric Properties
The measurement of dielectric constant, Er and dielectric loss (tangent delta) has been performed for selected samples of tape made from the tapes indicated in Table 2. These measurements were performed using a (non-metallized) split cavity method in a range of frequency from 3.3 GHz to 16 GHz. A reference to the measurement method is given in “Full-Wave Analysis of a Split-Cylinder Resonator for Nondestructive Permittivity Measurements” by Michael Janezic published in IEEE Transactions on Microwave Theory and Techniques, Vol 47, No. 10, October 1999. Data for two frequencies are provided in Table 2. The data, (Er and loss), for all measured samples shows a very slight increase with frequency.
The dielectric constant reduced to approximately 50% however and dielectric loss is increased slightly for the LTCC tape in the current invention.
The dielectric properties of the fired film of this invention, which is a “devitrified glass-ceramic-glass composite”, depend on the quantity and/or quality of total crystals and glasses present in the composite. The LTCC dielectric properties also depend on the conductor film which is a “metal-devitrified glass-ceramic composite”.
It was stated earlier, one of major contributions of this invention should give freedom for circuit designers to incorporate different layers of LTCC tapes for different function in a composite format.
Tape Shrinkage and Refire Stability
The shrinkage values have been measured then calculated using the “Hypotenuse” method, known to those skilled in the art. All parts were fired at 850° C. following a standard green tape firing profile. Several composite test format have been made to demonstrate the shrinkage of the tape of this invention and its ability to constrain other commercially available tapes if incorporated within the composite.
Details of a some typical eight layer composite structures are given below. A refers to 951 and B refers to 943 are commercial tapes of DUPONT COMPANY, Wilmington, Del. E refers to the tape described in EL#518 and C refers to tape of this invention. Table 4 is a representation of some typical test pattern builds and Table 5 is the shrinkage of 8 layer composites after firing in a typical green belt furnace profile. All results show up to approximately 80% more constraining than the shrinkage of some of the commercially available LTCC tape. The shrinkage of the LTCC tape of this invention has a shrinkage of ˜1%.
#1, #2 & #4 are internally constraining Composite Format and #3 is externally constraining composite format.
#5 & #6 are typical circuit systems using the tape of this invention in composite format.
“C” is the Tape of this invention.
“A”, “B” and “E” are commercially available tape or tapes described in U.S. Pat. App. No. 11/543742 Microstructures taken using Scanning Electron microscope of the fired film of all the composites show (1) no delamination between the layers (2) good microstructures in terms of grain and grain boundaries and (3) no significant increase in the level of porosity
*944 (1) and 944 (2) are based on two different tape chemistries convered in U S Patent application 11/543742 (Attorney Docket # EL-0518USNA). Finer silica-based based tape of this invention (#3) gave lower shrinkage when used in the composite structure described here compare to tape contains coarser silica (#1 & #2)
**Two layers of “tape of this invention” (TTI) are inserted anywhere in a symmertical manner as ahown in the table within the 8 layers of commecially available green tapes, 951, 943, & 944. TTI layers are “circuit functional layers” of the overall circuit and need not to be removed. TTI layers could be connected with other tape layers through via-fill conductors, and circuit conductor lines in the conventional manner.
Microstructure of the Fired Composites
The microstructral analysis on Scanning Electron Micrographs of several combinations of low K tape and other LTCC tapes composites has shown (1) complete interfacial microstructral compatibility (2) good densification of the low k tapes and (3) no significant microstructural defects of any kind.
Dielectric constant of two different tape formulations in a buried composite form is measured. Results show the effect of conductor binders on the K values.
K values of the low K tape as measured within the buried form for three different conductors are given below. Used vias to measure the cap values. Results show the conductor binder effect on the dielectric constant K values are calculated from the measured capacitor value at frequency 1 KHz and calculated thickness values using tape shrinkage data from table 6.
Claims
1. A low k thick film dielectric composition comprising, based on weight percent total inorganic composition:
- (1) 40-80 percent glass frit with a log viscosity range of 2-6 Poise;
- (2) 20-60 percent ceramic oxide selected from the group consisting essentially of silica, silicates, and mixtures thereof, wherein said ceramic oxide has a dielectric constant in the range of 2 to 5 k.
2. The low k thick film dielectric composition of claim 1 further comprising: up to 5 weight percent inorganic oxides selected from the group consisting of copper oxide, silicon dioxide, aluminum oxides and mixed oxides.
3. A method of using a low k thick film in the formation of a low temperature cofired ceramic structure for high frequency applications comprising the steps:
- providing two or more layers of a low k thick film dielectric tape, having dielectric constant in the range of 2 to 5 and comprising, based on solids: (a) 40-80 weight percent glass composition; (b) 20-60 weight percent ceramic oxide; dispersed in a solution of (c) organic polymeric binder;
- providing two or more layers of a high k thick film dielectric tape having a dielectric constant in the range of 5 to 8;
- collating the layers of low k and high k thick film dielectric tapes wherein said dielectric tapes are not separated by a buffer layer;
- laminating the layers of low k thick film and high k thick film to form an assembly; and
- processing the assembly to form a low temperature cofired ceramic structure.
4. The method of claim 3 wherein said glass composition consists essentially of, based on mole percent, 50-6% B2O3, 0/5-5.5% P2O5, SiO2 and mixtures thereof, 20-50% CaO, 2-15% Ln2O3 where Ln is selected from the group consisting of rare earth elements and mixtures thereof;
- 0-6% MI2O where MI is selected from the group consisting of alkali elements; and
- 0-10% Al2O3, with the proviso that the composition is water millable.
5. A thick film tape comprising the composition of claim 1.
6. A LTCC device comprising one or more tapes of claim 5, wherein the one or more tapes form a signal processing section.
7. The LTCC device of claim 6, wherein the tape provides X-Y constraining.
8. The LTCC device of claim 6, wherein the device further comprises a constraining tape.
9. A method of using a low k thick film tape in the formation of a low temperature cofired ceramic structure for high frequency applications comprising the steps:
- (a) providing two or more layers of a low k thick film dielectric tape, wherein the tape comprises the composition of claim 1 dispersed in a solution of organic polymeric binder;
- (b) applying a conductor track on the two or more layers, and applying vias connecting the two or more layers, forming a functional layer;
- (c) collating multiple functional layers;
- (d) laminating the collated functional layers; and
- (e) Processing the assembly to form a low temperature cofired ceramic structure.
10. The method of claim 9, wherein the LTCC device further comprises one or more high k thick films.
11. The method of claim 9 wherein, after the lamination of step (d), the two or more layers of a low k thick film dielectric tape form a single signal processing section.
12. An LTCC device made by the method of claim 9.
13. A beamformer, filter, antenna, or coupler comprising the LTCC device of claim 12
14. The beamformer of claim 13, wherein the beamformer is used in an application selected from the group consisting of: high frequency sensors, multi-mode radar modules, telecommunications components, telecommunications modules, and antennas.
15. An electrically functioning circuit comprising one or more functional layers, wherein a functional layer comprises:
- (a) two or more layers of a low k thick film dielectric tape of claim 5; and
- (b) a conductor track portion, wherein the conductor track portion is on the two or more layers of low k thick film dielectric tape, wherein vias connect the two or more layers.
16. The electrically functioning circuit of claim 15, wherein the circuit is a microwave module, package, or board.
17. The electrically functioning circuit of claim 15, wherein the circuit further comprises a surface metallization.
18. The electrically functioning circuit of claim 26, wherein the two or more tape layers form a signal processing section.
19. The electrically functioning circuit of claim 15, wherein the tape provides X-Y constraining.
20. The electrically functioning circuit of claim 15, wherein the circuit further comprises a constraining tape.
Type: Application
Filed: Jun 29, 2007
Publication Date: Jan 31, 2008
Applicant: E.l.du Pont de Nemours and Company (Wilmington, DE)
Inventors: Kenneth Hang (Hillsborough, NC), Kumaran Nair (Head of the Harbor, NY), Mark McCombs (Clayton, NC), Timothy Mobley (Cary, NC)
Application Number: 11/824,116
International Classification: H05K 1/03 (20060101); C03C 3/078 (20060101);
