TEM Investigation of Silicon Devices
As the layers of materials in a silicon device decrease in thickness in order to improve the performance of the device, analytical techniques with nanometre resolution are required to characterise the materials. The aim of this project was to evaluate and demonstrate the TEM for use in materials characterisation in device technology. In doing so, new TEM techniques have been devised and existing techniques have been developed to describe the fine structure of a material and what happens at the interfaces between different materials.
To understand the areas of device technology which require materials characterisation the relevant principles behind the operation of a device have been explained. The performance of existing analytical techniques which are used in the industry have been described for future comparison to the TEM techniques used in this work. The theory behind these TEM techniques was then explained and the properties of the electron beam were investigated both theoretically and experimentally. The beam properties included the probe diameter, current and current density, as these often define the limits of useful microanalysis.
The capability of TEM imaging as a method of investigating the coarse and fine structure of device components was demonstrated. The diffraction contrast encountered in TEM imaging provides information on the atomic structure of materials. Using TEM imaging and diffraction, a new technique was devised to determine the grain size distribution of a conducting film involving the application of Voronoi statistics and other models. This technique enables the grain size distribution of a film in a fully-processed device to be determined at any level through the film thickness. A description of the principal mechanisms for normal and abnormal grain growth was provided to explain the experimental results. A critical evaluation of the existing statistical models which mimic grain growth was given to determine their accuracy. The grain size distribution was calculated for two Al films with columnar grains and the Voronoi model was shown to be applicable to the analysis.
Two diffraction techniques were developed to enable a quantitative description of the distribution of orientations, or texture, in a film. The first uses Kikuchi diffraction patterns for the identification of individual grain orientations, and the other uses a new scanning technique to measure the level of texturing in a film. The analyses used for both diffraction techniques are original and a definition of the ‘degree of texture’ has been proposed. The scanning technique can be modified to evaluate the degree of texture at a particular depth through the thickness of a film thereby allowing a description of the development of the texture. The texture development was determined for a polycrystalline Si film and scanning dark field imaging was used to verify the results and provide a visual description of the grain growth in a textured film.
The limits and capabilities of TEM microanalysis have been explored using Si devices and GaAs samples. The techniques involved were EDX and PEELS. EDX was investigated for its spatial resolution and its sensitivity to traces of dopants both theoretically and experimentally. In its various forms, EDX was then was used to identify a source of unknown material which appeared to be a result of a flaw in the fabrication of the device. Compositional profiles of Al, Si and Ti across a Ti/TiN diffusion barrier were used to determine the effectiveness of the barrier in separating the Al and Si to either side of it. The possible roles of PEELS in device technology were also discussed. It was then used to determine the N composition through the same Ti/TiN barrier to complement the earlier EDX results . The results from both PEELS and EDX analysis demonstrated the high level of quantitative information available in identifying the composition of a material on a nanometre scale.