Sputtering Target’s Material Selection for Magnetron Ion Sputter / Evaporating Coating Coater of SEM Samples and Materials Science
Introduction
This article describes target material options for sputter coaters, magnetron sputter coaters, thermal evaporation carbon coater that deposit a thin metal or carbon coating on non-conductive SEM samples or other substrates. Coating a sample with a conductive metal renders an insulating 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.
History of SEM and Sputter Coater
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 until nowadays. The 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
The problem is as follows. Negative charge builds up on a non-conductive specimen at normal electron accelerating voltages (kV), particularly above 10 kV, because more electrons land on the specimen that leave as secondary electrons (SEs) or backscattered electrons (BSEs). This can produce in the SEM image strong bright areas and scan raster shifts. These image artifacts can be so severe that the resulting image has no relationship to the object being scanned. While charging 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, operating 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. The 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 guns. There are a range of metals for sputter coating, some for use at low magnifications and others for use at high magnifications 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.
Coating selection
The coating metal should be selected to achieve optimum performance based on the type of analysis to be performed: for example, low-magnification, high-magnification imaging, or microanalysis. Most SEM sputter coaters permit quick target changes, allowing the microscopist to select an appropriate coating metal for the task at hand. The sputtered coating should have a high secondary electron emission yield so that the signal-to-noise ratio will be high. The ideal coating should have no structure (grains or islands) that would interfere with the details of specimen features. Thus, 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 coatings suitable for high-magnification imaging, deposit at slower rates; but, this is not a problem because useful coating thicknesses are quite small, typically 1–3 nm. Some coating materials 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 (VPI – Model 900M) with pure argon as the process gas. Some coatings require “high-resolution” sputter coaters (VPI – SD650MH) 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. Carbon is commonly used as a conductive coating for microanalysis samples, but this material should be deposited by vacuum evaporation or ion-beam sputtering.
Instruments
In general, there are two types of sputter coaters. The system above could be described as a “high-resolution” sputter coater like VPI High Vacuum Magnetron Sputtering Coater 650MH because a turbo pump is employed to obtain a higher (and cleaner) vacuum environment, and pure argon gas is backfilled in the chamber to remove air and increase sputter efficiency. The second type of sputter coater may be described as a more basic unit, developing only a modest vacuum with a mechanical pump and sometimes replacing argon backfill gas with room air, like VPI’s Magnetron Sputtering Coater SD-900M Model. This basic sputter coater is acceptable for coating Au, Au/Pd, Ag films, but not for coatings with finer grain sizes. Use of a system with a poorer vacuum and air backfill results in lower sputter efficiency and deposited films that are not as clean. Coating film thickness monitor also called thickness measurements were obtained using the quartz thickness monitor (operating at 4~6 MHz) inherent with the system (VPI’s 900M, 650MH, Carbon Coater could provide optional accessories like thickness monitor measurements)
Target Metal Selection - Gold
Gold is perhaps the most widely used coating material for non-conductive SEM samples, but it is not recommended as a sputter coating for research purposes where high-magnification images are required . Gold has a high secondary electron yield and sputters relatively rapidly, but the coating structure is composed of large islands (grains) that can be observed at high magnifications in most modern research-level SEMs. Thus, it should only be used for imaging at low magnifications, say less than 5000×, where the coating structure will not interfere with the structural details of the sample. An advantage shared by most other precious-metal coatings, Au coatings do not oxidize in laboratory air. X-ray emission lines of the Au may interfere with X-rays from S and Nb, while the Au L-alpha line may interfere with X-rays from Ge. If the Au coating is appropriately thin, however, there should not be significant problems with qualitative X-ray microanalysis.
Target Metal Selection - Gold/palladium
Gold/palladium sputtered alloys have smaller grain size and are the recommended metal coatings for general research purposes. Secondary electron yields are high, and sputter rates for Au/Pd are only slightly lower than for pure Au. The Pd X-ray lines do not overlap important lines from other elements; thus, no additional interference with X-ray microanalysis would be expected beyond that mentioned above for Au.
Target Metal Selection - Platinum
Platinum has a finer grain size than either Au or Au/Pd, which makes it more suitable for higher-magnification applications. A sputtered Pt coating exhibits a high SE yield, but Pt has a lower sputtering rate than Au. Pt has been observed to crack. This effect could be “stress cracking” and could be attributed to oxygen deposition in the sputtered coating, indicating the need for a sputter coater with better vacuum. The characteristic X-rays of Pt 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 has a similar small grain size and high SE yield as pure Pt, but it is less sensitive to “stress cracking.” The Pt/Pd alloy is a suitable all-round coating material for high-magnification applications.
Target Metal Selection - Chromium
Chromium has a very fine grain size, but the sputtering rate is only about half that of Au. Thin 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 (VPI’s coater for example) for target conditioning to remove the oxide prior to coating. The better vacuum, in combination with pure argon flushing of the chamber, reduces the partial pressure of oxygen enough to avoid oxidization of the sputtered Cr layer. The thin Cr film on the sample surface will oxidize in air, and samples must be viewed immediately after 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 oxygen.
Target Metal Selection - Iridium
Iridium exhibits a fine grain size on virtually all specimen 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. This 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 like VPI’s high vacuum magnetron sputtering coater 650MH. Since specimens for microanalysis are often coated with evaporated carbon by VPI’s SD-980 pulsing thermal evaporation carbon coater, Ir is a good alternative coating material when carbon must be analyzed by X-ray microanalysis. Interference of the Ir could occur for P and Ga, respectively. Again, a 1–2 nm thick coating will provide adequate conductivity while not interfering with X-ray microanalysis.
Target Metal Selection - Tungsten
Tungsten is an excellent coating for high-resolution coating since it has an extremely fine grain size. But W oxidizes rapidly and requires the same stringent turbo-pumped high-resolution coater (VPI’s high vacuum magnetron sputtering coater 650MH) 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 after coating because of rapid oxidization in laboratory air. The W X-ray spectrum has a wide range of potential microanalysis interferences, but the extremely thin coating (< 1 nm) minimizes the problem.
Other metals. Alternative precious metals (silver, tantalum, 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 advantage not found with other coatings: it can be dissolved, returning the surface to the uncoated state. VPI’s high vacuum magnetron sputtering DC and RF power supplier coater could coat all kinds of metals and non-metals (discussed above) on the materials.
Summary and Review
This article has shown here are only valid when using a modern turbo-pumped DC magnetron SEM sputter coater (VPI SD-650MH or SD-900M, SD-980 carbon 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. This problem can usually be rectified with a rotating sample stage (VPI could provide optional sample horizontal rotation design stage or other ratations) 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 absolute in value. Also visual evaluation of the film color and opacity can be useful in estimating the thickness of the sputtered film. If X-ray microanalysis of the sample is required, select a coating (target) material that is not present in the sample. This should avoid interfering peaks in the X-ray spectrum of the sample. Also consider all the X-ray lines possible from your sample and from the sputtered film. It must be kept in mind not only what X-ray lines might be present, but also what lines could be enhanced at the electron beam accelerating voltage (kV) to be used in your study. If all possible interferences must be avoided, then classic carbon deposition (thermal evaporation) is the recommended approach to render a non-conductive sample amenable for X-ray microanalysis. The rule of thumb for selecting a sputter target for coating an SEM specimen is to choose the metal that produces the smallest grain size consistent with capabilities of the SEM available. Thus, Au may be acceptable for a table-top SEM for magnifications below 5000×; Au/Pd and Pt would be useful for general purpose SEM imaging; and Cr or W would be appropriate for high-resolution, high-magnification imaging with an FE-SEM. Once the target metal has been selected, the effort should be to produce the thinnest metal film that mitigates charging effects, ideally in the range of 1–2 nm.
Suggestions
The ease with which sputter coater target metals can be changed allows for flexibility in preparing SEM specimens for imaging and microanalysis or materials science coatings. VPI Coaters are suitable and cost effective for all kinds of clients globally. Meanwhile targets are available for low magnification and high-magnification work, and they may be changed to facilitate elemental analysis. Use the metal coating that produces the smallest grain structure consistent with the capabilities of the SEM available. Different sputter targets vary in cost, the performance of sputtering targets VPI provides is very good and stable, and some require additional infrastructure such as the increased pumping/vacuum capabilities of a high-resolution sputter coater.
