Microelectromechanical micro-relay with liquid metal contacts

Details Microelectromechanical micro-relay with liquid metal contacts Microelectromechanical micro-relay with liquid metal contacts

2001-02-01

2002-05-28

Donovan, Lincoln (Department: 2832)

Electricity: magnetically operated switches, magnets, and electr

Electromagnetically actuated switches

Polarity-responsive

C335S047000, C335S058000

Reexamination Certificate

active

06396371

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to electrical and electronic circuits and components. More specifically, the present invention relates to micro-electromechanical (MEM) relays with liquid metal contacts.
BACKGROUND OF THE INVENTION
A MEM switch is a switch operated by an electrostatic charge, thermal, piezoelectric or other actuation mechanism and manufactured using micro-electromechanical fabrication techniques. A MEM switch may control electrical, mechanical, or optical signal flow. Conventional MEM switches are usually single pole, single throw (SPST) configurations having a rest state that is normally open. In a switch having an electrostatic actuator, application of an electrostatic charge to the control electrode (or opposite polarity electrostatic charges to a two-electrode configuration) will create an attractive electrostatic force (“pull”) on the switch causing the switch to close. The switch opens by removal of the electrostatic charge on the control electrode(s), allowing the mechanical spring restoration force of the armature to open the switch. Actuator properties include the required make and break force, operating speed, lifetime, sealability, and chemical compatibility with the contact structure.
A micro-relay includes a MEM electronic switch structure mechanically operated by a separate MEM electronic actuation structure. There is only a mechanical interface between the switch portion and the actuator portion of a micro-relay. When the switch electronic circuit is not isolated from the actuation electronic circuit, the resultant device is usually referred to as a switch instead of a micro-relay. MEM devices are typically built using substrates compatible with integrated circuit fabrication, although the electronic switch structure disclosed herein does not require such a substrate for a successful implementation. MEM micro-relays are typically 100 micrometers on a side to a few millimeters on a side. The electronic switch substrate must have properties (dielectric losses, voltage, etc.) compatible with the desired switch performance and amenable to a mechanical interface with the actuator structure if fabricated separately.
MEM switches are constructed using gold or nickel (or other appropriate metals) as contact material for the device. Current fabrication technology tends to limit the type of contact metals that can be used. The contacts fabricated in a conventional manner tend to have lifetimes in the millions of cycles or less. One of the problems encountered is that microscale contacts on MEM devices tend to have very small regions of contact surface (typically 5 micrometers by 5 micrometers). The portion of the total contact surface that is able to carry electrical current is limited by the microscopic surface roughness and the difficulty in achieving planar alignment of the two surfaces making mechanical and electrical contact. Thus, most contacts are point contacts even on a surface that would seem to have hundreds or thousands of square micrometers of contact surface available. The high current densities in these small effective contact regions create microwelds and surface melting, which if uncontrolled results in impaired or failed contacts. Such metallic contacts tend to have short operational lifetimes, usually in the millions of cycles.
The state of the art in macro-scale relays/switches is well developed. There has been a considerable effort in developing long life contact metallurgy for the signal contacts. The signal contact life and the appropriate contact metallurgy tends to be rated by the application, such as “dry” signals (no significant current or voltage), inductive loads and high current loads.
It is known in the art, that electrical contacts using mercury (chemical symbol Hg) as an enhancement for switch contact conductivity yields longer contact life. It is also known that the Hg enhanced contacts are capable of operating at higher current than the same contact structure without mercury. Mercury wetted reed switches are an example. Other examples or mercury wetted switches are described in U.S. Pat. Nos. 5,686,875, 4,804,932, 4,652,710, 4,368,442, 4,085,392 and Japanese application 03118510 (Publication No. JP04345717A).
The use of mercury droplets in a miniature relay (a device which is much larger than a MEM relay) controlled by a high voltage electrostatic signal is taught in U.S. Pat. No. 5,912,606. U.S. Pat. No. 5,912,606 uses the electrostatic signal on a gate to attract liquid metal drawn from a first contact to liquid metal drawn from a second contact or to draw liquid metal from both contacts to a shorting conductor mounted on the gate in order to electrically connect the contacts.
A conventional vertically activated surface micromachined electrostatic MEM micro-relay
10
structure is shown in FIG.
1
. The MEM micro-relay
10
includes a single substrate
30
on which is micromachined a cantilever support
34
. A first signal contact
50
, a second signal contact
54
, and a first actuator control contact
60
a
are disposed on the same substrate
30
. The contacts have external connections (not shown) in order to connect the micro-relay to external signals. One end of a cantilever
40
is disposed on cantilever support
34
. Cantilever
40
includes a second actuator control contact
60
b.
A second end of the cantilever
40
includes a shorting bar
52
. The two conductive actuator control contacts
60
a
and
60
b
control the actuation of the MEM micro-relay
10
.
Without a control signal, the shorting bar
52
on the cantilever
40
is positioned above the substrate
30
by the support
34
. With the cantilever
40
in this position, the first and second signal contacts
50
and
54
on the substrate
30
are not electronically connected. An electrostatic force created by a potential difference between the second actuator control contact
60
b
and the first actuator control contact
60
a
on substrate
30
control connection is used to pull the cantilever
40
down to toward the substrate
30
. The MEM micro-relay
10
uses the conductive shorting bar
52
to make a connection between the two signal contacts
50
and
54
attached to the same substrate
30
as the cantilever
40
and cantilever support
34
. When pulled to the substrate
30
, the shorting bar
52
touches the first and second signal contacts
50
and
54
and electrically connects them together. The cantilever
40
typically has an insulated section (not shown) separating the shorting bar
52
from the cantilever electrostatic actuator control contact
60
b.
Thus, the first and second signal contacts
50
and
54
are connected by the cantilever
40
shorting bar
52
, which is operated by an isolated electrostatic force mechanism using the two actuator control contacts
60
a
and
60
b
surfaces. The contacts
50
,
54
and the shorting bar
52
typically have short operational lifetimes due to the problems described above.
The micromachined electrostatic MEM micro-relay
10
is shown as a normally open (NO) switch contact structure. The open gap between the actuator control contact
60
a
and the cantilever beam
40
is usually a few microns ({fraction (1/1,000,000)} meter) wide. The gap between the shorting bar and the signal contacts is approximately the same dimension. When the switch closes, the cantilever beam
40
is closer to but not in direct electrical contact with actuator control contact
60
a.
If the signal contact metal is wettable with mercury, and the rest of the micro-relay is not wettable, then the mercury could be deposited on the signal metalization and allowed to flow into the active contact area under the cantilever by capillary action. The problem of mercury bridging at these close spacings must be addressed. When the mercury contacts are not contained, the contacts are subject to all the problems described in the above referenced patents including splashing and the need for liquid metal replenishment.
Mercury contacts represent a major challenge for the conventional MEM switch. The typical physical sep

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