Stress reduction in silicon dioxide layers by pulsing an oxygen/silane helicon diffusion plasma

A low pressure high density helicon reactor used to deposit silicon dioxide (SiO2) from a mixture of oxygen/silane has been pulsed with a constant "on'' time of 500 mu s and a duty cycle varying from 10% to 100%. Over this range, the deposition rate changes by only a factor of 2.5 implying that deposition is continuing in the postdischarge with a time constant of 1 ms. For duty cycles of 30% and above, the films show good characteristics but the 10% duty cycle has a somewhat higher "p etch" implying some porosity. The pulsing reduces the compressive stress by at least a factor of two. This is correlated with the reduction in the measured plasma potential and density implying that for the present conditions, the stress is determined by the energy and number of ions striking the growing film. For the low ion energies considered here (less than or equal to 50 eV) a simple model using a temporal evolution of the ion energies and fluxes measured in an argon plasma suggests that the compressive stress would decrease in the postdischarge with a time constant of about 80 mu s, assuming that the total stress is the integral of the stress over the on and "off" periods. The experimental results in oxygen/silane plasmas show that this is probably an upper limit and the actual decay time may be considerably less. (C) 1998 American Institute of Physics.