Useful Information for Selecting Target Material for Coating Non-conductive SEM Samples

Most SEM sputter coaters allow for using a choice of sputter target material to optimize the coating material for the sample and or application. Below are some useful tips for selecting sputter target metal to coat non-conductive samples. Please be aware that each sample is different and the coating quality is also affected by the interaction of sample material and target material. The metallization of the surface creates a thin conductive layer on a non-conductive sample An additional advantage is that most coating materials have a higher secondary electron (SE) yield as the sample material. Coating material with lower atomic numbers are more suitable for backscattered electron (BE) imaging of the sample. The information below is valid when using modern DC magnetron SEM sputter coaters.

Aluminum	The sputter yield for Aluminum is very low and it oxidizes very quickly. Not practical with most SEM sputter coaters. SDS (108KB PDF)
Gold	Excellent and most widely used target material for standard SEM coating. Due to the low work function it is very efficient and when using an ultra-cool sputter coaters (such as the Cressington 108auto) there is virtually no heating of sensitive samples. Grain size is visible with high magnifications on modern SEMs. SDS (121KB PDF)
Gold/Palladium	Has a higher work function than pure gold and hence lower sputtering rates. Au/Pd gives smaller grain size when evaporated, but in SEM sputter coaters the difference between Au and Au/Pd is small if practically visible at all. Less suitable for heat sensitive specimens and less suitable for EDS analysis due to two sets of peaks for the coating material.
Palladium	Can be used as a lower cost alternative for low to medium magnification range. Lower SE signal. Alternative to avoid overlapping peaks as with Au when using EDS. SDS (118KB PDF)
Platinum	Fine grain size; smaller than Au or Au/Pd. Higher work function than Au result in slower sputtering rates. Sensitive to "stress cracking" if oxygen is present. More expensive. High SE yield. SDS (119KB PDF)
Platinum/Palladium	Similar grain size as Pt, but less sensitive to stress cracking. Good for FESEM. Best results are achieved in a high resolution coater such as the Cressington 208HR.
Silver	Lower cost alternative for lower magnification range. Large grain size than Au. Suitable for table top SEMs. Lower SE yield than Au or Pt. Coating can oxidize if samples are not stored under vacuum. Alternative for special EDS work. SDS (92KB PDF)
Chromium	Very fine grain size, useful for FESEM. Required high vacuum such as the Cressington 208HR coater to eliminate oxygen. Coating oxidizes and sample can only be stored in high vacuum. Image directly after coating. Excellent alternative for BE imaging of biological samples. Low sputtering rates. SDS (100KB PDF)
Iridium	Very fine grain size. Excellent all-round coating material for FESEM samples. Requires high vacuum coater such as Cressington 208HR. Slower sputtering rates. SDS (92KB PDF)
Tungsten	Alternative for Cr. Very fine grain size, but oxidizes very rapidly. Low sputtering rates
Titanium	SDS (113KB PDF)
Tantalum	SDS (121KB PDF)
Nickel	Alternative for special EDS application, not ideal for SE imaging. Suitable for BE imaging. Very low sputtering rates. Coating oxidizes. SDS (161KB PDF)
Niobium	SDS (125KB PDF)
Cobalt	SDS (145KB PDF)
Copper	Alternative for EDS applications. Suitable for medium/low magnifications, special EDS applications and BE imaging.
Aluminum	SDS (108KB PDF)

 

Note 1: Carbon is best used for EDS application but it is difficult to sputter - carbon evaporation yields much better and quicker results.