The objective of this work was to develop an improved wet etch-stop technology for silicon micromachining. To establish a reference for process improvement, the diffusion process currently used to fabricate p++ Si:B etch stops was comprehensively investigated. Subsequently, a novel germanium- based epitaxial etch-stop technology was developed.
A range of techniques were used to study p++ silicon layers created by boron diffusion into (001) silicon wafers. The results revealed gradients in boron and lattice constant, as well as a graded three- dimensional dislocation array from lattice-mismatch stress. The gradients in boron concentration and dislocation density can lead to curl in micromachined structures. Although annealing steps can remove the boron gradient, a flat membrane will be a tenuous balance.
Epitaxial films of p++Si:B and strain-compensated p++Si1-xGex:B can remove composition gradients and improve process control. However, it is undesirable to depend on p-type layers doped at levels near the solubility limit to prevent etching. We have therefore developed a unique etch stop created from relaxed SiGe alloys. Etch-stop behavior quite similar to heavily boron-doped silicon has been demonstrated in undoped silicon-germanium. Neither strain nor defects are responsible for the etch-stop behavior. A model is proposed based on energy band structure and a SiOx passivation mechanism.