Active solid-state devices (e.g. – transistors – solid-state diode – Integrated circuit structure with electrically isolated... – Passive components in ics
Reexamination Certificate
2002-03-28
2004-10-19
Nguyen, Cuong (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Integrated circuit structure with electrically isolated...
Passive components in ics
C257S296000, C257S295000
Reexamination Certificate
active
06806553
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a tunable thin film capacitor with a thin film dielectric layer whose dielectric constant is changed by the voltage applied across a lower electrode layer and an upper electrode layer.
2. Description of the Related Art
It is so far known that a strontium titanate (SrTiO
3
) thin film that is a paraelectric substance or a strontium barium titanate (Ba, Sr) TiO
3
thin film that is a ferroelectric substance exhibits a non-linear change in the dielectric constant thereof upon application of a predetermined voltage to the dielectric thin film thereof (A. Walkenhorst et al., Appl. Phys. Lett. 60 (1992) 1744 and Cem Bascriet. al., J. Appl. Phys 82 (1997) 2497).
Thin film capacitors using as their thin film dielectric layers perovskite structure ferroelectric oxide thin films of these strontium titanate and strontium barium titanate, etc. are proposed (Japanese Unexamined Patent Publication JP-A 11-260667 (1999)).
These tunable thin film capacitors are each constituted in such a manner that, on a support substrate
161
, a lower electrode layer
162
, a thin film dielectric layer
163
and an upper electrode layer
164
are successively formed by deposition as shown in a sectional view of FIG.
24
. More specifically, on the support substrate
161
, a conductor layer that will become the lower electrode layer
162
is formed by deposition over approximately the whole surface of the substrate
161
, and then, patterning is performed to form the lower electrode layer
162
of a predetermined shape. Next, the dielectric layer
163
is formed on the lower electrode layer
162
. This dielectric layer
163
is formed, by the thin film method, with a mask placed at a predetermined position, or formed by the spin coating process, and then patterned into a predetermined shape. Further, the layer is heated for hardening as required. The upper electrode layer
164
is formed in such a manner that a conductor layer that will become the upper electrode layer
164
is formed on the dielectric film
163
and then pattern-processed. Here, the facing region of the dielectric layer
163
which is actually held between the lower electrode layer
162
and the upper electrode layer
164
becomes a capacitance-producing region.
In the dielectric layer
163
in this capacitance-producing region, the dielectric constant that the dielectric layer
163
has is changed by an external control voltage fed across the lower electrode layer
162
and the upper electrode layer
164
.
Accordingly, in case that the mutually facing areas of both electrodes layers
162
and
164
and the thickness of the dielectric layer
163
are made constant, the capacitance value obtained between both electrode layers
162
and
164
can be varied by setting the external control voltage at a predetermined voltage.
Further, the thin film capacitor shown in
FIG. 24
is constituted such that, in order to prevent the lower electrode layer
162
and the upper electrode layer
164
from short-circuiting to each other, the upper electrode layer
164
is extended onto the support substrate
161
so as to form an air bridge
165
. In other words, by extending the upper electrode layer
164
onto the support substrate
161
, as pace is formed around the dielectric layer
163
.
This space can be formed in such a manner that an organic resist member that is formed by heat treatment or the like is formed, and thereafter, the upper electrode layer
164
is formed and heat-treated.
In such a variable capacitor, the capacitance thereof is changed by a control voltage applied across the lower electrode layer
162
and the upper electrode layer
164
, and therefore, it is important that this voltage be uniformly applied to the dielectric layer
163
.
This external control voltage is, for example, ten to several tens of volts, but this external control voltage is, actually, hard to be uniformly applied to the capacitance-producing region; and thus, it is difficult to stabilize the dielectric constant of the dielectric layer
163
to a predetermined value. For example, by taking the maximum capacitance into consideration, the facing area of the capacitance-producing region is set, but, in case that an attempt is made to obtain this capacitance value from one capacitance-producing region, the area of the capacitance-producing region is increased. As a result, the voltage drop of the control voltage occurs in the upper electrode layer
164
and the lower electrode layer
162
; and, of the lower electrode layer
162
and the upper electrode layer
164
, the portions adjacent to portions
162
a
,
164
a
extended from the capacitance-producing region become high in potential, while, in the central portion of the capacitance-producing region and portions away from the extension portions
162
a
,
164
a
shown in
FIG. 25
, the potential becomes relatively low. In other words, in the same capacitance-producing region, a distribution of potential takes place, resulting in the problem that the sufficient dielectric constant control, i.e., a sufficiently wide variable range, cannot be obtained; due to this, there has been the problem that a capacitance corresponding to the external voltage cannot be obtained as according to the specifications.
Further, in order to structurally prevent the upper electrode layer
164
and the lower electrode layer
162
from short-circuiting to each other, the air bridge
165
is provided. The existence of this hollow structure results in giving a large restriction when the capacitor is mounted on a mother board and in lacking in reliability. Further, as for the method of manufacturing the upper electrode layer
164
, an organic resist is used for the formation of the air bridge
165
, and thus, a manufacturing method deviating from the thin film technology must be used, so that the manufacturing steps become very complicated.
To a tunable thin film capacitor that operates in a high-frequency region, it is important to reduce the loss of the respective electrode layers. For this, it necessary to increase the thickness of a lower electrode layer
162
and an upper electrode layer
164
. In practice, however, it is difficult to increase the thickness of the lower electrode layer
162
.
It is because, in the case of increasing the thickness of the lower electrode layer
162
, the coverage of the lower electrode layer
162
to the dielectric layer
163
is deteriorated. Further, separation by peeling takes place between a substrate
161
and the lower electrode layer
162
. Moreover, separation by peeling takes place between the dielectric layer
163
and the lower electrode layer
162
. These inconveniences are caused because, by increasing the thickness of the lower electrode layer
162
, the stress resulting from the difference between the thermal expansion coefficients of the substrate
161
and the dielectric layer
163
is increased.
Particularly, in the case of such a variable capacitance element, the increase in thickness of both electrode layers
162
,
164
is very important in view of the fact that a relatively high capacitance controlling voltage of about 10 V must be applied to the dielectric layer
163
between the lower electrode layer
162
and the upper electrode layer
164
.
Further, in the case of the known tunable thin film capacitor, an air bridge
165
was formed around the dielectric layer
163
in order to prevent the upper electrode layer
164
from short-circuiting to the lower electrode layer
162
. Thus, a portion of the upper electrode layer
164
is positioned in the air like a bonding air, so that it is very difficult to actually mount the capacitor onto a mother board or the like; and thus, the known tunable thin film capacitor could hardly be put to practical use.
Concerning a tunable thin film capacitor constituted such that, by application of an external voltage, the dielectric constant of the dielectric layer is changed to vary the capacitance, the important problems to be solved are to reduce the voltage loss
Kishino Tetsuya
Kusabe Kazuhiro
Mishima Tsuneo
Yashima Yukihiko
Kyocera Corporation
Nguyen Cuong
LandOfFree
Tunable thin film capacitor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Tunable thin film capacitor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Tunable thin film capacitor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3286041