Formation of N+P Junctions Using In-situ Phosphorus Doped Selective Si1-xGex Alloys for CMOS Technology Nodes Beyond 50nm.

Abstract

As CMOS integrated circuits are scaled beyond the 50nm regime, conventional source/drain junction and contact technologies can no longer satisfy the requirements of MOSFETs, which require super-abrupt doping profiles and extremely low contact resistivities. To address these challenges, selective Si1-xGex source/drain technology was proposed by this laboratory. In this approach, in-situ doped Si1-xGex layers are selectively deposited in recessed source/drain regions. Since the dopants occupy substitutional sites during epitaxial growth, high temperature annealing is not required for dopant activation, which eliminates diffusion and provides abrupt doping profiles. Furthermore, smaller bandgap of Si1-xGex reduces the metal-semiconductor barrier height, an essential requirement for achieving a substantial reduction in contact resistivity.

This thesis focuses on selective rapid thermal chemical vapor deposition of in-situ phosphorus doped Si1-xGex alloys intended for this application. Experiments were carried out to study electrical properties of the in-situ doped layers with emphasis on maximizing the active carrier concentration. Active phosphorus levels in the range of 2 — 5 x 1020 cm-3 were obtained.

The deposited layers were used to fabricate pn junctions with excellent reverse leakage characteristics. Junctions fabricated on lightly doped substrates exhibited behavior equivalent to best junctions in spite of the lattice mismatch between the Si substrate and the phosphorus doped Si1-xGex. Junctions fabricated on heavily doped substrates suffered from band to band tunneling, which is expected regardless of the junction formation technique. Deposition selectivity of the process was studied and determined that high flows of PH3 could degrade the selectivity. An alternative deposition process based on alternating periods of deposition and etching was developed, which provided substantial improvements in deposition selectivity.