Al capping layers for nondestructive x-ray photoelectron spectroscopy analyses of transition-metal nitride thin films

Abstract

X-ray photoelectron spectroscopy (XPS) compositional analyses of materials that have been air exposed typically require ion etching in order to remove contaminated surface layers. However, the etching step can lead to changes in sample surface and near-surface compositions due to preferential elemental sputter ejection and forward recoil implantation; this is a particular problem for metal/gas compounds and alloys such as nitrides and oxides. Here, the authors use TiN as a model system and compare XPS analysis results from three sets of polycrystalline TiN/Si(001) films deposited by reactive magnetron sputtering in a separate vacuum chamber. The films are either (1) air-exposed for ≤10 min prior to insertion into the ultrahigh-vacuum (UHV) XPS system; (2) air-exposed and subject to ion etching, using different ion energies and beam incidence angles, in the XPS chamber prior to analysis; or (3) Al-capped in-situ in the deposition system prior to air-exposure and loading into the XPS instrument. The authors show that thin, 1.5–6.0 nm, Al capping layers provide effective barriers to oxidation and contamination of TiN surfaces, thus allowing nondestructive acquisition of high-resolution core-level spectra representative of clean samples, and, hence, correct bonding assignments. The Ti 2p and N 1s satellite features, which are sensitive to ion bombardment, exhibit high intensities comparable to those obtained from single-crystal TiN/MgO(001) films grown and analyzed in-situ in a UHV XPS system and there is no indication of Al/TiN interfacial reactions. XPS-determined N/Ti concentrations acquired from Al/TiN samples agree very well with Rutherford backscattering and elastic recoil analysis results while ion-etched air-exposed samples exhibit strong N loss due to preferential resputtering. The intensities and shapes of the Ti 2p and N 1s core level signals from Al/TiN/Si(001) samples do not change following long-term (up to 70 days) exposure to ambient conditions, indicating that the thin Al capping layers provide stable surface passivation without spallation.

Venting temperature determines surface chemistry of magnetron sputtered TiN films

“…We employ the previously developed Al-cap technique 16 to separate the effects of residual gas exposure in the highvacuum (HV) environment during the post-deposition phase from those introduced by the following venting sequence and air exposure. With the help of x-ray photoelectron spectroscopy (XPS) analyses performed on a series of TiN samples as a function of T v , we find that the majority of surface reaction products, including TiO 2 , TiO x N y , and N 2 previously detected after prolonged annealing experiments, form shortly after vent, provided that T v is sufficiently high.…”

Native target chemistry during reactive dc magnetron sputtering studied by ex-situ x-ray photoelectron spectroscopy

“…23 To circumvent these problems, we recently demonstrated that a few nm thick metal capping layers deposited immediately after film growth prevent oxidation of polycrystalline TM nitride surfaces during transport in air from magnetron sputtering system into the XPS instrument. 24 In this way, high quality core level spectra directly comparable to those acquired from films grown and analyzed in situ in a ultrahigh-vacuum (UHV) XPS system can be acquired. 25 Here, we report high-energy resolution core-level photoelectron spectra obtained ex situ from native Ti targets after magnetron sputtering in Ar/N2 atmospheres.…”

“…33 In the present case, however, the satellite contribution was smeared out, because of ion-induced damage of the target surface. 24 In the case of the target sputtered at pN2 = 200 mPa, the increase in main peak full-width-at-half-maximum (FWHM) together with high-intensity tail on the high BE side were likely the result of N overstoichiometry with nitrogen atoms occupying interstitial sites.…”

SiNx coatings deposited by reactive high power impulse magnetron sputtering: Process parameters influencing the residual coating stress

“…Compared to mid-frequency and direct current magnetron sputtering, HiPIMS was reported to yield thin films with comparatively low stress levels, 20,21 while the film hardness and density were not sacrificed. 22,23 The reduced stress in films deposited by HiPIMS is commonly attributed to the method's inherent increased flux of ionized target material together with a decreased flux of ionized working gas and implies a moderate total flux of ions.…”