Particles in Plasma Processing Reactors: Modeling Nucleation, Transport, and Interparticle Effects to Reduce Wafer Contamination By

Plasma processing contamination continues to affect device yield in the microelectronics industry. Particles as small as tens of nanometers in radius may cause killer defects due to small feature sizes, which are decreasing to < 0.1 µm. In low pressure discharges (tens to hundreds of millitorr), particles charge negatively and are subject to charged particle forces (ion-drag and electrostatic), and momentum transfer phenomena (neutral fluid-drag forces, thermophoresis, and self-diffusion), as well as gravity. Methods to prevent particle bombardment to the wafer are investigated using a numerical model to simulate particle transport. Creating temperature gradients greater than 100 K/cm by heating the wafer allows thermophoresis to accelerate particles away from the wafer while the plasma is active, as well as in the afterglow. In Inductively Coupled Plasmas (ICPs), increasing the applied rf bias to the powered electrode and lowering the ICP power aid in shielding the wafer from particles. Another method of preventing particle deposition is curtailment of particle growth above ~10 nm. We investigated Ar/SiH 4 radio-frequency (rf) discharges and found that by decreasing the rf power, the growth rate for particles decreases as the radical density responsible for polymerization decreases. Increasing the flow rate lowers the particle residence time and aids in the reduction of particles impacting surfaces. iv Particle transport is also affected by Coulomb interactions between particles. Experimental observations showed that in trapping sites particles form clouds and move collectively. We numerically investigated Coulomb interactions between particles. In rf discharges, ordered systems (Coulomb liquids or solids) occur at low particle temperatures. Dislocations in lattices result because of impurities of differently sized particles, due to the disparity in particle charges. Waves may be induced by impacting a Coulomb solid with energetic particles. Repeated impacts can liquefy the lattice if the particles do not dissipate enough energy as a result of fluid drag effects or Coulomb collisions.