Mineral and Materials Characterization Facility – Field Emission-Scanning Electron Microscope

Field Emission-Scanning Electron Microscope

The Mineral and Materials Characterization Facility is host to a state-of-the-art TESCAN MIRA3 LMH Schottky field emission-scanning electron microscope (FE-SEM) that is used for imaging at magnifications of 10x to over 300,000x at a spatial resolution down to the nanometer scale. In addition to imaging, the instrument is used for semiquantitative chemical analysis by energy-dispersive X-ray spectroscopy. The instrument forms an integral part of the undergraduate education in petrology and mineralogy and can be accessed by undergraduate students during scheduled class and laboratory activities or as part of their supervised research. The instrument is widely used by graduate students in economic geology, petroleum geology, geological engineering, and hydrogeology.  Diverse research is conducted by faculty members from Mines and external users, including government agencies, museums, and private businesses.

The FE-SEM was aquired by Drs. Thomas Monecke and Katharina Pfaff in September 2015. Purchase of the instrument was made possible through a partnership agreement between TESCAN and Mines. Major donors who contributed to the instrument purchase included Apache Corporation, Tim Bartshe, Marshall and Jane Crouch, Hugh and Michelle Harvey, John and Erica Lockridge, and Jim Weber.

Specifications of the MIRA3 LMH Schottky FE-SEM

Internal size of chamber:230 mm
Width of door:148 mm
Maximum specimen height:81 mm
Stage:Compucentric, 5-axis fully motorized
Resolution of SE detector:1.2 nm at 30 kV and 2.5 nm at 3 kV
Chamber vacuum: <9×10-3 Pa
Gun vacuum:<3×10-7 Pa
Field of view:6.4 mm at working distance of 10 mm (Easy Mode training level)
Electron gun:High-brightness Schottky emitter
Acceleration voltage:200 V to 30 kV (20 kV in Easy Mode training level)
Probe current:2 pA to 200 nA

Secondary Electron Imaging

The FE-SEM is also equipped with a TESCAN In-Beam secondary electron detector that detects secondary electrons emitted back into the objective allowing specimen examination at a very short working distance. This detector is not available to users at the Easy Mode training level.

The In-Beam SE detector has an outstanding resolution of 1 nm at an acceleration voltage of 30 kV and of 2 nm at an acceleration voltage of 3 kV.

Backscattered Electron Imaging

The FE-SEM is equipped with a TESCAN motorized retractable annular, single-crystal YAG backscatter electron (BSE) detector. BSE images show variations in the mean atomic number in the sample region imaged. The full functionality of the BSE detector is available to user at the Easy Mode training level. The image to the left shows a view of the sample chamber. The retractable BSE detector in operating position is located immediately below the pole piece of the FE-SEM. The EDS detector can be seen in the left behind the pole piece.

At the Easy Mode training level, users are able to acquire BSE images at a working distance of 10 mm, typically using an acceleration voltage of 20 kV. The detector is able to reach a resolution of 2 nm. The system is characterized by a high sensitivity and atomic number resolution of 0.1 Z. 

Cathodoluminescence Imaging

The FE-SEM is equipped with a TESCAN Rainbow detector that is used to image visible light emitted from a sample impacted by electrons. One of the channels of the detectors is panchromatic while the other three channels collect red, green, and blue (RGB) signals that are electronically recombined into a live color CL image. Color calibrations of the electronically combined RGB channels is easy to perform as the RGB acquisition channels of the detector can be independently adjusted using the control software. Depending on the application, panchromatic or live color CL images, or both at the same time, can be collected.

The minimum working distance of the Rainbow CL detector is 13 mm. The detector has a unique design (Kolosova & Jiruse, 2014) that allows a field of view of ~4 mm at a working distance of 15 mm and ~5 mm at 20 mm working distance. The large field of view is exceptional and is highly beneficial for applications that require a combination of standard optical microscopy, optical CL, and SEM-CL. Panchromatic imaging with the Rainbow CL detector is conducted over a spectral range of 350-850 nm. 

Energy-Dispersive X-Ray Spectroscopy

The FE-SEM is equipped with a Bruker XFlash® 6/30 silicon drift detector for energy-dispersive X-ray spectroscopy (EDS). EDS is a technique that is based in the detection of X-rays emitted from the sample during bombardment by the electron beam and allows the elemental composition of the analyzed sample volume to be determined. The full functionality of the EDS system is available to users at the Easy Mode training level.

Active area:30 mm2
Silicon thickness:0.45 mm
Window material:Super-light element window
Collimator:Multi-layer, 6.0 mm aperture
Radiation entry:Axial
Elevation angle:30°
Working distance:10 mm

User Policies

Training Level Policy

Based on the level of experience in electron microscopy and familiarity with the instrument and laboratory-specific procedures, users are given access to the software of the FE-SEM at different training levels. Most users of the laboratory facility have access privileges at the Easy Mode training level, which provides full functionality of the instrument at a fixed working distance of 10 mm and an acceleration voltage of 20 kV. Training of new users is currently only provided at the Easy Mode training level. A presentation for new users at the Easy Training level can be downloaded here.

To minimize the risk of a collision between the sample/stage with the detectors, only users with significant experience are given full software access to all five motions of the motorized sample stage (i.e., z-axis and stage tilting). Similarly, usage of the BSE and in-beam SE detectors at working distances below 10 mm is restricted to a limited number of users only. Users will be granted higher access privileges on a case by case basis.

Sign-Up and Billing Policy

After users have been authorized to operate the instrument, users will be granted access to the online calendar to schedule time on the instrument. Users at the Easy Mode training level can only sign-up during prime time hours, which are 9 am to 5 pm Monday through Friday. Certain prime time slots are reserved for teaching and instrument development work and cannot be booked using the online calendar. Users are strongly encouraged to sign-up only for the amount of instrument time required. Users may reserve time only for themselves and are not allowed to turn-over time slots to other users without permission.

Reserved time will be forfeited if a user does not begin within 10 minutes of the scheduled start time. If users cancel a reservation less than 24 hours prior to a scheduled start time or do not use reserved time, they will be charged for a minimum of one hour of instrument time. Billing is performed in 30 min increments and commences at the reserved starting time. When reserving time, users should leave 30 min between the end of the previous session and the start of their own session to allow sufficient time to get ready (mounting of samples etc.).