From molecules to particles in silane plasmas

Abstract; Particle formation has been investigated experimentally from the initial molecular precursors up to the final micron-sized particles in a low pressure silane rf capacitive discharge. Neutrals and ions were studied by quadrupole mass spectro-metry in power-modulated plasmas: Whole series of negative ions were observed, ranging from monosilicon anions through to nanometric clusters. Anion confinement results in particles and conversely, anion de-trapping can inhibit particle formation. Plasma polymerisation is considered in terms of neutral and ionic species. Laser light scattering measurements show that particles appear during a rapid coalescence phase and possible mechanisms are discussed. INTRODUCI'ION Particles during silane plasma processing are a source of contamination [ 11, all the more serious as device areas increase and feature sizes diminish. Despite clean room precautions, however, particles may still ultimately form during the process itself by homogeneous gas-phase nucleation inherent in the plasma chemistry. This is an example of plasma state polymerisation which limits the maximum process rate for particle-free deposition [2]. If formed, micrometric particles are negatively-charged by electron-ion flu