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LA-WATAP

LASER-Assisted
3D Atom Probe for Semiconductors and Materials


The CAMECA LA-WATAP instrument is the next generation of Tomographic (or 3D) Atom Probe,  providing quantitative atomic scale 3D elemental mapping of chemical heterogeneities in materials.
The LA-WATAP offers the following advantages:
  • Analysis of  semiconductor materials with near-atomic depth resolution. This is possible by using a flexible (IR/ visible/ UV) ultrafast (400fs) laser setup. The unique hybrid evaporation mode combines reduced heating of the sample with ultra-fast surface polarisation promoting ion evaporation under the high DC electrical field.
  • Excellent mass resolution even on low thermal conductivity materials. This is obtained by reducing the heating of the sample with ultra-fast laser pulsing. Conventional picosecond laser based 3DAP work in thermal evaporation mode leading to thermal effects and peak tails.
  • Analysis of thin electrically insulating layers by using fs-laser pulses of short wavelength (UV).
  • Highest analyzer transmission (detected ions/evaporated atoms >60%). Reflectron-based 3DAP instruments reduce this typically to 35% due to the use of grids. Transmission is a key parameter when applying the (destructive) 3DAP technique to the quantification of nanoscale volumes. 
  • Large analysis area (100nm in diameter) for a better statistics on composition measurements.
  • Fast acquisition, up to 1E6 atoms per minute depending on sample strength (flux of at./pulse).
  • Best quantitative results with the exclusive Advanced Delay Line Detector (ADLD) and its benchmark multi-hit performance.
  • Flexible and fast dedicated FIM (Field Ion Microscopy) detector for metallurgical application with highest S/N performance and fast switch between FIM and 3DAP modes.
A six-page introduction leaflet of the new  LA-WATAP 3D Atom Probe can be downloaded.

Flexible Laser setup.
The response of a material under Laser illumination depends on its nature, its shape and on the wavelength of the Laser. The LA-WATAP offers flexible Laser setup, ensuring optimized analysis conditions for all materials. The operator can easily select the most appropriate wavelength for any given sample: I.R. (1030nm), visible (515nm), or U.V. (343nm).
Analysis of a 12nm thick SiO2 layer. The use of UV facilitates the analysis and quantification on electrically  insulating materials. Using IR the analysis was not possible. Using green the interfaces were of unphysical shape and the stoichiometry of the oxide was wrong and with a loss of mass resolution.
Atomic scale depth resolution in semiconductors.

The interaction of ultrafast (400fs) pulses of polarized light with a small tip (radius 10-100nm) results in non-linear effects confining the polarization to the skin of the tip apex. Combining this with a DC electrical field and a moderate heating of the tip, the evaporation of ions can take place without dramatic loss of spatial resolution.

 
Excellent depth resolution is evidenced in  the 3D AP analysis of a <111> silicon sample.
The <111> atomic planes are directly visible in the reconstructed volume (4x4x20nm), without any data treatment. Such data quality is not available using conventional ps-laser technology.


Improvement of Atom probe mass resolving power with LASER evaporation.


Mass spectra  of an aluminium-silicon-magnesium alloy obtained with HV pulses (green curve) and laser pulses (blue curve). On the right, 3D image of the Mg distribution.
The use of femto-second Laser technology and CAMECA design result in a higher mass resolving power compared to HV pulsing (inducing an energy spread) or pico-second conventional Laser technology (inducing thermal evaporation effect with longer heating of the tip). 



A larger Field Of View
The 3DAP Field Of View is determined by the instrument geometry (tip-detector distance and detector size) but also by the tip itself (radius, cone angle and material evaporation field).
Smaller tip radius and lower evaporation field will result in lower FoV. Note that the radius increases during the analysis and the DC voltage is progressively raised in order to regulate the evaporation flux.
The LA-WATAP typically records volumes of 50-100nm diameter by 100-200nm depth, resulting in files of several tens of millions ions, in a few hours (depending on requested evaporation flux that is sample-dependent).

Te: blue, Fe: green, Co: black.

82 x 82 × 140 nm3  (17 M atoms)
The Laser evapoartion mode allowed analysis of the above TbFe/Co multilayer, too fragile to be analyzed in HV mode. Zooming inside the Large Field Of View reveals atomic plane depth resolution and dissymetric interfaces (sharp and diffused) linked to layer growth conditions.


Analysis of semiconductors with the Atom Probe:
Drain silicide interface in MOS transistor structure

The concentration profiles of Ni, Si and Pt below clearly reveal the presence of both Ni2Si and NiSi phases as a consequence of the reactive diffusion between Ni and Si substrates. Furthermore, Pt enrichment at both Ni/Ni2Si and Ni2Si/NiSi interfaces is clearly evidenced.

This example illustrates the importance of quantification to understand a physical process. Two NiSi phases are assigned without ambiguity, mainly thanks to the Advanced Delay Line Detector’s superior multi-hit capabilities and optimized Laser evaporation mode. Moreover, due to the unique in-depth resolution, boundaries between phases are clearly revealed without matrix effect as observed in SIMS with this type of sample.

The CAMECA LA-WATAP is developed through a scientific and technical collaboration with the GPM-University of Rouen-CNRS, France.