RF or DC oxygen plasmas are composed of a number of highly energetic and reactive species that will readily clean most organics off wafers and strip photoresist.
While operating with prolonged high power plasmas residual gas deposition and chamber sputtering collects on most surfaces.
A small, modular, and versatile chamber was constructed for plasma and other research experiments. The main vacuum manifold can be configured with multiple feedthroughs in either standalone vacuum mode or connected to a large chamber. The front is a 6″ CF viewport that can be swapped for a gas feedthrough assembly.
The chamber is being used right now to study simulate vacuum conductance and pressure gradients across larger chamber systems such as an ion implanter with a turbo pump on one end and an ion source and MFC gas flow far away from the pump. The gas flow creates a higher pressure in the ion source chamber and, in theory, allows for low energy beam transport and acceleration into a much lower pressure substrate/target chamber (in the case of an ion implanter).
A quick attempt making a titanium sublimation pump. Ti welding rod was bent into a coil around aluminum round stock and placed across 30-50 amps in high vacuum yielding successful results. Chamber was roughed down to 20mTorr then pumped with turbo to 1e-5 and briefly baked out. More testing will be done and results will be posted along with an updated design with shielding to make it into an actual pump rather than depositing all over the chamber walls.
During testing I experienced a strange pumping curve a few times as shown in the last picture where the filament while heated at 50 amps seemed to out gas twice before reaching sublimation.
(Click on image to enlarge)
The basic idea of a TSP or getter pump is that the filament is heated past 900 degrees c with a high current across it. The filament first out gases and raises the chamber pressure, but then reaches sublimation pressure where it begins to form a thin volatile coating of Ti on the chamber walls.
Titanium, in this heated state, will readily combine with gaseous specious in the chamber to form a more stable coating and the gas molecules in the chamber basically get incorporated into a thin film on the chamber walls and trapped. The filament current is cycled for highest effectiveness. A rewound MOT is used for high current supply.
Two internal radiant infrared heat lamps and external resistive heating elements are used to bakeout the vacuum system for 24 hours allowing for ion pump operation. Normally when the chamber is full of unwanted moisture and impurities ionizing gas in the chamber will lead to a sharp increase in pressure due to plasma cleaning of the chamber walls and subsequent release of gas molecules.
I machined a delrin insulator with fiberglass center rod for the CF 4 pin electrical feedthrough to provide power to the two incandescent substrate heaters in my new sputtering setup and to allow for instrumentation hookup.
New sputtering setup has dual substrate heaters as well as insulated feedthroughs for both target and substrate so that it can operate in DC, RF, Bias sputtering modes, or any combination therein. A quartz microbalance thickness monitor will be added soon.
The second to last image shows multiple colors in glow discharge during sputtering possibly due to DC bias. Bottom and top electrodes are both at different potentials with respect to chamber ground, so it is possible that different gasses are ionized due to differences in ionization energies of the residual gaseous species.
Thin films of copper were prepared on non-conductive samples to be observed under the Electron Microscope. This process is called DC Diode Sputtering and took place at just below 100mTorr and 2000v @ 150watts.
This same setup can be used at lower powers to plasma clean or etch the top layer surface of the sample for better images under the SEM. Oxygen (and sometimes a small amount of Nitrogen) is usually a much more effective gas for this cleaning than Argon and leads to less unwanted sputtering as well.
Sputtering system v2 was made to support DC and RF sputtering so I can make dielectric coatings as well and it supports larger target and substrate samples. An insulator will be constructed to mount the target holder to the main chamber walls so that RF can be used more effectively.
(Click on image to enlarge)
Miscellaneous Upgrades – Vent Valve/TC Gauge CF Tee Construction, Gas Cylinder Holder Fabrication, and Ion Gauge Controller Analog Output Hack
My HP (Granville Phillips) hot cathode ionization gauge controller (59822b) had provisions for an analog output however the optional connector was not installed on the back panel and the connection points on the main PCB of the controller did not show any voltage deflection proportional to pressure so I took the pressure output from one of the stages in the electrometer which happened to be one of the outputs of a quad opamp chip. This output is fed to a NI DAQ USB6009 and displayed in a LabView VI so that the pressure of the chamber while pumping down can be held constant when requested by PID feedback loop with one or more of the mass flow controllers.
A pressurized gas cylinder holder was designed and fabricated by Diversatech Manufacturing, INC (www.diversatech.com) for safer operation of my vacuum chamber when using gasses such as Oxygen, Nitrogen, or Argon for processes such as sputtering or plasma cleaning.
Quick connect fitting block off made by the extremely talented J.D Guerard machining.