Abstract:
A micro electro mechanical system (MEMS) device is provided. The MEMS device includes: a stage which operates in a vibration mode; an axle which supports the stage and allows rotation of the stage; and a capacitive sensor which detects rotation of the stage. The capacitive sensor includes: a sensing arm which extends from the stage; driving combs which extend from the sensing arm and rotated together with the stage; fixed combs which are fixedly supported for engagement with the driving combs, the fixed combs including surfaces overlapping opposite surfaces of the driving combs in accordance with the rotation of the driving combs; and a capacitance sensing portion which detects a capacitance change of the driving combs and the fixed combs. Therefore, the MEMS device performs precise scanning by structurally preventing deformation of the stage having a light reflecting surface.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This application claims priority from Korean Patent Application No. 10-2006-0053551, filed on Jun. 14, 2006, 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]     Apparatuses consistent with the present invention relate to a micro electro mechanical system (MEMS) device, and more particularly, to an MEMS device that performs precise scanning by structurally preventing deformation of a stage having a light reflecting surface and includes a precise sensing structure for detecting the rotation speed and direction of the stage.  
         [0004]     2. Description of the Related Art  
         [0005]     MEMS devices are frequently used in display devices, laser printers, precise measuring instruments, precise machining devices, etc. For example, in a display device, an MEMS device is used as an optical scanner for reflecting or deflecting a scanning light beam onto a screen.  
         [0006]     A related art MEMS device includes a stage formed with a light reflecting surface, a driving coil wound around the light reflecting surface of the stage, and a magnet forming a magnetic field across the driving coil. During the operation of the MEMS device, the stage is rotated in a direction determined by the Lorentz law due to the interaction between a current applied to the driving coil and the magnetic field formed by the magnet, and thus light incident onto the light reflecting surface of the stage is reflected onto a screen in a predetermined scanning direction. Since the driving coil is wound around the light reflecting surface of the stage, the size and inertia mass of the driving coil increase, thereby decreasing the driving efficiency of the MEMS device. Furthermore, since the driving coil is wound around a relatively large area, the magnetic field should travel a relatively long distance. This lowers the strength of the magnetic field and causes a driving power loss.  
         [0007]     To address these problems, the light reflecting surface is formed on a top surface of the stage, and the driving coil is formed on a bottom surface of the stage within an inner region of the light reflecting surface. However, in this case, the light reflecting surface directly receives heat generated from the lower driving coil and thus undergoes thermal deformation. Therefore, incident light is not precisely reflected from the light reflecting surface of the stage in a desired scanning direction. For this reason, there is a need for an improved structure that can increase the efficiency of the MEMS device and keep the stage flat.  
         [0008]     Meanwhile, to realize a high-resolution display device, an optical scanner should scan a screen very precisely. For this, there is a need for a sensor that can precisely detect the rotation of a reflecting mirror of the optical scanner.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention provides an MEMS device that performs precise scanning by structurally preventing deformation of a stage having a light reflecting surface.  
         [0010]     The present invention also provides an MEMS device that includes a precise sensing structure for detecting the rotation speed and direction of a stage.  
         [0011]     According to an aspect of the present invention, there is provided an MEMS device including: a stage which operates in a vibration mode; an axle which supports the stage and allows rotation of the stage; and a capacitive sensor which detects a rotational state of the stage, wherein the capacitive sensor includes: a sensing arm extending from an end of the stage in parallel with the axle, the sensing arm being spaced a predetermined distance from the axle; a plurality of driving combs extending from the sensing arm in a direction crossing the axle, the driving combs being rotated together with the stage; a plurality of fixed combs arranged in parallel with each other and fixedly supported at a predetermined location for engagement with the driving combs, the fixed combs including surfaces overlapping opposite surfaces of the driving combs in accordance with the rotation of the driving combs; and a capacitance sensing portion which detects a capacitance change of the driving combs and the fixed combs via the overlapping surfaces of the driving combs and the fixed combs.  
         [0012]     According to another aspect of the present invention, there is provided an MEMS device including: a stage which operates in a vibration mode; an axle which supports the stage and allowing rotation of the stage; and a capacitive sensor which detects a rotational state of the stage, wherein the capacitive sensor includes: a first sensing arm and a second sensing arm that extend from the stage in parallel with the axle, the first and second sensing arms being spaced a predetermined distance from the axle in opposite directions; a plurality of driving combs extending from each of the first and second sensing arms in a direction crossing the axle, the driving combs being rotated together with the stage; a plurality of fixed combs arranged in parallel with each other and fixedly supported at a predetermined location for engagement with the driving combs, the fixed combs including surfaces overlapping opposite surfaces of the driving combs in accordance with the rotation of the driving combs; and a capacitance sensing portion which detects a capacitance change of the driving combs and the fixed combs via the overlapping surfaces of the driving combs and the fixed combs.  
         [0013]     When the stage is in a horizontal position, the fixed combs and the driving combs may face each other at the same height and overlap each other to a maximum level, and when the stage and the driving combs are rotated, an overlapping area between the fixed combs and the driving combs may decrease.  
         [0014]     When the stage is in a horizontal position, the fixed combs and the driving combs may be located at different heights, and when the stage and the driving combs are rotated, the driving combs of the first sensing arm or the second sensing arm may overlap corresponding fixed combs according to a rotational direction of the stage and the driving combs.  
         [0015]     When the stage is in a horizontal position, the driving combs of the first sensing arm may be located at a different height from the fixed combs corresponding to the driving combs of the first sensing arm, and the driving combs of the second sensing arm may be located at the same height as the fixed combs corresponding to the driving combs of the second sensing arm.  
         [0016]     According to a further another aspect of the present invention, there is provided an MEMS device including: a stage which operates in a vibration mode; a driving body rotatably disposed to upwardly or downwardly face the stage; a spacer column disposed between the stage and the driving body to keep the stage and the driving body at a predetermined distance from each other; a driving coil wound along an edge of the driving body; and a magnet which forms a magnetic field across the driving coil.  
         [0017]     According to a still further another aspect of the present invention, there is provided an MEMS device including: a stage which operates in a vibration mode; deformation absorbing springs extending outward from both sides of the stage and having a thin ring shape; a driving body rotatably disposed to upwardly or downwardly face the stage; spacer columns disposed between the driving body and the deformation absorbing springs so as to keep the driving body and the deformation absorbing springs at a predetermined distance from each other in a vertical direction; a driving coil wound along an edge of the driving body; and a magnet which forms a magnetic field across the driving coil. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The above and other aspects of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings, in which:  
         [0019]      FIG. 1  is a perspective view illustrating main parts of an MEMS device according to an exemplary embodiment of the present invention;  
         [0020]      FIG. 2  is an enlarged plan view illustrating a capacitive sensor of the MEMS device of  FIG. 1  according to an exemplary embodiment of the present invention;  
         [0021]      FIG. 3  is a sectional view taken along line III-III of  FIG. 2  to illustrate a vertical arrangement of the capacitive sensor according to an exemplary embodiment of the present invention;  
         [0022]      FIG. 4  is a graph showing a linear relationship between the capacitance of the capacitive sensor and the rotation angle of a stage, according to an exemplary embodiment of the present invention;  
         [0023]      FIG. 5  is a profile graph showing the rotation angle of the stage as a function of driving time, according to an exemplary embodiment of the present invention;  
         [0024]      FIG. 6  is a capacitance-time graph of the capacitive sensor when the stage is driven as illustrated in  FIG. 5 , according to an exemplary embodiment of the present invention;  
         [0025]      FIG. 7  illustrates a vertical sectional structure of the stage according to an exemplary embodiment of the present invention;  
         [0026]      FIGS. 8A and 8B  show results of a thermal deformation analysis performed on the stage of  FIG. 7 , according to an exemplary embodiment of the present invention;  
         [0027]      FIG. 9  is an exploded perspective view illustrating an MEMS device according to another exemplary embodiment of the present invention;  
         [0028]      FIGS. 10A and 10B  are views illustrating a coupling structure of the stage and the driving body of  FIG. 9 , according to an exemplary embodiment of the present invention;  
         [0029]      FIGS. 11A and 11B  show bending deformation analysis results to compare a conventional stage and driving body with the stage and driving body of the present invention, according to an exemplary embodiment of the present invention;  
         [0030]      FIGS. 12A and 12B  show modification examples of  FIG. 10A , according to another exemplary embodiments of the present invention;  
         [0031]      FIGS. 13A and 13B  show different structures that can be employed for effectively maintaining the flatness of the stage according to exemplary embodiments of the present invention;  
         [0032]      FIG. 14  shows bending deformation analysis results for the driving bodies of  FIGS. 13A and 13B , according to an exemplary embodiment of the present invention;  
         [0033]      FIG. 15  shows bending deformation analysis results for the stages of  FIGS. 13A and 13B , according to an exemplary embodiment of the present invention;  
         [0034]      FIGS. 16A and 16B  show an upper structure and a lower structure of the MEMS device of  FIG. 9  according to exemplary embodiments of the present invention;  
         [0035]      FIG. 17  is a sectional view taken along line XVII-XVII of  FIG. 16B  to illustrate a vertical arrangement of a capacitive sensor according to an exemplary embodiment of the present invention;  
         [0036]      FIG. 18  is a plan view illustrating a driving coil according to an exemplary embodiment of the present invention;  
         [0037]      FIGS. 19A and 19B  show an i th  turn of a driving coil having rounded corners and an i th  turn of a driving coil having non-rounded corners, according to an exemplary embodiment of the present invention; and  
         [0038]      FIGS. 20A and 20B  show a plan view of an MEMS device and a sectional view taken along line XX-XX of the plan view, according to another exemplary embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0039]     An MEMS device will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown.  
         [0040]      FIG. 1  is a perspective view illustrating main parts of an MEMS device according to an exemplary embodiment of the present invention. The MEMS device includes a stage  110  operating in vibration mode, an axle  150  supporting the stage  110  and functioning as a rotating center of the stage  110 , and an outer frame  120  (illustrated only in the enlarged view of  FIG. 1  for clarity) surrounding the stage  110 . The MEMS device further includes a capacitive sensor detecting the rotation of the stage  110 .  
         [0041]     The capacitive sensor includes a sensing arm  113  extending from an end of the stage  110  at a predetermined distance from the axle  150  in parallel with the axle  150  (in an x-axis direction), a plurality of driving combs  114  evenly extending from the sensing arm  113  in an y-axis direction perpendicular to the axle  150 , and a plurality of fixed combs  124  extending in parallel from the outer frame  120  for interlocking with the driving combs  114 . The driving combs  114  are rotated together with the stage  110  about the axle  110 . As the driving combs  114  rotate, the driving combs  114  approaches or departs from the fixed combs  124  in a z-axis direction according to the rotation direction of the driving combs  114 .  
         [0042]      FIG. 2  is an enlarged plan view illustrating the capacitive sensor of the MEMS device of  FIG. 1  according to an exemplary embodiment of the present invention. Referring to  FIG. 2 , the fixed combs  124  and the driving combs  114  are spaced a predetermined gap C gap  in the x-axis direction and have an overlapping length C L  in the y-axis direction. When a predetermined electrical potential difference is applied between the fixed combs  124  and the driving combs  114 , the overlapping fixed combs  124  and the driving combs  114  are electrically charged. Reference numeral P denotes the distance between the axle  150  and an end of the sensing arm  113 .  
         [0043]      FIG. 3  is a sectional view taken along line III-III of  FIG. 2 . Referring to  FIG. 3 , an overlapping area between the mutually-facing fixed combs  124  and the driving combs  114  increases or decreases as the driving combs  114  rotate. When the stage  110  is in a horizontal position, the driving combs  114  supported by the stage  110  are placed in parallel with the fixed combs  124  and thus overlap the fixed combs  124  maximally. Therefore, the overlapping area between the driving combs  114  and the fixed combs  124  is maximal, and thus the capacitance of the driving combs  114  and the fixed combs  124  is also maximal. When the stage  110  rotates a predetermined angle, the driving combs  114  incline with respect to the fixed combs  124  and thus overlap the fixed combs  124  less compared with the case where the stage  110  is in the horizontal position. That is, the overlapping area of the combs  114  and  124  and the capacitance of the combs  114  and  124  dependent on the overlapping area can be expressed as a function of a rotation angle Ψ of the stage  110 .  
         [0044]      FIG. 4  is a graph showing a relationship between the capacitance (C) of the combs  114  and  124  and the rotation angle Ψ of the stage  110 . When the rotation angle Ψ of the stage  110  increases from zero degree, the capacitance (C) of the combs  114  and  124  decreases in linear proportion to the increase of the rotation angle Ψ. This linear relationship between the capacitance (C) and the rotation angle Ψ can be expressed by “y=−ax+b”, where (a) denotes a proportional constant determined by system variables of the capacitance sensor. The proportional constant (a) relates to the sensitivity or resolving power of the capacitive sensor. When the capacitive sensor has a great proportional constant (a), the capacitive sensor can perform precise sensing with a great sensitivity.  FIG. 5  is a graph showing the rotation angle Ψ of the stage  110  with respect to driving time (t). The rotation angle Ψ of the stage  110  periodically changes with different rising and falling intervals in a given period. The rotation angle Ψ of the stage  110  periodically changes between +Ψ max  and −Ψ max  based on the horizontal position of the stage  110  (Ψ=0° when the stage  110  is in a horizontal position).  
         [0045]      FIG. 6  is a capacitance-time graph of the combs  114  and  124  when the stage  110  is driven as illustrated in  FIG. 5 . Referring to  FIG. 6 , when the rotation angle Ψ of the stage  110  is zero (when the stage is in a horizontal position), the capacitance (C) of the combs  114  and  124  is at a maximum C Ψ     max   . When the rotation angle Ψ of the stage  110  is at a maximum +Ψ max  or minimum −Ψ min , the capacitance (C) of the combs  114  and  124  is at a minimum C 0 . A driving margin C margin  corresponding to the minimum capacitance C 0  may be maintained at a sufficiently large value for the linear relationship between the rotation angle Ψ and the capacitance (C). Meanwhile, the capacitive sensor further includes a detecting circuit (not shown) to electrically measure the capacitance (C) of the combs  114  and  124 . The detecting circuit can be configured in a conventional manner, and thus a detailed description of the detecting circuit will be omitted.  
         [0046]      FIG. 7  illustrates a vertical sectional structure of the stage  110  depicted in  FIG. 1 . Referring to  FIG. 7 , the stage  110  includes a silicon layer  111  forming a body of the stage  110 , a driving coil pattern  115  filled in trenches formed in a bottom surface of the silicon layer  111 , a coil insulating layer  117  formed between the silicon layer  111  and the driving coil pattern  115  to surround the driving coil pattern  115 , an insulating layer  119  covering the bottom surface of the silicon layer  111  and exposed surfaces of the driving coil pattern  115  to insulate the silicon layer  111  and the driving coil pattern  115 , and a reflective metal layer  112  formed on top of the stage  110 . The insulating layer  119  forming a bottom portion of the stage  110  may be formed of an insulating material having a low thermal expansion coefficient. For example, the insulating layer  119  may be formed of a silicon oxide, like the coil insulating layer  117 . The reflective metal layer  112  forming a top portion of the stage  110  may be formed of a metal having a high thermal expansion coefficient and reflectivity, so as to be used as a bimetal together with the insulating layer  119 . For example, the reflective metal layer  112  may be formed of aluminum.  
         [0047]     Magnets (not shown) form a magnetic field across the driving coil pattern  115 . Therefore, when a driving current is applied to the driving coil pattern  115 , an electric field is formed around the coil pattern  115  and interacts with the magnetic field, thereby rotating the stage  110  in a direction determined by the Lorentz law. Here, when the driving current is applied, the driving coil pattern  115  experiences thermal expansion due to its resistance. In this case, since the driving coil pattern  115  is formed on the bottom of the stage  110 , the stage  110  can deflect upward. To prevent the upward deflection of the stage  110 , the reflective metal layer  112  having a high thermal expansion coefficient is formed on top of the stage  110 , and the insulating layer  119  having a low thermal expansion coefficient is formed on the bottom of the stage  110 . That is, the upper reflective metal layer  112  and the lower insulating layer  119  function as a bimetal deflecting the stage  110  downward, so that the upward deflection of the stage  110  by the driving coil pattern  115  can be prevented. Therefore, the reflective metal layer  112  of the stage  110  can be kept flat, and thus a scanning line distortion can be prevented.  FIGS. 8A and 8B  show results of a thermal deformation analysis performed on the stage  110  for explaining the effects of the exemplary embodiment of the present invention. Referring to  FIG. 8A , the deflection of the stage  110  in the z-axis direction is indicated. Due to this deflection (bending deformation), an end of the stage  110  was deflected by 4.611 μm in the z-axis direction when compared with a center of the stage  110 . Referring to  FIG. 8B , in the effective area of the stage  110  actually used for reflecting light, the maximum deflection of the stage  110  was 3.052 μm.  
         [0048]      FIG. 9  is an exploded perspective view illustrating an MEMS device according to another exemplary embodiment of the present invention. Referring to  FIG. 9 , the MEMS device includes a stage  210  and a driving body  220  that face each other, an axle  250  supporting the driving body  220  and functioning as a rotating center of the driving body  220 , an outer frame  221  supporting the driving body  220  via the axle  250  and allowing rotation of the driving body  220 , and a frame base  230  receiving and supporting the outer frame  221 .  
         [0049]     The stage  210  and the driving body  220  may have shapes similar to each other and be arranged vertically. When the MEMS device is used as an optical scanner in a display apparatus, the stage  210  includes a reflecting surface on one side. The driving body  220  supports the stage  210  and operates the stage  210  in a vibration mode. While operating in the vibration mode, the stage  210  deflects incident light in a scanning direction. It is not necessary that the stage  210  have the same area as the driving body  220 . The stage  210  can have a smaller area than the driving body  220  as long as the stage  210  can have a sufficient light reflecting area for satisfying a given scanning angle.  
         [0050]     A driving coil  225  is wound along the edge of the driving body  220 . The driving coil  225  runs along a curve at rounded portions of the edge of the driving body  220 . A driving current is applied from an outer power source (not shown) to an end of the driving coil  225 . Magnets (not shown) generate a magnetic field across the driving coil  225 , and interaction between the applied driving current and the magnetic field causes the driving body  220  to rotate about the axle  250  in a direction determined by the Lorentz law. In the current exemplary embodiment, the driving body  220  is provided separate from the stage  210 , and the driving coil  225  is wound around the driving body  220 . In other words, since the driving coil  225  is formed under the stage  210 , thermal deformation of the stage  210  by the driving coil  225  can be prevented. Therefore, the driving coil  225  can be wound around a relatively small diameter, thereby reducing the moment of inertia of rotary parts such as the driving body  220  and improving the driving efficiency of the MEMS device.  
         [0051]     Meanwhile, in addition to the outer frame  221 , another outer frame  211  is provided around the stage  210  at a predetermined distance from the stage  210 . The outer frames  211  and  221  have shapes similar to each other and are arranged vertically. The outer frame  221  surrounding the driving body  220  supports the driving body  220  via the axle  250  and allows the rotation of the driving body  220 , and a connection terminal (not shown) is formed on the outer frame  221  in electrical connection with the driving coil  225  so as to supply electricity to the driving coil  225  from an outside power source. The outer frame  221  is stably supported by the frame base  230 . The frame base  230  supports the driving body  220  and spaces the driving body  220  a predetermined distance from a floor, thereby allowing a vibration motion of the driving body  220 . An opening  230 ′ is formed in a center portion of the frame base  230 , such that the driving body  220  can vibrate without interference.  
         [0052]      FIG. 10A  shows planar and sectional structures of the stage  210  supported by the driving body  220 . Referring to  FIG. 10A , a spacer column  215  is interposed between the stage  210  and the driving body  220 . The spacer column  215  keeps the stage  210  and the driving body  220  at a predetermined distance from each other so as to prevent transmission of bending deformation of the driving body  220  to the stage  210 . In detail, the driving coil  225  undergoes thermal expansion due to its resistance when a current is applied, and thus the driving body  220  in which the driving coil  225  is formed can be deflected upward as shown in  FIG. 10B . However, since the stage  210  is spaced apart from the driving body  220 , the stage  210  can be kept flat. That is, the driving body  220  can be freely deflected without interference with the stage  210  owing to the gap formed by the spacer column  215  between the stage  210  and the driving body  220 . The stage  210  can be thermally or mechanically affected by the spacer column  215 . Therefore, a contact area between the stage  210  and the spacer column  215  is maintained minimal as long as the stage  210  can be stably supported.  
         [0053]      FIGS. 11A and 11B  show bending deformation analysis results to compare a related art case where the entire bottom surface of the stage  210  is supported, with the case where a portion of the bottom surface of the stage  210  is supported by the spacer column  215  and the other portion of the bottom surface is let free in the current exemplary embodiment of the present invention. In  FIGS. 11A and 11B , like reference numerals denote like elements. Referring to  FIG. 11A  showing the analysis result of the related art structure, a deformation of the driving body  220  was directly transmitted to the stage  210 , causing the maximum vertical deformation of the stage  210  to be 6.57 μm. However, referring to  FIG. 11B  showing the analysis result of the present invention, under the same conditions, the maximum vertical deformation of the stage  210  was 0.45 μm (about 1/10 of the maximum value measured in a related case). Therefore, the analytical results show that the MEMS device according to the current exemplary embodiment of the present invention is more suitable for precise optical scanning.  
         [0054]     In the current exemplary embodiment, one spacer column  215  is provided on a center (C) of the stage  210  to stably support the stage  210 . However, referring to  FIGS. 12A and 12B , two or more spacer columns  215  can be provided in a predetermined arrangement. In this case, even when the spacer columns  215  are not precisely aligned, the stage  210  can be stably supported by the spacer columns  215 . The center of the arranged spacer columns  215  may be aligned with the center (C) (particularly, the center of gravity) of the stage  210 . For this, the spacer columns  215  can be symmetrically arranged with respect to the center of the stage  210 .  
         [0055]     In the current exemplary embodiment, the stage  210  having a reflecting surface is disposed above the driving body  220  providing a driving force, and the spacer column  215  is interposed between the stage  210  and the driving body  220  to keep the stage  210  and the driving body  220  at a predetermined distance from each other.  FIGS. 13A and 13B  show different structures that can be employed for effectively maintaining the flatness of the stage  210 . In  FIGS. 13A and 13B , like reference numerals denote like elements. Referring to  FIGS. 13A and 13B , the stage  210  and the driving body  220  face each other and are spaced a predetermined distance apart from each other. The stage  210  includes a light reflecting surface smaller than the driving body  220 . Referring to  FIG. 13A , the driving body  220  has a plate shape. However, referring to  FIG. 13B , the driving body  220  includes a circular opening  220 ′ and is divided into a center region A 1  and an outer region A 2  by the circular opening  220 ′. The circular opening  220 ′ prevents thermal deformation from being transmitted between the center region A 1  and the outer region A 2 .  
         [0056]      FIG. 14  shows bending deformation analysis results for the driving bodies  220  of  FIGS. 13A and 13B . In  FIG. 14 , the horizontal axis denotes the distance from the center of the driving body  220  in the x-axis direction, and the vertical axis denotes deflection of the driving body  220  measured in a vertical direction (perpendicular to the x-axis and y-axis directions) when the driving body  220  was heated to 100° C. Referring to  FIG. 14 , both the driving bodies  220  of  FIGS. 13A and 13B  were deflected upward. In detail, the driving body  220  of  FIG. 13B  (indicated by a dashed line) was deflected much more than the driving body  220  of  FIG. 13A  (indicated by a solid line). The reason for this is that the center region A 1  and the outer region A 2  of the driving body of  FIG. 13B  are not firmly supported by each other due to the circular opening  220 ′ formed between the center region A 1  and the outer region A 2 . Therefore, the driving body  220  of  FIG. 13B  is more easily deflected by thermal stress.  
         [0057]      FIG. 15  shows bending deformation analysis results for the stages  210  of  FIGS. 13A and 13B . In  FIG. 15 , the horizontal axis denotes the distance from the center of the stage  210  in the y-axis direction, and the vertical axis denotes deflection of the stage  210  measured in a vertical direction (perpendicular to the x-axis and y-axis directions) when the driving body  220  was heated to 100° C. Referring to  FIG. 15 , both the stages  210  of  FIGS. 13A and 13B  were deflected downward. In detail, the maximum deflection of the stage  210  of  FIG. 13A  (indicated by a solid line) was 450 nm, and the maximum deflection of the stage  210  of  FIG. 13B  (indicated by a dashed line) was 50 nm. That is, the deflection of the stage  210  can be maintained at a low level of several tens of nanometers (for example, one tenth of incident light wavelength or less) by separating the center region A 1  of the driving body  220  using the opening  220 ′. The reason for this is that since the outer region A 2  of the driving body  220  where the driving coil  225  is wound is separated from the center region A 1  of driving body  220  where the stage  210  is connected, the propagation of thermal stress from the outer region A 2  to the stage  210  is prevented.  
         [0058]     A capacitive sensor can be, provided to monitor the rotation of the stage  210  of the MEMS device according to another exemplary embodiment of the present invention. The capacitive sensor will now be described in detail.  FIGS. 16A and 16B  show an upper structure of the MEMS device including the stage  210  and the outer frame  211  and a lower structure of the MEMS device including the driving body  220  and the outer frame  221 , respectively. Referring to  FIG. 16A , sensing arms  213   a  and  213   b  extend from the stage  210  in parallel with the axle  250  and are spaced a predetermined distance from the axle  250  in opposite directions. Each of the sensing arms  213   a  and  213   b  includes a plurality of driving combs  214  uniformly extending in a direction perpendicular to the axle  250 . The driving combs  214  are rotated together with the stage  210  about the axle  250 . Referring to  FIG. 16B , a plurality of fixed combs  224  is formed at each corner of the outer frame  221 . The fixed combs  224  extend in parallel with each other for interlocking with the driving combs  214 . The driving combs  214  of the sensing arm  213   a  and the fixed combs  224  form a first sensing portion S 1  at one side of the axle  250 , and the driving combs  214  of the other sensing arm  213   b  and the fixed combs  224  form a second sensing portion S 2  at the other side of the axle  250 . Referring to enlarged portion of  FIG. 16B , the driving combs  214  and the fixed combs  224  are adjacent to each other and are arranged to overlap each other. The capacitance of the driving combs  214  and the fixed combs  224  vary depending on the rotation of the driving combs  214 .  
         [0059]      FIG. 17  is a sectional view taken along line XVII-XVII of  FIG. 16B . Referring to  FIG. 17 , the first sensing portion SI and the second sensing portion S 2  are respectively formed at the left and right sides of the axle  250 . That is, the first sensing portion S 1  and the second sensing portion S 2  are symmetric with respect to the axle  250 . When the stage  210  is in a horizontal position, the driving combs  214  and the fixed combs  224  are placed at different heights and are spaced a small distance from each other. In this state, the combs  214  and  224  are not electrically charged even when a predetermined voltage is applied to the combs  214  and  224 . However, when the stage  210  and the driving combs  214  integrally formed with the stage  210  are rotated a predetermined angle Ψ about the axle  250 , one of the driving combs  214  overlaps one of the fixed combs  224 . In more detail, when the stage  210  is rotated counterclockwise about the axle  250 , an overlapping area is formed at the first sensing portion S 1  and thus the first sensing portion S 1  is electrically charged. However, in this case, the second sensing portion S 2  is not electrically charged. Although now shown, when the stage  210  is rotated clockwise about the axle  250 , an overlapping area is formed at the second sensing portion S 2  and thus the second sensing portion S 2  is electrically charged. In this case, the capacitance of the first sensing portion S 1  is not electrically charged. The rotation direction of the stage  210  can be easily determined by measuring changes in the capacitances of the first and second sensing portions S 1  and S 2 . Furthermore, the rotation angle of the stage  210  can be easily determined using the measured capacitances of the first and second sensing portions S 1  and S 2  and the capacitance-rotation angle relationship of  FIG. 4 .  
         [0060]     Alternatively, the first and second sensing portions S 1  and S 2  can be asymmetric with respect to the axle  250 . For example, when the stage  210  is in a horizontal position, the driving combs  214  and the fixed combs  224  of the first sensing portion S 1  can be placed at the same height, and the driving combs  214  and fixed combs  224  of the second sensing portion S 2  can be placed at different heights.  
         [0061]      FIG. 18  shows a driving coil pattern of the MEMS device of  FIG. 9  according to an exemplary embodiment of the present invention. Referring to  FIG. 18 , a driving coil  225  is wound to run from an inside area to an outer area. Inner and outer ends of the driving coil  225  are respectively connected to connection terminals  240  to receive a driving current.  FIGS. 19A and 19B  show an i th  turn of a driving coil  225  having rounded corners and an i th  turn of a driving coil  225 ′ having non-rounded corners. Corners of the driving coil  225  are rounded to increase efficiency. According to experimental results, when the corners of the driving coils  225  are rounded, the power consumption of the driving coil  225  is reduced by 17% as compared with the case where the corners of the driving coil  225  are not rounded.  
         [0062]      FIG. 20A  shows a plan view of an MEMS device and a sectional view taken along line XX-XX of the plan view according to another exemplary embodiment of the present invention. Referring to  FIG. 20A , the MEMS device includes a stage  310 , a deformation absorbing springs  312  supporting both sides of the stage  310 , a driving body  320  facing a bottom surface of the stage  310 , and spacer columns  315  keeping the driving body  320  at a predetermined distance from the deformation absorbing springs  312 . The deformation absorbing springs  312  is formed on both sides of the stage  310  to support the stage  310 . The stage  310  is spaced a predetermined distance apart from the driving body  320 . The deformation absorbing springs  312  has a thin ring shape suitable for elastic deformation. While being elastically deformed, the deformation absorbing springs  312  keeps the stage  310  flat. The spacer columns  315  keep the stage  310  and the driving boy  320  at a predetermined distance from each other. The driving body  320  provides a rotation force to the stage  310 . For this, a driving coil  325  having a predetermined pattern is formed on a bottom surface of the driving body  320 . When a predetermined current is applied to the driving coil  325 , the driving body  320  and the stage  310  coupled to the driving body  320  are rotated about an axle  350  by the interaction between the applied current and a magnetic field formed by magnets (not shown). Here, the driving coil  325  is heated and expanded due to its electric resistance, thereby causing upward deflection of the driving body  320  as shown in  FIG. 20B . However, since the stage  310  is spaced a predetermined distance apart from the driving body  320 , the stage  310  can be kept flat without interfering with the driving body  320 . Furthermore, while being elastically deformed, the deformation absorbing springs  312  absorbs the bending stress transmitted from the driving body  320  through the spacer columns  315 , thereby preventing the deformation of the stage  310 .  
         [0063]     According to the MEMS device of the exemplary embodiments of the present invention, the stage having the light reflecting surface is spaced a predetermined distance from the driving body providing a driving force, so that the flatness of the stage is not affected by the bending deformation of the driving body. Therefore, the MEMS device can perform scanning precisely in a desired direction.  
         [0064]     Further, in the related art stage where both the light reflecting surface and the driving coil are formed, the driving coil is inevitably formed around the light reflecting surface, thereby decreasing the driving efficiency of the MEMS device. However, in the exemplary embodiments of the present invention, since the driving coil is formed at the additional driving body, the driving coil can be located inside the light reflecting surface of the stage, thereby increasing the driving efficiency of the MEMS device.  
         [0065]     Furthermore, the MEMS device according to the exemplary embodiments of the present invention can include the capacitive sensor. Therefore, the rotation speed and direction of the stage can be precisely measured. For example, when the capacitive sensor is applied to an MEMS device used for an optical scanner, a stage can be precisely controlled and thus a high-resolution display device can be provided.  
         [0066]     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.