Patent Number: 
Section: claims

1. A substrate holder assembly for holding substrates to be exposed to an ion beam during implantation in an ion implanter, the substrate holder assembly comprising:a base rotatable about a first axis of rotation;at least two support arms extending substantially in parallel with said first axis of rotation from the base to ends provided with substrate holders;wherein rotation of the base allows the substrate holders to adopt designated positions, with the at least two support arms extending from points on the base displaced from the first axis of rotation by substantially equal distances and separated by a substantially equal separation angle such that rotation of the base through the separation angle causes the support arms to move between designated positions. 2. The substrate holder assembly of claim 1, wherein the at least two support arms are rotatable about their longitudinal axes. 3. The substrate holder assembly of claim 1, wherein the substrate holders are each provided with a support surface for supporting the substrate, the support surface being rotatable about its centre axis. 4. The substrate holder assembly of claim 3, wherein the support surface's centre axis is substantially normal to the support arm's longitudinal axis. 5. The substrate holder assembly of claim 1, wherein the at least two support arms are moveable along their longitudinal axes such that the distance of each substrate holder from the base may be varied. 6. An ion implanter comprising an ion source, optics operable to guide ions produced by the ion source along an ion beam path to a process chamber for implantation in a substrate, a substrate transfer mechanism, and the substrate holder assembly of claim 1 positioned to hold substrates in the process chamber such that the at least two support arms extend substantially normal to the ion beam path; and wherein a first of the designated positions corresponds to the substrate holder being disposed to face into the ion beam, and a second of the designated positions corresponds to the substrate holder being disposed to be clear of the ion beam path and to co-operate with the substrate transfer mechanism thereby allowing substrates to be placed on and removed from the substrate holder. 7. The ion implanter of claim 6, wherein the at least two support arms are rotatable about their longitudinal axes. 8. The ion implanter of claim 6, wherein the substrate holders are each provided with a support surface for supporting the substrate, the support surface being rotatable about its centre axis. 9. The ion implanter of claim 8, wherein the support surface's centre axis is substantially normal to the support arm's longitudinal axis. 10. The ion implanter of claim 6, wherein the at least two support arms are moveable along their longitudinal axes such that the distance of each substrate holder from the base may be varied. 11. The ion implanter of claim 10, wherein the base has an associated scanning unit that is operable to scan the support arm back and forth along its longitudinal axis. 12. The ion implanter of claim 11, wherein the scanning unit is operable to scan the support arm in a direction substantially normal to both the longitudinal axis of the support arm and the ion beam path. 13. The ion implanter of claim 12, wherein the scanning unit is operable to scan the support arm such that the ion beam traces a raster pattern across a substrate when held by the substrate holder.