Ever wondered what makes semiconductor devices work like magic? Well, the answer lies in the thin films that coat them, and one of these processes is the rotary metal target. These cylindrical, rotating forms of metal (like tantalum, niobium, and molybdenum) are used in sputtering, a key process in semiconductor manufacturing. If you’re involved in purchasing for semiconductor production, this article is for you. Let’s break down why these metals are crucial and how to choose the right one for your needs.
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In a nutshell, rotary metal targets are circular metal forms used in sputtering—a process where ions bombard a target material, ejecting particles that then deposit onto a substrate, creating a thin, functional film. The catch? The target rotates to ensure uniform wear and deposition. Pretty cool, right?
Now, why are materials like tantalum (Ta), niobium (Nb), and molybdenum (Mo) so special for these tasks? Let’s dive into it.
When you think of durability, tantalum is probably top of mind. Why? This metal’s a powerhouse—perfect for high-performance devices that require top-tier electrical conductivity, corrosion resistance, and thermal stability.
So, why use tantalum in semiconductor sputtering?
Corrosion Resistance: Tantalum resists corrosion even in harsh environments. This makes it ideal for devices exposed to moisture and heat (think high-performance circuits).
High Melting Point: With a melting point over 3,000°C, tantalum won’t break a sweat in high-energy sputtering environments.
Great Electrical Conductivity: Essential for creating those thin films that make electronics work smoothly.
So, if you’re looking for a reliable, durable thin film, tantalum’s your guy.
Niobium is a bit of a secret weapon in semiconductor sputtering. Why? Because it has superconducting properties and can withstand extreme temperatures—perfect for specialized applications like quantum computing.
What makes niobium stand out?
Superconductivity: Niobium is essential for devices where superconductivity matters. This is a must for next-gen tech like quantum computers.
Strength in Cold: It’s strong even at ultra-low temperatures, which is crucial for technologies used in cryogenic environments.
Excellent Bonding: Niobium forms strong bonds with other materials, helping to produce durable and reliable thin films.
So, if you’re working with cutting-edge quantum or cryogenic tech, niobium will be your go-to.
Molybdenum is the Swiss army knife of metals—it’s versatile, reliable, and performs well in high-stress environments. Whether you’re dealing with high temperatures, electrical conductivity, or thermal stability, molybdenum is a safe bet.
Why choose molybdenum for sputtering?
Thermal Stability: It’s got amazing thermal conductivity, which is perfect for high-temperature semiconductor applications.
Electrical Conductivity: Molybdenum is great at conducting electricity, so it’s ideal for making thin films that need consistent electrical performance.
Wear Resistance: It’s built to last—molybdenum resists wear, so your rotary targets last longer, saving you costs in the long run.
Need a metal that’s reliable, tough, and doesn’t quit? Molybdenum’s got you covered.
Choosing the right rotary metal target isn’t as simple as picking the first one you see. Here are a few things to keep in mind:
Material Composition: Is purity important? The composition of the target material affects film quality and performance.
Deposition Requirements: Are you after high deposition rates or super-thin films? Different targets work better for specific deposition needs.
Compatibility: Make sure your target material is compatible with the process gases (like argon or oxygen) and substrate materials (like silicon).
Temperature Tolerance: Check that the target can handle the temperatures expected during sputtering. You don’t want your target to degrade prematurely.
Cost vs. Performance: Yes, budget matters. But it’s essential to balance cost with performance. Sometimes, spending a little more up front can save you big-time down the road.
Tantalum, niobium, and molybdenum are the stars of the rotary metal target world, each bringing something unique to the sputtering table. Whether you need high durability, superconductivity, or thermal stability, these metals have got your back. When selecting a rotary metal target for semiconductor applications, keep in mind the key factors that can make or break your process, from material composition to temperature tolerance.
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.
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.
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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
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.
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