Patent Publication Number: US-2007120207-A1

Title: Torsion spring for MEMS structure

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
      This application claims priority from of Korean Patent Application No. 10-2005-0115058, filed on Nov. 29, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a torsion spring for a micro-electro-mechanical system (MEMS) structure, and more particularly, to a torsion spring with a great ratio of bending stiffness to torsion stiffness.  
      2. Description of the Related Art  
      Micro-electro-mechanical system (MEMS) structures are built using a semiconductor process. In general, MEMS structures include a stage suspended above a substrate and torsion springs that support both sides of the stage so that the stage can seesaw about the torsion springs. MEMS structures can be applied to, among other things, MEMS gyroscopes, optical scanners of flat panel displays, or the like.  
      The torsion springs should make the stage or a driving frame pivot only in a specific direction. To this end, the torsion springs should have a great ratio of bending stiffness to torsion stiffness, a resistance to deformation in a direction perpendicular to the axis of rotation, and a resistance to torsion around the axis of torsion.  
      Torsion springs used for macro structures can have a great ratio of bending stiffness to torsion stiffness by being manufactured to have a circular or cross-shaped section. However, this approach is difficult to be applied to torsion springs used for MEMS structures and requires many additional processes.  
       FIG. 1  is a perspective view of a conventional torsion spring  10  for a MEMS structure, which has a beam shape with a rectangular section. Referring to  FIG. 1 , bending stiffness and torsion stiffness of the conventional torsion spring  10  are determined by a ratio of width b 0  to length L 0  and a ratio of width b 0  to height h 0  of the beam. For example, when the ratio of the length L 0  increases, both the bending stiffness and the torsion stiffness decrease. Accordingly, it is difficult to increase the ratio of the bending stiffness to the torsion stiffness for the conventional torsion spring  10  constructed as shown in  FIG. 1 .  
      To solve this problem, Lilac Muller, Albert P. Pisano, and Roger T Howe suggested in “Microgimbal Torsional Beam Design Using Open, Thin-Walled Cross Section” Journal of MEMS, Vol. 10, NO. 4, December 2001, a torsion spring  20  as shown in  FIG. 2 . The torsion spring  20  has a horizontal beam  23  that is formed on top surfaces of a pair of parallel vertical beams  21  to connect the vertical beams  21 . The torsion spring  20  can significantly increase the bending stiffness without a substantial increase of the torsion stiffness. However, it is difficult to form a trench with a predetermined width and depth using etch lag during an etching process of the torsion spring  20  of  FIG. 2  .  
     SUMMARY OF THE INVENTION  
      The present invention provides a torsion spring for a MEMS structure, which can be simply manufactured to have a great ratio of bending stiffness to torsion stiffness.  
      According to an aspect of the present invention, there is provided a torsion spring for a MEMS structure, in which the torsion spring is connected between a pivoting member and a fixed member and supporting the pivoting member so that the pivoting member can pivot about the torsion spring, the torsion spring comprising: a horizontal beam; at least one vertical beam formed on the horizontal beam; and a plurality of auxiliary beams formed on the horizontal beam and parallel to the vertical beam.  
      The auxiliary beam may have a plate shape extending in a longitudinal direction of the horizontal beam.  
      The auxiliary beam may have a bar shape formed along a longitudinal direction of the horizontal beam.  
      The vertical beam may be formed at the center of the horizontal beam, and the auxiliary beams may be formed at both sides of the vertical beam.  
      The vertical beam may be a pair of vertical beams formed on both edges of the horizontal beam, and the auxiliary beam may be formed between the vertical beams.  
      The vertical beam may be a pair of vertical beams spaced apart from both edges of the horizontal beam, and the auxiliary beam may be formed at both sides of the vertical beams.  
      The vertical beam may be three vertical beams formed at regular intervals on the horizontal beam, and the auxiliary beam may be formed between the vertical beams.  
      According to another aspect of the present invention, there is provided a torsion spring for a MEMS structure, wherein the torsion spring is connected between a pivoting member and a fixed member and supporting the pivoting member so that the pivoting member can pivot about the torsion spring, the torsion spring comprising: a horizontal beam; an upper vertical beam and a lower vertical beam formed on top and bottom surfaces of the horizontal beam, respectively, to correspond to each other; and a plurality of upper and lower auxiliary beams formed on the top and bottom surfaces of the horizontal beam and parallel to the upper and lower vertical beam, respectively.  
      The horizontal beam may be a stack comprising a first conductive layer, an insulating layer, and a second conductive layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and aspects of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
       FIG. 1  is a perspective view of a conventional torsion spring for a MEMS structure;  
       FIG. 2  is a perspective view of another conventional torsion spring for a MEMS structure;  
       FIG. 3  is a perspective view of a torsion spring for a MEMS structure according to an exemplary embodiment of the present invention;  
       FIG. 4  is a perspective view of a modification of the torsion spring of  FIG. 3 ;  
       FIGS. 5A through 5D  are sectional views of other modifications of the torsion spring of  FIG. 3 ;  
       FIG. 6  is a perspective view of an optical scanner having a torsion spring of an exemplary embodiment of the present invention;  
       FIGS. 7A through 7C  are perspective views illustrating a method of manufacturing the torsion spring of  FIG. 3 ;  
       FIG. 8  is a perspective view of a torsion spring according to another exemplary embodiment of the present invention;  
       FIGS. 9A through 9D  are perspective views illustrating a method of manufacturing the torsion spring of  FIG. 8 ; and  
       FIGS. 10A through 10D  are sectional views of a modification of the torsion spring of  FIG. 8 . 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION  
      The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The described exemplary embodiments are intended to assist the understanding of the invention, and are not intended to limit the scope of the invention in any way. The thickness of layers and regions shown in the drawings may be exaggerated for clarity.  
       FIG. 3  is a perspective view of a torsion spring for a MEMS structure according to an exemplary embodiment of the present invention.  
      Referring to  FIG. 3 , a torsion spring  30  includes a pair of vertical beams  33 , a horizontal beam  31  formed on bottom surfaces of the vertical beams  33  to connect the vertical beams  33 , and a plurality of auxiliary beams  35  perpendicularly formed on the horizontal beam  31 . Although the horizontal beam  31 , the vertical beams  33 , and the auxiliary beams  35  are integrally formed, they are given different reference numerals for convenience of explanation.  
      The horizontal beam  31  and the vertical beams  33  extend in a longitudinal direction of the torsion spring  30 . The horizontal beam  31  and the vertical beams  33  have plate shapes with a rectangular section. The auxiliary beams  35  may have rectangular bar shapes. The horizontal beam  31  and the vertical beams  33  are perpendicular to each other. Ends of the horizontal beam  31  and the vertical beams  33  are connected to predetermined portions on a substrate (not shown), for example, connected between a fixed member like an anchor and a pivoting member like a stage.  
      A gap G 2  between the auxiliary beams  35 , a gap GI between one of the pair of vertical beams  33  and the auxiliary beams  35 , and a gap G 3  between the other of the pair of vertical beams  33  and the auxiliary beams  35  may be formed to have micrometer dimensions, and cause etch lag during an etching process. The etch lag enables the horizontal beam  31  to be formed.  
       FIG. 4  is a perspective view of a modification of the torsion spring  30  of  FIG. 3 . Reference numerals identical to those in  FIG. 3  denote like elements, and a detailed description of these elements will not be repeated.  
      Referring to  FIG. 4 , auxiliary beams  36  formed between the vertical beams  33  have plate shapes and extend in a longitudinal direction of a torsion spring  30 ′, similarly to the vertical beams  33  and the horizontal beam  31 .  
       FIGS. 5A through 5D  are sectional views of other modifications of the torsion spring  30  of  FIG. 3 .  
      Referring to  FIG. 5A , a torsion spring  40  includes a horizontal beam  41 , three vertical beams  43  perpendicularly formed on the horizontal beam  41  and spaced at regular intervals from one another, and auxiliary beams  45  formed on the horizontal beam  41  between the vertical beams  43 . The auxiliary beams  45  may have plate shapes or bar shapes parallel to the vertical beams  43 .  
      Referring to  FIG. 5B , a torsion spring  50  includes a horizontal beam  51 , a vertical beam  53  perpendicularly formed on the horizontal beam  51 , and auxiliary beams  55  formed on the horizontal beams  51  at both sides of the vertical beam  53 .  
      Referring to  FIGS. 5C , a torsion spring  60  includes a horizontal beam  61 , two vertical beams  63  formed on the horizontal beam  61  and spaced apart from both edges of the horizontal beam  61 , and auxiliary beams  65  formed on the horizontal beam  61 .  
      Referring to  FIG. 5D , a torsion spring  70  includes a horizontal beam  71 , two first vertical beams  73  perpendicularly formed on both ends of the horizontal beam  71 , three second vertical beams  74  formed between the first vertical beams  73 , and auxiliary beams  75  formed between the first and second vertical beams  73  and  74 . A gap Gi′ between the auxiliary beams  75  and each of the vertical beams  73  and  74  is greater than a gap G 2 ′ between the second vertical beams  74 , and thus a depth of a trench formed due to the gap G 2 ′ is smaller than a depth of a trench formed due to the gap G 1 ′. These depths can be adjusted by controlling the widths of the gaps Gi′ and G 2 ′.  
       FIG. 6  is a perspective view of an optical scanner having a torsion spring according to an exemplary embodiment of the present invention. A general description of the type of optical scanner such as that shown in  FIG. 6  is provided in U.S. Patent Application Publication No. 2006/0082250, which is hereby incorporated by reference in its entirety.  
      Referring to  FIG. 6 , an optical scanner includes a first torsion spring  81  connected between a stage  80  and a driving frame  82 , and a second torsion spring  84  connected between the driving frame  82  and a fixed frame  83 . Each of the first and second torsion springs  81  and  84  may have a similar structure to a torsion spring of one of the exemplary embodiments of the present invention. Because the first and second torsion springs  81  and  84  are structured according to one of the exemplary embodiments of the present invention, the torsion springs of the optical scanner of  FIG. 6  have good bending stiffness. The horizontal beam can be easily formed using etch lag of the auxiliary beams when the stage  80 , the driving frame  82 , the fixed frame  83 , and the vertical beams of the torsion spring of an exemplary embodiment of the present invention are formed.  
       FIGS. 7A through 7C  are perspective views illustrating a method of manufacturing the torsion spring  30  of  FIG. 3 .  
      Referring to  FIG. 7A , an insulating mask  91  is formed on a silicon substrate  90 . At this time, a gap G 1 ″ between a vertical beam portion and a frame portion should be greater than each of a gap G 2 ″ between the vertical beam portion and an auxiliary beam portion and a gap G 3 ″ between the auxiliary beams  35 .  
      Referring to  FIG. 7B , when areas not covered by the mask  91  are etched in a reactive ion etching (RIE) process, an etch rate of the gap G 1 ″ is faster than that of each of the gaps G 2 ″ and G 3 ″ because the gap G 1 ″ is greater than each of the gaps G 2 ″ and G 3 ″.  
      Referring to  FIG. 7C , after the etching process is performed for a predetermined period of time, the frame portion and the vertical portion are completely separated from each other by the gap G 1 ″ to form a frame  92  and the torsion spring  30 . The torsion spring  30  includes the vertical beams  33  and the auxiliary beams  35  formed on the horizontal beam  31 .  
       FIG. 8  is a perspective view of a torsion spring  100  according to another exemplary embodiment of the present invention.  
      Referring to  FIG. 8 , the torsion spring  100  includes a horizontal beam  101 , upper and lower vertical beams  111  and  112  perpendicularly formed at centers of top and bottom surfaces of the horizontal beam  101 , respectively, to correspond to each other, and upper and lower auxiliary beams  115  and  116  perpendicularly formed on the horizontal beam  101  such that the upper auxiliary beams  115  are disposed at both sides of the upper vertical beam  111  and the lower auxiliary beams  116  are disposed at both sides of the lower vertical beam  112 .  
      The horizontal beam  101  may be composed of a first conductive layer  102 , an insulating layer  103 , and a second conductive layer  104 . The horizontal beam  101  may be manufactured by etching a silicon-on-insulator (SOI) substrate. In this case, the torsion spring  100  fabricated using the multi-layered silicon substrate can have paths through which voltages are separately applied to upper comb electrodes and lower comb electrodes as shown in  FIG. 6 .  
      The auxiliary beams  115  and  116  cause etch lag such that the first and second conductive layers  102  and  104  can be formed while the other elements, such as the frame  92  in  FIG. 7C , are formed as described above.  
      The torsion spring  100  constructed as above is configured in a ribbed structure, thereby increasing bending stiffness.  
       FIGS. 9A through 9D  are perspective views illustrating a method of manufacturing the torsion spring  100  of  FIG. 8 . Reference numerals identical to those in  FIG. 8  denote like elements, and a detailed description of the elements will not be repeated.  
      Referring to  FIG. 9A , an SOI substrate  120  is prepared. A frame to which the torsion spring  100  is connected is partially illustrated for convenience of explanation. The substrate  120  is formed by stacking a first conductive layer  122  made of silicon, an insulating layer  123  made of silicon oxide, and a second conductive layer  124  made of silicon. Next, a mask  126  is formed on the first conductive layer  122 . Here, a gap G 1 ′″ between an auxiliary beam portion and a frame portion is greater than each of a gap G 3 ′″. between a vertical beam portion and the auxiliary beam portion and a gap G 2 ′″ between auxiliary beam portions.  
      Referring to  FIG. 9B , when areas not covered by the mask  126  are etched in an RIE process, an etch rate of the gap G 1 ′″ is faster than that of each of the gaps G 2 ′″ and G 3 ′″. Accordingly, while the gap G 1 ′″ is etched to the insulating layer  123  that is used as an etch stop layer, etch lag occurs at the gaps G 2 ′″ and G 3 ′″ to form the first conductive layer  102  of the horizontal beam  101 , the vertical beam  111 , and the auxiliary beams  115  on the first conductive layer  102 .  
      Referring to  FIG. 9C , the second conductive layer  124  of the substrate  120  is etched to form the vertical beam  112  and the auxiliary beams  116  respectively corresponding to the vertical beam  111  and the auxiliary beams  115  formed at the first conductive layer  122  of the substrate  120 .  
      Referring to  FIG. 9D , an exposed portion of the insulating layer  123  is etched to form the torsion spring  100  and the frame  128 .  
      Although the upper and lower auxiliary beams  115  and  116  have bar shapes in the present exemplary embodiment, the present invention is not limited thereto. That is, the upper and lower auxiliary beams  115  and  116  may have plate shapes like the upper and lower vertical beams  111  and  112 .  
       FIGS. 10A through 10D  are sectional views of modifications of the torsion spring  100  of  FIG. 8 .  
      Referring to  FIG. 10A , a torsion spring  130  includes a horizontal beam  131 , upper and lower vertical beams  137  and  138  perpendicularly formed on both edges of top and bottom surfaces of the horizontal beam  131 , respectively, to correspond to each other, and upper and lower auxiliary beams  135  and  136  perpendicularly formed on the top and bottom surfaces of the horizontal beam  131 , respectively, such that the upper auxiliary beams  135  are disposed between the upper vertical beams  137  and the lower auxiliary beams  136  are disposed between the lower vertical beams  138 . The torsion spring  130  has an “H” shape, and accordingly the horizontal beam  131  increases the bending stiffness of the torsion spring  130 .  
      The horizontal beam  131  may be composed of a first conductive layer  132 , an insulating layer  133 , and a second conductive layer  134 .  
      Referring to  FIG. 10B , a torsion spring  140  includes a horizontal beam  141 , upper and lower vertical beams  145  and  146  perpendicularly formed on top and bottom surfaces of the horizontal beam  141 , respectively, to correspond to each other, and upper and lower auxiliary beams  147  and  148  perpendicularly formed on the top and bottom surfaces of the horizontal beam  141  such that the upper auxiliary beams  147  are disposed between the upper vertical beams  145  and the lower auxiliary beams  148  are disposed between the lower vertical beams  146 . The horizontal beam  141  may be composed of a first conductive layer  142 , an insulating layer  143 , and a second conductive layer  144 .  
      Referring to  FIG. 10C , a torsion spring  150  includes a horizontal beam  151 , two upper and lower vertical beams  155  and  156  perpendicularly formed on top and bottom surfaces of the horizontal beam  151 , respectively, to correspond to each other and be spaced apart from both edges of the horizontal beam  151 , and upper and lower auxiliary beams  157  and  158  perpendicularly formed on the top and bottom surfaces of the horizontal beam  151 . The horizontal beam  151  may be composed of a first conductive layer  152 , an insulating layer  153 , and a second conductive layer  154 .  
      Referring to  FIG. 10D , a torsion spring  160  includes a horizontal beam  161 , two first upper and lower vertical beams  165  and  166  perpendicularly formed on top and bottom surfaces of the horizontal beam  161  to be disposed on both sides of the top and bottom surfaces of the horizontal beam  161 , three second upper and lower vertical beams  167  and  168  formed such that the second upper vertical beams  167  are disposed between the first upper vertical beams  165  and the second lower vertical beams  168  are disposed between the first lower vertical beams  166 , and upper and lower auxiliary beam  169  and  170  formed such that the upper auxiliary beams  169  are disposed between the first upper vertical beams and the second upper vertical beam  167  and the lower auxiliary beams  170  are disposed between the first lower vertical beams  166  and the second lower vertical beams  168 . A gap G 1 ″″ between the auxiliary beam  169  and  170  and each of the first and second vertical beams  165 ,  166  and  167 ,  168  is greater than a gap G 2 ′″ between the second vertical beams  167  and  168 , and thus a depth of a trench formed due to the gap G 2 ′″ is less than a depth of a trench formed due to the gap G 1 ′″. The horizontal beam  161  may be composed of a first conductive layer  162 , an insulating layer  163 , and a second conductive layer  164 .  
      As described above, the torsion spring for a MEMS structure according to exemplary embodiments of the present invention has increased bending stiffness due to the horizontal beam. Also, the horizontal beam of the torsion spring of the exemplary embodiments of the present invention can be easily formed using etch lag that occurs at the region where the trench is narrow.  
      While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.