Tunable high-frequency capacitor

Details Tunable high-frequency capacitor Tunable high-frequency capacitor

2002-10-16

2004-10-12

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrostatic capacitors

Variable

C361S290000, C361S303000

Reexamination Certificate

active

06804107

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a tunable capacitor and in particular to a steplessly tunable micromechanic high-frequency capacitor.
BACKGROUND OF THE INVENTION
Tunable capacitors of a high quality factor Q are required, for example, in tunable oscillators, so-called VCOs=voltage control oscillators, and in high-frequency and microwave circuits. Capacitance diodes, also referred to as varactor diodes, are responsible for changing the capacitance. Application of a reverse-biased voltage changes the space charge distribution in the barrier layer of these diodes, and thus their capacitance. In this way, the capacitance decreases as the reverse-biased voltage increases. Typically a change in capacitance of 10 pF to 2 pF is achieved with a change in the reverse-biased voltage from 1 V to 10 V. These diodes, however, have drawbacks that become apparent in particular in microwave oscillators. Thus, for example, the high series resistance leads to low resonance quality factors Q, and noise increases with higher frequencies. A further drawback is that the capacitance's swing of the diodes is limited to about 2 pF at the bottom end.
In the high-frequency or microwave range and in designing appropriate components, it is further always desirable to have adjustable or variable components such as, for example, adjustable capacitors, adjustable resistors or adjustable inductive devices. Such components provide a developer in the high-frequency and microwave ranges with high flexibility in relation to the design of high-frequency and microwave circuits. In re-configurable radio systems of the future, such as, for example, software-based radio systems, these components are referred to as enabling components/technology. This is due to the fact that software-based radio systems require a programmable high-frequency receiver unit (HF receiver unit) which, in addition to other requirements placed upon the high-frequency components, supports at least different frequency bands. In conventional components technology, in particular when mobile radio applications are considered, this represents a big challenge since power consumption, space requirements, weight, as well as electrical behavior, represent very demanding challenges.
The well-known varactor diode described above is, for example, one type of an adjustable capacitor. In addition to the drawbacks mentioned above, a further drawback is that the capacitance of the varactor diode changes in a non-linear manner with the control voltage applied, and that the Q factor of such a varactor diode is generally very low, at about 40. These features limit the range of employment of varactor diodes to applications where the requirements with regard to linearity and the Q factor (the quality) are not demanding. Typical examples of the use of varactor diodes are the above-mentioned voltage-controlled oscillators (VCOs) as well as some variable filters.
In view of these drawbacks, manifold research activities have recently been developed worldwide so as to find solutions for the above adjustable components.
Several concepts for adjustable capacitors have been suggested. These concepts are based on so-called MEMS technology, MEMS standing for “micro-electro-mechanical systems”. MEMS technology is an up-and-coming technology that has attracted more and more attention in the last few years. The adjustable MEMS capacitor can be seen as a counterpart of the semiconductor varactor diode.
There are numerous requirements placed upon an MEMS capacitor for use in programmable high-frequency circuits. Ideally, an MEMS capacitor should first of all be a linear device. This means that the capacitance of the MEMS capacitor does not change with the high-frequency signal when a fixed control voltage or a fixed control current is applied. Secondly, the MEMS capacitor should have a high Q factor (high quality), i.e. low losses. Thirdly, an MEMS capacitor should have a broad variability range. The tuning range typically required for mobile phones is from 0.5 pF to 20 pF. In addition, the MEMS capacitor should not consume any power and be readily integratable with other circuits and circuit components.
No MEMS capacitor is currently known which might be available from any manufacturer. However, there are several concepts, such as, for example, the approach described in the essay by T. T. C. Nguyen, “Micromachining Technologies for Miniaturized Communication Devices”, Proc. SPIE Conference on Micromachined Devices and Components IV, SPIE volume 3514, September 1998, pp. 24-37. This essay describes a continuously variable capacitance which is actuated in an electrostatical manner and wherein a movable electrode consists of a metallic membrane suspended on a spring beam.
Generally speaking, an MEMS capacitor principally consists of two metal plates, the distance and/or area (overlap area) is controlled by actuators. These actuators are controlled by a voltage in the case of electrostatic actuators, or by a current in the case of thermal actuators.
An example of an MEMS capacitor known from the prior art is described in more detail with reference to FIG.
4
.
FIG. 4A
shows a plan view representation of a known MEMS capacitor, and
FIG. 4B
shows a cross-sectional representation of the capacitor shown in FIG.
4
A.
The MEMS capacitor
400
is formed by a fixed electrode
402
and a movable electrode
404
, those electrodes
402
and
404
having essentially the same surface area A. The movable electrode
404
is arranged to substantially overlap the fixed electrode
402
. The fixed electrode
402
and the movable electrode
404
are spaced apart by a distance x. The movable electrode
404
is supported by a first suspension
406
and a second suspension
408
, the suspensions
406
and
408
being designed so as to move the movable electrode
404
such that the distance x between the electrodes
402
and
404
changes.
The first suspension
406
is fastened on a first fastening element
410
, and the second suspension
408
is fastened on a second fastening element
412
. These elements
410
and
412
in turn are fastened on surface areas (not shown), such as on a substrate or the like. The first suspension
406
includes a first portion
406
a
, a second portion
406
b
and a third portion
406
c
. The second portion
406
b
includes an actuator connected between the first portion
406
a
and the third portion
406
c
. The first portion
406
a
is further connected to the first fastening element
410
, and the third portion
406
c
is connected to movable electrode
404
. Similarly, the second suspension
408
includes a first portion
408
a
, a second portion
408
b
as well as a third portion
408
c
; the portion
408
b
also including an actuator. The first portion
408
a
is further connected to the second fastening element
412
, and the third portion
408
c
is further connected to the movable electrode
404
. As can be seen in
FIG. 4A
, the portions
406
b
and
408
b
are arranged in an inclined manner so that the electrode
404
is arranged to be spaced apart from the electrode
402
.
The electrode
402
includes a high-frequency input connection
414
, as is illustrated by the arrow
416
. The second fastening element
412
includes a high-frequency output connection
418
, as is illustrated by the arrow
420
. The path of a high-frequency signal through the capacitor
400
thus extends from the connection
414
via the electrode
402
to the electrode
404
to the output
418
. With regard to the suspensions
406
and
408
it is to be noted that they are not insulated for the high-frequency signal path.
For driving the actuators
406
b
and
408
b
, a control connection
422
is provided which is connected to the second fastening element
412
via a line
424
. Application of an appropriate control signal to the connection
422
leads to an actuation of the actuators
406
b
and
408
b
, whereby a displacement of the movable electrode
404
is caused, so that the distance x between the movable electrode
404
and the fixed electro

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