Electricity: circuit makers and breakers – Electrostrictive or electrostatic
Reexamination Certificate
2001-06-20
2003-05-20
Mai, Anh (Department: 2832)
Electricity: circuit makers and breakers
Electrostrictive or electrostatic
C200S013000
Reexamination Certificate
active
06566617
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a micro machine switch and a method for manufacturing thereof and in particular to a micro machine switch which allows the on/off control of a wide range of signal frequency, that is, from DC (direct current) signal frequency to signal frequency of one or more giga Hertz and a method for manufacturing thereof.
BACKGROUND ART
In the following the prior art will be described taking the case of the invention described in “Micro Electromechanical Switch”, by Yun Jason Yao, ROCKWELL INTERNATIONAL CORPORATION (Japanese Patent Laid-Open No. Hei9-17300).
FIGS.
16
(
a
) and
16
(
b
) are a plan view of the micro machine switch disclosed in Japanese Patent Laid-Open Hei9-17300 and a cross-sectional view of the same taken along the line D-D′, respectively. As shown in the same figures, an anchor structure
52
comprising thermosetting polyimide, a lower electrode
53
composed of gold and signal lines
54
comprising gold are provided on a substrate
51
comprising gallium arsenide.
And on the anchor structure
52
provided is a cantilever arm
55
consisting of silicon oxide, which extends to the signal lines
54
across the lower electrode
53
and faces the same with a space left between them.
An upper electrode
56
comprising aluminium is formed on the top of the cantilever arm
55
in such a position so as to face the anchor structure
52
and the lower electrode
53
. And a contact electrode
57
comprising gold is provided on the bottom of the cantilever arm
55
in such a position so as to face the signal line
54
.
In the micro machine switch having such construction, when applying a voltage of 30 V between the upper electrode
56
and the lower electrode
53
, the force of attraction is applied to the upper electrode
56
in the direction of the substrate (downward in the direction shown by the arrow
58
by the electrostatic force). Therefore, the cantilever arm
55
is strained toward the substrate side; as a result, the contact electrode
57
comes in contact with both ends of the signal lines
54
.
In the normal state, a space is provided between the contact electrode
57
and the signal lines
54
, as shown in FIG.
16
(
b
); accordingly, the two signal lines
54
are separated from each other. Thus, in state where no voltage is applied to the lower electrode
53
, no current flows through the signal lines
54
.
On the other hand, in state where voltage is applied to the lower electrode
53
and the contact electrode
57
is in contact with the signal lines
54
, the two signal lines
54
short-circuit, which allows current to flow between them. Thus, the application of voltage to the lower electrode
53
allows the on/off control of the electric current or signals passing through the signal lines
54
.
However, in order to reduce the switch loss particularly when using the switch for the signals in the microwave range, it is important that the upper electrode
56
and the contact electrode
57
are well insulated from each other. In other words, if the upper electrode
56
and the contact electrode
57
short-circuit, signals (including DC) flowing through the signal lines
54
flow out even to the upper electrode
56
.
Even if the upper electrode
56
and the contact electrode
57
do not short-circuit, in state where electrostatic capacity is significantly large, alternate signals flowing through the signal lines
54
also flow out to the upper electrode
56
and leak outside.
As described above, when the upper electrode
56
and the contact electrode
57
are not well insulated, the leak of signals becomes large, and switching characteristics deteriorate. From this viewpoint, the prior art described above uses an insulating material (silicon oxide) as the material constituting the cantilever arm
55
.
The micro machine switch of the prior art described above has the following problems.
The cantilever arm
55
is contact with the upper electrode
56
and the anchor structure.
52
, both of which differ from the cantilever arm
55
in the material, over a wide range. Further, the cantilever arm
55
is designed to have a mechanically flexible construction so as to control the driving voltage of the switch and to move only by applying micro voltage.
As described above, since the upper electrode
56
, the cantilever arm
55
and the anchor structure
52
are formed of different materials, their thermal expansion coefficients are also different, and particularly in the cantilever arm
55
, warps are likely to be caused due to distortion.
For example, when comparing silicon dioxide, aluminium and polyimide, the thermal expansion coefficient of silicon dioxide is about 1/100 times as small as those of the other two. Therefore, the metal portion of the upper electrode
56
etc. expands with the changes in the processing temperature and the atmospheric temperature after the completion of a device, and thereby warps are easily caused in the cantilever arm
55
.
The existence of such warps has a bad influence on switching characteristics regardless of their direction relative to the substrate
51
. In cases where the cantilever arm
55
warps upward, even if the bottom side surface of the cantilever arm
55
comes in contact with the lower electrode
53
by the application of voltage, a state is likely to occur in which the contact electrode
57
does not come in contact with the signal lines
54
. In that case, even if the contact electrode
57
comes in contact with the signal lines
54
, the intensity of pressure applied to the contact portion is very small, and such a very light contact gives rise to a problem of increasing the contact resistance of the switch.
On the other hand, in cases where the cantilever arm
55
warps downward, although the contact electrode
57
surely comes in contact with the signal lines
54
due to application of a voltage, the entire contact electrode does not come in planar contact with the signal lines
54
, and what is called single contact (both come in contact with each other only at one area) is very likely to occur. Thus, even in this case, there arises a problem of increasing the contact resistance of the switch.
As described above, in any case, a problem is created such that warps caused in the cantilever arm
55
increase the contact resistance, resulting in increasing the switch resistance when the switch is in the on state.
In fact, in the micro machine switch according to the prior art, the switch production process is performed at a low temperature of 250° C. or lower, and thereby warps caused due to the processing temperature are controlled.
To be concrete, a silicon dioxide film for making up the cantilever arm
55
is formed by the plasma enhanced CVD (PECVD) process. A PECVD oxide film offers the advantage of being able to be formed at low temperatures, and keeping the processing temperature low is important when decreasing the influence of the big difference in thermal expansion coefficient from material to material.
On the other hand, it is well known that mechanical properties (distortion, rigidity, reliability, etc.) and electrical properties (dielectric constant, maximum breakdown voltage, etc.) of materials can be remarkably improved by the optimization of, particularly, temperature conditions.
However, in the micro machine switch according to the prior art as described above, since the processing temperature needs to be kept low, the temperature parameter cannot be made good use of for optimizing the materials. In this respect, the prior art can be said to be limited largely in material.
Generally, a cantilever arm has the advantage that the width of its arm can be decreased when increasing its thickness so as to keep the rigidity constant. Thus, it has another advantage that the dimensions of the entire switch can be decreased, therefore, multiple switches can be formed in a limited area.
However, in the micro machine switch according to the prior art, which utilizes silicon dioxide for the cantilever arm, it is limited largely in the thickness of the cantilever ar
Ara Youichi
Chen Shuguang
Suzuki Ken-ichiro
Klaus Lisa N.
Mai Anh
NEC Corporation
Young & Thompson
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