Stable negative ions containing up to sixteen silicon atoms have been measured by mass spectrometry in RF power-modulated silane plasmas for amorphous silicon deposition. These hydrogenated silicon cluster ions reach much higher masses than the positive ions, which have no more than six silicon atoms. This supports the view that negative ions are the precursors to particulate formation in silane plasmas. The time-dependent fluxes of positive and negative ions from the plasma are shown with a 5 mus time resolution. Possible cluster reaction sequences are discussed and the effect of visible light on the negative ion signal is commented upon.