Ion implantation has become a standard, commercially-accepted technique for introducing conductivity-altering impurities into semiconductor wafers.  The desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the semiconductor wafer.  The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded in the crystalline lattice of the semiconductor material to form a region of desired conductivity. 
In commercial ion implanters, several factors are crucial in achieving an effective system.  One crucial factor is throughput in terms of wafers processed per unit time.  Wafer transfer time, implant time and downtime all contribute to the total processing time.  In order to reduce the implant time, efforts are constantly made to increase the ion beam current applied to the semiconductor wafer while maintaining the wafer below a prescribed maximum temperature.  Other crucial factors in ion implantation include dose accuracy and dose uniformity over the surface of the wafer, since semiconductor devices fabricated by ion implantation must have controlled and repeatable operating characteristics.  Furthermore, minimizing particulate contamination is extremely important, since semiconductor devices with microminiature features are extremely susceptible to damage by such contamination. 
Ion implanters have generally fallen into two major categories, serial and batch.  In serial systems, wafers are processed one at a time.  Since the wafer is continuously in the ion beam during implantation, the maximum ion beam current is limited.  In batch systems, a number of semiconductor wafers are typically mounted on wafer mounting sites in an annular region near the periphery of a disk.  The disk intercepts the ion beam in a small area of the annular region, and the disk is rotated so that all of the wafers successively intercept the ion beam.  Since each individual wafer intercepts the ion beam for only a fraction of the disk revolution time, the average power applied to each wafer is relatively low.  As a result, significantly higher ion beam currents can be utilized in batch systems than in serial systems.  Since the ion beam is typically smaller in cross-section than the surface area of a wafer, it is necessary to provide additional movement, either of the rotating disk or of the beam, in order to uniformly distribute ions over the wafer surface. 
In one prior art batch ion implanter, the ion beam is magnetically scanned in one direction, and disk rotation provides movement in a second direction.  Such a system is disclosed in Enge U.S. Pat.  No.  4,276,477 issued June 30, 1981.  One disadvantage of magnetic beam scanning is that the required scan magnets are large and heavy. 
A second approach to batch ion implantation has been to utilize a stationary ion beam and mechanical scanning of the wafers in two dimensions.  However, two-dimensional mechanical scanning in vacuum is difficult because the drive mechanisms are preferably located outside the vacuum chamber for operational reasons and to prevent contamination generated by moving mechanical parts.  A two-dimensional mechanical scan system for ion implantation is disclosed in Ardnt, Jr. et al.  U.S. Pat.  No.  3,983,402 issued Sept. 28, 1976.  The disclosed system utilizes a pair of bellows for transmitting reciprocating motion into the vacuum chamber.  Another prior art system that utilizes a rotating disk and a sliding seal, or linear air bearing, to provide reciprocating motion of the rotating disk relative to the ion beam is described in Ryding U.S. Pat.  No.  4,229,655 issued Oct. 21, 1980.  Still another prior system for providing two-dimensional motion is described in European Patent Application No.  178,803 published Apr. 23, 1986.  A rotating disk is carried on an arm that precesses back and forth in an arc within the vacuum chamber. 
A further complication in batch ion implantation systems is that provision must be made for loading wafers onto the disk prior to implantation and for removing wafers from the disk after implantation is complete.  Wafer exchange is preferably performed automatically in a manner which reduces the possibility of particulate contamination to a minimum.  In one wafer loading technique, the disk is flipped or pivoted from a more-or-less vertical orientation during implantation to a horizontal orientation for loading and unloading of wafers.  This technique is utilized in the Varian Model 160-10 Ion Implanter and is shown in U.S. Pat.  Nos.  4,276,477; 3,983,402; and 4,229,655.  In another technique disclosed in European Patent Application No.  178,803, the disk is maintained in the implant position during loading and unloading of wafers. 
It is desirable during ion implantation of semiconductor wafers to control the angle of incidence of the ion beam on the wafer surface, since the ion penetration depth is a function of the angle-of-incidence (implant angle).  The variation in penetration depth with angle-of-incidence, commonly known as "channeling," depends on the orientation of the crystal axes of the semiconductor wafer relative to the ion beam.  Channeling depends not only on the angle-of-incidence, but also the crystal structure of the semiconductor wafer.  To control channeling, it is customary to utilize a prescribed implant angle for a given crystal structure and crystal orientation.  It is therefore necessary that ion implantation systems have the capability to vary the implant angle.  Tapered heat sink inserts positioned underneath each wafer are disclosed in European Pat.  application No.  178,803.  In order to alter the angle of implant, it is necessary to vent the vacuum chamber to atmosphere, to change the heat sink insert at each wafer location and then to vacuum pump the chamber back to high vacuum. 
Prior art systems have utilized both peripheral clamping and centrifugal clamping of wafers during ion implantation.  Peripheral clamping with a clamping ring is generally satisfactory, but an annular portion of the wafer near its peripheral edge is shadowed by the clamping ring, thus rendering that portion of the wafer useless.  Centrifugal clamping exposes the entire wafer surface for treatment, but requires that the disk be designed so that sufficient centrifugal force for clamping and wafer cooling is applied to the wafer. 
It is a general object of the present invention to provide improved ion implantation systems. 
It is another object of the present invention to provide improved apparatus for workpiece translation in a vacuum chamber. 
It is another object of the present invention to provide apparatus for workpiece translation in a vacuum chamber with drive mechanisms isolated from the vacuum chamber. 
It is still another object of the present invention to provide apparatus for workpiece translation in two dimensions relative to a stationary beam. 
It is yet another object of the present invention to provide apparatus for two-dimensional workpiece translation that is compatible with a horizontal wafer transfer system. 
It is still another object of the present invention to provide apparatus for workpiece translation in a vacuum chamber wherein the angle-of-incidence of an ion beam on a workpiece can be changed without opening or venting the vacuum chamber. 
It is a further object of the present invention to provide apparatus for workpiece translation in a vacuum chamber wherein the angle-of-incidence of an ion beam on a workpiece can be changed without readjusting the system which transfers workpieces to and from the translation apparatus. 
It is a further object of the present invention to provide apparatus for workpiece translation in a vacuum chamber wherein the translation apparatus can easily be accessed.