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The input is designed to accept large overloads in excess of 100?A without sustaining damage.
The content of this website (content being images, text, sound and video files, programs and scripts) of this website is copyright © Deben UK Limited unless specifically mentioned. Size distributions of airborne nano- to micrometer particles with high sensitivity and resolution. Both components (CPC and DMA) are carefully manufactured and calibrated, and thoroughly tested within our TQM for superior quality so that you can rely on the data that you obtain with them. Our company started in 2013 selling updated versions of the emission control system WinEmag that respects the full requirements of the standard EN 14181, including the QAL 2 QAL 3 and verification. An improved Faraday cup array for determining the dose of ions delivered to a substrate during ion implantation and for monitoring the uniformity of the dose delivered to the substrate. The (e,2e) spectrometer vacuum chamber (figure 1) is manufactured from 304 grade stainless steel, the construction being carried out by Vacuum Generators using a design of Jones, Read and Cvejanovic, full details of which may be found in T. 3mm thick mu-metal magnetic shielding is placed both internally and externally to decrease extraneous external magnetic fields to less than 5mG at the interaction region. All inputs and outputs to the spectrometer pass through the vacuum chamber via feedthroughs on the top flange. The top flange of the vacuum system is a 25mm stainless steel flange with 11 conflat CF70 flanges arranged around a central 200mm flange designed for locating a hydrogen source. Two flanges are used for the high voltage channeltron and photomultiplier tube inputs and pulse outputs. Three rotary motion feedthroughs allow the analysers and electron gun angles to be changed externally via stepper motors for both analysers and for the electron gun. The ionisation gauge is placed at right angles to the main flange to prevent light from the thoriated iridium filament entering the vacuum system, which would increase noise on the photomultiplier tube. The ionisation gauge feedthrough is sooted, and a loose cover is placed over the entrance in the chamber flange to prevent stray light from the ion gauge entering the vacuum system. The target gas enters the system via a CF70 flange which has two 6mm stainless steel tubes welded into the flange. The other gas feed is connected to a hypodermic needle set at 90A° to the incident beam for measurements in other geometries.
The gas jet which is not currently used in the interaction region can be used to non-locally fill the chamber with gas for background counting measurements.
The final CF70 flange is used when the photomultiplier tube is cooled, usually during baking of the system following exposure to atmosphere. The tube is cooled by passing vapour from a liquid nitrogen source through expansion coils wrapped around the photomultiplier tube housing. The electron gun is a two stage non-selected gun designed by Woolf (Ph.D thesis, University of Manchester) (see figure 3). The gun consists of a filament, grid and anode source which is focussed using a triple aperture lens through two 1mm defining apertures in the centre of the gun. The apertures define the electron beam pencil and beam angles, and are focussed onto the interaction region using a second triple aperture lens located between the apertures and interaction region. The electron gun is shielded using advance (constantin) sheet which is grounded to the body of the gun mounting.
The gun is mounted onto an x-y translation table to allow placement of the electron beam as accurately as possible to the centre of rotation of the analysers and electron gun rotation axis.
The gas jet is mounted from the electron gun using a narrow support of 310 stainless steel, and the Faraday cup can be attached to this support. The original unselected electron gun built by Woolf was completely rewired and a custom built PTFE 25 way inline wire-wrap plug and socket was placed between the 52 way vacuum electrical feedthrough and the electron gun. The wiring from the 52 way feedthrough to the inline plug is shielded colour coded 30 awg PTFE coated advance wire, and the inline plug and socket connectors are non-magnetic gold plated wire wrap pins. The electron analysers are constructed from molybdenum and are mounted from two rotating gear plates controlled externally via the rotation feedthroughs on the top flange (see figure 2). The input electrostatic lenses for each analyser are three element cylindrical lenses as shown in figure 4.
The interaction region is focussed onto the entrance aperture of the hemispherical energy analysers with a beam angle of around zero degrees, the image of the analyser entrance aperture being larger than the size of the interaction region. Field gradients around the input and exit apertures are corrected using standard Jost correctors. The energy selected electrons passing through the exit apertures are finally collected by Mullard X919BL channel electron multipliers which amplify the single electron to produce a current pulse at their output.
A principle source of noise on the signal pulses arises from the turbo-molecular pump motor. The noise from the turbo pump is reduced to ~ 5mV using these techniques, and as the average pulse height from the saturated multiplier is around 20-40 mV, this allows adequate discrimination following amplification. The pre-amplifiers located external to the vacuum system are commercial 100X preamplifiers with 400pS rise-times, manufactured by Phillips Scientific. The NIM signals produced by the discriminators drive two ORTEC 437 Time to Amplitude Converter's via appropriate delay cables and an ORTEC pulse delay unit.
The main TAC output feeds an ORTEC Multichannel Analyser card located in the PC controlling the experiment, whereas the second monitor TAC drives an external MCA, allowing convenient monitoring of the coincidence signal as the experiment progresses.
The high voltage supplies to the electron multipliers are two Brandenberg 5kV supplies controlled by the main PC via serial DAC's. Principally the filter is used to reduce pickup noise from entering via the HT cable onto the multiplier output. Secondly, the filter has a slow response time for high voltage spikes, and reduces the probability of damage to the pulse electronics should the EHT supply suddenly turn on or off. Capacitor C2 decouples the 1kW load resistor to earth for high frequency pulses from the CEM's, and the voltage appearing across this load is connected via the capacitor C3 to the input of the 6954 preamplifier.
Since the input impedance of the preamplifier is 50W and the CEM is effectively an ideal constant current supply, most of the current passes through to the preamplifier.
Finally, the 10M resistor prevents the output to the preamp from rising above ground when the preamp is disconnected from the circuit and is therefore a safety device for the preamplifier when initially connected. The original design placed the atomic beam nozzle and Faraday cup on the same support structure fixed to the body of the electron gun, with the atomic beam direction being in the detection plane when the perpendicular plane geometry is chosen (see figure 2). A second configuration is adopted for exclusively perpendicular plane coincidence experiments, the atomic beam nozzle being oriented at an angle of 45A° to the detection plane so that the gas effusing from the nozzle does not fill the analysers when they oppose the nozzle (see figure 6).
In this configuration the support strut for the atomic beam nozzle is terminated lower than the detection plane, as is the nozzle itself. For non-perpendicular plane measurements the Faraday cup is once mounted from the support strut and the gas effuses perpendicular to the electron beam direction. One difficulty when aligning an (e,2e) coincidence experiment lies in the focussing the incident electron beam and analysers onto the interaction region.
The throughput of electron current as detected by the Faraday cup is insensitive to both position and focusing of the electron beam, since the cup opening is usually made quite large (in this case 6 - 10mm) and the bias voltage applied to the cup to prevent electrons returning to the interaction region allows all electron trajectories within this opening to be detected. Conventionally, following maximisation of the Faraday cup current, the beam is steered and focussed onto the gas jet as detected by counts in the analysers.
This has the disadvantage that the overlap volume is maximised for both atomic and electron beam density, which tends to place the interaction volume as close as possible to the output of the atomic beam nozzle, which is almost certainly not in the centre of the detection plane. To eliminate the need for time consuming iterative techniques and the associated errors due to detection geometry, a separate method for tuning the electron gun is adopted that does not depend upon the analysers. The interaction between the target and the electron beam not only produces ionisation, but the valence states of the target also are excited with various probabilities depending upon the incident energy of the electron beam.
These photons are detected by the photomultiplier tube, by placing a 450nm A± 50nm optical filter in front of the photocathode. The interaction region is therefore defined where an aperture in front of the photocathode is focused by the lens. The plano-convex focussing glass lens is covered by a grounded fine tungsten mesh to eliminate surface charging. The photomultiplier tube may be cooled by circulating nitrogen gas boiled off LN2 through copper coils encircling the photomultiplier tube housing.
The photomultiplier tube used in the experiment is a 9789QB EMI photomultiplier tube with a bi-alkali photocathode peaking in the blue region of the optical spectrum. As with the electron multiplier circuitry, it is important to optimise the load of the photomultiplier tube so that reflections along the feeder line to the counting electronics are minimised.
The 1nF 6kV capacitors across the dynodes close to the output stage are used as charge storage pumps that quickly refurbish the surface charge on the dynodes, allowing increased counting rates. The dynode resistors are 1% tolerance metal film resistors as these are more stable with temperature. The EHT supply to the photomultiplier tube is a Brandenberg 2kV supply operating at 1500V for this tube. The negative going output pulses from the tube measure typically 200-500mV in height, and therefore are able to directly drive an ORTEC 473A constant fraction discriminator located in one of the NIM crates. All resistors and capacitors for the dynode chain are soldered to the photomultiplier tube base internal to the vacuum system, using standard 5-core solder.
Typical pulse heights from the electron multipliers in saturation mode are 25-40mV, whereas the typical height of the photomultiplier tube pulses is of the order of 250mV. The electron multiplier pulses are obtained from the multiplier supply circuitry as explained above. The amplified pulses from the 6954 amplifiers pass along doubly shielded RG-58 cable to the input of the ORTEC 473A discriminators. The two analyser discriminators are located in separate NIM crates to increase isolation and reduce common mode pickup. The slow rise-time positive outputs from the 473A discriminators pass to three ORTEC 441 rate-meters which allow counting and monitoring of the pulses. The fast NIM pulses from the analyser 473A CFD's pass through appropriate delay lines to two 437 TAC's.
This second multichannel analyser allows the operator to monitor the total accumulated coincidence signal during operation. The lens supplies that control the voltages impressed onto the lens elements and deflectors in the experiment were developed over a number of years, and therefore take different forms. Three of these supplies feed the deflectors in the electron gun, the other two supply the input deflectors in the analysers. In a second 19" crate is located the gun lens high voltage supplies together with the high voltage supplies for the Faraday cup.
In a third 19" crate is located the analyser voltage supplies together with a set of serial loading supplies that control the electron multiplier EHT supplies. A further supply controls the current boost circuitry located in the filament constant current supply unit (see below). The outputs of each computer controlled supply that drives an individual lens element or deflector are connected to a 70 relay switching board. The Digital voltmeter is in turn accessed via an IEEE-488 interface which is addressed by an 8086 slave PC which converses with the central PC which controls the experiment.
The outputs from the relay board connect to the spectrometer via shielded multicore cables which take the gun voltages to the 52 way feedthrough and the analyser voltages to the two 19 way feedthroughs located on the vacuum flange. The gun supply multicore also carries the filament supply current to the 52 way feedthrough. The Faraday cup current is measured by the Keithley Digital voltmeter when under computer control but this can also be measured by an external pico-ammeter.
In addition to the analyser electrostatic lens and deflector voltage connections, the 19 way CF70 feedthroughs are used to control signals from internal sensing opto-isolators which control the stepper motor interlocks. The stepper motors that drive the analysers and electron gun around the detection plane are five phase motors with integral 10X reduction gearboxes driving the rotary motion feedthroughs. The intelligent stepper motor drivers are addressed via a serial port located on the controlling PC bus. An additional interference problem was traced to the RS232 serial line from the controlling PC.
The PC bus was found to be poorly earthed due to very little copper being used for the bus earth. This was cured by decoupling the 0V RS232 line from the main 0V line for the stepper motors, as is shown in figure 10.
The analyser stepper motor supplies have a facility for halting the motors using limit switches which are optically coupled to the internal 68000 logic circuits driving the supplies.
The SD5443-3 phototransistor normally illuminated by the TEMT88PD photodiode during operation ensured a fail-safe mechanism should any of these components fail (this automatically prevent the motors from running). Should the analysers move into a position as defined by the four criteria (a) - (d) above, a shutter located on the analyser turntables moves between the appropriate photodiode and phototransistor, shutting off the light and thereby shutting down the stepper motor current. Only motion in the opposite direction is allowed by controlling logic, allowing the analyser to move away from the point of imminent collision but no closer. The six pairs of optocouplers that prevent the analysers moving into the Faraday cup, electron gun and gas jet support are located on the mounting struts supporting the main body of the spectrometer. Two optocouplers are located on the side of analyser 2, and two shutters are located on the sides of analyser 1 to prevent the analysers colliding with each other. Finally a ninth optocoupler is located over the electron gun counterweight which monitors when the gun is at an angle less than 65A°, where the sweep angle around the detection plane is reduced.
The current boost circuit operates from a serial driven supply card as detailed previously.
The main filament drive is through six power diodes that feed the current from the constant current supply to the filament. The Darlington transistor T1 feeds additional current to the filament via three parallel 10 turn 100W trim resistors through a 1N4007 diode. As the constant current supply acts as an infinite resistance source, the boost circuit supplies additional current only when the voltage at the anode of the feed 1N4007 exceeds the output voltage of the constant current supply. Finally, the filament bias voltage (and hence energy of the electrons) is set to the midpoint of the filament by two 100W balancing resistors located across the terminals. In addition to the ADC buffer card, three independent isolated regulated supplies drive REF01 voltage reference sources for measurement of the positions of 10 turn potentiometers located on the analyser and electron gun rotary drive shafts. The angular position of the electron gun and analysers is calibrated when the system is opened using visible laser diodes that define these angles to an accuracy of A±0.2A°. Figure 13 shows an overall block diagram detailing the computer control and high speed pulse interfacing to the spectrometer.
The following information pertains to experiments presently being conducted in the laser collisions lab of the above laboratory. In these experiments atoms and molecules excited by electron impact are studied when they are in a metastable state following the collision.
Many different processes can occur in this reaction, depending upon the energetics and kinetics of the reaction.
The experiments described here look at the process of inelastic scattering, where the target is left in a metastable state. The experiments are precursors to measuring stepwise laser excitation of metastable atoms and molecules.
An electron which has well defined energy Einc, and momentum Pei is incident on either an atom or a molecular target which is in its ground state. The velocity of the gas is constrained to a narrow distribution by the supersonically expanding shock wave.
The target beam enters a second vacuum chamber where it travels ~ 100mm prior to interacting with a well defined beam of electrons.
These electrons are produced in an electron gun that controls both the direction and energy of the electrons.
Only a very small number of these electrons will interact with the target beam, the majority passing through the interaction region and being collected by a Faraday cup 200mm downstream. The majority of the target beam is also unaffected during the reaction, and passes through the interaction region. The majority undergo an elastic collision where only the direction of the incident electron may change and the target remains in its ground state.
A small number of targets will be excited, and depending upon the energy that can be exchanged between the electrons and the target, excitation to various states can occur.
Depending upon the cross section for the state, which is a function of both the energy and the angle through which the electron is scattered, a small number of these targets will be excited to the metastable state. Since these metastable excited targets cannot decay by photon emission, and since there are very few collisions between targets in the gas beam (their relative velocity is close to zero in the beam and therefore their relative position to each other does not change markedly), most of these will remain in the excited state as they pass out of the interaction region. Scattered Atoms (Molecules) in a Metastable State with internal energy Eexc and which carry momentum Paf away from the reaction.
Studying the momentum of the targets that have been deflected by the electron collision, since these are related to the electron momentum by conservation of momentum. It should be noted that in conventional electron scattering experiments which measure excitation cross sections, the target momentum is not usually considered. There are a number of complications that can arise in these experiments where the target momentum is exploited for measurement. If the incident electron is of sufficient energy to excite not only the metastable state but also higher lying states which through photon emission can decay to the metastable state, there will be a contribution to the target signal due to these cascades. For a significant number of targets there is more than one metastable state, and these may be sufficiently close in excitation energy to be unresolvable when exploiting their individual momentum transfer contributions.
This effect is particularly significant when the target is a molecule since the metastable vibrational and rotational states will be much closer together than can be resolved by even the highest resolution electron impact studies.
The difficulties associated with cascading can be overcome by confining the study to the region close to threshold for excitation of the metastable state. For studies of excitation at energies where contributions from cascading become significant, a time resolved coincidence experiment between the scattered electrons and the deflected targets becomes necessary. Unlike conventional coincidence studies, the direction of the coincident scattered electron is known since the momentum of the excited target is known, and hence the collection rate of the experiments can be enhanced by placing the electron detector at the appropriate point in space. The difficulties associated with state selectivity can be overcome by employing a further stepwise laser excitation step from the metastable state to an upper state using a high resolution laser probe. Since the laser radiation transfers information about the metastable state to this upper state, measurement of the upper state can in principal yield information about the lower state once the laser excitation mechanism is well understood. Allows the electron excitation mechanism to be studied by analysing the stepwise excited state. Field Ionisation if the laser excited state has sufficiently high principal quantum number (high yields, in principal 100% efficient). The latter Field Ionisation technique is adopted in these experiments, although photon detection can be easily installed into the vacuum system if this proves advantageous to measurement of the signal. Molecular Vibrational & Rotational states can be studied in detail for a wide range of targets.

A detailed analysis of the laser excitation process is required to fully characterise the electron excited state.
The processes where the linear momentum transferred from the electron to the target is exploited to determine cross sections for excitation of the state are now considered. The square root in the above expression shows the origin of the two different momenta at any given deflection angle of the targets as described in figure 4. Note that for each atom detection angle qa there are 2 contributing momenta Paf1, Paf2 excepting the case of Forward & Backward scattered electrons, as shown in figure 5.
Hence with sufficient time resolution and detection angle resolution the experiments should reveal 2 distinct peaks at any given detection angle.
It should be noted that the lifetimes of the low l highly excited helium Rydberg targets, typically only a few microseconds, exclude them from being detected in the present experiments where the targets drift for around 300 microseconds from the interaction region to the metastable detector. The chamber is lined with a double layer of m-metal which reduces external magnetic fields to < 5mG at the interaction region. Fast output deflector circuit only allows electrons to exit electron gun, preventing any ions created in field free region or other high potential region from reaching interaction region.
Angle selectivity is achieved using 1mm apertures 1 & 2 spaced 60mm apart prior to a 719BL CEM metastable detector at end of detector (LHS). Electrostatic deflection plates remove unwanted electrons and ions from entering the detector and possibly reaching the detectors. Visible Laser Diodes (VLD's) align the detector to the interaction region formed by electron beam, gas beam and rotation axis of detector. A control pulse (0 - 150Hz) drives the gas nozzle HT pulse & the electron gun Grid following an appropriate delay.
The Turbo-MCS is gated to either sweep over ~100ms for each gas pulse or is set to count events for a set number of gas pulses at each detection angle.
The following graphs show the metastable angular distribution when the target gas is excited using a continuous electron beam, rather than a pulsed electron beam for four different excitation energies from 20eV to 50eV. As the incident energy increases forward scattering starts to dominate over backscattering as a collision mechanism as might be expected. The evolution from single atom momentum at q'a = 4A°through double momentum peaks at intermediate energies then back to a single peak at the highest scattering angles can clearly be seen as is discussed in figure 4.
Gaussian profiles have been fitted to the peaks to establish the amplitude, width and position of each peak as a function of the deflection angle.
Note that a 4ms electron excitation peak is visible at higher detection angles, which is thought to be due to either photon or stray electron contributions. This accurately yields the time difference between excitation of the target and collection of the metastables. For these experiments the atoms excited by the incident electrons are counted and fitted to a Gaussian of width w and height h as shown in figure 14. Figure 15 shows the results of these least squares fit calculations at 40eV Incident Energy, showing the amplitude h and width w together with the associated peak positions. Note that this method allows the DCS to be determined over the complete set of scattering angles from forward to 180A° backward scattering of the electrons. Note that some discrepancies exist between the calculations from the fast and the slow atoms.
This effectively acts as a high resolution timing probe of the target gas beam which for helium has a time spread of the order of 6ms from the interaction region to the detector.
Exploring the Helium (e,2e) Differential Cross Section at 64.6eV with Symmetric Scattering Angles and Unequal Outgoing Electron Energies.
In the experiments described here the gun angle y has again been varied from coplanar geometry (y = 0A°) to the perpendicular geometry (y = 90A°) and the scattering angles x are again symmetric (as shown in figure 1), but the symmetry of the outgoing electron energies has been relaxed.
One purpose of the experiments described here was to investigate the dependence of the dip on the symmetry of the outgoing electron energies. The spectrometer is a fully computer controlled and real-time computer optimised (e,2e) coincidence spectrometer. The spectrometer is maintained at its optimum working condition during data accumulation, with automatic adjustment for any long term drifts.
The vacuum pressure during operation was typically 10E-5 torr, with a background pressure of 10E-8 torr.
Two hemispherical deflection analysers with acceptance half angles of 3A° rotate in the detection plane as shown in figure 1. The discriminators feed a time to amplitude converter via appropriate delay lines, resulting in an 8ns FWHM coincidence signal accumulating in a multi-channel analyser located on the controlling computer bus. The computer optimises the spectrometer at regular intervals as the analysers sweep around the detection plane using a modified simplex technique.
An additional check of the apparatus has been made to verify that the measured differential cross section is unchanged when the detected energies E1 and E2 are exchanged. Figures 2(a) - (d) show the helium (e,2e) differential cross sections obtained for the four selected energy pairs. At higher gun angles the angular range is limited only by the physical size of the analysers. A trend that can be noticed in the data is that the differential cross section becomes more isotropic as the energy asymmetry increases. For all energy sharing the forward scatter peak evolves into the lower angle peak in the perpendicular plane, while the backscatter peak evolves into the central peak. In the perpendicular plane the central peak becomes broader as the asymmetry increases, and the side peaks become less distinct, almost disappearing at the highest asymmetry. The peak at xi = 90A° can arise either from collision with a valence electron whose momentum is equal in magnitude but opposite in direction to the incident electron, or via multiple collision processes due to long range Coulombic interactions between the outgoing electrons, as becomes increasingly important in the threshold region. The existence of the sharp dip at y = 67.5A°, xi = 70A° and E1 = E2 has been noted previously.
For asymmetric energy sharing the deconvolved cross section at the minimum is insensitive to the width of the estimated experimental angular function, the deconvolved cross section being almost unchanged from the measured cross section. The sharpness of the dip clearly depends critically on the energy sharing of the detected electrons.
The free-standing meter is battery powered and measures from below 1pA to 2?A in 5 switched ranges. Negatively charged particles are attracted by a positive electrode and for a given voltage, only particles of a certain mobility pass through the DMA via an exit slit in the positive electrode.
The particles deposit their charge on the filter and initiate a small current that is converted to a voltage using a high impedance resistor. It includes a complete data inversion algorithm and allows data to be displayed and exported in many different ways to meet a variety of needs. This participation is important point for our company and for the future sale of our system on the this MEMA region. Also new is the central system with multiple user displaying all connected sources, including applications for environmental inspections.
The improved Faraday cup array incorporates a variable size ion beam aperture by changing only an insertable plate that defines the aperture without changing the position of the Faraday cups which are positioned for the operation of the largest ion beam aperture. The improvement of claim 1, wherein said removable, insertable member includes a peripheral or stepped edge by which it is retained in said aperture.
The improvement of claim 1, wherein said removable, insertable member is constructed of material selected from the group consisting of graphite, molybdenum and Al alloys. The assembly of claim 5, additionally including a spacer ring located intermediate said mounting ring and said mask, and extending around said bias ring. The assembly of claim 5, wherein said holder for said Faraday cups includes double counter bores in an end adjacent said bias ring. The assembly of claim 5, wherein said removable insert includes a peripheral edge for mounting in said mask.
The assembly of claim 5, wherein said opening in said, removable insert may be composed of different configurations and of different areas.
The assembly of claim 5, wherein each of said mounting ring, mask, bias ring, cup holder, and the Faraday cups is constructed of electrically conductive material. The assembly of claim 5, wherein said mounting ring is composed of stainless steel, said mask is composed of graphite, said cup holder is composed of MACOR ceramic, and said Faraday cups are composed of tantalum or stainless steel.
The method of claim 15, additionally including providing the member with a peripheral or stepped edge of reduced thickness for retaining the member in the irradiation defining aperture. The method of claim 15, additionally including forming the member from materials selected from the group consisting of graphite, molybdenum, and Al alloys. One of these is internally connected to a gas jet which can be set at 45A° to the incident electron beam and is used in the perpendicular plane experiments. These stainless steel feeder tubes are connected to the nozzle using grounded shielded PTFE hose. One follows the anode, another corrects the beam direction between the apertures and a third steers the beam onto the interaction region. This shield is coated with aerodag colloidal graphite to reduce patch fields on the surface of the metal, and is made as small as possible to allow the analysers to approach the gun as close as is feasible when non-perpendicular plane geometries are selected (see figure 2).
The XY translation table is mounted from the rotating yoke, to which is attached the photomultiplier tube. This allows the gun to be dismounted from the system without the need for rewiring of the feedthrough when the need arises (filament changes, cleaning etc).
The wiring between the gun and inline socket is also shielded colour coded PTFE coated advance wire facilitating spot welding to the electrostatic lens elements of the gun. The analysers are mounted onto x-z translation tables that allow them to view the interaction region accurately over the full range of angles accessible. These lenses have a capability for energy zooming of around 10 : 1, and have a maximum entrance acceptance angle of A±3A° as defined by the entrance apertures. This ensures that the overlap volume accepted by each analyser encompasses the interaction volume for all detection angles as the analysers move in the detection plane.
These devices use an activated lead glass internal surface which has a high coefficient of secondary electron emission when a high voltage (around 3000V) is applied across the multiplier.
This noise may be picked up by the channel electron multiplier and the high voltage feed to the multiplier, and is generated internally in the vacuum system. The CFD's are operated in NaI mode as this has a 2ms dead time following the initial pulse, thus reducing the possibility of false triggering due to any mismatching impedance reflections along the delay cable.
These EHT supplies are connected via VERO 200 high voltage couplings to high voltage cable connecting to the pickoff circuitry located on the high voltage feedthroughs. This allowed the analysers to roam the detection plane over a complete 360A° angular range. The configuration of the target source hypodermic when perpendicular plane measurements are conducted. The common procedure is to initially maximise the current from the gun onto the Faraday cup, then onto analyser counts as the electron beam is moved through the gas jet. Maximising the Faraday cup current is therefore only a crude preliminary alignment of the incident electron gun beam.
This therefore assumes that the analysers are initially focused onto the interaction region, and a slow iterative technique must be adopted where the analysers and electron gun are tuned so as to maximise the signal from both analysers.
In this case a true differential cross section is not obtained, since the scattered and ejected electron trajectories lie in a different plane than defined in the initial experimental alignment. An EMI 9789QB reduced photocathode photomultiplier tube is installed in the vacuum system together with optical focussing and defining elements (figure 2). A group of these excited states decay back to lower valence states with the emission of visible photons at wavelengths around 450nm for both helium and argon targets. Tuning of the electron gun to PMT counts therefore guarantees that the electron gun is steered and focussed to the point at the centre of the detection plane independent of the analysers or gas beam density.
This is mainly used when the vacuum system is baked at temperatures above 55A°C, since the photocathode is rapidly damaged by temperatures exceeding this. The tube is a venetian blind dynode construction, and has a reduced photocathode to decrease the noise on the output of the tube to less than 20Hz. The photomultiplier tube base was thoroughly cleaned in an ultrasound following location of these components, to displace any flux residue from the solder joints.
It is therefore necessary to amplify the channeltron pulses prior to transmission to the discriminators, whereas the photomultiplier tube pulses are sufficiently large to not require further amplification.
These pulses are amplified by Phillips Scientific 6954 100X amplifiers located external to the EHT feedthroughs connected by 10cm lengths of RG-58 cable. These crates are further decoupled from the AC mains supply using filter circuitry on each supply line. Internal to these rate-meters buffer 7400 TTL chips have been added to send to 32 bit counters located in the main PC via RG-58 cable.
These supplies are fully floating, their mid-point voltage being equal to the lens element voltage in which the deflectors are housed.
All these supplies are fully computer controlled isolated units with low output impedance to reduce noise and eliminate current loops.
This relay switching unit allows either the voltage or the current on each lens element to be measured by a Keithley Digital voltmeter. This drive is further reduced internally by 10X reduction gears, resulting in a reduction ratio of 100:1. This led to the ground line of the RS232 serial line moving around with respect to the 0V stepper motor reference as drive current to the motors returned along the ground line. The optocoupler logic circuit circuit that controls the stepper motors and avoids collisions between the analysers, the electron gun, the Faraday cup and the hypodermic support.
Added to this filament supply is a computer controlled current boost circuit that can be used to increase the filament current during electron gun tuning. The additional current then returns to the 12V DC regulated supply via a second 1N4007 diode. This therefore depends upon both the current driving the filament, as well as the resistance of the filament when hot (typically 4W - 6W). This supply feeds current to the filament without changing the characteristics of the main constant current supply used while operating the spectrometer. These high stability reference sources ensure that the analyser and electron gun positions are accurately measured with time. A datalogger internal to the controlling PC establishes a cubic spline relationship between the measured angles and the corresponding ADC conversion from the potentiometers. This PC also controls the selection of the relay motherboard for measurement of the required lens element either in voltage or current mode when a request is sent from the controlling computer via an RS232 serial line.
The electron impact occurs when a well controlled electron beam interacts with a well defined gas beam comprising ground state atoms or molecules under investigation. Here the electrons do not lose any energy in the reaction, however the momentum of the electrons may change. Here the incident electron loses energy in the reaction to the target, which is subsequently excited to one of its (usually discrete) states. In these experiments the target is prepared in some way so that it has internal energy prior to interaction with the electron beam.
This is here defined as a state that cannot decay to a lower state of the target via a single photon emission (dipole allowed) transition. State selectivity is obtainable by exploiting momentum transferred to the atom by the electron impact.
This is brought about in these experiments by employing a supersonically expanded jet of target gas which is skimmed by an expontially flared aperture in a source chamber.
These ground state targets eventually enter the high vacuum pump located at the top of the interaction chamber from where they are ejected. They can then drift for many hundreds of microseconds before a collisional de-excitation occurs either with each other or with a surface or stray molecule inside the vacuum chamber.
Note that if the targets are molecules they may share this internal energy between rotational & vibrational states in the metastable electronic manifold.
This can be achieved using conventional electron energy analysers that select electrons of a specific energy and scattering angle for measurement. As the possible target deflection angles following the collision is confined to a much smaller range than the possible scattering angles of the electrons (which range from 0A° through to 180A° in space), in principle the cross section for excitation of the state can be completely mapped out over all possible electron scattering angles. In these experiments only the momentum of the electron is measured, both for the incident beam and for the scattered electrons using energy and angle selective analysers. The signal will then be an incoherent summation of the contributions fom these excited states. Of particular interest in this region is the study of temporary negative ion states that show up as resonances. Measurements of energy selected scattered electrons in coincidence with measurement of the excited targets then exclude any contribution from upper states, since the electrons which excite these states are excluded from the coincidence measurement.
Tunable laser radiation has an energy resolution typically 1,000 to 1,000,000 times better than is possible using electron spectrometers, and so resonant excitation can individually select the metastable state for further excitation following electron impact excitation.
The coplanar momentum transfer scheme, showing the relationship between the extrema of the electron scattering angles together with the corresponding extrema of the target deflaction angles.
The graphs show the effects as a function of the momentum, velocity and deflection angle of the target helium atoms. Variation of the maximum and minimum target deflection angle as a function of the incident energy of the electrons. This aperture together with the 1mm exit orifice of the nozzle are aligned using visible laser diodes. All are aligned using Visible Laser Diodes apertured to accurately define their individual axes. This is important as the detector may measure ions created as the electron beam interacts with the gas beam.
The calibrated angle plate which accurately measures the deflection angle of the target beam. This additionally allows the zero angle to be calibrated wrt the apertures in the source chamber which define the ground state target beam direction. Since the interaction region to detector distance is accurately known, this immediately yields the value of final atom momentum Paf(qa) to substitute into the appropriate equations. Gaussian fitting parameters as a function of the Detection Angle for the time resolved metastable atom deflections at 40eV Incident Energy. It can be seen that the DCS is forward peaked as expected, and that the DCS has been determined at the complete set of scattering angles from forward to backward scattering.
This may be attributable to errors associated with the solid angle of detection of the analyser which has not been considered in the above calculation. The Experimental Geometry for asymmetric energies, where the scattered and ionised electrons do not take equal energy away from the reaction.

The outgoing electrons were selected to have equal energy and the same scattering angle x with respect to the projection of the incident electron trajectory onto the detection plane spanned by the electron analysers. Deconvolution of the experimental results using an estimated angular response function indicated that this dip is five or more orders of magnitude smaller than the maximum differential cross section at this energy. Following energy selection the electrons are detected by channel electron multipliers, whose pulses are amplified and noise discriminated. As discussed elsewhere, the side peaks near x = 45A° and 135A° are thought to arise from a double scattering process in which the incident electron scatters elastically from the nucleus and then collides with the valence electron in a quasi-free interaction. The (e,2e) differential cross-section at a gun angle y of 67.5A°, showing the evolution of the dip as a function of the energy sharing of the detected electrons. Variation of the minimum of the dip shown in figure 3 as a function of the energy difference (E1 - E2) when the experimental angular function is deconvolved from the data. Figure 3 shows how the energy sharing affects the shape of the differential cross section for the four different detection energy pairs, and figure 4 shows the dependence on the energy sharing of the minimum differential cross section at the dip, deconvoluted as described elsewhere. As the degree of asymmetry decreases this sensitivity correspondingly increases, until for symmetric energy sharing the calculated deconvolved differential cross section changes by two orders of magnitude compared to the measured cross section. The meter has automatic zero correction but there may be small residual error not exceeding 5pA at 25°C.
1990), well known for highest resolution and lowest diffusional losses even for small particles. The software collects data with 10x per second and can display averaged data with 0.5 to 60 seconds averages.
During the first 10 months we register more than twenty licenses sold,and so we would like to thank our customers. Typically, this assembly consists of four Faraday cups located at the corners of the ion beam raster pattern (e.g.
The spectrometer is shown configured in the perpendicular plane, the electron gun being placed in the vertical position with respect to the analysers which span the horizontal detection plane.
The electron source is a heated tungsten hairpin filament heated by passing around 2A of DC current through the filament.
The saturated gain of these devices yields around 1E8 electrons in the output current pulse for a single electron which enters the input cone. This allows the analyser to roam over a complete 360 degrees of the detection plane when in this geometry (apart from the position where the analysers would clash wth each other).
The analysers are then adjusted for maximum signal count and the procedure is iterated until a maximum is found.
The dark noise count is also reduced when cooled, however since the reduced cathode PMT only produces dark counts of less than 20Hz at room temperature, this is not necessary during routine operations.
The 1.82M resistor was chosen to optimise the single photon counting efficiency of the tube. It is not found that these solder connections significantly affect the ultimate vacuum pressure in the system. Isolated DC supplies to these amplifiers is obtained from a standard +15V supply, the circuit diagram being given in figure 8.
The power supply to these TTL buffers is derived from the rate-meter supplies via 7805 5V regulators located internally. This supply boosts the current without changing the main supply, thus ensuring supply stability before and after the electron gun is tuned. Thus the electrons may be scattered throughout space from the reaction zone, the distribution of these electrons then giving information on the elastic differential cross section for the target in question. These states may be below the ionisation limit of the target, in which case the target usually remains neutral, or higher order excitation processes can occur where more than one electron in the target is excited.
The interaction then removes energy from the target and so the electrons emerge from the reaction with greater energy than they had prior to the reaction.
The excited target therefore has a very long lifetime, since the probability of 2 photon decay is much smaller than for single photon decay processes.
For light atoms and molecules (He & H2 for example) the metastable targets are deflected through angles from ~5A° for forward electron scattering to in excess of 30A° for backward scattering depending upon the impact energy. An electron of well defined momentum Pei collides with a target atom or molecule in its ground state that has a well defined initial momentum Pai. These experiments are conventionally limited to a range of scattering angles that does not include either the forward scattering region of the backward scattering region. These experiments do not have the spatial or temporal resolution to define either the incident or the final target momentum, since they mostly produce a target beam via continuous effusive flow from a capillary. These resonances occur since the scattered electron following excitation can remain in the region of the excited target for sufficient time to form a pseudo-stable state which has its own unique properties. As before the electron of incident momentum Pei is scattered inelastically from the target of incident momentum Pai, leaving the target in a metastable state. Electrons incident along the z-axis have a well defined energy Einc and thus a well defined momentum Pei.
Electrons incident along the z-axis have a well defined energy Einc and a well defined momentum Pei.
Electrons incident along the z-axis with well defined energy Einc and well defined momentum Pei can scatter throughout the range of angles from 0A° (forward scattering) through to 180A° (backward scattering) as shown in figures (a) and (b).
Amplitude h, peak position and peak width w are all calculated using a least squares fit to the data, a sample of which is presented in figure 14. The dip is less pronounced at other incident energies where the excess energy is equally shared.
If the two electrons emerge from this reaction in the perpendicular plane then they have an angle of approximately 90A° to each other, as observed. Where absolute readings are required on low ranges this small error may be subtracted from the reading obtained.
By stepping through the voltage from 10,000V to 5V a particle size distribution is obtained. Vienna-Type DMA offers flexible design with two electrodes of different length to accommodate a variety of experimental needs. The electron gun is a two stage non-energy selected gun that produces an incident electron beam current of up to 4 microamps at an incident energy from around 30eV to 300eV. The 470k resistors bias the dynodes to achieve saturation of the current pulse at the output of the tube, hence biasing the tube for single photon counting applications. This was found to be necessary to eliminate problems associated with current loops from the 20A peak motor current feeding the common point of the logic circuitry.
The energy required to excite these processes is usually above the ionisation limit, and so these excitation mechanisms often lead to ionisation via photon emission or through autoionisation.
In these experiments the initial target state is almost always prepared by excitation with a laser beam from the ground state, since very high yields (theoretically up to 50% for CW laser excitation) of target excitation can be achieved prior to electron collision. This long lifetime (which can be up to seconds if there are no other mechanisms for de-excitation) is exploited in these experiments, which measure the quantity of metastable targets as a function of the target deflection angle following electron collision. The correspondence (via momentum and energy conservation equations) between the deflection angle and arrival time at the detector of the excited target with the electron scattering angle therefore allows differential cross sections to be measured over the full range of possible scattering geometries from forward to backward scattering.
The target is excited to a metastable state, the electron losing energy Eexc during the reaction. Any variation in cross section due to variations in the target momentum is therefore averaged out in the signal that is collected. The target is then further excited using resonant laser radiation to a higher lying state which is then studied. These electrons interact with a beam of target atoms or molecules prepared with a well defined momentum Pai travelling along the x-axis with an initial thermal energy Eai. These electrons interact with a beam of target atoms or molecules prepared with a well defined momentum Pai travelling along the x-axis with an initial energy Eai.
Since momentum must be conserved in the reaction, the change of electron momentum delta Pe along the z-axis for these cases must be balanced by a corresponding change in momentum Delta Pa of the targets along this axis.
The graph shows the effect discussed in figure 4, where for each deflection angle there are two distinct atom momenta corresponding to unique electron scattering directions. These are normally pinched off from exiting the region of the filament using a -15V grid potential wrt the tip of the filament.
The analysers are identical, and consist of a 3-element electrostatic lens focussing the interaction region onto the entrance of a hemispherical electrostatic energy analyser.
The Anode deflector pair corrects for any misalignment of the filament emission point with respect to the axis of the electron gun. Some experiments have also been conducted where the target beam is prepared in a metastable state using electron bombardment, but these experiments are very preliminary. The final momentum of the inelastically scattered electron is given by Pef, whereas the final momentum of the target is given by Paf. The high resolution of the laser excitation allows details of the electron excitation to be studied in higher detail than is possible using electron spectroscopy alone. When requested the grid potential is rapildly raised so that the positive anode potential field reaches the filament to extract electrons which are accelerated and focussed onto the interaction region using 2 triple aperture electrostatic lenses. The Chamber is lined internally with 3mm thick mu-metal, and a mu-metal cylinder encloses the chamber externally as shown. The electrons selected in angle and energy are amplified using Mullard 919BL channel electron multipliers.
The defining apertures limit the pencil and beam angles of the electron beam following focussing by lens 2 onto the interaction region. The load is set by the 50W resistor across the anode to the HT feed, the pulse being decoupled to ground via the 1nF capacitor.
The scattered electrons leave the interaction region with momentum Pef at scattering angles (thetae, phie), whereas the deflected excited targets leave the interaction region with linear momentum Paf at polar angles (thetaa, phia). The scattered electrons are confined to the xz plane and leave the interaction region with momentum Pef at a scattering angles thetae. In general as shown in figure (c), for a particular target deflection angle between these two extrema there are two possible electron scattering angles that can result in targets deflected to this angle, as shown.
A further set of output deflectors pulsed simultaneously with the grid allows the final focussed beam to pass through the exit aperture and onto the target beam.
This reduces the magnetic field at the interaction region to less than 5mG, an acceptable level for the electron energies used in this experiment. The interaction region defined by the interaction of the gas target beam and the electron beam is focussed onto the photocathode of an EMI 9789QB photomultiplier tube via a lens, 450nmA±50nm optical filter and defining aperture. Placement of the apertures ensures that the beam angle is close to zero degrees at the interaction region over a wide range of energies, whereas the aperture opening reduces the pencil angle to around A±2 degrees over this energy range. The deflected excited targets therefore must also leave the interaction region in the xz-plane and are deflected to the angle thetaa.
In the example shown, electrons scattered in the lower xz-plane experience a momentum change larger than electrons scattered into the upper xz-plane.
This set of deflectors also removes any ions created inside the electron gun in the high potential regions from reaching the interaction chamber, since their flight times are much slower than the associated electron pulse. 1 includes a movable Faraday flag which allows setup of the implantation conditions while precluding implantation of the wafers or substrates mounted on a research cube. The target gas effuses from a 0.6mm ID hypodermic needle constructed from platinum iridium alloy. The 10MW safety resistor prevents the output charging up to a high voltage if the feed is disconnected. The deflectors between the apertures correct for space and surface charge beam variations inside the field free region, whereas the final deflectors allow the beam to be steered onto the interaction region defined by focussing a 1mm aperture in front of the photomultiplier tube onto the gas target beam.
Observation of the targets at any intermediate deflection angle will therefore display two different momenta for these targets. The flag is placed into the implant position to allow the projection of the scanned area onto the wafers or substrates. The electrons passing through the interaction region which are not scattered are collected in a Faraday cup opposite the electron gun.
The gun, photomultiplier tube, gas hypodermic and Faraday cup all rotate on a common yoke with an axis through the centre of the horizontal detection plane, allowing all geometries from the perpendicular plane to the coplanar geometry to be accessed. In addition, commercially available Faraday cup assemblies are designed with a single projection aperture size for each assembly. While this is desirable for high-dose implantations, it is not necessary for low-dose implantations. Also, existing Faraday cup assemblies are quite complicated, not easily scaleable to larger aperture areas, and not compatible with ultra-high vacuum (UHV) applications. The present invention is a versatile, high sensitivity Faraday cup array or assembly for ion implantation.
The present invention allows for varying the aperture size by changing only an insertable plate that defines the aperture without changing the positions of the Faraday cups, which are selected for the operation of the largest aperture.
1 schematically illustrates an exploded view of a prior art Faraday cup array for ion implantation dose measurement and monitoring.FIG. 3 is a partial cross-sectional view of an embodiment of a Faraday cup assembly similar to FIG. 2 and incorporating the insertable plate for defining the aperture area.DETAILED DESCRIPTION OF THE INVENTIONThe present invention is directed to a versatile, high-sensitivity Faraday cup array, particularly for dose measurement and uniformity monitoring during ion beam implantation. In addition the present invention is structurally simple and the aperture area can be varied by changing only an insertable plate that defines the aperture without changing the positions of the Faraday cups, which are placed to allow operation using the maximum required aperture.FIG.
1 schematically illustrates an exploded view of a typical prior art Faraday cup assembly or array for ion beam implementation in combination with a research tube containing a substrate and a movable Faraday flag. As shown, the Faraday cup assembly generally indicated at 10 basically includes a mounting ring or member 11, a mask 12, a scanned area on the mask 13, four Faraday cups 14 having openings 15, and an aperture 16 through which an ion beam 17 passes.
The Faraday flag generally indicated at 23 includes a plate 24 hinged at 25 to enable plate 24 to be movable over a 90° swing as indicated by arrow 26, with plate 24 in the upright or setup position containing a scanned area 27 having a Faraday cup 28 therein, and the Faraday flag 23 allows setup, as indicated by legend, of the implantation conditions while precluding implantation of the wafers or substrates 21.
Moving the movable plate 24 to the implant or lowered position, indicated by legend, a portion of the scanned ion beam 17 passes through aperture 16 of Faraday cup assembly 10, and is projected onto wafers or substrates 21 of research cube 20 for ion implantation thereof.
2 and 3 illustrate an embodiment of a complete Faraday cup assembly or array designed to fit inside a tube.
This embodiment of, the Faraday cup assembly, generally indicated at 30, consists of the following six (6) major components; a mask 31, a mounting ring 32, an isolation cup holder 33, Faraday cups 34, a bias ring 35, and a cup shield 36.
The mask 31 which may be composed of electrically conductive material, such as aluminum and graphite, defines the sizes of apertures 37 for the Faraday cups 34 and the maximum size of the irradiation defining apertures 38 of a square configuration but may be rectangular, etc., through which ion beam axis 38? passes, and shields the Faraday cups 34 from extraneous ion bombardment and electrical signals.
The mounting ring 32 which may be composed of electrically conductive structural material such as Al alloys, stainless steel, etc., shields the Faraday cups 34 from extraneious electrical signals and has screws 39 to fix the components of the Faraday cup assembly inside a tube. The isolation cup holder 33, which may be composed of MACOR ceramic, alumina or other electrically isolating dielectric material, electrically isolates the four Faraday cups 34 from the rest of the structure. The Faraday cups 34 which may be composed of electrically conductive material, such as stainless steel, Al alloys, tantalum, etc.
The bias ring 35, may be composed of electrically conductive material such as stainless steel land Al alloys, is biased, for example, at about a ?300V to preclude low energy electrons from passing through the cup defining aperture 37, and entering the Faraday cups 34 and to assure proper collection of low energy electrons generated in the Faraday cups 34.
The cup shield 36 shields the Faraday cups 34 from ion and electron bombardment, either direct or after the ions hit the substrates, such as 21 in FIG.
The mask 31, mounting ring 32, bias ring 35, cup shield 33, and the Faraday cups 34 should be fabricated from electrically conductive materials.FIG. 3 is a partial, cross-sectional view of an embodiment of a Faraday cup assembly or array, such as illustrated in FIG. 2, which incorporates the means of the present invention for varying the aperture area of the assembly without changing the locations of the Faraday cups. The aperture area varying means comprises an insertable plate that defines the aperture of the Faraday cup assembly. 3 illustrates only a section of the Faraday cup assembly from a center line, such as ion beam axis 38? of FIG. As shown, the Faraday cup assembly or array, generally indicated at 40 is composed of the same major components as FIG. 2 which includes a mask 31?, a mounting ring 32?, an electrically isolating cup holder 33?, a Faraday cup 34? having a cup defining aperture 37? with a tapered hole, a bias ring 35?, and a cup shield 36?. All the components but the cup holder are constructed of electrically conductive materials. A spacer ring 41 is located intermediate to the mask 31? and the mounting ring 32?, extends around the bias ring 35, and may be composed of electrically conductive materials such as stainless steel and Al alloys. A signal wire 42 connected to Faraday cup 34? extends through an opening 43 in cup shield 36?. The irradiation defining aperture 44 includes a maximum aperture edge 45 and has therein a removable aperture insert (insertable plate or member) 46 which includes a stepped edge 47 which is removably inserted to a matching edge 48 of mask 31?.
3 are constructed of materials which are UHV compatible thus enabling use in UHV applications.
By the present invention, the irradiation or implantation pattern on the substrate or wafers can be varied by just installing the appropriate aperture inserts and, yet, the correct dosimetry will still be achieved by the same Faraday cups because the raster pattern will include the Faraday cups and all areas inside the cups.The Faraday cup aperture 37? is usually chosen to be comparable to the ion beam diameter. The maximum angle of the rastered beam with the normal to the mask is used to select the diameter of the holes 50 in the bias ring 35?.
3, the diameter of the holes 50 were chosen to ensure that ions rastered at angles, as indicated by arrows 52, 53 and 54, of up to 5° could pass into the Faraday cups 34? without scattering (and changing charge) on the inside of the hole 50 of the bias ring 35?. The inside diameter of the Faraday cups 34? should be chosen to be essentially the same as the diameter of the holes 50 in the bias ring 35?, and the inside length of the Faraday cups 34? should equal one to a few times the inside diameter of the cups. The double counter bores indicated as 55-56 in the cup holder 33? (preferably composed of MACOR ceramic) near the top of the Faraday cups are required to electrically isolate the Faraday cup from surface leakage current resulting from the potential applied to the bias ring 35?.
Such double counter bores 55-56 forces the electrons creating the surface leakage to make abrupt directional changes. Consequently, the surface leakage currents are restricted to values much smaller than the ion beam current even when the time-averaged beam current is as small as 10?12A.The Faraday cups 34? and the cup holder 33? must fit inside the cup shield 36? which has an inside diameter and shape to allow the ions to pass by without scattering from the cup shield and then impinging on the substrate. 3, an inner end 57 of cup shield 36? is tapered to allow for passage of rastered ions at an angle of 2° from centerline 58 as indicated at arrow 59. Similarly, the inside diameter of the projection aperture or hole 60 in the bias ring 35? must be large enough to ensure that no ions scatter from this diameter even for the maximum aperture size.It has thus been shown that the present invention provides a means for improving Faraday cup arrays by extending the sensitivity range, providing the irradiation defining aperture with a varying area whereby its size is easily scaleable, and can be utilized for UHV applications. The invention is structurally simple land utilizes an insertable plate in the irradiation defining aperture whereby the aperture area can be easily changed without changing the positions of the Faraday cups.While an embodiment of the invention has been illustrated and described, and specific materials, parameters, etc.
Modifications and changes may become apparent to those skilled in the art, and it is intended that the invention be limited only by the scope of the appended claims.

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