Wave transmission lines and networks – Long line elements and components – Switch
Reexamination Certificate
2001-08-23
2003-01-28
Wamsley, Patrick (Department: 2819)
Wave transmission lines and networks
Long line elements and components
Switch
C333S259000
Reexamination Certificate
active
06512432
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a microswitch and to a method of fabricating the microswitch, and in particular, to a microswitch for turning ON and OFF signals ranging from DC to AC current having a broad range of signal frequencies up to several hundred GHz, and to a method of fabricating such a microswitch.
2. Description of the Related Art
The prior art will first be explained taking as an example the invention disclosed in Micro Electromechanical RF Switch (Japanese Patent Laid-open No. 17300/97, U.S. Pat. No. 5,578,976, by Jun J. Yao of Rockwell International Corporation.
A plan view of the microswitch of the invention is shown in
FIG. 1
, and a cross section (taken along line C-C′) is shown in FIG.
2
. In this microswitch, anchor structure
144
composed of thermo-setting polyimide, lower electrode
146
and signal lines
148
both composed of gold are provided on gallium arsenide (GaAs) substrate
149
. Cantilever arm
140
composed of a silicon oxide film provided on anchor structure
144
extends as far as the position of lower electrode
146
and signal lines
148
and confronts these components with an interposed spatial gap. Upper electrode
141
composed of aluminum is provided on the upper surface of cantilever arm
140
from a position opposite anchor structure
144
to a position confronting lower electrode
146
. In addition, contact
142
composed of gold is provided on the lower surface of cantilever arm
140
at a position confronting signal lines
148
.
When a voltage of 30 V is applied between upper electrode
141
and lower electrode
146
, an electrostatic force works to attract upper electrode
141
toward the substrate, whereby cantilever arm
140
bends downward and contact
142
contacts signal lines
148
. As shown in
FIG. 1
, a gap is provided in signal lines
148
at a position confronting contact
142
.
Current does not flow in signal lines
148
in the state in which voltage is not applied between upper electrode
141
and lower electrode
146
, but current can flow in signal lines
148
in the state in which voltage is applied between upper electrode
141
and lower electrode
146
and contact
142
contacts signal lines
148
. In this way, ON/OFF control of the flow of a current or signal through signal lines
148
can be effected by the application of voltage.
In this case, sufficient electrical isolation between upper electrode
141
and contact
142
is critical for reducing loss in the switch. In other words, the problem exists that a portion of the signal (including DC) that flows through signal lines
148
flows to upper electrode
141
in the event of an electrical short-circuit between upper electrode
141
and contact
142
. Even without a short-circuit between upper electrode
141
and contact
142
, a considerably large electrostatic capacity between these two components inevitably results in the flow of a portion of the AC signal that flows through signal line
148
to upper electrode
141
and to the outside. When these two components are not adequately isolated, signal leakage increases and the characteristics of the switch deteriorate.
Typically, an electrostatic switch is required to exhibit high impedance when the switch is OFF and further, to allow switching between ON and OFF by the application of a low voltage. A switch that implements switching of an RF signal therefore necessitates an increase of the distance between signal line
148
and contact
142
of the switch that is provided above signal line
148
to increase impedance when OFF. In the above-described prior art, the distance between contact
142
and signal line
148
is less than the distance between upper electrode
141
and lower electrode
146
. Such a construction entails the problem that, when the distance between signal line
148
and contact
142
of the switch that is positioned above signal line
148
is increased, the distance between the voltage application components (upper electrode
141
and lower electrode
146
) must also be increased, thereby necessitating the application of greater voltage to drive the voltage application components. Since electrostatic force decreases in inverse proportion to the square of the gap, reducing the distance between these voltage application components is crucial for reducing the drive voltage of the switch.
In the above-described example of the prior art, moreover, cantilever arm
140
is shaped so as to extend parallel to substrate
149
. In this case, when electrostatic attraction works between lower electrode
146
and upper electrode
141
that is provided in the central portion of cantilever arm
140
to bend cantilever arm
140
toward substrate
149
, the distance that contact
142
moves in the direction of substrate
149
is greater than the amount of deflection of upper electrode
141
at the position that confronts lower electrode
146
, this distance being the product of the amount of deflection of upper electrode
141
and a spring ratio (the distance between contact
142
and the base of anchor structure
144
of cantilever arm
140
divided by the distance between lower electrode
146
and the base of anchor structure
144
of cantilever arm
140
). Thus, when the switch is turned ON, contact
142
first contacts signal line
148
on the right side of FIG.
2
. If cantilever arm
140
is constructed with sufficient flexibility, all of contact
142
can be caused to contact signal line
148
. However, it has been found that generally, the electrostatic force that drives the switch is small while the rigidity of cantilever arm
140
is rather great. As a result, the problem was encountered in the prior-art construction that contact between contact
142
and signal lines
148
was insufficient, impedance was not sufficiently low when the switch was turned ON, and signal loss was therefore great. Furthermore, in the construction of above-described example of the prior art, there also occurred the problem of one-sided contact in which contact
142
made contact with only one of signal lines
148
and did not contact the other signal line
148
when the switch was turned ON. It was found that this problem is related to the fact that the dimensions of narrow portion
143
of the connecting portion between cantilever arm
140
and anchor structure
144
are determined by the voltage that is applied between upper electrode
141
and lower electrode
146
and the lack of freedom in the design of contact
142
. This point will be explained in greater detail in the embodiments of the present invention.
In addition to these problems, it was found that the prior-art example has the following problems that arise from materials and fabrication processes. Cantilever arm
140
(made from silicon dioxide) of the prior-art example contacts the different materials of upper electrode
141
(made from aluminum) and anchor structure
144
(made from polyimide) over an extensive area. Because this cantilever arm
140
is designed as a mechanically flexible construction in order to decrease the drive voltage, the slight strain that arises between these different materials tends to cause a high degree of warping. The strain that causes warping largely depends on the differences in thermal expansion coefficients of the different materials and on differences in processing conditions. The silicon dioxide in the prior-art example has a thermal expansion coefficient that differs by approximately 100 times from those of aluminum and polyimide. Warpage tends to occur easily due to processing temperatures as well as to temperature changes in the ambient atmosphere after completion of the device. Fabrication conditions such as the film thicknesses of the cantilever arm and anchor structure therefore must be accurately controlled to control warpage during fabrication, and these requirements result in increased fabrication costs.
Furthermore, since the completed device is subject to the influence of temperature changes in the atmosphere, problems have arisen relating to
NEC Corporation
Wamsley Patrick
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