Thick film capacitor
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This patent application is a Continuation-In-Part of our copending patent application, Ser. NO. 383,280, filed on July 27, 1973 and assigned to the same assignee as this patent application and now U.S. Pat. No. 3,878,443. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to thick film capacitors and, in particular, to a high dielectric constant material for making such capacitors. 
     2. Background of the Invention 
     In the semiconductor art, the use of discrete (separate) devices mounted on a printed circuit board is increasingly giving way to the fabrication of complete circuits in a single package containing a ceramic substrate. Devices, such as resistors and capacitors, as well as their electric interconnections, are formed on the substrate by screen printing. 
     Screen printing, as is known in the art, entails forcing a liquid mixture through a patterned screen to print the components on the substrate. The substrate is then dried and fired to harden the materials in place. The screen may comprise silk, stainless steel, or other appropriate material depending upon the device being printed. The liquid mixture, herein referred to as an &#34;ink&#34;, comprises finely divided particles, a binder, and a liquid vehicle. 
     Screen printing, however, poses special problems for forming capacitors. The dielectric material for the capacitors must be in the form of finely ground particles suspended in a liquid vehicle. The electric properties of the dielectric material are not the same for the bulk and the finely ground, printed and fired conditions. Thus, there is a problem producing high dielectric constants in screen printed capacitors due to the fact that the deposited material must be finely ground particles which cannot be brought back to their original, high density. 
     Other difficulties arise from the binders needed to hold the articles together during firing. The material used as the binder can contribute to poor dielectric constant and high dissipation factor by reacting with the dielectric during firing. Further, the firing temperatures of the less costly electrode materials are generally lower than ideal for binders which would not detract from dielectric properties. Noble metal electrodes, which can withstand higher firing temperatures, are generally too expensive for most applications. Additionally, the firing of the dielectric material in contact with the less costly electrical conductors may produce a high temperature chemical reaction between the electrode and the dielectric because of their incorporated binders. 
     Yet another difficulty that must be overcome is the aging characteristics of the dielectric, i.e., change in dielectric constant on the shelf and also at elevated temperatures or at high applied voltages. For example, presently used high dielectric constant materials change in dielectric constant by approximately 22% in the temperature range of from 100°C to 150°C, and by approximately 33% over an applied voltage range of from 0 to 167 volts per mil thickness. 
     It has been difficult up until this time to overcome these difficulties simultaneously with presently available dielectric inks, particularly in finding a suitable binder material for incorporation therein. 
     In view of the foregoing, it is therefore an object of the present invention to provide an improved dielectric ink for thick film capacitors. 
     Another object of the present invention is to provide an improved binder for dielectric inks that does not deteriorate the dielectric constant of the resulting material. 
     A further object of the present invention is to provide an improved dielectric that is more resistant to aging effects. 
    
    
     DESCRIPTION OF THE DRAWING 
     The drawing is a side elevation view in cross-section of a capacitor made in accordance with the teachings of this invention 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to the Figure, there is shown a thick film capacitor 10 embodying the high dielectric constant dielectric material of this invention. The capacitor 10 comprises a substrate 12 having opposed major surfaces 14 and 16 being the respective top and bottom surfaces thereof. The substrate 12 is made of a suitable material such, for example, as alumina, beryllia, mullite and forsterite. 
     A layer 18 of a suitable electrically conducting metal such, for example, as a palladium-silver alloy, platinum, a platinum-palladium alloy and a platinum-gold alloy is deposited on the surface 14 of the substrate 12. The layer 18 functions as the first electrode of the capacitor. Preferably, the layer 18 is deposited on the surface 14 by screen printing and firing at the recommended temperature for the selected electrode material. 
     A layer 20 inch a dielectric material 18, made in accordance with the teachings of this invention, is disposed on the layer 18 by printing, preferably through a 140 mesh U.S. series stainless steel screen. A thickness of from 0.005 inch to 0.0015 inches is preferred for the layer 18 prior to firing in place to assure one of a pin-hole free layer. The layer 20 is substantially a uniform thickness throughout the layer. 
     The dielectric material is fired in place at a temperature of from 850°C ± 25°C to 1075°C ± 25°C. 
     A layer 22 of an electrically conductive metal is then disposed on the layer 20 of dielectric material in the same manner as the layer 18. Preferably, the metal of the layer 22 is the same as the metal of the layer 18. Alternately the metal of the layer 22 may be of another metal as long as it is able to withstand the temperatures of the manufacturing processes and it does not detrimentally affect the material of the dielectric layer 20. The metal layer 22 functions as the second electrode of the capacitor 10. 
     The dielectric material of the layer 20 comprises an alkali-free glass binder including at least one electro-positive element common to the dielectric material. The dielectric material as a result of the particular glass binder has a high dielectric constant and a better resistance to aging effects. The exact high temperature mechanism and what produces the aging characteristics is not fully understood by the Applicants. However, it is believed that the presence of the common electro-positive element in the glass binder prevents, or acts as a substitute for, reactions with that common element in the dielectric material. 
     The improvement in the dielectric constant of the dielectric material is noted when a second firing of the dielectric material occurs. The second firing occurs because the layer 22 of metal comprising the second electrode is fired in place to form a good electrical conductive relationship between the electrode 27 and the dielectric material 20. The variation in dielectric constant K with the firing of the second electrode, that is to say, the improvement in the dielectric constant with a second firing has been tabulated and is shown in Table I. 
     
                       Table I                                                     
______________________________________                                    
BaTiO.sub.3 (CP)                                                          
        Second electrode firing                                           
Ba Glass  Temp. (°C)                                               
                      Time (Min)  K                                       
______________________________________                                    
3%         850        10          580                                     
5%         850        10          570                                     
7%         850        10          530                                     
3%        1050        12          790                                     
5%        1050        12          930                                     
7%        1050        12          760                                     
______________________________________                                    
 
    
     The dielectric material of Table I was prepared from chemically pure (CP) barium titanate and different percentages by weight of glass binder relative to the dielectric material. 
     Preparation of the capacitor ink is as follows: 
     chemically pure barium titanate is milled in acetone to break down agglomerated particles. After drying, the particles are mixed with a solution of ethyl cellulose in pine oil, by means of a mortar and pestle, in the following proportions: 
     
         Barium Titanate                                                           
            (CP)     24 gms.                                              
                               (60 cc pine oil                            
Ethyl Cellulose      10 gms.   ( 8 gms. ethyl                             
Solution                       cellulose                                  
 
    
     The barium zinc glass used as the binder is prepared by weighing out the proportions of oxides and wet mixing them in acetone. After drying, the mixture is smelted in a platinum crucible heated to an elevated temperature of approximately 1390°C ± 25°C using a platinum stirrer to insure uniformity. The melt is then poured on a steel chill plate after which is it pulverized and ground to 1 to 2 micron particle size and mixed with the barium titanate and ethyl cellulose solution. 
     The capacitor ink thus formed, when printed through a 165 mesh stainless steel screen, produces a pin-hole free dielectric of approximately uniform thickness. The dielectric exhibits high dielectric constant, as shown in Table I, and good aging characteristics. For example, the dielectric constant varies by approximately 19% over a temperature range of 100°C to 150°C and varies by approximately 22% over an applied voltage range of 0-167 volts per mil. 
     The glass binder utilized in Table I is fired at 1050°C ± 25°C for 12 minutes for all samples and comprises a barium zinc glass having the following composition by oxide percent: 
     
         Material   Range-% by oxide                                               
                          Preferred-% by oxide                            
______________________________________                                    
BaO        38 - 46        42                                              
ZnO         7 - 11        9                                               
SiO.sub.2  35 - 45        40                                              
B.sub.2 O.sub.3                                                           
           6 - 8          6                                               
Al.sub.2 O.sub.3                                                          
           2 - 5          3                                               
______________________________________                                    
 
    
     For Tables II and III, the dielectric material comprises modified barium titanate, that is, a mixture of barium titanate and barium stannate. The modified barium titanate ranges from 82 percent by weight to 86 percent by weight and the remainder is barium stannate. Preferably, the dielectric material comprises 84% barium titanate and 16% barium stannate by weight. 
     As is known, chemically pure barium titanate exhibits a variation in dielectric constant with temperature. Specifically, in a curve of the variation of dielectric constant with temperature, a pronounced peak occurs at approximately 125°C. The dielectric constant varies from about 1500 at 25°C to from 6,000 to 10,000 at 125°C. This wide variation in dielectric constant must be allowed for in circuitry utilizing the chemically pure barium titanate as a dielectric. Modified barium titanate on the other hand, has a dielectric constant that is relatively uniform, approximately 2000, over a wide temperature range from about 50°C to about 150°C. 
     In Table II examples of the present invention utilizing the preferred composition of modified barium titanate as the dielectric material with 3 and 7 percent by weight of the total weight of the composition barium zinc glass binder. Table II further shows that the dielectric constant is electrode dependent, i.e., the choice of electrode material may increase or decrease the dielectric constant, even with the same dielectric material. Further, even with the preferred palladium-silver electrodes, some variation in dielectric constant is obtained depending upon the electrode paste or ink utilized. 
     
                       Table II                                                    
______________________________________                                    
Modified BaTiO.sub.3                                                      
(84% BaTiO.sub.4 + 16% BaSnO.sub.3)                                       
Ba Glass           Metal Type  K                                          
By Weight Total Composition                                               
______________________________________                                    
3%                 8151         1002                                      
7%                 8151        873                                        
3%                 8228        723                                        
7%                 8228        557                                        
______________________________________                                    
 
    
     It is evident from the results obtained as tabulated in Table II that although both materials were palladium-silver compositions, that something else was present in the composition material. Each metal material employed a different glass binder to enhance adherence to the dielectric material layer. It is believed that a difference in the glass binder material caused the difference in the K values. 
     In Table III, which follows, the dielectric material comprises modified barium titanate, 84% BaTiO 3  and 16% BaSnO 3 , but the glass binder comprises a glass containing both barium and titanate. The glass contains the one electro-positive element in common with a titanium dioxide additive in the dielectric material. A similar variation in dielectric constant with the electrode material employed is to be noted. 
     
                       Table III                                                   
______________________________________                                    
BaTiO.sub.3 Bearing Glass                                                 
                   Metal Type  K                                          
By Weight of Total Composition                                            
______________________________________                                    
3%                 8151        845                                        
7%                 8151        696                                        
3%                 8228        746                                        
7%                 8228        696                                        
______________________________________                                    
The Ba-TiO.sub.3 bearing glass composition by oxide                       
percent is as follows:                                                    
Range              Preferred                                              
Material % by oxide                                                       
                   % by oxide                                             
______________________________________                                    
BaO       38 to 46     42.05                                              
ZnO        7 to 11     9.00                                               
SiO.sub.2 35 to 45     35.59                                              
B.sub.2 O.sub.3                                                           
           6 to 8      5.96                                               
Al.sub.2 O.sub.3                                                          
           2 to 5      2.67                                               
TiO.sub.2  3 to 10     4.73                                               
______________________________________                                    
 
    
     In addition to modifying chemically pure barium titanate with barium stannate small amounts of metal oxide additives may also be employed. Such suitable metal oxide additives are lanthanum oxide, cerium compounds, stannates, zirconates, neodymium oxide, bismuth compounds and the like. These metal oxide additives comprise from 0.125 to 5% by oxide weight of the modified dielectric material. 
     There is thus provided by the present invention an improved binder for thick film printed capacitor dielectrics that does not contribute to poor dielectric constant or react as readily with the dielectric material during firing. Further, the glass binder provides a resulting dielectric having improved aging characteristics, enabling a capacitor formed therewith to withstand higher voltages than obtainable in the prior art. 
     Having thus described the invention, it will be apparent to those skilled in the art that various modifications may be made within the spirit and scope of the present invention.