Abstract:
A vertical comb-type electrode structure capable of a large linear-displacement motion. The vertical comb-electrode structure includes: a first substrate including a plurality of vertical static comb-electrodes; and a second substrate stacked on an upper surface of the first substrate, the second substrate including a plurality of vertical moving comb-electrodes, wherein the static comb-electrodes are vertically moved or positioned a predetermined distance toward the moving comb-electrodes in the initial state of the electrode structure so that no gaps between the static comb-electrodes and the moving comb-electrodes exist.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
       [0001]     This application claims the benefit of Korean Patent Application No. 10-2005-0125454, filed on Dec. 19, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a vertical comb-type electrode structure provided by a micro-electromechanical system (MEMS) technique, and more particularly, to a vertical comb-type electrode structure which can perform a large linear-displacement motion.  
         [0004]     2. Description of the Related Art  
         [0005]     Vertical comb-type electrode structures in which moving comb-electrodes (or a rotor) and static comb-electrodes (or a stator) are formed on a silicon-on-insulator (SOI) substrate are generally used in electrostatic sensors, micro light scanners, or microactuators.  
         [0006]      FIG. 1  is a perspective view of a conventional vertical comb-type electrode structure  10 . Referring to  FIG. 1 , in the conventional vertical comb-type electrode structure  10 , an upper silicon substrate  14  having moving comb-electrodes  17  is stacked on a lower silicon substrate  11  having static comb-electrodes  12 . An insulation layer  13 , for example, an oxide layer, is interposed between the lower silicon substrate  11  and the upper silicon substrate  14 . The moving comb-electrodes  17  are vertically aligned on opposite sides of a driving plate  15  connected to the upper silicon substrate  14  through a spring  16 . The static comb-electrodes  12  are formed on the lower silicon substrate  11  and alternate with the moving comb-electrodes  17 . When voltages are applied to the moving comb-electrodes  17  and the static comb-electrodes  12 , the driving plate  15  moves in a vertical direction or rotates due to an electrostatic force generated between the moving comb-electrodes  17  and the static comb-electrodes  12 .  
         [0007]     However, when a large displacement in a conventional vertical comb-type electrode structure occurs, it is accompanied by a significantly non-linear motion. When the conventional vertical comb-type electrode structure achieves linear motion, the displacement therein is quite small.  
         [0008]      FIG. 2A  is a cross-sectional view of a two-layered vertical comb-type electrode structure in which a relatively large displacement can be performed. Referring to  FIG. 2A , since moving comb-electrodes  17  are formed in an upper silicon substrate  14  and static comb-electrodes  12  are formed in a lower silicon substrate  11 , there is a gap T BOX  between the moving comb-electrodes  17  and the static comb-electrodes  12 . The thickness of the gap T BOX  is the same as the thickness of an insulation layer  13 , formed between the upper silicon substrate  14  and the lower silicon substrate  11 .  FIG. 2B  illustrates relative positions of static comb-electrodes  12  and moving comb-electrodes  17 . When the moving comb-electrodes  17  move up and down, the moving comb-electrodes  17  overlap the static comb-electrodes  12 , so that the capacitance between the static comb-electrodes  12  and the moving comb-electrodes  17  changes. Accordingly, as illustrated in  FIG. 2C , the vertical comb-type electrode structure can be represented as an equivalent circuit where variable capacitors are connected in parallel. Referring to  FIG. 2C , C 1  denotes a capacitance between the right static comb-electrode  12  and the right moving comb-electrode  17 , and C 2  denotes a capacitance between the left static comb-electrode  12  and the left moving comb-electrode  17 .  
         [0009]     Since the capacitance increases as the overlapping area between the static comb-electrodes  12  and the moving comb-electrodes  17  increases, when the driving plate  15  moves in a vertical direction, the capacitance changes, as illustrated in  FIG. 3A . That is, from the time when the static comb-electrodes  12  overlap the moving comb-electrodes  17 , the capacitance linearly increases. In addition, when an applied voltage is constant, the electrostatic force Fe generated between the static comb-electrodes  12  and the moving comb-electrodes  17  is proportional to the capacitance change rate. Accordingly, the electrostatic force F e  is drastically changed from the time when the static comb-electrodes  12  overlap moving comb-electrodes  17  (z=−T BOX ), and then becomes constant, as illustrated in  FIG. 3B . Since the electrostatic force F e  is proportional to the square of the applied voltage (V), the displacement of the driving plate  15  can be controlled by controlling the applied voltage. However, since the electrostatic force F e  exhibits a discontinuity at z=−T BOX , the displacement of the driving plate  15  does not change as the applied voltage reaches a threshold value thereof, as illustrated in  FIG. 3C . When the applied voltage is greater than the threshold value, the driving plate  15  radically moves to a position of z=−T BOX , and then linearly moves. Accordingly, the two-layered vertical comb-type electrode structure of  FIG. 2A  can provide a relatively large displacement, but cannot provide a linear motion at an applied voltage less than the threshold value.  
         [0010]      FIG. 4A  is a cross-sectional view of another conventional vertical comb-type electrode structure for obtaining linear motion. In the conventional vertical comb-type electrode structure illustrated in  FIG. 4 , static comb-electrodes  12  and moving comb-electrodes  17  are formed in the same plane and overlap each other. Then the static comb-electrodes  12  are displaced downwards by an upper cover  18  by a distance TD.  FIG. 4B  illustrates a relative position of the moving comb-electrodes  17  and the static comb-electrodes  12 .  
         [0011]     In such a structure,  FIG. 5A  shows the change of capacitance corresponding to the movement of a driving plate  15 . When the moving comb-electrodes  17  are moved the distance T D  to entirely overlap the static comb-electrodes  12 , the capacitance is maximized. Unlike the vertical comb-type electrode structure of  FIG. 2A , since there is no initial gap between the static comb-electrodes  12  and the moving comb-electrodes  17 , non-linear motion does not occur in the vertical comb-type electrode structure of  FIG. 4A . However, as illustrated in  FIG. 5B , when the driving plate  15  moves a distance greater than T D , the direction of the electrostatic force F e  becomes opposite, so that the driving plate  15  cannot be moved a distance greater than T D , as illustrated in  FIG. 5C . Accordingly, the vertical comb-type electrode structure of  FIG. 4A  cannot provide a large displacement.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention provides a simple vertical comb-type electrode structure that provides a large linear displacement motion.  
         [0013]     The present invention also provides an electrostatic sensor, a microactuator, or a micro light scanner using the vertical comb-type electrode structure.  
         [0014]     According to an aspect of the present invention, there is provided a vertical comb-electrode structure including: a first substrate including a plurality of vertical static comb-electrodes; and a second substrate stacked on an upper surface of the first substrate, the second substrate including a plurality of vertical moving comb-electrodes, wherein the static comb-electrodes are vertically moved a predetermined distance toward the moving comb-electrodes so that no gaps between the static comb-electrodes and the moving comb-electrodes exist.  
         [0015]     The vertical comb-electrode structure may further include: a base substrate disposed under the first substrate, wherein protruding portions vertically pressing the static comb-electrodes toward the moving comb-electrodes are formed on the base substrate so that the static comb-electrodes at least partially overlap the moving comb-electrodes  
         [0016]     An insulation layer may be interposed between the first substrate and the second substrate.  
         [0017]     A thickness of the protruding portions formed on the surface of the base substrate may be greater than at least a thickness of the insulation layer.  
         [0018]     The first substrate and the static comb-electrodes may be integrally formed in the same plane, and a spring may be integrally formed between the first substrate and the static comb-electrodes so that the static comb-electrodes are vertically displaced with respect to the first substrate.  
         [0019]     The second substrate may further include a driving plate integrally formed therewith in the same plane, and a spring may be integrally formed between the second substrate and the driving plate so that the driving plate is moved in a vertical direction or rotated with respect to the second substrate.  
         [0020]     The plurality of moving comb-electrodes may be vertically aligned and parallel to each other on sides of the driving plate.  
         [0021]     According to another aspect of the present invention, there is provided a micro light scanner includes the vertical comb-type electrode structure.  
         [0022]     According to another aspect of the present invention, there is provided a micro actuator includes the vertical comb-type electrode structure.  
         [0023]     According to another aspect of the present invention, there is provided an electrostatic sensor includes the vertical comb-type electrode structure.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0025]      FIG. 1  is a perspective view of a conventional vertical comb-type electrode structure;  
         [0026]      FIG. 2A  is a cross-sectional view of a conventional vertical comb-type electrode structure;  
         [0027]      FIG. 2B  illustrates relative positions of static comb-electrodes and moving comb-electrodes in the conventional vertical comb-type electrode structure of  FIG. 2A ;  
         [0028]      FIG. 2C  is an equivalent circuit for the conventional vertical comb-type electrode structure of  FIG. 2A ;  
         [0029]      FIGS. 3A through 3C  illustrate characteristics of the conventional vertical comb-type electrode structure of  FIG. 2A ;  
         [0030]      FIG. 4A  is a cross-sectional view of another conventional vertical comb-type electrode structure;  
         [0031]      FIG. 4B  illustrates relative positions of static comb-electrodes and moving comb-electrodes in the conventional vertical comb-type electrode structure of  FIG. 4A ;  
         [0032]      FIG. 5A through 5C  illustrate characteristics of the conventional vertical comb-type electrode structure of  FIG. 4A ;  
         [0033]      FIG. 6A  is a cross-sectional view of a vertical comb-type electrode structure that vertically moves, according to an embodiment of the present invention;  
         [0034]      FIG. 6B  illustrates relative positions of static comb-electrodes and moving comb-electrodes in the vertical comb-type electrode structure of  FIG. 6A ;  
         [0035]      FIGS. 7A through 7C  illustrate characteristics of the vertical comb-type electrode structure of  FIG. 6A ;  
         [0036]      FIG. 8  is a cross-sectional view of a vertical comb-type electrode structure that rotates, according to an embodiment of the present invention;  
         [0037]      FIGS. 9A through 9C  illustrate characteristics of the vertical comb-type electrode structure of  FIG. 8 ; and  
         [0038]      FIG. 10  is an exploded perspective view of a vertical comb-type electrode structure according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0039]     Hereinafter, the present invention will be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown.  
         [0040]      FIG. 6A  is a cross-sectional view of a vertical comb-type electrode structure that vertically moves, according to an embodiment of the present invention. Referring to  FIG. 6A , a silicon-on-insulator (SOI) substrate including a lower silicon substrate  21  and an upper silicon substrate  24  is bonded to a base substrate  30 . The bonding method is, for example, an anodic bonding method, a silicon direct bonding (SDB) method, or a metallic bonding method. Like the conventional art, an insulation layer  23 , for example, an oxide layer, is interposed between the lower silicon substrate  21  and the upper silicon substrate  24  so that electric shorts between the lower silicon substrate  21  and the upper silicon substrate  24  are prevented. A plurality of vertical static comb-electrodes  22  are integrally formed with the lower silicon substrate  21  in the same plane. In addition, a driving plate  26  and a plurality of vertical moving comb-electrodes  27  are integrally formed with the upper silicon substrate  24  in the same plane. As illustrated in  FIG. 6A , the plurality of moving comb-electrodes  27  are vertically aligned and parallel to each other on opposite sides of the driving plate  26 .  
         [0041]     Protruding portions  31  are formed on the surface of the base substrate  30  to correspond to the static comb-electrodes  22 , and press the static comb-electrodes  22  toward the moving comb-electrodes  27 . According to an embodiment of the present invention, the thickness of the protruding portions  31  formed on the surface of the base substrate  30  may be greater than at least the thickness of the insulation layer  23 . Accordingly, as illustrated in  FIG. 6A , the static comb-electrodes  22  are vertically moved toward the moving comb-electrodes  27  to partially overlap the moving comb-electrodes  27 .  
         [0042]      FIG. 6B  is a cross-sectional view of the static comb-electrodes  22  and the moving comb-electrodes  27  of  FIG. 6A .  FIG. 6B  illustrates relative positions of the plurality of static comb-electrodes  22 , which are vertically moved by the protruding portions  31 , and the plurality of the moving comb-electrodes  27 . Referring to  FIG. 6B , in the vertical comb-type electrode structure according to the current embodiment of the present invention, the static comb-electrodes  22  vertically overlap the moving comb-electrodes  27  by a predetermined distance T (T≧0).  
         [0043]     Accordingly, when the moving comb-electrodes  27  are vertically moved, that is, in the downward direction of  FIG. 6B , capacitance generated between the static comb-electrodes  22  and the moving comb-electrodes  27  changes, as illustrated in  FIG. 7A . That is, when the direction of the vertical motion of the moving comb-electrodes  27  is set as the z-axis, and the position of a bottom end portion of the moving comb-electrodes  27  before moving is defined as the origin of the z-axis, the capacitance linearly increases as the moving comb-electrodes  27  move from z=+T along the -z direction. In addition, if an applied voltage is constant, electrostatic force (F e ) generated by the capacitance change is constant when the moving comb-electrodes  27  move from z=+T along the -z direction, as illustrated in  FIG. 7B . Accordingly, in the vertical comb-type electrode structure according to the current embodiment of the present invention, the moving comb-electrodes  27  overlap the static comb-electrodes  22  by a distance T at an initial position (z=0), and thus the moving comb-electrodes  27  can linearly move. That is, as illustrated in  FIG. 7C , the vertical movement of the moving comb-electrodes  27  is proportional to a square of the applied voltage. To increase the displacement range of the moving comb-electrodes  27 , the distance T is small. In particular, when a top end portion of the static comb-electrodes  22  lines up with the bottom end portion of the moving comb-electrodes  27 , that is, T=0, the moving comb-electrodes  27  can linearly move.  
         [0044]     Thus, in the vertical comb-type electrode structure according to the current embodiment of the present invention, linear motion is possible, compared with the conventional art in  FIG. 2A . In addition, larger displacement can be obtained, compared with the conventional art in  FIG. 4A .  
         [0045]      FIG. 8  is a cross-sectional view of a vertical comb-type electrode structure that rotates. The driving plate  26  in the vertical comb-type electrode structure of  FIG. 6A  moves in a vertical direction, as indicated by an arrow. However, the driving plate  26  in the vertical comb-type electrode structure of  FIG. 8  rotates, as indicated by an arrow. Vertical motion and rotational motion can be selected according to how a voltage is applied to the vertical comb-type electrode structures of  FIG. 6A  and  FIG. 8 , respectively. For obtaining the motion in a vertical direction, in the vertical comb-type electrode structure of  FIG. 6A , the same voltages are applied to both sides of the static comb-electrodes  22  and the moving comb-electrodes  27 . Meanwhile, for obtaining the rotational motion, as illustrated in  FIG. 8 , opposite directional voltages are applied to both sides of the static comb-electrodes  22 , or a voltage is alternately applied to either side of the static comb-electrodes  22 .  
         [0046]     When the driving plate  26  rotates as illustrated in  FIG. 8 , capacitance between the static comb-electrodes  22  and the moving comb-electrodes  27  changes, as illustrated in  FIG. 9A . In  FIG. 9A , C 1  denotes a capacitance between a right static comb-electrode  22  and a right moving comb-electrode  27 , and C 2  denotes capacitance between a left static comb-electrode  22  and a left moving comb-electrode  27 . When the driving plate  26  is horizontally disposed, the angle (θ) is 0°. When the driving plate  26  rotates in a clockwise direction, θ&gt;0°. When the driving plate  26  rotates in a counter-clockwise direction, θ&lt;0°. The capacitance C 1  between the right static comb-electrode  12  and the right moving comb-electrode  17  linearly increases, when the driving plate  26  rotates in a clockwise direction, that is, when θ increases. Meanwhile, the capacitance C 2  between the left static comb-electrode  12  and the left moving comb-electrode  17  linearly increases, when the driving plate  26  rotates in a counter-clockwise direction, that is, when θ decreases. The static comb-electrodes  22  overlap the moving comb-electrodes  27  at an origin position. Thus, as illustrated in  FIG. 9A , C 1  increases starting from an angle less than 0°, and C 2  increases from an angle greater than 0°.  
         [0047]     Accordingly, electrostatic torquete caused by the capacitance change changes, as illustrated in  FIG. 9B . For example, a first torqueτ e1 , acting in a clockwise direction is constant when θ&gt;0, and a second torqueτ e2  acting in a counter-clockwise direction is constant when θ&lt;0. As illustrated in  FIG. 9C  the driving angle θ in the clockwise or counter-clockwise direction is linearly proportional to the square of applied voltage.  
         [0048]      FIG. 10  is an exploded perspective view of a vertical comb-type electrode structure according to an embodiment of the present invention. Referring to  FIG. 10 , a lower silicon substrate  21  having static comb-electrodes  22  is stacked on a base substrate  30 , and an upper silicon substrate  24  having moving comb-electrodes  27  is stacked on the lower silicon substrate  21 . Although not illustrated, an oxide layer is interposed between the lower silicon substrate  21  and the upper silicon substrate  24  for insulation therebetween.  
         [0049]     As described above, the lower silicon substrate  21  and the static comb-electrodes  22  are formed in the same plane. For example, a single silicon substrate is etched so that the lower silicon substrate  21  is integrally formed with the static comb-electrodes  22 . As illustrated in  FIGS. 6A and 8 , since the protruding portions  31  are formed on an upper surface of the base substrate  30  corresponding to the static comb-electrodes  22 , the static comb-electrodes  22  are upwardly moved by a thickness of the protruding portion  31  when the lower silicon substrate  21  is stacked. To move the static comb-electrodes  22  with respect to the lower silicon substrate  21 , the lower silicon substrate  21  may be connected to the static comb-electrodes  22  through a plate spring  25 , as shown in an enlarged portion of  FIG. 10 . The plate spring  25  may be integrally formed with the lower silicon substrate  21  and the static comb-electrodes  22  using an etching process.  
         [0050]     The upper silicon substrate  24  includes a driving plate  26  which moves in a vertical direction or rotates, and a plurality of moving comb-electrodes  27  are vertically aligned and parallel to each other on opposite sides of the driving plate  26 . The driving plate  26  is connected to the upper silicon substrate  24  through a torsion spring  29  for vertical motion or rotational motion with respect to the upper silicon substrate  24 , as illustrated in  FIG. 10 . Like the lower silicon substrate  21 , the upper silicon substrate  24 , the driving plate  26 , the moving comb-electrodes  27 , and the torsion spring  29  are integrally formed in the same plane by etching a single silicon substrate.  
         [0051]     As described above, in the vertical comb-type electrode structure according to the current embodiment of the present invention, large displacement and linear motion are both possible. Accordingly, the vertical comb-type electrode structure can be properly applied to a micro light scanner, a microactuator, or an electrostatic sensor. For example, when the vertical comb-type electrode structure is used in a micro light scanner which scans images at high speed in a laser TV, a mirror is formed on the surface of the driving plate  26 , and voltages are applied to the static comb-electrodes  22  and the moving comb-electrodes  27  so that the driving plate  26  having the mirror rotates at high speed. In addition, when the vertical comb-type electrode structure is used as a microactuator, voltages are applied to the static comb-electrodes  22  and the moving comb-electrodes  27  so that the driving plate  26  moves in a vertical direction. Alternatively, instead of driving the driving plate  26  by applying voltages to the static comb-electrodes  22  and the moving comb-electrodes  27 , a capacitance change between the static comb-electrodes  22  and the moving comb-electrodes  27  caused by the vibration of the driving plate  26  can be measured to sense inertia, etc. That is, the vertical comb-type electrode structure can be used as an electrostatic sensor.  
         [0052]     In present invention, static comb-electrodes overlap moving comb-electrodes due to protruding portions of a base substrate so that a vertical comb-type electrode structure in which large displacement and linear motion are possible, is provided in a simple manner and at low cost.  
         [0053]     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.