Abstract:
Techniques for providing a ribbon-shaped gas cluster ion beam are disclosed. In one particular exemplary embodiment, the techniques may be realized as an apparatus for providing a ribbon-shaped gas cluster ion beam. The apparatus may comprise at least one nozzle configured to inject a source gas at a sufficient speed into a low-pressure vacuum space to form gas clusters. The apparatus may also comprise at least one ionizer that causes at least a portion of the gas clusters to be ionized. The apparatus may further comprise a beam-shaping mechanism that forms a ribbon-shaped gas cluster ion beam based on the ionized gas clusters.

Description:
FIELD OF THE DISCLOSURE 
   The present disclosure relates generally to semiconductor manufacturing equipment and, more particularly, to techniques for providing a ribbon-shaped gas cluster ion beam. 
   BACKGROUND OF THE DISCLOSURE 
   Traditional ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with energized ions. In semiconductor manufacturing, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels. 
   There has been a continuing effort to shrink feature sizes of semiconductor devices. As semiconductor devices are scaled down in size, the depth of related P-N junctions must be reduced accordingly. Such reduced depth P-N junctions are sometimes referred to as shallow or ultra-shallow junctions. In order to form shallow or ultra-shallow junctions, it is necessary to implant dopants with low-energy ions. However, due to fundamental limitations in the extraction and transport of low-energy ions, conventional ion implantation systems may not perform satisfactorily to form shallow or ultra-shallow junctions. In response to this problem, gas cluster ion implantation has been developed to achieve shallow or ultra-shallow implants. 
     FIG. 1  shows a typical gas cluster ion implantation system  100 . The system  100  is typically enclosed in a vacuum housing (not shown). A source gas may be introduced into the vacuum housing via a properly shaped nozzle  102 . A suitable source gas may include, for example, one or more inert gases (e.g., argon), oxygen-containing gases (e.g., oxygen and carbon dioxide), nitrogen-containing gases (e.g., nitrogen), and other dopant-containing gases (e.g., diborane). The nozzle  102  may inject the source gas at a high speed (e.g., supersonic speed). Since the vacuum chamber is at a much lower pressure than the source gas, the injected source gas will experience an instant expansion that results in cooling and condensation of the injected source gas. That is, the source gas will condense into a jet  10  of gas clusters wherein each gas cluster may have a few to several thousands atoms or molecules. The cluster jet  10  may then go through a skimmer  104  that removes stray atoms or molecules from the cluster jet  10 . The resulting cluster jet  12  may be ionized in an ionizer  106 . The ionizer  106  typically produces thermo-electrons and causes them to collide with the gas clusters in the cluster jet  12 , thereby ionizing the gas clusters to form a gas cluster ion beam  14 . Each gas cluster typically has one positive charge. The gas cluster ion beam  14  may further pass through one or more sets of electrodes  108  that may focus the gas cluster ion beam  14  and/or accelerate it to a desired energy. The gas cluster ion beam  14  may also be filtered through a mass analyzer  110  that selects gas clusters of desired mass(es). For example, the mass analyzer  110  may deflect all monomer ions and other light ions and only allow more massive gas clusters to pass through. Finally, the gas cluster ion beam  14  may be directed to a wafer (not shown) which is typically housed in an end station (not shown). The wafer may be mechanically scanned and/or tilted during an implantation with the gas cluster ion beam  14 . A neutralizer  112  may generate electrons to offset charge buildup on the wafer. 
   The adoption of gas cluster ion implantation significantly improves the performance of ultra shallow junctions. It is now possible to implant atoms to a depth of 5-100 angstroms. So far, however, gas cluster ion implantation has been limited to the use of spot beams of gas clusters. To use a single spot beam in a uniform implantation, the spot beam has to be scanned multiple times across an entire wafer, which may not be efficient for large wafers (which may be up to 300 mm in diameter these days). In addition, the use of spot beams requires a complex design of end stations in order to accommodate two-dimensional wafer movements. 
   In view of the foregoing, it would be desirable to provide a solution for gas cluster ion implantation which overcomes the above-described inadequacies and shortcomings. 
   SUMMARY OF THE DISCLOSURE 
   Techniques for providing a ribbon-shaped gas cluster ion beam are disclosed. In one particular exemplary embodiment, the techniques may be realized as an apparatus for providing a ribbon-shaped gas cluster ion beam. The apparatus may comprise at least one nozzle configured to inject a source gas at a sufficient speed into a low-pressure vacuum space to form gas clusters. The apparatus may also comprise at least one ionizer that causes at least a portion of the gas clusters to be ionized. The apparatus may further comprise a beam-shaping mechanism that forms a ribbon-shaped gas cluster ion beam based on the ionized gas clusters. 
   In accordance with other aspects of this particular exemplary embodiment, the at least one nozzle may comprise an array of nozzles, wherein the array of nozzles are so arranged as to cause the gas clusters to form a ribbon-shaped jet. The at least one ionizer may cause at least a portion of the ribbon-shaped jet to be ionized, thereby forming a static ribbon-shaped gas cluster ion beam. And, the beam-shaping mechanism may coordinate the formation of the gas clusters by the array of nozzles to shape the static ribbon-shaped gas cluster ion beam. 
   In accordance with further aspects of this particular exemplary embodiment, the at least one nozzle may comprise a single nozzle that forms a stream of gas clusters. The at least one ionizer may cause at least a portion of the stream of gas clusters to be ionized. And, the beam-shaping mechanism may deflect the ionized stream of gas clusters back and forth at a sufficiently high frequency to form a scanned ribbon-shaped gas cluster ion beam. 
   In accordance with additional aspects of this particular exemplary embodiment, the at least one nozzle may comprises a single nozzle having an elongated nozzle opening that causes the gas clusters to form a ribbon-shaped jet or a single nozzle having an array of nozzle openings so arranged as to cause the gas clusters to form a ribbon-shaped jet. 
   In another particular exemplary embodiment, the techniques may be realized as a method for providing a ribbon-shaped gas cluster ion beam. The method may comprise injecting, through at least one nozzle, a source gas at a sufficient speed into a low-pressure vacuum space to form gas clusters. The method may also comprise causing at least a portion of the gas clusters to be ionized. The method may further comprise forming a ribbon-shaped gas cluster ion beam based on the ionized gas clusters. 
   The present disclosure will now be described in more detail with reference to exemplary embodiments thereof as shown in the accompanying drawings. While the present disclosure is described below with reference to exemplary embodiments, it should be understood that the present disclosure is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present disclosure as described herein, and with respect to which the present disclosure may be of significant utility. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to facilitate a fuller understanding of the present disclosure, reference is now made to the accompanying drawings, in which like elements are referenced with like numerals. These drawings should not be construed as limiting the present disclosure, but are intended to be exemplary only. 
       FIG. 1  shows a traditional gas cluster ion implantation system. 
       FIG. 2  shows an exemplary system for generating a static ribbon-shaped gas cluster ion beam in accordance with an embodiment of the present disclosure. 
       FIG. 3  shows another exemplary system for generating a static ribbon-shaped gas cluster ion beam in accordance with an embodiment of the present disclosure. 
       FIGS. 4   a  and  4   b  show exemplary nozzle designs for providing a ribbon-shaped gas cluster ion beam in accordance with an embodiment of the present disclosure. 
       FIG. 5  shows an exemplary skimmer that facilitates a ribbon-shaped gas cluster ion beam in accordance with an embodiment of the present disclosure. 
       FIG. 6  shows an exemplary system for generating a scanned ribbon-shaped gas cluster ion beam in accordance with an embodiment of the present disclosure. 
       FIGS. 7A and 7B  illustrate exemplary scanner plates in accordance with embodiments of the present disclosure. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Referring to  FIG. 2 , there is shown an exemplary system  200  for generating a static ribbon-shaped gas cluster ion beam in accordance with an embodiment of the present disclosure. The system  200  may be enclosed in a vacuum chamber (not shown). The system  200  may comprise a plurality of nozzle-skimmer pairs that are arranged in an array  21  having a desired geometry. Each nozzle  202  may be capable of injecting a source gas at a sufficiently high speed (e.g., supersonic speed). The source gas may comprise any of a variety of gases, such as, for example, inert gases (e.g., argon), oxygen-containing gases (e.g., oxygen and carbon dioxide), nitrogen-containing gases (e.g., nitrogen), and other dopant-containing gases (e.g., diborane). The source gas may also be provided by heating up one or more solid substances until they vaporize. Upon injection into the vacuum chamber, the source gas may quickly expand, cool, and condense into a jet of gas clusters near each nozzle. As used hereinafter, a “gas cluster” refers to a group of atoms or molecules that are typically held together by surface tensions or van der Waals forces rather than molecular bonds or covalent bonds. Each corresponding skimmer  203  may deflect uncondensed source gas from the cluster jet. The plurality of nozzle-skimmer pairs may be positioned in close proximity with one another such that, in aggregate, the cluster jets they produce will form a substantially uniform jet of gas clusters ( 20 ) that has a two-dimensional cross section of a desired geometry. According to a preferred embodiment, the nozzle-skimmer pairs may be arranged in an elongated rectangular array to produce a ribbon-shaped jet  20 . The positioning of the array of nozzle-skimmer pairs may be adjusted to cause the ribbon-shaped jet  20  to have a desired size and gas cluster density. 
   The gas cluster ion implantation system  200  may also comprise one or more ionizers  204  that ionize at least a portion of the gas clusters in the ribbon-shaped jet  20 . There may be a single ionizer  204  adapted to accommodate the ribbon shape of the jet  20 . Alternatively, there may be multiple ionizers  204  arranged in an array that spans the width of the ribbon-shaped jet  20 . The one or more ionizers  204  may employ any of a variety of electron-generating techniques. For example, traditional thermionic filaments may be used to produce thermo-electrons that can ionize the gas clusters through impact. According to one embodiment, one or more plasma flood guns (PFG&#39;s) may be used to provide the electrons needed for ionization of the gas clusters. An array of PFG&#39;s may be provided, or a single PFG with one or more slit apertures or an array of exit apertures may be utilized. Ideally, each gas cluster in the ribbon-shaped jet  20  may become ionized with a single positive charge. In practice, some gas clusters may not acquire any charge and some might become overcharged. After passing through the one or more ionizers  204 , the ribbon-shaped jet  20  becomes a ribbon-shaped gas cluster ion beam  22  that may continue traveling down the original beam path. 
   The gas cluster ion beam  22  may be subject to electrostatic manipulation by a series of electrodes  206 . The electrodes  206  may accelerate, decelerate, and/or focus the gas cluster ion beam  22 . The electrodes  206  may be the same as or similar to those developed for conventional, non-gas cluster ion beams. After passing through the electrodes  206 , the gas cluster ion beam  22  may have a more refined shape and a desired energy. 
   Next, the gas cluster ion beam  22  may pass through a mass analyzer  208  that is adapted to accommodate a ribbon-shaped ion beam. The analyzer  208  may comprise a conventional C or H magnet or a window frame magnet. A main function of the analyzer  208  may be to remove light-weight ions (e.g., monomers) from the gas cluster ion beam  22 . According to embodiments of the present disclosure, the analyzer  208  may be so configured to cause negligible deflection of heavy gas clusters. 
   The gas cluster ion beam  22  may then be directed towards a target wafer in an end station (not shown). One or more neutralizers  210  may be positioned near both the beam path and the target wafer. The one or more neutralizers  210  may provide low-energy electrons that help offset charge buildup on the target wafer. The end station does not need to accommodate two-dimensional scans of the target wafer as required for spot beam ion implantations. Instead, the end station may have a simplified design which facilitates one-dimensional scans and tilting of the target wafer. The ribbon width of the gas cluster ion beam  22  is typically wider than the width of a target wafer. As a result, a single scan of the target wafer perpendicular to the ribbon width may be sufficient to cover the entire wafer surface. 
   According to embodiments of the present disclosure, it may be desirable to control the ribbon-shaped gas cluster ion beam  22  with additional beam-shaping mechanisms (not shown in  FIG. 2 ). For example, in order to achieve a desired uniformity in gas cluster distribution and/or charge distribution within the ion beam  22 , it may be beneficial to coordinate the generation of gas clusters by the plurality of nozzles  202 . The amount of source gas supplied to each nozzle  202  as well as the injection speed and angles may be programmed and fine-tuned to ensure a desired output of the ribbon-shaped jet  20  of gas clusters. Similar programming and coordination may be applied to other components such as the ionizers  204  and neutralizers  210 . Other known beam-shaping techniques applicable to traditional ribbon-shaped ion beams may also be adapted to shape the ribbon-shaped gas cluster ion beam  22 . 
     FIG. 3  shows an exemplary system  300  for generating a static ribbon-shaped gas cluster ion beam in accordance with an embodiment of the present disclosure. Compared with the system  200  in  FIG. 2 , the system  300  comprises a different array of nozzles  302 . The nozzles  302  may not be independently positioned as the individual nozzles  202 . Rather, each row of the nozzles  302  may be pre-arranged along a gas tube  31 . One or more gas tubes  31  may collectively produce a ribbon-shaped jet of gas clusters ( 30 ). One skimmer (not shown) may be used for each row of nozzles  302  or for all rows. One or more ionizers  304  may ionize gas clusters in the ribbon-shaped jet  30 , thereby forming a ribbon-shaped gas cluster ion beam  32 . The gas cluster ion beam  32  may then pass through electrodes  306 , a mass analyzer  308 , and a neutralizer  310  before striking a target wafer (not shown). 
     FIGS. 4   a  and  4   b  show exemplary nozzle designs for providing a ribbon-shaped gas cluster ion beam in accordance with an embodiment of the present disclosure.  FIG. 4   a  shows a gas tube  402  having an elongated nozzle opening  404  along one side. The elongated shape of the nozzle opening  404  may help create a ribbon-shaped jet of gas clusters without requiring multiple nozzles as shown in  FIGS. 2 and 3 .  FIG. 4   b  shows a gas tube  406  with multiple elongated nozzle openings  408 . 
     FIG. 5  shows an exemplary skimmer  502  that facilitates a ribbon-shaped gas cluster ion beam in accordance with an embodiment of the present disclosure. The skimmer  502  may have an elongated slit aperture  504  to allow a ribbon-shaped cluster jet  50  to pass through and to deflect particles or clusters whose directions of movement are not sufficiently aligned with the slit aperture  504 . 
     FIG. 6  shows an exemplary system  600  for generating a scanned ribbon-shaped gas cluster ion beam in accordance with an embodiment of the present disclosure. The system  600  may comprise components  602 , similar to those illustrated in  FIG. 1 , that generate a single gas cluster ion beam  60  (i.e., a spot beam). The gas cluster ion beam  60  may be controllably deflected by a set of parallel plates  604  or other deflection mechanisms. The deflection may be achieved with an electrostatic force, a magnetic force, an electromagnetic force, or a combination thereof. The gas cluster ion beam  60  may be scanned, i.e., deflected back and forth, at a sufficiently high frequency (e.g., 100-1000 Hz) such that the parallel plates  604  (or other deflection mechanisms) output a plurality of beamlets. According to one embodiment, one or more additional sets of parallel plates may be implemented for the scanning. According to another embodiment, multipoles may be used in place of the parallel plates  604 . According to yet another embodiment, the parallel plates  604  may be replaced by a pair of plates that are at an angle with each other. One exemplary embodiment of the non-parallel scanner plates is shown in  FIG. 7A , where a pair of plates  702  are initially parallel to each other and then fan out to give a scanned ion beam  70  a large exit. Another exemplary embodiment is shown in  FIG. 7B , wherein a pair of plates  704  have their fan-out portion curved to prevent the ion beam  704  from hitting the plates  704 . 
   The scanned gas cluster ion beam may be further shaped by an electrostatic collimator  606  having multiple electrodes that are individually biased and/or shaped to produce a desired electrostatic field configuration. The electrostatic field configuration may accelerate the gas cluster ions to a desired final energy and may produce a scanned ribbon-shaped gas cluster ion beam  62  with substantially parallel beamlets. The scanned ribbon-shaped gas cluster ion beam  62  may then impact a target wafer  608  at a uniform, controlled incident angle. Exemplary electrostatic collimation techniques may be found in U.S. Pat. Nos. 5,091,655, 5,177,366, 6,774,377, 5,180,918 and 4,942,342, each of which is incorporated by reference in its entirety. 
   The scanned ribbon-shaped gas cluster ion beam  62  may be wider than the target wafer  608 , such that a 1-D mechanical scanning of the target wafer  608  may be sufficient for a beam coverage of the entire wafer surface. 
   The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.