Dopant-induced stress in microfabricated silicon devices

The modification of material characteristics by introducing dopant atoms into a crystal lattice is a fundamental basis for modern micro- and nanosystems technology. In this work, the uneven distribution of dopants is shown to have a remarkable effect on the residual stress and the consequent deformation of released, mechanical silicon structures. In particular, the focus is on segregation of initial dopants inside the bulk silicon which takes place in such fabrication processes as thermal oxidation. A theoretical model based on perceiving the dopant-induced change in Si crystal lattice parameter is developed. We experimentally investigate a series of silicon-on-insulator wafers, including samples with dopant types B, P, and Sb, and concentrations in the range from 10¹⁵ to 5 × 10¹⁹ atoms cm ^-3. Released cantilevers are fabricated as test structures and the residual stress is determined by measuring their final curvature. Experimental results are compared with the modelled values obtained utilizing the dopant profiles determined by secondary ion mass spectrometry and concentration distribution simulations. The use of lightly doped substrates or the selection of processes not modifying the underlying Si surface (e.g., plasma enhanced chemical vapour deposition PECVD or metal deposition) is shown to be an effective solution for minimizing the dopant redistribution-induced stress. Besides the scientific impact, knowledge of the stress generated by dopants is of great significance for industrial manufacturing of a wide range of micro- and nanomechanical systems.