Transmission Electron Microscopy in Micro-nanoelectronics

Chapter 4

Dark-Field Electron Holography for Strain Mapping

4.1. Introduction

Strained silicon is now an integral feature of the latest generation of transistors and electronic devices [ITR 11, GHA 03, ANT 06] because of the associated enhancement in carrier mobility [LEE 05, THO 06]. Strain is also expected to have an important role in future devices, for example optoelectronic components [JAC 06]. Different strategies have been used to engineer strain in devices, leading to complex strain distributions in two and three dimensions [ACO 06, PAR 06]. Developing methods of strain measurement at the nanoscale has, therefore, been an important objective in recent years because, at the time, none of the existing techniques combined the necessary spatial resolution, precision and field of view [ITR 11, FOR 06]. For example, Raman spectroscopy or X-ray diffraction techniques can map strain at the micrometric scale, whereas transmission electron microscopy (TEM) allows strain measurement at the nanometer scale but only over small sample areas. The technique described in this chapter, dark-field electron holography (DFEH), was developed specifically to solve this problem and measure strain to high precision, with nanometer spatial resolution and for micrometer fields of view [HYT 08, HYT].

TEM is the only tool capable of measuring strain at the nanoscale; the techniques fall into two broad categories: diffraction-based and image-based. Convergent beam electron diffraction (CBED) is the ...