EQUIPMENT & ACCESSORIES CATALOG EDITION IV SPUTTER COATERS, SEM/TEM CARBON COATERS


Techniques and Applications


increasing the interaction path length of the high energy electrons in the discharge. Deflection and retardation of electrons result in increased ion yield and sputtering efficiency.


It was indicated previously that while imperical design may be in evidence, it should now be apparent that effective production of positive ions for glow discharge is required. The sputter head and its associated power supply should be a primary objective of design and development.


All modern SEM sputter coaters use heads fitted with an arrangement of magnets and often an associated shroud assembly, with a disc target. Power supplies generally employ solid state switching for applied voltage control. See Figure 3.


The overall result is a low mean voltage head with low energy input. The possibility of thermal damage due to radiant heating and electron bombardment is considered negligible.


For a typical modern magnetron sputter coater Vacuum


8 x l0-2 Current


Sputtering Voltage 100V to 0 0


Deposition Grain size


to 2 x l0-2 3Kv


to 50mA to 25 nm/min Less than 5nm Temperature rise Less than 10C mbar


It is, of course, possible to satisfy very precise parameters by the selection of target material, 'voltage' 'deposition'. 'current' and 'vacuum'. Under these conditions, it is possible to achieve thin films to I0nm with grain sizes better than 2nm and temperature rises of less than 1°C.


Choice of Sputtering Material


As stated many times, metal coating is an indispensable technique for SEM. The development of high resolution FEG SEMs has brought about more wide spread use of specialized techniques such as Ion Beam Sputtering, Penning Sputtering, E-Beam Evaporation and Planar magnetron ion-sputtering.


More lately Chromium coating has become the "fashionable" material to use. It offers a thin continuous film and emits less back scattered electrons than other sputter materials. However it is not free of its own problems. To operate it requires a high vacuum and ideally vacuum transfer (or vacuum storage) of the sample to avoid oxidation problem. Cr coated samples may often have a "see through" look as there is the possibility of images generated from electrons from sub surface structures. More recently iridium films have been shown to give excellent fine grain (sub nanometer) films that compare favorably with those generated with Cr. Both metals require high vacuum sputter coaters for


Figure 3 – Diagram of a “cool” sputtering head effective deposition.


Application data collected has shown that a high quality well designed rotary pumped magnetron sputter coater, such as the EMS 550X, is capable of producing a continuous Pt (platinum) film with a grain size in the order of 2 nm. It also has the benefit of being a good secondary electron emitter, unlike chromium. Some images of chromium show bright high contrast images. Many workers, and our own studies have led us to consider the possibility of each grain of chromium being oxidised before sample is coated and hence the film is not truly continuous and indeed each metal grain is individually charging. This is another reason to consider iridium as an alternative.


Silver as a sputter material is often ignored but is a very satisfactory method for ensuring conductivity of the SEM sample but has a major advantage the whole process is reversible as the metal may be removed by the neutral aqueous reagent known as "Farmers reducer". This enables many samples to be viewed and then returned to their original condition. Beware. ...Silver may form crystalline deposits on the surface of the sample in the presence of active Halogens


• Sputtered silver offers smaller grain size than evaporated silver.


• Sputtered Gold and Silver have similar grain size but the silver has larger reticulation after storage.


• Silver is the most conductive metal known.


• Silver has a high secondary electron coefficient.


• X-ray emission lines are well separated from the biologically important sulphur and phosphorous.


• Cost effective.


Gold/Palladium (80:20) targets are now a popular standard choice for the routine coating of a wide range of samples. The idea behind using this alloy is that the palladium will act as a physical barrier to the gold which will attempt to conglomerate into large islands and restrict ultimate resolution performance.


The minimal loss in secondary electron emission performance from the palladium is not seen as significant with current SEMs.


Other target choices are generally made based on the requirement for X-ray analysis of samples or back scattered electron detection.


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