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Description  |
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TECHNICAL FIELD
The present invention relates generally to mechanical switches and more
particularly to devices for switching electrical signals.
BACKGROUND ART
Microminiature mechanical switches offer an alternative to semiconductor
electronic components as a means for signal switching. U.S. Pat. No.
5,047,740 to Alman describes a miniature switch for controlling microwave
signal transmission. A spring-loaded mechanism is controlled by a magnetic
solenoid to connect a first microwave signal line to either a second or a
third microwave signal line. Solenoid activation pivots an armature which
determines the positioning of jumpers relative to the microwave signal
lines.
An electrostatically actuated micromachined rotary switch is described in
U.S. Pat. No. 5,121,089 to Larson. The rotary switch is fabricated on an
integrated circuit wafer using integrated circuit fabrication processing.
Microwave transmission lines are positioned to contact a rotating blade of
the switch when the rotating blade is properly aligned. Rotation of the
blade is controlled by electrostatic fields created by control pads and
other switch elements formed on a substrate that also contains the
microwave transmission lines.
In a paper entitled "Thermo-Magnetic Flexure Actuators," 0-7803-0456-X/92,
1992 IEEE, Guckel et al. of the University of Wisconsin describe an
actuator that utilizes one or both of thermal effects and magnetic forces
to cause deflection of beams when an electrical current is applied. While
this structure functions well in certain applications, there are
difficulties. For example, if the Guckel et al. actuator were to be used
as a switch to conduct a signal from the beams to structure that contacts
the beams following deflection, signal transmission would be susceptible
to feedthrough from the actuator-deflection current. Another difficulty
involves inconsistent and even conflicting design requirements for
different components of a transmission scheme. A signal line design
requires the selection of materials and dimensions to yield a suitable
impedance and to minimize signal loss. On the other hand, the actuator of
Guckel et al. is designed to achieve a desired deflection in a reliable
and efficient manner.
The previously identified patent to Alman lists a number of concerns in the
design of a micromachined switch. The switch must be non-particulating and
must be adjustable to compensate for changes in the forces which initiate
the switching action, e.g., magnetic forces. Moreover, the switch must be
reliable over many switching cycles.
What is needed is a microminiature signal switch which minimizes the
compromises between fabricating a switch and fabricating signal lines, and
which reduces space and cost requirements over conventional combinations
of micromachined switches and signal lines.
SUMMARY OF THE INVENTION
The invention provides a thermally actuated signal switch having an
actuator supported for reciprocating movement between a raised position in
which an electrical circuit is electrically opened and a lowered position
in which the circuit is electrically closed. The reciprocating movement is
achieved by forming at least a portion of the actuator of materials that
have sufficiently different coefficients of thermal expansion to induce
displacement in response to the input or release of thermal energy.
Signal lines are formed on a first substrate. The actuator is defined by a
substrate that is parallel to the first, or signal, substrate. Typically,
the actuator substrate is micromachined to form "bi-metallic" legs that
are controlled to selectively move a suspended contact region between the
raised and lowered positions that achieve electrical switching. In the
preferred embodiment, the actuator substrate is a semiconductor, so that
one of the "metals" is semiconductor material. The other material should
be one having a sufficiently different coefficient of thermal expansion
that the legs are caused to arc as a result of stresses induced by the
expansion differential.
The suspended contact region may be formed as an element of the
micromachined actuator substrate, but there are thermal, electrical, and
mechanical advantages to forming the contact region as a part of a third
substrate that is positioned between the signal and actuator substrates.
The contact region is aligned with respect to the bi-metallic legs such
that arcing of the legs causes motion of the contact region relative to
the signal lines of the signal substrate.
The suspended contact region may include a conductive line formed on a side
adjacent to the signal substrate. The conductive line is aligned to
electrically connect with at least one signal line on the signal substrate
when the actuator substrate is in a condition in which the contact region
is in the lowered position. For the less desirable embodiment in which the
contact region is integral with the bi-metallic legs, the conductive line
on the contact region should be on the side of the actuator substrate
opposite to the metal layer that is heated to induce displacement. In
either embodiment, the conductive line should be electrically isolated
from the actuating signal that is used to control displacement of the
legs.
The switch may be utilized to control signal transmission at microwave
frequencies. Thus, the three substrates may be formed to provide an
environment for microwave signal transmission. By "microwave environment"
what is meant is that the structure is designed so as to minimize
introduction of signal reflections, losses and noise within the microwave
frequency range, while maintaining desired isolation between unconnected
signal lines and while obtaining reproducible contact between signal lines
when connected. Ground planes are formed on the first, signal substrate
and on the center substrate. Optionally, a stop is formed on either the
contact region or the first substrate to prevent direct contact of the
movable conductive line with a signal line on the first substrate. The
stop should be positioned to limit movement of the actuator to a position
in which the signal lines are sufficiently close to allow passage of high
frequencies, i.e., allow electrical connection, but sufficiently spaced
apart to filter low frequencies.
The suspended contact region may be a boss extending from the actuator
substrate or, if used, from the center substrate. Moreover, the actuator
substrate may be micromachined to form more than one actuator on the same
substrate. That is, separately operated actuators may be operatively
associated with a single electrical circuit.
An advantage of the invention is that because the transmission circuitry is
formed on one substrate and the actuator is formed from a second
substrate, there is a reduction in the space requirements over
conventional combinations of circuitry and mechanical switches.
Furthermore, as will be described more fully below, an interposer can be
placed between actuator and transmission line circuitry to provide thermal
and electrical isolation and to add "scrubbing action." The switches can
be fabricated at a low cost and with a high degree of integration.
Parasitic capacitance and inductance are significantly reduced, relative
to structures in which signals are conducted from one substrate to another
in order to undergo a switching process, and then conducted back onto the
first substrate. Moreover, fewer compromises need to be made in forming a
combination of electrical circuitry and mechanical switches. Particularly
in the embodiment in which one or more contact region is formed of a
center substrate that is between the signal and actuator substrates, a
high degree of thermal isolation is achieved.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side sectional view of a first embodiment of a thermally
actuated switch in accordance with the invention.
FIG. 2 is a bottom view of an actuator substrate of the thermally actuated
switch of FIG. 1, taken along lines 2--2.
FIG. 3 is a side sectional view of a second embodiment of a thermally
actuated switch in accordance with the invention.
FIG. 4 is a spring member of the actuator substrates of FIGS. 1-3.
FIG. 5 is a top view of a 1.times.4 switch in accordance with the
invention.
FIG. 6 is a side sectional view of a third embodiment of a thermally
actuated switch in accordance with the invention.
FIG. 7 is a top view of a 1.times.n switch adapted for use with attenuators
.
BEST MODE FOR CARRYING OUT THE INVENTION
With reference to FIG. 1, a switch 11 includes a signal substrate 10 having
first and second signal lines 12 and 14. The signal lines may be formed on
the substrate using conventional techniques. Photolithographic processing
may be utilized to pattern and etch a metallic layer, but the choice of
techniques for fabricating signal lines and any electronic devices on the
signal substrate 10 may vary according to factors known by persons skilled
in the art. In the preferred embodiment, the signal lines are formed of a
layer of gold. However, other materials may be utilized.
The selection of a signal substrate 10 is based upon achieving electrical
and mechanical characteristics for a particular application. In the
preferred embodiment, a microwave environment is formed for transmitting
microwave frequencies via the signal lines 12 and 14. Silicon, quartz and
semi-insulating gallium arsenide are acceptable materials for the
substrate. A flexible copper on a polyimide substrate, commonly known as a
"flex circuit," is another substrate candidate, as is sapphire.
A conductive layer 16 is formed on a side of an upper insulating layer 17
opposite to the signal lines 12 and 14. The conductive layer is
electrically connected to form a ground plane, thereby establishing a
microstrip structure for transmitting microwave signals. The upper
insulating layer isolates the signal lines from the ground plane. A lower
insulating layer 18 isolates the ground plane 16 from a substrate 10.
Supported at the top of the signal switch 11 is an actuator substrate 20.
The length of the actuator substrate is shown as matching that of the
signal substrate 10, but a portion of the signal substrate may be left
exposed to allow access to input/output pads on the signal substrate. The
selection of an actuator substrate is based upon compatibility with
micromachining techniques and upon thermal expansion properties. A
preferred material is silicon, since silicon possesses the desired
mechanical characteristics. However, gallium arsenide may be the preferred
material for microwave applications.
Silicon nitride layers 22 and 24 are deposited onto the opposed major
surfaces of the actuator substrate 20. The silicon nitride layers are
utilized to pattern the actuator substrate. For example, a pair of legs
may be formed to provide a central region 26 between suspensions 28 and
30. The first silicon nitride layer 22 is patterned to selectively etch
the semiconductor material to form a sloping wall 32 to the central
region. The second silicon nitride layer 24 is patterned to etch the
actuator substrate 20 from the opposite direction. Other materials may be
used instead of silicon nitride, as is readily understood by persons
skilled in the art.
The lower surface of the actuator substrate 20 includes a chromium/nickel
layer 34 and a thick electroplated nickel layer 36. In one embodiment,
chromium is sputtered onto the silicon nitride layer 24, whereafter nickel
is sputtered onto the chromium. The chromium/nickel layer 34 then acts as
a seed layer for electroplating the thick nickel layer 36.
The chromium-nickel layer 34 and the plated nickel layer 36 are patterned
to form the configuration shown in FIG. 2. The suspensions 28 and 30 are
each shown as a pair of spring members that extend from the suspended
central region 26 to the thicker outer region of the actuator substrate
20. Dashed lines are included to show the sloped walls 32 formed by
etching the substrate to provide the suspended central region 26.
Optionally, the two spring members of a suspension may be connected
together to increase the stiffness of the suspension.
The plated nickel layer includes a pair of input/output pads 38 and 40. In
operation, a source of current is connected to the input/output pads to
induce current flow between the pads via the central region 26. The
electrical pathway may be considered as originating at pad 38 and
extending along a first U-shaped nickel region 42 to the suspension 28.
Because the spring members of the suspension 28 are thick plated nickel,
the current flow through the central region 26 progresses without a
significant voltage drop. However, the central region includes four
serpentine heaters 44, 46, 48 and 50 that are formed of the thin,
chromium/nickel layer described above. Two of the heaters 44 and 48 are
shown in FIG. 1. The metal on the opposed sides of the four heaters is
removed to define each heater as a flow path for conducting current from
one area of plated nickel to another area of plated nickel. The electrical
current through the thinner heaters generates localized heating which then
is conducted through the surrounding plated nickel and the semiconductor
material of the actuator substrate 20. The difference in coefficients of
thermal expansion of the plated nickel layer and the semiconductor
material generates stresses that induce deflection of the central region
26. The central region may be considered as comprising two legs and a
reciprocating center. A first leg includes heaters 44 and 46 that are
electrically parallel and the second leg includes electrically parallel
heaters 48 and 50, but the legs are in series. Arrows are shown in FIG. 2
to indicate current flow through the heaters.
The current flow path from the heaters 48 and 50 includes the second
suspension 30 and a second U-shaped nickel region 52. The input/output pad
40 is connected to the second U-shaped nickel region 52.
The serpentine heaters 44-50 may be formed to provide 200 mW power at under
2 V, with an electrical current under 200 mA, but the desired voltage,
current and heater resistance will vary according to the desired distance
of travel for achieving the switching in accordance with the invention.
More fundamentally, heat may be generated by other means. While it is
convenient to conduct current through the plated and unplated areas of the
metallization, it is possible to form separate thin film resistors above
or below the "bi-metallic layers" comprising the nickel and the
semiconductor material of the central region. Current flow would be then
primarily through the thin film resistors.
As previously noted, the central region 26 may be considered as having two
legs. Optionally, a greater number of legs may be incorporated. Increasing
the number of legs will increase radial symmetry. However, an increase in
the number of legs will also require additional area.
Returning to FIG. 1, heat generated as a result of current flow through the
heaters 44 and 48 is conducted to the plated nickel 36 and to the
semiconductor material of the central region 26. The difference in
coefficients of thermal expansion results in deflections that cause the
central region to bow downwardly. While the preferred materials include a
silicon substrate and include electroplated nickel on a nickel/chromium
seed layer, other materials may be used, as long as coefficients of
thermal expansion are sufficiently different to ensure that arcing is
induced by the "bi-metallic effect" when the actuator substrate is heated.
The Young's moduli of the bi-metallic layers 26 and 36 should be
sufficiently high to ensure that the force generated by the bi-metallic
effect achieves the desired electrical switching. Another concern in
selecting the layer 36 regards the melting point of the material. The
layer 36 should have a sufficiently high melting point to ensure that
plastic deformation does not occur during deflection of the central region
26 of the actuator substrate 20. Electroplated nickel is the preferred
material. Theoretically, copper may be substituted. While aluminum has
been used in other bimorphic structures, aluminum has a low yield strength
in comparison to nickel and copper and is therefore less suitable over
time.
While not critical, the thickness of the nickel layer is approximately
equal to the thickness of the actuator substrate at the flexible legs. An
acceptable thickness of the nickel layer and the silicon "layer" within
the legs is approximately 20 .mu.m. The end-to-end length of the central
region 26 may be 10,000 .mu.m. The width of a leg may be 2,800 .mu.m.
Between the signal substrate 10 and the actuator substrate 20 is an
interposer substrate 54. The design and the choice of materials for
forming the interposer substrate are largely dictated by thermal
considerations. When the interposer substrate 54 is connected to the
actuator substrate 20, the flow of heat to the interposer substrate should
be minimal. Any heat flow to the interposer substrate will increase the
power requirements of the switch 11. Moreover, the microwave performance
of the signal substrate 10 may be compromised, particularly if electrical
circuitry is incorporated onto the signal substrate. Thus, the interposer
substrate adds more ground planes to the overall device and provides more
flexibility to the designer when setting impedance values and isolation.
The most likely candidates for forming the interposer substrate 54 are
polyimide, quartz and silicon. Polyimide may be the preferred material,
since it has a low thermal conductivity and is inexpensive. However, there
is some difficulty with proper alignment of the polyimide substrate with
the signal substrate 10, which is formed of a different material.
Alignment will be addressed more thoroughly below. A quartz substrate has
a thermal conductivity between that of polyimide and silicon. A concern in
the use of a quartz substrate is the ability of a quartz suspension to
withstand long-term fatigue. It is likely that use of a quartz interposer
would include formation of a central boss, as will be described with
reference to FIG. 3. A silicon substrate may also include a central boss,
so that thermal losses from the central region of the interposer substrate
outwardly and downwardly to the signal substrate 10 can be controlled.
Further control of such thermal losses can be achieved by depositing or
growing a dielectric, such as a thermal oxide having a thickness of at
least 2 .mu.m, on both the top and bottom of the substrate and by limiting
the contact area between the interposer substrate and the actuator
substrate. Another property that the interposer substrate 54 can provide
is scrubbing action. If a central boss is formed and is connected to the
body of the interposer substrate via spiral members, the boss will rotate
as it is pressed downwardly by the actuator substrate 20 into contact with
the signal lines 12 and 14 of the signal substrate 10. The scrubbing
action will occur as a signal line 62 on the interposer substrate contacts
the signal lines 12 and 14.
In FIG. 1, a raised region 56 is designed to make contact with the portion
of the actuator substrate that is to be deflected. The raised region 56
better ensures that the displacement of the actuator substrate is
transferred to the interposer substrate 54. Preferably, the raised region
should minimize the thermal connection between the two substrates 20 and
54. Thus, the raised region should have a minimal width and should be made
of a material having a low thermal conductivity, such as polyimide.
Optionally, the raised surface may be replaced with a structure extending
downwardly from the central region 26 of the actuator substrate 20.
On the upper surface of the polyimide interposer substrate 54 is a ground
plane 58. The ground plane is a conductive film, such as a gold film, to
provide shielding for the transmission of microwave signals. Atop the
ground plane 58 is a dielectric layer 60, such as an additional layer of
polyimide, that electrically insulates the ground plane from the plated
nickel layer 36. The signal line 62 defines a contact region of the
interposer substrate 54. The signal line 62 is shown as being formed
directly onto the polyimide interposer substrate, on a side of the
interposer substrate opposite to the ground plane 58. If the interposer
substrate were made of silicon, the signal line should be separated from
the substrate by a dielectric layer, in order to ensure proper electrical
isolation from the ground plane 58.
Coupled to the interposer substrate 54 is an alignment member 64. The
alignment member may be formed of polyimide, but this is not critical. The
raised region 56 is preferably formed at the same time as the alignment
member 64. However, it is possible to form the raised region on the plated
nickel layer 36 of the actuator substrate 20, rather than on the
interposer substrate.
The alignment member 64 includes projections 66 and 68 that are spaced
apart by a distance to receive an alignment ridge 70 formed in the plated
nickel layer of the actuator substrate 20. The alignment ridge 70 is also
shown in FIG. 2. This structure provides the desired alignment of the
interposer substrate to the actuator substrate. While not shown, alignment
between the interposer substrate and the signal substrate 10 is achieved
similarly by means of downward projections from a spacer 72, with the
downward projections being fitted into a patterned structure on the signal
substrate. The patterned structure on the signal substrate should be
positioned to avoid the signal traces 12 and 14. Alternatively, the three
substrates can be aligned by positioning the substrates in an alignment
frame.
When assembled, the signal substrate 10, the interposer substrate 54, and
the actuator substrate 20 are connected together and deflection of the
central region 26 of the actuator substrate causes the signal line 62 of
the interposer substrate to come into contact with both of the signal
lines 12 and 14 on the signal substrate. Current through the heaters 44-50
causes deflection of the central region 26 by means of the bi-metallic
effect, with the deflection being in a downward direction to press the
signal line 62 into electrical contact with the signal lines 12 and 14.
Termination of the actuating current to the heaters allows the materials
of the actuator substrate to contract, returning the structure to the
relaxed position shown in FIG. 1.
A second embodiment of the invention is shown in FIG. 3. A signal substrate
10 is identical to the one described with reference to FIG. 1, so that the
reference numerals are repeated. The actuator substrate 20 includes most
of the features of the actuator substrate of FIG. 1. However, the
chromium/nickel layer and the electroplated nickel layer are shown as a
single layer 74. This layer 74 has a uniform thickness, other than at
heaters 76 and 78, which comprise merely the chromium/nickel seed layer in
order to provide the necessary resistance for generating heat. A portion
of a suspension 28 is shown in greater detail in FIG. 4. A spring member
79 includes an elbow 80 that connects first and second arm portions 81 and
82. The suspension serves three roles. Firstly, the suspension provides a
degree of thermal isolation of the legs from the stationary portion of the
actuator substrate 20. This reduces the amount of thermal energy needed
for a desired deflection of the legs. Secondly, the suspension provides
rotational flexibility at the end of a leg. The flexibility accommodates
the movement experienced as the leg expands and arcs during heating cycles
and contracts during relaxation. Thirdly, the suspension provides lateral
flexibility in addition to the rotational flexibility, so that the
tendency of a leg to pull inwardly as the leg arcs can be accommodated.
The embodiments of FIGS. 1-3 are structures in which the switch is normally
open. Optionally, a normally closed switch can be fabricated, in which an
actuator substrate moves away from a signal substrate when current is
caused to flow through one or more heaters. In a normally closed
embodiment, the suspensions are moved away from the outside boundaries of
the legs. When the suspensions are at the ends of the legs nearest the
center, arcing of the legs will cause movement in the opposite direction
of the embodiments of FIGS. 1 and 2.
An interposer substrate 84 of FIG. 3 includes a downwardly depending boss
85 that is aligned with the central region 26 of the actuator substrate
20. Deflection of the central region in a downward direction causes the
boss 85 to move in the direction of the signal substrate 10. A pair of
legs 86 and 87 connect the boss to the stationary portion of the
interposer substrate. Suspensions 89, similar to the suspensions 28 and 30
of FIGS. 1-3, connect the legs 86 and 87 to the stationary portion of the
interposer substrate.
On the opposed major surfaces of the interposer substrate 84 are silicon
nitride layers 91 and 93. In a microwave environment, the silicon nitride
layer 91 on the upper surface insulates the plated layer 74 of the
actuator substrate 20 from a ground plane 95 on the interposer substrate.
The ground plane is a conductive film that provides shielding for the
transmission of microwave signals along signal lines 12 and 14.
The downwardly depending boss 85 forms a contact region for selectively
connecting the two signal lines 12 and 14 on the signal substrate 10. A
signal line 97 is aligned with respect to the signal substrate to
electrically connect the signal lines when the legs 86 and 87 of the
interposer substrate 84 are flexed by deflection of the central region 26
of the actuator substrate 20. That is, current through the heaters 76 and
78 on the actuator substrate causes deflection of the central region by
means of the bi-metallic effect, with the deflection being in a downward
direction to press the signal line 97 into electrical contact with both
the signal line 12 and the signal line 14. Termination of the current
through the heaters 76 and 78 allows relaxation of the materials, thereby
opening the current path.
In the embodiment of FIG. 3, the interposer substrate 84 is more
efficiently thermally isolated from the actuator substrate 20 than in the
embodiment of FIG. 1. The suspensions 89 and the downwardly depending boss
85 limit contact between the interposer substrate and the signal substrate
10 to contact with a spacer 99, with the suspensions 89 limiting thermal
communication between the boss 85 and the spacer 99. When the switch is in
the closed position, there is also thermal contact between the conductor
area of the interposer substrate and the signal lines 12 and 14 of the
signal substrate, which can be reduced by the use of the boss shown in
FIG. 3 and by the use of additional dielectric layers, such as thick (>2
.mu.m) silicon dioxide. The embodiment of FIG. 3 adds some complexity, but
may be preferred if the interposer substrate does not have a sufficiently
low thermal conductivity to ensure proper thermal isolation of the signal
substrate.
As previously noted, the preferred embodiments have actuator central
regions that include a plurality of legs. A less desirable embodiment is
one in which the central region is a circular structure. In general, the
force required to deflect a circular diaphragm includes both a term that
increases linearly with displacement and a term that increases as the cube
of displacement. For displacements less than approximately the thickness
of the diaphragm, the linear term is dominant and the diaphragm is
considered to act as a rigid plate. However, for displacements
significantly greater than that of the thickness of the diaphragm, the
cube term dominates and the element is considered to act as a thin,
flexible diaphragm. In the cube-law region, the force required to achieve
a given increment of additional deflection builds up rapidly. To double a
deflection, the deflection force must be increased as a factor of eight.
Because the preferred embodiments include legs, a structure is formed
which substantially avoids the cube-law disadvantage. However, with
appropriate modifications, the diaphragm approach may be used.
Referring now to FIG. 5, a top view of an actuator 88 of a 1.times.4 switch
is shown schematically. A single input line 90 can independently be
connected to any one of four output lines 92, 94, 96 and 98. The input
line, the output lines and lines 100, 102, 104, 106 and 108 are all shown
in phantom, since in the preferred embodiment the lines are conductive
traces on a signal substrate positioned below the actuator substrate 88
and an interposer substrate, not shown.
Each of the output lines 92, 94, 96 and 98 extends beneath the first leg
110 of an individually activated actuator 112, 114, 116 and 118. The
traces 102-108 that are connected to the input signal 90 on the signal
substrate extend below second legs 120 of the actuators 112-118.
Between the first leg 110 and the second leg 120 of each actuator 112-118
is a bridge 122. Each of the first and second legs includes a heater of
the type described above, while the bridge may have a uniform coating of
metal. Optionally, the bridge 122 is void of conductive material.
The bridge 122 of each actuator 112-118 is aligned with the contact region
of the interposer substrate, not shown. As described above, the contact
region has a conductive trace that is brought into electrical contact with
signal traces on the signal substrate. Conductive traces 124, 126, 128 and
130 are shown in phantom to represent the conductive traces of the four
contact regions aligned below the four actuators 112-118. Thus, when the
bridge of one of the actuators is caused to move downwardly by means of
the bi-metallic effect created by conducting current through the heaters
on the first and second legs 110 and 120, the bridge presses the
operatively associated conductive trace 124-130 to connect the input line
90 with one of the output lines 92, 94, 96 or 98.
For simplicity, the heaters are omitted from FIG. 5 and the suspensions 132
and 134 of the first and second legs to the stationary portion of the
actuator substrate 88 are shown schematically. In practice, the heaters
will be on the lower surface of each of the first and second legs 110 and
120. The heaters will only occupy a portion of each leg. While not
critical, the heater may be centered on the lower portion of the leg.
As shown in FIG. 5, the conductive trace 124-130 of each contact region is
larger than the two lines 92, 94, 96 and 98 and 102-108 to which the
conductive trace must connect. Optionally, the signal lines on the signal
substrate may include enlarged areas, or stubs, at the regions which are
to connect to the conductive traces 124-130. The stubs increase the
likelihood that proper electrical connection is obtained. The signal lines
should be designed according to known microwave principles for achieving
desired characteristics, e.g., proper isolation of signals and proper
avoidance of reflection. For example, terminators may be used to prevent
reflections along an open circuit.
While the input traces 102-108 and the output traces 92, 94, 96 and 98 are
shown as extending parallel to the length of the four actuators 112-118,
this is not critical. Optionally, the traces to be connected may enter at
angles to the lengths of the actuators. For example, traces 92 and 102 may
be perpendicular to the length of actuator 112 and may not enter the
regions below the two legs 110 and 120. In this case, the conductive trace
124 on the contact region below the actuator 112 will extend across the
contact region in a direction perpendicular to the direction illustrated
in FIG. 5.
A third embodiment of a signal switch is shown in FIG. 6. Because the
signal substrate 10 is identical to the signal substrate of FIG. 1,
reference to a ground plane layer 16, upper and lower insulating layers 17
and 18, and signal traces 12 and 14 are made using the same numerals as
employed in FIG. 1.
An actuator substrate 136 is positioned atop the signal substrate 10. In
this embodiment, the actuator substrate remains in an "upright" position,
rather than in the inverted position of FIG. 1. Silicon nitride layers 138
and 140 are deposited onto the opposed major surfaces of the actuator
substrate. In a microwave environment, the silicon nitride layer 140 on
the lower surface remains in the form of a flexible diaphragm when the
actuator substrate is etched to define a pair of legs 142 and 144. A
conductive film 146, such as a gold film, on the lower surface of the
silicon nitride layer 140 is electrically grounded to provide shielding
for the transmission of microwave signals. Openings 148 in the layer 140
and the conductive film 146 permit the flow of etchant used to define the
legs 142 and 144.
A dielectric layer 150 isolates a signal line 152 from the conductive film
146 that operates as a ground plane. A downwardly depending boss 154 and
the signal line 152 are aligned to electrically connect the two signal
lines 12 and 14 on the signal substrate 10 when the legs 142 and 144 are
flexed by the bi-metallic effect of heating the legs and a plated nickel
layer 156 and 158 in the same manner as described with reference to FIG.
1.
The signal line 152 moves upwardly and downwardly in correspondence with
movement of the boss 154. In the relaxed condition of FIG. 6, a spacer 160
prevents contact of the signal line 152 with the signal lines 12 and 14 on
the signal substrate 10. A suspension structure connects the inward ends
of the legs 142 and 144 to the boss. Thus, in contrast to FIGS. 1-3, the
suspensions are at the ends of the legs opposite to the stationary portion
of the actuator substrate 136. Because the suspension structure is at the
end of the legs nearer the boss 154, heating the legs and the nickel layer
156 and 158 causes a downward arcing of the legs, moving the signal line
152 toward the substrate 10.
As previously noted, a power-to-open embodiment may be fabricated, so that
a relaxed actuator substrate electrically connects signal lines. For
example, a power-to-open switch may be formed by reducing | | |
