Patent Publication Number: US-2010111770-A1

Title: Microfluidic Chip and Method of Fabricating The Same

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is a continuation-in-part application of U.S. patent application Ser. No. 11/934,811 filed on Nov. 5, 2007, which claims the benefit of Korean Patent Application No. 10-2007-0055716, filed on Jun. 7, 2007; this application claims the benefit of Korean Application No. 10-2008-110004, filed on Nov. 6, 2008, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     One or more embodiments relate to microfluidics, and more particularly, to a microfluidic chip and a method of fabricating the microfluidic chip. 
     2. Description of the Related Art 
     Microfluidic chips, that is, chip-shaped devices, are used in microfluidics to perform various biochemical reactions using a small amount of biochemical fluid or to process a biochemical fluid for biochemical reactions. In general, a microfluidic chip includes an inlet hole for injecting a biochemical fluid into the microfluidic chip, an outlet hole for discharging the biochemical fluid out of the microfluidic chip, a channel through which the biochemical fluid can flow, and a chamber in which the biochemical fluid is received. 
     Microfluidic chips could have the organic thin films on an inner surface of the chamber using an organosilane-based material in order to capture the cells present in a biochemical fluid or to purify DNA extracted from the cells, which are well known. Such a conventional microfluidic chip includes a lower substrate formed of silicon (Si) and an upper substrate formed of a transparent glass material, and the lower substrate and the upper substrate are attached to each other using an anodic bonding method. The anodic bonding may destroy an organosilane-based material since it requires a high temperature condition of 400° C. or higher. Therefore, after attaching the lower substrate and the upper substrate using the anodic bonding method, the organic thin film is formed through the holes on inner surfaces of the chamber and the channel using a chemical vapor deposition (CVD) method. 
     Microfluidic devices are used in various fields. For example, the microfluidic device may be used as an analyzing apparatus of a high throughput. The microfluidic device includes microfluidic structures such as channels and chambers. The microfluidic device may be fabricated in various ways. Microfluidic fabricating technologies such as lithography, etching, depositing, micromachining, and Lithographie, Galvanoformung, and Abformung (LIGA) processes may be used to fabricate the microfluidic devices. 
     The microfluidic device may be fabricated by forming microfluidic structures on two substrates and coupling the substrates to each other. For example, the microfluidic structures may be formed on two glass substrates, and the glass substrates are coupled to each other to fabricate the microfluidic device. Each of the two substrates includes entire or a part of the microfluidic structures. 
     The conventional microfluidic chip uses the expensive inorganic materials such as silicon or glass, and the lower substrate and the upper substrate are attached to each other using the anodic bonding method that requires the high temperature condition. In addition, since the organic thin film should be formed through the holes after attaching the lower substrate and the upper substrate to each other, the fabrication costs of the conventional microfluidic chip increase and the uniformity of generated organic thin film is not guaranteed. In addition, a method for effectively forming the microfluidic structures on the substrate and coupling the substrates on which the microfluidic structures are formed, and a microfluidic device fabricated by the above method are required. 
     SUMMARY 
     One or more embodiments provide a microfluidic chip including a lower substrate and an upper substrate attached to each other using a novel bonding method instead of an anodic bonding, and including an organic thin film formed on an inner surface of a chamber, and a method of fabricating the microfluidic chip. 
     According to an aspect, there is provided a microfluidic chip including: a lower substrate including a channel, through which a biochemical fluid can flow, and a chamber, in which the biochemical fluid can be received, formed on an upper surface of the lower substrate; an upper substrate formed of a silicon resin, and having a lower surface attached to the upper surface of the lower substrate; and an organic thin film formed on the upper surface of the lower substrate except for portions on which the lower substrate and the upper substrate are attached to each other, wherein the lower surface of the upper substrate is activated by an O 2 -plasma process, and the lower surface of the upper substrate is adhered to the upper surface of the lower substrate so that the lower substrate and the upper substrate can be attached to each other. 
     The microfluidic chip may further include: a unit for enlarging a contact surface area with the biochemical fluid in the chamber. 
     The unit for enlarging the contact surface area may include a plurality of pillars protruding from the lower substrate so that they contact the lower surface of the upper substrate, and they are separately arranged from one another. 
     The organic thin film may be formed on a surface of the unit for enlarging the contact surface area. 
     The silicon resin of the upper substrate may be PDMS (polydimethylsiloxane). 
     The lower substrate may include Si, SiO 2 , SiN, or a polymer. 
     The organic thin film may be a SAM (self-assembled monolayer). 
     The organic thin film may include an organosilane-based material. 
     The organosilane-based material may have an alkoxysilane group or a chlorosilane group. 
     A photocatalyst layer including a photocatalyst material may be disposed between the lower substrate and the organic thin film. 
     The photocatalyst material may be TiO 2 , ZnO, SnO 2 , SrTiO 3 , WO 3 , B 2 O 3 , or Fe 2 O 3 . 
     The lower substrate may include a photocatalyst material. 
     The photocatalyst material may be TiO 2 . 
     An oxide layer or a nitride layer may be formed on portions of the upper surface of the lower substrate, which contact the lower surface of the upper substrate. 
     The oxide layer may include SiO 2  or TiO 2 . 
     The nitride layer may include SiN. 
     According to another aspect, there is provided a method of fabricating a microfluidic chip, the method including: forming a lower substrate including a channel, through which a biochemical fluid can flow, and a chamber, in which the biochemical fluid can be received, on an upper surface of the lower substrate; forming an upper substrate including a silicon resin; forming an organic thin film on the upper surface of the lower substrate; removing the organic thin film that is formed on portions of the lower substrate, which will be attached to the upper substrate; and activating a lower surface of the upper substrate using an O 2 -plasma process, and adhering the upper substrate to the lower substrate to attach the upper and lower substrates to each other. 
     The formation of the organic thin film may include: coating the lower substrate with a solution including the material forming the organic thin film. 
     The removal of the organic thin film may include: forming a photo mask including a flat transparent plate, a photoresist layer including a pattern corresponding to the portions, from which the organic thin film will be removed, on the transparent plate, and a photocatalyst layer including a photocatalyst material formed on a lower surface of the transparent plate; arranging the photo mask on the upper surface of the lower substrate so that the photocatalyst layer can contact the organic thin film; and irradiating ultraviolet (UV) rays to the photo mask so that the organic thin film that contacts the photocatalyst layer and is exposed to the UV rays can be decomposed. 
     The removal of the organic thin film may include: placing a flat photocatalyst plate including a photocatalyst material on the lower substrate on which the organic thin film is formed; and irradiating the UV rays to the photocatalyst plate to decompose the organic thin film that contacts the photocatalyst plate and is exposed to the UV rays. 
     The method may further include: forming a photocatalyst layer including a photocatalyst material on the upper surface of the lower substrate before forming of the organic thin film, and forming the organic thin film on the photocatalyst layer in the process of forming the organic thin film, wherein the removal of the organic thin film includes: forming a photo mask including a flat transparent plate and a photoresist layer including a pattern corresponding to portions, from which the organic thin film will be removed, on the transparent plate; arranging the photo mask on the upper surface of the lower substrate; and irradiating the UV rays to the photo mask so that the organic thin film that contacts the photocatalyst layer and is exposed to the UV rays can be decomposed. 
     One or more embodiments may include a microfluidic structure including a polysiloxane layer. 
     One or more embodiments may include a method of fabricating a microfluidic structure by using a polysiloxane layer. 
     According to one or more embodiments, the microfluidic structure includes: a first substrate; a second substrate; and a polysiloxane layer disposed between the first and second substrates, wherein the polysiloxane layer is coupled to the first and second substrates via an SiO 2  layer. 
     According to one or more embodiments, the method of fabricating a microfluidic structure includes: providing a first substrate and a second substrate on which micro-structures are formed; depositing an SiO 2  layer on surfaces of the first and second substrates; and coupling the first and second substrates to each other by interposing a polysiloxane layer between the surfaces, on which the SiO 2  layer is deposited, of the first and second substrates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a partially cut exploded perspective view of a microfluidic chip according to an embodiment; 
         FIGS. 2A through 2G  are cross-sectional views illustrating a method of fabricating the microfluidic chip of  FIG. 1 ; 
         FIGS. 3A through 3E  are cross-sectional views illustrating a method of fabricating a microfluidic chip according to another embodiment; 
         FIG. 4  is a partially cut perspective view of a microfluidic chip according to another embodiment; 
         FIGS. 5A through 5G  are cross-sectional views sequentially illustrating a method of fabricating the microfluidic chip of  FIG. 4 , according to another embodiment; and 
         FIG. 6  is a partially cut perspective view of a microfluidic chip according to another embodiment; 
         FIGS. 7A and 7B  are diagrams of a microfluidic structure according to another embodiment; 
         FIG. 8  is a diagram illustrating a method of fabricating a microfluidic structure according to another embodiment; 
         FIGS. 9A through 9C  are diagrams of a microfluidic structure according to another embodiment; and 
         FIGS. 10A and 10B  are diagrams of a pump which is fabricated by using film valves. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a microfluidic chip and a method of fabricating the same will be described with reference to accompanying drawings. 
       FIG. 1  is a partially cut exploded perspective view of a microfluidic chip  100  according to an embodiment. 
     Referring to  FIG. 1 , the microfluidic chip  100  of the current embodiment includes a lower substrate  101  and an upper substrate  115 , which are attached to each other. The lower substrate  101  is formed of a Si material, and includes a channel  102 , through which a fluid can flow, and a chamber  105  receiving the fluid in a center portion of the channel  102  on an upper surface thereof. A plurality of pillars  107  are formed in the chamber  105 . The pillar  107  is a unit for enlarging an surface area contacting the fluid induced in the chamber  105 . The pillars  107  are separated from each other in the chamber  105 , and protrude out of the upper surface of the lower substrate  101  so that they contact a lower surface of the upper substrate  115 . 
     The surface of the lower substrate  101  formed of the Si material is oxidized by oxygen in the air, and thus, an oxide layer  109  including SiO 2  is formed. The oxide layer  109  has a function of attaching the upper substrate  115  and the lower substrate  101  to each other. On the other hand, the lower substrate  101  may be formed of a polymer resin such as PDMS (polydimethylsiloxane), PMMA (polymethylmetaacrylate), PC (polycarbonate), and PE (polyethylene). If the lower substrate  101  is formed of the polymer resin, the oxide layer  109  is not formed. Therefore, an oxide layer including SiO 2  or TiO 2  or a nitride layer including SiN should be specifically formed. In order to form the oxide layer or the nitride layer, a CVD method or a physical vapor deposition (PVD) method can be used. In addition, the lower substrate  101  may be formed of SiO 2  or SiN. In this case, since the lower substrate  101  is formed of the oxide material or the nitride material, an additional oxide layer or a nitride layer is not required. 
     An organic thin film  110  is formed on the upper surface of the lower substrate  101 . The organic thin film  110  is coated to capture in the chamber  105  certain cells such as bacteria included in a biochemical fluid injected into the microfluidic chip  100  or to purify DNA extracted from the cells in the chamber  105 . The organic thin film may include an organosilane based material, and can be stacked as a self-assembled monolayer. The organic thin film  110  is also formed on surfaces of the plurality of pillars  107 . The organosilane-based material can be an alkoxysilane group material or a chlorosilane group material. The alkoxysilane group material can be octadecyldimethyl(3-trimethoxysilyl propyl) ammonium chloride, polyethyleneiminertrimethoxysilane, and aminopropyltriethoxysilane, and the chlorosilane group material can be octadecyltrichlorosilane. 
     The organic thin film  110  is mostly formed of a hydrophobic material, and thus, interferes with the attachment between the lower substrate  101  and the upper substrate  115 . Therefore, the organic thin film formed on areas  112  on the upper surface of the lower substrate  101 , which are attached to the upper substrate  115 , is removed. Hereinafter, the area  112  will be referred to as an attaching area. 
     The upper substrate  115  is formed of a silicon resin, for example, PDMS (polydimethylsiloxane). The upper substrate  115  includes an inlet hole  116  connected to a side of the channel  102  of the chamber  105  so as to inject the biochemical fluid into the microfluidic chip  100 , and an outlet hole  117  connected to the other side of the channel  102  of the chamber  105  so as to exhaust the biochemical fluid out of the microfluidic chip  100 . The method of attaching the lower substrate  101  and the upper substrate  115  will be described later. 
       FIGS. 2A through 2G  are cross-sectional views sequentially illustrating a method of fabricating the microfluidic chip of  FIG. 1 . Hereinafter, the method of fabricating the microfluidic chip  100  will be described in detail with reference to  FIGS. 2A through 2G . 
     The method of fabricating the microfluidic chip  100  may include into a first process (refer to  FIG. 2A ) of forming the lower substrate  101  on which the channel  102  and the chamber  105  are formed, a second process (refer to  FIG. 2F ) of preparing the upper substrate  115  formed of Si, a third process (refer to  FIG. 2B ) of forming the organic thin film  110  on the upper surface of the lower substrate  101 , a fourth process (refer to  FIGS. 2C through 2E ) of removing the organic thin film  110  formed on the attaching area  112  of the lower substrate  101 , and a fifth process (refer to  FIGS. 2F and 2G ) of attaching the upper substrate  115  and the lower substrate  101  to each other. 
     Referring to  FIG. 2A , the lower substrate  101  formed of the Si material is prepared, and the channel  102 , the chamber  105 , and the plurality of pillars  107  are formed on the upper surface of the lower substrate  101 . An etch prevention layer (not shown) having patterns corresponding to the channel  102 , the chamber  105 , and the pillars  107  is formed on the upper surface of the lower substrate  101  using a photolithography method, and the upper surface of the lower substrate  101  is selectively removed using a wet etching process or a dry etching process to form the channel  102 , the chamber  105 , and the pillars  107 . On the other hand, the channel  102 , the chamber  105 , and the pillars  107  can be formed using a general machining process such as a press process or a milling process. 
     The surface of the lower substrate  101 , on which the channel  102 , the chamber  105 , and the pillars  107  are formed, is oxidized by the oxygen in the air, and the oxide layer  109  including SiO 2  is formed. The oxide layer  109  helps the attachment between the upper substrate  115  and the lower substrate  101 . Meanwhile, the lower substrate  101  can be formed of a polymer resin such as PDMS (polydimethylsiloxane), PMMA (polymethylmetaacrylate), PC (polycarbonate), and PE (polyethylene). If the lower substrate  101  is formed of the polymer resin, the oxide layer  109  is not formed, and thus, the oxide layer including SiO 2  or TiO 2  or the nitride layer including SiN should be specifically formed. The oxide layer or the nitride layer can be formed using the CVD method or the PVD method. 
     In the second process, a mixed solution including the PDMS resin and a linking agent is injected into a mold (not shown) corresponding to the shape of the upper substrate  115  and is cured, and then, the cured shape is separated from the mold to form the upper substrate  115  (refer to  FIG. 2F ) formed of the PDMS. In more detail, Sylgard® 184 of Dow Corning Inc. is injected into the mold, and disposed under an optimal curing condition to form the upper substrate  115 . For example, the optimal curing condition of Sylgard® 184 is to keep the product for 45 minutes at a temperature of 100° C., 20 minutes at a temperature of 125° C., or 10 minutes at a temperature of 150° C. Sylgard® 184 is an example of the mixed solution of the PDMS resin and the linking agent. 
     The inlet hole  116  and the outlet hole  117  can be formed using a general machining process such as a pressing process or a drilling process. Otherwise, a structure corresponding to the inlet hole  116  and the outlet hole  117  is disposed in the mold, and the mixed solution of the PDMS resin and the linking agent is injected into the mold to form the inlet hole  116  and the outlet hole  117 . 
     Referring to  FIG. 2B , the third process includes a coating process of dipping the upper surface of the lower substrate  101  into a solution including a material forming the organic thin film  110 . In more detail, the upper surface of the lower substrate  101  is dipped into a solution including ethanol and octadecyldimethyl(3-trimethoxysilyl propyl) ammonium chloride that is the organosilane-based material for one hour, and after that, the lower substrate  101  is washed and disposed for 50 minutes at a temperature of 100° C. to form the organic thin film  110  on the upper surface of the lower substrate  101 . The octadecyldimethyl(3-trimethoxysilyl propyl) ammonium chloride is an example of material forming the organic thin film  110 , and can be substituted by other materials such as polyethyleneiminertrimethoxysilane, aminopropyltriethoxysilane, or octadecyltrichlorosilane. 
     The fourth process includes forming of a photomask  10  (refer to  FIG. 2C ), arranging the photomask  10  on the upper surface of the lower substrate  101  and irradiating ultraviolet rays onto the photomask  10  (refer to  FIG. 2D ), and removing the organic thin film  110  on the attaching area  112  on substrate  101  (refer to  FIG. 2E ). 
     Referring to  FIG. 2C , the photomask  10  includes a flat transparent plate  11  formed of a transparent material such as glass, a photoresist layer  12  formed on the transparent plate  11 , and a photocatalyst layer  15  formed on a lower surface of the transparent plate  11 . The photoresist layer  12  includes a pattern  13  corresponding to the area, from which the organic thin film  110  will be removed, of the lower substrate  101 , that is, the attaching area ( 112 , refer to  FIG. 1 ). The photoresist layer  12  including the pattern  13  by spin coating a liquid type photoresist on the transparent plate  11  and performing an exposure, a development, and a baking process to remove a certain area, or by laminating a film type photoresist on the transparent plate  11 , and performing the exposure and the development process to remove a certain area. 
     The photocatalyst layer  15  is formed of a photocatalyst material. The photocatalyst material is a material causing a reaction of decomposing the organic thin film  110  when it is exposed to ultraviolet rays when contacting the organic thin film  110 . For example, the photocatalyst material can be TiO 2 , ZnO, SnO 2 , SrTiO 3 , WO3, B 2 O 3 , or Fe 2 O 3 . The photocatalyst layer  15  can be formed by spin coating TiO 2 -sol solution on the lower surface of the transparent plate  11 , and baking the coated layer. The TiO 2 -sol solution can be formed by mixing titanium isopropoxide, isopropanol, and HCl of 0.1N, and stabilizing the mixed solution. Otherwise, the photocatalyst layer  15  can be formed using the CVD method or the PVD method. 
     Referring to  FIG. 2D , the photomask  10  is arranged on the upper surface of the lower substrate  101  so that the pattern  13  of the photoresist layer  12  can overlap with the attaching area  112  (refer to  FIG. 1 ), and ultraviolet rays (UV) are irradiated thereon. The regions of the lower substrate  101  contacting the photocatalyst layer  15  coincide with the attaching area  112 . When the UV rays are irradiated onto the photomask  10 , the photocatalyst layer  15  and the organic thin film  110  are partially exposed to the UV rays through the pattern  13 . Therefore, some part of the organic thin film  110 , which contacts the photocatalyst layer  15  and is exposed to the UV rays, is decomposed by the photocatalyst material. 
     Referring to  FIG. 2E , when the photomask  10  is separated from the lower substrate  101  after irradiating the UV rays, the attaching area  112 , from which the organic thin film  110  is removed by the decomposition operation of the photocatalyst material, is exposed. 
     Referring to  FIG. 2F , the fifth process includes activating the lower surface of the upper substrate  115  so as to be easily attached to the lower substrate  101  by performing an O 2 -plasma process on the lower surface of the upper substrate  115 . In the O 2 -plasma process, O 2 -plasma particles are collided on the lower surface of the upper substrate  115 . Next, as shown in  FIG. 2G , the lower surface of the upper substrate  115  is adhered to the upper surface of the lower substrate  101  to be attached, and thus, the microfluidic chip  100  is formed. When the oxide layer  109  exposed on the attaching area  112  (refer to  FIG. 2E ) of the lower substrate  101  is adhered to the lower surface of the upper substrate  115  that is O 2 -plasma processed, the contact surfaces of the substrates  101  and  115  are attached to each other by a dehydration-condensation. 
       FIGS. 3A through 3E  are cross-sectional views sequentially illustrating a method of fabricating a microfluidic chip according to another embodiment of the present invention. The fabrication method shown in  FIGS. 3A through 3E  may include a first process of preparing a lower substrate  201  on which a channel  202  and a chamber  205  are formed, a second process of forming an upper substrate  215  formed of Si, a third process of forming an organic thin film  210  on an upper surface of the lower substrate  201 , a fourth process of removing the organic thin film  210  formed on an attaching area  212  of the upper surface of the lower substrate  201 , and a fifth process of attaching the upper substrate  215  and the lower substrate  201  to each other. The first and third processes are shown in  FIG. 3A , the fourth process is shown in  FIGS. 3B and 3C , and the second process and the fifth process are shown in  FIGS. 3D and 3E . 
     The first process and the third process are the same as the first and third processes for fabricating the microfluidic chip  100  described with reference to  FIGS. 2A and 2B , and detailed descriptions for the above processes are omitted. Reference numeral  207  of  FIG. 3A  denotes a pillar, and reference numeral  209  denotes an oxide layer including SiO 2 . The second process is the same as the second process for fabricating the microfluidic chip  100  described with reference to  FIG. 2F , and detailed descriptions of the above process are omitted. Reference numeral  216  of  FIG. 3D  denotes an inlet hole, and reference numeral  217  denotes an outlet hole. 
     The fourth process includes placing a flat photocatalyst plate  20  on the lower substrate  201  and irradiating UV rays onto the photocatalyst plate  20  (refer to  FIG. 3B ), and washing the lower substrate  201  to remove the organic thin film  210  from the attaching area  212  (refer to  FIG. 3C ). Referring to  FIG. 3B , the photocatalyst plate  20  includes a photocatalyst material. The photocatalyst material causes a reaction of decomposing the organic thin film  210  when it is exposed to UV rays when contacting the organic thin film  210 , for example, can be TiO 2 , ZnO, SnO 2 , SrTiO 3 , WO 3 , B2O 3 , or Fe 2 O 3 . Portions of the lower substrate  201 , which contact the photocatalyst plate  20 , coincide with the attaching area  212  (refer to  FIG. 3C ). 
     When the UV rays are irradiated onto the photocatalyst plate  20 , the photocatalyst plate  20  is exposed, and at the same time, some parts of the organic thin film  210  contacting the photocatalyst plate  20  are decomposed by the photocatalyst material. Referring to  FIG. 3C , when the photocatalyst plate  20  is separated from the lower substrate  201  after irradiating the UV rays, the attaching area  212  formed by removing the organic thin film  210  from the lower substrate  201  due to the decomposition operation of the photocatalyst material is exposed. 
     On the other hand, according to the method of removing the organic thin film  210  shown in  FIGS. 3B and 3C , since the decomposition of the organic thin film  210  is diffused on a peripheral portion of the contact area between the photocatalyst plate  20  and the organic thin film  210 , an error of the attaching area  212  may be larger than an error of the attaching area  112  that is formed by the method of removing the organic thin film  110  shown in  FIGS. 2C through 2E . Therefore, if a highly accurate microfluidic chip has to be fabricated, the microfluidic chip may be fabricated using the method shown in  FIGS. 2A through 2G . 
     The fifth process includes activating a lower surface of the upper substrate  215  by performing an O 2 -plasma process, in order to collide O 2 -plasma with the lower surface of the upper substrate  215 , as shown in  FIG. 3D , and attaching the lower surface of the upper substrate  215  onto the upper surface of the lower substrate  201  to form the microfluidic chip  200  as shown in  FIG. 3E . 
       FIG. 4  is a partially cut perspective view showing a microfluidic chip  300  according to another embodiment. 
     Referring to  FIG. 4 , the microfluidic chip  300  of the current embodiment also includes a lower substrate  301  and an upper substrate  315 , which are attached to each other. The lower substrate  301  is formed of Si, and includes a channel  302 , a chamber  305 , and a plurality of pillars  307  on an upper surface thereof. The pillars  307  are arranged to be separated from each other in the chamber  305 , and protrude from the upper surface of the lower substrate  301  so that they contact the lower surface of the upper substrate  315 . 
     The surface of the lower substrate  301  formed of Si is oxidized by the oxygen in the air, and thus, an oxide layer  309  including SiO 2  is formed. On the other hand, if the lower substrate  301  is formed of a polymer such as PDMS (polydimethylsiloxane), PMMA (polymethylmetaacrylate), PC (polycarbonate), and PE (polyethylene), an oxide layer including SiO 2  or TiO 2  or a nitride layer including SiN can be specifically formed. 
     A photocatalyst layer  311  including a photocatalyst material is deposited on the oxide layer  309 . The photocatalyst material can be TiO 2 , ZnO, SnO 2 , SrTiO 3 , WO 3 , B 2 O 3 , or Fe 2 O 3 . An organic thin film  310  is formed on the photocatalyst layer  311 . The organic thin film  310  is the same as the organic thin film  110  included in the microfluidic chip  100  of  FIG. 1 , and detailed descriptions for the organic thin film  310  are omitted. The organic thin film formed on an attaching area  312  (refer to  FIG. 5E ) on the upper surface of the lower substrate  301  is removed. 
     The upper substrate  315  is formed of PDMS (polydimethylsiloxane) that is a silicon resin. The upper substrate  315  includes an inlet hole  316  and an outlet hole  317 . 
       FIGS. 5A through 5G  are cross-sectional views illustrating a method of fabricating the microfluidic chip shown in  FIG. 4 . The method of fabricating the microfluidic chip  300  includes a first process of preparing the lower substrate  301  including the channel  302  and the chamber  305 , a second process of preparing the upper substrate  315  formed of Si, a third process of forming the organic thin film  310  on the upper surface of the lower substrate  301 , a fourth process of removing the organic thin film  310  formed on the attaching area  312  of the lower substrate  301 , and a fifth process of attaching the upper substrate  315  and the lower substrate  301  to each other. In addition, the method of the current embodiment can further include a process of forming a photocatalyst layer  311  on the upper surface of the lower substrate  301  before the third process. 
     Referring to  FIG. 5A , the first process includes preparing the lower substrate  301  formed of Si, and forming the channel  302 , the chamber  305 , and the plurality of pillars  307  on the upper surface of the lower substrate  301 . The first process is the same as the first process for fabricating the microfluidic chip  100  described with reference to  FIG. 2A , and thus, detailed descriptions for the first process are omitted here. In the second process, a mixed solution of the PDMS resin and the linking agent is injected into a mold (not shown) corresponding to the shape of the upper substrate  315 , and is cured and separated from the mold to form the upper substrate  315  (refer to  FIG. 5F ) including the PDMS. In addition, the inlet hole  316  and the outlet hole  317  can be formed in the upper substrate  315 . The second process is also the same as the second process for fabricating the microfluidic chip  100  described with reference to  FIG. 2F , and thus, detailed descriptions for the second process are omitted. 
     Referring to  FIG. 5B , the photocatalyst layer  311  including the photocatalyst material is formed on the upper surface of the lower substrate  315 . The photocatalyst layer  311  can be formed by spin-coating a solution including the photocatalyst material onto the lower substrate  301 , and then, baking the coated solution. Otherwise, the photocatalyst layer  311  can be formed using the CVD method or the PVD method. In the third process, the organic thin film  310  is formed on the photocatalyst layer  311 . The process of forming the organic thin film  310  is the same as the third process for fabricating the microfluidic chip  100  described with reference to  FIG. 2B , and thus, detailed descriptions for the process are omitted. 
     The fourth process includes forming a photo mask  30  (refer to  FIG. 5C ), arranging the photo mask  30  on the upper surface of the lower substrate and irradiating the UV rays onto the photo mask  30  (refer to  FIG. 5D ), and washing the lower substrate  301  to remove the organic thin film  310  from the attaching area  312  (refer to  FIG. 5E ). 
     Referring to  FIG. 5C , the photo mask  30  includes a flat transparent plate  31  and a photoresist layer  32  formed on the transparent plate  31 . The photoresist layer  32  includes a pattern  33  corresponding to portions of the lower substrate  301  from which the organic thin film  310  will be removed, that is, corresponding to the attaching area  312  (refer to  FIG. 5E ). A method of forming the photoresist is the same as the method described with reference to  FIG. 2C , and detailed descriptions for that are omitted. 
     Referring to  FIG. 5D , the photo mask  30  is arranged on the upper surface of the lower substrate  301  so that the pattern  33  of the photoresist layer  32  can overlap with the attaching area  312  (refer to  FIG. 5F ), and the UV ray is irradiated on the photo mask  30 . When the UV rays are irradiated onto the photo mask  30 , the organic thin film  310  and the photocatalyst layer  309  under the organic thin film  310  are partially exposed to the UV ray through the pattern  33 . Therefore, some parts of the organic thin film  310 , which contact the photocatalyst layer  309  and are exposed to the UV ray, are decomposed by the photocatalyst material. 
     Referring to  FIG. 5E , when the photo mask  30  is separated from the lower substrate  301  after irradiating the UV rays to the photo mask  30 , the attaching area  312  that is formed by removing the organic thin film  310  from the lower substrate  301  due to the decomposition of the photocatalyst material is exposed. In the fifth process, an O 2 -plasma process, by which O 2 -plasma is collided with the lower surface of the upper substrate  315  formed in the second process, is performed to activate the lower surface of the upper substrate  315  as shown in  FIG. 5F , and then, the lower surface of the upper substrate  315  is adhered to the upper surface of the lower substrate  301  to be attached to the lower substrate. Thus, the microfluidic chip  300  is formed as shown in  FIG. 5G . 
       FIG. 6  is a partially cut perspective view showing a microfluidic chip according to another embodiment. 
     Referring to  FIG. 6 , the microfluidic chip  400  according to the current embodiment also includes a lower substrate  401  and an upper substrate  415 , which are attached to each other. The lower substrate  401  includes a photocatalyst material, and the photocatalyst material may be TiO 2 . A channel  402 , a chamber  405 , and a plurality of pillars  407  are formed on an upper surface of the lower substrate  401 . As described with reference to  FIG. 2A , the channel  402 , the chamber  405 , and the pillars  407  can be formed using an etching process or a machining process. 
     Since TiO 2  is an oxide material that can help the attachment between the upper substrate  415  and the lower substrate  401 , the lower substrate  401  does not require an additional oxide layer like the oxide layer  109  shown in  FIG. 1 . An organic thin film  410  is formed on the upper surface of the lower substrate  401 . The organic thin film  410  can be formed using the same process for forming the organic thin film  110  described with reference to  FIG. 2B , and thus, detailed descriptions for the process of forming the organic thin film  410  are omitted. The organic thin film  410  formed on an attaching area  415  of the upper surface of the lower substrate, which is attached to the upper substrate  415 , is removed. The method of removing the organic thin film  410  is the same as the method described with reference to  FIGS. 5C through 5E , that is, a photo mask including a flat transparent plate and a photoresist layer formed on the transparent plate is arranged on the lower substrate  401  and the UV ray is irradiated to the photo mask to partially decompose the organic thin film. 
     The upper substrate  415  is formed of a silicon resin, for example, PDMS (polydimethylsiloxane), and includes an inlet hole  416  and an outlet hole  417 . As described with reference to  FIGS. 5F and 5G , the lower surface of the upper substrate  415  is activated to be easily attached by performing the O 2 -plasma process, and then, the lower surface of the upper substrate  415  is adhered to the upper surface of the lower substrate  401  to attach the upper and lower substrates  415  and  401  to each other. Then, the microfluidic chip  400  is formed. 
     On the other hand, cell capture experiments and polymerase chain reaction (PCR) experiments were performed using the microfluidic chip  100  of the present invention and the conventional microfluidic chip having the lower substrate formed of Si and the upper substrate formed of a glass material, and the results of the experiments were compared. Since equivalent results were obtained within an acceptable error range, and thus, it could be determined that the microfluidic chip  100  can be used instead of the conventional microfluidic chip in microfluidics. 
     The microfluidic chip, in which the organic thin film is formed on the inner surfaces of the chamber, can be fabricated using silicon resin that can be easily formed and is cheaper than the glass material. Therefore, the costs for fabricating the microfluidic chip can be reduced, and a defect rate can be reduced and a production yield can be improved by generating the organic thin film before the bonding process. 
     According to another embodiment, a microfluidic structure includes a first substrate, a second substrate, and a polysiloxane layer disposed between the first and second substrates, wherein the polysiloxane layer is coupled to the first and second substrates via a SiO 2  layer. 
     The SiO 2  layer may be deposited on the first and second substrates. The first and second substrates may be formed of a solid support, for example, a material selected from the group consisting of plastic, silicon, and glass. The plastic may have a hydrophilic or a hydrophobic surface, for example, may be one of selected from the group consisting of polyethylene, polypropylene, polystyrene, polyurethane, polysulfone, PTFE, PVC, polycarbonate, and PMMA, however, the embodiments of the present invention are not limited thereto. 
     One or more of the first and second substrates may include a micro-structure. The micro-structure may not be in micro-meter level, but may have a small structure. For example, at least one cross-section of the micro-structure, that is, a diameter, an width, and a height of the micro-structure may be in ranges of about 10 nm to about 1000 mm, from about 10 nm to about 100 mm, or from about 10 nm to about 10 mm. The micro-structure may provide the fluid with a path. For example, the micro-structure may be selected from the group consisting of a channel, a chamber, an inlet, and an outlet. A part of the micro-structure may be formed on the surface or inner space of the substrate, or on the surface and in the inner space of the substrate. 
     The microfluidic structure includes a polysiloxane layer disposed between the first substrate and the second substrate, and the polysiloxane layer is coupled to the first and second substrates via the SiO 2  layer. 
     The polysiloxane may be one of PDMS and diphenylsiloxane. 
     The polysiloxane layer may be formed as a film. The film may have a thickness of about 10 to about 500 μm, or about 100 to about 300 μm. 
     The polysiloxane layer may be coupled to entire surfaces of the first and second substrates. The polysiloxane layer may be simple membrane without including the micro-structure. Otherwise, the polysiloxane layer may be coupled to a part of the surfaces of the first and second substrates. 
     The SiO 2  layer is strongly adhered to the polysiloxane. Therefore, the SiO 2  layer may be deposited onto the first and second substrates and fixed on the substrates, and then, may be adhered to the polysiloxane layer. The deposition of SiO 2  layer onto the first and second substrates may be performed using a method selected from the group consisting of the liquid phase deposition, evaporation, and sputtering method. 
     The microfluidic structure is a device including one or more micro-structures. The micro-structure is described above. The microfluidic structure may be a microfluidic apparatus, an inlet and an outlet of which are connected to each other through one or more channels. The microfluidic apparatus may further include an additional structure of a valve, a pump, or a chamber. 
     The microfluidic structure includes the first substrate having a surface on which a pneumatic channel is formed, and the second substrate having a surface on which a fluid channel is formed. In addition, the polysiloxane layer is disposed between the above surfaces of the first and second substrates so that the polysiloxane layer is deflected to control the flow of the fluid in the fluid channel when a pressure or vacuum is applied to the pneumatic channel. The polysiloxane layer generally blocks the flow of the fluid in the fluid channel, and when a pressure or vacuum is applied to the pneumatic channel, the polysiloxane layer may be deflected to flow the fluid in the fluid channel. The microfluidic structure may further include an additional surface and a layer. The additional surface may be an additional channel for providing the fluid with a flow path. The second substrate may include a plurality of bias channels for providing the fluid with flow paths. The microfluidic structure may include a plurality of valves realized by the polysiloxane layer, which are disposed as parts of the pumps. 
     The microfluidic structure may include the first substrate having a surface on which a pneumatic channel is formed, and the second substrate having a surface on which a fluid channel is formed. In addition, the polysiloxane layer is disposed between the above surfaces of the first and second substrates so that the polysiloxane layer is deflected to activate a plurality of valves which may be switched pneumatically when a pressure or vacuum is applied to the pneumatic channel. In addition, the valves which may be pneumatically switched may control the flow of the fluid in the microfluidic apparatus. Here, the first substrate may include a plurality of etched channels, and the etched channels may distribute the pressure applied to the polysiloxane layer. In the microfluidic structure, three successive valves which may be pneumatically switched may form a pump. The three valves may include an input valve, a diaphragm valve, and an output valve. 
     According to another embodiment of the present invention, a method of fabricating a microfluidic structure includes providing a first substrate and a second substrate, on which micro-structures are formed, depositing a SiO 2  layer on surfaces of the first and second substrates, and coupling the first substrate and the second substrate by interposing polysiloxane between the surfaces of the first and second substrates on which SiO 2  layer is deposited. 
     The above method includes an operation of providing the first and second substrates on which micro-structures are formed. The micro-structures on the first and second substrates may be formed of a well-known method, for example, an injection molding, a photolithography, or a LIGA method. 
     The substrates may be formed of a solid support, for example, one selected from the group consisting of plastic, silicon, and glass. The plastic may include a hydrophilic surface or a hydrophobic surface, for example, may be one of selected from the group consisting of polyethylene, polypropylene, polystyrene, polyurethane, polysulfone, PTFE, PVC, polycarbonate, and PMMA. 
     The micro-structure may not be in micro-meter level, but may have a small structure. For example, at least one cross-section of the micro-structure, that is, a diameter, an width, and a height of the micro-structure may be in ranges of about 10 nm to about 1000 mm, from about 10 nm to about 100 mm, or from about 10 nm to about 10 mm. The micro-structure may provide the fluid with a path. For example, the micro-structure may be selected from the group consisting of a channel, a chamber, an inlet, and an outlet. A part of the micro-structure may be formed on the surface or inner space of the substrate, or on the surface and in the inner space of the substrate. 
     The above method also includes an operation of depositing SiO 2  on the surfaces of the first and second substrates. The depositing process may be performed by the method selected from the group consisting of the liquid phase deposition (LPD), evaporation method, sputtering method, and chemical vapor deposition (CVD) method. The LPD method includes an operation of forming hydrofluosilicic acid aqueous solution which is saturated at a room temperature by dissolving silicon dioxide powder in an aqueous solution including 34% of hydrofluosilicic acid (H 2 SiF 6 ). The silicon dioxide powder which is not dissolved in the solution may be removed from the aqueous solution of hydrofluosilicic acid by using a filter paper. The saturated hydrofluosilicic acid solution may be changed into a supersaturated solution by adding water, boric acid aqueous solution, or ammonium hydroxide in the saturated aqueous solution. In addition, the substrates are dipped into the supersaturated solution to grow a silicon dioxide film on the surfaces of the substrates. The deposition may be performed in a temperature range of about 10° C. to about 50° C. The above deposition is a method of depositing the silicon dioxide film on the plastic substrates in a previously evacuated chamber by a glow discharge, and includes forming an air flow on outer portion of the chamber by evaporating organic silicon component and mixing the evaporated organic silicon component with an oxidizing agent and an inert gas; flowing the air flow to the chamber to be adjustable; establishing glow discharge plasma in the chamber from the air flow; flowing the air flow in the plasma to be adjustable while locking a part of the plasma therein; depositing a first coating of silicon dioxide on the substrates; removing and/or redistributing external surface particles from the substrates; and repeatedly performing the above operations to deposit a second coating of silicon dioxide on the substrates. The oxidizing agent may be oxygen. The organic silicon may be selected from the group consisting of 1,1,3,3-tetramethyldisiloxane, hexamethyldisiloxane, vinyltrimethylsilane, methyltrimethoxysilane, vinyltrimethoxysilane, and hexamethyldisilazane. However, the above method of depositing SiO 2  layer is an example, and other well known deposition methods may be used in one or more embodiments of the present invention. The operation of depositing SiO 2  layer may be performed before providing the substrates. That is, before forming the micro-structures on the substrates, the SiO 2  layer may be deposited, and then, the micro-structures may be formed. 
     The above method also includes an operation of coupling the first and second substrates to each other by interposing the polysiloxane between the surfaces, on which the SiO 2  layer is deposited, of the first and second substrates. 
     The operation of coupling the first and second substrates may include arranging the surface of the first substrate, the polysiloxane layer, and the surface of the second substrate to correspond to each other, and coupling them by compressing the first and second substrates. 
     The polysiloxane may be selected from the group consisting of PDMS and diphenylsiloxane. 
     The polysiloxane layer may be formed as a film. For example, the polysiloxane layer may have a thickness of about 10 μm to about 500 μm, or about 100 μl in to about 300 μm. 
     The polysiloxane layer may be coupled to entire surfaces of the first and second substrates. That is, the polysiloxane layer may not include the micro-structure. Otherwise, the polysiloxane layer may be coupled to a part of the surfaces of the first and second substrates. 
     The microfluidic structure is a device including one or more micro-structures. The micro-structure is described above. The microfluidic structure may be a microfluidic apparatus, an inlet and an outlet of which are connected to each other through one or more channels. The microfluidic apparatus may further include an additional structure of a valve, a pump, or a chamber. 
     The microfluidic structure includes the first substrate having a surface on which a pneumatic channel is formed, and the second substrate having a surface on which a fluid channel is formed. In addition, the polysiloxane layer is disposed between the above surfaces of the first and second substrates so that the polysiloxane layer is deflected to control the flow of the fluid in the fluid channel when a pressure or vacuum is applied to the pneumatic channel. The polysiloxane layer generally blocks the flow of the fluid in the fluid channel, and when a pressure or vacuum is applied to the pneumatic channel, the polysiloxane layer may be deflected to flow the fluid in the fluid channel. The microfluidic structure may further include an additional surface and a layer. The additional surface may be an additional channel for providing the fluid with a flow path. The second substrate may include a plurality of bias channels for providing the fluid with flow paths. The microfluidic structure may include a plurality of valves realized by the polysiloxane layer, which are disposed as parts of the pumps. 
     The microfluidic structure may include the first substrate having a surface on which a pneumatic channel is formed, and the second substrate having a surface on which a fluid channel is formed. In addition, the polysiloxane layer is disposed between the above surfaces of the first and second substrates so that the polysiloxane layer is deflected to activate a plurality of valves which may be switched pneumatically when a pressure or vacuum is applied to the pneumatic channel. In addition, the valves which may be pneumatically switched may control the flow of the fluid in the microfluidic apparatus. Here, the first substrate may include a plurality of etched channels, and the etched channels may distribute the pressure applied to the polysiloxane layer. In the microfluidic structure, three successive valves which may be pneumatically switched may form a pump. The three valves may include an input valve, a diaphragm valve, and an output valve. 
     Hereinafter, one or more embodiments will be described in more detail. However, one or more embodiments of the present invention are exemplary embodiments, and the scope of the invention is not limited thereto. 
       FIGS. 7A and 7B  are diagrams showing an example of the microfluidic structure according to an embodiment.  FIG. 7A  is a side view of the microfluidic structure and  FIG. 7B  is an exploded view of the structure shown in  FIG. 7A . Referring to  FIGS. 7A and 7B , the microfluidic structure  1000  includes a first substrate  2000 , a second substrate  3000 , and a polysiloxane layer  4000  disposed between the first and second substrates  2000  and  3000 . The polysiloxane layer  4000  is coupled to the first and second substrates  2000  and  3000  via a SiO 2  layer  5000 . The first and second substrates  2000  and  3000  include micro-structures, for example, channels, on surfaces thereof. The SiO 2  layer  5000  is deposited on the surfaces of the first and second substrates  2000  and  3000 . The first and second substrates  2000  and  3000  are coupled to each other while interposing the polysiloxane layer  4000  to form the microfluidic structure  1000 . The polysiloxane layer  4000  may be formed of PDMS or diphenylsiloxane. 
     Referring to  FIG. 7B , the SiO 2  layer  5000  is deposited on the micro-structure, that is, deposited after forming the micro-structure. However, the SiO 2  layer  5000  may be deposited before forming the micro-structures or during forming the micro-structures so that the SiO 2  layer  5000  may not be deposited on portions corresponding to the micro-structures. 
       FIG. 8  is a diagram illustrating a method of fabricating the microfluidic structure according to an embodiment of the present invention. 
     Referring to  FIG. 8 , the first and second substrates  2000  and  3000  are provided. The micro-structures may be formed on the first and second substrates  2000  and  3000 , and the micro-structures may be fabricated by the injection molding method or the photolithography method. The first and second substrates  2000  and  3000  are formed of plastic, and the micro-structures may be fabricated by the injection molding. In addition, the SiO 2  layer  5000  is deposited on the surfaces, on which the micro-structures are formed, of the first and second substrates  2000  and  3000 . The deposition may be performed by the liquid phase deposition, the evaporation, and the sputtering. Next, the polysiloxane layer  4000  is arranged between the surfaces on which the SiO 2  layer  5000  is deposited, and then, the first and second substrates  2000  and  3000  are compressed to be coupled to each other so as to fabricate the microfluidic structure. 
     In the microfluidic structure fabricated by the method illustrated in  FIG. 8 , the micro-structures may be a pneumatic channel  2400  and a pneumatic valve  2200  formed on the first substrate  2000 , and a fluid channel  3400  and a fluid valve  3200  formed on the second substrate  3000 . The first and second substrates  2000  and  3000  are arranged and coupled to each other in a way to render the pneumatic valve  2200 , the polysiloxane layer  4000 , and the fluid valve  3200  may perform as a diaphragm valve or a pump. The micro-structures performing as the pump or the valve will be described with reference to  FIG. 9 . 
       FIGS. 9A through 9C  are diagrams of a microfluidic structure according to another embodiment.  FIGS. 9A through 9C  show a film valve which may be installed in the microfluidic structure.  FIG. 9A  is a plan view of the film valve, and  FIGS. 9B and 9C  are side views showing the film valve in a closed state and in an open state. The microfluidic structure includes the polysiloxane layer disposed between two plastic substrates  2000  and  3000 . The polysiloxane layer may be HT-6135 or HT-6240 having a thickness of about 254 μm manufactured by Bisco, Inc. The polysiloxane layer is strongly coupled to the surfaces of the two substrate on which SiO 2  layer is deposited. The fluid channel  3400  is used to convey the fluid. The pneumatic channel  2400  and the valve region  2200  are etched to convey air or other fluids in order to activate the valve under a pressure or a vacuum status. In general, the pneumatic channels  2400  and  2200  are located in a substrate  2000  (referred to as pneumatic substrate), and the fluid channel  3400  is located in the other substrate  3000  (referred to as fluid substrate). The pneumatic substrate may include a port providing the pneumatic channel with a pressure or vacuum. 
     The valve illustrated in  FIGS. 9A through 9C  is operated as follows. Activated vacuum is provided to the valve region  2200  through a pneumatic channel  2400 . The applied vacuum deflects the polysiloxane layer  4000  at the portion which locates at or around the valve region  2200 , toward a portion which is apart from the discontinued point  3410  of the fluid channel to provide a path through which the fluid may flow. Therefore, the valve is opened as shown in  FIG. 9C . The surface of the discontinued point ( 3410 ) may not contain SiO 2  layer so that the polysiloxane layer  4000  may contact with the surface of the discontinued point ( 3410 ) to block the flow of fluid in the fluid channel when a pressure or a vacuum pressure is not applied to the pneumatic channel, and to be deflected to control a flow of the fluid in the fluid channel when a pressure or a vacuum pressure is applied to the pneumatic channel. The valve which may be opened and closed by using the pneumatic pressure is referred to as a switchable valve or a pneumatically switchable valve. When the pressure or the vacuum is not applied to the polysiloxane layer  4000 , the polysiloxane layer  4000  blocks the fluid channel as shown in  FIG. 9B . 
     The film valve may form various modes of controlling the fluid.  FIGS. 10A and 10B  are diagrams of a pump formed by using the film valve.  FIGS. 10A and 10B  are a plan view and a side view of a film pump. Referring to  FIGS. 10A and 10B , three successive film valves form a diaphragm pump  6000 . Pumping operation is performed by activating the valves according to five-periods. The diaphragm pump  6000  includes an input valve  2000 , a diaphragm valve  6000 ′, and an output valve  2200 ″. Since the diaphragm pump  6000  may operate in any direction, the input valve  2200 ′ and the output valve  2200 ″ may be changed with each other. The diaphragm pump  6000  includes the fluid substrate  3000  including the etched fluid channel  3400 , the polysiloxane layer  4000 , and the pneumatic substrate  2000 . The polysiloxane layer  4000  is coupled to the fluid substrate  3000  and the pneumatic substrate  2000  via the SiO 2  layer. The pumping operation may be performed in series of processes. The output valve  2200 ″ is closed, and the input valve  2200 ′ is opened. Then, the diaphragm valve  6000 ′ is opened. In addition, the input valve  2200 ′ is closed. After that, the output valve  2200 ″ is opened. Also, the diaphragm valve  6000 ′ is closed, and the fluid is pumped through the opened output valve  2200 ″. The valve may perform as a pump, a mixer, or a router. 
     According to the microfluidic structure of one or more embodiments, the microfluidic structure may be fabricated in a simple way. Therefore, the microfluidic structure including various micro-structures on the substrates of various materials may be fabricated efficiently. 
     According to the method of fabricating the microfluidic structure of one or more embodiments, the microfluidic structure may be fabricated effectively and easily. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, 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 invention as defined by the following claims.