Abstract:
An ion injecting apparatus has an ion source, a mass-analyzing magnet, an accelerating/decelerating element, and deflecting elements. The mass analyzing magnet mass-analyzes an ion beam extracted from the ion source. The accelerating/de-celerating element accelerates and decelerates the ion beam at a post-stage. The deflecting elements are arranged between the mass analyzing magnet and the accelerating/decelerating element. Each direction angle of the deflecting element is determined such that a final beam trajectory in the predetermined area before being introduced into a wafer substrate is matched to each other in both an operating mode and a non-operating mode of the deflecting elements.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an ion implantation apparatus and, in particular, to an ion implantation apparatus which is capable of producing an ion beam having no energy contamination. 
     In a recent ion implantation process for semiconductor device production, implantation energy has been getting lower and lower to make the depth of implanted ions shallower with scale reduction of micro patterning of the semiconductor device. 
     Extraction voltage of ion source is lowered to generate the low energy ions. However, the lower the extraction voltage is, the worse the extraction efficiency of ions from ion source is. 
     Further, the ions repel each other due to electric charge of themselves. This mutual repulsion causes a rapid increase of the ion beam diameter. This is what is called “Space charge effect”. This effect becomes stronger with the decrease in ion energy. Consequently, transport efficiency is also reduced in low energy region. As a result, enough ion beam current can not be obtained. 
     To compensate for this difficulty, so called, post-deceleration technology has been used conventionally. In this technology, the ions are extracted from the ion source at a relatively high extraction voltage, and are mass analyzed and transported near to the target wafers, then the ions are decelerated down to the desired energy by a reverse electric field. 
     In such a deceleration method, there is an advantage that relatively higher beam current of low energy ions can be obtained easily. However, if the ions are neutralized by the reaction with residual gas before the deceleration position, such neutralized particles can not be decelerated by the reverse electric field. Consequently, such neutralized particles are implanted into the target wafer with original energy that is different from desired energy. This phenomenon is what is called “Energy contamination”. 
     Similar phenomena occur also in post-acceleration method in which the ions are accelerated by a forward electric field after mass analyzing to obtain higher energy ion beam. 
     If the ions are neutralized by the reaction with residual gas before the acceleration position, such neutralized particles can not be accelerated by the forward electric field. Consequently, such neutralized particles are implanted into the target wafer with original energy that is different from desired energy. 
     On the contrary, if the electrons of the ions are stripped more by the reaction with residual gas before the acceleration position, ions become higher valence (multi-charged) ions. Such multi-charged ions are accelerated by the forward electric field the valence times more than the single charged ions and implanted into the target wafer with different energy that is higher than the desired energy. 
     Thus, the energy contamination often occurs in the apparatus equipped with post-acceleration as well. 
     Referring to FIG. 1, description will be made about a related ion implantation apparatus equipped with post-deceleration or post-acceleration. 
     In FIG. 1, an ion beam extracted from an ion source  41  is mass-analyzed by a mass analyzing magnet  42  and a mass analyzing slit  43  to select desired ion species. 
     Specifically, immediately after the ion beam passes the mass analyzing magnet  42  at a point A, only desired ions exist on a trajectory that can pass through the mass analyzing slit  43 . 
     In this case, energy of the desired ions at the Point A is determined in dependence upon the extraction voltage of the ion source  41  and the valence number of the ion. Therefore, the desired ions, which are on the trajectory towards the mass analyzing slit  43 , have no energy dispersion at the point A. 
     After the ions pass through the mass analyzing slit  43 , the ions are decelerated or accelerated in a post-stage electrode portion  44 . In this event, the ions are decelerated or accelerated in accordance with a direction of an electric field applied to the post-stage electrode portion  44 . 
     Namely, when a reverse electric field is applied, the ions are decelerated. On the other hand, when a forward electric filed is applied, the ions are accelerated. 
     The mass analyzing slit  43  is generally located nearby a downstream side of the post-stage electrode portion  44 , hence an electrode part of the post-stage electrode portion  44  may perform the function of the mass-analyzing slit  43  in many cases. 
     As a specific example, description will be made about such a case that boron ions (B+) having one valance are implanted into a silicon wafer  46  in a wafer-processing chamber  45  with the energy less than 1 keV by using the post-deceleration. 
     In this event, a mode is classified into a first mode (drift mode) and a second mode (deceleration mode). 
     In the first mode, the ions are extracted from the ion source  41  with an extraction voltage less than 1 kV, and are implanted without the post-deceleration. 
     In the second mode, the ions are extracted with a relatively higher voltage (for example, n kV), and a reverse electric field is applied to the post-deceleration electrode portion  44  to finally produce the ion having energy less than 1 keV. 
     In the first mode, the extraction efficiency is degraded because the extraction voltage is low. Further, the transport efficiency is not high because the beam diverges by the space charge effect. Consequently, the beam current becomes small. 
     In the first mode, the energy contamination does not occur, however the beam current becomes small. In consequence, implantation time becomes long to implant the predetermined implantation quantity. 
     In the second mode, the current is increased in comparison with the first mode because of relatively higher extraction voltage than the first mode. 
     For example, when the ions are extracted from the ion source  41  with the voltage of several to 10 kV, the ions are transported towards the post-stage electrode portion  44  with the initial energy of several to 10 keV. 
     Further, the reverse electric field is applied such that the ions are decelerated down to energy of ½-{fraction (1/10)} at the post-stage electrode portion  44  to finally produce the ion beam with the energy less than 1 keV. 
     However, a part of ions lose their charge by reaction with residual gas and become neutral particles in an area (an area B in FIG. 1) between the exit position (Point A) of the mass analyzing magnet  42  and a deceleration position. 
     In consequence, the part of the ions as the neutral particles are not affected by the reverse electric field for the deceleration. Thereby, the ions are implanted with the initial energy of several to 10 keV. As a result, not only the desired boron with the energy less than 1 keV but also the boron ions with the initial energy are inevitably implanted. 
     Thus, the beam current in the second mode of the deceleration is higher than the first mode, and the implantation time is advantageously short. 
     On the contrary, the particles not having the desired energy are inevitably mixed. This phenomenon is referred to as the energy contamination. 
     From the view point of getting the beam current as much as possible, deceleration ratio (namely, a ratio of energy before the deceleration to the energy after the deceleration) is to be desirably higher. However, as the deceleration ratio is higher, content or quantity of the energy contamination is generally higher. 
     FIG. 2 shows a typical example of implanted profile when the energy contamination takes place by the deceleration of the second mode in comparison with the profile of the first mode. 
     It is found out that the implanted profile of the second mode has higher energy component which is implanted to deeper position with the energy before deceleration. 
     To eliminate the components of energy contamination on the ion implantation apparatus with post-deceleration or post-acceleration, additional element for re-deflecting by electric field or magnetic filed has been conventionally used on the downstream trajectory of post-stage electrode. 
     In particular, this conventional method has been widely used on the apparatus equipped with the post-acceleration. 
     A structure of another related art is schematically illustrated in FIG. 3 to show the conventional method. 
     In FIG. 3, an ion beam extracted from an ion source  61  is mass-analyzed by a mass-analyzing magnet  62  and a subsequent mass-analyzing slit  63  to select desired ion species. 
     More specifically, immediately after the ions pass the mass analyzing magnet  62  at a point A, only the desired ions exist on such a trajectory that they can pass through the mass analyzing slit  63 . 
     After the ions pass through the slit  63 , the ions are decelerated or accelerated at a post-stage electrode portion  64 . 
     With such a structure, neutralized particles and multi-charged ions, which are generated in the area B and have the different energy from the desired energy, are separated by a re-deflecting element  67  after post-deceleration/post-acceleration. 
     Consequently, only ions having the desired energy passes along a desired ion beam trajectory  68  to be introduced into a wafer substrate  66  in a wafer processing chamber  65 . 
     In this case, a deflected ion beam having high valence passes along a trajectory  69   a  while a beam including neutral particles passes along a trajectory  69   b.    
     The related arts have the following disadvantages. 
     (1) The additional deflecting element is located at the downstream side of the post-deceleration/post-acceleration element. Thereby, an additional space is required, and a whole size of ion implantation apparatus becomes large. 
     (2) When low energy ions are obtained in the deceleration (namely, the second mode for the deceleration operation, the transport distance becomes long after the ions are decelerated to the low energy. 
     Thereby, the beam diverges by the space charge effect. Finally, the beam current decreases, and advantage is lost in the second mode of the deceleration operation. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide an ion implantation apparatus which is capable of obtaining an ion beam having no energy contamination in both deceleration/acceleration modes of a post-stage. 
     An ion implantation apparatus according to this invention has an ion source, a mass-analyzing magnet, an accelerating/decelera-ting element, and deflecting elements. 
     With this structure, the mass analyzing magnet mass-analyzes an ion beam extracted from the ion source. The deflecting elements deflect the ion beam on the trajectory. 
     The deflecting elements are arranged between the mass-analyzing magnet and the accelerating/decelerating element. 
     In this case, the deflecting elements have a certain deflection angle each and have an operating mode and a non-operating mode. Each deflection angle is determined such that a final beam trajectory in a predetermined area before being introduced into a wafer substrate is matched to each other in both the operating mode and the non-operating mode. 
     The ion implantation apparatus further includes a separating slit arranged at the downstream side of the accelerating/decelerating element to eliminate undesirable particles causing energy contamination. 
     Further, the deflecting planes of the deflecting elements are substantially equivalent to the deflecting plane of the analyzing magnet. 
     In this case, deflection due to the deflecting elements are carried out by the use of electric field. 
     Alternatively, the deflection may be carried out by the use of magnetic field generated by an electromagnet preferably. 
     A beam trajectory is constituted such that the ion beam is transported to the wafer substrate when the post-acceleration/post-deceleration is not performed by the accelerating/decelerating element and the deflecting elements are not operated. 
     In the event, the deflecting elements are preferably composed of three stages such that the deflection operation is carried out three times at least. Thereby, the final beam trajectory in the predetermined area before being introduced into the wafer substrate is matched to the trajectory of non-operating mode. 
     Instead, the deflecting elements may be composed of two stages such that the ion beam is extracted from the mass-analyzing magnet on the trajectory offset from an original trajectory. In the event, preliminary deflection can be carried out by the mass-analyzing magnet. 
     The beam trajectory is offset by setting magnetic field of the mass-analyzing magnet weaker or stronger than a normal value. 
     The deflecting elements preferably include a first stage deflecting element and a second stage deflecting element. The mass-analyzing magnet deflects an ion beam trajectory around 90 degrees. 
     With such a structure, the first stage deflecting element is arranged as close to the mass-analyzing magnet as possible while the final stage deflecting element is arranged as close to the post-stage electrode portion as possible. 
     The accelerating/decelerating element is desirably arranged immediately after the mass-analyzing slit. 
     Further, the separating slit may be arranged at the predetermined distance from the accelerating/decelerating element. 
     In this condition, the mass-analyzing slit is substantially matched with a focal point of the ion beam through the mass-analyzing magnet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic view showing an ion implantation apparatus according to a related art; 
     FIG. 2 shows a graph showing an example of energy contamination in an ion implantation apparatus according to a related art; 
     FIG. 3 shows a schematic view showing an ion implantation apparatus according to another related art; 
     FIG. 4 shows a schematic view showing an ion implantation apparatus according to a first embodiment of this invention; 
     FIG. 5 shows a schematic view showing an ion implantation apparatus according to a second embodiment of this invention; and 
     FIG. 6 shows a schematic view showing an ion implantation apparatus according to a third embodiment of this invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     First Embodiment 
     Referring to FIG. 4, description will be made about a first embodiment of this invention. 
     In FIG. 4, three stages or more of deflecting elements  107   a ,  107   b , and  107   c  are arranged between a mass-analyzing magnet  102  and a post-stage electrode portion  104 . 
     A desired ion beam extracted from an ion source  101  is coming off from an initial trajectory by the effect of the first stage deflecting element  107   a  immediately after the mass-analyzing magnet  102 . Thereafter, the ion beam passes through the deflecting element  107   b  for curving and returning towards the initial trajectory. 
     Finally, the ion beam is returned onto an initial beam trajectory  112  by the deflecting element  107   c  immediately before the post-stage electrode portion  104 . 
     On halfway, at least one deflecting element  107   b  is required to curve and return the ion beam. Consequently, three deflecting devices  107   a ,  107   b , and  107   c  must be prepared at least. 
     In the meantime, particles neutralized from ions in the ion beam are not deflected by both electric field and magnetic field. 
     Particles, which are neutralized from ions in an area between the first deflecting element  107   a  located immediately after the mass-analyzing magnet  102  and the final deflecting element, are coming off from the trajectory of other normal ions, and are eliminated or lost before the post-stage electrode portion  104 . 
     Alternatively, the neutralized particles collide with slits  111  even when they pass through the post-stage electrode portion  104 . In consequence, the neutralized particles finally do not reach the wafer substrate  106  in the wafer-processing portion  105 . Herein, it is to be noted that the trajectory  109  represents a beam trajectory of the neutralized particles which are “not deflected” by the deflecting elements. 
     On the contrary, when the valence of the ion is increased by electron stripping, the ion receives stronger deflecting force than the ion having the desired valence. Consequently, the ions are coming off from the trajectory of other normal ions, and are eliminated or lost before the post-stage electrode portion  104 . 
     Alternatively, the ions collide with the slits  111  even when the ions pass the post-stage electrode portion  104 , and finally do not reach the wafer substrate  106  in the wafer-processing chamber  105 . Herein, it is to be noted that the trajectory  110  represents a beam trajectory of the higher valence ions which are “largely deflected” by the deflecting elements. 
     The neutralized particles, which can be eliminated by this method, are generated between the first-stage deflecting element  107   a  and the final-stage deflecting element  107   c.    
     Therefore, the first-stage deflecting element  107   a  is arranged as close to the mass-analyzing magnet  102  as possible while the final-stage deflecting element  107   c  is provided as close to the post-stage electrode portion  104  as possible. 
     In this method, when the ion implantation apparatus is not operated in post-deceleration or post-acceleration mode, it is unnecessary that the deflecting elements  107   a ,  107   b , and  107   c  are operated. In this case, the beam trajectory after the mass-analyzing magnet  102  becomes a linear trajectory  112 . In this event, the mass analyzing slit  103  may have the function of a part of the post-stage electrode portion  104 . 
     Second Embodiment 
     Referring to FIG. 5, description will be made about a second embodiment of this invention. 
     In FIG. 5, two stages of deflecting elements  207   a  and  207   b  are arranged between a mass-analyzing magnet  202  and an after-electrode portion  204  in minimum. 
     When the ion implantation apparatus is operated in post-deceleration or post-acceleration mode, a magnetic field of the mass-analyzing magnet  202  is set at a weaker state (or a stronger state) than the normal state. 
     In consequence, an ion beam from an ion source  201  comes from the mass analyzing magnet  202  along a trajectory  213  having a certain offset from the original beam trajectory  212 . 
     Thereafter, the ion beam is deflected towards the original trajectory  212  by the first stage deflecting element  207   a , and is returned back onto the original beam orbital  212  by the deflecting element  207   b  immediately before the post-stage electrode portion  204 . 
     Particles neutralized from ions in the ion beam are not deflected by both electric field and magnetic field. 
     Particles neutralized from ions in an area of the upstream side are eliminated before the post-stage electrode portion  204 . Alternatively, the neutralized particles collide with slits  211  even when they pass through the post-stage electrode portion  204 . 
     In consequence, the neutralized particles finally do not reach a wafer substrate  206  in a wafer-processing portion  205 . Herein, it is to be noted that the trajectory  209  represents a beam trajectory of the neutralized particles which are “not deflected” by the deflecting elements. 
     On the contrary, when the valence of the ion is increased by electron stripping, the ion receives stronger deflecting force than the ion having the desired valence. 
     Consequently, the ions are coming off from the trajectory of other normal ions, and are lost or eliminated before the post-stage electrode portion  104 . 
     Alternatively, the ions collide with the slit  211  even when the ions pass the post-stage electrode portion  204 , and finally do not reach the wafer substrate  206  in the wafer processing chamber  205 . Herein, it is to be noted that the trajectory  210  represents a beam trajectory of the higher valence ions which are “largely deflected” by the deflecting elements. 
     In this method, the neutralized particles or higher valence ions generated until the final deflecting element  207   b  can be eliminated. Therefore, the first deflecting element  207   a  may be arranged far from the mass-analyzing magnet  202 . However, the final deflecting device  207   b  must be arranged as close to the post-stage electrode portion  204 . 
     In this method, when the ion implantation apparatus is not operated in post-deceleration or post-acceleration mode, it is unnecessary that the deflecting elements  207   a  and  207   b  are operated. In this case, the magnetic filed of the mass analyzing magnet  202  is set to a normal value such that ions are deflected on the normal trajectory  212 . In this event, the mass analyzing slit  203  may have the function of a part of the post-stage electrode portion  204 . 
     In the first and the second embodiments, a deflecting plane of the deflecting elements  107  and  207  may not always correspond to the deflecting plane of the mass analyzing magnets  102  and  202 . This fact will not cause a principle problem. 
     However, in general, a deflecting direction of the deflecting elements  107  and  207  is most advantageously determined such that the ion beam is deflected on the same plane as the deflecting plane of the mass-analyzing magnets  102  and  202 . This reason will be explained below. 
     Herein, it is assumed for explanation that the deflecting plane of the mass-analyzing magnets  102  and  202  is horizontal (namely, a height thereof is constant). 
     The ion beam converges on the deflecting plane after the beam passes the magnet in the general mass analyzing magnets  102  and  202 . Further, narrow mass-analyzing slits  103  and  203  are arranged at the focal point. Thereby, the ions having different momentum can be efficiently analyzed by the slits. 
     Although a beam size (width) in the deflecting plane becomes minimum at the focal point, a beam size (a height) in a vertical direction for the deflecting plane is not always small. 
     Therefore, the mass-analyzing slit  103 ,  203  is normally provided as a narrow aperture whose height dimension is larger than the width. 
     When the deflecting direction of the deflecting elements  107 ,  207  is determined such that the ion beam is deflected in the same plane as the deflecting plane of the mass-analyzing magnets  102  and  202 , the ion beam can not pass through the mass-analyzing slit  103 ,  203  having the narrow width even if deflecting angle by the deflecting elements is small. 
     By contrast, when the ion beam is deflected in the vertical direction for the deflecting plane of the mass analyzing magnet  102 ,  202 , the ion beam can not be separated by the mass-analyzing slit  103 ,  203 , unless the beam is deflected as much as the height of the mass-analyzing slit  103 ,  203  having a larger height dimension in comparison with the width. 
     Further, both magnetic field and electric field can be used for the additional deflecting elements  107 ,  207  in principle. However, the magnetic field is generally more advantageous. This reason will be described as follows. 
     That is, when the deflecting electric field is applied, the electrons, which can reduce the positive space charge of the ion beam, can not exist in the region the deflecting electric field is applied. 
     Consequently, the space charge effect strongly affects the ion beam itself, and the ion beam diverges. As a result, the transport efficiency decreases. In particular, the beam remarkably diverges by the space electric charge in the low energy region. Therefore, the magnetic field is essential for the apparatus equipped with post-deceleration to generate the low energy ion beam. 
     Third Embodiment 
     Referring to FIG. 6, description will be made about a third embodiment of this invention. 
     A single deflecting element  307  is arranged between a mass-analyzing magnet  302  and the post-electrode portion  304  in the third embodiment. 
     The deflecting element  307  is provided immediately before the mass-analyzing slit  303  in an area (region) B as illustrated in FIG. 6 while separating slits  311  are arranged in an area (region) A. 
     In FIG. 6, an ion beam is extracted from an ion source  301  and passes through the mass analyzing magnet  302 . The ion beam is transported through the area (region) B, and is deflected to a desired beam trajectory  310  by the deflecting element  307  immediately before the mass analyzing slit  303 . 
     The ion beam passing along a desired beam trajectory  301  reaches a wafer substrate  306  in a wafer-processing portion  305 . 
     Particles neutralized from the ions are neither deflected by magnetic field nor electric field. Consequently, the particles, which neutralized in the upstream area of the deflecting device  307 , are eliminated or lost before the post-electrode portion  304 . 
     Alternatively, the neutralized particles collide with the slits  311  even when the particles pass the post-electrode portion  304 , and finally do not reach the wafer substrate  306  in the wafer processing portion  305 . Herein, it is to be noted that the trajectory  309  represents a beam trajectory of the neutralized particles which are “not deflected” by the deflecting element. 
     On the contrary, when the valence of the ion is increased by electron stripping, the ion receives stronger deflecting force than the ion having the desired valence. 
     Consequently, the ions are coming off from the trajectory of other normal ions, and are eliminated or lost before the post-stage electrode portion  304 . 
     Alternatively, the ions collide with the slits  311  even when the ions pass the post-stage electrode portion  304 , and finally do not reach the wafer substrate  306  in the wafer-processing chamber  305 . Herein, it is to be noted that the trajectory  308  represents a beam trajectory of the higher valence ions which are “largely deflected” by the deflecting elements. 
     In this event, the mass analyzing slit  303  may have the function of a part of the post-stage electrode portion  304 . 
     According to this invention, the energy contamination does not occur on the ion implantation apparatus equipped with post-deceleration/post acceleration almost. 
     In this event, the length of beam trajectory after post-deceleration/post-acceleration does not become long. 
     In particular, the generation of low energy ions by the post-deceleration can be freed from the problem of the energy contamination. In consequence, the deceleration ratio can be increased to increase the beam current. 
     Further, a switching operation, among the normal drift mode, the post-acceleration/post-deceleration mode and the post-acceleration/post-deceleration mode with the additional deflection, can be readily performed in order to cope with various implantation conditions widely. 
     Moreover, the deflecting elements can be arranged between the mass-analyzing device and the analyzing slit on the condition that the total trajectory length and structure of each component (member) is not changed. Thereby, the entire structure of the ion implantation apparatus is not almost changed, and the energy contamination can be eliminated. 
     While this invention has thus far been described in conjunction with several embodiments thereof, it will be readily possible for those skilled in the art to put this invention into practice in various other manners.