Abstract:
Micro-electromechanical systems (MEMS) actuators and switches exhibiting geometries and configurations providing superior operating characteristics and longer lifetimes.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of Micro-Electromechanical Systems (MEMS) and in particular to actuators for chip level MEMS devices including switches. 
       BACKGROUND OF THE INVENTION 
       [0002]    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. 
         [0003]    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. 
         [0004]    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. 
         [0005]    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. 
         [0006]    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. 
       SUMMARY OF THE INVENTION 
       [0007]    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. 
         [0008]    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. 
         [0009]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0010]    Further features and advantages of the invention will become apparent upon review of the detailed description in conjunction with the drawing in which: 
           [0011]      FIG. 1(A)  is a plan view of a representative MEMS actuator; 
           [0012]      FIG. 1(B)  is a side view of the MEMS actuator of  FIG. 1(A)  disposed upon a substrate; 
           [0013]      FIG. 1(C)  is a plan view of a MEMS switch constructed from a pair of MEMS actuators of  FIG. 1(A) ; 
           [0014]      FIG. 2(A)  is a plan view of a pair of MEMS actuators having asymmetric hot arm lengths according to the present invention; 
           [0015]      FIG. 2(B)  is a perspective view of the MEMS actuators of  FIG. 2(A) ; 
           [0016]      FIG. 3  is a plan view of a pair of MEMS actuators having asymmetric hot arm widths according to the present invention; 
           [0017]      FIG. 4  is a plan view of a pair of MEMS actuators having a tapered portions of a hot arm according to the present invention; 
           [0018]      FIG. 5(A)  is a plan view of a pair of MEMS actuators having a tapered cold arm according to the present invention; 
           [0019]      FIG. 5(B)  is a perspective view of the MEMS actuators of  FIG. 5(A) ; 
           [0020]      FIG. 6  shows a series of individual configurations  6 ( a )- 6 ( d ) of tip/flange configurations according to the present invention; 
           [0021]      FIG. 7  is a plan view of an angled contact geometry for MEMS actuators according to the present invention; 
           [0022]      FIG. 8  shows a series of individual operations  8 ( a )- 8 ( e ) on two actuator tip configurations including the angled geometry and conventional non-angled geometry—according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    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. 
         [0024]    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. 
         [0025]    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. 
         [0026]    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. 
         [0027]    Referring simultaneously to  FIGS. 1A ,  1 B, and IC (collectively  FIG. 1 ) there is shown an example of a representative MEMS cantilever actuator  10  mounted on a substrate  12 . Such actuators are generally known in the art (See, for example U.S. Pat. No. 7,036,312 by the present inventors—the entire contents of which are incorporated by reference as if set forth at length herein) and have an immovable end  15  and a free end  13 . 
         [0028]    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. 
         [0029]    As shown in  FIG. 1 , the MEMS actuator  10  comprises a hot arm member  20  including two spaced-apart portions  22 , each being provided at one end with a corresponding anchor pad  24  connected to a substrate  12 . The spaced-apart portions  22  may be substantially parallel as shown in the  FIG. 1  and connected together at a common end  26  that is opposite the anchor pads  24  and overlying the substrate  12 , as shown in  FIG. 2 . 
         [0030]    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 . 
         [0031]    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. 
         [0032]    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 . 
         [0033]    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  FIG. 1 . 
         [0034]    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. 
         [0035]    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. 
         [0036]    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. 
         [0037]    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. 
         [0038]    In the embodiment shown in  FIG. 1  the cold arm member  30  comprises a narrower section  36  adjacent to its anchor pad  32  in order to facilitate the movement between the deflected position and the neutral position. The narrower section  36  has a width laterally decreased from the exterior compared to a wider section  38  of the cold arm member  30 . In one exemplary embodiment, the width decrease is at a square angle. Other shapes and geometries are possible, as will be shown later. 
         [0039]    The actuator  10  in the embodiment shown in  FIG. 1  includes a set of two spaced-apart additional dielectric tethers  50 . These additional dielectric tethers  50  are transversally disposed over the portions  22  of the hot arm member  20  and over the cold arm member  30  and adhere to these parts. 
         [0040]    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. 
         [0041]    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 . 
         [0042]    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 . 
         [0043]    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. 
         [0044]      FIG. 1  further shows that the actuator  10  comprises a tip member  60  attached to the free end of the cold arm member  30 . While an actuator may be constructed without a tip member, as we shall show such tips facilitate the construction of MEMS switches from actuators. 
         [0045]    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 . 
         [0046]    With continued reference to  FIG. 1 , it may be observed that the tip member  60  of the actuator  10  of a preferred embodiment include a lateral contact flange  62 . This flange  62  is useful for connecting two substantially-perpendicular actuators  10 , as particularly shown in  FIG. 1C . Such arrangement creates a MEMS switch  100 . 
         [0047]    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 . 
         [0048]    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  FIG. 1C . Those skilled in the art will of course recognize that different physical arrangements and relationships are possible, so a particular direction of movement is referenced for exemplary purposes only. 
         [0049]    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 . 
         [0050]    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. 
         [0051]    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. 
         [0052]    Turning our simultaneous attention now to  FIG. 2A  and  FIG. 2B  (collectively “FIG.  2 ”) there is shown an alternative embodiment of the present invention. In particular, the embodiment shown therein is that exhibiting an asymmetric hot arm length. 
         [0053]    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  FIG. 2 , one of the portions is longer than the other portion by a length ΔL as shown in the inset of  FIG. 2A . In the preferred configuration shown, it is the outer portion that is longer by the amount ΔL. Operationally, by making the outer portion longer, the actuator exhibits better stress distribution over an actuator in which all of the members are the same length. Additionally, it also provides a more efficient actuation mechanism which reduces stress along the structure and reduces the temperature (current) required for actuation in the latched position. 
         [0054]    More particularly, when a pair of actuators such as those shown in the perspective drawing  FIG. 2B , are latched, the asymmetric configuration such as that shown here exhibits a much lower stress in that latched position. Also shown in this  FIG. 2 , both of the portions  222  of the hot arm member  220  are longer than the cold arm member, whose anchor pad s designated by  232 . 
         [0055]      FIG. 3  shows yet an alternative configuration of the hot arm member wherein the two portions thereof do not exhibit the same width. In particular, one of the portions is shown having a width w 1 , while the other portion is shown having a width w 2  where w 1  ≠w 2 . Advantageously, narrowing the outer hot beam produces an effect similar to increasing its length. 
         [0056]      FIG. 4  shows yet another hot arm member configuration according to the present invention. In particular, the hot arm member  400  shown in that  FIG. 4  has a portion where one end of the portion is wider than the other end of that portion. In the configuration shown, the end closes to the free end has a width w[ 2 ] while the end closest to the anchor pads has a width w[ 1 ] where w[ 1 ]&lt;w[ 2 ]. When so configured, the taper serves as a “choke” to the electrical energy. As a result, the temperature of a hot arm member so configured will exhibit more uniform temperature distribution across its length and therefore a lower peak temperature for a given displacement. 
         [0057]    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. 
         [0058]    Turning simultaneously now to  FIG. 5A  and  FIG. 5B  (collectively “FIG.  5 ”) there is shown an actuator configuration according to the present invention whereby a cold arm member exhibits a tapered profile. In this configuration, the width of the cold arm member closest to the anchor pad has a width w[ 1 ] which is larger than the width of that cold arm member closes to its free end. Advantageously, this tapered cold arm profile distributes more uniformly any stresses introduced into that member. As a result, greater reliability is one result. More particularly, mechanical creep performance is enhanced. 
         [0059]    Further variations to the MEMS actuators of the present invention are shown in  FIG. 6 . More particularly,  FIG. 6  shows a series of individual configurations  6 {a}- 6 ( d ) wherein variations to the tip member flange(s) are shown. 
         [0060]    With reference to  FIG. 6(   a ) a one-bump configuration is shown. According to the present invention, one flange of the two tip members which latch has disposed thereon a “bump”  602  of material such as gold which advantageously improve contact resistance of the switch. This improvement is attributed—in part—to the fact that a much smaller surface area and therefore higher contact pressure is exhibited. In this exemplary configuration, the bump exhibits a substantially hemispherical geometry. 
         [0061]    Similarly, the configuration shown in  FIG. 6(   b ) is that of a “double bump” wherein each of the latch components of the tip members has a bump  603 ,  604 , respectively. As can be appreciated, when so configured and properly aligned, such a configuration further minimizes the surface area of the latches that contact one another. As before, gold or other materials may preferably be used for the bumps. Additionally, it should be noted that while only a single bump was shown in  6 ( a ) and one bump on each flange is shown in  FIG. 6(   b ) those skilled in the art will appreciate that one or more bumps may be disposed upon a given flange as an application requires. 
         [0062]    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. 
         [0063]      FIG. 6(   c ) shows yet an alternative tip member flange configuration wherein one of the flanges exhibits a “positive” angle. As can be observed from this  FIG. 6(   c ), the positive angle is characterized by an angle  605  that is greater than 90 degrees between the inner flange face  606  and the main tip member. This positive angle configuration may advantageously be combined with a bump configuration, such as the single bump configuration shown previously wherein a bump  610  is disposed on the inside face of the mating flange. 
         [0064]    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. 
         [0065]    Finally,  FIG. 6(   d ) shows a configuration having a “negative” angle. As can again be observed from the figure, the negative angle is characterized by an angle  608  that is less than 90 degrees between the inner-flange face  609  and the main tip member. Like the other configuration just shown, this negative angle configuration may be combined with other bump configurations, such as the single bump configuration. 
         [0066]    Turning now to  FIG. 7 , there is shown yet another contact configuration of mating tip members and their flanges. In particular, shown therein is a configuration wherein each of the mating flanges have a negative angle thereby producing an angled contact. When configured in this manner, a MEMS switch constructed from two such actuators will have a minimal stroke. 
         [0067]    Shown in the inset of  FIG. 7  is a distance w that is substantially the width of a given flange and any associated bumps disposed thereon. As noted before, the bump and/or the entire flange may be made from gold or other suitable materials. As can be appreciated by those skilled in the art, a minimal actuator stroke will produce lower stress in the actuators. Lower stroke permits a lower temperature to actuate and smaller deformations. Advantageously, the negative angle may be of a variety, depending upon the application. More particularly, negative angles of between 10 and 45 degrees are particularly useful. In other words, the negative angle (the angle between the flange and its respective tip member) will be substantially from 45 degrees to 80 degrees. Advantageously, the angled geometry provides a more positive latch while requiring fewer movements which may advantageously provide a longer, less stressful operating lifetime. 
         [0068]    This lower stroke may be appreciated and understood by those skilled in the art with reference to  FIG. 8  which shows a series of illustrations depicting the actuation latching of a representative actuator having an angled latch and straight latch. With reference to that  FIG. 8 , it can be seen that the stroke for the angled latch is depicted W[ 1 ] while that for the straight latch is depicted by W[ 2 ]. Those skilled in the art will readily recognize that not only are fewer movements required to engage the latch of the angled embodiment, but the displacement or stroke through which it must move is less as well. Advantageously, while a straight latch must first move apart, the angled latch may first move towards one another ( FIG. 8(   b )). Because they do not have to move apart to engage, fewer movements are required as well. 
         [0069]    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.