When selecting a suitable target for SEM sputter coating, metal grain size is an important consideration. The following provides a guide to this and some other key features.
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FE-SEM
W-SEM
Coating for tungsten SEM ' using a standard vacuum (i.e.rotary pumped) sputter coater: platinum, gold, gold/palladium platinum/palladium
See also our target selection guide by brand and model. Target selection by brand and model.
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Chromium (Cr) sputterjng targets
Chromium is widely used for high resolution sputter coating for FE-SEM applications, typically giving a sub-nanometre grain size. Chromium is an oxidising metal and requires a high vacuum sputter coater (e.g. Safematic high vacuum sputter coater .SEM samples that have been newly coated with Cr will quickly oxidise on contact with air and should therefore be observed immediately or stored under vacuum. See: EM-Storr vacuum storage containers.
Gold (Au) sputtering targets
A typical sputtered grain size is around 5nm and the metal gain structure can often be visualised in an SEM at magnifications of 30,000X and above. Historically gold sputter coating was popular for SEM and is still widely used in many laboratories, even though other metals offer finer grain coating characteristics.
Gold/Palladium (Au/Pd) sputtering targets
Has the same properties (sputtering rate, secondary electron yield etc.) as gold, but the sputtered grain size of the film is usually smaller. Palladium is thought to inhibit the growth of gold grains on the specimen surface during the sputtering process.
Iridium (Ir) sputtering targets
Sputter coated iridium films exhibit a very fine grain size on virtually all materials and is an excellent all-round fine grain coating material for FESEM applications. For many it is the material of choice for high and ultra-high resolution FE-SEM imaging. Iridium has the added benefits of being a non-oxidising material (no storage issues) and having a high SE yield ' it is replacing chromium for high resolution SEM coating. To sputter iridium successfully requires the use of a high vacuum, high resolution sputter coater. Iridium sputtering rates are lower than chromium. Iridium targets are thicker due to fabricated constraints (it is a brittle metal), but overall costs are normally lower than for Pt or Pt/Pd.
Iridium is also an excellent alternative material for coating SEM samples which have to be analyzed for carbon by EDX or WDX. A thin layer is enough to give excellent conductivity and since the material is very thin it hardly interferes with EDX or WDX analysis.
Indium Tin Oxide (ITO) sputtering targets
ITO targets consist of indium oxide and tin oxide with a 90/10 wt% composition. ITO is an interesting material since it is both electrically conductive and light transparent. Applications in SEM include Cathodeluminescence imaging and/or the analysing of non-conductive samples with a detector/spectrometer. ITO is also useful for coating glass coverslips for correlative microscopy and array tomography applications.
ITO sputter coating for Correlative Microscopy ' Tips & Tricks for Scanning Electron and Fluorescence Imaging ' see: this link
Nickel (Ni) sputtering targets
Nickel is an alternative sputter coating material for SEM EDX applications and BSE imaging. It is not ideal for SE imaging and the coating oxidises slowly. It has a (very) low sputtering rate due to its low work function and the fact that as a magnetic material it 'short circuits' the magnet in the DC magnetron sputter head, resulting in a less dense plasma. In a standard vacuum (rotary pumped) SEM sputter coater, the coating contains a mixture of Ni and Ni-oxide. The Ni coating layer can enhance elements through X-ray fluorescence. If needed, nickel coatings can be removed with hydrochloric acid or nitric acid.
Palladium (Pd) sputtering targets
Palladium sputtering is sometimes used instead of carbon for x-ray microanalysis (application dependent).
Platinum/Palladium (Pt/Pd) sputtering targets
A Pt/Pd alloy (80/20) has a similar grain size and SE yield as pure Pt, but is less senstive to 'stress cracking' under the SEM beam. Platinum/Palladium is a suitable all-round sputter coating material for FESEM applications when thin coatings are used.
Platinum (Pt) sputtering targets
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Sputter coated platinum typically gives a grain size of around 2-3nm and offers the best routine fine grain coating from a rotary pumped SEM coater, such as the Safematic CCU-010LV. Platinum has an excellent SE yield, but is higher work function results in slower sputtering than gold (typically 60% slower). Platinum sputtered films tend to be sensitive to 'stress cracking' when oxygen is present (oxygen can be released from some porous SEM samples).
Silver (Ag) sputtering targets
Silver sputtering is a most suitable and low cost alternative to gold sputtering in many SEM imaging applications in the low and medium magnifications ranges. For SEM and FIB/SEM silver is a widely underestimated coating material and has the highest conductivity of all metals. When using EDS, silver can be an alternative to gold coating for many biological samples if P, Cl and S need to be analysed. The sputtering rate of silver is similar to that of gold, but the SE yield is somewhat lower than for Pt, Au or Ir.
The grain size of sputtered silver is similar or slightly larger than gold, except for samples containing halogens which can result in coarser sputtered grains. Sputtered silver coatings can tarnish (in the presence of halogens) and are less suitable for long term storage. Generally silver is an excellent low cost coating material for many less demanding SEM imaging applications and for table top SEMs.
An additional advantage of silver sputtering is that coatings can be dissolved and the sample surface studied in the original conditions. Removal of Au, Au/Pd, Pt and Ir coatings is more difficult and involves very harsh chemicals. Silver coatings can be removed with Farmer's reducer (a mixture of potassium ferricyanide and sodium thiosulfate).
Tungsten (W) sputtering targets
Tungsten is an excellent alternative for ultra-high resolution SEM coating. Sputter coated tungsten has a very fine grain size and tends to be less visible than chromium, but like chromium it oxidises rapidly on contact with air. For this reason samples must be imaged immediately after coating or stored under vacuum (See: EM-Storr vacuum storage containers). Tungsten has a low sputtering rate, but due to its high atomic number the SE yield tends to be higher.
Examples of other sputtering materials ' although not commonly used in EM
Carbon (C) targets
Although it is possible to sputter carbon, the deposition rate is very low and therefore impractical for most applications. Thermal evaporation using instruments such as the Safematic CT-010 carbon coater is recommended for SEM and TEM carbon coating applications.
Cobalt (Co) targets
Cobalt is not generally used for SEM coating applications. It has a very low sputtering rate due to its low work function and the fact that as a magnetic materials it 'short circuits' the magnet in the DC magnetron sputter head with a less dense plasma as a result. In a standard SEM sputter coater, the sputtered films contains a mixture of Co and Co-oxide. Cobalt coated layers can enhance elements through X-ray fluorescence.
Indium (In) targets
Indium is not generally used for SEM coating applications. It has a very low sputtering rate due to its low work function. In a standard SEM coater, the coating contains a mixture of In and In-oxide. Indium coated layers can enhance elements through X-ray fluorescence.
Tin (Sn) targets
Tin is not generally used for SEM coating applications. It has a very low sputtering rate due to its low work function. In a standard SEM coater, the coating contains a mixture of Sn and Sn-oxide. Tin coated layers can enhance elements through X-ray fluorescence.
Zinc (Zn) targets
Zinc is not generally used for SEM coating applications. It has a very low sputtering rate due to its low work function. In a standard SEM coater, the coating contains a mixture of Zn and Zn-oxide. Zinc coated layers can enhance elements through X-ray fluorescence.
Intended application: Selecting the right target material first requires an in-depth understanding of the application requirements of the film. Different applications (e.g., photovoltaics, semiconductors, decorative coatings) have different requirements for the electrical, optical and mechanical properties of thin films. Therefore, defining the application purpose and performance requirements of the film is the first step in target selection.
Evaluate target properties: After determining the application needs of the film, the target needs to be selected based on its purity, compositional homogeneity, size and shape. For example, for highly transparent optical films, it is particularly important to select targets with uniform composition and high purity. For large-area coatings, target size and shape uniformity are critical to improving deposition efficiency and film uniformity.
Consider cost-effectiveness: When selecting targets, there is also a balance of cost-effectiveness to consider. While high-quality targets may cost more, they may be more economical in the long run by providing superior film performance and longer service life.
Equipment and target compatibility: Ensure that the specification, size and shape of the target purchased matches the equipment requirements, and that the physical and chemical properties of the selected material are suitable for the required application and equipment to ensure that it can be loaded and used correctly.
Sputtering method: For reactive sputtering or the preparation of thin films of specific compounds, targets matching the gases used in the sputtering process are required. High-frequency sputtering or magnetron sputtering are generally more suitable for using pure, homogeneous targets for stable deposition. Some sputtering methods may require targets of special shapes, sizes, or structures, such as magnetron sputtering, which improves target utilization. The choice of sputtering method needs to take into account the physical and chemical properties of the target to ensure that it is compatible with the process and that the desired film properties are obtained.
Conductivity: The higher the purity of the target, the higher the conductivity. In semiconductor and conductive film applications, trace impurities can significantly affect the conductivity of the material. High purity targets reduce these impurities and provide higher conductivity, which is critical for high performance electronics.
Specialized resources and guidance are critical to selecting the right target, and by partnering with AEM, a supplier of specialized sputtering targets, we will utilize our years of technical knowledge and experience to help you more accurately select the target that meets your needs.
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