Abstract:
An ion implanter includes a sample stage for setting a sample having a main surface, an ion generating section configured to generate a plurality of ions, the ion generating section including a container into which an ion source gas is introduced and a filament for emitting thermal electrons provided in the container, an implanting section configured to implants an ion beam containing the plurality of ions in the main surface of the sample, and a control section configured to control a position of the sample or a spatial distribution of electrons emitted from the filament so that a direction of eccentricity of a center of gravity of the ion beam coincides with a direction of a normal line of the main surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-193034, filed Jun. 30, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an ion implanter and a method of manufacturing a semiconductor device including an ion implanting process.  
         [0004]     2. Description of the Related Art  
         [0005]     In recent years, large scale integrated circuits (LSI) have often been used in important parts of computers and communication apparatuses; the large scale integrated circuits are formed by connecting a large number of transistors and resistors together as electric circuits and integrating the circuits together on one chip. Thus, the performance of the whole apparatus depends strongly on the performance of a single LSI. The performance of the single LSI can be improved by increasing the degree of integration, that is, miniaturizing the elements.  
         [0006]     Miniaturization of elements can be formed by reducing the junction depth of a diffusion layer, for example, a source/drain diffusion layer. The junction depth can be reduced by optimizing ion implantation and the subsequent heat treatment step (annealing). This serves to realize, for example, a MOS transistor having a shallow source/drain diffusion layer of junction depth not higher than 0.2 μm.  
         [0007]     To form a shallow diffusion layer by doping impurities, it is necessary to make a reduced heat budget so as to shallowly distribute impurity atoms during ion implantation and to prevent the impurity atoms from diffusing deeply during the subsequent heat treatment. Further, to use impurity doping to form wells in which elements such as MOS transistors are formed and areas (channel doping layers) in which channels of MOS transistors are induced, it is necessary to accurately control the amount of impurities implanted.  
         [0008]     On the other hand, with a miniaturization of elements, for example, a reduction in gate processing size, offset is more likely to occur in a source/drain area owing to shadowing of a gate electrode and deviation of the incident angle of ion beam. Such offset makes notable transistor characteristics asymmetric.  
         [0009]     A cone angle has been considered to be the cause of the asymmetry of the transistor characteristics which may occur if a batch type high current ion implanter is used. Thus, attempts have been made to eliminate the asymmetry of the transistor characteristics by adjusting an α angle and a β angle (Extended Abstracts of International Workshop on Junction Technology 2002, S2-3.).  
         [0010]     However, at present, the asymmetry of the transistor characteristics is not necessarily eliminated simply by adjusting the α and β angles.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     An ion implanter according to an aspect of the present invention comprises a sample stage for setting a sample having a main surface; an ion generating section which generates a plurality of ions, the ion generating section including a container into which an ion source gas is introduced and a filament for emitting thermal electrons provided in the container; an implanting section which implants an ion beam containing the plurality of ions in the main surface of the sample; and a control section which controlles a position of the sample or a spatial distribution of electrons emitted from the filament so that a direction of eccentricity of a center of gravity of the ion beam coincides with a direction of a normal line of the main surface.  
         [0012]     A method of manufacturing a semiconductor device according to an aspect of the present invention comprises generating a plurality of ions by an ion generating section including a container into which an ion source gas is introduced and a filament for emitting thermal electrons provided in the container, the plurality of ions being generated in the container; controlling a position of a sample or a spatial distribution of electrons emitted from the filament so that a direction of eccentricity of the center of gravity of ion beam containing the plurality of ions coincides a direction of a normal of a main surface; and implanting the ion beam into the main surface of the sample set on a sample stage. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0013]      FIG. 1  is a diagram schematically showing a semiconductor wafer on which a vertical MOS transistor and a horizontal MOS transistor are formed;  
         [0014]      FIG. 2  is a diagram schematically showing an ion implanter according to an embodiment;  
         [0015]      FIG. 3  is a diagram schematically showing a center-of-gravity eccentricity measuring mechanism of the ion implanter according to the embodiment;  
         [0016]      FIGS. 4A and 4B  are plan views showing slits of the center-of-gravity eccentricity measuring mechanism of the ion implanter according to the embodiment;  
         [0017]      FIG. 5  is a sectional view showing the slits of the center-of-gravity eccentricity measuring mechanism of the ion implanter according to the embodiment;  
         [0018]      FIG. 6  is a diagram illustrating a variation of a current detecting mechanism of the center-of-gravity eccentricity measuring mechanism of the ion implanter according to the embodiment;  
         [0019]      FIG. 7  is a diagram illustrating another variation of the current detecting mechanism of the center-of-gravity eccentricity measuring mechanism of the ion implanter according to the embodiment;  
         [0020]      FIG. 8  is a diagram illustrating a method of adjusting a stage of the ion implanter according to the embodiment;  
         [0021]      FIG. 9  is a diagram illustrating the method of adjusting the stage of the ion implanter according to the embodiment;  
         [0022]      FIG. 10  is a diagram illustrating a method of adjusting a stage of the ion implanter according to the embodiment;  
         [0023]      FIG. 11  is a diagram illustrating the method of adjusting the stage of the ion implanter according to the embodiment;  
         [0024]      FIG. 12  is a diagram illustrating the method of adjusting the stage of the ion implanter according to the embodiment;  
         [0025]      FIG. 13  is a diagram illustrating the method of adjusting the stage of the ion implanter according to the embodiment;  
         [0026]      FIG. 14  is a diagram illustrating the method of adjusting the stage of the ion implanter according to the embodiment;  
         [0027]      FIG. 15  is a diagram illustrating the method of adjusting the stage of the ion implanter according to the embodiment;  
         [0028]      FIG. 16  is a diagram schematically showing the ion implanter according to an embodiment; and  
         [0029]      FIG. 17  is a diagram schematically showing an ion chamber of the ion implanter according to the embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     Embodiments of the present invention will be described below with reference to the drawings.  
       First Embodiment  
       [0031]      FIG. 1  is a diagram schematically showing a semiconductor wafer  3  on which a vertical MOS transistor  1  and a horizontal MOS transistor  2  are formed. Each of the vertical MOS transistor  1  and horizontal MOS transistor  2  comprises a gate electrode  4  and a source/drain diffusion layer  5 . The vertical MOS transistor  1  and the horizontal MOS transistor  2  have a gate length (channel length) of not higher than 65 nm. The semiconductor wafer  3  is, for example, a Si wafer.  
         [0032]     Studies by the present inventors indicate that a gate length (channel length) of not higher than 65 nm makes the asymmetry of the characteristics of the horizontal MOS transistor  2  more significant and that the asymmetry of the characteristics of the horizontal MOS transistor  2  cannot be sufficiently eliminated by adjusting the α and β angles.  
         [0033]     The cause of the asymmetry of the transistor characteristics has been found to be a variation in the incident angle of ion beam, caused by eccentricity of the center of the gravity of the ion beam. Moreover, the eccentricity of the center of the gravity of the ion beam is considered to result from the spatial distribution of thermal electrons emitted from a cathode filament in an ion source chamber.  
         [0034]     An ion implanter and a method of manufacturing a semiconductor device according to the present embodiment are provided taking the above cause into account.  FIG. 2  is a diagram schematically showing the ion implanter according to the present embodiment.  
         [0035]     First, an ion source chamber  11  generates a plurality of ions, which are then drawn by a drawer electrode  12  to become a beam. A separation electromagnet  13  subjects the plurality of ions to mass separation. The ions are selected from, for example, As +  ions, B +  ions, BF 2+  ions, P +  ions, Ge +  ions, Sb +  ions, In, Ga +  ions, F +  ions, N +  ions, C +  ions, BF +  ions and cluster ions including one of the those ions.  
         [0036]     The ion source chamber  11  comprises a chamber into which an ion source gas is introduced and a cathode filament provided in the chamber and to which a voltage (power) is applied by a voltage source (power source) to emit thermal electrons.  
         [0037]     Subsequently, the ions are completely separated by a slit  14  into beam-like ions (ion beam). An accelerator  15  then uses acceleration and deceleration or no loads to control the ion beam so that they have a predetermined final energy. Then, the ion beam is focused by a quadrupole lens  16  so as to have a focal point on a front surface (main surface) of a semiconductor wafer  21 .  
         [0038]     Subsequently, the ion beam is scan by scanning electrodes  17  and  18  so that the amount of beam to be implanted is uniformly distributed all over an implantation surface of the semiconductor wafer  21 . To remove neutral particles resulting from collision with a residual gas, the ion beam is bent by a polarizing electrode  19 . The ion beam  20  bent by the polarizing electrode  19  are implanted into the semiconductor wafer  21  set on a stage.  
         [0039]     The present embodiment further comprises a center-of-gravity eccentricity measuring mechanism  23  that measures the amount of eccentricity of the center of gravity of the ion beam  20  and a stage driving mechanism  24  that changes the three-dimensional posture (position) of the stage  22  so that the direction of the eccentricity of the center of gravity of the ion beam  20  coincide with a direction perpendicular to the surface of the semiconductor wafer  21 , on the basis of the amount of eccentricity of the center of gravity of the ion beam  20  measured by the center-of-gravity eccentricity measuring mechanism  23 .  
         [0040]     The stage driving mechanism  24  comprises a mechanism that can accurately drive the stage  22  in the directions of an X and Y axes or an X, Y, and Z axes. The mechanism has a gonio with respect to each operation axis. The center-of-gravity eccentricity measuring mechanism  23  is stored, using a moving mechanism (not shown), in an apparatus located away from an ion beam line except during measurements.  
         [0041]     For a single wafer processing type ion implanter, the normal stage driving mechanism is used as the stage driving mechanism  24 . On the other hand, a batch type ion implanter generally does not comprise the stage driving mechanism  24 . Even if the batch type ion implanter comprises a stage driving mechanism, its control range is narrow, so that an exclusive stage driving mechanism  24  must be provided.  
         [0042]     The center-of-gravity eccentricity measuring mechanism  23  comprises two slits  26   u  and  26   d  and current detecting mechanisms  27  provided below the respective slits  26   u  and  26   d  as shown in  FIG. 3 . The slit  26   u  comprises a plurality of openings  28  arranged in a matrix (6×3). In the slit  26   u , reference numeral  28  is shown only at one opening for simplification. Similarly, the slit  26   d  comprises a plurality of openings  29  arranged in a matrix (6×3).  
         [0043]      FIGS. 4A and 4B  are plan views of the slits  26   u  and  26   d , respectively.  FIG. 5  is a sectional view of the slits  26   u  and  26   d  as viewed from the direction of arrows in  FIGS. 4A and 4B . As shown in  FIGS. 4A, 4B , and  5 , the slits  26   u  and  26   d  are the same. The slits  26   u  and  26   d  are arranged so that each opening  29  in the slit  26   d  is located below the corresponding opening  28  in the slit  26   u . Accordingly, ions vertically entering the opening  28  also vertically enter the opening  29  located below the opening  28 . In this case, the openings  28  and  29  appear to be rectangular in the plan views but may have a different shape such as a circle.  
         [0044]     The current detecting mechanism  27  comprises a plurality of Faraday cup  27   f  arranged in a line in an X direction. In  FIG. 3 , reference numeral  27   f  is shown only at one Faraday cup for simplification. The current detecting mechanism  27  is scanned by a scanning mechanism (not shown) in a Y direction. Thus, the current of the ion beam passing through the slits  26   u  and  26   b  can be detected using Faraday cups  27   f  the number of which corresponds to only one row.  
         [0045]     In this case, the plurality of Faraday cups  27   f  are arranged in a line in the X direction. However, as shown in  FIG. 6 , the plurality of Faraday cups  27   f  may be arranged in a line in the Y direction. In this case, the current detecting mechanism  27  is scanned by the scanning mechanism (not shown) in the X direction. Alternatively, as shown in  FIG. 7 , it is allowable to use only one Faraday cup  27   f . In this case, the current detecting mechanism  27  is scanned by the scanning mechanism (not shown) in the X and Y direction as shown in  FIG. 7 .  
         [0046]     Now, description will be given of a method of determining the amount of eccentricity of the center of gravity of the ion beam  20 .  
         [0047]     FIGS.  8  to  15  are diagrams illustrating a method of adjusting the stage  22  of the ion implanter according to the present embodiment.  
         [0048]      FIG. 8  is a diagram schematically showing how the ion beam  20  with no eccentricity of the center of gravity passes through the slits  25   u  and  25   d . Reference numerals  28   a  to  28   e  and  29   a  to  29   e  denote openings. The ion beam  20  passing through the opening  28   b  in the slit  25   u  passes directly through the opening  29   b  in the slit  25   d.    
         [0049]     Thus, the amount of current in the ion beam  20  under the opening  28   b  detected by the current detecting mechanism  27  is substantially the same as the amount of current in the ion beam  20  under the opening  29   b  detected by the current detecting mechanism  27 .  
         [0050]     With this result of measurement, the direction in which the ion beam  20  flies is determined to be parallel to a line (normal) perpendicular to the surface (main surface) of the semiconductor wafer  21  as shown in  FIG. 9 . Accordingly, the controller  25  gives the stage driving mechanism  24  no instructions on adjustment of the posture (position) of the stage  22 .  
         [0051]      FIG. 10  is a diagram schematically showing how the ion beam  20  with a certain amount of eccentricity of the center of gravity passes through the slits  25   u  and  25   d . In  FIG. 10 , the ion beam  20  passing through the opening  28   b  in the slit  25   u  flies downward and rightward and then passes through the opening  29   c  in the slit  25   d.    
         [0052]     Thus, the amount of current in the ion beam  20  under the opening  28   b  detected by the current detecting mechanism  27  is larger than that (in this case, substantially zero) in the ion beam  20  under the opening  29   b  detected by the current detecting mechanism  27 . Further, the amount of current (in this case, substantially zero) in the ion beam  20  under the opening  28   c  detected by the current detecting mechanism  27  is smaller than that in the ion beam  20  under the opening  29   c  detected by the current detecting mechanism  27 .  
         [0053]     With this result of measurement, the direction in which the ion beam  20  flies is determined to be unparallel to the normal of the semiconductor wafer  21  at its initial position (the position shown by a broken line) as shown in  FIG. 11 . Thus, the controller  25  controls the stage driving mechanism  24  so that the flying direction of the ion beam  20  is parallel to the normal of the semiconductor wafer  21  as shown in  FIG. 11 . That is, in accordance with an instruction from the controller  25 , the stage driving mechanism  24  changes the posture (position) of the stage  22  so that the flying direction of the ion beam  20  is parallel to the normal of the semiconductor wafer  21 .  
         [0054]     In the description of FIGS.  8  to  11 , for simplification, the ion beam  20  comprises one ion beam having a single amount of eccentricity of the center of gravity, and the ion beam passes through the one opening in the slit. In the description below, the ion beam  20  comprises two ion beams having different amounts of eccentricity of the center of gravity, and the ion beams pass through the different openings of the slit.  
         [0055]      FIG. 12  is a diagram schematically showing how the ion beam  20  containing an ion beam  20   1  with no eccentricity of the center of gravity and an ion beam  20   2  with a certain amount of eccentricity of the center of gravity passes through the slits  25   u  and  25   d . The amount of current in the ion beam  20   1  entering the opening  28   b  is substantially the same as the amount of current in the ion beam  20   2  entering the opening  28   c.    
         [0056]     In  FIG. 12 , the ion beam  20   1  passing through the opening  28   b  passes directly through the opening  29   b , while the ion beam  20   2  passing through the opening  28   c  flies downward and rightward and then passes through the opening  29   d.    
         [0057]     Thus, the amount of current in the ion beam  20   1  under the opening  28   b  detected by the current detecting mechanism  27  is substantially the same as the amount of current in the ion beam  20   1  under the opening  29   b  detected by the current detecting mechanism  27 . On the other hand, the amount of current in the ion beam  20   2  under the opening  28   c  detected by the current detecting mechanism  27  is larger than that (in this case, substantially zero) in the ion beam  20   2  under the opening  29   c  detected by the current detecting mechanism  27 . Further, the amount of current (in this case, substantially zero) in the ion beam  20   2  under the opening  28   d  detected by the current detecting mechanism  27  is smaller than that in the ion beam  20   2  under the opening  29   d  detected by the current detecting mechanism  27 .  
         [0058]     Here, as shown in  FIG. 13 , a first vector  20   1 V is defined to have a magnitude and a direction corresponding to the amount of current and the flight direction, respectively, of the ion beam  20   1  under the opening in the slit  25   u  before passage through the slit  25   d . Likewise, a second vector  20   2 V is defined to have a magnitude and a direction corresponding to the amount of current and the flight direction, respectively, of the ion beam  20   2  under the opening in the slit  25   u  before passage through the slit  25   d . The direction D of the synthetic vector of the first and second vectors can be considered to be the flying direction of the whole ion beam  20  containing the ion beams  201  and  202 . That is, in the present embodiment, the flying directions of the ion beams  20   1  and  20   2  are weighted by the amounts of current to calculate the flying direction of the whole ion beam  20 .  
         [0059]     As shown in  FIG. 13 , the direction D of the synthetic vector is determined to be unparallel to the normal of the semiconductor wafer  21  at the initial position (the position shown by a broken line). Thus, as shown in  FIG. 13 , the stage driving mechanism  2 . 4  changes the posture (position) of the stage  22  so that the flying direction D of the synthetic vector is parallel to the normal of the semiconductor wafer  21 .  
         [0060]      FIG. 14  is a diagram schematically showing how the ion beam  20   1  with no eccentricity of the center of gravity and the ion beam  20   2  with a certain amount of eccentricity of the center of gravity pass through the slits  25   u  and  25   d . The amount of current in the ion beam  20   1  is larger than the amount of current in the ion beam  20   2 . The amount of current in the ion beam  20   1  under the opening  29   b  is larger than the amount of current in the ion beam  20   2  under the opening  29   d.    
         [0061]     In this case, as shown in  FIG. 15 , the direction D of the synthetic vector is determined to be unparallel to the normal of the semiconductor wafer  21  at the initial position (the position shown by a broken line). Thus, in accordance with an instruction from the controller  25 , the stage driving mechanism  24  changes the posture (position) of the stage  22  so that the flying direction D of the synthetic vector is parallel to the normal of the semiconductor wafer  21  as shown in  FIG. 15 .  
         [0062]     As described above, in general, the posture (position) of the stage  22  may be controlled as follows. If the ion beam  20  enter n (≧1) openings of the slit  25   u , when the magnitude of a vector corresponds to the amount of current in the ion beam passing through each opening and the direction of the vector corresponds to the direction of the ion beam passing through the opening, then the posture (position) of the stage  22  is controlled so that the direction of the synthetic vector of the vectors passing through the openings is parallel to the normal of the semiconductor wave  21 .  
         [0063]     When ions are implanted by controlling the posture (position) of the stage  22  as described above, the asymmetry (the degradation of the element characteristics) of the characteristics of the horizontal MOS transistor  2  of gate length (channel length) not higher than 65 nm is confirmed to be sufficiently improved.  
         [0064]     In the prior art, the asymmetry is notable if acceleration energy is not higher than 3 keV or if a dose amount is not less than 1×10 14  cm −2  or if the acceleration energy is not higher than 3 keV and the dose amount is not less than 1×10 14  cm −2 . However, the present embodiment has been confirmed to sufficiently suppress the asymmetry under any of the above ion implantation conditions.  
         [0065]     The asymmetry is sufficiently suppressed because the ion implanter and method of ion implantation according to the present embodiment sufficiently eliminate a variation in the incident angle of ion beam caused by eccentricity of the center of gravity of the ion beam. It has also been confirmed to be possible to reduce the above variation below, for example, a variation in the angle between a side of a gate electrode which is processed to extend in a substantially vertical direction and the normal line of the surface (main surface) of the wafer or a variation in the angle between that side of a connection hole formed in an insulating film which is processed to extend in a substantially vertical direction and the normal line of a bottom surface of the connection hole.  
       Second Embodiment  
       [0066]      FIG. 16  is a diagram schematically showing an ion implanter according to the second embodiment of the present invention.  FIG. 17  is a diagram schematically showing the configuration of an ion source chamber of the ion implanter according to the present embodiment. Parts of this ion implanter which correspond to those in  FIG. 2  are denoted by the same reference numerals, with their detailed description omitted.  
         [0067]     As previously described, eccentricity of the center of gravity of ion beam is considered to result from the spatial distribution of thermal electrons emitted from the cathode filament in the ion source chamber. Thus, in the present embodiment, the eccentricity of the center of gravity of the ion beam  20  is controlled by controlling the spatial distribution  37  of thermal electrons in the ion source chamber  11 . The present embodiment will further be described below.  
         [0068]     In  FIG. 17 , reference numeral  30  denotes a chamber. A gas introduction pipe  32  is provided in a wall of the chamber  30  to introduce an ion source gas  31  such as BF 3  or AsF 3 . Discharge apparatuses  34   u  and  34   d  are provided in the chamber  30  and each comprise a cathode filament  33  that emits thermal electrons when a voltage source (power source; not shown) applies a voltage (power) to the filament  33 . The discharge apparatus  34   u  is provided at a upper position in the chamber  30 , while the discharge apparatus  34   d  is provided at a lower position in the chamber  30 . The discharge apparatus  34   u  can be moved up and down by a driving mechanism  35 . Likewise, the discharge apparatus  34   d  can be moved up and down by a driving mechanism  36 .  
         [0069]     In a state that the ion source gas  31  has been introduced into the chamber  30  from the gas introduction pipe  32 , the discharge apparatuses  34   u  and  34   d  causes discharge to generate B, As, or other ions. The ions are drawn by the drawer electrode  12 .  
         [0070]     According to the present embodiment, the spatial distribution  37  of thermal electrons can be controlled by using the driving mechanisms  35  and  36  to control the positions of the discharge apparatuses  34   u  and  34   d , that is, the cathode filaments  33 . On the other hand, the amount of eccentricity of the center of gravity of ion beam can be measured using the center-of-gravity eccentricity measuring mechanism  23 .  
         [0071]     The amount of eccentricity of the center of gravity of ion beam measured by the center-of-gravity eccentricity measuring mechanism  23  is transmitted to the controller  25 . The controller  25  controls the driving mechanisms  35  and  36  so that the amount of eccentricity becomes equal to or smaller than a predetermined value. Such feedback control enables the spatial distribution  37  of thermal electrons to be controlled so that the eccentricity of the center of gravity of ion beam becomes sufficiently small.  
         [0072]     When ions are implanted by controlling the spatial distribution  37  of thermal electrons as described above, the asymmetry of the characteristics of the horizontal MOS transistor  2  of gate length (channel length) not higher than 65 nm is confirmed to be sufficiently improved.  
         [0073]     In the prior art, the asymmetry is notable if the acceleration energy is not higher than 3 keV or if the dose amount is not less than 1×10 14  cm −2  or if the acceleration energy is not higher than 3 keV and the dose amount is not less than 1×10 14  cm −2 . However, the present embodiment has been confirmed to sufficiently suppress the asymmetry under any of the above ion implantation conditions.  
         [0074]     The asymmetry is sufficiently suppressed because the ion implanter and method of ion implantation according to the present embodiment sufficiently reduce a variation in the incident angle of ion beam caused by eccentricity of the center of gravity of the ion beam. It has also been confirmed to be possible to reduce the above variation below, for example, a variation in the angle between a side of a gate electrode which is processed to extend in a substantially vertical direction and the normal line of the surface (main surface) of the wafer or a variation in the angle between that side of a connection hole formed in an insulating film which is processed to extend in a substantially vertical direction and the normal line of a bottom surface of the connection hole.  
         [0075]     The present invention is not limited to the above embodiments. For example, the first and second embodiments may be combined together. Moreover, the present invention is applicable to diffusion layers other than the source/drain diffusion layer of the MOS transistor.  
         [0076]     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.