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Sputtering Target Selection for SEM Sample Preparation

Posted by Chia-yu Chen on

Sputter coating has been widely used in SEM sample preparation to make non-conductive SEM sample conductive. To achieve optimal SEM performance, it is critical to control sputter coating quality which is affected by target material, interaction with sample and process parameters. In this article, selection guides for target materials will be discussed. This information is only valid for modern DC magnetron SEM sputter coater.

MSE Supplies offers high purity sputtering targets and sputter coaters. Compact Magnetron Ion Sputtering Coater and High Vacuum Magnetron Ion Sputtering Coater are suitable for SEM Sample preparation.

MSE Supplies Target material selection

Gold (Au)

Gold is the most widely used materials for SEM sample coating. However, it is not suitable for using in high magnification applications since gold had large grains which can be observed under high magnifications. Therefore, gold should be selected when using for low magnification image which is typically less than 10K. The X-ray emission lines of the Au M-series and Au L-alpha line can interfere with EDX analysis.  

Gold/ Palladium (Au/Pd)

Au/Pd alloys have smaller grain size than Au. The Au/ Pd sputtering rate is slightly lower than pure Au. Au/Pd is also not suitable for EDX analysis.

Platinum (Pt)

Platinum has a finer grain size than Au and Au/Pd, which can be used for higher magnification applications. Pt has a lower sputtering rate than Au. The presence of oxygen can result in cracks which is called “stress cracking” .

Hence, it requires a better vacuum to achieve Pt coating. Pure Ar purge also helps to remove residual air. Pt is not suitable for EDX analysis.

Platinum/Palladium (Pt/Pd)

Pt/Pd alloy has similar grain size as Pt but less sensitive to “stress cracking”. Pt/Pd (80/20) can provide higher magnification applications for all-round coating, which is widely used in FE-SEM sample preparation. Pt/Pd coating also requires high vacuum system, which is an important factor to choose a right sputter coater.

Iridium (Ir)

Iridium has very fine grain size and suitable for all-round coating material. Ir is good for high and ultra-high resolution FESEM imaging, but has a lower sputter rate. Besides, it requires high vacuum system.
A thin layer of Ir is enough to create high conductivity and hardly interfere with EDX analysis.

Chromium (Cr)

Chromium exhibits a very fine grain size and is proven to be a useful coating material for FESEM applications. However, the sputtering rate is only half rate of Au. In addition, since Cr is easily oxidized, Cr requires the use of high resolution sputter coater with high vacuum system to remove oxygen before coating. The Cr surface on the sample will oxidize in air, and therefore, the coated sample needs to be view immediately after coating. Cr can be a good option for EDX analysis.

Tungsten (W)

Tungsten has extremely fine grain size and good for high magnification applications. Tungsten has even lower sputtering rate than Cr. Like Cr, Tungsten is easily oxidized and therefore, requires the use if high resolution sputter coater with a high vacuum system.

The table shows the summary of the above targets’ properties and applications.

Target material

Grain size (nm)

Typical Maximum Magnification

Relative SE Yield

Relative Sputter Rate

Vacuum Requirements

X-ray Microanalysis

Au

10-12

10K

High

10

Modest

 

Au/Pd

4-8

25K

High

9

Modest

 

Pt

2-3

50K

High

6

Stringent

 

Ir

1-2

100K

High

4

Stringent

 

Cr

1-2

100K

Moderate

5

Stringent

Y

W

<1

200K

High

2

Stringent

 

(Heu et al. 2019)

 

For high purity sputtering target, please visit MSE Supplies. If you need customized targets such as different purity, diameter and thickness, please email us at sales@msesupplies.com and we will prepare a quote for the customized products for you.

MSE Supplies (msesupplies.com) is a major global supplier of sputtering targets. Both standard and customized products are available from MSE Supplies. If you need something not listed on our website, please email us at sales@msesupplies.com and we will prepare a quote for the customized products for you.  If you have any questions, please email us at tech@msesupplies.com.

Reference

Heu, Rod, Sina Shahbazmohamadi, John Yorston, and Patrick Capeder. 2019. 'Target Material Selection for Sputter Coating of SEM Samples', Microscopy Today, 27: 32-36.

 

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