Abstract:
An ion implanter comprises an ion source and a wafer support device having a rotary disk that supports a plurality of wafers thereon and is rotated about its center axis, and capable of being swung alternately in opposite directions. An ion beam emitted by the ion source is projected on the wafers for ion implantation. The wafer support device is supported so that the center of gravity of the wafer support device lies below an axis about which the wafer support device is swung alternately in opposite directions and a component of the gravitational acceleration imparted to the wafer support device acts in the same direction as a force applied to the wafer support device to reverse the same.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ion implanter and, more particularly, to an ion implanter provided with a to-and-fro wafer disk for supporting a wafer to be subjected to ion implantation. 
     2. Description of the Related Art 
     In a conventional ion implanter, a plurality of wafers are mounted in a circumferential arrangement on a rotary disk, the rotary disk is rotated and moved alternately in opposite directions to implant ions uniformly in all the wafers. A metronome type swinging mechanism for swinging the rotary disk alternately in opposite directions disclosed in JP-A No. Hei 8-329879 drives a reversible shaft disposed on a level below the center of gravity of the rotary disk by a motor or the like to swing the rotary disk alternately in opposite directions in a vertical plane. This metronome type swinging mechanism varies the moving speed of the rotary disk according to the position of the wafer subjected to ion implantation in order that ions are implanted uniformly in all the wafers. 
     An ion implanter, particularly, a SiMOX ion implanter that implants oxygen ions in silicon wafers is required to be capable of implanting oxygen ions in wafers in a highly accurate uniformity at a high throughput. When reversing the rotary disk in a to-and-fro motion, the direction and magnitude of the speed of the rotary disk change suddenly, and consequently, the speed of the rotary disk varies. Therefore, ion implantation is interrupted in a reversing range in which the moving direction of the rotary disk is reversed, whereby the ions can be implanted in the wafers supported on the rotary disk in a highly accurate uniformity only in a moving range in which the rotary disk moves at a predetermined speed. However, the interruption of ion implantation while the rotary disk is moving in the reversing range causes a time loss and reduces throughput. 
     Accordingly, it is an object of the present invention to provide an ion implanter capable of implanting ions in wafers in a highly accurate uniformity and to carry out an ion implantation process at a high throughput. 
     SUMMARY OF THE INVENTION 
     With the foregoing object in view, the present invention provides an ion implanter including an ion source, and a wafer support device having a rotary disk that supports a plurality of wafers thereon and is rotated about its center axis, and capable of being swung alternately in opposite directions, wherein an ion beam emitted by the ion source is projected on the wafers for ion implantation, the wafer support device is supported so that a component of the gravitational acceleration imparted to the wafer support device acts in the same direction as that of a force applied to the wafer support device to reverse the same. 
     Since the gravitational acceleration is imparted at all times to the wafer support device, the gravitational acceleration assists the force applied to the wafer support device when reversing the wafer support. Consequently, load that acts on the wafer support device when the wafer support device is reversed is reduced, time loss is reduced and the throughput of the ion implanter is improved. 
     According to the present invention, it is preferable that the center of gravity of the wafer support device should lie below an axis about which the wafer support device is swung alternately in opposite directions. 
     When the wafer support device is thus disposed, the component of the gravitational acceleration imparted to the wafer support device acts in the same direction as that of the force that is applied to the wafer support device to reverse the same. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a side elevation of an ion implanter in a preferred embodiment according to the present invention; 
     FIG. 2 is a front elevation of the ion implanter shown in FIG. 1; 
     FIG. 3 is a typical view of assistance in explaining the to-and-fro motion of a rotary disk included in the ion implanter shown in FIG.  1  and holding wafers; and 
     FIG. 4 is a graph of assistance in explaining the variation of the speed of the rotary disk holding wafers when the rotary disk is swung alternately in opposite directions. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Construction and operation of an ion implanter in a preferred embodiment according to the present invention will be described with reference to FIGS. 1 to  4 . 
     The ion implanter will be described as applied to an ion implantation process for SiMOX with reference to FIGS. 1 and 2. 
     Referring to FIG. 1, only ions of a predetermined mass are separated from an ion beam IB emitted by an ion source  10  by a mass separator  20  to provide an ion beam of ions of a single kind. The ion beam thus processed by the mass separator  20  is introduced into an end station  30 . The end station  30  has a rotary disk  33  fixedly mounted on the output shaft  32  of a motor  31 . The motor  31  is placed in a scan box  34 . The rotary disk  33  is provided with a plurality of wafer holders  35 . A plurality of wafers  36  are held on the wafer holders  35 . The wafer holders  35  are arranged at equal angular intervals on the circular rotary disk  33 . As shown in FIG. 2, the wafers  36  are arranged at equal angular intervals on the rotary disk  33 . The rotary disk  33  mounted on the output shaft  32  of the motor  31  is driven for rotation in the direction of the arrow A by the motor  31 . 
     The scan box  34  is attached to the lower end of a swing shaft  37  fixedly held on a shaft  38 . The shaft  38  is turned by a scanning motor  39  in which the direction of rotation is reversible. The output shaft of the scanning motor  39  is rotated alternately in opposite directions to turn the shaft  38  alternately in opposite directions and to move the rotating rotary disk  33  holding the wafers  36  alternately in opposite directions as shown in FIG.  2 . 
     The axis of rotation of the shaft  38  that swings the rotary disk  33  alternately in opposite directions extends on a level above the center of gravity of the rotary disk  33  (position of the output shaft  32 ). Therefore, the rotary disk  33  swings about the axis of the shaft  38  alternately in opposite directions indicated by the arrows B like a pendulum. 
     As shown in FIG. 2, the ion beam IB-R introduced into the end station  30  has a substantially rectangular cross section. The ion beam IB-R is able to implant ions in the entire surfaces of all the wafers  36  when the wafer holders  35  of the rotary disk  33  holding the wafers  36  are revolved in the direction of the arrow A and the rotary disk  33  is swung alternately in opposite directions indicated by the arrows B. Since the wafer holders  35  holding the wafers  36  are attached to the rotary disk  33  in a circumferential arrangement, the velocity (linear velocity) relative to the ion beam IB-R of a portion of the wafer  36  nearer to the center of rotation of the rotary disk  33  is higher than that of a portion of the wafer  36  farther from the center of rotation of the rotary disk  33 . Therefore, if the rotary disk  33  is swung in the directions of the arrows B at a fixed speed, the dose of the portion nearer to the center of rotation of the rotary disk  33  is smaller than that of the portion farther from the center of rotation of the rotary disk  33 . Therefore, ions cannot be implanted in the wafers  36  in a uniform dose distribution. The dose uniformity of the wafers  36  can be improved by reducing the moving speed of the rotary disk  33  when the portion of the wafer  36  nearer to the center of rotation of the rotary disk  33  crosses the ion beam IB-R below the moving speed of the same when the portion of the wafer  36  farther from the center of rotation of the rotary disk  33  crosses the ion beam IB-R. Therefore, a stepping motor is used as the scanning motor  39  and the value of a speed command given to the scanning motor  39  is varied to control the speed of the to-and-from motion of the rotary disk  33 . 
     The variation of the speed of the to-and-from motion of the rotary disk  33  will be explained with reference to FIGS. 3 and 4. Referring to FIG. 3, the rotary disk  33  holding the wafers  36  is suspended by the shaft  37  from the shaft  38 . The rotary disk  33  is swung alternately in opposite directions indicated by the arrows B about the axis P of rotation of the shaft  38 . In FIG. 4, the position of the center O of the rotary disk  33  is measured on the horizontal axis and the speed v of the center O of the rotary disk  33  is measured on the vertical axis. The center O of the rotary disk  33  moves from a position V through positions W and X to a position Y and then moves from the position Y through the positions X and W to the position V. The movement of the rotary disk  33  is controlled so that a speed v 1  at the position W is higher than a speed v 2  at the position X, and the speed of the rotary disk  33  varies continuously from the speed v 1  to the speed v 2 , and from the speed v 2  to the speed v 1 . The ion beam IB-R is projected on the wafers  36  while the rotary disk  33  is moving between the positions W and X. 
     FIG. 3 shows the rotary disk  33  at the position W shown in FIG.  4 . The shaft  38  is stopped upon the arrival of the center O of the rotary disk  33  at the position W. When the scanning motor  39  applies a force FM through the shaft  38  to the rotary disk  33  to reverse the rotary disk  33 , the rotary disk  33  is moved beyond the position W in the direction of the arrow {circle around ( 1 )} by an inertial force F 1 . Suppose that the center of gravity of the rotary disk  33  coincides with the center O of the rotary disk  33  and the gravity is denoted by G. Then, a component G 1  of the gravity G acts on the center O in the same direction as the force FM. When a condition: F 1 &lt;FM+G 1  is satisfied, the movement of the rotary disk  33  in the direction of the arrow {circle around ( 1 )} is stopped. FIG. 4 shows that the foregoing condition is satisfied upon the arrival of the center O of the rotary disk  33  at the position V. 
     On the other hand, in the conventional ion implanter, in which the rotary disk is moved alternately in opposite directions by a motor or the like about an axis extending on a level below that of the center of gravity of the rotary disk, a component of the gravitational acceleration imparted to the rotary disk acts in the same direction as the inertial force. Therefore, the rotary disk stops when a condition: F 1 +G 1 &lt;FM is satisfied. The variation of the speed of the rotary disk before the condition is satisfied is indicated by a chain line in FIG.  4 . When the ion implanter of the present invention uses the force FM equal to that used by the prior art ion implanter, the rotary disk  33  can be stopped earlier than the rotary disk of the prior art ion implanter because the component G 1  of the gravity G assists the force FM in stopping the rotary disk  33 . 
     The rotary disk  33  of the ion implanter embodying the present invention moves beyond the position W and stops at the position V in a time T 3 , whereas the rotary disk of the prior art ion implanter takes a time T 4  longer than the time T 3  before stopping after passing the position V. The rotary disk  33  of the ion implanter embodying the present invention takes a time T 1 =T 2 +2×T 3  to move from the position V to the position Y. During the time 2×T 3 , the ion beam is not projected and the time 2×T 3  is a time loss. Therefore, the time loss ratio is: 2×T 3 /(T 2 +2×T 3 ). In the operation of the prior art ion implanter, time loss is 2×T 4  and hence the time loss ratio is: 2×T 4 /(T 2 +2×T 4 ). Thus, the time loss in the ion implanter embodying the present invention is less than that in the prior art ion implanter. 
     Suppose that twenty 200 mm diameter wafers are held on the rotary disk and the ion implantation time T 2  is 1.5 s, T 3  is 0.25 s and T 4  is 0.5 s. Then, the time loss ratio in the operation of the ion implanter embodying the present invention is 25% (=0.5/2.0) whereas the time loss ratio in the prior art ion implanter is 40% (=1.0/2.5). 
     Large wafers have been widely used in recent years. When processing, for example, 300 mm diameter wafers for ion implantation, the ion implantation time T 2 =2.25 s and the time loss T 4 =0.5 and the time loss ratio=31% (=1.0/3.25) when the prior art ion implanter is used, and the ion implantation time T 2 =2.25 s, the time loss T 3 =0.25 s and the time loss ratio=18% (=0.5/2.75) when the ion implanter embodying the present invention is used. 
     When starting reversing the rotary disk  33  after stopping the same, i.e., when starting moving the rotary disk  33  from the position V in the direction of the arrow {circle around ( 2 )}, the scanning motor  39  is controlled so as to apply a force FM to the rotary disk  33 , both the force FM and a component G 1  of the gravitational acceleration G acts on the center of the rotary disk  33 , so that the rotary disk  33  can be accelerated to the speed v 1  at which the rotary disk  33  must be moved for ion implantation in a short time. 
     As mentioned above, since the axis of rotation of the shaft  38  about which the rotary disk  33  swings alternately in opposite directions like a pendulum extends on a level above the center of gravity of the rotary disk  33 , the gravitational acceleration imparted to the rotary disk  33  assists the driving force of the scanning motor  39  in swinging the rotary disk  33 , so that the time loss can be reduced and the throughput of the ion implanter can be improved. The speed of swinging motion of the rotary disk  33  is thus controlled to improve the uniformity of dose distribution and the throughput can be improved. 
     Although the invention has been described in its preferred embodiment with a certain degree of particularity, obviously many changes and variations are possible therein. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein without departing from the scope and spirit thereof.