Abstract:
A method for carrying out pulsed laser deposition is disclosed. The method comprises providing a target having a desired composition; irradiating the target with a pulsed laser beam to provide a plume of target material; and directing the plume in a desired direction by use of an inert carrier gas. The plume of target material is passed through an aperture to create an atomic beam. One or both of the plume or the atomic beam is irradiated to reduce the amount of agglomerated particles in the atomic beam. The atomic beam is directed onto a substrate to produce a deposition product. An apparatus for carrying out the method is also disclosed.

Description:
STATEMENT REGARDING FEDERAL RIGHTS 
       [0001]    This invention was made with government support under Contract No. DE-AC52-06NA25396 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     BACKGROUND OF INVENTION 
       [0002]    The present application relates generally to atomic, molecular, and nanoparticle deposition techniques. More particularly, the application relates to high-pressure, atomic, molecular, and nanoparticle beam deposition. 
         [0003]    A variety of methods for depositing thin-film coatings have been developed over the last half-century, including chemical vapor deposition, physical vapor deposition (e.g. sputtering, e-beam evaporation, IBAD, etc.), and pulsed laser deposition. Each has its advantages/disadvantages, depending on the material to be deposited, the substrate involved, and the desired material structure. 
         [0004]    Of these coating methods, Pulsed Laser Deposition (PLD) has the advantage that materials can be stochiometrically-deposited, so that the composition of the target and deposit remain nearly identical. However, PLD has the disadvantage of “spall,” i.e. ejecting nano- to micro-scale particulates that travel with the plume to become embedded in the newly-formed deposit. And at high-pressures (i.e. atmospheric pressures and above), homogeneous nucleation tends to occur, where atoms within the plume recombine into nuclei, clusters, and aggregates. In addition, increasing the operating pressure and throughput of a PLD system tends to result in more recombination events. Ultimately, this limits the deposition rate and quality of the deposits that can be created by PLD systems. 
         [0005]    Many different chemical vapor deposition (CVD) techniques exist, including Thermal CVD, plasma enhanced CVD, laser assisted CVD techniques, etc. All of these approaches use chemical precursors that decompose when exposed to elevated temperatures and/or electromagnetic fields—leaving behind a deposit material. The primary disadvantages of CVD methods are: (1) the deposition rate may be very slow, as the process is often maintained at low pressures to reduce homogeneous nucleation, (2) undesired byproducts can be incorporated into the deposit material, (3) obtaining a correct deposit stochiometry is difficult with multi-component systems, (4) the deposit uniformity, composition and crystal structure may depend strongly on the local substrate temperature or other process conditions, and (5) it is difficult (even with plasma enhanced CVD), to ensure that only certain chemical species are present during the reaction, which can influence the ultimate composition/structure of the deposit material. The growth of diamond vs. graphite is an important example, where the species present during the reaction directly influence the resulting crystal structure. 
         [0006]    Accordingly, it would be desirable to provide a method and/or apparatus capable of conducting deposition and etching where specific atomic/molecular species can be directed at a substrate to produce a uniform deposit of desired composition and crystal structure—and at rapid rates. It would also be desirable to provide a method and/or apparatus capable of controlling the stochiometry of the deposit without introducing undesired spall-related particles/agglomerates and/or impurities at high deposition rates. 
       SUMMARY OF INVENTION 
       [0007]    One embodiment relates to an apparatus for carrying out laser-induced atomic/molecular beam deposition. The apparatus comprises: a chemical precursor delivery system with laser window, a target within the precursor delivery system, a first optic configured to direct a laser beam at the target, a first wall having a aperture, a substrate configured to receive an atomic/molecular beam, a second optic configured to direct a laser beam at either a point proximate to or at the aperture in the first wall, and a third (optional) optic, directing a laser beam at the point where the atomic/molecular beam impinges on the substrate. 
         [0008]    Another embodiment relates to a method for carrying out laser-induced atomic/molecular beam deposition. The method comprises: (1) providing a flow of chemical precursor, (2) providing a target within the flow of chemical precursor, having a desired composition, (3) irradiating the target with a laser beam to provide a plume of target material, (4) directing the plume in a desired direction by use of the flow of chemical precursor, (5) passing the precursor and plume of target material through an aperture into a region of lower pressure, (6) irradiating the plume and precursor to create an atomic/molecular beam, (e.g. to reduce the amount of agglomerated particles in the plume or to partially decompose the precursor), (7) directing the atomic/molecular beam onto a substrate to produce a deposition product, and (8) (optional) irradiating the location where the atomic/molecular beam impinges on the substrate to further decompose the atomic/molecular beam or to produce a specific deposit composition, stochiometry, or crystal structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0009]      FIG. 1  is a schematic view of an apparatus for carrying out pulsed laser deposition. 
       
    
    
     DETAILED DESCRIPTION  
       [0010]    Using a focused short-pulse laser beam (i.e., less than about 1 fs to more than about 100 ps), a very high-temperature plasma is created at an aperture through which chemical precursors are flowing. While the chemical precursors are usually gases, they may also be liquids or super-critical fluids). These chemical precursors would typically be Chemical vapor deposition precursors or etchants, e.g. methane, trimethylamine alane, silane, chlorine, etc. Reactive gases, e.g. hydrogen or nitrogen, or inert carrier gases, such as Argon or Krypton may also be used. Adjacent to the aperture, a target comprised of a desired deposition material is placed such that a portion of the short-pulse beam is focused on the target and a plume of target material enters the flow of precursor/reactive/inert gases. Alternatively, nanoparticles of desired deposit material can be created through homogeneous nucleation using a laser beam in a separate chamber (e.g. through photolysis) and carried by the gas flow. 
         [0011]    The plume is oriented such that it will not coat the laser window and/or the precursor delivery system more than necessary. The plume or nanoparticles of desired deposit material are carried along with the flow of precursor and pass through the aperture. A laser beam is focused near or at the aperture, such that some, if not all, molecular bonds are broken on the precursor/reactive gases, and particulates from the plume (or otherwise) are re-heated and potentially evaporated into their atomic components. Some components may be ionized, depending on the temperature. 
         [0012]    During irradiation and while passing through the aperture into a region of relative vacuum, the precursor/plume becomes an atomic/molecular beam, where some of the (thermal) energy is converted into linear motion toward a target substrate or fiber to produce a deposition product. Specific species are produced in the atomic/molecular beam through the properties of the laser beam, the precursor flow rate, the aperture size, the pressure differential across the aperture, the precursors/materials used, etc. The potential for recombination to occur in the atomic/molecular beam is also controlled through similar parameters, so that, if desired, nanoparticles/clusters of specific composition and structure can be grown (or eliminated) in the beam. 
         [0013]    Several apertures may be used to “align” this beam, so that the atomic/molecular beam motion is collimated—and the region were deposition occurs is limited (transverse to the beam). Where atomic/molecular ionized species are present, a magnetic field may be used to select particular species to pass through successive apertures, so that only particular species and/or particles may be present at the final deposition zone. Additional reactive gases may be added in subsequent chambers to form intermediate or desired species within this atomic/molecular beam. 
         [0014]    Next, a bias voltage may be applied between the aperture (where the atomic/molecular beam is created) and the substrate/deposit material. In this way, species can be accelerated toward the substrate/deposit, such that they arrive with a desired energy, and the deposit can act as field-or thermionic-emission sources (e.g. when the deposit is in the form of fibers). Finally, a third laser beam (or a portion of the first/second beams) is directed at the location where the atomic/molecular beam impinges on the substrate/deposit—either to decompose any remaining precursors, desorb undesired by-products off the surface, re-evaporate forming nuclei within the molecular beam, induce the formation of a particular deposit microstructure by controlling the deposit temperature, etc. 
         [0015]    One goal in designing an apparatus is to control the propagation distance of the beam; for example, if it is shorter than the time it takes for components within the beam to recombine into undesireable species/nuclei, such species/nuclei may not be present in the deposition product. More than one atomic beam can also be used to obtain multiple desired species at the reaction zone. One atomic beam can be split into several atomic beams for parallel deposition. Acoustic waves can also be used to separate species, retard or enhance the flow velocity at and beyond the aperture(s), etc. to create desired species or eliminate undesirable species. 
         [0016]    Some benefits include a very high flux of materials along the atomic/molecular beam, so that very high deposition rates are possible. In addition, many disadvantages of the PLD and CVD techniques are eliminated, as species in the beam can be selected, homogeneous nucleation can be controlled, and undesired particulates can be broken into their atomic constituents within the atomic/molecular beam. In addition, optimal precursors and deposition compounds can be created within the atomic/molecular beam, such that particular stochiometries, compositions, and crystal structures can be created and undesired impurities and particulates are eliminated within the deposit material. 
         [0017]    Consider, for example, the deposition of diamond-like carbon at high rates, using hydrogen as a carrier gas and methane or adamantane (for example) as precursor gases. The laser-heated plasma at the aperture will crack the hydrocarbon(s), and given the correct conditions, leave some sp 3  bonded constituents in the gas flow. Using species selectivity, optimal species for diamond growth can be selected through the apertures—and accelerated toward the substrate/deposit. The hydrogen may also be decomposed into atomic hydrogen and selected through the apertures, enhancing the rate of graphite etching. This should allow diamond-like carbon and single-crystal diamond to be grown at large rates. 
         [0018]    Referring to  FIG. 1 , apparatus  10  includes a target  12  that is irradiated with a pulsed laser beam  14 . A plume of material  16  is ejected from the target which is configured such that plume  16  travels in the direction of wall  18 . A carrier or precursor gas may be used to direct the flow of plume  16 . Wall  18  includes an aperture  20  through which at least a portion of plume  16  may pass through. A second laser beam  22  is directed toward the aperture  20  to at least partially irradiate plume  16 . While second laser beam  22  is directed at a point up stream of aperture  20 , it may be desirable to direct another beam at a point on the downstream side of aperture  20 . Alternatively, only a beam directed at a point on the downstream side of aperture  20  may be used. The region downstream of aperture  20  may be at a pressure lower than the region upstream of aperture  20  to accelerate the atomic beam. The carrier or precursor gas may be introduced through an aperture and directed in the direction of wall  18  and aperture  20 . 
         [0019]    The second laser beam  22  energizes the contents of plume  16  to break down agglomerates and nuclei resulting in an atomic or molecular beam. The beam passes between wall  18  and wall  24 . In the region between walls  18  and  24 , a reactive gas may be introduced into the atomic beam to change the composition of the beam. Again, wall  24  includes an aperture  26  and a third laser beam  28  may be directed toward the aperture  26  to at least partially irradiate the atomic beam. While third laser beam  28  is directed at a point up stream of aperture  26 , it may be desirable to direct another beam at a point on the downstream side of aperture  26 . Alternatively, only a beam directed at a point on the downstream side of aperture  26  may be used. The region downstream of aperture  26  may be at a pressure lower than the region upstream of aperture  26  to accelerate the atomic beam. In some embodiments, an electrical potential may be provided between wall  18  and wall  24  to accelerate or slow charged species. Also, a magnetic field may be used to divert charged species of a certain mass and velocity away from aperture  26 , thus selecting what species will advance towards the substrate  36 . 
         [0020]    Another optional chamber is shown between walls  24  and  30 . The atomic beam passes between wall  24  and wall  30 . In the region between walls  24  and  30 , a reactive gas may be introduced into the atomic beam to change the composition of the beam. Again, wall  30  includes an aperture  32  and a fourth laser beam  34  may be directed toward the aperture  32  to at least partially irradiate the atomic beam. While fourth laser beam  28  is directed at a point up stream of aperture  32 , it may be desirable to direct another beam at a point on the downstream side of aperture  32 . Alternatively, only a beam directed at a point on the downstream side of aperture  32  may be used. The region downstream of aperture  32  may be at a pressure lower than the region upstream of aperture  32  to accelerate the atomic beam. In some embodiments, an electrical potential may be provided between wall  24  and wall  30  to accelerate or slow charged species. Also, a magnetic field may be used to divert charged species of a certain mass and velocity away from aperture  32 , thus selecting what species will advance towards the substrate  36 . 
         [0021]    The beam may exit the chamber between walls  24  and  30  through aperture  32 . The atomic beam is then directed toward substrate  36  where fibers, thin films, and other useful structures may be grown. A fifth laser beam  38  (or portion of another beam) is directed at the substrate/fibers at the point of deposition. The fifth laser beam  38  may be used to decompose any remaining precursors, desorb undesired by-products off the surface, re-evaporate forming nuclei within the molecular beam, and/or to induce the formation of a particular deposit microstructure by controlling the deposit temperature. 
         [0022]    While the apparatus is shown having three walls, other numbers may be used. A single wall may be used in conjunction with a laser beam such as beam  38  directed to a point near the point of deposition. Alternatively, a greater number of walls may be used to provided additional chambers for the introduction of reactive gasses.