TEM strain measurement methods on SiGe/Si films for accuracy tests

Transmission electron microscopy (TEM) is still considered a key analysis technique when high spatial resolution is required, even considering the generally lengthy, difficult and sometimes troublesome sample preparation. Such important advantage applies to imaging, structural and diffraction analysis and analytical investigations. The analysis and quantification of strain fields in crystalline materials also takes advantage of the TEM spatial resolution and over the last decades several TEM based techniques were developed [1]. In the framework of the CHALLENGES project financed by the EC (Grant agreement 861857 [2]), several methods have been used to validate the results of Tip Enhanced Raman Spectroscopy (TERS) in the characterisation of semiconductors (Si based) wafers and structures. Strain was one of the physical quantities measured in either thin film or patterned structures and TEM methods were used as standard reference. A series of three Si1-xGex heterostructutres, deposited as thin films onto a Si substrate, at different Ge concentration and thickness (see Table I), were used as ‘standard strain’ samples in order to compare all the techniques adopted (that includes optical based as well as diffraction based ones, such as TEM and X-ray Diffraction, XRD) onto the same samples. The Ge nominal concentration and SiGe film thickness were #1: 20%/45 nm, #2: 30%/29 nm and #3: 40%/22nm, respectively. We therefore took this opportunity to test different standard TEM strain analysis methods, namely Convergent Beam Electron Diffraction (CBED), Geometric Phase Analysis (GPA) and standard Selected Area Diffraction (SAD), in such samples using a conventional TEM, with the aim of evaluating the accuracy of the measurements with particular reference to the strain relaxation related to the lamellae thickness. The application of well established TEM techniques for crystal strain evaluation in simple SiGe/Si heterostructures showed some limitations. Despite the High strain sensitivity, CBED confirmed to be ineffective in samples regions were an high strain gradient or a strong strain relaxation occur. Both GPA and conventional diffraction were able to measure the uniaxial deformation, but best results were obtained in thick lamellae (>450 nm), thus confirming that the relation between strain vs. local sample thickness plays an important role in measurement accuracy.