The nature of stress in nanoscale refractory metal sputtered films

Residual stress has been shown to affect the mechanical, optical, and electronic properties of thin layers. The intent of this research was to investigate very thin, or "nanoscale", Mo, Ta, and W films in order to study the residual stress behavior within the first few nm of growth. The films ranged in thickness from 2.5 nm to 500 nm, and were examined using a variety of advanced characterization techniques to obtain data on the residual stress, impurity level, crystal structure, and microstructure as a function of layer thickness.; The 2.5 nm thick Mo and 2.5 to 10 nm thick Ta films showed very high compressive stresses of {dollar}{lcub}sim{rcub}{lcub}-{rcub}{dollar}3 GPa. The stress for both systems relaxed substantially with increasing thickness. The high compressive stresses were caused by incorporation of O and C impurities from the substrates into the first few nm of the growing films. The relaxation in stress was caused by changes in the average impurity concentration as a function of thickness, increases in grain size, and, for Ta, a phase transformation. The W films exhibited a relatively uniform tensile stress for all thicknesses, but the thinner films became more compressive with time due to incorporation of impurities from the atmosphere.; The residual stress of these nanoscale films was different in both sign and magnitude from the stress of "thick" films; thus, the stress of thin layer could not be extrapolated from available data on thicker coatings. Furthermore, the stress behavior of the three metals studied was very dissimilar, in spite of the fact that the metals have comparable bulk properties. This was probably due to the rapid changes in impurity content, microstructure, and grain size during the initial stages of film growth. Clearly, a careful study of residual stress is prerequisite to using a nanoscale film in order to prevent undesirable properties. These large stresses offer the potential of creating high quality structures by deliberately varying the stress in different layers to obtain an average stress of zero.