,,Atomic Scale characterization of nano scale structures (FinFETs) using Atom Probe Tomography
Kambham, Ajay Kumar; S0201348
Introduction of 3D device structures, such as fin field effect transistors (FinFETs), nanowire based transistors and novel materials is inevitable to sustain scaling and continuous performance improvement in future technology. The gate stacks of 3D structures consist of non-planar layers; some of which have a sub-nanometer thickness. Their development and understanding is intimately linked with the ability to obtain information on their structure, composition and dopant distribution. The latter is a complex task as these structures have nanometer scale dimensions and are composed of heterogeneous materials including insulators. Therefore, metrology tools must be developed that are capable of measuring these structures at near atomic scale and in particular be able to perform 3D elemental mapping of their chemical distributions with sub-nanometer resolution.The objective of this thesis is therefore to establish a technique capable of compositional characterization of 3D-nanostructures using the Atom Probe Tomography (APT) with a focus on the analysis of FinFETs. APT works on the basis of peeling of ionized atoms from a needle shaped specimen and projected onto a position sensitive single ion detector. The process of peeling off ionized atoms from the surface of a solid is called field evaporation. In APT, field evaporation is achieved by applying a standard voltage of several (positive) kilo-volt to the needle shape specimen (APT-tip) with an apex radius of 10-100 nm. This creates an electric field of several 10 V/nm around the tip enabling the ionization of atoms on the surface of the tip. The positive ions thus created are repelled from the surface by the electric field and follow a trajectory pointing perpendicularly away from the surface. These ions are projected on to a position sensitive detector which is a few centimeters away from the tip apex. This approach allows us to magnify the atomic positions on the surface of detector by more than a million times enabling sub-nanometer lateral resolution. The reconstruction of the evaporated ion will be performed by using projection principle, which is accurate enough to reconstruct with sub-nanometer precision.This work demonstrated the capability of laser assisted wide angle tomographic atom probe (LAWATAP) to characterize 3D structures like FinFETs with near atomic resolution. In the discussion of optimization of sample preparation (using focused ion beam), an explanation has been provided for the necessity to avoid Ga+ beam interaction as it destroys the region of interest during the sample preparation for 2D and 3D device structures. The results in this study have highlighted in depth understanding of the 3D dopant distribution in nanoscaled devices (FinFETs). Therefore, it is concluded that laser assisted APT to date provides 3D atomic scale mapping of nanoscale semiconductor devices in its current state.