Abstract:
Techniques for reducing contamination during ion implantation is disclosed. In one particular exemplary embodiment, the techniques may be realized by an apparatus for reducing contamination during ion implantation. The apparatus may comprise a platen to hold a workpiece for ion implantation by an ion beam. The apparatus may also comprise a mask, located in front of the platen, to block the ion beam and at least a portion of contamination ions from reaching a first portion of the workpiece during ion implantation of a second portion of the workpiece. The apparatus may further comprise a control mechanism, coupled to the platen, to reposition the workpiece to expose the first portion of the workpiece for ion implantation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is related to U.S. patent application Ser. No. 11/329,761, filed Jan. 11, 2006, which claims priority to U.S. Provisional Patent Application No. 60/660,420, filed Mar. 9, 2005, each of which is hereby incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates generally to semiconductor manufacturing and, more particularly, to techniques for reducing contamination during ion implantation. 
       BACKGROUND OF THE DISCLOSURE 
       [0003]    Ion implanters are widely used in semiconductor manufacturing to selectively alter the conductivity of materials. In a typical ion implanter, ions generated from an ion source are transported as an ion beam downstream through a series of beamline components which may include one or more analyzer and/or corrector magnets and a plurality of electrodes. The analyzer magnets may be used to select desired ion species and filter out contaminant species or ions having undesirable energies. The corrector magnets may be used to manipulate the shape of the ion beam or otherwise adjust the ion beam quality before it reaches a target wafer. Suitably shaped electrodes can be used to modify the energy and the shape of the ion beam. After the ion beam has been transported through the series of beamline components, it may be directed into an end station to perform ion implantation. 
         [0004]      FIG. 1  depicts a conventional ion implanter system  100 . As is typical for most ion implanters, the system  100  is housed in a high-vacuum environment. The ion implanter system  100  may comprise an ion source  102  and a series of beamline components through which an ion beam  10  passes. The series of beamline components may include, for example, an extraction manipulator  104 , a filter magnet  106 , an acceleration or deceleration column  108 , an analyzer magnet  110 , a rotating mass slit  112 , a scanner  114 , and a corrector magnet  116 . Much like a series of optical lenses that manipulate a light beam, the ion implanter components can filter and focus the ion beam  10  before steering it towards a target wafer  118 . The target wafer  118  is typically housed in a wafer end-station (not shown) under high vacuum. 
         [0005]    In semiconductor manufacturing, ion implantation of a target wafer is often performed on only selected areas of the wafer surface, while the rest of the wafer surface is typically masked with a photosensitive material known as “photoresist.” Through a photolithography process, the target wafer may be coated with a patterned layer of photoresist material, exposing only selected areas of the wafer surface where ion implantation is desired. During ion implantation, an ion beam makes its impact not only on the exposed portion of the wafer surface, but also on the photoresist layer. The energetic ions often break up chemical bonds within the photoresist material and release volatile organic chemicals and/or other particles into the vacuum chamber (i.e., wafer end-station) that houses the target wafer. This phenomenon is known as “photoresist outgassing.” Photoresist outgassing in an ion implanter can have several deleterious effects on an ion beam. For example, the particles released from the photoresist may cause a pressure increase or pressure fluctuations in the high-vacuum wafer end-station. The outgassed particles may also migrate upstream from the wafer end-station to other beamline components, such as the corrector magnet  116  and the scanner  114  as shown in  FIG. 1 , and may affect vacuum levels in those portions of the ion implanter as well. 
         [0006]    The outgassed particles and/or contamination particles from other sources often interact with an incident ion beam by exchanging charges with beam ions. For example, an ion with a single positive charge may lose its charge to an outgassed particle and become neutralized; a doubly charged ion may lose one positive charge to an outgassed particle and become singly charged; and so on. As a result, the outgassing-induced charge exchange can interfere with an ion dosimetry system in the ion implanter. 
         [0007]    A typical ion dosimetry system determines ion doses by integrating a measured beam current over time and converting the integrated beam current (i.e., total ion charges) to a total dose based on an assumption that a particular ion species has a known charge state. The outgassing-induced charge exchange, however, randomly alters the charge state of the ion species, thereby invalidating the charge-state assumption that the ion dosimetry system relies on. For example, if the outgassed particles tend to rob positive charges from positive ions, then such charge exchange will cause the dosimetry system to undercount that ion species, which in turn leads to an over-supply of that ion species to a target wafer. 
         [0008]    Due to the above-mentioned upstream migration of outgassed particles, as well as other sources of contamination, charge exchange may occur in or near a corrector magnet. Charge-altered ions are subject to a different Lorentz force as compared to those same species of ions that experience no charge exchange. As such, the charge-altered ions will deviate from the main ion beam path, resulting in non-uniform dosing of the target wafer. Beamlets formed by streams of the charge-altered ions are referred to hereinafter as “parasitic beamlets.” 
         [0009]      FIG. 2  illustrates ion trajectories for charge-altered ions during ion implantation with multiple-charged ions. In this example, doubly-charged phosphorous ions (P 2+ )  20  are generated for ion implantation of a target wafer  202 . Charge exchange occurring in a corrector magnet  204  may cause the p 2 +ions  20  to either lose or gain a positive charge, introducing contamination ions P + 22 and P 3+ 24 respectively. Compared to the P 2+  ions  20 , the P +  ions  22  will be bent less by the magnetic field in the corrector magnet  204  and therefore tend to deviate towards the “outside” of the target wafer  202 . In contrast, the P 3+  ions  24  will be bent more by the magnetic field in the corrector magnet  204  and therefore tend to deviate towards the “inside” of the target wafer  202 . Note that the contamination ions  22  and  24  may either miss the target wafer  202  completely or hit the target wafer  202  at angles different from the P 2+  ions  20 . These contamination ions at unintended angles will affect an ultimate dopant profile in the target wafer  202 . 
         [0010]    In view of the foregoing, it would be desirable to provide techniques for reducing contamination during ion implantation which overcomes the above-described inadequacies and shortcomings. 
       SUMMARY OF THE DISCLOSURE 
       [0011]    Techniques for reducing contamination during ion implantation are disclosed. In one particular exemplary embodiment, the techniques may be realized by an apparatus for reducing contamination during ion implantation. The apparatus may comprise a platen to hold a workpiece for ion implantation by an ion beam. The apparatus may also comprise a mask, located in front of the platen, to block the ion beam and at least a portion of contamination ions from reaching a first portion of the workpiece during ion implantation of a second portion of the workpiece. The apparatus may further comprise a control mechanism, coupled to the platen, to reposition the workpiece to expose the first portion of the workpiece for ion implantation. 
         [0012]    In accordance with other aspects of this particular exemplary embodiment, the second portion may comprise the remaining portion of the workpiece. 
         [0013]    In accordance with further aspects of this particular exemplary embodiment, the first portion may comprise one half of the workpiece, and the second portion may comprise the other half of the workpiece. Accordingly, the mask may have a semi-circular shape. 
         [0014]    In accordance with additional aspects of this particular exemplary embodiment, the mask may remain in a fixed relative position with respect to the ion beam during the ion implantation of the first portion and the second portion of the workpiece, and the fixed relative position may be chosen based on a likelihood of impact by the contamination ions. 
         [0015]    In accordance with another aspect of this particular exemplary embodiment, the mask may be made from one or more materials selected from a group consisting of: silicon, carbon, and silicon carbide. 
         [0016]    In accordance with yet another aspect of this particular exemplary embodiment, the control mechanism may reposition the workpiece by rotating the workpiece by a predetermined angle. 
         [0017]    In accordance with still another aspect of this particular exemplary embodiment, the ion implantation on the first portion of the workpiece may be based on a recipe different from the ion implantation on the second portion of the workpiece. 
         [0018]    In accordance with a further aspect of this particular exemplary embodiment, the ion beam may be a ribbon beam, and the ion implantation of the first portion and the second portion of the workpiece may be performed by translating the workpiece and the mask relative to the ribbon beam. 
         [0019]    In another particular exemplary embodiment, the techniques may be realized by a method for reducing contamination during ion implantation. The method may comprise positioning a mask in front of a workpiece to block an ion beam and at least a portion of contamination ions from reaching a first portion of the workpiece during ion implantation of a second portion of the workpiece. The method may also comprise repositioning the workpiece, after the ion implantation of the second portion, to expose the first portion of the workpiece for ion implantation. 
         [0020]    In accordance with other aspects of this particular exemplary embodiment, the mask may remain in a fixed relative position with respect to the ion beam during the ion implantation of the first portion and the second portion of the workpiece, and the fixed relative position may be chosen based on a likelihood of impact by the contamination ions. 
         [0021]    In accordance with further aspects of this particular exemplary embodiment, the second portion may comprise remaining portion of the workpiece. 
         [0022]    In accordance with additional aspects of this particular exemplary embodiment, the first portion may comprise one half of the workpiece, and the second portion may comprise the other half of the workpiece. Accordingly, the mask may have a semi-circular shape. Alternatively, the mask may have a rectangular shape. 
         [0023]    In accordance with another aspect of this particular exemplary embodiment, the step of repositioning the workpiece may comprise a step of rotating the workpiece by a predetermined angle. 
         [0024]    In accordance with yet another aspect of this particular exemplary embodiment, the ion implantation on the first portion of the workpiece may be based on a recipe different from the ion implantation on the second portion of the workpiece. 
         [0025]    In accordance with still another aspect of this particular exemplary embodiment, the ion beam may be a ribbon beam, and the ion implantation of the first portion and the second of the workpiece may be performed by translating the workpiece and the mask relative to the ribbon beam. 
         [0026]    In yet another particular exemplary embodiment, the techniques may be realized by at least one signal embodied in at least one carrier wave for transmitting a computer program of instructions configured to be readable by at least one processor for instructing the at least one processor to execute a computer process for performing the method as recited above. 
         [0027]    In still another particular exemplary embodiment, the techniques may be realized by at least one processor readable carrier for storing a computer program of instructions configured to be readable by at least one processor for instructing the at least one processor to execute a computer process for performing the method as recited above. 
         [0028]    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 
         [0029]    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. 
           [0030]      FIG. 1  shows a conventional ion implanter. 
           [0031]      FIG. 2  illustrates ion trajectories for charge-altered ions during ion implantation with multiple-charged ions. 
           [0032]      FIG. 3  shows an exemplary system for reducing contamination during ion implantation in accordance with an embodiment of the present disclosure. 
           [0033]      FIG. 4  illustrates an exemplary method for reducing contamination based on a half-moon shaped mask in accordance with an embodiment of the present disclosure. 
           [0034]      FIG. 5  shows an exemplary mask having a half-moon shaped aperture in accordance with an embodiment of the present disclosure. 
           [0035]      FIG. 6  illustrates an exemplary method for reducing contamination based on two complementary masks or an adjustable mask in accordance with an embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0036]    Embodiments of the present disclosure may reduce contamination from undesired ions, especially during ion implantation with multiple-charged ions, by splitting the ion implantation process of a workpiece (e.g., a semiconductor wafer) into two or more phases. A portion of the workpiece having a relative position with respect to an incoming ion beam may be identified as most likely to be affected by contamination ions or parasitic beamlets. That portion of the workpiece may be masked during a first phase of the ion implantation process while the rest of the workpiece is exposed to the incoming ion beam. In a second phase of the ion implantation process, the workpiece may be rotated such that the previously masked portion may be exposed to the incoming ion beam. In this way, it may be ensured that only a trusted portion of the ion beam is used for ion implantation of the workpiece. 
         [0037]    Referring to  FIG. 3 , there is shown an exemplary system  300  for reducing contamination during ion implantation in accordance with an embodiment of the present disclosure. 
         [0038]    In this example, multiple-charged ions  30  are generated for ion implantation of a target wafer  302 . Charge exchange occurring in or near a corrector magnet  304  may cause the ions  30  to either lose or gain a positive charge, introducing contamination ions  32  and  34 , respectively. Since the ions  32  are bent less by the magnetic field in the corrector magnet  304  and therefore tend to hit the “outside” half of the target wafer  302 , while the ions  34  are bent more by the magnetic field in the corrector magnet  304  and therefore tend to miss the target wafer  302  on the “inside,” it may be recognized that the “outside” half of the target wafer  302  is more likely to see contamination ions or parasitic beamlets (i.e., ions  32 ). Therefore, a mask  306  may be positioned in front of the target wafer  302  to prevent all ions (including ions  30  and  32 ) from reaching the “outside” half of the target wafer  302  during ion implantation. 
         [0039]    To block the “outside” half of the target wafer  302 , the mask  306  may have a half-moon shape and may be at least as large as half of the target wafer  302 . The mask  306  may be made from one or more materials that have little or no contamination effect on the target wafer  302 . For example, the mask  306  may be made of silicon, carbon, or silicon carbide. 
         [0040]    With the mask  306  in position, only the “inside” half of the target wafer  302  is exposed for ion implantation. The ion beam  30  may be typically a static or scanned ribbon beam with a beam width of at least the radius (or diameter) of the target wafer  302 . A static ribbon beam may typically comprise a plurality of parallel beamlets that span the beam width. A scanned ribbon beam may be formed by scanning a spot beam, typically with an electrostatic or magnetic scanner, back and forth over the “beam width” at a relatively fast frequency. The target wafer  302  may be translated relative to the ion beam  30  in one or more scan passes to ensure uniform beam coverage of the exposed wafer surface. 
         [0041]    Once the “inside” half of the target wafer  302  has been implanted, the target wafer  302  may be rotated 180° such that the previous “inside” and “outside” halves have their positions reversed. That is, after the rotation and with the mask  306  still in position, the previously masked half-wafer may now be exposed, and the previously exposed half-wafer may now be masked. After the rotation, the ion implantation may be repeated with either the same or different recipes and/or parameters. 
         [0042]    Therefore, in a two-phase ion implantation process, both halves of the target wafer  302  may be implanted. And, both phases are based on a “trusted” portion (i.e., “inside” half) of the ion beam  30 . As a result, at least the contamination ions or parasitic beamlets in the “outside” half of the ion beam  30  may be avoided. 
         [0043]      FIG. 4  illustrates an exemplary method for reducing contamination based on a half-moon shaped mask  404  in accordance with an embodiment of the present disclosure. In this top view of a wafer  402 , the half-moon shaped mask  404  is positioned to block the “outside” half of the wafer  402 . A ribbon beam  40 , which is slightly wider than the wafer  402 , is generated and extends horizontally across the wafer  402 . 
         [0044]    During a first phase of ion implantation, the wafer  402  (and the mask  404 ) may be translated vertically with respect to the ion beam  40 . Typically, the first phase may be completed with two full scans of the wafer  402  by the ion beam  40 , e.g., by translating the wafer  402  (and the mask  404 ) up and down. 
         [0045]    Upon completion of the first phase, the ion beam  40  may be turned off or otherwise kept off the wafer  402 , and the wafer  402  may be rotated 180° around an axis  42 . Then, a second phase of the ion implantation may be performed, wherein the half-wafer that was masked during the first phase may be implanted and the other half-wafer that was implanted during the first phase may be masked. 
         [0046]      FIG. 5  shows an exemplary mask  504  having a half-moon shaped aperture  54  in accordance with an embodiment of the present disclosure. The mask  504  may serve the same purpose of masking a wafer  502  as the mask  404  shown in  FIG. 4 . However, since the mask  504  has an overall shape and size similar to the wafer  502 , the mask  504  may be more easily handled by the same automated wafer handling system (not shown) that handles the wafer  502 . 
         [0047]      FIG. 6  illustrates an exemplary method for reducing contamination based on two complementary masks or an adjustable mask in accordance with an embodiment of the present disclosure. The exemplary embodiments illustrated in  FIGS. 3-5  all involve half-moon shaped masks that block exactly one half of a target wafer during each phase of ion implantation. However, a wafer mask in accordance with embodiments of the present disclosure does not have to be one half the wafer size or in the shape of a semi-circular disk. 
         [0048]      FIG. 6  shows one scenario where it may be determined that only a small area  60  on the right side (“outside”) of a wafer  602  is affected by contamination ions. Accordingly, during a first phase of ion implantation, a mask  604   a  may be positioned in front of the wafer  602 . The mask  604   a  may have a solid portion that blocks the area  60  and may have an aperture  64   a  that exposes the rest of the wafer surface  602 . Upon completion of the first phase of ion implantation, the wafer  602  may be rotated 180° such that the previously blocked area  60  is now on the left side (“inside”) . A mask  604   b  having a complementary shape of the mask  604   a  may be provided for a second phase of ion implantation. The mask  604   b  may be an entirely different wafer mask from the mask  604   a.  Alternatively, the mask  604   b  may be the same wafer mask as the mask  604   a,  wherein the wafer mask is adjustable to create the different masks  604   a  and  604   b.  The mask  604   b  may expose the previously masked portion (i.e., area  60 ) of the wafer  602  via an aperture  64   b,  and mask the portion of the wafer  602  that was already implanted during the first phase. After the second phase, the entire wafer  602  will have been implanted. 
         [0049]    At this point it should be noted that the techniques for reducing contamination during ion implantation in accordance with the present disclosure as described above typically involves the processing of input data and the generation of output data to some extent. This input data processing and output data generation may be implemented in hardware or software. For example, specific electronic components may be employed in an ion implanter or similar or related circuitry for implementing the functions associated with contamination reduction in accordance with the present disclosure as described above. Alternatively, one or more processors operating in accordance with stored instructions may implement the functions associated with contamination reduction in accordance with the present disclosure as described above. If such is the case, it is within the scope of the present disclosure that such instructions may be stored on one or more processor readable carriers (e.g., a magnetic disk), or transmitted to one or more processors via one or more signals. 
         [0050]    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.