Decomposition of CrN induced by laser-assisted atom probe tomography.

It is known that measurement parameters can significantly influence the elemental composition determined by atom probe tomography (APT). Especially results obtained by laser-assisted APT show a strong effect of the laser pulse energy on the apparent elemental composition. Within this study laser-assisted APT experiments were performed on Cr0.51N0.49 and thermally more stable (Cr0.47Al0.53)0.49N0.51, comparing two different base temperatures (i.e. 15 and 60 K), laser wavelengths (i.e. 532 and 355 nm) and systematically modified laser pulse energies. Absolute chemical compositions from laser-assisted APT were compared to data obtained from ion beam analysis. The deduced elemental composition of CrN exhibited a strong increase of the Cr content when the laser pulse energy was increased for both laser wavelengths. For low laser pulse energies Cr, CrN, N and N2 ions were identified, while the amount of detected Cr ions increased and the amount of N ions strongly decreased at higher laser pulse energies. Further, increased detection of more complex Cr-containing ions such as Cr2N at the expense of CrN was observed at higher pulse energies. At the highest pulse energy levels used within this work, the resulting Cr content was > 80 at%, dominated by the amount of detected elemental Cr ions. The change of the mass spectrum of the detected ions with increasing laser pulse energy provides evidence that high laser pulse energies initiate the decomposition of CrN during the APT measurement, consistent with the known thermal decomposition path into Cr2N and subsequently into Cr and gaseous N. In contrast, variation of the laser pulse energy for the thermally more stable CrAlN resulted only in a slight increase of Cr and a decrease of the resulting concentrations of Al and N with increasing laser pulse energy and no change in the type of detected ions. In conclusion, within the present study, the decomposition of a coating material with low thermal stability induced by laser-assisted APT was reported for the first time, emphasizing the importance of the selection of suitable measurement parameters for metastable materials, which are prone to thermal decomposition.

Efficient preparation of microtip arrays for atom probe tomography using fs-laser processing.

Microtip arrays, also called microtip coupons, are routinely used in atom probe tomography (APT) as specimen carriers. They are commercially available consumables, usually made of Si with high electrical conductivity, produced via dedicated shaping techniques. Their purpose is to act as a specimen mount after focused ion beam (FIB) based lift-out procedures. Within this work, an alternative approach to prefabricated microtip coupons is presented, by directly creating a microtip array on the sample to be investigated utilizing fs-laser processing. An exemplary array of microtip posts was fs-laser processed from a TiN coating on Si substrate and subjected to final preparation via annular FIB milling. Subsequently, APT specimen of the TiN coating as well as of the Si substrate were successfully measured in laser assisted mode, using a commercial local electrode APT system. To further emphasize the versatility of the proposed approach, additional voltage measurements of highly conductive B doped Si arrays as well as exemplarily fs-laser processed microtip arrays of various other materials are provided as supplementary material to this article. The presented methodology bypasses the lift-out and avoids the necessity of a Pt weld between specimens and coupon posts which is frequently considered to represent a weak spot. It reduces consumables consumption and provides a high number of specimens in short time, while it is applicable for a wide range of materials and has thus the potential to revolutionize APT specimen preparation.

Anisotropic wet-chemical etching for preparation of freestanding films on Si substrates for atom probe tomography: A simple yet effective approach.

A major drawback of atom probe tomography (APT) experiments of complex samples is the demanding and rather time consuming specimen preparation via the lift-out process. It usually requires a skilled operator for focused ion beam (FIB) preparation, and frequently overbooked FIB workstations represent a major bottleneck in sample throughput. Within this work, the authors present an alternative approach for APT specimen preparation of functional films and coatings on Si substrates via anisotropic wet-chemical etching. Utilizing this simple, yet effective approach, a freestanding section of the film to be investigated can be fabricated in a few steps. After the etching procedure, freestanding film posts and subsequently APT specimen can be easily prepared by basic FIB milling operations without the need for a lift-out process. Hence, this approach reduces FIB efforts to a minimum in terms of complexity and required machine utilization.

Enhanced Ti0.84Ta0.16N diffusion barriers, grown by a hybrid sputtering technique with no substrate heating, between Si(001) wafers and Cu overlayers.

We compare the performance of conventional DC magnetron sputter-deposited (DCMS) TiN diffusion barriers between Cu overlayers and Si(001) substrates with Ti0.84Ta0.16N barriers grown by hybrid DCMS/high-power impulse magnetron sputtering (HiPIMS) with substrate bias synchronized to the metal-rich portion of each pulse. DCMS power is applied to a Ti target, and HiPIMS applied to Ta. No external substrate heating is used in either the DCMS or hybrid DCMS/HiPIMS process in order to meet future industrial thermal-budget requirements. Barrier efficiency in inhibiting Cu diffusion into Si(001) while annealing for 1 hour at temperatures between 700 and 900 °C is investigated using scanning electron microscopy, X-ray diffraction, four-point-probe sheet resistance measurements, transmission electron microscopy, and energy-dispersive X-ray spectroscopy profiling. Ti0.84Ta0.16N barriers are shown to prevent large-scale Cu diffusion at temperatures up to 900 °C, while conventional TiN barriers fail at ≤700 °C. The improved performance of the Ti0.84Ta0.16N barrier is due to film densification resulting from HiPIMS pulsed irradiation of the growing film with synchronized Ta ions. The heavy ion bombardment dynamically enhances near-surface atomic mixing during barrier-layer deposition.