Treating work pieces with electro-magnetically scanned ion beams

Abstract

A magnetic scanning technique for sweeping an ion beam across an implantation target, such as a semiconductor wafer, by means of modulating the energy of a beam and directing it through an analyzer magnet, which effects a scanning motion of the beam of constant intensity, the wave form for the modulation being selected to take into account that the areal density of the ions in the scanned beam varies dependently with the amount of displacement of the beam from a reference point. An ion scan can be obtained in which the ions travel in parallel paths and enter the target at a constant desired angle throughout the scan. The technique is applicable to targets held stationary or rotated during implant. By employing predetermined modulation wave forms which are adapted to other parameters of the selected system, a desired scan distribution of ions can be obtained, for instance a uniform distribution in X and Y directions. As applied to a semiconductor wafer rotated on a disc past the ion beam, the technique solves the problem of compensating for the fact that the area of a ring on the spinning disc depends linearly on the radius of the ring. The technique makes use of the properties of ion beams in uniform magnetic fields to produce a radial dependence of the density of the ion beam on the wafer which precisely cancels the radial dependence of circumferential length as a function of radius. The magnetic scanning technique is well adapted for use with large wafers held on rotating discs and has advantages over the conventional techniques of either electrostatic or mechanical scanning. The technique is readily adapted to a variety of other wafer transport systems including rotating conveyors and linear transports.