How to create DC glow discharge
In DC glow discharge, there is a fixed cathode and a fixed anode. Electrons will be accelerated from cathode to anode and acquire more energy. Along the way to the anode, electrons will experience many collisions with other ions and neutrals. During the impacts, electrons will transfer their energy to bonded electrons within ions and neutrals. Once the electric field strength reaches a certain level, free-moving electrons can acquire enough energy to knock out bonded electrons from neutral particles. A process similar to avalanche breakdown will happen in the gas chamber. Electron and ion density will be multiplied and eventually the whole space will be filled with positive, negative ions and electrons. Sometimes a heated filament is used as a cathode electrode to emit a constant flow of free electrons to help to ignite the plasma and improve the stability. Unfortunately, evaporated metals from heated filament can sometimes contaminate the sample. There is an optimum voltage range between cathode and anode to generate plasma. If the voltage is too low, free electrons won’t have enough energy to ionize the neutrals. If the voltage is too high, electrons will move too fast to avoid any collisions with neutrals. The optimum voltage range is a function of gas species and pressure.
Application and limitation of DC glow discharge
Glow discharge has been used by many cryo-EM microscopists to treat the TEM grids and make the surface hydrophilic. DC glow discharge can only generate extremely weak plasma, which happens to be suitable for surface treatment of fragile holey carbon grids. RF plasma system can generate much stronger plasma for high-speed surface cleaning and ashing. But if the intensity of the plasma is too strong, it may damage the fragile carbon and graphene grids. The Tergeo-EM plasma cleaner made by PIE Scientific LLC solved this problem on the traditional rf plasma system by integrating a dedicated downstream/remote plasma source and pulsed rf power supply. Tergeo-EM can generate plasma that is much gentler than glow discharge by combining the unique downstream mode and pulsed mode. Tergeo-EM systems can easily treat ultra-thin TEM grids without damaging them. Tergeo-EM can also generate strong plasma that can easily remove the photoresist and thick layer of carbon deposition on apertures inside the electron microscope column.