An apparatus in combination with a load lock of an ion implanter comprises a cover adjacent an isolation valve slot of the load lock. The cover defines an aperture generally conforming to the size and shape of the load, or wafer, within the load lock with sufficient clearance for a robot arm to pick the wafer from within the load lock and transfer the wafer to the implant chamber. The cover masks a portion of the slot so as to reduce the opening between the load lock and the implant chamber of the ion implanter. The smaller opening reduces the pressure burst from the load lock to the implant chamber when the isolation valve and slot is opened. By reducing the pressure burst, the cover can shorten the recovery time for the implant chamber to reach operating pressure.