Abstract:
A capacitor structure is fabricated by forming a pattern of first dielectrics over a foil, forming first electrodes over the first dielectrics, and co-firing the first dielectrics and the first electrodes. Co-firing of the dielectrics and the electrodes alleviates cracking caused by differences in thermal coefficient of expansion (TCE) between the electrodes and the dielectrics. Co-firing also ensures a strong bond between the dielectrics and the electrodes. In addition, co-firing allows multi-layer capacitor structures to be constructed, and allows the capacitor electrodes to be formed from copper.

Description:
BACKGROUND  
         [0001]    1. Technical Field  
           [0002]    The technical field is ceramic capacitors. More particularly, the technical field includes co-fired ceramic capacitors that may be embedded in printed wiring boards.  
           [0003]    2. Background Art  
           [0004]    Passive circuit components embedded in printed wiring boards formed by fired-on-foil technology are known. Known components are separately fired-on-foil. “Separately fired-on-foil” capacitors are formed by depositing a thick-film dielectric material layer onto a metallic foil substrate and firing under thick-film firing conditions, and subsequently depositing a top electrode material over the thick-film dielectric material layer. U.S. Pat. No. 6,317,023 B1 to Felten discloses such a process.  
           [0005]    The thick-film dielectric material should have a high dielectric constant (K) after firing. A high K thick-film dielectric is formed by mixing a high dielectric constant K powder (the “functional phase”) with a glass powder and dispersing the mixture into a thick-film screen-printing vehicle. High K glasses can be wholly or partially crystalline, depending on their composition and the amount of high K crystal they precipitate. These glasses are often termed “glass-ceramics.” 
           [0006]    During firing of the thick-film dielectric material, the glass component of the dielectric material softens and flows before the peak firing temperature is reached, coalesces, encapsulates the functional phase, and subsequently crystallizes, forming the glass-ceramic. The glass-ceramic, however, does not re-soften and flow on subsequent firings, and its surface is often difficult to adhere to.  
           [0007]    Silver and silver-palladium alloys are preferred metals for forming capacitor electrodes because of their relatively small differences in thermal coefficient of expansion (TCE) from the dielectrics used in fired-on-foil capacitors. Small TCE differences result in low stress in the electrode upon cooling from peak firing temperatures. However, silver and silver-containing alloys may be undesirable in some applications because of the possibility of silver migration. In addition, the relatively low melting points of silver and silver alloys preclude their use at higher firing temperatures.  
           [0008]    Copper is a preferred material for forming electrodes, but the large TCE differences between copper and thick-film capacitor dielectrics lead to post-firing stresses in the electrodes. The stresses result in electrode cracking. In addition, because pre-fired glass ceramics do not re-soften and flow on subsequent firings, a copper electrode fired on a pre-fired glass-ceramic surface may not adhere well to the glass-ceramic. The electrode may therefore separate from the dielectric. Both cracking and separation result in high dissipation factors.  
         SUMMARY  
         [0009]    According to a first embodiment, a method for making a fired-on-foil ceramic capacitor structure comprises forming first dielectrics over a metallic foil, forming first electrodes over the first dielectrics, and co-firing the first dielectrics and the first electrodes. In the first embodiment, cracking and separation of the electrode from the dielectric caused by differences in thermal coefficient of expansion (TCE) between the electrodes and the dielectrics is avoided by co-firing the electrodes and the dielectrics. Alleviation of the TCE problem also allows the use of preferred materials, such as copper, to form the electrodes.  
           [0010]    According to a second embodiment, a two-layer capacitor structure comprises a metallic foil, dielectrics disposed over the foil, first electrodes disposed over the first dielectrics, and second electrodes disposed over the dielectrics and over the first electrodes. In the second embodiment, the capacitance density of the capacitor structure is increased because of the additional dielectric/electrode layer. Additional layers may also be added, further increasing capacitance density. Also according to the second embodiment, the capacitor structure may comprise a copper foil and copper electrodes.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The detailed description will refer to the following drawings, wherein like numerals refer to like elements, and wherein:  
         [0012]    [0012]FIGS. 1A to  1 D schematically illustrate steps in manufacturing a first embodiment of a capacitor structure shown in front elevation;  
         [0013]    [0013]FIG. 1E is a top plan view of the first capacitor structure embodiment;  
         [0014]    [0014]FIGS. 2A to  2 J schematically illustrate steps in manufacturing a second embodiment of a capacitor structure shown in front elevation;  
         [0015]    [0015]FIG. 2K is a top plan view of the second capacitor structure embodiment; and  
         [0016]    [0016]FIG. 3 illustrates a third embodiment of a capacitor structure. 
     
    
     DETAILED DESCRIPTION  
       [0017]    [0017]FIGS. 1A-1D illustrate a general method of manufacturing a capacitor structure  100  (FIG. 1E) having a single-layer capacitor on metallic foil design. FIG. 1E is a plan view of the finished capacitor structure  100 . Specific examples of the capacitor structure  100  are also described below.  
         [0018]    [0018]FIG. 1A is a side elevational view of first stage of manufacturing the capacitor structure  100 . In FIG. 1A, a metallic foil  110  is provided. The foil  110  may be of a type generally available in the industry. For example, the foil  110  may be copper, copper-invar-copper, invar, nickel, nickel-coated copper, or other metals that have melting points in excess of the firing temperature for thick film pastes. Preferred foils include foils comprised predominantly of copper, such as reverse treated copper foils, double-treated copper foils, and other copper foils commonly used in the multilayer printed circuit board industry. The thickness of the foil  110  may be in the range of, for example, about 1-100 microns, preferably 3-75 microns, and most preferably 12-36 microns, corresponding to between about ⅓ oz and 1 oz copper foil.  
         [0019]    The foil  110  may be pretreated by applying an underprint  112  to the foil  110 . The underprint  112  is a relatively thin layer applied to a component-side surface of the foil  110 . In FIG. 1A, the underprint  112  is indicated as a surface coating on the foil  110 . The underprint  112  adheres well to the metal foil  110  and to layers deposited over the underprint  112 . The underprint  112  may be formed, for example, from a paste applied to the foil  110 , and is then fired at a temperature below the softening point of the foil  110 . The paste may be printed as an open coating over the entire surface of the foil  110 , or printed on selected areas of the foil  110 . It is generally more economical to print the underprint paste over selected areas of the foil. When a copper foil  110  is used in conjunction with a copper underprint  112 , glass in the copper underprint paste retards oxidative corrosion of the copper foil  110 , and it may therefore be preferable to coat the entire surface of the foil  110  if oxygen-doped firing is utilized.  
         [0020]    In FIG. 1A, a dielectric material is screen-printed onto the pretreated foil  110 , forming a first dielectric layer  120 . The dielectric material may be, for example, a thick-film dielectric ink. The dielectric ink may be formed of, for example, a paste. The first dielectric layer  120  is then dried. In FIG. 1B, a second dielectric layer  125  is then applied, and dried. In an alternative embodiment, a single layer of dielectric material may be deposited through a coarser mesh screen to provide an equivalent thickness in one printing.  
         [0021]    In FIG. 1C, an electrode  130  is formed over the second dielectric layer  125  and dried. The electrode  130  can be formed by, for example, screen-printing a thick-film metallic ink. In general, the surface area of the dielectric layer  125  should be larger than that of the electrode  130 .  
         [0022]    The first dielectric layer  120 , the second dielectric layer  125 , and the electrode  130  are then co-fired. The thick-film dielectric layers  120 ,  125  may be formed of, for example, a high dielectric constant functional phase such as barium titanate and a dielectric property-modifying additive such as zirconium dioxide, mixed with a glass-ceramic frit phase. During co-firing, the glass-ceramic frit phase softens, wets the functional and additive phases and coalesces to create a dispersion of the functional phase and the modifying additive in a glass-ceramic matrix. At the same time, the copper electrode powders of the layer  130  are wetted by the softened glass-ceramic frit phase and sinter together to form a solid electrode. The layer  130  has a strong bond to the high K dielectric  128  that results from the co-firing. The post-fired structure is shown in front elevation in FIG. 1D.  
         [0023]    [0023]FIG. 1E is a plan view of the finished capacitor structure  100 . In FIG. 1E, four dielectric/electrode stacks  140  on the foil  110  are illustrated. Any number of stacks  140 , in various patterns, however, can be arranged on a foil  110  to form the capacitor structure  100 .  
         [0024]    Examples 1-3 illustrate particular materials and processes used in practicing the general method illustrated by FIGS. 1A-1E.  
         [0025]    [0025]FIGS. 2A-2J illustrate a method of manufacturing a capacitor structure  200  having a double-layer capacitor on metallic foil design. FIG. 2K is a plan view of the finished capacitor structure  200 .  
         [0026]    [0026]FIG. 2A is a front elevational view of first stage of manufacturing the capacitor structure  200 . In FIG. 2A, a metallic foil  210  is provided. The foil  210  may be pretreated by applying and firing an underprint  212 , as discussed above with reference to FIG. 1A. A dielectric material is screen-printed onto the pretreated foil  210 , forming a first dielectric layer  220 . The first dielectric layer  220  is then dried.  
         [0027]    In FIG. 2B, a second dielectric layer  225  is then applied, and dried. A single layer of dielectric material may alternatively be used.  
         [0028]    In FIG. 2C, a first electrode  230  is formed over the second dielectric layer  225  and dried. The first electrode may be formed by, for example, screen-printing a thick-film metallic ink. The first electrode  230  is formed to extend to contact the foil  210 .  
         [0029]    The first dielectric layer  220 , the second dielectric layer  225 , and the first electrode  230  are then co-fired. The dielectric layers  220 ,  225  may have similar compositions to the materials discussed above with reference to FIGS. 1A-1E, and the co-firing process imparts the advantages of adhesion and defect-free processing discussed above. A resulting dielectric  228  is formed from the co-firing step, as shown in FIG. 2D.  
         [0030]    In FIG. 2E, a third layer of dielectric material is screen-printed onto the co-fired structure of FIG. 2D, forming a third dielectric layer  240 . The third dielectric layer  240  is then dried. In FIG. 2F, a fourth dielectric layer  245  is applied and dried. A single layer of dielectric material may alternatively be used.  
         [0031]    In FIG. 2G, a second electrode  250  is formed over the fourth dielectric layer  245  and dried. The second electrode  250  extends to contact the foil  210 . The structure is then co-fired. FIG. 2H illustrates the structure after co-firing, with the resulting dielectric  260  and dielectric/electrode stack  265 . After co-firing, the dielectric  260  securely adheres to both electrodes  230 ,  250 , and the electrodes  230 ,  250  are crack-free.  
         [0032]    As an alternative to two separate firing steps as discussed with reference to FIGS. 2D and 2H, a single co-firing can be performed after forming the second electrode  250 . A single co-firing is advantageous in that production costs are reduced. Two separate firings, however, allow inspection of the first electrode  230  for defects such as cracks and for printing alignment issues after the first firing.  
         [0033]    In FIG. 2I, the structure may be inverted and laminated. For example, the component face of the foil  210  can be laminated with laminate material  270 . The lamination can be performed, for example, using FR4 prepreg in standard printing wiring board processes. In one embodiment, 106 epoxy prepreg may be used. Suitable lamination conditions are 185° C. at 208 psi for 1 hour in a vacuum chamber evacuated to 28 inches of mercury. A silicone rubber press pad and a smooth PTFE filled glass release sheet may be in contact with the foil  210  to prevent the epoxy from gluing the lamination plates together. A foil  280  may be applied to the laminate material  270  to provide a surface for creating circuitry. The embodiments of the capacitor structure  100  discussed above with reference to FIG. 1E may also be laminated in this manner. The dielectric prepreg and laminate materials can be any type of dielectric material such as, for example, standard epoxy, high Tg epoxy, polyimide, polytetrafluoroethylene, cyanate ester resins, filled resin systems, BT epoxy, and other resins and laminates that provide insulation between circuit layers.  
         [0034]    Referring to FIG. 2J, after lamination, a photo-resist is applied to the foil  210  and the foil  210  is imaged, etched and stripped using standard printing wiring board processing conditions. The etching produces a trench  215  in the foil  210 , which breaks electrical contact between the first electrode  230  and the second electrode  250 . FIG. 2K is a top plan view of the completed capacitor structure  200 . A section  216  of the foil  210  is one electrode of the resulting capacitor structure  200 , and may be connected to other circuitry by a conductive trace  218 . A section  227  is coupled to the second electrode  230  and may be connected to other circuitry by a conductive trace  219 .  
         [0035]    The capacitor structure  200  discussed above has high capacitance density due to its two-layer capacitor structure. In addition, the capacitor structure  200  can be produced crack-free by co-firing of the dielectric layers and the electrodes.  
         [0036]    [0036]FIG. 3 illustrates a third embodiment of a capacitor structure. The capacitor structure  300  is a three-layer embodiment having a high capacitance density. The capacitor structure  300  comprises a foil  310  and a plurality of dielectric/electrode stacks  365  (only one stack  365  is illustrated). The dielectric/electrode stack  365  include a first electrode  330  and a second electrode  350  separated by a dielectric  360 , similar to the first and second electrodes  230 ,  250  of the capacitor structure  200  discussed above. Each dielectric/electrode stack  365  also has a third electrode  335  formed over the dielectric  360 . A trench  315  breaks electrical contact of a portion  316  of the foil  310  and the electrode  350 , from a portion  317  of the foil  310 , the first electrode  330 , and the third electrode  335 . A laminate material  370  and a second foil  380  may be included in the capacitor structure  300 .  
         [0037]    The capacitor structure  300  can be manufactured in a manner similar to the capacitor structure  200 . The third layer portion of the dielectrics  360  in the stacks  365  may be formed from one or more dielectric ink layers, as discussed above, and the electrodes  335  can be formed over the dielectrics  360 .  
         [0038]    The dielectric/electrode stacks  365  can be co-fired in three individual steps, or in a single step. Firing of each electrode/dielectric layer allows inspection of the product for defects. A single firing, however, reduces the cost of producing the capacitor structure  300 .  
         [0039]    The additional layer in the dielectric/electrode stacks  365  provides a high capacitance density for the capacitor structure  300 . Co-firing of the dielectric layers and the electrode provides a low dissipation factor and crack-free structure.  
         [0040]    In other embodiments, four or more layer capacitor structures can be produced by alternatively forming dielectric and electrode layers, and co-firing the layers.  
         [0041]    In the embodiments discussed in this specification, the term “paste” may correspond to a conventional term used in the electronic materials industry, and generally refers to a thick-film composition. Typically, the metal component of the underprint paste is matched to the metal in the metal foil. For example, if a copper foil were used, then a copper paste could be used as the underprint. Examples of other applications would be pairing silver and nickel foils with a similar metal underprint paste. Thick film pastes may be used to form both the underprint and the passive components.  
         [0042]    Generally, thick-film pastes comprise finely divided particles of ceramic, glass, metal or other solids dispersed in polymers dissolved in a mixture of plasticizer, dispersing agent and organic solvent. Preferred capacitor pastes for use on copper foil have an organic vehicle with good burnout in a nitrogen atmosphere. Such vehicles generally contain very small amounts of resin, such as high molecular weight ethyl cellulose, where only small amounts are necessary to generate a viscosity suitable for screen-printing. Additionally, an oxidizing component such as barium nitrate powder, blended into the dielectric powder mixture, helps the organic component burn out in the nitrogen atmosphere. Solids are mixed with an essentially inert liquid medium (the “vehicle”), then dispersed on a three-roll mill to form a paste-like composition suitable for screen-printing. Any essentially inert liquid may be used as the vehicle. For example, various organic liquids, with or without thickening and/or stabilizing agents and/or other common additives, may be used as the vehicle.  
         [0043]    High K thick-film dielectric pastes generally contain at least one high K functional phase powder and at least one glass powder dispersed in a vehicle system composed of at least one resin and a solvent. The vehicle system is designed to be screen-printed to provide a dense and spatially well-defined film. The high K functional phase powders can comprise perovskite-type ferroelectric compositions with the general formula ABO 3 . Examples of such compositions include BaTiO 3 ; SrTiO 3 ; PbTiO 3 ; CaTiO 3 ; PbZrO 3 ; BaZrO 3  and SrZrO 3 . Other compositions are also possible by substitution of alternative elements into the A and/or B position, such as Pb(Mg 1/3  Nb 2/3 )O 3  and Pb(Zn 1/3 Nb 2/3 )O 3 . TiO 2  and SrBi 2 Ta 2 O 9  are other possible high K materials.  
         [0044]    Doped and mixed metal versions of the above compositions are also suitable. Doping and mixing is done primarily to achieve the necessary end-use property specifications such as, for example, the necessary temperature coefficient of capacitance (TCC) in order for the material to meet industry definitions, such as “X7R” or “Z5U” standards.  
         [0045]    The glasses in the pastes can be, for example, Ca—Al borosilicates, Pb—Ba borosilicates, Mg—Al silicates, rare earth borates, and other similar glass compositions. High K glass-ceramic powders, such as lead germanate (Pb 5 Ge 3 O 11 ), are preferred.  
         [0046]    Pastes used to form the electrode layers may be based on metallic powders of either copper, nickel, silver, silver-containing precious metal compositions, or mixtures of these compounds. Copper powder compositions are preferred.  
         [0047]    The capacitor structure embodiments described in this specification have many applications. For example, the capacitor structure embodiments can be used within organic printed circuit boards, IC packages, applications of said structures in decoupling applications, and devices such as IC modules or handheld device motherboards.  
         [0048]    In the above embodiments, the electrode layers are described as formed by screen-printing. Other methods, however, such as deposition by sputtering or evaporation of electrode metals onto the dielectric layer surface may also be used.  
         [0049]    The foregoing description of the invention illustrates and describes the present invention. Additionally, the disclosure shows and describes only the preferred embodiments of the invention, but it is to be understood that the invention is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings, and/or the skill or knowledge of the relevant art. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments.  
       EXAMPLES  
     Example 1  
       [0050]    Referring to FIGS. 1A-1E, a specific embodiment of the capacitor structure  100  was described. In this embodiment, the foil  110  was a copper foil. The type of copper foil  110  can be any commercial grade of copper foil used in the printed wiring board industry, and may be in the range of ⅓ oz copper foil (approximately 12 microns thickness) to 1 oz copper foil (approximately 36 microns thickness). The copper foil  110  was pretreated by applying a copper underprint paste over selected areas of the foil  110 . The resulting product was then fired in nitrogen at 900° C. for 10 minutes at peak temperature, with a total cycle time of approximately 1 hour, forming the underprint  112 .  
         [0051]    In FIG. 1B, a thick-film dielectric ink was screen-printed onto the pretreated copper foil  110  through  400  mesh screen to create a pattern of ½ inch by ½ inch first dielectric layers  120 . The wet printed thickness of the first dielectric layers  120  is approximately 12-15 microns. The first dielectric layers  120  were dried at 125° C. for approximately 10 minutes, and second dielectric layers  125  were applied by screen-printing, followed by another drying step at 125° C. The thick-film dielectric ink included a barium titanate component, a zirconium oxide component, and a glass-ceramic phase.  
         [0052]    Referring to FIG. 1C, thick-film copper electrode ink layers  130  was printed through 400 mesh screens onto the dielectric squares  120 , and dried at 125° C. for approximately 10 minutes to form a 0.9 cm by 0.9 cm square electrode. In general, the printed electrode  130  thickness was limited only by the need for a pinhole-free film, and was typically in the range of 3 to 15 microns. The resulting structure was co-fired to 900° C. for 10 minutes at peak temperature using a thick film nitrogen profile. The nitrogen profile included less than 50 ppm oxygen in the burnout zone, and 2-10 ppm oxygen in the firing zone, with a total cycle time of 1 hour. Co-firing resulted in the dielectric/electrode stacks  140  illustrated in FIG. 1E.  
         [0053]    In this example, the thick film dielectric material had the following composition:  
                                                       Barium titanate powder   64.18%           Zirconium oxide powder    3.78%           Glass A   11.63%           Ethyl cellulose    0.86%           Texanol   18.21%           Barium nitrate powder    0.84%           Phosphate wetting agent     0.5%.                      
 
         [0054]    Glass A comprised:  
                                                       Germanium oxide   21.5%           Lead tetraoxide    78.5%.                      
 
         [0055]    The Glass A composition corresponded to Pb 5 Ge 3 O 11 , which precipitated out during the firing, and had a dielectric constant of approximately 70-150. The thick film copper electrode ink comprised:  
                                                       Copper powder   55.1%           Glass A    1.6%           Cuprous oxide powder    5.6%           Ethyl cellulose T-200    1.7%           Texanol    36.0%.                      
 
         [0056]    After firing, the capacitor structure was crack free and had the following electrical characteristics:  
                                                       capacitance density   approximately 150 nF/in 2             dissipation factor   approximately 1.5%           insulation resistance   &gt;5 × 10 9  Ohms           breakdown voltage   approximately 800 volts/mil.                      
 
         [0057]    In this example, the use of copper as the material to form the foil  110  and the electrodes  130  was advantageous because copper was not subject to a large degree of migration. In conventional, separately fired-on-foil methods, the large TCE difference between copper and dielectric materials leads to cracking and separation of the electrode from the dielectric, and high dissipation factors. However, by co-firing the electrodes and dielectrics, cracking did not occur and low dissipation factors were achieved.  
       Example 2  
       [0058]    A process as described in Example 1 was repeated, except that the thick-film dielectric  128  was printed through 325 mesh screen, with a wet thickness of each of the two layers of approximately 15-20 microns. Results were similar to the embodiment of Example 1, except that the capacitance density was approximately 120 nF/inch 2 .  
       Example 3  
       [0059]    A process as described in Example 2 was repeated using a variety of dielectric and electrode dimensions shown in the table below:  
                                           Dielectric Size mils   Electrode Size mils   Dielectric Size mils   Electrode Size mils                     250 × 250     210 × 210    36 × 338     20 × 320         56 × 340     40 × 320    96 × 340     80 × 320         176 × 340     160 × 320    36 × 178     20 × 157         96 × 180     80 × 158   336 × 180     320 × 158         26 × 180     10 × 159    56 × 180     40 × 158         176 × 180     160 × 158    26 × 100     10 × 74         36 × 98     20 × 77    56 × 100     40 × 78         56 × 100     240 × 78    96 × 100     80 × 78         176 × 100    3160 × 78    26 × 60     10 × 39         36 × 58     220 × 37    56 × 60     40 × 38         96 × 60     280 × 38    26 × 40     10 × 18         36 × 38     216 × 17    56 × 40     40 × 18         26 × 30     210 × 9    36 × 28     20 × 7         26 × 340     210 × 318   336 × 340     320 × 318         90 × 90     270 × 70   170 × 170     150 × 150         330 × 330     310 × 310   240 × 240   229.5 × 229.5       119.5 × 119.5   109.5 × 109.5                  
 
         [0060]    Capacitance in these embodiments was proportional to the area of the printed copper electrode, but the calculated capacitance densities were essentially identical to that of Example 1.