Mechanical Scan System For Ion Implantation

Abstract

Workpieces to be implanted with a uniform ion dose are attached to a major surface of a target positioned in the path of an ion beam. The target is rotated and traversed so that the ion beam traces segments of spiral paths on each workpiece. The uniformity of the ion dose in the workpieces is controlled by regulating the position of the target during each traversal according to the accumulated ion dose, or by regulating the traversing speed according to the ion beam current.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

Broadly speaking, this invention relates to ion implantation. More particularly, this invention relates to a rotary scanning system which distributes the ion dose from an ion beam over a plurality of workpieces, and which regulates the uniformity of the ion dose implanted in the workpieces.

2. Description of the Prior Art

Slices of semiconductor material to be used in the fabrication of semiconductor devices can be treated by implanting the slices, under vacuum, with ions from an ion beam. In production, it is typically necessary to implant ions into large quantities of slices. For this purpose, it is desirable to use an ion implantation system which is capable of handling a large batch of slices, and which has an ion beam current of sufficient magnitude to implant a desired ion dose in a short time. However, prior art ion implantation systems are limited by the number of slices which can be handled in each batch, and by the ion beam current which can be controlled.

A typical prior art ion implantation system is shown in U. S. Pat. No. 3,117,022, which issued on Jan. 7, 1964 to G. A. Bronson et al. In the Bronson patent, an ion source generates a ion beam which is directed towards a stationary target, and which is deflected by horizontal and vertical electrostatic deflection plates to distribute the ions over the surface of the target. The target can be a slice of semiconductor material. A control circuit generates voltages which cause the deflection plates to deflect the ion beam in such a manner that a raster is produced on the target. The target, deflection plates, control circuit, and other associated apparatus are maintained at a negative potential of a few hundred kilovolts with respect to the ion source, to attract the positively charged ions in the ion beam and to impart the energy necessary to implant the ions in the surface of the target.

When an ion beam is electrostatically deflected, the charge on the constituent ions reacts with the potential on the deflection plates to deflect the ions. However, a small percentage of the ions become neutralized in transit from the ion source, and are not deflected. These so-called neutral ions all impinge on a small area of the target, increasing the ion dosage in that area, and creating an undesirable "hot spot." The hot spot can be eliminated by separating the charged ions from the neutral ions by means of additional deflection plates, but this remedy requires a longer beam path to accommodate the additional deflection plates.

The maximum current in an ion beam is related to the length of the beam path. The ion beam is spread by the effects of the space charge surrounding the constituent ions; the longer the beam path, the greater the spread. Therefore, it is desirable to keep the beam path as short as possible.

In an alternative prior art method of deflecting an ion beam, the ion beam is held stationary and the raster is produced by moving the target with respect to the ion beam by means of an X-Y scanning system. Such a method eliminates the need for deflection plates, their associated drift spaces, and driving circuits, and permits the target to be placed closer to the ion source. A higher beam current is possible because of the shorter beam path. Because the beam is not deflected, irregularities caused by the non-deflection of neutral particles are no longer significant. However, the scanning speed in an X-Y system is limited because of the need to reverse the motion of the scanning system at the ends of the lines comprising the raster. Therefore, an X-Y scanning system cannot take full advantage of the higher beam current made possible by the elimination of deflection apparatus if a low or moderate implanted dose is required.

The problem is, therefore, to provide a method of moving a plurality of workpiece workpieces high speed with respect to a stationary, high-current ion beam, and to regulate the movement to control the ion dose implanted into the workpieces.

SUMMARY OF THE INVENTION

As a solution to the above problem, this invention discloses a method of moving workpieces with respect to a stationary ion beam by first, rotating and traversing a target to which the workpieces are affixed so that the ion beam traces segments of spiral paths on each workpiece, and second, regulating the traversals to regulate the ion dose implanted into the workpieces.

In a preferred embodiment, apparatus is disclosed for practicing the above method by which the target is positioned during each traversal according to the accumulated ion dose. In another embodiment, apparatus is disclosed for controlling the traversing speed of the target according to the ion beam current.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partially schematic, partially diagrammatic view of a target for holding workpieces to be implanted with ions;

FIG. 2 is a diagram useful for deriving an equation for a translating velocity V;

FIG. 3 is a partly schematic, partly diagrammatic representation of a rotating and traversing apparatus for the target of FIG. 1, and a first embodiment of a control apparatus therefor; and

FIG. 4 is a partly schematic, partly diagrammatic representation of the rotating and traversing apparatus shown in FIG. 3 and a preferred, second embodiment of control apparatus therefor.

DETAILED DESCRIPTION

Throughout the following description, identical reference numerals are used to identify identical elements in different figures.

Referring now to FIG. 1, workpieces 10 are temporarily attached to a target 11, which is rotated at an angular velocity .omega. and traversed at a velocity V by means which will be described in conjunction with the explanation of FIG. 3. The workpieces 10 are preferably evenly distributed on the target 11 so that the target is balanced and can be rotated at speeds as high as 1,200 r.p.m. A typical target is a disc 20 inches in diameter holding 60 workpieces. Each workpiece is typically a disc of semiconductor material 0.010 inch thick and 2 inches in diameter.

An ion beam 13 from an ion source 14 is directed through analyzing, accelerating and focusing apparatus (not shown) onto an area 15 of the workpieces 10 spaced at a radial distance R from the axis 12. Ions from the ion beam 13 are implanted in the workpieces 10 in a spiral path 16 as the target 11 is rotated and traversed by means (not shown) which will be disclosed in conjunction with the description of FIG. 3. The workpieces 10 and the target 11 are maintained at a high potential with respect to the ion source 14 by a high voltage source 17 to impart the necessary implantation energy to the ions in the ion beam 13. The movement of ions in the ion beam is measured as a current I which flows in the connections to the high-voltage source 17.

Typically the ion-source end of an ion implantation apparatus is maintained at ground potential, as indicated by a ground connection 18. Alternatively, the target end of the ion implantation apparatus can be maintained at ground potential, as shown in phantom representation by a ground connection 19.

An ion dose D is implanted in the workpieces 10 during each traversal of the target 11 with respect to the ion beam 13. The magnitude of the ion dose D can be regulated by controlling either the ion beam current I or the traversing velocity V. The ion beam current is difficult to control; therefore, the ion dose D is more readily regulated by controlling the traversing velocity V.

The ion dose D is independent of the rotational velocity .omega. if the rotational velocity is sufficiently high that fluctuations in the ion beam current I are averaged over all the workpieces 10.

The ion density within the ion beam 13 is non-uniform, and generally follows a Gaussian distribution. Therefore, each trace of the ion beam 13 implants a non-uniform ion dose in the workpieces 10. However, the traversing velocity V is scaled to the rotational velocity .omega. so that the pitch of the spiral path 16 is small, preferably less than 2 percent of the radial dimension of the area 15. This small pitch results in a large overlap of adjacent traces.

A typical implantation run comprises several successive traversals. Because the traversing velocity V is not precisely synchronized with the rotational velocity .omega., and because alternate traversals are opposite in direction, the traces from successive spirals tend to overlap randomly. Because of the overlap between adjacent traces within each spiral, and the random overlap between successive spirals, the non-uniformities in the overlapping traces average so that the total ion dose in the workpieces 10 is essentially uniform.

FIG. 2 is useful in explaining the interrelationship of the traversing velocity V, the ion beam current I, the radial distance R, and the ion dose D. For the purpose of this explanation, the ion beam 13 is assumed to be focused onto a point 20.

Each region of the workpieces 10, such as an arbitrarily small region 21, accumulates the ion dose D during each traversal of the target 11. The region 21 has a radial dimension .DELTA.R and a circumferential dimension .DELTA.C.

During each traversal, the point 20 is within an annulus having the radial dimension .DELTA.R for a time

T = .DELTA. R/V (1)

because the target 11 is rotating, however, the point 20 is within the circumferential dimension .DELTA.C during a fraction

F = .DELTA. C/2.pi.R (2)

of each rotation. Therefore, the point 20 is within the region 21 for a time

t = FT = .DELTA. R.DELTA.C/2.pi.RV (3)

during each traversal.

The implanted ion dose D in the region 21 is a function of both the time t during which the point 20 is within the region 21, and the ion beam current I during the time t. Thus,

D = tI/.DELTA.R.DELTA.Cq (4)

where q is the ion charge. Combining equations (3) and (4)

D = I/2.pi.RVq (5)

and rearranging to solve for V gives

V = I/2.pi.RDq (6)

Equation (6) can also be used where the ion beam 13 is focused on a larger area of the workpieces 10, such as a circular, elliptical or rectangular area, if the workpieces 10 pass through the entire larger area during each traversal.

Referring to FIG. 3, apparatus is shown for rotating and traversing the target 11, and a first embodiment of control apparatus is shown for controlling the traversing velocity of the target 11 according to equation (6). The target 11 is mounted on the shaft 30 of motor 31 which rotates the target 11 at an essentially constant speed. The motor 31 is mounted on a carriage 32 which is slidably supported on ways 33. The carriage 32 is moved along the ways 33 by a traveling nut 34 which is fixed to the carriage 32 and threaded on a leadscrew 35. The leadscrew 35 is rotated by a motor 36. A high voltage connection is made to the target 11 by means of a slip-ring 37 mounted on the shaft 30.

In the first embodiment of the control apparatus, a current measuring circuit 40 is connected between the slip-ring 37 and the high voltage source 17. A potentiometer 41 is mechanically linked to the carriage 32, as indicated by a dotted line 42. The potentiometer 41 provides a signal R', which represents the radial position R, on a lead 43. A manually operable potentiometer 44 provides a signal D', which represents the ion dose to be implanted in the workpieces 10, on the lead 45. The current measuring circuit 40 provides a signal I', which represents the ion beam current I, on the lead 46.

A calculating circuit 47 combines the signals I', R' and D' to generate a signal V' according to the equation

V' = K I'/R'D' (7)

which is analogous to equation (6) when

K = 1/2.pi.q (8)

The signal V' is connected through an amplifier 48 to drive the motor 36.

The calculating circuit 47 can comprise analog computing elements to perform the necessary multiplication and division steps. Alternatively, the calculating circuit 47 can comprise analog-to-digital converters to transform the analog signals on the leads 43, 45 and 46 to digital form, digital computing elements to perform the necessary multiplication and division steps, and a digital-to-analog converter to transform the digital result to an analog signal to drive the motor 36 through the amplifier 48. The digital computing elements can be provided in the form of a small digital computer. Many possible implementations of the apparatus of FIG. 3 will be apparent to one skilled in the art.

In operation, the potentiometer 44 is manually set to represent the ion dose D to be implanted in the workpieces 10; the carriage 32 is positioned at one limit of its travel; the motor 31 is powered to rotate the target 11; and the ion beam 13 is energized. The calculating circuit 47 generates the signal V', which is then amplified by the amplifier 45 to cause the motor 36 to move the carriage 32 at the traversing velocity V.

Additional apparatus (not shown) can be provided to reverse the traversing direction when each traversal is completed, and to control the number of traversals.

It is typically more practical to regulate the position instead of the velocity of a movable object. Therefore, in a second, and preferred, embodiment of the control apparatus, shown in FIG. 4, the radial position R of the target 11 is controlled instead of the traversing velocity V. Before FIG. 4 is described in detail, the equation necessary to define the radial position R will be developed.

Since

V = dR/dt (9)

the change in radial distance R with respect to time, and

I = dQ/dt (10)

the change in charge Q with respect to time, equation (6) can be rewritten, by substitution and rearrangement, as

R (dR/dt) = [(dQ/dt)/2.pi.Dq] (11)

Cancelling the dt term and integrating gives

.intg.RdR = 1/2.pi.Dq .intg. dQ (12) R.sup.2 = Q/.pi.Dq + C (13)

where C is the constant of integration.

The radial distance R can, therefore, be determined as a function of the charge Q, which is the time integral of the current I in the ion beam 13. If R is assumed to have an initial value R.sub.I when Q is zero, then

R.sub.I.sup.2 = Q + C (14)

and equation (13) can be rewritten

R.sup.2 = Q/.pi.Dq + R.sub.I.sup.2 (15)

in the second embodiment of the control apparatus shown in FIG. 4, carriage 32 is linked to a non-linear potentiometer 50 as indicated by the dotted line 51. The non-linear potentiometer 50 is tapered to provide a signal on a lead 52 which is proportional to the square of the radial distance R. A current integrator 53 is connected between the slip-ring 37 and the high potential source 17.

A pulse output from the current integrator 53 is connected by a lead 54 to the "count" input of a dose counter 55. The counting elements of the dose counter 55 are connected by a cable 56 to the inputs of a digital-to-analog converter 57. The analog output of the digital-to-analog converter 57 is connected by a lead 60 to a dose-setting potentiometer 61. A potential source 62 is connected to a tap of the potentiometer 61 and by a lead 63 to a summing junction 64. The lead 52 from the potentiometer 50 is also connected to the summing junction 64. The output from the summing junction 64 is connected to an amplifier 65, which is further connected by a lead 66 to the motor 36.

A limit switch 70, which is operated when the carriage 32 is positioned so that the radial distance R is a minimum R.sub.I, is connected by a lead 71 to the "reset" input of a bistable circuit 72, and to a first input of an OR-gate 73. A limit switch 74, which is operated when the carriage 32 is positioned so that the radial distance R is maximum, is connected by a lead 75 to the "set" input of the bistable circuit 62 and to a second input of the OR-gate 73. the 0 output of the bistable circuit 72 is connected by a lead 76 to an "up-count enable" input of the dose counter 55. The 1 output of the bistable circuit is connected by a lead 77 to a "down-count enable" input of the dose counter 55. The output of the OR-gate 73 is connected by a lead 80 to the "count" input of a traversal counter 81.

The current integrator 53 generates pulses on the lead 54 which correspond to units of charge passing therethrough. A typical commercial instrument, which can be used for the current integrator 53, is a Brookhaven Instruments Model 1000 Current Integrator. At a typical range setting, this instrument emits pulses at a rate of 833 pulses per second to represent a current I of 500 microamperes. The dose counter is typically a 12-bit binary counter accommodating a maximum count of 4096 pulses. The traversal counter 81 can be any convenient counting instrument. The remaining electrical elements shown in FIG. 4 are well known to a skilled practitioner.

The motor 36, the potentiometer 50, the summing junction 64, and the amplifier 65 comprise a well-known servo-positioning system in which the desired position of the carriage 32 is applied as a command signal to the summing junction 64 on the lead 63. Because the signal on the lead 52 is proportional to the square of the radial distance R, the command signal must also be proportional to the square of the radial distance R. The use of the non-linear potentiometer 50 is merely a convenient method of obtaining the second-order relationship indicated in equation (15). A skilled practitioner could accomplish the same result by using a linear position feedback means in place of the non-linear potentiometer 50, and by performing the necessary square-root calculation by other well-known analog or digital means.

The analog signal LQ' on the lead 60 represents the charge Q which has passed through the current integrator 53 during a traversal. The potentiometer 61 divides the signal LQ' by D', where D' represents the ion dosage D and is determined by the setting of the potentiometer 61. The magnitude of the potential source 62 is (R'.sub.I).sup.2, determined from the minimum value R.sub.I of the radial distance R. When the servo-positioning system is balanced, the signals at the summing junction add so that

LQ/D' + (R'.sub.I).sup.2 - (R').sup.2 = 0 (16)

which can be rearranged

(R').sup.2 = LQ'/D' +(R'.sub.I).sup.2 (17)

which is analogous to the equation (15) when

L = 1/.pi.q (18)

Before ion implantation is started, the carriage 32 is moved to an initial position in which the limit switch 70 is operated, and in which the radial distance R is R.sub.I ; the dose counter 55 and the traversal counter 81 are initialized at zero; and the bistable circuit 72 is reset, to activate the 0 output thereof and to enable the dose counter 55 as an up-counter. To begin implantation, the motor 32 is powered to rotate the target 11; and, when the target 11 is rotating at the proper speed, the ion beam 13 is energized.

As ions are implanted in the workpieces 10, the current I flows, and the current intergrator 53 generates pulses which increment the count in the dose counter 55. The digital-to-analog converter 57 generates an analog signal proportionate to the count in the dose counter 55, and applies the analog signal to the potentiometer 61. The voltage across the potential source 62 adds to the output signal from the potentiometer 61 to result in the command signal on the lead 63. The summing junction 63, the amplifier 64, the motor 36, and the potentiometer 50 co-operate to move the carriage 32 so that the radial distance R is proportionate to the square root of the magnitude of the command signal on the lead 63.

When the carriage 32 operates the limit switch 74, a signal on the lead 75 sets the bistable circuit 72 which activates the 1 output thereof, and enables the dose counter 55 as a down-counter. Further pulses from the current integrator 53 then decrement the count in the dose counter 55.

As the count decreases, the signals on the leads 60 and 63 decrease proportionately, and the servo-positioning apparatus returns the carriage 32 toward the initial position.

When the count in the dose counter again reaches zero, the carriage 32 again operates the limit switch 70, and the signal on the lead 71 again resets the bistable circuit 72, re-enabling the dose counter 55 as an up-counter. The above-described traversal cycle is repeated until a desired number of traversals has been completed.

When the carriage 32 operates either the limit switch 70 or the limit switch 74, the resulting signal on either the lead 71 or the lead 75 increments the traversal counter 81 through the OR-gate 73. Thus, the traversal counter 81 counts completed traversals of the carriage 32. Apparatus (not shown) can be provided to terminate ion implantation when the count in the traversal counter 81 reaches a preset value.

The invention described above is disclosed in the context of using an ion beam to treat semiconductor workpieces. However, the invention is not limited to use with ion beams. Other electrically or magnetically propelled particle beams, such as electron beams, charged droplet beams, or the like; gas propelled beams, such as abrasive particle beams, sprayed-coating beams, or the like; or radiant energy beams, such as X-ray beams, light beams, infrared beams, or the like, can be used with the invention if it is desired to controllably treat workpieces therewith.

One skilled in the art may make other changes and modifications to the embodiments of the invention disclosed herein, and may devise other embodiments thereof, without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. A method of controllably treating a workpiece with a radiation beam, the workpiece being mounted on the major surface of a target located in the path of the beam, which comprises the steps of:

rotating the target about an axis essentially parallel to the beam,

traversing the target along a path substantially perpendicular to the beam to trace a spiral path on the target and the workpiece with the beam, and

varying the traversing velocity as a function of the distance between said axis and the beam.

2. A method of uniformly treating a workpiece with a particulate beam, the workpiece being mounted on a major surface of a target located in the path of the beam, which comprises the steps of:

rotating the target about an axis essentially parallel to the beam,

traversing the target along a path substantially perpendicular to the beam to trace a spiral path on the target and the workpiece with the beam, and

varying the traversing velocity to be inversely proportional to the distance between said axis and the beam.

3. The method according to claim 2 in which said varying step further comprises:

simultaneously varying the traversing velocity to be inversely proportional to a desired particle dose in the workpiece.

4. The method according to claim 2 in which said varying step further comprises:

simultaneously varying the traversing velocity to be directly proportional to the rate of particle flow in the beam.

5. A method of implanting a uniform ion dose from an ion beam into a workpiece, the workpiece being mounted on a major surface of a target located in the path of the ion beam, which comprises the steps of:

rotating the target about an axis essentially parallel to the ion beam,

traversing the target along a path substantially perpendicular to the ion beam to trace a spiral path on the target and the workpiece with the ion beam, and

varying the traversing velocity to be inversely proportional to the distance between said axis and the ion beam.

6. The method according to claim 5 in which said varying step further comprises:

simultaneously varying the traversing velocity to be inversely proportional to a desired ion dose in the workpiece.

7. The method according to claim 5 in which said varying step further comprises:

simultaneously varying the traversing velocity to be directly proportional to the current in the ion beam.

8. A method of implanting a uniform ion dose from an ion beam into a workpiece, the workpiece being mounted on a major surface of a target located in the path of the ion beam, which comprises the steps of:

rotating the target about an axis essentially parallel to the ion beam;

traversing the target along a path substantially perpendicular to the ion beam at a velocity

V = K I/DR

where:

I is the ion beam current,

D is the desired ion dose in the workpiece,

R is the distance between the ion beam and said axis, and

K is a constant of proportionality.

9. Apparatus for moving a workpiece with respect to a radiation beam to controllably treat the workpiece with the beam, the workpiece being mounted on a major surface of a target located in the path of the beam, comprising:

means for rotating said target about an axis essentially parallel to the beam, and

means for traversing the target along a path substantially perpendicular to the beam at a velocity which varies as a function of the distance between the beam and said axis.

10. Apparatus for moving a workpiece with respect to a particle beam to uniformly treat the workpiece with particles from the particle beam, the workpiece being mounted on a major surface of a target located in the path of the particle beam, comprising:

means for rotating said target about an axis essentially parallel to the particle beam, and

means for traversing the target along a path substantially perpendicular to the particle beam at a velocity inversely proportional to the distance between the particle beam and said axis.

11. Apparatus for moving a workpiece with respect to an ion beam to uniformly implant the workpiece with ions from the ion beam, the workpiece being mounted on a major surface of a target located in the path of the ion beam, comprising:

means for rotating said target about an axis essentially parallel to the ion beam; and

means for traversing said target along a path substantially perpendicular to the ion beam at a velocity

V = K I/DR

where:

I is the current in the ion beam,

D is the desired ion dose in the workpiece,

R is the distance between the ion beam and said axis, and

K is a constant of proportionality.

12. A method of uniformly treating a workpiece with an ion beam, the workpiece being mounted on the major surface of a target which is located in the path of the ion beam, which comprises the steps of:

rotating the target about an axis essentially parallel to the ion beam;

traversing the target along a path substantially perpendicular to the ion beam to trace a spiral path on the target and the workpiece with the ion beam;

accumulating in a counting means a count which is directly proportional to the number of ions implanted into said workpiece during said traversing step; and

during said traversing step, maintaining the distance between the ion beam and said axis at a value which is a direct function of said accumulated count.

13. The method according to claim 12 in which said accumulating step further comprises:

generating a plurality of pulses, the repetition rate of which is directly proportional to the magnitude of the current in the ion beam; and

counting said pulses to accumulate said proportional count.

14. The method according to claim 12 which further comprises:

maintaining said distance at a value which is also a function of a desired ion dose in the workpiece.

means for rotating the target about an axis essentially parallel to the ion beam;

means for traversing said target along a path substantially perpendicular to the ion beam; and

means, coupled to said traversing means, for maintaining a distance between the ion beam and said axis according to the equation

R.sup.2 = L Q/D + C

where:

R is said distance,

Q is the time integral of the current in the ion beam,

D is the desired ion dose in the workpiece,

L is a constant of proportionality, and

C is a constant relating to an initial value of said distance.

15. A method of implanting a uniform ion dose from a ion beam into a workpiece, the workpiece being mounted on a major surface of a target which is located in the path of the ion beam, which comprises the steps of:

rotating the target about an axis essentially parallel to the ion beam;

traversing the target along a path substantially perpendicular to the ion beam to trace a spiral path on the target and the workpiece with the ion beam;

during said traversing step, maintaining a distance between the ion beam and said axis according to the equation

R.sup.2 = L Q/D + C

where:

R is said distance,

Q is the time integral of the current in the ion beam,

D is the desired ion dose in the workpiece,

L is a constant of proportionality, and

C is a constant relating to an initial value of said distance.

16. Apparatus for moving a workpiece with respect to an ion beam to uniformly implant the workpiece with ions from the beam, the workpiece being mounted on a major surface of a target which is located in the path of the ion beam, comprising:

means for rotating the target about an axis essentially parallel to the ion beam;

means for traversing said target along a path substantially perpendicular to the ion beam; and

means, coupled to said traversing means, for maintaining a distance between the ion beam and said axis according to the equation

R.sup.2 = L Q/D + C

where:

R is said distance,

Q is the time integral of the current in the ion beam,

D is the desired ion dose in the workpiece,

L is a constant of proportionality, and

C is a constant relating to an initial value of said distance.

17. The apparatus according to claim 16 in which said traversing means further comprises:

means for detecting when the target is traversed to preselected limits; and

means, responsive to said detecting means, for reversing the direction of said traversing means.

18. The apparatus according to claim 16 in which said maintaining means further comprises:

current integrating means for generating a series of pulses, the repetition rate of which is proportional to the magnitude of the ion beam current;

means for counting said pulses;

a digital-to-analog converter for generating an analog signal proportional to the count in said counting means;

analog dividing means for reducing the magnitude of said analog signal by a predetermined ratio;

a non-linear potentiometer, coupled to said traversing means, to generate a position signal representative of the square of said distance; and

means, responsive to the difference between said reduced analog signal and said position signal for operating said traversing means.

19. The apparatus according to claim 18 in which said counting means further comprises:

a bidirectional counter;

bistable means for conditioning said bidirectional means to function as an up-counter or as a down-counter;

first detecting means, activated when the target is traversed to a limit of travel at which the count in said bidirectional counter is a minimum, and connected to said bistable means to condition said bidirectional counter as an up-counter; and

second detecting means, activated when the target is traversed to a limit of travel at which the count in said bidirectional counter is a maximum, and connected to said bistable means to condition said bidirectional counter as a down-counter.

20. A method of controlling the exposure of a workpiece to a radiation beam, the workpiece being mounted on a target which is located in the path of the beam, comprising the steps of:

rotating the target about an axis substantially parallel to the beam; while simultaneously

moving the target, relative to the beam, along an axis transverse to said beam to trace a spiral path on the target with the beam; while simultaneously

varying the pitch of said spiral path as a function of the distance between said axis and the beam to uniformly expose the workpiece to the beam.

21. A method of imparting a uniform dose of ions to a workpiece which comprises the steps of:

directing a beam of ions onto the workpiece,

establishing relative motion between said beam and the workpiece to scan the workpiece with a plurality of spiral beam-trace segments having differing scanning velocities, and

varying the spacing between said beam-trace segments to compensate for the differing scanning velocities to regulate the uniformity of the ion dose.

22. A method of uniformly treating a workpiece with a radiation beam, the workpiece being mounted on a major surface of a target located in the path of the radiation beam, which comprises the steps of:

rotating the target about an axis,

traversing the target across the radiation beam to trace a spiral path with the radiation beam on the target and the workpiece, and

varying the traversing velocity as a function of the distance between the point where the radiation beam impinges on the target and the axis of rotation.