Sputter Coating Preparation Techniques and Advantages
Here you can find information and downloads to explain the techniques and advantages of sputter coating. Details on this page cover:
Annular sputtering targets: how to remove
Target replacement
In normal use, erosion of the target will occur. Initially this will be seen as a ring on the surface of the target. With further use, holes will be worn through the target material (generally in a circular ‘racetrack’ pattern) exposing the aluminium backing plate. When this occurs the target should be replaced.
Target removal
(a) Ensure power supplies to the sputter coater are set to OFF.
(b) Open the top plate to obtain access to the target.
(c) Unscrew and discard the old target ring, if the target has seized tight and cannot be unscrewed, lubricate lightly with isopropanol.
(d) If necessary clean vicinity of the target, using a gentle abrasive material (Scotchbrite or similar) and a lint-free tissue moistened with isopropanol.
NB: In most cases the above removal procedure will be successful. However, if the target will still not unscrew, and target is worn out and is being removed for replacement, the follow procedure can be tried:
Take a look at the surface of the old target. It will be eroded and in places the aluminium backing plate may be exposed. If this is the case you may be able to see two small holes in the surface of the backing plate (if covered by the sputtering metal, then gently press on the surface to find the indent of the holes).
Once located you should be able to insert two makeshift ‘turning handles’ into the holes (eg two small screwdrivers) and use these as levers to turn the target. But, as mentioned above, proceed carefully as the fine threads of the aluminium backing plate are easily cross-threaded.
Refit new target
(a) Screw the new target ring onto the target support.
(b) Clean the target and surrounding area using a lint-free tissue moistened with isopropanol.
(c) Lower the top plate onto the vacuum chamber and reconnect any services previously removed.
E5100, E5150, SC510 and SC515: replacing glass cylinder
The E5100 has a glass chamber fitted into an aluminium collar. The collar is seated onto the base plate of the E5100.
Should the glass cylinder need to be replaced, we will need to know the exact diameter of the glass chamber (to the nearest 0.1mm). This information is engraved onto the inner wall of the aluminium collar, either at the bottom (more usually) or the top, in which case the glass cylinder must first be removed.
We keep some E5100 cylinders of slightly varying sizes and it may therefore be possible to supply an exact replacement from stock. If not, the outer rim of the glass cylinder must be specially ground down to size. This may increase delivery time and cost. If you are in any doubt, please contact us.
Background
Due the method of manufacture, glass cylinders have a size tolerance of +/-0.1mm. For this reason the aluminium collar needed to be tailored to suit the diameter of the glass chamber.
The E5100, E5150, SC510 and SC515 are discontinued products. When they were manufactured, we used to keep a large stock of glass chambers. The problem now is that as a discontinued item we keep a much smaller stock, which means the chance of having a exact size glass chamber is small.
SC7620: target replacement
In normal use, erosion of the target will occur. Initially this will be seen as triangular patterns on the surface of the target. With further use, holes in the target (generally in a triangular pattern) will become apparent. When this occurs the target should be replaced.
Target replacement
(a) Ensure power supplies to the sputter coater are all set to OFF.
(b) Hinge the top plate back, to expose the target assembly.
(c) To release the shroud, remove 2 x M3 socket screws (pictured), then remove the shroud.
(d) Unscrew and remove the target clamp (if the target clamp will not unscrew, lubricate lightly with isopropanol). Remove and discard the old target. If necessary clean the magnet holder and target clamp, using a gentle abrasive material (Scotchbrite), rinse with isopropanol and dry thoroughly.
Refit new target
(a) Position the new target in the target clamp ring, ensure target is lying flat.
(b) Carefully fit the clamp over the magnet holder, tighten until the target is securely clamped.
(c) Re-fit the shroud and secure using the 2 x M3 socket screws.
(d) Position the top plate assembly on the vacuum chamber and reconnect the services previously removed. Care to be taken when lowering the top plate onto the glass chamber (pictured).
(e) When taking the system back into service carry out the test procedure (see manual chapter 6.1), this will ensure the system is thoroughly dried out.
SC7640: high tension (HT) failure
If a high-tension failure occurs on the SC7640, the coater will often display the symptoms of going through its automatic or manual cycle in the normal way, with no HT (ie no sputtering).
The SC7640 is fitted with a large, black, mechanical break interlock switch (located on the upright sputtering head support pillar). This device ensures safe operation of the optional add-on carbon evaporation heads. The switch is normally activated when the sputtering head is in the down (operational) position. However, when the sputtering head is in the raised position, with the optional evaporation head in place, the interlock switch is ‘open’ and the exposed sputtering head cannot be powered.
If the black mechanical break interlock switch is moved (this can sometimes happen during transit to the customer's site or if the user has made adjustments to the switch) it may need to be repositioned. These switches are operated by a ‘key’ on the hinged top plate and must be fully engaged to enable the high-voltage power supply.
The ‘fix’ is simple: with the SC7640 under vacuum, loosen the two side screws on the black switch and slide the switch body upwards until it butts against the actuator then tighten the screws.
SC7640: locating a vacuum leak
If a vacuum leak occurs on an SC7640 it will often display the symptoms by going through the operating cycle as far as the I/Lock (interlock) position and no further. This usually means that the unit is not pumping far enough to engage the vacuum trip to enable the leak valve and the sputtering power supply. Under these circumstances the following test should be performed:
(a) Fold the top plate back and remove the glass chamber. Put a lightly greased laboratory rubber bung (stopper) into the base-plate pumping port and a small piece of rubber sheet over the gas inlet (again on the base-plate).
(b) Ensure the system is in MAN (ie manual mode) and press START SEQUENCE. The unit should pump very quickly, due to the small gas volume, and follow the vacuum sequence. With a leak-tight system the unit should obviously complete the sequence and, providing the LEAK valve is closed, also pump to base pressure (which is approximately 10-2mbar).
(c) If the unit still only reaches mid-scale on the meter with just the I/Lock light on, then there is a vacuum leak somewhere on the vacuum side of the base-plate.
(d) If the unit pumps OK, then there is a vacuum problem with the sputtering head or chamber seals.
SC7640: short circuit in the sputtering head
This can coincide with the changing of the sputtering target, in which case it may be that a ‘whisker’ of metal from the target thread is causing the head to ‘short’ out. Usually, blowing around the target area with a gas jet duster removes unwanted sputtering target fragments and solves the problem. If this does not solve the problem then there are two fairly easy checks that can isolate the fault to either the sputtering head or to the high voltage power supply.
Set the SC7640 to MANUAL mode of operation and allow the unit to complete the pumping sequence such that the I/Lock LED is on solid. Ensure that the LEAK valve is closed and that the unit has reached it’s ultimate vacuum, typically better than 3x10-2mbar. Turn the voltage control to Zero.
Test 1
Press the SET HT button and slowly increase the voltage control. There should be no, or minimal, discharge current indicated on the meter. At the full 3kV voltage, a ‘dirty’ sputtering head may indicate up to about 5mA. If the current avalanches out of control then this does indicate a problem and the 630mA fuse may well blow.
Test 2
Remove the high voltage cord to the sputtering head at the back panel. Repeat Test 1. If it is now possible to increase the voltage to 3kV without any indicated current then the fault is likely to be in the sputtering head or in the high voltage cord connector. If, however, the unit still shows an avalanching current then the fault may be the back panel high voltage connector or the HV power supply.
Silver as a removable coating for SEM
Acknowledgement: The following abstract and method results (introduction only) is reproduced by kind permission of A.A. Mills, Scanning Microscopy, Vol. 2, No.3, 1988 (Pages 1265-1271)
Abstract
A thin film of silver, applied by sputtering or vacuum evaporation, provides an excellent conformal conductive coating for scanning electron microscopy of insulating specimens. When no longer required it is easily removed with Farmer’s Reducer - a dilute aqueous solution of potassium ferricyanide and sodium thiosulphate.
No damage was apparent to fine structure in the calcite matrix of ostracode shells, or to other biological tissues. No problems have been encountered with grain in the silver film at magnifications up to x15,000, or in the storage of coated specimens in a desiccator for periods exceeding six months.
Introduction
Many specimens for which scanning electron microscopy (SEM) is invaluable are electrical insulators, for example microfossils and dried biological preparations. To promote the emission of secondary electrons, and to prevent charging of the surface (with consequent repulsion of bath incoming and secondary electrons) it is usual to coat such specimens with a very thin layer of metal.
Nowadays gold (sometimes over a thin undercoat of carbon) is commonly employed for the majority of work, although refractory metals have been recommended for the very highest magnifications. These coatings are normally applied by sputtering in a glow discharge, for this technique is omni-directional and tends to give a fine-grained deposit, while the apparatus required is comparatively simple and inexpensive since a high vacuum is not required.
An alternative, older technique (which also allows aluminium to be deposited) is evaporation of a molten bead of the chosen metal in a high vacuum. The inherent directionality of this method means that specimens must generally be moved continuously by a rotating/nodding table.
Problems arise when it is desired to return a specimen to its original uncoated condition, for example to allow successive treatments or because too thick a coating has been accidentally applied. Even specimens which have been correctly coated may be rendered unsuitable for subsequent optical and analytical examination, due to the highly reflective nature of the gold film and its interference with x-ray emission. For these reasons there is frequently a reluctance to allow SEM examination of certain material, eg type specimens and archaeological artefacts.
Removal of Gold and Aluminium Coatings
Attempts have therefore been made to remove the metal film by suitable reagents, which must obviously not attack the substrate. It is well-known that gold is recovered from siliceous ores by complexing with aqueous cyanide under oxidising (aerobic) conditions, and two groups, have independently utilised this reaction.
A major obstacle is the highly toxic nature of cyanides, necessitating efficient fume hoods and a high degree of supervision and control unwanted in most laboratories. A less objectionable reagent is ferric chloride in alcohol, but it requires some six hours on a gold/palladium film from a smooth PTFE surface, and appears likely to attach many specimens. Mercury amalgamates gold, but does not remove it completely and adds its own background.
Aluminium dissolves in weak acids and alkalies with the evolution of hydrogen. Sylvester and Bradley therefore hoped that soaking in a dilute solution of sodium hydroxide would enable this metal to be removed from calcite microfossils without damage to the matrix. Unfortunately, they were later obliged to acknowledge that insufficiently careful exposure to alkali could result in dissolution of fine structure.
Advantages of a silver film
Silver would appear to have much to commend it as an alternative to gold. It is the most conductive metal known, possesses a high secondary electron coefficient, and is readily applied by sputtering or evaporation to follow irregular contours better than any other material.
Unlike gold, its x-ray emission lines are well-separated from those of the biologically important sulphur and phosphorus. Its cost is only a fraction of gold and the platinum metals. The unique applicability of silver to photography has resulted in extensive research upon its complex ions and their solubility.
Quite early in the history of photography it was found that a dark, over-exposed negative could be rendered less opaque (‘reduced’) by aqueous oxidising agents in the presence of sodium thiosulphate. The metallic silver forms the Ag ion, which is promptly complexed by the thiosulphate so that still more silver dissolves. No gas is evolved. The negative would be removed from the reagent and thoroughly washed when a sufficient amount of silver had been abstracted from the image.
Materials and methods
One of the mildest of these ‘reducers’ is that formulated by Farmer in 1884, employing very dilute potassium ferricyanide as the oxidising agent. As paper, albumen and gelatine were apparently unaffected, it was thought that this reagent might well prove suitable for dissolving silver from a variety of coated specimens without damage to the matrix. Ferricyanides do not possess the extreme toxicity of the simple cyanides, and may be purchased and used in the same way as ordinary laboratory and photographic chemicals.
Farmer’s Reducer - the formulation used is based on that given by Jacobson:
Solution A
25g sodium thiosulphate (crystals)
250ml water
2 drops of Kodak ‘Photoflo’
Solution B
10g potassium ferricyanide
100ml water
These solutions appear to be stable indefinitely at room temperature if kept in securely stoppered amber glass bottles. Immediately before use, the following mixture is to be prepared:
50ml water
50ml Solution A
3ml Solution B
It was found that the resulting pale yellow solution had a pH of about 5, the same as the CO2-equilibrated tap water used for its preparation. It was unstable, losing activity and colour after about two hours at room temperature.
A neutral mixture may be prepared by substituting pH 7 phosphate buffer (conveniently prepared from a BDH tablet) for water in the above dilution. However, all the tests to be described in the paper were conducted with the ordinary solution prepared with tap water.
It should be noted that calcium carbonate has a significant solubility in water. In nature, calcite microfossils are protected against percolating groundwater by the sacrificial dissolution of fossils above and around them. Once removed from this environment to the laboratory, such fossils should presumably be washed only with distilled water that has been allowed to stand in contact with CaCO (eg marble chips) and filtered. Otherwise needles and similar fine structures will be particularly at risk.
This equilibrated ‘hard’ water could be used to prepare and dilute the Farmer’s Reducer. A very brief final rinse in distilled water is probably permissible; the common practice of ‘soaking overnight’ is not.
Results - silver mirror on glass
A silver mirror was made by evaporating the metal on to a microscope slide cleaned with chromic acid. Sufficient was deposited to give a semi-transparent film: silvery when placed on a dark background and viewed by reflected light, but behaving as a blue filter when examined by transmitted light.
The coated glass slide was immersed in freshly-prepared Farmer’s Reducer. The silver was gently dissolved in a controlled manner, as shown by the gradual and uniform loss of colour in transmitted light, until none remained after three minutes. No gas was evolved. It was decided that a 10 minute immersion should allow an ample margin to deal with specimens with convoluted surfaces. The reagent had no effect upon gold films. Alloys of silver and gold have not been investigated.