Characterizing the effect of uniaxial strain on the surface roughness of Si nanowire MEMS-based microstructures
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Characterizing the effect of uniaxial strain on the surface roughness of Si nanowire MEMS-based microstructures. E. Escobedo-Cousin1, S.H. Olsen1, T. Pardoen2, U. Bhaskar2, J.-P. Raskin2 1 2
Newcastle University, Newcastle upon Tyne, United Kingdom Université Catholique de Louvain, Louvain-la-Neuve, Belgium
ABSTRACT This work addresses the paucity of roughness measurements by reporting on roughness parameters in uniaxial strained Si beams relevant for state of the art MOSFETs, nanowire and MEMS devices, with varying degrees of strain. Roughness is characterized by high resolution AFM and strain is characterized by Raman spectroscopy. Microstructures comprising a silicon nitride actuator are used to induce a wide range of stress levels in Si beams. The microstructures also allow the comparison of surface evolution in the strain direction (along the Si beam) compared with the unstrained direction (across the Si beam). A gradual reduction in rms roughness amplitude and increase in roughness correlation length in the direction of the applied stress are found for increasing values of strain. In contrast, surface roughness in the direction perpendicular to the applied stress remained largely unchanged from the unstrained initial state. INTRODUCTION This work uses an original MEMS concept to strain released silicon beams in order to analyze the relationship between on-chip applied strain and nanoscale surface roughness in Si. Roughness affects carrier mobility through surface roughness scattering at high electric field operation in metal-oxide-semiconductor field-effect transistors (MOSFET). Nanoscale roughness parameters such as rms height (Δ) and correlation length (Λ) are key parameters to model the carrier mobility at high electric fields. Simulations indicate that a reduction in Δ and Λ compared with bulk Si values can explain the high values of electron mobility observed experimentally in tensile strained silicon devices. However, due to the limited characterization techniques available to measure roughness accurately on a nanoscale, to date such assertions have remained largely unconfirmed. Δ can be calculated by atomic force microscopy (AFM) and accurate values of Δ are available to device modellers for several degrees of biaxial strain. In contrast, the determination of Λ requires further work and its actual impact on mobility is not fully understood, therefore, Λ is often used in simulations as an adjustment parameter between theoretical and experimental data. Two recent works have successfully found experimental correlation between surface roughness parameters and the device performance of biaxially strained Si MOSFETs. Bonno et al used high resolution AFM measurements to compare the surface of the strained and unstrained Si, and a statistical analysis of the images was used to extract the roughness parameters 1. Both Δ and Λ were found to reduce in the presence of strain, although only one level of strain was studied (0.8%). A similar reduction of roughness parameters in strained Si layers was found by Zhao e
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