No abstract

Aberration-corrected scanning transmission electron microscopes (STEMs) can resolve single atoms, probe them spectroscopically, and produce elemental and chemical maps with atomic resolution in 2D and 3D. When equipped with a monochromator and operating in an aloof mode, STEMs can record vibrational electron energy loss spectra (vib-EELS) to create nm-scale vibrational maps while avoiding radiation damage. These techniques require excellent short-term (vibration) and long-term (drift) stability of the microscope's sample stage.Typical side-entry stages are mounted on the side of the microscope in such a way that the sample is connected to the objective lens (OL) polepiece through a~30 cm long mechanical path. This means that if the temperature of the microscope (or of the sample rod) changes by just 0.1°C then the sample can drift by hundreds of nm. This results in image drift rates of the order of nm per minute being common with side-entry stages in all but the most stable environments.In order to minimize these kinds of problems, Nion's cartridge-based sample stage was designed to have an OL polepiece-to-sample mechanical path of 10 cm and a thermal expansion center that coincides with the microscope's optic axis [1]. The detachable sample cartridges have no direct link to the microscope's outside. These two innovations have resulted in drift rates smaller than 1 nm per hour and sample vibration amplitudes <0.1 Å r.m.s. [2]. Unfortunately, compared to a side-entry sample rod that supports the sample at one end and exits outside the microscope vacuum at the other end, a detachable cartridge in not as well suited for bringing in "services" to the sample such as cooling, heating, electrical, gases, liquids, straining, tilting, and nano-indenting.To combine the stability and freedom from drift of our centro-symmetric design with the flexibility of a side-entry sample rod, Nion has developed a new side-entry sample stage. The first version of the stage has been built for Orsay's CHROMATEM project: a high energy resolution monochromated STEM-EELS (HERMES) instrument, whose requirements include cathodo-luminescence (CL) capabilities with both light detection and injection [3], and a liquid nitrogen-cooled sample holder. A CL mirror can be introduced on the entrance side of the sample, in a 6 mm OL polepiece gap. The mirror is UHV-compatible and precisely moveable, so that the mirror's focal spot can be made to coincide with the "optimum probe formation spot" of the objective lens, about 0.5 x 0.5 x 2 µm in size (w x d x h). The mirror can couple out CL, or bring in laser illumination, as needed for instance for energy-gain spectroscopy. The cathodo-luminescence capabilities of the system are complemented by Nion's sub-10 meV energy resolution ground-potential monochromator [4], and a new Nion electron energy loss spectrometer optimized for vibrational studies.