Ion Implantation Profile Optimization in Piezoresistive Diaphragms
Implantation energy and post-anneal thermal budgets set piezoresistive sensitivity and offset stability in silicon pressure sensor diaphragms.
Ion Channeling
Implant Path
Subatomic trajectories through a crystalline lattice depend on the alignment of the incoming particle beam with the open pathways between rows of atoms. In semiconductor doping, ion channeling occurs when dopant ions penetrate much deeper into the silicon crystal than predicted by amorphous material models. This effect arises when ions travel along the open channels of the crystal lattice without suffering major collisions.
Junction Alteration
Deep penetration of dopants modifies the electrical profile of the resulting transistor or sensor structure. When ion channeling remains uncontrolled, the deep concentration profile causes the activation region to shift away from the surface. This alters the threshold voltage and increases leakage current.
Prevention Technique
Process engineers prevent this unwanted penetration by tilting the wafer during implantation. A typical tilt angle of seven degrees randomized with a twist angle misaligns the lattice channels from the ion beam.
Metrology Method
Secondary ion mass spectrometry is used to measure the depth profile of the implanted species. By analyzing the concentration of dopants at various depths, engineers can verify if ion channeling has occurred during the implantation step. If the profile shows a long tail of deep dopants, the implanter alignment must be adjusted to ensure correct device performance.