Laser-fired contact formation on metallized and passivated silicon wafers under short pulse durations
Open Access
- Author:
- Raghavan, Ashwin Sreekant
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 08, 2014
- Committee Members:
- Tarasankar Debroy, Dissertation Advisor/Co-Advisor
Todd Palmer, Dissertation Advisor/Co-Advisor
Suzanne E Mohney, Committee Member
S Ashok, Committee Member
Edward William Reutzel, Special Member - Keywords:
- laser-fired contact
silicon wafer
photovoltaic device
heat transfer and fluid flow
laser-material interaction behavior - Abstract:
- The current process used to fabricate localized aluminum-silicon (Al-Si) ohmic contacts on the rear side of dielectrically passivated Si-based photovoltaic (PV) devices has a negative impact on manufacturing throughput and yield. Laser processing is an alternative, non-contact method that can be used to rapidly fabricate laser-fired contacts (LFCs) without sacrificing energy conversion efficiency. However, laser firing is a highly transient process governed by a number of complex physical phenomena, such as rapid heating, melting, and alloying element vaporization, that occur simultaneously over extremely short time and length scales. Therefore, determining the impact of laser processing on LFC formation is not a trivial undertaking. Recent development of the LFC process has centered on the use of nanosecond pulse durations, despite the need for costly control systems, to prevent significant ablation of the Al metallization layer. The use of laser systems capable of microsecond laser pulses can be used to mitigate these issues. In addition, longer pulse durations can be used to form larger, more heavily-alloyed LFCs while preventing significant liquid metal expulsion when laser parameters are carefully selected. In order to assess the viability of using microsecond pulse durations for LFC formation, it is important to determine the influence that laser processing parameters have on contact geometry and passivation layer quality since these factors will directly impact device performance. In addition, due to the complexity of the simultaneously occurring physical processes, a quantitative framework is required to evaluate the physical phenomena occurring during contact formation over a wide range of processing parameters without the need for costly experimentation. The objective of this work is to develop a comprehensive understanding of the physical processes governing 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. In order to achieve these objectives, a combination of experimental characterization and numerical simulations are employed. Advanced materials characterization tools are used to section, polish, and determine the impact of laser processing on the LFC and the surrounding passivated regions. Electrical analysis is performed using physical measurements of contact resistance as well as device simulations to evaluate the influence that laser processing and contact geometry have on contact resistance. In addition, a heat transfer and fluid model is developed to quantitatively understand how changes in processing parameters affect the LFC geometry, temperature, velocity, and concentration fields. Finally, in order to predict the impact of contact geometry on device performance, analytical expressions are developed to account for the contacts produced using microsecond pulses that have with larger interfacial contact areas to determine the rear surface series resistance and effective rear surface recombination velocities. 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 µs 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.