Application of Stress Measurement to the Study of Thermally Activated Processes in Thin-Film Materials
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APPLICATION OF STRESS MEASUREMENT TO THE STUDY OF THERMALLY ACTIVATED PROCESSES IN THIN-FILM MATERIALS F. J. VON PREISSIG AND W. D. NIX Department of Materials Science and Engineering, Stanford University, Stanford, CA 94309 ABSTRACT Changes in mechanical stress occurring in unpatterned thin films on substrates at elevated temperatures can yield information about the kinetics of the controlling processes. In this study, a specialized furnace allowed stress at elevated temperatures to be measured by the optical-lever method. The furnace was designed for good temperature control, short thermal transient periods, accurate stress measurement with low scatter, and control of the gas ambient. Stress changes in phosphosilicate glass films measured during exposure to steam at 110- 150*C allowed water diffusion coefficients to be calculated. Diffusivity decreased with increasing phosphorus content and with pre-annealing treatments. The effective activation energy for water sorption was about 0.43 eV. Analysis of stress changes associated with crystallization of amorphous silicon at temperatures near 600*C yielded an effective activation energy of 3.1 eV and a density change of about 1% for the transformation. INTRODUCTION The biaxial stress in a film on a substrate is the sum of intrinsic stress, thermal-expansion stress, and all other changes in stress that have occurred since the film was formed. Most processes that cause stress to change at constant temperature exert their effect via plastic deformation or volume change. Plastic deformation is a response to the stress itself and results in a decreasing absolute value of stress (stress relaxation). Some responsible mechanisms are dislocation formation and motion,1 ,2 bulk and grain boundary diffusion of atoms, 2 and viscous flow. 3 Volume change is constrained laterally by the substrate, resulting in a proportional change in stress. It can5 be caused by the filling in of micro-voids with surrounding material,'4 phase transformations, the incorporation of additional matter, 6.7 and the evolution of volatile 8 species. A popular way to measure biaxial stress in unpatterned films is by the optical-lever method. 9 -11 In this method, the component of curvature that the film stress induces in the substrate is measured by means of a laser beam that reflects off of the substrate. The average stress through the film thickness is then calculated. 12 Stress measurements can be used to probe the kinetics and other aspects of material processes that effect stress. In this study, an optical-lever system coupled with a specialized furnace was used to measure stress changes occurring over time at elevated temperatures in controlled environments. These measurements were applied to the investigation of two thermally activated phenomena that cause changes in film stepper volume: sorption of water in phosphosilicate glass and crystallization of -------amorphous silicon.
APPARATUS
sensor
Optical-Lever System The optical-lever stress-measurement system used in this study is illustrated sch
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