Mechanical Stresses During Solid State Amorphization of Zr/Co Multilayers
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deposition the substrate was rotated with 20 rpm to maintain a constant thickness throughout the whole wafer (thickness variations less than 2%). The evaporation rates are 0.7 nm/sec and typical individual layer thicknesses are 33 nm for Zr and 17 nm for Co, giving a mean composition of Zr4 sCo 5 2. The wafers are cut to obtain three pieces of size 55 mm x 15 mm, allowing the measurement of three identical samples at three different temperatures. Annealing and simultaneous stress measurement are performed inside a high vacuum chamber at base pressure 2*10-6 mbar, filled with 10 mbar purified 5N Argon gas. The gas atmosphere establishes thermal equilibrium inside the measuring cell even at high heating rates. The samples are ramped up to the annealing temperature at 20 K/min and then held within +- 0.5 K for the specified time interval. The mechanical stress in the film is monitored during the annealing reaction using a bending beam technique with capacitance sensors. In this measuring system the substrate is clamped vertically on one side and the bending of the substrate is monitored as a capacitance change between the substrate electrode and three fixed electrodes on the holder with guard ring configuration. Here the condensed film itself was used as one electrode, eliminating the influence of an additional electrode on the backside of the substrate. The three capacitance sensors are positioned in line with equal distance (here 15 amm) along the length of the substrate. Thus, we measure three distances which can define a curvature and we eliminate in this way any effects from substrate tilting and distance changes of the spacer. The stress a in the film of thickness df is calculated by: (Y= (ds Es/(3 (1 - vs) 12) *Az/df
(1)
Az denotes the determined substrate displacement due to the curvature change of the substrate at the position of the middle electrode (distance I = 25 mm from the substrate clamping). Inserting values of the substrate thickness ds, elastic modulus E. and its Poisson's ratio vs, we obtain the proportionality S= 15.53 MPa * Az/df. The sensitivity for distance changes of each electrode is about I nm. Nevertheless, the experimental resolution for substrate bending is mainly influenced by oscillations due to the temperature control. In separate measurements this was determined to be in the order of +-10 nm according to changes in temperature within +-0.5 K. This corresponds to +-3 MPa for a 50 nm thick film. In order to compare the influence of background gas pressure on the film reaction we performed in situ measurements in UHV in a different system, which also allows the simultaneous measurement of mechanical stress and electrical resistivity on the same substrate with four probe method ** [9]. In this case the metal films are condensed directly at the reaction temperature, the substrate used here is single The measurement of the double layer resistivity change during the crystalline quartz of thickness 500 intm. reaction allows the determination of the thickness of the amorphous layer [101 ena
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