Positional control of crystal grains in silicon thin film utilizing cage shaped protein

Abstract

We have proposed a new crystallization method for silicon thin films utilizing a cage-shaped protein (ferritin), called ‘‘bio-nano crystallization’’, which combines semiconductor processing technology and biotechnology. We utilized nickel nanoparticle-accommodated ferritins as metal catalysts, and succeeded in performing the crystallization. When the ferritin was adsorbed randomly onto the film, crystal nuclei were formed at random places, thus grain position was randomly distributed. In this study, we performed the positional controlled deposition of ferritin by electrostatic interaction for location control of crystal grains. Positively charged areas were formed on negatively charged SiO2 using 3-aminopropyltriethoxysilane (APTES) as the electrostatic pattern. As a result, we could optimize Ni ferritin concentration to make a large adsorption difference between APTES and amorphous silicon. Therefore, nickel nanoparticles adsorption areas were controlled using APTES patterns. Furthermore, the location control of crystallized areas was achieved by optimizing the concentration of Ni ferritin and the APTES pattern. # 2011 The Japan Society of Applied Physics Recently, polycrystalline silicon (poly-Si) thin film transistors (TFTs) are widely used as driving circuits for various kinds of displays. Further technical innovations on display devices such as realization of higher resolution, higher quality, and downsizing are expected for the realization of system on panel. The device performance depends on the quality of poly-Si thin films; thus, improvement of grain size and quality is important. Therefore, crystallization methods have been studied. Metal-induced lateral crystallization (MILC) 1) is one of the crystallization methods utilizing metal catalysts for poly-Si fabrication, where lateral crystallization of amorphous silicon (a-Si) is advanced after crystalline nucleus formation by annealing of a catalyst. As an improved MILC process, we have proposed and demonstrated the utilization of a cage-shaped supramolecular protein. In nature, ferritin accommodates ferrihydrite (5Fe2O3� 9H2O) nanoparticles in its cavity. In addition, it is well known that different kinds of inorganic nanodots, such as iron oxide, cobalt oxide, and compound semiconductors, can be formed in its cavity by biomineralization. 2–5) Our novel MILC utilizes nickel oxide-accommodated ferritins (Ni-Fers) as metal catalysts for the formation of crystalline nuclei. We named this method as ‘‘bio-nano crystallization (BNC)’’. Ni deposition in the BNC process merely requires a very simple solution process such as just applying a drop of Ni ferritin solution. It does not require complicated and expensive process equipment such as high-vacuum systems. Ni nanoparticle accommodated ferritins are adsorbed on the substrate by the self-assembly capability of ferritin. This process achieves very low-concentration Ni catalyst thinfilm fabrication; in this film, the concentration of Ni is very low but sufficient to work as a catalyst for MILC; as a result, we can drastically decrease metal impurity concentration. 6) In addition to these advantages, BNC is highly compatible with conventional patterning process. It will enable the formation of micrometer small patterns by combination of the lift-off method and BNC for position selective crystallization. We previously demonstrated the fabrication of poly-Si thin films possessing a several tens of m crystal grain size by BNC with normal thermal annealing 6) and pulsed rapid thermal annealing. 7,8) However, Ni-Fers were adsorbed at random places on the a-Si surface; thus, crystal grains were also formed in random positions. In this study, we combine BNC and the selective adsorption of ferritin to realize the positional control of crystal grains. We observed the dependence of Ni adsorption density on Ni-Fer concentration with or without electrostatic interaction. The crystal grain density dependence of Ni-Fer concentration and the number of Ni-Fers for single-crystal grain formation has been revealed. The positions of crystal grains have been controlled utilizing electrostatic interaction patterns.