Accurate reporting of ion time-of-flight during HiPIMS with gated front-end mass spectrometry

The quality of high-power impulse magnetron sputtering (HiPIMS) deposited films can often improve through the effective use of metal-ion acceleration, requiring precise measurements of time-of-flight (ToF). These measurements are commonly done using time- and energy-resolved mass spectrometry but require careful consideration of the transit time of ions inside. The transit time is typically calculated by considering the travel length in various parts of the spectrometer (e.g., from orifice to detector), but errors associated with these estimations can lead to nonphysical values in a HiPIMS process (e.g., negative ToFs). Here, we report a practical approach to determine ion ToF directly at the substrate position without any need for transit-time corrections, by synchronizing a gating pulse to the front-end of a HIDEN Analytical EQP-300 mass spectrometer. The ToF is measured by applying +70 V to repel ions and a 5 μs gating pulse of 0 V to accept them. By adjusting the pulse delay, different 5 μs intervals can be measured. To prevent interference with the HiPIMS plasma, a grounded shield is placed in front of the mass spectrometer head with a variable slit-opening (0.5–3 mm). The effectiveness of the shielding is verified by Langmuir probe measurements, noting negligible shifts in plasma potential. Measurements of the ToFs of Ar+, Al+, Sc+, Y+, and W+ ions are presented; Al+ and Ar+ ions were also compared to ToF calculated using mass spectrometry flight tube equations, noting a 15%–20% error in calculated transit times, resulting in a 14 μs offset for Al ions and negative ToF for Ar ions.