Semiconductor device manufacturing: process – Having integral power source
Reexamination Certificate
2001-07-07
2003-02-04
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Having integral power source
C438S253000, C257S218000, C257S420000
Reexamination Certificate
active
06514781
ABSTRACT:
FIELD OF THE INVENTION
This invention is related to MEMS devices. More particularly, this invention is related to maintaining the state of a MEMS device in the event of a power failure.
BACKGROUND OF THE INVENTION
Modern communications systems require a level of robustness that protects the state of the optical switches from being lost in the event of a power failure. Recently, microelectromechanical systems (MEMS) devices have been developed for optical switching. MEMS devices are miniature mechanical devices manufactured using the techniques developed by the semiconductor industry for integrated circuit fabrication. MEMS optical switches typically include an array of mechanically actuatable mirrors that deflect light from one optical fiber to another. The mirrors are configured to translate or rotate into the path of the light from the fiber. Mirrors that rotate into the light path generally rotate about a substantially horizontal axis, i.e., they “flip up” from a horizontal position into a vertical position. MEMS mirrors of this type are usually actuated by magnetic interaction, electrostatic interaction, thermal actuation or some combination of these. The MEMS mirrors may be retained in the “up” position by an electrostatic clamping voltage. In the event of a power failure, the clamping voltage may be lost and any MEMS mirrors that were clamped may return to the “down” position under the influence of mechanical restoring forces. In this manner, the state of the switch may be lost in the event of a power failure.
The problem is illustrated through an example shown in
FIG. 1
, which depicts a schematic diagram of a MEMS apparatus according to the prior art. The depicted apparatus generally includes a MEMS optical switch
100
. The optical switch
100
has a substrate
102
, and a moveable element
104
moveably coupled to the substrate
102
. The moveable element
104
may be one of several such moveable elements that are moveably coupled to the substrate
102
. The moveable element
104
moves between a horizontal “OFF” position (shown in phantom) and a vertical “ON” position. In the “ON” position, the moveable element
104
is retained against a top chip
106
. In this example, the top chip
106
is electrically isolated from the substrate
102
, and all other MEMS elements, and a clamping voltage, e.g., +40 V, is applied between the moveable element
104
and the top chip
106
. In the apparatus shown in
FIG. 1
the clamping voltage difference is supplied by a high voltage source, such as a DC-DC converter
130
and a high voltage driver
120
. The high voltage driver
120
is essentially an electronic switch for addressing and selectively coupling a plurality of moveable elements
104
to the voltage potential output by the DC-DC converter
130
or to ground. In this example, the output of the DC-DC converter is also coupled directly to the top chip
106
. Thus, the top chip
106
sustains a clamping voltage as long as power is supplied to the DC-DC converter
130
. The high voltage driver
120
may be controlled by a microcontroller
110
, e.g., a PIC microcontroller to set a voltage potential for each movable element
104
configured in an optical cross-connect switch matrix. Depending on the required state of the switching element
100
, a voltage difference may exist between the moveable element
104
and one or more clamping structures. The clamping structure may clamp the movable element in a state and may also provide a mechanical stop to accurately align and fix the movable element in the required state. A top chip may be assimilated herein for purposes of examples shown, as an electrostatic clamping surface having a global mechanical stop to accurately align the movable element in the ON state. In this example, the top chip
106
, charged to some electrostatic potential (V
clamp
), provides the mechanical stop and clamps the moveable element
104
when the moveable element
104
is electronically connected to zero voltage (ground) through the high voltage driver. Alternatively, when the output of a high voltage driver coupled to the movable element
104
is set to V
clamp
through the high voltage driver
120
, no clamping voltage difference is present between the top chip
106
and the moveable element
104
and thus the moveable element
104
is allowed to fall back to the OFF state. It is also important to note in this example that in the clamped state, a small insulating gap, such as an air gap, is maintained between the top chip and the moveable element in order to maintain electrical isolation between the two surfaces.
In the event of a power failure in the example shown, the microcontroller
110
no longer receives the logic voltage Vcc and, therefore, can no longer control the high voltage driver
120
. Although the top chip
106
is electrically isolated from the other MEMS elements, the DC-DC converter
130
and high voltage driver
120
, both sharing the same circuit node as the top chip
106
, may be resistively coupled to ground. The coupling of the top chip
106
to these circuits causes charge to leak from the top chip
106
to ground. If the leakage of charge is sufficiently large, the voltage difference between the top chip
106
and the moveable element
104
will quickly be reduced to a level insufficient to retain the moveable element
104
against the top chip
106
. The moveable element
104
then returns to the “OFF” position interrupting any optical signal that may be deflected by the moveable element
104
. Even when power is restored, the state of the MEMS device
100
will not be recovered since the clamping voltage does not actuate the moveable element
104
.
Thus, there is a need in the art, for a method of maintaining the state of a MEMS device in the event of a power failure and an apparatus for implementing such a method.
SUMMARY OF THE INVENTION
The disadvantages associated with the prior art are overcome by a method and apparatus for maintaining the state of a MEMS device in the event of a power failure. The MEMS device generally has one or more MEMS elements moveably coupled to a substrate and a clamping surface that may be electrically isolated from all other MEMS elements. According to the method, an adequately sized charge storage device is connected between the clamping surface and an electrical ground. A clamping voltage applied between a clamping surface and at least one MEMS element retains the at least one MEMS element against the clamping surface. In the event of a power failure, all potentially leaky circuit paths to ground are isolated from the clamping surface, with the exception of the charge storage device that serves to maintain the electrostatic clamping voltage.
The apparatus generally comprises a charge-storing circuit, e.g., a capacitive circuit or battery permanently connected between the clamping surface and an electrical ground and an isolator element electrically connected between the clamping surface and all other circuits sharing the same node as the clamping surface (e.g., a top chip). The isolator element is configured to electrically isolate all potentially leaky circuit paths from the clamping surface in the event of a power failure. The isolator element may include an opto-isolator, diode or other circuit capable of providing low-leakage electrical isolation.
REFERENCES:
patent: 6112273 (2000-08-01), Kau
patent: 6122232 (2000-09-01), Schell
patent: 6266306 (2001-07-01), Schell
Beerling Timothy
Chang Mark W.
Dalton Scott D.
Daneman Michael J.
Panyko Stephen F.
Isenberg Joshua D.
JDI Patent
Niebling John F.
Onix Microsystems, Inc.
Stevenson André C.
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