Mechanical stress

• excessive stress causes cracks, bending, loss of adhesion, electrical failure, stress in network, forces on atoms, bond stretching
• thermal stress: caused by the difference in thermal expansion when layers are deposited at higher temperature,
  • when a lyer is deposited at high temperature as in the animation below, during cooling the layer and substrate will both contract
  • because the two materials have different values for the thermal expansion coefficient (α , β), they will have different lengths at the end of the cooling process (l₁ = l₂, l₀₁ > l₀₂ if α < β or l₀₁ < l₀₂ if α > β)
  • mecahnical stress will appear due to the fact that the two layers will exert forces upon each other and the layers will curb in order to be able to stick to one another at the new lengths

• In case of growing silicon oxide on top of silicon: silicon oxide is compressive, thus it stretches the silicon
• In case of growing silicon nitride on top of silicon: silicon nitride is tensile, thus it compresses the silicon (see picture below)
• the thermal stress can be expressed as: ; the thermal coefficients (α_film, α_sub) depend on the temperature
• effects of thermal stress can be seen in the next picture: hillock, cracks:

• intrinsic stress - due to lattice mismatch, incorporation of atoms, vacancies; annealing reduces the intrinsic stress

Other parameters for choosing the suitable method :

• capacity to deliver the desired material
• safety
• costs
• effectiveness
• damage
• thermal budget