Abstract:
The invention provides methods and apparatus for measuring the distribution of cluster ion sizes in a gas cluster ion beam (GCIB) and for determining the mass distribution and mass flow of cluster ions in a GCIB processing system without necessitating the rejection of a portion of the beam through magnetic or electrostatic mass analysis. The invention uses time-of-flight measurement to estimate or monitor cluster ion size distribution either before or during processing of a workpiece. The measured information is displayed and incorporated in automated control of a GCIB processing system.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     This application claims priority of U.S. Provisional Application Serial No. 60/218,247 entitled GAS CLUSTER ION BEAM CLUSTER SIZE DIAGNOSTIC, filed Jul. 14, 2000, the provisional application being incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to the measurement of the size distribution of gas clusters in gas cluster ion beams, and, more particularly to apparatus and methods for the measurement of the size distribution of gas clusters concurrently with workpiece processing. 
     The use of a gas cluster ion beam (GCIB) for etching, cleaning, and smoothing of material surfaces is known (see for example, U.S. Pat. No. 5,814,194, Deguchi et al.) in the art. For purposes of this discussion, gas clusters are nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such clusters typically consist of aggregates of from a few to several thousand molecules loosely bound to form the cluster. Such clusters can be ionized by electron bombardment or other means, permitting them to be formed into directed beams of known and controllable energy. The larger sized clusters are the most useful because of their ability to carry substantial energy per cluster ion, while yet having only modest energy per molecule. The clusters disintegrate on impact, with each individual molecule carrying only a small fraction of the total cluster energy. Consequently the impact effects of large clusters are substantial, but are limited to a very shallow surface region. This makes ion clusters effective for a variety of surface modification processes, without the tendency to produce deeper subsurface damage characteristic of monomer ion beam processing. 
     Means for creation of and acceleration of such GCIB&#39;s are described in the reference (U.S. Pat. No. 5,814,194) previously cited. Presently available ion cluster sources produce clusters ions having a wide distribution of sizes, N (where N=the number of molecules in each cluster—in the case of monatomic gases, an atom of the monatomic gas will be referred to as a molecule and an ionized atom of such a monatomic gas will be referred to as a molecular ion throughout this discussion). 
     To a first order approximation, the surface modification effects of an energetic cluster is dependent on the energy of the cluster. However, second order effects are dependent on the velocity of the cluster which is dependent on both the energy of the cluster and it&#39;s mass (and hence the cluster size, N.) In order to maximize the utility of a gas cluster ion beam for surface processing, it is useful to know and control both the energy of the cluster and the cluster size or cluster size distribution. In certain applications gas cluster ion beams are used for deposition or growth of surface films. When thus used it is important to know the mass flow to the workpiece. However, unless the average size or size distribution (average mass or mass distribution) is also known, the total mass flow to the workpiece is not known. In the usual case, ionized clusters from a practical ionized cluster source, do not necessarily all carry the same number of electrical charges. Under certain conditions it can be arranged that the cluster ions predominately carry a single electrical charge, and in such case it is accurately assumed that each charge corresponds to a single cluster, but unless the average size or size distribution (average mass or mass distribution) is also known, the total mass flow to the workpiece is not known. It is possible, by controlling the source conditions, to influence the ratio of cluster ions to molecular ions and as well as to influence the cluster size distribution. However, unless a means is available to measure and monitor the cluster size distribution and the ratio of cluster ions to molecular ions, optimal adjustment and control of the source conditions for influencing ionized cluster size is difficult. When the ionized clusters do not all predominately carry a single charge or a known number of charges, knowledge of the ionized clusters&#39; mass per charge can also be used as a useful parameter to control or adjust the beam&#39;s effectiveness for smoothing, etching, or other processing. 
     For these and other reasons it is useful to have a means of measurement that can provide cluster size distribution information about a gas cluster ion beam or that can provide information about a gas cluster ion beam&#39;s cluster-size-per-charge distribution or cluster-mass-per-charge distribution. 
     Because molecular ions, as well as cluster ions, are produced by presently available cluster ion beam sources, those molecular ions are accelerated and transported to the workpiece being processed along with the cluster ions. Molecular ions, having high energy with low mass results in high velocities, which allow the light molecular ions to penetrate the surface and produce deep damage which is likely to be detrimental to the process. 
     It has become known in the ion cluster beam art that many GCIB processes benefit from incorporating means within GCIB processing equipment for eliminating molecular ions from the ion cluster beams. Electrostatic (see U.S. Pat. No. 4,737,637, Knauer) and electromagnetic (see Japanese laid open application JP 03-245523, cited as prior art in U.S. Pat. No. 5,185,287, Aoyagi et al.) mass analyzers have been employed to remove light ions from the beam of heavier cluster ions. Electrostatic and electromagnetic mass analyzers have also been employed to select ion clusters having a narrow range of ion masses from a beam containing a wider distribution of masses. 
     Presently practical GCIB sources produce a broad distribution of ion cluster sizes with limited cluster ion currents available. Therefore it is not practical to perform GCICB processing by selecting a single cluster size or a narrow range of cluster sizes since the available fluence of such a beam is too low for productive processing. It is preferred to eliminate only the molecular ions and other lowest mass ions from the beam and use all remaining heavier ions for processing. Practical experience has shown that it is often sufficient to provide filtering to eliminate molecular ions while depending on the typical cluster size distribution characteristics (few small size clusters are created by typical sources) to limit the small clusters (N=2 to ˜10) in the beam. Clusters of size N&gt;10 are adequately large to provide acceptable results in most processes. Since the typical cluster distribution contains clusters of up to N=several thousand and there are few clusters of mass less than 100, it is not significantly detrimental if clusters up to size 100 are removed from the beam in the process of eliminating the molecular ions. However, in order to adequately predict the processing effectiveness of a gas cluster ion beam, it is very desirable to know the distribution of masses or cluster sizes in the beam and to know whether molecular ions and the smallest size cluster ions are present or not. 
     It is therefore an object of this invention to provide a way of measuring the distribution of cluster ion sizes in a GCIB. 
     It is also an object of the invention to provide means to determine the mass distribution and mass flow of cluster ions in a GCIB. 
     It is also an object of the invention to determine the presence or absence in a GCIB of undesirable molecular ions. 
     It is still another object of this invention to perform cluster size and mass measurement concurrently with workpiece processing. 
     SUMMARY OF THE INVENTION 
     The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow. 
     In the present invention it is preferably arranged that the molecular and cluster ions produced in an ionization system for gas clusters predominantly carry a single electrical charge and such ions are accelerated through a known acceleration potential. Based thereon, the ions, both molecular ions and cluster ions, in a GCIB produced in the apparatus of the invention have known and controllable energies per ion, regardless of cluster size or whether the ion is a cluster or a molecular ion. Since the ions are all generated from a pure gas, the molecular weight of each molecule is known. Therefore, by measuring the times-of-flight of the known ions of known energy over a known distance, it is possible according to the invention to calculate the mass and the cluster size of the ions. Alternatively, when it is not practical or not desired to arrange that all molecular and cluster ions produced in the gas cluster ionization system predominately carry a single electrical charge, in the present invention, by measuring the times-of-flight of the ions it is possible to calculate the mass per charge or size per charge of the ions. 
     By providing a selected scanning waveform and introducing appropriate sensing apparatus in a conventional GCIB processor, the invention makes an in situ capability for measurement of the mass and cluster size distribution of the ions in a GCIB. The invention separates the GCIB pathway for processing from the GCIB pathway for mass and size measurement, making it possible to perform the cluster size and mass measurement concurrently with workpiece processing. 
     For a better understanding of the present invention, together with other and further objects thereof, reference is made to the accompanying drawings and detailed description and its scope will be pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing typical ion cluster size distribution for a GICB source; 
     FIG. 2 is a schematic showing the basic elements of a prior art GCIB processing system; 
     FIG. 3 is a schematic showing a GCIB processing system employing an embodiment of this invention; 
     FIG. 4 is a schematic of a scan generator for y-scan plates having beam-switching provision according to the invention; 
     FIG. 5 is a schematic of a time-of-flight analyzer system of the invention; 
     FIG. 6 is a time-of-flight spectrum as can be shown on the time-of-flight analyzer display of the invention; 
     FIG. 7 shows a schematic of a GCIB processing system employing an alternative embodiment of the invention; and 
     FIG. 8 shows a positive scanning signal for an alternate embodiment of the GCIB processing system of this invention, having a deflected time-of-flight analysis beam path. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the typical ion cluster size distribution produced by a commonly available GCIB source. The cluster formation process has been shown (in N. Kofuji, et al., Development of gas cluster source and its characteristics,  Proc.  14 th Symp. on Ion Sources and Ion - Assisted Technology , Tokyo (1991) p. 15) to produce few small size clusters (values of N from 2 to about 10), but molecular ions (N=1) are produced in abundance as are larger clusters (N&gt;a few tens, up to a few thousands.) It is known from the teachings of U.S. Pat. No. 5,459,326, Yamada, that molecules in a cluster are not individually energetic enough (on the order of a few electron volts) to significantly penetrate a surface to cause the residual surface damage typically associated with the other types of ion beam processing in which individual molecular ions may have energies on the order of thousands of electron volts. Nevertheless, according to Yamada and Matsuo, Cluster ion beam processing,  Matl. Science in Semiconductor Processing I , (1998) pp. 27-41, the clusters themselves can be made sufficiently energetic (some thousands of electron volts), to effectively etch, smooth or clean surfaces. 
     FIG. 2 shows a typical configuration for a GCIB processor  100  of a form known in prior art, and which may be described as follows: a vacuum vessel  102  is divided into three communicating chambers, a source chamber  104 , an ionization/acceleration chamber  106 , and a processing chamber  108 . The three chambers are evacuated to suitable operating pressures by vacuum pumping systems  146   a ,  146   b , and  146   c , respectively. A condensable source gas  112  (for example argon or N 2 ) is admitted under pressure through gas feed tube  114  to stagnation chamber  116  and is ejected into the substantially lower pressure vacuum through a properly shaped nozzle  110 . A supersonic gas jet  118  results. 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 a few to several thousand weakly bound molecules. A gas skimmer aperture  120  separates the gas products that have not been formed into a cluster jet from the cluster jet so as to minimize pressure in the downstream regions where such higher pressures would be detrimental (e.g., ionizer  122 , high voltage electrodes  126 , and process chamber  108 ). Suitable condensable source gases  112  include, but are not limited to argon, nitrogen, carbon dioxide, oxygen. 
     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 . The electron impact ejects electrons from the clusters, causing a portion the clusters to become positively ionized. 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  having an initial trajectory  154 . Filament power supply  136  provides voltage V F  to heat the ionizer filament  124 . Anode power supply  134  provides voltage V A  to accelerate thermoelectrons emitted from filament  124  to cause them to bombard the cluster containing gas jet  118  to produce ions. Extraction power supply  138  provides voltage V E  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 V ACC  to bias a high voltage electrode with respect to the ionizer  122  so as to result in a total GCIB acceleration potential equal to V ACC . One or more lens power supplies ( 142  and  144  shown for example) may be provided to bias high voltage electrodes with potentials (V L1  and V L2  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. 3 shows one possible embodiment of the invention utilizing of a basic GCIB apparatus  200  having a modified configuration to implement the invention. In this embodiment, a fixed (DC) deflection voltage has been added to the beam scanning voltage waveforms at the Y-scan deflection plates  202  to impose a fixed offset angle  210  between the initial beam trajectory  154  and the central path  206  of the scanned GCIB  148 , the offset being imposed in the region between the scan plates  202 . At scan plates  202 , in addition to being deflected in the amount of offset angle  210 , the GCIB also is scanned through a scanning angle  226 . One half of the scan angle  226  is referred to as the scan half angle  208 . Any uncharged molecules or clusters that may be present in the GCIB, are not responsive to the electrostatic field between scan plates  202 , and continue in a straight line along path  204 , which extends to a region now separated from the workpiece  152  processing region. A Faraday enclosure  222  having a suppressor ring electrode  216  and a grounded ring electrode  218  at the beam entrance opening of the Faraday enclosure  222  is disposed in the path of the straight line path  204 , which is an extension of the initial beam trajectory  154 . The suppressor ring electrode  216  is negatively biased with respect to the Faraday enclosure  222  by a bias power supply, E S , so as to permit entrance of any energetic ions or uncharged particles traveling along path  204  with minimal influence, but yet also prevent the exit of low energy electrons, thus retaining all secondary electrons that may be produced in the Faraday enclosure  222 . Thus, if any uncharged molecules or clusters are stopped in the Faraday enclosure  222 , they have no electrical effect, but any ions that are stopped in the Faraday enclosure have their charged conducted by lead  220  to a time-of-flight analysis system  400 . Workpiece  152  is held and positioned in the path of the scanned GCIB  148  and away from the Faraday enclosure  222  and the undeflected path  204  by workpiece holder  150 . 
     Still referring to FIG. 3, the beam entrance to scan plates  202  is separated from the entrance to the Faraday enclosure  222  by a drift distance, D, which may be any convenient distance, preferably many times greater than the length of scan plates  202  and also preferably greater than 30 cm. A scan generator  300  provides y-axis scanning voltages and deflection voltages to scan plates  202  through leads  212  and  214 . Scan generator  300  also provides synchronization pulses to the time-of-flight analysis system  400  through lead  224 . 
     FIG. 3 further shows, for purposes of example, but not for limitation, a fixed angular deflection of the central path  206  from the scanned GCIB  148  of approximately 15 degrees from the initial beam trajectory  154  and its extended undeflected path  204 , however, it is apparent that any deflection slightly larger than the scan half-angle  208  is potentially adequate to separate the scanned GCIB  148  from the undeflected path  204 . 
     FIG. 4 shows details of scan generator  300  and of the signals it produces. The scan generator  300  produces scanning, deflection, and beam switching signals for y-scan plates  202  and produces synchronization signals for use by the time-of-flight analysis system  400 . A digital sequence generator  302  produces sequences of binary number and control signals that flow to a digital-to-analog converter (DAC)  306  through signal bus  304 . The digital sequence generator  302  may consist of a microprocessor, micro-controller, small computer, or other digital logic system capable of generating the necessary digital number sequences and control signals for the DAC  306 . It is easily appreciated that the digital sequence generator and its control may be implemented by any of a variety of techniques that are well known to those skilled in the art of digital and computer logic. The digital sequence generator may furthermore communicate by way of a communication bus  324  with an optional remote digital system controller  326 . Although such a digital system controller may be part of an overall automated control system for a GCIB processing apparatus and may be desirable from a point of convenience and efficiency in integrating and coordinating the functions of a GCIB processor, it is not a necessary part of the invention. Accordingly, the digital system controller  326  and the communication bus  324  are shown for purposes of example and not for limitation. 
     Still referring to FIG. 4, the binary number sequences and control signals provided by the digital sequence generator  302  enable the DAC  306  to produce a y-scanning signal that is amplified by inverting amplifier  308  and non-inverting amplifier  310  and by high-voltage amplifiers  312  and  314  to produce high level scanning signals  316  and  318  which are connected to the y-scan plates  202  by leads  214  and  212  respectively. During normal processing operations, the scan generator  300  produces a periodic saw-tooth waveform for each plate, having a period, t scn , (which could, for example, but without limitation, be on the order of 1 to 100 milliseconds) and an amplitude suitable to produce the desired scan angle  226 . Consider the scanning signal  318 : The periodic saw-tooth waveform is superimposed on a DC deflection voltage, V D , which is chosen to impose a fixed offset angle  210  between the initial beam trajectory  154  and the central path  206  of the scanned GCIB  148  as the beam travels through the region between the scan plates  202 . 
     From time to time as required, either at least once before the start of GCIB processing, or preferably, repeatedly during GCIB processing, the digital sequence generator intersperses a beam switching pulse among the periodic saw-tooth scan waves. The timing of the waveform segments shown in the plots for signals  316  and  318  is chosen to include one such beam switching pulse. In the illustrated scanning signals,  316  and  318 , the onset of the beam switching pulse is labeled t 0  and the duration of the beam switching pulse is labeled t pw  During the beam switching pulse duration, the y-scanning signal voltage is zero, and for that reason, the GCIB is not deflected by offset angle  210  and is not scanned through scan angle  226 , but, rather, travels undeflected through the region of y-scan plates  202  along the straight line path  204 , which is an extension of the initial beam trajectory  154 . Accordingly the undeflected GCIB flies in a straight line to the Faraday enclosure  222 , where it is stopped and the charge in the ions is converted into a detected current which flows into time-of-flight analyzer  400  through lead  220 . At time t 0 +t pw , the beam switching pulse ends, periodic saw-tooth scanning begins, the beam switches back to the offset path which is directed at the workpiece  152  and GCIB processing resumes. Note that scanning signal  316  is the negative of scanning signal  318 . A sample of the scanning signal  308  is conducted to pulse shaping circuit  320 . Pulse shaping circuit processes the scanning signal to extract and provide a synchronization pulse signal  322  which has a leading edge occurring at time t 0 , synchronous with the onset of the beam switching pulse to the scan plates  202 . The synchronization pulse signal  322  is shown, for example, but not for limitation, to have a pulse duration equal to t spw , which may be equal to or different from t pw . Synchronization pulse signal  322  is conducted by lead  224  to the time-of-flight analyzer  400 . During the beam switching pulse duration, t pw , molecular ions and cluster ions entering the entrance receive no deflection and so form a pulse of ions directed toward the Faraday enclosure  222 . The undeflected ion pulse may contain molecular ions and cluster ions of various masses and correspondingly various velocities. As the ions fly toward the Faraday enclosure  222 , their velocity differences cause them to become dispersed along the path  204  to the Faraday enclosure  222  and the ion pulse becomes extended. Consequently, the arrival times of the ions at the entrance to the Faraday enclosure  222  are delayed by different amounts related to their velocities. 
     FIG. 5 shows one embodiment of a time-of-flight analyzer  400  to be used in the invention. The arrival of the electrically charged ions in the Faraday enclosure  222  results in a corresponding current waveform induced in lead  220  to the time-of-flight analyzer  400 . A short delay time, t p , after t 0 , the lightest, fastest ions arrive at the Faraday enclosure and are converted to an electrical current signal. According to their dispersion, the heavier, slower ions arrive later, with the heaviest, slowest ions arriving at the delayed time, t s . The total current signal  402  collected by the Faraday enclosure as a result of the flight of a single dispersed ion pulse resulting from a single beam switching pulse may appear, for example, as shown the time amplitude plot  402  (current signal) in FIG.  5 . In the example chosen for illustration of the principle, there is a current peak at time t p  corresponding to the prompt arrival of the molecular ions and a broader peak occurring between t p  and t s  corresponding to the arrival of the slower cluster ions. Current-to-voltage converter  404  receives the current signal  402  and produces a voltage signal subsequently amplified by amplifier  406 . The time-of-flight analyzer  400  also receives a synchronization pulse signal  322  conducted by lead  224  from scan generator  300 . A digital storage oscilloscope  408 , having a trigger signal input  414 , at least one vertical signal input  412 , internal time base generator for generating a display sweep signal, and a visual display  410  receives the synchronization pulse signal  322  and the amplified voltage signal derived from the current signal  402 . Oscilloscope digitizes, stores, and displays the signal presented at vertical input  412 , which is a time-of-flight spectrum for the ions in the single beam pulse resulting from one beam switching pulse. 
     Oscilloscope  410  may also have data communications capabilities and remote readout capabilities and may communicate by way of a communication bus  416  with an optional digital system controller  326 . Although such a digital system controller may be part of an overall automated control system for a GCIB processing apparatus and may be desirable from a point of convenience and efficiency in integrating and coordinating the functions of a GCIB processor, it is not a necessary part of the invention. Accordingly, the digital system controller  326  and the communication bus  324  are shown for purposes of example and not for limitation. When an optional digital system controller is employed, is often has sufficient computational power to permit more sophisticated analysis and processing of the information in the time-of-flight signal. 
     The time of flight resolution of the current signal  402  is dependent on the flight distance D and the duration of the beam switching pulse t pw  for a given beam energy. It is preferable that the duration of the beam switching pulse be shorter by at least a factor of 10 than the flight time of the ions over the distance D. With a short t pw , the current signal  402  approaches the true time-of-flight spectrum. With longer values of t pw , the beam switching pulse duration t pw  becomes mathematically convolved with the time-of-flight spectrum. In such a case, it must be deconvolved in order to provide a spectrum with acceptable time-of-flight resolution. It is preferable that the duration of the beam switching pulse width t pw  be less than one tenth of the time-of-flight of the lightest ion for which good resolution is desired. When t pw  is short compared to the flight time of the molecular ions, the oscilloscope display  410  will have good resolution for even the molecular ions, and a time-of-flight spectrum  550  such as shown in FIG. 6 is produced on the display  410 . 
     When t pw  is short compared to the time-of-flight, the ion or cluster ion mass is related to the time-of-flight as follows:                m   i     =         2      E       v   2       =         2      E         (     D     t   d       )     2       =         2          E        (     t   d     )       2         D   2       =       2            qeV   ACC          (     t   d     )       2         D   2                     (Eqn.  1)                                
     where m l =mass of ion (or cluster ion) 
     E=GCIB energy 
     v=velocity of ion (or cluster ion) 
     D=ion (or cluster ion) flight distance 
     t d =t−t 0 =ion (or cluster ion) time of flight 
     V ACC =total beam acceleration potential 
     q=cluster charge state 
     e=unit charge (electronic charge) and              N   =       m   i       m   m               (     Eqn   .              2     )                                
     where N=cluster size 
     m l =mass of ion (or cluster ion) 
     m m =mass of a molecule of the gas forming clusters 
     Using Eqn. 2 and the value of m l  from Eqn. 1 it is possible to determine the relationship between t d , the time of flight of a cluster ion and its cluster size, thus making it possible to label the x-axis of the display  550  shown on the display  410  in units of cluster size, N, as well as in time-of-flight time units. 
     When the ionized clusters all bear a single charge equal in magnitude to the unit charge (electronic charge), e, then q=1 and Eqn. 1 and Eqn. 2 calculate the cluster size N. However, in general the ionized clusters may have a distribution in both size N and in charge state q, q being the number of electronic charges held by the ion. At the present, there is no easy separation of these distributions. Consequently in this case the time-of-flight measurement is used to measure the distribution of N′:                N   ′     =     N   q             (Eqn.  3)                                
     where q=cluster charge state. 
     While this generalization somewhat reduces the utility of the measurement, the time-of-flight derived N′ distribution is very useful in determining consistency of operation in commercial production equipment and for facilitating the set-up of equipment operating parameters to achieve consistent processing. Moreover, it is often possible to infer the size distribution separately by operating the cluster ionizer at low levels so that it is unlikely that the clusters acquire more than a single charge each—in such case the approximation that q=e (ions are singly ionized) is justified. 
     The GCIB processing system  200  shown in FIG. 3 incorporates an embodiment of the invention which although operable, may be viewed as having a minor drawback. As drawn in FIG. 3, straight-line path  204  passes very near to the x-scan plates  132 . As a result the pulses of GCIB that are undeflected by scan plates  202  and therefore travel along straight line path  204  may be influenced by any x-scanning electrical field between the x-scan plates  132 , causing the GCIB pulse which was not deflected at plates  202  to receive a small amount of scanning at scan plates  132 . This condition may be tolerated by choosing the shape Faraday enclosure  222  to have a shape that is elongated in the x-scan direction so as to collect all of the GCIB pulse even though it is somewhat extended in the x-scanning axis direction. Alternatively, the condition of concern, namely that the straight line path  204  passes near to the x-scan plates  132 , can be avoided by increasing the fixed offset angle  210 , or by increasing the spacing between scan plates  202  and scan plates  132  by moving scan plates  132  further downstream the central beam path  206 . Although these are various means to deal with the proximity of the straight-line path to the x-scan plates  132 , FIG. 7 shows an alternate embodiment of the invention, which avoids the problem in a different way. 
     The GCIB processing system  250  shown in FIG. 7 is similar to the GCIB processing system  200  shown in FIG. 3 except that rather than disposing the Faraday enclosure  222  along the straight line path  204 , which is an extension of the initial beam trajectory  154 , it is instead disposed along a deflected path  252  which is deflected away from the central path  206  by a separation angle  254  that is greater than the offset angle  210  by an amount that separates the deflected path  252  from the x-scan plates  132  sufficiently for the beam pulse traveling along the deflected path  252  to avoid any scanning effects from x-scan plates  132 . Such deflection can be achieved by utilizing a scanning signal  500  as shown in FIG.  8 . 
     In FIG. 8, note that the scanning signal  500  is identical to the scanning signal  318  shown for the first embodiment except that the beam switching pulse switches to a value of −V D2  rather than zero. The negative value of −V D2  causes the switched beam pulse to travel along deflected path  252  rather than along straight-line path  204 . Scanning signal  500  represents the positive scanning signal applied to lead  212 . Of course, there is a corresponding signal that is the negative of the scanning signal  500 , which is generated and placed on lead  214 . Scan generator  300  can generate signal  500  and its negative provided that the DAC  306 , amplifiers  308 ,  310 ,  312 , and  314  all have bipolar output capabilities and provided that the digital sequence generator  302  generates the corresponding digital sequences having negative values for the beam switching pulse period. 
     Once gas cluster ion beam cluster mass and/or cluster size and/or cluster size per charge and/or cluster mass per charge is determined by the present invention as described in the various embodiments set forth above, appropriate adjustments (for example adjusting source gas flow or adjusting ionizer filament voltage VF or anode voltage VA) can be made to the GCIB apparatus via conventional manual operator controls or via conventional feedback circuitry connected to conventional automation controls in order to maintain preferred gas cluster ion beam cluster mass and/or cluster size and/or cluster size or mass per charge during workpiece processing to assure proper quality or rate of processing, and thereby improving the processing of the workpiece. Alternatively, during workpiece processing, gas cluster ion beam cluster characteristics may be determined to assure proper continued workpiece processing. Should cluster characteristics drift out of desired specifications, conventional control and alarm circuitry can suspend processing and signal for operator attention to restore proper processing conditions, and thereby improve the processing of the workpiece. 
     Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments within the scope of the claims.