Fabrication of Film Circuits Having a Thick Film Crossunder and a Thin Film Capacitor

This invention relates to film circuits and in particular to a method of fabricating such circuits which permits thick film crossunders and thin film capacitors to be formed on the same substrate.

Thin film circuits offer significant advantages over most thick film and silicon integrated circuits where precision components are needed. In fabricating thin film circuits, just as in the integrated circuit field, a great deal of effort and attention has been directed to packing more components onto a substrate. This trend has necessitated use of some type of structure for bridging conductors when high density circuits are desired. Although several alternatives are available for this function, the most reliable appears to be the thick film crossunder. As known in the art of thin film circuit fabrication, such structures are typically formed by depositing a thick film electrode such as gold coupled to the thin film conductors to be bridged. One or more layers of a thick film dielectric such as glass layers are then deposited and fired and a thin film conductor formed thereover in a direction essentially orthogonal to the conductor being bridged.

While thick film crossunders have been used for thin film resistor circuits, it does not appear that they have been successfully incorporated into circuits including thin film capacitors. Among other reasons, this was apparently due to the incompatibility of firing cycles for the crossunder dielectrics and thick film electrodes with the standard capacitor components, especially the glass layer used for compensation of the roughness of the circuit substrate underlying the capacitor and the oxide underlay used to protect the glass layer.

It is therefore an object of the invention to provide a technique for the fabrication of film circuits which permits fabrication of thick film crossunders with thin film capacitors on the same substrate. It is a further object of the invention to provide a technique for incorporating thin and thick film components on a single substrate with a minimum number of processing steps.

These and other objects are achieved in accordance with the invention which is a method of fabricating a film circuit on an insulating substrate. A thick film electrode is formed on one portion of the substrate. A plurality of dielectric layers are deposited and fired on a portion of the electrode, and at least one layer capable of smoothing irregularities on the substrate is deposited and fired on a portion of the substrate remote from the electrode where a capacitor is to be formed. The firing of all layers is compatible with previously deposited and fired layers. A metal layer is then deposited over essentially the entire substrate and a portion of the metal is removed from the area of the thick film electrode to be contacted. The metal layer is oxidized to form a protective oxide layer. This is followed by fabricating over the oxide at least one thin film capacitor and a thin film conductor electrically coupled to the thick film electrode.

These and other features of the invention will be delineated in detail in the following description. In the drawing:

FIGS. 1-6 are cross-sectional views of a portion of a film circuit during various stages of fabrication in accordance with one embodiment of the invention.

It will be appreciated that for purpose of illustration, these figures are not necessarily drawn to scale.

One embodiment of the invention will be described with reference to the sequence of steps illustrated in FIGS. 1-6. It will be appreciated that these figures show only one portion of a film circuit which would usually include many more elements, including one or more integrated circuit chips bonded to the substrate to form a hybrid integrated circuit.

As illustrated in FIG. 1, the circuit was formed on one surface of an insulating substrate, 10, which in this case is alumina. Any type of substrate compatible with film circuit fabrication may be utilized with the present invention. Deposited on a selected portion of the surface of the substrate by standard screen printing techniques was a thick film electrode, 11. In this particular example, the electrode was deposited as a paste comprising fritted gold and then fired at a temperature of 975 degrees Celsius for 10 minutes to achieve the desired conductivity. Typically, the electrode will be fired at a temperature in the range 900-1000 degrees Celsius for 10-20 minutes to achieve a conductivity on the range 0.002-0.010 .OMEGA./.quadrature.. Other standard thick film compositions could be used. The thickness of the electrode was approximately 1 mil and the thickness will usually lie within the range 0.5-1.5 mils.

As illustrated in FIG. 2, a first dielectric layer, 12, was then deposited by standard screen printing on a portion of the electrode. In this example the layer was deposited from a paste comprising glass powders dispersed in an organic vehicle where the powders comprise the oxides of silicon, lead, calcium, aluminum, boron, potassium, magnesium and sodium, and the organic vehicle comprises cellulose based polymers dissolved in a solvent system based on pine oil derivatives and a mixed ester. Such a paste is sold, for example, by DuPont under the designation 9615. The glass powders comprised approximately 70 percent by weight of the paste. After deposition, the paste was fired at a temperature of approximately 875 degrees Celsius for approximately 10 minutes. In general, a temperature in the range 800-950 degrees Celsius for a time within the range 10-20 minutes would be used to produce the appropriate adhesion and mechanical strength for the crossunder. The thickness of the layer 12 was approximately 0.5 mil, though thicknesses in the range 0.4-1.0 mils will generally be preferred.

It will be appreciated that although a particular preferred composition of layer 12 is disclosed, the invention could be used with other dielectric layers which are capable of electrically insulating the crossunder and crossover conductors, can withstand subsequent thin film processing, and have a thermal expansion coefficient which is compatible with the other deposited layers.

As noted in FIG. 3, a second dielectric layer, 13, was deposited, again by standard screen-printing, over the first layer 12 and also in selected portions of the substrate remote from the thick film electrode (only one of which is shown). The latter portions are the areas chosen for fabrication of thin film capacitors described later. Thus, in this embodiment, the second layer serves the dual function of serving as the dielectric protection for the thick film electrode and also compensating for irregularities in the substrate to permit capacitor fabrication. Typically, in prior art processes, such functions were performed by different compositions.

In this particular example, the layer 13 was deposited from a paste which, again, comprised glass powders dispersed in an organic vehicle. For this layer, however, the glass powders comprised oxides of silicon, boron, calcium, barium, aluminum, lead, bismuth, titanium and potassium. The organic vehicle comprised cellulose based polymers dissolved in alpha terpineol. The glass powders comprised approximately 30-35 percent by weight of the paste.

The firing of the second layer, 13, must be chosen so as to be compatible with the previously deposited layer, 12. That is, the firing must be sufficient to achieve the appropriate adhesion and mechanical strength for the second layer without melting the first layer. In this particular example, the firing of the first layer actually increases its melting point from 850 to 950 degrees Celsius. Thus, the second layer was fired at a temperature of approximately 950 degrees Celsius for a time of 10 minutes. Firing in the range 850-975 degrees Celsius for 10-20 minutes will generally be useful. The thickness of the layer, 13, was approximately 0.5 mils, but will generally lie within the range 0.3-1.0 mil. Again, other compositions may be suitable.

The previous step has the advantage of combining deposition of the top crossunder glass layer and the underlying capacitor glass layer. Alternatively, the two layers could comprise different compositions and be deposited and fired separately. In such a case the firing of each layer must be compatible with all the previously deposited layers. In a typical sequence, after deposition and firing of layer 12, the top dielectric for the crossunder would be deposited and fired, followed by deposition and firing of another glass layer in the area of capacitor formation. The top dielectric would typically be the same as the bottom dielectric (12), and the capacitor underlayer (13) would be the composition previously described for both the top dielectric and capacitor underlayer. The bottom layer would typically be fired at 800-875 degrees Celsius for 10-20 minutes, and the top layer at a temperature of 800-850 degrees Celsius for 10-20 minutes. The capacitor underlayer would typically be fired at 850-950 degrees Celsius for 10-20 minutes.

It will be appreciated that the crossunder dielectric layers are deposited so as to leave two edge portions, 21, of the bottom electrode uncovered for later electrical contact. In the direction perpendicular to the plane of the figures, the glass layers can extend over the edges of the bottom electrode.

Formation of the crossunder dielectric layers and capacitor underlayers was followed by deposition over essentially the entire circuit of a layer of oxidizable metal such as beta-tantalum (14 of FIG. 4). This layer was deposited by sputtering, but any standard deposition method could be employed. The layer was approximately 500 Angstroms thick, but could have a thickness of 200-600 Angstroms in order to serve its function as a protective layer for the glass layers (12+13). Next, as shown in FIG. 4, the metal layer, 14, was etched so as to uncover the edge portions, 21, of the bottom electrode for later electrical contact. This was accomplished by photolithographic etching employing a standard photoresist (not shown) as a mask. Care must be taken, however, in etching the layer 14 without also attacking the thick film electrode (11). It was discovered that an etchant comprising hydrofluoric acid, nitric acid and water in equal proportions by volume would satisfy this condition. Other etchants which could be used in the inventive method may also be identified.

Next, the layer 14 was oxidized to completely convert the tantalum metal to Ta.sub.2 O.sub.5 layer as shown in FIG. 5. This was accomplished by heating at 550 degrees Celsius for approximately 16 hours in air. The oxide layer in this example was approximately 500 Angstroms thick and should preferably be within the 200-600 Angstroms in order to adequately protect the underlying glass layers 12 and 13 during subsequent processing. It will be appreciated that other metal oxide layers could be used, but tantalum oxide is preferred since it is chemically resistant and has a thermal expansion similar to subsequently formed layers.

With the formation of the above-described dielectric layers and capacitor underlayers, the circuit can then be processed in accordance with standard thin film fabrication techniques in order to produce thin film resistors and capacitors interconnected with the thick film crossunder connection. The sequence used in this particular example will be described only briefly with reference to the final structure shown in FIG. 6, since the techniques employed are previously known and the invention is not limited to use of a particular thin film processing. (See, for example, U.S. Pat. No. 3,607,679 issued to Melroy et al., and U.S. patent application of F. R. Arcidiacono et al., Ser. No. 974,162 filed Dec. 28, 1978 and assigned to Western Electric.)

A film of alpha tantalum was deposited to a thickness of approximately 3000 Angstroms and then selectively etched to form the capacitor bottom electrode 16 in the areas of the glass layer 13 remote from the thick film electrode. The etchant was a mixture of 5:1:1 by volume of HF, HNO.sub.3 and H.sub.2 O. A portion of the capacitor electrode was then anodized to form the capacitor dielectric, 17, comprising tantalum pentoxide. This was followed by sputtering a layer of tantalum nitride, 18, to a thickness of approximately 200 Angstroms over essentially the entire substrate. The thin film conductor, 19, was then formed over selected portions of the circuit including the exposed portions, 21, of the thick film electrode, the dual dielectric layers 12 and 13, and the capacitor dielectric, 17, to form, respectively, the thick film crossunder connection, the conductor crossover, and the top electrode of the capacitor, as well as various interconnections between components. The conductor in this case was formed by successively evaporating a layer of titanium to a thickness of 2500 Angstroms and a layer of palladium to a thickness of 2500 Angstroms, followed by selective plating of gold in the desired conductive pattern to a thickness of approximately 20,000 Angstroms. The areas of palladium left uncovered were then etched with a solution comprising ferric chloride and the titanium was etched with a solution comprising 1:1:20 by volume of HF, HNO.sub.3 and H.sub.2 O. The uncovered tantalum nitride layer, 18, was then etched to complete conductor formation and also form the pattern for the thin film resistor, 20. The etchant was, again, a 1:1:1 by volume solution of HF, HNO.sub.3 and H.sub.2 O.

The combination of thin film components and thick film crossunder in accordance with the invention permits very high density circuits. For example, in a typical circuit a substrate measuring 0.550 inches.times.2.00 inches contained 60 thin film resistors, 15 thin film capacitors and 7 silicon integrated circuit chips.

Various modifications will become apparent to those skilled in the art. All such variations which basically rely on the teachings through which the invention has advanced the art are properly considered within the spirit and scope of the invention.