Patent Publication Number: US-2012027946-A1

Title: Direct deposit and removal of nanoscale conductors

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to provisional application Ser. No. 61/415,525, filed on or about Nov. 19, 2010, entitled “Method and Apparatus for Direct Deposit and Removal of Nanoscale Conductors” naming the same inventors as in the present application. The contents of this provisional application are incorporated by reference, the same as if fully set forth. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     The subject matter of this patent application was invented under the support of at least one United States Government contract. Accordingly, the United States Government may manufacture and use the invention for governmental purposes without the payment of any royalties. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present disclosure relates to nanoscale conductors and, more particularly, to deposition and removal of conductive material at the nanoscale level. 
     2. Description of Related Art 
     The deposit and removal of conductive traces in nanoscale conductors is of importance in various fields. For example, in the field of integrated circuit editing, current circuit edit practice may involve using a high energy (30 keV) gallium ion beam to add and remove conductive traces in integrated circuits. This addition and removal may be performed as part of the manufacturing debugging process. 
     Material removal may be achieved by direct sputtering with the ion beam, or by ion-induced chemistry with precursor gases, resulting in local etching of the surface. 
     Focused ion beams have been used for direct deposition of nanoscale conductors. However, there have been difficulties associated with direct focused ion beam (FIB) deposition of nanoscale conductors, which has led to the development of FIB-induced chemical vapor deposition (CVD) as an alternative to direct deposition, as discussed in U.S. Pat. No. 5,104,684. In this technique, the target surface may be exposed to a gaseous precursor compound, which contains the desired deposition metal, as it is bombarded by the ion beam. The technique can produce yields of about one metal atom on the surface per ion in the beam. The width of a line of material deposited in this way is roughly equal to the size of the trenches which can be milled into the target with the same beam (in the absence of the precursor). 
     Several disadvantages may be associated with this technique. The deposited material is typically less than fifty percent (50%) metal precursor by atom count, with the balance consisting of ions from the focused beam and other elements in the precursor compound. As a result, the conductivity of the deposited material is lower by a factor of 10 to 100 than that of the bulk metal. Additionally, since the technique is often performed at high beam energies (&gt;2 keV) in order achieve a small focal spot size, large quantities of beam ions are implanted into the target below the surface to depths on the order of 10 nanometers. This unwanted contamination can be a problem during circuit editing processes since they often take place very close to circuit components which must not be damaged. 
     Other problems may occur with current circuit edit practice. For example, during the chemical deposition process, careful balancing of beam chemistry (which adds material) and sputtering by high energy gallium ions (which removes material) must be exercised. This makes it difficult to develop robust processes. There is further a need for deposition and removal of nanoscale conductors that does not require balancing of beam chemistry. 
     In addition, circuit editing may require a careful deposition or removal of material very close to other materials. It may be desirable that these other materials are not damaged. Accordingly, there is yet further a need for deposition and removal of nanoscale conductors that reduces or eliminates damage to other materials when adding or removing material very close to the other material. 
     BRIEF SUMMARY OF DISCLOSURE 
     The present disclosure addresses the needs described above by providing a method and apparatus for deposition and removal of nanoscale conductors. The apparatus comprises a focused ion beam source, including a magneto-optical trap ion source; and a beam energy control device configured to selectively and controllably produce a low energy beam, the energy control device being further configured to selectively and controllably produce a high energy beam. The apparatus further comprises an ion-optical column configured to receive the beam from the beam energy control device and direct the low energy beam onto the target surface in order to deposit nanoscale conductive material onto the target surface, the ion optical column being further configured to direct the high energy beam onto the target surface in order to remove nanoscale conductive material from the target surface. 
     The method comprises the steps of providing a focused ion beam from a source that includes a magneto-optical trap ion source. The method further comprises receiving the focused ion beam at a beam energy control device and selectively and controllably producing a low energy beam with the beam energy control device and/or selectively and controllably producing a high energy beam with the beam energy control device. The method still further comprises directing the low energy beam from an ion optical column onto the target surface in order to deposit nanoscale conductive material onto the target surface, and/or directing the high energy beam from an ion optical column onto the target surface in order to remove nanoscale conductive material from the target surface. 
     These, as well as other objects, features and benefits will now become clear from a review of the following detailed description of illustrative embodiments and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an apparatus for direct deposit and removal of nanoscale conductors in accordance with one embodiment of the present disclosure. 
         FIGS. 2A ,  2 B,  2 C and  2 D illustrate the apparatus of  FIG. 1  when applied to creation of a vertical interconnect in accordance with one embodiment of the present disclosure. 
         FIG. 3  is a magneto-optical trap ion source in accordance with one embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     An apparatus is described for direct deposition and removal of nanoscale conductors onto a target surface such as a substrate. The apparatus may include magneto-optical trap ion source (MOTIS) combined with a number of well-known ion-optical elements to produce a beam of focused metal ions that form nanoscale conductive material. The focused ion beam may be capable of selective rapid and simple switching between deposition and removal of conductive material. The focused metal ions may either deposit directly at low energy or sputter material away at high energy. The beam energy may be controlled such that it is of a particular energy within the low energy or high energy spectrum. The size of the features which can be produced by these techniques will be similar in size to that of the ion beam at the target. 
     For purposes of the present disclosure, included within the definition of conductive materials are any one of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), chromium (Cr), silver (Ag), erbium (Er), aluminum (Al), dysprosium (Dy) and ytterbium (Yb). Generally, a conductive material may be defined herein as one having a conductivity of 10 6  Siemens per meter (S/m) or greater. 
     The ions needed to deposit and remove nanoscale conductors may be created with kinetic energies ranging from less than 0.5 keV (low energy) to greater than 2 keV (high energy), and may be focused to nanometer-scale dimensions and directed at a surface. When the ions have a low kinetic energy, i.e., energy less than about 0.5 keV, they may deposit directly on the surface in a spot with nanoscale dimensions, thus leaving a small quantity of conductive material on the surface. When this deposition spot is scanned across a surface, a pattern of arbitrary design may be created in directly deposited conductive material. When the ions have a high kinetic energy, i.e., energy greater than about  2  keV, the metal ions may sputter more material from the surface than is deposited. Using the same scanning methods as in the deposition process, it may be possible to remove existing conductive patterns from the surface. 
     Referring now to  FIG. 1 , illustrated is an apparatus for deposit and removal of nanoscale conductors in accordance with one embodiment of the present disclosure. Ions may be produced by the MOTIS  10 . Unlike other high-brightness, low-emittance ion sources, the MOTIS  10  may provide a choice of ionic species, several of which may be suitable for direct deposition of conductive material, e.g., those mentioned hereinabove. Also, because the energy spread in the MOTIS  10  may be proportional to the acceleration energy, chromatic aberrations in the ion focusing optics may be constant over the entire range of beam energies. Therefore, both low and high energy beams may be easy to create using the same apparatus. Thus, rapid switching between deposition and removal may be possible, greatly reducing the total beam time required for a given task. 
     In lieu of a MOTIS  10 , other ion sources may be used when a high current, low resolution source is needed. This type of ion source may be needed for fine editing of small circuit components over large areas. Where an ionic species not provided by the MOTIS  10  is needed, another ion source may also be used. For example, it may be that the desired conductive material (e.g., gold or platinum) is not compatible with use in a MOTIS  10 . It should be noted, however, that the MOTIS  10  may indeed offer a much broader choice than conventional ion sources. 
     After ions are produced by the MOTIS  10 , they may be accelerated by the electric field created by the difference in electric potential V top −V mid . The beam  20  may then experience lensing action as it passes through apertures in each of the middle electrode  54  and bottom electrode  58 . More particularly, beam  20  may experience defocusing and/or focusing as it passes an aperture  35  in the middle electrode  54 . 
     Beam  20  may also experience defocusing and/or focusing as it passes through the aperture  45  in the bottom electrode  58 . The actual lensing action experienced by the beam as it passes through aperture  45  will depend on the differences in the electric field in the space between top electrode  50  and middle electrode  54  and the field in the space between middle electrode  54  and bottom electrode  58 . If these fields are the same, then there may be no lensing action. If the fields are different then there may be lensing action. The electric fields arise as a function of voltage differences between the electrodes. For example, the electric field in the space between top electrode  50  and middle electrode  54  separated by distance L 1  is (V top −V mid )/L 1 . The electric field in the space between middle electrode  54  and bottom electrode  58  is (V mid −V bot )/L 2 . 
     The MOTIS  10  may be situated halfway between V top  and V mid . Accordingly, the beam energy may be expressed as follows after exiting the extraction electrodes  50 ,  54 , and finally  58 : 
         U   beam   =e* ( V   top   −V   mid /2 
     where U beam  represents beam energy, e represents electron charge, V top  represents the voltage of the top electrode  50  and V mid  represents the voltage of the middle electrode  54 . Optimization of the potentials and spacing between these electrodes  50 ,  54  and  58  may depend on the particular aberration coefficients of the elements in a given system. 
     After exiting the extraction electrodes  50 ,  54 ,  58 , beam  20  may enter ion focusing column  60 . An aperture  70  may be employed to define a hard edge for the virtual source size. Aperture  70  defines a size for the beam  20  as it enters the ion optical column  60 . 
     The beam  20  may then enter a deflector  75 . Such deflectors are well known in the art. An example of such a deflector is the double-octupole type described in U.S. Pat. No. 5,393,985. The deflector  75  may be programmed to direct the beam  20  to the area of the target surface  85  the user desires. Target surface  85  may be a substrate. Finally the beam  20  may pass through a three element lens  80  (or objective lens) which has its first and third elements  81 ,  83  grounded. Voltage V lens  may be selected to focus the beam  20  at the target surface  85 . 
     Secondary particles emitted by the target surface  85  in response to ion beam bombardment may be collected by a detector  90  such as a micro-channel plate or continuous dynode. The secondary particles collected by the detector may be used for imaging, particularly with an ion or electron microscope. The secondary particles detected may be any of the following: secondary electrons emitted from the surface, substrate secondary ions, or backscattered ions. Which of these will give the best signal will depend on the ionic species being employed, the beam energy, and the surface composition. An electron ‘flood gun’ may also be employed to aid in surface neutralization and improve secondary electron detection. 
     With the direct deposit and removal apparatus of the present disclosure, beam  20  may have the same energy when it strikes the surface as it does when leaving extraction electrode  58 . If the extraction electrode geometry is held fixed with the three extraction electrodes  50 ,  54 ,  58 , two of them having apertures and each of the voltages in the extraction electrodes are multiplied by a common factor a, then the ratio ΔU/U may remain unchanged. 
     If the voltages on the deflector and lens ion column elements are also multiplied by α, then the ion trajectories will also remain unchanged. This allows the user to change the beam energy to suit the task without shifting the beam on the target or changing the focal properties of the final lens. Additionally, since ΔU/U is unchanged, the chromatic aberration contribution to spot size will not depend on beam energy. This is markedly different from the performance of a liquid metal ion source (LMIS) or other emission source, which have fixed values for ΔU. In those systems, the chromatic aberration focal spot size contributions may grow inversely with the beam energy. 
     A conductive connection to a metal line buried beneath an insulating layer may be facilitated by the deposition and removal of nanoscale conductors. Referring now to  FIGS. 2A-2C , illustrated are the steps required to create a conductive connection to a metal line buried beneath an insulating layer. Such a conductive connection may prove useful when using the apparatus of the present disclosure in the field of circuit editing. In a first step as shown in  FIG. 2A , the area to be edited may be imaged with a medium to high energy beam (e.g., &gt;1 keV). Then a high energy beam (e.g., &gt;2 keV) may be concentrated in an area above the metal line  240  to which the connection will be made. The high energy beam may sputter away the insulating material  210 . During this process, the image can be monitored and milling should be stopped once the metal line  240  has been uncovered ( 2 B). The ion beam can then be switched into a low energy mode (e.g., &lt;0.5 keV) and concentrated in the same region to fill the trench with the atomic species from the ion beam ( 2 C). The process can be repeated above a different metal line  250 , and the two connected with a conductive layer  220  deposited over the insulating top surface  210 , if desired ( 2 D). 
     Referring now to  FIG. 3 , illustrated is a magneto-optical trap ion source in accordance with one embodiment of the invention. This MOTIS is also described in U.S. Pat. No. 7,709,807, which is assigned to the same assignee as the present invention and incorporated by reference herein. Ion source  300  may comprise a magneto-optical trap designated as dashed line  310 . The magneto-optical trap may include a plurality of laser beams, e.g., the six laser beams  320 ,  322 ,  324 , and  326  for cooling and trapping. 
     In this configuration of  FIG. 3 , two of the six laser beams project into and from the plane of the figure, and thus are depicted by the dashed circle and represented as items  330  and  332 . The noted laser beams may be emitted from one or more lasers (not shown in  FIG. 3 ). The six laser beams for the magneto-optical trap may be generally formed by one laser. 
     The magneto-optical trap may also comprise one or more components for providing a magnetic field, which as noted may be current carrying coils for producing the magnetic field, represented as  340  in  FIG. 1 . The coils can be disposed within a vacuum chamber of the magneto-optical trap. However, the present invention includes embodiments in which the coils are located outside of the vacuum chamber. The particular sizes and configuration of the laser beams and magnetic coils may depend upon the particular set up and characteristics desired for the magneto-optical trap. The laser beams  320 ,  322 ,  324 ,  326 ,  330 , and  332 , and the coils  340 , serve to retain a cloud  305  of cold neutral atoms. 
     One or more permanent magnets for the component may provide the magnetic field. That is, instead of using coils through which electrical current is passed to generate magnetic field(s), one or more permanent magnets may be used instead of, or in conjunction with, the current carrying coils. The magnetic field providing component can use one or more permanent magnets or electromagnets, or combinations thereof. 
     The magneto-optical trap ion source  300  may further comprise an ionization laser  350  which emits a beam  352  as depicted in  FIG. 1 . The ionization laser  50  may be separate from the cooling and trapping laser beams  320 ,  322 ,  324 ,  326 ,  330 , and  332 . The orientation of the laser  350  and laser beam  352  emitted therefrom with respect to the other laser beams is not critical. However, it may be necessary that the laser beam  352  intersect the cloud  305  of cold atoms. The laser  352  may be focused to converge at a location inside the cloud  305  of cold atoms, but the present invention includes other configurations. The size of the laser beam  352  may depend upon the particular application. For example, a relatively tightly focused beam having a diameter of about 1 to 5 micrometers can be used for .low emittance applications. And, a relatively large focused beam, such as up to the size (i.e. diameter or span) of the cloud of atoms can be used for applications requiring high current and geometry characteristics. 
     The ionization laser  350  may be a separate unit from the laser of the magneto-optical trap. The ionization laser may use a wavelength significantly different from the wavelength of the cooling laser used in the trap. However, potentially for certain applications, it may be possible to use a laser beam from the trap, shift its wavelength, and then use the shifted wavelength beam as the ionization laser. 
     Instead of the extraction electrodes being separate from the magneto-optical trap ion source as described in  FIG. 1 , the extraction electrodes may be a part of the magneto-optical trap ion source as shown in  FIG. 3 . For example, the magneto-optical trap ion source  300  may comprise an extraction element, which may be an electrode assembly  360 ,  370 . First electrode  360  may have a reflective layer  362 . An aperture  364  may also be included. Electrode  360  may be maintained or may be otherwise in electrical communication with ground. Electrode  360  may include a reflective layer  362  in order to reflect laser beams  320 ,  322 ,  324 , and  326 . The electrode assembly may also include a second electrode  370  that may be transparent or substantially so. Electrode  370  may be maintained or held at some electrical bias potential. This potential depends upon the particular application. For example, a positive potential may be used to extract ions and a negative potential may be used to extract electrons. However, it may include any configuration in which two or more electrodes are appropriately electrically biased such that the resulting electric field extracts ions from the system. 
     Electrode  370  may be transparent or substantially so because the laser beams  320 ,  322 ,  324 , and  326  may pass through the electrode  370 . However, in the present configuration, none or only a portion of these lasers pass through the electrode  370 . Electrode  370  may be in the form of a silica window coated with indium tin oxide (ITO), a transparent electrical conductor. Electrodes  360 ,  370  may be disposed within the vacuum chamber of the magneto-optical trap. The extraction element can be in nearly any form. If one or more electrodes are used for the extraction element, the electrodes can be in nearly any configuration. Generally, any arrangement that creates an extraction electric field is suitable. In this embodiment, the vacuum chamber may be maintained at ground potential and the single electrode may be held at a negative electrical potential to thereby extract ions. In other embodiments, two electrodes may be used so that one of the electrodes is held at ground while the other electrode is held at a positive potential to thereby create an electric field that extracts ions. The present disclosure includes the use of one, two, or more appropriately biased electrodes to extract ions from within the system. 
     Beam  380  may pass through the aperture  364  defined in the electrode  360  and propagate outward as desired. 
     While the specification describes particular embodiments of the present invention, those of ordinary skill can devise variations of the present invention without departing from the inventive concept.