Abstract:
A self-assembly structure of micro electromechanical optical switch utilizes residual stresses of three curved beams. The first curved beam pushes the base plate away from the substrate. The second curved beam lifts up the mirror slightly. Then, the third curved beam rotates the mirror vertical to the base plate and achieves self-assembly. In another embodiment, magnetic force and magnetic-activated elements are used.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to micro electromechanical optical switches applicable to optical communication industry, and especially relates to a self-assembly structure of micro electromechanical optical switch utilizing residual stress.  
       BACKGROUND OF THE INVENTION  
       [0002]     Micro electromechanical systems are the kind of microminiaturized mechanical, optical or electronic devices that can operate with sound, light, electricity, magnetism, taste, cooling, heating as well as motion and so on, and mainly include mechanical and electronic components. In recent years, optical systems utilize optical micro electromechanical (optical MEMS) technology of semiconductor manufacturing and other related micro machining process and technology to get various types of high accuracy, high optical quality miniature electromechanical elements, like lens, mirror, diffraction grating and so on. In accompany with built-in micro actuators, the micro electromechanical elements may directly manipulate the light, and extremely arouse people&#39;s attentions in the optical communication domain. However, the micro electromechanical elements made by semiconductor manufacturing or other micro machining processes are mostly of thin film structure. Therefore, it is a very important topic to develop a mirror actuator that can be self-assembled to provide a larger displacement.  
         [0003]     U.S. Pat. No.  6 , 292 , 600  discloses a free-rotating hinged micro-mirror switching element operated in “open” and “close” states. The micro-mirror comprises a mirror connected to the substrate by free-rotating micro-hinges. The hinges include one or more hinge pins and one or more hinge staples. Pushrods are connected at one end to the mirror and at the opposite end to the translation stage with hinge joints. And the actuated component is the scratch-drive actuator (SDA). Through applying appropriate voltage to the SDA, the SDA can be deformed or moved to a certain extent. The deformation or movement in turn causes the pushrods to act upon the mirror and rotate it to a predetermined position or angle from the substrate. Such design is able to turn the linear movement of the pushrods into rotation of the mirror and relatively reduce the entire device dimensions. However, because the degree of freedom of the mirror rotation is extremely sensitive to the optical fiber coupling efficiency, the mirror rotation angle has to be precisely controlled and encounters much difficulty.  
         [0004]     Besides, in U.S. Pat. Nos. 6,526,198 and 6,556,741, the micro-mirror switch includes a substrate, an electrode coupled to the substrate, and a micromachined plate rotatably coupled to the substrate about a pivot axis. The micro-mirror has an orientated reflective surface mounted to the micromachined plate. An electrical source is coupled to the electrode and the micromachined plate. When voltage is applied, the electrostatic force causes the actuator move downward. The high reflection mirror surface is assembled to the actuator and perpendicular to the substrate. In order to avoid causing electrical short when the mirror moves down, a landing electrode with a buckle beam is specially used. Though the structure may provide precise movement, but each optical switch element needs additional assembly, which consumes time and a lot of work.  
       SUMMARY OF THE INVENTION  
       [0005]     In order to solve the above problems, the invention provides a micro electromechanical optical switch having a self-assembly structure. The assembly cost and error are reduced by the self-assembly mechanism.  
         [0006]     The self-assembly structure of micro electromechanical optical switch of the invention mainly includes a substrate, a base plate, a mirror and three curved beams (respectively as first curved beam, second curved beam and third curved beam). One end of the base plate is anchored on the substrate. Another end is free. A side of the mirror is pivoted on the base plate. One end of the first curved beam is fixed on the substrate; the other end is located between the base plate and the substrate so that the residual stress of the first curved beam lifts the base plate upward and away from the substrate. On the other hand, one end of the second curved beam is anchored on the base plate; the other end is plate; the other end is located at the pivot potion of the mirror pivoted to the base plate. The residual stress of the second curved beam lifts the mirror slightly. Then, the third curved beam rotates the mirror up about vertical to the base plate, and accomplish the self-assembly.  
         [0007]     Furthermore, the invention includes magnetic-activated elements on the mirror and the base plate. So that, an external magnetic force can be applied to control the relative position of the mirror and the base plate and achieve self-assembly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The invention will become more fully understood from the detailed description given herein below. However, this description is for purposes of illustration only, and thus is not limitative of the invention, wherein:  
         [0009]      FIG. 1A  is an exploded view of an optical switch of the invention;  
         [0010]      FIG. 1B  is an assembly of an optical switch of the invention;  
         [0011]      FIG. 2  is an assembly view of the invention where the mirror is lifted slightly;  
         [0012]      FIGS. 3A and 3B  are finished assembly views of the invention;  
         [0013]      FIG. 4  is an explanatory view of function of the curved beam that lifts the base plate by residual stress;  
         [0014]      FIGS. 5A and 5B  are explanatory views of a curved beam with residual stress in a first embodiment;  
         [0015]      FIGS. 6A and 6B  are explanatory views of a curved beam with residual stress in a second embodiment; and  
         [0016]      FIGS. 7A and 7B  are functional views of self-assembly of the invention applying magnetic-activated elements.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]      FIGS. 1A and 1B  illustrate a self-assembly structure of a micro electromechanical optical switch according to the invention. The optical switch includes a substrate  10 , a base plate  20 , a mirror  30 , a first curved beam  40 , a second curved beam  60 , and a third curved beam  50 . One end of the base plate  20  is anchored on the substrate  10 ; the other end (the left end in the drawing) is free. One side of the mirror  301  is pivoted on the base plate  20 .  
         [0018]     One end of the first curved beam  40  is fixed to the substrate  10 ; the other end extends to a position between the substrate  10  and the base plate  20 . One end of the second curved beam  601  is anchored on the base plate  20 ; the other end extends to a position under the mirror  30  and about the central portion. One end of the third curved beam  50  is anchored on the base plate  20 ; the other end extends to the pivot portion of the mirror  30  connecting to the base plate  20 . The front end of the third curved beam  50  is formed with an opening  51  and a tenon  511 .  
         [0019]     As shown in  FIG. 2 , by residual stress of the second curved beam  60 , the end of the second curved beam bends up and lifts the mirror  30  rotating slightly with the pivot portion and away from base plate  20 . As shown in  FIGS. 3A and 3B , the residual stress of the third curved beam  50  bends up the front end of the beam, causes the tenon  511  engaging with a cutoff  31  formed on the mirror  30  to lift the mirror  30  vertically to the base plate  20 . As shown in  FIG. 4 , the front end of the first curved beam  40  bends upward, lifts the base plate  20  to rotate from the substrate  10  by its pivot portion. Because the bend-up angle of the first curved beam  40  is limited, if the base plate  20  needs a great angle of lift-up, then we can make the front end of the first curved beam  40  approach the pivot portion of the base plate  20  that links with the substrate  10 , and use a drawbridge  70  to assist the required positioning angle.  
         [0020]     The design of the curved beam is shown in  FIGS. 5A, 5B . On a substrate  91 , there are a first material layer  92  and a second material layer  93  fixed together. The two layers have different thermal-expansion coefficients. When applying electric current to heat up material layer  94 , there is a sacrifice material layer  95  to be removed. Because a stress gradient is distributed on the material layer  94 , the material layer  94  warps after being released.  
         [0021]     The residual stresses of two materials enable a micro cantilever beam to have a stress gradient distribution and cause the micro cantilever beam bend according to the moment of force and get a displacement. By theory of material mechanics, a double-layer plate structure with known thickness, Young&#39;s modulus and uniform stress values σ 1  and σ 2  respectively, then its radius of curvature ρ and the displacement δ are calculated:  
         1   ρ     =       6   ⁢     (       m   ⁢           ⁢     σ   2       -     σ   1       )           hE   2     ⁡     (       2   ⁢   m     +       K   ⁡     [       n   ⁡     (     1   +   n     )       2     ]         -   1         )             
       δ   =     ρ   ⁡     (     1   -     cos   ⁡     (     L   /   ρ     )         )           
       K   =     1   +     4   ⁢   mn     +     6   ⁢     mn   2       +     4   ⁢     mn   3       +       m   2     ⁢     n   4             
        in which m is the Young&#39;s modulus ratio of the double-layer plate; n is thickness ratio the doubling plate and L is the cantilever length.        
 
         [0023]     Table 1 shows some examples of calculation. The materials respectively are silicon rich nitride and poly-silicon. The Young&#39;s modulus of silicon rich nitride is 300 Gpa and mean residual stress value is 100 Mpa. The Young&#39;s modulus of poly-silicon is 160 Gpa and mean residual stress value is 0 Mpa. The thickness is 0.4 um for silicon rich nitride and 2 um for poly-silicon. There have been a lot of related researches of prior arts. The listed examples are just embodiments. They are certainly not limited to the two materials.  
                       TABLE 1                           Displacement   Displacement       Cantilever length   (theoretical)   (finite element)                    500 um   12.132 um   12.9492 um        750 um    27.29 um   28.6235 um       1000 um   48.499 um   50.4397 um       1250 um   75.746 um   78.3963 um                  
 
         [0024]     On the other hand, please refer to  FIGS. 7A and 7B , there are magnetic-activated elements  32  and  21  (such as permalloy) added on the mirror  30  and the base plate  20  respectively. By applying an exterior magnetic field (as shown in the drawing a coil  81 ), thorough different volumes and assembly resilience of the magnetic-activated elements  32  and  21 , the mirror  30  and the base plate  20  are self-assembled. This mechanism substitutes the above first curved beam  40 , second curved beam  60  and third curved beam  50  (in  FIGS. 1A and 1B ) and directly utilizes the magnetic force for assembly instead of using residual stress. The permalloy material can be chosen from soft magnetic materials such as nickel (Ni), ferronickel (Ni—Fe), nickel cobalt alloy (Ni—Co) and so on, or retentive materials such as ferro—neodymium boron (Nd—Fe—B), samarium cobalt alloy (Sm—Co) and so on.  
         [0025]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.