Generally, a substrate and thin film over it have different thermal expansion coefficients, so stress is produced during temperature changes occurring in deposition and annealing. In microelectronic devices, the stress can be a major cause for yield and reliability problems. Even it can alter the actual device properties (for optoelectronic devices). A piece of solid is said to be under stressed while displaced from their equilibrium positions by a force. The displacement is governed by the inter-atomic potentials.