Abstract:
A method of forming nanofluidic enclosed channels includes providing a first substrate having a layer of a first material disposed thereon. A plurality of nanoscale slots is formed along a second substrate using nanolithography, etching, or other disclosed techniques. The first substrate is then bonded to the second substrate such that the layer of the first material on the first substrate is adjacent the plurality of slots on the second substrate to define a plurality of enclosed nanofluidic channels therethrough.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/379,878, filed on May 13, 2002. The disclosure of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention generally relates to the construction of fluidic channels and, more particularly, relates to a simple and convenient method of constructing nanofluidic channels of various types. The method of the present invention is particularly useful in the manipulation and detection of bimolecular (especially DNA molecules) using nanostructured fluidic channels. 
   BACKGROUND OF THE INVENTION 
   Nano-scale fluidics (hereinafter “nanofluidic”) is an emerging field of study that has significant technological advantages. For example, the interaction of biomolecules (such as DNA) with nanostructured channels having dimensions close to the persistence length of a molecule (˜50 nm) permits an entirely new way of detecting and separating molecules. In fact, the unique fluid behavior at nanoscale dimension promises many new applications, assuming fabrication of nanofluidic channels can be simplified and made more cost effective. 
   Despite the relative ease of constructing nanoscale structures, the sealing of these nanoscale structures into functional nanofluidic channel devices often leads to many technological challenges. For instance, known methods of constructing sealed “micron-scale” fluidic channels typically include anodic bonding of a glass coverslip or soft elastomeric material to prefabricated channels on a substrate. The high temperature and high voltage typically used in the anodic bonding process greatly limit the process to commercial applications; while the bonding of soft elastomeric material, such as PDMS, to nanofluidic channels being about 100 nm or less in size often results in the partial or complete filling of the channel due to the rubber-like behavior of the soft elastomeric material. 
   As is known, sacrificial layer etching can also be used to form nanofluidic channels. However, the removal of this sacrificial layer in nano-channels is non-trivial. In fact, via holes are often necessary to reduce the time needed to remove the sacrificial layer to a reasonable duration, which consequently increases the device complexity and fabrication cost. Recent progress using non-uniform depositions, such as e-beam evaporation and sputtering, provides a flexible solution to this issue. Still this involves deposition machines and is a time-consuming and complex process that requires careful control of the non-uniformity during the deposition process. 
   Accordingly, there exists a need in the relevant art to provide a simple, convenient, and cost effective method of manufacturing nanofluidic channels. Furthermore, there exists a need in the relevant art to provide a simple, convenient, and cost effective method of fabricating nanofluidic channels having dimensions down to approximately tens of nanometers capable to being used in low-cost and high-volume manufacturing. Still further, there exists a need in the relevant art to overcome the disadvantages of the prior art. 
   SUMMARY OF THE INVENTION 
   It is therefore the object of this invention to provide a simple method of fabricating nanofluidic channels with dimensions down to tens of nanometers, which facilitates low-cost and high-volume manufacturing of nanofluidic channels for a wide range of applications. 
   According to the principles of the present invention, an advantageous method of forming nanofluidic enclosed channels is provided. The method includes providing a first substrate having a layer of a first material disposed thereon. A plurality of nanoscale slots is formed along a second substrate using nanolithography, etching, or other disclosed techniques. The first substrate is then bonded to the second substrate such that the layer of the first material on the first substrate is adjacent the plurality of slots on the second substrate to define a plurality of enclosed nanofluidic channels therethrough. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIG. 1  is a series of perspective views illustrating the method of fabricating nanofluidic channels according to the principles of the present invention; 
       FIGS. 2(   a )–( b ) is a series of photographs illustrating nanofluiclic channels fabricated by a polymer bonding technique representing a nanoscale depth according to the principles of the present invention; 
       FIG. 3  is a photograph illustrating application of the present invention in connection with DNA stretching; 
       FIGS. 4(   a )–( d ) is a series of perspective views illustrating the method of fabricating nanofluidic channels with integrated artificial gel structure according to the principles of the present invention; 
       FIGS. 5(   a )–( b ) is a series of perspective views illustrating the method of fabricating micro- and nanofluidic channels according to the principles of the present invention; 
       FIGS. 6(   a )–( b ) is a series of perspective views illustrating the method of fabricating all-polymer nanofluidic channels using a stamping method according to the principles of the present invention; 
       FIGS. 7(   a )–( b ) is a series of perspective views illustrating the method of fabricating nanofluidic channels and an optical slit according to the principles of the present invention; and 
       FIGS. 8(   a )–( c ) is a series of photographs, with portions shown schematically, illustrating nanofluidic channels having Si “grass” according to the principles of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
   According to the teachings of the present invention, a method is provided that includes directly forming fluidic channels by a 2-step nanoimprinting technique. With particular reference to  FIG. 1 , which illustrates a schematic of the preferred method, a nano-channel template  10  with nanoscale trenches  12  is fabricated by a first nanoimprinting and reactive ion etching (RIE) process in oxide. Then nano-channel template  10  is used as a mold (preferably without surfactant coating) to imprint into a thin polymer layer  14  on a substrate  16  through a second nanoimprinting process. A thickness of polymer layer  14  is carefully chosen such that polymer layer  14  will not completely fill nanoscale trenches  12  of nano-channel template  10 . The portions of nanoscale trenches  12  of nano-channel template  10  that remain unfilled by polymer layer  14  define sealed channels  18 . The lateral width dimension of sealed channels  18  is determined by the width of nanoscale trenches  12  of nano-channel template  10 , while the height of sealed channels  18  is determined by the initial thickness of polymer layer (PMMA)  14  and/or by the initial depth of nanoscale trenches  12 . By controlling the initial thickness of polymer layer  14  or the initial depth of nanoscale trenches  12 , it is possible to accurately control the height of sealed channels  18 , as illustrated in  FIGS. 2(   a )–( b ). That is, in the current example, the width of sealed channels  18  is fixed at 350 nm. It should be understood, however, that the width of sealed channels  18  might be made to smaller dimensions rather straightforwardly. By choosing different initial widths and depths for nanoscale trenches  12 , the volume of sealed channels  18  may be easily varied. 
   In this illustration, polymer layer  14  is made of PMMA, which is a thermal plastic material that possesses good optical properties and low auto-fluorescence level. Low auto-flourescence levels are particularly desirable for bimolecular detection using fluorescent labeling techniques. However, it should also be understood that the method of the present invention should not be regarded as being limited to the use of either PMMA or thermalplastic polymer for fluidic channel sealing. A wide range of materials, such as UV-curable, heat-curable polymers, sol-gel materials, and the like may be applied. With certain materials, the sealing of sealed channels  18  can be achieved at room temperature. Coating of the film onto the flat substrate can also be done by techniques other than spin coating, such as dip coating, and possibly spray coating depending on the properties of the material being used. 
   The fabrication of nano-fluidic channels by direct nanoimprinting techniques has many advantages. For example, nanoimprinting is generally regarded as being a relatively simple and cost effective process. The channels are sealed in one single nanoimprinting process. Additionally, the surface properties may be tailored by selecting different polymer materials or function. Furthermore, the complexity of fabricating the fluidic channels does not vary substantially as the size of the nanochannel changes. Therefore, in principle, the present method is equally applicable to the fabrication of larger-scale fluidic channels. 
   It is also possible to create a thick protection layer on top of the self-sealed nanofluidic channels by applying a second layer of coating that has solvent compatibility with the material used for sealing the channels. This technique also allows direct integration of optical element, such as near-field apertures on top of the nanofluidic channels, and optical waveguide to the side of the channel for optical excitations. 
   The methods described in the present application may be used in a wide variety of advantageous applications. For example, straight nanofluidic channels may be used in DNA stretching and, when combined with an integrated near-field scanner, they can be used for ultrahigh spatial resolution dynamic mapping of long chain polymers. An image of such application is provided in  FIG. 3 . Fluidic channels with dense arrays of nanoscale posts may also serve as artificial gels, which are commonly used in DNA electrophoresis. Such structures can be easily fabricated using a similar method as described above and a schematic of the fabrication process is illustrated in  FIG. 4 . 
   Briefly, with reference to  FIG. 4 , a channel  50  and nanoscale posts  52  may be formed and patterned inside a portion of a substrate  56  by nanolithography—such as electron beam lithography, deep UV lithography, or nanoimprinting technique—and reactive ion etching. Subsequently, a second substrate  58  is coated with a polymer  60  and is brought into contact and pressed against substrate  56  having fabricated channel  50  under elevated temperature. Channels  50  are sealed and good adhesion of polymer  60  to channel  50  is enhanced by the presence of the dense array of nanoscale posts  52  that act as artificial gels or entropic barriers for DNA strands. Substrate  58  may also be removed from the assembly when necessary if substrate  58  is made to have low surface energy. 
   In addition, electro-osmotic flow in nanofluidic channels can be used to create large pressure difference that is difficult to achieve with other techniques; this is very useful for fluidic pumping applications. In addition, nanofluidic channels are a new emerging area where many potential applications can be exploited. A simple and low cost fabrication technique will undoubtedly speed up such explorations. Further extension of this technique can create vertically integrated nanofluidic channels, and even more complex fluidic matrix. 
   Referring now to  FIGS. 5(   a )–( b ), a schematic is provided that illustrates a simple technique of creating micro- and nanofluidic channels with device input and output according to the principles of the present invention. First, fluidic channels  100 , having both micro-dimensions  102  and nanoscale dimensions  104 , are defined on a substrate  106  by standard lithography and etching processes. A standard microscope coverslip  108  is also prepared that includes an inlet/outlet hole  110  etched through its entire thickness. A thin layer of polymer  112  (or other materials suitable for bonding and compatible with bimolecular detection) is uniformly coated over coverslip  108 , by a conventional process such as spin coating. Coverslip  108  is flipped and brought on top of fluidic channels  100  and substrate  106 . As seen in  FIG. 5(   b ), under suitable temperature and pressure, a firm bond is formed between coverslip  108  and substrate  106 , which at the same time seals fluidic channels  100 . During this process, polymer material  112  will be displaced according to the pattern of fluidic channels  100 . Similar as above, the height of nanofluidic channels  100  is determined by the initial etched channel depth and the amount of polymer  112  being squeezed into channels  100 . Rigid coverslip  108  or other flat hard surface guarantees that no sagging will occur during nanochannel formation. It is anticipated that additional or auxiliary channels (not shown) may be positioned on sides of fluidic channels  100  to aid in the controlled displacement of polymer  112  during the bonding process. In the wide channel regions  102 , auxiliary posts (not shown but similar to above) may be used to ensure that coverslip  108  does not bend and/or sag to block fluidic channels  100 . Coverslip  108  preferably has a thickness of about 175 μm, which is compatible with many experimental setups using optical microscopes. 
   Alternatively, as seen in  FIGS. 6(   a )–( b ), fluidic channels  100  on substrate  106  may be “stamped” out directly by using an imprinting technique using a template  118  and a substrate  120  having a first polymer layer  122 . An additional polymer layer  124  and substrate  126  is then used to form an all-polymer fluidic channel  100 ′. The advantage of forming all-polymer fluid channel  100 ′ is to be able to easily change its surface properties by surface chemical modification. 
   Referring now to  FIGS. 7(   a )–( b ), a simple approach of forming an (or an array of) optical near field slits above fluidic channels is illustrated. In this case, a coverslip  200  is first coated with metal  202  and patterned using Electron-beam lithography or nanoimprinting and etching, or alternatively by a lift-off process. Next, a polymer layer  204  is coated over metal layer  202  that has nanoscale opening slits  206 . Subsequently, one may then employ the method described above in connection with  FIG. 6  to form sealed fluidic channels  208 . Laser beam incident from above metal nano-slits  206  will be attenuated and spatially localized due to the near-field effect, which can be used to provide highly localized excitations for fluorescent detection. 
     FIGS. 8(   a )–( c ) illustrate an alternative method to implement artificial gel structure that may be used in DNA electrophoresis. In this method, instead of patterning and etching posts with nanoscale separations as described above, a specially developed reactive ion etching process is used to create Si “grass”  300  in a trench region  302 . The grass density can be controlled by the etching parameter as can be seen in  FIGS. 8(   b ) and ( c )). The position and separation between Si pillars  300  are random, mimicking a gel pore structure. Si grass  300  and channel  302  can be oxidized to facilitate electrophoresis process. To complete the fluidic channel, polymer material is used to provide the sealing from above as described above. Due to the soft nature of the polymer at temperatures above Tg, the tips of Si pillars  300  will penetrate into the polymer, which ensures that in DNA electrophoresis applications the biomolecules can only flow in between the narrow spacings between Si posts  300  at the bottom part of the channels. 
   Accordingly to the principles set forth above, the fabrication of nano-fluidic channels by direct nanoimprinting technique has many advantages. It is a simple and low-cost process. The channels are sealed in one single nanoimprinting process. The surface properties may be tailored by selecting different polymer materials or function. The complexity of the fluidic channel fabrication does not scale inversely to the size of the nano-channel. In principle, the present invention is also equally applicable to the fabrication of larger-scale fluidic channels. 
   The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.