Why Vacuum Coating?

Non-Conductive samples often have a charging effect from electrons collecting on the sample surface causing issues with SEM imaging.  While many SEM systems have a “low vacuum”  or “charge reduction” mode that can overcome this issue, that mode usually has limitations in magnification and image quality that is sample dependent.  Images collected in “low vacuum” modes usually have a flat contrast resulting in unappealing images.

For the best imaging, non-conductive samples are commonly coated by using a Sputter Coater that applies a nano-thin layer of conductive metal.  Even conductive samples benefit from a sputter coated layer of highly electron emitting materials like gold, platinum or gold/palladium. This coating allows imaging at higher beam energies to obtain the highest resolution and magnification without concern of electron charge effects or beam damage to sensitive samples.

EDS analysis can still be done on coated samples as most EDS software allows defining the coating metal so it is ignored during analysis.  A metal coating type is selected that is not present in the sample.  Alternately, for the most precise EDS analysis, a carbon coater can be used to produce an x-ray transparent conductive coating. Carbon coating is popular in mineralogical analysis of resin mounted polished samples and slide mounted polished thin sections.

We offer multiple choices for sample coating with either metals, carbon or both.  The economical SPT-20 coater is an ideal entry level coater that is best suited for flat samples.  The more advanced DESK series coaters are available in both Low and High Vacuum models.  Since the level of vacuum plays a role in the resulting grain size, the High Vacuum coaters are mostly used only Field Emission or FE-SEM and TEM that produce images often above 100,000X magnification where the potential exists to observe the artifact of coating grains.  High Vacuum coating is also recommended when coating with oxidizing metals like Chrome and some others.

For further reading about Sample Preparation and Sputter Coating, we suggest Echlin’s “Handbook of Sample Preparation for SEM and EDS“.  An on-line version of the book is also available HERE.

Thermal Evaporation

We offer multiple choices for applying thin films to samples with using both Thermal Evaporation or Sputtering or in combination.  While sputter coating uses a “target” of pure metal,  A thermal evaporator can have multiple sources that allow it to create alloys of different metals in the resulting film.  Likewise, through the coaters control panel, sequential coatings can be performed using either evaporation or sputtering depending on the coaters configuration.

Thermal evaporation is a method of physical vapor deposition (PVD) and typically uses a resistive heat source to evaporate a solid material in a vacuum environment to form a thin film. The coating material is often contained in a “boat”, “basket” or “coil” made of materials capable of withstanding temperatures higher than evaporated material. The materials to be applied with Thermal Evaporation techniques can be pure atomic elements including both metals and non metals, or can be molecules such as oxides and nitrides. The material is heated in a high vacuum chamber until vapor pressure is produced allowing the evaporated material to traverse the vacuum chamber and coat a substrate

For further reading about Thermal Evaporation for deposition of thin films and coating, a very nice summary of published works is available here at Science Direct.