RF microelectromechanical systems device

Details RF microelectromechanical systems device RF microelectromechanical systems device

2003-02-24

2004-03-30

Vortman, Anatoly (Department: 2835)

Electricity: circuit makers and breakers

Electrostrictive or electrostatic

C333S262000

Reexamination Certificate

active

06713695

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to RF microelectromechanical systems (MEMS) devices for use in, for example, RF circuits.
2. Description of the Related Art
FIG. 9A
is a plan view of a micro-machine switch
40
.
FIG. 9B
is a sectional view of the micro-machine switch
40
taken along line A—A in
FIG. 9A
(refer to Japanese Patent No. 3119255). In the micro-machine switch
40
, a first signal line
42
a
and a second signal line
42
b
are arranged on a substrate
41
. One end of each of the first signal line
42
a
and the second signal line
42
b
face each other, with a gap G therebetween. A bottom electrode
43
is arranged on the substrate
41
and is separated from the first signal line
42
a
and the second signal line
42
b.
A fixing unit
45
is arranged near the bottom electrode
43
on the substrate
41
. A movable element
44
is arranged above the substrate
41
. The movable element
44
faces, with a space therebetween, an area of the substrate covering the area extending from the one end of the first signal line
42
a
and the one end of the second signal line
42
b
to the bottom electrode
43
. The movable element
44
is supported by the fixing unit
45
and beams
46
(
46
a
and
46
b
).
An insulating film
47
is arranged on substantially the entire surface of the movable element
44
at the substrate side. A movable electrode
48
and a contact electrode
50
are arranged on the insulating film
47
. The movable electrode
48
faces the bottom electrode
43
and the contact electrode
50
faces an area extending from the one end of the first signal line
42
a
to the one end of the second signal line
42
b
, with the gap G therebetween.
In the micro-machine switch
40
having the arrangement described above, for example, a DC voltage applied between the bottom electrode
43
and the movable electrode
48
causes electrostatic attraction to occur between the bottom electrode
43
and the movable electrode
48
. This electrostatic attraction causes the beams
46
a
and
46
b
to be bent, thereby allowing the movable element
44
to be attracted towards the substrate
41
. The displacement of the movable element
44
causes contact between the contact electrode
50
and the one end of the first signal line
42
a
and the one end of the second signal line
42
b
, and therefore the contact electrode
50
connects the first signal line
42
a
and the second signal line
42
b
. The contact between the contact electrode
50
and the first signal line
42
a
and the second signal line
42
b
causes signal conduction of the first signal line
42
a
and the second signal line
42
b
to be switched on. In contrast, removing the contact electrode
50
from the first signal line
42
a
and the second signal line
42
b
causes the signal conduction of the first signal line
42
a
and the second signal line
42
b
to be switched off. That is to say, the micro-machine switch
40
performs a switching operation, as described above.
In the arrangement of the micro-machine switch
40
described above, the movable element
44
is made of low-resistivity silicon which is heavily doped with boron. Since the low-resistivity silicon has large a dielectric loss, an RF signal made to flow in the contact electrode
50
by the switch-on operation disadvantageously experiences large propagation loss.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide an RF-MEMS device that effectively minimizes propagation loss of an RF signal.
According to a first preferred embodiment of the present invention, an RF-MEMS device includes a substrate, an RF signal-conducting unit disposed on the substrate, a movable element arranged above the substrate with a space therebetween and facing at least part of the RF signal-conducting unit with a space therebetween, a movable electrode arranged on the movable element and facing at least part of the RF signal-conducting unit, and a fixed electrode arranged on the substrate and facing at least part of the movable element. The movable element includes a high-resistivity semiconductor functioning as an insulator for an RF signal and functioning as an electrode for a low-frequency signal and a DC signal. The fixed electrode and the movable element functioning as the electrode together function as a movable element displacing unit for displacing the movable element towards the fixed electrode by electrostatic attraction caused by a DC voltage applied between the fixed electrode and the movable element. The movable element displacing unit displaces the movable element towards or away from the substrate due to the electrostatic attraction. The displacement of the movable element by the movable element displacing unit changes the space between the RF signal-conducting unit and the movable electrode, thereby varying the capacitance between the RF signal-conducting unit and the movable electrode.
An RF-MEMS device according to a second preferred embodiment of the present invention includes a substrate, an RF signal-conducting unit disposed on the substrate, a movable element arranged above the substrate with a space therebetween and facing at least part of the RF signal-conducting unit with a space therebetween, a movable electrode arranged on the movable element and facing the RF signal-conducting unit, an upper member facing at least part of the top of the movable element with a space therebetween, and a fixed electrode arranged on the upper member to face at least part of the movable element. The movable element includes a high-resistivity semiconductor functioning as an insulator for an RF signal and functioning as an electrode for a low-frequency signal and a DC signal. The fixed electrode and the movable element functioning as the electrode together function as a movable element displacing unit for displacing the movable element towards the fixed electrode by electrostatic attraction caused by a DC voltage applied between the fixed electrode and the movable element. The movable element displacing unit displaces the movable element towards or away from the substrate due to the electrostatic attraction. The displacement of the movable element by the movable element displacing unit changes the space between the RF signal-conducting unit and the movable electrode, thereby varying the capacitance between the RF signal-conducting unit and the movable electrode.
Preferably, the RF-MEMS device further includes a protective insulating film provided on at least one of a surface of the RF signal-conducting unit and a surface of the movable electrode, the surface of the RF signal-conducting unit being disposed opposite to the surface of the movable electrode.
Preferably, an RF signal with a frequency of about 5 GHz or more flows in the RF signal-conducting unit.
Preferably, the high-resistivity semiconductor functioning as the movable element has a resistance ranging from about 1,000 &OHgr;cm to about 10,000 &OHgr;cm.
The RF-MEMS device may be a variable capacitor defined by the RF signal-conducting unit and the movable electrode and the capacitance of the variable capacitor varies by displacement of the movable element.
The RF signal-conducting unit may be a coplanar line. The RF-MEMS device may be a switching device in which an increased capacitance between the coplanar line and the movable electrode causes an RF short circuit between the coplanar line and the movable electrode, thereby switching off the conduction of the coplanar line, and in which a decreased capacitance between the coplanar line and the movable electrode causes an RF-open circuit between the coplanar line and the movable electrode, thereby switching on the conduction of the coplanar line.
The RF signal-conducting unit may include divided signal lines which are divided by an intermediate gap provided in an area that faces the movable electrode, and one end of each of the divided signal lines at each side of the intermediate gap faces the movable electrode with a space t

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