Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-07-02
2003-04-22
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S535000, C257S532000, C257S306000, C257S303000, C257S763000
Reexamination Certificate
active
06552384
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thin-film capacitor element for use in small electronic circuits and an electronic circuit board over which, together with this thin-film capacitor element, thin-film circuit elements, such as inductor elements, are to be formed.
2. Description of the Prior Art
In recent years, along with the advancement of integrated circuit technology, downsizing of electronic circuits is further proceeding, and there have been developed small electronic circuit boards over which capacitors, resistors, inductors and the like are to be formed as thin films.
Substrates for such electronic circuit boards can be made of a monocrystalline material such as sapphire or a sintered material such as alumina. As alumina is relatively inexpensive and excels in high frequency performance among these available materials, high frequency devices extensively use alumina substrates. Over an alumina substrate, various required circuit elements are formed as a set of thin films (hereinafter referred to as being formed in a filmy state). For instance, a capacitor element is configured over an alumina substrate by successively stacking layers of a lower electrode, a dielectric layer and an upper electrode. A resistor element is configured by forming in a filmy state a resistance layer of a desired shape over an alumina substrate and forming, also in a filmy state, electrodes at its two ends. An inductor element is configured by forming in a filmy state a metallic film of a desired shape over an alumina substrate and forming, also in a filmy state, electrodes at its two ends. Further over the alumina substrate path, a transmission line is formed as an electroconductive pattern, and this transmission line is connected to the respective electrodes of the capacitor element, resistor element and inductor element.
FIG. 12
is a plan of a conventionally known thin-film capacitor element, and
FIG. 13
, a sectional view along line
13
—
13
in FIG.
12
. As these drawings show, a conventional thin-film capacitor element consists of a stacked structure of a lower electrode
31
, a dielectric layer
32
and an upper electrode
33
formed over a substrate
30
, and the capacitance of the capacitor is determined by the range in which the lower electrode
31
and the upper electrode
33
overlap each other.
The lower electrode
31
is formed of Cu or the like sputtered or plated over the substrate
30
and etched into a desired pattern shape. The dielectric layer
32
is formed of SiO
2
or the like sputtered or chemical vapor-deposited over the lower electrode
31
and the substrate
30
and etched into a desired pattern shape. The patterned dielectric layer
32
extends to over the substrate
30
via the top face and side faces of the lower electrode
31
. The upper electrode
33
is formed of Cu or the like sputtered or plated over the dielectric layer
32
and the substrate
30
and etched into a desired pattern shape. The patterned upper electrode
33
extends to over the substrate
30
via the top face and side faces of the dielectric layer
32
.
Incidentally, one of the requirements for such a thin-film capacitor element concerns the breakdown voltage between the lower electrode
31
and the upper electrode
33
. If this breakdown voltage is below the required level, the thin-film capacitor element will be broken down at a low voltage and can no longer operate as such. The breakdown voltage is heavily dependent on the thickness of the dielectric layer
32
intervening between the two electrodes
31
and
33
. Increasing the thickness of the dielectric layer
32
would raise the breakdown voltage, but in the conventional thin-film capacitor element described above the dielectric layer
32
becomes thinner near the corners of the lower electrode
31
(see section P in
FIG. 13
) and thereby invites a drop of the breakdown voltage. This is due to the circumstance that when the dielectric layer
32
is formed over a level gap resulting from the etching of the lower electrode
31
, the coverage of the dielectric layer
32
is adversely affected near the corners of the lower electrode
31
. Especially where the dielectric layer
32
is formed by sputtering, it is difficult for sputtered atoms to adhere to the vertical face of the substrate
30
with the consequence that a drop or fluctuations of the breakdown voltage become conspicuous.
Although the dielectric layer
32
can be improved by thinning the lower electrode
31
to reduce the level gap, the thinner the lower electrode
31
and the upper electrode
33
, the greater the resistance component of conductors in series, which would give rise to another problem of an lowered Q value of the thin-film capacitor element. Or if the overall thickness of the dielectric layer
32
is increased, the breakdown voltage can be prevented from dropping, but the thicker the dielectric layer
32
, the smaller its capacitance per unit square measure. Accordingly, the thin-film capacitor element will become bigger and, moreover, as a greater thickness of the dielectric layer
32
does not serve to stabilize the shape of coverage, fluctuations of the breakdown voltage will remain.
In the conventional thin-film capacitor element described above, the capacitance of the capacitor is determined by the range in which the lower electrode
31
and the upper electrode
33
overlap each other (the rectangle of B×C in FIG.
12
). When the lower electrode
31
and the upper electrode
33
are etched into a desired pattern shape, the size and alignment of the two electrodes
31
and
33
tend to be adversely affected in accuracy by fluctuations in side etching quantity and etch rate, giving rise to a problem of fluctuations in the capacitance of the capacitor element. This problem is more serious with thin-film capacitor elements of smaller capacitances, because a smaller capacitance means a smaller area of overlap between the lower electrode
31
and the upper electrode
33
and accordingly the impact of accuracy fluctuations of etching and overlapping on the capacitance increases in relative terms.
Although an alumina substrate used in this kind of electronic circuit board has the aforementioned advantages, at the same time it is inferior in surface smoothness to sapphire and other monocrystalline substrates. The surface roughness (Ra) of an alumina substrate of 99.5% in purity, for instance, is 30 to 100 nm in terms of unevenness. As a consequence, when a capacitor element film is formed over an alumina substrate whose surface smoothness is so poor, the thickness of the dielectric layer formed over the lower electrode becomes partly thin, resulting in a problem of a significantly lowered breakdown voltage.
In view of this problem, in order to smoothen the alumina substrate surface, there are known such methods as mirror-grinding the whole surface of the alumina substrate or coating it with an insulating film of a polymer material or glass. However, the grinding method involves the disadvantages of leaving small dents between crystals in the alumina substrate and moreover, as an alumina substrate is hard, taking a long time. On the other hand, coating with an insulating film creates no big problem to the capacitor element, which is a capacitive one among the various filmy circuit elements and transmission lines formed over the insulating film, because the dielectric loss of the insulating film of a polymer or glass, it does, however, invite increased dielectric losses in other resistance elements, inductor element or layers under the transmission lines, which might lead to a deterioration the high frequency performance of the high frequency device.
SUMMARY OF THE INVENTION
In a thin-film capacitor element according to the present invention, a lower electrode and a dielectric layer are successively stacked over a substrate, the periphery of this dielectric layer is covered with an insulating layer having an opening, and an upper electrode formed over this insulating layer is stacked over the dielectric lay
Murata Shinji
Tokuta Mitsuru
Yamamura Ken
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Flynn Nathan J.
Tran Tan
LandOfFree
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