1.6 Glow Discharges and Plasma 1.6.1 DC Glow Discharges cathode glow : highly luminous layer, excited atoms - secondary electrons neutralize the incoming discharge ions and positive cathode ions

Crookes dark space : sheath  net positive space charge, low e- density - most of the applied voltage is dropped - positive gas ions are accelerated toward the cathode Negative glow - accelerated electrons acquire enough energy to impact – ionize the neutral gas molecules All of these charge effects stem from the fact that electrons and ions have such different masses and, hence, velocities and energies.

Gas collision with e- Ionization Excitation Dissociation Dissociative ionization Degree of ionization : 10-4 T = E/K  2 eV for e-  23,000 K T = 0.025 eV (290 K) for neutral T = 0.04 eV (460 K) for ions

1.7.1 Ion-Surface Interactions 1.7 Sputtering 1.7.1 Ion-Surface Interactions Each depends on the type of ion (mass, charge), the nature of surface atoms, and ion energy.

1.7.2 Sputter Yield S values are typically in the range of 0.01 and 4 and increase with the mass of metals and energy of the sputtering gas(Noble sputtering gases with energies of 0.5KeV and 1.0 KeV) See Table 3-4

S from collision cascade theory by Sigmund *  : a measure of the efficiency of momentum transfer in collision Sn(E) : stopping power - a measure of the energy loss per unit length due to nuclear collision

M2 M1 * Elastic binary collisions: potential energy is conserved  “Energy transfer function” if  = 0 & M2 = M1  E2 = E1 If  = 0 & M2 >> M1  E2=E1x10-4 (e-, N2) * Inelastic binary collisions: potential energy is not conversed U is the change in internal energy All of an electron’s kinetic energy is transferred to heavy N2

1.7.3 Sputtering of alloys 1.7.4 Substrate heating Thermal evaporation  loss of stoichiometry Sputtering  same composition much greater difference in vapor pressures compared with the difference in sputter yield Melts homogenize readily due to rapid atomic diffusion and convection effects in the liquid phase 1.7.4 Substrate heating T t

: density c : heat capacity d : thickness dT/dt : rate of temperature rise p : incident power flux Hc : heat of condensation Ek : average kinetic energy of incident adatoms Ep : plasma energy from electron, ion bombording L : heat loss d• : deposition rate : atomic volume : Stephan-Boltzman const.  : emissivity

1. 8 Sputtering Processes - DC, RF, Magnetron, Reactive Sputtering - Target is a plate of the materials to be deposited. 1. 8. 1 DC Sputtering (Diode, Cathodic sputtering)

- Film deposition rate depends on the sputtering pressure and current variables P : sheath(cathode) is wide  Ions are far from target (lost) : Electron mean free path   ionization efficiency is low  no plasma below 10 mTorr P : electron mean free path and large ion current but sputtered atoms undergo increased scattering  not efficiently deposited

1.8. 2 RF Sputtering - Suitable mean of depositing insulating thin films (eq) For DC sputtering of SiO2 Target : Quartz disk with 0.1cm thick and   1016 cm To draw a current density J=1mA/cm2, V=? V=RI For practical application, V=100 V and  should be less than ~106 um f > 50KHz : enough electrons to ionize gases(5~30MHz) f < 50KHz : essentially DC sputtering at both electrons (ions follow the frequency changes) Typically 13.56 MHz is used : FCC (federal communications commission) recommended

Concept of self-bias at RF electrodes The disparity in electron & ion mobilities  positiveley charged electrode draws more e- current but  no charge transfer through capacitor  self – bias voltage (negative) at target electrode (capacitively coupled electrode)

Both electrodes in RF sputtering system should sputter. cause a contamination in the sputtered film. The ratio of the voltage across the sheath at the small capacitively coupled electrode(Vc) to that across the large directly coupled electrode(Vd) including substrate and chamber walls, etc is given by The fourth- power dependence means large Ad is very effective in raising the target sheath potential while minimizing ion bombardment of grounded fixtures.