Patent Number: 
Section: claims

1. A method of scanning a substrate through an ion beam in an ion implanter, comprising:causing relative motion between the substrate and the ion beam; androtating the substrate substantially about its centre while causing the relative motion, such that the ion beam would pass over all of the substrate even if the substrate were not rotating, the relative motion between the substrate and the ion beam being at a constant speed. 2. The method of claim 1, comprising rotating the substrate substantially about its centre at a constant angular velocity. 3. The method of claim 1, wherein the ion beam is a ribbon beam, and the method comprises causing a relative motion between the substrate and the ribbon beam such that all of the substrate passes through the ribbon beam in a single pass. 4. The method of claim 3, comprising rotating the substrate such that it performs at least a complete revolution during the pass. 5. The method of claim 4, comprising rotating the substrate such that it performs at least fifteen to twenty revolutions during the pass. 6. The method of claim 1, wherein the ion beam is a spot ion beam, the method comprising causing the relative motion between the substrate and the ion beam such that the ion beam passes over all of the substrate by causing a series of translations of the substrate relative to the ion beam such that the ion beam traces a series of scan lines over the substrate. 7. The method of claim 6, comprising causing the translations to form the series of scan lines to be parallel or substantially parallel. 8. The method of claim 7, comprising causing the translations to form a raster pattern or saw-tooth pattern of scan lines. 9. The method of claim 7, comprising causing the translations to form a series of overlapping scan lines, such that the edges of the path of the ion beam across the substrate at the start of adjacent scan lines overlap. 10. The method of claim 7, comprising forming the scan lines to be spaced with a pitch P and wherein the ion beam has a dimension D in the direction of the pitch spacing, such that P is greater than 0.9 D. 11. The method of claim 7, comprising forming the scan lines to be spaced with a pitch P and wherein the ion beam has a dimension D in the direction of the pitch spacing, such that D is substantially equal to P. 12. The method of claim 6, comprising rotating the substrate such that it performs at least a complete revolution as the ion beam scans across each scan line. 13. The method of claim 12, comprising rotating the substrate such that it performs at least fifteen to twenty revolutions as the ion beam scans across each scan line. 14. The method of claim 12, comprising rotating the substrate and/or causing the relative motion between the substrate and the ion beam such that the resulting spirals traced by the ion beam over the substrate overlap on adjacent revolutions. 15. The method of claim 1, comprising causing relative motion between the substrate and the ion beam to form a scan line such that a point in the ion beam, having an average value of the total ion beam current along a section taken through the ion beam orthogonal to the direction of relative motion, passes over the centre of the substrate. 16. An ion implanter controller arranged to perform the method of claim 1. 17. An ion implanter comprising the controller of claim 16. 18. A computer program stored on a computer-readable medium and comprising computer program instructions that, when executed by a computer in an ion implanter, cause the ion implanter to perform the method of claim 1. 19. A computer readable medium carrying thereon a computer program according to claim 18.