Patent Publication Number: US-7721397-B2

Title: Method for fabricating capacitive ultrasonic transducers

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an ultrasonic transducer and, more particularly, to a flexible capacitive ultrasonic transducer and a method of fabricating the same. 
   With the advantages of non-invasive evaluation, real-time response and portability, ultrasonic sensing devices have been widely used in medical, military and aerospace industries. For example, echographic systems or ultrasonic imaging systems are capable of obtaining information from surrounding means or from human body, based on the use of elastic waves at ultrasonic frequency. An ultrasonic transducer is often one of the important components in an ultrasonic sensing device. The majority of known ultrasonic transducers are realized by using piezoelectric ceramic. A piezoelectric transducer is generally used to obtain information from solid materials because the acoustic impedance of piezoelectric ceramic is of the same magnitude order as those of the solid materials. However, the piezoelectric transducer may not be ideal for obtaining information from fluids because of the significant impedance mismatch between piezoelectric ceramic and fluids, for example, tissues of the human body. The piezoelectric transducer may generally operate in a frequency band from 50 KHz to 200 KHz. Furthermore, the piezoelectric transducer may generally be fabricated in high-temperature processes and may not be ideal for integration with electronic circuits. In contrast, capacitive ultrasonic transducers may be manufactured in batch with standard integrated circuit (“IC”) processes and therefore are integrable with IC devices. Furthermore, capacitive ultrasonic transducers are capable of operating at a higher frequency band, from 200 KHz to 5 MHz, than known piezoelectric transducers. Consequently, capacitive ultrasonic transducers have gradually taken the place of the piezoelectric transducers. 
     FIG. 1  is a schematic cross-sectional view of a capacitive ultrasonic transducer  10 . Referring to  FIG. 1 , the capacitive ultrasonic transducer  10  includes a first electrode  11 , a second electrode  12  formed on a membrane  13 , an isolation layer  14  formed on the first electrode  11 , and support sidewalls  15 . A cavity  16  is defined by the first electrode  11 , the membrane  13  and support sidewalls  15 . When suitable AC or DC voltages are applied between the first electrode  11  and the second electrode  12 , electrostatic forces cause the membrane  13  to oscillate and generate acoustic waves. The effective oscillating area of the conventional transducer  10  is the area defined by the first electrode  11  and second electrode  12 . In this instance, the effective oscillating area may be determined by the length of the second electrode  12  because the second electrode  12  is shorter than the first electrode  11 . Furthermore, the membrane  13  may generally be fabricated in a high-temperature process such as a conventional chemical vapor deposition (“CVD”) or low pressure chemical vapor deposition (“LPCVD”) process at a temperature ranging from approximately 400 to 800° C. 
     FIGS. 2A to 2D  are cross-sectional diagrams illustrating a conventional method for fabricating a capacitive ultrasonic transducer. Referring to  FIG. 2A , a silicon substrate  21  is provided, which may be heavily doped with impurities in order to serve as an electrode. Next, a first nitride layer  22  and an amorphous silicon layer  23  are successively formed over the silicon substrate  21 . The first nitride layer  22  may function to protect the silicon substrate  21 . The amorphous silicon layer  23  is used as a sacrificial layer and will be removed in subsequent processes. 
   Referring to  FIG. 2B , a patterned amorphous silicon layer  23 ′ is formed by patterning and etching the amorphous silicon layer  23 , exposing portions of the first nitride layer  22 . A second nitride layer  24  is then formed over the patterned sacrificial layer  23 ′, filling the exposed portions. 
   Referring to  FIG. 2C , a patterned second nitride layer  24 ′ with openings  25  is formed by patterning and etching the second nitride layer  24 , exposing portions of the patterned amorphous silicon layer  23 ′ through the openings  25 . The patterned amorphous silicon layer  23 ′ is then removed by a selective etch. 
   Referring to  FIG. 2D , a silicon oxide layer is deposited through the openings  25  to form plugs  26 . Chambers  27  are thereby defined by the plugs  26 , the patterned second nitride layer  24 ′ and the first nitride layer  22 . A metal layer  28  is then formed over the patterned second nitride layer  24 ′ to serve as a second electrode. 
   However, the conventional capacitive ultrasonic transducer is inflexible due to the utilization of a silicon-based substrate. The inflexibility restricts the conventional capacitive ultrasonic transducer to a limited application. It may therefore be desirable to have a flexible capacitive ultrasonic transducer and a method of fabricating the same. 
   BRIEF SUMMARY OF THE INVENTION 
   Examples of the present invention may provide a capacitive ultrasonic transducer that comprises a flexible layer, a first conductive layer on the flexible layer, a support frame on the first conductive layer, the support frame including a flexible material, a membrane over the support frame being spaced apart from the first conductive layer by the support frame, the membrane including the flexible material, a cavity defined by the first conductive layer, the support frame and the membrane, and a second conductive layer on the membrane. 
   Some examples of the present invention may provide a method for fabricating capacitive ultrasonic transducers, the method comprising providing a substrate, forming a flexible layer on the substrate, forming a first conductive layer on the flexible layer, forming a patterned sacrificial layer on the first conductive layer, forming a first polymer layer over the patterned sacrificial layer, patterning the first polymer layer to provide a patterned first polymer layer, exposing portions of the patterned sacrificial layer through openings, forming a second conductive layer on the patterned first polymer layer, patterning the second conductive layer to provide a patterned second conductive layer, forming a second polymer layer over the patterned second conductive layer, patterning the second polymer layer, exposing portions of the patterned sacrificial layer through the openings, and removing the patterned sacrificial layer through the openings. 
   Examples of the present invention may also provide method of forming capacitive ultrasonic transducers, the method comprising forming a flexible layer on a substrate, forming a first conductive layer on the flexible layer, forming a patterned metal layer on the first conductive layer, forming a first polymer layer on the patterned metal layer and the first conductive layer, patterning the first polymer layer to provide a patterned first polymer layer, exposing portions of the patterned metal layer through openings, forming a patterned second conductive layer on the patterned first polymer layer, forming a patterned second polymer layer on the patterned second conductive layer and the patterned first polymer layer over the patterned metal layer, and removing the patterned metal layer through the openings. 
   Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The features and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings examples which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
     In the drawings: 
       FIG. 1  is a schematic cross-sectional view of a conventional capacitive ultrasonic transducer; 
       FIGS. 2A to 2D  are cross-sectional diagrams illustrating a conventional method for fabricating a capacitive ultrasonic transducer; 
       FIG. 3  is a schematic cross-sectional view of a flexible capacitive ultrasonic transducer consistent with an example of the present invention; and 
       FIGS. 4A to 4J  are schematic cross-sectional diagrams illustrating a method of fabricating a flexible capacitive ultrasonic transducer consistent with an example of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the present examples of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     FIG. 3  is a schematic cross-sectional view of a flexible capacitive ultrasonic transducer  30  in accordance with one example of the present invention. Referring to  FIG. 3 , the flexible capacitive ultrasonic transducer  30  includes a flexible base  39 , a first electrode  31 , a support frame  35 , a membrane  38  and a second electrode  32 . The flexible base  39  may be made of a material such as, for example, polymer or other suitable material that may allow the capacitive ultrasonic transducer  30  to conform to a surface of an object. In one example, the flexible base  39  may have a thickness of approximately 0.45 micrometer (μm), the first electrode  31  may have a thickness of approximately 0.2 μm, and the second electrode  32  may have a thickness of 0.5 μm. The first electrode  31  may include a metal film made of platinum (Pt) or aurum (Au), and the second electrode  32  may include a metal film made of aluminum (Al). The first electrode  31  and the second electrode  32  may serve as a positive electrode and a negative electrode, respectively, of the capacitive ultrasonic transducer  30 . The support frame  35  and the membrane  38  may be made of polymer. In one example, the membrane  38  has a thickness of approximately 2 μm and the support frame  35  separates the first electrode  31  and the membrane  38  by a distance of approximately 2 μm. A cavity  36  is defined by the first electrode  31 , the support frame  35  and the membrane  38 . 
     FIGS. 4A to 4J  are schematic cross-sectional diagrams illustrating a method for fabricating a flexible capacitive ultrasonic transducer in accordance with one example of the invention. Referring to  FIG. 4A , a substrate  40  is provided to serve as a supporting base on which flexible capacitive ultrasonic transducers may be fabricated. The substrate  40  may include a silicon substrate having a thickness of approximately 550 μm. A flexible layer  49 , which may eventually serve as a flexible base like the flexible base  39  illustrated in  FIG. 3 , is formed on the substrate  40  by a conventional coating process or other suitable processes. A conductive layer  41  is formed on the flexible layer  49  by a conventional sputtering process of other suitable processes. The conductive layer  41 , which eventually serves as a first electrode for a capacitive ultrasonic transducer, may include a metal film such as a gold film. 
   Referring to  FIG. 4B , a patterned photoresist layer  42  is formed on the flexible layer  49  by a conventional patterning and etching process, exposing portions of the flexible layer  49  through openings  47 . The patterned photoresist layer  42  may include a polymeric material such as, for example, AZ4620. The pattern of the openings  47  may include but is not limited to a hexagon. 
   Referring to  FIG. 4C , a sacrificial metal layer  45  is formed to fill the openings  47  by a conventional electroplating process or other suitable processes. The sacrificial metal layer  45  may be substantially coplanar with the patterned photoresist layer  42 , and will be removed in a subsequent process so as to define a cavity. In one example according to the present invention, the sacrificial metal layer  43  includes copper (Cu). 
   Referring to  FIG. 4D , the patterned photoresist layer  42  is stripped and a first polymer layer  46  is formed over the sacrificial metal layer  43 . In one example according to the present invention, the first polymer layer  46  includes a polymeric material such as, for example, SU8-2002. 
   Referring to  FIG. 4E , the first polymer layer  46  illustrated in  FIG. 4D  may then be lapped or polished by a conventional lapping or chemical machine polish (CMP) process. Next, a patterned first polymer layer  46 - 1  is formed by a conventional patterning and etching process, exposing portions of the sacrificial metal layer  43  through openings  43 . The patterned first polymer layer  46 - 1  subsequently serves as a support frame and at least a portion of a membrane for the capacitive ultrasonic transducer. 
   Referring to  FIG. 4F , a conductive layer  44  is formed over the patterned first polymer layer  46 - 1  and the sacrificial metal layer  45  by a sputtering, evaporating or PECVD process. In one example, the conductive layer  44  includes Al. Next, a photoresist layer  48  is formed over the conductive layer  44 . In one example, the photoresist layer  48  may include a positive photoresist, such as, for example, AZ5214E. 
   Referring to  FIG. 4G , a patterned conductive layer  44 - 1  is formed on the patterned first polymer layer  46 - 1  by a conventional patterning and etching process. The patterned conductive layer  44 - 1  subsequently becomes a second electrode for the capacitive ultrasonic transducer. 
   Referring to  FIG. 4H , a patterned second polymer layer  51  is formed over the patterned first polymer layer  46 - 1  and the patterned conductive layer  44 - 1 . The sacrificial metal layer  45  illustrated in  FIG. 4G  is removed via the openings  43  through an etching process. In one example, the sacrificial metal layer  45  is removed by a wet etching process using ferric chloride (FeCl 3 ) as an etchant solution, which is etch selective so that the sacrificial metal layer  45  is removed without significantly removing the conductive layer  41 . Cavities  50  are therefore defined, but not sealed, by the conductive layer  41  and the patterned first polymer layer  46 - 1 . 
   Referring to  FIG. 4I , a patterned layer  52  may be formed to fill the openings  43  illustrated in  FIG. 4H . The patterned layer  52  may include a polymer layer. Cavities  50 - 1  are therefore defined and sealed by the conductive layer  41 , the patterned first polymer layer  46 - 1  and the patterned layer  52 . Next, referring to  FIG. 4J , the substrate  40  is removed after the capacitive ultrasonic transducers are formed. The method illustrated in  FIGS. 4A to 4J  may be controlled at a temperature lower than approximately 150° C. (Celsius). 
   It will be appreciated by those skilled in the art that changes could be made to the examples described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular examples disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims. 
   Further, in describing representative examples of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.