Capacitor and method for making same with high yield
BACKGROUND OF THE INVENTION 
     The present invention relates to a capacitor and a method of making it, and more particularly, to a thin film MIM (metal-insulator-metal) capacitor and a method for making it that results in a high yield. 
     When making thin film capacitors, a first electrode is formed on a substrate, an insulator is formed overlying the first electrode, and a second electrode is formed overlying the insulator. However, during the formation of the insulator, void-type defects, such as cracks, thin areas, and pin holes, occur, which reduce the electrical dielectric strength, thereby reducing the yield of the process. 
     It is, therefore, desirable to have a capacitor and a method for making it that results in a high yield. 
     SUMMARY OF THE INVENTION 
     A capacitor comprises a first electrode, an insulator overlying said first electrode, said insulator including a primary dielectric having at least one void and a fill dielectric disposed in said void, and a second electrode overlying said insulator. 
     A method for making a capacitor comprises forming a first electrode, forming an insulator overlying said first electrode by forming a primary dielectric having at least one void and forming a fill dielectric in said void, and forming a second electrode overlying said insulator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an embodiment of a capacitor in accordance with the invention; and 
     FIGS. 2-4 are cross-sectional views illustrating successive steps of an embodiment of a method in accordance with the invention for making the capacitor of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 there is shown an insulating substrate 10. Overlying the substrate 10 is a first electrode 12, while overlying the first electrode is an insulator 14 comprising a primary dielectric 16 having voids, such as a crack 18, a thin area 20 and a pin hole 22, that communicate with an upper surface 17 of the primary dielectric 16. Within each void 18, 20, and 22 is a fill dielectric 24. Overlying the insulator 14 is a second electrode 26. Preferably, the second electrode 26 is recessed from the insulator 14, which is recessed from the first electrode 12, which in turn is recessed from the substrate 10, all as shown in FIG. 1, to avoid undercutting during manufacture (described below). 
     The substrate 10 can comprise an insulator, such as Al 2  O 3 , BeO, AlN, or BaTi0 3 . The first and second electrodes 12 and 26 can comprise a heavily doped semiconductor, such as Si, GaAs, or InP, or a metal or combination of metals, such as Al, Cr-Cu, or Ti-Pt-Au having a typical thickness between about 1 to 3 micrometers (μm). The primary dielectric 16 can comprise Si0 2 , Si 3  N 4 , Al 2  O 3 , BaTi0 3 , MgO, or Ta 2  O 5 . In general, the lower thickness limit for the primary dielectric is determined by the applied voltage stress, i.e., it is necessary to avoid voltage breakdown, while the upper thickness limit is determined by mechanical stress. For Si 3  N 4  typical satisfactory thicknesses are between about 0.25 to 1μm, while for Ta 2  O 5  satisfactory thickness are between about 0.15 to 0.215μm. The fill dielectric can comprise an curable liquid material, such as a polyimide, e.g., the polyimides sold under the trademarks &#34;Pyralin 25 45&#34;  or &#34;Pyralin 25 55&#34; by E. I. Dupont Company, Wilmington, Del. 
     FIG. 2 shows the first steps in making the capacitor of FIG. 1. For a substrate 10 of Al 2  O 3 , the first electrode 12 is formed by vapor deposition such as by evaporating Cr in an O 2  atmosphere having a pressure 2×10 -5  Torr to form a Cr 2  O 3  layer (not shown) having a thickness of about 15 nanometers (nm) to obtain good adherence to the substrate 10. Then Cr is evaporated in a vacuum to form a first Cr layer (not shown) of about 25 nm thickness in order to provide a good conductance and transition to a Cu layer (not shown). Thereafter, the Cu layer is formed with a thickness of about 2 μm by evaporating Cu in a vacuum, and then a second Cr layer (not shown) is formed with a thickness of about 12 nm to provide good adhesion to the primary dielectric 16. Other forms of vapor deposition such a sputtering can also be used to form all of the layers the first electrode 12. 
     To form the primary dielectric 16 by sputtering, a target of Si 3  N 4  can be bombarded by ions of N 2  and Ar. Alternately, the primary dielectric 16 can be formed by plasma enhanced chemical vapor deposition by reacting SiH 4  and NH 3  at about 250° C. Whatever method is used, the voids 15, 20, and 22 will occur during the formation of the primary dielectric 16 due to imperfections thereof. Then a first photoresist layer (not shown) is deposited and defined in order to be able to define the primary dielectric 16 (described below). Thereafter, the primary dielectric 16 of Si 3  N 4  is defined using a wet etchant, such as HF, or dry etching using a plasma of CF 4 . The first photoresist layer is then removed using a photoresist etchant, such as KOH, NaOH, an amine, or a plasma of O 2 . 
     Thereafter, the second Cr layer is removed using a Cr etchant, such as a mixture of KOH and K 3  FeO(CN) 6 , and a second photoresist layer deposited and defined. Then the Cu layer is etched using a Cu etchant such as FeCl 3  and the second photoresist layer removed. The second photoresist layer is removed and then the first Cr layer and the Cr 2  O 3  etched. 
     As shown in FIG. 3, a liquid polyimide layer 24a is deposited by spin coating, with a thickness between about 1 to 2 μm thereby filling in the voids 18, 20, and 22. The layer 24a is then partially cured between about 125° to 130° C. for about 1/2 hour if photoresist strippers are used in a step described below. The layer 24a is then etched using a photoresist developer, typically for between about 30 to 60 seconds. During the etching the polyimide remains in the voids 18, 20, and 22 since it is thicker, etchant accessibility is limited, and due to the partial curing (if used). Thus the fill dielectric 24 as shown in FIG. 4 is formed, while that portion of the layer 24a outside of the voids 18, 20, and 22 is removed. A photoresist stripper is then applied if definition of the polyimide at locations not shown is desired. The fill dielectric 24 is then cured between about 300° to 400° C. for about 1/2 to 11/2hours in an inert atsmophere such as N 2 . 
     The second electrode 26 is then formed. It can be similar to the first electrode, i.e., a first layer of Cr 2  O 3  (not shown), a layer of Cr, and then a layer of Cu or Ti-Cu. Thickness can be similar except that the layer of Cu or Ti-Cu has a typical thickness of up to about 3μm. These layers can be formed by vapor deposition. A third photoresist layer is then deposited and defined. Thereafter, the Cu, Cr, and Cr 2  O 3  layers are successively etched, and the third photoresist layer removed. 
     EXAMPLE 
     Capacitors made as described above were subject to a test using an electrical field of 1 Megavolt/cm. They were considered to have passed the test if no breakdown occurred and if their capacitance increased by less than 10% compared with their initial capacitance at zero volts. A yield of 91% was obtained. 
     COUNTEREXAMPLE 
     In contradistinction, identical capacitors except that no fill dielectric 24 was used had a yield of 73%.