Patent Publication Number: US-6664885-B2

Title: Thermally activated latch

Description:
FIELD OF THE INVENTION 
     The invention is directed to a microelectromechanical device and a method for latching a device, more particularly to a device having a component that can be latched and remains latched in an unpowered state. 
     BACKGROUND OF THE INVENTION 
     Microelectromechanical systems (MEMS) have recently been developed as alternatives for conventional electromechanical devices such as relays, actuators, valves and sensors. MEMS relays having lower contact-to-contact resistance are needed. In addition, it is advantageous to have a relay that does not require power to maintain the relay in a latched position, but merely uses power to actuate the relay between the positions. 
     SUMMARY OF THE INVENTION 
     Generally, the present invention provides a device for latching an actuator to a substrate where the substrate includes a thermally activated material located on the substrate. The device also includes a heater coupled to the thermally activated material that is capable of heating the thermally activated material until it softens. The actuator includes a contact area and the actuator is movable between a contact position and a non-contact position. In the non-contact position, the contact area is spaced apart from the thermally activated material on the substrate. In the contact position, the actuator contacts the thermally activated material at the contact area. 
     A method of latching the actuator on a device is also provided including the steps of heating a thermally activated material until it softens. A next step is moving an actuator having a contact area from a non-contact position to a contact position where the contact area is in contact with the softened thermally activated material. The thermally activated material is allowed to cool and resolidify so that the thermally activated material retains the actuator in the contact position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more completely understood by considering the detailed description of various embodiments of the invention which follows in connection with the accompanying drawings. 
     FIG. 1 is a side view of one embodiment of a microelectromechanical system (MEMS) device, shown in an OFF or non-contact position. 
     FIG. 2 is a top view of the device of FIG.  1 . 
     FIG. 3 is a front view of the device of FIG. 1 in the non-contact position. 
     FIG. 4 is a side view of the device of FIG. 1 in an ON or contact position. 
     FIG. 5 is a side view of a second embodiment of a MEMS device, shown in an OFF or non-contact position. 
     FIG. 6 is a top view of the MEMS device of FIG.  5 . 
     FIG. 7 is a front view of the device of FIG. 5 in the non-contact position. 
     FIG. 8 is a side view of a third embodiment of a MEMS device in an OFF or non-contact position. 
     FIG. 9 is a top view of the MEMS device of FIG.  8 . 
     FIG. 10 is a front view of the device of FIG. 8 in the non-contact position. 
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The invention is believed to be applicable to a variety of systems and arrangements for microelectromechanical system (MEMS) devices. The invention has been found to be particularly advantageous in application environments where an actuator is needed, such as in telecommunications. While the invention is not so limited, an appreciation of various aspects of the invention is best gained through a discussion of various application examples operating in such an environment. 
     FIG. 1 illustrates a side view of one particular embodiment of a MEMS device  10 . FIG. 2 illustrates a top view of device  10  and FIG. 3 illustrates a front view of device  10 . The device  10  includes a substrate  16 , an actuator  20  and a spacer or anchor  24  between the substrate  16  and the actuator  20 . The actuator  20  is fixed to the spacer  24  at a first end  34  and is spaced from and suspended over the substrate  16  at a second end  36  in a non-contact position illustrated in FIG.  1 . The substrate  16  includes a thermally activated material  40  and a heating element  44  positioned underneath the second end  36  of the actuator  20 . The second end  36  of the actuator  20  includes a contact area  46  (shown in FIG. 1) that will contact the thermally activated material when the actuator  20  is in a contact position. 
     The actuator  20  is movable between the non-contact position illustrated in FIG. 1 and a contact position illustrated in FIG.  4 . In the contact position, the actuator  20  contacts the substrate at the contact area  46  at its second end  36 . The thermally activated material  40  is used to hold the actuator in the contact position. To accomplish this latching, the thermally activated material  40  is heated by the heating element  44  until it at least softens. Often, the material  40  softens at a melting point, but some materials have a softening point that is lower than the melting point, as discussed further herein. The thermally activated material should be softened sufficiently so that the actuator can establish good contact with the thermally activated material over a significant area. The actuator  20  is then brought into contact with the softened thermally activated material  40 . The heating element  44  is turned off and the thermally activated material  40  is allowed to cool to a temperature below the melting temperature, which causes the contact area  46  of the actuator  20  to be fused to the material  40 . The thermally activated material  40  retains the actuator in place as it stiffens and holds the actuator to the substrate. Thus no power is needed to keep the actuator in a latched position. To move the actuator  20  from the contact position to the non-contact position, the thermally activated material  40  is heated, softens, and releases the actuator  20 . In one preferred embodiment, the actuator  20  has a spring force that returns it to its noncontact position when the thermally activated material  40  is softened. In alternate embodiments, other actuating mechanisms are used to move the actuator  20  to the noncontact position, such as thermal, mechanical, electrostatic, magnetic, electromagnetic or other mechanisms. 
     The thermally activated material  40  may include many different materials that are softened at a temperature that is achievable by the device and is compatible with the use of the device. A softening temperature that is as low as possible is preferred because it requires less power to heat the thermally activated material. Other characteristics of the thermally activated material  40  should also be considered when selecting a material, such as the heat of melting transformation, the viscosity and any vapor release that will occur during heating or melting. Preferably, the thermally activated material will not run off of the substrate  16  when heated to the point where it softens. The thermally activated material may include additives to prevent it from running off of the substrate when heated. 
     For many choices for the thermally activated material, such as solder materials, the material softens at its melting point. Other materials may have a softening point that is lower than its melting point. Some materials have a softening temperature range over which they become increasingly pliant. The thermally activated material will be heated to a point where it is soft enough to allow the contact area of the actuator to establish good surface area contact with it, so that the actuator will be held in place when the thermally activated material cools. This point may be at the softening point, at the melting point, or somewhat beyond the softening point depending on the material. 
     Examples of materials that can be used as the thermally activated material are described in RAGNAR HOLM, ELECTRIC CONTACTS (4th ed. 1967), which is hereby incorporated herein by reference in its entirety. Table X,1 of ELECTRIC CONTACTS provides melting temperatures and softening temperatures where appropriate for several materials that could be used for a thermally activated material, such as gold or copper. 
     Where the thermally activated material softens at its melting temperature, a preferred range for a melting temperature of the thermally activated material is about 250° C. (482° F.) or less, more preferably about 220° C. (420° F.) or less, still more preferably about 190° C. (374° F.) or less, and most preferably about 160° C. (320° F.) or less. One example of such a thermally activated material  44  is solder. Solder is an alloy of tin, lead and bismuth that enables a melting temperature as low as 135° C. (275° F.). Solder may include flux to prevent the solder from running off of the substrate when heated. The following Chart 1 shows material composition and melting temperatures for three common solder types. 
     
       
         
           
               
             
               
                 CHART 1 
               
             
            
               
                   
               
               
                 Melting Temperatures for Common Solder Type 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 Melting Temp. 
                 Melting Temp. 
               
               
                 Solder Type 
                 % Lead 
                 % Tin 
                 (° C.) 
                 (° F.) 
               
               
                   
               
               
                 50-50 
                 50 
                 50 
                 218 
                 425 
               
               
                 60-40 
                 60 
                 40 
                 188 
                 371 
               
               
                 63-37 
                 63 
                 37 
                 183 
                 361 
               
               
                   
               
            
           
         
       
     
     The actuator  20  may be moved between the contact position and non-contact position in many different ways. For example, in the first embodiment illustrated in FIGS. 1-4, the actuator  20  is a bi-material cantilever arm including a first material  50  and a second material  51 , shown in FIG.  1 . The materials have different coefficients of thermal expansion causing each material to expand differently when heated, so that the actuator moves into the contact position shown in FIG.  4 . The first material  50  is connected to a first heating element  54  and the second material  51  is connected to a second heating element  55 , shown in FIG.  2 . In an alternate embodiment only one heating element is used to heat the actuator  20 . In yet another alternate embodiment, a current source is coupled to the actuator to heat the actuator  20 . An actuator heated by current may include two layers separated by an insulating material  60  along most of its length, but with a conductive bridge  61  between the layers at the end  36 . A current source could be applied to the actuator to heat the actuator. In this alternative, the dimensions and resistance of the actuator components are selected so that sufficient heat to move the actuator is generated by the application of current. The actuator  20  may be configured so that a restoring force acts to restore it back to the non-contact position from the contact position. 
     Many different configurations for a bi-material cantilever are possible. In addition, other types of thermally activated actuators are possible. Other alternative actuating mechanisms are also possible. For example, electrostatic, magnetic, electromagnetic, mechanical or other forces may be used to move the actuator  20  between the contact and non-contact positions. 
     A MEMS device  100  is shown in FIGS. 5-7 that is similar in many ways to the device  10  shown in FIGS. 1-4. The device  100  of FIGS. 5-7 includes a substrate  116 , an actuator  120  and a spacer  124  between the substrate  116  and a first end  134  of the actuator  120 . A second end  136  of the actuator  120  is spaced away from the substrate  116 . The substrate  116  includes a thermally activated material  140  and a heating element  144 . The actuator  120  is movable between a non-contact position illustrated in FIG. 5 and a contact position where the actuator  120  is touching the thermally activated material  140 . The actuator  120  includes a contact area  146  that will contact the thermally activated material  140  when the actuator  120  is in a contact position. The contact position for actuator  120  is similar to the contact position of the actuator  20  shown in FIG.  4 . 
     The actuator  120  may be a bi-material cantilever beam including a first material  150  and second material  151 , shown in FIG. 5, where the first and second materials  150 ,  151  are connected to first and second heating elements  154 ,  155 . The actuator  120  may operate like the embodiment of FIGS. 1-4 having a bi-material cantilever, as described above. The alternative actuating mechanisms for the actuator  120  that were described above are also available for the embodiment of FIGS. 5-7. 
     The device  100  also includes an input line  160  and an output line  162 , separated by a gap  164 , shown in FIGS. 6-7. The actuator  120  includes a crossbar  166  at a second end  136  of the actuator  120 . When the actuator  120  is in a contact position, similar to the contact position illustrated in FIG. 4, the crossbar  166  contacts both the input and output lines  160 ,  162 , bridging the gap  164 . The crossbar  166  is an electrically conductive material that completes a microrelay between the input and output signal lines  160 ,  162 . 
     Preferably, the actuator  120  also includes a connector device  170  joining the crossbar to the remainder of the actuator  120 . In a preferred embodiment, the connector device  170  is somewhat flexible, so that it is possible for the crossbar  166  to be held flush against the input and output lines  160 ,  162  although the remainder of the actuator  120  is not horizontally orientated. This will allow the contact area between the crossbar  166  and the input and output lines  160 ,  162  to be as large as possible. 
     The connector device  170  includes a top piece  172  and a bottom piece  174 , shown in FIG.  5 . The connector device  170  can function without the top piece  172 . The connector device  172  may have many different configurations than the configuration illustrated in FIG. 5 as long as the connector device  170  allows the crossbar  166  to contact the input line  160  and output line  162  when the actuator  20  is in the contact position. Where current is applied to the actuator to move the actuator, connector device  170  is preferably an electrical insulator. 
     The MEMS device  200  illustrated in FIGS. 8-10 includes a substrate  16 , an actuator  220 , and a spacer  24  between the actuator  220  and the substrate  16 . Many elements of the device  200  are the same as the elements of the device  10  shown in FIGS. 1-4, and like reference numbers are used to refer to these elements. The actuator  220  can move between a non-contact position shown in FIGS. 8-10 and a contact position similar to that illustrated in FIG.  4 . In addition to the elements described in relation to FIGS. 1-4, the device  200  shown in FIGS. 8-10 includes a mirror  225  at a second end  36  of the actuator  20 . The movement of the mirror  225  along with the actuator  220  allows the device  200  to be used as a switch or relay in an optical device. 
     The devices described herein are preferably fabricated using batch processing techniques for advantages in cost and ease of assembly. 
     The various embodiments described above are provided by way of illustration only and should not be construed to limit the invention. Those skilled in the art will readily recognize various modifications and changes which may be made to the present invention without strictly following the exemplary embodiments and applications illustrated and described herein, and without departing from the true spirit and scope of the present invention which is set forth in the following claims.