Ajay Kumar Kambham

atom probe analysis of a 3D-finfet with high-k metal gate

The atom probe analysis of a full gate stack (metal gate/high-k dielectric) in a 3D finFET is reported. The measurement reliability in this kind of heterogeneous structure is discussed in the light of different artefacts, i.e. mass overlap and 3D reconstruction artefacts.

Atom probe for FinFET dopant characterization

With the continuous shrinking of transistors and advent of new transistor architectures to keep in pace with Moores law and ITRS goals, there is a rising interest in multigate 3D-devices like FinFETs where the channel is surrounded by gates on multiple surfaces. The performance of these devices depends on the dimensions and the spatial distribution of dopants in source/drain regions of the device. As a result there is a need for new metrology approach/technique to characterize quantitatively the dopant distribution in these devices with nanometer precision in 3D. In recent years, atom probe tomography (APT) has shown its ability to analyze semiconductor and thin insulator materials effectively with sub-nm resolution in 3D. In this paper we will discuss the methodology used to study FinFET-based structures using APT. Whereas challenges and solutions for sample preparation linked to the limited fin dimensions already have been reported before, we report here an approach to prepare fin structures for APT, which based on their processing history (trenches filled with Si) are in principle invisible in FIB and SEM. Hence alternative solutions in locating and positioning them on the APT-tip are presented. We also report on the use of the atom probe results on FinFETs to understand the role of different dopant implantation angles (10° and 45°) when attempting conformal doping of FinFETs and provide a quantitative comparison with alternative approaches such as 1D secondary ion mass spectrometry (SIMS) and theoretical model values.

In-situ observation of non-hemispherical tip shape formation during laser-assisted atom probe tomography

It is shown by SEM imaging of the tip and by observing the emission pattern of the evaporated atoms that laser assisted evaporation in an atom probe can lead to nonhemispherical tip shapes and time-dependent nonuniform emission. We have investigated this nonuniformity by observing the change in field of view when using laser wavelengths of 515 nm and 343 nm on silicon. The change is monitored in situ by 0.5 nm thick silicon oxide. We demonstrate that the field of view can easily be changed by more than 10 nm and that the apparent oxide layer thickness can deviate substantially from its correct value. The dependence of the tip shape deformations and the reconstruction artifacts on the laser wavelength are explained through simulations of the laser-tip interaction and nonhomogeneous heating effects.

Characteristics of cross-sectional atom probe analysis on semiconductor structures

The laser-assisted Atom Probe has been proposed as a metrology tool for next generation semiconductor technologies requiring sub-nm spatial resolution. In order to assess its potential for the analysis of three-dimensional semiconductor structures like FinFETs, we have studied the Atom Probes lateral resolution on a silicon, silicon-germanium multilayer structure. We find that the interactions of the laser with the semiconductor materials in the sample distort the sample surface. This results in transient errors of the measured dimensions of the structure. The deformation of the sample furthermore leads to a degradation of the lateral resolution. In the experiments presented in this paper, the Atom Probe reaches a lateral resolution of 1-1.8 nm/decade. In this paper we will discuss the reasons for the distortions of the tip and demonstrate that with the present state of data reconstruction severe quantification errors limit its applicability for the quantitative analysis of heterogeneous semiconductor structures. Our experiments show that reconstruction algorithms taking into account the time dependent nanostructure of the tip shape are required to arrive at accurate results.

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.

Dopant and carrier profiling in FinFET-based devices with sub-nanometer resolution

Atom probe tomography (APT) in conjunction with scanning spreading resistance microscopy (SSRM) is demonstrated for the first time to profile dopant and carrier distributions in FinFET-based devices with sub-nanometer resolution. These two techniques together provide information on the degree of conformality, the dose retention and the dopant activation. These results are also compared with a methodology involving secondary ion mass spectrometry (SIMS). Ion implantation for increased conformality of source/drain extensions is demonstrated for tilted implants, which clearly leads to improved device performance.

High performance n-MOS finFET by damage-free, conformal extension doping

A solution for conformal n-type finFET extension doping is demonstrated, yielding ION values of 1.23 mA/µm at IOFF=100 nA/um at 1V. This high device performance results from 40% reduced external resistance, which in term is stemming from 130% increased fin sidewall doping (confirmed by SIMS, SSRM and Atom Probe) relative to ion implant process. In this work we also report lowered gate leakage due to the damage-free extension doping.

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).