MINIATURE MONOLITHIC ELECTRICAL CAPACITOR

Abstract: A miniature monolithic capacitor (10) and a composite sheet (11) from which such capacitors are cut, in which very thin layers of electrode material (15) are stacked between coatings (16) of dielectric with the electrode layers (15) being substantially thickened through their edge portions (19) so that the termination surfaces (21) of the capacitor includes dielectric (16) and the cross section of the thickened edges (19), thus facilitating edge termination of the capacitor using conventional techniques.

Description: This invention relates generally to facilitating completing the construction of electrical capacitors and more particularly concerns forming a monolithic capacitor to facilitate edge termination. Examples of such capacitors are described in the following US patents: Patent No 4 499 520, issued February 12, 1985, entitled "Capacitor With Dielectric Comprising Poly-Functional Acrylate Polymer and Method of Making"; Patent No 4 490 774, issued December 25, 1984, entitled "Capacitors Containing Polyfunctional Acrylate Polymers as Dielectrics"; Patent No 4 533 710, issued August 6, 1985, entitled "1,2-Alkanediol Diacrylate Monomers and Polymers Thereof Useful as Capacitor Dielectrics"; Patent No 4 513 349, issued April 23, 1985, entitled "Acrylate-Containing Mixed Ester Monomers and Polymers Thereof Useful as Capacitor Dielectrics"; and Patent No 4 515 931, issued May 7, 1985, entitled "Polyfunctional Acrylate Monomers for Polymers Thereof Useful as Capacitor Dielectrics". It has become possible to make miniature monolithic capacitors having alternate layers of electrode material and dielectric material adding up to thousands of layers but with each layer being very thin. For instance, it is possible to make capacitors in which the electrode layers defining the plates of the capacitor are in the range of 2 - 5 x 10⁻⁸m (200-500 angstroms) thick, and the electrode layers are separated by dielectric coatings of about 10⁻⁶m (one micron) thick. Such capacitors are described in Patent Application No EP 0 147 696A. Subsequent investigation has shown that dielectric coatings can be thinner, in the range of 10⁻⁷ - 10⁻⁶m (0.1 to 1 micron) thick. Edge termination of such a capacitor structure is difficult because there is virtually no edge to such extremely thin electrode layers and, if the layers are partially exposed, they are structurally weak. FR-A-2 117 567 discloses a capacitor of the rolled or wound type maufactured from layers detached from supporting foils. The thickness of such layers cannot be as low as that of monolithic capacitors and edge termination presents no particular problems. US 4 453 199 discloses a method of forming capacitors in which the edges are coated, eg by sputtering with a conductive layer, so that, again, edge termination presents no particular problems. Accordingly, it is an object of the invention to provide an improved miniature monolithic capacitor that permits conventional end termination procedures, such as schooping, to be followed despite having extremely thin electrode layers in the active capacitance region of the capacitor. A related object is to provide a capacitor of the foregoing type that can be made at the same high speeds and substantial volumes contemplated for prior designs not embodying the invention. According to the present invention there is provided a composite sheet from which individual minature monolithic capacitors can be formed comprising, in combination, a first plurality of strips of electrode material disposed parallel to one another and in one plane, a second plurality of strips of electrode material disposed parallel to one another and in a second plane closely spaced from and parallel to said one plane, said strips being offset in said planes so that portions of the strips intermediate the ends of the strips in one plane are adjacent the spaces between the strips in the other plane, dielectric coating surrounding and spacing said strips, and said strip portions intermediate the ends being three to five times thicker than the remainder of said strips so that cutting through said thicker portions to form capacitor blocks from the sheet exposes an edge of electrode material for edge termination that is accordingly thicker than the rest of the strips. According to the preent invention there is further provided a method of forming minature monolithic capacitors comprising the steps of forming first and second pluralities of strips of electrode material in adjacent parallel planes separated by a layer of dielectric coating, said strips being offset so that portions intermediate their ends of the strips in one plane are adjacent the spacing between the strips in the other plane, the thickness of each strip at said portion intermediate the ends being formed thicke than the remainder of each strip, and cutting through said thicker portions to form capacitor blocks having edges of electrode material three to five times thicker than the rest of the strips. Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which; Figure 1 is a fragmentary perspective, somewhat diagrammatic, view of a composite sheet embodying one form of the invention from which capacitors can be formed; Figure 2 is an enlarged fragmentary section of a portion of the sheet shown in Figure 1; and Figure 3 is a perspective view of a capacitor formed from the sheet of Figure 1 embodying the invention and prior to edge termination. Turning to the drawings, there is shown a miniature monolithic capacitor 10 formed from a composite sheet 11 which is cut along lines 12 to form capacitor block 13. The sheet 11 can be formed by depositing alternate layers 15 of electrode material such as aluminium, and coatings 16 of dielectric material such as curable resin, on a substrate 17 defined by a copper sheet. Each block 13 has a central capacitance region 18 in which the layers 15 are stacked in spaced parallel relationship. The layers 15 have edge portions 19 extending from the capacitance region 18 to an edge surface 21 of the block 13, with the edge portions 19 being staggered so that every other layer has an edge portion extending to one block edge surface 21 while the intermediate layers have edge portions extending to the opposite block edge surface. In composite sheet form, the layers 15 are defined by long strips 22 of electrode material. To form single electrode layer pairs, a plurality of such strips 22 are disposed parallel to one another and in one plane, and a second plurality is disposed parallel to one another and in a second plane closely spaced from and parallel to the adjacent plane. The strips 22 are offset in the two planes so that the mid-portions of the strips 22 in one plane are adjacent the spaces between the strips in the other plane. When the blocks 13 are cut along the line 12, a single electrode layer pair in each block is created by the strips in each plane. The total block 13 is formed of a plurality of additional similarly formed electrode layer pairs. Indeed, although only five such pairs are shown in the drawings, in practice hundreds of electrode layers can be deposited. In accordance with the invention, the portions of the strips 22 intermediate their ends are made substantially thicker than the remainder of the strips so that the resulting layer edge portions 19 have substantially increased thickness from the thin layers 15 in the capacitance regions 18 to much thicker layers whose edges are exposed in the block edge surfaces 21. More specifically, the electrode layer thickness is increased 3 to 5 times. Considering the dimensions stated above, increasing the thickness of a 2.5 x 10⁻⁸m (250 angstroms) electrode layer four times produces an exposed edge of the layer portions 19 that is 10⁻⁷ (0.1 microns) thick. With the dielectric coatings being 10⁻⁶m (1 micron) in thickness, the edge surfaces 21 become 10% metal making the edge surface configuration very similar to what is conventionally found in ceramic chip capacitors and conventionally terminated by joining the metal together with electrically conductive material. The process of schooping is typically used for such an end termination formation step. The capacitor 10 is shown ready for edge termination, having had the substrate 17 slotted at 24 to electrically separate the terminal ends 21 and the centre or capacitance region of the capacitor surrounded by a passivation layer 25. The thickened central portions of the layer strips 22 can be formed by forming the aluminium vapour shadow mask that defines the shape of the strips being deposited with a vapour opening extension of appropriate length, the mask being adapted to deposit more aluminium at the centre of each strip to form the thickened layer mid-portions of strips 22. It can now be seen that the miniature monolithic capacitor 10, despite having extremely thin electrode layers in the active capacitance region, can be provided with end termination through conventional techniques since sufficient metal is exposed at the termination surfaces 21. Nevertheless, the capacitor can be made at the same high speeds and substantial volumes contemplated from the manufacture of such capacitors which do not embody this end termination characteristic. It will be understood that the foregoing description is only of a certain preferred embodiment, and the invention is to be determined by giving the following claims their proper scope and interpretation.

Claims: A composite sheet from which individual miniature monolithic capacitors can be formed comprising, in combination, a first plurality of strips of electrode material (15) disposed parallel to one another and in one plane, a second plurality of strips (15) of electrode material disposed parallel to one another and in a second plane closely spaced from and parallel to said one plane, said strips being offset in said planes so that portions of the strips intermediate the ends of the strips in one plane are adjacent the spaces between the strips in the other plane, dielectric coating (16) surrounding and spacing said strips, and said strip portions (19) intermediate the ends being three to five times thicker than the remainder of said strips so that cutting through said thicker portions to form capacitor blocks from the sheet exposes an edge of electrode material for edge termination that is accordingly thicker than the rest of the strips. The combination of Claim 1 in which the strips of the two pluralities form single electrode layer pairs, and including a plurality of additional similarly formed electrode layer pairs to create multi-layer capacitors. A method of forming miniature monolithic capacitors comprising the steps of forming first and second pluralities of strips (15) of electrode material in adjacent parallel planes separated by a layer of dielectric coating (16), said strips being offset so that portions (19) intermediate their ends of the strips in one plane are adjacent the spacing between the strips in the other plane, the thickness of each strip at said portion (19) intermediate the ends being formed thicker than the remainder of each strip, and cutting through said thicker portions to form capacitor blocks having edges of electrode material three to five times thicker than the rest of the strips.

Inventors: CICHANOWSKI, STANLEY WILLIAM
Applicants: SPECTRUM CONTROL, INC. (A PENNSYLVANIA CORPORATION)
Application year: 1986
Main classification: H01G4
IPC codes: H01G4