2, Intel Corp., Hillsboro, Oregon, United States
Strain is widely employed in semiconductor technology due to its ability to enhance carrier mobility, which provides an alternative to scaling for higher performance. For the length scale of latest generation devices like FinFET, electron nanodiffraction is the method of choice for strain characterization. A nano-sized probe is scanned across desired sample area and a series of diffraction patterns are acquired over a region of interest. With this technique, strain measurement can be done with high spatial resolution as well as a large field of view. The spatial resolution can be further improved by increasing the convergence angle of the electron beam. Thus, the technique eventually becomes scanning CBED (convergent beam electron diffraction), where diffraction spots are no longer sharp peaks but are disks. Identification of the disk position is crucial for strain analysis. Here we report on a systematic study of scanning CBED for high-accuracy strain measurement. We propose a new method to find the center of diffraction disk using circular Hough transform, and calculate the strain by fitting multiple disks. The method was applied to do strain mapping on a SiGe MOSFET. The limiting factors of measurement precision and accuracy are discussed to optimize the experimental conditions. Multislice simulations were done to examine the effects of 3D strain and curved lattices. The results are used to pave the way for strain characterization in complex crystals and nanostructures with high spatial resolution and accuracy.