Quickly Switchable Angular and Spatially Resolved Cs-corrected STEM

In this paper, we discuss a methodology to set up quickly switchable angular and spatially resolved Cs-corrected STEM, equipped with a hexapole-type probe corrector. With this method, the convergence semi-angle (Alpha) can be varied or tuned, though with instrument specific upper and lower limits. The method allows us to conduct angular and spatially resolved STEM, by simply changing the CL aperture size, without rigorous geometric aberrations corrections between these two imaging conditions. Although the angle could be either increased or decreased, the smaller angles in the nanobeam regime, where diffraction disks are separated, seem to offer more advantages, particularly when combined with 4D-STEM, or even precession electron diffraction (PED), for example, strain [1], orientation mapping [2] and differential phase contrast imaging (DPC) [3]. To vary the convergence semi-angle, Alpha, in a hexapole-type probe corrector, two condenser lenses above the corrector, in our case, JEOL Grand ARM 300CF with a widegap polepiece, the condenser lens 2 (CL2) and the condenser lens 3 (CL3) need to be adjusted as a pair. Only one condition needs to be met, the CL2's crossover must be at the front focal plane of CL3 so that CL3 produces a parallel illumination to the corrector, to prevent significant increase in geometric aberrations. The condition also ensures that all the scan calibrations, including magnification stay the same. For a larger angle, CL2 must be stronger, moving its crossover up. In order to compensate, CL3 needs to be weaker to move up its front focal plane to the CL2 crossover position, as shown in Fig. 1 (b). For a smaller angle, CL2 is to be weaker, bringing its crossover down. CL3 must be strengthened to move its front focal plane (closer to CL3) to where CL2 crossover is located as in Fig.1 (c). In practice, CL2 and CL3 pair-adjustment can be done by keeping the Ronchigram in focus. As one of the CLs is changed, the Ronchgiram goes out of focus. Refocusing the Ronchigram by changing the other CL satisfies our condition. The smallest attainable angle was limited by the maximum excitation of CL3 in our microscope. Once the optimum CL2 and CL3 values are determined, it is best to record them so that they can be recalled later. It is likely that geometric aberrations increase more than tolerable, especially for the first time. The star aberration and four-fold astigmatism increased in our study. After some manual corrections, followed by JEOL's COSMO iterations, which utilizes the SRAM method [4], geometric aberrations are corrected as good as our original setting. Figure 2 (b) is a typical Ronchigram in our standard setting, where Alphas of 100um and 10um CL apertures are 75mrad and 7.7mrad, respectively. Once CL2 and CL3 are adjusted and aberrations are corrected, as shown in Fig. 2 (c), Alphas of 100um and 10um are decreased to 20.3mrad and 2.06mrad, respectively. It means that simply changing the CL aperture between 100um and 10um, we could switch Alphas between 20.3mrad (spatially resolved) and 2.06mrad (angular resolved), except for one key aspect, a high probe current in the 20.3mrad setting. The spatially resolved setting uses a 100um CL aperture and its current was measured to be 312pA, although the angular resolved setting, using a 10um CL aperture, had a current of 3.2pA. The probe size of spatially resolved setting is dominated by the finite source size due to its high current [5], whereas the probe size of the angular resolved setting is dominated by the diffraction limit, as shown in Fig. 2 (f). Reducing the probe current below the finite source size limit would not only improve the attainable resolution by reducing both the finite source size effect and the energy spread (dE), as plotted in Fig. 2 (f), but also decrease the radiation damage that the high probe current might inflict on the samples. It is rapidly achieved by adjusting the emission current from 15uA (full emission) to 1∼4uA range. A methodology for setting up a quickly switchable high-angle and -resolution probes in Cs-corrected STEM is presented. By adjusting two CLs before the probe-corrector, the convergence semi-angles of 75mrad and 7.7mrad are decreased to 20.3mrad (high resolution) and 2.06mrad (nanobeam/high angle resolution), respectively without breaking the aberration corrections. These two imaging conditions can be easily and quickly switched by changing the CL aperture and the emission current, without rigorous aberration corrections [6]. Simplified ray diagrams for (a) standard, (b) larger and (c) smaller convergence semi-angles. Ronchigrams with 100um and 10um CL apertures (a) before and (b) after CLs are adjusted; (c) typical diffraction pattern from gold particles with Alpha of 2.06mrad; (d) STEM ADF image of cold particles with a 100um Aperture (Alpha =20.3mrad, Ip = 35pA); (e) STEM ADF image of gold particles with a 10um aperture (Alpha = 2.06mrad, Ip = 3.3pA); (f) Calculated probe diameters with Cc, diffraction, and finite source size limits.