Table of Contents
The Leica EM ACE200
Introduction
The Leica EM ACE200 coating system is used for precise coating of samples for subsequent examination with a scanning electron microscope (SEM). Samples are metal coated using the sputtering method where argon plasma erodes a target material. Carbon coating is achieved by carbon thread evaporation. Any material can be processed as long as it is not sensitive to vacuum, argon plasma, or the heat generated during carbon coating. Sputter coating prevents charging the specimen with an electron beam in the SEM high-voltage mode. Metal coatings are also useful for increasing signal-to-noise ratio. The ACE200 can be configured as a sputter coater, carbon coater, or both, where the two processes can be switched over from one to the other.
With the SEM, sputter coating is not required for all samples. Images up to a magnification of 4000x on microfossils are of often high quality without any sputter coating. For detailed observations in magnifications greater than 4000x, for very delicate fossils, for any analyses of thin-sections or smear slides, or for EDS work, sputter coating before using the SEM is used to produce higher quality images.
The Leica EM ACE200 coating system includes the following main functional units:
The following targets are used for sputter coating:
The working gas (argon) must be supplied at a pressure of ~500 mbar ±100 mbar (5.6—8.4 psi above atmosphere). The gas should be at least 99.99% pure.
Caution! Do not use a standard two-stage regulator for the sputter coater unless it is equipped with a low-pressure delivery-side gauge. Most regulators don't effectively pick up pressure until 10 or even 15 psi or higher. It may be necessary to put a second low-pressure-range regulator downstream from the main line regulator. Make sure you have the right tool for the job!
Sputter coating is performed using ionized argon to create a plasma. The argon ions are accelerated by high voltage and directed toward the source via a magnet where they collide with the target and displace surface atoms. Due to this collision the surface atoms are directed toward the area below the target and coat the sample. This coating process can be set to directional or diffuse (which provides more even coating on a larger surface and is better for fissured samples), depending on the process pressure. This also influences the coating rate (diffuse means slower rate) and the grain size (directional means finer grains).
The illustrated procedure can be found starting on p. 43 of the operating manual linked above. There is also an ACE200 youtube video that shows the loading of the target at http://www.youtube.com/watch?v=Kd1lyKIDT8.
The sputtering procedure is summarized as follows:
Note: if the vacuum cannot stabilize, make sure the argon supply valve is open and pressure is on the line.
The coating process can be started at any time (system evacuated or not) as long as the door and source cover are closed and the source is connected. The Start button will turn into a Stop button when the system starts the coating cycle.
Once the Start button is activated the coater automatically runs the complete coating cycle. The system either stays under vacuum or vents automatically (if Vent after process is activated).
Pressing the Stop button terminates the process after confirmation, regardless of the step of the process. The system either stays under vacuum or vents automatically.
After setting coating parameters and pressing Start the system performs the following steps automatically:
For Au/Pd coating, the following settings are recommended:
The carbon coating process is carried out by evaporating a carbon thread. It is possible to coat using short pulses of 150 milliseconds or evaporating the thread completely with maximum power, a so-called flash.
The carbon thread can be loaded as a single thread or as a double thread. Thin layers from 1 to at least 20 nm can be achieved (there is variance in threads).
To minimize carbon thread waste, when loading a double thread, cut a piece of thread twice as long as the width of the black door frames of the coater. Fold the thread into half and load it as illustrated in the linked manual (p. 34). Be sure to wear gloves during this process.
Also note that there is an ACE200 youtube video where the loading of the carbon thread can be seen at http://www.youtube.com/watch?v=yKd1IyKIDT8.
After setting coating parameters and pressing Start the system performs the following steps:
For C coating, the following settings are recommended:
To switch between carbon thread and sputter coating, exchange the source (sputter or carbon thread) and choose the process on the main screen of the instrument. The instrument automatically detects if the correct source is installed and gives and error message if it does not match the selected process.
Leica service manual: ACE200 service manual.pdf
This document originated from Word document SEM Sput_Coat_374.docx (see Archived Versions below for a pdf copy) that was written by S. Herrmann in 2013 and edited during Expedition 374 by A. DeLoach. Credits for subsequent changes to this document are given in the page history.