Abstract:
A Faraday assembly of an ion implantation apparatus includes a Faraday cup in a vacuum chamber, a driving shaft to which the Faraday cup is connected, a motor for inserting the driving shaft further into and drawing the driving shaft out of the vacuum chamber to cause the Faraday cup to advance and retreat within the chamber, and an auxiliary supplier of power for exerting a force that acts on the driving shaft as the driving shaft is being extracted by the motor from the vacuum chamber. Therefore, the force of suction, due to a pressure difference between interior and exterior of the vacuum chamber, is prevented from overloading the motor as the Faraday cup retreats within the vacuum chamber. As a result, the Faraday cup is positioned precisely and efficiently within the vacuum chamber.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an ion implantation apparatus. More particularly, the present invention relates to a Faraday assembly of an ion implantation apparatus, for moving a Faraday cup within a vacuum chamber of the apparatus. 
   2. Description of the Related Art 
   Generally, ion implantation apparatus have been used for a long time in the manufacturing of semiconductor devices. In particular, an ion implantation apparatus forcibly implants ions into a selected region of a wafer made of silicon, for example. The ion implantation apparatus includes a particle accelerator to accelerate the ions as a beam towards the wafer. The ions collide with the silicon atoms of the wafer, thereby gradually losing energy, and stop at a certain depth in the crystal lattice structure of the silicon. 
   The ion implantation apparatus also includes a Faraday cup. The Faraday cup functions as a sensor for measuring the current, energy, and shape of the ion beam as well as the absorption of ion beam energy at the end of the ion beam. The Faraday cup is typically located in front of a target (the wafer) in a vacuum chamber sometimes referred to as the “target chamber”. The location of the Faraday cup in the vacuum chamber can be adjusted according to the region of the target selected for ion implantation. The apparatus for changing the location of the Faraday cup is referred to as a Faraday assembly. The Faraday assembly thus includes a Faraday cup located in a vacuum chamber and power means for moving the Faraday cup in the vacuum chamber in the direction of propagation of the ion beam. 
   The power means includes a reversible motor, and a ball screw connected to the motor for converting the rotary output of the motor into rectilinear motion. Accordingly, the ball screw can be rotated in forward and reverse directions according to the rotary direction of the output of the motor, so that the Faraday cup can be moved rectilinearly back and forth within the vacuum chamber. 
   The interior of the vacuum chamber is maintained at a low vacuum pressure of about 10 −3  torr to minimize contamination of the wafer during the ion implantation process. On the contrary, the environment outside the vacuum chamber is at atmospheric pressure. Accordingly, it is relatively easy to advance the Faraday cup within the vacuum chamber. However, a relatively large load is applied to the power means by the vacuum in the vacuum chamber when the Faraday cup is retracted within the vacuum chamber. In this case, the motor is overloaded and draws excessive current, whereupon the Faraday cup is positioned at an incorrect location within the vacuum chamber. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to solve the aforementioned problems of the prior art. 
   More specifically, an object of the present invention is to provide a Faraday assembly of an ion implantation apparatus that can precisely and reliably position a Faraday cup within a vacuum chamber of the apparatus. 
   Another object of the present invention is to provide a Faraday assembly of an ion implantation apparatus having an assist for the motor of the assembly so that the motor is not overloaded during an operation of the assembly in which a component of the assembly is being drawn out of a vacuum chamber by the motor. 
   According to one aspect of the present invention, there is provided a Faraday assembly of an ion implantation apparatus comprising: a Faraday cup located in a vacuum chamber, a driving shaft connected to the Faraday cup and extending outwardly from the vacuum chamber, main power means for moving the driving shaft further into and further out of the vacuum chamber to cause the Faraday cup to advance and retreat within the vacuum chamber, and auxiliary power means to exert an independent force that acts on the driving shaft as the driving shaft is being drawn out of the vacuum chamber by the main power means. 
   The main power means comprises a motor, such as a reversible motor having a rotary output or a cylinder having a reciprocating piston. The main power means may also include a power transmission mechanism connecting the motor to the driving shaft so as to transmit the power output by the motor to the driving shaft. The power transmission mechanism preferably comprises a ball screw. In the case of the motor being of the type that has a rotary output, the power transmission mechanism also preferably includes a belt and pulley system or the like connecting an output shaft of the motor to the ball screw. 
   According to yet another aspect of the present invention, there is provided a Faraday assembly of an ion implantation apparatus comprising: a Faraday cup located in a vacuum chamber, a driving shaft connected to the Faraday cup and extending outwardly from the vacuum chamber, a motor operatively connected to the driving shaft so as to move the driving shaft in a first direction in which the driving shaft is extended further into the vacuum chamber and a second direction in which the driving shaft is drawn out of the vacuum chamber, and auxiliary power means for exerting a force acting on the driving shaft in the second direction when the driving shaft is moved by the motor in said second direction. 
   Preferably, the auxiliary power means comprises a spring. 
   According to another aspect of the present invention, there is provide a Faraday assembly of an ion implantation apparatus comprising: a Faraday cup located in a vacuum chamber, a driving shaft connected to the Faraday cup and extending outwardly from the vacuum chamber, a motor, a ball screw having a ball nut and a lead screw and driven by the motor, a carrier constituted by the ball nut of the ball screw and to which the driving shaft is fixed such that the driving shaft is moved as the ball nut of the ball screw translates along the lead screw, and a spring supported in the assembly so as to exert a biasing force on the carrier. The carrier may also include a support plate protruding from the ball nut and to which the driving shaft is fixed. The driving shaft can be moved via the carrier in both a first direction in which the driving shaft is extended further into the vacuum chamber and a second direction in which the driving shaft is drawn out of the vacuum chamber so as to reposition the Faraday cup within the vacuum chamber. The biasing force exerted by the spring acts in the second direction as the driving shaft is moved via the carrier in the second direction. 
   A support shaft may be disposed outside of the vacuum chamber and extend through the support plate parallel to the driving shaft. In this case, the spring is disposed around the support shaft as interposed between the vacuum chamber and the support plate. 
   Alternatively, the spring may be disposed around the driving shaft as interposed between the vacuum chamber and the support plate. 
   As still another alternative, the spring may be disposed around the lead screw as interposed between the ball nut and the vacuum chamber. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments thereof made with reference to the attached drawings in which: 
       FIG. 1  is a perspective view of a Faraday assembly of an ion implantation apparatus according to the present invention; 
       FIG. 2  is a side view of the Faraday assembly of the ion implantation apparatus according to the present invention, and shows a state in which the Faraday cup of the assembly is being retracted within a vacuum chamber; 
       FIG. 3  is a side view of the Faraday assembly of the ion implantation apparatus according to the present invention, and shows a state in which the Faraday cup of the assembly is being advanced within the vacuum chamber; 
       FIG. 4  is an enlarged view of a support plate of the Faraday assembly of the ion implantation apparatus according to the present invention; 
       FIG. 5  is a side view of another embodiment of the Faraday assembly of the ion implantation apparatus according to the present invention; 
       FIG. 6  is a side view of yet another embodiment of the Faraday assembly of the ion implantation apparatus according to the present invention; and 
       FIG. 7  is an enlarged view of a portion of the embodiment of the Faraday assembly shown in  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of a Faraday assembly of an ion implantation apparatus according to the present invention will be described in detail hereinafter with reference to the attached drawings. 
   Referring to  FIGS. 1-3 , a first embodiment of the Faraday assembly of the ion implantation apparatus according to the present invention includes: a Faraday cup  110  located in a vacuum chamber  100  which is maintained at a low vacuum pressure of about 10 −3  torr; and a driving shaft  130  extending into the vacuum chamber  100  through a wall thereof. One end of the driving shaft  130  is connected to the Faraday cup  110  within vacuum chamber  100  and the other end of the driving shaft  130  is disposed outside the vacuum chamber  100 . 
   Also, the wall of the vacuum chamber  100  is provided with an annular seal  120  for preventing loss of the vacuum pressure within the vacuum chamber  100  and for preventing external air from entering the vacuum chamber  100 . The driving shaft  130  is centered in the seal  120 . The seal  120  may be cylindrical or may be a gasket fitted to the outer surface of the driving shaft  130 . 
   The Faraday assembly also includes a ball screw whose lead screw  150  extends parallel to and beneath the driving shaft  130  outside the vacuum chamber  100 . One end of the lead screw  150  is rotatably supported by a bearing  101   a . The bearing  101   a  is mounted to a pedestal  101  provided on the outside of the wall of the vacuum chamber  100 . 
   In addition, the Faraday assembly comprises a carrier  140  disposed outside the vacuum chamber  100 . The carrier  140  includes a conveying block  141  and a support plate  142  that protrudes from the top of the conveying block  141 . The conveying block  141  comprises the ball nut of the ball screw, i.e., a nut whose inner threads are engaged with those of the lead screw  150  through a series of balls. The support plate  142  is connected to the end of the driving shaft  130  disposed outside the vacuum chamber  100 . The driving shaft  130  and the support plate  142  may be connected by fixing the end of the driving shaft  130  directly to the support plate  142  or by mounting a bracket to the support plate  142  and fixing the end of the driving shaft  130  to the bracket. 
   Accordingly, as the lead screw  150  is rotated, the conveying block  141  moves along the lead screw  150 . As a result, the driving shaft  130  is moved rectilinearly by the support plate  142  in the direction of movement of the conveying block  141 . 
   Furthermore, the Faraday assembly includes a motor, and a power transmission mechanism comprising a belt and pulley system for transmitting the power of the motor to the ball screw. More specifically, the motor may be a reversible motor  180  having a rotary output shaft. The belt and pulley system comprises a power delivering pulley  160  fixed to the outermost end of the lead screw  150 , and a power delivering belt  170  wrapped around the power delivering pulley  160  and the output end (pulley on the output shaft) of the motor  180  to transmit the power of the motor  180  to the ball screw. Alternatively, the motor of the Faraday assembly may be a cylinder having a reciprocal piston, and the power transmission mechanism may be a rack and pinion. 
   Still further, the Faraday assembly has a support shaft  200  extending parallel to and above the driving shaft  130  outside-the vacuum chamber  100 . One end of the support shaft  200  is fixed to a pedestal  201  provided on the outside of the wall of the vacuum chamber  100 . The other end of the support shaft  200  is fixed to another pedestal  210  such that the support shaft  200  extends through a hole  142   a  in the upper portion of the support plate  142 , as shown best in  FIG. 4 . The inner diameter D 2  of the hole  142   a  is larger than the outer diameter D 1  of the support shaft  200 . The support shaft  200  is supported by the pedestals  201 ,  210  so that a predetermined spacing is maintained between the support shaft  200  and the support plate  142 . Accordingly, the support plate  142  can be slid freely relative to the support shaft  200 . 
   Therefore, the movement of the support plate  142  does not produce friction with support shaft  200  nor will the support plate  142  bind with the support shaft  200 , which friction or binding could otherwise overload the motor  180 . In this respect, a separate lubricating member, such as a bearing or bushing, may be provided within the hole  142   a  to support the support shaft  200  in a manner that essentially prevents friction as the support plate  142  is moved along the support shaft  200 . 
   The Faraday assembly also includes an auxiliary power means for urging the driving shaft  130  in the direction which retracts the Farady cup  100  within the vacuum chamber  100 . In the present embodiment, the auxiliary power means is a coil (helical) spring  220 . The coil spring  22  extends around the support shaft  200 . One end of the spring  220  butts up against the pedestal  201 , and the other end thereof butts up against the support plate  142 . Preferably, the maximum restoring force of the spring  220  (force exerted by the spring upon its maximum contraction) is smaller than the difference in pressure between interior and exterior of the vacuum chamber  100 . 
   Now, the operation of the above-described Faraday assembly of the present invention will be described in more detail. 
   During an ion implantation process, a target within the vacuum chamber  100  is scanned with an ion beam. The Faraday cup  110  of the Faraday assembly is moved in the direction of the scan in the vacuum chamber  100  to sense characteristics of the ion beam in the vacuum chamber  100 . To this end, the driving shaft  130  adjusts the location of the Faraday cup  110 . 
   For example, the motor  180  is rotated in one direction, e.g., a forward direction. As a result, the rotary power is delivered to the lead screw  150  through the power delivering belt  170  and the power delivering pulley  160 . Thus, the lead screw  150  begins to rotate, whereby the driving shaft  130  advances into the vacuum chamber  100 . That is, the conveying block  141  of the carrier  140  advances toward the vacuum chamber  100  along the lead screw  150 . Accordingly, the driving shaft  130  is extended into the vacuum chamber  100  by the support plate  142  of the carrier  140 . At this time, the interior of the vacuum chamber  100  is in a high vacuum state, and the environment outside the vacuum chamber  100  is at atmospheric pressure. Accordingly, a relatively small load is exerted on the motor  180 . Also, at this time, the spring  220  is contracted. 
   Therefore, some load is applied to the motor  180  by the restoring force exerted by the spring  220  on the motor via the support plate  142 , ball screw and power transmission mechanism. However, this load is mostly offset by the suction created due to the pressure difference between the interior and exterior of the vacuum chamber  100 . Accordingly, the motor  180  is not overloaded even though the spring  220  is contracted. 
   On the other hand, the motor  180  is rotated rotates in the opposite direction, e.g. in reverse, when the driving shaft  130  is to be drawn from the vacuum chamber. In this case, as well, power is delivered to the ball screw through the power delivering belt  170  and the power delivering pulley  160 , and the lead screw  150  rotates in the direction opposite to the direction used to advance the driving shaft  130  within the vacuum chamber  100 . 
   Accordingly, the carrier  140  retreats from the vacuum chamber along the lead screw  150 . At this time, suction created by the pressure difference between the interior and exterior of the vacuum chamber  100  is exerted on the driving shaft  130  and the Faraday cup  110  connected to the driving shaft  130 . Conventionally, the motor  180  would experience the force of the suction as a load. 
   However, in the present invention, the force of the suction is offset by the restoring force of the spring  220 . That is, the spring  220  functions as an auxiliary power means to reduce the load exerted on the motor  180  while the driving shaft is being drawn out of the vacuum chamber  100 . Accordingly, the Faraday cup  110  can be positioned reliably for a long time. 
     FIG. 5  shows another embodiment of according to the present invention. In this embodiment, a spring  300  is disposed around the driving shaft  130 . One end of the spring  300  is supported by the sealing member  120  and the other end thereof is supported by the support plate  142  of the carrier  140 . 
   According to another embodiment shown in  FIG. 6 , a spring  400  is disposed around the ball screw  150 . One end of the spring  400  is supported by the pedestal  101 , and the other end thereof is supported by the conveying block  141  of the carrier  140 . 
   In this embodiment, bearings  101   a  and  141   a  prevent the spring  400  from rubbing against the ball screw  150  and thereby damaging or impeding the rotation of the ball screw  150 . The bearings  101   a  and  141   a  protrude from opposing ends of the pedestal  101  and the conveying block  141 , respectively. The ends of the spring  400  are supported by the outer races of the bearings  101   a  and  141   a.    
   As mentioned above, according to the Faraday assembly of the ion implantation apparatus of the present invention, the force of the suction due to the pressure difference between interior and exterior of the vacuum chamber is prevented from being applied to the motor as the Faraday cup is drawn in a direction towards the outside of the vacuum chamber. Accordingly, the operation, stability and efficiency of the Faraday assembly are improved so that the efficiency of the ion implantation process is enhanced. 
   Finally, although the present invention has been described in detail above with respect to the preferred embodiments thereof, the present invention is not so limited. For example, springs other than a helical spring can be used as the auxiliary power means. Alternatively, a buffer piston may be employed as the auxiliary power means. Also, various alternatives to the disclosed main power means, power transmission mechanisms, auxiliary power means, and relative dispositions of these components will be readily apparent to those of ordinary skill in the art. Accordingly, such modifications of and changes to the disclosed embodiments are seen to be within the true spirit and scope of the present invention as defined by the appended claims.