Abstract:
An charge exchanger for changing the polarity of an ion beam includes an charge exchange member provided in the path of an ion beam, and through which the ion beam can pass, and charge conversion material coating the surface of the charge exchange member. The charge conversion material is, for example, a fine powder of particles of magnesium against which the ion beam collides. The charge exchange rate caused by such collisions is controlled based on the measured dosage of ions injected into a target or on predetermined ion injection conditions. First, a desired ion dosage is determined based on the specifics of the ion implantation to be carried out. Then, an ion beam is injected onto a wafer after having been passed through the charge exchanger. Finally, the charge exchange member is advanced at a speed which establishes an charge exchange rate that gives rise to conditions necessary for the specific ion implantation process to occur.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an charge exchanger, to a method by which the polarity of an ion beam is changed by the charge exchanger, and to an ion implantation system comprising the charge exchanger. More particularly, the present invention relates to an charge exchanger which produces Mg (magnesium) molecules for collision with an ion beam, which collision acts to change the polarity of the beam, and to a method of controlling the amount of the Mg colliding with the ion beam so as to regulate the rate at which the polarity of the ions of the beam is changed. 
     2. Description of the Related Art 
     An charge exchanger changes the polarity of ions from positive to negative ions, or from negative to positive. One of the main factors in assessing the efficiency of the charge exchanger is the rate at which the polarity of the ions is changed. One type of system that employs an charge exchanger is an ion implantation system used in the fabricating of semiconductor devices. 
     In general, ion implantation refers to technology in which ions are injected into a given target. In a conventional ion implantation technique, the energy supplied to the ions is sufficient to accelerate the ions to such a degree that they can penetrate the target surface. 
     With such a technique, the concentration of impurities can be maintained within a range of 10 14  to 10 18  atoms/cm 3 . Such a technique is widely used for implanting ions into a given target material because it controls the concentration of impurities better than other impurity implantation techniques. 
     An ion implantation system generally includes at least the following components: a vacuum apparatus, an ion source, an ion extractor, an charge exchanger, a mass analyzer, an accelerator tube, and a final process station. The system is designed to supply varying levels of high voltage to effect ion decomposition, extraction, and acceleration. During the ion implantation process, the gas molecules supplied from the ion source collide with hot electrons and are extracted by an electric field formed by the applied voltage. The extracted ions form an ion beam. The ions are selectively analyzed for their charge while the ion beam is diffracted, and the ion beam is then accelerated sufficiently to penetrate a wafer to an intended depth. 
     The above-described conventional ion implantation system generally has a structure as shown in FIG.  1 . In the ion implantation system  100 , a positive ion beam  3  is extracted from an ion source  1  through an extractor  2 , and the positive ion beam  3  is converted into a negative ion beam at an charge exchanger  4 . The negative ion beam is supplied to a mass analyzer  5 . In the mass analyzer  5 , the ion beam is diffracted by a magnetic field and is analyzed. 
     Next, the ion beam  3  is accelerated by an accelerator  6 . The energy by which the ion beam is made to travel is doubled by the accelerator  6 . The accelerated ion beam passes through a stripper  7  which changes the polarity of the ion beam from negative to positive. The accelerating ion beam then passes through a magnet  9  of a final processing (ion implantation) station  8 . The magnet  9  deflects the beam onto a wafer  11  held on a disk  10 . 
     While some of the ions are injected into the wafer, the remainder of the ion beam  3  passes through the disk  10  and collides with a Faraday Cup  12 . Here, the dosage is measured by the flow of the electrons supplied from the ground. 
     As described above, the implantation system  100  effects two ion exchanges, one at the charge exchanger  4  where the polarity of the ion beam is changed from positive to negative, and the other at the stripper  7  where the polarity of the beam is changed from negative to positive. 
     One example of a conventional charge exchanger  20  is shown in FIG.  2 . In this device, a solid piece of Mg  26  is installed in a casing  22  through a hole in the casing  22 . A heater  28  is inserted through the center of the piece of Mg  26 . 
     The heater  28  is heated to produce a temperature of 450° C. or more. At this time, a vacuum is formed inside the charge exchanger  20 . The heat vaporizes the Mg. The resulting gaseous magnesium molecules collide with the ion beam  3  extracted from the ion source  1 . Then, the ion beam  3  receives the electrons from the magnesium molecules and consequently acquires a negative net charge. For example, BF 2+  ions are converted to BF and F −  ions, and BF +  ions to B and F −  ions, B +  to B −  ions, and F +  to F −  ions. 
     The ion exchange rate can be measured as the percentage of the ions making up the positive ion beam which become negative. The conventional charge exchanger  20  effects a very low ion exchange rate of less than 5%. 
     A low ion exchange rate means that a small amount of the magnesium molecules bonded with the extracted ions. The magnesium molecules, which have not bonded with the extracted ions, flow inside the charge exchanger  20 , and when the ion implantation is completed, and the inner temperature of the charge exchanger  20  cools to less than 250° C., the molecules attach to the inside of the charge exchanger  20  and contaminate the same. The molecules especially contaminate an area around the vacuum seal (not shown). The charge exchanger must therefore be cleaned periodically to remove the contaminants. 
     In addition, heating the magnesium  26  to a high enough temperature to form gaseous magnesium molecules requires a large amount of power. This contributes to the high cost of running the facility. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an charge exchanger capable of establishing a high exchange rate which contributes to the efficiency of the facility in which the charge exchanger is employed. 
     To achieve this object, the charge exchanger of the present invention comprises an ion exchange member provided in the path of an ion beam, and through which member the ion beam can pass, and a charge conversion material coating the member. The charge conversion material is made up of charged particles that will change the polarity of the ion beam when ions of the ion beam collide with them. 
     The charge exchanger further comprises a drive means in the form of a motor or the like for advancing the member while it is being impinged by the ion beam in order to provide new particles of charge conversion material for collision with the ions of the ion beam. 
     The ion exchange member is in the form of tape wound around a pair of rotary shafts. The motor is connected to one of the shafts to rotate the same and thereby advance the ion exchange member. Means may be provided for controlling the speed of the motor. In this case, the ion exchange member is provided with indicia, and a sensor senses the indicia as the ion exchange member is being advanced. The indicia can be magnetic imprints formed on the tape or a series of holes formed in the tape. Information from the sensor is used as feedback to control the speed of the motor. 
     Another object of the present invention is to provide a method in which the ion exchange rate can be controlled to effect a precise ion implantation, based on the dosage of ions injected into a target or on preselected ion injection parameters. 
     To achieve this object, the present invention provides a method in which the ion exchange rate is controlled during ion implantation by predetermining an ion dosage on the basis of the desired ion implantation process to be carried out, directing an ion beam to the wafer through the ion exchange member, and controlling the speed at which the ion exchange member is advanced to establish an ion exchange rate that produces an ion beam of a dosage that will effect the desired ion implantation process. 
     The speed is selected by measuring the dosage of the ion beam injected into the wafer or determining the ionization state of the ion beam required to effect an appropriate ion exchange rate, and based on this information, advancing the ion exchange member at a speed which establishes the necessary ion exchange rate. 
     A further object of the present invention is to provide an ion implantation system in which the ion exchange rate is high, the contamination of the charge exchanger is kept low, and which requires a relatively low amount of power to operate. 
     To achieve this object, the present invention provides an ion implantation system comprising an extractor for forming an ion beam from a source of ions, a mass analyzer for mass-analyzing the ion beam, an charge exchanger for changing the polarity of the ion beam, an accelerator for supplying the ion beam with energy in order to accelerate the ion beam, and an ion implantation station at which an ion beam is injected into a wafer, the charge exchanger comprising a casing having an inlet and an outlet through which the ion beam passes, an ion exchange member disposed in the path of the ion beam and having charge conversion material which will change the polarity of the ion beam when the ion beam collides therewith, and drive means for advancing the ion exchange member to provide a new surface of charge conversion material against which the ion beam will collide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a schematic diagram of a typical ion implantation system; 
     FIG. 2 is a cross-sectional view of a prior art charge exchanger used in such an ion implantation system; 
     FIG. 3 is a cross-sectional view of an charge exchanger according to the present invention; 
     FIG. 4 is a detailed perspective view of the charge exchanger; 
     FIG. 5 is a perspective view of interior components of the charge exchanger, showing one form of indicia-bearing charge exchange tape and an associated sensor according to the present invention; 
     FIG. 6 is a perspective view of interior components of the charge exchanger, showing another form of indicia-bearing charge exchange tape and an associated sensor according to the present invention; 
     FIG. 7 is a block diagram of control equipment of an ion implantation system according to the present invention; and 
     FIG. 8 is a flow chart of a first embodiment of a method of controlling an charge exchange rate during ion implantation according to the present invention; and 
     FIG. 9 is a flow chart of a second embodiment of a method of controlling an charge exchange rate during ion implantation according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The overall structure of the charge exchanger  40  according to the present invention is shown in FIG.  3 . Such an charge exchanger  40  can be employed in the type of ion implantation system shown in FIG. 1, such as a Deep Process ion implantation system in which ions are injected deeper than 0.2 μm from the surface of a wafer, as well as in similar types of systems. 
     Referring now to FIG. 3, an ion beam exchanger housing  44  is held in place by two fixing members  46  inside a casing  42  of the charge exchanger  40 . The housing  44  has a beam inlet  48 , a beam outlet  50 , and a vertically extending coated tape  52  interposed therebetween. The coated tape  52  comprises a base of a synthetic resin having minute magnesium particles deposited thereon. The tape  52  is wound around a main rotation shaft  54  and a sub-rotation shaft  56  of the ion beam exchanger housing  44 . The shafts  54  and  56  can be interlocked for co-rotation by any appropriate means. The casing  42  includes a cable inlet  60  through which a cable  58  passes. The cable inlet  60  is provided between the fixing members  46 . The cable  58  is connected to a motor connected to the main rotation shaft  54 . In addition, a vacuum pump  62  forms a vacuum inside the charge exchanger  40 . 
     In the present invention as described above, an ion beam from an extractor  2  is introduced into the charge exchanger  40 , where it travels through the inlet  48  of the ion beam exchanger housing  44 . The ion beam is converted from a positive ion beam to a negative ion beam while passing through the coated tape  52 . The negative ion beam is supplied to a mass analyzer  5  via the outlet  50  of the ion beam exchanger housing  44 . 
     The ion beam exchanger housing  44  is illustrated in more detail in FIG.  4 . The inlet  48  is a cylinder made of a metallic material such as molybdenum (Mo) or tungsten (W). The cylinder protrudes from a front wall of the ion beam exchanger housing  44 . A variable speed motor  64  is connected to the main rotation shaft  54  to apply a driving force to the coated tape  52 , whereby the coated tape  52  is wound in one direction at a certain speed. 
     The coated tape  52  is wider than the diameter of the inlet  48  through which the ion beam is induced. The tape  52  thus provides a sufficient area over which the ions of the ion beam can become bonded to the magnesium molecules. The base of the coated tape  52  is coated with a dense thin film of fine magnesium powder to ensure a high probability of collision between ions of the ion beam and the molecules of magnesium. 
     During this process, the coated tape  52  is wound on the main rotation shaft  54  and is simultaneously unwound from the sub-rotation shaft  56  by the motor  64 . The speed at which the motor  64  is operated is selected considering the desired depth of ion implantation, energy of the ion beam, and dosage, etc. 
     Referring to FIG. 5, a tensioner  66  in the form of a tensioned roller serves to tension the coated tape  52  as it is unwound from the sub-rotation shaft  56 . The coated tape  52  has a track of indicia by which the speed at which the tape is wound around the rotation shaft  54  by the motor  64  can be determined. The indicia can be made up of marks  68  of a certain size and spaced apart at fixed intervals. The marks  68  are formed at the edge of the coated tape  52 . A sensor head  70  is provided to sense the marks  68 . The marks  68  can be magnetic in which case the sensor head can sense the presence of the magnetic fields emanating from the marks. 
     When the motor  64  is turned on, the sensor head  70  is moved opposite the track to sense the marks  68  spaced apart from one another at uniform intervals. If the motor  64  stops, the sensor head  70  is moved away from the coated tape  52 . 
     FIG. 6 shows another embodiment of the charge exchanger. In this case, the coated tape  52  has a series of holes  72  formed at constant intervals alongside an edge thereof. A light-emitting member  74  faces the holes  72  at one side of the tape  52 , whereas a light-receiving member  76  faces the holes at the other side of the tape  52 . Therefore, the speed of the coated tape  52  can be determined from the rate at which light signals are received by the light-receiving member  76 . 
     FIGS. 7-9 show how the motor  64  is controlled to thereby control the charge exchange rate. In FIG. 7, a data input device  80 , into which the data for ion-implantation is keyed, is connected to a controller  82 . The data input device  80  has a program that establishes the operating conditions of the ion implantation system based on the data input thereto. The ion extractor  2 , and a servo  86  of the motor  64  are connected to the controller  82 . Furthermore, a dose counter  88 , installed in the final processing station  8  to measure the dosage of ions directed thereto, is also connected to the controller  82 . Finally, a speed calculating device  92  is connected to the controller  82  and to the sensor  70  or  74 . The speed calculating device generates a signal corresponding to the speed of the coated tape  52  based on signals issued by the sensor  70  or  74 . 
     To control the charge exchange rate, the speed of the motor  64  can be controlled based on measurements of the dosage or the speed of the coated tape  52 . 
     First, a method of controlling the speed of the motor  64  based on the dosage can be performed as follows. Referring to FIG. 8, the parameters under which ion implantation is to be carried out are set (S 2 ) by the data input device  80 , and the ion extractor  2  and the motor  64  are driven (S 4 ) at rates calculated to effect ion implantation within the established parameters. Subsequently, the dose counter  88  measures the dosage (S 6 ) of ions reaching the target, and if the measured value (S 8 ) is within an allowable range of the value calculated in step (S 2 ), the process is continuously performed without any changes (S 10 ). However, if the measured value of the dosage is outside the allowable range of values, the controller  82  issues a control signal to the motor servo  86  which causes the motor  64  to rotate (S 12 ) at a different speed calculated by a program stored in the controller  82  to produce ion implantation within the parameters established by the data input device  80 . 
     That is, if the measured dosage is less than a value necessary to effect a desired implantation of ions in the target, the charge exchange rate is determined to be too low. That is, the coated tape  52  is determined to be moving too slowly to cause a high enough probability of collision between ions of the ion beam and magnesium molecules on the tape  52 . In this case, the controller  82  supplies control signals to the motor servo  86  which in turn increases the rotational speed of the motor  64 . As a result, the speed of the coated tape  52  is increased, and more magnesium molecules collide with the ion beam thereby increasing the charge exchange rate. 
     Of course, if the dosage of ions being implanted is too high, the controller  82  can gradually decrease the speed of the motor  64  until the ions are being implanted with a desired dosage. 
     Next, a method of controlling the charge exchange rate based on the rotational speed of the motor  64  will be described with reference to FIG.  9 . The parameters under which ion implantation is to be carried out are set (S 20 ) by the data input device  80 , and the ion extractor  2  and the motor  64  are driven (S 22 ) to implant ions. The depth to which the ions will be injected into a wafer depends on the energy of the ion beam. In this method, the specific energies needed to inject the ions to certain depths are stored as statistical data in the data input device  80 . This data is used to calculate the speed at which the coated tape  52  should travel to be in accord with the energy supplied to the ion beam. 
     Indicia on the coated tape  52  in the form of the marks  68  or holes  72  is sensed by the sensor  70  or  74 . The speed of the coated tape  52  is measured (S 24 ) by the number of the marks  68  or holes  72  sensed per unit of time, e.g., per hour. Then, the speed of the coated tape  52 , as measured by the speed calculating device  92 , is monitored by the controller  82  (S 26 ). 
     As long as the speed of the coated tape  52  is proper, the controller  82  allows the process to continue uninterrupted (S 28 ). However, if the measured speed is too high or too low, a driving signal is issued by the controller  82  to the motor servo  86  to establish a motor speed (S 30 ) which causes the coated tape  52  to travel at the predetermined speed. 
     According to the embodiments of the present invention as described above, the extracted ions collide with a high density of magnesium molecules. In addition, the power consumption of the ion convertor is relatively low. Therefore, the charge exchange rate is high, the efficiency of the facility in which the charge exchanger is employed is correspondingly high, and the process is economical to carry out. Moreover, the molecules are situated on a medium, i.e., a plastic tape, whereby they will not contaminate the inside of the charge exchanger. 
     Although the present invention has been described in detail above, various changes, substitutions and alterations thereto will become apparent to those of ordinary skill in the art. For instance, although the charge exchanger has been described in connection with ion implantation equipment, the charge exchanger can be employed by any equipment in which it is necessary to change the polarity of an ion beam. Therefore, all such changes, substitutions and alterations are seen to be within the true spirit and scope of the present invention as defined by the appended claims.