Single-crystal silicon films on glass
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We present a new process based on the electrolysis of glass, which allows the transfer of a single-crystal silicon film while creating an in situ barrier layer free of mobile ions in the glass. This barrier layer consists only of network-forming elements (i.e., aluminum, silicon, and boron) and is free of modifiers. The barrier layer glass is unusual and cannot be synthesized via any of the known glass-forming processes. The barrier layer is thermally stable and thus allows the fabrication of displays with ultimate performance. The process consists of the hydrogen ion implantation of silicon to create a defect structure followed by bringing the glass and the silicon wafer in contact, and finally applying electrical potential to cause the electrolysis of glass.
As displays such as those in cellphones, laptop computers, and flat-panel televisions, become more ubiquitous, there is a need for even better performance. The performance of the displays is determined by the quality of the silicon film that is deposited on glass. To improve performance, amorphous silicon is being replaced with polycrystalline silicon film or polysilicon, which is formed by laser crystallization of the former to grow micron-sized grains. The polysilicon films are limited by the nonuniformity and low electron mobility that is associated with grain boundaries. An ultimate goal for this technology is to obtain a single-crystal silicon film on glass that would offer the best attributes. Attempts to develop such a film based on the hydrogen ion implantation of silicon followed by thermal bonding to glass present problems such as the contamination of the silicon by the elements in the glass via diffusion at the high process temperatures encountered during the fabrication of thin-film transistors (TFTs).1–4 One of the glasses of interest for display application is a boro-aluminosilicate glass containing MgO, CaO, SrO, and trace alkali impurities (Na ∼ 200 parts per million). Network formers such as B, Al, and Si are firmly bound in the glass network; therefore, the diffusion of these elements out of glass is not a concern. However, alkaline earth components such as Ca, Mg, and Sr and alkaline impurities are network modifiers and may diffuse into silicon during high-temperature processing. This problem necessitates the deposition of a barrier film on glass to prevent the diffusion of mobile species. Such a depos-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0330 J. Mater. Res., Vol. 22, No. 9, Sep 2007
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ited film, typically silicon–oxynitride deposited via plasma-enhanced chemical vapor deposition, inherently contains many defects that act as charge-trapping centers and makes the fabrication of high-performance devices in silicon difficult. In addition, it is very difficult to transfer silicon films via thermal processes on large-area display glass substrates with long-range waviness. Alternatively, the alkaline earth components and trace
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