Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
1999-04-19
2001-05-01
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S306100, C361S313000, C361S302000, C438S003000, C438S254000
Reexamination Certificate
active
06226170
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a thin-film capacitor comprising an electrically insulating substrate which is provided with at least two inner electrodes which are separated from each other by means of a dielectric layer, and with two end contacts which each electroconductively contact one of the inner electrodes.
A thin-film capacitor of the type mentioned in the opening paragraph is known per se, for example from U.S. Pat. No. 4,453,199. Said patent document more particularly describes a thin-film capacitor which is provided with an electrically insulating substrate which is preferably made of glass. Said glass substrate is provided, by means of thin-film techniques, with successively a sub-layer of phosphor-doped silicon dioxide, a first inner electrode of aluminium, a dielectric layer of silicon dioxide, a second inner electrode of aluminium and an overlayer of silicon dioxide. The known capacitor is further provided with two sputtered end contacts. Via one of the ends, each of the inner electrodes electrically contacts one of the end contacts of the capacitor.
It has been found that the known thin-film capacitor has an important drawback. It has namely been found that in the case of mass-production of the known thin-film capacitor, the value of the contact resistance between the end contacts and the inner electrodes is relatively high. In addition, it has been found that this value is not the same for all capacitors manufactured in a single batch. This relatively high and variable value of the contact resistance may cause problems in high-frequency applications of this type of thin-film capacitor. Under said conditions, the contribution of the contact resistance to the ESR (electrical series resistance) becomes unacceptably high. The value of the ESR is determined by the resistance of the inner electrodes, the resistance of the end contacts and by the contact resistance between the inner electrodes and the end contacts.
SUMMARY OF THE INVENTION
It is an object of the invention to reduce this disadvantage. The invention more particularly aims at providing a thin-film capacitor in which the contact resistance between the inner electrode and the end contact is relatively low, so that the value of the ESR is also relatively low. In addition, the value of the contact resistance of each of the capacitors manufactured in a single batch should be substantially equal.
These and other objects of the invention are achieved by a thin-film capacitor of the type mentioned in the opening paragraph which is characterized in accordance with the invention in that the electroconductive contact takes place via a through-connection which is in communication exclusively with the main surface of the inner electrode.
The invention is also based on the recognition that the ends of the inner electrodes, which in the known capacitor make contact with the end contacts, are very sensitive to oxidation during the production process. This oxidation occurs, in particular, during the provision of the end contacts. As a result, the contact resistance between the inner electrodes and the end contacts becomes undesirably high and the value thereof varies uncontrollably, even if the thin-film capacitors are manufactured in a single batch.
By ensuring that the electric contact with the end contacts no longer takes place via the ends, but instead exclusively via the main surface of the inner electrodes, the problem of the relatively high and uncontrolled contact resistance is reduced substantially. In this construction, the through-connection communicates with the part of the end contact which extends substantially parallel to the substrate. It has been found that, as a result of the inventive measure, the ESR value of the capacitor in accordance with the invention is relatively low in comparison with that of the known capacitor.
A preferred embodiment of the invented thin-film capacitor is characterized in that the material of the through-connection is identical to the material of at least a part of the end contacts. This measure contributes substantially to the desired low contact resistance between the end contacts and the through-connection. An additional advantage of this measure is that the through-connection and a part of the end contacts can be provided simultaneously.
Another preferred embodiment of the thin-film capacitor in accordance with the invention is characterized in that the through-connection is channel-shaped, the longitudinal direction of the channel extending substantially parallel to the longitudinal direction of that end of the inner electrode with which the through-connection is in electric contact. By providing channel-shaped through-connections, a relatively large contact surface between the through-connection and the inner electrode is achieved. This has a favorable effect on the desirable low contact resistance between both parts of the capacitor. The channel-shaped through-connections preferably extend substantially as far as the side faces of the inner electrodes of the thin-film capacitor. If the channels extend as far as or beyond the side faces of the inner electrodes, problems may arise in the etch process of the channels during the manufacture of these capacitors.
Another favorable embodiment of the invented capacitor is characterized in that the capacitor also comprises one or more floating electrodes which do not electroconductively contact the end contacts. The advantageous effect of the low and controlled contact resistance between the end contact and the inner electrode is obtained also in this type of thin-film capacitor.
Yet another attractive embodiment of the invented thin-film capacitor is characterized in that the end contacts are U-shaped. Capacitors in accordance with this embodiment can be used as so-called SMD components. In such an embodiment, the through-connection is in communication with one of the two limbs of the U-shaped end contact.
Another interesting embodiment of the invented thin-film capacitor is characterized in that a layer of a polymer material is situated between one of the limbs of each of the U-shaped end contacts and the main surface of the substrate. By virtue of this layer, which preferably consists of polyimide, an improved mechanical strength of the end contacts is obtained. In this embodiment, the risk of fracture between the end contacts and the substrate is reduced if the capacitor is exposed to a mechanical load, such as vibrations.
REFERENCES:
patent: 4453199 (1984-06-01), Ritchie et al.
patent: 5347423 (1994-09-01), deNeuf et al.
patent: 5479316 (1995-12-01), Smrtic et al.
patent: 5736448 (1998-04-01), Saia et al.
patent: 6033919 (2000-03-01), Gnade et al.
patent: 6125027 (2000-09-01), Klee et al.
De Wild Willem R.
Nellissen Antonius J. M.
Van Der Beek Gerrit P.
Ha Nguyen
Reichard Dean A.
Spain Norman N.
U.S. Philips Corporation
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