L. Clément

Quantitative evaluation of process induced strain in MOS transistors by Convergent Beam Electron Diffraction.

Convergent Beam Electron Diffraction (CBED) experiments and simulations associated with Finite Element calculations were performed in order to measure strain and stress in a complex device such as periodic MOS transistors with a spatial resolution of about 2 nm and a sensitivity that could reach 50 MPa. A lamella of a thickness of about 475 nm was extracted from the wafer with the transistors by Focus Ion Beam (FIB) and was observed in cross-section in a Transmission Electron Microscope (TEM). When approaching the transistors, the HOLZ lines of the CBED patterns acquired in the silicon substrate, become broader and broader. This HOLZ line broadening, which is due to the stress relaxation in the thin foil, was used to determine quantitatively the strain and stress in the lamella and then in the bulk device. We showed that this procedure could be applied to a complex device. Two parameters, the intrinsic material strains--or equivalently the intrinsic material stresses--in the nickel silicide (NiSi) and nitride (Si(3)N(4)) layers on the top of the transistors gate, were successfully fitted by trial and error, in the procedure.

Measurement of nanograin orientations: Application to Cu interconnects

Nowadays the orientation maps of polycrystalline material are necessary for a better understanding of, for example, the formation of voids in the interconnects of modern electronic devices. As new generation of devices has dramatically reduced in size, new tools are required to meet these spatial resolution specifications. In this work two electron microscopy techniques are used to study the voids nucleation sites. Orientation maps were acquired with Electron BackScattered Diffraction (EBSD) technique and with NanoBeam Electron Diffraction (NBED) coupled with the ASTAR technique [1]. Experiments were performed on a Zeiss LEO1530 Scanning Electron Microscope (SEM) and on a JEOL 2010 FEF Transmission Electron Microscope (TEM), both equipped with a FEG (Field Emission Gun). The orientation maps were acquired with a probe size of 15 nm for EBSD and 2.7 nm for NBED. The present study is carried out on polycrystalline copper interconnections as used in the 45 nm technological node. The orientation maps were acquired on the same cross-section sample allowing a precise comparison of the EBSD and ASTAR techniques. This study confirms the localization of the voids nucleation sites (along the Cu/dielectric interface and grain boundaries) and gives new information concerning the orientation in the neighborhood of the voids.

Dealing With Multiple Grains in TEM Lamellae Thickness for Microstructure Analysis Using Scanning Precession Electron Diffraction

Materials microstructure is a source of variability in devices performance considering today’s transistor feature sizes. As the well-known Electron BackScatter Diffraction (EBSD) technique [1] shows limitations to characterize grains orientation smaller than a hundred nanometers, new tools have been developed over the past few years to characterize the texture of crystalline materials at nanometer scale. Among them is the ASTAR tool based on the acquisition and indexation of Precession Electron Diffraction (PED) patterns acquired in scanning mode [2]. Although its use has proved to be efficient to analyze nano-crystalline materials [3], indexation issues appear when crystal grains size is significantly smaller than the lamella thickness. For such cases, the acquired images are composed of a superimposition of several diffraction patterns. Strong expertise and time is required to prepare ultra-thin TEM lamellae adapted for the analysis of current nanometer scale transistor devices so that only the feature of interest remains in the thickness while left crystalline. As an alternative, a dedicated procedure is proposed to overcome crystal overlapping issues and exploit volume information: each diffraction pattern is iteratively re-indexed after subtraction of the reflections related to the main—or previous— solution. In this work, this technique is used to study the microstructure of a material when several crystalline phases remain in the TEM lamella thickness.

Retrieving overlapping crystals information from TEM nano-beam electron diffraction patterns

The diffraction patterns acquired with a transmission electron microscope (TEM) contain Bragg reflections related to all the crystals superimposed in the thin foil and crossed by the electron beam. Regarding TEM‐based orientation and phase characterisation techniques, the nondissociation of these signals is usually considered as the main limitation for the indexation of diffraction patterns. A new method to identify the information related to the distinct but overlapped grains is presented. It consists in subtracting the signature of the dominant crystal before reindexing the diffraction pattern. The method is coupled to the template matching algorithm used in a standard automated crystal orientation mapping tool (ACOM‐TEM). The capabilities of the approach are illustrated with the characterisation of a NiSi thin film stacked on a monocrystalline Si layer. Then, a subtracting‐indexing cycle applied to a 70 nm thick thin foil containing polycrystalline tungsten electrical contacts shows the capability of the technique to recognise small nondominant grains.

Measurement of Nanograin Orientations: Application to Cu Interconnects and Nanoparticle Phase Identification

Today, orientation maps of polycrystalline material are necessary for a better understanding of, for example, the formation of voids in the interconnects modern electronic devices. In this context, EBSD (Electron BackScattering Diffraction) has proved to be a powerful tool to measure grain orientation, but its spatial resolution is limited at the best to a diameter of about 10 nm. As new generations of devices have dramatically reduced in size, new tools are required to meet these spatial resolution specifications. In this work the NanoBeam Electron Diffraction (NBED) coupled with the ASTAR system is used to obtain orientation maps. The ASTAR system is an automatic crystallographic orientation indexing tool developed for the transmission electron microscopes [1]. It can operate using the precession diffraction mode to provide quasikinematical patterns. Experiments were performed on two different microscopes: a JEOL 2010 FEF with a FEG (Field Emission Gun) and a JEOL 3010 equipped with a LaB6 filament. With these respective microscopes, diffraction patterns using a beam size of 3 nm (JEOL 2010 FEF) and 10 nm (JEOL 3010) can be achieved and indexation of grains or nanoparticles around 10-20 nm can be obtained. Orientation maps obtained with different configurations (microscopes, voltage, camera length, with and without precession) will be compared. Two studies will be presented: the first one deals with polycrystalline copper interconnections as used in the 45nm technological node, the second one, illustrates the phase identification in nanoparticles.