Main chain liquid crystal elastomers exhibit several interesting phenomena, such as three different regimes of elastic response, unconventional stress-strain relationship in one of these regimes, and the shape memory effect. Investigations are beginning to reveal relationships between their macroscopic behavior and the nature of domain structure, microscopic smectic phase structure, relaxation mechanism, and sample history. These aspects of liquid crystal elastomers are briefly reviewed followed by a summary of the results of recent elastic and high-resolution X-ray diffraction studies of the shape memory effect and the dynamics of the formation of the smectic-C chevron-like layer structure. A possible route to realizing auxetic effect at molecular level is also discussed.

Using a five-step atomic layer deposition (ALD)-anneal (DADA) process, with 20 ALD cycles of metalorganic precursors followed by 40 s of rapid thermal annealing at 1073 K, we have developed highly crystalline Hf_1−xZr_xO₂ (0 ≤ x ≤ 1) thin films (<7 nm) on ∼1 nm ALD Al₂O₃ passivated Ge and Si substrates for applications in higher-k dielectric metal oxide semiconductor field effect transistors below 10 nm technology node. By applying synchrotron grazing incidence x-ray d-spacing maps, x-ray photoelectron spectroscopy (XPS), and angle-resolved XPS, we have identified a monoclinic to tetragonal phase transition with increasing ZrO₂ content, elucidated the role of the Ge vs Si substrates in complete tetragonal phase formation (CTPF), and determined the interfacial characteristics of these technologically relevant films. The ZrO₂ concentration required for CTPF is lower on Ge than on Si substrates (x ∼ 0.5 vs. x ∼ 0.86), which we attribute as arising from the growth of an ultra-thin layer of metal germanates between the Hf_1−xZr_xO₂ and Al₂O₃/Ge, possibly during the first deposition and annealing cycle. Due to Ge-induced tetragonal phase stabilization, the interfacial metal germanates could act as a template for the subsequent preferential growth of the tetragonal Hf_1−xZr_xO₂ phase following bottom-up crystallization during the DADA ALD process. We surmise that the interfacial metal germanate layer also function as a diffusion barrier limiting excessive Ge uptake into the dielectric film. An ALD Al₂O₃ passivation layer of thickness ≥1.5 nm is required to minimize Ge diffusion for developing highly conformal and textured HfO₂ based higher-k dielectrics on Ge substrates using the DADA ALD process.

Mesoporous materials with tailored properties hold great promise for energy harvesting and industrial applications. We have synthesized a novel tungsten bronze mesoporous material (K_xWO₃; x ∼ 0.07) having inverse FDU-12 type pore symmetry and a crystalline framework. In situ small angle X-ray scattering (SAXS) measurements of the mesoporous K_0.07WO₃ show persistence of a highly ordered meso-scale pore structure to high pressure conditions (∼18.5 GPa) and a material with remarkable mechanical strength despite having ∼35% porosity. Pressure dependent in situ SAXS measurements reveal a bulk modulus κ = 44 ± 4 GPa for the mesoporous K_xWO₃ which is comparable to the corresponding value for the bulk monoclinic WO₃ (γ-WO₃). Evidence from middle angle (MAXS) and wide angle X-ray scattering (WAXS), high-resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy shows that the presence of potassium leads to the formation of a K-bearing orthorhombic tungsten bronze (OTB) phase within a monoclinic WO₃ host structure. Our ab initio molecular dynamics calculations show that the formation of the OTB phase provides superior strength to the mesoporous K_0.07WO₃.

In this paper, we report on Muller Matrix (MM) based scatterometry (aka. optical critical dimension or OCD) simulation strategies for two sub-7nm fin structures: one with a SixGe1-x/Si/SixGe1-x/Si/SixGe1-x/Si nanosheet structure and its comparison with a hypothetical Si-only fin structure of similar dimensions at sub-7nm semiconductor technology nodes. Si-fins are providing the performance improvements necessary for the transistors used in current generation integrated circuits. Development of sub-7nm technology nodes requires further performance improvements including advanced structures including new materials. This demand for improved performance has created the need for new fabrication processes such as extreme ultraviolet lithography, self-aligned quadruple patterning etc., along with new metrology challenges for adequately monitoring the semiconductor process control during fabrication of these advanced nanostructures.1 New materials and structures, such as, Si/Si1-xGex (0≤x≤1) stacked nanosheet structures have recently been developed as one of the potential replacement for Si-based FinFETs.2,3 We have simulated two fin structures: one with Si-fins and another with 3 alternating stacks of Si1-xGex/Si (x=0.3). We have used Rigorous Coupled Wave Approximation (RCWA) to simulate OCD spectra for 0° – 360° azimuthal angles in 10° steps by keeping the fin pitch fixed at 24nm while systematically changing (1) the fin critical dimension (CD) from 5.0–7.5nm in 0.5nm steps, (2) thickness of the Si and Si1-xGex (x=0.3) nanosheets (NST) from 8.0–1.5nm in 0.5nm steps, (3) the fin bending angle (FBA) from 0°–2.5° in 0.5° steps, and (4) the undercut angle (ED) of the Si1-xGex NSTs from 0°–10° in 2° steps. Difference in etch-rate of Si and Ge could give rise to non-zero ED during NST formation. Both the fin structures possess mirror symmetry about two orthogonal planes perpendicular to the substrate and with one of the planes along the fin long-axis. For angles 0°, 90°, 180°, and 270°, this mirror symmetry leads to absence of the cross-polarization terms in the simulated OCD spectra such that off-diagonal MM elements are zero over the simulated wavelength range. The broken mirror symmetry in other azimuthal angles leads to cross-polarization of the electrical field vectors which is seen in the off-diagonal MM elements becoming non-zero. Additionally, the off-diagonal MM elements show increased sensitivity and reduced correlation between the parameters under study which leads to a powerful metrology means for studying the critical fin-parameters for fast and reliable semiconductor process control in sub 7-nm technology nodes. All the non-zero MM-elements are found to be sensitive to changes in CD and NST with the off-diagonal elements showing greater sensitivity to the changes. At azimuthal angles <15°, MM23, MM31, and MM34 are highly sensitive to changes in etch undercut and fin bending angle defects. References: [1] D. Dixit et al., “Advanced applications of scatterometry based optical metrology,” Proc SPIE 10145, 101451H (2017). [2] N. Loubet et al., “Stacked nanosheet gate-all-around transistor to enable scaling beyond FinFET,” VLSI Technol. 2017 Symp., T230–T231, IEEE (2017). [3] R. Muthinti et al., “Advanced in-line optical metrology of sub-10nm structures for gate all around devices (GAA),” Proc SPIE 9778, 977810 (2016).

Structural studies by synchrotron x-ray diffraction on two main-chain smectic-C elastomers reveal the presence of two different relaxation mechanisms in these systems at low and high strains. At low strains, the smectic layers are reoriented with layer-normals distributed in a plane perpendicular to the stretch direction. The system relaxes relatively slowly (time-constant ~ 45 minutes) which is attributed to the flow properties of the LC layers embedded in the elastomer network. At high strains, the equilibration time (~ 4 - 8 minutes) conforms to a faster relaxation and appears to have its origin in the polymer components of the system. Due to misaligned microdomains at small strains, the value of global orientational order parameter S for the mesogenic parts is initially small (~ 0.15). With increasing strain, the local domain-directors, the mesogens, and the polymer chains, all tend to align parallel to the stretch direction giving rise to a higher measured value of S ~ 0.83 at a strain of 4.0. The siloxane segments remain less ordered, attaining a value of S ~ 0.4 for a strain of 4.0. The layers gradually become oblique to the stretch direction conforming to the structural property of the smectic-C phase. The system finally assumes a chevron-like optically monodomain structure. Both elastomers are locked-in this state even after removal of the external stress giving rise to strain retention and the shape memory effect. The presence of a transverse component in the main-chain leads to higher strain retention in the second elastomer. A preference for the orientation of the smectic layer-normals toward the stretch direction persists after removal of external stress. Upon thermal annealing, the chevron-like microstructure gradually melts via a different path to the initial polydomain structure.