Si and Al Etching and Product Detection in a Plasma Beam under Ultrahigh Vacuum

Abstract

The study of plasma etching mechanisms is made difficult by the inaccessability of the wafer surface to analysis during etching. In the present work, a “plasma beam” is extracted from a miniaturized plasma etching chamber through an array of molecular‐flow orifices in an Mg‐foil electrode. The beam effuses into ultrahigh vacuum and onto either a mass spectrometer for ionic and neutral plasma species analysis or onto an Si or Al film on a quartz crystal thickness monitor for etching studies. Desorbing etch product can be mass‐analyzed before encountering gas or wall collisions. The plasma beam source and its rf power supply can be d‐c biased relative to the mass spectrometer and the etching film to accelerate or retard the ionic component without affecting neutral plasma species, thus allowing energy analysis of effusing ions and measurement of ion energy effect on etching. For plasma maximum ion energy was ≅100 eV at 27 MHz and ≅400V at 100 kHz. Beam etching of Si using and plasma (27 MHz) and of Al using plasma has been carried out, and , ,, and desorption has been observed. Ion bombardment is required to initiate etching in all cases. Subsequently, the effect of ion energy variation from zero (ions reflected) to 500 eV on etching rate and product desorption rate is large (≅x18 enhancement) for Si in , much less (x4) for Si in and zero for Al in . These results correlate with the degree of anisotropy obtained for these etching systems in conventional plasma etching, assuming that the anisotropy is produced by vertical etching rate enhancement by the normally incident ions. They also explain the much higher etching rate observed for Si and at the lower frequency, where ion energy is higher.