Abstract:
Beam-defining apparatus and methods for defining a gas cluster ion beam used to process a workpiece. The beam-defining apparatus includes a second member projecting from a first member in a direction away from the workpiece and an aperture defined in the first and second members that is configured to transmit at least a portion of the gas cluster ion beam to the workpiece.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/831,100, filed Jul. 14, 2006, the disclosure of which is hereby incorporated by reference herein in its entirety. 

   TECHNICAL FIELD 
   The invention relates generally to gas cluster ion beam (GCIB) apparatus and methods for processing the surface of a workpiece and, in particular, to methods and apparatus for reducing workpiece contamination in a high current GCIB processing tool. 
   BACKGROUND OF THE INVENTION 
   Gas cluster ion beams (GCIBs) have been used for etching, cleaning, and smoothing surfaces on workpieces, and for assisting the deposition of films from vaporized carbonaceous materials, and for depositing and/or infusing dopants, semiconductor materials, and other materials. For purposes of this discussion, gas clusters are considered nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such gas clusters may consist of aggregates of from a few molecules to several thousand molecules or more that are loosely bound to form a cluster. 
   The gas clusters can be ionized by electron bombardment, permitting them to be formed into directed beams of controllable energy. Such ions each typically carry positive charges given by the product of q·e (where e is the magnitude of the electronic charge and q is a positive integer having a value of from one to several, representing the charge state of the cluster ion). The larger sized cluster-ions are often 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 gas cluster ions effective for a variety of surface modification processes, without the tendency to produce deeper subsurface damage, which is characteristic of conventional ion beam processing. 
   Presently available cluster-ion sources produce cluster-ions having a wide distribution of sizes, N, up to N of several thousand (where N=the number of molecules in each cluster). In the case of monatomic gases like argon, an atom of the monatomic gas will be referred to as either an atom or a molecule and an ionized atom of such a monatomic gas will be referred to as either an ionized atom, a molecular ion, or a monomer ion. Because of their low mass, molecular ions and/or monomer ions and other very light ions in an accelerated GCIB are often considered undesirable, because when accelerated through an electrical potential difference they acquire much higher velocities than the larger cluster ions. 
   When used to process a workpiece surface, such high velocity monomer ions tend to penetrate the surface much more deeply than the larger clusters and produce unwanted sub-surface damage, detrimental to the desired process. Accordingly, it has been common practice to incorporate a monomer beam filter in GCIB processing equipment. Such a filter typically uses a magnetic field applied by a (preferably permanent) magnet to the beam to deflect the monomer ions and other low mass ions out of the main GCIB to eliminate their undesired effects on the GCIB process. The monomer and other low mass ions are typically analyzed out of the main GCIB using a downstream aperture that intercepts the deflected light ions, while allowing the heavier ions (which are essentially undeflected) to pass to the workpiece. Commonly-assigned U.S. Pat. No. 6,635,883 to Torti et al. teaches the use of a magnet and aperture for removing monomer and low-mass cluster ions and is incorporated by reference herein in its entirety. 
   A current measuring device, as for example a Faraday cup, is typically used in GCIB processing equipment to measure the dose of GCIB applied during processing and/or to control the amount of GCIB dose delivered to a workpiece. Such a current measuring device often has an entrance aperture for accepting the beam to be measured. Occasionally, the envelope of a GCIB is ill defined and may tend to fluctuate slightly, so it is useful and desirable to use a beam defining aperture to cleanly define the shape and/or extent of a GCIB prior to current measurement with a Faraday cup. Such a defining aperture assures that the GCIB measured and the GCIB utilized in workpiece processing are the same in extent and that the entire beam used in processing is accepted for measurement by the Faraday cup or other current measuring means, for precise process dosimetry purposes. Commonly-assigned U.S. Pat. No. 6,646,277 to Mack et al. teaches the use of a defining aperture for beam definition prior to workpiece and/or the dosimetry Faraday cup, and is incorporated by reference herein in its entirety. 
   Many useful surface-processing effects can be achieved by bombarding surfaces with GCIBs. These processing effects include, but are not limited to, smoothing, etching, film growth/deposition, and infusion of materials into surfaces. In many cases, it is found that in order to achieve industrially practical throughputs in such processes, GCIB currents on the order of hundreds or, perhaps, thousands of microamperes are required to supply the necessary surface processing doses. In general the processing effects tend to increase with increasing GCIB current and/or dose. 
   Several emerging applications for GCIB processing of workpieces on an industrial scale are in the semiconductor field and in other high technology fields. Due to yield and performance considerations, such applications typically require that processing steps contribute only very low levels of contamination. Although GCIB processing of workpieces is done using a wide variety of gas cluster source gases, many of which are inert gases, in many GCIB processing applications it is desirable to use GCIBs comprising reactive source gases and source gases that can be used to deposit metals, ceramics, semiconductor, and other films, sometimes in combination or mixture with inert or noble gases. 
   Often halogen-containing gases, oxygen, metals-containing gases, semiconductor-materials-containing gases and other reactive gases or mixtures thereof are incorporated into GCIBs, sometimes in combination or mixture with inert or noble gases. These gases pose a problem for gas cluster ionizer design for semiconductor processing because of their corrosive nature, because they result in etching, sputtering, or deposition of films on impacted surfaces. Often such etching, sputtering, or deposition is part of the intended and desired workpiece processing. 
   However, apertures such as those used for beam definition and for separating molecular, monomer, and low-mass ions from the processing beam also are irradiated by the GCIB. After extended processing periods involving the processing of many workpieces, the apertures can acquire huge GCIB doses. Such incidental dosing of the apertures can result in formation of contamination of the aperture surfaces due to sputtering, corrosion, and deposition of GCIB components or materials sputtered and/or chemically etched from other surfaces due to GCIB incidence effects. The contaminating materials accumulate on the aperture surfaces, often in the form of poorly-adhered films or accumulations. 
   Normal thermal cycling, vibrations, or other effects can cause the release of particles of the contaminants from the aperture surfaces. The proximity of such apertures to the workpiece and/or transport of the particles by electrostatic transport effects or other effects can result in very undesirable transport of contaminating particles to the workpiece(s) being processed in the GCIB equipment resulting in spoiled product or low product yields. 
   With reference to  FIGS. 1A and 1B , a conventional beam-defining apparatus  10  for a GCIB processing tool includes an aperture plate  12  and an aperture  14  extending through the aperture plate  12 . Aperture plate  12  is supported, held in alignment, electrically grounded and thermally heat sunk by aperture plate support (not shown). Aperture plate  12 , which is typically electrically conductive, has a front surface  16  that is struck by a GCIB  20  traveling in the direction of axis  18 . The aperture  14  defines the beam and analyzes the beamlet traveling along axis  18 , so that monomer, molecular and/or low mass cluster ions are eliminated from the GCIB  20  and only a collimated or filtered portion  19  is transmitted for irradiating and processing a workpiece  22  and for purposes of dosimetry. The aperture  14  has a round cross-sectional profile and is generally disposed within the plane of the aperture plate  12  between the front and rear surfaces  16 ,  17  of the aperture plate  12 . 
   A portion of the GCIB  20  is intercepted by the front surface  16  of the aperture plate  12  at a roughly annular region  24  surrounding the aperture  14 . The angle of incidence is approximately normal to the plane of the front surface  16  of aperture plate  12 . After prolonged use, and as a result of sputtering, etching, and/or deposition, contaminants  26  accumulate on the annular region  24  on the front surface  16 . Eventually, some of the contaminants  26  may be shed from the front surface  16  in the form of particles that may be transported to the workpiece  22  causing undesirable particulate contamination of workpiece  22 . Particles shed from the aperture plate  12  are predominately shed into the GCIB  20  where electrostatic forces and other beam forces facilitate transport to the workpiece  22 . 
   What is needed, therefore, is a beam-defining apparatus for a GCIB processing tool that includes an aperture constructed to reduce the release of contaminant particles of from surfaces near the aperture. 
   SUMMARY OF THE INVENTION 
   A beam-defining apparatus is provided for defining a gas cluster ion beam used to process a workpiece. In one embodiment, the beam-defining apparatus comprises a first member adapted to be supported in a spaced relationship with the workpiece and a second member projecting from the first member in a direction away from the workpiece. The first and second members include an aperture configured to transmit at least a portion of the gas cluster ion beam to the workpiece. 
   The beam-defining apparatus may be used in conjunction with a gas cluster ion beam apparatus for processing a workpiece with a gas cluster ion beam. The gas cluster ion beam apparatus comprises a vacuum vessel and a gas cluster ion beam source within the vacuum vessel. The gas cluster ion beam source configured to produce the gas cluster ion beam. The beam-defining apparatus is disposed in the vacuum vessel between the gas cluster ion beam source and the workpiece. 
   In another embodiment, a method is provided for processing a workpiece with a gas cluster ion beam. The method comprises directing the gas cluster ion beam through an inlet opening to a beam-defining aperture and orienting a surface surrounding the beam-defining aperture relative to a travel direction of the gas cluster ion beam so that the surface is inclined relative to the travel direction. The method further comprises impinging the surface with the gas cluster ion beam to reduce a cross-sectional area of the gas cluster ion beam transmitted through the beam-defining aperture and exposing the workpiece to the gas cluster ion beam after the gas cluster ion beam exits an outlet opening of the beam-defining aperture. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1A  is a front view of a conventional GCIB beam-defining apparatus for a GCIB processing apparatus. 
       FIG. 1B  is a cross-sectional view taken generally along line  1 B- 1 B of  FIG. 1A . 
       FIG. 2  is a schematic view of a GCIB processing apparatus incorporating a GCIB beam-defining apparatus in accordance with an embodiment of the invention. 
       FIG. 3A  is a front view of the GCIB beam-defining apparatus of  FIG. 2 . 
       FIG. 3B  is a cross-sectional view taken generally along line  3 B- 3 B of  FIG. 3A  with the gas cluster ion beam depicted. 
       FIG. 4  is a cross-sectional view similar to  FIG. 3B  of a GCIB beam-defining apparatus in accordance with an alternative embodiment of the invention. 
       FIG. 5  is a cross-sectional view similar to  FIGS. 3B and 4  of a GCIB beam-defining apparatus in accordance with an alternative embodiment of the invention. 
       FIG. 6  is a graph showing particle contamination performance of a GCIB processing apparatus with a conventional beam-defining apparatus as shown in  FIGS. 1A and 1B . 
       FIG. 7  is a graph showing improved particle contamination performance of the GCIB beam-defining apparatus of  FIGS. 3A , and  3 B. 
   

   DETAILED DESCRIPTION 
   With reference to  FIG. 2 , a GCIB processing apparatus  100  includes 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 , which is stored in a gas storage cylinder  111  is admitted under pressure through gas metering valve  113  and gas feed tube  114  into stagnation chamber  116  and is ejected into the substantially lower pressure vacuum through a properly shaped nozzle  110 . A supersonic gas jet  118  results. Cooling, which results from the expansion in the gas jet  118 , causes a portion of the gas jet  118  to condense into clusters, each consisting of from several to several thousand weakly bound atoms or molecules. A gas skimmer aperture  120  partially separates the gas molecules that have failed to condensed 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 , suppressor electrode  142 , and processing chamber  108 ). Suitable condensable source gases  112  include, but are not limited to argon, nitrogen, carbon dioxide, oxygen, NF 3 , GeH 4 , B 2 H 6 , and other gases and/or gas mixtures. 
   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 impacts with clusters eject electrons from the clusters, causing a portion the clusters to become positively ionized. Some clusters may have more than one electron ejected and may become multiply ionized. 
   Suppressor electrode  142 , and grounded electrode  144  extract the cluster ions from the ionizer exit aperture  126 , accelerate them to a desired energy (typically with acceleration potentials of from several hundred V to several tens of kV), and focuses them to form a gas cluster ion beam (GCIB)  128 . The axis  129  of the supersonic gas jet  118  containing gas clusters is substantially the same as the axis of the GCIB  128 . Filament power supply  136  provides filament voltage V f  to heat the ionizer filament  124 . Anode power supply  134  provides anode voltage V A  to accelerate thermoelectrons emitted from filament  124  to cause the thermoelectrons to bombard the cluster-containing gas jet  118  to produce cluster ions. Suppression power supply  138  provides suppression voltage V S  to bias suppressor electrode  142 . Accelerator power supply  140  provides acceleration voltage V Acc  to bias the ionizer  122  with respect to suppressor electrode  142  and grounded electrode  144  so as to result in a total GCIB acceleration potential equal to V Acc . Suppressor electrode  142  serves to extract ions from the ionizer exit aperture  126  of ionizer  122 , to prevent undesired electrons from entering the ionizer  122  from downstream, and to form a focused GCIB  128 . 
   A magnet  132 , which may have the construction of a permanent magnet, has a clear aperture  222  to allow GCIB passage and applies a magnetic field in a direction transverse to the travel direction of the GCIB  128  along axis  128 . The magnetic field of the magnet  132  deflects the monomer ions, molecular ions, and perhaps some other of the lighter ions in the GCIB  128  forming a beamlet of undesired monomer, molecular and other low-mass ions traveling in a direction  130  slightly deflected from axis  129  and separating the undesired monomer, molecular and/or other low-mass ions from the heavier and larger cluster ions traveling in GCIB  128  along axis  129 . 
   A filtered GCIB  131  consists of the high-mass essentially undeflected portion of GCIB  128  and passes through an aperture  404  in an aperture plate  402  of a beam-defining apparatus  400 . Aperture plate  402  defines the beam and analyzes the beamlet traveling in direction  130 , so that monomer, molecular and/or low mass cluster ions are eliminated from the GCIB and only the filtered GCIB  131  is passed for workpiece processing and for dosimetry. Aperture plate  402  is typically electrically conductive. Aperture plate  402  is supported, held in alignment, electrically grounded, and thermally heat sunk by aperture plate support  206 . 
   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 , which can be disposed in the path of the filtered GCIB  131 . Since most applications contemplate the processing of large workpieces with spatially uniform results, a scanning system is desirable to uniformly scan a large-area workpiece  152  through the stationary filtered GCIB  131  to produce spatially homogeneous workpiece processing results. 
   An X-scan actuator  202  provides linear motion of the workpiece holder  150  in the direction of X-scan motion  208  (into and out of the plane of the paper). A Y-scan actuator  204  provides linear motion of the workpiece holder  150  in the direction of Y-scan motion  210 , which is typically orthogonal to the X-scan motion  208 . The combination of X-scanning and Y-scanning motions moves the workpiece  152 , held by the workpiece holder  150  in a raster-like scanning motion through filtered GCIB  131  to cause a uniform (or otherwise programmed) irradiation of a surface of the workpiece  152  by the filtered GCIB  131  for processing of the workpiece  152 . The workpiece holder  150  disposes the workpiece  152  at an angle with respect to the axis  129  of the filtered GCIB  131  so that the filtered GCIB  131  has an angle of beam incidence with respect to a workpiece  152  surface. The angle of beam incidence may be 90 degrees or some other angle, but is typically 90 degrees or near 90 degrees as shown in  FIG. 1 . During Y-scanning, the workpiece  152  and the workpiece holder  150  move from the position shown to the alternate position “A” indicated by the designators  152 A and  150 A respectively. Notice that in moving between the two positions, the workpiece  152  is scanned through the filtered GCIB  131  and in both extreme positions, is moved completely out of the path of the filtered GCIB  131  (over-scanned). Though not shown explicitly in  FIG. 1 , similar scanning and over-scan is performed in the (typically) orthogonal X-scan motion  208  direction (in and out of the plane of the paper). 
   A beam current sensor  218  is disposed beyond the workpiece holder  150  in the path of the filtered GCIB  131  so as to intercept a sample of the filtered GCIB  131  when the workpiece holder  150  is scanned out of the path of the filtered GCIB  131 . The beam current sensor  218  is typically a Faraday cup or the like, closed except for a beam-entry opening, and is typically affixed to the wall of the vacuum vessel  102  with an electrically insulating mount  212 . 
   A controller  220 , which may be a microcomputer based controller, connects to the X-scan actuator  202  and the Y-scan actuator  204  through electrical cable  216  and controls the X-scan actuator  202  and the Y-scan actuator  204  so as to place the workpiece  152  into or out of the filtered GCIB  131  and to scan the workpiece  152  uniformly relative to the filtered GCIB  131  to achieve desired processing of the workpiece  152  by the filtered GCIB  131 . Controller  220  receives the sampled beam current collected by the beam current sensor  218  by way of lead  214  and thereby monitors the GCIB and controls the GCIB dose received by the workpiece  152  by removing the workpiece  152  from the filtered GCIB  131  when a predetermined desired dose has been delivered. 
   With reference to  FIGS. 3A and 3B , the beam-defining apparatus  400  includes a tubular protrusion  412  that projects outwardly from a front surface  406  of the aperture plate  402  toward the GCIB upstream direction and in a direction facing away from the workpiece  152 . The aperture  404 , which is defined as a bore partially inside the protrusion  412  and partially inside aperture plate  402 , collimates and shapes the GCIB  128  so that, after passing through the beam-defining apparatus  400 , the filtered GCIB  131  impinges workpiece  152 . The aperture  404  extends between an inlet opening  420  near the apex  428  of the protrusion  412  and an outlet opening  422  in the aperture plate  402  downstream of the inlet opening  420  in a direction toward workpiece  152 . In use, the outlet opening  422  is positioned along a central axis  430  between the inlet opening  420  and the workpiece  152 . 
   The protrusion  412  includes an outer surface  412   b  that intersects the front surface  406  of the aperture plate  402  at a corner. Likewise, the aperture plate  402  and protrusion  412  define an inner surface  412   a  that surrounds the aperture  404  and that intersects a rear surface  417  of the aperture plate  402  at another corner defined at the outlet opening  422 . The inner and outer surfaces  412   a ,  412   b  converge and intersect at an apex  428 , which is remote from the workpiece  152  and spaced from aperture plate  402   a  along central axis  430 , at the apex  428  to define inlet opening  420  to the aperture  404 . 
   The outlet opening  422  is typically larger in cross-sectional area than the inlet opening  420  to limit interactions between the GBIC  128  and the inner surface  412   a . The protrusion  412  may be dimensioned such that the length, L, as shown in  FIG. 3B , that the protrusion  412  projects from front surface  406  is greater than or equal to R B . In an alternative embodiment, the front and rear surfaces  406 ,  417  of the aperture plate  402  may be non-planar, as opposed to the planar surfaces  406 ,  417  of the representative embodiment. 
   The GCIB  128  may be distributed symmetrically about axis  129  and, in particular, the GCIB  128  may be substantially cylindrical with a round cross-sectional profile from a perspective along the axis  129  and a beam radius, R B , as best shown in  FIG. 3B , measured radially from the axis  129 . The aperture  404  and its openings  420 ,  422 , as well as inner surface  412   a , are aligned relative to the central axis  430  that, in the representative embodiment, is shown aligned substantially collinear with the axis  129  of the GCIB  128 . Typically, the aperture  404  and its openings  420 ,  422 , as well as inner surface  412   a , has a concentric arrangement relative to the central axis  430 . As understood by a person having ordinary skill in the art, the axes  129 ,  430  are not limited to being collinear but may merely be parallel or may be angularly inclined relative to each other. 
   A portion of the GCIB  128  is intercepted by the outer surface  412   b  of protrusion  412  and another portion by an annular region  410  on the front surface  406  of the aperture plate  402 , although the latter impingement is contingent upon value of the beam radius, R B , and the spatially relationship between the axes  129 ,  430 . Typically, the axes  129 ,  430  are approximately collinear, which is assumed for purposes of description. The portion of the GCIB  128  that impinges the outer surface  412   b  of protrusion  412  impacts at a glancing angle (i.e., an acute angle) rather than at an approximately normal angle (i.e., 90°), as occurs in conventional beam-defining apparatus when the GCIB impinges the aperture plate  12  ( FIGS. 1A ,  1 B). Because of the glancing incidence, the sputtering rate of the constituent material of the outer surface  412   b  and deposition of the sputtered material on the outer surface  412   b  is lower than the sputtering rate if the angle of incidence of the GCIB was normal to the surface, as in conventional beam-defining apparatus. 
   Furthermore, material removed from the outer surface  412   b  by sputtering or etching tends to redeposit at a roughly annular region  410  on the front surface  406  but remote from the inlet opening  420  to aperture  404 . After prolonged use, and as a result of sputtering, etching, and/or deposition, contaminants  408  accumulate on the annular region  410  on the front surface  406 . Eventually some of the contaminants  408  are shed from the front surface  406  in the form of particles but, in this instance, are not efficiently transported to the workpiece  152  because, at least in part, of the remoteness of the annular region  410  from the inlet opening  420  to aperture  404  and, possibly, because at least in part of the shielding of the shed particles from electrostatic, and other, beam forces provided by the protrusion  412 . 
   In the representative embodiment, the inner and outer surfaces  412   a ,  412   b  of the protrusion  412  are conical or frustoconical so that the inner and outer surfaces  412   a ,  412   b  taper in an upstream direction toward opening  420 . The angle, θ 1 , formed by the conical outer surface  412   b  with the central axis  430  may be less than or equal to about 45° and greater than about 0°. The angle, θ 2 , formed by the inner conical surface  412   a  with the central axis  430  is greater than 0° and, in specific embodiments, may be about 3° or more. In another embodiment, the edge radius, R Edge , may be a sharp edge having a radius of less than about 1 millimeter. 
   With reference to  FIG. 4  in which like reference numerals refer to like features in  FIGS. 3A ,  3 B and in accordance with an alternative embodiment, the protrusion  412  of a beam-defining apparatus  500  includes serrations  504  on the outer surface  412   b . The serrations  504  comprise a series of concentric ridges extending about the circumference of the protrusion  412  and encircling the aperture  404  of the beam-defining apparatus  500 . For gas cluster ion beams characterized by a low sputtering rate when incident normal to a surface, the serrations  504  may have benefits in comparison with a smooth outer surface  412   b  of the protrusion  412  ( FIG. 3B ). An additional benefit of this profile is that the serrations  504  may interfere with gravity transport of shed particles toward the inlet opening  420  of the aperture  404 . 
   With reference to  FIG. 5  in which like reference numerals refer to like features in  FIGS. 3A ,  3 B, and  4  and in accordance with an alternative embodiment, the protrusion  412  of a beam-defining apparatus  600  includes a feature  603  projecting from the outer surface  412   b . The feature  603 , which extends about the circumference of the protrusion  412  and encircles the aperture  404 , projects from the outer surface  412   b  to define a circular pocket or indentation  604  generally between the outer surface  412   b  and a surface of the feature  603  that is shadowed from the GCIB  128 . The feature  603  and circular indentation  604  may serve to collect particles shed from the conical outer surface  412   b  that may tend to transport by gravity or other forces toward the inlet opening  420  of aperture  404 . Such particles are caught in the circular indentation  604 , where they are shielded from the influence of the GCIB  128 . The feature  603  may be continuous and unbroken. In the representative embodiment, the feature  603  is located closer to the inlet opening  420  than to the aperture plate  402  and outlet opening  422 . 
     FIG. 6  is a graph illustrating the particle contamination performance of a GCIB processing apparatus incorporating a conventional beam-defining apparatus, substantially as shown in  FIGS. 1A and 1B , that includes a flat aperture plate with a conventional round, planar aperture. The GCIB processing apparatus was configured for processing clean 200 mm diameter silicon wafers for semiconductor applications. Numerous wafers were processed by irradiating them with a gas cluster ion beam composed from a B 2 H 6  source gas, accelerated with a 5 kV accelerating potential. 
   Particles of size greater than 0.16-micron diameter were measured on the wafers both before, and after, GCIB processing with a dose of 1×10 15  gas cluster ions per cm 2 . The number of particles added to the wafer by the GCIB process was calculated for each wafer and plotted in  FIG. 6  as a function of total operating time of the GCIB processing apparatus. The plotted data in  FIG. 6  shows that, when processed with a beam-defining apparatus including a conventional aperture, particulate contamination rates on the processed wafers started out at a low level of about thirty (30) particles added per wafer. However, with cumulative operating time, contamination rates grew rapidly (in about ten (10) hours) to very high levels of more than 400 particles added per wafer. 
     FIG. 7  is a graph showing improved particle contamination performance of a GCIB processing apparatus outfitted an improved beam-defining apparatus substantially as shown in  FIGS. 3A and 3B  . The GCIB processing apparatus was again configured for processing clean 200 mm diameter silicon wafers for semiconductor applications. Numerous wafers were processed using the same processing conditions as used for  FIG. 6  by irradiating them with a gas cluster ion beam composed from a B 2 H 6  source gas and accelerated with a 5 kV accelerating potential. Particles of size greater than 0.16-micron diameter were measured on the wafers both before, and after, GCIB processing with a dose of 1×10 15  gas cluster ions per cm 2 . The number of particles added to the wafer by the GCIB process was calculated for each wafer and plotted as a function of total operating time of the GCIB processing apparatus. A twenty-five (25) wafer rolling average was also plotted on the graph. 
   As is apparent from the data in  FIG. 7 , the improved beam-defining aperture reduced the observed particulate accumulation. Particulate contamination rates on the processed wafers were observed to remain at a low average contamination rate of about twenty-five (25) particles added per wafer. The particulate contamination rates did not increase with cumulative operating time up to at least 192 hours, which represents a substantial improvement over the behavior observed for a beam-defining apparatus having a conventional aperture. 
   The various embodiments of the beam-defining apparatus feature an improved beam aperture geometry that increases the distance over which contaminants must be transported to the aperture so as to be transported to, and thereby contaminate, the workpiece. The improved beam aperture geometry presents an increased surface area impinged by the GCIB that causes contamination to accumulate at a lower development rate on the surfaces bounding the aperture than observed in conventional beam-defining apparatus. The improved beam aperture geometry shields particles of contamination shed by the beam-defining apparatus from beam-induced electrostatic transport effects that would, if not mitigated, potentially transfer particles from the beam-defining apparatus to the workpiece. 
   While the invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, the aperture may have a cross-sectional geometrical shape that is not round, but is instead rectangular, slit-shaped, elliptical, and another non-round aperture shape. Thus, the invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicants&#39; general inventive concept.