Abstract:
The present invention relates to an imprint method for manufacturing micro capacitive ultrasonic transducer, which uses a mold with a particularly patterned surface to imprint into a flexible material thus forming the oscillation cavities of the ultrasonic transducer. Such imprint method not only realizes the volume manufacturing and reduces the cost, but also can precisely control the geometrical size of the oscillation cavities and thus shorten the distance between the upper and the lower electrodes to the micro/nano level, largely improving the sensitivity of the transducer. Moreover, the present invention further changes the procedure for manufacturing micro capacitive ultrasonic transducer of the prior art, which can both save the process steps and overcome the disadvantages in the prior art.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method for manufacturing ultrasonic transducer, in particular to a method for manufacturing micro capacitive ultrasonic transducer. In detail, the present invention employs the nanoimprint lithography method to manufacture the micro capacitive ultrasonic transducer.  
       BACKGROUND OF THE INVENTION  
       [0002]     The technology of ultrasonic inspection has been developed since the World War II. In the beginning, it is used for the national defense and the military affairs. Until 1950s, the ultrasonic inspection technology started to be widely employed on the medical treatments. In the area of the ultrasonic inspection, ultrasonic transducer plays a very important role thus attracting the industry/government/academia to plunge into the research in the past decades, and the related technologies are also getting more and more mature now. Among all the ultrasonic transducers, the piezoelectric transducer was kept the main stream for a long time.  
         [0003]     The so-called piezoelectric effect includes both of the direct piezoelectric effect and the converse piezoelectric effect. Under the direct piezoelectric effect, a piezoelectric body, when put into an electric field, will be elongated along the direction of the electric field according to the elongating of the electrical dipole moment thus transferring the mechanical energy into electric energy. On the contrary, under the converse piezoelectric effect, if the piezoelectric body was pressed, the electrical dipole moment thereof will be shortened. In order to resist this tendency, the piezoelectric body thus will induce voltage for trying to keep the original state. With such character, the piezoelectric transducer transfers the electrical signals into the sonic signals, and also can transfer the sonic signals into the electrical signals thus being able to regard as a probe in the ultrasonic inspection. The common material of the piezoelectric body can be the ceramic, such as BaTiO3 and PZT, and the single crystal materials, such as quartz, tourmaline, tantalates, and columbate. However, the piezoelectric transducer still exit some disadvantages, for example the cost of such piezoelectric transducer is too high, and the oscillation of the crystal lattice will easily debase the bandwidth and the sound pressure. Moreover, the difference between the impedances of the piezoelectric material and that of the air is so large as to cause the unmatched phenomenon thus resulting in large reflection of the sonic signals in the contact interface and diminish the inspection efficiency. In addition, for the limitation of the resolution and the bandwidth, the piezoelectric transducer is hardly to be used for the precise inspection in nano-level.  
         [0004]     Instead of the piezoelectric transducer, the micro capacitive ultrasonic transducer has become the main stream of the ultrasonic transducer research. The related patents have also been gradually accumulated recently, such as U.S. Pat. No. 6,426,582, U.S. Pat. No. 6,004,832, and U.S. Pat. No. 6,295,247 and so on. Please refer to  FIG. 1 , which shows the basic structure of the micro capacitive ultrasonic transducer. A plurality of the support pedestals  12  is formed on the substrate  11 , and the oscillation film  13  with an upper electrode  14  thereon is formed on the support pedestal  12 . Wherein, the substrate  14  doped with impurities to get conductivity is used to be the lower electrode for forming a capacitance structure with the upper electrode  14 . The oscillation cavity  15  composed of the substrate  11 , the support pedestal  12 , and the oscillation film  13  is used to provide the space for oscillation when the oscillation film  13  is vertically oscillating. Such micro capacitive ultrasonic transducer possesses the following merits: (1) larger bandwidth; (2) easily to form large density array; (3) simply to be integrated with the front-end circuits on the same wafer; and (4) being able to largely manufacture thus reducing the cost.  
         [0005]     In fact, the important character of the micro capacitive ultrasonic transducer is the design of the oscillation cavity and the oscillation film, so the geometric parameters of the oscillation cavity and the oscillation film, such as the radius and the thickness of the oscillation film and the distance between the upper electrode and the lower electrode, are rigidly related to the efficiency of the ultrasonic transducer. Therefore, it is very important to control all of such geometric parameters more stably and more uniformly in the manufacturing process. Please refer to  FIG. 2A  to  2 C, which are the schematic views showing the traditional method for manufacturing the micro capacitance ultrasonic transducer of the prior art. Firstly, a substrate  21  is provided, and then a support film  22 , an oscillation film  23  and a conductive layer  24  are successively formed on the substrate  21 . A plurality of holes  25  that penetrates the oscillation film  23  and the conductive layer  24  then is  1 o generated after the procedure of photolithography and etching. Finally, through the plurality of holes  25 , the support film  22  can be etched to form a plurality of oscillation cavities  221  thereon. Because of the character that the selectivity of the etching rate on the support film  22  and the oscillation film  23  is different, the etching solution that preferentially etches the support film  22  rather than the oscillation film  23  is used to form a plurality of oscillation cavities  221  thus completing the whole ultrasonic transducers. Wherein, the shape of the oscillation cavities  221  is approximate cylinder that expanded from the center of the holes  25 . However, such method is hardly to control the precise shape of the oscillation cavities and cannot provide the check mechanism. It only depends on the experience so that many vibrations in the process, such as the variation of the etching solution concentration, will very easily cause the variation of the geometrical size of the oscillation cavities  221  further affecting the character of the whole transducers.  
         [0006]     Moreover, the plurality of holes  25  used for the entries of the etching solution and the exits of the etching by-products will easily cause the contamination of the oscillation cavities  221 , remaining certain residues on the wall of the cavities thus affecting the characters of the transducer. The present invention thus provides a new method not only for overcoming the aforesaid disadvantages but also for improving the character of the ultrasonic transducers.  
       SUMMARY OF THE INVENTION  
       [0007]     The primary object of the present invention is to provide an imprint method for manufacturing micro capacitive ultrasonic transducer. The method employs a particularly patterned mold to form the oscillation cavities of micro capacitance ultrasonic transducer thus obtaining the purposes of large batch manufacturing, uniform control, and cost reduction.  
         [0008]     The secondary object of the present invention is to precisely control the size of the oscillation cavities of micro capacitive ultrasonic transducer, reducing the distance between the top and the bottom electrodes thus increasing the sensitivity of the ultrasonic transducer.  
         [0009]     The third object of the present invention is to provide an imprint method for manufacturing micro capacitive ultrasonic transducer, which can improve the cleanness of the oscillation cavities without generating entry holes in the prior art that provide the entries of the etching liquid and the exits of the byproducts.  
         [0010]     In order to achieve the aforesaid objects, the present invention provides an imprint method for manufacturing ultrasonic transducer, including the following steps: 
        a) Providing a substrate with electric conductance.     b) Forming a support film layer on the substrate.     c) Providing a mold with a patterned surface, wherein the patterned surface having an array pattern with projections and recesses arranged in order.     d) Imprinting the mold into the support film layer with the patterned surface thus transferring the array pattern into the support film layer.     e) Removing the mold, a plurality of recessions corresponding to the array pattern thus formed within the support film layer.     f) Providing a polymer film, the polymer film having an obverse side and a reverse side     g) Forming a plurality of upper electrodes corresponding to the recessions and a plurality of conductor lines between any two adjoining upper electrodes on the polymer film.     h) Sticking the reverse side of the polymer film onto the support film layer to seal the recessions and become a plurality of cavities thus completing a plurality of ultrasonic transducers.        
 
         [0019]     In order to achieve the aforesaid objects, the present invention also provides another method including the following steps: 
        a) Providing a substrate with electric conductance.     b) Forming a support film layer on the substrate.     c) Providing a cylindrical mold with a patterned outer surface, the patterned outer surface having an array pattern with projections and recesses arranged in order.     d) Rotating the cylindrical mold over the support film layer thus transferring the array pattern into the support film layer, forming a plurality of recessions.     e) Providing a polymer film, the polymer film having an obverse side and a reverse side     f) Forming a plurality of upper electrodes corresponding to the recessions on the polymer film and a plurality of conductor lines between any two adjoining upper electrodes.     g) Sticking the reverse side of the polymer film onto the support film layer to seal the recessions and become a plurality of cavities thus completing a plurality of ultrasonic transducers.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  is the schematic view showing the basic structure of the micro capacitive ultrasonic transducer.  
         [0028]      FIG. 2A  to  FIG. 2C  are the schematic views showing the method for manufacturing the micro capacitive ultrasonic transducer in the prior arts.  
         [0029]      FIG. 3A  to  FIG. 3E  are the schematic views showing the nanoimprint lithography method applied in the semiconductor process.  
         [0030]      FIG. 4A  to  FIG. 4G  are the schematic views showing the first embodiment of the present invention.  
         [0031]      FIG. 4H  is the top view of the micro capacitive ultrasonic transducer of the present invention.  
         [0032]      FIG. 5A  to  FIG. 5G  are the schematic views showing the second embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     Matched with corresponding drawings, the preferable embodiments of the invention are presented as following and hope they will benefit your esteemed reviewing committee members in reviewing this patent application favorably.  
         [0034]     Nanoimprint lithography has developed since 1996 when Dr. Stephen Y. Chou published the related papers. Nanoimprint lithography is quiet different from the traditional lithography in semiconductor process; it does not need to use any energy beams, so the resolution in nanoimprint lithography will not be limited by the phenomenon of diffraction, scattering, and interference when optical wave entering into the photoresist and by the effect of back scattering from the substrate. In fact, the imprint method was disclosed at least as early as in 1970s, and the related researches as well as lots of the related patents have been accumulated, such as U.S. Pat. No. 4,035,226, U.S. Pat. No. 5,259,926, U.S. Pat. No. 5,772,905, and U.S. Pat. No. 6,375,870.  
         [0035]     Please refer to  FIG. 3A  to  FIG. 3E , which are the schematic views showing the technology of nanoimprint lithography applied in the semiconductor manufacturing process. Firstly, an isolation film  32  and a flexible film  33  keeping the state of plasticity are successively formed on a substrate  31 . Then a mold  34  with relative projection and recess patterns formed on the surface thereof is pressed into the flexible film  33  thus transferring the pattern into the flexible film  33 . In the process of imprinting, the projection portion of the patterned surface will not directly touch to the isolation film  32  thus forming a relative thin region  331  above the isolation film  32  and generating a relative high-low pattern corresponding to the pattern on the mold surface. Then, the relative thin region  331  is removed by the method of etching to reveal a partial isolation region  321  under the thin region  331 . Finally, the partial isolation region  321  and the flexible film  33  are removed, and then the remaining portion of the isolation film  32  corresponding to the mold surface pattern can be used as the mask for the follow-up steps in semiconductor process such as ion implantation.  
         [0036]     Obviously, the nanoimprint lithography employed in the semiconductor manufacturing process can save quiet a number of process steps. Moreover, the using of the mold not only accelerates the manufacture procedure, but also saves the high cost of the mask fabricating and maintaining. Besides, the array pattern is so practicable in mold manufacturing that nanoimprint lithography technology can be easily applied to the ultrasonic transducer manufacturing, providing the quiet innovation of the industry. The advantages of the nanoimprint lithography technology are: 
        1) Volume manufacturing.     2) Lower cost.     3) Variety choices of the polymer materials used for the oscillation film and the oscillation cavity, such as Bio-compatible material, which makes the micro capacitive ultrasonic transducer more beneficial to apply in the biomedical science.     4) Shorting the height of the oscillation cavity and well controlling the uniformity thus improving the sensitivity of the ultrasonic transducer.     5) Employing the polymer material, instead of the silicon, in the oscillation cavity thus diminishing the effect of Lamb wave.     6) Unifying the materials of the oscillation film and the oscillation cavity, which are different in the conventional process and cause the different expansion coefficient, to overcome the problem of the stability of the transducer.     7) Precisely controlling the size of the ultrasonic transducer in micro or even nano level thus improving the efficiency of the transducer and enlarging the application thereof.        
 
         [0044]      FIG. 4A  to  FIG. 4G  are the schematic views showing the first embodiment of the present invention. As shown in the figures, the substrate  41  doped with impurity for electric conductivity is provided as the lower electrode of the ultrasonic transducer. In the preferable embodiment for strengthening the lower electrode, a plurality of conductive plates can be formed on the substrate  41 ; wherein between any two of the adjoining plates is connected with a conductor line. Then, a support film layer  42  is formed on the substrate  41 . To operate in the nanoimprint technology, the material of the support film layer  42  has to be a flexible polymer such as PMMA. In order to improve the sensitivity of the ultrasonic transducer, the support film layer  42  used to be the wall of the oscillation cavities of the transducer is better to be controlled as thin as possible. Further, a mold  51  with a patterned surface  511  is provided, and wherein the patterned surface  511  has an array pattern  512  with projections and recesses arranged in order. By using a driving apparatus, the mold  51  can be imprinted into the support film layer  42  with the patterned surface  511  thus transferring the array pattern  512  to the support film layer  42 . After removing the mold  51 , a plurality of recessions  421  corresponding to the array pattern  512  thus is formed on the support film layer  42 . In the process of imprinting, the projection portion of the patterned surface will not directly touch to the surface of substrate  41 ; in other words, the bottom of the recessions  421  formed by the mold  51  will not touch to the substrate  41  thus remaining a relative thin region above the substrate  41 . Next, the relative thin region is removed by using the method of etching to reveal the substrate  41  on the recession bottom. Such method can prevent the mold from damaging the surface thereof and the substrate surface. Besides, the imprint method can be hot stamping, laser imprint, nanoimprint, and any other technologies that can generate the imprint-like effect.  
         [0045]     Next, a polymer film  43  is provided on a platform, and a plurality of particularly arranged upper electrode plates  441  is formed on the polymer film  43 . The upper electrode plate  441  is used as the upper electrode of the capacitive ultrasonic transducer, and between any two of the adjoining upper electrode plates is connected with a conductor line. Finally, the polymer film  43  with the upper electrode plates  441  thereon is stuck on the support film layer  42  thus sealing the recessions  421  becoming a plurality of closed cavities  422 . Wherein the materials of the polymer film  43  and the support film layer  42  can be the same, which can prevent the problem of different expansion coefficient resulting in the instability of the ultrasonic transducer. On the closed cavities  422  is the polymer film  43 , and on the polymer film  43  is the plurality of upper electrode plates  441 ; wherein the upper electrode plates are respectively corresponding to the closed cavities  422 . Please refer to  FIG. 4H , which is the top view showing the micro capacitive ultrasonic transducer of the present invention. The upper electrode plates  441  are respectively located onto the central area of the corresponding closed cavities  422 , and the cross section area of the upper electrode plate  441  is about 60%˜70% of that of the closed cavity  422 ; besides, between any two of the adjoining electrode plates is connected with a conductor line.  
         [0046]     Moreover, the formation of the aforesaid upper electrode plates  441  can be the traditional semiconductor manufacturing process, including the following steps: 
        1) Forming a conductive layer  44  on a polymer film  43 , then coating a photoresist film on the conductive layer  44 .     2) Using photolithography technology to form a photoresist mask arranged in order on the photoresist film.     3) Etching the conductive layer  44  to form the upper electrode plates  441  corresponding to the photoresist mask. 
 
 Such method is employed when the material of conductive layer  44  is solid film, such as metal film or polycide. But if the material of conductive layer  44  is a flexible material, the nanoimprint technology can also be employed in the formation of the upper electrode plates  441 , including the following steps: 
    1′) Forming a conductive layer  44  onto the polymer film  43 .     2′) Proving a second mold with a patterned surface, wherein the patterned surface having a second array pattern with projections and recesses arranged in order.     3′) Imprinting the second mold into the conductive film  44  thus transferring the second array pattern to the surface of the conductive film  44 .     4′) Removing the second mold, a plurality of the upper electrode plates  441  thus being formed on the polymer film  43 .        
 
         [0054]     Please refer to  FIG. 5A  to  FIG. 5G , which are the schematic views showing the second preferable embodiment of the present invention. First, the substrate  61  doped with impurity for electric conductivity is provided as the lower electrode of the ultrasonic transducer. In the preferable embodiment for strengthening the lower electrode, a plurality of conductive plates can be formed on the substrate  61 , and between any two of the adjoining plates is connected with a conductor line. Then a support film layer  62  is formed on the substrate  61 . To operate in the nanoimprint technology, the material of the support film layer  62  has to be a flexible polymer, such as PMMA. Then, a cylindrical mold  71  with an array pattern  712  formed on the outer surface thereof is provided to press to and roll across the support film layer  62  thus forming a plurality of the particularly arranged recessions  621  on the support film layer  62 . Similarly, in the rolling process of the cylindrical mold  71 , the projection portion of the mold outer surface will not touch to the surface of the substrate  61 . In other words, the bottom of the recessions  621  formed by the mold  71  will not touch to the substrate  61  thus remaining a relative thin region above the substrate  61 . Next, removing the relative thin region by the etching method to reveal the portion of the substrate  61 .  
         [0055]     Next, a polymer film  63  is provided on a platform, and a plurality of particularly arranged upper electrode plates  641  is formed on the polymer film  63 . The upper electrode plate  641  is used as the upper electrode of the capacitance ultrasonic transducer, and between any two of the adjoining upper electrode plates is connected with a conductor line. Finally, the polymer film  63  with the upper electrode plates  641  thereon is stuck on the support film layer  62  thus sealing the recessions  621  becoming a plurality of closed cavities  622 . Wherein, on the closed cavities  622  is the polymer film  63 , and on the polymer film  63  is the plurality of upper electrode plates  641  corresponding to the closed cavities  622 . The upper electrode plates  641  are respectively located onto the central area of the corresponding closed cavities  622 , and the cross section area of the upper electrode plate  641  is about 60%˜70% of that of the closed cavity  622 ; besides, between any two of the adjoining electrode plates is connected with a conductor line.  
         [0056]     In addition, as described in the first embodiment of the present invention, the formation of the upper electrode plates  641  can be the tradition semiconductor manufacturing process if the material of the conductive film is solid film, such as metal film or polycide. However, if the material of the conductive film is also the flexible material, the imprint method thus can be employed, such as hot stamping, laser imprint, nanoimprint, the imprint methods described in the aforesaid two embodiments, and any other technologies that can generate the imprint-like effect.  
         [0057]     Moreover, the formation of the upper electrode plates both in the first and the second embodiment can be carried out after the polymer film stuck onto the support film layer. In other words, after forming a plurality of recessions of the support film layer on the substrate, the polymer film can be struck onto the support film layer in advance thus sealing the plurality of recessions to become a plurality of the closed cavities for micro capacitive ultrasonic transducer. Finally, a plurality of the upper electrode plates corresponding to the closed cavities is formed on the polymer film thus completing a plurality of the micro capacitive ultrasonic transducers.  
         [0058]     Although the present invention has been described with reference to a preferred embodiment, it should be appreciated that various modifications and adaptations can be made without departing from the scope of the invention as defined in the claims.  
         [0059]     In summary, from the structural characteristics and detailed disclosure of each embodiment according to the invention, it sufficiently shows that the invention has progressiveness of deep implementation in both objective and function, also has the application value in industry, and it is an application never seen ever in current market and, according to the spirit of patent law, the invention is completely fulfilled the essential requirement of new typed patent.