Abstract:
According to an aspect of an embodiment, a micro-electro-mechanical systems (MEMS) device comprises a substrate, a MEMS and a movable absorber. 
     The MEMS has a movable part having a resonance frequency on the substrate. The movable absorber absorbs a vibration in accordance with the resonance frequency so as to vibrate itself.

Description:
FIELD OF THE INVENTION 
       [0001]    This art relates to devices using micro-electro-mechanical systems (MEMS) elements. More specifically, a MEMS device accommodates MEMS element(s), which has a vibration-proof structure. 
         [0002]    MEMS devices have movable parts, i.e., mechanical elements fabricated by a semiconductor integrated circuit fabrication technology or the like, that are mechanically driven by using electricity. Optical switches are one example of systems using MEMS devices. Optical switches having a variety of structures have been proposed. Among those devices, an optical switch using a MEMS mirror array, which can provide a compact-sized multi-channel switch, is currently under development for practical application. 
         [0003]    However, because MEMS mirrors are movable mechanical structures, they may be moved out of position when they are subject to an externally applied vibration or impact. This disturbs optical paths. Therefore, as disclosed in Japanese Laid-open Patent Publication No. 2006-35375, MEMS mirrors are typically protected from an externally applied vibration or impact using a damper made of vibration-proof rubber. The rubber exchanges a gel being a more flexible material than the rubber. 
         [0004]    Examples of known structures of MEMS mirrors for optical switches include a structure in which each of the MEMS mirrors pivots about a single axis, which is disclosed in U.S. Pat. No. 6,753,960, and a structure in which each of the MEMS mirrors pivots about two axes, which is disclosed in U.S. Pat. No. 6,591,029. In both structures, the MEMS mirrors are arranged in an array on a semiconductor substrate. 
         [0005]    Because MEMS mirrors are movable mechanical structures, they have a resonance frequency (a natural frequency) determined by their shape and material. If the frequency components of an externally applied vibration or impact include the resonance frequency of the MEMS mirrors, the MEMS mirrors resonate. This significantly disturbs optical paths. Accordingly, parameters such as hardness of a vibration-proof damper need to be adjusted to sufficiently attenuate the vibration. 
         [0006]    U.S. Pat. No. 6,591,029 discloses that light emitted from an input fiber is reflected at an input MEMS mirror, and then reflected at an output MEMS mirror to an output fiber. That is, in one optical path, light is reflected at two MEMS mirrors having four pivot axes. When the MEMS mirrors having four pivot axes, which are disclosed in U.S. Pat. No. 6,591,029, are subject to an externally applied vibration or impact, they may be moved by an amount four times greater than an amount by which the MEMS mirrors having a single pivot axis, which are disclosed in Japanese Unexamined Patent Application Publication No. 2006-35375 and U.S. Pat. No. 6,753,960, are moved. Accordingly, for example, the structure disclosed in U.S. Pat. No. 6,591,029 requires a vibration absorbing damper for absorbing an externally applied vibration having a vibration attenuation capability four times larger than the vibration attenuation capability of the vibration absorbing dampers required by the structures disclosed in the other patent documents. A MEMS device having MEMS mirrors, each having four pivot axes, requires a vibration transmissibility of about −80 dB if the resonance frequency of the MEMS mirrors is in the range of about 1 kHz to 2 kHz. 
         [0007]      FIG. 1  shows a measurement example of the vibration transmissibility of a simple damper made of vibration-proof rubber, which is a known vibration-proof structure. The known damper made of vibration-proof rubber provides a vibration attenuation of about −40 dB to −50 dB. When the frequency component of an externally applied vibration or impact includes the resonance frequency of the MEMS mirrors, as described above, the vibration attenuation capability is insufficient. 
         [0008]    Therefore, there is a problem in that the vibration attenuation capability becomes insufficient when the MEMS mirrors are subject to an externally applied vibration having the resonance frequency of the MEMS mirrors, whereby the MEMS mirrors undergo resonant vibration. In addition, the resonant vibration of the MEMS mirrors degrades the optical properties of the optical switch. 
         [0009]    In a device such as an optical switch, a plurality of MEMS mirrors are arranged along an optical path. Therefore, light propagating in the optical switch tends to be influenced by the resonance of the MEMS mirrors. 
       SUMMARY 
       [0010]    According to an aspect of an embodiment, a micro-electro-mechanical systems (MEMS) device comprises a substrate, a MEMS and a movable absorber. 
         [0011]    The MEMS has a movable part having a resonance frequency on the substrate. The movable absorber absorbs a vibration in accordance with the resonance frequency so as to vibrate itself. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a graph showing the vibration transmissibility of a known vibration-proof structure; 
           [0013]      FIG. 2  shows a structure of a MEMS device of the embodiment; 
           [0014]      FIG. 3  shows a structure of an optical switch; 
           [0015]      FIGS. 4A and 4B  show a structure of a movable part in a MEMS mirror array; 
           [0016]      FIG. 5  shows a first exemplary structure of a vibration absorber; 
           [0017]      FIG. 6  shows a second exemplary structure of the vibration absorber; 
           [0018]      FIG. 7  shows a third exemplary structure of the vibration absorber; 
           [0019]      FIG. 8  shows a fourth exemplary structure of the vibration absorber; 
           [0020]      FIG. 9  is a graph showing the vibration transmissibility of the vibration absorber shown in  FIG. 5 ; 
           [0021]      FIG. 10  shows a structure of a vibration absorber for a casing; and 
           [0022]      FIGS. 11A to 11C  show a structure of a second MEMS device including a MEMS element having a single axis structure. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    These embodiments provide a MEMS device capable of sufficiently attenuating mechanical resonance, even when the MEMS device includes a plurality of MEMS elements having the same resonance frequency, and even when the MEMS device is subject to an externally applied vibration having a frequency the same as the resonance frequency of movable parts of the MEMS elements. 
         [0024]    Embodiments will now be described with reference to the drawings. It is to be noted that configurations of the following embodiments are exemplary, and these embodiments are not limited thereto. 
       Structure of MEMS Device 
       [0025]      FIG. 2  shows a structure of a MEMS device of the embodiment. The MEMS device includes a casing  1 , an optical input/output portion  2 , an optical path bending mirror  3 , a MEMS mirror array  4  (MEMS elements), vibration-proof rubber members  5  and  5 ′, a vibration absorber  6 , a movable absorbing portion  7 , and a base  8 . 
         [0026]    The casing  1  has two slant surfaces arranged to form a substantially V-shaped structure with a horizontal bottom portion. The optical input/output portion  2  is arranged on one of the slant surfaces of the casing  1 . The optical path bending mirror  3  is arranged on the other one of the slant surfaces of the casing  1 . The MEMS mirror array  4  is arranged on the horizontal portion at the bottom of the V-shaped structure of the casing  1 . The vibration absorber  6  and the vibration-proof rubber members  5  and  5 ′ are arranged between the casing  1  and the base  8 . The base  8  serves as a foundation for fixing the MEMS device to an external member. The vibration-proof rubber members  5  and  5 ′ may be made of a gel. The vibration-proof rubber members  5  and  5 ′ are elastic members for absorbing externally applied vibration by being elastically deformed. The movable absorbing portion  7  is arranged within the vibration absorber  6 . 
         [0027]    An externally applied vibration or impact (propagated from the base  8 ) is partially absorbed by the vibration-proof rubber members  5 ′, similarly to the known structure, and then the remaining portion of the vibration having a frequency component the same as that of the MEMS mirrors is absorbed at the vibration absorber  6 . The vibration-proof rubber members  5  further attenuate the vibration. The vibration-proof rubber members  5  prevent both MEMS mirrors  4  and movable absorbing portions  7  of the vibration absorber  6  from vibrating. 
       Structure of Optical Switch 
       [0028]      FIG. 3  shows an optical structure of the MEMS device shown in  FIG. 2 . In  FIG. 3 , components the same as those shown in  FIG. 2  are denoted by the same reference numerals. First, structures of respective portions will be described. 
         [0029]    The optical input/output portion  2  includes an input optical fiber array  21 , an output optical fiber array  22 , an input lens array  23 , and an output lens array  24 . In the optical input/output portion  2 , each of the fibers of the input optical fiber array  21  corresponds to one of the lenses in the input lens array  23 . Similarly, each of the fibers of the output optical fiber array  22  corresponds to one of the lenses in the output lens array  24 . 
         [0030]    The MEMS mirror array  4  includes the MEMS mirrors  45 . The number of the MEMS mirrors  45  of the MEMS mirror array  4  equals the number of the fibers of the input fiber array  21  and the output fiber array  22  of the optical input/output portion  2 . Some of the MEMS mirrors  45  correspond to the input lens array  23 , and the others correspond to the output lens array  24 . 
         [0031]    The optical path bending mirror  3  includes a first mirror and a second mirror. The first mirror reflects light beams from the MEMS mirror array  4  onto the second mirror, where the light beams are reflected back in the direction of the MEMS mirror array  4 . 
         [0032]    Referring to  FIG. 3 , optical paths in the optical switch will be described. The input lens array  23  converts light propagated in the input optical fiber array  21  into light beams, which are suitable for propagation through space. The input lens array  23  emits the converted light beams onto the MEMS mirrors  45  of the MEMS mirror array  4  corresponding to the input fiber array  21 . The MEMS mirror  45  reflects the light beams emitted from the input optical lens array  23  onto the optical path bending mirror  3 , where the light beams are reflected back onto the MEMS mirrors  45  of the MEMS mirror array  4  corresponding to the output fiber array  22 . The MEMS mirrors  45  reflect the light beams from the optical path bending mirror  3  onto the output lens array  24  corresponding to the MEMS mirrors  45 . The output lens array  24  converges the light beams from the MEMS mirrors  45  so that the light can propagate through the output optical fiber array  22 . The output optical fiber array  22  allows the light to propagate to the outside of the optical switch. By controlling the angles of the MEMS mirrors  45 , the optical switch can change optical paths. Thus, the optical switch can output light to a desired output optical fiber. 
         [0033]    In the structure in which the MEMS mirrors bend the optical path several times, as described above, the MEMS mirrors, each having a plurality of pivot axes, are arranged in an array. This produces mechanical vibrations in the same direction, having the same resonance frequency. 
       Structure of Movable Part of MEMS Mirror Array 
       [0034]      FIGS. 4A and 4B  show a structure of a movable part  40  of the MEMS mirror array  4 . The movable part  40  includes a pair of Y-axis rotational hinges  41 , a pair of X-axis rotational hinges  42 , a first frame  43 , a second frame  44 , and the MEMS mirror  45 . The first frame  43  supports the second frame  44  with the pair of Y-axis rotational hinges  41 . The second frame  44  supports the MEMS mirror  45  with the pair of X-axis rotational hinges  42 . The MEMS mirror  45  is capable of rotation (movement) about the X-axis rotational hinges  42 . An actuator rotates the MEMS mirror  45 . The second frame  44  is capable of rotation (movement) about the Y-axis rotational hinges  41 . The actuator rotates the second frame  44 . The X-axis rotational hinges  42  and the Y-axis rotational hinges  41  are provided perpendicular to each other. 
         [0035]      FIG. 4A  shows the MEMS mirror  45  rotated about the X-axis.  FIG. 4B  shows the second frame  44  rotated about the Y-axis. The movable part  40  of the MEMS mirror  45  can be rotated about the X-axis and the Y-axis in combination, if necessary. 
       Structures of Vibration Absorber 
     1. First Exemplary Structure of Vibration Absorber: 
       [0036]      FIG. 5  shows a first exemplary structure of the vibration absorber  6  shown in  FIG. 2 . The vibration absorber  6  includes the movable-absorbing-portion array  7  and a housing  56 . The movable-absorbing-portion array  7  includes movable absorbing portions  50 .  FIG. 5  shows four movable absorbing portions  50 . The housing  56  has vibration-proof-rubber-members attaching portions  59  to which the vibration-proof rubber members  5  and  5 ′ will be attached. The vibration-proof-rubber-members attaching portions  59  are provided on both the top and bottom surfaces of the housing  56 . The vibration absorber  6  is arranged parallel to the MEMS mirror array  4 , as shown in  FIG. 2 , whereby the movable absorbing portions  50  of the vibration absorber  6  are arranged substantially parallel to the movable parts  40  of the MEMS mirror array  4 . 
         [0037]    The movable absorbing portions  50  are pseudo-MEMS elements, i.e., the movable absorbing portions  50  have the same oscillation characteristics as the MEMS mirrors  45 . Therefore, the movable absorbing portions  50  have the same resonance frequency and similar pivot axes as the MEMS mirrors  45 . That is, each of the movable absorbing portions  50  has a pair of Y-axis rotational hinges  51 , a pair of X-axis rotational hinges  52 , a first frame  53 , a second frame  54 , and an oscillating body  55 . The first frame  53  supports the second frame  54  with the pair of Y-axis rotational hinges  52 . The second frame  54  supports the oscillating body  55  with the pair of X-axis rotational hinges  52 . The X-axis rotational hinges  52  and the Y-axis rotational hinges  51  are provided perpendicular to each other. 
         [0038]    The oscillating body  55  is capable of movement about the X-axis rotational hinges  52 . A vibration propagated from the base  8  oscillates the oscillating body  55  about the X-axis rotational hinges  52 . This oscillation has the same resonance frequency as the MEMS mirror  45 . Accordingly, vibration energy propagated from the base  8  can be reduced by oscillating the oscillating body  55 . 
         [0039]    The second frame  54  is capable of movement about the Y-axis rotational hinges  51 . A vibration propagated from the base  8  oscillates the second frame  54  about the Y-axis rotational hinges  51 , along with the oscillating body  55 . This oscillation has the same resonance frequency as the MEMS mirror  45 . Accordingly, vibration energy propagated from the base  8  can be reduced by oscillating the oscillating body  55  through the second frame. 
         [0040]    The vibration absorption capability of the vibration absorber  6  depends on the number of the movable absorbing portions  50 . Thus, the number of the movable absorbing portions  50  should be determined according to the vibration transmissibility required by the system. 
         [0041]    Because the movable absorbing portions  50  and the movable parts  40  have the same resonance frequency, their shapes are geometrically similar. In other words, the structures of the movable absorbing portions  50  and the movable parts  40  are the same. Further, the MEMS mirror array itself may serve as the vibration absorber  6 . When the MEMS mirror array serves as the vibration absorber  6 , the actuator is not necessary. 
       2. Second Exemplary Structure of Vibration Absorber: 
       [0042]      FIG. 6  shows a second exemplary structure of the vibration absorber  6  shown in  FIG. 2 . If the movable parts  40  have a multi-axis structure having a plurality of moving directions (the X-axis and Y-axis directions) as shown in  FIG. 4A  and  FIG. 4B , the movable absorbing portions  50  of the vibration absorber  6  may be structured to have a plurality of vibration absorbing structures corresponding to the moving directions of the movable parts  40 . A structure as shown in  FIG. 6  simplifies the process of manufacturing the axes of the movable absorbing portions  50  of the vibration absorber  6 . 
         [0043]    In  FIG. 6 , components the same as those shown in  FIG. 5  are denoted by the same reference numerals so as to make explanation thereof unnecessary. In  FIG. 6 , the movable-absorbing-portion array  7  includes two types of movable absorbing portions  50 ′: one supporting the oscillating body  55  on the first frame  53  with Y-axis rotational hinges  51 ′; and the other supporting the oscillating body  55  on the first frame  53  with X-axis rotational hinges  52 ′. Where the resonance frequency=f 0 , the structure of a single-axis, simple rotational hinge as shown in  FIG. 6  can be expressed by the following Expression 1. 
         [0000]    
       
         
           
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                       h 
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         [0000]    h: thickness of mirror and hinge; W: width of mirror; Lc: length of mirror; Lt: length of hinge; b: width of hinge; G: modulus of rigidity; B: constant determined by thickness and width of hinge; and ρ: density 
         [0044]    The vibration absorption capability of the vibration absorber  6  depends on the number of the movable absorbing portions  50 . Thus, the number of the movable absorbing portions  50  should be determined according to the vibration transmissibility required by the system. 
       3. Third Exemplary Structure of Vibration Absorber: 
       [0045]      FIG. 7  shows a third exemplary structure of the vibration absorber  6  shown in  FIG. 2 . In  FIG. 7 , components the same as those shown in  FIG. 5  are denoted by the same reference numerals so as to make explanation thereof unnecessary. Each of movable absorbing portions  50 ″ in the movable-absorbing-portion array  7  has a hinge  63 . The hinge  63  extends from the first frame  53  and supports an oscillating body  55 ″. In  FIG. 7 , the oscillating body  55 ″ positioned at an end of the hinge  63 , as a cantilever structure, may be provided with a weight so that the oscillating body  55 ″ has the same resonance frequency as the MEMS mirrors  45 . The oscillating bodies  55 ″ are arranged radially from the center of the movable-absorbing-portion array  7  with the hinges  63 , so as to correspond to the moving directions of the movable parts  40  of the MEMS mirrors. If the movable parts  40  of the MEMS mirrors have two-axis rotational hinges as shown in  FIG. 4 , it is more effective that the movable absorbing portions  50  of the vibration absorber  6  have the two-axis rotational hinges as shown in  FIGS. 5 and 6 . However, the hinge  63  of the cantilever structure as shown in  FIG. 7  is easier to manufacture than the two-axis rotational hinges as shown in  FIGS. 5 and 6 . The oscillating bodies  55 ″ can oscillate at a frequency the same as the resonance frequency of the MEMS mirrors, even with the cantilever-shaped hinge  63 . 
         [0046]    The vibration absorption capability of the vibration absorber  6  depends on the number of the movable absorbing portions  50 ″. Thus, the number of the movable absorbing portions  50 ″ should be determined according to the vibration transmissibility required by the system. 
       4. Fourth Exemplary Structure of Vibration Absorber: 
       [0047]      FIG. 8  shows a fourth exemplary structure of the vibration absorber  6  shown in  FIG. 2 . If the resonance frequency of the movable absorbing portions  50 ,  50 ′, and  50 ″ shown in  FIGS. 5 ,  6 , and  7 , respectively, and the resonance frequency of the MEMS mirrors  45  are different, the effect of the movable absorbing portions  50 ,  50 ′, and  50 ″ for absorbing vibration is decreased. Accordingly, it is preferable that the half width of the resonance frequency of the movable absorbing portions  50 ,  50 ′, and  50 ″ of the vibration absorber  6  be large.  FIG. 8  shows a structure for reducing the Q-value of the resonance of the movable absorbing portions  50 ,  50 ′, and  50 ″ arranged in the vibration absorber  6 . More specifically, the movable absorbing portions  50 ,  50 ′, and  50 ″ are provided with attenuators  64  to reduce the Q-value. This increases the range of frequency of the vibration that the movable absorbing portions  50 ,  50 ′, and  50 ″ can absorb. The attenuators  64  may be made of rubber sheets, gel sheets, or the like, and sandwich the movable-absorbing-portion array  7  from above and below. This structure successfully attenuates vibration of the movable absorbing portions  50 ,  50 ′, and  50 ″. If the peak value of the resonance of the movable absorbing portions  50 ,  50 ′, and  50 ″ decreases, the vibration attenuation capability decreases. However, this may be compensated for by increasing the number of the movable absorbing portions  50 ,  50 ′, and  50 ″. 
       5. Exemplary Structure 5 of Vibration Absorber: 
       [0048]    Although the resonance frequencies of the movable absorbing portions  50 ,  50 ′, and  50 ″ are determined by Expression 1, there is a certain freedom for modifying Expression 1 to derive the same resonance frequency f 0 . For example, if the width of mirror is increased to 2W and the width of hinge is increased to 2b, the weights or the moments of inertia of the movable absorbing portions  50 ,  50 ′, and  50 ″ can be changed while maintaining the resonance frequencies of the movable absorbing portions  50 ,  50 ′, and  50 ″. The movable absorbing portions  50 ,  50 ′, and  50 ″ response to an externally imposed impact in various ways according to the magnitudes of their moments of inertia, whereby they become capable of coping with disturbances of any strength and rate. Accordingly, it is preferable that the vibration absorber  6  have a plurality of movable absorbing portions having different moments of inertia. 
       Vibration Transmissibility of Vibration Absorber 
       [0049]      FIG. 9  is a graph showing the vibration transmissibility of the vibration absorber shown in  FIG. 5 , in which the resonance frequencies of the MEMS mirrors and the movable absorbing portions  50  of the vibration absorber  6  are both 1.2 kHz. A dashed line depicts the vibration transmissibility of a known structure, which is the same as that shown in  FIG. 1 . The structure shown in  FIG. 5  has a damper structure, in which the double-layered vibration-proof rubber members  5  and  5 ′ are used. Thus, the vibration attenuation capability is more than double that of the known structure. Vibration is sufficiently attenuated at a frequency region  60 , which corresponds to the resonance frequency of the MEMS mirrors  45 . 
       Structure of Vibration Absorber for Casing 
       [0050]      FIG. 10  shows a structure of a vibration absorber for the casing. In  FIG. 10 , components the same as those shown in  FIG. 2  are denoted by the same reference numerals so as to make explanation thereof unnecessary. The structure shown in  FIG. 10  differs from that shown in  FIG. 2  in terms of the function of the vibration absorber  6 . The vibration absorber  6  has a vibration absorbing portion  9  for the casing. The casing  1 , when it receives a vibration having a frequency equal to the resonance frequency of the substantially V-shaped inclined surface structure from the outside, resonates and vibrates. The vibration of the casing I disturbs the optical path. To prevent this, the vibration absorber  6  in  FIG. 10  has the vibration absorbing portion  9  for the casing therein, which absorbs a vibration having a frequency the same as the resonance frequency of the casing  1 . The structure of the vibration absorbing portion  9  for the casing may be the same as those of the movable absorbing portions  50 ,  50 ′, and  50 ″ shown in  FIGS. 5 ,  6 , and  7 , respectively, for example, as long as their resonance frequencies are the same as the resonance frequency of the casing  10 . Further, the vibration absorber  6  of  FIG. 10  may have the movable absorbing portions  50 ,  50 ′, and  50 ″ having the same resonance frequency as the MEMS mirrors  45 . 
       Structure of Second MEMS Device 
       [0051]      FIGS. 11A to 11C  show a structure of a second MEMS device, which is a wavelength selective switch using MEMS elements.  FIGS. 11A and 11B  are a top view and a perspective view of the wavelength selective switch, respectively, and  FIG. 11C  shows the structure of the vibration absorber  6 . The second MEMS device includes MEMS elements each having a movable part having a single axis structure. Light entering from the optical input/output portion  2  is split by a spectroscopic device  11 . The light split by the spectroscopic device  11  enters the MEMS mirror array  4  through a lens  12 . The MEMS mirrors of the MEMS mirror array  4  reflect the light back onto the lens  12 . The light reflected at the MEMS mirror array  4  returns to the optical input/output portion  2  via the spectroscopic device  11 . The MEMS mirror array  4  includes a plurality of MEMS mirrors arranged in directions in which the spectroscopic device  11  splits the light. By moving the MEMS mirrors in the direction perpendicular to the direction in which the light is split, the position in the spectroscopic device  11  at which the light beam is reflected back can be changed. By adjusting the control angles of the MEMS mirrors, light beams can be reflected back to different output ports of the optical input/output portion  2 , in accordance with their wavelengths. The vibration absorber  6  is disposed between the lens  12  and the MEMS mirror array  4 . In  FIG. 11A , the vibration absorber  6  is provided near the MEMS mirror array  4 . 
         [0052]      FIG. 11C  is a front view of the vibration absorber  6 . The vibration absorber  6  includes an aperture  62  at the center thereof for allowing the optical path to extend to the MEMS mirror array  4 . The vibration absorber  6  further includes movable absorbing portions  61  arranged on both sides of the aperture  62 . The structure of the movable absorbing portion  61  of the vibration absorber  6  is the same as that of the movable absorbing portions  50 ,  50 ′, and  50 ″ shown in  FIGS. 5 to 7 , respectively. When these pseudo-MEMS elements of the vibration absorbing structure are subject to an externally applied vibration or impact, they oscillate and absorb the vibration energy. This reduces the resonance amplitude of the MEMS mirrors, and reduces influence of the vibration or impact on optical paths. 
         [0053]    The above-described embodiments can be combined, if necessary.