Engineering oxygen precipitation for simultaneous denuded zone and bulk microdefect formation in pre-annealed heavily boron-doped Czochralski silicon substrates for p/p+ epitaxial wafers

Abstract

Heavily boron-doped Czochralski (HB-Cz) silicon substrates (10–20 mΩ·cm) for p/p⁺ epitaxial wafers face difficulty in simultaneously forming an oxide-precipitate-free denuded zone (DZ) and a high-density bulk microdefect (BMD) zone during device thermal processing, due to oxygen precipitation enhanced by the heavy boron-doping. The as-received HB-Cz silicon substrates, whether from the seed-end or the tail-end of the silicon crystal ingot, fail to simultaneously form both a DZ and a high-density BMD zone after the simulated device thermal processing (780 °C/3 h + 1000 °C/16 h, L-H annealing) of the corresponding epitaxial wafers. This work investigates the pre-annealing processes involving rapid thermal annealing (RTA), conventional furnace annealing (CFA), and their combinations for HB-Cz silicon substrates to address the aforementioned technical dilemma. As a result, we establish a dedicated pre-annealing process: the non-interchangeable, sequential application of RTA (1250 °C/30 s, cooling rate: 25–100 °C/s) and CFA (1200 °C/3 h, ramping-up from 700 to 1200 °C at 5 °C/min). The RTA step dissolves the grown-in oxide precipitates and introduces the vacancy-oxygen (VO_m, m ≤ 4) complexes into the bulk region of substrate, serving as the heterogeneous nucleation precursors for oxygen precipitation during the subsequent CFA step, while occurring vacancy out-diffusion near the surface of substrate to facilitate DZ formation. The subsequent CFA step enables the oxide precipitate nucleation and growth based on the RTA-induced VO_m complexes, while simultaneously promoting the oxygen out-diffusion to secure the DZ width. The optimized RTA + CFA pre-annealing process, ensures both a well-defined DZ (>40 μm) and high-density BMDs in HB-Cz silicon substrates from any crystal ingot position after the L-H annealing of corresponding p/p⁺ epitaxial wafers. Therefore, this RTA + CFA pre-annealing strategy provides a robust and universal technical foundation for manufacturing high-performance p/p⁺ epitaxial wafers essential for advanced power devices and integrated circuits.