Abstract:
A gas-cluster-jet generator with improved vacuum management techniques and apparatus is disclosed. The gas-cluster-jet generator comprises a substantially conically shaped vacuum chamber for housing the nozzle and jet exit portions of the gas-cluster-jet generator. A skimmer may be located at the narrow end of the conical chamber and a close-coupled vacuum pump is located at the wide end of the conical chamber. Support members for the nozzle are high conductivity “spider” supports that provide support rigidity while minimizing gas flow obstruction for high pumping speed. The conically shaped vacuum chamber redirects un-clustered gas in a direction opposite the direction of the gas-cluster-jet for efficient evacuation of the un-clustered gas. The nozzle and a skimmer may have fixed precision relative alignment, or may optionally have a nozzle aiming adjustment feature for aligning the gas-cluster-jet with the skimmer and downstream beamline components. Also disclosed are various configurations of gas-cluster ion-beam processing tools employing the improved gas-cluster-jet generator.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application Ser. No. 61/221,720, filed Jun. 30, 2009 and incorporated herein by reference in its entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to cluster jet formation and the use of an improved cluster-jet generator in a gas-cluster ion-beam apparatus for workpiece processing. 
     BACKGROUND OF THE INVENTION 
     Gas-cluster-jet nozzles are employed as a means of generating a neutral beam of gas-clusters for use in, for example, molecular beam epitaxy and gas-cluster ion-beam formation. 
     Gas-cluster-jets are typically formed by ejecting a high-pressure (typically about 2 atmospheres or more) condensable source gas into a vacuum through a nozzle. Various nozzle forms have been employed, including conical, sonic, and Laval forms. In each case, as the high-pressure gas expands into the vacuum through the nozzle, adiabatic expansion occurs and the source gas at least partially condenses into a beam of gas-clusters. The clusters may range in size of from as few as 2 to as many as tens of thousands of molecules (atoms in the case of monatomic gases) loosely bound together into clusters. In general the gas-cluster-jet contains a wide distribution of gas-cluster-sizes. Additionally, a large quantity of un-clustered gas atoms/molecules may also flow into the vacuum through the nozzle. 
     Many practical applications of gas-cluster-jets are best implemented in a low-pressure vacuum (as are the cluster generation, ionization, and acceleration processes), so it is important to be able to remove un-clustered gas from the vacuum system continuously and efficiently, so as to maintain the integrity of the vacuum in the system generating and employing the gas-cluster-jet. Conventionally, this has been done by the use of skimmers and collimators to separate the gas-cluster-jet from the un-clustered gas, by the use of differential vacuum pumping techniques, and by brute force application of large vacuum pumps with high pumping speed (typically, all three techniques employed in combination). 
     A field of application for gas-cluster-jets that has emerged as a practical industrial process in recent years has been in the formation of a gas-cluster ion-beam (GCIB). When a gas-cluster-jet is ionized using a conventional ionization process such as electron impact ionization, a fraction of the gas-clusters become ionized and can be accelerated and otherwise manipulated by electric and magnetic fields and may thus be employed in various useful industrial and scientific applications. 
     Gas-cluster ion-beams have been used to process surfaces for purposes of cleaning, etching, smoothing, film growth, doping, infusion, and the like. Gas-cluster ions are ionized, loosely bound, aggregates of materials that are normally gaseous under conditions of standard temperature and pressure, typically consisting of from a few hundred atoms or molecules to as many as a few ten thousands of atoms or molecules. Gas-cluster ions can be accelerated by electric fields to considerable energies of tens of thousands of eV or even more. However, because of the large number of atoms or molecules in each gas-cluster ion, and because of the loose binding of the clusters, their effect upon striking a surface is very shallow—the cluster is disrupted at impact and each atom or molecule carries only a few eV of energy. At the surface, instantaneous temperatures and pressures can be very high at gas-cluster ion impact sites, and a variety of surface chemistry, etching, shallow infusion, and cleaning effects can occur. Gas-cluster ion-beams have been used to clean and smooth medical implants and to adhere drugs to the surfaces of medical devices including stents (See U.S. Pat. No. 7,105,199 granted Sep. 12, 2006 to Blinn et al. and U.S. Pat. No. 6,676,989, granted Jan. 13, 2004 to Kirkpatrick et al.) 
     Other applications of GCIB include numerous uses in the field of electronics, including film formation, surface etching, surface smoothing, surface modification, shallow doping, and production of strained semiconductor materials. 
     Numerous prior art patents have disclosed details of GCIB apparatus, including the means of forming the neutral gas-cluster-jet. As examples see U.S. Pat. No. 5,814,194, Deguchi et al.; see JP 25093312A2, Toshihisa et al.; see U.S. Pat. No. 6,486,478, Libby et al.; see US 2006/0118731A1, Saito et al.; and see US 2003/0109092A1, Choi et al. All have employed the concepts: nozzle, skimmer, differential vacuum pumping, and large vacuum pumps. 
     Therefore it is an object of this invention to provide methods and systems for improved generation of a gas-cluster-jet by employing improved vacuum chamber geometry. 
     Another object of this invention to provide a GCIB processing system employing and benefiting from methods and systems for improved generation of a gas-cluster-jet with improved vacuum chamber geometry. 
     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 invention described hereinbelow. 
     The present invention provides a gas-cluster-jet generator with improved vacuum management techniques and apparatus. The gas-cluster-jet generator comprises a substantially conical shaped vacuum chamber for housing the nozzle and skimmer portions of the gas-cluster-jet generator. The skimmer may be located at the narrow end of the conical chamber and a close-coupled vacuum pump is located at the wide end of the conical chamber so that skimmed gases are evacuated in a direction opposite to the flow direction of the gas-cluster jet. Support members for the nozzle are high vacuum conductivity “spider” supports that provide support rigidity while minimizing gas flow obstruction for high pumping speed. The nozzle and skimmer may have precision, fixed relative alignment, or may optionally have an adjustable nozzle aiming capability for aligning the gas-cluster-jet with the skimmer. 
     The system may optionally employ a collimator for improved separation of gas-cluster-jet from un-clustered gas atoms/molecules. When employed as a gas-cluster-jet generator for a GCIB processing apparatus, the system may additionally employ an ionizer, an accelerator, an optional beam filter to remove monomer and low-mass ions from the GCIB, and a target holder and/or manipulator. 
     One embodiment of the present invention provides an apparatus for generating a gas-cluster beam, comprising a gas expansion nozzle mounted in a chamber to cause gas clusters from the expansion nozzle to form a beam passing through the chamber in a predetermined direction and through an aperture at an end of the chamber, wherein the chamber is formed by one or more surfaces surrounding the beam and aperture and located to deflect gas clusters and molecules from the nozzle that are not traveling within and aligned with the beam away from the beam and towards an opposing predetermined direction 
     The one or more surfaces may include a conical first surface coaxially surrounding the beam and angled towards the opposing predetermined direction. The one or more surfaces include a flat second surface surrounding the aperture and facing the opposing predetermined direction. 
     The one or more surfaces may include one or more third surfaces facing away from the beam and located immediately surrounding the beam to deflect gas molecules and clusters traveling at more than a predetermined distance from the beam away from the beam. The apparatus may further comprise a vacuum apparatus located behind the expansion nozzle for evacuating deflected gas molecules and clusters that are not part of the beam from the chamber in the opposing predetermined direction. 
     The gas expansion nozzle may be mounted at opposing input and outlet ends using a limited number of elongated members extending from sides of the chamber to allow easy flow of gas molecules and clusters that are not part of the beam in the opposing predetermined direction. The gas expansion nozzle may be adjustably mounted at the outlet end of the nozzle to enable adjustment of the predetermined direction. The gas expansion nozzle may be tiltably mounted at the input end of the nozzle to support adjustment of the predetermined direction at the outlet end of the nozzle. 
     The one or more surfaces may have substantially the shape of a cone or a pyramid or a elliptic paraboloid or an ellipsoid. The one or more surfaces may surround substantially all of the beam located within the chamber. 
     The apparatus may further comprise a second chamber surrounding the gas cluster beam beyond the aperture and the first said chamber and having a second aperture located for allowing further flow of the gas cluster beam. The apparatus may still further comprise one or more fourth surfaces facing away from the beam and located immediately surrounding the beam at the second aperture for deflecting gas molecules and clusters traveling at more than a predetermined distance from the beam away from the beam. The gas expansion nozzle may be mounted at input and outlet ends, and the outlet end may be adjustably mounted to enable adjustment of the predetermined direction. The second chamber may be formed by at least one plane surface oriented at an angle of from 30° to about 60° with respect to the gas cluster being and adapted to direct gas molecules and clusters that are not part of the beam away from the beam. 
     In another embodiment, the present invention provides a gas-cluster ion-beam processing apparatus comprising the gas-cluster beam generator apparatus for generating a gas-cluster beam as described above, an ionizer for ionizing at least a portion of the gas-cluster beam to form a gas-cluster ion-beam having a path, and a workpiece holder for supporting a workpiece in the path of the gas-cluster ion-beam. The gas-cluster ion-beam processing apparatus may further comprise a differential pumping chamber having a plane surface oriented at an angle of from about 30 degrees to about 60 degrees with respect to a gas-cluster beam trajectory and adapted to direct at least a portion of un-clustered gas into a vacuum pump. 
     Yet another embodiment of the present invention provides a method for generating a gas-cluster beam, comprising the steps of directing a gas expansion nozzle into a chamber to cause gas clusters from the expansion nozzle to form a beam passing through the chamber in a predetermined direction and through an aperture at an end of the chamber, deflecting gas clusters and molecules from the nozzle that are not traveling within and aligned with the beam away from the beam and towards an opposing predetermined direction using walls of the chamber that surround the beam and aperture, and creating a vacuum behind the expansion nozzle for evacuating deflected gas molecules and clusters that are not part of the beam from the chamber. 
     The step of directing may include adjustably mounting the outlet end of the nozzle and adjusting the predetermined direction. 
     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 schematic view of a prior art GCIB apparatus of conventional design; 
         FIG. 2  is a cross-sectional view of a portion of an improved gas-cluster-jet generator according to a first embodiment of the invention; 
         FIGS. 3A ,  3 B, and  3 C are detail views of a conical vacuum chamber enclosure employed in the improved gas-cluster-jet generator of the first embodiment of the invention; 
         FIGS. 4A and 4B  are detail views of the first nozzle support spider employed in the improved gas-cluster-jet generator of the first embodiment of the invention; 
         FIGS. 5A and 5B  are detail views of the second nozzle support spider employed in the improved gas-cluster-jet generator of the first embodiment of the invention; 
         FIG. 6  is a schematic view of a GCIB processing system including the improved gas-cluster-jet generator of the first embodiment of the invention; 
         FIG. 7  is a cross-sectional view of a portion of an improved gas-cluster-jet generator according to a second embodiment of the invention that incorporates a nozzle adjustment; 
         FIG. 8  is a cross-sectional view of a rotated portion of an improved gas-cluster-jet generator according to the second embodiment of the invention; 
         FIG. 9  is a schematic view of a GCIB processing system incorporating the improved gas-cluster-jet generator of the second embodiment of the invention; 
         FIG. 10  is a schematic view of an alternative configuration of a GCIB processing system incorporating the improved gas-cluster-jet generator of the first or second embodiment of the invention; 
         FIG. 11  is a schematic view of another alternative configuration of a GCIB processing system incorporating the improved gas-cluster-jet generator of the second embodiment of the invention; 
         FIG. 12  is a schematic view of yet another alternative configuration of a GCIB processing system incorporating the improved gas-cluster-jet generator of the second embodiment of the invention; and 
         FIGS. 13A ,  13 B, and  13 C are detail views of an alternative shaped vacuum chamber enclosure employed in the improved gas-cluster-jet generator of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is made to  FIG. 1  of the drawings, which shows a typical GCIB processor  100  of a type known in prior art for surface processing. Although not limited to the specific components described herein, the processor  100  is made up of a vacuum vessel  102  which is divided into three communicating chambers: a source chamber  104 , an ionization/acceleration chamber  106 , and a processing chamber  108  which includes therein a workpiece holder  150  capable of positioning a workpiece  10  for processing by a gas cluster ion beam. 
     During use, 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, O 2 , CO 2 , or N 2  or other condensable gas) stored in a 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 suitably shaped nozzle  110 , resulting in a supersonic gas jet  118 . Cooling, which results from the adiabatic expansion in the jet, causes a portion of the gas jet  118  to condense into gas clusters, most consisting of from a few hundred to several thousand (or even tens of thousands) weakly bound atoms or molecules. A gas skimmer aperture  120  partially separates the gas molecules that have not 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 , high voltage electrodes  126 , and processing chamber  108 ). Suitable condensable source gases  112  include, but are not necessarily limited to inert gases (such as argon), nitrogen, carbon dioxide, and oxygen. 
     After the supersonic gas jet  118  containing gas clusters has been formed, the gas clusters are ionized in an ionizer  122 . The ionizer  122  may be 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 . Other conventional types of electron sources may alternatively be employed as sources of electrons for impact ionization. The electron impact on the gas clusters 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  122 , forming a beam, then accelerates the cluster ions with an acceleration potential (typically from 1 kV to as much as several tens of kV) 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  volts (V). One or more lens power supplies ( 142  and  144 , 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  10  to be processed by the GCIB processor  100  is held on a workpiece holder  150 , disposed in the path of the GCIB  128 . In order to accomplish uniform processing of the workpiece  10 , the workpiece holder  150  may be designed to appropriately manipulate workpiece  10 , as may be required for uniform processing. 
     Any workpiece surfaces that are non-planar, for example, spherical or cup-like, rounded, irregular, or other un-flat configuration, may be oriented within a range of angles with respect to the beam incidence to obtain optimal GCIB processing of the workpiece surfaces. This employs a workpiece holder  150  with the ability to be fully articulated for orienting all non-planar surfaces to be processed in suitable alignment with the GCIB to provide processing optimization and uniformity. More specifically, when the workpiece  10  being processed is non-planar, the workpiece holder  150  may be rotated and articulated by an articulation/rotation mechanism  152  located at the end of the GCIB processor  100 . The articulation/rotation mechanism  152  preferably permits  360  degrees of device rotation about longitudinal axis  155  (which may be coaxial with the initial trajectory  154  of the GCIB  128 ) and sufficient articulation about an axis  157  perpendicular to axis  155  to maintain the workpiece surface to within a desired range of beam incidence. 
     Under certain conditions, depending upon the size of the workpiece  10 , a scanning system may be desirable to produce uniform irradiation of a large workpiece. Although often not necessary for GCIB processing, two pairs of orthogonally oriented electrostatic scan plates  130  and  132  may be utilized to produce a raster or other scanning pattern over an extended processing area. When such beam scanning is performed, a scan generator  156  provides X-axis and Y-axis scanning signal voltages to the pairs of scan plates  130  and  132  through lead pairs  158  and  160  respectively. The scanning signal voltages are commonly triangular waves of different frequencies that cause the GCIB  128  to be converted into a scanned GCIB  148 , which scans the entire surface of the workpiece  10 . 
     When beam scanning over an extended region is not desired, processing is generally confined to a region that is defined by the diameter of the beam. The diameter of the beam at the surface of the workpiece can be set by selecting the voltages (V L1  and/or V L2 ) of one or more lens power supplies ( 142  and  144  shown for example) to provide the desired beam diameter at the workpiece. Although not specifically shown, such prior art GCIB processing systems typically employ sensors and circuits for measuring and controlling the GCIB parameters (as for example acceleration potential, beam current, beam focus, gas flow, beam dose applied to the workpiece, workpiece manipulation, etc.) important to processing and also employ additional controls and automation for automatic processing and management of processing recipe selection and control. 
     Although  FIG. 1  shows a workpiece holder and manipulator suitable for holding and manipulating certain types of planar and simply shaped non-planar workpieces, it will be understood by those familiar with the prior art that other types of simpler and more complex holders and manipulators are known. For example, U.S. Pat. No. 6,676,989, Kirkpatrick et al. teaches a holder and manipulator optimized for processing tubular or cylindrical workpieces such as vascular stents. Manipulators for exposing multiple surfaces of biological materials to GCIB irradiation will be known to those skilled in the art and/or may readily be constructed using no more than ordinary skill. Simple workpiece holders without manipulation may be employed when manipulation is not required. 
     In the following description, for simplification of the drawings, item numbers from earlier figures may appear in subsequent figures without discussion. Likewise, items discussed in relation to earlier figures may appear in subsequent figures without item numbers or additional description. In such cases items with like numbers are like items and have the previously described features and functions and illustration of items without item numbers shown in the present figure refer to like items having the same functions as the like items illustrated in earlier numbered figures. 
       FIG. 2  is a cross-sectional view  200  of a portion of an improved gas-cluster-jet generator according to a first embodiment of the invention. A conical gas-cluster-jet generator chamber enclosure  203  encloses a gas-cluster-jet generator chamber  204 . The conical gas-cluster-jet generator chamber enclosure  203  is substantially conical and has an inner surface  201  that is conically coaxial with gas-cluster-jet trajectory  218 . Gas-cluster-jet trajectory  218  has a flow direction  222 . The inner surface  201  forms a circular cone with conical half-angle θ, with respect to gas-cluster-jet trajectory  218 . The conical half-angle θ may be in the range of from about 30 degrees to about 50 degrees dependent on other geometrical considerations of the application, but it is preferably about 35 degrees. First nozzle support spider  205  and second nozzle support spider  207  support the nozzle  210 . A high-pressure source gas is delivered to the nozzle  210  through a flexible gas feed tube  214  and gas flange  217 , attached to the nozzle  210  and sealed with O-ring  219 . The nozzle  210  has an upstream end  224  and a downstream end  226 . An opening in the gas flange  217  forms a stagnation chamber  216 . The conical gas-cluster-jet generator chamber enclosure  203  supports a gas skimmer  220  at its narrow end. Nozzle  210  and gas skimmer  220  are supported with respect to one another in precision alignment such that the gas-cluster-jet trajectory for the gas-cluster-jet formed by the nozzle  210  passes through the gas skimmer  220 . The conical gas-cluster-jet generator chamber  203 , the first nozzle support spider  205 , the second nozzle support spider  207 , the nozzle  210 , and the gas skimmer  220  are all precision machined with close tolerance mating surfaces so that the nozzle  210  and gas skimmer  220  are positioned with precision alignment with respect to one another, such that the gas-cluster-jet trajectory  218  of the gas-cluster-jet formed by the nozzle  210  passes through the gas skimmer  220 . 
     The nozzle  210  may preferably be a conical metal nozzle having an inlet throat of about  50  micrometers diameter and an outlet opening of about 6.4 millimeters diameter, and an overall length of about 60 millimeters. Alternatively nozzles of other forms, materials, and dimensions can be employed as will be known to those skilled in the art. 
       FIGS. 3A ,  3 B, and  3 C are detail views of the conical gas-cluster-jet generator chamber enclosure  203 .  FIG. 3A  is a bottom view  300 A.  FIG. 3B  is a cross-sectional view  300 B.  FIG. 3C  is a top view  300 C. The  FIGS. 3A-3B  show that the shape of the inner surface  201  of the conical gas-cluster-jet generator chamber enclosure  203  is substantially a solid, truncated right circular cone coaxial with the gas-cluster-jet trajectory  218  (of  FIG. 2 ). 
       FIGS. 4A and 4B  are detail views of the first nozzle support spider  205 .  FIG. 4A  is a top view  400 A, and  FIG. 4B  is a side view  400 B. The  FIGS. 4A and 4B  show that first nozzle support spider  205  has an open structure with spider support of a central portion such the central portion is rigidly supported, but yet provides an open, high transparency, high conductance pathway for evacuation of skimmed un-clustered gas from the gas-cluster-jet generator chamber  204  (of  FIG. 2 ). 
       FIGS. 5A and 5B  are detail views of the second nozzle support spider  207 .  FIG. 5A  is a bottom view  500 A, and  FIG. 5B  is a side view  500 B. The  FIGS. 5A and 5B  show that second nozzle support spider  207  has an open structure with spider support of a central portion such the central portion is rigidly supported, but yet provides an open, high transparency, high conductance pathway for evacuation of skimmed un-clustered gas from the gas-cluster-jet generator chamber  204  (of  FIG. 2 ). 
       FIG. 6  is a schematic view of a GCIB processing system  600  including the improved gas-cluster-jet generator of the first embodiment of the invention. Although the improved gas-cluster-jet generator is implemented as a gas-cluster-jet generator for a GCIB processing apparatus, it is understood and intended by the inventors that the same concept is not limited thereby and is employable in other gas-cluster-jet applications such as for example molecular beam epitaxial growth apparatus, or other applications employing gas-cluster-jet generation. Such applications are intended to be included within the scope of the invention. 
     The GCIB processing system  600  includes the portion of an improved gas-cluster-jet generator  200  shown in  FIG. 2 . Referring again to  FIG. 6 , a GCIB system enclosure  602  encloses the GCIB processing system  600 , and includes an intermediate chamber  605  and may include a collimator  666 . It also includes a beamline chamber  606 . A processing chamber  608  is enclosed by a processing chamber enclosure  680  for receiving a GCIB  628  for processing a workpiece  670 . An isolation valve  668  controllably isolates or connects the processing chamber  608  with the beamline chamber  606 . When isolated, the processing chamber may be vented to atmosphere for maintenance or for inserting and/or removing workpieces for processing. When vented, a processing chamber access/viewing port  662  facilitates access to the processing chamber. When evacuated, the processing chamber access/viewing port  662  serves as a window for observation. A vacuum system (not shown) is present to evacuate the processing chamber  608  to rough vacuum before opening the isolation valve  668  between the processing chamber  608  and the beamline chamber  606 . A workpiece holder  650  is provided in the processing chamber  608  for holding the workpiece  670  in the path of the GCIB  628  for workpiece processing. Although a simple workpiece holder  650  is illustrated, it is understood that more complex manipulating or scanning workpiece holders as will readily be known to or devised by those skilled in the art may be employed and it is intended that such be included within the scope of the invention. 
     The intermediate chamber  605  and the beamline chamber  606  have an opening  688  between them. Opening  688  is normally closed by blank-off plate  664 , so that the only communication between the intermediate chamber  605  and the beamline chamber  606  is the aperture of the collimator  666 . An intermediate chamber vacuum pump  646   b  evacuates the intermediate chamber  605 . A beamline chamber vacuum pump  646   c  evacuates the beamline chamber  606 . Optionally, the blank-off plate  664  can be removed so that the intermediate chamber  605  and the beamline chamber  606  communicate through opening  688  and the system can be operated with one of the vacuum pumps  646   b  and  646   c  removed and blanked off or disabled. 
     Any un-clustered gas from the gas-cluster-jet generator chamber  204  (of  FIG. 2 ) that is not skimmed by the skimmer  220 , may be separated from the gas-cluster-jet by the collimator  666  and evacuated by the vacuum pump  646   b . The interior wall portion of the intermediate chamber  605  formed by blank-off plate  664  is oriented at an angle φ with respect to the gas-cluster-jet trajectory  218  so as to position the interior (to intermediate chamber  605 ) surface of blank-off plate  664  in such a way as to direct at least a portion of un-clustered gas separated from the gas-cluster jet by the collimator  666  into the vacuum pump  646   b  and to facilitate coupling of a large mouth, high pumping speed vacuum pump  646   b  to the relatively smaller intermediate chamber  605  if so desired to improve the vacuum in intermediate chamber  605 . In such configuration, and with blank-off plate  664  closing the opening  688 , the intermediate chamber  605  can serve as a highly effective differential pumping chamber to improve the downstream vacuum in beamline chamber  606 . The angle I is preferably in the range of from about 30 degrees to about 60 degrees and directs un-clustered gas to the vacuum pump  464   b  more effectively than if the angle (I) were 90 degrees. 
     Beamline chamber  606  encloses an ionizer  622  for ionizing a gas-cluster-jet following gas-cluster-jet trajectory  218 . The ionizer  622  converts the gas-cluster-jet to a GCIB  628 . A set of high voltage electrodes  626  (two electrodes shown for example, not for limitation) serves to extract the GCIB  628  from the ionizer  622 , to accelerate the GCIB  628  to a desired energy, and optionally to focus the GCIB  628 , according to conventional GCIB technology. An optional beam filter  674  selectively removes monomer ions and optionally small cluster ions from the GCIB  628  when very small clusters or monomers are undesirable. The beam filter  674  may be a magnetic beam filter that deflects low mass cluster ions out of the main GCIB  628 . A beamline component support bracket  672  supports the ionizer  622 , the high voltage electrodes  626 , and the optional beam filter  674  in proper location relative to the gas-cluster-jet trajectory  218  and the GCIB  628 . 
     Generation of the gas-cluster-jet is done in the source chamber  604 . An external conventional source gas supply (not shown but typical to that of  FIG. 1 ) provides a high pressure gas to the gas-cluster-jet generator through flexible gas feed tube  214  by connection at gas coupling  660 . The external source gas supply supplies gas and provides flow control to control the gas flow rate of gas through the nozzle  210 . The GCIB system enclosure  602  serves to closely couple the mouth  690  of the source chamber vacuum pump  646   a  to the large diameter end of the conical gas-cluster-jet generator chamber enclosure  203 . Thus the source chamber  604  includes the volume between the conical gas-cluster-jet generator chamber enclosure  203  and the mouth  690  of the source chamber vacuum pump  646   a . The source chamber  604  volume includes and is somewhat greater than the volume of the gas-cluster-jet generator chamber  204  (of  FIG. 2 ). Referring again to  FIG. 6 , the conical shape of the inner surface of the conical gas-cluster-jet generator chamber enclosure  203  serves to efficiently direct un-clustered gas atoms/molecules skimmed from the gas-cluster-jet by the gas skimmer  220  in a direction opposite to the flow direction  222  of the gas-cluster-jet trajectory  218  and into the mouth  690  of the source chamber vacuum pump  646   a  for evacuation thereby. The open, transparent, high conductivity constructions of the first nozzle support spider  205  and the second nozzle support spider  207  and the close coupling of the mouth  690  of the source chamber vacuum pump  646   a  also facilitate the efficient transport of gas to the source chamber vacuum pump  646   a . The source chamber vacuum pump  646   a  may be a turbo-molecular vacuum pump with a mouth diameter approximately the same as the diameter of the large end of the conical gas-cluster-jet generator chamber enclosure. The improved source chamber  604  including the improved gas-cluster-jet generator results in better vacuum attainment, lower vacuum pump performance requirements and/or a combination of both. Thus it can offer improved performance and/or reduced cost. 
     Although the invention has been described above in terms of a gas-cluster-jet generator comprising a substantially conically shaped inner surface of the conical gas-cluster-jet generator chamber enclosure  203  that serves to efficiently direct un-clustered gas atoms/molecules skimmed from the gas-cluster-jet by the gas skimmer  220  in a direction opposite to the flow direction  222  of the gas-cluster-jet trajectory  218  and into the mouth  690  of the source chamber vacuum pump  646   a  for evacuation thereby, it is recognized by the inventors that other shapes including, without limitation, substantially pyramidal and substantially elliptic paraboloid and substantially ellipsoid shapes (or truncated portions of those shapes) for the inner surface of the gas-cluster-jet generator chamber enclosure  203  will also serve to efficiently direct un-clustered gas atoms/molecules skimmed from the gas-cluster-jet by the gas skimmer  220  in a direction opposite to the flow direction  222  of the gas-cluster-jet trajectory  218  and into the mouth  690  of the source chamber vacuum pump  646   a  for evacuation thereby. When a truncated pyramidal gas-cluster-jet generator chamber is used as an alternative to the conical gas-cluster-jet generator chamber enclosure  203 , the cross section appears identical to the conical gas-cluster-jet generator chamber enclosure  203 . It is intended that such alternate embodiments are included within the scope of the invention. 
       FIG. 7  is a cross-sectional view  700  of a portion of an improved gas-cluster-jet generator according to a second embodiment of the invention that incorporates a nozzle alignment adjustment. A conical gas-cluster-jet generator chamber enclosure  203  encloses a gas-cluster-jet generator chamber  204 . The conical gas-cluster-jet generator chamber enclosure  203  is substantially conical and has an inner surface  201  that is conically coaxial with gas-cluster-jet trajectory  218 . The inner surface  201  forms a circular cone with conical half-angle θ, with respect to gas-cluster-jet trajectory  218 . The conical half-angle θ may be in the range of from about 30 degrees to about 50 degrees dependent on other geometrical considerations of the application, but it is preferably about 35 degrees. First nozzle support spider  205  and second nozzle support spider  207  support the nozzle  210 . First nozzle support spider  205  movably supports the nozzle  210  near its outlet or downstream end  226  and second nozzle support spider  207  movably supports nozzle  210  near its input or upstream end  224 . A high-pressure source gas is delivered to the nozzle  210  through a flexible gas feed tube  214  and gas flange  217 , attached to the nozzle  210  and sealed with O-ring  219 . An opening in the gas flange  217  forms a stagnation chamber  216 . The conical gas-cluster-jet generator chamber enclosure  203  supports a gas skimmer  220  at its narrow end. Nozzle  210  and gas skimmer  220  are aligned with respect to one another in an adjustable alignment such that the gas-cluster-jet trajectory for the gas-cluster-jet formed by the nozzle  210  passes through the gas skimmer  220 . The downstream end  226  is the exit end of the nozzle  210  and passes through the first nozzle support spider  205  with a loose clearance fit that permits motion of the nozzle  210  with respect to the first nozzle support spider  205 . The throat end (gas input end  224 ) of the nozzle  210  fits into a recess in the second nozzle support spider  207  with a small amount of clearance that permits a slight tilting motion of the upstream end  224  of nozzle  210  with respect to the second nozzle support spider  207 . A compressed coil spring  718  biases the nozzle input end  224  against the second nozzle support spider  207 . This arrangement allows a force acting laterally to the outlet end  226  to displace the outlet end  226  and to tilt the nozzle  210  slightly with respect to the resting position of the nozzle  210 . Thus the gas cluster jet trajectory  218  from an initially misaligned nozzle  210  can be steered to optimize the gas-cluster flow that passes through the entrance aperture of the gas skimmer  220 , without necessity of inherently precise fixed alignment (as in the earlier-discussed first embodiment). This is particularly useful in the embodiments of  FIGS. 6 ,  9 ,  11  and  12 , wherein a skimmer or collimator is located at greater distance from the nozzle. 
     A nozzle steering clamp  702  is attached to the first nozzle support spider  205  and encloses an O-ring carrier  704 . An O-ring  708  is held in an internal diameter groove in the O-ring carrier  704  and tightly but flexibly engages the outer diameter of the exit end of the nozzle  210 . A compressed first coil spring  706  biases the O-ring carrier  704  against an opposing first steering shaft  710 . When first steering shaft  710  moves longitudinally, it moves the O-ring carrier  704  and the exit end of the nozzle  210  in the direction of the longitudinal motion of the first steering shaft  710 , increasing or decreasing the compression in first coil spring  706 . First steering shaft  710  has a threaded portion  714  that engages a first threaded opening  712  in the conical gas-cluster-jet generator chamber enclosure  203  and has a first control shaft coupler  716  for connecting to a rotary motion shaft for adjusting the longitudinal motion of first steering shaft  710 . 
     The nozzle  210  may preferably be a conical metal nozzle having an inlet throat of about 50 micrometers diameter and an outlet opening of about 6.4 millimeters diameter, and an overall length of about 60 millimeters. Alternatively nozzles of other forms, materials, and dimensions can be employed as will be known to those skilled in the art. 
       FIG. 8  is a rotated cross-sectional view  800  of the improved gas-cluster-jet generator also shown in  FIG. 7 . In  FIG. 8 , the gas-cluster-jet generator has been rotated 90 degrees about the gas-cluster-jet trajectory  218  with respect to the position illustrated in  FIG. 7 . Thus  FIG. 8  permits viewing a second steering shaft  810  that steers the nozzle  210  in a direction orthogonal to that of the first steering shaft  710 . A compressed second coil spring  806  biases the O-ring carrier  704  against an opposing second steering shaft  810 . When second steering shaft  810  moves longitudinally, it moves the O-ring carrier  704  and the exit end of the nozzle  210  in the direction of the longitudinal motion of the second steering shaft  810 , increasing or decreasing the compression in second coil spring  806 . Second steering shaft  810  has a threaded portion  814  that engages a second threaded opening  812  in the conical gas-cluster-jet generator chamber enclosure  203  and has a second control shaft coupler  816  for connecting to a rotary motion shaft for adjusting the longitudinal motion of second steering shaft  810 . 
       FIG. 9  is a schematic view of one configuration of a GCIB processing system  900  including the improved gas-cluster-jet generator of the second embodiment of the invention. The GCIB processing system  900  includes the portion of an improved gas-cluster-jet generator shown in  FIGS. 7 and 8 . Referring again to  FIG. 9 , a first control shaft  902  connects with the first steering shaft  710  via first control shaft coupler  716 . First control shaft  902  passes through a first rotary motion vacuum feedthrough  904  and has attached a first control shaft adjustment knob  906  for adjusting the longitudinal motion of first steering shaft  710  to control the alignment of nozzle  210  (and thus the gas-cluster-jet trajectory  218 ) with respect to gas skimmer  220  and other downstream beamline components. Like (not visible in this view) elements connected with second steering shaft  810  (not visible in this view) provide for adjusting the longitudinal motion of second steering shaft  810  and thus for controlling the alignment of nozzle  210  in an orthogonal direction. 
       FIG. 10  is a schematic view of a second configuration of a GCIB processing system  920  including the improved gas-cluster-jet generator of the second embodiment of the invention. In this configuration, the GCIB processing system  920  employs skimmer  220  but does not employ any collimator (previously shown as item  666  in  FIGS. 6 and 9 ). Referring again to  FIG. 10 , this second configuration employs the features of the second embodiment of the invention for steering the gas-cluster-jet  218  through the skimmer  220 . Though not shown, this second configuration of a GCIB processing system  920  may alternatively employ the gas-cluster-jet generator of the first embodiment of the invention (with fixed alignment of nozzle  210  and skimmer  220 ). Since no collimator is employed, a beam opening  922  between the intermediate chamber  605  and the beamline chamber  606  facilitates passage of the gas-cluster-jet along the gas-cluster-jet trajectory  218  into the beamline chamber  606 . No blank-off plate (as previously shown as item  664  of  FIG. 9 ) is employed in this configuration of  FIG. 10 . Referring again to  FIG. 10 , the opening  668  between intermediate chamber  605  and beamline chamber  605  provides fluid communication between intermediate chamber  605  and beamline chamber  606 . No intermediate chamber vacuum pump (as previously shown as item  646   b  in  FIG. 9 ) is employed in this configuration of  FIG. 10 . Referring again to  FIG. 10 , a blank-off plate  924  seals the intermediate chamber  605  from atmospheric pressure. Thus the intermediate chamber  605  is evacuated primarily by the beamline chamber vacuum pump  646   c  and may operate at a higher pressure than it does in the configuration shown in  FIG. 9 . In the configuration of  FIG. 10 , the cost of an intermediate chamber vacuum pump is eliminated, which is an economic advantage that may be traded off with any gas-cluster-jet intensity reduction resulting from a higher pressure in the intermediate chamber  605 . 
       FIG. 11  is a schematic view of a third configuration of a GCIB processing system  940  including the improved gas-cluster-jet generator of the second embodiment of the invention. In this configuration, there is no skimmer (as previously shown as item  220  in  FIG. 9 ) at the exit of the source chamber  604 . Referring again to  FIG. 11 , instead a skimmer  944  is located at the exit of the intermediate chamber  605 . An opening  942  in the source chamber  604  facilitates passage of the gas-cluster-jet along the gas-cluster-jet trajectory  218  from the source chamber  604  to the intermediate chamber  605 . A first control shaft  902  connects with the first steering shaft  710  via first control shaft coupler  716 . First control shaft  902  passes through a first rotary motion vacuum feedthrough  904  and has attached a first control shaft adjustment knob  906  for adjusting the longitudinal motion of first steering shaft  710  to control the alignment of nozzle  210  (and thus the gas-cluster-jet trajectory  218 ) with respect to gas skimmer  944  and other downstream beamline components. Like (not visible in this view) elements connected with second steering shaft  810  (not visible in this view) provide for adjusting the longitudinal motion of second steering shaft  810  and thus for controlling the alignment of nozzle  210  in an orthogonal direction. 
       FIG. 12  is a schematic view of a fourth configuration of a GCIB processing system  960  including the improved gas-cluster-jet generator of the second embodiment of the invention. This fourth configuration is like the third configuration (as previously shown in  FIG. 11 ) except that no intermediate chamber vacuum pump (as previously shown as item  646   b  in  FIG. 11 ) is employed in this configuration. Referring again to  FIG. 12 , a blank-off plate  924  seals the intermediate chamber  605  from atmospheric pressure. Thus the intermediate chamber  605  is evacuated primarily by the source chamber vacuum pump  646   a  and may operate at a higher pressure than it does in the configuration shown in  FIG. 11 . Optionally (not shown), the blank-off plate  664  may be omitted, permitting the beamline chamber vacuum pump  646   c  to assist in the evacuation of the intermediate chamber  605  through opening  688 , permitting the intermediate chamber  605  to operate at lower pressure. By pre-selection of the size of the opening  688  and of the beamline chamber vacuum pump  646   c , considerable control of the operating pressure of the intermediate chamber  605  may be obtained. In the configuration of  FIG. 12 , the cost of an intermediate chamber vacuum pump is eliminated, which is an economic advantage that may be traded off with any gas-cluster-jet intensity reduction resulting from a higher pressure in the intermediate chamber  605 . 
       FIGS. 13A ,  13 B, and  13 C are detail views of an alternative shaped gas-cluster-jet generator chamber enclosure  983 .  FIG. 13A  is a bottom view  980 A.  FIG. 13B  is a cross-sectional view  980 B.  FIG. 13C  is a top view  980 C. The  FIGS. 13A-13B  show that the shape of the inner surface  981  of the gas-cluster-jet generator chamber enclosure  983  is substantially a solid, elliptic paraboloid or similar surface of revolution such as an ellipsoid coaxial with the gas-cluster-jet trajectory  218  (of  FIG. 2 ). Though not shown in both forms, the elliptic paraboloid or similar surface of revolution may optionally be truncated by any suitable amount. 
     Although specific applications of the improved gas-cluster-jet generator has been described employing a conical nozzle, it is understood that alternate nozzle forms, including without limitation, sonic and Laval forms are compatible with the practice of the invention and it is intended that such alternate forms are encompassed within the scope of the invention. Although certain specific examples employing the improved gas-cluster-jet-generator as gas-cluster-jet sources for GCIB apparatuses, it is understood that the invention is applicable to a wide variety of other systems that employ gas-cluster jets, including without limitation, gas-cluster-jet deposition systems and molecular beam epitaxy systems, and it is intended that such other applications are included within the scope of the invention. Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments within the spirit and scope of the invention and of the appended claims.