Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Of specified configuration
Reexamination Certificate
2001-03-01
2003-03-04
Clark, Jasmine J B (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Of specified configuration
C257S773000, C257S731000, C257S734000
Reexamination Certificate
active
06528887
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally microelectromechanical systems (MEMS). More particularly, forming conductive landing pads on MEMS structures.
BACKGROUND ART
Microelectromechanical systems (MEMS) are miniature mechanical devices manufactured using the techniques developed by the semiconductor industry for integrated circuit fabrication. Such techniques generally involve depositing layers of material that form the device, selectively etching features in the layer to shape the device and removing certain layers (known as sacrificial layers, to release the device. Such techniques have been used, for example, to fabricate miniature electric motors as described in U.S. Pat. No. 5,043,043.
Recently, MEMS devices have been developed for optical switching. Such systems typically include an array of mechanically actuatable mirrors that deflect light from one optical fiber to another. The mirrors are configured to translate and move into the path of the light from the fiber. Mirrors that move into the light path generally use torsion flexures to translate mirror position vertically while changing its angular position from a horizontal to a vertical orientation. MEMS mirrors of this type are usually actuated by magnetic interaction, electrostatic interaction, thermal actuation or some combination of these. The design, fabrication, and operation of magnetically actuated micromirrors with electrostatic clamping in dual positions for fiber-optic switching applications are described, e.g., by B. Behin, K. Lau, R. Muller in “Magnetically Actuated Micromirrors for Fiber-Optic Switching,” Solid-State and Actuator Workshop, Hilton Head Island, S.C., Jun. 8-11, 1998 (p. 273-276).
When the mirror is in the horizontal position, it rests against a substrate that forms a base. Often, the mirror is subject to electromechanical forces, sometimes referred to as “stiction” that cause the mirror to stick to the substrate and prevent the mirror from moving. The same stiction forces can also prevent the mirror from being properly released from the substrate during manufacture. To overcome stiction problems, landing pads (also called dimples or bumps have been used extensively in MEMS devices to minimize or otherwise control the contact area between the device and the underlying substrate. In the prior art, such landing pads are formed prior to deposition of a device layer either by etching pits in an underlying sacrificial layer or by depositing pads of another material prior to the deposition of the layer forming the device.
Recently, silicon on insulator (SOI) techniques have been developed for fabricating MEMS devices. In SOI, an oxide layer is grown or deposited on a silicon wafer. A second silicon wafer is then bonded to the oxide layer, e.g. by plasma bonding. After bonding, the second silicon wafer is cleaved such that a thin layer of silicon is left attached to the oxide layer to form an SOI substrate. However, when that thin silicon layer is a MEMS device layer it is generally not possible to process the underside of the device layer prior to bonding the device layer to the oxide layer. Any processing of the device layer must therefore be done after it is attached to the underlying substrate. However since the underside of the device layer is firmly attached to the oxide layer it is not normally possible to deposit material on or etch material from the underside of the device layer. Currently, no technology exists for forming pads on the underside of the device layer of a MEMS device fabricated using SOI.
There is a need, therefore, for an SOI MEMS device having landing pads on an underside of the device layer and a method of fabricating same.
The problem of stiction with respect to an exemplary MEMs mirror device
800
is shown in FIG.
8
. The device
800
includes a mirror
811
formed from the device layer
812
of a substrate
810
. The mirror
811
may be movably attached to the device layer by a flexure
814
, actuated by an. off-chip electromagnet, and individually addressed by electrostatic clamping either to a surface of the substrate
810
or to a vertical sidewall
804
of a top mounted chip
806
. Magnetic actuation may move the mirror
811
between a rest position parallel to the substrate
810
and a position nearly parallel to the vertical sidewall
804
of the top-mounted chip
806
, while the application of electrostatic field may clamp the mirror
811
in the horizontal or vertical position. The electrostatic field used to hold the mirror
811
in a position regardless of whether the magnetic field is on or off can increase the level of stiction between the mirror
811
and each landing surface.
When clamped to either the substrate
810
or the vertical sidewall surface
804
, the mirror rests on a set of landing pads or dimples
822
,
824
, which may protrude below or above the mirror surface, respectively. These landing pads
822
,
824
minimize the physical area of contact between the mirror
811
and the clamping surface, thus reducing stiction effects. However, since the mirror
811
and clamping surface (either the side wall
804
or the substrate
802
) are at different potentials, the landing pads
822
,
824
are made of an insulating material in order to prevent an electrical short between the mirror
811
and the clamping surface. While the insulating landing pad material does, indeed, prevent an electrical short, its inherent properties can lead to other problems. Firstly, most insulating materials have the capacity to trap electrical charge and can, in some cases, maintain that charge for long periods of time—sometimes indefinitely. As a result, the potential of the landing pads
822
,
824
can drift to an arbitrary value, resulting in either parasitic clamping potential between the mirror
811
and the clamping surface, even when both are externally driven to the same voltage, or a reduced clamping force by shielding the mirror potential. Second, since the insulating landing pads
822
,
824
will typically be at a potential close to the mirror potential when not in contact with the clamping surface, a rapid discharge can occur when the landing pads
822
,
824
first come into the contact with the clamping surface that is a kept at a potential different than the mirror
811
. This rapid discharge may be exhibited as arcing or short pulses of high current. Such surges can lead to physical damage to the landing pads
822
,
824
or the clamping surface, or may produce micro-welding, where the landing pad is welded to the clamping surface—resulting in the mirror
811
being stuck.
There is a need, therefore, for a MEMS device having stiction resistant landing pads and a method of operating a MEMS device configured in a stiction reduced mode.
SUMMARY
The disadvantages associated with the prior art are overcome by a MEMs design having electrically isolated conductive landing pad structures that can be set to the same electrical potential as the landing surface. The design is enabled by providing a substrate having a sacrificial layer disposed between a base layer and a device layer. One or more vias are etched through the device layer and the sacrificial layer is etched forming depressions in the sacrificial layer at locations corresponding to vias in the device layer. The vias and depressions are filled with an electrically conductive landing pad material forming an isolated structure having landing pads that may be coupled to a voltage potential substantially equal to that of the landing surface.
The various embodiments of the present invention include methods of production and inventive devices having a device layer with at least one landing pad on an underside of the device layer attached to the device layer by a plug passing through an opening in the device layer. The device may be attached to the device layer by one or more compliant flexures, which allow the device to move in and out of a plane defined by the device layer.
The various embodiments are well suited to use with silicon on insulator substrates since the patterning of a sacri
Behin Behrang
Daneman Michael J.
Clark Jasmine J B
Onix Microsystems
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