Permanently mounted reference sample for a substrate measurement tool

A method and apparatus for measuring a reference sample in order to collect a reference characteristic, without moving the reference sample, is disclosed. In one embodiment, the method of the present invention comprises the following steps. An operator places a cassette of unprocessed wafers into a processing chamber of a processing tool that also includes a holding chamber. While the wafers are being processed, the holding chamber, which is coupled to a measurement tool, measures the reference sample that is mounted on a stage in the holding chamber. The resulting reference characteristic value (e.g., spectrum to determine film thickness) is then stored in the measurement tool's computer system. After a film is grown/formed on the wafers, the processed wafers are moved one by one into the holding chamber to be measured. A first wafer is placed on the stage in the holding chamber and a characteristic value for the first processed wafer is obtained using the measurement tool. The computer system of the measurement tool uses an algorithm to compare the reference characteristic value to the first wafer characteristic value to obtain a first differential value. The first differential value is then used to help determine the characteristic (e.g., film thickness) of the film formed on the first processed wafer. In another embodiment, a similar process is followed to measure another characteristic of a wafer, such as resistivity. These examples are illustrative and not limiting. Thus, the present invention can be used whenever a reference sample is to be measured to help determine a selected characteristic of a substrate or wafer.