Abstract:
A method of fabricating nanosized holes with controlled geometries employs tools and methods developed in the microelectronics industry. The method exploits the fact that epitaxially grown film thicknesses can be controlled within a few atomic monolayers and that by using etching techniques, trenches and channels can be created that are only a few nanometers wide. The method involves bonding two shallow channels at an angle such that a nanopore is defined by the intersection. Thus, a nanopore-defining device includes a nanopore with dimensions that are determined by the dimensions and orientations of the intersecting channels, with the dimensions being accurately controlled within a few monolayers.

Description:
TECHNICAL FIELD  
         [0001]    The invention relates generally to molecular detection and characterization and more particularly to fabricating reproducible, single-molecule nanopores having controlled geometric properties.  
         BACKGROUND ART  
         [0002]    Devices and methods for detecting the passage of a single macromolecule through a nanopore are known. For example, U.S. Pat. No. 5,795,782 to Church et al. describes a nanopore that is created by the insertion of a bacterial pore-forming protein (α-hemolysin) in a lipid membrane. Since protein geometry and physiochemical characteristics are genetically determined, the pore-forming protein is able to form nanopores having a predetermined geometry.  
           [0003]    One concern with the prior art techniques is that the process of verifying the existence and proper formation of each nanopore fabricated by using bacterial pore-forming proteins is slow and potentially expensive. Another concern is that because the lipid membrane in which the nanopores are made degrades with time, the resulting nanopores cannot be mass produced for use over an extended period of time.  
           [0004]    What is needed is a method for efficiently and consistently forming nanopores with controlled geometries such that the resulting nanopores can be formed in controlled arrays and are capable of being stored for extended periods of time without degrading. What is further needed is such a method that enables precise positioning of the nanopores.  
         SUMMARY OF THE INVENTION  
         [0005]    Integrated circuit (IC) fabrication techniques are employed to form precisely dimensioned and positioned nanopores. Film-thickness control within a few atomic monolayers can be achieved by a variety of IC fabrication techniques, including photolithography, epitaxial growth and plasma enhanced chemical vapor deposition (PECVD).  
           [0006]    A first embodiment of a method of forming nanopores uses a pair of three-layer segments. Each such multi-layer segment includes a substrate layer, such as a silicon substrate, and includes an intermediate layer and a top layer that are grown or deposited to predetermined and uniform thicknesses. The chemical compositions of the three layers are selected such that the substrate layer and top layer are selectively etchable with respect to the intermediate layer, while the intermediate layer is selectively etchable with respect to the substrate and top layers. In alternate embodiments, each layer is selectively etchable with respect to the other layers. Furthermore, it is contemplated that more than three layers are used to form the segments, with one or more of the layers being selectively etchable with respect the other layers.  
           [0007]    The top layer of each three-layer segment is patterned so that at least one supply conduit extends through the top layer to the intermediate layer. Next, the intermediate layer of each segment is patterned using a wet etch process to remove a portion of the intermediate layer. This wet etchant is introduced through the supply conduit and the process is allowed to proceed sufficiently long to form an “undercut” in the intermediate layer, with the undercut extending beyond the boundaries of the supply conduit through the top layer.  
           [0008]    Following the selective patterning of the top layer and the intermediate layer, excess matter is removed from the edges of each three-layer segment, so that at least one edge of the segment is smooth and is located at a controlled distance from the supply conduit. The smoothed edge is then masked with a photoresist, while leaving a controlled width of the intermediate layer exposed. A narrow slot in the intermediate layer along the smoothed edge of the segment is created using conventional photolithography and etching techniques. After etching, a second selective etch process is conducted to form a path in the intermediate layer from the undercut to the slot. Thus, the completed path within each segment extends from the supply conduit through the intermediate layer to the smooth edge of the segment.  
           [0009]    To create a single nanopore, the slots of the pair of three-layer segments are abutted, with the axes through the slots being coaxial while the corresponding layers of the two segments are in non-parallel relationships. The segments are wafer bonded using known techniques. Finally, the exposed portions of the slots are filled. Thus, the resulting structure contains a single nanopore at the interface of the two segments. The geometry of the nanopore is controlled by the orientation of the two segments and the thicknesses of the intermediate layers.  
           [0010]    Using modern microchip manufacturing techniques (e.g., epitaxial growth, PECVD, thermal growth, sputtering, evaporation or molecular beam epitaxy (MBE)), the thickness of the intermediate layer can be controlled to the nearest nanometer. Therefore, the dimensions of the resulting nanopore can also be controlled to the nearest nanometer. Furthermore, because the method of manufacture of the individual nanopores involves error-tolerant steps (i.e., process steps that achieve desired results despite process imperfections), both high batch yield and mass production are possible.  
           [0011]    In the second embodiment, each of two multi-layer segments is etched to create a recess (or “tub”) in its substrate layer. Preferably, the etching process is conducted such that the walls of the recesses intersect the front surface of the substrate layer at steep angles. The recesses are then etched so that the walls are smooth. A second selectively etchable material is blanket deposited or grown to a controlled thickness on each front surface and on the sides and bottom of each recess, thereby forming a coated tub. Each coated tub is then filled with a substance, such as the substrate layer material or a third etchable material. The upper surfaces of the segments are polished to a uniform level, typically past the original surface on which the second etchable material was blanket deposited or grown. As one example, the polishing of the segments may be performed using chemical mechanical polishing (CMP) to remove the materials (e.g., the second and third etchable materials) from the front surfaces of the segments. This step is error-tolerant, since polishing into the front surface of a segment does not adversely affect the process, providing the step leaves a “filled tub.” 
           [0012]    For each multi-layer segment, a portion of the second selectively etchable material on the upwardly extending side walls of the filled tub is masked and the exposed portion is at least partially etched. The surrounding substrate material may also be etched to facilitate etching of the second selectively etchable material. The resulting void is filled with a bonding material to bond the substrate to the “block” of material at the center of the recess. The mask is removed and the surface is again polished to a uniform level using polishing techniques known in the IC manufacturing art.  
           [0013]    The two multi-layer segments having the same or different nanopore-defining patterns are then aligned such that the second selectively etchable layer of the first segment intersects the second selectively etchable layer of the second segment. The intersection satisfies predetermined geometric criteria. Specifically, the thicknesses of the walls of the tubs determine the area of the nanopore and the shape of the nanopore is determined by the wall thicknesses and by the orientations of the segments. Once aligned, the two segments are wafer bonded using techniques known in the IC manufacturing art.  
           [0014]    The back of each multi-layer segment is etched to create a supply conduit. Each supply conduit is etched in a controlled manner from the back side of the segment in a position such that the supply conduit intersects the second selectively etchable layer of the segment. The second selectively etchable layer is then etched out of each segment, thereby creating a nanopore with predetermined dimensions and geometry. “Nanopore” is defined herein as including pores that have a cross sectional dimension as large as 0.1 millimeter.  
           [0015]    An advantage of the invention is that nanopore capability is achieved using techniques that are conventionally considered to be inadequate for such purposes. Since film thicknesses are a key to setting the dimensions of the nanopores and since techniques for controlling film thicknesses to within a few atomic monolayers are known, “coarser” integrated circuit techniques (e.g., photolithography) may be employed for other steps without a sacrifice in end results. Another advantage is that the surfaces of the various channels can be easily modified to optimize their properties for the intended applications. For example, an oxide layer can be created in the channels by performing a baking step within an oxygen-rich environment or by performing anodic oxidation in order to adjust the surface charge for compatibility with DNA. The oxide layer can be further modified using well-known silalyation agents to add chemical functionality and to vary the degree of hydrophobicity. Moreover, the oxide layer can be modified to include affinity probes, such as Biotin and antibodies, enzymes, and/or surface-bound polymers. By tailoring the oxide layer to include agents and/or probes, the invention may be used in chemical analysis and characterization of macromolecules, synthetic and naturally occurring, colloidal micro and nanoparticles, based on interactions of such molecules and particles with the nanopore. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a perspective view of an exemplary three-layer segment having a supply conduit in accordance with a first embodiment of the invention.  
         [0017]    [0017]FIG. 2 is a perspective view of the three-layer segment of FIG. 1 having the supply conduit, an undercut and a slot in a smoothed edge.  
         [0018]    [0018]FIG. 3 is a process flow for manufacturing a single nanopore having a well controlled geometry in accordance with the first embodiment.  
         [0019]    [0019]FIG. 4 is a front view of the three-layer segment of FIG. 2 having a path from the undercut to the smoothed edge.  
         [0020]    [0020]FIG. 5 is a perspective view of a pair of three-layer segments of FIG. 3 abutted in a non-parallel manner, thus creating a single nanopore.  
         [0021]    [0021]FIG. 6 is a process flow for manufacturing a nanopore having a well controlled geometry in accordance with a second embodiment of the invention.  
         [0022]    [0022]FIG. 7 is a perspective view of a substrate layer having an etched recess in accordance with the method of FIG. 6.  
         [0023]    [0023]FIG. 8 is a perspective view of the substrate layer of FIG. 7 having an intermediate layer.  
         [0024]    [0024]FIG. 9 is a perspective view of the substrate layer of FIG. 8 having material formed within the recess.  
         [0025]    [0025]FIG. 10 is a perspective view of the substrate layer of FIG. 9 wherein a portion of the intermediate layer is masked.  
         [0026]    [0026]FIG. 11 is a perspective view of the substrate layer of FIG. 10 wherein a portion of the intermediate layer has been etched out and filled.  
         [0027]    [0027]FIG. 12 is a perspective view of the alignment of the intermediate layers of two multi-layer segments in accordance with the method of FIG. 6.  
         [0028]    [0028]FIG. 13 is a top view illustrating the overlap of two channels of FIG. 12, thereby defining the position of a single nanopore.  
         [0029]    [0029]FIG. 14 is a side sectional view of supply conduits and intermediate layers of multi-layer segments formed in accordance with the method of FIG. 6. 
     
    
     DETAILED DESCRIPTION  
       [0030]    The following describes a method of fabricating one or more nanosize or microsize pores (i.e., “nanopores”) with well controlled geometries and locations, as illustrated in FIGS. 1 through 14. The invention has two primary embodiments. Briefly, the first embodiment includes the steps of selectively etching portions of intermediate layers and top or bottom layers of a pair of three-layer segments. The edges of the two segments are then wafer bonded at a predetermined angle to each other, such that a single nanopore is formed at the segment-to-segment interface. The width of the nanopore is determined by the original thickness of the intermediate layer of the first segment. The height of the nanopore is determined by the original thickness of the intermediate layer of the second segment. The geometry of the nanopore is further controlled by the angle formed by the abutment of the two segments, which preferably is more orthogonal than parallel.  
         [0031]    [0031]FIG. 1 illustrates a single three-layer segment  2 . The three-layer segment  2  is comprised of a substrate layer  10 , an intermediate layer  12  and a top layer  14 . While not critical, the substrate layer  10  may be a portion of a Silicon (Si) wafer that is standard to the integrated circuit (IC) manufacturing art. Providing the substrate is represented by step  30  in FIG. 3. The intermediate layer  12  is grown or deposited on the substrate layer  10  to a predetermined and uniform thickness, as shown in step  32 . In the present embodiment, the intermediate layer  12  is composed of Silicon Germanium (Si x Ge 1−x , hereinafter SiGe) and is grown using techniques known in the IC manufacturing art. Thermal growth, epitaxial growth, sputtering, evaporation, PECVD, and MBE are all possibilities. Finally, the top layer  14  is deposited or grown on the intermediate layer  12  to a predetermined and uniform thickness using techniques known in the IC manufacturing art, as shown in step  34 . In the present embodiment, the top layer  14  is composed of Si. Although the substrate layer  10  and the top layer  14  are stated as having known or predetermined and uniform thicknesses, this is not critical.  
         [0032]    Additionally, although the substrate and top layers  10  and  14  are described as being Si and the intermediate layer  12  is described as being SiGe, this is not critical. The chemical compositions of the three layers  10 ,  12  and  14  should be selected such that the substrate layer  10  and top layer  14  may be selectively etched with respect to the intermediate layer  12 , or the intermediate layer  12  may be selectively etched with respect to the substrate and top layers  10  and  14 . Thus, materials in the III-V system, as known in the IC manufacturing art, or other materials such as polymers, glasses and insulators, could be used to form the three-layer segment  2 . In alternate embodiments, the materials for the three layers may be selected such that each layer is selectively etchable with respect to the other two layers. It is also contemplated that more than three layers may be used to form the segment of FIG. 1, with each layer being selectively etchable with respect to the other layers.  
         [0033]    Once the three-layer segments are formed, the top layer  14  of each three-layer segment  2  is patterned at step  36  by etching at least one supply conduit  20  to extend through the top layer  14  or the substrate layer  10  to the intermediate layer  12 . In the present embodiment, the etched supply conduit  20  is approximately 50 μm in diameter, but this is not critical. Moreover, this step is error-tolerant, since neither the exact position nor the exact dimensions of the supply conduit are critical to the performance of the final product. A selective etch of the top layer  14  may be performed using a mixture of HNO 3 , HF and acetic acid. Nevertheless, selective etching of the top layer  14  may be performed using any etching process known in the IC manufacturing art which allows selective etching of the top layer  14  with respect to the intermediate layer  12 . Furthermore, although in this embodiment the supply conduit  20  is etched through the top layer  14 , in alternative embodiments, the supply conduit  20  may be formed by etching the substrate layer  10 .  
         [0034]    With reference to FIG. 2 and FIG. 3, after etching the supply conduit  20 , the intermediate layer  12  of each three-layer segment  2  is selectively time etched using a wet etch process at step  38 , as known in the IC manufacturing industry. The wet etch process is performed such that a portion of the intermediate layer  12  is removed in a pattern that is defined by the shape of the supply conduit  20 . The process continues sufficiently long to allow an undercut  22  to form in the intermediate layer  12 . The “undercut” is that portion of the etched region which extends beyond the boundaries of the supply conduit  20  in the top layer  14 . While etching of the undercut  22  is described as being performed using a wet etch process, this is not critical. Any etch process that selectively etches the intermediate layer  12  with respect to the top layer  14  and the substrate layer  10  may be employed. Because the exact dimensions of the undercut are not critical, performance of step  38  is error-tolerant.  
         [0035]    At step  40 , excess matter of the three-layer segment  2  is removed from the perimeter of the segment such that at least one edge of the segment  2  is smooth and is at a controlled distance from the supply conduit  20 . In the present embodiment, the distance from a smooth edge  24  to the boundary of the supply conduit  20  is 100 μm, however this distance is not critical. The removal of matter and smoothing of the edge of the segment may be accomplished by sawing the segment and using Chemical Mechanical Polishing (CMP), as known in the IC manufacturing art. However, any method known in the IC manufacturing art that will produce a smooth edge  24  at a controlled distance from the undercut  22  may be employed.  
         [0036]    The smooth edge  24  is masked at step  42  with a patterned photoresist to expose at least a portion of the length of the intermediate layer  12 . In the present embodiment, the length of the exposed intermediate layer  12  is on the order of 100 μm, however this dimension is not critical. A slot  26  in the intermediate layer  12  at the smooth edge  24  is created using conventional photolithography combined with selective etching techniques. The slot has a width dimension (e.g., w=100 μm) perpendicular to the thicknesses of the three layers  10 ,  12  and  14 , where w is precisely controlled to the resolution of the selected photolithography techniques (100 nm to 1 μm). The slot has a depth, or thickness, dimension (d) measured in the direction parallel to the thicknesses of the three layers, where d is precisely controlled by the selected techniques for forming the intermediate layer (1 nm to 10 nm). With reference to FIGS. 3 and 4, after the slot is completed, a second selective etch process is performed at step  44  to form a path  52  in the intermediate layer  12  from the slot  26  in the smooth edge  24  to the undercut  22 .  
         [0037]    In reference to FIGS.  3 - 5 , a pair of three-layer segments  2  and  53  are oriented in a non-parallel manner, such that the interface of the slots  26  in the pair of three-layer segments forms a single nanopore  54  (which is represented by a black region). In step  46 , the segments are abutted and wafer bonded using aligning and wafer bonding techniques known in the IC manufacturing art. If the widths (w) of the slots  26  exceed the thickness of the three layers  10 ,  12  and  14  that form each segment  2  and  53 , it may be necessary to fill the portions of the slots that are not part of the nanopore  54  with a bonding material, as shown in step  48 . Thus, when in use, matter will only proceed from the etched path  52  of the first segment  2  to the etched path of second segment  53  via the resulting nanopore  54 . Filling the portions of the slots  26  that are not part of the nanopore  54  may be performed using a thermoset or photocurable polymer or using other techniques known in the IC manufacturing art, such as PECVD or sputtering. The geometry of the nanopore  54  is controlled by the orientation of the three-layer segments relative to each other and the thicknesses of the intermediate layers of the two segments. In a preferred application, the segments  2  and  53  are at right angles to each other and the slots are 20 μm features, but some applications may benefit from segment alignments between zero and ninety degrees.  
         [0038]    At step  50 , fixtures (i.e., plumbing) may be attached to the supply conduits  20  of the three-layer segments  2  and  53  to enable the introduction and exit of matter passed through the nanopore  54 , as indicated by the flow arrows in FIG. 5. A probe  55  and an analysis system  57  are schematically shown to represent the use of the device. The device may be used for DNA sequencing, for example, by performing DNA injection at the IN supply conduit and monitoring properties within the OUT supply conduit. Ideally, properties (e.g., conductivity) are monitored within the nanopore  54  itself, but this is not an issue in all applications.  
         [0039]    [0039]FIGS. 6 through 14 illustrate the second embodiment of the invention. In reference to FIG. 6 and FIG. 7, a substrate layer  90 , such as a Si substrate, is provided (step  60 ) and patterned (step  62 ). The pattern into the substrate layer should have the appropriate angles and orientation to follow the crystal planes of the substrate material. As an example, for a Si substrate layer with a ( 110 ) orientation, the angle should be approximately 109 degrees.  
         [0040]    In step  62 , a recess  92  is etched in the substrate layer  90  to a predetermined depth using etching techniques known in the IC manufacturing art. In the preferred embodiment, each recess in an array of recesses is etched to a depth of approximately 100 μm using a KOH etch process. Also in the preferred embodiment, the recesses have sides of approximately 100 μm. However, the depth of the etch, the dimensions of the pattern, and the etch process are not critical. Etching is conducted such that the walls of the recesses intersect the plane of the front surface  98  at steep angles. It should be noted that while only one substrate layer  90  is shown in FIG. 7, a second substrate layer (e.g., a second Si substrate) is similarly processed.  
         [0041]    The recesses  92  in both substrate layers  90  are then cleaned at step  64 , so that the walls of the recesses are smooth. Cleaning of the recesses can be accomplished by any means known in the IC manufacturing art.  
         [0042]    In reference to FIG. 6 and FIG. 8, a thin intermediate layer  94  of SiGe is then grown at step  66  on at least the walls and base of the recess  92  of both substrate layers. In the preferred embodiment, the SiGe layer is grown to a controlled thickness of 2 nm within each recess and on the front surface of each substrate layer (i.e., a blanket deposition). However, selective deposition to merely coat the recesses may be achieved by covering the front surfaces with a sacrificial oxide during the SiGe growth, with the sacrificial oxide being removed at a later time (step  70 ). As will be explained below, the thickness of the intermediate layer  94  plays an important role in determining the dimensions of the nanopore to be formed. However, it is not critical that the intermediate layer  94  be 2 nm thick, nor is it critical that intermediate layer  94  be composed of SiGe. The thickness of the intermediate layer  94  should be determined by the size of the desired nanopore, and any material that is selectively etchable with respect to the substrate layer  90  may be used.  
         [0043]    In reference to FIG. 6, each recess  92  is then filled in step  68  with Si, Poly-silicon or any other material selectively etchable with respect to the intermediate layer  94 . Similar to forming the intermediate layer in step  66 , the material is preferably blanket deposited or grown at step  68 . However, if the sacrificial oxide was used to limit the growth of the intermediate layer to a coating of the recess, the same sacrificial oxide will limit the growth at step  68  to growth within the recess.  
         [0044]    Referring now to FIGS. 6 and 9, the top of each multi-layer segment is polished in step  70 , such that the substrate layer is coplanar with the intermediate layer  94  and a “block”  96  within the recess  92 , where the “block material” is the material that was grown or deposited in step  68 . In the preferred embodiment, CMP is used to polish the segments, but this is not critical. This step is error-tolerant, since over-polishing into the substrate layer will not affect performance of the end product.  
         [0045]    In reference to FIG. 6 and FIG. 10, a portion of the intermediate layer  94  that is exposed at the front surface  98  of the substrate layer  90  is masked at step  72  with an etch resistant mask  100 . In the preferred embodiment, the intermediate layer  94  is masked to expose all but a “C” shape of the SiGe intermediate layer. As will be explained below, it is the protected portion that is used to form the nanopore. The unmasked portion of the intermediate layer  94  is etched at step  74  to a depth of approximately 10 μm. Additionally, a portion of the surrounding substrate layer  90  may be etched during the etching of the intermediate layer material. Again, the process is error-tolerant. The mask  100  may then be removed.  
         [0046]    At step  76 , the resulting trench in the SiGe material is filled with “bonding material”  102  (FIG. 11) to attach the block  96  to the substrate layer  90 . This prevents the block from “floating” after the nanopore-fabrication process is complete. In one embodiment, the bonding material  102  is silicon or Poly-silicon. However, any material that is capable of bonding the substrate layer  90  to the block  96  may be used. It should be noted that although the etch is described as being performed to a depth of 10 μm, this is not critical. The selective etch need only be sufficiently deep such that when the void is filled, the resulting bond has sufficient strength to secure the block in place relative to the substrate layer after the front surface  98  is again polished to a uniform level using CMP. In reference to FIG. 6 and FIG. 11, upon completion of this second polishing step  78 , the front surface  98  is smooth, but has an exposed channel  104  of SiGe in a “C” shape. For clarification, the SiGe intermediate layer is shown by hatching.  
         [0047]    At step  80 , first and second multi-layer segments are aligned and wafer bonded. The alignment of the blocks  96  and  106  of the two segments is isolated in FIG. 12, while the alignment of the exposed SiGe channels  104  and  108  is isolated in FIG. 13. The alignment of the blocks  96  and  106  provides one location  110  at which the SiGe channel  104  of the lower block  96  contacts the SiGe channel  108  of the upper block  106 . It is at this location  110  that the nanopore will subsequently be formed. The portion of the SiGe that is covered by the bonding material following the etch-and-fill process described with reference to FIGS. 10 and 11 is not shown in FIG. 13, since it is not exposed. Thus, only one nanopore will be formed. As can be seen from FIG. 13, the alignment does not need to be precise, since the channels  104  and  108  will overlap even if they are not centered relative to each other.  
         [0048]    In step  82 , supply conduits are etched from the back sides of the two bonded segments. This is represented in FIG. 14. In this figure, the supply conduits  112  and  114  have been completed, but the SiGe material of the exposed channels  104  and  108  remains. The etching of the supply conduit  112  through the upper segment  90  will be described, but the same process is followed in forming the supply conduit  114  through the lower segment  116 . As one possible approach, a selective KOH etch step is allowed to anisotropically etch the silicon of the substrate layer  90 . The etch progresses at least until the block  106  is reached. Because of the configuration of the SiGe material, the etching will intersect the SiGe without requiring exacting tolerances in the location of the etching.  
         [0049]    In another approach to the etching process, the size of the hole at the back side of the substrate layer is set to create an etch pit that will stop within 100 μm of the front surface of the substrate layer. As a consequence, the etch will stop within 20 μm to 50 μm of the portion of the SiGe that is parallel with the front surface of the substrate layer. This only partially completes the formation of the supply conduits. Next, the hole is etched with a non-selective etchant, until the supply conduit reaches the SiGe sidewall as shown in FIG. 14. Again, the tolerances are relaxed, since significant over etching can occur without adversely affecting the process.  
         [0050]    The SiGe material is then etched at step  84 . As a result, the supply conduit  112  is connected to the supply conduit  114  via the channels  104  and  108  that previously contained the SiGe material. Referring to FIGS. 13 and 14, the connection between the two supply conduits will have dimensions that are dictated by the dimensions and the alignment of the exposed channels  104  and  108 . For the nanopore location  110  in which the exposed channels intersect at right angles and have a thickness of 2 nm, the nanopore will have a square cross section of 2 nm×2 nm. However, other dimensions may be more desirable in specific applications.  
         [0051]    In step  86  of FIG. 8, plastic moldings or plumbing fittings are glued or otherwise attached to the two segments of FIG. 14. The moldings or plumbing fittings provide fluidic connections to the supply conduits  112  and  114 . The device may then be used in a manner that was described with reference to FIG. 5. As an alternative to the fabrication sequence in which the supply conduits are first etched and then attached to moldings or plumbing fittings, the steps can be reversed. Within this alternative approach of attaching the moldings or plumbing fittings and then etching the supply conduits, the attached moldings/fittings are effectively defining the etching regions.  
         [0052]    While the second embodiment of FIGS.  6 - 14  has been described as being one in which a single “block” is formed in each multi-layer segment, the process is easily adapted to forming an array of nanopores. That is, a number of blocks can be formed in a first segment and a corresponding number of slightly offset blocks can be formed in a second segment, so that the arrangement of FIGS. 12, 13 and  14  is repeated across the surfaces of the segments when the two segments are aligned and bonded. The pattern of nanopores may be selected for use in array-based nanopore DNA sequencing or similar processing.  
         [0053]    As an optimal feature for either or both of the embodiments described above, the surfaces of the channels can be modified to tailor their properties for the intended applications. For instance, an oxide layer can be formed in the channels by performing a bake in an oxygen-rich environment or by performing anodic oxidation. The oxide layer will vary the surface charge for compatibility with DNA. The oxide layer can be further modified using well-known silalyation agents to add chemical functionality and to vary the degree of hydrophobicity. As another possibility, the oxide layer may be modified with affinity probes, such as Biotin and antibodies, enzymes, and surface-bound polymers.  
         [0054]    The devices that are formed using the process steps that have been described will have well-defined arrays of holes in the nanometer to micron range. While the method has been described as used to provide holes that are in the nanometer range, “nanopore” is defined herein as including holes which have dimensions as great as 0.1 millimeter.