Dopant Segregation at Silicon-oxide Interfaces
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2007-05-26
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With the fast scaling of MOSFET devices, interfaces between silicon and dielectric layers are becoming increasingly important. However, a physical understanding of dopant segregation at such interfaces using atomic resolution remains elusive in spite of intensive study. In this dissertation, As and Sb are selected as dopants to achieve different levels of segregation in equilibrium conditions. This study utilizes a combination of theoretical and experimental concepts. Due to the fact that each experimental method has its own artifacts, we use a combination of three different methods (SIMS, GI-XRF and Z-contrast imaging⁄EELS) to allow accurate determination of position and concentration of dopants. Additionally, ab initio calculations provide appropriate structure model by calculating the energy of different preferred segregation sites.
After implanting As (10¹⁵ and 10¹⁶ cm⁻²) into Czochralski Si (100) wafer at 32keV, a SiO₂ layer is thermally grown. Then Si⁄SiO₂ samples are annealed at 900°C for 360min in N₂, with a final SiO₂ thin film less than 15nm measured by ellipsometry. Combining the above three experimental methods, the segregation of As to the Si⁄SiO₂ interface is observed. The As concentration profiles of both samples are analyzed close to the interface region by EELS, and compared with those measured by GI-XRF and SIMS. A maximum of 4˜5x10²¹ cm⁻³ arsenic (10¹⁶ cm⁻²) and 1.2x10²¹ cm⁻³ arsenic (10¹⁵ cm⁻²) are observed at the last monolayer of Si. The total dose loss at the interface of the 10¹l⁶ cm⁻² As doped sample is 8˜9%. With the incorporation of ab initio calculations, a physical explanation of the segregation mechanism is given based on both theoretical and experimental results.
Using Z-contrast imaging, Sb segregation at Si-SiO₂ interface is also observed on Sb doped Si⁄SiO₂ samples. Unlike the As doped samples, pentagon-shaped Sb precipitates are detected 8nm from interface on the Si side.
For the As doped Si⁄Hf[subscript x]Si[subscript 1-x]O samples, an unexpected silicate interfacial layer is observed between hafnium oxide thin film and silicon substrate. Therefore, As segregation at the novel interface turns out to be exactly same as As at Si⁄SiO₂ interfaces.
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Dopand segregation
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Degree
PhD
Discipline
Materials Science and Engineering
