Zeiss Gemini 460VP SEM

Zeiss Gemini 460 SEM with Oxford Instruments Ultim Max 100 mm² EDS and Symmetry S3 EBSD detectors

Zeiss Gemini 460VP SEM

Ideal for:

• High resolution imaging, especially for beam sensitive samples under low kV imaging conditions.
• Phase analysis by BSE imaging.
• Elemental analysis for material and phase identification using EDS.
• Phase identification and grain size analysis using EBSD.

Equipped with an “UltimMax” 100 mm² large area analytical silicon drift detector from Oxford Instruments for Energy Dispersive Spectroscopy (EDS) analysis of elemental compositions. The system is capable of point, line and area scans, as well as “live” elemental mapping. The Symmetry S3 Electron Back Scatter Diffraction (EBSD) detector and associated Aztec HKL advanced software from Oxford Instruments, enables phase identification, phase mapping, and grain size analysis at high magnification, on polished surfaces or ion beam prepared cross-section samples.

The Zeiss Gemini 460VP SEM is fitted with a state-of-the-art Gemini 2 Electron Optical Column which enables a number of unique imaging functions over and above the traditional in chamber SE and BSE detectors.

Everhart-Thornley Secondary Electron (SE) Detector
InLens Secondary Electron Detector
Variable Pressure Secondary Electron Detector
Low Accelerating Voltage (Low kV) Imaging
Back Scatter Electron (BSE) Imaging
InLens Back Scatter Detector
Elemental Analysis by Energy Dispersive X-ray Spectrometry (EDS)
Electron Back Scatter Diffraction (EBSD)

Everhart-Thornley Secondary Electron (SE) Detector

This is the most widely used SE detector in SEM’s for general purpose topographical imaging. It is located within the chamber, between the bottom of the objective lens/pole piece and the sample. The “collector grid” in front of the detector is typically operated under a positive applied voltage (typically set @ +300V). The electric field generated by this voltage attracts low energy secondary electrons which are emitted from the sample surface due to interactions between the primary beam electrons and the atoms of the sample material. It also detects higher energy backscattered electrons that hit the detector due to their angle of escape from the sample i.e. the image is a composite, dominated by topographical information from the secondary electrons but with contributing atomic mass information from the backscattered electrons. Note: Other detectors are used for dedicated BSE only imaging.

InLens Secondary Electron Detector

The InLens SE detector is a high efficiency detector for high resolution imaging and detects secondary electrons directly in the beam path. The high detection efficiency arises from its geometric location in the beam path combined with the electrostatic field generated by the electrostatic lens beneath the objective lens, which accelerates the electrons from the sample surface up to the detector.

The high surface resolution is generated by the preferential attraction of SE1 electrons. These are secondary electrons emitted at the point of impact between the electron beam and the specimen. Because they are generated only at the very surface of the sample, they are ideal for displaying surface structures at high resolution. The efficiency of the InLens SE detector is mainly determined by the electric field of the electrostatic lens, which decreases exponentially with distance. Therefore, the “working distance”, between the sample and lens/pole piece, is one of the most important factors affecting the signal to noise ratio of this detector.

For accelerating voltages of 20V to 10kV, working distances of 0-5 mm are recommended. For accelerating voltages of 10 kV -20 kV, working distances of 2-5 mm are recommended for optimum image generation.

Variable Pressure Secondary Electron Detector

A specialised SE detector used in variable pressure mode where the standard Everhart-Thornley SE detector cannot be used (Note: this detector cannot be used under high vacuum conditions). It is a separate detector that sits within the chamber beneath the lens/pole piece and the sample. The variable pressure mode enables the imaging and analysis of nonconducting specimens without charging artefacts. This is because positively ionised gas molecules stabilise local charging. As such, variable pressure mode can also be used for imaging and analysis of strongly outgassing or moist specimens without any need for specimen preparation.

The detector has a positive potential for attracting secondary electrons. As these electrons move towards the detector they interact with gas molecules within the chamber that are present under variable pressure conditions. The accelerated secondary electrons excite gas molecules in their path, which emit a photon as they de-excite to the ground state. It is these photons which are detected and used to generate the image. Note: The contribution of BSE electrons to image formation is <1% in this mode. However, this mode reduces the resolution that is possible compared to imaging under high vacuum conditions.

Low Accelerating Voltage (Low kV) Imaging

The accelerating voltage determines the kinetic energy of the primary beam electrons before they interact with the sample. When the primary beam electrons impact the surface they undergo scattering events due to their interaction with the sample atoms. If they have a high kinetic energy (high kV) they will undergo multiple scattering events and at greater depths within the sample compared to low kinetic energy (low kV) electrons. This means that they will generate a large number of SE electrons, but from a large volume of analysis in the sample. This means very high signal generation and, therefore high signal-to-noise ratios which allow higher quality images. However, because of the large analysis volume, the information from the surface of the sample is “washed out” by the information generated within the material. The high kinetic energy electrons may also damage the surface of sensitive samples.

Lowering the accelerating voltage reduces the kinetic energy of the incoming electrons. The analysis volume of the incoming electron beam in the sample is dramatically reduced such that the detected secondary electrons come mostly from the surface/near surface region of the sample. As a result, the images are much more sensitive to surface topography.

Small topographical features and very thin surface films that would be “invisible” under high kV imaging can be successfully imaged at low kV.

In addition, the high energy electrons generated under high kV conditions can generate significant damage to the sample surface, potentially distorting or transforming the surface features and morphology that users may be trying to detect and characterise. Low kV imaging also offers advantages in imaging nonconductive samples, as it helps to mitigate “charging” of the sample surface.

These characteristics make low kV imaging especially suited to biological samples, carbon-rich materials such as polymers, and very thin surface layers such as in electronics or oxidised/contaminated surfaces.

Back Scattered Electron (BSE) Imaging

The Variable Pressure (VP) Back Scatter Detector (BSD) is a pneumatically retractable detector capable of operating under high vacuum and variable pressure conditions. When inserted pneumatically, it sits around the bottom of the pole piece. It consists of five separate diodes segments detectors – a central ring detector, surrounded by four quadrant detectors. The central ring detector provides primarily material contrast information, while the quadrant detector signals can be added to give additional topographical contrast.

The detectors are only “activated” by high energy electrons such as BSE’s and not low energy SE’s, making it very sensitive to sample phase contrast.

The emission of BSE’s from the specimen is related to the atomic mass of the material interacting with the electron beam. Higher atomic mass elements generate more backscattered electrons, generating a higher signal which is seen as higher brightness in the image. Lower atomic mass elements appear darker. The contrast of compounds and alloy phases is determined by the relative wt% of the elements in the analysis volume.

One disadvantage of this detector design is that, under low kV operating conditions, a lot of the BSE’s are lost in the central hole and cannot be detected. In such instances, users should shift to using the InLens BSE detector.

InLens Back Scatter Detector

The InLens Back Scatter Detector is referred to by Zeiss as the Energy-Selective Backscatter (EsB) detector. It is located within the column, above the InLens SE detector. BSE electrons generated at the surface are accelerated up the column in the same way as SE electrons for the InLens SE detector. A small percentage of SE electrons pass through this detector and would be detected by the EsB detector. To prevent this, a filtering grid voltage is installed in front of the EsB detector which repels the low energy SE’s and only enables the higher energy BSE’s to be detected.

By varying the filtering grid voltage it is also possible to select the desired energy of the BSE’s to enhance contrast and resolution.

The signals from the InLens SE and EsB detectors can be combined to allow simultaneous imaging and mixing of high contrast topography (SE) and compositional contrast (BSE).

Elemental Analysis by Energy Dispersive X-ray Spectrometry (EDS)

Interaction of the primary electron beam with the surface generates a number of different output signals which provide information about the sample. One interaction type occurs when an incoming primary beam electron interacts with an electron in an inner shell of the atom, exciting it and ejecting it from the shell to leave an electron vacancy or electron “hole”. When an electron from an outer, higher energy shell drops down to fill this hole, it releases an x-ray due to its change in energy. The magnitude of the energy change depends upon the difference in energy between the outer, higher energy shell and the inner, lower energy shell, as well as on the atomic structure of the element. Critically, the x-rays emitted in this way have different energies that are characteristic for each element. Detection of these x-rays by an EDS detector, therefore, enables identification of elements in the sample. The intensity of the x-rays can be used to quantify how much of each element is present within the analysis volume. The data is presented as a graph of counts per second/electron volt (eV) versus energy of the x-ray in kilo electron volts (keV).

The Oxford Instruments Ultim Max 100 mm² Silicon Drift EDS detectors on the CEMMS SEM’s combine extremely large sensor sizes (100 mm² in this case) with extreme electronics, to deliver unparalleled speed and sensitivity. The extremely large area of detection enables up to 10 X more data to be collected in the same time as “conventional” systems, with no loss of accuracy. This enables much faster data collection, superior analysis of small features and nanostructures, superior detection of low atomic mass elements (Li, B, C, N etc) and better statistics per analysis data point. The speed of analysis enables extremely large areas to be accurately mapped within a standard SEM session, while also enabling “live” elemental imaging for the first time.

The advanced AZtecLive software has a number of innovative, next-generation features to advance your analysis, particularly the “Tru” analysis quantification which analyses the spectra quantitatively to eliminate artefacts, automatically corrects for element peak overlaps and removes false variations due to the x-ray background.

Electron Back Scatter Diffraction (EBSD)

Electron Back Scatter Diffraction (EBSD) is an ancillary technique within the SEM that uses the interaction of the electron beam with the sample to study the crystallographic structure of the material. In this analysis the sample is tilted 70° from the horizontal. When the primary electron beam hits the sample, backscattered electrons are generated in the conventional manner. When these BSE’s leave the sample, they interact with atoms of the material and are elastically diffracted and lose energy, leaving the surface at various scattering angles. The scattered BSE electrons form characteristic “Kikuchi” patterns on the EBSD detector. These can be indexed to generate phase identification (when combined with elemental data from the EDS), grain structure, orientation and size, as well as crystallographic texture and defect and stress densities.

The Oxford Instruments Symmetry S3 EBSD detectors on both SEM’s use customised CMOS detectors and fibre optics to generate a powerful combination of analysis speed, sensitivity and diffraction pattern detail.

The high analysis speed (in excess of 5700 patterns per second) enables texture and grain size characterisation in a matter of seconds, even at low beam currents and without sacrificing pattern resolution. This is particularly significant in the analysis of challenging, real-world samples, such as multiphase alloys and plastically deformed steels.

In addition, the Symmetry S3 detector can collect distortion-free, megapixel resolution patterns to enable detailed strain and phase analysis.