Ion implanters are well known and generally conform to a common design as follows. An ion source produces a mixed beam of ions from a precursor gas or the like. Only ions of a particular species are usually required for implantation in a substrate, for example a particular dopant for implantation in a semiconductor wafer. The required ions are selected from the mixed ion beam using a mass-analysing magnet in association with a mass-resolving slit. Hence, an ion beam containing almost exclusively the required ion species emerges from the mass-resolving slit to be transported to a process chamber where the ion beam is incident on a substrate held in place in the ion beam path by a substrate holder.
Often, the cross-sectional profile of the ion beam is smaller than the substrate to be implanted. For example, the ion beam may be a ribbon beam smaller than the substrate in one axial direction or a spot beam smaller than the substrate in both axial directions. In order to ensure ion implantation across the whole of the substrate, the ion beam and substrate are moved relative to one another such that the ion beam scans the entire substrate surface. This may be achieved by (a) deflecting the ion beam to scan across the substrate that is held in a fixed position, (b) mechanically moving the substrate whilst keeping the ion beam path fixed or (c) a combination of deflecting the ion beam and moving the substrate. For a spot beam, relative motion is generally effected such that the ion beam traces a raster pattern on the substrate.
Our U.S. Pat. No. 6,956,223 describes an ion implanter of the general design described above.
To achieve a desired implant within the tight tolerances now required by the semiconductor industry requires very good control of the ion beam through the ion implanter, right up to the point of incidence on the wafer being implanted. As a result, it is desirable to know the properties of the ion beam at one or more points along its path through the ion implanter. In particular, it is useful to have a measure of the intensity of the ion beam across its cross-section (often measured as the ion beam current) and the spread of directions of ions in the cross-section of the ion beam, generally known as the emittance. Emittance is a measure of the confusion of the beam, i.e. how much angular variation exists within the ion beam. For an ideal ion beam, the angular variation is minimal and so emittance is low.
Emittance is often represented graphically as shown in FIG. 1. Distance from the centre of the ion beam is plotted as the y co-ordinate on the abscissa and y′, the measured angle from the longitudinal axis, is plotted on the ordinate. FIG. 1 shows a plot for a typical, diverging ion beam that produces a diagonal from lower left to upper right on such a plot. In this graph, the crosses indicate values obtained, i.e. an angle y′ measured for a particular position in the beam y. As can be seen, the crosses generally describe an ellipse. A slice taken vertically through any particular y value provides the range of angles of ions at that particular position in the ion beam. Hence, the wider the ellipse, the larger the spread in angles and so the greater the confusion in the ion beam.
It is also useful to measure not only angles at different positions in the ion beam, but also the intensity that angle contributes. Such measurements allow contour plots like that shown in FIG. 2 to be realised, which provide a convenient presentation of detailed information relating to the emittance of the ion beam. FIG. 2 shows a diverging ion beam with most of its intensity at its centre. Hence, the contours that indicate intensity define an elongate hill that extends from lower left to upper right.