Size Effect on Raman Measured Stress and Strain Induced Phonon Shifts in Ultra-Thin Film Silicon

TL;DR

This paper investigates how the size of ultra-thin silicon films affects the accuracy of stress measurements using Raman spectroscopy, highlighting the importance of accounting for size-dependent mechanical properties.

Contribution

It demonstrates the deviation of mechanical properties in silicon films thinner than 5 nm and provides guidelines for accurate Raman stress analysis in nano-scale structures.

Findings

Mechanical properties deviate from bulk values below 5 nm thickness.

Ignoring size effects can cause up to 400% errors in stress measurement.

Best practices for interpreting Raman data in ultra-thin films are proposed.

Abstract

The fabrication of complex nano-scale structures, which is a crucial step in the scaling of (nano) electronic devices, often leads to residual stress in the different layers present. This stress gradient can change many of the material properties and leads to desired or undesired effects, especially in the active part of the transistor, its channel. Measuring, understanding, and, ultimately, controlling the stress fields is hence crucial for many design steps.The level of stress can in principle be measured by micro-Raman spectroscopy. This, however, requires \emph{a priori} knowledge of the mechanical properties of the material. The mechanical properties start to deviate from the bulk values when film dimensions become thinner than 5 nm. If this effect is ignored, errors of up to 400\% can be introduced in the extracted stress profile. In this work, we illustrate this effect for a range of Si (001) slabs with different silicon film thickness, ranging from 5 to 0.7 nm and provide best practices for the proper interpretation of micro-Raman stress measurements.