Abstract:
Provided are a laser-induced ultrasound generator and a method of manufacturing the laser-induced ultrasound generator. The laser-induced ultrasound generator includes: a substrate including a plurality of nanostructures provided on a first surface of the substrate; and a thermoelastic layer provided on the first surface of the substrate, the thermoelastic layer being configured to generate an ultrasound by absorbing a laser beam incident onto a second surface of the substrate, the second surface facing the first surface. The nanostructures may be cylinder-shaped nano-pillars.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Korean Patent Application No. 10-2013-0136302, filed on Nov. 11, 2013, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     The exemplary embodiments relate to laser-induced ultrasound generators and methods of manufacturing the same. 
     2. Description of the Related Art 
     When a laser is irradiated onto a material such as a liquid or a solid, the irradiated material absorbs light energy to generate instant thermal energy, and the thermal energy generates an acoustic wave due to thermoelasticity of the material. 
     As an absorption ratio and a thermoelastic coefficient of materials vary according to a light wavelength of the materials, ultrasound waves generated by different materials differ in amplitude in response to the same light energy. The generated ultrasound waves are used in an analyzer of materials, a non-destructive tester, and a photoacoustic tomography, or the like. 
     A laser-induced ultrasound generator (hereinafter referred to as an ultrasound generator) is an apparatus for generating an ultrasound wave by using a laser. By using the ultrasound wave, it may be diagnosed as to whether, for example, tumors are formed in the body of a patient, that is, in an object. The ultrasound wave is generated based on the principle that energy of absorbed light is converted into pressure. 
     A conventional laser-induced ultrasound generator uses a thermoelastic material layer having a low light absorption ratio, and thus, a low ultrasound generation efficiency. 
     SUMMARY 
     Provided are laser-induced ultrasound generators with an increased ultrasound generation efficiency. 
     Provided are methods of manufacturing the laser-induced ultrasound generators. 
     Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented exemplary embodiments. 
     According to an aspect of an exemplary embodiment, there is provided a laser-induced ultrasound generator including: a substrate including a plurality of nanostructures provided on a first surface of the substrate; and a thermoelastic layer provided on the first surface of the substrate, the thermoelastic layer being configured to generate an ultrasound by absorbing a laser beam incident onto a second surface of the substrate, the second surface facing the first surface. 
     The plurality of nanostructures may include a plurality of cylinder-shaped nanopillars. 
     Each of the plurality of nanopillars may have a diameter of about 10 nm to about 1000 nm. 
     A gap between adjacent nanopillars may be about 10 nm to about 1000 nm. 
     The thermoelastic layer may include a metal or a polymer material. 
     The substrate may include a laser beam-transmitting material. 
     The laser-induced ultrasound generator may further include a matching layer provided on the thermoelastic layer, wherein a surface of the matching layer faces the first surface of the substrate. 
     The matching layer may include a polymer. 
     The laser-induced ultrasound generator may further include a laser oscillator configured to irradiate the laser beam onto the second surface of the substrate. 
     According to another aspect of an exemplary embodiment, there is provided a method of manufacturing a laser-induced ultrasound generator, the method including: forming a thin metal film on a substrate; converting the thin metal film into a plurality of metal dots by annealing the substrate; forming a plurality of nanostructures on the substrate by dry-etching the substrate, the dry-etching comprising using the plurality of metal dots as a mask; removing the plurality of metal dots; and forming a thermoelastic layer on the substrate to cover the plurality of nanostructures. 
     The forming of the thin metal film may include forming a thin metal film having a thickness of about 10 nm to about 1000 nm. 
     The converting of the thin metal film into the plurality of metal dots may include forming metal dots, each having a diameter of about 10 nm to about 1000 nm, as the plurality of metal dots. 
     The forming of the plurality of nanostructures may include forming a plurality of nanopillars, each having a diameter corresponding to a size of one of the plurality of metal dots, as the plurality of nanostructures. 
     The method may further include forming a matching layer on the thermoelastic layer, a surface of the matching layer facing a surface of the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic structural view of a ultrasound generator according to exemplary embodiments; 
         FIG. 2  is a scanning electron microscope (SEM) photographic image of nanopillars formed on a glass substrate; 
         FIG. 3  is a simulation graph showing light absorption ratios of an ultrasound generator having nanostructures according to exemplary embodiments and a conventional ultrasound generator without nanostructures; and 
         FIGS. 4A through 4E  are cross-sectional views illustrating a method of manufacturing an ultrasound generator according to exemplary embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings, wherein thicknesses of layers or regions illustrated in the drawings are exaggerated for clarity of description. In this regard, the present exemplary embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element, or intervening elements may also be present. Like reference numerals refer to the like elements throughout and a detailed description thereof will be omitted. 
       FIG. 1  is a schematic structural view of an ultrasound generator  100  according to exemplary embodiments. 
     Referring to  FIG. 1 , the ultrasound generator  100  may include a substrate  110  through which a laser beam L is transmitted, and a thermoelastic layer  130  formed on the substrate  110 . A matching layer  150  may be further formed on the thermoelastic layer  130 . A laser oscillator  170  irradiates the laser beam L onto the substrate  110 . 
     The substrate  110  may be formed of a material having a relatively high light transmittivity so that a laser beam L may be incident onto the thermoelastic layer  130  without any loss. The substrate  110  may be formed of quartz, fused silicon, glass or the like. The laser beam L may be incident onto a first surface  110   a  of the substrate  110 , and a plurality of nanostructures may be formed on a surface of the substrate  110  opposite to the first surface  110   a . The nanostructures may be cylinder-shaped nanopillars  114 . The nanopillars  114  may be formed by etching the substrate  110  and thus, the nanopillars may be formed to be expanded from the substrate  110 . 
     Although the nanopillars  114  are illustrated as the nanostructures according to the current exemplary embodiment, the exemplary embodiments are not limited thereto. For example, nano-cone structures may be formed as the nanostructures instead of the nanopillars  114 . 
     The nanopillars  114  may have a diameter of about 10 nm to about 1000 nm, and a gap between adjacent nanopillars  114  may be about 10 nm to about 1000 nm. 
       FIG. 2  is a scanning electron microscope SEM photographic image of the nanopillars  114  formed on the substrate  110  which is formed of glass. Referring to  FIG. 2 , each of the nanopillars  114  may have an average diameter of about 100 nm, and a gap between adjacent nanopillars  114  may be about 100 nm. As illustrated in  FIG. 2 , the nanopillars  114  may have different diameters from one another. 
     The thermoelastic layer  130  expands upon absorbing an irradiated laser beam L, and an ultrasound U is generated according to the expansion of the thermoelastic layer  130 . The thermoelastic layer  130  may be formed of a material having a relatively high thermal expansion coefficient. The thermoelastic layer  130  may be a thin film so as to easily thermally expand or contract. For example, the thickness of the thermoelastic layer  130  may be several μm or less. The thermoelastic layer  130  may be formed of a metal or a polymer material. For example, the thermoelastic layer  130  may be formed of a metal such as Cr, Ti, Au, or Al or of a polymer material such as black polydimethylsiloxane (PDMS) mixed with carbon or carbon tapes. 
     The thermoelastic layer  130  may fill spaces between the nanopillars  114 . The thermoelastic layer  130  may completely fill spaces between the nanopillars  114  as illustrated in  FIG. 1 . However, exemplary embodiments are not limited thereto. For example, the thermoelastic layer  130  having a small thickness may be formed to partially fill spaces between the nanopillars  114 . 
     If the thermoelastic layer  130  is formed of the metal, the thermoelastic layer  130  may be formed as a double layer. For example, the thermoelastic layer  130  may include an adhesive layer formed of Ti or Cr and a metal layer including a material such as Au or Al on the adhesive layer. 
     The matching layer  150  may modify acoustic impedance of an ultrasound U generated in the thermoelastic layer  130  stepwise so that the acoustic impedance of the ultrasound U is similar to that of an object. The thermoelastic layer  130  may be a single layer or may be formed of a plurality of layers. The matching layer  150  may be formed of a polymer material. For example, the matching layer  150  may be formed of parylene, polydimethylsiloxane (PMDS) or polyimide. 
     The matching layer  150  on the thermoelastic layer  130  may be omitted. In particular, if the thermoelastic layer  130  is formed of a polymer material, the matching layer  150  may be omitted. 
     The laser oscillator  170  irradiates the laser beam L onto the substrate  110 , from which an ultrasound U is generated. For example, the laser oscillator  170  may be a pulse laser, and a pulse width of the laser may be in the range of nanoseconds or picoseconds. 
     After the laser beam L is transmitted through the substrate  110  and then is irradiated onto the thermoelastic layer  130 , an ultrasound U is generated in the thermoelastic layer  130  due to thermoelasticity. The ultrasound U is irradiated onto an object, a portion of the ultrasound U is absorbed by the object, and the remainder of the ultrasound U is reflected. By receiving a signal reflected by the object, that is, an echo signal of the ultrasound U, a shape of the object and characteristics of tissues of the object may be measured. 
     The ultrasound generator  110  may convert light into the ultrasound U based on the following principle. When light having an energy density of I(x, y, z, t) is irradiated onto the thermoelastic layer  130 , the thermoelastic layer  130  generates heat H as expressed as in Equation 1 below.
 
 H =(1− R )· I·μe   μz   ([Equation 1]
 
     Here, R denotes a reflection coefficient of a thermoelastic layer with respect to the light, and μ denotes an absorption coefficient of the thermoelastic layer with respect to the laser beam, and z denotes a vertical distance between the thermoelastic layer and a surface onto which the laser beam is incident. 
     In the thermoelastic layer, a variation in temperature (ΔT) as expressed in Equation 2 below is generated. 
     
       
         
           
             
               
                 
                   
                     
                       
                         k 
                         
                           C 
                           2 
                         
                       
                       ⁢ 
                       
                         
                           
                             ∂ 
                             2 
                           
                           ⁢ 
                           T 
                         
                         
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                             t 
                             2 
                           
                         
                       
                     
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                       ⁢ 
                       
                         C 
                         P 
                       
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                           T 
                         
                         
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                         ( 
                         
                           k 
                           · 
                           
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                             T 
                           
                         
                         ) 
                       
                     
                     + 
                     H 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     Here, k denotes a thermal conductivity of the thermoelastic layer, C denotes a heat propagation speed in the thermoelastic layer, ρ denotes a density of the thermoelastic layer, and Cp denotes a specific heat of the thermoelastic layer. 
     Due to the variation in temperature (ΔT), a variation in volume (ΔV) as in Equation 3 below is generated in the thermoelastic layer. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           ∂ 
                           2 
                         
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                       ( 
                       
                         
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                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     Here, β denotes a thermal coefficient of volume of the thermoelastic layer. 
     An ultrasound having a pressure P as expressed in Equation 4 below is generated according to the variation in volume (ΔV) of the thermoelastic layer. 
     
       
         
           
             
               
                 
                   
                     
                       1 
                       ρ 
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           
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                             - 
                             
                               1 
                               
                                 v 
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                     P 
                   
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                     ⁢ 
                     
                       ( 
                       
                         
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                       ) 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     Here, v s  denotes a speed at which the ultrasound travels. 
     If the same material is used for the thermoelastic layer in the ultrasound generator, an ultrasound generation efficiency may be improved only by increasing the light absorption ratio of the thermoelastic layer. 
     According to exemplary embodiments, the nanopillars  114  are formed between the substrate  110 , which is an insulation material, and the thermoelastic layer  130 , and thus, light irradiated onto the nanopillars  114  generates surface plasmon polaritons between the substrate  110  and the thermoelastic layer  130 . If the nanopillars  114 , which are nanostructures in a three-dimensional shape, are formed between the substrate  110  and the thermoelastic layer  130 , the surface plasmon polaritons become trapped in the nanostructures, and a light absorption ratio in the thermoelastic layer  130  is increased. Thus, an ultrasound generation efficiency may be improved. 
       FIG. 3  is a simulation graph showing light absorption ratios of an ultrasound generator having nanostructures according to exemplary embodiments and a conventional ultrasound generator without nanostructures. The ultrasound generator according to the current exemplary embodiment includes a thermoelastic layer formed by depositing a 50 nm thick Au layer, and 2 μm thick parylene layer as a matching layer, and glass is used as a substrate. Nanopillars have a width, height, and interval which are each 100 nm. The conventional ultrasound generator has the same structure as the current exemplary embodiment except that the substrate and the thermoelastic layer are flat. 
     Referring to  FIG. 3 , a first curve C 1  denotes a light absorption ratio of the ultrasound generator according to the current exemplary embodiment, and a second curve C 2  denotes a light absorption ratio of the conventional ultrasound generator. A light absorption ratio of the ultrasound generator having nanostructures is larger than that of the conventional ultrasound generator. When a laser beam wavelength is 550 nm, a light absorption ratio of the conventional ultrasound generator is about 0.3, while that of the ultrasound generator according to the current exemplary embodiment is about 0.7. Thus, the light absorption ratio of the ultrasound generator according to the current exemplary embodiment is greater than that of the conventional ultrasound generator. 
     Therefore, the thermoelastic layer of the ultrasound generator of the current exemplary embodiment has an increased light absorption ratio due to a function of the nanopillars formed between the substrate and the thermoelastic layer. Furthermore, when the same laser energy is used in the ultrasound generator of the current exemplary embodiment and the conventional ultrasound generator, the ultrasound generator of the current exemplary embodiment generates an ultrasound having a pressure greater than that of an ultrasound generated by the conventional ultrasound generator. 
       FIGS. 4A through 4E  are cross-sectional views illustrating a method of manufacturing an ultrasound generator according to exemplary embodiments. 
     Referring to  FIG. 4A , a metal layer  220  having a first thickness H 1  is deposited on a substrate  210 . The metal layer  220  may be formed of a typical metal such as Ag, Au or Pb. If a metal for the metal layer  220  has contracting properties upon being heated, then the metal for the metal layer  220  is not limited to a predetermined material as above. The first thickness H 1  may be about 10 nm to about 1000 nm. The substrate  210  may be formed of, for example, quartz, fused silica or glass. 
     Referring to  FIG. 4B , the substrate  210  is annealed. An annealing temperature may vary according to the material of the metal layer  220  and the first thickness H 1 . After the annealing, a plurality of metal dots  222  is formed on the substrate  210 . Each of the metal dots  222  may have a size of about 10 nm to about 1000 nm, and a distance between the metal dots  222  may also be about 10 nm to about 1000 nm. 
     Referring to  FIG. 4C , the metal dots  222  are used as a mask to dry-etch the substrate  210 . After etching, a plurality of cylinder-shaped nanopillars  214  is formed on the substrate  210 . An aspect ratio of the nanopillars  214  may be about 1. The nanopillars  214  may have a diameter of about 10 nm to about 1000 nm, and a gap between adjacent nanopillars  214  may be about 10 nm to about 1000 nm. 
     The substrate  210  is dipped into a solution which is capable of removing the metal dots  222 , thereby removing the metal dots  222  from the substrate  210 .  FIG. 4C  illustrates the substrate  210  before the metal dots  222  are removed. 
     Referring to  FIG. 4D , a thermoelastic layer  230  covering the nanopillars  214  is formed on the substrate  210 . The thermoelastic layer  230  may be formed of a metal or a polymer material. For example, the thermoelastic layer  230  may be formed of a metal such as Cr, Ti, Au, or Al or of a polymer material such as black polydimethylsiloxane (PDMS) mixed with carbon or carbon tapes. If the thermoelastic layer  230  is formed of a metal, the thermoelastic layer  230  may be formed as a double layer. For example, the thermoelastic layer  230  may include an adhesive layer formed of Ti or Cr and a metal layer including Au or Al on the adhesive layer. 
     Referring to  FIG. 4E , a matching layer  250  may be formed on the thermoelastic layer  230 . The matching layer  250  may be formed of a polymer material. For example, the matching layer  250  may be formed of parylene, PMDS, or polyimide. The matching layer  250  may have a thickness of about several μm. The matching layer  250  may be formed of a plurality of layers. Also, the matching layer  250  may be formed of a plurality of layers that are formed of different materials. 
     When the thermoelastic layer  230  is formed of a polymer material, the matching layer  250  may be omitted. 
     According to the method of manufacturing a laser-induced ultrasound generator, as metal dots formed by annealing are used in forming nanopillars, an additional mask process involving a nano-sized mask is not required. 
     It should be understood that the exemplary embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each exemplary embodiment should typically be considered as available for other similar features or aspects in other exemplary embodiments. 
     While one or more exemplary embodiments have been described with reference to the figures, 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 disclosure as defined by the following claims.