SEM Carbon coater

SEM Carbon coater, Agar

This is used to cover non-conducting samples with a conducting carbon layer for use inside a Scanning Electron Microscope (SEM).
Two carbon tips are making electrical contact and a current is running through them, heating them to a red/white glow. Carbon molecules are emitted in all directions inside the vacuum chamber.

The vacuum chamber, glass cylinder to the left, is evacuated by a rotary oil pump located in the service room.
The sample to be covered is placed on the small table underneath the carbon tips. The height of the table can be adjusted to change the thickness of the coating.

The top of the vacuum chamber can be opened and lifted backwards. The glass cylinder can be removed to more easily access the sample table and adjust its height.
Care should be taken when removing the glass cylinder, its top and bottom edges make a vacuum seal with O-rings. If edges are chipped or damaged vacuum cannot be achieved.

Detailed practices

More detailed practices using holder to avoid carbon particles

SEM images, carbon coated sample

Frontpanel controls, from left to right:

Mains switch, starts the evacuation of the chamber.
A-meter/vacuumeter, shows current through carbon tips or vacuum in chamber.
A-meter/vacuum switch, makes the meter show either current or vacuum.
Current control knob, sets how much current is run through carbon tips.
Start button, activate current through carbon tips.
Pulse ON/Off switch, alternates between continuous mode and pulsed mode.
Stop button, stops current through carbon tips.
Pulse button, for pulsed mode, turns on current when pressed.

Current/vacumeter, upper scale is current in Amps, lower scale is vacuum in mbars.
The meter-switch is depressed, causing the meter to show vacuum, 0.05 mBar.
Normal pump-down to 0.1 mBar should take about 30 sec. Then vacuum is good enough for operation.
If pump-down takes much longer something is wrong.

Pump-down is ready, the start-button is lit, system is ready for operation.

Press the start-button and rather decidedly and quickly turn up the current control knob to about 100 A. The carbon pins should start to glow. Do not turn up the current too slowly, it causes the carbon tips to sinter and may not start to glow. The current is not quite stable and may need some after adjustment. The current to be used for coating is dependent on the desired coating thickness and quality.

Here the approximately the same current has been running a little longer.
As you can see the glow instensity of the carbon tips increase with time.
To stop the current, press the Stop-button.

When you are finished, the chamber is vented by room air. On top of the
chamber is a valve that is opened by tilting it by hand.
Warning! The top might be hot due to running the current for some time.
It is VERY important to ALWAYS vent the chamber!
Otherwise oil might seep backwards from the oil-pump up into the chamber.

Detailed practices

Never run a continuous deposition for more than five - six seconds. There is an internal overheating/overcurrent protection that shuts off after that. To get the coater working again you have to shut off, vent and start again.

It is recommended to always use pulse mode.

Deposition rate is about 1.5 - 2 nm/s when pulsing in one-second intervals. This rate depends of distance between sample table and source.

It is recommended to always use the holder described in document linked to below.
More detailed practices using holder to avoid carbon particles
The carbon rods emits larger particles which falls down. Putting your sample above the carbon rods alleviates this problem.

SEM images of carbon coated sample

Images of a 400 nm silicon wire coupled to a photonic crystal.

Carbon coater: Six pulses of one second each.

Enough to avoid charging effects during SEM imaging.

Images courtesy of Ziyang Zhang, IMIT, KTH-Kista.

Click image for a full resolution version

Anders Liljeborg Nanostructure Physics, KTH.