| 摘要 |
PURPOSE: To increase a mechanical strength by making a wall thickness larger at the portion where the bending moment of a wafer board holding portion becomes the largest and its neighboring portion over a particular portion of the overall length of the wafer board holding portion, and also giving essentially the same thickness in the potion other than said particular portion. CONSTITUTION: A thick wall portion 3 is formed at A (a boundary between a straight line and a circular arc of a notched portion having a longitudinal cross section with a circular arc shape and a straight line shape) where the bending moment becomes the largest in a wafer board holding portion 2 and its vicinity. Also, the length of the thick wall portion is set to about 1/4 to 1/2 of the overall length of the wafer board holding portion and located toward the tip portion from the vicinity of the portion A where the maximum bending moment occurs. As stated above, the portion where the bending moment of the wafer board holding portion 2 becomes the largest and its vicinity have a larger wall thickness compared to other portions, so that any breakage and bending of the wafer board holding portion 2 can be prevented and moreover deflection at the tip of the wafer board holding portion 2 can be decreased. |
