For illustration in FIG. 3B, the deposited material M forms a uniform layer with an even spread surface. Accordingly, the layer has a uniform thickness d. After the deposition process, the wafer is transferred by the robot blade 132 of the robot 131 of the buffer chamber 130. The weight meter 210 is equipped in the robot blade 132. The second measurement is performed to the wafer with the layer of the material M.

The measured data of the first measurement and the second measurement is sent to the processor 240. The processor 240 calculates the weight difference after and before the deposition process. According to the density of the material M, the thickness d is calculated by the following equation:

$d=\frac{\delta }{\pi ??r^2?\rho }$
δ is the weight difference of the first measurement and the second measurement. ρ is the density of the material M. π is the circular constant.

Based on the discussion above, the thickness of the material M is able to be calculated from the weight measurements. In some embodiments, when the material M is non-evenly deposited on the wafer, for example, the thickness is different from different locations on the wafer W, an average thickness of the material M is obtained from the above discussion.