Abstract:
Methods for fabricating ultra-sharp nanoprobes can include the steps of providing a wafer, and patterning a silicon layer on the wafer with a plurality of geometric structures. The geometric structures can be patterned using electron-beam lithography or photolithography, and can have circular, triangular or other geometric shapes when viewed in top plan. The methods can further include the step of depositing a non-uniform cladding on the geometric structures using plasma enhanced chemical vapor deposition (PECVD) techniques, and then wet-etching the wafer. The non-uniform nature of the cladding can result in more complete etching in the areas where the cladding has lower density and incomplete etching in the areas of higher density of the non-uniform cladding. The different etching rates in the proximity of at least adjacent two geometric structures can result in the formation of ultra-sharp nanoprobes.

Description:
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     This invention (Navy Case No. 103009) is assigned to the United States Government and is available for licensing for commercial purposes. Licensing and technical inquires may be directed to the Office of Research and Technical Applications, Space and Naval Warfare Systems Center, Pacific, Code 72120, San Diego, Calif. 92152; voice (619) 553-5118; email ssc_pac_T2@navy.mil. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains generally to nanoprobes. More specifically, the invention pertains to ultra-sharp nanoprobes and methods for fabricating ultra-sharp nanoprobes of any desired shape, which can allow for more precise chip-scale placement of the nanoprobe for microelectronic and nanophotonic applications. 
     BACKGROUND OF THE INVENTION 
     Ultra-sharp nanoprobes can offer many applications in the areas of high-efficiency field emission flat panel displays, biochemistry (biochemical sensing), biomimetics, electronics, atomic/scanning probe microscopy, scanning tunneling microscopy, nanolithography, optics, and nanoimprinting. 
     The most important nanoprobes parameters for some of the above-cited applications can include the apex size, the probe location and the probe shape. Nanoprobes can be fabricated from a multitude of materials including silicon, silica, metal, polymer, and carbon nanotube. For atomic force microscopy (AFM) applications, the radius of curvature of the probe end is the point of contact between the probe and the sample, and the radius of curvature can be responsible for the lateral distortion that is seen in the resulting AFM images. In order to get clearer, more accurate pictures, the AFM probe radius should be as small as possible. Also, the probe&#39;s access to the depths of the sample and the microscope depth of field can be determined and can be limited by the shape of the probe&#39;s body. 
     Nanoprobe fabrication is fast becoming an attractive research field in nanoengineering. Many groups are trying to find ways of minimizing the probe radius, and many groups are also exploring methods of engineering the shape of the nanoprobe body. The current state of the art in nanoprobes fabrication can often rely on techniques that utilize elaborate precursor chemicals, catalysts, vacuum conditions, or equipment, and any combination thereof. To realize their ultimate potential, synthesized nanoprobes may require simpler fabrication techniques that can allow for control over the final nanomorphology of the nanoprobe. 
     One such nanoprobe fabrication technique can be the Bosch process, which alternates repeatedly between two modes to achieve nearly vertical structures. The first mode can be a standard, nearly isotropic plasma etch. The plasma contains some ions, which attack the wafer from a nearly vertical direction. Sulfur hexaflouride SF 6  is often used for silicon. The second mode can be deposition of a chemically inert passivation layer. For instance, C 4 F 8  source gas can yield a substance similar to a Teflon® material. Each mode can last for several seconds. The passivation layer can protect the entire substrate from further chemical attack and prevents further etching. However, during the etching phase, the directional ions that bombard and etch the substrate also attack the passivation layer at the bottom of the trench (but not along the sides). The directional ions can collide with the passivation layer and can sputter portions of the passivation layer off, which can expose the substrate to the chemical etchant. Further, these etch/deposit steps are repeated many times over, which can result in a large number of very small isotropic etch steps taking place only at the bottom of the etched pits. 
     The Bosch process described above can be time consuming and it can result in spiral shaped nanoprobes. Other alternative fabrication methods can include plasma etching with Cl 2 /HBr or anisotropic wet etching with KOH. The Cl 2 /HBr etch method can require nanoscale patterning using electron beam lithography prior to the etching process, which can be time consuming and expensive. Also, the delicate processing care required for these methods can result in low production yield. Anisotropic wet etching is a relatively simple fabrication method, but this method can typically result in pyramidal shape structures, which are not tall, and which lack very sharp ends. Thus, this method of etching can result in nanoprobes with limited effectiveness. 
     In view of the above, it can be an object of the present invention to provide a method for manufacturing ultra-sharp nanoprobes which can be used more effectively for AFM applications. Another object of the present invention can be to provide a method for manufacturing ultra-sharp nanoprobes, which can result in a relatively high yield of nanoprobes, when compared to the prior art. Still another object of the present invention is to provide a method for manufacturing ultra-sharp nanoprobes that uses low temperature deposition techniques to fabricate the nanoprobes. Another object of the present invention to provide a method for manufacturing ultra-sharp nanoprobes that take advantage of non-uniform cladding “defects” to fabricate the nanoprobe. Yet another object of the present invention is to provide a method for manufacturing ultra-sharp nanoprobes which is easy to manufacture, that is inexpensive, and that is easy to use. 
     SUMMARY OF THE INVENTION 
     Methods for fabricating an ultra-sharp nanoprobes in accordance with several embodiments of the present invention can include the initial steps of providing a wafer, and patterning the wafer with a plurality of geometric structures. The geometric structures can made of materials selected from the group consisting of silicon, metals, GaAs, GaN, Sapphire, Germanium, InP, GaP, GaSb, InSb, InAs, CdS, CdTe, ZnO, ZnSe, LiNbO 3  and LiTaO 3 , and can further have circular or triangular geometric shapes. Other shapes are possible. The geometric structures can be patterned using electron-beam lithography or photolithography. 
     Methods in accordance with several embodiments can further include the step of depositing a non-uniform cladding on the wafer and then wet-etching the wafer. Deposition of the non-uniform cladding can be accomplished using plasma enhanced chemical vapor deposition (PECVD) techniques. The PECVD can be accomplished using materials selected from the group consisting of SiO x , SiN x , SiO x N y . Amorphous silicon (a-Si:H), SiC, and Diamond-like carbon (DLC). The non-uniform nature of the cladding can result in more complete etching in the areas where the cladding has lower density and incomplete etching in the areas of higher density of the non-uniform cladding. At least two geometric structures can be shaped and arranged so that at least two geometric structures can cooperate with the cladding to form the ultra-sharp nanoprobes due to the different rates of etching of the non-uniform cladding. Nanoprobes having a radius of less than twenty nanometers can be realized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the present invention will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similarly-referenced characters refer to similarly-referenced parts, and in which: 
         FIG. 1  is a block diagram of methods that can be taken to accomplish the methods of the present invention according to several embodiments; 
         FIG. 2  is a greatly enlarged side elevational view of a silicon-on-insulator (SOI) wafer from the providing step in  FIG. 1 ; 
         FIG. 3  is the same view as  FIG. 2  after the dry-etching step; 
         FIG. 4  is the same view as  FIG. 2  after the depositing step; 
         FIG. 5  is the same view as  FIG. 2  after the wet-etching step: 
         FIG. 6  is a top plan view of the wafer of  FIG. 5 ; 
         FIG. 7  is a top plan view of alternate geometries for the geometric structures of  FIG. 6 ; 
         FIG. 8  is a top plan view of alternate geometries for the geometric structures of  FIG. 6 ; 
         FIG. 9  is a black and white scanning electron microscope (SEM) photograph of the wafer of  FIG. 7 . 
         FIG. 10  is a larger scale black-and-white SEM photograph of the nanoprobe portion of the SEM photograph of  FIG. 9 ; 
         FIG. 11  a black-and-white SEM photograph of an alternative embodiment of the geometric structures and nanoprobe shown in  FIG. 9 ; and, 
         FIG. 12  is a black-and-white SEM photograph of a nanoprobe portion of the photograph of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     This invention can exploit the properties of low density films deposited with plasma enhanced chemical vapor deposition (PECVD) in the fabrication of ultra-sharp nanoprobes. PECVD can be performed at lower deposition temperatures than CVD. The resulting PECVD oxides produced at lower temperatures (250° C.-350° C.) are typically significantly more porous than those deposited at higher temperatures such as those from normal CVD processes. This material nonuniformity is particularly noticeable when PECVD oxides are deposited around silicon strip structures. Typically for SiO2 materials, the hydrofluoric acid (HF) etch rate can correlate well with the resulting film density. 
     This non-uniform oxide cladding is typically thought of in the prior art as an impediment or defect, which can profoundly and negatively affect microelectronic and nanophotonic circuit performance. The present invention according to several embodiments can take advantage of this porous, non-uniform oxide “defect” to fabricate extremely small, ultra-sharp nanoprobes structures. 
     Using the fabrication process steps described more fully below can result in lower density PECVD deposited SiO2 films within 1 μm on each side of a silicon strip structure. These low density regions can etch faster when the regions are subject to an HF solution. Silicon strip structures of various shapes can be placed in close proximity, within 2 um of each other, allowing for sharp oxide tips to be formed when the PECVD oxide layer is etched. The tip shape can be controlled by designing appropriate gaps between the silicon structures and by designing circular silicon type structures of various radii of curvature (as depicted in the Figures). Other geometric structures could be used. The final probe radius can reach sizes as small as approximately ten nanometers (10 nm). 
     Referring initially to  FIGS. 1 and 2 , a block diagram, which shows steps that can be taken to accomplish the methods of the present invention according to several embodiments, can be shown and can be generally designated with reference character  10 . As shown, method  10  can include the initial step  12  of providing a wafer  30 . The wafer  30  can be a 680 μm thick silicon on insulator (SOI) wafer composed of a silicon layer  32 , a 3 μm buried oxide (BOX) layer  34  and a 250 nm patterned silicon layer  36  can be placed on top of the BOX layer  36 . A side elevational view of wafer  30  can be seen in  FIG. 2 . Other materials for layers  32 ,  34  and  36  could be used, depending of the desired applications selected by the end user. 
     Referring again to  FIG. 1 , the methods according to several embodiments can include the step of patterning the wafer  30  with a plurality of geometric structures as indicated by box  14  of  FIG. 1 . As shown in  FIGS. 6-8 , the geometric structures  40  can be selected from the group consisting of lines, circles, squares, triangles, or any other complex polyhedral shape when viewed in top plan, so long as the geometric structures are arranged and spaced to form the nanoprobe once the methods of the embodiments are performed. A FOX-16 electron beam (e-beam) resist from Dow Corning® can be diluted in methyl isobutyl ketone (MIBK), one part FOX-16 to two parts MIBK (by weight), and spun at 4000 rpm, which can result in a 180 nm thick coat. A hot plate bake step at 175° C. for 4 minutes aids in removing the solvent. The patterning step can be accomplished in some of the embodiments with an e-beam lithography system, for instance a EBPG 5200 from VisTec®, using a dosage of 5,120 μC/cm 2 , and developed in Tetramethylammonium hydroxide (TMAH) for 1 minute. Other patterning methods such as photolithography could also be used, depending on the material that is used for patterned silicon layer  36 . 
     After the wafer  30  is patterned, and as shown in  FIGS. 1 and 3 , the methods  10  can include the step of dry etching the patterned silicon layer  36  of wafer  30 , as shown by block  16 . In some embodiments, this can be accomplished using an Plasmalab® 100 reactive-ion etching/inductively coupled plasma (RIE/ICP) from Oxford Instruments Plasma Technology, Ltd, with a mixture of 25 standard cubic centimeters per minute (sccm) of sulfur hexafluoride (SF 6 ) and 50 sccm of Octafluorocyclobutane (C 4 F 8 ) at a temperature of 15° C., and with a RIE power of 30 W and ICP power of 1200 W. As shown in  FIG. 3 , once the dry etching step occurs, all that remains is the portion of patterned silicon layer  36  that forms the aforementioned geometric structures. 
     Referring now to  FIGS. 1 and 4 , after wafer  30  has been dry-etched, a non-uniform cladding  38  (see  FIG. 4 ) can be deposited over structures  40 , as shown by step  18  in  FIG. 1 . Non-uniform cladding  38  can be a 1800 nm layer of SiO2 cladding deposited via any lower temperature method the can cause non-uniformity, such as plasma-enhanced chemical vapor deposition (PECVD), for example (250° C.-350° C.). An example tool for performing this step can be the Oxford Plasmalab® 80Plus system, the step can be accomplished at a temperature of 350° C. Other tools and temperatures could be used to establish non-uniform cladding  38 , depending on the materials selected for layers,  32 ,  34  and  36 . Etching can be accomplished using a mixture of 5% silane (SiH 4 ) and 95% nitrogen (N 2 ) at 117 sccm with 710 sccm of nitrous oxide (N 2 O) at a deposition rate of 72 nm/minute. The PECVD chamber pressure is 1000 mT and the RF power can be 20 W at 13.56 MHz. 
     Referring next to  FIGS. 1, 5 and 6 , the methods according to several embodiments can include the step  20  of wet-etching the non-uniform cladding  38  portions of the wafer. To do this, the resulting silicon waveguides can be patterned with Shipley S1805 photoresist, then exposed with a mask aligner (such as an Hybrid Technology Group (HTG) Mask Aligner, for example), and etched in a complementary metal-oxide-semiconductor (CMOS) grade J.T. Baker buffered oxide solution (BOE) consisting of 33.5% ammonium (NH 4 ), 7% hydrogen fluoride (HF), and 59.5% water (H 2 O), for a duration of 195 seconds. The remaining S1805 photoresist can be removed with Shipley Microposit Remover 1165. 
     By cross-referencing  FIGS. 5 and 6 , it can be seen that after step  20  is performed, the placement and arrangement of geometric structures  40  when viewed in top plan (the structure  40  were patterned from patterned silicon layer  36 ) can cooperate with non-uniform cladding  38  to form ultra-sharp nanoprobes  42 . This is because the less dense portions of non-uniform cladding  38  were completely etched (resulting in portions of layer  34  which are visible in  FIG. 6 ), while the denser portions of non-uniform cladding  38  etched slower, which left significant portions of cladding  38  remaining on wafer  30 . Adjacent geometric structures  40  can cooperate with the cladding  38  to establish a nanoprobe  42 . For example, and referring to  FIG. 6 , structure  40   a  (circle) and structure  40   b  (line) in  FIG. 6  cooperate with cladding  38  to establish nanoprobe  42   a . It should be appreciated that the methods of the present invention can accomplish the fabrication of a great many nanoprobes  42  (for example, nanoprobes  42   b  and  42   c  are also visible in  FIG. 6 ). The number of nanoprobes fabricated is limited only the user&#39;s placement, arrangement and selection of geometric structures. As shown in  FIGS. 7 and 8 , triangles could also be used as well as circles, squares (not shown) or complex polyhedral shapes ( FIGS. 11 and 12 ), and lines to generate the nanoprobe  42 , provided the geometric structures are close enough to cause the nanoprobes  42  to emerge from non-uniform cladding  38  once steps  16 ,  18  and  20  of the methods according to several embodiments are accomplished. 
     Referring now to  FIGS. 9 and 10 , the nanoprobes  42  can be seen in greater detail. The nanoprobes  42  can be imaged via a scanning electronic microscope (SEM).  FIG. 9  is an SEM photograph wherein visible SiO2 nanoprobes  42  are visible on the sample.  FIG. 10  is a close-up of the nanoprobe portion of the SEM photograph of  FIG. 9 . For the methods of the present invention according to several embodiments, the nanoprobes  42  illustrated in  FIG. 7  can have a radius  44  of twenty nanometers (20 nm). With nanoprobes of the shape and size, methods such atomic force microscopy (AFM) can be accomplished more effectively with the device of the present invention. As mentioned above, the size, shape and positioning (via the patterning step) of structures can determine the shape and location of the nanoprobes  42  that are formed, as well as the yield of nanoprobes (the number of nanoprobes  42  for a given surface area of wafer  30 ) when using the methods of the present invention according to several embodiments. 
       FIGS. 11 and 12  are SEM photographs resulting from the use of alternative geometries for the geometric structures  40  of the present invention.  FIGS. 11 and 12  utilized complex polyhedral shapes to perform the ultra-sharp nanoprobes. As shown in  FIGS. 11 and 12 , the use of different geometries  40  can result in a greater yield of nanoprobes per unite areas of wafer  30 , or the use can result in nanoprobes with different radii  44 , depending on the applications of the end user. 
     In several alternative embodiments, a thin layer of metal (not shown in the Figs.) can be deposited on cladding  38  using electron beam evaporation or atomic layer deposition. In other alternative embodiments, a number of alternative materials that may be used in the design of nanoprobes  42 . For example, silicon, metals, GaAs, GaN, Sapphire, Germanium, InP, GaP, GaSb, InSb, InAs, CdS, CdTe, ZnO, ZnSe, LiNbO 3  and LiTaO 3  could be used for layer  36 . Other PECVD oxides may also be used, such as SiO x , SiN x , SiO x N y , Amorphous silicon (a-Si:H), SiC, and Diamond-like carbon (DLC), for example. As mentioned above, there are several ways to pattern the wafer, including but not limited to e-beam lithography and photolithography. The nanoprobes can be combined with microfluidic chambers for biochemical sensing applications. Similarly, the nanoprobes may be combined with materials responsive to electron interactions, such as phosphors or organic compounds for photonic applications. 
     The use of the terms “a” and “an” and “the” and similar references in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.