Pulsed radiofrequency glow discharge time-of-flight mass spectrometry: Depth profile analysis of multilayers on conductive and non-conductive substrates

Combination of a pulsed glow discharge (PGD) source with time-of-flight mass spectrometry (TOFMS) enables for fast multielemental transient signals detection of solid samples. Such instrument is ideal for depth profiling of layered materials with depth resolution down to the low nm range. In this work, depth profile capabilities of radiofrequency (rf) PGD-TOFMS system have been investigated when dealing with thin layers (GexNi100-x/Ni/NdyNi100-y with total thicknesses between 15 nm and 75 nm) deposited on two substrates with different electrical properties (non-conductive glasses and silicon wafers). Attention has been paid towards the selection of the different PGD operational parameters aiming at achieving best instrument performance for each substrate in terms of depth resolution of the thin coating layers. Particularly, the applied pulse width has proven to have a direct influence in the temporal regions (prepeak, plateau and afterglow) responsible of the analytical signals detected by TOFMS. Experimental results showed that shorter pulses (0.16 ms was finally selected) and periods (1.32 ms) allowed for better depth profile resolution of the layers deposited onto the glass substrate. However, such selection for the nonconductive substrate hindered obtaining analytical information from the prepeak and plateau regions. On the other hand, the effect of pulse width on depth resolution was not so critical when coatings were deposited onto the silicon wafer.
Work supported by Spanish MINECO under grant FIS2016-76058 (AEI/FEDER, EU).