Fast STEM Spectrum Imaging Using Simultaneous EELS and EDS

Introduction Up to now, researchers performing analytical investigations in the transmission electron microscope typically had to choose between analytical spectroscopy techniques. They might choose energy dispersive X-ray spectroscopy (EDS) analysis when working with thick samples containing high-Z elements or choose electron energy-loss spectroscopy (EELS) when studying low-Z materials in thin samples [1]. With the advent of STEM instruments possessing both highmechanical stability and high-brightness probes [2], coupled with the latest generation of fast, efficient X-ray and EELS detectors [3, 4], choosing between complementary techniques is a significant restriction. The acquisition systems available up to now have forced a choice because EELS, EDS, and fast scanning have not been designed to work together, resulting in inefficient data collection. In this article, we present a data collection system that is designed to allow EELS, EDS, and dark field detectors to operate efficiently at spectral rates over 1 kHz. This is achieved by allowing each detector system to acquire data independent of the other. The data collection systems are, however, linked together by a synchronization clock pulse to ensure the resulting datasets are in exact spatial alignment. The system described here was recently implemented using DigitalMicrograph together with the Bruker Esprit EDS system. The necessary timing precision is achieved by synchronizing the EELS and the EDS spectrometers with the beam scanning system as shown schematically in Figure 1. During simultaneous spectral acquisition, the EELS spectrometer provides system synchronization by means of a clock pulse at the end of each acquired frame. This pulse is used to advance the beam position and to inform the EDS system that the pixel has advanced. The EELS and EDS data are stored locally in software buffers and transferred asynchronously to the host application to ensure no data is lost. This mode of acquisition can be performed with any combination of the available signals and is also compatible with other features such as DualEELSTM [3, 5, 6] acquisition and spatial drift correction.