Stress and Strain Evolution in Stacked Gate-All-Around Transistors for Sub-7nm Node Studied By Advanced Transmission Electron Microscopy Techniques and Finite Element Method Modelling
Authors: Reboh Shay , Coquand Rémi , Loubet Nicolas , Bernier Nicolas , Chao Robin , Audoit Guillaume , Rouviere Jean-Luc , Barraud Sylvain , Augendre Emmanuel , Li Juntao , Muthinti Raja , Gaudiello John , Gambacorti Narciso , Yamashita Tenko , Faynot Olivier 
Published: February 2020

In recent years there was a major structural transition from planar 2D to 3D FinFETs in Si-technology of transistors providing improvement of electrical characteristics for advanced devices. A further evolution to stacked gate-all-around channels (SGAA) as NanoWires or NanoSheets (NS), is expected to overcome the limitations of FinFETs for sub-7nm nodes. The manipulation of the elastic strains in the channel of the transistor is in all cases expected to improve devices characteristics by increasing the mobility of charge carriers. As strain engineering in such devices is based on the manipulation of structure and material properties, the intricate 3D structure of SGAA renders the task particularly challenging. The fabrication flow of SGAA is similar to FinFETs, but based in the formation of fin-patterned Si/SiGe multilayers instead of a single semiconductor. Therefore, the lattice mismatch between Si and SiGe generates significant mechanical stress and interactions from the beginning of the integration process (left of Fig 1a). In this framework, here we explore the unique mechanics of SGAA transistors from a theoretical and experimental perspective.
In a first moment, to understand the strain/stress generation and relaxation effects along the integration we developed models based on Finite Element Method and identified the critical steps in the integration. We show that a major component of strain in the Si-channels is generated after dummy gates and spacers are done and the multilayer is recessed in the unprotected regions of the fin for the later fabrication of source and drain by epitaxy. At this step, the relaxation of the SiGe layers induce a tensile stress in the Si-channels as shown in the central image of Fig. 1a. A second important transition occurs at the step that characterizes the principal difference from SGAA with FinFET integration. It comes after the dummy gate is removes and the sacrificial SiGe layers are etched resulting in suspended Si-channels. According to our model, the tensile stress previously generated is significantly relaxed (right of Fig. 1a). However, a tensile characteristic tend to be present when the channels will be wrapped by the gate providing a favorable situation for NMOS but detrimental for PMOS.
In a second moment, we present our progress to validate, adjust or correct the proposed models based on experimental results. For this, we use geometric phase analysis and precession electron diffraction (PED) techniques of advanced transmission electron microscopy (TEM) to assess the strains with nanometer spatial resolution. The protocols needed for interpretation of the results and comparison with models will be discussed. Structures for SGAA fabricated in different substrates such as bulk-Si, virtually relaxed SiGe, strained Silicon-On-Insulator and compressive SiGe-On-Insulator are examined in this work.
As an example of our results and methodology, in Fig. 1b we display a dark-field TEM micrograph of a fin-patterned multilayer Si/SiGe on virtually relaxed SiGe substrate along the fin-direction. It is composed by a relaxed substrate Si0.8Ge0.2, a 50 nm thick Si layer (dark contrast) and the 10nm Si/7nm Si0.65Ge0.35 (bright contrast) multilayer ended by a top-layer of 17 nm Si0.65Ge0.35. Figures 1c corresponds to a map of the vertical strain εzz obtained by PED. Strain values are given with respect to the Si0.8Ge0.2 substrate taken as reference in the experiment. The Si layers have therefore a smaller lattice because of the tensile strain in the horizontal direction and the different composition of the layer with respect to the reference. The Si0.65Ge0.35 naturally present higher values due to the higher Ge content compared to Si0.8Ge0.2, but also due to an elastic reaction to the compressive horizontal strain associated to lattice mismatch. In Fig. 1d the results from model calculation of the structure considering the experimental spatial resolution and convolution effects for PED. The agreement with experiment allows the deconvolution of the map to present the real strain in the structure (Fig. 1e). The map in Fig. 1f shows the lattice rotations in the structure. The top portion of the stack rotates outward due to the predominant relaxation of the Si0.65Ge0.35. The bottom part rotates inward in association to the contraction of the Si layers.
A particularly interesting result considering SOI-type substrates will also be presented. We reveal that for compressive SiGe-On-Insulator the patterning process of the multilayer at room temperature induces stress relaxation by a gliding mechanism along the interface with the insulator. As a consequence, elastic energy is transferred to the Si layers creating high tensile uniaxial strain, a favorable configuration to improve the mobility of electrons in NFETs. The phenomenon is modelled and discussed in terms of its possible physical origin/mechanism.

Figure 1