In-situheatingoftheNSNstack.avi (Restricted Access)
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Thesis (Ph. D.)--University of Rochester. Materials Science Program, 2014
Porous nanocrystalline silicon (pnc-Si) membrane is a new class of materials that is promising for a wide range of applications from biofiltration to cell culture substrate. Nano-size pores are spontaneously formed in a silicon film sandwiched between two silicon dioxide layers during rapid thermal annealing. Previous research has shown that pore formation in the pnc-Si membrane is thermally driven and closely connected to silicon crystallization, however, the process by which pore are formed is still not well understood.
In this thesis, the fabrication and characterization process of pnc-Si membranes is first introduced to understand their basic structure and properties, followed next by a study on the effects silicon dioxide capping films have on pore formation. The results show that both the top and bottom silicon dioxide films are essential to pore formation in pnc-Si membranes. The top oxide layer prevents the silicon film from agglomerating while the bottom oxide layer acts as a barrier layer to prevent homoepitaxy of the silicon film during annealing.
To study the effects of capping materials on pore formation, silicon nitride, is incorporated into the sandwich structure to replace the capping silicon dioxide layers. From this study, it was found for the first time that nanopores can still be formed in silicon films sandwiched between two silicon nitride layers during rapid thermal annealing. Pore formation in these new silicon nitride capped structures is then discussed, along with the resulting pore characteristics.
In the final part of this thesis, ex-situ and in-situ annealing studies of pore formation in pnc-Si membranes are discussed. The ex-situ study shows that both the silicon crystallization and associated pore formation are enhanced in the silicon film in the nitride/silicon/nitride (NSN) stack compared to that in the oxide/silicon/oxide (OSO) stack. The in-situ heating studies demonstrate that pore growth in the NSN stack follows a pearl-necklace pattern while no clear pattern is observed from the OSO stack. The energy-dispersive X-ray spectroscopy (EDS) study shows that Ar atoms embedded during sputtering move together, forming Ar bubbles to occupy the porous areas in the silicon film during the heating process. These Ar bubbles are held in the pores of the silicon film when silicon nitride is used to cap the membrane, but when silicon dioxide is used as the capping layer the Ar diffuses out of the structure. This may lead to the different pore growth patterns between the NSN and OSO stacks. Finally, pore formation process can be understood in two stages which are void nucleation and pore growth. Nano-voids are nucleated in the silicon film near interface with the oxide or nitride layer, and grow in both the vertical and lateral directions during crystallization process. This process is attributed to the diffusion of silicon atoms during the transition from amorphous silicon to nanocrystalline silicon. These voids become through pores when their growth spans the entire silicon film in the vertical direction. The pore growth is strongly affected by the silicon thickness, temperature ramp rate and capping layers.
