Description

The EVACTRON^® Anti-Contaminator is a RF small plasma device consisting of a cylindrical electrode with multiple apertures housed in a Aluminum cylinder about 40mm in diameter and 50 mm long with standard vacuum fittings (KF 40 or CF 2.75) at each end. The Evactron electrode is made from Al sheet with 3mm round holes punched into it. This electrode assembly was developed empirically as a device that would ignite a RF plasma at low power and showed exceptional cleaning ability when used with air at low RF power. The electrode is operated at 13.56 MHz with typical power levels between 5 and 20 Watts in air. A nearby RF matching network matches the impedance of the load with the RF supply to maximize power transfer to the plasma. Typically the plasma will ignite at between 3 and 5 Watts of input power. With the variations in the matching networks we have had one device that would ignite at 1 Watt. The low wattage of the plasma generator produces a very low temperature plasma.

The Evactron Electrode operates inside a typical metal vacuum chamber with chamber walls at ground. The design is neither a pure capacitive nor a inductive design. The classical parallel plate capacitive and inductive coil designs require to much power to ignite a plasma and do not produce the low temperature plasma required for a air plasma device to be successful as an Oxygen radical source.

The theory behind the Evactron electrode is not fully worked out. It is believed that each of the holes in the cylinder provides a small hollow cathodes that are enhanced by the RF currents circulating around the circumference of the holes to create small magnetic traps for the excited electrons therein. The interior of the cylinder provides and additional hollow cathode area to trap the electrons. The net effect is to create a device that has a high density of low energy electrons. This allows the plasma to ignite a plasma at very low RF power below 10 Watts.

US Patent 6,105,589 has been issued to Ronald Vane for the method and apparatus of the Evactron for cleaning electron microscopes and further patents are pending on the electrode and EVACTRON assembly described herein.

The Evactron is operated as a downflow plasma ashing device. Gas is fed into to the plasma chamber and the reactive species carried into the instrument by convection flow. The main chamber and specimen is not subject to direct ion or electron bombardment. The plasma stays inside the Evactron chamber. The pressure used is between .4 and 1.2 Torr. At these pressures there is viscous flow of the gases, high electrical conductivity, and the mean free path is too short for ion sputtering. Sputtering becomes possible below about 0.15 Torr. Above 1.2 Torr recombination of the radicals by three body collisions stops the ashing process.

Theory of Operation

The Evactron™ system’s ability to create a low temperature plasma is an important part of the method for generation the oxygen radicals from air. When oxygen is ionized a series of reactions lead to the formation of oxygen radicals:

  O2 + O+ > O2+ + O

  O2+ + e- > O + O

Compared to the ions these radicals are long-lived species and may leave the plasma region. Oxygen radicals are lost in reactive collisions with surfaces and other gases. They are not lost in collisions with O₂ and N₂. The recombination of two radicals does not occur unless there is a third body to remove the excess energy.

The ionization potential of oxygen is 12.1 eV and nitrogen is 15.6 eV. Thus oxygen ionization takes place in a lower temperature or lower energy plasma than nitrogen. By lowering the average temperature of the electron-energy- distribution oxygen ionization is favored. When nitrogen ions are produced in an air plasma they react with O radicals by the following fast reactions:

  N2+ + O > NO+ + N

  N + O > NO+ + e-

Thus two oxygen radicals are destroyed by every nitrogen ion produced. Because nitrogen is the major constituent of air, this destruction takes place quickly once nitrogen ionization begins. In addition NO+ is a stable ion with a low ionization potential (9.5 eV). It is unable to react with the neutral diatomic gases in air and reacts with hydrocarbons to form nitrogen oxide polymers that are resistant to further oxidation and removal. The transition from an oxygen-dominated plasma to a nitrogen ion-dominated plasma is function of the plasma temperature. In the Evactron system an operating pressure and plasma temperature are adjusted such that the oxygen radical flux to the surfaces is maximized.

Convention plasma ashing for hydrocarbon removal is done either with argon, Oxygen, or argon Oxygen mixture. For the removal of photoresists Fluorine containing molecules such as CF₄ are often added to the gas mixture. Fluorine is thought to speed the process by hydride extraction from the hydrocarbons. The ashing chambers are typically either parallel plate or inductively coupled producing high temperature plasmas. With argon the ashing mechanism is sputter etch. For oxygen plasma the dominate reactive species is Oxygen radicals. Oxygen ions may be involved in reactive ion etch mechanisms and these result in the formation of higher energy oxides. Low temperature Oxygen radicals are involved in the important reaction with hydrocarbons to form CO, CO2, and H2O which can be removed as gases from the system. Higher temperature O radicals and O metastables are more able to convert hydrocarbons into volatile species. For low temperature O the C-C single bond is hard to break. The result is the formation of surface carbonyl groups rather than the breaking away of CO gas. Another effect is cross linking of the hydrocarbons as H and C are removed if the density and temperature of O radicals is two low.

The use of Argon as a dilutant gas with Oxygen allows higher temperature O radicals to be formed. These have more reactive ability than the low temperature O in air plasmas. This allows faster oxidation and removal of the hydrocarbons. It also allows for more oxide formation and damage to other materials.