Thermally actuated microelectromechanical systems including thermal isolation structures

Microelectromechanical structures include a microelectronic substrate and spaced apart supports on the microelectronic substrate. A beam extends between the spaced apart supports and expands upon application of heat thereto, to thereby cause displacement of the beam between the spaced apart supports. The application of heat to the beam creates a thermal conduction path from the beam through the spaced apart supports and into the substrate. A thermal isolation structure in the heat conduction path reduces thermal conduction from the beam, through the spaced apart supports and into the substrate, compared to absence of the thermal isolation structure. The thermal isolation structure preferably has lower thermal conductivity than the beam and the supports. The heat that remains in the beam thereby can be increased. The thermal isolation structure may include a thermally insulating structure at each end of the beam, a thermally insulating structure in each spaced apart support, a thermally insulating structure in the substrate adjacent each spaced apart support, and/or at least one thermally insulating structure in the beam. Accordingly, improved thermal efficiency for microelectromechanical structures may be provided, to thereby allow lower power, higher deflection, larger force and/or higher speed.

FIELD OF THE INVENTION
 This invention relates to electromechanical systems, and more particularly
 to microelectromechanical systems.
 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. No.
 5,909,078 entitled Thermal Arched Beam Microelectromechanical Actuators to
 Wood et al., the disclosure of which is hereby incorporated herein by
 reference. Disclosed is a family of thermal arched beam
 microelectromechanical actuators that includes 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 heat to the arched beam to cause further arching of the beam
 as a result of thermal expansion thereof, to thereby cause displacement of
 the arched beam.
 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. 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.
 Notwithstanding the above-described advances, there continues to be a need
 to further increase the thermal efficiency of MEMS devices. By increasing
 the thermal efficiency of MEMS devices, lower power, larger deflection,
 higher forces and/or higher speed operations may be provided.
 SUMMARY OF THE INVENTION
 It therefore is an object of the present invention to provide improved
 microelectromechanical structures.
 It is another object of the present invention to provide improved thermal
 arched beam microelectromechanical devices.
 It is yet another object of the present invention to provide
 microelectromechanical devices that can have higher thermal efficiency.
 It is still another object of the present invention to provide thermal
 arched beam devices that can have higher thermal efficiency.
 These and other objects may be provided, according to the present
 invention, by microelectromechanical structures that include a
 microelectronic substrate, at least one support on the microelectronic
 substrate and a beam that extends from the at least one support and that
 expands upon application of heat thereto, to thereby cause displacement of
 the beam. Application of heat to the beam also creates a thermal
 conduction path from the beam, through the at least one support and into
 the substrate. A thermal isolation structure in the heat conduction path
 reduces thermal conduction from the beam through the at least one support
 and into the substrate, compared to absence of the thermal isolation
 structure. The thermal isolation structure preferably has lower thermal
 conductivity than the beam and the at least one support. The heat that
 remains in the beam thereby can be increased. Higher thermal efficiency
 may be obtained, to thereby obtain lower power, larger deflection, higher
 force and/or higher speed operation.
 Microelectromechanical structures according to the present invention
 preferably comprise a microelectronic substrate and spaced apart supports
 on the microelectronic substrate. A beam extends between the spaced apart
 supports and expands upon application of heat thereto, to thereby cause
 displacement of the beam between the spaced apart supports. The
 application of heat to the beam creates a thermal conduction path from the
 beam through the spaced apart supports and into the substrate. A thermal
 isolation structure in the heat conduction path reduces thermal conduction
 from the beam, through the spaced apart supports and into the substrate,
 compared to absence of the thermal isolation structure. The thermal
 isolation structure preferably has lower thermal conductivity than the
 beam and the at least one support. The heat that remains in the beam
 thereby can be increased.
 The thermal isolation structure may comprise a thermally insulating
 structure at each end of the beam, between the beam and the spaced apart
 supports, to thereby thermally isolate the beam from the supports and the
 substrate. Alternatively, or in addition, the thermal isolation structure
 may comprise a thermally insulating structure in each spaced apart
 support, between the beam and the substrate, to thereby thermally isolate
 the beam from at least a portion of the supports and from the substrate.
 Alternatively, or in addition, the thermal isolation structure may
 comprise a thermally insulating structure in the substrate adjacent each
 spaced apart support, to thereby thermally isolate the beam and the
 supports from at least a portion of the substrate. Alternatively, or in
 addition, the thermal isolation structure can include at least one
 thermally insulating structure in the beam, to thermally isolate a portion
 of the beam from remaining portions of the beam, from the supports and
 from the substrate.
 The beam may be heated externally by an external heater, or internally by
 passing current through the beam. When current is passed through the beam,
 and the thermal isolation structure includes a thermally insulating
 structure at each end of the beam, an electrically conductive structure
 also may be provided on each of the thermally insulating structures, to
 provide an electrically conductive path from the beam to the spaced apart
 supports. A thermally insulating structure in each spaced apart support
 may be provided when it is difficult to thermally isolate a portion of the
 beam. For example, when the beam comprises metal, a silicon nitride tether
 may be provided in each spaced apart support, between the metal beam and
 the substrate. Finally, a thermally insulating structure in the substrate
 may comprise an insulator-containing trench, such as an oxide-filled
 trench, in the substrate beneath each spaced apart support.
 In all of the above embodiments, a trench also may be provided in the
 microelectronic substrate beneath the beam, to provide increased spacing
 between the beam and the surface of the substrate beneath the beam. The
 heat that remains in the beam thereby may be increased by providing an
 increased air gap between the beam and the substrate, to thereby allow
 reduced thermal conduction and/or convection directly from the beam to the
 substrate through the air gap.
 Microelectromechanical structures according to the present invention
 preferably are employed with thermal arched beams as described in the
 above-cited U.S. patents, that comprise an arched beam that is arched in a
 predetermined direction and that further arches in the predetermined
 direction upon application of heat thereto. The predetermined direction
 preferably extends generally parallel to the face of the microelectronic
 substrate. A valve plate, coupler, capacitor plate, relay contact and/or
 other structure may be mechanically coupled to the thermal arched beam,
 for example as described in the above-cited patents.
 One preferred embodiment of microelectromechanical structures according to
 the present invention includes an arched silicon beam that extends between
 spaced apart supports on a microelectronic substrate. The arched silicon
 beam is arched in a predetermined direction and further arches in the
 predetermined direction upon application of heat thereto, to thereby cause
 displacement of the arched silicon beam in the predetermined direction. A
 silicon dioxide link is provided at each end of the arched silicon beam,
 between the arched silicon beam and the spaced apart supports. An
 electrically conductive structure preferably is provided on each of the
 silicon dioxide links, to provide an electrically conductive path from the
 arched silicon beam to the spaced apart supports. The silicon dioxide link
 preferably is fabricated by thermally oxidizing the ends of the arched
 silicon beam. The electrically conductive structure may be provided by
 forming a metal film on the silicon dioxide link, that electrically
 connects the arched silicon beam to the spaced apart supports.
 In another preferred embodiment, an arched metal beam extends between the
 spaced apart supports. A silicon nitride or other thermally insulating
 tether is provided in each spaced apart support, between the arched metal
 beam and the substrate, since it may be difficult to provide a thermally
 insulating link in the arched metal beam itself. An electrically
 conductive structure may be provided on each of the silicon nitride
 tethers, to provide an electrically conductive path from the arched metal
 beam to the spaced apart support. The electrically conductive structure
 preferably comprises a metal film on the silicon nitride tether that
 electrically connects the arched metal beam to the spaced apart supports.
 In another preferred embodiment of the present invention, an arched metal
 beam is provided and an insulator-containing trench is provided in the
 substrate beneath each spaced apart support. The insulator-containing
 trench preferably comprises an oxide-filled trench in the substrate
 between each spaced apart support. Thus, where it is desirable to provide
 the metal beam and the spaced apart supports as one continuous metal
 structure, an insulator-containing trench in the substrate beneath each
 spaced apart support can reduce the thermal conduction from the continuous
 metal beam and spaced apart supports into the substrate, to thereby allow
 increased heat to remain in the beam.
 The present invention also may be used with other thermally actuated beams
 such as cantilever beams and/or bimorphs that extend from a support on a
 microelectronic substrate, and that expand upon application of heat
 thereto, to thereby cause displacement of at least part of the beam. A
 thermal isolation structure in the thermal conduction path from the beam
 through the support and into the substrate, can increase the heat that
 remains in the beam. As was described above, thermally insulating
 structures may be placed at the end of the beam adjacent the support, in
 the support, in the substrate and/or in an intermediate portion of the
 beam. Electrically conductive links and/or trenches in the substrate
 beneath the beam also may be provided. Accordingly, improved thermal
 efficiency for microelectromechanical structures may be provided, to
 thereby allow lower power, larger deflection, higher force and/or higher
 speed.

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 and 1B are top and side cross-sectional views, respectively, that
 illustrate creation of thermal conduction paths in thermal arched beam
 MEMS actuators according to the present invention. According to the
 present invention, it has been realized that these thermal conduction
 paths can reduce the thermal efficiency of MEMS devices, and thereby
 increase the power and/or decrease the deflection, force and/or speed of
 operation thereof.
 Referring now to FIGS. 1A and 1B, a thermal arched beam MEMS device
 includes a microelectronic substrate 100 and spaced apart supports 102a
 and 102b on the microelectronic substrate. An arched beam 110 of width W
 and length L extends between the spaced apart supports 102a and 102b.
 Multiple beams may be provided in all embodiments. The arched beam may be
 fabricated of metal, silicon or other material that has a positive
 coefficient of thermal expansion, so that it expands upon application of
 heat thereto. As shown in FIG. 1A, the arched beam is arched in a
 direction 104 that preferably extends parallel to the microelectronic
 substrate 100. Thus, upon application of heat thereto, further
 displacement of the beam 110 between the spaced apart supports 102a and
 102b in the direction 104 is produced. Heat may be applied by passing a
 current i through the beam 110 and/or by means of an external heater 124.
 The heater 124 may be adjacent the substrate 100 as shown in FIGS. 1A and
 1B or may be a floating heater 124' that is spaced apart from the
 substrate 124 and anchored thereto by heater anchors 126. The design and
 operation of thermal arched beams are described in the above-cited U.S.
 patents and need not be described in detail herein.
 As shown in FIGS. 1B and 1D, two thermal paths are created upon application
 of heat to the beam. A first thermal conduction path Qa extends from the
 beam 110 through the spaced apart supports 102a and 102b and into the
 substrate 100. In FIG. 1B, a second thermal convection path Qb extends
 from the beam 110 through the air gap between the beam 110 and the
 substrate 100 and into the substrate 100. In FIG. 1D, the second thermal
 conduction path Qb extends from the heater 124' and into the substrate 100
 and from the beam 110 into the substrate. These thermal paths can increase
 the power that is used to power the actuator, and/or reduce the
 deflection, force and/or speed of operation thereof.
 According to the invention, at least one thermal isolation structure is
 provided in the first thermal conduction path Qa, that reduces thermal
 conduction from the beam 110, through the spaced apart supports 102a and
 102b and into the substrate 100, and thereby can increase the heat that
 remains in the beam 110. In addition or alternatively, at least one
 thermal isolation structure may be provided in the second thermal
 convection path Qb that reduces thermal conduction from the beam through
 the air gap between the beam 110 and the substrate 100 and into the
 substrate. The thermal isolation structure preferably has lower thermal
 conductivity than the beam 110 and the spaced apart supports 102a and
 102b.
 In general, most of the heat may be lost through the supports 102a and 102b
 in the first thermal conductive path Qa compared to through the second
 thermal convection path Qb. In other words, Qa generally is greater than
 Qb for geometries that generally are used. Moreover, the thermal arched
 beams may be fabricated of nickel, silicon and/or other materials with
 high thermal expansion coefficient. They also may have high thermal
 conductivity. Thus, increasing the beam length L may improve the thermal
 efficiency somewhat.
 FIGS. 2A and 2B are top and side cross-sectional views of first embodiments
 of MEMS structures according to the present invention. As shown in FIGS.
 2A and 2B, a thermally insulating structure, such as thermal isolation
 links 200a and 200b, is included at each end of the beam 110'. The thermal
 isolation links 200a, 200b reduce thermal conduction from the beam 110'
 through the spaced apart supports 102a, 102b and into the substrate 110a,
 to thereby allow increased heat to remain in the beam. The thermal
 isolation links 200a and 200b preferably have lower thermal conductivity
 than the beam 110' and the spaced apart supports 102a and 102b.
 In a preferred embodiment, the thermal arched beam 110' comprises silicon
 and the thermal isolation links 200a and 200b may be fabricated by
 thermally oxidizing end portions of the beam 110', to thereby create
 silicon dioxide links at each end of the arched silicon beam, between the
 arched silicon beam and the spaced apart supports. Thermal oxidation may
 be obtained by masking the beam to expose the ends thereof, and then
 thermally oxidizing using conventional techniques. It will be understood,
 however, that the thermal isolation links 200a and 200b may comprise other
 materials that provide thermal isolation between the beam 110' and the
 spaced apart support 102a and 102b.
 The thermal conductivity of silicon dioxide is approximately 1.4
 W/m.degree. K and is much smaller than the thermal conductivity of silicon
 (150 W/m.degree. K). Thus, by providing silicon dioxide thermal isolation
 links 200a, 200b of length L.sub.0 /2 and a silicon beam 110' of length L,
 a thermal equivalent to a silicon beam of length L+L.sub.0
 (150/1.4).apprxeq.L+107 L.sub.0 may be provided. Accordingly, improved
 isolation may be provided.
 As was described above, in some embodiments, it may be desirable to heat
 the beam 110 by passing a current i therethrough. In this case, it may be
 desirable to provide an electrically conductive structure, such as
 conductive links 210a and 210b, on the respective thermal isolation links
 200a and 200b, to thereby electrically connect the beam 110' with the
 supports 102a and 102b. The conductive structures 210a and 210b may be
 provided by a metal film on the thermal isolation links 200a, 200b.
 Although the conductive links are shown above the beam, other locations
 may be used. The conductive links 210a and 210b may increase the thermal
 conduction in the thermal conduction path Qa somewhat. However, since the
 conductive structures may be provided by thin metal films, minimal
 increase in the thermal conduction path Qa may be obtained.
 It will be understood that portions of the beam 110' and supports 102a,
 102b also may be metallized and that one continuous metallization layer
 may stretch from the supports, onto the thermal isolation links 200a' and
 the beam 110'. Accordingly, as shown in FIG. 2B, a current i may pass
 through the beam 110' from the supports 102a and 102b, notwithstanding the
 presence of the thermal isolating links 200a, 200b. Thus, thermal
 conduction in path Qa may be reduced. It also will be understood that if
 the beam 110' carries an electrical signal, the electrically conductive
 structure may be provided, even though indirect heating may be employed.
 Still referring to FIGS. 2A and 2B, an optional trench 220 may be formed in
 the substrate 100 beneath the beam 110', to provide increased spacing
 between the beam 110' and the surface of the substrate 100 beneath the
 beam. The thermal convection in path Qb thereby may be reduced, to thereby
 allow increased heat to remain in the beam. The trench 220 may be used in
 a self-heated beam, wherein a current i is passed through the beam. It
 also may be used in an embodiment where the beam is heated indirectly by a
 heater 124' that is spaced apart from the substrate 100 as shown in FIGS.
 2C and 2D. It will be understood that the trench 220 may be used
 independent of the thermal isolation links 200a and 200b.
 It also will be understood that the thermal isolation links 200a, 200b need
 not be provided at the ends of the beam 110'. Thus, as shown in FIGS. 3A
 and 3B, thermal isolation links 300a and 300b may be provided at
 intermediate portions of the beam 110, to isolate at least a first portion
 of the beam from remaining portions of the beam. The thermal isolation
 links 300a and 300b preferably have lower thermal conductivity than the
 beam 110' and the spaced apart supports 102a and 102b. An electrically
 conductive structure 310a, 310b also may be provided to allow a current to
 pass across the thermal isolation links 300a, 300b. Indirect heating also
 may be provided.
 FIGS. 4A and 4B illustrate other embodiments of the present invention. As
 shown in FIGS. 4A and 4B, thermally insulating structures 400a and 400b
 are provided in each spaced apart support, to thermally isolate the beam
 110 from at least a portion of the supports 102a' and 102b' and from the
 substrate 100. These embodiments preferably may be utilized when the beam
 110 comprises nickel or other metal that is not readily oxidized in
 contrast with silicon. Accordingly, thermally insulating structures 400a
 and 400b may be provided in the respective spaced apart supports 102a' and
 102b'. For example, silicon nitride tethers may be provided. In this
 regard, the thermally insulating tethers 400a and 400b in the spaced apart
 supports 102a' and 102b' also may be located on the surface of the spaced
 apart supports 102a', 102b', between the supports 102a', 102b' and the
 beam 110. The thermal insulating structures 400a and 400b preferably have
 lower thermal conductivity than the beam 100 and the spaced apart support
 102a and 102b'. When the beam 110 is directly heated by passing a current
 therethrough, an electrically conductive structure 410a and 410b may be
 provided across the respective thermally insulating structure 400a and
 400b to allow current to pass from the support 102a, 102b to the beam 110.
 Also, when direct heating is provided by passing current through beam 110,
 a trench 220 may be provided in the substrate 110, as was described above.
 When indirect heating is provided for the beam 110, electrically
 conductive structures 410a and 410b may be eliminated, but the trench 220
 may be provided.
 FIGS. 5A and 5B are top and side cross-sectional views of yet other
 embodiments of the present invention. As shown in FIGS. 5A and 5B, a
 monolithic structure 540, such as a monolithic nickel structure, may
 provide the beam 110", the spaced apart supports 102a" and 102b" and
 conductive connectors 530a and 530b for the microelectromechanical
 structure. When a monolithic structure 540 is provided, it may be
 difficult to provide thermally isolating links within the beam or between
 the beam and the supports. Accordingly, as shown in FIGS. 5A and 5B, a
 thermally insulating structure 500a and 500b is included in the substrate
 100 adjacent each respectively spaced apart support 102a" and 102b", to
 thereby thermally isolate the beam 110" and the supports 102a" and 102b"
 from at least a portion of the substrate 100. The thermal insulating
 structures 500a and 500b preferably have lower thermal conductivity than
 the beam 110" and the spaced apart supports 102a" and 102b".
 In a preferred embodiment, the thermally insulating structures 500a, 500b
 in the substrate comprise a thermal insulator-containing trench in the
 substrate beneath each spaced apart support. More preferably, the thermal
 insulator-containing trench comprises a silicon dioxide-filled trench in
 the substrate beneath each spaced apart support. The trench may be about
 20 .mu.m deep. Thermal efficiency of directly or indirectly heated beams
 thereby may be increased. When direct heating is used, a trench 220 also
 may be provided in the substrate 100 as was already described. The
 thermally isolating links, conductive structures and silicon
 dioxide-filled trenches may be fabricated using conventional MEMS
 fabrication processes.
 It will be understood by those having skill in the art that the thermal
 isolation structures of FIGS. 2A-2B, 3A-3B, 4A-4B and 5A-5B may be used in
 combination, to provide enhanced thermal isolation in various MEMS
 structures. Subcombinations of these thermal isolation structures also may
 be used. For example, FIGS. 2A-2B and 5A-5B may be combined to provide a
 thermal insulating link at each end of an arched beam and an
 insulator-containing trench in the substrate beneath each spaced apart
 support. The beam may comprise high thermal expansion material, such as
 nickel, silicon, gold, other materials and combinations of materials. The
 thermally isolating link may comprise silicon dioxide, silicon nitride,
 organic dielectrics, other materials and combinations thereof. The thermal
 isolation trench may comprise silicon dioxide, silicon nitride, organic
 dielectrics, other materials and combinations thereof. For direct heating
 with an isolated support and beam, it may be preferred for the beam to
 include a high resistivity material. A thin metal film can be formed on
 the thermal isolating link for electrical connection, as was described
 above.
 Thermal isolation structures according to the present invention may be
 integrated into existing MEMS fabrication processes, such as the well
 known LIGA and MUMPS fabrication processes. Moreover, thermal isolation
 structures according to the present invention may be used for thermal
 beams that are not arched and also may be used for cantilever beams such
 as bimorph beams, that are supported at a single end thereof. Stated
 differently, one of the supports of FIGS. 2A-5B may be eliminated. As is
 well known to those having skill in the art, in any of these embodiments,
 the beam may be coupled to a valve plate, capacitor plate, mirror, relay
 contact, coupler and/or other structure, such as was described in the
 above-cited U.S. patents. Moreover, multiple beams may be coupled together
 to provide increased force and/or increased efficiency. Thermal isolation
 structures according to the present invention thereby can provide higher
 thermal efficiency microelectromechanical structures, to thereby allow
 reduced power, larger deflection, higher force and/or higher speed of
 operation.
 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.