Abstract:
A technique for isocentric ion beam scanning is disclosed. In one particular exemplary embodiment, the technique may be realized by an apparatus for isocentric ion beam scanning. The apparatus may comprise an end station having a mechanism for holding and translating a wafer. The apparatus may also comprise a deflector that tilts an ion beam to a predetermined angle and directs the ion beam into the end station. The wafer may be translated with respect to the ion beam for isocentric scanning at least a portion of a surface of the wafer, and wherein the ion beam is maintained at the predetermined angle during isocentric scanning.

Description:
FIELD OF THE DISCLOSURE  
       [0001]     The present disclosure relates generally to semiconductor manufacturing and, more particularly, to a technique for isocentric ion beam scanning.  
       BACKGROUND OF THE DISCLOSURE  
       [0002]     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, at different energy levels, and sometimes at different incident angles.  
         [0003]     For a uniform distribution of dopant ions, an ion beam is typically scanned across the surface of a target wafer.  FIG. 1   a  shows a typical beam path  10  formed by scanning an ion beam spot  102  across a wafer  104 . The ion beam spot  102  may be scanned back and forth in the X direction while the wafer  104  is translated in the Y direction, thereby forming the zigzag pattern of the beam path  10 .  FIG. 1   b  shows another typical beam path  11  formed by scanning the ion beam spot  102  across the wafer  104 . Here, the ion beam spot  102  is scanned along straight lines in the X direction and makes square turns at the end of a sweep.  
         [0004]     As the semiconductor industry is producing devices with smaller and smaller feature sizes, ion beams with lower energies are required for wafer implantation. Compared with high- or medium-energy ion beams, low-energy ion beams present some unique challenges. For example, a low-energy ion beam usually has a large beam spot, which can cause problems for beam utilization and uniformity tuning. In addition, the shape or current density of a low-energy ion beam can change substantially as it propagates down a beam line. As a result, if different portions of a wafer meet the low-energy ion beam at different positions along the beam line, different dopant profiles may be created for the different portions of the wafer. This problem is illustrated in  FIG. 2 , wherein an ion beam  202  is propagating along the Z direction. If a wafer  204  meets the ion beam  202  at a first position Z=Z 1 , the wafer  204  will see a beam spot  206 . If the wafer  204  meets the ion beam  202  at a second position Z=Z 2 , for example, when the ion beam  202  is scanned across the surface of the wafer  204 , the wafer  204  will see a beam spot  208  that may be quite different from the beam spot  206 . Therefore, the portion of the wafer  204  implanted with the ion beam at beam spot  208  may have a different dopant profile from the portion implanted with the ion beam at beam spot  206 . The problem of beam-line variation is not unique to low-energy ion beams but can also be observed in some high-current ion beams.  
         [0005]     As a countermeasure to the beam-line variation problem, a concept known as “isocentric scanning” has been proposed. Isocentric scanning generally involves moving a wafer with respect to an ion beam in such a way that the point in space where the ion beam strikes the wafer surface remains the same no matter which part of the wafer surface meets the ion beam. It should be noted that isocentric scanning does not have to cover the entire surface of a wafer but may cover only a portion of the wafer surface.  
         [0006]      FIG. 3  illustrates isocentric scanning of a wafer  304 . An ion beam  302  may propagate along the Z direction, and the wafer  304  may be translated within the X-Y plane to perform the isocentric scanning. Effectively, the wafer  304  is moved in such a way that a beam spot  306  (i.e., a cross section of the ion beam  302 ) having the same shape and current density distribution will strike different portions of the surface of the wafer  304  at a same point (X 0 , Y 0 , Z 0 ) in space.  
         [0007]      FIG. 3  only shows the simplest example of isocentric scanning wherein the ion beam  302  has a normal incidence on the wafer  304 . These days, however, it is often necessary to implant a wafer at one or more different incident angles other than the normal incident angle, which technique is known as “angled ion implantation.” Sometimes, it may also be desirable to implant different portions of a wafer at different incident angles. In existing ion implantation systems, angled ion implantation is typically accommodated by maintaining an ion beam along a fixed reference direction and tilting a target wafer with respect to the ion beam. To achieve isocentric scanning on a tilted wafer, the wafer has to be moved in a complex pattern with respect to the ion beam.  
         [0008]      FIG. 4  illustrates a complex movement of a tilted wafer  404  for isocentric scanning by an ion beam  402 . The ion beam  402  propagates along the Z direction. For isocentric scanning, the wafer  404  is moved in such a way that the same beam spot (or ion beam cross section)  406  strikes the surface of the wafer  404 . Since the wafer  404  is tilted with respect to the ion beam  402 , the wafer  404  is moved not only in the X-Y plane, but also in the Z direction. For example, at position A, the center of the tilted wafer  404  may be at Z=Z A . At position B, the center of the tilted wafer  404  may be at Z=Z B  (Z B ≠Z A ). The movement in the Z direction has to precisely coordinated with the movement in the X-Y plane in order to meet the isocentric scanning requirement.  
         [0009]     To precisely control the tilting and the three-dimensional (3-D) translation of a wafer for isocentric scanning, existing ion implantation systems have to be equipped with a fairly complex end station to hold, tilt, and move the wafer.  FIG. 5  illustrates a typical setup for isocentric scanning in existing ion implantation systems. An end station  508  may be a process chamber or similar component that houses a mechanism (not shown) for holding a wafer  504  and for controlling wafer movements. An ion beam  502  may propagate along a reference direction  50  and may be directed into the end station  508  via an aperture  510 . With respect to the end station  508 , the angle of the ion beam  502  is relatively fixed. Isocentric scanning is achieved by tilting and moving the wafer  504 , all inside the end station  508 . For example, the wafer  504  may be tilted by an angle E so that the normal direction  52  of the wafer  504  is at an angle θ with respect to the reference direction  50 . The mechanism in the end station  508  is capable of moving the wafer  504  in all three dimensions based on the tilt angle θ and the isocentric scanning requirement. Such capabilities necessitate a sophisticated design of the end station  508  that is often expensive to build and maintain.  
         [0010]     In view of the foregoing, it would be desirable to provide a technique for isocentric ion beam scanning which overcomes the above-described inadequacies and shortcomings.  
       SUMMARY OF THE DISCLOSURE  
       [0011]     A technique for isocentric ion beam scanning is disclosed. In one particular exemplary embodiment, the technique may be realized by an apparatus for isocentric ion beam scanning. The apparatus may comprise an end station having a mechanism for holding and translating a wafer. The apparatus may also comprise a deflector that tilts an ion beam to a predetermined angle and directs the ion beam into the end station. The wafer may be translated with respect to the ion beam for isocentric scanning at least a portion of a surface of the wafer, and wherein the ion beam is maintained at the predetermined angle during isocentric scanning.  
         [0012]     In accordance with other aspects of this particular exemplary embodiment, the deflector may tilt the ion beam with an electrostatic field, a magnetic field, or an electromagnetic field.  
         [0013]     In accordance with further aspects of this particular exemplary embodiment, the wafer may be translated in a two-dimensional movement during isocentric scanning.  
         [0014]     In accordance with additional aspects of this particular exemplary embodiment, the end station may further comprise a plasma flood gun that can be repositioned based on the predetermined angle of the ion beam.  
         [0015]     In accordance with another aspect of this particular exemplary embodiment, the end station may further comprise an aperture that can be repositioned to admit the ion beam into the end station based on the predetermined angle. The aperture may block one or more particles in the ion beam that are not tilted by the deflector.  
         [0016]     In another particular exemplary embodiment, the technique may be realized as a method for isocentric ion beam scanning. The method may comprise tilting an ion beam to a predetermined angle. The method may also comprise directing the ion beam into an end station holding a wafer. The method may further comprise translating the wafer with respect to the ion beam for isocentric scanning at least a portion of a surface of the wafer, wherein the ion beam is maintained at the predetermined angle during isocentric scanning.  
         [0017]     In accordance with other aspects of this particular exemplary embodiment, the deflector may tilt the ion beam with an electrostatic field, a magnetic field, or an electromagnetic field.  
         [0018]     In accordance with further aspects of this particular exemplary embodiment, the wafer may be translated in a two-dimensional movement during isocentric scanning.  
         [0019]     In accordance with additional aspects of this particular exemplary embodiment, the method may further comprise preventing charge buildup on the wafer surface with a plasma flood gun that can be repositioned based on the predetermined angle of the ion beam.  
         [0020]     In accordance with another aspect of this particular exemplary embodiment, the method may further comprise repositioning an aperture to admit the ion beam into the end station based on the predetermined angle of the ion beam.  
         [0021]     In accordance with yet another aspect of this particular exemplary embodiment, the method may further comprise blocking one or more particles in the ion beam that are not tilted by the deflector.  
         [0022]     In yet another particular exemplary embodiment, the technique may be realized as 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.  
         [0023]     In still another particular exemplary embodiment, the technique may be realized as 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.  
         [0024]     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  
       [0025]     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.  
         [0026]      FIG. 1   a  shows a typical setup for scanning an ion beam across a wafer.  
         [0027]      FIG. 1   b  shows another setup for scanning an ion beam across a wafer.  
         [0028]      FIG. 2  illustrates a beam-line variation problem in an ion beam.  
         [0029]      FIG. 3  illustrates isocentric scanning of an ion beam.  
         [0030]      FIG. 4  illustrates a complex movement of a tilted wafer for isocentric ion beam scanning.  
         [0031]      FIG. 5  shows a typical setup for isocentric scanning in existing ion implantation systems.  
         [0032]      FIG. 6  illustrates an exemplary method for isocentric ion beam scanning in accordance with an embodiment of the present disclosure.  
         [0033]      FIG. 7  shows an exemplary system for isocentric ion beam scanning in accordance with an embodiment of the present disclosure.  
         [0034]      FIG. 8  shows another exemplary system for isocentric ion beam scanning in accordance with an embodiment of the present disclosure.  
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0035]     Embodiments of the present disclosure provide a new approach for isocentric ion beam scanning wherein an ion beam is tilted to a predetermined angle before the ion beam is directed into an end station for isocentric scanning of a target wafer. With the ion beam maintained at the predetermined angle, there may be no need to tilt the target wafer itself. As a result, isocentric scanning of the target wafer may involve a two-dimensional (2-D), rather than a three-dimensional (3-D), translation of the target wafer. Since it is no longer necessary to tilt the target wafer or to coordinate its 3-D movements, the end station may have a simpler and therefore less expensive design.  
         [0036]      FIG. 6  illustrates an exemplary method for isocentric ion beam scanning in accordance with an embodiment of the present disclosure. In an ion implantation system, an end station  608  may hold a wafer  604 . The normal direction of the wafer  604  may be aligned with the Z direction.  
         [0037]     First, an ion beam  602  may be generated and an incident angle (e.g., θ′) may be determined for an angled ion implantation on the wafer  604  or a portion thereof.  
         [0038]     Next, the ion beam  602  may be tilted to the predetermined angle θ′ with respect to the Z direction before the ion beam  602  is directed into the end station  608  via an aperture  610 . Tilting of the ion beam  602  may be achieved in a number of ways. Typically, the ion beam  602  may be tilted with an electrostatic, magnetic, or electromagnetic field. The tilt angle may also be achieved, for example, by adjusting the relative position of the end station  608  with respect to a reference direction  60  of the ion beam  602 . Meanwhile, the wafer  604  may maintain its position inside the end station  608  without tilting.  
         [0039]     Then, with the ion beam  602  stationary (i.e., at the angle θ′), the wafer  604  may be translated in the X-Y plane, without any movement in the Z direction, so that the portion(s) of the wafer  604  to be implanted will meet the same beam spot  606 . Thus, an isocentric scanning of the wafer  604  may be achieved for different incident angles without tilting the wafer  604  or moving it in a complex 3-D pattern.  
         [0040]      FIG. 7  shows an exemplary system  700  for isocentric ion beam scanning in accordance with an embodiment of the present disclosure. The system  700  may comprise an end station  708  and a deflector device  712 .  
         [0041]     The deflector device  712  may employ an electrostatic field, a magnetic field, or a combination thereof to change the propagation direction of an ion beam  702 . The deflector device  712  may cause the ion beam  702  to be tilted at an angle θ″ with respect to a reference direction (e.g., the Z direction).  
         [0042]     The end station  708  may comprise a wafer handling mechanism  706  that can hold a wafer  704  and control its movements (e.g., translation or rotation within the X-Y plane). The end station  708  may also comprise a sliding aperture  710  that can be repositioned to admit the ion beam  702  into the end station  708 . The position of the sliding aperture  710  may depend on the tilt angle θ″ of the ion beam  702 . Particles in the ion beam  702  that have not been deflected to the angle θ″ may be blocked by a sidewall of the aperture  710  or a sidewall of the end station  708 . The end station  708  may further comprise a plasma flood gun (PFG)  714  that prevents charge buildup on the wafer  704 .  
         [0043]     In operation, the deflector device  712  may be adjusted to tilt the ion beam  702  to a predetermined angle (e.g., θ″). Then, the sliding aperture  710  (or a relative position between the deflector  712  and the end station  708 ) may be adjusted to allow the properly tilted ion beam  702  to pass therethrough and enter the end station  708 . Then, the wafer handling mechanism  706  may be activated to move the wafer  704  with respect to the ion beam  702  to effectuate isocentric scanning.  
         [0044]     According to embodiments of the present disclosure, during an isocentric scanning of a wafer as described herein, the resulting beam path on the wafer is not limited to any particular pattern. When performing an isocentric scanning with a spot beam, for example, the beam path may have a zig-zag pattern as illustrated in  FIG. 1   a  or a raster pattern as illustrated in  FIG. 1   b.    
         [0045]     When performing an isocentric scanning with a ribbon beam, only one-dimensional translation of the wafer may be needed if the ribbon beam is at least as wide as the wafer. One example is shown in  FIG. 8 .  FIG. 8  shows an exemplary system  800  for isocentric ion beam scanning in accordance with an embodiment of the present disclosure. The system  800  may be similar to the system  700  shown in  FIG. 7 , except that the system  800  employs a ribbon beam  802  for isocentric scanning of the wafer  704 . The ribbon beam  802  may be deflected by the deflector  712  to a tilt angle (e.g., θ′″) before being directed into the end station  708  via the sliding aperture  710 . The ribbon beam  802  may be made no narrower than the diameter of the wafer  704 . Since the ribbon beam  802  is wide enough to cover the width of the wafer  704  in the X direction, there may be no need to translate the wafer  704  in the X direction. Instead, an isocentric scanning of the ribbon beam  802  on the wafer  704  may only involve translation of the wafer  704  in the Y direction.  
         [0046]     At this point it should be noted that the technique for isocentric ion beam scanning 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 isocentric ion beam scanning 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 isocentric ion beam scanning 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.  
         [0047]     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.