Accurate Reporting of Ion Time-of-Flight during HiPIMS with Gated Front-End Mass Spectrometry

Abstract

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 experimentally, using a bipolar HiPIMS power supply to synchronize a gating pulse to the front-end of a HIDEN Analytical EQP-300 mass spectrometer, placed at the working distance. The ToF is measured by applying a +70 V bias to repel ions, and a 5 us gating pulse of 0 V to accept them. To prevent interference with the HiPIMS plasma, a grounded shield is placed in front of the mass-spec 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 for a DC sputter discharge. The gate is then synchronized to a HiPIMS pulse and data collected at 5 us intervals by adjusting the pulse delay. Measurements of the time-of-flights 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.