Metal sputter and carbon evaproation coater
Why we sometimes coat SEM samples
The SEM works by sweeping a hair-thin, perfectly straight lightening bolt called an electron beam across the surface of the sample.
Some samples are electrically conductive, so extra electrons conduct off the surface to ground. These are relatively easy samples.
However, some samples are electrically non-conductive, so static electric charge can build up on the surface of the sample (like the static electric buildup that happens when you rub a balloon on your hair). This is bad for us because the negative charge of the static electricity repels the negative charge of the electron beam.
One way to prevent static electric charging of non-conductive samples is to coat the surface with a very thin layer of conductive material: either metal or carbon. Our Quorum Q150R ES plus coater deposits a 5-20 nanometer thick layer of gold or graphene on top of samples to prepare them for scanning electron microscopy.
Why use metallic gold or platinum?
Noble metals like gold, platinum, palladium, and iridium are expensive, so why would we coat samples with these instead of cheap iron?
Gold and platinum group elements are much more conductive to electricty than iron.
Gold and do not oxidize easily, so they do not form new oxide crystals on the surface of the sample after coating (this would create features not characteristic of the sample itself and would reduce the conductivity of the coating).
Gold and platinum group elements have more electrons in each atom, so these metals interact more with the electron beam to create a stronger signal (brighter image) than iron would.
This is a photo of the surface of the 2-inch diameter gold foil after it has been used for a couple of years. It started out smooth and shiny, but the coating process erodes gold atoms from the surface one atom at a time.
How does the coater deposit gold?
Sputter coaters deposit metal by:
ionizing argon gas atoms using a magnetron
argon gas ions to collide with a gold foil at top of chamber because they're attracted by a combination of magnetic field and electric field
argon ions slam into gold foil and dislodge individual gold atoms
gold atoms rain down onto sample surface below
The magnetron is a circular device, which is why the gold eroded off the foil in the photo above in a circular pattern (called "racetrack erosion").
There is a beautifully illustrated explanation of the physical vapor deposition (PVD) process posted by Plansee High Performance Materials on youtube. Their configuration is huge compared to ours and they're depositing molybdenum on glass instead of gold on random samples, but the concepts are the same.
What does a vapor of gold and argon look like?
During the metal coating process, the ionized argon and vaporized individual gold atoms glow a beautiful purple. Note the floating ring of glowing ionized argon gas and gold hovering a few millimeters from the gold foil (the upper purple ring is a reflection of the ionized ring on the shiny gold surface). The samples are on the wheel below the purple glow. The wheel rotates slowly so the samples get a nice even coating.
How do we measure the coating thickness?
In this video of the sputter coater depositing gold, you can see the glowing ring beneath the gold foil at the top and the rotating sample holder below.
In the foreground on the left is a little brass drum which measures the thickness of the coating as metal deposits. The drum contains a little piece the mineral quartz. Quartz vibrates when electricity flows through it (which is why it was used for making watches in the olden days). The frequency of vibration depends on 1) amount of electric current, and 2) mass of quartz (because it's harder to vibrate a bigger thing). The thickness monitor runs current through the quartz and measures the vibration frequency. As the gold deposits on the sample and on the quartz, the heavier quartz slows its vibration. The amount of slowing tells us the thickness of gold on the surface.
Why coat with carbon instead of gold?
As a geochemist, I prefer to coat with graphene carbon instead of gold because graphene is electrically conductive, but does not create confusion when I'm using the EDS x-ray analyzer for measuring the chemical compositions of my crystalline samples.
The process for coating samples with carbon is different than metal sputtering. To coat with carbon, we string a thread of graphite string between two electrodes. The sample is put in the vacuum chamber, then high voltage electricity flows through the carbon thread. The electricity heats the thread so hot that it glows brightly and carbon evaporates off the thread, then rains down onto the sample below.
What does carbon coating look like?
This video shows a sample being coated with carbon. The sample holder turns so the samples get an even exposure to the carbon atom rain. The electric current pulses through the carbon thread rather than flowing continuously so the carbon thread doesn't get so hot that it melts through like an old fashioned fuse.