This invention relates generally to the field of Micro-Electromechanical Systems (MEMS) and in particular to actuators for chip level MEMS devices including switches.
MEMS devices are small movable mechanical structures advantageously constructed using conventional semiconductor processing methods. Oftentimes MEMS devices are provided as actuators—which have proven quite useful in a wide variety of applications.
A MEMS actuator is oftentimes configured and disposed in a cantilever fashion. Accordingly, it thus has an end attached to a substrate and an opposite free end which is movable between at least two positions—one being a neutral position and the other(s) being deflected positions.
Common actuation mechanisms used in MEMS actuators include electrostatic, magnetic, piezo and thermal—the last of which is the primary focus of the present invention. The deflection of a thermal MEMS actuator results from a potential being applied between a pair of terminals—commonly called “anchor pads” in the art—which potential causes a current flow thereby elevating the temperature of the structure. This in turn causes a part thereof to either elongate or contract, depending upon the particular material(s) used.
A known use of thermal MEMS actuators is to configure them as switches. Such MEMS switches offer numerous advantages over alternatives and in particular they are extremely small, relatively inexpensive, consume little power and exhibit short response times.
Given the importance of thermally actuated MEMS devices, structures that enhance their performance, reliability and/or manufacturability would represent a significant advance in the art.
In accordance with an aspect of the invention, a MEMS actuator is provided with an improved latch which imparts less stress on cantilever members while exhibiting less creep than prior-art structures.
In accordance with another aspect of the invention, a MEMS actuator is provided with an improved hot beam having a tapered profile that advantageously exhibits a more uniform temperature profile across its length, thereby improving its reliability and operating life over prior art structures.
In accordance with yet another aspect of the invention, a MEMS actuator is provided with an improved cold beam having a tapered profile that advantageously distributes stress along its length more uniformly than with prior art structures.
Further features and advantages of the invention will become apparent upon review of the detailed description in conjunction with the drawing in which:
The following merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope.
Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions.
Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.
Thus, for example, it will be appreciated by those skilled in the art that the diagrams herein represent conceptual views of illustrative structures embodying the principles of the invention.
Referring simultaneously to
As its name implies, the free end 13 of the actuator 10 is capable of being moved. Such movement is effected by the actuation mechanism(s) inherent in the device. In this representative MEMS device shown in FIG. 1—and as shall be discussed in greater detail—the actuation mechanism is assumed to be thermal.
As shown in
The actuator 10 also comprises a cold arm member 30 adjacent and substantially parallel to the hot arm member 20. The cold arm member 30 has at one end an anchor pad 32 connected to the substrate 12, and a free end 34 that is opposite the anchor pad thereof 32. The free end 34 is overlying the substrate 12.
Although these exemplary structures show substantially parallel members, it is noted that various shapes and geometries are possible—as shall be discussed in the context of the present invention.
In the representative embodiment shown, a dielectric tether 40 is attached over the common end 26 of the spaced-apart portions 22 of the hot arm member 20 and the free end 34 of the cold arm member 30. As can be appreciated, the dielectric tether 40 mechanically couples the hot arm member 20 to the cold arm member 30 while keeping them electrically isolated, thereby maintaining them in a spaced-apart relationship with a minimum spacing between them to avoid a direct contact or a short circuit in normal operation as well as to maintain the required withstand voltage, which voltage is roughly proportional to the spacing between the members 20, 30.
The dielectric tether 40 is typically molded directly in place at a desired location and is attached by direct adhesion. Direct molding further allows having a small quantity of material entering the space between the parts before solidifying. Of course those skilled in the art will readily understand that the dielectric tether 40 can be attached to the hot arm member 20 and the cold arm member 30 in different manner(s) than the one shown in
As shown, the dielectric tether 40 is located over the actuator 10, namely on the opposite side of the members with reference to the substrate 12. This has many advantages over previous MEMS actuators for which the dielectric tether, usually made of glass, was provided under the member. In such configurations, the dielectric tether was typically made of glass and located under the members and constructed from thin layers of silicon oxide or nitride, which layers were very fragile. As can be readily appreciated, such prior-art dielectric tethers generally increased the complexity of the manufacturing process.
When constructed in this manner, the dielectric tether 40 is preferably made entirely of a photoresist material. A suitable material for this purpose is known in the trade as SU-8 which is a negative, epoxy-type, near-UV photo resist based on EPON SU-8 epoxy resin (from Shell Chemical). Other suitable materials include polyimide, spin on glass or other polymers or a combinations thereof. Moreover, combining different materials is also possible.
With these structural relationships outlined, we may now describe the operation of this representative MEMS actuator. In particular, when a control voltage is applied at the anchor pads 24 of the hot arm member 20, an electrical current flows into both the first and the second portions 22 thereby heating the member. In the illustrated embodiment, the material used for making the hot arm member 20 is selected such that it increases in length as it is heated.
The cold arm member 30, however, does not elongate since there is no current initially flowing through it and it therefore is not actively heated. As a result of the hot-arm increasing in length and the cold arm staying substantially the same length, the free end of the actuator 10 is deflected sideward, thereby moving the actuator 10 from a neutral position to a deflected position. Conversely, when the control voltage is removed, the hot arm member 20 cools and shortens in length. As a result, the actuator 10 returns to its neutral position. Advantageously both movements may occur very rapidly.
In the embodiment shown in
The actuator 10 in the embodiment shown in
It has been advantageous to provide at least one of these additional dielectric tethers 50 on an actuator 10 to provide additional strength to the hot arm member 20 by reducing their effective length in order to prevent distortion of the hot arm member 20 over time. Since the gap between the parts is extremely small, the additional tethers 50 reduce the risk of a short circuit between the two portions 22 of the hot arm member 20 or between that portion 22 of the hot arm member 20 which is the closest to the cold arm member 30 and the cold arm member 30 itself by keeping them in a spaced-apart configuration.
In those applications where the cold arm member 30 is used to carry high voltage signals, the portion 22 of the hot arm member 20 closest to the cold arm member 30 will deform, moving it towards the cold arm member 30, due to an electrostatic force between them which is caused by the high voltage signal. As can be appreciated, if the portion 22 of the hot arm member 20 gets too close to the cold arm member 30, a voltage breakdown can occur, possibly destroying the MEMS switch 100. Additionally, since the two portions 22 of the hot arm member 20 are relatively long, they tend to distort when heated to create the deflection, thereby decreasing the effective deflection stroke of the actuators 10.
As can be readily appreciated, using one, two or more additional dielectric tethers 50 may offer a number of advantages, including increasing the rigidity of the portions 22 of the hot arm member 20, increasing the deflection stroke length of the actuator 10, while decreasing the risk of shorts between the portions 22 of the hot arm member 20 and increasing the breakdown voltage between the cold arm member 30 and hot arm members 20.
The additional dielectric tethers 50 may advantageously be made of a material identical or similar to that of the main dielectric tether 40. When preparing the tethers, small quantities of materials are flowed between the parts before solidifying in order to improve the adhesion. In addition, one or more holes or voids 52 may be provided in the cold arm member 30 to receive a small quantity of material before it solidifies—thereby improving its adhesion thereto.
When tip members are used to conduct electrical current, the surface of the tip member 60 may be preferably designed so as to lower the contact resistance when two of such tip members 60 make contact with each other. Those skilled in the art will recognize that this characteristic may be realized by employing tip members made of gold, or gold over-plated. Other possible tip materials for electrical conduction will be recognized in the art and include gold-cobalt alloys, palladium, etc. Generally, all that is required for such materials is that they provide a lower electrical resistance as compared to Ni, which is a preferred material for the cold arm member 30. Of course, other materials may be used for the hot arm member 20 and/or the cold arm members 30.
With continued reference to
As can now be understood and appreciated the MEMS switch 100 has two static positions, namely a closed position in which the first actuator 10 and the second actuator 10′ are mechanically engaged at and by their lateral contact flanges 62. Conversely, an open position is that in which they are not mechanically engaged at and by their lateral contact flanges. As can be appreciated, when an electrical potential is applied to one of the mechanically engaged actuators, they are effectively electrically engaged as well and as such an electrical current may flow thorough the two engaged actuators. Stated alternatively, when disengaged they are electrically isolated, there is no electrical continuity between the cold arm members 30.
With these structural relationships described, we may now explain how MEMS actuators operate. Note that when describing a direction of movement, it is with reference to the exemplary arrangements shown in this
Returning to FIG. IC, it is noted that to move from one position to the other (i.e., from open to closed or closed to open), the actuators 10,10′ are operated in sequence. Briefly stated, the tip member 60 of the second actuator 10′ is deflected upward (away from actuator 10). Then, the tip member 60 of the first actuator 10 is deflected to its right. The control voltage which initiated the upward deflection of second actuator 10′ is removed or sufficiently diminished such that it (the second actuator) moves downward toward the first actuator 10 sufficiently to permit its flange 62′ to engage the back side of the flange 62 of the first actuator 10.
Continuing, the control voltage which initiated the rightward deflection of the first actuator 10 is then similarly removed or diminished, thereby causing it to return toward its neutral, undeflected position while causing the two flanges (62, 62′) to become mechanically engaged and permitting electrical engagement therebetween. When the cold arm members are so connected, an electrical signal or current then be transmitted between both corresponding anchor pads 32 of the two cold arm members 30. Advantageously, opening and closing the MEMS switch 100 is very rapid—typically occurring in only a few milliseconds.
When so operated, the MEMS switch 100 is effectively “latched” into position and will remain so unless specifically “unlatched” As can now be understood and appreciated however, re-setting or “unlatching” the MEMS switch 100 to its open (“unlatched”) position is done by reversing the above-described operations.
Turning our simultaneous attention now to
More particularly, hot arm 220 is that member of the actuator 200 through which an electrical current is flowed and subsequently elongates and thereby deflects. The hot arm 220 includes two portions 222 each of the two having an anchor pad 224. As shown in that
More particularly, when a pair of actuators such as those shown in the perspective drawing
As with the variations shown earlier, this tapered hot arm member 400 may have one or both of the portions exhibiting this tapered characteristic in one form or another. Once again, the particular materials chosen and the application will dictate the taper characteristics and which—if any—of the hot arm member portions will have the taper.
Turning simultaneously now to
Further variations to the MEMS actuators of the present invention are shown in
With reference to
Similarly, the configuration shown in
As can be appreciated, such configurations affect the “wiping” or cleaning of the latches as they become engaged/disengaged. As a result, the contact effectiveness and lifetime, is potentially improved. Advantageously, additional “self-wiping” configurations are possible according to the present invention.
c) shows yet an alternative tip member flange configuration wherein one of the flanges exhibits a “positive” angle. As can be observed from this
As can be readily understood, such angular flanges may increase the amount of friction between the moving flanges. As a result, a more forceful, self-wiping action is produced thereby enhancing its operational characteristics as noted above.
Finally,
Turning now to
Shown in the inset of
This lower stroke may be appreciated and understood by those skilled in the art with reference to
At this point, while the present invention has been shown and described using some specific examples, those skilled in the art will recognize that the teachings are not so limited. In particular, and according to the present invention, various permutations of the individual aspects of the present invention—for example angled geometry, bumps, tapered members, etc, may be used alone or in any useful combinations. Accordingly, the invention should be only limited by the scope of the claims attached hereto.