Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Physical stress responsive
Reexamination Certificate
2001-11-02
2004-06-15
Coleman, W. David (Department: 2823)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Physical stress responsive
C257S414000
Reexamination Certificate
active
06750078
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to microelectromechanical systems (MEMS), and in particular to MEMS switches that have a connecting beam with a high resonance frequency to provide high-speed switching.
BACKGROUND OF THE INVENTION
A microelectromechanical system (MEMS) is a microdevice that integrates mechanical and electrical elements on a common substrate using microfabrication technology. The electrical elements are formed using known integrated circuit fabrication techniques while the mechanical elements are fabricated using lithographic techniques that selectively micromachine portions of a substrate. Additional layers are often added to the substrate and then micromachined until the MEMS device is in a desired configuration. MEMS devices include actuators, sensors, switches, accelerometers, and modulators.
MEMS switches have intrinsic advantages over conventional solid-state counterparts, such as field-effect transistor switches. The advantages include low insertion loss and excellent isolation. However, MEMS switches are generally much slower than solid-state switches. This speed limitation precludes applying MEMS switches in certain technologies, such as wireless communications, where sub-microsecond switching is required.
MEMS switches include a suspended connecting member called a beam that is electrostatically deflected by energizing an actuation electrode. The deflected beam engages one or more electrical contacts to establish an electrical connection between isolated contacts. When a beam is anchored at one end while being suspended over a contact at the other end, it is called a cantilevered beam. When a beam is anchored at opposite ends and is suspended over one or more electrical contacts, it is called a bridge beam.
The key feature of a MEMS switch that dictates its highest possible switching speed is the resonance frequency of the beam. The resonance frequency of the beam is a function of the beam geometry. The beams in conventional MEMS switches are formed in structures that are strong and easy to fabricate. These beam structures are suitable for many switching applications, however the resonance frequency of the beams is too low to perform high-speed switching.
FIG. 1
illustrates a prior art MEMS switch
10
that includes a cantilever beam
12
. The beam
12
consists of a structural portion
14
and a conducting portion
16
. High-speed MEMS switches require both strength and high conductivity making it necessary to use the composite beam
12
. The MEMS switch
10
further includes an actuation electrode
18
and a signal contact
20
that are each mounted onto a base
22
. One end
24
of the beam
12
is connected to the base
22
and the other end
26
of the beam
12
is suspended over the signal contact
20
. The suspended end
26
of the beam
12
moves up and down when a voltage is applied to the actuation electrode
18
. As the end
26
of the beam
12
moves up and down, the conducting portion
16
of the beam
12
engages and disengages the signal contact
20
.
FIG. 2
illustrates the prior art MEMS switch
10
during fabrication. The MEMS switch
10
includes a release layer
28
that is removed by conventional techniques such as etching. Removing the release layer
28
exposes the actuation electrode
18
, the signal contact
20
, and the conducting portion
16
of the beam
12
. The conducting portion
16
of the beam
12
and the contacts
18
,
20
are usually made of the same acid resistant metal because they must withstand the process of removing the release layer
28
. Gold is the most commonly used material for the conducting portion
16
, the actuation electrode
18
, and the signal contact
20
.
The MEMS switch
10
typically needs to operate in excess of 10 billion switching cycles such that the repeated contact between the signal contact
20
and the conducting portion
16
of the beam
12
is a critical design consideration. There are many mechanisms that contribute to the aging and failure of contacts. These mechanisms include mechanical impact damage, sliding-friction induced damage, current-assisted interface bonding, solid-state reaction, and even local melting. When the conducting portion
16
and signal contact
20
are made of the same metal, they tend to bond each other such that the switch
10
oftentimes does not open at the appropriate time, especially if the contacts are made of a very soft material such as gold. Gold bonding can easily occur at room temperature such that the operating life of existing MEMS switches is typically below 1 billion switching cycles.
REFERENCES:
patent: 6396368 (2002-05-01), Chow et al.
patent: 6535663 (2003-03-01), Chertkow
Chris Keller, Microfabricated High Aspect Ratio Silicon Flexures, HESXIL, RIE, and KOH Etched Desgin & Fabrication, MEMS Precision Instruments, CA 1998, pp. 23-44, 133-139 and 141-153.
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