Measurements of stress during vapor deposition of copper and silver thin films and multilayers

Abstract

Copper and silver single layer and multilayered thin films were thermal vapor deposited onto cantilevered substrates [Si(100) with native oxide] near room temperature in ultrahigh vacuum. The total force per unit width (F/w) in the film during and after deposition was determined from the change in substrate curvature measured in situ by a laser scanning technique. The intrinsic component of F/w was obtained by subtraction of the thermal component, which was obtained by measuring the product of the biaxial modulus of the film (Yf) and the difference in coefficients of thermal expansion of the substrate and the film (Δαs−f) while each sample was still in the ultra-high vacuum deposition chamber. For all samples, the measured value of YfΔαs−f was substantially lower than the calculated value based on the {111} biaxial modulus and the coefficients of thermal expansion of the bulk materials, even though x-ray diffraction indicated strong {111} film texture. During deposition, a general trend in F/w was found regardless of the initial surface material as long as a heterogeneous interface was formed: following an initial compressive transient, F/w goes through a broad tensile maximum, then decreases and eventually becomes net compressive. This work conclusively demonstrated the presence of a compressive component of F/w. The evolution of the general features is unaffected by stopping and then resuming deposition of the same material. When a Ag/Cu multilayer is deposited continuously or intermittently (i.e., with periods of time without deposition between deposition of each layer), the evolution of the features of F/w is different during deposition of Cu layers from that during deposition of Ag layers, but is the same for each Cu layer and each Ag layer. After deposition ends, F/w changes with time in the tensile direction regardless of whether the final F/w during deposition is tensile or compressive. This change is reversed upon further deposition of the same material and therefore is a surface change from a dynamic to a static configuration (not a bulk change such as grain growth). The magnitude of the change in F/w after deposition increases linearly with the thickness of the top layer (the layer that was most recently deposited). The (111) Ag/Cu interface stress was determined for the first time, to the best of our knowledge, based on measured discontinuities in F/w associated with interface formation during deposition of multilayers. These discontinuities were different for Cu onto Ag and Ag onto Cu, and were repeatably reproduced for each interface during continuous deposition of multilayered films. A value of −0.21±0.10 N/m was found for the (111) Ag/Cu interface stress. The sign (compressive) is significant; it is the same as that of earlier experimental determinations, but opposite to that of theoretical calculations. The absolute value of the interface stress is low due to the incomplete formation of each interface at the early stage of layer formation.