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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - APT-Met (Atom Probe Tomography (APT) Metrology for future 3D semiconductor devices)

Teaser

Summary

The objective behind the APT Met project was a training by research proposal for the emerging and exciting field of Atom Probe Tomography (APT). Fundamental to this project were the metrology and training advances needed to underpin its future for 3-dimensional (3D) characterization of next generation semiconductor devices. Additionally, it aimed to enhance the skills and knowledge of the beneficiary in this novel research field which is of strong industrial importance, as well as enhance their future career prospects. Although APT has established itself as a reliable metrology for metallurgy, it remains an emerging concept for semiconductor applications. The focus was therefore placed on addressing the fundamental issues related to APT metrology, developing relevant protocols and advancing it towards a more optimized, reliable and efficient technique for the wide spread study of semiconductor structures.

Work performed

The APT met proposal consisted of several work packages, each covering a specific area underpinning an atom probe measurement.
WP1 focused on sample preparation. A key objective of this WP was to improve the sample preparation for ATP of III V and III nitride materials. As the region of interest (ROI) layer can be just nanometers below the sample surface, any protective capping layer employed for the needle sample shape preparation must not damage the underlying sample structure. The typical approach of using ion assisted platinum deposition (iPt) was found to etch our materials. A new method combining an initial electron beam deposited Pt (ePt) layer with a subsequent iPt layer was shown to be extremely successful. This solution meant that the benefits offered by a FIB SEM to locate and cover small regions was retained. Additionally, high failure rates associated with poor adhesion to the sample carrier using iPt were reduced by reconfiguring the sample/carrier geometry and resulted in a significant measurement yield.
For WP2, the impact of the laser power/energy and wavelength combined with electric field to stimulate surface atom emissions were studied. The objective was to establish the link between electric field (at evaporation), local and temporal heating from the laser-tip interaction, and stoichiometric emission and/or detection of the constituents of the compound. A significant finding was that III V semiconductor materials not only emit singular ions but cluster ions too, with the single/cluster ion ratio being dependent on the experimental conditions. Cluster ions have a strong impact on quantification since their formation leads to mass spectrum peak overlaps for the same element but with a different charge state. Unambiguous assignment and accounting becomes impossible unless the species of interest has multiple isotopes and correction procedures based on expected abundancies can be developed. For single isotope compounds, stoichiometric analysis cannot be readily achieved while experimental conditions leading to a complete absence of clusters was never found for all the III V materials tested. Based on our extensive exploration of experimental conditions, operational parameters for an accurate stoichiometric analysis of GaN was determined. An important conclusion of the data analysis is that the apparent composition is predominantly influenced by the electrical field at the tip apex with a correlation virtually invariant between different instruments and independent of the laser conditions (different wavelengths, laser power,..).
In WP3 the shape of the tip was investigated to evaluate its impact on the detection probabilities and composition analysis. For higher laser energy/power, significant variations in the III V stoichiometry across the tip was observed which indicates a spatially varying tip shape and varying electric field. The tip shape arises from a localized absorption (and heat diffusion) within the tip apex, causing spatially variant heating and locally varying evaporation probabilities. Since the detailed tip shapes are a result of the laser-tip interaction, they do have a wavelength and power/energy dependence. As the detailed tip controls the local electric field, the emission probability of single ions/clusters and the concentration quantification is affected. The non hemispherical shape invariably impacts on the ion emission trajectories with deleterious consequences on spatial resolution. Lower laser energies/powers resulted in a more homogenous tip emission, minimizing such inaccuracies. For GaN, the materials crystallographic nature was observed in the form of poles and zone lines. These features are associated with low field regions leading to nanoscale field variations over the tip surface. Investigation suggested a moderate dependence on their presence to the ion trajectories and spatial resolution attainable.
WP4 and WP5 focused on data quantification and reconstruction of real device

Final results

Substantial advances from the APT Met project have been made towards understanding and exploiting APT for the analysis of III V and III nitride semiconductor materials. A new and reproducible FIB SEM preparation approach has been created and disseminated. This approach finally enables III V and III nitride samples to be prepared and measured whilst continuing to exploit the accurate sample preparation derived from FIB SEM. A different sample geometry and tip mounting approach was also developed which significantly increased sample measurement yields from <20% to >75%, a prerequisite for wider industrial exploitation and adoption.
For GaN, measurement protocols leading to accurate stoichiometric analysis have been determined. The developed procedure is also widely applicable across different instruments, laser wavelengths and energies/powers. This shows a high level of transferability and reproducibility which is an important stepping stone towards APT playing a similar and uncontested role like SIMS. With the concurrent and widespread deployment of 3D-technology, it also demonstrates its emergence towards becoming a replacement for SIMS as the industrial metrology of choice for next generation technologies.
The project concluded by demonstrating its applicability to real device layers through establishing a link between structural observations and electrical performance. These findings have made a real impact on the understanding of new device layers being developed, with APT now being implemented more routinely for this type of analysis within IMEC.