In this part, surface adsorption-desorption reactions and secondary electron emission process are explained in detail. Surface processes have a key role on surface etching, for example. The gas phase on top of the surface interacts with the surface layer. Adsorption-desorption reactions affect gas-phase species concentration. Also, positive ion neutralization and secondary electron emission have effects on gas discharge. Therefore, gas and surface reactions are coupled.
Positive ions near the metal surface can be neutralized by an electron located in the conduction band or cause secondary electron emission of an electron if they have sufficient energy. Electrons occupy the energy levels up to a limiting energy level, called Fermi energy. To emit an electron from metal, free electrons – electrons above the Fermi level – should pass an energy barrier which defines the work function. The work function is calculated by the method of images. The Coulomb force acting on an electron outside the metal and an imaginary electron inside the metal is integrated over a range from order an atomic radius to infinity. The work function is roughly related to square root of the ionization energy of the metal. If an ion closes the metal surface within an atomic radius, an electron at the conduction band can tunnel through the potential barrier and creates recombination neutralization or Auger mechanisms. If there is an excess of energy after an electron tunnel to neutralize the ion, a secondary electron gains that energy and be emitted from the metal. Secondary electron emission occurs in very short time because the tunneling time is smaller than the ion collision time with the surface. Therefore, secondary emission is independent of ion energy and affected by atomic ion species and surface composition of the metal.
At low thermal energies, physisorption, chemisorption and desorption can be observed on the surface with heavy particles. Adsorption is the result of attractive forces between molecules and surfaces. In physisorption, the dominant mechanism is weak van der Waals force. The atom should be in the order of 1-3 Angstrom from the surface and oscillate with a certain vibration frequency. Chemisorption occurs closer distance to the surface. It is mostly observed with physisorption in a system. The molecules are captured by physisorption and diffuse from the surface through the vacant sites for chemisorption. For the dissociative chemisorption, the single bonded molecules interacted with the surface separately. Molecules can only be absorbed if they lose their energy in the collisions with the surface. The flux of incident particles is considered with the sticking coefficient which is a function of a fraction of region covered by the incident molecules, the gas and surface temperature. Desorption is the opposite of adsorption and its rate can be calculated with an Arrhenius form which includes the energy of the potential well and temperature. The initial rate represents the number of attempted escapes per second. In thermal equilibrium, absorption and desorption rates are balanced.
Reference: Lieberman, Michael A., and Alan J. Lichtenberg. Principles of plasma discharges and materials processing. John Wiley & Sons, 2005.
Course: AME 60637