Elastic Constants and Viscosity of Amorphous PdSi/PdSiFe Multilayers
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ELASTIC CONSTANTS AND VISCOSITY OF AMORPHOUS PdSi/PdSiFe MULTILAYERS ANN WITVROUW, PAMELA CAMPOS AND FRANS SPAEPEN Division of Applied Sciences, Harvard University, Cambridge, MA 02138.
ABSTRACT The biaxial elastic constants and viscosity between 100 and 300 'C of amorphous PdSi/PdSiFe multilayered thin films with repeat lengths between 2.0 and 4.9 nm have been determined using substrate curvature measurements. No dependence on repeat length has been seen for either property. Linear increases of the multilayer viscosities with time are observed. An analysis of the viscous flow of multilayers shows that in the present case this regime is reached after a very short transient period.
INTRODUCTION Multilayered thin films exhibit a variety of interesting properties (optical [1], mechanical [2], diffusional [3]...), which are often a function of the repeat length. The effect of the repeat length on the viscosity of amorphous multilayers has never been studied, as it is difficult to make a freestanding multilayered film thick enough for the conventional tensile creep experiments. However, recently we have shown that it is possible to measure viscous flow in thin films from stress relaxation by monitoring changes in substrate curvature at elevated temperatures using a laser scanning technique [4,5]. This method is applied here on PdSi/PdSiFe multilayered thin films with repeat lengths between 2.0 and 4.9 nm for temperatures ranging from 100 to 300 'C. To obtain absolute values of the viscosity the biaxial elastic constant EJ(l-vm) of the multilayered thin film must be known. These have been determined for the different repeat lengths by measuring stress versus temperature for multilayers on two different substrates.
EXPERIMENTAL DETAILS Sample Preparation Amorphous PdSi/PdSiFe multilayered thin films, I to 2jim thick, with repeat lengths between 2.0 and 4.9 nm were ion beam sputtered onto 1"xl/4" fused quartz (510 gim) and Si(100) (370 jtm) substrates from a PdSi and an elemental Fe target on a rotating target block [6]. Exact thickness and variation in thickness of the substrates was determined by a 1000-point thickness measurement across each 3" wafer by the supplier (ZITI-corporation) for the Si substrates and by manual micrometer measurements for the fused quartz substrates. The repeat lengths of the multilayers were determined from the low angle X-ray diffraction peaks [7], with the number of harmonics ranging from two for the lowest to five for the highest repeat length. Multiplying the repeat length with the number of bilayers gives an accurate measurement of the thickness of the film. The average composition of the films was determined by electron microprobe: 83 at% Pd~nSi 23 and 17 at% Fe. The films were amorphous as checked by isothermal DSC measurements [8] on as-sputtered films and high angle X-ray measurements on the films before and after all the anneals. The compressive stress in the as-sputtered multilayer is about 40 MPa.
Mat. Res. Soc. Symp. Proc. Vol. 239. (1992 Materials Research Society
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