In order to acquire the best analytical results for microstructure, metrology, micro-hardness or micro-elemental analysis; specimens are often mounted in a resin “puck” and polished to a mirror finish. The resin mount is often precision sectioned to expose critical areas of interest that lie within the specimen. This process assures that a flat oxide-free layer is exposed for accurate imaging and chemical analysis.
Specimen types suitable for resin mounting include:
- Metals (for inclusion and grain analysis)
- Semiconductor Wafers and Components
- Metal Plated Components
- Circuit Boards for plated pathways and via’s
- Multi-Layer Plastic Films
- Minerals (both whole and ground for extraction)
- Seeds and hard or woody plant material
- Ceramics, Composites and Polymers
The shape, size, and numbers as well as the hardness, brittleness, porosity, heat and pressure sensitivity of the specimens have to be considered when resin mounting. A suitable mounting media (resin) with sufficient hardness, adhesion and chemical resistance is needed. A variety of acrylic and polyester resins with different curing times and properties are commercially available.
When selecting a resin, it is important to utilize resins that do not outgas and create vacuum problems. There are many tricks and tips available to guide users on how to prepare a great SEM mount. For instance, low viscosity resins and the use of vacuum impregnation are very useful to help support friable and porous specimens. Watch for publications on these topics and a future FAQ posts on Ion Mill Cross Section Polishing to achieve the absolute best defect free surface for imaging at the nanometer scale.
Polished sections are suitable for examination by optical and/or scanning electron microscopy. For SEM examination, sample need to be condcutive to get the best results and since the sample is often an “island” surrounded by the resin mount, one of numerous treatments needs to be undertaken. Often, a thin conductive layer is often applied to the surface of the polished section to improve its conductivity. While coatings can be removed, it is recommended to observe the polished section optically prior to coating the sample for SEM examination.
Since the resin is obviously non-conductive by nature, making the resin mounted sample conductive for SEM analysis can be achieved in a number of ways:
- If a hot mounting Phenolic resin can be used with a conductive sample, then adding a suitable amount of “carbon filler” will provide a ground path to the sample and eliminate charging. This is the ideal technique as the mount is equally suitable for Optical analysis. This works well with samples that are already conductive, however, for non-conductive samples like ceramics, plastics or some minerals, this will not be adequate and other treatments work better. Carbon fillers do not work well usually with cold mounting epoxies.
- Techniques used with cold mounting epoxies of specimens that are inherently conductive include the following:
- For a larger specimen like a metal object, a simple piece of copper tape or silver paint is often used to ground it to the sample holder or stage to create a conductive “bridge” over the insulating resin.
- Alternately, adding “copper shot” to the epoxy resin is a simple method to ground the sample. Likewise, simply drilling a hole in the side of the mount and adding a metal screw that touches the sample and extends to the outside is another simple grounding technique.
- Using a carbon evaporator or coater to apply a thin electron transparent layer (approximately 20-25nm) to the polished surface is an ideal technique for almost any sample, especially mineral grain mounts. While the carbon can render the sample unusable for optical examination, the carbon layer can be easily removed.
- Carbon is the preferred coating material for the chemical characterization by EDS due to high transmission of x-rays.
- For non-conductive samples, using a metal sputter coater to apply a nano-thin layer of electron transparent pure metal will create a conductive layer to dissipate charging electrons. Conductivity and grain size of the coating is variable based on the type of metal sputtered. High resolution images require a thin highly conductive, fine-grained coating
- For EDS analysis through a metal coating, it is important to keep the layer ultra-thin (less than 10nm) and use a metal that will not interfere with the x-ray spectral signature of the sample. The reader is referred to other information available regarding choice of metal coating via sputtering technique.
- Most EDS software allows defining the coating material and its thickness to compensate for its effect on determination of the quantitative composition.
* Pay attention to the sides of the resin mount as a conductive path is needed to reach the SEM sample holder and stage. If the carbon coating does not contact a conductive part of the stage or sample holder, use a bit of carbon or silver paint or a small piece of copper tape from the coated surface to the bridge the gap.
There are many practical reasons for utilizing resin mounted samples in microscopic analysis. It is usually not difficult to find a method others have found that works well. Once you have the proper equipment and supplies, achieving the highest quality results is a simple process. As the saying goes in Electron Microscopy:
“The imaging and data will only be as good as the sample preparation”
Spending appropriate effort in sample preparation is paramount to achieving satisfactory electron microscopy results.
For further reading on this subject and to locate quality supplies for resin mounting – visit the SAMPLE PREP tab at our Microscopy Links page. Buehler, Struers and others provide comprehensive guides with methods outlined for most any type of material. This serves as the perfect starting point for determination of the best practice and supplies needed.
Buehler: SumMet™ – The Sum of Our Experience
Buehler: Methods by Material
Struers: How to Cold Mount