Two-step rapid thermal diffusion of phosphorus and boron into silicon using solid diffusion sources have been performed. In the first step, the glass of $HBO_2$ or $P_2O_5$ is transferred from the solid diffusion sources to the processing wafer and in the second step, the predeposition of the dopants into the processing wafer from the glass is performed. By separating the glass transfer and the predeposition in the two-step rapid thermal diffusion, extremely shallow junctions with junction depths of about 25nm as well as deep junctions with junction depths of about 530nm are achieved.
By using a pyrometer to read the temperatures of a wafer in the rapid thermal diffusion system in which tungsten-halogen lamps are placed at one side of the wafer, the reproducibility of the sheet resistance is enhanced to be within ±2%. Using a patterned heating wafer during the glass transfer and using a 4 inch silicon ring during the predeposition, the uniformity of the sheet resistance of about ±10% in a 3 inch wafer except outer 5 mm rim has been achieved.
In the phosphorus diffusion, the diffusion characteristics during the glass transfer process are analyzed. The electrically active doping concentration at the surface after the predeposition at 1100℃ for 2 sec is about $3\times10^{20}cm^{-3}$. By comparing the electrically active doping profile measured by ASR and the chemical doping profile measured by SIMS, it is found that most of the phosphorus atoms contribute to the electrical conduction uniformly. Diodes with a junction depth of about 150nm have been fabricated. The areal and edge leakage current densities at 5V reverse bias are 1.06 nA/㎠ and 90.2 pA/cm, respectively, The ideality factor is about 1.04.
In the boron diffusion, ambient gas during the predeposition affects sheet resistances and junction depths. The predeposition at 1100℃ in $O_2$ ambient makes sheet resistances lower and junction depths deeper when compared with the predeposition in $N_2$ ambient. It is observed that the electrically active doping concentration at the silicon surface measured by ASR is increased as the ambient changes from $N_2$ to $O_2$ during the predeposition at 1100℃. By comparing the electrically active boron profile measured by ASR with the chemical doping profile measured by SIMS, it is found that a boron rich layer is formed at the silicon surface during the predeposition in $N_2$ ambient. Thus, the electrical conduction becomes less than a diffused layer where the surface concentration is determined by the solid solubility. It is also found from the ESCA results that the boron rich layer formed at the silicon surface is composed of $SiB_6$ compound plus some oxidized silicon.
AES has also been used to examine the phenomena occurring in the glass layer and silicon during the predeposition of boron at 1000℃ in $N_2$ to $O_2$ ambients. The boron rich layer is also formed at the silicon surface for the predeposition in $N_2$ ambient. However, when the predeposition is performed in $O_2$ ambient, the boron rich layer is not formed. It is thought that the oxide grown at the silicon surface by $H_2O$ which is formed in $O_2$ ambient prevents the formation of the boron rich layer.
Also, the predeposition of boron is performed at 900℃ in $N_2+O_2$ ambient to achieve low doping concentrations at the surface by removing part of the boron rich layer by oxidation. The sheet resistance of about 10 KΩ/sq with the uniformity of about 15% is achieved.
