Microscopy 101
Table 1: Some characteristics of common sputter target materials. Sputter Material
Au
Au/Pd Pt Ir
Cr W
Grain Sizea 10–12 nm 4–8 nm 2–3 nm 1–2 nm 1–2 nm < 1 nm
Typical Maximum Magnificationb
10,000× 25,000× 50,000× 100,000× 100,000× 200,000×
Relative SE yieldc High High High High
Moderate High
aMeasured from images similar to Figure 1, from discussions with Jack Vermeulen, and from [2]. bEstimates for typical SEMs: tabletop to 10 kx; workhorse W-SEM to 50 kx; FE-SEM above 50 kx.
cEstimated from secondary electron coefficients at 20 keV given in [4] and the DC Joy electron database [
web.utk.edu/∼srcutk/database.doc]. dEstimated from data in [3].
the need for a sputter coater with better vacuum. Te character- istic X-rays of Pt (particularly the M-series at 2.05 keV) have the potential to overlap with lines from P and Zr, but interference should be minimal for 1–2 nm thick coatings. Platinum/palladium alloy (80/20) has a similar small
grain size and high SE yield as pure Pt, but it is less sensitive to “stress cracking.” Te Pt/Pd alloy is a suitable all-round coating material for high-magnification applications. Iridium exhibits a fine grain size on virtually all speci-
men materials and is an excellent all-round coating material for high-magnification applications. It is also usually the most expensive coating metal, typically about twice the price of Au/ Pd and Pt. Tis non-oxidizing material has a high SE yield, and for some applications it has been replacing chromium for high-resolution sample coating. It sputters at a lower rate and requires the use of a turbo-pumped high-resolution sputter coater. Since specimens for microanalysis are oſten coated with evaporated carbon, Ir is a good alternative coating material when carbon must be analyzed by X-ray microanalysis. Inter- ference of the Ir M-series (1.98 keV) and L-series (9.18 keV) could occur for P and Ga, respectively. Again, a 1–2 nm thick coating will provide adequate conductivity while not interfer- ing with X-ray microanalysis. Chromium has a very fine grain size, but the sputtering
rate is only about half that of Au. Tin Cr films have proven to be a useful coating material for high-magnification imaging in FE-SEMs. Because it oxidizes easily, Cr requires the use of a turbo-pumped, high-resolution sputter coater with a target shutter for target conditioning to remove the oxide prior to coating. Te better vacuum, in combination with pure argon flushing of the chamber, reduces the partial pressure of oxy- gen enough to avoid oxidization of the sputtered Cr layer. Te thin Cr film on the sample surface will oxidize in air, and sam- ples must be viewed immediately aſter coating. Samples may be stored in high vacuum. Chromium is an excellent coating material for high-resolution backscattered electron imaging of low Z materials and biological samples. Chromium can be a good choice for-Xray microanalysis because its X-ray lines do not interfere with common specimen elements except for oxy- gen, where a near overlap occurs between the Cr L-series (0.573 keV) and the oxygen K-line (0.525 keV). Tungsten is an excellent coating for high-resolution
coating since it has an extremely fine grain size (Figure 1 and Table 1). But W oxidizes rapidly and requires the same
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stringent turbo-pumped high-resolution coater described for Cr. As a refractory metal like Cr, it has a low sputtering rate, but the SE yield is high. Samples must be imaged immediately aſter coating because of rapid oxidization in laboratory air. Te W X-ray spectrum has a wide range of potential microanalysis interferences, but the extremely thin coating (< 1 nm) mini- mizes the problem. Other metals. Alternative precious metals (silver, tanta-
lum, and palladium) and common metals (nickel, copper, and titanium) have been used for special purposes. However, the possibility of coating oxidation may still be a problem for some of them (Ag, Ta, Ni, Cu, and Ti). Silver has a particular advan- tage not found with other coatings: it can be dissolved with Farmer’s reducer, returning the surface to the uncoated state.
Results Figure 1 and Table 1 show the variation in grain size with
the most common sputtering targets. Te grain size values in Table 1 are for comparison purposes and for defining trends. Te images in Figure 1 reflect the trends. Images were obtained on a Zeiss Merlin FE-SEM at the same magnification for all coatings. Figure 2 shows a practical demonstration of how sputter coating can reduce, if not eliminate, charging on a low-Z, non-conducting material such as expanded polytetra- fluoroethylene (ePTFE). Figure 2a shows the severe charging that occurs with no coating applied. Figure 2b shows the same sample coated with approximately 5 nm of Au/Pd, and charg- ing appears to have been eliminated.
Discussion Te examples shown here are only valid when using a mod-
ern turbo-pumped DC magnetron SEM sputter coater with argon as the process gas. Grain size of the coating depends on coating thickness and the coating/sample material interaction. As a rule, the thinner the coating, the smaller the grain size. If the surface has irregular topography with cavities, a uniform coating might be difficult to achieve. As a result, localized surface charging could degrade image quality. Tis problem can usually be rectified with a tilting or rotating sample stage deployed within the sputter-coating system. Coating thickness was determined using a quartz thickness monitor. As a rule coating thickness monitors register values that are not abso- lute in value. Te operator sets a defined thickness end point, and the sputter process will cease. Te actual thickness could be greater. If a thickness monitor is not available, the sputter
35
Relative Sputter Rated
10 9 6 4 5 2
Vacuum
Requirements Modest Modest
Stringent Stringent Stringent Stringent
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