Abstract:
A capacitive micromachined ultrasonic transducer (CMUT) is described, including a substrate, a conductive film disposed over the substrate, a conductive membrane suspended over the conductive film with a vacuum space underneath, and at least one anchoring post disposed under a middle of the conductive membrane and supporting the conductive membrane.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    This disclosure relates to structures of capacitive micromachined ultrasonic transducer (CMUT) having a large volumetric displacement. 
         [0003]    2. Related Art 
         [0004]    CMUT uses deformable membranes to transmit and receive ultrasound. When an AC signal with a proper DC bias is applied across the membrane and the counter electrode, the alternating electrostatic force drives the membrane to vibrate and generate ultrasound. The same CMUT also works as an ultrasound receiver. In the reception mode, the membrane is agitated by impinging ultrasound and changes the capacitance. With a DC bias, this change of capacitance generates an electrical signal, which carries the amplitude and phase information of the impinging ultrasound. A CMUT normally has a broader acoustic bandwidth than its piezoelectric counterpart and can be fabricated using IC processes. CMUT is a promising alternative for ultrasound transducers and is useful in medical imaging and nondestructive evaluation of material structures. 
         [0005]    In the reception mode, a CMUT is sensitive to ultrasound frequency around the resonance frequency of the membrane but insensitive to the ultrasound in the rest of the spectrum. As a result, the mechanical structure of a CMUT generally is designed such that the fundamental mode of the membrane falls in the frequency window of interests to the application. The sensitivity of a CMUT as a receiver depends on the volumetric displacement of the membrane under a specific ultrasound pressure. 
         [0006]    Taking a circular membrane of uniform thickness as an example; when its circumference is completely anchored, its resonance frequency is 
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         [0000]    wherein v is the Poisson&#39;s ratio, ρ is the density, E is the Young&#39;s modulus of the membrane material, h is the membrane thickness and r o  is the radius of the membrane. Under a uniform pressure p, the center of the membrane is deformed by 
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         [0007]    According to Eq. (1), at the same h/r o   2  ratio and the same resonance frequency, a smaller membrane (smaller radius r 1 ) is thinner (smaller thickness h 1 ) while a larger membrane (larger radius r 2 ) is thicker (larger thickness h 2 ). Therefore, according to Eq. (2), a thinner and smaller membrane has a larger central deformation (h 1 &lt;h 2 →1/h 1 &gt;1/h 2 ) than its thicker and larger counterpart of the same fundamental frequency (the same h/r o   2  ratio). This concludes that for a higher reception sensitivity, a thinner and smaller membrane is preferred. 
         [0008]    For most imaging and detection applications, due to the small aperture size, a transducer does not work alone, but multiple transducers are connected in an array to form an element as shown in  FIG. 1 . Each transducer  10  includes a dielectric layer  110  on a substrate  100 , a conductive film  120 , and a conductive membrane  140  suspended over the film  120  by anchoring walls  130  with a vacuum space  150  underneath. In such design, the spaces between the transducers  10  serve to accommodate release holes  160  and some of the anchoring walls  130 . The spaces do not contribute to volumetric displacement in ultrasound transduction and limit the overall volumetric displacement of the element, especially when the frequency of the transducer gets higher so that the membrane area of each cell is reduced and more non-contributive spaces are needed. Though the non-contributive spaces can be reduced by forming a single large and thick membrane instead of multiple small and thin membranes, the volumetric displacement would still be limited due to the thickness of the large and thick membrane. 
       SUMMARY 
       [0009]    This disclosure provides structure designs of capacitive micromachined ultrasonic transducer (CMUT) that make larger overall volumetric displacements. 
         [0010]    The CMUT according to an embodiment of this disclosure includes a substrate, a conductive film disposed over the substrate, a conductive membrane suspended over the conductive film with a vacuum space underneath, and at least one anchoring post disposed under the middle of the conductive membrane and supporting the conductive membrane. 
         [0011]    The CMUT according to another embodiment includes a substrate having a V-shape cavity therein, a conductive film disposed over the substrate and in the V-shape cavity, and a conductive membrane suspended over the conductive film and in the V-shape cavity with a vacuum space underneath. 
         [0012]    The CMUT according to still another embodiment includes a substrate, a corrugated conductive film over the substrate, and a corrugated conductive membrane suspended over and conformal with the conductive film with a vacuum space underneath. 
         [0013]    In order to make the aforementioned and other objects, features and advantages of this disclosure comprehensible, a preferred embodiment accompanied with figures is described in detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  illustrates a top view and a cross-sectional view of conventional CMUTs. 
           [0015]      FIG. 2  illustrates a top view and a cross-sectional view of a CMUT according to a first embodiment. 
           [0016]      FIGS. 3A-3B  illustrate cross-sectional views of two CMUT structures according to a second embodiment and perspective views of the V-shaped cavities in the two CMUT structures. 
           [0017]      FIG. 4  illustrates a cross-sectional view of a CMUT according to a third embodiment and a perspective view of the conductive film therein. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       [0018]      FIG. 2  illustrates a top view and a cross-sectional view of a CMUT according to the first embodiment. 
         [0019]    Referring to  FIG. 2 , the CMUT includes a substrate  200 , a dielectric layer  210  on the substrate  200 , a conductive film  220  disposed on the dielectric layer  210 , and a conductive membrane  240  suspended over the conductive film  220  by anchoring walls  230   a  and anchoring posts  230   b  with a vacuum space  250  underneath. 
         [0020]    The anchoring walls  230   a  define the area of the CMUT. Each of the anchoring posts  230   b  is disposed under the middle of the conductive membrane  240  and supports the conductive membrane  240 . The anchoring posts  230   b  may be hollow as illustrated, wherein each hollow anchoring post may surround a release hole  260  in the conductive membrane  240 . The number of the anchoring posts  230   b  may be three as illustrated, or any other number larger than one. It is noted that the release holes  260  have been sealed after the release process that removes the sacrificial material under the conductive membrane  240  through the release holes  260  to form the vacuum space  250 . 
         [0021]    The anchoring walls  230   a  and the anchoring posts  230   b  may be formed from the same material. It is even possible that the anchoring walls  230   a,  the anchoring posts  230   b  and the conductive membrane  240  are all formed from the same material, such as doped polysilicon or germanium. The dielectric layer  210  may be a silicon oxide/silicon nitride/silicon oxide (ONO) composite layer. The conductive film  220  may include doped polysilicon or germanium or silicon carbide (SiC). In addition, the conductive film  220  may be defined in a manner such that the anchoring walls  230   a  and the anchoring posts  230   b  defined later are disposed on portions of the conductive film  220  that are separated from all the other portions of the conductive film  220 , as shown in  FIG. 2 . 
         [0022]    With the above structural design, the conductive membrane  240  can be made large and thin to have a large volumetric displacement. For example, the dimension of the conductive membrane  240  surrounded by the anchoring walls  230   a  may range from tens of square micrometers to about tens of thousands of square micrometers while the thickness of the same ranges from about tens of nanometers to tens of micrometers. 
         [0023]    The anchoring posts  230   b  act as stationary nodes in the vibration to boost the resonance frequency of the element to the higher part of the spectrum and improve the volumetric displacement, and may also provide release holes  260  in their hollow forms such that a reasonable short release time is made. Thereby, a better electromechanical coupling efficiency is achieved. The frequency of ultrasound suitably detected by such CMUT ranges from sub-kHz to above 100 MHz. 
       Second Embodiment 
       [0024]      FIGS. 3A-3B  illustrate cross-sectional views of two CMUT structures according to the second embodiment and perspective views of the V-shaped cavities of the two CMUT structures. 
         [0025]    Referring to  FIG. 3A , the CMUT structure includes a substrate  300  having a V-shape cavity  305  therein, a dielectric layer  310  disposed on the substrate  300  and in the V-shape cavity  305 , a conductive film  320  disposed on the dielectric layer  310  and in the V-shape cavity  305 , a conductive membrane  340  suspended over the conductive film  320  and in the V-shape cavity  305  by anchoring walls  330  with a vacuum space  350  underneath, and a polymer layer  370  covering the conductive membrane  340  for protecting the membrane from shorting to its ambient. 
         [0026]    The materials of the dielectric layer  310 , the conductive film  320 , the anchoring walls  330  and the suspended conductive membrane  340  may refer to those mentioned in the 1 st  embodiment. In addition, the anchoring walls  330  may be disposed on layers that are patterned along with the conductive film  320  and separated from the same. 
         [0027]    Referring to  FIG. 3B , the CMUT structure is different from that in  FIG. 3A  only in that the V-shape cavity  305  has a flat bottom  307  that has the effect of having a smaller plate area. 
         [0028]    For each of the above two CMUT structures, the angle θ between each sidewall of the V-shape cavity  305  and the horizontal plane may range from be any value larger than 0° and smaller than or equal to 90°, preferably about 54.7°, to increase the volumetric displacement. The dimension of the conductive membrane  340  surrounded by the anchoring walls  330  may range from about tens of square micrometers to about tens of thousand of square micrometers while the thickness of the same ranges from about tens of nanometers to about hundreds of micrometers. 
         [0029]    Each of the above CMUT structures according to the second embodiment provides larger volumetric displacement in some particular frequency domain, for example, in low-frequency domain. The frequency of ultrasound suitably detected by such CMUT ranges from about sub-kHz to about 100 MHz. 
       Third Embodiment 
       [0030]      FIG. 4  illustrates a cross-sectional view of a CMUT according to the third embodiment and a perspective view of the conductive film therein. 
         [0031]    Referring to  FIG. 4 , the CMUT includes a substrate  400  having a corrugated surface  403 , a correspondingly corrugated dielectric layer  410  on the substrate  400 , a correspondingly corrugated conductive film  420  on the dielectric layer  410 , and a corrugated conductive membrane  440  suspended over and conformal with the conductive film  420  by anchoring walls  430   a  with a vacuum space  450  underneath. Accordingly, the corrugated surface  403  of the substrate  400  shapes the corrugated dielectric layer  410 , the corrugated conductive film  420  and the corrugated conductive membrane  440 . 
         [0032]    The materials of the dielectric layer  410 , the conductive film  420 , the anchoring walls  430  and the conductive membrane  440  may refer to those mentioned in the 1 st  embodiment. In addition, the anchoring walls  430  may be disposed on layers that are patterned along with the conductive film  420  and separated from the same. 
         [0033]    The dimension of the conductive membrane  440  surrounded by the anchoring walls  430  may range from about tens of square micrometers to tens of thousands of square micrometers while the thickness of the same ranges from about tens of nanometers to about tens of thousand of micrometers. 
         [0034]    The CMUT according to the third embodiment provides a larger electromechanical coupling efficiency in some particular frequency domain, e.g., in low frequency domain. The frequency of ultrasound suitably detected by such CMUT ranges from about sub-kHz to about 100 MHz. 
         [0035]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the disclosed embodiments without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the disclosure cover modifications and variations of this disclosure provided they fall within the scope of the following claims and their equivalents.