Laser-fired contact formation on metallized and passivated silicon wafers under short pulse durations.

摘要

The objective of this work is to develop a comprehensive understanding of the physical processes governing laser-fired contact (LFC) formation under microsecond pulse durations. Primary emphasis is placed on understanding how processing parameters influence contact morphology, passivation layer quality, alloying of Al and Si, and contact resistance. In addition, the research seeks to develop a quantitative method to accurately predict the contact geometry, thermal cycles, heat and mass transfer phenomena, and the influence of contact pitch distance on substrate temperatures in order to improve the physical understanding of the underlying processes. Finally, the work seeks to predict how geometry for LFCs produced with microsecond pulses will influence fabrication and performance factors, such as the rear side contacting scheme, rear surface series resistance and effective rear surface recombination rates.;The characterization of LFC cross-sections reveals that the use of microsecond pulse durations results in the formation of three-dimensional hemispherical or half-ellipsoidal contact geometries. The LFC is heavily alloyed with Al and Si and is composed of a two-phase Al-Si microstructure that grows from the Si wafer during resolidification. As a result of forming a large three-dimensional contact geometry, the total contact resistance is governed by the interfacial contact area between the LFC and the wafer rather than the planar contact area at the original Al-Si interface within an opening in the passivation layer. By forming three-dimensional LFCs, the total contact resistance is significantly reduced in comparison to that predicted for planar contacts. In addition, despite the high energy densities associated with microsecond pulse durations, the passivation layer is well preserved outside of the immediate contact region. Therefore, the use of microsecond pulse durations can be used to improve device performance by leading to lower total contact resistances while preserving the passivation layer.;A mathematical model was developed to accurately predict LFC geometry over a wide range of processing parameters by accounting for transient changes in Al and Si alloy composition within the LFC. Since LFC geometry plays a critical role in device performance, an accurate method to predict contact geometry is an important tool that can facilitate further process development. Dimensionless analysis was also conducted to evaluate the relative importance of heat and mass transfer mechanisms. It is shown that convection plays a dominant role in the heat and mass transfer within the molten pool. Due to convective mass transfer, the contacts are heavily doped with Al and Si within 10 is after contact formation, which contributes to the entire resolidified region behaving as the electrically active LFC. The validated model is also used to determine safe operating regimes during laser processing to avoid excessively high operating temperatures. By maintaining processing temperatures below a critical temperature threshold, the onset of liquid metal expulsion and loss of alloying elements can be avoided. The process maps provide a framework that can be used to tailor LFC geometry for device fabrication.;Finally, using various geometric relationships for the rear side contacting scheme for photovoltaic devices, it is shown that by employing hemispherical contacts, the number of LFCs required on the rear side can be reduced 75% while doubling the pitch distance and increasing the passivation fraction. Reducing the number of backside contacts required can have a noteworthy impact of manufacturing throughput. In addition, the analytical models suggest that device performance can be maintained at levels comparable to those achieved for planar contacts when producing three-dimensional contacts. The materials and electrical characterization results, device simulations, and design considerations presented in this thesis indicate that by forming three-dimensional LFCs, performance levels of Si-based photovoltaic devices can be maintained while greatly enhancing manufacturing efficiency. The research lays a solid foundation for future development of the LFC process with microsecond pulse durations and indicates that device fabrication employing this method is a critical step moving forward.