Abstract:
An apparatus includes a primary electrode and an acceleration electrode. The acceleration electrode or, alternatively, an additional secondary electrode contains a slot that extends obliquely through the acceleration electrode or through the secondary electrode. This measure allows secondary electrons to be produced in a highly effective manner.

Description:
BACKGROUND OF THE INVENTION  
       FIELD OF THE INVENTION  
         [0001]    The invention relates to an apparatus for producing secondary electrons. The apparatus includes a primary electrode used for producing primary electrons, and an acceleration electrode used for accelerating the primary electrons.  
           [0002]    The apparatus is, for example, arranged in a free space used, for example, for an ion beam to pass through, for holding a plasma and/or for holding a workpiece or a material to be processed.  
           [0003]    The ion beam contains, for example, boron, phosphorus or arsenic ions. In a plasma, electrons are extracted from atoms so that the ions which are produced in the process have positive charges. Positively charged ions are also referred to as cations.  
           [0004]    The primary electrode is, for example, a very thin wire with a very small radius of curvature. The radius of curvature results in a corona discharge. Only charges of the same polarity as the electrode are produced, because of the corona discharge, at a sufficient distance from the primary electrode. This means that the primary electrode has a negative potential in order to produce electrons. By way of example, the diameter of the primary electrode wire is 30 μm (micrometers).  
           [0005]    The acceleration electrode is at a different potential than the primary electrode, for example at ground potential. The electric field which is formed between the acceleration electrode and the primary electrode accelerates the primary electrons toward the acceleration electrode. The primary electrodes pass through openings in the acceleration electrode, and retain their direction because of the lack of any electric acceleration field downstream from the acceleration electrode.  
           [0006]    By way of example, apparatus such as these for producing secondary electrons are used in ion implantation systems for producing integrated circuits, in order to counteract charges that can occur on a wafer due to the cations.  
         SUMMARY OF THE INVENTION  
         [0007]    It is accordingly an object of the invention to provide an apparatus for producing secondary electrons, which effectively counteracts charges, or effectively prevents such charges.  
           [0008]    Further objects of the invention are to provide a secondary electrode and an acceleration electrode that can be used in an apparatus for producing secondary electrons.  
           [0009]    The invention is based on the idea that the primary electrons have comparatively high energy, for example, several hundred electron volts (eV), because of the acceleration voltage. The high energy makes it possible, for example, for the primary electrons which reach a semiconductor wafer directly to produce a strong negative charge there, which is likewise not wanted. For example, the primary electrons can be carried by an ion beam. In order to prevent such processes or similar processes, in a first embodiment of the apparatus, the acceleration electrode contains at least one aperture opening, which extends obliquely through the acceleration electrode. The primary electrons thus arrive at a side wall of the aperture opening before they leave the apparatus and, for example, reach free space. Furthermore, when the primary electrons arrive, they produce secondary electrons which have comparatively low energy. In addition, the aperture opening has a greater length than the material thickness of the electrode, since it is at an inclined angle.  
           [0010]    Based on the first embodiment of the apparatus, a secondary electrode is constructed to perform the function of an acceleration electrode in addition to other functions. The secondary electrode has three functions:  
           [0011]    producing the acceleration voltage;  
           [0012]    producing secondary electrons; and  
           [0013]    interrupting the direct path for primary electrons to the exterior, for example, to free space.  
           [0014]    In a second embodiment of the invention, the apparatus for producing secondary electrons contains a separate acceleration electrode for accelerating the primary electrons emitted by a primary electrode. In this case, the secondary electrode produces secondary electrons when accelerated primary electrons arrive. The secondary electrode is preferably arranged on the side of the acceleration electrode which faces away from the primary electrode. The secondary electrode is thus located between the acceleration electrode and free space. Primary electrons arrive at the secondary electrode, and produce secondary electrons. Because the secondary electrode is physically separated from the acceleration electrode, the secondary electrons pass to the exterior, for example to free space, without being disturbed by the electric field for producing and accelerating the primary electrons. Since the secondary electrode is also located on the same side of free space as the primary electrode, the number of primary electrons which will reach free space is reduced.  
           [0015]    In one development based on the second embodiment of the apparatus, the secondary electrode contains at least one aperture opening, which extends obliquely through the secondary electrode. This development is based on the same ideas as the first embodiment of the invention. The secondary electrode has two functions:  
           [0016]    producing the secondary electrons when primary electrons arrive; and  
           [0017]    interrupting the direct path for primary electrons to the exterior, for example to free space.  
           [0018]    Other measures in order to use the aperture opening to impede the passage of primary electrons or to prevent them from passing through may also be taken instead of or in addition to an aperture opening which extends obliquely through the acceleration electrode or through the secondary electrode. For example, an electrode having an aperture opening which runs in the normal direction to the electrode surface can be inclined with respect to free space or to the ion beam or can be arranged with a specific offset from the aperture opening and primary electrode, in order to prevent primary electrons from passing through and thus to achieve the same effects as those explained above.  
           [0019]    In one development which is applicable to both aspects, the aperture opening is an elongated hole which preferably extends parallel to the primary electrode or to a wire of the primary electrode. The elongated hole can be produced in a simple manner, for example, using a milling machine. Oblique elongated holes can also be produced without any problems by using a milling machine. Alternatively, laminates are used.  
           [0020]    In one refinement, the side surfaces of the aperture opening overlap one another or are aligned with the arrival direction at which the majority of primary electrons reach the aperture opening. An aligned arrangement ensures that, on the one hand, no primary electrons can reach the exterior, for example, free space, directly, and that, on the other hand, the impact surface in the aperture opening is large in comparison to the length of the aperture opening. In consequence, a large number of secondary electrons can pass to the exterior, for example, to free space, through the aperture opening in a simple manner.  
           [0021]    In one development, the aperture opening is arranged inclined at an angle of between 30° and 70° to the surface normal of the electrode in the area of the aperture opening. The appropriate angle depends on the thickness of the electrode. For example, an angle of 55° is particularly suitable for a thickness of about 5 mm. In one refinement, the angle is defined by:  
           tan (90°−w)=d/b,  
           [0022]    where w is the angle, d is the thickness of the electrode with the aperture opening, and b is the width of the aperture opening.  
           [0023]    In one refinement, the distance between the side walls of the aperture opening is between 2 mm and 6 mm. The appropriate distance once again depends on the thickness of the electrode with the aperture opening. For example, a distance between the side walls of 4 mm is particularly suitable for an electrode thickness of about 5 mm and an inclination angle of 55°.  
           [0024]    In another development, only one aperture opening is provided for each primary electrode. This results in a physically simple apparatus. In one refinement, the apparatus contains only one primary electrode. This measure further simplifies the configuration of the apparatus.  
           [0025]    Alternatively, however, the apparatus contains two or more primary electrodes, each of which has at least one associated aperture opening. The use of aperture openings with different aperture directions means that secondary electrons which are produced by the primary electrons from different primary electrodes can be concentrated at a specific point outside the apparatus, for example, in an electron cloud in free space. While the physical length and the neutralization effect remain the same, this ensures that fewer primary electrons can, for example, reach the impact location of the ion beam, that is to say, for example, the semiconductor wafer or a material to be irradiated.  
           [0026]    In another development, aluminum, an aluminum alloy, graphite, a graphite compound or aluminum oxide Al 2 O 3  is used as the material for the electrode with the aperture opening. Aluminum Al and graphite C have a yield of 1, or else of the order of magnitude of 1, with an energy of 300 eV. This means that every primary electron that arrives produces a secondary electron. The yield is also referred to as the secondary electron emission coefficient. The secondary electron emission coefficient of aluminum oxide is at least twice as great as that of aluminum or graphite.  
           [0027]    A roughened surface on the electrode with the aperture opening leads to greater secondary electron emission. In one refinement, the mean roughness of the surface is 6.3 μm.  
           [0028]    In another development with a separate acceleration electrode and a secondary electrode, the acceleration electrode contains more than 100 openings, more than 500 openings, or even more than 1000 openings. The openings are, for example, holes with a diameter of about 1 mm. However, wire meshes may also be used as an alternative.  
           [0029]    In one development, aluminum or an aluminum alloy is used as the material for the acceleration electrode. In one refinement, aluminum with a purity of 99.9% or more than 99.9% is used. This material is particularly suitable for use as the electrode material for producing integrated circuit arrangements. The mean surface roughness of the acceleration electrode in one refinement is 4.3 μm, that is to say the acceleration electrode is smoother than the secondary electrode.  
           [0030]    In one development, the primary electrode or the primary electrodes is or are arranged parallel to the propagation direction of the ion beam. This type of arrangement is used in particular for so-called high-current ion implantation systems. The ion beam current in these systems is greater than 1 mA. For example, the ion current is in the range between 1 and 25 mA. In one development, the primary electrodes may, for example, be arranged around the ion beam.  
           [0031]    In an alternative development, the primary electrode or the primary electrodes is or are arranged transversely with respect to the propagation direction of the ion beam, preferably at an angle of 90°. This arrangement is particularly suitable for systems in which the ion beam is, for example, guided in the form of a scanning movement, that is to say it is scanned. A scanning movement such as this is used in particular for medium-current ion implantation systems, in which the ion current has values of less than 1 mA. However, high-current ion implantation systems also exist in which the ion beam carries out a scanning movement.  
           [0032]    The invention also relates to a secondary electrode having the features of the secondary electrodes mentioned above, that is to say in particular with an aperture opening which extends obliquely through the secondary electrode. The technical effect described above thus also applies to the secondary electrode.  
           [0033]    The invention additionally relates to an acceleration electrode which, in particular, has the features of the acceleration electrode mentioned above. In particular, the acceleration electrode contains more than 100 openings, more than 500 openings or more than 1000 openings. The technical effects mentioned above thus likewise apply to this acceleration electrode.  
           [0034]    By way of example, the inventive apparatus or the inventive electrode can be used in a simple manner in an ion implantation system of the EATON implanter type, in particular in a system of the NV 8250 P type. The dimensions stated below relate to this type of system. However, other types of systems can also be modified in a similar way in order to use the invention.  
           [0035]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0036]    Although the invention is illustrated and described herein as embodied in an apparatus for producing secondary electrons, in particular a secondary electrode and an acceleration electrode, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
           [0037]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]    [0038]FIG. 1 is a view of an apparatus for producing secondary electrons using a corona discharge;  
         [0039]    [0039]FIGS. 2A and 2B are views of a secondary electrode; and  
         [0040]    [0040]FIG. 3 is a plan view of an acceleration electrode. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]    Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a configuration of an electron production apparatus  10  which contains an aperture area  12  or a chamber for an ion beam  14  to pass through. The ion beam  14  is produced using non-illustrated units of an ion implantation system, specifically by using an ion source, for example, an RF (radio frequency) source, a mass separator arrangement, for example, a magnet, and an acceleration path.  
         [0042]    Before or after passing through the aperture area  12 , the ion beam  14  is deflected, for example, using a deflection system, so that it carries out a scanning movement.  
         [0043]    The aperture area  12  is bounded at the bottom by a shielding pivoting electrode  16 . The pivoting electrode  16  is mounted such that it can pivot about a point P. When the production apparatus  10  is in a normal mode of operation, the pivoting electrode  16  is in the position illustrated in FIG. 1, that is to say the ion beam  14  can pass through the aperture area  12  without any impediment. Conversely, in a test operating mode, the pivoting electrode  16  is pivoted upwards into the aperture area  12 , so that the ion beam  14  can no longer strike a wafer  18 .  
         [0044]    The aperture area  12  is bounded at the top by a secondary electrode  20  which extends parallel to the pivoting electrode  16  in the normal operating mode. The configuration of the secondary electrode  20  will be explained in more detail below.  
         [0045]    A primary electron production apparatus  26 , which is used for producing primary electrons, is arranged a short distance away on the outside of the secondary electrode  20 . The electron production apparatus  26  contains three glow wires  28 ,  30  and  32 , which are arranged in a row parallel to one another, for example, with uniform spacings between them. The glow wires  28 ,  30  and  32  run at an angle of 90° with respect to the propagation direction of the ion beam  14 . The glow wires  28 ,  30  and  32  are surrounded by a triple reflector  34 , which has three mutually separated cutouts  36 ,  38  and  40 . A respective glow wire  28 ,  30  and  32  extends in the center of each cutout  36 ,  38 , and  40 . An acceleration electrode  42 , which lies in a plane, is, for example, a grating formed by holes or a wire mesh. The acceleration electrode  42  is located opposite the openings of the cutouts  36 ,  38 , and  40  and at a short distance from the triple reflector  34 .  
         [0046]    The pivoting electrode  16 , the secondary electrode  20  and the acceleration electrode  42  are, in the exemplary embodiment, composed of aluminum. The glow wires  28  to  32  are composed, for example, of tungsten. Aluminum is likewise suitable as the material for the triple reflector  34 .  
         [0047]    The secondary electrode  20  contains three slots  50 ,  52  and  54  which are arranged obliquely in the secondary electrode  20  and are adjacent to the glow wires  28 ,  30  and  32 , in this sequence. The slots  50  and  52  are thus located closer to the point at which the ion beam  14  enters the aperture area  12  than the slot  54 . The side walls of the slots  50  and  52  are parallel to one another. The side walls of the slots  50  and  52  thus define an angle of −60° with respect to a normal to the surface of the secondary electrode  20  which faces away from the aperture area  12 . In this case, minus means that the angle from the normal runs in a counterclockwise direction. The side walls of the slot  54  are parallel to one another, but are inclined in a direction which opposes the inclination direction of the side walls of the slots  50  and  52 . The side walls of the slot  54  thus define an angle of +60° with respect to the normal to the surface of the secondary electrode  20  which faces away from the aperture area  12 . The thickness of the secondary electrode  20  and the angle are of such magnitudes that pointed edges of the slots  50 ,  52  and  54  are each aligned with one another in the direction of the normal to the surface of the secondary electrode  20 . Inlet openings in the slots  50 ,  52  and  54  are arranged opposite the glow wires  28 ,  30  and  32 , in this sequence.  
         [0048]    A potential of −300 V (volts) is applied to the triple reflector  34  and to the glow wires  28  to  32 . A voltage which is superimposed on this potential, in particular a DC voltage of, for example, 30 volts, leads to a current flow which causes the glow wires  28  to  32  to glow. Then, by way of example, 5 A (amperes) per glow wire  28  to  32  flows through the glow wires  28  to  32 . There is a potential of 0 V on the pivoting electrode  16 , on the secondary electrode  20  and on the acceleration electrode  42 . Thus, for example, the primary electrons are accelerated from the glowing filament  28  to the side surface of the slot  50  which is located closer to the entry point of the ion beam  14 . As a result of passing through the acceleration electrode  42 , these primary electrons  60  arrive at this side surface, where each of them produces a secondary electron. The second electrons  62  which are produced in this way are attracted by the ion beam  14  and, because of the directional effect of the slot  50 , arrive in an electron cloud  64  on a path that is inclined with respect to the propagation direction of the ion beam.  
         [0049]    Primary electrons  66  pass in the same way from the glow wire  30  through the acceleration electrode  42  to the side surface of the slot  52  which is closer to the entry point of the ion beam  14 . The primary electrons  66  produce secondary electrons  68 , which are attracted by the ion beam  14 , and enter the electron cloud  64 .  
         [0050]    Primary electrons  70  are accelerated from the glow wire  32  towards the acceleration electrode  42 . The primary electrons  70  pass through the acceleration electrode  42  and reach the side surface of the slot  54  which is further away than the point at which the ion beam  14  enters the aperture area  12 . The primary electrons  70  produce secondary electrons  72 , which likewise enter the electron cloud  64 . While the secondary electrons  62  and  68  mainly enter the electron cloud  64  parallel to one another, the secondary electrons  72  are in the opposite direction to the secondary electrons  62  and  68 , because of the different inclination direction of the slot  54 . Primary electrons  70  which pass randomly through the slot  54  are also directed away from the wafer  18  due to the inclination of the slot  54 .  
         [0051]    [0051]FIG. 2A shows a secondary electrode  100  for an EATON implanter of the NV-8250 type, that is to say in particular for 6-inch wafer implantations or, after conversion, for 8-inch wafer implantations. The secondary electrode  100  is used instead of the secondary electrode  20  in an electron production apparatus which contains only one glow wire. A slot  102  which is arranged in the secondary electrode  100  corresponds, for example, to the slot  50 ,  52  or  54 . The secondary electrode  100  has a rectangular base body, on whose longitudinal faces two attachment webs  104  and  106  are arranged centrally. The attachment webs  104  and  106  have a length which corresponds approximately to ⅓ of the length of the base body of the secondary electrode  100 . The attachment web  104  projects about 10 mm beyond the base body of the secondary electrode  100 . In contrast, the attachment web  106  projects only about 3 mm beyond the base body of the secondary electrode  100 . The base body of the secondary electrode  100  has length of 162 mm and a width of about 36 mm. An opening of the slot  102  extends close to the attachment web  106 , virtually over the entire length of the base body. In the exemplary embodiment, the length of the slot  102  without including rounded areas at the ends of the slot  102  is 150 mm.  
         [0052]    [0052]FIG. 2A also shows a section plane A in the center of the secondary electrode  100 . The section plane A is located transversely with respect to the longitudinal axis of the secondary electrode  100 .  
         [0053]    [0053]FIG. 2B shows the secondary electrode  100  along the cross section lying in the section plane A. The thickness of the secondary electrode  100  is about 5 mm. As is illustrated in FIG. 2B, side walls  110  and  112  of the slot  102  are inclined at an angle of −55° with respect to a normal N. The distance between the side walls  110  and  112  is 4 mm.  
         [0054]    The secondary electrode  100  is composed of aluminum, that is to say of the material Al  99 . The mean roughness of the surface is 6.3 μm.  
         [0055]    [0055]FIG. 3 shows an acceleration electrode  150  which is used instead of the acceleration electrode  42  in the same system in which the secondary electrode  100  is also used. The acceleration electrode  150  has a length of 162 mm and a width of 11 mm. A large number of holes  160  to  166 , for example, more than 500 holes, are located within a frame  152 . The holes  160  to  166  each have a diameter of 1 mm, and are arranged in six rows  170  to  180  which run parallel to one another. The holes  160 ,  162  of two adjacent rows  170 ,  172  are offset by 0.65 mm with respect to one another. The distances between the inner rows  174  and  176 , between the central rows  172 ,  178  and between the outer rows  170  and  180  are 1.13 mm, 3.38 mm and 5.63 mm, respectively. Mutually adjacent holes  164 ,  166  within one row  170  to  180  each have a separation of 1.3 mm between the center points of the holes  164  and  166 .  
         [0056]    The acceleration electrode  150  is illustrated greatly enlarged in FIG. 3. The acceleration electrode  150  is composed of aluminum Al 99.9, so that only a small number of stray atoms emerge from the acceleration electrode  150 . Implantation processes can thus be carried out with a high yield. The mean roughness of the surface is 4.3 μm. The thickness of the frame  152  is 1.5 mm. The acceleration electrode  150  has a thickness of 1 mm within the frame  152 .  
         [0057]    In another exemplary embodiment, the electron production apparatus  10  has no acceleration electrode  42 . The functions of the acceleration electrode  42  are then additionally carried out by the secondary electrode  20 .  
         [0058]    In a further exemplary embodiment, laminates which, for example, are held in a frame are used instead of the oblique slots. Two adjacent laminates each form one aperture opening, which extends obliquely through the frame. Each laminate is an elongated thin metal platelet, for example, composed of aluminum.