Selecting a Target Material
Precious metals can be sputtered in any kind of sputter coater. Oxidizing metals should only be used in turbo-pumped high vacuum coaters. The residual oxygen can create an undesired mixture of metal and a non-conductive oxide coating.
Coating material with lower atomic numbers is more suitable for backscattered electron (imaging) whereas coating with higher atomic number materials is more suitable for SE imaging.
If EDX analysis of the sample is needed, then always choose a material which is not present in the sample, to prevent overlapping peaks in the EDX spectrum.
Here you find some guidance information for selecting the target material. The information is mainly valid when using modern DC magnetron sputter coaters with Argon as process gas.
Here you find some guidance information for selecting the target material. The information is mainly valid when using modern DC magnetron SEM sputter coaters and Argon as process gas.
|Gold is an excellent and most widely used coating material to coat non-conductive samples for standard SEM applications. It is very efficient to coat. For thin layers there is hardly any heating of the sample surface. The grain size is visible when using high magnifications on modern SEMs.
|The Au/Pd alloy (60/40 or 80/20) is less efficient for coating than pure gold, it results in lower sputter rates. Au/Pd produces smaller grain sizes when evaporated in high vacuum, but when used in rotary pumped sputter coaters the difference between Au and Au/Pd is hardly visible. It is less suitable for heat sensitive specimens and less suitable for EDX analysis due to the extra set of peaks for Pd.
|Ag is a good low-cost alternative for Au in many imaging applications for low and medium magnifications. Ag has the highest conductivity of all metals. When using EDX, Ag is an alternative for Au on many biological samples if P, Cl and S need to be analyzed. The sputtering rate is comparable to the one of Au, whereas the SE yield is somewhat lower than for Pt, Au or Ir. The grain sizes are similar or slightly larger than Au, except for samples containing halogens which can cause coarser grains.
Ag coatings can be removed with “Farmer's reducer” (i.e., a mixture of Potassium Ferricyanide and Sodium Thiosulfate), so, the sample surface can be studied once again in the original state.
|Pt yields finer grain sizes than Au or Au/Pd and is therefore more suitable for higher magnifications. Excellent SE yield. Expect slower sputtering rates compared to Au. Pt tends to be susceptible for “stress cracking” when oxygen is present (oxygen can come from porous samples or residual gas). More expensive than Au due to higher fabrication costs.
|Pt/Pd alloy (80/20) results in a similar grain size and SE yield as pure Pt but is less prone to “stress cracking”. All-round coating material for FESEM applications when thin coatings are needed. Best results are achieved when using high resolution sputter coaters.
|Ir exhibits a very fine grain size on virtually all materials and is an excellent all-round fine grain coating material for FESEM applications. It is the material of choice for high and ultra-high resolution FESEM imaging. With the added benefit of being a non-oxidizing material and resulting in a high SE yield, it is replacing Chromium for high resolution sample coating. It still requires the use of a high resolution sputter coater and has lower sputtering rates. Ir targets are thicker due to fabrication constraints, but overall costs are lower than for Pt or Pt/Pd. Ir is also an excellent alternative material for coating samples which will be analyzed for carbon by EDX or WDX. A thin layer is enough to create excellent conductivity and since the material is very rare it hardly interferes with EDX or WDX analysis.
|Cr has a very fine grain size, especially on semiconductor type materials and is proven to be a useful coating material for FESEM applications. Cr requires the use of a turbo pumped, high vacuum, high resolution sputter coater with a target shutter since the presence of oxygen causes oxidization during coating. Cr on the sample surface will oxidize with air and therefore, samples must be viewed immediately after coating. Samples can be stored in high vacuum or in inert gas. Cr has lower sputtering rates and the target tends to heat up. Lower SE yield than Pt, Pt/Pd or Ir. Excellent coating material for high resolution BE imaging of low Z and biological samples.
|W is an excellent alternative for ultra-high resolution coating. W has a very fine grain size and tends to be less visible than Cr. W oxidizes rapidly, similar to Cr. High vacuum coaters have to be used. Low sputtering rates. Due to the high atomic number, the SE yield tends to be higher than for Cr. Samples must be imaged immediately after coating or stored under high vacuum.
|Ta is also a candidate for high resolution coating (most refractory and high melting materials exhibit fine grain size). It oxidizes quite rapidly, similar to Cr. High vacuum coaters have to be used. Low sputtering rates. Due to high atomic number the SE yield tends to be higher than with Cr. Samples must be imaged immediately after coating or stored under high vacuum.
|Pd can be used as a lower cost alternative for low to medium magnification ranges. Gives a lower SE signal than Au. When using EDX analysis, Pd can be an alternative.
|Ni is an alternative coating material for EDX applications and BE imaging. Not ideal for SE imaging, the coating oxidizes slowly. It has (very) low sputtering rate. As a magnetic materials it ‘shields’ the magnet in DC magnetron sputter coaters with a less dense plasma as a result. In a standard SEM coater the coating contains a mixture of Ni and Ni-oxide. The Ni coating layer can enhance elements through X-ray fluorescence. The Ni coating can be removed if needed with a Hydrochloric acid or Nitric acid.
|Cu is an alternative low cost material for EDX applications and BE imaging. Suitable for low and medium magnification ranges. Lower SE yield. Coatings will slowly oxidize. In a standard SEM coaters the coating consists of a mixture of Cu and Cu-oxide. However, it is a low cost alternative for educational applications to demonstrate and to investigate the influence of coating parameters. The Cu coating layer can be used to enhance the analysis of transition materials through X-ray fluorescence. The copper coating can be removed with Ferric Chloride or Nitric Acid.
|Ti is seldom used as coating material, but has applications where it chosen to avoid any interference with EDX analysis. Low atomic number gives less interference with BE imaging. Ti oxidizes rapidly and samples need to be imaged directly after coating.
|Carbon is the material of choice for coating non-conductive samples to allow for EDX analysis and BE imaging. It is has a low atomic number, is conductive and is inert at room temperature. It can't be sputtered in DC magnetron sputter coaters; high energy is needed and if sputtered it tends to deposit as DLC material which is non-conductive. Carbon can be used as a target in ion-beam coaters.
Usually Carbon it is deposited in carbon evaporators by thermal evaporation to coat SEM samples or to produce carbon support films for TEM.