Patent Publication Number: US-6700127-B2

Title: Point source for producing electrons beams

Description:
FIELD OF THE INVENTION 
     An apparatus for producing mono-atomic point source electron beams comprises a tip disposed within a first vacuum chamber with a volume of less than about 1 cubic millimeter and a pressure of less than 10 −8  Torr and an electrode. 
     BACKGROUND OF THE INVENTION 
     Many analytical devices, such as electron microscopes, utilize a focused beam of electrons to illuminate a substrate. Sources of these electron beams are often contained in the tips of the analytical device. 
     Electron point sources, which may be utilized in these analytical devices, are well known. These electron point sources, often on the order of the atomic scale and adapted to provide field emission of coherent electron beams, have been described in, e.g., “Coherent point source electron beams”, Hans-Werner Fink, Werner Stocker, and Heinz  Schmid, Journal of Vacuum Science and Technology B , Volume 8, Number 6, November/December 1990, pp. 1323-1324, in “Unraveling nanotubes: field emission from an atomic wire,” A. G. Rinzler, J. H. Hafner, P. Nikolaev, L. Lou, S. G. Kim, D. Tomanek, P. Nordlander, D. T. Colbert and R. E. Smalley, Science, 269, pp. 1550-1553 (1995), and in “Carbon nanotubes are coherent electron sources”, Heinz Schmid, Hans-Werner Fink,  Applied Physics Letters , Volume 70, Number 20, May 19, 1997, pp. 2679-2680. The first reference discloses a tungsten tip terminated with an atomically perfect pyramid of tungsten atoms as the electron emitter. The second and third references disclose a carbon nanotube as the electron emitter. 
     By way of further illustration, U.S. Pat. No. 5,654,548 (“Source for intense coherent electron pulses”) discloses how such sources can be used for one type of electron microscopy. The entire disclosure of this United States patents is hereby incorporated by reference into this specification. 
     Electron beams have been used in constructing microscopes. For example, U.S. Pat. No. 6,005,247 (Electron beam microscope using electron beam patterns) discloses “An electron beam microscope includes an electron beam pattern source, a vacuum enclosure, electron optics, a detector and a processor.” U.S. Pat. No. 6,043,491 (Scanning electron microscope) discloses “A scanning electron microscope in the present invention, by employing a retarding method and suppressing interferences between an electron beam and secondary electrons or back scattered electrons, makes it possible to obtain a clearer SEM image with a higher resolution.” The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     Field emitted electron beams are also useful in many types of vacuum microelectronic devices, as described in “Vacuum Microelectronics,” edited by Wei Zhu, (John Wiley &amp; Sons, New York, 2001). 
     Fabrication of specialized tips used in scanning electron microscopes and atomic force microscopes is well known to those skilled in the arts. For example, U.S. Pat. No. 6,020,677 (Carbon cone and carbon whisker field emitters) discloses “Carbon cone and carbon whisker field emitters are disclosed. These field emitters find particular usefulness in field emitter cathodes and display panels utilizing said cathodes.” U.S. Pat. No. 5,393,647 (Method of making superhard tips for micro-probe microscopy and field emission) discloses “Forming micro-probe tips for an atomic force microscope, a scanning tunneling microscope, a beam electron emission microscope, or for field emission, by first thinning a tip of a first material, such as silicon.” The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     The prior art sources of atomic point source electron beam emitters typically must be operated at very low pressures, on the order of about 10 −8  to 10 −10  Torr, to protect them from disruptive contamination, chemical degradation, or destructive ion bombardment by residual gas ions. This often requires the use of complicated, expensive, and cumbersome equipment. 
     It is an object of this invention to provide a device which allows electron beam point sources to be utilized with samples maintained at pressures in a wide range of vacuums from about atmospheric pressure to 10 −10  Torr. The mechanically protective ultra high vacuum enclosure of these delicate electron beam point sources in conjunction with the exceptionally good electron-optical qualities of such sources makes possible very small source to target distances, ranging from about 1 centimeter to 10 nanometers. This in turn reduces vacuum requirements needed for practical application of such electron beams, including scanning electron microscopy. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, there is provided an apparatus for providing an electron beam, comprised of a vacuum chamber and comprised of a proximal end and a distal end, wherein the distal end comprises electron transparent material, a source of said electron beam disposed within said chamber, and means for focusing said electron beam from said source of electron beam, wherein said vacuum chamber has a volume of less than about 1 cubic millimeter and wherein the vacuum within said vacuum chamber is greater than 10 −7  Torr. In one embodiment, an electrode is disposed outside of the vacuum chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described by reference to the following drawings, in which like numerals refer to like elements, and in which: 
     FIG. 1 is a schematic of an enclosed point source electron beam generator, 
     FIG. 2 is a schematic of an enclosed point source electron beam generator, 
     FIG. 3 is a schematic of an enclosed point source electron beam generator, 
     FIG. 4 is a schematic of a miniature scanning electron microscope using an enclosed point source electron beam generator, 
     FIG. 5 is a schematic of an electron beam focusing coupler for a superconducting nano-channel, 
     FIG. 6 is a schematic of a superconducting nano-channel Y junction and, 
     FIG. 7 is a schematic of a superconducting nano-channel Y junction and, 
     FIG. 8 is a schematic of a point source electron beam generator coupled to an electron beam focusing superconducting nano-channel and, 
     FIG. 9 is a schematic of a point source electron beam generator coupled to a superconducting nano-channel. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Atomic scale point source electron beams have many potential advantages for scanning electron microscopy, including higher resolution at lower voltages in much more compact configurations; these electron beam sources also are advantageously used in vacuum microelectronic devices. The primary disadvantage is the requirement for operation at ultra-high vacuum when used as electron field emitters to avoid damage by ion bombardment. By using a miniature ultra-high vacuum chamber to permanently enclose the field emission part, the vacuum requirements for the rest of a scanning electron microscope can be greatly relaxed, leading to major operational and economic advantages, and a much wider range of practical application of this uniquely advantageous point source of coherent electron beams. 
     In one embodiment, the invention of this patent application comprises the structure and utilization of a mono-atomic tip in place of conventional field emission sources, providing a far superior initial electron beam in terms of narrow beam divergence and narrow energy spread and greatly reducing the requirements for high beam voltages and expensive electron optical systems needed for very high resolution imaging. 
     The enclosed point source electron beam generator described in this specification may operate with a miniature ultra-high vacuum enclosure with an electron-transparent window. This enables the rest of the system to be operated under more conventional vacuum conditions. The rest of the system may comprise conventional or, due to the very narrow electron beam sources produced at relatively low voltages, greatly miniaturized versions of conventional scanning electron microscopes, scanning transmission microscopes, point projection Fresnel microscopes, electron beam lithography systems, and vacuum microelectronic devices. 
     An alternative means of generating very fine electron beams at low voltages (about 50 to 500 volts) from a conventional electron beam and coupling it to a superconducting nano-channel is also disclosed. Such beams can be used for the microscopy systems and vacuum microelectronic devices. 
     Very fine electron beams from any of the above sources may be guided and/or manipulated by superconducting nano-channels. 
     As is known to those in the field of electron beam technology, suitably oriented magnetic fields may be used to confine electron beams for some distance once they have been suitably created and formed. The small size of the electron beam source of this invention and the ability to position it close to the ultimate target makes it feasible to wholly immerse the entire source-to-target system in the bore of a powerful magnetic field generating system whose internal magnetic field is oriented parallel to the main electron beam axis. The magnetic field system, depending on system size and performance requirements, may employ permanent magnets or conventional electromagnets or superconducting electromagnets, optionally augmented with magnetic pole pieces, following common practices well known to those in the art. Immersing the entire system in this magnetic field has the net effect of causing electrons that would normally radially diverge from the main beam axis to instead spiral around it. For scanning electron microscopy or scanning electron beam surface modification applications, either the source or target would need to be mechanically scanned relative to the other. Such scanning may for instance be implemented by any of the lateral electromechanical scanning techniques that are used for scanning tunneling microscopes or atomic force microscopes, following common practices well known to those in the field. 
     In the remainder of this specification reference will be made to the use of single walled superconducting carbon nanotubes. However, it is to be understood that multi-walled superconducting carbon nanotubes may be utilized as well, as may be any other essentially atomically perfect nanotube structure, which, if not naturally superconducting, may be optionally externally coated with a thin film of superconducting material. 
     In the preferred embodiment illustrated in FIG. 1, there is illustrated a tip assembly  10  comprised of a-high quality electron-transparent thin wall  12  positioned at the distal end  14  of an ultra-high vacuum chamber  16 . 
     The thin wall  12  is electron-transparent, i.e., electron beams may be passed through it without significant dispersion or attenuation, relative to the intended application. Electron transparency is a function of electron energy and the type and thickness of the thin wall material. Using means well known to those skilled in the art, the initial electron beam energy would be set for attaining an acceptable level of electron transparency for a particular thin wall material, and then, if needed, the electron beam energy would subsequently be raised or lowered as appropriate for the intended application. 
     Electron-transparent thin-walls and structures and materials comprising them are well known to those skilled in the art. Reference may be had, e.g., to U.S. Pat. No. 6,300,631 (Method of thinning an electron transparent thin film membrane on a TEM grid using a focused ion beam), U.S. Pat. No. 6,194,720 (Preparation of transmission electron microscope samples), U.S. Pat. Nos. 6,188,068, 6,140,652, 6,100,639, 6,060,839, 5,986,264, 5,940,678 (electronic transparent samples), U.S. Pat. Nos. 5,633,502, 4,680,467, 3,780,334 (Vacuum tube for generating a wide beam of fast electrons), and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     Referring again to FIG. 1, and in the embodiment depicted, wall  12  is preferably a film that preferably has a thickness of from about 1 to about 50 nanometers. In one preferred embodiment, film  12  consists essentially of silicon nitride, boron nitride, or diamond. 
     The wall  12 , in combination with wall  18 , define a chamber  16 . The vacuum within chamber  16  is preferably greater than about 10 −7  Torr. In one aspect of this embodiment, the vacuum within chamber  16  is from about 10 −7  to about 10 −10  Torr. 
     The vacuum within chamber  16  may be created by conventional means. In one embodiment, (not shown) the tip assembly  10  is placed within an ultra high vacuum chamber (not shown) during its manufacturing assembly process and chamber  16  is vacuum sealed to the electron transparent wall  12  thus enclosing an ultra high vacuum within chamber  16 . 
     The chamber  16  has a relatively small volume, of preferably less than about 1 cubic millimeter. In one embodiment, the chamber  16  has a volume of less than about 0.1 cubic millimeters. 
     Referring again to FIG. 1, it will be seen that the tip assembly  10  is utilized within a sample vacuum chamber  20  whose volume may be at least about 1,000 times as great as the volume of chamber  16 . However, the vacuum in chamber  20  may be substantially lower than the vacuum in chamber  16 . The pressure in chamber  20  is typically at least about 10 to 1,000 times as great as the pressure within chamber  16 . 
     Referring again to FIG. 1, and in the preferred embodiment depicted therein, the tip assembly  10  is disposed above sample  22  and can be moved, by means described elsewhere in this specification, so that it is closer to or further away from sample  22 . 
     Referring again to FIG. 1, and in the preferred embodiment depicted therein, an extraction electrode assembly  24  is preferably disposed around chamber  16 . Electrode assembly  24  is electrically connected to external voltage supply  26  by means of conductors  28  and  30 . 
     In another embodiment, not shown, the extraction electrode assembly  24  is disposed within chamber  16 . 
     In one embodiment, the extraction electrode assembly  24  is electrically charged to an electrical potential typically in the range 50 to 500 volts with respect to the field emission tip  32  (which is the mono-atomic point source of electron beam  34 ). 
     In the embodiment depicted in FIG. 1, tip assembly  10  may comprise either a single or multi walled carbon nanotube  32  or a tungsten mono-atomic point emitter (not shown). Reference may be had to U.S. Pat. No. 6,159,742 (Nanometer-scale microscopy probes), U.S. Pat. No. 4,939,363 (Scanning tunneling microscope), and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     The extraction electrode assembly  24  may optionally be fashioned from a superconducting material to take advantage of the Meissner effect for narrowing the emission cone of electrons from the emitter due to the superconducting material&#39;s expulsion and thus confinement of the magnetic fields of the emerging electrons. The Meissner effect is the ability of a material in a superconducting state to expel all magnetic fields therefrom (i.e., such a superconductor is perfectly diamagnetic and exhibits a permeability of zero). Reference may be had, e.g., to U.S. Pat. No. 4,975,669 (Magnetic bottle employing Meissner effect). The entire disclosure of this United States patent is hereby incorporated by reference into this specification. 
     Referring again to FIG. 1, and in the preferred embodiment depicted therein, the emission tip  32  is attached to an electrically insulating tip enclosure  36  to isolate the tip  32  from electrode  24 . An electrical connection is made from the voltage source  26  to the electrode  24  by means of conductor  28 . An electrical connection is made from the voltage source  26  to the tip  32  by means of conductor  30 . The entire assemblage is attached to an electrically insulating supporting mount  40 . 
     In this preferred embodiment, the beam extraction voltage preferably is selected according to the type of ultra thin film material used for the electron window  12 , since, as is known to those skilled in the arts, transparency is energy dependent. After passage through the electron window  12 , the beam  34  can subsequently be accelerated or decelerated as needed to a target-relative voltage in the range of about 20 to 1,000 volts. 
     FIG. 2 illustrates another configuration of a tip assembly  50  in which tip  32  is in the shape of a carbon nanotube. In this embodiment, tip  32  has a relatively small diameter, in the range of 0.3 to 10 nanometers. In this embodiment, the carbon nanotube may be composed of single or multi-walled metallic-type carbon nanotube; alternatively, it may be composed of tungsten mono-atomic point emitter or other suitable material. 
     Referring again to FIG. 2, the tip  32  is preferably embedded in a support structure  42 , which also serves as a thermal sink and ultra-high vacuum seal to a superconducting single walled metallic-type carbon nanotube  44  of relatively larger diameter (in the range, e.g., of approximately 5 to 200 nanometers), which also serves as a field emission extraction electrode and as a miniature ultra-high vacuum chamber. Electrical lead  43  passes through the support structure  42  to provide a means for creating an electrical potential difference between tip  32  and wall  44 . In this embodiment, the electron beam  34  emerges from the field emitter  32  and is confined and focused by the superconducting nanotube  44 . Since the momentum of the electrons in beam  34  is largely parallel to the wall  44 , relatively little force is required to confine it within wall  44 . This beam penetrates and emerges from the semispherical end cap  46 . This end cap is less strongly superconducting, or may not be superconducting at all, than the rest of the carbon nanotube  44 . Since the momentum of the electron beam  34  is perpendicular to the middle of end cap  46 , the middle of end cap  46  serves as an electron window for certain material-dependent electron beam energies. An optional coating of material  48 , which may optionally be superconducting, may be used for purposes of vacuum sealing, enhanced mechanical strength, or enhanced superconducting focusing of electron beam  34 . In another embodiment (not shown), coating  48  may be connected to the electrical lead  43  and is then used as an electron extraction electrode, instead of nanotube  44 . 
     FIG. 3 illustrates another preferred embodiment of this invention. In this configuration, a fixed or dynamic emitter tip positioning system  60  is enclosed in a miniature ultra high vacuum chamber  62  and support structure  64 . The tip  32  preferably has a relatively small diameter, e.g. in the range of approximately 0.3 to 10 nanometers; single walled metallic-type carbon nanotube  32  serves as an atomic point source field emitter of electrons  34 . Alternatively, the atomic point source field emitter  32  may be a multi-walled carbon,nanotube or a tungsten mono-atomic point emitter or other suitable material. This electron emitter  32  is embedded in a positioning system  60 . The support structure  64  also serves as a thermal sink and ultra-high vacuum seal to a superconducting single walled metallic-type carbon nanotube  66  of relatively larger diameter, e.g. in the range of approximately 5 to 200 nanometers, which serves both as a field emission extraction electrode and as a miniature ultra-high vacuum chamber. The electron beam  34  emerges from the field emitter  32  and is confined and focused by the superconducting nanotube  66 . The electron beam  34  penetrates the semispherical end cap  46  and emerges from the end of it. This end cap is less strongly superconducting or may not be superconducting at all. Since the momentum of the electron beam is perpendicular to the end cap  46  it serves as an electron window. An optional coating of material  48 , optionally superconducting, may be used for purposes of vacuum sealing, enhanced mechanical strength, or enhanced superconducting focusing of the electron beam. In the embodiment depicted in FIG. 3, electrical leads  67 ,  68  are connected to a voltage supply (not shown) which provides the electrical potential difference between the tip  32  and the field emission extraction electrode  66 . Alternatively, an optional electrical lead  69  may be connected to a voltage supply (not shown) when the optional coating of material  48  is to be utilized as the field emission extraction electrode. 
     The relatively larger single walled carbon nanotubes in FIGS. 2 and 3 may be quite long compared to their diameter, e.g. on the order of a micron or more; in general, such nanotubes have aspect ratios of at least about 1:10 to 1:1000. The material properties (such as toughness and springiness of such nanotubes) may be adapted to allow the nanotubes to optionally be subjected to mechanical bending involving various high frequency resonant motion patterns, in the kilohertz through megahertz range, depending on specific geometry for purposes of directing, diverting, modulating, or scanning the emergent electron beam. 
     There are several forms of carbon nanotubes. In general, the most commonly studied forms of carbon nanotubes have physical properties such that they conduct electricity better than copper, they have a tensile strengths over 100 times that of steel, they become superconductors when cooled to extremely low temperatures, and they are exceptionally tough and resilient when subjected to mechanical bending. 
     The electron transparent structures illustrated in the Figures can be formed by the carbon nanotube end caps  46  shown in FIGS. 2 and 3. Alternatively, or additionally, these electron transparent structures may be replaced, in part or in whole, by mechanically attaching some other ultra thin film of suitably electron transparent material to the end of an uncapped carbon nanotube. 
     The micro-enclosed point source electron beam generators  10  of FIG. 1 and 32 of FIGS. 2 and 3 may be mechanically scanned near the target to be imaged or incorporated into the tip of an atomic force microscope for the purpose of very high resolution electron microscopy and spectroscopy; or such point source electron beam generators  10  of FIG. 1 and 32 of FIGS. 2 and 3 can be incorporated into an electron beam micro-column, such as described in “Fabrication of electron-beam microcolumn aligned by scanning tunneling microscope”, Jeong-Young Park, et al, Journal of Vacuum Science and Technology A, Volume 15, Number 3, May/June 1997, 1499-1502. 
     FIG. 4 illustrates the use of a micro-enclosed point source of electrons  70 , (which may consist of any of the systems shown in FIGS. 1,  2 , and  3 ) to substantially improve on other devices, such as, e.g., the device disclosed in Thomas George&#39;s “Miniature Electron Microscopes Without Vacuum Pumps”, NASA Technical Brief, Vol. 22, No. 8. (JPL NEW TECHNOLOGY REPORT NPO-20335). A low-to-medium vacuum enclosure  72  contains the whole system; in general, the pressure within enclosure  72  is from about 10 −3  to 10 −6  Torr. An optional superconducting cylinder  74  can be used for narrowing the conical emerging electron beam. An optional beam extraction electrode and/or beam acceleration or deceleration electrodes  76  may be used. Electrode pair  78  and electrode  80  are used for scan deflection and focus. Backscattered electron detectors  82  are placed above the observation and manipulation stage  84 . Secondary and backscattered electrons may be detected either by a micro channel plate, or a channeltron, or by other conventional means. 
     The use of superconducting channels for manipulating electron beams has been described in “High Tc bulk superconductor wigglers”, Hidenori Matsuzawa, et al,  Applied Physics Letters , Volume 59, Number 2, Jul. 8, 1991, 141-142. FIG. 5 shows how a relatively large (in the range of approximately 0.1 to 100 micron diameter) beam of electrons or positive ions  90  may be narrowed into a beam  100  by means of a superconducting channel assembly  88 . Beam  90  passes through superconducting material  92  with a converging funnel channel  94  to a channel  96  of dimensions in the range of approximately 1 to 100 nanometer diameter, and through a connected single walled superconducting carbon nanotube  98 . The superconducting structure  92  may optionally be split in planes perpendicular to the funnel axis into several mutually insulating segments that are mutually electrified so as to facilitate the attraction of electrons into each successive segment. 
     FIG. 6 illustrates the use of superconducting carbon nanotubes  110 ,  112  in the range of about 0.3 to 100 nanometers in diameter constructed into a Y-junction  114 . Because superconductivity is likely substantially reduced in the junction region itself, this region would normally be externally coated with a thin film of superconducting material  116 . The more general use of high temperature superconductors for such coatings and the coating of all channels removes the requirement that the carbon nanotubes be superconducting or be used at the temperature at which they are superconducting. This system-can be used to couple an electron beam  120  with an ion beam  122  or with another source of electrons at a different energy level, from inlets  110 ,  112  into the Y-junction  114  and to the single coaxial outlet  118 . One of several means of using such a system is to use the electron beam for target illumination and positioning purposes, and using the ion beam for transient milling or ion deposition purposes. 
     Alternatively, the Y-junction assembly  130  shown in FIG. 7 can be used to split an electron beam  132  entering inlet  134  into  2  beams  136 ,  138  exiting at outlets  140 ,  142 . Additional thin film coating  144  of a superconducting material may optionally be employed to enhance the superconducting property at the junction  148 . Such junctions need not be symmetric in branching angles or in terms of nanotube diameters. Multiple such splitting and merging junctions may be combined in practice, and may be structured so as to implement nano-scale electron beam analogs of fluidic technology, including feedback loops. Modulation mechanisms may be provided by external pulsed magnetic fields above the local superconducting shielding level, induction of trapped magnetic fields inside and along the axis of nano-channel loops, locally induced transient thermal excursions above the superconducting threshold temperature, mechanical bending, and the use of electrically insulated superconducting channel segments at differing potentials. These can be used in vacuum electronic device systems that dispense with individual solid state cathodes and individual solid state anodes. Such systems can also be realized without using carbon nanotubes, by exploiting the fabrication techniques that are used for micro-electro-mechanical systems. Such device systems can implement analog and digital types of transducer, signal processing, and computing functions. The highly modulated electron beam output of such systems can be used for subsequently miniaturized electron microscopy implementation, and for corollary use in spatially resolved electrochemistry processes. 
     FIG. 8 illustrates one preferred use of the electron beam emitter assembly  50  of FIG. 2 together with the superconducting channel assemble  88  of FIG. 5. A material  160  is used to attach assembly  50  to the assembly  88 . In one embodiment, material  160  is a non-conducting material, e.g. Nylon-6, Nylon-66, Teflon or the like, and electrically isolates assembly  50  from assembly  88 . In another embodiment, material  160  is a superconducting material. 
     Free standing flexible superconducting nanometer scale tubes and fixed superconducting nanometer scale channels formed on supporting substrates, manufactured by means well known to those skilled in the art of micro-lithography and related micro-fabrication techniques, may be used for conveying coherent electron beams with energies corresponding to wavelengths of a similar order of magnitude (e.g. a few electron volts) and provides a nanometer scale electron beam analog of micron scale fiber optical systems. FIG. 9 illustrates a preferred embodiment in which an electron beam emitter assembly  50  of FIG. 2 is coupled to superconducting channel  170  for conveyance of a coherent electron beam  180 . Referring to FIG. 9, superconducting channel  170  consists of a superconducting material  178  in the form of a tube. At one end  172  of channel  170  is attached the electron beam source  50 . The electron beam  180  passes through a 90 degree bend  176  in the channel  170  and exits at the channel end  174 . 
     It is to be understood that the aforementioned description is illustrative only and that changes can be made in the apparatus, in the ingredients and their proportions, and in the sequence of combinations and process steps, as well as in other aspects of the invention discussed herein, without departing from the scope of the invention as defined in the following claims.