Acceptance Angle Control
T in fl exible cantilevers also provide an improved measure of instrument protection; damage to the pole piece or other detectors is unlikely because the cantilevers will defl ect if unexpected contact occurs. With this holder, samples are not limited to 3 mm foils/ substrates; the cantilever can grip the edge of a self-supporting sample of arbitrary shape. Gripping a sample this way minimizes sample holder shadowing eff ects.
Figure 2 : (a) An illustration of the modular aperture system components, and (b) an assembly view including the STEM detector and the new cantilever-style sample holder with a sample at an arbitrary orientation. Primary electrons are shown emerging from the SEM pole-piece and scattering in a forward direction through the sample and to the detector.
STEM detector design . A Zeiss LEO 1525 SEM equipped with a Schottky fi eld emission electron gun was used to image the samples at 30 kV with a 30 μ m condenser aperture, resulting in a spot size of 4–5 nm and a probe current ~165 pA. Detectors used here included a KE-Developments STEM detector, an Everhart-T ornley secondary electron (SE) detector, and an ETP Semra Series 8.6 Robinson backscattered electron (BSE) detector.
The new STEM detector comprises two plates ( Figure 1a ): an upper plate for DF imaging with four rectangular diodes surrounding a 100 μ m diameter through-hole and a lower plate with a diode for BF imaging positioned under the through-hole. Note that angular selectivity built into this detector is minimal, and acceptance angle adjustments must be obtained through changes in CL, defined here as the distance between the sample and the detector diode. The STEM detector also has an xyz -positioning stage to align the diodes with the optic axis. This detector positioning feature can also be used to elicit unconventional and potentially useful image contrast.
One step toward comprehensive acceptance angle control involves moving the detector to an appropriate distance from the pole piece. For example, when the transmitted electron detector is at its lowest position ( Figure 1b ), the distance between the top of the detector and the bottom of the pole piece is ~20 mm, thereby maximizing the available CL and space for positioning the sample. When the transmitted electron detector is set at its highest position, the distance between the detector and pole piece is ~10 mm.
Cantilever specimen holder . Positioning a sample at any location and/or orientation between the detector and the pole piece is also essential for comprehensive STEM-in-SEM imaging. A new cantilever-clamp-style sample holder [ 10 ] can be used for that purpose ( Figure 1c ). In addition to allowing precise sample positioning, the cantilevers can be made very thin, allowing the sample to be located almost anywhere in the vacant space between the STEM detector and pole piece.
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Detector acceptance angles . Acceptance angle ranges are improved just by switching from the carousel-style to the cantilever- style holder and using the 20 mm of CL available when the detector is at its lowest position. For example, the acceptance half-angle range available for BF imaging with the
existing 100 μ m through-hole and the carousel-style holder is ~10 < β < 25 mrad. Substitution of the cantilever-style holder enables a BF range of ~2.5 < β < 50 mrad. For DF imaging with the vendor-supplied carousel-style holder, the acceptance half-angle range is ~85 < β < 1270 mrad. Substitution of the cantilever-style holder expands that range to ~20 < β < 1420 mrad. The minimum inner acceptance half-angle ( β i ≈20 mrad) is due to a small gap between the diodes and the 100 μ m through-hole. However, when a single diode is used, β can be much smaller than 20 mrad, thereby enabling marginal and annular BF imaging. Perhaps the key step toward comprehensive acceptance angle control is the mask/aperture system. One embodiment of the system includes two main components: (1) a support frame
Table 1 : STEM signal collection modes and their associated acceptance angles. The primary electron beam convergence semi-angle is α , and the inner and outer acceptance semi-angles are β i and β o (see Figure 3a ). Note that these defi nitions are not all-encompassing and that other defi nitions may exist.
STEM-in-SEM
Signal Collection Mode Brightfield (BF)
Annular Brightfield (ABF) Marginal Brightfield (MBF)
Low-Angle Annular Darkfield (LAADF) *
Medium-Angle Annular Darkfield (MAADF) *
High-Angle Annular Darkfield (HAADF) *
Acceptance Angle Range β i = 0, β o < α 0 < β i , β o < α β i ≈ α,
Thin Annular Detector (TAD) β o ≈ 1.1 β i
β i > α β o ≤ 50 mrad
β i > 50 mrad β o < 100 mrad
β i ≥ 100 mrad
* LAADF, MAADF, and HAADF distinctions are somewhat arbitrary, and the ranges provided are typically associated with high-energy STEM. Because lower-energy electrons scatter more strongly, these values will be somewhat higher for STEM-in-SEM.
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