Abstract
A method for removing contaminants from a graphene product uses an accelerated neutral atom beam to remove product contaminants without disruption of the product's crystalline lattice and morphology to enable usage in high purity devices/systems such as exemplified in semi-conductor and like high purity needs applications.
Claims
1. A method for enhancing purity of graphene product with surface contaminants comprising the steps of: providing a reduced pressure chamber and mounting therein a target graphene product per se or on a carrier layer for it, the product having contaminants at its exposed surface or surfaces; forming gas cluster comprising inert gas cluster ions and neutral atoms within the reduced pressure chamber and accelerating it as a beam along a path; promoting fragmentation and/or dissociation of at least a portion of the accelerated gas cluster ions along the beam path; removing charged particles from the beam path to form an accelerated beam of neutral atoms (Neutral Beam) along the beam path in the reduced pressure chamber; holding the target graphene product in the beam path; irradiating all or a portion of a surface of the graphene product with the Neutral Beam under controlled dosimetry and Neutral Beam velocity and energy conditions, whereby the Beam removes impurities to create a crystalline graphene surface free of the contaminants doing so without disrupting the lattice morphology of the irradiated surface(s).
2. The method of claim 1, wherein the step of removing removes essentially all charged particles from the beam path.
3. The method of claim 1, wherein the removing step forms an accelerated neutral beam that is fully dissociated.
4. The method of claim 1, wherein the step of promoting includes increasing the range of velocities of ions in the accelerated gas cluster ion beam.
5. The method of claim 1, wherein the step of promoting includes 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.
6. The method of claim 1, wherein the acceleration step accelerates the gas cluster ions through a potential of from 1 to 50 KV.
7. The method of claim 6, wherein the acceleration step accelerates the gas cluster ions through a potential of from 5 to 50 kV.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a schematic illustrating elements of a prior art apparatus for processing a workpiece using a GCIB;
[0037] FIG. 2 is a schematic illustrating elements of another prior art apparatus for workpiece processing using a GCIB, wherein scanning of the ion beam and manipulation of the workpiece is employed;
[0038] FIG. 3 is a schematic of an apparatus according to an embodiment of the invention, which uses electrostatic deflection plates to separate the charged and uncharged beam components;
[0039] FIG. 4 is a schematic of an apparatus according to an embodiment of the invention, using a thermal sensor for Neutral Beam measurement;
[0040] FIG. 5 is a schematic of an apparatus according to an embodiment of the invention which uses deflected ion beam current collected on a suppressed deflection plate as a component of a dosimetry scheme;
[0041] FIG. 6 shows a schematic of an apparatus according to an embodiment of the invention, employing mechanical scanning for irradiating an extended workpiece uniformly with a Neutral Beam;
[0042] FIG. 7 shows a schematic of an apparatus according to an embodiment of the invention with means for controlling the gas target thickness by injecting gas into the beamline chamber;
[0043] FIG. 8 shows a schematic of an apparatus according to an embodiment of the invention, which uses an electrostatic mirror to separate charged and neutral beam components;
[0044] FIG. 9 shows a schematic of an apparatus according to an embodiment of the invention wherein an accelerate-decelerate configuration is used to separate the charged beam from the neutral beam components;
[0045] FIG. 10 shows a schematic of an apparatus according to an embodiment of the invention wherein an alternate accelerate-decelerate configuration is used to separate the charged beam from the neutral beam components;
[0046] FIG. 11 is a schematic of a Neutral Beam processing apparatus according to an embodiment of the invention wherein magnetic separation is employed;
[0047] FIGS. 12A-12C were photomicrographic showing effects of full and charge separated beams;
[0048] FIGS. 13A and 13B are charts showing decontamination test results achieved by using the method and apparatus of the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
[0049] Reference is now made to FIG. 1, which shows a schematic configuration for a prior art GCIB processing apparatus 100. A low-pressure vessel 102 has three fluidly connected chambers: a nozzle chamber 104, an ionization/acceleration chamber 106, and a processing chamber 108. The three chambers are evacuated by vacuum pumps 146a, 146b, and 146c, respectively. A pressurized condensable source gas 112 (for example argon) stored in a gas storage cylinder 111 flows through a gas metering valve 113 and a feed tube 114 into a stagnation chamber 116. Pressure (typically a few atmospheres) in the stagnation chamber 116 results in ejection of gas into the substantially lower pressure vacuum through a nozzle 110, resulting in formation of a supersonic gas jet 118. Cooling, resulting from the expansion in the jet, causes a portion of the gas jet 118 to condense into clusters, each consisting of from several to several thousand weakly bound atoms or molecules. A gas skimmer aperture 120 is employed to control flow of gas into the downstream chambers by partially separating gas molecules that have not condensed into a cluster jet from the cluster jet. Excessive pressure in the downstream chambers can be detrimental by interfering with the transport of gas cluster ions and by interfering with management of the high voltages that may be employed for beam formation and transport. Suitable condensable source gases 112 including but are not limited to argon and other condensable noble gases, nitrogen, carbon dioxide, oxygen, and many other gases and/or gas mixtures. After formation of the gas clusters in the supersonic gas jet 118, at least a portion of the gas clusters are ionized in an ionizer 122 that is typically an electron impact ionizer that produces electrons by thermal emission from one or more incandescent filaments 124 (or from other suitable electron sources) and accelerates and directs the electrons, enabling them to collide with gas clusters in the gas jet 118. Electron impacts with gas clusters eject electrons from some portion of the gas clusters, causing those clusters to become positively ionized. Some clusters may have more than one electron ejected and may become multiply ionized. Control of the number of electrons and their energies after acceleration typically influences the number of ionizations that may occur and the ratio between multiple and single ionizations of the gas clusters. A suppressor electrode 142, and grounded electrode 144 extract the cluster ions from the ionizer exit aperture 126, accelerate them to a desired energy (typically with acceleration potentials of from several hundred V to several tens of kV), and focuses them to form a GCIB 128. The region that the GCIB 128 traverses between the ionizer exit aperture 126 and the suppressor electrode 142 is referred to as the extraction region. The axis (determined at the nozzle 110), of the supersonic gas jet 118 containing gas clusters is substantially the same as the axis 154 of the GCIB 128. Filament power supply 136 provides filament voltage Vf to heat the ionizer filament 124. Anode power supply 134 provides anode voltage VA to accelerate thermoelectrons emitted from filament 124 to cause the thermoelectrons to irradiate the cluster-containing gas jet 118 to produce cluster ions. A suppression power supply 138 supplies suppression voltage Vs (on the order of several hundred to a few thousand volts) to bias suppressor electrode 142. Accelerator power supply 140 supplies acceleration voltage VAcc to bias the ionizer 122 with respect to suppressor electrode 142 and grounded electrode 144 so as to result in a total GCIB acceleration potential equal to VAcc. Suppressor electrode 142 serves to extract ions from the ionizer exit aperture 126 of ionizer 122 and to prevent undesired electrons from entering the ionizer 122 from downstream, and to form a focused GCIB 128.
[0050] 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.
[0051] 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.
[0052] FIG. 2 shows a schematic illustrating elements of another prior art GCIB processing apparatus 200 for workpiece processing using a GCIB, wherein scanning of the ion beam and manipulation of the workpiece is employed. A workpiece 160 to be processed by the GCIB processing apparatus 200 is held on a workpiece holder 202, disposed in the path of the GCIB 128. In order to accomplish uniform processing of the workpiece 160, the workpiece holder 202 is designed to manipulate workpiece 160, as may be required for uniform processing.
[0053] 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.
[0054] 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.
[0055] FIG. 3 is a schematic of a Neutral Beam processing apparatus 300 according to an embodiment of the invention, which uses electrostatic deflection plates to separate the charged and uncharged portions of a GCIB. A beamline chamber 107 encloses the ionizer and accelerator regions and the workpiece processing regions. The beamline chamber 107 has high conductance and so the pressure is substantially uniform throughout. A vacuum pump 146b evacuates the beamline chamber 107. Gas flows into the beamline chamber 107 in the form of clustered and unclustered gas transported by the gas jet 118 and in the form of additional unclustered gas that leaks through the gas skimmer aperture 120. A pressure sensor 330 transmits pressure data from the beamline chamber 107 through an electrical cable 332 to a pressure sensor controller 334, which measures and displays pressure in the beamline chamber 107. The pressure in the beamline chamber 107 depends on the balance of gas flow into the beamline chamber 107 and the pumping speed of the vacuum pump 146b. By selection of the diameter of the gas skimmer aperture 120, the flow of source gas 112 through the nozzle 110, and the pumping speed of the vacuum pump 146b, the pressure in the beamline chamber 107 equilibrates at a pressure, PB, determined by design and by nozzle flow. The GCIB flight path from grounded electrode 144 to workpiece holder 162, is for example, 100 cm. By design and adjustment PB may be approximately 6105 torr (8103 pascal). Thus the product of pressure and beam path length is approximately 6103 torr-cm (0.8 pascal-cm) and the gas target thickness for the beam is approximately 1.941014 gas molecules per cm2 which combined with monomer evolution due to the initial ionization of the gas clusters in the ionizer 122 and collisions that occur between gas cluster ions in the GCIB 128 is observed to be effective for dissociating the gas cluster ions in the GCIB 128 and results in a fully dissociated accelerated Neutral Beam 314. VAcc may be for example 30 kV and the GCIB 128 is accelerated by that potential. A pair of deflection plates (302 and 304) is disposed about the axis 154 of the GCIB 128. A deflector power supply 306 provides a positive deflection voltage VD to deflection plate 302 via electrical lead 308. Deflection plate 304 is connected to electrical ground by electrical lead 312 and through current sensor/display 310. Deflector power supply 306 is manually controllable. VD may be adjusted from zero to a voltage sufficient to completely deflect the ionized portion 316 of the GCIB 128 onto the deflection plate 304 (for example a few thousand volts). When the ionized portion 316 of the GCIB 128 is deflected onto the deflection plate 304, the resulting current, I.sub.D flows through electrical lead 312 and current sensor/display 310 for indication. When VD is zero, the GCIB 128 is undeflected and travels to the workpiece 160 and the workpiece holder 162. The GCIB beam current 1B is collected on the workpiece 160 and the workpiece holder 162 and flows through electrical lead 168 and current sensor/display 320 to electrical ground. 1B is indicated on the current sensor/display 320. A beam gate 172 is controlled through a linkage 338 by beam gate controller 336. Beam gate controller 336 may be manual or may be electrically or mechanically timed by a preset value to open the beam gate 172 for a predetermined interval. In use, VD is set to zero, and the beam current, IB, striking the workpiece holder is measured. Based on previous experience for a given GCIB process recipe, an initial irradiation time for a given process is determined based on the measured current, IB. V.sub.D is increased until all measured beam current is transferred from 1B to I.sub.D and I.sub.D no longer increases with increasing V.sub.D. At this point a Neutral Beam 314 comprising energetic dissociated components of the initial GCIB 128 irradiates the workpiece holder 162. The beam gate 172 is then closed and the workpiece 160 placed onto the workpiece holder 162 by conventional workpiece loading means (not shown). The beam gate 172 is opened for the predetermined initial radiation time. After the irradiation interval, the workpiece may be examined and the processing time adjusted as necessary to calibrate the desired duration of Neutral Beam processing based on the measured GCIB beam current IB. Following such a calibration process, additional workpieces may be processed using the calibrated exposure duration.
[0056] 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.
[0057] FIG. 4 is a schematic of a Neutral Beam processing apparatus 400 according to an embodiment of the invention, which uses a thermal sensor for Neutral Beam measurement. A thermal sensor 402 attaches via low thermal conductivity attachment 404 to a rotating support arm 410 attached to a pivot 412. Actuator 408 moves thermal sensor 402 via a reversible rotary motion 416 between positions that intercept the Neutral Beam 314 or GCIB 128 and a parked position indicated by 414 where the thermal sensor 402 does not intercept any beam When thermal sensor 402 is in the parked position (indicated by 414) the GCIB 128 or Neutral Beam 314 continues along path 406 for irradiation of the workpiece 160 and/or workpiece holder 162. A thermal sensor controller 420 controls positioning of the thermal sensor 402 and performs processing of the signal generated by thermal sensor 402. Thermal sensor 402 communicates with the thermal sensor controller 420 through an electrical cable 418.
[0058] 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 I.sub.D 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 I.sub.D 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, I.sub.D, and determines a current ratio between the initial values of IB and I.sub.D. During processing, the instantaneous I.sub.D measurement multiplied by the initial IB/I.sub.D 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 I.sub.D for correction of any beam fluctuation during the process.
[0059] FIG. 5 is a schematic of a Neutral Beam processing apparatus 500 according to an embodiment of the invention that uses deflected ion beam current collected on a suppressed deflection plate as a component of a dosimetry scheme. Referring briefly to FIG. 4, the dosimetry scheme shown in FIG. 4 can suffer from the fact that the current, I.sub.D, includes the current due to the ionized portion 316 of the GCIB 128 as well as secondary electron currents resulting from ejection of secondary electrons emitted when the ionized portion 316 of the beam strikes deflection plate 304. The secondary electron yield can vary depending on the distribution of cluster ion sizes in the ionized portion 316. It can also vary depending on the surface state (cleanliness, etc.) of the impacted surface of the deflection plate 304. Thus, in the scheme described in FIG. 4, the magnitude of I.sub.D is not a precise representation of the current due to the ionized portion 316 of the GCIB 128. Referring again now to FIG. 5, an improved measurement of the ionized portion 316 of GCIB 128 can be realized at deflection plate 304 by adding an electron suppressor grid electrode 502 proximal to the surface of deflection plate 304 that receives the ionized portion 316. The electron suppressor grid electrode 502 is highly transparent to the ionized portion 316 but is biased negative with respect to the deflection plate 304 by second suppressor voltage Vs2 provided by second suppressor power supply 506. Effective suppression of secondary electrons is typically achieved by a Vs2 on the order of several tens of volts. By suppressing the emission of secondary electrons, the current loading of deflector power supply 440 is reduced and the precision of the I.sub.D representation of the current in the ionized portion 316 of the GCIB 128 is increased. Electron suppressor grid 502 is insulated from and maintained in proximity to deflection plate 304 by insulating supports 504.
[0060] 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 FIG. 5) is captured in a faraday cup. Sample current, I.sub.S, collected in the faraday cup is conducted via electrical lead to current sensor 562 for measurement, and the measurement is communicated to a dosimetry controller via electrical cable. Faraday cup provides a superior current measurement to that obtained by measuring the current I.sub.D collected by deflection plate 304 (as shown in FIG. 5). Current sensor operates substantially as previously described for the current sensor 422 (as shown in FIG. 5) except that current sensor has increased sensitivity to accommodate the smaller magnitude offs as compared to I.sub.D. Dosimetry controller operates substantially as previously described for dosimetry controller 432 (as shown in FIG. 5) except that it is designed to accommodate a smaller current measurement I.sub.S (as compared to I.sub.D of FIG. 5).
[0061] FIG. 6 is a schematic of a Neutral Beam processing apparatus 600 according to an embodiment of the invention that uses mechanical scanner 602 to scan a spatially extended workpiece 160 through the Neutral Beam 314 to facilitate uniform Neutral Beam scanning of a large workpiece. Since the Neutral Beam 314 cannot be scanned by magnetic or electrostatic techniques, when the workpiece 160 to be processed is spatially larger than the extent of the Neutral Beam 314 and uniform processing of the workpiece 160 is required, a mechanical scanner 602 is employed to scan the workpiece 160 through the Neutral Beam 314. Mechanical scanner 602 has a workpiece holder 616 for holding workpiece 160. The mechanical scanner 602 is disposed so that either the Neutral Beam 314 or the GCIB 128 can be incident on the workpiece 160 and/or the workpiece holder 616. When the deflection plates (302,304) deflect the ionized portion 316 out of the GCIB 128, the workpiece 160 and/or the workpiece holder 616 receive only the Neutral Beam 314. When the deflection plates (302, 304) do not deflect the ionized portion 316 of the GCIB 128, the workpiece 160 and/or the workpiece holder 616 receives the full GCIB 128. Workpiece holder 616 is electrically conductive and is insulated from ground by insulator 614. Beam current (IB) due to GCIB 128 incident on the workpiece 160 and/or the workpiece holder 616 is conducted to beam current measurement device 424 via electrical lead 168. Beam current measurement device 424 measures IB and communicates the measurement to dosimetry controller 628. Mechanical scanner 602 has an actuator base 604 containing actuators controlled by mechanical scan controller 618 via electrical cable 620. Mechanical scanner 602 has a Y-displacement table 606 capable of reversible motion in an Y-direction 610, and it has an X-displacement table 608 capable of reversible motion in an X-direction 612, indicated as in and out of the plane of the paper of FIG. 6. Movements of the Y-displacement table 606 and of the X-displacement table 608 are actuated by actuators in the actuator base 604 under control of the mechanical scan controller 618. Mechanical scan controller 618 communicates via electrical cable 622 with dosimetry controller 628. Function of dosimetry controller 628 includes all functions previously described for dosimetry controller 432, with additional function for controlling the mechanical scanner 602 via communication with mechanical scan controller 618. Based on measured Neutral Beam energy flux rate, dosimetry controller 628 calculates and communicates to mechanical scan controller 618 the Y- and X-scanning rates for causing an integral number of complete scans of the workpiece 160 to be completed during processing of a workpiece 160, insuring complete and uniform processing of the workpiece and insures a predetermined energy flux dose to the workpiece 160. Except for the use of a Neutral Beam, and the use of a Neutral Beam energy flux rate measurement, such scanning control algorithms are conventional and commonly employed in, for examples, conventional GCIB processing tools and in ion implantation tools. It is noted that the Neutral Beam processing apparatus 600 can be used as a conventional GCIB processing tool by controlling the deflection plates (302, 304) so that GCIB 128 passes without deflection, allowing the full GCIB 128 to irradiate the workpiece 160 and/or the workpiece holder 616.
[0062] FIG. 7 is a schematic of a Neutral Beam processing apparatus 700 according to an embodiment of the invention that provides active setting and control of the gas pressure in the beamline chamber 107. A pressure sensor 330 transmits pressure measurement data from the beamline chamber 107 through an electrical cable 332 to a pressure controller 716, which measures and displays pressure in the beamline chamber. The pressure in the beamline chamber 107 depends on the balance of gas flow into the beamline chamber 107 and the pumping speed of the vacuum pump 146b. A gas bottle 702 contains a beamline gas 704 that is preferably the same gas species as the source gas 112. Gas bottle 702 has a remotely operable leak valve 706 and a gas feed tube 708 for leaking beamline gas 704 into the beamline chamber 107 through a gas diffuser 710 in the beamline chamber 107. The pressure controller 716 is capable of receiving an input set point (by manual entry or by automatic entry from an system controller (not shown)) in the form of a pressure set point, a pressure times beam path length set point (based on predetermined beam path length), or a gas target thickness set point. Once a set point has been established for the pressure controller 716, it regulates the flow of beamline gas 704 704 by way of electrical cable 712 into the beamline chamber 107 to maintain the set point during operation of the Neutral Beam processing apparatus. When such a beamline pressure regulation system is employed, the vacuum pump 146b is normally sized so that in the absence of beamline gas 704 being introduced into the beamline chamber 107, the baseline pressure in the beamline chamber 107 is lower than the desired operating pressure. If the baseline pressure is chosen so that the conventional GCIB 128 can propagate the length of the beam path without excessive dissociation, then the Neutral Beam processing apparatus 700 can also be used as a conventional GCIB processing tool.
[0063] FIG. 8 is a schematic of a Neutral Beam processing apparatus 800 according to an embodiment of the invention that employs an electrostatic mirror for separation of the charged and neutral beam portions. A reflecting electrode 802 and a substantially transparent electrical grid electrode 804 are disposed displaced from each other, parallel to each other, and at a 45-degree angle to the beam axis 154. The reflecting electrode 802 and the substantially transparent electrical grid electrode 804 both have holes (836 and 838 respectively) centered on the beam axis 154 for permitting passage of the Neutral Beam 314 through the two electrodes. A mirror power supply 810 provides a mirror electrical potential VM across the gap between the reflecting electrode 802 and the substantially transparent electrical grid electrode 804 via electrical leads 806 and 808, with polarity as indicated in FIG. 8. VM is selected to be slightly greater than VAce+VR (VR being the retarding potential required to overcome the thermal energy the gas cluster jet has before ionization and accelerationVR is typically on the order of a few kV). The electric field generated between the reflecting electrode 802 and the substantially transparent electrical grid electrode 804 deflects the ionized portion 814 of the GCIB 128 through approximately a 90-degree angle with respect to the axis 154. A faraday cup 812 is disposed to collect the ionized portion 814 of the GCIB 128. A suppressor electrode grid electrode 816 prevents escape of secondary electrons from the faraday cup 812. The suppressor grid electrode 816 is biased with a negative third suppressor voltage VS3 provided by third suppressor power supply 822 by way of electrical cable 818. VS3 is typically on the order of several tens of volts. The faraday cup current, I.sub.D2, representing current in the deflected ionized portion 814 of the GCIB 128 (and thus the current in the GCIB 128) flows through electrical lead 820 to current sensor 824. Current sensor 824 measures the current 102 and transmits the measurement to dosimetry controller 830 via electrical lead 826. The function of dosimetry controller 830 is as previously described for dosimetry controller 432, except that dosimetry controller 830 receives 102 current measurement information from current sensor 824 and dosimetry controller 830 does not control deflector power supply 440, but instead controls mirror power supply 810 via electrical cable 840. By setting mirror power supply 810 to output either zero volts or VM, dosimetry controller 830 controls whether the full GCIB 128, or only the Neutral Beam 314 of GCIB 128 is transmitted to the workpiece 160 and/or workpiece holder 616 for measurement and/or processing.
[0064] FIG. 9 is a schematic of a Neutral Beam processing apparatus 940 according to an embodiment of the invention, which has the advantage of both the ionizer 122 and the workpiece 160 operating at ground potential. The workpiece 160 is held in the path of Neutral Beam 314 by electrically conductive workpiece holder 162, which in turn is supported by electrically conductive support member 954 attached to a wall of the low-pressure vessel 102. Accordingly, workpiece holder 162 and the workpiece 160 are electrically grounded. An acceleration electrode 948 extracts gas cluster ions from ionizer exit aperture 126 and accelerates the gas cluster ions through a voltage potential VAce provided by acceleration power supply 944 to form a GCIB 128. The body of ionizer 122 is grounded and VAcc is of negative polarity. Neutral gas atoms in the gas jet 118 have a small energy on the order of several tens of milli-electron-volts. As they condense into clusters, this energy accumulates proportional to cluster size, N. Sufficiently large clusters gain non-negligible energies from the condensation process and when accelerated through a voltage potential of VAcc, the final energy of each ion exceeds VAce by its neutral cluster jet energy.
[0065] 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.
[0066] FIG. 10 is a schematic of a Neutral Beam processing apparatus 960 according to an embodiment of the invention, wherein the ionizer 122 operates at a negative potential VR and wherein the workpiece operates at ground potential. An acceleration electrode 948 extracts gas cluster ions from ionizer exit aperture 126 and accelerates the gas cluster ions toward a potential of VAcc provided by acceleration power supply 944 to form a GCIB 128. The resulting GCIB 128 is accelerated by a potential VAcc-VR. A ground electrode 962 decelerates the ionized portion 958 of the GCIB 128 and returns it to the acceleration electrode 948.
[0067] FIG. 11 is a schematic of a Neutral Beam processing apparatus 980 according to an embodiment of the invention. This embodiment is similar to that shown in FIG. 7, except that the separation of the charged beam components from the neutral beam components is done by means of a magnetic field, rather than an electrostatic field. Referring again to FIG. 11, a magnetic analyzer 982 has magnetic pole faces separated by a gap in which a magnetic B-field is present. Support 984 disposes the magnetic analyzer 982 relative to the GCIB 128 such that the GCIB 128 enters the gap of the magnetic analyzer 982 such that the vector of the B-field is transverse to the axis 154 of the GCIB 128. The ionized portion 990 of the GCIB 128 is deflected by the magnetic analyzer 982. A baffle 986 with a Neutral Beam aperture 988 is disposed with respect to the axis 154 so that the Neutral Beam 314 can pass through the Neutral Beam aperture 988 to the workpiece 160. The ionized portion 990 of the GCIB 128 strikes the baffle 986 and/or the walls of the low-pressure vessel 102 where it dissociates to gas that is pumped away by the vacuum pump 146b.
[0068] FIGS. 12A through 12C show via experimental setups 1000, 1020, and 1040 the comparative effects of full and charge separated beams on a gold thin film. In an experimental setup, a gold film deposited on a silicon substrate was processed by a full GCIB at 1002, 1022, and 1042 (charged and neutral components), a Neutral Beam at 1004, 1024, and 1044 (charged components deflected out of the beam), and a deflected beam comprising only charged components at 1006, 1026, and 1046. All three conditions 1000, 1020, and 1040 are derived from the same initial GCIB, a 30 kV accelerated Ar GCIB. Gas target thickness for the beam path after acceleration was approximately 210.sup.14 argon gas atoms per cm2. For each of the three beams, exposures were matched to the total energy carried by the full beam (charged plus neutral) at an ion dose of 210.sup.15 gas cluster ions per cm2. Energy flux rates of each beam were measured at 1008, 1028, and 1048 using a thermal sensor and process durations were adjusted to ensure that each sample received the same total thermal energy dose equivalent to that of the full (charged plus neutral) GCIB dose.
[0069] A process for cleaning impurities from graphene by using ANAB according to an aspect of the present disclosure is described with reference to FIGS. 13A and 13B.
[0070] 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.
[0071] 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.
[0072] 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.
[0073] 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.
[0074] 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 S.sub.iO.sub.2 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.
[0075] 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.
[0076] 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.
[0077] Referring to FIGS. 13A and 13B, two sets of samples were irradiated at 5 KV for 2 seconds per square cm. During irradiation, the ANAB beam was fixed and the sample under test was moved underneath the ANAB beam.
[0078] FIG. 13A lists resistivity measurements when the back side of a sample, i.e., the side from which the polymer had been removed, was irradiated.
[0079] FIG. 13B shows resistivity measurements for samples in which the graphene had been removed from a polymer layer without depositing the graphene layer onto S.sub.iO.sub.2. In this example the ANAB beam irradiated the side of the graphene layer that had been in direct contact with the copper surface during formation of the graphene layer. The irradiation angle of incidence was 45 degrees relative to the sample being irradiated. The irradiation times for the measurements in FIG. 13A were identical to the irradiation times for the measurements in FIG. 13B.
[0080] It was observed that approximately the same amount of copper was removed from the graphene in the measurements shown in FIG. 13A as compared to the measurements shown in FIG. 13B. These measurements confirmed that both processes, i.e., ANAB irradiation from the front and back surface of a graphene layer, removed the same amount of copper.
[0081] 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.
