Evolving Electron Microscopes Push Even Deeper

 

Wed, 06/03/2015 - 2:58pm Tim Studt Technological advances in electron microscopes provide images and information of yet unknown materials and reactions. Image of FEI’s Titan.Electron microscopy is a multi-scale, multi-modal and multi-dimensional technique for imaging materials down to the atomic level. Developed in 1931 by German physicist Ernst Ruska and electrical engineer Max Knoll, the electron microscope (EM) has evolved from Ruska’s initial 400X capabilities to its current 10,000,000X performance. The much smaller wavelength of electron beams, compared to visible light for optical microscopes, allows EMs a much higher resolving power for imaging samples. EMs consist of two similar configurations—the transmission EM (TEM) and the scanning EM (SEM). The primary focus of a SEM is to image a sample surface, while that of a TEM is to look what is inside or beyond the surface. EM technologies continue to evolve, each year improving upon limitations inherent in the devices. It was long recognized the resolution of TEMs was limited by intrinsic imperfections in the device’s electromagnetic focusing lenses, commonly referred to as spherical aberrations. A five-year research collaboration between the U.S. Dept. of Energy’s Lawrence Berkeley National Laboratory, Argonne National Laboratory, Oak Ridge National Laboratory, the Univ. of Illinois at Urbana-Champaign, FEI Co. and CEOS (Corrected Electron Optical Systems) GmbH solved these issues with a 0.05-nm resolution research target obtained in 2009. FEI and CEOS recently announced a new collaboration with Germany’s Univ. of Ulm to develop a sub-Angstrom, low-voltage electron (SALVE) microscope. This multi-year collaboration will involve the development of a dedicated aberration-corrected TEM capable of imaging radiation-sensitive materials, such as 2-D and organic samples, and selected molecules with molecular or even atomic-scale resolution. The device is also expected to provide spectroscopic information at very low acceleration voltages. “After the commercial introduction of spherical aberration corrections in TEMs, the main interest lay in the development and routine application of aberration-corrected wide-field TEM,” says Don Kania, CEO of FEI. Over the past five years, researchers have focused on the combination of analytical methods with superior lateral resolution. “Through simultaneous acquisition of various signals from the sample, a better understanding of the sample properties now became possible,” says Kania. With the detection of four simultaneously acquired STEM (scanning TEM) signals, we can now image phase-contrast dependent structures together with z-number dependent large angle detection.” These STEM applications can be combined with EDS (energy dispersive x-ray spectroscopy) and/or EELS (electron energy loss spectroscopy) for new applications, such as 3-D imaging of proteins and associated complexes using reconstruction software. By utilizing a STEM detector divided into four quadrants, researchers can also measure the intrinsic magnetic and electrical fields of samples. “These electrical fields, in particular, can now be measured down to 50-pm resolution” says Kania. These differential phase-contrast imaged fields can be compared to electron holography and combined with simultaneously acquired EDS and EELS signals. Electron holography is holography created with electron waves that are created in an off-axis scheme. The holographic image is created by a split electron beam that produces and interference pattern of equidistant spaced fringes. Aberration-corrected TEM and STEM systems can also be used to image electron-beam-sensitive 2-D materials like graphene with atomic resolution. In these applications, the TEM/STEM systems are optimized toward lower high tensions. Direct electron detectors, such as FEI’s Falcon II CMOS camera which enhances the detector’s quantum efficiency, have initiated a revolution in cryo-microscopy, according to Kania. With these detectors, users can collect thousands of high-quality images per day with standard 1-sec exposures. This detector can operate continuously without interruption 24/7. High-end EMs Hitachi Ltd. recently announced its development of an atomic-resolution holography EM accelerated at 1.2 MV, with a claim for the world’s highest point resolution of 43 pm. The instrument incorporates the first spherical aberration corrector in an ultra-high-voltage EM. It also has a new field emission electron gun with a vacuum 100X greater than conventional field emission guns (3 x 10-10 Pa). The gun can thus operate stably for more than 10 hr, without the need for readjustments of the spherical aberration corrector. The solid base in which the instrument is installed is lined with acoustic damping materials to minimize the effects of vibration, which special alloys also surround the instrument to act as a magnetic barrier. The instrument is expected to be used for research in the R&D of advanced functional materials through the understanding of quantum phenomena relating to the functions and properties of high-performance materials such as magnets, batteries and superconductors. Carl Zeiss AG also recently introduced a new generation of field-emission SEMs (FE-SEMs) with a novel optical design. Their GeminiSEM includes a Nano-twin lens design providing high contrast with sub-nanometer resolution. Resolution improvements are also enhanced at low beam voltages. Zeiss’ NanoVP (variable pressure) design allows the use of in-lens detection at pressures up to 150 Pa, and ensures efficient signal detection by detecting secondary (SE) and backscattered (BSE) electrons in parallel, minimizing the time to image. Detector signals are boosted by up to 20X under low-voltage imaging conditions. The objective lens design combines electrostatic and magnetic fields to maximize the optical performance while reducing field influences at the sample to a minimum. This enables excellent imaging, even on challenging magnetic material samples. The high-resolution gun mode also minimizes aberrations and allows for smaller probe sizes. Hybrid systems WITec GmbH integrated its alpha300 confocal Raman imaging system with a SEM to create its RISE microscope. With this system, ultra-structural surface properties can be connected to molecular compound data. The instrument allows fast and easy switching between the SEM and Raman measurements, and enables automated sample transfers from one measuring position to the other. Once positioned on the instrument’s scan table, the sample is automatically transferred and repositioned between the EM and Raman measuring procedures. The sample remains inside the vacuum chamber during the complete measurements to ensure a convenient workflow with ease-of-use. The instrument has in-beam SE and BSE, as well as in-chamber SE and BSE detectors. “RISE was developed as part of the EU-funded project UnivSEM, which supports the development of supplementary analysis tools for SEMs and underlines the importance of hybrid microscopes,” says Philippe Ayasse, RISE product manager. Its integrated software interface provides easy measurement controls and doesn’t compromise on either of the SEM or Raman imaging capabilities. The device includes advanced 3-D beam technologies for true stereoscopic imaging, 3-D experience and 3-D navigation. It also has a diffraction limited lateral resolution of 200 to 300 nm. An ultra-fast Raman imaging option is also available with only 0.76 msec integration times per spectrum. Other options include a high- and low-vacuum operation and a combined electron beam and focus ion beam (FIB) system are available. Future developments for this system include the integration of a new automated Raman imaging system to RISE. “Its automated and user-friendly setup is an ideal complement to a SEM environment featuring a push-button instrument,” says Ayasse. “This system features pre-defined calibration routines, automated laser wavelength selection and automated absolute laser power determination that facilitates quick and user-friendly system maintenance.” HybriScan Technologies offers a similar Raman-SEM hybrid instrument, its HSCMM, which is an integrated Raman microscope and SEM. The HSCMM provides a correlation between chemical and morphology properties. The Raman-based device requires no optical realignment and it is directly coupled with the SEM, it can also work as a stand-alone Raman instrument. FEI offers a correlation system that leverages both light and electron microscopy in a workflow approach. “A light microscope is used to find a feature of interest and then the sample is seamlessly transferred to the electron microscope for high-resolution imaging,” says FEI’s Kania. Their system for doing this is CorrSight, the only purpose-built light microscope designed around the requirements in a correlative experiment. In the CorrSight, the automated imaging system is run by FEI’s MAPS (modular automated processing software) which manages the entire workflow from the light microscope to the EM, ensuring automated registration of the instruments and images together. Features on the user’s light microscopy images are used as the basis for correlation, including fiducials and cellular structures. Expected changes “Over the next five years in the electron microscopy arena, we expect to see advances primarily in software, workflows, usability and productivity,” says Kania. “The time to results (or data throughput) will also significantly improve due to advances in automation procedures and algorithms. Both hardware and software requirements will be boosted by user requests for more data within a shorter timeframe. “We also believe that the classical life science market for routine 2-D and 3-D imaging of plastic-embedded material will go more in the direction of SEM and small-stage Dual-Beam, due to the increased resolution and contrast in these tools. The life science TEM market will have more emphasis toward 2-D and 3-D cryo-TEM.” source : www.rdmag.com