Abstract: The angular position of a notch 17 is determined on the basis of data provided by second notch detectors 24a and 24b and an encoder 25. A controller 38 turns a rotational arm 22 for angular displacement such that holders 26 and 27 are displaced in a circumferential direction B from the notch 17, and then the holders 26 and 27 hold a wafer 19. Since the holders 26 and 27 are in touch with edges in which the notch 17 is not formed, the first notch detector 23 is able to detect the notch 17 without difficulty. Thus the position of the wafer 19 relative to the rotational arm 22 does not need to be changed several times depending on positions at which the holders 26 and 27 hold the wafer 19, which is necessary in the prior technology. The aligner 20 touches parts, not including the notch 17, of the wafer 19 and is capable of adjusting the position of the wafer 19 in a short time.

Abstract: A screw pump including a screw assembly in a sleeve, the assembly comprising a drive screw having protruding threaded sides, and multiple driven screws having concave threaded sides which are closely meshed with the drive screw, thus forming a series of sealed chamber. The screw pump further includes one or more channels which are formed in the circumferential direction around the periphery of the driven screws, so that the final sealed chamber adjacent a discharge port of the pump partially communicates with the port through the channels shortly before the chamber completely opens to the port.