Microscopy101
Target Material Selection for Sputter Coating of SEM Samples Rod Heu,1
* Sina Shahbazmohamadi,2
and Patrick Capeder4 1
John Yorston,3 Rave Scientific, 8 Heller Park Lane, Somerset, NJ 08873
2University of Connecticut, 159 Discovery Drive Storrs, CT 06269-3247 3Carl Zeiss Microscopy, One Zeiss Drive, Tornwood, NY 10594 4Safematic GmbH, Elestastrasse 12, CH-7310 Bad Ragas, Switzerland
*
rod@ravescientific.com
Abstract: This article describes target material options for sputter coaters that deposit a thin metal coating on non-conductive SEM samples. Coating a sample with a conductive metal renders an insu- lating sample conductive enough to minimize charging effects on the SEM image. In most cases, coating SEM samples with only a few nanometers of a metal results in crisp, clear images. Proper target material selection is dictated by overall
imaging requirements, the
SEM available, the specimen material being evaluated, and whether X-ray microanalysis will be required.
Keywords: sputter targets, SEM specimen coating, non-conductive specimens, grain size, sputter rate
Introduction Since its commercial introduction in 1965, the scanning
electron microscope (SEM) has evolved to incorporate many improvements in imaging and microanalysis capabilities, yet the problem of charging in non-conductive samples remains. Te SEM user is still required to cope with the examination of non-conductive samples on a case-by-case basis. Fortunately, there are a number of strategies to aid in this process. Charge mitigation. Te problem is as follows. Negative
charge builds up on a non-conductive specimen at normal elec- tron accelerating voltages (kV), particularly above 10 kV, because more electrons land on the specimen that leave as secondary electrons (SEs) or backscattered electrons (BSEs). Tis can pro- duce in the SEM image strong bright areas and scan raster shiſts. Tese image artifacts can be so severe that the resulting image has no relationship to the object being scanned. While charg- ing can be minimized by imaging at low beam energies near 1 keV, only recent SEM models, particularly those employing field emission electron guns (FE-SEMs), can maintain small electron beam probe sizes on the specimen at such a low accelerating voltage (kV). Alternatively, a variable-pressure SEM, operat- ing in low-vacuum mode (specimen chamber pressure about 1 torr=133 Pa), produces positive ions that can neutralize surface charging. A third method of suppressing charge buildup is to deposit on the non-conducting specimen surface an extremely thin conductive coating, typically a metal that adds minimal structure to the true specimen surface. Te latter method is easy, dependable, and can be used with any SEM. Some coatings exhibit a grain structure that can be observed in modern SEMs, especially those equipped with field-emission (FE) electron
32 doi:10.1017/S1551929519000610
guns. Tere are a range of metals for sputter coating, some for use at low magnifications and others for use at high magnifica- tions in an FE-SEM. An additional benefit of metal coating is that the yield of secondary electrons (SEs) is usually much higher than for the bare non-conducting surface [1]. Coating selection. Te coating metal should be selected
to achieve optimum performance based on the type of analysis to be performed: for example, low-magnification, high-mag- nification imaging, or microanalysis. Most SEM sputter coat- ers permit quick target changes, allowing the microscopist to select an appropriate coating metal for the task at hand. Te sputtered coating should have a high secondary elec-
tron emission yield so that the signal-to-noise ratio will be high. Te ideal coating should have no structure (grains or islands) that would interfere with the details of specimen features. Tus, coatings with large grains would be suitable only for low magnifications, where the structure of the coating would be too small to see. Some metals that produce fine-grained coat- ings suitable for high-magnification imaging, deposit at slower rates; but, this is not a problem because useful coating thick- nesses are quite small, typically 1–3 nm. Some coating mate- rials have X-ray lines that may interfere with the detection of elements in the specimen. However, at typical accelerating voltages, this should not be a problem when the coating is only 1–2 nm in thickness. If there is a serious interference, another coating metal could be selected to coat that specimen. Finally, there is a cost factor since the most useful coating materials are precious metals.
Materials and Methods While not exhaustive, the list of materials below describes
the most common metals used to sputter coat samples for the SEM. Keep in mind that this information is only valid when using a modern DC magnetron SEM sputter coater with pure argon as the process gas. Some coatings require “high-resolu- tion” sputter coaters that operate at better vacuum to reduce the possibility of oxidation during processing; in fact, some systems employ a shutter to shield the sample while oxide is sputtered off the target itself in a pre-conditioning step. Car- bon is commonly used as a conductive coating for microanaly- sis samples, but this material should be deposited by vacuum evaporation or ion-beam sputtering.
www.microscopy-today.com • 2019 July
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