A semiconductor wafer, which has chips formed thereon, is moved from a class 100 environment into a vacuum chamber in which pattern writing is performed on the chips by an electron beam without significantly affecting the vacuum level in the vacuum chamber. The wafer is initially disposed on an elevator, which is within the vacuum chamber, through an opening, which is vacuum sealed by the elevator at this time, in the top wall of the vacuum chamber. A lid is then placed over the opening from the exterior to cooperate with the elevator to form an ante chamber which the wafer disposed therein. The ante chamber, which is purged prior to the lid being placed over the opening, is then reduced to a vacuum of 5 .times. 10.sup..sup.-2 torr. The elevator then lowers the wafer into a horizonal plane in which a transfer mechanism is located to transfer the wafer from the elevator to an X-Y table on which the wafer is mounted for pattern writing of the chips. The transfer mechanism includes an arm, which is indexed 180.degree. during each antivation of an indexing means connected thereto, and gripping means at each end of the arm to simultaneously grasp and release wafers on the elevator and X-Y table to interchange the wafers between the elevator and the table.