The NCEM sample preparation lab hosts a variety of instruments use to prepare thin sections for transmission electron microscopy. The sample preparation methods can broadly be divided into 2 kinds of processes : physical thinning, or chemical thinning. Here is a description of the available tools in each category:
Wet (Chemical) sample preparation lab
The chemical lab is used to prepare TEM sections using wet processes such as :
1. Fume hoods to allow drop casting of nanoparticle solutions: Nanomaterials suspended in a liquid medium (such as a volatile organic solvent) can often be directly deposited on TEM grids. This process can be conducted in the fume hoods in the wet sample preparation lab.
2. Electrojet polishing: The wet lab is equipped with a Fischione 110 twin-jet polisher, which uses an electrolyte jet (typically a solution containing a mineral acid) to thin the sample via electrochemical thinning. The solutions can be cooled to liquid nitrogen temperatures to control the rate of thinning and an integrated sensor can be used to automatically stop the process once the sample in sufficiently thin. This method is suited for metallic samples, which are already polished down into foils of approximately 30-100 micron thickness, and which have been cut into 3mm discs.
3. Osmium and ruthenium staining: The lab includes a dedicated fume hood for staining low contrast TEM specimens (such as biological specimens and polymers). The staining process uses a controlled exposure to either Ruthenium oxide or Osmuim oxide vapors, which can improve sample contrast in the TEM.
Dry (Physical) sample preparation lab
The dry lab is used to prepare thin TEM sections as well as polishing samples for EBSD which requires a scratch free sample surface. The available instruments in this lab include:
1. Allied multiprep: The multiprep can be used to mechanically polish samples to electron transparency. Diamond lapping films can be mounted on the polishing surface, and the TEM sample can be positioned with an accuracy of a few microns on the polishing wheel. Samples may be prepared using flat polishing followed by ion milling, or alternately via wedge polishing.
2. Southbay diamond saw: The diamond saw can be used to cut wafers into smaller pieces for TEM sample preparation. The saw can be positioned accurately on the sample via a micrometer screw.
3. Buehler MiniMet grinder polisher: This is a semi automatic grinder/polisher provides a random polishing motion, and allows users to set the speed, pressure and time of polishing. The soft stop feature reduces the load as the sample reaches its final polishing sequence.
4. Gatan Dimple Grinder: A quick method to polish samples down to <5 microns of final thickness using loaded polishing wheels covered with a diamond paste which acts as an abrasive.
5. Buhler mechanical polisher: This mechanical polisher uses fine grit sand paper as an abrasive to provide smooth surfaces for scanning electron microscopy or electron backscatter diffraction. Sample positioning on the abrasive is done using a manually held fixture, which provides more flexibility than the multiprep, but allows the user less control over the sample position / thickness.
6. Dace vibrational polisher: This vibration polisher is designed to prepare smooth, scratch free samples for high quality EBSD imaging. Samples are mounted onto polishing cloths with colloidal silica slurries, and a high frequency horizontal vibration removes surface scratches from specimens.
7. Ultrasonic cutter: Allows users to cut brittle samples into 3mm diameter discs for further polishing using dimpling, mechanical polishing, or jet polishing.
8. Gatan PIPS ion mill: This argon ion mill uses two ion beams, with Ar+ ions with kinetic energy varying from 0.5-8keV. The ion beam is unfocused with a nominal waist of about 1mm, which produces large thin areas. Samples can also be cooled to liquid nitrogen temperatures to ensure local heating does not create sample preparation artifacts. Samples need to be mounted onto a Mo or diamond 3mm TEM grid, and thinned using mechanical polishing before ion milling.
9. Fischione Nano mill: This ion mill uses focused low energy Ar+ ions (from 2keV down to 50eV) which can be used to remove FIB damage, surface implantation, and amorphization layers on thin sections, typically prepared by FIB. The samples need to be mounted on half moon TEM grids. Unlike the PIPS, the Nano mill uses a focused ion beam with a spot size of ~ 1 micron to polish samples. The sample stage can be cooled by liquid nitrogen during the milling process.
10. Gatan Carbon/Gold sputter coater: This sputter coating chamber uses high energy, high current Ar+ ion beams to sputter coat samples. Currently, the available sputter targets include Carbon, Au, Cr and Pd. Thin films of sputter coating can be used to mitigate charging issues in the FIB or SEM.
11. Fischione plasma cleaner: This plasma cleaner uses Argon plasma to remove hydrocarbon impurities from samples. Typically, TEM samples are loaded onto either an FEI or JEOL holder, and plasma cleaned along with the TEM holder. Currently, there are adapters to accommodate either an FEI holder, a JEOL holder, or a small basket for larger samples up to an inch in size.
12. RMC Cryomicrotome: The microtome can be used to make thin sections of soft materials such as biological tissue, or polymer samples. A diamond knife can be used to make the sections, and samples can be mounted into molds of epoxy. An integrated optical microscope allows uses to align the sample and knife. Appropriate knives must be purchased by individual users depending on the requirements of their project. The lab has a glass knife maker, which can be used for some particularly soft samples, where section thickness is not critical. However, for high precision microtomy, the use of diamond knives is encouraged. Sections as thin as 50 nm can be produced using this method, which can then be transferred to TEM grids or glass slides for further imaging. A cryogenic stage allows uses to cool the sample to liquid nitrogen temperatures before microtomy, which can improve sections.
