Study of Stress in Microelectronic Materials by Photoelasticity
The study of stress is playing an important role in microelectronic technology. In comparison with other techniques, the photoelasticity technique has the advantages of having high spatial resolution and high sensitivity. It can be used in qualitative observation in real-time and quantitative determination of stress distribution in the microelectronic materials and devices. This dissertation presents a systematic study of photoelastic stress analysis in microelectronic materials and devices, ranged from theoretical study to practical system setup, from measurement methods to their applications. At first, based on the detailed survey on the piezo-optic properties of the crystals used in microelectronics, we apply the stress-optic law of engineering mechanics to study the stress in crystals, such as silicon, gallium arsenide, and diamond. We, for the first time, derive the relationship between the stress ellipsoid and the refractive index ellipsoid, and, derive the matrix forms of piezo -optic coefficient tensor for several commonly used coordinates. These theoretical results have laid a firm ground for the photoelastic stress analysis of microelectronic materials and devices. Second, based on exclusive experiments, we develop several effective methods of photoelastic analysis to determine the stress state in the samples. Some of them are successfully borrowed from the classic photoelastic mechanics, such as the Semarmont compensation which is used to determine the decimal fringe of the isochromatic line, and the shearing stress difference method which is used to separate two principal stresses and obtain the stress distribution of a sample. We also provide our origination to the photoelastic techniques, such as the three-direction observation method which is used to abstract the principal stresses from the secondary principal stresses; the Fourier analysis method and the intensity analysis method, both of which are especially suitable for precisely and automatically determining the principal stress distribution in a large area. Combining computer and digital image processing techniques with photoelastic techniques, we set up a photoelastic measurement system, which has the capability of qualitative observation of the photoelastic patterns and quantitative measurement of the stress distribution. We apply the photoelasticity principles and methods to investigate the stress state in microelectronic materials, including developing a series of feasible methods to measure the stress distribution of the microelectronic materials and devices, investigating the mechanisms of stress induction and stress change. For example, we investigate the stress distribution of a synthetic diamond substrate, study the stress induced during the typical impurity diffusion processes for manufacturing power diodes, and analyze the stress induced in the thin film/substrate structures. We also develop some models to explain the measurement results and provide theoretical discussions of the results.
- Pub Date:
- GALLIUM ARSENIDE;
- Physics: Condensed Matter; Engineering: Electronics and Electrical; Engineering: Materials Science