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Selecting suitable target material for SEM application

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Selecting suitable target material for SEM application

  The coating material for making a non-conductive SEM sample conductive should be selected to achieve optimum performance. There is no such thing as a universal coating materials and target materials might vary for different sample materials and desired results. China sputtering target manufacturer-----Baoji Oukai summarizes the tips of selecting suitable target material for SEM application

  Most coating materials have a (much) higher secondary electron (SE) yield than the non- conductive sample materials. Below some practical guidance information for selecting the target material and expected grain size of the coating is given. The information is only valid when using a modern DC magnetron SEM sputter coater with Argon as process gas. Grain size of the coating depends strongly on coating thickness and coating material/sample material interaction.

  Silver target

  Ag is a most suitable and lower cost alternative for Au in many imaging applications for low and medium magnifications ranges. It is a widely underestimated coating material. Ag has the highest conductivity of all metals. The Ag target coating can tarnish (in the presence of halogens) and is less suitable for long term storage. Excellent low cost coating material for less demanding imaging applications and table top SEMs.

  An additional advantage is that the Ag coating can be dissolved and the sample surface can be studied afterward in original conditions.

  Chromium target

  Cr has a very fine grain size, especially on semiconductor type materials and has proven to be a useful coating materials for FESEM applications. Cr requires the use of a turbo pumped, high vacuum, high resolution sputter coater with a target shutter for target conditioning. The higher vacuum in combination with pure Argon flushing of the chamber reduces the partial pressure of oxygen enough to avoid oxidisation of the sputtered Cr layer. The Cr on the sample surface will oxidise in air and samples must be viewed immediately after coating. Samples can be stored in high vacuum. Cr has lower sputtering rates and the target tends to heat up.

  Tungsten target

  W is an excellent alternative for high-resolution coating. W target has a very fine grain size and tends to be less visible than Cr. W oxidises rapidly, similar to Cr. Low sputtering rates, but SEM yield tends to be higher. Samples must be imaged immediately after coating.

  Tantalum target

  Ta is also candidate for high resolution coating (most refractory and high melting metals exhibit a finer grain size). Ta oxidises quite rapidly, similar to Cr. Low sputtering rates, but due to its high atomic number, the SE yield tends to be higher. Samples must be imaged immediately after coating or stored under high vacuum.

  Nickel target

  Ni is an alternative coating material for EDX applications and BE imaging. Not ideal for SE imaging, the coating oxidises slowly. It has (very) low sputtering rate due to the low work functions and the fact that as a magnetic materials it “short circuits” the magnet in the DC magnetron sputter 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.

  Copper target

  Cu is an alternative low cost material for EDX applications and BE imaging. Suitable for low and medium magnification ranges. Lower SEM yield. Coatings will slowly oxidise. In a standard SEM coater the coating consists of a mixture of Cu and Cu-oxide. The Cu coating layer can be used to enhance the analysis of transition materials through X-ray fluorescence.

  Titanium target

  Ti target is rarely used as coating material, but it has applications where it is chosen to avoid any interference with EDX analysis. Low atomic number gives less interference with BSE imaging. Ti oxidises quite rapidly and samples need to be imaged immediately after coating


 
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