Patent Number: 048048526
Section: summary

BACKGROUND OF THE INVENTION The doping of electrically active elements into semiconductors is at this time done almost exclusively by injecting the elements into the semiconductor material by means of instruments known as ion implanters. Ion implanters create highly controlled beams of the desired ions and drive these ions at relatively high energy into the semiconductor wafers so as to dope the wafers in a uniform manner, at a controlled depth and temperature, and with well-regulated dose strengths. The functional components of ion implanters are: a source, called the ion source, of the element to be implanted; an acceleration and control section whose function is to produce a beam of the homogeneous ions having the desired energy and intensity profile; a target system which holds the semi-conductor wafers which will be implanted; and a wafer handling system which loads fresh wafers into the target system and unloads the implanted ones. The wafers are generally much larger than the beam size so that it is necessary to also have a scanning system which either scans the beam over the wafers or mechanically scans the wafers across the beam. The present invention is directed towards the scanning function of the ion implantation system. It specifically concerns the technique for scanning the ion beam across wafers which are rotated or otherwise conveyed in front of the ion beam. Some early proposals for scanning are given in Freeman, U.S. Pat. No. 3,689,766. There the beam is scanned by varying the energy of the beam and passing it through a magnetic field. Reduction in variation of the ion beam intensity was to be achieved by sensing the ion beam current and feeding back that signal to control the amplitude of the sweep voltage applied to the beam. The scanning of the beam across the semi-conductor wafer has continued to be of concern throughout the evolution of the modern ion implanter. The problems of scanning get more severe as the wafers get larger. The 3 inch diameter wafers of a decade ago have been successively replaced with 4 inch, 5 inch, and 6 inch wafers; 8 inch diameter wafers are now becoming standard. A common requirement is to maintain spatial uniformity of the implanted ions over large surfaces even as the criteria for uniformity become more stringent; specificaions of &lt;1% non-uniformities of implant dose over the wafer area are now common. Examples of present day scanning techniques are given in Ryssel and Glawischnig, Ion Implantation Techniques, Springer-Verlag, pp. 1-20 (1982), see especially pages 4 and 9. Spinning Disc Scanning To implant wafers, manufacturers have adopted the rotating disc holding system in which the semiconductor wafers are held on the face of a large disc which is rotated in front of the beam. When, as is normally the case, the beam is much smaller than the diamer of the wafers, the beam must be scanned orthogonally to the circular motion, that is, there must be a scan across the wafer in the radial direction of the disc. A central problem which must then be overcome to produce a spatially uniform implant over the face of the wafer is that, in each revolution of the disc, the area of the wafer furthest from the center of the rotating disc is swept more rapidly than the area closest to the center of rotation. To obtain a uniform implant the scanning must compensate for this radial dependence. Radial scanning can be done by electromagnetic or mechanical techniques. A parallel beam, i.e. with rays having a constant angle, is recognized as a necessary feature for uniform implantaton dose density, or to avoid "channeling". Magnetic scanning of the wafers, for instance, as shown in Ion Implantation Techniques, supra, P. 9 and in Enge, U.S. Pat. No. 4,276,477, has involved a number of drawbacks and complexities to achieve a parallel beam. Mechanical scanning of the wafers across a stationary beam maintains the constancy in angle between the beam and the wafer-face in a natural way. Several techniques have been used to correct for the radial dependence of the circumferential length when the wafers are mounted on a spinning disk. When magnetic scanning is employed, a separate microcomputer has been used to compensate for radial deviaions, Ion Implantation Techniques, supra, p. 10. For mechanical scanning, Robertson, U.S. Pat. No. 3,778,626, varies the radial velocity in inverse proportion to measured radial position of the wafer with respect to the beam. Ryding, U.S. Pat. No. 4,234,797, by means of a slot in the rotating disc, samples the charge per unit area deposited by the beam in each revolution, and uses the sample values to vary the radial velocity of the disc so as to maintain a contant charge per unit area over the entire scan. Mechanical scanning of the rotating disc across the beam is, by its nature, relatively slow compared to electromagnetic scanning. And the technical difficulties associated with mechanical scanning generally increase as the wafer and hence the wafer-holding disc increase in size. Electromagnetic Scanning Electromagnetic scanning whether for radial scanning for a spinning disc system or for x or y scanning of a raster can be done much more rapidly than mechanical scanning, allowing more back-forth motions per implant. In principle, this has several advantages over mechanical scanning, but as presently practiced also has serious drawbacks. Among the advantages in respect of spinning disc arrangements are first, of course, one eliminates the need for the precision mechanical system needed for the highly controlled radial scan; second, the rapid radial scanning reduces the problems of wafer heating; third, the ability to control the rate of radial scan over a wide range of times makes practical the use of the full intensity of the beam over a wide range of total implant dose, without compromising the uniformity of the dose density. However, in the electrostatic or magnetic methods used until now, the problems in controlling the beam over the full scan get rapidly more severe as the wafers get larger; in particular, it becomes increasingly difficult to maintain a constant angle entry of the beam into the wafer. SUMMARY OF THE INVENTION The present invention uses electromagnetic scanning with an analyzer magnet to produce a scanned beam which enters the wafers at a fixed angle, independent of the size of the wafer. The technique can be employed to produce the proper radial dependence of ion dose to compensate for radial changes of circumferential area in a spinning disc system, and has advantages when applied to other techniques for scanning wafers. An ion implanter is provided in which an ion beam is electro-magnetically scanned across an implantation target by first modulating the energy of the beam in accordance with a predetermined, cyclical wave form, and passing the thus-modulated beam through an analyzer magnet and thence to the target. According to a broad aspect of the invention, under conditions of operation in which the intensity of the ion beam is unchanging, the wave form of the modulator voltage is adapted to take into account the non-linear relationship between the deflection of the beam by the magnetic field and the beam modulation energy of the beam, so as to produce a uniform areal distribution of the number of ions implanted into the target. Where a uniform magnetic field of constant magnitude is employed, acting upon an energy-modulated beam of constant intensity, the wave form is selected to take into account that the areal density of the ions in the scanned beam is directly proportional to the amount of the displacement. In embodiments with ions of uniform energy before modulation and directed normally to the entrance face of the magnet, the scanned ions can emerge from the exit face in parallel paths without correction, and can enter the target at a constant angle over the sweep of the beam. In embodiments employing a spinning disc, the direction of the scanning motion achieved with the analyzer magnet is arranged radially with respect to the disc to uniquely achieve compensation for radial changes of circumferential area. In embodiments employing a pair of analyzer magnets set orthogonally to one another, the beam energy modulation techniques can achieve a uniform X-Y scan.