Jay Mody

Evolution of metastable phases in silicon during nanoindentation: mechanism analysis and experimental verification

This paper explores the evolution mechanisms of metastable phases during the nanoindentation on monocrystalline silicon. Both the molecular dynamics (MD) and the in situ scanning spreading resistance microscopy (SSRM) analyses were carried out on Si(100) orientation, and for the first time, experimental verification was achieved quantitatively at the same nanoscopic scale. It was found that under equivalent indentation loads, the MD prediction agrees extremely well with the result experimentally measured using SSRM, in terms of the depth of the residual indentation marks and the onset, evolution and dimension variation of the metastable phases, such as beta-Sn. A new six-coordinated silicon phase, Si-XIII, transformed directly from Si-I was discovered. The investigation showed that there is a critical size of contact between the indenter and silicon, beyond which a crystal particle of distorted diamond structure will emerge in between the indenter and the amorphous phase upon unloading.

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.

Analysis and modeling of the high vacuum scanning spreading resistance microscopy nanocontact on silicon

Within this paper, the authors propose a refined high vacuum scanning spreading resistance microscopy (HV-SSRM) electromechanical nanocontact model based on experimental results as well as molecular dynamics (MD) simulation results. The formation under the tip of a nanometer-sized pocket of β-tin, a metastable metalliclike phase of silicon (also named Si-II), acting as a virtual probe is demonstrated. This gives a reasonable explanation for the superior SSRM spatial resolution as well as for the electrical properties at the Schottky-like SSRM contact. Moreover, the impact of the doping concentration on the plastic deformation of silicon for different species using micro-Raman combined with indentation experiments is studied. In order to elucidate the superior results of SSRM measurements when performed under high vacuum conditions, the impact of humidity on the mechanical deformation and Si-II formation is also analyzed using MD and SSRM experimental results.

Observation of diameter dependent carrier distribution in nanowire-based transistors

The successful implementation of nanowire (NW) based field-effect transistors (FET) critically depends on quantitative information about the carrier distribution inside such devices. Therefore, we have developed a method based on high-vacuum scanning spreading resistance microscopy (HV-SSRM) which allows two-dimensional (2D) quantitative carrier profiling of fully integrated silicon NW-based tunnel-FETs (TFETs) with 2 nm spatial resolution. The key elements of our characterization procedure are optimized NW cleaving and polishing steps, the use of in-house fabricated ultra-sharp diamond tips, measurements in high vacuum and a dedicated quantification procedure accounting for the Schottky-like tip-sample contact affected by surface states. In the case of the implanted TFET source regions we find a strong NW diameter dependence of conformality, junction abruptness and gate overlap, quantitatively in agreement with process simulations. In contrast, the arsenic doped drain regions reveal an unexpected NW diameter dependent dopant deactivation. The observed lower drain doping for smaller diameters is reflected in the device characteristics by lower TFET off-currents, as measured experimentally and confirmed by device simulations.

Experimental studies of dose retention and activation in fin field-effect-transistor-based structures

With emerging three-dimensional device architectures for advanced silicon devices such as fin field-effect-transistors (FinFETs), new metrology challenges are faced to characterize dopants. The ratio of dopant concentration in the top surface and sidewalls of FinFETs may differ significantly, thereby influencing the performance of these devices. In this work, a methodology involving secondary ion mass spectrometry (SIMS) is presented to study the dose conformality in fins. However, SIMS is limited to probe the quantitative chemical dopant concentration (i.e., top/sidewall of fins). The fraction of the active dopant concentration determining the performance of FinFETs would still be unknown. Additionally, the concept based on SIMS is unable to provide information on the lateral junction depth. Thus, to obtain the unknown active dopant concentration and their spatial distribution, the authors extend their study by measuring the cross section of the fins with scanning spreading resistance microscopy and extracting the quantitative active carrier concentration in the fins.With emerging three-dimensional device architectures for advanced silicon devices such as fin field-effect-transistors (FinFETs), new metrology challenges are faced to characterize dopants. The ratio of dopant concentration in the top surface and sidewalls of FinFETs may differ significantly, thereby influencing the performance of these devices. In this work, a methodology involving secondary ion mass spectrometry (SIMS) is presented to study the dose conformality in fins. However, SIMS is limited to probe the quantitative chemical dopant concentration (i.e., top/sidewall of fins). The fraction of the active dopant concentration determining the performance of FinFETs would still be unknown. Additionally, the concept based on SIMS is unable to provide information on the lateral junction depth. Thus, to obtain the unknown active dopant concentration and their spatial distribution, the authors extend their study by measuring the cross section of the fins with scanning spreading resistance microscopy and extr...

Physical degradation of gate dielectrics induced by local electrical stress using conductive atomic force microscopy

Local electrical stress in gate dielectrics using conductive atomic force microscopy (C-AFM) induces structural damage in these layers. To allow C-AFM to become a mature technique to study oxide degradation, the impact of this structural damage, i.e., protrusions and holes, on the electrical behavior must be well understood. The physical nature and growth mechanism of protrusions due to a negative substrate voltage (Vs<0) is, however, debated in literature. In this work, we have studied the chemical composition of the surface protrusions using various analysis techniques (atomic force microscopy, transmission electron microscopy, and electron energy loss spectroscopy) showing that it consists of oxidized Si. A mechanism is proposed to explain the correlation between the observed surface damage and the measured current during constant voltage stress.

Toward extending the capabilities of scanning spreading resistance microscopy for fin field-effect-transistor-based structures

In this work, the authors investigate the present capabilities of scanning spreading resistance microscopy (SSRM) to map the carrier distribution in fin field-effect-transistor- (FinFET) based structures. Whereas for a planar metal-oxide-semiconductor transistor the distance to the back-contact is noncritical, this no longer holds true for a FinFET-based device as the limited cross-section of the fin may induce an important series resistance. The authors examine theoretically and experimentally the influence of the back-contact distance and the fin dimensions on the dominance of the spreading resistance. Based on the study, the authors propose a maximum distance for the back-contact that is needed to obtain a reliable two-dimensional map of the spreading resistance of fins with uniform doping concentration and fins with junction using SSRM. As the back-contacts are FIB deposited, the authors also study the influence of the Ga+ beam energy on the back-contact resistance, which adds critically to the bulk r...

Impact of the environmental conditions on the electrical characteristics of scanning spreading resistance microscopy

Within this study, the authors have investigated scanning spreading resistance microscopy (SSRM) measurements on silicon samples under different environmental conditions. The authors have mainly focused on the possibility to reduce the required force for obtaining a stable electrical contact between the probe and the sample, and to improve the reproducibility of the technique by cleaving and measuring the samples in a controlled ambient. The authors demonstrate that, by measuring samples that were cleaved and maintained in nitrogen (N2) ambient, the force needed for a stable electrical contact between the probe and the sample was reduced by more than a factor of 3 when compared to the force required in air. This leads to an improved signal to noise ratio and enhanced reproducibility with remaining variations now below 10% for n-type as well as p-type samples. At the same time, tip requirements are relaxed and tip lifetime is improved. Our work has demonstrated that in situ SSRM is a very good candidate to...

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.

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).