Composition analysis of III–V materials grown in nanostructures: The self-focusing-SIMS approach
Authors: Franquet Alexis , Douhard Bastien , Conard Thierry , Melkonyan Davit , Vandervorst Wilfried 
Published: March 2016

One of the important growth drivers for the semiconductor industry is the shrinking of transistors to smaller dimensions. System performance and functionality increase as the density of transistors, which can be integrated onto a single chip, increases. To maintain the device performances for very small dimensions, additional improvements are needed such as the implementation of new materials (high-k's, metal gate, III–V or group IV alloys) and the introduction of new three-dimensional device architectures, e.g., field-effect transistor's. In this context, the metrology needs (determination of the exact composition and thickness of deposited/grown thin films) are more and more shifting from the analysis of blanket samples to more real devices where thin films are grown in very confined volumes (with dimensions below 20 nm). Their confined shape suggests that metrology with the standard analysis methods like x-ray photoelectron spectroscopy, secondary ion mass spectrometry (SIMS), and Rutherford backscattering spectroscopy should no longer be applicable due to a lack of spatial resolution. Moreover, even if one could obtain such a very fine focused beam (&lt;10 nm spot size), the resulting signal intensity will be so low that the accuracy when determining layer composition or dopant concentration becomes questionable due to the low counting statistics. The concept of self-focusing-SIMS (SF-SIMS) is a novel concept which overcomes not only the spatial resolution limitations of SIMS for these applications but also eliminates the sensitivity (and thus accuracy) problem. The concept is based on determining the composition of a compound film using a broad beam bombarding an array of identical features. The required spatial resolution is created through the use of cluster ions which contain only the constituents of the compound film and thus limit the information content to the confined volume. Indeed, the cluster formation mechanism implies that all cluster constituents must originate from the same collision cascade and thus are emitted in close proximity (&lt;0.5 nm) and cannot be formed by a recombination process from two very distant regions. As such, the composition information becomes confined (i.e., self-focused) to the areas where all constituents are simultaneously present. Recently, the authors demonstrated the use of SF-SIMS to quantify the Ge content of SiGe films grown in narrow trenches. In the present paper, the SF-SIMS approach is applied on III–V materials. Both InGaAs films grown on InP/Si in shallow trench isolation (STI) and more complex stacks, i.e., InGaAs/InAlAs/InP/Si grown in STI, are analyzed. The authors will show that the SF-SIMS method can accurately determine the In content in each of the layer, even for very confined volumes (width of the III–V structures around 40 nm). The obtained results will be compared to analyses performed with other complementary methods such as Auger electron spectroscopy, transmission electron microscopy/energy dispersive x-ray spectroscopy, and atom probe tomography.