Direct drives incapable of raising objects with nanometre-accuracy above large areas

Direct drives can position objects with high speed and precision, backlash-free and without any intervening gears or coupling. One of their uses is in semiconductor manufacture, for instance the manouevring of reticles. A positioning stage is usually part of the equipment for measuring and structuring substrates, wafers, reticles and the like. The slider of this stage moves objects with extreme accuracy on one plane. In production, this already happens with an accuracy of less than 10 nanometres.

But the manufacturing is carried out on different levels. Here an example is the structuring of wafer layers. Such layers are often as thin as a few nanometres; an entire wafer may be between 0.7 and 0.9 mm thick. Presently available systems of wafer measurement only achieve the extremely low level of measurement uncertainty over a mere few square millimetres. Across their full area, for whole wafers with diameter 150 to 300 mm the typical measurement uncertainty is only in the ± 0.5 to ± 1 micrometre range.

Solution to the problem: an integrated 6DoF direct drive with pneumatic gravity compensators for the lifting

In joint work with SIOS Meßtechnik GmbH and Ilmenau University of Technology, IMMS has developed a 6-dimensional direct drive with air bearing which can move objects along three spatial axes and freely in space around these axes in an area of travel appropriate to the application: planar diameter of 100 mm, ascent or descent range of 10 mm under active control with nanometre-accuracy. Pneumatic gravity compensation assists the raising and lowering by constantly approximating to zero the power to be supplied by the vertical electromagnetic actuators. As a result there is next to no current flowing in the actuator coils, which means there is no unwanted source of heat in the measuring space to interfere with the necessary precision of measurement.

The principle behind this drive is absolutely new in the field of six-dimensional direct drives and has been applied in no known drive system to date. Investigations have already been carried out into its scalability for other applications with a planar field diameter of 200 mm.