Microelectronic applications of in-situ FIB sectioning in the TOF-SIMS
Résumé
The increasingly complex architectures and diversity of materials used in modern semiconductor devices makes their characterization challenging. TOF-SIMS analysis is particularly well suited to dopant and multilayer analysis in such devices, but standard approaches such as surface imaging or dual-beam depth profiling can be limited when faced with deeply buried interfaces and very heterogeneous samples. This is the case for through-silicon vias and copper pillars. These objects are typically several tens of microns in dimension and are used in 3D integration approaches to connect chips together. To address this type of sample, the capabilities of a TOF-SIMS instrument can be extended by the use of an in situ focused ion beam (FIB) gun. This is often a gallium source FIB, but other sources can be used such as Xe plasma sources to obtain higher etch rates for larger samples. In fact, even in the absence of a dedicated FIB column, the LMIG source (Ga, Bi, Au etc) that is normally used for spectroscopy on the instrument can also be used without pulsing to produce a continuous high current beam for FIB milling. The drawback of this method is that the sample must be rotated between milling and imaging steps and the LMIG parameters will require changing. There may also be milling artefacts arising from the presence of a mix of monoatomic and cluster ions when a bismuth or gold source is used.
The use of a dedicated FIB column opens up the possibility of performing FIB-TOF-SIMS tomography experiments and limits sample drift and re-positioning errors as the sample may be kept in the same position. However, FIB-TOF-SIMS tomography experiments can be time-consuming (overnight analysis to several days) and the investigated volume is limited by the sputter rates obtainable. To overcome these limitations, a prescreening with a non-destructive X-ray imaging technique can help to identify positions of interest for FIB-TOF-SIMS tomography to be performed on. This can be TSVs containing filling defects (voids) that can then be investigated at high resolution and with compositional information by FIB-TOF-SIMS. Another important question is whether copper diffused out from the TSV, through the diffusion barrier (often a thin layer of metal nitride) to the surrounding silicon. The presence of copper is deleterious for electronic properties and once present in silicon can diffuse relatively fast to active areas of the device. The engineering of barrier layers is routinely performed by depth profiling on full wafer samples to maximize layer quality and presence of certain defects may vary between full sheet deposition vs conformal filling of high aspect ratio holes. The use of an in situ FIB allows both tomography experiments and depth profiles to be performed on the side of a FIB-cut parallel to the object of interest.