Note: Descriptions are shown in the official language in which they were submitted.
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BISTABLE MICRO-SWITCH AND METHOD
OF MANUFACTURING THE SAME
BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
This invention relates in general to micro-switches and, more particularly, to
a
micro-machined bistable switch using a shape memory alloy.
2. DESCRIPTION OF THE RELATED ART
The first electro-mechanical and solid state micro-switches were developed in
the late 1940's. Since that time, the electronics industry has pushed the
manufacturing and functional limits for producing such switches. In
particular,
current electro-mechanical micro-switches exhibit technical inadequacies in
size, cost,
function, durability, and connection techniques for high frequency
applications. In
turn, solid state switches exhibit a characteristically high off state to on-
state
impedance ratio, and for many applications, undesirably high values of on-
state
"contact" resistance in off state coupling capacitance. Consequently, the
electronics
industry is currently looking into new and innovative ways to manufacture
switches
that can be smaller, more reliable, durable, functional, and cost efficient.
In a variety of present day and predicted circuit applications, a need exists
for
low cost, micro-miniature switching devices that can be fabricated on
conventional
hybrid circuit substrates or boards and have bistable capabilities. In
addition, the
manufacturing process for these devices should be compatible with conventional
solid
state techniques such as thin-film deposition and patterning procedures used
to form
the conductive paths, contact pads and passive circuit elements included in
such
circuits.
A shape memory alloy ("SMA") is a known material capable of undergoing
plastic deformation from a "deformed" shape to a "memory" shape when heated.
If
the SMA material is then allowed to cool, it will return partially to its
deformed shape
and can be fully returned to the deformed shape. In other words, the SMA
material
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undergoes a reversible transformation from an austenitic state to a
martensitic state
with a change in temperature.
Research and development companies have only touched the surface of how
this controllable shape deformation material can be used in switching
structures. For
example, conventional electro-mechanical switches have used SMA wires as a
rotary
actuator and bent SMA sheets as a valve. The wire is twisted or torsioned
about its
longitudinal axis and the ends of the wire are then constrained against
movement.
The sheet actuators are mechanically coupled to one or more movable elements
such
that the temperature-induced deformation of the actuators exerts a force or
generates a
motion of the mechanical elements.
The problems with these and similar SMA switch configurations and
manufacturing techniques are similar to those described above for conventional
electro-mechanical switches. In particular, constraints of size, reliability,
durability,
functionality, and cost limit the usefulness of prior art SMA switches.
In closing, conventional switches and relays, with or without the use of shape
memory alloys, are normally large, bulky, or too fragile to be used for
industrial
purposes or mass production. Therefore, it would be advantageous to develop a
switch or relay that can benefit from the characteristics of a shape memory
alloy and
eliminate the problems listed above of current switching technologies that may
or may
not use a shape memory alloy.
The present invention is directed to overcoming, or at least reducing the
effects of, one or more of the problems set forth above.
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SUMMARY OF THE INVENTION
In one embodiment, the present invention provides a bistable switch. The
switch includes the following elements: a substrate having at least one power
source;
a flexible sheet having a first distal end attached to the substrate; a bridge
contact
formed at a second and opposite distal end of the flexible sheet; and at least
one heat
activated element connected to a first surface of the flexible sheet and
between the
second distal end and the power source, wherein current from the power source
passing through the heat activated element indirectly bends the flexible sheet
and
shorts the signal contacts on the substrate with a sustainable force.
Another embodiment of the present invention provides a process for
manufacturing a bistable switch for a substrate having signal line contacts
and a
power source. In particular, the process comprises providing a flexible sheet;
connecting at least one heat activated element between a first distal end of
the flexible
sheet and the power source; forming a conductive bridge contact at the first
distal end
I 5 of the flexible sheet; and mounting a second and opposite distal end of
the flexible
sheet to the substrate, wherein current from the power source passing through
the heat
activated element indirectly bends the flexible sheet and shorts the signal
contacts on
the substrate.
The inventive structure provides a relatively simple and inexpensive way to
produce bistable switches with performance levels not attainable with current
solid
state approaches using the standard semiconductor base unit, the transistor.
This new
and innovative micro-machine way of fabricating micro-switches will enable the
users
to build systems that can carry very high voltage, current, and frequency
signals. This
becomes possible since the micro-switch is conceptually equivalent to a micro-
relay.
In fact, this micro-switch is a mechanical micro-structure that moves to
connect or
disconnect conductive contacts. In addition, this design and method is
compatible
with standard silicon processing, allowing mass production at a reasonable
cost.
BRIEF DESCRIPTION OF THE DRAWINGS
Other aspects and advantages of the invention will become apparent upon
reading the following detailed description and upon reference to the drawings,
in
which:
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Figure 1 illustrates a perspective view of a bistable switch in accordance
with
one embodiment of the present invention;
Figure 2 illustrates a general schematic layout of the inventive bistable
switch
of Figure 1;
Figures 3A and 3B - SA and SB illustrate a process for manufacturing the
bistable switch of Figure 1;
Figures 6A and 6B illustrate an alternative process step for manufacturing the
bistable switch of Figure 1 to include a crimped arm portion;
Figures 7A and 7B shows the bistable switch of Figure 6A mounted and
activated to illustrate a first and a second switch position;
Figure 8 illustrates an alternative embodiment of the bistable switch of
Figure
1 to include multiple bridge contacts; and
Figures 9A and 9B illustrate still another embodiment of the inventive
bistable
switch.
I S While the invention is amenable to various modifications in alternative
forms,
specific embodiments thereof have been shown by way of example in the drawings
and are herein described in detail. It should be understood, however, the
description
herein of specific embodiments is not intended to limit the invention to the
particular
forms disclosed, but on the contrary, the intention is to cover all
modifications,
equivalents, and alternatives falling within the spirit and scope of the
invention as
defined by the appended claims.
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DETAILED DESCRIPTION OF THE INVENTION
The present invention employs the unique properties of a shape memory alloy
("SMA") with recent advances in micro-machining to develop an efficient,
effective
and highly reliable micro-switch. The use of an SMA in micro-switches
increases the
5 performance of switches or relays by several orders of magnitude. In
particular, this
is accomplished because both stress and strain of the shape memory effect can
be very
large, providing substantial work output per unit volume. Therefore, micro-
mechanical switches using SMA as the actuation mechanism can exert stresses of
hundreds of megapascals; tolerate strains of more than three percent; work at
common
TTL voltages that are much lower than electrostatic or PZO requirements; be
directly
powered with electrical leads on a chip; and survive millions of cycles
without
fatigue.
Shape memory alloys undergo a temperature related phase change beginning
at temperatures above TA, which can be characterized by the ability of the
alloy to
recover any initial shape upon heating of the alloy above a temperature TA and
below
TH, regardless of mechanical deformation imposed on the alloy at temperature
below
TA . In operation, when the SMA material is at a temperature TL, below
temperature
TA, the SMA possesses a particular crystal structure whereby the material is
ductile
and may be deformed into an arbitrary shape with relative ease. Upon heating
the
SMA to a temperature TH, above temperature TA, the crystal structure changes
in
order to restore the SMA back to an initial, undeformed shape, to resume the
originally imparted shape, thereby representing the onset of a restoring
stress.
Consequently, the transition temperature range of a shape memory alloy, over
which
the phase transformation occurs, is defined as being between TH and TA. The
SMA is
optimally deformed between 2 and 8% at temperatures below TA which deformation
can be fully recovered upon heating of the SMA to between TA and TH. One
preferred
deformation is 4%.
These memory materials have been produced in bulk form primarily in the
shape of wires, rods, and plates. The most conventional and readily available
shape
memory alloy is Nitinol, an alloy of nickel and titanium. However, other SMAs
include copper-zinc-aluminum, or copper-aluminum-nickel. With a temperature
change of as little as 18°C, Nitinol can go through its phase
transformation and exert a
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very large force when exerted against a resistance to changing its shape. As
discussed
earlier, conventional switches and relays that use shape memory alloys
generally
operate on the principle of deforming the shape memory alloy while it is below
phase
transformation temperature range. Heating the deformed alloy above its
transformation temperature range recovers all or part of the deformation, and
the
motion of the alloy moves the necessary mechanical elements.
Turning now to the drawings, Figure 1 illustrates a thermally-actuated
bistable
micro-mechanical switch 10 in accordance with one embodiment of the present
invention. Actuating arm 12 of switch 10 is micro-machined and secured to an
upper
substrate surface 14. Substrate 14 could include an insulated silicon or
gallium-
arsonide substrate, a printed circuit board, a flat plate of a ceramic
material such as
high density alumina (A1203) or beryllia (Be0), or a glassy material such as
fused
silica. However, persons of ordinary skill in the relevant arts should
appreciate that
the present inventive switch is not so limited, and therefore can be mounted
to nearly
any stable structure to provide the desired cantilever style bistable switch.
Upper surface 14 provides control contacts 16a, 16b and ground contact 18 to
securely interconnect the respective control and ground contacts of arm 12. In
addition, upper substrate surface 14 provides signal contacts 20a and 20b to
be
bridged or shorted by conductive bridge contact 22 of arm 12. Signal contacts
20a
and 20b may carry or support any electrical signal, including, for example,
conventional analog or digital data, or voice signals.
Top and bottom conductive path elements 24a and 24b couple to arm 12 by a
conventional technique, and the two SMA elements 26a and 26b mount between the
contact and ground vias on the top and bottom center beam of arm 12. In one
embodiment, SMA elements 26a and 26b are made from a wire of a titanium nickel
alloy having a diameter of between about 25 and 125 microns.
During operation the above inventive switch provides the basic circuit
structure as illustrated in Figure 2. In particular, when relay 30a is closed
and relay
30b is open, current passing through the top conductive horseshoe-type path,
composed of elements 16a, 24a, 26a, and 18, will move arm 12 upward. In
contrast,
when relay 30a is open and relay 30b is closed, current passing through the
bottom
conductive horseshoe-type path, composed of elements 16b, 24b, 26b, and 18,
will
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move arm 12 downward. The force present during the thermal cooling stage is
much
less than the force present while an SMA element is being heated. In other
words,
conductive means, to be described in detail below, transfers the necessary
power from
either control contact 16a or 16b through conductive path element 24a or 24b
and
SMA element 26a or 26b, respectively, to ground contact element 18. For the
below
embodiments, SMA elements 26a and 26b will preferably have a diameter of
between
about 25 and 125 microns and can be supplied with 40 to 160 milliamps during
operation.
Referring now to Figures 3A - 3B through 6A - 6B, the manufacturing process
for fabricating the bistable switch according to the present invention will
follow. In
particular, Figure 3A, 4A, SA and 6A illustrate the bottom surface of switch
10, and
Figures 3B, 4B, SB and 6B illustrate the respective side views of the same
Figures.
Figures 3A and 3B illustrate a stabilizing material 50 coated with a patterned
photoresist layer 52. In this particular embodiment, stabilizing material 50
is a
beryllium copper alloy that is manufactured in rolled sheets having a
thickness
between about 12 to 50 microns and a width of between about 300 to 1,200
microns.
However, other materials may be used that provide the desired elastic or
flexible
properties and thickness. For example, materials selected from the group
including
polyresin, plastic, wood composites, silicon, silicon resin, and various alloy
materials
such as a stainless steel alloy may be used.
In a preferred micro-machining process, a conventional photolithographic
technique is used to define the desired pattern onto the surface of
stabilizing material
50 (pattern represented by dotted lines). In particular, patterned photoresist
52 defines
a three beam structure having a tail portion 54 and a head portion 56, contact
vias 58a
and 58c, and two gaps 60a and 60b to define beams 62a, 62b, and 62c. A
conventional etching technique removes stabilizing material 50 unprotected by
pattern
photoresist 52 to form the desired three beam structure 12 as illustrated in
Figure 4A.
Persons of ordinary skill in the relevant art will appreciate that the desired
pattern can be formed by other conventional methods. For example, if the
desired
switch size is large enough to avoid micro-machining techniques, stabilizing
material
50 could be patterned by a conventional punch or molding process.
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Next, as illustrated in Figures 4A and 4B, a nonconductive insulation layer 64
coats the top and bottom surface of structure 12. This electrical insulator is
preferably
a paralene layer. In alternative embodiments, insulation material 64 could be
selected
from the group including silicon dioxide, polyimide, wet oxide, and silicon
nitride
layer. These alternatives will provide a similar structure having similar
operational
characteristics. Persons of ordinary skill in the art will appreciate that
insulation layer
64 may be eliminated if stabilizing material 50 is a nonconductive material.
On each side of coated structure 12, a conductive material, such as gold, is
deposited and patterned to create a portion of the desired horseshoe-type
path. More
specifically, the top surface of coated structure 12 (see Figure 1) provides
an L-shaped
conductive path 24a coupled between control via 58a and top contact pad. In
addition, the same conductive material forms ground via 58c. On the opposite
or
bottom side of structure 12, as illustrated in Figure 4A, coated structure 12
provides
another L-shaped conductive path 24b coupled between control contact 68b and
bottom contact pad 58b. In addition, the same material forms control contact
68a,
ground contact 70 and bridge contact 22. Persons of ordinary skill in the
relevant arts
should appreciated that the conductive material for conductive paths 24a and
24b,
control contacts 68a and 68b, ground contact 70, ground and control vias 58a
and
58c, top and bottom contact pads 58b, and bridge contact 22 may be selected
from the
group of gold, copper, palladium-gold alloy, nickel, silver, aluminum, and
many other
conductive materials available in the art.
With reference to Figures SA and SB, an actuator element 26a and 26b
securely couples to the top and bottom surfaces of arm 12 between each contact
pad
and ground via 58c. If desired, an adhesive material (not shown) can be used
to couple
actuator elements 26a and 26b to respective top and bottom arm surfaces. The
adhesive material could be selected from the group including cement, epoxy,
lock on
chip tap, solder, embedding, polyimide, and mechanical attachment such as a
clip or
clamp. This connection positions each actuator element 26a and 26b over a
central
portion of the top and bottom surface of middle beam 62B to complete the
conductive
horseshoe-type path. Actuator elements 26a and 26b are preferably a nickel-
titanium
SMA provided in a sheet, ribbon, or wire.form. For the above embodiments, SMA
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elements 26a and 26b will preferably have a diameter of between about 25 and
125
microns.
As disclosed earlier, SMA elements 26A and 26B extend or contract after
current passing through the material reaches a preestablished phase
transformation
temperature. With this particular embodiment, the phase transformation process
will
typically occur by one of two methods. A first phase transformation technique
reduces the bulk volume of the actuation material, and as a result, the length
of the
shape memory alloy will reduce, contracting stabilizing material 12. In a
second
phase transformation technique, SMA is stretched by a percentage not exceeding
8%
before and/or after it is installed to stabilizing structure 12. Upon phase
transformation, the length of SMA will reduce, going back to its original
length before
contracting the stabilizing material 12 layer even more, up to 8%. Depending
on the
requirements on the displacement of head portion 12a, contact force, number
cycles,
and manufacturing processes, the shape memory alloy may or may not be
stretched.
The last steps of the desired process includes crimping and mounting the
above structure. Without the crimping step, the above structure can be mounted
to a
desired substrate to form a reliable micro-machined bistable switch having a
cantilever structure as illustrated in Figure 1. In turn, the switch cannot
continuously
short the signal contacts unless power is active to generate the necessary
current and
transformation within the desired SMA element. Consequently, this final
coining or
crimping step will allow the active device to maintain a contact position,
even after
the power is deactivated. This coining or crimping, therefore, provides a snap
action
function to the arm that maintains the arm in a given position, except when
one of the
SMA elements flips the arm to the opposite position.
Referring to Figure 6A and 6B, the desired coined or crimped elements 80A
and 80B are illustrated. This snap action structure may be formed using a
conventional punch and dye method. More specifically, a central portion of
left and
right beams 62A and 62C are crimped to form a wave-type deformation or
ungulation.
To persons skilled in the relevant arts, this crimped area 80A and 80B will
create a
sustainable force when actuator element 26a or 26b transforms to move arm tip
12a
up or down. In turn, crimped areas 80A and 80B will allow bridge contact 22 to
maintain contact with or separation from signal contacts 20a and 20b even
after the
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source coupled to switch 10 is deactivated. In other words, by forming crimps
80A
and 80B, once arm 12 is positioned up or down, current must pass through the
appropriate SMA element to bend arm 12 to the other position, down or up
respectively. Otherwise, switch 10 will always be positioned up or down unless
it is
5 physically moved by the user.
With or without a crimp element formed on first and third beams 62A and
62C, the resultant structure must be secured to substrate 14, as illustrated
in Figures
7A and 7B or Figure 1. In particular, cantilever switch 10 couples to
substrate surface
14 by a conventional bonding method. In particular, solder or pressure slots
of a
10 printed circuit board are used to attach and secure power and ground
contacts 16a,
16b, and 18 to substrate surface 14 of switch 10. Consequently, when actuating
element 26b is heated by the bottom horseshoe-type conductive path, the
resultant
structure will bend downwards to couple bridge contact 22 with signal contacts
20a
and 20b. In turn, when actuating material 26A is heated by the top horseshoe-
type
conductive path, the connection between bridge contact 22 and signal contacts
20a
and 20b will be broken.
Another embodiment of the present invention would include the placement of
an additional bridging contact 22' on the top surface of tip 12a for shorting
complementary signal contacts 20a', 20b' on a multiple layer substrate. With
this
example as illustrated in Figure 8, if the top SMA element 26a is heated by an
electrical current passing through the top horseshoe-type conductive path, the
structure will move up to couple top bridging contact 22' with top signal
contacts
20a' and 20b'. On the other hand, if actuator element 26B is heated by an
electrical
current passing through bottom horseshoe-type conductive path 24b and 26b, the
structure will move down to couple bridging contact 22 with signal contacts
20a and
20b. With this particular embodiment, arm 12 is not crimped. Consequently,
bridge
contacts 22 or 22' will only be able to continually short signal contacts 20a,
20b or
20a', 20b' while the respective SMA 26a or 26b is heated to move tip 12a up or
down. However, those skilled in the art will recognize that crimping could be
used to
maintain the arm 12 in contact with one or the other of contacts 20a and 20b
or 20a'
and 20b'.
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Figures 9A and 9B illustrate another embodiment of the above inventive
switch. In this embodiment, sheet 50 is patterned and etched or punched to
form the
desired arm 12 as described above with reference to Figure 3B, and bridge
contact 22
is formed (as described above) on arm tip 12a. Next, a central portion of
actuator
element 60 is looped over or attached to arm 12 at a location adjacent to tip
12a and
electrically separated from bridge contact 22. Lastly, tail portion 54 of arm
12 is
attached to substrate surface 14 and ends 62a and 62b of actuator element 60
are
extended in a horizontally opposed direction adjacent the length of arm 12 to
connect
with a power source 64 adjacent substrate surface 14. In other words, the
conductive
L-shaped path and contacts formerly located on arm 12 to provide the necessary
circuit to activate SMA element (see Figure 1 ) has been moved to a location
off of
switch arm 12, to provide power source 64.
Referring now to Figure 9B, during operation, a current supplied to SMA 60
by source 62 contracts SMA 60 to move arm 12 down and short signal contacts
20a
and 20b with bridge contact 22. As described in the above disclosure, with
power
source 62 deactivated, SMA 60 will return to a position that will separate
bridge
contact 22 from signal contact 20a and 20b. The skilled artisan will
appreciate that
another SMA (not shown) may be attached in a similar way to arm 12, but on an
opposite side to SMA 60, and supplied current by a similar power source. In
turn arm
12 can be crimped to form a device that will function as described above with
reference to Figures 7A and 7B, and arm 12 can be patterned with or without
multiple
parallel beams. With this particular embodiment, a single coining or a
complete
surface crimp may be used if there are no beams on arm 12 and an additional
SMA
element is attached to or wrapped around the other side of arm 12.
With respect to the above embodiments, it will be appreciated by persons of
ordinary skill in the relevant arts that arm 12 can be patterned to form a
structure
having as many beams as necessary to hold any desired SMA element(s). In turn
arm
12 could be patterned to form only a rectangular structure having no beams. On
a
similar note, the thickness and number of SMA elements 26a and 26b can
increase or
decrease to accommodate the desired arm structure and force necessary to move
the
same when heated. Additionally, the number of crimps formed on flexible arm 12
will depend on the shape and functional characteristics of the resultant
switch.
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In summary, this invention provide a relatively simple and inexpensive way to
produce micro-switches and relays. This new and innovative micro-machine way
of
fabricating micro-switch and relays will enable a user to build systems that
can carry
very high voltage, current, and frequency signals. Additionally, this
inventive process
can conceptually be designed to be compatible with standard silicon processing
and
allow mass production of the device at very reasonable cost. Consequently, the
inventive structure provides a miniature bistable snap action electro-
mechanical
switch that can be activated by a shape memory alloy which possess a unique
capability for increase speed actuation and forces relative to any prior art
switching
mechanism. In addition, because of the advances in micro-machining, this
structure
can be produced to have a length similar to between about 500 - 3,000 microns,
a
width between about 200 - 1,200 and between about 25 -35 microns thick, which
is
smaller than any competing bistable switches on the market today. A skilled
artisan
will appreciate that these dimensions may change to obtain the desired size
and
functional characteristics for the inventive switch.
Other variations in design still coming within the inventive concept claimed
herein will be apparent to those skilled in the art. For example, the
illustrative
embodiments described herein employ SMA elements 26a and 26b as part of the
conductive path for heating the SMA elements to accomplish the same end. For
example, the SMA elements could be coupled to a separate electrically
conductive
element, or they could be coupled to an entirely different sort of heating
element (e.g.,
non-electrical).
Illustrative embodiments of the invention are described above. In the interest
of clarity, not all features of an actual implementation are described in the
specification. It will be of course appreciated that in the development of any
such
actual embodiment, numerous implementation-specific decisions must be made to
achieve a developer's specific goals, such as compliance with system-related
and
business-related constraints, which will vary from one implementation to
another.
Moreover, it will appreciated that, although such a development effort might
be
complex and time-consuming, it would nonetheless be a routine undertaking for
those
of ordinary skills in the art having the benefit of this disclosure.
