A method for controlling wafer uniformity in a polishing tool includes providing a plurality of carrier heads, determining a signature for each of the carrier heads, and installing carrier heads with similar signatures in a polishing tool. The at least one shut-off flap may selectively allow the product to be dispensed from the outlet orifice. Once the received reflected power rises or falls to the threshold, the emitting power is not further increased or decreased respectively, but is maintained constant. In consequence, in the X-ray CT apparatus provided with a slip ring, the wear debris produced in a contact space between the slip ring and a brush can be prevented from being scattered in the apparatus. Also provided are methods for expressing amplified siRNA expression cassettes in cells. A protective layer is formed over the stack.