A Mechanistic Study of CC12F2/02 RIE for Bipolar Trench Isolation

The etching of 1.25×3.5 micron trenches in silicon for use in bipolar device isolation using chemistries based on CC12F2 (freon-12) and 02 has been investigated.It was found that competitive reactions in the trench favor deposition of a Si02-like material on the sidewalls and promote lateral etching of the N+ subcollector under conditions necessary for high selectivity.In addition thermally activated processes were investigated by direct measurement of the sample surface temperature during etching.The temperature threshold for the initiation of subcollector undercut was determined to be 70C and the vertical etch rate exhibited an inverse activation indicating adsorption-limited etching.Results are discussed in the context of a qualitative model, based on the etchant-unsaturate model, which describes the contribution of several phenomena to the trench shape.These include sidewall passivation, species transport, deposition, N+ doping enhancement, Si loading, and ion scattering.