Patent Number: 050013513
Section: summary

The invention relates to an object holder for positioning an object in a radiation beam, comprising an X-Y translation mechanism. In known object holders an X-Y translation mechanism is constructed so that an X-transporter for translation in an X-direction carries a carriage for translation across the X-transporter in an Y-direction, the X-transporter acting as a reference and supporting face for the Y-transporter or vice versa. This results in a comparatively complex translation mechanism where errors in the second translation constitute a sum of errors in both translation systems and where an undesirable rotary movement and tilting can readily occur during the last translation motion. This is understandable because, considering the desirable low-friction movements, it is difficult to prevent a given play for each of the guides. This is extremely undesirable, however, in object carriers where angular orientation of an object is co-decisive for the accuracy of measurements to be performed. It is the object of the invention to mitigate these drawbacks. In accordance with the present invention each of two translatory movements separately refers directly to a supporting face of a supporting plate in a translation mechanism. A simple and inexpensive movement mechanism exhibited extremely accurate positioning, notably because of a force coupling, which minimizes undesirable rotation and tilting. In a preferred embodiment in accordance with the invention the pressure exerted towards the supporting plate is realized by a magnetic force of attraction, by means of permanent magnets, for example, which are coupled to an X-transporter and to a support for the object table, which magnets preferably bear on the supporting face of the supporting plate via an intermediate spacer which are preferably ductile to some extent. In addition to a smooth sliding contact, a desirable air gap is thus also achieved. In a further preferred embodiment, a rotation of the object table is added to the X-Y translation; this is comparatively simply achieved by mounting the supporting plate so as to be rotatable about an axis extending transversely of its supporting face. Driving can be realized by mounting a motor-driven rotation mechanism on a rear side of the supporting plate (with respect to the object table), the axis of rotation preferably extending through the centre of a substantially circular supporting plate. An object tilting mechanism for tilting the supporting face of the supporting plate can also be added to the supporting plate. If the angles of tilt need not be very large, for example no more than approximately plus and minus 10.degree., use can be made of a shaft in the form of a cylindrical bush which is partly open in the lateral direction, so that at that area a free passage is realized for a radiation beam extending parallel to the supporting plate.