Arul Kumar

Three-dimensional doping and diffusion in nano scaled devices as studied by atom probe tomography

Nowadays, technological developments towards advanced nano scale devices such as FinFETs and TFETs require a fundamental understanding of three-dimensional doping incorporation, activation and diffusion, as these details directly impact decisive parameters such as gate overlap and doping conformality and thus the device performance. Whereas novel doping methods such as plasma doping are presently exploited to meet these goals, their application needs to be coupled with new metrology approaches such as atom probe tomography, which provides the 3D-dopant distribution with atomic resolution. In order to highlight the relevant processes in terms of dopant conformality, 3D-diffusion, dopant activation and dopant clustering, in this paper we report on 3D-doping and diffusion phenomena in silicon FinFET devices. Through the use of atom probe tomography we determine the dopant distribution in a fully completed device which has been doped using the concept of self-regulatory plasma doping (SRPD). We extract the dopant conformality and spatial extent of this doping process and demonstrate that after annealing the resulting 3D-doping profiles and gate overlap are dependent on the details of the plasma doping process. We also demonstrate that the concentration-dependent 3D-diffusion process leads to concentration gradients which are different for the vertical versus the lateral direction. Through a statistical analysis of the dopant atom distributions we can identify dopant clustering in high concentration regions and correlate this with details of the dopant activation and, eventually, the device performance.

3D site specific sample preparation and analysis of 3D devices (FinFETs) by atom probe tomography

With the transition from planar to three-dimensional device architectures such as Fin field-effect-transistors (FinFETs), new metrology approaches are required to meet the needs of semiconductor technology. It is important to characterize the 3D-dopant distributions precisely as their extent, positioning relative to gate edges and absolute concentration determine the device performance in great detail. At present the atom probe has shown its ability to analyze dopant distributions in semiconductor and thin insulating materials with sub-nm 3D-resolution and good dopant sensitivity. However, so far most reports have dealt with planar devices or restricted the measurements to 2D test structures which represent only limited challenges in terms of localization and site specific sample preparation. In this paper we will discuss the methodology to extract the dopant distribution from real 3D-devices such as a 3D-FinFET device, requiring the sample preparation to be carried out at a site specific location with a positioning accuracy ∼50 nm.

Atom probe tomography analysis of SiGe fins embedded in SiO2: Facts and artefacts

We present atom probe analysis of 40nm wide SiGe fins embedded in SiO2 and discuss the root cause of artefacts observed in the reconstructed data. Additionally, we propose a simple data treatment routine, relying on complementary transmission electron microscopy analysis, to improve compositional analysis of the embedded SiGe fins. Using field evaporation simulations, we show that for high oxide to fin width ratios the difference in evaporation field thresholds between SiGe and SiO2 results in a non-hemispherical emitter shape with a negative curvature in the direction across, but not along the fin. This peculiar emitter shape leads to severe local variations in radius and hence in magnification across the emitter apex causing ion trajectory aberrations and crossings. As shown by our experiments and simulations, this translates into unrealistic variations in the detected atom densities and faulty dimensions in the reconstructed volume, with the width of the fin being up to six-fold compressed. Rectification of the faulty dimensions and density variations in the SiGe fin was demonstrated with our dedicated data treatment routine.

Atomic insight into Ge1−xSnx using atom probe tomography

Ge(1-x)Sn(x) is receiving a growing interest in the scientific community, as it has important applications in opto-electronic devices, ( as stressor) Source/Drain materials for Ge and SiGe MOSFETS. It is predicted that at 10% Sn concentration or even lower, unstrained Ge(1-x)Sn(x) will exhibit a direct band gap. Moreover, in strained Ge(1-x)Sn(x) the expected concentration of Sn for this cross-over is even lower. As the theoretical Sn incorporation in Ge(1-x)Sn(x) is less than 1%, and Ge(1-x)Sn(x) is prone to relaxation, routes towards the growth of metastable strained films has been extensively explored. Although Ge(1-x)Sn(x) films (with x up to 10%) have been grown using various methods like molecular beam epitaxy, CVD growth etc. there remain issues with tendency of these layers to relax. Detailed studies on the relaxation mechanisms and effects on the Sn-atoms require suitable characterization techniques. Various techniques have been used to study the surface of the film, crystallography or concentration of Sn in the film but none of them provides information at the atomic scale as they average over many layers and atoms. Atom probe tomography (APT) analysis, on the other hand, is one such method that can provide atomic scale resolutions (∼0.3 nm) due to its ability to perform atom by atom analysis. In this paper we explore the use of APT for characterizing Ge(1-x)Sn(x) layers. We comment on the difference of field evaporation values of Ge and Sn in Ge(1-x)Sn(x) layer by taking a closer look at the co-evaporation of the two elements and comment on the accuracy of depth reconstruction of APT for Ge(1-x)Sn(x) layer. Comparing the Sn-distributions and their local surroundings we saw a tendency for the Sn to locally enrich forming Sn clusters. Higher order clusters were observed for the relaxed sample.

Atom Probe Tomography for 3D-dopant analysis in FinFET devices

As the nano scale device performance depends on the detailed engineering of the dopant distribution, advanced doping processes are required. Progressing towards 3D-structures like FinFETs, studying the dopant gate overlap and conformality of doping calls for metrology with 3D-resolution and the ability to confine the analyzed volume to a small 3D-structure. We demonstrate that through an appropriate methodology this is feasible using Atom Probe Tomography (APT). We extract the 3D-dopant profile and important parameters such as gate overlap and profile steepness, from transistor formed with plasma doping processes. Analyzing samples with different doping processes, the APT results are entirely consistent with device performances (Ioff vs. Ion).

Laser-assisted atom probe tomography of semiconductors: the impact of the focused-ion beam specimen preparation

This paper demonstrates the increased light absorption efficiency of semiconducting atom probe tips resulting from focused-ion-beam (FIB) preparation. We use transmission electron microscopy to show that semiconducting tips prepared with FIB are surrounded with an amorphized shell. Photomodulated optical reflectance measurements then provide evidence that FIB-induced damage leads to an increase in both sub- and supra-bandgap light absorption efficiency. Using laser-assisted atom probe tomography (La-APT) measurements, we finally show that, for a nanoscale tip geometry, the laser-induced heating of a tip during La-APT is enhanced by the FIB preparation. We conclude that, upon supra-bandgap illumination, the presence of a FIB-amorphized surface dramatically increases the light-induced heat generation inside semiconducting tips during La-APT. Furthermore, we also deduce that, in the intriguing case of sub-bandgap illumination, the amorphization plays a crucial role in the unexpected light absorption.