CHALLENGES - Strain analysis in semiconductor devices using tip-enhanced Raman spectroscopy

The progressive downsizing of semiconductors is driving information processing technology into a broader spectrum of new applications and capabilities. Strained silicon has become one of the best solution for integrated circuits thanks to the advantages and efficiencies it brings. In these technologies, a biaxial tensile stress is applied to the silicon in the channel region of a MOSFET, increasing the mobility of carriers. This stress can be imposed by doping the silicon underneath with germanium, causing a mismatch between the lattice constant thus improving the electrons’ mobility. Over the years, there has been an increasing need, especially in the industrial sector, to develop faster and non-destructive characterization techniques to monitor strain during the manufacturing phases of semiconductor devices. Currently, Tip-Enhanced Raman Spectroscopy (TERS) is one of the best solution for strain characterization, as it is a non-contact and non-destructive technique with a lateral resolution of a few nanometers and the capability of analyzing and collecting signals from the most superficial layer of a sample. The enhanced field is strongly restricted to the surface plasmons region, just a few nanometers deep, thanks to the simultaneous use of a nanometric tip of an Atomic Force Microscope (AFM) and a laser from a Raman spectrometer. The analyzed sample consists of a (100) silicon substrate where an epitaxial layer of Si1-xGex with x=0.3 and thickness of 17 nm is grown following several patterns. The manufacturing processes of this sample are subject to trade secret under the EU CHALLENGES project https://www.challenges2020.eu/). The AFM probe employed is characterized by an innovative coating which enables its implementation in the clean room for in-line characterization. Its production is also to trade secret under the same project. This technique is used to map the variation in the position of the silicon peak (≈520.5 cm-1) along the sample pattern in order to identify the strain profile with a resolution of a few nanometers. The results confirm that TERS represent a powerful tool in monitoring the quality of production lines in the semiconductor industry and currently provides the best resolution among the Raman techniques for the strain characterization.