Nucleation and Growth at Reactive Interfaces Followed by Impedance Measurements
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** INTEL, Components Research, Santa Clara ABSTRACT To evaluate the thickness of different constitutive layers of a TiAI multilayer, two different experiments, based on impedance measurements on multilayers, have been performed. In the first one, the thickness of the layer is deduced from resistivity measurements, applied to a parallel resistance model. In the second one, the thickness of the layer is deduced from comparing of calculated and measured values of surface potentials. INTRODUCTION To study the process of interfacial reactions in a multilayer, one of our major concern is to follow the thickness evolution in multilayered samples during the reaction. Usually, thickness determinations of thin films, intended for microelectronics devices are performed by different experimental techniques. For example, for metallic layers, the square resistance method can be used [1]. This method provides fast information about the thickness of a monolayer. However the model used for the thickness interpretation take into account a single experimental measurement, therefore additional hypothesis are necessary to measure thickness of multilayer samples (number of equations should be at least equal to number of unknown). Furthermore, because of symmetry of the measurement device, classical square resistance method is not sufficiently sensitive to give precise information about the earlier stage of a new phase growth. In the first part, we present an attempt to improve the determination of the thickness or conductivity in multilayers with and without interfacial reaction, from electrical potential determinations and numerical fitting. This method allows accurate thickness determination in one step even in the case of multilayer sample, it has been called ( Potential Method o (PM). In the second part we compare the results obtained by the potential method to the results obtained by the classical square resistance measurements method (SRA). The potential method gives the thickness of the different layers by taking into account the electrical perturbations induced by the other layers of a sample. The method includes two main steps which are : * The numerical calculation of the electrical potential on the surface of the sample (direct method).
e An iterative procedure which compares the measured values, to the calculated ones, through an optimization algorithm (inverse method), which tends to produce a unique solution for a given structure. Schematically the procedure is shown in Figure 1. We may recall that a similar procedure is used in Geophysics to determine the conductivity and the thickness of underground layers. This method is called the electrical drilling [2], [3].
Optimization Optimiationof
V
algorithm
Potential
calculations Analytical potentials
m(inverse method) Figure 1 : Schematic representation of the procedure followed to determine different parameters of a multilayer, by applying the potential method. POTENTIAL METHOD THICKNESS MEASUREMENT Physical background This approach considers a n-stratified media, with
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