Large O2 Cluster Ions as Sputter Beam for ToF-SIMS Depth Profiling of Alkali Metals in Thin SiO2 Films.

A sputter beam, consisting of large O2 clusters, was used to record depth profiles of alkali metal ions (Me+) within thin SiO2 layers. The O2 gas cluster ion beam (O2-GCIB) exhibits an erosion rate comparable to the frequently used O2+ projectiles. However, because of its high sputter yield the necessary beam current is considerably lower (factor 50), resulting in a decreased amount of excess charges at the SiO2 surface. Hence, a reduced electric field is obtained within the remaining dielectric layer. This drastically mitigates the Me+ migration artifact, commonly observed in depth profiles of various dielectric materials, if analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) in dual beam mode. It is shown, that the application of O2-GCIB results in a negligible residual ion migration for Na+ and K+. This enables artifact-free depth profiling with high sensitivity and low operational effort. Furthermore, insight into the migration behavior of Me+ during O2+ sputtering is given by switching the sputter beam from O2+ to O2 clusters and vice versa. K+ is found to be transported through the SiO2 layer only within the proceeding sputter front. For Na+ a steadily increasing fraction is observed, which migrates through the unaffected SiO2 layer toward the adjacent Si/SiO2 interface.

The effect of bias-temperature stress on Na+ incorporation into thin insulating films.

The action of Na(+) incorporation into thin insulating films and transport therein under influence of a bias voltage and temperature (BT stress) is the subject of this work. Deposited onto highly n-doped Si wafers, the insulators get BT stressed and subsequently investigated by means of time-of-flight-secondary ion mass spectrometry (ToF-SIMS). A thin PMMA film, spin-coated onto the insulator, serves as host matrix for a defined amount of Na(+), provided via sodium triflate. Combining BT stress and ToF-SIMS depth profiling enables the unambiguous detection of Na(+), incorporated into the insulating material. The insulators of interest vary in their nitride content: SiO(2), SiO(x)N(y), and Si(3)N(4). For SiO(2), it is shown that once a threshold BT stress is exceeded, Na(+) gets quantitatively incorporated from PMMA into the underlying insulator, finally accumulating at the SiO(2)/Si interface. A quantitative assessment by combination of Butler-Volmer kinetics with hopping dynamics reveals activation energies of E(a) = 1.55 - 2.04 eV for Na(+) transport in SiO(2) with varying thickness. On the other hand, SiO(x)N(y) and Si(3)N(4) films show a different Na(+) incorporation characteristic in this type of experiment, which can be explained by the higher coordination of nitrogen and hence the reduced Na(+) permeability within these insulators.

Digital holographic reflectometry.

Digital holographic microscopy (DHM) is an interferometric technique that allows real-time imaging of the entire complex optical wavefront (amplitude and phase) reflected by or transmitted through a sample. To our knowledge, only the quantitative phase is exploited to measure topography, assuming homogeneous material sample and a single reflection on the surface of the sample. In this paper, dual-wavelength DHM measurements are interpreted using a model of reflected wave propagation through a three-interfaces specimen (2 layers deposited on a semi-infinite layer), to measure simultaneously topography, layer thicknesses and refractive indices of micro-structures. We demonstrate this DHM reflectometry technique by comparing DHM and profilometer measurement of home-made SiO(2)/Si targets and Secondary Ion Mass Spectrometry (SIMS) sputter craters on specimen including different multiple layers.