Invited Paper MC-TuM12
Strain Measurement using Electron Beam Techniques

Tuesday, October 20, 2015, 11:40 am, Room 114

Strain can modify deeply material properties such as optical emission, transport properties or structural strength. With the development of nanotechnologies, the need of tools that can measure strain with high accuracy (about 0.01%) and high spatial resolution (about 1 nm) has appeared. The demand of Microelectronics industry has been particularly strong since Intel has implemented strained channels to boost the transport performance of their devices, and during the last decade, many new TEM base techniques have been developed to reach these goals. Of course, not only the microelectronics industry, but also any fields involving nanomaterials will benefit from these developments.

In this presentation, after a short review of the different TEM techniques, we will focus on the solution we have developed and chosen: Nanobeam Precession Electron Diffraction (N-PED). Like in all TEM diffraction techniques, a small electron beam is made and diffraction patterns are acquired at different positions of the electron beam. In addition, in N-PED, the incident electron beam is rotated by a small angle around the observation direction and a descan is applied after the sample in order to bring back the diffracted beams to their unprecessed positions. In fact there is a compromise between spot size, beam convergence and precession angle. We adopted a setting where the beam convergence is about 2.2 mrad, the probe diameter is of about 1 nm, and the precession angle is below 0.5°. The advantages of this setting for strain measurement are mainyfold : (i) the diffraction spots have disk shapes and do not saturate, (ii) the intensity within the diffraction disks is more uniform (iii) more diffraction disks are visible (iii) a greater accuracty is obtained by locating the edges of the disks, (iv) the measurements are very stable versus changes in sample thickness or orientation and (v) strain maps of 4 components of the 3D strain tensor can be obtained with one zone axis orientation. We will show how this simple and robust N-PED technique has been used successfully for the analysis of microelectronics devices and nanostructures. In our FEI TITAN ultimate microscope where we used a Gatan Ultrascan CCD camera, the main drawbacks of N-PED are (i) its relatively slow speed and (ii) the amount of stored data to acquire large maps. For instance, to acquire 100x50 diffraction patterns containing 1Kx1K pixels, it took 90 minutes and 12 Gbytes on the hard disk. However with the new available fast cameras and larger disks, these issues are greatly reduced.