PureB diode fabrication using physical or chemical vapor deposition methods for increased back-end-of-line accessibility

Several methods of depositing pure boron (PureB) layers on silicon are examined with respect to their potential for fabricating advanced PureB (photo)diodes with back-end-of-line (BEOL) CMOS compatibility. PureB devices were fabricated in two different batch furnace chemical-vapor deposition (CVD) systems or by electron-beam-assisted physical-vapor deposition (EBPVD), and their electrical characteristics were found to be comparable to those of devices previously fabricated using single-wafer CVD and molecular beam epitaxy (MBE) systems. For all methods, the material properties of the B-layers and the I-V characteristics of the PureB diodes follow the same temperature dependence over the range 50 °C–400 °C. This was also the case for the EBPVD layers which were deposited at 50 °C and then annealed at higher temperatures, instead of being deposited at these temperatures as for the other methods. At 400 °C, the ability to achieve an optimal suppression of the electron injection into the PureB anode regions, corresponding to an electron current density of ∼20 pA/cm², was verified for all methods. The advantages and disadvantages of each deposition method is evaluated with respect to equipment availability, B-layer selectivity, conformality, and thickness control. The batch furnace systems could be attractive for high-volume production, but hardware improvements as discussed here would be needed to reduce the effects of gas depletion. On all points except conformality, EBPVD appears to be a very good option for fabricating nm-thin B-layers suitable for fabricating high-performance 400 °C PureB diodes.