Dielectric links for microelectromechanical systems

FIELD OF THE INVENTION

This invention relates to electromechanical systems, and more particularly to microelectromechanical systems and fabrication methods therefor.

BACKGROUND OF THE INVENTION

Microelectromechanical systems (MEMS) have been developed as alternatives to conventional electromechanical devices, such as relays, actuators, valves and sensors. MEMS devices are potentially low-cost devices, due to the use of microelectronic fabrication techniques. New functionality also may be provided, because MEMS devices can be much smaller than conventional electromechanical devices.

A major breakthrough in MEMS devices is described in U.S. Pat. 5,909,078 entitled

Thermal Arched Beam Microelectromechanical Actuators

to the present inventor et al., the disclosure of which is hereby incorporated herein by reference. Disclosed is a family of thermal arched beam microelectromechanical actuators that include an arched beam which extends between spaced apart supports on a microelectronic substrate. The arched beam expands upon application of heat thereto. Means are provided for applying beat to the arched beam to cause farther arching of the beam as a result of thermal expansion thereof, to thereby cause displacement of the arched beam.

Unexpectedly, when used as a microelectromechanical actuator, thermal expansion of the arched beam can create relatively large displacement and relatively large forces while consuming reasonable power. A coupler can be used to mechanically couple multiple arched beams. At least one compensating arched beam also can be included which is arched in a second direction opposite to the multiple arched beams and also is mechanically coupled to the coupler. The compensating arched beams can compensate for ambient temperature or other effects to allow for self-compensating actuators and sensors. Thermal arched beams can be used to provide actuators, relays, sensors, microvalves and other MEMS devices. Other thermal arched beam microelectromechanical devices and associated fabrication methods are described in U.S. Pat. No. 5,994,816 to Dhuler et al. entitled

Thermal Arched Beam Microelectromechanical Devices and Associated Fabrication Methods

, the disclosure of which is hereby incorporated herein by reference.

As MEMS devices become more sophisticated, there continues to be a need for MEMS structures that can be used in more sophisticated MEMS devices. Fabrication of these structures preferably should be accomplished using conventional MEMS fabrication process steps.

SUMMARY OF THE INVENTION

The present invention provides microelectromechanical structures that include first and second movable metallic members that extend along and are spaced apart from a microelectronic substrate and are spaced apart from one another, and a movable dielectric link or tether that mechanically links the first and second movable metallic members while electrically isolating the first and second movable metallic members from one another. The movable dielectric link preferably comprises silicon nitride. These microelectromechanical structures can be particularly useful for mechanically coupling structures that are electrically conducting, where it is desired that these structures be coupled in a manner that can reduce and preferably prevent electrical contact or crosstalk.

The movable dielectric link is attached to the first and second movable metallic members beneath the first and second movable metallic members. Alternately, the movable dielectric link can be attached to the first and second movable metallic members above the first and second movable metallic members, opposite the microelectronic substrate. When the movable dielectric link is attached to the first and second movable members beneath the first and second movable metallic members, a trench can be provided in the microelectronic substrate adjacent the movable dielectric link, to reduce and preferably prevent stiction between the movable dielectric link and the microelectronic substrate thereunder.

More than two movable metallic members can be mechanically linked to a single movable dielectric link. Moreover, a movable third conductive member can extend between the first and second movable metallic members and across the movable dielectric link. The third conductive member can be spaced apart from the first and second movable metallic members and the movable dielectric link, so that independent movement can be provided.

The movable dielectric link can be attached to the first and second movable metallic members due to the adhesion therebetween. Moreover, first and second anchors can be added to anchor the movable dielectric link to the first movable metallic member and to the second movable metallic member, respectively. The anchors can comprise an aperture in the movable metallic member, and a first mating protrusion that extends from the movable metallic member into the aperture. Alternatively, the aperture can be provided in the movable metallic member and the protrusion can be provided in the movable dielectric link. The anchor also can comprise a notch in the movable metallic member or the movable dielectric link. Other configurations of anchors can be used.

The dielectric link can link the first and second movable metallic members at respective first and second ends of the movable metallic member that are adjacent one another. Alternatively, one or more of the movable metallic members can be attached to the dielectric link at intermediate portions thereof. The movable metallic members preferably comprise electroplated members and more preferably electroplated nickel members. A plating base layer can be provided between the movable metallic members and the movable dielectric link.

Movable dielectric links according to the invention can be used with many microelectromechanical devices including microelectromechanical actuators and sensors that move at least one of the first and second movable metallic members. Movable dielectric links according to the present invention can be particularly advantageous when used with thermal arched beam microelectromechanical systems as described in the above-cited patents.

Microelectromechanical structures according to the present invention can be fabricated by forming a sacrificial layer on a microelectronic substrate and forming a dielectric link on the sacrificial layer. First and second spaced apart metallic members are electroplated on the sacrificial layer, such that the first and second spaced apart metallic members both are attached to the dielectric link. The sacrificial layer then is at least partly removed, for example by etching, to thereby release the dielectric layer and at least a portion of the first and second metallic members from the microelectronic substrate.

The dielectric link can be formed prior to electroplating the first and second spaced apart metallic members, such that the dielectric link is attached to the first and second metallic members beneath the first and second metallic members. In other embodiments, the electroplating step can precede the step of forming a dielectric link, such that the dielectric link is attached to the first and second metallic members above the first and second metallic members, opposite the microelectronic substrate.

When the electroplating is performed prior to forming the dielectric link, the dielectric link can be formed between the first and second spaced apart metallic members and extending onto the first and second spaced apart metallic members opposite the sacrificial layer. Prior to electroplating, a plating base preferably can be formed on the sacrificial layer. The first and second spaced apart metallic members are then plated on the plating base.

Alternatively, when the dielectric link is formed prior to electroplating, the first and second spaced apart metallic members can be electroplated on the sacrificial layer and extending onto the dielectric link, such that the first and second spaced apart metallic members both are attached to the dielectric link. A plating base preferably can be formed on the sacrificial layer and extending onto the dielectric link, prior to electroplating the first and second spaced apart metallic members on the plating base.

In preferred methods of the present invention wherein the dielectric link is formed prior to electroplating, the sacrificial layer can be a first sacrificial layer. A second sacrificial layer can be formed on the first sacrificial layer and spaced apart from the dielectric link. The first and second spaced apart metallic members then are electroplated on the second sacrificial layer, such that the first and second spaced apart metallic members both are attached to the dielectric link. The removing step then can be accomplished by etching the first and second sacrificial layers, to thereby separate the dielectric link and at least a portion of the first and second metallic members from the microelectronic substrate.

The etching step can be followed by the step of forming a trench in the microelectronic substrate beneath the dielectric link, to further separate the dielectric link from the microelectronic substrate. In particular, the dielectric layer can be formed by blanket forming a dielectric layer on the microelectronic substrate and on the sacrificial layer, and patterning the dielectric layer to form the dielectric link and a dielectric mask on the microelectronic substrate that is spaced apart from the dielectric link. The trench then can be formed by etching the microelectronic substrate beneath the dielectric link using the dielectric mask as an etch mask.

The dielectric link preferably can comprise silicon nitride, the metallic members preferably can comprise nickel and the sacrificial layers preferably can comprise silicon dioxide. However, in other embodiments, the movable metallic members can be replaced with movable conductive, nonmetallic members such as doped polysilicon, that can be formed using deposition and lithography and/or other processes for forming MEMS conductive layers. Accordingly, microelectromechanical structures and fabrication methods can be provided that can mechanically link members that are electrically conducting but can provide high dielectric isolation between the linked members.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D

are cross-sectional views of first microelectromechanical structures including dielectric links according to the present invention, during intermediate fabrication steps.

FIGS. 2A-2D

are cross-sectional views of second microelectromechanical structures including dielectric links according to the present invention, during intermediate fabrication steps.

FIGS. 3A-3D

are cross-sectional views of third microelectromechanical structures including dielectric links according to the present invention, during intermediate fabrication steps.

FIGS. 4A-4C

are top views of microelectromechanical structures according to the present invention.

FIGS. 5A-5I

are cross-sectional views of fourth microelectromechanical structures including dielectric links according to the present invention, during intermediate fabrication steps.

FIGS. 6A-6C

are top views of additional microelectromechanical structures according to the present invention.

FIG. 7

is a top view of a micro-relay that includes a dielectric link according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. It will be understood that when an element such as a layer, region or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. Also, when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

FIGS. 1A-1D

are cross-sectional views of first embodiments of microelectromechanical structures including dielectric links according to the present invention, during intermediate fabrication steps. Referring now to

FIG. 1A

, a sacrificial layer

110

such as a layer of silicon dioxide is formed on a microelectronic substrate

100

such as a monocrystalline silicon substrate. The silicon dioxide can be chemical vapor deposited silicon dioxide, spin-on-glass, thermally grown silicon dioxide or other conventional forms of silicon dioxide. Other layers such as low pressure chemical vapor deposited phosphosilicate glass (PSG) also may be used. A layer of silicon nitride and/or another dielectric

120

is formed on the sacrificial layer

110

. The layer

120

preferably is formed by low pressure chemical vapor deposition of silicon nitride or other conventional techniques, and preferably is a layer of low stress silicon nitride. Other dielectrics also may be used in layer

120

, such as organic insulators including polyimide, as long as the sacrificial layer

110

can be etched at different etch rates than the layer

120

.

Referring now to

FIG. 1B

, the layer

120

is patterned to form a dielectric link or tether

120

a

on the sacrificial layer

110

. Conventional photolithography can be used. The dielectric link

120

a

preferably comprises silicon nitride. However, other dielectric materials may be used.

Referring now to

FIG. 1C

, an optional plating base layer

130

then is formed on the sacrificial layer

110

and on the dielectric link

120

a

and patterned using conventional techniques. First and second spaced apart metallic members

140

a

and

140

b

then are electroplated on the plating base layer

130

using a conventional electroplating stencil or mold if necessary. The plating base layer

130

can be patterned on the dielectric link before or after the electroplating process is performed. In preferred embodiments of the present invention, the first and second spaced apart metallic members

140

a

and

140

b

comprise nickel and the plating base comprises copper. However, other materials also can be used. It also will be understood by those having skill in the art that other conductive layers, such as doped polysilicon can be formed and patterned on the sacrificial layer and on the dielectric link

120

a

using conventional patterning techniques, instead of or in addition to the first and second spaced apart metallic members

140

a

and

140

b

.

Still referring to

FIG. 1C

, it can be seen that the plating base

130

and/or the electroplated members

140

a

and

140

b

include a respective notch

142

a

and

142

b

therein that conforms to the dielectric link. This notch can provide an anchor to promote improved adhesion of the spaced apart metallic members

140

a

and

140

b

to the dielectric link

120

a

.

Finally, referring to

FIG. 1D

, at least part of the sacrificial layer

110

is removed, to thereby release or separate the dielectric link and at least a portion of the first and second metallic members

140

a

and

140

b

from the microelectronic substrate

100

. The removal can take place by an etch that etches the sacrificial layer

110

without substantially etching the dielectric link

120

a

or the spaced apart metallic members

140

a

,

140

b

. Hydrofluoric acid or other conventional etchants may be used. Other conventional MEMS fabrication steps also may be performed including metallization, dicing and packaging. See for example the

MUMPS Design Handbook Revision

4.0 by Koester et al., Cronos Integrated Microsystems, May 1999, the disclosure of which is hereby incorporated herein by reference.

Still referring to

FIG. 1D

, first microelectromechanical structures according to the present invention include a microelectronic substrate

100

, first and second movable metallic members

140

a

,

140

b

that extend along and are spaced apart from the microelectronic substrate

100

, and are spaced apart from one another. A movable dielectric link

120

a

mechanically links the first and second movable metallic members, while electrically isolating the first and second movable metallic members from one another. The movable metallic members

140

a

and

140

b

can move along the substrate face in the direction shown by arrows

144

.

In the fabrication methods and structures shown in

FIGS. 1A-1D

, the movable dielectric link

120

a

is beneath the first and second spaced apart metallic members

140

a

,

140

b

. In contrast, in

FIGS. 2A-2D

, the dielectric link extends above the spaced apart metallic members

140

a

,

140

b

, opposite the substrate

100

.

In particular, referring to

FIG. 2A

, the sacrificial layer

110

is formed on the microelectronic substrate

100

. Then, in

FIG. 2B

, the plating base

130

and the spaced apart metallic members

140

a

and

140

b

are formed using conventional electroplating techniques. As with

FIG. 1C

, the plating base

130

can be omitted and other conductive materials may be used.

Then, referring to

FIG. 2C

, a silicon nitride and/or other dielectric layer

120

′ is formed on the first and second metallic members

140

a

,

140

b

opposite the microelectronic substrate

100

. The layer

120

′ also preferably extends into the space between the spaced apart metallic members

140

a

,

140

b

. Although the layer

120

′ is shown filling the space between the spaced apart metallic members

140

a

,

140

b

,it need not fill the entire space.

Then, as shown in

FIG. 2D

, the layer

120

′ is patterned using conventional techniques to form a dielectric link

120

a

′ that extends on the spaced apart movable members

140

a

,

140

b

opposite the substrate

100

. The sacrificial layer

110

is then at least partially removed as was described in connection with FIG.

1

D. Accordingly, in the microelectromechanical structures of

FIG. 2D

, the movable dielectric link

120

a

is attached to the first and second movable metallic members

140

a

,

140

b

above the first and second movable metallic members, rather than beneath the members as was the case in FIG.

1

D.

FIGS. 3A-3D

illustrate other microelectromechanical structures and fabrication methods of the present invention.

FIG. 3A

corresponds to FIG.

1

A.

FIG. 3B

corresponds to

FIG. 1B

, except that vias or apertures

120

b

are patterned in the dielectric link

120

a

. The vias may be patterned simultaneous with the patterning of layer

120

or in a separate step.

Then, as shown in

FIG. 3C

, the plating base

130

and/or the plated metallic layers

140

a

,

140

b

also are formed in the vias

120

b

,to thereby form anchors

142

142

b

′. Stated differently, vias are formed in the dielectric link and mating protrusions are formed in the plating base and/or plated layers, to provide anchors, and thereby promote additional adhesion between the dielectric link and the spaced apart metallic members

140

a

and

140

b

. The remainder of the processing in

FIGS. 3C and 3D

corresponds to that of

FIGS. 1C and 1D

, and need not be described again. It also will be understood that alternatively, vias can be formed in the spaced apart metallic members and protrusions may be formed in the dielectric link. Other forms of anchors including ridges, roughened surfaces and/or adhesion promoting layers, can be used.

FIGS. 4A-4C

illustrate top views of various microelectromechanical structures according to the present invention. As shown in

FIG. 4A

, one or more microelectromechanical actuators and/or sensors and/or other microelectromechanical devices

400

a

,

400

b

move at least one of the first and second movable metallic members

140

a

,

140

b

. The dielectric link

120

a

mechanically links the first and second movable metallic members

140

a

,

140

b

while electrically isolating the first and second movable metallic members from one another. Although the dielectric link

120

a

is shown with a square shape, other shapes can be used.

As shown in

FIG. 4B

, more than two microelectromechanical devices can be included on a substrate

100

and linked to a single dielectric link

120

a

. For example, in

FIG. 4B

, three devices

400

a

-

400

c

are coupled to three movable members

140

a

-

140

c.

In

FIG. 4C

, four microelectromechanical devices

400

a

-

400

d

are used. The first and second movable metallic members

140

a

and

140

b

are mechanically coupled by a dielectric link

120

a

. A third movable member

410

extends across the dielectric link but is spaced apart therefrom, so that independent movement may be obtained for member

410

. It will be understood that member

410

can be fabricated by forming a sacrificial layer on the dielectric link and then forming the member

410

on the sacrificial layer opposite the dielectric link. When the sacrificial layer is removed, member

410

can move independent of members

140

a

and

140

b.

It will be understood that in all of the embodiments of

FIGS. 4A-4C

, additional microelectronic circuitry can be formed in the substrate

100

, and multiple sets of links and members can be formed on a single substrate. It also will be understood that in the embodiments described above, the dielectric link

120

a

is attached to the ends of the movable metallic members

140

a

-

140

c

. However, the dielectric link

120

a

can be attached to intermediate portions of one or more of the movable metallic members

140

a

-

140

c

, to thereby form a wide variety of microelectromechanical devices.

FIGS. 5A-5I

illustrate other microelectromechanical structures according to the present invention during intermediate fabrication steps. In general, the structures and fabrication methods of

FIGS. 5A-5I

add a trench in the microelectronic substrate beneath the movable dielectric link. It has been found that stiction can occur between the dielectric link and the microelectronic substrate due to surface adhesive forces. The trench can further space apart the dielectric link from the microelectronic substrate, to thereby reduce and preferably eliminate stiction.

More specifically, referring to

FIG. 5A

, a first sacrificial layer

110

is formed on a substrate

100

. The first sacrificial layer

110

is then patterned in

FIG. 5B

to form a patterned first sacrificial layer

110

a

. Then, in

FIG. 5C

, silicon nitride and/or another dielectric layer

120

is formed on the substrate including on the patterned first sacrificial layer

110

a

. The layer

120

is then patterned in

FIG. 5D

to form a dielectric link

120

a

and a mask

120

c

that will be used to form the trench as described below.

Referring now to

FIG. 5E

, a second sacrificial layer then is formed on the first patterned sacrificial layer

110

a

, on the dielectric link

120

a

and on the mask

120

c

. The second sacrificial layer

150

preferably comprises the same material as the first sacrificial layer

110

, such as silicon dioxide.

Referring now to

FIG. 5F

, the second sacrificial layer

150

is patterned to form a patterned second sacrificial layer

150

a

. A plating base

130

then is formed, and the first and second members

140

a

and

140

b

are plated on the second sacrificial layer and on the dielectric link

120

a

. Then, in

FIG. 5H

, the first and second sacrificial layers are at least partially removed, to thereby release the dielectric link and at least a portion of the first and second metallic members

140

a

and

140

b

from the microelectronic substrate

100

.

Finally, referring to

FIG. 5I

, the microelectronic substrate

100

is etched using the mask

120

c

as an etch mask, to form a trench

160

beneath the dielectric layer. The substrate may be etched to a depth of between about 10 &mgr;m and about 30 &mgr;m. Etching can take place by continuing the same etch that was used to etch the sacrificial layers or by using another etchant.

Microelectromechanical structures according to

FIG. 51

include a trench

160

in the microelectronic substrate adjacent the movable dielectric link

120

a

that is attached to the first and second movable metallic members

140

a

,

140

b

beneath the first and second movable metallic members. It also will be understood that the methods of

FIGS. 2A-2D

and

3

A-

3

D may be modified to form a trench

160

in the microelectronic substrate

100

.

FIGS. 6A-6A

are top views of other microelectromechanical structures according to the present invention.

FIGS. 6A-6C

correspond to

FIGS. 4A-4C

, except that the trench

160

also is shown.

FIG. 7

is a top view of a micro-relay that includes thermal arched beam actuators that were described in the above-incorporated U.S. Pat. Nos. 5,909,078 and 5,994,816, and includes a dielectric link

120

a

according to the present invention. As shown in

FIG. 7

, the micro-relay

700

includes first and second microelectromechanical actuators

400

a

′ and

400

b

′ in the form of thermal arched beam microelectromechanical actuators. Actuator

400

a

′ can be an active actuator that is heated by a heater

702

via control contacts

730

to cause movement of the first movable member

140

a

in the direction shown by arrows

144

. Actuator

400

b

′ can be a passive actuator that can provide thermal compensation and/or a load to the micro-relay. The dielectric link

120

a

mechanically links movable metallic members

140

a

and

140

b

while maintaining electrical isolation therebetween.

As shown in

FIG. 7

, the dielectric link

120

a

can include holes

120

e

therein which can be used to promote passage of the etchant that is used to release the sacrificial layers in

FIGS. 1D

,

2

D,

3

D and

5

H. The second movable metallic member

140

b

can be stabilized by one or more suspension beams

710

. A hysteresis loop

720

can be used to ensure that the micro-relay is not damaged if an overvoltage is applied, by allowing the hysteresis loop to absorb excess force. Load contacts

740

and switch contacts

750

also are shown.

Accordingly, structures and methods of the present invention can allow microelectromechanical devices such as micro-relays, sensors, switch matrices and/or variable capacitors to include a movable mechanical link that permits mechanical coupling of adjacent moving structures, while maintaining dielectric isolation between the structures. They may be particularly useful for mechanically coupling structures that are electrically conducting, where it is desired to couple these structures in a manner that reduces and preferably prevents electrical contact or crosstalk. Thus, for example, high dielectric isolation can be obtained between the control or drive side of a relay and the load side of a relay. Without such a link, it may be difficult to achieve useful isolation in a relay. The dielectric link and fabrication process preferably are used to connect structures that move in the plane of the substrate, such as are formed by surface micromachining of silicon wafers or other MEMS fabrication processes. improved microelectromechanical structures and fabrication methods thereby may be provided.

In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.