Dopant Introduction
Semiconductor doping processes introduce specific impurities into a silicon substrate to modify its electrical behavior. When creating p-type regions in a silicon substrate, boron ion implantation is the primary method to inject boron atoms at controlled depths and concentrations. The process is constrained by the acceleration voltage and the ion dose limitations.
Profile Distribution
The spatial distribution of the dopants determines the junction depth and the resulting concentration profile. In boron ion implantation, the light mass of boron ions causes them to penetrate relatively deep into the silicon lattice, often resulting in a broad distribution known as a Pearson profile. Channeling effects can occur if the ion beam aligns perfectly with the crystal planes, which requires tilting the wafer to prevent unwanted deep penetration.
Annealing Phase
Post-implantation thermal processing is required to repair the crystal damage caused by the high-energy ion bombardment and to activate the dopant atoms. During this step, the implanted boron ions move from interstitial positions to substitutional positions in the crystal lattice. This phase must be carefully controlled to prevent excessive thermal diffusion, which would broaden the transition regions and degrade the resolution of the sensor junctions.
Calibration Control
Secondary ion mass spectrometry is used to verify the depth distribution after the doping process. This measurement confirms that the active dose matches the target specification.
