It has been indicated that under conditions of glow discharge, ion bombardment of the cathode will occur. This results in the erosion of the cathode material and is termed plasma sputtering, with the subsequent omni-directional deposition of the sputtered atoms forming coatings of the original cathode material on the surface of the sample and work chamber.
This process is enhanced in sputter coaters for use in Scanning Electron Microscopy where one objective is to provide an electrically conductive thin film representative of the specimen to be viewed. Such films inhibit “charging”, reduce thermal damage, and enhance secondary electron emission.
The most common arrangement for a D.C. (Direct Current) sputter coater is to make-the negative cathode the target material to be sputtered (typically gold, platinum or with high vacuum sputter coaters, metals such as chromium and iridium), and to locate the specimens to be coated on the anode (which is usually “earthed” to the system, so the specimens are effectively at “ground” potential).
The desired operating pressure is obtained by a pump (usually a two-stage rotary vacuum pump, or a turbomolecular pumped “backed” by a rotary pump), with an inert gas, such as argon admitted to the chamber by a fine control (leak) valve.
Sputter Coater : General points for Improving Performance
- Cleanliness, the work chamber must be kept clean! We advise that a separate carbon coater be used in applications where the maximum performance of the sputter coater is required.
- Clean the glass chamber with hot soapy water and dry thoroughly, solvents can be used but we have found this unnecessary and has greater danger to health and safety. If the deposit is stubborn, use a kitchen scouring pad such as the green Scotch Brite variety.
- Use Isopropyl alcohol on metal surfaces, not acetone which has greater danger to health and safety. It will also take longer to out gas and reduce the vacuum performance.
- Vacuum, Never leave the chamber under vacuum without isolating the roughing pump from the coater, this is usually done with a manual valve (Quorum high vacuum sputter coaters have useful “pump hold” facility that allows the vacuum chamber to be held under vacuum when the instrument is not in use). Failure to do so will increase the risk of suck back of hydrocarbons (pump oil) into the sputter chamber and increase contamination.
- Always ensure the system is dry and pumping to its correct vacuum level before working with samples, failure to do so will result in poor sputter rate and contamination.
- Ballast rotary pumps on a regular basis and ensure they are serviced at regular intervals.
- Sputter gas, Always use high purity argon gas of the grade known as “White spot” this will ensure fast sputter rate and good pump down time.
- Rotary planetary specimen stages are essential for ensuring even coatings on specimens with irregular surfaces.