Energy transport in plasma etching of nanoporous dielectric materials

Abstract A Monte Carlo routine was developed to simulate the motion and energetics of ions in the pores of a xerogel material under plasma etching conditions. The simulation included the effects of an applied electric field and input conditions for the pore as a function of pressure and applied voltage in the plasma reactor. We were interested in the ion energy in a pore, the ion penetration depth and the effect of ion energy on etching. At low pressures the nanoporous material etches faster than dense silicon dioxide. This is to be expected given the decrease in density and increase in surface area that arises due to the porosity. However, as the pressure is increased, the etch rate decreases dramatically and, eventually, the dense oxide may etch faster than the porous material. CHF 3 was used as the etchant gas and, for this gas, we believe this behavior to be controlled by the ion energy and energy transport in the pores of the xerogel material. As the pressure in the plasma reactor is increased, the incoming ions switch over from etching activation to polymerisation activation. This agrees with the observed crossover in etch rate seen experimentally and with the cessation in etching as pressure is increased. The switch is affected by pore roughness and correlates with the average ion energy in the pore.