ANAlyse de Transitors MOS à l'Echelle atoMiquE – ANATEME

The MOS (metal-oxide-semiconductor) transistor is, by far, the most widely used device in the current semiconductor component production, since it is the basic component of CMOS technology (Complementary MOS). From the beginning, this technology is characterized by its capacity to scale down devices sizes in order to achieve greater transistor densities of integration and speed. Analyzing the chemical and spatial distribution in three dimensions of these devices at the atomic scale presents huge challenges for semiconductor researchers. However, among the characterisation techniques nowadays used to analyse semiconductor processes such as the high-resolution transmission microscope (HR-TEM), secondary ion mass spectroscopy (SIMS) or scanning probe instruments, none can reveal the position and identity of all atoms in three-dimension with atomic scale. To date, the only technique that may approach this ideal is the Atom Probe Tomography. The Tomographic Atom Probe (TAP), developed at the GPM laboratory (Rouen University), is based on the field evaporation of atoms from a very sharp tip. The TAP was until recently confined to the analysis of conducting materials (metal alloys) because of the use of a voltage pulse to initiate atom evaporation, and to a relatively low field of view (section of analysis) of 20x20 nm2 due to detector limitation. Then, the Laser-Assisted Wide-Angle Tomographic Atom Probe (so-called LAWATAP) was achieved in 2006 at the GPM laboratory through a partnership between the CAMECA Company and the instrumentation team. This evolution of the TAP consists of the implementation of a femto-second pulsed laser associated with a field of view enhancement that now enables the analysis of resistive materials like semiconductors or dielectrics and permits an increased field of view (up to 100x100 nm2). The LAWATAP's field of view should now enable the three-dimensional reconstruction of an entire MOS transistor. This is the final objective of this project. However several issues have to be overcome. First, sample preparation has to be optimised and especially damaging induced by the FIB (focused-ion-beam) gallium beam. The forecasted acquisition of a FIB and TEM should also enable the precision positioning of a transistor in a very sharp tip, necessary for sample field evaporation. Secondly, as far as the analysis is concerned, several key issues have to be addressed. The first step is to fully understand the physics of laser-assisted evaporation and determine the best analyses conditions on semiconducting or insulating materials. As an example, it has recently been demonstrated that the mass resolving power strongly depends on the laser wavelength. The second step is to demonstrate the feasibility of analysing progressively complicating MOS processes. 3D-reconstruction of the analysed volume is also of great importance. Indeed, the multiplicity and novelty of materials used in a MOS transistor will complicate the reconstruction process. A technological development to increase the mass resolving power of the LAWATAP, in order to clearly separate elements such as nitrogen and doubly ionized silicon (mass resolving power of 3000), will also be experimented. Third, the analyses of MOS processes will be carried out in order to: 1) Understand and optimise the evaporation of dopants within a silicon matrix. As an example, a loss in bore is observed as compared with SIMS profiles, which is not yet understood.2) Detect and quantify dopant clusters within the Si matrix as far as ultra-shallow junctions are concerned and compare with simulation and SIMS profiles. 3) Associate ultra-shallow junctions and silicides in order to study dopant segregation at semiconductor/metal interface. 4) Analyse thin (5 -10 nm) SiO2 on Si layers. This will also enable the study eventual dopant segregation at the oxide-semiconductor interface. 5) Analyse ultra-thin dielectric stacks (e.g. SiOx/HfO2) and even link results with the mobility degradation in MOS transistors containing high-k gate dielectric if achievable. 6) Study the whole gate/dielectric/substrate stack with silicided poly-silicon gate or with a metal gate. Finally, we should be in capacity to analyse an ultimate nano-transistor in a single TAP analysis and reconstruct it in 3 dimensions at the atomic scale. This would represent a major breakthrough in semiconductor science.