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.
Longitudinal Straggle
Ion Dispersion
Ion beam implantation generates a statistical spread of implanted species along the direction of the incoming beam. Longitudinal straggle represents the standard deviation of the depth distribution of these implanted ions. This value determines the abruptness of the resulting electrical junction in semiconductor devices.
Physical Modelling
Collisions between incoming ions and target atoms lead to energy loss and statistical scattering. Accurate predictions of longitudinal straggle utilize stopping and range of ions in matter calculations to simulate millions of ion trajectories. Heavier ions like arsenic exhibit a narrower depth spread than lighter dopants such as boron.
These simulations are verified through actual depth profiles to ensure the accuracy of the process models.
Metrological Evaluation
Secondary ion mass spectrometry profiles are obtained to evaluate the dopant distribution in a test wafer. By measuring the slope of the concentration profile, the longitudinal straggle can be extracted and compared with the simulated values. This verification step confirms the consistency of the ion implanter acceleration grid.
Deviations from the expected straggle indicate issues with energy purity or wafer tilt during the implantation step.
Processing Challenge
High-temperature annealing steps cause dopant diffusion that can blur the initial distribution. Thermal budgets are kept low to prevent the broadening of the profiles.