Influence of surface coatings on elasticity, residual stresses, and fracture properties of silicon microelements

Abstract

Microscale silicon cantilever beams of (100)〈100〉 and (100)〈110〉 orientations were magnetron sputtered with submicron layers of Al, Ti, or TiN, or thermally coated with SiO2. Theoretical expressions for the elastic deflection induced by residual stresses were derived, and utilized to deduce such stresses from observed deflections. A theory for the elastic stress distribution in coated beams exposed to external bending moments was utilized to deduce maximum stress levels at fracture in the coatings and in the substrates. The fracture tests were performed in situ in a scanning electron microscope by means of specially designed equipment. For uncoated beams, the average fracture stress was 6 GPa (maximum 13 GPa) for 〈100〉 beams, and 4 GPa (maximum 6 GPa) for 〈110〉 beams. Most coatings proved to have a strength-reducing effect, particularly the brittle, thin coatings of TiN, but also the Ti coatings (which displayed brittle fracture behavior). Ductile, thin coatings of Al were either neutral, or induced a small reinforcement. The influence of the thermal oxide was ambiguous: either a small strengthening or a small weakening effect. Five different types of fracture in coated silicon beams are discussed, and ‘‘reinforcement factors’’ associated with these fracture types are expressed in terms of fracture toughnesses. By comparing the experimental results with these expressions, it was possible to identify the probable types of fracture, and to explain why different strengthening or weakening effects occurred for different coatings.