In an embodiment, a method includes performing a first atomic layer deposition (ALD) process to form a first material layer over a first blank wafer, the first ALD process comprising: performing a first precursor sub-cycle using a first precursor; performing a first purge sub-cycle using a inert gas; and performing a second precursor sub-cycle using a second precursor and the inert gas; and performing a second purge sub-cycle for a first duration over a second blank wafer different from the first blank wafer using the inert gas to deposit first defects onto the second blank wafer.