Hysteretic MEMS two-dimensional thermal device and method of manufacture

A MEMS hysteretic thermal device may have two passive beam segments driven by a current-carrying loop coupled to the surface of a substrate. The first beam segment is configured to move in a direction having a component perpendicular to the substrate surface, whereas the second beam segment is configured to move in a direction having a component parallel to the substrate surface. By providing this two-dimensional motion, a single MEMS hysteretic thermal device may by used to close a switch having at least one stationary contact affixed to the substrate surface.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

STATEMENT REGARDING MICROFICHE APPENDIX

Not applicable.

BACKGROUND

This invention relates to a microelectromechanical systems (MEMS) thermal device, and its method of manufacture. More particularly, this invention relates to a MEMS thermal actuator which is constructed with at least two segments, each segment pivoting about a different point, and with motion hysteresis between the heating phase and the cooling phase.

Microelectromechanical systems (MEMS) are very small moveable structures made on a substrate using lithographic processing techniques, such as those used to manufacture semiconductor devices. MEMS devices may be moveable actuators, valves, pistons, or switches, for example, with characteristic dimensions of a few microns to hundreds of microns. A moveable MEMS switch, for example, may be used to connect one or more input terminals to one or more output terminals, all microfabricated on a substrate. The actuation means for the moveable switch may be thermal, piezoelectric, electrostatic, or magnetic, for example.

FIG. 1shows an example of a prior art thermal switch, such as that described in U.S. Patent Application Publication 2004/0211178 A1. The thermal switch10includes two cantilevers,100and200. Each cantilever100and200contains a passive beam110and210, respectively, which pivot about fixed anchor points155and255, respectively. A conductive circuit120and220, is coupled to each passive beam110and210by a plurality of dielectric tethers150and250, respectively. When a voltage is applied between terminals130and140, a current is driven through conductive circuit120. The Joule heating generated by the current causes the circuit120to expand relative to the unheated passive beam110. Since the circuit is coupled to the passive beam110by the dielectric tether150, the expanding conductive circuit drives the passive beam in the upward direction165.

Applying a voltage between terminals230and240causes heat to be generated in circuit220, which drives passive beam210in the direction265shown inFIG. 1. Therefore, one beam100moves in direction165and the other beam200moves in direction265. These movements may be used to open and close a set of contacts located on contact flanges170and270, each in turn located on tip members160and260, respectively. The sequence of movement of contact flanges170and270on tip members160and260of switch10is shown inFIGS. 2a-2d, to close and open the electrical switch10.

To begin the closing sequence, inFIG. 2a, tip member160and contact flange170are moved about 10 μm in the direction165by the application of a voltage between terminals130and140. InFIG. 2b, tip member260and contact flange270are moved about 17 μm in the direction265by application of a voltage between terminals230and240. This distance is required to move twice the 5 μm width of the contacts, a 4 μm initial offset between the contact flanges170and270, and additional margin for tolerances of 3 μm. InFIG. 2c, tip member160and contact flange170are brought back to their initial position by removing the voltage between terminals130and140. This stops current from flowing and cools the cantilever100and it returns to its original position. InFIG. 2d, tip member260and contact flange270are brought back to nearly their original position by removing the voltage between terminals230and240. However, in this position, tip member160and contact flange170prevent tip member260and contact flange270from moving completely back to their original positions, because of the mechanical interference between contact flanges170and270. In this position, contact between the faces of contact flanges170and270provides an electrical connection between cantilevers100and200, such that inFIG. 2d, the electrical switch is closed. Opening the electrical switch is accomplished by reversing the movements in the steps shown inFIGS. 2a-2d.

SUMMARY

The switch construction and method of manufacture may be simplified if a single MEMS actuator is capable of moving in two different directions, rather than having two MEMS actuators each moving in a single direction as shown. If a MEMS actuator is capable of moving in two different directions, then a MEMS switch using a fixed contact may be made using a single MEMS actuator. Furthermore, if the motion of the device is hysteretic, i.e. the motion is different upon heating than it is upon cooling, the actuator may be designed so as to latch in a détente position against the contact. If such an actuator can be designed, then the control of the switch may also be simplified, because only the single actuator may need to be controlled. Accordingly, it is desirable to design and fabricate a MEMS actuator which is capable of moving in two substantially different directions, and with motion which is hysteretic.

A MEMS device is described, which includes a cantilevered beam that bends about one or more points in at least two substantially different directions. The MEMS device also includes a driving means coupled to the cantilever, wherein the driving means may include a drive beam tethered to the cantilever by at least one tether. Upon heating the drive beam, the drive beam expands to deform the cantilever. Upon cooling the drive beam, a heat sink located near the anchor point causes the drive beam to cool with a different temperature profile than it did upon heating, and therefore the cantilever deflects along a different trajectory upon cooling than it did upon heating.

Embodiments of the MEMS device are described, which include a MEMS thermal actuator that may extend in two orthogonal directions by having at least two segments disposed orthogonally to each other. Each segment bends about a different point. Therefore, the MEMS hysteretic thermal actuator may have articulated motion, and be capable of moving in two substantially different directions.

Furthermore, the MEMS segmented thermal actuator may move along one trajectory while heating up, but may move in a second, substantially different trajectory while cooling down. In other words, the motion of the segmented thermal actuator may be hysteretic during the heating phase compared to the cooling phase. The segmented, hysteretic thermal actuator may therefore be used to close and latch an electrical switch, for example, as well as in any of a number of different applications, such as valves or pistons, which may require articulated, hysteretic motion.

Several embodiments of the MEMS segmented, hysteretic thermal actuator are disclosed. In a first embodiment, two substantially different directions of motion are achieved by including a substantially ninety-degree bend between two segments of a cool beam of the thermal actuator. A current-carrying element provides a hot driving beam, which expands relative to the cool beam. The current-carrying element is disposed adjacent to the two segments of the cool beam and heats up as current is driven through it. The current-carrying element expands upon heating, driving the first segment of the cool beam in one direction before the substantially ninety-degree bend, and driving the second segment of the cool beam in another direction after the substantially ninety-degree bend. Because the temperature profile of the beam depends on whether the beam is being heated or cooled, the beam moves differently upon heating than it does upon cooling, and therefore the motion is hysteretic.

In another exemplary embodiment, the MEMS segmented, hysteretic device consists of two segments and a rigid link joining the first segment to the second segment in an approximately rectilinear fashion. Upon heating an adjacent hot driving beam, the hot driving beam bends the first segment about its anchor point. Upon further heating, the hot driving beam bends the second segment about the rigid link. Upon cooling, the bending of the first segment about the anchor point relaxes before the second segment about the rigid link. Therefore, the motion of the MEMS segmented actuator is hysteretic, being different upon heating than upon cooling.

In yet another exemplary embodiment, the segments of the MEMS segmented, hysteretic device are oriented to move in two substantially different planes. A flexure joins the two segments. A driving beam is a circuit which is disposed adjacent to the segments, such that the driving beam drives the device in two different planes of motion, one about the anchor point and the other about the flexure.

These and other features and advantages are described in, or are apparent from, the following detailed description.

DETAILED DESCRIPTION

Although the systems and methods described herein are applied to an electrical switch, it should be understood that this is only one embodiment, and that the systems and methods may be appropriate for any number of devices, such as valves, pistons and other devices using actuators.

A MEMS hysteretic device is described, which includes a cantilevered beam that extends from an anchor point in at least two orthogonal directions. The MEMS hysteretic device also includes a driving means coupled to the cantilever, wherein the driving means may include a drive beam coupled to the cantilever by at least one tether. Upon heating the drive beam, the drive beam expands to deform the cantilever. Upon cooling the drive beam, a heat sink located near the anchor point causes the drive beam to cool with a different temperature profile than it did upon heating, and therefore the cantilever deflects along a different trajectory upon cooling than it did upon heating.

FIG. 3is a diagram illustrating a first exemplary embodiment of a MEMS hysteretic thermal actuator500. MEMS hysteretic thermal actuator500includes two beam segments300and400, which are joined at a substantially ninety-degree joint460. The first segment300includes a first drive beam portion320disposed adjacent, and coupled to a first cool, passive beam portion310. Similarly, the second segment400includes a second drive beam portion420disposed adjacent, and coupled to a second cool, passive beam portion410. Current is input to the drive beam portions320and420at contacts330and340, and the current circulating in the drive beam circuit heats portions320and420by joule heating. The drive beams320and420are mechanically coupled to the passive beam portions310and410by dielectric tethers350and450, respectively. The dielectric tethers350and450may be made of any convenient, non-conducting material which couples the drive beam portions320and420to segmented passive beam portions310and410mechanically, but not electrically. In one embodiment, dielectric tethers350and450may be made from an epoxy-based photoresist such as SU-8, a negative photoresist developed by IBM of Armonk, N.Y.

The heat generated in the drive beam circuit flows out predominantly through the contacts330and340, and to a lesser extent by radiation and convection to the closely spaced substrate, about 4 um from the drive beam circuit. Because heat is generated all along the drive beams320and420, and flows out predominantly through the contacts330and340which act as heat sinks, the point in the drive beam circuit which is at the maximum temperature starts out being located adjacent to the ninety degree joint460or at a location approximately midway to the distal end of the drive beam420. As the temperature continues to rise, however, the location of maximum temperature begins to move out along the drive beam circuit, away from contacts330and340. If the duration of the current pulse is long enough, the point of maximum temperature will occur near the distal end of the drive circuit. The heat generated causes the first drive beam portion320and the second drive beam portion420to expand, which bends the first segment300in the negative x-direction325about the anchor point360, and bends the second segment400in the positive y-direction425about the substantially ninety-degree joint460.

When the current pulse ceases, the drive beam begins to cool. Since the dominant heat sink is located at the contacts330and340, the first drive beam portion320, located closer to the heat sink330and340, cools faster than the second drive beam portion420, which is located further from heat sinks330and340. As a result, the first segment300of the MEMS hysteretic thermal actuator500relaxes before the second segment400. Therefore, when the MEMS hysteretic thermal actuator500is heating, it bends in a trajectory that is different from the trajectory upon cooling, resulting in hysteretic behavior when the trajectory is plotted on a graph, as described below.

FIG. 4is a plot showing the results of a mathematical simulation using an ANSYS multi-physics finite element model, which simulates the deflection of the tip of the passive beam that results from the heating of the drive beam with a square wave current pulse. The current pulse used forFIG. 4is 190 mA amplitude and 3 μsec duration. Each point in the plot corresponds to an equal increment of time. As shown inFIG. 4, the tip of the MEMS hysteretic thermal actuator500moves in the positive y-direction and the negative x-direction (the x- and y-axes are indicated inFIG. 3). The movement in the positive y-direction is accomplished largely by beam portion400, and movement in the negative x-direction is accomplished largely by beam portion300. The slope of the trajectory of the tip end is approximately −6, such that for every displacement of −1 μm in the negative x-direction, the y-displacement increases by about 6 μm. At the upper left of the displacement trajectory, at the point labeled A, the current pulse ceases, and the drive beam begins to cool. The cooling, as described above, relaxes the beam portion300first in the x-direction, followed by the beam portion400in the y-direction, so that the trajectory of the beams upon cooling is different than the trajectory of the beams upon heating. This is illustrated by the hysteresis seen in the curves shown inFIG. 4. The hysteresis is evident in the different slope of the upper trajectory compared to the lower trajectory. The slope of the upper trajectory is about 5.6 compared to the slope of about 6 for the lower trajectory. The nominal difference in the location of the tip end on the upper trajectory compared to the lower trajectory is on the order of about 5 μm for this current waveform. This hysteresis may be used to latch and unlatch the MEMS hysteretic actuator, when the actuator is used in a switch for example, as described further below.

Returning toFIG. 3, MEMS hysteretic thermal actuator500may be used to open and close an electrical switch. To implement this switch, MEMS hysteretic thermal actuator500is formed with a contact470, which is adjacent to another contact480which is rigidly affixed to the substrate. The two contacts470and480may be made of different material than segmented beams310,320,410and420. The contacts470and480may be made of a material which has a low contact resistance relative to the material of segmented beams310,320,410and420. In one embodiment, the contacts470and480are gold, however, other materials such as gold-cobalt alloy, palladium, etc., may be used as well. In this embodiment, an electrical signal may flow from segmented beams310and410to contact470and then to contact480when the switch is closed. However, in another alternative embodiment described below with respect toFIGS. 19,20aand20b, the electrical signal may flow between two contacts located beyond the tip of segmented beam410, rather than through segmented beam410to the contact.

In the quiescent state, the two contacts470and480of MEMS hysteretic thermal switch500may be located adjacent to each other, rather than one in front of the other as is the case with contact flanges170and270shown inFIG. 1.

Because of the location of contacts470and480may be adjacent to one another, contact470does not need to be retracted as was shown inFIG. 2a. Instead, the sequence of motion for the MEMS hysteretic thermal actuator500is shown as shown inFIG. 4, wherein upon energizing the drive beam, the tip of the cool beam moves up and to the left. Upon cessation of the drive current pulse, the cool beam relaxes on the upper trajectory shown inFIG. 4, whereupon it becomes engaged on contact480, because it relaxes more quickly in the x-direction than the y-direction. The spring constant of the MEMS hysteretic thermal actuator500causes the switch to remain latched, because it exerts a normal force on the contact surfaces470and480. The contact surfaces470and480remain engaged because of friction between the contact surfaces470and480. Alternatively, the contact surfaces470and480may be shaped so that they remain engaged even without friction. Techniques and design considerations for such a switch are described in U.S. patent application Ser. No. 11/263,912, herein the '912 application), which is incorporated by reference in its entirety for all purposes.

To unlatch the MEMS hysteretic actuator500, a square wave current pulse may again be applied to the drive beams320and420. The unlatching current pulse may be of a lower amplitude and/or shorter duration than the latching current pulse. The resulting movement of the MEMS hysteretic thermal actuator releases the MEMS hysteretic thermal actuator from its engagement with contact480. The restoring force of beam portion400may be designed to provide sufficient retraction of beam portion400to clear the engaging contact480. The unlatch pulse may also be tailored in pulse shape, magnitude and duration to assure that MEMS hysteretic actuator500is released from the latched position.

The hysteresis shown inFIG. 4may also be enhanced, if needed, by tailoring the shape of the current pulse applied to drive beams320and420. For example, if the first 3 μsec, 190 mA current pulse is followed immediately by a second, lower current pulse of 160 mA for another 3 μsec, the trajectory of the tip of the MEMS hysteretic actuator500is as shown inFIG. 5. InFIG. 5, the second current pulse is applied at the point labeled B in the graph. The current ceases at the point labeled C in the graph, and the upper trajectory describes the relaxation of the MEMS hysteretic actuator500. The result of the second current pulse is to hold the MEMS hysteretic thermal actuator500in approximately its deformed shape, while the additional heat provided by the additional current moves the point of maximum temperature from a location about ⅔ down the length of the MEMS hysteretic thermal actuator500to the tip end of the MEMS hysteretic thermal actuator500. As a result, the hysteresis experienced by the cooling MEMS hysteretic thermal actuator500may be exaggerated, because the heat built up in the tip end of the MEMS hysteretic thermal actuator500takes longer to dissipate through the far-removed contacts330and340.

AlthoughFIG. 5shows results for one particular example of a tailored pulse shape, it should be clear that a large number of alternative pulse shapes or pulse trains can be envisioned, such as a triangular, ramped or saw-toothed pulse shape, to accomplish other objectives with the MEMS hysteretic actuator500, or enhance its performance in other ways.

Additional features of the MEMS hysteretic actuator500may be used to adjust the deflection of the MEMS hysteretic actuator500. For example, areas in the passive beams310and410may be removed to form a flexible hinge, to enhance the deflection of the passive beams310and410about their respective anchor points. Design considerations and implementation of such features are described further in the incorporated '912 application.

FIG. 6is a diagram illustrating a second exemplary embodiment of the MEMS hysteretic thermal actuator800. In the second exemplary embodiment, as in the first exemplary embodiment, the MEMS hysteretic thermal actuator800includes two beam portions600and700coupled by a substantially ninety-degree joint760. Beam portions600and700are coupled to pivot anchor660and joint760, respectively. Each beam portion600and700includes a drive beam portion620and720and a cool beam portion610and710. However, in the second exemplary embodiment, the drive beam portions620and720are disposed on the opposite side of passive beam portions610and710, compared to the first exemplary embodiment. For this reason, MEMS hysteretic thermal actuator800bends in an opposite sense to MEMS hysteretic thermal actuator500, as drive beam portion620tends to bend passive beam portion610in the positive x-direction625rather than the negative x-direction. Similarly, drive beam portion720tends to bend passive beam portion710in the negative y-direction725rather than the positive y-direction. Upon cooling, because of its proximity to the heat sink of contacts630and640, the drive beam620cools more rapidly than drive beam720, resulting in hysteretic behavior of the MEMS hysteretic actuator800. Therefore, the behavior of this MEMS hysteretic actuator800, if plotted on a graph similar toFIGS. 4 and 5, would show the inverse behavior. The tip end of the MEMS hysteretic thermal actuator would therefore be driven to the lower right ofFIG. 6. Accordingly, to make an electrical switch using MEMS hysteretic thermal actuator800, the contacts770and780would be placed as shown inFIG. 6.

FIG. 7is a diagram illustrating a third exemplary embodiment of the MEMS hysteretic actuator1100. As with the previous embodiments, the MEMS hysteretic thermal actuator1100includes two beam portions900and1000coupled by a substantially ninety-degree joint1060. Each beam portion900and1000includes a drive beam portion920and1020and a passive beam portion910and1010. Drive beam portions920and1020may be coupled to passive beam portions910and1010by tethers950and1050, respectively. Tethers950and1050may be thermally insulating, though not necessarily electrically insulating.

In the second exemplary embodiment, the drive beam portions920and1020are disposed adjacent to a heater element930, which supplies heat to the drive beam portions920and1020. The heater element also has a heat sink940disposed at its base, which dissipates heat when the heater element930is disabled. The heater element930may include, for example, an electrical circuit arranged in a serpentine pattern within heater element930. For simplicity of depiction, however, the electrical circuit is not shown inFIG. 7, and the heater element930is shown as a simple outline overlaying drive beam portions920and1020. It should be understood, however, that the heater element930may generate heat in any of a number of other ways, for example, it may be an optically opaque element which absorbs incident light.

Upon becoming heated by the heater element930, drive beam portions920and1020expand, driving passive beam portions910and1010in directions925and1025, respectively. Upon cooling, because of its proximity to the heat sink940of heater element930, the drive beam920cools more rapidly than drive beam1020, resulting in hysteretic behavior of the MEMS hysteretic actuator1100. Accordingly, the behavior of MEMS hysteretic thermal actuator1100is similar to that of MEMS hysteretic thermal actuator500, and can be described qualitatively by the plots shown inFIGS. 4 and 5. A latching electrical switch may be made using MEMS hysteretic actuator1100, by disposing contacts1070and1080as shown inFIG. 7.

FIG. 8is a diagram illustrating a fourth exemplary embodiment of MEMS hysteretic actuator1400. As with the previous embodiments, the MEMS hysteretic thermal actuator1400includes two beam portions1200and1300coupled by a substantially ninety-degree joint1360. Each beam portion1200and1300includes a drive beam portion1220and1320and a passive beam portion1210and1310. The drive beam portions1220and1320are separated from the passive beam portions1210and1310by a dielectric barrier1230which extends out toward the end region of the second beam portion1300, but ends before the edge of second beam portion1300. The flexibility of the two segments1200and1300to bending about the anchor point1260and rigid link1360, respectively, may be adjusted by removing an area of material1230and1330, near their pivot points, which causes segments1200and1300to pivot more easily about these points.

The drive beam portions1220and1320may be formed of a material having a higher coefficient of thermal expansion (CTE), relative to passive beam portions1210and1310, which are formed of a material having a lower coefficient of thermal expansion. However, all of beam portions1220,1320,1210and1310are electrically conductive. A current is driven through drive beam portions1220and1320to the end of the second beam portion1300, whereupon the current reverses direction and flows out through passive beam portion1310and1210. The current causes joule heating in both beam portions1200and1300. However, because drive beam portions1220and1320are formed from a material having a higher coefficient of thermal expansion relative to passive beam portions1210and1310, drive beam portions expand relative to passive beam portions1210and1310. Accordingly, drive beam portions1220and1320bend the passive beam portions1210and1310about anchor point1260and substantially ninety-degree joint1360, respectively. Upon cooling, because of its proximity to the heat sink of anchor point1260, the drive beam1220cools more rapidly than drive beam1320, resulting in hysteretic behavior of the MEMS hysteretic actuator1400. Accordingly, the behavior of MEMS hysteretic thermal actuator1400is similar to that of MEMS hysteretic thermal actuator500, and can be described by plots similar to those shown inFIGS. 4 and 5. A latching electrical switch may be made using MEMS hysteretic actuator1400, by disposing contacts1370and1380as shown inFIG. 8.

FIG. 9is a diagram illustrating a fifth exemplary embodiment of MEMS hysteretic thermal actuator1700. As with the previous embodiments, the MEMS hysteretic thermal actuator1700includes two beam portions1500and1600coupled by a substantially ninety-degree joint1660. Each beam portion1500and1600includes a drive beam portion1520and1620and a passive beam portion1510and1610. The fifth exemplary embodiment is also similar to the fourth exemplary embodiment, in that the drive beam portions1520and1620are formed from a material having a higher coefficient of thermal expansion, and the passive beam portions1510and1610are formed from a material having a lower coefficient of thermal expansion. However, in the fifth exemplary embodiment, beam portions1510,1520,1610and1620need not be electrically conductive, because heat is supplied by a heater element1530. Heater element1530may be a conductive circuit with wires formed in a serpentine pattern, or may be any other device capable of generating heat. The heat generated by heater element1530is absorbed by drive beam portions1520and1620, as well as passive beam portions1520and1610. However, because drive beam portions1520and1620are formed from a material having a higher coefficient of thermal expansion relative to passive beam portions1510and1610, drive beam portions expand relative to passive beam portions1510and1610. Accordingly, drive beam portions1520and1620bend the passive beam portions1510and1610about anchor point1560and substantially ninety-degree joint1660, respectively. Upon cooling, because of its proximity to the heat sink of the anchor point1560, the drive beam1520cools more rapidly than drive beam1620, resulting in hysteretic behavior of the MEMS hysteretic actuator1700. Accordingly, the behavior of MEMS hysteretic thermal actuator1700is similar to that of MEMS hysteretic thermal actuator500, and can be described by plots similar to those shown inFIGS. 4 and 5. A latching electrical switch may be made using MEMS hysteretic actuator1700, by disposing contacts1670and1680as shown inFIG. 9.

FIG. 10is a diagram illustrating a sixth exemplary embodiment of the MEMS hysteretic thermal actuator2000. Like the previous embodiments, MEMS hysteretic thermal actuator2000includes two beam portions1800and1900coupled by a substantially ninety-degree joint1960. Each beam portion1800and1900includes a drive beam portion1820and1920and a passive beam portion1810and1910, which are coupled by tethers1850and1950, respectively. The drive beam portions1820and1920are formed from stiff, thermally conductive materials. Drive beam portion1820is in thermal communication with a circuit1805, which generates heat at the base of the drive beam portion1820. The heat generated by circuit1805is conducted by thermally conductive drive beam portion1820to thermally conductive drive beam portion1920, causing drive beam portions1820and1920to heat up. Accordingly, the drive beam portions1820and1920are required to be thermally conductive, but may not be electrically conductive.

The heating of drive beam members1820and1920causes drive beam portions1820and1920to expand. The expansion of drive beam portion1820causes driven beam1810to bend about anchor point1860in the negative x-direction1825. Similarly, the expansion of drive beam portion1920causes driven beam portion1910to bend about substantially ninety-degree joint1960in the positive y-direction1925. Upon cooling, because of its proximity to the heat sink of electrical circuit1805, the drive beam1820cools more rapidly than drive beam1920, resulting in hysteretic behavior of the MEMS hysteretic actuator2000. Accordingly, the behavior of MEMS hysteretic thermal actuator2000may be similar to that of MEMS hysteretic thermal actuator500, and may be described by plots similar to those shown inFIGS. 4 and 5. A latching electrical switch may be made using MEMS hysteretic actuator2000, by disposing contacts1970and1980as shown inFIG. 10.

FIG. 11is a diagram of a seventh exemplary embodiment of a MEMS hysteretic thermal actuator2300. Like the previous embodiments, MEMS hysteretic thermal actuator2300includes two beam portions2100and2200coupled by a rigid link2260. However, in this embodiment, the rigid link2260does not join the two beam portions2100and2200at a substantially ninety-degree angle. Instead, rigid link2260joins beam portion2100and2100in a rectilinear fashion. Rigid link2260provides a distinct pivot point for beam portion2200compared to beam portion2100, which may pivot about anchor point2160. Accordingly, the presence of rigid link2260allows MEMS hysteretic actuator2300to move in two substantially different directions, with at least about a five degree angle between these directions. Each beam portion2100and2200includes a drive beam portion2120and2220and a passive beam portion2110and2210, which are coupled by tethers2150and2250, respectively.

Heat is generated in drive beam portions2120and2220by applying a voltage between contacts2130and2140. Current flows in drive beam portions2120and2220as a result of the voltage, which heats drive beam portions2120and2220by joule heating. Drive beam portions2120and2220expand because of their increased temperature. Because drive beam portions2120and2220are tethered to passive beam portions2110and2210by tethers2150and2250, the expansion causes passive beam portion2110to bend about anchor point2160, and passive beam portion2210to bend about rigid link2260. Upon cooling, the drive beam portion2120cools faster than drive beam portion2220, because of its closer proximity to the heat sink of contacts2130and2140. As a result, the motion of MEMS hysteretic thermal actuator2300is hysteretic, as the thermal profile of the MEMS hysteretic thermal actuator2300is different upon heating than it is upon cooling. By disposing contacts in the appropriate locations on MEMS hysteretic thermal actuator2300, a latching electrical switch may be formed.

Although embodiments have been described wherein the first segment is joined to the second segment at an angle of about zero degrees (FIG. 11) and an angle of about ninety degrees (FIGS.3and6-10), it should be understood that any other angle greater than or equal to about zero degrees and less than or equal to about ninety degrees may also be used in the MEMS hysteretic actuator.

FIG. 12is a schematic side view of an eighth exemplary embodiment of a MEMS hysteretic thermal actuator2600. Like the previous embodiments, MEMS hysteretic thermal actuator2600includes two beam portions2400and2500coupled by a rigid link2560. Each beam portion2400and2500includes a drive beam portion2420and2520and a passive beam portion2410and2510, which are coupled by tethers2450and2550, respectively. However, in this embodiment, drive beam portions2420and2520are disposed such that they drive the driven beam portions2410and2510in two different planes. In particular, drive beam portion2420is oriented to bend passive beam portion2410about fixed anchor point2460in direction2425, indicated inFIG. 12. Drive beam portion2520is oriented to bend passive beam portion2510about the rigid link2560in the direction2525, which is into the paper as indicated inFIG. 12. Accordingly, MEMS hysteretic thermal actuator has one component bending in the plane of the paper, and another component bending in a plane orthogonal to the paper. Like the previous embodiments, current is input to drive beam portions2420and2520by applying a voltage between contacts2430and2440. The current heats the drive beams by joule heating, and the resulting expansion of the drive beam portions2420and2520causes the bending of the passive beams2410and2510described above. The contacts2430and2440also provide a heat sink for the drive beam portions2420and2520. Accordingly, drive beam portion2420located nearer to heat sink contacts2430and2440cools more quickly than drive beam portion2520located farther from heat sink contacts2430and2440. Therefore, the motion of MEMS hysteretic thermal actuator2600, like MEMS hysteretic thermal actuators500-2300is hysteretic between the heating phase and the cooling phase.FIG. 13is a perspective view of the eighth exemplary embodiment described above.

One of the issues with the eighth exemplary embodiment of MEMS hysteretic thermal actuator2600is difficulty of manufacturing. As shown inFIG. 13, in the first segment2400of MEMS hysteretic thermal actuator2600, the cantilevered drive beams2420are disposed adjacent to the passive beam portion2410, whereas in the second portion2500of MEMS hysteretic thermal actuator2600, the cantilevered drive beams2420are disposed above the passive beam portion2510. Because of the orientations of the drive beam portions2420and2520with respect to the passive beam portions2410and2510, the drive beam portions cannot be formed or deposited in a single step. This complicates the manufacturing process flow for making MEMS hysteretic thermal actuator2600.

FIG. 14illustrates a ninth exemplary MEMS hysteretic thermal actuator3000, which may be easier to manufacture than MEMS hysteretic thermal actuator2600. MEMS thermal device3000is similar to hysteretic MEMS thermal device2600, in that it is designed to move in two substantially orthogonal planes. In hysteretic MEMS thermal device3000, the drive beam is separated into two segments,3200and3300, which are separated by a flexure3250. The flexure3250serves to decouple the motion in the two orthogonal planes, which motion is driven by a first segment3200and a second segment3300. The first plane of motion is out of the plane of the paper, in direction3225, as indicated inFIG. 14. This motion is produced by fabricating the cantilevered drive beam3200slightly below the plane of the passive beam segment3500, relative to the substrate surface. That is, cantilevered drive beam3200is closer to the substrate surface than passive beam segment3500. More generally, the driving beam segment3200may have an average elevation different than the average elevation of the passive beam segment3500, wherein the average elevation is along the longitudinal axis of driving beam3200. Accordingly, when cantilevered drive beam segment3200is heated by passing a current through cantilevered drive beam segment3200by applying a voltage to pads3130and3140, it expands about the pads3130and3140anchored to the surface of the substrate. Because passive beam segment3500is not heated, it does not expand, so that cantilevered drive beam segment3200is forced to bend itself and passive beam segment3500upward, away from the substrate surface, about the anchor point3530of the passive beam segment3500.

Then, the second segment3300of cantilevered drive beam begins to heat as the current is passes through it. It also expands as a result of the heat, and begins to move in direction3325, bending passive beam segment3600in this direction. Cantilevered drive beam segment3300and passive beam segment3600move in direction3325because they are formed in the same plane, so that as cantilevered drive beam segment3300heats up, it bends toward the unheated passive beam segment3600in direction3325. Accordingly, driving beam segment3300has an average elevation substantially the same as passive beam segment3600.

At the joint between cantilevered drive beam segment3200and cantilevered drive beam segment3300is flexure3250. The flexure3250consists of a length of the hot driving beam which bends away from and then back toward a knee3255in the passive beam, thus adding length to the hot driving beam. This additional length adds to the heat produced in the hot driving beam, by increasing its resistance. The amount of heat created within the flexure3250is larger than in the driving beam segments3200and3300because of the larger distance flexure3250is from the passive beam segments3500and3600which may act as radiative heat sinks. Therefore, the flexure3250acts as a heat choke, which impedes the flow of heat from the tip of the driving beam segment3300back to the anchor points3130and3140, which act as the primary heat sink for the device. The presence of the flexure therefore enhances the hysteresis of the device, because heat built up in the tip of the driving beam segment3300has greater difficulty returning to the heat sink anchor point3130and3140than heat built up in the first driving beam segment3200. It should be understood that additional lengths of driving beam may also be added to driving beam segment3300to increase the temperature at the distal end of MEMS hysteretic thermal actuator3000, thus increasing the hysteresis of the actuator3000.

In addition, flexure3250acts as a mechanical component to decouple the out-of-plane motion of the first beam segments3200and3500, from the in-plane motion of the second beam segments3300and3600. This function is provided primarily by the presence of dielectric spacers3251and3253between the driving beam segment3200and the knee3255of the passive segment3500and dielectric spacers3355between driving beam segment3300and passive segment3600. Dielectric spacers3253act to transmit the out-of-plane torque produced by driving beam segment3200to passive beam segment3500by tethering the segments together at that point to bend the passive segment3500in direction3225. Dielectric spacers3251act as an anchor for the bending of driving beam segment3300in direction3325. The torque from driving beam segment3300is transmitted to the passive segment3600by dielectric spacers3355, to bend the passive beam in direction3325, at hinge flexure3650.

In one exemplary embodiment, flexure3250may have a width a of about 48 μm and a height b of about 32 μm. The length of driving beam segment3200may be 270 μm, with a beam segment width of about 5 μm. The length of driving beam segment3300may be about 128 μm, so that the flexure3250is located about ⅔ of the distance between the anchor points3130and3140and the tip of driving beam segment3300. However, it should be understood that these dimensions are exemplary only, and that other shapes and dimensions may be chosen depending on the requirements of the application. For example, the flexure3250may alternatively be located at about ⅓ or ½ of the distance between the anchor points3130and3140and the tip of the driving beam segment3300.

FIG. 15shows the ninth exemplary embodiment in perspective view, in which the construction of the ninth embodiment is more clearly evident. The first driving beam segment3200is fabricated on a lower plane than passive beam segment3500. This allows the driving beam segment3200to bend the passive beam segment3500up and out of plane. In contrast, driving beam segment3300is in the same plane as passive segment3600, in order to bend passive segment3600in the same plane as driving beam segment3300. Thus, a step up occurs in the driving beam between driving beam segments3200and3300. This step up may occur adjacent to dielectric tethers3253, as the driving beam segment3200enters flexure region3250. Accordingly, passive beam segments3500and3600may all be in the same plane, allowing the cool, passive beam segments3500and3600to be formed in a single process step, and simplifying manufacturing. Similarly, the driving beam segments3200and3300are all formed on the same side of passive beam segments3500and3600, allowing the driving beams segments to be formed in a single deposition step, as described below.

Some additional features shown inFIG. 15give the ninth exemplary embodiment several advantageous characteristics. First, passive beam segment3600has a relieved, hinged area3650, which gives it additional flexibility to bend in direction3325. Secondly, passive beam segment3500has a rectangular box shape. This gives passive beam segment3500a high moment of inertia in the in-plane direction but a low moment of inertia in the out of plane direction. This configuration enhances the ability of MEMS hysteretic thermal actuator3000to move out of plane rather than in-plane.

Additional structures, such as heat sink3550may be added to passive beam segment3500to assist with transferring heat through conduction, convection, and radiation away from passive beam segment3500. The temperature difference between driving beam segment3200and passive beam segment3500may be proportional to the magnitude of out of plane movement that can be achieved. Heat may be transferred to passive beam segment3500from driving beam segment3200and the insulators3253. Passive beam segment3500does not have a good thermal conduction path to anchor3530due to its thin width required to lower the out of plane stiffness. Additional heat sink3550may be added to increase the area of passive beam segment3500that can transmit heat without changing the stiffness. If more in-plane stiffness is required, heat sink3550can be designed to triangulate the box shape and increase the in-plane stiffness significantly more than the out of plane stiffness. These features thus enhance the ability of MEMS hysteretic thermal actuator3000to move out of plane rather than in-plane. Thus, the out-of-plane motion of passive beam segment3500is effectively decoupled from in-plane motion of passive beam segment3600by structures3550and3650, as well as by flexure3250.

The hysteretic effect in the ninth exemplary embodiment of MEMS hysteretic thermal actuator3000arises from the same effect as for MEMS hysteretic thermal actuators500-2600. The hysteresis arises from the proximity of the primary heat sink to one of the segments of MEMS hysteretic thermal actuator3000. This cools the segment nearest the heat sink faster than the segment further away from the heat sink, whereas during heating, the two segments are heated relatively uniformly. In the case of MEMS hysteretic thermal actuator3000, both segments3200and3300are heated relatively uniformly, moving the tip3600diagonally away from the surface of the paper, up from the surface of the paper because of driving beam segment3200and downward in direction3325because of the action of driving beam segment3300. However, upon cooling, driving beam segment3200cools faster than driving beam segment3300, because of its closer proximity to heat sinking anchor points3130and3140. MEMS hysteretic thermal actuator3000may therefore move back toward the substrate surface before relaxing back in the upward direction, opposite to direction3325. Accordingly, because MEMS hysteretic thermal actuator moves through different points upon heating as it does upon cooling, MEMS hysteretic thermal actuator has a different trajectory upon activation, for example upon heating, than it does upon relaxation, for example upon cooling. For this reason, MEMS hysteretic thermal actuator3000may be used to rise up and over a stationary contact3700, landing on the contact at the end of a latching pulse and remaining there because of frictional forces, or by forming a détente structure on the contact. The MEMS hysteretic thermal actuator3000may then be unlatched by applying a current pulse of appropriate amplitude and duration.

The ability of MEMS hysteretic thermal actuator3000to rise up and over a stationary contact3700, is illustrated inFIGS. 16-19.FIG. 16depicts MEMS hysteretic thermal actuator3000in its initial position, adjacent to a stationary contact3700. By applying a voltage to anchor points3130and3140, a current flows through driving beam segments3200and3300, heating them by Joule heating. This causes driving beam segment3200to expand relative to passive segment3500, such that driving beam segment3200bends passive segment3500about its anchor point3530, and away from the plane of the paper. At the same time, driving beam segment3300also heats and expands, bending passive segment3600about its pivot point at hinged flexure3650. This causes MEMS hysteretic thermal actuator3000to rise up and over stationary contact3700, as shown inFIG. 17.

When the voltage applied to anchor points3130and3140is removed, current ceases to flow and heat ceases to be generated. At this point, the heat flows back out of the device mainly through anchor points3130and3140. This cools driving beam segment3200before driving beam segment3300cools, which lowers MEMS hysteretic thermal actuator3000onto the stationary contact3700. This situation is depicted inFIG. 18. MEMS hysteretic thermal device3000may remain latched on stationary contact by frictional forces or a combination of frictional and normal forces. This latching may close, for example, an electrical switch.

Using a latching current pulse of about 3000 μsec in duration and about 160 mA in amplitude, MEMS hysteretic thermal actuator3000may be expected to move a total diagonal distance of about 11 μm, rising by about 8 μm and moving laterally about 8 μm.

When it is desired to open the electrical switch, a shorter duration and/or lower amplitude pulse may be applied to MEMS hysteretic thermal actuator3000, which will raise the driving beam segment3200sufficiently to free the tip of passive segment3600from the stationary contact, as described earlier with respect to MEMS hysteretic thermal actuator500. The unlatching is shown inFIG. 19. MEMS hysteretic thermal actuator3000will then immediately move back in direction3326. In one exemplary embodiment, the unlatching pulse has a duration of 1000 μsec and 160 mA.

Although the embodiments500-3000described above each have at least two substantially straight beam segments in the cantilever, it should be understood that a MEMS hysteretic device may also be formed using a single cantilevered arcuate beam. In this embodiment, the arcuate actuator is disposed adjacent to an arcuate drive beam, and tethered to the drive beam by at least two dielectric tethers, one at the tip of the arcuate actuator and one at an intermediate location. The amount of hysteresis provided by such an arcuate embodiment may depend on the curvature of the cantilever and the location of the dielectric tethers. However, in general, the cantilever having two segments disposed orthogonally to each other may have a larger amount of hysteresis, and may therefore be more suitable for making a latching switch.

An exemplary method for fabricating the MEMS hysteretic actuator3000will be described next. Although the method is directed primarily to the fabrication of MEMS hysteretic actuator3000, it should be understood that the method may be adapted to fabricate MEMS hysteretic actuators500-2600. The MEMS hysteretic actuator3000may be fabricated on any convenient substrate4000, for example silicon, silicon-on-insulator (SOI), glass, or the like.

Because inFIGS. 20-27, the MEMS hysteretic actuator is shown in cross section, only a few of the beam segments of the MEMS hysteretic thermal actuator can be seen in the figures, as the other segments may be disposed in a plane which is not easily depicted in the figures. For example, inFIG. 27, element4200may be understood to depict driving beam segment3200in MEMS hysteretic actuator3000. Element4300, corresponding to driving beam segment3300, lies partially in the same plane as element4200, and behind element4200, and is thus partially obscured by element4200. However, it should be understood that the driving beam segment3300may be formed at the same time as, and using similar or identical processes to those used to form the driving beam segment4200which is depicted inFIGS. 20-27. Similarly, only one structure4500, may represent passive segment3500of the MEMS hysteretic thermal actuator3000inFIGS. 20-27. However, it should be understood that all components of passive beam3500, for example heat sink3550may be formed at the same time, using similar or identical process steps used to form structure4500. Accordingly, both the second driving beam portion3300, the heat sink3550and second passive segment3600of MEMS hysteretic thermal actuator3000may be formed at the same time, using the same process steps and materials, as used to form segments4200,4300and4500, representing driving beam segment3200,3300and passive segment3500, respectively.

FIG. 20illustrates a first exemplary step in the fabrication of the MEMS hysteretic thermal actuator3000. The process begins with the deposition of a seed layer4010over the substrate4000. The seed layer4010may be chromium (Cr) and gold (Au), deposited by plasma vapor deposition (PVD) to a thickness of 100-200 nm. Photoresist (not shown) may then be deposited over the seed layer4010, and patterned by exposure through a mask. A sacrificial layer4020, such as copper, of a thickness of 2 μm may then be electroplated over the seed layer through the photoresist, as depicted inFIG. 21. The plating solution may be any standard commercially available or in-house formulated copper plating bath. Plating conditions may be particular to the manufacturer's guidelines. However, any other sacrificial material that can be electroplated may also be used. In addition, deposition processes other than plating may be used to form sacrificial layer4020. The photoresist may then be stripped from the substrate4000.

FIG. 22illustrates a second step in an exemplary process for fabricating MEMS hysteretic thermal actuator3000. In the second step, another sacrificial layer4030is formed over the substrate4000and the first sacrificial layer4020. The second sacrificial layer4030may also be copper, with a thickness of about 5 μm. The purpose of the two sacrificial layers4020and4030is to provide two levels which allow the offset between the elevations of driving beam segment3200and passive segment3500, which provides the out-of-plane movement of the first segment of MEMS hysteretic thermal actuator. The dashed line inFIG. 22indicates areas further behind the indicated cross section which require a higher elevation, such as for driving beam segment3300inFIGS. 14-16. The second sacrificial layer4030is also deposited in these areas to provide this higher elevation. This step second step may be performed in the same way as for the first sacrificial layer4020, by depositing and patterning photoresist, plating the sacrificial layer4030on the first sacrificial layer through the photoresist, and stripping the photoresist.

A third step in the exemplary method for fabricating the MEMS hysteretic thermal actuator3000is illustrated inFIG. 23. InFIG. 23, the substrate4000is again covered with photoresist, which is exposed through a mask with features corresponding to stationary contact3700and a corresponding second contact at the distal end of the passive segment3600. For simplicity, this second contact was not shown inFIGS. 14-19. A conductive material with good electrical transport properties and good contact resistance may then be plated in areas4600and4700. The additional contact material4600may be used at the distal end of passive segment3600because it may have lower contact resistance than the material that will form the beam3600. In one exemplary embodiment, gold (Au) is chosen as the contact material4600and4700. Additional gold features may also be plated in this step, such as a bonding ring, which may eventually form a portion of a hermetic seal which may bond a cap wafer over the substrate4000and MEMS hysteretic thermal actuator3000. Another of the gold features may be an external access pad that will provide access to the MEMS hysteretic actuator electrically, from outside the hermetically sealed structure.

Gold may then be electroplated in the areas not covered by the photoresist, to form gold features4600and4700, and any other gold structures needed. The photoresist is then stripped from the substrate4000. The thickness of the gold features4600and4700may be, for example, 5 μm. Because the gold contact features4600and4700may be plated over both the first sacrificial layer4020and the second sacrificial layer4030, the gold contacts may be formed with a lip4650at a lower elevation than the rest of gold contact feature4600. This lip4650may mate with a corresponding lip on the stationary gold contact4700, to form a détente position for the closed switch, so that the switch may be positively latched rather than relying on frictional forces to keep the switch closed. However, it should be understood that MEMS hysteretic thermal actuator3000may be formed without this lip as well, as shown inFIGS. 14-19.

If desired or needed, an optional step may be included in the process at this point, which is illustrated inFIG. 24. This optional step is the deposition of another contact material over the surface of the stationary gold contact3700or4700. This additional contact material may be chosen to improve the mechanical properties of the contact metals. For example, palladium or a gold-cobalt alloy may be sputtered over stationary contact4700, which may reduce the tendency of the two gold contacts4600and4700to adhere to one another after touching. Other additional metals may include ruthenium, platinum, gold-platinum alloy, and gold-nickel alloy, for example.

FIG. 25illustrates a fifth step in an exemplary method for fabricating the MEMS hysteretic thermal actuator3000. InFIG. 25, photoresist (not shown) is once again applied over the substrate4000, and patterned according to the features in a mask. The exposed portions of the photoresist are then dissolved as before, exposing the appropriate areas of the seed layer4010and sacrificial layers4020and4030. The exposed seed layer4010and sacrificial layers4020and4030may then be electroplated with nickel to form the features4200,4300and4500. As described above, feature4200may correspond to driving beam segment3200, and feature4500may correspond to passive segment3500. Feature4300may correspond to driving beam3300inFIGS. 14-16, and is deposited over the second sacrificial layer4030, shown as the dashed line inFIG. 22. Although only beam segments3200,3300and3500are shown in this step, it should be understood that passive beam segment3600may be formed using substantially similar or identical techniques, at the same time as passive beams segment3200,3300and3500. However, since passive beam segment3600may be located directly adjacent to, and in the same plane as driving beam portion3300and passive segment3500, only driving beam segments3200,3300and passive segments3500are depicted inFIG. 25. Furthermore, although passive beam3500is represented only by the single feature4500, it should be understood that any additional structures such as heat sink3550may be formed using similar or identical processes to those shown. The gold contact4600may be affixed to the nickel beam4500by the natural adhesion of the gold to the nickel, after deposition. Although nickel is chosen in this example, it should be understood that any other conductive material that can be electroplated may also be used. In addition, deposition processes other than plating may be used to form beams4200and4500. The photoresist may then be stripped from the substrate4000.

FIG. 26illustrates a fifth step in the fabrication of the MEMS hysteretic thermal actuator3000. InFIG. 17, a polymeric, nonconducting material4800such as the negative tone photo-patternable polymer, photoresist SU-8 is deposited over the substrate4000, and beams4200and4500. The photoresist4800is then cross-linked, by for example, exposure to UV light. The unexposed resist is then dissolved and removed from the substrate4000and structures4200and4500in all areas that the dielectric tether should be absent. This step may form the dielectric tethers3251and3253, as well as other tethers indicated in the precedingFIGS. 14-19. The remaining photoresist may then be cured by, for example, baking. While SU-8 is used in this embodiment, it should be understood that this is exemplary only, and that any other non-conducting material may be used to form the tethers4800.

FIG. 27illustrates a sixth step in the fabrication of the MEMS hysteretic thermal actuator3000. In this step, the beams4200,4500and4600may be released by etching the underlying sacrificial copper layers4020and4030. Suitable etchants may include, for example, an isotropic etch using an ammonia-based Cu etchant. The Cr and Au seed layer4010may then also be etched using, for example, a wet etchant such as iodine/iodide for the Au and permanganate for the Cr, to expose the SiO2surface of the substrate4000. Although not shown inFIG. 27, it should be understood that structures4200and4500may be anchored to the substrate4000by anchors plated in an area without sacrificial layers4010and4020. These anchors may form anchor points3130,3140and3530. However, because the cross section shown does not include any anchor points, they are not shown inFIG. 27. The substrate4000with the MEMS hysteretic thermal actuator3000may then be rinsed and dried, and its fabrication is essentially complete.

The resulting MEMS hysteretic actuator3000may then be encapsulated in a protective lid or cap wafer. Details relating to the fabrication of a cap wafer may be found in co-pending U.S. patent application Ser. No. 11/211,625, incorporated by reference herein in its entirety.

It should be understood that one gold feature may be used as an external access pad for electrical access to the MEMS hysteretic thermal actuator3000, such as to supply a signal to the MEMS hysteretic thermal actuator3000, or to supply a voltage the terminals3130or3140in order to energize the drive loops of the actuator, for example. The external access pad may be located outside the bond line which will be formed upon the bonding of a cap layer to the substrate4000. Alternatively, electrical connections to MEMS hysteretic actuator may be made using through-wafer vias, such as those disclosed in co-pending U.S. patent application Ser. No. 11/211,624, U.S. patent application Ser. No. 11/482,944, and U.S. patent application Ser. No. 11/541,774, each of which is incorporated herein by reference in its entirety.

In each of the previous embodiments, an electrical signal is presumed to flow along the cantilevered beam to a stationary contact located under the tip of the beam. However, it is also envisioned to configure the MEMS hysteretic device such that an electrical signal flows between two stationary electrodes located on the substrate surface under the MEMS hysteretic actuator itself. In this case, the contact material4600disposed on the distal end of the second beam segment may provide the electrical connection between the stationary electrodes. Such an exemplary embodiment is described in co-pending U.S. patent application Ser. No. 11/334,438, incorporated by reference herein in its entirety.

While various details have been described in conjunction with the exemplary implementations outlined above, various alternatives, modifications, variations, improvements, and/or substantial equivalents, whether known or that are or may be presently unforeseen, may become apparent upon reviewing the foregoing disclosure. For example, while MEMS hysteretic thermal actuators are described which have two segments, it should be understood that any number of additional segments may also be used. Furthermore, although the cantilevers are described as having straight segments, it should be understood that this is exemplary only, and that the cantilever may also have an arcuate shape. While the embodiments described above relate to a microelectromechanical actuator, it should be understood that the techniques and designs described above may be applied to any of a number of other microelectromechanical devices, such as valves and switches. Accordingly, the exemplary implementations set forth above, are intended to be illustrative, not limiting.