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Description  |
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BACKGROUND AND SUMMARY
The present invention relates to a microminiature force-sensitive switch
capable of being trimmed to detect changes in an external condition.
Microminiature switches responsive to changes in external conditions are
known in the art, as exemplified by U.S. Pat. No. 4,543,457 to Petersen.
The switch therein described includes a silicon wafer having a
reduced-thickness deflectable membrane which moves in response to changes
in external conditions and thereby establishes contact between a common
terminal and first one then progressively more confronting terminals of
the switch device.
The Petersen apparatus teaches one embodiment of the switch in which the
deflectable member takes the form of a diaphragm etched in a silicon
substrate which is adapted to bulge from its relaxed state to a more
strained state in response to an increase in an applied external force.
This force may comprise a pressure differential, which requires that one
side of the diaphragm remain at a constant pressure. This can be
accomplished by forming a hermetically sealed chamber around this side of
the diaphragm. When the switch is to be used to measure acceleration, a
mass may be attached to the diaphragm. The diaphragm also may be modified
to include a metal layer having a substantially different thermal
coefficient of expansion than that of the diaphragm to enable the
measurement of temperature changes.
Another embodiment of the switch taught by Petersen is an elongated beam
anchored at one or both ends which deflects in a manner analogous to the
function of the diaphragm in digitizing or monitoring temperature or
acceleration.
It is generally known in the art that switching devices may be trimmed so
as to set the threshold level of detection. Trimming has been accomplished
by chemical and laser techniques and is a major component of the total
cost of trimmed devices. The trimming techniques generally use a plurality
of external taps or pads for accessing the switching devices, and depend
for their effectiveness on the closing of switches in a particular order
for each individual device.
The prior art described above has not overcome several problems that are
addressed and solved by the present invention. For example, Petersen
teaches a switching device that is responsive to changes in external
conditions such as temperature, pressure or acceleration, yet the
fabrication and operation of the device to detect a particular threshold
level of the changing external condition is not achievable in the most
efficient and cost effective manner due to structural limitations in the
Petersen apparatus. Specifically, leads must be extended from each switch
to external taps or pads in order to calibrate the device. As a
preselected force is applied to the deflectable member, each switch must
be monitored for closure in order to determine which switch closes at a
desired deflection level. Such a necessary procedure is time consuming and
labor-intensive, which increases the cost of each device and renders the
device unsuitable for mass production.
The cost is also raised by the extra materials and parts required and by
the more frequent failure of the hermetic seal due to the need for
penetration of the seal by multiple switch leads. Moreover, because such
devices require more switch leads and taps or pads, the number of switches
on each device is physically limited. This limits the achievable precision
of detection and increases the user's cost by requiring the purchase of
more devices to accomplish a given operation. Furthermore, the probability
of failure of a device increases with increasing complexity and with the
addition of elements. The extra lead, taps, etc., also increase the
physical size of each device, which limits its application.
Although the use of fusible links in an integrated circuit is known, such
use has generally been limited to applications involving read only memory
elements or the fixing of an impedance value. In U.S. Pat. No. 4,016,483
to Rudin, for example, the impedance value in a microminiature integrated
circuit by the selective blowing of fuses is shown. The device therein
described includes fusible aluminum links in parallel with binary weighted
resistive elements. The application of electrical energy to selected
fusible links causes them to open, thereby inserting selected resistive
elements into the impedance circuit.
The prior art does not teach the use of fusible links to enable trimming of
a switching level, nor such a use that does not depend on the
designer-intended order of operation of the trimming switches.
To overcome these limitations and disadvantages of the prior art, the
present invention combines a deflectable member, a plurality of switches,
and fusible links in such a way so as to produce a device which is
compact, less costly to produce, and which can be trimmed or calibrated
after the device has been fabricated without regard to the order of
operation of the switching elements of the device as set by the design and
tooling and without requiring the leads from each switch to be available
for external connection during this calibration process.
It is therefore a general object of the present invention to provide a
microminiature switching device for detecting or monitoring of a threshold
level of an external condition such as pressure, temperature or
acceleration, the switching device being designed so that the threshold
level to be detected or monitored is easily and accurately calibrated and
so that the device is reliable in operation subsequent to its calibration.
Another object of this invention is to reduce the cost of manufacturing and
trimming such a switching device.
A further object of this invention is to provide a method for trimming the
switching device that is independent of the order of operation of the
switches of the device.
A more specific object of the invention is to provide a microminiature
switching device whose elements are contained within a hermetically sealed
chamber.
Another object of this invention is to minimize external connections to the
elements of the switching device to minimize the number of required
penetrations of the hermetic seal so as to reduce the possibility of seal
failure.
The present invention achieves these objects by providing a reliable,
low-cost, microminiature force-sensitive switching device which is capable
of being easily trimmed without the necessity of a plurality of additional
external connections to the apparatus and without regard to the
operational order of its elements, resulting in a device which accurately
and reliably senses a selected external condition within a desired range.
According to the present invention, the switching device includes a
deflectable member whose deflection is caused by changes in an external
condition to be detected, such as temperature, pressure or acceleration.
Movement of the deflectable member from a less strained to a more strained
condition causes the establishment of electrical contact between a common
contact and first one then progressively more switch terminals in the
device. The circuit is completed by the series connection of each switch
to a fusible link whose other terminal is tied in common with all other
fusible links in the switching device which is in a common terminal. The
common terminal connected to the fusible links is, in the preferred
embodiment, within the chamber formed by the hermetic sealing of a first
substrate, including the deflectable member, and a second substrate. From
the common terminal, a single conductor is passed through the seal to
enable electrical connection of the device to an external calibrating or
monitoring circuit. This reduction in the number of seal penetrations
reduces the seal failure rate.
The trimming of the device to preselect the threshold value of the external
condition to be detected is accomplished easily, accurately and reliably.
A chosen calibrating level of the external condition is applied to the
device. The resultant force causes the deflection of the deflectable
member and consequent closing of one or more switches. A voltage is then
applied to the common terminals of the device, causing a current to pass
through the closed switches and the fusible links connected in series with
them. These fusible links are permanently blown, while the fusible links
connected in series with the switches which did not close on application
of the calibrating condition are not blown.
The result is that the switches whose fusible links were open-circuited can
no longer be connected to the common terminal on subsequent application of
a force on the deflectable member. Only at a level of force greater than
the threshold defined by the externally applied calibrating condition will
the deflectable member move sufficiently to close an additional one or
more switches to create a conductive path between the external terminals
and thereby enable the generation of an electrical signal indicative of
the level of the external condition exceeding this threshold level. By
connecting each switch in said switching device to a fusible link, it is
possible automatically to trim the device, enabling multiples of the
switching devices to be trimmed at once, without requiring identification
of the particular switches that are closed or open at any level of the
external condition.
These and other objects, features and advantages of the present invention
will become more apparent to those skilled in the art from the following
detailed description of the invention in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a trimmable, force-sensitive switching
device constructed according to a preferred embodiment of the present
invention;
FIG. 2 is a cross-sectional view of the switching device according to the
present invention illustrating a preferred embodiment of a deformable
member;
FIG. 3 is an enlarged, fragmentary sectional view of the switching device,
taken generally in the region indicated by bracket 3--3 in FIG. 2;
FIG. 4 is a partially diagrammatic plan view of the switching device of
FIG. 2; and
FIG. 5 is an enlarged, fragmentary sectional view of the switching device
constructed according to a second embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 1-5 illustrate the present invention. Referring first to FIG. 1,
shown is a schematic of a switching device 10 according to the present
invention. Switching device 10 includes a plurality of switches 52A-52E
whose closure is a function of the operation of a deformable member, as
described below. A first electrical contact of each switch 52, identified
as 80A-80E for respective switches 52A-52E, is coupled to a common
terminal 46. Connected in series between said common terminal 46 and each
switch 52 is a fusible link 44. As seen, fusible links 44A-44E are
connected in series to corresponding switches 52A-52E. The opposite pole
of each switch 52A-52E, identified as 81A-81E, comprises a common terminal
28. Common terminal 28 is connected to an external terminal 34 and common
terminal 46 is connected to an external terminal 50.
With reference to FIGS. 2, 3 and 4, the switching device 10 includes a
first substrate 12, preferably of silicon, having a reduced-thickness,
deflectable member 14 formed in the central portion thereof. As best seen
in FIG. 2, the deflectable member 14 is integrally formed with and
bordered by relatively thick side regions 16 and 18, of the first
substrate 12. Deflectable member 14 includes an outer recess 20 and an
inner recess 22 which are formed from the first substrate 12 by etching.
The planar interior surface of the outer recess 20 forms the outer surface
24 of the deflectable member 14. The planar interior surface of the inner
recess 22 forms the inner surface 26 of the deflectable member 14.
A second substrate 30 is positioned with respect to deflectable member 14
so as to create a chamber 54 formed by the two substrates 12, 30 and
deformable member 14. Second substrate 30 is made preferably of glass.
Chamber 54 may be a hermetically sealed chamber formed in a conventional
manner.
FIGS. 2 and 3 show the deflectable member 14 in a relaxed, planar position
resulting from the absence of a net external force across the member 14.
Upon application of an increasingly greater net external force, such as
pressure, across the member 14, the member 14 deflects from the relaxed
position to increasingly strained positions. This movement produces an
inward bulge of the member initially from the central region of the
member, a bulge which spreads outwardly toward the sides of the member as
greater external force is applied.
FIG. 3 shows the surface portion 32 of the second substrate 30 confronting
the inner surface 26 of the deflectable member 14. A common contact 28,
preferably a planar conductive surface, is carried on the surface 32 and
is connected to the first external terminal 34 (shown in FIG. 4) by a
first external connecting means 36, preferably a single conductor, which
passes through the hermetic seal formed between the first substrate 12 and
the second substrate 30. FIG. 3 also shows a fragment of the deflectable
member 14, the inner surface of which is coated with a thin silicon
dioxide insulating layer 38. The spaced electrical contacts 80A-80E
preferably are carried in a spaced apart relation on the inner surface 26
of deflectable member 14 in a position confronting the common contact 28.
The spaced electrical contacts 80A-E comprise a plurality of contact
buttons, represented by contact button 40 in FIG. 3, which may be formed
integrally with insulating layer 38 and which have the generally truncated
conical shape shown. The buttons 40 are coated with a layer 42 of an
electrically conductive metal, preferably aluminum or gold. Alternatively,
contact buttons 4D may be formed as part of layer 42. A plan view of
electrical contacts 80A-80E is shown in FIG. 4.
As seen in FIG. 4, the boundary defining the inner surface 26 of
deflectable member 14 is shown by the dashed line 74. The boundary of the
outer surface 24 of deflectable member 14 is shown by the solid line 76.
The boundary of the common contact 28 is shown as the dashed line 78. The
inner boundary of the thick side regions 16 and 18 of the first substrate
12 is shown as line 82 and the outer boundary of the thick side regions 16
and 18 of the first substrate 12 is shown as line 84. The outer boundary
of the second substrate 30 is shown as line 86.
As also seen in FIG. 4, the common contact 28 is connected to the first
external terminal 34 by the first external connecting means 36 which
passes though the hermetic seal between the first substrate 12 and the
second substrate 30. Each of the spaced electrical contacts 80A-E is
connected by a conductor to the first terminal of one of a plurality of
fusible links 44A-44E. The fusible links 44A-44E are made of a conductive
material which is designed to open circuit upon the passage of a
preselected current through the links. The second terminals of the fusible
links 44A-44E are connected to the common terminal 46 within the sealed
chamber 54. The second external connecting means 48, preferably a single
conductor, leads from the common terminal 46 through the hermetic seal
between the first substrate 12 and the second substrate 30 to the second
external terminal 50 which is adapted for connection to external
detecting, monitoring or other circuits.
As the net external force on the deflectable member 14 increases and the
member bulges inwardly into the sealed chamber 54, the spaced electrical
contacts 80A-80E which are carried on the inner surface 26 of the member
14, make contact with the common contact 28 which is carried on the
confronting surface 32 of the second substrate 30. In general, the bulging
of the deflectable member 14 initially brings the one or more spaced
electrical contacts 80A-80E which are positioned closest to the center of
the member 14 into contact with contact 28. Progressively more and more of
the spaced electrical contacts 80A-80E thereafter come into contact with
the common contact 28. Note that this operation does not require that
switches 52A-52E close in any logical order, either based on relative
positions or otherwise. This would be represented schematically in FIG. 1
by the progressive closing of switches 52A-52E. The result is the
establishment of a complete circuit for current to flow from the first
external terminal 34 through the closed switches 52A-52E, through the
fusible links 44A-44E which are connected in series with those of the
switches 52A-52E that have closed, to the second external terminal 50.
One of the advantages of the present invention is that the switching device
10 may be automatically trimmed so as to enable the subsequent detection
or monitoring of a threshold external force applied to the deflectable
member. This is accomplished by applying a preselected force, representing
the threshold level of the force to be detected, just a little less, to
the deflectable member 14. As described above, the deflectable member 14
bulges inward, closing one or more of the switches 52A-52E, completing a
circuit through their associated fusible links 44A-44E. Then, a voltage is
applied to the first external terminal 34 and the second external terminal
50 so as to cause the flow of current in the completed circuit of an
amount sufficient to open circuit the fusible links 44A-44E which are
connected in series with those of the switches 52A-52E which have closed.
The preselected voltage may then be removed. The current is also selected
so that it is not large enough to damage any switch 52A-52E during this
fusing operation.
The result of this trimming operation is that, when the switching device 10
is placed in a detecting or monitoring circuit, it will not complete a
circuit within the switching device 10 upon application to the deflectable
member 14 of an external force which is the same as or less than the force
applied in the trimming operation. Only higher levels of force applied to
the deflectable member 14 will result in the closure of one or more of
switches 52A-52E whose associated fusible links have not been rendered
open by the trimming operation. Thus, the exceeding of the trimmed
threshold level of the external force is automatically detectable by the
switching device.
The present invention also has the advantage of enabling detection of a
predetermined threshold value of the externally applied force without
regard to the order of operation of the switches 52A-52E, or the identity
of the particular switches that are closed. This allows trimming and
operation that is more easily, more quickly, and more cheaply
accomplished.
Another advantage of the present invention is that, by connecting the
second terminals of each of the fusible links 44A-44E to a common terminal
46 within the hermetically sealed chamber 54, and by connecting the common
terminal 46 to the second external terminal 50 by a second external
connecting means 48 which is a single conductor, the number of
penetrations of the hermetic seal is reduced. This reduces the rate of
failure of the seal, thereby increasing the reliability of the present
invention compared to the prior art.
By reducing the number of external connections to the switching device, the
present invention reduces the quantity of some of the materials needed,
thereby reducing costs, and also reduces the physical size of the
switching device, making it adaptable to a greater number of applications.
Simplification of fabrication also reduces the total cost of the device.
FIG. 5 shows a microminiature, force-sensitive switching device 10'
constructed according to a second embodiment of the invention. FIG. 5 is
an enlarged, fragmentary cross-sectional view of a deformable beam 70. The
second embodiment is designed to respond to changes in acceleration forces
applied to the switching device 10'.
The construction of switch 10' is substantially the same as that of the
first embodiment, except that the deflectable member takes the form of a
reduced-thickness beam 70. The beam 70 may be formed integrally with, and
anchored to, opposite thick side regions 16' and 18' of the first
substrate 12' so that deflection occurs first in the central beam region,
then progressively outwardly toward the thick side regions 16' and 18' of
the first substrate 12'. Alternatively, the beam 70 may have a cantilever
construction, only one end of the beam 70 being integrally formed from,
and anchored to, the first substrate 12'. In this form, deflection occurs
first at the beam's free end, then occurs progressively toward the beam's
anchored end. FIG. 5 represents the latter type of beam construction.
The cantilever-beam member 70 may be etched from the silicon first
substrate 12'. A plurality of spaced electrical contacts 80'A-80'E are
carried on the inner surface 26' of the beam 70, the contacts being formed
by coating insulative buttons, such as buttons 40', with a conductive
layer 42'. Alterntively, buttons 40' may be formed as part of conductive
layer 42', rather than being a part of insulative layer 38'. A common
contact 28' is carried on the surface of the second substrate 30'
confronting the inner surface 32' of the beam 70 and the spaced electrical
contacts 80'A-80'E.
Each of the spaced electrical contacts 80'A-80'E is connected to the first
terminal of one of a plurality of fusible links represented in FIG. 5 by
fusible link 44'. The second terminal of each of the fusible links 44' is
connected to a common terminal 46.
The beam switch described may be adapted either for acceleration or
temperature detection. FIG. 5 illustrates the inclusion of a series of
mass elements 72 which make the beam 70 responsive to changes in
acceleration in the direction of arrow 73. Various sites of attachment of
the mass elements 72 to the beam 70, other than as shown in FIG. 5, would
be possible without departing from the present invention. The force of
acceleration acting on the masses 72 cause the beam 70 initially to
deflect from a relaxed condition toward a strained condition in which the
beam's free end begins to flex inwardly toward the common contact 28' on
the surface 32' of the second substrate 30'. As the acceleration force is
increased, the beam 70 is increasingly deflected so as to bring first one
then progressively more of the spaced electrical contacts 80'A-80'E
against the common contact 28'.
In a temperature-detecting application [embodiment not shown], the beam 70
would include a metallic inner layer whose temperature coefficient of
expansion is different than that of the first substrate 12' on which the
metallic layer is carried. The beam 70 would have a relaxed condition at a
selected lower temperature and the switches 80'A-80'E would be open. As
the temperature to which the beam is exposed increases, the relatively
greater thermal expansion in the metal layer causes the beam 70 to deflect
inwardly, closing first one then progressively other switches 80'A-80'E to
allow the detection or monitoring of temperature changes.
The second embodiment of the invention shown in FIG. 5 thus enables the
trimming of the switching device in the same manner as described in the
first embodiment.
A trimmable force sensitive switch according to the present invention may
also be constructed wherein a change in force deflects a deformable member
from a more strained to a less strained state to thereby cause one and
then more switches to close. Such a device would have utility, for
example, where the sealed chamber 54 is initially at a higher pressure
than externally thereto. Increasing external pressure in this case would
move the deflected member toward a more relaxed state defined to be when
all of the switches have closed.
From the foregoing, it can be appreciated how the objects of the invention
are met so as to gain advantages over the prior art. The invention
provides a microminiature, force-sensitive switching device for detecting
or monitoring of a preselected threshold level of an external condition
such as pressure, acceleration or temperature, the threshold level being
set easily, quickly, and economically, without regard to the order of
operation of the individual switches of the switching device. The device
reduces the failure rate of the hermetic seal, thereby increasing
reliability; it simplifies the fabrication and trimming of the device,
thereby making it suitable for mass production; and it reduces the cost of
manufacturing and trimming the device.
While various embodiments of the invention have been described herein, it
will be appreciated that various changes and modifications may be made
without departing from the spirit and scope of the invention.
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