construction and operation of an ion implanter in a preferred embodiment according to the present invention will be described with reference to fig1 to 4 . the ion implanter will be described as applied to an ion implantation process for simox with reference to fig1 and 2 . referring to fig1 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 fig2 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 fig2 . 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 fig2 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 fig3 and 4 . referring to fig3 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 fig4 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 . fig3 shows the rotary disk 33 at the position w shown in fig4 . 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 . fig4 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 fig4 . 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 t3 , whereas the rotary disk of the prior art ion implanter takes a time t4 longer than the time t3 before stopping after passing the position v . the rotary disk 33 of the ion implanter embodying the present invention takes a time t1 = t2 + 2 × t3 to move from the position v to the position y . during the time 2 × t3 , the ion beam is not projected and the time 2 × t3 is a time loss . therefore , the time loss ratio is : 2 × t3 /( t2 + 2 × t3 ). in the operation of the prior art ion implanter , time loss is 2 × t4 and hence the time loss ratio is : 2 × t4 /( t2 + 2 × t4 ). 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 t2 is 1 . 5 s , t3 is 0 . 25 s and t4 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 t2 = 2 . 25 s and the time loss t4 = 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 t2 = 2 . 25 s , the time loss t3 = 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 .