Patent Application
20240018003
The present invention is the teaching of adapting accelerated neutral atom beam (ANAB) technology to provide a solution to this problem by providing accelerated particles at significant density and controlled velocity so that the mechanical (impact) removal of impurities may be completed without damage to the carbon bonding lattice in the graphene film. It is known that gas cluster ion beams (GCIB) are formed from argon gas (or other inert gas) that is created in a first chamber and expanded into a vacuum through a shaped nozzle. Once the cluster has formed the cluster can be ionized by electron impact and then electrostatically accelerated to a useful velocity. The accelerated cluster can then be broken up by collision with un-accelerated gas atoms, which overcome Van der Waals forces, and then the charged portion of the cluster is electrostatically or magnetically deflected out of the remaining cluster stream forming the accelerated neutral atom beam (ANAB). The neutral accelerated atoms in the ANAB beam travel at the already accelerated velocity in tight formation just as when they were a part of the accelerated cluster. However their path is extremely straight and intense as there is no longer any charge repulsion pushing them away from each other as with a traditional charged particle beam. ANAB formation also allows for extremely uniform control of particle speeds as the ANAB atoms are all accelerated in the same cluster. For example, if a cluster of 1000 atoms is ionized and accelerated through 30 kilovolts, then the velocity attained will be equal to 30 volts potential (30 kV/1000 atoms=30 eV/atom). It is very difficult to transport a beam of particles with very low energies at high intensity due to charge repulsion effects. With ANAB, formed as described above, the particle velocity is highly controllable and can be tuned to provide impact energies just below the binding energy threshold of carbon atoms while having enough impact energy to remove metallics and polymer residues preferentially. ANAB irradiation of graphene films provides reduction of metallic and polymer impurities to acceptable levels to allow entry into semiconductor device manufacturing facilities and other manufacturing locations with strict contamination guidelines. This opens new technical uses for graphene where its properties can provide enhanced performance relative to other existing materials and opens usage of beneficial graphene to such areas as semiconductive components and integrated circuits and other electronic, magnetic, optical, chemical/biological/medical devices, batteries and more.
Gas cluster ion beams (GCIBs) have been employed to smooth, etch, clean, form deposits on, grow films on, or otherwise modify a wide variety of surfaces including for example, metals, semiconductors, and dielectric materials. In applications involving semiconductor and semiconductor-related materials, GCIBs have been employed to clean, smooth, etch, deposit and/or grow films including oxides and others. GCIBs have also been used to introduce doping and lattice-straining atomic species, materials for amorphizing surface layers, and to improve dopant solubility in semiconductor materials. In many cases such GCIB applications have been able to provide results superior to other technologies that employ conventional ions, ion beams, and plasmas. Semiconductor materials include a wide range of materials that may have their electrical properties manipulated by the introduction of dopant materials, and include (without limitation) silicon, germanium, diamond, silicon carbide, and also compound materials comprising group III-IV elements, and group II-VI elements. Because of the ease of forming GCIBs using argon (Ar) as a source gas and because of the inert properties of argon, many applications have been developed for processing the surfaces of implantable medical devices such as coronary stents, orthopedic prostheses, and other implantable medical devices using argon gas GCIBs. In semiconductor applications, a variety of source gases and source gas mixtures have been employed to form GCIBs containing electrical dopants and lattice-straining species, for reactive etching, physical etching, film deposition, film growth, and other useful processes. A variety of practical systems for introducing GCIB processing to a wide range of surface types are known. For example, U.S. Pat. No. 6,676,989 C1 issued to Kirkpatrick et al. teaches a GCIB processing system having a workpiece holder and manipulator suited for processing tubular or cylindrical workpieces such as vascular stents. In another example, U.S. Pat. No. 6,491,800 B2 issued to Kirkpatrick et al. teaches a GCIB processing system having workpiece holders and manipulators for processing other types of non-planar medical devices, including for example, hip joint prostheses. A further example, U.S. Pat. No. 6,486,478 B1 issued to Libby et al. teaches an automated substrate loading/unloading system suitable for processing semiconductor wafers. U.S. Pat. No. 7,115,511 issued to Hautala, teaches the use of a mechanical scanner for scanning a workpiece relative to an un-scanned GCIB. In still another example, U.S. Pat. No. 7,105,199 B2 issued to Blinn et al. teaches the use of GCIB processing to improve the adhesion of drug coatings on medical devices and to modify the elution or release rate of a drug from the medical devices. The teachings of these references are incorporated herein by reference as though set out at length herein.
Materials for optical devices include a wide variety of glasses, quartz, sapphire, diamond, and other hard, transparent materials. Conventional polishing and planarizing including mechanical, chemical-mechanical, and other techniques have not produced adequate surfaces for the most demanding applications. GCIB processing has in many cases been shown to be capable of smoothing and/or planarizing optical surfaces to a degree not obtainable by conventional polishing techniques, but alternative techniques that do not result in a rough interface between the smoothed surface and the underlying bulk material are needed to avoid creation of scattering layers embedded in the optical material.
Although GCIB processing has been employed successfully for many applications, there are new and existing application needs not fully met by GCIB or other state of the art methods and apparatus. In many situations, while a GCIB can produce dramatic atomic-scale smoothing of an initially somewhat rough surface, the ultimate smoothing that can be achieved is often less than the required smoothness, and in other situations GCIB processing can result in roughening moderately smooth surfaces rather than smoothing them further.
Other needs/opportunities also exist as recognized and resolved through embodiments of the present invention that provide some useful instruction for present purposes. In the field of drug-eluting medical implants, GCIB processing has been successful in treating surfaces of drug coatings on medical implants to bind the coating to a substrate or to modify the rate at which drugs are eluted from the coating following implantation into a patient. However, it has been noted that in some cases where GCIB has been used to process drug coatings (which are often very thin and may comprise very expensive drugs), there may occur a weight loss of the drug coating (indicative of drug loss or removal) as a result of the GCIB processing. For the particular cases where such loss occurs (certain drugs and using certain processing parameters) the occurrence is generally undesirable and having a process with the ability to avoid the weight loss, while still obtaining satisfactory control of the drug elution rate, is preferable.
Ions have long been favored for many processes because their electric charge facilitates their manipulation by electrostatic and magnetic fields. This introduces great flexibility in processing. However, in some applications, the charge that is inherent to any ion (including gas cluster ions in a GCIB) may produce undesirable effects in the processed surfaces. GCIB has a distinct advantage over conventional ion beams in that a gas cluster ion with a single or small multiple charge enables the transport and control of a much larger mass-flow (a cluster may consist of hundreds or thousands of molecules) compared to a conventional ion (a single atom, molecule, or molecular fragment.) Particularly in the case of insulating materials, surfaces processed using ions often suffer from charge-induced damage resulting from abrupt discharge of accumulated charges, or production of damaging electrical field-induced stress in the material (again resulting from accumulated charges.) In many such cases, GCIBs have an advantage due to their relatively low charge per mass, but in some instances may not eliminate the target-charging problem. Furthermore, moderate to high current intensity ion beams may suffer from a significant space charge-induced defocusing of the beam that tends to inhibit transporting a well-focused beam over long distances. Again, due to their lower charge per mass relative to conventional ion beams, GCIBs have an advantage, but they do not fully eliminate the space charge transport problem.
A further instance of need or opportunity arises from the fact that although the use of beams of neutral molecules or atoms provides benefit in some surface processing applications and in space charge-free beam transport, it has not generally been easy and economical to produce intense beams of neutral molecules or atoms except for the case of nozzle jets, where the energies are generally on the order of a few milli-electron-volts per atom or molecule, and thus have limited processing capabilities.
In U.S. Pat. No. 4,935,623 of Hughes Electronics Corporation, Knauer has taught a method for forming beams of energetic (1 to 10 eV) charged and/or neutral atoms. Knauer forms a conventional GCIB and directs it at grazing angles against solid surfaces such as silicon plates, which dissociates the cluster ions, resulting in a forward-scattered beam of atoms and conventional ions. This results in an intense but unfocused beam of neutral atoms and ions that may be used for processing, or that following electrostatic separation of the ions may be used for processing as a neutral atom beam by requiring the scattering of the GCIB off of a solid surface to produce dissociation, a significant problem is introduced by the Knauer techniques. Across a wide range of beam energies, a GCIB produces strong sputtering in surfaces that it strikes. It has been clearly shown (see for example Aoki, T and Matsuo, J, “Molecular dynamics simulations of surface smoothing and sputtering process with glancing-angle gas cluster ion beams,” Nucl. Instr. & Meth. in Phys. Research B 257 (2007), pp. 645-648) that even at grazing angles as employed by Knauer, GCIBs produce considerable sputtering of solids, and thus the forward-scattered neutral beam is contaminated by sputtered ions and neutral atoms and other particles originating in the solid surface used for scattering/dissociation. In a multitude of applications including medical device processing applications and semiconductor processing applications, the presence of such sputtered material contaminating the forward-scattered beam renders it unsuitable for use. The teachings of these references and all other patent and publications cited in this application are incorporated herein by reference as though set out at length herein.
In U.S. Pat. No. 7,060,989, Swenson et al. teach the use of a gas pressure cell having gas pressure higher than the beam generation pressure to modify the gas cluster ion energy distribution in a GCIB. The technique lowers the energy of gas cluster ions in a GCIB and modifies some of the surface processing characteristics of such modified GCIBs. Such gas modification of GCIB gas cluster ion energy distribution is helpful, but does not reduce problems caused by charges deposited in the workpiece by the ions in the GCIB and does not solve certain processing problems, as for example, the weight loss of drug coatings during GCIB processing. Although the techniques of Swenson et al. can improve the ultimate surface smoothing characteristics of a GCIB, the result is still less than ideal. The teachings of these references are incorporated herein by reference as though set out at length herein.
Gas clusters and gas cluster ion sizes are typically characterized by N, the number of atoms or molecules (depending on whether the gas is atomic or molecular and including variants such as ions, monomers, dimmers, trimers, ligands) comprising the individual cluster. Many of the advantages contributed by conventional GCIB processing are believed to derive from the low velocities of ions in the GCIB and from the fact that large, loosely bound clusters disintegrate on collision with a solid surface, causing transient heating and pressure but without excessive penetration, implantation, or damage to the substrate beneath the surface. Effects of such large clusters (having N monomers—as defined below—on the order of a few thousand or more) are generally limited to a few tens of Angstroms. However, it has been shown that smaller clusters (having N on the order of a few hundred to about a thousand) produce more damage to an impacted surface and are capable of producing discrete impact craters in a surface (see for example, Houzumi, H., et al. “Scanning tunneling microscopy observation of graphite surfaces irradiated with size-selected Ar cluster ion beams”, Jpn. J. Appl. Phys. V44(8), (2005), p 6252 ff). This crater-forming effect can roughen and remove material from surfaces (etch) in undesirable competition with the surface smoothing effects of the larger clusters. In many other surface processing applications for which GCIB have been found useful, it is believed that the effects of large gas cluster ions and smaller gas cluster ions may compete in counter-productive ways to reduce processing performance. Unfortunately, the readily applied techniques for forming GCIBs all result in generation of beams having a broad distribution of cluster sizes having size, N, ranging from around 100 to as much as several tens of thousands. Often the mean and/or peak of the size distribution lies in the range of from several hundred to a few thousand, with distribution tails gradually diminishing to zero at the size extremes of the distribution. The cluster-ion size distribution and the mean cluster size, NMean, associated with the distribution is dependent on the source gas employed and can be significantly influenced by selection of the parameters of the nozzle used to form the cluster jet, by the pressure drop through the nozzle, and by the nozzle temperature, all according to conventional GCIB formation techniques. Most commercial GCIB processing tools routinely employ magnetic or occasionally electrostatic size separators to remove the smallest ions and clusters (monomers, dimers, trimers, etc. up to around N=lO or more), which are the most damaging. Such filters are often referred to as “monomer filters”, although they typically also remove somewhat larger ions as well as the monomers. Certain electrostatic cluster ion size selectors (as for example the one employed in U.S. Pat. No. 4,935,623, by Knauer) require placing grids of electrical conductors into the beam, which introduces a strong disadvantage due to potential erosion of the grids by the beam, introducing beam contamination while reducing reliability and resulting in the need for additional maintenance to the apparatus. For that reason, monomer and low-mass filters are now typically of the magnetic type (see for examples, U.S. Pat. No. 6,635,883, to Torti et al. and U.S. Pat. No. 6,486,478, to Libby et al.) Aside from the smallest ions (monomers, dimers, etc.), which are effectively removed by magnetic filters, it appears that most GCIBs contain few or no gas cluster ions of sizes below about N=100. It may be that such sizes do not readily form or after forming are not stable. However, clusters in the range from about N=100 to a few hundred seem to be present in the beams of most commercial GCIB processing tools. Values of NMean in the range of from a few hundred to several thousand are commonly encountered when using conventional techniques. Because, for a given acceleration potential the intermediate size clusters travel much faster than the larger clusters, they are more likely to produce craters, rough interfaces, and other undesirable effects, and probably contribute to less than ideal processing when present in a GCIB. The teachings of these references are incorporated herein by reference as though set out at length herein.
Accelerated Neutral Beams are generated and employed to treat the surfaces of various materials (including metals and various materials such as semiconductor materials and dielectric materials as may for example be employed in microfabrication of integrated circuits or micro-mechanical devices) so as to form very shallow—1 to 10 nm or even less-amorphous and/or oxidized layers near the surfaces of such materials. These treated layers are modified in a way that facilitates chemical etching of materials that are otherwise not easily or controllably chemically etched. Thus, chemical etching can be performed using the original unmodified material to act as the etch-stopping layer for the process, making the etch depth controlled by the processing depth of the accelerated Neutral Beam This avoids over-etching and undercutting and other directional etching problems that often interfere with obtaining desired chemical etching results in many materials. Since the depth of penetration of an accelerated Neutral Beam can be controlled to be a preselected depth of from less than 1 nm to as much as 10 nm by controlling dose and energy during processing, a range of very shallow etching depths can be reliably obtained. The process consists of choosing accelerated Neutral Beam parameters to amorphize and/or oxidize a predetermined depth of surface modification for the material selected, irradiating the surface of the material (optionally through a mask or patterned template to control patterning) to form a shallow modified layer, chemically etching the surface of the material, using an etchant that has a high differential etch rate for the modified versus the unmodified material, and using the unmodified material as the etch stop layer for the process. Very shallow, repeatable and controllable etching of a variety of materials is thus enabled. These accelerated Neutral Beams are generated by first forming a conventional accelerated GCIB, then partly or essentially fully dissociating it by methods and operating conditions that do not introduce impurities into the beam, then separating the remaining charged portions of the beam from the neutral portion, and subsequently using the resulting accelerated Neutral Beam for workpiece processing. Depending on the degree of dissociation of the gas cluster ions, the Neutral Beam produced may be a mixture of neutral gas monomers and gas clusters or may essentially consist entirely or almost entirely of neutral gas monomers. It is preferred that the accelerated Neutral Beam is an essentially fully dissociated neutral monomer beam. An advantage of the Neutral Beams that may be produced by the methods and apparatus of the embodiments of this invention, is that they may be used to process electrically insulating materials without producing damage to the material due to charging of the surfaces of such materials by beam transported charges as commonly occurs for all ionized beams including GCIB. For example, in semiconductor and other electronic applications, ions often contribute to damaging or destructive charging of thin dielectric films such as oxides, nitrides, etc. The use of Neutral Beams can enable successful beam processing of polymer, dielectric, and/or other electrically insulating or high resistivity materials, coatings, and films in other applications where ion beams may produce unacceptable side effects due to surface charging or other charging effects. Examples include (without limitation) processing of corrosion inhibiting coatings, and irradiation cross-linking and/or polymerization of organic films. In other examples, Neutral Beam induced modifications of polymer or other dielectric materials (e.g. sterilization, smoothing, improving surface biocompatibility, and improving attachment of and/or control of elution rates of drugs) may enable the use of such materials in medical devices for implant and/or other medical/surgical applications. Further examples include Neutral Beam processing of glass, polymer, and ceramic bio-culture labware and/or environmental sampling surfaces where such beams may be used to improve surface characteristics like, for example, roughness, smoothness, hydrophilicity, and biocompatibility.
Since the parent GCIB, from which accelerated Neutral Beams may be formed by the methods and apparatus of embodiments of the invention, comprises ions, it is readily accelerated to desired energy and is readily focused using conventional ion beam techniques. Upon subsequent dissociation and separation of the charged ions from the neutral particles, the Neutral Beam particles tend to retain their focused trajectories and may be transported for extensive distances with good effect. When neutral gas clusters in a jet are ionized by electron bombardment, they become heated and/or excited. This may result in subsequent evaporation of monomers from the ionized gas cluster, after acceleration, as it travels down the beamline. Additionally, collisions of gas cluster ions with background gas molecules in the ionizer, accelerator and beamline regions, also heat and excite the gas cluster ions and may result in additional subsequent evolution of monomers from the gas cluster ions following acceleration. When these mechanisms for evolution of monomers are induced by electron bombardment and/or collision with background gas molecules (and/or other gas clusters) of the same gas from which the GCIB was formed, no contamination is contributed to the beam by the dissociation processes that results in evolving the monomers.
There are other mechanisms that can be employed for dissociating (or inducing evolution of monomers from) gas cluster ions in a GCIB without introducing contamination into the beam Some of these mechanisms may also be employed to dissociate neutral gas clusters in a neutral gas cluster beam One mechanism is laser irradiation of the cluster-ion beam using infra-red or other laser energy. Laser-induced heating of the gas cluster ions in the laser irradiated GCIB results in excitement and/or heating of the gas cluster ions and causes subsequent evolution of monomers from the beam Another mechanism is passing the beam through a thermally heated tube so that radiant thermal energy photons impact the gas cluster ions in beam The induced heating of the gas cluster ions by the radiant thermal energy in the tube results in excitement and/or heating of the gas cluster ions and causes subsequent evolution of monomers from the beam In another mechanism, crossing the gas cluster ion beam by a gas jet of the same gas or mixture as the source gas used in formation of the GCIB (or other non-contaminating gas) results in collisions of monomers of the gas in the gas jet with the gas clusters in the ion beam producing excitement and/or heating of the gas cluster ions in the beam and subsequent evolution of monomers from the excited gas cluster ions. By depending entirely on electron bombardment during initial ionization and/or collisions (with other cluster ions, or with background gas molecules of the same gas(es) as those used to form the GCIB) within the beam and/or laser or thermal radiation and/or crossed jet collisions of non-contaminating gas to produce the GCIB dissociation and/or fragmentation, contamination of the beam by collision with other materials is avoided.
Through the use of such non-contaminating methods of dissociation described above, the GCIB is dissociated or at least partially dissociated without introducing atoms to the dissociation products or residual clusters that are not part of the original source gas atoms. By using a source gas for initial cluster formation that does not contain atoms which would be contaminants for the workpiece to be processed using the residual clusters or dissociation products, contamination of the workpiece is avoided. When argon or other noble gases are employed, the source gas materials are volatile and not chemically reactive, and upon subsequent irradiation of the workpiece using Neutral Beams these volatile non-reactive atoms are fully released from the workpiece. Thus for workpieces that are optical and gem materials including glasses, quartz, sapphire, diamond, and other hard, transparent materials such as lithium triborate (LBO), argon and other noble gases can serve as source gas materials without contributing contamination due to Neutral Beam irradiation. In other cases, other source gases may be employed, provided the source gas atomic constituents do not include atoms that would result in contamination of the workpiece. For example, for some glass workpieces, LBO, and various other optical materials are oxygen containing, and oxygen atoms may not serve as contaminants. In such cases oxygen-containing source gases may be employed without contamination.
As a neutral gas cluster jet from a nozzle travels through an ionizing region where electrons are directed to ionize the clusters, a cluster may remain un-ionized or may acquire a charge state, q, of one or more charges (by ejection of electrons from the cluster by an incident electron). The ionizer operating conditions influence the likelihood that a gas cluster will take on a particular charge state, with more intense ionizer conditions resulting in greater probability that a higher charge state will be achieved. More intense ionizer conditions resulting in higher ionization efficiency may result from higher electron flux and/or higher (within limits) electron energy. Once the gas cluster has been ionized, it is typically extracted from the ionizer, focused into a beam, and accelerated by falling through an electric field.
The amount of acceleration of the gas cluster ion is readily controlled by controlling the magnitude of the accelerating electric field. Typical commercial GCIB processing tools generally provide for the gas cluster ions to be accelerated by an electric field having an adjustable accelerating potential, VAce, typically of, for example, from about 1 kV to 70 kV (but not limited to that range—VAcc up to 200 kV or even more may be feasible). Thus a singly charged gas cluster ion achieves an energy in the range of from 1 to 70 keV (or more if larger VAcc is used) and a multiply charged (for example, without limitation, charge state, q=3 electronic charges) gas cluster ion achieves an energy in the range of from 3 to 210 keV (or more for higher VAce). For other gas cluster ion charge states and acceleration potentials, the accelerated energy per cluster is qVAcc eV. From a given ionizer with a given ionization efficiency, gas cluster ions will have a distribution of charge states from zero (not ionized) to a higher number such as for example 6 (or with high ionizer efficiency, even more), and the most probable and mean values of the charge state distribution also increase with increased ionizer efficiency (higher electron flux and/or energy). Higher ionizer efficiency also results in increased numbers of gas cluster ions being formed in the ionizer. In many cases, GCIB processing throughput increases when operating the ionizer at high efficiency results in increased GCIB current. A downside of such operation is that multiple charge states that may occur on intermediate size gas cluster ions can increase crater and/or rough interface formation by those ions, and often such effects may operate counterproductively to the intent of the processing. Thus, for many GCIB surface processing recipes, selection of the ionizer operating parameters tends to involve more considerations than just maximizing beam current. In some processes, use of a “pressure cell” (see U.S. Pat. No. 7,060,989, to Swenson et al.) may be employed to permit operating an ionizer at high ionization efficiency while still obtaining acceptable beam processing performance by moderating the beam energy by gas collisions in an elevated pressure “pressure cell.” The teachings of these references are incorporated herein by reference as though set out at length herein.
When the Neutral Beams are formed in embodiments of the present invention there is no downside to operating the ionizer at high efficiency—in fact such operation is sometimes preferred. When the ionizer is operated at high efficiency, there may be a wide range of charge states in the gas cluster ions produced by the ionizer. This results in a wide range of velocities in the gas cluster ions in the extraction region between the ionizer and the accelerating electrode, and also in the downstream beam This may result in an enhanced frequency of collisions between and among gas cluster ions in the beam that generally results in a higher degree of fragmentation of the largest gas cluster ions. Such fragmentation may result in a redistribution of the cluster sizes in the beam, skewing it toward the smaller cluster sizes. These cluster fragments retain energy in proportion to their new size (N) and so become less energetic while essentially retaining the accelerated velocity of the initial unfragmented gas cluster ion. The change of energy with retention of velocity following collisions has been experimentally verified (as for example reported in Toyoda, N. et al., “Cluster size dependence on energy and velocity distributions of gas cluster ions after collisions with residual gas,” Nucl. Instr. & Meth. in Phys. Research B 257 (2007), pp 662-665). Fragmentation may also result in redistribution of charges in the cluster fragments. Some uncharged fragments likely result and multi-charged gas cluster ions may fragment into several charged gas cluster ions and perhaps some uncharged fragments. It is understood by the inventors that design of the focusing fields in the ionizer and the extraction region may enhance the focusing of the smaller gas cluster ions and monomer ions to increase the likelihood of collision with larger gas cluster ions in the beam extraction region and in the downstream beam, thus contributing to the dissociation and/or fragmenting of the gas cluster ions. The teachings of these references are incorporated herein by reference as though set out at length herein.
In an embodiment of the present invention, background gas pressure in the ionizer, acceleration region, and beamline may optionally be arranged to have a higher pressure than is normally utilized for good GCIB transmission. This can result in additional evolution of monomers from gas cluster ions (beyond that resulting from the heating and/or excitement resulting from the initial gas cluster ionization event). Pressure may be arranged so that gas cluster ions have a short enough mean-free-path and a long enough flight path between ionizer and workpiece that they must undergo multiple collisions with background gas molecules.
For a homogeneous gas cluster ion containing N monomers and having a charge state of q and which has been accelerated through an electric field potential drop of VAcc volts, the cluster will have an energy of approximately qVAcc/Nr eV per monomer, where Nr is the number of monomers in the cluster ion at the time of acceleration. Except for the smallest gas cluster ions, a collision of such an ion with a background gas monomer of the same gas as the cluster source gas will result in additional deposition of approximately qVAcc/Nr eV into the gas cluster ion. This energy is relatively small compared to the overall gas cluster ion energy (qVAcc) and generally results in excitation or heating of the cluster and in subsequent evolution of monomers from the cluster. It is believed that such collisions of larger clusters with background gas seldom fragment the cluster but rather heats and/or excites it to result in evolution of monomers by evaporation or similar mechanisms. Regardless of the source of the excitation that results in the evolution of a monomer or monomers from a gas cluster ion, the evolved monomer(s) have approximately the same energy per particle, qVAcc/Nr eV, and retain approximately the same velocity and trajectory as the gas cluster ion from which they have evolved. When such monomer evolutions occur from a gas cluster ion, whether they result from excitation or heating due to the original ionization event, a collision, or radiant heating, the charge has a high probability of remaining with the larger residual gas cluster ion. Thus, after a sequence of monomer evolutions, a large gas cluster ion may be reduced to a cloud of co-traveling monomers with perhaps a smaller residual gas cluster ion (or possibly several if fragmentation has also occurred). The co-traveling monomers following the original beam trajectory all have approximately the same velocity as that of the original gas cluster ion and each has energy of approximately qVAcc/Nr eV. For small gas cluster ions, the energy of collision with a background gas monomer is likely to completely and violently dissociate the small gas cluster and it is uncertain whether in such cases the resulting monomers continue to travel with the beam or are ejected from the beam.
To avoid contamination of the beam by collisions with the background gas, it is preferred that the background gas be the same gas as the gas constituting the gas cluster ions. Nozzles for forming gas cluster jets are typically operated with high gas flow on the order of 100-600 sccm. The portion of this flow that does not condense into gas clusters raises the pressure in the source chamber. In addition to the gas transmitted through the skimmer aperture in the form of gas clusters, unclustered source gas from the source chamber can flow through the skimmer aperture to the downstream beamline or beam path chamber(s). Selecting the skimmer aperture diameter to provide an increased flow of unclustered source gas from the source chamber to the beamline is a convenient way to provide the added beamline pressure to induce background gas collisions with the GCIB. Because of the high source gas flow (unclustered gas through the skimmer aperture and gas transported to the target by the beam) atmospheric gases are quickly purged from the beamline. Alternatively, gas may be leaked into the beamline chamber, or as pointed out above, introduced as a jet crossing the GCIB path. In such case, the gas is preferably the same as the source gas (or inert or otherwise non-contaminating). In critical applications a residual gas analyzer can be employed in the beamline to confirm the quality of the background gas when background gas collisions play a role in the evolution of monomers. Prior to the GCIB reaching the workpiece, the remaining charged particles (gas cluster ions, particularly small and intermediate size gas cluster ions and some charged monomers, but also including any remaining large gas cluster ions) in the beam are separated from the neutral portion of the beam, leaving only a Neutral Beam for processing the workpiece.
In typical operation, the fraction of power in the Neutral Beam relative to that in the full (charged plus neutral) beam delivered at the processing target is in the range of from about 5% to 95%, so by the separation methods and apparatus disclosed herein it is possible to deliver that portion of the kinetic energy of the full accelerated charged beam to the target as a Neutral Beam.
The dissociation of the gas cluster ions and thus the production of high neutral monomer beam energy is facilitated by: 1) Operating at higher acceleration voltages. This increases qVAcc/N for any given cluster size; 2) Operating at high ionizer efficiency. This increases qVAcc/N for any given cluster size by increasing q and increases cluster-ion on cluster-ion collisions in the extraction region due to the differences in charge states between clusters; 3) Operating at a high ionizer, acceleration region, or beamline pressure or operating with a gas jet crossing the beam, or with a longer beam path, all of which increase the probability of background gas collisions for a gas cluster ion of any given size; 4) Operating with laser irradiation or thermal radiant heating of the beam, which directly promote evolution of monomers from the gas cluster ions; and 5) Operating at higher nozzle gas flow, which increases transport of gas, clustered and perhaps unclustered into the GCIB trajectory, which increases collisions resulting in greater evolution of monomers.
For producing background gas collisions, the product of the gas cluster ion beam path length from extraction region to workpiece times the pressure in that region contributes to the degree of dissociation of the gas cluster ions that occurs. For 30 kV acceleration, ionizer parameters provide a mean gas cluster ion charge state of 1 or greater, and a pressure times beam path length of 6×10−3 torr-cm (0.8 pascal-cm) (at 25 deg C) provides a Neutral Beam (after separation from the residual charged ions) that is essentially fully dissociated to neutral energetic monomers. It is convenient and customary to characterize the pressure times beam path length as a gas target thickness. 6×10−3 torr-cm (0.8 pascal-cm) corresponds to a gas target thickness of approximately 1.94×1014 gas molecules/cm2. In one exemplary (not for limitation) embodiment the background gas pressure is 6×10−5 torr (8×10−3 pascal) and the beam path length is 100 cm, the acceleration potential is 30 kV, and in this case the Neutral Beam is observed to be essentially fully dissociated into monomers at the end of the beam path. This is without laser or radiant beam heating and without employing a gas jet crossing the beam. The fully dissociated accelerated Neutral Beam condition results from monomer evolution from cluster heating due to the cluster ionization event, collisions with residual gas monomers, and collisions between clusters in the beam. Using the dissociated Neutral Beam produces improved smoothing results on smoothing a gold film compared to the full beam In another application, using the dissociated Neutral Beam on a drug surface coating on a medical device, or on drug-polymer-mixture layer on a medical device or on a drug-poly-mixture body of a medical device provides improved drug attachment and modification of a drug elution rate without the drug weight loss that occurs when the full GCIB is used. Measurement of the Neutral Beam cannot be made by current measurement as is convenient for gas cluster ion beams. A Neutral Beam power sensor is used to facilitate dosimetry when irradiating a workpiece with a Neutral Beam. The Neutral Beam sensor is a thermal sensor that intercepts the beam (or optionally a known sample of the beam). The rate of rise of temperature of the sensor is related to the energy flux resulting from energetic beam irradiation of the sensor. The thermal measurements must be made over a limited range of temperatures of the sensor to avoid errors due to thermal re-radiation of the energy incident on the sensor. For a GCIB process, the beam power (watts) is equal to the beam current (amps) times VAcc, the beam acceleration voltage. When a GCIB irradiates a workpiece for a period of time (seconds), the energy (joules) received by the workpiece is the product of the beam power and the irradiation time. The processing effect of such a beam when it processes an extended area is distributed over the area (for example, cm2. For ion beams, it has been conveniently conventional to specify a processing dose in terms of irradiated ions/cm2, where the ions are either known or assumed to have at the time of acceleration an average charge state, q, and to have been accelerated through a potential difference of, VAce volts, so that each ion carries an energy of q VAcc eV (an eV is approximately 1.6×10−19 joule). Thus an ion beam dose for an average charge state, q, accelerated by VAcc and specified in ions/cm2 corresponds to a readily calculated energy dose expressible in joules/cm2. For an accelerated Neutral Beam derived from an accelerated GCIB as utilized in embodiments of the present invention, the value of q at the time of acceleration and the value of VAcc is the same for both of the (later-formed and separated) charged and uncharged fractions of the beam The power in the two (neutral and charged) fractions of the GCIB divides proportional to the mass in each beam fraction. Thus, for the accelerated Neutral Beam as employed in embodiments of the invention, when equal areas are irradiated for equal times, the energy dose (joules/cm 2 deposited by the Neutral Beam is necessarily less than the energy dose deposited by the full GCIB. By using a thermal sensor to measure the power in the full GCIB, PG, and that in the Neutral Beam, PN, (which is commonly found to be from about 5% to about 95% that of the full GCIB) it is possible to calculate a compensation factor for use in the Neutral Beam processing dosimetry. When PN is equal to a PG, then the compensation factor is, k=1/a. Thus, if a workpiece is processed using a Neutral Beam derived from a GCIB, for a time duration is made to be k times greater than the processing duration for the full GCIB (including charged and neutral beam portions) required to achieve a dose of D ions/cm2 then the energy doses deposited in the workpiece by both the Neutral Beam and the full GCIB are the same (though the results may be different due to qualitative differences in the processing effects due to differences of particle sizes in the two beams.) As used herein, a Neutral Beam process dose compensated in this way is sometimes described as having an energy/cm2 equivalence of a dose of D ions/cm2.
As described in more detail below, ANAB treatment applied to a graphene layer on a 2 inch diameter SiO2 wafer showed results of shrinkage of surface concentration of copper on the graphene decreased by P4 on orders of magnitude, i.e. from samples of 350×1010 atoms/cm2 using ANAB (Neutral Beam) processing and a one inch wafer treated to produce, as showing below a reduction of copper from 140×1010 atoms/cm2 to 95×1010 atoms/cm2. Significant enhancements were achieved as to other metal contaminants. There was no significant damage to the graphene layer as a result of such irradiation. This is the breakthrough that the semiconductor industry has sought in vain for the last two decades. The potential benefits of semiconductor usage of graphene were well recognized but intractable contamination hindered usage prospects.
Use of a Neutral Beam derived from a gas cluster ion beam in combination with a thermal power sensor for dosimetry in many cases has advantages compared with the use of the full gas cluster ion beam or an intercepted or diverted portion, which inevitably comprises a mixture of gas cluster ions and neutral gas clusters and/or neutral monomers, and which is conventionally measured for dosimetry purposes by using a beam current measurement. Some advantages are as follows:
As used herein, the term “intermediate size”, when referring to gas cluster size or gas cluster ion size is intended to mean sizes of from N=10 to N=150. As used herein, the terms “GCIB”, “gas cluster ion beam” and “gas cluster ion” are intended to encompass not only ionized beams and ions, but also accelerated beams and ions that have had all or a portion of their charge states modified (including neutralized) following their acceleration, the terms “GCIB” and “gas cluster ion beam” are intended to encompass all beams that comprise accelerated gas clusters even though they may also comprise non-clustered particles. As used herein, the term “Neutral Beam” is intended to mean a beam of neutral gas clusters and/or neutral monomers derived from an accelerated gas cluster ion beam and wherein the acceleration results from acceleration of a gas cluster ion beam in referencing a particle in a gas or a particle in a beam, the term “monomer” refers equally to either a single atom or a single molecule the terms “atom,” “molecule,” and “monomer” may be used interchangeably and all refer to the appropriate monomer that is characteristic of the gas under discussion (either a component of a cluster, a component of a cluster ion, or an atom or molecule). For example, a monatomic gas like argon may be referred to in terms of atoms, molecules, or monomers and each of those terms means a single atom. Likewise, in the case of a diatomic gas like nitrogen, it may be referred to in terms of atoms, molecules, or monomers, each term meaning a diatomic molecule. Furthermore, a molecular gas like CO2 or B2H6, may be referred to in terms of atoms, molecules, or monomers, each term meaning a polyatomic molecule. These conventions are used to simplify generic discussions of gases and gas clusters or gas cluster ions independent of whether they are monatomic, diatomic, or molecular in their gaseous form. In referring to a constituent of a molecule or of a solid material, “atom” has its conventional meaning.
The step of removing may remove essentially all charged particles from the beam path. The removing step may form an accelerated neutral beam that is fully dissociated. The neutral beam may consist essentially of gas from the gas cluster ion beam. The treating step may include irradiating the substrate through openings in a patterned template and the shallow modified layer may be patterned. The etching step may produce an etched pattern on the substrate. The patterned template may be a hard mask or a photoresist mask in contact with the surface of the substrate. The step of promoting may include increasing the range of velocities of ions in the accelerated gas cluster ion beam. The step of promoting may include introducing one or more gaseous elements used in forming the gas cluster ion beam into the reduced pressure chamber to increase pressure along the beam path. The etching step may be done using a suitable chemical etchant that has a differential etching rate for the shallow modified layer and the unmodified substrate. The chemical etchant may comprise hydrofluoric acid. The treating step may further comprise scanning the substrate to treat extended portions of the surface with the accelerated neutral beam. The substrate surface may comprise a metal, a semiconductor, or a dielectric material. The shallow modified layer may have a depth of 6 nanometers or less. The shallow modified layer may have a thickness of from about 1 nanometer to about 3 nanometers. The gas cluster ions may comprise argon or another inert gas. The gas cluster ions may further comprise oxygen. The method may further comprise forming an oxygen containing layer on the portion of the substrate prior to the step of irradiating. The oxygen containing layer may be less than 5 monolayers thick. The acceleration step may accelerate the gas cluster ions through a potential of from 5 to 50 kV.
According to another aspect of the present disclosure, a method for additively preparing a surface of a substrate is provided. The method for additively preparing a surface of a substrate includes steps of providing a reduced pressure chamber and forming a gas cluster ion beam within the reduced pressure chamber. In an illustrative embodiment, the gas cluster ion beam includes oxygen gas cluster ions. The method then includes steps of accelerating the oxygen gas cluster ions to form an accelerated oxygen gas cluster ion beam along a beam path within the reduced pressure chamber and promoting fragmentation and/or dissociation of at least a portion of the accelerated oxygen gas cluster ions along the beam path. The method further includes steps of removing charged particles from the beam path to form an accelerated neutral oxygen beam along the beam path in the reduced pressure chamber. According to an aspect of the present disclosure the substrate is held in the beam path. A portion of a surface of the substrate is then irradiated with the accelerated neutral oxygen beam[[.]], wherein the neutral oxygen beam reactively interacts with the substrate to form a stable oxide layer on the surface of the substrate.
For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawings, wherein:
Reference is now made to
A workpiece 160, which may (for example) be a medical device, a semiconductor material, an optical element, or other workpiece to be processed by GCIB processing, is held on a workpiece holder 162 that disposes the workpiece in the path of the GCIB 128. The workpiece holder is attached to but electrically insulated from the processing chamber 108 by an electrical insulator 164. Thus, GCIB 128 striking the workpiece 160 and the workpiece holder 162 flows through an electrical lead 168 to a dose processor 170. A beam gate 172 controls transmission of the GCIB 128 along axis 154 to the workpiece 160. The beam gate 172 typically has an open state and a closed state that is controlled by a linkage 174 that may be (for example) electrical, mechanical, or electromechanical. Dose processor 170 controls the open/closed state of the beam gate 172 to manage the GCIB dose received by the workpiece 160 and the workpiece holder 162. In operation, the dose processor 170 opens the beam gate 172 to initiate GCIB irradiation of the workpiece 160. Dose processor 170 typically integrates GCIB electrical current arriving at the workpiece 160 and workpiece holder 162 to calculate an accumulated GCIB irradiation dose. At a predetermined dose, the dose processor 170 closes the beam gate 172, terminating processing when the predetermined dose has been achieved.
In the following description, for simplification of the drawings, item numbers from earlier figures may appear in subsequent figures without discussion. Likewise, items discussed in relation to earlier figures may appear in subsequent figures without item numbers or additional description. In such cases items with like numbers are like items and have the previously described features and functions and illustration of items without item numbers shown in the present figure refer to like items having the same functions as the like items illustrated in earlier numbered figures.
Any workpiece surfaces that are non-planar, for example, spherical or cup-like, rounded, irregular, or other un-flat configuration, may be oriented within a range of angles with respect to the beam incidence to obtain optimal GCIB processing of the workpiece surfaces. The workpiece holder 202 can be fully articulated for orienting all non-planar surfaces to be processed in suitable alignment with the GCIB 128 to provide processing optimization and uniformity. More specifically, when the workpiece 160 being processed is non-planar, the workpiece holder 202 may be rotated in a rotary motion 210 and articulated in articulation motion 212 by an articulation/rotation mechanism 204. The articulation/rotation mechanism 204 may permit 360 degrees of device rotation about longitudinal axis 206 (which is coaxial with the axis 154 of the GCIB 128) and sufficient articulation about an axis 208 perpendicular to axis 206 to maintain the workpiece surface within a desired range of beam incidence.
Under certain conditions, depending upon the size of the workpiece 160, a scanning system may be desirable to produce uniform irradiation of a large workpiece. Although often not necessary for GCIB processing, two pairs of orthogonally oriented electrostatic scan plates 130 and 132 may be utilized to produce a raster or other scanning pattern over an extended processing area. When such beam scanning is performed, a scan generator 156 provides X-axis scanning signal voltages to the pair of scan plates 132 through lead pair 159 and Y-axis scanning signal voltages to the pair of scan plates 130 through lead pair 158. The scanning signal voltages are commonly triangular waves of different frequencies that cause the GCIB 128 to be converted into a scanned GCIB 148, which scans the entire surface of the workpiece 160. A scanned beam-defining aperture 214 defines a scanned area. The scanned beam-defining aperture 214 is electrically conductive and is electrically connected to the low-pressure vessel 102 wall and supported by support member 220. The workpiece holder 202 is electrically connected via a flexible electrical lead 222 to a faraday cup 216 that surrounds the workpiece 160 and the workpiece holder 202 and collects all the current passing through the defining aperture 214. The workpiece holder 202 is electrically isolated from the articulation/rotation mechanism 204 and the faraday cup 216 is electrically isolated from and mounted to the low-pressure vessel 102 by insulators 218. Accordingly, all current from the scanned GCIB 148, which passes through the scanned beam-defining aperture 214 is collected in the faraday cup 216 and flows through electrical lead 224 to the dose processor 170. In operation, the dose processor 170 opens the beam gate 172 to initiate GCIB irradiation of the workpiece 160. The dose processor 170 typically integrates GCIB electrical current arriving at the workpiece 160 and workpiece holder 202 and faraday cup 216 to calculate an accumulated GCIB irradiation dose per unit area. At a predetermined dose, the dose processor 170 closes the beam gate 172, terminating processing when the predetermined dose has been achieved. During the accumulation of the predetermined dose, the workpiece 160 may be manipulated by the articulation/rotation mechanism 204 to ensure processing of all desired surfaces.
The Neutral Beam 314 contains a repeatable fraction of the initial energy of the accelerated GCIB 128. The remaining ionized portion 316 of the original GCIB 128 has been removed from the Neutral Beam 314 and is collected by the grounded deflection plate 304. The ionized portion 316 that is removed from the Neutral Beam 314 may include monomer ions and gas cluster ions including intermediate size gas cluster ions. Because of the monomer evaporation mechanisms due to cluster heating during the ionization process, intra-beam collisions, background gas collisions, and other causes (all of which result in erosion of clusters) the Neutral Beam substantially consists of neutral monomers, while the separated charged particles are predominately cluster ions. The inventors have confirmed this by suitable measurements that include re-ionizing the Neutral Beam and measuring the charge to mass ratio of the resulting ions. The separated charged beam components largely consist of cluster ions of intermediate size as well as monomer ions and perhaps some large cluster ions. As will be shown below, certain superior process results are obtained by processing workpieces using this Neutral Beam.
Thermal sensor controller 420 communicates with a dosimetry controller 432 through an electrical cable 428. A beam current measurement device 424 measures beam current 1B flowing in electrical lead 168 when the GCIB 128 strikes the workpiece 160 and/or the workpiece holder 162. Beam current measurement device 424 communicates a beam current measurement signal to dosimetry controller 432 via electrical cable 426. Dosimetry controller 432 controls setting of open and closed states for beam gate 172 by control signals transmitted via linkage 434. Dosimetry controller 432 controls deflector power supply 440 via electrical cable 442 and can control the deflection voltage VD between voltages of zero and a positive voltage adequate to completely deflect the ionized portion 316 of the GCIB 128 to the deflection plate 304. When the ionized portion 316 of the GCIB 128 strikes deflection plate 304, the resulting current ID is measured by current sensor 422 and communicated to the dosimetry controller 432 via electrical cable 430. In operation dosimetry controller 432 sets the thermal sensor 402 to the parked position 414, opens beam gate 172, sets VD to zero so that the full GCIB 128 strikes the workpiece holder 162 and/or workpiece 160. The dosimetry controller 432 records the beam current 1B transmitted from beam current measurement device 424. The dosimetry controller 432 then moves the thermal sensor 402 from the parked position 414 to intercept the GCIB 128 by commands relayed through thermal sensor controller 420. Thermal sensor controller 420 measures the beam energy flux of GCIB 128 by calculation based on the heat capacity of the sensor and measured rate of temperature rise of the thermal sensor 402 as its temperature rises through a predetermined measurement temperature (for example 70 degrees C.) and communicates the calculated beam energy flux to the dosimetry controller 432 which then calculates a calibration of the beam energy flux as measured by the thermal sensor 402 and the corresponding beam current measured by the beam current measurement device 424. The dosimetry controller 432 then parks the thermal sensor 402 at parked position 414, allowing it to cool and commands application of positive VD to deflection plate 302 until all of the current Io due to the ionized portion of the GCIB 128 is transferred to the deflection plate 304. The current sensor 422 measures the corresponding ID and communicates it to the dosimetry controller 432. The dosimetry controller also moves the thermal sensor 402 from parked position 414 to intercept the Neutral Beam 314 by commands relayed through thermal sensor controller 420. Thermal sensor controller 420 measures the beam energy flux of the Neutral Beam 314 using the previously determined calibration factor and the rate of temperature rise of the thermal sensor 402 as its temperature rises through the predetermined measurement temperature and communicates the Neutral Beam energy flux to the dosimetry controller 432. The dosimetry controller 432 calculates a neutral beam fraction, which is the ratio of the thermal measurement of the Neutral Beam 314 energy flux to the thermal measurement of the full GCIB 128 energy flux at sensor 402. Under typical operation, a neutral beam fraction of about 5% to about 95% is achieved. Before beginning processing, the dosimetry controller 432 also measures the current, ID, and determines a current ratio between the initial values of IB and ID. During processing, the instantaneous ID measurement multiplied by the initial IB/ID ratio may be used as a proxy for continuous measurement of the IB and employed for dosimetry during control of processing by the dosimetry controller 432. Thus, the dosimetry controller 432 can compensate any beam fluctuation during workpiece processing, just as if an actual beam current measurement for the full GCIB 128 were available. The dosimetry controller uses the neutral beam fraction to compute a desired processing time for a particular beam process. During the process, the processing time can be adjusted based on the calibrated measurement of ID for correction of any beam fluctuation during the process.
An alternate schematic of a Neutral Beam processing apparatus according to an embodiment of the invention that uses a sample of deflected ion beam current collected in a faraday cup as a component of a dosimetry scheme. In this embodiment of the invention, a sample of the ionized portion 316 (as shown in
Downstream of the acceleration electrode 948, a retarding electrode 952 is employed to ensure deceleration of the ionized portion 958 of the GCIB 128. Retarding electrode 952 is biased at a positive retarding voltage, VR, by retarding voltage power supply 942. A retarding voltage VR of a few kV is generally adequate to ensure that all ions in the GCIB 128 are decelerated and returned to the acceleration electrode 948. Permanent magnet arrays 950 are attached to the acceleration electrode 948 to provide magnetic suppression of secondary electrons that would otherwise be emitted as a result of the returned ions striking the acceleration electrode 948. A beam gate 172 is a mechanical beam gate and is located upstream of the workpiece 160. A dosimetry controller 946 controls the process dose received by the workpiece. A thermal sensor 402 is placed into a position that intercepts the Neutral Beam 314 for Neutral Beam energy flux measurement or in the parked position for Neutral Beam processing of the workpiece under control of the thermal sensor controller 420. When thermal sensor 402 is in the beam sensing position, the Neutral Beam energy flux is measured and transmitted to the dosimetry controller 946 over electrical cable 956. In normal use, the dosimetry controller 946 closes the beam gate 172 and commands the thermal sensor controller 420 to measure and report the energy flux of the Neutral Beam 314. Next, a conventional workpiece loading mechanism (not shown) places a new workpiece on the workpiece holder. Based on the measured Neutral Beam energy flux, the dosimetry controller 946 calculates an irradiation time for providing a predetermined desired Neutral Beam energy dose. The dosimetry controller 946 commands the thermal sensor 402 out of the Neutral Beam 314 and opens the beam gate 172 for the calculated irradiation time and then closes the beam gate 172 at the end of the calculated irradiation time to terminate the processing of the workpiece 160.
A process for cleaning impurities from graphene by using ANAB according to an aspect of the present disclosure is described with reference to
Contamination of graphene layers results from the process of forming graphene layers in which the graphene is grown on a copper layer. A polymer is applied over the graphene layer and then removed to remove the graphene from the copper layer. The graphene side of the polymer is then placed on a SiO2 substrate to deposit the graphene onto the SiO2. The polymer is then removed, leaving the graphene layer on the SiO2 substrate. Impurities, such as copper atoms from the copper layer are inevidablly found on the graphene layer.
According to an aspect of the present disclosure, the energy of an ANAB beam was tuned to process a graphene surface for removing contaminants such as copper. Raman spectroscopic analysis was performed after ANAB irradiation to confirm that the graphene structure had not been damaged by the ANAB irradiation. This example reveals that ANAB can remove impurities from graphene films without the resorting to the use of chemical methods that have been attempted in the past and without damaging the graphene structure.
In this example, the graphene samples under test were provided on coupons of SiO2. Metalized bars had been formed on the SiO2 and the graphene had been laid over the metalized bars.
Initially, resistivity of the coupons was measured. The coupons were then subject to a vacuum environment. It was observed that resistivity of the coupons changed during application of the vacuum as the moisture and ambient atmosphere was pumped away. The resistivity of the coupons then became stable.
After the resistivity of the coupons stabilized under vacuum, the coupons were irradiated with ANAB at higher and higher energy levels, while observing how much the resistivity of the SiO2 coupons changed upon being radiated with different energy level ANAB beams. It was observed that irradiation with ANAB beams having an energy level of about 5 KV made only incremental changes to the resistivity up to a certain resistivity level. After reaching the certain resistivity level, it was observed that changes to resistivity of the coupon stopped, i.e., resistivity of the coupon stabilized even though application of the ANAB irradiation continued.
For samples in which it was observed that resistivity stabilized and ceased to increase with continued ANAB irradiation, Raman spectroscopy showed the graphene was still continuous and undamaged. For samples in which resistivity continued to change or increased more than a certain amount, Raman spectroscopy revealed that the graphene had been damaged.
Based on these observations applicants determined that ANAB irradiation of graphene samples would not damage the graphene if the energy of the ANAB beam is at or below 5 KV ANAB, i.e. range of 1-5 KV being preferred under these or similar conditions, without prejudice to higher energy under other sample conditions.
Referring to
It was observed that approximately the same amount of copper was removed from the graphene in the measurements shown in
Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments within the spirit and scope of the invention.
The present application claims priority to U.S. Provisional Patent Application No. 63/356,718, filed Jun. 29, 2022 and incorporated herein in its entirety. This invention relates generally to methods and apparatus for removing graphene production processing residuals (contaminants) from the finished graphene product using low energy, Neutral Beam processing and, preferably, using high beam purity methods and systems for deriving an accelerated neutral monomer (e.g. atom) beam from an accelerated gas cluster ion beam to remove the residuals without disrupting the basic crystalline structure and morphology of the graphene product. Graphene is an ultra-strong, flexible, and extremely conductive material usually manufactured by chemical vapor deposition (CVD) synthesis of carbon using copper substrates to initiate the formation of a monolayer of carbon atoms all bound to three other carbon atoms in overall hexagonal arrays forming a perfect two-dimensional sheet of flat or curved forms. Due to its intrinsic properties, graphene is finding many new uses that exploit its extreme strength, flexibility, and conductivity. Graphene often is grown using a CVD process involving decomposition of gasses on various metals (usually transition metals and preferably copper). The metallic surface provides a substrate surface on which carbon atoms condense in 2D form, as a hexagonal crystalline lattice, as they are released from hydrocarbon gasses used in the CVD process. Once the formation of graphene layer is complete it can be coated with an adhesive polymer and the graphene can be separated (delaminated) from the metallic substrate leaving a free-standing graphene OCT polymer membrane. The graphene can then be applied for use as a free-standing membrane or applied to another surface after which the polymer can be chemically dissolved to leave a bare graphene surface. Variations of this method have allowed graphene to be formed for many uses, but these have been limited by the eventual lack of necessary purity of the remaining graphene surface. The process of formation and then attachment with polymer and then removal leaves behind impurities at unacceptable levels for high purity applications (such as in semiconductor usage). Facilities such as those used in the formation of semiconductors and other devices that are manufactured to extremely high purity standards will not allow graphene contaminated with residual metals (copper in particular) to enter their manufacturing environment. Contaminants can also arise from etchants e.g., FeCl3, used in removing metallic substrates. These cautions also apply to protecting tools used in semiconductor manufacturing. This situation has greatly limited the uses of graphene in these high purity applications. Chemical methods have not been able to remove these impurities to acceptable levels and typical ion bombardment techniques have not been able to successfully remove contaminants without destroying the overall continuity of the graphene structure due to the excessive energies required to provide transport to these charged particles.
| Number | Date | Country | |
|---|---|---|---|
| 63356718 | Jun 2022 | US |
