The NIST Standard Reference Database 71 can be used to determine the inelastic mean free path (IMFP) of the Ir 4f photo-electrons using the Gries predictive model. Baselines were fit to the peaks and using peak heights and corresponding sensitivity factors that reflect photo-electron cross sections and the analyzer transfer function, the spectral regions were quantified to reflect atomic percentages. The sample was sputtered for 1 minute and then data taken in the Ir 4f, Fe 2p 3/2 and O 1s spectral regions using a constant 71.55 eV pass energy. The ion current was estimated to be 1 ♚ looking at the current deposited onto graphite which has minimal secondary electron yield, allowing the stage ammeter to more closely reflect the ion current. The Zalar rotation was set at a few RPM and at an incidence angle of 45 degrees. The intention is to guarantee the complete sputtering of the Ir-deposited region with minimum decrease in current density. A 1 keV Ar + ion beam was rastered across the sample to produce an approximately 13 mm diameter region of sputtering. In this study, a 4 nm iridium film was sputtered through a 10 mm diameter mask directly onto a polished 304 stainless steel XPS sample stub. To accomplish this, great effort must be made to align the ion-gun at the working stage height, calibrating the ion-gun rastering, and optimizing the ion-gun condenser and objective lens settings to provide a well defined and focused beam of specified current.
If this is not the case, then the analysis region will include the crater wall and will include information from different depths. The sputtering crater needs to be larger than the analysis area and the analysis area needs to be centered on the sputtering crater. The size of the sputtering crater relative to the analytical area is also important. Lower incidence angles are also more useful for rougher samples. When this angle is lower and more grazing, there is less transverse momentum transfer to the sample and then the sputtering yield is much lower. Another geometric concern is the angle of ion incidence. Azimuthal rotation, called Zalar rotation, will reduce this cratering- but at the cost of lower yield. If the sample is not rotated then it will crater asymmetrically towards the ion-gun because the ion density is larger closer to the ion gun. Thus while great for speed, high sputtering yield is bad for very fine depth resolution of thin layers.Īlso, geometric relationships matter. To accomplish this the sputtering conditions must be optimized for both the materials and depth ranges involved in the study.Īs an example, higher beam energies yield higher sputtering yields- the number of atoms sputtered per incident ion- but a high yield will just cause one to punch through a very thin layer without getting any data from that layer. In this case an Ar + ion beam is used to sputter the sample surface in between alternating XPS studies. In addition to angle-resolved XPS (ARXPS), the chemistry of surfaces and interfaces can be studied using depth profiling.