Sample Preparation
Overview
Always contact staff if you want to image a new type of specimen in a microscope.
In general, better prepared samples yield better images. This typically includes ensuring a sample is clean, dry, conductive, vacuum stable, and an appropriate size. The requirements depend on the microscope being used. If a sample does not meet the recommendations for a particular microscope, additional processing may be required.
You should know:
- the general composition of your specimen and substrate
- the overall size and geometry of your specimen
- the form of your specimen
- the approximate size of features you want to image
- the relative conductivity of the specimen and substrate
Specimens must be loaded onto an appropriate sample holder designed to interface with the microscope stage. Improper loading of a specimen may result in damage to the instrument, sample holder, or your specimen.
Scanning Electron Microscopy
Most specimens for SEM yield the best images when they are conductive. Specimens may be insulating if:
- the specimen is sufficiently small or thin (e.g. less than 100nm thick) and on a conductive (or semi-conductive) substrate. This means the bulk of the electron beam will scatter through the specimen and interact with the substrate, leaving only a small portion of electrons in the insulating specimen. These specimens may perform best in Low Voltage SEM
- the specimen is sputter coated. Sputter coating applies a thin coating of a conductive metal to the specimen surface, which gives the electron beam an “escape route,” reduces potential damage due to the beam, and increases the yield of scattered electrons for an improved signal-to-noise ratio. In some cases, a thin strip of copper tape may be added if the specimen is thick or on a less- to non-conductive substrate. Copper tape should make contact with the conductive part of the specimen and the stub or sample holder.
- the specimen is being imaged in Low Vacuum SEM.
Transmission Electron Microscopy
Most TEM specimens are prepared on 3mm grids. Specimens should ideally be a maximum of about 100nm thin, conductive, dry, and clean. If needed, a plasma cleaner is available for removal of hydrocarbon contamination.
Cross-section TEM specimens may be prepared with the lift-out technique in the FIB.
Always contact staff before loading a new type of sample into the TEM, or if you have any questions about your specimens.