Non-linear Raman shift-stress behavior in top-down fabricated highly strained silicon nanowires

Abstract

Strain engineering is a key technology to continue Moore's law with silicon or any other foreseen semiconductor in very large scale integration. The characterization of strain in nanostructures is important to determine the potential of these technologies, and it is typically performed using micro-Raman when investigating strained silicon. Here, we report on the Raman shift-stress behavior from the (001) silicon surface of highly strained ultra-thin (15 nm-thick) suspended nanowires with stresses in the range of 0–6.3 GPa along the [110] direction. We employ a strain technology that offers a precise control of stress values at large sampling while reducing variability. The stress level of the nanostructures has been accurately evaluated by the finite element method simulations and further correlated to the Raman spectra. For stresses below 4.5 GPa, the aforementioned behavior was linear and the extracted stress shift coefficient was in agreement with those reported in the literature. For stresses greater than 4.5 GPa, we show that the Raman shift-stress behavior resembles a quadratic function.

Effect of Native Oxide on Stress in Silicon Nanowires: Implications for Nanoelectromechanical Systems

“…The Raman shift, Δω, is commonly used to measure stress by linking the longitudinal optical (LO) mode obtained in backscattering configuration with stress components in the Si NW . In our case, Δω is defined as the shift between the unstrained Si peak, ω 0 , and the LO shifted strained Si peak, ω i , as given in eq . where S 11 and S 12 are the elastic constants and p and q are the phonon deformation profiles.…”

“…Relating the Raman shift to the uniaxial stress along the NW, the stress shift coefficient (SSC) of eq is an important parameter. A wide range of SSC is reported for Si NWs, where the SSC of −1.93 × 10 –9 cm –1 Pa –1 is recommended for NWs in sub-100 nm thickness . Therefore, the Raman shift given in eq can be written as …”

“…For this purpose, Raman spectroscopy is widely employed to measure uniaxial stresses in NWs − with a series of studies specifically addressing Si NWs. ,− Although Raman techniques have demonstrated a strain up to 4.5% in Si NWs patterned on highly strained silicon-on-insulator (sSOI) substrates, ,,, the level of strain reported for Si NWs fabricated on stress-free substrates is limited to 0.048% . In addition to Raman spectroscopy, X-ray diffraction approaches have been utilized to measure strain in Si NWs.…”

“…As native oxide is a very thin layer of amorphous SiO 2 forming on Si surfaces at ambient temperatures upon contact with air, it has an entirely different growth mechanism compared to that of thermal oxides. , In this respect, previous works reported a strain up to 0.05% in Si NWs, which increases to 0.5% or 160 MPa of stress in thermally oxidized Si NWs depending on their surface condition. The initial formation of the native oxide layer on Si surfaces and the kinetics of its growth have been the subject of various studies, where parameters such as Si doping level, NW shape, and crystal orientation were found to alter the thickness of the oxide layer in a range between 0.3 and 5 nm. − The intrinsic stress in Si NWs has been predominantly the main focus of various studies, where pre-strained substrates and thermally oxidized processes are used for fabrication. , Although a limited number of studies have presumed native oxide as a possible cause of intrinsic stress, ,− an in-depth study dedicated specifically to NWs is required to closely examine this link.…”

“…For this purpose, Raman spectroscopy is widely employed to measure uniaxial stresses in NWs 18−20 substrates, 11,21,22,24 the level of strain reported for Si NWs fabricated on stress-free substrates is limited to 0.048%. 23 In addition to Raman spectroscopy, X-ray diffraction approaches have been utilized to measure strain in Si NWs.…”