Novel Reactor Design And Method For Atmospheric Pressure Chemical Vapor Deposition Of Micro And Nano Sio2 X Films In Photovoltaic Applications

A laboratory-scale reactor and a novel method for the atmospheric pressure chemical vapor deposition (APCVD) of SiO2-x films are developed. The deposited films are investigated to synthesize heterogeneously upon the substrate surface with the elimination of the so-called gas-phase reaction, hence preventing parasitic oxide particles upon the substrate surface and the reactor inner walls. The films are extensively inspected in terms of chemical and optical properties and utilized for crystalline silicon solar cell applications. Simple reactor design with low safety measures, a wide range of deposition rates, high film resilience, and stability for the intended applications are successfully achieved. The newly developed APCVD SiO2-x is proven to protect the Si wafer surface against texturing in alkaline and acidic solutions. Electroplated metallization schemes of heterojunction and passivated emitter rear contact solar cells are examined with the use of the SiO2-x as a masking layer in the grid electrode-free area.

Iron pyrite (FeS2) is a promising material to act as the light absorbing layer in a thin film solar cell. This thesis focuses on the design and optimization of a chemical vapor deposition (CVD) chamber capable of depositing iron pyrite thin films by the reaction of iron acetylacetoneate and tert-butyl disulphide in argon at 300 oC at a base pressure ranging from 10 mTorr to 760 Torr. A custom, as-built 5" CVD system is first characterized by performing experiments attempting to deposit thin films of iron pyrite at a base pressure of 10 mTorr. After initial efforts are unsuccessful, a series of modifications are made to the system, and experiments at both low and atmospheric pressure are pursued. It is found that an external chamber for the iron acetylacetoneate precursor is necessary for better control over its vapor pressure, and that the growth rate must be slow to deposit homogeneous films. Optimal results at atmospheric pressure are achieved when the flow lines of the TBDS vapor, iron acetylacetoneate vapor, and argon carrier gas are combined prior to the deposition chamber.

ABSTRACT: The design and development of a reactor to make this process controlled and repeatable can be accomplished using theoretical and empirical tools. Fluid flow modeling, reactor sizing, low-pressure pumping and control are engineering concepts that were explored. Work on the design and development of an atmospheric pressure cold-wall CVD (APCVD) reactor will be presented. A detailed discussion of modifications to this reactor to permit hot-wall, low-pressure CVD (LPCVD) operation will then be presented. The consequences of this process variable change will be discussed as well as the necessary design parameters. Computational fluid dynamic (CFD) calculations, which predict the flow patterns of gases in the reaction tube, will be presented. Feasible CVD reactor design that results in laminar fluid flow control is a function of the prior mentioned techniques and will be presented.

The high cost of semiconductor fabrication equipment has traditionally represented a large barrier to entry for groups seeking to develop or commercialize novel micro- and nanoscale devices. Much of the cost barrier stems from the large size of the substrates processed in this equipment, and the associated complexity of maintaining consistent operation across the full substrate area. By scaling the substrate size down from the 150-300 mm diameter sizes commonly seen in today's production environments, the capital cost and physical footprint of tools for micro- and nanoscale fabrication can be dramatically decreased, while still retaining a similarly high level of performance. In this work, an ultra-low cost inductively-coupled plasma chemical vapor deposition (ICPCVD) system for processing substrates up to 50.8 mm (2") in diameter is presented. The ICPCVD system is built within a modular vacuum tool architecture that allows sections of the full tool to be easily and inexpensively replaced to adapt to new processing conditions or provide additional functionality. The system uses a non-pyrophoric mixture of silane (1.5% in helium) and low substrate temperatures ( : 150*C) to deposit uniform silicon-based films with a high quality comparable to films deposited in research-grade commercial tools. Using response surface methods, the performance of the ICP-CVD system has been characterized for both silicon dioxide and silicon nitride films, and repeatable control of the deposited film properties, including deposition rate, index of refraction, film stress, and density, has been demonstrated.

A low-pressure chemical vapor deposition (LPCVD) reactor was built in order to implement a low-temperature process to deposit thin-films of silicon and fabricate pn junction photovoltaic devices using disilane as the source gas. This work represents the first reported work on using disilane for fabrication of photovoltaic devices. Films doped with diborane showed high growth rates of approximately 45-150 Å/min for temperatures ranging from 450 to 550 °C. Undoped films were grown and found to have significantly lower growth rates and were not practical at temperatures less than 500 °C. The films were completely amorphous for growth temperatures of less than 500 °C, and crystallinity increased sharply above 500 °C. The optical properties of the films exhibited low optical bandgaps of approximately 1.4-1.1 eV. The conductivity of the doped films was found to be on the order of 10−3 S/cm. Devices were fabricated by depositing p-type layers on n-type crystalline silicon substrates to form pn junctions. Diodes and pn junction photovoltaic devices were fabricated, exhibiting modest but promising performance, and were limited by parasitic series resistance. This research represents the first reported work on fabricating pn junction photovoltaic devices in a low-temperature LPCVD process using disilane, and serves as a solid foundation for future work to improve the process and fabricate novel device structures.