Discover the basics of SEM and the unique advantages of ZEISS Gemini electron optics
The ZEISS GeminiSEM family stands for effortless imaging: get sub-nanometer resolution and high detection efficiency, even in variable pressure mode. GeminiSEM 500 combines proven Gemini technology with a novel electron optical design. Achieve better resolution, especially at low voltage. With 20 times greater Inlens detection signal, you will always acquire crisp images fast and with minimum sample damage. The new variable pressure mode makes you feel like you’re working in high vacuum.
This article highlights the history and principle of scanning electron microscopy as well as current applications of ZEISS Gemini technology for high-end nanoimaging at low voltages and advanced materials analysis. Download the knowledge posters and White Papers as free pdf files and contact us for further questions via our website!
A Small World of Huge Possibilities
Antonie van Leeuwenhoek created the first light microscope in the mid-1600’s. About 200 years later, Ernst Abbe, working with German engineer and entrepreneur Carl Zeiss, published the observation that the resolution of a microscope could be defined as the wavelength of the light used, divided by twice the numerical aperture. In 1931, Ernst Ruska built the first transmission electron microscope. Four years later, Max Knoll discovered a means to sweep an electron beam over the surface of a sample, creating the first scanning electron microscope (SEM) images.
It has been 50 years since the first commercial scanning electron microscope (SEM) was launched by the Cambridge Instrument Company, a UK-based predecessor company of ZEISS Microscopy. SEMs create surface images of bulk material by scanning an electron beam over the sample, recording the resulting echoes and electrical interactions point by point. Resolution in the nanometer range is routine. SEMs use electromagnetic “lenses” to focus an electron beam to a sharp point and raster-scan across the sample. They create images by recording the interactions of the electron beam with the sample surface, which could be a ceramic material, metal, or biological specimen. These interactions can take many forms, and SEM users can install a wide range of spezialized detectors around the sample chamber to explore and analyze them.
ZEISS Gemini optics: High resolution at low voltages on real world samples
The unique feature of ZEISS field emission scanning electron microscopes is the Gemini electron optical column. It consists of a beam booster, a Gemini objective and one or two Inlens detectors depending on the model. The Gemini concept ensures efficient signal detection and simultaneously guarantees small probe sizes and high signal-to-noise ratios. Real world samples are imaged easily with high resolution and high contrast, even magnetic or very beam sensitive samples.
The Gemini objective lens design combines electrostatic and magnetic fields to maximize optical performance while reducing field influences at the sample to a minimum. This enables excellent imaging, even on challenging samples such as magnetic materials. The Gemini Inlens detection concept ensures efficient signal detection by detecting secondary (SE) and backscattered (BSE) electrons in parallel minimizing time-to-image. Gemini beam booster technology guarantees small probe sizes and high signal-to-noise ratios. The Gemini II column in GeminiSEM 500 comes with a novel electron optics: the newly designed Nano-twin lens even further improves resolution at low beam voltages.
Novel Optical Design of Field Emission SEMs
High resolution imaging of sensitive samples can be a challenge. A novel optical design in ZEISS GeminiSEM 500 enables easy handling of sensitive samples. Imaging at low acceleration voltages – between 1 kV and 5 kV, or even lower than 1 kV – has become the golden standard in field emission scanning electron microscopy recently. In order to achieve excellent images from sensitive samples it is mandatory to avoid beam damage and to balance the SE yield and beam current for charge neutrality.
With the new optical design of the Gemini column it is easy to achieve optimal imaging conditions for charging samples like Al2O3 at 1 kV. In the Al2O3 sample surface, steps of one to several monolayers can be imaged (left image). The second possibility to image sensitive or outgassing samples is to work at higher pressures in the specimen chamber, under so-called variable pressure conditions. The driver for imaging samples under higher pressure has always been the possibility of examining insulating materials without modifying the surface by a conductive coating. The new variable pressure (VP) technology permits operation at higher pressures with high resolution and improved signal detection efficiency, allowing to use detectors that were previously available only for high vacuum conditions. With the pressure-limiting aperture in operation the natural fibers coated with silver nano-particles were imaged at variable pressure conditions with 80 Pa (right image) in the chamber with the Inlens SE detector (on the left) and the Inlens EsB detector (on the right), gaining topography and material contrast information. Sample courtesy of SBUK and Dr. F. Simon, Leibniz Institute of Polymer Research Dresden, Germany.
Surface Sensitive Imaging with a Bias on the Sample
Tandem decel is an approach applied in the ZEISS GeminiSEM family to decelerate the electron beam twice: once in the Gemini lens and a second time before the electron beam ultimately hits the specimen by feeding a negative bias voltage on the sample. The effect is an improvement in resolution and contrast, especially at low landing energies.
Gold particles on carbon are used as the standard sample to verify resolution in an FE-SEM. At a landing energy of 1 kV and a bias voltage of -5 kV the Inlens SE detector shows excellent resolution and additionally details on the surface of the particles (left). Furthermore, backscatter images, that always have an overall lower resolution than SE images, due to the information depth of the scattered electrons, show an effect when using Tandem decel. In a series, different landing energies were tested on Fe2O3/ZrO2 particles: 1 kV final landing energy produced images with best results regarding resolution and surface sensitivity with a backscatter detector (right image).
Variable Pressure Imaging
The motivation behind the development of variable pressure (VP) scanning electron microscopy was the ability to observe non-conductive samples without modifying the surface with a conductive coating. Unlike a conventional SEM, which only works in high vacuum, the VP SEM can operate under a gas pressure in the specimen chamber. A novel concept for this, the so-called NanoVP in ZEISS GeminiSEM instruments, significantly improves image quality with respect to resolution and signal collection, while a number of detectors can be selected under variable pressure.
The positive electrode of a Lithium-ion battery was imaged with the Inlens EsB detector to derive material contrast information at low acceleration voltage (5 kV, pressure = 80 Pa, BGPL = 0.8 mm, left image). The Inlens EsB collects pure backscattered electrons and is capable of filtering electrons according to their energy, thus delivering images with unique backscatter information. This was only available for high vacuum applications before it became accessible for variable pressure conditions with the introduction of the NanoVP now.
Benefits of Back Scatter Detectors at Low Voltages
Analyze materials properties with the help of backscatter detectors and appropriate imaging conditions in ZEISS field emission scanning electron microscopes. Discover how to differentiate between grain orientation and differences in atomic number and how to take topographical information into account when interpreting the images. This Technology Note describes channeling, materials and topography contrast comparing two detectors, one mounted on the objective lens, the second being an in-column detector.
The images of Fe2O3/ZrO2 nanoparticle composites deposited on carbon were imaged at 1.5 kV landing energy. The topography and material contrast are clearly separated on the Inlens SE (left) and EsB (center) detectors. When imaged with the BSD detector (right) with 5 kV bias, the material contrast is mixed with topography.
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