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1Accelerators and Ion Sources Department, Nuclear Research Center, Atomic Energy Authority, Cairo, P.O. In the present work, an investigation has been made for the extraction characteristics and beam diagnosis for a Pierce-type electron gun with spherical anode to acquire an electron beam suitable for different applications. A method and apparatus for gas cluster ion beam (GCIB) processing uses X-Y scanning of the workpiece relative to the GCIB.
Tracing the electron beam profile by X-Y probe scanner along the beam line at two different places reveals that the spherical anode affects the beam to be convergent. The acceleration voltage increases the electron beam currents up to 250 mA at Acceleration voltage 75kV and decreases the beam perveance, beam waist and beam emittance.
IntroductionThe development of electron beam devices has a long history and electron beam technology has undergone several changes over the years.
The apparatus of claim 1, wherein the current measurement means provides at least one beam current measurement signal to said control means, said at least one beam current measurement signal representing a sample of said gas cluster ion beam current measured while the workpiece and said workpiece holder are removed from said gas cluster ion beam trajectory. The apparatus of claim 2, wherein said control means uses the at least one beam current measurement signal representing a sample of said gas cluster ion beam current to control the scanning of the workpiece.
The apparatus of claim 2, wherein said control means uses the at least one beam current measurement signal representing a sample of said gas cluster ion beam current to control a dosage of said gas cluster ion beam applied to the workpiece during processing.
The apparatus of claim 1, further comprising a controllable neutralizer for providing electrons to reduce space charge in said gas cluster ion beam. The apparatus of claim 1, wherein said control means measures at least one sample of said total current available and uses said at least one measured sample of total current available to control said controllable neutralizer to reduce the total current available for said electrical charging of the workpiece to a predetermined safe level for processing. The apparatus of claim 1, wherein said control means measures at least one sample of said total current available and uses said at least one measured sample of total current available to prevent processing of the workpiece if said total current exceeds a predetermined safe level for said processing. The apparatus of claim 6, wherein said control means further measures at least one sample of said gas cluster ion beam current and uses the measurement of the at least one sample of said gas cluster ion beam current to control the scanning of the workpiece. The apparatus of claim 1, further comprising a controllable neutralizer for providing electrons to reduce electrical charging of the workpiece. The apparatus of claim 6, wherein said control means further measures at least one sample of said gas cluster ion beam current and uses the measurement of the at least one sample of said gas cluster ion beam current to control a dosage of said gas cluster ion beam applied to the workpiece.
The minimum beam radius could be found at the minimum beam perveance and maximum convergence angle. A variety of electron beam sources have been designed and utilized in the basic research as well as for the industrial applications[1-3]. A single Faraday cup sensor is used to measure the GCIB current for dosimetry and scanning control and also to measure and control the degree of surface charging that may be induced in the workpiece during processing. Zakhary , "Investigation of Beam Performance Parameters in a Pierce-Type Electron Gun", Science and Technology, Vol.
Means for creation of and acceleration of such GCIBs are also described in the Deguchi reference. The electron beam could be suited for the two suggested experiments in our lab, plasma acceleration and surface modifications of polymers.
Charged particle beams from different types of sources have to be transmitted without loss of the particles through the accelerator. Dhole, “Design, Development and Characterization of Tetrode Type Electron Gun System for Generation of Low energy Electrons”, Indian Journal of Pure & Applied Physics, Vol. It is very important that the particles strike the target should have the parameters required for a certain application.
For a beam transmission without loss of the particles, the cross-section of the beam must not exceed a given maximum value of a well-defined point.
To achieve this we will introduce an important quantity known as emittance which one wants to minimize for any given current[5]. 5,459,326, Yamada, “Method for Surface Treatment with Extra-Low-Speed Ion Beam”, 1995) that atoms in a cluster ion are not individually energetic enough (on the order of a few electron volts) to significantly penetrate a surface to cause the residual sub-surface damage typically associated with the other types of ion beam processing in which individual ions may have energies on the order of thousands of electron volts. The emittance is an important aspect for high-quality beam, which is basically de?ned by the product of the width and transverse velocity spread of the beam (region of phase space occupied by beam). Nevertheless, the cluster ions themselves can be made sufficiently energetic (some thousands of electron volts), to effectively etch, smooth or clean surfaces as shown by Yamada & Matsuo (in “Cluster ion beam processing”, Matl. Science in Semiconductor Processing I, (1998) pp 27-41).Since GCIBs contain ionized particles that carry electrical charge, a measure of the processing dose a workpiece receives is the amount of charge (amp-seconds) received by a unit area of the workpiece, measured in amp-seconds per square centimeter, for example. For insulating, partly insulating, or semiconductive workpieces, ion beam processing can induce charging of the workpiece undergoing ion beam processing. Knowing the beam properties before its injection into the accelerator saves the time needed for accelerator tuning and increases the overall efficiency of the beam transmission[9]. An advantage of GCIB processing over some more conventional ion beam processes is that, because of the relatively large mass to charge ratio of the cluster ions compared to conventional atomic or molecular ions, processing can often be effected with less transfer of charge to the workpiece.
At the present work, the electron beam properties will be defined and regulated to match the suggested applications (plasma Acceleration and electron beam treatments of different types of materials). The electron beam diagnostics done in this work include the measurement of electron beam intensity, perveance, profile, and hence emittance as a function the acceleration voltages.
A single Faraday cup sensor is used to measure the GCIB current for dosimetry and scanning uniformity control and also to measure and control the degree of surface charging that may be induced in the workpiece during processing.To insure uniform processing, X-Y mechanical scanning of the workpiece relative to the GCIB is used to distribute the beam effects over the surface or the workpiece. The mechanical scanning mechanism moves the workpiece in an orthogonal raster pattern through the GCIB and also out of the beam at least once each processing cycle.
Experimental SetupThe electron beam is produced from a Pierce-type electron gun (figure 1) with an indirectly heated cathode operating in a diode circuit.
To provide this charging current sensing feature, the suppression voltage on the Faraday cup bias ring is removed. The gun section is connected to the field free space chamber through an aperture in the anode electrode. 1 is a schematic showing the basic elements of a prior art GCIB processing apparatus that uses an electro-statically scanned beam;FIG.
2 is a schematic showing the basic elements of a GCIB processing apparatus of the present invention that uses mechanical scanning of a workpiece to distribute the effect of a GCIB over a surface of a workpiece;FIG.
3 is a schematic of a GCIB processing system having charging control and dose measurement and control improvements of the present invention;FIG.
4A is a normal view of the workpiece holder of the present invention, with a workpiece in place;FIG. Zakaullah, “Emission Characteristics of the Thermionic Electron Beam Sources Developed at EBSDL”, Nucl. 4B is a normal view of the workpiece holder of the present invention showing the relationship of a GCIB scanning pattern relative to workpiece holder and workpiece; andFIG. 5 represents a schematic of details of the dosimetry and scanning control portions of the present invention.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTSFIG. 9- Two perpendicular cylindrical rodsThe scanning probe moves in the direction of the beam (z-direction) and moves in a direction perpendicular to the beam direction. The three chambers are evacuated to suitable operating pressures by vacuum pumping systems 146a, 146b, and 146c, respectively. The scanning probe is placed at distances 20mm (at the beam waist) from the anode, and a set of currents Ip values is recorded for different Acceleration Voltages. A condensable source gas 112 (for example argon or N2) stored in a gas storage cylinder 111 is admitted under pressure through gas metering valve 113 and gas feed tube 114 into stagnation chamber 116 and is ejected into the substantially lower pressure vacuum through a properly shaped nozzle 110. Then the probe is shifted to a new distance 150mm (at the beam exit) from the anode, and the previous procedures are repeated. Cooling, which results 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. Studying Electron Beam Intensity and PerveanceFigure 3 shows the influence of the Acceleration voltages on the extracted electron beam current and perveance. Suitable condensable source gases 112 include, but are not necessarily limited to argon, nitrogen, carbon dioxide, oxygen, and other gases.After the supersonic gas jet 118 containing gas clusters has been formed, the clusters are ionized in an ionizer 122.
The ionizer 122 is typically an electron impact ionizer that produces thermoelectrons from one or more incandescent filaments 124 and accelerates and directs the electrons causing them to collide with the gas clusters in the gas jet 118, where the jet passes through the ionizer 122.
At higher acceleration voltages, the increase of the electron beam current is limited and the current seems to be quasi-saturated in this region.
The electron impact ejects electrons from the clusters, causing a portion the clusters to become positively ionized. This quasi-saturation effect is due to the fact that, the number of the backscattered electrons increases with increasing the electron energy[12,13].
A set of suitably biased high voltage electrodes 126 extracts the cluster ions from the ionizer, forming a beam, then accelerates them to a desired energy (typically from 1 keV to several tens of keV) and focuses them to form a GCIB 128.
Space charge effect in beams is conveniently characterized by the perveance taking into account the magnitude of the beam current and the acceleration voltage[14].Figure 1 shows that the perveance decreases with the increase of the acceleration voltage. Anode power supply 134 provides voltage VA to accelerate thermoelectrons emitted from filament 124 to cause them to irradiate the cluster containing gas jet 118 to produce ions. For values of the perveance P ? 10-2 the intrinsic electric field of the beam has an appreciable effect on the motions of the electrons. Extraction power supply 138 provides voltage VE to bias a high voltage electrode to extract ions from the ionizing region of ionizer 122 and to form a GCIB 128. Accelerator power supply 140 provides voltage VAcc to bias a high voltage electrode with respect to the ionizer 122 so as to result in a total GCIB acceleration energy equal to VAcc electron volts (eV). One or more lens power supplies (142 and 144 shown for example) may be provided to bias high voltage electrodes with potentials (VL1 and VL2 for example) to focus the GCIB 128.A workpiece 152, which may be a semiconductor wafer or other workpiece to be processed by GCIB processing, is held on a workpiece holder 150, disposed in the path of the GCIB 128. Since most applications contemplate the processing of large workpieces with spatially uniform results, a scanning system is desirable to uniformly scan the GCIB 128 across large areas to produce spatially homogeneous results. Two pairs of orthogonally oriented electrostatic scan plates 130 and 132 can be utilized to produce a raster or other scanning pattern across the desired processing area. When beam scanning is performed, the GCIB 128 is converted into a scanned GCIB 148, which scans the entire surface of workpiece 152.FIG. 2 shows a schematic of the basic elements of an improved configuration for a mechanical scanning GCIB processor 200, which uses a mechanical scanning technique to scan the workpiece relative to the GCIB. Shanker, “Contribution of Backscattered Electrons to the Total Electron Yield Produced in Collisions of 8-28 keV Electrons with Tungsten”, PRAMANA J. 2, the GCIB 128 is stationary (not scanned) and the workpiece 152 is mechanically scanned through the GCIB 128 to distribute the effects of the GCIB 128 over a surface of the workpiece 152. The electron beam is found to be wider and the convergence angle is larger and hence the beam emittance is larger at the Faraday Cup (FC) i.e. An X-scan actuator 202 provides linear motion of the workpiece holder 150 in the direction of X-scan motion 208 (into and out of the plane of the paper). A Y-scan actuator 204 provides linear motion of the workpiece holder 150 in the direction of Y-scan motion 210, which is orthogonal to the X-scan motion 208. So that the beam profile is traced at distance 150mm from the anode aperture as shown in figure 4b. The combination of X-scanning and Y-scanning motions moves the workpiece 152, held by the workpiece holder 150 in a raster-like scanning motion through GCIB 128 to cause a uniform irradiation of a surface of the workpiece 152 by the GCIB 128 for uniform processing of the workpiece 152.
The workpiece holder 150 disposes the workpiece at an angle with respect to the axis of the GCIB 128 so that the GCIB 128 has an angle of beam incidence 206 with respect to the workpiece 152 surface. The deduced fitting data are also summarized in table 1.Table 1 shows the variations of the beam parameters with different beam energies.
The angle of beam incidence 206 may be 90 degrees or some other angle, preferably 90 degrees or near 90 degrees. In such a table we can see the parameters; xc is the centre of the beam, w is the beam full width at half maximum (FWHM) and A is the area under the curve.


During Y-scanning, the workpiece 152 held by workpiece holder 150 moves from the position shown to the alternate position “A”, indicated by the designators 152A and 150A respectively. Notice that in moving between the two positions, the workpiece 152 is scanned through the GCIB 128 and in both extreme positions, is moved completely out of the path of the GCIB 128 (over-scanned). 2, similar scanning and over-scan is performed in the orthogonal X-scan motion 208 direction (in and out of the plane of the paper) as will be discussed later in discussion of FIG. The velocities of the electrons emitted perpendicular to the cathode surface have two components; axial and transverse. From such an equation, it is seen that ? is proportional to the electron temperature and inversely proportional to the accelerating voltage U, i.e. As the electrons approach the axis of the beam, the columbic repulsive force increases, this effect leads to the escape of electrons towards the beam boundaries. The workpiece 152, workpiece holder 150, X-scan actuator 202 and Y-scan actuator 204 are all disposed and function as described for the mechanical scanning GCIB processor 200 of FIG. A neutralizer 310 disposes one or more thermionic filaments (two shown for example, not for limitation, as first filament 312 and second filament 314, disposed radially about and parallel to the GCIB 128) near the path of the GCIB 128. After the beam escapes from the anode lens field, the beam splitting appears, figure 4b.Figure 6 shows the variation of the beam radius at waist and at the FC entrance with the variation of the electron beam perveance. Although the neutralizer 310 has been shown as a thermionic neutralizer for example, it is recognized that other types of neutralizers may be used as part of the present invention, provided that the neutralizer be controllable to increase or decrease the electron output for neutralization. Such alternate neutralizers known in the ion beam art include, but are not limited to, an accel-decel electron guns and various plasma devices such as a plasma electron flood. In the present example of a thermionic neutralizer 310, a controllable neutralizer power supply 346 having a control signal input 358 provides controllable filament current 318 symbolized by IF for heating the filaments 312 and 314 through leads 326 and 328.
These results are in a very close agreement with the computer simulation results previously reported[21].Finally, the increase of the beam width with the increase of z and the decrease of this width with the increasing of the extracting voltages are evident from figures 4 and 5. Some thermoelectrons 336 emitted by the heated thermionic filaments 312 and 314 are attracted to the positive space charge of the positively charged GCIB 128 and flow along the GCIB 128, reducing the space charge of GCIB 128 and providing a source of electrons to the workpiece 152 to reduce positive charging thereof by the GCIB 128, by neutralizing positive charges that could otherwise accumulate on the workpiece 152. A GCIB defining aperture 332 defines GCIB 128 to limit the extent of GCIB 128 downstream of the GCIB defining aperture 332 to assure that the entire extent of GCIB 128 can pass through GCIB sensor aperture 338 into a Faraday cup 306. Faraday cup 306 has a suppression electrode 308 and a grounded enclosure 304 and is disposed in the path of GCIB 128 downstream of the GCIB defining aperture 332 and the GCIB sensor aperture 338 so as to collect sensor current 342, symbolized as IS, for measurement.A first single-pole double-throw switch 322 having a charging measurement position identified “C” and having a dosimetry measurement position identified “D” controllably connects sensor current 342 to either of resistor 330 via lead 316, or through lead 354 to sensing input 360 of dosimetry and scanner control system 500.
Switches 320 and 322 are either both switched to position “C” or both switched to position “D”.When dosimetry measurement is required, switches 320 and 322 are both switched to position “D”.
The increase of the acceleration voltage decreases the width and the divergence angle of the electron beam and hence the electron beam emittance decreases. Hence sensor current 342 (IS) is connected to dosimetry and scanner control system 500 and suppression electrode 308 is connected to suppression power supply 344 and thereby biased negative with an electrical potential, suppressor voltage VS, which may be 1500 volts, for example. Table (2) confirms the principle which states that theminimum beam radius is found at minimum beam perveance and higher beam convergence. Hence sensor current 342 (IS) is connected through lead 316 to resistor 330 as well as to the non-inverting input of amplifier 348; also suppression electrode 308 is connected to ground and is without bias. This sum of GCIB and electron currents approximates the total current available for charging a workpiece. Amplifier 348 has a high input-impedance non-inverting input and amplifies the voltage drop across resistor 330 due to sensor current 342 (IS) flowing to ground through resistor 330. Amplifier 348 has a gain A1 and outputs charging signal SC proportional to the available workpiece charging current.
Electron Beam Fluence CalculationElectron beam absorbed dose is used to identify the electron beam radiation in the science of dosimetry. Lead 340 connects charging signal SC to the charging signal input 384 of charge alarm system 350 and to the sample signal input 366 of track-hold module 364. Also the following quantities are used to describe an ionizing radiation beam (electron beam): particle fluence, energy fluence, particle fluence rate and energy fluence rate.
Charge alarm system 350 has an alarm output 386 and produces an alarm signal SA at alarm output 386 if the magnitude of SC exceeds a preset value previously experimentally determined to be detrimental to the workpiece 152. Lead 352 connects alarm signal SA from alarm output 386 of charge alarm system 350 to alarm signal input 362 of dosimetry and scanner control system 500. In this work we will use the terms, electron beam fluence and fluence rate to describe the electron beam in the irradiation of materials surfaces experiments. According to the definitions of fluence and fluence rate, these parameters are calculated and shown in figures 7 and 8. The maximum number of electrons enter the FC are 1.56 X 1018 electrons at acceleration voltage 75 kV. Responsive to an increase in signal SCH at control input 358, neutralizer power supply 346, produces increasing filament current 318 (IF) in thermionic filaments 312 and 314, resulting in increased thermionic emission of thermoelectrons 336, with corresponding increasing availability of electrons to neutralize GCIB 128 space charge and to reduce the net current available for workpiece charging. Dosimetry and scanner control system 500 has scanner control outputs 378 for outputting scanner control signals on cable 334 to control the X-scan actuator 202 and the Y-scan actuator 204.
The functionality of dosimetry and scanner control system 500 is explained hereinafter during the discussion of FIG. ConclusionsDifferent types of electron guns have been designed for producing different electron beam energies. In this work, a Pierce-type electron gun with spherical anode has been investigated to produce an electron beam with suitable properties valid for the suggested application in our laboratory. 4A is a normal view 400 of the workpiece holder 150 of the present invention, with a workpiece 152 in place. Upon investigation of the designed gun, it has been found that the acceleration voltage increases the electron beam currents up to 250mA and decreases the beam perveance. The workpiece holder 150 may use electrostatic attraction (an electrostatic chuck) to retain the workpiece 152 or may use gravity or a clamp or other arrangement to hold the workpiece in position on the workpiece holder 150.
A first retaining pin 402 and a second retaining pin 404 may optionally be used to help retain the workpiece 152 on the workpiece holder.FIG. This effect is proved by tracing the electron beam density distribution at two different places along the beam line.
4B is a normal view 450 of the workpiece holder 150 of the present invention showing the relationship of a GCIB 128 (as shown in FIGS. Also it is found that the distribution of the electron beam affected by the acceleration voltage i.e. 2 and 3) scan path 452 (indicated by dotted serpentine path) relative to workpiece holder 150 and to a workpiece 152. For purposes of this figure and this discussion it has been assumed that the angle of beam incidence 206 as defined in FIG. 2 is 90 degrees, however the invention is not limited to 90 degrees angle of beam incidence 206. Electron beam emittance is preferably to be as low as possible for the suggested plasma acceleration experiment.
While the polymer treatment experiment requires a spread out beam with larger diameter i.e. The scan path 452 represents the path that the center of the GCIB takes shown relative to the workpiece holder 150 and workpiece 152 as the workpiece holder is mechanically scanned through the GCIB 128 by X-scan actuator 202 and Y-scan actuator 204 (both as shown in FIGS. It is proven that the minimum beam radius could be found at the minimum beam perveance and maximum convergence angle. A preliminary position 454 represents the position of the center of GCIB 128 prior to commencing processing. Also the electron beam fluence rate and electron fluence are considered and calculated, these two values depend on the acceleration voltage. Start position 456 represents the position of the center of GCIB 128 at the commencement of controlled processing. We can conclude that the resulting electron beam (density and shape) could be controlled by not only the gun geometry but also the acceleration voltages to produce a suitable electron beam for different applications. Finish position 466 represents the position of the center of GCIB 128 at the finish or end of processing. Dotted circles 458a, 458b, 458c, and 458d represent the envelope of the projection of the GCIB envelope (known as the beam spot) on the plane of the front surface of the workpiece holder 150. Along the bottom edge 468 of the workpiece holder 150, the GCIB 128 also completely over-scans the bottom edge 468 of the workpiece holder 150.
Between the start position 456 and the finish position 466, the scan path 452 covers a scanned area, AS=DX?DY, where DX and DY are indicated by the designators 460 and 462 respectively.
Between the start position 456 and the finish position 466, the scan path 452 of the center of the GCIB 128 makes a predetermined number, N, passes across the workpiece holder 150 in the Y-axis direction. The value of N is not particularly critical and may be chosen to provide adequate overlap of successive scan passes to result in an acceptable degree of uniformity of processing by the GCIB. In general, smaller beam diameters and larger workpieces require larger values of N to provide good processing uniformity. At the Y-position extremes near the bottom edge 468 of the workpiece holder 150, of each of the N passes of the scan path 452, the scan path makes X-axis distance increments 464i, 464i+1, 464i+2, .
The X-direction scan speed, VX, in the direction of the X-motion increments is not particularly important and is chosen for design simplicity, since the GCIB 128 is always off of the workpiece 152 during the X-scan motions because of the over-scan. At all of the times when the center of the GCIB 128 is at preliminary position 454, or at start position 456, or at finish position 466, or at any of the X-axis distance increments 464 near the bottom edge 468 of workpiece holder 150, the entire beam spot of GCIB 128 is scanned beyond the bottom edge 468 of workpiece holder 150 and the GCIB 128 passes downstream of the workpiece holder 150 and enters the Faraday cup 306 (as shown in FIG.
At any of those such times, sensor current 342, IS, may be used for measuring the GCIB 128 beam current, IB, or the total current, IT, (including electrons) available for charging the workpiece, depending on the “D” or “C” position selection of switches 320 and 322 as shown in FIG. Although complete overscan of the workpiece 152 on all sides is the preferred embodiment for achieving uniform processing of the entire workpiece 152, it is recognized that to it is only necessary to overscan the workpiece 152 and workpiece holder 150 in at least one location to practice the present invention.
For purposes of explanation, the scan path 452 has been shown as a serpentine path, with beam travel in X-scan and Y-scan directions and describing an overall rectangularly shaped scanned area AS.
Other two-axis scan paths describing scan patterns of area AS generated by constant or varying velocities in the two axes and producing rectangular or non-rectangular scanned area, even including spiral patterns may be utilized provided that the pattern includes at least one complete overscan such that the entire beam spot of GCIB 128 is scanned beyond an edge of the workpiece holder 150 and enters the Faraday cup 306 for measurement.FIG. 5 represents a schematic of details of the dosimetry and scanning control system 500 of FIG. 5, dosimetry and scanner control system 500 has a sensing input 360 for receiving sensor current 342, IS, on lead 354.
During dosimetry measurements, when switches 320 and 322 are set to their respective “D” positions, IS is a measure of the GCIB 128 current, IB. Sensor current 342 (IS) is connected through lead 354 to resistor 518 as well as to non-inverting input of amplifier 502.
Amplifier 502 has a high input-impedance non-inverting input and amplifies the voltage drop across resistor 518 due to sensor current 342 (IS=IB) flowing to ground through resistor 518. Amplifier 502 has a gain A2 and outputs dosimetry signal SD proportional to the beam current, IB, of GCIB 128. Accordingly, indicator device 356 and neutralizer power supply 346 receive SCH, which tracks SC.
If the system is functioning properly, charging signal SC is minimized and charge alarm system 350 does not output an alarm or alarm signal SA.
When the finish position 466 has been achieved, the processing of the workpiece 152 is complete and processing dose DP has been applied uniformly to the entire workpiece 152, and with minimized charging of the workpiece 152.Although the invention has been described with respect to various embodiments, it should be realized that this invention is also capable of a wide variety of further and other embodiments within the spirit and scope of the appended claims.



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